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a Dep. of Agronomy, Purdue University, West Lafayette, IN 47907
b Dep. of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907
* Corresponding author (dschulze{at}purdue.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: Ac, acetate • XANES, x-ray absorption near-edge structure • DCB, dithionite-citrate-bicarbonate • PPAC, Pinney Purdue Agricultural Center soil sample • SEPAC, Southeast Purdue Agricultural Center soil sample • SWPAC, Southwest Purdue Agricultural Center soil sample • WP, West Point soil sample
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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1000 mg Mn kg-1 of soil, but it can range from 20 to 10 000 mg kg-1 (Sparks, 1995). Colloidal Mn oxides, which make up a considerable amount of the total Mn in soils, typically have a small particle size, a large surface area, and a very high negative surface charge in the pH range of soils. Consequently, they exert chemical influence far out of proportion to their total concentrations (Bartlett, 1988). Because of their unique surface properties, Mn oxides act as scavengers for heavy metals in soil and water, enter into complexation reactions with organic matter, and play major roles in redox processes (Bartlett, 1988; McKenzie, 1989). There are indications that Mn oxides play a catalytic role in the formation of soil organic matter (Shindo et al., 1996). As an essential plant nutrient, Mn has been implicated in plant resistance or susceptibility to a variety of diseases (Huber and Wilhelm, 1988). Low concentrations and the transient nature of Mn oxide minerals in soils limits knowledge of Mn oxide mineralogy to occurrences in nodules and coatings. Manganese in soils occurs in three oxidation states: Mn(II), Mn(III), and Mn(IV). The solubilities of Mn(III) and Mn(IV) are too low for the species to remain in solution and Mn(II) is the major soluble species over the pe and pH range of soils (Norvell, 1988). Plant available Mn in soils has been analyzed by wet-chemical methods for almost a century. As early as 1935, a dynamic oxidation–reduction equilibrium was proposed between water-soluble Mn(II), exchangeable Mn(II), easily reducible MnO2, and relatively inert manganic oxides (Sherman et al., 1942). This hypothesis has been expanded recently to differentiate between Mn(II) adsorbed onto clay minerals, Mn(II) adsorbed onto or chelated with organic substances, and Mn(II) carbonates and phosphates. Of all the proposed pools, only the Mn(III) and Mn(IV) oxides are considered to be unavailable to plants. For all of the sequential extraction schemes, the extracted fractions are operationally defined, and there is no universally accepted method of validation (Ma and Uren, 1995).
Wet-chemical analysis of Mn oxidation state in soils is only suitable for bulk samples, and faces potentially large interferences from Fe (Amonette et al., 1994). X-ray photoelectron spectroscopy provides oxidation state information, but requires that the sample be placed in a high-vacuum environment (Hochella, 1988). Ultraviolet-visible spectroscopy can be used to study redox reactions, but the samples are limited to suspensions dilute enough to allow scattered light transmission (Risser and Bailey, 1992). X-ray absorption near-edge structure spectroscopy has potential for studying Mn oxidation states in soils without the need to dry or unrealistically dilute the specimen (Fendorf et al., 1994; Schulze and Bertsch, 1995; Schulze et al., 1995b; Fendorf and Sparks, 1996). Recent advances have resulted in synchrotron x-ray microprobes that can be used to determine trace element and oxidation state distributions with <10 µm resolution (Sutton and Rivers, 1999).
Pure Mn oxide minerals have a complex edge structure that contains information on the coordination environment as well as the oxidation state of the element of interest. If applied correctly, Mn XANES spectroscopy can provide valuable information on the form and oxidation state of Mn as compounds undergo oxidation and reduction reactions (Ammundsen et al., 1996, 1998; Fendorf, 1999; Fendorf et al., 1999).
The objective of this paper is to evaluate the use of XANES spectroscopy for determining the relative proportions of Mn(II), Mn(III), and Mn(IV) in Indiana surface soils, and to correlate the XANES spectra with Mn removed by different wet-chemical extractants.
