Revolutionizing Metal-organic Frameworks (MOFs) in Wastewater Treatment Applications

2.1 Mercury

Mercury is used in a wide variety of products, including batteries, fluorescent lighting, pigments, insecticides, and chemical catalysts. However, non-efficient treatment leads to significant mercury contamination. ²² Lung damage, psychological and neurological issues, restlessness, anxiety and depression are brought on by inorganic mercury. ²³ Herein, nitrogen-containing Sm-MOF was prepared by Wu et al., to detect and remove Hg²⁺ ions. It shows that 97 % of Hg²⁺ ions were removed in 2 h from the solution. With maximum adsorption amount (q_e) equal to 243 mg/g. The findings also demonstrate that the produced Sm-MOF, due to a low detection limit of 0.87 M and the broad linear range of 0.6–300 M, has excellent sensitivity and selectivity for the detecting of Hg²⁺. ²⁴ Furthermore, Halder et al., prepared a Ni-based MOF ([Ni(3-bpd)₂(NCS)₂]_n) for Hg²⁺ removal and detection, Removal can be observed as Hg atoms make coordination bonds with S atoms of the SCN group, the color of [Ni(3-bpd)₂(NCS)₂]_n turns from green to grey, the removal efficiency was found to be 94.30 % with Hg adsorption capacity equal 713 mg/g. ²⁵ Additionally, a zirconium-based MOF filter was prepared by Yan et al., this filter avoids the disadvantage that arises from the usage of MOF powder materials and shows a very high removal efficiency within a few minutes equal to 97.8 %, and saturated adsorption capacity similar to 427.3 mg/g at pH = 4.5. ²⁶ Ji et al., prepared thiol-functionalized MOF-808 by grafting thioglycolic acid on MOF-808 to remove Hg(II),that show impressively ultrafast adsorption kinetics (C₀ = 10 mg/L, removal > 99 % in 10 s) and high adsorption capacity equal to 977.5 mg/g, with a rate constant k equal to 36.89 g mg⁻¹ min⁻¹, and after six adsorption-regeneration cycles, MOF-808-SH demonstrates great recyclability with a low loss of capacity. ²⁷ Moreover, Mon et al., prepared eco-friendly and stable MOFs ({Cu₄^II [(S,S)-methox]₂}·5H₂O) as methox is bis [(S)-methionine]oxalyl diamide, it has been discovered to be capable of lowering the [Hg²⁺] concentration in drinking water from unsafe levels of 10 ppm to safe ranges of below 2 ppb with a capture efficiency of 99.95 % in 15 min. ²⁸

2.2 Copper

The human body needs copper, but too much of it can harm the brain, create gastric ulcers, or irritate the mucous membranes. Anthropogenic sources were piping for drinking water, kitchenware, pigments, roofing, alloys, and pharmaceuticals. ^{21 , 29} So Tan et al., prepared Zr-based metal organic framework (MOF-801) using solvothermal method, At an initial Cu²⁺ concentration of 10 mg/L, for 180 min, and an adsorbent dosage of 10 mg, the results of adsorption showed a removal efficiency equal 97.80 % and equilibrium adsorption capacity of Cu²⁺ from the process equal to 9.78 mg/g that demonstrated the efficiency of MOF-801 in removing Cu²⁺. ³⁰ Similarly, a cadmium terephthalate metal-organic framework MOF-2 (Cd), Cd-TPA have been synthesized using ultrasonic synthesis method by Ghaedi et al., for the removing of copper and lead ions by adsorption and 1.0 g adsorbent dose was used to achieve removal percentage values of 99.9 for lead and 97.9 % for copper with highest monolayer adsorption capacities 434.78 for copper and 769.23 mg/g for lead. ³¹ Moreover, Bakhtiari et al., used direct carbonization of zeolitic imidazolate framework (ZIF-8), to synthesis Nano porous carbons (NC) and studied (MOF-NC) particles as adsorbent for copper removal which shows adsorption capacity equal to 33.44 mg/g and rate of adsorption equal 5.87 × 10⁻⁶ g mg⁻¹ min⁻¹. ³¹ In another work Bakhtiari et al., prepared nano porous metal organic framework (MOF-5), The greatest removal efficiency of copper ion by MOF-5 is at pH = 5.2, which is shown to be more than 80 %, and a high value of the adsorption capacity of MOF-5 for copper ion (290 mg/g) suggests that MOF-5 is an effective adsorbent for Cu removal. ³² Additionally, a composite ZnO-NP@Zn-MOF-74 was prepared by Guo et al., which demonstrated improved copper removal capabilities compared to the usual Zn-MOF-74 replacing the traditional zinc ions with layered porous ZnO microspheres. It has maximal sorption capacities of 106.27 mg/g at pH = 5.5, and the removal mechanism involves π-π stacking, electrostatic interaction, surface complexation and ion exchange. ³³

