The current study deals with Cr(VI) removal using unmodified alumina nanoparticles of two different sizes (NA1: mean hydrodynamic diameter = 75.3 +/- 2.8 nm; NA2: mean hydrodynamic diameter = 229.8 +/- 3.3 nm). The equilibrium,adsorption capacities, 73.2 and 59.4 mg of Cr(VI)/g of adsorbent, were noted for NA1 and NA2, respectively, under optimized conditions (pH 7.0, temperature = 27 degrees C, initial Cr(VI) concentration = 20 mg L-1, adsorbent dosage = 0.1g L-1) but different contact times, (120 min for NA1, 180 min for NA2): For both sorbents, the equilibrium adsorption data fitted well with the Langmuir isotherm model. The adsorbent NA1 followed a pseudo-second-order kinetics, whereas NA2 followed pseudo-first-order kinetics. Surface characterization studies (zeta potential measurement, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared (FTIR) spectroscopy) substantiated oxyanionic binding on the sorbent surface. The EPR and XRD spectroscopy Confirmed the existence of reduced Cr(III) on the adsorbent surface. The applicability of the sorbent in Cr(VI)-contaminated water was studied.