Distinct Roles of Illite Colloid and Humic Acid in Mediating Arsenate Transport in Water-Saturated Sand Columns

Arsenate is a common groundwater contaminant that poses serious threat to human health. Although arsenate is mainly transported in dissolved form in groundwater under certain conditions, some studies found a large fraction of arsenate is associated with mineral and organic colloids in natural water, indicating co-transport of arsenate, and particulate matters may significantly alter arsenate migration. Silicate minerals are known to adsorb arsenate, and silicate-mineral colloids are the most abundant form of mineral colloids in many soil water and groundwater. However, the role of silicate-mineral colloids in mediating arsenate transport is not clear, especially when silicate-mineral colloids co-exist with natural organic matters. The objective of this study is to determine and compare the roles of silicate-mineral colloid and natural organic matters in arsenate transport and examine how natural organic matters influence mineral-colloid-facilitated arsenate transport. Arsenate-spiked solution with or without illite colloids and/or humic acid was injected into water-saturated sand columns, and effluent arsenate concentrations were measured. Our results showed that the roles of illite colloids and humic acid in arsenate transport were very different. While illite colloids did not influence arsenate transport due to their low affinity for arsenate under typical groundwater conditions, humic acid substantially increased arsenate transport via competition against arsenate for adsorptive sites on the transport medium and potential formation of non-adsorbing aqueous phase arsenate-humic acid complexes. Although illite colloids were highly mobile in the presence of humic acid, arsenate adsorption to illite colloids was low and transport of illite-associated arsenate was insignificant. Since the affinity of illite for arsenate is intermediate among different clay minerals, our results imply that clay-mineral colloids in general may not have a large potential to increase As transport, and that natural organic matters are the most important type of natural colloids that may influence arsenate transport in groundwater.