The design and preparation of multifunctional adsorbent for practical wastewater treatment is still an enormous challenge. To remove multiple metal ions from wastewater, we developed a broad-spectrum metal ions trap named UIO-67-EDTA by incorporation of ethylenediaminetetraacetic acid into robust UIO-67. The adsorption experiments for 15 kinds of heavy metal ions including hard acid (Mn2+, Ba2+, Al3+, Cr3+, Fe3+), borderline acid (Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Sn2+, Bi2+), soft acid (Ag+, Cd2+, Hg2+), and two kinds of dissolved minerals (Mg2+, Ca2+) show that the trap is very effective both in batch adsorption processes and breakthrough processes. At a pH value of 4.0, the removal efficiency for all metal ions was over 98% within 10 min, and the maximum static adsorption capacity for the representative metal ions Cr3+, Hg2+and Pb2+ was up to 416.67, 256.41, and 312.15 mg g−1, respectively. The adsorption kinetics fitted well with the pseudo-second-order model, indicating that the chemical adsorption was the rate-determining step in the adsorption process. Meanwhile, the material showed high stability and recyclability, the removal efficiency for the three representative metals was still maintained over 93% after five consecutive adsorption cycles.
Fabrication of a broad-spectrum heavy metal ion trap possessing strong chelating groups with high-density active binding sites
Serving as an extremely efficient adsorbent for multifarious and complex metal ions purification
High stability together with recyclability offering a potential possibility for practical applications.
Water pollution caused by metal ions has become a pressing global environmental issue, creating a serious threat to humans and aquatic organisms in view of their high toxicity, biological accumulation and persistency in nature (Vorosmarty et al. 2010; Bakker 2012). It is of great importance to search a simple and effective way for removal of heavy metal ions from waste water. Among the traditional techniques for metal ion removal, adsorption is regarded as one of the most promising methods due to its simplicity and efficiency (Li et al. 2014). However, the existing adsorbents, such as activated carbons (Nayak et al. 2017), carbonaceous materials (Ding et al. 2016), polymers (Ko et al. 2017), clay minerals (Hebbar et al. 2018), nanosized metal oxides (Jin et al. 2017) and layered metal sulfides (Manos & Kanatzidis 2016), have confronted either low adsorption capacities or low efficiencies (Madadrang et al. 2012). Although the complex functional groups can effectively boost adsorption capacity, most adsorbents still suffer from the disadvantage of specificity in heavy metal capture. That is to say, they are only sensitive to one or a few specific metals, but not to all metal ions co-existing (Li et al. 2018). Therefore, there is a need to develop a multifunctional adsorbent containing high-density binding sites with strong binding affinity to diverse metal ions.
Metal-organic-frameworks (MOF) are a new type of hybrid porous material composed of metal clusters and organic linkers (Li et al. 1999). Because of their ordered crystalline structure, high porosity, extraordinary surface area and pre-designed functional groups, MOF has been regarded as one of the most promising materials for potential application in separation, adsorption, and catalysis (Gao et al. 2019). In addition, by post-synthetic ligand exchange or modification methods, various functional groups can be easily fixed in the backbone of MOFs without changing their original structure (Li et al. 2018). Benefitting from this, some studies have demonstrated that MOF derivatives obtained by rationally introducing functional groups can improve its adsorption properties for pollutant removal (Wen et al. 2018).
It is well-known that ethylenediaminetetraacetic acid (EDTA) is a powerful chelating agent that can form stable coordination complexes with various metal species due to its six active binding sites: four hard carboxyl and two relatively softer tertiary amine groups (Jiang et al. 2019). However, these metal–ligand complexes are water soluble in general, which seriously restrict its application in solid phase extraction. To date, many researches have shown that assembling of EDTA on a supporting material can facilitate separation, recovery and maintain the active sites for metal ions chelating (Kumar et al. 2013; Cui et al. 2015).
