Redox Properties of Structural Fe in Smectite Clay Minerals

Chapter 17

Redox Properties of Structural Fe in Smectite Clay Minerals Downloaded by UNIV OF OKLAHOMA on April 26, 2013 | http://pubs.acs.org Publication Date (Web): September 2, 2011 | doi: 10.1021/bk-2011-1071.ch017

Anke Neumann,1 Michael Sander,1 and Thomas B. Hofstetter1,2,* 1Institute

of Biogeochemistry and Pollutant Dynamics (IBP), Swiss Federal Institute of Technology (ETH) Zürich, Universitätsstr. 16, 8092 Zürich, Switzerland 2Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstr. 133, 8600 Dübendorf, Switzerland *corresponding author: [email protected]

Redox reactions of structural Fe in clay minerals play important roles in biogeochemical processes and for the fate of contaminants in the environment. Many of the redox properties of Fe in clay minerals are, however, poorly understood, thus limiting the knowledge of the factors that make structural Fe participate in electron transfer reactions. This chapter summarizes the current state of knowledge on the redox properties of structural Fe in clay minerals. In the first part, we review the various spectroscopic observations associated with structural Fe reduction and oxidation and how changes in Fe oxidation state affect the clay mineral structure and the binding environment of Fe in the octahedral sheet of planar 2:1 clay minerals. In the second part, we show how information on the structural alterations and arrangement of Fe can be interpreted to assess the apparent reactivity and the thermodynamic redox properties of structural Fe in clay minerals.

Introduction The Fe2+/Fe3+ redox couple plays an important role in the biogeochemical cycling of elements, and is of direct relevance for the remediation of environmental systems contaminated with organic and inorganic pollutants (1–4). Clay-mineral-bound Fe is of particular importance in environmental © 2011 American Chemical Society In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Downloaded by UNIV OF OKLAHOMA on April 26, 2013 | http://pubs.acs.org Publication Date (Web): September 2, 2011 | doi: 10.1021/bk-2011-1071.ch017

electron transfer reactions because Fe-containing clay minerals are ubiquitous in subsurface environments and structural Fe in clay minerals is not subject to the same dissolution and re-precipitation processes as Fe in (oxyhydr)oxides (5–7). Structural Fe in clay minerals therefore can act as renewable source of redox equivalents in soils and sediments for the natural or enhanced attenuation of pollutants at contaminated sites (Figure 1, (2, 8)). Structural Fe3+ in clay minerals can be reduced to Fe2+ by microorganisms and surface-bound Fe2+ (9–11), which is a viable reductant for many organic pollutants (e.g., chlorinated solvents, nitroaromatic explosives; (12–14)) and metals (e.g., U, Tc, Cr; (15–17)). The reduction of organic compounds leads to transient products, which often are more susceptible to complete microbial degradation via oxidative pathways. Reduction of metals from radioactive waste repositories or re-processing sites by Fe2+ often results in the formation of sparingly soluble, and hence, less mobile metal species.

Figure 1. Schematic representation of redox cycling of structural Fe in clay minerals. The colored hexagons symbolize the octahedral binding environment of Fe3+ (red) and Fe2+ (blue) in the clay mineral structure. Structural Fe3+ can be reduced by electron donors such as dithionite, hydrazine, or microbes. The resulting structural Fe2+, in turn, reduces oxidized organic contaminants (e.g., carbon tetrachloride or nitroaromatic compounds) or metals (e.g., Tc7+).

Reduction of structural Fe3+ to Fe2+ increases the net negative charge in the clay mineral structure (6), which alters important mineral properties such as its cation exchange capacity (18, 19) and its swelling pressure (18, 20). These oxidation-state induced changes are highly relevant for the bioavailability of soil nutrient such as K+, Ca2+, Cu2+, Zn2+, and NH4+ (21) as well as for maintaining the integrity and stability of radioactive waste repositories that rely on clay mineral-based backfill material for radionuclide retention (22). A comprehensive understanding of Fe reduction and oxidation processes and their effects on clay mineral properties is therefore essential for assessing clay mineral mediated natural and engineered processes. The characterization of the redox properties of structural Fe in clay minerals has proven to be challenging. Some of the most fundamental properties are only poorly understood, including the fraction of total structural Fe available for reduction/oxidation and estimates of structural Fe3+/Fe2+-reduction potentials. The redox properties of structural Fe are affected by its bonding environment 362 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


