Distribution of hexavalent Cr species across the clay mineral surface–water interface

The adsorption isotherms of Cr(VI) on kaolinite, montmorillonite, and alumina were adequately treated with Langmuir model showing behavior characteristic of single-layer adsorption. The efficiency of the adsorbents in removing Cr(VI) from water follows the order alumina > kaolinite > montmorillonite > silica. Speciation studies indicate that hydrogen chromate ions were the major adsorbed species and simultaneous adsorption of dichromate ion occurred at concentrations greater than approximately 10−3 molL−1. It is most probable that the mechanism of adsorption of the hydrogen chromate ion at the surface of alumina is predominantly electrostatic adsorption, with outer sphere complex formation.

Graphical abstractChanges in Gibbs free energy of Cr(VI) species as a function of pH and the fraction of alumina on the surface, show that hexavalent Cr species interacting with claylike minerals containing a higher fraction of alumina and/or at lower pH are thermodynamically driven toward the adsorbed phase.Figure optionsDownload full-size imageDownload as PowerPoint slide