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(13) Nagao, T.; Neubert, D.; Loser, E. Chemosphere 1990,20, 1189-1192. (14) Schulze-Schalge,T.; Koch, E.; Schwind, K.; Hutzinger, 0.; Neubert, D. Chemosphere 1991,23, 1925-1931. (15) van Zorge, J. A.; van Wijnen, J. H.; Theelen, R. M. C.; Olie, K.; van den Berg, M. Chemosphere 1989,19, 1881-1895. (16) Dioxins and planar PCBs in food: Concentrations in food products and intake by the Dutch population: RIVM 730501.034; National Institute for Public Health and Environmental Hygiene: The Netherlands, 1991. (17) Leung, H.; Murray, F. J.; Paustenbach, J. Am. Ind. Hyg. ASSOC. J. 1988, 49, 466-474. (18) van Berkel, 0. M.; Olie, K.; van den Berg, M. Int. J. Environ. Anal. Chem. 1988, 34, 51-67. Billmeyer, F. W. Textbook of Polymer Science, 3rd ed.; J. Wiley & Sons: New York, 1986.Wegman, R. C. C.; Freudenthal, J.; de Korte, G. H. L.; Groenemeijer, G. S.; Japenga, J. Chemosphere 1986, 15, 1107-1 112. Donnelly, J. R.; Munslow, W. D.; Vonnahme, T. L.; Nunn, N. J.; Hedin, C. M.; Sovocool, G. W.; Mitchum, R. K. Biomed. Environ. Mass Spectrom. 1987, 14, 465-472. Unpublished results. Sovocool, G. W.; Munslow, W. D.; Donnelly, J. R.; Mitchum, R. K. Chemosphere 1987, 16, 221-224. Munslow, W. D.; Sovocool, G. W.; Donnelly, J. R.; Mitchum, R. K. Chemosphere 1987,16, 1661-1666.
(25) Donnelly, J. R.; Munslow, W. D.; Mitchum, R. KO; Sovocool, G. W. J. Chromatogr. 1987, 392, 51-63. (26) Hale, M. D.; Hileman, F.D.; Mazer, T.; Shell, T. L.; Noble, R. W.; Brooks, J. J. Anal. Chem. 1985,57, 640-648. (27) Unpublished results. (28) Meijer, H. E. H.; Elemans, P. H. M. Polym. Eng. Sci. 1988, 28, 275-290. (29) Timmons, L. L.; Brown, R. D. Chemosphere 1988, 17, 217-233. (30) Hileman, F.; Wehler, J.; Wendling, J.; Orth, R.; Ritchie, C.; McKenzie, D. Chemosphere 1989,18, 217-224. (31) Luijk, R.; Govers, H. A. J.; Eijkel, G. B.; Boon, J. J. J. Anal. Appl. Pyrolysis 1991, 20, 303-319. (32) Manas-Zloczower, I.; T a b o r , Z. Adv. Polym. Technol. 1983, 3, 213-221. (33) Winter, H. H. Polym. Eng. Sci. 1976, 20, 406-412. (34) Striebich, R. C.; Rubey, W. A.; Tirey, D. A,; Dellinger, B. Chemosphere 1991,23, 1197-1204. (35) Odian, G. Principles of Polymerization, 2nd ed.; John Wiley & Sons: New York, 1981; p 268. (36) Young, R. J. Introduction to Polymers;Chapman and Halk London, 1981; p 8. (37) Brauman, S. K.; Chen, I. J. J. Fire Retard. Chem. 1981,8, 28-36.
Received for review June 11, 1992. Accepted July 7, 1992.
