for metal removal. Some other authors stated that the proteinaceous part of EPS plays an important role in complexation of metal ions [10]. Several studies demonstrated that the metal resistance genes (MRGs) and antibiotic resistance genes (ARGs) co-occur in bacteria isolated from water bodies polluted with heavy metal wastes. These kinds of studies give a little clue about the heavy metal resistance potential of antibiotic resistance strains [11]. Metal transformation by microorganisms serves various biological functions. Anaerobic respiration of microbes reduces metals, causing in detoxification, and the reduced forms are less toxic and little soluble as well [12]. Some other methods like biosorption, bioleaching, and precipitation are found to be very efficient for detoxification [12–14]. The application of mono- or multi-species of biofilm gives the microorganisms a best existence slot and their metabolic abilities also increases in presence of high amount of lethal compounds [1]. Indeed, microorganisms can simply remediate polluted water in several water bodies and waste streams by removing metals, separating metals in soil and sediments by different processes including enzymatic actions [15]. Moreover, using bacteria over other microorganisms helps in reducing other contaminants present in waste materials. Therefore, biodetoxification of heavy metal by using biofilm bacteria is a more efficient process, eco-friendly, cost effective, and possess no side effect to living beings. Generally, heavy metals present everywhere such as air, water, soil, and sediment. In this chapter, we discuss about some biofilm bacteria and their role in detoxification of heavy metals.
3.2 Source and Toxicity of Heavy Metal Pollution
The multiple applications of heavy metals in industrial, domestic, agricultural, medical, and technology sectors is the main reason for their wide spreading in environment [16]. Generally, the heavy metals exist all over the earth surface. The social contact to environment results anthropogenic activities like mining and smelting operation, industrial manufacture and application, and metal and metal containing compound application in domestic and agriculture field [17, 18]. Sometimes, natural incidence like volcanic eruptions on land as well as on the ocean beds are reported to be responsible for heavy metal pollution in soil and water bodies [16]. Industrial sources have a large contribution toward heavy metal pollution from activities including metal melting out in processing plants, coal flaming in power plants, incineration of petroleum products, nuclear power stations and high-tension lines, textiles, plastics, wood conservation, microelectronics, and paper processing plant [19, 20]. The wastage from livestock systems can disturb the micro- and macro-environment such as water, soil, and food chain [21]. The metals’ presence in water reduces their quality and causes human disease, even the essential metals at high concentration gives negative effect and toxicity [21]. The metals and metalloids are common pollutant in waste water [22]. Soil accumulate heavy metals and metalloids by production from quickly growing industrial areas, mine tailings, high metal waste disposal, leaded gasoline and paint, fertilizers applied in land, animal manures, sewage sludge, pesticides in agriculture, coal incineration deposits, petrochemical spillage, and atmospheric deposition [23]. Heavy metals enter to ecosystem and hence human through direct contact with contaminated soil, food chain, and drinking of contaminated ground water. It causes significant reduction in food quality by phytotoxicity, decrease the quality, and hence the fertility of land used for cultivation purpose affecting food safety and land occupation difficulties [23]. Metal ions combine with biological factors such as DNA and nuclear protein result in deterioration of DNA and conformational change which may indicates to variation of cell cycle, carcinogenesis, or apoptosis [16]. The nonessential heavy metals have direct or indirect negative effect on human from tissue level to organ system and from nucleic acid to physiology level.
3.2.1 Non-Essential Heavy Metals
The toxicity and carcinogenicity potential of some frequently present non-essential heavy metals like mercury, chromium, lead, arsenic, and cadmium are described in this section.
3.2.1.1 Arsenic
Arsenic present in periodic table of period 4 and group VA in metalloid state. The inorganic form includes trivalent arsenite (AsIII) and pentavalent arsenate (AsV) and methylated metabolites are organic form of arsenic, e.g., monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), and trimethylarsine (TMA) oxide. Atmospheric pollutions occur through arsenic due to volcanic eruption, soil erosion, and anthropogenic activities [24]. In ores, arsenic generally exists in powdery amorphous and crystalline forms. It enters in to the environment through withstand of rocks, mining and smelting methods, pesticide practice in agriculture, and coal ignition. It causes ground water as well as surface water contamination and exists as arsenate (AsV) and arsenite (AsIII) in maximum groundwater. Its high concentrations in drinking water create toxic effect to animal and human [22].
Industrial product in agricultural application such as herbicides, insecticides, sheep dip, dye-stuffs, preservatives of wood and algicides contains arsenic compounds or components. Arsenic is also used in veterinary medicines and medical treatments in drugs to treat syphilis, yaws, amoebic dysentery, and Trypanosomiasis like diseases [25, 26]. Arsenic compound also create genotoxicity by inhibiting DNA repair, promote chromosomal aberration, exchanges of sister chromatid, and micronuclei development in both rodent and human cells [27, 28].
3.2.1.2 Cadmium
Cadmium is a highly toxic and nonessential heavy metal for environment. Moderate concentration of cadmium (around 0.1/kg) is commonly found in the soil crust. The maximum amount of cadmium compounds are accumulated in sedimentary rock and phosphates of marine (contain nearly 15 mg/kg) [29] and naturally released to environment by abrasion of rocks and soil, forest fires, and volcanic eruption. The anthropogenic activities are also responsible for cadmium pollution such as metal plating, metallurgical alloying, ceramics, mining, and other industrial operations. It is used as a protecting guard on alloys and steel, in paints and plastic solder and braces color and nickel-cadmium rechargeable batteries in stabilizer. It is also present in fungicides, super phosphate fertilizer, cigarette, and ash. Anthropogenically, their concentrations arise mainly by minerals used in agriculture and industries [30]. The humans are exposed to cadmium via the rout of inhalation or smoking of cigarette, ingestion contaminated food, working in cadmium contaminated place, but skin absorption is rare and smoking is the main influencer [31, 32]. Cadmium is moreover exist in trace amount in particular foods like green and leafy vegetables, potatoes, seeds, grains, mushrooms, and in some sea foods such as kidney and liver of mollusks and crustaceans, shellfish, mussels, cocoa powder, and dried seaweeds [16]. Cadmium causes severe health problems such as erosion in gastrointestinal tract and internal damage in pulmonary, hepatic, or renal systems, depending on the rout of contamination [33, 34]. Cadmium is highly carcinogenic but mainly it causes pulmonary cancer and other parts such as adrenals, testes, and the hematopoietic system [35].
3.2.1.3 Chromium
Chromium occupies a position in the first row of d-block in the periodic table and is a transition metal of group VIB. It does not exist in elemental form, so it forms compound and is less commonly available element [23]. Naturally, it is present in the earth with oxidation state ranging from chromium (II) to chromium (VI) [36]. The trivalent form of chromium compound [Cr(III)] is stable and accumulated in ores such as ferro-chromite. Hexavalent [Cr(VI)] is another form of chromium compound and is second most stable state form [37]. Chromium entered in different environments (air, water, and soil) through the release of waste from industrial and other anthropogenic activities. The different chromium industries such as metal melting out, tannery services, chromate manufacture, stainless steel repairing, and ferrochrome and chrome pigment manufacture industries are mainly responsible for chromium contamination in the environment [16]. Chromium is also used in paper, pulp, and rubber manufacturing applications [22]. The hexavalent form of chromium [Cr(IV)] is the toxic compound from industrial pollutant which is classified as human carcinogen by various regulatory and non-regulatory agencies [16]. For drinking water, World Health Organization (WHO) restricted 50 μg
