Raman spectroscopic study of aqueous alkali sulfate solutions at high temperature and pressure to yield precipitation

The shift of the Raman band of ν1(SO42−) in aqueous alkali (Li, Na and K) and ammonium sulfate solutions was studied over a wide temperature range up to 350 °C under 25 MPa. The temperature shift for Li sulfate that was in the direction to low wavenumber was smaller and its dependence on the molality was larger than for the other two alkali sulfates. These trends were discussed based on the solution structure controlled by the cation. The discontinuous change of the spectral position from the solution phase to the solid phase at 390 °C was observed during the precipitation progress of Li and Na sulfates. Based on the present experimental results, K+ cations are suggested to be most adequate among the investigated alkali cations as the neutralizing reagent in supercritical water oxidation (SCWO) process.