Access to clean drinking water is a critical global issue, particularly in regions lacking sanitation infrastructure and electricity. Millions of people, primarily in rural areas, consume fecal-contaminated water, resulting in significant morbidity and mortality. The World Health Organization (WHO) and UNICEF highlight the severity of this problem, projecting a worsening water crisis. This study addresses the need for simple, efficient, and cost-effective water purification methods suitable for electricity-poor areas. Photocatalytic water treatment using titanium dioxide (TiO2) and sunlight offers a potential solution. TiO2, a wide-band gap semiconductor, absorbs UV light, generating electron-hole pairs that produce reactive oxygen species (ROS) when in contact with water and oxygen. These ROS effectively attack organic contaminants and pathogens. However, challenges remain in using TiO2 nanoparticles (TiO2NPs) due to difficulties in recovery after purification. This research introduces a novel and durable photocatalytic filter based on a TiO2NWs/CNTs nanocomposite to overcome these challenges. The nanoporous structure of the composite mechanically traps pathogens, while the TiO2NWs generate ROS using UV light from sunlight. The CNTs enhance the photocatalytic action and contribute to pasteurization through visible light absorption and heating.

Extensive research since 1985 has explored the photocatalytic properties of TiO2 for water decontamination. The use of nano-sized TiO2, such as TiO2NPs and TiO2NWs, is particularly effective due to their large surface area. Various studies have demonstrated the efficacy of TiO2-based photocatalysis in removing microorganisms and pathogens. However, challenges in the practical application of TiO2NPs, such as difficulty in recovery and mechanical instability of immobilized films, have hindered the development of operational devices. This study builds upon this existing research by focusing on a novel composite material to overcome these limitations and create a practical, sunlight-powered water purification system.

The study involved the synthesis of TiO2NWs and CNTs, both individually and then as a nanocomposite material. TiO2NWs were synthesized through a multi-step process involving the formation of titanate nanowires followed by heat treatment to obtain anatase TiO2NWs. CNTs were produced via catalytic chemical vapor deposition (CVD) in a continuous-mode rotary tube furnace. The TiO2NWs and CNTs were then mixed in varying proportions and processed via doctor blading into freestanding, flexible filter papers. The filter papers were characterized using high-resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), and other techniques. A prototype solar-thermal water purification system was constructed using the nanocomposite filter paper. The flow rate of water through the filter was characterized, along with its photo-thermal properties (temperature increase upon sunlight exposure). The photocatalytic efficiency was evaluated using various methods: Electron spin resonance (ESR) spectroscopy with TEMPOL to monitor ROS generation; UV-Vis spectroscopy with methyl orange (MO) to assess photodegradation of organic pollutants; HPLC-MS to determine the removal of micropollutants (drugs and pesticides); and microbiological tests (Colilert, Quanti-Tray, and CFU methods) to evaluate the removal and inactivation of E. coli bacteria. The detailed methods for material synthesis, characterization, and photocatalytic efficiency testing are described in the supplementary materials.

The TiO2NWs/CNTs nanocomposite filter effectively removed waterborne pathogens. The nanoporous structure of the filter mechanically retained pathogens (bacteria, viruses, and worms), with a cutoff size of less than 1 µm. The TiO2NWs photocatalytically generated ROS under UV illumination, further inactivating the trapped pathogens. The CNTs enhanced photocatalysis and contributed to thermal pasteurization by absorbing visible light and heating the filter. ESR spectroscopy confirmed the efficient photocatalytic generation of ROS. The degradation of methyl orange dye under UV-A illumination demonstrated the photocatalytic activity of the filter, exceeding that of commercially available TiO2NPs. HPLC-MS analysis showed a reduction in the concentration of selected micropollutants (gabapentin and metformin), although the reduction was modest due to low concentrations and short residence time. Microbiological tests using E. coli demonstrated efficient pathogen removal and inactivation. The combination of mechanical filtration, ROS generation, and thermal pasteurization resulted in near-complete elimination of E. coli. Even sunlight exposure alone showed significant bacterial inactivation. A small prototype filter (0.3 m²) achieved a daily throughput of approximately 2 liters of purified water.

The findings demonstrate the feasibility of a simple, sunlight-powered water purification system using a TiO2NWs/CNTs nanocomposite filter. The combined mechanisms of mechanical filtration, ROS generation, and thermal pasteurization provide effective water decontamination. The superior performance of the composite filter compared to TiO2NPs highlights the synergistic effects of incorporating CNTs. The reduction in micropollutant concentrations suggests the potential for broader application, although further optimization may be needed for improved efficiency. The scalability of the system is a key advantage; increasing filter surface area can readily increase the water purification capacity. This technology offers a viable solution for providing clean drinking water in areas lacking electricity or advanced water treatment infrastructure.

This study successfully demonstrated a simple, cost-effective, and sustainable solar-powered water purification system. The TiO2NWs/CNTs nanocomposite filter effectively removed various contaminants, including pathogens and micropollutants. Future research could focus on incorporating gold nanoparticles (AuNPs) to further enhance photocatalytic activity through plasmonic effects, and optimizing filter surface structuring to improve light trapping and absorption. This approach holds significant promise for addressing global water scarcity and improving public health in resource-limited settings.

The study focused primarily on E. coli bacteria. Further research is needed to evaluate the filter's effectiveness against a broader range of pathogens and micropollutants. The reduction in micropollutant concentrations, while promising, was relatively modest; optimizing filter design and operating conditions could further enhance this aspect. The prototype's performance was tested under specific sunlight conditions; further testing is needed to assess its performance under varying environmental conditions. The long-term durability and stability of the filter under continuous operation need further investigation.