Role of chemical hardness in the adsorption of hexavalent chromium species onto metal oxide nanoparticles

Abstract

We employed chemical hardness to explain the adsorption behaviour of Cr(VI) anions on metal oxide sorbents. In the absence of electrostatic attraction at pH values around the point of zero charge, silica and titania nanoparticles adsorbed a very small amount of Cr(VI) anions. This lack of adsorption is because the magnitude of the chemical hardness is too high to pair with the less hard Cr(VI) anions. In contrast, maghemite nanoparticles with relatively weak values of hardness adsorbed 0.021mol of Cr(VI) per one mole of OH groups on the particle surface, 2.7 times that of titania. The magnitude of the chemical hardness decreases in the order of SiO2≫TiO2>γ-Fe2O3, which is determined by the hardness of constituting metal cations. This result indicated that the adsorption onto an oxide surface is governed by metal cations in that their chemical hardness determines the basicity of surface OH groups, which regulates adsorption as well as protonation/deprotonation. The hardness match between a sorbent and a sorbate may be interpreted as their chemical affinity. We demonstrated that the chemical affinity is the principal driving force for adsorption of Cr(VI) anions onto the oxides.