United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/SR-94/018 March 1994
EPA Project Summary
Abiotic Transformation of Carbon
Tetrachloride at Mineral Surfaces
Michelle Kriegman-King and Martin Reinhard
Transformation of carbon tetrachlo-
ride (CCL) by biotite, vermiculite, and
pyrite in the presence of hydrogen sul-
fide (HS-) was studied under different
environmental conditions. In systems
containing biotite and vermiculite, the
rate of CCI4 transformation was depen-
dent on the temperature, HS- concen-
tration, surface concentration, and Fe(ll)
content in the minerals. At 25°C, the
half-life of CCI4 with 1 mM HS- was
calculated to be 2600,160, and 50 days
for the homogeneous, vermiculite (114
m2/L) and biotite (55.8 m2/L) systems,
respectively. The transformation rate
with biotite and vermiculite was nearly
independent of pH in the range 6-10 at
constant HS- concentration. The rate
dependence on Fe(ll) content of the
sheet silicates suggested that the trans-
formation occurs at surface sites where
HS- is associated with Fe(ll).
CCI4 reacted relatively rapidly in
1.2-1.4 m2/L pyrite with >90% of the
CCL transformed within 12-36 days at
25°C. The observed rate law supports
a heterogeneous reaction mechanism.
The reactivity of CCI4 with pyrite in-
creased in the order: air-exposed py-
rite/aerobic, air-exposed pyrite/HS~,
air-exposed pyrite/anaerobic, and
acid-treated pyrite/anaerobic; but over-
all the reaction rate varied only by a
factor of 2.5. The CCI4 transformation
products varied under different reac-
tion conditions. In the sheet silicate
systems, approximately 80-85% of the
CCI4 was transformed to CS2 which hy-
drolyzed to CO2; whereas only 5-15%
of the CCI4 was reduced to CHCI3. In
the pyrite systems, CO2 was the major
transformation product formed under
aerobic conditions, whereas CHCI3 was
largely formed under anaerobic condi-
tions. Formation of some CS2 was ob-
served in all pyrite systems.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory, Ada, OK, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The objectives of this research were to
test the ability of ferrous iron-bearing min-
erals to abiotically transform carbon tetra-
chloride (CCI4) in sulfidic (containing HS~)
environments. CCI4 is a frequently found
groundwater contaminant. The work fo-
cused largely on biotite, vermiculite, and
pyrite as the ferrous iron-bearing miner-
als. Biotite and vermiculite are sheet sili-
cates that are commonly found as
detrital materials in sedimentary rocks.
Sulfidogenic conditions are often ob-
served in plumes of hazardous waste
and landfill leachate. Specific factors that
were studied include solid type and con-
centration, iron content, pH, temperature,
sulfide concentration, CCI4 concentration,
and the presence or absence of oxygen.
The CCI4 transformation products were
studied as a function of reaction condi-
tions.
Procedure
Kinetic Studies
The base case reaction conditions to
measure the disappearance rate of 1 jj. M
CCI4 in the biotite and vermiculite systems
were pH = 7.5-8.5, temperature = 50°C,
and [HS~] = 1 mM. The solids concentra-
tions for the base case were 55.8 m2
biotite/L, and 114m2 vermiculite/L. Con-
trols were established by reacting CCI4
Printed on Recycled Paper
-------
with HS- at the same temperature and
pH, but in the absence of the solids. Ex-
periments with biot'rte were conducted over
a pH range of 6-10, temperature range of
37.5-62.7°C, solids concentration of
11.2-280 m2biotite/L, and [HS-] = 0.02-4
mM. For all pyrite/CCI4 transformation rate
and product studies, 1 p.M CCI4 was re-
acted in aqueous systems containing
1.2-1.4 m2/L pretreated pyrite at pH 6.5
and 25°C, except the experiments con-
ducted with sutfide that were at pH 7.75.
