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

Abstract

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 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 Mg²⁺–Fe³⁺ 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 Ni²⁺–Fe³⁺ solution.