Recent Progress in Microfiltration/Ultrafiltration Membranes for Separation of Oil and Water Emulsions

Next-generation stimuli-responsive smart membranes: Developments in oil/water separation

Effective treatment of oil-contaminated wastewater is essential due to its severe environmental and health impacts. The membrane-based separation is cost-effective, energy-efficient, and eco-friendly; however, fouling has remained a pressing issue. Stimuli-responsive membranes, which adjust their pore structure and surface properties in response to external triggers such as light, pH, and temperature, offer enhanced fouling resistance and improved separation performance. This review provides a comprehensive analysis of stimuli-responsive membranes for oil/water separation, emphasizing the role of smart polymeric materials engineered for controllable separation processes. We critically assess the strengths of these advanced membranes, including their tuneable wettability and energy-efficient operation, while identifying key limitations such as long-term stability, response time, scalability, and cost-effectiveness. Furthermore, the review explores various polymer types, synthesis methods, and fabrication techniques, evaluating their effectiveness in separation applications. Finally, the review concludes by outlining the challenges and proposing future directions to enhance the performance of stimuli-responsive membranes. By offering valuable insights into the dynamic control of membrane structures and properties, this study aims to inspire the development of next-generation stimuli-responsive membranes, drive their commercialization, and promote sustainable water treatment solutions.

Tunning the Wettability of the PVDF Membrane using the PVA-Stabilized TA-UiO-66-NH2 MOF Membranes to Separate Layered Oil-Water Mixture and Surfactant-Stabilized Emulsion

This study introduces a UiO-66-NH2/Tannic acid/Polyvinylidene fluoride (UTP) composite membrane for efficient oil-water separation. Pristine polyvinylidene fluoride (PVDF) membranes, due to their hydrophobic nature, tend to foul during oil-in-water emulsion separation. By incorporating the metal-organic framework (MOF) UiO-66-NH2 and stabilizing it with tannic acid (TA) and polyvinyl alcohol (PVA), the membrane's hydrophilicity and antifouling properties were significantly enhanced. The water contact angle of the UTP membrane decreased from 121° to 3°, indicating a dramatic increase in hydrophilicity, while the underwater oil contact angle (UWOCA) of 119° demonstrated excellent oleophobicity. The modified membrane achieved over 99 % separation efficiency and improved flux by 15 times compared to the pristine PVDF. TA acted as a binder, ensuring uniform MOF dispersion and improving the composite's stability. The PVA further reinforced the structure, enhancing durability under operational conditions. Durability tests showed no significant MOF detachment after repeated use, confirming the stability of the UTP composite. The results highlight the potential of the UTP membrane for oil-water separation with superior permeability and fouling resistance.

Changes in the Separation Properties of Aged PVDF Ultrafiltration Membranes During Long-Term Treatment of Car Wash Wastewater

Car wash wastewater (CWW) is complex waste that may be effectively treated by the ultrafiltration (UF) process. However, one of the most important challenges in implementing this process on an industrial scale is the fouling phenomenon membrane aging. Indeed, these may lead to a reduction in UF performance possibly associated with a loss in integrity of the fouled/aged membrane. Therefore, the main aim of the current study was to provide a comprehensive investigation on the changes in the separation properties of aged FP100 ultrafiltration membranes made of polyvinylidene fluoride (PVDF) with respect to their application for long-term treatment of CWW. For this purpose, studies were conducted for new membranes and membranes previously used for over 5 years in a pilot plant. As a feed, solutions of dextran, solutions of model organism Escherichia coli and synthetic CWW were used. It has been found that PVDF membranes demonstrated poor stability when in frequent contact with chemicals periodically applied for membrane cleaning. Indeed, the aged membranes were characterised by the increased porosity. However, it is important to note that membranes aging had no significant impact on the permeate quality during the UF process of synthetic CWW. Indeed, the obtained permeate was characterised by the turbidity lower than 0.25 NTU. Likewise, with regard to the separation of E. coli, the aged PVDF membranes ensured the high process efficiency and over 99.99% bacterial retention. In the interest of the growing potential of PVDF membrane in CWW treatment, the results obtained in the current work complement the findings made in this field.

