Mechanisms of formation and transformation of Ni-Fe layered double hydroxides in SO32− and SO42− containing aqueous solutions

The oxidation of (Ni,Fe)(OH)2 suspensions in the presence of SO32− and SO42− anions can lead whether to the SO3-form or to the SO4-form of the Ni(II)-Fe(III) layered double hydroxides (LDHs). The hydroxysulphite, like the corresponding Fe(II)-Fe(III) compound, is characterised by a M(III)/M(II) ratio of 1/3 and forms from the initial hydroxide. If P=[Fe]/[Ni]=1/3, the end product is then Ni6IIFe2III(OH)16SO3·nH2O. If P>1/3, the SO3-LDH still contains Fe(II) and the oxidation process goes on. The SO3-LDH tends to transform into a mixture of ferrihydrite and SO4-LDH. The hydroxysulphate, like the corresponding Fe(II)-Fe(III) compound, is characterised by a M(III)/M(II) ratio of 1/2 and obeys to the composition Ni4IIFe2III(OH)12SO4·nH2O. The formation of the SO4-LDH is favoured by the oxidation of SO32− into SO42− anions, whereas the formation of ferrihydrite is the process that takes place if the initial anions are stable, e.g. Cl−.