Zeolitic Imidazolate Framework-8 with High Efficiency in Trace

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Zeolitic Imidazolate Framework-8 with High Efficiency in Trace Arsenate Adsorption and Removal from Water Jie Li, Yi-nan Wu, Zehua Li, Bingru Zhang, Miao Zhu, Xiao Hu, Yiming Zhang, and Fengting Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508381m • Publication Date (Web): 03 Oct 2014 Downloaded from http://pubs.acs.org on October 19, 2014

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The Journal of Physical Chemistry

Zeolitic Imidazolate Framework-8 with High Efficiency in Trace Arsenate Adsorption and Removal from Water Jie Li†,§, Yi-nan Wu†,§, Zehua Li†, Bingru Zhang†, Miao Zhu†, Xiao Hu†, Yiming_Zhang†, and Fengting Li†,* †. College of Environmental Science & Engineering, State Key Laboratory of Pollution Control and Resource Reuse Study, Tongji University, 1239, Siping Road, Shanghai, 200092(China)

§. Jie Li and Yi-nan Wu contributed equally to this work.

* Corresponding author E-mail: [email protected] (F.T.L.); Tel: +86 21 65980567; Fax: +86 21 65985059;

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Abstract: The development of highly efficient adsorbents, especially those aimed at the capture of trace (ppb, 10-9) arsenate, is one of the principal challenges in the water treatment field. In this article, zeolitic imidazolate framework-8 (ZIF-8) was explored for the removal of trace arsenate from water. Results showed that ZIF-8 outperformed some other adsorbents and owned the first and highest reported adsorption capacity (76.5 mg g−1) at a low equilibrium concentration (9.8 µg L−1). Satisfactory adsorption properties (adsorption capacity, adsorption rate, adaptability to water environment, regeneration capacity) demonstrated the feasibility of using ZIF-8 as an efficient adsorbent for the removal of aquatic trace arsenate. In addition, fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra revealed the proposed mechanism of As(V) adsorption onto ZIF-8: producing large amounts of external active sites (Zn–OH) through the dissociative adsorption of water and subsequently forming an inner-sphere complex with arsenate molecule. Insights into the adsorption process uncovered the key factors to the formation of this high removal efficiency: the hydration process to form surface hydrogen group by dissociative adsorption of water; the high accessible surface area; and the cooperative interaction (e.g., Van der Waals’ force and hydrogen bonding) between As(V) species at low surface coverage.

Keywords: adsorption properties, mechanisms, low equilibrium concentration, hydration process, cooperative interaction

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1. Introduction

Arsenic (As) is a metalloid element that exhibits excessive levels of toxicity. Prolonged exposure to arsenic-contaminated environment could cause chronic and acute toxicity, cancer, and even death1. Unfortunately, arsenic pollution of groundwater and surface water has frequently occurred in the past decades because of the geochemical processes and uncontrolled anthropogenic activities. According to statistics, a total of 105 countries and up to 226 million people worldwide are suffering from arsenic pollution2.

Given the absence of effective therapeutic measures for arsenic toxicity, the key prevention measure is to avoid exposure to arsenic sources and assure the arsenic concentration in drinking water below the safety limits (10 µg L−1). Thus, studies have focused on the removal of aquatic As(V) [As(V) in pentavalent oxidation state is the main existing form of arsenic in natural water]3. Traditional technologies such as coagulation, membrane technology, and adsorption are often used to treat As(V)-contaminated water. Thereinto, adsorption is an efficient method for purifying polluted water that contains recalcitrant organic pollutants and heavy metals. For the removal of trace contaminants (ppb, 10-9), the adsorption method is superior to the coagulation method which requires high doses of coagulants4. Adsorption method also has the merits of low cost and being easy to operate, while membrane technology requires high energy consumption, tedious operation, and complex

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instruments5. In terms of adsorption technology, the main focus is to seek and explore highly efficient adsorbents.

Zeolitic imidazolate frameworks-8 (we call this material ZIF-8), in which zinc ions act as coordination centers and link together with 2-methylimidazole through self-assembly processes, constitutes an emerging class of porous materials6. Featured by high chemical and thermal stability, large accessible surface area and abundant active surface sites7-9, ZIF-8 has attracted considerable attention in many applications. Typically, ZIF-8 is taken as hydrophobic material and widely used in gas storage (CO2, CH4, etc.) and separation of organic chemicals (benzotriazoles, organic vapors, etc.)10-13. The main adsorption and removal mechanism includes electrostatic enhancement, hydrophobic and π−π interaction, etc.

