Key role of pore size in Cr(VI) removal by the composites of 3-dimentional mesoporous silica nanospheres wrapped with polyaniline.

A series of three-dimensional silica nanospheres with different pore sizes was synthesized in a biphasic oil-water system and their pore dimensions were adjusted by controlling the composition of the oil phase. The silica nanospheres were then wrapped with polyaniline, characterized, and the obtained silica nanosphere-polyaniline composites were used for the removal of Cr(VI). Polyaniline was generated by the polymerization of aniline. The mesoporous silica has sufficient dendritic pore channels and offers a large contact surface for the polymerization of aniline. Furthermore, the mesoporous silica nanospheres are beneficial for dispersing polyaniline and transferring aqueous Cr(VI). The silica nanosphere-polyaniline composite with the largest pore size (~15.4 nm) showed the best Cr(VI) removal performance. We also investigated the kinetic characteristics and the result could be fitted to the pseudo-second-order kinetic model. Moreover, we demonstrate that the composites maintain a high Cr(VI) removal efficiency compared to other anions (HPO, SO, etc.), indicating their good prospect in practical wastewater treatment. Remarkably, the silica-polyaniline composites showed enhanced Cr(VI) removal efficiency under UV-irradiation. The effects of electrons and H on Cr(VI) reduction are also discussed based on the results of UV-vis and X-ray photoelectron spectroscopic studies and bath experiments (influence of pH on adsorption capacity). Mechanistic studies indicate that the Cr(VI) removal occurs in two stages-adsorption and reduction. The negatively charged aqueous Cr(VI) species first interact with the positively charged protonated amine groups via electrostatic attraction, and are then further reduced to less-toxic Cr(III) by the electrons and H donated by the amine groups on polyaniline.