2.3 Lead

One of the elements that is most harmful to people, animals, and plants is lead. It has an additive effect that can lead to severe hematologic, brain, and kidney issues. Additionally, it is nonbiodegradable, and eating food with excessive amounts of this element might have negative health effects on people. Therefore, it is crucial to precisely calculate the trace lead concentrations present in the environment and food. For cereals and fruit juices, the maximum acceptable concentration levels of lead are 0.05 mg kg⁻¹ and 0.2 mg kg⁻¹, respectively, ³⁴ Herein, Zhennan et al., designed Cu-MOFs/Fe₃O₄ a magnetic metal organic frameworks (MOFs) composite by in-situ growth of Cu-MOFs with Fe₃O₄ doping nanoparticles which reported adsorption capacities equal to 219.00 mg/g for Pb²⁺ Also Fe₃O₄/Cu-MOFs was found to be recyclable adsorbent for Pb(II) removal from water, ³⁵ Also Pb²⁺ ions was removed at PH = 5 using two metal-organic frameworks (MOFs) which is Cu-BTC and Zn-BTC, with benzene-1,3,5-tricarboxylic acid linker, which has been synthesized by Hasankola et al., maximum absorption capacity reported for these two MOFs was 333 mg g⁻¹ for Cu-BTC and 312 mg g⁻¹ for Zn-BTC, in <30 min. ³⁶ Zhang et al., reported and adsorbent rate of 99.35 % of lead and it is concentration in a few tens of seconds reached the safe level for drinking standard, using MIL-100 (Fe) which was prepared by hydrothermal method and treated with ultrasonic to achieve slow polymerization of dopamine hydrochloride in the cages of it. ³⁷ A combination between Amino-functionalized MOFs and ceramic membrane ultrafiltration (CUF) was reported by Yin et al., the process had the lowest flux fall, lowest level of membrane fouling, and the maximum removal of Pb (II) (61.4 %) at pH equal to 4.5. Additionally, the equilibrium adsorption capacity was exceptionally high (1795.3 mg/g). ³⁸ Zhu et al., synthesized Nanotube-like Tb-based metal–organic frameworks (Tb-MOFs) through the solvothermal assembly of carboxylate-based ligands and Tb³⁺, shown that the Tb-MOFs had good adsorption properties, With a maximal removal capacity of 547 mg g⁻¹ within 50 min in pH equal to 5.5. ³⁹ And by successfully grafting natural citric acid (CA), Yang et al., reported that an eco-friendly adsorbent MOF-808-CA with abundant free carboxylic sites to synergistically trap Pb²⁺ was successfully prepared. It has a superior adsorption capacity of 173.3 mg/g, removal efficiency can reach 99 % in 5 min, and the concentration of Pb²⁺ can be decreased from 10 mg/L to 7 g/L after equilibrium. ⁴⁰ Geisse et al., used thiophene functionalized metal–organic frameworks for Removal of lead ions with a maximum Pb²⁺ loading of 98.5 mg/g MOF,which was prepared using HCl as a modulator, DUT-67 can be recycled at least five times in flow conditions at relevant concentrations with no loss in adsorption activity. ⁴¹

