Molecular imprinting technology is a technology that produces template Molecular cavities with specific molecular selectivity and high affinity in polymers, which is similar to the principle of enzyme and substrate recognition system in biology (the latter is often called "lock and key" model)^[65]. Different molecules have different shapes, sizes and reaction characteristics, so they can be regarded as different templates, which are combined with functional monomers in a covalent, semi-covalent or non-covalent manner to form combinatorial units, and then polymerized or condensed with crosslinking agents to synthesize polymers with template-selective recognition sites.Removal of part or all of the template after polymer formation can be used for corresponding molecular recognition and adsorption (Figure 3), that is, the imprinted polymer has a unique template "memory" ability to quickly and accurately select the template (target molecule) from the mixture^[66]. Broadly speaking, atoms, ions, molecules, complexes and even microorganisms can be used as templates for the preparation of imprinted materials^[67]. Molecularly imprinted materials have three unique characteristics of structure predictability, recognition specificity and application universality, and are widely used in purification and separation, chemical/biological sensing, artificial antibodies, drugs and other industries due to their high stability, simple preparation methods, low cost and other advantages^[65]. The traditional preparation methods of imprinted polymers are divided into chain polymerization and sol-gel method, and the chain polymerization can be further divided into bulk polymerization, suspension polymerization, emulsion polymerization and precipitation polymerization^[67]. These traditional preparation methods have some problems, such as template leakage, low binding capacity, irregular shape of materials, incompatibility in aqueous medium, etc. In recent years, new molecular imprinting technologies, such as surface imprinting technology, multi-template imprinting technology and magnetic response technology, have been developed to broaden the application scope of molecularly imprinted polymers^{[65][68][69][70]}.

The earliest molecular imprinting phenomenon can be traced back to 1931. Polyakov et al. Added benzene and toluene additives in the preparation process of silica particles. After washing to remove the additives, they found that the adsorption capacity of the prepared silica particles for these additives was higher than that of other structural ligands (that is, the adsorbent was selective for benzene and toluene).Later, it was found that with the addition and removal of benzene and toluene, the structure of the prepared silica particles changed significantly (that is, the molecular cavity left by the washing of benzene and toluene in the template effect)^[71][67]. In 1972, Wulff and Klotz used molecular imprinting technology to prepare an organic polymer for selective adsorption, which made the study of molecular imprinting technology no longer limited to silica materials^[72]. Subsequently, researchers have developed non-covalent, semi-covalent and other preparation methods of molecularly imprinted polymers, which has promoted the rapid development of molecular imprinting technology. In 1976, Nishide et al. Prepared an ion-imprinted polymer adsorbent using ions as templates^[66]. Since then, molecular imprinting technology has become one of the mainstream technologies to realize the research and development of selective ion adsorption materials, and selective adsorption materials for metal ions such as Cu²⁺, Hg²⁺ and Pb²⁺ have been prepared in many fields such as environment, chemistry, materials and biology, thus realizing the selective removal or recovery of specific ions^{[73⇓~75][76⇓~78][79⇓~81][65]}. Aiming at the defects that the stability of the complex formed by common functional monomers (such as acrylamide, methacrylic acid and the like) and target ions is poor, so that the selectivity of the imprinted polymer of a target metal ion template is poor,Researchers used both metal ions and specific ligands that can form stable complexes as templates to synthesize double-imprinted polymers (double-imprinting technology), which improved the selectivity of molecularly imprinted materials for metal ions^[82][83].

According to the investigation, the prepared molecularly imprinted materials have been used for the selective adsorption of Cu²⁺, Hg²⁺, Pb²⁺, Cr (Ⅵ), and CN^-. The following are some cases^{[73][76][79][84][85]}.

