MECHANISMS OF OXIDATION OF NI(II)-FE(II) HYDROXIDES IN CHLORIDE-CONTAINING AQUEOUS MEDIA: ROLE OF THE PYROAURITE-TYPE NI-FE HYDROXYCHLORIDES

Abstract

Ni-Fe pyroaurite-type hydroxychlorides were prepared by aerial oxidation of Ni(II)-Fe(II) hydroxides precipitated in aqueous solution with various P = Fe/Ni ratios. When P≥ 1/3, Ni(II)-Fe(II)-Fe(III) hydroxychlorides characterized by a specific Fe(IIl)/[Fe(II)+Ni(II)] ratio of 1/3, corresponding to the idealized formula of NiII 3-x FeII x FeIII(OH)8Cl.nH2O (with 0 ≤ x ≤ 3), were obtained at the end of the first stage of oxidation, In a second reaction stage, these hydroxychlorides oxidize with deprotonation of hydroxyl ions into O2- ions, i.e. the remaining FeII(OH)2 groups are transformed into FeIIIOOH groups. Along with the Ni(II)-Fe(III) hydroxychloride which contains a part of the FeIIIOOH groups a second phase is obtained. It is an amorphous Fe(III) or Ni(II)-Fe(III) oxyhydroxide when 1/3<P≤3/2, and a ferric oxyhydroxide identified as γ-FeOOH (lepidocrocite) when P >3/2. On the other side of the domain, when P<1/3, the Fe(III)/[Fe(II)+Ni(II)] ratio cannot reach the specific value of 1/3; this gives rise to a pyroaurite-type Ni(II)-Fe(III) hydroxychloride with a lower chloride content, that is with an average composition of NiII 3+y FeIII 1-y (OH)8Cl1-y .nH2O where y = {[4/(1+P)] - 3}, down to minimum Fe(III) and Cl contents corresponding to y = 1/3 (P = 1/5). The in situ mechanisms of oxidation of Ni(II)-Fe(II) hydroxides into Ni(H)-Fe(II)-Fe(III) hydroxychlorides are discussed.