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Soil Science Society of America Journal 64:1608-1615 (2000)
© 2000 Soil Science Society of America

DIVISION S-2-SOIL CHEMISTRY

Solid-Phase Iron Characterization During Common Selective Sequential Extractions

Matthew J. La Force and Scott Fendorf

Dep. of Geological and Environmental Sciences, Stanford Univ., Stanford, CA 94305-2115 USA

laforce{at}stanford.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Selective chemical extractions provide semiquantitative information on elemental partitioning within operationally defined soil fractions. In this study, the efficiency of common extraction steps was determined for a mining-impacted soil by analyzing Fe transformations in the solid phase using x-ray diffraction, scanning electron microscopy, and x-ray absorption near edge structure (XANES) spectroscopy. Extractions involve the isolation of operationally defined double-deionized water (soluble), magnesium chloride (exchangeable), sodium hypochlorite (organic matter), sodium acetate–acetic acid (carbonate), hydroxylamine-hydrochloride–nitric acid (Mn-oxides), ammonium oxalate in the dark (AOD) (noncrystalline material), hydroxylamine-hydrochloride–acetic acid (Fe oxides), potassium perchlorate–hydrochloric–nitric acid (sulfidic), and hydrochloric–nitric–hydrofluoric acid (residual) fractions of the solid phase. Ferric Fe remained in the solid phase throughout the extraction sequence until its removal by hydrochloric–nitric–hydrofluoric acid (residual fraction). The hydroxylamine-hydrochloride (1.0 M in 25% [v/v] HOAc) extraction may underestimate Fe associated with crystalline materials. Thus, selective sequential extractions need to be optimized for a given soil prior to implementation and should be used in conjunction with spectroscopic techniques, when possible, to fully ascertain elemental partitioning within the solid phase.

Abbreviations: AOD, ammonium oxalate in the dark • EDS, energy dispersive spectroscopy • ICP, inductively coupled plasma • XANES, x-ray absorption near edge structure • SSE, selective sequential extraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
A WIDELY USED TECHNIQUE for understanding elemental distributions in the solid phase involves the use of selective sequential extractions (SSE); see reviews by Pickering (1981), Chao (1984), and Martin et al. (1987). Unfortunately, widespread and uncritical use of SSEs has called into question the validity of using such procedures for determination of elemental partitioning in soils (Martin et al., 1987; Nirel and Morel, 1990; Tessier and Cambell, 1991; Kim and Fergusson, 1991). Selective sequential extractions have been used on contaminated lake sediments (Tessier et al., 1985; Harrington et al., 1998; La Force et al., 1999), radioactive wastes (Trautmannsheimer et al., 1998; Dhoum and Evans, 1998; Arey et al., 1999), and mine waste–contaminated areas (Kuo et al., 1983; Hickey and Kittrick, 1984; Ramos et al., 1994; Ma and Rao, 1997; Fanfani et al., 1997). Chemical extractions are also used for mineralogical analyses and subsequent taxonomic delineation of soils (Jackson et al., 1986; Parfitt and Childs, 1988; Dahlgren, 1994; Soil Survey Staff, 1996).

The use of SSEs is based on the premise that chemical reagents can remove elements from specific fractions of the solid phase. In essence, the amount of any one given element extracted from a particular phase is dependent on the reagent concentration and type, extraction sequence, and solid/solution ratio, hence the term operationally defined (Miller et al., 1986). The effectiveness of extractions depends on the affinity and specificity of the extracting chemical for the target phase. Unfortunately, the selectivity of a given reagent for a specific phase may be limited or an extracted element may readsorb during an extraction sequence (Bunzl et al., 1999). Additionally, redox-sensitive elements may change oxidation states during an extraction sequence, resulting in an erroneous conclusion regarding its partitioning (Gruebel et al., 1988; Kim and Fergusson, 1991). Therefore, soil extractions are at best operationally defined, and are referred to by their method of extraction and not the target phase (Martin et al., 1987; Nirel and Morel, 1990; Tessier and Cambell, 1991; Kim and Fergusson, 1991). Nevertheless, in light of the shortcomings of selective extractions, they (i) provide valuable information regarding general elemental partitioning patterns (Shuman, 1979; Tessier and Cambell, 1991), (ii) allow for high sample throughput for robust statistical analysis (Luoma, 1981), (iii) provide semiquantitative estimates of contaminants within soils (Shuman, 1985; Lo and Yang, 1998), and (iv) are useful for monitoring relative changes in contaminant partitioning as a consequence of changing physicochemical soil conditions (Brannon and Patrick, 1987; La Force et al., 1999; Arey et al. 1999).

