The hydrolysis of Fe−Si systems with Si/Fe ratios between 0 and 4 leads to the formation of poorly crystalline or, more frequently, of long-range disorganized precipitates. The increase of Si/Fe molar ratios results in an dramatic change of Fe polymerization. The formation of double and single corner-sharing Fe linkages is reduced compared to pure Fe hydrolysis products. The growth regime depends on the Si concentration in the system. Three-dimensional and two-dimensional growth of Fe colloids occurs at low and high Si/Fe ratios, respectively, systems with Si/Fe ratios around 1 representing a crossover between these two regimes. Though Si neighbors cannot be detected unequivocally by Fe K-edge EXAFS, their presence in the close environment of Fe atoms is evident from the change in Fe speciation.

The growth mechanisms of imogolite-like aluminogermanate nanotubes have been examined at various stages of their formation. The accurate determination of the nucleation stage was examined using a combination of local- (XAS at the Ge−K edge and 27Al NMR) and semilocal scale technique (in situ SAXS). For the first time, a model is proposed for the precursors of the nanotubular structure and consist in rooftile-shaped particles, up to 5 nm in size, with ca. 26% of Ge vacancies and varying curvatures. These precursors assemble to form short nanotubes/nanorings observed during the aging process. The final products are most probably obtained by an edge−edge assembly of these short nanotube segments.

A simple aqueous synthesis yielded about 100 times more structurally well-organized single-walled aluminogermanate nanotubes than previously reported "standard" procedures. The structure analyses using XRD, IRTF, TEM, and XAS were greatly facilitated by the high concentrations available, and they ascertained the imogolite-like structure of the nanotubes. Simplicity and yield of the synthesis protocol are likely to favor commercial applications of theses materials as well as simplified syntheses of other nanophases.

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This work aims at a better understanding of the interactions existing between Fe and Si in freshly precipitated Fe(III)/Si phases (prepared at pH 3, 5, 7, and 10 with Si/Fe molar ratios in the range 0.25-4). By coupling the results of two spectroscopic techniques, FTIR and 29 Si NMR, interesting structural information emerges. We show that Si and Fe atoms do not form separate silica and FeOOH particles and that the presence of Si-O-Fe bonds hinders the formation of Fe oxyhydroxides. The ratio Si/Fe ) 1 constitutes a transition point between Si-O-Fe and Si-O-Si bond formation at pH ) 3 and 5, where Si-clusters appear once the maximum amount of Si-O-Fe bonds are formed. This is confirmed by 29 Si NMR which demonstrates the presence of Si pockets in three of the eight examined samples. The return to equilibrium of the 29 Si magnetization leads to a value for both dimensionality of the silica-rich pockets (D ) 2.2) and length over which dimensionality is observed (2 nm). By using both FTIR and 29 Si solid-state NMR, we clearly demonstrate how the pH of synthesis determines the structural properties of the formed samples. The results obtained are in good agreement with our previous study conducted by EXAFS.

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Zinc (Zn) is a potentially toxic trace element that is present in large amounts in organic wastes (OWs) spread on agricultural lands as fertilizer. Zn speciation in OW is a crucial parameter to understand its fate in soil after spreading and to assess the risk associated with agricultural recycling of OW. Here, we investigated changes in Zn speciation from raw OWs up to digestates and/or composts for a large series of organic wastes sampled in full-scale plants. Using extended X-ray absorption fine structure (EXAFS), we show that nano-sized Zn sulfide (nano-ZnS) is a major Zn species in raw liquid OWs and a minor species in raw solid OWs. Whatever the characteristics of the raw OW, anaerobic digestion always favors the formation of nano-ZnS (>70% of zinc in digestates). However, after 1 to 3 months of composting of OWs, nano-ZnS becomes a minor species (<10% of zinc). In composts, Zn is mostly present as amorphous Zn phosphate and Zn sorbed to ferrihydrite. These results highlight (i) the influence of OW treatment on Zn speciation and (ii) the chemical instability of nano-ZnS formed in OW in anaerobic conditions.