Preparation of bimetallic Pb/Zn metal-organic framework @graphene oxide composite and its adsorption performance for Pb(II) and Zn(II)

Zhuannian Liu; Mohammad Fahim Bakhtari; Xiaogang Han

doi:10.4491/eer.2024.198

Environ Eng Res > Volume 29(6); 2024 > Article

Liu, Bakhtari, and Han: Preparation of bimetallic Pb/Zn metal-organic framework @graphene oxide composite and its adsorption performance for Pb(II) and Zn(II)

Research

Environmental Engineering Research 2024; 29(6): 240198.

Published online: April 26, 2024

DOI: https://doi.org/10.4491/eer.2024.198

Preparation of bimetallic Pb/Zn metal-organic framework @graphene oxide composite and its adsorption performance for Pb(II) and Zn(II)

Zhuannian Liu^†

, Mohammad Fahim Bakhtari, Xiaogang Han

College of Geology and Environment, Xi’an University of Science and Technology, Xi’an, 710054, China

^†Corresponding author: E-mail: zhuannianliu@163.com, Tel: +8613892803449

Received April 2, 2024 Revised April 23, 2024 Accepted April 25, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The removal of harmful heavy metals from water bodies has emerged as a major concern owing to ecological challenges and the associated threats and dangers they pose. In this study, we used graphene oxide (GO) as an adsorbent to absorb Zn²⁺ and Pb²⁺ in simulated wastewater. The Pb²⁺and Zn²⁺ adsorbed on the surface of GO were utilized as metal ion centers, and benzene-1, 4-dicarboxylic acid (BDC) served as an organic linker to synthesize a bimetallic Pb/Zn-MOF@GO composite via the hydrothermal method. Pb/Zn-MOF@GO composite was characterized using SEM, XRD, FTIR and BET analytical methods. The Pb/Zn-MOF@GO shows a higher BET surface area of 460 m² g⁻¹ compared to that of Pb-MOF@GO (296 m² g⁻¹) and Zn-MOF@GO (408 m² g⁻¹). The maximum adsorption capacities of Pb/Zn-MOF@GO composite material for Zn²⁺ and Pb²⁺ were 114.68 and 132.62 mg L⁻¹, respectively. The adsorption kinetics of the composite materials for Zn²⁺ and Pb²⁺ follow the pseudo-second order kinetic model, and the adsorption isotherm best fits the Langmuir model. The adsorption mechanism was governed by hydrogen bonding, and the −C=O reaction was also involved in the adsorption process. The findings of this study provide new insights into water pollution and resource utilization.

Keywords: Adsorption, Graphene oxide, Heavy metal, Metal organic framework, Lead-zinc deposit, Wastewater

Graphical Abstract

Keywords: Adsorption, Graphene oxide, Heavy metal, Metal organic framework, Lead-zinc deposit, Wastewater

1. Introduction

Heavy metals are highly toxic and can accumulate in animals and plants. This accumulation can cause chemical reactions in the human body, posing a significant threat to human health [1, 2]. The main sources of heavy metal pollution is industrial activities such as manufacturing electronic equipment, lead-containing batteries, leather products, and mining [3]. China is one of the largest producers and consumers of metals/metalloids, which has contributed to the creation of approximately 1,500,000 hectares of barren land in China. The reserves of lead and zinc within China amount to 20.41 million tons and 44.23 million tons, respectively. As of 2021, China’s annual production of lead and zinc reached 7.36 million tons and 6.56 million tons, respectively. The mining activities associated with lead and zinc pose a significant threat to the environment and human health, and due to their high toxicities, it is critical to remove heavy metal ions before discharging them into the ecosystem [4]. Traditional methods have been developed to remove heavy metals, such as chemical precipitation [5], ion exchange [6], membrane separation [7], photocatalysis [8], artificial wetland treatment [9], electrochemical treatment [10], coagulation flocculation [11, 12]. However, these methods encounter limitations due to insufficient metal removal, extensive reagent and energy demand, generation of toxic waste, high costs, and complexity. Therefore, there is an urgent need to explore an environmentally friendly approach to mitigate the concentration of heavy metals. The adsorption separation method is considered a simple and effective approach due to its low cost, rich range of adsorbent materials to choose from, and ease of implementation [13, 14]. Various adsorbents, including graphene oxide (GO) and GO-based adsorbents, are extensively utilized in wastewater treatment. They are especially effective in removing organic matter and heavy metals due to their excellent surface activity, wettability, reactivity, high specific surface area, and good mechanical strength. Additionally, these adsorbents can be easily combined with small molecular inorganic compounds, macromolecular polymers, and other substances to form composite materials, resulting in strong chemical reactions. However, the adsorption capacity of pristine GO has been reported to be ineffective [15] in removing heavy metal ions, which hinders its wide application in industrial waste-water treatment.