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MATERIALS AND METHODS |
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In a separate experiment to assess the tendency of the Mn in a reduced soil to reoxidize, a reduced sample of the West Point (WP) soil was rapidly air-dried prior to analysis. The saturated reduced soil slurry was spread thinly onto a flat surface and a fan was used to blow air across the sample, drying it in <5 h. The air-dried soil was then crushed and stored in a plastic vial for 3 d prior to analysis.
Sequential Extraction
A sequential extraction procedure was designed to extract increasingly less soluble fractions of Mn from the samples. The extractants were chosen to measure the main hypothesized pools of soil Mn discussed above. The extraction times were adjusted such that one step of the extraction procedure could be completed in 1 h, the time required to collect a XANES spectrum of the residue remaining from the previous extraction. This allowed us to obtain XANES spectra of the residue immediately after extraction while making efficient use of our synchrotron beam time.
The extraction procedure consisted of the following sequence: (i) 1 M NH4Ac at pH 7, (ii) either 0.5 M CuSO4 at pH 2.9 or 1 M NH4Ac at pH 3, (iii) 0.018 M quinol in 1 M NH4Ac at pH 7, and (iv) DCB. The first extractant, pH 7 NH4Ac, was chosen to extract water-soluble and exchangeable Mn. For the second extractant, we initially chose 0.5 M CuSO4 because we believed that the higher affinity of Cu for organic ligands and the ability of Cu to lyse microbial cells, was best for extracting Mn from organic matter and microbial biomass (Norvell, 1988). (Extractants commonly used for organic Mn fraction determinations [NaOCl, H2O2] can also dissolve Mn oxides [Uren et al., 1988; Shuman, 1991] and thus are not appropriate.) The 0.5 M CuSO4, however, has a pH of 2.9 and the Mn released by CuSO4 may simply be an acid-soluble fraction. To test this, we repeated some of the experiments using 1 M NH4Ac acidified to pH 3 and found that the pH 3 NH4Ac extracted just as much Mn as the 0.5 M CuSO4. Therefore, we used only pH 3 NH4Ac in subsequent experiments. The third extractant, 0.018 M quinol, is a weak reductant chosen to dissolve readily reducible Mn (Leeper, 1947; Heintze and Mann, 1951). Hydroxylamine hydrochloride is also commonly used to extract soil Mn (Chao, 1972), but it typically extracts more Fe than the quinol extraction (Jarvis, 1984), indicating that quinol is more specific for Mn. The final extractant, DCB, is a strong reductant and was chosen to reduce and extract Mn oxides in more recalcitrant Mn oxide minerals, perhaps those in small concretions or accumulations, as well as any Mn substituted in Fe oxide minerals (Olsen, 1965).
Extraction Procedure
The <2-mm bulk sample was thoroughly mixed to assure uniform moisture content, then a 10-g aliquot was weighed, dried at 105°C, and reweighed to determine moisture content. An aliquot of moist soil sufficient to give 5 g of oven-dry soil was weighed into a 50-mL polypropylene centrifuge tube, and fractionated into forms designated as pH 7 NH4OAc, pH 3 CuSO4 or pH 3 NH4OAc, quinol, and DCB extractable Mn as described in Table 3. After each extraction, the residue was washed (by adding 20 mL of deionized water, shaking vigorously by hand, then centrifuging [900 x g] for 5 min) to remove entrained solution and minimize interaction between the different extracting solutions. The Mn contents of the supernatants from each extraction step were analyzed by atomic absorption spectroscopy (Varian SpectrAA 400, Varian Australia Pty. Ltd., Mulgrave, Victoria, Australia).
