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Division S-2—Soil Chemistry

Oxidizing Behavior of Soil Manganese

Interactions among Abundance, Oxidation State, and pH

^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 (Christine.Negra{at}uvm.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Due to their high degree of reactivity, Mn oxides in soil systems may exert a greater influence on trace metal chemistry than that suggested by their relatively low abundance. In particular, Mn is the only known oxidizer of trivalent Cr in soils. We investigated soil properties that influence the Cr oxidizing capacity of Mn oxides in eight well-aerated high Mn soils. Total and easily reducible Mn abundance were quantified by extraction with 1.5 M NH₂OH·HCl and 0.02 M hydroquinone. Relative average Mn oxidation state in soil samples was determined by x-ray absorption near edge structure spectroscopy (Mn-XANES) main edge energy position. Soils ranged in percentage of NH₂OH·HCl-extractable Mn between 0.14 and 1.27, pH between 4.4 and 7.2, and percentage of C between 9.0 and 27.2. Manganese-XANES spectra showed that most of the study soils had a high Mn(IV)/Mn(III) ratio with edge energy position intermediate to that of a synthetic birnessite and a synthetic pyrolusite. In these high Mn soils, Mn-XANES edge energy was positively correlated with soil pH, suggesting a linear increase, over the normal range of soil pH, in the Mn(IV)/Mn(III) ratio of soil oxides. Soils with more total reducible Mn generally demonstrated greater net Cr(VI) production, but this pattern was moderated by soil pH and relative Mn oxidation state. High Mn soils with low pH and Mn oxidation state were weaker Cr oxidizers than their Mn abundance would suggest. Our data provide evidence that greater Mn abundance and greater Mn(IV)/Mn(III) ratio in soil Mn oxides enhances Cr oxidation.

Abbreviations: XANES, x-ray absorption near edge structure

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE CHEMICAL INFLUENCE of Mn oxides in soil systems may be much greater than that suggested by their relatively low abundance. This is due to their surface characteristics (e.g., high negative surface charge, low point of zero charge, large surface area, low crystallinity) and dynamic redox behavior. Manganese oxides may act as scavengers of trace metals (McKenzie, 1989; Kay et al., 2001; Latrille et al., 2001; Palumbo et al., 2001; Liu et al., 2002) and evidence points to a catalytic role in creation of soil organic matter (Shindo and Huang, 1982; Bartlett and James, 1993). Manganese oxides are powerful oxidizers, known to oxidize Co(II) to Co(III) (Crespo and Lunar, 1997; Manceau et al., 1997; Brooks et al., 1999; Fendorf et al., 1999), Cr(III) to Cr(VI) (Bartlett and James, 1979; Risser and Bailey, 1992; Kozuh et al., 2000; Kim et al., 2002), and As(III) to As(V) (Scott and Morgan, 1995; Chiu and Hering, 2000). Tetravalent Mn oxide minerals can catalyze Fe precipitation by oxidizing Fe²⁺ ions (Golden et al., 1986). An understanding of the soil properties that affect Mn oxide reactivity is important to evaluating the potential for Mn-mediated transformations in soil systems.

There are substantial gaps in knowledge of Mn in the bulk soil due to difficulties in studying the typically small, low-abundance, low-crystallinity Mn oxides (McKenzie, 1989; McDaniel and Buol, 1991; Latrille et al., 2001). Occurring largely in the fine-grain fraction, soil Mn may be found as dispersed particles, small concretions or nodules, coatings on ped surfaces or in association with other inorganic constituents (Childs, 1975; Ross et al., 1976; Hudson-Edwards et al., 1996). Mineral forms of Mn oxides vary considerably, differing primarily in the arrangement and linkage of Mn octahedra. A range of layer charges and compositions are possible given the variety of redox conditions and chemical constituents present during soil Mn oxide formation (Post and Veblen, 1990).

