U(VI) sorption by silica: Effect of complexing anions

Sorption of U(VI) on silica surface was investigated in NaClO4 medium as a function of pH, ionic strength, and U(VI) concentration by employing batch sorption experiments and surface complexation modeling (SCM). The effect of complexing anions viz., carbonate and citrate on U(VI) sorption was also studied. The U(VI) sorption on silica was found to increase with increasing pH of the suspension and leveling off at pH ∼4.5 for [U(VI)] = 4.47 × 10−07–4.47 × 10−05 M. With further increase in U(VI) concentration, the sorption edge shifted to higher pH. The varying ionic strength has negligible effect on U(VI) sorption. In presence of citric acid (1 × 10−03 M) U(VI) sorption on silica decreased significantly in the pH range 2–6, while carbonate ions (1 × 10−03 M) have negligible effect on U(VI) sorption at least up to pH 8. Surface complexation modeling was satisfactorily able to explain the sorption phenomena in all the systems. Furthermore, the modeling approach was evaluated by fitting the literature data on U(VI) sorption onto silica.

Graphical abstractSorption of U(VI) by silica is significantly affected by the presence of citrate in the pH range of 3–6, while carbonate anion does not influence the U(VI) sorption by silica at least up to pH 8. Surface Complexation Modeling showed the evolution of surface complexes (SiO)2UO2 and (SiO)2UO2CO32− in presence of carbonate and as (SO)2UO2Cit3− in presence of citrate.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The shift in sorption edge to higher pH is observed with increasing U(VI) concentration. ► Dissolved silica dominates the speciation at lower U(VI) concentration. ► The varying ionic strength and carbonate has negligible effect on U(VI) sorption. ► Citrate decreases U(VI) sorption on silica in the pH range 2–6. ► U(VI) forms bidentate surface complexes at silica in presence of CO32− and citrate.