Environ. Sci. Techno/. 1995, 29, 1511-1517
Destruction of Organohalides in Water Using Metal Particles: Carbon TetrachloridelVVater Reactions with Magnesium, Tin, and Zinc TATYANA BORONINAt A N D KENNETH J. KLABUNDE* Department of C h e m i s m , Kansas State University, Manhattan, Kansas 66506
GLEB SERGEEV Department of C h e m i s m , Moscow State University, Moscow, Russia 119899
As a possible method for degrading chlorocarbons in contaminated water supplies, the reactions of metallic magnesium, tin, and zinc with CCIdH20 mixtures have been studied. In the case of Mg, oxidation by water overwhelmed the Mg-CC14 reaction. However, Sn and Zn were successfully used to degrade CC14. Major products in the Sn/CCIdH20 system were CO2, CHC13, SnOz, and HCI with smaller amounts of CHC13 and CH2C12. In the case of Zn/CCI$H20, the major products were ZnC12,Zn(OH)2, and CH4 with CHC13, CH2C12, and CH3CI as intermediate products. Thus, Sn and Zn behave quite differently with the final carbon-containing product, with Zn being CH4 but with Sn being COZ. This is rationalized by the competing reactions of a possible intermediate CI$MCI, which can be protonated by H20 to give CHC13 or eliminate CC12 (which subsequently reacts with water to form CO2 and HCI). Metal surface areas are also important, and the most active metal samples were prepared by a metal vapor-solvent codeposition method (SMAD cryoparticles). However, conventional Zn dust and Sn granules were also effective, onlywith lower reaction rates.
Introduction Background Literature. Gillham and O’Hannesin (1)and Reynolds and co-workers (2)have reported on the transformations of halogenated aliphatic compounds in the presence of metallic particles of several metals (especially transition metals). In particular, iron (steels), galvanized metals, aluminum, copper, and brass were studied. Their intriguing studies showed that in some cases over an 80% decline in the concentration of chlorocarbon occurred over, for example, 336 h. Galvanized metal (zinc)and mild steel consistentlygave the highest rates of degradation, and 50% reductions in chlorocarbon concentrations were sometimes found in as little as 45 min (3). These very encouraging results prompted our interest in exploring the underlying chemistry involved and in finding metal samples that would completely detoxify the chlorocarbon (rather than convert it to another toxic compound). In fact, several research groups have taken up this challenge as judged by very recent presentations (4-6). Herein, we describe some results utilizing Mg, Zn, and Sn metals. Earlier work has employed primarily zerovalent iron. At this time, it is not our intent to evaluate which metal system would be preferable from an economic or even environmental standpoint. Instead, our studies are aimed at gaining some chemical understanding at how different metals behave when reactingwith chlorocarbonl water mixtures. Further Rationale. The chemistry of metals reacting with chlorocarbons in aqueous solution is a rather new concept for chemists. In the past, it was generally thought that water would preclude such chemistry. Indeed, ever since the time of Frankland (7) and Barbier (81, the reactions of organohalides with metals have been of great interest and usefulness, but an important tenant of this work was to avoid water contamination at all costs, otherwise either the reactions could not be initiated or the products would be destroyed. However, as we have seen, recently a different thought process has been necessary. Due to long-term widespread use of chlorocarbons in commerce and industry, there are huge quantities of groundwater that are now contaminated bychlorocarbons. It is becoming clear that new approaches need to be developed for the decontamination of these water supplies, either in-situ or by pump-and-treat methods. Ultrafine metal particles offer a promising new approach to this problem. Their high reducing potential is a desired property since the contaminants (organohalides, nitro compounds, etc.) generally have reasonably high electron affinities. For example, we can consider the zinc/CCL reaction from a thermodynamic point of view:
2zn(s)
+ CC1,(1)
-
2ZnCl,(s)
+ C(s)
(1)
AH- = -702 kJ This is a very exothermic reaction. However, under the circumstances described, there are two problems: (1) this is a solidlliquid reaction, and so surface area and possibly t Visiting scientist from Moscow State University, Moscow, Russia, supported by the National Science Foundation.
0013-936X/95i0929-1511$09.00/0
c 3 1995 American Chemical Society
VOL. 29, NO. 6, 1995 i ENVIRONMENTAL SCIENCE & TECHNOLOGY
1511
morphology of the metal would play a major kinetic role, and (2) water will be present. Of course, water potentially changes reaction pathways in significant ways. Metals with high reducing potentials are often thermodynamically unstable in water, e.g.
