Technologies for POPs Treatment

#### **Chapter 5**

## Type, Sources, Methods and Treatment of Organic Pollutants in Wastewater

*Poslet Shumbula, Collet Maswanganyi and Ndivhuwo Shumbula*

#### **Abstract**

Persistent organic pollutants (POPs), which are synthetic organic chemical compounds, either intentionally or unintentionally produced, have widely aroused public concern in recent years. These chemicals are toxic and major environmental concern due to their persistence, long range transportability, bioaccumulation and potentially adverse effects on living organisms. Uncontrolled inputs combined with poor environmental management often result in elevated levels of persistent organic pollutants in affected estuaries. Since the Stockholm Convention on POPs was adopted, different techniques have been extensively developed. A major focus revealed the need for low cost methods that can be implemented easily in developing countries such as electrochemical techniques. Persistent organic pollutants are known to be resistant to conventional treatment methods such as flocculation, coagulation, filtration and oxidant chemical treatment. However, various advanced wastewater treatment technologies such as, activated carbon adsorption, biodegradation using membrane bioreactor and advanced oxidation processes (AOPs) have been applied in the treatment of POPs.

**Keywords:** persistent organic pollutants, environmental contaminants, dioxins, biodegradation, wastewater treatment

#### **1. Introduction**

In the past decades, the health effects of environmental pollution on the population have been a growing source of worry around the world. According to the WHO (World Health Organization), one-third of the diseases afflicting humanity are caused by extended exposure to pollution. Since World War II, scientists have identified a number of chemical contaminants that are toxic, persistent in the environment, bioaccumulative, and prone to long-range atmospheric transboundary migration and deposition, and are expected to have serious health consequences for humans, wildlife, and marine biota both near and far from their source of emission. These toxins are chemical contaminants, also called the dirty dozen [1]. Being volatile substances, POPs evaporate into the air in warm regions of the globe, are transported by air currents up to cold regions and in mountainous regions where they condense [2, 3].

#### **Figure 1.**

*Classification of persistent organic compounds according to their origin. Picture adapted from [9].*

Most POP chemicals are non-polar organic compounds, consequently hydrophobic, with extremely low water solubility. In marine and terrestrial systems, they bind strongly to solids, particularly organic matter, evading the aqueous segment [4]. They are also lipophilic, which means that they accumulate in the fatty tissue of living animals and human beings. The stockpiling in fatty tissue allows the compound to persevere in biota, where the metabolism rate is low [5–8]. Due to the bioaccumulation and biomagnification phenomena, the POP concentration may be much higher in the tissues of the organisms (up to 70,000 higher concentrations). POP concentrations tend to rise as you travel up the food chain, therefore species at the top of the food chain, such as fish, predatory birds, mammals, and humans, have the largest concentrations of these chemicals and are thus at the greatest danger of acute and chronic harmful effects. POPs are mostly man-made chemical products intended to be used in various areas, for an example, in agriculture and industry, or unintentional by-products resulting from industrial processes, or from waste incineration. Different classes of POPs substances such as organochlorinated pesticides (OCP), polychlorinated biphenyl (PCBs), perfluorinated compounds (PFCs), brominated compounds (BFR), dioxins and furans are known. Most of these substances are anthropogenic origin. However, substances such as dioxins and furans may have natural origin (**Figure 1**), such as volcanic activities and vegetation fires [10–17].

#### **2. Types of POPs**

Many POPs were widely used during industrial revolution after World War II. However, many of these chemicals proved to be beneficial in pest and disease control, but they had unforeseen effects on human health and environment. In Stockholm 2001, representatives from 92 countries have agreed to sign the Stockholm Convention on POPs to reduce and/or eliminate the release of 12 original POP substances. More contaminants have been discovered; the main concern is over the original 12. These contaminants are the 10 intentionally produced chemicals: aldrin, endrin, chlordane, DDT, dieldrin, heptachlor, mirex, toxaphene, hexachlorobenzene (HCB) and PCBs and the two unintentionally produced substances polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans

*Type, Sources, Methods and Treatment of Organic Pollutants in Wastewater DOI: http://dx.doi.org/10.5772/intechopen.101347*

#### **Figure 2.**

*Categories of persistent organic pollutants. Picture adapted from [20].*

(PCDFs) [18, 19]. Another type of interest also classified as persistent organic compounds is polycyclic aromatic hydrocarbons (PAHs). Combustion and burning of organic compounds produces these substances unintentionally. Their occurrences are related to anthropogenic processes, and contamination of PAHs in river sediment is especially serious in high-density industrial areas [18]. Persistent organic pollutants (POPs) are a group of chemicals that have been intentionally or unintentionally produced, and introduced into the environment as shown in **Figure 2**.

#### **2.1 Intentional persistent organic pollutants**

Intentionally produced chemicals currently or were once used in agriculture, manufacturing, disease control or industrial processes. These intentional POPs compounds, shown in **Figure 3**, will be produced as wanted products by different chemical reactions that include chlorine. These types are organic molecules with linked chlorine atoms, high lipophilicity and, usually, high neurotoxicity, and they are called organochlorine compounds. Some of the well-known examples of organochlorine compounds are the chlorinated insecticides, such as dichlorodiphenyltrichloroethane, and polychlorinated biphenyls. They have several compounds which can be divided into two types that are industrial chemicals and organochlorine pesticides [22, 23].

#### *2.1.1 Industrial chemicals*

Polychlorinated biphenyls, very stable mixtures that are resistant to extreme temperature and pressure, are a group of manmade chemicals, oily liquids or solids, clear to yellow in color, with no smell or taste. They have been discovered in water, sediments, avian tissue, and fish tissue all throughout the planet. These chemicals make up a significant subset of special wastes. PCBs are a group of chemical compounds in which the biphenyl molecule has 2–10 chlorine atoms linked to it. When explaining PCBs, monochlorinated biphenyls (i.e., one chlorine atom bonded to the biphenyl molecule) are frequently mentioned. The chemical structure of chlorinated biphenyls is depicted in **Figure 4**. There are 209 distinct PCB congeners in theory. Many of them are resistant to degradation, allowing them to survive for lengthy periods of time in the environment and spread via air and water transport mechanisms [25–27].

Many industrial applications, such as fire-resistant transformers and insulating condensers, relied heavily on PCBs. Prior to 1977, they were utilized as heat exchanger fluids and in the fabrication of aluminum, copper, iron, and steel [27]. Apart from

**Figure 3.** *Intentionally persistent organic pollutants chemical structures. Adapted from [21].*

**Figure 4.** *Industrial POPs chemical structure. Adapted from [24].*

their usage in the above applications, they were also applied as plasticizers in natural and synthetic rubber products, as well as adhesives, insulating materials, flame retardants, lubricants in the treatment of wood, clothing, paper, and asbestos, chemical stabilizers in paints and pigments, and as dispersing agents in aluminum oxide formulations. PCBs are frequently discovered in the effluent and sludge of municipal wastewater treatment plants. Although prohibited in the 1980s, PCBs are presently employed in transformers in some parts of the world, especially Brazilian [28, 29].

*Type, Sources, Methods and Treatment of Organic Pollutants in Wastewater DOI: http://dx.doi.org/10.5772/intechopen.101347*

#### *2.1.2 Organochlorine pesticides*

Organochlorine (OC) pesticides are typically man-made synthetic pesticides widely used all over the world. They belong to the group of chlorinated hydrocarbon derivatives, which have vast application in the chemical industry and in agriculture. Pesticides are a class of chemicals used to kill insects, weeds, fungi, bacteria, and other organisms. Insecticides, fungicides, bactericides, herbicides, and rodenticides are some of the terms used to describe them. The majority of pesticides may kill a wide range of pests and weeds, but some are targeted at specific pests or pathogens. Although these substances are typically man-made, plant derivatives and naturally occurring inorganic minerals are examples of exceptions that occur naturally. Since the first naturally occurring pesticide, nicotine derived from tobacco leaf extracts, was employed to control the plum curculio and the lace bug in the seventeenth century. Many chlorinated hydrocarbon insecticides were created in the 1940s, although they were not widely used until the 1950s. Aldrin, dieldrin, heptochlor, and endrin form part of the reported chlorinated hydrocarbon insecticides. However, in spite of their early promise, these organochlorine insecticides are now much less used because of their environmental pollution impact [30, 31].

Pesticides are employed for many different purposes. Pesticide use has increased due to increased agricultural production, resulting in increased pollution of environmental compartments such as soil, water, and air. Pesticide properties like high lipophilicity, bioaccumulation, long half-life, and potential for long-range transport have enhanced the risk of contamination in air, water, and soil, even after many years of use. This occurrence has the potential to become a long-term hazard to the ecosystem's plant and animal groups' coexistence. Pest problems result in the loss of nearly a third of the world's agricultural productivity each year, despite the fact that pesticide consumption exceeds two millio liters each year. A study by Pimentel showed that only a small percentage (0.3%) of applied pesticides goes into the target pest while 99.7% go somewhere else into the environment [32].

Although the application of organochlorine pesticides has been forbidden for a considerable period in many countries, the residues continue to induce a significant impact on the environment and its ecosystems [33]. Overuse or misuse of pesticides has a negative impact on environmental health as well as ecosystem services. Many aquatic and terrestrial animals, have been documented to be toxicated by pesticides. Pesticides have a negative impact on aquatic ecosystems, including microbes, animals, plants, and fish [34–38].

During the last three or four decades, insecticide manufacturing has been rather constant. Insecticides and fungicides, on the other hand, are the most important pesticides for human exposure in food since they are sprayed just before or after harvesting. Herbicide output has risen as chemicals have increasingly supplanted land cultivation in weed management, accounting for the majority of agricultural pesticides. Large amounts of pesticides have the ability to enter water either directly, as in mosquito control applications, or indirectly, as in drainage of agricultural lands [39–41].

DDT was widely employed during World War II to protect soldiers and civilians from malaria, typhoid, and other diseases caused by insects before its insidious effects on humans and wildlife were discovered. DDT was employed to manage disease after the war, and it was sprayed on a number of agricultural crops, particularly cotton. It did the job, reducing the threat of malaria and the loss of income to the agriculture industry [42]. DDT continues to be applied against mosquitoes in several countries to control malaria. Its stability, its persistence, and its widespread use have meant that DDT residues can be found everywhere; residual DDT has even been detected in the Arctic.

#### **2.2 Unintentionally POPs**

Unintentionally produced chemicals (see **Figure 5**) are a result of combustion of medical waste, incarnation and some industrial processes. They are divided into three types, viz., polycyclic aromatic hydrocarbons (PAHs), dioxin and furan compounds.

#### *2.2.1 Polycyclic aromatic hydrocarbons (PAHs)*

PAHs are ubiquitous group of several hundreds of chemicals that comprise two or more fused benzene rings in linear, angular or cluster arrangements, containing only carbon and hydrogen. The central molecular structure is held together by stable carbon-carbon bonds. They are mostly caused by incomplete combustion of natural or man-made fuels such as coal and wood, as well as vehicular pollutants and cigarette smoke [44]. Dietary exposure accounts for more than 70% of human exposure in non-smokers [45]. According to a dietary survey conducted in the United Kingdom, cereals and oils/fats account for a significant portion of PAH intake [46]. Typical PAH contamination occurs when food is subjected to combustion products in technical procedures such as direct fire drying [47]. High PAH concentrations in charcoal grilled/barbecued foods may also result from certain traditional home cooking methods such as grilling, roasting, frying, and smoking [48]. However, the greatest amounts of PAHs released into the environment are via anthropogenic processes like fossil fuel combustion and by-products of industrial processing. The Environmental Protection Agency (EPA) of the United States included 16 PAHs on a priority pollutants list because they are considered potential or probable human carcinogens. As a result, their dispersal and the likelihood of human exposure have received a lot of interest. PAHs have been found in soil, air, and sediments, as well as on a variety of food and beverage products [49–51].

#### *2.2.2 Dioxins and dibenzofurans*

Polychlorinated dibenzo-*p*-dioxins (PCDDs), dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) constitute three groups of relevant persistent organic pollutants with enhanced chronic toxicity. PCDD/Fs (**Figure 6**) are emitted by a

**Figure 5.** *Unintentional produced POPs chemical structures. Adapted from [43].*

*Type, Sources, Methods and Treatment of Organic Pollutants in Wastewater DOI: http://dx.doi.org/10.5772/intechopen.101347*

#### **Figure 6.**

*Molecular structures of chlorinated dibenzo-p-dioxins (dioxins, PCDDs) and dibenzofurans (furans, PCDFs). Adapted from [52].*

variety of human activities and industrial processes, and can be referred as undesirable by-products. PCBs are ubiquitous environmental pollutants as a result of their large-scale manufacture till the end of the 1980s and their continued use. PCDD/Fs and PCBs can also be released from stationary sources such as waste incineration and biomass and fossil fuel combustion. PCDD/Fs and PCBs can be considered environmental markers of anthropogenic activities in light of this information, as their occurrence is invariably linked to human activities. PCDDs and PCDFs, commonly called "dioxins", are two classes of "quasi-planar" tricycles aromatic ethers with 210 different compounds (congeners) in total [53].

PCDDs and PCDFs are solids at room temperature and have a rather low volatility. Dispersion in the atmosphere is thus likely to occur mainly in particulate aerosols. The PCDD/F have been of concern for decades because of their toxic properties. A structurally similar series of compounds, the chlorinated dibenzofurans (furans), have similar chemical properties and toxic effects. The most toxic and most extensively studied representative of the chlorinated dioxins (PCDDs) is 2,3,7,8-tetrachlorodibenzo-*p-*dioxin (TCDD). In recent years there has been a growing trend to include a specific subgroup of PCBs, the so-called dioxin-like PCBs (**Figure 5**) which has finally been added to methods along with the dioxins and furans. It is widely acknowledged that man-made sources and activities contribute far more to the environmental burden of PCDDs and PCDFs than natural processes, particularly since the 1930s, when environmental levels have steadily increased in tandem with the large-scale production and use of chlorinated chemicals [54, 55]. Chemical processes, combustion processes, and secondary sources are the three primary categories of man-made sources of PCDDs and PCDFs [56].

#### **3. Sources of POPs**

In the past decades, many reports on the dependents of POPs by industry and agricultural sectors were seen. POPs proved to be beneficial in pest and disease control, crop production, and industrial applications. Many were widely used commercially during the boom in industrial production after World War II, resulting in wide geographical distribution. **Figure 7** shows some of the sources related to POPs [57].

POPs are extremely stable in all environmental elements. They are discharged into the atmosphere through a variety of industrial sources, including power plants, heating plants, and incinerating facilities, as well as from domestic furnaces, transportation, agricultural sprays, and evaporation from water surfaces, soil, and landfills. Other sources of POPs compounds, such as inadvertent generation, can be present in incinerations, chemical plants, other combustions, forest fires, putrefaction, and PCB-containing wastes. This type of trash can be found in a variety of

#### **Figure 7.**

*Schematic depicting POPs in the environment and main environmental processes during long-distance atmosphere transport, bioaccumulation, and biomagnification. Adapted from [57].*

places and stems from a variety of activities, such as the use of old oil, equipment repair and maintenance, and building destruction [58, 59].

Wastewaters from plants generating or using POPs, as well as runoff from fields and roads, and atmospheric deposition, are the origins of pollutants, oil, fates, liquid fuels, dirt, ash, and silt entering the water system. Oceans and seas are their greatest reservoirs, where they collect from river sediments, air deposition, trash disposal, and accidents. They are retained in sediments on the bottoms of seas,

#### **Figure 8.**

*Conceptual model for the behavior of persistent organic pollutants in the air-plant soil system. Adapted from [60].*

oceans, and huge lakes, where they can be released and re-enter the atmosphere after a period of time, as indicated in **Figure 8** [42, 60].

