Taguchi orthogonal design was used for multivariate optimization of Cd(II) removal by Carpinus betulus tree leaves from an aqueous solution in a batch system. Effective factors on adsorption process, such as temperature, amount of sorbent, initial concentration of Cd(II), pH value, and particle size were considered using an L16 orthogonal array design. Taguchi analysis is based on choosing the best run by analyzing signal-to-noise ratio (S/N), whose form depends on the experiment objective. Thus, the removal percentage (R%) and capacity uptake (q) of Cd(II) were transformed into an accurate S/N ratio for a “high is better” response. The best conditions for biosorption of Cd(II) were determined by the Taguchi method and desirability approach as pH ¼ 4.8, sorbent dose of 2 g l1, initial Cd(II) concentration of 140 mg l1, temperature 25 C, and sorbent size of 0.12e0.17 mm. In addition, metal interactions with sorbent were analysed by FT-IR spectroscopy, SEM, and XRD methods. Potentiometric titration modeling also denotes that the C. betulus tree leaves may be characterized by two kinds of active sites (carboxylic and hydroxyl). A nonlinear regression was applied to fit the experimental data with Langmuir, Freundlich, DubinineRadushkevich, and Sips isotherms. As a result, the Sips isotherm showed the best concordance as an equilibrium model. The biosorption process was kinetically fast and followed by a pseudo-second order kinetic model. Thermodynamic investigations showed that the biosorption process was spontaneous and exothermic.
