25th International Meeting on Organic Geochemistry IMOG 2011

P-398
Stable carbon isotope fractionation of dissolved BTEX during
progressive volatilization
Yongqiang Xiong, Yun Li, Qianyong Liang, Chenchen Fang, Jingru Zhang
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of
Sciences, Guangzhou, China (corresponding author:xiongyq@gig.ac.cn)
Benzene, toluene, ethylbenzene, and xylenes
(BTEX) have attracted much attend because of their
significant hazard to human health and the
environment. In addition to biodegradation,
attenuation of BTEX contamination can be caused by
different processes, such as sorption, dispersion,
diffusion, and volatilization. Compound-specific stable
isotope analysis (CSIA) has been widely used to trace
the source of contaminants and monitor the
processes controlling their fate and transport in
subsurface environments. To accurately interpret
isotopic data of BTEX in the environment, the isotopic
effects of various subsurface processes should be
better understood, particularly in different environment
conditions. For example, in the saturated zone,
sorption and degradation are the primary processes
of contaminant attenuation. The equilibrium model is
probably appropriate for describing isotope
fractionation in the zone, because where the
movement of water and contaminants is slow enough
and contact times are long enough. In the unsaturated
zone, however, diffusion and volatilization are
important transport processes for VOCs. Thus, the
Rayleigh fractionation model is more compatible with
the open system.
An experiment of open, progressive volatilization
was conducted in order to investigate stable carbon
isotopic fractionation of dissolved BTEX in the
unsaturated zone. Headspace single-drop
microextraction (HS-SDME), coupled to gas
chromatography (GC) and gas chromatographyisotope
ratio mass spectrometry (GC-IRMS), was
employed to determine concentration and carbon
isotopic values of the residual BTEX in the aqueous
solutions by optimizing extraction conditions.
Fig. 1 demonstrates a good linear least-squares fit
between ln[(1000+δ 13 Cresidual) /(1000+δ 13 Cinitial)] and lnf
for each compound of BTEX, with R 2 being 0.9072,
0.9166, 0.8065, and 0.8064, respectively. This
suggests that the volatilization of dissolved BTEX
follows the Rayleigh distillation trend. Although the
isotope fractionation enrichment factors are small
(△ 13 Cvapor-liquid falling in the range of -0.2 ~ -0.1‰), a
significant shift of δ 13 C values (>0.5‰) are observed
when extensive volatilization of BTEX has taken place
(such as >75% for Benzene and > 95% for toluene,
ethylbenzene, and o-xylene, respectively). Therefore,
before the corresponding change points are reached,
� 13 C signature of BTEX can potentially be used to
trace their source and transport in the subsurface.
Moreover, it is also used to assess the remediation
implementation (such as soil vapor extraction) by
monitoring the stable isotope values vs. time.
Removal of most of the initial contaminants will result
in a remarkable shift of the isotopic composition of the
residual BTEX, which can provide useful information
on the efficacy of soil vapor extraction as a
remediation technique.
ln[(δ 13 Cr+1000)/(δ 13 Ci+1000)]
ln[(δ 13 Cr+1000)/(δ 13 Ci+1000)]
Fig. 1. Carbon isotope fractionation of BTEX, as ln[(1000+δ 13 Cr)
/(1000+δ 13 Ci)] vs. fraction of BTEX remaining, presented as lnf,
during progressive evaporation
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
α=0.9998
Benzene
y = -0.0002x + 0.0002
R 2 = 0.9072
-0.0002
-7.00 -5.00 -3.00 -1.00
lnf
1.00
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
Ethylbenzene
y = -0.0001x + 7E-05
R 2 = 0.8065
lnf
α=0.9999
0.0002
lnf
α=0.9999
-0.0002
-7.00 -5.00 -3.00 -1.00 1.00
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0.001
0.0008
0.0006
0.0004
0
α=0.9998
y = -0.0001x + 4E-05
R 2 = 0.8064
Toluene
y = -0.0002x - 8E-05
R 2 = 0.9166
-0.0002
-7.00
0.0014
-5.00 -3.00
lnf
-1.00 1.00
o- Xylene
0.0012
-0.0002
-7.00 -5.00 -3.00 -1.00 1.00
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