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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)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The Standard Net Cr Oxidation Test is a useful measure of Mn oxide surface reactivity (Bartlett and James, 1979). Manganese oxides are the only known oxidizers of chromium in soils, producing the highly carcinogenous chromate 
anion (Bartlett, 1996). The extent of Cr oxidation by Mn oxides has been linked to the abundance of hydroquinone-extractable Mn, trivalent Cr and competitors for surface sites, as well as to soil pH and oxide Mn(IV)/Mn(III) ratio (Bartlett and James, 1979; Risser and Bailey, 1992), while properties such as point of zero charge and surface area have not been demonstrated to influence Cr oxidation (Kim et al., 2002). The rate of Cr oxidation may be limited by accumulation of reaction products on Mn surfaces, specifically reduced Mn2+ (Risser and Bailey, 1992) or, under higher pH conditions, hydrolyzed species such as Cr(OH)3·nH2O (Fendorf and Zasoski, 1992). Previous work with our study soils showed a strong positive correlation among Cr oxidation, soil pH, and Mn(IV)/Mn(III) ratio, as well as NH2OH·HCl-extractable Mn (Negra et al., 2005).
The oxidizing power of Mn oxides has also been demonstrated with Co(II) (Crowther et al., 1983; Brooks et al., 1999; Fendorf et al., 1999), As(III) (Scott and Morgan, 1995), and Pu(V) (Duff et al., 1999) with important implications for the solubility and bioavailability of these metals. Oxidation of trace metals at the Mn oxide surface is probably due to electron transfer through surface oxygen bridges (Manceau et al., 1997). There is evidence for a predominant role of Mn(III) in the oxidation of Cr by birnessite (Nico and Zasoski, 2000) and Co by buserite (Manceau et al., 1997). Other work points to the importance of Mn(IV) in trace metal oxidation (Fendorf et al., 1999; Banerjee and Nesbitt, 2001). There is also the possibility of solid-state reduction within synthetic (Fendorf et al., 1999) and natural (Guest et al., 2002) Mn oxides, producing a stable Mn(III) phase. The absence of a common oxidation reaction mechanism may result from variability in oxide surface characteristics (Villalobos et al., 2003) and chemical conditions.
Substantial enrichment of naturally occurring and contaminant trace metals has been observed in Mn concretions and nodules (Dawson et al., 1985; Latrille et al., 2001; Liu et al., 2002; Manceau et al., 2002) due to sequestration during Mn oxide formation (Childs, 1975; Palumbo et al., 2001), sorption on external and internal surfaces, or incorporation into layer structures (Manceau et al., 2003). Preferential accumulation of trace metals by Mn oxides may derive from specific adsorption properties of Mn oxide surfaces (Lion et al., 1982; Fu et al., 1991), as well as from overall negative oxide charge. Manganese oxides have been found to specifically adsorb weakly hydrated cations, commonly in the order of preference Pb > Cu > Mn > Co > Zn > Ni (McKenzie, 1989). The low point of zero charge that is typical of many Mn oxides enables cation sorption even in low pH conditions (Stahl and James, 1991), unlike many other soil sorbents such as Fe oxides or organic matter. Negative Mn oxide charge also arises from vacant Mn4+ positions in the layer structure (estimated to be one out of seven in birnessite) or from substitution of layer Mn(IV) by Mn(III). In work by McKenzie (1980), oxide surface area did not influence sorption of divalent Pb, Co, Cu, Ni, Zn, or Mn, suggesting the predominance of specific adsorption in which sorbate metals form inner-sphere complexes of high-selectivity and low reversibility with valence-unsatisfied ligands on the oxide surface.
In this study, we investigate the interactions of trace metals with Mn in aerated, high-Mn soils by assessing their ability to interfere in chromium oxidation, a reaction known to be specific to Mn oxide surfaces. We also investigate the microdistribution of soil Mn and accumulation of added trace metals by Mn in situ using µSXRF. We interpret the observed trace metal behaviors in the complex, dynamic soil matrix through comparisons with relevant bulk soil chemical properties.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil pH was measured in 0.01 M CaCl2 (2:1, v/v) and total C by elemental analyzer (data were well correlated with Walkley-Black results). Total reducible Mn was determined using a modification of the hydroxylamine hydrochloride method reported by Gambrell (1996). 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 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 these soils, this procedure will extract most or all of the Mn present (Neaman et al., 2004; Negra et al., 2005). 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 of 0.02 M hydroquinone. Samples were diluted with 12.5 mL of 1 M CaCl2 and shaken 15 min. Extractable metals were measured using the modified Morgan's extractant (1.25 M ammonium acetate, pH 4.8) at a 5:1 solution/soil ratio and 15 min. shaking. Total abundance of Mn, Fe, and selected trace metals was determined by the hydrofluoric acid digestion method reported by Jackson (1958). Concentrated HCl (3 mL) and 48% HF (10 mL) were added to 0.5 g dried, ground (passed through a 150-µm sieve) sample, shaken for 3 h, and heated to 75 to 100°C. Samples were allowed to cool, treated with 100 mL of saturated H3BO3 solution and diluted to 200 mL. Elemental concentration in all extracts was measured by ICP–AES. Selected samples were analyzed for total elemental abundance by energy-dispersive x-ray fluorescence spectroscopy (Spectro-X-Lab 2000). Detection limits were 0.05 mg kg–1 for ICP–AES (0.5 mg kg–1 for Pb) and 1 mg kg–1 for XRF.
