Advanced analysis via statistical physics model to study the efficiency of catechol removal from wastewater using Brazil nut shell activated carbon

This paper presented the preparation, characterization, and adsorption properties of Brazil nut shell activated carbon for catechol removal from aqueous solutions. The equilibrium adsorption of catechol molecules on this activated was experimentally quantified at pH 6 and temperatures ranging from 25 to 55 °C, and at 25 °C and pH ranging from 6 to 10. These results were utilized to elucidate the role of surface functionalities through statistical physics calculations. All these experimental adsorption isotherms were fitted and interpreted via a monolayer model with one energy, which was chosen as the optimal model. Model physicochemical parameters, which may be categorized as stereographic parameters such as the maximum adsorbed quantity (Q_M), the number of adsorbed catechol molecules per one Brazil nut shell activated carbon binding site (n), and the number of effectively occupied binding sites (N_M) and energetic parameter such as the half saturation concentration (C_HS), were analyzed. Microscopically speaking, these modeling results were employed to stereographically and energetically investigate the phenol derivative adsorption mechanism. The maximum catechol adsorbed quantities on this activated carbon ranged from 89.98 to 103.16 mg/g under the tested operating conditions. The adsorption of catechol molecules was found to be exothermic where the maximum adsorbed quantity augmented with solution temperature and the maximum adsorption efficiency was found 103.16 mg/g at 55 °C. In addition, it was found that the catechol molecules were adsorbed with nonparallel orientations on the activated carbon adsorbent since the numbers of catechol molecules per site were superior to 1 (1.10 < n < 1.86). Moreover, the calculated molar adsorption energies, which varied between 19.04 and 22.37 kJ/mol, showed exothermic (ΔE > 0) and physical (ΔE < 40 kJ/mol) adsorption process involving hydrogen bonds, π-π interactions, electron donor-acceptor interactions, and dispersion forces. Finally, the tested adsorbent exhibited unimodal pore size and site energy distributions with peaks centered at pore radius ranging from 2.26 to 2.68 nm, and at adsorption energy ranging from 20.01 to 23.78 kJ/mol, respectively. Macroscopically speaking, three thermodynamic potentials, including the adsorption entropy, internal energy of adsorption, and Gibbs free energy, suggested that the adsorption of catechol on Brazil nut shell activated carbon was a spontaneous and exothermic mechanism.