Effects of chloride transport on the bioelectrochemical remediation of nitrate-contaminated groundwater†

Nitrate is a common groundwater contaminant, primarily caused by the leaching of fertilizers. It poses a risk to human health, prompting the USEPA to set a drinking water limit of 10 mg L⁻¹. Membrane-based bioelectrochemical systems (MBES) are effective treatment mechanisms for remediation of nitrate-rich groundwater. However, there is a knowledge gap surrounding how chloride ions as competing ions impact nitrate removal mechanisms and kinetics. In this study, nitrate-rich groundwater was fed into the cathode side of an MBES equipped with an anion exchange membrane (AEM). Nitrate ions were subsequently transported to the anolyte, where microbe-mediated reduction to N₂ was achieved. The system performance was evaluated under varied catholyte nitrate and chloride concentrations as well as with different applied current densities. The MBES consistently achieved nitrate removal efficiencies of at least 85% with catholyte nitrate concentrations ranging from 14 mg L⁻¹ NO₃⁻-N to 56 mg L⁻¹ NO₃⁻-N. Notably, the highest nitrate removal rate of 8.28 ± 0.01 mg NO₃⁻-N L⁻¹ h⁻¹ was achieved when the catholyte influent nitrate concentration was 56 mg L⁻¹ NO₃⁻-N. The nitrate removal behavior in the MBES can be characterized as a pseudo-first-order reaction. The presence of chloride ions, acting as model competing ions to nitrate, was found to decrease the rate of nitrate removal. Additionally, we found that diffusion is the primary driving force for nitrate removal, with electromigration slightly enhancing nitrate transport across the membrane in the MBES. When actual groundwater was used as the catholyte, 90.6 ± 12.1% nitrate was removed and the removal rate reached 5.3 ± 0.4 mg L⁻¹ h⁻¹ NO₃⁻-N, demonstrating the high efficiency of this MBES in treating nitrate-contaminated groundwater.