Experiments were conducted in a 1 mM
NaCI ionic medium. Controls were estab-
lished by reacting CCI4 under the same
conditions in the absence of pyr'rte in ad-
dition to reacting CCI4 in homogeneous
solutions of Fe2* or HS-. For the studies
of the pyr'rte oxidation products, 0.1-1 mM
CCI4 was reacted with large particles of
pretreated pyr'rte (0.2 g). Controls were
established by reacting pyrite under the
same conditions but in the absence of
CCI4.
Materials
BiotHe, vermicul'rte, muscovrte, and py-
rite were obtained from Ward's Scientific
Establishment, Inc. (Rochester, NY). All
transformation and adsorption studies were
conducted in flame-sealed glass ampules
because the reaction times were on the
Equation
order of weeks to months, often at el-
evated temperature (50°C). Ampules were
filled with approximately 13.5 ml of buffer
that was filtered through a sterilized 0.2
jim nylon filter (Nalgene Corp., Roches-
ter, NY).
Analytical Procedures
Reaction solutions were analyzed for
CCI4, CHCI3, and CO using gas chroma-
tography. Ion chromatography was used
to measure formate and the potential py-
rite oxidation products SO32-, SO42', S,O32-.
The CCI4 product distribution was deter-
mined using 14C-labeled substrate. Sur-
faces were characterized using XPS.
Mineral Characterization
Solids were characterized for specific
surface area, [Fe(ll)] and [Fe(lll)]. XPS
conducted on cleavage sheets of the bi-
otite and vermiculite did not show the pres-
ence of any redox-sensitive trace metals
besides iron. XPS studies indicated that
sulfide does interact with the biotite sur-
face, but the type and extent of effect
sulfide has on the surface were not deter-
mined. The near surface S:Fe ratio of
freshly cleaved pyrite was determined to
be 2.1. After pyrite was reacted in aque-
ous solution under all reaction conditions,
the near surface was depleted in iron with
Number
')[CCI4]a
(1)
(2)
(3)
(4)
[C02]=
(K obs~K COZ I
,^CS* [C.°/4]°
Vfobs~KCOz I
[exp(-A"c02 0-
01
[(1- expHf co, t ))-
obs
[Nonvot]
"•obs
t )}
(5)
•0)1 (6)
(7)
(8)
S:Fe>4. The oxidation state of the iron-
depleted pyrite surface appeared un-
changed when evaluated with XPS.
Results and Discussion
Transformation of CCI4 by
Vermiculite and Biotite
Kinetic Modeling and Product
Distributions
Observed pseudo-first-order rate con-
stants (k'obs) for the disappearance of CCI4
in the vermiculite and biotite systems were
calculated from regressions of ln([CCIA/
[CCIJ0) vs. time where [CCIJ0 and [CCIJ,
were the CCI4 concentrations at time = 0
and time = t, respectively. For experiments
with k'^O.OOl day1, the transformation
was considered negligible and was as-
sumed to equal zero.
In the sheet silicates systems, the dis-
appearance of CCI4 was hypothesized to
obey the laws shown in Equations 1
through 3, where a, p1, 02, yl, *£, and 8
represent the reaction order with respect
to reaction in solution and at the mineral
interface, k'homo and k'hetero are
pseudo-first-order rate constants, and
k|H2o, kHS- and khet are intrinsic rate
constants. Both the heterogeneous and
homogeneous rate constants were first
order with respect to [CCIJ (e.g. a = 1).
The mineral surface area concentration
(SC) was calculated from the product of
the solids loading (g/L) and the specific
surface area (m2/g) of the mineral. The
pKa for the first dissociation of H2S at the
reaction temperature was used to calcu-
late the HS- concentration based on the
pH and the amount of total sulfide added
to the system. For homogeneous systems,
k'obs = k'homo' wnere k'homo accounts *or t"6
reactions with H2O and HS" in solution. In
the heterogeneous systems, k'hetero = k'obs
- k1
Rate constants for the different CCI4
transformation pathways were evaluated
by considering the relationship shown in
Equation 4, where k'CS2> k'CHC|3, and
k'NV are pseudo-first-order rate constants
that describe the formation of CS2, CHCI3,
and the nonvolatile product, respectively.
