Υποστήριξη Διδακτορικής Διατριβής - Zhongda Liu

The global water crisis is one of the most pressing challenges facing humanity today. Uncontrolled water pollution has intensified the water scarcity caused by rapid population growth, urbanization, and industrialization. According to the United Nations, 42% of global household wastewater (113 × 10⁹ m³) was discharged without safe treatment in 2022. In the same year, more than 2.2 billion people still lacked access to safely managed drinking water. Industrial and household pollutants such as azo dyes, endocrine-disrupting chemicals, and pharmaceuticals have long-term environmental persistence and can cause chronic toxic effects on aquatic ecosystems. Through bioaccumulation in the food chain, such pollutants can ultimately pose risks to human health, causing problems like hormonal disruption, neurological disorders, and other chronic diseases. Considering the significant gap between current global wastewater treatment capabilities and demand, there is an urgent need to develop innovative, efficient, and sustainable wastewater treatment technologies.

Persulfate-based advanced oxidation processes (AOPs) are promising approaches to deal with such problems. Two types of persulfates, i.e., peroxydisulfate (PDS, S₂O₈²⁻) and peroxydisulfate (PMS, HSO₅⁻), can be activated by certain methods to generate various reactive species for the degradation of organic pollutants, involving hydroxyl radicals (^•OH), sulfate radicals (SO₄^•−), superoxide radicals (O₂^•−), singlet oxygen (¹O₂), high-valent metal (HVM)-oxo species, and mediate direct electron transfer (DET) pathways. Heterogeneous activation, electrochemical activation, and hybrid activation are of particular interest due to their multiple advantages. Delicate design strategies enable the simultaneous attainment of high activity and stability in heterogeneous catalysts. Besides, electrochemical processes exhibit high sustainability due to high treatment efficiencies and less reliance on chemicals. Hybrid strategies bring together the merits of faster degradation kinetics, deeper mineralization rate, improved catalyst lifetime, reduced chemical dosage, and reduced electricity energy consumption. 

This project aims to develop innovative and sustainable water purification technologies by employing persulfate activation via heterogeneous, electrochemical, and hybrid strategies. In the first process, a heterogeneous system was developed for PMS activation and acid orange 7 (AO7) degradation, in which the carbon-iron-cobalt co-doped graphitic carbon nitride (g-C₃N₄/C-FeCo) was prepared with a co-doping strategy. The co-doping boosted the PMS activation due to improved charge transfer mobility, and non-radical pathways dominated the AO7 degradation. Remarkably, the required catalyst dosages (5–50 mg/L) were one order of magnitude lower than commonly used levels, but ultrahigh degradation rate constants (up to 2.985 min⁻¹) were achieved with complete removal (>99.9%) finished within one minute. A stabilized performance was also demonstrated during a 36-h test in a flow reactor. Mechanistic study revealed that the PMS activation proceeded via the non-radical pathway, giving rise to O₂^•−, ¹O₂, high-valent metal-oxo species, and mediated electron transfer mechanism.

In the second process, another heterogeneous catalyst was developed for PMS activation and bisphenol S (BPS) degradation, in which the carbon-encapsulated cobalt ferrite (CoFe₂O₄/CoFe@C) was designed with a unique core-shell structure. The structure and activity were highly dependent on the preparation temperature. The hydrothermal process resulted in a core shell product with metal carbonate cores and amorphous carbon shells. A sufficiently high temperature (>300°C) is crucial for the decomposition of the metal carbonate cores to form the metal oxide cores. With increasing temperature up to 900°C, the amorphous carbon shell gradually graphitized and reduced the metal oxide to zero-valent CoFe. The optimal temperature of 500°C balanced the formation of CoFe and over-graphitization of the carbon shell and thus achieved the optimal activation performance. The BPS removal reached 97.0% in 15 min with a rate constant of 0.199 min⁻¹ and low metal leaching (0.66 mg/L of Co and 0.04 mg/L of Fe). Both radical and non-radical pathways contributed to the activation, with SO₄^•–, ¹O₂, and high-valent metal-oxo species as the main contributing reactive species.

 The third process is an electrochemical system for AO7 degradation assisted by both PMS and PDS. Reticulated vitreous carbon (RVC) cathode demonstrated excellent performance in generating H₂O₂ and activating persulfate, especially PMS. Abundant H₂O₂ (up to 117 mg/L) was produced from oxygen reduction with current efficiencies between 78–24% in 1 hour. Moreover, the addition of 0.5 mM of PMS boosted the degradation rate by 9.6 times reaching 0.4470 min⁻¹ and the energy consumption was reduced by 90.1% to 0.0243 kWh/m³/order. Also, the RVC cathode was very robust with little deterioration observed in 10 cyclic tests. The H-cell test, scavenging experiments and electrochemical tests demonstrated that more than 90% of degradation took place on the cathode, with the occurrence of O₂^•− and direct electron transfer (DET) pathways.

In the fourth process, a hybrid system was developed for PDS activation and ciprofloxacin (CIP) degradation by combining heterogeneous and electrochemical activation. Considering the low activity of CoFe₂O₄/CoFe@C catalyst for PDS activation, the catalyst was applied as the particle electrode assisted by the RVC cathode. PDS enhanced the generation of H₂O₂ from the RVC cathode, and both PDS and H₂O₂ were efficiently activated by the particle electrode in the electric field. As a result, a remarkable synergy was observed in the hybrid process, removing 95.5% of CIP in 10 min with a rate constant of 0.309 min⁻¹. The process showed a high sustainability with reduced electric energy consumption (0.31 kWh/m³/order) by making full use of cathodic electrons. Scavenging, probing, and electrochemical tests revealed that ^•OH, O₂^•−, and ¹O₂ were the major contributing reactive species.

For all the four processes, the validity of the catalyst or process design was first demonstrated, followed by the systematic investigation of key affecting process parameters, environmental applicability, role of reactive species, degradation pathways, and in silico toxicity assessment of the identified degradation products. A comprehensive conclusion of obtained results, limitations, and prospects of the developed processes was provided. In summary, the innovated heterogeneous, electrochemical, and hybrid strategies for persulfate activation in this project pave the way for the development of greener sustainable AOP technologies for water purification.