High flux and antifouling properties of negatively charged membrane for dyeing wastewater treatment by membrane distillation

Springtail-inspired omniphobic slippery membrane with nano-concave re-entrant structures for membrane distillation

Omniphobic membranes, due to their exceptional properties, have drawn significant attention for overcoming the bottleneck in membrane distillation (MD) technology. This study demonstrates an innovative method for fabricating an omniphobic membrane that is simple and facile compared to other methods such as wet/dry etching and photolithography. The surface morphology of springtails was imitated using electrospraying technique to coat a polyvinylidene fluoride substrate with concave-shaped polystyrene beads that were successfully developed by controlling the electrical traction (voltage) and air resistance (humidity). Then, the lipid coating of springtail surfaces was mimicked by dip-coating the membrane in a low-toxicity short-chain perfluoropolyether lubricant. The concave structure's tiny air pockets increased membrane hydrophobicity significantly, indicated by the fact that the first round of water bouncing took only 16.3 ms. Finally, in MD treatment of seawater containing 1.0 mM sodium dodecyl sulfate, the optimized omniphobic membrane maintained a stable 99.9% salt rejection rate.

Purifying surface water contaminated with azo dyes using nanofiltration: Interactions between dyes and dissolved organic matter

In this research, the interactions of two azo dyes, Methyl Orange (MO) and Eriochrome Black T (EBT), with dissolved organic matter (DOM) in surface water were studied, emphasizing their removal using nano-filtration membranes (NF-270 and NF-90). High-Performance Size Exclusion Chromatography (HPSEC) findings indicated that the dyes' molecular weight in deionized (DI) water ranged from 500 to 15k Dalton (Da), adjusting peak intensities with Jingmi River (JM) water Beijing. Notably, when dyes were diluted in JM water, ultraviolet (UV533 & 466, and UV254), together with total organic carbon (TOC) parameters, revealed color removal rates of 99.49% (EBT), 94.2% (MO), 87.6% DOM removal, and 86% TOC removal for NF-90. The NF-90 membrane demonstrated a 75% flux decline for 50 mL permeate volume due to its finer pore structure and higher rejection effectiveness. In contrast, the NF-270 membrane showed a 60% decline in flux under the same conditions. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) analysis of dye-treated membranes in JM water revealed that the NF-270 showed a CC bond peak at 1660 cm-1 across various samples, while analyzing NF-90, the peaks at 1400 cm-1, 1040 cm-1, 750 cm-1, and 620 cm-1 disappeared for composite sample removal. The hydrophobicity of each membrane is measured by the contact angle (CA), which identified that initial CAs for NF-270 and NF-90 were 460 and 700, respectively, that were rapidly declined but stabilized after a few seconds of processing. Overall, this investigation shows that azo dyes interact with DOM in surface waters and enhance the removal efficiency of NF membranes.

Progress in membrane distillation processes for dye wastewater treatment: A review

Textile and cosmetic industries generate large amounts of dye effluents requiring treatment before discharge. This wastewater contains high levels of reactive dyes, low to none-biodegradable materials and chemical residues. Technically, dye wastewater is characterised by high chemical and biological oxygen demand. Biological, physical and pressure-driven membrane processes have been extensively used in textile wastewater treatment plants. However, these technologies are characterised by process complexity and are often costly. Also, process efficiency is not achieved in cost-effective biochemical and physical treatment processes. Membrane distillation (MD) emerged as a promising technology harnessing challenges faced by pressure-driven membrane processes. To ensure high cost-effectiveness, the MD can be operated by solar energy or low-grade waste heat. Herein, the MD purification of dye wastewater is comprehensively and yet concisely discussed. This involved research advancement in MD processes towards removal of dyes from industrial effluents. Also, challenges faced by this process with a specific focus on fouling are reviewed. Current literature mainly tested MD setups in the laboratory scale suggesting a deep need of further optimization of membrane and module designs in near future, especially for textile wastewater treatment. There is a need to deliver customized high-porosity hydrophobic membrane design with the appropriate thickness and module configuration to reduce concentration and temperature polarization (CP and TP). Also, energy loss should be minimized while increasing dye rejection and permeate flux. Although laboratory experiments remain pivotal in optimizing the MD process for treating dye wastewater, the nature of their time intensity poses a challenge. Given the multitude of parameters involved in MD process optimization, artificial intelligence (AI) methodologies present a promising avenue for assistance. Thus, AI-driven algorithms have the potential to enhance overall process efficiency, cutting down on time, fine-tuning parameters, and driving cost reductions. However, achieving an optimal balance between efficiency enhancements and financial outlays is a complex process. Finally, this paper suggests a research direction for the development of effective synthetic and natural dye removal from industrially discharged wastewater.

