Transformation of γ-FeOOH to α-FeOOH in acidic solutions containing metal ions

The transformation reaction of γ-FeOOH to α-FeOOH in FeSO4 solutions dissolving Ti(IV), Cr(III), Cu(II) and Ni(II) at pH < 3 was examined by FT-IR. The TEM observation demonstrated that the transformation proceeds by dissolution of γ-FeOOH particles and recrystallization of γ-FeOOH particles. The transformation occurred in FeSO4 solutions but not in both FeCl2 and Na2SO4 solutions, indicating that the transformation needs the coexistence of Fe(II) and SO42−. This can be explained by a strong adsorption of SO42− promoting the reductive dissolution of γ-FeOOH particles by Fe(II). All the metal ions added in FeSO4 solutions before the reaction markedly interfered with the transformation in the order of Ti(IV), Cu(II) > Cr(III) > Ni(II), though the difference was not pronounced. The incorporation of the metal ions into the formed α-FeOOH particles was in the order of Ti(IV) > Cr(III) > Cu(II) > Ni(II) well according to the order of their hydrolysis constants. Ti(IV) and Cr(III) with a high hydrolysis constant lowered the solution pH after the reaction from 2.7 to 1.7, that is unfavorable to α-FeOOH formation. Taking every factor into consideration, the marked inhibitory effect of the metal ions is ascribed mainly to interruption of the reductive dissolution of γ-FeOOH particles.