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Kinetics of trichloroethene dechlorination with iron powder
ARTICLE IN PRESS
Water Research 39 (2005) 1165–1173 www.elsevier.com/locate/watres
Kinetics of trichloroethene dechlorination with iron powder Junko Haraa,, Hiroyuki Itob, Koichi Sutoa, Chihiro Inouea, Tadashi Chidaa a
Graduate Schools of Environmental Studies, Tohoku University, 20 Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b Soil Remediation Business Unit, Dowa Mining Co. Ltd., 1-8-2 Marunouchi, Chiyoda-ku, Tokyo 100-8282, Japan Received 24 May 2004; received in revised form 14 October 2004; accepted 20 December 2004
Abstract The dechlorination of trichloroethene (TCE) with metallic iron is an advantageous method for the remediation of contaminated groundwater and soil. The toxic reaction intermediates such as dichloroethenes (DCEs) and vinyl chloride (VC), however, occasionally accumulate in the pathway of the reaction. We have been trying to suppress these intermediates by using metallic iron powder containing impurities. In order to investigate the reaction pathways, we measured the production rates of the intermediates and the final products of the dechlorination of TCE such as DCEs, VC, ethyne or ethene. Ethyne, ethene, ethane and cis-DCE were observed as the major products, and trans-DCE, 1,1DCE, VC, C3-hydrocarbons (such as propane, propylene), C4-hydrocarbons (such as n-butane) and methane were observed as the minors. Also the rate constants of TCE to ethyne and ethyne to ethene were larger than any other constants. These fact show the production of ethene/ethane via ethyne is the main pathway of the dechlorination of TCE using the metallic iron powder. r 2005 Elsevier Ltd. All rights reserved. Keywords: Trichloroethene; Iron powder; Kinetics; Dechlorination; Hydrogenation
1. Introduction Chlorinated organic compounds, such as trichloroethene (TCE) and tetrachloroethene (PCE), have been widely used as industrial solvents, detergents for dry cleaning, etc. They are completely synthetic organic compounds having anesthetic and carcinogen properties and are known to cause damage to the human liver and kidney. Their releases into subsurface environments result in contamination of the soil and groundwater. Hence, their decontamination researches are currently very active. Several remediation techniques, such as physical (soil vapor extraction, pump and aeration), chemical (oxidative and reductive reaction) and biological treatment are used in contaminated sites depending Corresponding author. Tel./fax: +81 22 795 7402.
E-mail address: [email protected] (J. Hara).
on site characteristics. Among these techniques, dechlorination of organic chlorides using zero-valent metals has been frequently reported in recent years. Senzaki and Kumagai (1998) reported that TCE and 1,1,2,2-tetrachloroethane were reductively degraded during the oxidation of metals in laboratory experiments. Reductive dechlorination of TCE and PCE by zero-valent metals has also been known since Gillham and O’Hannesin (1994) proposed that metallic iron fillings could be used for an in situ, passive groundwater remediation (Matheson and Tratnyek, 1994; Orth and Gillham, 1996; Roberts et al., 1996). Iron powder are now recognized as a reduction media in the aqueous layer and applied to the foundation of PRBs. The reaction of TCE and zero-valent metal is broadly interpreted as a reductive dechlorination with producing non-chlorohydrocarbons such as ethene and ethane as the final products. Matheson and Tratnyek (1994)
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.12.011
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examined the chemical reaction rate of TCE dechlorination and zero-valent iron, and estimated its half-life to be 30 days. They reported that the overall reductive dechlorination of TCE with metallic iron powder took place accompanied by direct electron transfers from the metallic iron surfaces to the reactants. In the process, water plays an important role as a proton donor. The reaction seems to proceed via unstable reaction intermediates such as free radicals, which result in producing many minor products such as C3- and C4-hydrocarbons (Deng et al., 1997). Dichloroethene (DCE) isomers, vinyl chloride (VC), ethyne, ethene and ethane are regarded as the main products of the dechlorination process. Some of the observed intermediates, such as DCE isomers and VC, are toxic and carcinogenic, and are dechlorinated at a relatively slow rate in the presence of metallic iron (Arnold and Roberts, 2000). Roughly speaking, in the presence of metallic iron, TCE is dechlorinated to nontoxic ethene and ethane through two parallel pathways; one is the consecutive hydrogenolysis via DCE isomers and VC, and the other is the reductive b-elimination via ethyne (Roberts et al., 1996; Vogan et al., 1995; Ooshita et al., 2001). Recently, Campbell et al. (1997) revealed pathways of TCE and PCE dechlorination with metallic iron powder through batch experiments. Arnold and Roberts (1998) showed that reductive b-elimination played an important role in the reactions of chlorinated ethynes, and specifically 30% of the TCE, 85% of the cis-DCE, and 95% of trans-DCE are dechlorinated by the reductive belimination pathway in the presence of metallic zinc powder. However, toxic and carcinogenic DCE isomers and VC retained their remarkable amounts in the solution. This paper aims to estimate the rate of each elementary step involved in the reaction of TCE with metallic iron powder containing some impurities, by using a modified kinetic model based on the one originally proposed by Campbell et al. (1997).
Table 1 Chemical composition of the iron powder, E-200 Element
Total Fe FeO C
Content (wt%) 95.6
P
Mn
Si
S
18.4 0.32 0.03 0.02 0.27 0.03
evaluated with BET analysis was 0.47, 0.20, 0.13 m2 g1. VOCs such as TCE, DCEs, VC, ethyne and ethene were obtained as analytical grade chemicals from GL Science Co. Ltd., Japan. Reaction experiments were performed in a closed batch system of 24 ml vials each containing 10 ml of deionized water. Deoxygenated nitrogen gas was introduced into the water for 5 min to remove dissolved oxygen. A 100 mg of iron powder sample was added to the vials and nitrogen gas was introduced to the headspace of the vials for 1 min to remove oxygen. Iron powder of 100 mg supplied enough amount of electron (Ito et al., 1998). The vials were then sealed with a Teflon-faced butyl rubber septum and an aluminum seal. Subsequently, TCE, DCEs, VC, ethyne or ethane were injected into the vials to achieve a concentration of 100 mg l1. Benzene (1.0 ml) was added as an internal standard. The vials containing the assay mixture were then shaken on a shaker (Taitec Co. Ltd., Japan) at 400 rev min1. The reaction temperature was kept at 25 1C. At every predetermined time, headspace aliquots (10 ml) were sampled from the vials. The chlorinated hydrocarbon concentrations were determined by a gas chromatograph (GC-390; column: TC-5; detector: FID; GL Science Co. Ltd., Japan). The concentrations of non-chlorinated hydrocarbons were determined by the same system equipped with a SILICAPLOT column (GL Science Co. Ltd., Japan) instead of the TC-5 column.
3. Reaction pathways and kinetics 2. Materials and methods The metallic iron sample mainly used in this study was iron powder supplied by Dowa Iron Powder Co. Ltd., Japan (catalog code number: E-200), which was produced by coke reduction of iron one and has been used commercially to treat wastewater contaminated mainly with heavy metals. The iron powder sample had a porous structure with a specific surface area of 3.9 m2 g1 determined by BET analysis and included the impurities shown in Table 1. The other iron powder; reduced iron powder, electric iron powder and atomized iron was purchased from Wako Pure Chemical Industries Ltd. Specific surface area of the each iron powder
3.1. Reaction pathway Fig. 1 shows the reaction pathways based on the previous papers (Campbell et al., 1997; Arnold and Roberts, 1998) except for the pathways of chloroethyne to vinyl chloride, ethyne to Cn-hydrocarbon and ethyne to ethane, and the following assumptions: (1) the overall reaction involves the following three steps; hydrogenolysis, reductive b-elimination and a hydrogen addition, (2) reductive b-elimination occurs only in the case that two chlorine molecules are eliminated,
ARTICLE IN PRESS J. Hara et al. / Water Research 39 (2005) 1165–1173
TCE
DCEs (a) VC (a)
1,1-DCE
chloloethyne (b)
TCE
k3
(b)
(a)
(a)
k2
k1
trans-DCE k6
k7
k5
k8
k9
ethyne k11
k10
(C-1)
ethene
(C-3)
k4
cis-DCE
VC
ethyne
ethene
1167
k12
k13
(C-2)
ethane
ethane
k14
(d)
Cn hydrocarbons
Cn hydrocarbons
Fig. 2. Reaction pathway of TCE degradation. k1–k14 indicate the reaction rate constant of each reaction path.
