Arsenite and arsenate binding to dissolved humic acids: Influence of pH, type of humic acid, and aluminum

The fate of arsenic in the aquatic environment is influenced by dissolved natural organic matter (DOM). Using an equilibrium dialysis method, conditional distribution coefficients (D-om) for As(III) and As(V) binding onto two commercial humic acids were determined at environmentally relevant As/dissolved organic carbon (DOC) ratios and as a function of pH. At all pH values, As(V) was more strongly bound than As(III). Maximum binding was observed around pH 7, which is consistent with H+ competition for binding sites at low pH values and OH- competition for the arsenic center at high pH. For both oxidation states, D-om values increased with decreasing As/DOC ratios. D-om values were fitted as a function of the As/DOC ratio for As(III) and As(V). Compared to the aquatic humic acid, the terrestrial humic acid had a higher affinity for arsenic binding with 1.5-3 times higher D-om values under the same conditions. Al3+ in excess to arsenic successfully competed for strong binding sites at low As/DOC ratios. Under environmentally relevant conditions, about 10% of total As(V) may be bound to DOM, whereas > 10% of As(III) is bound to DOM at very low As/DOC ratios only. Binding of arsenic to DOM should be considered in natural systems.