A comparative study of Cd, Cr(III), Cr(VI), Hg, and Pb uptake by minerals and soil materials

The pH dependency of Cd, Cr(III), Cr(VI), Hg, and Pb uptake by 14 different types of minerals and soil materials has been studied. The solids were interacted with metal solutions separately in a batch procedure, and the percentage of metal uptake of different metal-solid combinations was compared and evaluated. The results were quantified by the pH values at which 10, 50 and 90% of the metal uptake took place. Physical and chemical characteristics of the solids were correlated with metal uptake. The results verify the importance of geochemical parameters of soils such as organic content, type of clay mineral, presence of complexing ions, and redox-potential for controlling metal uptake. Retention of Cd, Cr(VI), Hg, and Pb was found to be strongly dependent on organic content of the materials studied. Montmorillonite (in bentonite and smectite) showed the highest uptake of Cd, Cr(II) and Pb among all minerals and soil materials, while illite and kaolinite showed lower uptake than the soils. At low pH, the uptake percentage of Cr(VI) by organic soils was higher than that of any of the other metal ions. The uptake of Hg was low in comparison to other cations, which may be explained by formation of soluble Hg(Cl)(2) degrees or Hg(Cl)(4)(2-) complexes.