2.4 Hexavalent chromium Cr⁶

Hexavalent chromium Cr⁶⁺ is a class of major heavy metal contaminants that has drawn a lot of attention. It mostly enters the environment via industrial wastewater and then travels through several environmental pathways to reach the human body. Their anthropogenic causes included plating, data storage, leather, tanning of textiles and Wood treatment, and the production of ferroalloys, which inflicted varying degrees of harm on human organ function. Here to our knowledge, the pathway that is relatively significant is heavy metal enrichment. A mild Cr⁶⁺ overdose may result in rashes, allergies, and stomach ache. ^{42 , 43} As a result Amino-functionalized MIL-101(Cr) (AFMIL) prepared by Jalayeri et al., using hydrothermal method at pH3.51 and T = 60 °C, the greatest measured adsorption capacity of Amino-functionalized MIL-101(Cr) toward Cr(VI) was 44 mg/g for a liquid phase concentration of 140 mg/L, The outcome of the experiment shows that AFMIL has some very intriguing potential uses for removing Cr(VI) from water. ⁴⁴ Also Zhou et al., prepared Magnetic Zr–based metal organic framework (UiO-66) @Polypyrrole (magnetic UiO-66@Ppy) by in situ chemical oxidative polymerization method, that shows excellent adsorption capacity of 259.1 mg/g for Cr(VI) removing from water at pH equal 2. ⁴⁵ It was suggested by Gao et al., to use the metal organic framework (MOF) as a template to fabricate Porous Fe_0.72⁽⁰⁾ Fe_2.28^(II)C which used to remove hexavalent chromium, the resulting Fe_0.72⁽⁰⁾ Fe_2.28^(II)C shown excellent removal performance with a maximal capacity of 354.6 mg/g. ⁴⁵ In order to remove toxic chromium ions from ground water and industrial effluents, a novel nanocomposite adsorbent material was created by Samuel et al., using chitosan (CS), graphene oxide (GO), and metal organic framework (MOF [Zn(BDC) (DMF)]). At pH3, this material had an excellent potential candidate for adsorption capacity of 144.92 mg/g, which opens the door to the development of MOF/GO/CS based composite materials in the future. ³ Nguyen et al., created MOF-808, an ultra-stable Zr-MOF with a steady adsorption capability that is made up of 6-connected zirconium clusters and 1,3,5-benzene tricarboxylic acid. MOF-808’s crystal size ranges from 40 nm to about 1,000 nm. MOF-808 450 produced a maximum K₂Cr₂O₇ trapping adsorption of 141.2 mg/g. ³⁹

3.1 Mechanisms of heavy metal’s adsorption onto MOFs

One of the problems to be resolved by an adsorption mechanism is the potential interactions between adsorbents (adsorbent active sites) and those absorbed in MOFs. The evaluation of the studies’ findings in the literature has revealed that there are four common processes for MOF adsorption, including coordinative unsaturated sites. As demonstrated in Figure 2, hydrogen bonding, electrostatic interactions, and acid-base interactions, Due to the highly ordered structures of pristine MOFs, ion exchange serves as the primary mechanism for the adsorption of Cr (VI) oxoanions. Additionally, the Lewis acid/base principle demonstrates a key function in the attraction of heavy metal oxyanions. As Lewis bases, heavy metal oxoanions can interact with Lewis acidic sites in MOFs throughout an overlapped orbital process, increasing the adsorption capacity, Figure 2. ^{29 , 47}

Figure 2:

Mechanisms of heavy metal’s adsorption onto MOFs ¹ .

