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Published in J. Environ. Qual. 32:2033-2045 (2003).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Ground Water Quality

Mineralogical Characteristics and Transformations during Long-Term Operation of a Zerovalent Iron Reactive Barrier

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

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

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

Received for publication October 31, 2002.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Design and operation of Fe0 permeable reactive barriers (PRBs) can be improved by understanding the long-term mineralogical transformations that occur within PRBs. Changes in mineral precipitates, cementation, and corrosion of Fe0 filings within an in situ pilot-scale PRB were examined after the first 30 months of operation and compared with results of a previous study of the PRB conducted 15 months earlier using X-ray diffraction and scanning electron microscopy employing energy dispersive X-ray and backscatter electron analyses. Iron (oxy)hydroxides, aragonite, and maghemite and/or magnetite occurred throughout the cores collected 30 mo after installation. Goethite, lepidocrocite, mackinawite, aragonite, calcite, and siderite were associated with oxidized and cemented areas, while green rusts were detected in more reduced zones. Basic differences from our last detailed investigation include (i) mackinawite crystallized from amorphous FeS, (ii) aragonite transformed into calcite, (iii) akaganeite transformed to goethite and lepidocrocite, (iv) iron (oxy)hydroxides and calcium and iron carbonate minerals increased, (v) cementation was greater in the more recent study, and (vi) oxidation, corrosion, and disintegration of Fe0 filings were greater, especially in cemented areas, in the more recent study. If the degree of corrosion and cementation that was observed from 15 to 30 mo after installation continues, certain portions of the PRB (i.e., up-gradient entrance of the ground water to the Fe0 section of the PRB) may last less than five more years, thus reducing the effectiveness of the PRB to mitigate contaminants.

Abbreviations: EDX, energy dispersive X-ray • Fe0, zerovalent iron • PRB, permeable reactive barrier • SEM, scanning electron microscopy • XRD, X-ray diffraction


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A NUMBER OF IN SITU AND EX SITU Fe0 PRBs have been used in North America and Europe as a means to remediate hazardous organic compounds (Vogan et al., 1999; Roh et al., 2000c; Comfort et al., 2001), radionuclides (Gu et al., 1998, 2002a; Phillips et al., 2000; Roh et al., 2000b,c), and heavy metals (Blowes et al., 1997) from contaminated ground water and soils. In Fe0 PRBs, iron corrosion in oxygenated ground water produces Fe2+, Fe3+, and OH- ions, while Fe corrosion in anaerobic systems produce Fe2+, OH-, and H+ ions. Iron (oxy)hydroxides and other minerals will form as Fe corrosion continues in PRBs, which will decrease porosity and Fe0 reactivity (Puls et al., 1999b).

Numerous studies of different PRB systems in operation have shown that mineral precipitation within and between different PRBs will vary broadly because of differing ground water geochemistries (Liang et al., 2000). After eight months of operation, goethite was the most abundant Fe (oxy)hydroxide phase present in a full-scale Fe0 PRB at the U.S. Coast Guard Center at Elizabeth City, NC (Blowes et al., 1997). Within two years of operation of this PRB, Fe (oxy)hydroxides coated Fe surfaces from areas where the pH was between 9 and 10.7 and Eh was generally between -250 and -550 mV. Additionally, green rusts are thought to have formed where SO2-4 was partially reduced in the ground water (Puls et al., 1999a). Coatings of FeS were also observed on mineral surfaces under alkaline conditions (pH = 7.5–9.9) in a small-scale field Fe0 system near the full-scale Fe0 PRB at the U.S. Coast Guard Support Center near Elizabeth City, NC after 20 months of operation. Amorphous FeS and mackinawite were observed in the X-625 ground water treatment facility, Portsmouth, OH (Roh et al., 2000a). Calcium and Fe carbonate were identified in samples collected from a Fe0 gate about one year after installation of the PRB at the Denver (Colorado) Federal Center where bicarbonate was high in the ground water (McMahon et al., 1999) and Ca2+ concentration within the PRB decreased (Odziemkowski et al., 1998). After two years of operation, a funnel and gate Fe0 barrier system at a former industrial site in upstate New York had significant amounts of aragonite and calcite at up-gradient interfaces within the Fe0 portion of the barrier. This portion was slightly aerobic and pH was near neutral. Aragonite and calcite were also present in the midsection of the barrier where the pH was 9 to 10 and the Eh was generally less than -300 mV. Further down-gradient, there were fewer carbonate minerals. A few samples had green rust complexes of chloride, carbonate, and sulfate. Iron oxides, Fe (oxy)hydroxides, and minor amounts of siderite were randomly distributed (Vogan et al., 1999). However, the PRB at Bordon, ON, Canada showed no mineral precipitates after one year of operation even though losses of 105 mg L-1 Ca2+ and 82 mg L-1 of HCO-3 were detected across the barrier (Shoemaker et al., 1995).

The accumulation of these precipitates can result in clogging or plugging of PRBs, which generally occurs at the entrance of the system (Mackenzie et al., 1999; Phillips et al., 2000). Amorphous ferric oxyhydroxides bridge or cement the individual Fe0 filings together to effectively block pore space from water flow (Mackenzie et al., 1999). Other identified cementing agents are aragonite (CaCO3), siderite (FeCO3) (Roh et al., 2000a), and amorphous FeS (Phillips et al., 2000).