Performances of PTFE and PVDF membranes in achieving the discharge limit of mixed anodic oxidation coating wastewaters treated by membrane distillation

The mixed wastewater generated by anodic oxidation coating facilities contains high levels of various contaminants, including iron, aluminum, conductivity, chemical oxygen demand (COD), and sulfate. In this study, the effectiveness of the membrane distillation (MD) process using polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) membranes was investigated to treat mixed wastewater from an anodized coating factory. The results indicate that both hydrophobic membranes effectively removed targeted contaminants. However, the PTFE membrane achieved higher removal efficiencies, with over 99% removal of sulfate, conductivity, iron, and aluminum, 85.7% of COD, and 86% of total organic carbon (TOC). In contrast, the PVDF membrane exhibited a significant decline in removal efficiency as the temperature increased and performed well only at lower feed temperatures. The PTFE membranes outperformed the PVDF membranes in treating chemically intensive anodic oxidation wastewaters. This superiority can be attributed to the PTFE membrane's morphology and structure, which are less influenced by feed water temperature and chemicals. Additionally, its slippery surface imparts anti-adhesion properties, effectively preventing membrane fouling, and maintaining the treated water quality and flux for longer operation time.

Optimization of Chitosan Synthesis Process Parameters to Enhance PES/Chitosan Membrane Performance for the Treatment of Acid Mine Drainage (AMD)

Acid mine drainage (AMD) is an environmental issue linked with mining activities, causing the release of toxic water from mining areas. Polyethersulphone (PES) membranes are explored for AMD treatment, but their limited hydrophilicity hinders their performance. Chitosan enhances hydrophilicity, addressing this issue. However, the effectiveness depends on chitosan's degree of deacetylation (DD), determined during the deacetylation process for chitosan production. This study optimized the chitin deacetylation temperature, alkaline (NaOH) concentration, and reaction time, yielding the highest chitosan degree of deacetylation (DD) for PES/chitosan membrane applications. Prior research has shown that high DD chitosan enhances membrane antifouling and hydrophilicity, increasing contaminant rejection and permeate flux. Evaluation of the best deacetylation conditions in terms of temperature (80, 100, 120 °C), NaOH concentration (20, 40, 60 wt.%), and time (2, 4, 6 h) was performed. The highest chitosan DD obtained was 87.11% at 80 °C, 40 wt. %NaOH at 4 h of chitin deacetylation. The PES/0.75 chitosan membrane (87.11%DD) showed an increase in surface hydrophilicity (63.62° contact angle) as compared to the pristine PES membrane (72.83° contact angle). This was an indicated improvement in membrane performance. Thus, presumably leading to high contaminant rejection and permeate flux in the AMD treatment context, postulate to literature.

Durable Nano-Flower Structured Foam Coupled with Electrically-Driven in Situ Aeration Enable High-Flux Oil/Water Emulsion Separation with Dynamic Antifouling Ability

The conventional membranes used for separating oil/water emulsions are typically limited by the properties of the membrane materials and the impact of membrane fouling, making continuous long-term usage unachievable. In this study, a filtering electrode with synchronous self-cleaning functionality is devised, exhibiting notable antifouling ability and an extended operational lifespan, suitable for the continuous separation of oil/water emulsions. Compared with the original Ti foam, the in situ growth of NiTi-LDH (Layered double hydroxide) nano-flowers endows the modified Ti foam (NiTi-LDH/TF) with exceptional superhydrophilicity and underwater superoleophobicity. Driven by gravity, a rejection rate of over 99% is achieved for various emulsions containing oil content ranging from 1% to 50%, as well as oil/seawater emulsions. The flux recovery rate exceeds 90% after one hundred cycles and a 4-h filtration period. The enhanced separation performance is realized through the "gas bridge" effect during in situ aeration and electrochemical anodic oxidation. The internal aeration within the membrane pores contributes to the removal of oil foulants. This study underscores the potential of coupling foam metal filtration materials with electrochemical technology, providing a paradigm for the exploration of novel oil/water separation membranes.

Conductive nanofiltration membranes via in situ PEDOT-polymerization for electro-assisted membrane fouling mitigation

Nanofiltration (NF) membranes play a pivotal role in water treatment; however, the persistent challenge of membrane fouling hampers their stable application. This study introduces a novel approach to address this issue through the creation of a poly(3,4-ethylenedioxythiophene) (PEDOT)-based conductive membrane, achieved by synergistically coupling interfacial polymerization (IP) with in situ self-polymerization of EDOT. During the IP reaction, the concurrent generation of HCl triggers the protonation of EDOT, activating its self-polymerization into PEDOT. This interwoven structure integrates with the polyamide network to establish a stable selective layer, yielding a remarkable 90 % increase in permeability to 20.4 L m-2 h-1 bar-1. Leveraging the conductivity conferred by PEDOT doping, an electro-assisted cleaning strategy is devised, rapidly restoring the flux to 98.3 % within 5 min, outperforming the 30-minute pure water cleaning approach. Through simulations in an 8040 spiral-wound module and the utilization of the permeated salt solution for cleaning, the electro-assisted cleaning strategy emerges as an eco-friendly solution, significantly reducing water consumption and incurring only a marginal electricity cost of 0.055 $ per day. This work presents an innovative avenue for constructing conductive membranes and introduces an efficient and cost-effective electro-assisted cleaning strategy to effectively combat membrane fouling.

