Thiosemicarbazone modified zeolitic imidazolate framework (TSC-ZIF) for mercury(ii) removal from water

Zeolitic imidazolate frameworks (ZIF-8), and their derivatives, have been drawing increasing attention due to their thermal and chemical stability. The remarkable stability of ZIF-8 in aqueous and high pH environments renders it an ideal candidate for the removal of heavy metals from wastewater. In this study, we present the preparation of novel aldehyde-based zeolitic imidazolate frameworks (Ald-ZIF) through the integration of mixed-linkers: 2-methylimidazole (MIM) and imidazole-4-carbaldehyde (AldIM). The prepared Ald-ZIFs were post-synthetically modified with bisthiosemicarbazide (Bisthio) and thiosemicarbazide (Thio) groups, incorporating thiosemicarbazone (TSC) functionalities to the core of the framework. This modification results in the formation of TSC-functionalized ZIF derivatives (TSC-ZIFs). Thiosemicarbazones are versatile metal chelators, hence, adsorption properties of TSC-ZIFs for the removal of mercury(ii) from water were explored. Removal of mercury(ii) from homoionic aqueous solutions, binary and tertiary systems in competition with lead(ii) and cadmium(ii) under ambient conditions and neutral pH are reported in this study. MIM3.5:Thio1:Zn improved the removal efficiency of mercury(ii) from water, up to 97% in two hours, with an adsorption capacity of 1667 mg g−1. Desorption of mercury(ii) from MIM3.5:Thio1:Zn was achieved under acidic conditions, regenerating MIM3.5:Thio1:Zn for five cycles of mercury(ii) removal. TSC-ZIF derivatives, designed and developed here, represent a new class of dynamically functionalized adsorption material displaying the advantages of simplicity, efficiency, and reusability.


Introduction
Metal-organic frameworks (MOFs) are a class of adsorbent materials composed of metal cations, connected to polytopic organic linkers via coordination bonds. MOFs, 1-3 as porous crystalline materials, combine high porosity, large surface area, exible pore size and shape, 4,5 and in most cases, a high stability, 6 with simple, economical and convenient direct synthetic routes. [7][8][9] The porous structure exhibited by MOFs offers large surface areas, reaching $5200 m 2 g À1 , 10 and variety of pore dimensions and topologies. 11 All the before mentioned properties render MOFs suitable candidates for catalysis, [12][13][14][15][16] separation, 17,18 gas storage, [19][20][21] and drug delivery 19,22-28 among other applications. 16,[29][30][31][32] The exibility of the coordination bond, joining the organic linker to metal ion, permits chemical modulations through post-synthetic modication (PSM) of the metal-organic framework. This promotes MOFs to high performance, tailor-made materials. 33 PSM, ranging from carrying out chemical transformation [34][35][36] or exchange on pre-synthesized materials, 37,38 has emerged as a powerful method for functionalizing MOFs. 39,40 Zeolitic imidazolate frameworks (mainly, ZIF-8) received much attention due to their thermal and chemical stability which makes them ideal candidates for further adjustment of their physical and chemical features to attain satisfactory performances in a wide range of potential applications. [41][42][43] ZIF-8 structures have been prepared using different approaches, mainly hydro and solvothermal. [44][45][46] The remarkable stability of ZIF-8 in aqueous and high pH environments renders it an ideal candidate for the removal of heavy metals from wastewater. 47 Heavy metals, in general, are toxic to all living organisms. 48 Mercury, in particular, is considered to be extremely dangerous due to high solubility and bioaccumulation properties. 49,50 Different techniques have been developed for the removal of heavy metals from contaminated wastewater, 51,52 such as chemical precipitation, 53 membrane ltration, 54 electrochemical treatments, 55,56 adsorption 57,58 and ion exchange. 