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a School of Natural Resources, University of Nebraska-Lincoln, 362 Plant Sciences, Lincoln, NE 68583-0915
b Department of Environmental Science, Kasetsart University, Bangkok, Thailand 10900
* Corresponding author (pshea{at}unl.edu).
Received for publication July 12, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: Fe0, zerovalent iron • LANL, Los Alamos National Laboratory
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Soils located in TA-16 are grossly contaminated, with some soils containing high explosive concentrations of >20% (w/w). This magnitude of contamination is excessive and indicates that precipitated or solid-phase high explosives are present in the soil matrix. When soils contain solid-phase contaminants, soil solutions become saturated and natural attenuation processes are severely inhibited. Using 14C-RDX, Singh et al. (1998a) demonstrated that the presence of solid-phase RDX in the soil matrix prevented the formation of bound (unextractable) residue. Consequently, remediating highly contaminated soils requires aggressive soil treatments that are sustainable and can continue to remove contaminants from the soil solution as the solid-phase residues dissolve.
Zerovalent iron (Fe0) has an excellent potential to abiotically remediate RDX-contaminated water and soil (Hundal et al., 1997; Singh et al., 1998b, 1999). Hundal et al. (1997) found Fe0 effectively destroyed RDX in aqueous solution and soil slurries. Aqueous batch experiments indicated that as little as 1% Fe0 (w/v) effectively transformed 32 mg RDX L-1. Moreover, transformation products (measured as 14C activity) were water-soluble and not strongly sorbed to the iron surface. Producing non-adsorbing transformation products can be advantageous if these products are biodegradable. Singh et al. (1998b) tested this hypothesis by using 14C-RDX in static microcosms and measuring mineralization by trapping evolved 14CO2. Results showed that a single Fe0 amendment increased RDX mineralization, with greater than 60% of the 14C-RDX recovered as 14CO2. Considering carbon use efficiencies for most organic compounds, these data indicate that the Fe0–induced transformation products of RDX are highly biodegradable. Subsequent studies (Wildman and Alvarez, 2001; Oh et al., 2001; Oh and Alvarez, 2002) further support synergistic effects between Fe0 treatment and enhanced biological degradation of RDX.
Although Hundal et al. (1997) observed that Fe0 effectively transformed RDX in soil slurries, working with soil slurries is problematic for several reasons. The equipment required for continuous agitation is expensive and limits the volume of soil that can be treated at any given time. Dewatering of treated soil is also required. A desirable alternative to slurry treatment would be on-site treatment in soil windrows. Using soil windrows allows much greater volumes of soil to be treated and is constrained by only the size of the windrows and acreage available.
Initial work with RDX-contaminated soil from the abandoned Nebraska Ordnance Plant (Mead, NE) indicated that Fe0 could be effective in static unsaturated soil microcosms (Singh et al., 1998b). The effectiveness of Fe0 in transforming RDX in unsaturated soils opened the door for field-scale applications. Using zerovalent iron at the field scale requires machinery that can thoroughly mix the iron and other amendments into the soil matrix. The Microenfractionator (H & H Eco Systems, North Bonneville, WA) is the trade name of a soil mixing implement that can treat large volumes of soil (400–1000 m3 h-1). This machinery can also simultaneously spray liquids into the soil windrows during mixing to achieve any desired water content.
Our objective was to combine Fe0 treatment of RDX-contaminated soil with the machinery required for field-scale use. This was accomplished by conducting batch experiments in the laboratory to optimize Fe0 applications and then testing these treatments at the pilot scale with a table-top version of the field-scale mixer (Fig. 1) . Two high explosives–contaminated soils, indicative of the contamination found at LANL TA-16, were used for pilot-scale testing.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Two LANL soils were used for experimentation (hereinafter referred to as Soils A and B). Soil A used in the first batch and pilot-scale experiment was obtained from the center of a discharge pond approximately 100 m east of Building TA-16-260 at LANL TA-16. Soil B was from the same outwash pond but was mixed with material from the sides of the pond and discharge areas.
