The Effect of Nickel on the Conversion of Amorphous Iron(III) Hydroxide into more Crystalline Iron Oxides in Alkaline Media

The effect of nickel (Ni) on the transformation of amorphous iron(III) hydroxide into more crystalline iron oxides has been followed using X‐ray powder diffraction and chemical analysis. One aim of this study was to determine the fate of Ni that had been co‐precipitated with amorphous iron(III) hydroxide as the amorphous phase recrystallised. Ni/amorphous iron(III) hydroxide co‐precipitates transformed into Ni substituted α‐FeOOH or α‐Fe2O3, NiFe2O4 and α‐3Ni(OH)2.2H2O in alkaline media. The type of reaction products formed depended on the concentration of Ni in the system and on the pH. Ni retarded the crystallisation of amorphous iron(III) hydroxide by stabilising the co‐precipitate both against dissolution leading to α‐FeOOH and against the internal rearrangement process which leads to α‐Fe2O3. Chemical analysis showed that Ni was incorporated into the structure of α‐FeOOH to a maximum level of 5.5 mol% and into the structure of α‐Fe2O3 to up to 7 mol%. Excess Ni was adsorbed on the surface of the iron oxide or taken up by NiFe2O4 or the pure Ni phase. The b0 dimension of the unit cell increased from 0.9960 nm for α‐FeOOH to 0.9965 nm for α‐FeOOH with 5.5 mol% Ni substitution. The same maximum level of Ni could be incorporated in the α‐FeOOH structure in the presence of either Mn (up to 8 mol%) or Co (up to 7 mol%).