Biosorption of Cd(II), Cr(III), and Cr(VI) by saltbush (Atriplex canescens) biomass: thermodynamic and isotherm studies.

The biosorption data of Cd(II), Cr(III), and Cr(VI) by saltbush leaves biomass were fit on the Freundlich and Langmuir adsorption isotherms at 297 K. The Cd(II) and Cr(III) solutions were adjusted to pH 5.0 and the Cr(VI) solution was adjusted to pH 2.0. The correlation coefficient values indicated that the data fit better the Freundlich model. The maximal capacities (K(F)) were found to be 5.79 x 10(-2), 3.25 x 10(-2), and 1.14 x 10(-2) mol/g for Cr(III), Cd(II), and Cr(VI), respectively. Similar results were obtained using the Langmuir and the Dubinin-Radushkevick equations. Thermodynamic parameters calculated from the Khan and Singh equation and from the q(e) vs C(e) plot show that the equilibrium constants for the biosorption of the metals follow the same order of the maximal capacities. The negative Gibbs free energy values obtained for Cd(II) and Cr(III) indicated that these ions were biosorbed spontaneously. The mean free energy values calculated from the Dubinin-Radushkevick equation (10.78, 9.45, and 9.05 for Cr(III), Cr(VI), and Cd(II), respectively) suggest that the binding of Cd(II), Cr(III), and Cr(VI) by saltbush leaves biomass occurs through an ionic exchange mechanism.