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High-performance removal of mercury ions (II) and mercury vapor by SO3−-anchored covalent organic framework
Journal Pre-proof − High-performance removal of mercury ions (II) and mercury vapor by SO3 -anchored covalent organic framework Yuan Tao, Xiao Hong Xiong, Jian Bo Xiong, Li Xiao Yang, Ya Lin Fan, Han Feng, Feng Luo PII:
S0022-4596(19)30631-0
DOI:
https://doi.org/10.1016/j.jssc.2019.121126
Reference:
YJSSC 121126
To appear in:
Journal of Solid State Chemistry
Received Date: 30 October 2019 Revised Date:
4 December 2019
Accepted Date: 11 December 2019
Please cite this article as: Y. Tao, X.H. Xiong, J.B. Xiong, L.X. Yang, Y.L. Fan, H. Feng, F. Luo, High− performance removal of mercury ions (II) and mercury vapor by SO3 -anchored covalent organic framework, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/j.jssc.2019.121126. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Author Contributions Section Conceived and designed the experiments: Feng Luo. Performed the experiments: Yuan Tao and Xiao Hong Xiong. Analyzed the data: Yuan Tao and Xiao Hong Xiong. Contributed reagents/materials/analysis tools: Jian Bo Xiong, Li Xiao Yang, Ya Lin Fan and Han Feng.
High-Performance Removal of Mercury Ions (II) and Mercury Vapor by SO3--Anchored Covalent Organic Framework Yuan Tao,† Xiao Hong Xiong,† Jian Bo Xiong, Li Xiao Yang, Ya Lin Fan, Han Feng and Feng Luo*
1
High-Performance Removal of Mercury Ions (II) and Mercury Vapor by SO3--Anchored Covalent Organic Framework Yuan Tao,† Xiao Hong Xiong,† Jian Bo Xiong, Li Xiao Yang, Ya Lin Fan, Han Feng and Feng Luo* State Key Laboratory for Nuclear Resources and Environment, and School of Biology, Chemistry and Material Science, East China University of Technology, Nanchang, Jiangxi 344000, China, [email protected]. †
These authors are the co-first author.
ABSTRACT: In this work, we report a SO3--anchored covalent organic framework for the elimination of mercury pollution. The adsorbent of [NH4]+[COF-SO3−] enables ultrahigh Hg2+ adsorption capacity up to 1299 mg/g and a recorded uptake of 932.6 mg/g for Hg0 among all reported porous solid adsorbents for such use. The high selective adsorption towards Hg2+ is demonstrated by the large distribution coefficient (Kd) of 2.3× 105 mL/g. Moreover, the high selectivity tested by breakthrough experiment was achieved even among different disturbed metal ions (Pb2+, Cd2+, Zn2+, Ni2+, Co2+, Mg2+, K+). Moreover, the adsorbent also shows great recycle without detectable change of Hg2+ capture and structural damage even after four recycle. This remarkable adsorption capacity in conjunction with the fast adsorption kinetics, the strong binding affinity, high selectivity, and good recycle use makes it as a good solid adsorbent for mercury.
Keywords ： SO3--Anchored;
covalent
organic
framework;
mercury
ions(II);mercury vapor;
high-performance removal
1. INTRODUCTION In the present highly industrialized world, eliminate heavy metal pollution has become a globally unified destination.[1] Among these pollutants, mercury (Hg2+ and Hg0) is one of the most toxic species. For example, long-term exposure to mercury can cause various diseases in humans and animals such as the Minamata disease.[2] However, in our daily life the use of mercury-containing products like that of batteries, cosmetics and 1
electronic devices is ineluctable and prevailing.[3] Thereby, the removal of mercury has become a urgent issue but still a long-term and challenging target. In the previous reports, numerous methods have been explored for removing mercury, including chemistry precipitation, membrane separation, ion exchange and adsorption.[4-7] Compared with other methods, solid adsorption technology is one of the best choices due to its low cost, simple operation and high efficiency.[8-9] Nevertheless, this adsorption technology based on traditional porous adsorbents such as activated carbon,[10] zeolites,[11] hydrogels[12] was seriously restricted by some defects such as low adsorption capacity, poor selectivity, and slow adsorption kinetics. Recently, driven by the Hg-S coordination interactions, an effective approach with the porous adsorbent anchored by free-standing thio-/thiol-functionalized groups through the design of desired ligand or the post-synthesis modification (PSM) procedure was proposed.[13-19] For example, Xu et al through the design of a thiol-laced ligand constructed a thiol-functionalized UiO-66 like counterpart and the resulted metal-organic framework can effectively remove Hg2+ with residual concentration less than 0.01 mg L-1 form a 10 mg L-1 Hg2+ solution.16 On the other hand, PSM method was attested to be a powerful tool to anchor thiol-functionalized organic unit within the channel of porous adsorbents.[20-23] For example, Ma et al synthesized a thiol-modified COF of COF-S-SH through PSM method via thiol-ene “click” reaction. This material afforded ultrahigh mercury adsorption capacity, viz. 1350 mg/g for Hg2+ and 863 mg/g for Hg0.[21] Whereas the predesign of thiol-containing organic ligand or PSM with thiol-modification, both of them were restricted by the harsh conditions and expensive reagents in the synthetic aspect from the consideration of practical application. In this regard, we recently proposed a simple and general photoassisted multicomponent PSM method to prepare a sulfur-modified ZIF-90 derivate, based on very cheap materials. The resulted material gave 596 mg/g Hg2+ capture, and a 12.7-fold enhancement in the adsorption capacity was observed, relative to the pristine ZIF-90.[24] Moreover, Hg-S interactions provided by a inorganic part was also proposed to solve the above problem. Jiang et al prepared a composite material of In2S3@MIL-101, where the inorganic part of In2S3 affords Hg-S interactions to selectively capture Hg2+ for the solution, leading to a high adsorption capacity of 518.2 mg/g.[25] In contrast to the synthesis of sulfur-modified porous adsorbents, the synthesis of nitrogen- or oxygen-modified counterpart is more inexpensive and facile. In principle, Hg-N and Hg-O bonds are also strong coordination interactions. Hence, exploitation of nitrogen- or oxygen-modified porous adsorbents with excellent 2