AH- = -70 kJ The thermodynamics of such reactions is only one consideration. The reacting metal particles can form protective hydroxide and/or oxide layers that can inhibit even the most thermodynamically favorable processes. Thus, in order for a reaction to be successful, CCL must be able to penetrate this forming oxidelhydroxide layer and thereby continue to react at a rate higher than the metal can react with water itself. In addition, water could become involved in the chemistry secondarily as a medium for transport of intermediates or products away from reactive metal sites or by direct participation by providing protons. For example, we know that in organic solventscarbon-halogen bonds often oxidatively add to zero-valent metals (R-X M R-M-X, especially where M = Mg or Zn) (9). It is also known that CCL can behave similarly but that C13CMC1 usually is unstable, decomposing thermally to CC12+ MC12. In fact, compounds such as C ~ ~ C H ~ C are~ very H S useful as CC12 transfer reagents (10). Thus, it seems feasible that a short-lived intermediate such as shown in reaction 3 could be involved and that rapid protonation or carbene elimination could direct the reaction in different ways:
+
-
C1,CZnCl
+ H,O
-
+ HOZnCl - C02 + ZnC1, + 2HC1 C1,CH
(3)
With this background,we will now describe the initial results with Mg, Sn, and Zn reactions with CCLlH20 mixtures.
Experimental Section ReactantslSolvents. Distilled water; carbon tetrachloride, spectranalyzed grade (Fisher Chemical/Fisher Scientific); chloroform, 99.9% ACS, HPLC grade (Aldrich);methylene chloride, analytical grade (Fisher Chemical/Fisher Scientific); magnesium metal, ribbon, m-8 (Fisher Scientific); tin, metal, mossy, reagent ACS, Code 2390 (Baker and Adamson); tin, metal, granular, analyzed grade 0. T. Baker Chemical Co.);and zinc, metal dust (FisherScientific),were used to conduct the reactions between chlorocarbon and metal particles in water solution. Magnesium ribbon was cut into small pieces just prior to use, washed with acetone, and dried. Tin and zinc metal particles were used without any treatment. Surface areas for these metal samples were quite low ( < I m2/g). High Surface Area Metal Samples. In order to be able to compare extremely reactive metal powders with conventional samples, a metal vaporization procedure was employed. The method has been described elsewhere (11, 12) and has been called the SMAD method or the cry0 method. Synthesisof Zn and Sn Cryo-Particles. Cryo-particlesof Zn and Sn were prepared by codeposition under vacuum (1 x Torr) of the vapors of 2.5 g of the metal with 150 mL (liquid) of pure pentane at 77 K over a 1-2-h period. After completion of the codeposition, the pentane was pumped away under vacuo, and the cryo-particles were 1512
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6 , 1 9 9 5
collected and stored under argon gas. Surface areas are generally between 30 and 60 m21g. General Procedure for Reaction of Metal Particleswith Chlorocarbon and Water. Distilled water (100 mL) was placed into a 250-mL two-neck round-bottom flask, and argon was bubbled through it for at least 4 h, but usually overnight. Metal [2-5 g (0.08-0.2 mol) of magnesium, 1 or 10 g (0.008 or 0.08 mol) of tin, 1 or 5.2 g (0.015 or 0.08 mol) of zinc] was added under a flow of argon, and the flask was sealed immediately with a Teflon-faced silicon rubber septum. Chlorocarbon (100pL) was injected through the septum under the water level. The reaction mixture was magnetically stirred at room temperature. The reaction flask was covered to avoid chlorocarbon decomposition due to the light. When the experimentwith cryo-particleswas conducted, 1 g of tin or zinc cryo-particles was placed into the flask in a dry cabinet under argon. Then the flask was sealed with a septum and taken out; 100mL of distilled water saturated with argon (argon was bubbled through water overnight) was injected into the reaction flask through the septum, and 1OOpLof chlorocarbon was added with a syringe under the water level. Reaction headspace and water solution were sampled through the septa and analyzed by GC, GClMS, and FT-IR. Solid products [Mg(OH)2,Zn(OH),, Sn02]were analyzed by X-ray diffraction (XRD). Gas Chromatography/MassSpectrometry(GC/MS) and GCAnalyses. To identify gaseous and liquid products such as chloroform, methylene chloride, and methyl chloride in a single injection over the course of the reaction, GClMS analysis was performed. The reaction headspace was analyzed by a Hewlett Packard GUMS system (5890 Series I1 GCl5989A mass spectrometer) using an RTX-5 capillary column (Alltech, 5% diphenyl polysiloxane and 95% dimethyl polysiloxane),30 m x 0.25 mm i.d. x 0.25 pm film thickness. The column temperature was 35 "C, the injection port temperature was 230 "C, the flow rate was 30 mLlmin, and the split was 1:30. For each analysis, 1OOpLof