Effects of dissolved carbonate on arsenic adsorption and mobility

The effects of high aqueous carbonate concentrations on arsenic mobility and transport in the subsurface were studied in synthetic iron oxide-coated sand column experiments. Elevated aqueous carbonate concentrations in groundwater have been studied and linked, by some authors, to increased aqueous As concentrations in natural waters. This study found that increasing carbonate concentrations had relatively little effect on As(V) adsorption to the iron oxide-coated sand surface at pH 7. The adsorption of As(V) decreased marginally when the CO2(g) partial pressure increased from 10(-3.5) to 10(-1.8) atm, despite a 50-fold increase in total dissolved carbonate (0.072 to 3.58 mM). Increasing the CO2(g) partial pressure to 10(-10) atm resulted in only a slight decrease in As(V) adsorption and increase in mobility, despite a >300-fold increase in total dissolved carbonate (to 22.7 mM). When compared to phosphate, a known competitive anion, carbonate mobilized less adsorbed As(V) than was mobilized by phosphate, even when present in much higher concentrations than phosphate. This was also true for an experiment with lower pore water velocity and an experiment where As(II) was introduced instead of As(V). Our experiments conclude that while carbonate anions do compete with As for adsorption to iron oxide-coated sand, the competitive effect is relatively small with regard to the total concentration of adsorbed As and the potential competitive effects of phosphate.