Analytical strategies to evaluate polycyclic aromatic hydrocarbons (PAHs) bioavailability in soils; correlation with their biodegradability

International audiencePolycyclic aromatic hydrocarbons (PAHs) are persistent and toxic pollutants which present geo-accumulation characteristics. Depending on their chemical form and soil characteristics, they are more or less strongly sequestrated in soils, and the effectiveness of bioremediation processes is closely linked to their bioavailability for the soil degrading bacterial communities. The aim of this study was to develop a simple and rapid method to evaluate bioavailability of PAHs in soils and to show if a correlation exists with their biodegradability. The extraction methods usually performed (Soxhlet, Microwave Assisted Extraction MAE, or Accelerated Solvent Extraction ASE) give pseudo-total content of PAHs in soil samples, but do not allow to access the labile and so bioavailable fraction of PAHs. Soft extraction methods, using solubilizing agents in aqueous phase, are more likely to give the labile fraction of PAHs in soils. Additives such as synthetic surfactants, emulsions and cyclodextrins were tested to extract PAHs from five dissimilar natural soils, with very different geochemical properties (nature and content of organic matter and clays, pH…). New cyclolipopeptidic biosurfactants, produced by bacteria, were also tested, as well as original additives such as micellar ionic liquids. The critical micelle concentration (CMC) of our synthesized ionic liquids could be determined by a novel method using capillary electrophoresis. The nature of soil organic matter was determined using RockEval 6 pyrolysis, and the nature of clays was assessed using X-ray diffraction.Aqueous extraction capacities with these different additives were then compared to pseudo total extractions performed by MAE with organic solvents, in order to determine whether the ratio between labile PAHs and total PAHs was linked to the soil retention capacities. In parallel, we studied the degradation kinetics of 7 mixed PAHs, for three months, in the five soils. Thereafter, a principal component analysis (PCA) allowed us to correlate (or not) the biodegradation of low and high molecular weight PAHs to some characteristics of soils, to their bioavailability, but also to the number of cultivable bacteria degrading phenanthrene