Arsenate Adsorption by Unsaturated Alluvial Sediments

Arsenic can occur naturally in excess of drinking water standards. Earlier a field-scale recharge pond experiment in the Antelope Valley ground water basin, Mohave Desert, California, was performed to determine the effectiveness of naturally occurring minerals in the unsaturated zone to treat high As water. The extent to which the remediation can be continued on any site is unknown and depends on the As adsorption properties and capacity of the sediments. Therefore, arsenate adsorption reactions as a function of solution As concentration and solution pH were investigated on five alluvial sediments obtained from various depths of a borehole adjacent to the recharge pond. Arsenate adsorption increased with increasing solution pH, exhibited a maximum around pH 4 to 5, and then decreased with increasing pH. The constant capacitance model was able to fit arsenate adsorption on the sediments as a function of solution As concentration and solution pH. A general regression model was used to predict arsenate surface complexation constants from routinely measured chemical parameters. The predicted arsenate constants were used to predict adsorption on the sediments, thereby providing a completely independent evaluation of the ability of the model to describe arsenate adsorption. The prediction equations were able to satisfactorily predict arsenate adsorption on one of the five sediments whose chemical properties fell into the range for the set of soils used to develop the prediction equations. Equimolar phosphate concentrations did not affect the extent of arsenate adsorption indicating that the extent of arsenate remediation by the sediments will be unaffected by the low amounts of native phosphate.