Evaluation of a mercapto-functionalized silica binding phase for the selective sampling of SeIV by Diffusive Gradients in Thin films

Abstract

This study evaluates binding discs based on 3-mercaptopropyl-functionalized silica gel for the selective sampling of selenite (SeIV) using Diffusive Gradients in Thin films sampler (DGT). SeIV accumulation was quantitative and selective over SeVI and followed the theoretical linear accumulation with the exposure time up to 0.7 µg. The sampling was not affected by ionic strength variations down to 10−2 mol L−1 (as NaNO3) but SeIV accumulation was found to decrease significantly for pH greater than 5 and was nearly zero at pH 9. Both the limited accumulation range and the pH dependence were unexpected because they have not been reported in the literature related to the SeIV trapping by thiol-based solid phases. Our experiments showed that after SeIV was bound to thiol functional groups, a further pH-dependent reaction occurred with free thiols, resulting in the reduction of SeIV into elemental selenium (Se0) followed by its release and back-diffusion through the DGT sampler. Unfortunately, such a reversible accumulation is incompatible with the implementation of the mercapto-functionalized silica binding phase in DGT devices for SeIV selective sampling.