Inspired by these strategies, we synthesized a multifunctional MOF-based trap by incorporation of EDTA into a water stable MOF (UIO-67) to remove multiple metal ions from wastewater simultaneously. Just as expected, the UIO-67-EDTA exhibited prominent performance for all the selected metal ions both in single- and multi-component adsorption experiments. What's more, the material showed high stability and recyclability under the experimental conditions after five consecutive adsorption cycles.
MATERIALS AND METHODS
Unless otherwise stated, all reagents used in the experiments were of analytical grade and all solutions were prepared using double-distilled water (See in Supporting Information).
Synthesis of UIO-67
UIO-67 was synthesized according to previous publications with minor modification (Gutov et al. 2015; Pankajakshan et al. 2018). Briefly, 0.64 g ZrCl4·8H2O was dissolved in 100 mL N,N dimethylformamide (DMF), and 0.4 mL formic acid was added as the modulator. After sonicating for 5 min, 0.42 g 4,4-biphenyldicarboxylicacid (H2BPDC) was added and mixed well again. The homogenous solution was then placed into an electric oven and sustained at 120 °C for 24 hours. After cooling down to room temperature, the obtained precipitate was centrifuged and washed three times using DMF and acetone, respectively. To remove residual solvents, the material was evacuated 12 h at room temperature followed for 12 h at 120 °C.
Synthesis of UIO-67-EDTA
EDTA functionalized UIO-67 (UIO-67-EDTA) was synthesized through a solvent-assistant linker exchange method (Figure 1) (Bury et al. 2013). Generally, 0.10 g UIO-67 was added into 50 mL 0.10 mol L−1 EDTA-2Na aqueous solution and the mixture was stirred at 60 °C for 24 h. The white precipitate was then centrifuged and washed with water and acetone several times. Finally, the solid sample was dried at 60 °C under vacuum conditions.
In the evaluation of adsorption properties and removal performance of the UIO-67-EDTA, 17 different metal ions including hard acid (Mn2+, Ba2+, Al3+, Cr3+, Fe3+), borderline acid (Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Sn2+, Bi2+), soft acid (Ag+, Cd2+, Hg2+), and dissolved minerals (Mg2+, Ca2+) were considered as the targeted analytes. All experiments were conducted in triplicate and the experimental details are presented in Supporting Information.
Batch adsorption process
To investigate the adsorption kinetics of UIO-67-EDTA, 10 mg UIO-67-EDTA was dispersed in 100 mL single-component experimental solutions with the concentrations of 50, 100 and 200 mg L−1, respectively. The suspensions were stirred for 2 hours at room temperature. During the adsorption period, the mixture was filtered at intervals through a 0.22 μm cellulose syringe, and the filtrates were measured by ICP-MS to determine the residual metal ions content.
To investigate the adsorption isotherm of UIO-67-EDTA, 10 mg UIO-67-EDTA was dispersed in 100 mL single-component experimental solutions at different concentrations (varied from 10 mg L−1 to 1,000 mg L−1). The suspension was stirred for 2 hours at room temperature and filtered through a 0.22 μm cellulose syringe filter, and then the filtrates were measured by ICP-MS to determine the residual metal ions content.
The effect of pH and adsorption mechanism
To investigate the effect of pH, 10 mL 10 mg L−1 single-component experimental solutions were used. The pH of the experimental solutions was firstly adjusted to 2.0, 3.0, 3.5, 4.0, 4.5, 5.0 and 6.0 using 0.1 M HCl or 0.1 M NH3·H2O. Then 10 mg UIO-67-EDTA was added. After stirring for 2 hours at room temperature, the aqueous solution was filtered through a 0.22 μm cellulose syringe filter and the filtrate was measured by ICP-MS to detect the residual ions content.
