Natural formation of vinyl chloride in the terrestrial environment

Article abstract of DOI:10.1021/es015611l

Vinyl chloride is a highly reactive and toxic substance which is widely used in industry. It is the parent compound of poly(vinyl chloride) (PVC), one of the most important industrial polymers. Until now, it was thought that vinyl chloride found in the environment is exclusively man-made or results from the degradation of other anthropogenic substances, such as trichloroethylene and tetrachloroethylene. Here, we demonstrate that vinyl chloride also has natural sources. Soil air and ambient air from a rural area in Northern Germany were investigated for volatile chlorinated halocarbons. The concentrations of vinyl chloride in the soil air were significantly enhanced as compared to ambient air, indicating a natural formation of this compound in the soil. A series of laboratory experiments using different soils and model compounds was conducted, which clearly proved that vinyl chloride could be produced during soil processes. We propose that this highly reactive compound can be formed during the oxidative degradation of organic matter in soil, for example, in a reaction between humic substances, chloride ions and an oxidant (ferric ions or hydroxyl radicals). The redox-sensitive aromatic compounds in soil such as catechols and o-quinones can be degraded to CO₂, accompanied by the release of vinyl chloride and other volatile chlorinated compounds. This process could have started in the Late Silurian to Early Devonian, 400 million years ago, when the first soils on earth evolved.

Full text of DOI:10.1021/es015611l

Environ. Sci. Technol. 2002, 36, 2479-2483
roform, chlorophenols, and chlorinated dioxins, long believed
Natural Formation of Vinyl Chloride
in the Terrestrial Environment
to be only of anthropogenic origin, also have significant
natural terrestrial sources (10-12). Thus, the terrestrial
environment plays an important role for the emission of
chlorine into the atmosphere and the biogeochemical cycling
of chlorine in soil (13).
F R A N K K E P P L E R , * , †
R E I N H A R D B O R C H E R S , § J E N S P R A C H T , †
S T E F A N R H E I N B E R G E R , † A N D
H E I N Z F . S C H O¨ L E R †
In contrast to many organochlorine compounds, it is still
believed that vinyl chloride (VC) found in the environment
is solely man-made. VC is a known carcinogen and regulated
chemical (14, 15). Most VC produced is further polymerized
to poly(vinyl chloride) (PVC). A part of the industrially
produced VC (95%) enters the environment by air or
wastewater. VC in water or soil evaporates easily into the
troposphere, where it is rapidly oxidized by photochemically
produced hydroxyl radicals (lifetime, 2-3 days). Any VC which
does not evaporate may leach to the groundwater and may
be subject to biodegradation under aerobic and anaerobic
conditions (16-18). Another anthropogenic source of VC in
the environment comes from the degradation of chlorinated
solvents such as trichloroethylene (TCE) and tetrachloro-
ethylene (PCE) (19-22). Here, we present field investigations,
controlled laboratory studies, and model experiments which
imply that vinyl chloride also has natural sources.
Institute of Environmental Geochemistry,
Heidelberg University, Im Neuenheimer Feld 236,
D-69120 Heidelberg, Germany, and Max-Planck Institute for
Aeronomy, D-37191 Katlenburg-Lindau, Germany
Vinyl chloride is a highly reactive and toxic substance
which is widely used in industry. It is the parent compound
of poly(vinyl chloride) (PVC), one of the most important
industrial polymers. Until now, it was thought that vinyl
chloride found in the environment is exclusively man-made
or results from the degradation of other anthropogenic
substances, such as trichloroethylene and tetrachloroeth-
ylene. Here, we demonstrate that vinyl chloride also has
natural sources. Soil air and ambient air from a rural area
in Northern Germany were investigated for volatile
chlorinated halocarbons. The concentrations of vinyl
chloride in the soil air were significantly enhanced as
compared to ambient air, indicating a natural formation of
this compound in the soil. A series of laboratory experiments
using different soils and model compounds was conducted,
which clearly proved that vinyl chloride could be produced
during soil processes. We propose that this highly
Materials and Methods
Chem icals. Catechol (1,2-hydroquinone), potassium chlo-
ride, iron(III) sulfate, hydrogen peroxide (30%), and methanol
were purchased from Sigma-Aldrich (Steinheim, Germany).
