Environ. Sci. Technol. 2007, 41, 7736-7743
Reductive Dechlorination Pathways of Tetrachloroethylene and Trichloroethylene and Subsequent Transformation of Their Dechlorination Products by Mackinawite (FeS) in the Presence of Metals HOON Y. JEONG,† HAEKYUNG KIM,‡ AND K I M F . H A Y E S * ,† Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109-2125, and Department of Civil and Environmental Engineering, Michigan State University, East Lansing, Michigan 48824-1226
Because of frequent co-occurrence of metals with chlorinated organic pollutants, Fe(II), Co(II), Ni(II), and Hg(II) were evaluated for their impact on the dechlorination pathways of PCE and TCE and the subsequent transformation of the initial dechlorination products by FeS. PCE transforms to acetylene via β-elimination, TCE via hydrogenolysis, and 1,1-DCE via R-elimination, while TCE transforms to acetylene via β-elimination and cis-DCE and 1,1DCE via hydrogenolysis. Acetylene subsequently transforms in FeS batches, but little transformation of cis-DCE and 1,1DCE was observed. Branching ratio calculations indicate that the added metals decrease the reductive transformation of PCE and TCE via β-elimination relative to hydrogenolysis, resulting in a higher production of the toxic DCE byproducts. Nonetheless, acetylene is generally the dominant product. Production of highly water-soluble compound(s) is suspected as a significant source for incomplete mass recoveries. In the transformation of PCE and TCE, the formation of unidentified product(s) is most significant in Co(II)-added FeS batches. Although nearly complete mass recoveries were observed in the other FeS batches, the subsequent transformation of acetylene would lead to the formation of unidentified product(s) over long time periods.
Introduction Tetrachloroethylene (PCE) and trichloroethylene (TCE) are common contaminants in groundwaters (1). Under sulfatereducing conditions, mackinawite (FeS) generally forms (2). FeS has been shown to be an effective reagent in the reductive dechlorination of PCE and TCE under anoxic conditions (3, 4). In FeS-mediated transformation, the dechlorination rates are highly dependent on the solution composition (4-7). Variations in pH and the presence of some organic compounds affect the dechlorination rates by FeS (4, 5). Co* Corresponding author phone: (734)763-9661; fax: (734)763-2275; e-mail:
[email protected]. † University of Michigan. ‡ Michigan State University. 7736
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present metals also impact the dechlorination rates of hexachloroethane depending on their relative hardness (6). While hard metals decrease the dechlorination rates, intermediate to soft metals enhance the rates. Recently, Jeong and Hayes (7) reported the impact of Fe(II), Co(II), Ni(II), and Hg(II) on the dechlorination rates of PCE and TCE. The impact of Fe(II) on the dechlorination rates varied with its added concentration due to the formation of several Fe sorption phases. Co(II) and Hg(II) enhanced the reductive dechlorination of PCE and TCE, but Ni(II) slowed down the dechlorination. The observed results were attributed to different dechlorination reactivities of new sulfide phases formed by interaction of the metals with FeS. PCE and TCE are reductively dechlorinated via multiple pathways including hydrogenolysis (replacement of a halogen substitute by hydrogen) and β-elimination (formation of an additional carbon-carbon bond with loss of two vicinal halogens) in FeS systems (3, 4). These reactions involve a net transfer of two electrons. Hydrogenolysis of both chloroethylenes leads to the formation of more toxic and recalcitrant chlorinated byproducts including dichloroethylenes and vinyl chloride, whereas their β-elimination results in the formation of a relatively benign product, acetylene. Furthermore, the dechlorination products may undergo subsequent transformation. Thus, the examination of the disappearance rates of PCE and TCE and their dechlorination products is necessary for assessing their ultimate environmental impact. Considering the common occurrence of metals with chlorinated organic pollutants (Table S.1, Supporting Information (SI)), this study was initiated to investigate reductive dechlorination pathways of PCE and TCE and the subsequent transformation of their initial dechlorination products by FeS in the presence of metals. Fe(II), Co(II), Ni(II), and Hg(II) were chosen based on their pronounced impact on the dechlorination rates (6, 7) and the range of stabilities for the corresponding metal sulfides that they may form (8). Another objective was to verify the proposed reaction pathways and examine the causes of incomplete mass recoveries using mass balances analysis and comparison of reactant transformation rates with product generation rates. Also, the impact of metals on the competing pathways of PCE and TCE transformation was evaluated.
