ZIF‑8 and Its Derivative Adsorbents for Heavy Metal Removal in Water: A Review

Abstract

Zeolite imidazolium framework-8 (ZIF-8) is a typical MOF material, which has the advantages of high specific surface area, high crystallinity and polymetallic sites, and strong thermal and chemical stability. The excellent adsorption properties of ZIF-8 are also utilized to synthesize new adsorbent materials to enhance their adsorption performance, reducing production cost and improving recyclability. Considering the high feasibility and wide application of ZIF-8 and the composites adducted with ZIF-8 for heavy metal ion removal in aqueous solutions, this paper focuses on the specific synthesization, application, and development of ZIF-8 and its derivatives, as well as the key removal mechanism for heavy metal adsorption. This paper provides a better understanding and new ideas for effective preparation of ZIF-8 and ZIF-8-aided adsorbents and their successful utilization in heavy metal wastewater purification.

1. Introduction

Metal–organic frameworks (MOFs) are new organic–inorganic hybrid materials with large pore volume, highly ordered pores, low density, a high surface area, and favorable chemical and thermal stability. The most attractive feature of MOFs is the binding ability for various metals and organic ligands to produce many novel materials with different crystal structures and chemical compositions. Zeolitic imidazolium frameworks (ZIFs), classified as an important and large kind of MOFs, are highly ordered porous solids bridged by inorganic metal ions with imidazolium ligands in a tetrahedral environment. Their topologies and porous structures can be controlled by the imidazole unit types. They are a series of three-dimensional self-assembled products with nitrogen-containing organic ligands and divalent metal ions in tetrahedral coordination porous materials. ZIFs are called zeolite-like imidazolium ester skeleton materials because the angle of the metal–N–N–metal substructure formed after coordination has a similar 145° coordination angle and topology to that of Si–O–Si in zeolite molecular sieves, while a metal ion (i.e., Zn²⁺ or Co²⁺) is employed instead of silicon and imidazolium ligands are used in place of the oxygen bridges in zeolite. Suffix numbers are adopted to identify various ZIFs according to their topological crystal structures. Among them, ZIF-8 (Zn²⁺) and ZIF-67 (Co²⁺) are the most widely studied and applied ones.

Zeolitic imidazolate framework-8 (ZIF-8) is a particular kind of ZIF which is coordinated by Zn²⁺ and 2-methylimidazole with a pore size of 11.6 Å–3.4 Å. Early in 2006, Chen’s research group first synthesized a framework compound with identical composition and specific structure, named Metal Azolate Framework-4 (MAF-4). In fact, MAF-4 and ZIF-8 are structurally identical. Both are composed of Zn²⁺ ions coordinated with 2-methylimidazole ligands, forming a three-dimensional porous framework with the same topological structure and pore characteristics. Based on their findings, this new product was further systematically studied and officially named as ZIF-8. With the advantages of a large specific surface area, adjustable pore size, and good chemical stability, ZIF-8 has been widely utilized in gas storage and separation, drug delivery, selective extraction, and adsorptive treatment; especially, it also performs well in water pollutant removal. ,

In recent years, with the rapid development of construction and manufacturing, heavy metal ions are increasingly discharged into the environment through various industrial wastewaters. Heavy metals lead to much serious water pollution due to their toxicity, persistence, and nonbiodegradability. Heavy metals in aquatic environments mainly include copper (Cu), lead (Pb), arsenic (As), mercury (Hg), chromium­(Cr), etc. These heavy metals pose a major threat to the ecological environment, human health, and life safety. − Therefore, the removal of toxic heavy metals from aqueous solutions is essential to human health and society’s sustainable development. Currently, the typical techniques for removing heavy metal ions from aqueous solutions include chemical precipitation, ion exchange, electrodialysis, coagulation and flocculation, and adsorption, while adsorption is extensively applied because of its high efficiency, low cost, and easy operation.

