Bioremediation of Chlorinated Organic Pollutants in Anaerobic Sediments
Chlorinated organic compounds are designated as priority pollutants, and as persistent organic pollutants (POPs), because they are highly toxic, can bioconcentrate and persist in the environment. Various sources of chlorinated compounds include improper disposal of chemical wastes, agricultural run-off, use of herbicides, insecticides, fungicides, solvents, hydraulic and heat-transfer fluids, plasticizers, cleaning agents, fumigants, aerosol propellants, gasoline additives, degreasers, and intermediates for chemical syntheses (Rawford et al. 2005). In addition to anthropogenic sources, there are various natural sources of chlorinated organic compounds in the environment. Chlorinated compounds also occur naturally in the environment in low concentrations, such as volcanic eruption. Chlorobenzene, carbon tetrachloride, phenolic compounds (phenol, chlorophenols, nitro phenol, polychlorinated biphenyls (PCBs)) and dyes are some major chlorinated organic compounds prevalent in sediments. For example, in contaminated places, concentration of PCBs in sediments was reported as high as 104 mg/Kg, which is several orders of magnitude higher than the permissible level (Vasilyeva and Strijakova 2007).
Complete removal of POPs from the environment is difficult. Some POPs are still persistent, can accumulate in fatty tissues, and are present in higher concentrations at higher levels in the food chain, with long-range mobility (Bhatt et al. 2007). Chlorinated pollutants are recalcitrant under aerobic conditions because oxidative stress associated with biodegradation of chlorinated pollutants is a significant barrier for the evolution of aerobic pathways for chlorinated compounds, thereby allowing for the emergence of anaerobic counterparts (Nikel et al. 2013). Although anaerobic dechlorination does not result in complete degradation of xenobiotics, it contributes to the detoxification of the environment by forming less toxic products (Wiegel and Wu 2000). Aquatic sediments are typical examples of anaerobic conditions since oxygen diffusion is limited and microbial respiration depletes the oxygen when organic substrates are available. Aquatic sediments also act as sinks for recalcitrant and hazardous organic pollutants entering fr om various sources, most of which are antliropogenic. The problem is worldwide and this becomes alarming when these compounds, through various biogeochemical processes, can be available to the benthic organism as well as to the organisms in the water column through the sediment-water interface (Perelo 2010).
This chapter explores the anaerobic degradation of chlorinated organic pollutants in anaerobic sediments by natural attenuation.
Microbial degradation of chlorinated compounds
Although many of the chlorinated compounds escape from the degradation and persist in the environment, certain microorganisms exposed to these synthetic chemicals have evolved the ability to utilize some of them. Bacteiia can degrade chlorinated aromatic compounds in four ways (Haggblom 1992, Commandeur and Parsons 1994): (1) dehalogenation after ring cleavage, (2) oxidative dehalogenation, (3) hydrolytic dehalogenation and (4) reductive dehalogenation. Biodegradation of chlorinated compounds is of two types: aerobic degradation and anaerobic degradation. Degradation of a compound depends on the structure of the compound (either oxidized or reduced), the number of chlorine substituents in the structure, and the position of chlorine in the molecules. In aerobic degradation, molecular oxygen serves as the electron acceptor (e.g., chloroaliphatic compounds). Bhatt et al. (2006) reported that 4-chlorophenol (4-CP) can be partially or completely degraded aerobically by Pseudomonas, Alcaligenes, Rhodococcus, Azotobacter, etc. In anaerobic degradation, the electron acceptors are N03“, SO|~, Fe3+, H+, S, fumarate, trimethylamine oxide, an organic compound, or C02. In anaerobic conditions, chlorinated aromatic compounds become reductively dehalogenated in a process known as halo respiration, resulting in the accumulation of lower chlorinated congeners.
Degradation of most of the chlorinated compounds are not based on microbial energy metabolism. When microorganisms can transform one substrate to different substrates that is not associated with that organism’s energy production such as carbon assimilation, or any other growth process, that particular mode of activity is called “cometabolism”. Duiing cometabolism, in the presence of one organic material (primary energy source), degradation of another compound takes place. Several chlorinated hydrocarbons are transformed cometabolically by bacteiia, and degrade the chlorine unsubstituted hydrocarbons. Detoxification and complete mineralization of chlorinated compounds can be easily attained by using a sequential treatment process, that is, anaerobic followed by aerobic treatments (Bhatt et al. 2007). However, the difficulty in assessing the degree of biodegradation of compounds like PCBs in the natural environment like sediments is due to the great diversity of congeners as well as the difficulty of their chemical analysis. A slow decrease in higher-chlorinated content and accumulation of lower-chlorinated PCB congeners like ortho-substituted PCBs was observed in river and marine sediments by Vasilyeva and Strijakova (2007).