Most tests of sequential extraction procedures rely on solution measurements (Rendell et al., 1980; Rapin et al., 1986; Miller et al., 1986; Belzile et al., 1989; Kim and Fergusson, 1991; Bermond, 1992; Hall and Pelchat, 1999). However, a few studies have used x-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy (EDS), epithermal neutron activation analysis, and radioactive isotopes to analyze the solid phase after extraction with a reagent (Tessier et al., 1979, 1989; Tipping et al., 1986; Gruebel et al., 1988; Dhoum and Evans, 1998). Gruebel et al. (1988) extracted redox-sensitive elements such as As and Se with a two-step sequential extraction technique and determined that their extraction procedure inadequately estimated As and Se partitioning in soils. Other investigators, such as Kheboian and Bauer (1987) used model sediments wherein trace elements were doped at known concentrations to test the validity of an extraction sequence. Both studies suggested that the sequential extraction procedures had problems owing to readsorption and redistribution of some elements in the sediment. The overall effectiveness of sequential extractions has been criticized by Martin et al. (1987) and Nirel and Morel (1990). Nonetheless, Belzile et al. (1989) evaluated the effects of readsorption in trace element-amended soils and determined that readsorption may not be problematic (with the exception of Pb during extraction from the carbonate phase) when using sediments doped with naturally representative trace element concentrations.

The selective extraction procedure described here is designed to remove elements from mine waste–contaminated soils. The extraction procedure seeks to partition contaminants into: double-deionized water (soluble), magnesium chloride (exchangeable), sodium hypochlorite (organic matter), sodium acetate–acetic acid (carbonate), hydroxylamine-hydrochloride–nitric acid (Mn oxides), AOD (noncrystalline material), hydroxylamine-hydrochloride–acetic acid (Fe oxides), potassium perchlorate–hydrochloric–nitric acid (sulfides), and hydrochloric–nitric–hydroflouric acid (residual) extractable fractions of the solid phase. This extraction scheme is a hybrid of two well-established extraction sequences developed by Tessier et al. (1979) and Shuman (1985). The majority of the reagents and extraction order chosen for this work are described therein. However, we inferred that the soil used herein would be abundant in organic matter and therefore removed this fraction prior to removal of carbonates, a method similar to that described by Shuman (1985). In addition, we added an extraction step to remove sulfide minerals commonly associated with mine wastes (Chao and Sanzolone, 1977). By combining these two extraction sequences we intend to provide a more detailed analysis of the solid phase. To test the validity of this extraction sequence, and common extraction steps in general, we used x-ray diffraction, EDS, and XANES spectroscopy to characterize the remaining solid-phase Fe. These techniques allowed for the solid-phase products to be described in detail after each step in the sequential extraction scheme.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Sample Collection and Preparation
Six cores were taken from a mine waste–contaminated site located near the Cataldo wetlands on 4 Dec. 1997. Sediments were oxic at the time of collection and thus no special sample handling was required. Consequently, all chemical extractions transpired at ambient laboratory conditions. The cores were removed from the site using a 4-cm-diam. polyvinyl chloride piston coring device (Baudo, 1990). Upon returning to the laboratory, the top 2 cm of each core was homogenized, and 14 2-g wet sediment samples were placed into acid-washed 50-mL polyethylene centrifuge tubes; the SSE procedure was initiated immediately following sample preparation.

