Subcritical water extraction

Principles and Characteristics Water is an interesting alternative for an extraction fluid because of its unique properties and nontoxic characteristics. Two states of water have so far been used in the continuous extraction mode, namely subcritical (at 100 °C T 374 °C and sufficient pressure to maintain water in the liquid state) and supercritical (T 374°C, p 218 bar). Unfortunately, supercritical water is highly corrosive, and the high temperatures required may lead to thermal degradation of less stable organic compounds. However, water is also an excellent medium for extraction below its critical temperature [412], Subcritical water exhibits lower corrosive effects. 

The conditions in PHWE are typically harsh and, therefore, the method is not suitable for thermolabile compounds. Analytes may also react with each other or with the water molecules during the extraction. From an analytical point of view the most salient negative factors of SWE in the continuous mode are co-extraction of undesirable components of the matrix (usually polar components) and dilution of the analyte in the extract. This calls for a clean-up and concentration step prior to individual separation and detection of the target compounds. 

Miller and Hawthorne [416] have developed a chromatographic method that allows subcritical (hot/liquid) water to be used as a mobile phase for packed-column RPLC with solute detection by FID, UV or F also PHWE-LC-GC-FTD couplings are used. Before LC elution the extract is dried in a solid-phase trap to remove the water. In analogy to SFE-SFC, on-line coupled superheated water extraction-superheated water chromatography (SWE-SWC) has been proposed [417]. On-line sample extraction, clean-up and fractionation increases sensitivity, avoids contamination and minimises sources of error. 

With the emergence of SWE as an alternative to SFE opportunities exist for combining derivatisation reactions with aqueous extractions. Although extractions using superheated and supercritical water yield pleasing results, many instrumental problems will have to be overcome before this technique is ready to leave the (academic) research laboratories [77]. This approach might play a significant role in future analytical extractions. 

Applications Subcritical water extractions with suitable adjustments to the temperature (up to 250 °C) 

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CO2 extraction has been prevalent for the isolation of essential oils and other natural lipophilic pigments like carotenoids. Hot water and superheated water extraction methods are used for analytical preparation of polar pigments. The technique is commonly referred to as subcritical water extraction because the practitioners of this approach come from SEE backgrounds. 

Organic solvent-less techniques (e.g. subcritical water extraction, headspace SPME). 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

With suitable adjustments to the temperature, also subcritical water extraction (SWE) or pressurised hot water extraction (PHWE) allows selective extraction of polar (chlorinated phenols), low-polarity (PCBs and PAHs) and nonpolar (alkanes) organic compounds from industrial soils [418]. 

Although SFC fills a niche in what can be considered as a continuum of separation eluents from gases to liquids, it cannot claim a unique status subcritical water extraction (SWE, cf. Section 3.4.3) and pressurised fluid extraction (PFE, cf. Section 3.4.6) are other examples of eluents where altering the conditions cause a useful change in the solvation properties. 

Bruno F, Curini A, Di Corcia A, Fochi I, Nazzari M, Samperi R (2002) Determination of surfactants and some of their metabolites in untreated and anaerobically digested sewage sludge by subcritical water extraction followed by liquid chromatography-mass spectrometry. Environ Sci Technol 36 4156—4161... 

The procedure [56] involving subcritical water extraction of polychlorobiphenyls from soils described in section 5.6.1.4 has also been applied to sediments. 

S.B. Hawthorne, C.B. Grabanski, E. Martin and D.J. Miller, Comparison of Soxhlet extraction, pressurized liquid extraction, supercritical fluid extraction and subcritical water extraction for environmental solids recovery, selectivity and effects on sample matrix. J. Chromatogr.A 892 (2000) 421 133. 

The use of subcritical water extends beyond destruction. A method for remediation of PCB-contaminated soil and sediments has been described that uses zerovalent iron as the dechlorination agent and subcritical water extraction (SWE) as the transporting medium. The initial findings indicate that this technique may be a viable method for remediation of PCB-contaminated soil and sediments (Yak et al., 1999). 

Table 1.2. Applications of subcritical water extraction to the determination of organic compounds in soil (from author s own files)...

Determinand Subcritical water extractant Sorbent trap Analytical finish Reference... 

An example of the application of subcritical water extraction-solid-phase microextraction is that of Crescenzi et al. [122] (see above). 

