title: Intraparticle diffusion and adsorption of arsenate onto granular ferric hydroxide (GFH).
abstract: 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 approximately 0.0394+/-0.0056 cm(3)g(-1), mesopore volume approximately 0.0995+/-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 microg As/mg dry GFH at a liquid phase arsenate concentration of 10 microg As/L. The homogeneous surface diffusion model (HSDM) was used to describe the DCBR data. A non-linear relationship (D(S)=3.0(-9) x R(p)(1.4)) was observed between D(s) and GFH particle radius (R(P)) 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) 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.
authors.affiliation: Department of Civil and Environmental Engineering, Box 5306, Arizona State University, Tempe, AZ 85287-5306, USA. bzaman@asu.edu
authors.forename: Mohammad
authors.initials: M
authors.lastname: Badruzzaman
authors.forename: Paul
authors.initials: P
authors.lastname: Westerhoff
authors.forename: Detlef R U
authors.initials: DR
authors.lastname: Knappe
mesh_terms: D000327:Adsorption; D001149:Arsenates / Q000032:analysis* / Q000737:chemistry* / Q000302:isolation & purification; D004058:Diffusion; D005290:Ferric Compounds / Q000737:chemistry*; D006540:Herbicides / Q000032:analysis* / Q000737:chemistry* / Q000302:isolation & purification; D016062:Porosity; D014873:Water Pollutants / Q000032:analysis* / Q000302:isolation & purification; D018508:Water Purification / Q000379:methods*
chemical_list: D001149:Arsenates; D005290:Ferric Compounds; D006540:Herbicides; D014873:Water Pollutants; C021024:ferric hydroxide; C025657:arsenic acid