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Soil Science Society of America Journal 66:306-315 (2002)
© 2002 Soil Science Society of America

DIVISION S-9 - SOIL MINERALOGY

Oxidation of Chromium(III) to (VI) by Manganese Oxides

Jae Gon Kima, Joe B. Dixon*,a, Charles C. Chusueib and Youjun Denga

a Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474
b Dep. of Chemistry, P.O. Box 30012, Texas A&M Univ., College Station, TX 77842-3012

* Corresponding author (j-dixon{at}tamu.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 
Manganese oxides in soils oxidize Cr from a stable form to a more toxic mobile form. The influence of Mn oxide diversity on Cr oxidation is the subject of this report. Oxidation of Cr(III) to Cr(VI) by coarse clay-size natural Mn oxides, birnessite, todorokite, and lithiophorite was studied at pH 4 and 7, and at 200 and 400 µM initial Cr(III) concentrations. The oxidation rate obeyed the first-order rate law at the 400 µM Cr(III) initial concentration; the 200 µM concentration was too low and reverse reactions may have interfered. Rate law is valid only far from equilibrium. The rate of oxidation was greater for todorokite and birnessite that contained the most quadrivalent Mn and least for lithiophorite that contained a greater proportion of trivalent Mn determined by x-ray photoelectron spectroscopy (XPS). The deviation of Mn oxidation state from the frequently assumed quadrivalent form in Mn oxides suggests the need for a thorough assessment of the ion species of Mn in Mn oxides that occur in soils. At 400 µM Cr(III), the rate of Cr oxidation by the pyrolusite sample was intermediate between birnessite and todorokite suggesting that the sample was not pure pyrolusite. Apparently, the synthesis process did not reach equilibrium with pyrolusite. The sample used as a chemical standard was apparently more soluble and more effective in oxidizing Cr(III) than would be expected for the natural mineral pyrolusite.

Abbreviations: BE, binding energy • FT-IR, Fourier transform infrared spectroscopy • IR, infrared • PZC, point of zero charge • USNM, United States National Museum • XPS, x-ray photelectron spectroscopy • XRD, x-ray diffraction


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 
THE POTENTIAL TOXICITY of Cr in the environment has raised much interest in understanding how its chemical speciation is influenced by Mn oxides in soils and sediments. Chromium is a trace metal in most soils; yet, it has been added into local environments by various human activities, including mining and smelting of chromite ore, stainless steel (alloy) production, chrome plating, chrome tanning of leather, pigment production, oil well drilling, and petroleum refining. In the soil environment, it can exist as nonhazardous Cr(III) (similarly benign as Fe and Al oxides) or as a toxic and carcinogenic Cr(VI) form. Chromium(VI) is a soluble, mobile anion when oxidized. The most stable oxidation states of Cr under earth surface conditions are trivalent and hexavalent forms (Cotton and Wilkinson, 1988). Chromium(III) is 10 to 100 times less toxic than Cr(VI). Also, Cr(III)-picolinic acid is essential for human health especially for sugar metabolism (Katz and Salem, 1993). Chromium(VI), on the other hand, is a Class A human carcinogen, an irritant, and a corrosive substance. The mobility of Cr(VI) in soils and natural waters is much higher than Cr(III) because Cr(VI) forms anionic complexes and more soluble compounds.

The oxidation and reduction reactions of Cr in soil and natural water are dependent on the presence of oxidants and reductants (Bartlett and James, 1988). The main oxidants of Cr(III) to Cr(VI) in soils are Mn oxides (Bartlett and James, 1988). On the other hand, Fe(II) (Buerge and Hug, 1997; Eary and Rai, 1988), organic matter (Wittbrodt and Palmer, 1995), and reduced S (Patterson et al., 1997) can reduce Cr(VI) to Cr(III).

