Synthesized akhtenskites remove ammonium and manganese from aqueous solution: removal mechanism and the effect of structural cations

Abstract

Ammonium and manganese removal by tunnel-structured manganese oxide is still enigmatic. Herein, tunnel-structured akhtenskites with different structural cations (Na–MnO_x, Mg–MnO_x Ca–MnO_x, Fe–MnO_x) were synthesized by the KMnO₄ and Mn²⁺ reaction in the presence of different metal cations, and were used to remove ammonium and manganese from aqueous solution. The results of the batch adsorption experiments indicated that akhtenskites effectively removed NH₄⁺ and Mn²⁺, and the removal process fitted the pseudo-second-order model. By measuring the concentration of nitrate and nitrite, discriminating the adsorbed and oxidized Mn²⁺, and testing the zeta potential of the oxides, it can be concluded that NH₄⁺ was merely removed by electrostatic adsorption via Mn–O⁻; Mn²⁺ could also be adsorbed by ion exchange with Mn–OH, and the adsorbed Mn²⁺ could be partly oxidized. The structural properties of the akhtenskites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) specific area, and X-ray photoelectron spectroscopy (XPS). The experimental results showed that ions with higher valence can result in a higher percentage of Mn(III) in akhtenskite. Mg²⁺ can result in a lower proportion of lattice oxygen in the oxide, and Fe³⁺ can increase the pH of the point of zero charge. Both of them were unfavored for the oxidation of Mn²⁺. Moreover, it was found that Ca–MnO_x had optimal removal performance in the catalytic oxidation of Mn²⁺ owing to appropriate percentages of O_latt and Mn(III) and lower zeta potential. This study provides new insights into the synthesis and application of manganese oxides.