Editorial for Special Issue “Recent Advances in Photocatalytic Treatment of Pollutants in Water”

Water is the fundamental source of life on earth. Nowadays, the growth of urban populations and industrial development have resulted in a continuous increase in sewage discharge, which frequently contains pathogenic microorganisms, toxic chemicals, and pharmaceutical pollutants [1,2]. Traditional treatment methods struggle to effectively remove emerging organic micro-pollutants such as drug residues and pesticides [3,4]. When these harmful substances enter surface water and groundwater, they can cause eutrophication, algal blooms, and fish death, ultimately threatening human health. In the face of the urgent demand for safe water, advanced oxidation technologies are proving promising due to their ability to mineralize organic pollutants into non-toxic products, CO₂, and H₂O [5,6]. Among these technologies, photocatalysis—using solar energy as its sole power source—is regarded as the “greenest” solution for treating water containing organic micro-pollutants [7,8].

This Special Issue explores recent advancements in photocatalytic technology for water treatment, featuring diverse perspectives, from material design to practical applications. Contribution 1 presents a “urea-dicyandiamide” copolymerization strategy to synthesize porous V-doped g-C₃N₄ (V/CN), achieving a specific surface area of 64.6 m²/g and improved catalyst yield. The combination of vanadium doping and nanosheet and hollow tubular structures enhances the separation of photogenerated carriers, boosting the photocatalytic activity of g-C₃N₄ in the peroxymonosulfate (PMS) system. Under simulated sunlight using PMS as an oxidant, carbamazepine is completely degraded within 20 min, effectively balancing the advantages of “high activity” and “low cost”. The results reveal that this is an economical and efficient method for degrading pharmaceutical pollutants in aquatic environments.

TiO₂ has emerged as one of the most promising materials for degrading organic pollutants, offering multiple advantages, including non-toxicity, low cost, high photoreactivity, and stability. When TiO₂ is irradiated by UV light, highly reactive oxygen species (ROS) are produced and oxidize and mineralize organic pollutants into harmless products [9]. However, the low photocatalytic efficiency and wide bandgap of TiO₂ limit its practical application [10]. To improve catalytic efficiency, contribution 2 introduces a modified TiO₂ photocatalyst doped with bismuth (Bi) and fluorine (F), along with SnO₂ and SiO₂. This co-modification significantly enhances the photocatalytic degradation efficiency of TiO₂; specifically, RhB degradation efficiency reaches 100% within 20 min under simulated sunlight. The reaction rate constant is 41 times greater than that of Bi/TiO₂ alone, demonstrating that this synergistic strategy effectively improves light absorption and carrier separation efficiency.

To address the issue of the wide bandgap in TiO₂ (about 3.2 eV), which restricts its degradation efficiency under visible light, in Contribution 3, Li et al. successfully extend the light response of TiO₂ to the visible spectrum through interstitial nitrogen doping. Their study compares two nitrogen sources, thiourea and ammonium bicarbonate, finding that 5%N_T/TiO₂ prepared with thiourea exhibits superior degradation efficiency for methylene blue (MB) compared to both pristine TiO₂ and substitutive N-doped TiO₂ (5%N_AB/TiO₂). The benefit of interstitial doping over substitutional doping is that interstitial nitrogen atoms distort the TiO₂ lattice, reduce interplanar spacing, and enhance electron transport. For contribution 4, a triple-action effect was utilized to enhance the photocatalytic activity of TiO₂. TiO₂ nanoparticles were prepared and transformed into nanotubes during the doping progress, supported by gold nanoparticles to increase charge carriers and active sites while preventing recombination reactions. Consequently, the photocatalytic degradation rate of acid green 1 reached 100% after 17 min of light radiation. This study demonstrated that this triple-action effect addresses titanium oxide’s limitations by creating new photo-active sites and pathways for charge carriers, in addition to inhibiting recombination reactions. Contribution 5 introduces another material design concept by combining TiO₂ with phosphoric acid-treated peanut shell biochar (p-BC). The TiO₂/p-BC composite reduces the band gap to 2.73 eV, which significantly enhances tetracycline removal efficiency through synergistic adsorption and photocatalysis. Under optimal conditions, the tetracycline removal efficiency reaches 95.3%, remaining above 86% after five cycles, demonstrating excellent stability and reusability. It represents an interesting approach to developing an efficient TiO₂-based photocatalyst, using low-cost agricultural waste as a catalyst precursor while promoting resource utilization.

