DDT, DDD, and DDE dechlorination by zero-valent iron

Article abstract of DOI:10.1021/es9701669

Traditionally, destruction of DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane] for environmental remediation required high-energy processes such as incineration. Here, the capability of powdered zero-valent iron to dechlorinate DDT and related compound

Full text of DOI:10.1021/es9701669

Environ. Sci. Technol. 1997, 31, 3448-3454
thermodynamically favored reaction (eq 3) (1):
DDT, DDD, and DDE Dechlorination
by Zero-Valent Iron
Fe + RCl + H f Fe2+ + RH + Cl
0
+
-
(3)
G R E G O R Y D . S A YL E S , * ,
†
Recent work has shown that the rates of dechlorination
of chlorinated solvents (3, 5, 6) and of reduction of nitroaro-
matic compounds (8) by zero-valent iron are proportional to
the specific iron surface area (surface area per unit reactor
volume). These results imply that the reduction reactions
occur by electron transfer at the iron surface.
DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane] was
a widely used pesticide, banned in the United States in 1972,
but continues to be used in some countries. DDT is listed as
a priority pollutant by the U.S. Environmental Protection
Agency, and many DDT-contaminated sites are on the
Superfund National Priority List. Effective destructive tech-
nologies for the treatment of DDT-contaminated sites other
than incineration are lacking. Most DDT-contaminated sites
are also co-contaminated with DDD [1,1-dichloro-2,2-bis-
G U A N R O N G YO U , M A O X I U W A N G , ‡ A N D
‡
M A R G A R E T J . K U P F E R L E ‡
U.S. Environmental Protection Agency, National Risk
Management Research Laboratory, Cincinnati, Ohio 45268,
and Department of Civil and Environmental Engineering,
University of Cincinnati, Cincinnati, Ohio 45221-0071
Traditionally, destruction of DDT [1,1,1-trichloro-2,2-bis(p-
chlorophenyl)ethane] for environmental remediation
required high-energy processes such as incineration. Here,
the capability of powdered zero-valent iron to dechlorinate
DDT and related compounds at room temperature was
investigated. Specifically, DDT, DDD [1,1-dichloro-2,2-bis(p-
chlorophenyl)ethane], and DDE [2,2-bis(p-chlorophenyl)-
(
1
p-chlorophenyl)ethane] and DDE [2,2-bis(p-chlorophenyl)-
,1-dichloroethylene], both as impurities in DDT and as
products of natural DDT transformation. DDD and DDE are
also classified as priority pollutants and are also extremely
stable in the environment.
1,1-dichloroethylene] transformation by powdered zero-valent
iron in buffered anaerobic aqueous solution was studied
at 20 °C, with and without the presence of nonionic
surfactant Triton X-114. The iron was successful at dechlo-
rinating DDT, DDD, and DDE. The rates of dechlorination
of DDT and DDE were independent of the amount of iron,
with or without surfactant. The rates with surfactant
present were much higher than without. Initial first-order
transformation rates for DDT, DDD, and DDE were
determined. For example, the initial first-order rate of DDT
Reductively dechlorination of DDT can occur when
coupled to the oxidation of metals. Biological systems catalyze
reductive dechlorination of DDT using metal-coenzyme
complexes where cations of iron or cobalt are oxidized in the
reaction (9-11). The abiotic dechlorination of DDT was
observed in the presence of cooking utensils at 100 °C and
was attributed to zero-valent iron in the utensils (12). No
previous work describing the use of zero-valent iron to
remediate DDT contamination at room temperature was
found in the peer-reviewed literature.
