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-NH
2-CS aerogel monomers with hierarchical structure by covalent cross-linking, which showed effective and stable adsorption of Pb
2+ [42][83]. The aerogel was characterized by many pores and a low density. The adsorption of Pb
2+ by the material was 102.03 mg·g
−1. Among them, N and Pb
2+ 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).
Figure 1. (a–c) Synthesis route of UiO-66-NH2-CS aerogel (d) Adsorption of Pb2+ by UiO-66-NH2-CS (e) Synthesis equation of UiO-66-NH2-CS.
Azmi et al. proposed a new synthesis method to prepare MIL-96, which has different crystal habits and a larger particle size
[43][84]. 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@NH
2 composites and investigated their adsorption capacity for two drug contaminants, ibuprofen (IBF) and acetaminophen (ACE), in aqueous solutions
[44][85]. 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 -NH
2 on adsorption
[45][86]. The elimination capacity of BPS was greatly enhanced, and the adsorption capacity reached 513 mg·g
−1 when the -NH
2 group was added to MIL-101. The research reveals that hydrogen bonds exist in MIL-101-NH
2, 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-NH
2) that can efficiently remove Cr
6+ from water
[46][87]. According to the pseudo-second-order kinetic model, the initial stage of Cr
6+ adsorption is rapid, and the subsequent adsorption phase is slow. The adsorption capacity of Cr
6+ 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 Cr
6+ and UiO-66-NH
2 may be the principle of Cr
6+ adsorption by UiO-66-NH
2. Roushani et al. demonstrated in an experiment that TMU-16-NH
2 is a resultful adsorbent for the treatment of Cd
2+ from water solutions
[47][88]. 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 Cd
2+ adsorption capacity was 126.6 mg·g
−1. Mechanism studies show that the amino group of TMU-16-NH
2 is a good binding site for Cd
2+, and the adsorption of Cd
2+ is facilitated by the creation of coordination bonds between -NH
2 groups and Cd
2+. 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).
[48][89]. The adsorption quantity of NH
2-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 -NH
2 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.
Figure 2. Structural formula of NH2-MIL-68(In) and main adsorption mechanism of p-ASA by NH2-MIL-68(In).
Roushani et al. investigated the adsorption and removal of the anionic dye methyl orange (MO) by two MOF materials (TMU-16-NH
2 and TMU-16)
[49][90]. 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-NH
2 (393.7 mg·g
−1) is superior to that of the original TMU-16 because the hydrogen-bond between MO and -NH
2 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, NH
2-MIL-101(Al) and MIL-101(Al)
[50][91]. These two materials have been applied to dislodge methylene blue (MB) from aqueous solutions. The adsorption capacity of NH
2-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 -NH
2 was introduced. Liu et al. put forward a magnetic MOF substance for the adsorption of several cationic and anionic dye types
[51][92]. Adsorption isotherm and thermodynamic studies showed that the maximum adsorption capabilities of NH
2-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 NH
2-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 (NH
2-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, NH
2-MIL-125(Ti), and MIl-125(Ti) (the first three materials are obtained by adjusting the ratio of BDC to NH
2-BDC, respectively)
[52][93] (
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 NH
2-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.
Figure 3. Synthesis and structure diagram of NH2-MIL-125(Ti).
2.1.2. -NMe3+
Wu et al. created a novel quaternary amine anion-exchange MOF (UiO-66-NMe
3+) for the adsorption of 2,4-dichlorophenoxyacetic acid (2,4-d), a toxic pesticide that is extensively utilized
[53][94]. UiO-66-NMe
3+ had a maximum adsorption capacity for 2,4-d of 279 mg·g
−1, which was much higher than UiO-66 and UiO-66-NH
2. 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-NMe
3+. 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)-NMe
3+ and took advantage of it as a novel adsorbent to wipe off diclofenac sodium (DCF) from wastewater
[54][95]. The results showed that DCF quickly adsorbed onto MIL-101(Cr)-NMe
3+. 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)-NMe
3+ and DCF are the reasons for the efficacious adsorption. Based on the results of the regeneration experiment, MIL-101(Cr)-NMe
3+ could be recovered at least five times without suffering a substantial decrease in adsorption ability.
Based on the preceding discussion,
reswe
archers can conclude that MOFs modified with -NH
2 or -NMe
3+ 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, -NH
2 successfully acts as a Lewis base. Because metal ions are Lewis acids, they can interact with -NH
2 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 Hg
2+ from water
[55][96]. 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 Hg
2+ in water. Nevertheless, unfunctionalized HKUST-1 demonstrated no adsorption for Hg
2+. 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 Hg
2+ in aqueous solutions
[56][97]. After the introduction of -SH, the rate of UiO-66-SH’s adsorption to Hg
2+ 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 (Cu
2+, Mn
2+, Ni
2+, Cd
2+, Ba
2+, and Co
2+), the adsorbent exhibited selective adsorption of Hg
2+ (
Figure 4). Density functional theory (DFT) calculations revealed that this was primarily due to the strong coordination between Hg
2+ 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.
Figure 4. Schematic diagram of UiO-66-SH selective adsorption of Hg2+ at different pH values.
