Systematic evaluation of nitrate and perchlorate bioreduction kinetics in groundwater using a hydrogen-based membrane biofilm reactor

To evaluate the simultaneous reduction kinetics of the oxidized compounds, we treated nitrate-contaminated groundwater (∼9.4 mg-N/L) containing low concentrations of perchlorate (∼12.5 μg/L) and saturated with dissolved oxygen (∼8 mg/L) in a hydrogen-based membrane biofilm reactor (MBfR). We systematically increased the hydrogen availability and simultaneously varied the surface loading of the oxidized compounds on the biofilm in order to provide a comprehensive, quantitative data set with which to evaluate the relationship between electron donor (H₂) availability, surface loading of the electron acceptors (oxidized compounds), and simultaneous bioreduction of the electron acceptors. Increasing the H₂ pressure delivered more H₂ gas, and the total H₂ flux increased linearly from ∼0.04 mg/cm²-d for 0.5 psig (0.034 atm) to 0.13 mg/cm²-d for 9.5 psig (0.65 atm). This increased rate of H₂ delivery allowed for continued reduction of the acceptors as their surface loading increased. The electron acceptors had a clear hydrogen-utilization order when the availability of hydrogen was limited: oxygen, nitrate, nitrite, and then perchlorate. Spiking the influent with perchlorate or nitrate allowed us to identify the maximum surface loadings that still achieved more than 99.5% reduction of both oxidized contaminants: 0.21 mg NO₃-N/cm²-d and 3.4 μg ClO₄/cm²-d. Both maximum values appear to be controlled by factors other than hydrogen availability.