Analytical Procedures
For SSE, standard solutions of the metals were made for each extraction step in a background solution of the extracting reagents to reduce matrix interferences. After each extraction step, elements were measured using inductively coupled plasma (ICP) optical emission spectrophotometry (Thermo Jarrell Ash IRIS ICP-OES, Franklin, MA) with a 5% accuracy range, and quality control was checked every 15 samples. Detection limits were defined by 3{sigma} (where {sigma} is the standard deviation) of seven blanks (Klesta and Bartz, 1996). Detection limits were 0.05 mg Fe L-1 and 0.01 mg Mn L-1. X-ray diffraction and scanning electron microscopy were conducted on air-dried samples using a Siemens D5000K Diffractometer (Siemens, Cherry Hill, NJ) and a Hitachi S-2300 (Hitachi Instruments, Danbury, CT) equipped with a Kevex Super-Flex Energy Dispersive Spectroscope (Kevex, Foster City, CA). All glass and plastic ware used in the experiment were rinsed with 0.5 M HCl prior to use.

Selective Extraction Procedures
Sequential extractions were initiated using 14 2-g wet sediment subsamples ({approx}1 g dry weight equivalent) from six cores along with appropriate controls and blanks. After each extraction step, samples were centrifuged at 2400 g for 15 min and the supernatant was filtered through a 0.20-µm membrane filter and acidified with concentrated HCl prior to ICP analysis. Samples remaining after each extraction step were washed with 10 mL of double-deionized water, centrifuged, decanted, and discarded. At least four subsamples were taken from the solids remaining after exposure to a given extractant, homogenized, and used for spectroscopic analysis.

The sequential extraction procedure used for this experiment was as follows. Ten milliliters of double-deionized water was added to 1 g of dry weight equivalent sediment and shaken for 1 h. To the remaining sediment, 10 mL of 1 M MgCl2 (pH 7) was added and shaken for 1 h. The residual sediment was then exposed to 20 mL of sodium hypochlorite at pH 9.5, while being heated to 95 ± 5°C for 1 h. This was done three times to enhance removal of the organic fraction (Anderson, 1963; Lavkulich and Weins, 1970; Omueti, 1980; Shuman, 1983). The resulting solids were then leached for 5 h with 20 mL of 1 M NaOc–HOAc at pH 5 to remove the carbonate fraction of the sediment (Tessier et al., 1979, 1985). To the leftover sediment, 25 mL of 0.1 M NH2OH·HCl in 0.01 M HNO3 adjusted to pH 2 was added (Chao, 1972; Shuman, 1982) and repeated two times to enhance removal of Mn oxides. This procedure is expected to extract contaminants that are bound to crystalline Mn oxides and is estimated to be 85% efficient at selective solubilization of crystalline Mn oxides residing in the soil (Chao, 1972). Additionally, a few percent of the total amorphous Fe will probably be removed during this extraction (Chao, 1972, 1984). For the next extraction, the sediment was shaken for 4 h with 20 mL of 0.2 M AOD (pH 3) to remove the noncrystalline materials (McKeague and Day, 1966; Schwertmann, 1973; Fey and LeRoux, 1977; Hodges and Zelazny, 1980; Jackson et al., 1986). To the remaining residue, 30 mL of 1.0 M NH2OH·HCl solution in 25% (v/v) HOAc was added and then heated to 95 ± 5°C for 6 h to remove the crystalline Fe and Mn (hydr)oxides (Tessier et al., 1979). A KClO3–HCl treatment followed by 4 M boiling HNO3 was performed to remove recalcitrant sulfides (Chao and Sanzolone, 1977). At the end of the procedure, an aqua regia–hydroflouric digestion was used on the remaining solids. The procedure was initiated by using 10 mL 3:1:2 (by volume) HCl–HNO3–H2O kept at 95 ± 5°C for 1 h, then cooled, and followed by 10 mL of HF heated for 2 h at 95 ± 5°C, shaken, and then neutralized with 20.0 mL of saturated boric acid and 0.50 g of HBO3(s) (Sridhar and Jackson, 1974).