mercury capture is also highly desirable. However, in the literature just low adsorption capacity was observed in this category.[26] In this work, we report a breakthrough in this category. The porous adsorbent of [NH4]+[COF-SO3-] shows abundant free-standing SO3--units within the channel, providing strong Hg-O coordination interaction to selectively capture Hg2+ and Hg0 with ultrahigh adsorption capacity up to 1299 mg/g and 932.6 mg/g, respectively. 2. Experimental 2.1 Materials Reagents and solvents were commercially available and were used without further purification. 2,4,6-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid were purchased from Ark Pharm, inc.1,2-dichlorobenzene, n-butanol and other chemicals were purchased from Macklin (Shanghai) Inc and were used without further purification. Deionized water was used in the experiments. 2.2 Synthesis of COF-SO3H. 0.3 mmol (63 mg) of 2,4,6-triformylphloroglucinol, 0.45 mmol (84.7 mg) 2,5-diaminobenzenesulfonic acid was added into a Pyrex tube with 1.5 mL butyl alcohol and 1.5mL 1,2-dichlorobenzene. The mixture was sonicated for 20 min, followed by addition of 0.5 mL of 3 M aqueous acetic acid. After that, the tube was degassed by freeze-pump-thaw cycles for three times, sealed under vacuum and heated at 120 °C for 3 days. The reaction mixture was cooled to room temperature and washed with deionized water, dimethylacetamide and acetone. The resulting dark red powder was dried at 120 °C under vacuum for 12 hours. 2.3 Synthesis of [NH4]+[COF-SO3-]. 50 mg COF-SO3H was added into 20 mL glass bottles with 10 mL (1%) ammonium hydroxide and stir for 24 hours. Then the resultant materials were washed by deionized water and methanol. The resulting dark red powder was dried at 60 °C under vacuum for 12 hours. 2.4 Measurements
X-ray powder diffraction were collected by a Bruker AXSD8 Discover powder diffractometer at 40 kV, 40 mA for Cu Kλ ( λ= 1.5406 Å). The simulated powder patterns were calculated by Mercury 1.4. Infrared Spectra (IR) were measured by a Bruker VERTEX70 spectrometer in the 500-4000 cm-1 region. The gas adsorption isotherms were collected on a Belsorp-max. Ultrahigh-purity-grade (>99.999%) N2 3
gases were used during the adsorption measurement. SEM and EDS measurements were carried out using a Hitachi S-4800 microscope. The analyses of concentrations of metal ions in the solution was carried out by ThermoFisher iCapQ ICP-MS and ThermoFisher iCap7600 ICP-OES instruments. X-ray photoelectron spectra (XPS) were collected by Thermo Scientific ESCALAB 250 Xi spectrometer.
3. Methods 3.1 Mercury uptake via batch experiments. The starting Hg2+ stock solution was made by dissolving 1.7082 g Hg(NO3)2·H2O in 1000 mL deionized water to create an 1000 mg L-1 Hg2+ solution. The pH value was adjusted by HNO3(Guaranteed reagent) or NaOH aqueous solution. All the adsorption experiments were conducted at 298 K. In pH-dependent experiments, the Hg2+ solution with pH=3~8 was adjusted by HNO3(Guaranteed reagent) and NaOH . The dose of adsorbent is 10 mg, the Hg2+ solution is 30 mL (300 mg/L)，and the contact time is 2 h. In isotherm experiments, the Hg2+ solution with initial concentration of 50-450 mg L-1 and pH= 6 was used. The dose of adsorbent is 10 mg, the Hg2+ solution is 30 mL and the contact time is 2 h. In kinetics experiments, the Hg2+ solution with initial concentration of 400 mg L-1 and pH=6 was used. The dose of adsorbent is 10 mg, the Hg2+ solution is 30 mL. In determining affinity experiments, the Hg2+ solution with initial concentration of 10.25 mg L-1 and pH=6 was used. The dose of adsorbent is 10 mg, the Hg2+ solution is 100 mL. In selective adsorption experiments, a mixed solution contains Hg2+, K+, Mg2+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+ with initial concentration of 50 mg L-1 and pH= 6 was used. The dose of adsorbent is 10 mg, while the solution is 100 mL. In the recycle experiment, Hg2+ solution with initial concentration of 25 mg L-1 at pH=6 was used. The dose of adsorbent of 10 mg, V= 30 mL and the contact time of 2 h was used. 20 mL, 2 M HCl (Guaranteed reagent) was used for elution, the desorption time was 2 h. 3.2 Mercury uptake via breakthrough experiments.
4
As shown in Figure S6, the packed bed was prepared by filling [NH4]+[COF-SO3-] adsorbent (300 mg) into a glass tube with inner aperture of 0.8 mm and length of 1 m. The experiment was carried out at room temperature and a flow rate of 0.05 ml/min for the outflow solution was used. The mixed solution (pH=6) is composed of K+, Mg2+, Zn2+, Cd2+, Pb2+, Hg2+ and their initial concentration is 50 mg L-1. 3.3 Mercury vapor capture experiment. About 400 mg metallic mercury used for elements mercury capture was conducted in a closed pressure resistant glass tube. A small glass vial (4 mL) with 20 mg [NH4]+[COF-SO3-] was placed into a 25 mL pressure resistant glass tube, then capped and placed in the oil bath where the temperature was kept at 140℃for 12 h. Then it is cooled to room temperature. The Hg0-loaded samples was obtained and the Hg0 was isolated from COF material by 1 mol/L HCl. The obtained solution was filtered through a 0.22 um filtering membrane, and the concentration of mercury was detected by ICP-MS.
4 Results and discussion
4.1 Characterization The [NH4]+[COF-SO3-] adsorbent was obtained by immerging COF-SO3H in NH3·H2O. And the COF-SO3H was synthesized by common solvothermal reaction of 2,4,6-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid at 120 ℃ for 72 hours. The synthesis in detail was listed in the Supporting Information. Its structure (Figure 1a) has been discussed in our previous work.[27] The comparison shows that the color of COF-SO3H and [NH4]+[COF-SO3-] has changed slightly darker, and the color of [NH4]+[COF-SO3-] and loaded mercury basically unchanged (Figure S11).
5
Figure 1. (a) View of the structure of [NH4]+[COF-SO3-]. Due to the existence of SO3- group, the π-π interactions between adjacent layers shows somewhat offset, rather than common fashion. The SO3- groups in the adjacent layers show an opposite arrangement. The NH4+ ions are stabilized by the hydrogen bonds (highlighted broken line). The color code for these atoms is S/yellow, C/gray, H/green, O/red, N/blue. (b) The SEM images and the insert of the mapping of C, N, S, O elements by the energy dispersive spectrometry (EDS). (c) The IR spectrum of [NH4]+[COF-SO3-]. The COF-SO3H and [NH4]+[COF-SO3-] samples were further characterized by the scanning electron microscopy (SEM, Figure 1b), powder X-ray diffraction (PXRD, Figure S1), infrared spectrum (IR, Figure 1c), and X-ray photoelectron spectroscopy (XPS). All the above results were consistent with our recent report. The success of introducing [NH4]+ into the COF channel is approved by the emergence of new peaks in the IR at 3194 cm-1 (Figure S2) and in the XPS at 402 eV (Figure 2). Through the BET test, we found that COF-SO3H has a specific surface area of 300.2 m2/g, but the modified [NH4]+[COF-SO3−] has a lower specific surface area 110.6 m2/g, and the pores are also reduced by the introduction of ammonia ions (Figure S3).