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Total and Total Reductant Extractable Manganese
Samples for total Mn analysis were air-dried, ground to <150 µm, and Mn was determined by x-ray fluorescence spectroscopy. Total reductant extractable Mn was determined by exhaustive DCB extraction of the moist samples. For each soil, the equivalent of 5 g of oven-dry soil was shaken with 40 mL of 0.3 M Na citrate, 5 mL of 1 M NaHCO3, and 1 g of Na dithionite for 24 h at room temperature, followed by centrifugation and decantation of the supernatant (Olsen, 1965). This was repeated two additional times. Manganese in the supernatant was determined by atomic absorption spectroscopy as described above. We refer to this measurement as total-extractable Mn to differentiate it from the DCB-extractable Mn determined in the sequential extraction procedure (two 45 min extractions). This enabled us to determine the efficiency of the sequential extraction procedure at removing the reductant extractable Mn.
X-ray Absorption Near-Edge Structure Spectroscopy
X-ray absorption near-edge structure spectra were obtained at the x-ray fluorescence microprobe beamline X26A at the National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY, with a 30 by 30 µm x-ray beam spot size. Technical details of the beam line are given by Bajt et al. (1993), Schulze et al. (1995b), and Duff et al. (1999). The energy resolution is 1.5 eV.
After each extraction and washing, the residue was mixed and a small subsample (0.1 g) was removed. The standards and soil samples were mounted between two sheets of 4-µm thick Prolene x-ray film using 32-mm diameter x-ray fluorescence sample cups (Chemplex Industries, Stuart, FL). To minimize changes in the Mn oxidation state of treated samples, all sample preparation (grinding and dilution of standards, extraction of soils) was performed at the beamline. At the energy of the Mn K-edge, 95% of the fluorescent photons entering the detector emanate from the upper 150 µm of a moist soil sample. Thus, the spectra represent a sampled volume of 30 by 30 by 150 µm3, or 135 pL. Sample heterogeneity on this scale therefore becomes a concern. To assess heterogeneity, and to ensure that XANES spectra were collected at representative locations on the samples, a line scan was done on each sample before a spectrum was acquired. The line scan consisted of setting the incident x-ray energy above the Mn K-absorption edge, then recording the total counts under the Mn K
fluorescence peak at 41 points spaced 50 µm apart along a 2-mm transect across the sample (10 s counting time). We avoided collecting the full XANES spectrum at locations with particularly high Mn concentrations, likely to be Mn concretions or other Mn accumulations, or locations with particularly low Mn concentrations, likely to be the site of quartz or other primary mineral grains.
High-resolution XANES spectra were obtained by scanning the monochromator in 0.5-eV steps across the absorption edge, and in 5-eV steps before and after the edge. The fluorescence photons were measured with a SiLi solid state detector, and the counting time per step was 10 s for most samples (XANES spectra obtained on soil samples after DCB treatment had a counting time per step of 20 s). Energy calibration was relative to the preedge peak of KMnO4 at 6543.3 eV (Riggs-Gelasco et al., 1996). The spectra were normalized by setting the average intensity below the edge (6440–6520 eV) to zero, and the average intensity above the edge (6700–6870 eV) to one. The spectra are presented without additional smoothing. Since this is a microprobe beamline that illuminates a very small portion of a sample and sample Mn contents were close to the detection limits, the spectra are noisier than comparable spectra from a beamline that illuminates a large portion of a bulk sample.
Oxidation State Standards
Reagent grade manganese sulfate (MnSO4 · H2O) was the Mn(II) standard, a reagent grade Mn(III) oxide from Aldrich (Milwaukee, WI) (Mn2O3, shown to be bixbyite by x-ray diffraction—data not shown) was the Mn(III) standard, and a synthetic birnessite (Na4Mn14O27 · 9H2O) prepared as described by Golden et al. (1987) was the Mn(IV) standard. Birnessite is not a pure Mn(IV) standard, but also contains some Mn(II) and Mn(III) (Drits et al., 1997), however birnessite is an appropriate standard because it is frequently identified in soils, and is more likely to form in the cation-rich environment of soils than a mineral like pyrolusite (ß-MnO2). In addition, our experience has shown that XANES spectra from birnessite provide a better match to XANES spectra from oxidized soils than do spectra from minerals like pyrolusite. Bixbyite, which stochiometrically should contain only Mn(III), has a crest in its XANES spectrum at the same position as Mn(III) in goethite or Mn(III) pyrophosphate (Scheinost et al., 2001) and, although it is perhaps not ideal, it is the best Mn(III) standard that we are aware of. The standards were diluted with corundum (Buehler Micropolish C 1.0 micron alpha alumina, Buehler Ltd., Lake Bluff, IL) to 2500 mg Mn kg-1 sample to avoid self-absorption. Aqueous MnSO4 has a different XANES spectrum than solid MnSO4, so to avoid errors from comparing the spectra of dry standards with spectra of wet samples, all standards were moistened with deionized water prior to XANES analysis (Schulze et al., 1995a).