Recently, metal-organic frameworks (MOFs) formed by the self-assembly of metal clusters or metal ions with rigid organic ligands containing nitrogen and oxygen have received wide attention [16]. MOF materials have the characteristics of a low-density skeleton that can be designed, a rich structure, an ultrahigh specific surface area, permanent pores, and functional pore space. They are widely used for the removal of hazardous pollutants such as heavy metal ions [17, 18], organic dyes [19, 20] and etc. MOFs are mainly synthesized by solvothermal [21], hydrothermal [22], electrochemical [23], sonochemical [24], microwave [25], spray drying [26], flow chemistry [27], ionothermal [28], ultrasound [29] and slow evaporation [30] methods. Various synthesis methods offer unique advantages and can influence the properties of the resulting MOFs. The choice of synthesis method can impact the efficiency, scalability, and characteristics of the MOFs produced. For instance, You et al. [21] prepared a GO/MIL-101 (Fe, Cu) composite using the solvothermal method. The results indicate good properties such as a higher adsorption capacity and a specific surface area of 778.11 m² g⁻¹. Haso et al. [22] prepared a copper diphenylamine MOF via the hydrothermal method, and the results show higher adsorption capacities of 97.6%, 99.5%, and 99.5%, for Cd, Cr, and Pb, respectively. Pirzadeh et al. [23] developed a microstructure Cu₃(BTC)₂ MOF via electrochemical synthesis. Stawowy et al. [24] fabricated UiO-66(Ce) MOFs via sono-chemical method to adsorb CO₂. The results demonstrated that this method led to higher adsorption of irreversible CO₂ due to the smallest particle size and strong accessibility of Ce³⁺ for CO₂ adsorption. Thi Dang et al. [25] designed highly stable nanoscale Hf and Zr-based MOFs through microwave synthesis method. The prepared materials exhibited high porosity and a good structure compared to the original framework showing higher adsorption capacity (463.02 and 466.39 mg g⁻¹) for drugs containing an overdose of curcumin. Chaemchuen et al. [26] synthesized high-quality ZIF-8, ZIF-67, and bimetallic Zn/Co-ZIF MOFs through the spray drying method. The results demonstrate that the spray drying method is an efficient, simple, and continuous synthetic technique for producing MOFs (ZIFs) in larger quantities with high-quality properties with minimal effort and time. This efficiency is uncommon in other MOF synthesis techniques. Didriksen et al. [27] prepared a CPO-27-Ni MOF via flow chemistry. The result showed an outstanding specific surface area of 1085 m² g⁻¹.

Although MOFs have excellent properties, they are prone to skeleton collapse in aqueous environments due to weak coordination bonds, which limits their practical applications. Therefore, many researchers have employed various methods to enhance the stability of MOF materials, such as modifying or adjusting the structural of ligands, introducing metal dopants, and altering the composition. For instance, Mahmoodi et al. [31] fabricated a magnetic bio-nanocomposite metal-organic framework (MOF) utilizing a facile, efficient, and eco-friendly ultrasound-assisted technique. This composite material exhibits a considerable surface area of 1263.9 m² g⁻¹, with a maximum adsorption capacity of 344.82 and 250.81 mg g⁻¹ for Cu²⁺ and BR18, respectively. Daradmare et al. [32] prepared MOFs doped alginate beads to increase active sites for Cr (VI) adsorption from aqueous solution. Ma et al. [33] were able to enhance the recovery and adsorption capacity of the adsorbent by using calcium alginate beads containing alginate and MIL-121 composite. They created composite beads by immobilizing MIL-121 onto an alginate matrix, resulting in improved recycling and adsorption. The composite beads have a higher adsorption capacity than both MIL-121 and calcium alginate due to their hierarchical pore structure. Yang et al. [34] designed composite material consisting of Fe-BTC/PDA and employed it for the purpose of extracting Pb and Pd ions from actual water samples. It was found that a quantity of 1 gram of Fe-BTC/PDA beads possesses the capability to detoxify more than 10 L of freshwater, which contains exceedingly hazardous Pb concentrations of 600 ppm, while being subjected to a continuous flow. Yang et al. [35] developed a MnFe₂O₄ @MIL-53 @UiO-66 @MnO₂ composite material for removing Pb(II) and Cd(II). The hierarchical structure of the composite allowed for easy access of the ions. The composite had abundant binding sites for the ions. The adsorption capacities for Pb(II) and Cd(II) were 1018 and 440.8 mg g⁻¹ at 25 °C, respectively. The composite material could be easily recycled with the help of an external magnet. Rao et al. [36] prepared IRMOF-3/ GO composite exhibiting a remarkable adsorption capacity for Cu²⁺ (254.14 mg g⁻¹). The incorporation of GO has led to an increase in surface areas from 2588 m² g⁻¹ to 2884 m² g⁻¹, followed by a gradual decrease. The introduction of oxygen functional groups on the GO layers has introduced new crystallization sites during the crystallization process of IRMOF-3. As a result, the surface area and porosity have been significantly enhanced. These findings provide evidence that the incorporation of MOF with GO can greatly improve the adsorption performance and other properties of the composite. Xie et al. [37] successfully prepared GO-COOH/MOF-808 composite and effectively applied it to remove Pb²⁺, Cd²⁺, Co²⁺, Ni²⁺, and Cu²⁺ with higher adsorption efficiencies, which are 157.78, 135.96, 82.35, 90.99, and 91.49 mg g⁻¹, respectively. The synergistic effect of ion exchange, electrostatic adsorption, chemisorption, π-π interactions, and coordination complexation is attributed to the possible mechanisms of heavy metal ion adsorption. This study proposes that GO-COOH/ MOF-808 exhibits promising potential as an effective adsorbent for the elimination of heavy metal ions. Chen et al. [38] synthesized MOF-525 on the surface of GO to obtain MOF-525/GO composite materials with a BET surface area of up to 444.49 m² g⁻¹. The maximum adsorption capacities of GO, MOF-525, and the MOF-525/GO composite for tetracycline were 236.7, 371.5, and 413.6 mg g⁻¹, respectively. This indicates that GO enhances the adsorption performance of MOF-525. Dadashi et al. [39] used ultrasound to immobilize copper-based MOF onto GO to obtain GO/Cu-MOF adsorbents. The adsorption capacities of the GO/Cu-MOF composite for methylene blue (MB) were 173, 251, and 262 mg g⁻¹ at 25°C, 45°C, and 65°C, respectively. In comparison, the adsorption capacities of the Cu-MOF at the same temperatures were 106, 117, and 142 mg g⁻¹, respectively. The results confirmed that immobilizing Cu-based MOF onto GO can improve the adsorption performance of the composite material.

