Water pollution is one of the most serious environmental problems of our time. Industrial factories, farms, and even our households release a wide range of contaminants into lakes, rivers, and groundwater. These pollutants include:

• Organic dyes from textile and paper industries

• Antibiotics and pharmaceutical residues from hospitals, animal farms, and households

• Heavy metals such as lead, chromium, mercury, and cadmium from mining, electroplating, and other industrial processes

Many of these substances are not only toxic but also very stable. They do not break down easily in nature, and they can accumulate in the food chain, causing long-term harm to ecosystems and human health.

Traditional water treatment technologies—such as filtration, coagulation, chemical precipitation, and biological treatment—have improved a lot over the years. However, they still struggle with certain types of pollutants, especially complex organic molecules and low-concentration heavy metals. In some cases, these treatment processes can even produce more harmful by-products.

Because of these challenges, researchers have been looking for new materials and methods that can selectively capture or break down pollutants in a more efficient, economical, and environmentally friendly way. One of the most promising families of materials to emerge in this context is metal–organic frameworks (MOFs), and within this group, a specific material called ZIF-67 has attracted a great deal of attention.

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What Is ZIF-67?

ZIF-67 belongs to a subclass of MOFs known as zeolitic imidazolate frameworks (ZIFs). These materials combine:

• Metal ions (in the case of ZIF-67, mostly cobalt ions, Co²⁺)

• Organic ligands (typically imidazolate-type molecules such as 2-methylimidazole)

These building blocks self-assemble into a highly ordered, three-dimensional, porous structure. You can imagine ZIF-67 as a rigid sponge on the nanoscale, full of tiny, well-defined pores.

ZIF-67 stands out because:

• It has a very high specific surface area, often above 1700 m² per gram. That means there is a huge amount of surface available for pollutants to interact with.

• Its pore size is tunable, typically in the range of about 0.34–3.4 nm, which is suitable for many organic molecules and metal ions.

• It shows excellent chemical and thermal stability, remaining stable over a wide pH range (about pH 2–12) and under relatively harsh conditions.

• It contains active sites (such as cobalt centers and functional groups) that can interact strongly and selectively with pollutants.

Compared to traditional adsorbents like activated carbon:

• ZIF-67 often has 1–5 times higher adsorption capacity for certain pollutants.

• It can exhibit much better selectivity, targeting specific pollutants even in complex water matrices.

• Its structure can be chemically modified to tune its performance.

The main drawback is that ZIF-67 is typically more expensive to produce than activated carbon (roughly 30–50% higher in cost), mainly due to the use of metal precursors and organic ligands. However, its superior performance and selectivity make it a very attractive candidate for high-value applications, especially where specific pollutants must be removed efficiently.

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Why Do We Need Better Adsorbents?

Water pollutants such as antibiotics, dyes, and heavy metals pose multiple risks:

• Antibiotics in water can promote the development of antibiotic-resistant bacteria and resistance genes, creating a global health crisis.

• Dyes can be carcinogenic or mutagenic, and they also block light penetration in water, disrupting photosynthesis and oxygen balance in aquatic ecosystems.

• Heavy metals like Pb, Cr, and Hg are non-biodegradable and can cause neurological damage, cancer, and other serious diseases even at very low concentrations.

Many traditional adsorbents have limitations:

• Activated carbon has decent adsorption ability but relatively poor selectivity and limited capacity for certain complex molecules.

• Ion exchange resins can clog and may cause secondary pollution.

• Zeolites and other microporous materials often have pores that are too small for large organic molecules, and their structures are rigid and difficult to tune.

• Mesoporous materials have larger pores but lack the precise structural control and versatile functionality that MOFs offer.

MOFs, and especially ZIF-type materials such as ZIF-67, combine:

• Very high porosity

• Tailorable pore size and surface chemistry

• Good stability

• Possibility of introducing functional groups for specific pollutant recognition

This makes them ideal for advanced adsorption and photocatalytic applications.

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How Is ZIF-67 Synthesized?

The way ZIF-67 is synthesized has a strong influence on its final structure, pore size, surface area, and performance in water treatment. Over the past decade, several strategies have been developed. In all cases, the basic reaction involves a cobalt source (such as cobalt nitrate) and the organic ligand 2-methylimidazole (Hmim) in a suitable solvent.

1. Solvothermal Method

The solvothermal method is one of the most widely used approaches. In this method:

• Cobalt salt and 2-methylimidazole are dissolved in one or more organic solvents (often methanol, DMF, or a mixture).

• The solution is heated, sometimes in a sealed vessel, for a certain period.

This approach allows:

• Good control of crystal growth

• High crystallinity

• Formation of microporous ZIF-67 structures

The choice of solvent and reaction temperature has a decisive impact on the final morphology and pore structure. For example, mixtures of methanol and DMF can lead to specific combinations of micropores and mesopores, which is beneficial for adsorption.

2. Surfactant-Assisted Synthesis

In surfactant-assisted methods, surfactants are added to the reaction mixture. These molecules help:

• Improve particle dispersion

• Prevent agglomeration

• Influence particle size and shape

By adjusting the type and amount of surfactant, researchers can tailor the microstructure of ZIF-67, which in turn affects how it interacts with pollutants.

