United States Environmental Protection Agency Robert S. Kerr Environmental Research Laboratory Ada, OK 74820 Research and Development EPA/600/SR-94/018 March 1994 EPA Project Summary Abiotic Transformation of Carbon Tetrachloride at Mineral Surfaces Michelle Kriegman-King and Martin Reinhard Transformation of carbon tetrachlo- ride (CCL) by biotite, vermiculite, and pyrite in the presence of hydrogen sul- fide (HS-) was studied under different environmental conditions. In systems containing biotite and vermiculite, the rate of CCI4 transformation was depen- dent on the temperature, HS- concen- tration, surface concentration, and Fe(ll) content in the minerals. At 25°C, the half-life of CCI4 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 transformation rate with biotite and vermiculite was nearly independent of pH in the range 6-10 at constant HS- concentration. The rate dependence on Fe(ll) content of the sheet silicates suggested that the trans- formation occurs at surface sites where HS- is associated with Fe(ll). CCI4 reacted relatively rapidly in 1.2-1.4 m2/L pyrite with >90% of the CCL transformed within 12-36 days at 25°C. The observed rate law supports a heterogeneous reaction mechanism. The reactivity of CCI4 with pyrite in- creased in the order: air-exposed py- rite/aerobic, air-exposed pyrite/HS~, air-exposed pyrite/anaerobic, and acid-treated pyrite/anaerobic; but over- all the reaction rate varied only by a factor of 2.5. The CCI4 transformation products varied under different reac- tion conditions. In the sheet silicate systems, approximately 80-85% of the CCI4 was transformed to CS2 which hy- drolyzed to CO2; whereas only 5-15% of the CCI4 was reduced to CHCI3. In the pyrite systems, CO2 was the major transformation product formed under aerobic conditions, whereas CHCI3 was largely formed under anaerobic condi- tions. Formation of some CS2 was ob- served in all pyrite systems. This Project Summary was developed by EPA's Robert S. Kerr Environmental Research Laboratory, Ada, OK, to an- nounce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction The objectives of this research were to test the ability of ferrous iron-bearing min- erals to abiotically transform carbon tetra- chloride (CCI4) in sulfidic (containing HS~) environments. CCI4 is a frequently found groundwater contaminant. The work fo- cused largely on biotite, vermiculite, and pyrite as the ferrous iron-bearing miner- als. Biotite and vermiculite are sheet sili- cates that are commonly found as detrital materials in sedimentary rocks. Sulfidogenic conditions are often ob- served in plumes of hazardous waste and landfill leachate. Specific factors that were studied include solid type and con- centration, iron content, pH, temperature, sulfide concentration, CCI4 concentration, and the presence or absence of oxygen. The CCI4 transformation products were studied as a function of reaction condi- tions. Procedure Kinetic Studies The base case reaction conditions to measure the disappearance rate of 1 jj. M CCI4 in the biotite and vermiculite systems were pH = 7.5-8.5, temperature = 50°C, and [HS~] = 1 mM. The solids concentra- tions for the base case were 55.8 m2 biotite/L, and 114m2 vermiculite/L. Con- trols were established by reacting CCI4 Printed on Recycled Paper ------- with HS- at the same temperature and pH, but in the absence of the solids. Ex- periments with biot'rte were conducted over a pH range of 6-10, temperature range of 37.5-62.7°C, solids concentration of 11.2-280 m2biotite/L, and [HS-] = 0.02-4 mM. For all pyrite/CCI4 transformation rate and product studies, 1 p.M CCI4 was re- acted in aqueous systems containing 1.2-1.4 m2/L pretreated pyrite at pH 6.5 and 25°C, except the experiments con- ducted with sutfide that were at pH 7.75. Experiments were conducted in a 1 mM NaCI ionic medium. Controls were estab- lished by reacting CCI4 under the same conditions in the absence of pyr'rte in ad- dition to reacting CCI4 in homogeneous solutions of Fe2* or HS-. For the studies of the pyr'rte oxidation products, 0.1-1 mM CCI4 was reacted with large particles of pretreated pyr'rte (0.2 g). Controls were established by