Spinel-based ceramic membranes coupling solid sludge recycling with oily wastewater treatment

Recent advances in membrane technologies applied in oil-water separation

Effective treatment of oily wastewater, which is toxic and harmful and causes serious environmental pollution and health risks, has become an important research field. Membrane separation technology has emerged as a key area of investigation in oil-water separation research due to its high separation efficiency, low costs, and user-friendly operation. This review aims to report on the advances in the research of various types of separation membranes around emulsion permeance, separation efficiency, antifouling efficiency, and stimulus responsiveness. Meanwhile, the challenges encountered in oil-water separation membranes are examined, and potential research avenues are identified.

A comprehensive review on the behavior and evolution of oil droplets during oil/water separation by membranes

Membrane separation technology has significant advantages for treating oil-in-water emulsions. Understanding the evolution of oil droplets could reveal the interfacial and colloidal interactions, facilitate the design of advanced membranes, and improve the separation performances. This review on the characteristic behavior and evolution of oil droplets focuses on the advanced analytical techniques, and the subsequent fouling as well as demulsification effects during membrane separation. A detailed introduction is provided on microscopic observations and numerical simulations of the dynamic evolution of oil droplets, featuring real-time in-situ visualization and accurate reconstruction, respectively. Characteristic behaviors of these oil droplets include attachment, pinning, wetting, spreading, blockage, intrusion, coalescence, and detachment, which have been quantified by specific proposed parameters and criteria. The fouling process can be evaluated using Hermia and resistance models. The related adhesion force and intrusion pressure as well as droplet-droplet/membrane interfacial interactions can be accurately quantified using various force analysis methods and advanced force measurement techniques. It is encouraging to note that oil coalescence has been achieved through various effects such as electrostatic interactions, mechanical actions, Laplace pressure/surface free energy gradients, and synergistic effects on functional membranes. When oil droplets become destabilized and coalesce into larger ones, the functional membranes can overcome the limitations of size-sieving effect to attain higher separation efficiency. This not only bypasses the trade-off between permeability and rejection, but also significantly reduces membrane fouling. Finally, the challenges and potential research directions in membrane separation are proposed. We hope this review will support the engineering of advanced materials for oil/water separation and research on interface science in general.

Progress in alumina ceramic membranes for water purification: Status and prospects

Ceramic membranes have gained increasing attention in recent years for the removal of various contaminants from water. Alumina membrane is considered as one of the most important ceramic membranes, which plays important roles not only in separation processes such as microfiltration, ultrafiltration, and nanofiltration, but also in catalysis- and adsorption- enhanced separation applications in water purification and wastewater treatment. However, there is currently still lack of a comprehensive critical review about alumina membranes for water purification. In this review, we first discuss recent developments of alumina membranes, and then critically introduce the state-of-the-art strategies for lowering fabrication cost, improving membrane performances and mitigating membrane fouling. Especially, aiming to improve membrane performance, some emerging methods are summarized such as tailoring membrane structure, developing flexible membranes, designing nano-pores for precise separation, and enhancing multi-functionalities. In addition, engineering applications of alumina membranes for water purification are also briefly introduced. Finally, the prospects for future research on alumina membranes are proposed, such as economic preparation/application, challenging precise separation, enriching multi-functionalities, and clarifying separation mechanisms.

Recyclable structured toxic industrial nickel-containing sludge for efficient anionic contaminant adsorption

Safe, efficient, and simultaneous treatment of toxic industrial sludge and anionic contaminant crisis in one route still remains a persistent global challenge. Herein, we proposed a facile waste-control-waste conceptual design strategy to develop low-cost and high-performance sludge-based adsorbent for not only recycling of toxic waste nickel-containing sludge (NCS) but for the efficient removal of anionic contaminants in wastewater. The as-designed Ni-Al layered double oxides/calcined NCS (Ni-Al LDOs/CNCS) (216.96 m²/g, 0.44 cm³/g) with hierarchical porous structure possessed a larger specific surface area and well-developed porosity compared with raw NCS (60.52 m²/g, 0.26 cm³/g). It was proved that a higher hydrothermal temperature (180 °C) and a longer hydrothermal time (24 h) both promote the in situ assembly of LDHs nanosheets on CNCS surface. Significantly, the sludge-based adsorbent displayed high adsorption capacity towards five representative anions including F^- (~ 31.1 mg/g), SO₄^2- (~ 37.7 mg/g), NO₃^- (~ 21.8 mg/g), Cl^- (~ 28.0 mg/g), and H₂PO₄^- (~ 35.8 mg/g). Furthermore, the adsorbent maintained desirable adsorption capacity even after 6 adsorption/desorption cycles. Therefore, this study could be potentially extended toward design of other industrial waste sludge-derived high value-added advanced materials and for wastewater treatment applications.