X-Ray Absorption Near Edge Structure Spectroscopy
X-ray absorption near edge structure spectroscopy is useful for examining oxidation states and qualitatively assessing structural changes of minerals within soils. Furthermore, it can be conducted in a noninvasive environment and requires only small quantities of the solid. Samples were prepared to minimize self absorbance; after each extraction procedure the soil residue was ground with a mortar and pestle and vacuum filtered to create a thin soil residue. The thin-films were then mounted on an acrylic plate and sealed with Kapton polymide film (Dupont, Circleville, OH) to prevent moisture loss while minimizing x-ray absorption. Samples were cooled with dry ice until analysis 12 h later.

The XANES spectroscopy was performed on beamline 4-3 (an 8-pole wiggler), running under dedicated conditions at the Stanford Synchrotron Radiation Laboratory. The ring operated at 3 GeV with a current ranging from {approx}100 to {approx}50 mA. Energy selection was accomplished using a Si (220) monochromator with an unfocused beam. The XANES spectra were recorded by fluorescent x-ray production using a wide-angle ionization chamber for all samples (Lytle et al., 1984). A 3-µx Mn filter and Soller slits were used to minimize the effects of scattered primary radiation. Incident and transmitted intensities were measured with in-line ionization chambers. XANES spectra were recorded over the energy range of -200 to +500 eV about the K-edge of Fe (7111 eV). Each scan was calibrated internally by placing an Fe metal foil between the second and third in-line ionization chambers, with the first inflection point of Fe0 set to 7111 eV. Between three and six spectra were averaged for each sample. Spectral backgrounds were subtracted from the averaged XANES spectra using a low-order polynomial or Gaussian function fit to the pre-edge region. Spectra were normalized by setting the total atomic cross-sectional absorption to unity.

X-ray absorption spectra of model Fe compounds considered to be dominant species that may reside within these environments were recorded and used for comparative analysis with the unknowns. These materials include siderite (FeCO3), jarosite [KFe3(SO4)2(OH)6], amorphous iron sulfide (FeS), biotite [K(Mg,Fe)3(AlSi3O10)(OH)2], ferrihydrite (Fe5HO8·4H2O), goethite ({alpha}-FeOOH), lepidocrocite ({alpha}-FeOOH), magnetite (Fe3O4), and hematite ({alpha}-Fe2O3). The siderite (Aldrich Chemicals, Milwaukee, WI) and hematite (Stem Chemicals, Newburyport, MA) were reagent grade while goethite, ferrihydrite, and lepidocrocite were synthesized following the procedures of Schwertmann and Cornell (1991); amorphous iron sulfide was synthesized following the procedures of Patterson et al. (1997). The jarosite and biotite samples were from the Stanford Mineralogical collection. The purity of all our Fe standards was checked using x-ray diffraction.

Following data reduction, first-derivative curves were obtained using a Savitzky-Golay function; raw spectra, first-, and second-derivative curves were used for qualitative measurements. In addition, a linear combination of first- and second-derivative spectra obtained from Fe standards were fit to the unknown spectrum. Unfortunately, owing to the heterogeneous nature of the soil matrix and spectral similarity among the Fe standards, we could not unambiguously differentiate the various Fe(III) (hydr)oxide phases. However, we can monitor Fe oxidation states and solid-phase transformation in response to a given extraction, thus making XANES analysis suitable for our purposes.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Phase Partitioning of Fe by Selective Sequential Extractions
Homogenized soil removed from the site via piston coring was phase partitioned using SSE. The total Fe removed from the first two extractions was 904 ± 57 mg kg-1 (Table 1) . A significant portion (P < 0.05) of this Fe is from the magnesium chloride (exchangeable) extraction. Substantial Fe (4260 mg kg-1) was removed via a sodium hypochlorite extraction (Table 1) that is assigned to the organic pool. It is important to note that this extraction can oxidatively dissolve sulfidic fractions of the sediment, thereby overestimating Fe concentrations from the organic fraction (Hoffman and Fletcher, 1981). In addition, oxidation of FeS, or other Fe(II) phases, can lead to the formation of Fe(III) (hydr)oxides, which are then available for removal using a later extraction.