Curve Fitting
A linear combination of the Mn standards (MnSO4, Mn2O3, and birnessite) was used to quantify the changes in the XANES spectra after each step of the sequential extraction. The best fit was determined by minimizing the sum of the squares of the difference between the calculated and experimental spectra over the range from 6530 to 6570 eV. A 95% confidence interval, calculated from the best fit, was used to assess the range of values for each component that could be considered to be a good fit. The range is ±10% for the MnSO4 and birnessite, and ±20% for the Mn2O3.
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RESULTS |
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The sum of the Mn extracted is usually 80 to 90% of the total-extractable Mn, but occasionally it is as high as 125%. We attribute this to the presence of Mn concretions or other Mn accumulations. Three of the four soils are Aquolls or Aqualfs (Table 1), indicating that the soils are waterlogged (and reduced) for part of the year. Wetting and drying cycles in a soil lead to the mobilization, then reprecipitation of Mn, which can result in the formation of Mn accumulations (Gilkes and McKenzie, 1988). A visual examination of the soils revealed Mn concretions in the Southeast Purdue Agricultural Center (SEPAC) soil, and even though care was taken to homogenize the <2-mm soil material prior to analysis, these Mn accumulations apparently result in increased variability in our extraction data. Greater homogenization, however, by air-drying, grinding, and sieving the soil through a smaller sieve was not desirable because it would likely result in undesirable changes in Mn oxidation state and availability (Berndt, 1988; Warden, 1991).
The sum of the Mn extracted by pH 7 NH4Ac, CuSO4 (or pH 3 NH4Ac), and quinol was approximately the same for both the aerated field-moist soil and the saturated reduced soil, and, thus, the fraction extracted by DCB remained similar regardless of microbial reduction. This indicates that the quinol extracted the same, easily reducible Mn pool that, under the conditions of our experiment, is subject to microbial reduction.
The results of the sequential extractions conducted at the synchrotron beamline immediately prior to collection of XANES spectra are essentially identical to the results of the extractions conducted in the laboratory (Tables 4, 5, and 6).
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West Point and Southeast Purdue Agricultural Center Soils
The results for the WP soil (Table 5, Fig. 1) are similar to those for the SEPAC soil (Table 6, Fig. 2). Prior to the sequential extraction, the field-moist oxidized soil had a XANES spectrum with a crest at the Mn(IV) position, but the soil had a broader peak than birnessite, and a shoulder at about the energy of the Mn(III) position. The pH 7 NH4OAc extraction removed almost no Mn, and as expected, there was no appreciable change in the XANES spectra, or in the fit results. The pH 3 NH4OAc (or pH 3 CuSO4) extraction also removed almost no Mn, however, the fit results show a decrease of 20% in the proportion of Mn(IV), with most of the change being accommodated by an increase in the proportion of Mn(III) and only a small increase in the proportion of Mn(II). These are significantly different changes that indicate a reduction of Mn(IV) even though little Mn was extracted (Fig. 1 and 2; Tables 5, and 6). Following the quinol extraction, there was a clear loss of intensity at the Mn(IV) position, and a relative increase in the size of the Mn(II) peak, with no change in the proportion of Mn(III) present. This clearly indicates reduction of Mn(IV) by the quinol. After the two DCB extractions, there was complete loss of intensity at the Mn(IV) position, consistent with removal of Mn(IV) oxides. The XANES spectra of the soil remaining after DCB extraction had peaks at the Mn(III) position, but a clear shoulder appears at the position of Mn(II). The Mn(II) and Mn(III) that remains after DCB probably occurs in primary and secondary silicate minerals, and is inaccessible to the extractants. This is supported by the fact that the combination of MnSO4 and Mn2O3 spectra cannot reproduce the XANES spectrum after DCB extraction (Fig. 1 and 2), indicating that the local environment of the remaining Mn is considerably different from that of the standards.
