*2.3.4 Phytoextraction*

In phytoextraction, heavy metals are removed from contaminated materials (soil and water) by uptake into harvestable plant parts [54]. As a result, phytoextraction reduces soil contamination. Most plant species cannot sustain in heavily polluted environments, so phytoextraction is suitable for sites with low-moderate levels of metal pollution [55]. There are four characteristics of plant species that can be used effectively for phytoextraction: (a) high metal-accumulation capability in their aboveground parts, (b) tolerance to high metal concentrations, (c) ability to grow rapidly with high biomass, and (d) profuse root systems. A number of common

*New Advancements in the Field of Pollution Treatment, Including Contamination of the Soil… DOI: http://dx.doi.org/10.5772/intechopen.109955*

hyperaccumulator plants have been discovered and used to treat heavy metal-contaminated soil, including *Pteris vittata* L, *Sedum plumbizincicola*, *Solanum nigrum* L, *Polygnoum hyiper* L, *Thlaspi rave service* L, *Calendula officinalis*, and many others [56]. There are some limitations to heavy metal extraction by hyperaccumulating plants, including poor extraction efficiency, low biomass, easy environmental impact, heavy metal poisoning, and long repair times. However, they can be avoided by combining them with other technologies. Heavy metals in the soil can be effectively activated by adding chelators, creating a water-soluble metal chelator complex, which could change the occurrence form of heavy metals in the soil and then encourage the enrichment of heavy metals by plants, given the limitations of hyperaccumulation plants in the extraction of heavy metals [57]. The synthetic chelating agents EDTA, DTPA, EGTA, and EDDS are the most often utilized ones. Lead has the greatest ability to be activated by EDTA when compared to other heavy metal ions like Cu, Zn, and Cd.

### **2.4 Nanoremediation**

The use of nanoparticles in nanoremediation enables the removal of heavy metals contaminants from soils and other environments in a cost-effective and eco-friendly manner [58]. Through the use of this novel remediation method, heavy metals can be absorbed, reduced to a stable metallic state, and catalyzed to leave a site [59, 60]. Different technical processes are employed in nanoremediation, including adsorption, heterogeneous catalysis, electrical field deployment (electronanoremediation), photodegradation, and the use of microorganisms (nanobioremediation) to remove or immobilize heavy metals from contaminated soils [61]. Metal nanoparticles, metallic oxides nanoparticles, carbonaceous nanoparticles, polymeric nanoparticles, and nanocomposites have all been successfully used and applied to remove heavy metals. Through pore spaces, nanoparticles can also reach inaccessible areas, such as crevices and aquifers, eliminating the need for traditional methods. These remediation materials have three modes of action: (1) A physical process that involves the adsorption and immobilization of contaminants on the surface of the particles. In one study, iron oxide Fe3O4 particles (12 nm in diameter) were used to remove arsenic from water after sorption and magnetic separation. (2) Toxic compounds are transformed into less harmful products through the process of detoxification, which induces and/or catalyzes the initial chemical breakdown. The dominant mechanisms are oxidationreduction reactions, as in the photocatalytic oxidation of organics by titanium oxide TiO2 nanoparticles [62], or the reduction of organics by nanoscale zero-valent iron (nZVI). (3) In bio-cooperative degradation, the particles increase bioavailability while degrading pollutants into more bioremediable species [63]. As an illustration, Fe3O4 NPs were employed as ion suppliers to enhance the production of biogas during anaerobic digestion procedures [64]. The tendency of nanoparticles to biostimulate bacterial cells was highlighted in a recent review by Abdelsalam and Samer, which also highlighted how this increased bacterial activity and growth kinetics [65].

### **3. Water treatment technologies**

Reclaiming freshwater for use in agriculture and human activities requires wastewater treatment. Every year, as global water demands rise, many pollution schemes have threatened water sources [66]. Proper treatment and permanent removal of heavy metals are of immediate necessity. Many effective ways to remove pollutants

such as heavy metals from wastewater are currently available [67]. Conventional techniques include ion exchange, membrane filtration membrane filtration, and chemical precipitation. Due of its simplicity, the chemical precipitation process is extensively utilized. Other alternative treatment techniques like photocatalysis, electrochemical, flotation, coagulation, and adsorptions have garnered a lot of attention in recent years. In order to remove heavy metals from wastewater, this study analyzes numerous treatment systems, their mechanisms, and the most recent developments.