CO was detected in very small quantities
and was not considered in this analysis.
Data of CS2, CO2, CHCI3 and nonvolatile
concentrations as a function of time were
fit to Equations 5, 6, 7, 8, respectively.
The rate constants, k'pSj, k'Co2,
k'cHci3, and k'Ny were estimated using
both visual and nonlinear statistical
curve-fitting. The rate constant k'CO2 is
the pseudo-first-order rate constant for the
appearance of CO2, due to CS2 hydroly-
-------
sis. The curve-fitting results for a typical
experiment are shown in Figure 1. Good
agreement was found across experiments
in terms of the fraction of CCI4 reacting
via the three different pathways. A mass
balance of 95-100% was obtained in these
experiments.
At 50'C, PH 6-9, SCbNte = 0-280 m2/L
and low HS" concentrations ([HS"]<0.5
mM), the reaction orders from Equation 3
were determined to be cc= 1, (52 = 1.2,
and 8 = 1. The pH dependence in the
environmentally relevant pH range was
too low to be determined reliably and both
yl and -ft. were assumed to be zero. At
high HS-concentrations ([HS-]=0.5-4 mM)
and SCbiotite<55.8 m2/L, the rate of disap-
pearance of CCI4 in heterogeneous sys-
tems was independent of HS~
concentration (P2 = 0).
The major transformation pathway of
CCI4 with HS~ is the formation of CO2 via
CS2. It was proposed that CCI4 undergoes
reduction to form a trichloromethyl radical
which then reacts with HS", Sx2', or S2O32-
to form CS2 which hydrolyzes to CO2. At
50°C, the rate constant for the disappear-
ance of CS2 ranged from 0.03 to 0.06
day1. Reported Arrhenius constants for
the hydrolysis of CS2 to CO2 under reac-
tion conditions herein, result in a hydroly-
sis rate of 0.006-0.015 day1 at 25°C
(half-life of 45-110 days). About 85% of
the CCI4 is ultimately transformed to CO2
in these systems. Reductive dehalogena-
tion of CCI4 to CHCI3 contributed to 5-15%
of CCI4 transformation. Ferrous iron in
the sheet silicates appears to be play-
ing a role in the transformation of CCI4
with HS- It is most likely that the reaction
is occurring at sites where HS~ is associ-
ated with ferrous iron.
Adsorption of CCI4 onto biotfte and ver-
miculite was determined by control ex-
periments using radiolabeled CCI4 at 25°C.
Comparison of the aqueous CCI4 concen-
tration in the homogeneous and heteroge-
neous systems over four weeks showed
less than 3% adsorption. Because the ad-
sorption of CCI, was so small, CCI4 mea-
surements in the transformation studies
were not corrected for adsorption.
The rate of hydrolysis (kH2o) of CCI4
was calculated to be 0.002 day1 at 50°C.
In homogeneous systems, the CCI4 trans-
formation rate in the presence of sulfide is
at least an order of magnitude greater
than kHz0 when [HS~]>0.5 mM. In het-
erogeneous systems, the CCI. transfor-
mation rate is faster than KHzO wjth
[HS-]>0.05 mM and SCbj n, = 55.8 m2/L.
The very low reactivity of HS~ in homog-
enous solution was enhanced in the pres-
ence of minerals indicating a catalytic effect
of the surfaces.
