Scalable concentration process of graphene oxide dispersions via cross-flow membrane filtration

Emerging advances of composite membranes for seawater pre-treatment: a review

As the population continues to grow, the preservation of the world's water resources is becoming a serious challenge. The seawater desalination process is considered a sustainable option for the future. The two most common technologies used in desalination are reverse osmosis (RO) and membrane distillation (MD). However, membrane fouling caused by the accumulation of contaminants on the membrane surface is an emerging and growing problem. A pre-treatment stage is required to reach optimal efficiency during the desalination process since this stage is crucial for a successful desalination process. In this regard, the development of new material-based composite membranes has the potential to upgrade the anti-fouling features of RO membranes thereby enhancing desalination efficiency due to their high permeability, hydrophilicity, selectivity mechanical strength, thermal stability, and anti-bacterial properties. The objective of this review is to present various techniques for seawater pre-treatment. The results of the use of several membrane types and methods of modification have also been discussed. The performance of composite membranes for seawater pre-treatment is defined and the future perspectives have been highlighted.

Centrifuge-Free Separation of Solution-Exfoliated 2D Nanosheets via Cross-Flow Filtration

Solution-processed graphene is a promising material for numerous high-volume applications including structural composites, batteries, sensors, and printed electronics. However, the polydisperse nature of graphene dispersions following liquid-phase exfoliation poses major manufacturing challenges, as incompletely exfoliated graphite flakes must be removed to achieve optimal properties and downstream performance. Incumbent separation schemes rely on centrifugation, which is highly energy-intensive and limits scalable manufacturing. Here, cross-flow filtration (CFF) is introduced as a centrifuge-free processing method that improves the throughput of graphene separation by two orders of magnitude. By tuning membrane pore sizes between microfiltration and ultrafiltration length scales, CFF can also be used for efficient recovery of solvents and stabilizing polymers. In this manner, life cycle assessment and techno-economic analysis reveal that CFF reduces greenhouse gas emissions, fossil energy usage, water consumption, and specific production costs of graphene manufacturing by 57%, 56%, 63%, and 72%, respectively. To confirm that CFF produces electronic-grade graphene, CFF-processed graphene nanosheets are formulated into printable inks, leading to state-of-the-art thin-film conductivities exceeding 10⁴ S m^-1 . This CFF methodology can likely be generalized to other van der Waals layered solids, thus enabling sustainable manufacturing of the diverse set of applications currently being pursued for 2D materials.

Active Oxygen Functional Group Modification and the Combined Interface Engineering Strategy for Efficient Hydrogen Peroxide Electrosynthesis

Cathodic catalytic activity and interfacial mass transfer are key factors for efficiently generating hydrogen peroxide (H₂O₂) via a two-electron oxygen reduction reaction (ORR). In this work, a carbonized carboxymethyl cellulose (CMC)-reduced graphene oxide (rGO) synthetic fabric cathode was designed and constructed to improve two-electron ORR activity and interfacial mass transfer. Carbonized CMC exhibits abundant active carboxyl groups and excellent two-electron ORR activity with an H₂O₂ selectivity of approximately 87%, higher than that of rGO and other commonly used carbonaceous catalysts. Carbonizing CMC and the agglomerates formed from it restrain the restacking of rGO sheets and thus create abundant meso/macroporous channels for the interfacial mass transfer of oxygen and H₂O₂. Thus, the as-constructed carbonized CMC-rGO synthetic fabric cathode exhibits exceptional H₂O₂ electrosynthesis performance with 11.94 mg·h^-1·cm^-2 yield and 82.32% current efficiency. The sufficient active sites and mass-transfer channels of the cathode also ensure its practical application performance at high current densities, which is further illustrated by the rapid organic pollutant degradation via the H₂O₂-based electro-Fenton process.

Kenics Static Mixer Combined with Gas Sparging for the Improvement of Cross-Flow Microfiltration: Modeling and Optimization

The use of membrane filtration as a downstream process for microbial biomass harvesting is hampered due to the low permeate flux values achieved during the microfiltration of fermentation broths. Several hydrodynamic methods for increasing permeate flux by creating turbulent flow patterns inside the membrane module are used to overcome this problem. The main goal of this study was to investigate the combined use of a Kenics static mixer and gas sparging during cross-flow microfiltration of Bacillus velezensis IP22 cultivation broth. Optimization of the microfiltration process was performed by using the response surface methodology. It was found that the combined use of a static mixer and gas sparging leads to a considerable increase in the permeate flux, up to the optimum steady-state permeate flux value of 183.42 L·m−2·h−1 and specific energy consumption of 0.844 kW·h·m−3. The optimum steady-state permeate flux is almost four times higher, whilst, at the same time, the specific energy consumption is almost three times lower compared to the optimum results achieved using gas sparging alone. The combination of Kenics static mixer and gas sparging during cross-flow microfiltration is a promising technique for the enhancement of steady-state permeate flux with simultaneously decreasing specific energy consumption.