Humic acid was obtained from Fluka (Buchs, Switzerland).
Volatile organic compounds calibration standards were from
Supelco (Taufkirchen, Germany). The water used in all
experiments was freshly prepared doubly distilled, deionized
water.
Soil Sam ples for Laboratory Studies. Soil samples were
collected on September/ October 1998 at two different
sampling sites on the basis of their relatively uninfluenced
history: soil no. 1, a grassland soil of western Patagonia/
Chile (52°48′36′′S/ 72°55′45′′W); and soil no. 2, a peatland
soil from the natural reserve Rotwasser Odenwald/ Germany
(49°36′39′′N/ 8°53′11′′E). Using a clean spade, four samples
(500-1000 g) were collected from the top soil (O and A
horizon) in a square 1 ×1 m. The four samples were combined
to a bulk sample in a polyethylene plastic bag and transported
to the laboratory. The soil samples were freeze-dried, milled
(0.24 mm mesh), and stored in a freezer (-24 °C) until
chemical analyses or laboratory experiments were conducted.
Soil samples and the humic acid were analyzed for pH,
organic carbon (Corg), total iron, and halogens.
reactive compound can be formed during the oxidative
degradation of organic matter in soil, for example, in a
reaction between humic substances, chloride ions and an
oxidant (ferric ions or hydroxyl radicals). The redox-
sensitive aromatic compounds in soil such as catechols
and o-quinones can be degraded to CO , accompanied by
2
the release of vinyl chloride and other volatile chlorinated
compounds. This process could have started in the
Late Silurian to Early Devonian, 400 million years ago,
when the first soils on earth evolved.
Introduction
Experim ental Procedure. For laboratory experiments on
the formation of vinyl chloride from two different soils and
a commercially available humic acid, 20-mL glass vials were
used as reaction bottles (crimp-capped sealed with a PTFE-
lined butyl rubber septum). All experiments were conducted
at room temperature (22 °C). A total of 1000 mg milled soil
or humic acid was added to 10 mL of doubly distilled
deionized water. 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 volatile compounds in the flasks were purged by helium
(20 mL/ min) for either 15, 30, 45, or 60 min and focused on
a preconcentration trap. After the purge cycle was completed,
the volatile chlorinated compounds were measured by
capillary gas chromatography and mass spectrometric de-
tection as described in the following section below (analytical
methods). Three replicates were measured for each sample
Many halocarbons found in the environment have both
anthropogenic and natural sources. More than 1800 orga-
nochlorine compounds are known to be naturally produced
in a range of chemical, geological, or biological processes (1,
2), but current knowledge of the known sources, sinks, and
global burdens of these organochlorines is incomplete. In
some cases, natural sources exceed anthropogenic emissions.
Chloromethane (CH3Cl) is by far the most abundant volatile
halocarbon in the atmosphere (3) with mainly natural sources
(4). For the terrestrial environment, both biological and
abiotic sources of CH3Cl have been identified (5-9). Chlo-
* Corresponding author phone: ++49-6221-546003; fax: ++49-
6221-545228; e-mail: fkeppler@ix.urz.uni-heidelberg.de. Present
address: Institute of Environmental Geochemistry, Heidelberg
University, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany.
† Heidelberg University.
§
Max-Planck Institute for Aeronomy.
9
10.1021/es015611l CCC: $22.00
Published on Web 04/17/2002
2002 Am erican Chem ical Society
VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 4 7 9

(n ) 3), and the mean is shown in the figures. The coefficient
of variation (RSD) was in the range of 12-33%. Blanks were
analyzed according to the same procedure but without the
addition of soil or humic acid.
The experiments with the model compound catechol were
carried out in a 20 mL sealed glass vial containing 10 mL of
doubly distilled deionized 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.
