Reaction between chlorocarbon vapors and sodium carbonate

Article abstract of DOI:10.1021/es980077b

The kinetics of the reactions between tetrachloromethane (CCl₄), 1,2- dichloroethane (C₂H₄Cl₂), or chlorobenzene (C₆H₅Cl) and sodium carbonate were investigated using evolved gas analysis-Fourier transform infrared spectroscopy. Sodium carbonate reacted with CCl₄ between 600 and 900 K to form over 90% carbon dioxide (CO₂) and less than 10% tetrachloroethene (C₂Cl₄). This reaction followed the three-dimensional diffusion mechanism and had an activation energy of 105 ± 10 kJ/mol and a steric factor of 5000 ± 3000 min^-1. The reaction between C₂H₄Cl₂ and sodium carbonate produced CO₂, ethanal (C₂H₄O), water (H₂O), vinyl chloride (C₂H₃Cl), ethene (C₂H₄), and ethyne (C₂H₂) between 600 and 900 K from at least two different pathways. The product temperature profiles indicated that CO₂, C₂H₄O, and C₂H₃Cl were formed initially and that approximately 10% of the product is C₂H₄ at 900 K. The reaction kinetics followed the Ginstling- Brounshtein diffusion mechanism and had an activation energy of 100 ± 10 kJ/mol and a steric factor of approximately 10⁴ min^-1. Benzene was produced from the reaction between chlorobenzene and sodium carbonate at temperatures above 800 K. This reaction followed the three-dimensional diffusion mechanism and had an activation energy of 80 ±10 kJ/mol and a steric factor of approximately 500 min^-1. The kinetics of the reactions between tetrachloromethane (CCl₄), 1,2-dichloroethane (C₂H₄Cl₂), or chlorobenzene (C₆H₅Cl) and sodium carbonate were investigated using evolved gas analysis-Fourier transform infrared spectroscopy. Sodium carbonate reacted with CCl₄ between 600 and 900 K to form over 90% carbon dioxide (CO₂) and less than 10% tetrachloroethene (C₂Cl₄). This reaction followed the three-dimensional diffusion mechanism and had an activation energy of 105 ± 10 kJ/ mol and a steric factor of 5000 ± 3000 min^-1. The reaction between C₂H₄Cl₂ and sodium carbonate produced CO₂, ethanal (C₂H₄O), water (H₂O), vinyl chloride (C₂H₃Cl), ethene (C₂H₄), and ethyne (C₂H₂) between 600 and 900 K from at least two different pathways. The product temperature profiles indicated that CO₂, C₂H₄O, and C₂H₃Cl were formed initially and that approximately 10% of the product is C₂H₄ at 900 K. The reaction kinetics followed the Ginstling-Brounshtein diffusion mechanism and had an activation energy of 100 ± 10 kJ/mol and a steric factor of approximately 10⁴ min^-1. Benzene was produced from the reaction between chlorobenzene and sodium carbonate at temperatures above 800 K. This reaction followed the three-dimensional diffusion mechanism and had an activation energy of 80 ± 10 kJ/mol and a steric factor of approximately 500 min^-1.

Full text of DOI:10.1021/es980077b

Environ. Sci. Technol. 1999, 33, 1691-1696
systems (8, 9). Because sodium oxalate thermally decomposes
Reaction between Chlorocarbon
Vapors and Sodium Carbonate
to form sodium carbonate in this temperature region (10,
11), sodium carbonate may have contributed to the reaction
and may also react with chlorofluorocarbons in this tem-
perature range. Molten sodium carbonate has been reported
to react with chlorocarbons (12) and to remove fluorine from
dimethylperfluoro-3,6-dioxa1,7-1,8octanedioate (13).
Thermodynamic calculations for selected reactions be-
tween sodium carbonate and tetrachloromethane (CCl4),
trichloromethane (CHCl3), 1,2-dichloroethane (C2H4Cl2), or
chlorobenzene (C6H5Cl) are presented in Table 1. The
aliphatic chlorocarbon reactions have very favorable Gibbs
free energies of reaction at 298 K (∆rxnG(298)) and at 750 K and
should react nearly completely once the reaction is initiated.
J . W . P A R R E T T , J R . , J . P . S U M N E R , A N D
T . C . D E V O R E *
Department of Chemistry, James Madison University,
Harrisonburg, Virginia 22807
The kinetics of the reactions between tetrachloromethane
(CCl4), 1,2-dichloroethane (C H Cl2), or chlorobenzene (C H -
2
4
6 5
Cl) and sodium carbonate were investigated using evolved
gas analysis-Fourier transform infrared spectroscopy.
