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Slow
Desorption of Phenanthrene from Silica Particles:
Influence of Pore Size, Pore Water, and Aging Time
Michael
H. Huesemann,
Pacific Northwest National Laboratory, Marine Sciences
Laboratory, 1529 West Sequim Bay Road, Sequim, WA 98382,
Tel: 360-681-3618, Fax: 360-681-3699
Timothy J. Fortman,
Pacific Northwest National Laboratory, Marine Sciences
Laboratory, 1529 West Sequim Bay Road, Sequim, WA 98382,
Tel: 360-681-3621
Robert G. Riley,
Pacific Northwest National Laboratory, Battelle Blvd.,
Richland, WA 99352, Tel:
509-376-1935
Christopher J. Thompson, Pacific Northwest
National Laboratory, Battelle Blvd., Richland, WA 99352,
Tel: 509-376-6602
Zheming Wang, Pacific Northwest National
Laboratory, Battelle Blvd., Richland, WA 99352, Tel:
509-376-6119
Michael J. Truex, Pacific Northwest National
Laboratory, Battelle Blvd., Richland, WA 99352, Tel:
509-376-5461
Brent Peyton,
Washington State University, Chemical Engineering
Department, Pullman, WA 99164, Tel:
509-335-4002
In an
effort to better understand the environmental fate and
transport of aged petroleum hydrocarbons in aquifer
solids, we performed a series of sorption and desorption
experiments using phenanthrene as a model hydrocarbon
compound and porous silica particles as model aquifer
solids. When micro-porous and meso-porous silica particles
were exposed to aqueous phenanthrene solutions for various
durations it was observed that sorbed-phase phenanthrene
concentrations increased with aging time only for meso-porous
but not micro-porous silicas. Desorption equilibrium was
reached almost instantaneously for the micro-porous
particles while both the rate and extent of desorption
decreased with increasing aging time for the meso-porous
silicas. These findings indicate that phenanthrene can be
sequestered within the internal pore-space of meso-porous
silicas while the internal surfaces of micro-porous
silicas are not accessible to phenanthrene sorption,
possibly due to the presence of physi- or chemi-sorbed
water that may sterically hinder the diffusion of
phenanthrene inside water-filled micro-pores. By contrast,
the internal surfaces of these micro-porous silicas are
accessible to phenanthrene when incorporation methods are
employed which assure that pores are devoid of
physi-sorbed water. Consequently, when phenanthrene was
incorporated into these particles using either
supercritical CO2 or via solvent soaking, the
aqueous desorption kinetics were extremely slow indicating
effective sequestration of phenanthrene inside
micro-porous particles. A two-compartment conceptual model
is used to interpret the experimental findings and assess
the bioavailability and risk of aged petroleum
hydrocarbons in groundwater aquifers.
Degradation
of Persistent Organochlorine Pesticides in Rhizosphere
Soils
XM
Zhu,
Department of Geography and Resource Management, Chinese
University of Hong Kong, HK SAR, P.R. China
KC Lam,
Department of Geography and Resource Management, Chinese
University of Hong Kong, HK SAR, P.R. China
S. Tao,
Department of Urban and Environmental Science, Peking
University, Beijing, P.R. China
Persistent
organic environmental contaminants such as HCHs (hexachlorocyclohexanes),
DDTs (dichloro-diphenyl-trichloroethanes) and other
organochlorine pesticides were widely used before
prohibition and distributed globally by transport through
air and water. Due to their extensive use in agriculture
and industry in the past, environmental contamination with
organochlorine pesticides (OPs) has occurred widely.
Evidences
for enhanced microbial degradation of xenobiotic chemicals
in the rhizosphere have widely reported, suggesting that
vegetation may play an important role in facilitating
bioremediation of contaminated surface soils. While it is
know that these contaminants are more readily degraded or
biotransformed by plants or by their attendant rhizosphere
microbes in varying degrees, information on the
environmental behavior of persistent organic pollutants
has been very limited.
This
study has been conducted to determine the concentration of
r-HCHs and DDTs in the rhizosphere of wheat cultivated in
contaminated agricultural soil. The variations of (DDD+DDE)/DDT
ratios in rhizosphere and non-rhizosphere soil were
significantly different, indicating that the rhizosphere
can indeed enhance the DDT degradation in certain degrees.
The time trend of (DDD+DDE)/DDT ratios in rhizoshere and
non-rhizosphere soil were both evidently increased. The
significant differences of concentrations of r-HCHs and
DDTs between planted and unplanted soil were not seen,
probably due to experimental error or insufficient time
for the aging process. Similar trends were also not
evident in planted soils as the distance from the plant
roots increases. The study only confirmed that the
conversion from DDT to DDD and DDE could be enhanced by
the presence of plants, however it has not pinpointed
which factor plays a more vital role. Observations show
that pH was slightly higher in rhizoshere soil than in
non-rhizoshere soil. Further studies are needed to
elucidate the degradation mechanisms of persistent
organochlorine pesticides in the rhizoshere soil
environment.
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