Kargen Tech Blog | MOF for Water Purification and Pollutant Removal

Introduction

With the growth of industrial production and the fine-chemicals industry, the variety of pollutants in water bodies keeps increasing. Beyond suspended solids, organic matter, and inorganic salts, pollutants such as heavy-metal ions, PFAS, antibiotics, pesticides, and dyes have drawn increasing attention. These substances are often present at low concentrations, yet may be highly toxic, highly persistent, or difficult to biodegrade.

Activated-carbon adsorption, ion exchange, chemical precipitation, membrane separation, and advanced oxidation are already widely used in water treatment, but when facing complex water matrices they can still suffer from insufficient adsorption selectivity, difficult material regeneration, or the generation of concentrated waste streams. Metal-Organic Frameworks (MOFs) can tune their pore structure and surface chemistry, and have therefore become an important direction in water-purification materials research.

What Is a MOF, and Why Can It Be Used for Water Purification?

An MOF is a crystalline porous material built from metal ions or metal clusters and Organic Linkers through coordination bonding. The metal nodes act as connection points in the framework, and the organic linkers connect these nodes to form regular pores and cavities.

What makes MOFs useful in water treatment is not merely their large specific surface area, but more importantly their tunable pore environment. Changing the metal nodes and organic linkers can alter pore size, in-pore polarity, and surface charge; introducing thiol, amino, or carboxyl groups onto the linkers can add binding sites for specific pollutants. Once a pollutant enters a pore, it may simultaneously experience pore confinement, electrostatic interaction, coordination, hydrogen bonding, or π–π interaction. These different effects work together, allowing MOFs to adsorb selectively based on a pollutant's size, charge, and functional groups.

Selective Adsorption of Heavy-Metal Ions

Heavy metals such as lead, cadmium, mercury, chromium, and arsenic are difficult to remove by biodegradation. Chemical precipitation works for high-concentration heavy-metal wastewater, but at low concentrations for deep treatment it is easily affected by precipitation equilibrium, coexisting ions, and sludge disposal.

MOFs can capture metal ions by installing specific coordinating groups within their pores. Taking mercury as an example, researchers introduced amide and thiol sites onto the framework of the water-stable Uio-66 to produce UiO-66-A.T. This material shows a maximum adsorption capacity of about 691 mg/g for Hg²⁺ and high mercury selectivity in mixed systems containing multiple heavy-metal ions[1]. Its main effect is not simple pore filling, but the relatively strong coordination between thiol groups and mercury ions.

However, the maximum adsorption capacity is usually obtained at high initial concentrations and relatively ideal experimental conditions. Real wastewater also contains calcium, magnesium, salts, and organic matter, which may compete for adsorption sites. Therefore, for practical applications, more attention should be paid to removal at low concentrations, dynamic breakthrough capacity, material regeneration, and whether metals or linkers leach out.

Adsorptive Removal of PFAS

PFAS are a class of organofluorine compounds containing stable carbon–fluorine bonds, and are difficult to degrade naturally in the environment. Conventional activated carbon and ion-exchange resins can remove some long-chain PFAS, but short-chain PFAS are generally harder to adsorb.

A PFAS molecule has a polar or charged hydrophilic head group at one end and a hydrophobic fluorocarbon chain at the other. Effective adsorption must address both parts. The zirconium-based MOF NU-1000 has relatively stable zirconium-oxo clusters and large pores: the zirconium nodes can interact electrostatically or by coordination with the PFAS head groups, the hydrophobic regions in the pores can interact with the fluorocarbon chains, and pore confinement can further enrich the pollutant inside the material. Studies report that NU-1000 adsorbs various perfluorosulfonic acids and perfluorocarboxylic acids at capacities of about 400–620 mg/g and 201–604 mg/g, respectively, with fast adsorption kinetics[2].

These data show that MOFs can be designed synergistically around both the head groups and fluorocarbon chains of PFAS. But in real water bodies, PFAS concentrations are usually far below those in laboratory isotherm tests, and natural organic matter and inorganic anions also compete for sites. Therefore, when evaluating materials, emphasis should be placed on short-chain PFAS, dynamic breakthrough at trace concentrations, regeneration methods, and the subsequent treatment of PFAS in the regenerant.

