First step towards understanding the behavior of oxygenated polycyclic aromatic compounds (O-PACs) in soils and groundwater
Résumé
Objectives: the aim of this study is to assess O-PAC migration in groundwater in order to know if they could form large contamination plumes in groundwater and therefore trigger a risk for sensitive targets such as drinking water wells. Innovative nature of the proposed topic: O-PAC migration in groundwater and parameters controlling their behavior in soils have never been assessed whereas it is well established in literature that these compounds are toxic, persistent and always present in soils of PAH contaminated sites. Abstract Oxygenated Polycyclic Aromatic Compounds (O-PACs) are toxic, persistent, highly leachable and often abundant at PAH contaminated sites. Furthermore, many studies have proven that O-PACs could be formed during and after the application of some remediation techniques on PAH contaminated sites1,2. However, in contrast to the 16 US EPA PAHs classified as priority pollutants and due to the lack of regulations and data regarding their behavior in soils, O-PACs are not included in health risk assessment studies and monitoring programs of PAH contaminated sites. However, these aromatic compounds could as well have an impact and contribute to the risk for human beings and the Environment. This study constitutes an important step in the process of understanding the transfer of these compounds within the soil system and in determining the related parameters that could affect their behavior. Two PAH/O-PAC couples were chosen for this study: fluorene/fluorenone (FLU/FLUone) and acenaphthene/dibenzofuran (ACE/DBFUR). These compounds were primarily selected regarding their available data, the possibility of their laboratory manipulation as well as the similarity in their molecular structures. Sorption isotherms onto a non-contaminated soil were individually determined using controlled batch experiments for all four compounds. Effects of ionic strength and liquid to solid ratio (L/S) on the sorption of FLU and FLUone were furthermore investigated through controlled batch experiments. For both O-PACs and PAHs, experimental data showed that the sorption kinetics were designated by the occurrence of two distinct phases. A fast-initial phase followed by a second much slower sorption process. Sorption equilibrium was achieved within less than 24 hours of mixing while no degradation of the studied compounds was observed. For all studied compounds and in all experimental conditions, linear sorption models best fit the isotherm data. Results revealed that ACE and DBFUR were similarly adsorbed onto the soil where the values of organic carbon-water partition coefficient (Koc) were 1184 and 1153 L/kg, respectively. In the same experimental conditions, Koc of FLU (1931 L/kg) was higher than that of FLUone (1355 L/kg), showing a smaller affinity of FLUone towards the solid phase. Furthermore, decreasing the L/S ratio from 100 L/kg to 50 and 30 L/kg, increased the sorption of FLUone onto the soil by 64 and 77% respectively, while the sorption of FLU was slightly increased by 13 and 31% respectively. Moreover, increasing the ionic strength of the aqueous phase by a factor of 6 favored 2 the sorption of FLUone by 62% while the sorption of FLU slightly decreased by 13%. These results provided meaningful first information regarding O-PAC behavior in soils: highly soluble O-PACs such as FLUone could easily migrate in groundwater, form larger contamination plumes than PAHs and reach drinking water wells. In addition, the difference in PAH and O-PAC behavior when decreasing the L/S ratio and increasing the ionic strength is a first hint that mechanisms responsible for O-PAC fate and transport in soils could be different than the ones responsible for PAH retention in soils. Further studies are in progress at different scales (lab and field scales) in order to better understand the migration potential of O-PACs.