Bartlett and James (1979) were the first to demonstrate oxidation of Cr by Mn oxide in soils. Chromium oxidation by several synthetic Mn oxides in aqueous systems has been reported: pyrolusite (ß-MnO2) (Eary and Rai, 1987), birnessite (Na, K, Mg, Ca) {[Mn(IV, III)}]2O4)·nH2O (generalized formula, modified from Post and Veblen, 1990) (Fendorf et al., 1993; Fendorf and Zasoski, 1992), buserite (Na4Mn14O27·9H2O) (Silvester et al., 1995), manganite ({gamma}-MnOOH) (Johnson and Xyla, 1991), and hausmanite (Mn3O4) (Zasoski and Chung, 1992).

Oxidation of Cr(III) to Cr(VI) by Mn oxides in an aqueous system is complex and several factors have been credited with influencing the extent and rates of the processes involved. Adsorption mechanisms of Cr(III) on Mn oxides, competition with other cations such as Al and Fe for adsorption sites, mechanism of electron transfer, and desorption and readsorption of produced Cr(VI) and Mn(II) have been reported as controlling factors for the kinetics and the oxidation capacity of Mn oxides (Amacher and Baker, 1982; Eary and Rai, 1987; Fendorf et al., 1993; Fendorf et al., 1992; Fendorf and Zasoski, 1992; Johnson and Xyla, 1991; Manceau and Charlet, 1992; Silvester et al., 1995). In addition to these factors, pH, initial Cr(III) concentration, and the ratio of surface area of Mn oxide to solution volume also determine the kinetics and oxidation capacity (Eary and Rai, 1987; Fendorf et al., 1993). The kinetics of Cr oxidation are faster than originally thought, suggesting that the factors influencing Cr(VI)/Cr(III) ratios in natural systems deserve further investigation (Pettine et al., 1994).

Manganese oxides, which are the only proven natural oxidants for Cr(III) in the surface environment, are widely distributed as suspended particles in surface waters and as nodules or coatings in soils and sediments. The oxidation of Cr by Mn oxides has been well documented, but most studies were conducted with a uniform Mn oxide in terms of particle size, chemical composition, and crystallinity. Little literature is available on the relationships between Cr oxidation capacity of natural Mn oxides and the kinetics of the reaction, and their mineralogy and surface property. The objective of this study is to examine and compare how three Mn oxide minerals that commonly occur in soils affect Cr oxidation.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 
Preparation of Manganese Oxides
Natural todorokite (USNM #113967 from Cuba), birnessite (USNM #115513 from Mexico), and lithiophorite (USNM #R8811 from South Africa) obtained from the U.S. National Museum (USNM, Washington, DC) were used as models for soil minerals for the oxidation of Cr(III) to Cr(VI). Also, reagent grade MnO2 that x-ray diffraction indicates is mostly pyrolusite (J.T. Baker Chemical Co., Phillipsburg, NJ) was included for its chemical purity. The Mn oxide samples were ground to coarse clay (2–0.2 µm) with an agate mortar and pestle in a pH 10 solution of Na2CO3 to minimize the destruction of crystal structure and enhance dispersion during grinding. The ground Mn oxide samples were freed of interstitial salts by dialysis in distilled water, dried in an oven at 60°C, and stored in glass bottles for further use.

Characterization of Manganese Oxides
The four Mn oxides used (with ideal formulas in parentheses) are pyrolusite (Mn[III, IV]O2), lithiophorite [(Al, Li)Mn[III, IV]O2(OH)2], birnessite [(Na,K,Mg,Ca){[Mn(IV, III)]}2O4·nH2O] and todorokite [(Na)Mn4(IV, III)O12·3H2O]. The Mn oxide samples were analyzed by x-ray diffraction (XRD) and infrared (IR) using a Philips x-ray diffractometer (Philips X-ray diffractometer, Mahwah, NJ) employing a CuKa radiation and a Perkin Elmer 2000 FT-IR spectrometer (Perkin-Elmer Corp., Analytical Instruments, Norwalk, CT), respectively. The natural Mn oxides (lithiophorite, todorokite, and birnessite) were dissolved in 0.25 M hydroxylamine hydrochloride (NH2OH·HCl) in 0.1 M HNO3 (Chao, 1972). The undissolved material was weighed and analyzed with the x-ray diffractometer to identify and quantify the impurities. The external surface area of the Mn oxide samples was determined by the Brunauer-Emmett-Teller-method (Brunauer, 1957) using N gas (N2) adsorption with an Autosorb-6 (Quantachrome Co., Boynton Beach, FL). The point of zero charge (PZC) of the Mn oxides was determined by a coagulation method (Yopps and Fuerstenau, 1964).

