After a protracted battle, humans have triumphantly changed the course of nature and created social economics. In the meantime, environmental destruction has posed a realistic threat to mankind’s existence and progression. Water is the source of life. Oceans cover about 70 percent of the Earth. However, the freshwater resources that can be directly used by people contain less than 1% of the total water in the world. It is worth noting that the quality of available freshwater resources for humans is deteriorating due to demographic and environmental factors. [1,2,3]^[1][2][3]. At present, every year in some poor and backward countries, about 2 million people suffer from related diseases and eventually die because of a lack of access to clean water ^[4].

In water, most of the pollutants can be divided into heavy metal ions, acid ions, organic dyes, organic poisons, and drugs, which, to a certain extent, will cause allergy symptoms, nausea, headaches, heart disease, and other symptoms, and even lead to death ^[5]. A number of strategies have been used to treat these pollutants. They include nanofiltration ^[6], solvent extraction [7,8]^[7][8], resin use ^[9], photocatalytic degradation [10^[10][11],11], cooling, ion exchange, chemical precipitation, electrochemical deposition, reverse osmosis, biological treatment [12^{[12][13][14][15]},13,14,15], and others. However, most of these technologies have certain shortcomings, such as expensive equipment, complicated operations, and a long experimental period. Fortunately, adsorption, as one of the most promising pollutant treatment methods, has attracted the research interest of many researchers.

For the sake of adsorbing pollutants in water, it is necessary to choose the right material. Metal-organic frame materials were first proposed in 1999 by Omar Yaghi and his team at the University of California, Berkeley ^[16]. The material has the advantages of low density, a large specific surface area ^[17], ideal porosity [18^[18][19],19], big pore size, while being adjustable [20,21,22,23]^{[20][21][22][23]}. Therefore, it presents an attractive topic in the fields of catalytic conversion [24,25,26^{[24][25][26][27]},27], gas storage [28^[28][29][30],29,30], electrochemical biosensing [31,32^[31][32][33],33], drug slow-release control ^[34], molecular recognition [35^[35][36][37],36,37], magnetism ^[38], adsorption and separation of pollutants [39^[39][40][41],40,41], and so on. In order to fully exploit the advantages of MOFs in water purification, researchers modified the functional groups on MOFs so as to further boost the adsorption performance of MOFs. All these MOFs have achieved remarkable results in water purification.

