Adsorption of arsenate (V) and arsenite (III) on goethite in the presence and absence of dissolved organic carbon

The environmental fate of arsenic (As) is of utmost importance as the public and political debate continues with the USEPA's recent proposal to tighten the As drinking water standard from 50 to 10 mug L-1. In natural systems, the presence of dissolved organic C (DOC) may compete with As for adsorption sites on mineral surfaces, hence increasing its potential bioavailability. Accordingly, the adsorption of arsenate [As (V)] and arsenite [As (III)] on goethite (alpha -FeOOH) was investigated in the presence of either a peat humic acid (Hap), a Suwannee River Fulvic Acid (FA) (international Humic Substances Society, St. Paul, MN), or citric acid (CA). Adsorption edges and kinetic experiments were used to examine the effects of equimolar concentrations of organic adsorbates on As adsorption. Adsorption edges were conducted across a pH range of 3 to 11, while the kinetic studies were conducted at pH 6.5 for As (V) and pH 5.0 for As (III). Both Hap and FA decreased As (V) adsorption, while CA had no effect. Humic acid reduced As (V) between pH 6 and 9 by approximate to 27%. Fulvic acid inhibited As (V) adsorption between pH 3 and 8 by a maximum of 17%. Arsenite adsorption was decreased by all three organic acids between pH 3 and 8 in the order of CA > FA approximate to Hap. The different pH regions in which Hap and FA decreased As (V) adsorption suggest that more than one functional group on these complex organic polymers may be responsible for binding to the alpha -FeOOH surface. Similarly, the relative surface affinity of the As(Ill or V) species and that of the competing organic ligand as a function of pH may play a major role in the outcome of As adsorption on alpha -FeOOH. The results of these experiments suggest that DOC substances are capable of increasing the bioavailability of As in soil and water systems in which the dominant solid phase is a crystalline iron oxide.