Adsorption of Chromium(VI) from Water by Clays

Interactions of kaolinite and its modified forms, acid-activated kaolinite, poly(oxozirconium) kaolinite, and tetrabutylammonium kaolinite, have been investigated for their utilization as adsorbents for Cr(VI) in aqueous medium. The variables are Cr(VI) concentration, amount of kaolinite or its modified forms, pH, interaction time, and temperature. The adsorption is strongly dependent on the pH of the medium with Cr(VI) uptake increasing from pH 1.0 to pH 7.0, after which the uptake decreases. The process attains equilibrium within 240 min. The kinetics of the interactions is tested with respect to pseudo-first-order, second-order, Elovich, liquid film diffusion, and intraparticle diffusion models, and it is seen that the interactions do not follow a simple model. The adsorption process, however, gives a good fit with both the Langmuir and Freundlich isotherm equations. The Langmuir monolayer capacity of the clay adsorbents is from 10.6 to 13.9 mg g^-1. The adsorption process is endothermic ((delta)H = 30.4-63.9 kJ mol-1) accompanied by an increase in entropy ((delta)S = 88.4-198.1 J mol^-1 K^-1) and decrease in Gibbs energy. The results have shown that acid-activated kaolinite has the largest adsorption capacity followed by nonactivated kaolinite, ZrO-kaolinite, and TBA-kaolinite.