reaction headspace was injected using a gas-tight syringe. The water solutions were analyzed by a Perkin-Elmer GUMS system (Auto System Gas ChromatographlQ-Mass 910 Mass Spectrometer). An HP-1 (cross-linked methyl silicon gum) capillary column, 1.2 m x 0.2 mm i.d. x 0.33 pm film thickness, was used. The analysis conditions for direct water injection were column temperature 90 "C, injection temperature 230 "C, flow pressure 10 psi, split, the AUX zone temperature 200 "C, the sample volume 1pL. Gas chromatographic analysis was used to observe chlorocarbon degradation and appearance ofliquid and gaseous products-chloroform, methylene chloride, dichloroethane, carbon dioxide, methane, and methyl chloride. The GC analyses were carried out on the Series 580 GOW-MAC isothermal gas chromatograph with thermal conductivity dector (TCD) and argon as the carrier gas. For these analyses, 200 pL of reaction headspace or 1 pL of water or pentane solutionwas injected. Agas-tight syringewas used for headspace injections. For headspace analyses of volatile chlorocarbons, an Alltech 2.4 m x 3.2 mm x 2.2 mm i.d. stainless steel, Tefloncoated column packed with 1%ATIM-lOOOon GraphpacGB, 60l80 mesh, was used. The column temperature was 115 "C, the injection port temperature was 200 "C, and the detector temperature and the current were 120 "C and 120 mA, respectively. The carrier gas flow rate was 30 mllmin.
The measurements of the concentration of carbon dioxide generated over the reaction were carried out using anAlltechTELON 1.8m x 3.2mmo.d. x 2.4mmi.d.column packed with Chromosorb 107,801100 mesh. The column temperature was 76 "C, injector temperature was 200 "C, and the detector temperature and current were 100 "C and 120 mA, respectively, while the flow rate was 30 mL/min. The appearance of methane was detected by a Carbosphere, 80/100 mesh column, Alltech, stainless steel, 3 m x 3.2 mm x 2.2 mm. The column temperature of 120 "C, the injector temperature of 200 "C, detector temperature of 140 OC, a detector current of 120 mA, and a flow rate of 40 mL/min were used for these measurements. Portions of the water layer were analyzed by extraction with pentane (22 mL of water and 3 mL of pentane), and portions of the pentane extract were analyzed using a 3.7 m x 3.2 mm 0.d. x 2.2 mmi.d. stainless steel column packed with 10% UCON-50-HB-5100on Chromosorb W-HP, 801 100 mesh. The column temperature was 90 "C, injector temperature was 200 "C, detector temperature was 120 "C, detector current was 120 mA, and the flow rate was 20 mLI min. fl-hfiared (PT-IR) Measurements. The infrared transmission spectra were recorded at 2 cm-' resolution and signal averaging of 64 scans using a Bio-Rad FT-IR spectrometer (FTS-40). A homemade IR-gas cell of 10 cm length and 1.5cm diameter, equipped with a stopcock, was used to obtain FT-IR spectra of the reaction headspace. The cell was evacuated to Torr, and the background spectrum was recorded. Then 2-20-mL portions of headspace gas were injected through the septum and the spectrum of headspace obtained. X-rayPowder Diffraction (XFtD)Analysis. X-ray powder diffraction data were collected on a Scintag-XDS-2000 automatic diffractometer (Cu KQ radiation) set at voltage of 40 kV and a current of 40 mA. The range 20-80" 2 0 and the scanning speed 2" 2 0 min-' were used to obtain the diffractometer trace for solid product identification. To measure the diffraction peak broadening due to the small cryo-particle size, scans ranged from 40" to 46" 2 0 for zinc and from 42" to 48" 2 0 for tin with the scanning speed 0.02 2 0 min-'. Immediatelybefore the measurement, the cryo-particles were mixed with an oil in an inert atmosphere and then taken out and loaded onto the XRD-sample cell at ambient conditions. The commonly accepted Scherrer formula was used to calculate crystallite size from the diffraction peak broadening. The zinc dust was used as an external standard. The size of the zinc cryo-crystalliteswas determined to be 400-450 A, and the size of tin cryo-crystallites was more than 2000 A. For analysis of solid products from the reactions, several consecutive isolation steps had to be performed. Floating solids were collected by filtration on a fritted filter funnel F. Then the liquid reaction mixture was shaken, causing more flocculent material to rise in suspension while metal particles again sank to the bottom. The suspended material was decanted, filtered off, collected, and dried on the F funnel. Lastly, the remaining, unreacted metal particles were collected on another filter frit under ambient conditions. Calibration Curves. A series of blank experiments were carried out: known amounts of COZ,CC4, or CHC13 were injected into pure water in the reaction flask (injectedbelow water level with stirring). Analyses of the headspace by GC
TABLE 1
Summary of Reactions Carried Out and Products reaction.
