J. Phys. Chem. C 2010, 114, 16547–16555
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Co-Adsorption of Ga(III) and EDTA at the Water-r-FeOOH Interface: Spectroscopic Evidence of the Formation of Ternary Surface Complexes Katarina Nore´n and Per Persson* Department of Chemistry, Umeå UniVersity, SE-901 87 Umeå, Sweden ReceiVed: June 13, 2010; ReVised Manuscript ReceiVed: August 18, 2010
Co-adsorption reactions between metal ions and anionic ligands play important roles in controlling availability and transport of chemical species in natural aquatic environments as well as in industrial processes. A molecular understanding of the properties of the surface species formed provides means to model these reactions in a predictive manner and to exploit them in synthetic routes of modified surfaces. In this study, we have used EXAFS and infrared spectroscopies in combination with quantitative adsorption measurements to investigate the coadsorption of Ga(III) and EDTA on R-FeOOH (goethite) as a function of pH. The quantitative results showed a 1:1 stoichiometry between adsorbed Ga(III) and EDTA and a maximum in total adsorption around pH 5. EXAFS and infrared data showed that the molecular structures displayed pH-dependent characteristics, and within the studied pH range, these results were concurrent and indicated that Ga(III)EDTA formed ternary surface complexes on goethite. The collective results were fully consistent with the occurrence of both outer sphere Ga(III)EDTA and inner sphere ternary surface complexes of type A (i.e., a surface-Ga(III)-EDTA structure), where the latter was favored by increasing pH. This study showed that despite a macroscopic adsorption behavior that was seemingly ligand-like, a substantial fraction of Ga(III) may bond directly to surface hydroxyl groups. 1. Introduction Reactions that control ion speciation in natural systems include: adsorption of metal ions and anionic ligands on mineral surfaces, complexation of metal ions by ligands in the aqueous phase, coadsorption of cationic metals and anionic ligands, dissolution of minerals, subsequent interaction of dissolved metal ion and ligands, and readsorption of such complexes.1 The present study focuses on coadsorption reactions, which are believed to play important roles in dissolution/precipitation processes as well as in the speciation and reactive transport of metal ions and ligands in surface waters, soils, and groundwater. Furthermore, these reactions are of importance in industrial processes such as mineral processing by froth flotation, paper production, catalyst deposition and activation, and manufacturing of new electrode materials and paints. The coadsorption of metal ions and negatively charged ligands is commonly explained by one of three principally different mechanisms: surface precipitation, cooperative electrostatics, or ternary surface complexation. Surface precipitation often involves the formation of a 3-D surface phase containing both the metal ion and the ligand.2 However, a combined mechanism can also be envisaged, at least at higher pH values, where a metal hydroxide-like surface precipitate might form. This phase can contain ligand adsorption sites of higher activity than the underlying surface and thereby enhance the ligand adsorption. The mechanism of cooperative electrostatics has been suggested for several metal ion/ligand systems.3,4 This is solely a surfacecharge effect, where the adsorption of anionic species at low pH lowers the surface charge and consequently facilitates the adsorption of positively charged metal ions. Similar reasoning can be applied to a high pH scenario where metal ion adsorption increases the surface charge, making the adsorption of anionic * Corresponding author. Tel: +46-90-786 55 73. E-mail: Per.
[email protected].
species more favorable. In both cases, the metal ion and the ligand adsorb on different surface sites, and there is no direct interaction between them. In contrast, when coadsorption results in the formation of a ternary surface complex, then there is a direct interaction between the metal ion and the ligand.5 This can occur through the adsorption of an intact metal-ligand aqueous complex by purely electrostatic forces (outer-sphere type of interaction) or through the formation of new inner-sphere structures. These inner-sphere structures can be of either type A, where the metal is bonded to the surface and the ligand whereas the ligand is bonded only to the metal ion (i.e., surface-metal-ligand (S-M-L)), or of type B complex, where the ligand bridges the surface and the metal ion (S-L-M).5 The focus of the present work is the coadsorption of metal ions and ligands involving the strongly chelating EDTA molecule. EDTA forms strong aqueous complexes with a wide range of metal ions.6 Because of this strong complexing property, EDTA has become a widely used chemical in industrial processes and products such as detergents. Discharge of these substances to surface waters has resulted in widespread EDTA contamination, which has increased the mobility and bioavailability of metal ions and the solubilities of minerals. As a consequence, Me(II)EDTA and Me(III)EDTA reactions on Fe and Al oxides have been extensively investigated by macroscopic techniques.7-12 Adsorption measurements from a study by Nowack and Sigg7 indicated that Me(II)EDTA complexes adsorb in an inner-sphere mode as type B complexes, that is, by direct bonds between EDTA and the goethite surface. However, a spectroscopic study of Pb(II)EDTA ternary complexes on goethite showed, using a combination of EXAFS and infrared spectroscopy, that these complexes are more likely outer-sphere (i.e., no direct bonds to goethite surfaces) with all carboxylate and amine groups bonded to Pb(II) under all conditions examined.13 In a similar study, Pd(II)EDTA was shown to adsorb onto goethite via both outer sphere and inner
10.1021/jp1054233 2010 American Chemical Society Published on Web 09/07/2010
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J. Phys. Chem. C, Vol. 114, No. 39, 2010