Manganese oxides have been shown to oxidize Cr(III) in natural environments while Fe²⁺, organic matter and reduced S have been shown to reduce oxidized Cr (Eary and Rai, 1991; Fendorf and Li, 1996; Kim et al., 2002). There is substantial evidence that Mn abundance and oxidation state correspond to Cr oxidizing capacity in soils. In kinetic studies of Cr oxidation and reduction in soils by Kozuh et al. (2000), net Cr(VI) production was much higher in high Mn, low organic matter soils. Risser and Bailey (1992) observed that the rate and extent of reaction of Cr³⁺ with a synthetic birnessite was influenced by initial reactant concentration and proposed that removal of reduced Mn²⁺ is the rate-limiting step due to slow diffusion from the oxide surface or a slow rate of Mn²⁺ hydration. Work by Bartlett and James (1979) demonstrated that acidic, reduced, and low Mn soils were poor Cr oxidizers and that the net Cr oxidizing capacity of field-moist soils can be predicted by the abundance of hydroquinone-extractable Mn, a reactive oxidized Mn fraction.

Several studies have suggested that Mn(III) functions as the primary oxidant in oxidizing Mn systems (Johnson and Xyla, 1991; Manceau et al., 1997; Nico and Zasoski, 2000), while others have indicated that Mn(IV) has a more important role (Jardine and Taylor, 1995; Banerjee and Nesbitt, 1999; Fendorf et al., 1999; Guha et al., 2001). Kim et al. (2002) found a positive correspondence between the Mn(IV)/Mn(III) ratio of natural Mn minerals and amount of Cr(III) oxidized over 12 h. Ross et al. (2001a) demonstrated that the reducing effect of even short-term air-drying on soils was linked to an increase in exchangeable Mn(II) and a significantly lowered net Cr oxidizing capacity. The complex and dynamic chemical environment in a natural soil affects the degree to which Cr(III) is oxidized. For this reason, the Standard Net Chromium Oxidation Test relies on the use of field-moist soils in which the redox poise has been preserved.

Chromium oxidation may also be influenced by soil pH. Low pH has been observed to enhance Cr oxidation in dilute Cr(III) suspensions applied to naturally formed Mn oxides (Kim et al., 2002) and moist soils (Bartlett and James, 1979), and low pH enhanced Cr oxidation in dried soils (Chung and Sa, 2001). This may be attributed to the proton-consuming nature of most Mn reduction reactions or to the decreasing Cr(III) solubility and formation of Cr(III)-hydroxide phases on Mn surfaces with increasing pH (Manceau and Charlet, 1990; Johnson and Xyla, 1991; Fendorf and Zasoski, 1992). It is conceivable, however, that with more concentrated Cr(III) treatment, the abundance of reactive oxidized forms of Mn may become the limiting factor for Cr oxidation (Guha et al., 2001). If so, pH conditions conducive to oxidation of Mn (i.e., pHs of 6 to 7) could become a more important controlling variable. Decreasing pH is a precursor to an increasing proportion of exchangeable Mn, at the expense of the bound and structural oxide fractions (Marschner, 1988). For a given redox potential within reducing conditions, Mn reduction becomes more likely as pH decreases.

X-ray absorption near edge structure spectroscopy makes it possible to collect information related to elemental oxidation state and chemical environment in situ within the complex chemical matrix of a natural soil sample. Because it is sensitive only to the local environment of absorbing atoms, XANES is suitable for use with disordered minerals and elemental concentrations as low as 100 µg g^–1 or less (Schulze and Bertsch, 1995). When working with complex, unknown samples, XANES spectra must be assumed to reflect the combination of different oxidation states and coordinating environments for the element of interest. The XANES main edge energy position provides greater sensitivity to elemental valence than the preedge, which is more influenced by structural effects (McKeown and Post, 2001). Systematic determination of XANES "fingerprints" for elements in reference compounds has been used to determine oxidation state in unknown samples through spectral comparison, founded on the assumption that unknowns can be modeled as a linear combination of knowns. The wide variety of Mn minerals that may be present in a soil is an obstacle to determination of the relative abundance of the possible oxidation states from XANES spectra. Nevertheless, estimates of the relative elemental oxidation state may be useful in making comparisons with reference compounds and across soils (Manceau et al., 1992).

