1. Introduction

With the rapid development of economy and technology, the pollution of heavy metal ions, e.g., Cu(Ⅱ), Pb(Ⅱ), Zn(Ⅱ), Cd(Ⅱ), Ni(Ⅱ), Fe(Ⅲ), has become a potential threat to human health and the natural environment [1,2]. Researchers have developed various methods for the removal of heavy metals, such as flocculation [3], electrolysis [4], membrane separation [5], photocatalysis [6], precipitation [7], and adsorption [8]. Among them, adsorption technique has become the most commonly used method due to its advantages of high adsorption capacity, strong selectivity, high recycling rate and simple operation [9]. To this end, various adsorbents have been developed, typically zeolite [10], biomass [11], agricultural waste [12], and porous polymers [13]. To date, some review articles have been reported about the adsorption of metal ions, including carbon nanotubes (CNTs) [14], graphene [15], layered double hydroxide (LDHs) [16], and metal organic frameworks (MOFs) [17].

Microporous organic polymers (MOPs) have attracted enormous attention due to their highly designable structure coupled with the diversity of synthesis methodologies [18]. MOPs mainly include covalent organic frameworks (COFs) [19], conjugated microporous polymers (CMPs) [20], porous aromatic frameworks (PAFs) [21], and hyper crosslinked polymers (HCPs) [22], etc. HCPs are usually prepared by Friedel-Crafts alkylation reaction, endowing the polymers with high crosslinking density, which tends to hinder the shrinkage of the polymer chain and results in a stable porous structure [23]. They stand out from many MOPs and have broad application prospects in various fields including storage/separation of gases [24], adsorption/enrichment [25,26], catalysis [27], sensing [28], and chromatographic separation [29]. In our previous studies, various HCPs were designed for the adsorption of iodine [30-32], drugs [33,34], organic pollutants [26], and metal ions [35]. Compared with widely reported materials toward metal ions, such as carbon nanomaterials [14] and LDHs [16], HCPs are characterized by ultra-light framework, wide range of building monomers, easy modification and functionalization [36-38], indicating that HCPs can act as competitive materials for the adsorption of heavy metals.

A literature survey illustrates that reviews on HCPs for the adsorption of heavy metals are rare. In Waheed et al.'s review [37], the adsorption isotherms of dyes, nutrients, inorganic ions, organic contaminants, and toxic metals ions with HCPs were described. Masoumi et al. [39] focused on the evaluation of HCPs from the viewpoints of different polymer types, cross-linkers, substitution degree, and chemical modifications that are effective in the structure and efficiency of cross-linked adsorbents. No review has been comprehensively and systematically reported about the fundamentals including physicochemical properties, adsorption mechanisms, and fabrication strategies, and their applications toward heavy metal ion adsorption despite their extensive applications in this field.

In this review, we provide a detailed discussion of the latest advances in the adsorption of heavy metals by HCPs in recent years. The physicochemical properties, preparation of HCPs, active functional groups such as carboxyl groups, sulfonic groups, phosphate groups, amino groups, and amidoxime groups introduced in the HCPs synthesis process, adsorption mechanisms, applications to metal ions adsorption, and influencing factors are summarized. Finally, the opportunities and challenges are discussed. The schematic diagram for the whole review is displayed in Fig. 1.

2. Physicochemical properties of HCPs

HCPs represent an important class of porous polymer materials, which have become a research hot spot due to their cheap starting monomers and precursors, rich synthesis strategies, and moderate experimental conditions [40]. They generally have high specific surface area, low density, and plentiful permanent pore structure [22]. HCPs possess many advantages, including low cost, environmental friendliness, easiness to prepare, and high reusability. In addition, HCPs have excellent chemical and thermal stability due to their rigid benzene ring structure. These properties make HCPs an attractive candidate for the removal of heavy metal ions. After years of rapid development, there are more and more types of HCPs materials, and their application scope has been greatly expanded.

When HCPs are applied to the adsorption of heavy metal ions, they tend to show low selectivity and low adsorption capacity [41]. To improve the adsorption capacity, researchers have tried to introduce functional groups during the preparation process of HCPs. For example, a phosphate-containing HCP for U(Ⅵ) adsorption was developed by Tuo's research group with the maximum adsorption capacity as high as 800 mg/g [42]. Anito et al. [43] designed a HCP adsorbent containing carboxylic groups and amino groups. The maximum adsorption capacity of the polymer for Pb(Ⅱ) reached 1138 mg/g. Besides, functional groups such as sulfonic groups, amino groups, and amidoxime groups have also been widely used to enhance the adsorption capacity of HCPs toward metal ions [44].

3. Adsorption mechanism

The adsorption mechanism of heavy metal ions with HCPs adsorbents is complex. However, coordination, electrostatic attraction, and ion exchange are the most dominant mechanisms reported in the literature.

3.1. Coordination

Coordination (complexation and chelation) is an important mechanism in the adsorption of heavy metal ions with HCPs adsorbents. For example, Bai et al. [45] synthesized amidoxime-based HCPs (HCPA) adsorbent by a simple "knitting" synthetic strategy and investigated the adsorption process of HCPA on U(Ⅵ) and proposed a reasonable adsorption mechanism. The removal rate of HCPA reaches 80% at pH 6.0. Such a high adsorption ability can be attributed to the complexation of U(Ⅵ) with the imine and hydroxyl groups of the amidoxime group. Moreover, the removal rate of HCPA still remains at 60% after seven cycles, and the good reusability of the as-prepared HCPA is beneficial for the real-world applications.

Xiang et al. [46] synthesized a HCP by Friedel-Crafts alkylation reaction using 2,4,6-trichloro-1,3,5-triazine as raw materials. The material has strong adsorption capacity for Hg(Ⅱ) (604 mg/g) due to the strong coordination between electron-rich heteroatoms on the adsorbent and Hg(Ⅱ). He et al. [47] prepared a triazine and thiophene bifunctionalized HCP (TSP-NS) by a one-step Friedel-Crafts reaction using 2,4,6-trithiophene-1,3,5-triazine as the monomer, and the obtained TSP-NS was used for the removal of Cu(Ⅱ). Combining the experimental results and theoretical quantum calculations, it is clear that the N and S heteroatoms in the bifunctionalized triazine and thiophene groups on TSP-NS can easily form coordination complexes with Cu(Ⅱ) (Fig. 2). In short, this polymer has advantages such as strong adsorption capacity, high reusability, and low cost, making it an attractive adsorbent for toxic metal ion removal and recovery.

He et al. [48] synthesized a phosphonic acid-based functionalized triptycene-based hyper-crosslinked porous polymer (TPP-BPP) and a benzene-based hyper-crosslinked porous polymer (B-BPP), and used them to adsorb U(Ⅵ). According to the fitting degree of the kinetic model, the pseudo-second-order kinetic model obtained a very high correlation coefficient. And the adsorption capacity calculated by the pseudo-second-order kinetic model is also close to the experimental value. This indicates that the main mechanism of U(Ⅵ) adsorption on B-BPP and TPP-BPP is the chemical adsorption or strong chelation of phosphine with U(Ⅵ).

3.2. Electrostatic attraction

In the adsorption of heavy metal ions, electrostatic attraction adsorption mechanism also plays an important role, which is directly caused by positive and negative charges of metal ions and adsorption materials. For example, Yang et al. [49] designed a thiourea modified hyper crosslinked resin (TM-HPS) and investigated its adsorption performance for Pb(Ⅱ), Cd(Ⅱ), and Cu(Ⅱ). The results of FT-IR and XPS show that the electron binding energy and the intensity of the peaks related to amino groups changed significantly before and after adsorption. These indicate that the adsorption mechanism is the coordination and electrostatic attraction between amino groups and metal ions.

Masoumi et al. [25] prepared a series of polystyrene-based HCPs by controlling different reaction conditions and used them to recover Cd(Ⅱ). The analysis of zeta potential shows that the adsorption of Cd(Ⅱ) by polystyrene-based HCPs is the simultaneous action of electrostatic attraction and chelation. It is also clear that electrostatic attraction is generally not the only adsorption mechanism of adsorbents, but will be accompanied by other adsorption effects, such as complexation and chelation.

3.3. Ion exchange

Ion exchange is the process of replacing ions between an insoluble solid with exchangeable ions and ions with the same charge in solution. Recent studies suggest that under mild conditions, the imidazolium-N⁺ sites are more favorable for metal removal than other common anions, including NO₃⁻, Cl⁻, SO₄²⁻. This is attributed to the higher binding free energies at the imidazolium-N⁺ sites [50]. For instance, Yang et al. [51] halogenated ionic covalent organic polymers (iCOPs) to enhance the adsorption capacity for ⁹⁹TcO₄⁻ and ReO₄⁻ in simulated polluted water. The presence of imidazolium-N⁺ sites in iCOPs increases the adsorption ability for ⁹⁹TcO₄⁻ and ReO₄⁻ when halogen groups are introduced near the sites. Due to ion exchange, high adsorption performance of iCOPs for metal ions is achieved. The prepared adsorbents have strong adsorption capacity, high elution efficiency, and excellent selectivity, making them highly promising adsorbents.

In the process of removing heavy metal ions by using HCPs, the replacement of H ions with heavy metal ions usually occurs when carboxylic and sulfonic-based HCPs are employed. Tan et al. [41] synthesized sulfonic acid-modified microporous HCPs for the adsorption of Pb(Ⅱ), Cu(Ⅱ), and Ni(Ⅱ). High adsorption capacity for metal ions was achieved due to the synergic effect of ion exchange and the adsorption of micropore. Moreover, the adsorbent can be recycled several times with minimal loss of adsorption capacity and thus may have potential industrial applications.

