Organic Fouling of Graphene Oxide Membranes and Its Implications for Membrane Fouling Control in Engineered Osmosis

Application of high-dose UV irradiation as nanofiltration pretreatment for drinking water production: Organic fouling mitigation and micropollutant removal

Nanofiltration (NF) is being increasingly applied to produce high-quality drinking water; however, its cost-effective operation remains challenging due to the perennial membrane fouling. On account of the low tolerance of common NF membranes to chemical oxidants, this study proposed high-dose UV irradiation as a pretreatment strategy for organic fouling mitigation. Results showed that the permeate flux decline of the membrane with UV-treated feedwater (with a dose of 750 mJ cm-2) was less drastic than that with raw feedwater, but slightly faster as compared to that with UV/Cl2 pretreatment. The final normalized fluxes were 0.69, 0.79, and 0.82, respectively, after 10 h of operation with raw, UV- and UV/Cl2-treated feedwaters. With the characterization of feedwaters and membranes, the fouling was found to be initiated by the adsorption of hydrophilic biopolymers onto the membrane, followed by the deposition of hydrophobic humic substances. Reduction of the "glue" biopolymers was crucial to membrane fouling mitigation. The applicability of UV pretreatment in practice was testified with a pilot-scale UV-NF system where permeate flux of the NF module decreased by 37% after six-week continuous operation. Moreover, UV pretreatment could remove most of the identified pesticides in the feedwater with a removal efficiency over 80% for metolachlor and imidacloprid, but had no or even a negative effect on perfluorinated compounds. This work discloses the efficacy and mechanism of high-dose UV irradiation for NF membrane fouling control, which facilitates future research and application of NF technology.

Metal cation crosslinked, partially reduced graphene oxide membranes with enhanced stability for high salinity, produced water treatment by pervaporative separation

Graphene oxide (GO)-based membranes hold significant promise for applications ranging from energy storage to protective coatings, to saline water and produced water treatment, owing to their chemical stability and unique barrier properties achieving a high selectivity for water permeation. However, unmodified GO membranes are not stable when submerged in liquid water, creating challenges with their commercial utilization in aqueous filtration and pervaporation applications. To mitigate this, we develop an approach to modify GO membranes through a combination of low temperature thermal reduction and metal cation crosslinking. We demonstrate that Zn2+-rGO and Fe3+-rGO membranes had the highest permeation flux of 8.3 ± 1.5 l m-2h-1and 7.0 ± 0.4 l m-2h-1, for saline water separation, respectively, when thermally reduced after metal cross-linking; These membranes maintained a high flux of 7.5 ± 0.7 l m-2h-1, and 5.5 ± 0.3 l m-2h-1for produced water separation, respectively. All the membranes had a salt rejection higher than 99%. Fe3+crosslinked membranes presented the highest organic solute rejections for produced water of 69%. Moreover, long term pervaporation testing was done for the Zn2+-rGO membrane for 12 h, and only a minor drop of 6% in permeation flux was observed, while Zn2+-GO had a drop of 24%. Both modifiers significantly enhanced the stability with Fe3+-rGO membranes displaying the highest mechanical abrasion resistance of 95% compared to non-reduced and non-crosslinked GO. Improved stability for all samples also led to higher selectivity to water over organic contaminants and only slightly reduced water flux across the membrane.

Polyrhodanine-based nanomaterials for biomedical applications: A review

Rhodanine is a heterocyclic organic compound that has been investigated for its potential biomedical applications, particularly in drug discovery. Rhodanine derivatives have been examined as the medication options for numerous illnesses, including cancer, inflammation, and infectious diseases. Some rhodanine derivatives have also shown promising activity against drug-resistant strains of bacteria and viruses. One of these derivatives is polyrhodanine (PR), a conducting polymer that has gained attention for its biomedical properties. This review article summarises the latest advancements in creating biomaterials based on PR for biosensing, antimicrobial treatments, and anticancer therapies. The distinctive characteristics of PR, such as biocompatibility, biodegradability, and good conductivity, render it an attractive candidate for these applications. The article also explores obstacles and potential future paths for advancing biomaterials made with PR, including synthesis modifications, characterisation techniques, and in vivo evaluation of biocompatibility and efficacy. Overall, as an emerging research topic, this review emphasises the potential of PR as a promising biomaterial for various biomedical applications and provides insights into the contemporary state of research and prospective directions for investigation.

Fabrication of polydopamine/rGO Membranes for Effective Radionuclide Removal

In this work, a novel polydopamine/reduced graphene oxide (PDA/rGO) nanofiltration membrane was prepared to efficiently and stably remove radioactive strontium ions under an alkaline environment. Through the incorporation of PDA and thermal reduction treatment, not only has the interlayer spacing of graphene oxide (GO) nanosheets been appropriately regulated but also an improved antiswelling property has been achieved. The dosage of GO, reaction time with PDA, mass ratio of PDA to GO, and thermal treatment temperature have been optimized to achieve a high-performance PDA/rGO membrane. The resultant PDA/rGO composite membrane has exhibited excellent long-term stability at pH 11 and maintains a steady strontium rejection of over 90%. Moreover, the separation mechanism of the PDA/rGO membrane has been systematically investigated and determined to be a synergistic effect of charge repulsion and size exclusion. Results have indicated that PDA/rGO could be considered as a promising candidate for the separation of Sr2+ ions from nuclear industry wastewater.

