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Journal of Environmental Quality 32:1299-1305 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Ground Water Quality

Impact of Sample Preparation on Mineralogical Analysis of Zero-Valent Iron Reactive Barrier Materials

D. H. Phillips*, B. Gu, D. B. Watson and Y. Roh

Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, Oak Ridge, TN 37831

* Corresponding author (dhphillips2003{at}yahoo.com)

Received for publication March 11, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Permeable reactive barriers (PRBs) of zero-valent iron (Fe0) are increasingly being used to remediate contaminated ground water. Corrosion of Fe0 filings and the formation of precipitates can occur when the PRB material comes in contact with ground water and may reduce the lifespan and effectiveness of the barrier. At present, there are no routine procedures for preparing and analyzing the mineral precipitates from Fe0 PRB material. These procedures are needed because mineralogical composition of corrosion products used to interpret the barrier processes can change with iron oxidation and sample preparation. The objectives of this study were (i) to investigate a method of preparing Fe0 reactive barrier material for mineralogical analysis by X-ray diffraction (XRD), and (ii) to identify Fe mineral phases and rates of transformations induced by different mineralogical preparation techniques. Materials from an in situ Fe0 PRB were collected by undisturbed coring and processed for XRD analysis after different times since sampling for three size fractions and by various drying treatments. We found that whole-sample preparation for analysis was necessary because mineral precipitates occurred within the PRB material in different size fractions of the samples. Green rusts quickly disappeared from acetone-dried samples and were not present in air-dried and oven-dried samples. Maghemite/magnetite content increased over time and in oven-dried samples, especially after heating to 105°C. We conclude that care must be taken during sample preparation of Fe0 PRB material, especially for detection of green rusts, to ensure accurate identification of minerals present within the barrier system.

Abbreviations: Fe0, zero-valent iron • PRB, permeable reactive barrier • XRD, X-ray diffraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PERMEABLE FE0 REACTIVE barriers are emerging technologies that show great promise for passive, long-term treatment of contaminated ground water with redox-sensitive metals, radionuclides, and chlorinated organic compounds (Liang et al., 2000; Scherer et al., 2000; Gu et al., 2002). However, a potential limitation of the Fe0 reactive barrier technology is the deterioration of the Fe0 materials by surface passivation in ground water and the subsequent precipitation of minerals that may cause cementation and decreased permeability of the Fe0 barrier. A suite of these mineral precipitates have been reported, including akaganeite (ß-FeOOH), goethite ({alpha}-FeOOH), lepidocrocite ({gamma}-FeOOH), magnetite (Fe3O4), maghemite ({gamma}-Fe2O3), green rust I ([Fe3(II)Fe(III)(OH)8Cl], [Fe4(III)Fe2(II)(OH)12][CO3·2H2O]), green rust II ([Fe4(III)Fe3(II)(OH)12][SO4·2H2O]), amorphous FeS, mackinawite (FeS1-x), aragonite (CaCO3), calcite (CaCO3), and siderite (FeCO3) (Blowes et al., 1997; MacKenzie et al., 1999; McMahon et al., 1999; Gu et al., 1999; Phillips et al., 2000; Roh et al., 2000a, 2000b). Mineral precipitates in Fe0 PRBs may decrease surface reactivity, thus reducing the effectiveness and lifespan of the barrier (Phillips et al., 2000; Qui et al., 2000). The accumulation of precipitates—such as siderite, aragonite, green rusts, and iron oxyhydroxides—can cause cementation that may restrict flow and eventually clog the system (Liang et al., 2000; Roh et al., 2000a, 2000b; Phillips et al., 2000). Despite their negative impacts on Fe0 systems, research has shown that some of these precipitates such as green rusts can reduce the mobility of pollutants (i.e., NO-3 into NH+4, Se6+ into Se4+ or Se0, Cr6+ into Cr3+) and dechlorinate chlorinated organic compounds from ground water (Hansen et al., 1994, 1996; Erbs et al., 1999; Génin et al., 2001).

At present there are about 40 full- and pilot-scale in situ Fe0 PRBs installed in North America (Permeable Reactive Barriers Action Team, 1995). Mineralogical characterization plays a large role in evaluating the performance of these barriers. Transformations of certain Fe mineral phases can occur during sample preparation for mineralogical identification, affecting the mineralogical characterization of corrosion products and interpretations of barrier processes. This is of great concern with material from Fe0 barriers or treatment columns, because much of this material is removed from the reduced and labile conditions within these Fe0 systems during sampling (Phillips et al., 2000). These samples can oxidize in aerobic conditions during preparation and processing, resulting in changes in the mineralogical composition. Thus, accuracy of the identification of the Fe minerals present within the barriers can be problematic if care is not given during sample preparation. Mineralogical information taken from artifacts formed during sample preparation can result in inaccurate calculations of the rates of Fe0 passivation, mineral buildup, and thus the lifespan of a Fe0 PRB. Additionally, a better characterization of mineralogical composition may greatly improve our understanding of the mechanisms and kinetics of chemically and/or biologically mediated reactions that ultimately determine the performance and extend the use of reactive barriers for ground water treatment.

