Environ. Sci. Technol. 2006, 40, 130-134
De Novo Formation of Chloroethyne
in Soil
FRANK KEPPLER,† REINHARD
BORCHERS,‡ JOHN T. G. HAMILTON,§
G E R H A R D K I L I A N , * ,| J E N S P R A C H T , | A N D
H E I N Z F . S C H Ö L E R |
Max-Planck-Institut for Nuclear Physics, Saupfercheckweg 1,
69117 Heidelberg, Germany, Max-Planck-Institute for Solar
System Resarch, D-37191 Katlenburg-Lindau, Germany,
Department of Agriculture and Rural Development,
Agriculture, Food and Environmental Science Division,
Queen’s University Belfast, Newforge Lane, Belfast BT9 5PX,
United Kingdom, and Institute of Environmental
Geochemistry, Heidelberg University, Im Neuenheimer Feld
236, D-69120 Heidelberg, Germany
To date, chloroethyne in the environment has been
proposed to occur as a reactive intermediate during the
reductive dechlorination of tri- and tetrachloroethene with
zerovalent metals. Such artificial conditions might possibly
be found at organohalide-contaminated sites that are
surrounded by remediation barriers made of metallic iron.
In this paper, it is shown that the highly reactive
chloroethyne is also a product of natural processes in
soil. Soil air samples from three different terrestrial ecosystems
of Northern Germany showed significant chloroethyne
concentrations, besides other naturally produced monochlorinated compounds, such as chloromethane, chloroethane
and chloroethene. Measured amounts range from 5 to
540 pg chloroethyne in air purged from 1 L of soil. A possible
route of chloroethyne formation in soil is discussed,
where chloroethyne is probably produced as a byproduct
of the oxidative halogenation of aromatic compounds in
soil. A series of laboratory studies, using the redox-sensitive
catechol as a discrete organic model compound, showed
the formation of chloroethyne when Fe3+ and hydrogen
peroxide were added to the system. We therefore propose
that the natural formation of chloroethyne in soil proceeds
via oxidative cleavage of a quinonic system in the
presence of the ubiquitous soil component chloride.
Introduction
The triple bond containing chloroethyne (chloroacetylene,
C2HCl) is an unstable, highly explosive gas with a nauseating
odor. It has a very limited application in chemical industries,
but it could possibly be an intermediate during the course
of the remediation of chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene (PCE) at polluted sites.
These chlorinated solvents are widespread groundwater and
soil contaminants in Europe and the United States. Recent
* Corresponding author present address: Institute of Environmental Geochemistry, Heidelberg University, Im Neuenheimer Feld
236, D-69120 Heidelberg, Germany; phone: ++49 6221 544818; fax:
++49 6221 545228; e-mail: [email protected].
† Max-Planck-Institut for Nuclear Physics.
‡ Max-Planck-Institute for Solar System Resarch.
§ Queen’s University Belfast.
| Heidelberg University.
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studies (1-5) have shown that chloroethyne may be formed
as an intermediate during reductive dehalogenation reactions
of TCE and PCE. Trichloroethene and tetrachloroethene can
be reduced by both biotic and abiotic transformation
processessmicrobially under methanogenic, acetogenic, and
sulfate-reducing conditions (3-8) and abiotically in the
presence of iron sulfides (9-12), iron oxides (9, 13), and
zerovalent metals such as iron and zinc (1, 2). However, these
reaction pathways cannot account for any presence of
chloroethyne in soil air under aerobic conditions.
To try to understand the whole picture, we must also look
at possible natural formation processes of chloroethyne.
