ROBERSON Mark, AMRHEIN Christopher, HUNT Mathew. Dept. of Soil and Environmental Sciences, Univ. of Calif., Riverside, USA

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1 Scientific registration n o : 1942 Symposium n o : 6 Presentation: poster X-ray absorption near edge spectroscopy for the identification of selenium species on zero-valent iron Identification des espèces du sélénium sur Fe(0) par la XANES ROBERSON Mark, AMRHEIN Christopher, HUNT Mathew Dept. of Soil and Environmental Sciences, Univ. of Calif., Riverside, USA Introduction Reuse of drainage waters originating from irrigated agriculture is gaining in popularity due to greater demands for existing water resources. A portion of the drainage water originating in the Imperial Valley of California is used to support several thousand acres of constructed duck-clubs and wetlands adjacent to the Salton Sea. These wetlands are noted for the variety of birds supported on a yearly and seasonal basis. After flowing through the wetlands, the drainage water enters the Salton Sea where it supports a large fisheries population. Paramount of concern in the reuse of this irrigation water is the water quality, particularly contamination due to the trace element selenium. Therefore, in order to improve the water quality, effective selenium removal methods must be developed. One promising technique is the use of zero-valent iron which serves as both a catalyst and an electron source for the reduction of selenate to reduced forms of selenium. The use of zero-valent iron to remove, reducible, toxic compounds has increased dramatically in the last few years (Blowes and Ptacek, 1992). A typical treatment strategy is to expose contaminated water to a bed or reactive wall of iron filings. The solution moving through the reactive barrier goes through a redox process where oxidized compounds are reduced to less toxic and or less mobile compounds. A common application for reactive barriers is the reduction of chlorinated organic compounds to chloride and short chain hydrocarbons (Gillham and O Hannesin, 1994). However, there are several situations where oxy-anions are being treated with zerovalent iron (Blowes et al, 1997 and Bostick et al., 1997). The objectives of this research were to determine the final selenium products formed when zero-valent iron is used as the catalyst and electron source for the reduction of selenate. Also, the influence of environmental factors on the species of selenium formed were investigated. Variables included ph, O 2, ionic strength, and solution composition. Materials and Methods Reactions between zero-valent iron metal and aqueous solutions were carried out in 1.3 L polyethylene containers. Table 1 lists the aqueous conditions used to prepare metal samples. Selenate was added at an elevated concentration of 35 um to ensure XANES detection. The iron used in all reactions was coarse, 40 mesh, iron filings obtained from Fisher. To minimize surface area variations, the filings were size 1

2 separated into a mesh fraction. Prior to use, the filings were washed for 1 hour in 1 N HCl, rinsed with deoxygenated water and dried with low heat under N 2 gas. BET-N 2 surface area analysis gave a surface area of about 3 m 2 g -1. After filling the reactor with prepared solutions, prepared iron filings were added at a ratio of 5 g L -1. Reactions carried out without O 2 were continuously flushed with N 2 gas. Reaction ph was maintained with a Brinkmann ph stat model E655. Aqueous samples collected to determine selenate loss were stored in 6N HCl prior to analysis. Aqueous selenium analysis was done using hydride generation atomic absorption spectroscopy. To ensure adequate surface loading of selenium, all reactions were carried out for 24 hours. Iron and iron-hydroxide samples collected for X-ray absorption near edge spectroscopy (XANES) were obtained by collecting the filings after the reduction reaction. Filings were dried with low heat under N 2 gas. For reactions carried out in oxygenated conditions the iron oxide product was separated from the metal through mechanical agitation and filtration. XANES analysis was conducted using Beam line 4-2 at the Stanford Synchrotron Radiation Laboratory. The first inflection of the absorption edge for Se was assumed to be at ev. A Si(220) double crystal monochrometer with a 1 mm upstream aperture was used. Samples were scanned twice and an average value of the two scans was recorded. Standards included FeSe, hexagonal metallic elemental Se, and sodium salts of selenite and selenate. Table 1. Solution composition and environmental variables. Sample ph Gas Ionic Strength Salt 1 5 N 2 1 mm NaCl 2 7 N 2 1 mm NaCl 3 free drift O 2 20 mm NaCl 4 free drift O mm Salton Sea 5 5 O 2 1 mm NaCl 6 free drift O 2 1 mm NaCl 7 7 O 2 1 mm NaCl 9 Rust product, Sample 6 10 Rust product, Sample 4 11 Rust product, Sample 3 Results and Discussion Figure 1 shows that the reduction of selenate with zero-valent iron followed a pseudo first order reaction rate. In addition, an increase in ionic strength, O 2 concentration or ph decreased the rate of selenate loss. Reactions carried out using water obtained from the Salton Sea of California had a significantly lower rate of selenate reduction than samples reacted with synthetic drain waters. The absorption edge for the selenium standards are shown in Figure 2. Due to the loss of valence electrons, greater energy is required to eject core-level electrons as the oxidation state of selenium increases. Quantitative analysis using edge spectra consists of a least squares fit of linear combinations of the spectra of the standards to the spectra of the unknown (Tokunaga personal comm., Pickering, et al., 1995 and Wong et al., 1984). However, the fit between the standards and the samples was not exact so calculation of selenium distribution was achieved by matching curve shapes of 2

