Surface hydroxylation of SBA-15 via alkaline for efficient amidoxime-functionalization and enhanced uranium adsorption

A surface hydroxylation process with an alkali solution of low concentration was employed to recover surface silanol groups of SBA-15 prior to amidoxime (AO)-functionalization. The effect of concentration and temperature on surface hydroxylation were investigated, and the optimal conditions of 0.003 mol L-1 and 65 degrees C for the surface hydroxylation were determined. The silanol density was increased from 1.20 mmol g(-1) of the untreated SBA-15 to 1.45 mmol g(-1) of the hydroxylated SBA-15. Characterizations through scanning electron microscopy, transmission electron microscopy, X-ray diffraction, N-2 adsorption-desorption proved that the hydroxylation process did not destroy mesopore structure and morphology of SBA-15. The hydroxylated SBA-15 could anchor more AO groups than the untreated SBA-15, resulting in higher U(VI) adsorption capacity (709 mg g(-1)) of the pristine AO-functionalized SBA-15 following hydroxylation than that of untreated sample (601 mg g(-1)). The adsorption capacity of the regenerated samples slightly decreased after three sorption-desorption cycles. Compared with other coexisting cations (Na+, K+, Mg2+, and Ca2+) in artificial seawater with rich uranium, the distribution coefficient (K-d) and concentration factor (CF) value of UO22+ were far larger, suggesting high selectivity of AO-H-SBA-15 for UO22+. These results indicate that the surface hydroxylation process can enhance U(VI) adsorption maintaining the high adsorption selectivity and long-term adsorption stability.