Research

Research Overview

Laurent Charlet team research activity focus on the chemical reactivity of (nano) particles, whether natural nanoparticles (oxide and clay found in soil, sediment and surface waters) or engineered ones (as used in oncology, nanotechnology or environmental engineering). While continuing his life-long research on soil and deep underground storage issues regarding water quality, he is now collaborating with the Medical community, looking for nanoparticle disease induction (e.g. podoconiosis) and treatment (e.g. cancer). His research focuses trace elements, their speciation at the molecular level in therefore a variety of biological and environmental media. Full professor at the age of 35 when ESRF synchrotron facility was under construction in Grenoble, Laurent Charlet work gained global spotlight by performing research at drastically different scales : (i) large scale multidisciplinary field investigations (on the fate of toxics such as mercury in Amazonian forest and arsenic in SE Asian deltas) and (ii) molecular level investigations at synchrotron facilities with a special focus on the redox surface chemistry of nanoparticles, particularly for oxyanion trace elements and their importance in human health, origin of life and environmental safety.

Research Techniques

Laurent Charlet team research is based on the development of advanced chemical concepts, methodology and instrumentation methods to investigate biological and geochemical processes governing the chemical speciation and impact on mobility, bioavailability and toxicity of trace elements (Se, As, Sb, Re, Hg) or organic molecules (antibiotics and other WTP non treated molecules) in heterogeneous chemistry. This activity is best illustrated by the rationalization of electron transfer at nanoparticle-water or nanoparticle-cell interfaces. His studies combine field & toxicology observations with spectroscopic (µXAFS, ESR, Mössbauer), neutron and X-Ray diffractometric studies performed on laboratory model system. The aim is to develop general new concepts and new tools that are essential in offering geochemistry new entries in several problems related to natural/engineered nanoparticle reactivity in the environment, as a mean of predicting biogeochemical processes that are relevant to water quality, paleoenvironment reconstruction, environment risk assessment, or cancer development) or treatment.

Research Projects

• Cancer nanotherapeutics, nanotoxicity and trace element deficiency

Laurent Charlet team has worked on both the toxicity of nanoparticles (NP), and their use as therapeutic agents. In particular, while working on selenium deficiency impact on osteoarthrosis, Keshin Beck disease and thyroid cancer, the team also investigated the therapeutic use of selenium nanoparticles and toxicity of nanowires.

Theranostic metallic nanomaterials, aimed for tumor cell targeting (Se-NP) and bio-imaging (Gd-magnetite. These nanomaterials are carefully synthesized in O2-free conditions and characterized. SeNP are well tolerated in vivo instead of Se salts could increase safe dosing regime, and thus were evaluated for the growth inhibition and metastatic potential on ovarian and prostate cancer cell, and their nanomechanical cell response (surface roughness, membrane stiffness). As output example, Prof. Conlan (Swansea U.) and Charlet recently made the entirely novel discovery that SeNPs play a direct role in histone methylation via the one carbon cycle and transulfurication pathway.

Silver Nanowires are anticipated to be incorporated into numerous consumer products such as touchscreen display, wearable electronics, paper printed electronics and sensor medical devices that could be in direct contact with skin. The AgNW toxicity was evaluated by Charlet and Dr. Ben Gilbert (LBL) teams and a material property the nanowire-bending stiffness, that is a function of diameter, was shown to control the cytotoxicity of AgNWs to nonimmune cells from humans, mice, and fish without deterioration of critical conductive transparent network performance parameters : electrical conductivity and optical transparency. Both 30- and 90-nm-diameter AgNWs are readily internalized by cells, as shown by CT- X-ray nanoimaging and fluorescent labelling confocal imaging, but thinner NWs are mechanically crumpled by the forces imposed during or after endocytosis, while thicker nanowires puncture the enclosing membrane and release silver ions to cytoplasm, thereby initiating oxidative stress. Similar mechanisms with natural imogolite nanotubes could be at work in the development of podoconiosis, an asymmetric elephantiasis disease, unique as both a neglected tropical disease (NTD) and a non-communicable disease, now investigated by Laurent Charlet team.

Selenium poor soils leading to Se-deficient dietary intake are found in Central China and Western Algeria. Selenium is an essential component of antioxidant and anti-inflammatory-related proteins and Se-deficiency influences cartilage and thyroid metabolism, leading to the progression of osteoarthritis, such as Kashin-Beck disease (KBD), and to thyroid cancer. Even if the selenium is not the only factor in the development of degenerative joint disease, Research in Charlet and Dr. S. Bohic (INSERM) teams have shown that Se-deficiency impacts articulate cartilage growth and development and induces morphological and compositional changes in the cartilage matrix during the fast maturation-like process, which could be related to degenerative-like cartilage morphology e.g. in KBD patients during childhood. Research on Se deficiency induces the pathophysiology of thyroid cancer, as selenium is together with iodine a critical trace element to thyroid metabolism. Cu isotopes (d65Cu) and plasma trace elements level, were shown with Dr. A. Gourlan to be suitable biomarkers of early thyroid cancer diagnosis.