Bicarbonate and sulfate enhance Fe0 corrosion (Odziemkowski et al., 1998), which will cause Fe0 filings to disintegrate, such as cracking or breaking apart, and consequently deplete the reactive media (Phillips et al., 2000). After 15 months of operation, more corrosion occurred in cemented samples compared with loose individual Fe0 filings from the Y-12 in situ Fe0 PRB on the Oak Ridge (Tennessee) Reservation. Corrosion appeared as approximately 10- to 150-µm-thick rinds that degenerated approximately 15 to 30% of these Fe0 filings (Phillips et al., 2000).

Little is reported on the effects of mineral precipitation, cementation, and media depletion in field-scale in situ Fe0 PRB systems after 1.5 years of operation. The objectives of this research were to investigate the changes in mineral precipitates, cementation, and Fe0 corrosion within the Y-12 in situ field-scale PRB after the first 30 months of operation, and compare these with results of a previous study of this PRB conducted 15 months earlier.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Site
The source of the contamination, the S-3 disposal ponds, consists of four unlined ponds constructed in 1951 on the west end of the Y-12 plant, Oak Ridge, TN. The ponds had a storage capacity of 40 million liters. Liquid wastes, composed primarily of nitric acid plating wastes, containing various metals (e.g., Ni, Cr, Co, Cd, Zn, and Pb), radionuclides (e.g., uranium and technetium), and volatile organic chemicals (e.g., tetrachloroethene), were disposed in the ponds until 1983. In 1984, nitric acid wastes that remained were neutralized and denitrified and the ponds were capped with a large paved asphalt parking lot (Fig. 1a) .



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  		Fig. 1. (a) Location of the Fe0 permeable reactive barrier (PRB) at the study site; (b) aerial view of the PRB showing the sampling locations of the cores and ground water wells; and (c) side view of the PRB showing the angled cores through the barrier material.

 
Geo Con (Oak Ridge, TN) completed construction of the Fe0 PRB in late November 1997 at the Oak Ridge Y-12 site near the S-3 disposal ponds at the primary uranium migration pathway to Bear Creek (Fig. 1a). This barrier is 68.6 m long, 61.0 cm wide, and approximately 9.1 m deep and composed of Fe0 filings in a 7.9-m-long midsection between two 30.3-m sections of pea gravel (Fig. 1b). The Fe0 filings (Peerless Metal Powders and Abrasives, Detroit, MI) are approximately 3 to 24 mm long (majority at >10 mm) and 0.5 to 1.5 mm thick (about -1/2 and 25 mesh size). The Fe0 filings were composed of about 86% Fe0, 3 to 4% carbon, and 3% silicon and other elements, and small amounts of magnetite (Fe3O4) in Fe0 grains (Roh et al., 2000a). A heterogeneous mixture of fill materials (mainly soil), saprolite, and rock fragments surround the Fe0 PRB. Native Nolichucky shale saprolite occurs near the bottom of the PRB. Ground water flow toward the PRB is generally from the northeast direction; however, it also enters the Fe0 portion of the PRB from up-gradient northern and eastern flow directions. General flow patterns have not changed since PRB installation. Ground water exits at the southern and western down-gradient ends of the PRB. Other details on the hydrology are given in Gu et al. (2002a).

Uranium-contaminated ground water (approximately 1 mg L-1) migrates to the upper reach of Bear Creek (Fig. 1a) and discharges through adjoining seeps. Eastern up-gradient background well TMW12 has U ground water concentrations that average 0.2 ± 0.1 mg L-1 over the first 30 mo of operation (Table 1). However, U concentrations ranging from 0.4 to 1 mg L-1 were measured for DP13, a well about 3.5 m up-gradient from the Fe0 portion of the PRB (Gu et al., 2002a). The lowest U concentration is 0.006 ± 0.01 mg L-1 at DP19M in the deep western down-gradient where ground water exits the PRB. Uranium concentrations range from 0.06 ± 0.1 mg L-1 at DP19S to 0.3 ± 0.1 mg L-1 at DP22M in other sections of the PRB. Therefore, U appears to be scavenged from the ground water before it reaches DP19M. Nevertheless, uranium minerals were not detected by X-ray diffraction (XRD) or scanning electron microscopy with energy dispersive X-ray analysis (SEM–EDX) in the core samples collected after 15 mo (Phillips et al., 2000) and 30 mo of operation (data not shown). This may be due to a combination of the precipitation of U minerals below a detectable limit and low U concentration in the ground water. Therefore, more time may need to lapse before detectable amounts of U minerals form in the PRB.


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  		Table 1. Geochemistry of the ground water samples from the eastern up-gradient background well (TMW12) and the multilevel piezometers within the Fe0 portion of the barrier (DP19 and DP22) collected periodically during the 30 mo of permeable reactive barrier (PRB) operation.{dagger}

 
Nitrate concentrations average 66.2 ± 47.1 mg L-1 in the background TMW12 well. Compared with the NO-3 values from TMW12, the NO-3 concentrations decreased noticeably in the Fe0 portion of the PRB from 32.9 ± 30.7 mg L-1 at DP22M to 3.5 ± 9.9 mg L-1 at DP19M (Table 1). However, levels above 1000 mg L-1 were detected in some deeper piezometers due to an upward vertical hydraulic gradient (Gu et al., 2002a). Technetium is usually detected at levels below 600 pCi L-1, and total dissolved solid concentrations are approximately 1000 mg L-1 (Gu et al., 2002a).