The oxidation state of Mn can change rapidly in response to environmental conditions; when oxidation state information is of interest, care must be taken to minimize disruption of redox poise through sample handling and experimental protocols. Reduction of Mn, as reflected in an approximately 1.5 to 1.7 eV downward shift in main edge position, resulted from long-term air-drying of soil samples as well as extended exposure to an x-ray beam (Ross et al., 2001a, 2001b). In well-aerated soils, Mn²⁺ concentrations are highly variable and can range from approximately 10^–3 to approximately 10^–9 M, but are likely to comprise a minor proportion of total Mn (Guest et al., 2002). In synthetic birnessite, interlayer or surface exchange sites may be occupied by Mn²⁺ or Mn³⁺, and Mn(III) may lie within structural layers (Risser and Bailey, 1992; Appelo and Postma, 1999).

The purpose of this study was to investigate soil properties that influence the oxidizing capacity of Mn oxides in well-aerated, high Mn soil systems. We used Mn-XANES spectroscopy to determine relative average Mn oxidation state in eight high Mn soils and compared these results with basic soil properties to identify important controlling factors of Mn oxidation state. Using Cr oxidation as an indicator of Mn redox activity, the influence of Mn abundance and oxidation state on Mn oxidizing capacity was assessed. We also adjusted sample pH to ascertain whether Mn oxidation state and oxidizing activity would shift in predictable ways.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil Characteristics
Eight soil samples were gathered from high pH basal till or bedrock Typic and Lithic Eutrudepts within Farmington-mapped units (loamy, mixed, active, mesic) in northwestern Vermont. Most were from the same locations as those reported by Ross et al. (2001a)( 2001b). All soil samples came from surface horizons, with the exception of one B horizon (Maple-B). All were hardwood forest soils, with the exception of one pasture soil (Barber), and were selected to obtain samples with high levels of Mn and a broad range in pH. Samples were passed 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. Selected samples were treated with 3 M H₂SO₄ (Maple-B, Maple-O) and CaCO₃ (Barber, Pease, Ridge, Holiday), shaken vigorously at intervals over an approximately 300-d incubation period, double-bagged, and stored at 4°C to achieve a modification in equilibrium pH.

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 (NH₂OH·HCl) method reported by Gambrell (1996). This method is similar to that used by Neaman et al. (2004) which achieved nearly total Mn dissolution. Treatment consisted of intermittent shaking of 0.5 mL of 1.5 M NH₂OH·HCl and approximately 5 mL of 0.1 M HNO₃ with 40 to 90 mg of the dried, ground (passed through 100-µm sieve) sample. The sample was diluted to 25 mL with 0.1 M HNO₃ and allowed to settle overnight. For comparison, selected samples were analyzed for total elemental abundance by energy-dispersive x-ray fluorescence spectroscopy (Spectro-X-Lab 2000, Kleve, Germany). Total Mn abundance was determined by the hydrofluoric acid digestion method reported by Jackson (1958). Samples were allowed to cool, treated with 100 mL of saturated H₃BO₃ solution, and diluted to 200 g. Easily reducible Mn was determined using the hydroquinone method reported by Bartlett and James (1979). Moist soil (2.5 g dry weight) was shaken for 1 h with 12.5 mL 0.02 M hydroquinone. Samples were diluted with 12.5 mL of 1 M CaCl₂ and shaken 15 min. The concentration of Mn in all extracts was measured by ICP–AES.