3. Microwave- and Ultrasonic-Assisted Methods

Both microwave-assisted and ultrasonic-assisted methods aim to make synthesis faster, more energy-efficient, and more controllable:

• Microwave synthesis rapidly heats the reaction mixture using electromagnetic waves. This can shorten the reaction time from hours to minutes, while promoting uniform nucleation and growth.

• Ultrasonic synthesis uses acoustic cavitation to create tiny hot spots and high-pressure regions. This enhances mixing and nucleation, enabling fast synthesis of nanosized ZIF-67 particles and better control over pore size and surface area.

These methods help overcome some limitations of conventional solvothermal synthesis, such as long reaction times, high energy consumption, and particle aggregation.

4. Hydrothermal Synthesis

The hydrothermal method is similar to solvothermal synthesis but uses water as the main solvent, sometimes with different cobalt precursors. It can offer:

• Better environmental friendliness

• Potential cost reductions

• Good control over crystal size and morphology

Certain hydrothermal strategies have achieved high yields (e.g., around 94% based on cobalt content) and have produced ZIF-67 with high surface area and porosity, making them suitable for industrial-scale production.

Overall, factors such as solvent choice, metal-to-ligand ratio, type of cobalt salt, temperature, and aging time all influence whether ZIF-67 crystals form as spheres, granules, or more complex shapes like rhombohedral dodecahedra. These structural variations directly impact performance in water purification.

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How Does ZIF-67 Adsorb Pollutants?

To design and apply ZIF-67 effectively, it’s important to understand how it captures pollutants from water. The adsorption mechanism can be quite complex, involving multiple types of interactions.

Main Types of Interactions

Some key interactions between ZIF-67 and contaminants include:

1. Electrostatic interactions

   • ZIF-67 can carry a net surface charge depending on pH.

   • Pollutant molecules or ions with opposite charge are attracted to the surface, promoting adsorption.

2. Hydrogen bonding

   • Functional groups such as –OH, –NH₂, or others on the MOF or pollutant can form hydrogen bonds.

   • These bonds help anchor antibiotics, dyes, and other molecules to the ZIF-67 framework.

3. Acid–base interactions

   • Acidic and basic functional groups can interact, as seen with certain modified MOFs.

   • For example, MOFs with both acidic and basic groups can more effectively capture specific organic acids or drug molecules.

4. Van der Waals forces and physical adsorption

   • Weak, non-specific interactions contribute to the initial attraction and accumulation of pollutants in the pores.

5. Coordination bonding and chemisorption

   • Metal ions in ZIF-67 (Co²⁺) can form coordination bonds with electron-donating atoms in pollutants (such as N, O, or S).

   • This creates stronger, more selective adsorption, especially for heavy metals and some pharmaceuticals.

6. π–π interactions and hydrophobic effects

   • Aromatic pollutants, such as many dyes and antibiotics, can stack with aromatic rings in the organic ligands.

   • Hydrophobic pores enrich non-polar or weakly polar molecules.

Because these interactions work together, ZIF-67 can show very high adsorption capacities and strong selectivity. Depending on the pollutant and conditions, the adsorption process can involve both physical adsorption (reversible, weaker forces) and chemisorption (stronger, often less reversible bonds).

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ZIF-67 for Dye Removal

Synthetic dyes are widely used in textiles, printing, leather, and cosmetics. Their intense color, high stability, and sometimes toxic or carcinogenic nature make them particularly problematic in wastewater.

ZIF-67 has been shown to remove various dyes extremely effectively, including:

• Methyl orange (MO)

• Malachite green (MG)

• Other aromatic dyes and organic molecules

In some studies, ZIF-67 achieved very high adsorption capacities for malachite green, reaching more than 3000 mg of dye per gram of adsorbent at higher temperatures. The adsorption capacity increased with temperature, indicating that the process is endothermic.

The adsorption of malachite green on ZIF-67 often follows a monolayer adsorption model, meaning that dye molecules form a single layer on the available active sites, consistent with the Langmuir isotherm interpretation. One of the main reasons for this strong adsorption is the π–π interaction between the aromatic rings of the dye and the organic parts of ZIF-67, combined with other interactions such as electrostatic attraction and hydrogen bonding.

These results highlight that ZIF-67 is not just a generic adsorbent but a highly efficient and tunable material for dealing with difficult dye pollutants.

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ZIF-67 for Antibiotic Removal

Antibiotics are now detected in many parts of the water cycle: surface water, groundwater, drinking water, and wastewater. Their presence can promote antibiotic resistance, which is a major global health challenge.

