Three-Dimensional and Lamellar Graphene Oxide Membranes …
97
Another work on composite membranes based on GO and MXene found an even
greater water flux of ~72 L · m
−2 · h
−1 · bar
−1 , while the reference GO membrane
achieved 6.5 L · m
−2 · h
−1 · bar
−1 under the same experimental conditions [55].
To this end, the authors used a GO/MXene mass ratio of 1/4. Different proportions
of GO and MXene led to varied permeabilities and rejection rates. Nevertheless,
the rejection of conventional organic dyes of small molecules was found to exceed
99.5%, highlighting their excellent removal efficiency. The overall good performance
is a consequence of the increased interlayer spacing and an electrostatic effect due
to a decrease in the oxygen-containing functional groups.
As described throughout this section, GO species are materials with many advantages that enable lamellar GO/rGO membranes with excellent properties for water
purification. Fully understanding their structure–property relationship is fundamental
to improve performance, stability, selectivity, and anti-fouling effectively.
3.2 GO/rGO 3D Membranes
3D assembly of GO/rGO monolayers is one of the most promising strategies for
nanofiltration technologies [56]. Similar to the lamellar case, the morphology of 3D
membranes can be controlled during their synthesis. The hierarchical structures can
be divided into two families: the hollow and the 3D porous structures. The former
includes nanoshells, nanospheres, nanococoons, and nanocapsules, while the latter
is composed of aerogels, hydrogels, sponges, and foams.
3D-GO networks were first obtained by Xu and co-authors [57]. The foam structure obtained by a hydrothermal method showed high capacity and conductance,
substantial surface area, and high mechanical and thermal stability. Controlling
the amount of oxygen in the reduction process and employing distinct treatment
methods are both effective to tune the membrane’s adsorptive properties. In
fact, these structures have been widely employed in adsorption experiments with
promising results [58].
There are countless functionalization options for 3D GO/rGO materials. For
instance, the irradiation by ion beams generates wrinkles, folds, and defects in the
structure that are a perfect fit for adsorption of naphthalene [59], a carcinogenic agent
whose eradication from water is a current challenge. Ji et al. [60] explored the use
of three distinct carbohydrates as reducing and spacing agents in 3D-rGO aerogels.
They found distinct structural characteristics for each carbohydrate, which can be
further tailored for specific pollutant adsorption.
Polydopamine (PDA) can be easily employed as both reducing and functionalization agents. This is possible due to a spontaneous process of polymerization
by dopamine and the self-assembly of graphene to form a hydrogel during the
hydrothermal process. A high density of PDA functional groups in the graphene
surface leads to a high adsorption capacity of a large number of pollutants such as
heavy metals, synthetic dyes, and aromatic pollutants [61]. Other functional groups
have been added to GO hydrogels in order to enhance their adsorptive properties. For
97
Another work on composite membranes based on GO and MXene found an even
greater water flux of ~72 L · m
−2 · h
−1 · bar
−1 , while the reference GO membrane
achieved 6.5 L · m
−2 · h
−1 · bar
−1 under the same experimental conditions [55].
To this end, the authors used a GO/MXene mass ratio of 1/4. Different proportions
of GO and MXene led to varied permeabilities and rejection rates. Nevertheless,
the rejection of conventional organic dyes of small molecules was found to exceed
99.5%, highlighting their excellent removal efficiency. The overall good performance
is a consequence of the increased interlayer spacing and an electrostatic effect due
to a decrease in the oxygen-containing functional groups.
As described throughout this section, GO species are materials with many advantages that enable lamellar GO/rGO membranes with excellent properties for water
purification. Fully understanding their structure–property relationship is fundamental
to improve performance, stability, selectivity, and anti-fouling effectively.
3.2 GO/rGO 3D Membranes
3D assembly of GO/rGO monolayers is one of the most promising strategies for
nanofiltration technologies [56]. Similar to the lamellar case, the morphology of 3D
membranes can be controlled during their synthesis. The hierarchical structures can
be divided into two families: the hollow and the 3D porous structures. The former
includes nanoshells, nanospheres, nanococoons, and nanocapsules, while the latter
is composed of aerogels, hydrogels, sponges, and foams.
3D-GO networks were first obtained by Xu and co-authors [57]. The foam structure obtained by a hydrothermal method showed high capacity and conductance,
substantial surface area, and high mechanical and thermal stability. Controlling
the amount of oxygen in the reduction process and employing distinct treatment
methods are both effective to tune the membrane’s adsorptive properties. In
fact, these structures have been widely employed in adsorption experiments with
promising results [58].
There are countless functionalization options for 3D GO/rGO materials. For
instance, the irradiation by ion beams generates wrinkles, folds, and defects in the
structure that are a perfect fit for adsorption of naphthalene [59], a carcinogenic agent
whose eradication from water is a current challenge. Ji et al. [60] explored the use
of three distinct carbohydrates as reducing and spacing agents in 3D-rGO aerogels.
They found distinct structural characteristics for each carbohydrate, which can be
further tailored for specific pollutant adsorption.
Polydopamine (PDA) can be easily employed as both reducing and functionalization agents. This is possible due to a spontaneous process of polymerization
by dopamine and the self-assembly of graphene to form a hydrogel during the
hydrothermal process. A high density of PDA functional groups in the graphene
surface leads to a high adsorption capacity of a large number of pollutants such as
heavy metals, synthetic dyes, and aromatic pollutants [61]. Other functional groups
have been added to GO hydrogels in order to enhance their adsorptive properties. For