Aerobic oxidation of arsenite to arsenate by Cu(ii)-chitosan/O2in Fenton-like reaction, a XANES investigation

Abstract

Oxidation of inorganic molecules (e.g., arsenite (As(iii)) to arsenate (As(v))) by Cu(ii)-chitosan in the presence of dissolved oxygen is examined to elucidate the ability and mechanism of Cu(ii)-chitosan and Cu(ii)-n-TiO2-chitosan participation in Fenton-like reactions. To form the Cu(ii)-chitosan complex, Cu(ii) binds with the amine groups of the chitosan backbone resulting in a Cu(ii)-complex with cationic behavior. Arsenic is then adsorbed to the copper binding site through a combination of Lewis acid-base coordinate bonding and electrostatics. As K-edge XANES indicate that arsenite is fully oxidized to arsenate when adsorbed to a Cu(ii)-chitosan complex in the dark without introduction of a photo-or chemical-oxidant. The oxidation of arsenite by this complex is strongly controlled by the presence of dissolved oxygen as suggested by linear combination fitting where the % As(v) bound decreases from 100% ± 0.0% to 60.2 ± 0.1% upon removal of dissolved oxygen via the freeze pump thaw method. Cu K-edge XANES indicate that Cu(ii) acts as a catalyst rather than a reactant, as it remains present as Cu(ii) after As oxidation in each system condition examined. For Cu(ii)-n-TiO2-chitosan, the amount of As(iii) oxidized to As(v) is strongly controlled by the loading of Cu(ii), with a higher loading of Cu(ii) leading to more As(iii) oxidized and bound on the surface of the adsorbent as As(v). As(iii) removal by both Cu(ii)-chitosan and Cu(ii)-nTiO2-chitosan is significantly improved due to the Fenton-like oxidation of As(iii) to As(v). For Cu(ii)-n-TiO2-chitosan, higher loadings of Cu(ii) relative to n-TiO2 lead to greater improvement of As(iii) removal performance under oxic vs. anoxic conditions.