Current trends in textile wastewater treatment-bibliometric review

A bibliometric study using 1992 to 2021 database of the Science Citation Index Expanded was carried out to identify which are the current trends in textile wastewater treatment research. The study aimed to analyze the performance of scholarly scientific communications in terms of yearly publications/citations, total citations, scientific journals, and their categories in the Web of Sciences, top institutions/countries and research trends. The annual publication of scientific articles fluctuated in the first ten years, with a steady decrease for the last twenty years. An analysis of the most common terms used in the authors' keywords, publications' titles, and KeyWords Plus was carried out to predict future trends and current research priorities. Adsorbent nanomaterials would be the future of wastewater treatment for decoloration of the residual dyes in the wastewater. Membranes and electrolysis are important to demineralize textile effluent for reusing wastewater. Modern filtration techniques such as ultrafiltration and nanofiltration are advanced membrane filtration applications.

Modular Customization and Regulation of Metal-Organic Frameworks for Efficient Membrane Separations

Metal-organic frameworks (MOFs) are considered ideal membrane candidates for energy-efficient separations. However, the MOF membrane amount to date is only a drop in the bucket compared to the material collections. The fabrication of an arbitrary MOF membrane exhibiting inherent separation capacity of the material remains a long-standing challenge. Herein, we report a MOF modular customization strategy by employing four MOFs with diverse structures and physicochemical properties and achieving innovative defect-free membranes for efficient separation validation. Each membrane fully displays the separation potential according to the MOF pore/channel microenvironment, and consequently, an intriguing H2 /CO2 separation performance sequence is achieved (separation factor of 1656-5.4, H2 permeance of 964-2745 gas permeation unit). Taking advantage of this strategy, separation performance can be manipulated by a non-destructive modification separately towards the MOF module. This work establishes a universal full-chain demonstration for membrane fabrication-separation validation-microstructure modification and opens an avenue for exclusive customization of membranes for important separations.

Separation of dye from aqueous solution by a new gravity compression and aeration system

Filtration is one of the important technologies for separating suspended particles. Under the condition of gravity compression, the filtration density can be increased and the separation effect of suspended particles can be improved. Considering the complex composition and the difficulty in degrading dye in industrial wastewater, a gravity compression aeration system with a modified polyester fibre ball (denoted as MPFB) was evaluated for the separation of dye from water. Congo red azo dye solution (0-40 mg/L) was selected as the model treatment compound. The MPFB was prepared by adjusting the concentrations of alkali (Quality score 0-25%), β-cyclodextrin (0∼80 g/L), reaction temperature (40-90°C), and silane coupler concentration (Concentration fractions 0-0.8%). We used Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) to characterise the MPFB. The separation was affected significantly by adsorption conditions such as MPFB dose and pH. The lower the MPFB dose, the higher the expected adsorption capacity. For the treatment of a dye solution at 500 mg/L, 100% removal was achieved with 48 g/L MPFB, at pH 8 during adsorption under non-circulation aeration. For 24 h of reaction, the system could reach the maximum adsorption capacity of 11.2 mg/g, which followed the pseudo-first order kinetics model and the intraparticle diffusion model. We discovered that circulation aeration provided the best adsorption and electrostatic and hydrogen bonding were the dominant components of adsorption. Overall, the system is a promising technology and has the potential to treat large volumes of dye wastewater.

Electrospun nanofibrous membranes for membrane distillation application-A dynamic life cycle assessment (dLCA) approach

Membrane distillation (MD) for water desalination and purification has been gaining prominence to address the issues relating to water security and the destruction of aquatic ecosystems globally. Recent advances in electrospun membranes for MD application have improved antifouling and anti-wetting performance. However, the environmental impacts associated with producing novel electrospun membranes still need to be clarified. It is imperative to quantify and analyze the tradeoffs between membrane performance and impacts at the early stages of research on these novel membranes. Life Cycle Assessment (LCA) is an appropriate tool to systematically account for environmental performance, all the way from raw material extraction to the disposal of any product, process, or technology. The inherent lack of detailed datasets for emerging technologies contributes to significant uncertainties, making the adoption of traditional LCA challenging. A dynamic LCA (dLCA) is performed to guide the sustainable design and selection of emerging electrospun poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrospun membrane (E-PH) and hybridizing polydimethylsiloxane (PDMS) on E-PH membrane (E-PDMS) for dyeing wastewater treatment technologies. The associated environmental impacts are related to the high energy demands required for fabricating electrospun nanofibrous membranes. After LCA analysis, the E-PDMS membrane emerges as a promising membrane, due to the relatively low impact/benefit ratio and the high performance achieved in treating dyeing wastewater.