(a) hydrogenolysis ; RCl + 2e- + H+ RH + Cl(b) -elimination ; RCl=RCl + 2e- R R + 2Cl(c) hydrogen addition reaction ; RH=RH c-1 ; R R + 2e- + 2H+ c-2 ; R=R + 2e- + 2H+ RH-RH c-3 ; R R + 4e- + 4H+ RH2-RH2 (d) polymerization
dC c-DCE ¼ k1 C TCE ðk5 þ k8 ÞC c-DCE ; dt
cis-DCE;
Fig. 1. A schematic diagram of the reaction pathways for the dechlorination of TCE with metallic iron powder.
trans-DCE;
(2)
dC t-DCE ¼ k2 C TCE ðk6 þ k9 ÞC t-DCE ; dt (3)
(3) the hydrogen addition step occurs only in ethyne and ethene, and (4) the reaction from TCE to chloroethyne is the ratedetermining step from TCE to ethyne. Chloroethyne is assumed to be an intermediate, although it was not identified in this study. Arnold and Roberts (1998) reported that the rate constant for chloroethyne to ethyne was four orders larger than that for TCE to chloroethyne. The previous report on dechlorination experiments (Ooshita et al., 2001) showed that DCE isomers, VC, ethyne, ethene, ethane and Cn-hydrocarbons were the detectable products of TCE dechlorination with the E-200 metallic iron powder, and excessive ethane was produced in comparison with that from the pathway from ethene to ethane. Therefore, we assumed two additional pathways: (i) the step from ethyne to Cn-hydrocarbons (nX3) ((d) in Fig. 1), and (ii) the direct formation of ethane from ethyne ((C-3) in Fig. 1). 3.2. Reaction kinetics Assuming the first-order reaction with the reaction rate constants shown in Fig. 2, which take place in a batchwise, perfect mixing tank, the time derivatives of the concentrations of the species are given as follows: TCE;
dC TCE ¼ ðk1 þ k2 þ k3 þ k4 ÞC TCE ; dt
(1)
1; 1-DCE;
VC;
dC 1;1-DCE ¼ k3 C TCE k7 C 1;1-DCE ; dt
dC VC ¼ k5 C c-DCE þ k6 C t-DCE þ k7 C 1;1-DCE dt k10 C VC ;
ethyne;
ethene;
(4)
ð5Þ
dC ethyne ¼ k4 C TCE þ k8 C c-DCE þ k9 C t-DCE dt ðk11 þ k12 þ k14 ÞC ethyne ; ð6Þ dC ethene ¼ k10 C VC þ k11 C ethyne k13 C ethene ; dt (7)
ethane;
dC ethane ¼ k12 C ethyne þ k13 C ethene ; dt
(8)
dC HC ¼ k14 C ethyne ; dt
(9)
hydrocarbons;
where CTCE, Cc-DCE, Ct-DCE, C1,1-DCE, CVC, Cethyne, Cethene, Cethane and CHC are the concentrations (mol m3) of TCE, cis-DCE, trans-DCE, 1,1-DCE, VC, ethyne, ethene, ethane, and hydrocarbons, respectively, and k1 through k14 are the apparent first-order rate constants (h1) of each reaction shown in Fig. 2. These simultaneous differential equations, Eqs. (1)–(9), can be solved for concentrations of TCE and its reaction products by using the Runge–Kutta–Gill method (Lapidus, 1962). Among the 14 parameters in Eqs. (1)–(9), k7, k10 and k13 can be directly determined
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by experiments of 1,1-DCE, VC and ethene dechlorination. We have still eleven unknown parameters, k1, k2, k3, k4, k5, k6, k8, k9, k11, k12, k14. However, we also have the following four equations: k1 þ k2 þ k3 þ k4 ¼ kTCE ;
(10)
k5 þ k8 ¼ kc-DCE ;
(11)
k6 þ k9 ¼ kt-DCE ;
(12)
k11 þ k12 þ k14 ¼ kethyne ;
(13)
where kTCE, kc-DCE, kt-DCE, kethyne are the pseudo-firstorder rate constants (h1) of TCE, cis-DCE, trans-DCE and ethyne, respectively, all of which can be measured experimentally. At this point, we have still seven unknown parameters. These unknown parameters can be derived from the simultaneous equations for each degradation step by fitting as follows. At the condition of CTCE ¼ 0, Cc-DCE ¼ 0, Ct-DCE ¼ 0 and CVC ¼ 0, the Eqs. (6)–(8) are written as dC ethyne ¼ ðk11 þ k12 þ k14 ÞC ethyne ; dt
(14)
dC ethene ¼ k11 C ethyne k13 C ethene ; dt
(15)
dC ethane ¼ k12 C ethyne þ k13 C ethene : (16) dt Since k13 is already obtained from the degradation experiment of ethene when ethyne is used as a starting materials, there are three unknown parameters and the three equations. So we can get the values of k11, k12, k14, after that the concentration of hydrocarbons are decided by using the value of k14. Subsequently, at the condition of CTCE ¼ 0, Cc-DCE ¼ 0 and C1,1-DCE ¼ 0, the equations of (3), (5) and (6) are written as dC t-DCE ¼ ðk6 þ k9 ÞC t-DCE ; dt
(17)
dC VC ¼ k6 C t-DCE k10 C VC ; dt
(18)
dC ethyne ¼ k9 C t-DCE ðk11 þ k12 þ k14 ÞC ethyne : (19) dt When trans-DCE is used as a starting materials, the unknown parameters k6 and k9 are obtained from the three equations by fitting the trans-DCE dechlorination data using the already derived parameter.
On the same way, k5 and k8 are obtained from the simultaneous equations (2), (5) and (6) at the condition of CTCE ¼ 0, Ct-DCE ¼ 0 and C1,1-DCE ¼ 0 by fitting the dechlorination data of cis-DCE as a starting material. Finally, the remaining parameters, k1, k2, k3 and k4 are deduced using the simultaneous equations (1)–(9). The value of k1 is obtained from the formation of cis-DCE, k2 is from that of trans-DCE, k3 is from that of 1,1-DCE and k4 is from that of ethyne. In this procedure, we were able to evaluate all 14 parameters involved in the dechlorination for TCE and the intermediates involved.
