Kinetics of humics removal from water and wastewater using granular activated carbon, iron-coated activated alumina, and beta ferric oxihydroxide

Abstract

Application of kinetic models to represent the removal of humic substances from water and wastewater was investigated. An adsorbate-adsorbent system was represented by humic substances and three adsorbents, namely granular activated carbon (GAC), iron-coated activated alumina (AAFS), and ferric oxihydroxide in its beta form. The Elovich, internal diffusion, and external diffusion models were considered. The first model represents fairly well the experimental data for GAC and AAFS. The intraparticle diffusion model applies well for the second stage (stage 2) of adsorption only. Reliable diffusion coefficients were estimated for GAC and beta ferric oxihydroxide. A different phenomenon occurred with AAFS, as adsorption seemed to follow a second stage instead of stage 3, which was attributed to the flaking of the coating, thus generating more bare surface. As a result, the dominant mechanism on AAFS varies, starting with the adsorption being limited by internal diffusion, then diffusion accompanied by a chemical reaction and later by external diffusion. The external diffusion model applies well in different stages of adsorption, most likely because of attrition generating new grains and hence exposing a new external surface. Therefore, it is not applicable as such over the whole range of data, which is to be expected, because the mixing is not likely to limit external adsorption. The calculated diffusion coefficient is between 8.34 and 9-10-11 m 2s-1, representing molecular weights up to 50kDa.