Application of response surface methodology for the optimization of NaOH treatment on oil palm frond towards improvement in the sorption of heavy metals

NaOH treatment on oil palm frond (OPF) to improve its heavy metal sorption was investigated. OPF was treated with between 0.1 and 1.1 M of NaOH for up to 5 h. For the sorption study, 1.0 g of the sorbent was in contact with 250 ml of 100 mg/l Zn and Cu solutions. Response surface methodology (RSM) was employed to optimize the conditions of the base treatment process. The treatment resulted in a marked improvement in the sorption capacity. RSM study resulted in quadratic models demonstrating the weaker effect of treatment time against the dominant effect of the base concentration, indicating the fast rate of the base treatment process. The conditions for the optimized process were achieved by treating the OPF in a 1.0 M NaOH solution for 45 min to give a removal of 61.5% and 64.0% for Zn and Cu, respectively. The results obtained were found to be significant at 99% confidence interval. Compared to Langmuir isotherm, Freundlich isotherm fitted the experimental data better (R2 >0.99), indicating the heterogeneity of the binding sites in the NaOH modified OPF. Despite lower strength and energy of the metal–sorbent interactions, a higher number of metal binding sites after NaOH treatment led to an improved sorption process.