Figure 1 depicts the disappearance of
CCI4, the appearance and disappearance
of CS2, and the appearance of products in
a heterogeneous system (55.8 m2/L bi-
otite and 1 mM HS-). About 65% of the
CCI. was transformed to CO2 after 60 days.
At this time, approximately 20% of the
CS2 was remaining. Chloroform, formed
via reductive dehalogenation of CCI4,
reached a maximum of 10%. CHCI3 was
shown to be relatively persistent in these
systems when 5 u.M CHCI3 was reacted
under the same conditions as the CCI4
experiments. The half-life of CHCI3 in the
presence of 55.8 m2/L biotite and 1 mM
HS~ at 50°C was measured to be 172
days, whereas the half-life for hydrolysis
of CHCI3 at pH 7.75 and 50°C is 5000
days. Carbon monoxide and a nonvolatile
component were measured as products in
very small quantities (<5% combined) in
the CCl4 systems. The nonvolatile prod-
uct, detected by 14C fractionation mea-
surements, was not identified in these
systems.
Proposed Reaction Mechanism
The proposed chemical transformation
pathways for CCI4 under anaerobic condi-
tions are summarized in Figure 2. The
products and intermediates in the shad-
owed boxes were detected in our experi-
ments. The first step for the transformation
of CCl4 has been proposed to be a
one-electron reduction to form a
trichloromethyl radical and Cl~. This radi-
cal can follow several different pathways
such as additional electron transfer to form
a dichlorocarbene and Cl~, dimerization
to form hexachloroethane, or electron
transfer and protonation to produce CHCI3.
The only pathway previously suggested
to form CO2 under anaerobic conditions is
1.2
cci4
CHCI3
CS2
Non-Volatile
CO2
Mass Balance
0.0-
0 20 40 60
Time (Days)
Figure 1. CCI4 transformation products from reaction with [HS -] = 1mM, SCtfesI> = 55.8 rrf/L, pH = 8.8 at 50°C.
-------
icr a
Carbon Tetrachloride
2x X
X
.X
Hexachloroethane
(not detected)
Trichloromethyl
Radical
V
ci
Chloroform
(5-15%)
Dichlorocarbene
H2O\ t2H2O
vv^y ^\_^,Hc,
Carbon
Monoxide
(1-2%)
HCOOH
Formic Acid
(Nonvolatile product
detected (3-6%), formic acid?)
CI
Trichloromethanethiolate
Cn~'H
CH2CI2
Methylene
Chloride
(not detected)
Vci-
,
CI
Dichlorothionomethane
H++2CI-
Carbon
Disulfide
(intermediate)
20H-
2HS-
Carbon
Dioxide
(81-86%)
Figure2. Proposed CCI. transformation pathways in HS- solution containing biotite. Compounds in shadowed boxes were detected in this study.
Compounds in brackets are intermediates proposed in this study and from the literature (see Griddle and McCarty, 1991).
-------
direct hydrolysis (Griddle and McCarty,
1991). In our systems, CS2 appears to be
a major intermediate that is transformed
to CO2.
Temperature Dependence
From the temperature data collected at
37.5, 50.0, and 62.7°C, values for the
Arrhenius activation energy (Ea) and
pre-exponential (A) for the homogeneous,
Vermiculite, and biotite systems were cal-
culated (Table 1). Lower Ea values in the
heterogeneous systems indicate that these
reactions will dominate the homogeneous
reactions to an even greater extent at
environmentally relevant temperatures. For
example, at 50°C, CCI4, in the presence
of biotite and 1 mM HS~, reacts 8 times
faster than CCI4 in the absence of biotite,
whereas at 15°C, the biotite system re-
acts 125 times faster than the homoge-
neous system.
Table 1. Arrhenius Parameters for CClf Trans-
formation with 1 mMHS ~: Ea and In A
Were Calculated Using k'!hel forBiotite
and Vermiculite and k': for the Homo-
geneous Systems a
System
EJkJ/mol]
ln(A)
Homogeneous
Vermiculite
Biotite
122+32" 41.0+2.1'
91.3 ±8.4 31.4 ±0.5
59.9+13.3 19.9 ±0.9
" Data collected at pH 7.5and in the temperature range
37.5 - 62.7°C.
b 95% confidence intervals.
