Water scarcity is a global issue stemming from uneven distribution and contamination, despite sufficient global freshwater resources. Atmospheric water harvesting, particularly fog harvesting, offers a promising solution, with meshes capable of collecting significant water volumes daily. However, fog near urban areas is often polluted, limiting the harvested water's usability. This research focuses on developing a system that combines efficient fog harvesting with simultaneous water purification, addressing the challenge of polluted water sources.

Previous research in atmospheric water harvesting has explored various methods including fog, dew, and vapor collection. Fog harvesting, using meshes made of fibers or wires, is a particularly promising approach. Optimizing mesh design involves considerations like projected surface area, wire diameter, wire spacing, and wire pattern. Surface wettability engineering, aiming for low contact angle hysteresis and a low receding contact angle, is crucial to prevent droplet re-entrainment. While significant progress has been made in enhancing fog harvesting efficiency, most studies focus on uncontaminated fog. Limited research has explored integrating water purification into fog harvesting systems, primarily using TiO2 coatings that require continuous UV irradiation for effectiveness.

The researchers engineered two photocatalytically reactive coatings: a hydrophilic TiO2-PVB-PDMS and a hydrophobic TiO2-EC-PDMS. TiO2 nanoparticles were embedded in a polymer matrix (PVB or EC) with a grafted PDMS brush. UV light activated the coatings, rendering them reactive even in the dark. The coatings' performance was assessed using contaminated water droplets containing methyl orange, diesel, and bisphenol A. Epifluorescence microscopy visualized contaminant decomposition, enabling quantification of decay time. A diffusion-adsorption-reaction model was used to understand the effects of droplet volume and wettability on contaminant degradation. Fog harvesting and purification experiments used a laboratory-scale fogging setup to evaluate the coated meshes' performance. Real-time water harvesting and treatment efficiency were measured by capturing fog droplets, collecting the water, and analyzing the pollutant concentration before and after treatment. Outdoor tests under varying UV index conditions further evaluated the system's performance.

The hydrophilic TiO2-PVB-PDMS coating demonstrated superior water treatment and fog harvesting efficiency compared to the hydrophobic TiO2-EC-PDMS coating. The TiO2-PVB-PDMS coating exhibited 99.9% diesel and undetectable bisphenol A removal within 30 minutes. Epifluorescence microscopy showed faster contaminant degradation on the hydrophilic coating, consistent with the diffusion-adsorption model indicating adsorption-limited kinetics for hydrophilic and diffusion-limited kinetics for hydrophobic surfaces. Laboratory-scale fog harvesting tests showed the TiO2-PVB-PDMS mesh achieved an 8% fog harvesting efficiency and 85-94% organic pollutant reduction (depending on UV index). Outdoor experiments confirmed high water treatment efficiency (>90% at high UV index and 85% at zero UV index), showcasing the system's practicality under varying sunlight conditions. Total organic carbon content analysis corroborated the significant reduction in organic contaminants.

The study successfully demonstrated a passive, energy-neutral system for simultaneous fog harvesting and water purification. The rationally designed TiO2-PVB-PDMS coating's hydrophilic nature and high reactive site density contributed to its superior performance. The diffusion-adsorption model effectively explained the observed contaminant degradation kinetics. The high water treatment efficiency achieved in both laboratory and outdoor settings validates the system's potential for real-world applications, particularly in water-scarce regions with polluted fog. The outdoor tests highlight the system's ability to function effectively even under low UV irradiance conditions, relying on initial UV activation.

This research presents a promising solution for addressing water scarcity by combining efficient fog harvesting with passive water purification. The system's energy-neutral operation and high performance in both laboratory and outdoor settings demonstrate its potential for widespread deployment. Future research could focus on scaling up the system for larger-scale applications and exploring the system’s applicability to other water sources and a broader range of pollutants.

The study's laboratory-scale fogging setup might not perfectly replicate natural fog conditions. The tested organic pollutants represent a subset of contaminants present in fog; further research is needed to assess the system's efficacy against a wider range of pollutants. Long-term durability and the potential for coating degradation over time also require investigation.