Progress 10/01/03 to 09/30/09
Outputs
OUTPUTS: Conferences and Symposia (all except one of the following were invited presentations at international venues): Stucki, Joseph W. 2009. Redox biogeochemistry of iron in phyllosilicates - an overview. Symposium on Redox Biogeochemistry of Phyllosilicate Minerals, 2009 Annual Meeting of the American Chemical Society, Salt Lake City, Utah, March 22-26, 2009. Ribeiro, Fabiana R., Joel E. Kostka, and Joseph W. Stucki. 2009. Biotic and abiotic reduction of iron in smectite: Comparisons of the effects on clay structure and environmental chemistry. Symposium on Redox Biogeochemistry of Phyllosilicate Minerals, 2009 Annual Meeting of the American Chemical Society, Salt Lake City, Utah, March 22-26, 2009. Day, Zachary B. and Joseph W. Stucki. 2009. Abiotic nitrate reduction by redox-activated Fe-bearing smectites. Symposium on Iron in Clay Minerals (held in honor of Joseph W. Stucki), 2009 Annual Meeting of The Clay Minerals Society, Billings, Montana, June 7, 2009. Stucki, Joseph W. 2009. Iron redox processes in smectites. Lecture given on the occasion of receiving the 2009 Marilyn and Sturges W. Bailey Distinguished Member Award at the 2009 Annual Meeting of The Clay Minerals Society, Billings, Montana, June 8, 2009. Stucki, Joseph W. 2009. The effects of iron redox cycles on smectite properties. Symposium on Environmental Mineralogy, French Academy of Sciences, Paris, France, September 14-15, 2009. Stucki, Joseph W. 2009. Evolution of the study of redox reactions of iron in smectites. 2009 Annual Meeting of the Clay Minerals Group of the Mineralogical Society of Great Britain and Ireland, given as the 10th George Brown Lecture, December 14, 2009. PARTICIPANTS: The work performed this year included the writing of a manuscript for publication in Pure and Applied Chemistry. The student, Fabiana Ribeiro, prepared the first draft of the manuscript, then the PI completed it. She recently moved back to her home country of Brazil where she hopes to find employment in the specific area of her training. The main reason for the move was so her husband could assume a faculty position at the University of Sao Paulo. TARGET AUDIENCES: The primary target audience comprises soil, clay, and environmental scientists engaged in agriculture, industry, and environmental remediation. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
The results found in this year's project generated an invitation for the PI to participate in a special symposium organized by the French Academy of Science on Environmental Mineralogy. This symposium was specifically organized to include my work on the redox cycles of Fe in phyllosilicates. Clearly, the result showing that the pathway for electron transfer from bacteria to iron in the structure of the clay mineral has impacted the scientific community's thinking about differences between biotic and abiotic reduction processes.
Publications
- Ribeiro, F.R., Fabris, J.D., Kostka, J.E., Komadel, P. and Stucki, J.W. 2009. Comparisons of structural iron reduction in smectites by bacteria and dithionite: II. A variable-temperature Mossbauer spectroscopic study of Garfield nontronite. Pure and Applied Chemistry, 81, 1499-1509. doi:10.1351/PAC-CON-08-11-16.
|
Progress 01/01/08 to 12/31/08
Outputs
OUTPUTS: Engineered barriers for high-level nuclear waste (HLW) consist of excavated repositories in sub-surface rock formations where canisters holding the radionuclide are stored. Clay minerals, particularly the 2:1 types, are used as backfill material both in the canisters and in the bore hole in order to prevent radionuclide transport to surrounding groundwater. One of the most important risks that can occur is the corrosion of the canister, which could be coupled with reduction of iron (Fe) in the clay structure. Such changes could greatly decrease the long-term stability of the clay and, consequently, of the barriers themselves. In order to test the potential effects of such redox interactions, an Fe-bearing clay mineral from a commercial source located in the Kutch region, India, was selected for study. This particular mineral is one of the candidate clay minerals to serve as such a barrier material, and is the one with the largest structural Fe content. Results from it should, therefore, provide maximum insight into the potential effects of redox interactions between the barrier and its surroundings. The unaltered clay was characterized by X-ray powder diffractometry (XRD), thermal gravimetric analysis (TGA/DTGA), Fourier-transform infrared (FTIR) spectroscopy, and variable-temperature Mossbauer spectroscopy. The chemically reduced and reoxidized forms of the clay were characterized by variable-temperature Mossbauer spectroscopy and chemical analysis. In the unaltered state the clay is comprised of smectite, maghemite, superparamagnetic goethite, and hematite, with a possible trace of kaolinite. In the reduced state the Fe (oxyhydr) oxides were dissolved. Upon reoxidation no six-line pattern was observed, indicating that the Fe remained only in the structure of the silicates. The final structure of the reduced-reoxidized clay contained more defects than the original clay, as revealed by greater quadrupole splitting values for structural Fe(III) in the clay. These findings indicate that upon exposure to natural redox cycles, the Kutch clay could undergo permanent changes in its mineralogical composition and clay mineral structure, but further study is required to ascertain the effects that such changes would have on its long-term stability as a barrier material. PARTICIPANTS: Alexandre Anastacio, Ph.D. Graduate Student TARGET AUDIENCES: The target audiences for this project include nuclear waste disposal agencies and companies, research and development in the public and private sectors, and academic research institutions. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
