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 νs(SO3), νas(SO3), and ν(S–OH) Raman bands of HSO4−(aq) depend on the H+ activity. The asymmetric shape of the νs(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 νs(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 νs(SO2) and νas(SO2) at ∼1140 cm−1 and ∼1370 cm−1, to νs(SO4) and νas(SO4) at ∼970 cm−1 and ∼1220 cm−1, and to νs(S–(OH)2) and νas(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.