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

Mohammad Badruzzaman, Paul Westerhoff, Detlef R U Knappe

Research output: Contribution to journalArticle

281 Scopus citations

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 ∼0.0394±0.0056 cm3 g-1, mesopore volume ∼0.0995±0.0096 cm3g-1) adsorbent with a BET surface area of 235±8 m2g-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 x Rp 1.4) was observed between Ds and GFH particle radius (RP) with Ds> values ranging from 2.98 x 10-12 cm2 s-1 for the smallest GFH mesh size (100 x 140) to 64 x 10-11 cm2 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.

Original languageEnglish (US)
Pages (from-to)4002-4012
Number of pages11
JournalWater Research
Volume38
Issue number18
DOIs
StatePublished - Jan 1 2004

Keywords

  • Adsorption
  • Arsenic
  • Iron
  • Surface diffusion
  • Water treatment

ASJC Scopus subject areas

  • Ecological Modeling
  • Water Science and Technology
  • Waste Management and Disposal
  • Pollution

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