Migration of microplastics in soil to groundwater and effect on the transport of Polycyclic Aromatic Hydrocarbons (PAH).

The recent detection of microplastics (MP) in groundwater is of great concerns. As soils are highly polluted by PM, their vertical transport by water infiltration has been suggested but remains to be demonstrated and characterised. Moreover, soils containing PM often have high levels of other pollutants such as Polycyclic Aromatic Hydrocarbons (PAH). Since PM has a strong affinity for PAHs, it is likely to modify their migration in soils and into groundwater. In addition, PM are often exposed to sunlight before entering the soil layers, which changes their physico-chemical properties.

      In this context, the objective of the thesis project is to improve the understanding of the ecodynamics of the PM/PAH/UV/soil system as it appears crucial to better understand these phenomena to assess the real threat on soils and groundwater resources and the associated health risks.

      Thus, the transport of PM by water infiltration in alluvial deposits towards groundwater will be explored by considering the physico-chemical characteristics of the soil, PM and water infiltration parameters to access the main parameters guiding this phenomenon. The dynamic migration approach will also address the possible partitioning of PAHs present in soil with plastic particles during their vertical migration and vice versa. Classical factors that may control the transport of PM in soil, such as particle size or soil pH, will be explored. In addition, other influencing factors will be considered including water flow and wet-dry cycles. Finally, the effect of photochemical ageing of PM will be studied on their vertical migration and capacity to transport PAHs. In addition, for advanced PM photooxidation, the fragmentation of aged and embrittled particles during the migration will be evaluated.

      To achieve these objectives, laboratory column experiments will be necessary and the determination of PAHs in soil and plastic particles will be carried out during the migration experiments. In addition, a special effort will be made to characterise the soil and plastic particles after photooxidation (zeta potential, physical and chemical surface state, average molar mass). In the water collected at the column outlet, the number and type of particles will be determined by FTIR microscopy, Pyr-GC-MS or ATG-GC-MS and the particles will be characterised by SEM, SEC or DLS. In addition, PAH concentrations will be determined in eluted plastics, water and soil by extraction and LC analysis or Pyr-GC-MS and/or ATG-GC-MS analysis. The plastic particles will be photooxidised in an accelerated ageing chamber and characterised by FTIR spectroscopy, SEM (EDX), SEC and DLS. Finally, the possibility of modelling the transport of plastic particles in soil will be explored, by adapting numerical models that have been developed for the transport of microparticles or nanoparticles in soils, and experiments will then be undertaken at a pilot scale and under simulated and controlled conditions.

The LCE is a joint research unit (CNRS AMU UMR 7376). It is made up of two teams and the proposed thesis subject will be developed in the TRAME team (Transfert Réactivité et Analyse des Micropolluants dans l'Environnement). This team has been interested for years in organic pollution (pesticides, pharmaceuticals, PCBs, PAHs, etc.) in various environmental media (soil, sediment, water, biota, etc.). For the last sevent years, this team has been working on the detection and characterisation of microplastics and their content in PCBs and PAHs and recently focussed on their fate. Thus, it has been possible to show their transfer to the atmospheric phase, the effect of their photochemical degradation on this process and the evolution of their sorption properties for PAHs. This team has all the equipmentw for the preparation and characterisation of samples with techniques such as ASE, SPE, Soxtherm, etc., spectroscopic techniques (UV, IRTF, microIRTF, etc.) and chromatographic techniques (LC-UV-Fluo, LC-QTof, GC-MS, GC-MS-MS, pyrolysis-GC-MS, etc.) It should be noted that this thesis work will be realised in collaboration with a team from Ineris (Ardevie, Arbois) and a team from the Institute of Radical Chemistry of AMU

A recent master’s degree or an engineer diploma in analytical chemistry, chemical physics, environmental chemistry, or related discipline, obtained with a minimum grade of 14/20 is required. Solid knowledge in analytical chemistry is required, as well as a taste for exploratory research and field observations. Knowledge in polymers or photochemistry would be an advantage. Independence, drive, and collaboration are important and will be encouraged to develop the candidate’s career.