Indeed, due to its hydrophobic nature, the amounts of water adsorbed onto ZIF-8 are extremely low14. So, it seems to be unsuitable to use ZIF-8 for the adsorption and removal of aquatic arsenate. However, various degrees of hydrophilicity could be theoretically obtained with adequate functionalization of the linkers15. With the help of adsorption sites monitoring by DFT (density functional theory) calculations and GCMC (grand canonical Monte Carlo method) simulation, previous researches have demonstrated that acid-basic sites including the hydrogen group and amino group could be obtained at the external surface or at defects, through the dissociative adsorption of water16, 17. In addition, the effect of external surface area of ZIF-8 crystals, especially for water adsorption, is proved to be significant and

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non-negligible since the nano-sized crystals having a good dispersibility in aquatic environment18. Thus, despite the overall hydrophobic nature and the smaller aperture size (11 Å × 3.4 Å) of ZIF-8 which limits the diffusion of the arsenate molecule to the inner pore structure19, abundant active adsorption sites could be obtained at its external surface, which is helpful to the adsorption of polar arsenate molecules. This hypothesis is also supported by the results of our former that a certain adsorption capacity of ZIF-8 for micro arsenate (ppm, mg L−1) was discovered9.

In this study, as mentioned previously, we emphasize on the removal of trace arsenate (ppb, 10-9, µg L−1) which is one of the principal challenges in the water treatment field and the data obtained is more approaching to reality. Effects of reaction parameters, including equilibrium concentration, PH, reaction time, coexisting anions, and adsorbent regeneration on the adsorption capacity were studied by means of single factor experiment method. In addition, through microscopic study by FTIR and XPS spectra, the possible mechanisms involved in the adsorption process were discussed and the key factors to the formation of the high removal efficiency were investigated.

2. Experimental and Theoretical Methods

2.1 Materials and Synthesis

2-methylimidazole, and Zn(NO3)2·6H2O were purchased from Alfa Aresar; Methanol, Na3AsO4·12H2O, and other chemicals were supplied by Sinoreagent. All

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solvents and chemicals are of reagent quality and were used without further purification. De-ionized water was used throughout this work. Based on the previously published work20, 21, ZIF-8 was synthesized and activated with a rapid room-temperature synthetic route. Typically, 2-methylimidazole (2.2 g, 26.83 mmol) was added into methanol (100 mL) and stirred until the solid is fully dissolved. Then, Zn(NO3)2·6H2O (0.5 g, 1.68 mmol) was added into the mixture and stirred at 25 °C for 8 h. The white precipitates were separated by centrifugation and washed with methanol for 3 times to remove the organic residues. Finally, the sample was dried at 150 °C overnight for further use (Figure S1).

2.2 Adsorption Experiments Procedures Reaction volumes were 500 mL and the adsorbent dosage was 40 mg L−1 for ZIF-8. Arsenate solution with different initial concentration was prepared by dissolving Na3AsO4·12H2O into de-ionized water. The pH was adjusted by 1.0 M HNO3 and NaOH solutions. All experiments were conducted at room temperature (25 °C) and the flask was shaken at 150 rpm in a thermostatic shaker (WELL, HZQ-B, China). After the adsorption process, the suspension was separated by a 0.22 µm membrane. The As(V) concentration of the filtrate was determined by inductively coupled plasma (ICP) spectrometer (Agilent 720ES, USA) at ppm level (mg L−1) or ICP-MS spectrometer (Agilent 7700, USA) at ppb level (µg L−1). Details about batch experiments procedures were reported in supporting information. The adsorption capacity Q (mg g−1) was calculated by equation (1). 6


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Q = ሺC଴ − C୲ ሻV/m

(1)

where C0 and Ct are the initial and residual concentrations (mg L−1) of As(V), respectively; V is the volume of solution (L) and m is the mass of the adsorbent (g)

2.3 Material Characterization

The crystal structure of the adsorbents was obtained by an X-ray powder diffractometer (D8 Advance, Germany). The BET surface area of the adsorbents was obtained with ASAP2020M analyzer (Micromeritics, USA) at 77 K in the relative pressure range of 0.05–0.25. The scanning electron microscope (SEM) and transmission electron microscopy (TEM) images were obtained by Hitachi S4800, Japan and Tecnai G2 F20 S-TWIN, USA, respectively.