Dissecting the structure-property relationship of ceramic membrane with asymmetric multilayer structures for maximizing permselectivity

Robust ceramic membranes presented attractive features of easy cleaning and excellent stability compared to polymeric membranes. Nevertheless, their inherent relationships between the membrane microstructures and separation properties are not completely clear. In this work, we established a quantitative structure-property model using α-Al₂O₃ membrane on account of the theory of filtrated cake to predict the effects of membrane structure-controlled factors (i.e., α-Al₂O₃ particle size and layer thickness) on separation performances (i.e., solute rejection and water permeance). The simulation results show that membrane pore size mainly depends upon α-Al₂O₃ particle size rather than the layer thickness. When the microstructure of top layer in a double-layer asymmetric ceramic membrane is fixed, there exists optimum particle size and layer thickness that constitute the support layer to achieve maximum water permeance. For a triple-layer ceramic membrane, a similar matching relationship exists between top layer and intermediate layer, indicating that the intermediate layer has a vital role in determining water permeance. While the bottom layer has little effect on overall separation property. Finally, the upper-bound tradeoff relationship between permeance and selectivity is further established for the α-Al₂O₃ membrane. This study reveals the structure-property relationship of ceramic membrane and provides insights into performance enhancement.

Efficient removal of micropollutants from low-conductance surface water using an electrochemical Janus ceramic membrane filtration system

Electrochemical membrane filtration (EMF) technology is effective to remove the micropollutant in the wastewater but its efficacy is drastically compromised in treating the surface water having a typically low conductivity. In this work, a Janus Fe-Pt electrochemical ceramic membrane (ECM) was fabricated by depositing a thin Fe layer on the side of a ceramic membrane facing feed (cathode) and Pt layer on the other side facing permeate (anode). The low Fe-Pt electrode distance (∼1 mm) ensured a decent conductance of the EMF system even in the low-salinity surface water and thereby maintained the removal efficiency of the micropollutant. It was identified that hydroxyl radicals (•OH) generated via anodic water oxidation and cathodic heterogenous Fenton process on bilateral sides of ECM were the dominant reactive oxygen species. The EMF system not only achieved 74% removal of atrazine (ATZ) from the low-conductance synthetic surface water with a low energy consumption (3.6 Wh per gATZ or 7.2 Wh m ^- 3), but also realized a stable removal of ATZ from real surface water over a continuous filtration experiment of 168 h. The theoretical computations and experimental analysis identified the degradation pathway, i.e., the dechlorination and dealkylation of ATZ in the EMF system. This study highlights the great potential of the Janus ECM in removing micropollutants from low-conductance surface water and wastewater.

Cost and efficiency perspectives of ceramic membranes for water treatment

More robust ceramic membranes with tailorable structures and functions are increasingly employed for water treatment, particularly in some harsh applications for their ultra-long service lifespan due to their high mechanical, structural, chemical and thermal stability and anti-fouling properties. Decreasing cost and enhancing efficiency are two key but quite challenging application-oriented issues for broader and larger-scale engineering application of current ceramic membranes, and are required to make ceramic membranes a highly efficient and economic water treatment technique. In this review, we critically discuss these two significant concerns of both cost and efficiency for water treatment ceramic membranes, focusing on an overview of various advanced strategies and mechanism insights. A brief up-to-date discussion is first introduced about recent developments of ceramic membranes covering the major advances of novel membranes and applications. Then some promising strategies for decreasing the cost of ceramic membranes are discussed, including membrane material cost and processing cost. To fully address the issue of moderate efficiency with single separation function, valuable and considerable insights are provided into recent major progress and mechanism understandings in application with other unit processes, such as advanced oxidation and electrochemistry techniques, to significantly enhance treatment efficiency. Subsequently, a review of recent ceramic membrane applications emphasizing harsh operating environments is presented, such as oil-water separation, saline water, refractory organic and emerging contaminant wastewater treatment. Finally, engineering application, conclusions, and future perspectives of ceramic membrane for water treatment applications are critically discussed offering new insight based on understanding the issues of cost and efficiency.

