Stable isotope fractionation during vaporization of
trichloroethylene has been measured, with possible
application as a technique to investigate subsurface
behavior. The equilibrium value of Δ13Cvapor
-
liquid has been
measured between 5 and 35 °C, and Δ13Cvapor
-
liquid,
Δ37Clvapor
-
liquid, and ΔD
vapor
-
liquid have been measured
during progressive evaporation of liquid trichloroethylene
at 22 ± 2 °C. Equilibrium values of Δ13Cvapor
-
liquid show a total
range of 0.07−0.82‰, with a trend of decreasing
Δ13Cvapor
-
liquid with increasing temperature, from approximately
+0.7‰ at 5−15 °C to approximately +0.1‰ at 35 °C.
Progressive evaporation experiments yield values of
Δ13Cvapor
-
liquid = +0.35‰ and +0.24‰, Δ37Clvapor
-
liquid =
−1.64‰, and ΔD
vapor
-
liquid = +8.9‰. The positive values for
carbon and hydrogen isotope fractionation, while
unexpected, are consistent with available quantitative and
qualititative data for trichloroethylene and other contaminant
hydrocarbons, but a satisfactory explanation for these
observations, particularly in combination with the negative
value for chlorine, remains elusive. Vapor−liquid fractionation factors have application to the investigation of
the behavior of trichloroethylene at contaminated sites,
particularly sites undergoing remediation by techniques
such as soil vapor extraction and soil bioventing.
Abstract. Iron oxide minerals play an important role in stabilizing organic carbon (OC) and regulating the biogeochemical cycles of OC on the earth surface. To predict the fate of OC, it is essential to understand the amount, spatial variability, and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concentrations and characteristics of Fe-bound OC in soils collected from 14 forests in the United States and determined the impact of ecogeographical variables and soil physicochemical properties on the association of OC and Fe minerals. On average, Fe-bound OC contributed 37.8 % of total OC (TOC) in forest soils. Atomic ratios of OC : Fe ranged from 0.56 to 17.7, with values of 1–10 for most samples, and the ratios indicate the importance of both sorptive and incorporative interactions. The fraction of Fe-bound OC in TOC (fFe-OC) was not related to the concentration of reactive Fe, which suggests that the importance of association with Fe in OC accumulation was not governed by the concentration of reactive Fe. Concentrations of Fe-bound OC and fFe-OC increased with latitude and reached peak values at a site with a mean annual temperature of 6.6 °C. Attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliphatic than non-Fe-bound OC. Fe-bound OC also was more enriched in 13C compared to the non-Fe-bound OC, but C ∕ N ratios did not differ substantially. In summary, 13C-enriched OC with less aliphatic carbon and more carboxylic carbon was associated with Fe minerals in the soils, with values of fFe-OC being controlled by both sorptive and incorporative associations between Fe and OC. Overall, this study demonstrates that Fe oxides play an important role in regulating the biogeochemical cycles of C in forest soils and uncovers the governing factors for the spatial variability and characteristics of Fe-bound OC.
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