2. Application of MOFs by Modified Functional Groups in Water Purification

2.1. N-Containing Group

2.1.1. -NH₂

Liu et al. synthesized UiO-66-NH₂-CS aerogel monomers with hierarchical structure by covalent cross-linking, which showed effective and stable adsorption of Pb²⁺ [83]^[42]. The aerogel was characterized by many pores and a low density. The adsorption of Pb²⁺ by the material was 102.03 mg·g⁻¹. Among them, N and Pb²⁺ play a coordination role, and O plays a synergistic adsorption role to a certain extent. After three cycles, the material’s adsorption capacity remained at 90.12%. This strategy provides an effective and universal approach for using MOFs in the field of pollutant treatment (Figure 1). Azmi et al. proposed a new synthesis method to prepare MIL-96, which has different crystal habits and a larger particle size [84]^[43]. After synthesizing the material, they added hydrolyzed polyacrylamide (HPAM) to increase the size of the material. Activated carbon (AC) was used as a reference adsorbent to examine the removal impact of MIL-96-RHPAM2 on perfluorooctanoic acid (PFOA). The results showed that MIL-96-RHPAM2 had a good PFOA adsorption capability of 340 mg·g⁻¹. The amine group and the anionic carboxylate of PFOA in the HPAM skeleton generate electrostatic bonds, which contribute to the high adsorption of PFOA by MIL-96-RHPAM2. Samuel et al. synthesized Cu-BTC@NH₂ composites and investigated their adsorption capacity for two drug contaminants, ibuprofen (IBF) and acetaminophen (ACE), in aqueous solutions [85]^[44]. The course of adsorption accords with the pseudo-second-order kinetic model and Langmuir adsorption, which reveal that chemisorption is the prime adsorption process. Based on hydrophobic and electrostatic effects, the adsorption capacities of IBF and ACE were 187.97 mg·g⁻¹ and 125.45 mg·g⁻¹, accordingly. The laboratory findings display that the approach can be applied to eliminate harmful drugs from toxic wastewater. In a study, Park et al. used MIL-101 to adsorb bisphenol S (BPS) in water and investigated the effectiveness of the introduction of -NH₂ on adsorption [86]^[45]. The elimination capacity of BPS was greatly enhanced, and the adsorption capacity reached 513 mg·g⁻¹ when the -NH₂ group was added to MIL-101. The research reveals that hydrogen bonds exist in MIL-101-NH₂, as well as O=S=O and -OH in BPS. The material can still have good adsorption characteristics after routine ethanol washing. The method also provides a novel horizon for the adsorption of organics containing sulfonyl in wastewater. Wu et al. compounded an adsorbent (UiO-66-NH₂) that can efficiently remove Cr⁶⁺ from water [87]^[46]. According to the pseudo-second-order kinetic model, the initial stage of Cr⁶⁺ adsorption is rapid, and the subsequent adsorption phase is slow. The adsorption capacity of Cr⁶⁺ was determined by the Langmuir model to be 32.36 mg·g⁻¹, and it occurred spontaneously at pH = 6.5. The electrostatic attraction between Cr⁶⁺ and UiO-66-NH₂ may be the principle of Cr⁶⁺ adsorption by UiO-66-NH₂. Roushani et al. demonstrated in an experiment that TMU-16-NH₂ is a resultful adsorbent for the treatment of Cd²⁺ from water solutions [88]^[47]. The results show that the initial concentration and pH of the solution have a great influence on the adsorption, which is spontaneous. The adsorbent’s maximal Cd²⁺ adsorption capacity was 126.6 mg·g⁻¹. Mechanism studies show that the amino group of TMU-16-NH₂ is a good binding site for Cd²⁺, and the adsorption of Cd²⁺ is facilitated by the creation of coordination bonds between -NH₂ groups and Cd²⁺. The performance of the adsorbent has been greatly improved compared with the results of research. Lv et al. used two MOFs (original MIL-68(In) and NH2-MIL-68(In)) to investigate the adsorption efficiency of typical organic arsenic compounds, arsine acid (p-ASA). [89]^[48]. The adsorption quantity of NH₂-MIL-68(In) for p-ASA (401.6 mg·g⁻¹) is greater than that of pure MIL-68(In), which can be attributed to hydrogen bonding and the π–π effect (Figure 2). In addition, the result of the adsorption of p-ASA is exothermic and spontaneous, conforming to the pseudo-second-order intraparticle diffusion model. The addition of -NH₂ to MIL-68(In) accelerates adsorption performance by increasing hydrogen bonding. This research offers a crucial theoretical and experimental foundation for the use of MOFs in the elimination of organic arsenic compounds like p-ASA. Roushani et al. investigated the adsorption and removal of the anionic dye methyl orange (MO) by two MOF materials (TMU-16-NH₂ and TMU-16) [90]^[49]. Thermodynamic and kinetic outcomes display that the removal of MO by the two MOFs is a spontaneous pseudo-second-order adsorption, and the removal rate of MO by TMU-16-NH₂ (393.7 mg·g⁻¹) is superior to that of the original TMU-16 because the hydrogen-bond between MO and -NH₂ is formed and the material itself is stable. The adsorption properties of these materials for anionic dyestuffs are stronger than those of other previously reported porous materials. Electrostatic interaction and hydrogen bonding both contribute to the understanding of the adsorption mechanism. Haque et al. prepared two metal-organic frameworks, NH₂-MIL-101(Al) and MIL-101(Al) [91]^[50]. These two materials have been applied to dislodge methylene blue (MB) from aqueous solutions. The adsorption capacity of NH₂-MIL-101(Al) on MB at 30 °C was up to 762 mg·g⁻¹ due to the electrostatic interaction between the cationic dye MB and the amino group of MOFs. The original MIL-101(Al) exhibited lower adsorption capability, which explains why -NH₂ was introduced. Liu et al. put forward a magnetic MOF substance for the adsorption of several cationic and anionic dye types [92]^[51]. Adsorption isotherm and thermodynamic studies showed that the maximum adsorption capabilities of NH₂-MIL-101(Al) for indigo carmine (IC) and malachite green (MG) were 135 mg·g⁻¹ and 274.4 mg·g⁻¹, respectively. This is primarily the contribution of π–π interaction, electrostatic interaction, hydrogen bonding and hydrophobic interaction between magnetic NH₂-MIL-101(Al) and organic dyes. The method has the characteristics of a short separation time and a high reuse rate, which is expected to make it a new adsorbent for adsorbing and removing dyes from aqueous solutions. Oveisi et al. used Ti as a metal source, 2-amino-1,4-phthalate ester (NH₂-BDC), and 1,4-phthalate ester (BDC) as organic connectors to synthesize five kinds of materials in the MIL series, which are MIL-X1, MIL-X2, MIL-X3, NH₂-MIL-125(Ti), and MIl-125(Ti) (the first three materials are obtained by adjusting the ratio of BDC to NH₂-BDC, respectively) [93]^[52] (Figure 3). These materials were used to study the removal of the cationic dyes methylene blue (MB), basic blue 41 (BB41), and basic red 46 (BR46) from water. The adsorption of pollutants by the five materials of the MIL series conforms to the pseudo-second-order kinetic model and the Langmuir adsorption, and the synthesized MIL has high reuse and stability within the three cycle periods. Most importantly, the maximum electron density was found in NH₂-MIL-125(Ti), so the adsorption performance is particularly ideal. The adsorption of MB, BB41, and BR46 is respectively 862, 1257, and 1296 mg·g⁻¹.

