Water contamination is a significant global issue, with microbial contamination accounting for a large portion of water scarcity and related deaths. Traditional water disinfection methods, such as boiling using non-renewable fuels, contribute heavily to CO₂ emissions. Solar energy offers a sustainable alternative, but existing solar water disinfection technologies suffer from low efficiency, intermittency, and potential for heavy metal contamination. UV disinfection, while effective, is limited by turbidity and produces additional contaminants. Chlorination, though scalable, leads to microbial regrowth and the formation of carcinogenic byproducts. Contact-based methods using various antimicrobial materials suffer from biofouling and require regeneration. This research aims to address these limitations by developing a scalable, efficient, and environmentally friendly solar water disinfection system using nanostructured carbon materials.

The paper reviews existing water disinfection techniques, highlighting their limitations. Boiling water using fossil fuels is identified as a significant source of CO2 emissions. Solar water distillation is discussed, noting its low efficiency and potential for introducing heavy metal contaminants. UV disinfection is critiqued for its inefficiency with turbid water and the generation of additional plastic-derived contaminants. Chlorination is shown to have its own set of disadvantages like microbial regrowth and the production of harmful byproducts. Contact-based methods using materials like zeolites and metal nanoparticles are described, but their biofouling and regeneration needs are emphasized. The need for a high-efficiency, broadband solar absorber material with low thermal conductivity and high thermal effusance is established, highlighting the challenge in finding materials possessing these properties simultaneously.

Nanostructured porous hard-carbon florets (NCF) were synthesized using a template-based chemical vapor deposition (CVD) method, employing amorphous dendritic fibrous nanosilica (DFNS) as a sacrificial template. The DFNS was subsequently etched away using NaOH, resulting in porous, monodispersed NCF structures. These NCFs were characterized using various techniques, including SEM, TEM, XPS, Raman spectroscopy, and XRD, confirming their porous structure, short-range graphitic ordering, and long-range disorder, resulting in a hard-carbon structure. The NCFs were spray-coated onto helically tapered aluminum tubes to create a solar receiver. The photothermal performance of the NCF-coated aluminum tubes was evaluated under direct solar illumination (2000 W m⁻²). Water flow through the device was controlled using a peristaltic pump. The temperature of the water at the inlet and outlet was measured using thermocouples, and thermal imaging was used to confirm the surface temperature distribution. The bactericidal efficacy of the device was tested using water spiked with *Escherichia coli* at different concentrations, and the bacterial concentration was measured using standard agar plate counting methods. Untreated lake water was also tested. A portable prototype, termed SWAP (Solar Water Antimicrobial Purifier), was developed, incorporating the NCF-coated solar receiver, a peristaltic pump, a microcontroller-based flow controller, and a sand filter. The prototype was tested using Powai lake water, and microbial content was analyzed using a commercial detection kit (Bactaslyde).

The NCF coating exhibited strong broadband absorption (~95%) of sunlight and excellent photon thermalization efficiency (87%). The helical design of the solar receiver achieved high surface temperatures (up to 95 °C) under solar irradiation. This resulted in effective water heating (up to 82 °C) and simultaneous disinfection through thermal shock. The system was effective in disinfecting water with high turbidity (5 NTU) and high bacterial loads (10⁶ CFU mL⁻¹), achieving >99.99% bacterial disinfection (*E. coli*). The SWAP prototype demonstrated a continuous flow water disinfection capacity of 42 L m⁻² day⁻¹ with a very low CO₂ footprint (5 kg L⁻¹). The NCF coating showed excellent stability even after prolonged exposure to solar irradiation and operation (40h). Importantly, the device avoids direct contact between the water and the NCF coating, preventing biofouling. Experiments were conducted with various flow rates, demonstrating the tradeoff between throughput and temperature. An intermittent flow scheme was explored to further optimize the system. ICP-AES analysis showed no leaching of aluminum or other heavy metals from the device into the water. The SWAP prototype was successfully tested with real-world lake water, achieving complete microbial decontamination within 15 minutes.

The results demonstrate the successful development of a highly efficient, scalable, and sustainable solar water disinfection system. The use of NCF coating addresses the limitations of existing methods by providing high solar-thermal conversion efficiency, low thermal conductivity, and long-term stability. The non-contact nature of the disinfection process minimizes biofouling. The low CO₂ footprint makes it an environmentally friendly alternative to traditional methods. The successful demonstration of the SWAP prototype showcases the potential for widespread implementation of this technology in areas with limited access to clean water. The high efficiency also allows for potential reduction in land-area requirement compared to other technologies.

This study successfully demonstrates the fabrication and application of NCF for efficient solar-thermal water disinfection. The resulting SWAP prototype offers a low-cost, low-power, and sustainable solution for providing clean, hot water. Future research could focus on optimizing the design for even higher throughput and exploring the use of NCF in other water purification applications, such as desalination.

While the study demonstrates the effectiveness of the SWAP prototype under specific conditions, further research is needed to evaluate its performance under varying environmental conditions (e.g., different solar irradiance levels, water quality variations). The study primarily focused on bacterial disinfection; investigating its efficacy against a broader range of waterborne pathogens is recommended. Scalability to larger-scale applications needs to be further explored.