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

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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-

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 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).

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 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)

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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

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  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

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                                                        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.

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      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

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