Structurally Stable, Antifouling, and Easily Renewable Reduced Graphene Oxide Membrane with a Carbon Nanotube Protective Layer

Membrane modification with carbon nanomaterials for fouling mitigation: A review

This paper provides a comprehensive overview of recent advancements in membrane modification for fouling mitigation in various water treatment processes, employing carbon nanomaterials such as fullerenes, nanodiamonds, carbon quantum dots, carbon nanotubes, and graphene oxide. Currently, using different carbon nanomaterials for polymeric membrane fouling mitigation is at various stages: CNT-modified membranes have been studied for more than ten years and have already been tested in pilot-scale setups; tremendous attention has been paid to utilizing graphene oxide as a modifying agent, while the research on carbon quantum dots' influence on the membrane antifouling properties is in the early stages. Given the intricate nature of fouling as a colloidal phenomenon, the review initially delves into the factors influencing the fouling process and explores strategies to address it. The diverse chemistry and antibacterial properties of carbon nanomaterials make them valuable for mitigating scaling, colloidal, and biofouling. This review covers surface modification of existing membranes using different carbon materials, which can be implemented as a post-treatment procedure during membrane fabrication. Creating mixed-matrix membranes by incorporating carbon nanomaterials into the polymer matrix requires the development of new synthetic procedures. Additionally, it discusses promising strategies to actively suppress fouling through external influences on modified membranes. In the concluding section, the review compares the effectiveness of carbon materials of varying dimensions and identifies key characteristics influencing the antifouling properties of membranes modified with carbon nanomaterials.

Electrochemical Opening of Impermeable Nanochannels in Laminar Graphene Membranes for Ultrafast Nanofiltration

Reduced graphene oxide (rGO) could be theoretically used to construct highly permeable laminar membranes with nearly frictionless nanochannels for water treatment. However, their pristine (sp² C-C) regions usually restack into impermeable channels as a result of van der Waals interactions, resulting in a much low permeance. In this study, we demonstrate that the restacked regions could be electrochemically expanded to form ultrafast water transport nanochannels by providing a low positive potential (e.g., +1.00 V vs SCE) to the rGO membrane. Experimental investigations indicate that the structural expansion is attributed to the intercalation of water molecules into the restacked regions, driven by hydrogen bond interactions between water molecules and hydroxyl groups that are electrochemically produced on edges of rGO nanosheets. The structural expansion could be promoted by weakening the graphene-OH^- interactions through intermittent application of the potential. As a result of more ultrafast water transport nanochannels available, the electrochemically treated rGO membranes could have a permeance 2 orders of magnitude higher than that of the pristine one and ∼3 times higher than that of graphene oxide membranes. Because of their smaller average pore size, the rGO membranes also have a higher ionic/molecular rejection performance than graphene oxide membranes.

Partially Oxidized Carbon Nanomaterials with Ni/NiO Heterostructures as Durable Glucose Sensors

Conventional enzyme-based glucose biosensors have limited extensive applications in daily life because glucose oxidase is easily inactivated and is expensive. In this paper, we propose a strategy to prepare a new type of cost-effective, efficient, and robust nonenzymatic Ni-CNT-O for electrochemical glucose sensing. It is first followed by the pyrolysis of Ni-ABDC nanostrips using melamine to grow carbon nanotubes (CNTs) to give an intermediate product of Ni-CNT, which is further accompanied by partial oxidation to enable the facile formation of hierarchical carbon nanomaterials with improved hydrophilicity. A series of physicochemical characterizations have fully proved that Ni-CNT-O is a carbon-coated heterostructure of Ni and NiO nanoparticles embedded into coordination polymer-derived porous carbons. The obtained Ni-CNT-O exhibits a better electrocatalytic activity for glucose oxidation stemming from the synergistic effect of a metal element and a metal oxide than unoxidized Ni-CNT, which also shows high performance with a wide linear range from 1 to 3000 μM. It also offers a high sensitivity of 79.4 μA mM^-1 cm^-2, a low detection limit of 500 nM (S/N = 3), and a satisfactory long-term durability. Finally, this glucose sensor exhibits good reproducibility, high selectivity, as well as satisfactory results by comparing the current response of simulated serum within egg albumen.

Improving the Performance of the Lamellar Reduced Graphene Oxide/Molybdenum Sulfide Nanofiltration Membrane through Accelerated Water-Transport Channels and Capacitively Enhanced Charge Density

Graphene is promising in the construction of next-generation nanofiltration membranes for wastewater treatment and water purification. However, the application of graphene-based membranes has still been prohibited by their deficiencies in permeability and ion rejection. Herein, regulating the 2D channel and enhancing the charge density are co-adopted for simultaneous enhancement of the water flux and salt rejection of reduced graphene oxide (rGO) membranes through the intercalation of molybdenum sulfide (MoS₂) nanosheets and external electrical assistance. The fabricated rGO/MoS₂ membranes possess expanded nanochannels with less friction and a higher water molecule transport velocity gradient (from 8.57 to 14.07 s^-1) than those of rGO membranes. Consequently, their water permeance increases from 0.92 to 34.9 L m^-2 h^-1 bar^-1. Meanwhile, benefiting from the high capacitance and negative potential of -1.1 V versus the saturated calomel electrode given to the membranes, their rejection rates toward NaCl reach 87.2% and those toward Na₂SO₄ reach 93.7%. The Donnan steric pore model analysis indicates that the capacitively and electrically increased surface charge density make great contributions to the higher ion rejection rate. This work gives new insights into membrane design for high water flux and salt rejection efficiency.