For the homogeneous reaction at 25°C,
the half-life of CC14 with 1 mM HS ~ was
calculated to be 2600 days. In the pres-
ence of 1 mM HS- and Vermiculite (114
m2/L) or biotite (55.8 m2/L) 25°C, CCI
removal was first order with half-lives of
160 and 50 days, respectively. On a sur-
face area normalized basis, CCI. transfor-
mation due to the presence of biotite was
approximately six times greater than ver-
miculite.
Transformation of CCI' by
Pyrite
Kinetic Modeling
The pyrite treatments and reaction con-
ditions studied were as follows:
air-exposed pyrite reacted aerobically,
air-exposed pyrite reacted anaerobically,
air-exposed pyrite reacted in the presence
of sulfide, fresh-ground pyrite reacted
anaerobically, and acid-treated pyrite re-
acted anaerobically. These conditions (ex-
cept the acid treatment) were chosen to
simulate different geochemical conditions.
The data showed a much better adher-
ence to zero-order model than to the
first-order model. In the pyrite systems
zero-order rate constants (kpci ) were
calculated from linear regressions of
[CCy/fCCIJoVs. time wherein KCCI is
equal to -(slope)([CCyo). A poor fit was
found for the fresh-ground system (R2 =
0.65) presumably due to the heteroge-
neous nature of the freshly cleaved sur-
faces.
Rate constants for the disappearance
of CCI4 (k°cc\t) are shown in Table 2. The
rate constants were normalized by the
pyrite surface concentration, assuming the
reaction was first-order with respect to the
surface concentration, SC, according to
Equation 9.
In the acid-treated pyrite system,
>90% of 1 |iM CCI4 was transformed
within 12 days at 25°C, whereas
half-lives in homogeneous solution are
1400 days for 1 mM HS~ at 25°C and
105 days for 0.1 mM Fe2+a at 50°C.
Assuming an activation energy of 60-120
kJ/mol, the half life of CCI4 with 0.1 mM
Fe2+aq at 25°C ranges from 700-4500
days. In Table 2, the data show that CCI
reacts the fastest with the acid-treated
pyrite, although the air-exposed pyrite re-
acted anaerobically is not statistically
slower. As expected, the slowest transfor-
mation rate was observed when CCI4 was
reacted with pyrite under aerobic condi-
tions. However, the rate constant was only
2.5 times slower than the acid-treated sys-
tem. The large error associated with the
fresh-ground pyrite system precludes com-
parison with other rate constants. Reac-
tion of air-exposed pyrite in the presence
of sulfide shows that treatment of an oxi-
dized pyrite surface with HS~ does not
restore the reactivity of pyrite. Rather, sul-
fide appears to inhibit the transformation
of CCI4by pyrite relative to the air-exposed/
anaerobic pyrite system. At pH 7.75, 85%
of the sulfide is present as HS~; and more
than 100 u.M is present as H2S. Because
of the observed zero-order dependence
on CCI4, reaction sites on pyrite are in-
ferred to be saturated with CCI4 when
[CCIJ = 1 u,M. Additionally, since [H2S]
was 100 times more concentrated than
CCI4 in these experiments, it is conceiv-
able that H2S blocks CCI4 reaction sites.
Characterization of the pyrite surface
chemistry is necessary to understand the
interaction of sulfide species with the py-
rite surface.
CCI4 Transformation Products
As shown in Table 3, the CCI4 prod-
uct distribution varies greatly depending
on the reaction conditions even though
kcci4 only varies by a factor of 2.5.
Under aerobic conditions, the major prod-
uct was CO2 (60-70%). Including the hy-
drolysis of CS2 to CO2, CO2 accounts for
70-80% of the CCI4 transformed. In con-
trast, the fresh-ground pyrite system forms
approximately 50% CHCI3 and ultimately
only 10-20% CO2. The total CO2 amount
in the fresh-ground system is a rough
estimate because the speciation of the
adsorbed fraction was not measured. In-
terestingly, some CS2 was formed in all
systems suggesting that the CCI4 or reac-
tive intermediates react with S22' sites on
pyrite, even in the presence of O2. A frac-
tion of the CCI4 or its transformation prod-
ucts appeared to be adsorbed to pyrite.