The point of zero charge (pHPZC) value of UIO-67-EDTA was determined by salt addition method (Bakatula et al. 2018). Firstly, 0.1 g of UIO-67-EDTA was added to a series of 50 mL centrifuge tubes containing 20.0 mL of 0.1 M NaNO3 solution. The pH was adjusted using 0.1 M HCl or 0.1 M NH3·H2O to 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11 (± 0.1 pH units) and denoted as pHi. After shaking for 24 h, the pH values of the sample were measured again and denoted as pHf. The pHPZC was obtained from the plot of ΔpH (= pHf – pHi) against pHi.
Multi-component batch adsorption experiments
100 mg UIO-67-EDTA was dispersed in a 10 mL multi-component experimental solution containing 17 kinds of selected metal ions with each metal concentration of 5 mg L−1. After stirred for 2 hours at room temperature, the aqueous solution was filtered through a 0.22 μm cellulose syringe filter. Subsequently, the filtrate was measured by ICP-MS to detect the residual ions content.
Breakthrough experiment process
About 100 mg UIO-67-EDTA was packed in a glass column (20 cm × 5 mm i.d.) plugged with glass wool at the both ends. Before using, the column was washed with double-distilled water successively. A multi-component solution (10 mL, 5 mg L−1) was passed through the column after adjusting its pH to 4.0 with a certain flow rate controlled by a peristaltic pump. Then the filtrate was measured by ICP-MS to detect the residual ions content after filtering through a 0.22 μm cellulose syringe filter.
Regeneration and recyclability experiments
To investigate the recyclable performance of metal ions on UIO-67-EDTA, 20 mg of UIO-67-EDTA was suspended in 10 mL 10 mg L−1 multi-component solution containing the selected representative metal ions (Cr3+, Hg2+and Pb2+) and stirred for 30 min. After adsorption equilibrium, the adsorbent that pre-loaded metal ions were regenerated using 10 mL EDTA-2Na aqueous solution with different concentrations (0.10, 0.15, 0.20 and 0.25 M) for three times under stirring. Then the eluent was diluted to 50 mL and filtered by a 0.22 μm cellulose syringe filter. Subsequently, the filtrate analyzed by ICP-MS to detect the residual ions content.
To further measure the recyclability of the absorbent, the regenerated UIO-67-EDTA was then subjected to succeeding cycles for metal ion removal.
RESULTS AND DISCUSSION
Figure 2 shows the X-ray photoelectron spectroscopy spectra (XPS) of UIO-67 and UIO-67-EDTA. Obviously, the characteristic peaks at the binding energy of 399.2 eV were attributed to the N 1s of EDTA that appeared after modification (Figure 2(a) and 2(c)) (Zhao et al. 2017). Elemental quantitative analysis data showed that the content of element nitrogen was about 2.76%. Because the element nitrogen comes from EDTA, according to the mole ratio of element nitrogen and EDTA (M = 292.2), the content of EDTA can be calculated to be 28.8% in UIO-67-EDTA. Moreover, it can be found that the characteristic peaks of Zr 3d exhibited a slight shift after introducing EDTA (Figure 2(b) and 2(d)), indicating that the EDTA molecules are tightly attached to the Zr6 clusters, but not being adsorbed into the channels of UIO-67 (Saleem et al. 2016). All of these observed results confirm that EDTA was successfully assembled on the framework of UIO-67 with high loading amounts.
As shown in Figure 3(a), the experimental powder X-ray diffraction (XRD) patterns of the synthesized UIO-67 matched well with the previous reports, indicating that UIO-67 had been successfully synthesized (Gutov et al. 2015; Pankajakshan et al. 2018). In addition, the related peaks of UIO-67-EDTA are closely matched to the original UIO-67, confirming that the crystal structure of material remained intact after EDTA functionalization. The FTIR spectra of UIO-67-EDTA (Figure 3(b)) showed a new vibration peak related to the C-N stretching band that appeared at 1,246 cm−1 compared with pristine UIO-67, testifying that EDTA had successfully grafted onto the pore structure of UIO-67 (Dupont et al. 2014; Huang et al. 2014). The morphologies and microstructures of the materials were characterized by scanning electron microscopy (SEM). It can be seen from Figure 3(c) and 3(d) that UIO-67 possesses individual octahedral morphology. There was no marked change in morphology and size after modification with EDTA, which proves that the material had excellent stability in aqueous solution.