The Standard Net Chromium Oxidation test and a modification of the test were used (Bartlett and James, 1996). In the modified version, moist soil (equivalent of 5 mg Mn based on NH₂OH·HCl-extractable Mn) was shaken with 30 mL of 0.001 M CrCl₃ instead of 2.5 mL of soil shaken with 25 mL of 0.001 M CrCl₃ as in the standard test. Because of an order of magnitude difference in total reducible Mn among samples, soil/solution ratio ranged from 1:100 to 11:100.

XANES Experiments
All Mn-XANES spectra were obtained using the x-ray fluorescence microprobe at beamline X26A of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratories, Upton, NY. The energy of the incident x-ray beam was tuned using a channel-cut Si(111) monochromator. This beam was then collimated to 350 µm in diameter using a four-jaw slit assembly and then focused to roughly 10 x 10 µm using Rh-coated Kirckpatrick-Baez micro-focusing mirrors. Fluorescence x-ray intensity was measured by using a Si(Li) energy-dispersive detector 90° to the incoming x-ray beam. Experiments were performed during seven different beam-time allocations from 1999 to 2003. Spectra were obtained by stepping the incident beam energy across the range of approximately 40 eV below to approximately 300 eV above the main Mn absorption edge. Step size was 0.18 eV in the main-edge region but coarser above and below to obtain baselines. For all runs, collection time varied between 4 and 8 s, with longer times for lower Mn soils. Energy was calibrated to the pre-edge peak (6543.3 eV) of a 10% KMnO₄ (MnVII) standard (set as 0 eV relative energy) that was run immediately before and after each scan (Riggs-Gelasco et al., 1996). Relative intensities were calculated as the Mn intensity (normalized to an upstream ion chamber to correct for decay in beam intensity) minus the lower baseline (average of intensities from –40 to –20 eV relative energy) divided by the average upper baseline intensities between 160 and 300 relative eV. In one case, the upper baseline was not level and the relative intensity was calculated by normalizing to a consistent main peak crest intensity. Soil samples were air-dried for 2 to 3 d before XANES analysis. Standards and samples were scattered onto kapton tape attached to a cardboard slide frame and mounted on the sample stage so that the incident beam met the sample material with only air interference. Standards for oxidation state comparison were prepared to provide a Mn content between 1 and 5%. Divalent Mn solutions of 0.1 and 0.5 M were prepared with reagent grade MnSO₄. For Mn(III), the Mn-pyrophosphate reagent of Bartlett and Ross (1988) was dried under N₂ and analyzed in the crystallized state (4.2% Mn). Synthetic birnessite was prepared by the method of Golden et al. (1987) and confirmed with x-ray diffraction. Synthetic pyrolusite was prepared by the method of McKenzie (1971). The manganite sample (NMNH B7639) was obtained from The National Museum of Natural History. All minerals were ground in an agate mortar and diluted with synthetic corundum (Buehler Micropolish C, 1-µm -alumina). For each spectra, a regression of relative intensity (within the 0.3–0.8 range) vs. relative energy was used to calculate the main absorption edge energy at half-height relative to the upper baseline. This value was used as the reference energy for Mn-XANES and as an indicator of relative Mn oxidation state. Under our Mn-XANES experimental conditions, resolution is sensitive to a shift in energy of 0.15 eV or less.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Manganese Abundance
The soils used in this study are high in Mn (Table 1). The average hydroquinone-extractable Mn abundance in 50 field-moist soils studied by Bartlett and James (1979) was 5.5 mmol kg^–1 (0.03% by weight), compared with an average of 26.9 mmol kg^–1 (0.15% by weight) for the eight soils in this study. The total reducible Mn fraction was also high in the study soils (Table 1), representing 0.14 to 1.27% of the soil by weight, compared with a more typical 0.03 to 0.06% (Gambrell, 1996; Chen et al., 1999).