The same experiments were performed using 1000 mg of
milled grassland soil from Patagonia instead of catechol. The
volatile compounds in the flasks were monitored as described
previously. Three replicates were measured for each sample
(n ) 3), and the mean is shown in the figures. The coefficient
of variation (RSD) was in the range of 7-56%. Blanks were
measured by the use of doubly distilled deionized water,
which were, for vinyl chloride, very close to detection limit
of the analytical system (around 2 pg).
Am bient and Soil Air Sam pling in the Field. Three
sampling sites were selected in a rural area of Schleswig-
Holstein 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 forest). The sampling was carried out
in early spring, April 2001. Ambient air was sampled from
about 1 m above the soil surface (on a stand) by pumping
1 L of air through an adsorbent tube filled with Carbotrap
300 (Supelco) at a rate of 40 mL/ min. After sampling the
adsorbent tubes were sealed in stainless steel tubes and kept
at -24 °C in darkness until analysis. Three to five measure-
ments were done at each sampling site. For collecting air
from topsoil a stainless steel cylinder with a volume of 1 L
(r ) 5 cm, h ) 12.7 cm) was used (Figure 1) 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 headspace was present between the soil and
the caps. The tube of the top screw-cap was connected to
an adsorbent tube while the tube of the bottom screw-cap
was connected to a nitrogen stream. Then, the cylinder
(including the topsoil 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 in- 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 sample site. After the field measure-
ments, all of the samples were transported to the laboratory
and stored in a freezer until further analyses were carried
out.
Analytical Methods. The adsorbent tubes were analyzed
by mounting them to an automatic thermal desorption unit
(in-house preparation) which was connected to a gas
chromatograph and a ion trap mass spectrometer. The
adsorbed compounds were desorbed with helium gas at 200
°C for 15 min, and they were trapped -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 rate 8 °C min-1 to
175 °C, 5 min isothermal. Mass spectrometric detection was
performed over a scan range of 48-200 amu. Vinyl chloride
and other chlorinated compounds were identified from their
retention time and mass spectrum (Figure 2). External
calibration standards and multipoint calibration curves were
FIGURE 1. Apparatus for sampling soil air from the top soil layer
(O and A horizon).
used to quantify the following compounds: chloromethane,
vinyl chloride, trichloroethylene, tetrachloroethylene, and
1,1,1-trichloroethane. The detection limits of the compounds
were in the range of 2-5 pg. All samples were clearly above
the detection limit. Blanks of the adsorbent tubes were
measured after thermal conditioning.
Calculation of the Concentration in the Soil Air. The
concentrations (ng L-1) of the compounds in the soil air were
calculated from the amounts of adsorbed compounds on
the adsorbent tubes (ng) and the volume of air in the soil (L).
Therefore, it was assumed that all volatile chlorinated
compounds in the soil air were purged and trapped by the
adsorbent tube. Total air volume in the soil was calculated
from the porosity and the measured water content of the soil
samples. Porosity of the soil was determined from the
measured bulk (BD) density and the particle density (PD).
The air volume of the soils was in the range of 180-400 mL/ L
(n ) 3).
Results and Discussion
Volatile chlorinated hydrocarbons were measured in ambient
air and soil air from the coastal salt marsh, peatland, and
deciduous forest. The mean concentration of vinyl chloride
in the air of topsoil was in the range of 0.6-7.7 ng L-1 (Table
1) with a large variation among the replicates (RSD 10-65%).
Compared to ambient air, the vinyl chloride concentration
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FIGURE 2. GC-MS analysis of vinyl chloride: (a) external standard with 20 ng of VC; (b) adsorbent tube after thermal conditioning used
as a blank; (c) soil air from a salt marsh. The chromatograms show the response to the molecular ion m/z 62. The scale of the ordinate
is different for each chromatogram; the values at scan 520 for the peak area are 2 000 000, 370, and 120 000, for (a), (b), and (c), respectively.