Many halocarbons found in the environment have both
anthropogenic and natural sources. More than 3000 organochlorine compounds are known to be naturally produced
in a range of chemical, geological, or biological processes
[for detailed information see recent reviews (14-17)]. In some
cases, natural sources exceed anthropogenic emissions. For
example, chloromethane and trichloromethane (chloroform)
have significant natural terrestrial sources (18). It is also
noteworthy to mention that chlorine in soil is to a high extent
organically bound, often by more than 50% (15). Thus soils,
including peatlands, play an important role in the biogeochemical cycling of chlorine in the terrestrial environment
(15, 19, 20). Surprisingly, for most of the reported “natural
organochlorines” the underlying processes of formation are
unknown. Although our previous work (21) has shown that
chloroethene (vinyl chloride) is naturally formed in soil, the
mechanism of formation has not yet been clarified. To our
knowledge, natural chloroethyne formation has only once
been reported before from fumarole and lava gas samples (22). The authors suggested that chloroethyne and other
C2-chlorohydrocarbons are formed by a synthetic course,
including the thermolytic formation of acetylene from
hydrothermal methane, condensation reactions, and concomitant catalytic halogenation in the presence of highly
activated surfaces of cooling magma or juvenile ash.
The principal objective of this work was to investigate the
natural formation of chloroethyne in soil by measuring soil
air from different ecosystems. In addition, model experiments
were conducted to show a possible route of natural chloroethyne formation in soil.
Experimental Section
Materials and Methods. Chemicals. Catechol (1,2-dihydroxybenzene), potassium chloride, iron(III) sulfate, hydrogen peroxide (30%), and methanol, all chemicals of analytical
grade, were from Sigma-Aldrich (Steinheim, Germany).
Chemicals for the preparation of chloro- and dichloroethynes
cis-1,2-dichloroethene, trichloroethene, potassium hydride
(30% in mineral oil), tetrahydrofuranswere purchased from
Acros Organics (Geel, Belgium). Calibration standards of
volatile organic compounds in methanolic solution were from
Supelco (Taufkirchen, Germany). The water used in all
experiments was freshly prepared doubly distilled deionized
water.
Laboratory Experiments. The experiments with the
model compound catechol were carried out in a 20 mL sealed
glass vial containing 10 mL water and an initial concentration
of 2 mM catechol, 10 mM KCl, and either 10 mM Fe2(SO4)3
or 10 mM H2O2 or both 10 mM Fe2(SO4)3 and 10 mM H2O2.
All experiments were conducted at room temperature (22
°C). The vials were sealed and shaken on a rotary board (200
rpm) for 1 min. The helium carrier gas of the gas chromatograph was introduced by piercing the septum with two
stainless steel tubes, one as an inlet and one as an outlet. The
10.1021/es0513279 CCC: $33.50
 2006 American Chemical Society
Published on Web 11/23/2005
volatile compounds in the flasks were purged with helium
(20 mL/min) for 30 min and prefocused on a preconcentration trap, a stainless steel loop filled with glass beads at
-196 °C (liquid nitrogen). After the purge cycle was completed, the volatile chlorinated compounds were separated
by capillary gas chromatography and measured by mass
spectrometric detection as described below (Analytical
Methods). For each of the used chemicals single blanks were
measured with their respective working concentrations in
10 mL of doubly distilled deionized water. In all cases blanks
for chloroethyne were below the detection limit of the
analytical system (∼2 pg per 20-mL vial, derived from the
detection limit of chloroethene; see below).
Ambient and Soil Air Sampling in the Field. Three
sampling sites were selected in a rural area of SchleswigHolstein in Northern Germany (54°25′N/8°50′E) on the basis
of their vegetation and the proximity to the North Sea: a
coastal salt marsh, a peatland, and a deciduous forest
(predominantly birch). The sampling was carried out in early
spring, April 2001. Ambient air was sampled from about 1 m
above the soil surface by pumping 1 L of air through an
adsorbent tube filled with Carbotrap 300 (Supelco) at a rate
of 40 mL/min. Three to five measurements were done at
each sampling site. For collecting air from top soil, a stainless
steel cylinder with a volume of 1 L (r ) 5.0 cm, h ) 12.7 cm)
was used (for description see ref 21). The cylinder was
completely hammered into the soil and carefully removed
from the soil with a clean spade. Two stainless steel screw
caps (including a PTFE seal) with a connector (stainless steel
tube) were used to close the cylinder immediately from both
ends. Nearly no visible headspace was present between the
soil and the caps. The tube of the top screw cap was connected
to an adsorbent tube (see above), while the tube of the bottom
screw cap was connected to a nitrogen stream. Then the
cylinder containing the top-soil core was purged for 1 h at
a rate of 30 mL/min (1.8 L), and the volatile compounds of
the soil air were trapped by the adsorbent tube. The flow rate
was checked at both ends with a flow meter to make sure
that the soil core was permanently purged by a constant
nitrogen stream. For all samples no significant discrepancies
between the flow rate at the inlet and outlet of the cylinder
were observed, indicating that there was no overpressure
formed in the system. Three to five soil cores were used to
measure the soil air from each sampling site. After sampling,
the adsorbent tubes were sealed in stainless steel tubes and
kept at -24 °C in darkness until analysis.
