2.4 Desalination and Pollutants Removal Performances
Desalination and pollutants removal performances are important application aspects of solar steam generators. Fast evaporation rate of saline water and effective inhibition on salt crystallization are crucial for desalination applications. The water evaporation performances and salt-resisting properties of CA//PVB-based product and CA-based product are comparatively studied. The 3.5 % NaCl aqueous solution, which is close to the average salinity of seawater, is adopted as a model substance. As seen from Figure 6 a, in the first-hour SSG experiment, the water evaporation rates with CA//PVB-based product and CA-based product are 2.909 kg m-2h-1 and 1.916 kg m-2h-1, respectively. Compared with the evaporation rates of pure water when using the two kinds of solar steam generators, the evaporation rates of 3.5 % NaCl solution decrease 1.19 % and 9.84 %, respectively. From careful observation on the red curve in Figure 6a, the slope of the curve decreases with the extension of evaporation time, which means CA-based product occurs obvious performance attenuation during the process of treating saline water. In order to amplify the effect of saline water evaporation on the two solar steam generators, five-hour evaporation experiments are carried out, and the photographs of the two post-use solar steam generators are provided in Figure 6b and c. It is obviously seen that a lot of salt crystals deposit on CA-based product, whereas the situation of CA//PVB-based product is much better, demonstrating excellent salt-resisting property of CA//PVB Janus nanofibers aerogel. On the one hand, salt crystals block water transfer channels, and on the other hand, they reflect light. Both of these effects negatively impact on water evaporation. According to the references[32,46], it has been proved that hydrophobic materials can inhibit the shuttle of salt ions. Therefore, the excellent salt-resisting property of CA//PVB Janus nanofibers aerogel can be owing to the hydrophobic PVB components. To sum up, in terms of desalination property, the amphipathic Janus nanofibers aerogel also shows its superiority over the hydrophilic substrate.
The evaporation rates of saline water with different salinities when using the CA//PVB-based product are also studied. The NaCl solutions with different concentrations (1 %, 3.5 %, 5 %, 10 %, 20 %) are prepared as analytes, and the results are shown in Figure 6d. The evaporation rate decreases with the increase of salinity, but the difference is not great, which proves that the product can be able to effectively treat high-salinity water. The desalination performances of the CA//PVB-based product on saline water with different salinities are revealed in Figure 6f, and the desalination results on actual seawater (sampled from Bohai Sea, geological coordinate of sampling site: 40°53′23.90″N, 121°12′26.72″E) are given in Figure 6g. As revealed inFigure S7 , the SSG process is performed in a lab-made acrylic tank (18 cm × 9 cm × 12 cm) in which the water vapor is condensed and collected at a rate of 2.227 kg m-2h-1 (this value is obtained when pure water is treated with the amphipathic Janus nanofibers aerogel under 1 sun). The results demonstrate that the concentrations of salt ions in the collected liquids greatly decrease compared with those of the original saline water and actual seawater and are much lower than the standard of the World Health Organization (WHO) and the United States Environmental Protection Agency (EPA) for salt ion concentration in drinking water. Moreover, it is also found that the prepared solar steam generator is self-cleaning in the dark. As seen from Figure 6e, after ten-hour light irradiation (close to the total energy of sunlight in a day), some visible salt crystals deposit on the surface of product, and these salt crystals disappear automatically within 4 h in the dark. This character may be owing to the large pores in the aerogel that are conducive to the rapid downward diffusion of salt ions under the driving of concentration gradient between evaporation surface and bulk water. This self-cleaning property is beneficial for performance recovery of the product with day/night changes in outdoor applications. In addition, a faster way to eliminate the influence of salt deposition is to flip the solar steam generator. That is, after the evaporation surface is altered, the water evaporation rate can be recovered quickly.
The treatment on wastewater containing antibiotics or organic dyes is another essential application of SSG. To evaluate the related performance of CA//PVB-based product, TCH, RhB and MB solutions, with the concentrations of 0.01 g L-1, are selected as model substances. As reveal from Figure 6h and Figure S8 , the removal rates of these organic contaminants are all above 99 % in the condensed water.