Role of the Wild Carob as Biosorbent and as Precursor of a New High-Surface-Area Activated Carbon for the Adsorption of Methylene Blue

Wild carob (WC), an abundant and unused lignocellulosic waste in Algeria, has been used as an inexpensive biosorbent for the elimination of methylene blue (MB) from the aqueous solution and as a precursor for the preparation of a new activated carbon by chemical activation with ZnCl 2 (ACWC). The adsorbent materials were characterized by N 2 physisorption, SEM/EDX, FTIR spectroscopy and isoelectric point (pHpzc) measurements. The adsorption performance of WC and ACWC was estimated for MB at different adsorption variables, such as solution pH (2–11), MB initial concentrations (25–200 mg L −1 ), time (0–1500 min), adsorbent dose (0.25–2.00 g L −1 ), temperature (10–40 °C) and NaCl concentration (0.0–0.5 M). The obtained results indicate that WC has an acidic surface due to the presence of carboxyl and phenol groups that play an important role in the fixation of cationic dye molecules, obtaining a maximum monolayer adsorption capacity of 84 mg g −1 at natural pH (7.5) and 40 °C. However, this adsorption performance was much higher in the case of the ACWC sample (218 mg g −1 ). From the equilibrium data, the Freundlich, Langmuir, Sips and Redlich–Peterson isotherms parameters of both samples were calculated and compared. For WC, the experimental data fitted well with Redlich–Peterson, Langmuir and Sips isotherm models, while in the case of ACWC they were best represented by the Redlich–Peterson and Sips isotherms. The adsorption kinetics data were found to follow the pseudo-second-order model for WC and the pseudo-first-order model for ACWC. The thermodynamic parameters suggest that, for both materials, the process was endothermic and spontaneous in the range of temperatures studied.