By functionalizing MIL-88A with mercaptoethanol, Singh et al. created MIL-88A-SH, a new-style MOF-based adsorbent, and tested its removal of Hg
2+ from water and Hg from air
[57][98]. The absorption is constant in the pH range of 5.0–9.0, and the adsorption capacity of Hg
2+ is very high, about 1111.1 mg·g
−1. In addition, when there are other interfering metal ions (Cu
2+, As
3+, Zn
2+, Cr
6+, Pb
2+, and Cd
2+), MIL-88A-SH still shows good adsorption properties for Hg
2+. For Hg in the air, about 45.6 mg·g
−1 of Hg
2+ is adsorbed. This is mainly because Hg was oxidized to Hg
2+ 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 Pb
2+ and Cd
2+ in water
[58][99]. 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 Cd
2+ and Pb
2+ (
Figure 5).
Figure 5. Schematic diagram of the strength of HS-mSi@MOF-5 on metal ions at different pH values.
2.2.2. -SO3H
Moradi et al. synthesized and used sulfonated MOFs supported on Fe
3O
4 NPs as a Fenton-like catalyst to remove methyl orange (MO) from water
[59][100]. When the initial concentration is 100 mg·L
−1, the incipient concentration of H
2O
2 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 Fe
3O
4@MIL-100(Fe)-SO
3H degradation will be a wastewater dye removal technique with great promise. Hasan et al. first synthesized three Zr-MOF series materials (UiO-66-SO
3H, UiO-66-NH
2, and UiO-66) for the removal and adsorption of diclofenac sodium (DCF) in water and contrasted these three materials with activated carbon (AC)
[60][101]. 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 -SO
3H, the kinetics and capacity (263 mg·g
−1) of adsorption were significantly enhanced, which may result from the acid–base attraction of -SO
3H in UiO-66-SO
3H and -NH
2 in DCF. Interestingly, however, the opposite trend was observed when UiO-66-NH
2 was utilized as a sorbent. This is primarily due to the alkali-base rejection of -NH
2 in DCF and UiO-66-NH
2, on account of relevant results. Wang et al. immobilized -SO
3H on the surface of MOFs to synthesize sulfonic acid-functionalized MOFs to remove Cd
2+ from aqueous solutions
[61][102]. Cu
3(BTC)
2-SO
3H 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 Cd
2+ 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 Cd
2+ and is simple to regenerate and recycle without suffering a major reduction in Cd
2+ adsorption capacity. Liu et al. focused on the adsorption behavior of UiO-66@mSi-SO
3H, UiO-66@mSi-SH, and UiO-66 on Cd
2+ [62][103]. 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-SO
3H. The theoretical maximum adsorption capacities of UiO-66@mSi-SO
3H and UiO-66@mSi-SH for Cd
2+ are 409.96 and 212 mg·g
−1, respectively. In the presence of -SH and -SO
3H, Cd
2+ in water was replaced by adsorbents to accomplish the aim of getting rid of Cd
2+. -SO
3H has diverse and complex coordination forms, allowing UiO-66-mSi-SO
3H to have an excellent adsorption capacity for removing Cd
2+. Lv et al. synthesized a late-model magnetic nanocomposite with a core-shell structure of Fe
3O
4@UiO-66-SO
3H 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
[63][104]. A considerable affinity for MB may be shown in the synthesized Fe
3O
4@UiO-66-SO
3H, 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, Fe
3O
4@UiO-66-SO
3H 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).
Figure 6. Schematic diagram of selective adsorption of MB by Fe3O4@UiO-66-SO3H and corresponding adsorption mechanism.
2.2.3. -SO4
Kang et al. selected Zr-BTC-NH
2-SO
4 as an adsorbent to capture radioactive Ba
2+ in nuclear wastewater
[64][105] (
Figure 7). By reason of the plentiful -SO
4 groups in the skeleton, which are strong Ba
2+ chelating groups, and the fact that the binding site is completely exposed, in comparison to most adsorbents, Zr-BTC-NH
2-SO
4 has a greater adsorption capacity of 181.8 mg·g
−1. When the concentration of interfering ions is ten times that of Ba
2+, the material still shows excellent selectivity. Most crucially, the course of adsorption is irreversible for Ba
2+, which can availably avert secondary pollution. This work has contributed to the development of adsorbents for the handling of radioactive Ba
2+ from nuclear effluent.
Figure 7. Schematic diagram of experimental setup used for breakthrough experiments.
Peng et al. proposed a Ba
2+ defect concept based on MOFs by introducing -SO
4 functional groups into the pore structure of MOFs
[65][106]. The functionalized MOFs can remove more than 90% of Ba
2+ within 5 min, and the removal efficiency reaches 99% after equilibrium. Notably, the sulfate-based functionalized material showed a high Ba
2+ absorption capacity of 131.1 mg·g
−1, exceeding that of most reportorial adsorbents, and could selectively capture Ba
2+ from outlet water. This
respape
archr offers a fresh viewpoint on environmental remediation and the removal of radioactive Ba
2+ 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
[66][117]. For S-containing groups, -SH, -SO
3H, and -SO
4 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 -SO
3H, 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, -SO
3H is a hydrophilic group, which can increase the water solubility of MOFs. For -SH, it can also produce the same effect as -SO
3H, but the acidity is not that strong, so the electrostatic effect of -SH is weaker than that of -SO
3H. 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.