Manganese-XANES
For six of our soils, the shape of the Mn-XANES spectra (Fig. 1) was similar to that of synthetic birnessite, but with higher edge energy positions (up to 0.9 eV) and generally steeper main absorption edges. In XANES spectra of Mn(IV) oxide samples, a more gradual rise in the main edge has been attributed to the possible inclusion of Mn(II, III) in Mn oxides and/or stabilization of Mn(III) within organic complexes (Schulze et al., 1995). Steeper main edges and higher edge energy positions in these six soils show that they have a greater proportion of tetravalent Mn relative to synthetic birnessite. The lowest pH soil (Pease) had a slightly lower edge energy position than birnessite, but a steeper main absorption edge making it difficult to estimate the degree of mixed valency. The shape of the Mn-XANES spectra for the one pasture soil (Barber) included in the study follows that of the highly oxidized synthetic pyrolusite, although with lower edge energy position (0.5 eV) and a more gradual main absorption edge. This suggests a higher proportion of trivalent Mn in the Barber soil, relative to synthetic pyrolusite. Interestingly, other soils sampled previously in the same location as Barber did not show this pattern (Ross et al., 2001a, 2001b).

View larger version (32K): [in this window] [in a new window]   Fig. 1. Manganese-x-ray absorption near edge structure (XANES) spectra for aqueous MnSO4, Mn(III)-pyrophosphate, synthetic birnessite, eight recently air-dried, high Mn soils, and synthetic pyrolusite. Successive spectra are offset by 0.2 relative intensity. Energy is relative to the pre-edge peak of a Mn(VII) standard.

The Mn-XANES edge energy position determined for the study soils varied over a fairly narrow range (1.0 eV) but there is a clear linear shift toward higher edge energy with higher soil pH, over a range of 2.8 pH units (Fig. 2) . For these well-aerated, high Mn soils, edge energy position can be predicted fairly well by the equation (eV) = 0.31(pH) + 7.01 (R² = 0.98, p < 0.001). Overall, it is likely that an increase in pH will correlate with a higher Mn(IV)/Mn(III) ratio. Because the shape and energy position of the main edge is a function of both Mn oxidation state and coordinating environment, the relationship between edge energy position and pH is likely to be complicated by variation in Mn mineralogy. Birnessite has been shown to undergo structural change (e.g., abundance of Mn[III] substitution and vacancies in Mn octahedra) in response to changing pH (Drits et al., 1997) and this pH effect on mineralogy may be operational in our study soils as well.

View larger version (13K): [in this window] [in a new window]   Fig. 2. Relative Mn oxidation state (Mn-x-ray absorption near edge structure [XANES] main edge energy at half-height) vs. soil pH.

Factors Affecting Chromium Oxidation
Chromium oxidation in the Bartlett and James (1979) study was proportional to hydroquinone-extractable Mn (r = 0.94), although their study soils were much lower in Mn abundance than our very high-Mn soils. Working with ground natural Mn oxide minerals, Kim et al. (2002) found that Cr oxidation rates followed the order birnessite > todorokite > lithiophorite > pyrolusite and did not correlate with 0.002 M hydroquinone-extractable Mn abundance, point of zero charge or BET surface area. Greater Cr oxidation was associated with Mn valence; the strongest Cr oxidizers showed the greatest Mn(IV)/Mn(III) ratio, as determined by x-ray photoelectron spectroscopy (XPS). In our study soils, we did not detect a relationship between net oxidized Cr and hydroquinone-extractable Mn, however Cr oxidation did show a positive correspondence to NH₂OH·HCl-extractable Mn (R² = 0.60, p = 0.02) (Fig. 3) .

View larger version (20K): [in this window] [in a new window]   Fig. 3. Total reducible Mn (NH2OH·HCl-extractable Mn) vs. Cr oxidizing capacity (modified Cr Oxidation test with 5 mg Mn equivalent soil samples).

