Removal of tellurium(IV) from environmental aquatic systems using metal-organic framework material MIL-100(Fe)

The natural abundance of semi-metal tellurium (Te) in the earth's crust is extremely scarce, only 1–5 μg/kg (Belzile & Chen 2015). Te, as a strategic resource, plays a crucial role in promoting global development and contributing to national economies due to its high commercial and industrial value. It has been applied in numerous industries, comprising photovoltaic materials, colored glass, electronics, and ceramics (Chen et al. 2021; Liu et al. 2021). However, with the widespread mining of tellurium ores and the extensive usage of tellurium materials, a significant amount of toxic tellurium-contaminated wastewater has been generated, potentially causing severe pollution and disturbing the ecological balance. In environmental water bodies, tellurium primarily occurs as Te(IV) and Te(VI). Te(IV) exhibits 10 times the toxicity of Te(VI), and it demonstrates a certain degree of toxicity and teratogenicity (Li et al. 2021b). Therefore, from the perspective of sustainable development and environmental protection, the removal of highly toxic tellurous acid salts from tellurium-containing wastewater is an urgent issue that needs to be addressed.

Present techniques for removing Te from solutions comprise membrane separation (Li et al. 2021c), chemical precipitation (Xu et al. 2020), and adsorption (Narimani-Sabegh & Noroozian 2018; Yu et al. 2018). Among these, adsorption technology is a common method for removing pollutants from water, playing a crucial role in pollution removal due to its efficiency, simplicity, and reusability. However, the quality of pollutant adsorption performance depends on the characteristics of the adsorbent materials themselves. Common materials for Te removal include TiO₂ (Zhang et al. 2010; Qiu et al. 2020), ZrO₂ (Wu et al. 2018), and magnetic nanoscale zero-valent iron (NZVFe) (Yu et al. 2018). Nevertheless, the continuous pursuit of researchers is the discovery of new, efficient, and inexpensive adsorbent materials. A novel kind of organic-inorganic hybrid material that emerged in recent years is metal-organic framework (MOF), which is characterized by huge specific surface areas, large porosity, and tunable morphology (Li et al. 2018). MOFs, with the characteristics of both organic polymers and inorganic compounds, have attracted great interest among researchers. They are widely used in magnetic materials, gas storage, sensors, catalysis, and adsorption (Kreno et al. 2012; Dubey et al. 2017). The structure and properties of MOFs can be adjusted to form new selective adsorbent materials through the selection of distinct metal ions and organic bridging ligands (Hasan & Jhung 2015). In comparison to conventional solid adsorbents (including activated carbon and mesoporous silica materials) (Wang & Dai 2009), MOFs offer further advantages, comprising diverse structural compositions, exposed active sites, adjustable sizes of pores, and huge surface areas. These characteristics provide several occasions for guest molecules to enter the framework and interact with it (Gupta et al. 2011), which is conducive to capturing target pollutants to meet the requirement for satisfactory removal efficiency. Specifically, its exceptional performance in removing organic and inorganic pollutants from water bodies has been demonstrated. Extensive research has been conducted on the use of ZIF-8 for arsenic removal (Li et al. 2014), FJI-H9 (MOF) for cadmium removal (Xue et al. 2016), UiO-66 for mercury removal (Zhao et al. 2021), and Zn-MOFs for lead removal (Tahmasebi et al. 2015) and organic dyes elimination (Kim et al. 2020). Additionally, Fe-based MOF material, MIL-100(Fe), has been successfully applied for arsenic removal due to its unique mesoporous structure, high surface area, and abundant active sites (Cai et al. 2016). It has resulted in an impressive adsorption capacity of up to 110 mg/g and excellent performance (Cai et al. 2016). However, there is a scarcity of reports concerning the research field of Te removal with MOFs. As iron-based inorganic oxides have some affinity for Te due to their widespread abundance and Te naturally exists in pyrite (Borner et al. 2021), the search for the iron-based adsorbent with a unique structure and high capacity to remove highly toxic Te(IV) from environmental water bodies is highly meaningful.

In this experiment, for the first time, we used the MOF material MIL-100(Fe) to remove Te(IV) from the solution. We investigated the performance of this adsorbent material and mainly studied the impacts of the amount of adsorbent, solution's initial pH, initial Te(IV) concentration, and competitive ions on the adsorption performance of MIL-100(Fe). Moreover, to examine the removal of Te(IV) in environmental water bodies by this material, it was applied to remove high concentrations of Te(IV) in lake water, river water, and seawater sample matrices. The removal efficiency of Te(IV) by MIL-100(Fe) was determined. The kinetics and thermodynamics of Te(IV) adsorption were also discussed, providing a deeper understanding of its adsorption mechanism.