in the clay mineral’s lattice, which itself depends on the layer composition (23), the ordering of structural cations (24, 25), the total Fe content (26), and the Fe oxidation state (27, 28). While these structural parameters are well studied for natural clay samples, the redox reactions of structural Fe remain poorly understood on the molecular level. As a consequence, numerous macroscopic observations lack mechanistic explanations. For example, it is unclear why Fe-containing clay minerals of similar composition cannot be reduced to comparable extents (21, 29, 30) or why the extent of structural Fe reduction differs between chemical and microbial reduction (10, 31–33). Advances towards a more holistic understanding of electron transfer processes involving structural Fe in clay minerals requires linking Fe2+ and Fe3+ structural arrangements to Fe3+/Fe2+-reduction potentials and apparent reactivity of clay mineral particles. The objective of this chapter is to review the current state of knowledge on the effects of structural Fe reduction and oxidation on the clay mineral redox properties and structure, and on the reactivity of structural Fe in electron transfer reactions. We first summarize the most commonly used analytical approaches for elucidating the binding environment of structural Fe in clay minerals. The second part focuses on the effects of Fe redox changes on the Fe-binding environment. In the third section, spectroscopic observations are linked to the apparent reactivities and the thermodynamic redox properties of structural Fe in clay minerals. The discussion primarily focuses on smectites because these planar 2:1 clay minerals have been most intensely investigated. In these clay minerals, structural Fe is located in the octahedral and/or the tetrahedral sheets. Most experimental data is available on electron transfer to and from octahedrally coordinated Fe. We will therefore elaborate on the effect of octahedral sheet properties including the di- versus trioctahedral site occupancy, cis/trans vacancies in dioctahedral clay minerals, as well as the distribution of Fe and its neighboring cations Al, Mg, and Fe on electron transfer reactions to and from structural Fe (34–36). A schematic representation of the clay mineral properties addressed in this chapter is depicted in Figure 2.

Spectroscopic Approaches Used To Elucidate the Structural Environment of Fe in Clay Minerals Several spectroscopic techniques facilitate the study of clay mineral structures and, specifically, the binding environments of structural Fe in clay minerals. The most widely used techniques are 57Fe Mössbauer, X-ray absorption, infrared, and visible spectroscopy. Spectroscopic analyses are generally complemented with the quantification of the total structural Fe content as well as Fe2+/Fe3+ or Fe2+/total Fe ratios after acid digestion of clay minerals. Notice that erroneous digestion procedures may have led to inaccurate estimates for redox active Fe2+/Fe3+-species and total Fe (39).

363 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 2. Schematic representations of theoretical octahedral sheet compositions of, smectites showing a) trioctahedral and b, c) dioctahedral occupancies of the octahedral sheet with cations. Dioctahedral clay minerals (b, c) can be categorized as cis- and trans-vacant, depending on whether the octahedral OH groups are on the same or on opposite sides of the octahedral vacancies, respectively. Smectites with low Fe content are cis-vacant (b), whereas Fe-rich smectites are trans-vacant (c). In panel a) local clustering of Fe2+ is indicated, whereas panel b) depicts a cation distribution following an exclusion rule with Fe3+ cations neighboring Al3+ but not Mg2+ or Fe3+. For the Fe-rich clay mineral, c), a largely random cation distribution was chosen (25). Panels d) and e) depict structural changes in the octahedral sheet after Fe3+reduction for the model clay minerals given in panels b) and c), respectively. In cis-vacant dioctahedral smectite of low Fe content only small structural changes are observed (d; (37)). Fe reduction of Fe-rich, trans-vacant dioctahedral smectite leads to structural rearrangements through the formation of trioctahedral Fe2+ groups enclosing domains of vacancies and to the dehydroxylation of the octahedral sheet (indicated as open circles in panel d, (26, 38)). 364 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

