Environ. Sci. Technol. 1996, 30, 3642-3644
Comment on “Destruction of Organohalides in Water Using Metal Particles: Carbon Tetrachloride/Water Reactions With Magnesium, Tin, and Zinc” SIR: In a recent paper, Boronina et al. (1) published experimental results on the degradation of carbon tetrachloride in the presence of water, using zero-valent Sn and Zn. They reported the following equations as being representative of the degradation of carbon tetrachloride in their experiments. For the Sn system:
Sn(s) + CCl4(l) + 4H2O(l) f SnO2(s) + 4HCl(aq) + CO2(g) + 2H2(g) (1) Reaction 1 was supported by the observed precipitation of SnO2 and evolution of CO2 and the development of low pH, between 1 and 2, at the completion of the experiment. The main degradation products were CHCl3, SnO2, CO2, and HCl. For the analogous Zn system:
4Zn(s) + CCl4(l) + 4H2O(l) f 2ZnCl2(aq) + 2Zn(OH)2(s) + CH4(g) (2) After reaction, the pH of the solution remained nearly neutral, usually around 6.7. The final products of CCl4 degradation by zero-valent Zn include CH3Cl, CH4(g), ZnCl2, and Zn(OH)2. Boronina et al. (1) proposed that the differences in the degradation products of carbon tetrachloride as indicated by eqs 1 and 2 were due to different reaction mechanisms for the two reactions. However, the reported reaction products for the degradation of carbon tetrachloride in the presence of zerovalent Sn and Zn might be explained by thermodynamic analysis of the stabilities of the reactants and products in terms of their reduction/oxidation potentials and solution pH, i.e., Eh-pH, without having to resort to different reaction mechanisms for the dechlorination of carbon tetrachloride by the two metals. The Eh-pH stabilities of species in the system C-O-H-Cl are illustrated in Figure 1. At alkaline conditions, the stable forms of carbon in solution are the ions HCO3-(aq) and CO32-(aq). At oxidizing conditions and acid pH’s, CO2(g) is the stable form of carbon, while at elevated fugacities of H2(g) and at high reduction potentials, as exists below line a, the stable form of carbon in this system is CH4(g). It is apparent that CH4 gas and water are both stable only in a narrow band of Eh. In most of the Eh range in which water is stable, CH4(g) is unstable and, depending on the prevailing pH conditions, is predicted to decompose into either CO2(g), HCO3-(aq) or CO32-(aq) (Figure 1). The stepwise dechlorination of carbon tetrachloride can be illustrated by the following reactions: 1
1
/2CCl4(l) + 1/2H+ + e- ) 1/2CCl3H(l) + 1/2Cl- (3) 1
+
-
1
1
-
/2CCl3H(l) + /2H + e ) /2CCl2H2(l) + /2Cl
(4)
/2CCl2H2(l) + 1/2H+ + e- ) 1/2CClH3(g) + 1/2Cl- (5)
1
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FIGURE 1. Eh-pH diagram showing the fields of stability of species in the system C-O-H-Cl at 25 °C and 1 atm total pressure (C ) 0.01 M; Cl- ) 0.04 M); calculated using the HSC Chemistry thermodynamic database and software package published by Outokumpu Research Oy (5). Thermodynamic data for the chlorinated species CH2Cl2 and CH3Cl were taken from Stull et al. (6). The calculation of reduction and oxidation potentials for haloorganic compounds of common environmental concern is discussed in refs 7 and 8. The reaction boundaries represent the sequential dechlorination of carbon tetrachloride under these conditions and correspond to eqs 3-6. CH4(g) is the predicted final product of this dechlorination sequence. The short dashed lines labeled (a) and (b) define the reduction and oxidation equilibria, respectively, for water.
