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 (H2PO4−, SO42−, 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.