Arsenic (III) and arsenic (V) adsorption on three California soils

Inorganic arsenite (As(III)) and arsenate (As(V)) are toxic pollutants that are transported in surface and ground waters and can adsorb on soil and sediment mineral surfaces. Because of the importance of adsorption reactions in determining the overall mobility of arsenic, we investigated the adsorption of As(III) and As(V) on three soils at varying As concentration, pH, and ionic strength. The objectives were to characterize and compare As(III) and As(V) adsorption on three arid-zone soils from California (Wasco, Fallbrook, and Wyo) and to determine the relationship between soil properties and adsorption. Chromatographic speciation of As(III)/(V) revealed that the three soils contained low levels of background As(V). Oxidation of added As(III) to As(V) was not detected below pH 8 in soil suspensions during 16-h adsorption experiments; however, As(III) oxidation was detected at higher adjusted pH. The soil with the highest citrate-dithionite extractable Fe and %clay (Wyo) had the highest affinity for As(III) and As(V) and displayed adsorption behavior similar to pure ferric oxide. Adsorption isotherms indicated that As(V) species adsorbed more strongly than As(III) under most conditions. However, a pH-dependent reversal in the relative affinity of As(III) and As(V) for the soils was observed at low As surface coverage.