Contaminant Dynamics

This thematic axis involves site-specific and/or laboratory-scale approaches to better understand the dynamics or eco-dynamics of contaminants in the environment.

The multi-scale analysis of persistent and emerging pollutants, conducted through regional and even international observatories (e.g., the Mediterranean coast, the Red River in Vietnam), helps to better grasp the global behavior of persistent pollutants in the specific context of global changes such as climate change, population growth, megacity development, and industrial acceleration. The goal is to study the impact of these substances on water quality, coastal habitats, benthic ecosystems, and fishery resources—particularly in areas where fishing is economically important—as well as to investigate how these pollutants migrate through the environment.

For example, the transfer of microplastics from oceans to the atmosphere has been demonstrated through field measurements and laboratory experiments. The main process involved appears to be the bursting of air bubble plumes generated by breaking waves.

Furthermore, although microplastics (MPs) and nanoplastics (NPs) have recently been detected in groundwater, their transfer pathways into this compartment remain largely hypothetical. In this context, the TRAME team is studying the migration of MPs and NPs through PAH- and metal-contaminated soils toward groundwater. The aim is to validate this transfer process, identify the controlling parameters, and assess the potential transport of pollutants during this migration.

Another focus within this axis involves environmental reactivity within the marine surface microlayer (also known as the Sea Surface Microlayer, SSML). This microlayer represents the air-sea interface, comprising the first few hundred micrometers below the water surface. One of its key characteristics is the accumulation of organic matter from marine microbiota (e.g., exopolysaccharides, surfactants) and organic micropollutants—especially those with amphiphilic properties (e.g., PCBs, DDT, hydrocarbons). It thus acts as a unique photochemical reactor, whose behavior is still poorly described in the scientific literature.

Using lab experiments, the TRAME team seeks to:

• Assess the influence of the polarity/amphiphilic nature of pollutants and the composition of the SSML on the enrichment of organic pollutants,
• Determine whether the photodegradation processes of these pollutants are enhanced or inhibited in the SSML by elucidating the underlying reaction mechanisms,
• Evaluate air–water interactions, including pollutant transfer to the atmosphere and degradation induced by atmospheric radicals.

Once released into the environment, organic micropollutants undergo natural transformation (e.g., biodegradation, photooxidation) or are transformed during treatment processes. These transformations lead to the generation of transformation products (TPs) in aquatic environments—products that may be toxic to aquatic organisms.

The study of these TPs is rarely considered in environmental quality or health risk assessments of the parent compounds. Yet, TPs could pose an unrecognized threat to water quality. With over 100,000 substances authorized for market use in Europe, a case-by-case approach for each compound is not feasible.

This creates several scientific challenges:

• To develop methodological tools that can predict the kinetics and transformation pathways of organic contaminants based on their chemical structure;
• To evaluate how environmental variability (across different spatial and temporal scales) influences the significance of these transformations.

Developing such predictive models requires a molecular-level understanding of chemical, photochemical, and biological reaction mechanisms, as well as precise identification of transformation products. The aim is to determine the chemical and environmental properties that favor either persistence or degradation of organic contaminants, and to define the relative importance of various transformation pathways. A molecular approach is essential to clearly define the limits of applicability of transformation rules derived from a limited set of "probe" compounds.

Though originally fundamental, this thematic axis also has industrial applications, and is divided into two sub-axes:

• Natural attenuation: mainly through photooxidation, but also phytoremediation and phytostabilization.
• "Guided" attenuation: via the development of innovative treatment processes and assessment of the impacts of photochemical treatments on the elimination of chlorination by-products.