RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

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

>SO₂⁺ + IO₃⁻ → >SO₂⁺–IO₃⁻
2 >SO₂⁺ + IO₃⁻ → (>SO)₂IO⁺ + 2H₂O

where >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.