Soil Science Society of America Journal 65:744-752 (2001)
© 2001 Soil Science Society of America
DIVISION S-2 - SOIL CHEMISTRY
Sensitivity of Soil Manganese Oxides
XANES Spectroscopy May Cause Reduction
Donald S. Rossa,
Heidi C. Halesa,
Grace C. Shea-McCarthyb and
Antonio Lanzirottib
a Dep. of Plant and Soil Sciences, Hills Bldg., Univ. of Vermont, Burlington, VT 05405-0082
b Univ. of Chicago/CARS, National Synchrotron Light Source, Brookhaven National Lab., Upton, NY 11973-5000
Corresponding author (dross{at}zoo.uvm.edu)
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ABSTRACT
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The examination of soils by x-ray absorption near-edge structure (XANES) spectroscopy can provide valuable clues for determining the structure and composition of Mn oxides. XANES experiments can be conducted at ambient temperatures using field-moist samples. This feature is attractive in the study of soil Mn because of the well-documented effects of sample drying on reactivity. The present study was undertaken to determine whether the analysis of field-moist soils by XANES spectroscopy has an effect on the oxidation state of the Mn. Repeated scans were taken on the same spot using seven medium- to high-Mn soils. In all 10 experiments with untreated soils, the main absorption edge dropped in relative energy in successive scans, indicating reduction of Mn. The change in eV was as high as 1.7 eV after 150 min of exposure time. After a series of scans showing reduction, small movement of the sample in the x-ray beam and reexamination showed no reduction, indicating that the effect was limited to the x-ray spot. Modeling the change with Mn(II) was successful and showed increases in reduced Mn between 6 and 20% after three to five repeated scans. The slopes of eV change vs. exposure time were remarkably similar in most experiments, even with large differences in the density of photons between runs. Comparisons of initial moist scans with those of recently air-dried samples found little difference, suggesting minimal reduction in a single XANES scan under our experimental conditions.
Abbreviations: EXAFS, extended x-ray absorption fine structure • NSLS, National Synchrotron Light Source • XANES, x-ray absorption near-edge structure • XAS, x-ray absorption spectroscopy
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INTRODUCTION
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X-RAY ABSORPTION SPECTROSCOPY (XAS) has been used to study the structure and oxidation state of Mn oxides (e.g., Manceau et al., 1992; Schulze et al., 1995b). X-ray absorption near-edge structure spectroscopy is useful for examining metal oxides that cannot be successfully analyzed by diffraction techniques, either because of low concentrations or low crystallinity. Both of these features are common with soil Mn oxides (McKenzie, 1989). This technique has great promise for the in situ study of Mn reactions in soil also because, unlike many other techniques, the samples can be examined in their field-moist state. However, soil Mn oxides have been found to be sensitive to environmental conditions (e.g., Fujimoto and Sherman, 1945; Bartlett and James, 1980) and reduction has been found to occur as a result of sample heating, drying, wetting, and pasteurization. In biological systems, XAS experiments are usually run at extremely low temperatures to avoid both sample damage and photoreduction (Powers, 1982). Because of the sensitivity of soil Mn oxides and because of problems found in biological studies, it is prudent to determine if the x-ray beam has an effect on the oxidation state of Mn.
Manganese XANES spectra consist of an energy scan across the main absorption edge that has an energy corresponding to the ejection of inner K shell electrons. The position of this edge (near 6550 eV) is found at higher energy at higher oxidation states, relating to the electron binding strength. A pre-edge feature is also present that corresponds to transitions to bound state orbitals. This peak and the main crest height and shape can vary, depending on the oxide structure. Interpretation of both the pre-edge and main-edge features has some theoretical basis but mainly relies on correlation with known structures. Manceau et al. (1992) analyzed a series of oxides and found that the intensity and width of the pre-edge peak changed with differences in the tunnel size in their structure. Schulze et al. (1995b) used mixtures of Mn(II) and synthetic birnessite to derive equations for estimating the ratio of Mn(II)/Mn(IV) in unknown samples. These relationships have been used to map the spatial distribution of oxidation states in the rhizosphere (Schulze et al., 1995a). For other elements, the proportion of different oxidation states can be obtained by curve fitting with the spectra of pure end members. A problem with Mn XANES is that "higher" oxides, such as birnessite, may contain 20% Mn(III) (Drits et al., 1997). Thus a relevant Mn(IV) oxide standard is difficult to obtain. Additionally, coordination and mineral structure influence the position and shape of the main edge and crest. These factors create problems in modeling unknown soil Mn oxides. Obviously, accurate interpretation of Mn XANES spectra of soils also requires that reduction does not occur during analysis.