Core Sampling
Cores were collected from the barrier in February 1999 (15 mo after installation) (Phillips et al. 2000) and in May 2000 (30 mo after installation). The cores collected 30 mo after installation, Cores 1 and 2 (Fig. 1b,c) were taken from two sections (fans) along the barrier at shallow (4.6–6.4 m) and deep (6.4–8.2 m) levels near zones sampled after the first 15 mo of operation (Cores A and C) (Fig. 1b,c). The northern up- and southern down-gradient interfaces of the PRB and soil and fill material were intercepted by coring at 60° angles. The cores collected after 15 mo of operation were excavated similarly (Phillips et al., 2000). A core was taken from each sampling point by driving the corer containing a polyurethane tube (1.8 m long x 3.8 cm in diameter) through a hollow barrel into the PRB material. The polyurethane tubes were cut to size and sealed with plastic caps immediately after they were removed from the PRB. Argon gas was injected into the cores through small incisions at each end of the plastic caps. These incisions were immediately sealed with tape after injection. The cores were stored in Ar purged airtight PVC tubes to minimize iron oxidation. During the period between sampling and sample preparation (1–3 wk), the PVC tubes were stored at 4°C and were purged with Ar twice a week. Generally, 0.9 to 1.4 m of Fe core material and 0.6 m of soil and fill material from northern up- and southern down-gradient interfaces were collected in the tubes. A single core was sampled that intercepted both up- and down-gradient interfaces of the PRB material and soil and fill to avoid spillage and consequent mixing and loss of PRB material. During preparation, the PRB material from each coring point was separated into three to five segments (usually approximately 30.5 cm in length).

Mineralogical Characterization
For each sample, mineralogical analysis was carried out according to Phillips et al. (2003). About 50 g of Fe0 PRB core material was placed on Whatman (Maidstone, UK) 42 filter paper in a Buckner funnel and acetone-dried under vacuum. This was repeated on the samples from the cemented core material, after being crushed and ground under hand pressure for 5 min using an agate mortar and pestle. The coarse (>150 µm), medium (150–125 µm), and fine (<125 µm) fractions were separated through sieving by manual shaking for 5 min. Coarse and medium fractions were each hand ground and sieved through 150- and 125-µm sieves, respectively, two more times to obtain adequate removal of surface precipitates. The fine material collected was immediately analyzed by an X-ray diffractometer (XDS 2000 diffractometer; Scintag, Sunnyvale, CA) operated at 40 kV and 35 mA.

Selected samples of loose and cemented Fe0 PRB core material collected during the 15- and 30-mo sampling visits were placed in EPO-THIN low-viscosity epoxy (Buehler, Lake Bluff, IL) (100 parts resin to 36 parts hardener) and the resin was allowed to move into micropores under vacuum over night. After the epoxy hardened, the samples were sliced using an ISOMET low-speed saw (Buehler) and polished with 6- and 1-µm Buehler METADI diamond suspensions (Buehler). The polished sections were then carbon-coated using a Pelco CC-7A carbon sputter coater (Ted Pella, Redding, CA) and examined with a JEOL (Peabody, MA) JSM-35CF and a Philips (Eindhoven, the Netherlands) 30 XL scanning electron microscope (SEM) at 15 keV equipped with energy dispersive X-ray (EDX) and backscatter electron (BSE) analyzers. These polished sections were used to assess the lifespan of the PRB by (i) examining the orientation and amount of mineral precipitates that occur on individual Fe0 surfaces and act as cementing agents and (ii) conducting visual estimates of the percent of Fe0 loss due to corrosion.

Ground Water Sampling and Characterization
Forty-seven piezometers were installed around the PRB and other sampling ports near the study site. Other details on the construction of the piezometers are given in Watson et al. (1999) and Gu et al. (2002a). This study focused on geochemical analysis of ground water collected from an eastern up-gradient background well (TMW12) (10.7 m to the north) in the pea gravel and two multilevel piezometers (DP19, DP22) in the Fe0 filing portion of the PRB near where the cores were sampled (Fig. 1b). Ground water was collected using a 0.45-µm in-lined filter (Millipore, Billerica, MA) at western down-gradient DP19 and eastern up-gradient DP22 (at vertical depths of 5.5 and 7.9 m) and at TMW12 (at vertical depths of 5.2 to 8.2 m) every 2 to 4 mo over a 30-mo period.

Ground water pH and Eh were measured in line (without exposing to the air) at vertical depths of 5.5 and 7.9 m in the multilevel piezometers with a precalibrated Model 6000 XLM multiparameter probe equipped with an Ag–AgCl reference electrode (YSI, Yellow Springs, OH). Immediately after taking the ground water sample in the field, colorimetric analysis for Fe2+ and S2- using Hach kits equipped with a DR/2000 spectrophotometer (Hach Company, Loveland, CO) were performed. Ferrous Fe was measured using the 1,10 phenantholine procedure where 0.1015 g of 1,10 phenantholine (phenolphthalein indicator powder pillow) was added to 25 mL of ground water and measured at a wavelength of 510 nm (detection limit 0.008 mg L-1). Sulfide (S2-) was measured using the methylene blue method (1.0 mL of both sulfide reagents added to 25 mL of ground water) at a wavelength of 665 nm (detection limit = 2 µg L-1).