Recent studies have highlighted ZIF-8 and its derivatives as highly promising adsorbents for heavy metal wastewater treatment. Pure ZIF-8, characterized by its large specific surface area and abundant imidazole functional groups, achieves efficient heavy metal adsorption through both physical and chemical interactions. For instance, it demonstrates rapid copper ion adsorption with stable performance across a wide pH range.

Derivatives of ZIF-8 exhibit further enhanced properties. Composites such as ZIF-8/activated carbons not only retain high adsorption capacity but also improve mechanical stability, making them more suitable for practical applications. Thiol-functionalized ZIF-8 shows particularly strong affinity for mercury ions due to the high selectivity of thiol groups, significantly boosting adsorption performance. Additionally, optimizing synthesis conditions to control the particle size and pore structure can further enhance adsorption kinetics. For example, tailored ZIF-8 variants with smaller particle sizes and tuned porosity demonstrate improved lead ion removal efficiency.

Despite challenges such as high synthesis costs and unresolved mechanistic insights into certain composites, ZIF-8-based materials demonstrate much potential for heavy metal removal. Development of economical and efficient adsorptive materials in consideration of ZIF-8 assistance is very feasible and favorable. Currently, there is limited research on ZIF-8 and its derivatives for heavy metal wastewater purification, and systematic study is urgently needed.

This study is focused on ZIF-8 and ZIF-8-modified adsorbents in heavy metal wastewater treatment, mainly including specific synthesization, application and improvement, and key removal mechanisms. This work not only provides a detailed and systematic instruction of ZIF-8 and its modified composites but also presents a promising candidate for the effective treatment of heavy metal wastewaters.

2. ZIF-8

2.1. Preparation

ZIF-8 [Zn­(2-mim)₂] consists of zinc (Zn²⁺) ions coordinated to a 2-methylimidazole (2-mim) ligand. As shown in Figure , in this framework, each zinc ion is tetrahedrally coordinated with four bonds of the imidazolate ligand. These zinc ions are interconnected through the 2-methylimidazole ligand to form a strong and extensive three-dimensional network. ,

1.

Crystal structure of ZIF-8: Zn (polyhedral), N (sphere), and C (line). Reproduced from ref with permission. Copyright 2024, Inorganic Chemistry Communications.

The main synthesis methods for ZIF-8 include room temperature stirring, solvothermal, microwave hydrothermal, emulsification, ultrasonic-assisted, and mechanochemical methods (Table ).

1. Main Synthesis Methods of ZIF-8 .

The solvothermal method is the most widely used method for ZIF-8 synthesis. It is easy to operate, but there is a big limit of long reaction time, high energy consumption, and waste of solvent. Solvothermal synthesis in methanol at room temperature could largely reduce energy consumption and highly improve ZIF-8 structural properties and particle size, while low production yield is a drawback. ZIF-8 preparation in methanol at room temperature can be carried out without any auxiliary stabilizers because the synthetic reaction is facilitated by molecular interactions between the reagents and the solvent, which has the ability to give hydrogen bonding, thus facilitating ligand deprotonation and Zn²⁺ coordination.

The room temperature stirring method to synthesize ZIF-8 has the advantages of easy operation, low equipment requirement, low cost, and the ability to optimize the shape, size, specific surface area, and structure of the samples by varying the stirring time, zinc source, and molar ratio, while there are still defects in the irregular crystalline shape of the product and low yields.

The microwave hydrothermal method has a shorter reaction time and a more efficient and controllable reaction. However, the high equipment cost and large initial investment hinder its application, as well as the high technical and professional operation and equipment maintenance.

The emulsion method offers advantages in morphology control, creating ZIF-8 particles with relatively uniform sizes (e.g., spherical and irregular shapes) via a stable oil-in-water system. Surfactants in the process may also modify ZIF-8 surfaces, potentially enhancing the affinity for heavy metal adsorption. However, it demands precise control of oil, surfactants, and water to maintain emulsion stability, and extra steps are needed to remove surfactants/unreacted substances, increasing process complexity and environmental risks.

