Highly efficient catalytic ozonation for ammonium in water upon γ-Al2O3@Fe/Mg with acidic-basic sites and oxygen vacancies

Abstract

Catalytic ozonation has prospects in the advanced treatment of nitrogen removal, and solid base MgO can efficiently catalyze the ozonation of ammonium nitrogen. However, it is necessary to improve the problem of easy loss, difficult recovery, and low percentage of gaseous products. Here, MgO, amorphous Fe₂O₃, and γ-Al₂O₃ were selected as doping components and supports, respectively, to prepare γ-Al₂O₃@Fe/Mg composite catalysts with abundant acidic-basic sites and oxygen vacancies. The results show that γ-Al₂O₃@Fe/Mg₅ can efficiently catalyze the ozonation of ammonium nitrogen (98.73%) with 67.82% gaseous product selectivity under the conditions of initial pH = 7, catalyst dosage of 112.88 g/L, and ozone dosage of 2.4 mg/min. The doping of Fe₂O₃ and MgO with a weaker lattice oxygen binding energy improves the gaseous product selectivity. The mechanism of ammonium nitrogen removal for γ-Al₂O₃@Fe/Mg₅ is revealed, especially the intrinsic contribution of acidic-basic sites and oxygen vacancies. The pH and active sites play different roles in ozone decomposition for NH₄⁺ removal. Surface hydroxyl protonation on basic sites and oxygen vacancies and electron transfer on acidic sites are responsible for ozone decomposition to hydroxyl radicals. Moreover, γ-Al₂O₃@Fe/Mg₅ exhibits good stability, few leaching ions, and can be settled in water for easy recovery. This study suggests that γ-Al₂O₃@Fe/Mg₅ is a good candidate for the catalytic ozonation of ammonium nitrogen.

Keywords: Acidic sites; Ammonium; Basic sites; Catalytic ozonation; Oxygen vacancies.