Sodium carbonate reacted with CCl4 between 600 and 900
∆
rxnG(750) is not favorable for chlorobenzene unless oxygen
is present. The reaction in oxygen is favorable, but this
reaction would probably require incineration-type temper-
atures to completely destroy the chlorobenzene. Because
alkaline substances are already used in the petroleum
industry as HCl scavengers to reduce acidic emissions (14),
they would have the advantage of also removing the HCl
that is produced during the catalytic combustion of many
chlorocarbons.
Because the thermodynamics of these reactions are
favorable, the kinetics will determine the effectiveness of
these processes for destroying chlorocarbon vapors. The
kinetics of the reactions between sodium carbonate and
tetrachloromethane, 1,2-dichloroethane, or chlorobenzene
have been investigated using evolved gas analysis-Fourier
transform infrared spectroscopy. The products formed and
the Arrhenius parameters were established for each reaction.
K to form over 90% carbon dioxide (CO ) and less than
2
10% tetrachloroethene (C Cl4). This reaction followed the three-
2
dimensional diffusion mechanism and had an activation
energy of 105 ( 10 kJ/ mol and a steric factor of 5000 (
3000 min-1. The reaction between C H Cl2 and sodium
2
4
carbonate produced CO , ethanal (C H O), water (H O), vinyl
2
2
4
2
chloride (C H Cl), ethene (C H ), and ethyne (C H ) between
2
3
2
4
2 2
600 and 900 K from at least two different pathways.
The product temperature profiles indicated that CO , C H O,
2
2 4
and C H Cl were formed initially and that approximately
2
3
10% of the product is C H at 900 K. The reaction kinetics
2
4
followed the Ginstling-Brounshtein diffusion mechanism
and had an activation energy of 100 ( 10 kJ/ mol and a steric
factor of approximately 104 min-1. Benzene was produced
from the reaction between chlorobenzene and sodium
carbonate at temperatures above 800 K. This reaction followed
the three-dimensional diffusion mechanism and had an
activation energy of 80 ( 10 kJ/mol and a steric factor of
Experimental Section
Reagents. The tetrachloromethane, 1,2-dichloroethane, and
chlorobenzene were Fisher ACS. The sodium carbonate was
from Mallinckrodt. All chemicals were used as received.
Apparatus. The apparatus used in this investigation has
been described in detail previously (15-17). Briefly, it was
constructed by attaching KBr windows to two opposing arms
of a stainless steel four-way cross-vacuum flange (MDC, Inc.).
One of the remaining arms was connected to the vacuum
line and the other was attached to a 25-cm-long piece of 9
mm glass tube that served as the reactor. The open end of
this tube was attached to the chlorocarbon container. The
flow of the chlorocarbon into the reaction vessel was
controlled using a tetrafluoroethene needle valve, and the
intensity of the IR bands provided a convenient measurement
of the amount of chlorocarbon flowing through the cell. The
IR absorbance was measured for the chlorocarbon vapor in
equilibrium with the liquid at 298 K. The ratio of the IR
absorbance during the experiment to this absorbance
multiplied by the known vapor pressures at 298 K (15 kPa
for CCl4, 11 kPa for C2Cl4, and 1.6 kPa for chlorobenzene)
(18) gave the chlorocarbon vapor pressure during the
experiment. A 12 mm Vycor tube was wrapped with Nichrome
resistance heating wire and slipped over the reactor to heat
the sample. A Perkin regulated power supply provided the
power for this furnace. The temperature of the sodium salt
bed was measured using a chromel-alumel thermocouple
connected to a Keithly digital thermometer. The temperature
was measured to ( 3 K over the temperature range (298-900
K) that could be obtained using this apparatus.
approximately 500 min-1
.
Introduction
Chlorocarbons are widely used in industry as solvents, have
a high stability, and tend to accumulate in the environment.
Investigations into the toxicity and the carcinogenic proper-
ties of these compounds have raised awareness about the
need to find ways to properly dispose of these compounds
(1). Incineration of chlorocarbons in high-temperature
furnaces is currently the most commonly used disposal
method, but it is expensive because it requires temperatures
in excess of 1000 K to ensure complete combustion of the
material (2, 3). Fuel costs for incineration can be 40% of the
total cost of the plant operation (4). Incomplete combustion
can also generate undesirable products which complicate
the remediation effort. Consequently, there is considerable
interest in developing a lower temperature method that will
effectively destroy the chlorocarbons without generating
incomplete combustion products (5). Two possible methods
have been advanced. The more thoroughly investigated
method is catalytic combustion of the chlorocarbons (5-7).
