Role of surface chemistry and CO2 reactivation of activated carbons on their adsorption capacity and rate toward chlorphenamine

Abstract

In this investigation, chlorphenamine (CPA) adsorption on various activated carbons (ACs) was explored. The first part of the study focused on three ACs: Gama B (GB), Micro 10 (M10), and Megapol M (MM). The textural properties, active site concentrations, and charge distribution on the surface of the ACs were analyzed in detail. The surface areas (SBET) were 1107, 812, and 766 m2/g, corresponding to MM, M10, and GB, respectively. MM exhibited an acidic nature, while M10 and GB had basic surfaces. Adsorption experiments conducted at pH = 7 and pH = 11 showed that MM had the greatest capacity for adsorbing CPA. The CPA adsorption capacity increased in the order GB ≈ M10 < MM, which was associated with the SBET and acidic site concentrations of the ACs. Raising pH solution from 5 to 9 improved CPA adsorption onto MM, the interaction was due to the opposite charges: MM (negative) and CPA (positive). At pH 11 and 25 °C, MM reached its greatest capacity for adsorbing CPA (574.6 mg/g). Raising temperature led to a modest increase in CPA adsorption capacity. Zeta potential measurements indicated that electrostatic attractions were the main contributors to CPA adsorption on MM between pH 5 and 9, whereas at pH = 11, the interaction between CPA and basal planes of MM via π electrons governed the adsorption of CPA. The second part of the work consisted of the CO2 reactivation of MM samples treated at 800 °C for different durations. The CO2 reactivation altered the textural properties of MM4 (4 h), MM8 (8 h), and MM8A (8h accumulated), the latter having the highest SBET, pore volume (Vp), and micropore surface area (Smic) owing to an extra CO2 reactivation cycle. CO2 reactivation also led to the removal actives sites and micropore development. Carboxylic sites quantification showed a decrement consistent with the CO2 reactivation time. MM8A had a minor loss of carboxylic sites as its surface was reoxidized. The Raman spectra analysis of ACs indicated that as CO2 reactivation time increased, the intensity ratio of the Raman bands, D and G (ID/IG), also increased since CO2 reactivation promoted the presence of defects on their surfaces. MM8A exhibited the highest adsorption capacity toward CPA at pH 7. Factors contributing to its good adsorption capacity included (i) having a low concentration of carboxylic groups as they can hinder the interaction with CPA via π electrons and (ii) the disordered structure as a result of the detachment of carboxylic groups located at the basal plane edges. Adsorption of CPA onto MM8A was found to be more effective at higher pH values, with temperature having minimal influence. The isosteric heat of adsorption suggested that physical interactions governed the CPA adsorption onto MM8A. Analysis of zeta potential measurements and the CPA coefficient distribution diagram revealed that at pH 11, π − π and hydrophobic interactions dominated the CPA adsorption. Diffusional models were employed to evaluate the rate at which CPA adsorbed onto MM8A, the experimental data were successfully modeled using the external mass transport model. Furthermore, CPA adsorption occurred faster on MM8A compared to MM.