A critical review on graphene oxide membrane for industrial wastewater treatment

An important way to promote the environmental industry's goal of carbon reduction is to promote the recycling of resources. Membrane separation technology has unique advantages in resource recovery and advanced treatment of industrial wastewater. However, the great promise of traditional organic membrane is hampered by challenges associated with organic solvent tolerance, lack of oxidation resistance, and serious membrane fouling control. Moreover, the high concentrations of organic matter and inorganic salts in the membrane filtration concentrate also hinder the wider application of the membrane separation technology. The emerging cost-effective graphene oxide (GO)-based membrane with excellent resistance to organic solvents and oxidants, more hydrophilicity, lower membrane fouling, better separation performance has been expected to contribute more in industrial wastewater treatment. Herein, we provide comprehensive insights into the preparation and characteristic of GO membranes, as well as current research status and problems related to its future application in industrial wastewater treatment. Finally, concluding remarks and future perspectives have been deduced and recommended for the GO membrane separation technology application for industrial wastewater treatment, which leads to realizing sustainable wastewater recycling and a nearly "zero discharge" water treatment process.

Membrane Life Cycle Management: An Exciting Opportunity for Advancing the Sustainability Features of Membrane Separations

Membrane science and technology is growing rapidly worldwide and continues to play an increasingly important role in diverse fields by offering high separation efficiency with low energy consumption. Membranes have also shown great promise for "green" separation. A majority of the investigations in the field are devoted to the membrane fabrication and modification with the ultimate goals of enhancing the properties and separation performance of membranes. However, less attention has been paid to membrane life cycle management, particularly at the end of service. This is becoming very important, especially taking into account the trends toward sustainable development and carbon neutrality. On the contrary, this can be a great opportunity considering the large variety of membrane processes, especially in terms of the size and capacity of plants in operation. This work aims to highlight the prominent aspects that govern membrane life cycle management with special attention to life cycle assessment (LCA). While fabrication, application, and recycling are the three key aspects of LCA, we focus here on membrane (module) recycling at the end of life by elucidating the relevant aspects, potential criteria, and strategies that effectively contribute to the achievement of green development and sustainability goals.

A membrane-based green and low-cost system for ensuring safe drinking water in a selenium-affected region

Towards an efficient, low-cost solution to the problem of contamination of groundwater by selenium leached out from earth's mineral crust, a new system is developed using a novel graphene-based nanocomposite membrane. The system not only purified selenium-contaminated groundwater with high degree but also ensured safe disposal of the rejected selenium through algorithmic chemical stabilization in a mineral matrix. All experiments were conducted with live contaminated water from selenium affected area rather than using synthetic solution in a semi-pilot unit involving a largely fouling-free flat sheet cross-flow membrane module. Pure water flux of up to 190 Lm^-2h^-1(LMH) with 96-97% selenium rejection at an optimum operating pressure of only 14 bar could be achieved. Rejected selenium was stabilized in mineral matrix through chemical coagulation-precipitation using suitable coagulants following prior optimization of the critical operating parameters by Model-based calibration toolbox (MATLAB R2020a). A high degree of stabilization efficiency (99.8%) could be achieved as reflected in an error-index of only 1.13%. For selenium-affected region, the membrane-integrated hybrid treatment system proved to be a potential candidate technology offering safe drinking water at an approximate cost of only 1.77 $/m³ which was found to be affordable to the consumers in subsequent willingness to pay survey.

Efficacy of Electrospun Nanofiber Membranes on Fouling Mitigation: A Review

Despite the advantages of high contaminant removal, operational flexibility, and technical advancements offered, the undesirable fouling property of membranes limits their durability, thus posing restrictions on their usage. An enormous struggle is underway to conquer this major challenge. Most of the earlier reviews include the basic concepts of fouling and antifouling, with respect to particular separation processes such as ultrafiltration, nanofiltration, reverse osmosis and membrane bioreactors, graphene-based membranes, zwitterionic membranes, and so on. As per our knowledge, the importance of nanofiber membranes in challenging the fouling process has not been included in any record to date. Nanofibers with the ability to be embedded in any medium with a high surface to volume ratio play a key role in mitigating the fouling of membranes, and it is important for these studies to be critically analyzed and reported. Our Review hence intends to focus on nanofiber membranes developed with enhanced antifouling and biofouling properties with a brief introduction on fabrication processes and surface and chemical modifications. A summary on surface modifications of preformed nanofibers is given along with different nanofiller combinations used and blend fabrication with efficacy in wastewater treatment and antifouling abilities. In addition, future prospects and advancements are discussed.

