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Zirconium-loaded mesostructured silica nanoparticles (Zr/MSN) were developed for the removal of hexavalent chromium (Cr(VI)) from aqueous solutions. X-ray diffraction, transmission electron microscopy, and Brunauer− Emmett−Teller results revealed that both MSN and Zr/MSN showed a characteristic highly ordered hexagonal pore structure with a high specific area of 821−1219 m 2 g −1 . Fourier transform infrared analysis showed that the presence of Zr diminished the absorbance band that was assigned to silanol groups of the structural defect sites, while XPS analysis of the binding energy of Si−O−Zr indicated interactions between the silanol groups and zirconium. The interaction of zirconium and silanol groups from the structural defect sites generated bidentate zirconium, which acted as an active site for the adsorption. The experimental results showed that the modification of MSN with zirconium significantly enhanced the adsorption capacity for Cr(VI). The equilibrium isotherm data showed that the adsorption process was best described by the Langmuir isotherm with 104 mg g −1 maximum adsorption capacity, while the kinetics of Cr(VI) adsorption followed a pseudo-second-order kinetic model. The thermodynamic properties confirmed that the adsorption of Cr(VI) onto Zr/MSN was spontaneous and endothermic in nature, with an activation energy of 24 kJ mol −1 showing that the adsorption was a chemisorption process.

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