3.2 Removal of MOFs in hydrocarbons elimination for wastewater

Recovering water bodies, including the ocean, lakes, rivers, and aquifers, that have been contaminated by hydrophobic organic compounds (HOCs) is one of the main environmental issues that must be resolved today. Aliphatic and aromatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and phenols are some of these contaminants. ⁴⁸ Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants generated by both anthropogenic and natural activities linked with industrialization and urbanization, are over 100 different types of PAHs. ⁴⁹ Polycyclic aromatic hydrocarbons (PAHs) are a class of dangerous organic compounds composed of two or more condensed benzene rings joined in linear, cluster, or angular structures, Table 1. Most of them are solids that are colorless, white, or pale yellow. ⁵⁰ PAHs have been found in sediments, water, and crustaceans in mixtures, where they often coexist with other pollutants. Dry and wet deposition, road runoff, industrial wastewater, leaching from creosote-impregnated wood, petroleum spills and fossil fuel combustion are the main ways that PAHs can enter water bodies. The solubility of PAHs in water diminishes as their molecular weight increases, however their boiling and melting points rise in parallel. As known, aromatic hydrocarbons with four and five rings are practically insoluble in water. Furthermore, solubility declines with increasing ring structure, degree of substitution, vapor pressure, and molecular weight. PAHs are known to be carcinogenic and mutagenic, as well as to bioaccumulate in human and animal tissue. ⁵¹ Phenolic compounds exist in water bodies as a result of polluted wastewater discharged from industrial, agricultural, and domestic activities. These chemicals are known to be poisonous, causing severe and long-term damage to both people and animals. Even at low doses, they operate as carcinogens and cause harm to red blood cells and the liver. ⁵² Zakariyya Uba Zango et al. synthesized Metal-organic frameworks (MOFs) of UiO-66(Zr) and NH₂-UiO-66(Zr) in the removal of anthracene (ANT) and chrysene (CRY) by using adsorption technique. These MOFs showed high regeneration ability. The solvothermal method was used to successfully synthesis UiO-66(Zr) and NH₂-UiO-66(Zr) MOFs. They exhibit high surface area “1,420 m² g⁻¹ for UiO-66(Zr) and 985 m² g⁻¹ for NH₂-UiO-66(Zr)”, good thermal stability, with high adsorption capacity and excellent removals efficiency. For time effect, Maximum adsorption of anthracene (ANT) and chrysene (CRY) achieved at 30 min and 25 min respectively. Adsorption efficiency was higher in CRY for all adsorbents, reaching 98.6 and 96.4 % for UiO-66(Zr) and NH₂-UiO-66(Zr), respectively. The percentage elimination of ANT for UiO-66(Zr) and NH₂-UiO-66(Zr) was 97.9 and 95.7 %, respectively. For PH effect, it is worth noting that both MOFs removed more pollutants at all pH levels. For isotherms, the measured Q_max values were substantially higher and matched the equilibrium adsorption capacity well. ^{53 , 54}

3.3 Role of MOFs in pharmaceutical removal from wastewater

Pharmaceutical wastewater (PWW) is arguably a rising problem that threatens health, living organisms and the environment. The actual impact of these products on human health not fully figured out but presence associated with many endocrine disruptions that may change hormonal actions. ⁶⁰ Pharmaceuticals such as hormones, antibiotics, lipids-lowering agents and psychological. Pharmaceuticals possess an elevated degree of stability which makes them as a burden for wastewater treatment. ^{61 , 62 , 63} Sources of pollution can be categorized into two parts; pollution associated with manufacturing processes as well as the municipal use of pharmaceuticals, Table 2. Various methods have been utilized to solve (PWW) including, biological treatment, membrane technology adsorption method, advanced oxidation process, hybrid systems as well as MOFs. MOFs possess several promising features as illustrated in Figure 3.

Figure 3:

Different methods for pharmaceuticals removal from wastewater.

Pharmaceutical products can be treated via an adsorption approach by utilizing carbons, zeolites, carbon nanotubes, and mesoporous silica but with limitations such with some of pharmaceuticals and personal care products (PPCPs which have miscellaneous features, thus, MOFs with enhanced constituents able to point out advantage with aid of numerous interactions with PCPPs such as polar interaction hydrogen bond formation, acid–base interfaces as well as hydrophobic interactions which subsequently promote adsorption and finally PWW treatment. ^{64 , 65 , 66 , 67 , 68 , 69 , 70}

3.4 Role of MOFs in wastewater depollution via catalyzed advanced oxidation process

MOFs contribute in the enhancement of advanced processes such as Fenton-based, photocatalysis as well as sulphate-centered oxidation; promoted generation of reactive species with MOFs has the key role in this process. ⁸³ For instance, bio-MOF-11-Co has been utilized in sulfachloropyradazine and para-hydroxybenzoic acid. ⁸⁴ Iron-derived MOF (MIL-88-A) was used in naproxen (NPX) elimination from wastewater. ⁸⁵ Moreover, iron-grounded MOF (MIL-100(Fe)) pointed out significant role in antibiotics contaminants removal such as sulfamethoxazole, ibuprofen, theophylline and bisphenol A, Figure 4.86 87

Figure 4:

Activation methods of metal-organic frameworks (MOFs) and their applications in degrading contaminants in wastewater.

