3.5. Separation performances of NST-GO membranes
As it mentioned, the volume of Ni(OH)2 nanosheets dispersion used for sacrificing can control the microstructure andd -spacing of NST-GO membranes, which will further influence the separation performances of the membranes. Therefore, DY molecules were used as probes to estimate the separation performances of different NST-GO membranes. As shown in Figure 5a, it is found that the water permeance increases dramatically from 3.8 L m-2h-1 bar-1 to 49.2 L m-2 h-1 bar-1 when the Ni(OH)2 volume increases from 0 mL to 7 mL. However, the DY rejection decreases accordingly. Notably, the decrease of rejection also shows the three stages as mentioned above. In the sub-saturation and over-saturation stages, the rejection drops obviously with the d -spacing increasing. This opposite trend is because the expanded d -spacing causes the less filtration resistance for dye molecules. However, in the saturation stage, the rejection decreases indistinctively and keeps around 94%, which is due to the relatively stable d -spacing. Typically, the DY rejection of the membrane prepared from 5 mL GO dispersion mixed with the 5 mL Ni(OH)2 nanosheet dispersion is 94.0% with the water permeance of 32.9 L m-2 h-1bar-1, which still has over 6 times increasing than the reduced GO membrane. Figure 5b is the UV-vis absorption spectra of the retentate, feed and permeate of DY solution filtrated by the NST-GO membrane. It is clear that the concentration of retentate is higher than that of the feed, indicating few adsorption behaviors happening on the membrane.
Another effective way to improve the water permeance is decreasing membrane thickness. Therefore, keeping the volume ratio of Ni(OH)2 versus GO at 1.0, the total volume of raw dispersion for membrane fabrication is decreased to make the membrane thinner. The cross-section of as-prepared NST-GO membranes was observed and the thickness was measured from SEM images (Figure S9, Supporting Information). The separation performances of NST-GO membranes with different thicknesses were tested and the results are displayed in Figure 5c. It is found that the DY rejection decreases while the water permeance increases as the thickness decreases. Typically, the 880 nm-thick NST-GO membrane has not only the high water permeance of 120.3 L m−2 h−1 bar−1but also a sufficient DY rejection of 87.9%.
Furthermore, a series of organic dye molecules with different sizes (Figure S8, Supporting Information) and inorganic salts were employed to investigate nanofiltration separation of the 880 nm-thick NST-GO. The solution permeance and dye rejection were tested and the results are displayed as Figure 5d. From the UV-vis absorption spectra, all of the dyes are rejected instead of adsorption (Figure S10, Supporting Information). The membrane has the rejection of more than 90% for molecules that are larger than thed -spacing of 1.14 nm while a lower rejection for molecules that are smaller than the d -spacing. For example, the rejection for EB is as high as 90.1% due to its larger molecular size (1.2 × 3.1 nm2), which can hardly pass the membrane channels. However, the rejection for MB is just 58.3% because the smaller MB molecules (0.9 × 1.6 nm2) can easily pass through the channels. Moreover, the membrane has the poor rejection for both divalent salt and monovalent salt as the most GO membranes performing, which needs further improvements in the future.