Extraction PAHs from soil

In Figure 7.53 a flow-injection interface for fluorometric monitoring of focused microwave-assisted Soxhlet extraction is represented [195]. This assembly allows real-time online monitoring of the PAHs extracted from solid samples in each Soxhlet cycle and provides qualitative and semi-quantitative information from natural and spiked samples. The method has been applied to a certified reference material (CRM 524, BCR, industrial soil/organics) for quality assurance/validation. The proposed technique is as efficient as conventional Soxhlet to extract PAHs from soils but with a drastic reduction of both extraction time and organic solvent disposal. 

Microwave-assisted extractions (MAE) can be performed in open (focused MAE) or closed (pressurized MAE) flasks. This technique is commonly used for extractions from complex and difficult sample matrices, replacing time- and solvent-intensive Soxhlet extractions or hydrodistillations.46 MAE is also widely applied to environmental samples, for example, for extracting polycyclic aromatic hydrocarbons (PAH) from soil, methylmercury from sediments, and trace metals and pesticide residues from plant material47 48 The use of microwave treatment instead of hydrodistillation offers a solvent-free separation technique essential oils are heated and dry-distilled 46... 

Many microwave extractions can reach maximum recovery in 10 to 20 minutes. Longer extraction time is not necessary and may lead to the decomposition of thermolabile analytes. It was reported that the recovery of sulfonylurea from soil was not affected by extraction time in the range 5 to 30 minutes [79], Similar observation was made in the extraction of PAHs from soils and sediments [6], In the extraction of PAHs and LAHs (linear aliphatic hydrocarbons) from marine sediments, the extraction time was found to be dependent on the irradiation power and the number of samples extracted per run [81], When the irradiation power was 500 W, the extraction time varied from 6 minutes for one sample to 18 minutes for eight samples [74], The recovery of OCPs from spiked marine sediments increased from 30% at 5 and 10 minutes to 60% at 20 minutes and to 74 to 99% at 30 minutes [82],... 

Because water is a polar substance that can be heated by microwave irradiation, it can often improve analyte recovery. In a study of focused MAE of PAHs from soil and sediments [6], sample moisture level showed significant influence on extraction efficiency, and 30% water in the sample provided the highest recovery. Similarly, the maximum recovery of phenyl-urea herbicides was obtained with 10% water in soils [75], In the extraction of triazines from soil, water content in the range 10 to 15% yielded the highest recovery [7],... 

The results for the removal of PAHs from the 5 surface substrates are summarized in Table 16. In general, the 23 PAHs listed in the table averaged removal rates around 90% from the smooth surfaces and over 80% for the porous cast magnesium surface. In contrast, supercritical fluid extraction studies using CO2 for the removal of PAHs from soils for environmental applications have shown relatively poor removal efficiencies for many of the compounds listed in the table often requiring the addition of secondary solvents to the C02. However, it appears that from the results on the removal of the PAHs shown in Table 16, surface contamination is... 

For example, from 1995 to 2000, more than 15 published studies on ASE involved the extraction of PAHs from soil or sediments in a virtually identical manner. 

In addition to the previous three commercial extractors, some authors have developed custom models [78,79], adapted in most cases from a supercritical fluid extractor [25,58,80]. For example, Heemken et al. altered a Suprex SF extractor for use in ASE [81] they disconnected the syringe pump from the COj cylinder and filled it with a suitable ASE solvent. The restrictor was replaced with a stainless steel capillary tube leading into the trapping vial and an additional nitrogen pipe was installed at the inlet valve of the extraction vessel for purging after extraction. In the extraction of PAHs from soil [79], a custom extractor and commercially available equipment provided equivalent results on the other hand, in the extraction of benzene and toluene from soil [78], the former provided even better results than the latter. 

It has for some time been a common practice to compare new extraction techniques with Soxhlet extraction, and ASE has been no exception in this respect. The ASE technique has many advantages over Soxhlet extraction, especially prominent among which are the following reduced operating times and consumption of organic solvents, and partial automatability. In many cases, ASE provides results similar to (e.g. in the extraction of organophosphorus pesticides, organic matter, hydrocarbons and PAHs from soil... 

Miniaturized retention has been accomplished by using solid-phase microextraction (SPME) to collect PAHs from soil, air particulate matter and urban particulates [62,165], and pyrethrins from pyrethrum flower [165] in the fibre after quantitative extraction with superheated water [62,166]. 

In many of the previous applications, a combination of static and dynamic extraction was found to reduce the extraction time and provide better recoveries. Such is the case with the extraction of linear alkylbenzenesulphonates [24] and 4-nonylphenols [25] from sediments, and that of PAHs from soil [47], where a combination of 15 min static extraction and 10-20 min dynamic extraction ensured quantitative extraction of the analytes, thus minimizing the dilution effect arising from extended dynamic extraction. 

Supercritical fluid extraction (SFE), usually with carbon dioxide and, often, with a modifier, has become of increasing interest in the last few years because of its selectivity, preconcentration effect, efficiency, simplicity, rapidity, cleanness, and safety, mainly concerning the extraction of organic compounds prior to separation and detection by chromatographic techniques. It has several advantages over classical solvent extractions, in comparison with recent extraction techniques. Approaches to obtain quantitative extractions, including fluid choice, extraction flow rate, modifiers, pressure, and temperature, are presented, as well as the potential for SFE to extract polynuclear aromatic hydrocarbons (PAHs) from soils, sediments, and biota. Improvements and new environmental applications are also reported. 

