Harvesting Clean Fogwater by Using Nanoengineered Photocatalytically Reactive Fog Harvesters

The increasing mismatch in location of population density to the distribution of freshwater availability has aggravated the global freshwater stress. To handle this stress, researchers have looked into possibilities of tapping water from alternative resources like fog, dew, rain and desalination. Fog being a passive resource and is estimated to have a huge potential for supplying water, happens to be the most promising one. However, in many places the utility of fog as potable water becomes limited as air-borne contaminants (a major concern near urban areas) present in the surroundings get attached to the fog droplets and get collected. A possible passive way to mitigate the contaminants from being collected with fog will provide a sustainable solution for the different fog harvesting communities. The present study was focused to find such passive solution to this identified problem by leveraging the existing knowledge of both photocatalysis and fog harvesting. Photocatalysis is a process where metal oxides with semiconductor properties like titanium dioxide or zinc oxide get energized due to exposure to light in the ultraviolet wavelength and are capable of reducing organic molecules on their surface. We spray the fog harvester mesh fibers with the titanium dioxide nanoparticle coating and graft PDMS brush onto the nanoparticles after that. The coating thickness is optimized and is of the order of microns, thus not having any significant effect on the aerodynamic efficiency of the fog harvester. On the other hand, the PDMS brush helps in modulating the surface wettability of the coated fiber, besides imparting robustness to the surface coating. Later using inductively coupled plasma - optical emission spectrometry we confirmed that no additional concentration of titania or silica is found in the collected water, confirming the coating is safe for fog harvesting applications.<br/>Two types of coated surfaces (varied by wettability i.e. superhydrophilic and hydrophobic) were prepared and tested against contaminated artificial fog, methyl orange dye was used as a contaminant surrogate. The concentration of organic contaminants were also varied and it was seen that for concentration at maximum allowable limit (referring to US EPA guidelines), the decontamination of the organic molecules leads to an impressive removal of 91% of the contaminants while dripping along the hydrophilic reactive surface. The best case for the hydrophobic reactive surface was at 76%, meaning the process takes longer timescale to decay on a hydrophobic surface than hydrophilic. To investigate this, a fundamental study was done using epifluorescence microscopy at micro-droplet scale to find the deciding factors driving the photocatalytic reduction reaction. It was found that the interplay between surface reactivity and the diffusivity of the molecules drive the photocatalytic decay reactions. Following the trends by finding lines of best fit and considering the operating conditions, a theoretical framework could be developed. The theoretical framework can predict the extent of decay expected at the given time depending on the concentration and the size of water droplets intercepted at the surface. It also predicts whether the reaction falls in the reactivity limited or the diffusion limited regime. Additionally, with the epifluorescence microscopy we developed a new process based on imaging the intensity to determine the rate of decay of contaminants in water droplets, an easy and scalable way to quantify the extent of cleaning of the water. The collection efficiency for the fog incidents were measured and the rate of collection was comparable to outdoor fog harvesting. These coated meshes were also tested outdoors, interestingly the decontamination was seen to even get enhanced on continuous UV exposure from the sun. Overall, the study may be considered as the first step towards understanding how to obtain drinkable water from contaminated fog.