Hydrolysis of mixed Ni2+-Fe3+ and Mg2+-Fe3+ solutions and mechanism of formation of layered double hydroxides

The hydrolytic behavior of mixed metallic solutions containing Ni2+-Fe3+ and Mg2+-Fe3+ has been studied with respect to the relative proportion of the divalent and trivalent cations in solution as well as the quantity of NaOH added. The combination of X-ray diffraction and vibrational spectroscopy provides a deep insight into both the nature of the phases and the structure of the formed LDH. The relative abundance of each phase is determined by using a mass balance diagram and is in good agreement with the solid characterization. We showed that the slow hydrolysis of mixed metallic solutions involved first the precipitation of Fe3+ to form an akaganeite phase, and then the formation of a precursor on the iron oxyhydroxide surface, which transforms into LDH by diffusion of Fe-III species from the akaganeite phase to the precursor. Interestingly, whatever the iron content in solution, the same fraction of Fe-III is incorporated into the LDH phase which is correlated to the nature of the formed precursor. For Ni2+-Fe3+ solution, the precursor is an alpha-Ni hydroxide, which formed a LDH phase with a very low iron content (x(layer) = 0.1), but a high charge density provided by structural hydroxyl default. This result unambiguously demonstrated that the LDH phase is formed from the precursor structure. For Mg2+-Fe3+ solution, the precursor is structurally equivalent to a beta-Mg(OH)(2) phase, leading to a LDH with a higher x(layer) value of similar to 0.2. In both cases, at the end of the titration experiments, a mixture of different phases was systematically observed. Hydrothermal treatment allows the recovery of a pure LDH phase exclusively for the Ni2+-Fe3+ solution.