Mechanisms of oxidation of Ni(II)-Fe(II) hydroxides in chloride-containing aqueous media: role of the pyroaurite-type Ni-Fe hydroxychlorides

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 greater than or equal to 1/3, Ni(II)-Fe(II)-Fe(III) hydroxychlorides characterized by a specific Fe(III)/[Fe(II)+Ni(II)] ratio of 1/3, corresponding to the idealized formula of (Ni3-xFexFeIII)-Fe-II-Fe-II(OH)(8)Cl.nH(2)O (with 0 less than or equal to x less than or equal to 3), were obtained at the end of the first stage of oxidation. In a second reaction stage, these hydrochlorides oxidize with deprotonation of hydroxyl ions into O2- ions, i.e. the remaining Fe-II(OH)(2) groups are transformed into (FeOOH)-O-III groups. Along with the Ni(II)-Fe(III) hydroxychloride which contains a part of the (FeOOH)-O-III groups a second phase is obtained. It is an amorphous Fe(III) or Ni(II)-Fe(III) oxyhydroxide when 1/3<P less than or equal to 3/2, and a ferric oxyhydroxide identified as gamma-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 (Ni3+yFe1-yIII)-Fe-II(OH)(8)Cl1-y. H2O 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(II)-Fe(II)-Fe(III) hydroxychlorides are discussed.