A systematic review of metal organic frameworks materials for heavy metal removal: synthesis, applications and mechanism (2023)

Natural sources of heavy metals usually occur in the form of volcanic eruptions, rock weathering, biological sources, forest fires, biological processes, and sea salt, etc. Through these processes, heavy metals are introduced into the ecospheres. Natural disasters, climate change, soil matrix, hyporheic exchange and geological factors all have some influence on water environment. For example, natural disasters such as earthquake, hurricane, tornado, flood and tsunami, among which flood is the main source of heavy metal pollution in water environment [8]. However, compared with anthropogenic sources, natural sources of heavy metals are less harmful and can be borne by the ecological environment to a certain extent.

The influence of anthropogenic factors is the main source of heavy metal pollution in ecological environment. The heavy metals are introduced into the environment mainly by i) deposition of atmospheric particulates, e.g., fuel combustion; ii) disposal of sewage sludge and effluents, e.g., industries, emissions from mining, and automobiles; iii) the heavy metals from soil and air are also finally discharged into the waterbodies through rainfall. Anthropogenic sources of different heavy metals are summarized in Table 1. It can be seen that the sources of heavy metals are mainly concentrated in mining [9], [10], smelting [11], [12], battery [13], [14], chemical industry, agriculture and other fields. Among the anthropogenic sources, mining plays a key roleby generating and releasing huge quantities of heavy metals [15]. Smelter emissions of heavy metals are considered theirmain anthropogenic source and constitute about 40∼73% of the total anthropogenic heavy metals emissions [15], [16]. Although the harmfulness of anthropogenic sources is significant, they can also be controlled. Therefore, in the process of environmental protection, in addition to effectively controlling the pollution by existing technologies, human intervention behaviors such as optimizing the process can be used to reduce the possibility of pollutant generation.

The basic principle of electrolytic treatment of heavy metals is to introduce direct current into the electrolytic cell containing heavy metal ions, and achieve the effective removal of heavy metals through oxidation-reduction process at the cathode and anode [18]. Due to the electrochemical performance of metals, metal cation will be deposited in the cathode cell because of accepting electrons and separated from high-concentration solutions to be removed [19]. The corresponding electrode reaction can be as shown in the reaction formula. Electrolysis is often used in heavy metal wastewater generated by the electroplating industry to recover metals, and has achieved good treatment results. Long-term development experience shows that the advantages of electrolysis to remove heavy metals are metal selectivity, low requirements for chemicals, and easy operation, while the disadvantages are high energy consumption, high investment costs and frequent electrode plate replacement [20], [21].

Ion exchange is a technology of reducing the concentration of heavy metals in wastewater by utilizing the exchange reaction of heavy metal ions and ion exchange resins. Ion exchange resin is a kind of polymer material containing ion exchange groups, which can usually be divided into cation exchange, anion exchange and chelate resins. In the actual application process, the ion exchange resin can be reused repeatedly, which improves the utilization rate of the material itself [22]. Compared with other heavy metal removal methods, the ion exchange method has simpler operation, higher removal efficiency and outstanding kinetic performance [23], [24]. However, the disadvantage is that other ions may be introduced during the working process, causing secondary pollution and increasing the difficulty of ions removal in the later stage, which has also become an important factor limiting its development.

This technology refers to a technology that achieves selective separation at the molecular level when molecular mixtures of different particle sizes pass through a semipermeable membrane [25]. The separation principle mainly relies on the pressure difference on both sides of the membrane as the driving force to achieve the purpose of separation, and the separation efficiency is affected by substrate size, membrane pore size, solution concentration, and the applied pressure. At present, the main membrane separation technologies include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), electrodialysis (ED), among which UF and RO have received more attention [26]. In general, membrane separation technology has high efficiency, simple operation, small footprint, and can effectively separate heavy metals, but there are also some limitations, such as membrane fouling, clogging, and low permeability.

