PhD in geosciences (M/F) – Influence of intense rainfall episodes on the speciation, quantification, flux, and fate of remobilized metals

Population growth, energy transition, and rising living standards have led to an exponential increase in the quantities of metals extracted, while technological developments have resulted in a diversification of the metals required for industrial development, causing an unprecedented increase in metal discharges into the environment. Metals are considered either toxic (e.g., Cd) or essential trace elements (e.g., Cu), but their concentration beyond certain thresholds degrades ecosystem quality and poses a threat to human and animal health (the “One Health” concept) in contaminated areas.  

Climate models predict that the frequency, intensity, and number of short-duration extreme precipitation events, as well as flood variability, will increase as the global climate changes. This reality makes it increasingly difficult to accurately constrain the fate and source-sink balance of metals in the Critical Zone (CZ) and, consequently, the overall contributions from continents to the oceans. Within the CZ, soils act as metal sinks, particularly in wetland areas. Their hydrological cycle (high water/low water) promotes the formation of chemical gradients and biogeochemical processes that regulate the metal cycle. Extreme floods are characterized by water levels and flow rates significantly higher than the oscillatory levels typically observed over time. The source-sink balance of metals in wetlands then becomes disrupted, increasing the export of metals over very short timescales (“flash pollution”) in partially known physicochemical forms, which may be toxic, and in quantities and concentrations that need to be assessed.  

Objectives

• Characterization of material exported from wetlands during extreme rainfall events. Hypothesis: The colloidal fraction is primarily responsible for metal export from wetlands.
• Influence of organic matter origin and Fe speciation on metal remobilization. Hypothesis: Fe speciation and the physicochemical characteristics of organic matter control the ability of aggregates to transport metals and alter their speciation, mainly through electron transfer induced by microbial activity.
• Metal distribution, speciation, and isotopic composition. Hypothesis: Metal remobilization is mainly controlled by redox processes during precipitation and flooding events.
   

Methodology

The PhD research will include:

• Field experiments with multiple sampling campaigns and in situ and laboratory characterization of surface water samples (particulate, colloidal, and dissolved fractions), groundwater, and interstitial water, as well as sediments and soils.
• Laboratory experiments to replicate observed conditions under controlled settings to identify metal remobilization processes.

Fundamental tools from geochemistry and mineralogy will be combined with isotopic geochemistry, molecular-scale speciation (spectroscopic techniques), and microbiology.

The candidate will be affiliated with the Laboratory of Planetology and Geosciences (LPG) in Nantes, but with co-supervision between Nantes and Angers.

Nantes Université is a recognized institution and a major research center in Western France. With 42 000 students and 3 200 lecturers and researchers, Nantes Université seeks to meet today's major challenges. Located near the Atlantic coast and 2 hours from Paris by train, Nantes is a large city regularly ranked for its quality of life and well-known for its cultural scene.

PhD students in physics, chemistry and geosciences receive training in internationally recognized laboratories associated with the CNRS. They also benefit from a wage of €2 200 gross per month. Within the Graduate School 3MG, students are able to attend courses dedicated to research and innovation and take part in events and meetings in France or abroad to develop their professional network.

The candidate will have a Master's degree (M2) or equivalent in geosciences. He/she will have knowledge in geochemistry of surface environments (water, soils), experimental analysis methods (ICP AES, ICP MS), and a strong interest in fieldwork (sampling campaigns) and laboratory work in a controlled environment (clean room). He/she may also have a basic understanding of hydrology to work on metal fluxes.

A good command of English is expected to facilitate communication with foreign partners, ensure bibliographic monitoring, and support the dissemination of scientific results (conference presentations and article writing).