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Impact des oxydes de fer naturels et des nanoparticules manufacturées sur la dynamique des éléments traces dans les sols de zones humides

Abstract : Nanoscience is based on changes in particle properties when their diameter is below 100 nm (ie nanoparticles, NPs). Considering the increasing use of such NPs and their discharge into the environment, the assessment of their risks to human health and the environment is a major issue. Underneath the protection of waters and soils, the surface water assessment quality is particularly important, especially in wetlands, where the toxic metals dynamics (e.g. As, Pb, Ni, Cr, Hg) is complex and depends on the redox conditions of the environment. As magnetite (nano-Fe3O4), a natural or manufactured NP, is known for its significant adsorption capacity with heavy metals, their interactions in riparian wetlands with trace metals (TMs) remain critical concerning their direct of indirect impact on trace metals (TMs) mobility. The objective of this thesis was to study the role of manufactured nano-Fe3O4 (~ 10nm) and natural iron oxides on the TMs dynamics in wetland surface waters and soils. Therefore, in a first part considering natural colloidal precipitates from reoxidation products from riparian areas (subject to redox oscillations), an analysis of the spatial distribution of elements was performed using nanoSIMS isotope mapping (i.e. 75As-, 56Fe16O-, sulfur (32S-) and organic matter (12C14N), while the sulfur speciation was evaluated X-ray adsorption at K edge of the sulfur (S) (XANES). These analyzes allowed to highlight the interactions between natural iron oxides, natural organic matter (NOM) and a toxic metalloid, As. Our results suggest, with a statistical colocalization of nanoSIMS images, the existence of two interaction types: (1) 12C14N-, 32S-, 56Fe16O- and 75As-, and (2) 12C14N-, 32S- and 75As-. The coexistence of the oxidized and reduced forms of S, confirmed by the XANES analyses might be attributed to the slow oxidation kinetic of MON. Thus, this first part shows that in addition to the known interactions between MON, iron oxides and As, a possible direct interaction between As and NOM through sulfur functional groups (e.g. thiols) is also possible in oxidized environment. In a second part, the effect of nano-Fe3O4 (~ 10 nm) on trace elements (TEs) and colloids mobility in the organomineral horizon of a natural wetland soil was assessed using soil columns. Our results show that the nanoparticles coating influences the mobility of NOM and TMs. Indeed, the TMs mobility increases in presence of naked nano-Fe3O4, suggesting associations where NOM stabilizes the nanoparticles and increases the nanoparticles and associated TMs mobility. This mechanism seems less possible with coated nano-Fe3O4 where MON blocks the coating adsorption sites and therefore the adsorption of metals. In the third part, a coupling between a new generation of size exclusion chromatography (SEC) column and the induced coupled plasma mass spectrometry (ICPMS) allowed quantitative measurements of the direct interactions between NPs and TMs in complex natural matrices (i.e. sewage sludge, groundwater, tap water) and different artificial samples containing different concentrations of humic acid and toxic metals (eg As, Pb, Th). The application of this method allowed to observe that the naked nano-Fe3O4 stability is more affected by the metal content (ie As and Pb) and pH as those coated. Adsorption isotherms could also be possible with this method. Finally, in a last part, a nano-Fe3O4 bioreduction monitoring by Shewanella oneidensis showed very different bioreduction speeds for two nanoparticle types, nano-Fe3O4 and hematite (nano- Fe2O3). The different speeds might be explained by the different crystallography, still emphasizing the importance of the surface configuration in regard to the reactivity and behavior of nanoparticles. Overall, this project has highlighted how is influenced the potential of nano-Fe3O4 to sequester toxic metals, not only by the soil matrix and NOM, but also by their own surface configuration. For example, the very fast nano-Fe3O4 uncoated dissolution (i.e. 8 days) by bacterial activity illustrates the importance of their surface configuration in the subsequent metal mobility. Indeed, such dissolution leads to significant release of Fe2+ able to complex and mobilize toxic metals. Nano-Fe3O4 could a coating slow this nanoparticle reduction and therefore differently affect the TMs transfer into the aquatic compartment
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Submitted on : Wednesday, December 16, 2015 - 8:39:49 AM
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Maya Al-Sid-Cheikh. Impact des oxydes de fer naturels et des nanoparticules manufacturées sur la dynamique des éléments traces dans les sols de zones humides. Géochimie. université de Rennes 1, 2015. Français. ⟨tel-01244610⟩

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