Harnessing Photoelectrochemistry for Resource Recovery from Wastewater Nitrates

Abstract

Nitrates are present in industrial effluent streams from fertilizer production, iron/steel production, coal mining, and municipal wastewater streams. Typically, nitrates are eliminated from wastewater through biological nitrification-denitrification treatment processes, but these are energy and carbon intensive, are not applicable for all waste streams and have not prioritized nitrogen recovery. We propose a photoelectrochemical device is proposed to transform wastewater nitrates to ammonia, nitrous oxide, and dinitrogen, coupled with water oxidation. Numerical models are developed to quantify process efficiencies and nitrogen-removal as a function of light absorber band gaps, electrocatalytic kinetic parameters, competing oxygen reduction and hydrogen evolution reactions, and the reacting nitrate species concentrations. A device-scale model was also developed to capture the coupled effects of light absorption, species transport and reaction kinetics. With a single light-absorber and state-of-the-art catalysts, optimal solar-to-chemical efficiencies of 7% and 11% and nitrogen-removal rates of 251.3 and 451.6 gN m-2 day-1 are predicted for the formation of ammonia and nitrous oxide respectively. Theoretically predicted peak nitrogen removal rates and specific energy intensities are competitive with reported estimates for bioelectrochemical and Sharon-Anammox processes that recover ammonia from waste streams. This result, together with the added benefit of harnessing sunlight to effect the nitrate transformation to value-added products, indicates promise in the proposed photoelectrochemical pathway for wastewater nitrogen removal coupled with resource recovery. Figure 1