The transformation of ferrihydrite {(5Fe} {2O} {39H} {2O)} to hematite (??-Fe {2O} 3) under alkaline condition in the presence and absence of lead was for the first time investigated using in situ, time-resolved synchrotron-based energy dispersive X-ray diffraction combined with off-line chemical characterization and imaging. The results showed that the crystallization of hematite occurred via a two-stage process with goethite (??-{FeOOH)} as an intermediate phase. The presence of lead enhanced the formation of hematite and reduced the induction times ( 20'30\%) but had little effect on the mechanism of the transformation reactions. The reaction rates for the two systems (with and without lead) ranged from 12 to 259-10 .4 s '1 and 19 to 461-10 .5 s .1 for the first and second stage, respectively. The activation energies of nucleation of the two systems were 16(??3) and 9(??2) {kJ/mol}, while the activation energies for crystallization ranged from 41(??7) to 77(??14) {kJ/mol.} During the hematite crystallization, the majority of the lead in the system was rapidly and irreversibly incorporated into the final hematite, while only minor amounts of lead were released back into solution. ?? 2010 American Chemical Society. ; cited By (since 1996) 3