Prediction of iodate adsorption and surface speciation on oxides by surface complexation modeling

Aqueous iodine species occur mainly as iodide (I−) and iodate (IO3−), depending on redox conditions. The adsorption of IO3− on naturally occurring oxides under oxic conditions is of environmental concern. The adsorption behaviors of IO3− by hydrous ferric oxide (HFO), α-FeOOH, and γ-Al2O3 were examined in this study as functions of pH, ionic strength, and solid concentration. Adsorption data were analyzed using an extended triple-layer model (ETLM) for surface complexation modeling to infer IO3− adsorption reactions and equilibrium constants. Results of ETLM analysis suggest that adsorption of IO3− is both an outer-sphere and an inner-sphere process, as expressed by the following complexation reactions, which are consistent with the independent pressure jump kinetic results and adsorption enthalpy measurements>SOH2++IO3-=>SOH2+_IO3-2>SOH2++IO3-=(>SO)2IO++2H2Owhere >SOH denotes surface hydroxyl. The predicted model speciation of IO3− on these oxides showed that the inner-sphere species increase concomitantly with decreasing pH and increasing ionic strength and solid concentration. The outer-sphere species distribute over a wider range of pH conditions and are more important at lower ionic strengths. Additionally, the outer-sphere species are dominant for γ-Al2O3, whereas the inner-sphere species are dominant for HFO and α-FeOOH. Comparison of the adsorption equilibrium constants for HFO, α-FeOOH, and γ-Al2O3 based on site-occupancy standard states permitted prediction of IO3− adsorption equilibrium constants for many more oxides using the Born solvation theory. Batch adsorption data from previous studies for IO3− on α-Fe2O3 and γ-Al2O3 were reasonably reproduced using ETLM with the predicted equilibrium constants, thereby validating this approach.