Elucidating the adsorption mechanism of herbicide Diuron onto activated carbons via steric, energetic and thermodynamic investigations

The residues of the species Calophyllum inophyllum and Manihot esculenta were carbonized after impregnation with ZnCl2 to produce two different activated carbons (ACs). These adsorbents were subsequently used in the removal of the toxic herbicide Diuron. Both the produced adsorbents present functional groups characteristic of structures containing lignin and cellulose. These groups are positioned in amorphous and disorganized arrangements. The activation process formed a porous carbon exoskeleton with a surface area of 468 m2 g-1 (AC Calophyllum inophyllum) and 614 m2 g-1 (AC Manihot esculenta), respectively. Adsorption tests of herbicide Diuron are performed at different temperatures, indicating better performances for the adsorbent derived from Manihot esculenta (Qmax = 220 mg/g). A physical model derived from statistical physics, which assumes the formation of two layers of Diuron molecules on the ACs surface is successfully used to analyze the adsorption mechanism. The theoretical analysis of model parameters shows that the number of herbicide molecules captured per adsorption site increases with temperature for both the tested adsorbents, suggesting that an endothermic aggregation process occurs. The calculation of the adsorption energies confirms that Diuron adsorption on the surface of both the adsorbents is based on physical forces. Finally, three thermodynamic parameters are analyzed to attribute a new macroscopic description of the adsorption mechanism.