Upcycling wastewater nitrate into ammonia fertilizer via concurrent electrocatalysis and membrane extraction

Abstract

Electrocatalytic upcycling of nitrate (NO3−) in wastewater into the valuable ammonium-based fertilizer has been considered as a green and appealing alternative to biological nitrogen removal or the industrial ammonia (NH3) production processes. This work investigated an innovative and energy-efficient electrolysis flow-through cell consisting of a porous activated iron nickel (FeNi) alloy cathode and a hollow polypropylene fiber membrane extraction unit to realize the electrochemical NO3− reduction and simultaneous upcycling to ammonium sulphate salts ((NH4)2SO4). Cathodic and anodic electrochemical half-reactions were coupled to enable NO3− reduction to ammonia and in-situ acid/base productions to promote the membrane stripping of NH3. Our results show that after electrolysis operation for 14 h for a synthetic wastewater containing 150 mM NO3− under a cathodic current density of 30 mA·cm−2, 99 % of NO3− removal efficiency, 98 % of ammonia selectivity, 93 % of Faradic efficiency and 97 % of total ammonia nitrogen (TAN) recovery were achieved respectively. A NH3 recovery flux and a specific energy consumption reached 2050 g-(NH4)2SO4·m−2·d−1 and 11 kWh·kg−1-(NH4)2SO4, respectively, which outcompetes many reported processes. Direct electron transfer was the main mechanisms of electrochemical NO3− reduction to ammonia. Interfacial reaction thermodynamics and kinetics analysis of key intermediates (e.g., *NO3, *NO2, and *NO) shows that the NiFe2O4 (3 1 1)-Ni site on the thermally activated FeNi alloy surface exhibited higher reactivity and specificity toward electrochemical NO3− reduction to ammonia over nitrogen (N2) or hydrogen (H2) generation. Ultimately, this study aims to promote sustainable nitrogen nutrient recovery and ammonia fertilizer synthesis from wastewater treatment.