The presence of Fe-bearing minerals in the Kutch clay accounts for about one-fourth of the total Fe content of the sample. Upon reduction, partial dissolution of the Fe-bearing minerals and introduction of defects in the silicate structure occurred. With reoxidation the Fe(III) state in the smectite was largely restored, but the environment of the Fe was more distorted. The Fe (oxyhydr)oxides, on the other hand, were completely and irreversibly removed by the reduction and washing processes. In an actual high-level nuclear waste (HLW) repository the reductively dissolved Fe could, however, persist in the vicinity of the original backfill material due to slow leaching from the system. These findings indicate that the structure of Kutch clay -- one potential material for use as a barrier in HLW disposal -- likely will be affected and altered by redox cycles in the barrier environment, which in turn will adversely affect the long-term stability of the barrier.
Publications
- Anastacio, A.S., Sellin, P., Fabris, J.D. and Stucki, J.W. 2008. Characterization of a redox-modified clay mineral with respect to its suitability as a barrier in radioactive waste confinement. Applied Clay Science 39,172-179.
|
Progress 01/01/07 to 12/31/07
Outputs
Iron-bearing clay minerals are important in agriculture, industry, and the environment. Among the various applications in which these minerals play a significant role is in the formation of barriers in the disposal of high-level radioactive waste (HLW). Engineered barriers for HLW consist of excavated repositories in sub-surface rock formations where canisters holding the radionuclide are stored. Clay minerals, particularly the 2:1 types, are used as backfill material both in the canisters and in the bore hole in order to prevent radionuclide transport to surrounding groundwater. One of the most important risks that can occur is the corrosion of the canister, which could be coupled with reduction of iron (Fe) in the clay structure. Such changes could greatly decrease the long-term stability of the clay and, consequently, of the barriers themselves. In order to test the potential effects of such redox interactions, an Fe-bearing clay mineral from a commercial source
located in the Kutch region, India, was selected for study. This particular mineral is one of the candidate clay minerals to serve as such a barrier material, and is the one with the largest structural Fe content. Results from it should, therefore, provide maximum insight into the potential effects of redox interactions between the barrier and its surroundings. The unaltered clay was characterized by X-ray powder diffractometry (XRD), thermal gravimetric analysis (TGA/DTGA), Fourier-transform infrared (FTIR) spectroscopy, and variable-temperature Mossbauer spectroscopy. The chemically reduced and reoxidized forms of the clay were characterized by variable-temperature Mossbauer spectroscopy and chemical analysis. In the unaltered state the clay is comprised of smectite, maghemite, superparamagnetic goethite, and hematite, with a possible trace of kaolinite. In the reduced state the Fe (oxyhydr) oxides were dissolved. Upon reoxidation no six-line pattern was observed, indicating that
the Fe remained only in the structure of the silicates. The final structure of the reduced-reoxidized clay contained more defects than the original clay, as revealed by greater quadrupole splitting values for structural Fe (III) in the clay. These findings indicate that upon exposure to natural redox cycles, the Kutch clay could undergo permanent changes in its mineralogical composition and clay mineral structure, but further study is required to ascertain the effects that such changes would have on its long-term stability as a barrier material.
Impacts
The impact of the results from this study is that the disposal of high-level radioactive waste in clay formations and using smectite clays as backfill barrier material can proceed with a higher degree of confidence in the properties of the clay materials. In this study the highest iron-containing clay was examined for its stability during oxidation and reduction cycles, and was found to be quite stable in spite of some minor alterations that occurred. The iron in the clay was readily reduced by inorganic reducing agents, but resisted complete reduction. The associated maghemite iron oxide phase was mostly dissolved during the first reduction cycle. Subsequently, the environment around the iron in the clay structure itself was changed slightly, but not to the extent of destabilizing the clay.