reacting pyrite under the same conditions but in the absence of CCI4. Materials BiotHe, vermicul'rte, muscovrte, and py- rite were obtained from Ward's Scientific Establishment, Inc. (Rochester, NY). All transformation and adsorption studies were conducted in flame-sealed glass ampules because the reaction times were on the Equation order of weeks to months, often at el- evated temperature (50°C). Ampules were filled with approximately 13.5 ml of buffer that was filtered through a sterilized 0.2 jim nylon filter (Nalgene Corp., Roches- ter, NY). Analytical Procedures Reaction solutions were analyzed for CCI4, CHCI3, and CO using gas chroma- tography. Ion chromatography was used to measure formate and the potential py- rite oxidation products SO32-, SO42', S,O32-. The CCI4 product distribution was deter- mined using 14C-labeled substrate. Sur- faces were characterized using XPS. Mineral Characterization Solids were characterized for specific surface area, [Fe(ll)] and [Fe(lll)]. XPS conducted on cleavage sheets of the bi- otite and vermiculite did not show the pres- ence of any redox-sensitive trace metals besides iron. XPS studies indicated that sulfide does interact with the biotite sur- face, but the type and extent of effect sulfide has on the surface were not deter- mined. The near surface S:Fe ratio of freshly cleaved pyrite was determined to be 2.1. After pyrite was reacted in aque- ous solution under all reaction conditions, the near surface was depleted in iron with Number ')[CCI4]a (1) (2) (3) (4) [C02]= (K obs~K COZ I ,^CS* [C.°/4]° Vfobs~KCOz I [exp(-A"c02 0- 01 [(1- expHf co, t ))- obs [Nonvot] "•obs t )} (5) •0)1 (6) (7) (8) S:Fe>4. The oxidation state of the iron- depleted pyrite surface appeared un- changed when evaluated with XPS. Results and Discussion Transformation of CCI4 by Vermiculite and Biotite Kinetic Modeling and Product Distributions Observed pseudo-first-order rate con- stants (k'obs) for the disappearance of CCI4 in the vermiculite and biotite systems were calculated from regressions of ln([CCIA/ [CCIJ0) vs. time where [CCIJ0 and [CCIJ, were the CCI4 concentrations at time = 0 and time = t, respectively. For experiments with k'^O.OOl day1, the transformation was considered negligible and was as- sumed to equal zero. In the sheet silicates systems, the dis- appearance of CCI4 was hypothesized to obey the laws shown in Equations 1 through 3, where a, p1, 02, yl, *£, and 8 represent the reaction order with respect to reaction in solution and at the mineral interface, k'homo and k'hetero are pseudo-first-order rate constants, and k|H2o, kHS- and khet are intrinsic rate constants. Both the heterogeneous and homogeneous rate constants were first order with respect to [CCIJ (e.g. a = 1). The mineral surface area concentration (SC) was calculated from the product of the solids loading (g/L) and the specific surface area (m2/g) of the mineral. The pKa for the first dissociation of H2S at the reaction temperature was used to calcu- late the HS- concentration based on the pH and the amount of total sulfide added to the system. For homogeneous systems, k'obs = k'homo' wnere k'homo accounts *or t"6 reactions with H2O and HS" in solution. In the heterogeneous systems, k'hetero = k'obs - k1 Rate constants for the different CCI4 transformation pathways were evaluated by considering the relationship shown in Equation 4, where k'CS2> k'CHC|3, and k'NV are pseudo-first-order rate constants that describe the formation of CS2, CHCI3, and the nonvolatile product, respectively. CO was detected in very small quantities and was not considered in this analysis. Data of CS2, CO2, CHCI3 and nonvolatile concentrations as a function of time were fit to Equations 5, 6, 7, 8, respectively. The rate constants, k'pSj, k'Co2, k'cHci3, and k'Ny were estimated using both visual and nonlinear statistical curve-fitting. The rate constant k'CO2 is the pseudo-first-order rate constant for the appearance of CO2, due to CS2 hydroly- ------- sis. The curve-fitting results for a typical experiment are shown in Figure 1. Good agreement was found across experiments in terms of the fraction of CCI4 reacting via the three different