Minerals play a key role in the biogeochemical cycling of the elements at the Earth's surface, sequestering and releasing them as they undergo precipitation, crystal growth, and dissolution in response to chemical and biological processes. Manceau's research in this field focuses on the structure of disordered minerals (clays, iron (Fe) and manganese (Mn) oxides, including ferrihydrite and birnessite), on chemical reactions at their surface in contact with aqueous solutions, and on the crystal chemistry of trace metals in these phases.
Electron image of δ-MnO2 nanoparticles viewed parallel (a) and perpendicular (b) to the layer plane.[8]Structure of a cylindrically bent layer of δ-MnO2 nanosheet.[8]
In 1997, he and Victor Drits led the synthesis and resolution of the structure of hexagonal and monoclinicbirnessite, and they showed in 2002 that the monoclinic form possesses a triclinic distortion.[9][10][11] The hexagonal form prevails at the Earth's surface and owes its strong chemical reactivity to the existence of heterovalent Mn4+-Mn3+-Mn2+ substitutions and Mn4+ vacancies in the MnO2 layer. The Mn4+-Mn3+ and Mn3+-Mn2+ redox couples confer to this material oxidation-reduction properties used in catalysis, electrochemistry, and in the electron transfer during the photo-dissociation of water by photosystem II,[12] while the vacancies are privileged sites for the adsorption of cations. He has characterized and modeled a number of chemical reactions occurring at the birnessite-water interface, including those of complexation of transition metals (Ni, Cu, Zn, Pb, Cd...), and oxidation of As3+ to As5+, Co2+ to Co3+,[13][14] and Tl+ to Tl3+.[15] The oxidative uptake of cobalt on birnessite leads to its billion-fold enrichment in marine ferromanganese deposits compared to seawater.[16]
From 2002 to 2012, he applied the knowledge base acquired on the crystal chemistry of trace metals and biogeochemical processes at mineral surfaces and the root-soil interface (rhizosphere) to the phytoremediation of contaminated soils and sediments, and abandoned mine sites.[17][18][19] He contributed to improving the Jardins Filtrants® (Filtering Gardens) process for treating wastewater and solid matrices by phytolixiviation, phytoextraction, and rhizofiltration developed by the Phytorestore company.
Binding site of mercury (Hg) in selenoprotein P of grebe bird.[22][23]
Mercury (Hg) is a global pollutant that is generated both by natural sources, such as volcanic eruptions and wildfires, and human activities, such as coal combustion, gold mining, and the incineration of industrial waste. In aquatic and terrestrial food chains, mercury accumulates as methylmercury (MeHg), a potent toxin that affects the function of animal's and human's brain and reproductive system. Understanding the internal detoxification processes of MeHg in living organisms is essential for protecting wildlife and humans, and designing treatments against mercury poisoning.
The new Hg(Sec)4 species identified by Manceau and his collaborators was the main “missing intermediate” in the chemical reaction that helps animals to survive high levels of mercury. However, because Hg(Sec)4 has a molar ratio of selenium to mercury of 4:1, four selenium atoms are required to detoxify just one mercury atom. Thus, Hg(Sec)4 severely depletes the amount of bioavailable selenium. Selenium deficiency can affect the function of animals’ brain and reproductive system, as selenoproteins serve critical antioxidant functions in the brain and testes.[27] His works on the Hg-Se antagonism won him the ES&T 2021 Best Paper Award.
The stepwise MeHgCys → Hg(Sec)4 + HgSe demethylation reaction is accompanied by the fractionation of the 202Hg and 198Hg isotopes, denoted δ202Hg. The δ202Hg fractionation measured on whole animal tissues (δ202Hgt) is the sum of the fractionations of the MeHgCys, Hg(Sec)4, and HgSe species, weighted by their relative abundance:
δ202Hgt = f(Spi)t × δ202Spi
where δ202Spi is the fractionation of each chemical species, and f(Spi) their relative abundance, or mole fraction. Manceau and his co-authors found that δ202Spi can be obtained by mathematical inversion of macroscopic isotopic and microscopic spectroscopic data.[26][28]
The combination of isotopic and spectroscopic data on birds and cetacea revealed that dietary methylmercury and the Hg(Sec)4-SelP complex are distributed to all tissues (liver, kidney, sketetal muscle, brain) via the circulatory system with, however, a hierarchy in the tissular percentage of each species. Most of the detoxification process is carried out in the liver, whereas the brain, which is particularly sensitive to the neurotoxic effects of mercury, is distinguished from other tissues by a low mercury concentration and a high proportion of inert HgSe. These results appear to be transposable to humans.[29]
Publications
Manceau has published more than 200 scientific papers in Science Citation Index journals totalizing more than 24,000 citations and garnering an h-index over 90.[30] In 2020, he was ranked 111th out of a total of 70,197 researchers in Geochemistry/Geophysics in a bibliometric study by scientists of the Stanford University based on the Elsevier Scopus database.[31]
4_I-Tasser_model.jpg)