In recent years, new functional adsorbent materials like graphene, GO, and composites have offered opportunities for heavy metal adsorption and separation. However, due to high costs, adsorbents must be recycled or reused. Seeking recycling technology to separate and recycle heavy metal ions from water can address pollution and resource utilization. This study used GO as an adsorbent to separate lead-zinc from simulated wastewater. The adsorbed ions were used as metal center and benzene-1, 4-dicarboxylic acid (BDC) as an organic linker to synthesize bimetallic composite material named Pb/Zn-MOF@GO. The physical and chemical structures of the composite materials were characterized and analyzed using SEM, XRD, FTIR, BET, etc. The adsorption performance and mechanism of Pb²⁺, Zn²⁺ in simulated wastewater were compared and analyzed and satisfactory results were achieved. The composite materials enhance dispersion, stability, and pore structures. The study explores factors, the relationship between structure and performance, and the synthesis mechanism. The findings of this study provide insights into water pollution and resource utilization.

2. Method and Materials

2.1. Materials

The chemical substances used in this experiment, including graphite powder, phosphoric acid (H₃PO₄), sulfuric acid (H₂SO₄), benzene-1,4-dicarboxylic acid (BDC) C₈H₆O₄, dimethylformamide (DMF) [(CH₃)₂N–CH], were purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd., Zinc nitrate Zn (NO₃)₂, and nitrate Pb (NO₃)₂ were provided by Tianjin Fuchen Chemical Reagent Factory. All chemicals were analytically pure. Deionized (DI) water was used to prepare the solutions and rinse the prepared samples.

2.2. Synthesis of Material

2.2.1. GO synthesis

GO was prepared using an improved Hummers method [40].

2.2.2. Zn-MOF

The Zn-MOF was synthesized following a previous study [41]. Briefly, 1.894 g of Zn (NO₃)₂ (10 mmol) and 0.831 g of BDC (5 mmol) were dissolved in 30 mL DI water and stirred for 1 h. Subsequently, 2 mL of trimethylamine was added dropwise to rapidly deionize the organic BDC. The mixture was stirred at room temperature for another 3 h and then transferred to a 100 mL hydrothermal synthesis reactor for a reaction at 130°C for 18 h. Once the reactor was cooled to room temperature, the sample was centrifuged and separated. The product was washed three times with DMF, ethanol and DI water, respectively. The mixture was then transferred to an oven and dried overnight at 60°C. Then, it was activated to 100°C for 12 h. Finally, the Zn-MOF material was obtained. Pb-MOF and Pb/Zn-MOF were synthesis in the same way as Zn-MOF, with addition of 3.31 g.