• Mineral particles surface chemistry

Below every soil square meter one walks on, a million square meter surface area is developed in soil. Natural nanomaterial such as clays and oxides are the storage capacity for major and trace element essential to plant and human life, and they retard by their adsorption the migration of toxic elements and molecules to surface and underground water bodies. The surface chemistry and reactivity towards trace elements of these nanoparticles has been investigated using solute and gas analysis as well as synchrotron X-ray absorption spectroscopy (XAFS and RIXS), neutron scattering, Mössbauer and XPS spectroscopy, Fluorescence decay mapping, X-ray tomography and AFM microscopy. Insights provided by molecular simulation (DFT, MD) confronted to the spectroscopic/scattering data have also been critical to a fundamental understanding of the structure and surface chemistry of nanoparticles, and the distinction of mechanisms such as cation exchange, outer-sphere and inner-sphere surface complexation as well as irreversible (reductive) surface coprecipitation of toxic trace elements.

After many years spent on the study of clay, calcite and Fe and Mn oxide surface chemistry, Laurent Charlet shifted his research focus on redox active minerals (magnetite, mackinawite and pyrite) layered double hydroxide (a cement active component) and imogolite nanotube (a volcanic soil component). They determine the solid chemical reactivity in anoxic environments, concrete and volcanic soils. A combination of advanced characterizations and modelling precisely quantified the various active sites (edge sites vs. interlayer sites) of LDHs and the surface reducing species of pyrite. AFM microscopy and molecular dynamics quantified the structure and energetics of imogolite surface water molecules, the hydrophobicity (or lower hygroscopicity) and aggregation of imogolite nanotubes. The curvature of imogolite nanotubes prevents, in contrast to analogue flat kaolinite surface, favors the formation of in-plane H-bonds along the directions of the nanotube circumference, lowering the enthalpy of adsorption of water molecules. Determination of redox couples at the surface of various Fe oxides allowed the accurate determination of the redox potential imposed by complex mineral interfaces, e.g., steel corrosion product nano-layers. The surface reactivity of nanocomposite pyrite−greigite and mackinawite toward highly concerned aqueous contaminants (e.g., Se, As, and Sb oxyanions) has demonstrated the scavenging of Se and its long-lived isotopes as Se° nanowires. In an early AFM study, Laurent Charlet measured the in-situ dissolution rate of hectorite, a Li-rich trioctahedral clay. This supports the low waste extraction concept for Li trapped in hectorite in Nevada, the new world Li resource.

• Hydrogen, Water and Waste Geological Storage

Hydrogen. Efficient, large-scale, and long-duration energy storage for intermittent renewable energy sources is a critical unsolved problem for the expansion of carbon-free electricity on the grid. Underground hydrogen storage (UHS) coupled to reversible water splitting and hydrogen oxidation has the potential to play a role in grid-scale energy storage. It is currently the only approach for the storage of energy at the necessary scale and over the sufficiently long periods to enable supply-side management of energy from renewables, and their intermittent (solar or wind) energy production. The geologic storage of H2 in salt caverns is an established technology, but these subsurface formations are available only in a fraction of regions that either host substantial populations or that benefit from solar or wind resources. Low-cost natural clay geological materials could store large amounts of hydrogen. Laurent Charlet team has proposed an innovative approach to UHS, through the study of hydrogen on clay at various water content, that will provide new opportunities for deployment and integration into energy and electricity systems.

Water. The Sponge City concept, popularized by Prof. Yu Kongjian (Peking University), corresponds to future cities that do not act like an impermeable system, but allow water to filter through the ground, like a sponge absorbing rain water, particularly flash flood water (in dry areas) or water outflow from treatment plants (e.g. after reverse osmosis treatment in Los Angeles after 2035). This allows for the extraction of water from the ground through urban or peri-urban wells. This water can be easily treated and used for the city water supply. Laurent Charlet team has conducted research on natural contamination (via large field studies in SE Asia, on delta aquifer contamination by arsenic to understand the release mechanism of such toxic), on decontamination by engineered wetlands and by layered filters (containing biochar, iron oxides, manganese oxides and activated carbon) that will allow the removal of contaminants (respectively pharmaceuticals, perchlorate and arsenic) and the recharge urban aquifers with high quality waters.

