Complexation and Precipitation of Arsenate and Iron Species in Sodium Perchlorate Solutions at 25∘C

The complexation of As(V) in aqueous solutions in the presence of iron(III) was investigated spectrophotometrically with both variable and constant ionic strengths. The determined thermodynamic and stoichiometric formation constants of the FeHAsO₄⁺ species are log₁₀^∘β = 9.21± 0.01 and log₁₀^Iβ (1.0mol⋅dm⁻³ NaClO₄) = 7.78 ± 0.01, respectively. The numerical treatment of the obtained spectral data was performed with the SPECA program. The analysis required the consideration of the hydrolysis of Fe(III) and the protonation of As(V) in the pH range studied. No significant hydrolysis was observed because of the low pH values (pH < 2.5) involved. The stabilities of the solid Fe(III) arsenates was established by solubility experiments. All of the solubility experiments were performed in aqueous NaClO₄ solutions at constant ionic strength (1.0mol⋅dm⁻³) and at 25^∘C. The experimental data were consistent with FeAsO₄⋅2H₂O being the solid phase (log₁₀ ^∘ K_so = −24.30± 0.08). The corresponding thermodynamic constants were computed by means of the Modified Bromley's Methodology (MBM) that describes the variation of the activity coefficients of all of the ions involved in the complexation and precipitation equilibria with the medium and ionic strength. Finally, the solid phase obtained in this work was also characterized by FT-IR and FT-Raman spectroscopies, and the hydration of the solid iron arsenate was confirmed by X-ray diffraction data.