Simultaneous Adsorption of Tri- and Hexavalent Chromium by Organoclay Mixtures

Abstract

Organoclays possess unique adsorption behaviour towards hydrophobic organic contaminants. They can also remediate ionic contaminants such as heavy metals and metalloids. The objective of the present study was to prepare organoclay and organoclay mixtures efficient to adsorb both cationic and anionic contaminants. The adsorbents were characterised by X-ray diffraction and infrared spectroscopy. Trivalent (Cr³⁺) and hexavalent (Cr₂O₇ ²⁻) chromium were selected as the model contaminants representing cationic and anionic properties. Bentonite modified with cationic surfactant hexadecyl trimethylammonium bromide at double the cation exchange capacity of the clay remarkably improved Cr₂O₇ ²⁻ adsorption capacity (as high as 0.49 mmol g⁻¹). Similarly, its modification with anionic surfactant sodium dodecyl sulphate at the same dosage improved Cr³⁺ adsorption (as high as 0.36 mmol g⁻¹). When these two organoclays were physically mixed in equal proportions (1:1), the resultant organoclay mixture efficiently adsorbed both Cr³⁺ (as high as 0.21 mmol g⁻¹) and Cr₂O₇ ²⁻ (as high as 0.32 mmol g⁻¹) implying that the mixture could remediate both anionic and cationic contaminants simultaneously. The adsorption of Cr³⁺ by the organoclay and organoclay mixture fitted well to the Langmuir isothermal model whereas the adsorption of Cr₂O₇ ²⁻ fitted well to the Freundlich model.