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 (NH2-ASNs) and MSNs (NH2-MSNs) and their removal from aqueous environments and show the specific surface area (SSA) of NH2-MSNs is four times as larger as that of NH2-ASNs and that more than 70% of the total SSA of NH2-MSNs is due to the presence of nanopores. Analyses of Cr(VI) adsorption kinetics on NH2-ASNs and NH2-MSNs exhibited relatively rapid adsorption behavior following pseudo-second order kinetics as determined by nonlinear fitting. NH2-ASNs and NH2-MSNs exhibited significantly higher Cr(VI) adsorption capacities of 34.0 and 42.2 mg·g-1 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 NH2-ASNs and NH2-MSNs. The adsorption of Cr(VI) on NH2-ASNs and NH2-MSNs was an endothermic and spontaneous process according to the thermodynamic analyses of temperature-dependent adsorption isotherms. The removal efficiencies of NH2-ASNs and NH2-MSNs exhibited a moderate reduction of less than 25% of the maximum values after five regeneration cycles. Furthermore, NH2-MSNs were also found to reduce adsorbed Cr(VI) into less harmful Cr(III).