Huang et al. Synthesized a graphene oxide-based ion imprinted polymer using Pb²⁺ as a template and acrylamide as a functional monomer.Under the coexistence of Ca²⁺, Cd²⁺, Co²⁺, Ni²⁺ and Zn²⁺,Realizes the selective adsorption of Pb²⁺,The adsorption mechanism was then analyzed by X-ray photo electron spectroscopy (XPS) (Fig. 4), and the spectra of the adsorbent before and after adsorption showed that the eluent could completely elute the lead ions adsorbed in the imprinted cavity without destroying the chemical properties of the recognition site^[86]. The Pb 4F spectrum, N 1s spectrum and O 1s spectrum after adsorption showed that the binding energy of Pb 4f_5/2 and Pb 4f_7/2, the binding energy of neutral amine (NH) and protonated amine (NH⁺) of N 1s changed, indicating that the N in two chemical States participated in the coordination reaction with Pb²⁺. The binding energy of carboxyl oxygen and carbonyl oxygen did not change, indicating that there was no interaction between oxygen atom and lead ion. The above results indicate that ion-imprinted adsorbents are more compatible with Pb²⁺ in terms of coordination geometry, size, coordination number and ionic charge, and show higher affinity for template ions. Li et al. Prepared a new type of Pb²⁺ imprinted polymer adsorbent with Pb²⁺ as template and ethylenediamine as monomer.It is found that in K⁺, Na⁺, Mg²⁺,Ca²⁺ 、Cd²⁺ 、Cr³⁺ 、Co²⁺ 、Fe³⁺ 、Mn²⁺ 、Under the condition of Ni²⁺, U Zn²⁺ and UN Hg²⁺,The adsorption capacity of the adsorbent for Pb²⁺ is much higher than that of other ions^[79]. Liu et al. Prepared a novel Pb²⁺ imprinted polymer (Pb²⁺-IIP) using Pb²⁺ as template and chitosan as monomer.In Mn²⁺, Cr²⁺, Fe²⁺, Cu²⁺,Under the coexistence of Ni²⁺, Zn²⁺, Co²⁺ and Hg²⁺,The adsorption capacity of the adsorbent for Pb²⁺ was much higher than that for other ions^[80]. Zhu et al. Prepared a new type of Pb²⁺ imprinted microbeads using Pb²⁺ as a template and 1,12-dodecanediol diphenyl-phosphonic acid and 4-vinylpyridine as monomers, and the adsorption capacity of the adsorbent for Pb²⁺ was much higher than that of other ions under the coexistence of competing ions Co²⁺ and Cd^2+[81].

Birlik et al. Prepared a Cu²⁺ imprinted polymer bead using Cu²⁺ and succinic acid modified chitosan as templates.In the mixed solution of Cu²⁺, Co²⁺, Ni²⁺ and Zn²⁺, it was found that the adsorption capacity of the double imprinted polymer for Cu²⁺ was much higher than that for other ions^[73]. Mishra et al. Prepared a Cu²⁺ imprinted polymer bead using Cu²⁺ and picolinic acid as templates and methyl methacrylate as monomer.Under the coexistence of Ni²⁺, Co²⁺ and Zn²⁺, the adsorption capacity of the adsorbent for Cu²⁺ is higher than that of other ions^[74]. Ren et al. Prepared a Cu²⁺ imprinted polymer by sol-gel method using Cu²⁺ as template and N- [3- (2-aminoethylamino) propyl] trimethoxysilane (AAPTMS) as monomer.The adsorbent showed high selectivity for Cu²⁺ under the coexistence of Pb²⁺, Ni²⁺, Cd²⁺ and Co^2+[75]. Monier et al. Prepared a Cu²⁺ imprinted adsorbent with Cu²⁺ as template and isatin modified chitosan as monomer.The adsorbent exhibit a high Cu²⁺ selectivity much higher than that of a non-imprinted adsorbent unde that coexistence of Pb²⁺, Ni²⁺, Cd²⁺ and Co^2+[87].

Hajri et al. Prepared a Hg²⁺ imprinted adsorbent using Hg²⁺ as template and Schiff base modified chitosan as monomer.Under the coexistence of Zn²⁺, Cu²⁺, Pb²⁺, Co²⁺ and Cd²⁺,The adsorption capacity of the adsorbent for Hg²⁺ is much higher than that of other ions^[76]. Tarisai et al. Prepared a sulfur-containing Hg²⁺ imprinted polymer (S-IIP) adsorbent using Hg²⁺ as template and thiol ligand as monomer.Under the coexistence of Zn²⁺, Cu²⁺, Pb²⁺, Co²⁺ and Cd²⁺,The adsorption capacity of the adsorbent for Hg²⁺ was higher than that of other ions^[77]. Wu et al. Prepared a Hg²⁺ organic-inorganic hybrid imprinted adsorbent using Hg²⁺ and cetyltrimethylammonium bromide as templates and N- [3- (trimethoxysilyl) propyl] ethylenediamine as a monomer, which showed much higher Hg²⁺ selectivity than non-imprinted adsorbent under the coexistence of Cu²⁺ or Cd^2+[78].