4.1 Radioactive heavy metals

Nuclear energy is considered the main energy source beyond fossil fuels. The widespread use of nuclear power provides large convenience to mankind. However, the development of nuclear power is highly controversial, because nuclear fission generates large amounts of radioactive wastes. ⁹² Radioactive metal ions (¹²⁸Pd, ¹³⁷Cs, ⁸⁹Sr, ²³⁵U, ⁵⁹Fe, ⁵⁷Co, ⁶⁵Zn, ⁹⁰Sr, ⁹⁹Tc, ⁷⁹Se, ¹³³Ba, etc.), toxic heavy metal cations (Hg²⁺, Pb²⁺, Cd²⁺, Cu²⁺, etc.) and oxyanions (CrO₄²⁻, Cr₂O₇²⁻, AsO₄³⁻, SeO₃²⁻, SeO₄²⁻, etc.) are major pollutants in nuclear reactor operations and industrial effluents, which create a serious threat to humans and ecological system. ^{93 , 94} While, organic pollutants and heavy metal ions such as Hg(II), Pb(II), Cr (Ⅵ), and Cd(II) are difficult to decompose into small eco-friendly molecules, and their stubborn accumulation in living organisms leads to high toxicity and potential carcinogenicity. ^{93 , 95} Because of the ability of heavy metals to form compounds that can be carcinogenic and mutagenic within the body even at very low levels. Therefore, people need to avoid unnecessary exposure to heavy metals. ⁹⁶

Metal-organic frameworks (MOFs) are a class of crystalline porous materials, assembled by metal clusters or ions and organic linkers connected via coordination bonds. MOFs have been explored in many applications, such as catalysis, adsorption/separation, chemical sensing, drug delivery, gas storage, and luminescence sensing. ^{92 , 93} Many papers ^{97 , 98} reported the potential applications of MOFs and MOF-based composites for the removal of toxic and nuclear waste-related metal ions from wastewater. MOFs were considered as potential adsorbent choices for radionuclide capture and recovery that is attributed to owing high density of the active site, excellent porosity, and high stability. ⁹² Figure 5 illustrates some development of MOF-based materials for the adsorption of toxic and nuclear waste-related metal ions. ^{92 , 94 , 99 , 100}

Figure 5:

Various MOF-based materials for the adsorption of toxic and nuclear waste-related metal ions. ^{92 , 94 , 99 , 100}

4.2 Uranium

Uranium (U(VI)) and its compounds are radioactive and toxic, and they are highly polluting and harmful to the ecological environment, human survival, and development. ¹⁰¹ Lin’s group ¹⁰² extracted U(VI) from aquatic environments by using the stable and porous phosphorylurea-derived MOFs UiO-68-type MOFs (MOF-2 and -3) with maximum adsorption capacities of 217 mg g⁻¹. An amidoxime-appended zirconium metal-organic framework [Zr₆O₄(OH)₄(BDC-NHCH₂CH₂C(=N–OH)NH₂)₆](UiO-66-AO, BDC = 1,4-benzenedicarboxylate) for efficiently and rapidly removing uranium from aqueous solution was fabricated through post-modifications of UiO-66 MOFs. It was found that the maximum adsorption capacity reached 232.8 mg g⁻¹ at pH 5.0 and 328 K based on the Langmuir isotherm. The sorption experiment in a simulated nuclear industry effluent demonstrated a high adsorption efficiency (about 95.3 %) and selectivity toward uranium. ¹⁰²

