Introduction

One of the most common sources of water contamination is through industrial sewages and this has been mentioned as a major concern in environmental issues1. There are different types of water contaminants such as inorganic and organic pollutants including toxic metals, different types of nutrients, insecticides, chemical fertilizers, detergents, hospital wastewater and persistent organic pollutants (POPs)2. Heavy metal ions (Hg2+, Pb2+, Cd2+, Cu2+, Cr3+, etc.) in water, have been mentioned as a serious threat to human health and living organisms because of their dangerous effects on different body organs3, 4. Specifically, mercury ion (Hg2+) and lead ion (Pb2+) contaminations in water resources, can lead to various disorders including central nervous system disorder, kidneys dysfunction, and stomach problems5, 6. Thus, the importance of detection and removal of these toxic heavy metal ions from water has been greatly studied in order to have a healthier life and environment7, 8.

Recently, the emergence of metal–organic frameworks (MOFs) and coordination polymers (CPs) porous materials has been greatly regarded as a mean to tackle this problem. These porous structures which are basically constructed of metal ions or metal clusters and organic ligands, have gotten remarkable range of applications as adsorbents9,10,11,12,13. MOFs have various properties which make them a good candidate in adsorption including large surface areas, ultrahigh porosity, distributed functional sites, and high thermal/chemical stability14, 15. Specific functional groups with sulfur and nitrogen atoms on organic linkers of MOFs through Hg–S interactions or Hg–N interactions have created suitable materials as sorbents for Pb2+ and Hg2+ions16,17,18. Among them, Thiazole, a five-membered heterocyclic compound, having N and S atoms can adsorb heavy metal ions based on coordination interaction19, 20. Organic ligands containing the thiazole ring have been less used in the synthesis of MOFs and CPs.

In this research, 2, 5-di (4- pyridyl) thiazolo [5, 4-d] thiazole (DPTTZ) ligand having thiazole ring was used as linker to synthesize a novel Zn-MOF, [Zn2 (DPTTZ) (OBA)2] (IUST-2) [OBA = 4,4'-oxybis (benzoic acid)], later to be tested for the removal of heavy metal ions from water. In this study, the influence of parameters such as contact time, pH parameters, and also the concentrations of lead and mercury ions in the solutions, were evaluated. This work shows that MOFs containing thiazole ring can play a very effective role in the removal of heavy metal ions from water.

Experimental

Materials and apparatus

All chemicals used in this research were of analytical laboratory grade, provided by well-known commercial sources, and used as received. 2, 5 -di (4- pyridyl) thiazolo [5, 4-d] thiazole (DPTTZ) was synthesized by reported procedure21. Powder X-ray diffraction (PXRD) patterns were recorded on Philips X’pert diffractometer. Elemental analysis was performed with a CHNS Thermo Finnigan Flash 1112 series elemental analyzer. The absorption spectroscopy FT–IR was measured in the 400–4000 cm−1 range, by the KBr disc technique on a Shimaduz FT-IR-8400 spectrometer. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out with Perkin Elmer Pyris 1 thermo gravimeter at 10 °C.min−1 heating rate under the argon (Ar) atmosphere. Field emission scanning electron microscopy (FE-SEM) images were taken on the FE-SEM TESCAN MIRA3 microscope. Atomic absorption spectrometry (AAS) on a Shimadzu 6300 AA instrument was used to detect the metal ion concentration in aqueous solutions. Nitrogen adsorption–desorption measurements (BET method) were performed at liquid nitrogen temperature (− 196 °C) with a Micromeritics ASAP 2020 adsorption instrument.

Single crystal X-ray diffraction

Crystallographic data for the IUST-2 were collected using Mo Kα radiation (λ = 0.71073 Å) on a Marresearch 345 dtb diffractometer equipped with an image plate detector. The programs used to solve and refine the structure were SHELXT 2018/2 (Sheldrick, 2018) and SHELXL2016/6 (Sheldrick, 2016), respectively22. Data reduction and cell refinement were carried out with the Automar software package (3.3a, 2015). Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added at ideal positions and refined using a riding model. Table S1 summarized the single-crystal x-ray diffraction data and structure refinement for IUST-2.

