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

2004; Elsevier BV; Volume: 38; Issue: 18 Linguagem: Inglês

10.1016/j.watres.2004.07.007

1879-2448

Mohammad Badruzzaman, Paul Westerhoff, Detlef R.U. Knappe,

Mine drainage and remediation techniques

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 ∼0.0394±0.0056 cm3 g−1, mesopore volume ∼0.0995±0.0096 cm3 g−1) adsorbent with a BET surface area of 235±8 m2 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 (kf) and intraparticle surface diffusion (Ds). The pseudo-equilibrium (18 days of contact time) arsenate adsorption density at pH 7 was 8 μg As/mg dry GFH at a liquid phase arsenate concentration of 10 μg As/L. The homogeneous surface diffusion model (HSDM) was used to describe the DCBR data. A non-linear relationship (DS=3.0−9×Rp1.4) was observed between Ds and GFH particle radius (RP) with Ds values ranging from 2.98×10−12 cm2 s−1 for the smallest GFH mesh size (100×140) to 64×10−11 cm2 s−1 for the largest GFH mesh size (10×30). 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.