This study focused on DDT, DDD, and DDE transformation
by zero-valent iron in aqueous systems. Each molecule of
DDT, DDD, and DDE has two aryl chlorines and three, two,
and two alkyl chlorines, respectively (Figure 1). The ef-
fectiveness of zero-valent iron in alkyl dechlorination shown
in literature indicated a potential for dechlorination of the
alkyl group on the DDT-related compounds. DDT, DDD,
and DDE have water solubilities of 3 µg/ L (0.008 µM) (25 °C),
-
1
dechlorination was 1.7 ( 0.4 and 3.0 ( 0.8 day or,
normalized by the specific iron surface area, 0.016 ( 0.004
-
2
-1
and 0.029 ( 0.008 L m h , without and with surfactant,
respectively. A mechanistic model was constructed that
qualitatively fit the observed kinetic data, indicating that
the rate of dechlorination of the solid-phase (crystalline)
reactants was limited by the rate of dissolution into the aqueous
phase.
1
60 µg/ L (0.5 µM) (24 °C), and 40 µg/ L (0.11 µM) (20 °C),
respectively (13). These low solubilities may limit the
interaction of these chemicals with zero-valent (solid phase)
iron.
Introduction
Zero-valent iron can drive the dechlorination of chlorinated
aliphatics and aromatics. Many chlorinated aliphatics (1-6)
such as tetrachloroethene, trichloroethene, carbon tetra-
chloride, and chloroform can be dechlorinated reductively
by zero-valent iron at room temperatures and pressures. The
chlorinated aromatics (polychlorinated biphenyls) have been
completely dechlorinated to biphenyl using zero-valent iron
at elevated temperatures (7).
The influence of nonionic surfactant Triton X-114 on the
rate of transformation was also studied. Triton X-114, an
alkylphenol ethoxylate, greatly increases the apparent solu-
bility of DDT to 800 µg/ L using 200 mg/ L of surfactant (14)
and enhances the apparent rate of anaerobic biological
dechlorination of DDT (11). The critical micelle concentration
of Triton X-114 in water at 25 °C is 110 mg/ L (14).
The objective of this study was to determine the feasibility
of using zero-valent iron and surfactant to dechlorinate the
priority pollutants DDT, DDD, and DDE. Feasibility was
measured as the initial rate of dechlorination of the parent
species and the extent of dechlorination of DDT, DDD, and
DDE.
Some aspects of the mechanism of dechlorination by iron
are understood. Zero-valent iron can reduce redox-reactive
species including water (eq 1) and oxygen (eq 2) (1):
0
2+
-
Fe + 2H O f Fe + H + 2OH
(1)
(2)
2
2
0
2+
-
2
Fe + O + 2H O f 2Fe + 4OH
2 2
Experimental Procedures
Batch Experim ental Procedures. Test reactors were 40-mL
glass vials with Teflon-lined septum caps. Uncapped reactors
were filled under anaerobic conditions by continuously
flushing with nitrogen. Iron powder (0.3-3 g) was preweighed
in test reactors. Then, the reactors were filled with 20 mL of
deoxygenated buffer solution, the target compound (DDT or
In the presence of water, alkyl chloride is reduced in a
Corresponding author e-mail: sayles.gregory@epamail.epa.gov;
*
fax: 513-569-7105.
†
‡
U.S. Environmental Protection Agency.
University of Cincinnati.
3
4 4 8
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997
S0013-936X(97)00166-1 CCC: $14.00
 1997 Am erican Chem ical Society

FIGURE 1. Proposed pathway of anaerobic reductive metabolism of DDT in microorganisms (from ref 15, modified according to ref 16). An
underlined acronym (e.g., DDT) implies that the chemical was measured in this study. The dechlorination path of DDE to DDMU was added
here based on well-known biotic and abiotic transformation of chlorinated ethenes.
FIGURE 2. Transformation of DDT and its impurities (DDD, DDE, and DDMU) in reactors without iron and in reactors with iron and NaOH.
These reactors are negative controls run to measure effectiveness of sample extraction: (b) no iron; (O) 50 g/L iron with 25 mM NaOH.
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Transformation of DDT and related chemicals for various initial amounts of zero-valent iron: (b) 15 g/L iron; (O) 50 g/L iron; (1)
50 g/L iron.