The rate constants for the appearance
of CHCI3 and HCOOH (k'CHci3 and k°NV,
respectively) were evaluated assuming two
Table 2. Zero-Order Rate Constants forCCI4 Transformation with Pyrite (1.2-1 4 nf/L) Reacted under
Aerobic and Anaerobic Conditions at 25°C
Pyrite Conditions
Slope (d-')
R1
k°CCI4
(mol/m * • d)
95% Confidence
Interval
Air-exposed, Aerobic 0.025
Air-exposed, HS~ 0.031
Air-exposed, Anaerobic 0.057
Fresh-Ground 0.056
Acid-Pretreated 0.082
0.93
0.87
0.85
0.65
0.96
0.021
0.026
0.047
0.039
0.053
0.017-0.026
0.020 - 0.032
0.035 - 0.049
0.022 - 0.056
0.046 - 0.060
Equation
Number
(9)
-------
Table 3. CCI4 Product Distribution in Percent from Reaction with Pyrite under Aerobic and Anaerobic Conditions at25°C
Condition
(Tims in d)*
Air-exposed;
Aerobic (42)
Air-exposed,
HS- (31)
Air-exposed,
Anaerobic ( 20)
Fresh-Ground (13)
Ackl-Pre-treated (13 )
CCI4
0-1
0-10
0-1
1
6-10
CHC/3
5-6
21-22
28-30
48
20-21
CS2
11-15
NMe
0-3
2
19-20
C02
52-59
NM
26-30
10
17
Formate b
2
NM
7-9
5
4
Adsorbed °
NV+C02
To
(2 NV+ 8 CO2)
NM
12
(7NV+5C02)
12?
9(2NV+7C02)
Mass
Balance d
84-87
NM
82-84
78
78
* Reaction time In days Is In parenthe—
* Formate was not directly measured. Formate was assumed to equal nonvolatile concentration.
.
:ffiS2£S£^
wilhh SK. Missing fraction likely to be adsorbed volatiles.
• NM-not measured.
1 Breakdown of adsorbed products not measured.
Tab/a4. Rate Constants for the Disappearance of CCIf and Appearance of Intermediates
Products from Reaction with Pyrite under Aerobic and Anaerobic Conditions at25°C
ahd
Air-exposed Pyrite
Reacted Aerobically
Acid-treated Pyrite
Reacted Aerobically
Rate Constant •
No intermediate * Intermediatee
D 2 = f\ 7O d D2
n . j — f. / o n j.
No intermediateb Intermediatec
J,
kcs,
k'cs.
/f'cos
KCHCI,
k'NV
[mol/nf-d] 0.021
[mol/nf-d] —
[mol/nf-d] 0.0078
[L/nf-d] —
[L/nf-d] 0.040
[mol/nf-d] 0.00092
[mol/nf-d] 0.00040
0.021 0.053
0.020 —
— 0.022
0.072 —
0.012 0.72
0.00092 0.012
0.00040 0.0027
0.053
0.023
—
7.7
0.12
0.012
0.0027
* k' - zero-order rate constant; k' = first-order rate constant. For symbols see text.
* ccr.-»cs.-»co..
.-»-,-j.
- accounts for the number of fining parameters.
Equation
Number
d(lntermed.} =
{lntermedm}
(10a)
(10b)
(11)
(12)
different kinetic models (Table 4). Assum-
ing CO2 is formed only by the CS2 path-
way, Equations 10a and 10b can be used
to solve for [CS2] and [CO2] as a function
of time. In Equations 10aand 10b, fc°cs2is
a zero-order rate constant for the appear-
ance of CS2, and k'COz is a first-order
rate constant for the formation of CO2.
