Abstract:

Ferrihydrite occurs widely in nature, such as in water body, soils, sediments and organisms. It has large surface area and high surface activity, controlling and influencing the morphology, migration and transformation of some contaminations and nutrient elements in the environment. It is also the important precursor form of many crystalloid iron (hydr)oxides. However, environmental geochemical behaviors of ferrihydrite and its implications for environments are not fully understood, which mainly presents in following aspects: (1)Ferrihydrite is always confused with amorphous iron (hydr)oxides or hydrate iron oxides and it is difficult to obtain experimental ferrihydrite samples in the environment and make quantitative analyses for them, whose contents are always underestimated. (2)Ferrihydrite is poorly crystallized, which makes the determination of its structure more difficult. The distribution, composition, structure and nature are not yet very clear. The XRD diffraction lines of ferrihydrite are wide, few and weak in intensity. Based on the number of diffraction lines, ferrihydrite can be classified into 2LFh with poorer crystallization and 6LFh with better crystallization. At present, generally accepted chemical formula of ferrihydrite is Fe5HO8·4H2O. However, lots of experiments have shown that H2O is not necessarily in its structure and it is easy to be replaced by external adsorbed matters. Some people think that ferrihydrite has a dualistic structure, i.e.with both octahedral coordination of its central Fe and tetrahedral coordination of surface Fe, which needs to be proved. Recently, structure study with use of PDF (Pair Distribution Function) shows that ideal ferrihydrite structure contents 20% tetrahedral coordination Fe and 80% octahedral coordination Fe and the chemical formula is Fe10O14(OH)2. However, quite a few researchers are doubtful of this new structure model. In this paper, the distribution, composition, structure and environmental geochemical behaviors of ferrihydrite are reviewed. The prospective research areas on ferrihydrite are also proposed.

Key words:  ferrihydrite, distribution, composition, structure, environmental behaviors