#### **4. Methods for treatment of persistent organic pollutants**

POPs have adversely posed a health concern worldwide for ages. Due to their concerning health issues, some countries have resorted to reduce the use of chemicals or processes that produce POPs, while others have prohibited them entirely. However, most processes that result in the production of POPs are beneficial to both human and environmental health. This is because some POPs are produced during production of synthetic chemicals for crop production, medication, clothing etc. In addition, some POPs are unavoidable since they may be produced unintentional from simple combustions. Since most of these POPs end up in water streams, various methods for wastewater treatment have been implement and reported in literature. These methods ensure the conversion of wastewater into portable water by removing harmful and hazardous chemicals [61]. Conventional treatment refers to some of the most effective water treatment procedures used in the service and distribution of industrial or municipal potable water. At different stages of a typical treatment method, any of the physical, chemical, or biological channels provide good combination.

Preliminary, primary, secondary, and tertiary wastewater treatment stages are in sequence of increasing treatment level, with final pH adjustments as needed. The chosen conventional approach must be able to meet the regulatory authority's recommended microbiological and chemical criteria while operating and maintaining at a low cost [62].

Conventional treatment methods such as flocculation, coagulation, filtration, and oxidant chemical treatment are ineffective against POPs. The chemical properties of POPs, such as, low water and high fat solubility, stability to all degradation processes and low vapor pressure, are the main components for their efficiency as pesticides and for their persistence in the environment [63]. The inability in some instances to remove POPs from wastewater using conventional methods have prompted scientists to develop other methods. Various advanced wastewater treatment technologies such as, activated carbon adsorption, biodegradation using membrane bioreactor [64] and advanced oxidation processes [65] have been applied in the treatment of POPs. This is because of growing number of emerging POPs that are being identified in water and the concerns that are accompanied by human and environmental health hazard [66]. Various setbacks such as cost, sophisticated instrumentation, low degradation efficiency, generation of toxic secondary chemicals and massive sludge production have recently been addressed using advanced methods and technologies. Below is the short discussion of biodegradation and advanced oxidation processes wastewater treatment technologies.

#### **4.1 Biodegradation**

Biodegradation is an evolving technology that comprises the application of selected living microorganisms to degrade, metabolize/immobilize any unwanted substances such as pesticides, organic pollutants and hydrocarbons from soil and water, to improve its quality [67]. Although every microorganism has the ability to eradicate pollutants, only few particular or engineered microorganisms are used broadly to eradicate pollutants efficiently. Bioremediation technology, applied in perspective to POPs removal, takes into consideration the following methods: (1) bioventing: aerating water to stimulate *in situ* biodegradation of organic

contaminants and promote bioremediation, (2) biostimulation: modification of contaminated media to provide the nutrition to soil microbiota by adjusting pH, addition of limiting nutrient to improve C: N: P ratio, and (3) bioaugmentation: addition of microbial community (bacteria and fungi) and any biocatalyst (gene and enzyme) to degrade organic/inorganic pollutants [68]. One of the most important variables in the efficient breakdown of petrochemical wastes in a given ecosystem is microorganism selection. It is because only those microbial species are adapted to work in that specific habitat. Likewise, intermediates created during photocatalytic degradation processes are harmful to a variety of creatures in the environment [69].

Currently, the membrane bioreactor does not always achieve the desired results in the treatment of POPs, and it performs poorly in the removal of non-biodegradable aliphatic and aromatic hydrocarbon compounds, halogenated organic compounds, organic dyes, pesticides, and phenols and their derivatives. The process technicalities and economic feasibilities are the two most significant assessment elements for achieving the goal in wastewater treatment technology [70].

#### **4.2 Advanced oxidation processes**

The use of conventional methods is not wholly accepted nowadays because of the high costs and operational problems. Consequently, it is necessary to adopt modern systems like advanced oxidative processes (AOPs) [71]. Some of the AOPs' characteristics include: (1) potential capacity for mineralization of organic pollutants to carbon dioxide and water, as well as oxidation of inorganic compounds and ions such as chlorides, nitrates, and others; (2) non-selective reactivity with the vast majority of organic compounds, which is particularly appealing to avoid the presence of potentially toxic by-products from the primary pollutants that can be produced by other methods that do not achieve complete oxidation [65]. Some of the AOPs discussed below.

#### *4.2.1 Catalysts in advanced oxidation processes*

AOPs have successfully used both homogeneous and heterogeneous catalysts. Heterogeneous systems have obvious advantages over homogeneous systems, such as the ability to separate the catalyst easily for reuse from the treated water, the lack of a secondary treatment to remove dissolved metals from the treated water, and the ability to withstand extreme operating conditions. The system is also effective over a broader pH range including the common pH for natural water and wastewater (pH 2–9) [72].

The AOPs, as water treatment processes, are performed at pressure and temperature close to environmental conditions. They involve the generation of hydroxyl radicals in sufficient quantity to interact with the organic compounds of the medium. Hydroxyl radicals are the best of the powerful oxidants because they meet a number of criteria, including: (1) they do not generate additional waste; (2) they are not toxic and have a short lifetime; (3) they are not corrosive to equipment; and (4) usually produced by easy-to-manipulate assemblies [73]. The following are some of the most common approaches used for this purpose: UV alone, UV/H2O2, UV/Fe3+, UV/H2O2/Fe3+, UV/O3, UV/S2O8 2−, UV/TiO2, UV/chlorine and UV in combination with other photocatalysts. The major issue is the removal efficiency of specific target contaminants by the UV AOPs. UV AOP removal rates vary depending on the molecular structure of the pollutants, both in terms of direct photolysis and radical processes. Furthermore, water matrix effects have a significant influence on removal rates. As a result, each UV AOP system must be individually controlled

#### *Type, Sources, Methods and Treatment of Organic Pollutants in Wastewater DOI: http://dx.doi.org/10.5772/intechopen.101347*

in line with its water matrix and targeted contaminant removal for optimal POPs control [74]. In most situations, the UV/chlorine oxidation process outperformed UV alone or chlorination, according to Xiang et al. [75]. During the UV/chlorine reaction, hydroxyl and Cl radicals were produced, with the hydroxyl radical taking the lead in the oxidation process. Its contribution to the rate of diuron degradation was calculated to be 28.95%.

#### *4.2.2 Photo-Fenton oxidation*

Most AOPs use a combination of oxidants and irradiation (O3/H2O2/UV) or a catalyst and irradiation (Fe2+/H2O2; UV/TiO2) to achieve their goals. The disadvantages that make them economically undesirable vary depending on the AOP are: (1) high electricity demand (for example, ozone and UV-based AOPs), (2) relatively large volumes of oxidants and/or catalysts (for example, ozone, hydrogen peroxide, and iron-based AOPs), and (3) pH operating conditions (e.g. Fenton and photo-Fenton) [76]. Photo-Fenton oxidation system has been identified as a feasible oxidation system for treating these wastewaters. In Fenton and Fenton-like reactions, hydroxyl radicals are usually generated from H2O2 catalyzed by iron (Fe2+, α-Fe2O3, Fe3O4, H2Fe2O4, α-FeOOH, etc.) [77]. Nonetheless, the cost effectiveness is one of the major concerns. However, the cost reduction can be obtained through application of heterogeneous catalysts, chelating agent, solar energy and integration with biological treatment technologies [78].

#### *4.2.3 Electrochemical oxidation processes*

Electrochemical oxidation procedures, among the numerous AOPs, are gaining popularity for water and wastewater decontamination due to their low cost and high efficiency. Dissolved organic contaminants are primarily oxidized in electrochemical oxidation processes by (i) direct anodic oxidation on the anode surface via charge transfer, and (ii) interaction with physio- and/or chemisorbed hydroxyl radical produced during water oxidation [79]. Electrochemical AOPs have been widely explored for the total degradation of POPs. The electrochemical oxidation is an effective and environmentally friendly technology because it does not require chemicals, only electric current is consumed. The first one is direct oxidation which occurs when the compound reacts directly at the anode's surface or by physisorbed or chemisorbed •OH. The second mechanism is indirect oxidation, which is achieved through the electrochemical generation of a mediator in the bulk solution such as ozone (O3), hydrogen peroxide (H2O2), active chlorine, active bromine or S2O8 2−, among others [80].

Recently, coupling approaches including an electrochemical pre-treatment followed by a biological process have been proposed as cost-effective and reliable remediation methods for persistent chemicals mineralization. This opens the door to more selective electrochemical methods than those involving hydroxyl radicals do, because the goal of the pre-treatment is no longer to achieve total mineralization of non-biodegradable species, but rather to improve their biodegradability by focusing on functional groups that have been shown to reduce biodegradability [81].

#### *4.2.4 Nanofibers*

In the one-time elimination of POPs, nanofibers have demonstrated to be the most effective. These adsorbents, on the other hand, demonstrate adaptability in the collection of pollutants. The use of fiber layers with varied pore channels and surface chemistry to produce selectivity for a target chemical could be researched further. Because adsorption is a common water treatment method, the production and operational costs of adsorbent materials are crucial to the introduction of any new classes of materials [82]. Physically and chemically stable carbon-based materials alone (without metals) have also been successfully used as the electro-catalysts [83]. Inexpensive, non-noble transition metals or their oxides supported in carbon nanotube has been reported for treatment of POPs. Bismuth-based nanocomposites [84], copper-reduced graphene oxide electrode [85], boron-doped diamond [86], with different boron and substrate silicon or niobium content [87] have indicated to be an efficient technology for treating POPs wastewater.

#### **5. Conclusions**

The POPs are organic compounds of anthropogenic origin, and are resistant to environmental degradation through chemical, biological, or photolytic processes and as a result, accumulate in the food chain. Contamination by POPs is widespread, and circulate globally via the atmosphere, oceans, and other pathways. The Stockholm Convention defines criteria for new POP candidates in terms of their persistence, long-range transport, bioaccumulation and toxicity. Recognizing the dangers of POPs, countries began limiting their production, use, and release. This global, legally binding agreement is to reduce and eliminate the release of 12 POPs, including pesticides and industrial chemicals, as well as unintentionally produced POPs. Conventional water treatment facilities have failed to effectively degrade persistent contaminants from wastewater. However, advanced water treatment options such as activated carbons, membrane bioreactors and advanced oxidation processes are well documented for their capital intensive treatment of these recalcitrant pollutants.

#### **Acknowledgements**

The authors would like to thank the University of Limpopo for the financial assistance towards this project.

#### **Conflict of interest**

Authors report no conflict of interest.

*Type, Sources, Methods and Treatment of Organic Pollutants in Wastewater DOI: http://dx.doi.org/10.5772/intechopen.101347*

### **Author details**

Poslet Shumbula1 \*, Collet Maswanganyi1 and Ndivhuwo Shumbula2

1 University of Limpopo, Polokwane, South Africa

2 University of Witwatersrand, Johannesburg, South Africa

\*Address all correspondence to: poslet.shumbula@ul.ac.za

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 6**

## Nonthermal Mechanochemical Destruction of POPs

*Giovanni Cagnetta and Mohammadtaghi Vakili*

#### **Abstract**

The present chapter is dedicated to all relevant theoretical and application aspects of mechanochemical destruction technology for mineralization of POPs, both stockpiled ones and as contaminants in environmental and waste matrices. It will show that such solid-state technology, realized by high energy milling of POPs with a co-milling solid reagent, can achieve complete mineralization of haloorganics into graphitic/amorphous carbon, carbon oxides, and halides; it takes place at near environmental temperature, thus limiting unintentional formation of dioxins (if treatment conditions are selected carefully); and, in some cases, it can be used to produce useful materials instead of just detoxified waste. The chapter will also give a comprehensive picture of complex mechanochemical destruction mechanism, including mechanochemical activation of the co-milling reagent and the cascade of radical reactions that cause POP molecules mineralization. Finally, technological and economic considerations will be provided, which corroborate the validity and feasibility of the mechanochemical destruction as an effective and safe technology to treat POPs.

**Keywords:** mechanochemistry, high energy milling, POPs mineralization, nonthermal technology, waste detoxification

#### **1. Introduction**

As a consequence of their classification as POPs by the Stockholm Convention and prohibition of their use, a number of already manufactured toxic chemicals have become obsolete. Many countries, especially developing ones, do not have the economic and/or technological capacity to dispose such waste materials in proper manner, that is, ensuring their mineralization to non-toxic form. Hence, obsolete chemicals are just stockpiled, often in poor conditions that cannot avert secondary contamination.

Currently, (high temperature) incineration is the sole largely available technology for efficient and economic POPs destruction. However, during combustion notable amounts of hydrogen halide gases are generated, which corrode facility structural elements and, therefore, heighten maintenance cost. Most importantly, risk of unintentional formation of dioxins is never null for such kind of plants. Consequently, new technologies alternative to combustion are highly needed. They must insure complete mineralization of POPs, even at high concentrations, and prevent secondary formation of new POPs [1]. Among the nonthermal alternatives, mechanochemical treatment is considered a valid option for POPs destruction.

#### **1.1 Fundamentals of mechanochemistry**

Mechanochemistry is a branch of chemistry that deals with physical and chemical transformations undergone by materials (eminently solids) that are induced during the action of mechanical forces (shear and compression), or are triggered by them [2]. The earliest known example of mechanochemical reaction is friction of two flints that originates sparks. Flint is a variety of quartz, so its scraped surfaces expose radicals that violently react with air producing sparkling plasma. The first mechanochemical experiments are acknowledged to Walthère-Victor Spring (1880–1911), who obtained barium carbonate by solid-state reaction between barium sulfate and sodium carbonate under high pressure; and Mathew Carey Lea (1823–1897), who demonstrated that heating and friction can induce, in some cases, diverse chemical transformations [3]. Today knowledge on mechanochemical phenomena is rather advanced, as well as development of special mechanochemical reactors that can be used to provide high mechanical energy input to solid systems.

A number of physicochemical phenomena may occur to solids under the action of mechanical forces. Some of them are quite unique, like emission of electrons [4] and light [5], while others are commonly experienced such as particle breakage and heating [6]. In truth, a complete classification of the various mechanochemical phenomena is hardly compilable (although some attempts were done [2]). The reason is that often it is not facile to distinguish properly said mechanochemical phenomena (those that occur while the mechanical force acts on the solids) from those that are triggered and/or facilitated by mechanical forces. In addition, it should be noted that every phase transition or emergence of new phases with diverse specific volume (due, for example, to chemical reactions) causes mechanical stress in the solid, thus possibly having mechanochemical effects on the system [7].

Currently, the "mechanochemical activation" theory is considered a reliable explanation of the evolution of solid systems under the action of (sufficiently intense) mechanical forces. Briefly, it hypothesizes that atoms or molecules are shifted from their equilibrium crystal lattice positions by mechanical stress, thus accumulating potential energy (**Figure 1**). This brings the solid in a high-energy metastable state that must release exceeding energy. Common relaxation pathways are heating and particle fracture. But, when particles reach a critical size, solid materials begin to build up crystal defects, develop amorphous phases or other crystalline morphologies, and chemical reactions might take place. Such processes are jointly named "mechanochemical (or mechanical) activation" of solids [8]. Mechanochemically activated solids are more prone to react with other chemicals and can give origin to reactive species.

