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

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-Al_0.3Mg_1.5). 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.