Arsenic speciation and sorption kinetics in the As–hematite–humic acid system

This study characterized arsenic speciation, and adsorption/desorption kinetics in the presence of humic acid (Aldrich Chem Co.). Humic acid may exist as a soluble and absorbable form in aquatic and soil environments, and the complexation of arsenic with soluble humic acid formed approximately 40 and 15% of total arsenite and arsenate, respectively. The oxidation of arsenite was the dominant reaction between arsenic and humic acid, though a slight reduction of arsenate was found in the system involving arsenite and humic-coated hematite. Negatively charged functional groups on humic acid can compete with arsenic adsorbed onto hematite, and an excessive concentration of humic acid may produce a higher adsorption affinity for hematite. Thus, some arsenic was released from arsenate-presorbed hematite. Arsenic adsorption onto humic–coated hematite showed sluggish kinetics, and a lower adsorption capacity when compared with bare hematite. Therefore, humic acid can affect arsenic speciation and mobility in geochemical environments.