Peracetic acid integrated catalytic ceramic membrane filtration for enhanced membrane fouling control: Performance evaluation and mechanism analysis

Endowing ceramic membrane (CM) catalytic reactivity can enhance membrane fouling control in the aid of in situ oxidation process. Peracetic acid (PAA) oxidant holds great prospect to integrate with CM for membrane fouling control, owing to the prominent advantages of high oxidation efficacy and easy activation. Herein, this study, for the first time, presented a PAA/CM catalytic filtration system achieving highly-efficient protein fouling alleviation. A FeOCl functionalized CM (FeOCl-CM) was synthesized, possessing high hydrophilicity, low surface roughness, and highly-efficient activation towards PAA oxidation. Using bovine serum albumin (BSA) as the model protein foulant, the PAA/FeOCl-CM catalytic filtration notably alleviated fouling occurring in both membrane pores and surface, and halved the flux reduction degree as compared with the conventional CM filtration. The PAA/FeOCl-CM catalytic oxidation allows quick and complete disintegration of BSA particles, via the breakage of the amide I and II bands and the ring opening of the aromatic amino acids (e.g., Tryptophan, Tyrosine). In-depth investigation revealed that the in situ generated ^•OH and ¹O₂ were the key reactive species towards BSA degradation during catalytic filtration, while the organic radical oxidation and the direct electron transfer pathway from BSA to PAA via FeOCl-CM played minor roles. Overall, our findings highlight a new PAA/CM catalytic filtration strategy for achieving highly-efficient membrane fouling control and provide an understanding of the integrated PAA catalytic oxidation - membrane filtration behaviors.

Structured sludge derived multifunctional layer for simultaneous separation of oil/water emulsions and anions contaminants

The effective treatment of complex oily wastewater is of great significance but still a considerable challenge, since single-function, expensive reagents, and complicated process have emerged as shackles for practical applications. Herein, with the objective to waste-control-waste, we proposed a facile and sustainable strategy to fabricate a low-cost multifunctional layer from hazardous waste aluminum sludge (WAS) for complex oily wastewater management. The as-designed layered double oxides/WAS (LDOs/WAS) layer with three-dimensional (3D) hierarchical rough surface exhibited excellent underwater superoleophobicity even under corrosive conditions and low adhesion to oil without any chemical modification reagent treatment. Significantly, the layer can be applied to gravity-directed simultaneous efficient oil-in-water emulsions and anions (taking phosphate as an example) separation with a separation efficiency for emulsion and phosphate up to 99.4% and 99.1%, respectively, and a high separation flux of above 2585 L m^-2 h^-1. Notably, the flux can be controlled simply and flexibly by adjusting the thickness of the layer. Furthermore, the layer also displayed excellent thermal stability, chemical stability, durability and recyclability. Therefore, this work not only presents a promising approach to design sludge-based multifunctional materials for complex oily wastewater remediation, but also shows great potential and value in environmental pollutions reduction and industrial applications.

Silicon carbide microfiltration membranes for oil-water separation: Pore structure-dependent wettability matters

Both the pore size and surface properties of silicon carbide (SiC) membranes are demonstrated to significantly affect their separation efficiency when used for oily water treatment. However, the potential influences of open porosity together with the pore size of SiC membranes on their surface properties and oil-water separation performance have rarely been investigated. In this work, porous SiC ceramic membranes with tunable open porosity and pore size were purposely prepared and selected to systematically study the effect of pore structure-dependent wettability on the oil-water separation performance. The measured pure water flux of selected membranes as a function of open porosity (34-48%) and pore size (0.43-0.67 μm) was well-fitted by using a modified H-P equation. Interestingly, the hydrophilicity of SiC membranes was improved with the increase in open porosity and pore size, as evidenced by the gradually decreased dynamic water contact angle and underwater adhesion of oil droplets. Further, the open porosity of SiC membranes was found to contribute more to the improved surface wettability. As a result, the stable flux of SiC membranes in oil-in-water (O/W) emulsions was increased by 24% with the increased open porosity while the oil rejection rate remained above 90%. This work quantitatively reveals the contributions of the pore structure to the surface wettability of ceramic membranes, and thus provides an effective pathway to improve their performance in oil-water separation.