4. Results 4.1. Apparent reaction rate constants The TCE dechlorination rate with E-200 was firstly compared with those observed using the three samples of iron powder; reduced iron powder, electrolytic iron powder and atomized iron. For comparison, Table 2 shows the dechlorination rate constant of TCE with E200 powder and the other samples. These rate constants for TCE, kTCE, are calculated per specific surface area. E-200 has almost the same value as the other samples, and every rate is close to the value of 3.973.6 104 h1 m2, reported by Johnson et al. (1996). However, the dechlorination rate per unit weight for E-200 is higher than the others because of the fact that its specific surface area is about 10 times larger than the others. Therefore, E-200 is more efficient on a weight-for-weight basis. Secondly, to better understand the mechanism of TCE dechlorination, we measured the reaction rate constants of the key intermediates; three DCE isomers, VC, ethyne and ethene, with the E-200 metallic iron powder. The reaction rates of each compound (including TCE) were described by a pseudo-first-order reaction as shown in Fig. 3. Table 3 shows the apparent rate constants and half-lives. TCE decreases relatively rapidly, and the reaction rate constants of cis-DCE and 1,1-DCE are considerably lower than that of TCE. This means that these compounds are less reactive with E-200 than TCE. On the other hand, the reaction rate constant of transDCE is about 10 times larger than that of the other DCE isomers. Vinyl chloride is less reactive with E-200 as well as cis-DCE and 1,1-DCE. The apparent rate constant for ethyne is almost four times larger than that of TCE
Table 2 Apparent reaction rate constants per specific surface area (kTCE) for TCE using several iron species Iron species
E-200
Iron reduced A
Iron electrolytic
Atomized iron
kTCE (h1 m2)
1.2 104
3.3 105
4.8 105
2.7 104
ARTICLE IN PRESS J. Hara et al. / Water Research 39 (2005) 1165–1173
1
TCE cis-DCE trans-DCE 1,1-DCE VC ethyne ethene
C/C0 [-]
0.1
1169
0
100
200 300 Time [hour]
400
500
Fig. 3. The relations between concentration and time of DCEs, VC, ethyne and ethene.
Table 3 Apparent reaction rate constants for TCE and intermediates of TCE dechlorination. k denotes the reaction rate constant Reactant
kE-200 (h1 m2)
Half-life (day)
TCE cis-DCE trans-DCE 1,1-DCE VC Ethyne Ethene
8.5 103 9.2 104 5.4 103 6.2 104 4.9 104 3.1 102 o2.6 104
9 80 14 120 150 2.4 o6900
and is the largest among the compounds used in this study. Ethene reacted quite slowly with E-200 giving only a trace of ethane as a reaction product (data not shown), which was much smaller than that produced from TCE or ethyne used as the starting material.
4.2. Hydrogenation of ethyne and ethene Fig. 4 shows the data of the hydrogen addition on ethyne. It can be seen from Fig. 4 that ethene and ethane are major products at the very beginning of the experiment with ethyne. Methane, C3- and C4-hydrocarbons, which are collectively denoted as HC in Fig. 4, also detected. The total carbon mass of the products equivalent to ethyne is also shown in Fig. 4. The carbon mass at the end of our experimental period is about 60% of the initial value, and tends to decrease with increasing reaction time. This may be due to unidentified hydrocarbon products, some of which are adsorbed onto the metallic iron surface. Indeed, peaks of C5- and C6hydrocarbons were detected gas chromatographically.
The solid lines in Fig. 4 denote the calculated values of each compound using Eqs. (6)–(9) under the initial conditions, CTCE ¼ 0, Cc-DCE ¼ 0, Ct-DCE ¼ 0, C1,1DCE ¼ 0 and CVC ¼ 0. As shown in Fig. 3 and Table 3, the apparent rate constant of ethyne, kethyne, is 3.1 102 h1m2. Calculated values are in good agreement with the experimental values. The fitting values of k11, k12 and k14 are 1.3 102, 2.6 103 and 1.5 102 h1 m2, respectively. The apparent rate constant of ethene, kethene, the same as k13, is 2.6 105 h1, is too low to account for the formation of the large amount of ethane without considering the direct formation of ethyne from ethane. The good agreement of calculated and experimental results implies that the direct pathway from ethyne to ethane is assumed reasonably. 4.3. Dechlorination of vinyl chloride Ethene was the only product of the dechlorination of VC using the iron powder sample in the hydrogenolysis process. The apparent rate constant of hydrogenolysis for VC was quite small to be 4.9 104 h1 m2. 4.4. Dechlorination of DCEs Figs. 5(a)–(c) show the time course of the dechlorination of the cis-DCE, trans-DCE and 1,1-DCE used as starting materials instead of TCE. Ethyne and ethene are the major products in the case of cis-DCE and transDCE. Ethyne is detected as the reaction intermediate in the both cases but the amount of ethyne reacted from trans-DCE is greater than cis-DCE. The solid lines in Fig. 5 express the calculated values of each compound. From the proposed kinetic model,
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1.0 Experiments ethyne ethene ethane HC total carbon Calculation ethyne ethene ethane HC total carbon
C/C 0 [-]
0.8 0.6 0.4 0.2 0.0 0
20
40
60 80 100 Time [hour]
120
140
Fig. 4. The time course of the hydrogen addition of ethyne.
the dechlorination of cis-DCE proceeds to vinyl chloride by hydrogenolysis or to ethyne by b-elimination. The apparent reaction rate constants of cis-DCE to VC (k5) and cis-DCE to ethyne (k8) to be 7.7 105 and 8.5 104 h1 m2, respectively. Fig. 5(a) shows the calculations agree well with the experimental fittings. Also, Fig. 5(b) shows the calculation agree well with the experimental data, fitting the rate constants of transDCE to VC (k6) and trans-DCE to ethyne (k9) to be 5.1 105 and 5.4 103 h1 m2, respectively. The dechlorination rates of trans-DCE are almost one order greater than that of cis-DCE. On the contrary to cis-DCE and trans-DCE, ethyne was not detected in the dechlorination process of 1,1DCE. The major products were ethene, VC and ethane. Therefore, we assumed that hydrogenolysis was the main process of the dechlorination of 1,1-DCE to vinyl chloride. Calculating the proposed model using the assumption, the best fitting of the rate constant for the conversion of 1,1-DCE to VC (k7) was 6.2 104 h1 m2, but the calculated concentration of VC was, however, much greater than the experimental one, and the calculated concentrations of ethyne and ethene were small as shown in Fig. 5(c). Ethane and C3hydrocarbons were detected in the dechlorination experiment for 1,1-DCE as mentioned above. Thus, the expected reaction pathway for 1,1-DCE appears inadequate. This reason must be due to that: (i) since the chemical composition of the reacted solution in the experiment using VC and ethane as starting compounds differs from that produced by the dechlorination of 1,1DCE, k10 and k13 which estimated by the other experimental system cannot be used in here, and (ii) there may be another unknown pathway in the 1,1-DCE dechlorination process with E-200 metallic iron powder. Anyway, there was quite small amount of 1,1-DCE, say, 0.5% of the initial value of TCE in the distribution profile of the dechlorination of TCE. So, there is no effect of the difference between the experimental
concentration and the calculation on the overall kinetics of TCE dechlorination.