Predictive equations for Cr(VI) based on total reducible Mn and either Mn-XANES edge energy position or soil pH show strong potential for explaining variation in soil Cr oxidizing capacity in high Mn soils:

Given the small number of samples in our study, these equations are not as robust as would be desirable, but they point to the potential importance of Mn oxidation state and soil pH in controlling Mn reactivity with Cr.

Our observations support the conclusion that the influence of pH on Cr oxidation is mediated through change in Mn oxidation state, consistent with what would be predicted from simple thermodynamics, rather than through another mechanism. Higher pH might be expected to reduce reactive Mn oxide surface area due to precipitation of Cr hydroxide species thereby reducing Cr oxidation, but this was not shown in our data (Fendorf and Zasoski, 1992). Higher pH might be expected to enhance Cr(III) sorption due to an increase in total Mn oxide charge, but point of zero charge has not been shown to influence Cr oxidation rate or specific adsorption (McKenzie, 1989; Kim et al., 2002). However, we did observe a strong correlation between pH and Mn-XANES edge energy suggesting that the higher Mn oxidation state (greater proportion of tetravalent Mn) associated with higher soil pH enhanced Cr oxidation.

Response to pH Alteration
To more fully explore the influence of pH on Mn oxidation state and reactivity, we equilibrated six of our study soils with varying quantities of acid (3 M H₂SO₄) and base (CaCO₃) additions to alter the soil pH by up to ±2.5 pH units (Table 3). A plot of the change in sample pH vs. change in the Mn-XANES edge energy position (Fig. 4) suggests a positive linear relationship, however this is largely due to substantial reduction in edge energy with acid addition (Fig. 5) . Increase in pH produced very modest and erratic increases in edge energy position while decrease in pH produced substantial reduction in edge energy. The correlation between the pH and Mn-XANES edge energy position data for these adjusted samples (R² = 0.48, p < 0.001; data not shown) is much weaker than that observed in the untreated soils.

View larger version (23K): [in this window] [in a new window]   Fig. 4. Change in pH in acid- and base-treated soils vs. corresponding change in Mn-x-ray absorption near-edge structure (XANES) main edge energy position.

View larger version (23K): [in this window] [in a new window]   Fig. 5. Selected Mn-x-ray absorption near-edge structure (XANES) spectra for acid-treated Maple-O samples. Energy is relative to the pre-edge peak of a Mn(VII) standard.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The high degree of correspondence between total Mn (XRF) and total reducible Mn (NH₂OH·HCl) in our study soils suggests that most of the Mn present is redox reactive. Further evidence for this stems from the finding that total reducible Mn, rather than easily reducible Mn (hydroquinone) is an important predictor of Cr oxidizing capacity. The Mn-XANES spectra indicate that most of the study soils have a higher Mn(IV)/Mn(III) ratio than synthetic birnessite. Our finding that Cr oxidation was greater in soils with higher Mn-XANES main edge energy position suggests that Mn oxides with a greater proportion of Mn(IV) have a greater oxidizing capacity. Higher Mn-XANES edge energy was highly correlated with pH suggesting an important role for pH in controlling Mn oxidation state, probably during oxide formation. Alteration of equilibrium pH produced unexpected responses in Mn-XANES edge energy and Cr oxidation capacity. Manganese minerals demonstrated resistance to valence change except when treated with reducing acid additions. Our data demonstrate that there is an important effect of total reducible Mn abundance, soil pH, and Mn oxidation state on Cr oxidizing capacity, however deviation from predictive equations due to possible mineralogical differences or variation in pH effect depending on Mn abundance were observed.

	ACKNOWLEDGMENTS

 
We thank Heidi Hales and Patrick Keane for technical assistance; the Soil Chemistry Group at the Swiss Federal Institute of Technology; Bill Rao of the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886. Use of the Beamline X26A was supported by the Department of Energy, Basic Energy Science's Geosciences Research Program under grant number DE-FG02-92ER14244.

Received for publication February 27, 2004.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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