Design and Machine Learning Prediction of In Situ Grown PDA-Stabilized MOF (UiO-66-NH2) Membrane for Low-Pressure Separation of Emulsified Oily Wastewater

Significant progress has been made in designing advanced membranes; however, persistent challenges remain due to their reduced permeation rates and a propensity for substantial fouling. These factors continue to pose significant barriers to the effective utilization of membranes in the separation of oil-in-water emulsions. Metal-organic frameworks (MOFs) are considered promising materials for such applications; however, they encounter three key challenges when applied to the separation of oil from water: (a) lack of water stability; (b) difficulty in producing defect-free membranes; and (c) unresolved issue of stabilizing the MOF separating layer on the ceramic membrane (CM) support. In this study, a defect-free hydrolytically stable zirconium-based MOF separating layer was formed through a two-step method: first, by in situ growth of UiO-66-NH2 MOF into the voids of polydopamine (PDA)-functionalized CM during the solvothermal process, and then by facilitating the self-assembly of UiO-66-NH2 with PDA using a pressurized dead-end assembly. A stable MOF separating layer was attained by enriching the ceramic support with amines and hydroxyl groups using PDA, which assisted in the assembly and stabilization of UiO-66-NH2. The PDA-s-UiO-66-NH2-CM membrane displayed air superhydrophilicity and underwater superoleophobicity, demonstrating its oil resistance and high antifouling behavior. The PDA-s-UiO-66-NH2-CM membrane has shown exceptionally high permeability and separation capacity for challenging oil-in-water emulsions. This is attributed to numerous nanochannels from the membrane and its high resistance to oil adhesion. The membranes showed excellent stability over 15 continuous test cycles, which indicates that the developed MOFs separating layers have a low tendency to be clogged by oil droplets during separation. Machine learning-based Gaussian process regression (GPR) models as nonparametric kernel-based probabilistic models were employed to predict the performance efficiency of the PDA-s-UiO-66-NH2-CM membrane in oil-in-water separation. The outcomes were compared with the support vector machine (SVM) and decision tree (DT) algorithm. This efficiency includes various metrics related to its separation accuracy, and the models were developed through feature engineering to identify and utilize the most significant factors affecting the membrane's performance. The results proved the reliability of GPR optimization with the highest prediction accuracy in the validation phase. The average percentage increase of the GPR model compared to the SVM and DT model was 6.11 and 42.94%, respectively.

Highly Permeable Sulfonated Polydopamine Integrated MXene Membranes for Efficient Surfactant-Stabilized Oil-in-Water Separation

MXene is an incredibly promising two-dimensional material with immense potential to serve as a high-performing separating or barrier layer to develop advanced membranes. Despite the significant progress made in MXene membranes, two major challenges still exist: (i) effectively stacking MXene nanosheets into defect-free membranes and (ii) the high fouling tendency of MXene-based membranes. To address these issues, we employed sulfonated polydopamine (SPD), which simultaneously serves as a binding agent to promote the compact assembling of Ti3C2Tx MXenes (MX) nanosheets and improves the antifouling properties of the resulting sulfonated polydopamine-functionalized MX (SPDMX) membranes. The SPDMX membrane was tested for challenging surfactant-stabilized oil-in-water separation with an impressive efficiency of 98%. Moreover, an ultrahigh permeability of 1620 LMH/bar was also achieved. The sulfonation of PD helps in improving the antifouling characteristics of SPDMX by developing a strong hydration layer and enhancing the oleophobicity of the membrane. The underwater SPDMX membrane appeared superoleophobic with an oil contact angle of 153°, whereas the ceramic membrane exhibited an oil contact angle of 137°. The SPDMX membranes showed an improved flux recovery (31%) compared to the nonsulfonated counterpart. This work highlights the appropriate functionalization of MXene as a promising approach to developing MXene membranes with high permeation flux and better antifouling characteristics for oily wastewater treatment.