4.5. Dechlorination of TCE In the dechlorination of TCE with the metallic iron powder, it is important to know the amounts of toxic intermediates such as vinyl chloride and the three isomers of dichloroethene. Fig. 6(a) shows the time courses of the concentration of TCE and the major reaction products. Clearly, ethyne, ethene, and ethane are the major products. At the initial stage of the reaction, large amounts of ethyne are produced. Later ethane and ethane are gradually produced. After 300 h, the concentration of ethyne begins to decrease. The concentration profile for ethyne, which displays an initial increase followed by a decrease, shows the typical behavior of a reaction intermediate in a consecutive reaction. Fig. 6(b) shows the time courses of the minor reaction products. The DCE isomers, cis-DCE, transDCE, and 1,1-DCE, are observed but VC is detected below the detection level (detection limit: 0.001 mg l1) in this experiment. The amount of cis-DCE is the largest among these isomers but its concentration is very low, say, 1% of the initial concentration of TCE. Also, we can see quite a small amount of trans-DCE in this figure. The total carbon mass of the major and minor products equivalent to TCE is also shown in Fig. 6(a). The carbon mass as TCE is about 60% at the mass of the starting point, and tends to decrease with increasing reaction time. The previous work (Campbell et al., 1997) has shown the similar trends in the reaction behavior. This must be due to unidentified products being sorbed onto the metallic iron surface. We detected gas chromatographically some hydrocarbons (data not shown), such as methane, C3-hydrocarbons (e.g. propane, propylene), C4-hydrocarbons, C5-hydrocarbons, C6-hydrocarbons, but did not measure them quantitatively. The solid lines
ARTICLE IN PRESS J. Hara et al. / Water Research 39 (2005) 1165–1173
0.20
1. 0 0. 8
C/C 0 [-]
1171
0.15
0. 6 0.10 0. 4 0.05
0. 2
0.00 500
0. 0 0
100
(a)
200 300 Time [hour]
400
Exepriments cis-DCE ethane ethene ethyne VC Calculation cis-DCE ethyne ethene ethane VC
1.0 Experiments trans-DCE ethyne ethene ethane VC Calculation trans-DCE ethyne ethene ethane VC
C/C 0 [-]
0.8 0.6 0.4 0.2 0.0 0
100
(b)
200 300 Time [hour]
400
500
1.0
0.12 0.10
0.8
C/C0 [-]
VC
0.08
0.6 0.06 0.4 0.04 0.2
Experiments 1,1-DCE ethene ethane VC Calculation 1,1-DCE ethene ethane VC
0.02 ethane
0.0 0 (c)
100
200 300 Time [hour]
400
0.00 500
Fig. 5. The time course of the dechlorination of the DCEs.
in Figs. 6(a) and (b) show the calculations based on the proposed kinetic model and the rate constants of TCE dechlorination steps. Calculated values were very close to the experimental data. The apparent rate constants of each pathway are summarized in Table 4. 5. Discussion This paper examined the reaction mechanism, kinetics of TCE dechlorination and the behavior of the reaction
intermediates have been examined by the proposed model and the batch-type experiments. Arnold and Roberts (2000) reported the pathways and kinetics of chlorinated ethene reaction with zero-valent iron particles by use of a modified Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic model. Their data were in agreement with LHHW kinetic model. However, our experimental data can also be well explained by an apparent first-order reaction rate model without considering the adsorption. Although, reaction at the iron
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1.0 Experiments TCE ethyne ethene ethane total carbone Calculation TCE ethyne ethene ethane total carbone
C/C 0 [-]
0.8 0.6 0.4 0.2 0.0
0
100
200 300 Time [hour]
(a)
400
500
0.020 Experiments cis-DCE trans-DCE 1,1-DCE VC Calculation cis-DCE trans-DCE 1,1-DCE VC
C/C0 [-]
0.015
0.010
0.005
0.000
0
100
(b)
200 300 Time [hour]
400
500
Fig. 6. The time course of the dechlorination of TCE. Table 4 Reaction rate constants of each reaction path calculated by using the proposed model. HC denotes Cn-hydrocarbons Reaction path
Rate constant
kE-200 (h1 m2)
TCE-cis-DCE TCE-trans-DCE TCE-1,1-DCE TCE-Ethyne cis-DCE-VC trans-DCE-VC 1,1-DCE-VC cis-DCE-Ethyne trans-DCE-Ethyne VC-Ethene Ethyne-Ethene Ethyne-Ethane Ethene-Ethane Ethyne-HC
k1 k2 k3 k4 k5 k6 k7 k8 k9 k10 k11 k12 k13 k14
1.3 104 2.6 105 5.1 105 8.2 103 7.7 105 5.1 105 6.2 104 8.5 104 5.4 103 4.9 104 1.3 102 2.6 103 2.6 105 1.5 102
surface involves adsorption of the reactants, chemical reaction at the surface and desorption of products, the chemical reaction is the rate-determining step and the
adsorption and/or desorption are not in this study because the iron powder sample used in this study has relationship high specific surface area. The ratios of reaction rate between hydrogenolysis and b-elimination of TCE, cis-DCE and trans-DCE were evaluated as in Table 5. We determined the ratio of the rate constant of hydrogenolysis to b-elimination for cis-DCE to be 1:10. This fact means that b-elimination is the main pathway of the dechlorination process of cisDCE to ethyne. It appears that, as is also the case for cis-DCE, most of the trans-DCE is reduced to ethyne by b-elimination. The ratio of the rate constant of hydrogenolysis to that of b-elimination was 1:100 (Fig. 5(b)). From this facts, we conclude that b-elimination is regarded as the major pathway in the dechlorination of cis-DCE and trans-DCE using E-200 iron powder. The b-elimination tends to progress preferentially as the dissolution of E-200 brings about a reducing atmosphere and abundant electron effects. Formation of vinyl chloride by hydrogenolysis is the minor pathway. Although b-elimination is regarded as the major pathway in these processes, there is the great difference between the apparent rate constants of the various substrates.
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Table 5 Ratios of reaction rate constant of hydrogenolysis and b-elimination for DCEs Dechlorination
Ratio among reaction rate
E-200
TCE ! DCE ðk1 þ k2 þ k3 Þ
(k1+k2+k3):k4
1:40
k5:k8
1:10
k6:k9
1:100
! Ethyne ðk4 Þ cis-DCE ! VC ðk5 Þ ! Ethyne ðk8 Þ trans-DCE ! VC ðk6 Þ ! Ethyne ðk9 Þ
To decide the main pathway of the dechlorination of TCE, we pay attention to the ratios of kDCEs to kethyne in the TCE dechlorination reaction, the sum of which expresses the rate constant of TCE. This total ratio is about 1:40, and the calculated values express the experimental values very well both for ethyne and DCE isomers. In other words, the pathway from TCE to ethene/ethane via ethyne is the major pathways of the degradation of TCE with E-200 iron powder. E-200 excels in TCE dechlorination and restricts the accumulation of DCE isomers and VC.
6. Conclusion Dechlorination of trichloroethene using the metallic iron powder E-200 was estimated quantitatively using the first-order kinetic equation. TCE was mainly dechlorinated via ethyne by a b-elimination pathway. There were quite small amount of the toxic intermediates such as DCEs and VC, which were slowly dechlorinated by the iron powder E-200. As a next step, we need to investigate the acceleration of them.
Acknowledgements This work was financially supported by the Japan Society for the Promotion of Science ((A) (2)14205150). This work would not have been possible without the help of Takayuki Ooshita (Miyama Co. Ltd.) in the experiments involving the degradation of Trichloroethene using zero-valent iron.
References Arnold, W.A., Roberts, A.L., 1998. Pathways of chlorinated ethylene and chlorinated acetylene reaction with Zn(0). Environ. Sci. Technol. 32, 3017–3025.