The CS2 data were fit with Equation 11
to solve for the rate constants, fr°cs2 and
k'coz (Table 4). These constants were
then substituted into the equation for the
formation of CO2 to graphically verify the
fit of the data. This model under-predicted
CO2 production, suggesting that CO2 is
formed via the reaction of CCI3 with O2.
However, the curve for the appearance of
CS2 also did not fit the data well (R2adjusted
= 0.78).
To model the time lag before the onset
of the CS, increase, formation of a rela-
tively stable intermediate .in the path to
form CS2 was hypothesized. In this kinetic
model, the appearance of CS2 was mod-
eled using Equations 12 and 13, where
k°s is the zero-order rate constant for the
formation of the intermediate. In Equa-
tions 12 and 13, the rate constant for the
appearance of CS2 (k'cs,) is now as-
sumed to be first order. The expression
for the CS2 concentration is shown in
Equation 14.
The curve-fitting results are shown in
Table 4 and Figure 3. At the end of the
experiment, the model predicts that the
intermediate attains a steady-state con-
centration of approximately 15% which
agrees with the missing mass balance
(Table 3).
A similar fitting analysis was conducted
on the results from the acid-treated pyrite
system. In this, case, no significant differ-
ence in the CS2 fit was observed if the
-------
Equation
Number
[052]=-,
[Interned.]-k'COz [CS2]
}FeS -S-.- + CC/4 -» [)FeS - S • + CC/4 ~] -)• >FeS- S - CC/3
(13)
(14)
(15)
0.0
Time (Day)
Figure 3. Disappearance of CCI4 in the presence of pyrite under aerobic conditions at 25°C.
Appearance of the products, CS^andCO^ with model results assuming the only path to form
CO., from CCI4 is: CCI4-> Interned. -» CS2 -» CO
appearance of an unknown intermediate
was included. As shown in Table 4, the
rate constant for the disappearance of the
unknown intermediate I (kj) was relatively
large, indicating that the intermediate I is
very short-lived. Therefore, k] is approxi-
mately equal to the rate constant for the
appearance of CS2 (k°CSi).
Proposed Mechanism of CCI4
Degradation at the Pyrite Surface
The proposed pathway of CCI4 degra-
dation by pyrite is summarized in Figure
4. Sulfur is the proposed electron transfer
site in reactions of CCI4 with pyrite be-
cause the surface was depleted in iron
and CS2 was detected under all reaction
conditions. Since the pyrite surface was
negatively charged under the reaction con-
ditions in this study, the surface sites are
proposed to be predominantly of the form
>FeSS-. It is assumed that the amphot-
eric nature of the leached pyrite surface is
similar to that of pyrtte-S. In the absence
of oxidation of the pyrite surface, the elec-
tron transfer reaction with CCI.is pro-
posed to occur via the reaction in Equation
15. The path to form CO2 can occur via
hydrolysis of CS2 or reaction of the trichlo-
romethyl radical with O2. The former path-
way is assumed to prevail under anaerobic
conditions. However, under aerobic con-
ditions, CO2 is the major product in the
pyrite system, and both pathways are pos-
sible.
Summary and Conclusions
The results of this work provide insight
into the rates of CCI4 transformation un-
der different environmental conditions, the
CCI, transformation products, and the
mechanism of CCI4 transformation at min-
eral surfaces. Major conclusions that can
be drawn from this work include
(1) The disappearance of CCI4 in
sulfidic systems was significantly
faster in the presence of mineral
surfaces (biotite, vermiculite, and
pyrite) than in homogeneous solu-
tion.
(2) The rate of transformation of CCI4
with the sheet silicates and sulfide
depended on the following reaction
conditions: temperature, surface
concentration, sulfide concentration,
and ferrous iron content in the min-
erals. The CCI4 transformation rate
was investigated over the range of
6-10 at constant [HS~]and showed
a very shallow minimum at
near-neutral pH.
