Desorption of PAHs

The analysis of particle bound PAH involves collection of PAH bound to dust particles on 0.8 pm glass fiber or silver membrane filters, desorption of PAH from the particles into a suitable organic solvent, and analysis of the extract by a capillary GC using an FID. Between 500 and 1000 L air at a flow rate of 120 L/h is recommended for sampling, which can give a detection limit of 0.15 to 0.50 pg/m3 for each compound (Riepe and Liphard, 1987). The method suggests the installation of an absorber resin, such as XAD-2 or Tenax, after the... 

As explained above, PAHs are barely soluble in water and it is thus common to use facilitating agents which can improve the solubilization/desorption of PAHs from soil. There are four groups of relevant additives— surfactants, cosolvents, cyclodex-trins, and biosurfactants. Here, the characteristics of and general information on each type of facilitating agent are introduced. 

A number of batch and column studies have sought to enhance the solubilization and desorption of PAHs from soil. Surfactants are commonly used to remediate PAH-contaminated soil. Two methods are employed micellar solubilization and PAH mobilization by reduction of interfacial tension (West and Harwell, 1992). As surfactant toxicity became a significant issue, biodegradable and biocompatible surfactants have been more widely examined. For example, food-grade surfactants such as Tergitol 15-S-X (X = 7,9, and 12) (Li and Chen, 2002) and other surfactants with indirect food additive status, such as alkyl diphenyl disulfonate (DOWFAX) (Deshpande et /., 2000), have been investigated for use in solubilization/desorption of single PAHs or PAHs mixtures from contaminated soils. 

The chemical n-butylamine has been also used in electrokinetic tests employing contaminated sediment and gas plant soil. The cosolvent contributed to the solubi-lization/desorption of PAHs. In the sediment test, 20% n-butylamine was more effective for partial solubilization of PAHs than was 3% Tween 80, and PAH concentrations in the anodic region were reduced to levels equivalent to those seen when 5% Igepal CA-720 was employed (Reddy and Ala, 2006). Also, 20% n-butylamine did not effectively transport PAHs in gas plant soil, even though PAHs were solubilized in effluent samples. This low removal efficiency might be attributable to the high organic content of the soil (Reddy et /., 2006). 

Biosurfactants are useful in contaminant biodegradation because biosurfactants enhance HOC bioavailability by increasing HOC aqueous solubility Ju and Elektorowicz (2000) used rhamnolipids produced by P. aeruginosa to enhance electrokinetic remediation of phenanthrene-contaminated soil. This study showed the potential for on-site production of biosurfactant that was then directly introduced to the electrokinetic cell. Biosurfactants thus show promise as enhancers of biodegradation and for the solubilization and desorption of PAHs in electrokinetic processes. 

Cheng KY, Zhao ZY, Wong JWC (2004). Solubilization and desorption of PAHs in sod-aqueous system by biosurfactants produced from Pseudomonas aeruginosa P-CG3 under thermophilic condition. Environmental Technology 25(10) 1159-1165. 

Kogel-Knabner I. Totsche, K. U. Raber, B. (2000) Desorption of PAH from soil in the presence of dissolved organic matter Effect of solution composition and aging. J. Environ. Qual. 29, 906-916. 

Additional investigations to separate PAH from fly ash as ultrasonic extraction and sublimation have been applied. The special refractory desorptivity of PAH from fly ash, derived from fluidized bed combustion, made it necessary to get some more information about the surface stmcture. For this reason the fly ash was investigated by electron microscopy. To improve contact to solvents, cmshing by the pressirre drop method has been performed. 

Extremely slow desorption of PAHs from earbonaeeous particles were reported [44]. Moreover, the similar desorption behavior was found in natural sediments associated with eoal and eoal-derived partieles [45], In both studies, resulting desorption rate constants were in the order of 10 - 10 for ki and 10 - 10 for k2, which is one order of magnitude lower than the slow and very slow desorption rate reported in other studies [42,46]. These previous studies used Tenax beads and observed rate eonstants of lO - 10 for fast desorption, 10 -10 for slow desorption and 10 - lO for very slow desorption. Consequently no fast desorption of PAHs eould be observed eoal and eoal-derived particles. Desorption rates of PAHs displayed very slow or even extremely slow desorption. 

If sorption and partitioning mechanisms dominate the fate of PAHs in soils, then the PAHs remaining in SOM should be primarily parent compounds which are sorbed to organic surfaces. Slow rates of desorption become the primary limitation for biodegradation however, the presence of adapted PAH-minerali-zing communities in contaminated soils suggests that PAH desorption occurs at sufficient rates over time to establish and maintain adapted microbial communities [36,264,356]. PAH biodegradation appears to proceed, albeit at much slower rates than predicted or desired [264,278,279]. 

Ghosh, U. Talley, J.W. Luthy, R.G. Particle-scale investigation of PAH desorption kinetics and thermodynamics from sediment. Environ. Sci. Technol. 2001, 35 (17), 3468-3475. 

A number of less commonly used analytical techniques are available for determining PAHs. These include synchronous luminescence spectroscopy (SLS), resonant (R)/nonresonant (NR)-synchronous scan luminescence (SSL) spectrometry, room temperature phosphorescence (RTP), ultraviolet-resonance Raman spectroscopy (UV-RRS), x-ray excited optical luminescence spectroscopy (XEOL), laser-induced molecular fluorescence (LIMP), supersonic jet/laser induced fluorescence (SSJ/LIF), low- temperature fluorescence spectroscopy (LTFS), high-resolution low-temperature spectrofluorometry, low-temperature molecular luminescence spectrometry (LT-MLS), and supersonic jet spectroscopy/capillary supercritical fluid chromatography (SJS/SFC) Asher 1984 Garrigues and Ewald 1987 Goates et al. 1989 Jones et al. 1988 Lai et al. 1990 Lamotte et al. 1985 Lin et al. 1991 Popl et al. 1975 Richardson and Ando 1977 Saber et al. 1991 Vo-Dinh et al. 1984 Vo- Dinh and Abbott 1984 Vo-Dinh 1981 Woo et al. 1980). More recent methods for the determination of PAHs in environmental samples include GC-MS with stable isotope dilution calibration (Bushby et al. 1993), capillary electrophoresis with UV-laser excited fluorescence detection (Nie et al. 1993), and laser desorption laser photoionization time-of-flight mass spectrometry of direct determination of PAH in solid waste matrices (Dale et al. 1993). 

Probably greatest attention has been directed to hydrophobic compounds especially PAHs and PCBs. For example, the desorption of I4C-phenanthrene and 14C-chrysene preloaded onto previously contamined soils has been examined and correlated with the kinetics of mineralization (Carmichael et al. 1997). 

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