Mass transport scaling and the role of silica on arsenic adsorption onto porous iron oxide (hydroxide)

Arsenic contamination in drinking water sources is one of the major health concern in the human history. Throughout this study, inorganic arsenic removal using commercially available iron-based porous adsorbents has been studied. A large number of iron oxide/hydroxide based adsorbents are available in the market, but the evaluation of adsorbents in pilot/full-scale level is an expensive and time consuming option for most of the arsenic impacted water utilities. So this study focused to extend the application of rapid small-scale column tests (RSSCTs), as originally developed for organic adsorption onto granular activated carbon (GAC), for arsenic adsorption onto porous iron-based adsorbents. While the validation of RSSCTs on arsenic adsorption was the primary objective, several other important issues such as the mass transport mechanisms of arsenic adsorption onto porous adsorbents, comparative arsenic treatability of different adsorbents and the role silica and other major anions were also addressed. Commercially available GFH, E33, AAFS50 and newly developed HIX were used in this research.; To investigate the rate-limiting intraparticle diffusion for arsenic removal onto porous adsorbents, arsenate adsorption kinetics onto GFH have been studied. Bottle-point batch isotherm and differential column batch reactor (DCBR) experiments were used to estimate isotherm parameters (K and 1/n) as well as mass transfer coefficients (kf, Ds) respectively. The homogeneous surface diffusion model (HSDM) was used to describe the DCBR data. A relationship (DS=3.0-9 xRp1.4) was observed between estimated Ds and corresponding particle radius (RP ). The rate-limiting process of intraparticle surface diffusion for arsenate adsorption onto GFH appears analogous to organic compound adsorption by GAC, despite differences in adsorption mechanisms (inner-sphere complexes for arsenic, but hydrogen bonding for organics).; Both proportional diffusivity (PD) and constant diffusivity (CD) based RSSCTs were conducted and compared against corresponding pilot column/full scale data. The PD-RSSCTs simulated pilot/full-scale arsenic breakthrough curves reasonably well. The dependency of the surface diffusivity on particle size as observed in the kinetic study of GFH also indicates that the PD-RSSCT should be valid.; Energy micrographs of used media, obtained from scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) indicate silica as the major fouling agent occupying about 0.1-8% of total atomic sites depending on the water quality and the type of adsorbent. (Abstract shortened by UMI.)...