Oil-in-water emulsion separation: Fouling of alumina membranes with and without a silicon carbide deposition in constant flux filtration mode

Ceramic membranes have drawn increasing attention in oily wastewater treatment as an alternative to their traditional polymeric counterparts, yet persistent membrane fouling is still one of the largest challenges. Particularly, little is known about ceramic membrane fouling by oil-in-water (O/W) emulsions in constant flux filtration modes. In this study, the effects of emulsion chemistry (surfactant concentration, pH, salinity and Ca²⁺) and operation parameters (permeate flux and filtration time) were comparatively evaluated for alumina and silicon carbide (SiC) deposited ceramic membranes, with different physicochemical surface properties. The original membranes were made of 100% alumina, while the same membranes were also deposited with a SiC layer to change the surface charge and hydrophilicity. The SiC-deposited membrane showed a lower reversible and irreversible fouling when permeate flux was below 110 L m^-2 h^-1. In addition, it exhibited a higher permeance recovery after physical and chemical cleaning, as compared to the alumina membranes. Increasing sodium dodecyl sulfate (SDS) concentration in the feed decreased the fouling of both membranes, but to a higher extent in the alumina membranes. The fouling of both membranes could be reduced with increasing the pH of the emulsion due to the enhanced electrostatic repulsion between oil droplets and membrane surface. Because of the screening of surface charge in a high salinity solution (100 mM NaCl), only a small difference in irreversible fouling was observed for alumina and SiC-deposited membranes under these conditions. The presence of Ca²⁺ in the emulsion led to high irreversible fouling of both membranes, because of the compression of diffusion double layer and the interactions between Ca²⁺ and SDS. The low fouling tendency and/or high cleaning efficiency of the SiC-deposited membranes indicated their potential for oily wastewater treatment.

Piezoelectricity induced by pulsed hydraulic pressure enables in situ membrane demulsification and oil/water separation

Recovering oil from oily wastewater is not only for economic gains but also for mitigating environmental pollution. However, demulsification of oil droplets stabilized with surfactants is challenging because of their low surface energy. Although the widely used oil/water separation membrane technologies based on size screening have attracted considerable attention in the past few decades, they are incapable of demulsification of stabilized oil emulsions and the membrane concentrates often require post-processing. Herein, the piezoelectric ceramic membrane (PCM), which can respond to the inherent transmembrane pressure in the pressure-driven membrane processes, was employed to transform hydraulic pressure pulses into electroactive responses to in situ demulsification. The pulsed transmembrane pressure on the PCM results in the generation of considerable rapid voltage oscillations over 3.2 V and a locally high electric field intensity of 7.2 × 10⁷ V/m, which is capable of electrocoalescence with no additional stimuli or high voltage devices. Negative dielectrophoresis (DEP) force occurred in this membrane process and repelled the large size of oil after demulsification away from the PCM surface, ensuring continuous membrane demulsification and oil/water separation. Overall, PCM provides a further opportunity to develop an environmentally friendly and energy-saving electroresponsive membrane technology for practical applications in wastewater treatment.

Superhydrophobic Carbon Nanotube Network Membranes for Membrane Distillation: High-Throughput Performance and Transport Mechanism

Despite increasing sustainable water purification, current desalination membranes still suffer from insufficient permeability and treatment efficiency, greatly hindering extensive practical applications. In this work, we provide a new membrane design protocol and molecule-level mechanistic understanding of vapor transport for the treatment of hypersaline waters via a membrane distillation process by rationally fabricating more robust metal-based carbon nanotube (CNT) network membranes, featuring a superhydrophobic superporous surface (80.0 ± 2.3% surface porosity). With highly permeable ductile metal hollow fibers as substrates, the construction of a superhydrophobic (water contact angle ∼170°) CNT network layer endows the membranes with not only almost perfect salt rejection (over 99.9%) but a promising water flux (43.6 L·m^-2·h^-1), which outperforms most existing inorganic distillation membranes. Both experimental and molecular dynamics simulation results indicate that such an enhanced water flux can be ascribed to an ultra-low liquid-solid contact interface (∼3.23%), allowing water vapor to rapidly transport across the membrane structure via a combined mechanism of Knudsen diffusion (more dominant) and viscous flow while efficiently repelling high-salinity feed via forming a Cassie-Baxter state. A more hydrophobic surface is more in favor of not only water desorption from the CNT outer surface but superfast and frictionless water vapor transport. By constructing a new superhydrophobic triple-phase interface, the conceptional design strategy proposed in this work can be expected to be extended to other membrane material systems as well as more water treatment applications.