While several metal oxide or supported metal oxide catalysts
have shown promise for remediating a variety of chloro-
carbons, a general catalyst that will remediate all chloro-
carbons has not yet been found. The second method
suggested is to react the chlorocarbon with alkaline salts.
For example, sodium oxalate is reported to react quantita-
tively with chlorofluorocarbons at 550 K in circulating bed
IR spectra were obtained using a Nicolet Magna-750 FTIR.
This spectrometer collected and stored one spectrum every
two seconds for reaction times up to 1 h. Spectra were
obtained from 4000 to 400 cm-1 with 4 cm-1 resolution. The
9
10.1021/es980077b CCC: $18.00
Published on Web 04/02/1999
1999 Am erican Chem ical Society
VOL. 33, NO. 10, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1 6 9 1

FIGURE 1. Infrared spectra observed for the reaction between Na2CO and CCl4. The reaction temperature is given in the figure. The initial
3
CCl4 pressure was 5 kPa. The scale is expanded to show the product features.
TABLE 1: Thermodynamics (in kJ/mol) for the Most Likely Reactions Between Some Sodium Salts and Some Simple Aliphatic
Chlorocarbons (Calculated Using Data from Ref 21 and 22)
reaction
∆
rxnH
∆
rxnG
∆rxnG
750
298
298
2Na2CO3 + CCl4 f 4NaCl + 3CO2
-462
-684
-121
-31
+112
+751
-2642
-570
-836
-173
-122
+86
+697
-2673
-734
-1068
-252
-261
+47
+615
-2719
3Na2CO3 + 2HCCl3 f 6NaCl + 4CO2 + H2O + C
Na2CO3 + C2H4Cl2 f 2NaCl + CO2 + C2H4O
Na2CO3 + C2H4Cl2 f 2NaCl + 2NaOH + CO2 + CO + H2O + 2C
3Na2CO3 + C6H5Cl f NaCl + 5NaOH + + 2CO2 + 7C
8Na2CO3 + C6H5Cl + 5H2O f NaCl + 15NaOH + 7CO2 + 7C
3Na2CO3 + C6H5Cl + 7O2 f NaCl + 5NaOH + 9CO2
ments model a flow reactor and were done to determine the
Arrhenius parameters for the reaction.
TABLE 2: Products Observed for the Reactions of Simple
Chlorocarbons with Sodium Carbonate in the Flow System
(Relative Concentrations for <5 kPa Initial Hydrocarbon
Pressure at 900 K)
The second (constant temperature procedure) was used
to determine the order of the reaction. After the reactor was
loaded with the sodium carbonate, the cell was evacuated
and the sodium carbonate was heated to the desired
temperature. The cell was isolated from the vacuum and
filled with chlorocarbon vapor. The IR was used to monitor
the concentration of each species in the cell as the reaction
proceeded, which provided information about the order of
reaction. These experiments model the reactions in a batch
reactor or a circulating flow reactor.
solid
chlorocarbon
temp (K)
gaseous products
products
CCl4
C2H4Cl2
800-900 CO2 (.92); C2Cl4 (.08)
525-900 CO2 (<.60); C2H4 (.10)
C2H4O (.20); C2H3Cl (.10)
C2H2 (<.02)
NaCl, C
NaCl, C
C6H5Cl
800-900 C6H6 (∼1)
Analyses of both experiments use the extent of reaction
(R) (19). Here Ris a unitless parameter that can be determined
from the absorbance of the chlorocarbon by using eq 1.
Nicolet-Aldrich spectral library was used to help identify
the evolved molecules.
The composition of the residue was determined using
powder X-ray analysis. The ASTM X-ray library was used to
help identify the diffraction patterns.
R ) 1 - A / A0
(1)
t
where A is the absorbance at time )t and A is the absorbance
Procedure. Two procedures were used during this in-
vestigation. In the first (variable temperature procedure),
0.1 to 0.5 g of sodium carbonate was placed in the center of
the reaction tube and held in place with plugs of glass wool.
