Sorption of chromium (III) from aqueous solution using bentonitic clay

A local clay from Jebel Chakir (northwest of Tunis, Tunisia, North Africa) has been characterized. The mineralogical and chemical compositions of the materials were determined by X-ray diffraction (XRD), infrared spectroscopy (IR) and by the inductively coupled plasma technique (ICP). The cation exchange capacities (CEC) as well as the surface area were determined using the adsorption isotherm obtained with methylene blue. The specific surface area (SE) was calculated by applying the BET equation to the nitrogen adsorption isotherm at 77 K. The results show that the material is a smectite with a small proportion of kaolinite. The CEC and SE values for the natural fraction are 50 meq /100 g, 48.95 m2 /g respectively for the fired clay; and 77 meq /100 g and 76.78 m2/g for the Na-purified fired clay fraction. The adsorption properties of the natural clay and the Na-purified clay in a chromium rich aqueous solution have been studied by the batch technique. The amount of adsorbed chromium ions, Q, was determined for the adsorption systems as a function of the contact time, the pH, the sorbent, and the metal ion concentration. The results show that the uptake of Cr (III) at pH 4, by the purified clay is very rapid. The quantity removed from the solution reached a maximum value 15 min after mixing, and 1 h for the natural clay although the latter removes greater quantities of Cr (III) ions compared to the Na-purified clay. The amounts sorbed by the natural clay were about 117.5 mg of chromium per gram of clay and 61.4 per gram with the Na-purified clay. Moreover, the results show that the adsorption behaviour of both clays depended highly on the pH. The adsorption increased with the pH of the suspension in the range [3–5.3]. The pH was limited to values equal or less than 5.3 because of the precipitation of the hydroxide chromium at higher pH. The experimental data points have been fitted to the Langmuir and Frendlich models. They are well fitted to the linearized Freundlich isotherm for the natural clay and Langmuir model for the Na-purified clay.