Hydrolysis of iron (II) chloride in anoxic conditions and influence of soil ligands

The oxidation-reduction steps of iron are of a particular importance in the global geochemical cycling and industrial applications. Indeed, 17x 1020 moles of Fe are stored in the sedimentary rocks and 3.5x1012 moles/year are transformed from oxidized to reduced reservoirs and vice versa. Most of the studies dealing with the Fe(II)-Fe(III) system concern the composition and the structure of final products from the oxidation of a Fe(II) solution. In acidic conditions (pH 8), the end product is magnetite. 2 For slightly acidic to slightly alkaline conditions, intermediate Fe(II)-Fe(III) hydroxysalts phases are formed.2 The effects of a number of ions such as carbonate,3 aluminium4 and silicate5'6 have been extensively studied during the oxidation of FeC12 solutions, Fe(OH)2 or green rusts. The presence of chloride in the initial solution promotes lepidocrocite,7 whereas the presence of Si hinders the formation of lepidocrocite and ferrihydrite forms instead.5'6. Surprisingly little is known about the nucleation and growth mechanisms of Fe(II) polymers under anoxic conditions during the first steps of hydrolysis which precede crystallization. This study describes the evolution of the nano-scale aqueous ferrous species as a function of the hydrolysis ratio and the influence of SiO4 ligands. Samples were synthesized under anoxic conditions and studied by EXAFS at the Fe K-edge. The formation of the first nuclei occurs at R=0.1 (with R=[OH-]added/[Fe(II)]initial) and corresponds to the formation of small clusters (NFe-Fe 2, the local structure of samples is identical to the ferrous iron hydroxide. The presence of Si ligands hinders strongly the Fe(II) condensation at low hydrolysis ratios (RK-edge EXAFS, this study clearly indicates that the polymerization of Fe(II) is more affected by the presence of SiO4 than in the case of Fe(III). (Texte integral)