Intraparticle diffusion and adsorption of arsenate onto granular ferric hydroxide (GFH)

Porous iron oxides are being evaluated and selected for arsenic removal in potable water systems. Granular ferric hydroxide, a typical porous iron adsorbent, is commercially available and frequently considered in evaluation of arsenic removal methods. GFH is a highly porous (micropore volume similar to0.0394+/-0.0056cm(3) g(-1), mesopore volume similar to0.0995+/-0.0096 cm(3) g(-1)) adsorbent with a BET surface area of 235+/-8 m(2) g(-1). The purpose of this paper is to quantify arsenate adsorption kinetics on GFH and to determine if intraparticle diffusion is a rate-limiting step for arsenic removal in packed-bed treatment systems. Data from bottle-point isotherm and differential column batch reactor (DCBR) experiments were used to estimate Freundlich isotherm parameters (K and 1/n) as well as kinetic parameters describing mass transfer resistances due to film diffusion (k(f)) and intraparticle surface diffusion (D-s). The pseudo-equilibrium (18 days of contact time) arsenate adsorption density at pH 7 was 8 mug As/mg dry GFH at a liquid phase arsenate concentration of 10 mug As/L. The homogeneous surface diffusion model (HSDM) was used to describe was observed between D, and GFH particle radius (R-P) the DCBR data. A non-linear relationship (Ds = 3.0-9 x R-p(1.4)) with D-s values ranging from 2.98 x 10(-12)cm(2) s(-1) for the smallest GFH mesh size (100 x 140) to 64 x 10(-11) cm(2)s(-1) I for the largest GFH mesh size (10 x 30). The rate-limiting process of intraparticle surface diffusion for arsenate adsorption by porous iron oxides appears analogous to organic compound adsorption by activated carbon despite differences in adsorption mechanisms (inner-sphere complexes for As versus hydrophobic interactions for organic contaminants). The findings are discussed in the context of intraparticle surface diffusion affecting packed-bed treatment system design and application of rapid small-scale column tests (RSSCTs) to simulate the performance of pilot- or full-scale systems at the bench-scale. (C) 2004 Elsevier Ltd. All rights reserved.