A modification of the Standard Net Chromium Oxidation test (Bartlett and James, 1996) was used in which moist soil samples or synthetic oxides (3.4 mmol NH2OH·HCl-extractable Mn L–1 or 5 mg in 27 mL) were shaken for 15 min. with 0.001 mol L–1 MnCl2, CoCl2, Pb(NO3)2, CuCl2, or NiCl2 (control treatments received distilled water), followed by addition of 0.01 M CrCl3 [to achieve an initial Cr(III) concentration of 0.001 M] and 15 min. shaking. A phosphate buffer was added to remove adsorbed Cr(VI), the sample centrifuged, and Cr(VI) was determined in the supernatant by reaction with s-diphenyl carbazide (s-DPC) (Bartlett and James, 1996). Because of an order of magnitude difference in Mn abundance among soils, soil/solution ratio ranged from 1:100 to 11:100.
In soils, the oxidation state of Cr will reflect the complex, dynamic chemical environment; therefore the Cr Oxidation Test relies on the use of field-moist soils in which the redox poise of the natural soil has been preserved. Bartlett and James (1979) and Ross et al. (2001b) demonstrated that soil drying will lower the net production of Cr(VI). Positive interference in the Cr Oxidation Test can result from formation of inner-sphere complexes of certain metals with diphenylcarbazone, but these are unstable in the pH range of the s-DPC reagent (Bartlett and James, 1996).
Microfocused Synchrotron X-Ray Fluorescence (µSXRF)
Microfocused synchrotron x-ray fluorescence spectroscopy can be used to generate images of microscopic-scale elemental distributions for selected regions within a sample. Scanned regions represent a very small part of the total sample, however, these images are useful for comparison of the microdistribution of trace metals with that of potential host species such as Mn oxides. Although overlapping elemental microdistributions provide evidence of an association between elements, they cannot be used to confirm the existence of specific types of chemical bonds or structural relationships (Manceau et al., 2002).
All µSXRF 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 three different beam-time allocations in 2000 and 2001. Incident x-rays were scanned across selected sample regions of approximately 300 to 600 x 300 to 600 µm in steps of 15 to 25 µm in the x–y plane. For all runs, collection time was 5 s. For elemental mapping, incident beam energy was set at 16.6 to 18 keV. X-ray fluorescence counts were collected for specified energy regions defined by the fluorescence energies of the elements of interest (e.g., Mn, Fe, Pb, Co) at each data collection point and were used to construct elemental maps, which describe relative abundance across the scanned area. The degree of spatial overlap in microdistribution for various element pairs is described by the correlation coefficient (r) for spatially referenced XRF data, calculated using Matlab (The Mathworks, Inc., Natick, MA).
Before µSXRF analysis, soil samples were pretreated, in the same manner as described above, at two loadings, 0.03 and 0.3 mol metal/mol Mn. After 15 min metal equilibration with soils, samples were centrifuged for 10 min and filtrate was removed. To remove easily desorbed metals, samples were shaken with 30 mL 0.1 M CaCl2 for 15 min. Filtrate was removed and samples were shaken with 30 mL of 0.001 M CaCl2 to restore ionic strength. The final Ca rinse solution was removed under vacuum. All filtrates were analyzed for metal concentration by ICP–AES to determine retention of added metals and Mn2+ release. Recently air-dried soil 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.