1
DDD dissolved in acetone), and, if required, Triton X-114
surfactant at 250 mg/ L. The buffer solution was 20 mM MOPS
[
3-(N-morpholino)propanesulfonic acid] with initial pH
adjusted to 7 and deoxygenated by purging with N gas for
h. Two 6-mm glass beads were added to improve the mixing
2
1
in reactors spiked with DDT and DDD. The closed reactors
were mixed by a rotary shaker at 130 rpm at 20 ( 0.5 °C.
Control reactors without iron or with iron and 25mM NaOH
(
to quench the reaction, see eq 3) were established to monitor
the extraction efficiency. An appropriate number of equiva-
lent reactors were established to allow duplicate reactors to
be sacrificed at each sampling time.
Extraction for Measurem ent of DDT and Byproducts.
Extraction was performed in situ, i.e., in the sacrificed test
reactors. The extractant included 0.5 mL of 1 M NaOH, 10
mL of toluene, and 0.5 mL of ethanol. NaOH was included
to quench the dechlorination reaction (eq 3). Following
addition of the solvents and NaOH, the reactors were closed
FIGURE 4. Calculated initial first-order transformation rates of DDT,
without and with Triton X-114 surfactant, at the zero-valent iron
levels noted. Error bars are 95% confidence limits.
3
4 5 0
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997

FIGURE 5. Transformation of DDT and related compounds with 250 mg/L surfactant Triton X-114 and various amounts of zero-valent iron:
b) 15 g/L iron; (O) 50 g/L iron; (1) 150 g/L iron.
(
and mixed for 1 h by the same rotary shaker at 20 °C. Bottles
were then centrifuged, and the solvent phase portion was
taken for analysis.
DDMU) did not change significantly during the 20-day test,
indicating that NaOH was a good quenching reagent to use
during sample extractions and that iron had no measurable
influence on the extractability of the chlorinated compounds.
Similar results were found from control reactors where DDD
was used as the parent compound (Figure 7).
To test the efficacy of this extraction method, two types
of control reactors were established: reactors with no iron
and reactors with iron and NaOH. The controls were spiked
with DDT, were run for the entire length of the experiment,
and were sampled at the same time as the active reactors.
The iron with NaOH reactors were established to determine
if NaOH could quench the dechlorination reaction. The no-
iron control reactors were set up to determine if the presence
of iron influenced the extraction efficiency.
Measurem ent of DDT and Byproducts. DDT and its
intermediates were measured with a gas chromatograph
(
Hewlett Packard, Model HP5890II), equipped a flame
ionization detector (FID), an autosampler (HP7673), and a
0-m fused silica capillary column (Supelco SPB-1, 0.32 mm
3
internal diameter, 1 µm film). Helium was the carrier gas
with a flow rate of 1.1 mL/ min; nitrogen was the makeup gas.
Injection was carried out in splitless mode. Injection port
Figure 2 shows the results of these control tests. Con-
centrations of DDT and its impurities (DDD, DDE, and
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Proposed scheme of the important processes involved
in DDT and DDD dechlorination by zero-valent iron. Subscripts solid,
aq, and ads imply the chemical is crystalline, dissolved in the aqueous
phase, or adsorbed to the iron surface, respectively. X-Surf implies
that chemical X is associated with surfactant and surfactant micelles.
FIGURE 8. Oxidation-reduction potential (relative to the standard
hydrogen electrode) measured in reactors with various initial
conditions: (b) 50 g/L iron with 25 mM NaOH; (O) 15 g/L iron; (1)
50 g/L iron; (3) 50 g/L iron; (9) 50 g/L iron with 250 mg/L Triton X-114.
biotransformation pathway (Figure 1) and from the com-
mercial availability of the compounds for use as GC standards.
DDT, DDD, DDE, DDMU[1-chloro-2,2-bis(p-chlorophenyl)-
ethylene], DDOH [2,2-bis(p-chlorophenyl)ethanol], DBH
(
dichlorobenzhydrol), and DBP (dichlorobenzophenone)
concentrations were measured and are reported. DPM
dichlorodiphenylmethane), DM (diphenylmethane), BP (ben-
(
zophenone), and BH (benzhydrol) were measured but not
detected in any samples.
