The mechanisms of oxidation of ferrous hydroxychloride β-Fe2(OH)3Cl in aqueous solution: The formation of akaganeite vs goethite

β-Fe2(OH)3Cl is precipitated by mixing FeCl2 · 4H2O 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 × [FeCl2 · 4H2O]/[NaOH]with [NaOH]= 0.4 mol l−1) which varies from 5 to 9. Whatever the value of R′, the oxidation of β-Fe2(OH)3Cl first gives rise to the standard chloride-containing green rust one GR1(Cl−) with formula [FeII3FeIII(OH)8]+ [Cl · nH2O]−. Then, due to the large chloride concentration of the solution (from 2 mol l−1 to 3.6 mol l−1), the oxidation of GR1(Cl−), which usually leads to γ-FeOOH lepidocrocite, produces an over-chlorinated GR1 compound, GR1(C1−)∗, with the general composition of [FeII3 − xFeIII1 + x(OH)8](1 + x)+ [Cl1 + 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 13. Moreover, the oxidation process of these over-chlorinated GR1(C1−)∗ compounds changes with R′. It produces α-FeOOH goethite when R′ ⪯ 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′ ≥ 6.5 and alone when R′ ≥ 8.0, is to be correlated to the increase of the chloride-content of the GR1(C1−1)∗ compound. Finally, the measurements of the electrode potential and pH of the solution at the equilibrium conditions between GR1(C1−) and β-Fe2(OH)3Cl allow to estimate the standard chemical potential of the ferrous hydroxychloride at μ0[β-Fe2(OH)3Cl] = − 219 600 cal mol−1.