For each of the soils in Table 1, we present Mn x-ray absorption near edge structure (Mn-XANES) data gathered through a previous study (Negra et al., 2005), and use these values as an indicator of relative Mn(IV)/Mn(III) ratio within soil Mn oxides. Data gathered by Mn-XANES for natural soils provides information about the relative average Mn oxidation state, but absolute determination of the abundance of different Mn oxidation states is complicated by the possible simultaneous presence of three Mn oxidation states and variations in Mn oxide mineralogy. The data for the study soils are reported as the energy position of the main absorption edge at half-height (eV) relative to the upper baseline, as calculated by a regression of relative intensity vs. relative energy. Energy values of the Mn-XANES spectra were calibrated to the pre-edge peak (6543.3 eV) of a (MnVII) standard (10% KMnO4) that was set as 0 eV relative energy (Ross et al., 2001a). Most of the study soils appear to have a higher Mn(IV)/Mn(III) ratio than synthetic birnessite.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The Mn oxides in the study soils were robust Cr oxidizers (Negra et al., 2005). Net production of Cr(VI) ranged from 0.4 to 3.6 µmol when 30 µmol Cr(III) were applied to (5 mg NH2OH·HCl-extractable Mn equivalent) field-moist soil while samples of synthetic birnessite and pyrolusite, containing equivalent amounts of Mn, produced 4.5 and 0.2 µmol net Cr(VI), respectively (Table 2). Although the study soils were quite high in C (9.0–27.2%), there does not appear to be any consistent effect of soil org-C on net Cr(VI) production as the relationship between net Cr oxidation and soil C content was quite weak (R2 = 0.11, p = 0.42). For example, the Maple soil's O and B horizon samples were similar in total Mn content and Cr oxidizing capacity but had very different C content (9.0 vs. 20.5%). Differences among soils in net Cr oxidation have been attributed to variation in Mn(IV)/Mn(III) ratio and oxide mineralogy, as well as soil pH (Banerjee and Nesbitt, 2001; Chung and Sa, 2001; Kim et al., 2002; Negra et al., 2005).
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Soil pH varied from 4.4 to 7.2 (Table 2) and no pH adjustments were made during these experiments. While it is possible that the pretreatment solutions altered native soil pH and influenced metal speciation, these soils exhibited a strong buffering capacity in previous efforts to alter equilibrium pH (Negra et al., 2005). Formation of metal hydroxides increases with pH, suggesting that we might expect metal pretreatment in our high-pH soils to result in greater interference in Cr oxidation than in their low-pH counterparts (Fendorf et al., 1993). However, little correspondence was observed between soil pH and the ability of recently added Pb and Ni to interfere with Cr oxidation. Hydroxylated Mn(II) species are not expected to form in normal soil pH ranges (Norvell, 1988) so this is an unlikely explanation for enhanced Mn(II) sorption by Mn oxides in high pH soils. While more readily sorbed metal hydroxides may or may not have formed in high-pH samples, in the complex chemical environment of a soil sample, higher pH is also likely to enhance the diversity of sorption sites by which these metals may be accumulated, diverting them from Cr-oxidizing sites on Mn oxide surfaces. The lower abundance of competing sorption sites in low-pH soils may explain the stronger interference of Cu in Cr oxidation in the pH 4.4 Pease soil (Lion et al., 1982; Fu et al., 1991).
Greater decrease in Cr oxidation by Mn and Co interference in high pH soils (Fig. 1) suggests these metals exhibit a high degree of specificity for Mn surfaces given the potentially greater competition by other sorption sites that may occur at higher pH. In the case of Co, this may be due to preferential sorption for hydroxylated metal species, which form at higher pH or, alternatively, greater affinity of Mn and Co for Mn surfaces at high pH may arise from higher Mn oxide valence. Previous work with these soils demonstrated a strong correlation between soil pH and Mn-XANES main edge energy (Negra et al., 2005). High pH and high Mn oxide valence were shown to be strong predictors of Cr oxidizing capacity and may also contribute to Mn oxides' affinity for added Mn and Co. Positive correlations were observed for Mn and Co interference in Cr oxidation and Mn-XANES edge energy (Mn: R2 = 0.58, p = 0.03; Co: R2 = 0.46, p = 0.06), although the relationships were weaker than those observed for pH (Fig. 1).
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-MnO2, sorption of Co increased rapidly with pH above pH 6, possibly accelerated by the sorption of hydroxylated species (CoOH+) or nucleation of Co(OH)2 at the Mn oxide surface as surface density of sorbed Co greatly exceeded monolayer coverage (Loganathan et al., 1977). Greater Mn(II) interference in Cr oxidation observed in high pH soils is probably due to enhanced Mn(II) sorption at sites where oxidation will slowly occur. Scott and Morgan (1995) attributed the diminished As(III) oxidation by synthetic birnessite, which resulted from pretreatment with divalent Mn to a reduced number of reactive surface sites. Oxidation of Mn2+ by existing Fe and Mn oxide surfaces is believed to be the primary abiotic mechanism of Mn oxide formation in the soil (Norvell, 1988). Newly oxidized Mn surface species are important contributors to Mn oxide reactivity (Junta-Rosso et al., 1997) and enhanced Cr oxidation, proportional to preexisting Mn abundance, has been observed 1 to 2 d after Mn2+ addition (Ross and Bartlett, 1981; Ross et al., 2001a). Initially, however, Mn addition is likely to inhibit Cr oxidation by competitive sorption as Mn2+ is sorbed before oxidation at the oxide surface. The 15 min. reaction time before Cr(III) addition does not allow for formation of new reactive surfaces that could enhance Cr oxidation.