Silk Nanofibril-Palygorskite Composite Membranes for Efficient Removal of Anionic Dyes

To develop membrane materials with good performance for water purification that are green and low cost, this work reports an organic-inorganic composite membrane composed of silk nanofibrils (SNFs) and palygorskite (PGS). To improve the stability of the the composite membrane, genipin was used as a crosslinking agent to induce the conformational transition of SNF chains from random coils to β-sheets, reducing the swelling and hydrolysis of the membrane. The separation performance can be adjusted by tailoring the component ratio of the nanomaterial. The results showed that these membranes can effectively remove anionic dyes from water, and they exhibit excellent water permeability. The SNF-based membrane had strong mechanical and separation properties, and the PGS could tune the structure of composite membranes to enhance their permeability, so this green composite membrane has good prospects in water treatment and purification applications.

Direct Contact Membrane Distillation for Treatment of Mixed Wastewater of Humic Acid and Reactive Dye: Membrane Flux Decline and Fouling Analysis

The main waste stream from the textile industry is its wastewater with high color, organic matters, and other contaminants. This study aims to investigate the effect of humic acid in mixed wastewater of humic acid and reactive dye on the treatment performance and permeate flux of a direct contact membrane distillation (DCMD) system. In this research, feed temperature and humic acid concentration were the main input parameters for the analysis of DCMD system operation. The fouling resistances significantly increased with higher humic acid concentrations in the mixed wastewater. As compared with the DI water test, 23% of flux decline occurred when the humic concentration in the wastewater was increased up to 20 mg/L. After the DCMD treatment, the 25 ADMI residual color was detected in the permeate when the mixed wastewater contained 20 mg/L humic acid. The mathematical model, based on the Antione equation, was proposed to predict the membrane flux decline of the DCMD system. The reduced pore size of the cake layer by a dimensionless constant β from the Kelvin equation was also considered for the fouling calculation to describe the transport mechanism.

High-Flux Ultrafiltration Membranes Combining Artificial Water Channels and Covalent Organic Frameworks

Artificial water channels (AWCs) have been well investigated, and the imidazole-quartet water channel is one of the representative channels. In this work, covalent organic frameworks (COFs) composite membranes were fabricated through assembling COF layers and imidazole-quartet water channel. The membranes were synthesized by interfacial polymerization and self-assembly process, using polyacrylonitrile (PAN) ultrafiltration substrates with artificial water channels (HC6H) as modifiers. Effective combination of COF layers and imidazole-quartet water channels provide the membrane with excellent performance. The as-prepared membrane exhibits a water permeance above 271.7 L·m−2·h−1·bar−1, and high rejection rate (>99.5%) for CR. The results indicated that the composite structure based on AWCs and COFs may provide a new idea for the development of high-performance membranes for dye separation.

Removal of protein, histological dye and tetracycline from simulated bioindustrial wastewater with a dual pore size PPSU membrane

Wastewater from production of active pharmaceutical ingredients (APIs) often contains proteins, azo dyes or antibiotics, which cause severe water eutrophication and growth of drug-resistant bacteria. A series of polyphenylsulfone (PPSU) membranes was prepared to determine the relationships between pore structures and the abilities of different membranes to separate foulants, and the characteristics and performance of the ultrafiltration membranes were investigated. The structure of the skin layer and the cross-sectional texture were converted from dense and finger-like macrovoids to porous sponge shapes because of a delayed liquid-liquid (L-L) demixing time. Formation of novel PPSU membranes via noncovalent bonding interactions was evaluated, and this selectively affected the membrane surface pore structure, layer thickness, surface polarity and electronic repulsive force. All PPSU membranes demonstrated excellent rejection of organic foulants, including bovine serum albumin (BSA) (~100% rejection) and acid red 1 (AR1) (~90% rejection). Additionally, M5 provided an excellent tetracycline (TC) rejection efficiency of 89% in the 1^st cycle. Due to the small size of TC, pore size effects were displayed. Moreover, the pure water flux recovery rate (FRR) increased from 85% (M1, water/ethanol: 100/0) to 99.9% (M4, water/ethanol: 30/70) after BSA filtration because the weak nonsolvent decreased the roughness of the membrane surface, and the membrane made with added EtOH yielded excellent FRR values (99.9%) after AR1 filtration. Therefore, PPSU membranes successfully achieved over 90% rejection of organic foulants and excellent FRRs, indicating that they may be suitable for purifying wastewater from API plants that generate organic foulants with a wide range of sizes.

