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]