Optimization of hexavalent chromium biosorption onto Arabica-coffee waste using response surface methodology

Face centered cube central composite design (FCCCCD) methodology was applied to optimize the Cr(VI) adsorption capacity of Arabica-coffee pulp biomass (WAC), q_e, as a function of key-factors, such as pH, chromium concentration C₀ and WAC-dose. The quadratic dependence of q_e, with the considered factors, has shown a strong influence of pH. The optimal conditions found in this way were pH 2; C₀ = 75 mg/L and WAC-dose= 0.5 g/L. The characterization of WAC, performed by FTIR and SEM/EDX, allowed the identification of OH, CH, CO, COH functional groups and also showed significant changes, particularly on WAC surface morphology, after biosorption. The experimental isotherm data (q_e vs C_e) was well-fitted to three models: Langmuir, Freundlich and Temkin. Furthermore, thermodynamic and kinetic results showed that Cr(VI) biosorption using WAC is an endothermic and spontaneous reaction, following pseudo-2^nd order kinetics. Finally, this work shows the feasibility of effectively using coffee biomass-waste, a low-cost material, for chromium removal in contaminated aqueous media.