EFFECTS OF COMPETITIVE ANIONS AND NOM ON ARSENIC DESORPTION FROM GLACIAL TILL

Factors that influence arsenic (As) desorption from sediments affect As mobility in the subsurface. This research is attempted to compare the effects of competitive anions and natural organic matter (NOM) on As desorption from glacial till samples. Glacial till samples were collected and examined with acid digestion, sequential extraction, and batch desorption experiments to understand the mechanisms that control As desorption. Acid digestion experiments showed As concentration in till samples was 20 mg/kg. Sequential extraction experiments indicated considerable amounts of arsenic (25% of the total As) in till samples are in labile form, suggesting these As could release to pore water under proper conditions. Glacial till samples were treated with different competitive anions and NOM, and the results demonstrate that the As leaching efficiency of competitive anions and NOM decreased in the order of Na₂CO₃ > H₂NaO₄P.H₂O > NaHCO₃ > NOM, where carbonate produced strong pH (pH 10) than that of bicarbonate (pH 8) in solution, and phosphate and NOM concentration in solution were adjusted with pH 8. Carbonate and phosphate replaced As from adsorption sites three to five orders of magnitude more than bicarbonate and NOM. Sodium carbonate, phosphate, bicarbonate ions and NOM extracted arsenic most efficiently; Na₂CO₃ leached up to 119 ppb of As from glacial till after 16 days of incubation. Kinetic studies of arsenic release indicate that arsenic concentration increased with leaching time in carbonate, bicarbonate and NOM experiments, where phosphate showed decreasing trend after 4 days. In batch leaching experiments, with the increase of competitive anion and NOM concentrations in solution, leaching of arsenic was also increased. The competition between competitive anions (carbonate, phosphate and bicarbonate) and NOM, and As oxyanions on adsorption sites was presumably due to the substitution of adsorbed arsenic on mineral surface by these anions and NOM, which resulted in release of As to pore water. Our results showed competitive anions may play a key role for As mobilization in subsurface environments.