Raman spectra of oxidized sulfur species in hydrothermal fluids

Raman spectroscopic determination of sulfur species molalities in hydrothermal fluids requires correct assignment and knowledge of the scattering efficiencies of Raman bands. Therefore, we studied the Raman spectra of NaHSO₄ and H₂SO₄ solutions experimentally to 700 °C, and of Na₂SO₄, NaHSO₄, H₂SO₄, and H₂SO₃ solutions by ab initio molecular dynamics simulation at 727 °C. The results indicate that the scattering efficiencies of the ν_s(SO₃), ν_as(SO₃), and ν(S–OH) Raman bands of HSO₄⁻(aq) depend on the H⁺ activity. The asymmetric shape of the ν_s(SO₃) Raman band of HSO₄⁻(aq) becomes more symmetric with increasing temperature, which correlates with decreasing hydrogen bonding in the molecular environment. Proton activity and ion pairing do not have a large effect on the change in the band asymmetry with temperature, and a resonance effect on the band shape is not observed. Therefore, we attribute the asymmetric shape of the ν_s(SO₃) Raman band of HSO₄⁻(aq) mostly to hydrogen bonding of the proton in the H–OSO₃⁻ molecule with water in its environment. The AIMD simulations clarify assignments of Raman bands of H₂SO₄⁰, specifically to ν_s(SO₂) and ν_as(SO₂) at ∼1140 cm⁻¹ and ∼1370 cm⁻¹, to ν_s(SO₄) and ν_as(SO₄) at ∼970 cm⁻¹ and ∼1220 cm⁻¹, and to ν_s(S–(OH)₂) and ν_as(S–(OH)₂) at ∼750 cm⁻¹ and ∼840 cm⁻¹. In addition, the experiments showed that diamond is not inert to H₂SO₄ at high temperatures as reduction of S(VI) to S(IV) produces SO₂⁰ and oxidation of diamond generates CO₂⁰.