Publications
- Anastacio, A., Aouad, A., Sellin, P., Fabris, J.D., Bergaya, F. and Stucki, J.W. 2007. Characterization of a redox-modified clay mineral with respect to its suitability as a barrier in radioactive waste confinement. Applied Clay Science (In Press) doi: 10.1016/j.clay.2007.05.007.
|
Progress 01/01/06 to 12/31/06
Outputs
The reduction of structural Fe in smectite is mediated either abiotically, by reaction with dithionite, or biotically, by reaction with a dissimilatory Fe-reducing bacterium. The effects of abiotic reduction on clay surface chemistry are much better known than the effects of biotic reduction. Since bacteria are likely the principal agent for mediating redox processes in natural soils and sediments, further study is needed to ascertain the differences between biotic and abiotic reduction processes. The purpose of the present study was to compare the effects of dithionite (abiotic) and bacteria (biotic) reduction of structural Fe in smectites on the clay structure as observed by infrared spectroscopy. Three reference smectites, namely, Garfield nontronite, ferruginous smectite (SWa-1), and Upton, Wyoming, montmorillonite, were reduced to similar levels by either Shewanella oneidensis or by pH-buffered sodium dithionite, then each sample was analyzed by Fourier-transform
infrared spectroscopy (FTIR). Parallel samples were reoxidized by bubbling O2 gas through the reduced suspension at room temperature prior to FTIR analysis. Redox states were quantified by chemical analysis, using 1,10-phenanthroline. The reduction level achieved by dithionite was controlled to approximate that of the bacterial reduction treatment so that valid comparisons could be made between the two treatments. Bacterial reduction was achieved by incubating the Na-saturated smectites with the bacteria in a minimal medium including 20 mM lactate. After redox treatment, the clay was washed 4 times with deoxygenated 5 mM NaCl. The sample was then prepared either as a self-supporting film for O-H stretching and deformation bands or as a deposit on ZnSe windows for Si-O stretching bands and placed inside a controlled atmosphere cell also fitted with ZnSe windows. The spectra from bacteria-treated samples were compared with dithionite-treated samples having a similar Fe(II) content. The
changes observed in all three spectral regions (O-H stretching, M2-O-H deformation, and Si-O stretching) for bacteria-reduced smectite were similar to results obtained at a comparable level of reduction by dithionite. In general, the shift of the structural OH vibration and the Si-O vibration, and the loss of intensity of OH groups, indicate that the bonding and/or symmetry properties in the octahedral and tetrahedral sheets changes as Fe(III) reduces to Fe(II). Upon reoxidation, peak positions and intensities of the reduced smectites were largely restored to the unaltered condition with some minor exceptions. These observations are interpreted to mean that bacterial reduction of Fe modifies the crystal structures of Fe-bearing smectites, but the overall effects are modest and of about the same extent as dithionite at similar levels of reduction. No extensive changes in clay structure were observed under conditions present in our model system.
Impacts
This work has a great impact on our understanding of how the chemistry of soil minerals is changed when they react with indigenous bacteria in the soil. Such reactions are expected to change the cation exchange capacity, the surface area, and the fate of pesticides in the soil. Reaction with the bacteria increases when oxygen is excluded from the soil environment due to flooding of soil pores with water, such as by rain or irrigation. In the absence of oxygen, bacteria need an alternative electron acceptor in order to continue to metabolize. One element that they select under such conditions is iron in the soil minerals. The bacterial action decreases the electronic charge of the iron, which, in turn, causes other changes in the framework structure of the soil minerals. The most significant result from the current study is that these changes in the soil minerals are reversed when oxygen reenters the soil matrix (as occurs when the pores are drained). This means that
the effects of bacterial changes on soil minerals are largely reversible.
Publications
- Lee, K., Kostka, J.E. and Stucki, J.W. 2006. Comparisons of structural iron reduction in smectites by bacteria and dithionite: An infrared spectroscopic study. Clays and Clay Minerals 54:197-210.
