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The interactions between arsenic, iron and organic mater in anoxic environment

Abstract : Arsenic occurs naturally in groundwater used as drinking water. It is thus responsible of a great mortality in the world. Understand the As interactions with its environment and its transfer mode to the aquifers is therefore crucial. This work was focused on the direct and indirect binding mechanisms of As(III) by organic matter (OM) in anoxic environments, in particular via OM thiol groups and as ternary complexes involving ionic Fe.The first part of this work was dedicated to the complexation of As(III) by the OM thiol. Binding experiments of As(III) by a humic acid (HA) grafted or not by thiol were thus performed. Grafted or not OM were able to bind As(III) but bound As(III) concentrations were low and dependant on the thiol site density. Modeling with PHREEQC-Model VI modified to take into account thiol site demonstrated that As(III) was bound as monodentate complexes to OM thiol sites. Another indirect binding mechanism involving ternary complex via cationic bridge was however described to explain larger binding of As(III, V) to natural OM. Here under anoxic conditions, we speculated that this bridge was an ionic Fe(II) bridge. However, little information exists about the binding of Fe(II) by OM. Complexation experiments of Fe(II) by humic substances (HS) were thus conducted. The experimental results showed that Fe(II) was weakly complexed to HS at acidic pH, when the functional groups were protonated. By contrast, at basic pH, 100% of Fe(II) were complexed to HS. Modeling calculations demonstrated that Fe(II) formed mainly carboxylic bidentate at acidic pH and carboxy-phenolic and phenolic bidentate at basic pH. In the last part, the complexation of As(III) as As(III)-ionic Fe(II, III)-OM ternary complexes was tested. Experimental results showed that As(III)-Fe(II)-OM ternary complexes could form in anoxic environments. Modeling allowed to test several ternary complexes conformations. The most potential was the binding of As(III) as mononuclear bidentate complex onto a bidentate Fe(II)-AH complex. However, another definition of the model that should be constrained by XAS data is required. By contrast, at low concentrations of Fe (III), when the oxidizing and reduced species coexist, As(III) does not form As(III)-ionic Fe(III)-OM ternary complexes.Speciation of As and Fe is particularly important in the study of the As(III) transfer. When As(III) is bound to OM as ternary complexes, its transfer is entirely controlled by the own OM transfer mechanisms. Here, we calculated, however, that much of As(III) remains as labile species and can therefore reach underlying aquifers as long as anoxic conditions exist.
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Charlotte Catrouillet. The interactions between arsenic, iron and organic mater in anoxic environment. Earth Sciences. Université Rennes 1, 2015. English. ⟨NNT : 2015REN1S043⟩. ⟨tel-01236989⟩

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