The combination of magnetic nanoparticles and metal-organic frameworks (MOFs) has demonstrated their potential for pollutant sequestration. Fe₃O₄@SiO₂@UiO-66 core–shell as an example of magnetic microspheres were synthesized and used for the removal of U(VI) from an aqueous environment. The maximum adsorption capacity was calculated to be 616.5 mg g⁻¹ for U(VI) by fitting the equilibrium data to the Langmuir model. Thus, the excellent adsorption capacity and sensitive response of the Fe₃O₄@SiO₂@UiO-66 core–shell magnetic microspheres to a magnetic field make them a promising candidate for the removal of uranium (VI) from aqueous solutions. ¹⁰³ A well-defined magnetic metal-organic frameworks (MOFs)/graphene oxide (Fe₃O₄@HKUST-1/GO) consisting of magnetic Fe₃O₄ nanoparticles, HKUST-1 nanocrystal, and GO was synthesized through a simple and environmentally friendly approach. The introduction of GO enhanced the ability of particles to uptake U(VI) from an aqueous solution. The adsorption process can be better described by the Langmuir model and the pseudo-second-order kinetic model. The results showed that the Fe₃O₄@HKUST-1/GO exhibited good adsorption capacity towards U(VI) at the initial solution pH value of 4.0 and T = 318 K. ¹⁰⁴ Zhimin Lv ⁹⁹ eliminates U(VI) from radioactive wastewater by using A MgO/carbon adsorbent as a safe treatment in the nuclear power industry. A MgO/carbon adsorbent was prepared by one-step Mg-MOF-74 pyrolyzation and used for U(VI) removal from the aqueous solution. Characteristic results indicated that the as-prepared composite was a typical porous structure. It can rapidly and effectively remove U(VI) and showed an excellent adsorption capacity (777.51 mg g⁻¹), which is much higher than other reported adsorbent materials. ⁹⁹

4.3 Thorium

The release of Th (Ⅳ) into the water thereby induces a huge threat to humans and other living creatures. Thus, high-efficiency removal and further recovery of Th (Ⅳ) content from wastewater are essential. ⁹² MOF-303 was synthesized by the solvothermal reaction reported in the literature. ¹⁰⁵ It is synthesized from aluminum chloride hexahydrate (AlCl₃⋅6H₂O) and 3,5-pyrazoledicarboxylic acid monohydrate (H₂PDC H₂O). A stable aluminum-based metal-organic framework (MOF 303) with high-density ion traps was used to selectively capture thorium (Th) ions from an aqueous solution. The features of the aluminum center endow this material with excellent resistance from β-ray irradiation even with a high dose of 1,000 kGy. Further, due to the special chelation interaction from the ion trap, MOF-303 exhibits a large adsorption capacity for Th(IV) ion (461.7 mg g⁻¹) and excellent separation coefficients up to 97.6, 97.3, and 81.3 for Th(IV)/Pr(III), Th(IV)/Eu(III) and Th(IV)/Nd(III), respectively. ⁹² The combination of magnetic nanoparticles and metal-organic frameworks (MOFs) has demonstrated their perspective on pollutant sequestration. In this work, a magnetic metal-organic framework nanocomposite (Fe₃O₄@AMCA-MIL53(Al) was prepared and used for the removal of U(VI) and Th(IV) metal ions from an aqueous environment. The isotherm and kinetic data were accurately described by the Langmuir and pseudo-second-order models. The adsorption capacity was calculated to be 227.3 and 285.7 mg/g for U(VI) and Th(IV), respectively, by fitting the equilibrium data to the Langmuir model. ¹⁰⁶

4.4 Barium

For the ¹³³Ba with a long half-life (10.5 years), irreversible adsorption and long-term storage are highly required. The simultaneous high-capacity and fast removal of radioactive barium ion (Ba²⁺) from nuclear wastewater are necessary but still face a large challenge. Thus, strong host-guest interaction and high stability are necessary for an ideal adsorbent. ⁹⁴ Efficient decontamination of radioactive Ba²⁺ is of great significance to human health and environmental safety. Herein, an adsorbent based on the sulfonic acid functionalized Zr-MOF has been successfully developed, which could efficiently decontaminate radioactive Ba²⁺ with excellent selectivity, recyclability, a high adsorption capacity up to 60.8 mg g⁻¹ as well as a short adsorption kinetic time of less than 5 min. This outstanding adsorption performance is attributed to the strong affinity between Ba²⁺ and high-density –SO₃H active sites in MOFs. ¹⁰⁷ Another adsorbent containing a sulfonic group, MOF-808-SO₃H, was prepared based on the oxidation of the –SH group. The adsorbent exhibits excellent capture ability for Ba²⁺ due to strong hard acid-base and electrostatic interactions, with the adsorption capacity of 152.0 mg g⁻¹ and fast equilibrium adsorption time, superior to other reported materials. ⁹⁴

