Isotherm Modeling and Statistical Physics Interpretation of Dye and Pesticide Adsorption on Activated Carbons for Environmental Remediation

A multilayer adsorption model was used to explain the adsorption mechanism of two toxic organic pollutants, methylene blue (MB) and 2,4-dichloro-phenoxy-acetic acid (2,4-D), on a commercial activated carbon (CPAC) and olive stone activated carbon prepared with ZnCl₂ (ZnCl₂/OSAC). Statistical physics-based modeling showed that the adsorption of both pollutants on tested activated carbons was multimolecular where 2-4 layers can be formed on the adsorbent surface. The calculated saturation adsorption capacities of ZnCl₂/OSAC and CPAC for MB were 481-574 and 873-932 mg/g at 30-50 °C, respectively, while the saturation 2,4-D adsorption capacities were 392-607 and 526-675 mg/g for the same adsorbents. Endothermic adsorption was observed for MB, while 2,4-D removal was exothermic. It was found that the MB and 2,4-D removal implied an inclined adsorption geometry, where physical interaction forces contributed to separating these pollutants. Overall, this research provides new findings to explain the adsorption mechanism of these relevant water pollutants on carbonaceous-based materials.