Preparation and characterization of polysulfone-polyurethane membranes for recovery of simulated wastewater from industrial textile processes

Techniques using membranes for the treatment of wastewaters usually promote higher quality of treated water when compared to other processes. Among them, pervaporation has advantages in terms of selectivity in addition to low working pressure, which can prevent clogging problems. Polysulfone and polyurethane have complementary characteristics and are interesting in the context of membranes for industrial applications. In this sense, the aim of this work was to prepare and characterize polysulfone/polyurethane-based membranes and tested them with a simulated wastewater containing the reactive black dye and sodium chloride by pervaporation. In their manufacture, thermal treatment (at 60°C) and photo-radiation treatment (using ultraviolet light) were also applied. The characterizations were performed using different analytical tools. In general, it was possible to verify that all membranes have a dense layer. The thermal analysis allowed to define that the indicated working temperature is below 50°C. With respect to the simulated wastewater treatment, all membranes reached 100% selectivity. Concerning the saline solution, the mean selectivity was around 98.5%. Moreover, the permeate flow values were within the range presented by commercial membranes ranging from 1.6 to 2.4 kg m^-2 h^-1. Although for the photoirradiated membranes the photo-graft reaction has occurred, among all membranes, the blend without any treatment stood out from the others, presenting the highest permeate flow of the simulated wastewater. Finally, the results reveal that these membranes are capable of recovering wastewater from textile processes, in addition to having the potential to remove salts from water through the pervaporation process.Highlights Polysulfone/polyurethane-based membranes were not yet evaluated for wastewater recovery.Modifications in the membrane characteristics promoted variations in the permeate flow.Changes in physical-chemical properties of membrane as a result of a photoinitiation reaction.Removal efficiency achieved was 100% for reactive black dye and 98.5% for sodium chloride.A new way of performing pervaporation on the recovery of aqueous solutions.

Highly Efficient Oxygen-Activated Self-Cleaning Membranes Prepared by Grafting a Metal-Organic Framework-Derived Catalyst

In this study, an efficient oxygen-activated self-cleaning membrane was successfully prepared by grafting a metal-organic framework-devised catalyst (CuNi-C) onto a membrane surface, resulting in enhanced filtration performance and self-cleaning capability based on oxygen activation under mild conditions. The pore features, surface roughness, and surface hydrophilicity of the prepared membrane were analyzed and used to determine the causes of the enhanced filtration performance; the results showed that an increase in the porosity and surface roughness enhanced the permeate flux, and enhanced adsorption capacity and surface hydrophobicity improved the membrane removal efficiency. The self-cleaning mechanism was elucidated by identifying the reactive oxygen species (ROS) and detecting catalytic element valences. The results revealed that zero-valent Cu embedded into the membrane surface effectively activated natural dissolved oxygen (DO) to generate ROS that degraded organic pollutants. In this study, catalytic oxidation with DO as the oxidant was successively integrated with membrane separation to prevent membrane fouling, providing a novel direction for the development of multifunctional membranes.

A Comprehensive Review of Effective Adsorbents Used for the Removal of Dyes from Wastewater

Aim:

The objective of the review paper aims to explore and to provide the insight of various low-cost adsorbents prepared and used in the removal of hazardous dye pollutants from the contaminated industrial effluents.

Background:

The major untreated discharge from the textile industries constitutes a wide range of organic contaminants with the enhanced concentration of biological oxygen demand and chemical oxygen demand inthe water bodies. Dyes are considered as the major water contaminants and this quest the researchers to adopt various technologies to remove the hazardous dye pollutants from the aquatic environment. Dyes are the chemical compounds that tend to adhere themselves with metal or salts by covalent bond formation or complexes by mechanical retention or physical adsorption so as to impart colours to which it is being applied.

Objective:

Numerous treatment methodologies which have been applied to the degradation of dyes. The current study has been focused on the distinct low cost and cost-effective adsorbents used in the removal of various dye pollutants. Also, the application of nanoparticles in the removal of the hazardous dye pollutants had received great interest because of its size and high reactive nature.

Methods:

The treatment technologies used in the removal of dye pollutants from wastewater have been listed as adsorption, coagulation, electrocoagulation, flocculation, membrane filtration, oxidation and biological treatment.

Results:

The complex structure of the dyes causes a great harmful impact on the aquatic environment. Though numerous treatment technologies have been applied, adsorption has been preferred by various researchers because of its cost-effective nature.

Conclusion:

The various adsorbents are used in the removal of cationic, anionic and non-ionic dyes. The different types of adsorbent from agricultural waste, activated carbons, nanomaterials and biomaterials have been discussed with the advantages and limitations.

