**3. Removal of polycyclic aromatic hydrocarbons (PAHs) in BES**

Incomplete combustion of organic matter and fuels leads to creation of PAHs. Additionally, various industrial processes use large amounts of PAH compounds such and naphthalene, phenanthrene and chrysene. These PAHs are degraded into compounds which are less toxic in nature. This degradation takes place while these PAHs are being used as a source of carbon and energy in solid or liquid media. However, there is no evidence for the existence of a single microorganism species which can work on PAHs of several types. This is the reason why using microbial consortia can be very useful. Usage of microbial consortium is advantageous because, they are cheap, show higher applicability and produce lower negative environmental impacts [23].

Degradation activities of PAHs are extremely long, taking up to years or at least months [24]. All PAHs are "fat-loving" or "water-hating chemicals [25]. They become less available for biodegradation in water, as they have a tendency to adsorb to organic molecules. Even though various bioremediation techniques are used to treat PAHs in polluted environments, they have limitations due to the negative effect caused by activity and diversity of indigenous hydrocarbon degrading bacteria, low abundance, slow growth rates, less availability in aqueous solutions among others. Further problems can also be created due to addition of co-substrates and nutrients to improve indigenous microbial activity due to high cost & and the fact that the added chemicals having the ability to act upon anything other than the target compounds. Hence, using Microbial fuel cells remains as a viable solution circumvent some of these problems [24].

Less complex compounds can be formed from recalcitrant PAHs, if bio electrochemical activity can increase the metabolism of PAH degraders closer to the electrodes [26]. When a microbial fuel cell with a single chambered air-cathode working upon a Winogradsky solution was used, detoxification activity by microflora of a specialized variety growing on PAHs in that Winogradsky solution was found. Microbial Fuel Cells not only shows improved degradation activities, but it also show higher amount of microbial detoxification activity [23]. Inocula extracted from complex microbial communities such as soil or sediments to drive electrochemical reactions in BES and to degrade xenobiotic pollutants may confer other benefits such as simultaneous nitrogen and phosphorus removal from wastewater [27]. Many previous studies have demonstrated that various BES types employing many different electrochemically active microorganisms are effective in biodegradation of a range of PAH pollutants (**Table 2**).

### **4. Removal of pesticides and their residues using BES**

Pesticides are defined as substances or mixtures designed for destroying and mitigating any group of pests such as insects and plants. During the past decades, different types of pesticides had widely been used in agriculture for high yield productions. Applications of pesticides have secured almost one-third of crop

production in the world. Pesticides have led to the improvement of food production to secure the demands of an ever increasing human population [35]. Majority of pesticides and their residues can cause unintended splash damage to other organisms than the target pest. The widespread use of pesticides not only brought adverse influence on agro ecosystems but also caused alteration in physiological processes of non-target organisms [36].

The over and misuse of pesticide has precedence to immense health problems, economic loss and various environmental problems. The resultant health problems of pesticides include cancer, birth defects, reproductive problems, liver, kidney, and neural problems among others. In many developing countries majority of pesticides are associated with adverse effect on human health and environment due to the overuse of pesticide. On the other hand the overuse of pesticide also results in the environmental pollution such as water and soil pollution and cause imbalance of ecosystem [37].

Presently, the soil bioelectrochemical remediation system (BERS) can effectively remove pesticides, e.g., hexachlorobenzene [38], isoproturon [39] and Metalachlor [40] from soils by means of native functional microorganisms, with the advantage of the production of bioelectricity. This new technology has been paid more attention by the researchers and engineers in the field of environmental management. In the soil BERS, microorganisms is in charge of system performance since degraders and electroactive bacteria were responsible for pollutant degradation and electricity generation. Thus, the insights to the structure and evolution of microbial community in the remediation process of soil BERS is the most important for revealing the mechanism of microbial electrochemical degradation [41]. **Table 3** summarizes a range of different studies that have successfully utilized BES to bio-transform or biodegrade a variety of pesticides.

## **5. Removal of polychlorinated biphenyls (PCBs) using BES**

Polychlorinated biphenyls (PCBs) are among the persistent organic pollutants which mainly accumulate in soils and sediments [46]. These lipophilic compounds are of major concern due to their low biodegradation, high bio-concentration and persistence in environment [47]. Polychlorinated biphenyls (PCBs) is a kind of light yellow or deep yellow oily liquid with the properties of insulating ability, thermal stability, resistance to acids, oxidation, hydrolysis, and flame resistance. Due to these unique physical and chemical properties, PCBs were widely used in many products especially in transformers and power capacitors [48]. Polychlorinated biphenyls are a subset of the synthetic organic chemicals known as polychlorinated hydrocarbons. They are composed of two attached benzene rings and multiple bonded chlorine atoms. PCBs consist of 209 isomers and congeners, with a 10,000 fold range in n-octanol/water partition coefficient (Kow, indicator of lipophilicity and then, of potential bioaccumulation) values between the mono-substituted chlorobiphenyl and the fully-substituted decachlorobiphenyl [49].

It is known that PCBs induce a wide variety of toxic responses in human, wildlife, plants and laboratory animals. They can penetrate the human body through skin contact, by inhalation of PCBs contaminated vapors and by consuming food contaminated with PCB residues [50]. Toxicological studies of PCBs in human were found to increase the rate of melanoma, gall bladder cancer, brain cancer, liver cancer, biliary tract cancer, gastrointestinal tract cancer and possibly connected to breast cancer. The people exposed to high levels of PCBs through skin contact or by consumption experience skin irritations like severe acne and rashes, nose and lungs infections and eye issues [51]. The uptake of PCBs and their negative



#### *Expedited Biodegradation of Organic Pollutants and Refractory Compounds… DOI: http://dx.doi.org/10.5772/intechopen.99229*

effects on plants has also been examined. For instance, PCBs firmly bind to the soil organic matter and provides low bioavailability for plants and microorganisms. But accumulation of PCBs in soil is perilous to all kinds of organisms including plants. The higher PCB concentrations affect the biosynthesis, ultrastructure of plant cell, membrane stability and plant DNA. Also, the plants that are highly exposed to PCBs result in reduction of photosynthesis, water/nutrients uptake and show visible symptoms of growth inhibition, browning of root tips and even death [52].

Anaerobic biodegradation is a major process for PCBs removal in sediments. However, a lack of or deficiency in terminal electron acceptors such as sulfate and nitrate can result in a decreased removal rate of bioremediation or no biodegradation at all [53]. Therefore, bio-stimulation by introducing electron acceptors could improve the microbiological activity in sediments [54].

Sediment microbial fuel cells (SMFCs), as a bio-electrochemical system, have been demonstrated to enhance the biodegradation of organic compounds, i.e. PCB. Xu et al. found that the combined application of a sediment microbial fuel cell (SMFC) and surfactant led to the highest removal efficiencies (43.26% of PCBs) after 60 days of operation, and produced the maximum power output (0.821 V of voltage and 18.30 W m−3 of power density) [55]. The degradation of polychlorinated biphenyls (PCBs) by sediment microbial fuel cell (SMFC) with/without nano-scale zero-valent iron (NZVI) addition was investigated by [56]. It was found that the combined application led to the highest removal efficiencies of PCBs (37.55 ± 1.11%) and TOC (49.72 ± 1.54%) in all circumstances and produced a higher power density (108.89 mW/m2 ) [56]
