EXAFS/XPS analysis of the arsenite removal mechanism on the Fe/Mn oxide-based composite: pH effect of the long-term oxidative-precipitative sorption

This work investigates the atomic-scale/molecular mechanism of arsenite uptake on the promising Fe/Mn oxide-based composite under conditions of (the technologically relevant and rarely studied) full saturation of adsorption sites as a function of pH (4.5; 6.0; 7.0; 8.0). It provides deep insight into the physico-chemical processes ruling the interfacial reactions (involving the redox chemical elements from both the water adsorbate and the anion exchanger) over a long period of time resulting in the competitive removal of poisonous arsenic. Using (As K-edge) extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) allowed us to focus on both the adsorbate and adsorbent; to discover and describe the three main processes underpinning the removal of H₃AsO₃, their consequence and domination at a specific pH value. The uptake of aqueous arsenite begins from the chemisorption step of (the original species) H₃AsO₃ forming chemisorptive linkage to the primarily amorphous Fe(III) oxide octahedra via bidentate binuclear complexation. The second (later stage) process is the inner-sphere coordination of arsenate (H₂AsO₄⁻), emerging in adsorbate due to the As(III) oxidation, creating bidentate mononuclear complexes to the phases of mainly Fe₂O₃ hydrous oxides. The last (arsenic) removal step is the precipitation of Mn(II) arsenate. The pH dependence of the intensity of each removal process {As(III) chemisorption; As(V) inner-sphere complexation and precipitation of Mn₂(AsO₄)₂} has been revealed. The way how the precipitation of Mn(II) arsenate (the compound containing a metalloid) manifests in the 1 s O XPS and FTIR spectra of the (heavy) metal oxides has been discovered.