Procedures are needed on the preparation and analysis of mineral precipitates from Fe0 barrier material to identify accurately the minerals present in these barriers without introducing artifacts. Additionally, a generalized procedure should be developed so that comparable results from different PRBs can be made. Accordingly, this paper focuses on X-ray diffraction (XRD) analysis for mineral identification, as it is a readily available analytical tool that offers the convenience of quick processing (15 min) and detection of crystalline minerals. The objectives of this study are (i) to investigate a quick and convenient method to prepare material from Fe0 barriers for mineralogical analysis by XRD that accurately confirms the presence or absence of minerals without inducing artifacts, and (ii) to identify Fe mineral phases and their transformations brought on by different preparation techniques under aerobic conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Materials
Zero-valent iron material was collected by a split-spoon sampler from an in situ reactive Fe0 barrier by coring 4.5 to 9 m (15–30 feet) below the surface of the barrier in May 2000. One core was collected that was oxidized, and the Fe0 filings were cemented by precipitates and corrosion by-products. In contrast, another core was collected that was reduced and the Fe0 filings were loose (Fig. 1) . The barrier was installed in late November 1997 at the Y-12 site on the Oak Ridge Reservation, Oak Ridge, TN (Phillips et al., 2000). The Fe0 filings (between -8 and +25 mesh size) used in the reactive barrier were obtained from Peerless Metal Powders and Abrasives, Detroit, MI. These Fe0 filings were composed of 86% Fe0, 3 to 4% C, 3% silicon, and trace amounts of magnetite (Fe3O4) and other minerals (Roh et al., 2000a).



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  		Fig. 1. Location of (a) angled cores in the Fe0 portion of the barrier and (b) vertical core through the whole Fe0 PRB. Figure not to scale.

 
Immediately after the cores were removed from the PRB, the ends of the cores were sealed with plastic stoppers. To minimize oxidation, argon was purged through slits made at the ends of the stoppers, then the cores were stored from 3 d to 2 wk in Ar-purged, gas-tight PVC tubes. Argon was recharged in the PVC tubes every 2 to 3 d. The core material was prepared under ambient air conditions to examine the changes in mineralogy using equipment that can be found and used in most laboratories.

Sample Preparation
Drying
Samples (approximately 350 g) were removed from the two undisturbed cores containing Fe0 barrier material and mixed well (Fig. 2) . While preparing the samples for analysis, no special equipment was used, such as anaerobic glovebags/gloveboxes. The sample from the reduced core material was split into two portions. In one portion (Sample 1), duplicate samples of very fine material (<20 µm) that is easily suspended in water was plated onto millipore filter paper (0.45 µm) using a 2:1 acetone–water mixture under vacuum until dry (Roh et al., 2000a). The water was added to keep the filter paper from dissolving. The other portion, Sample 2, was split into four subsamples. Sample 2a was acetone-dried, sample 2b was air-dried overnight, and samples 2c and 2d were placed in ovens overnight at 65 and 105°C, respectively. For the acetone-dried sample (Sample 2a), the whole sample was placed on a Whatman 42 filter paper in a Buckner filter and acetone-dried under vacuum. During the process, the sample was stirred with a spatula as high purity acetone (99.7%) was applied with a plastic squeeze bottle. The sample was left on the filter under vacuum until completely dry after acetone was added. The sample color lightened from black (10YR 3/1) to gray (7.5YR 4/0) when completely dried. The oxidized, cemented sample had to be crushed by grinding with an agate mortar and pestle. The acetone-drying preparation procedure for the whole sample (Sample 2a) was repeated on the material from the crushed oxidized core material (Sample 3).



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  		Fig. 2. Flowchart of the different mineralogical preparation treatments.

 
One half of the acetone-dried Sample 2a (Sample 2a-1) was used to determine the mineralogy of the different size fractions of the Fe0 PRB material and to examine which minerals form on the surface of the Fe0 filings. The different size fractions include coarse (>150 µm), medium (125–150 µm), and fine material (<125 µm) samples, which were separately ground and sieved.

Grinding and Sieving
The other half of Sample 2a (Sample 2a-2), all of Samples 2b through 2d, and all of Sample 3 were ground with hand-pressure for 5 min with an agate mortar and pestle. The samples were then sieved by manual shaking for 5 min to separate coarse particles (>150 µm, mostly Fe0 filings) from the fine material (<125 µm, very small Fe0 filing fragments and larger precipitates that have detached from the larger Fe0 filings). The coarse fraction (>150 µm) was hand ground and sieved two more times to obtain adequate removal of surface precipitates. This process was also repeated on the medium fraction (125–150 µm). The fine material (<125 µm) was immediately placed on the XRD for analysis without further grinding and sieving. The total sample preparation time was about 15 min.