59,60 Removal of mercury cations from contaminated wastewater has been recently achieved using novel sulfur-functionalized MOFs, [61][62][63] adsorption parameters of these MOFs are presented in Table 1. Relevant parameters include maximum mercury adsorption capacity (mg g À1 ), retention time (minutes), and pH of the medium characterising HKUST-1, 62 thiolfunctionalized ZIF-90 (ZIF-90-SH), 64 UiO-66-NHC(S)NHMe, 65 FJI-H12 66 and other robust MOFs 67 are presented in the table. The most recent example of an efficient Hg(II) adsorption material is using hybrid material -ZnS with ZIF-8 on lter paper. The high sulfur content in the hybrid material exhibits outstanding adsorption of Hg(II), where the removal was achieved through simple ltration of contaminated water using the monolith ZnS-ZIF-8. 68 This study presents the preparation of a new class of aldehyde modied ZIF-8 derivatives (Ald-ZIF), which were further functionalized with thiosemicarbazone (TSC) groups for the removal of mercury(II) ions from water. These Ald-ZIF were prepared through the integration of mixed-linkers: 2-methylimidazole (MIM) and imidazole-4-carbaldehyde (AldIM). The linkers were combined in two ratios (x 1 ¼ 15, y 1 ¼ 1 and x 2 ¼ 3.5, y 2 ¼ 1, where x and y represent the relative contents of MIM and AldIM, respectively) to yield two Ald-ZIF: MIM 15 :AldIM 1 :Zn and MIM 3.5 :AldIM 1 :Zn. The major component in all prepared Ald-ZIF is MIM, to retain the chemical and physical properties originally exhibited by ZIF-8. Incorporation of AldIM allows for further functionalization of the ZIF's framework, through postsynthetic modication (PSM). Accordingly, the prepared MIM x :AldIM y :Zn were post-synthetically modied with two thiosemicarbazide based functionalities; bis (NH 2 -NH-CS-NH-NH 2 ) and thio (NH 2 -NH-CS-NH 2 ) semicarbazones, through the condensation of the aldehyde (in AldIM) to bis/ thiosemicarbazide. This successful PSM resulted in the formation of four new thiosemicarbazone zeolitic imidazole framework derivatives (TSC-ZIF), as demonstrated in Scheme 1.

Results and discussion
Aldehyde modied ZIF-8 (Ald-ZIF) derivatives were successfully prepared through modifying the synthetic procedure of ZIF-8. 69 Simultaneous incorporation of commercially available 2-methylimidazole (MIM) and imidazole-4-carbaldehyde (AldIM) in two different ratios (x 1 ¼ 15, y 1 ¼ 1 and x 2 ¼ 3.5, y 2 ¼ 1) yielded MIM x :AldIM y :Zn (please refer to the ESI †). MIM 15 :AldIM 1 :Zn was successfully prepared through hydrothermal conditions using Zn(OAc) 2 in water/methanol at room temperature, whereas Characterisation of Ald-ZIF and TSC-ZIF FTIR measurements. A band at 1690 cm À1 corresponding to the stretching n(C]O) vibration of the carbonyl group was observed in the IR spectra of MIM 15 :AldIM 1 :Zn and MIM 3.5 :-AldIM 1 :Zn. This band disappears upon introducing the TSCfunctionalities, indicating successful post synthetic modication of Ald-ZIF. The conversion of the aldehyde groups, in Ald-ZIFs, to imine groups in TSC-ZIFs, was further conrmed by the strong band at 1604 cm À1 corresponding to the C]N stretching vibration. 70 Two new IR bands are also observed at 1047 and (1864) cm À1 , indicative of the presence of the thiosemicarbazone group corresponding to the n(C-N) and n(C]S) stretching vibrations, respectively (Fig. S1 in the ESI †). Since the linker contains a thioamide -NH-C]S functional group, it can exhibit the thione-thiol tautomerism. 71 The thiol n(S-H) band around 2570 cm À1 is absent from the IR spectra of the TSC-ZIFs, while the n(N-H) band is present at 3153 cm À1 , indicating that, in the solid-state, the linker remains as the thione tautomer. The proposed IR assignments of the ZIFs are in good agreement with literature data. [72][73][74] The introduction of the thioamide groups in MIM 3.5 :Bisthio 1 :Zn and MIM 3.5 :Thio 1 :Zn resulted in new vibrational bands, with characteristic absorptions at 2122 cm À1 corresponding to the n as (NH-C]S) modes. 71 NMR analysis. The degree of functionalization of Ald-ZIFs and TSC-ZIFs was determined by digesting the ZIFs under acidic conditions. The imine bond (linking the AldIM and the bisthio/ thiosemicarbazide) does not get affected by the acidic conditions. This adopted method follows the general trend of Scheme 1 Schematic illustration of (I) preparation of the two Ald-ZIFs (MIM x :AldIM y :Zn) and (II) post-synthetic modification (PSM) of Ald-ZIFs to obtain TSC-ZIFs through the incorporation of thiosemicarbazone derivatives (bisthiosemicarbazone and/or thiosemicarbazone) in aqueous solution.