Experimental Procedures Procedures for anoxic condition maintenance and mackinawite synthesis are described in SI. Transformation Experiments. Kinetic experiments were performed for the transformation of PCE and TCE and their initial dechlorination products (acetylene, cis-DCE, and 1,1DCE) using 160 mL serum bottles crimp-sealed with Tefloncoated butyl rubber septa (Wheaton). The batch systems consisted of unamended FeS batches, metal-amended FeS batches, and Fe (hydr)oxide batches (Table 1). Unamended FeS batches were prepared by dispersing FeS particles in 0.1 M Tris buffer solution (a mixture of tris(hydroxymethyl)aminomethane (Tris) and Tris-HCl). The resultant FeS concentration ([FeS]0) was 10 g/L (0.11 M). For metalamended FeS batches, chloride salts of Fe(II), Co(II), Ni(II), and Hg(II) were added to 10 g/L FeS suspensions to obtain the amended concentrations ([Me(II)]0) of 0.01 M. The resultant molar ratio of [Me(II)]0/[FeS]0 (∼0.1) was chosen to mimic the low level metal contamination in sulfidic sediments. At this molar ratio, the added metals were quantitatively removed from the solution by forming insoluble sulfide phases (7). Fe (hydr)oxide batches were prepared by adding FeCl2 in 0.1 M Tris buffer solutions to obtain 0.01 M total Fe concentration. The precipitates formed 10.1021/es0708518 CCC: $37.00
2007 American Chemical Society Published on Web 10/19/2007
TABLE 1. Pseudo-First-Order Rate Constants (ki,j), Mass Recoveries (Rmass), and Rate Recoveries (Rrate)a kacetylene kacetylene,1 Rmass, Rrate (%) kcis-DCE kcis-DCE,1 kcis-DCE,2 Rmass, Rrate (%) k1,1-DCE k1,1-DCE,1 Rmass, Rrate (%) kTCE kTCE,1 kTCE,2 kTCE,3 Rmass, Rrate (%) kPCE kPCE,1 kPCE,2 kPCE,3 Rmass, Rrate (%)
FeS only
Fe(II)-FeS
Co(II)-FeS
Ni(II)-FeS
Hg(II)-FeS
Fe (hydr)oxide
(3.9 ( 0.5) × 10-4 0 78, 13 no reaction
(2.5 ( 0.2) × 10-4 0 87, 21 no reaction
(4.1 ( 0.8) × 10-3 0 6, 1 no reaction
(6.6 ( 0.9) × 10-4 0 67, 8 no reaction
(3.4 ( 0.5) × 10-4 0 81, 15 no reaction
no reaction
no reaction
no reaction
no reaction
(2.2 ( 0.1) × 10-3 (1.1 ( 0.1) × 10-4 0 (2.1 ( 0.0) × 10-3 97, 108 (9.4 ( 0.9) × 10-4 (9.2 ( 0.4) × 10-5 (2.2 ( 0.4) × 10-6 (6.9 ( 0.3) × 10-4 87, 107
(9.9 ( 0.7) × 10-4 (1.2 ( 0.1) × 10-4 0 (7.5 ( 0.3) × 10-4 89, 106 (6.7 ( 0.7) × 10-4 (1.3 ( 0.1) × 10-4 (1.0 ( 0.3) × 10-6 (3.6 ( 0.2) × 10-4 88, 105
(2.8 ( 0.7) × 10-4 0 82, 0 (4.3 ( 0.7) × 10-3 (2.8 ( 0.1) × 10-4 (1.3 ( 0.2) × 10-6 (2.3 ( 0.2) × 10-3 24, 65 (8.6 ( 1.3) × 10-3 (8.4 ( 0.4) × 10-4 (1.0 ( 0.1) × 10-4 (3.1 ( 0.1) × 10-3 16, 50
(3.1 ( 0.3) × 10-4 (2.3 ( 0.0) × 10-5 0 (8.6 ( 0.7) × 10-5 87, 91 (6.3 ( 0.7) × 10-4 (1.7 ( 0.0) × 10-4 (1.2 ( 0.2) × 10-6 (1.6 ( 0.1) × 10-4 75, 86
(1.3 ( 0.1) × 10-3 (1.9 ( 0.0) × 10-4 (5.5 ( 1.4) × 10-7 (1.1 ( 0.0) × 10-3 112, 110 (1.7 ( 0.2) × 10-3 (2.8 ( 0.1) × 10-4 (8.0 ( 0.4) × 10-6 (1.3 ( 0.0) × 10-3 93, 105
(3.6 ( 0.9) × 10-4 (3.3 ( 0.6) × 10-4 97, 105 (1.6 ( 0.2) × 10-3 (9.3 ( 0.5) × 10-4 (5.5 ( 1.0) × 10-4 97, 93 (1.2 ( 0.0) × 10-3 (1.2 ( 0.1) × 10-3 118, 104 (9.1 ( 0.6) × 10-3 (4.9 ( 0.1) × 10-4 (1.2 ( 0.1) × 10-4 (8.9 ( 0.7) × 10-3 106, 107 (2.6 ( 0.1) × 10-3 (4.6 ( 0.4) × 10-4 (9.3 ( 3.5) × 10-6 (1.9 ( 0.1) × 10-3 104, 102
a All rate constants are given in h-1, and uncertainties indicate 95% confidence limits. Rate constants for non-transformational losses are kacetylene,blank ) (5.1 ( 2.5) × 10-5 h-1, kcis-DCE,blank ) k1,1-DCE,blank ) ∼0, kTCE,blank ) (1.8 ( 0.6) × 10-4 h-1, and kPCE,blank ) (2.2 ( 0.9) × 10-4 h-1. Rmass values are given for the last sampling time (t ) 36 days).