Arnold, W.A., Roberts, A.L., 2000. Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles. Environ. Sci. Technol. 34, 1794–1805. Campbell, T.J., Burris, D.R., Roberts, A.L., Wells, J.R., 1997. Trichloroethylene and tetrachloroethylene reduction in metallic iron–water–vapor batch system. Environ. Toxicol. Chem. 16, 625–663. Deng, B., Campbell, T.J., Burris, D.R., 1997. Hydrocarbon formation in metallic iron/water systems. Environ. Sci. Technol. 31, 1185–1190. Gillham, R.W., O’Hannesin, S.F., 1994. Enhanced degradation of halogenated aliphatics by zero-valent iron. Ground Water 32, 958–967. Ito, H., Mitsuya, S., Suto, K., Inoue, C., Chida, T., 1998. Research on dechlorination of trichloroethene using iron powder. Kogyo-Yosui 40, 25–30. Johnson, T.L., Scherer, M.M., Tratnyek, P.G., 1996. Kinetics of halogenated organic compound degradation by iron metal. Environ. Sci. Technol. 30, 2634–2640. Lapidus, L., 1962. Digital Computation for Chemical Engineers. McGraw-Hill, New York, p. 407. Matheson, L.J., Tratnyek, P.G., 1994. Reductive dehalogenation of chlorinated methanes by iron metal. Environ. Sci. Technol. 28, 2045–2053. Ooshita, T., Ito, H., Inoue, C., Chida, T., 2001. Dechlorination of trichloroethylene (TCE) with metallic iron powder. Proceedings of the First IWA Asia–Pacific Regional Conference, Asian Waterqual, vol. I, pp. 61–66. Orth, W.S., Gillham, R.W., 1996. Dechlorination of trichloroethene in aqueous solution using Fe0. Environ. Sci. Technol. 30, 66–71. Roberts, A.L., Totten, L.A., Arnold, W.A., Burris, D.R., Campbell, T.J., 1996. Reductive elimination of chlorinated ethylenes by zero-valent metals. Environ. Sci. Technol. 30, 2654–2659. Senzaki, T., Kumagai, Y., 1998. Removal of chloroorganic compounds in wastewater by reductive treatment— reductive degradation of chlorinated organic hydrocarbons with iron powder. Kogyo-Yosui 357, 2–7 (in Japanese). Vogan, J.L., Gillham, R.W., O’Hannesin, S.F., Matulewicz, W.H., Rhodes, J.E., 1995. Site specific degradation of VOCs in groundwater using zero-valent iron. Natl. Meet.—Am. Chem. Soc., Div. Environ. Chem. 35, 800–804.
Water Research 39 (2005) 1165–1173 www.elsevier.com/locate/watres
Kinetics of trichloroethene dechlorination with iron powder Junko Haraa,, Hiroyuki Itob, Koichi Sutoa, Chihiro Inouea, Tadashi Chidaa a
Graduate Schools of Environmental Studies, Tohoku University, 20 Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b Soil Remediation Business Unit, Dowa Mining Co. Ltd., 1-8-2 Marunouchi, Chiyoda-ku, Tokyo 100-8282, Japan Received 24 May 2004; received in revised form 14 October 2004; accepted 20 December 2004
Abstract The dechlorination of trichloroethene (TCE) with metallic iron is an advantageous method for the remediation of contaminated groundwater and soil. The toxic reaction intermediates such as dichloroethenes (DCEs) and vinyl chloride (VC), however, occasionally accumulate in the pathway of the reaction. We have been trying to suppress these intermediates by using metallic iron powder containing impurities. In order to investigate the reaction pathways, we measured the production rates of the intermediates and the final products of the dechlorination of TCE such as DCEs, VC, ethyne or ethene. Ethyne, ethene, ethane and cis-DCE were observed as the major products, and trans-DCE, 1,1DCE, VC, C3-hydrocarbons (such as propane, propylene), C4-hydrocarbons (such as n-butane) and methane were observed as the minors. Also the rate constants of TCE to ethyne and ethyne to ethene were larger than any other constants. These fact show the production of ethene/ethane via ethyne is the main pathway of the dechlorination of TCE using the metallic iron powder. r 2005 Elsevier Ltd. All rights reserved. Keywords: Trichloroethene; Iron powder; Kinetics; Dechlorination; Hydrogenation
1. Introduction Chlorinated organic compounds, such as trichloroethene (TCE) and tetrachloroethene (PCE), have been widely used as industrial solvents, detergents for dry cleaning, etc. They are completely synthetic organic compounds having anesthetic and carcinogen properties and are known to cause damage to the human liver and kidney. Their releases into subsurface environments result in contamination of the soil and groundwater. Hence, their decontamination researches are currently very active. Several remediation techniques, such as physical (soil vapor extraction, pump and aeration), chemical (oxidative and reductive reaction) and biological treatment are used in contaminated sites depending Corresponding author. Tel./fax: +81 22 795 7402.
E-mail address: [email protected] (J. Hara).
on site characteristics. Among these techniques, dechlorination of organic chlorides using zero-valent metals has been frequently reported in recent years. Senzaki and Kumagai (1998) reported that TCE and 1,1,2,2-tetrachloroethane were reductively degraded during the oxidation of metals in laboratory experiments. Reductive dechlorination of TCE and PCE by zero-valent metals has also been known since Gillham and O’Hannesin (1994) proposed that metallic iron fillings could be used for an in situ, passive groundwater remediation (Matheson and Tratnyek, 1994; Orth and Gillham, 1996; Roberts et al., 1996). Iron powder are now recognized as a reduction media in the aqueous layer and applied to the foundation of PRBs. The reaction of TCE and zero-valent metal is broadly interpreted as a reductive dechlorination with producing non-chlorohydrocarbons such as ethene and ethane as the final products. Matheson and Tratnyek (1994)
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.12.011
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examined the chemical reaction rate of TCE dechlorination and zero-valent iron, and estimated its half-life to be 30 days. They reported that the overall reductive dechlorination of TCE with metallic iron powder took place accompanied by direct electron transfers from the metallic iron surfaces to the reactants. In the process, water plays an important role as a proton donor. The reaction seems to proceed via unstable reaction intermediates such as free radicals, which result in producing many minor products such as C3- and C4-hydrocarbons (Deng et al., 1997). Dichloroethene (DCE) isomers, vinyl chloride (VC), ethyne, ethene and ethane are regarded as the main products of the dechlorination process. Some of the observed intermediates, such as DCE isomers and VC, are toxic and carcinogenic, and are dechlorinated at a relatively slow rate in the presence of metallic iron (Arnold and Roberts, 2000). Roughly speaking, in the presence of metallic iron, TCE is dechlorinated to nontoxic ethene and ethane through two parallel pathways; one is the consecutive hydrogenolysis via DCE isomers and VC, and the other is the reductive b-elimination via ethyne (Roberts et al., 1996; Vogan et al., 1995; Ooshita et al., 2001). Recently, Campbell et al. (1997) revealed pathways of TCE and PCE dechlorination with metallic iron powder through batch experiments. Arnold and Roberts (1998) showed that reductive b-elimination played an important role in the reactions of chlorinated ethynes, and specifically 30% of the TCE, 85% of the cis-DCE, and 95% of trans-DCE are dechlorinated by the reductive belimination pathway in the presence of metallic zinc powder. However, toxic and carcinogenic DCE isomers and VC retained their remarkable amounts in the solution. This paper aims to estimate the rate of each elementary step involved in the reaction of TCE with metallic iron powder containing some impurities, by using a modified kinetic model based on the one originally proposed by Campbell et al. (1997).
Table 1 Chemical composition of the iron powder, E-200 Element
Total Fe FeO C
Content (wt%) 95.6
P
Mn
Si
S
18.4 0.32 0.03 0.02 0.27 0.03
evaluated with BET analysis was 0.47, 0.20, 0.13 m2 g1. VOCs such as TCE, DCEs, VC, ethyne and ethene were obtained as analytical grade chemicals from GL Science Co. Ltd., Japan. Reaction experiments were performed in a closed batch system of 24 ml vials each containing 10 ml of deionized water. Deoxygenated nitrogen gas was introduced into the water for 5 min to remove dissolved oxygen. A 100 mg of iron powder sample was added to the vials and nitrogen gas was introduced to the headspace of the vials for 1 min to remove oxygen. Iron powder of 100 mg supplied enough amount of electron (Ito et al., 1998). The vials were then sealed with a Teflon-faced butyl rubber septum and an aluminum seal. Subsequently, TCE, DCEs, VC, ethyne or ethane were injected into the vials to achieve a concentration of 100 mg l1. Benzene (1.0 ml) was added as an internal standard. The vials containing the assay mixture were then shaken on a shaker (Taitec Co. Ltd., Japan) at 400 rev min1. The reaction temperature was kept at 25 1C. At every predetermined time, headspace aliquots (10 ml) were sampled from the vials. The chlorinated hydrocarbon concentrations were determined by a gas chromatograph (GC-390; column: TC-5; detector: FID; GL Science Co. Ltd., Japan). The concentrations of non-chlorinated hydrocarbons were determined by the same system equipped with a SILICAPLOT column (GL Science Co. Ltd., Japan) instead of the TC-5 column.