4.5 Iodine

Radioiodine ¹³¹I, which emits beta and gamma radiation, is the most frequently used radionuclide in nuclear medicine, especially to treat hyperthyroidism and thyroid cancer ¹⁰⁰ In the application of nuclear technology, radioiodine, a key radionuclide is released into the environment. Its danger to environmental and human health, such as hyperthyroidism, high environmental mobility, and human metabolic damage, makes its capture and safe storage very important. ¹⁰⁸ Silver-functionalized UiO-66 MOF was prepared by an easy one-pot solvothermal reaction. The prepared functionalized MOF was tested for the removal of radioactive iodine from water, obtaining an outstanding extraction capacity of ¹³¹I (∼1 MBq g⁻¹) due to the excellent affinity of Ag particles to iodide. The developed UiO-66-SO₃H@Ag 3D device showed high efficiency for the removal of ¹³¹I, excellent reusability, and good performance for the extraction of ¹³¹I from sewage water and hospital waste samples, obtaining in all cases recoveries above 90 %. ¹⁰⁰

4.6 Selenium

Contamination of Se has attracted worldwide attention, and the WHO has set a guideline at 40.0 μg/L for Se in drinking water. Moreover, Se(IV) and Se (VI) reveal more than 40 times higher toxicity than other forms of Se, and the acute toxicity of Se(IV) is nearly 10 times higher than Se(VI). ¹⁰⁹ Metal-organic framework materials (MOFs) and MOFs-based composite materials have been extensively applied to adsorb heavy metals due to their high specific surface area and high porosity. Herein, the MOF-801 and its complex Fe₃O₄/MOF-801 were respectively prepared and employed as adsorbents to capture the Se (IV) in an aqueous solution. The experimental results showed that the maximum adsorption capacity of MOF-801 and Fe₃O₄/MOF-801 was 101.2 and 206.6 mg g⁻¹ under acidic conditions, respectively ¹¹⁰ High-quality Al@Fe-MOF was prepared by in situ modification of Fe-MOF with Al³⁺ to improve the adsorption performance for selenite (Se (Ⅳ)). Compared with those of Fe-MOF, the specific surface area (SSA) and microporosity of Al@Fe-MOF decreased to 1,368 m² g⁻¹ and 38.5 %, respectively. the adsorption capacity and efficiency of Fe-MOF Compared with SSA for Se (Ⅳ) were increased by 77 % and 112 %, and the average free energy of adsorption was increased to 11.62  kJ/mol in Al@Fe-MOF. ¹⁰⁹

4.7 Technetium

One extremely nuisance radionuclide is technetium-99 (⁹⁹Tc), a significant portion of which exists in the form of pertechnetate, TcO₄⁻. The half-life of TcO₄⁻ is 2.13 × 10⁵ years, which can be generated in a typical uranium fission reactor with a high fission yield (∼6 %), giving rise to a mass of stock in spent nuclear fuel and waste. Imidazolium-based ionic liquids (ILs) are a promising candidate for the efficient separation of radioactive pertechnetate (TcO₄⁻) from nuclear waste. The bisimidazolium-based ILs into porous metal-organic frameworks (MOFs) were incorporated via a combination of immersion grinding and in-situ polymerization. 3,3′-divinyl-1,1′(1,4-butanediyl) dimidazolium dichloride is tightly bound inside and outside the porous MOFs matrix by uniform immersion grinding, which facilitates the exposure of more adsorption sites and provides channels for the anions to travel through quickly. Solvent-free polymerization reduces environmental pollution and energy consumption. Notably, the composite P[C₄(VIM)₂]Cl₂@MIL-101 possesses an admirable removal efficiency (673 mg g⁻¹) compared with the pristine poly (ionic liquid)s (215 mg g⁻¹). ¹¹¹ Direct removal of TcO₄⁻ from the highly alkaline solution of nuclear fuel is a serious and challenging environmental issue. The efficient synthetic two-dimensional metal-organic framework, named Mn-MOF, is established using MnCl₂·4H₂O coordinating with neutral nitrogen-donor ligand, showing ultrahigh stability in alkaline aqueous even under 1 M NaOH. The Mn-MOF exhibits a high adsorption capacity (403 mg/g) and high selectivity to ReO₄⁻ even under the presence of excess competing ions. Inspired by the highly efficient adsorption ability for ReO₄⁻, the Mn-MOF was applied to remove the TcO₄⁻ from nuclear waste. ¹¹²