Modulating Fe sites by La in porous MnFe2O4 for enhanced removal of ROX: Synergy of efficient adsorption and PMS activation

Abstract

Catalytic-adsorption method is a promising strategy for degrading organoarsenic compounds and removing secondary inorganic arsenic. The method relies significantly on heterogeneous catalysts with selectively adsorption and enhanced peroxymonosulfate (PMS) activation capacity. In this study, active sites for selective adsorption and PMS activations were developed by modulating the Fe-sites in porous MnFe₂O₄ through La-doping. Synchrotron radiation, EPR, and XPS characterizations confirmed the presence of oxygen vacancies, metal hydroxyl groups M(Fe/Mn/La)-(OH) and the active Fe(II)/Mn(II,III), as well as the fine structure of La occupied sites. Theoretical calculations indicate that the generation of Vo would increase the local electron cloud density of La dopants, leading to the transfer of local electrons into the bulk phase. The electron transfer characteristics result in the raising the d-band center of MnFe₂O₄ and lowering the Gibbs free energy of the intermediate state, thus promoting ¹O₂ generation. In 3% La-MnFe₂O₄/PMS system, 96% ROX (10 mg/L) were removed within 35 min with the secondary inorganic arsenic levels below 10 μg/L. The rate coefficients k for ROX removal in porous 3%La-MnFe₂O₄/PMS is 4.05 times higher than that in MnFe₂O₄/PMS. ROX was effectively removed in different water matrices (Liao River, Hun River, and groundwater), demonstrating the practical application potential of 3% La-MnFe₂O₄/PMS system. Under continuous flow conditions, an average of 97.9% and 87.3% of ROX were removed from ultrapure water and groundwater, respectively, over a 10-hour continuous run. This study highlights the high-performance spinel La-MnFe₂O₄ for the synergistic enhancement of PMS activation, secondary arsenic adsorption, and improved mass transfer, contributing to green and safe water treatment strategies.