| Polychlorinated phenols have been used in large quantities, as pesticides, herbicides and wood preservatives. Inadequate handling, accidental spills and leaching from dumpsites have resulted in its contamination into groundwater and soil. Environmental contamination with polychlorinated phenols has become a major global concern due to their now widespread distribution and toxicity. In the environment, PCP is often present with co-contaminants such as heavy metals. Little is known of its fate under these conditions. Depending on the presence of various degrading microorganisms and the appropriate conditions for their activity, chlorophenols may or may not be degraded in the presence of heavy metals. Microorganisms are known to possess resistance to various heavy metals, and so this could also be used in the bioremediation of heavy metal-polychlorinated contaminated soils. In situ studies are, however, necessary before effective bioremediation processes can be developed for such mixed wastes.; The objective of this research was to study the anaerobic degradation of PCP under two different physiological conditions, sulfidogenic (SRB) and methanogenic (MET), in the presence and absence of a heavy metal, cadmium (Cd). The degradation and transformation of PCP and the co-precipitation of Cd from its initial soluble phase by the two distinct enrichments were monitored. The microbial communities were also analyzed for their ability to mineralize PCP and to incorporate the carbon from PCP into their biomass. The degradation of PCP in the presence and absence of Cd in contaminated sediments with established redox gradients under these two enrichment conditions were also studied in order to determine the influence of the redox gradients on the biodegradation of PCP and biotransformation of Cd. The specific objectives were to understand the influence of Cd on the degradation of PCP under oxic and anoxic conditions, determine the influence of added electron donors such as lactate and acetate (enriching for sulfate-reducing or methane-producing, respectively) on the degradation of PCP in sediments, determine the PCP intermediary degradation products produced under oxic and anoxic conditions, and determine the rate of PCP removal in the presence and absence of Cd under oxic and anoxic conditions. The mechanisms of heavy metal resistance used by adapted sulfidogenic and methanogenic enrichments degrading pentachlorophenol in the presence of cadmium were also determined in this study.; Results showed that removal of 91 to 93% of PCP occurred in both SRB and MET enriched cultures, in the absence of Cd, within 82 days. The presence of soluble Cd initially decreased the rate of PCP removal by the enrichment cultures, but PCP removal rates improved as the Cd precipitated. The studies confirmed the mineralization of PCP by both enrichment cultures, as well as the incorporation of PCP carbon into the microbial biomass. Under simulated sand sediment conditions, while PCP was removed in the presence or absence of Cd, redox conditions, the presence or absence of Cd, and the sand type strongly influenced the rate and pathways of PCP biodegradation. We observed that both enrichments developed Cd resistance by removal of Cd from the aqueous phase, although the Cd was removed by different mechanisms depending upon the enrichment. The sulfidogenic enrichments removed Cd by extracellular precipitation, as cadmium sulfide (CdS), while the methanogenic enrichment culture removed Cd by extracellular sequestration of the Cd into the exopolymer.; This research has demonstrated the capability of microbial populations to degrade PCP in the presence of a co-contaminant heavy metal, Cd, and it has provided a basis for future research that can be focused on identifying and comparing the microbial species involved in these transformations as influenced by these various factors, so that effective bioremediation processes can be developed for cleaning en... |
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