Silica magnetite nanoparticles (S-MNPs) as core were embedded in mesoporous silica shells by using cetyltrimethylammonium bromide (CTAB) as a surfactant. Then, the resultant mesoporous silicamagnetite nanoparticles (M-S-MNPs) were modified with 3-aminopropyltriethoxysilane (APTES) as a coupling agent in dry hexane solvent. APTES-grafted mesoporous silica magnetite nanoparticles (A-M-SMNPs) were characterized by XRD, FTIR spectroscopy, EDX, TEM, elemental analysis, TGA/DTA technique. Results demonstrate that the obtained A-M-S-MNPs were nearly spherical in shape with 25 nm thick mesoporous silica shell. The adsorption behavior of the nanocomposite was examined in removing of Cr(VI) ion with concentrations 20, 30 and 50 mg/L at optimum pH level of 2. In this study a pH swing adsorption was observed too. The adsorption kinetic data were modeled using pseudo-second-order kinetics and intraparticle diffusion equations. The obtained results for intraparticle diffusion model show that the adsorption mechanisms are different in low and in high concentrations of Cr(VI) ion. According to the parameters of the Langmuir isotherm, the maximum adsorption capacity (qm) of A-M-SMNPs for Cr(VI) increases as the temperature rises from 298 to 318 K. For better understanding of adsorption mechanism, quantum mechanical methods were applied. The results indicate that the electrostatic and hydrogen bond interactions between surface functional groups and HCrO
4 ions have an important role in adsorption process. The easy separation from aqueous solution by an external magnetic field, rapid adsorption, regeneration, and reusability of A-M-S-MNPs are interesting points as an effective adsorbent for the removal of Cr(VI) ions.

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