6
Figure 2. The XPS of N element for samples of COF-SO3H and [NH4]+[COF-SO3-]. The highlight is derived from [NH4]+.
4.2 Mercury adsorption. 4.2.1 pH affecting. The mercury adsorption performance of [NH4]+[COF-SO3-] was initially explored in the solution at pH=3-8 (Figure 3). Generally speaking, the adsorption performance of solid adsorbent is highly dependent on the pH value of solution, because this will significantly affect the external charge and consequently electrostatic interactions between target ions and adsorbent. As shown in Figure 3, the optimal pH value for both COF-SO3H and [NH4]+[COF-SO3-] is six, giving 725 mg/g and 806 mg/g respectively. The adsorption performance of [NH4]+[COF-SO3-] exceeds COF-SO3H, most possibly because the deprotonation from the replacement of [NH4]+ by H+ benefits the cation exchange between [NH4]+ and Hg2+ and next Hg-O coordination interactions between SO3- unit and Hg2+ ions.
7
Figure 3. The pH-dependent adsorption for samples of COF-SO3H and [NH4]+[COF-SO3-] (C0 = 300 mg L-1, madsorbent = 10 mg, V = 30 mL, t = 2 h).
4.2.2 Adsorption kinetics and isotherms. The rate of adsorption is an importantly decisive index for a solid adsorbent. Approximately within 10 min, the adsorption equilibrium was reached for COF-SO3H and [NH4]+[COF-SO3-], giving 928 mg/g and 1044 mg/g (Figure 4a, S4), respectively, suggesting fast adsorption kinetics. The adsorption data obeys the pseudo-second-order kinetic model with R2 more than 0.999, suggesting a chemical adsorption process (Figure 4a, Table S1),[20-24] in line with the structure containing abundant free-standing SO3- or SO3H units for binding Hg2+. This fast adsorption kinetics is also comparable with many previously reported sulfur-modified porous adsorbents.[13-24] The adsorption capacity of [NH4]+[COF-SO3-] was estimated from an aqueous solution with initial Hg2+ concentration among 50-450 mg L-1. The maximal adsorption capacity is about 1299 mg/g (Figure 4b), and the theoretical adsorption capacity, calculated from the fitting by the Langmuir model, is as high as 1550 mg/g (Figure 4b, Table S2). Such outstanding adsorption performance surpasses most reported sulfur-containing porous adsorbents composed of thiol-modified metal-organic frameworks such as SH-MiL-68(In)(450 mg/g),[23] [Cu3(BTC)2]-SH (714 mg/g),[28] CaII CuII6[(S,S)-methox]3(OH)2(H2O)} (900 mg/g),[13] thiol-modified covalent organic frameworks such as TAPB-BMTTPA-COF (734 mg/g),[22] POP-SH (1216 mg/g),[21] and PAF-1-SH 8
(1014 mg/g),[20a] as well as inorganic sulfur-anchored adsorbents like that of In2S3 @MIL-101 (518 mg/g),[25] KMS-1 (377 mg/g)[29]. Notably, the experimental maximal adsorption capacity in [NH4]+[COF-SO3-] is comparable with the best mercury adsorbent of COF-S-SH (1350 mg/g), while the theoretical adsorption capacity even exceeds this benchmark value. To emphasize the deprotonation effect, the adsorption isotherm of COF-SO3H was investigated under the same conditions (Figure S5). The experimental maximal adsorption capacity and the calculated saturation adsorption capacity is 1033 mg/g and 1041 mg/g, respectively. This value is far lower than that observed in[NH4]+[COF-SO3-], strongly supporting the deprotonation effect on the Hg2+ adsorption performance.
Figure 4. (a) The adsorption kinetics of Hg2+ upon [NH4]+[COF-SO3-] (C0=400 mg L-1, madsorbent=10 mg, V=30 mL, pH=6). The insert is the fitting results by the pseudo-second-order kinetic model. (b) The adsorption isotherms of Hg2+ upon [NH4]+[COF-SO3-] (t=2 h, madsorbent=10 mg, V=30 mL, pH=6). The insert is the fitting results by the Langmuir model.
4.2.3 Adsorption affinity. The above results imply strong binding affinity between [NH4]+[COF-SO3-] and Hg2+ ions. To quantificationally estimate the binding affinity, we further carried out the measurement of distribution coefficient (Kd) from a 10 mg L-1 solution. After a contact time of 2000 min, more than 99% Hg2+ ions were removed from the solution. And the Kd value is calculated to be 2.13×105 ml/g (Figure 5a), which is comparable with many reported benchmark adsorbents (Table S3) such as TAPB-BMTTPA-COF (7.82×105 ml/g),[22] Zr-DMBD (9.99×105 ml/g),[16] [Cu3(BTC)2]-SH (4.73×105 ml/g),[28] and CaIICuII6[(S,S)-methox]3(OH)2(H2O)} (6.67×105 9
ml/g)[13]. The results strongly suggest that the binding affinity from the present COF most possibly driven by Hg-O interactions between SO3- and Hg2+ ions is as powerful as observed in the sulfur-containing porous adsorbents featuring by the Hg-S interactions. And it can be seen from PXRD that the powder structure remains stable after adsorption (Figure S12). 4.2.4 Relative selectivity. Next, the selectivity tests were also performed on [NH4]+[COF-SO3-] from a mixed solution containing Pb2+, Cd2+, Zn2+, Ni2+, Co2+, Mg2+, K+. High selectivity (more than 160) was observed for almost all tested metal ions, except for Pb2+ with a selectivity of 9.9 (Figure 5b). To further confirm this remarkable selectivity , we further carried out the breakthrough experiments (Figure S6), and similar trend was observed. The selective adsorption of Hg2+ over Zn2+, Ni2+, Co2+, Mg2+, K+ is excellent, however, [NH4]+[COF-SO3-] also enables good removal for Pb2+ ions, as evidenced by the breakthrough experiments even after the contact time of 416 h (Figure 5c). Low Hg2+/Pb2+ selectivity was also a common issue for many reported sulfur-containing porous adsorbents, mainly due to their comparable physical and chemical properties.[19-24] 4.3 Cyclic sorption / desorption studies. Moreover, this [NH4]+[COF-SO3-] adsorbent also shows good recycle use. As shown in Figure 5d, the desorption efficiency by 2M HCl is more than 99.9%, and the adsorbent even after four recycle shows no detectable decrease in the Hg2+ capture.