The saturated reduced soil (Fig. 1 and 2; Tables 5 and 6) has a XANES spectrum similar to that of the Mn(II) standard. This is expected, because in saturated anaerobic soils, Mn(IV) is reduced to Mn(II). Extraction with pH 7 NH4OAc decreased the height of the Mn(II) peak relative to that of Mn(III). Extraction with pH 3 NH4OAc further decreased the size of the Mn(II) peak. The spectra of the soils after quinol extraction were almost the same as after pH 3 NH4OAc extraction. After the two DCB extractions, all that remained was Mn(II) and Mn(III), apparently in the primary silicate minerals. The XANES spectra (not shown) and fit results (Table 5) for the reduced WP soil after rapid air-drying are virtually identical to the results for the reduced soil analyzed without drying.
Southwest Purdue Agricultural Center Soil
The results for the SWPAC soil (Fig. 3, Table 6) are slightly different from those for the WP and SEPAC soils. Prior to the sequential extraction, the field-moist, oxidized soil had a XANES spectrum that is almost entirely Mn(IV). The pH 7 NH4OAc and pH 3 CuSO4 extractions removed almost no Mn and there was no appreciable change in the XANES spectra. Following the quinol extraction, there was a loss of intensity at the Mn(IV) position and an increase in the size of the Mn(II) and Mn(III) peaks. After the two DCB extractions, all that remained was Mn(II) and Mn(III), apparently in the primary silicate minerals.
The saturated reduced SWPAC soil (Fig. 3, Table 6) has a XANES spectrum similar to that of the Mn(II) standard. Extraction with pH 7 NH4OAc removed >40% of the total-extractable Mn, with no change in the height of the Mn(II) peak. Extraction with pH 3 CuSO4 removed an additional 20% of the Mn and decreased the size of the Mn(II) peak relative to that of Mn(III). The spectrum of the soil after quinol extraction was almost the same as that after CuSO4 extraction.
It is important to note that all of the spectra in the microbially reduced soil could be fit without including Mn(IV), indicating that there was no Mn(IV) in the microbially reduced samples, and that despite the absence of Mn(IV), a strong reductant (DCB) was still necessary to dissolve about one third of the total-extractable Mn.
As mentioned above, the chemical extraction results and XANES fit results for the WP and SEPAC soils are very similar, while the results for the SWPAC soil are slightly different. The WP and SEPAC soils have similar pHs (6.0 and 5.8, respectively), while the SWPAC soil is more alkaline (pH 6.8) (Table 2). Manganese oxide mineralogy is very pH dependant (Bricker, 1965; Gilkes and McKenzie, 1988), so the differences seen in the XANES and extraction results could be because of differences in Mn mineralogy from the differing soil pHs.
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DISCUSSION |
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Our results are consistent with these expectations. Reduction increases the exchangeable Mn (pH 7 and pH 3 NH4OAc) at the expense of the reducible Mn (quinol and DCB). The XANES spectra confirm this, with reduction increasing the proportion of Mn(II), at the expense of Mn(IV). The presence of appreciable Mn(II) that is only extractable at pH 3 suggests a carbonate phase, but our soil spectra do not show a shoulder or peak for rhodochrosite at 6549 eV (Fig. 1), and the curve fitting is not improved by including the rhodochrosite standard as a component. This excludes rhodochrosite as the acid-soluble phase, although it does not exclude the possibility of a Mn(II) or Mn(III) phosphate phase or a MnCO3-CaCO3 solid solution (McBride, 1979; Zaravin, 1999), for which we do not have spectra or standards.