Particle morphology was investigated with a JEOL 2010, 200 kV transmission electron microscope (JEOL Inc., Peabody, MA). Samples were dispersed in distilled water and drop mounted on type A silicon monoxide coated 300-mesh Cu grids (Ted Pella, Inc., Redding, CA).

X-ray photoelectron spectroscopy was performed in an ion-pumped (300 L s-1) Perkin-Elmer PHI 560 system using a PHI 25-270AR double pass cylindrical mirror analyzer (Perkin-Elmer Corp., Analytical Instruments, Norwalk, CT). A Mg K-{alpha} anode with a photon energy of h{nu} = 1253.6 eV was operated at 12 kV and 200 W. The vacuum system pressure during XPS analysis was ~1 x 10-6 Pa (~1 x 10-8 Torr). Survey scans were performed using 100 eV pass energy; high resolution scans were performed with a 50 eV pass energy. Core levels of the C 1s, O 1s, Na 1s, Al 2p, and Mn 2p orbitals were scanned and normalized with their respective atomic sensitivity factors (Wagner et al., 1979). The Mn oxide powders were not amenable for XPS analysis of Li because the Li 1s core level overlaps with the Mn 2p3/2 transition at ~50 eV binding energy (BE). The core level BEs of the Cu 2p3/2 (932.7 eV) and Au 4f7/2 (84.0 eV) orbitals from sputter-cleaned foils were used to calibrate the XPS BE range (Seah, 1989). The precision of the BE measurements are within ±0.2 eV. Signal from adventitious C at a BE = 284.7 eV for the C 1s level was used to correct for sample charging (Barr and Seal, 1995). Samples were mounted onto a 1.0 by 1.0 cm x 0.1 mm support using double-sided tape (Scotch 3M, Structure Probe Inc., West Chester) attached to a probe and introduced into the ultrahigh vacuum via a turbopumped antechamber. The probe was differentially pumped using sliding seals. Curve fitting of the XPS spectra was performed using Peakfit ver. 3.1B (Jandel Scientific, San Rafael,, CA) software. The four Mn oxide samples were mounted onto the sample support probe, evacuated as described above, and scanned with XPS at room temperature.

To estimate reactivity of surface Mn for Cr oxidation, the solubility of the Mn oxides in hydroquinone [C6H4(OH)2] was determined (Stone and Ulrich, 1989). We chose hydroquinone because it dissolves Mn oxides by oxidation to p-benzoquinone on the surface and it is too large a molecule to diffuse into the crystal structure of the Mn oxides. A mixture of 25 mL of 0.002 M hydroquinone and 0.02 g of Mn oxide sample was placed in a 40-mL polyethylene centrifuge tube, shaken for 1 h, and centrifuged at 11000 relative centrifugal force (RCF) for 10 min. After centrifugation, the supernatant liquid was carefully removed with a pipette and filtered with 0.025 µm Millipore membrane (Millipore, Bedford, MA). The Mn concentration of the filtrate was determined with a Perkin Elmer 3100 atomic absorption spectrometer (Perkin-Elmer Corp., Analytical Instruments, Norwalk, CT) employing an air-acetylene flame.