in these batches are characteristic of chloride green rust (7). In parallel, 0.1 M Tris buffer solutions were run as blanks to account for non-transformational losses of the tested compounds. The pH of all batches measured before and after kinetic experiments was 8.3 ( 0.1. The volume ratio of headspace to solution was 0.067. Prior to the initiation of kinetic experiments, the reaction vials were equilibrated in a reciprocating water bath for 4 days. Chloroethylenes were injected as methanolic solutions, resulting in less than 0.06% (by weight) methanol, at which reductive transformation is not affected (9). Acetylene was introduced as a gaseous stock. The batches were then submerged in a reciprocating water bath at 170 rpm at 25 °C in the dark. By measuring the aqueous concentrations in blanks after partitioning to the headspace, the initial aqueous concentrations of PCE, TCE, cis-DCE, 1,1-DCE, and acetylene were approximately 1.5 × 10-5, 1.6 × 10-5, 1.8 × 10-6, 5.0 × 10-7, and 1.2 × 10-5 M, respectively. The initial concentrations of DCEs and acetylene were their maximum concentrations observed during the reductive dechlorination of PCE and TCE. At intervals, an aliquot of the solution phase was analyzed for reactants and their transformation products. At the end of kinetic experiments (36 days), the headspace of the batches was also analyzed for highly volatile products. On the basis of the results, transformation experiments were repeated with several batches in efforts to detect chlorinated acetylenes and unidentified product(s). Procedures for GC analyses are described in SI. Data Analysis. Irreversible first-order kinetics was assumed to quantitatively describe reactant disappearance and product generation. Their mathematical descriptions are firstorder linear differential equations, whose analytical solutions can be derived by an integration method (10) or Laplace transform (11). Then, pseudo-first-order rate constants and associated errors were determined by fitting the kinetic data to the analytical solutions (see SI) using KaleidaGraph 3.51 (Synergy Software 2000). Because of the absence of hydrocarbons with more than two carbons as reaction products, the mass balance (Mtot) is given by the total moles of C2-hydrocarbons present in the aqueous phase and headspace
Mtot )
∑pV
i aqCi
i
)
∑ i
(
1+
Vg Vaq
)
Hi VaqCi
(1)
where Ci is the aqueous concentration of species i, pi is the partitioning coefficient for i, Vaq and Vg are the volumes of the aqueous phase and headspace, respectively, and Hi is the dimensionless Henry’s law constant for i. The Henry’s law constants used are reported in ref 7. Mass recovery (Rmass) can be defined as the ratio of the mass balance at time t to that at t ) 0 (i.e., (Mtot)t/(Mtot)t)0). Incomplete mass recoveries in reductive dechlorination are often encountered due to volatile and sorption losses, inaccuracy in Henry’s law constants, and unidentified product formation. By mass balance analysis alone, it is difficult to determine the cause(s) of incomplete mass recoveries. Even when the formation of unidentified product(s) is significant, mass balance analysis does not provide insight into the mechanistic basis for their formation. Moreover, the time-dependent nature of mass balances makes it difficult to assess the extent of unidentified product formation at different time scales. According to conservation of mass, Mtot must be constant, and its first moment should be zero (dMtot/dt ) 0). Assuming pseudo-first-order kinetics, the first moment analysis leads to the following relationship among rate constants:
ki )
∑k
i,j
) ki,1 + ki,2 + ... + ki,n
(2)
j
where ki is the pseudo-first-order rate constant for the overall disappearance of a reactant i, and ki,j is the pseudo-firstorder rate constant for the generation of the jth initial dechlorination product (j ) 1, 2, ..., n) in parallel reactions. Also, the overall disappearance of a reactant may be caused by its non-transformation losses, which can be quantified from blanks. Assuming pseudo-first-order kinetics of nontransformational losses, eq 2 is modified as
ki )
∑k
i,j
+ ki,blank ) ki,1 + ki,2 + ... + ki,n + ki,blank
(3)
j
where ki,blank is the pseudo-first-order rate constant for the disappearance of i in blanks. From eq 3, the rate recovery (Rrate) for the transformation of a reactant i can be defined as the ratio of the sum of the rate constants associated with the formation of its initial dechlorination products and nontransformational losses to ki (i.e., (Σki,j + ki,blank)/ki). Analysis of both mass balances and rate recoveries provides the information on sources for incomplete mass recoveries as VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Aqueous concentrations of acetylene (Cacetylene) (A) and mass recoveries (Rmass) (B) for transformation of acetylene. Rmass represent the ratios of (Mtot)t to (Mtot)t)0. Compositions of FeS batches were 10 g/L FeS, 0.01 M metal amendments, and 0.1 M Tris buffer; those of Fe (hydr)oxide batches were 0.01 M FeCl2 and 0.1 M Tris buffer. The pH of all batches was 8.3. Dashed lines represent kinetic model fits. Errors associated with acetylene concentrations were typically less than 3%.
FIGURE 2. Transformation of cis-DCE (A) and 1,1-DCE (B) in Fe (hydr)oxide batches. The batch compositions were 0.01 M FeCl2 in 0.1 M Tris buffer at pH 8.3. Dashed lines represent the kinetic model fits. For ethylene production in panel A, line i corresponds to the model prediction by eq 9, and line ii corresponds to the model prediction by eq 10. Errors associated with concentrations of cis-DCE, 1,1-DCE, acetylene, and ethylene were typically less than 3%. well as reaction pathways. For example, when reaction pathways are not correctly postulated, Rrate may not be complete even if the mass balance is satisfied. Also, when Rmass is incomplete, the rate constant associated with the formation of unidentified product(s) (ki,unidentified) can be determined by
(4)
tion of acetylene suggests that the acetylene production during the reductive dechlorination of chloroethylenes would be underestimated without accounting for its subsequent transformation. Although acetylene is known to inhibit the activities of dehalorespiring microorganisms (12), the observed transformation indicates that its adverse impact on the biodegradation of PCE and TCE by indigenous microorganisms would be temporary.
