Sonocatalytic reduction of nitrate using magnetic layered double hydroxide: Implications for removal mechanism

Abstract

In this study, magnetic layered double hydroxides (mag-LDHs) were synthesized through compositing magnetite with three different metals (Mg, Cu and Al) under ultrasound (US, 100 kHz frequency and 50 W power). For the first time, mag-LDHs were applied to sonocatalytic reduction of nitrate (NO3-) and the reduction mechanism were determined by conducting kinetic tests and various spectroscopic analyses. Based on the kinetic data, NO3- reduction and the selectivity for N-2 highly depends on the ratio between Mg/Al, solution pH and sonication frequency. The best condition for sonocatalytic denitrification was found to be pH 7 operated under 100 kHz (50% power) using the catalyst with lowest amount of Al (mag-LDH-Al0.3Mg1.5). As a proposed mechanism, NO3- is initially reduced to NO2 by Cu-0, and then further reduced to N-2/NH4+ by Mg-0. Hypothetically Al-0 could provide sorption sites for hydrogen radicals (center dot H) dissociated from ultrasound, hence served as reducing sites in denitrification process. The XPS analysis showed an increased peak of Cu-0 after the sonocatalytic reduction when catalyst has lower amount of Al. The excessive hydrogen adsorbed on Al-0 might spill-over to the adjacent Cu, thus reducing the CuO into Cu-0 at high temperature created by the implosion of the microbubbles. Without the use of consumable reducing agents (i.e. H-2 gas), sonocatalytic reduction could be a potential candidate of remediation method to treat NOT polluted water with high N-2 selectivity and easy magnetic recovery. (C) 2018 Elsevier Ltd. All rights reserved.