To deduce, coordination, electrostatic attraction, ion exchange mechanisms or their combination contribute to the adsorption capacity of HCPs toward heavy metal ions, which plays a critical role in the highly efficient adsorption of heavy metals.

4. Preparation of HCPs

In the preparation of HCPs, choosing judicious monomers and appropriate length crosslinkers plays an important role to yield a well-developed porous polymer. So far, the following three strategies can be used to synthesize HCPs, i.e., post-crosslinking, one-step self-polycondensation, and external crosslinker braiding [29]. Post-crosslinking is regarded as the earliest strategy [39], but the synthesis of polymer precursors is usually time-consuming and the monomers adapt to the method are limited. The second one suffers from the disadvantage that the monomers must bear special functional groups. The third strategy distinguishes itself from the other two by its remarkable advantages including wide range of monomers, so it has been extensively applied in many fields. A comparison of the three preparation strategies for HCP is summarized in Fig. S1 (Supporting information).

4.1. Post-crosslinking method

Post-crosslinking method is the main synthesis method of early hyper crosslinked polystyrene network (Davankov resin, Fig. 3A) [52], in which HCPs are prepared by Friedel-Crafts alkylation reaction with polystyrene-based mildly-crosslinked polymer as precursor, Lewis acid as catalyst and external electrophile as hyper crosslinkers [53]. A variety of monomers can be employed in this method, including carbazole, divinylbenzene (DVB), etc. Haratbar et al. [54] prepared a carbazole network-based HCPs (HCP-CN) using carbazole as monomer and Lewis acid as catalyst by post-crosslinking, by which over 99.8% of Pb(Ⅱ) was ultimately removed. The main advantage of the post-crosslinking method is that the polymer reaction precursor can be directly used to obtain the hyper crosslinked products by one-step crosslinking. However, this synthesis method often requires obtaining polymer precursors first, and then a microporous network is formed by hyper crosslinking, which limits the available types of monomers and the performance of HCPs [40]. Moreover, the synthesis process requires a large amount of reagents and is complex to operate, which greatly limits its application in the field of HCPs preparation [55].

4.2. One-step self-condensation method

One-step self-condensation, also known as direct polycondensation, was firstly reported by Tan's group [56] in 2007. They synthesized a series of HCPs by the self-condensation of bischloromethyl monomers including dichloroxylene, 4,4′-bis(chloromethyl)-1,1′-biphenyl, and 9,10-bis(chloromethyl)anthracene. The porosity in the polymers is controlled not only by the level of cross-linking, but also by subtle changes in the design of the rigid monomer unit. The self-condensation of bischloromethyl monomers can lead to materials with enhanced gas storage properties.

Similarly, Tan's group [57] synthesized two HCP networks by the self-condensation of bishydroxymethyl and monohydroxymethyl monomers, respectively (Fig. 3B). N₂ adsorption isotherms for the polymers show that both materials are predominantly microporous with Brunauer-Emmett-Teller (BET) surface areas of 847 and 742 m²/g. This result breaks through the traditional understanding that hyper crosslinked reaction monomers must contain multiple reaction groups, extending monomers of the self-condensation that can form pores to single functional aromatic compounds. Afterwards, the same group [58] extended the research to one-step self-condensation polymerization without functional monomers based on the Scholl coupling reaction. The main principle is that under the action of AlCl₃ catalyst, the hydrogen atoms on two adjacent benzene rings are eliminated, and a new C–C single bond is formed at the same time, thereby forming a cross-linked structure.

The one-step self-condensation method eliminates the need for special functional groups in the monomers involved in the reaction. Besides, due to the high activity of the catalysts used in this strategy, the range of monomers that can participate in the reaction can be greatly expanded, including monomers with high or low electron density, acidic or alkaline functional monomers, aromatic, fused or heterocyclic monomers, etc. [40]. Compared with the post-crosslinking method, the one-step self-condensation method expands the choice of monomers and effectively reduces the reaction time and improves the reaction efficiency. However, with the one-step self-condensation method, the building blocks used in the synthesis must contain eliminable functional groups, which not only limits the range of monomers that can participate in the reaction, but also increases the cost of preparing microporous polymers during the reaction process.

4.3. External crosslinker braiding method

In response to the limited range of available monomers in the post-crosslinking and one-step self-condensation methods, Tan's group [59] proposed a strategy for the synthesis of external crosslinker braiding method in 2011. They used formaldehyde dimethyl acetal (FDA) as the external crosslinker in the research and enabled various aromatic monomers were directly crosslinked to form the highly porous networks (Fig. 3C). This avoids the need for monomers with specific polymerisable groups and also avoids the use of precious metal coupling catalysts. The specific surface area and pore volume can also be finally controlled by adjusting the ratio of external crosslinking agent. Similarly, Bai et al. [45] designed amidoxime functionalized HCP adsorbent by the external crosslinker braiding method strategy, in which FDA was also used as the external crosslinker, biphenyl and amidoxime-functionalized 1-(benzyloxy)−4-ethylbenzene as the original monomers, respectively. The synthesized adsorbent exhibited excellent adsorption capacity for U(Ⅵ).

Compared with the post-crosslinking method and one-step self-condensation method, the external crosslinking method has the advantages of no need of specific functional groups in monomers, mild synthesis conditions, low cost of raw materials. In addition, different porous structures and functionalized polymer networks can be obtained by changing the building units in the external crosslinking progress.

5. Functional groups for HCPs fabrication

HCPs have found their extensive applications in the field of separation and enrichment of various target considering their attractive merits. To improve the adsorption capacity of HCPs toward heavy metal ions, researchers often introduce specific active functional groups in the synthesis procedure of HCPs to enrich the interactions between HCPs and target ions. Over the past years, the functional groups for HCPs fabrication are mainly focused on carboxyl, sulfonic, phosphate, amino, and amidoxime groups.

5.1. Carboxyl groups

Carboxyl groups can undergo ion exchange and coordination reactions with metal ions [39], so increasing carboxyl sites of an adsorbent can improve its adsorption capacity. Maleic acid is a well-known dicarboxylic acid that is often used to construct adsorbent materials focusing on metal ions [60]. It has also found wide applications for the preparation of carboxylic-based HCPs, especially based on its copolymer with styrene which has been extensively reported. Gonte et al. [8] synthesized hyper crosslinked styrene-maleic acid (SMA) copolymer using free radical initiator and suspension polymerization technology, and studied its adsorption capacities of Cu(Ⅱ), Ni(Ⅱ), Zn(Ⅱ), and Co(Ⅱ) from mimicked waste-water systems. Equilibrium isotherm, adsorption kinetics and mechanism were studied in detail. Results indicate that the carboxylic groups present in SMA matrix are active sites for metal adsorption, and they can chelate with metal ions. Additionally, SMA based HCPs beads can be potential candidates for the treatment of industrial wastewater.

Meng et al. [61] also used SMA copolymer to produce porous hyper crosslinked polymer bearing carboxyl groups (C-SMA) via Friedel-Crafts acylation reaction. The obtained C-SMA was then loaded with Fe₃O₄ by a solvothermal method to prepare a magnetic U adsorbent (Fe₃O₄@C-SMA, Fig. S2a in Supporting information). The adsorbent shows good U adsorption performance (178 mg/g at pH 6.0), and can be readily magnetically recovered. XPS analyses display that the presence of carboxyl groups contributes to the adsorption of U by Fe₃O₄@C-SMA as carboxyl groups participate in the coordination with U. In addition, Fe₃O₄@C-SMA is highly selective for U in the presence of multiple competing ions (e.g., Na, Mg, K, Ca, Co, and Cd).

Except maleic acid, other carboxyl acids including salicylic acid and 2-hydroxyterephthalic acid have been reported for the construction of carboxylic-based HCPs. Meng et al. [62] chemically modified chlorinated polystyrene (NDA-150) with salicylic acid to obtain a HCP adsorbent (NDA-1500) containing abundant carboxyl groups, which was used to adsorb Pb(Ⅱ), Cu(Ⅱ), and Ni(Ⅱ). The removal efficiency of NDA-1500 for the three metal ions is 2–3 times higher than that of NDA-150. This is attributed to the presence of numerous functional groups on the surface of NDA-1500, and superficial carboxylic and phenolic groups act as adsorption sites. Moreover, the captured metal ions could be completely recovered with 0.2 mol/L hydrochloric acid, indicating the high reusability of the obtained carboxylic-based adsorbent. Yang et al. [63] synthesized a carboxylic-rich HCP by modifying HCPs with 2-hydroxyterephthalic acid (2-HTA) and used to adsorb Ni(Ⅱ), Pb(Ⅱ), Hg(Ⅱ), and Cd(Ⅱ). The carboxyl and hydroxyl functional groups on the surface of the adsorbent contribute to the high adsorption ability. The adsorption capacities of Ni(Ⅱ), Cu(Ⅱ), Cd(Ⅱ), and Pb(Ⅱ) are 33.46, 50.33, 69.69, and 178.27 mg/g at 25 ℃, respectively.

Since most metal ions can interact with carboxyl groups via ion exchange or coordination reactions, studies about the introduction of carboxyl groups to HCPs attract more attention compared with other functional groups. Future researches can be centered to explore more efficient metal ions adsorbents focusing on HCPs containing more carboxylic groups.