2.2.3. Zn-MOF@GO composite

Initially, 0.05 g of BDC was added to 30 mL of DI water and stirred well to form a uniform solution. Then, the BDC was blended with GO that was adsorbed with varying concentrations of Zn²⁺ for 1 h. Next, add 2 mL of triethylamine to rapidly deionize the organic ligand. Following this, the mixture was stirred for an additional 3 h at room temperature to ensure proper distribution of the GO adsorbed with heavy metal ions in the solution. The resultant solution transferred to a 100 mL hydrothermal reactor and reacted at 130°C for 18 h. After the reaction is complete, the reactor is cooled to room temperature, centrifuged, and separated. The product was washed three times with DMF, ethanol, and then rinsed three times with DI water. The material was oven-dried overnight at 60°C followed by activation at 100°C for 12 h. Finally, the Zn-MOF@GO composite material was obtained. Pb-MOF@GO and Pb/Zn-MOF@GO composites were synthesized in the same way as the Zn-MOF@GO composite. The schematic diagram of the hydrothermal synthesis of the Pb/Zn-MOF@GO composite material is shown in Fig. 1.

2.3. Characterization

2.3.1. SEM analysis

The microstructure of the composite material was observed and analyzed using the JSM-6710F field emission scanning electron microscope, manufactured by a Japanese electronics company. Grind a small amount of the sample to be tested and apply it onto a conductive adhesive. Record the order of the sample arrangement, load the sample onto the sample stage, and insert it into the vacuum chamber for extraction until the vacuum level reaches 9.6 × 10–5 Pa. Then, activate the electron gun and adjust the focal length and astigmatism of the objective lens to observe the microstructure of the composite material.

2.3.2. XRD analysis

The XD-3 powder X-ray diffractometer was used to test the crystalline structure of the composite material. The sample is placed on a glass slide with a specific groove to flatten its surface. The slit attachment is selected for scanning testing, and the X-ray excitation source is Cu target with Kα, λ= 0.15418 nm. The working current and voltage are 40 mA and 40 kV, respectively, with a test step of 0.02 and 2θ= 5~80°.

2.3.3. FTIR analysis

A Perkin Elmer 550s Fourier Transform Infrared Spectrometer, produced by Parker Company in the United States, was used. The composite material and pure potassium bromide were mixed and ground evenly in a ratio of 1:150. The mixture was then pressed into a card slot for testing. The test was conducted using pure potassium bromide tablets as the background. It involved 64 scans and covered a wavenumber range of 4000 cm⁻¹ to 500 cm⁻¹.

2.3.4. Specific surface area and pore structure analysis

The ASAP2020 physical adsorption instrument, manufactured by Mac Company in the United States, was used for testing. 0.1 g of composite material was placed into a sample tube and subjected to vacuum degassing pretreatment at 150°C for 12 h. After removing surface moisture and residual organic small molecules from the sample to be tested, N₂ adsorption and desorption experiments were performed. The calculation methods for BET specific surface area, pore volume, and pore size were based on the Brunauer Emmett Teller (BET) and Barrett Joyner Helenda (BJH) methods, respectively.

2.4. Batch Adsorption Experiment

The impact of various parameters on the adsorption proficiency of composite materials was examined. For the adsorption isotherm experiment, 0.05 g of composite materials were added to 100 mL solution containing Pb²⁺ and Zn²⁺ with initial concentrations ranging from 50–300 mg L⁻¹. These mixtures were oscillated in a constant temperature water bath oscillator at 150 rpm for 3 h at 25°C. In the adsorption kinetics experiment, 0.05 g of composite materials were added to 100 mL solution containing Pb²⁺ and Zn²⁺ with initial concentration of 100 mg L⁻¹, at various time intervals ranged from 30–240 min. The solution pH was adjusted to 2–7 with 0.1 mol L⁻¹ HCl and NaOH. After these experiments, suspension was separated by centrifugation and filtration. Then, the concentration of residual metal ions in the solution was determined using a UV–Vis spectrophotometer, based on calibration curves. The concentration of Zn²⁺ was determined in accordance with GB/T 7472-1987, while the concentration of Pb²⁺ was measured based on colorimetric determination of lead using Xylenol Orange as a color reagent [42]. Briefly, 10 mL of pretreated wastewater was transferred into a 50 mL colorimetric tube. Then, two drops of 1 g L⁻¹ p-nitrophenol indicator was added, followed by the addition of 1% NaOH solution until the solution turned pale yellow. Next, 10 mL of 250 g L⁻¹ O-phenanthroline solution and 10 mL of acetic acid sodium acetate buffer solution with a pH of 6.0 were added. Finally, 2 mL of a 10 g L⁻¹ Xylenol orange solution was added, diluted to the desired volume with DI water, and shake it well. After 20 min of color development, the concentration of Pb²⁺ was measured using a 2 cm colorimetric cuvette at a wavelength of 535 nm using UV–Vis spectrophotometer. The adsorption capacity of the adsorbents was calculated according to Eq. (1) [43].

