Environ. Sci. Technol. 2003, 37, 519-528
Reactivity of Fe(II) Species Associated with Clay Minerals THOMAS B. HOFSTETTER,* R E N EÄ P . S C H W A R Z E N B A C H , A N D STEFAN B. HADERLEIN† Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), P.O. Box 611, CH-8600 Du ¨ bendorf, Switzerland
Mineral-bound Fe(II) species represent important natural reductants of pollutants in the anaerobic subsurface. At clay minerals, three types of Fe(II) species in fundamentally different chemical environments may be present simultaneously, i.e., structural Fe(II), Fe(II) complexed by surface hydroxyl groups, and Fe(II) bound by ion exchange. We investigated the accessibility and reactivity of these three types of Fe(II) species in suspensions of two different clay minerals containing either ferrous iron-bearing nontronite or iron-free hectorite. Nitroaromatic compounds (NACs) exhibiting different sorption behavior on clays were used to probe the reactivity of the various types of reduced iron species. The clay treatment allowed for a preparation of nontronite and hectorite surfaces with Fe(II) adsorbed by surface hydroxyl groups at the edge surfaces. Furthermore, hectorite suspensions with additional Fe(II) bound to the ion exchange sites at the basal siloxane surfaces were set up. We found that both structural Fe(II) and Fe(II) complexed by surface hydroxyl groups of nontronite reduced the NACs to anilines. An electron balance revealed that more than 10% of the total iron in nontronite was reactive Fe(II). Fe(II) bound by ion exchange did not contribute to the observed reduction of NACs. Reversible adsorption of the NACs at the basal siloxane surface of the clays strongly retarded NAC reduction, even in the presence of high concentrations of Fe(II) bound by ion exchange to the basal siloxane surfaces. Our work shows that in natural systems a fraction of the total Fe(II) present on clays may contribute to the pool of highly reactive Fe(II) species in the subsurface. Furthermore, this work may help to distinguish between Fe(II) species of different reactivity regarding pollutant reduction. Although structural iron in clays represents only a small fraction of the total iron pool in soils and aquifers, reactive Fe(II) species originating from the reduction of structural Fe(III) in clays may contribute significantly to the biogeochemical cycling of electrons in the subsurface since it is not subject to depletion by reductive dissolution.
Introduction Several recent studies have demonstrated the importance of mineral-bound iron species in the reduction of inorganic * Corresponding author phone: +41-1-823 50 76; fax: +41-1-823 52 10; e-mail:
[email protected]. † Present address: Centre for Applied Geosciences (ZAG), Eberhard-Karls University Tu¨bingen, Wilhelmstrasse 56, D-72074 Tu ¨ bingen, Germany. 10.1021/es025955r CCC: $25.00 Published on Web 12/18/2002
2003 American Chemical Society
and organic pollutants, particularly in the subsurface (1). Laboratory experiments showed that various mineral phases that contain structural Fe(II) (e.g., magnetite (2, 3), iron sulfide (4-7), green rusts (8-11), phyllosilicates (12-19)) reduce priority pollutants including nitrate, chromate, nitroaromatic explosives, and pesticides as well as polyhalogenated solvents at significant rates. In addition, Fe(II) sorbed and/or precipitated on Fe(III)-containing minerals (e.g., goethite, lepidocrocite, or amorphous ferric iron hydroxide) was found to be highly reactive with respect to reduction of contaminants (20-24). Depending on the type of mineral-bound Fe(II) and depending on the solution composition (particularly pH), very different transformation rates and, in some cases, different transformation products may be found for a given organic pollutant (23). General conclusions about the relative importance of the various mineral-bound ferrous iron species (e.g., structural Fe(II) and Fe(II) bound by surface complexation) with respect to pollutant transformation in the subsurface, however, cannot yet be drawn. To this end, methods need to be developed that allow one to assess the reactivities of different mineral-bound ferrous iron species, particularly when present in complex natural systems. Iron in clay minerals may be present in very different chemical environments within the clay structure and at the mineral surface (Figure 1). Three different types of potentially reactive Fe(II) species can be found on clays: (i) structural Fe(II), (ii) Fe(II) complexed by surface hydroxyl groups at edge surfaces, and (iii) Fe(II) bound by ion exchange at basal siloxane surfaces. In this paper, we demonstrate how information on the relative reactivities of the three types of ferrous iron species can be gained using nitroaromatic compounds (NACs) as probe molecules. In most soils and aquifers, clay surfaces make up a significant fraction of the surface area of the solid matrix. Since most natural clays contain structural iron, redox processes involving such structural ferrous iron are thought to play an important role in the biogeochemical cycling of electrons as well as in the reductive transformations of pollutants (12, 25). The scope of our study was to characterize the various Fe(II) species of clays in terms of accessibility to and reactivity for organic pollutants. To this end, batch experiments were conducted using chemically reduced clays and nitroaromatic probe compounds. The two acetyl