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  		Table 1 Iron and Mn distribution from selective sequential extractions of the top 0 to 2 cm of sediment

 
The effectiveness of the carbonate extraction has been questioned since it may not completely dissolve the target (carbonate) phase. Moreover, this extraction may instead dissolve some Fe oxides in the system (Belzile et al., 1989; Hickey and Kittrick, 1984). In this experiment, SSE data indicate that Fe (hydr)oxides were not dissolved by this reagent (Table 1). In addition, the SSE data suggest that Fe carbonate (siderite) was negligible in this soil.

Iron (3400 ± 300 mg kg-1) and Mn (398 ± 41 mg kg-1) were abundant in the hydroxylamine-hydrochloride-nitric acid extractable (Mn oxides) fractions. This extraction step released significantly more Fe than Mn; however, on a relative basis this is only a minor fraction (0.05) of total Fe but a substantial fraction (0.24) of total Mn (Table 1). The abundant release of Fe results as a consequence of amorphous Fe removal by the extractant (Chao, 1972, 1984). Consequently, the amount of amorphous Fe detected using the ensuing chemical extractant (AOD) will be underestimated by {approx}10% because a portion (0.05) is removed by the hydroxylamine-hydrochloride–nitric acid extraction. Therefore, predicting contaminant release solely from the hydroxylamine-hydrochloride–nitric acid extractable (Mn oxides) fraction proves difficult because Fe may be coprecipitated with Mn oxides. However, this extractant is still useful for assessing Mn concentrations in soils wherein Mn oxides are abundant (Chao, 1972, 1984).

During the AOD extraction procedure, acid dissolution of Fe, or more likely ligand enhanced dissolution, occurs (Arshad et al., 1972; McKeague and Day, 1966). The AOD extraction does not specifically remove any one particular phase, rather it extracts dominantly noncrystalline Al, Fe, and Mn all at once from the solid phase along with some crystalline material, particularly lepidocrocite, during the short reaction period employed (McKeague et al., 1971; Schwertmann, 1973; Chao and Zhou, 1983; Jackson et al., 1986). For example, AOD can extract some Fe from magnetite but little from goethite and hematite (McKeague et al., 1971; Chao and Zhou, 1983) within 4 h, whereas Schwertmann (1973) determined that oxalate extracted considerable amounts of lepidocrocite along with amorphous Fe. In the sediment investigated here, more than one-half (0.53) of the total Fe is removed by the AOD extraction, indicating that noncrystalline Fe is abundant.

The hydroxylamine-hydrochloride–acetic acid extraction used to reduce crystalline Fe oxides released approximately five times less Fe (0.09) than the AOD extraction (0.53). It appears that this extraction also removed some Mn (426 ± 43.2 mg kg-1), indicating that either the Mn oxide extraction step was not completely efficient at extracting all the Mn from the solid phase (Chao, 1972) or that Mn was associated (i.e., coprecipitated) with Fe (hydr)oxides in the system. This extraction was initially designed to remove crystalline Fe in lake bottom sediments (Tessier et al., 1979); however, for Fe-rich mine tailings Ribet et al. (1995) determined that this extraction underestimated Fe (hydr)oxides and 2.0 M hydroxylamine-hydrochloride–acetic acid followed by heating for 24 h was required to remove all crystalline Fe constituents. Thus, the hydroxylamine-hydrochloride–acetic acid extraction needs to be optimized for a given soil.

Mine tailings that exist in the region are sulfidic in origin, and thus we expected that some recalcitrant sulfides would exist in the soil (Mok and Wai, 1990; La Force et al., 1998). It appears that Fe (6100 ± 706 mg kg-1) removed using KClO3–HCl is found in this system as FeS. However, recall that the fraction of total Fe (0.09) removed during this extraction step is slightly underestimated since FeSx may become oxidized and removed during extraction with sodium hypochlorite (Hoffman and Fletcher, 1981).