The support material is a critical component of the catalyst, significantly influencing its activity, reusability, stability, and recovery. For Contribution 6, a TiO₂/CdS nanocatalyst was immobilized on flexible nickel foam to achieve efficient photocatalytic degradation of antibiotic pollutants in water. Under optimal conditions determined based on response surface methodology—specifically the Box–Behnken design (RSM–BBD)—which included 28 g of catalyst at a pH of 9.04 in 150 min, the removal rate for tetracycline hydrochloride (TCH) reached 53.89% at a scale of 10 L, thereby providing direct parameters for process scaling-up. Furthermore, five consecutive cycling experiments demonstrated remarkable stability with only a minimal catalyst loss of 4.44%.

The practical applications of photocatalytic technology must consider environmental influences; parameters optimized in the laboratory may require adjustment for actual water bodies, which is an important consideration in the transition of photocatalytic technology from the laboratory to engineering applications. Contribution 7 shifts the focus from the material to the application environment, systematically investigating how inorganic anions influence the UV/persulfate advanced oxidation process. This study shows that in pure water, the UV/persulfate system can completely degrade herbicides (terbutylazine and isoproturon) within 30 min. However, the presence of Cl⁻ and HCO₃⁻ significantly reduces degradation efficiency. Notably, HCO₃⁻ effectively scavenges HO· and SO₄⁻, producing CO₃⁻ with weaker oxidation capacity, thus inhibiting herbicide degradation.

With the widespread detection of organic pollutants such as neonicotinoid pesticides in water bodies, there is an urgent need for the development of efficient photocatalyst. Contribution 8 summarizes the research progress on various solid catalysts for degrading the pesticide imidacloprid. Metal oxides such as nano-ZnO and black TiO₂ demonstrate excellent degradation performance. Carbon-based materials (e.g., g-C₃N₄ and graphene) and metal–organic frameworks enhance catalytic efficiency due to their high specific surface area and superior photoelectric properties. Additionally, Z-scheme heterojunctions have gained attention for their ability to broaden light response, improve redox capabilities, and reduce carrier recombination, which surpasses that of traditional type-I and type-II heterojunctions. Zeolites possess a high specific surface area, tunable pore structures, and excellent ion exchange capacity. Modified zeolite materials have shown great potential as carriers or composite components of photocatalysts in the photocatalytic treatment of pollutants [11]. Contribution 9 reports that their photocatalytic activity can be significantly enhanced through doping with metals (such as Ag, Fe, or Cu) or combining them with semiconductor materials (such as TiO₂, ZnO, or g-C₃N₄). These composite materials not only extend light absorption into the visible light range but also enhance the degradation efficiency of pollutants through the synergy of adsorption and photocatalysis. High-silica zeolites, due to their hydrophobicity, have enhanced adsorption capacity for organic pollutants. Although modified zeolites have achieved remarkable results at the laboratory stage, their industrial application still poses challenges such as high synthesis costs, unstable regeneration efficiency, and difficulties in large-scale production. Future research should focus on precision material design and the optimization of regeneration technology to promote the application of zeolite-based photocatalytic materials in actual water treatment.

Photocatalytic technology also shows great potential for water disinfection as an alternative to traditional chlorination and ultraviolet treatment. Contribution 10 systematically reviews advancements in red phosphorus and black phosphorus for photocatalytic water disinfection, emphasizing their benefits in visible light response, ROS generation, and pathogen inactivation mechanisms. The authors report that both materials exhibit excellent light absorption and biocompatibility while significantly enhancing the separation efficiency of photogenerated carriers and antibacterial performance through heterostructures with graphitic carbon nitride or TiO₂. Current research primarily focuses on pure water systems; future studies should enhance performance evaluation and long-term stability in actual aquatic environments.

Overall, photocatalytic water treatment technology, a key branch of advanced oxidation processes, shows significant potential in the treatment of emerging pollutants and in disinfecting water. There have been notable advances in two key areas: improving photocatalytic efficiency through novel materials and optimizing processes for complex aquatic environments. However, challenges remain regarding large-scale application, such as the low utilization of visible light and unresolved issues relating to material stability over prolonged use. Additionally, the influence of diverse components in real water bodies on the photocatalytic process requires systematic investigation and effective solutions. In the future, with further integration of materials science, catalytic chemistry, and environmental engineering, it is expected that efficient, cost-effective, and stable photocatalytic water treatment technologies will play a crucial role in the purification of water environments.