ARSENITE AND ARSENATE ADSORPTION ON FERRIHYDRITE - KINETICS, EQUILIBRIUM, AND ADSORPTION ENVELOPES

Because of its toxicity, arsenic is of considerable environmental conc ern. Its solubility in natural systems is strongly influenced by adsor ption at iron oxide surfaces. The objective of this study was to compa re the adsorption behavior of arsenite and arsenate on ferrihydrite, u nder carefully controlled conditions, with regard to adsorption kineti cs, adsorption isotherms, and the influence of pH on adsorption. The a dsorption reactions were relatively fast, with the reactions almost co mpleted within the first few hours. At relatively high As concentratio ns, arsenite reacted faster than arsenate with the ferrihydrite, i.e., equilibrium was achieved sooner, but arsenate adsorption was faster a t low As concentrations and low pH. Adsorption maxima of approximately 0.60 (0.58) and 0.25 (0.16) mol(As), mol(Fe)(-1) were achieved for ar senite 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 greater than or equal to 0.27 mol(As) 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 mol(As), kg(-1) ferrihydrite, t he adsorption envelopes crossed at approximately pH 6-7.5, i.e., adsor bed arsenate concentrations were relatively greater than adsorbed arse nite concentrations at lower pH values whereas adsorbed arsenite was g reater at higher pH. At the highest initial arsenic concentration of 1 3.3 mol(As) kg(-1) ferrihydrite, a distinct adsorption maximum was obs erved for arsenite adsorption at approximately pH 9.0, which correspon ds closely to the first pK(a) (9.2) of H3AsO30, whereas there was a co ntinuous drop in arsenate adsorption with increasing pH from 3 to 11.