The cell was evacuated to a pressure of ∼0.1 Pa. The flow of
chlorocarbon was started and pumped through the cell at a
constant rate. The sodium carbonate was heated at a constant
rate (4-10 K/ min for most experiments) for reaction times
up to 60 min while the FTIR monitored the spectra of the
evolved gases as the reaction proceeded. No carrier gas was
used in these experiments. Initial chlorocarbon pressures
between 0.5 and 5 Pa. (as monitored by the IR spectrum of
the chlorocarbon) were used for each sample. These experi-
t
0
at time ) 0. The rate expressions in terms of R tabulated by
Brown (19) were used to determine the kinetic parameters
for the reaction.
Beer’s law was used to establish the partial pressures of
all products produced during the reaction. The molar
extinction coefficient must be known in order to obtain exact
values using this method. Because these values are not known
for all of the compounds observed, semiempirical molecular
orbital calculations (PM3 and AM1) were used to estimate the
values of the molar extinction coefficients. Because the
calculated values are not exact, this procedure produced
concentrations that are estimated to be good to (10%.
9
1 6 9 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 10, 1999

FIGURE 2. The fraction of the CCl4 converted to products and the
fraction of C Cl4 formed for the reaction between Na2CO and CCl4
2
3
as a function of temperature in the flow experiment. The remaining
product was CO . The initial CCl4 pressure was 5 kPa.
2
Results and Discussion
CCl4/Na2CO3. Samples of the IR spectra obtained from the
variable temperature experiments for the reaction between
Na2CO3 and CCl4 are presented in Figure 1. CO2 and C2Cl4
were identified in the evolved gases, and NaCl was identified
in the XRD of the residue. The extent of the reaction depended
somewhat on the amount of Na2CO3 present and the pressure
of the CCl4, but less than 40% conversion was obtained from
a single pass for most experiments. The relative amount of
C2Cl4 increased with temperature, but it was still less than
10% of the product at 900 K. (see Figure 2).
FIGURE 3. The integrated rate plot obtained using the infrared
absorbance of CCl4 from the reaction between Na2CO and CCl4 as
3
a function of time in a closed cell experiment. The temperature was
held constant at 825 K. The linear relationship observed for the
function [1- (1-r)1/3]2 indicated that the reaction followed the
three-dimensional diffusion mechanism.
Greater extents of reaction were obtained for CCl4 in the
constant temperature experiments, and the conditions could
be adjusted so that CO2 was the only product observed in
these experiments. Of the 15 possible equations that Brown
gives to model possible reaction types (18), the best fit for
the absorbance versus time data for the disappearance of
the CCl4 was obtained using the three-dimensional diffusion
model (19, 20) (see Figure 3). The activation energy found
for this process was 105 ( 10 kJ/ mol and the steric factor (A)
was 5000 ( 3000 min-1 (see Figure 4).
The formation of CO2 probably results from the following
reaction (eq 2):
CCl4 + 2Na2CO3 f 4NaCl + 3CO2
∆rH ) -462 kJ/ mol (2)
While it is reasonable to expect that COCl2 will be a reaction
intermediate, this compound was not observed in any
experiment reported here. While the C2Cl4 could result from
the thermal decomposition of CCl4,
2CCl4 f C2Cl4 + 2Cl2
∆rH ) +180 kJ/ mol (3)
FIGURE 4. The Arrhenius plot obtained for the reaction between
Na2CO and CCl4. The function on the y-axis is appropriate for the
three-dimensional diffusion mechanism.
3
no C2Cl4 was observed when CCl4 was passed through a tube
packed with glass fragments at 900 K, so it is more likely that
the C2Cl4 was formed from a reaction with the Na2CO3. One
possibility is
C2H4Cl2/Na2CO3. The IR spectrum of the reaction products
observed for the reaction between 1,2-dichloroethane and
sodium carbonate at 900 K are compared to selected library
spectra in Figure 5. CO2, ethanal (C2H4O), vinyl chloride (C2H3-
Cl), ethene (C2H4), and ethyne (C2H2) were clearly produced
during the reaction. Over 80% of the starting material was
converted to products by one pass through the sodium
carbonate under the conditions used in this experiment. The
2CCl4 + 2Na2CO3 f 4NaCl + C2Cl4 + O2 + 2CO2 (4)
Although ∆rH for reaction (4) is slightly positive (+9.6 kJ/
mol), ∆rS is also positive (+163 J/ (K mol)) and some C2Cl4
is expected at temperatures above 60 K at 101 kPa pressure
from this reaction.