Ultrasonic assistance shortens reaction times via cavitation effects, promoting reactant mixing and accelerating ZIF-8 crystallization. This yields smaller, uniformly dispersed particles (e.g., dodecahedral shapes) with increased specific surface area. Yet, it relies on costly ultrasonic equipment with energy and noise pollution risks. Improper ultrasonic parameter control (power and time) may also damage ZIF-8 structures.

The mechanochemical method offers a solvent-free or reduced-solvent approach, significantly minimizing the environmental impact. By the use of ball-milling mechanical force to drive solid-state reactions, this technique enables the synthesis of ZIF-8 with tailored structures and enhanced stability. However, the process demands precise control over critical parameters, such as ball-to-powder ratios, rotation speeds, and milling times, requiring specialized operational expertise. Additionally, the initial products often exhibit irregular morphologies, necessitating postsynthesis treatments to optimize their properties. Prolonged milling also poses a risk of damaging the ZIF-8 crystal framework, highlighting the need for careful optimization.

In addition, other synthetic methods include the high-temperature and high-pressure solvent-free method, microwave-assisted method, and continuous flow method. They have also been explored and applied to the preparation of ZIF-8.

2.2. ZIF-8 Adsorption on Heavy Metals

Compared to traditional adsorbents (zeolite, activated carbon, biochar, chitosan, silica, clay minerals, and graphene), ZIF-8 has much potential in removing heavy metal ions from contaminated waters due to its excellent adsorption capacity (Table ).

2. Comparison of Heavy Metal Adsorption Capacity for Different Adsorbents.

contaminant	materials	initial concentration (mg/L)	pH	contact time (min)	adsorption capacity (mg/g)	ref
Cu(II)	ZIF-8	120	5.0	90	378.5
	rice husk	128		20	2.30
	CS/SA/SiO	100	6.0	240	47.50
	PANI@APTS-Fe3O4/ATP-0.7	100	5.0	15	142.5
	SH/GG@MIL-100	100	6.0	300	311.48
Pb(II)	ZIF-8	100	6.0	15	475.54
	PANI@APTS-Fe3O4/ATP-0.7	100	5.0	15	270.27
	A-MZF	100	3.0	300	160
	IDS-Mt5	300	8.0	10	214.73
	Fe3O4@UiO-66-PDA	50	5.0	800	121.42
Cr(VI)	ZIF-8	50	5.0	45	34.3
	ZnO NPs	20	6.0	35	48.50
	AO–CS	200	2.0	10	130.0
	MAF-LDOs	100	3.0	75	26.76
	UiO-66-(OH)2	20	3.0	200	18.20
As(V)	ZIF-8	20	7.0	480	81.40
	CS-FMO	40	3.0	1200	26.65
	Zr–Mn	10	7.0	900	52
	MIL-100(Fe)/1%GO-400	10	6.0	120	26.55
Cd(II)	ZIF-8	100	7.0	15	209.79
	MRBC	100	7.0	480	49.93
	MgAl-LDH	50	7.0	60	4.18
	MCTS@GO@DIIP	50	6.0	30	39.35
	Fe3O4@UiO-66-Lcys	50	7.0	420	430.89
Hg(II)	ZIF-8	20	6.5	90	1271.27
	K–HAS	120	4.0	480	128.4
	Fe–Mn@SCAs	100	6.0	90	324.42
	PANI@SA-SNM	50	4.0	120	352.76
	NH2-MIL-101@PES	100	5.0	24 h	237.45

As shown in Table , ZIF-8 is superior in adsorbing heavy metals from aqueous solutions. Under approximate experimental conditions and compared with other adsorbents, ZIF-8 performs much better and presents a significant adsorption capacity for various heavy metals. Short adsorptive time, mild reaction conditions in neutral or weak acid, easy control, and convenient operation promote ZIF-8 to be very helpful on heavy metal ion adsorption. However, when compared to some advanced adsorbents like MIL-series MOFs or UiO-66, ZIF-8 exhibits several notable limitations: (1) lower adsorption selectivity due to lack of targeted functional groups, (2) reduced stability under extreme pH conditions, and (3) relatively higher synthesis costs. To address these challenges, strategic modifications of ZIF-8 through functionalization or the development of novel ZIF-8-based composite materials could simultaneously enhance its selectivity, chemical stability, and heavy metal removal efficiency while optimizing production costs.