Mineralogical Analysis
X-Ray Diffraction
Duplicate boxed mounts (Jackson, 1975) were made with the fine material from each sample. The acetone-dried samples from the reduced core material were analyzed 0, 2, 4, 6, 8 and 24 h after collection, excluding about 15 min of sample preparation time. The acetone-dried samples from the oxidized, cemented area of the barrier were analyzed at 0, 1, 3 and 11 d after collection. The oven- and air-dried samples were analyzed at T = 24 h. All XRD analyses for mineralogical characterization were performed with a Scintag XDS 2000 diffractometer (Scintag, Sunnyvale, CA) operating at a scan rate of 2° 2{theta}/min. Cobalt K-{alpha} radiation ({lambda} = 0.179026 nm) was used for the XRD analyses to minimize fluorescence of Fe-rich minerals by the usual Cu radiation.

Scanning Electron Microscopy–Energy Dispersive X-ray Analysis
Selected undisturbed samples were carbon coated with a Bio-Rad carbon sputter coater and immediately examined with a JEOL ISM 35CF scanning electron microscope equipped with an energy dispersive X-ray analyzer (EDX) (Tokyo, Japan) to examine the morphology of the minerals and confirm the mineralogy.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Mineral Identification of Precipitates in Various Size Fractions
Samples of precipitates from the Fe0 filings were finely ground for XRD analysis. Grinding samples in an agate mortar removed precipitates thoroughly and allowed for rapid analysis, which is particularly important when unstable minerals are present. The finely ground material was pressed into a boxed mount, which minimizes preferred orientation of the minerals. Random orientation of nonplaty minerals, such as those found in barrier material, works well for mineral identification by XRD.

Zero-valent Fe (52.34° 2{theta}), maghemite/magnetite (41.76° 2{theta}), aragonite (30.53° 2{theta}), and quartz (24.28° 2{theta}) were detected in the >150 µm corrosion products. Minerals identified from the 125 to 150 µm fraction included Fe0, maghemite/magnetite, calcite (34.28° 2{theta}), aragonite, quartz, and green rusts (13.76° 2{theta}) (Sample 2a-1) (Table 1 ; Fig. 3 and 4) . Products from the <125 µm fraction were similar to those identified in the 125 to 150 µm fraction. Mineralogy of Sample 1 collected on the filter (<20 µm) (Sample 1) appeared to be mainly green rusts, maghemite/magnetite, and quartz, and have been found previously in barrier material (Phillips et al., 2000; Roh et al., 2000a). Therefore, due to differences in mineralogy among various size fractions, whole sample preparation appears to be necessary.


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  		Table 1. Summary of selected minerals identified in X-ray diffractograms.

 


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  		Fig. 3. X-ray diffractograms showing the minerals present in different size fractions of acetone-dried, reduced loose core material (Samples 1 and 2a-1) (A, aragonite; C, calcite; F, zero-valent iron; GR, green rust; M, maghemite/magnetite; Q, quartz).

 


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  		Fig. 4. SEM micrographs of selected minerals (a) green rust, (b) siderite, (c) goethite with elongate aragonite crystals, and (d) aragonite from an unsieved whole sample. Arrows indicate points represented by EDX analysis (insets).

 
Acetone Drying vs. Air- and Oven-Drying
Green rusts, maghemite/magnetite, Fe0, quartz, and aragonite were detected in acetone-dried material from Sample 2a-2 immediately after collection (Fig. 5) . The peak intensities of green rusts (13.76° 2{theta}) decreased within 2 h of collection, then disappeared thereafter (Fig. 5). The peaks at 13.76° 2{theta} obtained at 4 and 24 h after collection may be attributed to akaganeite, which overlaps the green rusts peak. Peaks from akaganeite intensified, then broadened between 4 and 24 h after collection. A duplicate of Sample 2a-2 was prepared to confirm the disappearance of green rusts. In the duplicate sample, the green rusts peak decreased through 8 h. Samples containing green rusts are usually analyzed quickly or in anaerobic conditions (Simon et al., 1997) and green rusts transform to goethite ({alpha}-FeOOH), lepidocrocite ({gamma}-FeOOH), ferrihydrite (Fe2O3·2FeOOH·2.6H2O), maghemite ({gamma}-Fe2O3), or magnetite (Fe3O4), depending on the rate of oxidation and dehydration (Hansen et al., 1994; Abdelmoula et al., 1996; Doner and Lynn, 1977). Therefore, samples from Fe0 reactive barriers prepared in aerobic conditions should be analyzed as soon as possible to detect green rusts. However, Hansen (1989) reported that oxidation of green rusts in XRD samples can be delayed by mixing glycerol with the sample during preparation.