(1)

q_{e} = \frac{(C_{0} - C_{e}) V}{m}

where q_e (mg g⁻¹) represent the amount of Zn²⁺ and Pb²⁺ adsorbed per unit mass of adsorbent, C₀ (mg L⁻¹), and C_e (mg L⁻¹) represent the pre and post adsorption of the Zn²⁺ and Pb²⁺ concentration, V (ml) represent the sample volume, and m (g) represents the adsorbent mass. The percentage of Zn²⁺ and Pb²⁺ removal by the adsorbents was calculated using Eq. (2).

(2)

η = \frac{(C_{0} - C_{t})}{C_{0}} \times 100 %

where η is the removal percentage of Zn²⁺ and Pb²⁺, C₀ (mg L⁻¹), and C_t (mg L⁻¹) are the concentration of Zn²⁺ and Pb²⁺ before and after adsorption.

3. Results and Discussion

3.1. Morphology and Structure (SEM)

Fig. S1 illustrates GO, Pb-MOF@GO and Zn-MOF@GO composite materials as depicted in (Please refer to text S1 in Supplementary Material). Fig. 2 (a–b) shows the Pb-MOF@GO composite at Pb²⁺ initial concentration ranging from (50–100 mg L⁻¹) displaying a diamond and irregular shape. Fig. 2 (c–d) display hexagonal and rectangular shapes when the initial concentration of Zn²⁺ were 50–100 mg L⁻¹. Fig. 2 (e–i) reveals the bimetallic Pb/Zn-MOF@GO composite material, characterized by rod-shaped and blocky structures. From Fig. 2 (e–i), it can be observed that the size of the Pb/Zn-MOF@GO composite material nanoparticles increases as the total metal ion concentration increases. When the total metal ion concentration was below 100 mg L⁻¹, as shown in Fig. 2e, there were no noticeable Pb/Zn-MOF nanoparticles, and the GO nano-sheets maintained their intact morphological structure. As the total metal ion concentration increased, the crystals combined with GO to form rod-shaped and massive composite structures, and the surface exhibited a rough and irregular morphology. The above analysis indicates that after the adsorption of GO and the separation of Zn²⁺ and Pb²⁺ ions in water, Pb/Zn-MOFs with various morphological structures and dispersions can be formed on their surfaces. These SEM images confirm the successful synthesis of a single and bimetallic MOF on the surface of GO using the hydrothermal synthesis method.

3.2. Crystal Structure (XRD)

Fig. 3 shows the XRD patterns of GO, Zn-MOF, Pb-MOF, Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite materials. The XRD characteristic diffraction peaks of GO appear at 2θ = 10.3°, 20.3°, and 42.7°, indicating the successful preparation of GO material [40]. As shown in Fig. S2 the characteristic diffraction peaks and crystal planes of Zn-MOF appear at positions 9.5° (011), 11.6° (002), 33.8° (112), 44.5° (002), etc, This is consistent with the characteristic diffraction peaks of Zn-MOF previously reported [44, 45], demonstrating the successful synthesis of Zn-MOF. The characteristic diffraction peaks and crystal planes of Pb-MOF occurred at 29.3° (200), 38.16° (220) etc. which are consistent with the peaks reported in the literature [46, 47]. This indicates the successful synthesis of Pb-MOF. The characteristic diffraction peaks mentioned above are all present in the spectrum of Pb/Zn-MOF. The Zn-MOF@GO, Zn-MOF@GO, and Pb/Zn-MOF@GO composite materials also exhibited similar characteristic diffraction peaks as Zn-MOF and Pb-MOF, indicating successful loading of Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite materials onto the GO surface [44, 47]. However, the strong peak of Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite materials somewhat diminished due to the influence of GO. In summary, the XRD analysis shows the successful synthesis of single and bimetallic MOF@GO composite materials. Fig. S3 and S4 shows the XRD spectra of the synthesized Zn-MOF@GO, Pb-MOF@GO, under different initial concentrations of Pb²⁺ and Zn²⁺. Fig. S5 reveals the Pb/Zn-MOF@GO composite materials at different initial concentrations of mixed solution of Pb²⁺ and Zn²⁺. From Fig. 3 it can be seen that most of the characteristic diffraction peaks of Zn-MOF, Pb-MOF, and Pb/Zn-MOF appear in the Pb/Zn-MOF@GO composite. This indicates the successful synthesis of MOF@GO composite materials. While some peaks at 9.50° and 11.60° disappeared, especially when the concentration of metal ions was lower, this could be attributed to the influence of GO addition, which completely covered the structure of MOF in the composition.