(3) The rate of transformation of CCI4
with pyrite varied with reaction con-
ditions in the following order:
air-exposed pyrite/aerobic £
air-exposed pyrite/sulfide <
air-exposed pyrite/anaerobic <
acid-treated pyrite/anaerobic. The
rate constants varied by only a fac-
tor of 2.5 for all the conditions stud-
ied. HS~ inhibited the CCI4
transformation rate by pyrite rela-
tive to systems reacted anaerobi-
cally in the absence of HS~.
(4) The CCI4 tiansformation products
varied greatly as a function of the
reaction conditions. In the sheet sili-
cate/sulfide systems, CS2 was iden-
tified as a major intermediate that
hydrolyzed to CO2, accounting for
>85% of the CCI4 transformed. In
the pyrite systems, CS2 was de-
tected under all reaction conditions,
suggesting that CCI4 or an interme-
diate must react directly with the
pyrite surface. Under aerobic con-
ditions, CO2 was the major trans-
formation product (80%). In the
fresh-ground pyrite systems,
roughly 50% of the CCI4 was trans-
formed to CHCI3. In all systems
studied, formate and carbon mon-
oxide were minor products.
(5) The rate of transformation of CCI4
with the sheet silicates was depen-
dent on both the ferrous iron con-
tent and the sulfur concentration,
indicating that the reaction occurs
at sites where sulfide is associated
with structural ferrous iron.
(6) In the pyrite system, the
near-surface was depleted of iron
after reaction in water, while the
oxidation state of the pyrite-S ap-
peared to remain the same. The
high sulfur concentration at the
near-surface makes it likely that
-------
ci a
ci ci
Carbon Tatrachlorida
• >FeSS"
2HCI
CS2
•M
Carbon Disulfide
CO2
^m
Carbon Dioxide
Figure 4. Proposed CCI4 transformation pathways with pyrite. Compounds in shadowed boxes were measured. Compounds in brackets are proposed
Intermediates.
-------
pyrfte-S is the reductant of
rather than pyrite-Fe.
Recommendations
The results of this study shpw that min-
eral surfaces may play a significant role in
the fate of halogenated organics in the
environment. Although they can be quan-
titatively applied to natural systems only
with some difficulty, these data show
that rate constants measured in deion-
ized water will greatly under-predict the
actual transformation rates. In pyrite- or
sulfide-rich environments, abiotic transfor-
mation pathways may be significant on
the time scale of groundwater transport.
Predictive capabilities are complicated at
this point due to the confounding effects
of natural organic matter, cosolvents, com-
peting oxidants, and miorobial activity. To
address these issues, continued research
to further understand the surface chemis-
try of pyrite, the CCI4 transformation path-
way at the pyrite surface, and the reactivity
of CCI4 and other polyhalogenated ali-
phatics under field conditions is neces-
sary. This work will ultimately lead to
predictive capabilities.
The pyrite surface was relatively reac-
tive with CCI4 even under aerobic condi-
tions; therefore, it is conceivable that a
pyrite-based treatment system could be
engineered. Further studies would have
to be conducted in order to (1) identify the
rate determining step of the reaction, (2)
test the efficiency of the method in col-
umn and batch reactors, (3) control the
CCI4 product distribution, and (4) measure
the pyrite oxidation products and ensure
that they are harmless. In addition, the
engineered system would have to be
tested with other haloaliphatics .to see if
they could also be transformed and if they
inhibited or effected the transformation of
CCI4
Reference
Griddle, C. S., and P. L McCarty. 1991.
Electrolytic model system for re-
ductive dehalogenation in aqueous
environments. Environ. Sci.
Technol., 25:973-978.
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Michelle Kriegman-King and Martin Reinhard are with Stanford University,
Stanford, California 94305-4020.
Stephen R. Hutchins is the EPA Project Officer (see below).
The complete report, entitled "Abiotic Transformation of Carbon Tetrachloride at
MineralSurfaces,"(OrderNo.PB94-144698;Cost:$19.50;subjecttochange)will
be available only from
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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