Gao et al. Prepared a chromate ion surface imprinting material IIP-PVI/SiO₂ to selectively remove chromate from water^[84]. In the presence of phosphate ions, the adsorption capacity of the adsorbent for chromate ions was much greater than that of phosphate ions compared with non-imprinted materials. Kong et al. Prepared a novel Cr (Ⅵ) -imprinted polymer (Cr (Ⅵ) -IIP) using Cr (VI) as a template, 4-vinylpyridine and N, N '-diethylaminoethyl methacrylate as monomers, and the adsorption capacity of the adsorbent for Cr (VI) was much higher than that of other ions under the coexistence of Cu²⁺, Cd²⁺ and Cr (Ⅲ)^[88].

Say et al. Prepared a CN^- imprinted metal chelate bead using CN^- as template and Ni-methacryloylhistidine as monomer.In binary mixed solution (CN^-/SCN^-, CN^-/S^2-, CN^-/Cl^-,CN^-/ {{\mathrm{NO}}_3}^{-}, CN^-/ {{\mathrm{SO}}_4}^{2-}),The adsorption capacity of the adsorbent for CN^- is much higher than that of other ions^[85].

In general, molecular imprinting technology has been widely used in the field of ion selective removal, especially for the removal of specific cations. The excellent selectivity makes it one of the widely used selective treatment methods. At present, the research on molecular imprinting technology mainly focuses on the preparation and application of molecularly imprinted polymers, and the research on the binding mechanism is relatively small, so the future research can be carried out in the development of new functional monomers and the reusability of molecularly imprinted materials.

For a long time, the scope of understanding of acid-base types and acid-base reactions in academia has been expanding, which has gone through different stages, such as acid-base ionization theory, acid-base solvent theory, acid-base proton theory, acid and base electron theory and soft and hard acid-base theory, and has greatly expanded the scope of application of acid-base theory^[89]. In 1887, Arrhenius, a Swedish chemist, put forward the acid-base ionization theory from the point of view of chemistry: a substance in which all the cations ionized in aqueous solution are H⁺ is called an acid, and a substance in which all the anions ionized in aqueous solution are OH^- is called an alkali. The essence of acid-base reaction is the reaction in which H⁺ and OH^- are combined to form water^[89]. This theory describes the properties of acids and bases and their behavior in chemical reactions from a quantitative point of view, but the reactions are limited to the H⁺ and OH^- of specific substances in water (solvent). In 1905, Franklin, an American chemist, put forward the theory of acid-base solvent, which extended the type of solvent from water to non-aqueous solvents and superacid systems: the substance that can produce the characteristic cation of the solvent is acid, and the substance that produces the characteristic anion of the solvent is alkali. The essence of acid-base reaction is that the characteristic cation and anion of the solvent combine to form solvent molecules^[89]. For example, liquid ammonia is used as a solvent, and an ammonium salt such as NH₄Cl is used as an acid because it provides a characteristic cationic {{\mathrm{NH}}_4}^{+}; An amino compound, such as NaNH₂, is a base because it provides a characteristic anionic {{\mathrm{NH}}_2}^{-}. In 1923, Danish chemist Brnsted and British chemist Lowry put forward the acid-base proton theory, which extended the concept of acid-base to all proton systems: any substance that can give a proton (H⁺) is an acid; Any substance that can accept a proton is a base. For example, when HCl is dissolved in water, the H₂O accepts a proton to form a H₃O⁺, so the HCl that gives a proton is an acid and the H₂O that accepts a proton is a base^[89]. In 1923, Lewis, an American chemist, proposed the acid-base electron theory (also known as Lewis acid-base theory), which made the concept of acid-base get rid of the limitations of H⁺ and solvents: acid is the acceptor of electrons (commonly known as Lewis acid).A base is an electron donor (commonly known as a Lewis base), and an acid-base reaction is a process in which an acid accepts a pair of electrons from a base to form a coordination bond to give an acid-base adduct^[89]. Taking HCl as an example, in the acid-base proton theory, HCl is a proton acid, and in the acid-base electron theory, HCl is an acid-base adduct formed by combining the Lewis acid H⁺ with the Lewis base Cl^-. In 1963, American chemist Pearson further developed the Hard-soft-acid-base (HSAB) theory from the acid-base electron theory, and put forward the concept of "absolute hardness" η, which further divided acid (or base) into soft and Hard acids (or bases)^[90]. The "absolute hardness" values of some acids and bases are shown in Tables 1 and 2. Generally speaking, the central atom with small volume, high positive charge and low polarizability is called hard acid, otherwise it is called soft acid, the coordinating atom with high electronegativity, low polarizability and difficult oxidation is called hard base, otherwise it is called soft base, and the other acids and bases are called critical acids and bases. Common hard and soft acids and bases and critical acids and bases are shown in Table 3.