#### **1.2 Mechanochemical reactors**

Action of mechanical forces on solid materials is realized in special mechanochemical reactors. In general, the main effect of these forces is particle breakage and, therefore, the large majority of such machines were originally designed to comminute particles. Consequently, high energy mill is usually considered synonym of mechanochemical reactor. A large number of milling machine typologies is available on the market that can apply mechanical forces with different intensities and by diverse means [9]. Principally, these machines can be classified according to the main type of action in three groups [10]:

1.Shocking mills, where the force derives from high speed impact of the material particles onto elements of the mill. In pin disintegrators, particle hits rotating

#### **Figure 1.**

*Main processes occurring during the action of mechanical forces on solids according to the "mechanochemical activation" theory.*

blades; in jet mills, a fluid (air) accelerates the particles and throws them against a target.


Operating parameters of each type of mill mainly control the rate of provision of mechanical energy (i.e., milling intensity) to the milled material, as well as the efficiency of the energy transfer (i.e., the amount of energy that is effectively accumulated by the solid, compared to that dissipated by heat). The quantity of mechanical energy that is accumulated by a unitary mass of milled solid is often referred to as 'specific energy dose'. It has been amply ascertained that transformation degree undergone by a mechanochemical system mainly depends on the total energy dose that is transferred to the system by the high-energy mill, independently from the milling intensity. In other words, the accumulated energy is invariant for a specific mechanochemical system [11]. Indeed, it was also proved that the modality of energy provision, that is, the type of high energy mill has, within certain tolerance ranges, limited influence on the transformation degree [12]. This fact points out that scaling up of mechanochemical processes is relatively facile and is not necessarily done just by trial and error. Taking advantage of the invariance of mechanochemical systems, provision of the same amount of energy dose in high

energy mills with diverse scale is a good starting point to achieve similar transformations and transfer the process from small scale to large one.

#### **2. Mechanochemical destruction of POPs**

In 1994, Rowlands et al. [13] demonstrated that high energy milling (HEM) can effectively destroy dichlorodiphenyldichloroethane (DDT) in presence of CaO as co-milling reagent. They obtained almost entire dechlorination of DDT, which, as ascertained in subsequent works, was transformed into halides and amorphous/ graphitic carbon. Since that groundbreaking work, efficacy of mechanochemical destruction (MCD) obtained by HEM of many toxic organohalogens, included all POPs, has been confirmed [14] (**Figure 2a** shows some examples with CaO as comilling reagent). In particular, a number of other co-milling reagents and related optimal milling conditions have been investigated, as well as their mechanical activation. Moreover, key aspects of organics mineralization mechanism have been ascertained.

#### **2.1 Treatment conditions of mechanochemical destruction**

Co-milling reagent is certainly the most important component of MCD reactions. Theoretically, POPs can be degraded by the sole action of mechanical forces [22], but it would require a long time (i.e., high energy consumption) and could only achieve incomplete mineralization. Differently, co-milling reagent boosts the reaction rate and assures the complete transformation of POPs into inorganic form (usually, halides and carbon). HEM facilitates formation of fresh surfaces on reagent particle due to their breakage, as well as mixing and contact with POPs, thus accelerating the solid-state reaction. Moreover, reagents can be activated by the mechanical energy provided by the mechanochemical reactor, thus enabling or heightening their reactivity (see Section 2.2). Reagents can be classified in four groups:

1.Reducing reagents, like zero valent metals (e.g., Fe, Al, Zn, and Mg) and hydrides (e.g., CaH2, NaBH4, and LiAlH4).

2.Oxidants, as manganese dioxide (MnO2), persulfate (S2O8 <sup>2</sup>), and ferrate (FeO4 <sup>2</sup>).

#### **Figure 2.**

*(a) MCD of some haloorganics (Hexabromocyclododecane [15], Dechlorane plus [16], Hexachlorobenzene [17],Trichlorobenzene [18], and γ-Hexachlorocyclohexane [19]) co-milled with CaO (dashed lines are obtained by interpolation of experimental data with the model of ref. [20]). (b) MCD kinetics invariance respect to milling intensity of hexachlorobenzene co-milled with CaO [21].*


Reagent-to-pollutant ratio (often calculated as mass ratio) is one of the most critical parameters of MCD treatment. It governs the reaction kinetics: within certain ranges, the MCD kinetic constant for a specified POPs-reagent system is directly proportional to such ratio, that is, the higher is the ratio, the faster is the reaction [21]. An exceedingly high reagent ratio, however, decreases the energy efficiency (the amount energy spent to achieve a certain POPs dehalogenation/mineralization percentage) of the treatment, having a negative economic impact.

Milling operation parameters also have a notable influence on the reaction progress. Each HEM device has a number of such parameters that can be modified for the same equipment. In general, some of them are related to geometrical feature of the milling device such as milling chamber dimensions, milling tool (i.e., ball) dimension, and chamber filling ratio; and others are properly said operating parameters, like milling jar speed (e.g., rotation speed or vibration frequency) and ball-to-powder charge ratio. All of them control the amount of the mechanical energy that is inserted in the MCD system and, consequently, its kinetics, but the operating parameters have surely the most relevant effect. It was amply verified that each parameter has an optimal value that maximizes the reaction rate for a specified MCD system [14], which is due to the energetic efficiency of the milling tool impacts. Moreover, it was ascertained as well that MCD systems are invariant respect to the energy provided to the system. In other words, the pollutant degradation conversions are the same for the same amount of mechanical energy inserted, independently from the milling intensity (**Figure 2b**). Hence, MCD results, within certain ranges of tolerance, can be reproduced on any type of HEM device [21].

#### **2.2 Mechanochemical activation of the co-milling reagent**

Primary role of HEM is to ensure intimate contact among POPs molecules and the co-milling reagent. In particular, products of the mechanochemical reaction (e.g., carbon), in addition to side-products deriving from interaction with air, milling tools, etc. (e.g., passivating oxide layer on zero valent metals), are removed by the continuous particle fracture, thus exposing fresh surfaces available for further reaction. Such effect is sufficient for reactive materials, like zero valent metals, hydrides, and strong oxidants or bases, which, in some cases, can degrade haloorganics even by simple manual grinding [23].

Other kinds of reagent are efficient thanks to the physicochemical transformations they undergo during HEM. In these cases, the elevated energy input of HEM devices induces the formation of active species, mainly electrons and radicals, that are responsible for POPs mineralization. Metal oxides are known to be very efficacious co-milling reagents. The oxide anion on particle surfaces, thanks to the mechanical energy, generates in the crystal lattice an oxygen vacancy (*VO*) with two trapped electrons and an oxygen atom that is released in gaseous form (Eq. (1)). This reaction passes through a step of electron release from the oxide anion to form an oxide radical (O�<sup>∙</sup> ):

$$\bullet \text{ } \text{ $ } ^{2-} \xrightarrow{\Delta E\_M} \text{$  } \text{ $ } ^{-\bullet} + e^- \xrightarrow{\Delta E\_M} V\_O + 2 \text{ } e^- + \frac{1}{2} \text{O}\_2 \text{$  } ^\uparrow \tag{1}$$

Generation of trapped and free electrons in CaO + chlorobyphenyl system, as well as the existence of the oxide radical, was ascertained by electron paramagnetic resonance [24]. Moreover, using the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) as probe, it was verified that electron generation on metal oxide surfaces under HEM is constant and follows a pseudo-zeroth order kinetics [25]. Electrons and oxide radicals are both responsible of haloorganics mineralization, as illustrated in subsection 2.3.

Another class of co-milling reagents that are remarkably reactive under HEM conditions is that of quartz (SiO2), alumina (Al2O3) and silico-aluminates (including clays, etc.). Most of such minerals are known to be plasma-formers, that is, their new surfaces created by particle breakage are rich of electrons [26], which interact with the organic pollutant and mineralize it. For example, in silica-based minerals electrons come from the homolytic cleavage of Si–O bonds, producing silyl (Si<sup>∙</sup> ) and siloxyl (SiO<sup>∙</sup> ) radicals [17]. Experiments with DPPH probe suggest that generation kinetics of such species follows the pseudo-first order [27].

Persulfate, a strong oxidant that is being widely utilized for advanced oxidation of organic pollutants in solution, has been proficiently used as co-milling reagent for POPs destruction. As in solution, it is transformed in sulfate radicals (SO�<sup>∙</sup> <sup>4</sup> ) by the mechanical energy, which then oxidize the haloorganics. In presence of strong bases or electron donors, such conversion is faster, thus increasing the overall mineralization rate [28, 29].

#### **2.3 Mechanochemical mineralization of POPs**

The active species generated by mechanochemical activation of co-milling reagent, or the reagent itself, interact with POPs molecules triggering and sustaining their degradation, eventually to mineral form. It was recently proposed that the mechanochemical activation of co-milling reagents and the mineralization of haloorganics are kinetically independent, which, in turn, suggests that both processes do not interfere significantly one with the other during HEM [20]. Since reactive species are of two types (i.e., electrons and radicals), their attack generally provokes expulsion of a halide and transformation of the haoloorganics into radical form (**Figure 3**). For example, it was suggested that the first step of sulfonated perfluoroalkyl substances (e.g., perofluorooctane sulfonate) MCD with La2O3 is cleavage of the polar group by addition of oxide radical to form perfluorinated moiety radical and sulfate (Eq. (2)) [30]:

$$\bullet \text{C}\_{8}\text{F}\_{17} - \text{SO}\_{3}^{-} + \text{O}^{-\*} \rightarrow \bullet \text{C}\_{8}\text{F}\_{17} + \text{SO}\_{4}^{2-} \tag{2}$$

Then, the perfluorinated moiety could undergo further oxidation by reaction with the oxide radical through a so-called CF2 flaking-off process to form COx (or carbonates) and fluorides (Eq. (3)); or it could be reduced by electron addition followed by fluoride expulsion and generation of graphitic/amorphous carbon (Eq. (4)).

$$\cdot \text{C}\_{8}\text{F}\_{17} + 2 \text{ O}^{-\text{-}\text{}} \rightarrow \cdot \text{C}\_{7}\text{F}\_{15} + \text{CO}\_{2} + 2 \text{ F}^{-} \tag{3}$$

$$\cdot \text{C}\_{8}\text{F}\_{17} + 2\text{ }e^{-} \rightarrow \cdot \text{C}\_{7}\text{F}\_{15} + \text{C} + 2\text{ }\text{F}^{-} \tag{4}$$

Likewise, hexachlorobenzene dechlorination in presence of CaO might proceed by capture of an electron from the oxide surface to generate pentachlorobenzyl radical (Eq. (5)) or by substitution of one chlorine with oxide radical to form a pentachlorophenoxyl radical (Eq. (6)) [31]:

*Nonthermal Mechanochemical Destruction of POPs DOI: http://dx.doi.org/10.5772/intechopen.101088*

**Figure 3.** *Generic reaction scheme of MCD process.*

$$\rm{C}\_{6}\rm{Cl}\_{6} + e^{-} \rightarrow \rm{C}\_{6}\rm{Cl}\_{5} + \rm{Cl}^{-} \tag{5}$$

$$\rm{C}\_{6}\rm{Cl}\_{6} + \rm{O}^{-\cdot} \rightarrow \rm{C}\_{6}\rm{Cl}\_{5}\rm{O} + \rm{Cl}^{-} \tag{6}$$

The mineralization process proceeds in similar fashion by addition of electrons to generate chlorides and graphitic/amorphous carbon, or by oxide radical attack to produce carbon oxides and chlorides.

It can be seen that generation of anion and organic radical as products of active species attack appears to be the general rule for MCD triggering processes. Then, the organic radicals are ultimately transformed into graphitic and amorphous carbon under the attack of electrons, or carbon oxides by addition of oxide radicals. Both redox processes occur at the same time, which is a distinct feature of mechanochemical reactions of organics [32]. Nevertheless, the mineralization process is not that plain (**Figure 3**). A number of secondary radical reactions have been observed in MCD systems, like de�/hydrogenation, oligomerization or radical addition, rehalogenation, etc. They are typical radical reaction that take place

among the organoradicals generated during the MCD process. Such by-products, however, are eventually destroyed following the mineralization pathways mentioned above.

Finally, it should be reminded that some types of reagent are per se highly reactive, so they do not necessitate of any mechanochemical activation and the effective mixing realized in the HEM is sufficient to induce the reaction. Obviously, the mineralization depends on the specific reagent. For instance, highly electropositive metals (Na, Mg, etc.) dehalogenate POPs, keeping their organic structure almost intact [33], while less electropositive zero valent metals such as iron produce graphitic/amorphous carbon, probably due to a less effective electron transfer rate, so that the original carbon skeleton is destroyed [17]. Another example is the notable efficacy of KOH to defluorinate perfluoroalkyl substances under HEM. After splitting the polar group, hydroxide anions sequentially substitute fluorides in the perfluorinated moiety, causing shortening of the organic chain by CF2 flake-off to generate formate [34].

#### **3. Application to stockpiled POPs**

Laboratory results on MCD with various reagents can be easily applied to the disposal of stockpiled POPs, which often are a mixture of congeners and/or byproducts of the manufacturing process. Anyway, such components have similar reactivity under HEM conditions. Preliminary scaling-up from laboratory-scale to large one can be done by taking advantage of the energetic invariance of mechanochemical reactions (as mentioned in subsection 1.2). Yet, pilot-scale testing could be necessary, especially if the experimental results are translated to a large milling equipment with different type of action, compared to the laboratory one. Choice of the reagent is a vital issue: it should be cheap, easily suppliable, durable, etc., but, its most important feature is efficacy. Efficacious reagent can be utilized with low reagent-to-pollutant ratio (which has a correlation of direct proportionality with MCD rate), so that the energy consumption per mass of treated POPs is contained. Unfortunately, most of the cheapest and largely available co-milling reagents (e.g., CaO, SiO2, Al2O3, Fe, etc.) are not so efficacious and necessitate of large reagent ratios [14]. In order to obviate to this problem, two strategies have been proposed to dispose stockpiled POPs: multi-reagent approach and waste-to-materials one.

#### **3.1 Multi-reagent approach**

The multi-reagent approach is simply based on mixing two or more cheap comilling reagents that, because of their specific physical or chemical properties, have a synergistic interaction that boosts the MCD rate. A typical example of taking advantage of physical properties is the case of mixing a soft reagent (e.g., zero valent metal, metal oxides) with a hard material (e.g., silica, alumina) to improve millability of the former. During HEM, soft material particles reach rapidly the critical size and cannot be further comminuted, therefore their specific surface remains unvaried during the MCD treatment. Moreover, if the reagent is plastic (like metals), the phenomenon of cold-welding hinders particle size reduction and thus the reactivity. Addition of a hard component to the mixture helps an effective fracture of the soft material particles, which are crushed on those of the hard component, forming smaller particles that cover the hard ones. An example is given by the case of zero valent iron (Mohs hardness of 4-5), which had a very poor effectiveness in destroying hexachlorobenzene (**Figure 4a**) [17]. Addition of quartz sand (Mohs hardness of 7), which itself performed better than iron in mineralizing

*Nonthermal Mechanochemical Destruction of POPs DOI: http://dx.doi.org/10.5772/intechopen.101088*

**Figure 4.**

*Influence of multi-reagent composition on two MCD systems: (a) hexachlorobenzene high-energy milled with Fe-SiO2 [17], and (b) hexabromocyclododecane high-energy milled with Na2S2O8-NaOH [35].*

the haloorganic, enhanced notably degradation conversion of hexachlorobenzene. Varying the composition of the Fe-SiO2 mixture, it can be seen that with low silica fractions, such mixture is still scantily effective, since the few SiO2 crystals are incorporated into the iron cold-welded particles. Then, a range of maximum effectiveness is observed: within this interval, silica crystals are covered with tiny Fe particles, which are extremely reactive towards hexachlorobenzene. Further addition of silica has a negative effect because of redundant number of crystals that cover iron particles.

