25th International Meeting on Organic Geochemistry IMOG 2011

P-232
Some experimental results on the influence of clay minerals on
fossil organic matter transformation in soil and sediments
Coralie Biache, Thierry Chislain, Raymond Michels, Pierre Faure
UMR7566 G2R, CNRS, Nancy Université, Vandoeuvre-lès-Nancy, France (corresponding
author:Pierre.Faure@g2r.uhp-nancy.fr)
Because of their toxicity, the fate of Polycyclic
Aromatic Hydrocarbons (PAHs) is of uppermost
interest to agro- and hydrosystems investigations.
Polluted soils and sediments are the major source of
long term diffusion of PAHs in the environment.
Although artificial remediation solutions exist (thermal
desorption for instance) or are of current intense
research, their applications may often be problematic
in regards to the huge amount of polluted material to
be treated and their elevated cost. Hence, natural
recovery is often the only remaining perspective.
Therefore, it is important to understand the fate and
reactivity of PAHs during the evolution through space
and time of these complex systems.
Polluted soils and sediments are likely to be in
contact with air, where oxidative chemistry is active.
Thus, chemical air oxidation may be a major factor
influencing PAHs in soils and sediments.
In this perspective, we studied the artificial air
oxidation of coal tar, polluted soil and model
compounds. Air oxidation was conducted at 100°C in
sealed glass reactors periodically aerated after CO2
measurements.. Oxidation of a coal tar (essentially
composed by PAHs) reveals, after 6 months, a
decrease in the PAH concentration (~ 50% of 16 PAH
including in the US/EPA list). This decrease can not
be explained only by mineralization processes (CO2
production represents only 0,5% of initial COT).
Molecular investigations reveal the increase in the
oxygenated PAH concentration and mass balance
suggests the formation of a solid organic constituent.
The same experiments, carried out on a real soil
from an ancient coking plant site, reveal similar
results (decrease in PAH concentration, O-PAH
productions, weak mineralization and organic extract
decrease). However, the intensity of these
parameters is higher (especially PAH degradation and
CO2 generation). This more intense evolution in the
case of the soil can be explained by the occurrence of
reactive mineral phases absent in the coal-tar.
The air oxidation of the model compound
fluoranthene, a dominant PAH in coal tar, allowed to
investigate the major reactive pathways. Fluoranthene
was oxidized alone and in presence of different
mineral matrices representative of soils (quartz sand,
calcite, clay). The experiments show that very little
CO2 is formed. Instead, series of higher molecular
weight compounds could be evidenced by HLPC-
SEC—QTOF mass spectrometry. Indeed,
fluoranthene units follow an oxidative mineral
catalyzed polymerization process leading to the
formation of a carbonaceous residue. This result was
not expected, as in literature high activation energies
(and thus high pyrolysis temperature) are usually
considered in the oxidation process of PAHs.
We thus propose that the air oxidation catalyzed
by minerals may be a novel pathway to the
immobilization of PAHs in the subsurface.
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