3 the samples to the standards. The most probable reason for the poor fit was that model compounds of Se that might be found in the samples were not used as standards. For example, the standard for selenite was sodium selenite. A good model compound for selenite would have been an iron-hydroxide compound, such as geothite, with adsorbed selenite. Spectrum analysis indicates the absence of selenate from all samples. Selenite was present in both the metal and iron-hydroxide phases from the reactions carried out with O 2. As shown in Figure 3, with O 2 present, a shoulder appears around ev suggesting a shift in the ratio of selenite to elemental or selenide. Elemental selenium was the predominant species found on all metal samples however, with a decrease in ionic strength there was an increase in the ratio of reduced selenium compounds. Figure 4 shows the metal phase from reactions with three ionic strength solutions. As the ionic strength of the solution decreased, the oxidation state of the absorbed selenium was reduced. The spectra shows that the 1 mm NaCl solution had a greater ph 7, 1 mm NaCl, N2 ph 5, 20 mm NaCl, N2 ph 5, 1 mm NaCl, O2 ph 5, 1 mm NaCl, N Time (hours) Figure 1. Influence of ph, ionic strength and redox level on the reduction of selenate with zero-valent iron. Data shown is the linear fit for the various reactions. 3

4 selenate - SeO 4 2- selenite - SeO3 2- selenium - Se selenide - FeSe Figure 2. Normalized standard spectra for the model compounds of FeSe, hexagonal elemental selenium, sodium selenite and sodium selenate. FeSe was used with the permission of I. Pickering, selenite was used with the permission of T. Tokunaga. percentage of selenide than the 20 mm NaCl or Salton Sea water. The 20 mm NaCl solution had a greater portion of elemental selenium but less selenide than the 1 mm NaCl solution. Additionally, the spectra obtained for the Salton Sea water shows considerable noise when compared with the spectra for the 1 or 20 mm NaCl solutions due to the lower concentrations of selenium in the Salton Sea water. Sample 7 Sample 1 Figure 3. Sample 1 was under N 2 gas, ph 5. Sample 7 was under O 2, ph 7. Sample 7 shows an increase in the proportion of Se(-II) and a decrease in Se(0). 4

5 Sample 3 Sample 4 Sample 6 Figure 4. Metal phase samples from reactions free drift reactions containing O 2. Sample 6 had an ionic strength of 1 mm NaCl. Sample 3 was 20 mm NaCl, and Sample 4 (Salton Sea water) was approximately 100 mm of mixed salts. Figure 5 shows the spectra obtained for both the metal and ironhydroxide phases for an aerobic, free drift reaction of low ionic strength. The metal phase data indicates a greater proportion of selenide compared to the iron-hydroxide phase. The initial absorption edge for both phases indicates that the major component of the samples was elemental selenium. However, the greater prominence of the second peak in the metal phase is used to indicate the presence of selenide. Sample 6 Sample 9 Figure 5. Spectra obtained for both the metal (sample 6) and iron-hydroxide (sample 9) for an aerobic, ph free-drift reaction at low ionic strength. 5

6 Conclusions Reduction and precipitation of selenium by zero-valent iron can be an effective treatment option. Selenate was reduced to selenide and elemental selenium under various conditions. Further work is ongoing to characterize the mineralogy of the iron-hydroxide precipitates and their influence on the reduction reaction. References Bostick, W.D., R. Jarabek, J. Fiedor, J. Farrell, R. Helferich Zerovalent iron for the removal of soluble uranium in simulated DOE site groundwater. In Proceedings International containment conference. St. Petersburg, FL Feb Gillham, R. W. and S.F. O Hannesin Enhanced degradation of halogenated aliphatics by zero-valent iron. Ground Water, 32, Wong, J., F.W. Lytle, R.P. Messmer and D.H. Maylotte K-edge absorption spectra of selected vanadium compounds. Physical Review B. vol. 30: Blowes, D.W., C.J. Ptacek and J. Jambor In-situ remediation of Cr(VI)- contaminated ground water using permeable reactive walls: laboratory studies. Environ. Sci. and Tech. vol. 31. no 12, Pickering, I, G.E. Brown and T. Tokunaga Quantitative speciation of selenium in soils using x-ray absorption spectroscopy. Environ. Sci. and Tech. vol. 29. no 9, Keywords: selenium, zero-valent iron, reduction, drainage, XANES Mots clés : sélénium, Fe(0), réduction, drainage, XANES 6