When water is used as the solvent, PLE is referred to as superheated water extraction, subcritical water extraction (SWE), or pressurized (hot) water extraction (PWE). Hot water is very effective as an extraction solvent for PAHs from soil and sediment. Superheated water is water above the boiling point but below the supercritical point, and under sufficient pressure to maintain... 

SFE usually refers to the extraction of solid samples with CO2 above its critical temperature and pressure. SFE greatly reduces extraction time, is inexpensive and nontoxic, and its properties can be tuned by changing temperature and pressure or adding modifiers. USEPA Method 3561 provides a method for the extraction of PAHs from soils, sediments, fly ash, solid-phase media, and other solids. Two- and three-ring PAHs can be extracted in pure CO2 while the higher molecular mass PAHs require the addition of modifiers. Supercritical CO2 is similar in solvent properties to HEX and can be modified by the addition of water or organic solvents. 

Extraction of contaminants has been combined with oxidation either within the extraction cell or as a second step. These processes avoid the need for clean-up of the water effluent from an extraction process. Wet-air oxidation [2] has been carried out within the cell during the extraction of PAHs from soil [50]. In excess of 99% of the hydrocarbons were removed, which was much higher than the percentage removal when extraction only was carried out. PAHs from solids have also been treated by extraction followed by on-line wet-air oxidation in a second ceU [51,... 

Sams0e-Petersen et al. [701] used focused MAE to extract PAHs with a mixture of hexane and methylene chloride from fruits and vegetables grown in contaminated soils. A further purification step (often performed by SPE) is always needed before HPLC determination. 

The Microwave-Assisted Process (MAP ) technology uses microwaves, and solvents that are relatively transparent to microwaves, to extract chemicals from various matrices based on the temperature differential between the solvent and the target compound. According to the developers, the technology is applicable to soils and wastes containing polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), total petroleum hydrocarbons (TPH), and other organic compounds. 

The desorption and vapor extraction system (DAVES) uses a low-temperature fluidized bed to remove volatile and semivolatile organics such as polychlorinated biphenyls (PCBs), polynuclear aromatic compounds (PAHs), pentachlorophenol (PCP), volatile inorganics (tetraethyl lead), and some pesticides from soil, sludge, and sediment. The process generally treats waste containing less than 10% total organic contaminants and 30 to 95% solids. The process does not treat nonvolatile inorganic contaminants such as metals. 

The Basic Extractive Sludge Treatment (B.E.S.T. ) process is an ex situ solvent extraction technology. The B.E.S.T. process uses one or more secondary or tertiary amines, such as diisopropylamine, to separate contaminants from soil, sediment, and sludge. This technology is applicable to most organics or oily contaminants, including polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), pesticides, herbicides, dioxins, furans, and other organic compounds. 

PAHs in soil may partition into soil organic matter (SOM) or adsorb on soil minerals. The sorptive properties of SOM fractions for organic contaminants in soil play an important role on the transportation of PAHs in soil. Xiao et al. (2004) has reported that soil/sediment organic matter can be fractionated into four fractions with a combined wet chemical procedure and that kerogen and black carbon (BC) are major SOM components in soil/sediment samples collected from the industrialized suburban areas of Guangzhou, China. Phenanthrene and naphthalene were used as the sorbates to study PAH s sorption isotherms on four original and four Soxhlet-extracted soil/sediment samples, 15 isolated SOM fractions, and a char as the sorbents. The sorption isotherms of phenanthrene and naphthalene on all the sorbents were variously nonlinear. The particulate kerogen and black carbon (KB) fractions... 

It was reported that the recoveries of 17 PAHs from six certified reference marine sediments and soils [77] increased from 70 to 75% when the temperature was increased from 50° C to 115°C, and remained at 75% from 115 to 145°C. In the extraction of OCPs from sediments, recovery was unchanged from 100 to 120°C [74]. In the extraction of phenylurea herbicides from soils, the recovery peaked in the range 60 to 80°C and decreased at lower or higher temperatures [75]. In the extraction of sulfonylurea herbicides from soils, recovery dropped from 70 to 80% to 1 to 30%. due to decomposition when temperature increased from 70°C to 115°C [79]. The recovery of oligomers from poly(ethyleneterephthalate) increased as temperature rose... 

Matrix effects have been observed in MAE applications. It was reported that recoveries of OCPs from aged soils (24 hours of aging) were lower than those from freshly spiked samples [78]. Similar matrix effects were also reported in the extraction of sulfonylurea herbcides from aged soils [79], In another study, the average recoveries of 17 PAHs from six different standard reference materials (marine sediments and soils) varied from 50 to 100% [77],... 

Microwave-assisted extraction (MAE) has become a major choice for the extraction of PCBs from solid matrices. In fact, this technique has been used to extract PCBs from a wide range of samples including soil, sediments and animal tissues [254-257], as well as certified reference materials (CRMs) [258]. Normally, the extractant used is the same as that employed with PAHs, viz. a 1 1 hexane-acetone mixture [246] however, organized media [256] have also provided results similar to those obtained with conventional methodologies such as Soxhlet extraction [259]. For example, the results for a range of Aroclors (1254, 1260, 1016 and 1248) were quite consistent with their certified values. The microwave-assisted extraction of PCB Aroclors 1248, 1254 and 1260 followed... 

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