products found
+ Mg (ribbon) + H20 + S n (mossy)+ H20 CClp + S n (granular) + H20 CClp + Sn (cryo) + H20 CC14 + Zn (dust) + H20 CC14 + Z n (cryo, 360 A) + HzO CHCl3 + Z n (dust) + H20 CH2CIz + Z n (dust) + H20
Mg(OH)2; Hz; CHC13 trace CHC13; COz; HCI; SnOz (cassiterite) t h e same; CO trace t h e same; CHzCIz; CIH2C-CH2CI trace CHCI3; CH2C12; CH3CI; CHI; ZnClz; Zn(0H)z; CO trace t h e same; CIHzC-CH2CI CH2C12; CH3CI; CH,; ZnCI2; Zn(OH)2; CO trace CH3CI; CHI; ZnCIz; Z n ( 0 H h
CClp CC14
a 80-1OOpL of chlorocarbon and 8 x lo-* or 8 x in 100 mL of H20; under argon.
mol of metal
were carried out by extracting known volumes of gas and injecting into the GC. A series of such control experiments allowed construction of calibration curves that were used to determine gaseous concentrations of COz, CCL, and CHC13generated by the M/CC4/H20 reactions carried out.
Results The reactions studied were usually carried out at room temperature and are summarized in Table 1. Each metal/ CCL/HzO combination will now be described. MglCCLIH20. Analysis of the gaseous headspace of the reaction by FT-IR indicated the presence of CHC13and COz in addition to HzO and CCL. Interestingly, the Mg ribbon was transformed into a fine gray precipitate with the evolution of bubbles of gas (Hz). After 48 h at room temperature followed by heating for 2 h to about 40 "C, a great deal of fine, white precipitate was formed, and the pH of the solution was close to 10. Upon exposure of the reaction mixture to air, the remaining gray solid (fine Mg particles) was converted to more of the white precipitate [Mg(OH)zI. A blank experiment where no CC&was present indicated that the Mg/HZO reaction took place to form Mg(0H)Zand HZ,but at a slower rate, with the solution pH reaching about 8. The conclusions from the experiments are that the presence of CCL enhances the rate of the Mg/H20reaction, yielding Mg(0H)Z and Hz more quickly. Only a small amount of CCL was destroyed yielding CHC13. Sn/CC4/H*O. All three samples of tin including granular, mossy, and cryo-particles caused the decomposition of CCL over time. Chloroform, COZ,HCl, and solid SnOz (cassiterite) were the products formed. In this case, the pH of the solution went down to 1-2. The presence of C1in the aqueous solution was confirmed by precipitation with silver nitrate. Analysis of the headspace by GC, GUMS, and FT-IR (Figure 1) confirmed that the major gaseous product was CO2. The highly reactive cryo-particles caused the formation of CHzClz and CzH4C12 as well. Analysis of the aqueous phase by GUMS also showed the presence of these same products (COz,CHC13, CHzClz,and CzH4C1z). In order to evaluate reaction rates, the headspace was analyzed by GC with time. Chlorocarbon and COZconcentrations were calculated with the aid of a series of calibration curves (see Experimental Section). VOL. 29, NO. 6 , 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1513
co
I
il
100
'
95
r
; 90 n 85
f
;
80
'I
7
4000
3500
3000
2500 2000 Uavenunbers
IS00
1000
500
FIGURE 1. FT-IR of the headspace gas generated by the SnICCIJH20 reaction. Note the large COz band at 2348 cm-' and the absence of the CHo band at 3013.