9
VOL. 33, NO. 10, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1 6 9 3

FIGURE 5. The IR spectrum obtained for the products of the reaction between Na2CO and C H Cl2 at 875 K. This spectrum is compared
3
2 4
to spectra from the Aldrich/Nicolet library in the figure.
FIGURE 6. The fractional composition of the products observed for
the reaction between Na2CO and C H Cl2 as a function of temperature
3
2 4
in the flow experiment. The initial pressure of C H Cl2 was 5 kPa.
2
4
reaction followed the Ginstling-Brounshtein diffusion mech-
anism (Figure 7) and had an apparent activation energy of
100 ( 10 kJ/ mol and steric factor of approximately 104 min-1
.
The main products observed for this reaction are CO2
and C2H4O. The approximately equal amount of CO2 and
C2H4O between 700 and 800 K is consistent with the reaction
given in Table 1. The decrease in the amount of C2H4O at
temperatures above 800 K could indicate that this compound
is further oxidizing or that the 1,2-dicholoroethene is reacting
to give the other observed products (most likely C2H4). C2H4
forms at least 10% of the product at 900 K. However, it was
difficult to accurately measure the amount of this compound
because its stronger bands are blended with starting material
bands and the reliability of the calculated molar extinction
coefficient for this compound could not be established by
comparison with other systems. It is estimated that the values
presented for this compound in Figure 6 are good to ( 20%.
Approximately 10% of the product mixture at 900 K is vinyl
chloride.
FIGURE 7. The infrared absorbance of C H Cl2 during the reaction
2
4
between Na2CO and C H Cl2 as a function of time in a closed cell
3
2 4
experiment. The temperature was held constant at 800 K. The linear
relationship observed for the function [(1 - 2r/3) - (1 - r)2/3]
indicated that the reaction was following the Ginstling-Brounshtein
diffusion mechanism.
the yield of this compound. No matter what the mechanism,
the net reaction expected for the production of C2H3Cl when
Na2CO3 is present is
Na2CO3 + 2C2H4Cl2 f 2NaCl + CO2 + H2O + 2C2H3Cl
(5)
The C2H3Cl could have resulted from the pyrolysis of the
C2H4Cl2, as this compound was observed in the closed cell
pyrolysis of 1,2-dichloroethane over glass at temperatures
over 800 K. Because no C2H3Cl was observed in open cell
pyrolysis of C2H4Cl2 over glass, the Na2CO3 probably increased
This reaction is slightly endothermic (∆rH ) +3.8 kJ/ mol)
but it has a negative ∆rxnG at both 298 K (-95.23 kJ/ mol) and
750 K. Possible reactions for the formation of C2H4 and C2H2
are
9
1 6 9 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 10, 1999

Na2CO3 + C2H4Cl2 f 2NaCl + CO2 + 1/ 2O2 + C2H4
∆rH ) +96 kJ/ mol (6)
CC(g) + Na2CO3(s) T CC (in Na2CO3)
CC ) CCl4, C2H4Cl2, or C6H5Cl
(9)
where
Na2CO3 + C2H4Cl2 f 2NaCl + CO2 + H2O + C2H2
∆rH ) +30 kJ/ mol (7)
Because the reaction intermediates on the surface have
not been identified, the exact mechanism for these reactions
is not known, but some inferences about the general
mechanism were established. All of the systems investigated
had similar activation energies, and all systems had products
corresponding to the loss of two chlorine atoms. This suggests
that loss of chlorine is the initiation step and that this step
controls the rate of reaction. Because the reaction is occurring
near the Na2CO3 surface, the aliphatic reactions probably
involve ionic intermediates:
Although both reactions are endothermic, they also have
positive ∆rxnS (∆rxnS (6) ) 237 J/ (K mol) and ∆rxnS (7) ) 226
J/ (K mol)) and are expected to become thermodynamically
favorable at temperatures above 405 and 132 K for reactions
6 and 7, respectively.
C6H5Cl/Na2CO3. As predicted by the thermodynamic
calculations presented in Table 1, chlorobenzene did not
react extensively with sodium carbonate in this temperature
region. An approximately 10% decrease in the chlorobenzene
absorbance was observed at 875 K when an initial pressure
of 2 kPa was used in the flow experiments. Benzene and
carbon dioxide were observed in the product spectrum at
temperatures above 800 K and were the only clearly identified
gaseous product produced during the flow experiments. The
infrared spectra obtained during the closed cell experiments
indicated that CO2, CO, H2O, and C3O2 were also produced.
