Oxidation of 4-hydroxybenzoic acid in strongly alkaline and high-salt solutions via ultrasonic-assisted ozone: Helping radioactive waste disposal and environmental safety

4-Hydroxybenzoic acid (4-HBA), as a gibbsite dissolution inhibitor and typical personal care products (PPCPs), seriously troubles radioactive waste disposal and environmental safety. This work studies the degradation mechanism of 4-HBA in ultrasonic-assisted ozonation in strongly alkaline and high-salt solutions, and comprehensively evaluates its environmental lifetime. Ultrasonic can significantly increase the effect of ozone by 1.52 times, making the degradation rate of 4-HBA reach 63.49 % within 60 min. The better electrochemical performance indicates that the redox reaction between US/O₃ system and 4-HBA is more prominent. Experimental analysis and density functional theory (DFT) calculations show that the effects of OH, ¹O₂, O₂⁻, and others on the degradation of 4-HBA are 56.6 %, 17.8 %, 22.1 %, and 3.5 % respectively, and HO₃ is the most important precursor for OH evolution. Mechanistic exploration and DFT calculations show that degradation behavior of 4-HBA in strongly alkaline and high-salt solutions, i.e., decarboxylation reaction, ring opening, hydroxylation, aldol condensation, and hydrogenation, is significantly different from acidic or neutral solutions. Based on the environmental lifetime assessment of the intermediate product, US/O₃ technology can help reduce the toxicity of 4-HBA. The US/O₃ process is used to remove 4-HBA from strongly alkaline and high-salt solutions, which has huge potential economic benefits in the fields of nuclear waste chemistry and environment.