Soil A was sent to the University of Nebraska in a 208-L metal drum. Physical handling involved spreading the soil onto the stainless steel table of the pilot-scale mixer (Fig. 1A) and removing large stones (approximately >5 cm in diameter). A table-top fan was used to gently pass air across the soil surface to facilitate air-drying overnight. We then ran the Microenfractionator through the soil four times. The pilot-scale Microenfractionator is a one-sixth replica of the field-scale unit (Fig. 1B) and can mix 70 to 100 kg of windrowed soil in one pass. Much of the larger consolidated soil not removed by hand was broken apart and pulverized by the Microenfractionator, producing a homogeneous soil that was easy to handle. Once the soil had been mixed, it was divided into two piles and placed in plastic bins with covers. Five soil samples from each plastic bin were taken and analyzed for RDX. An additional subsample was sieved (<2 mm) before RDX analysis. Representative subsamples from each soil were also sent to Midwest Laboratories (Omaha, NE) for characterization (Table 1). Soil B was handled similarly using the same pilot-scale mixer but experiments were performed on-site at LANL.
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A separate experiment compared the effects of acids (HCl vs. CH3COOH) on Fe0–mediated destruction of RDX in aqueous solutions and soil washings under pH-stat conditions. The pH-stat experiments were conducted with a Metrohm Titrino (Model 718S; Brinkmann Instruments, Westbury, NY). Both the aqueous solutions of RDX (initial concentration = 20 mg L-1) and aqueous soil washing of Soil A (concentration = 40 mg RDX L-1) were treated with 1% (w/v) Fisher Scientific Fe0. The pH was held at 4.5 with HCl or CH3COOH and changes in solution RDX concentrations were determined at 2, 4, 8, and 24 h.
The effect of acidifying amendments on RDX destruction by Fe0 in static soil microcosms was also investigated. In this experiment, triplicate samples from Soil A were incubated in Teflon centrifuge tubes at 30°C for 17 d at a gravimetric soil water content of 0.33 kg kg-1. Treatments included: (i) Fe0 + H2O; (ii) Fe0 + H2SO4; (iii) Fe0 + Al2(SO4)3; and (iv) Fe0 + CH3COOH. Controls (no Fe0) were also included for each amendment [i.e., H2O, H2SO4, Al2(SO4)3, and CH3COOH]. To increase soil water content to 0.33 kg kg-1, 3 g of Soil A was treated with 0.15 g Fe0 (5%, w/w) and 1 mL of H2O, acidified water (H2SO4, pH 2.0), H2O + Al2(SO4)3 (6 mg), or CH3COOH (10%, v/v).
Pilot-Scale Experiments
Study I (Soil A)
Pilot-scale experiments were conducted with 70 kg (oven-dry) soil. The initial study was conducted with Soil A using two treatments: Fe0 + CH3COOH and a control (H2O only). The soil was placed on the stainless steel table of the mixer (Fig. 1) and Fe0 added to a V-shaped indentation along the top ridge of the soil windrow. Iron (3.5 kg) was added to the 70 kg of soil to yield a concentration of 5% (w/w). The soil and iron were then mixed by the Microenfractionator three times. The Fe0–amended soil was evenly divided into five 16.25-kg volumes and placed in layers in a large plastic bin where 2.65 L of water–CH3COOH was sprinkled onto the soil to raise the gravimetric water content to 0.30 kg kg-1. Glacial acetic acid (1.05 L) was mixed with water and added to the soil to theoretically yield an initial concentration of 5% CH3COOH (v/v) in the soil solution. This was determined by measuring the initial water content of the soil (0.11 kg kg-1), calculating the additional water needed to bring the soil water content to 0.30 kg kg-1 (13.25 L), and adding the acetic acid to the water. The control treatment (no Fe0) was handled similarly but only water was added. Soil bins were covered and placed in a controlled environment chamber at 30°C in the dark.
