Arsenite and Arsenate Adsorption on Ferrihydrite:  Kinetics, Equilibrium, and Adsorption Envelopes

Because of its toxicity, arsenic is of considerable environmental concern. Its solubility in natural systems is strongly influenced by adsorption at iron oxide surfaces. The objective of this study was to compare the adsorption behavior of arsenite and arsenate on ferrihydrite, under carefully controlled conditions, with regard to adsorption kinetics, adsorption isotherms, and the influence of pH on adsorption. The adsorption reactions were relatively fast, with the reactions almost completed within the first few hours. At relatively high As concentrations, arsenite reacted faster than arsenate with the ferrihydrite, i.e., equilibrium was achieved sooner, but arsenate adsorption was faster at low As concentrations and low pH. Adsorp tion maxima of approximately 0.60 (0.58) and 0.25 (0.16) molAs molFe-1 were achieved for arsenite and arsenate, respectively, at pH 4.6 (pH 9.2 in parentheses). The high arsenite retention, which precludes its retention entirely as surface adsorbed species, indicates the likelihood that ferrihydrite was transformed to a ferric arsenite phase, although this possibility has yet to be confirmed by spectroscopic studies. The general trend at initial arsenic concentrations ≥0.27 molAs kg-1 ferrihydrite within the pH range of 4−9 was increasing arsenite adsorption and decreasing arsenate adsorption with increasing pH. At initial As concentrations of 0.27−0.80 molAs kg-1 ferrihydrite, the adsorption envelopes crossed at approximately pH 6−7.5, i.e., adsorbed arsenate concentrations were relatively greater than adsorbed arsenite concentrations at lower pH values whereas adsorbed arsenite was greater at higher pH. At the highest initial arsenic concentration of 13.3 molAs kg-1 ferrihydrite, a distinct adsorption maximum was observed for arsenite adsorption at approximately pH 9.0, which corresponds closely to the first pKa (9.2) of H3AsO30, whereas there was a continuous drop in arsenate adsorption with increasing pH from 3 to 11.