Much of the previous XANES work on Mn oxides has been done on mineral standards, either synthesized or obtained from standard collections (Manceau et al., 1992; Schulze et al., 1995b). Previous reports on Mn XAS of soils and sediments have used dried samples or reduced, flooded samples (Schulze et al., 1995b; Friedl et al., 1997). The advantages of using dried samples include easier sample homogenization and mounting for analysis. However, the effect of sample treatment, such as oxidation of an added metal, cannot be examined over time. Additionally, structural changes in the Mn oxides may occur with extended air drying (Ross et al., 2001). This investigation was undertaken to examine the effect of the x-ray beam on Mn oxides in moist soils, using repeated scans on one spot to examine changes in XANES spectra.
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MATERIALS AND METHODS
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Samples of the A horizon were taken in northwestern Vermont from soils having high-pH basal till or bedrock. All were either Typic or Lithic Eutrudepts but land use varied among pasture (Supersoil and Subsupersoil), forest (Lordstown, Hardhack, and Hickory), and long-term garden (Bridport). Samples were put through a 4-mm polyethylene sieve and stored at 4°C in double 4-mil polyethylene bags with a moist paper towel placed between the layers. Soil pH was measured in 0.01 M CaCl2 (2:1, v/v) and total C by elemental analyzer. Total reducible Mn was determined using a modification of the hydroxylamine hydrochloride method reported by Gambrell (1996). The Standard Chromium Net Oxidation Test (Bartlett and James, 1996) was performed without modification. Extractable Mn was measured using the modified Morgan's extractant (1.25 M NH4OAc, pH 4.8) at a 5:1 solution/soil ratio and 15 min shaking. More details on the soils and their characterization are given in Ross et al. (2001).
XANES Experiments
XANES spectra were obtained with the x-ray fluorescence microprobe at beam line X26A of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratories, Upton, NY. The experiments were performed during five different beam-time allocations from 1996 to 1999. Initial experiments were performed using an 8:1 ellipsoidal focusing mirror (279 in Table 1), and subsequent experiments used Au- or Rh-coated Kirckpatrick-Baez microfocusing mirrors. Spot sizes varied between 27 by 26 µm to about 300 by 300 µm. The intensity of the x-ray beam varied between runs, due to changes in the focusing mirror setup and changes in the operating voltage of the storage ring (2.485–2.800 GeV). Within a run, the intensity declined with the normal loss of energy during the 12-h lifetime of the electrons in the storage ring. Incident photons s-1 were calculated, based on measured ion chamber currents adjusted for differences in ion chamber gas composition between runs. Photon flux on the sample (ph s-1 µm-2) was adjusted to account for mirror reflectivity and size of the focused incident beam.
Field-moist soil samples were packed into 23-mm-diam. polyethylene XRF cups (Chemplex Industries, Stuart, FL) and covered with 4.0-µm prolene film. Dried samples were mounted between two layers of prolene at the end of the same cups. Spectra were obtained by monochromator scans from about 40 eV below to 300 eV above the main edge. Except for the first experimental run (Table 1, Run 279), step size was 0.18 eV in the main-edge region but coarser above and below to obtain baselines. In the first run, step size was between 0.45 and 0.90 eV in the main-edge region (-15 to +75 eV relative to the Mn(VII) pre-edge). For all runs, collection time varied between 1 and 8 s, with longer times for soils lower in Mn. Most scans were of the time length typically needed to obtain quality spectra, although some experiments were done with unusually short scan times. Repeated scans were run automatically and the elapsed time between scans was only the few seconds necessary for repositioning the monochromator. The length of each scan and number of scans per experiment varied (shown in Table 1) but most were run as typical scans (30–60 min each) and repeated 3 to 5 times. Energy was calibrated to the pre-edge peak of a 10% KMnO4 (MnVII) standard (Riggs-Gelasco et al., 1996) at 6543.3 eV. This peak was set as 0 eV relative energy. The calibration standard was run immediately before and after each set of repeated scans unless otherwise indicated. Relative intensities were calculated as the Mn intensity (normalized to an upstream ion chamber to account for beam variation) minus the lower baseline (average of intensities from -40 to -20 eV) divided by the average upper baseline intensities between 160 and 300 eV.
Data Analysis
XANES spectra were fit with PeakFit version 4 (SPSS, Chicago, IL) using the residuals method with Gaussian/Lorentian curves. The fits were performed on spectra between -10 and +40 eV relative energy. For fitting purposes, the first scan in a series of repeats was modeled along with the aqueous MnSO4 from the same experimental run. The two fitted spectra were iteratively combined at differing ratios to produce the best visual fit with the last soil scan in the series. All fits had an r2 > 0.999 except for the Bridport soil (0.998).
