High-Performance Engineered ZIF-67@PES Beads for Uranium Extraction from Aqueous Solutions

[NNadir]

The paper to which I'll briefly refer in this post is this one: High-Performance Engineered ZIF-67@PES Beads for Uranium Extraction from Aqueous Solutions Krishan Kant Singh, Gourab Karmakar, Pallavi Singhal, Adish Tyagi, Amit Kanjilal, Kamlesh K. Bairwa, and Avesh K. Tyagi Industrial & Engineering Chemistry Research 2025 64 (14), 7548-7562.

In theory, albeit not in practice, it should not be necessary to mine uranium at all while still providing all of the world's primary energy using nuclear energy. This would involve converting all of the ²³⁸U now in reserve, known as "depleted uranium" to ²³⁹Pu (and higher isotopes) using a fast neutron spectrum. Alternatively, or perhaps in concert, recovering thorium from lanthanide (rare earth) mine tailings could extend these resources further.

The world inventory of depleted uranium is about 1.6 million tons. The energy content of a kg of fully fissioned ²³⁹Pu is about 80 trillion Joules. If we assume that world energy consumption can be stabilized at 650 EJ/year, perhaps with the judicious use of process intensification with heat networks, this suggests that mined and isolated uranium could last for about 250,000 years. (The caveat is a function of a parameter known as "doubling time, the time that depleted uranium must be in a neutron flux to be converted to plutonium.)

An alternative to mining uranium is to isolate it from aqueous solutions, most prominently seawater, an idea around which many thousands of papers have been written and published. The ocean contains about 4.5 billion tons of uranium, in dilute solution, part of a global uranium cycle involving the Earth's mantle, crustal uplift to mountains, weathering and transport by rivers to the sea. Uranium also naturally occurs in groundwater which is mined for water supplies.

An excellent monograph on this topic, which I have in my files is this one: S. Krishnaswami, J. Kirk Cochran Editors:U-Th Series Nuclides in Aquatic Systems (2008) Imprint: Elsevier Science. It has tables of concentrations of uranium in all of the world's major rivers.

Many of the aforementioned papers on extraction from seawater refer to a class of resins functionalized with amidoximes which have a high affinity for uranium.

The paper referenced above is different, the use of what are known as "MOFs," metal organic frameworks, the particular subclass being known as ZIF's ( Zeolitic imidazolate frameworks).

From the paper's introduction:

Uranium plays a significant role in nuclear power reactor industries, serving as a key material for electricity generation and the production of valuable isotopes for diverse applications. (1) Metallic uranium is used as an X-ray target to produce high-energy X-rays, while depleted uranium serves as a shielding material in many applications. (2) However, the natural source of uranium is limited, with its presence primarily restricted to trace quantities in specific rocks, soil, and seawater at an ultradilute concentration of approximately 3.3 ppb. Moreover, uranium poses chemical toxicity and radiological hazards, making its removal or recovery from aqueous sources a critical environmental and public health concern. (3,4) To address this challenge, numerous techniques have been explored for uranium remediation, including solvent extraction, solid-phase extraction, chemical precipitation, membrane filtration, and electrochemical methods. (5−10) While the solvent extraction technique is widely used in the nuclear industry to treat waste samples and remove uranium, it suffers from inherent drawbacks, such as third-phase formation, generation of secondary waste, difficulty in handling large volumes, and high operational costs. Therefore, for treating large volumes of radioactive waste with trace concentrations of uranium, the solvent extraction technique is not a viable option.

Solid–liquid extraction can address these limitations and is a more practical approach to treating large volumes of waste containing low levels of uranium. Moreover, it is eco-friendly, economic, simple to use, and efficient in treating large volumes of waste streams. Various organic and inorganic materials have been synthesized and used as solid-phase sorbents to treat uranium-containing waste effluents. (9−13) Among them, polymer-based composites like beads, films, membranes, fibers, and nanocomposites are more promising and widely used to remove uranium from different waste streams due to their superior mechanical, chemical, and radiation stability. (1,2,12,14−18) However, despite these advancements, a key research gap remains in developing materials that combine a high uranium adsorption efficiency with structural robustness and ease of recovery for large-scale applications.