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Figure 5. (a) The adsorption affinity (Kd value, C0=10 mg L-1, madsorbent=10 mg, V=100 mL, pH=6). (b) The selectivity calculated from a multi-component adsorption experiment (C0=50 mg L-1 for metal ions, madsorbent=10 mg, V=100 mL, t=120 min, pH=6). (c) The breakthrough experiment (madsorbent=300 mg, C0=50 mg L-1 for metal ions, pH=6). (d) The recycle use of [NH4]+[COF-SO3-] showing neglectable variety in both the adsorption efficiency and desorption efficiency.
4.4 Eliminating mercury vapor and potential application. All the above results imply its superior application in Hg2+ removal from the aqueous solution. In fact, the removal of mercury vapor (Hg0) is also highly important in the industrial flue gas detoxification. In this regard, we further evaluated the mercury vapor remediation upon [NH4]+[COF-SO3-]. The test method is referenced to the reported method by Ma et al (Figure S7).[20-21] After the contact time of 12 h, the Hg0 adsorption capacity is as high as 932.6 mg/g, exceeding the best Hg0 porous adsorbent of COF-S-SH (863 mg/g), highlighting its promising application in eliminating mercury vapor. 4.5 Mechanisms of mercury sorption. 11
Figure 6. A comparison of Hg adsorption among these samples of COF-SO3H, [NH4]+[COF-SO3-], and TpPa-1 with the sample COF skeleton.
To disclose the adsorption mechanism, the same COF skeleton without any functionalized organic units within the channel, namely TpPa-1 (Figure S8),[30] was also explored about Hg2+ capture for a comparison. As expected, the TpPa-1 material just shows 420 mg/g adsorption capacity, while [NH4]+[COF-SO3-] and COF-SO3H gives 1110 mg/g, 1050 mg/g, respectively, under the same conditions (Figure 6). This direct evidence suggests such significant enhancement (more than two-fold) is due to the introduction of free-standing -SO3H or -SO3- units. Furthermore, the disappearance of characteristic peaks for [NH4]+ in IR (Figure S9) and XPS spectrum (Figure 7) was observed for the samples after Hg2+ capture, mainly due to ion exchange between [NH4]+ and Hg2+ ions. In addition, clear electron transfer for S was observed for the Hg2+-loaded samples in the XPS spectrum of S element (Figure S10), relative to that observed in [NH4]+[COF-SO3-], mainly due to the Hg-O coordination interactions between -SO3- units and Hg2+ ions. Accordingly, the overall adsorption mechanism for the present porous adsorbent of [NH4]+[COF-SO3-] is first cation exchange between [NH4]+ and Hg2+ ions and then binding of Hg2+ ions in the channel via Hg-O coordination interactions between -SO3- units and Hg2+ ions.
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Figure 7. A comparison of XPS of N element for these samples of COF-SO3H, [NH4]+[COF-SO3-], and Hg-loaded samples. The highlight shows the distinct change derived from the replacement by [NH4]+ or by Hg2+.
Conclusion In conclusion, we have shown in this work a porous SO3-- anchored COF adsorbent for the application in the removal of mercury from the aqueous solution (Hg2+) and vapor(Hg0). The results show that [NH4]+[COF-SO3-] enables ultrahigh adsorption capacity for both Hg2+ and Hg0, fast adsorption kinetics, strong binding affinity, high selectivity, and good recycle use, making it as a promising porous adsorbent for practical application in pollution control of mercury. The adsorption mechanism, as unveiled by comparison experiment in conjunction with IR and XPS characterizations, is first ion exchange and next coordination interaction of Hg-O, very different form the well-known sulfur modification approach (Hg-S), thus outlining a new pathway to design porous adsorbents with excellent mercury removal performance.
Supporting Information This includes Experimental Procedures, 12 figures, and 3 tables.
13
Acknowledgement We thanks to the National Science Foundations of China (21871047, 21661001, and 21966002), the Natural Science Foundation of Jiangxi Province of China (20181ACB20003, 20192BAB203002).
Declaration of interests The authors declare no competing interests.
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Y. Xie, S. J. Yu, C. L. Xiao, F. Luo, J. Wang, X. L. Wang, C. L. Chen, W. S. Wu, W. Q. Shi, S. A. Wang, X. K. Wang, Synthesis of novel nanomaterials and their application in efficient removal of radionuclides, Sci. China Chem. 62 (2019) 933-967. [21] B. Aguila, Q. Sun, J. A. Perman, L. D. Earl, C. W. Abney, R. Elzein, R. Schlaf, S. Q. Ma, Efficient mercury capture using functionalized porous organic polymer, Adv. Mater. 29 (2017) 1700665. [22] N. Huang, L. P. Zhai, H. Xu, D. L. Jiang, Stable covalent organic frameworks for exceptional mercury removal from aqueous solutions, J. Am. Chem. Soc. 139 (2017) 2428-2434. [23] G. P. Li, K. Zhang, P. F. Zhang, W. N. Liu, W. Q. Tong, L. Hou, Y. Y. Wang, Thiol-functionalized pores via post-synthesis modification in a metal-organic framework with selective removal of Hg(II) in water, Inorg. Chem. 58 (2019) 3409-3415. [24] W. H. Yin, Y. Y. Xiong, H. Q. Wu, Y. Tao, L. X. Yang,; J. Q. Li, X. L. Tong, F. Luo, Functionalizing a metal-organic framework by a photoassisted multicomponent postsynthetic modification approach showing highly effective Hg(II) removal, Inorg. Chem. 57 (2018) 8722-8725. [25] L. F. Liang, L. Y. Liu, F. L. Jiang, C. P. Liu, D. Q. Yuan, Q. H. Chen, D. Wu, H. L. Jiang, M. C. Hong, Incorporation of In2S3 nanoparticles into a metal-organic framework for ultrafast removal of Hg from water, Inorg. Chem. 57 (2018) 4891-4897. [26] (a) F. Luo, J. L. Chen, L. L. Dang, W. N. Zhou, H. L. Lin, J.Q. Li, S. J. Liu, M. B. Luo, High-performance Hg2+ removal from ultralow-concentration aqueous solution using both acylamide- andhydroxyl-functionalized metal-organic framework, J. Mater. Chem. A 3(2015) 9616-9620. (b) Y. L. Hou, K. K. Yee, Y. L. Wong, M. Zha, J. He, M. Zeller, A. D. Hunter, K. Yang, Z. Xu, Metalation triggers single crystalline order in a porous solid, J. Am. Chem. Soc. 138 (2016) 14852-14855. (c) N. D. Rudd, H. Wang, E. M. A. Fuentes-Fernandez, S. J. Teat, F. Chen, G. Hall, Y. J. Chabal, J. Li, Highly efficient luminescent metal-organic framework for the simultaneous detection and removal of heavy metals from water, ACS Appl. Mater. Inter. 8 (2016) 30294-30303. [27] X. H. Xiong, Z. W. Yu, L, L. Gong, Y. Tao, Z. Gao, L. Wang, W. H. Yin, L. X. Yang, F. Luo, Ammoniating covalent organic framework (COF) for high-performance and selective extraction of toxic and radioactive uranium ions, Adv. Sci. 6 (2019) 1900547.
