Newly Emerging Metal–Organic Frameworks (MOF), MXenes and Zeolite …
225
Hg
2+ [50] and As
2+ [51] from wastewater. In spite of these achievements, great challenges still exist in large-scale purification and desalination for all the nanosheets
above because of deficiency in long-term stable, multifunctional, and cost-effective
fabrication procedures [15].
The perpendicular pores in MFI zeolite enable water transport in all directions
and thus endow them with great permeation in desalination. Besides, the appropriate
pore size provides highs salt rejection [52]. More importantly, MFI zeolites could be
derived from mineral resources, which is abundant in natural and the cost is very low
[53]. Actually, zeolite is considered to be more advantageous in liquid separation
[54]. Recently, molecular dynamics simulation was used to study the desalination
performance of MFI nanosheets [55]. Non-equilibrium MD in a constructed RO
system was applied to investigate the HgCl 2 and CuCl 2 removal by MFI nanosheets
under 10–200 MPa (Fig. 2a and b).
Figure 2c shows Cu
2+ , Hg
2+ , and Cl
− rejection by MFI nanosheets under various
pressures. Time-dependent simulated separation process under pressure of 50 and
100 MPa is displayed in Fig. 2d. Cu
2+ and Hg
2+ could be totally rejected under all
pressures, while the pressure was increased to 100 MPa some Cl
− could permeate
because of the overcoming of electrostatic interactions and Van der Waals. Figure 2d
indicated that all the salt ions were totally rejected under 50 MPa even in 10 ns. But
when the pressure was increased to 100 MPa, permeation of some Cl
− occurred. In
spite of the comparatively low salt rejections under high pressures, extremely high
water flux could make some compensation. For instance, water flux dramatically
increased from 25.6 ns
−1 to 173.6 ns
−1 from 10 to 200 MPa.
3.3 MXene Nanosheet
It was reported that the surrounding groups of MXene could significantly affect the
heavy metal ions adsorption. For example, F and H sites weakened the adsorption,
while the intercalation of X
+ (X = Li, Na, K) could strengthen the adsorption behavior
of Ti 3 C 2 (OH) 2 . And alkalization intercalation could act as an effective method to
accelerate the heavy metal ions adsorption [39]. Different transition metal ions of M
and X endow MXenes with different properties because of various electronic structures [56]. Taking this under consideration, based on their research on the lead adsorption on alkalization-intercalated MXene (Ti 3 C 2 (OH) 2 ) [40], Guo et al.systematically
studied the adsorption of Pb on various MXenes (M 2 X(OH) 2 , M = Sc, Ti, Cr, Zr,
Nb, Mo, Hf, Ta, and X = C or N) using DFT by first principle calculations [57].
The adopted M 2 X type MXene is the thinnest in MXene family. The alkalization
treatment after the exfoliating by HF always leads to the formation of terminated
OH groups at their outer layers [58]. It is composed of tri-layer sheets characterized
by a hexagonal-like unit in which the M is nearly closely packed, and X fills in
octahedral sites [59]. The OH groups are always added to enhance their stability.
Three kinds of M 2 X(OH) 2 were adopted in the research (Fig. 3a–c). For Model A,
two OH groups sited on the top of M hollow site. For model B, one OH group was
225
Hg
2+ [50] and As
2+ [51] from wastewater. In spite of these achievements, great challenges still exist in large-scale purification and desalination for all the nanosheets
above because of deficiency in long-term stable, multifunctional, and cost-effective
fabrication procedures [15].
The perpendicular pores in MFI zeolite enable water transport in all directions
and thus endow them with great permeation in desalination. Besides, the appropriate
pore size provides highs salt rejection [52]. More importantly, MFI zeolites could be
derived from mineral resources, which is abundant in natural and the cost is very low
[53]. Actually, zeolite is considered to be more advantageous in liquid separation
[54]. Recently, molecular dynamics simulation was used to study the desalination
performance of MFI nanosheets [55]. Non-equilibrium MD in a constructed RO
system was applied to investigate the HgCl 2 and CuCl 2 removal by MFI nanosheets
under 10–200 MPa (Fig. 2a and b).
Figure 2c shows Cu
2+ , Hg
2+ , and Cl
− rejection by MFI nanosheets under various
pressures. Time-dependent simulated separation process under pressure of 50 and
100 MPa is displayed in Fig. 2d. Cu
2+ and Hg
2+ could be totally rejected under all
pressures, while the pressure was increased to 100 MPa some Cl
− could permeate
because of the overcoming of electrostatic interactions and Van der Waals. Figure 2d
indicated that all the salt ions were totally rejected under 50 MPa even in 10 ns. But
when the pressure was increased to 100 MPa, permeation of some Cl
− occurred. In
spite of the comparatively low salt rejections under high pressures, extremely high
water flux could make some compensation. For instance, water flux dramatically
increased from 25.6 ns
−1 to 173.6 ns
−1 from 10 to 200 MPa.
3.3 MXene Nanosheet
It was reported that the surrounding groups of MXene could significantly affect the
heavy metal ions adsorption. For example, F and H sites weakened the adsorption,
while the intercalation of X
+ (X = Li, Na, K) could strengthen the adsorption behavior
of Ti 3 C 2 (OH) 2 . And alkalization intercalation could act as an effective method to
accelerate the heavy metal ions adsorption [39]. Different transition metal ions of M
and X endow MXenes with different properties because of various electronic structures [56]. Taking this under consideration, based on their research on the lead adsorption on alkalization-intercalated MXene (Ti 3 C 2 (OH) 2 ) [40], Guo et al.systematically
studied the adsorption of Pb on various MXenes (M 2 X(OH) 2 , M = Sc, Ti, Cr, Zr,
Nb, Mo, Hf, Ta, and X = C or N) using DFT by first principle calculations [57].
The adopted M 2 X type MXene is the thinnest in MXene family. The alkalization
treatment after the exfoliating by HF always leads to the formation of terminated
OH groups at their outer layers [58]. It is composed of tri-layer sheets characterized
by a hexagonal-like unit in which the M is nearly closely packed, and X fills in
octahedral sites [59]. The OH groups are always added to enhance their stability.
Three kinds of M 2 X(OH) 2 were adopted in the research (Fig. 3a–c). For Model A,
two OH groups sited on the top of M hollow site. For model B, one OH group was