This theme focuses on the understanding of trace elements as well as micro-pollutants cycling in environmental systems. This includes transfer of these elements to natural settings, the physic and chemical processes affecting their fate and behavior, as well as the mechanisms controlling their bioavailability and resulting in their migration from the geosphere to the biosphere.
- A central question raised by this framework is related to the fate of trace elements (Cd, Pb, Cu, Zn, Ni, Cr, Ra…) and metalloids (Se, Mo…) in soils, sediments and aquatic systems. As a result, to correctly evaluate the ecosystem status, this sub-topic is devoted to the good understanding of the various parameters controlling the metal(loid)s cycling in environmental systems and the impact of anthropic activity on these cycles. To accurately assess the metal ions biogeochemical cycling at all scales, the precise identification of their origin in pristine and contaminated systems is necessary, as well as the accurate knowledge of the physico-chemical reactions with mineral, organic and biological surfaces known to control to a significant extent the metal(loid)s transfer in environmental systems. Previous work performed by our group (Juillot et al., 2008, Jouvin et al., 2009) showed that the combination of speciation studies with metal isotopic fingerprints determination was a powerful tool to unravel parts of these questions. Therefore, this strategy is the basis of the project for the next 5 years. Specifically, we want to address this general question through a series of three focused issues that are: i) what are the geochemical background values and the sources of Zn, Cu, Pb and Mo elements for which the group has already a good understanding in various watershed and lake systems, our test sites will be as diverse as the Seine river, the Loire estuary, the Amazon basin and the lake Pavin, in connection with other IPGP teams (cf. potamology transversal project); ii) what are the concentrations, chemical and redox speciation and fluxes of the dissolved elements mentioned above, focus is made on the dissolved part since it is thought to be the most reactive toward living organisms; iii) what is the speciation of these metals on solid phases and does it control or modifies the original isotopic signature, in order to correctly discriminate between sources, mobilization effects and physico-chemical processes. Some efforts, we believe, are still needed to understand the role of the organic matter since it remains hardly predictable (see OM sorption section and previous results) especially for ultra trace elements and radionuclides. The group focused on Eu to address this issue we will expand our studies to Ra and its radioactive isotopes since its chemical reactivity is poorly known, despite its major importance to understand its fate and the fate of RN in general. This statement is particularly true in organic rich redox transition zones such as peat bog (low O2, low pH, high and DOC) due to Ra low geochemical background (femtomolL-1) resulting in analytical challenges. Thus, the affinity of Ra for peat bog organic matter from small organic compounds to large macromolecules will be investigated as it was done for Eu.
- Microorganisms can be found in almost every environment on Earth including soils and groundwater systems, and large biomasses have been reported for bacteria (107 to 1010 cells/gram of solid in soil environments, Yee and Fein, 2002) with a total amount of prokaryotic carbon close to the estimated total carbon content of plants (Whitman et al., 1998). Microorganisms exhibit highly functionalized cell walls that can complex metals, may induce redox changes or isotopic fractionation for some elements (Cr, Fe…). Thus, as a continuous effort, the interactions between trace elements and microorganisms remain a research topic of our group. For instance, the reactivity of mangrove sediments will be investigated to determine the rates and processes of Cr(VI) conversion into Cr(III) and conversely in the intertidal vegetated zone. Furthermore, microorganisms usually form microbial biofilms with specific physico-chemical reactivity compared to the corresponding planktonic cells (existence of specific micro-environments), an thus an important research part will expand to the biofilm interactions with metal(loid)s during the next 5 years.
Recent progresses in nanotechnology have led to the development of engineered nanoparticles (NPs) exhibiting high physico-chemical reactivity, and their incorporation into a vast number of devices and products. Given their size, NPs can interact with living organisms by crossing cell membranes, and they have strong redox properties, raising concerns about the potential impact of their release to natural environments (water and soils). In particular, some NPs are of specific concern as they are composed of toxic elements that could potentially impact ecosystems. As a result, NPs are today considered as a new class of contaminants. This raises the question of the fate and behavior of nanoparticles in environmental systems? Given their nanometric size allowing specific properties, their physico-chemical behavior cannot be extrapolated from the corresponding free ion or micro-size material properties (Waychunas and Zhang, 2008; Auffan et al., 2009). Whereas many studies have recently focused on the direct exposure of model organisms to NPs, only few of them have investigated their transfer to aquatic systems and their fate in soils and sediments, although being considered a major sink for pollutants.
- Fate of nanoparticles in natural waters: Their solubility characteristics, their dissolution kinetics as well as their aggregation processes, will be assessed at low concentration detection strategies (see “detection of nanoparticles in environmental systems”) since they will drastically affect their transport.
- Reactivity of nanoparticles in soils and sediments: NPs released in soils and sediments will be transported by advection and diffusion, and their reactivity will depend on the presence of organic ligands (i.e. humic substances) that can stabilize the particles, the existence of bacteria that can complex, breakdown or uptake them, and the abundance of sorption sites available on mineral surfaces. We want to develop studies on this topic to understand how molecular-scale physico-chemical processes, promoted by mineral surfaces, microorganisms and organic ligands, affect NPs transport, reactivity and dissolution in soils or sediments. Particularly the interaction dynamic between NPS and minerals, bacteria, and microbial biofilms will be investigated.
We will use a systematic determination of the different parameters constituents of the aquatic systems: alkalinity, major and trace elements concentrations, nutriments concentrations, pH, dissolved O2, conductivity. This systematic approach relies on instrumentation available in the group (High Resolution ICP-MS in clean room, ICP-AES Arcos in clean room) or on the campus (Multi Collector ICP-MS…). In addition, the development of in situ probes like DGT to access the labile part of metals in aquatic systems and DMT to access the free concentration will be pursued. In order to identify the metal(loid)s complexes and their evolution over time or biogeochemical conditions, XAS constitutes a powerful approach to investigate the nature of the processes controlling the trace element cycling. The use of non-traditional (i.e. Mo, Fe, Zn, Cu) stable isotopes will provide crucial information regarding the detection of metal sources, as well as the detection of specific biogeochemical processes at the molecular scale. Some efforts will be made for the development of separation techniques such as steric discrimination for organic matter (size exclusion chromatography) and FFF (Field Flow Fractionation) both coupled to High Resolution ICP-MS the single count mode with the HR-ICP-MS in the case of NPs. In addition isotopes exchange techniques to measure the exchangeable fraction of an element in a system will also be pursued (Sivry et al., 2011). In fine, the collected datasets will be incorporated into thermodynamic and reactive transport models (VisualMinteq, Ecosat, Aquasim…) to extract parameters controlling the metal(loid)s transfer.