Flux and Fouling Behavior of Graphene Oxide-Polyphenylsulfone Ultrafiltration Membranes Incorporating ZIF-67/ZIF-8 Fillers 🌊🧪



Water is life 💧, but access to clean water is a challenge in many parts of the world. Advanced membrane technologies have emerged as one of the most promising solutions for water purification, wastewater treatment, and industrial separations. Among these, ultrafiltration (UF) membranes have gained attention for their efficiency in removing contaminants while maintaining high water flux. Recently, researchers have been exploring graphene oxide-polyphenylsulfone (GO-PPSU) membranes enhanced with ZIF-67 and ZIF-8 fillers to improve performance, particularly addressing two major challenges: flux and fouling.

In this post, we’ll dive deep into what makes these membranes special, how ZIF fillers enhance their properties, and what recent studies reveal about their performance. 🚀

What Are Ultrafiltration Membranes? 🧩

Ultrafiltration membranes are thin barriers designed to separate molecules based on size. Typically, they have pore sizes ranging from 1–100 nm, allowing water and small molecules to pass through while retaining larger contaminants like bacteria, viruses, and colloids.

UF membranes are used in:

  • Drinking water purification 🚰

  • Wastewater treatment 🏭

  • Food and beverage industries 🍶

  • Pharmaceutical production 💊

However, two common challenges often limit their efficiency:

  1. Flux decline – reduction in water permeation over time.

  2. Fouling – accumulation of particles, proteins, or biofilms on the membrane surface, reducing performance.

Improving membrane flux while minimizing fouling is a key goal in membrane research.

Enter Graphene Oxide (GO) 🌐🖤

Graphene oxide is a derivative of graphene, a single layer of carbon atoms arranged in a honeycomb lattice. GO has unique properties that make it ideal for ultrafiltration membranes:

  • High hydrophilicity 💦 – attracts water molecules, increasing flux.

  • Mechanical strength 💪 – enhances membrane durability.

  • Functional groups (–OH, –COOH) 🧬 – allow interaction with polymers and fillers, improving structural stability.

By blending GO with polyphenylsulfone (PPSU), researchers can produce membranes that are both strong and permeable. PPSU is already a widely used polymer in UF membranes due to its thermal stability, chemical resistance, and mechanical strength. Adding GO improves hydrophilicity and antifouling properties.

The Role of ZIF-67 and ZIF-8 Fillers 🧱🔬

Zeolitic imidazolate frameworks (ZIFs) are a type of metal-organic framework (MOF) known for:

  • High porosity – allows faster water transport.

  • Selective adsorption – can trap pollutants or contaminants.

  • Chemical stability – withstand harsh operational environments.

Two popular ZIFs in membrane research:

ZIF-67 🟪

  • Composed of cobalt ions (Co²⁺) linked with 2-methylimidazole.

  • Offers high hydrophilicity, enhancing water flux.

  • Provides antibacterial properties – reduces biofouling.

ZIF-8 🟦

  • Composed of zinc ions (Zn²⁺) linked with 2-methylimidazole.

  • Known for chemical and thermal stability.

  • Improves structural integrity of the membrane.

When incorporated into GO-PPSU membranes, these fillers create nanochannels that enhance water transport while maintaining contaminant rejection.

Fabrication of GO-PPSU/ZIF Membranes 🏗️

The typical preparation process involves:

  1. Synthesis of GO – usually via modified Hummers’ method, producing sheets with oxygen-containing functional groups.

  2. Preparation of ZIF fillers – either ZIF-67 or ZIF-8 synthesized using solvothermal or room-temperature methods.

  3. Blending – GO and ZIF fillers are mixed with PPSU in a solvent (e.g., N-methyl-2-pyrrolidone).

  4. Casting and phase inversion – the mixture is cast onto a glass plate and immersed in a nonsolvent (usually water), forming a porous membrane.

This method ensures uniform dispersion of GO and ZIFs in the polymer matrix, which is critical for consistent performance.

Flux Behavior of GO-PPSU/ZIF Membranes 🌊⚡

Water flux refers to the volume of water passing through a membrane per unit area per unit time (L/m²·h). For UF membranes, higher flux is desirable for efficiency.

How ZIFs Improve Flux:

  1. Nanochannels 🕳️ – ZIFs create additional pathways for water molecules.

  2. Hydrophilic surfaces 💦 – GO and ZIFs attract water, reducing resistance.

  3. Reduced pore blockage 🚫 – ZIFs prevent polymer chain collapse, maintaining porosity.

Studies show:

  • GO-PPSU membranes without fillers have moderate flux (~50–100 L/m²·h).

  • Incorporating 0.5–1 wt% ZIF-8 can increase flux by up to 40%.

