The accelerated increase in freshwater demand, particularly among populations displaced in remote locations where conventional water sources and the infrastructure required to produce potable water may be completely absent, highlights the urgent need in creating additional freshwater supply from untapped alternative sources via energy-efficient solutions. Herein, we present a hydrophilic and self-floating photothermal foam that can generate potable water from seawater and atmospheric moisture via solar-driven evaporation at its interface. Specifically, the foam shows an excellent solar-evaporation rate of 1.89 kg m –2 h –1 with a solar-to-vapor conversion efficiency of 92.7% under 1-Sun illumination. The collected water is shown to be suitable for potable use because when synthetic seawater samples (3.5 wt %) are used, the foam is able to cause at least 99.99% of salinity reduction. The foam can also be repeatedly used in multiple hydration–dehydration cycles, consisting of moisture absorption or water collection, followed by solar-driven evaporation; in each cycle, 1 g of the foam can harvest 250–1770 mg of water. To the best of our knowledge, this is the first report of a material that integrates all the desirable properties for solar evaporation, water collection, and atmospheric-water harvesting. The lightweight and versatility of the foam suggest that the developed foams can be a potent solution for water efficiency, especially for off-grid situations.
anthropogenic activities that have led to saline intrusion and contamination of the existing freshwater sources, making the current approaches for water treatment even more difficult and costly. [2][3][4] This, in combination with the continuously rising global water demand, results in a rather pessimistic prediction that an additional 2000 billion m 3 of freshwater supply will be required by 2030 to meet the global demand, assuming no gains on the current state of water-production capacity and efficiency. [5,6] As the existing freshwater sources are being depleted, there is a growing need for utilization of a broader range of water sources beyond the conventional ones, that is, brackish water, seawater, wastewater, and atmospheric moisture. [7] On top of that, there is a call for a greater focus on enhancing water security, particularly for the poor and vulnerable populations, as water scarcity is more critical in these geographical regions due to the lack of access to conventional infrastructure and power sources to produce clean water. [1,8,9] This, in addition to the increasing intensity and frequency of climate-related disasters, present unique demands, such as simple and off-grid operations, for water technologies (WTs) that can be deployed to manage the constraints imposed by these circumstances. [10]
Herein, expanded graphite is successfully combined with waterborne polyurethane to develop porous foams with underwater oleophobic properties for the separation of surfactant-free, oil-in-water mixtures and emulsions. To obtain foams with different pore sizes and therefore with different performances in the oil−water filtration process, two solvent-free fabrication processes are adopted. In the first one, the expanded graphite granules are mixed with the waterborne polyurethane (PUEGr), and in the second method, calcium carbonate is introduced to the two-component mixture (PUEGr_t). In both cases, the obtained foams exhibit hydrophilicity and oleophilicity in air and oleophobicity underwater, and they have porous interconnected networks, while their pore size distribution differs significantly. The foams can be used as 3D filters, able to separate, through gravity, surfactant-free, oil-in-water mixtures (10% w/w oil in water) with high oil rejection efficiencies and flow rates that depend on the type of foam. In particular, in the gravitydriven filtration process using 100 mL of the feed liquid, the PUEGr foams have an oil rejection efficiency of 96.85% and flow rate of 9988 L m −2 h −1 , while for the PUEGr_t foams the efficiency is higher (99.99%) and the flow rate is lower (8547 L m −2 h −1 ) due to their smaller pore size. Although the PUEGr_t foams have slower separation performance, they are more efficient for the separation of surfactant-free emulsions (1% w/w oil in water) reaching an oil rejection efficiency of 98.28%, higher than the 95.66% of the PUEGr foams of the same thickness. The foams can be used for several filtration cycles, as well as in harsh conditions without deteriorating their performance. The nature of raw materials, the simple solvent-free preparation method, the effective gravity-driven filtration even in harsh conditions, and their reusability suggest that the herein engineered foams have great potential for practical applications in oil−water separation through highly energy-efficient filtration.
This paper reports the fabrication of photothermal cryogels for freshwater production via the solar-driven evaporation of seawater. Photothermal cryogels were prepared via in situ oxidative polymerization of pyrrole with ammonium persulfate on preformed poly(sodium acrylate) (PSA) cryogels. We found that the pyrrole concentration used in the fabrication process has a significant effect on the final PSA/PPy cryogels (PPCs), causing the as-formed polypyrrole (PPy) layer on the PPC to evolve from nanoparticles to lamellar sheets and to consolidated thin films. PPC fabricated using the lowest pyrrole concentration (i.e., PPC10) displays the best solar-evaporation efficiency compared to the other samples, which is further improved by switching the operative mode from floating to standing. Specifically, in the latter case, the apparent solar evaporation rate and solar-to-vapor conversion efficiency reach 1.41 kg m –2 h –1 and 96.9%, respectively, due to the contribution of evaporation from the exposed lateral surfaces. The distillate obtained from the condensed vapor, generated via solar evaporation of a synthetic seawater through PPC10, shows an at least 99.99% reduction of Na while all the other elements are reduced to a subppm level. We attribute the superior solar evaporation and desalination performance of PPC10 to its (i) higher photoabsorption efficiency, (ii) higher heat localization effect, (iii) open porous structure that facilitates vapor removal, (iv) rough pore surface that increases the surface area for light absorption and water evaporation, and (v) higher water-absorption capacity to ensure efficient water replenishment to the evaporative sites. It is anticipated that the gained know-how from this study would offer insightful guidelines to better designs of polymer-based 3D photothermal materials for solar evaporation as well as for other emerging solar-related applications.
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