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In our previous study, a novel method combining underwater high-voltage plasma discharge with acoustic cavitation (ACAP) was developed and implemented using rhodamine B (RhB) as a model organic pollutant. Results revealed that injecting argon gas into the ACAP reactor positively influences RhB decomposition efficiency, but there is still further potential for improvement. The aim of this study was therefore to further improve the efficiency of the ACAP process through Fenton reactions. Two options for ferrous ion supply were considered: the addition of FeCl2 or the dissolution of iron from ACAP reactor steel parts into the RhB-containing solution. The results revealed that the degradation efficiency is increased by 20% due to the Fenton reactions when the concentration of ferrous ions reaches an optimal value. Lower pH was found to be desirable for the effect of Fenton reactions. Based on measurements using high performance liquid chromatography, a plausible mechanism of RhB degradation by the ACAP process assisted by Fenton reactions is additionally proposed and discussed. [ABSTRACT FROM AUTHOR]

• Mass transfer across a gas–liquid interface of acoustic cavitation is investigated. • Time-averaged Sherwood number is decreased with sound pressure amplitude. • Time-averaged Sherwood number can be negative value due to area and shell effect. • The instantaneous Sherwood number is varied in the range of O (101) ∼ O (102). In the present study, mass transfer across a gas–liquid interface of acoustic cavitation bubble is numerically investigated. The volume of fluid (VOF) method coupled with a continuous species transfer model is used to simulate the bubble dynamic behavior and mass transfer across the gas–liquid interface. Chemical species are transferred repeatedly between the liquid and gas phases in accordance with the bubble oscillation, and the chemical species are transferred into the bubble during an expansion phase, and to the outside of bubble during a compression phase. Time-averaged Sherwood number is decreased with an amplitude of sound pressure due to area effect and shell effect , and its value becomes negative at high pressure amplitude. Meanwhile, the instantaneous Sherwood number is largely varied in accordance with the bubble oscillation, and its magnitude becomes O (101) ∼ O (102). [ABSTRACT FROM AUTHOR]

• Investigation on gas injection during ACAP process is conducted. • Argon provides a better efficiency of Rhodamine B degradation compared to air. • Hydrogen and oxygen-containing radicals are generated in larger amount during argon injection. • Argon dissolves in water that lowers breakdown voltage and improves cavitation treatment. Underwater plasma has been long recognized as a "green" alternative to conventional chemicals in the wastewater treatment processes. However, practical application of underwater plasma is still challenging due to insufficient treatment performance. Recently, we proposed a novel process named ACAP utilizing acoustic cavitation in order to stabilize the plasma generation and to enlarge the plasma processing region. This work continues our investigation regarding the ACAP treatment process focusing on effects of gas injection. Experiments were performed using an ultrasonic installation equipped with a specially designed sonotrode (Diam. 48 mm) operated at a frequency of 20 kHz and acoustic power of 120 W. The results revealed that the degradation efficiency of Rhodamine B, which was used as a model wastewater pollutant, remains almost unchangeable in the case of air injection, but it is doubled when argon is injected into the ACAP reactor. It was found that the argon injection enhances the degradation efficiency significantly even without ultrasound irradiation. Results of additional measurements suggest that the effect of argon is attributed to its ability to yield high temperature during cavitation, comparatively good solubility in water and a better ability to reduce the breakdown voltage in water compared to the air case. [ABSTRACT FROM AUTHOR]