FIGURE 7 Water evaporation rates of magnetic evaporator for
saline solutions with different concentrations and seawater from Bohai
Bay under 1-sun illumination (a). The evaporation rates and the
regeneration performance of magnetic evaporator in 8-hour evaporation
for simulated seawater (b). Long-term seawater evaporation performance
of magnetic evaporator 12-hour illumination alternating with 12-hour
darkness, 3.5 wt% of saline solution was used as simulated seawater
(c). Schematic diagram of condensate water collection device made of
PMMA for outdoor solar-driven desalination (d). The XRD spectra of the
Fe3O4@C after photothermal evaporation
in various conditions (e).
The stability of the evaporator in seawater evaporation was carefully
measured as Figure 7b and Figure 7 c shown. In Figure 7b, photothermal
evaporation with the Fe3O4@C evaporator
was sustained for 8 hours. Thereafter, the evaporator was regenerated by
washing it with a little water. During continuous evaporation, the
evaporation rate for simulated seawater (3.5 wt%) gradually decreased
from 1.31 kg m-3 h-1 at the
beginning to 1.21 kg m-3 h-1 with
the assistance of evaporator. The precipitation of salt crystals on
spiny surface could be considered as the reason for the slight
efficiency decline. By removing salt crystals through washing them in
water, Fe3O4@C nanoparticles could be
collected and assembled into the spiny surface again for next
measurement. After four-time regeneration, evaporator still showed an
evaporation rate around 1.26 kg m-3h-1 at the beginning, about 96% of the initial
result. It was demonstrated that the magnetic field-induced method is an
alternative reusable strategy to improve the cycling performance of
evaporator.
In the other measurement as shown in Figure 7c, the
Fe3O4@C evaporator was continuously used
for 12-hour evaporation under illumination and 12-hour regeneration by
itself in dark, which simulated the case in daytime and nighttime. The
evaporation rate was decreased from 1.30 kg m-3h-1 to 1.19 kg m-3h-1 with the accumulated salt crystals after 12-hour
evaporation of simulated seawater (3.5 wt%). During the regeneration in
dark, the salt crystals were not completely dissolved as Figure S8
shown, which might be due to the weak hydrophilicity of
Fe3O4@C. Salt ions were difficult to
diffuse rapidly into the bulk solution. Thus, the regeneration of
evaporator was not sufficient. The evaporation rate on Day 5 was just
about 1.12 kg m-3 h-1, which was
86% of the initial rate after four-time regeneration. The performance
decline of evaporator could be attributed to the accumulation of
crystalline salt that influenced the light harvesting. During the
application in seawater, Fe3O4@C still
kept stable without an observable change in the crystal structure as XRD
patterns shown in Figure 7e.
Furthermore, the outdoor experiment was conducted to investigate the
potential of magnetic photothermal evaporator for practical application.
The lab-made evaporator in above measurement had a small size which was
not appropriate for practical desalination. Consequently, a larger
magnet with a diameter of 60 mm was used to compose a larger evaporator.
A portable water collection device made of polymethyl methacrylate
(PMMA) was used for solar-driven desalination. The specific design of
device was shown in Figure 7d. The as-prepared larger evaporator was
placed inside the device and converted solar light to heat for vapor
generation. The formed vapor condensed into water droplets that flowed
along the side walls into the container located below. After actual
solar irradiation in an open field from 07:35 to 18:06 (Sunset time) on
March 14 (38°53′28′′N, 121°31′31′′E) (Figure S9), over 10 ml of water
was finally collected. Meanwhile, there was no visible salt
precipitation on the black Fe3O4@C
layer. Suffering from the interference elements including low
temperatures, clouds, condensed water droplets and PMMA cover, the
actual light intensity accepted by sample was far lower than 1000 W
m−2. As Figure S10 shown, the concentrations of
residual Na+, K+,
Mg2+, Ca2+, Cl-,
Br- and SO42- in
condensate water were down to about 30.54, 3.97, 1.95, 3.459, 41.52,
0.05 and 7.02 mg L-1, respectively, which were all
lower than the standard limits of desalinated water from WHO (World
Health Organization).39