Adsorption of chromium (III), chromium (VI) and silver (I) on bentonite

The sorption of Cr(III), Cr(VI) and Ag(I) from aqueous solutions on bentonite has been studied by a batch technique. Distribution coefficients (Kd) were determined for sorption systems as a function of contact time, pH, sorbent and sorbate concentration and temperature. Sorption data have been interpreted in terms of Freundlich, Langmuir and Dubinin-Raduskevich (D-R) equations. Thermodynamic parameters for the sorption systems have been determined at different temperatures. The sorption of Cr(III) and Ag(I) on bentonite is exothermic in nature while that of Cr(VI) is an endothermic process. The ΔH° values for Cr(III), Cr(VI) and Ag(I) were −51.94 kJ/mol at 298 K, 5.619 kJ/mol at 293 K and −7.64 kJ/mol at 303 K respectively. The mean free energy of sorption, E, for Cr(VI) is 10 kJ/mol which shows that chromate ions from aqueous solutions at 10−5 to 10−3 M are predominantly sorbed on bentonite by an ion-exchange process. The sorption data for Cr(III) and Ag(I) do not fit in the D-R equation. Negative values of ΔG° show the spontaneity of the sorption processes; ΔG° values for Cr(III) and Ag(I) become less negative at higher temperatures which shows that sorption is less favoured at higher temperatures, while the increase in the negative values of ΔG° for Cr(VI) with the increase in temperature indicates that sorption is favoured at higher temperatures.