Vibrational characteristics of outer-sphere surface complexes: example of sulfate ions adsorbed onto metal (hydr)oxides

The vibrational characteristics of outer-sphere complexes of sulfate at several mineral oxide – water interfaces were investigated by in situ attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy. In the IR spectra obtained from surface outer-sphere complexes only one peak of the antisymmetric stretching vibrational mode υ₃ similar to the free sulfate ion SO₄ ²⁻ in aqueous solution is observed. However, on the investigated (hydr)oxide surfaces of Al³⁺, Ti⁴⁺, Fe^2+/3+, Cr³⁺, Ni²⁺, Ce⁴⁺, Cu²⁺, Y³⁺, Zn²⁺, and Nd³⁺ a shift of up to 14 cm⁻¹ was found, correlated to the polarizing power of the metal cations. A high polarizing power was found to result in a stronger shift of υ₃ compared to the aqueous SO₄ ²⁻ ion. Furthermore, the impact of the metal oxide structure on the characteristics of the formed outer-sphere complex was negligible, since different Al and Fe (hydr)oxides did not show any changes in the respective IR spectra. A variation of ionic strength (1 – 10-4 M) and pH (6.8 – 3.1) during the formation of SO₄ ²⁻ outer-sphere complexes on g-Al₂O₃ showed that those changes directly influence the surface potential, without modifying the geometry of the outer-sphere complex.