Additional component(s) can be used to activate or potentiate chemical reactivity of the main co-milling reagent. In this case the synergistic effect depends on the specific chemical properties of the components. For instance, persulfate (S2O8 <sup>2</sup>) can be directly activated by sole HEM to generate sulfate radicals (SO4 – ) with strong oxidant power. Nevertheless, addition of strong bases or electron donors (e.g., Fe) has been proved to remarkably accelerate persulfate cleavage kinetics and, consequently, target organics mineralization rate. Experiments on hexabromocyclododecane MCD (**Figure 4b**) [35] revealed that the meager debromination capability of persulfate could be markedly improved by addition of 20% NaOH. Then, excessive NaOH interfered with sulfate generation (likely by reacting with sulfate radical), reducing the debromination rate to levels close to those obtainable by treatment with sole NaOH.

#### **3.2 Waste-to-materials approach**

A serious issue of the MCD technology is production of large amounts of HEM residue. In general, the residue is mainly composed of unreacted reagent (since it is often employed in large excess to ensure a rapid and complete destruction of the treated POPs) and mineralization products (graphitic/amorphous carbon and halides). Although detoxified, such material is still an economic burden, because it must be disposed properly. The waste-to-materials approach is aimed to solve such problem by generating a useful material instead of waste. In fact, such method is based on the employment of highly reactive (and rather expensive, too) reagents in stoichiometric amounts that, however, can mineralize POPs and produce a valueadded material at the same time.

So far, only two reagents have been ascertained to satisfy both such requirements, that is, bismuth oxide (Bi2O3) and lanthanum oxide (La2O3). These oxides were used in stoichiometric amount with some brominated and fluorinated POPs (i.e., metal-to-halogen atomic ratio of 1) to mechanosynthetize the corresponding oxyhalide [30, 36, 37]:

$$\text{Br}-\text{POPs} + \text{Bi}\_2\text{O}\_3 \rightarrow \text{BiOBr} + \text{C} + \text{BiCO}\_3\text{Br} \, (+\text{CO}\_2) \tag{7}$$

$$\text{Br}-\text{POPs} + \text{La}\_2\text{O}\_3 \rightarrow \text{LaOBr} + \text{C} + \text{LaCO}\_3\text{Br} \,(+\text{CO}\_2) \tag{8}$$

$$\text{F}-\text{POPs} + \text{La}\_2\text{O}\_3 \rightarrow \text{LaOF} + \text{C} + \text{LaCO}\_3\text{F} \,(+\text{CO}\_2) \tag{9}$$

POPs were entirely mineralized in graphitic/amorphous carbon and CO2 (mainly found as carbonate), thus ensuring detoxification. At the same time, an almost pure oxyhalide was obtained, after a short thermal treatment to remove C and reconvert the carbonate into the corresponding oxyhalide. Bismuth oxybromide is a material with excellent photocatalytic properties and the mechanosynthetized BiOBr was tested for removal of dye methyl orange in water under visible light irradiation. Lanthanum oxyhalides have excellent optical properties with actual application in X-ray imaging for medical devices (LaOBr) and potential one to produce doping host for transparent oxy-fluoride glass ceramics (LaOF). The production of such value-added materials could be a driving force to use toxic and obsolete POPs as source of halogens, achieving their detoxification.

#### **4. Application to contaminated waste**

MCD of POPs in contaminated waste is complicated by components of the waste matrix, making it almost unpredictable. Most of the components are mechanochemically activated by HEM, so they can interact with both POPs and comilling reagent(s). Such interaction could be positive or negative. Components such as aluminosilicates, metal oxides, carbonates, etc. can be more or less activated, thus supporting the mineralization process. Moreover, some of these components are known to acquire improved catalytic properties during and after HEM, frequently facilitating POPs degradation [38]. On contrary, radical scavengers, like organic matter, hinder the degradation of haloorganics. In the following subsections, three examples are discussed: soils and sediments, fly ashes, and plastic waste.

#### **4.1 Soils and sediments**

In general, soils and sediments are suitable matrices to obtain effective mineralization of POPs by MCD. Providing an adequate amount of mechanical energy (e.g., prolonging HEM for sufficiently long time) ensures destruction of haloorganics due to a number of phenomena that might occur during the treatment [39]. In the first place, decrease of particle size and the consequent enlargement of specific surface enhance adsorption capacity of soils and sediments towards POPs. Clays play a key role in this: aside from particle breakage, HEM induces delamination of aluminosilicates, as well as partial amorphization of their surfaces, thus exposing more dangling bonds [40]. Hence, POPs can be adsorbed mainly by Van der Waals interactions, and possibly undergo catalytic degradation, thanks to surface acidity of clays [38].

More relevantly, aluminosilicates, metal oxides, carbonates, and other inorganic components of soil and sediments can be mechanochemically activated to generate active species (as elucidated in Section 2.2). These are deemed to be the major responsible of POPs mineralization in such kinds of waste [27, 39]. Organic matter, on the other hand, might facilitate adsorption and catalytic degradation of POPs onto particles, but surely scavenges radical species generated by the mechanical activation of inorganic components and co-milling reagent(s).

Since HEM can activate several mineral components of soil and sediments, MCD can be potentially realized by taking advantage of the "self-healing" properties of

#### *Nonthermal Mechanochemical Destruction of POPs DOI: http://dx.doi.org/10.5772/intechopen.101088*

these matrices. Indeed, mechanochemical treatment of such contaminated waste without addition of co-milling reagent suffices, in some cases, to achieve entire degradation of POPs [41]. Using a reagent, typically in large amount, insures complete mineralization of the haloorganics in a reasonable time and sensibly boosts the MCD rate [42], but usually transforms the contaminated matrix in a useless waste (whose amount is in general conspicuous). In order to avoid excessive usage of reagent, but to keep energy consumption contained, coupling of MCD with other technologies, such as thermal desorption [43] and biological treatment [44] was also experimented. Such approach is probably the most promising to avert extreme denaturation of the contaminated soils and sediments and allow their relocation in the original geological position.

#### **4.2 Fly ashes**

Fly ashes, with their high content of silicates, is another matrix that responds well to MCD treatment. However, because of the notable concentration of PCDD/ Fs, such waste must be co-milled with a suitable amount of reagent to ensure entire mineralization of dioxins. Metal oxides (e.g., CaO, MgO), zero valent metals (e.g., Al), and their combination are inexpensive reagents that can efficaciously destroy PCDD/Fs, dioxin-like compounds, and their precursors [45, 46].

A key issue of fly ash detoxification by MCD is the compresence of carbon and chlorides, which determines a high potential for reformation of dioxins, also under low-temperature HEM conditions. In fact, de novo formation of PCDD/Fs was observed during the mechanochemical treatment [46, 47], in particular in presence of dioxin-formation catalysts, like copper compounds [45]. This might be caused by hits of the milling tools, which induce high local temperature increase, although for short time, on surfaces of the particle that are trapped between the hitting tools [45]. Despite such issue, it was proved that sufficiently long time milling, as well as temperature control, assures definitive dioxin removal and prevents their reformation [48]. This is owed to extensive amorphization of the fly ash components, especially carbonaceous matter, which averts de novo formation [45, 47].

#### **4.3 Plastic waste**

Some types of plastic waste contain high amounts of (brominated) flame retardants because of their utilization in electric and electronic devices. Removal of such chemicals from the polymeric matrix is a hard task. Nonetheless, it was realized by HEM with co-milling reagents such as zero valent metals (e.g., iron), metal oxides (e.g., CaO), plasma-formers (e.g., SiO2), and their combination [49, 50]. Reagents with relatively high hardness (i.e., Fe and SiO2) were found to be more efficacious to debrominate the plastic waste for the reason that they improve the mechanical and chemical degradation of the polymeric matrix, thus allowing a better contact between the reagent and the flame retardant.

Another relevant problem of brominated flame retardant MCD in plastic waste is the negative effect of the polymeric matrix, which slows down the degradation rate. Firstly, the impact energy is mainly absorbed by the matrix, and only a minor share is actually available for reagent activation and debromination of the POPs. Secondly, the mechanochemically activated radical species generated from the comilling reagent are scavenged by the polymer, leading to chain shortening and other degradation phenomena of the plastic. In fact, it was verified experimentally that decabromodiphenyl ether mechanochemical degradation rate in polypropylene matrix co-milled with Fe-SiO2 mixed reagent was 4.4 times slower than the rate observed for the pure flame retardant co-milled with the same reagent and under

similar HEM conditions [49]. Consequently, longer milling times and higher energy consumption are required for POPs mineralization in plastic waste, compared to the MCD of sole haloorganics.

#### **5. Technological and economic considerations**

The MCD technology can efficaciously destroy POPs, whether they are in almost pure form as (obsolete) chemicals, or they are present in environmental and waste matrices as contaminants. In both cases it is possible to transform the POPs in mineral form using HEM, often employing a co-milling reagent. Yet, effectiveness alone is not sufficient for large-scale application of this technology: greenness, safeness, and cost effectiveness are necessary requirements as well. Among nonthermal technologies, MCD is certainly one of the greenest, compared to other POPs destruction technologies. It does not require any solvent, since it is a solid-state treatment. And, it can be potentially utilized to prepare useful materials (instead of detoxified waste). More importantly, MCD phenomena occur only under the energy input provided by the HEM and can be terminated by simply turning off the milling device. Hence, in case of any non-mechanical accident (e.g., unintentional emission of toxic chemicals), the process can be interrupted immediately [37]. In addition, when treating hazardous waste with high potential of dioxin de novo formation, milling chamber temperature can be kept very close to environmental temperature to avert unintentional dioxin generation [48]. In sum, MCD is a safe and green technology.

Another substantial advantage of the MCD technology is the simplicity of the plant design and its versatility. **Figure 5** shows a block scheme of a mechanochemical plant for treatment of stockpiled POPs and POPs-contaminated waste. Stockpiled POPs are directly fed into the milling section, with a co-milling reagent. An air treatment section is included to prevent any emission due to volatilization of the POPs or their degradation by-products, as well as release of contaminated dust. MCD treatment of waste materials just includes (depending on the specific waste to be treated) a drying section to decrease humidity content, and a sieving section to remove debris that cannot be fed to the mill (e.g., stones in contaminated soil). Air deriving from each of these auxiliary sections is treated to avert possible POPs release in the environment. Obviously, such simple plant scheme can be used versatilely to treat any type of POPs waste.

**Figure 5.** *Block scheme of a mechanochemical plant for stockpiled POPs and POPs contaminated waste treatment.*

*Nonthermal Mechanochemical Destruction of POPs DOI: http://dx.doi.org/10.5772/intechopen.101088*

Despite the above-mentioned advantages, MCD technology is affected by two issues: noise and fine powder emission, which is related to worker and environmental safety, and high energy consumption, which is mainly an economic problem. The first one is easily overcome by constructing adequate containment facilities and utilizing individual protective devices inside such facilities. The issue of energy consumption can be managed by a few ways, which can be selected through an adequate economic assessment. Possible options comprise employment of large amounts of cheap and easily suppliable reagents to boost the MCD reaction rate, or, alternately, reduced quantities of strong/efficacious reagents; and coupling MCD with other non- or low-thermal technologies (e.g., biological treatment, thermal desorption, etc.).

Economic assessment is the sole way to evaluate effectively the various solutions for reduction of energy consumption, as well as other issues related to plant configuration. Typology of HEM device available on the market, electric energy cost, kind of the accessible reagent(s), and nature and concentration of the POPs waste are some of the major factors that have remarkable influence on the investment and operating cost of an MCD plant. Such factors depend more or less on the location, so it is not possible to execute a priori a generic economic assessment for this technology. But, a tentative economic feasibility study for MCD treatment for soil was carried out on the basis of data related to the US in 2016 [39]. This study highlighted that milling chamber volume is the key parameter that governs both investment and operating costs: the larger is the volume, the lower are the expenditures. Estimated operating costs were close to or less than those of the technologies currently available on the market for contaminated soil treatment. Actually, the chief economic issue of MCD technology is that most of HEM devices are relatively small, hence, it is for now more suitable to treat low volume waste, like stockpiled POPs. On the other hand, given the increasing interest in mechanochemical technology in various fields, it is expected that larger scale mills will be available on the market, so that MCD treatment of large volume waste such as contaminated matrices will become economical, too.

#### **6. Conclusions**

The present chapter illustrated that high energy ball milling of haloorganics in presence of co-milling reagent(s) can dismantle the structure of such organics to generate inorganic graphitic/amorphous carbon, carbon oxides, and halides. This transformation occurs at near-room temperature and pressure; therefore, the chance of unintentional formation of dioxins and dioxin-like compounds is almost null (if milling parameters are chosen judiciously). POPs can be efficaciously destroyed as both in almost pure form (e.g., stockpiled obsolete chemicals) and in contaminated matrices (e.g., soil, sediments, and hazardous waste). The high energy consumption and the lack of sufficiently large industrial high energy milling equipment hamper full-scale application of this technology. Nevertheless, specific approaches to reduce energy consumption, such as multireagent and waste-tomaterials strategies, and the increasing interest for mechanochemical methods in other fields, which is pushing also the development of low-priced large-scale mills, will facilitate application of mechanochemical treatment to POPs destruction.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Giovanni Cagnetta<sup>1</sup> \* and Mohammadtaghi Vakili<sup>2</sup>

1 State Key Joint Laboratory of Environment Simulation and Pollution Control (SKJLESPC), Beijing Key Laboratory for Emerging Organic Contaminants Control (BKLEOC), School of Environment, Tsinghua University, Beijing, China

2 Green Intelligence Environmental School, Yangtze Normal University, Chongqing, China

\*Address all correspondence to: gcagnetta@mail.tsinghua.edu.cn

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Nonthermal Mechanochemical Destruction of POPs DOI: http://dx.doi.org/10.5772/intechopen.101088*

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#### **Chapter 7**

## Physiochemical Properties and Removal Methods of Phenolic Compounds from Waste Waters

*Yesim Gucbilmez*

#### **Abstract**

In this chapter, phenol and chlorophenols are investigated in terms of their production histories, physiochemical properties, pollution resources, and removal methods. It is seen that both phenol and chlorophenols are highly toxic compounds, produced from natural and anthropogenic sources, which are hazardous to both humans and the environment even at very low concentrations. The typical industries which produce phenol and chlorophenol pollution are petrochemical, textile, plastics, resin, dye, pharmaceutical, iron and steel, pulp and paper industries as well as the petroleum refineries, and coal gasification operations. Phenol is a highly corrosive and nerve poisoning agent. It causes harmful health effects, such as sour mouth, diarrhea, and impaired vision. It is also toxic for the ecosystem with toxicity levels ranging between 10-24 mg/L for humans, 9-25 mg/l for fish, and lethal blood concentration around 150-mg/100 ml. Chlorophenols found in natural waters or drinking water also cause serious health problems such as histopathological alterations, genotoxicity, mutagenicity, and carcinogenicity among others. Due to the aforementioned reasons, the phenolic compounds in wastewaters or drinking water must be removed using a suitable wastewater treatment method such as adsorption, extraction, electrochemical oxidation, biodegradation, catalytic wet air oxidation, or enzyme treatment among others.

**Keywords:** phenol, chlorophenols, wastewater treatment, phenolic compounds, phenolics, organic pollutants

#### **1. Introduction**

Due to technological advances and rapid industrial growth, water systems around the world are under threat. In general, developing countries suffer from water pollution originating from agricultural sources whereas developed countries have chemical discharge problems. In most wastewaters and drinking water, one or more of the following toxic organic pollutants may occur [1]: Organochlorines, chlorobenzenes (CBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), phenols, chlorophenols, and other phenol-derived compounds.