2.1.2. -NMe³⁺

Wu et al. created a novel quaternary amine anion-exchange MOF (UiO-66-NMe³⁺) for the adsorption of 2,4-dichlorophenoxyacetic acid (2,4-d), a toxic pesticide that is extensively utilized [94]^[53]. UiO-66-NMe³⁺ had a maximum adsorption capacity for 2,4-d of 279 mg·g⁻¹, which was much higher than UiO-66 and UiO-66-NH₂. This is mainly attributed to the following two reasons. For one thing, the functionality of quaternary amine groups can effectively increase the electrostatic effect. For another, the π–π conjugation between 2,4-d and MOFs improves the adsorption properties of UiO-66-NMe³⁺. After seven cycles, the adsorption performance decreased only slightly. This technique offers the best alternative for effectively removing 2,4-d from complex water conditions by adsorption. Wei et al. created the functionalized quaternary ammonium salt MIL-101(Cr)-NMe³⁺ and took advantage of it as a novel adsorbent to wipe off diclofenac sodium (DCF) from wastewater [95]^[54]. The results showed that DCF quickly adsorbed onto MIL-101(Cr)-NMe³⁺. At 20 °C for 30 min, DCF can be adsorbed in large quantities (310.6 mg·g⁻¹), which conforms to the pseudo-second-order kinetic model and the Langmuir adsorption. According to the adsorption mechanism analysis, the electrostatic interaction and π–π interaction between MIL-101(Cr)-NMe³⁺ and DCF are the reasons for the efficacious adsorption. Based on the results of the regeneration experiment, MIL-101(Cr)-NMe³⁺ could be recovered at least five times without suffering a substantial decrease in adsorption ability. Based on the preceding discussion, wresearchers can conclude that MOFs modified with -NH₂ or -NMe³⁺ can be useful in water purification. For organic matter in water (such as organic poisons, dyes, and drugs), the general adsorption mechanism focuses on hydrogen bonding and electrostatic interaction. This is very understandable. Because N is a highly electronegative atom, it can be used as the acceptor of H, thus forming hydrogen bonds and realizing the adsorption of pollutants. For heavy metal ions in water, -NH₂ successfully acts as a Lewis base. Because metal ions are Lewis acids, they can interact with -NH₂ in Lewis acid–base reactions to produce the adsorption effect.