3.3. Fourier-Transform Infrared Spectroscopy Analysis

Fig. 4 shows the FTIR spectrum of various composite materials, such as GO, Zn-MOF, Pb-MOF, Zn-MOF@GO Pb-MOF@GO, and Pb/Zn-MOF@GO. From Fig. 3 and Fig. S6, it can be seen that the characteristic absorption peaks of GO at 3199 cm⁻¹ and 3358 cm⁻¹ are attributed to the O-H vibration. Furthermore, the peak at 1714 cm⁻¹ corresponds to C=O vibration, and the C-O vibration peak occurs at 1051 cm⁻¹ [48]. The peak vibration of O-H in the Zn-MOF@GO composite material shifted to 3431 cm⁻¹, which can be attributed to the vibration of O-H in the water molecules adsorbed by the composite material. The infrared absorption peaks at 1652 cm⁻¹, 1568 cm⁻¹, 1203 cm⁻¹, and 733 cm⁻¹ are attributed to the stretching vibration of O-H, C-OH, and Zn-O vibrations of C=O in the organic linker BDC, respectively [49–52]. These peaks indicate the successful synthesis and loading of Zn-MOF on the GO surface. The infrared absorption peaks observed in Pb-MOF@GO and Pb/Zn-MOF@GO are consistent with those observed in Zn-MOF@GO, indicating the successful synthesis and loading of Pb-MOF and Pb/Zn-MOF onto the GO surface. Fig. S7 and S8 shows the FTIR spectrum of Zn-MOF@GO, Pb-MOF@GO composite materials under different GO adsorption capacity conditions, ranging from 50 to 250 mg L⁻¹, respectively. According to the description above and the FTIR analysis in Fig. S7 and S8, it is evident from Fig. 4 that the infrared absorption peaks of Zn-MOF, Pb-MOF are present in the respective composite materials. Similarly, Fig. S9 depicts the FTIR spectrum of Pb/Zn-MOF@GO composite under different GO adsorption conditions, using a mixed solution of Pb²⁺ and Zn²⁺. The FTIR spectrum of Zn-MOF is also appeared in Pb/Zn-MOF@GO composite material, further confirming the successful synthesis of the corresponding Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite materials.

3.4. Analysis of Specific Surface Area and Pore Structure of Composite Materials

Fig. 5 shows the N₂ adsorption and desorption curves of the three MOF@GO composite materials synthesized under hydrothermal technique. Fig. S10 shows the pore size distribution curve of the composite materials. From Fig. 5, it can be seen that, except for Pb-MOF@GO, the N₂ adsorption curves of the other two composite materials rapidly increase at low relative pressure (P/P0 < 0.1), indicating the presence of micropores in the composite material. In addition, both exhibit significant hysteresis loops when the relative pressure is higher than 0.45–0.95, indicating the presence of mesopores in the composite material [53, 54]. The pore size distribution curve in Fig. S10 further confirms that the synthesized Zn-MOF@GO and Pb/Zn-MOF@GO composite materials belong to micro-mesoporous materials, with a mesoporous size of around 30 nm. In contrast, the Pb-MOF@GO composite material is predominantly mesoporous. Table S1 presents the specific surface area and pore structure parameters of the three MOF@GO composite materials. From the relevant data in Table S1 it is demonstrated that the Pb/Zn-MOF@GO synthesized by the hydrothermal method has a higher BET surface area of 460 m² g⁻¹ compared to that of Pb-MOF@GO and Zn-MOF@GO, which are 296 m² g⁻¹ and 408 m² g⁻¹, respectively. This indicates that the combination of bimetallic MOF with GO can enhance the specific surface area. In addition, combining multiple metals can effectively integrate the advantages of different metal ions combined with organic ligands. This is expected to prepare composite materials with outstanding performance.

3.5. Adsorption Kinetics

Adsorption kinetics plays a crucial role in studying the process of material adsorption. It enables the analysis of the speed and mechanism of adsorption. The experimental data were evaluated with pseudo-first order and pseudo-second order kinetic models, and the data were analyzed according to Eqns. (3) and (4) [55].

(3)

log (q_{e} - q_{t}) = log q_{e} - \frac{k_{1}}{2.303} t

(4)

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{1}{q_{e}} t

where q_t and q_e (mg g⁻¹) are the adsorption capacities of the composite meterial at time t, and at equilibrium, respectively. k₁ (min⁻¹) is the pseudo-first order adsorption rate constant, and k₂ (g mg⁻¹ min⁻¹) is the equilibrium pseudo-second-order rate constant.

