From Fe(III) ions to ferrihydrite and then to hematite

The pathways from soluble Fe(III)-aquo ions to various solid polymeric Fe(I II) oxides of increasing thermodynamic stability involve intermediate produ cts which are still only partly known. This paper combines results from the literature with own new results from X-ray diffraction, Mossbauer spectros copy, and electron microscopy. It is maintained that no intermediate phases were positively identified between mono-, di-, and trimers and a range of solid polynuclear phases. This indicates fast polymerisation as the OH/Fe r atio of the system increases. The immediate solid polynuclear phases are po orly crystalline Fe(III)-oxyhydroxy salts and a range of oxyhydroxides, cal led ferrihydrites in mineralogy, with varying crystallinity and magnetic or dering behavior. The rate of hydrolysis as affected by pH, rate of OH addit ion and temperature is of paramount importance for the nature of these poly meric phases. In the presence of free or surface-bound water, the transform ation of ferrihydrite into the stable end product hematite (alpha-Fe2O3) pr oceeds by crystallization within the ferrihydrite aggregate and is enhanced as the pH approaches the zero point of charge as well as by Al in the syst em. This mechanism is different from that in nonaqueous systems, (C) 1999 A cademic Press.