2.2. S-Containing Group

2.2.1. -SH

Ke et al. selected the classic 3D Cu-MOF (HKUST-1) and used a simple post-synthesis strategy to achieve sulfhydryl functionalization of MOFs to investigate their application in removing Hg²⁺ from water [96]^[55]. They prepared a sequence of sulfhydryl-modified HKUST-1 by ligand-bonding the coordination unsaturated metal center in HKUST-1 to the -SH group in disulfide glycol. Under the same conditions, sulfhydryl-functionalized HKUST-1 showed very high adsorption capacity (714.29 mg·g⁻¹) and fine adsorption affinity for Hg²⁺ in water. Nevertheless, unfunctionalized HKUST-1 demonstrated no adsorption for Hg²⁺. This method is considered to be a new and effective method for the treatment of heavy metal ions in water. Li et al. created sulfhydryl-modified MOFs (UiO-66-SH) by a simple tactic and used them for selectively extracting Hg²⁺ in aqueous solutions [97]^[56]. After the introduction of -SH, the rate of UiO-66-SH’s adsorption to Hg²⁺ was extremely rapid, showing high adsorption performance (3.91 mmol·g⁻¹) in a broad pH range (2.3–8.0), and the adsorption efficiency remained high (> 90%) after seven regenerations. Even though there are other heavy metal ions (Cu²⁺, Mn²⁺, Ni²⁺, Cd²⁺, Ba²⁺, and Co²⁺), the adsorbent exhibited selective adsorption of Hg²⁺ (Figure 4). Density functional theory (DFT) calculations revealed that this was primarily due to the strong coordination between Hg²⁺ and -SH. It is worth mentioning that UiO-66-SH possesses a unique attraction for organic mercury forms as well. (PhHg⁺, EtHg⁺, and MeHg⁺). This method can provide a new idea for removing mercury from ecological environments. By functionalizing MIL-88A with mercaptoethanol, Singh et al. created MIL-88A-SH, a new-style MOF-based adsorbent, and tested its removal of Hg²⁺ from water and Hg from air [98]^[57]. The absorption is constant in the pH range of 5.0–9.0, and the adsorption capacity of Hg²⁺ is very high, about 1111.1 mg·g⁻¹. In addition, when there are other interfering metal ions (Cu²⁺, As³⁺, Zn²⁺, Cr⁶⁺, Pb²⁺, and Cd²⁺), MIL-88A-SH still shows good adsorption properties for Hg²⁺. For Hg in the air, about 45.6 mg·g⁻¹ of Hg²⁺ is adsorbed. This is mainly because Hg was oxidized to Hg²⁺ and complexed with sulfhydryl groups during the adsorption process. These data indicate that MIL-88A-SH can be used as a highly effective adsorbent to solve mercury pollution. Zhang et al. made use of the hydrothermal method to create MOF-5. On this basis, mercaptan-functionalized MOFs (HS-mSi@MOF-5) were invented and applied to research the adsorption effects of Pb²⁺ and Cd²⁺ in water [99]^[58]. In view of the capacity and rate of adsorption, the adsorption performance of HS-mSi@MOF-5 (312.5 mg·g⁻¹, 65.2 mg·g⁻¹) is preferable to that of the original material (211 mg·g⁻¹, 4.2 mg·g⁻¹). In the water stability test, the unmodified MOF-5 will gradually dissolve in an acidic solution, but its stability increases after the introduction of -HS. Based on the mechanism of adsorption, the coordination interaction and electrostatic interaction between -HS and toxic metal ions are critical for the disposal of Cd²⁺ and Pb²⁺ (Figure 5).