Electro-Enhanced Separation of Microsized Oil-in-Water Emulsions via Metallic Membranes: Performance and Mechanistic Insights

Conventional separation membranes suffer from evitable fouling and flux decrease for water treatment applications. Herein, a novel protocol of electro-enhanced membrane separation is proposed for the efficient treatment of microsized emulsions (∼1 μm) by rationally designing robust electroresponsive copper metallic membranes, which could mitigate oil fouling and coenhance permeance (from ∼1026 to ∼2516 L·m^-2·h^-1·bar^-1) and rejection (from ∼87 to ∼98%). High-flux Cu membranes exhibit superior ductility and electrical conductivity, enabling promising electroactivity. Separation performance and the fouling mechanism were studied under different electrical potentials and ionic strengths. Application of negative polarization into a large-pore (∼2.1 μm) Cu membrane is favorable to not only almost completely reject smaller-sized oil droplets (∼1 μm) but also achieve antifouling and anticorrosion functions. Moreover, surfactants around oil droplets might be redistributed due to electrostatic repulsion, which effectively enhances the steric hindrance effect between neighboring oil droplets, mitigating oil coalescence and consequently membrane fouling. Furthermore, due to the screening effect of surfactants, the presence of low-concentration salts increases the adsorption of surfactants at the oil-water interface, thus preventing oil coalescence via decreasing oil-water interfacial tension. However, under high ionic strengths, the fouling mechanism converts from cake filtration to a complete blocking model due to the reduced electrostatic repulsion between the Cu membrane and oil droplets. This work would provide mechanistic insights into electro-enhanced antifouling for not only oil emulsion separation but also more water treatment applications using rationally designed novel electroresponsive membranes.

Inorganic-Organic Hybrid Membrane Based on Pillararene-Intercalated MXene Nanosheets for Efficient Water Purification

Discharge of antibiotic-containing wastewater causes environmental pollution and threatens biological and human health. An efficient treatment method for this wastewater is urgently required. We prepared inorganic-organic hybrid MXene-pillararene nanosheets with a large lateral size (5-8 μm). The hybrid nanosheets were stacked on supports via vacuum-assisted filtration to prepare membranes with regular parallel slits and an interlayer spacing of 1.36 nm, which were used to purify antibiotic-containing water. Permeance through the membrane increased 100-fold compared with most polymeric and other two-dimensional nanofiltration membranes with similar rejection. This high permeance and rejection was attributed to the large lateral size of the nanosheets, regular interlayer spacing, and electrostatic interaction between the membrane and antibiotics. These membranes will broaden the applications of lamellar materials for the separation of high-value-added drugs in academia and industry.

Treatment of oily wastewaters by highly porous whisker-constructed ceramic membranes: Separation performance and fouling models

Efficient treatment of challenging oily emulsion wastewater can alleviate water pollution to provide more chances for water reuse and resource recovery. Despite their promising application potential, conventional porous ceramic membranes have challenging bottleneck issues such as high cost and insufficient permeance. This study presents a new strategy for highly efficient treatment of not only synthetic but real oily emulsions via unexpensive whisker-constructed ceramic membranes, exhibiting exceptional permeance and less energy input. Compared with common ceramic membranes, such lower-cost mullite membranes with a novel whisker-constructed structure show higher porosity and water permeance, and better surface oleophobicity in water. Treatment performance such as permeate flux and oil rejection was explored for the oily emulsions with different properties under key operating parameters. Furthermore, classical Hermia models were used to reveal membrane fouling mechanism to well understand the microscopic interactions between emulsion droplets and membrane interface. Even for real acidic oily wastewater, such membranes also exhibit high permeance and less energy consumption, outperforming most state-of-the-art ceramic membranes. This work provides a new structure concept of highly permeably whisker-constructed porous ceramic membranes that can efficiently enable more water separation applications.