cleaving MOFs, where the disassembly of the MOF takes place without cleaving the imine bond. 75 Nuclear Magnetic Resonance (NMR) measurements were performed aer digesting ZIF-8, Ald-ZIFs, and TSC-ZIFs in 80% deuterated solvent (DMSO-d 6 13 C NMR spectra of MIM 3.5 :Bisthio 1 :Zn and MIM 3.5 :Thio 1 :Zn exhibit two peaks at 142.03 and 178.07 ppm attributable to the C]N and C]S groups, respectively. The total transformation of the carbonyl groups in MIM x :AldIM y :Zn to bis/ thiosemicarbazone groups was demonstrated by the absence of the aldehyde proton peak at 9.69 and 9.17 ppm, indicating a nearly complete conversion of post-synthetic modication. This was further conrmed by disappearance of the 13 C NMR peak at 183 ppm, corresponding to the carbonyl group of the parent MIM 3.5 :AldIM 1 :Zn, in TSC-ZIFs ( Fig. S9 and S11 †).
Powder X-ray diffraction (PXRD) measurements. Crystallinity pattern and cubic framework structure of ZIF-8 was retained in Ald-ZIFs and TSC-ZIFs, as indicated by their PXRD diffraction data (the consistent peak positions and relative intensities as displayed in Fig. 1). 12 The PXRD diffraction patterns of the hybrid Ald-ZIF and TSC-ZIF match the diffraction patterns of the single-linker ZIF-8 structures, with all ZIFs exhibiting virtually identical cubic unit cells. Furthermore, XRD details of the reported ZIFs indicate that all samples have relatively the same framework topology with small differences in electron density and lattice constant.
The prominent reections at 2q ¼ 7.4 , 12.7 and 18.0 for the resulting ZIFs are clear, and are in good agreement with the simulated patterns for ZIF-8 using single crystal data (Fig. S12 †), with a typical SOD structure. 68 N 2 sorption-desorption isotherm. The dinitrogen sorption isotherms of the Ald-ZIFs and TSC-ZIFs were measured at 77 K, and the Brunauer-Emmett-Teller (BET) and pore volume of all the samples were calculated (Table 2 and Fig. S13 †). ZIF samples were degassed overnight at 423 K before surface area determination. All ZIF samples, including the parent ZIF-8, were analysed using the same protocol since sorption behaviour for ZIFs is sensitive to handling and pre-treatment procedures.
As demonstrated in Table 2, the calculated BET surface area for ZIF-8 is 1555 m 2 g À1 , matching reported values in the literature (1580 m 2 g À1 ). Given that the degree of post-synthetic modication and the size of the substituents dictate the available volume for the dinitrogen adsorption within the ZIF, 37,76 we expected the BET surface area and pore volume to decrease in the mixed-linker ZIFs, relative to ZIF-8. Indeed, all the mixedlinker ZIFs exhibit lower surface areas, with the higher aldehyde incorporation (MIM 3.5 :AldIM 1 :Zn) showing a more significant reduction in surface area than the lower aldehyde incorporation species (MIM 15 :AldIM 1 :Zn). Thus, the BET surface area of MIM 15 :AldIM 1 :Zn was found to be 1397 m 2 g À1 , marginally lower than that of ZIF-8. Whereas, the surface area of MIM 15 :Bisthio 1 :Zn and MIM 15 :Thio 1 :Zn is reduced relative to that of MIM 15 :AldIM 1 :Zn due to the decrease of internal void space associated with the introduction of the carbonyl groups. Similarly, post-synthetic modication of MIM 3.5 :AldIM 1 :Zn results in a more signicant decrease in BET surface area to 623 and 679 m 2 g À1 for MIM 3.5 :Bisthio 1 :Zn and MIM 3.5 :Thio 1 :Zn, respectively. This can be attributed to the higher degree of modication with bisthiosemicarbazone and thiosemicarbazone groups.