  • ZIF-67 can further enhance flux due to its hydrophilic and antibacterial properties.

Fouling Behavior and Antifouling Performance 🧼🛡️

Fouling is a major problem in UF membranes. It can be caused by:

  • Organic matter (proteins, humic acids)

  • Inorganic salts (scaling)

  • Microbial growth (biofouling)

How GO-PPSU/ZIF Membranes Resist Fouling:

  1. Hydrophilicity 💧
    Hydrophilic surfaces repel hydrophobic foulants, reducing adhesion.

  2. Surface charge ⚡
    GO and ZIFs introduce negative charges, repelling negatively charged contaminants.

  3. Antibacterial effect 🦠❌
    ZIF-67’s cobalt content disrupts bacterial growth on the membrane surface.

  4. Smooth surface morphology 🪞
    Reduced surface roughness minimizes foulant trapping.

Experimental results indicate:

  • Pure water flux recovery after fouling is higher for GO/ZIF membranes (~90%) than for pristine PPSU membranes (~60–70%).

  • Protein fouling tests using bovine serum albumin (BSA) show lower adsorption rates for ZIF-incorporated membranes.

Comparative Performance: ZIF-67 vs ZIF-8 📊

FeatureZIF-67ZIF-8
Metal ionCo²⁺Zn²⁺
HydrophilicityHighModerate
AntibacterialYes ✅No/Minimal
Flux enhancementModerate-HighHigh
Chemical stabilityModerateHigh

In practice:

  • ZIF-8 is preferred for chemically harsh environments.

  • ZIF-67 is excellent for applications prone to biofouling.

  • Hybrid approaches, combining both ZIF-67 and ZIF-8, have shown synergistic benefits – enhanced flux, reduced fouling, and improved mechanical stability.

Characterization Techniques 🔬

To understand membrane performance, researchers use various analytical tools:

  1. Scanning Electron Microscopy (SEM) 🖼️ – visualizes surface morphology and pore structure.

  2. Atomic Force Microscopy (AFM) 🌄 – measures surface roughness and nanostructure.

  3. Fourier Transform Infrared Spectroscopy (FTIR) 📡 – confirms chemical interactions between GO, ZIFs, and PPSU.

  4. Water Contact Angle Measurement 💧 – assesses hydrophilicity; lower angles indicate better water affinity.

  5. Flux and Rejection Tests 🚰 – evaluates water permeability and contaminant removal efficiency.

These characterizations help researchers optimize filler content, membrane thickness, and operational conditions.

Real-World Applications 🌍

GO-PPSU/ZIF membranes are versatile and can be applied in:

  • Drinking water purification 🚰 – removal of bacteria, viruses, and organic pollutants.

  • Industrial wastewater treatment 🏭 – handling heavy metals, dyes, and oils.

  • Food and beverage industry 🍷 – clarifying juices, milk, and wine.

  • Pharmaceutical production 💊 – separation of proteins and biomolecules.

Their enhanced flux and fouling resistance make them particularly valuable in continuous and long-term operations, reducing maintenance costs and downtime.

Challenges and Future Perspectives 🔮

Despite their advantages, several challenges remain:

  1. Scalability 🏗️ – synthesizing ZIFs and uniformly dispersing them in membranes at industrial scale is complex.

  2. Cost 💰 – GO and ZIF materials are more expensive than conventional polymers.

  3. Long-term stability ⏳ – mechanical and chemical stability under prolonged operation must be ensured.

  4. Environmental impact 🌱 – lifecycle assessments are needed for sustainable implementation.

Future research may focus on:

  • Hybrid ZIFs and MOFs – combining multiple fillers for synergistic effects.

  • Green synthesis methods – reducing energy consumption and toxic solvents.

  • Smart membranes – responsive to pH, temperature, or pressure changes for adaptive filtration.

Key Takeaways ✅

  • GO-PPSU membranes provide excellent base performance due to hydrophilicity and mechanical strength.

  • ZIF-67/ZIF-8 fillers enhance flux and antifouling properties by creating nanochannels, improving hydrophilicity, and introducing antibacterial effects.

  • Flux improvements of up to 40% and significant fouling resistance have been demonstrated.

  • Hybrid approaches combining ZIF-67 and ZIF-8 are particularly promising for real-world applications.

  • Challenges remain in cost, scalability, and long-term stability, but ongoing research is addressing these issues.

Conclusion 🌟

The integration of graphene oxide and ZIF fillers into PPSU ultrafiltration membranes marks a significant advancement in water purification technology. These membranes not only improve water flux but also resist fouling, which is a critical limitation in traditional UF systems. With continued innovation, such as hybrid ZIF incorporation and green fabrication methods, GO-PPSU/ZIF membranes could play a major role in sustainable water treatment solutions worldwide. 💧🌍



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