Modeling autohydrogenotrophic treatment of perchlorate-contaminated water in the presence of nitrate
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Perchlorate contamination is widespread. Perchlorate, a water contaminant, disrupts iodide uptake to the thyroid, inhibiting growth and mental development. Recent studies have demonstrated autohydrogenotrophic perchlorate reduction to chloride. Hydrogen gas can be produced in-situ via the corrosion of zero-valent iron (ZVI), thereby avoiding problems related to the low aqueous solubility of hydrogen gas. The presence of nitrate has been shown inhibit autohydrogenotrophic perchlorate reduction. However, no studies have modeled the effects of nitrate on autohydrogenotrophic perchlorate biokinetics or developed a model to function as a design tool to predict long-term performance of ZVI/biotic perchlorate treatment systems in the presence of nitrate. Batch experiments demonstrated the presence of nitrate significantly inhibited perchlorate degradation by an autohydrogenotrophic microbial consortium. However, the consortium was capable of significant perchlorate reduction while the bulk of the nitrate was still present. A modified competitive inhibition model successfully predicted autohydrogenotrophic perchlorate degradation in the presence of nitrate. The model describes perchlorate degradation as a function of the biomass, perchlorate, hydrogen, and nitrate concentrations, as well as the single-component perchlorate, hydrogen, and nitrate half-saturation coefficients and perchlorate maximum substrate utilization rate. To obtain the single-component parameters, a series of batch experiments were performed under perchlorate-, nitrate-, and hydrogen-limiting conditions. The single-component biokinetic parameters and model predictions indicate the consortium could treat perchlorate-contaminated water with concentrations in the low hundreds of μg/L and in states with perchlorate treatment goals in the low μg/L range. The consortium biokinetic parameters and modified competitive inhibition model were used in the development of an AQUASIM based biofilm model. The model also integrated physical parameters, ZVI hydrogen production, and abiotic nitrate reduction. The model was calibrated using the long-term performance results of a laboratory-scale ZVI/biotic column. Both laboratory and modeling results showed when the column becomes hydrogen-limited, the presence of nitrate decreases perchlorate removal efficiency. Full-scale simulations demonstrated the model could prove useful as a predictive design tool. Simulations suggest that a permeable reactive barrier that includes 10% ZVI and additional media capable of pH buffering could remove typical contaminated ground water concentrations of perchlorate in the presence of typical oxygen and nitrate concentrations.