TABLE 1. Concentration of Volatile Chlorinated Compounds Measured in the Top Soil Layer of Coastal Marsh, Deciduous Forest,
and Peatland
-1
concentrations of compounds (ng L )
vinyl chloride
chloromethane
1,1,1-trichloroethane
trichloroethene
Coastal Marsh
53.5 (40.4; 42.3; 77.8)
top soil aira
am bient airb
7.68 (5.5; 8.4; 9.14)
0.009 ( 0.003
0.32 (0.18; 0.24; 0.54)
0.15 (0.10; 0.25; 0.10)
1.24 ( 0.25
0.51 ( 0.11
0.13 ( 0.05
Deciduous Forest
4.5 (3.7; 6.8, 3.9; 5.1; 3.0)
1.24 ( 0.25
top soil aira
am bient airb
0.58 (0.62; 0.63; 0.48)
0.009 ( 0.003
0.42 (0.42; 0.38; 0.47)
0.10 (0.12; 0.11; 0.07;)
0.51 ( 0.11
0.13 ( 0.05
Peatland
4.0 (3.4; 4.2; 2,9; 5.4)
1.24 ( 0.25
top soil aira
am bient airb
1.0 (0.35; 1.12 1.2; 1.33)
0.009 ( 0.003
0.19 (0.23; 0.2; 0.2; 0.13)
0.51 ( 0.11
0.17 (0.16; 0.17; 0.18; -)
0.13 ( 0.05
a
b
The m ean value is given (n ) 3-5) and each single num ber (in brackets). Am bient air concentration represents the m ean value of all sam ples
sites (n ) 9) ( standard deviation.
in the topsoil was around 850, 110, and 60 times greater for
the coastal salt marsh, peatland, and deciduous forest,
respectively. For chloromethane, the concentration of soil
air was in the range of 4.0-53.5 ng L-1, and the concentration
ratios of soil air/ ambient air were around 40, 4, and 3 for the
coastal marsh, peatland, and deciduous forest, respectively.
The corresponding CH3Cl/ VC ratios were 7, 8, and 4. In
contrast to VC and CH3Cl, the concentration of 1,1,1-
trichloroethane (CH3CCl3), which is ascribed to anthropo-
genic sources (23), was in the range of the tropospheric
background (0.51 ( 0.11 ng L-1); the concentration ratios
between soil air and ambient air were close to unity (0.4-
0.8). Trichloroethylene and tetrachloroethylene, which are
known to be precursors of VC under anaerobic conditions,
showed soil air concentrations similar to ambient air. The
discovery of significantly enhanced VC and CH3Cl concen-
trations in soil air, in the presence of background tropospheric
concentrations of CH3CCl3, TCE, and PCE, indicate a natural
formation of VC in soil. As the VC, TCE, and PCE concentra-
tions in ambient air are very low, the significantly enhanced
concentrations for vinyl chloride in soil air cannot be
explained with a general contamination of the ambient air.
However, in environmental analytical studies, the risk of
systematic errors can be high and may lead to wrong
conclusions. Therefore, careful attention has been paid to
the fact that VC, TCE, and PCE are common groundwater
and soil pollutants and that TCE and PCE are also atmospheric
pollutants. No indication of a contamination source was
found in the studied soils, and the contribution of contami-
nation from the sampling equipment and analytical proce-
9
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FIGURE 4. Proposed building block of humic acid.
FIGURE 3. Formation of vinyl chloride by two organic-rich soils
and humic acid. A total of 1000 mg of grassland, peatland, or humic
acid was added to 10 mL of water in a 20-mL glass vial (n ) 3; RSD
12-33%). The pH in the medium was 4.1, 4.2, and 5.4 for grassland,
peatland, and humic acid, respectively.
SCHEME 1
FIGURE 5. Production of VC and CH Cl by the reaction of catechol,
3
chloride, and Fe(III). Initial concentrations were 2 mM catechol, 10
mM KCl, and 10 mM Fe2(SO ) or 2 mM catechol, 10 mM KCl, 10 mM
4 3
dure was controlled by measuring blanks.
Fe2(SO ) , and H O (n ) 3; RSD 7-56%). The pH in the medium was
4 3
2 2
To evaluate the natural production of VC, we performed
several series of controlled laboratory experiments in an
aqueous medium containing soil material from pristine areas.