Next-generation graphene oxide additives composite membranes for emerging organic micropollutants removal: Separation, adsorption and degradation

In the past two decades, membrane technology has attracted considerable interest as a viable and promising method for water purification. Emerging organic micropollutants (EOMPs) in wastewater have trace, persistent, highly variable quantities and types, develop hazardous intermediates and are diffusible. These primary issues affect EOMPs polluted wastewater on an industrial scale differently than in a lab, challenging membranes-based EOMP removal. Graphene oxide (GO) promises state-of-the-art membrane synthesis technologies and use in EOMPs removal systems due to its superior physicochemical, mechanical, and electrical qualities and high oxygen content. This critical review highlights the recent advancements in the synthesis of next-generation GO membranes with diverse membrane substrates such as ceramic, polyethersulfone (PES), and polyvinylidene fluoride (PVDF). The EOMPs removal efficiencies of GO membranes in filtration, adsorption (incorporated with metal, nanomaterial in biodegradable polymer and biomimetic membranes), and degradation (in catalytic, photo-Fenton, photocatalytic and electrocatalytic membranes) and corresponding removal mechanisms of different EOMPs are also depicted. GO-assisted water treatment strategies were further assessed by various influencing factors, including applied water flow mode and membrane properties (e.g., permeability, hydrophily, mechanical stability, and fouling). GO additive membranes showed better permeability, hydrophilicity, high water flux, and fouling resistance than pristine membranes. Likewise, degradation combined with filtration is two times more effective than alone, while crossflow mode improves the photocatalytic degradation performance of the system. GO integration in polymer membranes enhances their stability, facilitates photocatalytic processes, and gravity-driven GO membranes enable filtration of pollutants at low pressure, making membrane filtration more inexpensive. However, simultaneous removal of multiple contaminants with contrasting characteristics and variable efficiencies in different systems demands further optimization in GO-mediated membranes. This review concludes with identifying future critical research directions to promote research for determining the GO-assisted OMPs removal membrane technology nexus and maximizing this technique for industrial application.

Color removal from wastewater using a synthetic high-performance antifouling GO-CPTMS@Pd-TKHPP/polyether sulfone nanofiltration membrane

Modified graphene oxide with 5,10,15,20-tetrakis-(4-hexyloxyphenyl)-porphyrin and palladium (II) (signified by GO-CPTMS@Pd-TKHPP) prepared as a novel antifouling polyether sulfone (PES) blended nanofiller membrane. The membrane efficiency has been analyzed such as pure water flux (PWF), hydrophilicity, and antifouling features. By increasing of modified graphene oxide percentage from 0 to 0.1 wt.% in the polymer matrix, the PWF was incremented from 14.35 to 37.33 kg/m2·h at 4 bar. The membrane flux recovery ratio (FRR) has been investigated by applying powdered milk solution; the FRR results indicated that the 0.1 wt.%-modified graphene oxide membrane showed a positive effect on fouling behavior with Rir and FRR value 8.24% and 91.76%, respectively. The nanofiltration membrane performance was assessed applying the Direct Red 16 dye rejection. It was demonstrated that the optimal membranes (0.1 wt.%-modified graphene oxide) had notable dye removal (99.58% rejection). The results are also verified by measuring the scanning electron microscopy (SEM), water contact angle (WCA), and atomic microscopy analysis (AFM).

Investigating the potential of ammonium retention by graphene oxide ceramic nanofiltration membranes for the treatment of semiconductor wastewater

Ceramic membranes with high chemical and fouling resistance can play a critical role in treating industrial wastewater. In the present study, we demonstrate the fabrication of graphene oxide (GO) assembled ceramic nanofiltration (NF) membranes that provide effective ammonium retention and excellent fouling resistance for treating semiconductor wastewater. The GO-ceramic NF membranes were prepared via a layer-by-layer (LbL) assembly of GO and polyethyleneimine (PEI) on a ceramic ultrafiltration (UF) substrate. The successful fabrication of the GO-ceramic NF membranes was verified through surface characterization and pore size evaluation. We also investigated the performance of GO-ceramic NF membranes assembled with different numbers of bilayers for the rejection of ammonium ions. GO-ceramic NF membranes with three GO-PEI bilayers exhibited 8.4- and 3.2-times higher ammonium removal with simulated and real semiconductor wastewater, respectively, compared to the pristine ceramic UF substrate. We also assessed flux recovery after filtration using real semiconductor wastewater samples to validate the lower fouling potential of the GO-ceramic NF membranes. Results indicate that flux recovery increases from 39.1 % in the pristine UF substrate to 71.0 % and 90.8 % for the three- and ten-bilayers GO-ceramic NF membranes, respectively. The low-fouling GO-ceramic NF membranes developed in this study are effective and promising options for the removal of ammonium ions from semiconductor wastewater.

Graphene Oxide Membranes for High Salinity, Produced Water Separation by Pervaporation

Oil and gas industries produce a huge amount of wastewater known as produced water which contains diverse contaminants including salts, dissolved organics, dispersed oils, and solids making separation and purification challenging. The chemical and thermal stability of graphene oxide (GO) membranes make them promising for use in membrane pervaporation, which may provide a more economical route to purifying this water for disposal or re-use compared to other membrane-based separation techniques. In this study, we investigate the performance and stability of GO membranes cast onto polyethersulfone (PES) supports in the separation of simulated produced water containing high salinity brackish water (30 g/L NaCl) contaminated with phenol, cresol, naphthenic acid, and an oil-in-water emulsion. The GO/PES membranes achieve water flux as high as 47.8 L m-2 h-1 for NaCl solutions for membranes operated at 60 °C, while being able to reject 99.9% of the salt and upwards of 56% of the soluble organic components. The flux for membranes tested in pure water, salt, and simulated produced water was found to decrease over 72 h of testing but only to 50-60% of the initial flux in the worst-case scenario. This drop was concurrent with an increase in contact angle and C/O ratio indicating that the GO may become partially reduced during the separation process. Additionally, a closer look at the membrane crosslinker (Zn2+) was investigated and found to hydrolyze over time to Zn(OH)2 with much of it being washed away during the long-term pervaporation.