|
Progress 01/01/05 to 12/31/05
Outputs
The success of sub-surface bioremediation strategies depends significantly on knowing the identity of the constituent mineral phases in the soil because these phases determine the reactivity of the soil with the polluting species. In the case of uranium, remediation is usually defined as immobilization, which occurs when the valence or electrical charge (oxidation state) of the metal is decreased by chemical or biologically reactions (oxidation-reduction reactions). Iron phases in the soil are particularly important for this process because they participate in and stimulate oxidation-reduction reactions in the soil. Selective dissolution techniques have been used for many years to estimate the concentrations of various (iron-containing) phases in soil samples. This method for identification of the mineral composition of the soil is imprecise because solubility is influenced by factors such as crystallinity and isomorphous substitution. Nevertheless, because of the
speed and simplicity of dissolution techniques, they are commonly used in the routine characterization of soil properties. The most commonly used reagents to extract iron from soils are acid ammonium oxalate (AAO) or ethylenediaminetetraacetic acid (EDTA) for a combination of organic complexes and poorly crystalline oxides, and citrate/bicarbonate/dithionite (CBD) for all of the free iron oxides. Despite the extent to which these methods have been used, they have not been fully authenticated, primarily because of the difficulty in properly characterizing poorly crystalline and minor phases in a complex specimen, such as a soil. In the current project, the objective is to perform this authentication using variable-temperature Mossbauer spectroscopy. Past efforts to perform this authentication have been limited to the use of Mossbauer spectra recorded at 77 K (-196 Celsius). In the current study, the levels of poorly crystalline iron oxides in soils estimated by acid ammonium oxalate
(AAO) were compared with Mossbauer spectra recorded at temperatures ranging between 4 K (-269 Celsius) and 298 K (25 Celsius). In the Mossbauer spectra, the poorly crystalline iron oxide phases are seen as magnetically hyperfine split components, with a magnetic hyperfine field of less than 45 Tesla, which appear at temperatures appreciably below 77 K. In the present work, the amount of poorly crystalline iron determined by Mossbauer spectroscopy was appreciably greater than the AAO-extractable iron, even though AAO would be expected to extract organically complexed iron in addition to the poorly crystalline oxides. Furthermore, Mossbauer spectra of the residues after AAO extraction revealed the presence of considerable quantities of poorly crystalline iron oxides remaining in the samples. This important finding would have been missed had spectra not been recorded at 4 K, as indicated by the fact that Mossbauer spectra recorded above 12 K failed to detect part of the poorly
crystalline iron oxide phase. This unextracted phase can be identified from the difference between the intensities of the magnetically ordered components in the Mossbauer spectra at around 4 K and 77 K.
Impacts
The impact of the results from this study is that the time-honored methods for identifying the iron mineral phases in soils are unreliable and a new strategy must be adopted. One of the most important inorganic forms of iron in the soil is the iron (hydr)oxides. This class of minerals consists of more than half a dozen different varieties, which differ significantly in their crystallinity, both within and between the specific varieties. Other metal cations may also substitute for iron in their crystal structures. The solubility and reactivity of these minerals is greatly altered by the crystallinity, so the strategy commonly used to identify which of these minerals are present in the soil is to perform selective dissolution using acid ammonium oxalate (AAO) to remove only the easily solubilized (poorly crystallized) phases and a mixture of citrate, bicarbonate, and dithionite (CBD) to dissolve all of the iron minerals regardless of crystallinity, from which the
relative amounts of poorly and well crystallized iron (hydr)oxides are estimated. In the present study the validity of this strategy was tested by comparing the dissolution results with results obtained by Mossbauer spectroscopy. This spectroscopic method is highly specific for identifying iron mineral phases. Results revealed significant discrepancies in the amounts of poorly crystallized iron (hydr)oxides predicted by the AAO method and that observed spectroscopically. A new strategy must, therefore, be developed.
Publications
- Lee, K., Goodman, B.A., Kostka, J.E. and Stucki, J.W. 2006. Mossbauer spectroscopic evaluation of the acid ammonium oxalate method for determining poorly crystalline iron oxides in soils (In Preparation).
|
Progress 01/01/04 to 12/31/04
Outputs
Bacterial reduction of Fe in oxides and clay minerals has been postulated as a method for sub-surface bioremediation, but little has been done to actually demonstrate it in the field. Because the potential for reduced-Fe minerals in sub-surface soils to promote immobilization of redox active metals such as U, Tc, Cr, and As is great, further investigation into this phenomenon was undertaken using sub-surface soils from the Field Research Center of the Department of Energy at Oak Ridge National Laboratory. The overall objective of this study was to determine the effects of biostimulation on the mineralogy and oxidation state of Fe in the soil. The principal method for evaluating changes in Fe mineralogy was variable-temperature Mossbauer spectroscopy between 4 and 298 K. Specific objectives were to ascertain the importance of carrying out the measurements over a range of temperatures from 4 K upward, to determine the change in Fe oxide content due to bioreductive
dissolution, to measure the change in silicate Fe oxidation state, and to identify, to the extent possible, which Fe oxides are present in the resulting biostimulated cores. Biostimulation caused extensive dissolution of the Fe oxides in the soil, decreasing them from 65% of the total Fe to only 45%. The Fe(II)/Fe(III) ratio in the silicate phase simultaneously increased from about 0.3 to 0.4, indicating that, in addition to dissolving Fe oxides, biostimulation also partially reduced the Fe in the silicate phase. Comparison of peak area ratios at 4 and 77 K revealed that measurement of the Mossbauer spectra at a sample temperature of 4 K is critical to obtaining reliable estimates of the relative amounts of Fe in oxide versus silicate phases. For example, the Fe distributed in the oxide phase in the original (unbiostimulated) soil was estimated from the 4 K spectra to be 65 % of the total Fe, whereas the corresponding value from the 77 K spectra was only 55%. After biostimulation, the
estimates from 4 and 77 K were 45% and 31%, respectively. In both samples the 77 K spectra underestimated the amount of Fe in the oxide phases by a large margin compared to the 4 K spectra. A previous strategy to obtain accurate Fe oxide contents from 77 K spectra (because 4 K was inaccessible with available cryostats) was to remove superparamagnetic Fe oxides by acid ammonium oxalate treatment. To test the efficacy of this strategy, samples were submitted to the oxalate treatment and Mossbauer spectra obtained at both 4 and 77 K. In the biostimulated sample, the resulting distributions for Fe in the oxide phases were 42% and 35%, respectively. Results at 4 K revealed that the oxalate treatment removed only a small amount of the Fe oxide (45% decreased to 42%). The 77 K spectrum after oxalate treatment reported Fe oxide content to be about 35%, which differs significantly from the 4 K value of 42%. Hence, spectra at 4 K are essential. More work is needed to identify the specific Fe
oxide minerals that are present, but at least three different components were detected.
