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

Wong, Kien Tiek, Saravanan, Pichiah, Nah, In Wook, Choi, Jaeyoung, Park, Chulhwan, Kim, Namchan, Yoon, Yeomin, Jang, Min
Chemosphere 2019 v.218 pp. 799-809
X-ray photoelectron spectroscopy, aluminum, catalysts, copper, cupric oxide, denitrification, free radicals, hydrogen, hydroxides, magnesium, magnetism, magnetite, microbubbles, nitrates, nitrites, pH, reducing agents, remediation, sonication, sorption, spectral analysis, temperature, ultrasonics, water pollution
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.