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 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 (Ni6Fe2III)-Fe-II (OH)(16)SO(3)center dot nH(2)O. 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 (Ni4Fe2III)-Fe-II(OH)(12)SO4-nH(2)O. The formation of the SO4-LDH is favoured by the oxidation of SO3(2-) into SO42- anions, whereas the formation of ferrihydrite is the process that takes place if the initial anions are stable, e.g. Cl-. (c) 2005 Elsevier Ltd. All rights reserved.