3. Reaction pathways and kinetics 2. Materials and methods The metallic iron sample mainly used in this study was iron powder supplied by Dowa Iron Powder Co. Ltd., Japan (catalog code number: E-200), which was produced by coke reduction of iron one and has been used commercially to treat wastewater contaminated mainly with heavy metals. The iron powder sample had a porous structure with a specific surface area of 3.9 m2 g1 determined by BET analysis and included the impurities shown in Table 1. The other iron powder; reduced iron powder, electric iron powder and atomized iron was purchased from Wako Pure Chemical Industries Ltd. Specific surface area of the each iron powder
3.1. Reaction pathway Fig. 1 shows the reaction pathways based on the previous papers (Campbell et al., 1997; Arnold and Roberts, 1998) except for the pathways of chloroethyne to vinyl chloride, ethyne to Cn-hydrocarbon and ethyne to ethane, and the following assumptions: (1) the overall reaction involves the following three steps; hydrogenolysis, reductive b-elimination and a hydrogen addition, (2) reductive b-elimination occurs only in the case that two chlorine molecules are eliminated,
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TCE
DCEs (a) VC (a)
1,1-DCE
chloloethyne (b)
TCE
k3
(b)
(a)
(a)
k2
k1
trans-DCE k6
k7
k5
k8
k9
ethyne k11
k10
(C-1)
ethene
(C-3)
k4
cis-DCE
VC
ethyne
ethene
1167
k12
k13
(C-2)
ethane
ethane
k14
(d)
Cn hydrocarbons
Cn hydrocarbons
Fig. 2. Reaction pathway of TCE degradation. k1–k14 indicate the reaction rate constant of each reaction path.
(a) hydrogenolysis ; RCl + 2e- + H+ RH + Cl(b) -elimination ; RCl=RCl + 2e- R R + 2Cl(c) hydrogen addition reaction ; RH=RH c-1 ; R R + 2e- + 2H+ c-2 ; R=R + 2e- + 2H+ RH-RH c-3 ; R R + 4e- + 4H+ RH2-RH2 (d) polymerization
dC c-DCE ¼ k1 C TCE ðk5 þ k8 ÞC c-DCE ; dt
cis-DCE;
Fig. 1. A schematic diagram of the reaction pathways for the dechlorination of TCE with metallic iron powder.
trans-DCE;
(2)
dC t-DCE ¼ k2 C TCE ðk6 þ k9 ÞC t-DCE ; dt (3)
(3) the hydrogen addition step occurs only in ethyne and ethene, and (4) the reaction from TCE to chloroethyne is the ratedetermining step from TCE to ethyne. Chloroethyne is assumed to be an intermediate, although it was not identified in this study. Arnold and Roberts (1998) reported that the rate constant for chloroethyne to ethyne was four orders larger than that for TCE to chloroethyne. The previous report on dechlorination experiments (Ooshita et al., 2001) showed that DCE isomers, VC, ethyne, ethene, ethane and Cn-hydrocarbons were the detectable products of TCE dechlorination with the E-200 metallic iron powder, and excessive ethane was produced in comparison with that from the pathway from ethene to ethane. Therefore, we assumed two additional pathways: (i) the step from ethyne to Cn-hydrocarbons (nX3) ((d) in Fig. 1), and (ii) the direct formation of ethane from ethyne ((C-3) in Fig. 1). 3.2. Reaction kinetics Assuming the first-order reaction with the reaction rate constants shown in Fig. 2, which take place in a batchwise, perfect mixing tank, the time derivatives of the concentrations of the species are given as follows: TCE;
dC TCE ¼ ðk1 þ k2 þ k3 þ k4 ÞC TCE ; dt
(1)
1; 1-DCE;
VC;
dC 1;1-DCE ¼ k3 C TCE k7 C 1;1-DCE ; dt
dC VC ¼ k5 C c-DCE þ k6 C t-DCE þ k7 C 1;1-DCE dt k10 C VC ;
ethyne;
ethene;
(4)
ð5Þ
dC ethyne ¼ k4 C TCE þ k8 C c-DCE þ k9 C t-DCE dt ðk11 þ k12 þ k14 ÞC ethyne ; ð6Þ dC ethene ¼ k10 C VC þ k11 C ethyne k13 C ethene ; dt (7)
ethane;
dC ethane ¼ k12 C ethyne þ k13 C ethene ; dt
(8)
dC HC ¼ k14 C ethyne ; dt
(9)
hydrocarbons;
where CTCE, Cc-DCE, Ct-DCE, C1,1-DCE, CVC, Cethyne, Cethene, Cethane and CHC are the concentrations (mol m3) of TCE, cis-DCE, trans-DCE, 1,1-DCE, VC, ethyne, ethene, ethane, and hydrocarbons, respectively, and k1 through k14 are the apparent first-order rate constants (h1) of each reaction shown in Fig. 2. These simultaneous differential equations, Eqs. (1)–(9), can be solved for concentrations of TCE and its reaction products by using the Runge–Kutta–Gill method (Lapidus, 1962). Among the 14 parameters in Eqs. (1)–(9), k7, k10 and k13 can be directly determined
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by experiments of 1,1-DCE, VC and ethene dechlorination. We have still eleven unknown parameters, k1, k2, k3, k4, k5, k6, k8, k9, k11, k12, k14. However, we also have the following four equations: k1 þ k2 þ k3 þ k4 ¼ kTCE ;
(10)
k5 þ k8 ¼ kc-DCE ;
(11)
k6 þ k9 ¼ kt-DCE ;
(12)
k11 þ k12 þ k14 ¼ kethyne ;
(13)
where kTCE, kc-DCE, kt-DCE, kethyne are the pseudo-firstorder rate constants (h1) of TCE, cis-DCE, trans-DCE and ethyne, respectively, all of which can be measured experimentally. At this point, we have still seven unknown parameters. These unknown parameters can be derived from the simultaneous equations for each degradation step by fitting as follows. At the condition of CTCE ¼ 0, Cc-DCE ¼ 0, Ct-DCE ¼ 0 and CVC ¼ 0, the Eqs. (6)–(8) are written as dC ethyne ¼ ðk11 þ k12 þ k14 ÞC ethyne ; dt
(14)
dC ethene ¼ k11 C ethyne k13 C ethene ; dt
(15)
dC ethane ¼ k12 C ethyne þ k13 C ethene : (16) dt Since k13 is already obtained from the degradation experiment of ethene when ethyne is used as a starting materials, there are three unknown parameters and the three equations. So we can get the values of k11, k12, k14, after that the concentration of hydrocarbons are decided by using the value of k14. Subsequently, at the condition of CTCE ¼ 0, Cc-DCE ¼ 0 and C1,1-DCE ¼ 0, the equations of (3), (5) and (6) are written as dC t-DCE ¼ ðk6 þ k9 ÞC t-DCE ; dt
(17)
dC VC ¼ k6 C t-DCE k10 C VC ; dt
(18)
dC ethyne ¼ k9 C t-DCE ðk11 þ k12 þ k14 ÞC ethyne : (19) dt When trans-DCE is used as a starting materials, the unknown parameters k6 and k9 are obtained from the three equations by fitting the trans-DCE dechlorination data using the already derived parameter.
