Using selectivity to evaluate aqueous- and resin-phase denitrification during biological ion exchange.

An increased fertilizer application for agricultural purposes has resulted in increased nitrate (NO3-) levels in surface water and groundwater around the globe, highlighting demand for a low-maintenance NO3- treatment technology that can be applied to nonpoint sources. Ion exchange (IEX) is an effective NO3- treatment technology and research has shown that bioregeneration of NO3- laden resins has the potential to minimize operational requirements and brine waste production that often prevents IEX application for decentralized treatment. In this work, batch denitrification experiments were conducted using solutions with low IEX selectivity capable of supporting the growth of denitrifying bacteria, while minimizing NO3- desorption from resins, encouraging resin-phase denitrification. Although only 15% of NO3- was desorbed by the low selectivity solution, this initial desorption started a cycle in which desorbed NO3- was biologically transformed to NO2-, which further desorbed NO3- that could be biotransformed. Denitrification experiments resulted in a 43% conversion rate of initially adsorbed NO3-, but biotransformations stopped at NO2- due to pH limitations. The balance between adsorption equilibria and biotransformation observed in this work was used to propose a continuous-flow reactor configuration where gradual NO3- desorption might allow for complete denitrification in the short retention times used for IEX systems.

Synergistic Zerovalent Iron (Fe0) and Microbiological Trichloroethene and Perchlorate Reductions Are Determined by the Concentration and Speciation of Fe.

Trichloroethene (TCE) and perchlorate (ClO4-) are cocontaminants at multiple Superfund sites. Fe0 is often used during TCE bioremediation with Dehalococcoides mccartyi to establish anoxic conditions in the aquifer. However, the synergy between Fe0 abiotic reactions and microbiological TCE and ClO4- reductions is poorly understood and seldom addressed in the literature. Here, we investigated the effects of Fe0 and its oxidation product, Fe2+, at field-relevant concentrations in promoting microbial TCE and ClO4- reductions. Using semibatch microcosms with a Superfund site soil and groundwater, we showed that the high Fe0 concentration (16.5 g L-1) expected during Fe0in situ injection mostly yielded TCE abiotic reduction to ethene/ethane. However, such concentrations obscured dechlorination by D. mccartyi, impeded ClO4- reduction, and enhanced SO42- reduction and methanogenesis. Fe2+ at 0.25 g L-1 substantially delayed conversion of TCE to ethene when compared to no-Fe controls. A low concentration of aged-Fe0 synergistically promoted microbiological TCE dechlorination to ethene while achieving complete ClO4- reduction. Collectively, these results illustrate scenarios relevant at or downstream of Fe0 injection zones when Fe0 is used to facilitate microbial dechlorination. Results also underscore the potential detrimental effects of Fe0 and bioaugmentation cultures coinjection for in situ treatment of chlorinated ethenes and ClO4-.