Pearson gave the general rule of hard and soft acid and base reaction, that is, hard acid reacts with hard base to form stable complexes, and soft acid reacts with soft base to form stable complexes. This theory can be used to predict the stability of metal complexes, and to design and prepare selective ion adsorption materials, which has important theoretical and practical significance. In the research and development of selective ion exchange materials, researchers can select the appropriate acid and base as ligands according to the soft and hard acid and base properties of the target ion.As a specific functional group, it is loaded on resin, chitosan, metal organic framework, covalent organic framework, mesoporous silica and other framework materials to prepare adsorption materials, and finally the selective removal of target ions is achieved by using the priority of soft and hard acid-base reactions^{[34,28][39⇓~41][91⇓~93][42⇓~44][94⇓~96][89]}. For example, sulfur-containing functional groups (soft bases) can selectively remove Hg²⁺ (soft acids) (Fig. 4)^{[97⇓⇓⇓⇓⇓⇓⇓⇓~106]}; Metal ions (hard acid) can selectively remove F^-, $\mathrm{PO}_{4}^{-}$, As (V), etc. (Hard base); -NH- and orthophosphate (hard base) can selectively remove Pb²⁺ in the presence of soft acid such As Hg²⁺ (critical acid), etc^{[107⇓⇓⇓⇓⇓⇓⇓⇓~116][117⇓⇓⇓~121][122⇓⇓~125][126⇓⇓~129]}.

According to the survey, soft and hard acid and base materials have been used in the selective adsorption of Hg²⁺, Pb²⁺, phosphate, F^- and plasma. The following are some cases^{[131][126][117][107]}.

The sulfur-containing ligand (soft base) selectively adsorbs Hg²⁺ (soft acid). Ji et al. Prepared a Hg²⁺ selective adsorbent by loading thioglycolic acid onto a covalent organic framework (MOF), and systematically characterized the structural changes of the adsorbent before and after absorbing Hg²⁺ using FTIR and XPS spectra^[132]. In Fig. 6a, the FTIR spectrum of the adsorbent shows that the -SH stretching vibration peak shifts from 2565cm^-1 to 2426 cm^-1 after the adsorption of Hg²⁺, which is caused by the complexation of Hg²⁺ with the -SH group on the adsorbent. In the wide scan XPS spectrum of Fig. 6 B, the characteristic peak of Hg 4F at 101.4 eV can be observed after the adsorption of Hg²⁺, indicating the successful adsorption of Hg²⁺. The new peaks at 100.6 eV (Hg 4F 7/2) and 104.6 eV (Hg 4F 5/2) in the Hg 4F XPS spectrum of Figure 6 C may also be attributed to the formation of -S-Hg complexes. Figure 6 d shows that the S 2p binding energy changes from 163.4 to 162.6 eV after the adsorption of Hg²⁺. The binding energy change of 0.8 eV indicates that a coordination bond with strong interaction between Hg²⁺ and the S atom of the adsorbent may be formed, and this strong interaction causes the high selectivity of this adsorbent for Hg²⁺ in a multi-ion mixture. Dias et al. Prepared TDD-organoclay by loading 1,3,4-thiadiazole-2,5-dithiol on the surface of HDTA-montmorillonite.In the presence of Hg²⁺, Cu²⁺, Pb²⁺,In the mixture of Cd²⁺, Ni²⁺ and Zn²⁺,The adsorption capacity of TDD-organoclay for Hg²⁺ was significantly stronger than that for other ions^[131]. Pan et al. Prepared a novel macroporous chelating resin polymerized from epoxy resin and triethylenetetramine (TETA) and modified with carbon disulfide,It is found that in Cu²⁺, Pb²⁺, Cd²⁺, Ni²⁺,In the presence of Fe²⁺, Zn²⁺ and Mn²⁺, respectively,The adsorption of Hg²⁺ on the resin remained stable (about 98%), indicating that the resin had high Hg²⁺ selectivity^[98]. Tzvetkova et al. Prepared a Hg²⁺ selective adsorbent by modifying silica gel with 5-amino-1,3,4-thiadiazole-2-thiol (S5A).In the presence of Hg²⁺, Cu²⁺, Co²⁺,In acidic solutions of Cd²⁺, Pb²⁺ and Ni²⁺,The selectivity of the adsorbent for Hg²⁺ is higher than that of other ions^[133]. Jiang et al. Prepared a novel composite nanosphere with thiol-functionalized mesoporous SiO₂ shell and Fe₃O₄-SiO₂ core,In the presence of Cu²⁺, Pb²⁺, Cr²⁺, Ni²⁺, Zn²⁺The removal effect of Hg²⁺ is obvious (from 109.8 mg/L to 0.3 mg/L), while the content of other ions is almost unchanged^[134]. The above selectivity mechanism can be explained by HSAB theory: the sulfur-containing ligand as a soft base has weak affinity for critical acids such as Cu²⁺ and Ni²⁺, and has strong affinity for soft acid Hg²⁺.

