Physically Coating Nanofiltration Membranes with Graphene Oxide Quantum Dots for Simultaneously Improved Water Permeability and Salt/Dye Rejection

Abstract

strategy to produce fresh water from seawater and polluted water, without generating harmful by-products. [3] Among them, thin-film composite nanofiltration (TFC NF) membranes are an important component. They can produce higher water flux with lower energy consumption compared with reverse osmosis membranes. They are efficient in removal of both conventional and emerging contaminants from water. In addition, the higher rejection of divalent ions compared with monovalent ones makes them promising in the application of water softening. [4][5][6] Generally TFC NF membranes are prepared through the interfacial polymerization (IP) method, forming a structure of an ultrathin polyamide (PA) selective layer on the surface of a porous substrate. The structure and physiochemical properties of the PA selective layer and the underlying substrate can be individually modified to improve the overall membrane performance. [7] However, the trade-off between water permeability and salt rejection, a so-called "upper bound" relation, is one of the key problems faced by TFC membranes. [8][9][10] It is highly desirable that we can fabricate TFC PA NF membranes with improved water permeability and salt/dye rejection for promoting both energy effectiveness and water quality.Incorporating hydrophilic nanoparticles (NPs), such as TiO 2 , [11] SiO 2 , [12,13] Al 2 O 3 , [14] Ag, [15] nanozeolite, [16,17] and graphene oxide quantum dots (GO QDs), [18] into the top PA selective layer to form thin-film nanocomposite (TFN) membranes has been demonstrated as an effective strategy to improve the water permeability of membranes. The improvement is resulted from the increased membrane hydrophilicity and additional water pathways introduced by the NPs. However, NPs tend to interfere the IP process of the PA selective layer, leading to undesired changes in pore structure, thickness, and surface roughness of membranes. [19,20] Besides, the incompatibility between inorganic NPs and the organic polymer matrix results in microporous defects. [21,22] Both of them account for the decrease in membrane salt rejection. Additionally, NPs are mainly embedded inside the polymer matrix, so that they are not effectively available for surface-based interactions.A physically bound coating of graphene oxide quantum dots (GO QDs) on thin-film composite polyamide nanofiltration (NF) membranes is prepared through a pressure-assisted filtration method to simultaneously improve the membranes' water permeability and salt/dye rejection. It is the first attempt to use carbon-based nanoparticles as surface coating to improve the efficiency of water filtration membranes. Influences of the GO QDs' coating on the hydrophilicity and surface charge of the membranes are investigated by water contact angle and zeta potential measurements. With the GO QDs' coating, the surface roughness and hydrophilicity of the membranes increase, and the membrane surface becomes more negatively charged. Water permeability and salt/dye rejection of GO QD-coated NF membranes increase simultaneously ...

Nanovoid-Enhanced Thin-Film Composite Reverse Osmosis Membranes Using ZIF-67 Nanoparticles as a Sacrificial Template

“…Because of the hydrophilic nature, these nanomaterials can endow the membranes with greater surface hydrophilicity, therefore, effectively enhancing the membrane performance. Moreover, the hydrophilic nanomaterials also provide the additional preferential water pathways, which improve the water permeability of the membranes. , …”

“…Moreover, the hydrophilic nanomaterials also provide the additional preferential water pathways, which improve the water permeability of the membranes. 21,23 Nanovoids are found to account for 15−32% of the volume within the polyamide selective layer, 24 and they can significantly improve the separation performance of TFC and TFN membranes especially for water permeability. This phenomenon may open up a new strategy to enhance the performance of the polyamide selective layer by introducing more nanovoids in it rather than the incorporation of nanomaterials.…”

Nanovoid Membranes Embedded with Hollow Zwitterionic Nanocapsules for a Superior Desalination Performance

“…For example, aquaporins (AQPs) had been incorporated into biomimetic membranes to facilitate water permeation using nanovoid water channels in the desalination processes . Interphase boundary nanovoids had also been built into mixed-matrix NF membranes via novel nanofillers with various structures and characteristics , including carbon-based nanoparticles (NPs), − metal organic frame (MOF) materials, − and metal oxide nanoparticles. , However, the nanovoids from the boundary spaces were difficult to be tuned in size and shape for an improved desalination performance. For example, Lee et al reported that the addition of graphene oxide (GO) tended to decrease the mechanical strength and ionic rejection of the PA NF membranes.…”

Nanoclays‐Incorporated Thin‐Film Nanocomposite Membranes for Reverse Osmosis Desalination

“…[ 5,6,20–22 ] Many nanoparticles, such as zeolites, [ 6,23–25 ] silica, [ 26,27 ] metal‐organic frameworks−covalent‐organic frameworks (MOFs−COFs), [ 28–30 ] carbon nanotubes, [ 31,32 ] and carbon−graphene oxide quantum dots, [ 33–35 ] have been incorporated into the polyamide matrix of TFN membranes to increase membrane hydrophilicity and provide additional water pathways. [ 36 ] Nevertheless, most of the nanoparticles used are produced in laboratory scale and the yield is relatively low, which is difficult to scale up for commercialization when taking economic viability into account.…”