Noninvasive Real-Time Monitoring of Wetting Progression in Membrane Distillation Using Impedance Spectroscopy

Membrane distillation (MD) is a promising technology for the treatment of high salinity wastewater using a hydrophobic membrane; however, the occurrence of wetting due to surfactants in polluted or low surface tension liquid impedes MD application. Common monitoring approaches, such as conductivity and flux measurement, cannot explain the wetting phenomenon that occurs during the wetting process in detail. Recently, impedance spectroscopy has been proposed for early wetting detection, as it depends on the change of water/air composition in the membrane pores. An earlier and larger variation was observed with precise signal detection. In this study, we proposed an analytical approach to estimate the wetting front, which is the average feed intrusion distance, by the impedance value recorded in real-time operation. With this proposed approach, the wetting mechanism in the presence of a surfactant and the effect of pore size on a commercial polyvinylidene fluoride membrane could be quantified, which cannot be explained in detail using conductivity and flux measurements.

Membrane Distillation for Wastewater Treatment: A Mini Review

Water serves as an indispensable part of human life and production. On account of the overexploitation of traditional water sources, the demand for wastewater recycling is expanding rapidly. As a promising water treatment process, membrane distillation (MD) has been utilized in various wastewater treatments, such as desalination brine, textile wastewater, radioactive wastewater, and oily wastewater. This review summarized the investigation work applying MD in wastewater treatment, and the performance was comprehensively introduced. Moreover, the obstructions of industrialization, such as membrane fouling, membrane wetting, and high energy consumption, were discussed with the practical investigation. To cope with these problems, various strategies have been adopted to enhance MD performance, including coupling membrane processes and developing membranes with specific surface characteristics. In addition, the significance of nutrient recovery and waste heat utilization was indicated.

In-situ 3D fouling visualization of membrane distillation treating industrial textile wastewater by optical coherence tomography imaging

Membrane fouling, which is caused by the deposition of particles on the membrane surface or pores, reduces system performance in membrane distillation (MD) applications, resulting in increased operational costs, poor recovery, and system failure. Optical Coherence Tomography enables in-situ foulant monitoring in both 2D and 3D, however, the 2D images can only determine fouling layer thickness in severe fouling. Therefore, in this study, an advanced 3D imaging analysis technique using intensity range filters was proposed to quantify fouling layer formation during MD through the use of a single 3D image. This approach not only reduces computational power requirements, but also successfully separated the fouling layer from the membrane at the microscale. Thus, the thickness, fouling index, and fouling layer coverage can be evaluated in real time. To test this approach, Polyvinylidene fluoride (C-PVDF) and polytetrafluoroethylene (C-PTFE) membranes were used to treat a feed consisting of industrial textile wastewater. Thin and disperse foulants was observed on the C-PTFE, with a 22 µm thick fouling layer which could not be observed using 2D images after 24 h. Moreover, the C-PTFE demonstrated better antifouling ability than the C-PVDF as demonstrated by its lower fouling index, which was also supported by surface energy characterization. This work demonstrates the significant potential of 3D imagery in the long-term monitoring of membrane fouling process to improve membrane antifouling performance in MD applications, which can lead to lowered operational costs and improved system stability.

Modification of porous polyetherimide hollow fiber membrane by dip-coating of Zonyl® BA for membrane distillation of dyeing wastewater

Wetting and fouling have significantly affected the application of membrane distillation (MD). In this work, a dip-coating method was used for improving surface hydrophobicity of the polyetherimide (PEI) hollow fiber membrane. An air gap membrane distillation (AGMD) process was applied for treatment of the methylene blue (MB) solution. The porous PEI membrane was fabricated by a dry-wet spinning process and the hydrophobic 2-(Perfluoroalkyl) ethanol (Zonyl^® BA) was used as the coating material. From FESEM, the modified PEI-Zonyl membrane showed an open structure with large finger-like cavities. The modified membrane displayed a narrow pore size distribution with mean pore size of 0.028 μm. The outer surface contact angle of the PEI-Zonyl membrane increased from 81.3° to 100.4° due to the formation of an ultra-thin coated layer. The pure water flux of the PEI-Zonyl membrane was slightly reduced compared to the pristine PEI membrane. A permeate flux of 6.5 kg/m² h and MB rejection of 98% were found for the PEI-Zonyl membrane during 76 h of the AGMD operation. Adsorption of MB on the membrane surface was confirmed based on the Langmuir isotherm evaluation, AFM and FESEM analysis. The modified PEI-Zonyl membrane can be a favorable alternative for AGMD of dyeing wastewaters.

Optimize the preparation of Fe3O4-modified magnetic mesoporous biochar and its removal of methyl orange in wastewater