The sum of the measured molar concentrations of DDT,
DDD, and DDE, defined as total priority pollutants, is also
presented. Because DDT, DDD, and DDE are the only
compounds in the known transformation pathway that are
U.S. EPA priority pollutants, a drop in total priority pollutants
with time indicates the overall success of the remediation
process.
The concentration reported are an average of the duplicate
measurements. Duplicate concentrations were consistent
with relative percent differences typically less than 15%.
Measurem ent of Oxidation-Reduction Potential. Oxi-
dation-reduction potential was measured with a Corning
redox combination electrode (Pt/ Ag/ AgCl). The electrode
was fitted with a test reactor cap and septum so that it could
be placed into the test reactor, and the reactor was closed
during measurement. The reading at 2 min without mixing
was used as the measurement. For presentation here, the
oxidation-reduction potential values were converted to a
standard hydrogen electrode (SHE) reference by adding +222
mV to the mV reading obtained vs the Ag/ AgCl reference
used experimentally.
Chem icals. The electrolytically produced zero-valent iron
(
size < 100 mesh, 99%+ total iron), toluene, and ethanol
were purchased from Fisher Scientific Co. (Pittsburgh, PA).
The specific surface area of this zero-valent iron product was
2
previously determined (3) to be 0.287 m / g. Surfactant Triton
X-114, DDT, DDD, DDE, DDOH, DBP, DBH, DM, BH and BP
were purchased from Aldrich Chemical Co. (St. Louis, MO);
DDMU and DDA were purchased from Crescent Chemical
Co. (Hauppauge, NY); and MOPS was purchased from Sigma
Chemical Co. (St. Louis, MO).
FIGURE 7. Transformation of DDD and related compounds for initial
DDD concentration of 15.6 mM and for various other initial
conditions: (b) no iron; (O) 50 g/L iron with 25 mM NaOH; (1) 50
g/L iron; (3) 50 g/L iron with 250 mg/L Triton X-114.
Results and Discussion
The observed dechlorination kinetics discussed below do not
necessarily represent the intrinsic kinetics because other
factors may influence the apparent rate of dechlorination
such as the dissolution rate of crystalline DDT. The con-
centrations reported are total reactor concentrations, i.e., the
total mass of compound extracted from the reactor divided
by the aqueous volume.
and detector temperatures were 250 and 300 °C, respectively.
The column temperature was programmed to ramp from 85
°
C (5 min hold) to 150 °C (7 min hold) at a rate of 30 °C/ min,
then ramp to 154 °C (4 min hold) at a rate of 1 °C/ min, and
finally ramp to 290 °C (8 min hold) at a rate of 5 °C/ min.
The choice of which DDT transformation products to
measure was determined from the known DDT reductive
DDT Transform ation Experim ent. In these reactors, DDT
was spiked as the primary parent compound at 120 µM (43
3
4 5 2
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997

mg/ L). DDD and DDE were present at t ) 0 as impurities
form, subsequent dechlorination of DDD would likely be
limited by the rate of dissolution, as was the rate of
dechlorination of DDT. At the higher levels of iron (50 and
150 g/ L), the data suggest that the intrinsic dechlorination
reaction was faster than the rate of desorption yielding little
DDD accumulation. In the tests using surfactant (Figure 5),
it seems that the two lowest levels of iron did not provide a
reaction rate fast enough to avoid significant accumulation
of DDD as crystals or partitioned into surfactant micelles.
However, at the highest level of iron, little DDD accumulated,
suggesting that the dechlorination rate was faster than
desorption.
in the DDT at 7.5% and 2.4% (mol/ mol), respectively. Figure
3
shows that zero-valent iron successfully transformed DDT
with over 90% of the original mass removed within 20 days.
The figure shows almost equivalent DDT transformation
patterns for all three levels of iron tested. No significantly
large portion of the data set could be satisfactorily fit to a
simple rate model (zero order, first order, or Langmuir).
