44
A. S. Kazemi and M. A. Abdol
osmosis (RO) technology, the energy required for RO itself, is within a factor
of two from the limit of a reversible thermodynamic process [15, 16]. However,
there has been not much progress in RO membrane technology based on the same
polyamide thin-film composite design in the past three decades [17]. The current
membrane design still suffers low permeability even with the most permeable thinfilm composite membranes. The membranes become damaged while in contact with
chlorine, and fouling tendency is still a major challenge in these designs [18]. RO
membrane technology requires new membrane design and fabrication approaches to
meet the needs of advanced desalination demands [19]. An ideal membrane should
easily allow the flow of the desired species, reject undesired species and exhibit
robustness in operation in addition to being economical and manufacturable [20].
Therefore, high permeance, high selectivity, high chemical, mechanical and thermal
stability, and low fouling are the critical margins that new membrane technology
should focus on. With the emergence of two dimensional (2D) materials in the past
decade and beyond, and the enormous amount of attention they captured in the
scientific community due to their unique structural, thermal, and electronic properties; the perception of using them as a replacement in RO membrane technology has
excelled more than ever. Since the water flux scales inversely proportional to the
membrane thickness, it is necessary for new generation of RO membranes to have
high aspect ratio, high mechanical stiffness, atomic thickness, nanosized pores and
reactivity toward polar and non-polar water pollutants [19, 21, 22]. These extraordinarily properties can be found all together in 2D materials and though still in the
early stages of development, they present characteristics that are potentially advantageous for addressing persistent challenges in membrane separations [20]. Monolayer
graphene has been one of the first 2D materials isolated in nature and employed in
immense variety of applications ever since 2004 [23]. While production of large area
graphene is still an ongoing and active research field [24, 25], many attempts have
been made to design and fabricate nanopores across individual graphene sheets [26].
Developing 2D nano-porous graphene with tuneable ranges of pore size distribution, pore density and uniformity are critical for water purification and desalination
[26–33]. These properties play an essential role in the engineering of the electronic
properties [34], surface interactions [35] and chemical reactivity [36] of perforated
graphene materials.
We first discuss the state-of-the-art of existing membrane technologies for water
purification and desalination, emphasize their inherent weaknesses and determine the
pressing constraints for next-generation membranes. Then, we illustrate molecularlevel design approaches theoretically towards pristine and functionalized graphene,
summarize simulation considerations, explore mechanical aspects, and transport
phenomena focusing on monolayer graphene, and in some case on multilayer
graphene. Experimentally fabricating highly selective membranes, developments on
appropriate support layer and perforation techniques and characterization methods
are addressed next. While other nano-porous 2D materials [37–48] and carbon-based
materials [49–54] have also attracted extensive attention in separation techniques,
they will be discussed in the consecutive chapters. At last, we review computational
and experimental gaps and prospects in this field of study.
A. S. Kazemi and M. A. Abdol
osmosis (RO) technology, the energy required for RO itself, is within a factor
of two from the limit of a reversible thermodynamic process [15, 16]. However,
there has been not much progress in RO membrane technology based on the same
polyamide thin-film composite design in the past three decades [17]. The current
membrane design still suffers low permeability even with the most permeable thinfilm composite membranes. The membranes become damaged while in contact with
chlorine, and fouling tendency is still a major challenge in these designs [18]. RO
membrane technology requires new membrane design and fabrication approaches to
meet the needs of advanced desalination demands [19]. An ideal membrane should
easily allow the flow of the desired species, reject undesired species and exhibit
robustness in operation in addition to being economical and manufacturable [20].
Therefore, high permeance, high selectivity, high chemical, mechanical and thermal
stability, and low fouling are the critical margins that new membrane technology
should focus on. With the emergence of two dimensional (2D) materials in the past
decade and beyond, and the enormous amount of attention they captured in the
scientific community due to their unique structural, thermal, and electronic properties; the perception of using them as a replacement in RO membrane technology has
excelled more than ever. Since the water flux scales inversely proportional to the
membrane thickness, it is necessary for new generation of RO membranes to have
high aspect ratio, high mechanical stiffness, atomic thickness, nanosized pores and
reactivity toward polar and non-polar water pollutants [19, 21, 22]. These extraordinarily properties can be found all together in 2D materials and though still in the
early stages of development, they present characteristics that are potentially advantageous for addressing persistent challenges in membrane separations [20]. Monolayer
graphene has been one of the first 2D materials isolated in nature and employed in
immense variety of applications ever since 2004 [23]. While production of large area
graphene is still an ongoing and active research field [24, 25], many attempts have
been made to design and fabricate nanopores across individual graphene sheets [26].
Developing 2D nano-porous graphene with tuneable ranges of pore size distribution, pore density and uniformity are critical for water purification and desalination
[26–33]. These properties play an essential role in the engineering of the electronic
properties [34], surface interactions [35] and chemical reactivity [36] of perforated
graphene materials.
We first discuss the state-of-the-art of existing membrane technologies for water
purification and desalination, emphasize their inherent weaknesses and determine the
pressing constraints for next-generation membranes. Then, we illustrate molecularlevel design approaches theoretically towards pristine and functionalized graphene,
summarize simulation considerations, explore mechanical aspects, and transport
phenomena focusing on monolayer graphene, and in some case on multilayer
graphene. Experimentally fabricating highly selective membranes, developments on
appropriate support layer and perforation techniques and characterization methods
are addressed next. While other nano-porous 2D materials [37–48] and carbon-based
materials [49–54] have also attracted extensive attention in separation techniques,
they will be discussed in the consecutive chapters. At last, we review computational
and experimental gaps and prospects in this field of study.