Study on Treatment of Low Concentration Oily Wastewater Using Alumina Ceramic Membranes

In this study, alumina ceramic plate microfiltration membranes (ACMs) were used for the treatment of oily wastewater with different concentrations. The permeate oil concentration of the system was basically less than 5 mg·L−1, and the oil rejection rate was up to 97.6%. The effects of raw oil concentration on permeation flux and oil rejection rate of oily wastewater were studied. The results showed that with the increase of raw oil concentration, the oil rejection rate increased slightly due to the existence of oil film on the surface of filtered ACMs. Moreover, the existence of oil film had little effect on the flux change of ceramic membranes. The results showed that the permeability of ACMs mainly depended on their own oleophobic properties. In this system, physical cleaning technology is used to remove oil droplets and particles blocked in membrane pores. The results showed that physical cleaning could significantly recover the permeation flux as well as improve the oil rejection rate. On this basis, a system is proposed as a potential technique for oily wastewater treatment.

Robust zirconia ceramic membrane with exceptional performance for purifying nano-emulsion oily wastewater

While membrane-based oil-water separation has been widely explored, using conventional membranes to treat oily wastewaters remains practically challenging especially when such wastewaters contain more stable nano-sized oil droplets and are of high oil content, and harsh chemical conditions. Herein, we report a novel protocol of efficiently separating both synthetic and real oil nano-emulsions via specially designed robust zirconia membranes. The best-performing zirconia membrane, fabricated at low sintering temperature, has relatively uniform sub-100 nm pores and is underwater superoleophobic. Such zirconia membranes possess not only outstanding separation performance under long-term operation but robust structural stability at harsh conditions. At different cross-flow velocities, a combined model of intermediate pore blocking and cake filtration dominated membrane fouling behavior. Specifically, at high pH value (especially > pH_(IEP)), membrane fouling was effectively mitigated due to a dominant role of electrostatic repulsion interaction at membrane-oil interface. Compared with conventional and commercial ceramic membranes, our zirconia membrane is the first reported in literature that can effectively reject nano-sized oil droplets (∼18 nm) with over 99% rejection. Moreover, the zirconia membrane has also been challenged with real degreasing wastewater with very high oil content (∼4284 mg L^-1) and pH (∼12.4) and delivered consistently high separation performance over many operation cycles.

State-of-the-Art Ceramic Membranes for Oily Wastewater Treatment: Modification and Application

Membrane filtration is considered to be one of the most promising methods for oily wastewater treatment. Because of their hydrophilic surface, ceramic membranes show less fouling compared with their polymeric counterparts. Membrane fouling, however, is an inevitable phenomenon in the filtration process, leading to higher energy consumption and a shorter lifetime of the membrane. It is therefore important to improve the fouling resistance of the ceramic membranes in oily wastewater treatment. In this review, we first focus on the various methods used for ceramic membrane modification, aiming for application in oily wastewater. Then, the performance of the modified ceramic membranes is discussed and compared. We found that, besides the traditional sol-gel and dip-coating methods, atomic layer deposition is promising for ceramic membrane modification in terms of the control of layer thickness, and pore size tuning. Enhanced surface hydrophilicity and surface charge are two of the most used strategies to improve the performance of ceramic membranes for oily wastewater treatment. Nano-sized metal oxides such as TiO2, ZrO2 and Fe2O3 and graphene oxide are considered to be the potential candidates for ceramic membrane modification for flux enhancement and fouling alleviation. The passive antifouling ceramic membranes, e.g., photocatalytic and electrified ceramic membranes, have shown some potential in fouling control, oil rejection and flux enhancement, but have their limitations.

Emerging graphitic carbon nitride-based membranes for water purification

Membrane separation is a promising technology that can effectively remove various existing contaminants from water with low energy consumption and small carbon footprint. The critical issue of membrane technology development is to obtain a low-cost, stable, tunable and multifunctional material for membrane fabrication. Graphitic carbon nitride (g-C₃N₄) has emerged as a promising membrane material, owing to the unique structure characteristics and outstanding catalytic activity. This review paper outlined the advanced material strategies used to regulate the molecule structure of g-C₃N₄ for membrane separation. The presentative progresses on the applications of g-C₃N₄-based membranes for water purification have been elaborated. Essentially, we highlighted the innovation integration of physical separation, catalysis and energy conversion during water purification, which was of great importance for the sustainability of water treatment techniques. Finally, the continuing challenges of g-C₃N₄-based membranes and the possible breakthrough directions in the future research was prospected.

