In this study, cladodes of Opuntia ficus indica (OFIC), chemically modified with NaOH (OFICM), have been prepared, characterized, and tested as an effective biomass to remove Pb(II) and/or Cd(II) from aqueous media. At an optimum pH of 4.5, the adsorption capacity, q_e, of treated OFICM was almost four times higher than that of untreated OFIC. The maximum adsorption capacities (q_max) in the single removal of Pb(II) and Cd(II) were 116.8 and 64.7 mg g⁻¹, respectively. These values were 12.1% and 70.6% higher than those for the corresponding q_max in binary removal, which indicates the strong inhibitive effect of Pb(II) on the co-cation Cd(II) in a binary system. Structural and morphological characterization have been carried out by FTIR, SEM/EDX, and point of zero charge (pH_PZC) measurements. The SEM/EDX results confirmed that the metals are adsorbed on the surface. The presence of C-O, C=O, and COO- functional groups were identified by FTIR on both OFIC and OFICM surfaces. On the other hand, we found that the adsorption processes followed the pseudo-second-order kinetics for both single and binary systems, with a fast biosorption rate of Pb(II) and Cd(II). The equilibrium data (adsorption isotherms) were better described by Langmuir and modified-Langmuir models for single and binary systems, respectively. A good regeneration of OFICM was obtained with an eluent of 0.1 M HNO₃. Therefore, OFICM can be efficiently reused to remove Pb or Cd, up to three times.

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.

Face Centered Cube Central Composite Design (FCCCCD) methodology was applied to optimize the Cr(VI) adsorption capacity of Arabica-coffee pulp biomass (WAC), qe, as a function of key-factors, such as pH, chromium concentration C0 and WAC-dose. The quadratic dependence of qe, with the considered...