Visual and Raman spectroscopic observations of hot compressed water oxidation of guaiacol in fused silica capillary reactors

Raman spectroscopic technique towards understanding the degradation of phenol by sodium persulfate in hot compressed water

Degradation of phenol by sodium persulfate (SPS) in hot compressed water (HCW) was investigated in a lab-built fused quartz tube reactor (FQTR) coupled with Raman spectroscopy system. The species of S₂O₈^2-, SO₄^2-, HSO₄^-, SO₃^2- and HSO₃^- in the reaction system were qualitatively and quantitatively analyzed by Raman spectroscopy. The hydrothermal stability of phenol and SPS at different temperature and the degradation of phenol by SPS were also studied. The results indicated that phenol was not stable in aqueous solution above 200 °C, and that only SO₄^2- was generated in the hydrolysis of SPS at temperatures below 50 °C, and SO₄^2- and HSO₄^- were generated at higher temperatures. The maximum conversion rate (90.93%) and mineralization efficiency (38.88%) of phenol by SPS was obtained at reaction temperature of 300 °C with 180 min reaction time. During the degradation of phenol by SPS, HSO₄^- was the main product and S∗ (not detected by Raman spectroscopy) exhibits a positive correlation with temperature. In addition, a degradation pathway of phenol by SPS was proposed. The degradation data for the kinetic analysis indicated that the reaction followed pseudo first-order kinetics, and the reaction rate constants (k_s) were given as k_{50 °C} = 0.0083 min^-1, k_100°C = 0.0197 min^-1, k_{200 °C} = 0.0498 min^-1, k_{300 °C} = 0.0619 min^-1 and k_400°C = 0.0505 min^-1 at 30 min reaction. Moreover, the activation energy (12.580 kJ mol^-1), the enthalpy change (9.064 kJ mol^-1) and the entropy change (-222.104 J mol^-1) of the reaction were also calculated.