pathways. A mass balance of 95-100% was obtained in these experiments. At 50'C, PH 6-9, SCbNte = 0-280 m2/L and low HS" concentrations ([HS"]<0.5 mM), the reaction orders from Equation 3 were determined to be cc= 1, (52 = 1.2, and 8 = 1. The pH dependence in the environmentally relevant pH range was too low to be determined reliably and both yl and -ft. were assumed to be zero. At high HS-concentrations ([HS-]=0.5-4 mM) and SCbiotite<55.8 m2/L, the rate of disap- pearance of CCI4 in heterogeneous sys- tems was independent of HS~ concentration (P2 = 0). The major transformation pathway of CCI4 with HS~ is the formation of CO2 via CS2. It was proposed that CCI4 undergoes reduction to form a trichloromethyl radical which then reacts with HS", Sx2', or S2O32- to form CS2 which hydrolyzes to CO2. At 50°C, the rate constant for the disappear- ance of CS2 ranged from 0.03 to 0.06 day1. Reported Arrhenius constants for the hydrolysis of CS2 to CO2 under reac- tion conditions herein, result in a hydroly- sis rate of 0.006-0.015 day1 at 25°C (half-life of 45-110 days). About 85% of the CCI4 is ultimately transformed to CO2 in these systems. Reductive dehalogena- tion of CCI4 to CHCI3 contributed to 5-15% of CCI4 transformation. Ferrous iron in the sheet silicates appears to be play- ing a role in the transformation of CCI4 with HS- It is most likely that the reaction is occurring at sites where HS~ is associ- ated with ferrous iron. Adsorption of CCI4 onto biotfte and ver- miculite was determined by control ex- periments using radiolabeled CCI4 at 25°C. Comparison of the aqueous CCI4 concen- tration in the homogeneous and heteroge- neous systems over four weeks showed less than 3% adsorption. Because the ad- sorption of CCI, was so small, CCI4 mea- surements in the transformation studies were not corrected for adsorption. The rate of hydrolysis (kH2o) of CCI4 was calculated to be 0.002 day1 at 50°C. In homogeneous systems, the CCI4 trans- formation rate in the presence of sulfide is at least an order of magnitude greater than kHz0 when [HS~]>0.5 mM. In het- erogeneous systems, the CCI. transfor- mation rate is faster than KHzO wjth [HS-]>0.05 mM and SCbj n, = 55.8 m2/L. The very low reactivity of HS~ in homog- enous solution was enhanced in the pres- ence of minerals indicating a catalytic effect of the surfaces. Figure 1 depicts the disappearance of CCI4, the appearance and disappearance of CS2, and the appearance of products in a heterogeneous system (55.8 m2/L bi- otite and 1 mM HS-). About 65% of the CCI. was transformed to CO2 after 60 days. At this time, approximately 20% of the CS2 was remaining. Chloroform, formed via reductive dehalogenation of CCI4, reached a maximum of 10%. CHCI3 was shown to be relatively persistent in these systems when 5 u.M CHCI3 was reacted under the same conditions as the CCI4 experiments. The half-life of CHCI3 in the presence of 55.8 m2/L biotite and 1 mM HS~ at 50°C was measured to be 172 days, whereas the half-life for hydrolysis of CHCI3 at pH 7.75 and 50°C is 5000 days. Carbon monoxide and a nonvolatile component were measured as products in very small quantities (<5% combined) in the CCl4 systems. The nonvolatile prod- uct, detected by 14C fractionation mea- surements, was not identified in these systems. Proposed Reaction Mechanism The proposed chemical transformation pathways for CCI4 under anaerobic condi- tions are summarized in Figure 2. The products and intermediates in the shad- owed boxes were detected in our experi- ments. The first step for the transformation of CCl4 has been proposed to be a one-electron reduction to form a trichloromethyl radical and Cl~. This radi- cal can follow several different pathways such as additional electron transfer to form a dichlorocarbene and Cl~, dimerization to form hexachloroethane, or electron transfer and protonation to produce CHCI3. The only pathway previously suggested to form CO2 under anaerobic conditions is 1.2 cci4 CHCI3 CS2 Non-Volatile CO2 Mass Balance 0.0- 0 20 40 60 Time (Days) Figure 1. CCI4 transformation products from reaction with [HS -] = 1mM, SCtfesI> = 55.8 rrf/L, pH = 8.8 at 50°C. ------- icr a Carbon Tetrachloride 