(Industrial Electrochemistry and Electrochemical Engineering Division Student Achievement Award) Reactive Separations to Remediate and Valorize Nitrogen-Polluted Wastewaters

Abstract

Anthropogenic perturbations to the global nitrogen cycle due to industrial Haber-Bosch fertilizer production threaten large-scale food production, energy inputs for chemical manufacturing, and protection of water quality. Enabling a sustainable food-energy-water nexus requires feeding a growing population while minimizing environmental impacts. In this talk, I introduce Electrodialysis and Nitrate Reduction (EDNR), a novel electrochemical process that couples water purification with electrified ammonia production from nitrogen-polluted wastewaters.The EDNR reactor consists of three chambers and operates in two stages (Figure 1), with the influent entering the middle chamber and products recovered from the left and right chambers. In Stage 1, influent nitrate () and ammonium () are separated via electrodialysis (ED) and ammonia is recovered in the right chamber. In Stage 2, ammonia is synthesized from the electrochemical nitrate reduction (NR) in the left chamber. EDNR enables rational design of electrochemical environments in each chamber (e.g., electrolyte pH; cationic and anionic constituents; species concentrations) through tunable operating parameters such as applied potential, electrolyte flow rate, and duration of the ED and NR stages. This modular, tunable design facilitates robust water remediation and ammonia production from wastewaters of transient composition.We have demonstrated proof-of-concept EDNR reactors using titanium foil (left chamber, NR electrode), Ti/IrO2-Ta2O5 mesh (left and middle chambers, ED electrode), and platinum foil (right chamber, ED electrode). With recirculating batches of simulated wastewater (100 ppm + 500 ppm ), 75% influent NH4 + was recovered into the right chamber and 25% influent was converted to in the left chamber after three EDNR cycles. As the rate-limiting stage, NR on titanium merited further fundamental investigation. In particular, the reasons for titanium’s electrocatalytic NR performance remain largely unclear to date, especially regarding the role of the role of titanium hydride (TiHx, 0<x<2), which forms during NR. Rationally implementing Ti-catalyzed NR requires improved understanding of how near-surface Ti-hydride forms and influences NR activity and selectivity. Through systematic synchrotron x-ray characterization of Ti-hydride electrodes, electrochemical testing, and density functional theory calculations, we found that near-surface hydride content plays a relatively minor role in steering NR performance compared to applied potential and electrolyte effects. Put in context, our results help prioritize how EDNR operation can be optimized for ammonia production. As a validated platform with ongoing work to improve performance, EDNR shows great potential in realizing sustainable and distributed water remediation and ammonia production.