Acetylene Transformation. Acetylene is the major product in the transformation of PCE and TCE by FeS (3, 7). The disappearance of acetylene with a pseudo-first-order model fit is shown in Figure 1A, where its non-transformational losses are indicated by the blank. The observed transforma-
Acetylene transforms to ethylene via hydrogenation or C4-hydrocarbons via coupling reactions (9). In this study, ethylene is the sole observed reaction product. Except for Fe (hydr)oxide batches, ethylene is present at trace quantities, accounting for less than 1% of the mass balance. Additional kinetic experiments with ethylene (data not shown) indicate that the subsequent transformation of ethylene is negligible. Thus, aqueous concentrations of acetylene and ethylene are given by
∑k
ki,unidentified ) ki - (
i,j
+ ki,blank)
j
Results and Discussion
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dCacetylene ) -kacetyleneCacetylene dt
(5)
dCethylene pacetylene ) k C dt pethylene acetylene,1 acetylene
(6)
where Cacetylene and Cethylene are the aqueous concentrations of acetylene and ethylene, respectively, pacetylene and pethylene are the partitioning coefficients for acetylene and ethylene, respectively, kacetylene is the pseudo-first-order rate constant for the overall disappearance of acetylene, and kacetylene,1 is the pseudo-first-order rate constant for ethylene production from acetylene. Using the analytical solutions to eqs 5 and 6, the rate constants associated with acetylene transformation were determined (Table 1). All FeS batches show incomplete Rmass at t ) 36 days. Consistent with this, Rrate values are low (less than ∼20%), indicating that acetylene transformation cannot be accounted for by ethylene production and its non-transformational losses. Thus, the formation of unidentified product(s) is suspected in FeS batches, with the associated rate constant (kacetylene,unidentified) given by eq 4. Then, the decrease of Rmass due to the formation of unidentified product(s) was estimated in Figure 1B, where accounting for the formation of unidentified product(s) in this way reasonably describes the measured Rmass. Furthermore, an exponential decrease of Rmass in Co(II)-added FeS batches supports that the unaccounted mass balance is due to the formation of unidentified product(s). Considering that acetylene is the major dechlorination product, its subsequent transformation may be a significant source for the incomplete Rmass in the transformation of PCE and TCE by FeS. In contrast, acetylene transformation by green rust-type precipitates in Fe (hydr)oxide batches can be fully accounted for by ethylene production, with a nearly complete Rmass (97% at t ) 36 days). Acetylene can form complexes with transitional metals by donating π-electrons of its unsaturated carbon-carbon bonds to the d-orbitals of metals (13). In this study, acetylenemetal complexes would be in surface-bound or dissolved forms, which were not detected by the analytical methods applied (see SI). In acetylene transformation by cobalamin (a cobalt-containing coenzyme), Semadeni et al. (14) attributed the unaccounted mass balance to the formation of vinylcobalamin. In their study, the total mass of volatile compounds rapidly decreased initially and then remained constant. The mass balance during chloroacetylene transformation followed a similar decrease due to the formation of nonvolatile metal complexes (14). However, the continuous decrease of Rmass in Figure 1B indicates that the formation of acetylene-metal complexes is not primarily responsible for the incomplete Rmass in this study. This is particularly apparent in Co(II)-added FeS batches, where an exponential decrease of Rmass is noted. Alternatively, acetylene-metal complexes could act as reactive intermediates, which subsequently transform to ethylene via hydrogenation, vinylacetylene (C2H3CtCH) by dimerization, C4C6 aliphatic hydrocarbons by oligomerization, and benzene by cyclotrimerization (15). If these volatile products had been produced at significant quantities, they would have been detected in this study. Instead, the formation of highly watersoluble products such as acetaldehyde and ethanol may explain the incomplete Rmass in acetylene transformation (14, 16). Glod et al. (16) have proposed that the acetylenecobalamin complex reacts with OH- to form hydroxylethylcobalamin, which decomposes to acetaldehyde or ethanol. cis-DCE and 1,1-DCE Transformation. cis-DCE and 1,1-DCE are minor products in the reductive dechlorination
of PCE and TCE by FeS (7). In Figure S.1, significant transformation of cis-DCE and 1,1-DCE is limited to Fe (hydr)oxide batches, where green rust-type precipitates were proposed to form (7). Consistent with this, the reductive dechlorination of cis-DCE by green rust was previously observed (17). No apparent transformation of both DCEs in 0.01 M Fe(II)-added FeS batches suggests that the Fe sorption phase formed in these batches differs from the bulk phase precipitates formed in Fe (hydr)oxide batches. The lack of cis-DCE and 1,1-DCE transformation by FeS would lead to the build-up of these toxic byproducts during the transformation of PCE and TCE. However, as discussed next, their production is generally insignificant compared to acetylene production in the transformation of PCE and TCE by FeS. In cis-DCE transformation, acetylene and ethylene are the major products in Fe (hydr)oxide batches, with ethane present at trace quantities. Since vinyl chloride (VC) was not detected, cis-DCE transformation via hydrogenolysis is negligible. Instead, acetylene may form via β-elimination of cis-DCE. Also, ethylene may form via subsequent hydrogenation of acetylene. Thus, aqueous concentrations of cisDCE, acetylene, and ethylene are described by
dCcis-DCE ) -kcis-DCECcis-DCE dt
(7)
