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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.