The experiments utilized only reagents of analytical grade or higher. Deionized water (DIW) was used throughout the experiment for solution preparation. Fe(NO₃)₃·9H₂O, H₃BTC, and Na₂TeO₃·2H₂O were supplied by Aladdin Industrial Corporation (Shanghai, China). Na₂TeO₃·2H₂O was used to prepare a 1,000 mg/L Te(IV) standard solution. NaOH, NaHCO₃, HCl, HNO₃, HF, KCl, NaNO₃, MgCl₂, Na₃PO₄, CH₃OH, and CaCl₂ were obtained from Cologne Chemicals Limited (Chengdu, Sichuan). 2 mol/L of HCl and 2 mol/L of NaOH were used for pH adjustment of the solution. Lake and river water samples were collected near Jianyang, Chengdu, and seawater samples were collected from Dalian and Hainan, respectively. Before collecting samples, it is essential to rinse the collection bottle several times with the respective environmental water sample. During sample collection, ensure that the sampling bottle is filled with the water sample to eliminate any air trapped inside. Following collection for later use, all water samples underwent filtration through a 0.45-μm filter membrane for subsequent use. In this experiment, a seawater matrix was employed, which was derived from actual seawater diluted by half. All containers used in this work were soaked overnight with 20% (v/v) HNO₃, followed by rinsed with DIW and dried at 60 °C.

Inductively coupled plasma atomic emission spectrometry (ICP-OES, Avio-500, Perkin-Elmer, USA) was employed for determining the Te(IV). As depicted in Table S1, Supplementary material, the operating parameters were set according to the manufacturer's recommendations. Employing X-ray diffraction (XRD, D8 Advance, Bruker, Germany), the crystal structure of MIL-100(Fe) was characterized and confirmed. N₂ adsorption and desorption analyses were conducted using a Micromeritics ASAP 2020 Plus. Specific surface areas and pore size distributions for the sorbent were estimated utilizing the Brunauer–Emmett–Teller (BET) and the Barrett–Joyner–Halenda (BJH) techniques, respectively. Scanning electron microscopy (SEM, ZEISS sigma500, Germany) and energy dispersive X-ray spectroscopy (EDS, Bruker XFlash 6130, Germany) were employed to determine the morphology and elemental composition of MIL-100(Fe). For the analysis of the alterations in the chemical valence of Te(IV) after adsorption by MIL-100(Fe), X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Escalab 250 Xi, USA) was employed. Fourier transform infrared spectroscopy (FTIR, Thermo Fisher IS20, USA) was utilized to investigate the changes in the chemical bonds of Te(IV) after adsorption by MIL-100(Fe).

Employing a hydrothermal synthesis technique, the synthesis of MIL-100(Fe) was done (Ke et al. 2015). The synthesis process was as follows: first, 3.2526 g of FeCl₃·6H₂O (12.03 mmol), 0.426 mL of HF, 0.326 mL of HNO₃, and 1.6932 g of H₃BTC (8.06 mmol) had a dissolution in 60 mL of DIW. Then, the solution was stirred for 10 min to ensure homogeneity and transferred to a PTFE-lined stainless steel autoclave, and for 12 h, heating was done at 150 °C. Following the reaction's cooling to room temperature, the product was rinsed three times each with DIW and methanol, and the yellow solid was recovered by filtration. To obtain the resulting MIL-100(Fe) polymer, the overnight drying was done at 150 °C. The XRD was employed to characterize the produced MIL-100(Fe), and its characteristic diffraction peaks were in line with those stated in the literature (as depicted in Figure S1, Supplementary material) (Ke et al. 2015). We conducted an analysis on the specific surface area and porosity of the material. As shown in Figures S2 and S3, Supplementary material, the N₂ adsorption-desorption isotherm revealed that the synthetized MIL-100(Fe) has a specific surface area of about 1,554 m²/g, and a major pore size of approximately 1.5 nm. It indicated that the MIL-100(Fe) possesses favorable mesoporous surface properties, which could provide plenty of active sites for targeted adsorption and promoted the transportation between the adsorbent and the target substance, proving beneficial for the adsorption of Te(IV).

