Light-Assisted Catalysis in Water and Indoor Air Cleaning: Challenges and Perspectives

The detrimental effects of environmental pollution on human health, combined with global climate change, make it a critical contemporary problem. Despite the fact that water covers more than 71% of the Earth’s surface, ensuring access to high-quality drinking water for everyone is a major concern that societies encounter in the 21st century [1]. Utilizing renewable solar light and a catalyst to mineralize various harmful chemicals present in indoor air and water sources into benign small molecules, such as H₂O and CO₂, is an attractive approach. In this context, photocatalytic processes have consistently offered smart, green, and eco-friendly scale-up methods for environmental remediation. Numerous photocatalysts have proven successful in achieving the mineralization of chlorinated pollutants, organic contaminants, dyes, or antibiotics [2,3,4,5,6]. An analysis of the existing literature reveals the need for research studies to focus on developing efficient photocatalysts capable of mineralizing contaminants into non-toxic CO₂. Only such photocatalytic materials should be envisaged for environmental remediation.

This Special Issue devoted to “Light-Assisted Catalysis in Water and Indoor Air Cleaning: Challenges and Perspectives” is a collection of 10 papers, including 3 reviews and 7 research articles. The aim of this Special Issue is to present recent advancements in the photocatalytic removal of pollutants, elucidating the main factors contributing to their mineralization and the implication of reactive oxygen species (ROS) through dedicated experiments. Furthermore, it encompasses the design and physicochemical characterization of various photocatalytic systems employed in environmental remediation.

In their review paper, Pavel et al. [7] presented a detailed and comparative study of the photocatalytic removal of organic (e.g., alcohols, carboxylic acids, volatile organic compounds, and phenols) and inorganic (e.g., NO₃⁻) contaminants. The efficiency of multiple UV–Vis-light active photocatalysts and their corresponding degradation pathways were described, emphasizing the key factors contributing to their mineralization. The reaction mechanisms, the identification and quantification of by-products, and the implication of reactive active species (ROS) were considered for each category of the model target pollutant. Applying BiOCl and g–C₃N₄–based photocatalysts for water purification was reviewed by Ren and co-workers [8]. The authors described the preparation methods of g–C₃N₄/BiOCl composites via hydrothermal, deposition–precipitation, solvothermal, and calcination techniques. Subsequently, the authors explained the potential application of g–C₃N₄/BiOCl heterojunctions (e.g., degradation of dyes, residual pharmaceutical agents, and plasticizers) and the distinct reaction mechanisms involved in improving their performance. A comprehensive review by Galloni et al. [9] addresses the latest contributions to olive mill wastewater treatments, highlighting the potentialities and drawbacks of each removal method discussed. The authors emphasized the necessity of developing sustainable, environmentally friendly photocatalysts that could serve as valid alternatives to conventional treatment methods when optimized. In this context, the authors proposed recoverable magnetic compounds, as well as floating- and membrane-based devices, as promising alternatives to conventional TiO₂–based systems.

Ali et al. [10] examined how the structural, morphological, and optical properties of pure TiO₂ and TiO₂-incorporated Fe₂O₃ influenced their respective photoelectrochemical performances. The study revealed that pure titania exhibited the highest activity in the photocatalytic degradation of Rose Bengal dye under UV light, along with a superior photocurrent response, compared to the incorporated hematite counterparts. Additionally, Yang et al. [11] proposed utilizing TiO₂/hectorite composites with varying molar ratios of lithium, magnesium, and silicon as photocatalysts for the photodegradation of methylene blue (MB) under UV light irradiation. Under optimized conditions (molar ratio of Li/Mg/Si = 1.32/5.34/8), the highest removal rate of MB dye (97.8%) was revealed, while reusability tests after five runs showed only a slight decrease in the photocatalytic activity. Patel and co-workers [12] provided further evidence that photocatalytic, sonocatalytic, and sonophotocatalytic approaches play an important role in wastewater treatment, using Mn-doped ZnS quantum dots (Qds) as catalysts to remove solochrome dark blue azo dye (SDB). The authors claim that the Mn²⁺:ZnS Qds sample showed high activity for the sonophotocatalytic degradation of SDB (89%), surpassing the rates observed for sonocatalysis (69.7%) or photocatalysis (55.2%) alone. This behavior was attributed to the enhanced electron-holes separation, increased reactive radicals, and augmented active surface area. Another application of light-assisted processes is by removing pesticides from wastewater. In this regard, Kobkeatthawin et al. [13] provide insights into the photocatalytic degradation of imidacloprid (IMI) using g–C₃N₄/TiO₂ systems. The composites were prepared by subjecting g-C₃N₄ nanosheets and Ti precursor to a hydrothermal treatment, with various weight percentages of g–C₃N₄ in relation to TiO₂ (0.5, 1, 4, 10, and 15 wt.% of g–C₃N₄). In this study, the authors obtained enhanced photocatalytic performance for the composite materials compared to bulk materials due to a synergistic effect between Ti³⁺–TiO₂ and g–C₃N₄. The 0.5C₃N₄/TiO₂ sample displayed the highest IMI removal efficiency, reaching up to 93% within 150 min, and good stability during multiple recycling tests. In addition, Sandulescu and co-workers [14] studied the photocatalytic oxidation of phenol under sunlight irradiation using both bare and noble metal-loaded TiO₂. The experiments revealed that the supported noble metals function as visible light absorbers, assisting the separation of photo-charges and the reduction of O₂ to O₂⁻. The O₂⁻ oxidizes mildly phenol to oxygenated products. In a parallel process, •OH radicals yielded by TiO₂ mineralized phenol to CO₂ via rapid reaction sequences. Ignat et al. [15] used active noble plasmonic metals/Ga-substituted MgAl–hydrotalcites for the photocatalytic degradation of p–dichlorobenzene and 4–nitrophenol under simulated solar irradiation. The results revealed the enhanced photocatalytic performances of the synthesized plasmonic heterostructures (Ag–MgGaAl and Au–MgGaAl) compared to both the calcined forms and hydrotalcite precursors. The paper also included a discussion on the kinetic models that governed the studied plasmonic catalysts. Zhou and co-workers [16] successfully coupled β-NaYF₄:Yb,Er,Gd fluorescent nanorods to a reduced TiO₂ nanocomposite and applied it to visible-light catalytic sterilization under 980 nm near-infrared (NIR) light illumination. The authors claimed that up to 98.1% of Escherichia coli were effectively eradicated within 12 min of NIR light irradiation at a minimum inhibitory concentration of 40 μg/mL. The high bacterial activity was attributed to the synergistic effect between the fluorescent nanorods and reduced TiO₂.

Collectively, these studies provide an overview of the latest advances in the development of photocatalytic materials for diverse chemical reactions utilized in the removal of pollutants from indoor air or wastewater. These research studies pave the way for improving catalytic systems and contributing to a cleaner environment.

Finally, we express our gratitude to the journal Catalysts for the opportunity to produce this Special Issue, and to the editorial team, especially Assistant Editors Mia Zhang and Janine Li, for their continuous support during the submission and publication process. We also extend our gratitude to the authors and referees for their diligence and contributions to the success of this Special Issue.