The hard base ligand selectively adsorbs Pb²⁺ (critical acid). Since the critical acid (Pb²⁺) is more "hard" than the soft acid, it is easier to combine with the hard base; It is more "soft" than hard acid and is easier to combine with soft base. Pan et al. Supported ZrP on chloromethylated polystyrene (CP) polymer in a mixture containing Pb²⁺, Na⁺, Ca²⁺ and Mg²⁺.The adsorption capacity of the adsorbent for Pb²⁺ (20 mg/G) is significantly higher than that of the common ion exchanger (0. 2 mg/G), and the removal capacity of Pb²⁺ remains stable with the increase of the concentration of coexisting ions^[126]. Jia et al. Prepared a new hybrid adsorbent (TiP-001) by impregnating titanium phosphate (TiP) nanoparticles onto a strong acidic cation exchanger (D-001), and found that TiP-001 exhibited high Pb²⁺ selectivity compared with D-001 in the presence of Ca^2+[128]. The above selectivity mechanism can be explained by HSAB theory that :Pb²⁺ is the critical acid, while Na (I), Ca²⁺ and Mg²⁺ are hard acids, so Pb²⁺ is easier to combine with soft base orthophosphate to form a stable complex. Liu et al. Copolymerized two polypyrromethanes loaded with hydroxyl functional groups (-OH), and found that the adsorption of Pb²⁺ by these two adsorption materials was not significantly affected in the presence of Cd²⁺ and Ni²⁺, respectively^[129]. The selectivity mechanism of the material can be explained by the HSAB theory that :Pb²⁺ is a critical acid, Cd²⁺ and Ni²⁺ are soft acids,The hydroxyl group (-OH) is a hard base, so the affinity of Pb²⁺ to hydroxyl group (-OH) is greater than that of Cd²⁺ and Ni²⁺.