To study the adsorption performance of the MOF material MIL-100(Fe) for Te(IV), batch experiments were conducted under different conditions. In the investigation of factors that may influence the removal efficiency of Te(IV) by MIL-100(Fe), a controlled variable method was used for the experiments. Unless specifically stated, all adsorption runs were performed in 250 mL conical flasks at 298 K. In general, 0.5 g/L of MIL-100(Fe) adsorbent was introduced into 50 mL of 50 mg/L Te(IV) solution, and the mixture was sonicated to ensure homogeneity. Subsequently, the mixture was agitated at 25 °C for 2 h on a thermostatic shaker (200 rpm). After shaking, the solid/liquid phases were separated by filtration through a 0.45 μm filter to obtain the supernatant, and 2 mL of this supernatant was taken and added to 2 mL of 4% (v/v) HNO₃ so that the test sample solution contained 2% (v/v) HNO₃. Finally, the solution was analyzed by ICP-OES, and after adsorption, the residual Te(IV) concentration in the sample solution was investigated using a standard curve. All experiments in this work were repeated three times and the obtained experimental results represent the average of the three trials.

The adsorption kinetics of Te(IV) on MIL-100(Fe) were determined by monitoring the adsorption efficiency at varying times. The experiments were conducted at an initial Te(IV) concentration of 50 mg/L and a MIL-100(Fe) concentration of 0.5 g/L over various adsorption durations (0–480 min). The equilibrium state of the adsorption process was described by adsorption isotherms. In this experiment, 25.0 mg of MIL-100(Fe) was added to 50 mL solutions of Te(IV) with concentrations ranging from 5 to 2,000 mg/L without adjusted pH. The mixture was shaken for 2 h at a constant temperature of 25 °C. After shaking, the sample solution was filtered through a 0.45 μm filter membrane, and the Te(IV) concentration in the supernatant was measured using ICP-OES. To examine the adsorption isotherms of MIL-100(Fe) for Te(IV), two of the most common isotherm models, the Langmuir and Freundlich models, were applied.

To investigate the effect of temperature on the removal efficiency of Te(IV) by MIL-100(Fe), it was studied at different temperatures of 25, 35, and 45 °C in the presence of 50 mL of 50 mg/L Te(IV) solution, 25.0 mg of MIL-100(Fe), a reaction time of 2 h, and without adjusted pH.

In the adsorption experiments, the controlled variable method was used to study the impact of various factors, such as solution pH, adsorption time, adsorbent concentration, and coexisting ions, on the removal efficiency of MIL-100(Fe) for Te(IV). By varying the factor under investigation while holding all other experimental conditions constant, the influence of each factor was explored.

The amount of adsorbent is a significant element in the analyte's quantitative adsorption, which directly determines the adsorption sites' quantity. Choosing the optimal amount of adsorbent also has significant implications for the adsorption process efficiency in terms of cost. Therefore, we investigated the impact of the concentration of MIL-100(Fe) on the removal of Te(IV) within the range of 0.2–1.5 g/L. With the elevation in the concentration of the MOF material, the number of adsorption sites also gradually increases, which is conducive to the removal of Te(IV) by the material. As depicted in Figure 1(b), when the amount of adsorbent reaches 0.5 g/L, the removal rate of Te(IV) can reach over 96%, and then kept almost constant. This phenomenon could be owing to the increase in the amount of adsorption sites and the contact area (Ma et al. 2020). During the process of Te(IV) adsorption, the number of adsorption sites escalates rapidly as the dosage of the adsorbent increases. However, a portion of these sites remains unsaturated, resulting in a reduced utilization rate of MIL-100(Fe). Considering the removal efficiency and the principle of using minimal reagents to reduce experimental costs, the amount of MIL-100(Fe) adsorbent chosen for subsequent experiments was 0.5 g/L.