5.2. Sulfonic groups

Sulfonic group is a typical ion-exchange group which can interact with many types of heavy metal ions via exchanging with the counter ions, H⁺ or Na⁺ [64]. Undoubtedly, the introduction of sulfonic group into the preparation process of HCPs will be beneficial to their adsorption performance toward metal ions. So far, reported sulfonic-based HCPs are generally synthesized directly using sulfonating agents. For example, Tan et al. [41] used acetyl sulfuric acid to synthesize sulfonic-modified microporous HCPs (SAM-HCPs) by direct sulfonation reaction of HCPs. The simple surface modification retains the microporous structure and improves hydrophilic nature, and thus significantly enhances the removal capability of toxic metal ions, Pb(Ⅱ), Cu(Ⅱ), and Ni(Ⅱ), from aqueous solution. The adsorption performance of the synthesized SAM-HCP is significantly higher than that of the unsulfonated HCP, which can remove 99% of toxic metal ions due to the synergic effect of ion exchange and microporous adsorption. In another study, Jia et al. [65] prepared sulfonated hyper crosslinked polystyrene (SHCP) via a direct sulfonation reaction between hyper crosslinked polystyrene resin with sulfuric acid. The as-synthesized SHCP adsorbent exhibits the adsorption capacity of 0.7 mmol/g for Cd(Ⅱ). The adsorption capacity of Cd(Ⅱ) on SHCP is influenced by the ion exchange and electrostatic attraction between Cd(Ⅱ) and immobilized sulfonic acid groups.

Except surface modification with sulfonating agents, sulfonic-based HCPs can also be prepared via employing reactive monomers containing sulfonic group. Zhu et al. [66] prepared hyper-crosslinked lignosulfonates (CLS, Fig. S2b in Supporting information) by Friedel-Crafts alkylation reaction using sodium lignosulfonate (LS) containing sulfonic groups and dimethoxymethane in the coexistence of 1,2-dichloroethane (DEA) and anhydrous AlCl₃. The maximum adsorption capacity of CLS is up to 74.45 mg/g, which is about 7 times of that of LS. The crosslink reaction between LS chains via F-C alkylation reaction increases the specific surface area and avoids the formation of intermolecular hydrogen bonds. The increase of specific surface area and abundance exposure of binding sites consequently strongly enhances the adsorption capacity. The work provides a feasible synthesis method to obtain sulfonic-based HCPs, and contributes to their further applications to heavy metal ions adsorption.

5.3. Phosphate groups

Phosphate groups has a strong coordination ability with many metal ions [67], so researchers have combined them with HCPs to improve their adsorption capacity of metal ions. Up to now, phosphate-based HCPs are mainly applied to the adsorption of heavy metal U(Ⅵ). For example, Di et al. [42] used bis(2-methacryloxyethyl) phosphate as the monomer to prepare hyper crosslinked phosphate-based polymers (HCPP) and investigated its adsorption performance of U(Ⅵ). Results indicate that the maximum adsorption capacity of HCPP for U(Ⅵ) is up to 800 mg/g at pH 6.0. Such a high adsorption capacity can be attributed to the followings. Firstly, bis(2-methacryloxyethyl)phosphate monomers are linked by free radical addition reactions to form a backbone with hydrophilicity and high cross-linkage, creating a stable chemical environment for HCPP to adsorb U(Ⅵ). Secondly, by comparing the infrared spectra of the materials before and after adsorption, it is found that the P=O peak changed, while NO₃^- and U(Ⅵ) signals appear after adsorption. Such results indicate the chemical coordination between O and U(Ⅵ) in the phosphate group, proving that the phosphate group plays a great role in the adsorption process.

Tian et al. [68] synthesized two phosphorylated HCPs adsorbents based on bisphenol A and fluorene-9-bisphenol (PHCP-1 and PHCP-2, Fig. S2c in Supporting information) and applied to the capture of U(Ⅵ). Phosphorus trichloride was employed to phosphorylate original HCP-1 and HCP-2, i.e., hyper crosslinked bisphenol A polymer and hyper crosslinked fluorene-9-bisphenol polymer, respectively. For PHCPs/U system, the adsorption capacities achieve 286.01 mg/g (PHCP-1) and 297.14 mg/g (PHCP-2), which are much higher than those of HCP-1 (134.84 mg/g) and HCP-2 (135.58 mg/g). The adsorption of U on PHCPs is mainly due to the complexation between U(Ⅵ) and O in P-OH/P=O of phosphate groups. Evidently, the introduction of phosphate groups provides more active functional group sites in the adsorbents, leading to the high adsorption capacity of the phosphorylated HCPs.

It is worthy to note that except U(Ⅵ), phosphorylated HCPs have been scanty reported for the removal of other metal ions, which leaves a great room for the exploration of HCP adsorbents since phosphate group has a strong coordination ability with many metal ions.

5.4. Amino groups

Amino group has good affinity with many metal ions, therefore, it is often employed to construct heavy metal adsorbents [69]. For example, Yang et al. [70] synthesized a melamine-based porous HCP (SNW-D, Fig. S2d in Supporting information) and used it to adsorb Hg(Ⅱ) in water. Results showed that the polymer exhibited high adsorption capacity of Hg(Ⅱ) (841 mg/g). FT-IR, Raman, and XPS spectra studies show that the adsorption mechanism of Hg(Ⅱ) by the polymer is that N-containing groups (triazine ring, primary amino, and secondary amino) on the polymer matrix coordinate with Hg(Ⅱ). In addition, adsorption kinetics and isotherm studies demonstrated that the Hg(Ⅱ) removal by these polymers is extremely rapid (90% being attained within 5 min for a 400 mg/L Hg(Ⅱ) solution) and highly efficient (up to 1172 mg/g).

Wang et al. [44] used polylactide-b-polystyrene-b-polyglycidyl methacrylate (PLA-b-PS-b-PGM) triblock copolymer as the precursor to prepare PGM-functionalized hollow porous nanospheres (HPNs-PGM) by hyper crosslinking mediated self-assembly method. Then, ethylenediamine (EDA) was used to further open the epoxy groups to obtain ethylenediamine-modified hollow porous nanospheres (HPNs-NH₂). Due to the special multistage pore structure, high specific surface area, and a large number of amino active sites which can generate electrostatic attraction with Cr(Ⅵ) after protonated, the obtained solid adsorbent HPNs-NH₂ exhibits a high adsorption capacity for Cr(Ⅵ) (493 mg/g). Sang et al. [71] synthesized a porous hyper crosslinked phenylalaninol (HCP-PAO) for U(Ⅵ) adsorption, in which phenylalaninol bearing amino and hydroxyl groups was used as the monomer, and biphenyl dichlorobenzyl was used as the external crosslinker. U(Ⅵ) can coordinate with N in the amino group and O in the hydroxyl group of HCP-PAO. The maximum adsorption capacity is achieved as high as 369.5 mg/g.

5.5. Amidoxime groups

Similar to phosphate groups, amidoxime groups also have strong chelation for U(Ⅵ) [72], hence they have been employed for the construction of HCPs focusing on U(Ⅵ) adsorption. In Bai et al.' study [45], a series of amidoxime-based HCP adsorbents (HCPAs) were prepared by a simple "knitting" synthetic strategy and used for the removal of U(Ⅵ). The adsorption thermodynamics, reconstruction, dynamics and selectivity of HCPAs are discussed and compared. Furthermore, the adsorption mechanism of HCPAs with U(Ⅵ) is deduced by XPS and FT-IR, demonstrating that the U(Ⅵ) removal mechanism by HCPAs is originated from the complexation between U(Ⅵ) and amidoxime. In addition, the same group [73] reported a hyper crosslinked polymer resin adsorbent for the adsorption of amidoxime of U(Ⅵ) from ocean. The matrix was prepared by suspension polymerization with styrene (St) and DVB, followed by post-polymerization modification to graft diaminomaleonitrile and conduct amidoximation to introduce amidoxime groups in its interior and surface. Amidoxime groups can enhance the affinity to U and hydrophilicity to accelerate the U adsorption. Meanwhile, O in the amidoxime group of the prepared material participates in the coordination with U(Ⅵ). The maximum adsorption capacity toward U(Ⅵ) is 256.4 mg/g, implying that the adsorbent can be considered as a potential adsorbent to perform U(Ⅵ) extraction from natural seawater.

In another study, Zhang et al. [74] grafted diaminomethane nitrile (DAMM) onto a benzoylimide hyper crosslinked porous organic polymer backbone by a simple Schiff base reaction, and then reacted with hydroxylamine hydrochloride to prepare amidoxime modified hyper crosslinked benzoyl polymers (HCP-AO, Fig. S2e in Supporting information), which were used for the adsorption of U(Ⅵ) in simulated seawater solutions. The adsorption results show that there is strong chelation and electrostatic interaction between the amidoxime group on HCP-AO and U(Ⅵ), and HCP-AO has a high adsorption capacity toward U(Ⅵ). At pH 6.0, its maximum U(Ⅵ) adsorption capacity is 370.9 mg/g, and the removal rate of U(Ⅵ) is as high as 66.35%.

In summary, incorporating which functional groups on the design of adsorbent materials is an important issue to be considered for the preparation of heavy metal adsorbents.

6. Application to heavy metal ions adsorption

HCPs adsorbents containing various functional groups possess stable physicochemical properties, rich microporous structure, high specific surface area and abundant active sites, which endow them with highly specific adsorption capacity toward heavy metal ions [52]. So far, HCPs have been extensively applied to the adsorption of various heavy metal ions, including U(Ⅵ), Cu(Ⅱ), Hg(Ⅱ), Pd(Ⅱ), Cr(Ⅵ), Cd(Ⅱ), Fe(Ⅱ), and Fe(Ⅲ).

