Abstract
The oxidation state of structural iron greatly influences the physical-chemical properties of clay minerals, a phenomenon that may have significant implications for pollutant fate in the environment, for agricultural productivity, and for industrial uses of clays. Knowledge of redox mechanisms is fundamental to understanding the underlying basis for iron’s effects on clays. Past studies revealed that the extent of Fe reduction varied depending on the reducing agent used, but this variation may not have been a simple function of the reduction potential of the reducing agent. The objective of this study was to identify the relationship between the Fe reduction mechanism and free radical activity in the reducing agent. Several reducing agents and their mixtures with the Na-saturated, 0.5 to 2 μm size fraction of ferruginous smectite (SWa-1) were analyzed by electron spin resonance (ESR) spectroscopy to determine the presence of unpaired electrons or free radicals. Only Na2S2O4 exhibited paramagnetic free-radical behavior with a signal at about g = 2.011, which was attributed to the sulphoxylate (S02− ·) free radical. The free radical was labile in aqueous solution, and the ability of Na2S2O4 solution to reduce structural Fe in the smectite decreased with age of the solution and paralleled the disappearance of the free radical signal in the ESR spectrum. The paramagnetic species was preserved and enhanced if Na2S2O4 was added to the clay suspension, indicating that either the clay surface stabilized the SO2− · radical or the additional unpaired electrons were produced in the clay structure.
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References
Douglas, B., McDaniel, D. H., and Alexander, J. J. (1983) Concepts and Models of Inorganic Chemistry: John Wiley and Sons, New York, 800 pp.
Dunitz, J. D. (1956) The structure of sodium dithionite and the nature of the dithionite ion: Acta Cryst. 9, 579–586.
Griffen, D. T. (1992) Silicate Crystal Chemistry: Oxford University Press, Oxford, 442 pp.
Hodgson, W. G., Neaves, A., and Parker, C. A. (1956) Detection of free radicals in sodium dithionite by paramagnetic resonance: Nature 178, 489.
Khaled, E. M. and Stucki, J. W. (1991) Effects of iron oxidation state on cation fixation in smectites: Soil Sci. Soc. Am. J. 55, 550–554.
Komadel, P. and Stucki, J. W. (1988) The quantitative assay of Fe2+ and Fe3+ using 1, 10-phenanthroline: III. A rapid photochemical method: Clays & Clay Minerals 36, 379–381.
Komadel, P., Lear, P. R., and Stucki, J. W. (1990) Reduction and reoxidation of nontronite: Reaction rates and extent of reduction: Clays & Clay Minerals 37, 203–208.
Lear, P. R. and Stucki, J. W. (1985) The role of structural hydrogen in the reduction and reoxidation of iron in nontronite: Clays & Clay Minerals 37, 539–545.
Lear, P. R. and Stucki, J. W. (1987) Intervalence electron transfer and magnetic exchange interactions in reduced nontronite: Clays & Clay Minerals 35, 373–378.
Lear, P. R. and Stucki, J. W. (1989) Effects of iron oxidation state on the specific surface area of nontronite: Clays & Clay Minerals 37, 547–552.
Lide, D. R., ed. (1992) 1992 CRC Press, Boca Raton.
Lynn, S., Rinker, R. G., and W. H. Corcoran. (1964) The monomerization rate of dithionite ion in aqueous solution: J. Phys. Chem. 68, 2363.
McBride, M. B., Pinnavaia, T. J., and Mortland, M. M. (1975) Perturbation of structural Fe3+ in smectites by exchange ions: Clays & Clay Minerals 23, 103–107.
Neta, P., Huie, R. E., and Rose, A. B. (1988) Rate constants for reactions of inorganic radicals and aqueous solution: J. Phys. Chem. Ref. Data 17, 1031–1032, 1128–1141.
Nickless, G., ed. (1968) Inorganic Sulfur Chemistry: Elsevier Publishing Co., Amsterdam, 519–522.
Rinker, R. G., Gordon, T. P., Mason, D. M., and Corcoran, W. H. (1959) The presence of the SO2− radical ion in aqueous solutions of sodium dithionite: J. Phys. Chem. 63, 302.
Roth, C. B. and Tullock, R. J. (1973) Deprotonation of nontronite resulting from chemical reduction of structural ferric iron: in Proc. Int. Clay Conf., Madrid, 1972, J. M. Serratosa, ed., Div. Ciencias C.S.I.C., Madrid, 107–114.
Rozenson, I. and Heller-Kallai, L. (1976a) Reduction and oxidation of Fe3 in dioctahedral smectite-I: Reduction with hydrazine and dithionite: Clays & Clay Minerals 24, 271–282.
Rozenson, I. and Heller-Kallai, L. (1976b) Reduction and oxidation of Fe3+ in dioctahedral smectite-II: Reduction with sodium sulphide solution: Clays & Clay Minerals 24, 283–288.
Stucki, J. W. (1988) Structural iron in smectites: in Iron in Soils and Clay Minerals, J. W. Stucki, B. A. Goodman, and U. Schwertmann, eds., D. Reidel, Dordrecht, 625–675.
Stucki, J. W. and Lear, P. R. (1989) Variable oxidation states of iron in the crystal structure of smectite clay minerals: in Structures and Active Sites of Minerals, L. M. Coyne, D. Blake, and S. McKeever, eds., American Chemical Society, Washington, D.C., 330–358.
Stucki, J. W. and Roth, C. B. (1976) Interpretation of infrared spectra of oxidized and reduced nontronite: Clays & Clay Minerals 24, 293–296.
Stucki, J. W. and Roth, C. B. (1977) Oxidation-reduction mechanism for structural iron in nontronite: Soil Sci. Soc. Am. J. 41, 808–814.
Stucki, J. W. and Tessier, D. (1991) Effects of iron oxidation state on the texture and structural order of Na-nontronite: Clays & Clay Minerals 39, 137–143.
Stucki, J. W., Roth, C. B., and Baitinger, W. E. (1976) Analysis of iron-bearing clay minerals by electron spectroscopy for chemical analysis (ESCA): Clays & Clay Minerals 32, 186–190.
Stucki, J. W., Golden, D. C, and Roth, C. B. (1984a) Preparation and handling of dithionite reduced smectite suspension: Clays & Clay Minerals 32, 191–197.
Stucki, J. W., Golden, D. C, and Roth, C. B. (1984b) The effect of reduction and reoxidation on the surface charge and dissolution of dioctahedral smectites: Clays & Clay Minerals 32, 350–356.
van der Heijde, H. B. 1953. Tracer studies on the exchange reaction of some oxygen acids of sulfur: Rev. Trans. Chim. Pays-Bes. 72, 95–96.
Vedrine, J. C. (1980) General theory and experimental aspects of electron spin resonance: in Advanced Chemical Methods for Soil and Clay Minerals Research, J. W. Stucki and W. L. Banwart, eds., D. Reidel, Dordrecht, 331–389.
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Gan, H., Stucki, J.W. & Bailey, G.W. Reduction of Structural Iron in Ferruginous Smectite by Free Radicals. Clays Clay Miner. 40, 659–665 (1992). https://doi.org/10.1346/CCMN.1992.0400605
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DOI: https://doi.org/10.1346/CCMN.1992.0400605