Other applications of subcritical water extraction-solid-phase microextraction are the determination of terbuthylazine and its metabolites [123], polycyclic aromatic hydrocarbons [124,125] and polychlorobiphenyls [63]. Yang and Her [193] collected 1-chloronaphthylene, nitrobenzene and 2-chloro-toluene in soil on a hydrophobic polyisobutylene disc prior to analysis by attenuated total reflectance Fourier transform infrared spectroscopy. 

Chlorpyrifos metabolite Polychlorobiphenyls Comparison with subcritical water extraction [157]... 

Hawthorne et al. [61] coupled subcritical water extraction of polycyclic aromatic hydrocarbons with extraction using styrene-divinyl benzene extraction discs. The discs can be stored in autosampler vials without loss of polycyclic aromatic hydrocarbons. 

Wennrich et al. [167] investigated the capabilities of coupling accelerated solvent extraction with water as the extraction solvent and solid-phase microextraction to determine chlorophenols in polluted soils. Subcritical water extraction was performed using a commercially available accelerated solvent extractor. This system solves the problem of the analytes partitioning back to the soil matrix, which can occur in straightforward subcritical water extraction because in the Wennrich et al. method [167] the aqueous phase and the soil are separated under the extraction conditions. 

Solvent extraction Microwave extraction Solid phase microextraction Subcritical water extraction Subcritical water extraction Supercritical fluid extraction... 

Combination of static subcritical water extraction and solid-phase microextraction Comparison of CHC1F2, N2O and CO2 extractants. CHC1 F2 gave highest recovery, methanol-modified CO2 gave 90% recovery Combination of supercritical fluid extraction with off-line Fourier transform infrared spectroscopy... 

Chloropyritos metabolite Supercritical fluid extraction and subcritical water extraction [530]... 

Crescenzi, C., G. D Ascenzo, A. Di Corcia, M. Nazzari, S. Marchese, and R. Samperi (1999). Multiresidue herbicide analysis in soil subcritical water extraction with an on-line sorbent trap. Anal. Chem. 71 2157-2163. 

Di Corcia, A., A.B. Caracciolo, C. Crescenzi, G. Guiliano, S. Murtas, and R. Samperi (1999). Subcritical water extraction followed by liquid chromatography mass spectrometry for determining terbuthlyazine and its metabolites in aged and incubated soils. Environ. Sci. Technol., 33 3271-3277. 

Richter, P., B. Sepulveda, R. Oliva, K. Calderon, and R. Seguel (2003). Screening and determination of pesticides in soil using continuous subcritical water extraction and gas chromatography-mass spectrometry. J. Chromatogr. A, 944 169-177. 

Curren, M.S.S. and J.W. King. 2001. Ethanol-modified subcritical water extraction combined with solid-phase microextraction for determining atrazine in beef kidney. J. Agric. Food. Chem. 49 2175-2180. 

Ramos, L., E.M. Kristenson, and U.A.Th. Brinkman. 2002. Current use of pressurised liquid extraction and subcritical water extraction in environmental analysis. J. Chromatogr. A 975 3-29. 

Turner, C., P. Turner, G. Jacobson, K. Almgren, M. Waldeback, P. Sjoberg, E.N. Karlsson and K.E. Markides, Subcritical Water Extraction and Beta-Glucosidase-Catalyzed Hydrolysis of Quercetin Glycosides in Onion Waste, Green Chemistry, 8, 949-959 (2006). 

Gamiz-Gracia and de Castro (2000) devised a subcritical extractor equipped with a three-way inlet valve and an on/off outlet valve to perform subcritical water extractions in a continuous manner for the isolation of fennel essential oil. The target compounds were removed from the aqueous extract by a single extraction with 5 ml hexane, determined by gas-chromatography-flame ionization and identified by mass spectrometry. This extraction method is superior to both hydrodistillation and dichloromethane manual extraction in terms of rapidity, efficiency, cleanliness and the possibility of manipulating the composition of the extract. 

Gamiz-Gracia, L. and de Castro, M.D.L. (2000) Continuous subcritical water extraction of medicinal plant essential oil comparison with conventional techniques. Talanta 51(6), 1179-1185. 

Yang, Y., Bowadt, S., Hawthorne, S.B., Miller, D.J. Subcritical water extraction of polychlorinated biphenyls from soil and sediment. Anal. Chem. 67, 4571 576 (1995)... 

Table 1 Conditions used to compare Soxhlet, ASE, SFE, and subcritical water extractions of PAHs...

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