Analytical Procedure
Gas-free water prepared by boiling distilled water for 30 min was used to prepare the solutions. The pH of the solutions was adjusted with 0.1 M KOH or 0.1 M HCl. Twenty-five milliliters of 0.01 M KCl adjusted to pH 4.7 and 0.02 g of each Mn oxide sample were placed in a 40-ml polyethylene centrifuge tube. The suspension was shaken for 12 h with a horizontal shaker to hydrate the Mn oxide. After hydration, the pH of the suspension was checked and adjusted to 4 or 7. A fresh 0.1 M CrCl3·6H2O stock solution adjusted to pH 4 was added to the suspension to yield either a 200 or 400 µM initial Cr(III) concentration and then the suspension was shaken for the desired period. For the kinetic study, suspension was shaken for 1 to 72 h after adding Cr(III). For the static studies (fixed time), the suspensions were shaken for 24 h after adding Cr(III). The kinetic study was conducted at pH 4 and the static studies at both pH 4 and pH 7. After shaking, the suspensions were centrifuged at 11000 RCF for 10 min and filtered with a 0.025 µm Millipore membrane (Millipore, Bedford, MA). The concentrations of Cr(VI) in the filtrates were determined using the diphenylcarbazide chromagen method (Bartlett and James, 1979).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 SUMMARY
 REFERENCES
 
Characterization of Manganese Oxides
The lithiphorite and todorokite are highly crystalline, indicated by narrow XRD peaks (Fig. 1) . The birnessite produced weak and some broad reflections in the XRD pattern. The natural Mn oxides contained impurities: kaolinite in lithiophorite, kaolinite and smectite in todorokite, and poorly crystalline silica in birnessite (Fig. 1 and Table 1). The pyrolusite XRD curve indicates the presence of the highly crystalline phase.



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  		Fig. 1. X-ray diffraction patterns of the coarse clay-size (0.2–2 µm) Mn oxides (synthetic pyrolusite and natural lithiophorite, todorokite, and birnessite). The unit of d-spaces in the figures is angstroms (Å). The XRD peaks marked with * stand for silicates.

 

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  		Table 1. Mineralogical, physical, and chemical properties of Mn oxide samples.

 
Fourier transform infrared (FT-IR) spectra of the four samples differ significantly as expected from the different structural and compositional properties of the minerals (Fig. 2) . All four samples have absorbance bands in the 400 to 700 cm-1 range that is generally characteristic of Mn oxides (Potter and Rossman, 1979). The major difference is in the birnessite spectrum, which has a strong absorbance at 1018 cm-1 attributed to a poorly crystalline silica impurity; the presence of this impurity was also shown by XRD of residue after the Mn oxide was dissolved. The pyrolusite and birnessite spectra are the smoothest and have the least number of bands. The sharper features of lithiophorite and todorokite spectra suggest that they are more uniform in composition than the birnessite as also indicated by the XRD data.



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  		Fig. 2. Infrared curves of the coarse clay-size Mn oxides (%T equals % transmission).

 
The lithiophorite curve has several strong infrared absorbances and several weak ones in accord with the more complex composition and bonding (e.g., Li, Al, Mn[III, IV]), of the structure than the other three Mn oxides. Also, a kaolinite impurity apparently contributed little or none to the curve with its absorbances at or near 432, 472, 540, 700, 915, 1012, and 1109 cm-1. The weakness of absorbance near 915 and 1109 cm-1 where kaolinite characteristically absorbs indicates that it does not materially influence the major features of the spectrum of the Mn oxide (Farmer, 1974). Also, only 12% kaolinite is present in the samples as indicated by the amount of selective dissolution residue (Table 1).

Todorokite has three strong FT-IR absorbances: 559, 1029, 3100 cm-1. The absorbance near 3100 cm-1, attributed to water sorption, is at a lower wavenumber than for the other three samples. This specimen also contains smectite and kaolinite impurities, and apparently contribute to some of the very weak absorbances albeit to a lesser degree than kaolinite did in the lithiophorite sample.