57Fe

Mössbauer spectroscopy allows for the determination of the bulk ratio and can distinguish between octahedrally and tetrahedrally coordinated Fe atoms (40, 41). Additionally, Mössbauer spectroscopy can be used on mixed-phase samples, as clay minerals can be distinguished from iron oxides and other impurities. Spectra collected at low temperatures (77 K to 4 K) of clay minerals with high structural Fe contents provide information on electronic interactions between neighboring Fe atoms, such as magnetic ordering (33, 41, 42). For some clay minerals, the population of Fe atoms in cis and trans configuration can be quantified, although this remains a controversial topic (43–46). One notable shortcoming is that fitting of clay mineral spectra can often result in non-unique fits, requiring secondary techniques (47). An additional drawback of Mössbauer spectroscopy is that the exact identity of the Fe-neighboring cations cannot be determined at an atomic level (41). An alternative Fe-sensitive spectroscopic technique is Fe K-edge X-ray absorption spectroscopy (XAS). These spectra are collected by measuring the absorbance of 7112 eV X-rays available in synchrotron facilities. Near edge spectra (XANES, energy range 7090-7210 eV) allow determining the Fe oxidation state and can distinguish between tetrahedral and octahedral Fe coordination (26, 28). EXAFS spectra are typically interpreted relative to computed spectra for model structures of clay minerals. The extended X-ray absorption fine structure (EXAFS, energy range 7000-8300 eV) contains information on the nearest octahedral and tetrahedral neighbors (25, 26, 28), but the spectra do not typically yield reliable information on Fe-neighboring cations exceeding the second coordination sphere due to multiple scattering and the structural heterogeneity of natural clay minerals (25, 48). Various forms of IR spectroscopy monitor absorbance of IR light by hydroxyl (OH) groups bound to octahedral cations in clay minerals. The positions of OH absorption bands in IR spectra are indicative of octahedral cations attached to the OH groups as well as to the oxidation state and the structural environment of octahedral Fe (37, 49–54). Absorption bands of OH bending vibrations (spectral range 600-950 cm-1) are usually better resolved than absorption bands observed in the stretching region (3500-3700 cm-1). The near IR region contains bands resulting from the combination of bending and stretching modes (spectral range 4100-4600 cm-1) as well as the first overtones of the stretching vibrations (spectral range 6900-7400 cm-1). The combined analysis of the absorption bands in these four IR regions facilitates a rigorous and accurate characterization of Fe structural environments (55–57). In contrast to Mössbauer spectroscopy and XAS, IR spectroscopy cannot detect tetrahedrally bound Fe because it is not directly bound to hydroxyl groups. Clay minerals with high amounts of tetrahedral Fe do, however, exhibit a characteristic absorption band for tetrahedral Fe-O entities (58). Visible light absorption spectroscopy is limited to studying the intervalence electron transition between Fe2+ and Fe3+ in adjacent octahedral sites at 720 nm (59). This method provides information on the oxidation state of octahedral Fe (29) but yields no insight into the Fe binding environment. Furthermore, this technique is limited to clay minerals with high structural Fe contents (see further discussion below).


Fe2+/Fe3+


Reduction and Oxidation of Structural Fe and Their Effects on Clay Mineral Properties


Numerous studies investigated the effect of structural Fe oxidation state on clay mineral properties in the context of Fe biogeochemistry in soils and sediments. For manipulating the structural Fe oxidation state, a variety of reductants and oxidants have been used that result in different extents of reduction and oxidation, and differences in redox cycling. Different mechanisms of structural Fe redox processes have been proposed based on spectroscopic analysis of clay minerals with manipulated Fe oxidation states.

Reduction of Fe3+ Various reducing agents, including sulfide species (60), tetraphenyl boron (61, 62), and hydrazine (63, 64) have been used to reduce structural Fe3+ in clay minerals. The most frequently used reduction methods are chemical reduction by dithionite (65, 66), and microbial reduction using pure cultures and enrichment cultures (9, 10, 31).

Chemical Reduction Using Dithionite The sulfoxylate radical species, SO2•−, resulting from the disproportionation of dithionite in aqueous solution, has a reduction potential, Eh (SO32−/SO2•−) of -0.66 V (vs. SHE; pH 7) (66). This potential is so low that >90% of the structural Fe in smectites can be reduced (29, 30, 50). Dithionite reduction in buffered citrate-bicarbonate solutions leads to minor, incongruent (i.e., not uniform) smectite dissolution with 12 wt% Fe) the observable biphasic probe compound reduction kinetics could be explained by the presence of two distinct and interconvertable Fe2+ entities, exhibiting rate constants, which differed by three orders of magnitude. The same biphasic reduction kinetics of probe compounds was found for chlorinated solvents (14) and Cr6+ (79). In the case of smectites exhibiting low structural Fe content (3 wt% Fe), the above kinetic scheme simplifies to a pseudo-first order rate law of probe compound reduction, indicating the presence of only one type of reactive Fe2+, which is of low reactivity (12, 30). The above observations confirm spectroscopic findings that the structural Fe content is the predominant factor responsible for the formation of various structural Fe2+ entities with different reactivity. The reactive probe compound approach has mostly included only a small portion of the available structural Fe (i.e., >p) and unit activity of structural water, OH-, and cations (84), equation 8 simplifies to