SEM-EDX measurements. Surface morphology and chemical composition of Ald-ZIFs and TSC-ZIFs were also investigated using SEM (Fig. 2) and EDX (Fig. S14 †). The crystals of original ZIF-8 and MIM 15 :AldIM 1 :Zn present cubic and rhombic   (Fig. 2). The EDX spectra of the TSC-functionalized ZIFs conrmed that the ZIF samples are composed of C, N, O, Zn, and S, as presented in Fig. S14. † The relative content of S in the functionalized TSC-ZIFs were determined by EDX spectra.
TGAs curves analysis. Thermal stability of the prepared Ald-ZIF and TSC-ZIF samples, relative to ZIF-8, was characterised by thermal gravimetric analysis, (TGA) (Fig. S15 †). Prepared Ald-ZIFs and TSC-ZIFs display relatively high thermal stability similar to that of ZIF-8. MIM 3.5 :AldIM 1 :Zn undergoes an initial weight loss at about 450 C, which can be attributed to the loss of carbonyl groups of the framework. A further weight loss at 550 C is observed for ZIF-8 and MIM 3.5 :AldIM 1 :Zn due to framework decomposition. MIM 3.5 :Bithio 1 :Zn and MIM 3.5 :-Thio 1 :Zn undergo weight loss at around 220 C, which is not present in the ZIF-8 and Ald-ZIF samples. This can be attributed to the decomposition of the bisthiosemicarbazone and thiosemicarbazone groups, respectively. However, MIM 15 -: Bithio 1 :Zn and MIM 15 :Thio 1 :Zn exhibit negligible percentage weight loss at this temperature due to the low percentage of the TSC-linker within the framework of the ZIF. DFT calculations. X-ray diffraction studies show that the incorporation of imidazole-4-carbaldehyde to the framework of ZIF-8 does not alter signicantly the structure of the ZIF. To get insight into the orientation of the imidazole-4-carbaldehyde (AldIM) and thiosemicarbazone group (Thio) within the structure of the ZIF, we performed DFT calculations at the b3lyp/6-31G(d,p) level. [77][78][79] The X-ray crystal structure of ZIF-8 was truncated to include 24 Zn(II) ions that dene the large cage of the structure, with 8 of the 60 2-methylimidazole (MIM) (supposed to be 1 to 3.5) groups being replaced by imidazole-4carbaldehyde. These calculations yielded the expected tetrahedral coordination of the Zn ions provided by the bridging imidazole groups, with Zn-N distances of 2.0-2.04Å (1.97Å in the X-ray structure). 80 Our DFT studies suggest that the carbaldehyde groups point inwards the six-membered hexagonal Zn rings, with the O atom being placed slightly below the mean plane dened by the six Zn ions (ca. 0.78Å, Fig. 3). Indeed, changing the orientation of one of the aldehyde groups of this model towards one of the pores, dened by four ZnN 4 tetrahedra, results in a signicant increase in energy of 7.8 kJ mol À1 . Subsequent calculations on the same model where two imidazole-4-carbaldehyde groups are replaced by thiosemicarbazone units suggest that the bulky thiosemicarbazone groups are also directed towards the large central pores of the structure.

Mercury(II) removal efficiency from water
The ability of the Ald-ZIF and TSC-ZIF derivatives to sequester mercury(II) from aqueous solutions was investigated at ambient conditions (room temperature and neutral pH). Adsorption studies were conducted over a wide range of known mercury concentrations (ppm), with the change in the adsorbent colour (yellow crystals in the case of MIM 3.5 :Thio 1 :Zn) to black at high mercury(II) concentrations serving as a preliminary indication of adsorption (Fig. S16 †).