Both acidified (pH < 2) samples for total U and metal analysis and unacidified samples for anions and carbon analysis were collected. Calcium (detection limit of <0.025 mg L-1) and Fe3+ (detection limit of <0.1 mg L-1) were analyzed using a Thermo Jarrell Ash inductively coupled plasma atomic emission spectrometer (Thermo Elemental, Franklin, MA). Sulfate and nitrate were analyzed with a Model 500 ion chromatograph equipped with a conductivity detector (Dionex, Sunnyvale, CA). Total organic and inorganic carbon were also measured with a total organic carbon analyzer (TOC-5000A; Shimadzu, Tokyo, Japan). Total inorganic carbon was converted to bicarbonate or carbonate concentrations.

A laser-induced kinetic phosphorescence analyzer (KPA-11; ChemChek Instruments, Richland, WA) was used to measure total U(VI) in the acidified samples (with a detection limit better than 0.01 µg L-1). Because of relatively low U(VI) concentrations in the ground water samples and the interference (or quenching effects) of elements such as Cl- and Ca2+, the samples were purified and preconcentrated using UTEVA resin (100–150 µm) 2-mL extraction columns (Eichrom Technologies, Darten, IL) (Gu et al., 2002a). Experimentally, 10 mL of ground water sample was acidified to a concentration of 3 M with nitric acid, which complexes uranyl (UO2+) with NO3- (Horwitz et al., 1992). The UTEVA resin sorbed these uranyl–nitrate complexes, while the other ground water constituents were unsorbed in the leachate. The column was then leached with 20 mL of 0.01 M HNO3, which dissociated the uranyl–nitrate complexes resulting in desorption from the resin column (Gu et al., 2002a). Total U(VI) was measured with the KPA-11 from a solution of 1 mL of leachate and 1.5 mL of URAPLEX (ChemChek Instruments) enhancing reagent.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Distribution of Mineral Precipitates
After 30 mo buried underground, the Fe0 filings and fine corrosion byproducts still appear black (reduced) and loose in most sections of the cores and there is abrupt contact between the Fe0 PRB and surrounding fill material. Therefore, there is little to no mixing of surrounding soil and fill with the PRB. This is similar to the findings of the first cores taken in February 1999 (15 mo after installation) (Phillips et al., 2000). However, iron oxidation, cementation of PRB material, and mineral precipitates appeared to be greater in the core collected after 30 mo compared with the cores sampled after 15 mo. Quartz and calcium feldspar were present throughout the cores and were probably transported into the barrier as fine particles in the ground water; therefore, these minerals are not considered precipitates.

Iron (Oxy)hydroxides and Oxides
There has been a striking change in the distribution and presence of the Fe (oxy)hydroxides in the core samples collected after 30 mo compared with 15 mo of operation. The cores sampled after 15 mo of operation had akaganeite throughout and sporadic occurrences of goethite, while the cores sampled after 30 mo had a greater distribution of goethite (Fig. 2a) . Figure 3 shows that goethite was detected throughout Core 1 at the depths of 6.4 to 8.2 m. Lepidocrocite is also detected in the cores collected after 30 mo of operation. Iron (oxy)hydroxides are present as fine particles, which maybe easily suspended in the PRB. Therefore, the distribution of the Fe (oxy)hydroxides may have been disturbed during the 15-mo core sampling event due to multiple samplings from a single 60° angle core hole (mixing during retrieval) (Phillips et al., 2000). Slight changes in sampling location may have also effected these changes; however, the samples were collected close to where the cores were collected after 15 mo of operation. Single cores were collected that included both northern up-gradient and southern down-gradient interfaces to reduce core mixing when the cores were sampled after 30 mo of operation.



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  		Fig. 2. Minerals present on the surface of a Fe0 filing from the shallow portion of Core 2 at 5.5 to 5.8 m collected after 30 mo of operation: (a) scanning electron microscopy (SEM) photomicrograph showing hexagonal-shaped goethite crystals and elongated aragonite crystals, (b) energy dispersive X-ray (EDX) analysis of the goethite from (a); (c) EDX analysis of the aragonite from (a); (d) SEM photomicrograph showing the distribution of rounded formations of FeS in relation to the goethite and aragonite shown in (a); and (e) EDX analysis of the FeS from (d). Arrows indicate the location of the EDX analysis areas.

 


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  		Fig. 3. X-ray diffractograms showing the minerals present in Core 1 (6.4–8.2 m) from a reduced zone at the up-gradient interface to a highly cemented oxidized zone at the down-gradient interface collected 30 mo after operation. Ak, akaganeite; Ar, aragonite; C, calcite; Ca Fd, calcium feldspar; Fe, zerovalent iron; IHC, iron hydroxy carbonate; G, goethite; L, lepidocrocite; Mg, maghemite and/or magnetite; Mk, mackinawite; Q, quartz; and S, siderite.

 
The Fe (oxy)hydroxides were the most widely occurring corrosion by-products throughout the PRB. In the Fe0 portion of the barrier, ground water conditions are usually anaerobic (Eh ranging from -83.4 ± 41.1 mV at DP22M to -248.4 ± 38.7 mV at DP19M) compared with eastern up-gradient background conditions at TMW12, which has Eh values that averaged 100.8 ± 88.7 mV (Table 1). The decrease in Eh may be a result of the consumption of O2 and the production of dissolved H2 as ground water reacts with Fe0. The standard deviations of the Eh were high due to interferences such as seasonal variations in ground water conditions and ground water flow that affect dissolved oxygen. Overall, there is a general increase in pH within the Fe0 portion of the barrier compared with TMW12. Under these geochemical conditions there is a simultaneous release of Fe2+ (Gu et al., 2002a). Concentrations of Fe2+ were notably higher in the shallow zone (4.3–6.7 m) compared with the deep zone at 6.4 to 8.2 m (Table 1), which signifies oxidation and contributes to the greater abundance of Fe (oxy)hydroxides in the shallow portion of the PRB. The Fe0 oxidizes in the presence of ground water in the PRB as shown in Eq. [1] and [2]. Iron corrosion and the increase in pH in the PRB results in the precipitation of hydroxides (Eq. [3]) that convert to Fe oxyhydroxides (Eq. [4]) (Roh et al., 2000a):