Sampling consisted of pushing a hand-held soil probe (i.d. = 1.905 cm; length = 30.48 cm) perpendicularly into the soil. The location of each soil core taken by date and replicate was recorded on a grid coordinate. Four soil cores were taken from each bin at 0, 10, 20, 40, 60, 80, and 110 d after treatment. Soil samples were held in plastic whirl top bags at 4°C until analysis. Soil analysis included pH (1:1 soil to H2O), RDX concentration, and soil water content. Once gravimetric soil water content fell below 0.25 kg kg-1, additional water and/or CH3COOH were added to the soil and allowed to infiltrate. This resulted in adding water to the Fe0 treatment on Day 10 (2.92 L) and water–CH3COOH (3.33 L, 10% CH3COOH, v/v) on Day 60. On Day 40, water (3.08 L) was added to the control.
Study II (Soil B)
A second pilot-scale experiment was conducted at the LANL using a second soil from the drainage pond (Soil B). In this experiment, procedures similar to Study I were followed but the number of treatments was expanded. These included: (i) control (H2O only); (ii) Fe0; (iii) Fe0 + CH3COOH; (iv) Fe0 + Al2(SO4)3; and (v) Fe0 + CH3COOH + Al2(SO4)3. In this experiment, Fe0 (5%, w/w) and acetic acid (1.05 L per 70 kg) were added at the same rate used in Study I and Al2(SO4)3 was added at 2.0% (w/w). Soil samples were taken at 0.5 h and 10, 20, 40, 60, 80, 100, and 120 d after treatment. The soil water content was increased to 0.34 kg kg-1 in Soil B and the soil bins were incubated in the dark at approximately 28°C.
Batch Experiments Using Witherite and Acetic Acid
When acetic acid was added as an amendment, differences in RDX destruction were observed between Soil A and B (see Results). Acetic acid extracts of Soil A and B revealed large differences in Ba concentrations. Subsequent batch experiments were conducted to determine the effects of Ba and acetic acid on Fe0 performance. This was accomplished by treating 100 mL of aqueous RDX (20 mg RDX L-1) and RDX in a humic acid matrix (50 mg L-1, International Humic Substances Society Reference #1R102H) with Fe0 and various additions of acetic acid and Ba. Treatments included: (i) Fe0 (2%, w/v); (ii) Fe0 + 0.1 g witherite (BaCO3); (iii) Fe0 + CH3COOH (0.5%, v/v); and (iv) Fe0 + witherite + CH3COOH. To test the effects of these treatments on long-term Fe0 performance, experiments were conducted in three cycles where treatment solutions were sampled during the first 24 h and then decanted, and fresh RDX solutions (with BaCO3 and CH3COOH treatment additions) were added to the original Fe0 followed by sampling for another 24 h. To test the residual effects of each treatment on Fe0 performance, only aqueous RDX or RDX + humic acid (without BaCO3 or CH3COOH) was added in the third cycle.
Chemical and Physical Analyses
RDX was extracted from soil (3 g) with 15 mL CH3CN by sonicating for 18 h at 30°C, centrifuging at 5000 x g, removing the supernatant, and centrifuging (12000 x g) before analyzing with high performance liquid chromatography (HPLC). Acetonitrile extracts (10–25 µL) were injected onto a Keystone Betasil NU(R) or NA column (Keystone Scientific, Bellefonte, PA) with an isocratic (50:50 or 30:70) mixture of methanol and H2O at a flow rate of 1.0 mL min-1 and quantified spectrophotometrically at 220 or 254 nm. In the second pilot-scale experiment (Study II, Soil B), TNT was also quantified using the same HPLC procedure.
Acetic acid extracts of Soil A and B were prepared by shaking 25 g soil with 50 mL of CH3COOH (5%, v/v) for 24 h and centrifuging at 5000 x g for 30 min. The supernatant was analyzed for Ba by inductively coupled plasma (ICP) (Midwest Laboratories).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The unexpected high pH of Soil A was in part the impetus for a second study using Soil B. Analysis of Soil B revealed a near-neutral pH with lower K, Ca, and Na concentrations but considerably greater RDX contamination (12100 ± 814 mg kg-1). Soil B also contained more organic matter (Table 1).