dCacetylene pcis-DCE k C ) dt pacetylene cis-DCE,1 cis-DCE kacetyleneCacetylene (8) dCethylene pacetylene ) k C dt pethylene acetylene,1 acetylene
(9)
where kcis-DCE is the pseudo-first-order rate constant for the overall disappearance of cis-DCE, and kcis-DCE,1 is the pseudofirst-order rate constant for acetylene production from cisDCE. Using the previously determined kacetylene value and the solutions to eqs 7-9, the rate constants associated with cisDCE transformation were determined (Table 1). A model fit of cis-DCE transformation is illustrated for Fe (hydr)oxide batches in Figure 2A. In Fe (hydr)oxide batches, the rate of acetylene production accounts for only 58% of the overall transformation rate of cis-DCE, in contrast to a nearly complete Rmass at t ) 36 days (97%). As shown in Figure 2A, the ethylene production predicted by eq 9 significantly underestimates the experimental data, suggesting that ethylene may be produced via an alternative pathway without acetylene as an intermediate. A possible pathway for this is a concerted reaction of β-elimination and hydrogenation. Previously, the concept of concerted reactions has been introduced to explain the ethane production in the reductive dechlorination of trichloroethanes (18, 19). A schematic diagram of ethylene production via the concerted pathway is presented in Figure 3. Taking the direct formation of ethylene from cis-DCE into account, eq 9 is modified as follows:
dCethylene pacetylene ) k C + dt pethylene acetylene,1 acetylene pcis-DCE k C (10) pethylene cis-DCE,2 cis-DCE where kcis-DCE,2 is the pseudo-first-order rate constant for ethylene production from cis-DCE. The solution to eq 10 is used to estimate kcis-DCE,2. As shown in Figure 2A, consideration of ethylene as both an initial and a subsequent reaction product in cis-DCE transformation gives a better fit VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. 3. Proposed pathways for cis-DCE transformation: (i) π-bonded cis-dichlorovinyl complex, (ii) π-bonded chlorovinyl radical, (iii) di-σ-bonded vinyl complex, (iv) π-bonded acetylenic complex, (v) mono-σ-bonded vinyl complex, and (vi) π-bonded vinyl complex. In addition to subsequent hydrogenation of acetylene, ethylene can be produced via a concerted pathway of β-elimination and hydrogenation (i f ii f iii f v f vi). Similar types of surface complexes were suggested by Arnold and Roberts (9). to the experimental data. Yet, the deviation of the model fit from the data may indicate the involvement of multiple transformation pathways and surface complexation reactions in ethylene production as illustrated by Figure 3. Comparable to a nearly complete Rmass at t ) 36 days, cis-DCE transformation in Fe (hydr)oxide batches is almost fully accounted for (Rrate ) 93%). To the best of our knowledge, this concerted pathway is first proposed for chloroethylene transformation. In 1,1-DCE transformation, ethylene is the sole product in Fe (hydr)oxide batches. Although 1,1-DCE transforms to VC via hydrogenolysis by Zn(0) (20), the absence of VC in this study suggests that ethylene may not be produced via successive hydrogenolysis of 1,1-DCE. Previously, R-elimination has been proposed for the direct formation of ethylene from 1,1-DCE (9, 14). Thus, aqueous concentrations of 1,1DCE and ethylene are described by
9
dCTCE ) -kTCECTCE dt
(13)
dC1,1-DCE ) -k1,1-DCEC1,1-DCE dt
(11)
pTCE dCcis-DCE k C - kcis-DCECcis-DCE ) dt pcis-DCE TCE,1 TCE
(14)
dCethylene p1,1-DCE ) k C dt pethylene 1,1-DCE,1 1,1-DCE
(12)
dC1,1-DCE pTCE k C - k1,1-DCEC1,1-DCE ) dt p1,1-DCE TCE,2 TCE
(15)
where k1,1-DCE is the pseudo-first-order rate constant for the overall disappearance of 1,1-DCE, and k1,1-DCE,1 is the pseudofirst-order rate constant for ethylene production from 1,1DCE. A model fit of 1,1-DCE transformation using the solutions to eqs 11 and 12 is illustrated for Fe (hydr)oxide batches in Figure 2B, with the associated rate constants in Table 1. Consistent with the complete Rmass at t ) 36 days, the Rrate in Fe (hydr)oxide batches is also complete, indicating that the formation of ethylene via R-elimination fully accounts for the overall transformation of 1,1-DCE. TCE Transformation. Acetylene is the major product in the reductive transformation of TCE by FeS, with cis-DCE, 7740
1,1-DCE, ethylene, and ethane present as minor products (7). The lesser chlorinated ethylenes (cis-DCE and 1,1-DCE) form via hydrogenolysis of TCE, while acetylene forms via β-elimination of TCE followed by hydrogenolysis: TCE f chloroacetylene f acetylene. Previously, Arnold and Roberts (9, 20) demonstrated that chloroacetylene was transformed much faster than produced by zerovalent metals, which may explain why chloroacetylene was not observed in this study. Also, acetylene forms via subsequent β-elimination of cisDCE. Ethylene and ethane form via subsequent transformation of the initial dechlorination products. Taken together, aqueous concentrations of TCE and its initial dechlorination products are given by
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 22, 2007
dCacetylene pTCE ) k C + dt pacetylene TCE,3 TCE pcis-DCE k C - kacetyleneCacetylene (16) pacetylene cis-DCE,1 cis-DCE where kTCE is the pseudo-first-order rate constant for the overall disappearance of TCE, kTCE,1 is the pseudo-first-order rate constant for cis-DCE production from TCE, kTCE,2 is the pseudo-first-order rate constant for 1,1-DCE production from TCE, and kTCE,3 is the pseudo-first-order rate constant for acetylene production from TCE. The other rate constants
FIGURE 4. Transformation of TCE (A) and PCE (B) in Co(II)-added FeS batches. Batch compositions were 10 g/L FeS, 0.01 M CoCl2, 0.1 M Tris buffer, and pH 8.3. Dashed lines represent the kinetic model fits. Errors associated with concentrations of PCE and TCE were typically less than 5%, and those associated with concentrations of cis-DCE, 1,1-DCE, and acetylene were typically less than 3%.
