2007_6_Nr6_EEMJ

Microwave assisted chemistry-a review of environmental application
Other applications of microwave heating effect
were focussed on environment-related heterogeneous
catalytic reactions such as the decomposition of
hydrogen sulphide into hydrogen and sulphur and
reduction of sulphur dioxide with methane (Zhang
and Hayard, 2006). Thus, to reduce the hydrogen
sulphide emissions level into the atmosphere, it has
been investigated the catalytic decomposition by
microwave heating. The reactions were performed
under continuous flow conditions in tubular quartz
reactors using as catalyst either an impregnated
molybdenum sulphide on γ-alumina or a
mechanically mixed sample of molybdenum sulphide
on γ-alumina. The temperature in the microwave
cavity was monitored using an optical fibre
thermometer. It was found that the H 2 S conversion
degree under microwave conditions was much higher
than those obtained with conventional heating at the
same temperature, especially with mechanically
mixed catalyst. The enhancement of the reaction rate
and product selectivity under microwave conditions
must be attributed to thermal effects which may result
because of differences between the real reaction
temperature at the reaction sites and the observed
average temperature.
Microwave-assisted extraction technique is a
new procedure used especially to recovery of POPs
from soils, sediments and sewage sludge (Basheer et
al., 2005; Horikoshi et al., 2006; Hsieh et al., 2007).
Many papers underlines the advantages of this
technique over the other new (sonication, pressurised
liquid extraction and supercritical fluid extraction) or
classical methods (Soxhlet extraction) but also their
limitations.
Microwave-assisted extraction (MAE) is based
on the nonionising radiation that causes molecular
motion by migration of ions and rotation of dipoles,
without changing the molecular structure
(Fountoulakis et al., 2005). Due to the principles of
microwave heating the choice of the solvent depends
on its ability to absorb microwaves, defined by its
dielectric constant ε (Budzinski et al., 1999). Nonpolar
solvents do not absorb microwave energy and
therefore such solvents have poor extraction
efficiencies compared to polar solvents or mixture of
solvents at least one of which must be polar.
It was showed that the addition of water
facilitates non-polar organic solvents to absorb
microwave energy and so improves the release of
target analytes from sample matrix (Basheer et al.,
2005). This is because at high pressure and
temperature its dielectric constant, viscosity, and
surface tension become low these facts facilitating the
extraction from solid samples of the organic
compounds having different polarities. Nevertheless,
because of low selectivity the main drawback of
MAE is the need of a cleanup procedure (Yafa and
Farmer, 2006; Pastor et al., 1997).
Thus, to overcome this disadvantage, a
microwave-assisted extraction and partition method
(MAEP) using water-acetonitrile and n-hexane was
studied to determine some pesticides (trifluralin,
metolachlor, chlorpyriphos and triadimefon) from
agricultural soils (Fuentes et al., 2006).
Studies were carried out using sieved soils (2
mm mesh) with diverse physico-chemical properties
collected (0-20 cm depth) in different agricultural
zones in Chile. Aliquots of spiked soil were weighed
and transferred to a microwave extraction vessel and
the extraction solution (water-acetonitrile) was added
in 1:1 sample-to-solvent ratio. After homogenisation
by manual shaking, hexane was added for
partitioning. The extraction vessel was covered with
pressure-resistant holders and preheated for 2 min at
250 W and then 10 min at 900 W, and 130°C
maximum temperature using a microwave oven
system (which allows the simultaneous heating of six
vessels). An optic-fibber probe inside the monitoring
cell was used to control temperature. After
microwave irradiation, vessel was water-cooled,
opened and hexane layer was evaporated at dryness;
the residue was re-dissolved and directly analysed by
gas chromatography electron capture detection. It was
found that the method is efficient and fast to
determine hydrophobic pesticides at ng g -1 level in
soil with different clay-to organic matter ratios.
Among all the studied parameters (time and
power of irradiation, nature of solvent, percentage of
water) the quantity of water is of primary importance
to maximise the recoveries of polycyclic aromatic
hydrocarbons (PAH) from soils and sediments by
microwave-assisted extraction technique (Budzinski
et al., 1999). The studied PAHs range from three-ring
aromatic compounds (phenanthrene, anthracene) to
six-ring aromatic compounds (benzo[ghi]perylene),
and the optimal conditions established by working
with 0.1 to 1.0 g of freeze-dried sediments and soils
were as follows: 30% water, 30 ml of
dichloromethane, 30 W, 10 min irradiation time. The
extracted aromatic compounds were analysed by gas
chromatography coupled to mass spectrometry (GC-
MS). In these conditions the recoveries for all the
tested samples are very good (more than 85%). In
comparison with Soxhlet extractions (SE) this
technique are proved important advantages like as
decreasing of solvents volumes (2x250 ml for SE up
to 30 ml for MWAE) and reduction of operational
time (at least 48 hours for SE and 10 minutes for
MWAE).
MWAE was tested at laboratory-scale for the
extraction of petroleum hydrocarbons from
contaminated soil in Canada (Punt et al., 1999). It was
found that microwaves could be used to enhance the
solvent extraction of the contaminants from the soil
and that the proprieties of soil greatly affected the
extent to which the contaminants are removed.
MWAE also was applied to analyse
organochlorine pesticides and polychlorinated
biphenyls (Horicoshi et al., 2006). Thus it was
developed a MWAE procedure coupled with a liquidphase
microextraction (LPME) using a porous
polypropylene hollow fibber membrane (HFM) for
cleanup, enrichment and extraction of these POPs
from marine sediments. The sediment samples of 1 g,
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