Nanofluidic Membranes to Address the Challenges of Salinity Gradient Power Harvesting

Salinity gradient power (SGP) has been identified as a promising renewable energy source. Reverse electrodialysis (RED) and pressure retarded osmosis (PRO) are two membrane-based technologies for SGP harvesting. Developing nanopores and nanofluidic membranes with excellent water and/or ion transport properties for applications in those two membrane-based technologies is considered viable for improving power generation performance. Despite recent efforts to advance power generation by designing a variety of nanopores and nanofluidic membranes to enhance power density, the valid pathways toward large-scale power generation remain uncertain. In this review, we introduce the features of ion and water transport in nanofluidics that are potentially beneficial to power generation. Subsequently, we survey previous efforts on nanofluidic membrane synthesis to obtain high power density. We also discuss how the various membrane properties influence the power density in RED and PRO before moving on to other important aspects of the technologies, i.e., system energy efficiency and membrane fouling. We analyze the importance of system energy efficiency and illustrate how the delicately designed nanofluidic membranes can potentially enhance energy efficiency. Previous studies are reviewed on fabricating antifouling and antimicrobial membrane for power generation, and opportunities are presented that can lead to the design of nanofluidic membranes with superior antifouling properties using various materials. Finally, future research directions are presented on advancing membrane performance and scaling-up the system. We conclude this review by emphasizing the fact that SGP has the potential to become an important renewable energy source and that high-performance nanofluidic membranes can transform SGP harvesting from conceptual to large-scale applications.

Coupling of Immobilized Photosynthetic Bacteria with a Graphene Oxides/PSF Composite Membrane for Textile Wastewater Treatment: Biodegradation Performance and Membrane Anti-Fouling Behavior

The membrane bioreactor (MBR), as one of the promising technologies, has been widely applied for treatments of wastewater. However, serious membrane fouling and low microbial activity have been reported as major problems hindering the development of the MBR. To overcome these drawbacks, we intend to improve the MBR process in the view of membrane surface modification and efficient granular bacteria cultivation. In the present study, immobilized photosynthetic bacteria integration with graphene oxide (GO)/polysulfone (PSF) composite membrane separation (IPMBR) was first applied for textile wastewater treatment. Due to the high activity of immobilized cells, the IPMBR system exhibited higher efficiency on the removal of color, ammonia-nitrogen, and chemical oxygen demand than the conventional MBR system. In comparison with a pure PSF membrane, GO/PSF composite membrane presented the higher hydrophilicity (water contact angles of 62.9°) and more attractive permeability (178.5 L/m²h) by reducing the adhesion of hydrophobic foulants. During the whole operation, the immobilized photobioreactor exhibited approximately seven times higher membrane permeability that that of the conventional MBR. Meanwhile, the effect of the structure and character of immobilized photosynthetic bacteria on the membrane fouling reduction was investigated in detail. The change of extracellular polymeric substance concentration, settleability and particle size of flocs was very beneficial to alleviate membrane fouling. As a result, this research will open a new avenue for developing efficient and anti-fouling MBR technology in the future.

Combined effects of Pseudomonas quinolone signal-based quorum quenching and graphene oxide on the mitigation of biofouling and improvement of the application potential for the thin-film composite membrane

Biofouling caused by the growth of the biofilm is the main bottleneck that limits the effective operation of thin-film composite (TFC) membrane in the forward osmosis (FO) process. This study investigated the combined effects of graphene oxide (GO) immobilized thin-film nanocomposite (TFN-S) membrane and Pseudomonas quinolone signal (PQS)-based quorum quenching on biofouling mitigation, especially under the operation of pressure-retarded osmosis (PRO) mode, and the influence of methyl anthranilate (MA) inhibitor on the composition and structure of biofilm was also evaluated. Synthetic wastewater was used as the feed solution, in which the model strain Pseudomonas aeruginosa was added to simulate biofouling. The results showed that GO modification and MA addition both efficiently mitigated flux decline and EPS secretion, but the interference of PQS pathway on biofouling control was better than GO embedding. TFN-S membrane with MA addition exhibited superior anti-biofouling performance based on the combined effects of GO and MA. The alleviated concentration polarization and enhanced hydrophilicity of the TFN-S membrane reduced the flux decline in the early stage. Additionally, the antibacterial property of GO inhibited the viability of the attached bacteria (under PRO mode) and MA further mitigated the EPS secretion and biofilm development in the later stage. In the presence of PQS inhibitor MA, live/total cells ratio was 15% and 13% higher than that of TFC membrane in FO and PRO modes, respectively. Furthermore, exogenous addition of MA led to a relatively loose biofilm structure, resulting in high membrane permeability in the biofouling formation process.