Metal ions (hard acids) selectively adsorb anions (hard bases). Common anions in water are usually hard bases, which are easy to react with hard acids (such as metal ions) to form stable complexes, and become one of the main research directions of specific anion selective adsorption materials in water. Wu et al. Supported La³⁺ on a polymeric ligand exchanger chelex-100 to selectively remove phosphate from water^[117]. In the presence of Cl^- and {{\mathrm{SO}}_4}^{2-}, the adsorption capacity of the adsorbent for phosphate remained stable (0.098 mmol·g^-1 without Cl^- and {{\mathrm{SO}}_4}^{2-}; 0.092 mmol·g^-1 in the presence of Cl^-, {{\mathrm{SO}}_4}^{2-}). Barsbay et al. Prepared a novel polymeric ligand exchanger (PLE) loaded with Cu²⁺ to selectively remove phosphate from water^[118]. It was found that the adsorption of phosphate was significantly higher than that of other ions in low concentration solutions (1 mg/L phosphate, 1 mg/L bromide, 1 mg/Lnitrite, 10 mg/L sulfate and 10 mg/Lnitrate). Viswanathan et al. Prepared a novel adsorbent (Fe-CCB) to selectively remove fluoride from water by loading Fe³⁺ onto carboxylated chitosan beads^[107]. It was found that in the presence of Cl^-, {{\mathrm{SO}}_4}^{2-}, {{\mathrm{HCO}}_3}^{-} and {{\mathrm{NO}}_3}^{-}, the removal capacity of the adsorbent for F^- only decreased from 4000 mg/kg to ~ 3500 mg/kg, enabling the selective removal of fluoride. Viswanathan et al. Prepared a new adsorbent by loading La³⁺ onto carboxylated chitosan beads, and found that in the presence of Cl^-, {{\mathrm{SO}}_4}^{2-}, {{\mathrm{HCO}}_3}^{-} and {{\mathrm{NO}}_3}^{-},The adsorbent showed a small decrease in the removal capacity of F^- (from 4500 mg/kg to ~ 3500 mg/kg), enabling the selective removal of fluoride^[108]. Vatutsina et al. Impregnated hydrated iron oxide (HFO) onto a fiber polymer ion exchanger, and the adsorption capacity of the adsorbent for As (Ⅲ) and As (V) did not decrease in the presence of Cl^- and {{\mathrm{SO}}_4}^{2-}^[135]. When phosphate is present, only the adsorption of As (V) is affected. Nilchi et al. Supported nanoscale sol-gel derived TiO₂-SiO₂ on porous polyacrylonitrile (PAN) polymer,In the presence of Cl^-, {{\mathrm{NO}}_3}^{-}, {{\mathrm{NO}}_2}^{-}, {{\mathrm{SO}}_4}^{2-}, and {{\mathrm{SO}}_3}^{2-} anions,The distribution coefficient of the adsorbent for As (Ⅲ) is greater than 4000, and the distribution adsorption for As (Ⅴ) is greater than 5000, which indicates that the adsorbent can selectively remove As (Ⅲ) and As (Ⅴ) from water^[136].

The acid-base theory has also been applied to the selective removal of radionuclides. Researchers have found that the amidoxime chelating group has a specific strong affinity for {{\mathrm{UO}}_2}^{2+}, and have developed a variety of adsorbents for the nuclear industry, such as the extraction of uranium from nuclear wastewater and the extraction of uranium from seawater. Sun et al. Prepared an amidoxime-functionalized covalent organic framework material (COF-TpDb-AO) to selectively adsorb {{\mathrm{UO}}_2}^{2+} in water, and the adsorbent material maintained high affinity and efficient removal capacity for {{\mathrm{UO}}_2}^{2+} ( {{\mathrm{UO}}_2}^{2+} content decreased from 1000 ppb to about 0.1 ppb in 30 min) in real water samples of drinking water, well water and river water^[55]. Hao et al. Prepared a covalent organic framework material with electron donor-acceptor sites and amidoxime nanotraps for simultaneous selective adsorption and photocatalytic reduction of uranium to treat seawater. In spiked seawater at pH = 8.1 (the concentration of {{\mathrm{UO}}_2}^{2+} is ~ 2 to ~ 20 ppm), the adsorption material maintained a high adsorption capacity for {{\mathrm{UO}}_2}^{2+}, which is comparable to other high-performance uranium adsorbents^[137]. Abney et al. Used density functional theory to study the principle of selective adsorption of {{\mathrm{UO}}_2}^{2+} by amidoxime group, and calculated the bond length and molecular orbital structure of [UO₂(H₂O)₅]²⁺ and [UO₂(AO)₂(MeOH)₂].It was found that the π-bonding orbital energy of the oxime ester functional group is higher than that of uranyl σ (d) and π (d), and that the π-conjugated orbital of O — N has a significant overlap with the 1π_g and 2π_u orbitals of {{\mathrm{UO}}_2}^{2+}, indicating a significant bonding interaction^[138]. The Wiberg bond index (WBI) was calculated by natural bond orbital (NBO) analysis, and it was found that the strength of the amidoxime ester bond and the energy of the p orbital of the oxygen lone pair electron in the amidoxime ester group increased with the increase of the electron-donating ability of the substituent.O that the amidoxime has stronger interaction with the σ (f) orbital of uranium, so the amidoxime adsorbent containing the substituent with strong electron-donating ability has stronger binding ability with {{\mathrm{UO}}_2}^{2+}. Finally, based on this principle, two new oxime-functionalized imidazole adsorbents with greater uranyl ion affinity were developed.