Polymers and Solvents Used in Membrane Fabrication: A Review Focusing on Sustainable Membrane Development

(1) Different methods have been applied to fabricate polymeric membranes with non-solvent induced phase separation (NIPS) being one of the mostly widely used. In NIPS, a solvent or solvent blend is required to dissolve a polymer or polymer blend. N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF) and other petroleum-derived solvents are commonly used to dissolve some petroleum-based polymers. However, these components may have negative impacts on the environment and human health. Therefore, using greener and less toxic components is of great interest for increasing membrane fabrication sustainability. The chemical structure of membranes is not affected by the use of different solvents, polymers, or by the differences in fabrication scale. On the other hand, membrane pore structures and surface roughness can change due to differences in diffusion rates associated with different solvents/co-solvents diffusing into the non-solvent and with differences in evaporation time. (2) Therefore, in this review, solvents and polymers involved in the manufacturing process of membranes are proposed to be replaced by greener/less toxic alternatives. The methods and feasibility of scaling up green polymeric membrane manufacturing are also examined.

Recent development of pressure retarded osmosis membranes for water and energy sustainability: A critical review

With the goal of zero-liquid discharge and green energy harvest, extraction of abundant green energy from saline water via pressure retarded osmosis (PRO) technology is a promising but challenging issue for water treatment technologies to achieve water and energy sustainability. Development of high performance PRO membranes has received increased concerns yet still under controversy in practical applications. In this review, a comprehensive and up-to-date discussion of some key historical developments is first introduced covering the major advances of PRO engineering applications and novel membranes especially made in recent years. Then the critical performance indicators of PRO membranes including water flux and power density are briefly discussed. Subsequently, sufficient discussion on four performance limiting factors in PRO membrane and process is presented including concentration polarization, reverse solute diffusion, membrane fouling and mechanical stability. To fully address these issues, an updated insight is provided into recent major progresses on advanced fabrication and modification techniques of novel PRO membranes featuring enhanced performance with different configurations and materials, which are also reviewed in detail based on the viewpoint of design rationales. Afterwards, antifouling strategies and engineering applications are critically introduced. Finally, conclusions and future perspective of PRO membrane for practical operation are briefly discussed.

Cross-linked Graphene Oxide Framework Membranes with Robust Nano-Channels for Enhanced Sieving Ability

It remains challenging for graphene oxide (GO) membranes to achieve highly efficient performance and sufficient stability for aqueous molecule/ion precise separations. Herein, a molecular-level rational structure design protocol was proposed to develop ceramic-based graphene oxide framework (GOF) membranes with significantly enhanced sieving performance and stability for efficient removal of salts and micropollutants. Via a molecular cross-linking strategy, interlayered nanochannels between GO nanosheets can be rationally designed, featuring precisely tailorable channel size, promising surface chemistries and remarkably robust stability suitable for aqueous separation. Due to a significantly decreased nanochannel size, cross-linking of TU (thiourea) molecule significantly improved monovalent salt rejection (95.6% for NaCl), outperforming existing state-of-the-art GO-based, commercial organic nanofiltration and emerging two-dimensional MoS₂ membranes, while moderately decreasing water permeability. In comparison, the GOF membranes cross-linked with MPD (m-phenylenediamine) exhibited a simultaneous increase in permeability and rejection for both salts and micropollutants (21.0% and 53.3% enhancement for chloramphenicol (CAP) solution), breaking their conventional trade-off issue. Cross-linking mechanism indicates that more robust nanochannels were formed by stronger covalent bonds via dehydration condensation between amine (TU/MPD) and carboxyl groups (GO), and nucleophilic addition between amine (TU/MPD) and epoxy groups (GO). Molecule/ion separation mechanism involved size sieving (steric hindrance), electrostatic interaction, Donnan effect, and partial dehydration effect. This work provides a novel protocol for rationally designing size and surface chemistry of highly robust GO nanochannels at a subnanometer level to construct water-stable functional GOF membranes with enhanced sieving performance for water treatment applications.