0 012 n
E,
A
CCI,
A
A i
0010
. I
N -
zj 0.008 IN
2 ijl 0 r 2
+
0,
A
0006
0004
CHCI, 0 002
0000
'
3 .
CHCI,
I
6e-6
1
0
50
100
200
150
250
300
I
n
r,
5e-6
time, [min] FIGURE 3. Degradation of CC14 by different Sn patticles in water: ( 0 )Sn (cryo); (A)Sn (granular). Curves drawn are simply to aid the eye.
. i
0.012
t,71,
n
E,
.,
0.010
n N
0008
i
IN
2
5
0 I 0
20
40
60
80
100
120
time, [hrs] FIGURE2. Appearance of COZduring the SnICCIJH20 reaction. Molar concentrations are given in exponential form. Curves drawn are simply to aid the eye.
2
0006
0004 0 002
5 0
2
0.000
I
Figure 2 compares the rate of C02 generated by tin (granular vs mossy). Both the samples exhibited slow COz generation over a 100-h period with granular Sn being the more active. In contrast, cryo-tinparticleswere much more reactive (Figure 3) such that about 40% of the CC4 was destroyed over a 3-h period. Both C02 and CHC13 were generated much more quickly in the presence of cryo-tin, and small amounts of CH2C12were also detected. Results with tin show that the surface area of the metal is very important for reactivity, and interestingly C02 is the major gaseous product. Zn/CC4/H20. Zinc proved to be the most promising metal, and so degradation of CC4 and CHC13 was studied over Zn dust and cryo-zinc particles. Carbon tetrachloride degraded rather quickly, forming CHC13 and CH2C12. Eventually, these products were also degraded and formed CHSC1, CH4, traces of CO, and zinc salts. Chloride ion was present in large amounts [ZnC12(aq)], and Zn(OH12 formed as a very fine, white solid (as identified by XRD). Figures 4 and 5 show degradation patterns for CC14 and CHC13with time. Gaseous products were identified by a variety of methods: CH3Clby GUMS, CO by FT-IR, and CH4 by GC and FT-IR. It was possible to observe the appearance of 1514 rn ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6. 1995
CH,CI,
0
50
,
1
I
,
100
150
200
250
300
time, [min] FIGURE 4. Degradation of CC14 by Zn dust in water. Curves drawn are simply to aid the eye.
intermediate products such as CHC13 and CH2C12. For example, in Figure 4, CC4 was degraded quickly, and the maximum CHC13 concentration occurred when about 2030% of the C C 4 remained. At that point, the degradation of CHC13began to exceed its production rate. After about 4 h, only a small amount of CCl, and CHC13 remained, and 33-38% CHZC12 (percent of starting CC4) was observed. These results indicate that CH2Cl2 is of lower reactivitythan CCl4 and CHC13. When CHC13 was the starting material, it was almost completely degraded in about 3 h, and by this time 6065% CH2C12 appeared. It was possible to follow this apparent stepwise degradation of CCb by FT-IR. In Figures 6 and 7, a series of spectra show the disappearance of CC4 and the gradual appearance of CH4. The spectra also suggest the appearance and then the disappearance of CHC13 along with CH2Cb appearance (CH3Cl could not be observed by this method). In particular the following
., 5
CHC'3
0.014
CCI,
0,012
. I
c,
5
0.010
2
0.008
C Q
0
8 0,006 5 I" 0.004 0 '0 0.002
c 0
0.000
CH,CI,
3 0
20
40
80 100 120 140 160 180 200 220
60
time, [min]
w
FIGURE 5. Degradation of CHC13 by Zn dust in water. Curves drawn are simply to aid the eye.
w 3000
2200
22.5 hr I'
I400
600
W A V E N U M B E R S (cm") f
601
'1
FIGURE 7. FT-IR of the headspace gas generated by the ZnICCIJHzO reaction with time. Note progressive appearance/disappearance of CHCIJ, CHZCII, and CH4.