Transformation of Carbon Tetrachloride in the Presence of Sulfide, Biotite, and Vermiculite Michelle R. Kriegman-Klng and Martin Reinhard" Department of Civll Engineering, Stanford University, Stanford, California 94305-4020
ICarbon tetrachloride is transformed in aqueous solutions
containing dissolved hydrogen sulfide more rapidly in the presence of the minerals biotite and vermiculite than in homogeneous systems. Approximately 8 0 4 5 % of the CC4 was transformed to COPvia the measured intermediate, CS2. Chloroform comprised 5-15% of the products. The remaining 5% of the products were an unidentified nonvolatile compound and CO. At 25 OC, the half-life of CCl, with 1 mM HS- was calculated to be 2600,160, and 50 days for the homogeneous, vermiculite (114 m2/L),and biotite (55.8 m2/L) systems, respectively. The CCl, transformation rate was found to be dependent on the type and quantity of the solid and the temperature, but was independent of pH and HS- concentration above a critical HSconcentration. The activation energies (f95% confidence intervals) were determined to be 122 f 32,91.3 f 8.4, and 59.9 f 13.3 kJ/mol for the homogeneous, vermiculite, and biotite systems, respectively. The CCl, transformation rate exhibited first-order behavior with respect to biotite surbelow 55.8 m2/L. The face area concentration (SCbiotite) rate of CCll transformation was independent of HS- concentration when [HS-] = 0.5-4 mM and SCbiotit,= 55.8 m2/L. Below [HS-] = 0.5 mM, the rate law was dependent on HS- concentration. Introduction Abiotic transformations, such as reductive dehalogenation and nucleophilic substitution, can influence the fate of halogenated aliphatic compounds in aqueous environments. Sulfide, commonly found in hypoxic environments such as landfill leachate, hazardous waste plumes, and salt marshes at levels ranging from 0.2 pM to 5 mM, can act 2198
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as electron donor (1-3) or as a nucleophile (4-6) to promote transformation of halogenated organics. In subsurface environments, the transformation rates of halogenated organic compounds in aqueous solution may be influenced by mineral surfaces ( I , 2, 7, 8). We investigated the effect of mineral surfaces in the presence of dissolved sulfide on the transformation rate of carbon tetrachloride. CCl, was chosen as the compound of interest because it is a contaminant which is frequently found in groundwater, it is a suspected human carcinogen (9),it causes ozone depletion, and it is a greenhouse gas (10). Laboratory studies were conducted to identify and quantify the environmental parameters that govern the transformation rate of CCl, in a heterogeneous aqueous environment containing sulfide and sheet silicates (biotite and vermiculite). The parameters studied were temperature, pH, mineral surface area, and sulfide concentration. The reaction rate was hypothesized to be a function of surface area because the reactions have been shown to be faster in the presence of mineral surfaces (2). Sulfide was also expected to play a role in the kinetics because it could act as either an electron donor or a nucleophile. Data are presented in terms of [HS-] instead of total sulfide because HS- is a much stronger nucleophile than H2S (5,11) and HS-is a stronger reductant than H2S (12). Lastly, pH dependence was studied because it affects the surface properties of the minerals, the aqueous speciation of sulfide, and probably the surface speciation of sulfide. In these systems, sheet silicates and sulfide were hypothesized to influence the mechanism of CCl, transformation in the following three ways (2): (1)CCl, can undergo electron transfer with ferrous iron in the sheet sil-
0013-936X/92/0926-2198$03.00/0
0 1992 American Chemical Society
icate and the oxidized iron can then be rereduced by sulfide; (2) CC14can react with sulfide that is adsorbed to the sheet silicate; and (3) sulfide can react with dissolved iron (released to solution from mineral dissolution) to form a secondary mineral, an iron sulfide, which can then react with CClk Structural ferrous iron in biotite is a stronger reductant than aqueous ferrous iron (13). In hypotheses 2 and 3, the adsorbed sulfide or iron sulfide sulfide can act as an electron donor or as a nucleophile. As far as we know, the reduction potential and/or nucleophilicity of adsorbed sulfide on any solid has not been measured or compared with the reduction potential and/or nucleophilicity of aqueous sulfide. Polysulfides (SZ2-, where x is the number of sulfur atoms) are as strong or stronger nucleophiles than HS- (5,12,14). In our systems, S,2- may be forming from reaction of HS- with ferric iron in the sheet silicates. In the pH range of 7-9, S42-and SS2-are the polysulfides that are predicted to be present in solution (12). As a fourth hypothesis, SZ2-may be reacting with CC14via electron transfer of nucleophilic substitution. If secondary