The XANES spectra of the reduced soil indicate that Mn(IV) is not present and that the sample contains only Mn(II) and Mn(III). The sequential extraction results, on the other hand, show that quinol, a mild reductant, removes almost no Mn from the sample, while DCB, a strong reductant, dissolves considerable additional Mn. This implies that the DCB-extractable Mn is present in a reasonably crystalline phase, but that these phases do not contain appreciable Mn(IV). Soil Mn occurs in a variety of situations, including in primary aluminosilicate minerals, in smectite or other secondary minerals, substituted into Fe oxides, or as discrete Mn oxides (Gilkes and McKenzie, 1988; Cornell and Schwertmann, 1996). Manganese in primary aluminosilicates and smectite would not be extracted by DCB, leaving Fe oxides or discrete Mn oxides as the possible phases. The curve fitting shows that the samples still contain some Mn(III) after the quinol extraction (Table 5), and that this Mn(III) is removed by the DCB extraction. Both Mn and Fe oxides are dissolved by DCB, so the Mn(III) extracted by DCB could be Mn(III) substituted in Fe oxide minerals, or Mn(III) that has been reduced within the structures of Mn oxide minerals without dissolution of the minerals themselves.
Results for the oxidized soil show that the pH 3 NH4Ac or CuSO4 extraction removes almost no Mn (Table 4), however, the curve fitting shows a drop in Mn(IV) that is out of proportion with the amount of Mn extracted (Tables 5 and 6, Fig. 1 and 2). This can be explained either by the tendency of Mn2+ to readily adsorb to Mn oxides (Fendorf et al., 1993a; 1993b), or by solid state reduction of the Mn oxides. Adsorption of Mn to oxide surfaces is decreased at low pH (Norvell, 1988), and the extractants used (1 M NH4Ac and 0.5 M CuSO4) are likely to extract any Mn adsorbed onto exchange sites. Therefore, our interpretation is that Mn(IV) is reduced to Mn(III) in situ without destruction of the crystal structure.
There is considerable evidence for this in the literature. Solid state reduction of 
-MnO2 (an intergrowth of pyrolusite and ramsdellite) occurs as electrons migrate into the structure to reduce Mn(IV) to Mn(III) while charge balance is maintained by diffusion of protons into the structure to combine with oxygens to produce hydroxyl ions. To accommodate the increased size of the Mn(III) ion, there is a continuous increase in unit cell dimensions with increased reduction (Feitknecht et al., 1960; Chabre and Pannetier, 1995). Studies of the oxidation of Co(II)EDTA2- by pyrolusite-coated silica sand in column displacement experiments (Jardine and Taylor, 1995; Fendorf et al., 1999), and the reaction of H3AsO3 with birnessite films (Nesbitt et al., 1998) both provide additional evidence for solid state reduction of Mn(IV) to Mn(III). This aspect of the chemistry of Mn oxides requires more research to fully investigate its implications in soil systems.
These results highlight the power of x-ray absorption spectroscopy for the study of soil systems. The XANES spectra in our study were obtained from soils at their ambient Mn contents of 1000 mg kg-1, and the samples were analyzed moist with a minimum of manipulation other than the sequential extraction procedure. The XANES spectra were obtained from spots as small as 30 by 30 µm (135 pL sample volumes), on three different soils, during three separate trips to the synchrotron facility, for a highly reactive element. Despite these potential limitations, the results are repeatable, and the correlation between the XANES spectra and the bulk chemical extractions indicate that the spectra are representative of the whole sample. Where duplicate analyses were obtained (field-moist WP, reduced WP, field-moist SEPAC [Tables 5 and 6]), differences in the extraction and fit results are within acceptable limits, and the changes in XANES spectra with the different extractions are the same between duplicate samples. Interestingly, the results for the WP and SEPAC soils are very similar, possibly because of their similar soil pH.
