PAHs, octanol-water partition coefficients

Logarithm of Octanol/Water Partition Coefficient (log K0w), Calculated Henry s Law Constant (H), and Calculated Logarithm of the Octanol-Air Partition Coefficient (log KOA) at 25°C for Selected PAHs of Atmospheric Interest" 6... 

PAHs have been found all over the globe in all compartments of the environment. They are ubiquitous because the are persistent. Recalcitrance in PAHs may stem, in part, from the delocalized electrons in the planar pi orbitals of the aromatic structure. Their relatively high octanol-water partition coefficients, Kows, make them rather lipophilic. The lipophilicity of PAHs forces them from the dissolved phase to particles and also into lipid rich organisms, but they can be metabolized in higher organisms. However, these metabolites are often more toxic than their parent PAHs. When combined with other stressors, particularly ultraviolet radiation, PAHs can exert enhanced toxicity. 

Fig. 6 Log-log plot of estimated bioaccumulation factor vs. octanol-water partition coefficient of PAHs in Lake Erie (data from [137])...

Particle-gas partitioning can be described by a variation of the Pankow absorption model, in which liquid-phase vapor pressure is replaced by the octanol-air partition coefficient as a fitting parameter (equations (22)-(25)). Section 10.3.4 states the advantages of doing so. Koa has been reported as a function of temperature for several PCB congeners, chlorobenzenes, PAHs, polychloronaphthalenes (PCNs), and p,p -DDT (Harner and Bidleman, 1996, 1998b Harner and Mackay, 1995 Komp and McLachlan, 1997). For others, Koa can be estimated from the ratio of the octanol-water partition coefficient to the Henry s law constant (H = Pa m3/ mol) (Finizio et al., 1997 Harner and Mackay, 1995 Simonich and Hites, 1995). 

The IPAH model incorporated a number of factors that can modify the toxicity of the sediment-borne PAHs. Equilibrium partitioning was used to estimate the concentration of each PAH in the pore water of the sediment. The assumption was that the pore water material is the fraction that is bioavail-able. QSAR was also used to estimate the interstitial water concentration based on the octanol-water partition coefficient of several PAHs. Amphipods were used as the test organism to represent environmental toxicity. A toxic unit (TU) approach was used and the toxicity is assumed to be additive. The assumption of additivity is justified since each of the PAHs has a similar mode of action. Finally, a concentration-response model was formulated using existing toxicity data to estimate the probability of toxicity. 

Table 1 Physico-chemical properties saturation vapour pressure, ps j, Henry s law constant, K, and octanol-water partitioning coefficient, K , and degradability in air as the reaction rate coefficients of the hydroxyl radical with the gaseous and the particulate phase molecules, kg oh and 1% oh> respectively, of 17 parent PAHs and 1 alkylated PAH (retene) (298 K ten Hulscher et al. 1992 Finlayson-Pitts and Pitts 2000 Perraudin et al. 2007)...

This study was undertaken to test the ability of our previous molecular connectivity models to accurately predict the soil sorption coefficients, bioconcentration factors, and acute toxicities in fish of polycyclic aromatic hydrocarbons (PAHs), alkylbenzenes, alkenylbenzenes, chlorobenzenes, polychlorinated biphenyls, chlorinated alkanes and alkenes, heterocyclic arid substituted PAHs, and halogenated phenols. Tests performed on large groups of such compounds clearly demonstrate that these simple nonempirical models accurately predict the soil sorption coefficients, bioconcentration factors, and acute toxicities in fish of the above compounds. Moreover, they outperform traditional empirical models based on 1-octanol/ water partition coefficients or water solubilities in accuracy, speed, and range of applicability. These results show that the molecular connectivity models are a very accurate predictive tool for the soil sorption coefficients, bioconcentration factors, and acute toxicities in fish of a wide range of organic chemicals and that it can be confidently used to rank potentially hazardous chemicals and thus to create a priority testing list. ... 

We will first test the predictive power of this simple nonempirical model on chlorobenzenes and PAHs not included in the original study and on alkylbenzenes, heterocyclic-PAHs, amino- and hydroxy-PAHs, and chlorinated phenols by comparing our predictions with the corresponding experimental values. The results of this test will then be compared with the corresponding soil sorption coefficients predicted from their water solubility (S ) and/or 1-octanol/water partition coefficients (Kq, )- The first-order molecular connectivity indices of the new set of molecules were calculated and their soil sorption coefficients were predicted from the molecular connectivity model, equation 6. These results are shown in TABLE 1 together with the corresponding experimental soil sorption coefficients. 

TABLE 2 The range of reported experimental 1-octanol/water partition coefficients (log Kq ) for some alkylbenzenes, chlorophenols, PAHs, chlorobenzenes, PCBs, and other chlorinated hydrocarbons. 

Because PAHs exhibit a range in lipophilic affinity, elimination that relies solely on passive diffusion loss should be slower for the more hydro-phobic PAHs. Such a correlation has been shown for the freshwater amphi-pod [Diporeia spp. (Pontoporeia hoyi)], which does not metabolize PAHs to any appreciable extent (Landrum 1988). This author found that for a series of PAHs with increasing octanol-water partition coefficients, the elimination constant decreased, which led to longer half-lives. The more water-soluble PAHs phenanthrene, anthracene, and fluoranthene showed nearly identical rapid elimination patterns (/./, 2 d) in mussels (Mytilus edulis) exposed to contaminated suspended sediments for 28 d, while the more hydrophobic PAHs produced half-lives in the range of 4-6 d (Fig. 8 see Table 2) (Lake et al. 1985). The most hydrophobic of the group, pery-lene, displayed the slowest elimination. 

Koc,t (sediment organic carbon-water partition coefficient for water concentration with free and bound PAH) = ([Sediment]//oc)/[Watertotai] iTow (octanol-water partition coefficient) = [Octanol]/[Water] (sediment-water partition coefficient) = [Sediment]/[Water]... 

The Octanol-water partition constant (Kow) is the ratio of the eoncentration of a substance in octanol to that in water at equilibrium at a speeified temperature (Asante-Duah 2002), (Kow) is used to predict the fate of PAHs either in the environment or in biological tissues. (Kow) increases with increasing benzene rings, from LMW PAHs to HMW PAHs. The (Kow) is used to determine the phase which the PAH shall partition to, this ability is very useful in human exposure analysis to PAHs (Asante-Duah 2002). Measurements of (Kow) of individual PAHs correlate perfeetly with measurements of solubility and hydrophobicity, with increasing hydrophobieity is low solubility and an increasing Octanol-Water partition coefficient. 

TABLE 3.55 Experimental Octanol-Water Partition Coefficients (logKOW) and the Retention Indices (RI) (van Den Dool Kratz, 1963) For the Studied PAHs ... 

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