At first inspection, the degree of Pb, Ni, and Cu interference in Cr oxidation appeared to correspond positively to the abundance of total reducible soil Mn (Pb: R2 = 0.76, p = 0.01; Ni: R2 = 0.66, p = 0.01; Cu: R2 = 0.55, p = 0.04). However, because soil sample size was selected to achieve a uniform quantity of NH2OH·HCl-extractable Mn (soil samples ranged from 0.4 to 3.5 g dry weight basis), much larger quantities of the lower-Mn soils were used, relative to higher-Mn soils, providing many more competing sorption sites for added metals (e.g., Fe oxides, organic particles). Negative correlations were observed between sample size and interference in Cr oxidation by Pb, Ni, and Cu (Pb: R2 = 0.69, p = 0.01; Ni: R2 = 0.60, p = 0.02; Cu: R2 = 0.79, p < 0.01) suggesting that, in the larger samples, these metals interacted with competing sorbents, substantially reducing accumulation by Mn oxide surfaces and interference in Cr oxidation. This suggests that Pb, Ni, and Cu may have only low to moderate specific affinities for reactive oxidative sites on soil Mn oxides. Previous work has provided evidence of substantial specific adsorption of Pb by surface and interlayer sites on synthetic Mn oxides (McKenzie, 1980). Matocha et al. (2001) presented spectroscopic evidence of inner-sphere binding of Pb above and below interlayer vacancies in birnessite, independent of pH. Our data suggest that, in soils, Pb sorption and interference with Cr oxidation may rival that of Mn and Co only in the absence of competition by other sorbing sites. In comparison, the absence of a correlation between either soil Mn abundance or sample size with Mn or Co interference in Cr oxidation suggests that these two metals have a particularly strong affinity for soil Mn oxides, blocking reactive sites on Mn surfaces even in the likely presence of abundant competing sorbent surfaces.
Interference in Cr oxidation requires sorption and retention at potential Cr-oxidizing sites, and in our study soils, Ni and Cu exhibited a lesser tendency to do this than Mn, Co, and Pb. This may result from competition for Ni and Cu by other soil sorbents or successful displacement from Mn surfaces by Cr(III). It is also possible that different types of sites on Mn oxides are engaged in non-oxidative sorption and oxidation, and that non-oxidizing Ni and Cu have an affinity for non-oxidizing sites. There is evidence that incorporation of Ni within Mn oxides is a major form of Ni sequestration in surface systems. Specific affinity of lithiophorite for Ni has been attributed to structural substitution of Ni2+ for the similarly sized Mn3+ (Manceau et al., 2002, 2003). The affinity of soil Mn oxides for Cu may be weaker than that for the other trace metals investigated, although, Cu has been shown to partition into Mn oxide fractions (Childs, 1975; Hudson-Edwards et al., 1996). Latrille et al. (2001) found a positive correlation between Cu and Mn at the nanometric scale in Fe-Mn concretions. In marine manganates, Cu atoms adsorbed at the mineral surface and were also found within the crystal structure above octahedral Mn4+ vacancies (Crane, 1981; Arrhenius et al., 1979). These types of interactions with Mn are unlikely to occur within the brief equilibration period of our experiment as Ni and Cu incorporation within Mn oxides occurs primarily during Mn oxide formation as well as through diffusion and substitution into existing oxides. Diffusion processes are likely to be highly variable, although diffusion within soil Mn oxides may be slower than that in Al and Fe oxides (Trivedi and Axe, 2001).
Microdistribution of Soil Manganese and Added Metals
In forty-one µSXRF scans of metal-treated soil samples, Mn and Fe exhibited discrete microdistribution and were not uniformly dispersed through the sample (examples shown in Fig. 2– 4). The microdistribution of Mn and Fe often demonstrated substantial overlap (average: r = 0.65) with considerable variability (range: r = 0.18–0.97) that was not related to abundance of NH2OH·HCl-extractable Mn. In their work with Fe-Mn concretions, Latrille et al. (2001) found Mn distribution to be highly contrasted and restricted to areas of moderate Fe concentration, although Fe-Mn correlation coefficients were negative. Discrete banding of poorly ordered Fe and Mn oxides within Fe-Mn nodules has also been observed (Palumbo et al., 2001; Liu et al., 2002).