2x X X .X Hexachloroethane (not detected) Trichloromethyl Radical V ci Chloroform (5-15%) Dichlorocarbene H2O\ t2H2O vv^y ^\_^,Hc, Carbon Monoxide (1-2%) HCOOH Formic Acid (Nonvolatile product detected (3-6%), formic acid?) CI Trichloromethanethiolate Cn~'H CH2CI2 Methylene Chloride (not detected) Vci- , CI Dichlorothionomethane H++2CI- Carbon Disulfide (intermediate) 20H- 2HS- Carbon Dioxide (81-86%) Figure2. Proposed CCI. transformation pathways in HS- solution containing biotite. Compounds in shadowed boxes were detected in this study. Compounds in brackets are intermediates proposed in this study and from the literature (see Griddle and McCarty, 1991). ------- direct hydrolysis (Griddle and McCarty, 1991). In our systems, CS2 appears to be a major intermediate that is transformed to CO2. Temperature Dependence From the temperature data collected at 37.5, 50.0, and 62.7°C, values for the Arrhenius activation energy (Ea) and pre-exponential (A) for the homogeneous, Vermiculite, and biotite systems were cal- culated (Table 1). Lower Ea values in the heterogeneous systems indicate that these reactions will dominate the homogeneous reactions to an even greater extent at environmentally relevant temperatures. For example, at 50°C, CCI4, in the presence of biotite and 1 mM HS~, reacts 8 times faster than CCI4 in the absence of biotite, whereas at 15°C, the biotite system re- acts 125 times faster than the homoge- neous system. Table 1. Arrhenius Parameters for CClf Trans- formation with 1 mMHS ~: Ea and In A Were Calculated Using k'!hel forBiotite and Vermiculite and k': for the Homo- geneous Systems a System EJkJ/mol] ln(A) Homogeneous Vermiculite Biotite 122+32" 41.0+2.1' 91.3 ±8.4 31.4 ±0.5 59.9+13.3 19.9 ±0.9 " Data collected at pH 7.5and in the temperature range 37.5 - 62.7°C. b 95% confidence intervals. For the homogeneous reaction at 25°C, the half-life of CC14 with 1 mM HS ~ was calculated to be 2600 days. In the pres- ence of 1 mM HS- and Vermiculite (114 m2/L) or biotite (55.8 m2/L) 25°C, CCI removal was first order with half-lives of 160 and 50 days, respectively. On a sur- face area normalized basis, CCI. transfor- mation due to the presence of biotite was approximately six times greater than ver- miculite. Transformation of CCI' by Pyrite Kinetic Modeling The pyrite treatments and reaction con- ditions studied were as follows: air-exposed pyrite reacted aerobically, air-exposed pyrite reacted anaerobically, air-exposed pyrite reacted in the presence of sulfide, fresh-ground pyrite reacted anaerobically, and acid-treated pyrite re- acted anaerobically. These conditions (ex- cept the acid treatment) were chosen to simulate different geochemical conditions. The data showed a much better adher- ence to zero-order model than to the first-order model. In the pyrite systems zero-order rate constants (kpci ) were calculated from linear regressions of [CCy/fCCIJoVs. time wherein KCCI is equal to -(slope)([CCyo). A poor fit was found for the fresh-ground system (R2 = 0.65) presumably due to the heteroge- neous nature of the freshly cleaved sur- faces. Rate constants for the disappearance of CCI4 (k°cc\t) are shown in Table 2. The rate constants were normalized by the pyrite surface concentration, assuming the reaction was first-order with respect to the surface concentration, SC, according to Equation 9. In the acid-treated pyrite system, >90% of 1 |iM CCI4 was transformed within 12 days at 25°C, whereas half-lives in homogeneous solution are 1400 days for 1 mM HS~ at 25°C and 105 days for 0.1 mM Fe2+a at 50°C. Assuming an activation energy of 60-120 kJ/mol, the half life of CCI4 with 0.1 mM Fe2+aq at 25°C ranges from 700-4500 days. In Table 2, the data show that CCI reacts the fastest with the acid-treated pyrite, although the air-exposed pyrite re- acted anaerobically is not statistically slower. As expected, the slowest transfor- mation rate was observed when CCI4 was reacted with pyrite under aerobic condi- tions. However, the rate constant was only 2.5 times slower than the acid-treated sys- tem. The large error associated with the fresh-ground pyrite system precludes com- parison with other rate