**2. Status of heavy metal pollution**

instance, lead (Pb), cadmium (Cd), and chromium (Cr), which are potentially hazardous to ecosystems. The types of microorganisms that are used in bioremedia‐ tion processes due to their natural capacity to biosorb toxic heavy metal ions are discussed in detail. This chapter summarizes existing knowledge on various aspects of the fundamentals and applications of bioremediation and critically reviews the

Environmental contamination by heavy metals from anthropogenic and industrial activities has caused considerable irreparable damage to aquatic ecosystems. Sources include the mining and smelting of ores, effluent from storage batteries and automobile exhaust, and the manu‐ facturing and inadequate use of fertilizers, pesticides, and many others. The metals and metalloids that contaminate waters and are most commonly found in the environment include lead, chromium, mercury, uranium, selenium, zinc, arsenic, cadmium, silver, gold, and nickel. These metals are the subject of concern due to their high toxicity. Apart from being hazardous to human health, they also have an adverse effect on the fauna and flora, and they are not biodegradable in nature. Thus, there is a need to seek new approaches in developing treatments

Several different physicochemical and biological processes are commonly employed to remove heavy metals from industrial wastewaters before their discharge into the environment [1]. Conventional physicochemical methods such as electrochemical treatment, ion exchange, precipitation, osmosis, evaporation, and sorption are not cost-effective, and some of them are not environmentally friendly [2, 3]. On the other hand, bioremediation processes show promising results for the removal of metals, even when present in very low concentrations where physicochemical removal methods fail to operate. Furthermore, this is an eco-compat‐ ible and economically feasible option. The bioremediation strategy is based on the high metal binding capacity of biological agents, which can remove heavy metals from contaminated sites with high efficiency. In this regard, microorganisms can be considered as a biological tool for metal removal because they can be used to concentrate, remove, and recover heavy metals from contaminated aquatic environments [4]. Several studies have been conducted using microorganisms for the uptake of heavy metals in polluted waters as an alternative strategy to conventional treatments [5–7]. Bioremediation by microorganisms is very useful due to the action of microorganisms on pollutants even when they are present in very dilute solutions, and they can also adapt to extreme conditions. Although the mechanisms associated with metal biosorption by microorganisms are still not well understood, studies show that they play an important role in the uptake of metals and that this action involves accumulation or resistance. In this chapter, previously published research data on the potential of the microorganisms that have been used for the bioremediation of metals is discussed. In-depth descriptive information

obstacles to its commercial success and future perspectives.

2 Advances in Bioremediation of Wastewater and Polluted Soil

to minimize or even eliminate metals present in the environment.

**1. Introduction**

**Keywords:** Metals, microorganisms, bioremediation, polluted water

The term "heavy metal" generally refers to metallic elements with an atomic weight higher than that of Fe (55.8 g mol–1) or density greater than 5.0 g cm–3, and these metals are naturally present in the environment. However, some metals with an atomic weight lower than that of Fe, for example, Cr, and others which are considering metalloids, such as As and Se, are also commonly referred to as heavy metals [8]. Heavy metals can play a role as micronutrients, such as Cu, Fe, Mn, Mo, Zn, and Ni, but they can also be toxic to humans, e.g., Hg, Pb, Cd, Cu, Ni, and Co [9], depending on the exposure levels.

Contamination by heavy metals causes many deleterious effects, which affect not only fauna and flora but also human health [10, 11]. Heavy metal ions have a strong electrostatic attraction and high binding affinities with the same sites that essential metal ions normally bind to in various cellular structures, causing destabilization of the structures and biomolecules (cell wall enzymes, DNA and RNA), thus inducing replication defects and consequent mutagenesis, hereditary genetic disorders, and cancer [12]. Heavy metals are notable contaminants because they are toxic, nonbiodegradable in the environment, and easily accumulated in living organisms [13]. The contamination of waters with heavy metals occurs through natural and anthropogenic activities, mainly related to industrialization. Table 1 shows the natural and anthropogenic sources of some of the most widespread study heavy metals as environmental pollutants, together with a brief list of their adverse health effects and their applications [14]. Although studies on bioremediation generally consider the total amount of metal present in the environment, the toxicity of these metals is dependent on their chemical form. The wide range of chemical forms in which heavy metals can be present in the environment includes cationic/anionic species and complexes (hydroxylated or complexed to Cl), and their oxidation state varies depending on the medium pH and composition.

Heavy metals contaminated in soil can accumulate and persist for long periods of time and may be harmful to vital processes involved in microbial nutrient cycling [15]. The toxicity and mobility of heavy metals are strongly dependent on their chemical form and specific binding properties. Changes in the environmental conditions in soils, such as acidification and variations in the redox potential, can cause the mobilization of heavy metals from the solid phase to the liquid phase, thereby allowing the potential contamination to the plants grown in these soils [16]. In water bodies, a heavy metal in relatively high concentrations affects the biota due to its toxicity, or it can be bioaccumulated, which increases its effect further along


**Table 1.** Contamination sources, uses, and adverse health effects of some heavy metals

the food chain. The progressive increase in the concentration of a contaminant such as a metal, as it advances in the food chain, is known as biomagnification. This occurs due to the need for a large number of organisms from lower trophic levels to feed a member of a higher trophic level and thus contaminants that cannot be metabolized but are fat soluble can accumulate in the fatty tissues of living organisms.