Effect of an ionic environment on membrane fouling: a molecular dynamics study

The effect of the ionic environment on membrane fouling was investigated for polyamide (PA) and graphene oxide (GO) membranes using equilibrium molecular dynamics (MD) simulations. For each of these membranes, bovine serum albumin (BSA) was considered as the model foulant. The effect of the foulant on the membranes is investigated at seawater concentration and also in a normal aqueous environment. We investigated the translational and rotational motion of the protein relative to the membrane, interaction energy between the protein and the membrane surface, structural changes in the protein, and ion distribution around the protein and the membrane surface for all the systems. We found that the effects of ions were very different on both the membranes. Specifically, with an increase in ionic strength, the repulsion between the protein and membrane was observed in the case of GO, while for PA, no significant changes were observed for the same. Also, the ion distribution around the protein and the membrane surface were found to be different. In particular, for GO, there were more number of chloride ions around the protein and the membrane than that of sodium ions, which was probably the reason for the repulsion in the case of GO. However, in the case of PA, the membrane surface did not exhibit any affinity towards a specific ion, and the protein in the case of PA was surrounded by more number of sodium ions than chloride ions.

Influence of Multidimensional Graphene Oxide (GO) Sheets on Anti-Biofouling and Desalination Performance of Thin-Film Composite Membranes: Effects of GO Lateral Sizes and Oxidation Degree

The influence of the lateral size and the content of graphene oxide (GO) flakes in specific oxygenate functional groups on the anti-biofouling properties and performance of thin-film composite membrane (TFC) was studied. Three different multidimensional GO samples were prepared with small (500-1200 nm), medium (1200-2300 nm), and large (2300-3600 nm) size distribution, and with different degrees of oxidation (GO3 > GO2 > GO1), varying the concentration of the hydrogen peroxide amount during GO synthesis. GO1 sheets' length have a heterogeneous size distribution containing all size groups, whilst GO2 is contained in a medium-size group, and GO3 is totally contained within a small-size group. Moreover, GO oxygenate groups were controlled. GO2 and GO3 have hydroxyl and epoxy groups at the basal plane of their sheets. Meanwhile, GO1 presented only hydroxyl groups. GO sheets were incorporated into the polyamide (PA) layer of the TFC membrane during the interfacial polymerization reaction. The incorporation of GO1 produced a modified membrane with excellent bactericidal properties and anti-adhesion capacity, as well as superior desalination performance with high water flow (133% as compared with the unmodified membrane). For GO2 and GO3, despite the significant anti-biofouling effect, a detrimental impact on desalination performance was observed. The high content of large sheets in GO2 and small sheet stacking in GO3 produced an unfavorable impact on the water flow. Therefore, the synergistic effect due to the presence of large- and small-sized GO sheets and high content of OH-functional groups (GO1) made it possible to balance the performance of the membrane.

Zwitterionic Hydrogel-Impregnated Membranes with Polyamide Skin Achieving Superior Water/Salt Separation Properties

Support-free nonporous membranes have emerged as a new material platform for osmotic pressure-driven processes due to its insusceptibility to internal concentration polarization (ICP). Herein, we demonstrate high-performance membranes of zwitterionic hydrogels impregnated in porous membranes with a skin layer of highly cross-linked polyamides on both sides prepared by gel-liquid interfacial polymerization (GLIP). Such a configuration eliminates the pores and thus ICP, while the thin polyamide layer provides high salt rejection but negligible resistance to the water transport compared with the hydrogels. The polyamide skin layers are characterized using scanning electron microscopy and atomic force microscopy. The effect of the hydrogel compositions and polyamide formation conditions on the water/salt separation properties is thoroughly investigated. Example membranes show water permeance and salt rejection comparable to state-of-the-art commercial forward osmosis membranes and essentially no ICP.

Polyacrylonitrile Nanofiber Membrane Modified with Ag/GO Composite for Water Purification System

Silver nanoparticle-modified graphene oxide (Ag/GO) was reliably prepared by using sodium borohydride (NaBH₄) in the presence of citric acid capping agent via a simple wet chemistry method. This rapidly formed Ag/GO composite exhibited good dispersity in a solution containing hydrophilic polyacrylonitrile (PAN). Subsequent electrospinning of this precursor solution resulted in the successful formation of nanofibers without any notable defects. The Ag/GO-incorporated PAN nanofibers showed thinner fiber strands (544 ± 82 nm) compared to those of GO-PAN (688 ± 177 nm) and bare-PAN (656 ± 59 nm). Subsequent thermal treatment of nanofibers resulted in the preparation of thin membranes to possess the desired pore property and outstanding wettability. The Ag/GO-PAN nanofiber membrane also showed 30% higher water flux value (390 LMH) than that of bare-PAN (300 LMH) for possible microfiltration (MF) application. In addition, the resulting Ag/GO-PAN nanofiber membrane exhibited antibacterial activity against Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive). Furthermore, this composite membrane exhibited outstanding anti-fouling property compared to the GO-PAN nanofiber membrane in the wastewater treatment. Therefore, the simple modification strategy allows for the effective formation of Ag/GO composite as a filler that can be reliably incorporated into polymer nanofiber membranes to possess improved overall properties for wastewater treatment applications.