The SWPAC sample is the exception, possibly because of a more alkaline pH than the WP and SEPAC soils. Unlike the WP and SEPAC soils, fit results for the field-moist SWPAC soil do not change until after extraction with quinol, and the fit results for one of the reduced samples do not change after extraction with pH 7 NH4Ac. The duplicate reduced SWPAC samples are also very different, with one sample containing only 50% Mn(II) and the other 100% Mn(II) (as is expected for a reduced soil). As the first sample in an experimental run, the sample with only 50% Mn may have been exposed to atmospheric O2 for longer than the other samples, resulting in oxidation of reduced Mn. There may also have been inadequate mixing of the sample before removal of the aliquot for XANES analysis, which may explain why the sample after extraction with pH 7 NH4Ac has a higher proportion of Mn(II) and Mn(III). This, however, was the only time that there was any indication of inhomogeneities on the scale of the 30 by 30 µm2 incident beam size. On the other hand, a slower rate of Mn reduction after 7 d may reflect the fact that the SWPAC soil had a higher pH than the others, making reduction more difficult.
Given the uncertainties in interpreting Mn K XANES discussed above, our results cannot be used to identify the mineral form of Mn (beyond excluding rhodocrosite as a possible solid phase), nor can they be used to determine the absolute concentrations of Mn(II), Mn(III), and Mn(IV). Our results do provide a semiquantitative measure of the changes in the distribution of Mn oxidation state resulting from different chemical extractions, providing, for the first time, a possible means of validating sequential extraction schemes for Mn. Our results also show that XANES spectroscopy can track changes in Mn oxidation state that do not show up in wet-chemical extractions. Given the generally good agreement between XANES spectra collected from 135 pL volumes of soil versus extraction results obtained on 5 g quantities of bulk soil, we conclude that our experimental protocol minimized variations in Mn-oxidation state on the scale of tens of micrometers for that fraction of the soil Mn that is easily reduced. This does not indicate, however, that such homogeneity exists under all soil conditions.
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CONCLUSIONS |
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Our conclusions are: (i) In aerated soil, lowering the pH from 7 to 3 can result in solid state reduction of Mn(IV) to Mn(III) within the soil Mn oxide minerals, presumably coupled with the oxidation of organic compounds, without release of soluble Mn(II). (ii) Microbial reduction over a 7-d period solubilizes about the same pool of Mn oxides as is removed by the quinol extraction. (iii) About 2/3 of the Mn(II) resulting from the 7-d microbial reduction treatment occurs as water soluble and exchangeable Mn extractable at pH 7, but the remaining 1/3 of the Mn(II) requires an acid extractant for removal. This acid soluble Mn (II) phase probably occurs as a solid solution in calcite (MnCO3 or CaCO3) or as an Mn(II)PO4 phase, but not as rhodocrosite (MnCO3). (iv) Rapid air-drying of a reduced soil prevents reoxidation of the reduced Mn, and preserves the distribution of Mn between the different pools. (v) A considerable fraction of the total Mn occurs as Mn(III), presumably incorporated into the structure of Fe oxide minerals, or in the structure of Mn oxide minerals where it has been reduced without dissolution of the minerals themselves. This Mn pool requires a strong reductant (dithionite) for its removal. (vi) Mn(II) and Mn(III) occur in the structures of phyllosilicate clays and primary soil minerals and this Mn is not extracted by a strong reductant (dithionite).
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ACKNOWLEDGMENTS |
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Received for publication June 8, 2000.
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-MnO2 system. Prog. Solid St. Chem. 23:1–130.This article has been cited by other articles:
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  C. Negra, D. S. Ross, and A. Lanzirotti Soil Manganese Oxides and Trace Metals: Competitive Sorption and Microfocused Synchrotron X-ray Fluorescence Mapping Soil Sci. Soc. Am. J., March 1, 2005; 69(2): 353 - 361. [Abstract] [Full Text] [PDF]
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C. Negra, D. S. Ross, and A. Lanzirotti Oxidizing Behavior of Soil Manganese: Interactions among Abundance, Oxidation State, and pH Soil Sci. Soc. Am. J., January 1, 2005; 69(1): 87 - 95. [Abstract] [Full Text] [PDF]
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