On the same way, k5 and k8 are obtained from the simultaneous equations (2), (5) and (6) at the condition of CTCE ¼ 0, Ct-DCE ¼ 0 and C1,1-DCE ¼ 0 by fitting the dechlorination data of cis-DCE as a starting material. Finally, the remaining parameters, k1, k2, k3 and k4 are deduced using the simultaneous equations (1)–(9). The value of k1 is obtained from the formation of cis-DCE, k2 is from that of trans-DCE, k3 is from that of 1,1-DCE and k4 is from that of ethyne. In this procedure, we were able to evaluate all 14 parameters involved in the dechlorination for TCE and the intermediates involved.
4. Results 4.1. Apparent reaction rate constants The TCE dechlorination rate with E-200 was firstly compared with those observed using the three samples of iron powder; reduced iron powder, electrolytic iron powder and atomized iron. For comparison, Table 2 shows the dechlorination rate constant of TCE with E200 powder and the other samples. These rate constants for TCE, kTCE, are calculated per specific surface area. E-200 has almost the same value as the other samples, and every rate is close to the value of 3.973.6 104 h1 m2, reported by Johnson et al. (1996). However, the dechlorination rate per unit weight for E-200 is higher than the others because of the fact that its specific surface area is about 10 times larger than the others. Therefore, E-200 is more efficient on a weight-for-weight basis. Secondly, to better understand the mechanism of TCE dechlorination, we measured the reaction rate constants of the key intermediates; three DCE isomers, VC, ethyne and ethene, with the E-200 metallic iron powder. The reaction rates of each compound (including TCE) were described by a pseudo-first-order reaction as shown in Fig. 3. Table 3 shows the apparent rate constants and half-lives. TCE decreases relatively rapidly, and the reaction rate constants of cis-DCE and 1,1-DCE are considerably lower than that of TCE. This means that these compounds are less reactive with E-200 than TCE. On the other hand, the reaction rate constant of transDCE is about 10 times larger than that of the other DCE isomers. Vinyl chloride is less reactive with E-200 as well as cis-DCE and 1,1-DCE. The apparent rate constant for ethyne is almost four times larger than that of TCE
Table 2 Apparent reaction rate constants per specific surface area (kTCE) for TCE using several iron species Iron species
E-200
Iron reduced A
Iron electrolytic
Atomized iron
kTCE (h1 m2)
1.2 104
3.3 105
4.8 105
2.7 104
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TCE cis-DCE trans-DCE 1,1-DCE VC ethyne ethene
C/C0 [-]
0.1
1169
0
100
200 300 Time [hour]
400
500
Fig. 3. The relations between concentration and time of DCEs, VC, ethyne and ethene.
Table 3 Apparent reaction rate constants for TCE and intermediates of TCE dechlorination. k denotes the reaction rate constant Reactant
kE-200 (h1 m2)
Half-life (day)
TCE cis-DCE trans-DCE 1,1-DCE VC Ethyne Ethene
8.5 103 9.2 104 5.4 103 6.2 104 4.9 104 3.1 102 o2.6 104
9 80 14 120 150 2.4 o6900
and is the largest among the compounds used in this study. Ethene reacted quite slowly with E-200 giving only a trace of ethane as a reaction product (data not shown), which was much smaller than that produced from TCE or ethyne used as the starting material.
4.2. Hydrogenation of ethyne and ethene Fig. 4 shows the data of the hydrogen addition on ethyne. It can be seen from Fig. 4 that ethene and ethane are major products at the very beginning of the experiment with ethyne. Methane, C3- and C4-hydrocarbons, which are collectively denoted as HC in Fig. 4, also detected. The total carbon mass of the products equivalent to ethyne is also shown in Fig. 4. The carbon mass at the end of our experimental period is about 60% of the initial value, and tends to decrease with increasing reaction time. This may be due to unidentified hydrocarbon products, some of which are adsorbed onto the metallic iron surface. Indeed, peaks of C5- and C6hydrocarbons were detected gas chromatographically.
The solid lines in Fig. 4 denote the calculated values of each compound using Eqs. (6)–(9) under the initial conditions, CTCE ¼ 0, Cc-DCE ¼ 0, Ct-DCE ¼ 0, C1,1DCE ¼ 0 and CVC ¼ 0. As shown in Fig. 3 and Table 3, the apparent rate constant of ethyne, kethyne, is 3.1 102 h1m2. Calculated values are in good agreement with the experimental values. The fitting values of k11, k12 and k14 are 1.3 102, 2.6 103 and 1.5 102 h1 m2, respectively. The apparent rate constant of ethene, kethene, the same as k13, is 2.6 105 h1, is too low to account for the formation of the large amount of ethane without considering the direct formation of ethyne from ethane. The good agreement of calculated and experimental results implies that the direct pathway from ethyne to ethane is assumed reasonably. 4.3. Dechlorination of vinyl chloride Ethene was the only product of the dechlorination of VC using the iron powder sample in the hydrogenolysis process. The apparent rate constant of hydrogenolysis for VC was quite small to be 4.9 104 h1 m2. 4.4. Dechlorination of DCEs Figs. 5(a)–(c) show the time course of the dechlorination of the cis-DCE, trans-DCE and 1,1-DCE used as starting materials instead of TCE. Ethyne and ethene are the major products in the case of cis-DCE and transDCE. Ethyne is detected as the reaction intermediate in the both cases but the amount of ethyne reacted from trans-DCE is greater than cis-DCE. The solid lines in Fig. 5 express the calculated values of each compound. From the proposed kinetic model,
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1.0 Experiments ethyne ethene ethane HC total carbon Calculation ethyne ethene ethane HC total carbon
C/C 0 [-]
0.8 0.6 0.4 0.2 0.0 0
20
40
60 80 100 Time [hour]
120
140
Fig. 4. The time course of the hydrogen addition of ethyne.
the dechlorination of cis-DCE proceeds to vinyl chloride by hydrogenolysis or to ethyne by b-elimination. The apparent reaction rate constants of cis-DCE to VC (k5) and cis-DCE to ethyne (k8) to be 7.7 105 and 8.5 104 h1 m2, respectively. Fig. 5(a) shows the calculations agree well with the experimental fittings. Also, Fig. 5(b) shows the calculation agree well with the experimental data, fitting the rate constants of transDCE to VC (k6) and trans-DCE to ethyne (k9) to be 5.1 105 and 5.4 103 h1 m2, respectively. The dechlorination rates of trans-DCE are almost one order greater than that of cis-DCE. On the contrary to cis-DCE and trans-DCE, ethyne was not detected in the dechlorination process of 1,1DCE. The major products were ethene, VC and ethane. Therefore, we assumed that hydrogenolysis was the main process of the dechlorination of 1,1-DCE to vinyl chloride. Calculating the proposed model using the assumption, the best fitting of the rate constant for the conversion of 1,1-DCE to VC (k7) was 6.2 104 h1 m2, but the calculated concentration of VC was, however, much greater than the experimental one, and the calculated concentrations of ethyne and ethene were small as shown in Fig. 5(c). Ethane and C3hydrocarbons were detected in the dechlorination experiment for 1,1-DCE as mentioned above. Thus, the expected reaction pathway for 1,1-DCE appears inadequate. This reason must be due to that: (i) since the chemical composition of the reacted solution in the experiment using VC and ethane as starting compounds differs from that produced by the dechlorination of 1,1DCE, k10 and k13 which estimated by the other experimental system cannot be used in here, and (ii) there may be another unknown pathway in the 1,1-DCE dechlorination process with E-200 metallic iron powder. Anyway, there was quite small amount of 1,1-DCE, say, 0.5% of the initial value of TCE in the distribution profile of the dechlorination of TCE. So, there is no effect of the difference between the experimental
concentration and the calculation on the overall kinetics of TCE dechlorination.