6.1. Uranium

Uranium (U) is a radioactive element with a long half-life, radiotoxic and chemically toxic, usually present in the environment in the hexavalent form (mainly uranyl ion, U(Ⅵ)) [75]. U is easily absorbed by plant roots and retained in the roots causing damage and may also cause genetic mutations bearing deformed fruits, while U easily enters animals and humans through the food web and food chain, depositing in the liver, kidneys and bones, causing genetic mutations and cancer, etc., which can irreversible damage to human, animal and plant health [76]. Therefore, from the perspective of energy recovery and environmental protection, it is urgent to study the separation process of U.

The commonly reported approaches always have various shortcomings [77]. To provide an efficient and economical method, Leng et al. [78] developed three covalent organic polymers (COPs) with different crosslinked structures for the removal of U(Ⅵ) (Fig. 4). Compared with the other two synthesized materials, the prepared tPF-AO by amidoxime-functionalizing tetrafluoroterephthalonitrile (TFTPN) crosslinked phloroglucinol (tP) has the best recyclability, and its removal performance remains almost unchanged for ten turns. Besides, tPF-AO exhibits excellent stability and maximum adsorption capacity, with a theoretical maximum adsorption capacity of 578.9 mg/g. Research indicates that the quantitative result is in agreement with the chemical stability test result by FTIR spectroscopy.

Among the various heavy metal ions, HCPs have been most frequently applied to the adsorption of U. The HCPs adsorbents toward U capture are mainly focused on amidoxime [22,75,78-80] and phosphoric-based HCPs [48,81-85]. For instance, Zhu et al. [79] synthesized an amidoxime-modified hyper crosslinked poly(styrene-co-acrylonitrile) adsorbents with adjustable porous structure through a simple Friedel-Crafts reaction and amidoxime reaction, and used it to adsorb U(Ⅵ) from seawater. In the real seawater samples, a large number of metal ions (K, Ca, Na, Mg, Sr, Zn, Ni) are also contained. The removal rate of U(Ⅵ) can reach 84.4%, far higher than that of other ions. This is mainly due to the higher chelating affinity of amidoxime groups with U(Ⅵ), which makes the adsorbent have strong competitive adsorption capacity toward U(Ⅵ). In another study, Yang et al. [80] synthesized amidoxime-modified hollow organic porous nanospheres (HOPNs-AO, Fig. S3a in Supporting information) via a one-pot hyper-crosslinking self-assembly method and subsequent post-modification strategy, and applied it to the removal of U(Ⅵ). Due to the special adsorption property of amidoxime groups toward U(Ⅵ), the maximum adsorption capacity of HOPNs-AO is 519.5 mg/g. XPS results confirm that both N and O in the amidoxime group participate in the coordination effect during the adsorption process. Recyclability study displays that when 1 mol/L Na₂CO₃ was employed as the desorption agent, the adsorption capacity of HOPNs-AO reached over 80% of the initial adsorption effect after five adsorption/desorption runs. In addition, amidoxime functionalized hyper crosslinked fluorene-9-bisphenol (HCP-AO, Figs. S4a-d in Supporting information) for the adsorption of U(Ⅵ) was also prepared by Tian et al. [22]. The maximum adsorption capacity of U(Ⅵ) by HCP-AO adsorbent can reach as high as 234.46 mg/g. Such a high adsorption capacity can be assigned to the abundant amidoxime groups which can easily coordinate with U on the surface of HCP-AO. Besides, with 0.5 mol/L HCl as the desorption agent, the adsorption capacity of HCP-AO only decreases by 36.7 mg/g after three runs, and the material maintains an adsorption capacity of around 200 mg/g. The above results signals that HCP-AO can serve as an adsorbent material with high chemical stability in uranium-bearing wastewater. In summary, HCP-AO has research significance in the extraction of U from wastewater due to its good adsorption ability and recyclability.

He et al. [48] prepared a HCP adsorbent using phenyl phosphonic acid as the functional monomer and the aromatic building blocks such as benzene and triptycene as the external crosslinker. Then two phosphoric-based HCPs (TPP-BPP and B-BPP) were synthesized by external crosslinking agents and used to adsorb U(Ⅵ). Under the optimum experimental conditions, the maximum adsorption capacity of TPP-BPP and B-BPP is 119.05 and 222.72 mg/g, respectively. In addition, Zhu et al. [81] also prepared a phosphoric-based HCPs for the adsorption of U(Ⅵ), i.e., a Fe(Ⅲ) complex of phosphorylated hypercrosslinked calix[4]arenes (C4HP-Fe(Ⅲ), Fig. S3b in Supporting information). The maximum adsorption capacity is 497.6 mg/g thanking to the chelation between U(Ⅵ) and phosphate (Fig. 3c in Supporting information). During the adsorption process, the binding energies of P-O and P=O both shift due to the chelation between U(Ⅵ) and phosphate and hydroxyl groups. The porous C4HP-Fe(Ⅲ) material can capture U(Ⅵ) under simulated wastewater and seawater conditions, with K_d^U reaching 4.1 × 10³ mL/g and 4.4 × 10⁴ mL/g, respectively. Additionally, C4HP-Fe(Ⅲ) was regenerated by treated with 1 mol/L HNO₃ solution. In the sixth cycle, the adsorbent maintains 92.2% of its original adsorption capacity, demonstrating its good reusability.

Other than that, amino-based HCPs, carboxylic-based HCPs and sulfonic-based HCPs are also used to adsorb U(Ⅵ). Sang et al. [71] synthesized an amino-based HCP (HCP-PAO) by Friedel-Crafts alkylation reaction with phenylalanine as the starting monomer and biphenyl dichlorobenzyl as the external crosslinking crosslinker. The maximum adsorption capacity of HCP-PAO reaches 369.5 mg/g due to the coordination between U(Ⅵ) and the amino group of N and the carboxyl group of O. Ahmad et al. [86] prepared a tubular HCP via self-polymerization using α,α'-dichloro-p-xylene (DCX) as the monomer, and then carbonized it to obtain hollow tubular nanofibers (HTnFs). HTnFs were further modified with carboxylic (COOH) and sulfonic (SO₃H) groups to obtain HTnF-SO₃H and HTnF-COOH, exhibiting incredible U(Ⅵ) adsorption capacities as high as 1928.59 and 1827.57 mg/g, respectively. With increasing pH values, the adsorption amount firstly increases, then reaches maximum, and finally decreases due to the negatively charges at pH values less than the isoelectric point, the complexation between oxygen-carrying groups and U(Ⅵ) species, and the difficulty of negatively charged U(Ⅵ) species to enter into the framework at low, medium, and high pH values, respectively. Moreover, reusability and sustainability studies suggest that the removal efficiencies of HTnF-SO₃H and HTnF-COOH remain basically unchanged in the first two runs when the experimental adsorbents were treated with 0.8 mol/L HNO₃. As the number of runs increases, the efficiencies of the two adsorbents decrease by no < 85% after five consecutive operations under seawater conditions. To sum up, these adsorbents owe the advantages of easy preparation, regeneration properties and high U(Ⅵ) adsorption ability, and have potential prospects for the removal of U(Ⅵ) from seawater.

The reported applications of HCPs and other famous adsorbents including CNTs, graphene, LDHs, and MOFs toward U(Ⅵ) adsorption are summarized in Table S1 (Supporting information), indicating that HCPs have high potential for adsorbing U(Ⅵ).

6.2. Copper

Copper (Cu) is mainly used in electroplating, papermaking, herbicides and other industries, and the wastewater of these industries usually contains Cu(Ⅱ), which causes serious environmental and toxicological problems [87]. For example, Cu(Ⅱ) can damage the brain, spleen, liver, pancreas and cardiac muscle [88]. Therefore, effective adsorption of Cu(Ⅱ) is one of the important research directions in HCPs adsorbents.

Wang et al. [89] synthesized a carboxyl-containing hyper crosslinked polystyrene adsorbent WJN-101 to remove Cu(Ⅱ), and compared its adsorption performance with industrial resins IRC-84 and D-110. The results show that WJN-101 exhibited better adsorption capacity for Cu(Ⅱ) than IRC-84 and D-110. The adsorption of Cu(Ⅱ) by IRC-84 and D-110 is only dependent on the chemical adsorption of carboxyl functional groups, while the adsorption capacity of WJN-101 for Cu(Ⅱ) is attributed to the high specific surface area, pore structure, and chelation of carboxyl groups with Cu(Ⅱ) of WJN-101. Meanwhile, 2% HCl solution was used as the eluent to desorb Cu(Ⅱ) from the polymeric adsorbent WJN-101. This acidic solution did not significantly reduce the removal rate of Cu(Ⅱ) of WJN-101 after five adsorption-desorption cycles, and WJN-101 was successfully used in actual water/wastewater treatment. Ni et al. [90] prepared ethylenediaminetetraacetic acid (EDTA) modified hollow microporous organic nanospheres (EDTA-HMONs-BA) by one-pot hyper crosslinked mediated self-assembly and used polylactide-b-polystyrene (PLA-b-PS) diblock copolymer and benzylamine (BA) monomer as precursors. EDTA-HMONs-BA was used to adsorb Cu(Ⅱ), and the maximum adsorption capacity is 236 mg/g. The adsorption of Cu(Ⅱ) by EDTA-HMONs-BA involves the electrostatic interaction between Cu(Ⅱ) and adsorption sites, and the diffusion of Cu(Ⅱ) into the pores of EDTA-HMONs-BA until each adsorption site is occupied by Cu(Ⅱ). After five recycles, the adsorptive capacity of EDTA-HMONs-BA maintains > 90%. As a result, EDTA-HMONs-BA can be considered as potential and economic adsorbents for dealing with complex wastewater.

