Synthesis of magnetic silica with quaternary ammonium salt and its application for chromium(VI) removal

The contamination of water bodies with heavy metals (e.g. chromium and arsenic) is a major problem. Thus, the demand for effective ways of removing these toxic ions is increasing. In this study, a novel quaternary ammonium salt (QAS) was designed and synthesised in situ on the surface of silica-encapsulated iron oxide nanoparticles (SEION) to be easily separated by an external magnet. The SEION grafted with QAS (QAS-SEION) were characterised by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and zeta potential. Adsorption experiments were conducted to evaluate the capacity of QAS-SEION for removing Cr(VI) from water. The results indicated that the adsorption of Cr(VI) on the QAS-SEION reached equilibrium within 120 min. A pseudo-second-order model was befitting to describe the kinetics of Cr(VI) adsorption. The adsorption isotherm of Cr(VI) on the QAS-SEION was observed to fit both the Langmuir isotherm equation and Freundlich isotherm equation notably well. The maximum adsorption amount calculated by the Langmuir isotherm equation was close to that obtained by experiment. Overall, QAS-SEION is a promising material for the removal of Cr(VI) from contaminated water.