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In µSXRF scans gathered for six soils treated with 0.03 mol Cu/mol Mn, overlap in microdistribution of Cu with Mn was comparatively low (average: r = 0.31, range: r = 0.07–0.61), and was not correlated with any measured soil properties, including abundance of C. Spatial overlap of Cu with Fe was higher than that for Cu and Mn (average: r = 0.42, range: r = 0.06–0.74) and did not correlate with the overlap of Fe with Mn. Scans of the Ridge and Beech soils are shown in Fig. 4. These soils have relatively low Mn content and Cr-oxidizing capacity and low to moderate pH in which added Cu decreased the Cr oxidation test by 25.9 and 3.6%, respectively, indicating a weak affinity of added Cu for soil Mn. This represents a divergence from observations in Fe-Mn nodules in which strong spatial correlations have been documented between Cu and Mn (Latrille et al., 2001; Liu et al., 2002; Palumbo et al., 2001). In scans where there was substantial overlap of Cu with Mn, a similar or greater degree of overlap was observed for Cu with Fe (R2 = 0.85, p < 0.001), suggesting that the mechanism of Cu sorption to Mn and Fe oxides is similar.
In contrast with a typical bulk soil, Mn concentration in Fe-Mn nodules and concretions can be very high (10% or more). Incorporation of trace metals into these features is believed to occur primarily during the sequential processes of formation, dissolution, and reprecipitation in response to fluctuating redox conditions. Our experiments used well-aerated highly oxidized surface soils, which may lack analogous redox fluctuations that would facilitate trace metal sequestration by Mn oxides. The short duration of metal equilibration used in this study would not allow for significant new Mn oxide formation and metal sequestration, therefore, metal accumulation is likely to occur through sorption to external and possibly internal surfaces, as well as possible rapid oxidation processes. This may explain the divergence of our observations of metal accumulation by soil Mn from previous work with Mn-rich nodules and concretions.
Although µSXRF is an extremely powerful technique, there are limitations on the utility of the information gathered for making determinations regarding the interactions of mapped species. When the microdistribution of a trace metal demonstrates substantial overlap with more than one potential sorbing element, it is difficult to ascertain which element, if any, is the primary sorbing agent, or whether there is an intimate association of all three elements. This is the case for overlapping microdistributions of Co and Pb with both Mn and Fe. For Co, there is other evidence that points toward a very strong specific affinity with Mn. For Pb, it is equally probable that spatial overlap with Mn is a by-product of sorption on Fe surfaces.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Microfocused synchrotron x-ray fluorescence scans revealed substantial spatial correlation of soil Mn and added Co and Pb, but only modest overlap of soil Mn with added Cu. This pattern diverges from other work with Mn-rich nodules and concretions in which Cu distribution has consistently been shown to be highly correlated with that of Mn, while the correlation of Mn with Pb and Co has been weaker. Sequestration of Cu within Mn oxides during formation may be substantial but accumulation of Cu by existing Mn under highly oxidized conditions appears to be relatively weak.
Our data suggest that divalent Mn and Co may sorb to sites on soil Mn oxides at which subsequent oxidation occurs. In these study soils, high pH and Mn valence has been strongly linked to Cr oxidation capacity. Added Mn and Co interfered in Cr oxidation to a greater degree in these high-pH, high-Mn-valence soils. Cobalt accumulation by soil Mn was also greater in these soils. Higher Mn oxide valence may enhance sorption and oxidation of these oxidizable metals. It appears that the specific affinity of Pb and Cu for soil Mn oxides is weaker than that of Mn and Co. Lead interference in Cr oxidation was much stronger in smaller samples and Cu interference was much stronger in the lowest pH soil, both conditions in which the abundance of potential competing sorbent surfaces is greatly reduced. The microdistribution of Cu was much more weakly correlated with that of soil Mn and the microdistribution of Pb was equally well correlated with soil Fe as with soil Mn. The degree to which our findings differ from previous observations of these trace metals' interactions with Mn may derive from differences among Mn oxides in synthetic samples, Mn-rich nodules and the chemically dynamic bulk soil.
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ACKNOWLEDGMENTS |
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Received for publication February 28, 2004.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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-MnO2. Environ. Sci. Technol. 34:3363–3367.This article has been cited by other articles:
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