### **3.1 Photocatalysis**

Photocatalysis is a photo-activated chemical reaction occurring when free radical mechanisms are initiated as contact is made between the compound and photons that have sufficiently high energy levels. The words photo, which has to do with photons, and catalyst, which is a chemical that affects the rate of a process when it is present, are combined to form the term "photocatalyst." As a result, photocatalysts are substances that, when exposed to light, alter the rate of a chemical reaction. The term "photocatalysis" refers to this occurrence [68]. This technique was created as a result of research to emulate photosynthesis and the evolution of hydrogen for use in environmental applications. Semiconductors known for their photocatalytic properties, such as TiO2, ZnO, CeO2, CdS, and ZnS, was used in photocatalytic processes [69]. Strong oxidizing power, the ability to destroy heavy metal complexes and release them from the metal ions, and the capacity to oxidize and degrade organic complexes simultaneously are the characteristics that define photocatalysis.

Three processes make up the basic mechanism of photocatalysis. The first step is the production of charge carriers, which happens when a semiconductor is exposed to light that has a high energy or is equal to its bandgap. Second, the produced electron-hole pair moves onto the semiconductor's surface as electrons transition from the photocatalyst's valence band (vb) to conduction band (cb). Thirdly, electrons decrease the O2 molecule to make superoxide radical anion (O2) in the conduction band while photogenerated holes oxidize the H2O molecule to yield OH in the valance band [70]. Various metal oxide-based photocatalytic materials such as TiO2, ZnO, CuO, CdS, etc. have been used to remove organic and inorganic pollutants present in wastewater.

## **3.2 Coagulation/flocculation**

Coagulation flocculation is a highly efficient physicochemical method for removing heavy metals [71]. In this process, fine particles and colloids agglomerate into larger particles, reducing turbidity, NOM and other wastewater pollutants. In the first stage, a coagulant added to the water stimulates the coalescence of colloidal material into small aggregates known as flocs [72]. The most commonly used coagulants include aluminum sulfate, ferrous sulfate, polyaluminum chloride (PACl), polymeric ferrous sulfate (PFS), and polyacrylamide (PAM) [73]. In the second stage, with gentle agitation, the flocs agglomerate, settle and are then disposed of as sludge. This process is used as a pre-treatment, post-treatment or main wastewater treatment due to its versatility [74]. This process is relatively economical and simple in operation, but limitations are incomplete removal of heavy metals, generation of sludge, and high operating costs due to chemical consumption.

*New Advancements in the Field of Pollution Treatment, Including Contamination of the Soil… DOI: http://dx.doi.org/10.5772/intechopen.109955*

### **3.3 Chemical precipitation**

Chemical precipitation is an effective technique for removing heavy metals, mainly from effluents from the papermaking and electroplating industries. In this process, chemical precipitants such as alum, lime, iron salts and some polymers react with heavy metals present in the wastewater, resulting in insoluble precipitates [75]. This reaction allows metals to be removed more easily. Removal capacity and efficiency can be improved by optimizing parameters such as pH, temperature, initial concentration and ionic charge [76]. The mechanism of heavy metal removal by chemical precipitation is given by Eq:

$$\text{M}^{2+} + 2\text{(OH)}^{\cdot} \rightarrow \text{M(OH)}\_{2} \tag{1}$$

where M2+ and OH are the metal ions and the precipitant, respectively, and M(OH)2 is the metal hydroxide. The pH is adjusted to basic conditions (pH 9–11), which has the greatest impact in this treatment. Chemical precipitation is divided into hydroxide and sulfide precipitation. The use of coagulants in hydroxide precipitation can improve heavy metal removal by filtration or sedimentation. On the other hand, the sludge generated in the metal sulfide precipitation is removed by gravity separation or filtration. This process requires pre- and post-treatment as well as precise control over the addition of reagents due to the toxicity of sulfide ions and H2S. Although this method has the following advantages: low capital investment, simple operation, and easily automated treatment method but it also brings problems that can be produce a large amount of sludge containing toxic compounds that require further treatment, requires a large number of chemicals to reduce metals to an acceptable level for discharge, slow metal precipitation, poor settling, and the long-term environmental impacts [74].

