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<p>The photosensitized degradation of paracetamol (PCT) mediated by humic acid (HA) was investigated under controlled experimental conditions to elucidate the role of pH, excited states, and reactive oxygen species (ROS). Steady-state and time-resolved fluorescence, coupled with photolysis and UHPLC–MS/MS analyses, revealed that HA acts as an effective natural photosensitizer promoting PCT transformation through both Type I and Type II pathways. The efficiency and mechanism of degradation were strongly pH-dependent. At acidic pH, PCT degradation proceeded mainly via direct interaction with the excited states of HA (¹HA* and ³HA*), whereas at alkaline pH, singlet oxygen (<sup>1</sup>O<sub>2</sub>) dominated the process, leading to complete removal of PCT. Quenching experiments confirmed that hydroxyl and superoxide radicals played negligible roles except under highly basic conditions, where <sup>1</sup>O<sub>2</sub> reactivity with the phenolate form of PCT was markedly enhanced. UHPLC–MS/MS analysis identified distinct sets of photoproducts at different pH values, including hydroxylated derivatives, open-ring species, and phenolic dimers, supporting the proposed mechanistic pathways. These results provide mechanistic insight into the pH-dependent roles of excited states and reactive oxygen species in humic acid–sensitized photochemical transformation pathways, and may contribute to understanding the fate of paracetamol in sunlit aquatic systems.</p> Graphical abstract <p></p>

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Photochemical transformation of paracetamol sensitized by humic acid: pH-dependent pathways and mechanisms

  • 期刊论文

  • 发表日期：

    2026年3月20日

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  • Alberto Barrera,
  • José Natera,
  • Eduardo Gatica,
  • Eduardo De Gerónimo,
  • Walter A. Massad

摘要

The photosensitized degradation of paracetamol (PCT) mediated by humic acid (HA) was investigated under controlled experimental conditions to elucidate the role of pH, excited states, and reactive oxygen species (ROS). Steady-state and time-resolved fluorescence, coupled with photolysis and UHPLC–MS/MS analyses, revealed that HA acts as an effective natural photosensitizer promoting PCT transformation through both Type I and Type II pathways. The efficiency and mechanism of degradation were strongly pH-dependent. At acidic pH, PCT degradation proceeded mainly via direct interaction with the excited states of HA (¹HA* and ³HA*), whereas at alkaline pH, singlet oxygen (1O2) dominated the process, leading to complete removal of PCT. Quenching experiments confirmed that hydroxyl and superoxide radicals played negligible roles except under highly basic conditions, where 1O2 reactivity with the phenolate form of PCT was markedly enhanced. UHPLC–MS/MS analysis identified distinct sets of photoproducts at different pH values, including hydroxylated derivatives, open-ring species, and phenolic dimers, supporting the proposed mechanistic pathways. These results provide mechanistic insight into the pH-dependent roles of excited states and reactive oxygen species in humic acid–sensitized photochemical transformation pathways, and may contribute to understanding the fate of paracetamol in sunlit aquatic systems.

Graphical abstract