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.