Environ. Sci. Technol. lQQ4,28,686-691
I n Situ Measurements of Tetraphenylboron Degradation Kinetics on Clay Mineral Surfaces by I R Douglas B. Hunter' and Paul M. Bertsch
Division of Biogeochemistry, University of Georgia. Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 29802 ~~
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An attenuated total reflectance Fourier transform infrared (ATR-IR) spectroscopic method has been developed to quantitatively measure, in situ, the surface-facilitated degradation of tetraphenylboron (TPB) in fully aquated clay pastes. Two pathways for degradation of TPB could be studied both independently and simultaneously. Surface-facilitated oxidation of TPB to diphenylboric acid (DPBA) at Lewis acid sites on clay mineral surfaces was investigated on three members of the smectite family of clays. No degradation of TPB occurred on hectorite, which contains no structural Fe. TPB degraded to DPBA on montmorillonite and nontronite. A color change in nontronite indicated the production of mixed valance Fe3+O-Fe2+ states and clearly demonstrates the reduction of structural iron during the course of the reaction. The degradation of TPB to triphenylboron (TriPB) at Bronsted acid sites could also be measured on aluminum-saturated clays either independently on hectorite or simultaneously to the Lewis acid reaction on montmorillinite by the ATRIR method. First-order rate models are developed for both reactions and describe the data well.
Introduction The fate of organic contaminants in soils is a primary concern in environmental science. The ability for soil constituents to interact and potentially react with contaminants can have profound effects on the assessment of risks associated with specific contaminants. In particular, clay mineral surfaces can facilitate oxidationlreduction, acid-base, and hydrolysis reactions ( I ) . An example where this may be important is the projected use of the tetraphenylboron anion (TPB) at the Savannah River Site (SRS),a nuclear materials processing facility, located near Aiken, SC. Tetraphenylboron is planned to be used in the precipitation of cesium-containing radioactive wastes at the Defense Waste Processing Facility at the SRS. An estimated 200 000 kglyear for the next 30 years will be needed to solidify 35 years of accumulated high-levelwaste at the SRS, thus requiring the first large-scale industrial production of this chemical (2). Although tetraphenylboron is relatively nontoxic to soil microorganisms and plants, the same is not necessarily true for daughter products resulting from its degradation. For instance, diphenylboric acid, one potential degradative product, has been shown to inhibit nitrification processes in soil microbes (ref 3 and Bertsch, unpublished results). With such potentially large-scaleindustrial usage and accidental releases of TPB to the environment, it is necessary to understand the behavior and ultimate fate of this chemical in soil and aquatic systems.
* Author to whom all correspondence should be addressed. Telephone: (803) 725-2472;Fax: (803) 725-3309;Electronic mail: Hunter @ SREL.EDU. 686
Environ. Scl. Technol., Vol. 28, No. 4, 1994
Two primary TPB degradation pathways have previously been described ( 4 ) . The first is a two-electron transfer oxidation reaction producing diphenylboric acid (DPBA) and biphenyl. A mineral surface can behave as a Lewis acid by accepting electrons from adsorbed organic molecules (I,5). We will define the degradation of TPB to DPBA as occurring at Lewis acid sites on clay surfaces ( I ) . The second pathway is an acidic (H+)degradation to triphenylboron (TriPB),which subsequently decomposes in the presence of 0 2 to phenyl diphenylborinate, with subsequent hydrolysis to yield phenol and DPBA. Clay minerals can also behave as strong Bronsted acids primarily attributable to the presence of acidic exchangeable cations ( 1 , 5 ) . We will define the acidic degradation of TPB to TriPB as occurring at Bronsted acid sites on the mineral surface. Previous work has demonstrated surface-facilitated degradation of tetraphenylboron in soils (3). These researchers concluded that TPB was oxidized to diphenylboric acid (DPBA) at Lewis acid sites in the soil since the alternative decomposition pathway to triphenylboron (TriPB) at Bronsted sites was not observed. I t was a reasonable conclusion that degradation was being facilitated by the clay fraction, since mineral surfaces can act as Lewisacids by accepting electrons from adsorbed species, and biotic degradation had been discounted as important ( 3 , 5 , 6 ) .An example of the Lewis reactivity of structural Fe3+, as in the mineral nontronite, (M,+.nH2O)Fe3+4(7), is the