Nevertheless, a first-order degradation curve fit to the first 8
h of data (the first three data points) yielded the initial first-
order rates for each level of iron (Figure 4). Clearly, the effect
of increasing the mass of iron (i.e., the specific surface area
of iron) in the reactors on the reaction rate was insignificant.
The independence of the DDT transformation rate on the
specific iron surface area indicates that the transformation
rate was limited by the mass transfer of DDT, either from
slow dissolution of crystalline DDT or by poor mixing in the
reactors. Since the rate of mixing (not quantified) was much
faster than the apparent dechlorination rate, it is hypothesized
that the rate of dissolution of DDT limits the observed rate.
This hypothesis is supported by the results shown in Figure
DDE, an impurity in the DDT formulation, was trans-
formed by zero-valent iron as a parent chemical (Figure 1).
As with DDT, the initial first-order rates were statistically
independent of the specific iron surface area. The first order
rates averaged across the iron levels were 1.6 ( 0.6 and 2.6
-1
(
1.2 day , without and with surfactant, respectively. The
specific iron surface area normalized rates were 0.016 ( 0.006
-2 -1
and 0.025 ( 0.12 L m
h , respectively, very similar to the
rates for DDT. The DDE data suggests that DDE behaves
very similar to DDT in that the rates of dechlorination of both
compounds appear to be limited by the rate of dissolution.
5
. Here, in the presence of surfactant Triton X-114, the rate
of transformation of DDT was faster than without surfactant
and was also independent of specific iron surface area. Figure
The dynamics of DDMU, DDOH, DBH, and DBP, the other
products of the dechlorination of DDT, showed some
dependence on the iron level. Without surfactant present,
the compounds were observed in low concentrations when
they were not below detection limit. The behavior of these
compounds fits the scheme discussed above for DDD, namely,
surface area dependent apparent rates because the com-
pounds were produced at the iron surface. Somewhat higher
concentrations of the dechlorination products DDMU, DDOH,
DBH, and DBP were observed with surfactant present
corresponding to lower residual concentrations of DDT and
DDD. However, the levels of these dechlorination products
remained insignificant relative to the initial DDT concentra-
tion.
4
shows the calculated initial first-order transformation rate
of DDT in the presence of surfactant at the various iron levels.
Averaging the rates across iron levels yield DDT first-order
-
1
transformation rates of 1.7 ( 0.4 and 3.0 ( 0.8 day , without
and with surfactant, respectively, where the errors are 95%
confidence limits. For comparison to other published data,
these rates can be normalized by the specific iron surface
area. Since the rate is not a function of the specific surface
area, the lowest level of iron may be used for the normaliza-
tion. The specific surface area for 15 g/ L iron of this zero-
2
valent iron formulation is 4.3 m / L yielding specific first-
order rates of 0.016 ( 0.004 and 0.029 ( 0.008 L m
-
2
-1
h ,
without and with surfactant, respectively. These specific rates
are within a factor of 3 of the specific rates quoted elsewhere
for various chlorinated ethanes (5). Note that since the rates
observed here are limited by mass transfer, the intrinsic
dechlorination rates are likely to be much faster.
DDD is the product of the reductive dechlorination of
DDT (Figure 1) and is present initially as an impurity of the
DDT formulation. The dynamics of the DDD concentrations
varied greatly with the amount of iron and surfactant (Figures
Total priority pollutants, i.e., the sum of DDT, DDD, and
DDE concentrations, is a measure of the success of zero-
valent iron in converting these regulated chemicals into non-
regulated chemicals (Figures 3 and 5). For the greatest level
of iron (150 g/ L), the percent loss of total priority pollutants
was 93% and >99%, without and with surfactant, respectively,
indicating that zero-valent iron can be a very effective means
of remediating DDT-contaminated media.
3
and 5). Since the dechlorination rate of DDT (i.e., the
production rate of DDD) was virtually independent of iron
level, the dechlorination rate of DDD must be dependent on
the specific iron surface area.