3. Adsorbents Modified with ZIF-8

Though considered to be an excellent adsorbent, traditional ZIF-8 materials are easy to aggregate in water, resulting in an increment of particle size and reduction of interfacial area and thus low adsorption performance. Otherwise, it is difficult to separate ZIF-8 from an aqueous solution, which further limits its regeneration and reuse. Finally, a large bandgap of ZIF-8 also makes it less responsive to visible light and thus affects the efficiency of solar irradiation. Therefore, it is very necessary to exploit new materials with ZIF-8 to overcome its disadvantages and make the best use of the advantages. Herein, ZIF-8 is studied as a modifier to enhance the adsorption performance of other adsorbent materials and to improve heavy metal ion removal in wastewaters.

3.1. Classification of ZIF-8-Aided Adsorbents

According to the specific chemical composition, there are two sorts of ZIF-8 composites: organic- and inorganic-participated composites. Organic substrates for ZIF-8 addition usually include sodium alginate, chitosan, cellulose, and other organic parts. Inorganic constituents with ZIF-8 doping are metal parts and inorganic nonmetallic parts, in which metal parts are mainly magnetic metals and nonmagnetic metals based on their magnetic properties (Table ).

3. Classification of ZIF-8-Aided Adsorbents.

Based on current research, the most used substrates with ZIF-8 modification are organic materials. ZIF-8 organic composites have been studied extensively, mainly because of the wide variety of organics and the large amount of carbon contained in the organic linkers. From SEM images of such composites, it could be seen that the main body of the product is based upon the organic material and its surface is basically smooth, while a large number of pore-like structures are present after the addition of ZIF-8. Then, its surface became rough, indicating that a large amount of ZIF-8 was successfully grafted. Deng et al. employed a direct in situ synthesis strategy to fabricate multisite adsorption functional composites, termed as an aminated chitosan@ZIF-8 (AmCs@ZIF-8), for the effective removal of Cu­(II) (ca. 94%) and Congo red (CR) (almost 100%).

Many metals and their compounds, including Au, Ag, Pt, Ru, Fe₃O₄, GaN, CdTe, etc., have been successfully complexed with ZIF-8. For the kind of ZIF-8 metal composites, the metal nanomaterials were nearly spherical particles, and the composites after the addition of ZIF-8 had irregular columnar, spherical, and cubic structured particles with rough surface. Among the ZIF-8 metal composites, magnetic metal composites have attracted much attention. Due to the difficulty of separating ZIF-8 from aqueous solution and the difficulty of regenerating it, the combination of magnetic metal can effectively enhance the separation performance of adsorbents and improve the adsorbents’ recyclability. Xiong et al. prepared an adsorbent with a specific core–shell structure using a layer-by-layer self-assembly method, and the product consisted of a magnetic core, a carbon-based inner shell, and a ZIF-8 outer shell. It can selectively adsorb and enrich Congo red (CR) and Cu (II) adsorption in complex wastewater systems.

Besides organic and metallic composites, ZIF-8 inorganic nonmetallic composites are also a large group; the matrix materials include graphene, diamond, and fly ash. According to their SEM images, the inorganic nonmetallic materials show a lamellar structure with a smooth surface before ZIF-8 modification. However, the composites after ZIF-8 addition presented a lamellar structure dotted with nanoparticles, indicating the successful growth of ZIF-8 on the surface of the matrix material. Mirzaei et al. produced a series of ZIF-8/OND hybridized nanostructures with different oxidized nanodiamond (OND) contents, and due to the synergistic effect between OND and ZIF-8, the adsorption capacity of ZIF-8 hybridized with OND was remarkably increased by nearly 4 orders of magnitude.

