Biosorption of Cr(VI) by Theobroma cacao pericarp

This paper reports a comprehensive study of Theobroma cacao pericarp (TCP) residues, which has been prepared, characterized, and tested as an inexpensive and efficient biosorbent of Cr(VI) from aqueous solutions. The maximum adsorption capacity of TCP obtained at optimal conditions (pH = 2, dose = 0.5 g L^-1, C₀ = 100 mg L^-1) was q_max = 48.5 mg g^-1, which is one of the highest values reported by the literature. Structural and morphological characterization has been performed by FTIR, SEM/EDX, and pH_PZC measurements. FTIR analysis revealed the presence of O-H, -NH, -NH₂, C = H, C = O, C = C, C-O, and C-C functional groups that would be involved in the Cr(VI) biosorption processes. The experimental equilibrium data of biosorption process were successfully fitted to non-linear Langmuir (R² = 0.95, χ² = 11.0), Freundlich (R² = 0.93, χ² = 14.8), and Temkin (R² = 0.93, χ² = 14.7) isotherm models. Kinetics experimental data were well adjustment to non-linear pseudo-2nd (R² = 0.99, χ² = 2.08)- and pseudo-1st-order kinetic models (R² = 0.98, χ² = 2.25) and also to intra-particle Weber-Morris (R² = 0.98) and liquid film diffusion (R² = 0.99) models. These results indicate that Cr(VI) biosorption on heterogeneous surfaces as well as on monolayers of TCP would be a complex process controlled by chemisorption and physisorption mechanisms. The thermodynamic results indicate that the Cr(VI) biosorption on TCP is a feasible, spontaneous, and endothermic process. TCP can be regenerated with NaOH and reused up to 3 times.