No 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).

Wastewater is an underleveraged resource; it contains pollutants that can be transformed into valuable high-purity products. Innovations in chemistry and chemical engineering will play critical roles in valorizing wastewater to remediate environmental pollution, provide equitable access to chemical resources and services, and secure critical materials from diminishing feedstock availability. This perspective envisions electrochemical wastewater refining—the use of electrochemical processes to tune and recover specific products from wastewaters—as the necessary framework to accelerate wastewater-based electrochemistry to widespread practice. We define and prescribe a use-informed approach that simultaneously serves specific wastewater-pollutant-product triads and uncover mechanistic understanding generalizable to broad use cases. We use this approach to evaluate research needs in specific case studies of electrocatalysis, stoichiometric electrochemical conversions, and electrochemical separations. Finally, we provide rationale and guidance for intentionally expanding the electrochemical wastewater refining product portfolio. Wastewater refining will require a coordinated effort from multiple expertise areas to meet the urgent need of extracting maximal value from complex, variable, diverse, and abundant wastewater resources.

Haber-Bosch ammonia production and utilization has sustained exponential population growth but exacerbated wastewater nitrate pollution. Abundant nitrate pollutants could be refined to purified nitrogenous chemicals with the electrochemical nitrate reduction reaction (NO3RR). However, the dilute and impure composition of nitrate-rich wastewaters presents barriers to realizing practical electrocatalytic systems that to date operate most efficiently in concentrated, pure electrolytes. These barriers inform our investigation of the ammonia-selective homogeneous molecular NO3RR catalyst Co(DIM). In this work, we elucidate interfacial mechanisms of catalysis inhibition that describe Co(DIM)-mediated NO3RR performance in complex electrolyte compositions. These mechanisms inform design principles for a novel reactive separations platform, electrocatalyst-in-a-box (ECaB), that exhibits the lowest reported energy consumption for purified ammonia production from a real wastewater (90.0 ± 2.7 kWh kg-N–1). This work demonstrates a use-informed design approach that iterates between mutually informative mechanistic insights and performance of electrochemical wastewater refining systems in complex aqueous streams.