3.2. Synthesis

In situ synthesis is a simple and commonly used method, but it requires special properties of the raw materials. Only based on certain specified functional groups or high surface properties of the component materials, the even distribution of biochemical reaction or interaction can be spontaneous to form stable complexes without further treatment. Feng et al. successfully synthesized ZIF-8@ALG composite hydrogel microspheres under in situ conditions using sodium alginate as a carrier, and zinc and methylimidazole were sequentially added. The steps are shown in Figure .

2.

(A) Hydrogel synthesis principle; (B) ZIF-8@ALG composite hydrogel preparation process. Reproduced from ref with permission. Copyright 2024, Colloids and Surfaces A: Physicochemical and Engineering Aspects.

Activated carbon is commonly used as an inexpensive adsorbent material, and activation is crucial to produce activated carbon, which includes physical activation and chemical activation. During physical activation, the raw material is first carbonized, and subsequently, the carbonized material is activated twice by steam or carbon dioxide. For chemical activation, the raw material is impregnated with an activator, and then the impregnated material is heat-treated in an inert atmosphere. The most widely used activators are zinc chloride (ZnCl₂), phosphoric acid (H₃PO₄), potassium hydroxide (KOH), and sodium hydroxide (NaOH). Typically, after activation, the products need to be washed to remove the excess activator. As zinc chloride (ZnCl₂) is a commonly used activator, the raw materials for the synthesis of ZIF-8 are Zn²⁺ and 2-methylimidazole; thus, a new strategy is suggested to optimize the use of Zn resources. A novel in situ synthesis method for the ZIF-8 composite is based on in situ activated carbon preparation and then further ZIF-8 growth on the produced carbon with the residual Zn²⁺ to get a new ZIF-8 composite.

Other modification methods for ZIF-8 derivatives are surface modification, template synthesis, polymer modification, in situ growth in gels, acoustic chemistry microreactor synthesis, and stepwise growth (LPE). Surface modification uses surfactants or coupling agents to form functional layers or introduce specific groups, with crystal seeds refining particles. , Template synthesis employs soft (e.g., block copolymers) or hard (e.g., SBA-15) templates to create hierarchical or ordered pores after template removal. , Polymer modification involves in situ polymerization or blending to form cross-linked networks or composites with enhanced properties. , In situ growth in gels produces beads with ZIF-8 grown, offering good stability and adsorption performance. − Acoustic chemistry uses ultrasound to shorten reaction time and generate uniform particles. Microreactor synthesis enables precise control for consistent, small ZIF-8. Stepwise growth builds layers to customize structures for targeted metals.

3.3. Adsorption Performance

ZIF-8 derivatives outperform many other adsorbents in heavy metal adsorption. They inherit ZIF-8’s porous structure and, after carbonization, gain a more developed pore system (micropores and mesopores) and larger specific surface area, providing more adsorption sites for heavy metal ions with varying hydrated radii (e.g., Cu²⁺, Pb²⁺, Hg²⁺). Their adsorption capacities are often several times higher than those of traditional adsorbents. Their interconnected pores accelerate mass transfer, leading to faster adsorption kinetics and shorter equilibrium times. Additionally, retained nitrogen-containing groups from ZIF-8 and oxygen-containing groups formed during carbonization enhanced selectivity for specific heavy metals. Importantly, these derivatives show better stability than pristine ZIF-8 in harsh acidic/alkaline environments; their carbonaceous structure resists dissolution, reducing Zn²⁺ release and maintaining efficiency over cycles. They are promising for heavy metal remediation, aligning with the enhanced performance of the ZIF-8 composites (Table ).