10 1" *-
40W
I 1
BW
I
3000
I
2600
I
1
I
I
2opO
1500
IOW
600
Ulvarmlbare
FIGURE 6. FT-IR of the headspace gas generated by a long-term (27 h) reaction of ZnICCIJHZO. Note the absence of COZ(2348cm-') and the large amount of CHI (3013 cm-'1.
adsorptionscould be monitored in this way CCL 773 cm-l; CHC13, 1215 cm-l; COZ,2348 and 666 cm-l; H20, 12502000 cm-'; CH2C12,1262-1279 and 746 cm-I; and CH4, 3013 and 1303-1342 cm-'. The CCh and CHC13 bands essentially disappeared while CH2C12and CH4 remained. In additional experiments, it was shown that CH2C12 degradation was slower than CCL and CHC13 (Figure 8). This results suggest a sequential degradation process. The rates of degradation of these compounds were quite dependent on metal surface area (Figure 9). Thus, 5 g of Zn dust caused much more rapid degradation than 1 g and also shortened the induction time before the onset of product formation. Cryo-zinc particles gave the best results (Figure 9). Extremely high reactivity was observed initially, and after about 1 h the rate of CC4 degradation slowed down somewhat.
Discussion Choice of Metal and Metal Morphology. The data clearly show that the choice of metal is very important. As might be expected, high surface area is beneficial. In addition,
nanocrystalline, high surface area metal particles are best, and these observations are also not surprising considering the substantial body ofliterature published on the chemistry of "activated metals" (13,141. However, what is particularly intriguing,regarding the use of such metal particles in aqueous media is that a balance must be struck between reactivity with water vs chlorocarbon. In the case of Mg, oxidation of the metal by water tends to defeat its high reactivity with the chlorocarbon. Therefore, tin and zinc appear to be better choices since their reactivitywith water is lower, and yet they still are sufficiently reactive with the chlorocarbons. Of course, the chemistry involved is complex. Metal hydroxide andlor oxide that forms on the surface of the metal tends to protect it from further water oxidation. In the best scenario, the chlorocarbon would still be capable of attacking the metal even in the presence of this protective hydroxideloxide coating. These considerations suggest that kinetic parameters will be the controlling features. On the other hand, perhaps thermodynamics can aid in at least making rough predictions of reactivity and selectivity. Table 2 lists a few AHf" values for metal chlorides and metal hydroxides. As was pointed out in the Introduction, perhaps more favorable AHfo for the metal chlorides is important. Thus, if there is any validity to this type of comparison, it might be predicted that Mg, Ca, Fe, Ni, Zn, Cd, and Pb could possibly react with CCb competitively with water. However, to carry this argument further, the AH," for CC4 and H20 should be compared for reactions VOL. 29, NO. 6 , 1 9 9 5 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1515
-
0.012
c"
0008
Zn(dust,lg)
0010
E, '
0
. I
c1
5 Y
0.006
+
C
0.004
8
A
'Zn(dust,Sg)
i
Q,
-*
e
0,002
0 0 0.000
I 0
50
100
150
200
250
time, [min] FIGURE 9. Degradation of CCI, by Zn patticles of different surface areas due to the amount of Zn dust added and the use of cryo-zinc. Curves drawn are simply to aid the eye. TABLE 2
Standard Heats of Fonnation of Selected Metal Chlorides and Oxide9 2200
3000
difference A/-( chloride (kJ/mol) A$ oxide (kJ/mol) (chloride - oxide)
600
1400
w A v E N u M B E R S (an-')
Mg
FIGURE 8. FT-IR of the headspace gas generated by the Zn/CH2CI/H20 reaction with time. Note the disappearance of CHZCIZand the appearance of CH4.
