Controlled bi-functionalization of silica microbeads through grafting of amidoxime/methacrylic acid for Sr(II) enhanced sorption

Abstract

Silica microspheres are functionalized through the grafting of either polymethacrylic acid or amidoxime groups (mono-functional composite, PMAA/SiO2 and PAO/SiO2, respectively) or in combination (bi-functional composite, PAOxMAA100-x/SiO2). The materials are characterized by textural analysis, TGA, SEM-EDX, elemental analysis, FTIR, XPS, pHPZC in order to confirm the synthesis route, identify the reactive groups and support the interpretation of sorption mechanisms toward Sr(II). The sorption of Sr(II), at optimum pH 8, involves amine groups and hydroxyl/carboxyl groups in the bi-functional composite sorbent. The comparison of sorption properties shows that maximum sorption requires a majority of carboxylic groups from polyacrylic compared with amidoxime functions; optimum formulation: PAO20MAA80/SiO2 sorbent. Sorption isotherms are efficiently fitted by the Langmuir and Sips equations: the maximum sorption capacity reaches around 1.38 mmol Sr g−1. Kinetic profiles for sorption are modeled using the pseudo-first order rate equation; equilibrium is achieved within 40–60 min. Complete desorption of Sr(II) using 1 M HCl solutions is even faster; equilibrium time being 15–20 min. The sorbent can be recycled for 5 cycles with a limited decrease in the sorption performance, and full desorption. In fixed-bed column, the breakthrough curves are fitted by the Thomas equation. In multi-component solutions, the sorbent shows a preferred selectivity according the sequence: K(I) > Ba(II) > Na(I) > Ca(II). Tested on seawater, the sorbent shows high distribution ratio for Sr(II), B(III) and Mo(VI)