Thermogravimetric analysis (TGA) was carried out to test the thermal stability of the materials. As shown in Figure 4(a), a plateau region without considerable weight loss under 360 °C was observed on the TG curve of UIO-67 (blue line), indicating that the UIO-67 displays high thermal stability. After post-modification, the stability of UIO-67-EDTA was still high up to 270 °C (pink line). The TGA curves indicated that the weight loss of UIO-67-EDTA is step by step from 270 to 800 °C, and the rate of the fastest weight loss for UIO-67 was much lower than UIO-67-EDTA. At 600 °C the weight losses of UIO-67-EDTA and UIO-67 were 52.3% and 44.1%, respectively. All of the results proved that EDTA had successfully grafted onto the pore structure of UIO-67 and was relatively thermally stable.
Considering that the appropriate adsorptive is critical for the micropore material's surface area calculations (Cychosz & Thommes 2018), argon adsorption–desorption isotherms (at 87 K) were applied to evaluate area–volume data of UIO-67 and UIO-67-EDTA. The obtained data presented in Figure 4(b) show type I isotherms with H4-hysteresis loops for the two materials, indicating a microporous nature (Lassig et al. 2011). The Brunauer-Emmett-Teller (BET) surface area decreased from 1952 to 1,167 m2 g−1 after EDTA was fixed in the framework. This may be due to some blocking of the microporous structure by EDTA. However, the average pore size distribution only showed a small reduction from 13.09 Å to 8.95 Å after modification (Figure 4(c)). Note that such a cage size is still big enough for efficient mass transfer during metal ion capture.
Batch adsorption experiments
Signal-component capture performance
As shown in Figure 5(a)–5(c), UIO-67-EDTA exhibits good effectiveness toward all three types of metal ions with the removal efficiencies exceeding 98%. UIO-67 only shows less than 40% removal efficiencies for all the selected metal ions (Figure S2). The conspicuous performance of UIO-67-EDTA can be ascribed to the ultrahigh coordination number as well as strong binding affinity toward metal ions associated with both hard and relatively softer active sites of the grafted EDTA. To further verify the results, the spent adsorbents were washed using ultrapure water several times and then verified by XPS. Here, Cr3+, Hg2+and Pb2+ were selected as the representative metal ions which belongs to hard acid, soft acid and borderline acid, respectively. As can be seen from Figure 5(d)–5(f), the specific peaks corresponding to the metal ions are detected in the respective XPS spectra of metal ions-loaded UIO-67-EDTA (M@UIO-67-EDTA, M = Cr3+, Hg2+and Pb2+), indicating that metal ions are tightly link to the UIO-67-EDTA. This result further illustrates the excellent adsorption ability of the UIO-67-EDTA.
The effect of pH
One of the most important factors affecting adsorption behavior of adsorbent is surface charge and existing functional groups of the material which depend on solution pH (Maslova et al. 2021). Therefore, the point of zero charge (pHPZC) value was investigated to determine the surface charge of adsorbent. As can be seen in Figure S3, the pHPZC value of UIO-67-EDTA was determined as 3.64, which means that at a solution pH < 3.64, the surface charge of the sorbent is positive, while at a solution pH >3.64 the surface charge of the sorbent is negative (Kosmulski 2002).
To evaluate the effect of pH on adsorption behavior of UIO-67-EDTA, the adsorption experiments were carried out at the pH levels ranging from 2 to 6. Alkaline solutions were not tested for metal ions to avoid the formation of metal hydroxides. The presented results (Figure 6 and Figure S4) show that the removal efficiency for all the select metal ions is below 45% at pH 2. By increasing the pH value, the removal efficiency increased concurrently. When the pH value reached 4.0, the maximum removal efficiency of the UIO-67-EDTA exceeded 98%. Therefore, we choose pH 4 for all subsequent adsorption experiments to get the best adsorption efficiency.
