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Iron oxyhydroxide, especially in its so-called "amorphous" form, plays a key role in the retention and migration of organic and inorganic compounds in soils and aquatic media. The local structure of these "amorphous" species can be directly investigated using synchrotron-based X-ray absorption spectroscopy. In order to study the nucleation mechanisms, FeC13 solutions were hydrolyzed by NaOH and the precursors obtained at different molar ratios (0 I r = (NaOH)/Fe I 2.7) were studied by EXAFS. For r 1 1.5, Fe polymers formed at equilibrium are hexacoordinated and their local structure is the same as 8-FeOOH. For r = 1.5, the spectra obtained at different aging times show that the starting nuclei are dimers with edge-sharing octahedra. From t -50 min, trimers with edge and corner-sharing octahedra are detected in solution. After 1 h, 8-Fe00H-like polycations, formed by the coalescence of the trimers, can be observed. These polymers are extremely stable because C1-ions are still incorporated in the structure and are easily displaced by OH-.

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A porous system may be characterized by using two statistical distributions of chord lengths: (l) (particle chords) and f(m) (pore chords). Calculations are presented giving a general relationship between the shape of small angle scattering and the distribution of segment lengths limited by particle and pore boundaries. This development represents a generalization of Porod's method. By means of an approximation, this general expression is simplified and can be applied in many cases. The properties of distributions (l) [or f(m)] are analysed and it is shown that the condition (0) = 0 (or f(0) = 0] means that particles (or pores) do not possess any sharp edges. The presence or absence of sharp edges allows the separation of small angle scattering curves into two characteristic forms. The functions (l) and f(m) corresponding to several simple geometrical forms are analysed.

The process of hydrolysis-precipitation of aluminum from aqueous aluminum chloride solution at 25 °C and for a concentration of 10"1 M has been studied by using solutions with a neutralization ratio r = (NaOH) / (A1T) equal to 2 and 2.5, by small-angle X-ray scattering using a synchrotron source. In the former case, the aluminum ion is embodied principally in a polymer with the formula AluO^OH)^3* with an experimental radius of gyration of 9.8 Á, which corresponds to an ionic radius of 12.6 Á. In the second case, the aluminum is embodied partly in the species described above and partly in a colloidal species of chemical composition similar to that of the trihydroxide. The particle morphology of the colloidal species changes as a function of time. After aging for 1.5 h, the particles are cylindrical with a radius of about 15 Á and a length of 310 Á. After 24-h aging, the cylinders have agglomerated into more homogeneous platelets of diameter 500 Á and thickness 60 Á.

The structures of fresh sols of partially hydrolyzed ferric nitrate salt were investigated by dynamic light scattering and small-angle X-ray scattering. The size of the colloids formed vs the hydrolysis ratio n = (NaOH)/(Fe) varied with the age of the solutions. Just after the end of the mixing of NaOH with Fe-(N0&9HzO ( t < 10 min) the size distribution is bimodal in n = 2, n = 2.2, and n = 2.5 with size particles in the range 40-500 nm. At t > 10 min, the size distribution is monomodal without further evolution with time. The sizes are close to 10-20 nm. Small-angle X-ray scattering curves yield two results: (1) the colloids are linear at the semilocal range order when n = 1.5 or 2.0, but with n = 2.2 or 2.5, they are semilinear or with a structure of fractal aggregates formed by small subunits; (2) the size of the subunits varies from 7 8, ( n = 1.5) up to 13.5 8, (n = 2.5). This size variation with n is contrary to observations when using ferric chloride salts. NOS-ions are rapidly exchanged for OH-or 0-ligands in a homogeneous nucleation process.

Speciation resulting from fast hydrolysis of dilute ferric chloride salt (0.1 M) by NaOH at 25 °C was studied versus the hydrolysis ratio n = NaOH/Fe from 0 to 3.0 at different aging time from t = 400 s up to 1 day by using quasi-elastic light scattering and small-angle X-ray scattering. Before the flocculation threshold ( ~2.7), large colloids are formed during the hydrolysis. Photon correlation spectroscopy showed that at t = 400 s the size range varied from ~10 nm ( = 1.0) up to 700 nm (n = 2.7). At larger times the colloid size rapidly decreased during the first 30 min. The mean sizes were centered around ~10 nm. Small-angle X-ray scattering experiments carried out with = 1.0 ton = 3.0 permitted description of the semilocal structure of the colloids. The scattering curves are characteristic of colloids formed by aggregates of subunits. Whatever the value of n and the time (t < 1 h), the subunit size is equal to 16 Á in diameter. It is not modified by pH variation for t < 1 h). The semilocal structure of the colloids depends on the time and n. For = 1.0 and t = 400 s, locally the colloids are formed of linear aggregates. This local shape is originated from long-range magnetic dipolar interactions, which are known to linearize the fractal aggregates. The same sols aged for 1 h exhibited slightly more branched aggregates.

RESUME: Nous proposons l'&ude du m~canisme de gonflement d'une smectite, bas6e sur l'exploitation des diagrammes de diffusion aux petits angles obtenus avec la source de rayons X synchrotron de Lure (Orsay). Cette technique a permis d'&udier le gonflement, en fonction de la temperature, d'une montmorillonite Na, cn suspension dans l'eau et concentr& ~ 20~o.L'analyse des diagrammes est ~ffectu~e par une m&hode bas6e sur la comparaison des courbes exp~rimentales fi des courbes th6oriques. Les mod61es de structure permettant le calcul des diagrammes th6oriques fait appel fi la notion de particule (d6finie comme un empilement de feuillets parall~les) et de structure interne de la particule (d6finie par un ensemble de translations di affect~es des probabilit& pi qui d&erminent la position mutuelle des feuillets).Les r~sultats montrent qu'fi basse temp6rature, les particules de tr& grande taille sont constitutes par la superposition de sous empilements, comportant 4 ft. 5 feuillets, hydrates d'une mani~re homog~ne et se d~sorientant mutuellement pour former la structure du gel. On explique ainsi que la structure du gel form6 est correl& avec celle du solide initial et que le ph~nom~ne de gonflement est quasi r~versible.