Bioremediation of soil contaminated with a mixture of chlorinated aliphatic hydrocarbons.
Rajpal, Deseree A.
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Chlorinated aliphatic hydrocarbons (CAH’s) are a diverse group of industrial chemicals that play a significant role as pollutants of soil and groundwater. They are recalcitrant and resist degradation in most waste treatment systems. Furthermore, physical removal techniques used for CAHs are often very expensive, labour intensive and time consuming. Microbial communities native to contaminated areas are known to participate in biodegradation of these CAHs to an extent. The main focus of this study was therefore to investigate the bioremediation of soil contaminated with a mixture of CAHs, namely carbon tetrachloride (CCl4), dichloromethane (DCM) and 1, 2 dichloroethane (1, 2-DCA). Two different laboratory-scale microcosm types, a stationary microcosm (Type S) and microcosms that received a continuous circulation of groundwater (Type C) were used to determine the effects of 3 different bioremediation approaches, viz, biostimulation, bioaugmentation and a combination of biostimulation and bioaugmentation on the degradation process. For both microcosm types, gas chromatography analysis revealed that the greatest decreases in CAH concentrations occurred in soil that was biostimulated. 1, 2-DCA was rapidly biodegraded in Type C microcosms that contained glucose, with a 57% net degradation in 15 days. Consortia comprising of aerobic Bacillus and Alcaligenes sp. were used for bioaugmenting contaminated soil. However, this approach did not promote biodegradation as significantly as biostimulation experiments. A combination of biostimulation and bioaugmentation revealed that the addition of nutrients was still unable to induce the degradative ability of the introduced microorganisms to produce degradation values comparable to those of biostimulated soil microcosms. Common intermediates of CAH metabolism viz., chloroform, dichloromethane and carbon dioxide were detected by gas chromatography/mass spectrometry. The detection of chloroform and dichloromethane is sufficient evidence to assume that anaerobic conditions had developed, and that biodegradation was occurring under oxygen-limiting or oxygen-free conditions. An aerobic environment was initially created, but soil microbial respiration had probably led to the rapid development of anaerobic conditions and in all likelihood, enhanced degradation. The prevalence of anaerobic conditions can also account for the lack of appreciable degradation by the bacterial consortium used during bioaugmentation. Phospholipid phosphate analysis was conducted and used as an indicator of microbial biomass. It was noted that phospholipid phosphates did not always correlate with the degradation of CAHs in some microcosms. In this regard, different patterns were noted for Type S and Type C microcosms. Microbial biomass patterns for Type C biostimulated and bioaugmented soil microcosms increased within the first 5 days of sampling. This could have been as a result of the larger volume of groundwater required for the circulating microcosm possibly concealing actual CAH concentrations. In contrast, in Type S microcosms, for most treatments, a sharp decline in biomass within the first week was observed. This study clearly demonstrates that the bioremediation of certain chlorinated solvents can be a function of their water solubility. It must also be emphasized that the biodegradation of some CAHs in a mixture can affect the concentrations of others present in the mixture as well, warranting further study with mixtures of CAHs. Furthermore, the development and use of bioreactors, similar to the Type C microcosm can provide novel, simple ways to hasten remediation of chlorinated solvents like 1, 2-DCA.