Preparation of [Zn2 (DPTTZ) (OBA)2] (IUST-2)

A mixture of Zn(NO3)2.6H2O (0.010 g, 0.034 mmol), 4,4'-oxybis (benzoic acid) (0.009 g, 0.034 mmol), 2, 5-di (4- pyridyl) thiazolo [5, 4-d] thiazole (0.005 g, 0.017 mmol) and N,N-dimethylformamide (7 mL) was sealed in a 15 mL stainless steel reactor with Teflon liner and directly heated to 120 °C for 72 h and then cooled to 25 °C during 12 h. The single clear yellow crystals suitable for X-ray diffraction experiments were obtained with a good yield of 71.8%. FT-IR (KBr, cm-1): 3424(m), 3057(m), 2923(m), 1676(s), 1635(s), 1610(s), 1593(s), 1499(s), 1396(s), 1230(s), 1158(s), 1091(m), 1065(s), 1012(s), 877(s), 830(s), 782(s), 696(s), 661(s), 618(s), 504(s), 444(s). Elemental Anal. calc. for C42H24N4O10S2Zn2: C, 53.69; H, 2.57; N, 5.96; S, 6.83%. Found: C, 53.46; H, 3.34; N, 6.88; S, 6.62%.

Adsorption experiments

A series of batch absorption experiments were performed to determine the capability of the IUST-2 to adsorb targeted heavy metal ions (i.e. Pb2+ and Hg2+) from aqueous solutions. To do this, 5 mg of IUST-2 was placed into 25 mL of aqueous solutions with varied concentrations of targeted heavy metals and stirred at room temperature. The pH and adsorption time were investigated on the elimination of Pb2+ and Hg2+ cations onto the IUST-2. HCl/NaOH solution (0.1 M) was used to regulate the solution pH. The remaining concentration of these metal ions in each of the stock solutions were determined by atomic absorption spectrometry.

To investigate the kinetics of the sorption process, 5 mg IUST-2 was placed in a round-bottomed flask and then left to interact with 25 mL of aqueous solutions at 200 mg L−1 concentration of Pb2+ and Hg2+, separately, for various time intervals between 0.5 to 120 min. Moreover, the maximum adsorption capacities of IUST-2 for lead and mercury ions were estimated separately, by adsorption isotherms in a series of Pb2+ and Hg2+ solutions with the concentration range of 50–600 mg L−1. In all the aforementioned tests, pH values were set to be 5 and 4 for lead and mercury solutions, respectively.

Results and discussion

Crystal structure and characterization of IUST-2

According to the single crystal X-ray analysis, IUST-2 crystallizes in an orthorhombic crystal system and the space group is Pbcn with the asymmetric unit comprising of two Zn2+ ions, one DPTTZ ligand and two OBA ligands. As illustrated in Fig. 1a, each Zn2+ ion displays a square-pyramidal coordination geometry surrounded by four carboxylate O atoms from four OBA ligands at the equatorial positions and one pyridine N atom from one DPTTZ ligand at the axial position. The Zn − O bond lengths are in the range from 2.254(1) to 2.361(2) Å and the Zn − N bond distance is 2.271(2) Å. The O − Zn − O and N − Zn − O angles are in the range of 86.36(18)° − 161.79(15)° and 94.48(17)° − 103.81(17)°, respectively. From a crystallography point of view, four carboxyl groups from four OBA ligands, act as a bridging ligands to link independent Zn2+ ions, making a paddle-wheel shaped secondary building unit (SBU) Zn2(CO2)4. The carboxylate groups of OBA ligands adopting bismonodentate coordination modes μ2–η1:η1 link Zn nodes to form a two- dimensional wave-like sheets expanded by DPTTZ ligands, eventually resulting a 3D structure (Fig. 1b and c).

Figure 1
figure 1

(a) The Zn2+ ion Coordination environment in IUST-2; Symmetry codes: (i) − x + 1/2, − y + 3/2, z + 1/2; (ii) − x + 1/2, − y + 1/2, z − 1/2; (iii) − x + 1, y, − z + 1/2; (iv) − x, − y + 1, − z + 2; (v) − x + 1/2, − y + 3/2, z − 1/2; (vi) − x + 1/2, − y + 1/2, z + 1/2, (b) 2D sheet of {Zn2(OBA)2}n along the bc plane, (c) View of 3D framework of the IUST-2 along c axis.