mineral formation of pyrite (FeSJ occurs, the 5:- group on pyrite may exhibit a reactivity similar to that of the lower polysulfides, which is slightly less reactive than aqueous HS- (22). However, the iron sulfides, pyrite and marcasite, have been shown to increase the rate of CC14 transformation compared with homogeneous rates in systems containing aqueous ferrous iron or HS- (2). In this paper we will also present the results of several studies on the reactivity of sheet silicates containing different Fe(I1) contents and on the reactivity of HS- and the synthetic solids, Aerosil200 (an amorphous Si02)and gibbsite [aAl(OH),]. These studies give some'insight into the reactivity of the sheet silicate components. Methodology Minerals and Reagents. Biotite, vermiculite, and muscovite were obtained from Ward's Scientific Establishment, Inc. (Rochester, NY). The minerals were wetground with an Osterizer blender (using deoxygenated Milli-Q water, Millipore Corp., Bedford, MA) in an anaerobic glovebox (Coy Products, Ann Arbor, MI) which had a 90% Nz/lO% Hz atmosphere. After grinding, the minerals were freeze-dried, dry-sieved, and stored in the anaerobic glovebox. The 200-50 mesh U.S.standard (75-300 pm) size fraction was used for the experiments. Aerosil200 (>99.8% Si02,Degussa, Ridgefield Park, NJ) and gibbsite (Alcoa, Alcoa Center, PA) were rinsed twice with Milli-Q water before being used. The following compounds were obtained from commercial sources and used as received carbon tetrachloride (>99%), trichloroethylene (99%), and chloroform (99%). Radiolabeled CC14was obtained from Sigma Chemical Co. (St. Louis, MO), diluted in Milli-Q water, and stored in flame-sealed glass ampules at 4 "C in the dark. Sodium sulfide (Na2S-9H20,98%) and carbon disulfide (>99%) were stored in an Ar-filled glovebag. Individual crystals of sodium sulfide were rinsed with deoxygenated Milli-Q water to remove oxidation products and wiped dry with a cellulose tissue before use. Anhydrous pentane (>99%) was stored under a nitrogen atmosphere to prevent contamination by halogenated organics. Filling and Sealing of Ampules. All transformation and adsorption studies were conducted in flame-sealed glass ampules because the reaction times were on the order of weeks to months at elevated temperature. The ampule method was based on and adapted from Barbash and Reinhard (6). Ampules were washed with 10% nitric acid, rinsed with deionized water, and oven-dried overnight at 110 "C. Ampules were weighed and filled with a known
amount of sheet silicate. To minimize Fe(I1) oxidation in the autoclave, mineral-filled ampules were autoclaved under a nitrogen atmosphere. The nitrogen atmosphere was achieved by placing the ampules in a pressure cooker that contained 100 mL of water. The pressure cooker was evacuated three times to 75 mmHg, backfilled with 99.99% nitrogen, and autoclaved at 121 "C for 20 min (8). After being autoclaved, the ampules were placed in the anaerobic glovebox to allow oxygen to exsolve for a minimum of 2 days. Stock solutions of sulfide were made fresh daily by transferring preweighed and washed sulfide crystals into 50 mL of deoxygenated water in the glovebox. The amount of HC1 or sulfide stock used to make the sulfide solution was determined by the parameters of a given experiment. This sulfide solution will herein be referred to as the "buffer". A pH buffer or background electrolyte was not used to eliminate potential confounding effects on the CC14 transformation rate. Ampules were filled with approximately 13.5 mL of buffer that was cold-sterilized by filtration through a sterilized 0.2-pm nylon filter (Nalgene Corp., Rochester, NY). Each ampule was covered with a poly(viny1idene chloride) (Saran) sheet, secured at the neck with a 3/lsin.-inner-diameter ring of latex tubing, removed from the glovebox, spiked with an appropriated volume of an aqueous solution saturated with CC14, and flame-sealed under a stream of nitrogen. After being sealed, ampules were weighed to obtain the exact amount of buffer added and placed in the dark in a constant-temperature water bath ( i O . 1 "C) for the duration of the experiment. Weight loss upon sealing was less than 0.005% and was not considered significant. Daily to weekly, ampules were removed from the bath, shaken by hand, and returned to the bath. At each sampling time, two ampules for each experimental condition were removed from the constant-temperature bath and preserved at 4 "C in the dark until they could be extracted. Before extraction, ampules were centrifuged at 4 "C and 1400g for 20 min. Experiments with Aerosil 200 and gibbsite used a procedure similar to that described above, except the synthetic materials were added to the ampules as a sterilized slurry and ampules were centrifuged at 2800g for 30 min. Chemical Analyses. (a) CCll and CHCI:,Analysis. Reaction solutions were extracted in 2.9-mL glass vials with PTFE/silicone