Various studies have been conducted to minimize or eliminate the heavy metals present in the environment. Conventional processes include precipitation, reverse osmosis, adsorption onto activated carbon or alumina, and redox processes [17]. However, these technologies are considered to be inefficient because of expensive cost [12]. In bioremediation by microorgan‐ isms typically employing one type of organism or a consortium of microorganisms, high toxic chemicals are converted into less toxic chemicals by biological means [18]. The technology makes use of the metabolic potential of microorganisms to clean up contaminated environ‐ ments [19] and has been proposed as an attractive alternative owing to its lower cost and higher efficiency [20] compared with other physicochemical methodologies [12]. Microorganisms can decompose or transform hazardous substances into less toxic metabolites or degrade them to nontoxic end products. Microorganisms can also survive in contaminated habitats because they are metabolically able to exploit contaminants as potential energy sources [11].

In bioremediation, microorganisms with biological activity, including algae, bacteria, fungi, and yeast, can be used in their naturally occurring forms.

The number of publications on the use of microorganisms for the removal of heavy metals in contaminated environments has been increasing steadily over the past 10 years. Figure 1 shows the main types of microorganisms used in these processes, based on a search for papers reporting microorganisms and bioremediation studies, indexed in the ISI Web of Science for the period of 2004 to 2014. It can be observed in Figure 1 that the microorganisms that have been most commonly used are bacteria and fungi, although yeast and algae are also frequently applied.

**Figure 1.** Types of microorganisms used in bioremediation processes.

the food chain. The progressive increase in the concentration of a contaminant such as a metal, as it advances in the food chain, is known as biomagnification. This occurs due to the need for a large number of organisms from lower trophic levels to feed a member of a higher trophic level and thus contaminants that cannot be metabolized but are fat soluble can accumulate in

**Element Contamination sources Uses Adverse health effects**

Automobile exhaust Respiratory, cardiovascular,

Pesticides, detergents Mental disturbance, cancer,

Most uses are based on electrical conductor

Batteries, electronics,

Fertilizers, plastics,

Batteries, alloys Neurotoxic

properties

catalysts

pigments

renal effects

ulcer, hypokerotosis

other nutrients

system

Anemia and other toxicity effects induced indirectly through interaction with

Skin allergies, lung fibrosis, diseases of the cardiovascular

Abdominal pain, nausea, vomiting and diarrhea, gastric irritation, headache, irritability, lethargy, anemia

Mining waste, electroplating,

Electroplating, metal alloys, domestic and industrial waste, mining waste,

coal, gasoline, pigments

Metal alloys, pigments, electroplating, industrial waste, pipelines

**Table 1.** Contamination sources, uses, and adverse health effects of some heavy metals

of vegetable oils

industrial waste, production

battery plants

industrial sewage, anticorrosive products

pesticides

Natural Anthropogenic

4 Advances in Bioremediation of Wastewater and Polluted Soil

Cr Chromite mineral Electroplating, metal alloys,

Pb Galena mineral Battery plants, pipelines,

Ni Soils Metal alloys, battery plants,

Cd Zn and Pb

Cu Sulfides, oxides

Zn Minerals (sulfides,

oxides, silicates)

carbonates

minerals, phosphate rocks

Various studies have been conducted to minimize or eliminate the heavy metals present in the environment. Conventional processes include precipitation, reverse osmosis, adsorption onto activated carbon or alumina, and redox processes [17]. However, these technologies are considered to be inefficient because of expensive cost [12]. In bioremediation by microorgan‐ isms typically employing one type of organism or a consortium of microorganisms, high toxic chemicals are converted into less toxic chemicals by biological means [18]. The technology makes use of the metabolic potential of microorganisms to clean up contaminated environ‐ ments [19] and has been proposed as an attractive alternative owing to its lower cost and higher efficiency [20] compared with other physicochemical methodologies [12]. Microorganisms can decompose or transform hazardous substances into less toxic metabolites or degrade them to nontoxic end products. Microorganisms can also survive in contaminated habitats because

they are metabolically able to exploit contaminants as potential energy sources [11].

the fatty tissues of living organisms.

Figure 2 gives some indication of which metals are used in bioremediation processes employ‐ ing microorganisms, and chromium, copper, cadmium, and lead together account for 70% of applications, although nickel and zinc are also used. Other metals that are used to a lesser extent include arsenic and mercury.

**Figure 2.** Metals used in bioremediation process employing microorganisms.