The adsorption property of UIO-67-EDTA relevant to treatment time toward all of the selected heavy metal ions was investigated by varying the treatment time from 2 to 120 min (Figure S5) at solution pH of 4. For convenience, the discussion was focused on Cr3+, Hg2+and Pb2+ as well. As shown in Figure 7, the removal efficiencies are above 92% within 5 min and exceed 98% within 10 min. Such a fast adsorption rate of UIO-67-EDTA can be attribute to its big enough surface area and pore size, which can facilitate the diffusion of metal ions to active sites of EDTA assembled in UIO-67 frameworks.
The kinetics results and related parameters are shown in Table 1. It can be observed that the calculated qe (qe,cal) are well agree with the experimental qe (qe,exp) companied with high correlation coefficients (R2) values for all the representative heavy metal ions at three different concentrations. The results suggest that the kinetics of adsorption metal ions on UIO-67-EDTA are fitted well with the pseudo-second-order model, indicating that the chemical adsorption was the rate-determining step in the adsorption process (Hokkanen et al. 2013). In additionally, the small k values shown fast adsorption kinetics for the three kinds of heavy metal ions, implying that the UIO-67-EDTA is suitable for application in water disposal in a flow system.
|Pollutants .||qe, exp (mg g−1) .||pseudo-second-order model|
|qe, cal (mg g−1) .||k (g mg−1 min−1) .||R2 .|
|Pollutants .||qe, exp (mg g−1) .||pseudo-second-order model|
|qe, cal (mg g−1) .||k (g mg−1 min−1) .||R2 .|
The isotherm curves and relevant parameters calculated from the Langmuir model are presented in Figure 8 and summarized in Table 2. It is clear that Langmuir model shows a good agreement with the experimental data with high R2 values. The saturated adsorption capacities of UIO-67-EDTA for Cr3+, Hg2+and Pb2+ are calculated to be 416.67, 256.41, and 312.15 mg g−1, respectively. As shown in Tables S1, UIO-67-EDTA performs best as a whole.
|Pollutants .||Langmuir model|
|Qmax (mg g−1) .||KL (L mg−1) .||R2 .|
|Pollutants .||Langmuir model|
|Qmax (mg g−1) .||KL (L mg−1) .||R2 .|
The adsorption mechanism
It could be inferred from Equation (6) that solution pH is a key parameter that affect the adsorption efficiency of metal ions (Repo et al. 2011). Too high H+ concentration of the solution was adverse for the release of H+ from EDTA, that is to say the adsorption of metal ions would be limited more seriously at lower pH. However, too high pH of the solution was also not good for adsorption of metal ions, which was because of the hydrolytic products of metal ions (such as MOH+ and M(OH)2) could be formed (Guo et al. 2014).
It's worth noting that the adsorption behavior of UIO-67-EDTA in regard to Al3+, which is the most hydrolysable cation among studied ones. To insight into the adsorption mechanism of Al3+, the final pH (after adsorption) was measured. At pH of 2 and 3,the removal efficiency to 35 and 50%, and the final pH becomes 2.11 and 3.16, respectively. Under these conditions, Al3+ is the only existence form, and the adsorption mechanism is mainly following as the Equation (6). With a further increase in the initial pH to 4 the final pH value reaches 4.24 and the removal efficiency up to 97.6%. On this pH condition, a fraction of Al(OH)2+ and Al(OH)2+ hydrolytic species were formed in the solution, which is difficult to remove by the ion-exchange mechanism. But at given final pH, the surface of UIO-67-EDTA is negatively charged that is favorable for electrostatic interaction between Al(OH)2+ or Al(OH)2+ species and sorbent surface. Accordingly, the adsorption mechanism is mainly controlled by ion-exchange mechanism and the surface complexation mechanism. At the initial pH of 5 the final pH achieves value of 5.39. Evidently, in that case Al3+ totally precipitates as hydroxide.