sphere type A mechanisms, and the distribution among the complexes was indicated to be pH-dependent.14 These results emphasized that the structures of coadsorbed complexes cannot be deduced from adsorption data alone and that additional molecular-level spectroscopic information is critical. Herein we present the results from adsorption and spectroscopic measurements of Ga(III)EDTA complexes adsorbed at the water-R-FeOOH (goethite) interface. A combination of EXAFS and infrared spectroscopies has been used. These methods are capable of providing molecular-level information on adsorbates at low metal and ligand concentrations in an in situ fashion; that is, data can be recorded with bulk water present at ambient pressures and temperatures. Another advantage in combining these methods is the highly complementary information provided. EXAFS spectroscopy provides structural information about the adsorbed metal ion and how it is attached to the mineral surface, whereas infrared spectroscopy primarily monitors the structure of the ligand. 2. Experimental Section 2.1. Chemicals, Solutions, and Suspensions. Deionized water (Milli-Q Plus), boiled to remove dissolved CO2, was used for all solutions and suspensions. Solution ionic strengths were adjusted to 0.1 M (Na)Cl with NaCl (Merck, p.a.) dried at 180 °C. In all experiments, pH adjustments were made with standardized NaOH (0.2 M) and HCl (0.1 M). We prepared a stock ligand solution by dissolving weighed amounts of dried (80 °C) Na2H2EDTA (Merck). In adsorption experiments, the EDTA was spiked with the 14C isotope of H4EDTA (Sigma) to obtain a total activity of ∼300 Bq/mL in suspension. For quantitative analysis, the scintillation cocktail Optiphase “High Safe 3” (Wallac) was used. An acidic (pH ∼1.5) gallium(III) stock solution ([Ga]tot ) 45.5 mM) was prepared from GaCl3 (Aldrich, 99.999%). The stock solution was standardized by an indirect EDTA-Pb(NO3)2 titration using xylenol orange as the indicator. For the adsorption experiments, a solution of the Ga(III)EDTA aqueous complex was prepared from stock solutions of Ga(III) and EDTA spiked with 14C isotope. All experiments were conducted at 25 °C and in the absence of light. The synthesis of goethite (R-FeOOH) and characterization of the needle-like particles have been described by Boily et al.15 In brief, goethite was prepared in polyethylene bottles by the addition of 2.5 M KOH (EKA, p.a.) to 10 L of 0.15 M Fe(NO3)3 (Merck, p.a.) at an approximate rate of 10 mL/min. The precipitates were aged for 96 h at 60 °C and dialyzed for 3 weeks. The resulting particles were identified to be goethite by X-ray powder diffraction, and the surface area was determined to be 94 m2/g using a N2 BET analysis. A stock goethite suspension was made with a background electrolyte concentration of 0.1 M NaCl. The total concentration of goethite was kept constant at 10 g/L in all adsorption experiments. 2.2. Adsorption Experiments. Adsorption experiments were carried out in batch mode in the pH range of 3 to 10 and at a background electrolyte concentration of 0.1 M (Na)Cl. The stock suspensions of goethite that were used for batch sample preparation were acidified to pH ∼5 and purged overnight with Ar(g). We prepared each batch sample by transferring an aliquot of a stock goethite suspension to a 15 mL polypropylene centrifuge tube, adding a volume of stock ligand solution, and adjusting the pH to a target value between 3 and 10 using standardized acid or base. All samples were diluted so that the total Ga(III)EDTA concentration was 2.3 µmol/m2 (2.16 mM) and the goethite concentration was 10 g/L. During batch sample
Nore´n and Persson preparation, the centrifuge tubes were continuously purged with Ar(g) to avoid carbonate contamination. After equilibrating at 25 °C on an end-over-end rotator for 7 days, the pH of each batch sample was measured with a combination electrode (Orion) that was calibrated with commercial buffers (Merck). The outer reference cell of this electrode was filled with 0.1 M NaCl. Prior to infrared and quantitative adsorption measurements, the samples were centrifuged at a relative centrifugal force (rcf) of 3240g for 20 min, and the supernatant was filtered through a 0.22 µm Millipore filter. As described below, the supernatant was analyzed to determine the concentration of adsorbed EDTA and Ga(III) and to check for dissolved iron. Small amounts of the supernatant and the wet mineral paste were also analyzed using infrared spectroscopy. 2.3. Analysis. We determined the amount of EDTA adsorbed at the water-goethite interface by measuring the concentration of ligand remaining in the supernatant and subtracting this value from the total ligand concentration. The concentration of EDTA in the supernatant was measured using liquid scintillation counting (LSC). Each unknown sample was first acidified with concentrated HCl (analytical grade) to pH ∼2 and then mixed with the scintillation cocktail Optiphase “High Safe 3” (Wallac). Samples were kept in the dark overnight before being analyzed with a Beckman LS6500 scintillation counter. For the gallium and iron analyses, duplicate samples were acidified to pH below 2 with concentrated HCl (analytical grade). Total metal ion concentrations were measured using flame atomic absorption spectrometry (Perkin-Elmer AAS 3110). In the case of iron, no significant dissolution of goethite was detected; all samples were around or below the detection limit for the analytical technique of 2 µM [Fe]tot. 2.4. Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. 2.4.1. EXAFS Sample Preparation. The EXAFS samples were prepared according to the procedure for the adsorption experiments described above. The sealed centrifuge tubes containing the suspensions were equilibrated for 6 to 7 days. Immediately prior to EXAFS data collection, pH was measured and the suspensions were centrifuged. The wet pastes were loaded into 2 mm thick Teflon cells, sealed with Kapton tape, and subsequently brought to the experimental hutch. The time between centrifugation and the beginning of the EXAFS data collection was always