ZIF-67 and its composites have shown strong performance in removing antibiotics such as:

• Sulfonamides

• Tetracyclines

• Ciprofloxacin and other broad-spectrum drugs

Here, ZIF-67’s advantages include:

• High specific surface area, offering many active sites

• Adjustable pore size, suitable for different antibiotic molecules

• Good chemical stability in aqueous media across a wide pH range

• Simple synthesis, sometimes even at room temperature

In some composite systems, ZIF-67 is combined with other materials (for example, certain bismuth-based photocatalysts). In such cases, the ZIF-67 component:

• Concentrates antibiotic molecules in its pores through adsorption

• Facilitates interactions with other active phases for photocatalytic degradation

• Enhances both adsorption and catalytic activity through synergy

For instance, a ZIF-67-based heterostructure has reached antibiotic removal rates around 98% under optimized conditions, thanks to a combination of:

• π–π stacking between aromatic ring structures

• Coordination of antibiotics with cobalt centers

• Enhanced charge transfer in the composite for photocatalytic degradation

This shows that ZIF-67 can function not only as an adsorbent but also as a partner in adsorption–photocatalysis hybrid systems.

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ZIF-67 for Heavy Metal Removal

Heavy metals such as lead, mercury, cadmium, chromium, and arsenic are persistent and dangerous toxins. They do not degrade and can accumulate in organisms, causing long-term damage.

ZIF-67-based materials can remove heavy metals through:

• Coordination between metal ions and cobalt centers or functional groups on the framework

• Ion exchange processes with grafted groups (such as amino groups)

• Adsorption in hierarchical pore structures that allow fast mass transfer

Certain ZIF-67 composites have reached lead adsorption capacities of over 600 mg of Pb²⁺ per gram of material, while still maintaining more than 90% of their performance after multiple regeneration cycles. This combination of high capacity and good reusability is especially important for real-world applications.

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Desorption and Reusability

For any adsorbent to be practical in water treatment, regeneration and reuse are critical. ZIF-67 can typically be regenerated using:

• Heat treatment, which helps decompose or desorb organic contaminants

• Chemical elution, using solutions such as salts, acids, or organic solvents to wash out the pollutants

The key is to remove contaminants while preserving:

• The structural integrity of the framework

• The porosity and specific surface area

• The active sites needed for adsorption

Studies have shown that after several cycles (for example, five regeneration cycles), ZIF-67 can often retain around 80–90% of its initial adsorption capacity, depending on the pollutant and regeneration method. There may be some gradual decline due to pore blockage or partial deactivation of active sites, but performance remains relatively high.

Combining ZIF-67 with other materials (e.g., carbon-based supports, layered double hydroxides, or protective coatings) can further improve mechanical strength and fouling resistance, enhancing long-term stability.

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Challenges and Future Directions

Although ZIF-67 has excellent potential, several challenges remain before it can be widely adopted in large-scale, real-world water treatment systems.

1. Greener and Cheaper Synthesis

Many current synthesis methods use organic solvents like DMF or methanol, which can be costly and may cause secondary environmental impacts. Future efforts need to focus on:

• Water-based synthesis routes

• Low-temperature, low-energy processes

• Use of waste-derived or bio-based precursors

These improvements could reduce costs and make ZIF-67 more attractive for large-scale deployment.

2. Improved Stability in Harsh Conditions

Although ZIF-67 is stable over a wide pH range, extreme pH or very high temperatures can still cause structural collapse. Strategies to enhance stability include:

• Surface coatings (for example, thin silica layers)

• Metal doping (adding Zn, Fe, or other metals)

• Core–shell architectures (combining ZIF-67 with other MOFs like ZIF-8)

• Concepts such as self-healing frameworks with dynamic bonds

3. Selectivity in Complex Water Matrices

Real wastewater usually contains mixtures of:

• Multiple antibiotics

• Various dyes

• Heavy metals

• Natural organic matter and salts

These competing species can interfere with adsorption. To tackle this, researchers are designing:

• Hierarchical pore structures (micro–meso–macro)

• Multiple functional groups (e.g., –SH, –NH₂)

• Molecular imprinting strategies

The goal is to achieve selective adsorption of target pollutants in the presence of many others.

4. Low-Energy Regeneration

Conventional regeneration methods can be energy-intensive or produce secondary waste. Future work is exploring:

• Photothermal regeneration, using solar energy to trigger desorption

• Electrochemical regeneration, applying electric fields to control adsorption/desorption

• Careful study of degradation pathways to ensure that pollutants are transformed into safe products

5. Environmental and Health Risk Assessment

Because ZIF-67 contains cobalt, there is a need to:

• Evaluate potential leaching of metal ions

• Study the ecotoxicity of particles and degradation products

• Understand long-term behavior in natural water bodies

This includes developing degradable or responsive MOFs that can safely break down after use, minimizing risks.

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Final Thoughts

ZIF-67 and its derivatives represent a powerful new class of materials for water purification. With their high surface area, tunable pores, rich active sites, and strong adsorption and catalytic capabilities, they offer clear advantages over traditional adsorbents for removing dyes, antibiotics, and heavy metals.

At the same time, there is still work to be done: greener synthesis routes, better long-term stability, more selective adsorption in complex waters, low-energy regeneration, and full environmental risk assessments are all active areas of research.

As these challenges are gradually addressed, ZIF-67-based materials may become key components in next-generation water treatment technologies—helping us move towards cleaner water and a more sustainable future.