Phenolic compounds are phytochemicals found in nature that cannot be synthesized in the human body. They are mainly obtained from food and medicinal herbs and are present in most fruits and vegetables [2]. They are found in water systems

due to the discharge streams of industrial, agricultural, and domestic activities as well as the result of natural phenomena. They cause severe and long-lasting health hazards including damage to red blood cells and the human liver. Their interaction with the aquatic ecosystem can produce new compounds, which can be as toxic as the original phenolic molecules [3].

Chlorophenols (CPs) are common contaminants that can be found in surface, ground, and drinking waters [4–8]. Thermal and chemical decomposition of chlorophenols leads to the formation of harmful compounds which cause public health problems such as genotoxicity, mutagenicity, and carcinogenicity among others. In addition, some electrophilic molecules may occur as a result of the transformation of chlorophenols, which may connect and harm the DNA or gene products [5, 6].

#### **2. History of phenol and chlorophenols**

The academic and industrial chemist F.F. Runge was born in 1794 in Billwerder near Hamburg. Runge isolated phenol from coal tar in the year 1834 in an impure form and gave it the name "carbolic acid" "Karbols¨aure" (carbolic acid) [7–9].

British surgeon Joseph Lister used phenol as a disinfectant for the first time in 1865 for sterilizing surgical dressings, instruments, and wounds [10]. However, the phenol sprays used during the surgeries were dangerous for the lung's mucous membrane when inhaled. Thus, by 1890, the phenol spray was abandoned by the medical community [11].

During the Boer War, England placed an embargo on phenol causing phenol shortage on the continent. This led F. Raschig works at Ludwigshafen, Germany, to produce synthetic phenol on a large scale and in 1940, Hooker Chemical Corporation built a plant based on the so called the Raschig-Hooker process for the commercial production of phenol [12, 13].

During World War I and World War II, the existing sulfonation process for the production of phenol was further improved and other processes such as chlorination process and the Raschig Process were commercialized for the first time [14]. In 1924, Dow Chemical started the commercial production of synthetic phenol using direct chlorination of benzene to chlorobenzene which was called the Dow Process [11].

In terms of patent literature, since the middle of the twentieth century, researchers have mainly tried to improve the existing processes rather than developing new ones. One such example was the 1989 European patent of Mitsui Petrochemical Limited which used a recycle loop to reduce the amount of the side product acetone [15, 16].

Presently, phenol is used in the manufacture of phenolic and epoxy resins [17, 18], plastics [19], plasticizers [20], polycarbonates [21], nylon [22], dyes [23, 24] disinfectants [25], herbicides [26], polymers, drugs, pesticides [27] wood preservatives [28], and fungicides [29].

Chlorophenols (CPs) are produced by the electrophilic halogenation of phenol with chlorine. There are five basic types and 19 different CPs. CPs are mostly used as pesticides, herbicides, antiseptics, and disinfectants [30]. They are very toxic and their presence is dangerous for humans as well as aquatic life. They are found in the wastewaters of textile, pharmaceutical, petrochemical, pesticide, paper, and other industries [31].

The processes given in **Figure 1** have been mainly used in the industry for the production of phenol up to date [10, 32–35] and among them, only the Hock and Toluene Oxidation Processes are important for the phenol industry, the others have been discarded for economic reasons [10].

*Physiochemical Properties and Removal Methods of Phenolic Compounds from Waste Waters DOI: http://dx.doi.org/10.5772/intechopen.101545*

**Figure 1.**

*Processes used up to date for the industrial production of phenol [10, 32–35].*

The global phenol market reached a value of 23.17 billion US Dollars (USD) in the year 2020. The phenol market is further expected to grow at a compound annual growth rate (CAGR) of 5.3% between the years 2021 and 2026 to reach a value of about 30 billion USD by the year 2026 [36].

In the case of CPs, their production and usage have caused the presence of persistent toxic components into the water systems which are resistant to biological degradation. However, microorganisms exposed to these pollutants have obtained the ability to biologically degrade some of them and the biodegradation routes were seen to depend on the physicochemical and biological properties of the particular wastewater system under question. Understanding the genetic basis of catabolism of CPs may increase the efficiency of naturally occurring microorganisms or help to raise new microorganisms which can degrade CPs successfully [37].

#### **3. Physiochemical properties of phenol and chlorophenols**

Phenol is an odorous chemical compound, found either as a colorless liquid or white solid at room temperature, and maybe highly toxic and corrosive. Phenols are similar to alcohols but form stronger hydrogen bonds. The presence of stronger hydrogen bonds makes them more water-soluble than alcohols and their boiling points higher than those of alcohols [38]. They are widely used as raw materials in the manufacture of phenolic resins, automotive parts, nylon, epoxy resins, polycarbonate engineering thermoplastics, wood preservatives, heavyduty surfactants, pharmaceuticals, disinfectants, tank linings, and coating materials among others. They are also widely used in household products; for instance, phenol-derived *n*-hexylresorcinol, is used in cough drops and other antiseptic applications and butylated hydroxytoluene (BHT) is a common antioxidant in foods. Also, in the dye industry, substituted phenols are used to make intensely colored azo dyes [38, 39].

CPs, produced by chlorinating phenol or hydrolyzing chlorobenzenes, contain the benzene ring, the OH group, and chlorine atoms. All CPs, except 2-CP, are solids with melting points in the range of 33–191° C. They are weakly acidic and their acidity is slightly lower than that of phenols. In reactions with alkaline metals in water solutions, they yield highly soluble metal salts. Their level of toxicity depends on the chlorination degree and the place of the chlorine atoms with respect to the hydroxyl group [40].

#### **4. Sources of phenol and chlorophenol pollution**

The presence of phenolic compounds in wastewaters stem from two main sources: Natural and human-based activities. Natural activities include the decomposition of dead plants and animals, synthesis by microorganisms and plants in the aquatic ecosystem. Human activities, on the other hand, include industrial, domestic, agricultural, and municipal activities [10].

Phenol enters water systems in effluents from major industries, such as petrochemical, textile, plastics, resin manufacturing, dye, pharmaceutical, iron and steel, pulp and paper as well as petroleum refineries, and coal gasification operations. It is very important to remove phenols and aromatic compounds from industrial streams before discharging them because of their toxicity to aquatic organisms [41, 42].

The CPs are among the most important environmental pollutants. They are mostly used in the production of paper and pesticides and also as intermediate materials in the production of dyes, plastics, and pharmaceuticals [43–45]. These industries often cause wastewater and groundwater pollution. In addition, as a result of tap water chlorination treatment, CPs have also been detected in drinking water [46].

#### **5. Removal methods of phenol and chlorophenols**

Phenol is a nerve poisoning agent and it is highly corrosive. It causes health hazards, such as diarrhea, sour mouth, and impaired vision. It is also toxic for fish and the toxicity levels are in the range of 10–24 mg/L for humans and 9–25 mg/L for fish while the lethal blood concentration is around 150-mg/100 ml [47].

The available removal methods used for phenol can be separated into two main groups: Traditional and advanced. Traditional methods include steam distillation, extraction, adsorption, ion exchange [42] and advanced methods include wet air oxidation, catalytic wet air oxidation, ozonation, membrane processes, electrochemical oxidation, biological processes/biodegradation, and enzymatic treatment among others [42, 48].

Using the relative volatility of phenol, steam distillation can be carried out in order to remove phenol from aqueous mixtures. The phenol–water mixture forms a minimum azeotrope at 9.21% (w/w) phenol [42, 49–51]. Using this property, azeotropic distillation or steam distillation can be used to treat phenol wastewaters to obtain effluent concentrations as low as 0.01 mg/L [48].

Liquid–liquid extraction is a commonly employed technique used for the removal of phenolic compounds. It can be used for a wide range of phenol concentrations and is economical in some cases. Benzene and butyl acetate have been popular as solvents in this process in the past, however, presently, the most used solvent is di-isopropyl ether, which is used in the phenosolvan process [50, 51]. For the extraction process, the solubility of the preferred solvent in water should be tolerable so that further purification steps will not be necessary. The selectivity of the solvent depends mainly on the type of the solvent, the system temperature, and the amount of phenol in the wastewater [50].

The adsorption method has been found to be successful for the removal of phenols from wastewaters for a large concentration scale, depending on the adsorbent, recycling, and economics. Among the used adsorbents, activated carbon (AC) is the most preferred one in the industry. It is expensive but has been shown to be efficient for the removal of even very low amounts of organic pollutants [51–53].

#### *Physiochemical Properties and Removal Methods of Phenolic Compounds from Waste Waters DOI: http://dx.doi.org/10.5772/intechopen.101545*

Chemical oxidation processes can turn phenolic compounds into smaller molecules that are less toxic and easier to process or mineralize [54, 55]. Among the chemical oxidation processes, advanced oxidation processes (AOPs) such as the Fenton process, ozonation, photolysis, or their combinations are recommended for low-concentration wastewaters. Incineration, on the other hand, is suitable for wastewaters for which the COD values are higher than 100 g/L, however, it is no more commonly used since it is not an eco-friendly process [56].

Wet air oxidation (WAO) is a very clean technology since no additive is added to cause secondary pollution. The reaction is carried out at moderate temperatures (175–320°C) but at high pressures (2.17–20.71 MPa); the organic pollutants in the wastewater are oxidized into small organic acid molecules which are likely to biodegrade [57]. This method can be used for the treatment of wastewaters with initial COD values in the concentration range of 20–200 g/L [58].

Catalytic wet air oxidation (CWAO) offers an alternative path to treat refractory wastewaters. CWAO gained a lot of interest over the past 20 years due to its ability to oxidize toxic wastewaters and complete their mineralization [59–63]. In addition, it is a heterogeneous process, thus, an extra catalyst separation step is not necessary in most cases making the process more economical to apply [64].

Many researchers have studied the electrochemical oxidation of phenolic compounds [65, 66]. In this process, the electrode should be electrochemically stable, economically viable, and very efficient for the removal of organic pollutants [67]. There are different researches carried out using several anodic materials like Ti = SnO2, Pt [68], vitreous carbon [69], and PbO2 [70]. Among them, PbO2 electrodes have been successfully applied due to their high electrical conductivity values, strong oxidizing properties, and low costs [71, 72].

Biological treatment or biodegradation is the most widely employed method for the removal of phenols from water systems. The treatment is inexpensive, simple to design and maintain, and transforms the phenolic solutions into simple end products. Phenolic molecules such as Bisphenol A (BPA) can also be successfully treated with biological treatments such as activated sludge [73–75].

CPs, on the other hand, can be removed from wastewaters by a variety of methods including biological treatment [14], advanced oxidation processes [76], and adsorption [77–79].

Although various traditional and advanced methods are possible to apply for the removal of phenol from wastewaters; the two important parameters which define the suitable method are the initial and final phenol concentrations as seen in **Table 1** [42].


#### **Table 1.**

*Initial and Final Phenol Concentrations of Waste Waters and Corresponding Removal Methods (Modified from [42]).*

#### **6. Comparison of different methods for the removal of phenol and chlorophenols**

Researchers still extensively focus on phenol removal methods from wastewaters, considering both traditional methods such as adsorption and steam distillation and advanced processes, such as wet air oxidation and biodegradation. The traditional methods mostly have the drawbacks of low efficiency and high operational cost which can be cured by using low-cost adsorbents or increasing the surface area of the existing adsorbents. In the case of advanced methods, enzymatic treatment which uses different peroxidases seems to be an efficient method with removal efficiencies above 95% [49].

As for CPs, adsorption, biodegradation, and oxidation by AOPs seem more widely used than other methods [76–79]. AOPs involve the formation and use of hydroxyl ions (OH− ) through chemical, photochemical, or photocatalytic methods [77]. Adsorption has been reported to be one of the most successful methods for CP removal from wastewaters since it is a simple method with a low-maintenance cost, high efficiency, and less toxic by-product generation [79].

#### **7. Conclusions**

Phenols and CPs are important compounds used in the manufacture of epoxy resins, plastics, polycarbonates, nylon, dyes, disinfectants herbicides, drugs, pesticides, wood preservatives, and fungicides among others. However, they are quite toxic chemicals, even at low concentrations, causing harmful effects ranging from sour mouth, diarrhea, and impaired vision to histopathological alterations, genotoxicity, mutagenicity, carcinogenicity, and death. Thus, wastewaters containing phenol and CPs have to be treated thoroughly before being discharged into water systems. The water treatment processes used in industry depend on the type of wastewater and initial and final concentrations of the phenol and CPs. Traditional methods such as adsorption and distillation or advanced methods such as wet air oxidation and biological treatment can be used to remove phenol and CPs from wastewaters. For the removal of phenol from wastewaters, among traditional methods, the adsorption method is efficient on a large scale of concentrations, depending on the economics, recycling, and the adsorbent properties. Among advanced methods, on the other hand, the inexpensive and simple to design biological treatment is the most commonly applied method and the enzymatic treatment yields more than 95% phenol removal efficiency using different peroxidases. Finally, for the removal of chlorophenols from wastewater systems, the most used methods are adsorption, biological treatment, and advanced oxidation processes.

*Physiochemical Properties and Removal Methods of Phenolic Compounds from Waste Waters DOI: http://dx.doi.org/10.5772/intechopen.101545*

### **Author details**

Yesim Gucbilmez Department of Chemical Engineering/ESTU, Eskisehir, Turkey

\*Address all correspondence to: ygucbilmez@eskisehir.edu.tr

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 8**

## Occurrence and Removal of Persistent Organic Pollutants (POPs)

*Siyabonga Aubrey Mhlongo, Linda Lunga Sibali, Kholofelo Clifford Malematja and Peter P. Ndibewu*

#### **Abstract**

Since the revelation in the detection of the persistent organic pollutants (POPs) in industrial wastewater in the early 1990s, a notable progress has been achieved on the research and different removal applications or methods of this challenge at hand. This book chapter entails a decent understanding on the occurrence, effects, and amputation of POPs in the water sector in advancement of municipal performances of treating industrial wastewaters and environment at large. This current chapter also presents an overview of research associated to the amputation of persistent organic pollutants (POPs) from various water bodies, i.e., river sediments, sewage plants, industrial sludges, and wastewater. Also, discussing the relationships with actual pre-treatment and removal rates. Vital characteristics such as the wastewater matrix, location, sources of POPs, materials and modules, operational parameters and problems are presented with a clear focus on removal of these organic pollutant's different sources (like, textile wastewater). The particular methods to the removal of POPs can be associated with the application of ultrafiltration, nanofiltration and reverse osmosis as advanced treatment stages are considered in correlation with the textile wastewater characteristics and removal efficiencies requirements. This gives significance to the amalgamation of physico-chemical and biological treatment with membrane processes which is likely to represent an efficient solution for the removal of POPs from textile wastewater. However, since membrane fouling and hydrophilicity are apparent in the execution of this process, this chapter also covers the effective strategies like fabrication of membrane with a suitable additive to counterattack these challenges, which are often used in membrane technological research. This chapter also proposes an updated understanding of fouling and improvement of membrane properties.

**Keywords:** persistent organic pollutants (POPs), ultra-filtration (UF) membranes, blending, fouling, hydrophilicity

#### **1. Introduction**

There has been an advanced progress regarding the persistent organic pollutants (POPs) - i.e., elongated-lived, lethal organic composites such as PCBs, PAHs, OCPs and dioxins which have predominantly pursued their way into the environmental sector - constitute the theme of a research programme launched in the early 1990s

by the Swedish Environmental Protection Agency (SEPA). This environmental programme has raised funds estimated to SEK 50 million for the research facilities in Sweden to bring focus only on persistent pollutants [1]. Equally concerning, the data obtained by World Health Organization (WHO) in late 2016, an estimated that 1.2 billion public does not have access to clean water [2, 3]. The occurrence of persistent organic pollutants (POPs) in river water and water treatment plants has raise serious concerns, especially due to the high costs and energy consumption that comes with mitigation of these challenges – because it involves variety of steps, and over thirty processes have been primarily used [4, 5]. The apparency or the occurrence of POPs in industrial wastewaters and textile industries have led to more of ecological negative effects, these includes, i.e., good taste and odor issues of the downstream water supplies, and further forming foam. This results in inhibition of the natural self-purification processes, and worse case - negative effects on the marine life and living organism in the society [6].