2.2.2. -SO₃H

Moradi et al. synthesized and used sulfonated MOFs supported on Fe₃O₄ NPs as a Fenton-like catalyst to remove methyl orange (MO) from water [100]^[59]. When the initial concentration is 100 mg·L⁻¹, the incipient concentration of H₂O₂ is 40 mg·L⁻¹, the microwave power is 500 W, and the pH value is 3.0, the rate of the elimination of MO is very fast. Only when the microwave radiation time is 6 min, as much as 99.9% of MO can be removed. Finally, microwave-induced Fe₃O₄@MIL-100(Fe)-SO₃H degradation will be a wastewater dye removal technique with great promise. Hasan et al. first synthesized three Zr-MOF series materials (UiO-66-SO₃H, UiO-66-NH₂, and UiO-66) for the removal and adsorption of diclofenac sodium (DCF) in water and contrasted these three materials with activated carbon (AC) [101]^[60]. These three materials showed better adsorption performance than AC. The adsorption kinetics between UiO-66 and DCF are faster due to possible π–π interaction and electrostatic interaction. After the introduction of -SO₃H, the kinetics and capacity (263 mg·g⁻¹) of adsorption were significantly enhanced, which may result from the acid–base attraction of -SO₃H in UiO-66-SO₃H and -NH₂ in DCF. Interestingly, however, the opposite trend was observed when UiO-66-NH₂ was utilized as a sorbent. This is primarily due to the alkali-base rejection of -NH₂ in DCF and UiO-66-NH₂, on account of relevant results. Wang et al. immobilized -SO₃H on the surface of MOFs to synthesize sulfonic acid-functionalized MOFs to remove Cd²⁺ from aqueous solutions [102]^[61]. Cu₃(BTC)₂-SO₃H showed a relatively good absorption capacity of 88.7 mg·g⁻¹, exceeding that of the reference adsorbent. Additionally, it has quick kinetics for the adsorption of Cd²⁺ from aqueous solutions that are 1–3 orders of magnitude higher than those of current adsorbent materials. Furthermore, despite the existence of other interfering ions, it indicates a strong selectivity for Cd²⁺ and is simple to regenerate and recycle without suffering a major reduction in Cd²⁺ adsorption capacity. Liu et al. focused on the adsorption behavior of UiO-66@mSi-SO₃H, UiO-66@mSi-SH, and UiO-66 on Cd²⁺ [103]^[62]. In consideration of the data of the Elovich and the Langmuir models, the adsorption capacity can be UiO-66 < UiO-66@mSi-SH < UiO-66@mSi-SO₃H. The theoretical maximum adsorption capacities of UiO-66@mSi-SO₃H and UiO-66@mSi-SH for Cd²⁺ are 409.96 and 212 mg·g⁻¹, respectively. In the presence of -SH and -SO₃H, Cd²⁺ in water was replaced by adsorbents to accomplish the aim of getting rid of Cd²⁺. -SO₃H has diverse and complex coordination forms, allowing UiO-66-mSi-SO₃H to have an excellent adsorption capacity for removing Cd²⁺. Lv et al. synthesized a late-model magnetic nanocomposite with a core-shell structure of Fe₃O₄@UiO-66-SO₃H by means of a viable stepwise assembly tactic and then made the most of it as an adsorbent to wipe off methylene blue (MB) from aqueous solutions [104]^[63]. A considerable affinity for MB may be shown in the synthesized Fe₃O₄@UiO-66-SO₃H, which has a maximum adsorption capacity of 297.3 mg·g⁻¹. The adsorption process is basically completed within 15 min. Because of the effect of charge selectivity, Fe₃O₄@UiO-66-SO₃H has good reusability and can selectively adsorb MB from the mixed solution. The π–π interaction and electrostatic interaction are important during the adsorption process. (Figure 6).

2.2.3. -SO₄

Kang et al. selected Zr-BTC-NH₂-SO₄ as an adsorbent to capture radioactive Ba²⁺ in nuclear wastewater [105]^[64] (Figure 7). By reason of the plentiful -SO₄ groups in the skeleton, which are strong Ba²⁺ chelating groups, and the fact that the binding site is completely exposed, in comparison to most adsorbents, Zr-BTC-NH₂-SO₄ has a greater adsorption capacity of 181.8 mg·g⁻¹. When the concentration of interfering ions is ten times that of Ba²⁺, the material still shows excellent selectivity. Most crucially, the course of adsorption is irreversible for Ba²⁺, which can availably avert secondary pollution. This work has contributed to the development of adsorbents for the handling of radioactive Ba²⁺ from nuclear effluent. Peng et al. proposed a Ba²⁺ defect concept based on MOFs by introducing -SO₄ functional groups into the pore structure of MOFs [106]^[65]. The functionalized MOFs can remove more than 90% of Ba²⁺ within 5 min, and the removal efficiency reaches 99% after equilibrium. Notably, the sulfate-based functionalized material showed a high Ba²⁺ absorption capacity of 131.1 mg·g⁻¹, exceeding that of most reportorial adsorbents, and could selectively capture Ba²⁺ from outlet water. This preseaperrch offers a fresh viewpoint on environmental remediation and the removal of radioactive Ba²⁺ from nuclear effluent. MOFs modified with functional groups containing S atoms are generally used in the catalytic field, but due to some special properties, they can also be used to remove pollutants from water sources [117]^[66]. For S-containing groups, -SH, -SO₃H, and -SO₄ are the most representative. Since these three are nucleophiles, they can be regarded as Lewis bases, which explains their ability to adsorb metal ions from water through the Lewis acid–base process. For -SO₃H, when it is modified into organic molecules, it can increase the acidity of an aqueous solution, making the pH drop, thus promoting the electrostatic interaction between it and pollutants. In addition, -SO₃H is a hydrophilic group, which can increase the water solubility of MOFs. For -SH, it can also produce the same effect as -SO₃H, but the acidity is not that strong, so the electrostatic effect of -SH is weaker than that of -SO₃H. In addition, thanks to the presence of S atoms, MOFs modified by functional groups can also form hydrogen bonds with organic matter, but the effect is less obvious than the previous two.