Figure 4. (a) Schematic illustration of the improved solar
salt-water separation system, which consists of a rthogonal prismatic
table structure MF@HPB-PPy10-OA, a filter paper
diffusion layer, a polystyrene foam insulation layer and a cotton swab
brine transport line. The radius of the filter paper diffusion layer is
0.2 cm larger than that of the MF@HPB-PPy10-OA
evaporator; (b) Salt-water convection including the evaporator interior
and the whole system is enhanced by combining the unique hydrophilic
nano-channels of the 3D evaporator surface and the original macroporous
structure as well as the size effect of the diffusion layer, achieving
improved anti-salt fouling performance; (c) Water evaporation rate and
salt harvesting efficiency of the evaporators in salt-water separation
for high salinity sample (10 wt%). Insets correspond to digital
photographs of the respective salt-water separation final results; (d)
Photothermal water evaporation performance of the evaporator at wide
range of salinities compared to recent literature reports. Circles and
triangles refer to the reports of salt local crystallization and
PPy-based evaporators, respectively; MF@HPB-PPy10-OA
executes (e) cycles of salt-water separation and (f) up to 100 h
photothermal water evaporation measurementsfor high salinity brine
samples.
the hydrophilic surface. It is satisfactory that
MF@HPB-PPy10-OA exhibits excellent photothermal water
evaporation and salt-water separation capabilities. Under the identical
conditions, its evaporation rate and salt harvesting efficiency are up
to 3.3 kg m-2 h-1 and 96.5% (Figure
4c and Figure S15), respectively, which belongs to the record-high of
high-salinity or local salt crystallization systems reported so far and
reaches zero liquid discharge. Simultaneously, the
MF@HPB-PPy10-OA is quite stable and maintains the high
levels of evaporation rate and salt harvesting efficiency even after 5
consecutive cycles of salt-water separation demonstrated in Figure 4e,
Figure S16 and Figure S17. Continuous operation for up to 100 h in 10
wt% brine that reveals stable evaporation efficiency and the ability to
self-
clean in real time further confirms the outstanding of
MF@HPB-PPy10-OA (Figure 4f).
Such outstanding water evaporation, salt-water separation performance
and continuous operation capability for high salinity brine derives
primarily from the contribution of the unique structure and the
hydrophilic-hydrophobic engineering of MF@HPB-PPy10-OA
surface. For one, the nano water channels and the photothermal active
site HPB in the hydrophilic regions guarantee the excellent water
transport and high-efficiency water evaporation performance at the
interface. On the other hand, the OA molecular region imparts excellent
real-time self-cleaning ability to the whole porous 3D structure by
significantly improving the hydrophobicity of the surface, effectively
suppressing the salt crystallization on surface (Figure 4c and Figure
S18). Besides, according to the Hagen-Poiseuille law, the hydraulic
conductivity of micropores is ∼ 1012 times higher than
that from nano channels. The salt concentration in the nanoscale water
channels of hydrophilic regions is effectively diluted by the rapid
salt-water convection with the original macroporous water transport
channels, resembling a ”drilling effect”,[58]which boosts the anti-salt fouling performance. Benefit from the
self-assembly of HPB, OA and PPy, MF@HPB-PPy10-OA is
characterized by excellent photothermal stability in high-salinity
salt-water separation. This may be attributed to the unique function of
excited state electron transfer between HPB and PPy, which substantially
inhibits the photobleaching of PPy and the photo-oxidation of HPB. It is
worth mentioning that the above mentioned brine convection effect is
reinforced in the modified evaporation system by an innovative
technology, i.e. the diffusion layer size effect. The slightly larger
size of the hydrophilic filter paper diffusion layer accelerates the
brine exchange between it and the 3D porous
MF@HPB-PPy10-OA, altering the radial concentration
gradient of salt in the evaporation system and allowing the salt to
preferentially crystallize at the periphery of the farthest diffusion
layer (See Figure 4b, Figure S19 and Discussion section for details).
The appreciable convective effect ensures the rapid diffusion and
transport of brine in the MF@HPB-PPy10-OA, and
suppresses the crystalline precipitation of salt on the surface to some
extent.[9,52,59-60]