The residue was dark, suggesting that other products were
formed. These products were amorphous to X-rays and have
not been identified.
XCl2 + Na+ (in Na2CO3) f NaCl + XCl+ (in Na2CO3)
(10)
XCl+ (in Na2CO3) + Na+ (in Na2CO3) f
NaCl + X2+ (in Na2CO3) (11)
where X ) CCl2 or C2H4. These ionic intermediates could
then react with the CO32- (in Na2CO3) to pick up an oxygen
to form CCl2O or C2H4O.
The reaction that produced the benzene has not been
established. Possible reactions like
X
2+ (in Na2CO3) + CO32- (in Na2CO3) f XO + CO2 (12)
The CCl2O formed in this step for the CCl4 reaction could
react following similar steps to form CO2. The transfer of
electrons in eq 12 would account for the formation of C2Cl4
or C2H4.
Na2CO3 + 2C6H5Cl f 2NaCl + C6H6 + C7H4O3 (8)
require products (in this case C7H4O3) that were not observed.
The NIST database (20, 21) gives a C7H4O3 compound that
could reasonably form, but no thermodynamic data is given
for it, so the feasibility of this reaction could not be
established. The C7H4O3 could then decompose or react
further to generate the carbon and the carbon oxides observed
in the closed cell experiments.
X2+ (in Na2CO3) + CO32- (in Na2CO3) f
X + 1/ 2O2 + CO2 (13)
The mechanism for the aromatic hydrocarbon reaction
is probably similar to the mechanism for the aliphatic
chlorocarbons with the removal of a chlorine by the sodium
ion being the rate-limiting step.
This reaction followed the three-dimensional diffusion
mechanism. The small extent of reaction in the temperature
range investigated made it difficult to obtain an accurate
activation energy for this reaction, but both the variable
temperature method and the constant temperature experi-
ments indicated that the activation energy was 80 ( 10 kJ/
mol and the steric factor was approximately 500 min-1. These
values should be regarded as tentative, and additional
investigations will be needed before the kinetics of this
reaction are understood.
Na+ (in Na2CO3) + C6H5Cl f
NaCl + C6H5+ (in Na2CO3) (14)
However, a more complete characterization of the products
will be needed before the remaining steps in this mechanism
can be established.
The temperature range observed for the Na2CO3 chlo-
rocarbon reactions is similar to the effective temperature
ranges reported for metal oxide catalyzed systems (5-7, 23).
It is difficult to determine which system is more effective in
destroying chlorocarbons. In most cases, reaction kinetics
have not been reported for the metal oxide systems and
differences in reactor design make it difficult to directly
compare yield data because the percent conversion increases
as the dwell time in the reactor increases. The kinetics for
reactions between V2O5 and CCl4 and V2O5 and 1,2-dichlo-
roethane have been investigated using this system (23). The
products formed were similar, with the one clear difference
being that this metal oxide catalyst system produced HCl as
a product while the Na2CO3 reaction did not. The Na2CO3
reactions have slightly lower activation energies, but the
vanadium oxide system had larger steric factors and produced
larger extents of reaction under similar conditions.
Mechanism s. All of the reactions investigated followed a
three-dimensional diffusion type mechanism under condi-
tions where neither the surface area of the solid nor the
chlorocarbon is in large excess. The three-dimensional
diffusion model indicates that the solid product is collecting
on the surface of the compound and inhibiting the reaction
since the gaseous reactant must diffuse through this product
layer to reach the reactive front (20). The three-dimensional
diffusion model assumes quasi-equilibrium in the reaction
region, is often found for reactions involving ionic system,
and usually works best for systems where the extent of
reaction (R) for the solid is small (20). The Ginstling-
Brounshtein diffusion mechanism observed for 1,2-dichlo-
roethane is a special case of the three-dimensional diffusion
mechanism. Although derived for spherical particles, this
mechanism has been found to model reactions on other
shaped particles reasonably well. It also works better for larger
extents of reaction than does the three-dimensional diffusion
model (20). These mechanisms indicate that the first steps
are the adsorption of the chlorocarbon on the surface followed
by diffusion into the sodium carbonate.
Acknowledgments
The authors are grateful to Cynthia Kerns for assistance with
the X-ray analysis and to the ACS Petroleum Research Fund,
9
VOL. 33, NO. 10, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1 6 9 5

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(21) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.;
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the JMU-NSF-REU program, and the NSF-ILIP program for
supporting this project.
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