Simulation Study on Direct Contact Membrane Distillation Modules for High-Concentration NaCl Solution

Membrane distillation technology, as a new membrane-based water treatment technology that combines the membrane technology and evaporation process, has the advantages of using low-grade heat, working at atmospheric pressure with simple configuration, etc. In this study, heat and mass transfer were coupled at the membrane surfaces through the user-defined function program. The effects of feed temperature, feed velocity and permeate velocity on temperature polarization were mainly investigated for a high-concentration NaCl solution. The temperature polarization was increased with the increase of feed temperature and the decrease of feed and permeate velocity. The effects of temperature, inlet velocity and solution concentration on the evaporation efficiency of the membrane module for co- and counter-current operations were investigated in detail. The counter-current operation performed better than co-current operation in most cases, except for the condition where the NaCl concentration was relatively low or the module length was long enough. In addition, the optimal membrane thickness for both PVDF and PTFE was studied. The optimal membrane thickness was found in the range of 10 to 20 μm, which corresponded to the highest permeate flux for the selected materials, pore size distribution, and operation conditions. Membrane material with lower thermal conductivity and larger porosity was prone to get higher permeate flux and had larger optimal membrane thickness. Increasing feed velocity or feed temperature could decrease the optimal membrane thickness.

Flexible Superhydrophobic Metal-Based Carbon Nanotube Membrane for Electrochemically Enhanced Water Treatment

Treatment of highly saline wastewaters via conventional technology is a key challenging issue, which calls for efficient desalination membranes featuring high flux and rejection, low fouling, and excellent stability. Herein, we report a high-strength and flexible electro-conductive stainless steel-carbon nanotube (SS-CNT) membrane, exhibiting significantly enhanced anticorrosion and antifouling ability via a microelectrical field-coupling strategy during membrane distillation. The membrane substrates exhibited excellent mechanical strength (244.2 ± 9.8 MPa) and ductility, thereby overcoming the critical bottleneck of brittleness of traditional inorganic membranes. By employing a simple surface activation followed by self-catalyzed chemical vapor deposition, CNT was grown in situ on SS substrates via a tip-growth mechanism to finally form robust superhydrophobic SS-CNT membrane. To address the challenging issues of significant corrosion and fouling, using a negative polarization microelectrical field-coupling strategy, simultaneously enhanced antifouling and anticorrosion performance was realized for treatment of organic high salinity waters while exhibiting stable high flux and rejection via an electrostatic repulsion and electron supply mechanism. This application-oriented rational design protocol can be potentially used to extend toward high performance composite membranes derived from other electro-conductive metal substrates functionally decorated with CNT network and to other applications in water treatment.

Engineering a Nanocomposite Interlayer for a Novel Ceramic-Based Forward Osmosis Membrane with Enhanced Performance

Rational design of a high-performance defect-free polyamide (PA) layer on a robust ceramic substrate is challenging for forward osmosis (FO) water treatment applications. In this study, we first demonstrated a robust ceramic-based thin-film composite (TFC) FO membrane by engineering a novel nanocomposite interlayer of titanium dioxide and carbon nanotube (TiO₂/CNT). The structural morphologies and properties were systematically characterized for different substrates (without interlayer, with TiO₂ interlayer, or with TiO₂/CNT interlayer) and the corresponding ceramic-based TFC-FO membranes. Introduction of low roughness nanocomposite interlayers with decreased pore size created an interface with improved surface characteristics, favoring the formation of a defect-free nanovoid-containing PA layer with high cross-linking degree. The resulting ceramic-based FO membrane had a water permeability of approximately 2 L/(m² h bar) and a NaCl rejection of 98%, showing simultaneous enhancements in both compared to the control membrane without an interlayer. Mechanism analysis indicates that such a special nanocomposite interlayer not only provided more active sites for the formation of a thinner defect-free nanovoid-containing PA layer without penetration into substrate but also acted as a highly porous three-dimensional network structure for rapid water transport. This work provides a novel protocol for rational design and fabrication of a high-performance multilayered inorganic FO membrane as well as extended applications in water treatment with enhanced performance.