constants. Reac- tion of air-exposed pyrite in the presence of sulfide shows that treatment of an oxi- dized pyrite surface with HS~ does not restore the reactivity of pyrite. Rather, sul- fide appears to inhibit the transformation of CCI4by pyrite relative to the air-exposed/ anaerobic pyrite system. At pH 7.75, 85% of the sulfide is present as HS~; and more than 100 u.M is present as H2S. Because of the observed zero-order dependence on CCI4, reaction sites on pyrite are in- ferred to be saturated with CCI4 when [CCIJ = 1 u,M. Additionally, since [H2S] was 100 times more concentrated than CCI4 in these experiments, it is conceiv- able that H2S blocks CCI4 reaction sites. Characterization of the pyrite surface chemistry is necessary to understand the interaction of sulfide species with the py- rite surface. CCI4 Transformation Products As shown in Table 3, the CCI4 prod- uct distribution varies greatly depending on the reaction conditions even though kcci4 only varies by a factor of 2.5. Under aerobic conditions, the major prod- uct was CO2 (60-70%). Including the hy- drolysis of CS2 to CO2, CO2 accounts for 70-80% of the CCI4 transformed. In con- trast, the fresh-ground pyrite system forms approximately 50% CHCI3 and ultimately only 10-20% CO2. The total CO2 amount in the fresh-ground system is a rough estimate because the speciation of the adsorbed fraction was not measured. In- terestingly, some CS2 was formed in all systems suggesting that the CCI4 or reac- tive intermediates react with S22' sites on pyrite, even in the presence of O2. A frac- tion of the CCI4 or its transformation prod- ucts appeared to be adsorbed to pyrite. The rate constants for the appearance of CHCI3 and HCOOH (k'CHci3 and k°NV, respectively) were evaluated assuming two Table 2. Zero-Order Rate Constants forCCI4 Transformation with Pyrite (1.2-1 4 nf/L) Reacted under Aerobic and Anaerobic Conditions at 25°C Pyrite Conditions Slope (d-') R1 k°CCI4 (mol/m * • d) 95% Confidence Interval Air-exposed, Aerobic 0.025 Air-exposed, HS~ 0.031 Air-exposed, Anaerobic 0.057 Fresh-Ground 0.056 Acid-Pretreated 0.082 0.93 0.87 0.85 0.65 0.96 0.021 0.026 0.047 0.039 0.053 0.017-0.026 0.020 - 0.032 0.035 - 0.049 0.022 - 0.056 0.046 - 0.060 Equation Number (9) ------- Table 3. CCI4 Product Distribution in Percent from Reaction with Pyrite under Aerobic and Anaerobic Conditions at25°C Condition (Tims in d)* Air-exposed; Aerobic (42) Air-exposed, HS- (31) Air-exposed, Anaerobic ( 20) Fresh-Ground (13) Ackl-Pre-treated (13 ) CCI4 0-1 0-10 0-1 1 6-10 CHC/3 5-6 21-22 28-30 48 20-21 CS2 11-15 NMe 0-3 2 19-20 C02 52-59 NM 26-30 10 17 Formate b 2 NM 7-9 5 4 Adsorbed ° NV+C02 To (2 NV+ 8 CO2) NM 12 (7NV+5C02) 12? 9(2NV+7C02) Mass Balance d 84-87 NM 82-84 78 78 * Reaction time In days Is In parenthe— * Formate was not directly measured. Formate was assumed to equal nonvolatile concentration. . :ffiS2£S£^ wilhh SK. Missing fraction likely to be adsorbed volatiles. • NM-not measured. 1 Breakdown of adsorbed products not measured. Tab/a4. Rate Constants for the Disappearance of CCIf and Appearance of Intermediates Products from Reaction with Pyrite under Aerobic and Anaerobic Conditions at25°C ahd Air-exposed Pyrite Reacted Aerobically Acid-treated Pyrite Reacted Aerobically Rate Constant • No intermediate * Intermediatee D 2 = f\ 7O d D2 n . j — f. / o n j. No intermediateb Intermediatec J, kcs, k'cs. /f'cos KCHCI, k'NV [mol/nf-d] 0.021 [mol/nf-d] — [mol/nf-d] 0.0078 [L/nf-d] — [L/nf-d] 0.040 [mol/nf-d] 0.00092 [mol/nf-d] 0.00040 0.021 0.053 0.020 — — 0.022 0.072 — 0.012 0.72 0.00092 0.012 0.00040 0.0027 0.053 0.023 — 7.7 0.12 0.012 0.0027 * k' - zero-order rate constant; k' = first-order rate constant. For symbols see text. * ccr.-»cs.-»co.. .-»-,-j. - accounts for the number of fining parameters. Equation Number d(lntermed.