Langmuir 2006, 22, 2961-2965
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Hydration/Expansion and Cation Charge Compensation Modulate the Brønsted Basicity of Distorted Clay Water Javiera Cervini-Silva,*,†,| Richard A. Larson,‡ and Joseph W. Stucki§ Department of Earth and Planetary Science, UniVersity of California, Mulford Hall no. 3114, Berkeley, California 94720-3114, Department of Natural Resources and EnVironmental Sciences, UniVersity of Illinois, 372 National Soybean Research Center, 1101 West Peabody DriVe, Urbana, Illinois 61801, and Department of Natural Resources and EnVironmental Sciences, UniVersity of Illinois, W-321 Turner Hall, 1102 South Goodwin AVenue, Urbana, Illinois 61801 ReceiVed January 22, 2006 This letter addresses how iron redox cycling and the hydration properties of the exchangeable cation influence the Brønsted basicity of adsorbed water in 2:1 phyllosilicates. The probe pentachloroethane undergoes facile dehydrochlorination to tetrachloroethene, attributed to increases in the Brønsted basicity of near-surface hydrating water molecules following the reduction of structural Fe(III) to Fe(II). This dehydrochlorination process is studied in the presence of Na+- or K+-saturated Upton montmorillonite [(Na0.82(Si7.84Al0.16)(Al3.10Fe3+0.3Mg0.66) O20(OH)4] or ferruginous smectite [(Na0.87(Si7.38Al0.62)(Al1.08Fe3+2.67Fe2+0.01Mg0.23)O20(OH)4 ]. The effect of iron redox cycling on pentachloroethane dehydrochlorination is studied using reduced or reduced and reoxidized smectite samples saturated with Na+ (fully expanded clay) or K+ (fully collapsed clay). Variations in the clay Brønsted basicity following Na+-for-K+ exchange are explained by cationic charge compensation or interlayer hydration/expansion imposed by the nature of the exchangeable cation. Inverse relations between K+ fixation and clay water content as well as trends in pentachloroethane transformation indicate that increases in the Brønsted basicity result from increases in the clay hydrophilicity and shifts in the local activity of distorted clay water. Potassium fixation causes partially collapsed smectites bearing low amounts of structural Fe(II) to have a similar reactivity to that of fully expanded smectites (Na+ form) bearing higher amounts of structural Fe(II). In particular, the conversion of up to 80% of the pentachloroethane to tetrachloroethane by K+-saturated, reoxidized Upton was explained because the fixation of K+ causes nonreversible expansion and incomplete reoxidation of structural Fe(II), which contributes to the stabilization of charge density near sites bearing Fe(II). Higher pentachloroethane conversions by Upton montmorillonite over ferruginous smectite, however, suggest that charge dispersion rather than site specificity contributes predominantly to clay reactivity. Thus, clay interlayer hydration/expansion imposed by the nature of the exchangeable cation alters water dissociation and proton exchange in Fe(II)-Fe(III) phyllosilicates susceptible to iron redox cycling.
Introduction The fixation of potassium in 2:1 phyllosilicates is a ubiquitous process with implications for the bioavailability of this essential nutrient in soils and aquifers. Because of the hydrating properties of K+, the replacement of K+ by other cations within the clay interlayer can influence interlayer spacing and thus the accessibility of solutes to basal planes. Furthermore, because of its larger ionic radius and lower hydration energy,1,2 K+ adsorbs more strongly than Na+ to clay surface sites sharing high charge densities, typically ditrigonal cavities.1-4 K+-for-Na+ exchange provokes the exclusion of water from the clay interlayer and contributes to the collapse of clay layers,1,2 reducing the accessibility of basal surfaces while maintaining the exposure of clay edges (3-5% of the total surface) to solution (Figure 1). The hydrating properties of exchangeable cations influence the sorption of solutes on Fe-bearing clay minerals as well as * Corresponding author. E-mail:
[email protected]. Tel: (510) 643-2155. Fax: (510) 643-5098. † University of California. ‡ Department of Natural Resources and Environmental Sciences, National Soybean Research Center, University of Illinois. § Department of Natural Resources and Environmental Sciences, Turner Hall, University of Illinois. | Present address: Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico D.F., CP 04510. (1) Sposito, G.; Prost, R. Chem. ReV. 1982, 82, 554-573. (2) Low, P. F. Soil Sci. Soc. Am. J. 1980, 44, 667-676. (3) Bleam, W. F.; Hoffman, R. Inorg. Chem. 1988, 27, 3180-3186. (4) Fripiat, J. J. In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L. M., McKeever, S. W. S., Blake, D. F., Eds.; ACS Symposium Series No. 415; American Chemical Society: Washington, DC, 1990; Vol. 415, p 360.
