Adsorption of cadmium(II) onto goethite and kaolinite in the presence of benzene carboxylic acids

The effect of benzene carboxylic acids on the adsorption of Cd(II) (5×10⁻⁵ M) by goethite and kaolinite has been studied in 0.005 M NaNO₃ at 25°C. The concentrations of phthalic (benzene-1,2-dicarboxylic acid), hemimellitic (1,2,3), trimellitic (1,2,4), trimesic (1,3,5), pyromellitic (1,2,4,5) and mellitic (1,2,3,4,5,6) acids varied from 2.5×10⁻⁵ to 1×10⁻³ M. Mellitic acid complexes Cd(II) strongly above about pH 3, but the other acids only at higher pH, phthalic acid forming the weakest complexes. Phthalic, trimesic and mellitic acids adsorbed strongly to goethite at pH 3, but adsorption decreased at higher pH; however, mellitic acid was still about 50% adsorbed at pH 9, by which the other two were almost entirely in solution. At 10⁻³ M all the acids enhanced the adsorption of Cd(II) to goethite, the higher members of the series being the most effective. The higher members of the series suppressed Cd(II) adsorption onto kaolinite, but phthalic and trimesic acids caused slight enhancement. The effects of mellitic acid on Cd(II) adsorption depended strongly on its concentration. The maximum enhancement of Cd(II) adsorption onto goethite was at 10⁻⁴ M. The greatest suppression of Cd(II) adsorption onto kaolinite was at 10⁻³ M, and at 2.5×10⁻⁵ M mellitic acid enhanced Cd(II) adsorption onto kaolinite at intermediate pH. The results are interpreted in terms of complexation between metal and ligand (acid), metal and substrate, ligand and substrate, and the formation of ternary surface complexes in which the ligand acts as a bridge between the metal and the surface.