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

摘要

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>SOH2++IO3-=>SOH2+_IO3-2>SOH2++IO3-=(>SO)2IO++2H2Owhere >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.