FIGURE 5. Rate constants of product generation (ki,j) vs rate constants of reactant disappearance (ki) in transformation of PCE (A) and TCE (B). Error bars indicate 1 SD. are as previously defined and estimated. A model fit of TCE transformation using the solutions to eqs 13-16 is illustrated for Co(II)-added FeS batches in Figure 4A, with the rate constants associated with TCE transformation summarized in Table 1. The Rrate values for TCE transformation are nearly complete (more than 90%) in all FeS batches except for Co(II)-added FeS batches, indicating that TCE transformation is fully accounted for by its transformation to cis-DCE, 1,1DCE, and acetylene. As previously discussed, unidentified product(s) may form via subsequent transformation of acetylene. However, the nearly complete Rmass at t ) 36 days in these batches indicates that the formation of unidentified product(s) is limited by a slow conversion of TCE to acetylene. Consistent with an exponential decrease of Rmass (Figure S.2A), the Rrate for TCE transformation in Co(II)-added FeS batches is only 65%, indicating a significant formation of unidentified product(s) (kTCE,unidentified ) 1.5 × 10-3 h-1). In addition to its hydrogenolysis to acetylene, chloroacetylene (the β-elimination product of TCE) has been proposed to be hydrolyzed to a highly water-soluble compound, acetate (21,
22). This might explain the incomplete Rrate for TCE transformation in Co(II)-added FeS batches. As previously discussed, unidentified product(s) may also form via subsequent transformation of acetylene. Thus, using kacetylene,unidentified as well as kTCE,unidentified, the mass fraction of unidentified product(s) resulting from both pathways was estimated in Figure S.3A, where the transformation of TCE in Co(II)-added FeS batches is mainly responsible for unidentified product formation at early reaction times, with the subsequent transformation of acetylene being more important later. PCE Transformation. TCE, cis-DCE, 1,1-DCE, acetylene, ethylene, and ethane are the dechlorination products of PCE by FeS (7). TCE forms via hydrogenolysis of PCE. cis-DCE forms via successive hydrogenolysis of PCE (PCE f TCE f cis-DCE) (3, 23) or β-elimination of PCE followed by hydrogenation of dichloroacetylene (PCE f dichloroacetylene f cis-DCE) (9). Since trans-DCE is produced by hydrogenation of dichloroacetylene (9, 20), its absence as a reaction product suggests that cis-DCE production via the second pathway is highly unlikely. Also, the modeling of cisVOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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DCE production using this pathway did not give a reasonable fit. 1,1-DCE forms via successive hydrogenolysis of PCE (PCE f TCE f 1,1-DCE). Time profiles of 1,1-DCE production in the dechlorination experiments using PCE and TCE as parent compounds are compared in Figure S.4. After normalization to the initial concentrations of both reactants, 1,1-DCE production is much higher in the experiments with PCE than TCE, suggesting that 1,1-DCE likely forms via another pathway. A possible pathway for this may involve the sequence of PCE f dichloroacetylene f 1,1-DCE, but the second step is unlikely due to an infeasible structure rearrangement. R-Elimination has been proposed for the transformation of 1,1-DCE to ethylene (9, 14, 24). Similarly, 1,1-DCE may form via R-elimination of PCE. This pathway involves the formation of 1,1-dichlorocarbene (CCl2dC:) by a two-electron transfer, which would be subsequently reduced to 1,1-DCE via stepwise or concerted transfer of two electrons and two protons as proposed for R-elimination of 1,1-DCE (9). To the best of our knowledge, R-elimination of PCE has not been previously proposed. Identification of carbene intermediates using 2,3-dimethyl-2-butene as a trapping agent (25) would support this proposed pathway. Acetylene forms via multiple pathways: PCE f dichloroacetylene f chloroacetylene f acetylene, PCE f TCE f chloroacetylene f acetylene, and PCE f TCE f cis-DCE f acetylene. Since cis-DCE transformation was not observed in any batches except for Fe (hydr)oxide batches, where the transformation of TCE to cis-DCE was insignificant as compared to its transformation to acetylene, the last pathway should not be significant. Thus, only the first two sequences were considered. Given that dichloroacetylene and chloroacetylene are far more degradable than chloroethylenes (9, 20), the detection of these reactive species as reaction products of chloroethylenes was not expected. Ethylene and ethane are present only as subsequent reaction products. Thus, aqueous concentrations of PCE and its initial dechlorination products are described by
dCPCE ) -kPCECPCE dt
(17)
dCTCE pPCE ) k C - kTCECTCE dt pTCE PCE,1 PCE
(18)
dC1,1-DCE pTCE pPCE k C + k C ) dt p1,1-DCE PCE,2 PCE p1,1-DCE TCE,2 TCE k1,1-DCEC1,1-DCE (19) dCacetylene pPCE pTCE ) kPCE,3CPCE + k C dt pacetylene pacetylene TCE,3 TCE kacetyleneCacetylene (20) where kPCE is the pseudo-first-order rate constant for the overall disappearance of PCE, kPCE,1 is the pseudo-first-order rate constant for TCE production from PCE, kPCE,2 is the pseudo-first-order rate constant for 1,1-DCE production from PCE, and kPCE,3 is the pseudo-first-order rate constant for acetylene production from PCE. The other rate constants are as previously defined and estimated. A model fit of PCE transformation using the solutions to eqs 17-20 is illustrated for Co(II)-added FeS batches in Figure 4B, with the rate constants associated with PCE transformation in Table 1. The Rrate for PCE transformation is almost complete in all FeS batches except for Co(II)-added FeS batches. Consistent with an exponential decrease of Rmass (Figure S.2B), a low Rrate (50%) was observed in Co(II)-added FeS batches, which suggests a significant formation of unidentified product(s) during PCE transformation (kPCE,unidentified ) 4.4 × 10-3 h-1). 7742