ring cleavage of pyrogallol, a (Si~~Al,)020(0H)4 precursor in the formation of humic polymers, was facilitated (8, 9). Also, structural Mn(II1,IV) can be reduced in the oxidation of phenolic compounds on manganese oxides, MnOOH (10, 11). Infrared spectroscopy has previously been shown to be a powerful tool for studying surface acid sites by a variety of surface-sorbed probe molecules. For example, the protonation of NHB to NH*+when adsorbed to Bronsted sites is clearly distinguished from NH3 retained to Lewis sites using IR (12). Similarly, the formation of pyridinium ions upon the adsorption of pyridine to Bronsted sites is spectrally distinct from pyridine sorbed to Lewis sites (12). We have taken advantage of IR spectroscopy to identify and quantify the degradation products, TriPB and DPBA, as site-specificprobes of the reactivity of TPB at respective Bronsted and Lewis acid sites. In this current study, we investigated the nature of the reactive site(@on aquated reference clay minerals where the electron acceptor in the case of Lewis acid degradation can be detailed. Three reference clay mineral specimens were chosen from the smectite family with known structural iron contents varying from -zero to high Fe substitution (hectorite, montmorillonite, and nontronite, respectively). Their respective abilities to facilitate degradation of TPB to DPBA were then observed. The degradation of TPB to TriPB could then be further 0013-936X/94/0928-0686$04.50/0
0 1994 Amerlcan Chemical Society
manipulated according to the Bronsted acidity of the saturating cation (Na, Ca, or Al). The reaction kinetics of TPB on these clay surfaces were studied in situ by attenuated total reflectance IR, wherein the organo-boron reactants and products could be monitored in real time. The ability of ATR-IR to quantitatively monitor the reaction was compared to traditional extraction methods employing subsequent analysis by HPLC.
Experimental Section The clay minerals used in this investigation (KGA-1, well crystalline kaolin, Washington County, Ga; SWy-1, Na-Montmorillonite, Crook County, WY; SHCa-1, hectorite, San Bernardino Valley, CA; and NG-1, nontronite, Holen Hagen, Germany) were obtained from the Clay Minerals Society's Source Clay Minerals Repository (University of Missouri, Columbia, MO). The physical properties, surface area (SA),and cation-exchangecapacity (CEC), of these reference minerals are well characterized: KGa-1, SA = 40 m2 g-l, CEC = 1-10 cmol kgl; SWy-1, SA = 800 m2 gl,CEC = 135 cmol kg-l; SHCa-1, SA = 63 m2 gl,CEC = 44 cmol kg-1; NG-1, SA = 800 m2g-l, CEC = 115 cmol k g l (13). Laboratory-synthesized, highly crystalline goethite had a zero point charge of 7.3 and a surface area of 45 m2 g-1. The mineral surfaces were cleaned of organic (via oxidation by H20)2 and iron oxide (via dithionite-citrate system) residues and were then sizefractionated by sedimentation to particles smaller than 2 pm (14). Residual citrate was then removed by an additional treatment of HzOz. The clays were saturated with 1M solutions of NaC1, CaClz (pH 6-01,and AlC13 (pH 3.2) and then extensively washed with de-ionized water. X-ray diffraction analysis of Al-saturated SWy-1 gave no evidence that the interstitial layers of these samples had become occupied with polynuclear components during washing with non-pH adjusted water (15). The clays were dried at 95 "C for 3 days and then stored in a dessicator until use. Typically, 1.63 g of Aldrich Gold Label tetraphenylboron (TPB) was dissolved in 5 mL of DzO. An aliquot (0.7 mL) of this solution was added to 1.4 g of clay (time zero) and mixed for 5 min, creating a 50 % by weight D20: clay paste with an initial concentration of 0.4 mMol g1of TPB. The paste was then analyzed by IR and HPLC simultaneously for 40 h. For IR analysis, approximately 0.4 g of clay paste was then packed into two 50 X 10 X 2 mm Teflon plaques milled with 30 X 5 X 1 mm grooves. The outer edges of the plaques were then coated with a thin film of silicon grease before the plaques were clamped to either side of a 50-mm ZnSe ATR prism, which was mounted onto a vertical ATR attachment (SpectraTech, Stamford, CT). Silicon grease was required to seal the clays in contact with the prism to prevent the evaporation of DzO over the time course of the experiment. The area of the D20 bending mode at 1250 cm-l was measured to ensure that the pastes did not dehydrate in the purged environment of the optical bench. The ZnSe prism was placed in the vertical ATR cell adjusted to 45'. This geometry results in a calculated depth of penetration of 0.73 pm into the sample by the evanescingwave at 1580cm-l with an overall effective path length of 12.2 pm (16). All samples were analyzed with a Nicolet 740 FT-IR spectrometer equipped with a liquid-nitrogen cooled