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RESULTS AND DISCUSSION
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Soils used in this study had total reducible Mn contents between 0.05 and 1.75% (Table 2). In general, the range of Mn in soils is wide but a typical content of 0.06% has been reported (Gambrell, 1996). Soils were sampled at varying times before analysis (from 2 d up to 5 mo) but associated work (Ross et al., 2001) established that long-term cold storage in a field-moist condition did not cause changes in the high oxidative capacity of these soils. However, sample drying did have a reductive effect and did cause changes in XANES spectra, observable after extended drying.
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Table 2. Characteristics of the soils used. Reducible Mn was estimated by treatment with NH2OH · HCl, and extractable Mn was measured in 1.25 M NH4OAc, pH 4.8
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There was a downward movement toward lower energy in the main edge in all samples examined (10 experiments on seven soils). This, along with the change in the shape of the main edge (Fig. 1– 4)
, suggest an increase in Mn(II), and this increase can be successfully modeled as discussed below (Fig. 5) . Although the energy position was not calibrated between scans, the change over the time of the scans was not large. In all experiments, the absolute change in energy position averaged 0.15 eV (Table 1). At the energy of Mn XANES and under our experimental conditions, accurate resolution of a shift of 0.15 eV or less is difficult because of the size of the incident beam on the monochromator, the monochromator motor resolution, and mechanical reproducibility. The change in calibration represents the potential error in energy and is minor compared with the overall shift in edge position seen in the soil samples. In fact, in cases in which the calibration standard shifted in the negative direction, the downward movement in the soil scans should have been slightly greater.
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Fig. 1. Repeated Mn XANES scans of a Supersoil sample. The first series of scans were run successively on the same 310- by 300-µm spot. After energy calibration, the second series was run on a different spot, 1.4 mm offset from the initial scans. The energy is relative to the pre-edge peak of a Mn(VII) standard
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Fig. 4. Short-cycle repeated Mn XANES scans of a Supersoil sample. Each scan of a 310- by 300-µm spot lasted 8.0 min. The energy position was calibrated after the sixth run. The energy is relative to the pre-edge peak of a Mn(VII) standard
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Fig. 2. Repeated Mn XANES scans of a Lordstown-I sample run successively on the same 77- by 90-µm spot. After energy calibration, the sixth scan was taken on a different spot, 1.0 cm offset from the original. The energy is relative to the pre-edge peak of a Mn(VII) standard
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Fig. 3. Repeated Mn XANES scans of a Lordstown-II sample run successively on the same 26- by 27-µm spot. The fifth scan was taken at the same spot 15 h after the fourth scan (some error in the relocation of the sample is possible). The energy is relative to the pre-edge peak of a Mn(VII) standard
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Fig. 5. X-ray absorption near-edge structure (XANES) spectra and modeled fits with Mn(II) from the repeated-scan studies. The spectra shown are the first and last in each series. Dotted lines show the model fit to the first scan and the combination of this model with Mn(II) that best fits the last scan. A spectra of aqueous Mn(II) is given in (a) for comparison
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The series of longer scans of the Supersoil (Fig. 1) displayed the most visually dramatic changes. Each successive scan showed about 0.4 eV decrease in the edge position at half-height, and the presence of a shoulder, presumably from Mn(II), becomes increasingly pronounced. After the first series of repeated scans, the sample was moved 1 mm both horizontally and vertically and a new spot was examined. The fact that reduction is occurring in the x-ray beam is confirmed by the coincidence of the two scans of this series with the first two scans of the previous series. If other conditions of the experiment were causing reduction, it should have been occurring throughout the sample.
A similar experiment was conducted with the Lordstown-I soil (Fig. 2). In this experiment, the sample was moved 1 cm in the horizontal direction after the fifth scan, and again the spectra matched that of the original scan. In this and most other series of scans, the greatest change occurred between the first and second scans (the nonlinearity of the curves in Fig. 6
reflect this). This pattern is expected since one would not predict that the oxide would convert to Mn(II) in a linear fashion. We did not conduct the experiment on any sample long enough to determine how far the reduction would proceed.
