Sorption of Fe(II) and As(III) on goethite in single- and dual-sorbate systems
Experiments were conducted to quantify Fe(II) sorption onto goethite in the absence and presence of As(III). The experimental data obtained in single-sorbate experiments were modeled using a diffuse double layer surface complexation model and used to predict and compare sorption in dual-sorbate systems. The sorption process was shown to be reversible by the complete recovery of sorbed Fe(II) upon extraction with 0.5 N HCl. Sorption of Fe(II) increases with increasing pH, as observed previously for various iron oxides. Sorption isotherms obtained between pH 6.0 and 7.5 showed continuous increase in sorption density with increase in dissolved Fe(II) concentration; under these conditions, surface saturation was approached but not reached. Experiments conducted in the absence and presence of 500 and 1000 μM total As(III) did not show any significant difference in the Fe(II) sorption density. As(III) sorption density did not change with increasing sorbed Fe(II) concentration when the total arsenic concentration was 500 μM. However, when the total As(III) concentration was 1000 μM, As(III) sorption densities increased almost linearly with increasing sorbed Fe(II) concentrations. The model provided a good-to-adequate description of Fe(II) and As(III) sorption in single-sorbate systems over a range of experimental conditions but failed to predict the experimental observations in dual-sorbate systems. The predicted sorption densities for both As(III) and Fe(II) were lower than those observed. These discrepancies illustrate problems that may arise when model parameters obtained in single-sorbate systems are used to predict sorption in multi-sorbate systems where all sorbates are presumed to compete for the same sites. The lack of competition observed between As(III) and Fe (II) for sorption sites indicate that the concurrent release of Fe(II) and As(III) during reductive dissolution of iron oxides, inferred as the mechanism of arsenic mobilization in many reducing ground waters, may have relatively minor effects on the subsequent resorption of As(III) to residual iron oxides remaining in the sediment.