Changes in kaolinite, vermiculite, and smectite clays after adsorption by hydroxy-chromium species

Abstract

Kaolinite, vermiculite, and montmorillonites were treated with solutions containing hydroxy-chromium (OH-Cr) species. The OH-Cr solution was prepared by adding 0·2 М NaOH to 0·1 М chromium nitrate solution and allowing the solution to stand at 60°C for 1 day. The samples were characterised by chemical analyses, N₂ adsorption-desorption isotherms, and X-ray diffraction. The textural and structural behaviours of kaolinite, vermiculite, and the montmorillonites were analysed in the original samples and after treatment with the polymeric OH-Cr species. The montmorillonites showed higher retention of chromium (19·20%), higher basal spacing (2·06 nm), and higher micropore surface area (276 m²/g) than the vermiculite (3·70%, 1·49 nm, 13 m²/g) and kaolinite (1·15%, 0·73 nm, ~1 m²/g) clays after treatment with the OH-Cr species. In contrast, the external surface area increased from 6 to 9 m²/g for kaolinite and from 18 to 24 m²/g for vermiculite, and decreased from 7 to 4 m²/g for montmorillonite after treatment with the OH-Cr solution. The residual chromium, basal spacing, and texture of the clays after treatment with the OH-Cr species were primarily related to the magnitude of the negative charge originating from the octahedral sheet.