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[28] F. Ke, L. G. Qiu, Y. P. Yuan, F. M. Peng, X. Jiang, A. J. Xie, Y. H. Shen, J. F. Zhu, Thiol-functionalization of metal-organic framework by a facile coordination-based postsynthetic strategy and enhanced removal of Hg2+ from water, J. Hazard. Mater. 196 (2011) 36-43. [29] M. J. Manos, M. G. Kanatzidis, Sequestration of heavy metals from water with layered metal sulfides, Chem. - Eur. J. 15 (2009) 4779-4784. [30] S. Karak, S. Kandambeth, B. P. Biswal, H. S. Sasmal, S. Kumar, P. Pachfule, R. Banerjee, Constructing ultraporous covalent organic frameworks in seconds via an organic terracotta process, J. Am. Chem. Soc. 139 (2017) 1856-1862.
17
1. [NH4]+[COF-SO3-] enables ultrahigh adsorption capacity (1299 mg/g) for Hg (II).
2. The work give the first report about removing Hg2+ via coordination interaction of Hg-O.
3. [NH4]+[COF-SO3-] shows the record adsorption capacity (932.6 mg/g) of mercury vapor.
1
The authors declare no competing interests.
S0022-4596(19)30631-0
DOI:
https://doi.org/10.1016/j.jssc.2019.121126
Reference:
YJSSC 121126
To appear in:
Journal of Solid State Chemistry
Received Date: 30 October 2019 Revised Date:
4 December 2019
Accepted Date: 11 December 2019
Please cite this article as: Y. Tao, X.H. Xiong, J.B. Xiong, L.X. Yang, Y.L. Fan, H. Feng, F. Luo, High− performance removal of mercury ions (II) and mercury vapor by SO3 -anchored covalent organic framework, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/j.jssc.2019.121126. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Author Contributions Section Conceived and designed the experiments: Feng Luo. Performed the experiments: Yuan Tao and Xiao Hong Xiong. Analyzed the data: Yuan Tao and Xiao Hong Xiong. Contributed reagents/materials/analysis tools: Jian Bo Xiong, Li Xiao Yang, Ya Lin Fan and Han Feng.
High-Performance Removal of Mercury Ions (II) and Mercury Vapor by SO3--Anchored Covalent Organic Framework Yuan Tao,† Xiao Hong Xiong,† Jian Bo Xiong, Li Xiao Yang, Ya Lin Fan, Han Feng and Feng Luo*
1
High-Performance Removal of Mercury Ions (II) and Mercury Vapor by SO3--Anchored Covalent Organic Framework Yuan Tao,† Xiao Hong Xiong,† Jian Bo Xiong, Li Xiao Yang, Ya Lin Fan, Han Feng and Feng Luo* State Key Laboratory for Nuclear Resources and Environment, and School of Biology, Chemistry and Material Science, East China University of Technology, Nanchang, Jiangxi 344000, China, [email protected]. †
These authors are the co-first author.
ABSTRACT: In this work, we report a SO3--anchored covalent organic framework for the elimination of mercury pollution. The adsorbent of [NH4]+[COF-SO3−] enables ultrahigh Hg2+ adsorption capacity up to 1299 mg/g and a recorded uptake of 932.6 mg/g for Hg0 among all reported porous solid adsorbents for such use. The high selective adsorption towards Hg2+ is demonstrated by the large distribution coefficient (Kd) of 2.3× 105 mL/g. Moreover, the high selectivity tested by breakthrough experiment was achieved even among different disturbed metal ions (Pb2+, Cd2+, Zn2+, Ni2+, Co2+, Mg2+, K+). Moreover, the adsorbent also shows great recycle without detectable change of Hg2+ capture and structural damage even after four recycle. This remarkable adsorption capacity in conjunction with the fast adsorption kinetics, the strong binding affinity, high selectivity, and good recycle use makes it as a good solid adsorbent for mercury.
