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1. Printing ultrathin graphene oxide nanofiltration membranes for water purification

   https://doi.org/10.1039/c7ta06307e  

   Fathizadeh, Mahdi; Tien, Huynh Ngoc; Khivantsev, Konstantin; Chen, Jung-Tsai; Yu, Miao (January 2017, J. Mater. Chem. A)

   We demonstrated for the first time that inkjet printing can be a low-cost, easy, fast, and scalable method for depositing ultrathin (7.5–60 nm) uniform graphene oxide (GO) nanofiltration membranes on polymeric supports for highly effective water purification. A large area (15 × 15 cm 2 ) GO nanofiltration membrane was printed successfully on a modified polyacrylonitrile (M-PAN) support. Water permeance and rejection of small organic molecules (<1 nm, charged and uncharged) of printed GO membranes can be adjusted by controlling the GO “ink” concentration and/or printing time. Compared with commercial polymeric nanofiltration membranes, printed GO membranes, after optimization, showed approximately one order of magnitude higher water permeance and much higher rejection (>95%) of small organic molecules. Printed GO membranes also showed excellent performance in removing pharmaceutical contaminants, with ∼95% rejection and <10% water permeance decline over extended-period permeation testing. We believe that inkjet printing could be an effective method for preparing ultrathin GO membranes for effective water nanofiltration purification. 

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2. Transport properties of graphene oxide nanofiltration membranes: Electrokinetic modeling and experimental validation

   https://doi.org/10.1002/aic.17865  

   Wang, Zhongzhen; Xu, Chunyan; Fu, Qiang; Nair, Sankar (August 2022, AIChE Journal)

   Abstract There is a need for developing reliable models for water and solute transport in graphene oxide (GO) membranes for advancing their emerging industrial water processing applications. In this direction, we develop predictive transport models for GO and reduced‐GO (rGO) membranes over a wide solute concentration range (0.01–0.5 M) and compositions, based on the extended Nernst–Planck transport equations, Donnan equilibrium condition, and solute adsorption models. Some model parameters are obtained by fitting experimental permeation data for water and unary (single‐component) aqueous solutions. The model is validated by predicting experimental permeation behavior in binary solutions, which display very different characteristics. Sensitivity analysis of salt rejections as a function of membrane design parameters (pore size and membrane charge density) allows us to infer design targets to achieve high salt rejections. Such models will be useful in accelerating structure‐separation property relationships of GO membranes and for separation process design and optimization. 

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3. 2D graphene oxide channel for water transport

   https://doi.org/10.1039/c8fd00026c  

   Mi, Baoxia; Zheng, Sunxiang; Tu, Qingsong (January 2018, Faraday Discussions)

   Layer-stacked graphene oxide (GO) membranes, in which unique two-dimensional (2D) water channels are formed between two neighboring GO nanosheets, have demonstrated great potential for aqueous phase separation. Subjects of crucial importance are to fundamentally understand the interlayer spacing ( i.e. channel height) of GO membranes in an aqueous environment, elucidate the mechanisms for water transport within such 2D channels, and precisely control the interlayer spacing to tune the membrane separation capability for targeted applications. In this investigation, we used an integrated quartz crystal mass balance (QCM-D) and ellipsometry to experimentally monitor the interlayer spacing of GO, reduced GO and crosslinked GO in aqueous solution and found that crosslinking can effectively prevent GO from swelling and precisely control the interlayer spacing. We then used molecular dynamics simulations to study the mass transport inside the 2D channels and proved that the chemical functional groups on the GO plane dramatically slow down water transport in the channels. Our findings on GO structure and water transport provide a necessary basis for further tailoring and optimizing the design and fabrication of GO membranes in various separation applications. 

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4. Emulsion Assembly of Graphene Oxide/Polymer Composite Membranes

   https://doi.org/10.1021/acsami.3c02636  

   Ngo, Quynh P; Yuan, Weize; Wu, You-Chi Mason; Swager, Timothy M (May 2023, ACS Applied Materials & Interfaces)

   Graphene oxide/polymer composite water filtration membranes were developed via coalescence of graphene oxide (GO) stabilized Pickering emulsions around a porosity-generating polymer. Triptycene poly(ether ether sulfone)-CH2NH2:HCl polymer interacts with the GO at the water−oil interface, resulting in stable Pickering emulsions. When they are deposited and dried on polytetrafluoroethylene substrate, the emulsions fuse to form a continuous GO/polymer composite membrane. X-ray diffraction and scanning electron microscopy demonstrate that the intersheet spacing and thickness of the membranes increased with increasing polymer concentration, confirming the polymer as the spacer between the GO sheets. The water filtration capability of the composite membranes was tested by removing Rose Bengal from water, mimicking separations of weak black liquor waste. The composite membrane achieved 65% rejection and 2500 g m−2 h−1 bar−1. With high polymer and GO loading, composite membranes give superior rejection and permeance performance when compared with a GO membrane. This methodology for fabrication membranes via GO/polymer Pickering emulsions produces membranes with a homogeneous morphology and robust chemical separation strength. 

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5. Interactions of graphene oxide with the microbial community of biologically active filters from a water treatment plant

   https://doi.org/10.1016/j.watres.2024.122155  

   Ahamed, Tanvir; Li, Chao; Li, Mengyan; Axe, Lisa (October 2024, Water Research)

   With widespread occurrence and increasing concern of emerging contaminants (CECs) in source water, biologically active filters (BAF) have been gaining acceptance in water treatment. Both BAFs and graphene oxide (GO) have been shown to be effective in treating CECs. However, studies to date have not addressed interactions between GO and microbial communities in water treatment processes such as BAFs. Therefore, in the present study, we investigated the effect of GO on the properties and microbial growth rate in a BAF system. Synthesized GO was characterized with a number of tools, including scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectrometry. GO exhibited the characteristic surface functional groups (i.e., C-OH, C=O, C-O-C, and COOH), crystalline structure, and sheet-like morphology. To address the potential toxicity of GO on the microbial community, reactive oxygen species (ROS) generation was measured using nitro blue tetrazolium (NBT) assay. Results revealed that during the exponential growth phase, ROS generation was not observed in the presence of GO compared to the control batch. In fact, the adenosine triphosphate (ATP) concentrations increased in the presence of GO (25 μg/L - 1000 μg/L) compared to the control without GO. The growth rate in systems with GO exceeded the control by 20 % to 46 %. SEM images showed that GO sheets can form an effective scaffold to promote bacterial adhesion, proliferation, and biofilm formation, demonstrating its biocompatibility. Next-generation sequencing (Illumina MiSeq) was used to characterize the BAF microbial community, and high-throughput sequencing analysis confirmed the greater richness and more diverse microbial communities compared to systems without GO. This study is the first to report the effect of GO on the microbial community of BAF from a water treatment plant, which provides new insights into the potential of utilizing a bio-optimized BAF for advanced and sustainable water treatment or reuse strategies. 

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