Equations eqn (S1) and (S2) † were used to calculate the metal removal (%) from an aqueous solution where C i and C e represent the initial and equilibrium metal ion concentrations (mg L À1 ), respectively. The results for treating Hg(II) solutions with ZIF-8, Ald-ZIF and TSC-ZIF derivatives are presented in Fig. 4(a) and S17. † Treatment of a 100 mg L À1 aqueous Hg(II) solution with MIM 15 :Thio 1 :Zn and MIM 15 :Bithio 1 :Zn led to a 92.0% and 91.8% reduction in Hg(II) content within 30 min at ambient conditions. However, treating a Hg(II) solution of the same concentration, and under the same conditions, with ZIF-8 and MIM 15 :AldIM 1 :Zn resulted in 15% and 12% reduction, respectively (Fig. S17 †). An obvious increase in the adsorptive removal of mercury cation was observed in TSC-ZIFs incorporating a higher degree of functionality (ratio X 2 ¼ 3.5:  the treatment of a Hg(II) solution (C i ¼ 400 mg g À1 ) with MIM 3.5 :Thio 1 :Zn and MIM 3.5 :Bithio 1 :Zn resulted in 98.9% and 94.4% removal of the Hg(II) ion, respectively, with an unprecedented adsorption capacity (q m ) of 1667 mg g À1 and 1250 mg g À1 . This suggests that TSC-ZIFs possess both a high adsorption capacity and adsorption efficiency for the removal of mercury cations from water, in less than 2 hours and at ambient conditions (Fig. 4).
Adsorption isotherms for mercury(II) removal from water. The Langmuir (eqn (S3) and (S4) †), and Freundlich (eqn (S5) †) adsorption models were applied to analyse the obtained adsorption data for TSC-ZIFs. The experimental data t well the Langmuir equilibrium adsorption isotherm with a correlation coefficient of R 2 > 0.99 (Fig. S18, † 4(b) and Table 3). However, the tted Freundlich model resulted in a lower correlation coefficient (R 2 ¼ 0.92, Table S1 †) indicating that the adsorption process, follows a spontaneous single-layer chemical adsorption. 67,81 The maximum adsorption capacities of ZIFs reported in this study are presented in Table 3. The separation factor (R L ) was calculated to be between 0 and 1, indicating favourable adsorption of mercury cations into the prepared ZIFs (see Table  3). This can be attributed to the so sulfur donor atoms incorporated in two different ratios within the pores of ZIF structures. In particular, the incorporation of bisthiosemicarbazone and thiosemicarbazone containing groups enhances mercury extraction performance with respect to the parent ZIF-8. Adsorption capacity of MIM 3.5 :Thio 1 :Zn exceeds the values recently reported for porous functionalized ZIFs (see Table  1). 64,65,67,68 Adsorption kinetics. In order to evaluate the kinetic mechanism controlling the adsorption process, the effect of contact time between Hg(II) and the adsorbents on the adsorption process was investigated.
The adsorption rate constant (k 2 ¼ 0.32 Â 10 À2 g mg À1 min À1 ) of MIM 3.5 :Thio 1 :Zn exceeds many other reported porous absorbents in the literature. 65 This can be attributed to the higher degree of thiosemicarbazone incorporation. The steric demands of the thiosemicarbazone group occupying the inner surface of the pores and the high density of MIM 3.5 :-Thio 1 :Zn adsorption sites give this TSC-ZIF the best performance.
Characterisation of TSC-ZIFs aer removal of mercury(II). PXRD patterns of MIM 3.5 :Thio 1 :Zn did not change aer the adsorption of Hg(II) (Fig. 6(b)).The co-existence of Hg(II) with the TSC-ZIFs is observed in SEM images (Fig. S21 †) and EDX analysis (Fig. S22 †). Similarly, TGA measurements of the Hg(II) adsorbed onto MIM 3.5 :Thio 1 :Zn show one mass loss step at about 350 C. This temperature is higher than that observed for MIM 3.5 :Thio 1 :Zn, conrming that the adsorbent maintained a stable framework structure aer the adsorption process (Fig. S23 †).
Competitive binding (binary and tertiary systems). To evaluate the selectivity of MIM 3.5 :Thio 1 :Zn for Hg(II) ion adsorption, we performed experiments in the presence of Pb(II) and Cd(II) as potential interfering species.