} = {lntermedm} (10a) (10b) (11) (12) different kinetic models (Table 4). Assum- ing CO2 is formed only by the CS2 path- way, Equations 10a and 10b can be used to solve for [CS2] and [CO2] as a function of time. In Equations 10aand 10b, fc°cs2is a zero-order rate constant for the appear- ance of CS2, and k'COz is a first-order rate constant for the formation of CO2. The CS2 data were fit with Equation 11 to solve for the rate constants, fr°cs2 and k'coz (Table 4). These constants were then substituted into the equation for the formation of CO2 to graphically verify the fit of the data. This model under-predicted CO2 production, suggesting that CO2 is formed via the reaction of CCI3 with O2. However, the curve for the appearance of CS2 also did not fit the data well (R2adjusted = 0.78). To model the time lag before the onset of the CS, increase, formation of a rela- tively stable intermediate .in the path to form CS2 was hypothesized. In this kinetic model, the appearance of CS2 was mod- eled using Equations 12 and 13, where k°s is the zero-order rate constant for the formation of the intermediate. In Equa- tions 12 and 13, the rate constant for the appearance of CS2 (k'cs,) is now as- sumed to be first order. The expression for the CS2 concentration is shown in Equation 14. The curve-fitting results are shown in Table 4 and Figure 3. At the end of the experiment, the model predicts that the intermediate attains a steady-state con- centration of approximately 15% which agrees with the missing mass balance (Table 3). A similar fitting analysis was conducted on the results from the acid-treated pyrite system. In this, case, no significant differ- ence in the CS2 fit was observed if the ------- Equation Number [052]=-, [Interned.]-k'COz [CS2] }FeS -S-.- + CC/4 -» [)FeS - S • + CC/4 ~] -)• >FeS- S - CC/3 (13) (14) (15) 0.0 Time (Day) Figure 3. Disappearance of CCI4 in the presence of pyrite under aerobic conditions at 25°C. Appearance of the products, CS^andCO^ with model results assuming the only path to form CO., from CCI4 is: CCI4-> Interned. -» CS2 -» CO appearance of an unknown intermediate was included. As shown in Table 4, the rate constant for the disappearance of the unknown intermediate I (kj) was relatively large, indicating that the intermediate I is very short-lived. Therefore, k] is approxi- mately equal to the rate constant for the appearance of CS2 (k°CSi). Proposed Mechanism of CCI4 Degradation at the Pyrite Surface The proposed pathway of CCI4 degra- dation by pyrite is summarized in Figure 4. Sulfur is the proposed electron transfer site in reactions of CCI4 with pyrite be- cause the surface was depleted in iron and CS2 was detected under all reaction conditions. Since the pyrite surface was negatively charged under the reaction con- ditions in this study, the surface sites are proposed to be predominantly of the form >FeSS-. It is assumed that the amphot- eric nature of the leached pyrite surface is similar to that of pyrtte-S. In the absence of oxidation of the pyrite surface, the elec- tron transfer reaction with CCI.is pro- posed to occur via the reaction in Equation 15. The path to form CO2 can occur via hydrolysis of CS2 or reaction of the trichlo- romethyl radical with O2. The former path- way is assumed to prevail under anaerobic conditions. However, under aerobic con- ditions, CO2 is the major product in the pyrite system, and both pathways are pos- sible. Summary and Conclusions The results of this work provide insight into the rates of CCI4 transformation un- der different environmental conditions, the CCI, transformation products, and the mechanism of CCI4 transformation at min- eral surfaces. Major conclusions that can be drawn from this work include (1) The disappearance of CCI4 in sulfidic systems was significantly faster in the presence of mineral surfaces (biotite, vermiculite, and pyrite) than in homogeneous solu- tion. (2) The rate of transformation of CCI4 with the sheet silicates and sulfide depended on the following reaction conditions: temperature, surface concentration, sulfide concentration, and ferrous iron content in the min- erals. The CCI4 transformation rate was investigated over the range of 6-10 at constant [HS~]and showed a very shallow minimum at near-neutral pH. (3) The rate of transformation of CCI4 with pyrite varied with reaction con- ditions in the following order: air-exposed pyrite/aerobic £ air-exposed pyrite/sulfide < air-exposed pyrite/anaerobic < acid-treated pyrite/anaerobic. The rate constants