Effects of anion competitive adsorption on arsenic enrichment in groundwater.

Long-term intake of high arsenic groundwater has caused serious endemic disease on people in Datong Basin, northern China. The high arsenic groundwater has medium to high alkalinity with a mean pH value of 8.1; the water generally belongs to HCO(3)-Na type water and contains HPO(4)(2-) in most samples. In this study, batch experiments and geochemical modeling were conducted to reveal the geochemical processes responsible for arsenic enrichment in the groundwater due to desorption, and to examine the effect of pH, phosphate, bicarbonate and silicate on this desorption in the studied groundwater system. The experimental pH ranging from 7.35 to 8.72 as observed in the high arsenic groundwater, was favorable for arsenic desorption from the aquifer sediments. Arsenic concentration in the aqueous phase significantly increased with the increase of added PO(4)(3-) concentration. An elevated arsenic content of 13.6 μg/L was observed with the highest phosphate loading of 30 mg/L. Although bicarbonate addition caused less desorption of arsenic than phosphate on the mole basis, an elevated arsenic concentration of 56 μg/L in the batch solutions was observed when the added bicarbonate contents was up to 1500 mg/L. The contribution of silicate to arsenic desorption is lower than that of phosphate and bicarbonate.