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2007_6_Nr6_EEMJ

Surpateanu et al.

Surpateanu et al. /Environmental Engineering and Management Journal 6 (2007), 6, 521-527 reorientates to align itself with the field, the field is already changing and a phase difference exists between the orientation of the field and that of dipole. This phase differences cause energy to be lost from the dipole in random collisions, and to give rise to dielectric heating (Whittaker, 1997). Conduction mechanisms are related to movement of charge carriers (electrons, ions etc.) under the influence of the electric field. As a result of ion movements and collisions the conversion of kinetic energy to thermal energy occurs. In conclusion, the mechanism of microwave heating in the case of an electric conductor includes both dipolar polarisation of polar substances and conduction mechanism of ions in liquid phase or locked up in solids interstices. 3. Microwave equipment and sample preparation First experiments concerning the application of microwave heating effect to chemical reactions were performed with multi-mode domestic oven. In these ovens microwaves are heterogeneously distributed, and less-defined regions of high and low energy intensity are produced. Actually, a large variety of microwave equipment is found on the market, which assures well-defined regions of maximum and minimum field strength (Larhed and Haldberg, 2001). These equipments must be provided with reliable temperature control system to assure an efficient application of microwave irradiation as energy source. Also, an adequate vessel must be used. Open vessel systems may be used but the closed vessel has some advantages. This is because microwaves only heat the liquid phase, while vapours do not absorb microwave energy. The temperature of the vapour phase is therefore lower than the temperature of liquid phase and vapour condensation on cool vessel walls takes place (Agazzi and Pirola, 2000). As a result, the actual vapour pressure is lower than the predicted vapour pressure and this thermal non-equilibrium is a key advantage of microwave technology, as very high temperatures can be reached at relatively low pressures. The most limiting factor of microwave closed vessel are related to sample amount. This is because the larger sample amount, the higher is the pressure generated by the reaction. Thus, the microwave degradation of phenolcontaining polymeric compounds was accomplished by placing closed vessel inside a commercial oven (Chang et al., 2004). The microwave system was equipped with a Teflon-coated cavity and a removable 12-position sample carousel, a sensor for pressure measurements and an optical fibber to monitor and control the digestion temperature. Sometimes the heating microwave power is associated with UV irradiation to assure the degradation of refractory pollutants e.g. chlorophenols and so the equipment is completed with a low- or medium-pressure mercury lamp as UV light source (Cirva et al., 2005; Horicoshi et al., 2006). 4. Environmental applications of microwave chemistry The most important and directly applications of microwaves assisted chemistry into environmental field are related to extraction of some pollutants (especially persistent organic pollutants, POP) from liquid or solid media in the view of their analysis (Basheer et al., 2005; Fountoulakis et al., 2005), in the treatment of hazardous and infectious wastes (Gan, 2000; Diaz et al., 2005) and, also, in a range of environment-related heterogeneous catalytic reaction systems like as: the decomposition of hydrogen sulphide, reduction of sulphur dioxide with methane, reforming of methane with carbon dioxide etc. (Zhang and Hayard, 2006). Generally, chemical reactions based on microwave heating are characterised by a good reproducibility, are cleaner that those by traditional way (water or oil bath) and many times lead to high efficiency. Supplementary advantage of microwave assisted chemistry is their applicability both to homogenous reactions in aqueous or non-aqueous solutions and dry media reactions. This ultimate aspect was considered for the application of heating microwave effect to improve the yield of metal extracted from minerals simultaneous with the increasing demand for more environmental friendly processes. Many papers deals with the extraction of metals such as copper, gold, nickel etc. from their ores by microwave-assisted leaching, process more attractive comparatively with pyrometallurgy due to economical, technical and environmental reasons (Al- Harahsheh and Kingman, 2004). Thus, it was reported that by heating chalcopirite with concentrated sulphuric acid in an adapted domestic microwave oven (20 minutes, 200-260°C, 2,45 GHz), the leaching reaction product in water; the copper extraction was between 90-99% (Hsieh et al., 2007). In the same time the elemental sulphur was captured and only low volumes of sulphur dioxide was produced. The improvement of copper extraction was explained by the thermal convention currents generated as the result of different rate heating of liquid and heterogeneous reaction system. Similarly results were obtained for gold leaching from its refractory ores, especially pyrite and arsenopyrite. The enhancement of gold leachability after microwave pre-oxidation was explained by the formation of a porous (hematite) structure, which favorize gold extraction in a cyanide solution (Huang and Rowson, 2000). The same microwave heating effect was applied for coal desulphuration as pre-treatment to minimise SO 2 emissions during burning (Johnes et al., 2002). 522

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, 523

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