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.
Article: Removal of arsenite and arsenate using hydrous ferric oxide incorporated into naturally occurring porous diatomite.[show abstract] [hide abstract]
ABSTRACT: In this study, a simplified and effective method was tried to immobilize iron oxide onto a naturally occurring porous diatomite. Experimental resultsfor several physicochemical properties and arsenic edges revealed that iron oxide incorporated into diatomite was amorphous hydrous ferric oxide (HFO). Sorption trends of Fe (25%)-diatomite for both arsenite and arsenate were similar to those of HFO, reported by Dixit and Hering (Environ. Sci. Technol. 2003, 37, 4182-4189). The pH at which arsenite and arsenate are equally sorbed was 7.5, which corresponds to the value reported for HFO. Judging from the number of moles of iron incorporated into diatomite, the arsenic sorption capacities of Fe (25%)-diatomite were comparable to or higher than those of the reference HFO. Furthermore, the surface complexation modeling showed that the constants of [triple bond]SHAsO4- or [triple bond]SAsO4(2-) species for Fe (25%)-diatomite were larger than those reference values for HFO or goethite. Larger differences in constants of arsenate surface species might be attributed to aluminum hydroxyl ([triple bond]Al-OH) groups that can work better for arsenate removal. The pH-controlled differential column batch reactor (DCBR) and small-scale column tests demonstrated that Fe (25%)-diatomite had high sorption speeds and high sorption capacities compared to those of a conventional sorbent (AAFS-50) that is known to be the first preference for arsenic removal performance in Bangladesh. These results could be explained by the fact that Fe (25%)-diatomite contained well-dispersed HFO having a great affinity for arsenic species and well-developed macropores as shown by scanning electron microscopy (SEM) and pore size distribution (PSD) analyses.Environmental Science and Technology 04/2006; 40(5):1636-43. · 5.23 Impact Factor
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ABSTRACT: The present research evaluates the efficacy of granular ferric hydroxide (GFH) for perchlorate removal from aqueous solutions. Laboratory scale experiments were conducted to investigate the influence of various experimental parameters such as contact time, initial perchlorate concentration, temperature, pH and competing anions on perchlorate removal by GFH. Results demonstrated that perchlorate uptake rate was rapid and maximum adsorption was completed within first 30 min and equilibrium was achieved within 60 min. Pseudo-second-order model favorably explains the sorption mechanism of perchlorate on to GFH. The maximum sorption capacity of GFH for perchlorate was ca. 20.0 mg g−1 at pH 6.0–6.5 at room temperature (25 °C). The optimum perchlorate removal was observed between pH range of 3–7. The Raman spectroscopy results revealed that perchlorate was adsorbed on GFH through electrostatic attraction between perchlorate and positively charged surface sites. Results from this study demonstrated potential utility of GFH that could be developed into a viable technology for perchlorate removal from water.Chemical Engineering Journal.
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ABSTRACT: The study was carried out to evaluate the feasibility of synthetic siderite for As(V) removal from aqueous solution. Batch experiments were performed to investigate effects of various experimental parameters such as contact time (10 min-8 h), initial As(V) concentration (0.5-60.0 mg/L), temperature (15, 25, 35 and 45 degrees C), pH (2.0-10.0) and the presence of competing anions on As(V) adsorption on the synthetic siderite. Kinetic data reveal that the uptake rate of As(V) was rapid at the beginning and 90% adsorption was completed within 10 min at 45 degrees C and equilibrium was achieved within 3 h. The adsorption process was well described by pseudo-second-order kinetics model. The adsorption data better fitted Langmuir isotherm at low temperatures (i.e., 15 and 25 degrees C), while Freundlich isotherm at relatively high temperatures (35-45 degrees C). The maximum adsorption capacity calculated from Langmuir isotherm model was up to 31 mg/g. Thermodynamic study indicates an exothermic nature of adsorption and a spontaneous and favorable process. The optimum pH for As(V) removal was broad, ranging from 3.0 to 10.0. The As(V) adsorption was impeded by the presence of SiO(3)(2-), followed by PO(4)(3-) and NO(3)(-). The adsorption process appeared to be controlled by the chemical process. The high As uptake may attribute to both coprecipitation of As with goethite and lepidocrocite forming during the reaction and subsequent adsorption of As on these minerals.Journal of hazardous materials 11/2009; 176(1-3):174-80. · 4.14 Impact Factor