In addition, organic pollutants such as antibiotics and dyes usually contain aromatic rings and oxygen- or nitrogen-bearing functional groups, and can interact with MOFs through pore filling, hydrogen bonding, electrostatic interaction, and π–π interaction. Taking tetracycline as an example, researchers built hierarchically porous Zif-8, adding larger mass-transfer channels beyond the original micropores to shorten tetracycline's diffusion path into the material. The maximum fitted adsorption capacity of this material in a model water system approached 1 g/g[3].

Case Study: From Powder Adsorption to Processable Materials

When MOF powders are used directly in water treatment, they can be easily washed away with the flow or clog pipelines. Wang et al. used a thermally induced phase separation–hot pressing process to make a composite membrane of MOF and polyethylene, with an MOF loading as high as 86 wt%. Under cross-flow conditions, this membrane rejected organic dyes at about 99% while maintaining a water flux of about 125.7 L·m⁻²·h⁻¹·bar⁻¹[4]. Here, the MOF provides adsorption sites, the micron-scale channels between particles transport water, and the polymer supplies overall strength. These two types of research address material design and material processing, respectively: the former improves mass transfer through hierarchical pores, while the latter solves powder recovery and continuous-operation problems through composite membrane formation.

Figure 1. Fabrication and separation application of the MOF-polyethylene composite membrane by TIPS-HoP (ref. [4])

How Kargen Addresses the Industrial Challenges

Using MOFs for water treatment cannot stop at static adsorption tests on powder materials. Once they enter fixed-bed, cartridge-filter, or continuous-flow devices, issues of large-scale manufacturing, particle recovery, bed pressure drop, material strength, and cyclic regeneration must also be solved.

Guangdong Kargen Advanced Carbon Materials Co., Ltd. (Kargen) has carried out large-scale preparation of dozens of MOF materials and optimized processes such as mechanochemistry, low-solvent synthesis, organic-solvent-free cleaning, and cyclic purification, so as to reduce solvent use, production cost, and solvent residues in the product. For different water qualities and pollutants, materials can be selected from zirconium-, aluminum-, or iron-based and functionalized MOFs, and adsorption selectivity can be improved through functional-group modification, defect engineering, or composite design.

To address the difficulty of directly packing powders, Kargen has established a large-scale MOF granulation scheme and achieved ton-scale shipments of MOF granular products. During shaping, the binder, forming pressure, particle pore structure, and activation conditions must be controlled to avoid covering or compressing the pores, while ensuring the particles have mechanical strength, resistance to water scouring, and a low attrition rate.

For real devices, the material must also be evaluated by particle-size distribution, bulk density, bed pressure drop, dynamic breakthrough capacity, and post-regeneration strength. Through the coordinated adjustment of material synthesis and particle engineering, the material formulation and particle specifications can be determined according to water quality, pollutant concentration, and equipment structure, gradually transforming MOFs from laboratory powders into adsorbent materials usable in continuous water treatment.

References

[1] Lam, I. T. Y.; Yuan, Y.; Bang, K.-T.; et al. Towards the Fastest Kinetics and Highest Uptake of Post-Functionalized UiO-66 for Hg²⁺ Removal from Water. Nanoscale, 2023, 15, 10558–10566. DOI: 10.1039/D3NR01959D.

[2] Li, R.; Alomari, S.; Stanton, R.; et al. Efficient Removal of Per- and Polyfluoroalkyl Substances from Water with Zirconium-Based Metal–Organic Frameworks. Chemistry of Materials, 2021, 33, 3276–3285. DOI: 10.1021/acs.chemmater.1c00324.

[3] Zhang, Z.; Chen, Y.; Hu, C.; et al. Efficient Removal of Tetracycline by a Hierarchically Porous ZIF-8 Metal Organic Framework. Environmental Research, 2021, 198, 111254. DOI: 10.1016/j.envres.2021.111254.

[4] Wang, H.; Zhao, S.; Liu, Y.; et al. Membrane Adsorbers with Ultrahigh Metal-Organic Framework Loading for High Flux Separations. Nature Communications, 2019, 10, 4204. DOI: 10.1038/s41467-019-12114-8.