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 N2 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 Cu0, and then further reduced to N2/NH4+ by Mg0. Hypothetically Al0 could provide sorption sites for hydrogen radicals (·H) dissociated from ultrasound, hence served as reducing sites in denitrification process. The XPS analysis showed an increased peak of Cu0 after the sonocatalytic reduction when catalyst has lower amount of Al. The excessive hydrogen adsorbed on Al0 might spill-over to the adjacent Cu, thus reducing the CuO into Cu0 at high temperature created by the implosion of the microbubbles. Without the use of consumable reducing agents (i.e. H2 gas), sonocatalytic reduction could be a potential candidate of remediation method to treat NO3- polluted water with high N2 selectivity and easy magnetic recovery.
Keywords: Denitrification; Layered double hydroxide; Nitrate; Reduction; Sonocatalysis; Ultrasound.
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