Fig. 6 shows the adsorption kinetics equilibrium of Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite materials for Zn²⁺ and Pb²⁺ metal ions. From Fig. 6, it can be observed that the adsorption of Zn²⁺ and Pb²⁺ by the composite materials increases rapidly over time and reaches equilibrium at 180 min. At this point, the adsorption capacity of the Zn-MOF@GO composite for Zn²⁺ and Pb²⁺ was 70.93 and 64.53 mg g⁻¹, respectively. The adsorption capacities of Pb-MOF@GO and Pb/Zn-MOF@GO composite materials at equilibrium time were 72.72 and 78.95, 93.35 and 111.49 mg g⁻¹ for Zn²⁺ and Pb²⁺, respectively. The results indicate that the Pb/Zn-MOF@GO composite material has a better adsorption capacity for the removal of Zn²⁺ and Pb²⁺ metal ions compared to the Zn-MOF@GO and Pb-MOF@GO composite materials. Fig. S11 and S12 shows the pseudo first-order and pseudo second-order kinetics for the adsorption of Pb²⁺ and Zn²⁺ metal ions, respectively.

The data in Fig. 6, is fitted linearly with the first-order adsorption rate Eq. (3) and the second-order adsorption rate Eq. (4), as shown in Fig. 11S and S12. According to the slope and intercept of the adsorption kinetics equation fitting curve, the adsorption kinetics parameters obtained are shown in Table S2. It is obvious that the linear correlation coefficient R² value of the second-order adsorption kinetics equation is greater than that of the first-order adsorption kinetics. Additionally, the equilibrium adsorption amount (q_e.c) calculated from the fitting curve is close to the equilibrium adsorption amount (q_e) obtained from the experiment. These results indicate that the adsorption process of Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite materials for Pb²⁺ and Zn²⁺ solutions is more consistent with the second-order adsorption kinetics model. Additionally, chemical adsorption influences the rate changes during the adsorption process.

3.6. Adsorption Isotherm

The adsorption mechanism of heavy metal ions in solution was studied using the Langmuir and Freundlich isotherm models. The Langmuir adsorption isotherm model assumes that the adsorption sites of the adsorbent are uniform, the adsorption process is monolayer adsorption, there is no interaction between the adsorbates, and the adsorption process is a dynamic equilibrium process. The Freundlich adsorption isotherm model can represent the non-uniform adsorption process on the surface of the adsorbent, making it more suitable for low concentration reaction conditions. The Langmuir models can be calculated according to Eq. (5) [56].

(5)

\frac{1}{q_{e}} = \frac{1}{Q^{0}} + (\frac{1}{b Q^{0}}) (\frac{1}{C_{e}})

where Q⁰ (mg g⁻¹) is the unit saturated adsorption capacity when the monolayer adsorption is formed; C_e (mg L⁻¹) describes the amount of Zn²⁺ and Pb²⁺ adsorbed and the Zn²⁺ and Pb²⁺ concentration in the solution at equilibrium; q_e(mg g⁻¹) is the equilibrium adsorption capacity, b is the Langmuir equilibrium constant. The Freundlich isotherm could be expressed as Eq. (6).

(6)

log q_{e} = log K_{F} + \frac{1}{n} log C_{e}

where K_f and n are the adsorption constants, which correspond to factors such as temperature and the specific surface area of the sorbent. The n value describes the type of isotherm, 1 n⁻¹ <1 (favorable), 1 n⁻¹ > 2 (unfavorable).

Fig. 7 shows the adsorption isotherm of single Pb²⁺and Zn²⁺adsorbed by Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite materials. From Fig. 7, it is evident that the equilibrium adsorption capacity q_e increases with the increase of the initial concentration of heavy metal ions in the solution. Based on the data in Fig. S13 and S14, combined with equations (5) and (6), create a linear quasi-cooperative graph as depicted in Fig. S13 and S14. By combining Fig. S13 and S14, we obtained the slope and intercept of the linear fitting equation, as well as the theoretical values of Q₀, b, K_F, and n corresponding to the adsorption isotherm model. The adsorption isotherm parameters of Pb²⁺and Zn²⁺ for the composite materials are shown in Table S3. Accordingly, it can be seen that the R² values of the Langmuir model are greater than the R² values of the Freundlich model equation. Therefore, the Langmuir adsorption isotherm model can better describe the adsorption process of the composite materials for Pb²⁺and Zn²⁺. This indicates that the adsorption process for Pb²⁺and Zn²⁺ follows a single-layer adsorption pattern more closely. Based on the adsorption isotherm data, the maximum adsorption capacity of Pb/Zn-MOF@GO composite for Zn²⁺ and Pb²⁺ were found to be 114.68 and 132.62 mg g⁻¹, respectively. To enhance comprehension of the adsorption characteristics of the Pb/Zn-MOF@GO composite in the context of heavy metal removal in comparison to other GO-based adsorbents, a comparative list is presented in Table S4.