Except for the above-mentioned HCPs adsorbents for carboxyl and amino groups, researchers also employed other groups for the construction of HCP adsorbents focusing on Cu(Ⅱ) capture. Li et al. [41] prepared SAM-HCPs containing sulfonic groups with the adsorption capacity of 57.68 mg/g for Cu(Ⅱ). Besides ion exchange process, the adsorption process of Cu(Ⅱ) raised from the micropore of the adsorbent also exists. In addition, the SAM-HCPs adsorbent can be reused multiple times with a decrease in adsorption efficiency of no more than 3% after five adsorption-desorption cycles by using 1 mol/L HCl as the elution agent. He et al. [47] synthesized a triazine and thiophene bifunctional HCP (TSP-NS) and applied as the absorbent for Cu(Ⅱ), reaching a maximum adsorption capacity of 98.33 mg/g. This is because that N and S heteroatoms of bifunctional TSP-NS can easily form coordination complexes with Cu(Ⅱ), and high adsorption capacity can thus be achieved. Meanwhile, the regeneration results display that TSP-NS has an excellent recyclability and can be recycled at least five times. To deduce, thiophene and triazine groups are good functional ligands for metal ions, the use of these two groups to synthesize the bifunctionalized TSP-NS for the removal of Cu(Ⅱ) provides new ideas for potential technological applications for removing heavy metal ions from aqueous solutions.

Yang et al. [49] prepared a heavy metal ions adsorbent (TM-HPS) by directly loading thiourea groups onto hyper crosslinked resin, and the adsorption capacities of Cu(Ⅱ), Cd(Ⅱ), and Pd(Ⅱ) are 290.69, 432.90, and 689.65 mg/g, respectively. The adsorption ability of Cu(Ⅱ) with TM-HPS is not as competitive as that of Pd(Ⅱ) and Cd(Ⅱ). Such a result can be explained with the influence of the size of hydrated ions and the free energy of hydration. The smaller the hydration radius, the stronger the affinity; and the hydration radius of metal ions obeys the order: Cu(Ⅱ) > Cd(Ⅱ) > Pd(Ⅱ). Additionally, the higher the level of hydration free energy of metal ions, the easier they are retained in the aqueous solutions, making them less likely to be removed by the adsorbent; and the free energy of hydration follows the order: Cu(Ⅱ) > Cd(Ⅱ) > Pd(Ⅱ). It is worth mentioning that, although the adsorption capacity of TM-HPS for Cu(Ⅱ) is much lower than that for Pd(Ⅱ) and Cd(Ⅱ), TM-HPS is still a preferable HCP adsorbent for Cu(Ⅱ) due to its fascinating adsorption capacity. Therefore, the introduction of thiourea groups has a good research prospect in the design of HCPs as Cu(Ⅱ) adsorbents. In addition, the adsorption of Pd(Ⅱ), Cd(Ⅱ), and Cu(Ⅱ) by TM-HPS comprises two ways. The first approach is caused by the complexation between the surface functional groups of TM-HPS and metal ions, while the second is electrostatic interaction between sulfur of the thiourea group and metal ions. A summary of Cu(Ⅱ) adsorption with HCPs together with other materials is displayed in Table S2 (Supporting information), which declares that HCPs can serve as promising adsorbents for Cu(Ⅱ) capture.

6.3. Mercury

Mercury (Hg), as a heavy metal extensively used in industrial production, is one of the most hazardous elements to human beings [46]. The excessive intake of Hg mainly harms the central nervous system, digestive system and kidney of the human body. It also has great negative influence on respiratory system, skin, blood and eyes [91]. Therefore, it is of great significance to develop an adsorbent material that can effectively remove Hg(Ⅱ). Since Hg is a thiophilic element [92], HCPs with sulfur-containing functional groups have attracted much attention for the adsorption of Hg(Ⅱ).

For instance, Abadast et al. [93] prepared thiol-rich 3D-network porous HCPs for types of heavy metal ions adsorption by using the strong affinity of thiol groups with metals through Lewis acid-base interactions. The surveys show that the removal capacity of Hg(Ⅱ) is highest, reaching > 96% at pH 4.0. The high removal ability of thiol-rich 3D-network porous HCPs for Hg(Ⅱ) can be attributed to the complexation between the adsorbent and Hg(Ⅱ). The sustainability study of this adsorbent indicates that it can be repeated for five cycles while maintaining stable activity. Besides, the experimental results demonstrate the high selectivity of the adsorbent by selecting Hg(NO₃)₂ as the model pollutant for removing Hg(Ⅱ), and through this study, it is proved to be a highly efficient adsorbent for purifying polluted water. Varyambath et al. [94] used polycyclic aromatic hydrocarbon pyrene as the reactive monomer to synthesize a rigid highly cross-linked polymer (HCPPy) via Friedel-Crafts alkylation reaction. HCPPy is then modified with cystamine hydrochloride to obtain Cys-HCPPy. When used for the adsorption of Hg(Ⅱ), an adsorption capacity of 1124.82 mg/g is achieved at pH 6.0. And the coordination reaction between Hg and disulfide-containing cystamine ligands leads to the high Hg(Ⅱ) adsorption capacity of Cys-HCPPy. Ramezani et al. [95] synthesized a covalent sulfur-modified hyper crosslinked microporous organic polymer of coal tar (CTHP-SES, Fig. 5) and studied its application in the removal of low concentration Hg in water and gas phase. The adsorption capacity of CTHP-SES for Hg(Ⅱ) and Hg⁰ is 1037 mg/g and 416 mg/g, respectively. The high removal efficiency of CTHP-SES is due to the strong coordination of electron-rich sulfur with Hg(Ⅱ). In addition, reusability and sustainability studies indicate that the removal efficiency after four runs is almost constant without decline, demonstrating its good recyclability. Besides, although coal tar has been widely recognized as the starting material for synthesizing HCPs for gas adsorption [96,97], this study expands its application and develops a novel surface-modified HCPs based on coal tar as a practical adsorbent for removing heavy metals. The practice of continuously developing material applications like this is worth promoting in scientific research.

At present, the application of HCPs in the field of Hg adsorption is still in its infancy. So far, the research on HCPs adsorbents for Hg is limited to materials based on sulfur-based ligands. More HCPs Hg adsorbents with attractive adsorption performance still need to be studied and developed, which will undoubtedly contribute to the removal of Hg, the well-known hazardous element to human beings. The comparison of adsorption capacities of Hg(Ⅱ) with HCPs, CNTs, graphene, LDHs and MOFs is summarized in Table S3 (Supporting information), demonstrating that HCPs can act competitive materials toward Hg(Ⅱ) adsorption.

6.4. Chromium

Chromium (Cr) is a highly toxic heavy metal ion and is listed by the International Agency for Research on Cancer (IARC) as a Group 1 human carcinogen [98]. Cr in water mainly exists in the forms of Cr(Ⅲ) and Cr(Ⅵ). Cr(Ⅲ) has stable properties and low toxicity [99]; while Cr(Ⅵ) has strong oxidizing properties and high solubility, which leads to its high toxicity. Long-term human exposure can damage nerves, circulatory systems and even cause lung cancer [100]. Therefore, it is very important to remove or recover Cr(Ⅵ) from the environment. However, there are currently only a few reports on HCPs as Cr(Ⅵ) adsorption materials [98].

The HCPs used to capture Cr(Ⅵ) are mainly concentrated on the introduction of amino-based HCPs adsorbents [44,101]. Wang et al. [44] prepared a hyper crosslinked hollow porous nanospheres (HPNs-PGM) using PLA-b-PS-b-PGM as the precursor, and then further opened the epoxy group with EDA to obtain ethylenediamine-modified hollow porous nanospheres (HPNs-NH₂). The maximum adsorption capacity for Cr(Ⅵ) is as high as 493 mg/g. The enhanced adsorption efficiency of HPNs-NH₂ is not only associated with the unique hollow porous structure and the large specific surface area, but also attributed to the electrostatic attraction. Moreover, recyclability study indicates that the adsorption capacity of HPNs-NH₂ retains above 65% of the initial adsorption capacity after eight consecutive runs. Sun et al. [101] also prepared a series of amino-based porous HCPs (HCP-TTDs) by the Friedel-Crafts alkylation reaction of 1,1,1-trimethyl-3,3,3-triphenyldisiloxane and FDA and employed to the adsorption of organic dyes and Cr(Ⅵ). The adsorption capacity of Cr(Ⅵ) is 76.9 mg/g, suggesting that HCP-TTDs are promising candidates for the removal of Cr(Ⅵ) from wastewater. Table S4 (Supporting information) summarizes the different adsorption performances of HCPs, CNTs, graphene, LDHs, and MOFs for Cr(Ⅵ), which reflects the great attraction of HCPs in capturing Cr(Ⅵ).

6.5. Lead

Lead (Pb) is a non-biodegradable hazardous heavy metal that easily accumulates in human body. Pb in water mainly exists in the forms of Pb(Ⅱ) [102]. The major source of Pb in human body is the drinking water, containing substantial amount of Pb. The presence of large quantity of Pb in drinking water will cause anemia, cancer, renal kidney disease, nervous system damage and mental retardation [103]. So far, the adsorption of Pb by HCPs materials is mainly based on the coordination reaction between Pb and amino-based HCPs [43,54].