The birnessite curve has three major absorbances and several smaller ones. As noted above, the presence of poorly-crystalline silica apparently contributed to the absorbance near 1018 cm-1. The high adsorption capacity for water is evident from the large absorption in the 3400 to 3500 and 1600 cm-1 ranges (data not shown). An accessible interlayer surface in the birnessite probably accounts for some of the water sorption as does the tendency for the mineral to occur as small particles. These FT-IR and XRD curves establish the identity of the major minerals in the four samples.

Pyrolusite has a simple curve with major features at 414 and 639 cm-1. Two less intense absorbances are observed at 1104 and 1295 cm-1, and may be because of traces of impurities in the sample.

Particle Morphology
Todorokite has an overall platy particle shape and the interior of the particle is composed of laths that are characteristic of the mineral. The particle has a distinctive structurally controlled triangular shape and interior fabric. The evidence of structural control of particle shape is more for this mineral than for any of the other specimens. It has numerous internal examples of coarse lattice fringes and trilling, a triangular pattern of twinning that is characteristic of todorokite (Fig. 3a) .



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  		Fig. 3. Transmission electron micrographs of (a) todorokite (b) birnessite, (c) lithiophorite, and (d) pyrolusite.

 
Birnessite has an overall platy morphology that differs from most layer silicates in being mottled, suggestive of thick and thin spots and irregular surface features. The particle has an irregular array of the twisted fragments protruding from the edges and surfaces of the particle. The irregularities of the sheets add appreciably to the exposed surface area of a nominally platy particle. The particle apparently contains many thin curved rolls of sheets particularly in the lower half of the particle shown (Fig. 3b).

Lithiophorite has platy particles with many small laths generally parallel to the surface of the whole particle. The small laths are visible at several places near the edges of the particle indicating that they are a major component of the whole particle. The mottled surface indicates variability in thickness. Some weak moiré fringes are visible; a few of them are parallel near the bottom of the particle. A few holes in the particle are visible indicating the diversity of thickness of the particle. The lithiophorite is relatively smooth on two of the edges and irregular on the other two (Fig. 3c).

Pyrolusite differs from the three natural minerals in particle shape (Fig. 3d). The larger particles tend to be thick and rounded, suggestive of a conchoidal fracture as commonly observed in broken glass. Yet there are many smaller fragments with various shapes. This pyrolusite is devoid of cleavage surfaces or straight lines indicative of structurally controlled features. There are some particles with moiré fringes indicative of heterogeneity in composition where the whole particle is relatively smooth. The pyrolusite sample presented here has a wide range of particle sizes up to about 0.4 µm. The small particles near or on the larger ones illustrate the diversity in particle size that may account for some of the solubility in the dilute hydroquinone solution. Such contrasting particle size is often encountered where coarse mineral particles are crushed to clay size. The pyrolusite sample is almost devoid of morphological evidence of crystallinity yet both XRD and electron diffraction (data not shown) clearly indicate the presence of crystallinity. The pyrolusite has mostly smooth particles in contrast to the irregularity of the birnessite and lithiophorite that have irregular edges and basal surface features.

Other Properties
The PZC data place the four oxides in pairs analogous to the XPS data, todorokite and birnessite and pyrolusite and lithiophorite (Table 1). Oxides with a low PZC (i.e., todorokite and birnessite) would logically be negatively charged in the pH range of most soils and attract cations such as Cr(III).

Surface area values were about the same for the natural minerals, higher than the particle size would imply because of the complex fabric evident in the varied morphology (Fig. 3) especially in view of the higher intrinsic density of these oxides than layer silicates. Pyrolusite has the most diverse particle-size range and the lowest surface area values. It is lower than implied by the hydroquinone dissolution data for Mn.

