Investigation on the thermal activation of peroxydisulfate by using the hydrodynamic cavitation: A case study on tetracycline degradation

The synergetic technology of hydrodynamic cavitation (HC) and peroxydisulfate (PDS) has been adopted for the treatment of organic pollutants, while the rationale behind the thermal-activation of PDS in this process remains lacking. This paper presented investigation on the degradation of tetracycline under two types of operating conditions, including “internal reaction conditions” (pH value and TC/PDS molar ratio) and “external physical conditions” (hole shape, solution temperature and inlet pressure). Special emphasis was paid on the analysis of thermal effects through a robust modeling approach. The results showed that a synergy index of 6.26 and a degradation rate of 56.71 % could be obtained by the HC-PDS process, respectively, when the reaction conditions were optimized. Quenching experiment revealed that •OH and •SO₄^- were the predominant free radicals and their contribution to the degradation was 75.4 % and 24.6 % respectively, since a part of •SO₄^- was transformed into •OH in the solution. The thermal activation of PDS mainly occurred near the hole where the fitting temperature was around 340 K, while •OH was generated in the bubble collapse region downstream the hole, where the temperature was much higher and favorable for the cleavage of water molecular. The average temperature under different external physical conditions was in good consistence with the degradation rates. This research developed a useful method to effectively evaluate the activation extent of PDS by HC and could provide reliable guidance for further development of cavitational reactors to treat organic pollutants based on this hybrid approach.