such as 4-6. Of course, reaction 4 would not occur in
+ CC1,(1) - 2ZnClz(s) + C(s) Zn(s) + H,O(l) - ZnO(s) + H,(g) Zn(s) + 2Hz0(1) Zn(OH)(s) + H,(g) 2Zn(s)
,
-
Ca
Ti Cr
Fe Ni
(4)
(5)
CU Zn Cd
Ge (6)
water, but it could be used to evaluate the exothermicity of ZnClz formation. This really comes down to looking at the difference in AHf" of the metal chlorides minus AHp(CC&) and the metal oxides minus AHfo(H,O),and so the trends from Table 2 would not change. What would be more helpful would be to look at AHf" of the hydroxides, but few of these are reported in the literature. It seems pointless to try to carry these considerations further until complete product analyses can be carried out and thermodynamics of aqueous species can be considered. Dominant Chemical Reactions. Perhaps the most striking finding from this work is that the chemistry of the MICC14IH20 system can change dramatically depending on the metal. The two metallic elements that were successful in degrading CCl, over time were Sn and Zn. However, the dominantfinal product in the case of SnICCld H 2 0 was CO2, while the dominant final product in the case of ZnICC1dH20 was CH,. Obviously, these products are "complete opposites" in terms of oxidation vs reduction. How can this be rationalized? With Zn, the stepwise production of CHCl3, CHZC12, and eventually CHI was rather evident by following product 1516 a ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6, 1995
Sn
Pb
-641 -795 -804 (Tic141 -514 (TiC12) -557 (CrC13) -400 (FeC13) -342 (FeC12) -305 (NiC12) -137 (CUCI) -220 (CuCI2) -415 -392 -532 -511 (SnC14) -325 (SnC12) -359 (PbC12)
-602 -635' -944 (Ti021 -519 (TiO) - 1 140 0 2 0 3 ) -824 (Fez031 -272 (FeO) -489 (Ni203) -169 (CUZO) -303 (CUO) -351 -258d -580 -577 (Sn02) -281 (SnO) -217 (PbO)
-39 -160 $140 +5 +I3 +12c -70 -60' - 53c $83 -64 -134 +48 +66 -44 -142
a Linde, D. R., Ed.; CRC Handbook of Chemistry and Physics, 75th ed., CRC Press: Boca Raton, FL, 1994; pp 5-4-5-30. Ca(OH)2 AHfo = -985 kJ/mol. AHf"for Cr203/2 = -570; for Fez03/2 = -412; for Ni203/2 = -245; for Cu20/2 = -84.5. Cd(OH)2AHf' = -561.
formation with time. Therefore, it seems reasonable to propose a multistep reduction sequence as shown in Figure 10. Here, CCl, oxidatively adds to Zn forming C13CZnC1, a reactive intermediate that would be susceptible to rapid protonation by water. In a similar way, CHC13,CH2C12,and CH3Cl could be attacked and protonated. Overall, a balanced equation for complete reaction could be written as follows: 4Zn(s)
+ CC1,(1) + 4H20(1)- ZZnCl,(aq) +
2Zn(OH),(s) + CH,(g)
Note that no HC1 is formed according to this equation. This seems reasonable since the pH of the solution did not fall very much (pH - 6 after reaction). Also, the formation of ZnC12(aq) would explain the copious amounts of C1precipitated by the addition of AgN03. Zn(OH12 was also found as a solid, fine precipitate.
M + CC14 C13C-H
-
C13CMCI
“Y
+
HOMCI
\ C12C:
IM
MCI2
t
1
H C12C-M-CI
+
CI I
MO;MC12
I
H
1
CI ‘.C=O
i
b l
CHA _.
-7
+ HCI
I
C02 + 2 HCI
FIGURE 10. Proposed reaction intermediates in the M/CCIdhO reactions (where M = Zn, Sn) that help explain product differences with Zn and Sn.
With Sn, a similar reaction could begin the sequence (Figure 10). Oxidative addition of CC& to Sn could yield C13CSnC1. This species would also be somewhat susceptible to protonation by water, but it would be expected that this would be less favorable since Sn-Cl bonds are less ionic than Zn-C1 bonds. If the protonation of C13CSnC1is slower,then an expected decomposition pathway might dominate, that of CC12 elimination. If this happened the reaction of CClp with water would be expected to beveryfast, forming anunstable alcohol that would eliminate HCl and ClZCO (phosgene) (15). However, phosgene would be susceptibleto hydrolysis by excess water in the same way that other organic acyl chlorides are. The final products would indeed be COZ and more HC1. An overall balanced equation might be written as follows: Sn(s)
+ CC1,(1) + 4H20(1)- Sn02(s)+ 4HCl(aq) + Cop(@
+ 2H2(@
This reaction is supported by the observation of SnOz and Copand the development of low pH (between 1 and 2 after reaction). We have not yet confirmed the presence of hydrogen. Of course, these balanced equations are idealistic. In the tin system, small amounts of CHC13 and CH2Clz were also observed, and occasionally two carbon products such as C2H4ClZ were also detected. Although a complete mass balance study remains to be completed and it is possible that other (undetected) products exist, it seems that the formation of C13CMC1 as an initial reactive intermediate is a reasonable working hypothesis. The competition between protonation by water or elimination of CCh can be used to rationalize product mixtures observed. Further work may delineate if other reaction pathways exist, for example, electron transfer processes involving radical anion intermediates.