Impacts
The assessment of changes in iron mineralogy in sub-surface soil contaminated with uranium and other radioactive elements is a crucial aspect to the underlying knowledge needed to make accurate predictions of the fate of the readioactive material. The tools being utilized in this study are at the forefront of current scientific knowledge and are yielding important understanding of the behavior of iron minerals in the contaminated environment.
Publications
- Sorensen, K.C., Stucki, J.W., Warner, R.E. and Plewa, M.J. 2004. Alternation of mammalian-cell toxicity of pesticides by structural iron(II) in ferruginous smectite. Environmental Science Technology 38:4383-4389.
- Huo, D., Fialips, C.-I., Yan, L. and Stucki, J.W.. 2004. Effects of structural Fe oxidation state on physical-chemical properties of smectites: Evidence from infrared spectroscopy. Journal of the Japan Society of Soil Physics 96:3-9.
- Swearingen, C., Wu, J., Stucki, J.W. and Fitch, A. 2004. Use of ferrocenyl surfactants of varying chain lengths to study electron transfer reactions in native montmorillonite clay. Environmental Science and Technology 38:5598-5603.
- Ribeiro, F.R., Stucki, J.W., Larson, R.A., Marley, K.A., Komadel, P. and Fabris, J.D. 2004. Degradation of oxamyl by redox-modified smectites: Effects of pH, layer charge, and extent of reduction. Pp. 471-474. In: Pecchio, M. et al. (Eds.), Applied Mineralogy, Developments in Science and Technology, Volume 1. ICAM 2004 Brazil, Sao Paulo.
|
Progress 01/01/03 to 12/31/03
Outputs
The application of fertilizers, pesticides, other organic chemicals, heavy metals, and other potentially toxic substances to soils is a rather common practice in many parts of the world, which creates a significant risk to human health and the environment. The quality of soil and water resources depends in large measure on the fate and behavior of these pollutants and additives to the soil as they interact with soil constituents. Among the most reactive of these constituents are the phyllosilicate clay minerals and much of their reactivity is derived from the presence of iron (Fe) in their crystal structures. The charge on the Fe can change in the field due to bacterial activity, and the chemistry of the system is thereby changed significantly. The long-range goal of this project is to understand the role of oxidation-reduction (redox) reactions in determining the chemical and physical properties of natural soils and sediments and the chemical processes occurring
therein. The objective is to identify the underlying cause(s) for the effects of structural Fe oxidation state on clay surface properties, and to characterize the nature of the resulting interactions with surface species such as water, metals, and organic compounds. The central hypothesis for the proposed research is that reduction of structural Fe by either biotic (bacterial reduction) or abiotic (chemical reduction) means not only yields a new redox potential and electrostatic charge at the clay surface, but invokes in situ changes in crystal site occupancy of structural Fe and introduces structural defects which, in turn, further alter the clay surface chemistry. Progress has been made with the installation of a new Mossbauer spectrometer. This instrument will enable us to obtain more extensive and precise characterizations of the behavior of the Fe in the soil minerals. Its unique capability is to obtain results at very low temperatures (four degrees above absolute zero) without
the use of liquid air cryogens.
Impacts
No impact can yet be measured due to the short duration of this project thus far.