### **3.4 Ion exchange**

A reversible ion exchange takes place between the solid and liquid phase. In particular, an insoluble substance removes the ions from an electrolyte solution and releases other ions of similar charge in chemically equivalent amounts. The most common ion exchange materials are synthetic organic resins [77], inorganic three-dimensional matrix and new generation hybrid materials [78]. Using an adequate replacement resin can provide an effective and economical solution to contamination control requirements. In the case of heavy metals, more highly concentrated metals are obtained by elution with suitable reagents after separating the loaded resin. The acid functional resin contains sulfonic acid in its structure. Therefore, the physicochemical interactions occurring during the removal of metal ions. Various optimization goals can be investigated for ion exchange. For example, use less resin to achieve a greater removal rate and optimize contact time with a smaller device size [79]. Anionic resins are generally used at a lower pollutant concentration, while cationic resins contain strong and weak acidic resins with more extensive use [80]. Weakly acidic resins with (COOH), while acidic resins with (∙SO3H) group are among the most popular cation exchangers [81]. However, ion exchange has some disadvantages, such as B. the need for a pre-treatment process, for example to remove fat or oil, as well as the need for chemical reagents to recover resins, which also cause secondary pollution [82].

### **3.5 Electrochemical technologies**

Heavy metal ions from water sources can be effectively removed using electrochemical treatment techniques. These techniques involve recovering metals in their elemental metal state by employing cathodic and anodic processes in an electrochemical cell. Electrochemical treatments include electrocoagulation, electroflotation, and electrodeposition [83]. Traditional chemical coagulation is where electrocoagulation gets its start [84]. In this procedure, anode and cathode electrode sets serve as the sites for the oxidation and reduction reactions, respectively. An appropriate anode material is electrolytically oxidized to produce the coagulant as the charged ionic metals react with the anion in the effluent. By depositing pollutants on the cathode or removing them via flotation, the simultaneous cathodic reaction enables the removal of contaminants [74]. This method produces less sludge, is simple to use, and does not require any chemicals. The recovery of harmful metal ions from industrial wastewaters, such as Pb, Cd, Cu, Ni, Zn, or Cr, or the recovery of valuable metals from solutions, such as Ag, Pt, Au, etc., both involve considerable use of electrodeposition. The cost of treating water electrochemically has been reduced through a number of initiatives. In this regard, a comparison between platinum plate and stainless steel AISL904L was described. When treating Cu (II) from industrial contaminants, these plates are employed in place of three-dimensional electrodes. Cu foam can be used as an alternative because it has a wide surface area and performs better for the removal of effluents, but it makes the process more expensive. It was discovered that treating industrial water with tin dioxide anodes during the electrochemical process reduced water and electrolyte consumption by up to 70% [85].

### **3.6 Membrane technologies**

A membrane acts as a barrier, allowing some substances to pass through while obstructing others. This technology is controlled by the Donnan exclusion effect (charge-charge repulsion), the size exclusion or steric hindrance mechanism, and the adsorption capacity of particular pollutants [86]. This form of treatment can be used to get rid of organic and inorganic pollutants, suspended solids, and other things. Membranes are categorized as either organic (made of synthetic organic polymers like polyethylene or cellulose acetate) or inorganic (made of ceramics, metals, zeolites, silica, among other materials) depending on the substance used to make them [87]. Microfiltration, ultrafiltration, and distillation are examples of low-pressure membrane processes. Nanofiltration, reverse osmosis, and electrodialysis are examples of highpressure membrane processes. Direct osmosis, electrodialysis, and liquid membrane processes are examples of osmotic pressure-driven membrane processes. The removal performance of a membrane is greatly influenced by a number of variables, including the size and distribution of the pores, surface charge, degree of hydrophilicity, solution flow, and the presence of functional groups. These variables must be taken into account.

### **3.7 Adsorption**

One of the finest ways to remove heavy metals and other impurities from water is adsorption. Its benefits include the potential to prevent significant secondary pollutants, a high removal capacity, relatively low energy consumption, and technical requirements for operation [88]. Adsorbents should possess a number of desirable qualities, including a sizable specific surface area, high mechanical strength, strong