their ability to undergo electron transfer. Early work by Solomon5 indicates that the presence of water accelerated both the reduction of tetracyanoethylene to the tetracyanoethylene radical anion and the oxidation of benzidine to benzidine blue by unaltered montmorillonite. The adsorption of nitroaromatics and their reduction by swelling clay minerals occurs preferentially if the clay is saturated with nonhydrating (e.g., K+; K+ hydration energy ca. -314 kJ mol-1) rather than hydrating cations (e.g., Na+; Na+ hydration energy ca. -397 kJ mol-1). The coordinating water molecules of hydrating cations reportedly hinder clay basal planes from adsorbing (planar) nitroaromatics, thereby precluding charge transfer from the siloxane surfaces to sorbent NO2 groups.6-9 Nonhydrating cations can influence electron transfer in Fe(II)Fe(III) phyllosilicates either by facilitating the reduction of sorbents via edge sites, such as that of U(VI) by micas10 or Cr(VI) by biotite,11-13 or by altering the charge distribution. (5) Solomon, D. H. Clays Clay Miner. 1968, 16, 31-36. (6) Weissmahr, K. W.; Haderlein, S. B.; Schwarzenbach, R. P.; Hany, R.; Nu¨esch, R. EnViron. Sci. Technol. 1997, 31, 240-247. (7) Johnston, C. T.; De Oliveira, M. F.; Teppen, B. J.; Sheng, G.; Boyd, S. A. EnViron. Sci. Technol. 2001, 35, 4767-4772. (8) Haderlein, S. B.; Weissmahr, K. W.; Schwarzenbach, R. P. EnViron. Sci. Technol. 1996, 30, 612-622. (9) Li, H.; Teppen, B. J.; Johnston, C. T.; Boyd, S. A. EnViron. Sci. Technol. 2004, 38, 5433-5442. (10) Ilton, E. S.; Haiduc, A.; Moses, C. O.; Heald, S. M.; Elbert, D. C.; Veblen, D. R. Geochim. Cosmochim. Acta 2004, 68, 2417-2435. (11) Ilton, E. S.; Veblen, D. R.; Moses, C. O.; Raeburn, S. P. Geochim. Cosmochim. Acta 1997, 61, 3543-3563. (12) Scott, A. D.; Amonette, J. E. In Iron in Soils and Clay Minerals; Stucki, J. W., Ed.; Reidel Publishing Company: Boston, 1988; p 537. (13) Amonette, J. E.; Scott, A. D. In Clays Controlling the EnVironment; Churchman, G. J., Fitzpatrick R. W., Eggleton, R. A., Eds.; CSIRO Publishing: Melbourne, 1995; p 355.
10.1021/la0602113 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/22/2006
2962 Langmuir, Vol. 22, No. 7, 2006
Letters
Figure 1. Illustration of interlayer clay collapse following K+-for-Na+ exchange and the reduction of structural {Fe(II)} in swelling clay minerals.