The contact time between the solid-phase material and the target analyte to some extent affects the efficiency of element removal. Therefore, this study examined the impact of adsorption time on the Te(IV) removal process. As shown in Figure 1(c), within 0–60 min, the removal efficiency of Te(IV) by MIL-100(Fe) rapidly increases with the extension of adsorption time, and at 60 min, a removal efficiency exceeding 90% is achieved. It may be due to the rich adsorption sites on the surface of MIL-100(Fe), making for a rapid increase in the adsorption efficiency of MIL-100(Fe). And the adsorption equilibrium was attained with a maximum adsorption efficiency of 98% after 2 h. This was caused by the exhaustion of numerous adsorption sites on the MIL-100(Fe) surface. Besides, as the reaction proceeds, spatial resistances was increased and electron transfer rates were reduced, which in turn diminishes the ability to bind with Te(IV), resulting in a slight increase in adsorption time. Therefore, a reaction time of 2 h was chosen for subsequent experiments. Furthermore, the initial concentration of Te(IV) is a crucial factor involved in assessing the adsorption performance of the adsorbent and its performance in a real-world setting. Subsequently, we studied the impact of high-concentration Te(IV) initial concentration on the adsorption performance of MIL-100(Fe) under the condition of a removal time of 2 h. Figure 1(d) displays the adsorption capacity of MIL-100(Fe) under different Te(IV) concentrations. As the initial Te(IV) concentration continuously rises, the amount of Te(IV) adsorbed by the nanomaterial gradually increases in the range of 5–500 mg/L Te(IV). As the concentration of Te(IV) rises to 1,000 mg/L, the amount of Te(IV) adsorbed by 0.5 g/L MIL-100(Fe) stabilizes, attaining the adsorption capacity of 503.8 mg/g. This could be as a result of the adsorbed Te(IV) covering the adsorption sites on the materials surface as the adsorption amount increased, thereby preventing further adsorption to the unused active sites. According to the above findings, the prepared MIL-100(Fe) material has superior removal capacity for Te(IV).

Various types of anions exist in environmental waters, and the removal effect of the adsorbent on Te(IV) will be influenced by the types and ion strength in the water. Therefore, we investigated the impact of competitive ions in the water (Ca²⁺, Mg²⁺, K⁺, Na⁺, NO₃⁻, HCO₃⁻, PO₄³⁻, CO₃²⁻) on the removal of Te(IV) by MIL-100(Fe), as depicted in Table 1. Considering the complexity of the seawater matrix, the types and concentrations of coexisting ions refer to the standard seawater values. The findings show that under the existence of 20,000 mg/L of Na⁺, 5,000 mg/L of Mg²⁺, 1,000 mg/L of Ca²⁺ and K⁺, 400 mg/L of NO₃⁻, CO₃²⁻, and HCO₃⁻, and 100 mg/L of PO₄³⁻, the removal rate of Te(IV) by this adsorbent, MIL-100(Fe), still exceeds 94%. This indicates that MIL-100(Fe) has strong resistance to interference in the system for removing Te(IV), and is expected to be applied to environmental waters of different matrices.

Under the selected experimental conditions, we investigated the performance of this material in removing high-concentration Te(IV) in actual environmental water samples. Therefore, we chose lake water, river water, and high-salinity seawater as sample matrices. The seawater matrix was used after diluting the actual seawater by half. To better demonstrate the effect of MIL-100(Fe) in removing Te(IV), we added an additional 20 mg/L Te(IV) into each type of environmental water sample and then conducted the removal experiment. As shown in Table 2, the findings demonstrate that the removal efficiency of this material for Te(IV) in general river or lake water was as high as 99%. And for seawater matrix, the removal efficiency of Te(IV) was over 94%. It may be attributed to the excessive salinity and ionic intensity in the sample solution. This indicates that the adsorbent successfully applies to the effective removal of Te(IV) in environmental waters (including seawater).

This research proposed the first-time use of the MOF material MIL-100(Fe) to remove Te(IV) from environmental aquatic systems. The material achieved adsorption equilibrium within 2 h and showed a broadly applicable pH range (pH = 6–12). It demonstrated high adsorption capacity (531.9 mg/g) and excellent selectivity for Te(IV) removal, with strong resistance to interference. The adsorption process was primarily a monolayer chemical adsorption and an endothermic reaction. When applied to the removal of high concentrations of Te(IV) in actual lake water, river water, and seawater, the material achieved excellent removal results, confirming its applicability. The material holds the potential for the removal of Te(IV) in other complex matrices.

Additional information as noted in text, including XRD patterns of the synthesized MIL-100(Fe), N₂ adsorption and desorption isotherms of MIL-100(Fe), pore diameter distribution profiles of MIL-100(Fe), FTIR spectra of MIL-100(Fe) microspheres before and after adsorption, ICP-OES operating conditions and kinetics and isotherm model parameters of adsorption of Te(IV) by MIL-100(Fe).

Sichuan Province Science and Technology Support Program (No. 2023NSFSC0753) and Natural Science Foundation of Sichuan Province (No. 2022NSFSC1902) are acknowledged for their financial support.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.