X-ray Photoelectron Spectroscopy
XPS data are presented as bar graphs depicting the BE peak centers (Fig. 4) . The core level shifts provide information regarding the electronic environment of the Mn 2p3/2 atomic orbitals of the four Mn oxides studied. The Mn 2p3/2 peak position appears at 641.8 eV for a pure Mn(III) chemical oxidation state (Stranick, 1999) and a pure Mn(IV) state, giving rise to a peak position at 642.6 eV (Di Castro and Polzonetti, 1989). The relative core level shifts, obtained from curve fitting the overall Mn 2p3/2 peak envelopes, were used to quantify the relative amounts of Mn(III) and (IV). A higher BE denotes relatively more Mn(IV) (greater electron deficiency) and a lower BE denotes more Mn(III) (greater electron density). As apparent in Fig. 4, all of the Mn oxide minerals analyzed show a mixture of Mn(III) and Mn(IV). Todorokite and birnessite have predominantly Mn(IV), whereas, pyrolusite and lithioporite consist chiefly of Mn(III). These oxidation states are indicated in the formulas written earlier in this paper although the pyrolusite studied is not typical of the natural mineral as discussed below. It should be noted that differences in BE measurements of todorokite and birnessite are within the experimental error and relative amounts of Mn(III) and Mn(IV) within this pair are not exact. The same imprecision applies to the lithiophorite and pyrolusite BEs. However, quantitative analysis of relative amounts of the Mn oxidation states can readily be made between these two pairs.



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  		Fig. 4. Bar graphs of relative binding energy of Mn 2p3/2 for todorokite (T), birnessite (B), lithiophorite (L), and pyrolusite (P).

 
From the empirical formula of pyrolusite, MnO2, a predominant population of Mn(IV) within the mineral is expected, but this is not observed. There appears to be coordination on the Mn, such that electron density is pulled away from the nucleus of the atom, possibly as a result of higher coordination with O. The atom mole fraction of pyrolusite, obtained from the normalized XPS peak area intensities of the O 1s and Mn 2p3/2 core levels, show an O/Mn ratio of 3.49 (Table 2). This higher coordination with O may have resulted from heating the mineral. An early XPS study (Oku and Hirokawa, 1975) showed that when ß-MnO2 was heated to 200°C, the Mn 2p3/2 core level was lowered (by -0.9eV) and was indistinguishable from Mn3O4 (having a mixture of both [III] and [IV] states), which is considered to be the most thermodynamically stable species. Thus, our XPS measurement of the pyrolusite strongly suggests that the sample had been subjected to heat treatment transforming the mineral oxide powder to a predominantly Mn(III) state. The pyrolusite was heated at 200°C during synthesis as confirmed by the manufacturer (see materials section above).


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  		Table 2. Core level shifts and atom ratios obtained by x-ray photoelectron spectroscopy (XPS).

 
XPS also revealed varying amounts of adsorbed O (perhaps originating from the hydroxyl groups) on the solid surfaces of these minerals (Fig. 5) . The O 1s core level peaks were curve-fitted using Gaussian line shapes to deconvolute contribution from the Mn metal oxide (solid line) and adsorbed O (dotted line). The pyrolusite has the highest metal oxide content (529.1 eV) relative to adsorbed O (531.2 eV), which is consistent with its molecular formula. The high O/Mn ratio is most likely because of O chemically bonded to the Mn rather than O adsorbed onto the surface. From examining the peak shapes and in particular noting relative changes in intensity marked by the dotted line, there appears to be a trend of increasing amounts of adsorbed O from pyrolusite to lithiophorite to birnessite. (Todorokite does not appear to fit this trend.) Perhaps the relative increase in the adsorbed O moiety may be involved in the overall populations of the Mn(III) and Mn(IV). Increased withdrawal of electron density from the Mn (resulting in increased Mn[IV]) may accompany greater amounts of adsorbed O on the surface.



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  		Fig. 5. Binding energy curves O 1s electrons in the four Mn oxides. The abbreviations are todorokite (T), birnessite (B), lithiophorite (L), and pyrolusite (P).