Structural K+ in Fe(II)-Fe(III) phyllosilicates reportedly causes a smearing of electron density over the surface of individual sheets.14 Iron redox cycling in smectite clay minerals occurs as a result of burial, submersion, wetting, drying, and other events in natural soils and sediments. The reduction of structural Fe(III) to Fe(II) in swelling clays causes changes in swellability,15,16 texture,16 color,17 cation exchange capacity,18 and specific surface area.15 The reduction of structural Fe(III) to Fe(II) also results in electron delocalization throughout siloxane groups and the formation of channels and holes19 as well as a smaller gap between the clay conduction and valence band energies.20 A consequence is the polarization of adsorbed water.21 As the surface of smectite clay minerals is hydrated, water molecules reorganize to participate in hydrogen bonding interactions with other water molecules, just as in bulk water, and to form hydrogen-bridging interactions with organic solutes. The hydration properties of exchangeable cations controlling interlayer spacing in 2:1 phyllosilicates can thus influence both the redox cycling of structural Fe and the fate of organic solutes. To our knowledge, however, little is known regarding how coupled iron redox cycling and clay hydration/expansion alter the transformation of organic solutes instigated by clay water. Linear free-energy relationship analyses indicate that the adsorption of polychlorinated alkanes and alkenes by Na+-saturated reduced smectites and their hydrolysis in bulk water share a common reaction-limiting step.23 Related considerations show that the thermodynamic propensity for solute adsorption and transformation by reduced iron-rich swelling clays can be predicted from their hydrolysis properties and susceptibility to participate in electron-transfer reactions.24 (14) Rosso, K. M.; Ilton, E. S. J. Chem. Phys. 2003, 119, 9207-9218. (15) Lear, P. R.; Stucki, J. W. Clays Clay Miner. 1989, 37, 547-552. (16) Gates, W. P.; Jaunet, A.-M.; Tessier, D.; Cole, M. A.; Wilkinson, H. T.; Stucki, J. W. Clays Clay Miner. 1998, 46, 487-497. (17) Wu, J.; Roth, C. B.; Low, P. F. Soil Sci. Soc. Am. J. 1988, 52, 295-296. (18) Stucki, J. W.; Golden, D. C.; Roth, C. B. Clays Clay Miner. 1984, 32, 350-356. (19) Manceau, A.; Lanson, B.; Drits, V. A.; Chateigner, D.; Gates, W. P.; Wu, J.; Huo, D. F.; Stucki, J. W. Am. Mineral. 2000, 85, 133-152. (20) Aronowitz, S.; Coyne, L.; Lawless, L.; Rishpon, J. Inorg. Chem. 1982, 21, 3589-3593. (21) Yan, L.; Stucki, J. W. Langmuir 1999, 15, 4648-4657. (22) Chen, S. Z.; Low, P. F.; Roth, C. B. Soil Sci. Soc. Am. J. 1987, 51, 82-86. (23) Cervini-Silva, J. EnViron. Toxicol. Chem. 2003, 10, 2298-2305. (24) Cervini-Silva, J. Langmuir 2004, 20, 9878-9881.
In this study, we report the dehydrochlorination of pentachloroethane to tetrachloroethane by Na+- or K+-saturated, redoxmanipulated Upton montmorillonite [(Na0.82(Si7.84Al0.16)(Al3.10Fe3+0.3Mg0.66) O20(OH)4, referred to as Upton] 25 or ferruginous smectite [(Na0.87(Si7.38Al0.62)(Al1.08Fe3+2.67Fe2+0.01Mg0.23)O20(OH)4, referred to as SWa-1]. Pentachloroethane has been selected as a probe compound because it undergoes facile base-promoted hydrolysis leading to dehydrochlorination to tetrachloroethene:26-28
Cl2HCR-CβCl3 f Cl2CRdCβCl2 + H+ + Cl-
(1)
Experimental Section Clay Preparation (Na+ Form). Two smectite clays with different contents of structural iron were used in this study. Samples of the