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PXRD

The phase purity of IUST-2 was checked by powder X-ray diffraction (PXRD) experiments. The simulated and experimental PXRD patterns of the IUST-2 correspond to each other, demonstrating the phase purity of the IUST-2 crystals prepared using the solvothermal method. Comparison of powder x-ray diffraction patterns before and after the activation process with methanol showed that the IUST-2 structure remains intact after activation (Fig. S1). Considering the target application for IUST-2 as an adsorbent to remove metal ions in aqueous media, the water-stability of the IUST- 2 was further studied. The IUST-2 was immersed in water at RT for 48 h. Slight changes in the PXRD pattern were observed after 48 h of soaking in water, which returned to the initial state, after 24 h soaking in DMF (Fig. S2). This observation is possibly attributed to the MOFs’ breathing effect23.

TGA and DTA

In order to investigate the thermal stability of IUST-2, thermogravimetric and differential thermal analysis experiments were carried out under the Ar atmosphere (Fig. S3). A weight loss with a mild rate (approximately 20%) in the beginning, corresponds to the evaporation of trapped solvent molecules inside the IUST-2 during the washing process. Decomposition and pyrolysis of the IUST-2 starts with an endothermic process at 363 °C, which was the main thermal loss of 72%. The remaining weight (approximately 8%) may be attributed to the formation of zinc oxide, or zinc sulfide. This indicates that IUST-2 has rather good thermal stability.

BET

To measure N2 adsorption/desorption isotherm, a sample IUST-2 was degassed at 110 °C for 4 h. Fig. S4 shows that the N2 adsorption–desorption isotherm of the IUST-2 is of type IV, according to the IUPAC classification, having an H3 type hysteresis loop. The Brunauer–Emmett–Teller (BET) specific surface area of the IUST-2 is 105.636 m2.g−1. The pore volume and average pore size are 0.081 cm3.g−1 and 30.553 Å, respectively.

FE-SEM

Figure S5 shows the FE-SEM images of IUST-2 obtained by the solvothermal process. As could be observed from these figures, IUST-2 has a nanostructure with an average diameter of 31.4 nm.

Selective adsorption behaviors

Batch sorption tests on an aqueous solution with a range of different heavy metal ions such as (Hg2+, Pb2+, Cu2+, Cd2+, Ni2+, Co2+) were carried out to determine the capacity and behavior of IUST-2 in selective adsorption. From the results, it can be noticed that IUST-2 shows much higher removal efficiency in the presence of the Pb2+ (%97.08) and Hg2+ (%87.55) compared to other heavy metal ions, when applied to single solutions of different ions (Fig. 2a). In another test, and to investigate the interfering effects, the removal efficiency was studied by the IUST-2 on a mixed aqueous solution of different heavy metal ions, with a concentration of 100 mg.L−1 each. Still, the removal efficiency was much higher for Pb2+ and Hg2+ (88% and 79%, respectively), even in the presence of other interfering ions in the solution (Fig. 2b). It can be observed that IUST-2 revealed excellent uptake performance toward lead and mercury ions, which is assumed to be in relevance to thiazole ring. Thiazole is a unique heterocyclic compound containing sulfur and nitrogen atoms. Because of the strong affinity between N and S atoms of the thiazole ring with lead and mercury ions, compared to other background metal ions, selective removal for Pb2+ and Hg2+ is outperformed24,25,26,27,28.

Figure 2
figure 2

(a) The removal efficiency of heavy metal ions for the IUST-2: (a) on single solutions, (b) on coexisting solutions.

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The pH effect

The pH is a key variable that can both controls and affect the adsorption process. The pH of the solution may have effects on the surface charge of the adsorbate as well as on the adsorbent. Therefore, the impact of the pH ranging from 1.0 to 7.0 on the adsorption of Pb2+ and Hg2+ ions was investigated (Fig. 3). At pH < 3.5, the ability of the IUST-2 to remove heavy metal ions was degraded, because hydrogen ions can compete and coordinate better on the available adsorption sites on IUST-2. The ability of the IUST-2 to remove lead and mercury ions is the highest at pH = 5 and 4, respectively. With the pH value increasing, the concentration of hydrogen ions is diminished, and stronger electrostatic interaction occurs between metal ions (Pb2+ and Hg2+) and active sorption sites of the IUST-2. As a result, the optimum pH values of 5.0 and 4.0 were chosen for lead and mercury removal experiments, respectively, throughout this study.

Figure 3
figure 3

Influence of pH on the removal of lead and mercury ions using the IUST-2.

