Photocatalytically reactive surfaces for simultaneous water harvesting and treatment

We demonstrate a rationally designed system that can capture fog at high efficiency while degrading organic pollutants simultaneously.
Published in Materials
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Link of the paper in Nature Sustainability: https://www.nature.com/articles/s41893-023-01159-9

Synopsis:

We demonstrate a rationally designed system that can capture fog at high efficiency while degrading organic pollutants simultaneously. At the heart of our design is a wire mesh coated by anatase titanium dioxide (TiO2) nanoparticles embedded in a polymer matrix. Once activated by sunlight, the photoactive TiO2 layer decomposes organic molecules even in the absence of sunlight—an energy-free solution to address water scarcity.

 

Content:

A majority of the global human population is presently living under extreme freshwater scarcity due to uneven distribution of the freshwater resources or contamination. To overcome this challenge, researchers have focused on alternative freshwater resources such as rain, fog and dew, ways to reduce freshwater usage and treat contaminated water sources. While adopting new technologies, attempts are to have energy neutral processes like deploying meshes to collect fog, radiative cooling surfaces to condense dew,, and solar activated sorbents to adsorb and release water vapor. Considering the potential of availability, fog harvesting is one of the most promising methods: a large-scale installation of 5,000 m2 fog harvester mesh can harvest up to 100,000 L of water in a single day. The fog harvesters are typically made of woven fibers (e.g., Raschel mesh), wire meshes, or wire patterns (e.g., harp) that are placed perpendicularly to the oncoming fog laden wind stream. The fog droplets (radius 10–20 µm) collide with the fibers/ wires and attach due to capillarity, grow due to coalescence, and drain due to gravity to a collector below.

Based on the interplay of fog droplet hydrodynamics and mesh structures or wettability, attempts to improve fog harvesting yield has been tried. Variation of wire pattern and mesh material were considered to improve the mechanical robustness of the system. Through these modifications, researchers could have rationally design fog harvesters for a maximum collection efficiency of 17% in laboratory-scale experiments. Interestingly, this work was done using only deionized water in fog generator although the utility of fog harvested water at the communities is limited by atmospheric pollution or the water collecting surface is fouling over time. When fog harvesting is done near urban areas, organic pollutants contaminate the fog droplets with concentrations ranging from 0.3 to 25 ppm.

Here we show the possibility of harvesting fog water and also instantaneously treating the water on the surface of the fibers/ meshes. Inspired from previous work that used anatase TiO2, the most photocatalytically active phase, to clean surface contaminants and microbial organic deposits, we coat the surfaces with anatase TiO2 nanoparticles. Certain metal oxides known as photocatalysts like TiO2 when irradiated with a specific wavelength of light, which for TiO2 is ultraviolet light, causes defects in the lattice which drives the catalytic decay reaction, Figure 1(a). We embedded the TiO2 nanoparticles in polymer coatings on mesh wire surfaces (Figure 1(b)) and developed a combined fog harvesting and water purification device that operates outdoors completely passively, requiring sunlight only for activation. What is fascinating about the study is that the coating remains reactive even when not irradiated for several hours, enabling one to simultaneously collect and treat polluted fog even when solar irradiance is low as it would be under foggy conditions. This showcased the possibility of applying the concept in a real-world environment to passively remove a range of pollutants.

We engineered photocatalytic reactive coatings, one hydrophilic and one hydrophobic, to have different water treatment and fog harvesting performance. To demonstrate that our coatings decompose water-borne contaminants, Figure 1(c) and (d) show image sequences of contaminated water droplets (V = 5 µL), initially containing methyl orange at a concentration of, C0 = 25 ppm, on the reactive hydrophilic and hydrophobic surfaces, respectively. We see that the droplets transition from light orange to clear after 30 min and 250 min, respectively, indicating that the organic contaminants were. Additionally, the treatment of diesel (Figure 1(e)) and bisphenol A (Figure 1(f)), more realistic contaminants, present in water droplets provided a test for the actual performance of the reactive surface. Before these experiments, the coatings were irradiated with UV light, photocatalytically activating them; but the coatings and droplets were not exposed to UV light during these experiments.

Figure 1. Reactive coatings that purify with intermittent UV irradiation. (a) Schematic showing the working principle of the coating. (b) Scanning electron microscopy, scale bar 200 nm.  Image sequences showing how the (c) hydrophilic and (d) hydrophobic reactive coatings can treat contaminated water droplets (dispensed droplet size 5µl). (e) The plot of C vs. t (black line with circular markers) for diesel dispersed in water and placed on the pre-activated hydrophilic surface. (f) Plot of C vs. t (blue line with square markers) for bisphenol A-water solution on a similar pre-activated hydrophilic surface.

 

Guided by wetting and diffusion-adsorption-reaction theories, and using epifluorescence and ultraviolet-visible spectroscopy, we investigate the effect of contaminant type and concentration on the decay of organic compounds in water droplets and films and identify the rate limiting steps, providing the necessary basis for rational surface design for fog harvesters. Further the epifluorescence microscopy helps to explore the effect of wettability and droplet volume with respect to the time it takes to degrade contaminants in a single droplet and explain the results using a diffusion-adsorption-reaction model. We see the respective hydrophobic and hydrophilic reactive coatings lie in diffusion and adsorption limiting regimes. The laboratory-scale fogging setup was used to study the real-time water harvesting and treatment performance of the coated meshes. We did our experiments at a constant low velocity of 1.5 m s-1, as at this lower speed range for fog there is maximum concentration of contaminants. We studied the decay experiments over a range of contaminant concentrations, starting with the maximum allowable limits to very high levels. With the optimized experimental setup, Figure 2(a), we did our demonstration experiments in the outdoor both when the UV index was high (Figure 2(b)) and low (Figure 2(c), cloudy day), respectively.

Figure 2. Outdoor demonstration of fog harvesting and treatment on sunny and cloudy days.  (a) Setup exposed to sunlight (b) The decay efficiency for the dye solution when the UV index was 7 (meaning high UV exposure from sun). (c) The decay efficiency on a cloudy day where the UV index was 0 (meaning low UV exposure from the sun). Here the coated mesh was pre-activated by UV irradiation before it was used in the outdoor experiment.

 

Overall, we demonstrated a fully passive fog harvesting and water treatment system capable of capturing fog at high efficiency while simultaneously removing organic contaminants in an energy neutral way. Such performance at these concentrations is notable as 0.5 ppm is the maximum allowable limit for the most abundantly available airborne organic contaminants thereby demonstrating the promise of this technology. Thus, our study shows promise towards the development of water harvesting systems with passive water purification properties, in this case harnessing intermittent solar resources to harvest and treat water on foggy days, setting a pathway towards creating potable water from underutilized water sources and addressing the global challenge of water scarcity.

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