 
Similar to the pyrolusite, todorokite has a relatively high fraction of metal oxide O on the solid surface (Fig. 5). The BE of the O 1s (denoted by the arrow at 532.1 eV) is shifted closer to the value for H2O at 533.0 eV (Wagner et al., 1980) than the other Mn oxide minerals analyzed. This pronounced shift may result from todorokite's internal tunnel structure that enables this particular mineral to retain larger amounts of hydroxyl containing moieties e.g., OH–H2O. From XPS analysis, lithiophorite, and todorokite seem to have about equal amounts of adsorbed and metal oxide O on the surface. The asymmetric peak shape of the birnessite showing greater amounts of adsorbed O is consistent with its molecular formula, having the greatest number of H2O molecules in its structure.

A mineral with a relatively high Mn(IV)/Mn(III) ratio would be a better oxidizing agent than a lower ratio for the oxidation of Cr(III) to Cr(VI). XPS analysis of the relative amounts of Mn(IV) and Mn(III) would give reason to predict that todorokite and birnessite would be better oxidizing agents than lithioporite. This trend correlates with the relative reaction rates for Cr(VI) formation (Table 3) for the initial 400 µM concentration of Cr(III).


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  		Table 3. Chromium(VI) production per hour during initial 7 h except as noted.{dagger}

 
Chromium Oxidation by Manganese Oxides
The Cr oxidation data were plotted according to the first-order rate law equation (Fig. 6) . The oxidation experiments initiated at 200 µM Cr were curve fitted with log regression lines and those at 400 µM obeyed linear regression lines. The R2 values for the regression lines were from 0.93 to 0.99. The oxidation rates (slope of line) for the Mn oxides in 400 µM solutions were todorokite greater than pyrolusite greater than birnessite. The data points for lithiophorite had too much scatter for a rate line to be reliably plotted. The amount of Cr oxidation was less for lithiophorite than for the other Mn oxides. This sequence corresponds with the relative low Mn(IV/III) ratio based on the electron BE except for the pyrolusite that had the least ratio and still was second in reaction rate (see discussion below).



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  		Fig. 6. Kinetics of Cr oxidation by the coarse clay-size Mn oxides at pH 4. The oxidation tests were conducted at (a) 200 µM and at (b) 400 µM of initial Cr(III) concentration. Data were plotted according to the first-order rate equation (Sparks, 1989). Concentration: Kt = Cr(III) at time t; K{infty}= Cr(III) at equilibrium.

 
As noted above the oxidation rate curves obey the rate law at the higher concentration and better fit the log curve at the lower concentration (Fig. 6). The rate law requires a great excess of ions or molecules to be effective in evaluating the rate of a reaction (Sparks, 1989). These results indicate that 400 µM is sufficient to evaluate the rate of the reaction and 200 µM is not enough. Furthermore, the oxidation reaction of Cr(III) to Cr(VI) showed a first-order reaction dependence on the Cr(III) concentration.

Birnessite and todorokite consistently oxidize the most Cr in the 12-h fixed-time experiment and the acid pH maximized the extent of Cr oxidation (Fig. 7) . The three Mn oxide minerals oxidized about the same amount of Cr(III) at pH 7. Pyrolusite oxidized the least Cr in this experiment except at the neutral pH and the lower metal concentration where it was about equal to lithiophorite. The greater Mn(IV)/(III) ratios for birnessite and todorokite correlate with the greater Cr oxidation of these two minerals among the four oxides investigated.



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  		Fig. 7. Chromium oxidation by four Mn oxides in 12 h at pH 4 and 7. The four Mn oxides are abbreviated as follows: todorokite (T), birnessite (B), lithiophorite (L), and pyrolusite (P).

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
SUMMARY
 REFERENCES
 
Birnessite, todorokite, and lithiophorite were chosen for study because they frequently occur in soils. Birnessite and todorokite have been observed in Alfisols and Vertisols, and lithiophorite in Ultisols and Oxisols (Dixon, 1988; Dixon et al., 1994). Pyrolusite was represented in a reagent grade chemical standard and has been used in earlier published papers on Cr oxidation. All four samples gave characteristic x-ray diffraction and infrared patterns (Fig. 1 and 2). The two platy structured minerals, birnessite and lithiophorite have rough particle surfaces caused by curling of thin layers of birnessite and pitting of the lithiophorite evident in electron micrographs (Fig. 3). The intraparticle complexity of birnessite and lithiophorite is a marked contrast to the relatively smooth and uniform surfaces of kaolinite and other layer silicates usually found in soils and in model specimens. Todorokite, on the other hand, is more orderly than the other two Mn oxide minerals; yet, it is known to be inconsistent in the number of linked octahedra in the walls of the structural tunnels.