Binary adsorption with Pb(II). Binary metal containing systems were prepared using a xed concentration of [Pb(II)] ¼ 1000 mg L À1 and a mercury concentration [Hg(II)] ranging from 100 to 400 mg L À1 . The percentage removal of both metal ions,  existing in the binary system, is presented in Fig. 5(a). As depicted in the gure, MIM 3.5 :Thio 1 :Zn exhibits high removal efficiency for Hg(II) and low removal for Pb(II) ions, demonstrating a higher selectivity for Hg(II). Meanwhile, at higher mercury cations concentration, a co-adsorption induces a decrease in its adsorption. Previous studies have explained the removal of the metal ions in the competitive adsorption system is based on the comparative assessment of their initial adsorption rates. 82,83 Tertiary system with Pb(II) and Cd(II). The concentration of Pb(II) and Cd(II) ions in the mixed solution was set each to 1000 mg L À1 and the [Hg(II)] ranging from 100 to 400 mg L À1 . As illustrated in Fig. 5(b), interference of the two metal ions minimally disturbs the removal efficiency for Hg(II) ions, given that MIM 3.5 :Thio 1 :Zn exhibits lower removal efficiency towards Cd(II) and Pb(II) ions. Analysis of the removal efficiency values revealed that the order of adsorption was Hg(II) > Pb(II) [ Cd(II). Removal efficiency for Cd(II) and Pb(II) decreases when the concentration of [Hg(II)] increases, which demonstrates the selective adsorption for Hg(II). Besides, the presence of Cd(II) in the tertiary solution enhanced the removal efficiency for Hg(II) and Pb(II) (Fig. 5(b)).
This selective adsorption for Hg(II) ions can be attributed to the higher affinity of thiosemicarbazone groups for Hg(II) compared to other metal ions. 67 Regeneration of MIM 3.5 :Thio 1 :Zn. In actual applications, reusability of adsorbents is crucial and reects on the sustainability of the developed adsorbent. The reusability of MIM 3.5 :-Thio 1 :Zn was assessed through cycles of regeneration of the ZIF in solution using p-toluene sulfonic acid (pH ¼ 4) as a desorbent. Inspired by the literature, acidic conditions are expected to weaken the interaction between the adsorbate and adsorbent allowing for the regeneration of the TSC-ZIF. 68 The relative efficiency of the removal of Hg(II) in each through cycles of adsorption-desorption of mercury by MIM 3.5 :Thio 1 :Zn are presented in Fig. 6(a).
Over the ve cycles, the amounts of mercury adsorbed decreased slightly with increasing the number of cycles, which might be caused by the loss of material during the recycling process. However, the adsorption efficiency was maintained at approximately 75% for the highest concentration of mercury(II)/ ([Hg(II)] i ¼ 700 ppm) of each cycle, indicating that MIM 3.5 :-Thio 1 :Zn can be regenerated for cycles of mercury removal without compromising its removal efficiency. PXRD patterns of the recycled MIM 3.5 :Thio 1 :Zn (aer the ve cycle) were in good agreement with their PXRD patterns before adsorption ( Fig. 6(b)). This demonstrates the high stability of the TSC-ZIF aer the removal of mercury.

Conclusions
A new class of aldehyde-based zeolitic imidazolate frameworks (Ald-ZIF) was developed to serve as a precursor, which can be modied for the removal of mercury cations from water. Bisthiosemicarbazone and thiosemicarbazone are the functional groups introduced, through post-synthetic modication, to the new class of Ald-ZIF resulting in the formation of four classes of TSC-ZIF derivatives. TSC-ZIF contain pendent thiosemicarbazone groups within the pores of the material. The degree of functionalization of Ald-ZIF was monitored using IR and NMR spectroscopies. Structural and thermal integrity of the TSC-ZIF were conrmed using PXRD studies, SEM-EDX and TGA analysis. The porosity of the TSC-ZIF derivatives (as measured using BET surface area calculations) are reduced relative to ZIF-8, depending on the degree of functionalization and size of introduced substituents. Sequestration of mercury(II) from water at room temperature and neutral pH was achieved when treating Hg(II) contaminated water with TSC-ZIF derivatives. Among TSC-ZIF derivatives, MIM 3.5 :Thio 1 :Zn showed the highest capacity for mercury(II) ions due to the higher ratio of pore functionality, combined with the lower steric demands of the TSC group. Moreover, MIM 3.5 :Thio 1 :Zn showed selectivity for Hg(II) in solutions containing competitive Pb(II) and Cd(II) metal ions. MIM 3.5 :Thio 1 :Zn was regenerated for up to four cycles of mercury(II) removal without compromising the efficiency or structure of the ZIF. Therefore, TSC-ZIFs, as a new class of zeolitic frameworks, demonstrate promising adsorption capacity for heavy metals. Further work may provide a plethora of opportunities for generating new functionalized materials for other heavy metals and anions adsorption.

Conflicts of interest
There are no conicts to declare.