| Perchlorate contamination of the environment has received increasing recognition since the late 1990s. Despite being a very thermodynamically favorable electron acceptor, perchlorate remains recalcitrant in many soils and waters contaminated with this compound. The most effective method for treating perchlorate-contaminated sites appears to be anaerobic bioremediation where perchlorate is used as an electron acceptor for microbial growth. Several mixed- and pure-culture bacteria were used to assess various aspects of perchlorate reduction.; Seven novel perchlorate-degrading bacteria were isolated from microcosms developed from material collected at a perchlorate-contaminated site, the Longhorn Army Ammunition Plant, outside Karnack, Texas. Two of these isolates were capable of growth on hydrogen, perchlorate, and inorganic carbon. Five additional novel heterotrophic bacteria were also isolated from these microcosms. A comparison of 16S rDNA sequences recovered from these seven isolates showed that they are distributed within the beta and epsilon subclasses of Proteobacteria. The two isolates capable of autotrophic perchlorate degradation appear to be within the genus Dechloromonas.; Zero-valent iron (Fe(0)) should be a very favorable electron donor for abiotic and biotic perchlorate degradation. While abiotic reduction did not occur, enhanced microbial perchlorate reduction was observed in microcosms containing Fe(0). This activity was limited, however, by two inhibitory effects. Degradation ceased above pH 8.7, a pH range encountered during Fe(0) corrosion. In addition, Fe(0) indirectly inhibited perchlorate reduction over time by release of Fe(II), which likely affected bacteria by adsorption of Fe(II) on bacterial surfaces or by deposition of precipitates such as vivianite or siderite on bacterial surfaces.; A highly enriched mixed culture was used to assess the role of electron donor addition and redox potential on perchlorate degradation. Lactate, acetate, hydrogen, and poplar tree root products supported perchlorate reduction. Under conditions tested, the optimal electron donor requirements were approximately 1.2 mg COD per mg perchlorate. Perchlorate degradation was observed to decrease in microcosms where redox potential was increased. However, significant perchlorate reduction occurred at redox potentials as high as +180 mV (versus a standard hydrogen electrode). Rate coefficients were determined for this enrichment culture. |