Keywords ： SO3--Anchored;
covalent
organic
framework;
mercury
ions(II);mercury vapor;
high-performance removal
1. INTRODUCTION In the present highly industrialized world, eliminate heavy metal pollution has become a globally unified destination.[1] Among these pollutants, mercury (Hg2+ and Hg0) is one of the most toxic species. For example, long-term exposure to mercury can cause various diseases in humans and animals such as the Minamata disease.[2] However, in our daily life the use of mercury-containing products like that of batteries, cosmetics and 1
electronic devices is ineluctable and prevailing.[3] Thereby, the removal of mercury has become a urgent issue but still a long-term and challenging target. In the previous reports, numerous methods have been explored for removing mercury, including chemistry precipitation, membrane separation, ion exchange and adsorption.[4-7] Compared with other methods, solid adsorption technology is one of the best choices due to its low cost, simple operation and high efficiency.[8-9] Nevertheless, this adsorption technology based on traditional porous adsorbents such as activated carbon,[10] zeolites,[11] hydrogels[12] was seriously restricted by some defects such as low adsorption capacity, poor selectivity, and slow adsorption kinetics. Recently, driven by the Hg-S coordination interactions, an effective approach with the porous adsorbent anchored by free-standing thio-/thiol-functionalized groups through the design of desired ligand or the post-synthesis modification (PSM) procedure was proposed.[13-19] For example, Xu et al through the design of a thiol-laced ligand constructed a thiol-functionalized UiO-66 like counterpart and the resulted metal-organic framework can effectively remove Hg2+ with residual concentration less than 0.01 mg L-1 form a 10 mg L-1 Hg2+ solution.16 On the other hand, PSM method was attested to be a powerful tool to anchor thiol-functionalized organic unit within the channel of porous adsorbents.[20-23] For example, Ma et al synthesized a thiol-modified COF of COF-S-SH through PSM method via thiol-ene “click” reaction. This material afforded ultrahigh mercury adsorption capacity, viz. 1350 mg/g for Hg2+ and 863 mg/g for Hg0.[21] Whereas the predesign of thiol-containing organic ligand or PSM with thiol-modification, both of them were restricted by the harsh conditions and expensive reagents in the synthetic aspect from the consideration of practical application. In this regard, we recently proposed a simple and general photoassisted multicomponent PSM method to prepare a sulfur-modified ZIF-90 derivate, based on very cheap materials. The resulted material gave 596 mg/g Hg2+ capture, and a 12.7-fold enhancement in the adsorption capacity was observed, relative to the pristine ZIF-90.[24] Moreover, Hg-S interactions provided by a inorganic part was also proposed to solve the above problem. Jiang et al prepared a composite material of In2S3@MIL-101, where the inorganic part of In2S3 affords Hg-S interactions to selectively capture Hg2+ for the solution, leading to a high adsorption capacity of 518.2 mg/g.[25] In contrast to the synthesis of sulfur-modified porous adsorbents, the synthesis of nitrogen- or oxygen-modified counterpart is more inexpensive and facile. In principle, Hg-N and Hg-O bonds are also strong coordination interactions. Hence, exploitation of nitrogen- or oxygen-modified porous adsorbents with excellent 2
mercury capture is also highly desirable. However, in the literature just low adsorption capacity was observed in this category.[26] In this work, we report a breakthrough in this category. The porous adsorbent of [NH4]+[COF-SO3-] shows abundant free-standing SO3--units within the channel, providing strong Hg-O coordination interaction to selectively capture Hg2+ and Hg0 with ultrahigh adsorption capacity up to 1299 mg/g and 932.6 mg/g, respectively. 2. Experimental 2.1 Materials Reagents and solvents were commercially available and were used without further purification. 2,4,6-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid were purchased from Ark Pharm, inc.1,2-dichlorobenzene, n-butanol and other chemicals were purchased from Macklin (Shanghai) Inc and were used without further purification. Deionized water was used in the experiments. 2.2 Synthesis of COF-SO3H. 0.3 mmol (63 mg) of 2,4,6-triformylphloroglucinol, 0.45 mmol (84.7 mg) 2,5-diaminobenzenesulfonic acid was added into a Pyrex tube with 1.5 mL butyl alcohol and 1.5mL 1,2-dichlorobenzene. The mixture was sonicated for 20 min, followed by addition of 0.5 mL of 3 M aqueous acetic acid. After that, the tube was degassed by freeze-pump-thaw cycles for three times, sealed under vacuum and heated at 120 °C for 3 days. The reaction mixture was cooled to room temperature and washed with deionized water, dimethylacetamide and acetone. The resulting dark red powder was dried at 120 °C under vacuum for 12 hours. 2.3 Synthesis of [NH4]+[COF-SO3-]. 50 mg COF-SO3H was added into 20 mL glass bottles with 10 mL (1%) ammonium hydroxide and stir for 24 hours. Then the resultant materials were washed by deionized water and methanol. The resulting dark red powder was dried at 60 °C under vacuum for 12 hours. 2.4 Measurements
X-ray powder diffraction were collected by a Bruker AXSD8 Discover powder diffractometer at 40 kV, 40 mA for Cu Kλ ( λ= 1.5406 Å). The simulated powder patterns were calculated by Mercury 1.4. Infrared Spectra (IR) were measured by a Bruker VERTEX70 spectrometer in the 500-4000 cm-1 region. The gas adsorption isotherms were collected on a Belsorp-max. Ultrahigh-purity-grade (>99.999%) N2 3
gases were used during the adsorption measurement. SEM and EDS measurements were carried out using a Hitachi S-4800 microscope. The analyses of concentrations of metal ions in the solution was carried out by ThermoFisher iCapQ ICP-MS and ThermoFisher iCap7600 ICP-OES instruments. X-ray photoelectron spectra (XPS) were collected by Thermo Scientific ESCALAB 250 Xi spectrometer.
3. Methods 3.1 Mercury uptake via batch experiments. The starting Hg2+ stock solution was made by dissolving 1.7082 g Hg(NO3)2·H2O in 1000 mL deionized water to create an 1000 mg L-1 Hg2+ solution. The pH value was adjusted by HNO3(Guaranteed reagent) or NaOH aqueous solution. All the adsorption experiments were conducted at 298 K. In pH-dependent experiments, the Hg2+ solution with pH=3~8 was adjusted by HNO3(Guaranteed reagent) and NaOH . The dose of adsorbent is 10 mg, the Hg2+ solution is 30 mL (300 mg/L)，and the contact time is 2 h. In isotherm experiments, the Hg2+ solution with initial concentration of 50-450 mg L-1 and pH= 6 was used. The dose of adsorbent is 10 mg, the Hg2+ solution is 30 mL and the contact time is 2 h. In kinetics experiments, the Hg2+ solution with initial concentration of 400 mg L-1 and pH=6 was used. The dose of adsorbent is 10 mg, the Hg2+ solution is 30 mL. In determining affinity experiments, the Hg2+ solution with initial concentration of 10.25 mg L-1 and pH=6 was used. The dose of adsorbent is 10 mg, the Hg2+ solution is 100 mL. In selective adsorption experiments, a mixed solution contains Hg2+, K+, Mg2+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+ with initial concentration of 50 mg L-1 and pH= 6 was used. The dose of adsorbent is 10 mg, while the solution is 100 mL. In the recycle experiment, Hg2+ solution with initial concentration of 25 mg L-1 at pH=6 was used. The dose of adsorbent of 10 mg, V= 30 mL and the contact time of 2 h was used. 20 mL, 2 M HCl (Guaranteed reagent) was used for elution, the desorption time was 2 h. 3.2 Mercury uptake via breakthrough experiments.
4
As shown in Figure S6, the packed bed was prepared by filling [NH4]+[COF-SO3-] adsorbent (300 mg) into a glass tube with inner aperture of 0.8 mm and length of 1 m. The experiment was carried out at room temperature and a flow rate of 0.05 ml/min for the outflow solution was used. The mixed solution (pH=6) is composed of K+, Mg2+, Zn2+, Cd2+, Pb2+, Hg2+ and their initial concentration is 50 mg L-1. 3.3 Mercury vapor capture experiment. About 400 mg metallic mercury used for elements mercury capture was conducted in a closed pressure resistant glass tube. A small glass vial (4 mL) with 20 mg [NH4]+[COF-SO3-] was placed into a 25 mL pressure resistant glass tube, then capped and placed in the oil bath where the temperature was kept at 140℃for 12 h. Then it is cooled to room temperature. The Hg0-loaded samples was obtained and the Hg0 was isolated from COF material by 1 mol/L HCl. The obtained solution was filtered through a 0.22 um filtering membrane, and the concentration of mercury was detected by ICP-MS.