In this paper, Eichhornia Crassipes stems were used as biomass feedstock, and Fe²⁺ was used as the precursor solution to prepare Fe₃O₄-modified magnetic mesoporous biochar (Fe₃O₄@BC). By using Box-Behnken design (BBD) response surface methodology, the influences of three preparation parameters (X₁ = Fe²⁺ concentration, X₂ = pyrolysis temperature and X₃ = pyrolysis time) on the adsorption of methyl orange (MO) by Fe₃O₄@BC were investigated, and a reliable response surface model was constructed. The results show that X₁X₂ and X₁X₃ have a significant influence on the adsorption of MO by Fe₃O₄@BC. The surface area and pore volume of Fe₃O₄@BC are controlled by all preparation parameters. The increase of pyrolysis time will significantly reduce the -OH on the surface of Fe₃O₄@BC and weaken its MO adsorption capacity. Through the numerical optimization of the constructed model, the optimal preparation parameters of Fe₃O₄@BC can be obtained as follows: Fe²⁺ concentration = 0.27 mol/L, pyrolysis temperature = 405 °C, and pyrolysis time = 3.2 h. The adsorption experiment shows that the adsorption of Fe₃O₄@BC to MO is a spontaneous exothermic process, and the adsorption capacity is maximum when pH = 4. The adsorption kinetics and adsorption isotherms of Fe₃O₄@BC to MO conform to the pseudo-second-order kinetics and Sips model, respectively. Mechanism analysis shows that electrostatic interaction and H bond formation are the main forces for Fe₃O₄@BC to adsorb MO. This research not only realizes a new way of resource utilization of Eichhornia Crassipes biomass but also enriches the preparation research of magnetic biochar.

Electrochemical removal of RRX-3B in residual dyeing liquid with typical engineered carbonaceous cathodes

Electro-catalytic activities of carbonaceous cathodes including graphite plate, graphite felt, carbon felt, activated carbon felt (ACF) and carbon fiber felt (CFF) for degradation of Reactive Red X-3B (RRX-3B) in residual dyeing liquid were compared. The best electrochemical performance was obtained using dimensional stable anode (DSA) and CFF cathode due to the higher capacity for electro-generation of H₂O₂ by selective two-electron oxygen reduction. The CFF/DSA electrolysis system realized 78.2% COD removal and complete decolorization over a wide pH range. The efficacy of RRX-3B degradation was found to be dependent on the nature of carbonaceous materials. Electrochemical measurements showed that CFF possessed higher electrochemical surface area and hydrogen evolution reaction over-potential. Furthermore, the intrinsic graphitic N in CFF was proved to be catalytic active site by DFT calculations. Reactive Red X-3B degradation intermediates with benzene structures and carboxylic acids via hydroxylation in RRX-3B oxidation were identified by GC-MS. It was found that S/Cl/N-containing groups in RRX-3B molecule were mineralized to SO₄^2-, NO₃^- and Cl^- ions in the electrolysis.

Membrane Surface Functionalization with Imidazole Derivatives to Benefit Dye Removal and Fouling Resistance in Forward Osmosis

Water contaminated with low concentrations of pollutants is more difficult to clean up than that with high pollutant content levels. Membrane separation provides a solution for removing low pollutant content from water. However, membranes are prone to fouling, losing separation performances over time. Here we synthesized neutral (IM-NH₂) and positively charged (IL-NH₂) imidazole derivatives to chemically functionalize membranes. With distinct properties, these imidazole grafts could tailor membrane physicochemical properties and structures to benefit forward osmosis (FO) processes for the removal of 20-100 ppm of Safranin O dye-a common dye employed in the textile industry. The water fluxes produced by IM-NH₂- and IL-NH₂-modified membranes increased by 67% and 122%, respectively, with DI water as the feed compared to that with the nascent membrane. A 39% flux increment with complete dye retention (∼100%) was achieved for the IL-NH₂-modified membrane against 100 ppm of Safranin O dye. Regardless of the dye concentration, the IL-NH₂-modified membrane exhibited steadily higher permeation performance than the original membrane in long-term experiments. Reproducible experimental results were obtained with the IL-NH₂-modified membrane after cleaning with DI water, demonstrating the good antifouling properties and renewability of the newly developed membrane.

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

A Mini Review on Antiwetting Studies in Membrane Distillation for Textile Wastewater Treatment

The textile industry is an important contributor to the growth of the global economy. However, a huge quantity of wastewater is generated as a by-product during textile manufacturing, which hinders the ongoing development of textile industry in terms of environmental sustainability. Membrane distillation (MD), which is driven by thermal-induced vapor pressure difference, is being considered as an emerging economically viable technology to treat the textile wastewater for water reuse. So far, massive efforts have been put into new membrane material developments and modifications of the membrane surface. However, membrane wetting, direct feed solution transport through membrane pores leading to the failure of separation, remains as one of the main challenges for the success and potential commercialization of this separation process as textile wastewater contains membrane wetting inducing surfactants. Herein, this review presents current progress on the MD process for textile wastewater treatment with particular focuses on the fundamentals of membrane wetting, types of membranes applied as well as the fabrication or modification of membranes for anti-wetting properties. This article aims at providing insights in membrane design to enhance the MD separation performance towards commercial application of textile wastewater treatment.