/2CClH3(l) + 1/2H+ + e- ) 1/2CH4(g) + 1/2Cl- (6)
1
Many of the chlorinated species reported by Boronina et al. (1) are recognized as intermediate dechlorination products of carbon tetrachloride. The final dechlorination product of carbon tetrachloride predicted by these reactions is methane gas. The relative stabilities of reactants and products in eqs 3-6 may also be considered in terms of their Eh-pH stabilities (Figure 1). Metal corrosion reactions that result in the evolution of hydrogen gas, such as Zn + 2H+ ) Zn2+ + H2, are the combination of two electrochemical reactions:
M f M2+ + 2e2H+ + 2e- f H2 +
M + 2H f M
2+
(7)
+ H2
Evolution of hydrogen gas occurs at the solution-metal interface, with a concomitant dissolution of the metal. The general reaction that describes the reductive dechlorination reaction of carbon tetrachloride by a zero-valent metal is a combination of the following two reactions:
M f M2+ + 2eCCl4 + H + 2e- f CCl3H + Cl+
(8)
M + CCl4 + H+ f M2+ + CCl3H + ClThe preceding discussion provides a useful framework in which to consider the results of experiments on the degradation of carbon tetrachloride by Sn and Zn in aqueous solutions (1, 2). Figures 2 and 3 show the Eh-pH equilibrium diagrams at 1 atm and 25 °C for the Sn-H2O-Cl and S0013-936X(96)00585-8 This article not subject to U.S. copyright. Published 1996 by the American Chemical Society.
the Sn-SnO2 stability boundary in Figure 2. Depending on the Eh-pH conditions, however, the aqueous Sn species SnOH+(aq), SnOOH+(aq), and Sn2+(aq) may also form (Figure 2). SnO2 is essentially insoluble and forms a coating on Sn metal that will resist corrosion perfectly in moderately acid, neutral, and slightly alkaline solutions free from oxidizing agents (3). In terms of Figure 2, therefore, the entire area of the diagram in which SnO2 is stable, Sn is perfectly passivated, i.e., its corrosion is prevented, evolution of H2(g) ceases, and solution Eh-pH conditions are no longer constrained to lie along the Sn/SnO2 boundary. The presence of CO2(g) in eq 1 indicates that CH4(g) was not stable in the aqueous solution and was oxidized according to the following reaction: FIGURE 2. Eh-pH diagram showing the fields of stability of species in the system Sn-Cl-H2O at 25 °C and 1 atm total pressure (Sn ) 1.0 × 10-4 M; Cl- ) 0.04 M). Note that SnO2(s) is stable over the entire pH range. The fields of predominance of aqueous Sn species are defined by the long dashed lines. Note that the invariant point assemblage of Sn2+/SnOH+/SnOOH+ fixes both Eh and pH at 0.4 V and 1.6, respectively. These predicted solution conditions are compatible with the stable coexistence of SnO2(s), CO2(g), and CHCl3, consistent with experimental observation (1). Lines (a) and (b) are as defined in Figure 1.
FIGURE 3. Eh-pH diagram showing the stability fields of species in the system Zn-Cl-H2O, at 25 °C and 1 atm total pressure (Zn ) 0.01 M; Cl- ) 0.04 M). Note that the invariant point assemblage Zn/ Zn2+/Zn(OH)2 fixes both Eh and pH at -0.8 V and 6.5, respectively. This assemblage is in the stability field of CH4(g) (Figure 1), the predicted and observed reaction product for complete dechlorination of carbon tetrachloride (eq 6 in the text). Lines (a) and (b) are as defined in Figure 1.