nitrobenzene isomers chosen (2- and 4-acetyl nitrobenzene) showed almost similar reactivities, that is, reduction rates in laboratory model systems containing surface-bound Fe(II) species (20, 26, 27) but very different sorption behavior on clays (28-31). Planar NACs may form coplanar electron donor-acceptor (EDA) complexes with the siloxane surface of phyllosilicates. Due to their electron-withdrawing substituents, planar NACs such as 4-acetyl nitrobenzene (sorbing NAC) have a very high affinity for siloxane sites on clays and can therefore be used to probe directly for the reactivity of Fe(II) bound by ion exchange to the clay surface. In contrast, factors that prevent coplanarity and/or optimal resonance of NO2 groups with the aromatic ring (e.g., ortho substitution or bulky substituents) diminish the adsorption of NACs on clays. 2-Acetyl nitrobenzene (nonsorbing NAC), which is not enriched significantly at any of the potentially reactive Fe(II) surface sites of clays, is thought to react with these Fe(II) sites without discrimination due to availability constraints. Note that exchangeable cations present at the basal siloxane surfaces of the clay have a strong effect on the sorption behavior of NACs. Strongly hydrated cations such as Na+, Mg2+, and Ca2+ efficiently hinder the access of sorbing NACs to the siloxane VOL. 37, NO. 3, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic representation of different Fe(II) species in the experimental systems examined in this study: structural Fe(II), Fe(II) complexed by surface hydroxyl groups, and Fe(II) bound by ion exchange. The assays with iron-rich nontronite and iron-free hectorite were prepared with a procedure that allowed for a selective exchange of cations (Fe2+, Na+, K+) at the basal siloxane surface and at the edge surface giving rise to different reactive Fe(II) species. The prefix of an assay’s name (e.g., Na+-nontronite) indicates the type of cation bound by ion exchange to the basal clay surface. Note that although only ferrous iron is mentioned here, ferric iron is also present within the clay structure even after intensive chemical reduction (37). sites and thus diminish their adsorption. This effect is less pronounced if more weakly hydrated cations such as K+ or NH4+ are bound to the basal siloxane surfaces (28-31). We investigated the potentially different reactivity of Fe(II) species present at different sites of a clay mineral (Figure 1) in model systems containing aqueous suspensions of ironcontaining and iron-free clays subjected to various pretreatments. Nontronite (ferruginous smectite, SWa-1), which was reported to contain significant amounts of Fe(II) after either chemical or microbial reduction (32-37), served as a model clay for the investigation of the reactivity of structural iron and Fe(II) complexed by surface hydroxyl groups. Figure 1 schematically illustrates the presence of potentially reactive Fe(II) species at the surface and within the structure of reduced nontronite and describes the different clay suspensions examined in this work (the prefix of an assay’s name, e.g., Na+-nontronite, indicates the type of cation bound by ion exchange at the basal siloxane surface). The presence of either Na+ or K+ at the basal siloxane surfaces and within the clay interlayers was used to study the influence of NAC adsorption on the reactivity of the two types of reactive Fe(II) species. In addition, experiments with iron-free hectorite (California hectorite, SHCa-1) were conducted to examine the reactivity of Fe(II) bound by surface complexes to hydroxyl groups and by ion exchange in the absence of structural iron. To differentiate between the three major Fe(II) sites, we applied a procedure that allowed for the chemical reduction of the clays as well as the selective exchange of Fe(II) and other cations (Na+, K+) present at the basal clay surface. Information about the contribution of the different Fe(II) species to the observed overall reduction of the probe molecules was obtained from mass and electron balances as well as from kinetic studies.
Experimental Section Chemicals. All chemicals were of analytical grade or higher purity and were used without further purification. The compounds used in this study and companies from which they were purchased follow. Fluka AG (Buchs, Switzerland): 2-acetyl nitrobenzene (2-COCH3-NB), 4-acetyl aniline (4520
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COCH3-An), 1,3-dinitrobenzene (1,3-DNB), 1,2- dinitrobenzene (1,2-DNB), 2- and 3-nitroaniline, KHCO3, Na2S2O4, FeCl2, CaCl2, KCl, NaCl, sodium citrate, FerroZine (3-(2-pyridyl)5,6-diphenyl-1,2,4-triazine-4′,4′′-disulfonic acid monosodium salt), and sodium acetate. Sigma, Division of Fluka AG (Buchs, Switzerland): 2-acetyl aniline (2-COCH3-An). Scharlau (Barcelona, Spain): methanol (MeOH). Merck AG (Dietikon, Switzerland): 4-acetyl nitrobenzene (4-COCH3NB), NaHCO3, CaCO3, HCl, NaOH, acetic acid, Fe(0). Carbagas (Ru ¨ mlang, Switzerland): N2, N2/H2 (g99.999%). Minerals. Nontronite (ferruginous smectite, SWa-1) and hectorite (California hectorite, SHCa-1) were purchased from the Source Clay Minerals Repository (University of Missouri, Columbia, MO). Some properties of the clay minerals relevant in this study are listed in Table 1. Nontronite was ground with an agate mortar and dispersed in aqueous suspension with supersonic treatment. The clay particles were fractionated to