The method refers to the removal of heavy metals by adding chemicals into the heavy metal wastewater to convert the dissolved heavy metals into water-insoluble compounds through chemical reactions [27]. In the actual application process, the main operation is to add a precipitant to the wastewater containing heavy metal ions, so that the metal ions can be precipitated and removed by the formation of hydroxides or carbonates with low solubility. To ensure the successful application of this technology, the solubility of metal precipitates needs to be accurately determined [28]. At present, the chemical precipitation method has a relatively mature development, with high efficiency, low investment, and easy operation. However, this technology has a good removal effect in high-concentration heavy metal wastewater, and is not suitable for low-concentration wastewater treatment. In addition, a large amount of chemicals need to be added during the operation of the process, and heavy metals are separated as sediments, which easily leads to secondary pollution.

Adsorption is a physical and chemical separation technology for dissolved pollutants, which mainly refers to the process of removing various pollutants in wastewater by using the physical and chemical adsorption properties of solid adsorbents, and the treatment objects are highly toxic substances and biologically refractory pollutants. Generally, adsorption methods include physical, chemical, and semi-chemical adsorption [29]. In most cases, adsorption is the most effective purification method compared to those inefficient, complex and expensive techniques. The method is also considered preferred for heavy metal removal due to its outstanding flexibility in design, reversibility, and high quality wastewater treatment [30], [31]. The technical characteristics of different purification methods are summarized in Table 2. In general, the development of ion exchange, chemical precipitation, membrane separation and electrolysis is limited due to factors such as secondary pollution, high investment, and energy consumption. In contrast, adsorption method is more simple, economic, environmental protection and energy saving, and has excellent prospects for development.

Activated carbon is one of the adsorbents commonly used to remove contaminants from water. Due to its excellent porous structure, specific surface area and abundant functional groups, it can effectively remove heavy metals. At present, the conversion of cheap and abundant biological resources into activated carbon material has become the focus of research. Singh et al. prepared AC by treating Tamarind wood with sulfuric acid for Pb(Ⅱ) adsorption [45]. The adsorption equilibrium was reached after 50 min, and the saturated adsorption capacity of Pb(Ⅱ) reached 134.22 mg/g. Although the material has considerable adsorption capacity, it lacks selectivity and reproducibility. Sajjadi et al. converted raw pistachio wood wastes into AC by chemical activation, and conducted Hg(Ⅱ) absorption study under the best conditions, reaching the maximum capture capacity of 202 mg/g, but still lacked selectivity and repeatability [46].

Zeolite is a kind of natural porous aluminosilicate mineral with frame structure, which has large specific surface area and unique pore structure. Due to its strong hydrophilicity and electronegativity, its adsorption ability to non-polar organic matter and anionic pollutants in wastewater is insufficient, which limits its application in water treatment. Suazo-Hernandez et al. used nano zero-valent iron to functionalize zeolite, and the removal effect of the adsorbent on As(Ⅴ) was 38.28 mg/g [47]. Araki et al. prepared zeolite hollow fiber by a phase inversion-assisted extrusion method, and the adsorption capacity 47 mg/g in 100 ppm Cd²⁺ solution [48]. In order to improve their adsorption performance, more research has begun to focus on the modification of them.

Clay is generally formed by the weathering of silicate minerals, and is an important mineral raw material. It is mainly divided into three categories: kaolin, montmorillonite and mica, among which bentonite has great cation exchange capacity, strong selectivity and renewable. Compared with other adsorbents, clay has the characteristics of large specific surface area, high plasticity and bond strength, high Zeta potential and corrosion resistance, and has been widely studied in heavy metal removal. Al-Harahsheh et al. chemically modified the clay after washing with sulfuric acid with 3-chloropropyl triethoxylsilane and sodium hydroxide, and the capture amount of the obtained adsorbent was only 13.32 mg/g for Pb(Ⅱ) [49]. Olu-Owolabi et al. used sulphate and phosphate modified bentonite, resulting in a reduction in specific surface area. Although the adsorption capacity of Zn²⁺ increased compared with that of unmodified, it was only 98.04 mg/g [50].