Polysulfone-Polyvinyl Pyrrolidone Blend Polymer Composite Membranes for Batik Industrial Wastewater Treatment

Batik wastewater, in general, is colored and has high concentrations of BOD (biological oxygen demand), COD (chemical oxygen demand), and dissolved and suspended solids. Polysulfone (PSf)-based membranes with the addition of polyvinyl pyrrolidone (PVP) were prepared to treat batik industrial wastewater. PSf/PVP membranes were prepared using the phase inversion method with N-methyl-2 pyrrolidone (NMP) as the solvent. Based on the membrane characterization through FESEM, water contact angle, porosity, and mechanical tests showed a phenomenon where the addition of PVP provided thermodynamic and kinetic effects on membrane formation, thereby affecting porosity, thickness, and hydrophilicity of the membranes. The study aims to observe the effect of adding PVP on polysulfone membrane permeability and antifouling performance on a laboratory scale through the ultrafiltration (UF) process. With the addition of PVP, the operational pressure of the polysulfone membrane was reduced compared to that without PVP. Based on the membrane filtration results, the highest removal efficiencies of COD, TDS (total dissolved solid), and conductivity achieved in the study were 80.4, 84.6, and 83.6%, respectively, on the PSf/PVP 0.35 membrane operated at 4 bar. Moreover, the highest color removal efficiency was 85.73% on the PSf/PVP 0.25 operated at 5 bar. The antifouling performance was identified by calculating the value of total, reversible, and irreversible membrane fouling, wherein in this study, the membrane with the best antifouling performance was PSf/PVP 0.25.

Performance Evaluation and Kinetic Analysis of Photocatalytic Membrane Reactor in Wastewater Treatment

The objectives of the current study are to assess and compare the performance of a developed photocatalytic membrane reactor (PMR) in treating industrial waste (e.g., organic dye waste) against membrane distillation. The current PMR is composed of a feed tank, which is a continuous stirred photocatalytic reactor containing slurry Titanium dioxide (TiO₂) particles that are activated by using ultraviolet lamp irradiation at a wavelength of 365 nm, and a poly-vinylidene flouride (PVDF) membrane cell. The experimental setup was designed in a flexible way to enable both separate and integrated investigations of the photocatalytic reactor and the membrane, separately and simultaneously. The experimental work was divided into two phases. Firstly, the PVDF membrane was fabricated and characterized to examine its morphology, surface charge, and hydrophobicity by using a scanning electron microscope, surface zeta potential, and contact angle tests, respectively. Secondly, the effects of using different concentrations of the TiO₂ photocatalyst and feed (e.g., dye concentration) were examined. It is found that the PMR can achieve almost 100% dye removal and pure permeate is obtained at certain conditions. Additionally, a kinetic analysis was performed and revealed that the photocatalytic degradation of dye follows a pseudo-first-order reaction.

Biopolymer extracted from Moringa oleifera Lam. in conjunction with graphene oxide to modify membrane surfaces

A novel modification through pressurized filtration technique of commercial polyethersulfone membranes was proposed using a biopolymer extracted from Moringa oleifera Lam. (MO) along with graphene oxide (GO), in order to decrease fouling and improve the membrane selectivity for the removal of methylene blue dye. The effect of these agents over the morphology and performance of the membranes were studied through the evaluation of permeability, dye removal and fouling parameters. The characterization of the membrane indicates a significant decrease in pore size, as confirmed by the filtration experiments. Also, according to FTIR and SEM analysis the modification was effectively accomplished. All modified membranes presented low fouling rates (<10.55%) and high dye removal rates (from 2.85% to 96.73%). Furthermore, it is the first time that MO has been used as a natural polymer to improve and confer new characteristics on membranes, creating new possibilities for further study of this promising environment-friendly biopolymer in membrane separation processes.

Dye synthetic solution treatment by direct contact membrane distillation using commercial membranes

The reuse of treated dyeing wastewater has become a viable option to minimizing water scarcity problems and environmental impacts in the textile industry. The potentiality of commercial flat sheet membranes of polytetrafluoroethylene (PTFE) and polypropylene (PP) in direct contact membrane distillation (DCMD) for dye synthetic solution treatment has been explored in this work. DCMD is interesting for the textile industry since a recovery of heat by hot dyeing wastewater for thermal energy is possible. Moreover, DCMD enables water and dye reclamation with possible reuse in the textile process. The commercial availability of membranes may expedite the DCMD commercialization in the textile industry. Experiments were conducted in a laboratory-scale circulating unit with synthetic solutions containing reactive or disperse dye. High mean permeate flux up to 18.8 kg m^-2 h^-1 with complete colour rejection was obtained. The dyes tested in this study are not able to completely wet the membranes and the increase of the permeate flux when compared to distilled water is attributed to electrostatic interactions between the dyes and the membranes. Moreover, a partial wetting reduced vapour diffusion path and the permeate flux was increased. PP membrane showed higher performance due to higher porosity when compared to the PTFE membrane. In addition, an influence of dye class on permeability was observed. The results were promising when compared to other studies, which used commercial or lab-scale membranes.