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• Human life quality requires both industrial/agricultural fast development and high environmental quality. However, the two aspects are conflicted as some contaminants are accidentally released into the environment, potentially harmful to human health even at extra-low concentrations. Huge amounts of different pollutants including inorganic pollutants such as heavy metals, radionuclides, and organic pollutants such as pesticides, antibiotic contaminants and dye pollutants have been released into the water body in the last several decades with the development of human activities in industry and agriculture. Some contaminants are highly toxic even at extremely low concentrations, especially their accumulation in agricultural products or living organisms through food chains, and finally enter the human body. Thereby, it is a worldwide problem to eliminate the target pollutants from complex systems through sorption, catalytic mineralization strategies or to decontaminate the pollutants through in-situ treatment such as inactivation at solid particles, degradation of organic chemicals into CO₂ and H₂O, or solidification of metal ions into precipitates. The techniques for pollutant elimination are not only related to the properties of the target pollutant itself and environmental conditions (such as pH value, salinity, temperature, pollutant concentration, competitive ions) but also depend on the physicochemical properties of materials. Sorption is mostly applied as it is a simple operation on a large scale [1]. However, the selective elimination of pollutants from complex water systems and the separation and regeneration of the adsorbents should be considered. Photocatalysis is efficient for organic chemical degradation [2, 3]. While, the stability, efficient light absorption, and photo-generation/separation of electron-hole pairs are critical parameters for the photocatalytic degradation of organic pollutants. Electrocatalysis could efficiently separate target pollutants such as metal ions/radionuclides through the electrodeposition/reduction on the electrode. Still, the materials used as the electrode, and voltage and current are crucial for selective separation of target elements (Fig. 1) [4, 5].

  Figure 1

  

  Figure 1.  Porous nanomaterials and technologies for the removal of environmental pollutants. Adapted with permission [4]. Copyright 2021, John Wiley and Sons. Adapted with permission [1]. Copyright 2022. Elsevier. Adapted with permission [2]. Copyright 2022, John Wiley and Sons. Adapted with permission [5]. Copyright 2024, John Wiley and Sons.

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  The design and synthesis of materials are crucial to achieve highly selective removal of pollutants including heavy metal ions, radionuclides, and organic contaminants. Currently, porous nanomaterials, such as covalent organic frameworks (COFs), metal-organic frameworks (MOFs), porous aromatic frameworks (PAFs), hydrogen-bonded organic frameworks (HOFs) and porous organic polymers (POPs) have aroused extensive research interest due to the high stability, tunable pore sizes, porous structures, high specific surface area, active sites and abundant functional groups [1-3]. Fig. 1 shows the development milestones of porous nanomaterials in recent years [1, 2, 4, 5]. The applications of porous nanomaterials and their derivatives in environmental pollution cleanup have been studied extensively, and showed that the porous nanomaterials could efficiently remove pollutants through sorption, photocatalysis, electrocatalysis or oxidation–reduction reactions etc. [4, 5]. To achieve selective elimination of target chemicals, grafting special functional groups, adjusting active sites or modification of porous structures are most critical to selectively bind or react with the target molecules according to the properties of target pollutants.

  The rational design of sorption sites in porous nanomaterials is critical to achieving efficient removal of pollutants from aqueous solutions through sorption technology. For example, the Cu-based COFs were prepared and could bind I₂ through charge transfer and electron-rich π-conjugated sites, and the sorption capacity for I₂ was up to 2.99 g/g. Theoretical calculation further evidenced the formation of I_x⁻ (polyiodide anions) on Cu-N₄ structures with the adsorption energy of −22.4 kcal/mol [1]. Besides the metal-center sites, the construction of multi-N units could also enhance I₂ capture through the charge transfer between I₂ and hydrazine N/pyridine-N sites to form I₃⁻ and I₅⁻ on the multi-N nanotrap sites. Numerous studies have shown that introducing hydroxyl groups, phosphate group, amidoxime group, and other similar groups on the surface or within the pores of porous materials can achieve high selectivity in capturing uranyl ions. Among them, amidoxime-modified porous nanomaterials with crystalline and porous chemical structures are reported as the best adsorbents for U(Ⅵ) selective separation. The strong chemical complexation between the amidoxime group and U(Ⅵ) contributes to the highly selective extraction of U(Ⅵ) from complex water systems, especially from seawater. The high coordination of amidoxime with U(Ⅵ) enhanced U(Ⅵ) uptake, selectivity and stability, especially the effect of V(Ⅵ) ions, which have very similar structures to U(Ⅵ). Feng et al. [2] introduced uranyl ions into multivariate MOFs using a molecular imprinting strategy, thereby constructing in situ nanocage structures with high affinity for uranyl ions. Benefiting from the high affinity of this adsorbent for uranyl ions, the sorption capacity for uranium in natural seawater reached up to 7.35 mg/g, with an 18.38 times higher selectivity against vanadium.