Persistent organic pollutants (POPs) remains nothing else but a bunch of different chemical compounds that constitutes of different pedigrees but have common traits, viz., semi-volatility, hydrophobicity, bio-accumulative, high toxicity, and alarming persistency in the environment, and they can also drift into food chains [7]. Research have indicated major contributors of POPs in the environment, these are typically chemical industry, textile industry [8], pulp and paper industry [9], and treatment of landfill leachate [10, 11].

#### **1.1 Textile industries contribution to POPs in waste streams**

During the early months of 1990, several studies reported on textile industrial sector being the major contributor of POPs, and worse, discharging very high absorptions of different hazardous POPs, i.e., polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) [12–14]. A bigger portion of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans from textile industry are sourced all through - washing into sewage sludge, which is often used as an agricultural fertilizer, and are also the source of dioxins in the food chain [15]. A Few of these POPs like polychlorinated biphenyl, phenols, benzenes and dichloro-diphenyl-trichloroethane (DDT) are purposely formed in different of commercial applications for their significant nature or properties they have intermediates or pesticides **Table 1**.

Persistent organic pollutants are toxic chemicals which belong to the families of chemicals such as aliphatic and polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), organochlorines (OCPs), and organophosphorus pesticides (OPs) [17]. They tend to accumulate in the environment and have shown to resist photolytic, chemical, and biological degradation [18]. Persistent organic pollutants have been described by Stockholm convention as a wide range of chemicals which poses a greater risk to human life and biota due to their toxic, persistent and bio-accumulative nature [19]. Their exposure may lead to birth defects, dysfunctional immunity, change on the reproductive and/or nervous systems [20]. Therefore, their continuous detection in the environment even in low concentrations has been a genuine concern for many years. This chapter aims at giving a deep understanding on the occurrence of POPs, their nature and detection methods.

#### **1.2 Occurrence of persistent organic pollutants (POPs)**

The industrial application of POPs can be traced back during the early 1900s when these chemicals were commercialized, and used for pest and diseases control [7]. For a number of years, researchers have focused their attention on studying the *Occurrence and Removal of Persistent Organic Pollutants (POPs) DOI: http://dx.doi.org/10.5772/intechopen.100387*


#### **Table 1.**

*Processes and effects of some persistent organic pollutants from textile industry assembled by Mustereţ and Teodosiu [16].*

persistence factor, bioaccumulation, and toxicity of the common POPs such as, viz., PCBs (polychlorinated biphenyls, PAHs (polycyclic aromatic hydrocarbons) and OCPs (organochlorine pesticides) [21]. Although many POPs have been prohibited due their adverse effects however, they are still detected in considerable levels in the environment around the globe [22]. Several studies around the research space have reported significantly elevated levels of POPs in various matrices, including biota, sediment, soil, surface water, and drinking water [23–25]. It is no doubt that the rapid increase in human population, urbanization and industrialization have had a great impact in the rapid increase of the POPs in the environment [26]. Moreover, farming practices such as discharge of pesticides and fertilizers into the environment also lead to significant increase of POPs in the environment [27].

#### *1.2.1 Compositional patterns and properties of different POPs*

i.Sources of PCBs, their toxicity and nature in the environment

Polychlorinated biphenyls (PCBs) have been identified as a group of chlorinated organic pollutants consisting of 209 isomers and congeners that resulted from the variation in number and position of the chlorine atoms connected to the biphenyl rings [28, 29]. Most of these chemicals which are synthetic, have been used as coolants and lubricants mainly in electrical equipment such as electrical capacitors, generators and transformers owing to their insulating properties [30, 31].

They are characterized as persistent pollutants due to their low water solubility, high fat solubility, resistance to degradation and bioaccumulation in the environment [32]. The major concern associated with PCBs is their high level of toxicity even in extreme low concentrations. Despite their prohibition and also classified as one of the "dirty dozen" in the grouping of POPs, they are still detected in the different environment matrix [33]. Research conducted on monitoring of PCBs in the environment show that sediments are the major sources of PCBs [26]. This is because POPs such as PCBs have high organic carbon partition coefficients (Koc), making them to easily adsorb to sediments. Polycyclic biphenyls are often discharged into the environment from industrial discharge, storage leaks, volatilization, urban discharge [34].

ii.Sources of PAHs, their toxicity and nature in the environment

Polycyclic aromatic hydrocarbons are a group of lipophilic chemicals which exist in the environment in different forms (colorless, white, or yellow solids). These

chemicals exist as a mixture containing two or more benzene rings fused together in linear, cluster, and angular arrangements as shown below in **Figure 1** [35]. They have been listed under Stockholm convention as POPs due to their bio-accumulative and toxic nature in the environment, while they also have been found to exhibit toxic properties such as; carcinogenic, mutagenic and teratogenic making them harmful to human health and aquatic life [36]. Naturally, PAHs can be produced from incomplete combustion of renewable materials (e.g. wild fires) [37], and volcanic eruptions [38]. However, literature shows that anthropogenic activities such as garbage burning, coal combustion, exhaust from motor vehicles, etc. dominate the sources of PAHs in the environment [39]. The persistency of PAHs tends to increase with increasing molecular weight **Figure 2** [40].

iii.Sources of OCPs, their toxicity and nature in the environment

Organochlorine pesticides are chemicals often used in agricultural activities mainly for pest control purposes [41]. They have been listed as POPs owing to their toxic, bioaccumulation and non-biodegradability nature [42]. Improper disposal from domestic use such as indoor residual spraying of pesticides plays a significant role in the increased levels of OCPs in the environment [43]. The increasing demand of agricultural practices and the persistent fight against pests mean more pesticides residues produced **Figure 3**.

According to a study by Jayaraj, Megha [44], only 0.3% of the pesticides used on crops interact with the target pest while the rest becomes excess. Therefore, these chemicals end up in different environment matrix including soil, sediments, and air. Of all the environmental matrix contamination, sediment contamination has reported to have detrimental effect on the source of food chain [45]. Furthermore, literature shows that considerable levels of OCPs have been detected in various honey samples [46–48], which is a proof of the impact that OCPS have on the food chain.

#### iv.Removal of persistent organic pollutants (POPs) in wastewater

Due to the continuous released of POPs into the environment, this has prompted researchers across the globe to find solutions for treating POPs. Physico-chemical methods such as coagulation, ion exchange, oxidation and adsorption have over many years been applied for removal of wide variety of POPs in the environment [18, 49]. However, many of these methods have been associated with several setbacks such as high cost. Equally important, POPs have been reported to be resistant to physico-chemical methods such as flocculation, coagulation, filtration, and oxidation process [50]. More so, bioremediation has proved to have more advantages over some physico-chemical methods due to its cost effectiveness, wide variety of the microorganisms or bio-sorbents and non-destruction of the material site [51–54]. The *in situ* bioremediation process which involves carrying out treatment

**Figure 1.** *Structure of PCBs.*

*Occurrence and Removal of Persistent Organic Pollutants (POPs) DOI: http://dx.doi.org/10.5772/intechopen.100387*

**Figure 2.** *Different arrangements of PAHs [35].*

process at the contaminated site, has been regarded as a cheap, non-destructive and reliable method for degrading POPs in polluted sites [55].

Advanced oxidation process, defined as oxidation process in which hydroxyl radicals acts as oxidants, has drawn considerable recognition as a potential method for treating POPs in the environment [56]. An advanced oxidation process such as heterogeneous photocatalysis, has been widely used in degradation of POPs in the environment due to its cost effectiveness, wide availability and non-toxic properties [57]. In this heterogeneous photocatalysis, the decomposition and mineralization of contaminants using TiO2 as photocatalyst is based on the principle of the separation of light-induced electrons/holes (e<sup>−</sup> /h+ ) pairs [58].

#### **1.3 Removal of persistent organic pollutants (POPs) in wastewater membrane method**

Membrane technology have caught so much attention in the research sector due to the drastic growth over a short space of time. This is due to its approach with advantages of using reasonable energy, less chemical matrix, good film forming ability, flexibility, robustness, separation properties, and recently, they can easily integrate with a number of methods [59, 60]. Ultrafiltration (UF) membranes are likely to be the approach having replaced macromolecular separation technique such as proteins – and apart from being the newest approach, UF membranes have some very good attributes like, low energy consumption, mild operating conditions, no phase change and they are environmentally friendly [61]. There are many polymeric materials that have been used before in the membrane processes,

however, poly(ether)sulfone (PES) is mostly preferred material in (UF) membranes because of excellent properties (mechanical, thermal, and chemical stability). Some of the famously highlighted challenge about PES is the factor of hydrophobicity. This shortcoming have led to the announcements of membrane fouling from previously reported studies [62, 63]. The other polymer material previously investigated are cellulose [64], poly(vinylidene fluoride (PVDF) [65, 66], polyetherimide [67, 68], polysulfone (PS) [69] and polyethersulfone (PES) [70]. Nevertheless, PES remains the preferable membrane materials in the synthesis of UF membranes, for decades because of its convenient features.

Now, surface modification of polymeric membranes can be physical, chemical, or said to be bulky modified (i.e., polymer blends) [71]. Any type of membrane modification, be physical or chemical method - after the membrane is formed, it creates a more hydrophilic surface. These vast modification techniques can be classified into three processes, (i) graft polymerization, this is when smaller particles with hydrophilic nature are smoothly distributed or chemically infused onto the membrane scaffold; (ii)physical pre-adsorption of hydrophilic components to the membrane surface plasma treatment, this is slightly different because, there is rather a selected or a change of a functional group to the membrane surface [i.e., sulphonation, carboxylation, etc]; and (iii) Former studies confirms different kinds of modification procedures for the modification of PES membranes, namely, physical methods like blending and surface-coating methods [72, 73], and chemical methods including photo-induced grafting [74], and plasma treatment and plasmainduced grafting [75, 76].

#### *1.3.1 Membrane blending method as an effective technique for the removal of POPs*

#### *1.3.1.1 Blending method*

Usually, for an improved PES polymer property, blending method should be taken into considerations because of its simplicity and efficiency it has shown over the years. In order to observe a noticeable change in the performance of the membrane, blending method should be a necessity – this is when both PES polymer is mixed together with poly vinyl pyrrolidone (PVP), and thawed in *N*-methyl-2 pyrrolidone (NMP). The resultant polymer resin formed from the mixture should be left to be stable until handled further as normal casting technique [77]. Nearly, the idea of blending is to consortium or improve a material in a hydrophobic nature into a good mechanically hydrophilic material. This is achieved by directly blending a hydrophilic polymer like as PVP [78, 79] and poly (ethylene) glycol (PEG) [80], in that way, PES membranes are easily modified.

In this case, PVP is considered for the formation of micropores, in that way, the hydrophilicity and the antifouling properties of the membrane are increased [80]. Therefore, polymer blending technique gives rise to polymeric membrane with much improved performances and improved properties in reference to the pristine or bare PES membrane. Some researchers have encountered significant shortcomings, based on miscibility of the polymer [81]. In one way or other, there are going to be unexpected challenges with the miscibility which is limited to a narrow concentration range of vinyl pyrrolidone. These challenges are eventually resolved by blending sulfonated PES with the original PES, this is what has been done before [82, 83]. This positively outcomes the higher water permeability, and high rejections in the synthesized membrane – hence the confirmation by the sudden appearance of smaller pore sizes [84, 85]. Hence a clear indication that hydrophilicity can be wide-ranging by changes in the composition ratios of blending.

*1.3.2 Considerations affecting the removal of POPs by NF/UF PES membranes*

#### *1.3.2.1 The membrane characteristics*

Throughout the process of eliminating POPs from the source of waterbodies (or wastewater samples), PES material membranes become a vital factor if you consider the nature of the apparent POPs. This accurate selection of a PES membrane largely plays a role in the removal mechanism since the process is strongly related to the type and functional groups in the membrane chosen. Subsequently, there is also a significant aspect to contemplate in a suitable membrane selection, and that is - the molecular weight cut-off (MWCO), normally articulated in Dalton. This indicates the molecular weight of a hypothetical non-charged solute lying between 85 and 90% rejection, the porosity of a membrane, the surface charge, and the membrane material (polymer composition) as well as the degree of ionic species rejection [86]. In conclusion, the effect of each constraint on the removal of POPs is specifically related to the actual solute properties (molecular weight, molecular size, acid disassociation constant-pKa, and hydrophobicity/hydrophilicity — logKow), with which this governs the strength of the POPs-membranes physical and chemical interactions.

#### *1.3.2.2 Membrane charge*

Usually referred to as zeta potential, membrane surface charge is another vital factor to primarily study in membrane properties. The fundamental principle of the above factor lies in the fabrication of the membrane where you have to consider if the membrane has either a negatively or a positively charged surface [87]. Sometimes a membrane is pre-known to reject negatively charged pollutants (in this case, anions), such as nitrates, sulphates, and sulphites, henceforth, these fictional membranes should be negatively charged for them in order to be effectively repel the pollutants. This phenomenon therefore results into a reduced membrane fouling [88, 89]. This genius analysis of a membrane charge was discovered PVP micro particles were dispersed onto PES membrane for the membrane to give rise into an increased water permeability [90]. Thus, the zeta potential could result to many functional groups, such as, O=S=O, that comes with PES, and O=C-N of PVP that was dispersed across the scaffold of the membrane. The practical functional groups become the primary source of a negatively charged membrane [91, 92]. Consequently, this boldly confirms that an increase in the PVP particles likely to increase the hydrophilicity of the synthesized membrane – which by default leads to high permeability. Hence, the membrane charge increases as the PES and PVP dosages are varied.

#### *1.3.2.3 Persistent organic pollutants (POPs) hydrophobicity or hydrophilicity*

Hydrophilicity and hydrophobicity extremely defines the adsorption on the rejection of POPs during membrane applications process [93, 94]. Studies clearly shows that the interface between the non-polar hydrocarbon segments of POPs and the used membrane is primarily the cause of hydrophobic bonding - this has advanced the membrane progress on the extensive adsorption of POPs and of other organic pollutants onto the membrane technology [94–96]. A book published in the early 2000s vividly show that beyond hydrophobic interactions, adsorption could possibly occur over hydrogen bonding between the organic molecules and the hydrophilic groups of the membrane material [97]. Henceforth, hydrogen bonding and hydrophobic interactions may both occur independently

or concurrently. Therefore, according to the studied POPs in this book, the literature approves that the hydrophobic interactions is the driving force for the organic pollutants adsorption on the membrane surfaces - this constitutes the primary step of the rejection mechanism as Nghiem and Schäfer [97] have indicated in his study. In both ways, these observations have implicitly concluded that the rejection of hydrophobic compounds should (by experimentation) be examined after the used membrane is saturated with the target compounds, otherwise, the rejection could be incorrectly mistook for adsorptions are misread [98].

#### *1.3.3 Alternative method - Surface modification*

Several studies have been done on surface modification of membranes, and it has shown decent suitability for PES material for the amputation of POPs in wastewater. This includes, self-assembling nanoparticles [99] and/or nanotubes in PES membranes. Nonetheless, this book chapter solely focuses on PES polymer as an adsorbent blended for the improvement of the membrane properties. Another method still to advance in the membrane technology is - surfactant modification. However, a little progress has been observed in literature and still requires more work to be reported on using PES membranes. In the late 2000s, Boussu, Van Baelen [100] showed an increased in the flux for the nanofiltration membrane of waterbodies comprising of surfactants. Lastly, a brief study confirms that hydrogen fluoride could be considered to advance the membrane performances. Fourteen [14] days of immersion in a hydrogen fluoride solution, an increased permeability was obtained without any loss of rejection capacities [101–103].

