Adsorption kinetic study: effect of adsorbent pore size distribution on the rate of Cr (VI) uptake

Adsorbents were synthesized to obtain novel silica nanoparticles with a broad pore-size distribution (herein referred to as USG-41). The material demonstrated fast adsorption rates with highest adsorption capacities following Langmuir adsorption. Kinetic data best fit the intraparticle diffusion model demonstrating a two-step, surface and pore, adsorption process with pore diffusion being the rate determining step. This data provides key evidence of internal pore chelation of dichromate ions by USG-41. In contrast silica adsorbents (SBA-15 and MCM-41) prepared with similar average pore sizes to USG-41, but with narrow pore-size distributions, had lower adsorption capacities and their kinetic date best fit pseudo-second order diffusion models indicating a one-step, surface only, adsorption process. This study clearly demonstrated that pores size distribution, not the surface area or the average pore size, was central to ensure optimum adsorbent performance for removal of Cr (VI) from contaminated water.