Selective ion removal technology based on the theory of hard and soft acids and bases is one of the most widely used selective technologies, and the main application of this principle is to load specific acid and base ligands on the carrier material. At present, the selective research based on the HSAB theory focuses on the use of acid-base ligands and support materials, especially with the development of nanotechnology, the treatment effect and adsorption capacity of the selective materials developed have also been improved. However, there are relatively few studies on the theory of hard and soft acids and bases, and further studies can be made on the binding mechanism between ligands and target ions in the future.

Hydration energy (hydration enthalpy) refers to the energy released when one mole of ions is hydrated. Generally speaking, the greater the hydration energy, the more hydrophilic the ions are, and the smaller the hydration energy, the more hydrophobic the ions are^[144]. Table 4 shows the values of hydration energy for some common anions. Compared with the active group trimethylamine (i.e., -N(CH₃)₃) of the standard strong base anion resin, the active group with longer carbon chain means stronger hydrophobicity and is able to selectively adsorb more hydrophobic ions^[8]. Therefore, the selective adsorption of strongly hydrophobic ions can be achieved by selecting active groups with longer carbon chains, and the smaller the hydration energy of anions is, the stronger the hydrophobicity is, and the easier it is to be adsorbed by adsorbents with long-chain structural groups. For example, Behnsen and Riebe modified bentonite with three long-chain groups, cetylpyridinium, cetyltrimethylammonium, and benzethonium,The order of affinity of the three adsorbents for the six anions was found to be {{\mathrm{ReO}}_4}^{-}.>I^-> {{\mathrm{NO}}_3}^{-} >Br^->Cl^-> {{\mathrm{SO}}_4}^{2-} > {{\mathrm{SeO}}_3}^{2-} ,Corresponding to the order of increase in the combined energy of these anions^[145].

Based on this principle, many studies have developed selective adsorption materials for strong hydrophobic anions (such as $\mathrm{NO}_{3}^{-}$, $\mathrm{TcO}_{4}^{-}$, $\mathrm{ClO}_{4}^{-}$, etc.)^{[139⇓141][142][143]}. For example, triethylamino polystyrene resin NDP-2 prepared by Song et al. Can selectively adsorb {{\mathrm{NO}}_3}^{-} in the coexistence solution of Cl^-, {{\mathrm{HCO}}_3}^{-} and {{\mathrm{SO}}_4}^{2-};Ethylenediamine-based polystyrene resin ADVEGR prepared by Sowmya et al. Preferentially adsorbs {{\mathrm{NO}}_3}^{-} in the coexistence solution of {{\mathrm{NO}}_3}^{-}, Cl^-, {{\mathrm{HPO}}_4}^{2-} and {{\mathrm{SO}}_4}^{2-}^[140][141]. On the other hand, too long carbon chain of active group will lead to the increase of crosslinking density and pore size of adsorbent, which will reduce the capacity and ion exchange efficiency of adsorbent^[142]. Kang et al. Prepared quaternary aminated mesoporous silica adsorbents (C1Q- to C18Q-SBA-15) with active groups containing C numbers of 1, 4, 8, 12 and 18, respectively, and found that in the presence of {{\mathrm{HCO}}_3}^{-}, Cl^-, {{\mathrm{HPO}}_4}^{2-} and {{\mathrm{SO}}_4}^{2-},C8Q- and C12Q-SBA-15 were more effective in removing {{\mathrm{NO}}_3}^{-} than C1Q- and C4Q-SBA-15, while the adsorption capacity of C18Q-SBA15 for {{\mathrm{NO}}_3}^{-} was less than that of the other four adsorbents^[139].

