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.