Radical and nonradical oxidative degradation of acetaminophen in ultraviolet light activated persulfate systems: comparison of PDS and PMS

Due to differences in physical and chemical properties, peroxymonosulfate (PMS) and peroxydisulfate (PDS) show distinct reactivity under ultraviolet (UV) activation. Consequently, selecting the appropriate persulfate concentration is critical for optimising the degradation efficiency of the system. This study compared the degradation efficiency of acetaminophen (ACE) in UV light-activated PDS (UV/PDS) and PMS (UV/PMS) systems. Under optimal conditions ([ACE]₀ = 10 mg·L^-1, UV light power = 6 W, [pH]₀ = 7), the ACE removal rate reached 97.8% within 60 min when the dosage of PDS was 8 mM. 91.7% within 60 min with 10 mM PMS in the UV/PMS system, and only 82.4% when the PMS dosage was 8 mM. Quenching experiments and electron paramagnetic resonance (EPR) analysis revealed that the UV/PDS system exhibited higher characteristic peak signal intensities for sulfate radical (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>S</mml:mi><mml:msubsup><mml:mi>O</mml:mi><mml:mn>4</mml:mn><mml:mrow><mml:mo>⋅</mml:mo><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math>), superoxide radical (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>O</mml:mi><mml:mn>2</mml:mn><mml:mrow><mml:mo>⋅</mml:mo><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math>), singlet oxygen (¹O₂), and hydroxyl radical (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>OH</mml:mi><mml:mo>⋅</mml:mo></mml:math>) compared to the UV/PMS system. In the UV/PDS system, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>S</mml:mi><mml:msubsup><mml:mi>O</mml:mi><mml:mn>4</mml:mn><mml:mrow><mml:mo>⋅</mml:mo><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> and ¹O₂ were identified as the primary contributors to ACE removal, whereas <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>S</mml:mi><mml:msubsup><mml:mi>O</mml:mi><mml:mn>4</mml:mn><mml:mrow><mml:mo>⋅</mml:mo><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> was the predominant species in the UV/PMS system. Moreover, the intermediate products generated during the UV/PDS reaction were analyzed using liquid chromatography-mass spectrometry (LC-MS). This study demonstrated the feasibility of UV light-activated persulfate systems for ACE treatment, providing a systematic comparison of the performance and mechanisms of ACE degradation between the two systems. The findings offer valuable insights and practical guidance for selecting persulfates in UV light-activated research and applications.