Publications
- No publications reported this period
|
Progress 01/01/02 to 12/31/02
Outputs
The oxidation state of iron (Fe) in the crystal structure of smectite clay minerals profoundly alters their physical-chemical properties. Among the properties affected are layer charge, cation exchange and fixation capacity, swelling in water, particle size, specific surface area, layer stacking order, magnetic exchange interactions, octahedral site occupancy, surface acidity, and reduction potential. Also affected is the surface chemistry of the clay, which alters clay-water and clay-organic interaction mechanisms. Rates and extents of degradation of pesticides are increased in the presence of reduced smectites compared to oxidized and reduced-reoxidized counterparts. An hypothesis regarding the mechanism for Fe reduction in clay minerals was first developed in 1963, and subsequent modifications have been proposed periodically through the present time. Recent studies clearly reveal that the process of Fe reduction involves more than the mere transfer of an electron
to octahedral Fe(III) in the clay crystal. Ancillary reactions occur which produce significant structural modifications, some of which are reversible and others which are not. Such changes in the crystal-chemical environment of structural Fe are thought to play a dominant role in altering the clay surface chemistry. The effects of Fe oxidation state on the infrared (IR) spectra of dioctahedral smectites were studied using a purified and sodium-saturated fraction of the Garfield nontronite reference clay. The nontronite was first reduced with sodium dithionite for a period of 10 to 240 minutes to obtain various Fe reduction levels. The reduced samples were then reoxidized by bubbling O2 through the suspensions for 8 to 12 hours. IR spectra of the initially unaltered, reduced, and reduced-reoxidized nontronites were collected. After reduction, changes were observed in the spectral regions of O-H stretching, O-H deformation, and Si-O stretching, indicating that the clay structure was
significantly modified beyond merely a change in Fe oxidation state. Furthermore, a new component band in the O-H stretching region of the reduced samples exhibited a pleochroic effect, indicating the possible existence of trioctahedral domains. A large (up to 43 wavenumbers) downward shift of the main Si-O stretching band of the reduced samples was also observed. Such a large shift indicates that the change in Fe oxidation state in the octahedral sheet strongly affects the structural properties of the tetrahedral sheet, which might further affect physical and chemical properties of the mineral surface. The spectral differences across all three studied regions between unaltered and reoxidized samples after up to 240 minutes of reduction indicated that the redox process involving sodium dithionite is in some respects irreversible, even though virtually all structural Fe(II) can be reoxidized. Further structural information is being gathered using other physical-chemical techniques,
including Mossbauer spectroscopy and X-ray absorption spectroscopy.
Impacts
The clay minerals control a large fraction of the chemical processes taking place in the soil. Changes in the electrical charge (oxidation state) of iron in these ubiquitous soil minerals occurs readily in the soil through bacterial activity over short periods of time, causing significant modifications in the underlying chemical behavior of the soil. Understanding the associated changes in mineral structure, chemical composition, and surface reactions provides the basis for predicting more accurately the sustainability of certain cultural practices in the field, including fertilization and pest-control strategies.
Publications
- Fialips, C.-I., Huo, D., Yan, L., Wu, J. and Stucki, J.W. 2002. Effect of iron oxidation state on the IR spectra of Garfield nontronite. American Mineralogist 87:630-641.
- Fialips, C.-I., Huo, D., Yan, L., Wu, J. and Stucki, J.W. 2002. Infrared study of reduced and reduced-reoxidized ferruginous smectite. Clays and Clay Minerals 50:455-469.
- Stucki, J.W., Lee, K., Zhang, L. and Larson, R.A. 2002. The effects of iron oxidation state on the surface and structural properties of smectites. Pure and Applied Chemistry 74:2079-2092.
|
Progress 01/01/01 to 12/31/01
Outputs
The oxidation state of structural iron (Fe) in layer silicates profoundly alters their physico-chemical properties, including exchange and fixation of interlayer cations and surface hydration. Fixation of potassium (K) removes it from the plant-available pool in soils, and thereby creates an agricultural, economic, and environmental problem. A method to mitigate the effect of Fe oxidation state on K fixation would, therefore, be a great benefit. A recent study found that the addition of trimethylphenylammonium cations (TMPA) prevented clay layers from collapsing during Fe reduction, and increased surface hydration, which is opposite the effect observed when sodium is the interlayer cation. The objectives of this study were (1) to study cation selectivity between K and TMPA on redox-treated ferruginous smectite (Source Clays sample SWa-1), and (2) to examine the effect of Fe oxidation state and intercalated TMPA on K fixation in SWa-1. After samples of Na-, K-, and
mixed K/TMPA-SWa-1 were chemically reduced with sodium dithionite at room temperature, each clay was washed under inert-atmosphere conditions and the amounts of exchangeable (by 0.5 M MgCl2) and fixed (by digestion of residual after exchange) cation were determined. Results revealed that the selectivity of the oxidized surface of SWa-1 was much greater for TMPA than for K. In the reduced, higher-charged state, however, the selectivity was reversed and K dominated the exchange complex over TMPA. The K was partitioned between exchangeable and fixed forms. K fixation was observed only if this cation was present on the exchange complex prior to or during reduction. As the ratio of TMPA to K in the reducing medium increased from 0 to 4, the amount of K that became fixed upon reduction decreased by an order of magnitude (from more than 30 to about 3 meq/100 g). We propose, therefore, that the reduction of structural Fe in SWa-1 causes interlayers to fully or partially collapse, except TMPA
present during the reduction process functions as pillars to keep the smectite interlayers open, thereby decreasing metal cation fixation. This is consistent with previous studies which showed that the water retention capacity of reduced-TMPA-smectite was greater than reduced-Na- smectite. A method may, therefore, be devised whereby a type of pillaring molecule could be introduced along with K fertilizer to ameliorate the adverse effects of redox processes in soil on K fixation.