[1]

[2]

[3]

[4]

Akaganeite (ß-FeOOH) was detected in most of the oxidized zones from the cores collected 30 mo after installation, except in some cemented areas, and was absent from the more reduced zones of the PRB where green rusts were detected. Akaganeite was detected throughout the cores collected after 15 mo of operation (Phillips et al., 2000). Akaganeite has commonly been observed as a corrosion product of iron in chloride-containing surroundings, such as in marine environments (Refait and Génin, 1997).

Goethite ({alpha}-FeOOH) was present throughout in the shallow cores 4.6 to 6.4 m and the deep southern down-gradient zone of Core 1 at 7.0 to 8.2 m and the deep northern up-gradient zone of Core 2 at 7.0 to 8.2 m collected 30 mo after installation (Fig. 2a). Some akaganeite may have dehydrated to goethite (Schwertmann and Cornell, 1991). Lepidocrocite ({gamma}-FeOOH), which was not detected in the core material collected after 15 mo of operation, was observed sporadically throughout the shallow and deep zones mainly where cementation was present in the cores sampled 30 mo after installation. The increase in the occurrence of Fe (oxy)hydroxides in the Y-12 PRB is probably related to an increased oxidation in the shallow zone as observed by a general increase in Eh values (usually greater than -200 mV) in this portion of the barrier since January 1999 (Gu et al., 2002a).

Maghemite and/or magnetite was detected throughout the PRB by XRD. Magnetite and/or maghemite was originally present in the Fe0. However, Fe hydroxides can convert to magnetite in reduced environments at low temperatures from filings as shown by Eq. [5] (Roh et al. 2000a):

[5]

Maghemite and magnetite are indistinguishable by XRD analysis, although maghemite is usually an oxidation product of magnetite. Some magnetites contain an excess of Fe2O3 and may transform into the end-member maghemite (Deer et al., 1983).

Carbonate Minerals
Clusters of needle-shaped aragonite (CaCO3) crystals were observed by SEM–EDX and detected by XRD throughout the cores collected in the 30-mo sampling event from the PRB (Fig. 2a,c and Fig. 4a,b) . Similar aragonite crystalline morphology has been observed in Fe0 column material flushed with Ca2+ and CO2-3 (Schuhmacher et al., 1997; Mackenzie et al., 1999; Roh et al., 2000a). Additionally, calcite (CaCO3) that was not detected in the cores collected 15 mo after installation was detected sporadically throughout the cores sampled after 30 mo, particularly within the oxidized and cemented zones. Therefore, after 15 mo of operation, some aragonite transformed into calcite. Aragonite is less common than calcite at normal temperatures and pressures and it is metastable and readily transforms into calcite (Deer et al., 1983).



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  		Fig. 4. Carbonate mineral precipitates from the Fe0 permeable reactive barrier (PRB) sampled 30 mo after operation: (a) scanning electron microscopy (SEM) photomicrograph of clusters of needle-shaped aragonite crystals from the deep down-gradient portion of Core 1 at 5.8 to 6.1 m; (b) energy dispersive X-ray (EDX) analysis of the aragonite from (a); (c) SEM photomicrograph of cube-shaped siderite crystals from the shallow cemented interface from Core 2 at the up-gradient interface at 4.6 to 4.9 m; (d) EDX of the siderite from (c); (e) SEM photomicrograph of the euhedral hexagonal-shaped carbonate form of green rusts from the deep down-gradient interface from Core 2 at 7.9 to 8.2 m; and (f) EDX analysis of the green rusts from (e).

 
The ground water at the S-3 site contained both high Ca2+ and bicarbonate concentrations because of the presence of CaCO3–rich Nolichucky shale bedrock (Phillips et al., 1998) and strong nitric acid leachate from the S-3 ponds, and from the neutralization of the acid by limestone. Calcium concentrations have remained relatively consistent for the last 15 mo of operation (Gu et al., 2002a). Calcium concentrations decreased from an average of 189.5 ± 26.9 mg L-1 at the background TMW12 well to 95.3 ± 44.7 mg L-1 at shallow DP22S and 177.3 ± 10 mg L-1 at deep DP22M at the eastern up-gradient entrance in the Fe0 part of the PRB (Table 1). In western down-gradient well DP19, Ca2+ concentrations were 82.8 ± 29.1 mg L-1 in the shallow 5.5 m section and 9.8 ± 5.7 mg L-1 in the deep 7.9-m section. The drastic reduction of Ca2+ concentration in DP19M is due to precipitation of carbonate minerals up-gradient from DP19M. Hydroxide ions produced by Fe corrosion increase pH and react with dissolved carbonic acid (Eq. [6]) and bicarbonate (Eq. [7]) in the PRB ground water to produce carbonate ions (Roh et al., 2000a):

[6]

[7]

