(Invited) The Use of a Proton-Mediated Redox Couple for Ammonia Recovery

Abstract

Electrochemical techniques provide an efficient means to selectively recover ammonia nitrogen present in wastewater, of which nitrogen could potentially supply 30% of global fertilizer demand. The selective recovery can be achieved by converting ammonium into ammonia using electrochemical reactions to elevate the solution pH, instead of adding chemicals typically implemented in conventional air stripping processes. While electrolysis of water represents the most common electrochemical approach, its theoretical (1.2 V) and practical voltage (>2.5 V) requirements lead to a high level of energy consumption. To make the electrochemically driven ammonia recovery a more viable option, we examined the use of a proton-mediated redox couple for elevating solution pH. Our proof-of-concept study using a synthetic domestic wastewater (5 mM NH4Cl + 20 mM NaCl) showed that NH4 + was removed at a nitrogen flux of 34 g N m−2 d−1 through a cation exchange membrane at a constant current density of 9.6 A m−2. The solution pH simultaneously increased to 9.6 by the electrochemical reaction (O + H+ + e− → R) at a cell voltage of 0.47 V, which was sufficient to drive NH4 + to NH3. These results show that the use of a proton-mediated redox couple could provide a new strategy for ammonia recovery.