Groundwater Quality: Natural and Enhanced Restoration of Gioundwater Pollution (Proceedings ofthe
Groundwater Quality 2001 Conference held at Sheffield. UK. June 2001). IAHS Publ. no. 275. 2002.
175
Role of electron acceptors and donors in the
breakdown of chlorinated hydrocarbons
XIAOXIA LU, G U A N G H E LI
Department of Environmental
Beijing 100084, China
Science and Engineering,
Tsinghua
University,
e-mail: [email protected]
SHU T A O
Department of Urban and Environmental
Science, Peking
University,
Beijing 100871, China
J O H N A. DIJK
Wageningen University, 6703 CT Wageningen,
The
Netherlands
T O M N. P. B O S M A & JAN C E R R II SE
TNO Environment,
Energy and Process Innovation,
7300 API Ape/doom,
The
Netherlands
Abstract Soil column experiments were performed to investigate the trans­
formation of chlorinated hydrocarbons in the presence of different electron
acceptors or electron donors including N a N 0 , Fe(OH) , M n 0 , or a mixture
of volatile fatty acids (VFAs), respectively. The results indicated that 1,2dichloroethane (1,2-DCA) and vinyl chloride (VC) could be oxidized
anaerobically under manganese reducing and denitrifying conditions. Profile
measurements in the Mn0 -amended column revealed half-lives of 1.3 h for
1,2-DCA, and 4.7 h for VC degradation. Under iron reduction, only 1,2-DCA
was degraded. In a control column, without the addition of donors and
acceptors, not only 1,2-DCA and VC, but also chlorobenzene (MCB) was
removed. In the column supplied with VFAs, sequential reductive dechlorin­
ation of tetrachloroethene (PCE) to ethene was observed, and profile measure­
ments revealed that half-lives of PCE, trichloroethene, cis-l,2-dichloroethene
and VC were 0.8 h, 1.3 h, 1.6 h and 3.4 h, respectively.
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Key words a n a e r o b i c soil c o l u m n s ; c h l o r i n a t e d h y d r o c a r b o n s ; e l e c t r o n a c c e p t o r s ;
electron d o n o r s ; half lives; oxidative degradation; reductive dechlorination
INTRODUCTION
Soil and groundwater contamination by chlorinated hydrocarbons is a well-recognized
environmental problem. Studies showed that these toxic compounds might be
degraded biologically through their use by microorganisms as electron donors and/or
electron acceptors, depending on the redox conditions predominating in the soil
environment (Wiedemeier et al, 1998).
In anaerobic circumstances, particularly under sulphate reducing and methanogenic
conditions, reductive dechlorination is a well-established process and usually more
effective for higher chlorinated hydrocarbons than lower chlorinated ones (Suarez &
Rifai, 1999). Recent research reported that vinyl chloride (VC) and cis-dichloroethene
(cis-DCE) can also be degraded through anaerobic oxidative pathways, where oxidized
Xiaoxia Lu et al.
176
iron or manganese serves as the electron acceptor (Bradley et al., 1996, 1997, 1998).
Recently, we demonstrated the anaerobic oxidation of 1,2-dichloroethane (1,2-DCA)
with nitrate as the electron acceptor (Gerritse et al, 1999), and revealed the potential
for anaerobic oxidation of various (chlorinated) hydrocarbons (Dijk et al., 2000).
The aim of this study was to further investigate the influence of different electron
acceptors and donors, including N a N 0 3 , M n 0 2 , Fe(OH)3, or volatile fatty acids
(VFAs), on the transformation of selected chlorinated hydrocarbons.
