The mechanisms of oxidation of ferrous hydroxychloride beta-Fe-2(OH)(3)Cl in aqueous solution: The formation of akaganeite vs goethite

beta-Fe-2(OH)(3)Cl is precipitated by mixing FeCl2 . 4H(2)O and NaOH aqueous solutions with a large excess of ferrous chloride. This excess is determined by the experimental ratio R' of the initial concentrations of reactants (R'=[Cl-]/[OH-] or 2 x[FeCl2 . 4H(2)O]/[NaOH] with [NaOH]=0.4 moll(-1)) which varies from 5 to 9. Whatever the value of R', the oxidation of beta-Fe-2(OH)(3)Cl first gives rise to the standard chloride-containing green rust one GR1(Cl-) with formula [(Fe3FeIII)-Fe-II(OH)(8)](+)[Cl . nH(2)O](-). Then, due to the large chloride concentration of the solution (from 2moll(-1) to 3.6moll(-1)), the oxidation of GR1(Cl-), which usually leads to gamma-FeOOH lepidocrocite, produces an over-chlorinated GR1 compound, GR1(Cl-)*, with the general composition of [(Fe3-xFe1-xIII)-Fe-II(OH)(8)]((1+x)+)[Cl-1+x.(n-y)H2O]((1+x)-). The excess x of intercalated Cl- ions increases with R', i.e. with the chloride concentration, up to a maximum around 1/3. Moreover, the oxidation process of these over-chlorinated GR1(Cl-)* compounds changes with R'. It produces alpha-FeOOH goethite when R'less than or equal to 6.0 but the formation of this compound is not connected to the modifications occurring in the precursor GR1 compound since it is induced by a dissolved Fe(II) species, the complex FeCl2aq. In contrast, the formation of akaganeite, which is obtained along with goethite when R'greater than or equal to 6.5 and alone when R'greater than or equal to 8.0, is to be correlated to the increase of the chloride-content of the GR1(Cl-)* compound. Finally, the measurements of the electrode potential and pH of the solution at the equilibrium conditions between GR1(Cl-) and beta-Fe-2(OH)(3)Cl allow to estimate the standard chemical potential of the ferrous hydroxychloride at mu(0)[beta-Fe-2(OH)(3)Cl] = - 219 600 cal mol(-1). (C) 1997 Elsevier Science Ltd.