Adsorption of Cr(VI) onto <i>Elaeagnus</i> Tree Leaves: Statistical Optimization, Equilibrium Modeling, and Kinetic Studies

Abstract

A 24−1(IV) half-fractional factorial design was performed to identify significant variables for the biosorption of Cr(VI) by Elaeagnus tree leaves. A face-centered central composite design (FCCD) was carried out to find a suitable response surface relating all significant variables to R (removal) and q (sorption capacity). Simultaneous optimization of both responses (R and q) was carried out, and 80 % of the goal of desirability function was achieved. Simultaneous optimization of R and q and simple optimization of q was more favorable than that of R from an environmental and economical view. A kinetics study was performed by examining pseudofirst-order, second-order, and intraparticle diffusion kinetic models, and the best fit was obtained for the pseudosecond-order kinetics model with qe = (0.624 and 2.657) mg·g−1 for (10 and 50) mg·L−l Cr(VI), respectively. Langmuir, Freundlich, and Dubinin−Radushkevich models were used for the equilibrium study. The equilibrium data had the best fit with the Langmuir isotherm. Biosorption mean free energy (E) was calculated to be 16.2 kJ·mol−1. Considering kinetics and equilibrium studies, one can suggest that adsorption onto the sites is the rate-limiting step and that biosorption goes through chemisorption mechanisms. Fourier transform infrared (FTIR) spectra were recorded to identify functional groups involved in the biosorption.