Advanced Catalytic Processes for Wastewater Treatment

1. Introduction

The increasing demand for clean water, along with the persistent release of organic pollutants such as pharmaceuticals, pesticides, and dyes into aquatic environments, underscores the urgent need for efficient wastewater treatment technologies. Conventional processes often fail to eliminate recalcitrant contaminants completely, leading to the accumulation of harmful substances in natural water systems. Advanced oxidation processes—including persulfate activation, catalytic ozonation, and semiconductor photocatalysis—are crucial for pollutant degradation and mineralization under environmentally friendly conditions.

This Special Issue, “Advanced Catalytic Processes for Wastewater Treatment”, highlights the latest innovations in catalyst design, process integration, and mechanistic understanding, with an emphasis on sustainable solutions for water remediation. The five included contributions cover biomass-derived catalysts, electro-assisted ozonation, semiconductor heterojunctions, and nanostructure engineering, offering diverse yet complementary approaches to address pressing water pollution challenges.

2. Overview of Contributions

Ioannidi et al. developed biochars derived from pomegranate peel waste for catalytic persulfate activation to remove the pharmaceutical losartan. High-temperature biochar (850 °C) exhibited superior performance due to its increased surface area and mineral content. Via radical quenching experiments, sulfate radicals, hydroxyl radicals, and singlet oxygen were identified as active species. Importantly, in a continuous-flow reactor, the system achieved ~90% removal over 114 h, demonstrating both stability and scalability. This work illustrates how agro-waste valorization can yield efficient, sustainable catalysts for advanced oxidation processes (Contribution 1).

Zhou et al. reported a three-dimensional electrode-enhanced ozone catalytic oxidation system (3DE-GAC-O₃) for degrading thiamethoxam, a neonicotinoid pesticide of environmental concern. Using granular activated carbon as the particle electrode, the system significantly improved ozone utilization and hydroxyl radical formation. Under optimized conditions, chemical oxygen demand (COD) removal exceeded 93%, with the kinetics fitting a second-order model. This study underlines the potential of combining electrochemical enhancement with catalytic ozonation to remove pesticides from wastewater (Contribution 2).

Gomaa et al. synthesized ZnO–NiO nanofibers with varying NiO content to enhance the photocatalytic degradation of acetaminophen. The optimized sample (ZN1.5) achieved 92% degradation within 3 h under visible light in the presence of peroxymonosulfate. Its superior performance was attributed to ZnO-NiO heterojunctions, which promoted charge separation and the generation of reactive oxygen species (ROS). This work emphasizes the importance of heterojunction engineering and nanostructure optimization in efficiently removing pharmaceutical pollutants (Contribution 3).

Lou et al. fabricated ZnO nanorods selectively decorated with Bi₂O₃ quantum dots using different synthesis methods. Photochemical deposition proved most effective, yielding highly coupled heterojunctions that enhanced charge separation and prolonged carrier lifetime. The optimized Bi₂O₃/ZnO catalyst achieved 89.3% Rhodamine B degradation in 120 min, far outperforming pure ZnO or Bi₂O₃. The study revealed that the interfacial quality and synthesis method, rather than quantum dot size, were decisive in driving photocatalytic performance (Contribution 4).

Trabelsi et al. investigated the use of iron sludge-derived catalysts in photo-Fenton processes for laundry wastewater treatment. This approach proved effective in achieving efficient oxidation and pollutant removal, embodying the principles of industrial ecology by reusing iron-rich waste as a catalyst precursor, and process optimization ensured its successful performance while reducing chemical consumption. This contribution highlights circular economy strategies, exemplifying how waste materials can be valorized into effective catalysts for wastewater remediation (Contribution 5).

3. Future Perspectives

While significant progress in this field has been made, further efforts are required to bridge the gap between laboratory innovation and full-scale implementation. Future research should focus on improving long-term catalyst durability and regeneration under complex wastewater conditions at pilot and industrial scales. Moreover, establishing a comprehensive assessment of their by-products and toxicity is essential to ensure their safe application. The integration of catalytic processes with renewable energy sources, such as solar-driven systems, should be prioritized to enhance sustainability, and the development of multifunctional catalysts capable of addressing diverse contaminant mixtures could further advance practical applications.