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).