A systematic study of hexavalent chromium adsorption and removal from aqueous environments using chemically functionalized amorphous and mesoporous silica nanoparticles.

We report on the synthesis and characterization of highly monodisperse amorphous silica nanoparticles (ASNs) and mesoporous silica nanoparticles (MSNs) with particle sizes of 15-60 nm. We demonstrate adsorption of Cr(VI) ions on amino-functionalized ASNs (NH-ASNs) and MSNs (NH-MSNs) and their removal from aqueous environments and show the specific surface area (SSA) of NH-MSNs is four times as larger as that of NH-ASNs and that more than 70% of the total SSA of NH-MSNs is due to the presence of nanopores. Analyses of Cr(VI) adsorption kinetics on NH-ASNs and NH-MSNs exhibited relatively rapid adsorption behavior following pseudo-second order kinetics as determined by nonlinear fitting. NH-ASNs and NH-MSNs exhibited significantly higher Cr(VI) adsorption capacities of 34.0 and 42.2 mg·g and removal efficiencies of 61.9 and 76.8% than those of unfunctionalized ASNs and MSNs, respectively. The Langmuir model resulted in best fits to the adsorption isotherms of NH-ASNs and NH-MSNs. The adsorption of Cr(VI) on NH-ASNs and NH-MSNs was an endothermic and spontaneous process according to the thermodynamic analyses of temperature-dependent adsorption isotherms. The removal efficiencies of NH-ASNs and NH-MSNs exhibited a moderate reduction of less than 25% of the maximum values after five regeneration cycles. Furthermore, NH-MSNs were also found to reduce adsorbed Cr(VI) into less harmful Cr(III).