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