4 Results and discussion
4.1 Characterization The [NH4]+[COF-SO3-] adsorbent was obtained by immerging COF-SO3H in NH3·H2O. And the COF-SO3H was synthesized by common solvothermal reaction of 2,4,6-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid at 120 ℃ for 72 hours. The synthesis in detail was listed in the Supporting Information. Its structure (Figure 1a) has been discussed in our previous work.[27] The comparison shows that the color of COF-SO3H and [NH4]+[COF-SO3-] has changed slightly darker, and the color of [NH4]+[COF-SO3-] and loaded mercury basically unchanged (Figure S11).
5
Figure 1. (a) View of the structure of [NH4]+[COF-SO3-]. Due to the existence of SO3- group, the π-π interactions between adjacent layers shows somewhat offset, rather than common fashion. The SO3- groups in the adjacent layers show an opposite arrangement. The NH4+ ions are stabilized by the hydrogen bonds (highlighted broken line). The color code for these atoms is S/yellow, C/gray, H/green, O/red, N/blue. (b) The SEM images and the insert of the mapping of C, N, S, O elements by the energy dispersive spectrometry (EDS). (c) The IR spectrum of [NH4]+[COF-SO3-]. The COF-SO3H and [NH4]+[COF-SO3-] samples were further characterized by the scanning electron microscopy (SEM, Figure 1b), powder X-ray diffraction (PXRD, Figure S1), infrared spectrum (IR, Figure 1c), and X-ray photoelectron spectroscopy (XPS). All the above results were consistent with our recent report. The success of introducing [NH4]+ into the COF channel is approved by the emergence of new peaks in the IR at 3194 cm-1 (Figure S2) and in the XPS at 402 eV (Figure 2). Through the BET test, we found that COF-SO3H has a specific surface area of 300.2 m2/g, but the modified [NH4]+[COF-SO3−] has a lower specific surface area 110.6 m2/g, and the pores are also reduced by the introduction of ammonia ions (Figure S3).
6
Figure 2. The XPS of N element for samples of COF-SO3H and [NH4]+[COF-SO3-]. The highlight is derived from [NH4]+.
4.2 Mercury adsorption. 4.2.1 pH affecting. The mercury adsorption performance of [NH4]+[COF-SO3-] was initially explored in the solution at pH=3-8 (Figure 3). Generally speaking, the adsorption performance of solid adsorbent is highly dependent on the pH value of solution, because this will significantly affect the external charge and consequently electrostatic interactions between target ions and adsorbent. As shown in Figure 3, the optimal pH value for both COF-SO3H and [NH4]+[COF-SO3-] is six, giving 725 mg/g and 806 mg/g respectively. The adsorption performance of [NH4]+[COF-SO3-] exceeds COF-SO3H, most possibly because the deprotonation from the replacement of [NH4]+ by H+ benefits the cation exchange between [NH4]+ and Hg2+ and next Hg-O coordination interactions between SO3- unit and Hg2+ ions.
7
Figure 3. The pH-dependent adsorption for samples of COF-SO3H and [NH4]+[COF-SO3-] (C0 = 300 mg L-1, madsorbent = 10 mg, V = 30 mL, t = 2 h).
4.2.2 Adsorption kinetics and isotherms. The rate of adsorption is an importantly decisive index for a solid adsorbent. Approximately within 10 min, the adsorption equilibrium was reached for COF-SO3H and [NH4]+[COF-SO3-], giving 928 mg/g and 1044 mg/g (Figure 4a, S4), respectively, suggesting fast adsorption kinetics. The adsorption data obeys the pseudo-second-order kinetic model with R2 more than 0.999, suggesting a chemical adsorption process (Figure 4a, Table S1),[20-24] in line with the structure containing abundant free-standing SO3- or SO3H units for binding Hg2+. This fast adsorption kinetics is also comparable with many previously reported sulfur-modified porous adsorbents.[13-24] The adsorption capacity of [NH4]+[COF-SO3-] was estimated from an aqueous solution with initial Hg2+ concentration among 50-450 mg L-1. The maximal adsorption capacity is about 1299 mg/g (Figure 4b), and the theoretical adsorption capacity, calculated from the fitting by the Langmuir model, is as high as 1550 mg/g (Figure 4b, Table S2). Such outstanding adsorption performance surpasses most reported sulfur-containing porous adsorbents composed of thiol-modified metal-organic frameworks such as SH-MiL-68(In)(450 mg/g),[23] [Cu3(BTC)2]-SH (714 mg/g),[28] CaII CuII6[(S,S)-methox]3(OH)2(H2O)} (900 mg/g),[13] thiol-modified covalent organic frameworks such as TAPB-BMTTPA-COF (734 mg/g),[22] POP-SH (1216 mg/g),[21] and PAF-1-SH 8
(1014 mg/g),[20a] as well as inorganic sulfur-anchored adsorbents like that of In2S3 @MIL-101 (518 mg/g),[25] KMS-1 (377 mg/g)[29]. Notably, the experimental maximal adsorption capacity in [NH4]+[COF-SO3-] is comparable with the best mercury adsorbent of COF-S-SH (1350 mg/g), while the theoretical adsorption capacity even exceeds this benchmark value. To emphasize the deprotonation effect, the adsorption isotherm of COF-SO3H was investigated under the same conditions (Figure S5). The experimental maximal adsorption capacity and the calculated saturation adsorption capacity is 1033 mg/g and 1041 mg/g, respectively. This value is far lower than that observed in[NH4]+[COF-SO3-], strongly supporting the deprotonation effect on the Hg2+ adsorption performance.
Figure 4. (a) The adsorption kinetics of Hg2+ upon [NH4]+[COF-SO3-] (C0=400 mg L-1, madsorbent=10 mg, V=30 mL, pH=6). The insert is the fitting results by the pseudo-second-order kinetic model. (b) The adsorption isotherms of Hg2+ upon [NH4]+[COF-SO3-] (t=2 h, madsorbent=10 mg, V=30 mL, pH=6). The insert is the fitting results by the Langmuir model.