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Fig. 6. Change in oxidation state vs. time of exposure to the x-ray beam for all the repeated-scan experiments. The elapsed time was measured from the beginning of each scan. The Lordstown-II + Mn(II) sample had 20 mmol kg-1 Mn(II) added just before the first scan. Only the first 4 of the 21 scans in this series are shown
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The series of scans that showed the greatest tendency toward an end point were the Lordstown-II (Fig. 3). In this experiment, there was only slight downward movement between the second and fourth scans. The sample was reexamined 15 h after the fourth scan by returning that sample stage to the same location. The accuracy of the relocation was tested with a ZnS crystal (that fluoresces in the x-ray beam) mounted on a sample cup, and results showed that it was accurate to within 10 µm. With a spot size of 26 by 27 µm for this set of scans, it is not possible to show that the same spot was examined. However, the fifth scan does show one of two possibilities: (i) if a portion of the same spot was reexamined, then the soil outside the original spot was not reduced because the fifth scan is much closer to the first scan; or (ii) if the exact same spot was reexamined, then partial reoxidation occurred over the 15-h period. A combination of the two interpretations is also possible. This discussion is pertinent to the results, discussed below, on soils with added Mn(II). At a minimum, the 15-h scan confirms the results discussed above for the Supersoil and Lordstown-I series.
The scans shown in Fig. 1 to 3 were all run for about the length of time needed to obtain a useable spectra under the experimental conditions at the NSLS. The data on the position of the main edge are collected relatively early in a scan and the effects shown may be insignificant if subsequent scans are not going to be performed. To test the effect over shorter periods, soils were run with shorter collection times and, in some cases, coarser energy steps above the main edge. When the Supersoil was run on an 8-min cycle, downward movement of the edge was still apparent (Fig. 4). The overall rate of movement was similar to that found with longer scans in the same soil, about -0.6 eV after nearly 40 min (shown also in Fig. 6). In this series of scans, the energy position was recalibrated between the fifth and sixth scans. Because the spot size in this case was 300 by 300 µm, a 10-µm error in repositioning should not have affected the results. Similar changes were found in a short-scan series with a Lordstown-I sample. These results show that some reduction will occur within the time needed to collect data on the edge position. The magnitude of this change, however, may be minor compared with the precision of the monochromator and normal differences between samples.
A combination of the spectra of aqueous Mn(II) and the original scan can successfully model the change between the first and last scan in the longer series (Fig. 5). The lower portion of the edge can be precisely matched but there is some error evident in the upper portion. This may just be experimental error but it may also reflect a change in the oxide toward greater Mn(III) content. Manganite (
-MnOOH) can be added as an additional model component and this slightly improves the fit. The use of manganite alone, instead of Mn(II), did not give good results. The fits show an increase in Mn(II) between 8 and 14%. Similar changes can be obtained by using the method of Schulze et al. (1995b) to estimate Mn(II)/Mn(IV) ratios based on the peak intensities at 6552.6 and 6560.9 eV (9.3 and 17.6 relative eV). However, absolute numbers derived using this equation are negative because of differences in energy calibration and differences in standards assumed to represent Mn(IV) (synthetic birnessite vs. soil for our study). Although the exposure time was lengthy for these experiments, the magnitude of the reduction is large enough to present concerns for shorter experiments.
The rate of reduction was not related to variation in the photon flux on the sample, which ranged more than two orders of magnitude (Table 1). In fact, the rate was remarkably similar for all soils (Fig. 6) under a variety of conditions (shown in Table 1). Especially over the first hour, there is extremely close agreement between five of the series, representing four different soils and three different experimental sessions. Apparently, exposure to the x-ray beam at any photon flux within our experimental conditions was sufficient to cause reduction. This is consistent with the ease of reduction by other treatments such as heating or drying. These results indicate that the reduction might not be greater at brighter synchrotron x-ray sources, such as the Advanced Photon Source at Argonne National Laboratory.
The only instances in which we have not observed reduction with repeated scans are in experiments in which Mn(II) was mixed with the soil just before collecting spectra. In most of these experiments (data not shown), the initial XANES scan clearly showed the Mn(II) addition and subsequent scans, taken at 
12-h intervals over 2 to 3 d, showed oxidation. In a few experiments, not enough Mn(II) was added to result in a detectable downward shift of the main edge relative to untreated samples (i.e., the added Mn(II) could not be observed). No reduction was observed in subsequent scans but there were hours between scans instead of seconds as in the scans shown in Fig. 1 through 5. However, one experiment with added Mn(II) was run in a constant loop, collecting 21 scans over a 14-h period. This Lordstown-II sample (Fig. 7)
had been pretreated twice with 40 mmol kg-1 of Mn(II) in the month before analysis and then with 20 mmol kg-1 of Mn(II) just before obtaining the spectra. Apparently, the initial additions were mostly oxidized and masked the subsequent addition of reduced Mn. In this set of spectra, there may have been greater movement in the energy calibration than other series, both because of a much longer time span (more than 14 h) and because the storage ring was refilled during the experiment. However, there was almost no difference in the scans with >14 h of exposure time. The slight decrease in the edge position (Fig. 6) could be real or simply a function of experimental variation due to the lack of energy calibration. These results suggest that the reactive surface sites may behave differently after the addition of Mn(II). Previous work has shown that added Mn(II) is rapidly oxidized but that the soil's ability to oxidize added Cr is decreased in the first hours after Mn(II) addition (Ross and Bartlett, 1981). Apparently, the surface sites that adsorb Cr(III) are initially blocked by Mn(II). If the observed reduction of Mn is taking place at the same reactive sites, it is possible that the adsorption of Mn(II) at these sites is shielding the surface and preventing the transfer of e-.