Equations (8) and (9) have been proposed to relate EhFe to the oxidation state of structural Fe. These equations, however, were derived assuming a constant Eh0 Fe for all structural Fe in a given clay mineral, reversible electron transfer, and independent electron transfer to and from each structural Fe atom. These assumptions are unlikely fulfilled for most clay minerals for numerous reasons. (i) Structural Fe atoms in clay minerals are expected to be bound in various, chemically different microenvironments, resulting in a distribution of EhFe rather than a constant EhFe value. (ii) Fe3+ reduction in trans-vacant smectites involves dehydroxylation reactions and cation migration in the octahedral sheets (Figure 2; (26, 28, 84)). It is likely that some of these structural changes are at least partially irreversible, resulting in irreversible alterations of the binding environment of structural Fe in a reduction-oxidation cycle. Electron transfer is then no longer fully reversible and EhFe at a given Fe oxidation state will depend on the "redox history" and the direction of electron transfer (i.e., reduction or oxidation). (iii) Changes in the oxidation state of a given structural Fe atom in clay minerals with high structural iron content may affect EhFe of adjacent Fe3+/Fe2+ couples. This interdependence was suggested from monitoring the intervalence electron transfer band in Fe-rich dioctahedral smectites which showed sequential reduction of Fe3+-O-Fe3+ to Fe3+-O-Fe2+ up to Fe2+/total Fe ratios of 45%, followed by Fe2+-O-Fe2+ formation upon further reduction (29, 38). Similar to Fe (oxyhydr-)oxides, there may also be electron transfer between adjacent Fe sites in Fe-rich smectites (11, 87), which would violate the above assumption of independent electron transfer to and from individual structural Fe atoms. These considerations suggest that trends in EhFe predicted by equations (8) and (9) in dependence of the extent of reduction and solution chemistry may deviate from experimental trends of EhFe, which are yet to be measured. Advances towards improving our understanding of electron transfer to and from structural Fe rely on experimental approaches to directly quantify EhFe as a function of the oxidation state, the redox history, and solution chemistry. Based on the coordination chemistry of structural Fe in layer silicates, the standard reduction potential for structural Fe (Eh0 Fe) was estimated to range between Eh0 Fe of 0.71 to 0.74 V (pH 0, SHE, (88, 89)). Experimental validation of these Eh0 Fe is, however, still scarce. Attempts to characterize EhFe by using redox-active surfactants adsorbed to the clay mineral interlayers were unsuccessful (90). In principle, traditional batch reactivity assays may be used to estimate EhFe. However, the electron transfer in these systems is only indirectly monitored via the kinetics and extent of probe compound transformation. Instead, homogeneous electrocatalysis, successfully used to characterize the redox properties of natural organic matter (91), has great promise. In this approach, electron transfer to and 372 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

from particulate environmental phases at desired reduction potentials and solution pH is directly quantified by chronocoulometry (i.e., integration of reductive and oxidative currents). Electron transfer between the solid phase and the working electrode is mediated by mobile, redox-active organic radicals. Current work is directed towards exploring the possibilities of homogeneous electrocatalysis to elucidate the electron transfer mechanism to and from structural Fe in clay minerals.


Conclusion and Outlook Assessing the role of Fe-containing clay minerals in biogeochemical processes and pollution dynamics requires a fundamental understanding of the mineral properties that make structural Fe participate in environmental electron transfer reactions. While the available spectroscopic approaches for the description of Fe binding environments are elaborate, methods for the characterization of the clay particle’s essential redox properties are currently being developed. Quantifying the Fe3+/Fe2+ reduction potential as well as the electron donating and accepting capacities, for example by electrochemical methods, will be key to improve the understanding of the redox chemistry of structural Fe in clay minerals. Future work should also include a wider range of clay mineral types and structures. Most studies to date have addressed Fe-rich, planar 2:1 clay minerals to illustrate the relevance of Fe-mediated redox processes of clay minerals. In soils, sediments, and waste repositories, however, a variety of Fe-containing clay minerals are present or form as a result of weathering processes and their contribution to the redox cycling of Fe is essentially unknown. Finally, the proposed characterization of clay mineral redox properties should provide the basis for delineating the relevance of redox processes catalyzed by Fe in clay minerals versus that involving Fe (oxyhydr)oxides and other redox active species in the aquatic environments (see other chapters in this volume).

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Redox Properties of Structural Fe in Smectite Clay Minerals

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