Mechanisms of Reactive Species Generation in Persulfate-based Advanced Oxidation Process in Response to Environmental Conditions and Pollutant Degradation Pathway Regulation

過硫酸鹽高級氧化體系中活性物質生成的環境響應機制及污染物降解途徑調控方法

Student thesis: Doctoral Thesis

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Author(s)

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Detail(s)

Awarding Institution
Supervisors/Advisors
  • T C LAU (Supervisor)
  • Jianxiong ZENG (External person) (External Supervisor)
  • Wenwei LI (External person) (External Supervisor)
Award date3 Jul 2020

Abstract

Persulfate-based advanced oxidation processes (AOPs), which produce various oxidative species from activating of persulfate (i.e, peroxymonosulfate (PMS) or peroxydisulfate (PDS)), are appealing for environmental pollution control due to high efficiency for pollutant degradation. The resulting reactive species and the performances of such processes are strongly dependent on the environmental conditions (e.g., pH and the presence of ions and dissolved organic matters) and the activity, stability and recyclability of catalysts, which remain critical issues to be clarified and addressed. In particular, compared with the AOPs that rely on the generation of sulfate radicals (SO4•−) for pollutant degradation, non-radical degradation pathways are more preferable for practical application due to high selectivity towards target pollutants and high process robustness in complicated water environment. However, information regarding the detailed degradation pathways in environmental matrix is limited, and effective strategies to enhancing such non-radical pathways is still lacking. In this thesis, we provide insights into the impacts of pH and chorine ions on the pollutant degradation pathways in persulfate-based advanced oxidation process and developed a highly selective and efficient molecular catalyst to enhance the non-radical degradation pathway of organic pollutants in persulfate-based AOPs. The major research contents and results are listed below.

(1) To overcome the drawbacks of metal leaching and restricted lower-valent metal regeneration in conventional heterogeneous catalyst for PMS activation, we developed a novel CoMn2O4 catalyst by using metal organic framework compounds (MOFs) as the precursor. The catalyst exhibited significantly improved catalytic activity and stability due to the Co-Mn synergy (i.e. Co2+ plays a key role in the PMS activation while Mn3+ is mainly responsible for Co2+ regeneration). However, the predominance of SO4•− pathway still restricts the environmental application of this process due to various competitive reactions in complicated environment. Some of these reactions lead to formation of hazardous disinfection by-products (DBPs)). To improve the catalytic activity and selectivity of metal oxides for oxidative degradation of pollutants and avoid formation of DBPs, the interactions of PMS, pollutants and environment factors should be elucidated.

(2) We next investigated the PMS activation and degradation pathways of phenol in response to chlorine ions and pH. The formation of toxic DBPs mainly occurred at acidic pH. Different reactive species were generated in the PMS-Cl-phenol system in response to pH. At acidic pH reactive chlorine species are abundantly formed and can efficiently attacked phenol to form chlorophenols as the main products, while at alkaline pH singlet oxygen (1O2) was generated as the primary active species.

(3) We further explored the possibility ways to control the formation DBPs in PMS/Cl-system by enhancing their degradation via optimizing the environmental conditions. Our results show that chlorophenols are resistant to PMS under acidic pH, but could be efficiently degraded under basic condition due to a direct interaction between the pollutant and PMS. The 1O2 was generated from the interaction between PMS and benzoquinone (the pollutants degradation intermediate), and contributed to the non-radical degradation of chlorophenols.

(4) Lastly, we developed an efficient molecular catalyst (Ru(bpy)32+) to strengthen the non-radical pathway of pollutant degradation in persulfate-based AOPs. Ru(bpy)32+ was found to serve as a self-regenerative catalyst to efficiently activate PDS under visible light for pollutants degradation, and its regeneration was ascribed to the one electron transfer from Ru(bpy)3+ to the pollutant. Due to the efficient cycling of Ru(bpy)33+/Ru(bpy)32+, the system exhibited remarkable catalytic activity and stability for pollutant degradation.

    Research areas

  • persulfate, sulfate radical, non-radical, reactive chlorine species, singlet oxygen, electron transfer, molecular catalyst