#### *1.3.4 Factors to consider during the removal of POPs*

#### *1.3.4.1 Effect of the feed water composition*

A modified membrane performance with real water normally consists of (i) solutions containing salts, (ii) other organic matters, (iii) pesticides, hence, POPs rejection value is likely to vary significantly depending on the feed water composition. Importantly so, pH of the water becomes a prominent influent in the POPs rejection. Below is a brief discussion of how pH is an important parameter as the driving force in rejection values.

*Influence of water pH:* pH in the rejection role is vitally imperative in these experiments – as it directly involved in the membrane surface and membrane charge because of the dissociation phenomenon of functional groups throughout the adsorption of POPs. Different researchers have found membrane charge (zeta potential) suddenly leaning more to negative charge whilst the pH of the water body is increased, thus, resulting in functional group deprotonation [104–106]. Moreover, another prominent researcher, Freger, Arnot [107] verified about the varying of the pore sizes likely to take place reliant on the electrostatic interactions amongst the dissociated functional groups within the membrane material. Pang, Gao [108], also showed a study where high pH ranges seemed to cause reduced rejection rates, with permeate flux also going up. And this this was ascribed by the increased number of pore sizes at high pH values. These tests were conducted during the removal of one of the top four POPs (PCBs, OCPs and DDT, etc), and expectedly, the outcome exhibited that membrane rejection achieved the highest value at pH 7, and repeatedly gave lower rejection values at pH 2.5 [109]. This clearly indicates, the ion adsorption on the membrane scaffold, and predominantly at higher pH, OH− ion adsorption is increased - which automatically leads to an increase in the zeta potential of the membrane.

#### *1.3.4.2 Effect of membrane fouling*

Fouling of a membrane is apparent and continue to be a challenge within the membrane technology scope and industrial applications – this includes the wastewater treatment processes [86, 110]. This takes place as the undesirable particles accumulate to cause clogs in the water flow across the membrane. This results in the shortening of membrane life. Membranes looks at advancing the progress by creating membranes with better or improved properties – this means fabrication or modification to create low fouling propensity. The achievement relies on transformation of hydrophobic polymers into hydrophilic nature [111, 112]. However, it is of emergency that cost-efficiency efforts be applied in order to mitigate membrane fouling as much as possible. For this to be counter-attacked effectively, mechanisms of membrane fouling should be studied expansively, hence, to develop dynamic anti-fouling methodologies.

#### **2. Conclusions**

The contamination regulator of persistent organic pollutants (POPs) due to industrial and textile discharged effluents has become more severe and, clearly demands for interventions of more efficient wastewater advanced treatment. This leads to a combination of physico-chemical and biological treatment using membrane methods – which in-fact, embodies an efficient solution for the removal of POPs from these industrial and textile wastewaters.

In conclusion, application of membrane methods could successfully rely on several factors for its optimum use, i.e., material composition membrane selection, type of modules, wastewater characteristics and the interactions between contaminants (POPs) and the synthesized membrane. Membrane procedures potential use, for the removal of significance organic pollutants in industrial water bodies and from textile effluents are ultrafiltration (UF), reverse osmosis (RO) and nanofiltration (NF). These employed methods are cost-effective and easier to carry out. However, for fiscal and monetary reasons, these applications remain a disadvantage in the case where the effluents or wastewaters can be recuperated for re-use.

Application of PES polymeric membranes for these procedures or the removal of POPs contains increased removal rates, and the choice of a membrane material becomes paramount important considering properties of PES like, permeability, selectivity, chemical and mechanical resistance. But PES also have some integral operational challenges, such as: fouling and concentration/polarization phenomena. This further leads to an unexpected decrease of the permeate flux, and the vital aspect in the operational procedure of PES membranes and the performance of the membrane inevitably decrease. However, washing of membrane by the use of physical and chemical procedures could discreetly recover the permeate flux between the membrane processes cycles, yet fundamentally irreversible fouling could possibly emerge.

#### **Acknowledgements**

The authors would like to firstly appreciate the IntechOpen for the invitation to collaborate on this book, and also the editors, Dr. Mohamed Nageeb Rashed. Not forgetting to thank National research foundation (NRF) for funds to carry out this research (Ref No. MND190619448884). A Tshwane University of Technology (TUT) chemistry team for support and encouragement. Special thanks to supervisors, Prof Peter P. Ndibewu and Prof Linda L. Sibali for their outstanding guidance and exposing us to the greater side of science.

### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Siyabonga Aubrey Mhlongo1 \*, Linda Lunga Sibali1 , Kholofelo Clifford Malematja2 and Peter P. Ndibewu2

1 Department of Environmental Sciences, University of South Africa, South Africa

2 Department of Chemistry, Tshwane University of Technology, Pretoria, South Africa

\*Address all correspondence to: siyamhlongo1456@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Occurrence and Removal of Persistent Organic Pollutants (POPs) DOI: http://dx.doi.org/10.5772/intechopen.100387*

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### **Chapter 9**

## Recent Developments in the Application of Advanced Oxidative Processes for Remediation of Persistent Organic Pollutants from Water

*Ifeoluwa Oluwafunmilayo Daramola and Matthew Ayorinde Adebayo*

### **Abstract**

Environmental pollution as a result of industrialization is a continuous menace. In our precious environment, Persistent organic pollutants (POPs) are constantly present and these pollutants are of great concern because of their high level of toxicity, persistency and bioaccumulation. Therefore, this chapter discusses different types and sources of POPs in the environment. The chapter also introduces Advanced oxidative processes (AOPs) and the classes of AOPs. Removal of selected POPs from aqueous solutions by AOPs, such as sulfate radical, ionizing radiation, heterogeneous photocatalysis, electrohydraulic discharge system, ozonation, and Fenton processes, were discussed. The major aim of the chapter is to make available to environmental scientists the recent developments in the removal of POPs by AOPs.

**Keywords:** advanced oxidation processes, persistent organic pollutants, degradation, removal, environment

### **1. Introduction**

Today, global industrialization has resulted in the development of a variety of chemicals that, while useful, have attracted scientific attention because of their hazardous effects on humans and environment. Among these chemicals are Persistent organic pollutants (POPs) that are of serious concern because of their level of toxicity, long-persistent nature and bio-accumulation. The earth's ecology is currently being continuously contaminated by various pollutants. Pollutants of various forms are found in many locations. Some of these POPs are resistant to environmental deterioration (chemical, biological, and photolytic reactions) and exist for a lengthy period of time in our environment [1]. Persistent organic pollutants belong to a category of organic chemicals that are persistent, toxic, bioaccumulative, and are likely to have negative impacts on human health and the environment (persistent, bioaccumulative, and toxic substances) [2].

Persistent organic pollutants are defined by the Stockholm Convention as carbon-based chemicals that persist in the environment for a long period and are extensively disseminated. Persistent organic pollutants originate from manmade sources associated with the production, use, and disposal of some organic chemicals. Due to their persistence, ability to bioaccumulate in tissues, long-range transportability, and severe toxicity (even at low concentrations), POPs are a serious global hazard [3]. Persistent organic pollutants can also be produced unintentionally as by-products of combustion or chemical processing. Persistent organic pollutants are released into the environment on a regular basis, whether purposefully or unintentionally. The hydrophobicity of POPs is usually linked to halogenated compounds and these pollutants have low solubilities in water and high lipophilicities. They partition aggressively to solids, particularly organic matter, in aquatic systems and soils, avoiding the aqueous phase. These chemicals partition into lipids in organisms and are stored in fatty tissue instead of entering the aqueous milieu of cells. These chemicals exist persistently in plants and animals as a result of low metabolism [4]. Some of the POPs, such as polycyclic aromatic hydrocarbons (PAHs), can be produced from natural sources, however, POPs originate from the industries that are manufacturing a wide range of goods, such as agrochemicals, solvents, and flame-retardants [5].

#### **2. Sources and fate of POPs in the environment**

There are a number of POP chemicals, coming from certain series or 'families' of chemicals. Among the important classes of POP chemicals are many families of chlorinated (and brominated) aromatics, including polychlorinated dibenzop-dioxins and-furans (PCDD/Fs), polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), and different organochlorine pesticides (for instance DDT and its metabolites, chlordane, toxaphene, among others). Some are accidental by-products of combustion or the industrial synthesis of other chemicals (e.g., the PCDD/Fs) not produced deliberately. Many POPs have been synthesized for industrial uses (e.g., PCBs, PBDEs, and chlorinated paraffins) or as agrochemicals (e.g., chlordane, Dichlorodiphenyl trichloroethane (DDT), and Lindane). Examples of more polar POPs are phenols (e.g., polyethoxylated alkylphenols which are non-ionic surfactants), and chlorinated phenols [6].

The three main causes of rising POP levels in ecosystems are industrial and agricultural activities as well as municipal populations. Sources of POPs are mainly from anthropogenic activities and can be introduced into the environment through many pathways. These pollutants can reach the environment through urban runoff, agricultural runoff, drainage system, industrial effluent, landfill leachate and deposition from atmosphere. Waste incineration, consumer goods production, transportation, energy generation, mineral and metal mining (ferrous and nonferrous). Chemical synthesis also emits alarmingly large amounts of POPs into our precious environment [7].

It has been shown in literature that POPs can assimilate in the environment within weeks, but it will take years or decades for POPs to naturally decompose [8]. Persistent organic pollutants are known for their semi-volatility, which is a trait of their physicochemical properties that allows them to exist in the vapor phase or adsorbed on air particles, allowing for long-range movement in the atmosphere. Persistent organic pollutants are everywhere; they possess the ability to move through air, soil and water before being naturally decomposed [9]. They have been found in both industrialized and non-industrialized places, in urban and rural settings, in heavily populated and poorly populated areas, without significant

#### *Recent Developments in the Application of Advanced Oxidative Processes for Remediation… DOI: http://dx.doi.org/10.5772/intechopen.101304*

human aids. Persistent organic pollutants have been measured on every continent at locations that reflect every major climatic zone and geographic sector. These include places such as the Arctic, the open oceans, deserts, and the Antarctic, where there are no substantial local sources and long-distance movement from other areas of the world is the only conceivable explanation for their presence. Polychlorinated biphenyls have been found in the air in rates of up to 15 ng/m3 in all parts of the world; in industrialized areas, concentrations can be several orders of magnitude higher. Rain and snow have also been found to contain PCBs [10].

Global cycling of POPs under the influence of climate change primarily demonstrates that global warming promotes secondary emission of POPs; for example, temperature rise will cause POPs to be re-released from soils and oceans while melting glaciers and permafrost will cause POPs to be re-released into freshwater ecosystems. Extreme weather events around the world, such as droughts and floods, cause POPs to be redistributed due to strong soil erosion. The global transport of POPs has been considerably influenced by changes in atmospheric circulation and ocean currents. Climate change has affected marine biological productivity, affecting the ocean's POP storage capacity. The patterns of aquatic and terrestrial food chains have changed dramatically, potentially amplifying POP toxicity in ecosystems. Generally, global warming speeds up the process of POP volatilization and increases the number of POPs in the environment, while also facilitating their breakdown. The future of environmental behaviors of POPs has been forecasted using models such as G-CIEMS (Grid-Catchment Integrated Environmental Modeling System), Berkeley-Trent Global Model (BETR-Global), and Globo-POP. Governments make use of these models to analyze the influence of global warming on the fate of POPs in the environmental and, as a result, properly control POPs [11, 12].

All human beings are being exposed to POPs at some point in their life, regardless of age, tribe or location. As proved by current epidemiological evidence, it has been suggested that early-life exposure to POPs can adversely affect the development of immune and respiratory systems [13]. Persistent organic pollutants infiltrate the human system as early as infancy, and these pollutants have been detected in variable amounts in baby meals, which are popular around the world for giving nutrients to infants [14]. According to studies, the proportion of POPs in the human body increases with age, with elderly population often having the greatest amount of POPs in the system, which is due to the fact that the metabolism of elderly people is normally slow [15]. Among the diseases associated with POPs are endocrine disturbance, obesity, diabetes, cardiovascular problems, cancer, reproductive and other health-related issues [1].

The toxicity and persistence of POPs to humans have created a need to develop effective POPs' cleanup methods. In the first instance, the production and use of POPs should be controlled. To address this, the United States joined forces with the European Community and 90 other countries to sign a groundbreaking United Nations treaty in Stockholm, Sweden on the 23 May 2001, and a convention on POPs known as the Stockholm Convention on POPs was signed. The Convention entered into force on the 17 May 2004 [16]. The major aim of the Convention is to protect the environment as well as human health from POPs by controlling the usage of POPs with the view to phasing them out. The Convention requires that each party should prohibit and/or take any administrative or legal action required for the reduction/elimination of POPs production and usage, export and import, as well as to take actions to prevent or minimize POPs' release into the environment. The Convention identified twelve POPs chemicals known as the "*dirty dozen*" for intervention, and new chemicals are considered for listing at each Conference of the parties. These chemicals are presented in **Table 1**. Nine of the initial POPs are pesticides, one is an industrial chemical and two are unintentionally produced


#### **Table 1.**

*The twelve convention-identified POPs: The dirty dozen.*


*Recent Developments in the Application of Advanced Oxidative Processes for Remediation… DOI: http://dx.doi.org/10.5772/intechopen.101304*

#### **Table 2.**

*The POPs listed in May 2004.*

through certain industrial processes. Nine new POPs (**Table 2**) were listed in May, 2004 which includes unintentionally produced and released POPs, which result from some industrial processes [17].

Another approach to prevent the proliferation of POPs is the development of novel advanced technologies for remediation of water pollution. Water for human and animal consumption must be adequately treated to ensure excellent health, thus pollutants must be removed. Freshwater resources are exposed to a number of organic pollutants, including dyes, medicines, industrial chemicals, pesticides, and personal care products, which are discharged directly into natural water systems on a daily basis. Treatment of industrial effluents before being released into natural water bodies is crucial for effective protection of natural water resources. Some of the successful water treatment procedures used in the service and provision of

industrial or municipal potable water include flocculation, coagulation, filtration, and chlorination. Conventional treatment, however, is ineffective against removal of POPs. Despite their established lipophilicity, POPs are unlikely to adsorb on organic matter, and application of treatment chemicals frequently produce undesirable intermediates, making conventional treatment methods unfruitful [18].

However, various advanced wastewater treatment technologies such as activated carbon adsorption, membrane bioreactor (MBR) and advanced oxidation processes (AOP) have been applied in the treatment of POPs to counter the difficulty in conventional methods [19].

#### **3. Advanced oxidation processes (AOP)**

Advanced oxidative processes are aqueous phase oxidation systems that produce highly efficient oxidizing agents, such as hydroxyl and sulfate radicals that are mainly generated as the dominating species, which have high ability to destroy POPs. The radicals ensure that soluble organic pollutants are effectively degraded into biodegradable and simple molecules. The solution pH, water turbidity, duration of reaction, the amount/volume of the organic component sensitive to degradation, and the presence of OH radical scavengers or activator chemicals can all have impacts on the OH radical's degradation activity. The distinctiveness of these processes is their diversified creation of extremely reactive OH radicals, which oxidize organic contaminants in wastewater non-specifically and quickly. Depending on the treatment goals and the features of the wastewater stream, AOPs can be used as a single procedure, in combination with other AOPs, or in combination with conventional treatment techniques [20]. The benefits, which include ease of operation, high performance, low selectivity, strong reproducibility, minimal by-products' generation, and total degradation of polluants, make AOPs viable techniques for removal of POPs from aqueous effluents [21]. Classifications of AOPs for POPs removal are depicted in **Figure 1**.

