Thermodesorption coupled with molecular analyses as a new approach to estimate bio− and chemo−availability of PAHs in contaminated soils

International audiencePolycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants affecting about 200,000 sites in Europe, according to the European Environment Agency. Due to toxic, mutagenic and carcinogenic properties, 16 PAHs are listed as priority pollutants. The monitoring of these compounds in contaminated soils is either for risk assessment purpose or to anticipate the efficiency of remediation treatments. However, in aged materials such as historically contaminated soils, the total (16 listed) PAH concentration is not necessarily representative of the risk inherent to these materials or of the degradable fraction. The fraction to consider in these cases is the available fraction. Availability of organic contaminants, including PAHs, can be estimated using several methods (mild-solvent extraction, Tenax extraction, cyclodextrin extraction, passive sampling, biosensors…). However, several studies aiming to compare some of these methods point out inconsistencies in the results [1-4], underlining the need of a standardized method. Another challenge is to determine the available fraction in relation to a specific treatment/process (e.g. bioremediation, oxidation with specific oxidants…). In this study we propose a new method based on thermodesorption (Td) coupled with molecular analyses to determine several availabilities (bio- and chemo-). This method is based on linking the desorption temperature to the binding strength between the contaminant and the soil constituents. In other words, the aim of this study was to define such desorption temperatures for the 16 regulated PAHs, using the Td coupled with molecular analyses, with several desorption steps and comparing the results with the efficiency of several remediation treatments (microbial incubation, Fenton-like oxidation and KMnO4 oxidation).PAH contaminated soils presenting different properties, exhibiting varied ranges of PAH concentrations and different levels of PAH availability were analyzed. The soils were desorbed from 100 to 450 °C via seven 50 °C-temperature stages. The desorbed compounds were analyzed by gas chromatography-mass spectrometry/flame ionization detection (GC-MS/FID) allowing separation, identification and quantification of PAH, respectively.The desorption steps allowed to determine a temperature profile for each PAH in each soil which was then compared to the amount of degraded PAHs during several chemical and biological remediation treatments performed in the lab. From the first results, it was possible to determine, for each compound and for each treatment, the desorption temperature range corresponding to the degraded fraction of the contamination. Therefore, these desorption temperatures could be used to estimate the different bio- and chemo-availability in PAH contaminated soils. This method could prove to be a powerful tool to assess the risks associated with the presence of contamination and/or to select the appropriate remediation treatment for sites contaminated with organic compounds.[1] M. Barthe , É . Pelletier, G.D. Breedveld, G. Cornelissen, Chemosphere 71, pp. 1486-1493, 2008.[2] M. Bergknut, E. Sehlin, S. Lundstedt, P.L. Andersson, P. Haglund, M. Tysklind, Environ. Pollut. 145, pp. 154-160, 2007.[3] A.L. Swindell, B.J. Reid, Chemosphere 62, pp. 1126-1134, 2006.[4] J.D. MacRae, K.J. Hall, Environ. Sci. Technol. 32, pp. 3809-3815, 1998