Aging as the main factor controlling PAH and polar-PAC (polycyclic aromatic compound) release mechanisms in historically coal-tar-contaminated soils
In industrial sites, historically contaminated by coal tar (abandoned coking and manufactured gas plants), other families of organic pollutants than the 16 PAHs (polycyclic aromatic hydrocarbons) classified by the US-EPA can occur and induce potential risk for groundwater resources. Polar PACs (polycyclic aromatic compounds), especially oxygenated and nitrogenated PACs (O-PACs and N-PACs), are present in the initial pollution and can also be generated over time (i.e., O-PACs). Their aqueous solubilities are much greater than those of the PAHs. For these reasons, we need to increase our knowledge on polar PACs in order to better predict their behavior and the potential on-site risk. Batch leaching tests were carried out under various conditions of temperature, ionic strength, and availability of pollutants to determine the mechanisms and key parameters controlling their release. The results show a release of low-molecular-weight PAHs and polar PACs mainly by dissolution, while higher molecular weight PAHs are mainly released in association with colloids. Aging mainly controls the former mechanism, and ionic strength mainly controls the latter. Temperature increased both dissolution and colloidal mobilization. The Raoult law predicts the PAC equilibrium concentration for soils presenting high pollutant availability, but this law overestimates PAC concentration in aged soils (low pollutant availability). This is mainly due to limitation of PAC diffusion within coal-tar particles with aging. The most soluble PACs (especially polar PACs) are the most sensitive to aging. For better prediction of the PAC behavior in soils and water resources management, aging needs to be taken into account.
KeywordsAvailability Ionic strength Oxygenated PACs Nitrogenated PACs Groundwater Colloidal mobilization
The authors acknowledge Y. Duclos from the Agence de l’Environnement et de la Maîtrise de l’Énergie (ADEME) for many scientific discussions. Arcelor Mittal company (especially P. Charbonnier) and the GISFI (French scientific interest group on soil pollution (http://www.gisfi.univ-lorraine.fr) and especially N. Enjelvin) are acknowledged for providing the two coking plant soils used in this work. D. Billet and A. Razafitianamaharavo from the LIEC laboratory are acknowledged respectively for the dissolved organic carbon measurements and the specific area determination by BET.
This work was supported by ADEME, INERIS, and CNRS.
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