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Previously, the transformation of dichloroacetylene (the β-elimination product of PCE) to chloroacetate by hydrolysis was proposed to explain the unaccounted mass recoveries during PCE transformation (21, 22). The formation of this highly water-soluble product might also explain the incomplete Rrate for PCE transformation in Co(II)-added FeS batches. Additionally, unidentified product(s) may form via subsequent transformation of TCE (kTCE,unidentified) and acetylene (kacetylene,unidentified). Using this information, the mass fraction of unidentified product(s) was estimated for Co(II)-added FeS batches in Figure S.3B, where the formation of unidentified product(s) results primarily from PCE transformation at early reaction times and acetylene transformation subsequently. The formation of unidentified product(s) via TCE transformation is insignificant due to lower transformation of PCE to TCE. Impact of Metals on Pathways in PCE and TCE Transformation. As shown in Table 1, co-present metals significantly impact the dechlorination pathways of PCE and TCE by FeS. Branching ratios were calculated to evaluate the relative importance of the multiple pathways (3). Since the formation of 1,1-DCE accounts for less than 1% of total product generation in the transformation of PCE and TCE, only branching ratios of kPCE,3/kPCE,1 (acetylene to TCE) and kTCE,3/kTCE,1 (acetylene to cis-DCE) were determined for the transformation of PCE and TCE, respectively. The branching ratios of kPCE,3/kPCE,1 and kTCE,3/kTCE,1 in unamended FeS batches are 7.5 and 18.9, respectively. As compared to these, metal-amended FeS batches show lower branching ratios, indicating that the transformation of PCE and TCE via β-elimination becomes less important over hydrogenolysis in the presence of metals. Also, the branching ratios of kPCE,3/ kPCE,1 in PCE transformation are smaller than those of kTCE,3/ kTCE,1 in TCE transformation, implying that β-elimination is more important in the transformation of TCE than PCE by FeS. Similar results have been found in the dechlorination of chloroethylenes by zerovalent metals (9, 20). In those studies, the increasing contribution of β-elimination with lesser chlorinated ethylenes was explained by the fact that a two-electron reduction potential for β-elimination becomes higher than that for hydrogenolysis with lesser chlorinated ethylenes. Despite the different impacts of metals on the reaction pathways, acetylene production is dominant for PCE transformation in all FeS batches except for Ni(II)-added FeS batches where TCE production is slightly greater than acetylene production. Similarly, acetylene production is dominant for TCE transformation in all FeS batches, with cis-DCE production accounting for less than 20% of its overall transformation. As compared to unamended FeS batches, Fe(II)- and Ni(II)-added FeS batches show that the decreased PCE transformation is largely due to lower acetylene production. In contrast, the enhanced PCE transformation in Co(II)- and Hg(II)-added FeS batches is largely associated with higher production of acetylene and/or unidentified compounds. Similarly, the change of the TCE dechlorination rates by metal amendments is strongly related to that of the acetylene production rates. Thus, the reductive transformation of PCE and TCE via β-elimination is more susceptible to metal amendments (Figure 5). In TCE transformation by FeS, Butler and Hayes (4) observed that acetylene production via β-elimination was greatly affected by pH, whereas cis-DCE production via hydrogenolysis was relatively constant. In the transformation of carbon tetrachloride by magnetite, dihaloelimination was more susceptible to pH changes than hydrogenolysis (26). The variation of product distributions with experimental conditions has been related to the reduction potential and availability of electrons and the structure of surface atoms on reductants (27, 28). However, the basis for the higher susceptibility of β-elimination over
hydrogenolysis in metal-added FeS batches is complicated by the formation of secondary sulfide phases (7) and remains to be explored. Significant formation of unidentified product(s) was observed for the transformation of acetylene in all FeS batches and the transformation of TCE and PCE in Co(II)-added FeS batches. While attempts were made to identify some potential products (acetaldehyde, ethanol, chloroacetate, and acetate), their production was not observed within the detection limits of the methods used. Use of 14C-labeled reactants and radioactivity measurements following product separation by gas or liquid chromatography would aid in the identification of unknown products. The results presented here are relevant to the application of FeS to metal-contaminated sites. The results presented here indicate that co-present metals may impact the abiotic transformation of chloroethylenes where FeS is applied or biogenically produced in situ.
Acknowledgments Funding for this research was provided by NIEHS Grant P42 ES04911-12. The authors thank three anonymous reviewers for their insightful comments.
