Fraxinus tree leaves were successfully used to remove ternary mixture of Cu(II), Cd(II), and Pb(II) from an aqueous solution in a batch system. The simplexcentroid mixture design was used for optimization of the biosorption process. The effective factors on biosorption process, such as pH, s (amount of biosorbent), and Ci initial concentrations of metal ions, were considered via a crossed mixture–process design. Optimal conditions were found to be as follows: pH = 5, s (sorbent mass) = 0.05 g, CCu (initial Cu(II) concentration) = 100.0 mg/L, CCd (Initial Cd(II) concentration) = 129.1 mg/L, and CPb (Initial Pb(II) concentration) = 70.9 mg/L. The results clearly show competitive effects between mixture ingredients in favor of Pb(II), and also an interaction between process and mixture variables was observed. It was found that, with increasing Pb(II) contribution, the removal efficiency increases to its highest value. The pH has a positive effect and sorbent mass a negative effect on the response. To characterize the biosorption, Fourier transform infrared analysis was performed and, according to the results, the main functional groups of sorbent were involved in the biosorption process.
