Extended X-ray Absorption Fine Structure Revealed the Mechanism of Arsenate Removal by the Fe/Mn Oxide-Based Composite under Conditions of Fully Saturated Sorption Sites

Abstract

Molecular mechanism of arsenate removal by a promising inorganic composite based on Fe/Mn oxides and MnCO 3 was studied under the rarely investigated conditions of fully saturated sorption sites (characteristic of dynamic sorption, such as water treatment plants) at the pH of 4/6/7/8 using As Kedge extended X-ray absorption fine structure (EXAFS)/X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). Comparison of arsenic speciation in the initial adsorbate solution (calculated by Visual MINTEQ) and after sorption (determined by As 3d XPS) allowed the interpretation of the initializing forces of the interfacial processes. Contribution of various solid phases of this composite anion exchanger to the removal of arsenate was disclosed by examining the Fe 2p 3/2 and Mn 2p 3/2 XPS spectra supported by FTIR. As K-edge EXAFS simulation not only proved the chemisorptive binding of aqueous As(V) anions to the Fe/Mn oxide-based adsorbent but also demonstrated the presence of a variety of sorption sites in this complex structured porous material, which became available step-wise upon an increasing pressure on the interface with high arsenate loading during the long-term sorption process. The type of innersphere complexation of As(V) on the saturated surface discovered by As K-edge EXAFS modeling was a function of pH. Analysis of EXAFS fitting data resulted in suggestion of a methodological idea on how the EXAFS-derived coordination numbers can be used to distinguish the localization of adsorbed ions (surface versus structure emptiness). This work also provides more insights into the superiority of composite adsorbents (compared to the materials based on individual compounds) in terms of their capability to adapt/change the molecular sorption mechanism in order to inactivate (remove) more toxic aqueous anions. KEYWORDS: sorption, arsenate (H 2 AsO 4 − ), inorganic anion exchanger, composite based on Fe/Mn oxides and MnCO 3 , extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), pH effect