4.5. Dechlorination of TCE In the dechlorination of TCE with the metallic iron powder, it is important to know the amounts of toxic intermediates such as vinyl chloride and the three isomers of dichloroethene. Fig. 6(a) shows the time courses of the concentration of TCE and the major reaction products. Clearly, ethyne, ethene, and ethane are the major products. At the initial stage of the reaction, large amounts of ethyne are produced. Later ethane and ethane are gradually produced. After 300 h, the concentration of ethyne begins to decrease. The concentration profile for ethyne, which displays an initial increase followed by a decrease, shows the typical behavior of a reaction intermediate in a consecutive reaction. Fig. 6(b) shows the time courses of the minor reaction products. The DCE isomers, cis-DCE, transDCE, and 1,1-DCE, are observed but VC is detected below the detection level (detection limit: 0.001 mg l1) in this experiment. The amount of cis-DCE is the largest among these isomers but its concentration is very low, say, 1% of the initial concentration of TCE. Also, we can see quite a small amount of trans-DCE in this figure. The total carbon mass of the major and minor products equivalent to TCE is also shown in Fig. 6(a). The carbon mass as TCE is about 60% at the mass of the starting point, and tends to decrease with increasing reaction time. The previous work (Campbell et al., 1997) has shown the similar trends in the reaction behavior. This must be due to unidentified products being sorbed onto the metallic iron surface. We detected gas chromatographically some hydrocarbons (data not shown), such as methane, C3-hydrocarbons (e.g. propane, propylene), C4-hydrocarbons, C5-hydrocarbons, C6-hydrocarbons, but did not measure them quantitatively. The solid lines
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0.20
1. 0 0. 8
C/C 0 [-]
1171
0.15
0. 6 0.10 0. 4 0.05
0. 2
0.00 500
0. 0 0
100
(a)
200 300 Time [hour]
400
Exepriments cis-DCE ethane ethene ethyne VC Calculation cis-DCE ethyne ethene ethane VC
1.0 Experiments trans-DCE ethyne ethene ethane VC Calculation trans-DCE ethyne ethene ethane VC
C/C 0 [-]
0.8 0.6 0.4 0.2 0.0 0
100
(b)
200 300 Time [hour]
400
500
1.0
0.12 0.10
0.8
C/C0 [-]
VC
0.08
0.6 0.06 0.4 0.04 0.2
Experiments 1,1-DCE ethene ethane VC Calculation 1,1-DCE ethene ethane VC
0.02 ethane
0.0 0 (c)
100
200 300 Time [hour]
400
0.00 500
Fig. 5. The time course of the dechlorination of the DCEs.
in Figs. 6(a) and (b) show the calculations based on the proposed kinetic model and the rate constants of TCE dechlorination steps. Calculated values were very close to the experimental data. The apparent rate constants of each pathway are summarized in Table 4. 5. Discussion This paper examined the reaction mechanism, kinetics of TCE dechlorination and the behavior of the reaction
intermediates have been examined by the proposed model and the batch-type experiments. Arnold and Roberts (2000) reported the pathways and kinetics of chlorinated ethene reaction with zero-valent iron particles by use of a modified Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic model. Their data were in agreement with LHHW kinetic model. However, our experimental data can also be well explained by an apparent first-order reaction rate model without considering the adsorption. Although, reaction at the iron
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1.0 Experiments TCE ethyne ethene ethane total carbone Calculation TCE ethyne ethene ethane total carbone
C/C 0 [-]
0.8 0.6 0.4 0.2 0.0
0
100
200 300 Time [hour]
(a)
400
500
0.020 Experiments cis-DCE trans-DCE 1,1-DCE VC Calculation cis-DCE trans-DCE 1,1-DCE VC
C/C0 [-]
0.015
0.010
0.005
0.000
0
100
(b)
200 300 Time [hour]
400
500
Fig. 6. The time course of the dechlorination of TCE. Table 4 Reaction rate constants of each reaction path calculated by using the proposed model. HC denotes Cn-hydrocarbons Reaction path
Rate constant
kE-200 (h1 m2)
TCE-cis-DCE TCE-trans-DCE TCE-1,1-DCE TCE-Ethyne cis-DCE-VC trans-DCE-VC 1,1-DCE-VC cis-DCE-Ethyne trans-DCE-Ethyne VC-Ethene Ethyne-Ethene Ethyne-Ethane Ethene-Ethane Ethyne-HC
k1 k2 k3 k4 k5 k6 k7 k8 k9 k10 k11 k12 k13 k14
1.3 104 2.6 105 5.1 105 8.2 103 7.7 105 5.1 105 6.2 104 8.5 104 5.4 103 4.9 104 1.3 102 2.6 103 2.6 105 1.5 102
surface involves adsorption of the reactants, chemical reaction at the surface and desorption of products, the chemical reaction is the rate-determining step and the
adsorption and/or desorption are not in this study because the iron powder sample used in this study has relationship high specific surface area. The ratios of reaction rate between hydrogenolysis and b-elimination of TCE, cis-DCE and trans-DCE were evaluated as in Table 5. We determined the ratio of the rate constant of hydrogenolysis to b-elimination for cis-DCE to be 1:10. This fact means that b-elimination is the main pathway of the dechlorination process of cisDCE to ethyne. It appears that, as is also the case for cis-DCE, most of the trans-DCE is reduced to ethyne by b-elimination. The ratio of the rate constant of hydrogenolysis to that of b-elimination was 1:100 (Fig. 5(b)). From this facts, we conclude that b-elimination is regarded as the major pathway in the dechlorination of cis-DCE and trans-DCE using E-200 iron powder. The b-elimination tends to progress preferentially as the dissolution of E-200 brings about a reducing atmosphere and abundant electron effects. Formation of vinyl chloride by hydrogenolysis is the minor pathway. Although b-elimination is regarded as the major pathway in these processes, there is the great difference between the apparent rate constants of the various substrates.
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Table 5 Ratios of reaction rate constant of hydrogenolysis and b-elimination for DCEs Dechlorination
Ratio among reaction rate
E-200
TCE ! DCE ðk1 þ k2 þ k3 Þ
(k1+k2+k3):k4
1:40
k5:k8
1:10
k6:k9
1:100
! Ethyne ðk4 Þ cis-DCE ! VC ðk5 Þ ! Ethyne ðk8 Þ trans-DCE ! VC ðk6 Þ ! Ethyne ðk9 Þ
To decide the main pathway of the dechlorination of TCE, we pay attention to the ratios of kDCEs to kethyne in the TCE dechlorination reaction, the sum of which expresses the rate constant of TCE. This total ratio is about 1:40, and the calculated values express the experimental values very well both for ethyne and DCE isomers. In other words, the pathway from TCE to ethene/ethane via ethyne is the major pathways of the degradation of TCE with E-200 iron powder. E-200 excels in TCE dechlorination and restricts the accumulation of DCE isomers and VC.
6. Conclusion Dechlorination of trichloroethene using the metallic iron powder E-200 was estimated quantitatively using the first-order kinetic equation. TCE was mainly dechlorinated via ethyne by a b-elimination pathway. There were quite small amount of the toxic intermediates such as DCEs and VC, which were slowly dechlorinated by the iron powder E-200. As a next step, we need to investigate the acceleration of them.
Acknowledgements This work was financially supported by the Japan Society for the Promotion of Science ((A) (2)14205150). This work would not have been possible without the help of Takayuki Ooshita (Miyama Co. Ltd.) in the experiments involving the degradation of Trichloroethene using zero-valent iron.
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