Aragonite was not present in the deep Core C after 15 mo of operation (Phillips et al., 2000), but was present in the deep Core 2 samples after 30 mo of operation. The occurrence of CaCO3 precipitates and the lowering of Ca2+ in water samples at 4.6 to 6.4 m may be attributed to an increased ground water pH, which favored the precipitation of CaCO3. These data support SEM–EDX analysis and observations of greater precipitation of CaCO3 in the shallow zone compared with the deep zone. The carbonate (Eq. [8]) and bicarbonate (Eq. [9]) species that accumulate combine with Ca2+ to form aragonite and calcite:

[8]

[9]

Cube-shaped siderite (FeCO3) was also detected by XRD and SEM–EDX mainly in the northern up-gradient cemented zones in the shallow portion of the barrier and in the deeper portions of the barrier in Core 1 from 7.0 to 8.1 m in and near zones of cementation (Fig. 3 and Fig. 4c,d). Siderite appears to have precipitated in the more oxidized environment within this PRB. Ground water HCO-3 and CO-3 concentrations ranged from 142.8 ± 110.0 mg L-1 for DP19M in the deep western down-gradient portion of the Fe0 PRB near Core 2 to 481.5 ± 55.2 mg L-1 for DP22M in the shallow eastern up-gradient portion of the PRB near Core 1. Background well TMW12 (438.9 ± 139.0 mg L-1) had similar concentrations of HCO-3 and CO-3 to the eastern up-gradient piezometer DP22 in the shallow (DP22S) (454.3 ± 51.9 mg L-1) and deep (DP22M) zones of Core 1 (Gu et al., 2002a). The Fe2+ species produced during corrosion also combined with the carbonate (Eq. [10]) and bicarbonate (Eq. [11]) species to form siderite, which is also reported by Roh et al. (2000a):

[10]

[11]

Iron hydroxy carbonate [Fe2(OH)2CO3] was identified throughout the core material collected 30 mo after installation. This mineral was not detected in the cores collected after the first 15 mo of operation. The formation of carbonate minerals within the Fe0 barrier is undesirable because it reduces Fe0 reactivity, but was fully expected due to local ground water geochemistry.

Green Rusts
Green rusts were mainly detected by XRD in the deep portion of the barrier at 6.4 to 7.3 m in Core 1 and 7.0 to 8.2 m in Core 2 (Fig. 3 and Fig. 4e,f). Green rusts, which appear as hexagonal-shaped crystals, were present generally in areas where lepidocrocite, goethite, mackinawite, and siderite were not detected. Higher oxidation in certain areas of the barrier where goethite, lepidocrocite, mackinawite, and siderite are present may have hampered the formation or caused the transformation of green rusts to below a detectable amount by XRD analysis. The chloride (Eq. [12]), carbonate (Eq. [13]), and sulfate (Eq. [14]) forms of green rusts can transform from Fe oxides in moderately neutral solutions (6.5 < pH < 8.0). Green rusts can also easily precipitate from partially oxidized Fe2+ solutions at a pH of 7 or above (Hansen et al., 1996), and green rusts also convert to Fe(OH)2. However, green rusts are unstable (Simon et al., 1997) and rapidly oxidized in contact with air (Hansen et al., 1996; Phillips et al., 2003). During the hydrolytic oxidation of Fe2+, green rusts are produced as intermediate products and often transforms to Fe (oxy)hydroxides (Olowe and Génin, 1991), such as akaganeite (ß-FeOOH) (Phillips et al., 2003), goethite ({alpha}-FeOOH), lepidocrocite ({gamma}-FeOOH), maghemite ({gamma}-Fe2O3), and magnetite (Fe3O4) (Myneni et al. 1997). Only small patches of green rust were observed by SEM in the deep cores sampled 15 mo after installation; however, green rusts were not detected by XRD in these samples. Researchers report that the green rust precipitates are a useful component of PRB systems in the remediation of contaminants (Erbs et al., 1999; Gu et al., 1999):

[12]

[13]

[14]

Sulfides
In PRB material sampled after 30 mo of operation, crystalline (mackinawite) and amorphous FeS were detected by XRD and SEM–EDX in samples from across the shallow zone of the barrier (Fig. 2d,e). However, there appeared to be a decrease in amorphous FeS compared with findings of the cores sampled 15 mo after installation. The amorphous FeS observed in the cores sampled after 15 mo may have crystallized into mackinawite. Mackinawite, which was not detected in the PRB material collected 15 mo after installation (Phillips et al., 2000), was detected in PRB material sampled after 30 mo mainly in the shallow zone and in the deep sections where cementation or greater oxidation occurred according to XRD analysis. Amorphous FeS was observed as bytrodial or rounded shape formations and coatings in the PRB material sampled after approximately 15 and 30 mo (Fig. 2d,e).

Similar to the findings from the cores collected after 15 mo of operation, precipitation of Fe2+ with S2-, which forms FeS as well as other iron oxyhydroxides, has resulted in depletion of Fe2+ from the Fe0 filing portion of the barrier. Low Fe2+ ground water values were observed for eastern up-gradient background well TMW12 (0.6 ± 1.7 mg L-1), and most of the peizometers within the Fe0 portion of the PRB where the ground water conditions are favorable to oxidation [DP19S (9.1 ± 5.1 mg L-1), DP19M (0.3 ± 0.7 mg L-1), DP22S (17.8 ± 11.6 mg L-1), DP22M (1 ± 0.6 mg L-1)]. The larger amount of Fe2+ present in the shallow portion of the barrier is probably related to the greater abundance of Fe (oxy)hydroxides in that section of the PRB. According to peizometers DP19 and DP22 and well TMW12, sulfate concentrations generally decrease within the Fe0 portion of the barrier, except in the deep eastern up-gradient peizometer DP22M. However, sulfide concentrations are similar in the Fe0 portion of the PRB to concentrations from the background well TMW12. The formation of FeS appears to be independent of S2- formation. The formation of FeS precipitates may be generated by microbial reduction of SO2-4 to S2- (Puls et al., 1999b; Scherer et al., 2000; Gu et al., 2002b) as shown in Eq. [15] and [16] (Roh et al., 2000a). An increase in microbial populations, particularly sulfate-reducing bacteria, was observed in and within the vicinity of the Fe0 portion of the PRB compared with the background samples (Gu et al., 2002b):