  Photocatalysis strategy is an efficient technique to reduce high valent metal ions to low valent metal ions or to degrade organic pollutants under light irradiation. Besides the selective reaction of target pollutants on porous materials, the efficient light harvesting, generation/separation of electron-hole pairs and easy charge transfer are also crucial to improve the photocatalytic activity. The construction of porous structures with suitable bandgap for light absorption, intramolecular donor-acceptor, charge transport pathway, abundant active sites and abundant special functional groups is the best method for the porous nanomaterials to enhance the binding of organic molecules or metal ions and thereby increase the photoreduction efficiency of metal ions or organic pollutants. Cui et al. [3] synthesized MoS₂/COF-wood heterojunctions, which could inhibit charge recombination and thereby improve the photocatalytic degradation efficiency of dyes for wastewater purification. The generation of hydroxyl radical (^•OH) and superoxide radical (^•O₂⁻) free radicals enables pollutant degradation. Porous nanomaterials have been widely applied as photocatalysts for uranium extraction from seawater.

  Electrocatalysis separation of pollutants from complex systems can be used extensively as it is not considered the disadvantages of the sorption method such as sorption capacity, regeneration and reusability, photocatalysis technique such as light adsorption, bandgap adjustment and e⁻-h⁺ pairs generation/separation. Through grafting special functional groups (amidoxime, carboxyl, hydroxyl, amino-group, phosphate group, and hydrophobic group) and constructing an electron-transfer platform in the electrode, the target pollutants could be selectively deposited on the electrode through the electrochemical process. Yang and his colleagues reported an adsorption-electrocatalyst derived from MOFs, which includes an amidoxime group that specifically binds U(Ⅵ) and single-atom Fe active sites [4]. The amidoxime group achieves selective enrichment of U(Ⅵ) and the single-atom Fe acts as an electron transfer platform to catalytically convert adsorbed U(Ⅵ) into solid Na₂O(UO₃·H₂O)_x for collection. Additionally, electrocatalytic technology can be used for the reduction and immobilization of uranium from various systems such as groundwater, uranium mine wastewater, and radioactive effluents. For example, Wang et al. developed a supramolecular organic framework (MPSOF) electrode material containing selective binding sites to uranium and catalytic sites for the extraction of uranium from simulated high-salt radioactive effluents [5]. MPSOF exhibited a high extraction capacity of 7311 mg/g from simulated high-salt radioactive effluents.

  Despite the remarkable achievements of porous nanomaterials in the removal of environmental pollutants, some difficulties and challenges are still in their future development in this field. (Ⅰ) Most reported works focused on the synthesis and properties of porous nanomaterials in the laboratory. Real applications on a large scale, especially the in-situ treatment of real wastewater, are still limited. Future research should focus more on practical applications under real-world conditions. (Ⅱ) Most porous nanomaterials are in powder forms, which limits their recycling ability and results in secondary potential pollution. The fabrication of powders into films, gels, particles, foams or membranes could avoid the release of powders into solutions. (Ⅲ) Till now, most porous nanomaterials are synthesized in the laboratory on a small scale. How to synthesize porous nanomaterials on a large scale using a simple synthesis process at low cost, is critical to the actual applications. (Ⅳ) Although most porous nanomaterials are very stable under general conditions. The stability under extreme conditions such as strong acidic or base solutions, under high radio irradiation conditions etc. should be considered. (Ⅴ) The selectivity of target pollutants from complex systems is crucial for the elimination/separation of target pollutants from the actual environment. The porous structure, functionalization, and binding site adjustment etc. would help improve the selectivity of porous nanomaterials. (Ⅵ) How to recover and concentrate the extracting contaminants, while also enhancing the recyclability of the porous nanomaterials, remains a challenge. In conclusion, the rapid development of science and technology will effectively solve the challenges, and porous nanomaterials should have bright prospects in environmental pollution treatment.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Xiaolu Liu: Writing – original draft, Investigation, Data curation, Conceptualization. Suhua Wang: Writing – review & editing. Xiangke Wang: Writing – review & editing, Investigation.

  Acknowledgment

  This work was supported by National Natural Science Foundation of China (No. U21A20290).

  
  
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     Z. Cui, J. Wu, H. Li, et al., Sci. China. Chem. 67 (2024) 2111–2120. doi: 10.1007/s11426-023-1961-3

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     H. Yang, X. Liu, M. Hao, et al., Adv. Mater. 33 (2021) 2106621.

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     C. Wang, M. Xu, W. Wang, D. Hua. Adv. Funct. Mater. 34 (2024) 2402130.

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  Figure 1  Porous nanomaterials and technologies for the removal of environmental pollutants. Adapted with permission [4]. Copyright 2021, John Wiley and Sons. Adapted with permission [1]. Copyright 2022. Elsevier. Adapted with permission [2]. Copyright 2022, John Wiley and Sons. Adapted with permission [5]. Copyright 2024, John Wiley and Sons.

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