Zn-H2O-Cl systems, respectively. For calculation purposes, the molality of carbon in solution was assumed to be the same as the initial concentration of carbon tetrachloride (0.01 M). Also, the total chloride concentration was set at 0.04 M, assuming complete dechlorination of the carbon tetrachloride. The total concentration of zero-valent metal was always in excess of the molality of carbon tetrachloride (1). It should be noted, however, that the conclusions of this work are insensitive to the molality of species chosen for the calculations discussed here. Results of Calculations. The stability field of zero-valent Sn lies below that of water, indicating that metallic Sn should oxidize in the presence of water to produce SnO2 and evolve hydrogen gas, in accordance with eq 7. From Figure 1, it can be seen that CH4(g) and not CO2(g) is the stable form of carbon predicted to form at the Eh-pH conditions along
CH4(g) + 2H2O(l) ) CO2(g) + 4H2(g)
(9)
Passivation of the metallic Sn surface and subsequent loss of H2(g) would result in oxidation of the solution and lead to an increase in Eh from that defined by the Sn/SnO2 boundary, toward that of the CH4/CO2 boundary, and finally into the stability field of CO2(g). This also implies that solution pH will be acidic (Figure 1). One of the main reaction products remaining at the conclusion of the dechlorination reaction of carbon tetrachloride with Sn is CHCl3, indicating that complete dechlorination in the system CCl4/Sn/H2O did not occur. Figure 1 shows that the stability boundary for CCl4/CHCl3 lies at relatively oxidizing conditions in the field of stability of CO2(g), SnO2(s), and the aqueous Sn species SnOOH+(aq). These observations are consistent with an increase in Eh due to passivation of Sn metal. In the absence of measured solution species data, we may speculate as to the final solution Eh-pH conditions. If the aqueous Sn species SnOH+ is produced, as is predicted from thermodynamic considerations, then solution pH will tend toward the Sn2+(aq)/SnOH+(aq) boundary (Figure 2). Combined with loss of hydrogen due to the oxidation of CH4, which would continue to increase Eh, it is plausible that the solution Eh-pH conditions would equilibrate at the Sn2+(aq)/SnOH+(aq)/SnOOH+(aq) invariant point in the field of stability of SnO2(s), CO2(g), and CHCl3. This solution Eh-pH is also consistent with a measured final pH of between 1 and 2 (1). The triple point pH lies in this range (pH ) 1.6). The interpretation of the dechlorination reaction of carbon tetrachloride with zero-valent Zn is more straightforward than the analogous Sn system. Zn metal is unstable in aqueous solutions and will dissolve with the evolution of hydrogen gas in acid, neutral, and alkaline solutions (Figure 3). Zn metal, in the presence of moderately alkaline solutions (pH > 6.2 in Figure 3), can also become passivated by a film of a hydroxide, Zn(OH)2 (3). Figure 3 shows then that, as long as zero-valent Zn is present, it will continue to dissolve with the evolution of H2(g) and also form Zn2+, according to eq 7, and in the absence of any passivating film, i.e., at pH < 6.5 (Figure 3) will continue to dechlorinate carbon tetrachloride according to the general reaction given by eq 8. The products of the dechlorination of carbon tetrachloride in the system CCl4/Zn/H2O were ZnCl2(s), Zn(OH)2(s), and CH4(g) as opposed to CO2(g) in the analogous Sn experiments (1). The precipitation of ZnCl2(s) shows that the solubility product for this salt was exceeded
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during the dechlorination reaction with carbon tetrachloride. In Figure 3, the field of predominance of Zn2+ is shown as representing the formation of ZnCl2(s). At the conclusion of the experiment, the final solution pH was usually around 6.7. All of these observations are readily reconciled by the equilibrium relationships shown in Figure 3. The coexistence of Zn metal (present in excess), ZnCl2, and Zn(OH)2 constitutes an invariant point assemblage in this system and fixes both Eh and pH. For the concentration of Zn used to calculate stability relations in Figure 3 (0.01 M), the pH is fixed at approximately 6.5. This is in excellent agreement with a pH of approximately 6.7 reported by Boronina et al. (1). The solution Eh value defined by this triple point assemblage is at a relatively low oxidation potential of -0.8 V. The presence of CH4(g) as a product of the dechlorination of carbon tetrachloride involving Zn metal indicates that the dechlorination reaction has proceeded to the end point predicted by eq 6. These results also suggest that CH4(g) is the stable form of carbon at these conditions; i.e., 25 °C, 1 atm total pressure, solution Eh ) -0.8 V, and a carbon concentration of 0.02 M, consistent with thermodynamic relationships in the system C-O-H-Cl (Figure 1) and observations reported by refs 1 and 2. The formation of CH4(g) is favored by the relatively high fugacity of hydrogen generated at the solution interface with Zn:
C + 2H2(g) f CH4(g)
(10)
The preceding discussion shows that both metallic Sn and Zn have reduction potentials that lie below the field of thermodynamic stability of water at 25 °C and 1 atm total pressure (Figure 1). Both metals should, therefore, corrode in water with the evolution of H2(g). According to thermodynamic calculations, carbon tetrachloride should be spontaneously reduced and dechlorinated by both metals (eqs 6-9). At the reduction potentials (Eh’s) fixed by the Sn/SnO2 and Zn/Zn2+/Zn(OH)2 assemblages, the final product of the dechlorination reaction of carbon tetrachloride should be CH4(g), as both of these assemblages lie in the stability field of CH4(g) and not CO2(g) (Figure 1). The results of experiments by refs 1 and 2 in the system CCl4/Zn/H2O conform to this scenario. However, results of the dechlorination reaction in the system CCl4/Sn/H2O are more complex and indicate that carbon tetrachloride was only partially dechlorinated. Reasons for this may be attributable to the formation of a passivating film of SnO2, which acts as a kinetic barrier to further reductive dechlorination of carbon tetrachloride, and results in significant concentrations of intermediate dechlorination products remaining in solution (1). This is an area with a great deal of research potential, particularly if the present trend of applying zero-valent metal to the remediation of groundwater and soil systems continues. As demonstrated here and by ref 1, zero-valent
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Zn has several attributes that render it an ideal candidate for the application of this technology. It has a significant range of Eh-pH stability in which it is not passivated. It will, therefore, actively corrode in aqueous solutions and be able to reductively dechlorinate a wide range of haloorganic compounds including the common chloro and bromo alkanes and alkenes. The coexistence of Zn, Zn(OH)2, and Zn2+ will act as an Eh-pH buffer (until the supply of Zn metal is exhausted), which will help maintain stable conditions in most in situ groundwater and soil remediation applications. This is especially important if it is necessary to maintain a constant, near-neutral pH. Other transition metals such as Fe and Mn that have similar phase relations to Zn are predicted to behave in an analogous manner, but application to dechlorination of haloorganic compounds in groundwater and soil systems could be complicated by different phase relations and regions where Eh-pH conditions may result in passivation of the metals (3) or affect the reactivities of metal surfaces through fouling (4). Zero-valent Sn, however, is not suited to the application of this technology.
Acknowledgments I thank Drs. W. Davis-Hoover, S. Porter, J. A. Ryan, and L. Vane of the U.S. EPA/NRMRL and L. C. Murdoch, University of Cincinnati, for reviews and discussions that helped to enhance the clarity of this presentation.
Literature Cited (1) Boronina, T.; Klabunde, K. J.; Sergeev, G. Environ. Sci. Technol. 1995, 29, 1511-1517. (2) Assaf-Anid, N.; Loring, N. Contaminant Remediation With ZeroValent Metals. Preprints of Papers Presented at the 209th ACS National Meeting, Anaheim, CA, April 2-7, 1995; ACS: Washington, DC, 1995; Vol. 35, No. 1. (3) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; National Association of Corrosion Engineers: 1974. (4) Harms, E.; Lipczynska-Kochany, E.; Millburn, R.; Sprah, G.; Nadarajah, N. Contaminant Remediation With Zero-Valent Metals. Preprints of Papers Presented at the 209th ACS National Meeting, Anaheim, CA, April 2-7, 1995; ACS: Washington, DC, 1995; Vol. 35, No. 1. (5) Roine, A. Outokumpu HSC Chemistry for Windows; Outokumpu Research Oy: Helsinki, 1994. (6) Stull, D. R.; Westrum, E. F.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; John Wiley & Sons, Inc.: New York, 1969. (7) Vogel, T. M.; Criddle, C. S.; McCarty, P. L. Environ. Sci. Technol. 1987, 21, 722-736. (8) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry: John Wiley & Sons, Inc.: New York, 1993.
L. Taras Bryndzia U.S. Environmental Protection Agency National Risk Management Research Laboratory 5995 Center Hill Avenue Cincinnati, Ohio 45224 ES960585Q