Elements removed during the residual extraction are expected to be associated with the silicate matrix of the soil. These elements are considered nonlabile and are presumed to be bound tightly in the silicate fraction. This extraction step is used to provide estimates of residual Fe partitioning after an extraction scheme and when summed with the previous extractions creates an operationally defined total Fe concentration (e.g, 66515 mg kg-1). A significant portion (0.18) of Fe (12131 ± 1017 mg kg-1) remained in the solid phase until the residual extraction.

X-Ray Diffraction and Energy Dispersive Spectroscopy Analysis of the Solid Phase
After each extraction, the solid phase was analyzed for its crystalline components using x-ray diffraction. Unfortunately, we could not conclusively identify any crystalline Fe minerals; however, we were able to identify quartz (SiO2) that was present in the original unextracted sediment and remained throughout the extraction sequence until it was removed via the residual extraction, as expected.

Energy dispersive spectroscopy confirmed that Fe existed in the solid phase throughout the extraction sequence. Additionally, EDS data verified that FeAsS was detected after the organic matter extraction. This indicates that although amorphous sulfides may be removed via hypochlorite (Hoffman and Fletcher, 1981), some sulfides remain in the solid phase after this extraction until their removal using KClO3–HCl. Unfortunately, EDS does not provide oxidation state or crystallographic information. Consequently, XANES spectroscopy was used to determine the electronic state of Fe in the solid phase.

X-Ray Absorption Near Edge Structure Analysis of the Solid Phase
The XANES spectra of the sediment remaining after each step of the extraction sequence were qualitatively analyzed based on the energy position of the first-derivative pre-edge, main-edge, and post-edge spectral peaks (Fig. 1) . Iron pre-edge features contain electronic transitions (1s to 3d) and are used to provide insight into the coordination environment of Fe (Waychunas et al., 1983; Combes et al., 1989; Zhao et al., 1994). In general, Fe that is tetrahedrally coordinated (magnetite) has a greater pre-edge than octahedrally coordinated Fe (Waychunas et al., 1983; Combes et al., 1989; Manceau and Gates, 1997) (Fig. 2) . The main-edge contains information on the electronic state of the absorbing element and is used for oxidation state determination (higher oxidation states occur at greater energies), and the post-edge may be used to differentiate Fe species (Fig. 1). For example, Fe(III) (hydr)oxide standards (ferrihydrite, lepidocrocite, goethite, and hematite) have three consistent peaks within their first-derivative XANES spectra (Fig. 2). These peaks are (i) pre-edge peak that existed at 7111 eV, (ii) main-edge peak at 7124 eV, and (iii) post-edge peak at 7145 eV (Fig. 2). Jarosite had a main-edge peak at slightly higher energies {approx}7125 eV with a less pronounced post-edge peak at 7147 eV than the Fe(III) (hydr)oxide standards (Fig. 2). Siderite, amorphous FeS, and biotite have a 2 to 3 eV lower main-edge position and different post-edge spectral features than the Fe(III) (hydr)oxide standards (Fig. 2). Therefore, based on different spectral peak energy positions in our standard compounds we can differentiate between Fe(II) in these compounds and Fe(III) (hydr)oxides in our unknown samples. This information can be used for monitoring Fe oxidation state changes through the extraction sequence to determine if Fe is changing in response to chemical extractants. Furthermore, subtle, but distinct, features within the post-edge region can be used to determine the fraction of each specific phase.



