Cr(VI) adsorption on activated carbon: Mechanisms, modeling and limitations in water treatment

• Published in: Journal of environmental chemical engineering 2020-08, Vol.8 (4), p.104031, Article 104031
• Main Authors: Wang, Yongmei, Peng, Changsheng, Padilla-Ortega, Erika, Robledo-Cabrera, Aurora, López-Valdivieso, Alejandro
• Format: Article
• Language: English
• Subjects: Activated carbon; Adsorption mechanisms; Adsorption modeling; Chromium removal; Water treatment
• ISSN: 2213-3437; 2213-3437
• DOI: 10.1016/j.jece.2020.104031

[Display omitted] •Cr(VI) adsorption mechanisms on activated carbon (AC) were studied and modeled.•Adsorption occurred in two stages, successfully described by pore and surface diffusion models.•A thin Cr2O3(s) layer rapidly coated the AC surface lowering the adsorption rate.•Scattered Cr2O3(s) phases formed inside AC in the slow adsorption stage.•The Cr2O3 layer is responsible of the low Cr(VI) up-take per unit mass of AC. Adsorption of Cr(VI) on highly porous granular activated carbon (GAC) was studied through batch adsorption tests at a very long time, various temperatures and pH 3.5, in order to delineate the adsorption mechanisms, model the adsorption process and assess the applicability of GAC in water treatment. Adsorption of Cr(VI) occurred through a redox reaction with the formation of Cr2O3(s) (Cruz-Espinoza et al., 2012). It is reported for the first time that Cr(VI) adsorption proceeded in two steps. Furthermore, adsorption increased with temperature and reached equilibrium after several hours. In the first step, very fast adsorption occurred with the formation of a nano-size continuous layer of Cr2O3(s) coating on the GAC surface, as revealed by SEM analysis. In the second stage, adsorption occurred at a very slow rate due to the slow diffusion rate through the continuous Cr2O3(s) layer on the GAC surface. The two adsorption steps were successfully described by the surface diffusion model and pore volume diffusion model. The first step is controlled by surface diffusion while the second step by both pore volume diffusion and surface diffusion. A pseudo-activation energy was determined for each adsorption step and found that more adsorption energy is required in the first step than the second step. The adsorption isotherms for various temperatures were fitted well by the Langmuir model. The Cr2O3 layer on the GAC is responsible for its low Cr(VI) adsorption capacity, so a large amount of GAC would be required to treat water with Cr(VI).