Investigating the effects of plants and their root exudates on PAHs bioaccessibility to soil microorganisms and their impact on aged-contaminated brownfields remediation.

In Wallonia, there are 6,000 estimated brownfields (rising to over 3.5 million in Europe) that require remediation. Polycyclic aromatic hydrocarbons (PAHs) are persistent organic compounds of major concern that tend to accumulate in the environment, threatening ecosystems and health. They represent 17% of all encountered pollutants. The thesis, articulated around several axes, focused on improving less aggressive PAHs remediation techniques in brownfields, such as bioremediation and phytoremediation, as alternatives to environmentally aggressive, expensive and often disruptive current soil remediation strategies. More specifically, it was built on several authors' observation that PAHs soil contents decrease in the presence of plants. The hypothesis was made that this could be related to the production of plant root exudates. A bioaccessibility measurement protocol was adapted to two brownfield soils using Tenax® beads in order to compare PAHs bioaccessibility in soil samples. In both experimental soils, PAHs desorption kinetics were established, described by site distribution models, and common extraction times were calculated (respectively 48 h and 24 h for both experimental soils). The potential of saponin (a natural surfactant) as extracting agent and as a bioremediation enhancer on an aged-contaminated soil was investigated. In a first experiment, soil samples were extracted with saponin solutions (0; 1; 2; 4 and 8 g L-1). In a second experiment conducted in microcosms (28°C), soil samples were incubated for 14 or 28 days in presence of saponin (0; 2.5 and 5 mg g-1 DW). CO2 emissions were monitored throughout the experiment. After the incubation, dehydrogenase activity was measured as an indicator of microbiological activity, and the bioaccessible and residual PAHs contents were determined. The 4 g L-1 saponin solution globally extracted significantly more PAHs than water. Neither PAHs dissipation nor bioaccessibility were enhanced in presence of saponin compared to control samples after 28 days. However, CO2 emissions and dehydrogenase activities were significantly more important in presence of saponin, suggesting no toxic effect of this surfactant towards soil microbiota. The role of two Fabaceae (Medicago sativa L. or Trifolium pratense L.) root exudates in enhancing PAHs bioaccessibility and dissipation in an aged-contaminated soil was investigated during an incubation experiment (28°C). The CO2 emissions were significantly higher in presence of T. pratense exudates; the dehydrogenase activities (measured after 14 and 28 days) showed improvements of the soil microbial activity in presence of both types of root exudates compared to untreated soil samples; the PAHs residual contents decreased more in untreated samples than in the presence of T. pratense exudates; and M. sativa exudates lowered PAHs bioaccessibility but not residual contents. The effects of Medicago sativa L. and Trifolium pratense L. on the PAHs’ bioaccessibility and dissipation in an aged-contaminated soil throughout a rhizoremediation trial were investigated. The bioaccessible and residual PAH contents were quantified after three, six and twelve months of culture. The rhizoremediation results show that M. sativa developed better than T. pratense on the contaminated soil. When plants were absent (control) or small (T. pratense), the global PAHs’ residual contents dissipated from the rhizosphere to 8% and 10% of the total initial content, respectively; in the presence of M. sativa, dissipation after 12 months was only 50% of the total initial content; and the PAHs bioaccessible content increased more significantly in the absence of plants. Finally, a review of three scientific trends that lead PAHs contaminated soils/sediments remediation studies and management was conducted. (i) The choice of PAHs compounds that are being studied and targeted in scientific literature were discussed, and it was suggested that the classical 16 PAHs from the American Environmental Protection Agency’s (US-EPA) watch list might no longer be sufficient to meet actual environmental challenges. (ii) The choice of experimental material in remediation studies was discussed. Bibliometric measures were used to show the lack of PAHs remediation trials based on co-contaminated or aged-contaminated material. (iii) The systematic use of the recently validated bioaccessibility measurement protocol (ISO/TS 16751:2018) in remediation trials was discussed, and it was suggested that such measurement should be implemented as a tool to improve remediation processes and management strategies