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  		Fig. 1 First-derivative XANES spectral features common to standard and unknown samples. The three main XANES spectral peaks used for qualitative Fe analysis are the pre-edge, main-edge, and post-edge

 


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  		Fig. 2 First-derivative XANES spectra of Fe standards

 
We compared first- and second-derivative spectra of all unknown samples (SSE 0 through SSE 8) to determine if spectral shapes and Fe mineral fractions were altered in response to chemical extractants. The first-derivative spectra of samples SSE 0 (soil prior to extractions) through SSE 4 are similar, indicating that Fe was not removed at sufficient quantities to detect a change in the distribution of Fe species that comprise the XANES spectrum (Fig. 3) . However, in sample SSE 5, we notice a slight peak at 7172 eV in the first-derivative post-edge spectra that remained until it was removed after the residual extraction (Fig. 3). Close inspection of the second-derivative post-edge spectrum of SSE 5 also reveals a peak shoulder at 7140 eV (detected SSE 0 through SSE 4) has been removed by the addition of ammonium oxalate (Fig. 4) . Unfortunately, the shoulder at 7140 eV existed in all of the second-derivative Fe(III) (hydr)oxide standards; thus, it is difficult to decipher precisely which Fe(III) (hydr)oxide (most likely ferrihydrite) was removed during this extraction.



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  		Fig. 3 First-derivative Fe XANES spectra of solids remaining after each extraction step. SSE 0 is initial soil prior to extraction; SSE 2 solids remaining after water soluble, magnesium chloride, and sodium hypochlorite extractions

 


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  		Fig. 4 Second-derivative Fe XANES spectra of solids remaining after each extraction step

 
The spectra for the standards were qualitatively compared with the spectra obtained from the solids after each extraction step. The pre-edge peak in SSE Samples 3 through 7 was 7111 eV, with the main-edge occurring at 7124 eV, and an additional post-edge peak at 7144 eV (Fig. 3). Comparing the spectral peak energy positions in the unknown samples to the standards illustrates that Fe(III) predominates in the solid phase and remains throughout the extraction sequence until its removal during the residual extraction. Thus, XANES data indicate that the hydroxylamine-hydrochloride extraction designed to remove crystalline Fe(III) (hydr)oxides may not entirely remove Fe(III) (hydr)oxides from the solid phase. Therefore, a higher hydroxylamine-hydrochloride concentration coupled with a longer reaction time or repetitive treatments may be required to completely remove crystalline Fe(hydr)oxides from soils when using this extractant.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Selective sequential extractions can provide information regarding elemental associations in the solid phase. Understanding Fe transformations in the solid phase provides much needed detail about the efficiency and limitations of selective extractions. Additionally, using chemical extractions coupled with spectroscopic analysis provides information on the target phase response to a given reagent. In the extraction sequence employed here, the sodium hypochlorite (organic matter) extraction can oxidize Fe(II) species and sulfidic materials, causing overestimation of Fe in the organic and oxide occluded pools with subsequent underestimation of Fe residing in the sulfidic fractions. Extractions designed to target manganese oxide removed small amounts of amorphous Fe (0.05), thereby underestimating the quantity of amorphous Fe detected in the ensuing AOD extraction. The AOD extraction step removed the majority of Fe (0.53) from the solid phase. Addition of ammonium oxalate resulted in a change in the first- and second-derivative XANES spectra, indicating that Fe mineral fractions were changing in response to the extractant. Moreover, the hydroxylamine-hydrochloride extraction may not remove all the Fe(III) (hydr)oxides from the solid phase. Ferric (hydr)oxides were completely removed from the solid phase only after exposure to hydrochloric–nitric–hydroflouric acids. Nevertheless, the extraction sequence employed here appears to work reasonably well on this aerated, Fe-rich soil. Its efficiency on reduced soils or ones having vastly different mineralogy, however, has not been tested and may be limited. Chemical extractions in general need to be evaluated on a site basis for a given soil, and a given extraction should not be universally applied to different soils until its efficiency is optimized first (e.g., by varying extraction concentration, type, sequence, and reaction time).Lavkulich Wiens 1970


   ACKNOWLEDGMENTS
 
We gratefully acknowledge support provided by the Exploratory Research Program of the US EPA (grant number R-825399). We also thank Benjamin C. Bostick and Colleen M. Hansel for helpful discussions. Special thanks are given to the staff at the Stanford Synchrotron Radiation Laboratory (SSRL). SSRL is supported by Department of Energy, Office of Basic Energy Sciences; funding for the Biotech operation is provided by the National Institute of Health.

Received for publication June 4, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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