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G.-R. Xu
composite membranes, PA-TFC membrane) has gained extensive research in recent
years [3]. With the ongoing improvements and adjustments via various strategies,
such as surface modifications [4], nanoparticles incorporations [5], and sublayers
tuning [6], and interfacial synthesis controlling [7]; PA-TFC-based RO membranes
have obtained great advancements in both fundamental research and engineering
applications. Nevertheless, despite the comparatively excellent performance, RO
membranes are still limited by the low water permeability and membrane fouling
[8]. Currently, one of the great challenges for RO membranes is to further increase
their permeations in order to decrease the energy consumption, which is the main
cost component of RO engineering [9]. For example, Grossman et al. reported that
RO membranes with three times higher permeations could reduce the energy cost by
15–46% [8].
Not as that of salty water, wastewater is characterized by more complicated
components, sizes, charges, physical, and chemical properties [10]. Traditional
methods for the purification of wastewater are flocculation, coagulation, membrane,
advanced oxidation processes, filtration, biological processes, adsorption, chemical
precipitation, and so forth. Among these methods, because of complex instruments
and space-consuming facilities with high maintenance costs of other ones [11],
adsorption has always been the most versatile and facile way. Adsorbents are the
core of adsorption. Traditional adsorbents, such as ion-exchange resins [12], activated carbon [13], and functional clays [14], are always challenged by their limited
functional groups and not satisfied with adsorption ability. It is of great necessity to
explore more advanced adsorbents to meet the wastewater treatment desire.
In recent years, two-dimensional (2D) nanosheets and the corresponding assembled layered nanomembranes (2D nanosheet materials) have shown great potential in both desalination and wastewater treatment, especially after the discovery of
graphene. Various 2D nanosheet materials such as graphene [15], graphene oxide
[16], molybdenum disulfide (MoS 2 ) [17], boron nitride (BN) [18], silicon carbide
[19], MXene, and zeolite [20] have been widely used in desalination mainly theoretically, accompanied by some experimental works. Meanwhile, unique physical and
chemical properties (e.g., unsaturated surface atoms, greater surface energy fractions,
etc.) enable 2D nanosheet materials to effectively adsorb heavy metal ions [21].
Together with zeolite nanosheets, MXene and MOF nanosheets have attracted
great attention in recent years in liquid separation area. Compared with other 2D nanomaterials, these types demonstrate some unique characters. For example, although
possessing similar features like GO, MXene shows superiority in its hydrophilic
nature [22]. In this chapter, we focus on the applications of MOFs, zeolites, and
MXene nanosheets in wastewater treatment by adsorption and filtration (mainly RO).
Although some of these 2D nanomaterials also show great potential in removing
pollutants via other routes (e.g., organic pollutants removal by photocatalysis with
MOFs [23]), herein we do not include these issues because they are totally different
disciplines.
G.-R. Xu
composite membranes, PA-TFC membrane) has gained extensive research in recent
years [3]. With the ongoing improvements and adjustments via various strategies,
such as surface modifications [4], nanoparticles incorporations [5], and sublayers
tuning [6], and interfacial synthesis controlling [7]; PA-TFC-based RO membranes
have obtained great advancements in both fundamental research and engineering
applications. Nevertheless, despite the comparatively excellent performance, RO
membranes are still limited by the low water permeability and membrane fouling
[8]. Currently, one of the great challenges for RO membranes is to further increase
their permeations in order to decrease the energy consumption, which is the main
cost component of RO engineering [9]. For example, Grossman et al. reported that
RO membranes with three times higher permeations could reduce the energy cost by
15–46% [8].
Not as that of salty water, wastewater is characterized by more complicated
components, sizes, charges, physical, and chemical properties [10]. Traditional
methods for the purification of wastewater are flocculation, coagulation, membrane,
advanced oxidation processes, filtration, biological processes, adsorption, chemical
precipitation, and so forth. Among these methods, because of complex instruments
and space-consuming facilities with high maintenance costs of other ones [11],
adsorption has always been the most versatile and facile way. Adsorbents are the
core of adsorption. Traditional adsorbents, such as ion-exchange resins [12], activated carbon [13], and functional clays [14], are always challenged by their limited
functional groups and not satisfied with adsorption ability. It is of great necessity to
explore more advanced adsorbents to meet the wastewater treatment desire.
In recent years, two-dimensional (2D) nanosheets and the corresponding assembled layered nanomembranes (2D nanosheet materials) have shown great potential in both desalination and wastewater treatment, especially after the discovery of
graphene. Various 2D nanosheet materials such as graphene [15], graphene oxide
[16], molybdenum disulfide (MoS 2 ) [17], boron nitride (BN) [18], silicon carbide
[19], MXene, and zeolite [20] have been widely used in desalination mainly theoretically, accompanied by some experimental works. Meanwhile, unique physical and
chemical properties (e.g., unsaturated surface atoms, greater surface energy fractions,
etc.) enable 2D nanosheet materials to effectively adsorb heavy metal ions [21].
Together with zeolite nanosheets, MXene and MOF nanosheets have attracted
great attention in recent years in liquid separation area. Compared with other 2D nanomaterials, these types demonstrate some unique characters. For example, although
possessing similar features like GO, MXene shows superiority in its hydrophilic
nature [22]. In this chapter, we focus on the applications of MOFs, zeolites, and
MXene nanosheets in wastewater treatment by adsorption and filtration (mainly RO).
Although some of these 2D nanomaterials also show great potential in removing
pollutants via other routes (e.g., organic pollutants removal by photocatalysis with
MOFs [23]), herein we do not include these issues because they are totally different
disciplines.