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Fig. 7. Successive Mn XANES scans of a Lordstown-II sample that had been pretreated with Mn(II). Scans are averaged into groups of four and offset by 0.05 relative intensity (scan 5 was interrupted by a beam dump and is not shown). The soil had 80 mmol kg-1 of Mn(II) added in two doses 3 and 4 wk before analysis, and 20 mmol kg-1 of Mn(II) added just before analysis
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While there are apparently no published reports of photoreduction in the literature on Mn mineral oxides, the problem is well known in biological studies, and x-ray absorption experiments are routinely run at liquid helium temperature 17 to 20 K (e.g., Yachandra et al., 1986; Schiller et al., 1998). Early work showed that these temperatures were needed to prevent sample damage and photoreduction (Powers, 1982). In other x-ray absorption spectroscopic techniques, such as extended fine structure x-ray absorption spectroscopy, the samples are run at extremely low temperatures to minimize molecular movements and photoreduction is coincidentally avoided. The obvious difference between the mineral oxides and biological systems is the presence of reducing organics. Indeed, this is one of the main differences between soil and unconsolidated rock material. If one considers the soil as a biological system, then the results are not surprising. It is probable that the soil Mn oxide surfaces are not free of organics. An association between humus formation and Mn oxides has been hypothesized (Shindo and Huang, 1982; Bartlett, 1990). Naidja (1998) showed the accumulation of the reaction products of catechol on birnessite. Some of the soils used in this study were relatively high in C. No mention has been made of microorganisms but it is conceivable that bacteria colonize the manganese oxide surfaces. Some would argue that the oxides themselves are products of microbial oxidation (e.g., Nealson et al., 1988). It is thus probable that organics are intimately associated with Mn oxides in these soils. The source of reduced Mn may be Mn(III) or Mn(IV) complexed to organic matter, either at the Mn oxide surface or possibly as separate organic complexes.
An alternate source of electrons for reduction is the materials used in sample preparation for XANES analysis. In the absence of any organic compounds in the sample, Zavarin (1999) found reduction of selenate, sorbed to calcite, after extended exposure to a synchrotron x-ray beam. The hypothesis put forth was that the adhesive on Kapton or Mylar tapes, used to mount the sample, was the probable source of electrons causing reduction to selenite. While we used a prolene film with no adhesive, contribution of electrons from sample-holding materials cannot be ruled out.
Fortunately, the magnitude of reduction does not appear to be large enough to cause a problem in a single scan. Comparison of XANES spectra from moist and recently dried samples (Fig. 8) showed little difference. The spectra in Fig. 8 are from different subsamples of the same soil and the reductive effect is probably less than experimental error and differences between subsamples. This comparison assumes that air-dried samples do not show reduction with exposure to the x-ray beam and this assumption may not be valid. Overall, our results demonstrate that repeated scans on a single spot will cause reduction and experimental artifact. Longer collection times for samples lower in Mn or for extended x-ray absorption fine structure (EXAFS) experiments may cause reduction that will adversely affect results and interpretations. Soil manganese oxides appear to be susceptible to change by any method of study, similar to living organisms.
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Fig. 8. Comparisons of Mn XANES spectra of moist and air-dried soils. Different subsamples of each soil were analyzed. A spectra of synthetic birnessite is given in (a) for comparison
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ACKNOWLEDGMENTS
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We thank Darrell Schulze of Purdue Univ. for generous assistance with synchrotron techniques; many members of NCR-174, Synchrotron x-ray Sources in Soil Science Research, for their advice on techniques and data analysis; and the Smithsonian National Museum of Natural History for Mn oxide mineral standards.
Received for publication February 17, 2000.
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TOP
ABSTRACT
INTRODUCTION
