Experiment-based geochemical modeling of Arsenic(V) and Arsenic(III) adsorption onto aquifer sediments from an inland basin

Natural occurrence of groundwater arsenic (As) exists in many aquifers and threatens the health of hundreds of millions of people. Adsorption of As is critical to its distribution in aquifer groundwater, as it can lower dissolved As concentrations and provide a source for As mobilization. However, little is known about As adsorption onto aquifer sediments in inland basins. To fill this gap, we investigated As(V) and As(III) adsorption onto a brown sediment hosting low-As groundwater and two gray sediments hosting high-As groundwater sampled from an approximate flow path of the Hetao Basin, China. Those three sediments had total Fe contents of around 1.3% and As contents of around 9.0 μg/g. Arsenic(III) adsorption were kinetically faster than As(V) adsorption on the sediments. The characteristics of As(V) and As(III) adsorption both exhibited nonlinear isotherms, with adsorption decreasing with increases in pH from around 7 to 9 (typical ambient groundwater pH of inland basins). Generally, As(V) adsorption was greater than As(III) adsorption at pH < 7.5, whereas the opposite was true at pH > 7.5. Phosphate competed more strongly with As(V) adsorption than As(III) adsorption, while HCO₃ competed more strongly with As(III) adsorption. Aqueous As(III) was oxidized to As(V) by interaction with the brown sediment, mainly due to its high content of extractable Mn(IV) oxides. Arsenic adsorption onto the sediments was primarily controlled by the content and crystallinity of their Fe(III) oxides. The brown sediment showed weaker affinity for As(V) and As(III) than the gray sediments, possibly due to the better crystallinity of the Fe(III) oxides in the brown sediment. In the two gray sediments containing Fe(III) oxides with similar crystallinities, the As adsorption capacity was related to the content of Fe(III) oxides. Geochemical models were developed to quantitatively simulate As(V) and As(III) adsorption onto aquifer sediments. Experimental data were approximated well by these models, indicating that As(V) adsorption decreases at elevated pH and PO₄ concentration, while As(III) desorption is favored at high pH and concentrations of HCO₃ and PO₄. According to the models, the extent and kinetics of As(III) oxidation are controlled by the number of free oxidation sites in Mn(IV) oxides, pH, and As(III) concentration. This study suggests that the characteristics of As adsorption are highly dependent on the mineral phases existing in the sediments, which must be fully understood to quantify the reactive transport of groundwater As in aquifer systems.