2.2. Elimination of PCBs

Although the Stockholm Convention on Persistent Organic Pollutants (POPs) (of which PCBs are part of) signed in 2001 was aimed at eliminating and/or restricting the production and use of POPs [23], more of these ubiquitous substances are still being introduced into the environment through various human activities [24]. Water has become a widely used environmental matrix for monitoring POPs [20, 25, 26], although most studies on PCBs have been carried out on contaminated soils than water [27].

PCBs may not be readily biodegradable, but studies have shown that some bacteria species such as Vibrio cholera, Acinetobacter 1woffnii, Aeromonas hydrophila, Pseudomonas aeruginosa, Pseudomonas putida, Rhodococcus sp., Bacillus sp., and Burkholderia sp. have the ability to breakdown these compounds, although it is through a very long route [11, 13, 21, 36, 37, 51]. This may be achieved through co-metabolism and mineralization [8, 35]. They use of a metabolic pathway similar in all these bacteria, which comprise four steps catalyzed by enzymes BphA, BphB, BphC, and BphD [37]. The pathway, according to Petric et al., is initiated by insertion of two oxygen atoms at the carbon positions 2, 3 of one aromatic ring [37]. This is followed by dehydrogenation meta-cleavage and hydrolysis forming a 5-carbon compound [37]. The pro-

The Role of Bacteria on the Breakdown of Recalcitrant Polychlorinated Biphenyls (PCBs) Compounds in Wastewater

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According to Heider and Rabus, xenobiotic compound due to its recalcitrant nature is hard to break down [38]. The recalcitrant nature of these compounds is a result of the complexity of its chemical composition [8]. Breakdown of these compounds occurs when enzymes act on certain groups present in the compound [38]. The halocarbons, for example, the halogen group, are targeted, with enzymes such as oxygenases playing a major role in their breakdown [8]. The enzymes target the bonds such as ester-, amide-, or ether bonds present in the compounds leading to break down of these compounds [39, 40]. The enzymes may target the aliphatic chains and in aromatic compounds, the aromatic components may be targeted [40]. The mode of attack as well as the site depends primarily on the action of enzyme, its concentration, and favorable conditions [40]. According to Abor-Amer [40], the xenobiotics do not act as a source of energy to microbes and as a result, they are not degraded while the presence of a suitable substrate induces its breakdown [39]. These substrates are known as co-metabolites, and the process of degradation is known asco-metabolism [39]. Gratuitous metabolism is another process in which xenobiotics serve as substrates and are acted upon to release energy [8].

The processes described cannot be achieved through the use of Moringa oleifera in treating wastewater to remove PCBs. It is evident from literature that the removal of these compounds using plant protein has not been fully studied [52]. Plant protein has been found to be slightly efficient with the reduction of fecal coliforms and other bacteria [41, 42], which has made Moringa treatment to be applicable. The use of Moringa oleifera seed powder in water treatment plants has been found to target mainly microorganisms, thus reducing turbidity [43]. Although this mode of water treatment has been used, especially in rural areas of the developing countries, synthetic polymers, aluminum sulfate, ferric chloride, and poly aluminum chlorides used together with this powder have been reported to be unsafe [41, 43, 44]. The action of Moringa oleifera seed powder has been reported to be based on the ability of the protein contained in the seeds to be able to form coagulants, which reduce water turbidity by acting on coliforms [45]. The bacteria found to be mainly involved in biodegradation of POPs and PCBs have been found mostly not to be coliforms [11, 13]. After treating water with

tree, sustainability of tree growth and productivity, which relies on environmental conditions, may not be viable. This will therefore affect production and maintenance of the Moringa tree

–105 of bacteria is left [45]. Taking into cognizance that Moringa is a

cess follows a biphenyl catabolic pathway [37].

2.5. Biodegradation of xenobiotic compounds

Moringa seed powder, 10<sup>1</sup>

The destruction of PCBs by chemical, thermal, and biochemical processes is extremely difficult and presents the risk of generating extremely toxic dibenzodioxins and dibenzofurans through partial oxidation [12, 16, 27].