[15]

[16]

The organic guar gum used during the construction of the PRB may have encouraged growth of the sulfate-reducing bacteria by serving as a carbon source and consequently the reduction of sulfate to sulfide. Mackinawite may be a desirable mineral in Fe0 PRB systems as it is able to entrap heavy metals such as Cr and As (Patterson et al., 1997; Holms, 1999).

Depletion and Cementation of Zerovalent Iron Barrier Material
When compared with fresh Fe0 filings (Fig. 5a) , there has been a drastic increase in corrosion of the Fe0 filings over the first 30 mo of operation (Fig. 5b–d), based on SEM observation of polished sections of barrier material with SEM. Much of the corrosion has occurred within the last 15 mo. After the first 15 mo of operation, Phillips et al. (2000) reported that much of the surfaces of the Fe0 filings were not corroded, except in the northern up-gradient Core A and southern down-gradient Core C. However, the individual Fe0 filings from the cores sampled 15 mo after installation exhibited more corrosion compared with fresh Fe0 filings (Fig. 5a,b). The cores collected 30 mo after installation have corrosion rinds that are thicker (25–150 µm) and more continuous on individual Fe0 filings (Fig. 5c) compared with the rinds on the Fe0 filings from the cores sampled after the first 15 mo of operation (Fig. 5b), which were mostly thin (2–50 µm) and/or discontinuous from Cores A and C (Phillips et al., 2000). Additionally, about 25% of the Fe0 filings have corroded into smaller fragments (Fig. 5d) in the highly oxidized zones of Core 1 and shallow Core 2. Almost 30 to 80% of the Fe0 filings were observed to have been replaced by Fe (oxy)hydroxide and oxide rinds in the cemented section of the barrier where the rinds are 10 to 300 µm thick. The morphology of the rinds on these Fe0 filings appears mainly as the Fe (oxy)hydroxide replaced portions of the filings with smaller areas of precipitates of CaCO3 and FeS (Fig. 6a–d) . From the appearance of the Fe0 filings in the cemented oxidized zones, the Fe0 filings will last about another five years or less in these zones. However, approximately five years is a rough estimate because it is based on the visual inspection of the corrosion rinds of the polished sections. Similar rinds, described as 10- to 20-µm-thick oxyhydroxide layers, were also present on Fe0 filings in a column study where HCO-3 and SO2-4 were flushed through the system (Gu et al., 1999). The thickness of the corrosion rinds or the rate of iron corrosion will depend largely on the geochemistry of ground water. For example, Agrawal and Tratnyek (1996) report that corrosion of Fe0 can be increased by the reduction of H2CO3 and HCO-3. The Fe0 filings from the less oxidized zone of the Y-12 PRB where green rusts is present shows little corrosion and are similar to the fresh Fe0 filings.



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  		Fig. 5. Polished sections of (a) a fresh Fe0 filings, (b) loose Fe0 filing from the shallow up-gradient Core A collected after the first 15 mo of operation showing thin Fe (oxy)hydroxides corrosion rinds and aragonite crystals on the filing surface; (c) loose Fe0 filing from the shallow midsection of Core 1 collected 30 mo after installation showing thick Fe (oxy)hydroxide corrosion rinds from the depletion of the underlying Fe0 filing; (d) loose Fe0 filing corroding into smaller pieces from the shallow midsection of Core 1 collected 30 mo after installation; (e) scanning electron microscopy (SEM)–backscatter electron (BSE) analysis of cemented Fe0 filings from the up-gradient shallow Core A collected after the first 15 mo of operation; and (f) SEM–BSE of cemented Fe0 filings from the up-gradient shallow Core 1 collected after 30 mo of operation. [Note the smaller Fe0 filings and greater disintegration of the sample collected after 30 mo of operation (f) compared with the sample collected after the first 15 mo of operation (e).]

 


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  		Fig. 6. Cemented areas of the permeable reactive barrier (PRB): (a) backscatter electron (BSE) analysis of an undisturbed sample collected after the first 15 mo of operation at the shallow up-gradient interface of Core A showing Fe0 filings cemented together by Fe (oxy)hydroxides, Fe2O3, aragonite, and FeS; (b) map showing the distribution of the different minerals in (a); (c) BSE of an undisturbed sample collected 30 mo after installation at the shallow up-gradient interface from Core 1 showing the buildup of precipitation on an Fe0 surface; and (d) map showing the different minerals in (c). (Note the layering of mineral sequences, the transformation of calcite from aragonite, and the goethite crystals on the surface.)

