Reactive Separation of Ammonia from Wastewater Nitrate via Molecular Electrocatalysis

Abstract

Selective electrochemical conversion of nitrate to ammonia with ammonia capture can simultaneously remediate nitrate-polluted wastewater and supplement Haber-Bosch ammonia production. Homogeneous molecular catalysts remain underexplored in wastewater treatment and, more generally, in reactive separation processes. Despite the tunable reactivity of molecular catalysts, two barriers prevent their widespread implementation for wastewater nitrate treatment and ammonia recovery. The first barrier is the lack of reports of reaction activity and selectivity of nitrate reduction molecular catalysts in real wastewaters. The second barrier is the need to separate the catalyst, catalytic product, and treated wastewater. In this study, we employ electrochemical stripping in several configurations as a reactive separation unit process to address both barriers to implementation. Using the homogeneous molecular nitrate reduction catalyst Co(DIM), we demonstrate >70% removal of nitrate from municipal wastewater treatment plant secondary effluent with >98.5% selectivity to ammonia, leading to the generation of a high-purity ammonia product (ammonium sulfate). These experiments constitute a novel demonstration of molecular catalysis for nitrate reduction in real wastewater and highlight electrochemical engineering opportunities in reactive separations to valorize wastewater resources. This work advances environmental research toward United Nations Sustainable Development Goals (SDGs) for Clean Water (SDG 6) and Responsible Consumption and Production (SDG 12).