State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates

Nitrate pollution of groundwater, which is mainly caused by the application of nitrogen-based fertilizers in intensive agriculture, is a widespread problem all over the world and a potential risk for public health. Reverse osmosis, ion exchange and electrodialysis are currently used for water denitrification, yielding a highly concentrated reject water that requires economic and environmental costs for disposal. Nitrate reduction technologies that are able to convert nitrate into inert nitrogen gas have appeared that are promising, cost effective and environmentally friendly. Among these technologies, attention has been focused on i) the chemical reduction over mono- and bimetallic catalysts with hydrogen as the reducing agent and ii) electrocatalytic reduction processes over metallic anodes. Although selectivity values towards N2 of greater than 90% are achieved with both technologies, the undesired formation of ammonium as a reaction by-product is still the main drawback preventing their implementation at larger scales. For this reason, the development of new catalytic and electrodic materials as well as novel reactor configurations to avoid ammonium formation have been extensively investigated in the last few years to increase the effectiveness and competitiveness of both technologies. In this paper, an overview of the current state-of-the-art of both catalytic reduction and electroreduction of nitrates is presented, highlighting their potential and their main drawbacks along with guidelines for future research.