Sulfate radical (SR, SO4 − ) based AOPs have recently shown promising potentials in the degradation of non-biodegradable chemicals, with peroxydisulfate

**Figure 1.**

*Types of AOPs for POPs removal and degradation.*

#### *Recent Developments in the Application of Advanced Oxidative Processes for Remediation… DOI: http://dx.doi.org/10.5772/intechopen.101304*

(PDS, S2O8 2−) or peroxymonosulfate (PMS, HSO5) as oxidants. Sulfate radical has a half-life longer than **·**OH (hydroxyl radicals); SO4 − has a greater redox potential (2.5e3.1 V) than **·**OH that has a standard redox potential of 1.8e2.7 V; and SR is more selective for the oxidation of organic contaminants over a wide pH range. Sulfate radicals react with organic molecules by removing hydrogen, adding to double bonds, and transferring electrons. Electron-donating groups will have a faster reaction rate than electron-withdrawing groups with the SR because SR is electrophilic. Some organics will react immediately with persulfate, creating SRs that propagate secondary reactions or organic radicals that decompose the desired pollutants [22]. Peroxydisulfate is not costly, and has high aqueous solubility and stability at room temperature, that is why it is often utilized as a source of SO4 − [23, 24]. SBA-15 silica has been used as promising support for metal catalysts due to its outstanding hydrothermal and mechanical stabilities. Similarly, Fe- and Co-based catalysts supported on SBA-15 are widely used in catalytic degradation of non-biodegradable organic compounds [25]. Electro-activated persulfates could remove various POPs, such as pesticides, dyes, and pharmaceuticals, from simulated water at the laboratory scale. Wu and co-authors reported that the EC/Fe2+/S2O8 2− process removed 65.8% of acid orange 7 from wastewater in 60 min [26]. It has been shown that the combination of Electrochemical (EC) and heterogeneous activation of PDS using Fe–Co/SBA-15 as catalyst is a satisfactory technique for POPs removal—excellent in real water treatment (e.g., groundwater and wastewater) [27]. Electrochemical technique has recently been used in various investigations for persulfate activation due to its advantages in creating less sludge and so reducing both reactor volume and investment costs. Some refractory organic pollutants can be completely removed from contaminated sites under ideal conditions [28].

One of the homogenous phase methods of AOPs, the application of ionizing radiation, is considered as one of the most favorable and effective AOPs in the removal of POPs. Water and wastewater treatment with ionizing radiation is favorable because both eaq− and • OH are generated in the process of water radiolysis when diluted aqueous solutions (natural waters/most types of wastewaters of various origins) are irradiated. Depending on the chemical reactivity of the target species, water radiolysis products may participate in oxidative reactions with organic contaminants [29]. The most common process is water radiolysis, which produces compounds that react with dissolved species. Physical (≈ 1 fs), physico-chemical (10−15 – 10−12 s), which involves numerous processes, and chemical stage (10−12 – 10−6 s) are the three primary stages that occur at distinct rates [30]. Ionization radiation such as electromagnetic and gamma ones, high energy electrons (electron beam, EB) and charged particles and neutrons, virtually only *γ*-rays, and EB irradiations are employed in water and wastewater treatment [31]. Untreated (standard blue) chemical effluent from dye manufactures treatment with EB irradiation was effective in significantly decreasing toxicity and color; p < 0.0001 was obtained for the sample treated by 2.5 kGy [32]. The hyphenation of established original AOPs with biological treatment for ordinary applications is one of the most recent advances [33]. A combination of electron-beam of radiation source 1 MeV EB accelerator, 400 kW and biological treatment was used for purification of dyeing complex wastewater under continuous flow conditions in South Korea by Han et al. [34].

Heterogeneous photocatalysis, one of the most AOPs for water purification, is an effective, cost-effective, and environmentally friendly method of eliminating organic pollutants. During the process, a semi-conductor irradiated with an appropriate wavelength of light produces active species, which oxidize organic compounds dissolved in water. The primary benefit of this approach is that it is inherently destructive; it does not require mass transfer; can be performed at ambient conditions (atmospheric oxygen is utilized as an oxidant); and can

result in complete mineralization of organic carbon into CO2 [35]. The pollutant, catalyst, and source of illumination must all be in close proximity or in contact for photocatalysis to work efficiently and effectively. Advanced oxidation technology's ability to remove minimal concentrations of POPs from water has been thoroughly demonstrated, and the technology is gradually being implemented in many parts of the world, including developing countries [36]. Semiconductor photocatalysts commonly used include TiO2, CeO2, ZnO, Fe2O3, CdS, and so on. Due to its strong photocatalytic capability, chemical and biological inertness, excellent photochemical stability, and inexpensive price, titanium dioxide (TiO2) is a widely utilized photocatalyst in environmental degradation of organic molecules which led to complete mineralization into CO2, H2O, and harmless inorganic anions [37]. After 180 min of irradiation (λ > 400 nm), about 99% pentachlorophenol (PCP) was removed by Ag-deposited TiO2 nanotubes (TNTs) under simulated solar light [38]. Several other photocatalysts have been utilized for removal of POPs in wastewater. Lopes da Silva et al. [39] investigated the degradation of perfluorooctanoic acid (PFOA) in water using Indium oxide and the results showed a good potential of nanosized In2O3 photocatalyst in degradation. Nanocrystalline ZnO particles doped with different concentrations of Fe impurity was able to degrade methylene blue (MB) dye in aqueous solution under UV/sunlight exposure [40]. Zn/TiO2 catalyst synthesized from the hydrothermal method removed *ca.* 80% of paraquat from aqueous solution (using 4g L−1 of catalyst) under UV and solar light irradiation [41].

Electrohydraulic discharge system is one of the most advanced oxidation methods for degrading hazardous organic contaminants in water and wastewater. Electrical plasma technology, as one of AOPs, has sparked a lot of interest in the removal of organic pollutants, owing to the absence of external chemicals, environmental compatibility, ability to kill microbes, non-generation of secondary pollution high removal efficiency, efficacy, and ease of operation at ambient temperature and pressure [42, 43]. Depending on the solution pH, conductivity, and discharge magnitude, an electrohydraulic discharge system can activate both the physical process and the chemical reaction mechanism, which subsequently generates free active species such as H2O2, OH radical, O, O3, and O2**·** − . This system could be combined with a number of AOPs including chemical, photolysis, ultrasonic irradiation, electrical, and supercritical water oxidation in water. Oxidative degradation of medicinal drug diclofenac (DCF) in water was investigated using a pulsed corona discharge generated above liquid. Efficient removal of DCF in water was achieved after 15 min of non-thermal plasma treatment as DCF in solution was totally removed [44]. Two different non-thermal plasma dielectric barrier discharge (DBD) reactors (planar and coaxial) at atmospheric pressure were assessed for the removal of organic micropollutants (Atrazine, Chlorfenvinfos, 2,4-Dibromophenol, and Lindane) from aqueous solutions (1–5 mg L−1) at laboratory scale. The parent compounds disappeared as the plasma treatment time increased, and the degradations in both DBD devices followed first-order kinetics (*k*) in distilled water. The highest *k* value was recorded for 2,4-dibromophenol in the planar reactor, whereas the lowest *k* value was obtained for atrazine in the coaxial reactor [45]. The degradation of 13 distinct textile colors in an experimental DBD plasma batch reactor was investigated, during a 600-second treatment in the batch reactor, between 90 and 99% of most dyes were degraded. The investigation revealed that the DBD approach could be utilized to remediate a range of synthetic dye polluted water at low concentrations (up to 50 mg L−1) with success [46].

Ozonation is another AOP that aims to degrade a wide range of organic pollutants in water by targeting their unsaturated hydrocarbon bonds [47]. Ozonation is the most promising method for pollutant degradation, according to laboratory and pilot-scale research, because it successfully eliminates a variety of substrates

#### *Recent Developments in the Application of Advanced Oxidative Processes for Remediation… DOI: http://dx.doi.org/10.5772/intechopen.101304*

and by-products. Degradation, by ozonation, of 20 mg L−1 of sulfadimethoxine (SDM) at pH 7.0 in different water matrices was investigated. Water treatment *via* ozonation was proven to be effective as 100% removal of SDM was achieved within 10 min [48]. Dar et al. [49] used two different ozone generators (sources: water and air) to test the degradation of pentabromophenol (PBP) in an aqueous media. The water-source ozone generator achieved complete degradation of 50 μmol L−1 PBP after 5 min, and the air-source ozone generator achieved complete degradation of 10 μmol L−1 PBP after 45 min. The authors found out that ozonation is an effective and suitable procedure for PBP degradation in real water systems. The effectiveness of ozone treatment to eliminate the 16 priority Polycyclic aromatic hydrocarbons (PAHs) in waste activated sludge (WAS) was investigated by optimizing ozonation performance by varying key operating variables, including ozone gas flow rate, inlet concentration and dosage and then explore the pH dependent behavior of ozone-oxidation. The PAHs removal efficiency increased with ozone dosage and was strongly pH dependent. Even at ozone dosage of 40 mg O3·g−1, the PAHs removal efficiency at pH 9.0 (44.5%) was significantly higher than the one observed at pH 5.0 and 200 mg O3·g−1 (41.7%). The research indicates the need of WAS disintegration during ozonation to make PAHs more accessible to O3 molecules and **·**OH to initiate oxidation reactions and recommended to adopt a sequential batch operation for ozonation to mitigate the negative effect of soluble organic compounds generated by sludge solubilization so as to practically use ozonation for elimination of PAHs in WAS [50]. Nanotechnology can also be incorporated into ozonation for more efficient removal. The elimination of five organochlorine pesticides {hexachlorobutadiene (HCHBD), pentachlorobenzene (PCHB), hexachlorobenzene (HCHB), lindane (LIN), and heptachlor (HCH)} using integrated O3/nZVI procedures in water solution was explored. Except for LIN and HCHB, the ozonation method showed high removal efficiency of >90% after 60 min. The O3/UV procedure yielded somewhat higher removal rate and efficiency. Within 5 min of starting the nZVI procedure, high removal efficiency for LIN, PCHB, and HCH were measured. The findings imply that the O3/nZVI mechanisms have great potential for increasing organochlorine pollutants breakdown and elimination [51]. Ozone-based processes, i.e., single ozonation, O3/UVA, O3/UVA/Fe3+ and O3/UVA/ magnetite have been shown to be suitable technologies to deal with POPs [52].

Fenton oxidation is an AOP that is cost-effective and efficient method of removing POPs from water because of the low toxicity of the reagents, absence of mass transfer limitation due to its homogeneous catalytic nature (i.e., Fe2+ and H2O2), and the simplicity of the technology. The standard Fenton reaction, on the other hand, has a number of drawbacks, such as a narrow pH range, the formation of Fe-containing sludge, and a low hydrogen peroxide usage rate [53]. The Fenton system involves combining ferrous ions with hydrogen peroxide to produce hydroxyl radicals, which have a strong oxidizing ability and can breakdown organic pollutants. Ferric ions are generated during the reaction, which can be reacted to yield ferrous ions. A Fenton-like reaction is a reaction that occurs when hydrogen peroxide reacts with ferric ions. The main disadvantage of this method is the high cost of the reactants, H2O2 and Fe2+. As a result, numerous methods have been developed to utilize Fe3+ salts rather than Fe2+ salts, resulting in the photo-Fenton and electro-Fenton approaches [54]. Amorphous FeOOH quantum dots (QDs) were coupled with polymeric photocatalysts *g*-C3N4 which was developed as a visible light driving photo-Fenton catalyst. Highly dispersed FeOOH QDs anchored on *g*-C3N4 showed enhanced visible light driving photo-Fenton degradation of methylene orange (MO) and phenol—an indication that the FeOOH QDs coupled with g-C3N4 is a promising visible light driving photo-Fenton catalyst for organic pollutants treatment. S-doped NiFe-based particles were prepared by a solvothermal method and used


*Recent Developments in the Application of Advanced Oxidative Processes for Remediation… DOI: http://dx.doi.org/10.5772/intechopen.101304*


#### **Table 3.** *POPs removal using various AOPs.*

for degradation of methylene blue (MB) from aqueous solutions with visible light in photo Fenton reaction. Results showed that NiFe2S4 has a great performance of MB degradation in the photo-Fenton oxidation process; 100 mL of 30 mg L−1 MB could be completely degraded (99.8%) within 6 min under optimal reaction conditions [55]. Xiang and co-authors employed yolk-shell ZnFe2O4 as photo-Fenton catalyst to investigate antibiotics degradation. The yolk-shell ZnFe2O4 not only exhibited excellent photocatalytic activity for the removal of popular pollutants, but also for the co-existing pollutants consisted of tetracycline (TC) and Ciprofloxacin (CIP). They reported a novel technique for preparing high efficiency of photo-Fenton catalysts for decontamination of refractory pollutants in aqueous solutions [56].

Various studies conducted on the usage of AOPs for elimination of POPs from water are summarized in **Table 3**.

#### **4. Conclusion**

Persistent organic pollutants get into the environment through municipal populations, industrial and agricultural activities. A number of chemicals have been identified as POPs, and these chemicals include, but not limited to, Aldrin, Endrin, Heptachlor, Mirex, Chlordane, Toxaphene, Dieldrin, Hexachlorobenzene, Furans (polychlorinated dibenzofurans), Polychlorinated biphenyls, Dioxins (polychlorinated dibenzo-p-dioxins) and DDT. Advanced oxidative processes (AOPs) are aqueous phase oxidation systems that have high ability to eliminate POPs from water systems. Although several advanced oxidation processes (AOPs)

have recently achieved success in the treatment of POPs in wastewater, the successes of POPs treatment using various advanced technologies are not without downsides, such as low degradation efficiency, toxic intermediate generation, massive sludge production, high energy expenditure and high operational cost. Combination of AOPs is recommended for effective elimination of POPs from water.

### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Ifeoluwa Oluwafunmilayo Daramola1 and Matthew Ayorinde Adebayo2 \*

1 Department of Chemistry, University of Fort Hare, Alice, South Africa

2 Department of Chemistry, The Federal University of Technology, Akure, Nigeria

\*Address all correspondence to: adebayoma@futa.edu.ng

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Recent Developments in the Application of Advanced Oxidative Processes for Remediation… DOI: http://dx.doi.org/10.5772/intechopen.101304*

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### *Edited by Mohamed Nageeb Rashed*

Persistent organic pollutants (POPs) are toxic organic compounds that resist environmental degradation through biological, chemical, and photolytic processes. Many POPs are currently used as pesticides, pharmaceuticals, solvents, and industrial chemicals. Because of their persistence, POPs bioaccumulate and adversely affect human health and the environment. *Persistent Organic Pollutants (POPs) - Monitoring, Impact and Treatment* deals with several aspects of POP monitoring, occurrence, impact, and treatment technologies. The book is divided into two sections containing nine chapters that address such topics as the effect of POPs on wildlife, their role in hepatocarcinogenesis, treatment of POPs in wastewater, and much more.

### *J. Kevin Summers, Environmental Sciences Series Editor*

Published in London, UK © 2022 IntechOpen © Weedezign / iStock

Persistent Organic Pollutants (POPs) - Monitoring, Impact and Treatment

IntechOpen Series

Environmental Sciences, Volume 1

Persistent Organic

Pollutants (POPs)

Monitoring, Impact and Treatment

*Edited by Mohamed Nageeb Rashed*