Impacts
As a result of these studies we now know that potential availability of potassium to plants may be increased if very small amounts of quaternary ammonium cations are present in soils that undergo oxidation and reduction. The oxidation and reduction process is common in soils, and occurs by the activity of microorganisms under wet and warm moisture regimes.
Publications
- No publications reported this period
|
Progress 01/01/00 to 12/31/00
Outputs
Reduction of structural Fe in Na-exchanged dioctahedral smectites decreases swellability in water, but because clay interlayers also collapse in the process the concomitant effect on surface hydration energy is uncertain. This study examined the hydration behavior of oxidized and reduced dioctahedral smectite clays exchanged with polar (Na) and weakly-polar (organic) cations to determine the nature of the surface before and after Fe reduction, and to determine if clay surfaces are hydrophilic or hydrophobic. The H2O content in various dioctahedral smectites decreased if Na was replaced by tetramethylammonium (TMA), trimethylphenylammonium (TMPA), or hexadecyltrimethylammonium (HDTMA). Among the organo-clays, H2O adsorption decreased with increasing complexity of the cation. For oxidized smectites, those exchanged with TMPA retained less H2O than those exchanged with Na at all pressures. The extent of this difference depended on the clay and decreased with increasing
applied pressure. Reduction of Fe(III) to Fe(II) in the octahedral sheets decreased the swelling of Na-saturated smectites, apparently causing some previously swelling interlayers to collapse. If the Na interlayer cation was exchanged to alkylammonium after reduction, but prior to swelling pressure measurements, the swelling increased or remained near constant, suggesting that the organo-cation disrupted the collapse process of the interlayers associated with the reduced smectite layers. Reduced TMPA-saturated smectite surfaces are more strongly hydrated if the octahedral sheet is reduced than if oxidized. Thus, reduction of structural Fe increases the hydration energy of smectite basal surfaces, but swellability could decrease or increase depending on the extent of interlayer collapse occurring with different exchangeable cations. The behavior of pesticides at clay surfaces was also investigated. The fate of pesticides in the environment depends to a large extent on their
interactions with clay mineral surfaces in soils and sediments. Most studies of these interactions typically employ batch suspension methods. Another method, which may be much faster and simpler, is to cast the clay into the form of a self-supporting film immobilized between two porous nitrocellulose filters. The objective of this study was to investigate the kinetics of alachlor transport through such a membrane. We found that at least two kinetic phases (rapid and slow) exist. Herbicide disappearance from donor solutions, its appearance in acceptor solutions (including time lag and the rate at steady state), and its retention by the membrane revealed that adsorption on the clay surface plays a prominent role in trans-membrane transport processes. The oxidation state of structural Fe in the clay film changed the surface chemical properties of the clay and modified both the fast adsorption and slow transport of alachlor through the membrane. Transport through the reduced clay was
accompanied by chemical alterations of alachlor. No chemical alterations were observed if the clay film was in its oxidized state.
Impacts
As a result of these studies we now know that pesticides are degraded if soil clay minerals are in a reduced state, and we have demonstrated and affirmed that water molecules interact strongly and directly with clay mineral surfaces. This understanding is of fundamental importance to predicting fate and behavior of chemicals in the soil environment.
Publications
- Stucki, J.W., Wu, J., Gan, H., Komadel, P. and Banin, A. 2000. Effects of Fe oxidation state and organic cations on smectite hydration. Clays and Clay Minerals 48: 290-298.
- Kocherginsky, N.M. and Stucki, J.W. 2000. Sorption, diffusion, and desorption of alachlor in oxidized and reduced smectite membranes. Environmental Science and Technology 34:3574-3578.