The surface charges (Zeta potential) of the adsorbent at different pH were measured by a Zeta potential analyzer (Zetasizer Nano ZS 90, United Kingdom). The Fourier transform infrared (FTIR) spectra of the adsorbent before and after adsorption were measured on a FTIR spectrophotometer (Nicolet 5700, USA). The adsorbent samples before and after arsenic adsorption were analyzed using an X-ray photoelectron spectroscope (AXIS Ultra DLD, Kratos, Japan) with a 1486.6 eV Al Kα X-ray source. The energy step size for the wide-scan spectra and high-resolution scan were 1 and 0.05 eV, respectively. To compensate the charging effects, all binding energies were calibrated with neutral C1s at a binding energy of 284.6 eV. The spectra peaks were fitted using XPSPEAK41 software.

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3. Results and Discussion

3.1 Adsorption Isotherms

Figure 1. Adsorption isotherms of As(V) onto ZIF-8. The inset shows the high-resolution isotherm at extremely low equilibrium concentrations.

The adsorption isotherms data with fitting models are shown in Figure 1. High correlation coefficients (R2, in Table S2) suggest that Langmuir model is suitable for describing As(V) adsorption onto ZIF-8. The maximum removal capacity of ZIF-8 from the corresponding model is 106.7 mg g−1, and in good agreement with the actual experiment results. The equilibrium adsorption isotherm can directly yield the conditional removal capacity to enable comparison between various adsorbents22. However, the removal capacity is affected by many factors, such as reaction time, competition ions, pH and, equilibrium concentration; the latter considerably affects the adsorption capacity because an increased adsorbate concentration implies a high 8


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driving force and an enhanced interaction process23. Too often, there’s no comparison between the arsenic concentrations in natural water and those that are employed in various previously reported work24. As a result, there is a wide gap between the model-obtained result and the actual removal capacity in real practice. To conclude, directly comparing the adsorption capacity and ignoring the differences in equilibrium concentrations are not objective evaluations.

To further study the removal capacity of trace arsenate, the adsorption isotherms of As(V) onto ZIF-8 under extremely low equilibrium concentrations is shown as inset in Figure 1. At an equilibrium concentration of 9.8 µg L−1 (below the safety limits for arsenic in drinking water revised by the World Health Organization and U.S Environmental Protection Agency, meaning that the treated water does not need any further purification after the adsorption), ZIF-8 exhibited a capacitiy of 76.5 mg g−1 which is the first and highest reported adsorption capacity (Table 1), indicating that ZIF-8 is a promising alternative adsorbent for trace As(V) removal from water. In addition, in the first half of the “S”-shaped nonlinear isotherms, the increased ratio of the adsorbed As(V) amount (mg g−1) to the equilibrium As(V) concentration (µg L−1) suggests increased adsorbate affinity as a function of surface As(V) concentration. The above phenomena indicates that cooperative interactions between As(V) species (e.g., van der Waals force and hydrogen bonding) may occur at low surface coverage and depicts an active capture process of As(V) species instead of the random adsorption25. This hypothesis is contrary to the Langmuir model that a uniform monolayer adsorption and no reactions between As(V) species are assumed. 9



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But in fact, only in wide concentration range can Langmuir model obtain a stable maximum adsorption capacity and a good simulation result. The adsorption isotherms under extremely low concentration range just make up for the inapplicability of the Langmuir model at low concentration range and give a detailed display of the adsorption performance in the whole concentration range. For the cooperative interaction between As(V) species, it is conspicuous at low surface coverage (shown through in the form of “S”-shaped isotherm) and gradually diminished by increasing amounts of chemical adsorption with higher adsorbates concentration (shown through in the form of stable capacity line of Langmuir model isotherm). Table 1. Comparison of Adsorption Capacities of As(V) on Various Adsorbents. Adsorbent

Equilibrium

Capacity

concentration

(mg g−1)

References

Fe3O4 nanocrystals

7.8 µg L−1

0.98

2006,SCIENCE26

Nano Fe-Cu binary oxide

9.7 µg L−1

37.0

2013,WATER RES27

Powder Ce–Ti sorbent

10 µg L−1

7.5

2010,CHEM ENG J24

Granular Ce–Ti sorbent

10 µg L−1

2.5

2010,CHEM ENG J24

Granular ferric hydroxide (GFH) 10 µg L−1

8.0

2004,WATER RES28

76.5

This study

ZIF-8

9.8 µg L−1

3.2 Proposed Mechanism of As(V) Adsorption onto ZIF-8

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Figure 2. FTIR spectra of ZIF-8 before and after As(V) adsorption.

