Sorption Kinetics of Lead and Zinc Ions by Clay-Fly Ash Geopolymers

As sorption process in porous materials is mainly controlled by the diffusional effects; this research aims at defining the rate-controlling steps in the sorption process of lead, Pb(II), and zinc, Zn(II) by the clay-fly ash geopolymers as a sustainable active-passive liner material. Bulk-type clay-fly ash geopolymers are synthesized from the mixtures containing 60% fly ash to the total solid mass and then activated by 10 M NaOH solutions. Several batch experiments are conducted at the ambient temperature of 24 degrees C for 90 min of contact time. The significant effects of contact time and initial concentration of solutes are investigated. Several reaction- and diffusion-controlled kinetic models are used for the kinetic analyses of the experimental data. The results show the significant contribution of the physical adsorption, film, and intraparticle diffusivity mechanisms in the sorption process. Microstructural analyses exhibited the heterogeneous matrix of the prepared clay-fly ash geopolymer with macropores, mesopores, and micropores. Thus, a heterogeneous intraparticle model, derived from Fick's second law, is employed so as to evaluate the separate effects of pore and surface diffusion on the internal mass transfer of the solutes. The results reveal the domination of surface diffusion on the Pb(II) and Zn(II) removal through internal mass transfer.