Laboratory Experiments
Initial efforts to remove RDX from Soil A with Fe0 failed and it was believed that the high soil pH was rapidly passivating the Fe0 and reducing electron transfer from the iron surface. We hypothesized that the soil pH needed to be lowered before Fe0 would be effective. To accomplish this, various concentrations of acetic acid were added to a soil slurry. Following 20 h of equilibration, the initial pH of the slurry declined from 11.1 (no CH3COOH added) to 4.7 (2.13% CH3COOH, v/v). To determine how lowering pH would affect RDX destruction by Fe0, we used a pH-stat to maintain constant pH (4.5) and determined destruction kinetics. We observed that for both pure aqueous solutions and aqueous extracts of Soil A (soil washings), RDX destruction was faster when CH3COOH was used to control pH rather than the mineral acid HCl (Fig. 2)
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10.5 (Table 3).
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Monitoring soil water status during the experiment revealed that gravimetric water content was maintained between 0.25 and 0.30 kg kg-1 (data not shown). It is noteworthy that treatments were initiated by mixing the soil with the high-speed mixer but no mixing was performed when additional H2O and/or CH3COOH was added. Rather, the solutions were added to the top of the soil and allowed to infiltrate. In the field, this procedure would be recommended to minimize aeration of the soil windrow and accelerated aging (passivation) of the Fe0.
One qualitative difference between the treatments was the abundant growth of fungal hyphae on the Fe0–CH3COOH-treated soil. This occurred within a few days after treatment and continued throughout the course of the experiment. No fungal growth was observed on the control soil.
We also noticed that the Fe0–CH3COOH-treated soil became more dense and difficult to probe for sampling. Because of the added acidity, some of the carbonates dissolved and effervesced following CH3COOH addition. Consolidation and cementation of the precipitates probably resulted in a denser soil matrix. Additional mixing with the Microenfractionator following treatment would probably eliminate this problem.
Study II (Soil B)
High explosive concentrations in the control treatments (control and control + H2O) indicated fairly constant RDX concentrations throughout the experiment with an average RDX concentration of 12300 ± 634 mg kg-1 (Table 4). TNT concentrations, however, slowly declined with time indicating TNT degradation in the control. The variability and fluctuation in RDX concentrations is a function of the heterogeneity of contamination, which included solid-phase RDX intermixed throughout the soil matrix. Comparing results between the controls and the Fe0–based treatments revealed that TNT and some RDX were transformed shortly after mixing (t = 0.5 h; Table 4). This demonstrates that abiotic transformations induced by Fe0 can occur fairly quickly even in static, unsaturated soils. Moreover, relative declines in TNT concentration versus RDX immediately after Fe0 treatment support that TNT is a preferential electron acceptor and more prone to reductive transformation by Fe0.
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Results from the second pilot-scale experiment were not consistent with the first study (using Soil A) or laboratory batch studies. Using Soil B, we found that the largest destruction of RDX (and TNT) occurred with the Fe0–only and Fe0 + Al2(SO4)3 treatments. Using Fe0 alone, RDX concentrations decreased to 540 mg kg-1 (t = 120 d) resulting in a 96% reduction from the average initial concentration of the control (12100 mg RDX kg-1). The Fe0 + Al2(SO4)3 treatment was also equally effective and produced the lowest average concentration after 120 d (210 mg kg-1, 98% decline; Table 4). Adding CH3COOH, which had a positive effect on Fe0–induced RDX destruction in solution (Fig. 2) and static soil microcosms (Soil A, Table 2), negatively affected RDX and TNT loss compared with Fe0 alone in the second pilot-scale experiment (Table 4). When CH3COOH was added, decreases in RDX and TNT concentrations occurred only within the first 10 d. This is in contrast to the Fe0 and Fe0 + Al2(SO4)3 treatments where RDX loss continued to decline after 20 d. While a beneficial effect was observed by adding Al2(SO4)3, this effect was negated when CH3COOH was also added.