Fine quartz, kaolinite, and smectite impurities identified by XRD after dissolving the Mn oxides in the three natural mineral samples indicate they are similar to Mn nodules in some soils. The association of smectite with todorokite in these samples and kaolinite with lithiophorite imply a more advanced weathering environment for the lithiophorite sample and that is consistent with the findings for soils (Dixon, 1988). Also, fine silica is labile to weathering and its abundance in the birnessite sample suggests that it formed in a relatively mild or early weathering environment in comparison to the other two natural Mn oxides.

The XPS BE values indicate that todorokite and birnessite contain more Mn(IV) than lithiophorite. Lithiphorite oxidized Cr to a lesser degree than the other two Mn oxide minerals (Table 3).

Although, the pyrolusite sample contains that crystal phase as determined by XRD, the sample has several properties and a history that indicate that it contains other, perhaps noncrystalline, Mn oxide. The sample was prepared by an industrial process within a matter of days. A temperature of 200°C was employed for a relatively short time to equilibrate its crystal growth and Mn oxidation, in contrast to a long formation time for most natural crystals. Natural pyrolusite has a perfect (110) cleavage (Hurlbut, 1971) and it grows as needles (Sainieidukat et al., 1993) yet neither cleavage nor crystal faces was observed in the synthetic pyrolusite sample investigated. Thus, it is not surprising that the sample has properties of a glass [conchoidal fracture, low Mn(IV)/(III)] and greater solubility than expected for pyrolusite (Table 1)].

The XPS data suggest that the surfaces of the Mn oxides may have Mn ions in different coordination sites from the ideal structure. A modified surface structure has been recently reported for {alpha}-Al2O3 when it is hydrated (Eng et al., 2000). These two findings suggest that surface modification of oxides deserve further investigation to better understand their behavior.


   SUMMARY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
REFERENCES
 
The birnessite, todorokite, and lithiophorite samples contained silica, smectite, and kaolinite that indicate the model minerals formed under weathering conditions similar to those for the Mn oxides in soils. Thus their kinetic properties should be relevant to soil systems. Also, the irregular surface configuration of the birnessite and lithiophorite particles indicate marked surface complexity suggestive of their history of weathering, precipitation, and dissolution analogous to the formation of these two minerals in soil nodules. The excessive solubility of the synthetic pyrolusite sample indicates that a reference material needs to attain chemical equilibrium before it should be utilized as an oxidant to represent the crystal phase. Morphological data obtained by transmission electron microscopy, synthesis conditions, and solubility in hydroquinone suggest that the chemical standard chosen as a mineral sample is not pure pyrolusite and oxidized Cr faster than would be expected for pyrolusite in view of its stability in natural environments.


   ACKNOWLEDGMENTS
 
This research was supported by Texas Agricultural Experimental Station and by Interdisciplinary Research Initiatives Program and Dr. Arnold Vedlitz for collaborative research support in the George Bush School of Government and Public Service, Texas A&M University.

Authors acknowledge Drs. B.G. Shipezer and A. Clearfield of Chemistry Dept., Texas A&M University, College Station, TX for their help in the surface area determinations. Thanks to Dr. L.R. Hossner for helpful advice on kinetics.

CCC gratefully acknowledges support from the Associated Western Universities, Inc. and Pacific Northwest National Laboratories operated by Battelle Memorial.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 
Contribution from Texas Agriculture Experimental Station, Texas A&M University, College Station, TX.

Received for publication March 30, 1999.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 




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