Electrochemical reduction of aqueous nitrate using metallic silver particles as spatially suspended catalyst

Increasing nitrate concentrations in water sources is a serious concern. The current study demonstrates the electrochemical reduction of nitrate in the presence of silver metallic particles (AgMPs). Using Ti/RuO2 anode and a Ti cathode, nitrate removal in undivided and divided cells was limited to 18% and 77%, respectively, in the absence of AgMPs. Total Nitrogen (TN) removal in undivided and divided cells was nil and 29.5%, respectively, in the absence of AgMPs. Under identical conditions, the presence of 8 mM AgMPs increased nitrate removal to 63% and 99%, respectively, with significant improvement in nitrate conversion to N2. The effect of various parameters on nitrate removal and selectivity of products was studied using a divided cell: concentration of AgMPs, inter-electrode distance, cathode material, current density, initial nitrate concentration, and reusability of AgMPs. In the presence of 8 mM AgMPs, nitrate removal using different cathodes was Ti ≈ Fe > Cu > SS > Fe/Sn. The cathode type also affected the selectivity, with the maximum conversion to N2 achieved with the Ti cathode. Nitrate removal and conversion to N2 improved with the increase in current density up to 10 mA/cm2. Overall, the presence of AgMPs substantially improved nitrate removal and conversion to N2. The production of reductive species (AgHx) by the intercalation of nascent hydrogen on metallic silver appears to promote catalytic nitrate reduction. AgMPs could be reused up to 10 times with consistent performance. This work offers a new approach to nitrate reduction using an efficient and reusable catalyst.