DDD Transform ation Experim ent. Since DDD was a
product not the parent species in the DDT transformation
experiment above, a simple test using DDD at 11.0-12.6 µM
(
3.5-4.0 mg/ L) as the parent compound, with and without
Figure 6, adapted from Bizzigotti et al. (6), shows a simple
schematic model of the various processes competing in the
dechlorination of DDT and DDD. The data above suggest
that the dissolution of crystalline DDT into the aqueous phase
is slow relative to adsorption to iron and to dechlorination.
The presence of surfactant increases the apparent rate of
dechlorination. Thus, surfactant appears to speed the transfer
of DDT from the solid phase to the aqueous phase, i.e., the
rate of transfer from DDT solid to surfactant micelles to the
aqueous phase appears to be faster than direct dissolution
into the aqueous phase.
The scheme in Figure 6 also assists in understanding the
DDD concentration dynamics in Figures 3 and 5. Because
the amount of DDD present at t ) 0 is relatively small, most
of the DDD present is generated by dechlorination of DDT.
A molecule of DDD formed on the iron surface can feed either
into the next dechlorination reaction on the surface or can
desorb into the aqueous phase. In the surfactant-free test,
the DDD data in Figure 3 suggest that at the lowest iron level
surfactant, at 50 g/ L iron was conducted to determine the
rate of dechlorination of DDD. DDMU was present at t ) 0
as an impurity in the DDD at 5.8% (mol/ mol). Figure 7 shows
the dynamics of DDD and DDMU dechlorination. Zero-
valent iron was successful in transforming DDD and DDMU
with DDOH as the only product observed. Initial first-order
rates of DDD dechlorination were 0.95 ( 0.66 and 8.0 ( 0.8
-
1
day , without and with surfactant, respectively. DDMU is
a potential product of the dechlorination of DDD. However,
since DDMU never increased, apparently DDD was trans-
formed to DDMS and onto other products (Figure 1). No
transformations were observed in reactors lacking iron or
with addition of NaOH to quench the reaction.
The initial first-order rates of disappearance of DDMU
-1
were 1.5 ( 0.2 and 2.9 ( 2.4 day , without and with surfactant,
respectively. Since the kinetics as a function iron level were
not investigated in this test, the influence of iron level is not
known. However, because DDD and DDMU were introduced
into the reactors as solids, it is likely that the observed rates
of dechlorination for DDD and DDMU were limited by
dissolution rate, as discussed above.
(
15 g/ L) the intrinsic DDD dechlorination rate was slower
than the desorption rate resulting in DDD accumulation in
the aqueous phase as crystals of DDD. Once in the crystalline
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Other Results. Despite buffering, the pH increased with
time in all the reactors containing iron. In most reactors, the
pH increased to nearly 8 within 1 day and finished in the
range of 8-8.3. The oxidation-reduction potential decreased
rapidly from a level of +200 mV vs SHE to less than -300 mV
vs SHE within several hours and remained low for the duration
of the experiment (Figure 7). The amount of iron and the
addition of Triton X-114 had no significant effect on the
oxidation-reduction potential. In reactors to which NaOH
was added, oxidation-reduction potential remained above
mV vs SHE at all times and above +150 mV vs SHE most
of the time.
During the test, a black-colored film developed on the
reactor Teflon septa. The film was extracted in situ with the
other components in the bottle. The film was detachable by
sonication, but not all detached film was solubilized in 1%
HCl solution.
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This work was supported in part by the U.S. Environmental
Protection Agency’s National Risk Management Research
Laboratory Cooperative Agreements 816700 and 821029.
Although the research described in this paper has been funded
in part by the U.S. Environmental Protection Agency, it has
not been subjected to Agency review. Therefore, it does not
necessarily reflect the official views of the Agency.
Received for review February 25, 1997. Revised manuscript
X
received August 29, 1997. Accepted September 9, 1997.
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4 5 4
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