Fe°-enhanced bioremediation for the treatment of perchlorate in groundwater
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Perchlorate, a water contaminant that interferes with the thyroid’s ability to use iodine to produce growth hormones, has been found in groundwater in a number of states including Texas. Currently, the State of Texas has identified detectable levels of perchlorate in 28% of water systems sampled (66 out of 231 water sources sampled). Very few research studies have addressed treatment alternatives for perchlorate, especially for treatment of waters that contain only the part per billion levels of contamination recently identified in groundwaters. The purpose of this research was to construct a treatment process capable of reducing perchlorate encountered in groundwater using a combination of a biological and a chemical component. The biological component provides the microbial population capable of reducing perchlorate. Then, the chemical component provides hydrogen as the energy source for the bacterial population. The strategy to develop the process consisted of initially and independently developing each of these components. After understanding each component, the complete treatment process was constructed. First, the applicability of the biological component was evaluated by obtaining different mixed microbial cultures capable of degrading perchlorate, using hydrogen as the energy source and inorganic carbon as the source of carbon. Then, the rates of degradation for the cultures were obtained and one of the populations was selected for use in the combined treatment system. Second, production of hydrogen during anaerobic corrosion of a zero-valent iron verified the potential for the chemical component of the process. The hydrogen production rates were evaluated, and the influence of different dissolved species on the rates of hydrogen production was determined to evaluate the impact of water chemistry. Third, the two components were combined in batch and continuous-flow reactors, both resulting in successful removal of perchlorate from water. Fourth, a computer model was developed to provide a tool to help design continuous-flow reactors by incorporating the different parameters involved in the treatment process. A sensitivity analysis for the model was carried out to identify the relationship between the different parameters in the model and the removal of perchlorate. This analysis helped to determine the relative importance of the different parameters in the performance of the system. The research demonstrated that a mixed autotrophic bacterial population was easily isolated and capable of degrading perchlorate using hydrogen produced by zero-valent iron corrosion in batch and continuous-flow reactors. Perchlorate removal in continuous-flow reactors required hydrogen concentrations in excess of 10-3 mM and dissolved oxygen concentrations below 1 mg/L. In addition, the treatment process was found to be susceptible to process failure due to passivation of the iron surface. Mathematical modeling of the treatment system suggested that the technology shows promise for application in the in-situ scenario. The preliminary information provided by the model suggests that the contact times required for the technology in the lab scale were significantly smaller than those residence times typical of in-situ applications. However, the relatively long detention times required to achieve regulatory levels may limit ex-situ applications.