To elucidate the As(V) adsorption mechanism, the FTIR and XPS spectra were obtained before and after As(V) adsorption. Figure 2a illustrates the FTIR spectra of pristine and As-loaded adsorbent. The primary peaks match well with previously reported results7. The spectra of ZIF-8 before and after adsorption are similar except for some new peaks. The broad peak at 3387 cm−1 is ascribed to the stretching vibration of hydroxyl and amino groups, which are difficult to distinguish and separate completely. Abundant surface active sites including Zn–OH and N–H are produced through the dissociative adsorption of water8, 16, in good agreement with the hypothesis previously mentioned. The intense peak at 844 cm−1 is assigned to the stretching vibration of As-O, indicating the successful adsorption of As(V) species onto ZIF-829. The absorption peak at 500 cm−1 is characterized by metal-oxygen vibration24.

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Figure 3. XPS spectra of ZIF-8 before and after As(V) adsorption. (a) Wide scan spectra; (b) O1s spectra with high resolution; (c and d) Deconvoluted O1s spectra of ZIF-8 before and after As(V) adsorption. Full and dashed lines represent the raw intensity data and the deconvolution fitting intensities, respectively.

The wide-scan XPS spectra (Figure 3a) and EDS results (Figure S3) reveal that some arsenic atoms are detected on the surface after adsorption, confirming the successful adsorption of As(V). Among the binding energy peaks, only O1s peak decreased considerably from 531.6 to 531.1 eV (Figure 3b). Figure 3c and 3d illustrate the deconvoluted spectra of the O1s region to analyze the adsorption mechanism. The O1s spectra of virgin ZIF-8 (Figure 3c) produced H2O and M-OH (Zn-OH) peaks at 533.55 and 531.73 eV, respectively30. After adsorption (Figure 3d), a new peak occurred with a binding energy of 530.90 eV and represented M-O-M24. Despite the 12


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fact that As-OH group contained in the arsenic species (H3AsO4, H2AsO4−, and HAsO42−) should have increased the fraction of surface hydroxyl group24, the area ratio of M-OH decreased from 91.40% to 34.40% after As(V) adsorption, indicating the involvement of hydroxyl group and chemical adsorption process of As(V) onto ZIF-8.

In summary, producing large amount of external active sites (Zn–OH) through the dissociative adsorption of water and subsequently forming an inner-sphere complex (e.g., monodentate and bidentate complexes) is the proposed mechanism of As(V) adsorption onto ZIF-8 (Figure 4)31. To explain the previous study of FTIR spectra, a new peak at 500 cm−1 is attributed to the M-O-M bond (Zn-O-As) and implies the formation of arsenic-contained inner complex.

Figure 4. Proposed adsorption mechanism and simulated locations of As(V) molecules onto ZIF-8. Colour scheme: Zn, blue; O, yellow; N, gray; C, dark; H, red; and As, brown. All metal ions and arsenate are present as the polyhedral.

3.3 Conditional adsorption properties

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Figure 5. Adsorption properties of arsenate onto ZIF-8: (a) Adsorption kinetics. (b) pH effects on adsorption capacity and surface charge of ZIF-8. (c) Effects of competition anions on adsorption capacity. (d) Adsorption capacity ratios in three recycle times.

Figure 5a illustrates the effects of adsorption time on the removal of As(V). Higher correlation coefficients (R2, in Table S3) indicates that the experimental data fits better with the pseudo-second-order model, which assumes chemical adsorption process and that the adsorption rate is controlled by two factors: adsorbates concentration and the surface properties of the adsorbent. The adsorption rate was initially high and slowed down thereafter. ZIF-8 reached 50% of its adsorption capacity within 3 h and up to 80% of the maximum removal capacity was finished within 12 h. In general, the adsorption rate is lower compared to the reported 14


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adsorption process of organic chemicals13. The following reasons might be responsible for this relatively slow rate: the chemical arsenate adsorption process is usually slower than physical adsorption process; in addition, before the chemical adsorption process, the hydration process to form surface hydrogen group by dissociative adsorption of water also consumes much time32.