Analytical Methods. The adsorbent tubes were analyzed
by mounting them into an automatic thermal desorption
unit (in-house preparation) that was connected to a Varian
3400 series gas chromatograph with a Saturn 2000 ion trap
mass spectrometer. The adsorbed compounds were desorbed
with helium gas at 200 °C for 15 min and refocused at -196
°C (liquid nitrogen) in a stainless steel loop filled with glass
beads. For injection, the cold trap was heated to 90 °C. Gas
chromatographic separation was carried out on a DB1
column (60 m × 0.32 mm, film thickness 1 µm) using the
following temperature program: initial oven -65 °C, increasing at 8 °C min-1 to 175 °C, 5 min isothermal. Mass
spectrometric detection was performed in scan mode over
a range of 48-200 amu.
Quantitation. Volatile chlorinated compounds were
identified by their retention times and mass spectra. External
calibration standards and multipoint calibration curves were
used to quantify the following compounds: chloroethene,
trichloroethene, and tetrachloroethene. The detection limits
of the compounds were in the range of 1-10 pg per desorption
step.
Soil Air. Although we calculated the porosity and thus an
estimated soil air volume (range 180-400 mL/L) for each
soil sample, from the measured bulk and particle densities,
FIGURE 1. Mass spectra: assumed chloroethyne peak from a
peatland soil air sample chromatogram, scanned from m/z 48 (A),
and chloroethyne, qualitatively synthesized reference standard, 0.5
µL headspace from the reaction vial (B).
respectively, we do not feel justified to give results in
concentration units. Due to unknown desorption time
constants and considering the analytical procedure (1.8 L of
purge gas vs less than 0.4 L of purged volume within 1 h) it
seems sound to stick to absolute amounts: units in pg, purged
and trapped from 1-L soil volume under these given
experimental conditions.
Preparation, Identification, and Quantitation of Chloroethyne. Qualitative standards of chloroethyne and dichloroethyne were prepared according to ref 23 from 0.1 mmol
of cis-1,2-dichloroethene and trichloroethene, respectively,
through elimination of HCl with potassium hydride in 2 mL
of tetrahydrofuran, enclosed in a 10-mL septum vial. Mass
spectra obtained by analysis of the 0.5-µL headspace agreed
with the NIST database entries. Whereas the identity of
chloroethyne in samples could thus be confirmed by its mass
spectrum (Figure 1) and retention time (Figure 2), quantification was not straightforward, due to the lack of a
quantitative reference standard. We tried a reasonable best
estimate for the response factor of chloroethyne by comparison with chloroethene, assuming an identical recovery
during the adsorb/desorb step for both analytes. The mass
spectrometric response is determined by the ionization cross
section for 70 eV electrons and the relative abundance of
those ions in the spectrum that are selected for quantitation.
The first factor is largely dependent on the ionization energy
of the molecule. For both analytes the molecular ion countss
at m/z 60 + 62 for chloroethyne and m/z 62 + 64 for
chloroethenesgave the best signal-to-noise ratio and were
chosen for quantitation. The small difference in ionization
energies can be neglected, with regard to the 70 eV electron
kinetic energy (Table 1). Therefore, the desired response
factor for chloroethyne was estimated to be 64.3/36.5 ) 1.76
times the instrumental response of chloroethene and is used
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FIGURE 2. Chromatogram of a peatland soil air sample with total
ion current (A) and m/z 60 + 62 (B).