New insight into the irreversible membrane fouling in different pore-sized ultrafiltration ceramic membrane bioreactors (UCMBRs) for high-strength textile wastewater treatment

Despite great achievements in ceramic membrane bioreactor applications, membrane fouling, which decreases the permeability and separation performance of bioreactors and is associated with increased operational costs and energy consumption, remains a problem. The aim of this study was to expand our understanding of the fouling behavior in the long-term performance of ultrafiltration ceramic membrane bioreactors (UCMBRs) for high-strength textile wastewater reclamation. Using real textile wastewater effluent, the effects of ultrafiltration (UF) membrane pore sizes, cleaning strategies, and foulant distribution were systematically evaluated over more than three months of continuous operation. The results showed that UCMBR system achieved chemical oxygen demand and total nitrogen removal efficiencies as high as 91-95% and 39-43%, respectively. The high PN concentration can easily increase the viscosity of mixed liquor samples, contributing to a fouling layer on the membrane surface. In addition, the fouling layer formed on the surface of small-pore-sized ceramic UF membranes was not completely reversible but was difficult to eliminate by simple physical cleaning. Soluble extracellular polymeric substances, especially proteins and low molecular weight neutrals, remained, resulting in irreversible fouling on the UF membrane. However, saturated CO₂ backwash showed great potential for enhancing the system through efficient fouling control without using environmentally unfriendly cleaning chemicals. The cake-intermediate and complete-standard models were suitable for explaining the fouling mechanism in the large- and small-pore-sized UF membranes, respectively.

Hierarchically Structured Hybrid Membranes for Continuous Wastewater Treatment via the Integration of Adsorption and Membrane Ultrafiltration Mechanisms

Growing environmental concerns are stimulating researchers to develop more and more efficient materials for environmental remediation. Among them, polymer-based hierarchical structures, attained by properly combining certain starting components and processing techniques, represent an emerging trend in materials science and technology. In this work, graphene oxide (GO) and/or carbon nanotubes (CNTs) were integrated at different loading levels into poly (vinyl fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and then electrospun to construct mats capable of treating water that is contaminated by methylene blue (MB). The materials, fully characterized from a morphological, physicochemical, and mechanical point of view, were proved to serve as membranes for vacuum-assisted dead-end membrane processes, relying on the synergy of two mechanisms, namely, pore sieving and adsorption. In particular, the nanocomposites containing 2 wt % of GO and CNTs gave the best performance, showing high flux (800 L × m^-2 h^-1) and excellent rejection (99%) and flux recovery ratios (93.3%), along with antifouling properties (irreversible and reversible fouling below 6% and 25%, respectively), and reusability. These outstanding outcomes were ascribed to the particular microstructure employed, which endowed polymeric membranes with high roughness, wettability, and mechanical robustness, these capabilities being imparted by the peculiar self-assembled network of GO and CNTs.

Prospects of Polymeric Nanocomposite Membranes for Water Purification and Scalability and their Health and Environmental Impacts: A Review

Water shortage is a major worldwide issue. Filtration using genuine polymeric membranes demonstrates excellent pollutant separation capabilities; however, polymeric membranes have restricted uses. Nanocomposite membranes, which are produced by integrating nanofillers into polymeric membrane matrices, may increase filtration. Carbon-based nanoparticles and metal/metal oxide nanoparticles have received the greatest attention. We evaluate the antifouling and permeability performance of nanocomposite membranes and their physical and chemical characteristics and compare nanocomposite membranes to bare membranes. Because of the antibacterial characteristics of nanoparticles and the decreased roughness of the membrane, nanocomposite membranes often have greater antifouling properties. They also have better permeability because of the increased porosity and narrower pore size distribution caused by nanofillers. The concentration of nanofillers affects membrane performance, and the appropriate concentration is determined by both the nanoparticles' characteristics and the membrane's composition. Higher nanofiller concentrations than the recommended value result in deficient performance owing to nanoparticle aggregation. Despite substantial studies into nanocomposite membrane manufacturing, most past efforts have been restricted to the laboratory scale, and the long-term membrane durability after nanofiller leakage has not been thoroughly examined.