Optimization of Pb(II) adsorption onto modified walnut shells using factorial design and simplex methodologies

The removal of Pb(II) ions from aqueous solutions by chemically modified walnut shells was studied. A 24 full factorial design analysis was performed to screen the variables affecting Pb(II) removal efficiency. The effects of solution pH, adsorbent dose, initial concentration of Pb(II) ions, and temperature on metal removal efficiency were examined in a batch system. Using the experimental results, a linear mathematical model representing the influence of the different variables and their interactions was obtained. Analysis of variance (ANOVA), F-test and Student's t-test showed that Pb(II) ions adsorption is only slightly temperature dependent, but markedly increases with adsorbent dose and solution pH. The initial concentration of Pb(II) ions had a relatively small negative effect on removal efficiency. The optimization of the statistically significant factors was carried out using a modified simplex method. The recommended optimum conditions were: adsorbent dosage of 13.5 g/L, solution pH of 6.3, initial metal concentration of 45.3 mg/L, with the Pb(II) removal efficiency of 98.2%.

► A model showed the influence of variables and their interactions on Pb removal.
► Pb sorption was not temperature dependent, but increased with adsorbent dose and pH.
► Interactions of pH × dose, pH × Pb conc., dose × Pb conc., pH × dose × Pb conc. were significant.
► The modified simplex method improved Pb(II) removal efficiency by 15%.
► Maximum Pb removal efficiency obtained from the optimization procedure was 98.24%.