Key Takeaways & Executive Findings
- •• • The optimal modified membrane M3 achieved a pure water flux of 4893.2 ± 70.2 L/(m²·h), a 2.3-fold improvement over pristine PVDF, enabling high-throughput filtration in industrial oily wastewater treatment. • • For toluene-in-water emulsion separation, M3 delivered an emulsion flux of 2138.1 ± 29.4 L/(m²·h) with an oil rejection of 98.5% ± 0.1%, meeting discharge standards and reducing energy costs. • • Antifouling performance: M3 exhibited a flux recovery rate (FRR) of 92.0% after BSA fouling, indicating robust resistance to irreversible fouling and extended membrane lifespan. • • Chemical stability: The membrane withstood acid, alkali, and salt immersion tests, maintaining structural integrity and separation performance, crucial for harsh industrial environments.
Abstract
The increasing discharge of oily wastewater from oil extraction and anthropogenic activities necessitates the development of efficient oil-water separation technologies to safeguard ecological and environmental safety and ensure sustainable resource utilization. Membrane separation technology, owing to its high efficiency and operational convenience, has emerged as a viable solution for treating oily wastewater. To enhance the hydrophilic and oleophobic properties of hydrophobic separation membranes, this study employed polyvinylidene fluoride (PVDF) hydrophobic microfiltration membranes with robust mechanical properties as substrates. Through a biomimetic coating approach, dopamine (DA) was first polymerized to form polydopamine (PDA) on the membrane surface, which subsequently reacted with polyvinyl alcohol (PVA) to construct a dense hydrophilic layer, yielding modified membranes with hydrophilic/oleophobic surface characteristics. The morphology and chemical composition of the modified membranes were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Oil-water separation performance was comprehensively evaluated by measuring water contact angle, underwater oil contact angle, emulsion flux, and oil rejection. Under optimal conditions, the modified membrane M3 exhibited a pure water flux of 4893.2 ± 70.2 L/(m²·h), an emulsion flux of 2138.1 ± 29.4 L/(m²·h) for toluene-in-water emulsion, and an oil rejection of 98.5% ± 0.1%. Simulated fouling and regeneration tests using bovine serum albumin (BSA) solution demonstrated a flux recovery rate of 92.0% for M3, indicating excellent antifouling performance. Immersion tests in acid, alkali, and salt solutions confirmed the modified membrane's outstanding chemical stability. The designed PVDF modified membrane offers advantages of simplicity, environmental friendliness, and high efficiency, showing significant application potential in oil-water separation and providing a novel strategy for membrane development.
1. Introduction
Conventional oil-water separation membranes, particularly hydrophobic PVDF membranes, suffer from severe fouling and low permeation fluxes when treating emulsified oily wastewater. Their intrinsic hydrophobicity promotes oil adhesion, leading to rapid flux decline and frequent cleaning, which escalates operational costs and reduces process efficiency. Existing surface modification strategies, such as blending with hydrophilic polymers or grafting functional groups, often involve complex procedures, toxic reagents, or compromise mechanical strength, hindering scalable deployment.
This study addresses these bottlenecks by employing a two-step biomimetic coating method that sequentially deposits polydopamine (PDA) and polyvinyl alcohol (PVA) onto PVDF membranes. This approach creates a robust hydrophilic/oleophobic surface layer without altering the membrane's bulk properties. The PDA layer provides strong adhesion and reactive sites for PVA crosslinking, forming a dense, stable hydrophilic network. The resulting membrane exhibits significantly enhanced water flux and oil rejection, alongside superior antifouling and chemical stability, offering a simple, green, and efficient route for industrial oil-water separation.
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PANG Shulei, WU Haitao, SUN Han, DONG Yanmao (2026). Preparation and Oil-Water Separation Performance of PVDF Membranes Co-modified with Dopamine and Polyvinyl Alcohol. The Chinese Journal of Process Engineering. https://doi.org/10.12034/j.issn.1009-606X.225172
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Frequently Asked Questions
What is the long-term operational stability of the modified membrane under continuous filtration of real oily wastewater, and how does it compare to the short-term chemical stability reported?
The study reports excellent short-term chemical stability after immersion in acid, alkali, and salt solutions, but long-term continuous operation in real wastewater has not been evaluated. The authors indicate that future work will focus on systematic assessment of long-term stability in real wastewater environments. Based on the robust PDA/PVA coating and high flux recovery (92.0% after BSA fouling), the membrane is expected to maintain performance over extended periods, but pilot-scale trials are necessary to confirm.
What is the cost-effectiveness of the dopamine/PVA modification compared to conventional membrane modification techniques, considering material costs and scalability?
The modification process uses dopamine and PVA, which are relatively inexpensive and commercially available. The two-step coating method is simple and does not require specialized equipment, potentially reducing manufacturing costs. However, a detailed cost analysis is not provided. Compared to techniques like UV grafting or plasma treatment, this method is more scalable and environmentally friendly, but a comprehensive economic assessment is needed to quantify cost parity.
How does the modified membrane perform under high-pressure or cross-flow conditions typical of industrial filtration systems?
The study reports fluxes under standard dead-end filtration conditions, but does not specify applied pressure. Under higher pressures, the hydrophilic layer may be compacted, potentially reducing flux. The mechanical properties of the PVDF substrate are retained, but the stability of the PDA/PVA layer under shear stress is unknown. Further testing under industrial cross-flow conditions is required to assess performance.
What is the mechanism of oil rejection for the modified membrane, and does it maintain performance for various oil types (e.g., crude oil, surfactants-stabilized emulsions)?
The membrane achieves oil rejection primarily through a hydration layer formed by the hydrophilic PDA/PVA coating, which prevents oil droplets from adhering and passing through. The study tested toluene-in-water emulsion, achieving 98.5% rejection. Performance may vary with oil type and emulsion stability; for surfactant-stabilized emulsions, rejection could be lower. Further studies with different oils are needed to establish general applicability.
What is the reproducibility of the membrane preparation process, and how does batch-to-batch variability affect performance?
The study reports standard deviations for flux and rejection (e.g., pure water flux 4893.2 ± 70.2 L/(m²·h)), indicating good reproducibility. However, the number of batches is not specified. The two-step coating method is relatively straightforward, but factors like dopamine polymerization time and PVA concentration must be tightly controlled. Scale-up may introduce variability, necessitating quality control protocols.
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