3.7. Effect of pH on the Adsorption Performance

The chemical stability of the composite material was tested over a pH range of 2–7. Fig. S15 shows the effect of pH on the adsorption performance of Zn²⁺ and Pb²⁺ by the composite materials. From Fig. S15, it can be observed that the adsorption capacity of Pb/Zn-MOF@GO composite material for Zn²⁺ and Pb²⁺ increases gradually with an increase in pH. It reaches its maximum at pH 6 and then gradually decreases. It might be due to the competitive adsorption of Zn²⁺, Pb²⁺, and H⁺ on the Pb/Zn-MOF@GO composite. At a low pH (pH < 3.0), an excess of H⁺ ions may hinder the adsorption of heavy metal ions (Zn²⁺ and Pb²⁺) by the Pb/Zn-MOF@GO composite, resulting in a decrease in the adsorption capacity [57]. As the pH value increased, the adsorption efficiency of metal ions gradually increased because H⁺ had lost its dominant position in the competition. The results suggested that the adsorption of Zn²⁺ and Pb²⁺ by the Pb/Zn-MOF@GO composite was sensitive to pH, and the highest adsorption efficiency was achieved at pH 5–6.

3.8. Adsorption Mechanism of Zn²⁺ by Pb/Zn-MOF@GO Composite

Fig. 8 shows the comparison diagram of FTIR for the Pb/Zn-MOF@GO composite material before and after the adsorption of Zn²⁺. It can be observed that the infrared absorption peak of −OH at 3431 cm⁻¹ disappears after adsorption, suggesting that the adsorption process of Zn²⁺ involves hydrogen bonding. Similarly, the disappearance of the −C=O infrared absorption peak at 1652 cm⁻¹ indicates that −C=O is also involved in the reaction process of Zn²⁺ adsorption. Wang et al. [58] studied the reaction mechanism between Zn²⁺ and sulfur heterocyclic quinone dibenzo [b, i]thian-threne-5,7,12,14-tetraone (DTT) and found that the −C=O of two adjacent DTT molecules are bound through a Zn²⁺ coordination reaction, which can improve the stability of the composite. Based on the above analysis, the adsorption mechanism of Zn²⁺ by the composite material can be represented by Fig. S16.

4. Conclusions

In this study, GO was used as adsorbent to adsorb and separate Pb²⁺and Zn²⁺ from simulated wastewater. Next, GO was utilized as the matrix, with Pb²⁺ and Zn²⁺ adsorbed onto its surface act as metal ion centers, and BDC functioned as the organic linker to prepared bimetallic Pb/Zn-MOF@GO composite under hydrothermal method. The SEM analysis indicates irregular and rod like crystal structures. As the concentration of heavy metal ions adsorbed by GO increases, GO combines with Zn-MOF, Pb-MOF, and Pb/Zn-MOF crystals to form composite structures with rod-like or blocky shapes. The XRD spectrum and FTIR results confirmed the successful synthesis of Pb-MOF@GO, Zn-MOF@GO, and Pb/Zn-MOF@GO composite materials. The specific surface areas of Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite materials are 408, 296, and 460 m² g⁻¹, respectively. The composite materials possess a structure consisting of both micropores and mesopores. The adsorption kinetics and isotherm best fit the second-order adsorption kinetics model and Langmuir model, respectively. The maximum adsorption capacities of Zn-MOF@GO, Pb-MOF@GO, and Pb/Zn-MOF@GO composite for the removal of Zn²⁺ and Pb²⁺ were 83.47, 127.38, 127.22, 127.38, 114.68, and 132.62 mg g⁻¹, respectively. In summary, this study explores the factors that influence adsorption separation and the synthesis of composite materials. The findings of this study provide new insights into water pollution and resource utilization.

Supplementary Information

eer-2024-198-Supplementary.pdf

Acknowledgments

This research is financially supported by the Science and Technology Planning Project of Shaanxi Provincial Water Resources Department (2022slkj-5). We also gratefully acknowledge the support provided by Key R&D plan of Shaanxi province (2022SF-578)

Notes

Author Contributions

Z.L. (Professor) developed the methodology, Review and Edit the manuscript. M.F.B. (PhD student) conducted all the experiments, wrote the original manuscript, and designed the methodology. X.H. (PhD) revised the manuscript and participated in data analysis.

Conflicts of Interest Statement

The authors declare no conflict of interest.

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Fig. 1

Schematic diagram of Pb/Zn-MOF@GO composite material via hydrothermal synthesis.

Fig. 2

(a–d) synthesis of Pb-MOF@GO, Zn-MOF@GO after the adsorption of Pb²⁺ and Zn²⁺ by GO at 50–100 initial concentrations of Pb²⁺ and Zn²⁺, respectively; (e–i) illustrates the synthesis of Pb/Zn-MOF@GO after the adsorption of a mixed solution of Pb/Zn by GO with 50–250 mg L⁻¹ initial concentration, respectively.