Solubility and hydrolysis of Fe(II) under hyperalkaline conditions

The speciation and solubility of iron under reducing, hyperalkaline conditions was investigated through undersaturation solubility experiments in combination with solid phase characterization by XRD. The solubility of a well-defined Fe(OH)$_2$(cr) solid phase was studied at $T$ = (25 ± 2) °C in aqueous solutions containing 0.10 to 4.20 m KOH and 1.06 to 22.73 m NaOH. Experimental data were used to derive chemical, thermodynamic and (Specific Ion Interaction Theory (SIT), Pitzer) activity models that accurately describe iron solubility under these conditions. These results support Fe(OH)$_3$$^−$ as the main aqueous iron(II) species up to 1 m NaOH/KOH, while Fe(OH)$_4$$^{2−}$ dominates at higher hydroxide concentrations.