Haratbar et al. [54] prepared a series of amino-based HCPs by Friedel-Crafts alkylation reaction using formaldehyde dimethyl acetal and carbazole as the initial materials, and investigated their ability to remove Pb(Ⅱ). The findings show that HCP-CN can remove > 99.8% of Pb(Ⅱ), and the adsorption required only a few minutes to reach equilibrium. In addition, HCP-CN was reused up to five adsorption-desorption runs with methanol and HCl as the elution agent. The research reveals that HCP-CN possesses the ability to efficiently remove Pb(Ⅱ) from wastewater under different operating conditions. Hefnawy et al. [103] synthesized an amino-containing hyper crosslinked nanometer-sized chelating resin (HCNSCR) by using N-methacryloxyphthalimid (NMP) and methylenebisacrylamide (MBA) as reaction monomers. The adsorption capacities of the synthesized HCNSCR for Pb(Ⅱ), Cd(Ⅱ), and Zn(Ⅱ) is 1.2, 1 and 0.9 mmol/g, respectively. Both electrostatic attraction and chelation contribute to the removal of Pb(Ⅱ), Cd(Ⅱ), and Zn(Ⅱ). The surface of HCNSCR has a greater negative charge at higher pH, consequently, the electrostatic attraction between the adsorbent and metal ions is enhanced. Furthermore, the sustainability study of HCNSCR implies that after five runs, the adsorption capacity decreases from 100% to 93% with a slight decrease in adsorption capacity. These features suggest that HCNSCR has high potential for removing heavy metals from polluted water.

Besides, Anito et al. [43] designed a functionalized HCPs adsorbent (IDA-HCP, Figs. S3d and S4e-h in Supporting information) ligated with carboxylic groups and amino structures on the surface of HCPs using iminodiacetic acid as the monomer. IDA-HCP is found to be effective in adsorption of Pb(Ⅱ) with a maximum adsorption capacity of 1138 mg/g. The high removal efficiency of Pb(Ⅱ) by IDA-HCP can be assigned to the coordination of carboxylate and amino active sites on the surface of the adsorbent. Moreover, it can not only adsorb Pb(Ⅱ), but also has good capture efficiency for Hg(Ⅱ) and Cd(Ⅱ) in mixed solutions (containing Pb, Hg, Cd, Co, Fe, Zn, Mg, and Na) due to their stronger binding with iminodiacetic acid functional groups. However, the adsorption efficiency of Pb(Ⅱ) with IDA-HCP is highest, implying that the adsorbent exhibits the attractive competitiveness toward Pb(Ⅱ). Meanwhile, after four cycles of usage, IDA-HCP still expresses strong fixity in adsorbing Pb(Ⅱ), and its uptake capacity does not display an obvious decrease. The obtained IDA-HCP is competitive compared with adsorbents reported by some functional polymers for the removal of toxic metal ions such as amine functionalized porous polymer (POP-NH₂) [104] and sulfonic acid-modified microporous hypercrosslinked polymers (SAM-HCPs) [41]. Table S5 (Supporting information) presents a summary of the adsorption of Pb(Ⅱ) by HCPs and other materials, implying that HCPs have sufficient prospects for the removal of Pb(Ⅱ).

6.6. Cadmium

Cadmium (Cd) ion is one of the extremely poisoning heavy metals which remains in the tissues of the human body for a long time and causes severe diseases such as osteocalcin, stomach insufficiency, heart failure and blood pressure even at low concentration [49]. It is also reported as the main factor in hormonal deficiency by USEPA [105]. Although HCPs have been widely used as adsorption materials for U, Cu, Hg, etc., their application in Cd is relatively lagging behind. Currently, only a few studies have been explored focusing on the feasibility of HCPs as Cd adsorbents [106].

Jia et al. [65] used waste polystyrene foam as raw material, low-cost 1,2-dichloroethane as organic solvent and crosslinking agent, and concentrated sulfuric acid as sulfonating agent to carry out Friedel-Crafts reaction and sulfonation reaction to obtain hydrophilic SHCP adsorbent, which was used to remove Cd(Ⅱ) in wastewater by fixed bed column experiment. The study shows that the maximum adsorption capacity of SHCP for Cd(Ⅱ) reaches 0.7 mmol/g at 25 ℃ and pH 7.0. And the adsorption process involves the ion exchange between the immobilized sulfonic acid groups and Cd(Ⅱ). When 0.3 mol/L HNO₃ was used as the desorption agent for recovering SHCP, outstandingly, SHCP could be utilized up to four times in the treatment of wastewater containing Cd(Ⅱ), during which the desorption efficiency was greater than 86%. Besides, Masoumi et al. [25] also synthesized a range of polystyrene-based HCPs using Friedel-Craft reaction under different experimental parameters including initial concentration, synthesis temperature, FDA, polystyrene ratio and synthesis time (Fig. 6). The maximum adsorption capacity and removal efficiency of Cd(Ⅱ) is 950 mg/g and 92% at the temperature of 20 ℃, respectively. Notably, the adsorption process is contributed by both electrostatic interaction and chelation. Additionally, reusability tests were conducted by selecting 1 mol/L NaOH solution as the desorption agent, illustrating that adsorption capacities of 814 mg/g and 812.6 mg/g toward Cd(Ⅱ) in the first and the last cycle were achieved. The comparison of Cd(Ⅱ) adsorption capacity between HCPs and CNTs, graphene, LDHs, and MOFs is summarized in Table S6 (Supporting information), which demonstrates that HCPs are fascinating candidates toward Cd(Ⅱ) adsorption.

6.7. Iron

Iron (Fe) is one of the most abundant metals of the earth's crust, which exists naturally in water in soluble form as Fe(Ⅱ) or complexed form like the Fe(Ⅲ) [107]. Fe is also an essential nutrient for plant, bacterial and animal growths. However, excessive Fe can lead to health problems, such as hemochromatosis, coronary heart disease, hypertension and myocardial infarction [108]. Therefore, the effective adsorption of Fe is one of the important research issues. However, so far, there have been only a few studies on the design of HCPs adsorbents toward Fe.

Ratvijitvech et al. [108] synthesized catechol-based HCP (Catechol-HCP) by Friedel-Crafts alkylation reaction with catechol as monomer and FDA as external crosslinking agent. The maximum adsorption ability of Fe(Ⅱ) by the Catechol-HCP adsorbent is 40 mg/g (94%). The chelation between catechol structure and Fe(Ⅱ) heightens the adsorption capacity of the material for Fe. Additionally, HCl solution was used as the desorption agent, exhibiting that after four cycles, the adsorption efficiency of Catechol-HCP did not markedly diminish, reflecting the satisfactory reusability of the material. Liao et al. [109] synthesized hyper crosslinked microporous functional polystyrene (HCPMOS) with methoxy group by coordination polymerization using poly(p-methoxystyrene) containing oxygen atoms as reactive monomer and FDA as external crosslinker through Friedel-Crafts alkylation reaction. The results demonstrate that HCPMOS has a higher ability to remove Fe(Ⅲ) from aqueous solution than HCP without methoxy group and reaches a maximum adsorption capacity of 97.1 mg/g for Fe(Ⅲ) at 298 K. The adsorption mechanism analysis shows that the increase in the surface area and the high density of methoxy functional groups endow the adsorbent with high selectivity and adsorption capacity for Fe(Ⅲ). Therefore, the enhanced adsorption capacity of HCPMOS is a joint result of the complexation of between the methoxy and Fe(Ⅲ) and physical adsorption. Moreover, the adsorption-desorption experiments were repeated for five cycles. Briefly, HCPMOS was immersed and washed with 1 mol/L HCl and 1 mol/L NaOH for 1 h each in the desorption experiment. The decrease in equilibrium capacity does not exceed 1% and the desorption efficiency of HCPMOS overtakes 97.8%. The high adsorption ability of HCPMOS toward Fe(Ⅲ) provides a powerful method for adsorbing Fe(Ⅲ) in polluted water.

To sum up, HCPs have high specific surface area, attractive mechanical stability and chemical stability, and the introduction of functional groups during the synthesis process provides HCPs with a large number of active sites beneficial to the adsorption of heavy metal ions. HCPs can also remove heavy metal ions with high adsorption capacity, and have marvellous recyclability after adsorbing metals, making them an ideal choice for water treatment. But so far, the HCPs-related heavy metal adsorbents have been mainly focused on the application to U, Cu, Cr, Pb, Cd, and the adsorption of other heavy metal ions with this type of promising adsorbent remains undeveloped.

7. Factors affecting adsorption capacity

Obviously, the functional groups of HCPs and types of heavy metal ions can determine the adsorption capacity. On the other hand, various experimental parameters may significantly affect the adsorption behaviors of metal ions with HCPs, mainly including initial solution pH, adsorbent dosage, ionic strength, contact time, and temperature.

7.1. Initial solution pH

Generally, the solution pH is recognized as a very influential parameter that dominates the adsorption process, which not only governs the species of heavy metal ions present in the solution, but also affects the adsorption performance of metal ions by HCPs. Moreover, pH of the solution also affects the chemical properties of the solution, the degree of ionization protonation, and the surface charge of the adsorbent used [110]. For instance, when the solution is under strong acidic conditions, the protonation of functional groups on the surface of HCPs leads to a weakening of the electrostatic effect between HCPs and heavy metal ions, and reduces the adsorption of HCPs [44]. With the increase of pH in a certain range, the surface electronegativity of HCPs improves, the negative charge on the surface of HCPs increases, and the adsorption performance gradually enhances, thereby promoting the adsorption process. However, as the solution gradually becomes alkaline, the functional groups gradually lose their protonation ability and the problem of precipitation may occur, leading to a decrease in the adsorption capacity as well [111].