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  		Fig. 5. X-ray diffractograms showing the minerals present in samples analyzed at different times since sampling of the acetone-dried treatment on the reduced loose core material (Sample 2a-2) (A, aragonite; F, zero-valent iron; GR, green rusts; K, akaganeite; M, maghemite/magnetite; Q, quartz).

 
Sample 3 was from a cemented area within the barrier. The oxidized samples contain iron minerals of greater crystallinity than minerals from reduced samples as shown by the sharpness and number of peaks (Fig. 5 and 6) . Lepidocrocite (16.81° 2{theta}), goethite (24.73° 2{theta}), mackinawite (20.09° 2{theta}), and siderite (28.68° 2{theta}) are present in the oxidized environment of the barrier, but were not detected in the reduced core material. Quartz and aragonite are present throughout the barrier in both oxidized and reduced samples (Fig. 5 and 6). Siderite, which occurs in hydromorphic soils and sediments, oxidizes when these reduced zones are drained (Doner and Lynn, 1977). However, in the samples collected from this cemented and oxidized area within the Fe0 barrier, siderite is stable and present, even after >11 d in the acetone-dried samples as well as in air- and oven-dried samples.



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  		Fig. 6. X-ray diffractograms showing the minerals present for samples analyzed at different times since sampling (T = 0 h and T = 11 d) of acetone-dried oxidized cemented core material (Sample 3) (A, aragonite; F, zero-valent iron; G, goethite; L, lepidocrocite; M, mackinawite; S, siderite; Q, quartz).

 
As expected, green rusts were not observed in the air- and oven-dried samples (2b, 2c, and 2d) 24 h after collection (Fig. 7) . The air- and oven-dry samples have a similar assemblage of minerals containing maghemite/magnetite, calcite, aragonite, quartz, and Fe0 (Fig. 7). The maghemite/magnetite peak intensity increased in the 105°C oven-dried sample (Sample 2d) compared with the 65°C oven-dried (Sample 2c) and air-dried (Sample 2b) samples because the oxidation and dehydration of amorphous iron oxyhydroxides increased its crystallinity (Doner and Lynn, 1977). Sample color also became redder with drying temperature from 10YR 3/3 for the air-dried sample, 7.5YR 3/2 for the 65°C, and 5YR 3/3 for the sample dried at 105°C.



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  		Fig. 7. X-ray diffractograms showing the minerals present for the air-dried (25°C) and oven-dried (65 and 105°C) treatments of the reduced loose core material (Samples 2b–d) (A, aragonite; C, calcite; F, zero-valent iron; M, maghemite/magnetite; Q, quartz).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
There is a need for procedures in preparing and analyzing mineral precipitates from Fe0 barrier materials because some iron minerals are sensitive to oxidation and affected by sample preparation techniques. We found that different sample preparation techniques affected the apparent mineralogy of the Fe0 PRB material (Table 1). Hand grinding and sieving whole samples for analysis was necessary because there was a variation in mineral precipitates in the PRB material within the different size fractions of the samples. Transformations of iron oxides caused by contact with oxygen were slowed by drying from acetone under vacuum and rapid analysis immediately after sample collection. For example, green rusts were detected in acetone-dried samples of loose and reduced core material only within 2 h of sampling and not detected in the air- and oven-dried samples taken from the same material. In the acetone-dried samples the green rusts transformed into akaganeite after 2 h. Maghemite/magnetite XRD peaks intensified with time and increased with drying temperature in the loose and reduced core material. Therefore, to identify accurately those minerals present within barrier systems, care must be taken during sample preparation of Fe0 PRB material, especially to detect green rusts. It is of practical importance to monitor these minerals because green rusts have been found to be beneficial in the removal of sequestration of certain contaminants such as nitrate, radionuclides, and chlorinated organic compounds from ground water. Additionally, an accurate representation of minerals in the Fe0 barrier may improve our predictive capabilities of the rates of Fe0 passivation, mineral buildup, and thus the lifespan of Fe0 PRBs.


   ACKNOWLEDGMENTS
 
The authors thank N.D. Farrow, K. Hyder, and G. Houser for the collection of the core material. Funding for this research was supported by the Subsurface Contaminants Focus Area, Office of Environmental Management (EM) of the U.S. Department of Energy. Oak Ridge National Laboratory (ORNL) is managed by UT-Battelle LLC under contract DE-AC05-00OR22725. D.H. Phillips was supported by the postdoctoral research program administered by Oak Ridge Institute for Science and Education.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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