MCT-B detector. A total of 128 interferograms for each spectrum were collected, coadded, Happ-Genzel apodized, and transformed with final resolution of 2 cm-l. The time required to collect each spectrum was 139 s. A computer program was used to automatically collect and process spectra approximately every hour (every 0.33 h for aluminum-saturated clays). Peak areas were calculated by using the Lorentzian curve-fitting algorithm contained in Spectracalc (Galactic Industries, Salem, NH). The clay pastes were also sampled at regular time intervals by extraction of the organo-boron compounds with acetonitrile, and these extractions were analyzed by HPLC (3). Four replicates of approximately 10 mg of clay mixture were extracted at each time period with a 600-fold addition of acetonitrile and by shaking for 0.25 h. The mixture was centrifuged, decanted, extracted a second time with water (HPLC grade, acidified to pH 3.2 with H3P04),shaken for 0.25 h, and then centrifuged again. The solution was then decanted and mixed with the first extraction forming a 5050 acetonitri1e:watermixture with a final extractant dilution of 1200. No residual TPB, DPBA, TriPB, or biphenyl was detected when this procedure was repeated a second time and analyzed by HPLC. HPLC was performed on a Dionex 4040i ion chromatograph fitted with a Macmode C18 reverse-phase column (Chadds Ford, PA). The solvent system was water (HPLC grade) adjusted to pH 3.2 with H3P04 and acetonitrile. The elution program was a 0.25-h gradient from 50:50 acetonitri1e:water to 100% acetonitrile at a flow rate of 1 mL min-I. The UV detector was monitored at 254 nm. Injection volumes were 50 pL. Extraction efficiencieswere comparable to those reported by Mills et al. (3).
Results IR peak assignments for TPB and daughter compounds in a reactive aqueous clay paste mixture could be reconstructed by making immediate measurements on pastes to which each of the compounds had been individually added (Figure 1). The organo-boron modes of interest lie in the 1560-1620-cm-' spectral range. The positions of TPB, TriPB, and DPBApeaks (1580,1590,and 1605crn-', respectively) correspond to v4 mode assignments (17)but were shifted by 5 cm-' in the presence of all the minerals investigated from the peaks observed in aqueous solution (1575,1585,and 1600cm-1,respectively). The TriPB peak is observed to be verystrong versus TPB and DPBA which are both medium-strong vibrational modes, as has been reported previously (17). The relative absorptivities of the DPBA and TriPB were 4.3 and 11.6, respectively, compared to TPB, which was used in the quantitative temporal measurements discussed below. An example of time-resolved IR spectra of tetraphenylboron degradation on calcium-saturated SWy-1 demonstrates the appearance of the band associated only with DPBA over a 30-h period (Figure 2A). When the SWy-1 clay is saturated with aluminum, then two bands, those associated with DPBA and TriPB, arise over a similar time period (Figure 2B). The spectra reveal the ultimate loss of TriPB after a very rapid initial appearance. The time-resolved spectra of Na-NG-1 and Na-SWy-1 were qualitatively similar to Ca-SWy-1 in that only DPBA was observed to form as a daughter product. The IR spectra Envkon. Sci. Technot., Vol. 28, No. 4. 1994 887
B
A
n
DIPHENYLBORIC ACID
h
PHENYLBORON
4
1
2
.\ AJ
\
‘620
I
,
I
,
I
,
,
1610 1600 1590 1580 1570 1560 1550
WAVENUMBERS (cm-1)
B
I
1630 1620 1610 1600 1590 1580 1570 1560 1550
WAVENUMBERS
TRIPHENYLBORON
(cm-1)
Figure 1. Trace 1 is a representative I R spectrum of TPB on AlSWy-1 where substantialdegradationto reactant productshad already occurred. Trace 2 is an overlay of three I R spectra of Na-SWy-1 to which one each of the following had been added: A, DPBA; B, TriPB; C, TPB.
Table 1. Summary of Tetraphenylboron Degradation Pathways and Calculated First-Order Rate Constants,
4 W)
mineral Na-SHCa-1 A1-SHCa-1 Na-SWy-1 Ca-SWy-1 AI-SWy-1 Na-NG-1 Na-KGa-1 Na-goethite
octahedral Feu ( % ) CO.01 montmorillonite > hectorite). The change in color of nontronite during the reaction demonstrated that iron was the final electron acceptor in the oxidation of TPB. In contrast, the structural iron in goethite was not reactive to TPB degradation. The introduction of sorbed A1 to the surfaces of smectites created Bronsted sites that degraded TPB to TriPB. Both the Lewis and Bronsted degradation pathways could be measured on smectites containing structural iron and sorbed Al. The IR measurements of reactant and products in situ and in real time provided a more accurate history of the reaction than the extraction method.