Nuclear waste. The safety of radioactive waste geological storage (RGWS) is a major challenge to any French geochemist, as 70% of electricity in France is still today of nuclear origin. In the European concept of RWGS, vitrified waste steel canister will be stored either in a > 300 m thick clay rock, or in granite surrounded by a clay and concrete “near field” barrier. Once the repository is closed and water saturation restored, steel corrosion will lead for 20k-150k years to Hydrogen gas and ferrous iron release. Laurent Charlet team has shown that both species are sorbed on edge face of clay minerals and LDH concrete component, leading via redox reaction to the immobilization of otherwise extremely mobile radionuclides such as 79Se and high valence 235,238U. In addition, iron sulfides, that both exist in granitic and claystone host rocks or are formed during canister during steel corrosion, are key actors in controlling the redox potential and inhibiting the transport of redox-sensitive radionuclides, e.g. under hyperalkaline concrete environments. Such comprehensive investigations pave the way for a reliable disposal, by advancing our understanding of the interplay between radionuclides and barriers in the environment of geological repository.

• Ecological engineering

The treatment of urban and industrial wastewater is investigated by Laurent Charlet team, focusing on two major worldwide environmental issues : (i) the immobilization by clay and biochars of antibiotics and other organics common organic micropollutants (OMP) of surface water not addressed by WWTP and (ii) the decontamination of phosphogypsum stack effluents.
The immobilization of target emerging organic pollutants (antibiotics, anti epileptic, analgesic and anti corrosion OMP) by biochar for waste water treatment plant or groundwater recharge pretreatment was shown to combine mechanisms depending on biochar physico-chemical properties such as specific surface area dependent sorption, hydrophobic interaction with graphene like structures, cation exchange capacity and ternary surface complexation by Ca2+ bridging. Binding of antibiotics to clay minerals can also decrease both their physical and biological availability in soils. Binding mechanisms of oxytetracycline antibiotics (OTC) on smectite clays was shown by X-ray diffraction (XRD), infrared (IR), and solid-state nuclear magnetic resonance (NMR) spectroscopies, and Monte Carlo molecular simulations include interlayer OTC cation exchange involving protonated amine group and cation bridging ternary complex, via the deprotonated carboxyl group.
The phosphate fertilizer industry produces huge amounts of a waste by-product, known as phosphogypsum (PG) stacks, residues of the wet chemical digestion of apatite phosphate ore by sulfuric acid. PG stacks are often located in coastal areas, under tide influence, leading to diurnal water saturation and redox oscillations leading to possible natural attenuation of the contaminants via sulfide precipitation, investigated by Laurent Charlet team. The PG leaching waters are highly acidic, mostly due to residual phosphoric acid, and toxic, due to presence in PG of U, Cd, As and Sb that strongly limit the potential for PG recycling, e.g., as agricultural additives or building materials. Laurent Charlet team is investigating the treatment of these effluent water in two steps : (i) phosphate precipitation and pH increase by reaction with zero valent iron (ZVI) or with lime, and (ii) removal of the most problematic redox active contaminants (U, As, Sb) via magnetite surface reductive immobilization. In a circular economy fashion, we use steel industry waste derived magnetite produced by Hymag’In startup company.

• Paleoenvironments and Archeology

Particle are active records of environmental changes, and preservation agent of archeological artefacts. Metal ions stored at their surface (and stored in lake sediment or peat bog) record the air contamination, e.g., by mercury or copper emitting early metallurgy, and Laurent Charlet team to quantify and date, with Dr. S. Guédron early bronze age and medieval metallurgy in the Alps and later the rise (and impact) of cement industry. Particle surface reactions contribute also to the preservation of archeological artefacts. For instance, bones and teeth are preserved by underground substitution of Ca ions by Fe, Mn or Zn ions (and when heated such teeth were shown with I. Reiche (C2RMF) to produced Mn(V)-rich fake turquoise used by middle age monks) with diffusion demonstrated by PIXE U-shape profiles, while prehistoric cave painting are preserved by calcite precipitation which, depending on crystal size (and thus on paleoenvironment) act as a varnish - or conversely - as an obscuring layer of these archeological artefacts. Calcite in various records (coral, shell, speleothelm) 13C and 18O is often used to reconstruct paleoclimate, but Laurent Charlet team has shown that the signal could be somehow blurred by diffusion of 13CO3 within calcite, when the latter is at equilibrium with the atmosphere. So paleoenvironmental and archeological artefacts are transformed, after their burial, through reactions occurring at the particle/water interface.