4. Adsorption Capacity of ZIF-8-Based Adsorbents for Heavy Metal Removal in Water.

contaminants	materials		initial concentrations (mg/L)	pH	contact time	adsorption capacity/adsorption efficiency (mg/g)		ref
	raw materials	ZIF-8 based composites				before	after
Cu(II)	Fe3O4	Fe3O4@ZIF-8	40	5.0	24 h	5.7	305
	GO	ZIF-8@GO-7.83%	40	6.0	120 min	5	380
	CS	CS-ZIF-8–1:2	100	5.0	5 h	16	38
	MS	ZIF-8/MS	30	5.0	25 min	50	140
Pb(II)	Fe3O4	Fe3O4@ZIF-8	35	5.0	24 h	14.4	714.7
	SF	SFZIF-8(1:15)	100	7.0	12 h	75	380
	GO	ZIF-8@GO	25	5.0	100 min	273	356
	MMT	ZIF-8/MMT	400	5.0	120 min	79	297
	GO	ZG3	100	5.5	10 min	150	500
	BC	SA@ZIF-8/BC	200	5.0	90 min	126.1	300.3
	CS	CS-ZIF-8–1:2	100	5.0	120 min	23	38
	CS	ZIF-8@CS/HAP	200	6.0	50 min	98.74	291.2
	MS	ZIF-8/MS	30	5.0	25 min	40	150
Cr(VI)	Fe3O4	Fe3O4@PmPD(Z)	200	2.0	14 h	37.5	255
	Mg(OH)2/GO	ZIF-8/NH2/Mg(OH)2/GO	10	7.0	60 min	1.82	4.88
	PAN	ZIF-8@ZIF-8/PAN-m3	20	2.0	90 min	10.19	39.68
	CL-2	CLZ-2	20	5.0	30 min	225	378
As(V)	Fe3O4	Fe3O4@ZIF-8	50	3.0	24 h		76
	CS	ZIF-8/CS-N	20	6.0	60 min	15.6	39.2
As(III)	Fe3O4	Fe3O4@ZIF-8	27	8.0	4 h	8	100
Ni(II)	FA	ZIF-8/FA	100	5.5	4 h	1.6 %	36.1%
	PAA/PVDF	PAA/ZIF-8/PVDF-0.05	50	5.5		16.54	219.09

After doping, the adsorption performance of ZIF-8-aided composites on heavy metal removal is significantly improved compared to that without modification. As listed in Table , the adsorptive capacity/efficiency of the composites after ZIF-8 addition was 2–80 times higher than the unmodified ones due to remarkably strengthened matrix materials, while the best modification effect is gained on Cu­(II) adsorption. The enhanced action of the adsorbent after ZIF-8 addition is different based on various substrates. The best modification effect is gained with a metal material’s understructure, and the enhancement effect on organic materials is fairly good. Thus, exploring new base materials and optimizing adsorption performance on raw materials by ZIF-8 doping are still worthy of study.

The definite improvement on ZIF-8-aided composites is greatly attributed to their large specific surface areas, abundant functional groups, and surface-active sites. , Moreover, ZIF-8 can effectively prompt the dispersibility of matrix materials and play a synergistic role in heavy metal adsorption. And the adducted composites have physicochemical properties such as multiple functional groups, high average pore size, and strong negative charge. For example, the experimental results of Ren et al. showed that the modified composites had higher adsorption capacity and better removal rates for three different pollutants by increment of the functional groups and surface active sites. Yang found that the enhanced adsorption of Pb²⁺ by ZIF- 8/MMT composites was mainly attributed to the homogeneous dispersion of ZIF-8 on the product surface, the enhanced interaction of charges between the two materials, and the improvement of the pore structure.

4. Adsorptive Mechanism

The adsorption mechanism of ZIF-8 and its modified composites for heavy metal removal is mainly physical adsorption (electrostatic attraction) and chemical adsorption (ligand interactions and ion exchange) (Figure ).

3.

Adsorption mechanism of ZIF-8 and its modified composites on heavy metal removal.