Improving stability and separation performance of graphene oxide/graphene nanofiltration membranes by adjusting the laminated regularity of stacking-sheets

The laminated graphene oxide (GO) membranes are promising alternatives in the field of nanofiltration due to their unique stacked interlayer structure and controllable molecular transport channels. However, it is still challenging to obtain satisfactory physical stability and separation performance to meet its practical application. In this study, a novel GO/Gr (graphene) nanofiltration membrane with high stability was engineered by post-hot-pressure treatment, following forward pressure filtration. The impact of GO/Gr loading ratio of the composites nanofiltration membranes for the permeability, selectivity, hydrophilicity and physical stability was investigated. The GO/Gr nanofiltration membranes exhibited high stability and separation performance because of the enhanced regularity and smoothness of the overall stacking layers. It was demonstrated that the satisfactory permeability (12.8-20 L·m^-2·h^-1) of GO/Gr nanofiltration membranes could be achieved. Compared with the pure GO membranes, GO/Gr-0.5 membranes exhibited a higher Na₂SO₄, NaCl, MgCl₂, and MgSO₄ rejection rate of approximately 78.3%, 51.2%, 34.5% and 32.6%, respectively. Meanwhile, the rejection rate (99.5%, 99.9%, 97.3% and 98.6%) of composite membranes for Methylene blue, Congo red, Rhodamine B and Methyl orange could be achieved. This facile way reveals the potential of stacked GO/Gr membranes in developing GO-based nanofiltration membranes.

Optimization of Spray-Drying Process with Response Surface Methodology (RSM) for Preparing High Quality Graphene Oxide Slurry

The “Drying-redissolution” method is promising for the industrial production of high-concentration well-dispersed graphene oxide slurry (GOS). As the potential key step in this method, the spray drying process requires a statistical investigation which guides the large-scale preparation of graphene oxide powder (GOP). This work systematically studies the effects of operating parameters, including nozzle airflow rate (439–895 L·h−1), atomization pressure (0.5–0.7 MPa), and liquid feed rate (3.0–9.0 mL·min−1), by using the response surface methodology integrated Box–Behnken design (RSM–BBD), aiming to produce GOP with high yield and easy re-dispersion. The optimized spray drying condition is predicted to be 439 L·h−1, 0.59 MPa, and 9.0 mL·min−1, at which a powder yield of 70.45% can be achieved. The experimentally obtained GOP has an average particle size of 11.65 μm and the low crumpling degree of the particle morphology results in the good re-dispersibility (97.95%) and excellent adsorption performance (244.1 mg·g−1) of GOP. The GOS prepared by the spray-dried GOP possess low viscosity and high exfoliation efficiency with a single-layer fraction up to 90.8%, exhibiting good prospects for application. This work first applied the RSM–BBD model on the spray drying process of GO, and evidenced the possibility of producing high-quality GO slurry with the “drying-redissolution” method.

High-Strength GO/PA66 Nanocomposite Fibers via In Situ Precipitation and Polymerization

The uniform dispersion of graphene oxide (GO) and strong interfacial bonding are the key factors in achieving the high mechanical strength of GO/polymer composites. It is still challenging to prepare GO/PA66 composites with uniform GO dispersion by the in situ polymerization method. In this paper, we prepare GO/PA66 salt nanocomposite by in situ precipitating PA66 salt with GO in ethanol. The GO/PA66 nanocomposite fibers are then fabricated using the as-prepared GO/PA66 salt by in situ polymerizing and melt spinning. By tuning the GO content, the tensile strength and Young's modulus of the GO/PA66 fibers are increased from 265 ± 18 to 710 ± 14 MPa (containing 0.3 wt% GO) and from 1.1 ± 0.08 to 3.8 ± 0.19 GPa (containing 0.5 wt% GO), respectively. The remarkable improvements are attributed to the uniform dispersion of GO in the GO/PA66 salt nanocomposite via ionic bonding and hydrogen bonding in the in situ precipitation process, and the covalent interfacial bonding between the GO and PA66 during the in situ polymerization process. This work sheds light on the easy fabrication of high-performance PA66-based nanocomposites.

A Review on Reverse Osmosis and Nanofiltration Membranes for Water Purification

Sustainable and affordable supply of clean, safe, and adequate water is one of the most challenging issues facing the world. Membrane separation technology is one of the most cost-effective and widely applied technologies for water purification. Polymeric membranes such as cellulose-based (CA) membranes and thin-film composite (TFC) membranes have dominated the industry since 1980. Although further development of polymeric membranes for better performance is laborious, the research findings and sustained progress in inorganic membrane development have grown fast and solve some remaining problems. In addition to conventional ceramic metal oxide membranes, membranes prepared by graphene oxide (GO), carbon nanotubes (CNTs), and mixed matrix materials (MMMs) have attracted enormous attention due to their desirable properties such as tunable pore structure, excellent chemical, mechanical, and thermal tolerance, good salt rejection and/or high water permeability. This review provides insight into synthesis approaches and structural properties of recent reverse osmosis (RO) and nanofiltration (NF) membranes which are used to retain dissolved species such as heavy metals, electrolytes, and inorganic salts in various aqueous solutions. A specific focus has been placed on introducing and comparing water purification performance of different classes of polymeric and ceramic membranes in related water treatment industries. Furthermore, the development challenges and research opportunities of organic and inorganic membranes are discussed and the further perspectives are analyzed.