Acknowledgments This research was funded by Contract DE-ACOS76SR00819 between the University of Georgia and the US. Department of Energy. We would like to thank the technical assistance of Patterson Nuessle and Cameron Nuessle; Dr. C. Strojan for his comments on an earlier version of the manuscript; and Dr. G. Mills for his thoughtful comments during the course of the study as well as commenting on the manuscript. Literature Cited (1) Laszlo, P. Science 1987,235, 1473. (2) Bertsch, P. M. Borax Rev. 1991, 9, 16.
Mills, G. L.; Kaplan, D.; Schwind,D.;Adriano, D. J . Environ. Qual. 1990, 19, 135. Geske, D. H. J. Phys. Chem. 1959,63, 1062. Vouldrias, E. A.; Reinhard, M. In Geochemical Processes at Mineral Surfaces; Davis, J. A,, K. F. Hayes, K. F., Eds.; ACS Symposium Series 323; American Chemical Society: Washington, DC, 1986; pp 462-486. Mills, G. L.; Schwind, D. Environ. Toxicol. Chem. 1990,9, 569.
Brindley, G. W. In Crystal Structures by Clay Minerals and Their X-ray Identification; Brindley, G. W., Brown, G., Eds.; Minerological Society: London, 1984; Chapter 2, p 170. Wang, M. C.; Huang, P. M. Clays Clay Miner. 1989,37,525. Wang, M. C. Clays Clay Miner. 1991,39, 202. Ukrainczyk, L.; McBride, M. B. Clays Clay Miner. 1992, 40, 1992.
Stone, A. T.; Morgan, J. J. Environ. Sci. Technol. 1984,18, 617.
Peri, J. B. In Catalysis Science and Technology;Anderson, J. R., Boudart, M., Eds.; Springer Verlag: New York, 1984; Vol. 5, Chapter 3. Newman, A. C. D. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; John Wiley & Sons: New York, 1987; Chapter 5. Gee, G. W.; Bauder, J. W. In Methods of Soil Analysis, Part 1; Klute, A., Ed.; Soil Science Society of America, Inc.: Madison, WI, 1986; Vol. 19, Chapter 15, pp 393-394. Barnhisel, R. I.; Bertsch, P. M. In Minerals in Soil Environments, 2nd ed.; Dixon, J. B., Weed, S. B., Eds.; Soil Science Society of America: Madison, WI, 1989; Chapter 15, pp 729-779. Grifiths, P. R.; deHaseth, J. A. Fourier Transform Infrared Spectroscopy; John Wiley and Son: New York, 1986; Chapter 5, pp 191-194. Costa, G.; Camus, A.; Marsich, N.; Gatti, L. J . Organomet. Chem. 1967,8,339. Newman, A. C. D.; Brown, G. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; John Wiley & Sons: New York, 1987; Chapter 1. Lasaga, A. C. In Kinetics of Geochemical Processes;Lasaga, A. C., Kirkpatrick, R. J., Eds.; Minerological Society of America: Washington, DC, 1981; Chapter 1. Johnson, C. T.;Tipton, T.;Trabue, S. L.;Erickson, C.; Stone, D. A. Environ. Sci. Technol. 1992, 26, 382. Aochi, Y. 0.;Farmer, W. J.; Shawney, B. L. Environ. Sci. Technol. 1992, 26, 329. Lear, P. R.; Stucki, J. W. Clays Clay Miner. 1987,35,373. Rozenson, I.; Heller-Kallai, L. Clays Clay Miner. 1976,24, 271.
Lear, P. R.; Stucki, J. W. Clays Clay Miner. 1989,37,547. Ainsworth, C. C.; McVeety, B. D.; Smith, S. C.; Zacchara, J. M. Clays Clay Miner. 1991,39, 416. Tennakoon, D. T. B.; Thomas, J. M.;Tricker, M. J. J . Chem. SOC.,Dalton Trans. 1974, 2211, Taylor, R. M. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; John Wiley & Sons: New York, 1987; Chapter 2. Pusino, A.; Micera, G.; Gessa, C.; Petretto, S. Clays Clay Miner. 1989,37, 558. Received f o r review July 30,1993. Revised manuscript received December 2, 1993. Accepted December 9, 1993.' ~
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Abstract published in Advance ACS Abstracts, February 1,1994.
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