Adsorption of Selenium and Strontium on Goethite: EXAFS Study and Surface Complexation Modeling of the Ternary Systems

Knowledge of the geochemical behavior of selenium and strontium is critical for the safe disposal of radioactive wastes. Goethite, as one of the most thermodynamically stable and commonly occurring natural iron oxyhydroxides, promisingly retains these elements. This work comprehensively studies the adsorption of Se(IV) and Sr(II) on goethite. Starting from electrokinetic measurements, the binary and ternary adsorption systems are investigated and systematically compared via batch experiments, EXAFS analysis, and CD-MUSIC modeling. Se(IV), forms bidentate, inner-sphere surface complexes, while Sr(II) is assumed to form, outer-sphere complexes at low and intermediate. pH and inner-sphere complexes at high pH. Instead of a direct interaction between Se(IV) and Sr(II),our results indicate an electrostatically,driven mutual enhancement of adsorption. Adsorption of Sr(II) is promoted by an average factor of 5 within the typical groundwater pH range from 6 to 8 for the concentration range studied here. However, the interaction between Se(IV) and Sr(II) at the surface is two-sided, Se(IV) promotes Sr(II) Outer-sphere adsorption, but competes for inner-sphere adsorption sites at high pH. The complexity.of surfaces is highlighted by the inability of adsorption models to predict isoelectric points without additional constraints.