2.1. Physio-chemical properties of polychlorinated biphenyls (PCBs)

Polychlorinated biphenyls (PCBs) consist of two benzene rings with a carbon-to-carbon bond between carbon 1 on one ring and carbon 1 on the other ring [12]. PCBs have varying number of chlorines in their structure [12–14], as shown in Figure 1.

The Role of Bacteria on the Breakdown of Recalcitrant Polychlorinated Biphenyls (PCBs) Compounds in Wastewater http://dx.doi.org/10.5772/intechopen.75400 141

Figure 1. Structure of PCBs (source: [14]).

1. Background

140 Wastewater and Water Quality

One of the key areas in sustainable development entails the promotion of environmental management and introduction of new technologies to treat large quantities of waste. This includes treatment of wastewater for recycling purposes [1]. The adverse effects of global warming have mostly been experienced by countries in Africa, resulting in scarcity of water as a natural resource. This has prompted a great global concern to recycle and conserve water, especially in sub-Saharan Africa where the problem of water scarcity has affected most countries [2]. South Africa is faced with freshwater scarcity, which is exacerbated by its increasing

The presence of chemicals in the environment calls for quantification of such so as to come up with a risk analysis posed by these chemicals [4, 5]. According to Guillen et al., substances such as pharmaceuticals, perfluorinated acids, perfluorosulfonates, PAHs, PCBs, pesticides, and surfactants are mostly found in wastewater [4]. Ying et al. also noted that the presence of pharmaceutically active compounds in wastewater is a major concern [6]. Several methods may be used to determine quantitatively, these substances from wastewater, which is mainly from sewage treatment plants [6]. According to Ying et al., although much research has been done regarding the removal of these substances, it was mainly on activated sludge and no

Some strains of organism Acinetobacter have the ability to degrade pollutants such as biphenyls from wastewater [7]. Enterobacter cloacae secretes an emulsifier that increases the hydrophobicity of the bacterial cell surface and also neutralizes the surface charge of cells [8]. This as a result increases the ability of the bacteria to degrade PCBs [8, 9]. Biosurfactants are also effective at extremes of temperature, pH, and salinity [9, 10], a property that is essential in the biodegradation of PCBs as they are hydrophobic organic compounds [11]. This property causes these recalcitrant compounds to be removed through physico-chemical means or treatments, limited bioavailability to microorganisms, and limited availability to oxidative and reductive chemicals

This study is very important in contributing toward addressing sustainable development goal 6. With the global emphasis of conservation of natural resources and the three Rs, that is, reduce, reuse, and recycle, this research is very important. The research is my original proposal which was stimulated by quite a number of issues, such as the scarcity of water although there is a lot of water that is being let to waste. Also the high prevalence of cancer cases with no direct link to water but with a view to eliminate the possibility of such cancer causing chemicals with direct attention on PCBs, from sewage which in most cases the effluent is released into the environment.

Polychlorinated biphenyls (PCBs) consist of two benzene rings with a carbon-to-carbon bond between carbon 1 on one ring and carbon 1 on the other ring [12]. PCBs have varying number

2.1. Physio-chemical properties of polychlorinated biphenyls (PCBs)

of chlorines in their structure [12–14], as shown in Figure 1.

demand, pollution, unsustainable use, and climate change [3].

work has been done on wastewater [6].

when applied in treatments [8].

2. Brief literature

Toxicity of PCBs is dependent upon the number of chlorines present on the biphenyl structure and their position, that is, the co-planar congeners [13, 14]. The PCB congeners that have been deemed to be highly toxic were those that had chlorine atoms attached to the 3,4-ortho positions, followed by those with 5–10 chlorine atoms in the para and meta positions [13].

PCBs toxicity has been largely associated with their structure. This has resulted in PCBs being placed into categories, namely coplanar or non-ortho-substituted "arene" substitution patterns or noncoplanar or ortho-substituted congeners [15]. The coplanar group members are characterized by a fairly rigid structure, with the biphenyl rings in the same plane giving them a molecule structure similar to polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans. Based on this structure, this group of PCBs act in the same way as these molecules as an agonist of the aryl hydrocarbon receptor (AhR) in organisms [16]. This group of PCBs is considered contributors to overall dioxin toxicity [16].

On the other hand, the other group of PCBs, noncoplanar PCBs, has chlorine atoms at the ortho positions. According to Ross [17], they have not been found to activate the AhR and are not considered part of the dioxin group; however, they have been implicated in having some neurotoxic and immunotoxic effects, although at levels much higher than normally associated with dioxins, and thus of much less concern to regulatory bodies [18, 19].

According to Rudel et al. PCBs are very stable compounds and do not decompose readily [18]. Their chemical inability to oxidize and reduce in the natural environment gives them this characteristic; they have a long half-life (8–15 years) and are insoluble in water, thus the recalcitrant property [18]. The biodegradability (and solubility in water) of PCBs is also dependent upon the number of chlorine molecules it has [12, 13]. The more chlorine molecules contained in a compound renders that compound less biodegradable [12]. PCBs are mostly hydrophobic; some are less hydrophilic [20, 21]. These properties result in bioaccumulation of these compounds as they do no dissolve in water, and thus, they render them difficult to be biodegraded [12, 14, 20, 22].
