135 research outputs found
Normal and Inverse Diffusive Isotope Fractionation of Deuterated Toluene and Benzene in Aqueous Systems
Diffusive isotope fractionation of
organic contaminants in aqueous
solution is difficult to quantify, and only a few experimental data
sets are available for compounds of environmental interest. In this
study, we investigate diffusive fractionation of perdeuterated and
nondeuterated benzene and toluene. Multitracer experiments were carried
out in 1-D gel dissection tubes and in a quasi-2-D flow-through porous
medium. The experiments allowed us to simultaneously and directly
compare the diffusive and dispersive behavior of benzene and toluene.
We observed an unexpected, opposite behavior of the two monoaromatic
hydrocarbons. Toluene showed a normal diffusive isotope effect (<i>D</i><sub>C7D8</sub>/<i>D</i><sub>C7H8</sub> = 0.96)
with enrichment of the nondeuterated isotopologue in the direction
of the diffusive and transverse dispersive fluxes. Conversely, the
measured trends for benzene indicate inverse diffusive fractionation
(<i>D</i><sub>C6D6</sub>/<i>D</i><sub>C6H6</sub> = 1.02), with a remarkably faster diffusion rate of the perdeuterated
isotopologue that was enriched in the downgradient portion of the
diffusion tubes and at the fringes of the contaminant plumes in the
flow-through setup. These outcomes can neither be interpreted as mass-dependent
fractionation nor be described as purely hydrodynamic (i.e., mass
independent) effects. The results of this study are relevant for the
use of labeled/nonlabeled mixtures of organic compounds as conservative
and (bio)Âreactive tracers in environmental applications
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