To determine why acetic acid had an inhibitory effect on Fe0 performance in Soil B, an acetic acid extract of both soils was analyzed by inductively coupled plasma. This analysis revealed large differences in Ba concentrations (Table 1). Past activities in sector TA-16 at LANL indicate that the likely source of Ba was Ba(NO3)2, which was mixed with TNT to produce the explosive baritol. Analysis of soils at LANL by X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that most of the Ba is now in the form of barite (BaSO4) and witherite (BaCO3) (Don Hickmott, LANL, personal communication, 2001). Because BaSO4 and BaCO3 are not readily soluble in water (28.5 mg L-1 for BaSO4 and 24 mg L-1 for BaCO3; Dean, 1992), we believe less Ba was present in the soil solution of the Fe0–only treatment. One notable observation from the acetic acid extracts was differences in color. The extract from Soil A was amber while Soil B was clear. Aqueous extracts from both soils were also amber. This qualitative difference indicates precipitation of humic materials in the acetic acid extract of Soil B.
Batch Experiments with Barium and Humic Acid
Our batch experiments confirmed that Ba (witherite) did not directly interfere with the Fe0 treatment in an aqueous matrix because small additions of BaCO3 actually increased RDX destruction. In the humic acid matrix, however, Ba decreased the effectiveness of Fe0 (Cycle 1, Fig. 3)
. Acetic acid facilitated RDX destruction in both the aqueous and humic acid matrices during the first two cycles but RDX destruction was hindered when fresh acetic acid was not added to the humic acid matrix (Cycle 3, Fig. 3). Following Cycle 2, a brown floc (slime) was observed on the iron surface in the acetic acid treatments, which we believe was precipitated humic material. Without the addition of more acetic acid this precipitated material probably remained with the iron and prevented efficient RDX destruction. There is some evidence that the combination of Ba and CH3COOH was also inhibitory. Results from Cycle 3 showed a residual effect of witherite + CH3COOH, which was least effective in removing RDX while CH3COOH alone was the most effective (Cycle 3, Fig. 3). Another notable observation is increased RDX destruction by Fe0 and Fe0 + witherite in the humic acid matrix compared with the aqueous matrix (Cycle 1, Fig. 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Addition of H2SO4, Al2(SO4)3, and CH3COOH significantly improved RDX destruction (82–99%) compared with Fe0 alone (22%). Although all three amendments lowered soil pH, which would favor reductive transformations, it is unlikely that they all acted similarly in improving RDX destruction. While the initial effect of H2SO4 is probably due to acidification, a high sulfate concentration may favor formation of ferrihydrite over goethite (Brady et al., 1986), which can produce green rust as the pH increases (Taylor and McKenzie, 1980). Our previous research with Fe0 and Al2(SO4)3 indicated that Al2(SO4)3 in the soil solution during iron corrosion can facilitate metolachlor dechlorination by increasing the concentration of available Fe(II) and favoring green rust formation (Comfort et al., 2001). The early work of Taylor and Schwertmann (1978) also revealed that a high concentration of aluminum slows down the oxidation of Fe(II) and results in precipitation of an Al-ferrihydrite, which can then transform to green rust.
Acetic acid can facilitate RDX destruction either in solution or in a soil matrix if Ba contamination is not a problem. As demonstrated in the pH-stat experiment (CH3COOH vs. HCl), the beneficial effect of CH3COOH appears to be more than simple acidification of the RDX solution. Studies have shown that the reduction of compounds that have a weak interaction with the iron surface can be blocked by strong ligands (e.g., catechol, ascorbate, citrate) occupying surface sites (Cornell and Schwertmann, 1979; Johnson et al., 1998). The surface complexation model suggested by Scherer et al. (1999) indicates that if ligand competition is operative, destruction kinetics should rapidly decline toward zero as ligand concentration increases. However, this was not observed for CCl4 dechlorination by Fe0 in the presence of acetate (Johnson et al., 1998) nor in our experiments (RDX destruction increased). Because our experimental units were open to the atmosphere, formation of Fe(III) oxides was inevitable. Considering that the acetate ligand will complex with Fe2+ (log K1 = 3.2, log K2 = 6.1, log K3 = 8.3; Dean, 1992), this complex appears more resistant to oxidation. Visual evidence for this was observed in Fe0–RDX–CH3COOH mixtures where less Fe(III) species (i.e., rusting) occurred. High concentrations of CH3COOH were used in our treatments; high concentrations of organic acids are known to inhibit crystallization of Fe(III) oxides (Cornell and Schwertmann, 1979). Acetic acid will also inhibit precipitation of Fe(OH)2 and may promote formation of electron-conducting magnetite (FeIII2FeIIO4) on the iron surface through reaction of Fe2+ with amorphous Fe (hydr)oxides or 
-FeO(OH) (Tamaura et al., 1981, 1984). Unlike citrate and phosphate, acetate does not appear to suppress the crystal growth of magnetite (Sidhu et al., 1978). Thus by slowing down ferric oxyhydroxide formation on the iron surface and promoting magnetite formation, acetate would indirectly facilitate electron transfer from the iron. In addition, the increased microbial activity of Soil A after acetic acid addition (evidenced by fungal growth) may have further promoted RDX degradation.