4.1. Electrostatic Attraction

In aqueous solution, due to the protonation of the N–, –NH–, and –NH₂ groups of the imidazolate ligands in the composites, electrostatic interactions naturally occur for heavy metal ion adsorption. According to Hao’s research, during the adsorption of Cr­(VI), under the condition of strong acid (pH = 2), the emergent –NH–⁺ was electrostatically attracted to the negatively charged HCrO₄ ^–, and thus, adsorption was achieved.

pH of the original solution also plays a crucial role in electrostatic attraction as it has a significant effect on the surface charge of the composite. The isoelectric point (pH_ZPC) of the typical ZIF-8 is close to 9.6, but the isoelectric point of ZIF-8 composites is about 1.5–4. ,,, When the solution pH is greater than the pH_ZPC of the material, the surface of the material is negatively charged, which is very favorable for positively charged metal ion adsorption. Hence, it performs well for ZIF-8 composites on heavy metal adsorption in neutral or weak acid solutions. As the result of Kim et al., the surface of SFZIF-8 was negatively charged at pH values greater than 3, whereas Pb­(II) was present as Pb²⁺ and/or Pb­(OH)⁺ at pH values less than 9.5, suggesting an electrostatic attraction between SFZIF-8 and Pb­(II).

4.2. Ligand Interaction

During adsorption, various functional groups in ZIF-8 composites are helpful through coordination with heavy metal ions. The functional groups involved are N-containing functional groups (N–, –NH–, and –NH₂) on 2-methylimidazole in ZIF-8 and oxygen-containing functional groups (−OH, –COOH) in the matrix material. The coordination equation is as follows [–]:

I. Hydroxyl coordination

$$-\mathrm{O}\mathrm{H}+{\mathrm{M}}^{2+}\to -\mathrm{O}-\mathrm{M}+{\mathrm{H}}^{+}$$ 1

II. Carboxyl coordination

$$-\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{M}}^{2+}\to -\mathrm{C}\mathrm{O}\mathrm{O}-\mathrm{M}+{\mathrm{H}}^{+}$$ 2

III. Amino coordination

$$-{\mathrm{N}\mathrm{H}}_2+{\mathrm{M}}^{2+}\to -\mathrm{N}-\mathrm{M}+{\mathrm{H}}^{+}$$ 3

After M²⁺ adsorption by the ZIF-8 composite, the binding energy of the CN and C–N bonds of 2-methylimidazole in the ZIF-8 shell layer increased due to the coordination reaction between M²⁺ and the imine group of 2-methylimidazole in the ZIF-8 shell layer. In aqueous solution, ZIF-8 was hydrolyzed from Zn–N to Zn–OH and N–H. Upon adsorption of heavy metal ions on the surface of the composite material, a new peak formed by the movement and reduction of Zn–OH as well as the disappearance of −C–O, which confirmed the coordination reaction of the heavy metals with the hydroxyl groups on the surface of the adsorbent. After adsorption of Pb­(II) on ZIF-8@GO, the oxygen-containing functional groups all showed a decrement (the hydroxyl group had the most reduction), confirming that the oxygen-containing functional groups in ZIF-8@GO had liganded with Pb­(II).

4.3. Ion Exchange

Based on ICP-OES analyses, the release of Zn (II) from ZIF-8 into solution indicates an ion exchange between heavy metal ions and ZIF-8 during adsorption. The ion exchange equation is ()­

$${\mathrm{Z}\mathrm{n}(\mathrm{m}\mathrm{i}\mathrm{m})}_2+{\mathrm{M}}^{2+}\to {\mathrm{M}(\mathrm{m}\mathrm{i}\mathrm{m})}_2+{\mathrm{Z}\mathrm{n}}^{2+}$$ 4

After Cu²⁺ adsorption on the ZIF-8@ALG composite hydrogel, the Zn–N peak of the ZIF-8@ALG composite disappeared, and the Cu–N peak appeared. Therefore, this proved that ion exchange might occur during the adsorption process. The results showed that the ZIF-8@ALG composites could effectively adsorb Cu²⁺, and there was a loss of Zn²⁺ during the adsorption process.

