Organic cation exchanged montmorillonite and vermiculite as adsorbents for Cr(VI): Effect of layer charge on adsorption properties

The binding of anions at interlayer sites of organo-clay minerals is supposed to be stronger and more specific than on the external surfaces. In this study the Cr(VI) adsorption properties of two hydrous phyllosilicates, a montmorillonite and a vermiculite modified with Hexadecylpyridinium (HDPy), Hexadecyltrimethylammonium (HDTMA) and Benzethonium (BE) were investigated at pH 3–9. Structural features were determined by X-ray diffraction. The surface charge of the organo-clays at different pH was analyzed by polyelectrolyte titration in the cell of a particle charge detector. The effect of competing anions Cl−, NO3− and SO42− on Cr(VI) adsorption was determined.The interlayer space structures of the organo-vermiculites were paraffin-type, whereas it was pseudotrimolecular for the organo-montmorillonites. Besides organic cations also considerable amounts of inorganic cations were present in the interlayer space. As a consequence of organic cation treatment the surface charge of the clays shifted from negative to positive values up to 269 mmolc kg− 1; it was highly affected by the specific surface area, layer charge and pH. Depending on the kind of organic cation, organo-vermiculites and -bentonites showed comparable adsorption capacity for Cr(VI), which was at pH 4.5 0.38 and 0.30 mol kg− 1 for HDPy- and HDTMA-vermiculites, respectively and 0.37 and 0.26 mol kg− 1 for the corresponding bentonites. For all organo-clays the adsorbed amount of Cr(VI) decreased with increasing pH. The effect of competing anions, Cl−, NO3− and SO42− on Cr(VI) adsorption appeared to be small. For organo-vermiculites a higher portion of anion adsorption sites was located in the interlayer space. In context with the high share of adsorption sites in the interlayer space it is suggested that both organo-vermiculites and organo-bentonites are promising adsorbents of Cr(VI) in water purification methods.

This is the first study on Cr(VI) adsorption in the interlayer space of organo-clays, which considers layer charge
► Cr(VI) adsorption in interlayer of organo-clays considering layer charge.
► Anion adsorption sites in interlayer of vermiculite with more specific binding.
► Effect of competing anions Cl–, NO3– and SO42– on Cr(VI) adsorption is small.
► Cr(VI) adsorption sites in the interlayer are more protected against degradation.
► Structure and diffusion ways for anions in the interlayer need further attention.