Although Soil B had a high concentration of Ba (BaSO4 or BaCO3), it is unlikely that Ba alone reduced the effectiveness of the Fe0 treatment because RDX destruction increased when witherite was added with Fe0 in an aqueous solution. Adding BaCO3 may have promoted the carbonate form of green rust and slowed down Fe0 corrosion because Ba does not readily hydrolyze (log K = 0.5; Dzombak and Morel, 1990, p. 105, 187) and adsorbs to the >FeOH surface as >FeOHBa2+ (log K = 5.46; Dzombak and Morel, 1990, p. 105, 187). Adding acetic acid promoted dissolution of witherite and saturation of the iron surface with Ba. When acetic acid was no longer present in the matrix (Cycle 3), the witherite + CH3COOH treatment removed the least RDX. This residual effect may be due to passivation of the iron surface.
Our solution experiments indicate that RDX destruction by Fe0 was greater in the humic acid matrix than in aqueous solution (Cycle 1). Weber (1996) similarly observed that a Suwannee humic acid isolate acted as an electron transfer mediator in Fe0 treatment of 4-aminoazobenzene. This mediating effect, however, may be compound specific because natural organic matter (NOM) had an inhibiting effect on Fe0–mediated reduction of CCl4 (Tratnyek et al., 2001). In our experiments, Fe0 was less effective when BaCO3 was added to the humic acid matrix. Considering that Ba is commonly used to determine soil acidity by displacing hydrogen on organic functional groups and results in flocculation of organic matter, we believe an indirect effect of high Ba in Soil B was precipitation of humic material at or near the iron surface, resulting in physical blocking and hindrance of electron transfer. Competition for surface sites on the iron may also be occurring, as observed between trichloroethylene and NOM (Tratnyek et al., 2001).
Because of the equilibrium between the soil solution and solid or crystalline phase, remediating soils containing solid-phase RDX will not only require treatments that demonstrate rapid destruction kinetics in solution but also those that continue to remove RDX as the solid phase dissolves. Dissolution of solid-phase RDX will depend on temperature and surface area (Lynch et al., 2002), crystal size and concentration gradients (Stumm and Morgan, 1996), as well as soil water content and saturation–desaturation cycles. Consequently, days to months may be required for all of the solid-phase RDX to enter the soil solution. For Fe0 to be effective, it must continue to act as a reductant and engage in electron transfer reactions. Our experiments demonstrated that acetic acid greatly facilitated Fe0–mediated RDX destruction but this destruction rate may not be sustainable in all soils. Based on differing results with Soils A and B, we found that soil physicochemical properties can profoundly affect Fe0 performance, necessitating site-specific soil characterization before treatment. Alkaline soils such as Soil A will probably require pH adjustment whereas precipitation of humic material must be minimized in soils containing high concentrations of humic matter and/or Ba (as in Soil B). Moreover, variability arising from the nonuniform deposition of explosives and soil heterogeneity must be considered when treating munitions-contaminated soils.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-FeO(OH) by air oxidation of Fe(OH)2 suspensions. J. Chem. Soc. Dalton Trans. 981:1807–1811.-FeO(OH) to Fe3O4 and Green Rust II in an aqueous solution. Bull. Chem. Soc. Jpn. 57:2417–2421.Related articles in JEQ:
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