 
Continuous cementation of Fe0 filings was observed in the shallow northern up-gradient portions of Cores 1 (Fig. 5f) and 2 (approximately 25.4 cm thick), the southern down-gradient (approximately 10.2 cm thick) interfaces of shallow Core 1, and the deep southern down-gradient interface of Core 1 (approximately 10.2 cm thick) (data not shown). Less continuous cementation was also observed in the midsection of shallow Core 1 (approximately 25.4 cm thick). Based on XRD and SEM–EDX analysis, the primary component of the cementation was Fe oxides and Fe (oxy)hydroxides (particularly goethite), with coatings of siderite (FeCO3), mackinawite (FeS), and smaller amounts of aragonite (CaCO3) and calcite (CaCO3). The cementation is very hard and dense at the interface zones and had to be broken apart with a hammer. The cores collected after the first 15 mo of operation showed cementation only in the shallow southern down-gradient interface of Core A (near Core 1) and in the northern up-gradient deep interface of Core A (between 5.5 and 6.4 m deep). The continual corrosion observed in the core material sampled 30 mo after installation is accompanied by mineralogical changes of the cemented material such as aragonite transforming into calcite and amorphous FeS transforming into mackinawite over the 15 mo between sampling events (Fig. 6a–d). There was also a dramatic increase in the presence of siderite and the appearance of lepidocrocite, which was not present in any of core material collected after the first 15 mo of operation. Based on comparisons of Fe0 filings sampled in the PRB after 15 and 30 mo of operation, the coatings of Fe (oxy)hydroxides, carbonates, and FeS do not appear to be ceasing or slowing down the corrosion of the Fe0 filings. Continual formation of this cementation may eventually restrict the ground water flow rate and pattern through the barrier, thus reducing the long-term performance of this Fe0 barrier to immobilize uranium and other contaminants. Gradient fluctuations within the barrier indicate that cementation within the iron may be affecting the flow through the iron. In spring of 1999 (500 d after installation), a few months after the 15-mo core sampling visit, recharge events appear to have a noticeable effect on hydraulic gradients (or fluctuation) between wells up-gradient, within, and down-gradient of the Fe0 portion of the barrier. This suggests that the connectivity of the iron and gravel in the up-gradient and down-gradient portions of the PRB may be decreasing over time due to mineral precipitation and cementation in the Fe0 section of the barrier. Also, nitrate concentrations in eastern up-gradient piezometer DP22M, in the Fe0 portion of the barrier, have an increasing trend starting spring of 1999 (data not shown). Therefore, cementation of the Fe0 filings in the up-gradient portion of the barrier may be resulting in a decrease in iron reactivity and hydraulic connectivity and the start of system clogging (Gu et al., 2002a).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The in situ Fe0 PRB at the Bear Creek Valley Y-12 Plant, Oak Ridge, TN showed an increase in mineral precipitation, iron oxidation, and cementation from approximately 15 to 30 mo after installation. In the shallow portion of the PRB and the highly oxidized interfaces of the deep zone, there was greater precipitation of goethite, lepidocrocite, aragonite, mackinawite, and amorphous FeS, compared with the more reduced environments in the deeper zones of the PRB where green rusts formed. Many Fe minerals transformed into more crystalline structures, and corrosion and cementation of Fe0 filings increased within the last 15 mo of operation, especially in the up-gradient interface where ground water enters the Fe0 portion of the PRB. In this portion of the PRB, HCO-3, CO-3, and SO2-4 concentrations in the ground water are high, which is conducive to corrosion and cementation. Also, Ca2+, Fe3+, and Fe2+ concentrations in the ground water are higher, compared with other areas of the barrier, which promotes the precipitation of minerals. The Fe0 at the up-gradient interface section of the PRB is predicted to last only a total of eight years, as almost 30 to 80% of the Fe0 filings have been replaced by Fe (oxy)hydroxide rinds. The mineral coatings do not appear to be stopping or slowing down the corrosion of the Fe0 filings, which is consistent with how Fe rusts. Continual formation of mineral precipitates and cementation may eventually decrease iron reactivity and restrict ground water flow rates and patterns through the barrier, reducing the long-term performance of this PRB to remove uranium and other contaminants. However, in the deep up-gradient Core 1 and down-gradient Core 2, green rusts are present and there is less corrosion of Fe0 filings in the less-oxidized zone where the Eh is low and the pH are higher compared with other areas in the barrier.

In situ field-scale Fe0 PRBs are increasingly being used as a cost-effective means to remediate contaminated ground water compared with pump and treat systems. Because corroded and cemented areas in PRBs are difficult and expensive to repair and replace, prior knowledge of ground water geochemical characteristics of a potential treatment site and what effect these geochemical characteristics may have on Fe0 PRB material are useful to know before barrier installation. As we have demonstrated, the long-term performance issues and potential problems of PRB systems may be predicted with information gathered from mineralogical and geochemical studies of existing PRB systems.


   ACKNOWLEDGMENTS
 
The authors thank N.D. Farrow, K. Hyder, and G. Houser for assistance with the collection of the core material. We also thank Ed Kenik from the Materials Sciences and Engineering Division at Oak Ridge National Laboratory (ORNL) for his help with the scanning electron microscopy (SEM)–backscatter electron (BSE) analysis at the ShaRE Collaboration Research Center. Funding for this research was supported by the Office of Environmental Management of the U.S. Department of Energy. ORNL is managed by UT-Battelle LLC under Contract DE-AC05-00OR22725. D.H. Phillips was supported by the Oak Ridge Institute for Science and Education.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The submitted manuscript has been authored by a contractor of the U.S. Government under Contract no. DE-AC05-96R22464. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.


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


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