3.5. High cation permselective separation performance
A single laboratory-scale ED cell was used to evaluate the monovalent
permselectivity of the prepared membranes in
Na+-Mg2+ and
Li+-Mg2+ mixed solutions. The TFC
and TFN membrane surface layers with effective surface areas of 7.07
cm2 were orientated facing the concentrated chamber,
and testing was performed at a current density of 10 mA
cm−2. The cation permeation of all the membranes
decreased as follows: Na+ >
Li+ > Mg2+ (Figures 6c
and d), which was in agreement with the diameters of the hydrated
Na+ (0.72 nm), Li+ (0.76 nm), and
Mg2+ (0.86 nm) ions. The increase in monovalent cation
permeation through MOF-based membranes could be attributed to the
additional ion transport channels of the MOF particles embedded in the
MOF-containing surface layers. Moreover, the increase in the
UiO-66(Zr)-NH2 nanoparticle loading from 0.01% (w/v)
for the TFN-(Zr)-1 membrane to 0.03% (w/v) for the TFN-(Zr)-2 membrane
led to the further increase in \(J_{\text{Na}^{+}}\) from 5.32 ×
10−8 to 6.35 × 10−8 mol
cm−2 s−1, respectively, and in\(J_{\text{Li}^{+}}\) from 4.65 × 10−8 to 5.26 ×
10−8 mol cm−2 s−1,
respectively. In contrast (but consistent with the aforementioned
reason), \(J_{\text{Mg}^{2+}}\) decreased
from
0.56 × 10−8 mol cm−2s−1 for the TFN-(Zr)-1 membrane to 0.48 ×
10−8 mol cm−2 s−1for the TFN-(Zr)-2 membrane for the
Na+-Mg2+ system and from 0.69 ×
10−8 mol cm−2 s−1for the TFN-(Zr)-1 membrane to 0.43 × 10−8 mol
cm−2 s−1 for the TFN-(Zr)-2 membrane
for the Li+-Mg2+ system.
Consequently, the mono- over divalent cation separation performance of
the TFN-(Zr) membranes, surpassed that of the TFC membrane, as
illustrated in Figures 6c and d, for the
Na+-Mg2+ and
Li+-Mg2+ systems, respectively. In
addition, the separation performance of the TFN-(Zr) membranes further
improved with the increase in the MOF nanoparticle loading. Moreover,
the separation performance of the TFN-(Zr) membranes for the
Na+-Mg2+ system was higher than that
for the Li+-Mg2+ system. That was
ascribed to the diameter of the hydrated Na+ ion being
relatively smaller than those of the Li+ and
Mg2+ ions, which led to the slightly faster permeation
of the Na+ ions compared with the
Li+ and Mg2+ ions. The TFN-(Zr/Ti)
membranes outperformed the TFN-(Zr) membranes in terms of cation
permeation. \(J_{\text{Na}^{+}}\) of the TFN-(Zr/Ti)-2 membrane was
12.60% higher than that of the TFN-(Zr)-2 membrane. The higher cation
permeation of the TFN-(Zr/Ti) membranes was attributed to the
electrostatic assistance of the
UiO-66(Zr/Ti)-NH2-containing ions separating surface
layer. Consequently, the combined effect of physico-electrical such as
size-sieving and electrostatic assistance in
UiO-66(Zr/Ti)-NH2-containing membranes (particularly the
TFN-(Zr/Ti)-2 membrane) caused\(J_{\text{Na}^{+}}\) and \(J_{\text{Li}^{+}}\) of the TFN-(Zr/Ti)-2
membrane to be 30% and 21% higher, respectively, and its\(P_{\text{Na}^{+}{/\text{Mg}}^{2+}}\)and\(P_{\text{Li}^{+}/\text{Mg}^{2+}}\) to be 3.8 and 5.1 times
higher,\(\ \)respectively, than those of the standard state-of-the-art
Selemion CSO (AGC Engineering Co., Japan) MCPM, and significantly higher
than those of several recently reported permselective membranes
(Supporting Information, Table S2). In addition, HPAN substrate membrane
showed nearly no obvious selectivities for
Na+/Mg2+ (~1.5) and
Li+/Mg2+ (~1.3).
Therefore, the excellent cation permselectivity could be attributed to
the MOF-containing surface layers (Supporting Information, Figure S13).
Because the changes in cation permeation and permselectivity of the
TFN-(Zr/Ti)-2 membrane after five consecutive cation separation cycles
were negligible, it was concluded that the representative TFN-(Zr/Ti)-2
membrane presented excellent stability (Supporting Information, Figure
S14).