Special Issue on “Catalysis in Advanced Oxidation Processes for Environmental Remediation”

Current industrial development and other anthropogenic activities are leading to a dramatic increase in the concentration of pollutants in the environment and a strong dependence on non-renewable energy sources [1,2,3]. As a result, environmental remediation, the transformation of renewable substrates and plastic waste into valuable chemicals and clean fuels, and global climate change are becoming increasingly important challenges. Photocatalytic processes, which belong to the advanced oxidation processes (AOPs), enable the environmentally friendly activation of photocatalysts, especially when carried out under sunlight or energy-efficient radiation sources. This field of research offers numerous opportunities for both science and industry, although some weaknesses of current systems still limit a wider application of photocatalytic technologies [4,5,6]. Due to the growing number of new pollutants in the environment, modern research is focused on overcoming existing challenges, such as photocatalyst and system design; improving the photocatalytic performance of existing and developing new photocatalysts for various purposes; optimizing process conditions and reactor design; improving irradiation distribution; analyzing the relationship between structure, mechanism, and performance; investigating new visible light-driven systems, etc. [7,8,9,10].

In this Special Issue, seven papers have been accepted and published to highlight the recent progress in the field and provide answers to many open questions that need to be addressed to meet the stringent pollutant emission standards cost-effectively and efficiently. The contributions are listed below:

Benjak et al. (Contribution 1) demonstrate the application of heterogeneous photocatalysis in air purification. They investigate the possibility of using rubber tiles obtained from secondary raw materials as a carrier for the immobilization of titanium dioxide and the preparation of a new photocatalyst for passive air protection. In this way, they are trying to solve several urgent problems at the same time, such as the need to recycle waste rubber, solve problems related to air pollution, and protect human health. In this study, the rubber tiles obtained from granules are successfully modified by a surface application of a thin film from commercial TiO₂ P25 by a modified sol–gel process. The results of this study suggest that rubber substrates with the addition of TiO₂ are an example of an integrated approach to environmental protection, as they offer a way to simultaneously solve the problem of recycling waste materials and produce new catalytic materials for the photocatalytic degradation of air pollutants.

Kurajica et al. (Contribution 2) focus their attention on the development of photocatalysts for the removal of synthetic dyes from textile wastewater. Their contribution aimed to explore the simple hydrothermal technique for the direct preparation of silver nanoparticles deposited on anatase and to investigate the physico-chemical and optical properties as well as the photocatalytic efficiency of the obtained photocatalyst based on the degradation of the C.I. Reactive Violet 2 (RV2) azo dye. The prepared material was characterized using different instrumental techniques. It was found that the prepared nano Ag/anatase photocatalysts exhibit excellent photocatalytic efficiency for the decolorization of C.I. Reactive Violet 2 aqueous solution under both UV-A and visible light illumination.

The study by Bogdan et al. (Contribution 3) deals with the environmentally friendly synthesis of TiO₂/ZIF-8 composites. The syntheses aimed to combine titanium dioxide (TiO₂) as a proven photocatalytically active component and ZIF-8 as a representative of zeolitic imidazolate frameworks, which have unique structural, textural, and physicochemical properties and are therefore promising materials for the preparation of the hybrid photocatalysts. The hybrid photocatalysts were prepared by an innovative method that combines the advantages of eco-friendly mechanochemical synthesis and hydrothermal synthesis. The prepared hybrid photocatalysts with different TiO₂ contents were used as potential photocatalysts for the low-cost and effective removal of the neonicotinoid insecticide imidacloprid from wastewater. Based on the results obtained, the authors concluded that hybrid TiO₂/ZIF-8 materials with favorable adsorption properties and reduced band gap energies compared to ZIF-8 can be successfully prepared using the method described in this study.

Focusing on the need for the development and application of sustainable energy sources, Bratovčić and Tomašić (Contribution 4) provide an overview of photocatalytic CO₂ conversion into high-value fuels and chemicals. Special emphasis is placed on the development of promising photocatalysts responsible for the selectivity and efficiency of CO₂ photoreduction to C₁ products, a basic understanding of the reaction mechanism, and an overview of different photoreactor configurations. The authors point out that the synthesis of new photocatalytic composites and the proper design of photoreactors, as well as the prediction of the reaction mechanism of the reaction, can improve photocatalytic performance regarding the mass transfer and absorption of light by the photocatalyst. In the final part of this review, the authors list key challenges and give recommendations for future research in the aforementioned area that will ensure high solar-to-full efficiency and ensure low operational costs.

Kovačić et al. (Contribution 5) focus on improving the photocatalytic performance of BiVO₄ for the degradation of ciprofloxacin (CIP) by Fe and Ag photomodification. In this article, the results of the modification of an iso-type homojunction BiVO₄ are published for the first time. Based on XPS and Mott–Schottky analyses, the authors conclude that Fe photomodification of the original BiVO₄ iso-type homojunction leads to the formation of oxygen vacancies that allow increased electron mobility and increased oxidizing power of the photogenerated holes, although neither a pronounced Fe-containing phase nor Fe doping was detected. On the other hand, Ag photomodification led to the formation of Ag₂O/AgO and Ag nanoparticles on the surface of BiVO₄, which generated hot electrons by surface plasmon resonance and increased the mobility of the photogenerated electrons. This study highlights the central role of photogenerated electrons for CIP degradation by BiVO₄-based materials and emphasizes the importance of appropriate band-edge engineering for the optimization of contaminant degradation.

Viskovic et al. (Contribution 6) investigate the possibility of removing selected pesticides, such as acetamiprid, clothianidin, and thiacloprid, under the influence of simulated solar radiation with and without the presence of hydrogen peroxide. Acetamiprid and thiacloprid proved to be resistant to photodegradation, while clothianidin is the only active ingredient that is almost completely degraded under the influence of simulated solar radiation. Based on the results obtained, the main conclusions of the study can be summarized as follows: hydrogen peroxide alone is not sufficient for the complete removal of xenobiotics from aqueous solutions; for the fastest possible removal of xenobiotics from the aqueous solution, it is necessary to perform photolysis with the highest available concentration of hydrogen peroxide. In addition, the presence of various decomposition or transformation products, whose structure is still unknown, indicates the risk of increased toxicity in the environment.

Ćurković et al. (Contribution 7) present the results of photolytic and photocatalytic degradation of ciprofloxacin under UV-A and solar light simulator radiation. For this purpose, they use nanostructured cerium-doped TiO₂ films with different amounts of cerium. The best photocatalytic activity was achieved with the TiO₂ film doped with 0.08 wt.% Ce. Comparing the photocatalytic activity of the same films with both radiation sources, the authors conclude that the application of sunlight simulator radiation leads to higher photocatalytic efficiency and a higher degradation rate constant.

I hope that readers of this Special Issue of Processes on “Catalysis in Advanced Oxidation Processes for Environmental Remediation” will enjoy the interesting contributions from outstanding experts and scientists and gain new ideas for future research and new scientific achievements.