*New Advancements in the Field of Pollution Treatment, Including Contamination of the Soil… DOI: http://dx.doi.org/10.5772/intechopen.109955*

thermal stability, predictable morphology, and processing that is ecologically benign. Given the high adsorption capacity and efficiency, selectivity, low cost, and reusability, this should result in a high performance. Some of the most popular adsorbents are activated carbon (AC), polymer-based materials, biomaterials, magnetic materials, and industrial and agricultural wastes. Agricultural waste (fruit peels, bagasse, coir pith, cobs of corn, sawdust, and bark); Activated carbon (wood peat, coconut shells, coals); polymeric substances (Lignin, Chitosan, Cellulose, Alginate, Silk, and Cyclodextrin); sludge, metal hydroxide, red mud, fly ash, and other industrial byproducts; Ilmenite, Hematite, Magnetite, Spinel ferrite, and other magnetic adsorbents, Metal oxide particles/graphene composites, polymer matrix composites, and lignocellulosic residues/magnetic particles are all examples of composite adsorbents that are utilized for the removal of metals from wastewater [74].

### **3.8 Nanotechnology**

Treatments based on nanotechnology make use of nanomaterials, which have drawn interest in recent years due to their high surface-to-volume ratios and distinctive electrical, optical, and magnetic capabilities [89, 90].

One of the most popular nanotechnology technologies for heavy metal removal is nanofiltration. Chemisorption is a highly effective method for eliminating dissolved heavy metals in systems made of alumina nanofibers. Additionally, low dimensional structures like nanoclays, magnetic nanoparticles, single or multi metal oxides, non-metal oxides, and nanocarbon are the most frequently used for the purification, disinfection, and removal of heavy metals from water [91, 92]. All of these nanostructures have huge, highly reactive surfaces, and many of them may be produced synthetically or using abundant natural resources. Similar technologies for wastewater treatment include nano-assemblies, nanoplates, microspheres with nanosheets, and hierarchical ZnO nano-rods. However, the dearth of knowledge regarding the toxicity, effects on the environment, and health of nanomaterials is significant and prevents their full utilization [93]. Nanocarbon (carbon nanotubes, graphene and other carbon derivatives), 2D materials, also known as single-layer materials, include graphene and borophene, germanene, silicene, 2D metal carbides (MXenes), MoS2 nanosheets, silicon, iron, titanium, zinc, magnesium, and manganese oxides are commonly used for heavy metal remediation in the form of nanoparticles [94, 95].

### **3.9 Cold plasma technology**

Cold plasma is characterized by a range of temperatures that correlate to various types of particles. A considerable number of energetic and chemically reactive species, such as free radicals, excited atoms, ions, and molecules, are produced in cold plasmas thanks to the high energy of the electrons (up to 1–10 eV), which serves as the catalyst for the start and spread of plasma chemical reactions. One of the method's most significant benefits is that it does not require high temperatures, which lowers energy use [96, 97].

Due to the possibility of using various operating gases (air, Ar, O2, N2, etc.) or types of plasma discharges (such as glow discharge, corona discharge, radio frequency discharge, gliding arc discharge, and dielectric barrier discharge), various plasma properties may develop, leading to the emergence of a number of applications. Due to the special characteristics of cold plasma, it is widely used in various fields [98]. Different settings have been studied for the efficient degradation of contaminants

depending on the type of electrical discharges and reactor layouts. Three steps could be used to summarize the cold plasma process: Highly energetic electrons, OH radicals, ozone, O- and N-contained excited species, as well as other reactive species, are produced during the first step, contributing to the initiation and progression of the plasma chemical reactions; the second step entails the intrusion of the reacted species on the soil surface or soil pores or the dissolution or diffusion of the reacted species [99]. The capacity of easy mass transfer has a significant impact on the efficiency of remediation in both soil and water treatment as well as the effectiveness of the contact between the reactive species and the soil/water. When making electrical discharges during water cleanup, whether in a liquid or at a gas-liquid interface, the transport is changed since it is carried out by the slow aquatic ions, which is significantly impacted by the liquid conductivity. While other influencing factors (such as the impact of ionic charges present in water on the RONS/pollutant interaction) that also affect the process are not fully understood, it is discovered that the dissolved gases that create plasma micro-bubbles inside the liquid play a significant role in the process. The final step in the cleanup of contaminated sites is the chemical reaction of the reactive species with the organic pollutants. The process is impacted by the pollutants' type. For example, during soil treatment, highly volatile molecules are broken down via a two-path decomposition (evaporation of the contaminants into the gas phase where gas phase reactions are occurring or/and direct oxidation in soil due to the presence of the active species), whereas in the case of less volatile compounds, the oxidation processes are primarily occurring on the soil granules (the reactive species get in touch with the soil through diffusion or adsorption) [96].