QCM-D assay for quantifying the swelling, biodegradation, and protein adsorption of intelligent nanogels

Environmentally responsive nanomaterials have been developed for drug delivery applications, in an effort to target and accumulate therapeutic agents at sites of disease. Within a biological system, these nanomaterials will experience diverse conditions which encompass a variety of solute identities and concentrations. In this study, we developed a new quartz crystal microbalance with dissipation (QCM-D) assay, which enabled the quantitative analysis of nanogel swelling, protein adsorption, and biodegradation in a single experiment. As a proof of concept, we employed this assay to characterize non-degradable and biodegradable poly(acrylamide-co-methacrylic acid) nanogels. We compared the QCM-D results to those obtained by dynamic light scattering to highlight the advantages and limitations of each method. We detailed our protocol development and practical recommendations, and hope that this study will serve as a guide for others to design application-specific QCM-D assays within the nanomedicine domain.

Study on the Transport Mechanism of a Freestanding Graphene Oxide Membrane for Forward Osmosis

Graphene oxide membranes (GOMs) are promising separation technologies. In forward osmosis (FO), we found that the water flux from the feed solution to the draw solution can prevent ions from diffusing to the feed solution in a highly tortuous and porous GOM. In reverse osmosis (RO), we found that the salt rejection is low compared to that in commercially available RO membranes. While this prohibits the use of GOMs for RO and FO water desalination, we believe that such membranes could be used for other water treatment applications and energy production. To examine the transport mechanism, we characterized the physical and chemical properties of GOMs and derived mass transfer models to analyze water and salt transport inside freestanding GOMs. The experimental reverse salt flux was between the largest and smallest theoretical values, which corresponds to the lowest and highest tortuosity, respectively, in FO. Furthermore, the concentration profile for the reverse salt flux shortened as the NaCl draw concentration increased because the water flux increased and the electrical double layer (EDL) decreased with increasing NaCl in the draw solution. We provide insights into the transport mechanisms in GOMs and provide guidance for future exploration of GOMs in efficient water treatment and energy production processes.

Two-Dimensional Ti3C2Tx MXene/GO Hybrid Membranes for Highly Efficient Osmotic Power Generation

Osmotic power has emerged as one of the promising candidates for clean and renewable energy. However, the advancement of present osmotic power-harvesting technologies, specifically pressure-retarded osmosis (PRO) in this work, is hindered by the unsatisfactory membrane transport properties. Herein, we demonstrate the freestanding transition-metal carbides and graphene oxide hybrid membranes as high-performance PRO membranes. Due to the elimination of internal concentration polarization, the freestanding hybrid membrane can achieve a record-high power density up to approximately 56.4 W m^-2 with 2.0 M NaCl as the draw solution and river water (0.017 M) as the feed water at an applied hydraulic pressure difference of 9.66 bar. In addition, the hybrid membranes exhibit enhanced antifouling potential and antibacterial activity. The facile fabrication of the hybrid membranes shed light on a new membrane development platform for the highly anticipated osmotic power-harvesting technologies.

Recent advances in mitigating membrane biofouling using carbon-based materials

Biofouling is the Achilles Heel of membrane processes. The accumulation of organic foulants and growth of microorganisms on the membrane surface reduce the permeability, shorten the membrane life, and increase the energy consumption. Advancements in novel carbon-based materials (CBMs) present significant opportunities in mitigating biofouling of membrane processes. This article provides a comprehensive review of the recent progress in the application of CBMs in antibiofouling membrane. It starts with a detailed summary of the different antibiofouling mechanisms of CBM-containing membrane systems. Next, developments in membrane modification using CBMs, especially carbon nanotubes and graphene family materials, are critically reviewed. Further, the antibiofouling potential of next-generation carbon-based membranes is surveyed. Finally, the current problems and future opportunities of applying CBMs for antibiofouling membranes are discussed.

Enhanced covalent p-phenylenediamine crosslinked graphene oxide membranes: Towards superior contaminant removal from wastewaters and improved membrane reusability

The increasing depletion of freshwater necessitates the re-use and purification of wastewaters. Among the existing separation membrane materials, graphene oxide (GO) is a promising candidate, owing to its tunable physicochemical properties. However, the widening of GO membranes pore gap in aqueous environments is a major limitation. Crosslinking agents can be incorporated to alleviate this problem. This study describes a comparative analysis of uncrosslinked and p-Phenylenediamine (PPD) crosslinked GO membranes' water purification performance. Dip-coating and dip-assisted layer-by-layer methods were used to fabricate the uncrosslinked and crosslinked membranes respectively. The covalent interaction between GO and PPD was confirmed by Fourier Transform Infra-Red and X-ray Photoelectron Spectroscopy. The excellent membrane topographical continuity and intactness was assessed by means of Scanning Electron Microscopy, while water contact angle measurements were undertaken to evaluate and confirm membrane hydrophilicity. The improvement impact of the crosslinker was manifested on the enhancement of the stability and performance of the membranes during nanofiltration tests of aqueous solutions of methylene blue in a homemade nanofiltration cell operated at 1 bar.