varied by only a fac- tor of 2.5 for all the conditions stud- ied. HS~ inhibited the CCI4 transformation rate by pyrite rela- tive to systems reacted anaerobi- cally in the absence of HS~. (4) The CCI4 tiansformation products varied greatly as a function of the reaction conditions. In the sheet sili- cate/sulfide systems, CS2 was iden- tified as a major intermediate that hydrolyzed to CO2, accounting for >85% of the CCI4 transformed. In the pyrite systems, CS2 was de- tected under all reaction conditions, suggesting that CCI4 or an interme- diate must react directly with the pyrite surface. Under aerobic con- ditions, CO2 was the major trans- formation product (80%). In the fresh-ground pyrite systems, roughly 50% of the CCI4 was trans- formed to CHCI3. In all systems studied, formate and carbon mon- oxide were minor products. (5) The rate of transformation of CCI4 with the sheet silicates was depen- dent on both the ferrous iron con- tent and the sulfur concentration, indicating that the reaction occurs at sites where sulfide is associated with structural ferrous iron. (6) In the pyrite system, the near-surface was depleted of iron after reaction in water, while the oxidation state of the pyrite-S ap- peared to remain the same. The high sulfur concentration at the near-surface makes it likely that ------- ci a ci ci Carbon Tatrachlorida • >FeSS" 2HCI CS2 •M Carbon Disulfide CO2 ^m Carbon Dioxide Figure 4. Proposed CCI4 transformation pathways with pyrite. Compounds in shadowed boxes were measured. Compounds in brackets are proposed Intermediates. ------- pyrfte-S is the reductant of rather than pyrite-Fe. Recommendations The results of this study shpw that min- eral surfaces may play a significant role in the fate of halogenated organics in the environment. Although they can be quan- titatively applied to natural systems only with some difficulty, these data show that rate constants measured in deion- ized water will greatly under-predict the actual transformation rates. In pyrite- or sulfide-rich environments, abiotic transfor- mation pathways may be significant on the time scale of groundwater transport. Predictive capabilities are complicated at this point due to the confounding effects of natural organic matter, cosolvents, com- peting oxidants, and miorobial activity. To address these issues, continued research to further understand the surface chemis- try of pyrite, the CCI4 transformation path- way at the pyrite surface, and the reactivity of CCI4 and other polyhalogenated ali- phatics under field conditions is neces- sary. This work will ultimately lead to predictive capabilities. The pyrite surface was relatively reac- tive with CCI4 even under aerobic condi- tions; therefore, it is conceivable that a pyrite-based treatment system could be engineered. Further studies would have to be conducted in order to (1) identify the rate determining step of the reaction, (2) test the efficiency of the method in col- umn and batch reactors, (3) control the CCI4 product distribution, and (4) measure the pyrite oxidation products and ensure that they are harmless. In addition, the engineered system would have to be tested with other haloaliphatics .to see if they could also be transformed and if they inhibited or effected the transformation of CCI4 Reference Griddle, C. S., and P. L McCarty. 1991. Electrolytic model system for re- ductive dehalogenation in aqueous environments. Environ. Sci. Technol., 25:973-978. ------- ------- ------- Michelle Kriegman-King and Martin Reinhard are with Stanford University, Stanford, California 94305-4020. Stephen R. Hutchins is the EPA Project Officer (see below). The complete report, entitled "Abiotic Transformation of Carbon Tetrachloride at MineralSurfaces,"(OrderNo.PB94-144698;Cost:$19.50;subjecttochange)will be available only from National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at Robert S. Kerr Environmental Research Laboratory U.S. Environmental Protection Agency Ada, OK 74820 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 Official Business Penalty for Private Use $300 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 EPA/600/SR-94/018 ------- |