Abstract
The release of nitrate to the environment from wastewater effluent and agricultural runoff contributes to groundwater contamination, harmful algal blooms, and disruption of biogeochemical nitrogen flows. Typical treatment methods are based on nitrate separation, which produces waste streams that are often discharged to the environment. Alternatively, nitrate conversion via electrochemical reduction eliminates the production of concentrated waste streams while avoiding the addition of reductant or hole scavenger chemicals to accomplish the reaction. However, major challenges for nitrate removal from water via electrochemical conversion involve reducing the use of expensive precious metal electrocatalysts while also improving the reaction activity and selectivity, catalyst stability, and mass transport of nitrate to electrocatalyst active sites. The use of electrochemical membranes as multifunctional porous flow-through electrodes could potentially address these challenges based on improved mass transport and altered kinetics under flow conditions within membrane pores.Conductive membranes were fabricated using polymers combined with carbonaceous materials such as reduced graphene oxide (rGO) and carbon nanotubes. The rGO was functionalized with non-precious transition metal oxynitride electrocatalysts, where these catalysts showed higher nitrate conversion activity compared to the unsupported transition metal nitrides. The influence of catalyst materials, membrane fabrication process, and filtration conditions on nitrate reduction activity and selectivity were evaluated. In addition to the environmental impacts of closing the nitrogen loop by converting nitrate into innocuous N2, selective nitrate reduction to ammonia provides opportunities for recovery as fertilizer or carbon-free renewable energy storage. The prospects for reactive nitrogen recovery based on nitrate electrochemical conversion to ammonia were analyzed for various potential source waters.
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