In recent years, agricultural by-products have been widely developed in heavy metal removal applications due to their porous structure, large specific surface area and abundant surface active groups. These include corn bracts, orange husks, rice husks and coconut husks. The excellent surface properties of agricultural by-products create conditions for its modification. Therefore, the modification of agricultural by-products have been widely reported in recent years. Villen-Guzman et al. modified lemon leaves to absorb Ni(Ⅱ) ions, with a maximum adsorption capacity of 36.74 mg/ g. Geupta et al. prepared magnetic nano-adsorbent by combining Fe₃O₄ nanoparticles with orange peel, with a capture capacity of 71.43 mg/g for Cd(Ⅱ).

Metal-organic frameworks (MOF), an inorganic-organic hybrid materials with porous crystalline structures, large specific surface area, sufficient active sites, structural diversity and easy modification, formed by self-assembly of metal ions (or clusters) and organic ligands [51], [52]. According to different material structures, the most studied types of typical MOFs can be divided into: (1) University of Oslo (UiO) series; (2) Zeolitic Imidazolate Frameworks (ZIF) series; (3) Porous Coordination Network (PCN) series; (4) Materials of Institute (MIL) series; (5) Isoreticular Metal-Organic Frameworks (IRMOF) series, etc. Fig. 1 intuitively shows the development of MOF in removing heavy metals from wastewater. The concept of MOF was first proposed by Robson et al in 1989, and then Yaghi's team successfully synthesized the first MOF in 1995 [53]. Subsequently, MOF has been widely used in detection [54], catalysis [55], sensing [56], gas adsorption [57] and drug delivery [58] after Li et al's research [59]. Until 2009, the water stability of MOF was improved, and then studies on MOF removing heavy metals from wastewater began to be widely reported. Ji et al. modified MOF-808 by organic ligands with -SH functional group, the adsorbent has excellent adsorption capacity for Hg(Ⅱ), not only has a large adsorption capacity (977.5 mg/g), but also can show outstanding selectivity and repeatability to Hg(Ⅱ) [60]. Wang et al. synthesized a novel Zr-MOF by one-pot method from zirconium chloride and mercaptosuccinic acid [61]. The adsorbent can capture Hg(Ⅱ) and Pb(Ⅱ) in wastewater by chelation and electrostatic interaction, and can reduce Hg(Ⅱ) and Pb(Ⅱ) below international standards. The maximum adsorption capacity of Hg(Ⅱ) and Pb(Ⅱ) under optimum conditions was 1080 and 510 mg/g, respectively. Goyal et al. doped Fe in HKUST-1 MOF to obtain a bimetallic MOF adsorbent for removing Pb(Ⅱ), with a maximum capture capacity of 565 mg/g [43]. Tripathy et al. synthesized a composite material MU-2 by combining Fe3O4 wrapped with citric acid and UIO-66-NH₂, which has outstanding performance in removing Cr(Ⅵ), and the maximum capture capacity is as high as 773.3 mg/g [45]. As shown in Table 3, compared with traditional adsorbents, MOFs have shown outstanding performance in the treatment of heavy metals, with obvious competitive advantages and broad development space in the future.

Metal-organic framework as an emerging adsorbent for the removal of heavy metals from wastewater has shown surprising competitive advantages over traditional adsorbents, which means that MOF will occupy a dominant position in the field of water purification. In order to have a clearer understanding of MOF in water purification, the application of MOF adsorbents in heavy metal pollution was reviewed in this paper, and the synthesis, modification and adsorption mechanism of MOF was described in detail, aiming to provide a feasible reference for the design of MOF and the optimization of adsorption application, and expand the development of MOF in removing pollutants from wastewater (Fig. 2).