Hydrolysis of mixed Ni(2+)-Fe3+ and Mg(2+)-Fe3+ solutions and mechanism of formation of layered double hydroxides.
The hydrolytic behavior of mixed metallic solutions containing Ni(2+)-Fe(3+) and Mg(2+)-Fe(3+) 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 Fe(3+) 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 Ni(2+)-Fe(3+) solution, the precursor is an α-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 Mg(2+)-Fe(3+) solution, the precursor is structurally equivalent to a β-Mg(OH)2 phase, leading to a LDH with a higher x(layer) value of ~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 Ni(2+)-Fe(3+) solution.