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The effect of concentration

The influence of initial Hg2+ and Pb2+ concentrations on the adsorption efficiency by the IUST-2 was examined at room temperature. The adsorption isotherm curves are demonstrated for the adsorption of Pb2+ (Fig. 4a) and Hg2+ ions (Fig. 4b) at different initial concentrations (50–600 mg L−1) by 5 mg of the IUST-2 in 25 mL solution of heavy metal ions. It was observed that the adsorption capacity of the IUST-2 for Pb2+ and Hg2+ increased gradually to 1450 mg g−1 and 900 mg g−1, respectively, at an initial concentration of 500 mg L−1. These values for adsorption capacity are so much larger compared to other reported values for other MOFs29,30,31,32,33.

Figure 4
figure 4

The adsorption isotherm curves for the IUST-2 at different initial concentrations: (a) Pb2+, (b) Hg2+, and fitted lines with the Langmuir isotherm model: (c) Pb2+, (d) Hg2+.

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The experimental adsorption isotherms data fitted well with the Langmuir model, which is evident from the high correlation coefficient (R2 = 0.9933 for Pb2+ and 0.9980 for Hg2+) values. The equation of the Langmuir isotherm is given as Eq. (1):

$$\frac{{c}_{t}}{{q}_{t}} = \frac{1}{({q}_{e}.b)}+ \frac{{c}_{t}}{{q}_{e}},$$
(1)

where qt (mg g−1) represents the adsorption capacity at time t, ct (mg L−1) is the metal ion concentration at time t, b (L mg−1) is related to the energy of adsorption that demonstrates the affinity between the solute and the adsorbent, and qe (mg g−1) represents the maximum monolayer adsorption capacity.

Since the Langmuir model is compatible to the adsorption behavior, it is concluded that the distribution of active sites on the surface of IUST-2 is homogeneous34, 35. The results of the Langmuir model for lead (Fig. 4c) and mercury (Fig. 4d) ions adsorption on the IUST-2 is exhibited in Table S2. The theoretical maximum monolayer adsorption (qe) of Pb2+ and Hg2+ ions were computed to be 1430 and 900 mg g−1, respectively, which are very close to the values of experimental maximum adsorption capacity (1450 and 900 mg.g−1).

Adsorption kinetics and mechanism

As shown in Fig. 5a and b, variation of the removal capacity for Pb2+ and Hg2+ ions on the IUST-2 with time were achieved. Due to the presence of specific functional groups on the IUST-2 and high metal ions affinity for the sulfur and nitrogen of the thiazole ring, the removal of targeted ions is very quick and the maximum adsorption capacity is acquired after 3 min. The activated IUST-2 (5 mg) was placed in an aqueous solution of Pb(NO3)2 and HgCl2 (25 mL, 100 mg L−1 initial concentration, pH = 5 and 4 for lead and mercury ions, respectively) and then the achieved data were fitted with pseudo-second order kinetic model (Fig. 5c for lead and Fig. 5d for mercury ions) using the Eq. (2):

$$\frac{t}{{q}_{t}} = \frac{1}{({k}_{2}.{q}_{e}^{2})}+ \frac{t}{{q}_{e}},$$
(2)

where qe and qt (mg.g−1) are the removal capacity at equilibrium and time t (min), respectively; k2 (g (mg min)−1) is the pseudo-second order rate constant of the adsorption rate. The high coefficient of determination (R2˃ 0.9999) value was obtained, suggesting that the pseudo-second-order model was suitable for the adsorption kinetics of the IUST-2 and sorption process was mostly governed by chemical reactions between the metal ions and the active adsorption sites of the IUST-2 (Table S3)36, 37.

Figure 5
figure 5

Time-dependent adsorption capacity of IUST-2 toward: (a) Pb2+, (b) Hg2+, and Adsorption kinetics fitted with pseudo-second-order models: (c) Pb2+, (d) Hg2+.