As well-known, the initial pH value of uranium solution plays a crucial role in the adsorption of uranium by HCPs because it affects the surface charge of the HCPs adsorbents and the existing form of U(Ⅵ) in the solution [83]. For example, Meng et al. [61] prepared the magnetic uranium adsorbent by loading Fe₃O₄ into copoly(styrene/maleic anhydride) (C-SMA) through a solvothermal method and used Fe₃O₄@C-SMA to adsorb U(Ⅵ) under different conditions. Due to the uranyl ions mainly exist in the form of positive ions and the surface of Fe₃O₄@C-SMA has a positive charge when the pH is < 6.0, the two generate electrostatic repulsion force at low pH value, and hence the adsorption efficiency of the adsorbent gradually increases at pH 3.0–6.0. The lower the pH value, the lower the adsorption performance. As the pH value increases in the range of 3.0–6.0, the overall positive charge on the surface of Fe₃O₄@C-SMA decreases. The change from positive charge to negative charge increases the electrostatic interaction between the adsorbent and uranyl ions, thereby improving the adsorption property. When the pH value exceeds 6.0, with the increase of the pH value, carbonate ions compete with negatively charged Fe₃O₄@C-SMA to occupy the limited adsorption site. This leads to a decrease in the adsorption capacity of Fe₃O₄@C-SMA. In summary, the optimal pH for the adsorption of U(Ⅵ) by the adsorbent was selected as 6.0.

In general, the adsorption capacity of HCPs for Cu(Ⅱ) is often controlled by the pH value of the solution. The change in pH value can affect both the activity of adsorption sites and the interaction of ions in the solution. The removal of Cu(Ⅱ) and other metal ions by SMA was found to be dependent on the solution pH [8]. The study reported that the adsorption capacity of SMA gradually increased within the pH range of 4.0–7.0. The maximum adsorption capacity occurs at a pH value of 6.7. When the pH value is below 6.0, the adsorption efficiency of SMA for Cu(Ⅱ) and other metal ions is relatively lower, because H₃O⁺ ions in the aqueous solution near the active site of the carboxyl group compete fiercely with free metal ions. The lower adsorption is also due to the protonation of the carboxyl groups blocking the adsorption sites and resulting in steric hindrance.

pH value of the aqueous solution is a key factor affecting the ability of HCPs to adsorb Hg(Ⅱ), as it changes the state of Hg(Ⅱ) in the aqueous solution, resulting in significant changes in the process of Hg(Ⅱ) adsorption. Modak et al. [91] kept other parameters unchanged while pH values varied within the range of 2.0–11.0 in the experiment of using Th-2 to adsorb Hg(Ⅱ). The final experimental results imply that the adsorption capacity decreases at both higher and lower pH values. On the one hand, at lower pH, due to the high concentration of hydrogen ions in the solution, Hg(Ⅱ) competes with hydrogen ions for adsorption, resulting in a decrease in the adsorption capacity of Th-2 for Hg(Ⅱ). On the other hand, at the pH higher than 7.0, Hg(Ⅱ) precipitates into its hydroxides, leading to low adsorption. In the light of these results, a pH of 7.0 was chosen as the optimal value and kept constant for further studies. It is important to note that due to the presence of the thiophene ring, Th-2 exhibits higher stability than other competitive porous adsorbents.

Determining the optimal pH value plays a noticeable role in the adsorption of Cr(Ⅵ), since the pH value not only affects the surface charge of HCPs, but also influences the degree of protonation of functional groups. A study reported that the adsorption capacity of HPNs-NH₂ for Cr(Ⅵ) was improved when the pH of the solution increased from 1.0 to 4.0 [44]. There is a peak value of the maximum adsorption capacity at pH 4. The adsorption capacity of HPNs-NH₂ gradually reduces within the pH range of 4.0–10.0. This is because N in HPNs-NH₂ contain lone pair electrons, which can bind to protons. The surface of the adsorbent is positively charged when the pH value is low, and thus in an extremely acidic environment with the pH value below 2.0, amino protonation leads to weak electrostatic effects between the adsorbent and the corresponding metal ions, resulting in a smaller adsorption capacity value. Besides, when the solution pH is between 2.0 and 6.0, the removal ability of HPNs-NH₂ on Cr(Ⅵ) gradually improves with the increase of overall electrostatic attraction at low pH. At the same time, as the acidity continues to weaken, the degree of amino protonation continues to decrease, and the zeta potential on the surface of the adsorbent gradually reduces, leading to a decrease in electrostatic attraction compared to before. When the pH value is greater than 6.0, the positive charge on the surface of the adsorbent becomes negative charge, and it repels the negatively charged Cr(Ⅵ), resulting in low adsorption. As the solution gradually becomes alkaline, the amino group gradually loses its protonation ability, and the zeta potential of HPNs-NH₂ further decreases, leading to a greater rejection of negatively charged Cr(Ⅵ) and a decrease in adsorption value. Finally, a pH value of 4.0 was chosen as the optimal pH value in the experiments.

Due to the influence of solution pH on the charge profile of adsorbates and the functional groups of HCPs, the pH has a prominent impact on the adsorption of Cd(Ⅱ) by HCPs. As a proof-of-concept, the adsorption pattern of Cd(Ⅱ) onto Cd-SII-MH and NI-HM was studied in the pH range of 3.0–7.0 [111]. As the pH of the solution increases from 3.0 to 5.0, the adsorption capacity of Cd-SII-MH improves from 88.6 nmol/g to 156.4 nmol/g, and the adsorption capacity of NI-HM improves from 37.2 nmol/g to 52.5 nmol/g. The significant change is mainly related to the protonation/deprotonation balance of the functional groups in the polymeric chain. Also, as the pH value continues to increase to 7.0, there is no significant improvement in adsorption capacity. The maximum adsorption capacity of the adsorbents is achieved at pH 5.0. Meanwhile, the authors considered that precipitation problems may occur at higher pH values, which can affect the removal of heavy metal ions, and thus the experiment was not conducted in solutions with higher concentrations of OH^- ions.

7.2. Adsorbent dosage

In the adsorption process of heavy metal ions, the adsorbent dosage affects the surface area and adsorption sites of the adsorbent, which further affects the adsorption percentage. Generally, as the adsorbent dose increases, the accessibility of surface area or adsorption sites improves, leading to an increase in the adsorption capacity of heavy metal ions [110]. However, further increase of adsorbent amount and the aggregation of adsorption sites tends to cause the increase in the path length for metal diffusion and decreases the adsorption capacity [85]. That is, using excess adsorbent will result in surplus adsorption sites and reduces the utilization ratio of adsorbent.

In uranium adsorption studies, a variety of researchers have devoted to investigate the effects of HCPs adsorbent amount on the adsorption capacity since it is a key parameter from a practical application standpoint. For instance, Bai et al. [75] synthesized the HCR-AO resin through the post crosslinking method and applied it to remove U(Ⅵ) from simulated seawater. Results suggest that the removal rate improves with the increase of HCR-AO dosage from 0.01 g/L to 0.8 g/L. This can be justified due to the availability of more surface area and adsorption sites at larger HCR-AO dosage. When the adsorbent dosage is between 0.8 g/L and 1.2 g/L, the removal rate is almost constant. Hence, 0.8 g/L was chosen as the optimal adsorbent dosage. Similar results can also be found in Zhu's work, where the removal rate of U(Ⅵ) rapidly increases when the phosphorylated hyper-crosslinked calix[4]arene (C4HP) dosage ranges from 0.08 g/L to 0.12 g/L [86]. Further increasing the dosage of C4HP does not enhance the adsorption capacity of U(Ⅵ). Thus, 0.12 g/L was ultimately selected as the optimal value.

To understand the influence of adsorbent dosage on the copper capture, Ahmad et al. [112] conducted experiments by changing the dosage of magnetic tubular carbon nanofibers (MTCFs). When the adsorbent dosage is 0.5 g/L, the removal rate of Cu(Ⅱ) is highest, reaching 99.9% ± 0.1%. Initially, as the amount of MTCFs expands, the number of active adsorption sites in the solution also increases accordingly, which is more conducive to the adsorption of Cu(Ⅱ) on more active sites. However, with the further increase of MTCFs, the adsorption capacity of MTCFs inevitably presents a downward trend. The reason for this adsorption performance may be as followings. As the adsorbent amount increases, the possibility of MTCFs combination with Cu(Ⅱ) increases. The decreased Cu(Ⅱ) adsorption when further increasing the dosage can be assigned to the agglomeration behavior, decreasing the functional groups and the number of active sites.

In the industrial-scale application of HCPs, high doses of HCPs can lead to surplus adsorption sites, minimizing the utilization rate of the adsorbent, which is not conducive to reducing the cost of the adsorbent with the highest removal efficiency. As a representative, Masoumi et al. [106] investigated the effect of benzene-based HCPs dosage on the adsorption performance of Cd(Ⅱ) to enhance the interaction between active sites of benzene-based HCPs and Cd(Ⅱ). When the mass of the adsorbent exceeds 0.25 g, the removal efficiency of Cd(Ⅱ) does not significantly improve, indicating that the amount of the material larger than 0.25 g is uneconomical for Cd(Ⅱ) adsorption. Meanwhile, when the dosage of the adsorbent is in the range of 0.08–0.32 g, the uptake capacity of benzene-based HCPs toward Cd(Ⅱ) decreases with the increase of adsorbent dosage, and the uptake capacity shows a decreasing trend, which may be because the active adsorption sites are blocked. Therefore, the optimal dosage for this experiment was selected as 0.08 g.