Conclusions (1) Sn (mossy, granular, cryo-particles) causes the degradation of CC&in water solutions with CHC13, Sn02,CO2, and HCl as the main products.
(2) Zn (dust, cryo-particles) is more reactive than Sn and causes the degradation of CC&,CHC13, and CHpClp in water solutions. The final products are CH3C1, CH4, ZnCh and Zn(OH)2. (3) The different products found in the Sn and Zn reactions possibly indicate different mechanisms of the chlorocarbon degradation. (4)The ability of Zn and Sn particles to decompose the chlorocarbons depends on the quantity of the metal and its surface properties and increases in the order: Sn (mossy) < Sn (granular) < Sn (cryo-particles) < Zn (dust) < Zn (cryo-particles). (5) The metal particles obtained by the SMAD method (cryo-particles) allowed the shortest induction time and the most rapid degradation of chlorocarbons.
Acknowledgments The partial support of the National Science Foundation and the Army Research Office is acknowledged with gratitude. Partial support of the Hazardous Substance Research Center (through the U.S. Environmental Protection Agency under Assistance Agreement R-815709 to Kansas State University) is also acknowledged.
Literature Cited (1) (a) Gillham, R. W.; O’Hannesin, S. F. Ground Water 1991, 29, 749. (b) Gillham, R. W.; O’Hannesin, S. F. Ground Water 1994, 32, 958. (2) Reynolds, G. W.; Hoff, J. T.; Gillham, R. W. Enuiron. Sci. Technol. 1990, 24, 135-142. (3) Gillham, R. W.; O’Hannesin, S. F. Paper presented at the IAH Conference Modern Trends in Hydrogeology,Hamilton, Ontario, Canada, May 10-13, 1992. (4) (a)Helland, B. R.; Schnoor, J. L.; Alvarez, P. J.AbstractsofPapers; 9th Annual Conference on Hazardous Waste Remediation, June 8- 10,1994,Montana State University,Bozeman, MT Hazardous Waste Remediation Center: Manhattan, KS, 1994;Abstract 45. (b) Matheson, L. J.; Tratneyek, P. G.Environ. Sci. Technol. 1994, 28, 2045. (5) (a) Betterton, E. A.; Warren, K. D.; Arnold, R. G. Abstracts of Papers; 9th Annual Conference on Hazardous Waste Remediation, June 8- 10,1994,Montana State University, Bozeman, MT; Hazardous Waste Remediation Center: Manhattan, KS, 1994; Abstract 12. (b)Lipozynska-Kochany,E.; Harms, S.; Miburn, R.; Sprah, G.; Nadarajah, N. Chemosphere 1994, 29, 1477. (6) (a) Boronia, T.; Klabunde, K. J. Abstracts ofpapers; 9th Annual Conference on Hazardous Waste Remediation, June 8- 10,1994, Montana State University, Bozeman, MT; Hazardous Waste Remediation Center: Manhattan, KS, 1994; Abstract 14. (b) Schreier, C. G.; Reinhard, M. Chemosphere 1994, 29, 1743. (7) Frankland, E. J. Chem. SOC. 1849, 2, 263. (8) Barbier, P. J. Chem. SOC. 1899, 76, 323. (9) March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure; McGraw Hill: New York, 1968; pp 684,687, and references cited therein. (10) Elschenbroich, C.; Salzer, A. Organometallics, 2nd ed.; VCH Publishers: Weinheim, Germany, 1992; p 54. (11) Klabunde, K. J.; Murdock, T. 0. J. Org. Chem. 1979, 44, 3901. (12) (a) Klabunde, K. J. Chemistry of Free Atoms and Particles; Academic Press: San Diego, 1980. (b)Wabunde, K. J. FreeAtoms, Clusters, and Nanoscale Particles; Academic Press: San Diego, 1994. (13) Cintas, P. ActivatedMetakin OrganicSynthesis; CRC Press: Boca Raton, FL, 1993. (14) Rieke, R. D. Acc. Chem. Res. 1977, 10, 301. (15) See Criddle, C. S.; McCarty, P. L. Environ. Sci. Technol. 1991,25, 973 for a somewhat similar formation/decomposition sequence for phosgene.
Received for review August 3, 1994. Revised manuscript received February 23, 1995. Accepted March 6, 1995.@
ES940492D @Abstractpublished in Advance ACS Abstracts, April 15, 1995.
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