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

Abstract

Aqueous iodine species occur mainly as iodide (I −) and iodate (IO 3 −), depending on redox conditions. The adsorption of IO 3 − on naturally occurring oxides under oxic conditions is of environmental concern. The adsorption behaviors of IO 3 − by hydrous ferric oxide (HFO), α-FeOOH, and γ-Al 2O 3 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 IO 3 − adsorption reactions and equilibrium constants. Results of ETLM analysis suggest that adsorption of IO 3 − 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 > SOH 2 + + IO 3 - = > SOH 2 + _ IO 3 - 2 > SOH 2 + + IO 3 - = ( > SO ) 2 IO + + 2 H 2 O where >SOH denotes surface hydroxyl. The predicted model speciation of IO 3 − 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-Al 2O 3, whereas the inner-sphere species are dominant for HFO and α-FeOOH. Comparison of the adsorption equilibrium constants for HFO, α-FeOOH, and γ-Al 2O 3 based on site-occupancy standard states permitted prediction of IO 3 − adsorption equilibrium constants for many more oxides using the Born solvation theory. Batch adsorption data from previous studies for IO 3 − on α-Fe 2O 3 and γ-Al 2O 3 were reasonably reproduced using ETLM with the predicted equilibrium constants, thereby validating this approach.