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Terminal electron acceptors and electron donors during degradation

In sediments, a series of redox zones is prevalent in which microbial respiration proceeds based on the availability of terminal electron acceptors (TEA). Hence, the type of microorganisms and their metabolisms depend on the availability of electron acceptors and their corresponding energy yield. Once oxygen in the sediment is depleted by aerobic respiration, nitrate is used as a TEA by denitrifying bacteria, followed by manganese and iron reduction. The degradation of halogenated aromatic compounds and carbon tetrachloride has been reported under these conditions (Kazumi et al. 1995, Boopathy 2002). After iron, sulphate anions are used as TEA. Several chlorinated compounds have been reported in the sulphate reducing conditions (Drzyzga et al. 2001, Savage et al. 2010, van Eekert and Scliraa 2001). In the absence of other inorganic electron acceptors, methanogenesis has an important role for the biodegradation of organic matter by using C02 as TEA. Degradation of several chlorinated organic compounds has been reported in methanogenic conditions.

There exists a wide source of electron donors or substrates for dechlorination by microorganisms, and several lines of evidences show that once the electron donor is depleted, dechlorination does not proceed further (Villarante et al. 2001). Nies and Vogel (1990) observed that methanol was the most effective electron donor, followed by glucose, acetone and acetate to reductively dechlorinate polychlorinated biphenyls in an anaerobic sediment enrichment. In addition, other substrates like lactate for dechlorination of trichloroethylene to ethane, butyrate, propionate, and ethanol for dechlorination of trichlorophenoxyacetic acid were also observed (Gibson and Sulfita 1990). Ammonia was an effective electron donor for Nitrosomonas sp. capable of degrading several chlorinated aliphatic compounds.

Mechanism of anaerobic degradation: dechlorination or dehalorespiration

Dechlorination is a more promising anaerobic degradation mechanism for halogenated compounds. Since chlorinated compounds are electron deficient, they act as electron acceptors, thus generating energy in the anaerobic respiration process. The chlorinated organic compounds are used as TEA and the specific microorganisms gain energy dining the reductive chlorination, wherein, a chlorine atom is replaced by a hydrogen atom. The energy available from reductive dechlorination reactions is similar, irrespective of the position of the chlorine atoms or its oxidation state and the free energy obtained varies from -130 to -171 kJ/Cl atom removed (Dolfing and Harrison 1992).

Several chlorinated aromatic compounds and pesticides have been reported to be degraded under anaerobic conditions. Under anaerobic conditions, chlorinated volatile organic compounds (VOCs) can be degraded by the reductive dehalogenation pathways to less chlorinated products. For example, perchloroethylene (PCE) is degraded sequentially by reductive chlorination to vinyl chloride (VC) and ethylene/ethane (Lorah and Olsen 1999) (Figure 1). The chlorinated organic carbon compounds are used as electron acceptors by the microorganisms, and the available organic matter in the sediments would serve as a continuous supply of electron donors, which is required to drive the reaction. The strictly anaerobic conditions promote reductive dechlorination by cleavage of ether bonds, which are more prevalent (Ghattas et al. 2017).

It is difficult to generalize the dechlorination pathways because the dechlorinating microbial species depend on the position of the chlorine atom. Several mechanisms were proposed for the dechlorination of chlorobenzene and pentachlorophenol. For chlorobenzenes, chlorine atoms with two or one adjacent chlorine atoms were preferentially cleaved under sulphate reducing conditions in a contaminated river sediment (Susarla et al. 1998). However, for chlorophenols, ortho-chlorine removal was preferr ed by both sulphate reducers and methanogens (Takeuchi et al. 2000). Contrastingly, preferential para- and meta-chlorine removal was observed by Haggblom et al. (1993) under both sulphate reducing and methanogenic conditions, respectively. Halorespiring bacteria are a group of microorganisms which grow independently of the above inorganic electron acceptors. They are highly relevant in the bioremediation of chlorinated organic pollutants as they utilize these compounds itself as TEA. They utilize an array of chlorinated organic compounds including chlorinated biphenyls and dioxins (Cutter et al. 2001, Wu et al. 2002, Bunge et al. 2003). In natural sediments, microbes have different types of dechlorinating enzymes based on the type of

Anaerobic dechlorination pathways of perchloroethylene (PCE)

Figure 1. Anaerobic dechlorination pathways of perchloroethylene (PCE).

the chlorinated organic compounds as well as their congeners. For example, the chlorine atoms in PCBs are replaced one by one by hydrogen atoms and are removed as chloride ions by dehalogenase activity. Moreover, the rate of dechlorination in the sediments decreases from higher chlorinated congeners to less chlorinated congeners (Vasilyeva and Strijakova 2007). Around 70% of arachlor introduced in pure sediments of Lake Michigan was found to be dechlorinated after six months. It was also observed that low-chlorinated congeners were completely dechlorinated (Natarajan et al. 1998).