Metal–organic frameworks (MOFs) are a versatile class of novel porous materials, which have been extensively explored for energy and environmental remediation applications. (19−22) They show wide application in many fields, such as gas storage, (23) sensors, (24) catalysts, (25) and drug delivery. (26) Recently, MOFs have shown promising results for uranium recovery from water. This timely and crucial application of MOFs has led to an overwhelming number of scientific studies for uranium recovery through a number of MOFs such as HKUST-1, (27) MOF-76, (28) UiO-66, (29) MIL-101(Cr), (30) MIL-100(Fe), (31) UiO-66-NHPOPH₂, (32) UiO-68, (33,34) and MOF Zn(HBTC)(L)(H₂O)₂. (35) This marks a significant step toward mitigating the treatment of radioactive waste and also recovery of uranium from seawater. Zeolite imidazolate frameworks (ZIFs), a promising member of the MOF family, have shown potential for the adsorption of heavy metals...

The authors refer to a particular ZIF, ZIF-67, which, addressing the problem that it is in a powder form. To address this issue, the authors suspend the ZIF-67 in a polysulfone polymer.

Some graphics from the paper:

A cartoon on the synthesis:



The caption:

Figure 1. (A) Schematic Illustration for the synthesis of ZIF-67. The crystal structure of ZIF-67 (41) is reproduced with permission from ref (41).

The caption:

Figure 5. Optical microscopy images of the beads: (a) 10× and (b) 200×. SEM images of the beads: (c) surface 5000× and (d) cross-section 500×.

The caption:

Figure 11. Reusability study of the synthesized ZIF-67@PES beads.

The caption:

Figure 12. Influence of equilibration time on uranium sorption and extraction efficiency from seawater. (Exp. cond.: wt of beads = 15 mg; exp. conc. = 1 ppm; vol. = 5 mL of seawater; pH = 7.5; at room temp.; equilibration time: 5–120 min).

The caption:

Figure 14. Influence of the solution pH on uranium sorption. (Exp. cond.: wt of beads = 15 mg; exp. conc. = 1 ppm; vol. = 5 mL; pH= 2–9; at room temp.; equilibration time: 2 h).

The paper's conclusion:

The study presents a novel approach for developing ZIF-67-embedded poly(ether sulfone) (PES) composite beads using a simple yet effective phase inversion technique. These polymeric beads exhibit a porous internal structure, as confirmed by microscopic studies and BET analysis, with a surface area of 27.69 m² g^-1 and an average pore volume of 0.04 cc g^-1.Their thermal stability up to 550 °C further enhances their applicability in high-temperature environments. The significance of this study lies in demonstrating the potential of ZIF-67@PES composite beads as an efficient adsorbent for uranium extraction. It was observed that maximum adsorption can be achieved at pH 6-8, mirroring the pH range of most naturally occurring water bodies. The beads exhibit rapid sorption kinetics following a pseudo-second-order model, while sorption equilibrium aligns with the Langmuir isotherm, yielding a monolayer capacity of 83.26 mg g^-1comparable to the experimental capacity (approx. 80 mg g^-1). Furthermore, reusability tests confirm 0.1 M HNO₃ as an effective stripping agent for uranyl ion desorption, reinforcing the beads’ practical applicability in uranium recovery from aqueous systems. Despite these promising findings, the long-term stability and recyclability of these beads over multiple adsorption–desorption cycles as well as the scalability of the beads need further attention to translate them into technology.

Again, the recovery of uranium from seawater (which is currently more expensive than mined uranium) conceivably might not be necessary for hundreds of thousands of years, although such recovery might be a side product of removing natural uranium from drinking water and other related sources. The reprocessing of used nuclear fuel relies on solvent extraction which can lead to uranium contaminated water, but personally I would choose to supplant solvent extraction with fluoride volatility methods. Therefore this approach can be used to clean up process water, as well as runoff from abandoned uranium mines.

It is hardly the only approach, but to my knowledge is somewhat unique, although outside of my purview, the use of ZIF-67 for uranium capture has been known for some time. This is a nice spin on the idea, in my view.

Have a nice day tomorrow.