In a breakthrough process, the flow rate of the solution passing through the packed column is an important parameter for controlling the time of adsorption and analysis.
The effect of flow rate on adsorption of the selected metal ions were studied under optimum condition (pH = 4) by passing 10 mL of multi-component solution (5 mg L−1 for every ion) through the column with a peristaltic pump. The result in Figure 9(a)–9(c) shown that the flow rate had a great influence on the adsorption of the selected metal ions except for Cr3+ and Fe3+. With the increase of flow rate, the removal efficiency decreases gradually. High removal efficiency (>98%) of all target metal ions were obtained with 1.5–2.5 mL min−1. When above 2.5 mL min−1, the removal efficiency decreases lower than 95%. Thus, a flow rate of 2.5 mL min−1 was selected for breakthrough procedure for all of the tested metal ions. The residual concentrations in solution after flowing through the packed column of UIO-67-EDTA were reduced from 5 mg L−1 to excessively low level (0.05–2.1 μg L−1). These values are much lower than the limitation in standards for drinking water quality of GB/T 5750. In addition, the adsorption capacity in breakthrough process were also calculated and the detailed data presented in Table S2. This result demonstrated that UIO-67 still has an excellent adsorption capacity for all metal ions, even if in the case of coexistence of 17 kinds of metal ions. All the dramatic results demonstrate that UIO-67-EDTA is a commendable absorbent for capture multiple metal pollutants from water simultaneously that maybe put into practical application in flow system.
Regenerated and recyclability
From a practical perspective, recyclability is a key factor for an advanced adsorbent. In this work, EDTA was selected as eluent solution aimed to prevent the adsorbent from being destroyed at highly acidic condition. It is interesting to note that the metal ions-loaded M@UIO-67-EDTA can be easily regenerated by washing with 10 mL 0.20 M (pH = 4.4) EDTA-2Na solution for three times with the regeneration efficiency up to 96% (Figure 10(a)). The cause might be the affinity of the metal ions for the free EDTA (EDTA in the eluent) stronger than for it when it is fixed on the solid material. One can see that the color of UIO-67-EDTA turns from white to blue after Cu2+ loading and returns to white after regeneration (Figure S6). For testing the reusability of UIO-67-EDTA, the regenerated materials were then subjected to consecutive adsorption cycles for metal ions removal. As shown in Figure 10(b), the removal efficiency for three representative metal ions can be retained over 93% after five consecutive adsorption cycles. Furthermore, the SEM images shown in Figure S7 reveal that the UIO-67-EDTA retains its octahedral morphology, and no apparent collapse happened during the whole adsorption and desorption processes. The results indicate that UIO-67-EDTA is a recyclable and stable adsorbent for heavy metal ions that can be applied into practical use.
In summary, a multifunctional adsorbent of UIO-67-EDTA has been successfully synthesized by a facile solvent-assistant linker exchange method. Due to the strong chelation of EDTA groups together with large surface area and suitable pore size of the framework, the material exhibits fantastic capture performance toward a total of 17 kinds of common contaminants metal ions covering hard Lewis acid, borderline Lewis acid and soft Lewis acid both in batch adsorption and breakthrough process. What is more, the material also shows high stability and recyclability. All the features confirmed that the UIO-67-EDTA is an outstanding absorbent and has promising potential utility in industrial application.
This work was funded by the Doctoral Research Initiation Foundation of Lanzhou City University (LZCU-BS2019–17), the National Natural Science Foundation of China (NSFC 22062010) and Natural Science Foundation of Gansu Province (18JR3RA220).
CONFLICT OF INTEREST
The authors declare no conflict of interest.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.