TABLE 1. Factors Determining the Mass Spectrometric
Response
analyte
ionization
energy (eV)a
wt of M+ ion
count (%)b
chloroethyne
chloroethene
dichloroethyne
10.6
9.99
9.9
64.3
36.5
56.7
a Data from ref 24. b Computed from m/z 10-99 scans taken with the
same mass spectrometer at 70 eV.
in the following. A similar computation was performed for
dichloroethyne (m/z 94 + 96) in order to give an estimation
of its detection limit.
Results and Discussion
Field Measurements of Soil Air. Chloroethyne and chlorinated ethenes were determined in soil air of three different
ecosystems: coastal salt marsh, peatland, and deciduous
forest. Due to the relatively large variations, which can be
expected considering matrix heterogeneity and sampling
procedure, data of all individual measurements are shown
(Figure 3). In the air of topsoil, chloroethyne could be found
in the range of 5-540 pg purged from 1 L of soil, whereas
chloroethyne in ambient air at the sampling sites was not
detectable (detection limit ∼ 4 pg L-1). No dichloroethyne
was found in soil or ambient air (detection limit ∼ 3 pg per
sample). As TCE and PCE are common air, soil, and
groundwater pollutants, they were inevitably present in all
samples and showed background concentrations in soil air
between 16 and 200 pg purged from 1 L soil. Both TCE and
PCE were mainly monitored to exclude any conspicuously
elevated levels, which could indicate a contamination of the
sampling sites with these halocarbons. As mentioned earlier,
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the only conceivable pathway from TCE and PCE leading to
the chlorinated ethynes in the aerobic soil environment is
the dechlorination, e.g. by certain metals, under locally
strongly reducing conditions. In principle, enzymatically
mediated chloroethyne-generating processes, proceeding in
anoxic spots of soil crumbs or in deeper layers under
anaerobic conditions, cannot be ruled out completely without
further experimental work. The measured concentrations,
however, do not support the assumption of TCE or PCE being
precursors of chloroethyne found in soil air. In this case, it
is more likely that chloroethyne was formed naturally, by
processes different from the known degradation pathways
of TCE or PCE. This hypothesis gets support from the fact
that other monochlorinated compounds, such as chloromethane and chloroethene, have been found to occur
naturally in soils (21, 25). Accordingly, compared to ambient
air (9 pg L-1), concentrations of chloroethene in the topsoil
were strongly enhanced (100-1900 pg purged from 1 L of
soil). There were larger variations in the measured chloroethyne concentration among the replicates (RSD 98-125%)
of all sample sites than observed for the other analytes. This
could be caused either by the high reactivity of this compound
and thus fast reactions with other soil and air components
or analytical problems, such as poor trapping behavior or
partial decomposition of chloroethyne on the adsorbent
material used during the adsorb/desorb procedure. However,
these issues are not further investigated in this study.
Laboratory Experiments with Catechol. For model
experiments we presume the functional motifs of redoxsensitive aromatic 1,2-dihydroxy groups present in humic
substances as possible precursors for the formation of
chloroethyne in soil. Phenols and catechols, in particular,
are also important intermediates in the microbial degradation
of most aromatic compounds. Enzymes such as catechol
dioxygenases play a key role in the metabolism of those
molecules. The catalytic cleavage of catechols yields aliphatic
products by insertion of oxygen atoms into the aromatic
ring (26). According to their catalysis of the ring cleavage,
either between or outside the two o-hydroxy groups, catechol
dioxygenases are subdivided into intradiol and extradiol
cleaving enzymes. Both enzymes are non-heme iron enzymes: intradiol-cleaving enzymes contain Fe3+ in their active
site, whereas extradiol-cleaving dioxygenases possess an Fe2+
center (27). An alternative abiotic cleavage of the aromatic
ring has been observed recently (28). Laboratory experiments
with phenolic compounds have shown that catechol can be
oxidized abiotically by Fe3+ to produce CO2. Presumably,
2,4-hexadienedioic acid (muconic acid) and 2-oxo-6-hydroxyhexanoic acid are intermediates of this reaction, but
the pathway is not established. Remarkably, neither sunlight
nor microbial mediation is required for this process. For this
reason catechol was selected as a model compound to
investigate natural chloroethyne formation.