4.2.3 Adsorption affinity. The above results imply strong binding affinity between [NH4]+[COF-SO3-] and Hg2+ ions. To quantificationally estimate the binding affinity, we further carried out the measurement of distribution coefficient (Kd) from a 10 mg L-1 solution. After a contact time of 2000 min, more than 99% Hg2+ ions were removed from the solution. And the Kd value is calculated to be 2.13×105 ml/g (Figure 5a), which is comparable with many reported benchmark adsorbents (Table S3) such as TAPB-BMTTPA-COF (7.82×105 ml/g),[22] Zr-DMBD (9.99×105 ml/g),[16] [Cu3(BTC)2]-SH (4.73×105 ml/g),[28] and CaIICuII6[(S,S)-methox]3(OH)2(H2O)} (6.67×105 9
ml/g)[13]. The results strongly suggest that the binding affinity from the present COF most possibly driven by Hg-O interactions between SO3- and Hg2+ ions is as powerful as observed in the sulfur-containing porous adsorbents featuring by the Hg-S interactions. And it can be seen from PXRD that the powder structure remains stable after adsorption (Figure S12). 4.2.4 Relative selectivity. Next, the selectivity tests were also performed on [NH4]+[COF-SO3-] from a mixed solution containing Pb2+, Cd2+, Zn2+, Ni2+, Co2+, Mg2+, K+. High selectivity (more than 160) was observed for almost all tested metal ions, except for Pb2+ with a selectivity of 9.9 (Figure 5b). To further confirm this remarkable selectivity , we further carried out the breakthrough experiments (Figure S6), and similar trend was observed. The selective adsorption of Hg2+ over Zn2+, Ni2+, Co2+, Mg2+, K+ is excellent, however, [NH4]+[COF-SO3-] also enables good removal for Pb2+ ions, as evidenced by the breakthrough experiments even after the contact time of 416 h (Figure 5c). Low Hg2+/Pb2+ selectivity was also a common issue for many reported sulfur-containing porous adsorbents, mainly due to their comparable physical and chemical properties.[19-24] 4.3 Cyclic sorption / desorption studies. Moreover, this [NH4]+[COF-SO3-] adsorbent also shows good recycle use. As shown in Figure 5d, the desorption efficiency by 2M HCl is more than 99.9%, and the adsorbent even after four recycle shows no detectable decrease in the Hg2+ capture.
10
Figure 5. (a) The adsorption affinity (Kd value, C0=10 mg L-1, madsorbent=10 mg, V=100 mL, pH=6). (b) The selectivity calculated from a multi-component adsorption experiment (C0=50 mg L-1 for metal ions, madsorbent=10 mg, V=100 mL, t=120 min, pH=6). (c) The breakthrough experiment (madsorbent=300 mg, C0=50 mg L-1 for metal ions, pH=6). (d) The recycle use of [NH4]+[COF-SO3-] showing neglectable variety in both the adsorption efficiency and desorption efficiency.
4.4 Eliminating mercury vapor and potential application. All the above results imply its superior application in Hg2+ removal from the aqueous solution. In fact, the removal of mercury vapor (Hg0) is also highly important in the industrial flue gas detoxification. In this regard, we further evaluated the mercury vapor remediation upon [NH4]+[COF-SO3-]. The test method is referenced to the reported method by Ma et al (Figure S7).[20-21] After the contact time of 12 h, the Hg0 adsorption capacity is as high as 932.6 mg/g, exceeding the best Hg0 porous adsorbent of COF-S-SH (863 mg/g), highlighting its promising application in eliminating mercury vapor. 4.5 Mechanisms of mercury sorption. 11
Figure 6. A comparison of Hg adsorption among these samples of COF-SO3H, [NH4]+[COF-SO3-], and TpPa-1 with the sample COF skeleton.
To disclose the adsorption mechanism, the same COF skeleton without any functionalized organic units within the channel, namely TpPa-1 (Figure S8),[30] was also explored about Hg2+ capture for a comparison. As expected, the TpPa-1 material just shows 420 mg/g adsorption capacity, while [NH4]+[COF-SO3-] and COF-SO3H gives 1110 mg/g, 1050 mg/g, respectively, under the same conditions (Figure 6). This direct evidence suggests such significant enhancement (more than two-fold) is due to the introduction of free-standing -SO3H or -SO3- units. Furthermore, the disappearance of characteristic peaks for [NH4]+ in IR (Figure S9) and XPS spectrum (Figure 7) was observed for the samples after Hg2+ capture, mainly due to ion exchange between [NH4]+ and Hg2+ ions. In addition, clear electron transfer for S was observed for the Hg2+-loaded samples in the XPS spectrum of S element (Figure S10), relative to that observed in [NH4]+[COF-SO3-], mainly due to the Hg-O coordination interactions between -SO3- units and Hg2+ ions. Accordingly, the overall adsorption mechanism for the present porous adsorbent of [NH4]+[COF-SO3-] is first cation exchange between [NH4]+ and Hg2+ ions and then binding of Hg2+ ions in the channel via Hg-O coordination interactions between -SO3- units and Hg2+ ions.
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Figure 7. A comparison of XPS of N element for these samples of COF-SO3H, [NH4]+[COF-SO3-], and Hg-loaded samples. The highlight shows the distinct change derived from the replacement by [NH4]+ or by Hg2+.
Conclusion In conclusion, we have shown in this work a porous SO3-- anchored COF adsorbent for the application in the removal of mercury from the aqueous solution (Hg2+) and vapor(Hg0). The results show that [NH4]+[COF-SO3-] enables ultrahigh adsorption capacity for both Hg2+ and Hg0, fast adsorption kinetics, strong binding affinity, high selectivity, and good recycle use, making it as a promising porous adsorbent for practical application in pollution control of mercury. The adsorption mechanism, as unveiled by comparison experiment in conjunction with IR and XPS characterizations, is first ion exchange and next coordination interaction of Hg-O, very different form the well-known sulfur modification approach (Hg-S), thus outlining a new pathway to design porous adsorbents with excellent mercury removal performance.
Supporting Information This includes Experimental Procedures, 12 figures, and 3 tables.
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Acknowledgement We thanks to the National Science Foundations of China (21871047, 21661001, and 21966002), the Natural Science Foundation of Jiangxi Province of China (20181ACB20003, 20192BAB203002).
Declaration of interests The authors declare no competing interests.
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1. [NH4]+[COF-SO3-] enables ultrahigh adsorption capacity (1299 mg/g) for Hg (II).
2. The work give the first report about removing Hg2+ via coordination interaction of Hg-O.
3. [NH4]+[COF-SO3-] shows the record adsorption capacity (932.6 mg/g) of mercury vapor.
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The authors declare no competing interests.