Membrane photo-bioreactor coupled with heterogeneous Fenton fluidized bed for high salinity wastewater treatment: Pollutant removal, photosynthetic bacteria harvest and membrane anti-fouling analysis

In this study, efficient photosynthetic bacteria (PSB)-GO/PVDF membrane photo-bioreactor (MPBR) combined with heterogeneous Fenton fluidized bed was built and successfully applied for treatment of actual refractory seafood-processing wastewater with extremely high salinity. As effective pre-treatment, heterogeneous Fenton was designed for removing non-biodegradable organics and reducing iron-sludge discharge. In MPBR, GO/PVDF membrane fabricated by chemical grafting GO nanosheets was first used for salt-tolerated PSB harvest. Compared with original PVDF membrane, GO/PVDF membrane exhibited enhanced hydrophilicity, better permeability (4.4 times) and attractive flux recover rate (94%), which was attributed to remarkable reduction in hydrophobic proteins amount of extracellular polymeric substances (EPS). Importantly, COD and NH₃-N removal efficiency of MPBR with GO/PVDF membrane were kept about 95 and 98%, respectively, and average biomass productivity reached as high as 105 mg/L·d. This study provides a promising and economical way to build efficient MBR combined with new materials for high salinity hazardous wastewater treatment.

Impacts of mono/divalent cations on the lamellar structure of cross-linked GO layers and membrane filtration performance for different DOM fractions

This study evaluated the effects of co-existing cations (Na⁺ or Ca²⁺) on the lamellar structure of cross-linked graphene oxide (GO) layers and GO modified membrane performance in terms of their fouling behaviours and retention for single-model organic matter, namely, bull serum albumin (BSA), sodium alginate (SA), humic acid (HA) and tannic acid (TA). In the absence of co-existing cations, the GO layers mitigated membrane fouling for large molecules (SA, BSA, and HA) but led to severer pore blocking for small molecules (TA) compared with pristine membrane. Na⁺ and Ca²⁺ altered the performance impacts of the GO modified membrane due to different interactions with the cross-linked GO layers. Low concentrations of Na⁺ (<0.4 mM) enlarged interlayer spacing of the GO layers and caused a decrease in flux after physical cleaning, but the GO layers maintained the uniform lamellar structure. High concentrations of Na⁺ (>0.4 mM) promoted the aggregation of cross-linked GO layers through charge shielding and reduced the uniformity of lamellar structure, which weakened the antifouling performance for large molecules and promoted the passage of small molecules through the membrane. However, Ca²⁺ complexed with GO sheets and reinforced the uniform lamellar structure of the GO layers, leading to a better antifouling performance for the filtration of large molecules than the pristine membrane but aggravated TA fouling.

Nanoscale zero-valent iron (nZVI) immobilization onto graphene oxide (GO)-incorporated electrospun polyvinylidene fluoride (PVDF) nanofiber membrane for groundwater remediation via gravity-driven membrane filtration

Nanoscale zero-valent iron (nZVI), with its high reactivity towards a broad range of contaminants, has been a promising material for groundwater remediation. Membrane-supported nZVI can both avoid nZVI agglomeration for better reactivity and recycle nZVI to lower the risk of secondary pollution. In this study, we successfully fabricated a PVDF-GO membrane via electrospinning technology and employed the functionalized nanofiber membrane to immobilize nZVI particles. The addition of GO into PVDF nanofibers can both increase the hydrophilicity to improve membrane flux and offer -COOH as a binder to immobilize nZVI particles. PVDF-GO-nZVI membranes with different GO loadings (0%, 0.5%, 1%, 3% of PVDF) were tested with two typical nZVI-targeted contaminants (Cd(II) and trichloroethylene (TCE)) via gravity-driven membrane filtration. The results show that membrane with 1% GO had the best nZVI distribution against the aggregation and a better performance in both Cd removal (100%) and TCE removal (82%). The nZVI membrane had a high flux in gravity-driven filtration at 255 LMH for Cd(II) and 265 LMH for TCE respectively. Generally, the developed PVDF-GO-nZVI electrospun nanofiber membrane had an excellent performance in the gravity-driven membrane filtration system for groundwater remediation.

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

The excellent permeability and selectivity of reduced graphene oxide (rGO) membranes have been demonstrated both theoretically and experimentally; however, strategies for the fabrication of highly stable, antifouling rGO membranes with facile recovery after fouling have rarely been investigated. In this work, we report a structurally durable rGO-based hollow fiber membrane that allows high-pressure (at least 1 bar) back-flushing. This is achieved by sandwiching the rGO layer between a carbon nanotube (CNT) protective layer and a polyacrylonitrile (PAN) support. The CNT layer could also function as a prefiltration and pre-adsorption microsystem and endow a higher resistance against fouling. This is experimentally confirmed by the much higher normalized permeance (0.82-0.92) of the CNT/rGO/PAN membranes than the simple rGO/PAN membranes (0.42-0.53) under the same operating conditions. Additionally, under a low cathode potential (0.9 V), the membrane could easily be renewed after fouling by simple back-flushing with a flux recovery ratio of ∼96%. An investigation of the mechanism indicates that electrostatic repulsive forces promote the desorption of charged organic foulants (e.g., humic acid and dyes) from the rGO and CNT layers, and they can subsequently be removed from the membrane with water.