The reaction of catechol and Fe3+ (Figure 4) in the presence
of chloride demonstrates the formation of the triple bonded
chloroethyne. Concentrations of the contaminants TCE and
PCE were in the same range as measured for the blanks,
indicating that these compounds were not formed in the
catechol-Fe3+-chloride system. Chloroethyne formation is
accompanied by further volatile chlorinated breakdown
products, such as chloroethene and monochlorinated alkanes
(see also ref 21). No dichloroethyne was generated in any of
the experiments (detection limit < 2 pg per vial). Enhanced
production of chloroethyne could be observed when hydrogen peroxide (H2O2) was added to the catechol-Fe3+chloride system. Chloroethyne production increased drastically by a factor of 20, whereas the amount of chloroethene
produced was only slightly enhanced. No formation of
chloroethyne was observed by using only H2O2 as an oxidant
(data not shown). We suppose that hydroxyl radicals (•OH)
FIGURE 3. Selected volatile C2-organochlorines measured in the top soil air of three different ecosystems (A-C) and in ambient air at
the sampling sites (D).
FIGURE 5. Suggested pathway of chloroethyne formation.
FIGURE 4. Production of chloroethyne and chloroethene by the
reaction of catechol, chloride, Fe(III), and H2O2. Initial concentrations: 2 mM catechol, 10 mM KCl, and 10 mM Fe2(SO4)3, or 2 mM
catechol, 10 mM KCl, 10 mM Fe2(SO4)3, and H2O2 (n ) 3; RSD 7-56%).
The pH in the medium was in the range 2.5-3.0.
were formed in the system, when both Fe3+ and H2O2 were
combined. This would enable the Fenton reaction, where
H2O2 reacts with Fe2+ to generate hydroxyl radicals. Fe2+ is
rapidly provided by the reaction of catechol with Fe3+. The
•OH radical is a powerful, nonspecific oxidant that can react
with both organic compounds and chloride. Organic radicals
and chlorine easily form organochlorine compounds.
We also could measure several longer chained triple
bonded compounds such as butenyne and butadiyne (not
quantified), which probably helps to better clarify the
underlying process of chloroethyne formation in soil. The
actual mechanism, both in vitro and in soil, is still unknown
at this time. However, we can suggest a draft reaction pathway
for the natural production of chloroethyne (Figure 5). It is
likely produced as a byproduct of ring cleavage processes of
susceptible aromatic molecules during the decomposition
of organic matter. The widespread environmental compound
catechol acts as the organic matter source. Oxidation of
catechol, induced by Fe3+, could either yield phenolic
polymers (due to oxidative coupling processes) and/or
saturated and unsaturated aliphatic compounds. The initial
intermediate oxidation products are thought to be semiquinone and quinone. The oxidation may proceed, in the
presence of Fe3+, by effectively introducing oxygen atoms
into the aromatic ring and eventually forming aliphatic
compounds. Subsequent oxidation and/or reduction of these
compounds could form a variety of aliphatic molecules. A
fraction of these will be chlorinated when chloride is present
in the system. Up to now we have found several monochlorinated volatile compounds, e.g. chloroethyne, chloroethene,
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and (data not shown here; see also ref 21) chloromethane,
chloroethane, chloropropane, and chlorobutane, which
could, at least partially, originate from the same type of
geochemical process. The similarity between the spectrum
of halogenated volatiles produced in laboratory experiments
and that which is found in soil air may support the idea of
catechol being the key substrate in these reactions.
We see our work as a preliminary study showing the
natural occurrence of an unexpected highly unsaturated
organochlorine compound in soil. It might also add to our
understanding of natural chlorination processes.
Acknowledgments
This work was supported by grants from the Deutsche
Forschungsgemeinschaft (GRK 273) and through a European
Community Marie Curie Fellowship (MCFI-2002-00022) to
F.K.
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Received for review July 8, 2005. Revised manuscript received
October 11, 2005. Accepted October 25, 2005.
ES0513279