- Kocherginsky, N.M. and Stucki, J.W. 2000. Supported clay membrane: New way to characterize water and ion transport in clays. Adv. Environ. Res. (In Press).
|
Progress 01/01/99 to 12/31/99
Outputs
Variation in the oxidation state of iron (Fe) in the crystal structure of clay minerals profoundly alters the chemical and physical properties of the clays and the surrounding soils and sediments. The oxidation state is also a property that can be changed in situ, so an understanding of the controlling mechanisms may offer opportunities to modify the properties of soils. One probe of the mechanism relating Fe oxidation state to chemical behavior is the magnetic interaction between Fe atoms in the clay structure. The purpose of this study was to characterize magnetic exchange interactions in oxidized, reduced, and reoxidized smectite clays. Four smectites with different total Fe contents (two nontronites, one ferruginous smectite, and one montmorillonite) were reduced to obtain a range of Fe(II)/Fe(III) ratios and their magnetic properties were measured with a SQUID (Superconducting QUantum Interference Device) as a function of applied magnetic field strength at 5 K
and as a function of temperature in a field of 0.1 T. The unaltered nontronite and ferruginous smectite specimens showed antiferromagnetic coupling, whereas the coupling in the reduced samples was ferromagnetic; the paramagnetic Curie temperature increased with increasing Fe(II) content. Data collected after cooling samples both in the presence and absence of an external magnetic field of 0.1 T showed that at low temperatures the reduced (ferromagnetic) nontronite and ferruginous smectite samples exhibit a memory effect of previous magnetic field exposure consistent with superparamagnetic or spin glass behavior. The superparamagnetic/ferromagnetic transition temperature, Tf, increased linearly with increasing Fe(II) content for each of the nontronites, but the relationship between Tf and Fe(II) content differed for different clays, thus demonstrating that Tf is sensitive to isomorphous substitutions in the clay structure. The montmorillonite was paramagnetic in both oxidized and
reduced forms.
Impacts
The organization and distribution of Fe throughout the clay mineral structure has great bearing on the surface properties of the clay. This study reveals that Fe is located in adjacent sites and that it is not distributed in a single, symmetrical site; but rather exists in two or more types of site. Evidence may also suggest that the magnetic domains are rather small.
Publications
- SCHUETTE, R., GOODMAN, B.A., and STUCKI, J.W. 1999. Magnetic properties of oxidized and reduced smectites. Phys. Chem. Miner. (In Press).
|
Progress 10/01/97 to 09/30/98
Outputs
The electrical charge of iron in clay minerals potentially exerts a dominant influence on soil behavior with respect to water holding capacity, plant-availability of potassium fertilizers, pesticide degradation, and fate of heavy metals. How iron does this is still poorly understood. A better understanding would enable the design of soil management practices to deal more successfully with many problems in agriculture and the environment. Clay minerals assert their influence because of their large, chemically active surfaces. So iron undoubtedly must affect the surface in some manner. The purpose of this study was to discover how. Clays are comprised of individual layers, each consisting of four parallel planes of negatively charged oxygen ions. These ions are neutralized mostly by much smaller and positively charged silicon, aluminum, and iron ions occupying the gaps between oxygens within the layer. All of these ions have a fixed charge, except iron, which can change
between 3+ and 2+. In the soil this happens by natural bacterial activity. In this study, application of highly sophisticated x-ray and infrared spectroscopic techniques revealed that if the charge on iron is reduced from 3+ to 2+, the iron changes its location, water molecules become tightly bound, and distances between metal and oxygen ions change within the clay layer. These alterations in chemical structure drastically change the bond angles and energies that tie the clay layer together, which in turn modify the factors that control surface chemistry, namely, electronic orbitals in and the geometric configuration of surface oxygens. These results and methods will now be used to test a variety of common clays to discover patterns and general principles that can be converted to practical use.
Impacts
(N/A)
Publications
- Huo, D. 1997. Infrared study of oxidized and reduced nontronite and Ca-K competition in the interlayer. Ph.D. Thesis, University of Illinois.
- Manceau, A.; Lanson, B.; Drits, V.A.; Chateigner, D.; Gates, W.P.; Wu, J.; Huo, D. and Stucki, J.W. 1999. Oxidation-reduction mechanism of iron in dioctahedral smectites. 1. Structural chemistry of oxidized reference nontronites. American Mineralogist (Accepted).
- Manceau, A.; Drits, V.A.; Lanson, B.; Chateigner, D.; Wu, J.; Huo, D.; Gates, W.P. and Stucki, J.W. 1999. Oxidation-reduction mechanism of iron in dioctahedral smectites. 2. Structural chemistry of reduced Garfield nontronite. American Mineralogist (Accepted).
|
Progress 01/01/97 to 12/31/97
Outputs
This project began on October 1, 1997. There is no progress to report.
Impacts
(N/A)
Publications
- No publications reported this period
|
|