Recent advances have further discussed the synergistic interplay between ZIF-8’s structural and chemical properties, elucidating their collective role in enhancing heavy metal adsorption and effectively translating theoretical understanding into practical applications:

4.3.1. Pore Confinement Effect

ZIF-8’s uniform micropores (3.4–11.6 Å) enable size-selective adsorption of heavy metal ions through pore confinement. Smaller hydrated ions (e.g., Pb²⁺, ∼4.01 Å) diffuse more easily into ZIF-8’s pores, where van der Waals forces and spatial confinement enhance adsorption, while larger complexes are excluded. Modified composites (e.g., ZIF-8@SiO₂) with tailored mesopores extend selectivity to larger ions such as Cr₂O₇ ^2–, making them suitable for multicomponent wastewater treatment. ,

4.3.2. Competitive Exchange between Zn²⁺ and Heavy Metals

Beyond ion exchange, Zn²⁺ in ZIF-8 competitively interacts with heavy metals based on imidazole binding affinities. High-affinity ions (e.g., Cu²⁺, logK≈10.8) displace Zn²⁺ more readily than low-affinity ones (e.g., Pb²⁺, log K ≈ 8.2). pH modulates this process: acidic conditions weaken Zn²⁺-ligand bonds, enhancing exchange, while alkaline conditions stabilize the framework. ,

4.3.3. DFT Calculation

Density functional theory (DFT) calculations reveal atomic-level adsorption mechanisms in ZIF-8. Hg²⁺ shows stronger binding (−5.2 eV) than Pb²⁺ (−3.8 eV) due to shorter Hg–N bonds (2.1 vs 2.3 Å). Charge density maps confirm electron transfer from imidazole to metals, enhanced by –SH modification (Hg²⁺: −0.35 e^– vs – 0.21 e^–). Diffusion simulations demonstrate size selectivity, with Pb²⁺ facing a lower energy barrier (0.4 eV) than Ca²⁺ (0.6 eV).

Together, these mechanismselectrostatic attraction, ligand coordination, ion exchange, pore confinement, competitive displacement, and DFT-revealed atomic interactionswork synergistically to enable efficient heavy metal adsorption, guiding the design of high-performance, targeted adsorbents.

5. Development and Prospect

ZIF-8 has the advantages of a large specific surface area, high porosity, abundant surface active centers, stable crystal structure, etc., and shows excellent adsorption performance on heavy metal ion removal in wastewaters. Due to the high cost of pure ZIF-8, the preparation of ZIF-8-aided adsorbents in other matrix materials can effectively improve the adsorption capacity and operational cost. The classification and preparation of ZIF-8 and ZIF-8-adducted composites, as well as the adsorption mechanism for heavy metal ions, are discussed and elucidated in this work. This study hopes to give a better understanding and in-depth exploration for ZIF-8 and its modified composites in heavy metal wastewater treatment.

ZIF-8 composites maintain the properties of the pristine material but also exhibit enhanced performance due to synergistic effects, which has an immeasurable application prospect in heavy metal wastewater treatment. However, several key challenges need to be addressed for their potential utilization:

• (1)Material Optimization: Exploring ZIF-8 composites with precisely controlled size, composition, dispersion, and spatial distribution to maximize adsorption efficiency is necessary.
• (2)Cost-Effective Synthesis: The development of innovative, low-cost synthesis methods using economical matrix materials is crucial. Particularly, the utilization of residual Zn²⁺ from activated carbon production presents a sustainable approach for in situ preparation of ZIF-8/activated carbon composites.
• (3)Practical Applications: Intensive study is required to validate the performance of ZIF-8 composites in real wastewater systems, including their potential application in treating complex effluents containing pharmaceuticals, dyes, antibiotics, and pesticides.
• (4)Regeneration and Recycling: Effective strategies for the regeneration and reuse of exhausted ZIF-8 composites must be developed to prevent secondary pollution and improve economic viability.
• (5)Mechanistic Understanding: Advanced theoretical calculations (density functional theoryDFT, radial distribution functionRDF) and molecular simulations (molecular dynamicsMD, Monte CarloMC) should be employed to gain deeper insights into the adsorption mechanisms and guide the rational design of improved ZIF-8 composites.

Addressing these challenges will significantly advance the practical implementation of ZIF-8-based materials in wastewater treatment, contributing to more sustainable and efficient water purification technologies.

The authors declare no competing financial interest.