C O L U M N SETUP
Five glass columns (35 cm length, 3.6 cm inner diameter) were packed in an anaerobic
glove box with a mixture of sediments sampled from four different contaminated sites
in The Netherlands (Rotterdam, Arnhem, Tilburg and Uden). The columns were
percolated with mineral medium at an up-flow rate of 1.9 cm h" . The medium
consisted of NH C1 ( l O O r n g f ) , M g S 0 - 7 H 0 ( l O m g l " ) , C a C l - 2 H 0 ( 5 m g l " ' ) ,
trace elements solution (0.1 ml F ), and phosphate buffer (2 m M , pH 7.0). It was
constantly stirred and flushed with N / C 0 (90:10, v/v) and pumped up-flow into the
columns by a peristaltic pump (505S, Watson Marlow Ltd, UK). One column was used
as a control (without external electron acceptors or donors), and the other four columns
were supplied with N a N 0 , M n 0 , Fe(OH) and VFAs (prepared by mixing acetic,
propionic, valeric, isobutyric, isovaleric and butyric acids in a concentration ratio of
18:1.5:1.2:1.2:1:1, respectively). N a N 0 and VFAs were constantly injected by
syringe, with both influent concentrations being 5 m M . While M n 0 and Fe(OH)
were amended in the sediments at the beginning of the experiment, with the initial
concentrations being - 0 . 1 4 mol g" of soil and - 0 . 2 2 mol g" of soil, respectively. For
the control and electron acceptor amended columns, a mixture of chlorinated
compounds—containing VC (10 uM), trans-1,2-dichloroethene (trans-DCE) (100 uM),
cis-DCE (100 uM), 1,2-DCA (100 uM), 1,1,1- trichloroethane (1,1,1-TCA) (10 uM),
trichloroethene (TCE) (50 u M ) and M C B (10 u M ) — w a s injected into the medium
with a syringe pump (sp200i, WPI, USA), just before entering the column. For the
VFAs-amended column, a mixture of 1,2-DCA (10 pM), 1,1,1-TCA (10 uM), PCE
(10 uM) and M C B (10 uM) was injected into the column. All the columns were
operated in a temperature controlled room (20°C).
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4
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RESULTS
Oxidative degradation
The influent and effluent concentrations of the studied chlorinated hydrocarbons were
monitored over a period of approximately 200 days. Table 1 summarizes the observed
removal extent (%) of each compound after six months of operation.
In the column supplied with Fe(OH) , only 1,2-DCA was degraded. The 7 4 %
removal could not be attributed to adsorption, because compared to some of the other
compounds such as 1,1,1-TCA and M C B , the adsorption potential of 1,2-DCA is much
lower. In the M n 0 - a m e n d e d and NaN03-amended columns, more than 7 5 % of both
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Role of electron acceptors and donors in the breakdown of chlorinated hydrocarbons
177
Table 1 Extent of removal (%) of the studied chlorinated hydrocarbons in different columns.
Chlorinated hydrocarbons
Columns:
NaNO,
Mn0
VC
cis-DCE
Trans-DCE
1,2-DCA
1,1,1-TCA
TCE
MCB
89
<10
<10
100
<10
<20
<40
76
<40
<40
97
<40
<40
<40
2
Fe(OH)
<40
<40
<40
74
<40
<40
<40
3
Control
100
<20
<20
100
<40
<20
73
1,2-DCA and VC were removed. The relatively high outlet/inlet ratios (0.7-0.9) found
for the other chlorinated compounds supplied, indicated that the observed removal of
1,2-DCA and VC was mainly due to (microbial) degradation. Interestingly, in the
control column, not only 1,2-DCA and VC, but also M C B appeared to be removed.
These observations suggest that the sediments sampled from the contaminated sites
contained bacteria capable of transforming 1,2-DCA, VC and M C B . However, it was
still not clear what would be the terminal electron accepting processes (TEAP) in the
control column. Significant concentrations of reductive products, such as ethene,
ethane and benzene, were not detected, suggesting that the observed removal may have
proceeded through anaerobic oxidative pathways.
To further investigate the role of electron acceptors in the breakdown of selected
chlorinated hydrocarbons, profile measurements of both substrates and redox species
over the length of columns were performed. In addition, the dissolved hydrogen (H2)
concentrations were determined to characterize the redox conditions (Jakobsen et al.,
1998). Figure 1 displays profile measurements in the MnCVamended column on day 180.