septum-lined screw-top caps. Ampules were cracked open, 2 mL of aqueous solution was transferred to an extraction vial containing 0.5 mL of pentane, and the vial was shaken for 15 s. Extraction vials were stored inverted at -5 "C in the dark for up to 24 h before GC analysis. Extraction vials were removed from the freezer, defrosted to room temperature, and spiked with an appropriate amount of internal standard (trichloroethylene). Vials were mixed on a shaker table for 30 min at 350 rpm. The extractant was analyzed for CC14 and CHC13using a Hewlett-Packard 5890 gas chromatograph equipped with a 63Nielectron capture detector and a DB-1 column (J&W Scientific, Rancho Cordova, CA), 15 m x 0.535 mm, with a film thickness of 1.5 pm. Helium was used as the carrier gas and Ar/CH4 as the make-up gas. The gas chromatograph was calibrated daily with a minimum of five calibration standards, and duplicate measurements were made for each sample or standard. If the measurements did not agree within lo%, a third injection was made. (b) CS2Analysis. A 2.9-mL aliquot of reaction solution from the ampules was transferred to an extraction vial (described above) to fill the vial with no headspace. The vials were stored at 4 "C in the dark for up to 48 h before Envlron. Sci. Technol., Vol. 26, No. 11, 1992 2190
GC analysis. Aqueous samples (2-2,4 mL) were analyzed for CS2using a Tekmar Model ALS purge and trap with a Hewlett-Packard 5890 gas chromatograph equipped with a 10.0 eV Tracor photoionization detector and a Quadrex 007-502 column (Quadrex Corp., New Haven, CT), 75 m X 0.535 mm, with a film thickness of 2.5 pm. Helium was both the carrier gas and the make-up gas. The GC was calibrated daily using external standards at a minimum of four calibration levels. (c) CO Analysis. CO was determined by transferring a 7.1-mL aliquot of aqueous solution from an ampule to an 8.7-mL vial with a PTFE/silicone septum-lined screwtop cap. The vial was shaken at 350 rpm for 10 min to equilibrate the CO partitioning between the headspace and the aqueous phase. A 0.4-mL headspace sample was analyzed on a reduction gas detector (Trace Analytical, Menlo Park, CA) using 30 mL/min air as the carrier gas. The gas-phase CO concentration was calculated by comparing the peak height to CO gas standards (Scott Specialty Gases, San Bernadino, CA). The total amount of CO was calculated using a dimensionless Henry’s constant of 40 at 20 “C (15). (d) Radiolabeled Products Analysis. To determine the product distribution in experiments with radiolabeled substrate, three 1-mL aliquots of reaction solution were taken from each ampule. Aliquot 1was acidified with 0.3 mL of 1N H2SO4 and purged with N2for 10-15 min. This procedure stripped the volatiles and C02 from solution, leaving the nonvolatiles behind. After purging, 10 mL of Universol (ICN Biomedicals, Inc., Irvine, CA) liquid scintillation cocktail was added to the sample. Aliquot 2 was treated with 0.3 mL of 1 N NaOH and purged for 10-15 min, thereby stripping only the non-C02volatiles. Again, 10 mL of scintillation cocktail was added to the sample after purging. Aliquot 3 was immediately added to 10 mL of scintillation cocktail containing 0.3 mL of 1 N NaOH (not purged) to determine the total radioactivity. The CO, fraction was then calculated by subtracting the counts in aliquot 1from those in aliquot 2, and the volatile fraction was calculated as the difference between the counts in aliquot 3 and those in aliquot 2. The efficiency of this method to strip C02 under acidic conditions and to retain C02under basic conditions was tested by adding 0.13-0.56 mL of 1N H2S04or 1N NaOH, respectively, to the 1-mL aliquota. There was no significant difference in the results with increasing amounts of acid or base added. All samples were then counted twice on a Packard Tricarb Model 4530 liquid scintillation counter for 10 min. Measured counts per minute were converted to disintegrations per minute using the external standard method. Evaluation of Data. Observed pseudo-first-order rate constants (k\bs) for the disappearance of ccl4 were calculated from regressions of In ([CCl4]JCCl4l0) vs time, where [CCl4l0and [CC14],were the CC14concentrations at time 0 and time t, respectively. The rate law for the disappearance of CC14was hypothesized as follows:
where a,pl, p2, y l , 72, and 6 represent the reaction order with respect to the given parameter, k Lornoand k ;letero are pseudo-first-orderrate constants, and kHzO, km-,and kh,,, are intrinsic rate constants. The surface area concentratlon (SC) was calculated from the product of the solids loading 2200
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Table I. Specific Surface Area Determined by BET and Iron Content for Solids mineral
spec surface area (m2/g)
biotite vermiculite muscovite gibbsite Aerosil 200
1.45 2.94 0.75 11.4 200c
Fe(I1)
Fe(II1)
w t % (g/g)
w t % (g/g)”
3.1 1.3 0.6
3.1 0.8 1.1
b d
b
