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

Abstract

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.