Degradation in sulphate reducing conditions

Dining anaerobic degradation of halogenated compounds or dehalogenation in sulphate reducing conditions, the most important electron acceptor is sulphate. Sulphate was found to be an important electron acceptor for transformation of TCE and chlorinated benzenes than oxygen or nitrate (Cobb and Bower 1991). The degradation capacity of chlorinated aromatic compounds coupled to sulphate reduction is considerably more favourable than degradation coupled with methane production (Haggblom and Young 1995). Desulfitobacterium chlororespirans gains energy from the reductive ortho-dechlorination of 3-chloro-4-hydroxy benzoate and 2,3-di and polychloro-substituted phenols (Loeffler et al. 1996). Reductive dechlorination was found to related to respirator}' growth in Desulfitobacterium multivorans (Neumann et al. 1994).

It was observed that 4-chlorophenol w'as used as a sole source of carbon in a sulfidogenic consortium enriched with an estuarine sediment (Haggblom and Young 1995). After reductive dechlorination, 4-chlorophenol wras dechlorinated to phenol under sulphate-reducing conditions, and mineralization of the phenol ring to C02 occurred only with the coupling of sulphate reduction. The most common mechanisms of dehalogenation are hydrogenolysis and hydrolysis. Dehalococcoides sp. was found to degrade chlorinated ethenes, PCB and dioxins (Bedard et al. 2007, Bunge et al. 2003). Certain species of sulphate reducers are strictly halorespiring like Dehalococcoides and Dehalobacter. Desulfomonas michiganensis, an acetate-oxidizing anaerobic bacteria, can utilize perchloro ethylene as electron acceptor (Sung et al. 2003).

Degradation in niethanogenic conditions

Under niethanogenic conditions also, the common biotransformation pathway is reductive dechlorination or reductive dehydrogenolysis for chloroaliphatics containing one or two carbon atoms. Numerous chlorinated aromatic compounds have been revealed to be degraded by reductive dehalogenation under niethanogenic conditions (e.g., 2,4,5-trichlorophenoxyacetate, 3-chlorobenzoate, 2,4-dichlorophenol, 4-chlorophenol, 2,3,6-trichlorobenzoate, and dichlorobenzoates) (Haggblom 1992, Molm and Tiedje 1992, Commandeur and Parsons 1994). Bow'er and McCarty (1983) observed that under niethanogenic conditions, numerous 1- and 2-carbon halogenated aliphatic organic compounds present at low' concentrations (< 100 ug/liter) were degraded in batch bacterial cultures and also in a continuous-flow' niethanogenic fixed-film laboratory-scale column. Acetate was used as a primaiy substrate. Within two-day detention time, more than 90% degradation w'as observed. Although trichloroethylene (TCE) is degraded under different types of facultative anaerobic and anaerobic conditions such as nitrate-reducing, iron-reducing, and sulphate-reducing, the degradation is fast and complete under niethanogenic conditions, w'here the degradation of TCE forms non-toxic end products such as ethane and ethylene (McCarty and Semprini 1994).

Anaerobic degradation of tricolosan w'as reported under both sulphate reducing and niethanogenic conditions, where triclosan was converted to catechol and phenol by dechlorination. Phenol and catechol w'ere also further anaerobically degraded to reduce the aromatic ring (Veetil et al. 2012). As methanogens caimot directly degrade complex organic compounds, they depend on other organisms such as hydroloytic, fermentative and syntrophic acetogenic microbes for the supply of electron donors and substrates like H„ CO„ formate and acetate. Hence, methanogens co-exist as a microbial consortium (Stams et al. 2005).

334 Bioremediation Science: From Theory to Practice

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