Supporting Information Available Table of contaminant mixtures at U.S. DOE sites; procedures for anoxic condition maintenance, mackinawite synthesis, and GC analysis; and solutions to differential equations, cisDCE and 1,1-DCE transformation, Rmass in TCE and PCE transformation, estimated Rmass loss in TCE and PCE transformation, and 1,1-DCE production from PCE and TCE transformation. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Westrick, J. J.; Mello, J. W.; Thomas, R. F. The groundwater supply survey. Am. Water Works Assoc. J. 1984, 76, 52-59. (2) Berner, R. A. Iron sulfides formed from aqueous solution at low temperatures and atmospheric pressures. J. Geol. 1964, 72, 293306. (3) Butler, E. C.; Hayes, K. F. Kinetics of the transformation of trichloroethylene and tetrachloroethylene by iron sulfide. Environ. Sci. Technol. 1999, 33, 2021-2027. (4) Butler, E. C.; Hayes, K. F. Factors influencing rates and products in the transformation of trichloroethylene by iron sulfide and iron metal. Environ. Sci. Technol. 2001, 35, 3884-3891. (5) Butler, E. C.; Hayes, K. F. Effects of solution composition and pH on the reductive dechlorination of hexachloroethane by iron sulfide. Environ. Sci. Technol. 1998, 32, 1276-1284. (6) Jeong, H. Y.; Hayes, K. F. Impact of transition metals on reductive dechlorination rate of hexachloroethane by mackinawite. Environ. Sci. Technol. 2003, 37, 4650-4655. (7) Jeong, H. Y.; Hayes, K. F. Reductive dechlorination of tetrachloroethylene and trichloroethylene by mackinawite (FeS) in the presence of metals: Reaction rates. Environ. Sci. Technol. 2007, 41, 6390-6396. (8) Morel, F. M. M.; Hering, J. G. Principles and Applications of Aquatic Chemistry; Wiley-Interscience: New York, 1993. (9) Arnold, W. A.; Roberts, A. L. Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles. Environ. Sci. Technol. 2000, 34, 1794-1805. (10) Ritger, P. D.; Rose, N. J. Differential Equations with Applications; McGraw-Hill: New York, 1968.
(11) Eykholt, G. R. Analytical solution for networks of irreversible first-order reactions. Water Res. 1999, 33, 814-826. (12) Pon, G.; Hyman, M. R.; Semprini, L. Acetylene inhibition of trichloroethene and vinyl chloride reductive dechlorination. Environ. Sci. Technol. 2003, 37, 3181-3188. (13) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Sausalito, CA, 1987. (14) Semadeni, M.; Chiu, P. -C.; Reinhard, M. Reductive transformation of trichloroethene by cobalamin: Reactivities of the intermediates acetylene, chloroacetylene, and the DCE Isomers. Environ. Sci. Technol. 1998, 32, 1207-1213. (15) Pruchnik, F. P. Organometallic Chemistry of the Transition Elements; Plenum Press: New York, 1990. (16) Glod, G.; Brodmann, U.; Angst, W.; Holliger, C.; Schwarzenbach, R. P. Cobalamin-mediated reduction of cis- and transdichloroethene, 1,1-dichloroethene, and vinyl chloride in homogeneous aqueous solution: Reaction kinetics and mechanistic considerations. Environ. Sci. Technol. 1997, 31, 31543160. (17) Lee, W.; Batchelor, B. Abiotic reductive dechlorination of chlorinated ethylenes by iron-bearing soil minerals. 2. Green rust. Environ. Sci. Technol. 2002, 36, 5348-5354. (18) Fennelly, J. P.; Roberts, A. L. Reaction of 1,1,1-trichloroethane with zero-valent metals and bimetallic reductants. Environ. Sci. Technol. 1998, 32, 1980-1988. (19) Song, H.; Carraway, E. R. Reduction of chlorinated ethanes by nanosized zero-valent iron: Kinetics, pathways, and effects of reaction conditions. Environ. Sci. Technol. 2005, 39, 62376245. (20) Arnold, W. A.; Roberts, A. L. Pathways of chlorinated ethylene and chlorinated acetylene reaction with Zn(0). Environ. Sci. Technol. 1998, 32, 3017-3025. (21) Roberts, A. L.; Totten, L. A.; Arnold, W. A.; Burris, D. R.; Campbell, T. J. Reductive elimination of chlorinated ethylenes by zerovalent metals. Environ. Sci. Technol. 1996, 30, 2654-2659. (22) Burris, D. R.; Delcomyn, C. A.; Smith, M. H.; Roberts, A. L. Reductive dechlorination of tetrachloroethylene and trichloroethylene catalyzed by vitamin B12 in homogenous and heterogeneous systems. Environ. Sci. Technol. 1996, 30, 30473052. (23) Gantzer, C. J.; Wackett, L. P. Reductive dechlorination catalyzed by bacterial transition-metal coenzymes. Environ. Sci. Technol. 1991, 25, 715-722. (24) Lesage, S.; Brown, S.; Millar, K. A different mechanism for the reductive dechlorination of chlorinated ethenes: Kinetic and spectroscopic evidence. Environ. Sci. Technol. 1998, 32, 22642272. (25) McCormick, M. L.; Adriaens, P. Carbon tetrachloride transformation on the surface of nanoscale biogenic magnetite particles. Environ. Sci. Technol. 2004, 38, 1045-1053. (26) Danielsen, K. M.; Hayes, K. F. pH dependence of carbon tetrachloride dechlorination by magnetite. Environ. Sci. Technol. 2004, 38, 4745-4752. (27) Balko, B. A.; Tratnyek, P. G. Photoeffects on the reduction of carbon tetrachloride by zero-valent iron. J. Phys. Chem. B 1998, 102, 1459-1465. (28) O’Loughlin, E. J.; Kemner, K. M.; Burris, D. R. Effects of AgI, AuIII, and CuII on the reductive dechlorination of carbon tetrachloride by green rust. Environ. Sci. Technol. 2003, 37, 2905-2912.
Received for review April 11, 2007. Revised manuscript received September 5, 2007. Accepted September 6, 2007. ES0708518
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