Column length (cm)
Column length (cm)
Fig. 1 Profile measurements of chlorinated hydrocarbons (a), dissolved H and ratio of
Mn(ll) to Mn(lV) (b), in the Mn0 -amended column.
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In the first 4 cm, 1,2-DCA and VC concentrations rapidly declined from 50 u M
and 20 pJVl to below 4 JLIM with half-lives of 1.3 h and 4.7 h, respectively. In contrast,
M C B gradually declined over the distance o f t h e whole column according to a half-life
of 5.7 h. Since M C B has a high soil adsorption potential (K 245 1 kg" ), it was not
clear whether microbial activities were involved in its removal. For the other
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178
Xiaoxia Lu et al.
compounds, concentrations along the column did not change significantly. In view of
the heterogeneous distribution of M n 0 in the column (amorphous MnO? was
amended in the sediments at the start of the experiment), the ratio of Mn(II) to Mn(IV)
was used to describe the production of Mn(II). The gradual relative increase of Mn(II)
over the column distance suggested that microbial MnC>2 reduction occurred. Further­
more, the dissolved H concentrations in the range of 0.4 n M were within the reported
H thresholds for MnC>2 reduction (Jakobsen et al., 1998). Similar phenomena occurred
in the N a N 0 - a m e n d e d column where the concentration of nitrite increased from 6 u M
to 12 p M over the length of the column, confirming the presence of nitrate reduction.
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Reductive dechlorination
In the column amended with VFAs (mixture of acetate (3 mM), propionate (1 m M ) ,
valerate (0.2 mM), isobutyrate (0.1 mM), isovalerate (0.1 mM), and butyrate (0.5 mM)),
cis-DCE, VC and ethene were detected in the effluent after two months of operation,
demonstrating the reductive dechlorination of PCE. Figure 2 illustrates the profile
measurements of chlorinated hydrocarbons and VFAs along the column on day 120.
Column length (cm)
Column length (cm)
Fig. 2 Profile measurements of chlorinated hydrocarbons (a), and volatile fatty acids
(b), in the VF As-amended column.
The detection of TCE in the influent indicated that dechlorinating microbes were
already active around the sampling port. Within a distance of 8 cm, PCE and TCE
were completely converted to cis-DCE and VC. In the effluent, ethene was the main
end product together with a trace amount of VC. The half-lives calculated for PCE,
TCE, cis-DCE and VC were 0.8 h, 1.3 h, 1.6 h and 3.4 h, respectively. Stoichiometric
calculations showed that more ethene was yielded than possible from the degradation
of PCE alone. This suggests that 1,2-DCA was also dechlorinated to ethene.
Propionate, butyrate and valerate were rapidly consumed in the column. Corresp­
ondingly, the concentration of acetic acid was increased, most likely due to the
fermentation of the organic acids. N o consumption of isobutyrate and isovalerate was
observed. Apparently, propionate, butyrate and valerate were used as electron donors
for dechlorination, either as a direct substrate for the dechlorinating bacteria or via H
formed during VFA-degradation.
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Role of electron acceptors and donors in the breakdown of chlorinated
hydrocarbons
179
CONCLUSIONS
Anaerobic oxidative degradation of 1,2-DCA and VC occurred in the presence of
N a N 0 and M n O i . Profile measurements in the M n O j - a m e n d e d column showed that
the half-lives of 1,2-DCA and VC were 1.3 h and 4.7 h, respectively, In the presence of
oxidized iron, only 1,2-DCA was degraded, whereas in a control column (no electron
donor or acceptor supply), not only 1,2-DCA and VC, but also M C B , were partially
removed. Additional research is required to prove the apparent biodégradation of
MCB. In a VF As-amended column, PCE was reductively dechlorinated to ethene, and
propionate, butyrate, and/or valerate were consumed.
3
A c k n o w l e d g e m e n t s This project was a cooperation between Peking University and
T N O (Netherlands Organization for Applied Scientific Research) and was financially
supported by the Royal Netherlands Academy of Arts and Science, and the Chinese
Major State Basic Research Development Program: Study on Environmental Pollution
Mechanisms and Control Theory (G1999045711).
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