7.3. Contact time

The contact time is an important parameter in adsorption applications, which also has a significant impact on the adsorption of heavy metal ions by adsorbents [63]. The adsorption speed determines the time required for adsorption equilibrium, and the faster the speed, the shorter the required time. Numerous researchers studied the adsorption kinetics of heavy metals on HCPs at different contact time to explore the adsorption mode of HCPs. Generally, with increase in contact time, the removal rate of heavy metals enhances until reaches a plateau [90]. When the optimal adsorption time is exceeded, the adsorption capacity of the adsorbent used will not undergo further changes.

Tian et al. [113] prepared a novel porous uranium adsorbent (PA-HCP) containing numerous amine and phosphoryl groups to efficiently remove U(Ⅵ) from wastewater. The contact time for adsorption of U(Ⅵ) by PA-HCP was investigated in a range of 10–480 min at 298.15 K. Within the first 60 min, a faster adsorption rate of U(Ⅵ) was observed due to the abundant active sites on the surface of PA-HCP. Then, the availability and abundance of the active sites on the adsorbent surface gradually decrease, the adsorption rate gradually decreases and reaching equilibrium within 360 min.

The adsorption time of HCPs for Hg(Ⅱ) is an important influencing factor for the removal of toxic Hg(Ⅱ) from industrial wastewater. As a proof-of-concept, Ramezani et al. [95] detailed the effect of contact time on the adsorption of Hg(Ⅱ) by sulfur-decorated hyper-cross-linked coal tar (CTHP-SES). Notably, the rapid removal rate of CTHP-SES at the beginning is due to the high sulfur content and high surface area of the adsorbent, which have a high proximity to Hg(Ⅱ). With an increase of contact time, the adsorption of Hg(Ⅱ) increases steadily until reaching adsorption equilibrium at approximately 480 min.

Haratbar et al. [114] increased the contact time from 10 min to 50 min while keeping all other parameters fixed. The performance of the polystyrene-based and expanded polystyrene-based hypercrosslinked polymers (P-HCP, EPS-HCP) throughout the adsorption time is as follows: P-HCP < EPS-HCP for Pb(Ⅱ). In the early stage of adsorption, there are more vacant active sites on the surfaces of P-HCP and EPS-HCP that can be used to adsorb Pb(Ⅱ), and the adsorption rate increases rapidly. As the adsorption process progresses, the accessibility of active sites decreases, leading to a decrease in the later adsorption rate. The active adsorption sites of the adsorbents reach saturation within 40 min. Hence, the optimal adsorption time for the adsorbents was chosen as 40 min.

After reaching the optimal contact time for the adsorption of Cd(Ⅱ) by HCPs, further increasing the time will result in an inapparent and uneconomical impact of HCPs on the adsorption of Cd(Ⅱ). The case in point is benzene-based HCPs [106]. The adsorption capacity of benzene-based HCPs for Cd(Ⅱ) gradually heightens at the first 60 min, and then the effect of increasing contact time on the absorption capacity is insignificant. At the beginning of the adsorption process, this higher adsorption is endorsed by the availability of most adsorption active sites. As the contact time increases in the later stage, the pores of benzene-based HCPs gradually reduce, the potential gradient between benzene-based HCPs and Cd(Ⅱ) solution decreases, and the adsorption rate of Cd(Ⅱ) also diminishes. Therefore, the optimal contact time for the adsorbent was selected as 60 min.

7.4. Temperature

Another affecting the removal efficiency of heavy metal ions by adsorbents is temperature [84]. When conducting adsorption thermodynamics research, there are three important parameters, the values of enthalpy (ΔH⁰), entropy (ΔS⁰), and Gibbs energy (ΔG⁰) can be used to predict properties of the adsorption [115]. Through temperature experiments, their values can be determined.

For example, Metwally et al. [116] obtained poly(NMTPA-co-DVB) by suspension polymerization of N-methacryloxytetra chlorophthalimide (NMTPA) and dvinyl benzene (DVB), and then treated them with triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) respectively to synthesize two types of hypercrosslinked polymers nanoparticles (HCPNs), HCPNs (1) and HCPNs (2). When exploring the effect of temperature on the adsorption of Cu(Ⅱ) in wastewater by HCPNs (1) and HCPNs (2), three temperatures of 25, 40, and 60 ℃ were set in the experiment. Temperature and thermodynamic studies imply that the sorption process is endothermic process due to the positive values of ΔH⁰_ads. ΔS⁰_ads is a positive value in this study, indicating that the adsorption process is random. Meanwhile, ΔG⁰_ads of the adsorption process is negative value, hence the sorption process is also spontaneous. And the higher the temperature, the more effective the adsorption process.

In addition, Aljboar et al. [115] evaluated the performance of a hypercrosslinked poly(aniline-co-benzene) copolymer (PAB HCPs) in adsorbing Hg(Ⅱ) in polluted water in terms of adsorption thermodynamics. The experiments were conducted at 25, 35, 45, and 55 ℃, respectively. And the sign of ΔG⁰ is negative at these temperatures, which reflects the feasibility and spontaneity of the adsorption process. Moreover, ΔH⁰_ads and ΔS⁰_ads are all positive values, indicating that the adsorption process is endothermic and has randomness. The research results reveal that high temperature is more conducive to improving the adsorption ability of PAB HCPs.

Zhu et al. [66] found that the process of removing Pb(Ⅱ) using hypercrosslinked functionalized lignosulfonates (CLS) at 30, 40, and 50 ℃ resulted in the positive values of enthalpy and entropy of adsorption. These indicate a slight endothermic and random nature of the adsorption process. The adsorption of Pb(Ⅱ) is considered spontaneous, and higher temperatures are favorable for adsorption.

8. Summary and outlook

Up to now, HCPs adsorbents with various functional groups have attracted wide attention in the field of heavy metal adsorption due to their advantages of large specific surface area and abundant adsorption sites. This review focuses on the summary of HCPs containing different functional groups (carboxyl, sulfonic, phosphate, amino, amidoxime groups) and their applications as adsorbents toward heavy metal ions. Although considerable progresses have been made in the study of HCPs as heavy metal adsorbent materials, there are still some challenges that need to be addressed.

(1) In terms of the construction of HCPs, post-crosslinking, one-step self-condensation and external braiding have been proposed to prepare HCPs, in which the external crosslinking method offers a wider selection of monomers. To achieve the goal of highly effective adsorption of metal ions, the porous structure and active sites of HCPs are two important parameters. So far, the synthesis of HCPs with precisely controllable porous structure is still a challenge remained. Future works may focus on the selection of precursors, crosslinkers, solvents and catalysts, as well as the control of reaction conditions (time, temperature, pH, etc.). On the other hand, it is feasible to improve the adsorption performance of heavy metal ions by increasing the active sites of HCPs. However, it should be noted that a compromise needs to be achieved considering the functional groups and porous structure.

(2) In terms of the adsorption mechanism of HCPs toward metal ions, coordination, electrostatic attraction and ion exchange are the most dominant mechanisms. In fact, other mechanisms such as physisorption have also been reported, and the combination of two or more adsorption mechanisms also exists. Accordingly, more detailed explanations are expected for the adsorption mechanism of HCPs toward heavy metal ions, which is conducive to guiding us to reasonably design and develop efficient HCPs materials with adjustable reactivity, porous structure, morphology, and physical and chemical properties. For this goal, computational calculation and molecular modeling can act as the theory guideline for the design of HCPs adsorbents. Expanding the basic knowledge through more theoretical calculations and experimental results will actively bring more detailed observation results in this field.

(3) In terms of the applications of HCPs toward heavy metal adsorption, there is still a large space remained. According to currently reported studies, the HCPs-related heavy metal adsorbents have been mainly focused on the application to U, Cu, Cr, Pb, Cd, while other heavy metal ions are rarely tested or evaluated for their adsorption capacities. Furthermore, in adsorption-desorption experiments, solvent desorption is the most commonly used method for the regeneration of HCPs. Although the regeneration tests have been carried out in some reports, the limited cycles do not meet the requirements of practical applications. In addition, reported studies of HCPs for metal ions adsorption are mostly about the impact of HCPs on wastewater treatment, the adsorption of metal ions with HCPs from other matrices, such as soil, vegetables, crops, needs to be explored.

Despite these challenges, the significant progress in the development of HCPs and the growing interest of researchers in this area have paved the way for future studies. Further research along these lines is needed for the successful applications of HCPs for the remediation of heavy metal ions or other pollutants. Developments and efforts should be concentrated on the following aspects: (1) HCPs with more attractive structures and functional groups are worth further investigations, especially based on theoretical calculation and molecular simulation, which will benefit to expand the application of such a promising material. (2) The fabrication of HCPs by more economical and scalable strategies should be investigated. At present, the preparation of HCPs is usually limited in laboratories, and the feasibility of their mass production in industrial level remains unevaluated. (3) The adsorption of more heavy metal ions is expected, and more attention should be focused on the application potential of HCPs for practical wastewater containing multiple heavy metal ions. We believe that this will be great promotion for further explorations of HCPs adsorbents.

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

Hui Liu: Methodology. Baoying Xiao: Validation, Conceptualization. Yaming Zhao: Conceptualization. Wei Wang: Supervision. Qiong Jia: Supervision.

Acknowledgments

This work was supported by Innovation Platform (Base) and Talent Special Project, Jilin Provincial Science & Technology Department, China (No. 20230508033RC).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110619.