After suspending the soil samples in distilled water (no
additives were used), in vitro production of vinyl chloride
was observed (Figure 3). Both soil samples showed a
significant release of vinyl chloride, at rates of up to 120 pg
g-1 h-1. Besides VC, the production of CH3Cl, C2H5Cl, and
C3H7Cl could also be measured (Scheme 1) (data not shown).
CH3CCl3, TCE, and PCE were within the range of the blank
values, indicating that there was no in vitro production of
these compounds. The average CH3Cl/ VC ratio was around
7 for grassland soil and around 12 for peatland. Furthermore,
the formation of vinyl chloride was also observed with the
commercially available humic acid. The production rate of
VC from humic acids was similar to those of the grassland
soil.
VC could be formed by a variety of biotic and abiotic soil
processes. In this study, we focused our investigations on
abiotic processes. Our previous work (5) has shown that
methyl halides are produced naturally in soil during the
degradation of organic matter. In the presence of Fe(III) and
halide ions, monomeric constituents of humic substances
(HS) such as 2-methoxyphenol (guaiacol) can undergo
oxidation to quinones with concomitant formation of the
corresponding monohalomethane. A second reaction that
may occur in soil during the oxidation of organic material
is the formation of a highly reactive chlorinated compound
such as VC. In this case, we considered the redox-sensitive
functional aromatic groups of HS such as catechols and
o-quinones (Figure 4) as precursors for VC. Next to being
aromatic groups of HS, catechols also play a key role in the
biological degradation of aromatic structures. Laboratory
experiments with phenolic substances have shown that
in the range of 2.5-3.0.
catechol can be oxidized by Fe(III) to produce CO2 (24).
Neither sunlight nor microbial mediation are required for
this process. For this reason, catechol was selected for model
experiments. The reaction of catechol and Fe(III) in the
presence of chloride demonstrates that CO2 production is
accompanied by additional volatile breakdown products such
as VC and monochlorinated alkanes (CH3Cl, C2H5Cl, C3H7-
Cl).
Fe(III)-induced formation of VC from the model com-
pound catechol is illustrated in Figure 5. The CH3Cl/ VC ratio
was ∼8. There was no production of VC and CH3Cl when
Fe(III) was absent. Moreover, no VC production was observed
by using H2O2, another naturally occurring oxidant. When
both Fe(III) and H2O2 were used to oxidize catechol, the VC
production increased significantly. This is probably caused
by the Fenton reaction, where H2O2 reacts with Fe(II) to
generate hydroxyl radicals (•OH); Fe(II) is provided by the
reaction of catechol with Fe(III). The •OH radical is a powerful
oxidant and could be responsible for the increased concen-
tration of VC generated. In addition to the volatile com-
pounds, a variety of semivolatile aromatic and aliphatic
oxidation products could be produced, but these compounds
were not examined in this study. However, our reaction
scheme describes a novel chemical pathway from catechol
to VC and also a new source of monochlorinated alkanes
(Scheme 2).
In additional experiments, the influence of Fe(III) on VC
formation was confirmed with natural soils. The enhanced
Fe(III)-mediated production of VC is shown in Figure 6. After
adding Fe(III) in water containing soil and chloride, the
concentration of VC was twice that of the untreated sample.
9
2 4 8 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 11, 2002

relevant to many soil processes such as the formation of
chlorinated high molecular substances (chlorinated humus),
metabolic processes, or as an energy source for microorgan-
isms. However, to understand the formation and significance
of naturally produced VC in soil, more detailed investigations
are necessary.
SCHEME 2
Acknowledgments
The authors express their gratitude to I. Fahimi, B. Shotyk,
and S. Walter for reviewing the manuscript; to I. Levin for
the instrumental support; and to V. Niedan for collecting the
soil samples from Patagonia. This work was supported by
the Deutsche Forschungsgemeinschaft (DFG).
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Received for review July 16, 2001. Revised manuscript re-
ceived January 16, 2002. Accepted March 14, 2002.
ES015611L
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