Highly Permeable Sulfonated Graphene-Based Composite Membranes for Electrochemically Enhanced Nanofiltration

A sulfophenyl-functionalized reduced graphene oxide (SrGO) membrane is prepared. The SrGO membranes have a high charge density in water and could provide many atomically smooth nanochannels, because of their strong ionized-SO3H groups and low oxygen content. Therefore, the SrGO membranes have an excellent performance in terms of high permeance and high rejection ability. The permeance of SrGO membranes could be up to 118.2 L m−2 h−1 bar−1, which is 7.6 times higher than that of GO membrane (15.5 L m−2 h−1 bar−1). Benefiting from their good electrical conductivity, the SrGO membranes could also function as an electrode and demonstrate a significantly increased rejection toward negatively charged molecules and positively charged heavy metal ions such as Cu2+, Cr3+ and Cd2+, if given an appropriate negative potential. The rejection ratios of these metal ions can be increased from <20% at 0 V to >99% at 2.0 V. This is attributed to the enhanced electrostatic repulsion between the SrGO membrane and the like-charged molecules, and the increased electrostatic adsorption and electrochemical reduction in these heavy metal ions on the membranes. This study is expected to contribute to efficient water treatment and the advance of graphene-based membranes.

"Attacking-Attacking" Anti-biofouling Strategy Enabled by Cellulose Nanocrystals-Silver Materials

The development of high-performance anti-biofouling surfaces is paramount for controlling bacterial attachment and biofilm growth in biomedical devices, food packing, and filtration membranes. Cellulose nanocrystals (CNCs), a carbon-nanotube-like nanomaterial, have emerged as renewable and sustainable antimicrobial agents. However, CNCs inactivate bacteria under contact-mediated mechanisms, limiting its antimicrobial property mostly to the attached bacteria. This study describes the combination of CNCs with silver nanoparticles (CNC/Ag) as a strategy to increase their toxicity and anti-biofouling performance. CNC/Ag-coated surfaces inactivated over 99% of the attached Escherichia coli and Bacillus subtilis cells compared to 66.9 and 32.9% reduction shown by the pristine CNC, respectively. CNC/Ag was also very toxic to planktonic cells, displaying minimal inhibitory of 25 and 100 μg/mL against B. subtilis and E. coli, respectively. CNC/Ag seems to inactivate bacteria through an "attacking-attacking" mechanism where CNCs and silver nanoparticles play different roles. CNCs can kill bacteria by piercing the cell membrane. This physical membrane stress-mediated mechanism is demonstrated as lipid vesicles release their encapsulated dye upon contact with CNCs. Once the cell membrane is punctured, silver ions can enter the cell passively and compromise the integrity of DNA and other organelles. Inside the cells, Ag⁺ may damage the cell membrane by selectively interacting with sulfur and nitrogen groups of enzymes and proteins or by harming DNA via accumulation of reactive oxygen species. Therefore, CNC/Ag toxicity seems to combine the puncturing effect of the needle-like CNC and the silver's ability to impair the cell membrane and DNA functionalities.

Confining peroxymonosulfate activation in carbon nanotube intercalated nitrogen doped reduced graphene oxide membrane for enhanced water treatment: The role of nanoconfinement effect

Coupling membrane filtration with peroxymonosulfate (PMS) activation is promising to overcome the selectivity-permeability trade-off in membrane-based water treatment. However, the PMS catalytic efficiency of membrane still needs improvement to offset the insufficient reaction time during filtration process. Herein, an oxidized carbon nanotube intercalated nitrogen doped reduced graphene oxide (NRGO-OCNT) membrane with PMS activation function was firstly designed and prepared, which confined PMS activation in membrane interlayer for enhanced water treatment. The influence of confinement scale on membrane performance was studied through changing the OCNT intercalation ratio. Under the optimal confinement condition, the NRGO-OCNT membrane filtration integrated with PMS activation (MFPA) could realize 100% 4-chlorophenol removal at a high permeate flux of 290.2 L m^-2 h^-1 bar^-1 (retention time of only 0.36 s), whose performance was 2.8, 1.7 and 5.0 times higher than that of filtration alone, NRGO MFPA (excessive confinement) and NRGO-OCNT powder-based batch reaction (no confinement), respectively. Moreover, NRGO-OCNT MFPA preferentially removed smaller-sized organics which easily entered and diffused in confined interlayer. The outstanding performance of NRGO-OCNT MFPA was owing to the nanoconfinement effect in appropriate confined interspacing, where the mass transfer rate of reactants was greatly boosted for enhanced generation of SO₄^- and OH towards pollutant.