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To examine the mechanism of Hg2+ and Pb2+ ions sorption, the FT-IR spectrum of IUST-2 before and after the adsorption of metal ions was studied (Fig. 6). There are obvious changes in the infrared spectra of C − S and C − N vibrations of thiazole ring. There are strong interactions between Hg2+ and Pb2+ ions with S and N atoms of the thiazole ring that could limit C − S and C − N vibrations and consequently decrease their vibrational frequency. The peak at 833 cm−1 assigned to C − S stretching vibration that showed a red shift of the 833 cm−1 peak to 820 cm−1 peak and 824 cm−1 peak after treatment with Pb and Hg ions, respectively. Moreover, a significant red shift from 1407 cm−1 to 1392 cm−1 and 1384 cm−1 was observed for the characteristic C − N stretching vibration after the adsorption process, for lead and mercury ions, respectively, which confirmed the coordination of Pb2+ and Hg2+ ions to the nitrogen of the thiazole ring. Also, a new bond has appeared in 542 cm−1, which can attribute to the Pb–O vibration38. The Hg-O bond for mercury does not exist in the FT-IR spectrum, so the reason for the increase in the percentage of removal efficiency of IUST-2 for lead compared to mercury can be attributed to the presence of more adsorption sites for lead. The results of the powder x-ray diffraction before and after the adsorption process reveal that the IUST-2 can retain its crystallinity and structure after the adsorption of Pb2+ and Hg2+ metal ions, so the possibility of structural collapse should be removed (Fig. S6).

Figure 6
figure 6

The FT-IR spectra; for the IUST-2 before and after the adsorption Pb2+ and Hg2+ ions.

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Reusability

As a general rule, reusability and stability are two essential factors for adsorbent to work effectively. When the adsorption process was finished, the IUST-2 was regenerated by using an EDTA.2Na solution. Three cycles of adsorption/desorption were carried out to evaluate the reusability of the IUST-2 and this process was monitored with AAS. Figure 7 demonstrates that the IUST-2 possesses reversibility in the process of removing Pb2+ and Hg2+ ions.

Figure 7
figure 7

Reusability experiments of IUST-2 implemented with (a) Pb(II) ions, (b) Hg(II) ions.

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BET of IUST-2 before and after removal of targeted metal ions

The N2-BET analysis represents that the surface area of the pristine IUST-2 is 105.63 m2/g m2/g. The comparison of the BET surface area of virgin IUST-2 and IUST-2 after the adsorption of lead and mercury ions from the water is seen to have decreased from 105.63 m2/g to 6.33 m2/g, and 18.298 m2/g, respectively (Table S4). Also, a decrease was observed in the total pore volume from 0.081 to 0.020 cm3/g, and 0.066 cm3/g for the IUST-2 after the adsorption of Pb (II) and Hg (II) ions, respectively (Table S4). This might result from filling the volume of the pores. Hence, it can be concluded that IUST-2 is a good absorbent towards lead and mercury ions from the aqueous solution. In addition, the results of the BET surface area and pore volume after the removal of metal ions confirmed the reusability of IUST-2.

Comparison with other MOF-based adsorbents

Table 1 indicates the maximum sorption capacity of IUST-2 for the removal of lead and mercury ions from water compared to other MOF-based adsorbents in the literature. According to Table 1, the maximum sorption capacities of IUST-2 for Pb (II) and Hg (II) are higher than that of most other MOF-based adsorbents reported in the literature. Totally, these results demonstrated that IUST-2 is a promising adsorbent for the effective removal of Pb (II) and Hg (II) ions from polluted water.

Table 1 The compared results of the sorption capacity of IUST-2 for Pb (II) and Hg (II) ions with different absorbents based on MOFs.
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Conclusions

In conclusion, metal–organic frameworks containing the thiazole ring can be a good option for the adsorption of heavy metal ions. In this article, a new Zn-MOF, the IUST-2, based on the thiazole ligand was synthesized by solvothermal method. According to the single crystal x-ray diffraction, the IUST-2 is a 3D bipillared-layer framework structure. The IUST-2 displayed remarkable application in the adsorption of lead and mercury ions from water. The removal efficiency as high as 97% and 87% was obtained for Pb2+ and Hg2+ ions at an initial concentration of 100 mg L−1 after 3 min, respectively. Maximum adsorption capacity is 1450 mg g−1 for Pb2+ and 900 mg g−1 for Hg2+ ions. The results from Langmuir and pseudo-second order rate models reveal the removal of metal ions by the IUST-2 is a monolayer adsorption by the interaction between the active adsorption sites of the IUST-2 and Pb2+ and Hg2+ ions. This study indicates that metal–organic frameworks based on thiazole ligands can be good adsorbents for mercury and lead ions elimination of aqueous solution.