Mobility of arsenic in aquifer sediments at Datong Basin, northern China: Effect of bicarbonate and phosphate

Abstract

Effects of phosphate and bicarbonate concentration on mobilization of arsenic in aquifer sediments have not been extensively studied in flow-through experiments, where the dynamic-release behavior of elements can be monitored. In this work, these effects were investigated both through batch and column experiments, using a high-arsenic sediment sample from the Datong Basin, northern China. A solution of 1.0mM Ca(NO3)2 was first applied to a water-saturated column packed with the sediment; this was followed by leaching with either 0.10mM Na2HPO4 or 8.2mM NaHCO3 solutions. When the sediment was initially leached with 1.0mM Ca(NO3)2 solution, a rapid spike of As, Fe, Mn, and major cations was observed in the first 20 column pore volumes, reflecting the initial equilibration of the sediment to the influent Ca(NO3)2 in response to cation exchange (for cations) and sorption equilibrium (for Mn, Fe, As).After the Ca(NO3)2 solution was replaced with either the Na2HPO4 or NaHCO3 solution, a rapid increase in As concentration in the effluent was observed, consistent with the batch test results. Induced desorption of As from oxyhydroxide sorbents by Na2HPO4 suggests that phosphate competition was the major factor responsible for the observed increase of As, as indicated by a coinciding increase of K+ replaced in the exchange process. In the column leached with 8.2mM NaHCO3, high concentrations of sodium result in greater clay dispersion, and the transport of Fe/Mn oxyhydroxides with adsorbed arsenic may account for the enhanced arsenic concentration in the effluent, as suggested by the concurrently observed increase in Fe and Mn concentrations in the effluent.