Antifouling Double-Skinned Forward Osmosis Membranes by Constructing Zwitterionic Brush-Decorated MWCNT Ultrathin Films

Pressure retarded osmosis (PRO) process is hindered by severe fouling occurring within the porous support of the forward osmosis (FO) membranes. We designed a novel double-skinned FO membrane containing a polyamide salt-rejecting layer and a zwitterionic brush-decorated, multiwalled carbon nanotube (MWCNT/PSBMA) foulant-resisting layer on the back side. Our results demonstrated that the coating of the MWCNT/PSBMA layer on the porous polyketone (PK) support imparted enhanced hydrophilicity and smaller membrane pore size, thereby providing excellent resistance toward both protein adhesion and bacterial adsorption. We also further evaluated this resultant double-skinned membrane (i.e., TFC-MWCNT/PSBMA) in dynamic PRO fouling experiments using protein and alginate as model organic foulants. Compared to the pristine TFC-PK and hydrophobic TFC-MWCNT membranes, the TFC-MWCNT/PSBMA membrane exhibited not only the lowest water flux decline but also the highest water flux recovery after simple physical flushing. These results shed light on fabrication of antifouling PRO membranes for water purification purposes.

Ion Transport Behavior through Thermally Reduced Graphene Oxide Membrane for Precise Ion Separation

The cation transport behavior of thermally treated reduced graphene oxide membranes (GOMs) is reported. The GOMs were reduced by heat treatment at 25, 80, and 120 °C and then characterized by Fourier transform infrared spectroscopy, X-ray powder diffraction, and X-ray photoelectron spectroscopy to determine oxygen group content, C/O ratio, and layer spacing. The permeation rates of various cations with different sizes and charge numbers through these membranes were measured to understand the effect of the cations on transport behavior. The results indicated that the cation transport through the membranes depended on the layer spacing of the membrane and ion size and charge. Cations of the same valence permeating through the same GOM could be differentiated by their hydration radius, whereas the same type of cation passing through different GOMs could be determined by the spacing of the GOM layers. The cation valence strongly affected permeation behavior. The GOM that was prepared at 120 °C exhibited a narrow layer spacing and high separation factors for Mg/Ca, Mg/Sr, K/Ca, and K/Fe. The cations moving through the membrane could insert into the membrane lamellas, which neutralized the negative charge of the membrane, enlarged the layer spacing of the GOMs, and affected cation permeation.

Comparison of performance and biofouling resistance of thin-film composite forward osmosis membranes with substrate/active layer modified by graphene oxide

In this study, the influence mechanisms of graphene oxide (GO) on the membrane substrate/active layer for improving the water flux and anti-biofouling ability of thin-film composite (TFC) membranes in forward osmosis (FO) were systematically investigated. We fabricated a pristine TFC membrane, a TFC membrane in which the substrate or polyamide active layer was modified by GO (TFN-S membrane or TFN-A membrane), and a TFC membrane in which both the substrate and active layer were functionalized by GO (TFN-S + A membrane). Our results showed that the TFN-S membrane possesses a higher water flux (∼27.2%) than the TFN-A because the substrate that contained GO could improve the porous structure and porosity, while the TFN-A membrane exhibited a lower reverse salt flux and higher salt rejection than the TFN-S membrane, indicating that the surface properties played a more important role than the substrate for the salt rejection. Regarding the biofouling experiment, the TFN-A and TFN-S + A membranes facilitated a higher antifouling performance than the TFN-S and TFC membranes after 72 h of operation because of the greater hydrophilicity, lower roughness and facilitated higher bactericidal activity on the GO-modified surface. In addition, the biovolume and biofilm thickness of the TFN-A and TFN-S + A membranes were found to follow the same trend as flux decline performance. In conclusion, the substrate modified by GO could greatly improve the water flux, whereas the GO-functionalized active layer is favorable for salt rejection and biofouling mitigation. The advantage of TFN-A in biofouling mitigation suggests that the antibacterial effect of GO has a stronger influence on biofouling control than the changes of hydrophilicity and roughness.

The substrate modified by GO could greatly improve water flux, whereas the GO-functionalized active layer is favorable for biofouling mitigation.

Antimicrobial and antifouling properties of versatile PPSU/carboxylated GO nanocomposite membrane against Gram-positive and Gram-negative bacteria and protein

Biofouling is a serious issue in membrane-based water and wastewater treatment as it critically compromises the efficacy of the water treatment processes. This investigation demonstrates the antimicrobial and antifouling properties of a nanocomposite membrane system composed of carboxyl-functionalized graphene oxide (COOH-GO) and polyphenylsulfone (PPSU). The PPSU/COOH-GO nanocomposite membrane exhibited excellent antimicrobial properties, achieving maximum bacteriostasis rates of 74.2% and 81.1% against the representative Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa, respectively) and 41.9% against the representative Gram-positive bacterium (Staphylococcus aureus). The PPSU/COOH-GO nanocomposite membrane inhibited the attachment, colonization, and the biofilm formation of three species. Antifouling was assessed through filtration experiments using a model foulant bovine serum albumin (BSA). The fouling mechanisms were investigated by Hermia's models (complete blocking, intermediate blocking, standard blocking, and cake formation), and the analysis involved fitting the volumetric flux decline experimental data to models. The fouling study revealed a less irreversible fouling and increased flux recovery ratio for the PPSU/COOH-GO nanocomposite membrane. Complete blocking of pores and cake formation were the major fouling mechanisms for the membrane.