Removal of Nitrate from Drinking Water by Ion-Exchange Followed by nZVI-Based Reduction and Electrooxidation of the Ammonia Product to N2(g)

Abstract

Ion-exchange (IX) is common for separating NO 3− from drinking water. From both cost and environmental perspectives, the IX regeneration brine must be recycled, via nitrate reduction to N 2(g) . Nano zero-valent iron (nZVI) reduces nitrate efficiently to ammonia, under brine conditions. However, to be sustainable, the formed ammonia should be oxidized. Accordingly, a new process was developed, comprising IX separation, nZVI-based nitrate removal from the IX regeneration brine, followed by indirect ammonia electro-oxidation. The aim was to convert nitrate to N 2(g) while allowing repeated usage of the NaCl brine for multiple IX cycles. All process steps were experimentally examined and shown to be feasible: nitrate was efficiently separated using IX, which was subsequently regenerated with the treated/recovered NaCl brine. The nitrate released to the brine reacted with nZVI, generating ammonia and Fe(II). Fresh nZVI particles were reproduced from the resulting brine, which contained Fe(II), Na + , Cl − and ammonia. The ammonia in the nZVI production procedure filtrate was indirectly electro-oxidized to N 2(g) at the inherent high Cl − concentration, which prepared the brine for the next IX regeneration cycle. The dominant reaction between nZVI and NO 3 − was described best (Wilcoxon test) by 4Fe (s) + 10H + + NO 3 − → 4Fe 2+ + NH 4 + + 3H 2 O, and proceeded at >5 mmol·L −1 ·min −1 at room temperature and 3 < pH < 5.