Some researchers use both long-chain and short-chain active groups to achieve selective adsorption through long-chain active groups and to improve adsorption kinetics through short-chain active groups^[142]. For example, Gu et al. Prepared trihexylamine and triethylamine bifunctional polystyrene resin, which selectively adsorbed more hydrophobic radioactive ion {{\mathrm{TcO}}_4}^{-} under the coexistence of Cl^-, {{\mathrm{NO}}_3}^{-}, {{\mathrm{SO}}_4}^{2-} and {{\mathrm{HCO}}_3}^{-}.When applied to underground treatment, the penetration volume exceeds 250 000 bed volume, the resin can selectively adsorb {{\mathrm{ClO}}_4}^{-} with stronger hydrophobicity in the coexistence of {{\mathrm{NO}}_3}^{-} and {{\mathrm{SO}}_4}^{2-}, and the penetration volume can reach 38 000 bed volume when treating groundwater with high concentration of {{\mathrm{ClO}}_4}^{-}^[142][143]. Through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis of the material (Fig. 7), it was found that compared with the {{\mathrm{TcO}}_4}^{-} selective adsorbent (Purolite A-520E) with only long-chain functional groups, the adsorbent with both long-chain and short-chain active groups had more pores and larger specific surface area, thus achieving {{\mathrm{TcO}}_4}^{-} selective adsorption and higher ion exchange efficiency.

The application of hydrophilic/hydrophobic-based selective ion adsorption technology is closely related to the hydration energy of the target ion, and only the ions with strong hydrophobicity can be treated by this technology, which limits the range of ions suitable for this technology. At present, only three ions, {{\mathrm{NO}}_3}^{-}, {{\mathrm{TcO}}_4}^{-} and {{\mathrm{ClO}}_4}^{-}, have been studied as target ions. In the future, other target ions suitable for this mechanism can be found for selectivity related studies.

Molecules containing hydrogen bonding functional groups (such as hydroxyl, carbonyl, and nitrile) can selectively adsorb guest substances through hydrogen bonding between the more electronegative atom X and a hydrogen atom that is already covalently bonded to the more electronegative atom Y (X-H … Y), such as two O atoms or an O atom and an N atom^{[147][148][149]}. Adsorption materials prepared according to this principle have been used for selective adsorption of $\mathrm{H}_{2}\mathrm{PO}_{4}^{-}$, $\mathrm{HAsO}_{4}^{2-}$, etc. For example, Yu et Al prepared organic-inorganic composite adsorption materials by intercalating pyromellitic acid (PMA) into Zn-Al layered double hydroxide.In $\mathrm{H}_{2}\mathrm{PO}^{-}_{4}$, {{\mathrm{SO}}_4}^{2-}, {{\mathrm{CO}}_3}^{2-},The high-selectivity adsorption of the $\mathrm{H}_{2}\mathrm{PO}_{4}^{-}$ is realized under the coexistence condition of the {{\mathrm{NO}}_3}^{-} and the Cl^-,XPS spectrum analysis before and after $\mathrm{H}_{2}\mathrm{PO}_{4}^{-}$ adsorption found that (Fig. 8A),The electron density of the carbon atom in the carboxyl group (— COOH) decreases, which may be due to the electron transfer under the formation of hydrogen bonds, resulting in the decrease of the electron density in C = O^{[149][51][149]}. Then the electron density in C with C = O adjacent to O increases. Compared with the non-PMA-modified layered double hydroxide, the P 2p electron density in the $\mathrm{H}_{2}\mathrm{PO}_{4}^{-}$ adsorbed by the selective material decreased (Fig. 8 B), which may be due to the formation of hydrogen bonds to increase the electron density of — OH and decrease the electron density in the adjacent P. XPS analysis shows that the hydrogen bonding occurs between the oxygen dissociated from the carboxyl group (-COOH) in PMA and the hydroxyl group (-OH) of phosphate. Huang et al. Prepared triethylenetetramine activated lignin, and realized the highly selective adsorption of As (Ⅴ) under the coexistence of Cr (Ⅵ), P (Ⅴ) and As (Ⅴ), and the hydrogen bonding occurred between the N atom of triethylenetetramine and the hydroxyl group of {{\mathrm{HAsO}}_4}^{2-}^[51]. Liang et al. Prepared microporous carbon materials (MCM) and achieved highly selective adsorption of phosphate under the coexistence of {{\mathrm{SO}}_4}^{2-}, {{\mathrm{CO}}_3}^{2-} and Cl^-, and the hydrogen bonding occurred between the carboxyl group of MCM and the hydroxyl group of phosphate^[150].

In general, compared with other selective techniques, the selective ion removal technique based on hydrogen bonding has a unique effect on the selective adsorption of oxygen anions with hydroxyl or carboxyl groups, and is an effective method for the selective removal of some oxygen anions. In the future, further research can be done on the development of adsorption materials and the improvement of their stability and adsorption capacity.