The adsorption of arsenic onto hydrous ferric oxide (HFO) was examined for both As(III) and As(V). The effects of adsorbate/adsorbent ratios and of the presence of sulfate and calcium as co-occurring solutes were examined. The observed results were compared with the results of calculations obtained using the generalized two-layer model (a surface complexation model in which the coulombic term is fixed by double layer theory) to elucidate the factors affecting arsenic adsorption. The extent of arsenic adsorption onto HFO observed over the pH range 4-9 was comparable to that observed in previous studies when similar total arsenic/total iron conditions were examined. These results could be quantitatively described by the model. At decreasing adsorbate/adsorbent ratios, however, observed adsorption of As(III) was underpredicted by the model. This effect of total As(III) concentration (at fixed total iron concentration) could be accounted for by introducing surface heterogeneity, as is common in modeling transition metal cation adsorption. Decreased adsorption of both As(III) and As(V) were observed in the presence of sulfate. The effect of sulfate was greatest at lower pH. These trends were qualitatively consistent with model predictions (excluding site heterogeneity) but the magnitude of the competitive effect was less than predicted by the model, particularly for As(V). A cooperative effect of calcium on adsorption of As(V) at high pH was observed. This enhancement of As(V) adsorption could be attributed to favorable electrostatic effects arising from adsorption of calcium. However, quantitative agreement between the observed results and model predictions under these conditions was poor. Such considerations are crucial in modeling the adsorption behavior of arsenic and other inorganic contaminants, which occur, in natural and engineered aquatic systems, as trace constituents of complex mixtures of inorganic and organic solutes. | An investigation was carried out to examine the adsorption of arsenic onto hydrous ferric oxide (HFO) for both As(III) and As(V). The effects of adsorbate/adsorbent ratios and of the presence of sulfate and calcium as co-occurring solutes were determined. By comparing the observed results with the results of calculations obtained using generalized two-layer model, the factors affecting arsenic adsorption were elucidated.