Introduction:
By growing the population and expanding industries, the need for sustainable and efficient wastewater treatment methods has become increasingly urgent. Among the different technologies, advanced photocatalytic oxidation is a promising technology, gaining attention for its ability to disinfect water and wastewater with minimal formation of harmful by-products. This study explored the use of titanium dioxide (TiO₂) nanoparticles for photocatalytic disinfection of water contaminated with Escherichia coli (E. coli), focusing on using different stainless-steel substrates to enhance disinfection performance. The primary objective was to identify optimal conditions for achieving maximum bacterial removal efficiency while ensuring the stability and reusability of the coated substrates.
Materials and Methods:
In this research, TiO₂ nanoparticles were deposited on various stainless-steel substrates, including the inner surface of stainless-steel pipes, brushed stainless-steel sheets, and stainless-steel meshes. The TiO₂ nanoparticles were coated at different dosages (5, 10, 15, and 35 mg/cm²) on surfaces, and their performance was evaluated under UV light (6 W) for a residence time of 90 minutes. The brushed stainless-steel sheet was coated using physical vapor deposition (PVD), providing a durable and uniform TiO₂ layer. In contrast, stainless-steel meshes and the inner surfaces of stainless-steel pipes were coated using solution-based methods by brushing, followed by heat treatment to stabilize the coating. To assess the effectiveness of these coatings, laboratory experiments were conducted using ultrapure water contaminated with E. coli at an initial concentration of approximately 10⁹ CFU/ml. The photocatalytic disinfection process was carried out in a reactor equipped with a UV light source (6 W). Samples were collected at intervals (2, 5, 10, 20, 30, 60, and 90 minutes), and bacterial concentrations were determined using standard agar plate culture methods. Disinfection efficiency was calculated based on the bacterial log reduction and removal percentages of E. coli. The pseudo-first-order kinetic model was applied to analyze the disinfection data, and reaction rate constants (k) were determined to assess the effectiveness of different coatings. In addition, statistical analyses, including the Mann-Whitney U test and Kruskal-Wallis test, were employed to evaluate differences in performance between various coating conditions.

Result and Discussion:
The study revealed that TiO2 coatings on the inner surfaces of stainless-steel pipes achieved the highest bacterial removal efficiencies, reaching 99.99% disinfection within 20 minutes at dosages of 35 mg/cm² and 5 mg/cm². However, the coatings on stainless-steel pipes exhibited stability issues, as TiO₂ particles gradually shedd over time, which could reduce their long-term effectiveness. In contrast, stainless-steel mesh coated with TiO₂ nanoparticles at a dosage of 10 mg/cm², combined with UV irradiation, achieved a removal efficiency of 99.96% within 30 minutes. This configuration demonstrated a balance between high efficiency and stability, making it a reliable choice for sustained applications. Furthermore, the stainless-steel mesh showed better resistance to particle shedding compared with the inner surfaces of the pipes. The disinfection process was effectively described by the pseudo-first-order kinetic model, as indicated by high correlation coefficients (R²). The highest reaction rate constants (k) were observed for coatings on stainless-steel pipes (35 mg/cm²) and stainless-steel meshes (10 mg/cm²). Statistical analyses revealed no significant differences in disinfection efficiency between these two configurations. This result suggested that stainless-steel mesh coated with 10 mg/cm² dosages of TiO₂ offers an equally effective and more stable alternative. The findings established the critical role of nanoparticle dosage in determining the performance of photocatalytic systems. The findings from this research provided valuable insights into the design and optimization of sustainable water treatment technologies that align with global efforts to address water scarcity and pollution challenges. Currently, numerous water treatment plants worldwide are equipped with UV lamps that use ultraviolet light for water disinfection. However, studies have shown that relying solely on UV light can lead to the production of smaller-than-average colony phenotypes in surviving bacteria, which can pose a serious health risk. Therefore, the industrial-scale application of TiO₂ nanoparticles as a sustainable and effective disinfection method in water treatment should be considered. However, challenges such as costs, nanoparticle stability, and the need for advanced technologies to separate and recover photocatalysts must still be addressed. Therefore, it is recommended to investigate the durability and reusability of TiO₂ coatings under continuous operation, to evaluate the effectiveness of alternative light sources, including solar or energy-efficient LEDs, to reduce operational costs, to extend the findings to pilot-scale studies to assess the feasibility of implementing TiO₂-based photocatalytic systems in industrial water treatment facilities, and to examine the potential environmental risks associated with TiO₂ nanoparticle shedding and develop strategies for nanoparticle recovery and reuse.