As(V) uptake is analyzed as a function of the initial pH (Figure 5b) to determine the optimum pH for As(V) adsorption. The removal capacity steadily increased with reduced pH and reached maximum capacity at pH 4.0. Solution pH affects the performance of the adsorbents because it would not only influence the surface charge (Zeta potential) of the material but would also control the As(V) speciation during adsorption. Aquatic As(V) mainly exists as H3AsO4, H2AsO4−, HAsO42−, and AsO43− for pH below 2.3, 3–6, 8–10.5, or above 11, respectively, for various ionization degrees33, 34. The point of zero charge (pHzpc) for ZIF-8 is at pH 9.3. Reducing the pH could reverse the surface charge from negative to positive, which is conducive to the adsorption of negatively charged pollutants. However, the negative charges of As(V) species reduces from divalent to monovalent upon reduction in pH and even reaches zero valence from the reduction. The above beneficial and converse effects are supposed to be compromised and yield an optimum pH value. What is interesting is that, for ZIF-8, the removal capacity steadily increased with the decrease of pH; the capacity depended neither on the surface charges of the adsorbent nor on the negative charges of As(V) species. Thus, some other forces should have diminished the impact of electrostatic attraction. Considering the 15



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chemical adsorption process that through ion exchange to form inner-complex, the growing adsorption capacity at low pH mainly benefits from increased amount of hydroxyl group in As(V) species under acidic conditions.

In natural water, various ions often coexist with arsenate. In this section, several common anions (Cl−, F−, NO3−, SO42−, and PO43−) were selected to analyze their effects on the removal efficiency (Figure 5c). Only 18.8% of the maximum removal capacity was maintained in the presence of PO43− (0.2 m mol L−1), indicating marked inhibitory effect of PO43− for its similar nature with AsO43−, and their competition for binding sites. However, it should be noted that the adverse effects would not be that serious since the mass ratio of PO43− to AsO43− amount was much lower in natural water. The existence of F− also exhibited a negative impact on As(V) removal. Other ions failed to show remarkable influence on the adsorption performance of ZIF-8. The inhibitory effects of the ions on the adsorption capacity followed the order: PO43− > F− > SO42− > NO3− > Cl−. To conclude, the coexisting anions do have some negative effect on the adsorption efficiency and the results are generally in agreement with other studies34. The difference is that, compared to other materials, the impact on adsorption efficiency is greater with the existence of the coexisting anions, especially for PO43−.

To study the regeneration ability of ZIF-8, HNO3, HCl, NaCl, NaOH, and methanol solutions were used to regenerate As(V)-loaded material; 0.02 M NaOH was chosen finally. Figure 5d reveals that ZIF-8 maintained more than 80% of its capacity after

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three recycling times. The desorption of As(V) from ZIF-8 is ascribed to the ion exchange between As(V) species and abundant hydroxyl groups. The loss of adsorption capacity results from some inactive sites occupied by some undesorbed arsenate molecular since the desorption cannot be achieved completely; Also, despite the overall stability of the crystalline structure after adsorption (Figure S2), some minor collapse of the crystalline structure, confirmed by the zinc ions leakage detected in the filtrate, leds to the decrease of zinc centers which can produce active surface hydrogen group. In general, after three regeneration cycles, the adsorption capacity of ZIF-8 is maintained at a comparatively high level with only a slight decrease of its adsorption capacity (17%).

4. Conclusions

The main criteria for adsorbents includes the adsorption capacity, uptake rate, good adaptability to water environment (concentration range of pollutants, pH, competition ions, etc.), and regeneration ability. To this end, we have studied the characteristics of arsenate removal from water by ZIF-8 and discussed the possible involved adsorption mechanisms. The high efficiency is attributed to three key factors: the hydration process to form surface hydrogen group by dissociative adsorption of water, which helps to bind As(V) species tightly onto the surface; the high accessible surface area, fundamental to supply abundant adsorption sites; and the cooperative interaction (e.g., Van der Waals’ force and hydrogen bonding) between As(V) species at low surface coverage. Using ZIF-8 as an efficient

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adsorbent for the removal of trace As(V), presents a new material with high application potential for trace As(V) removal in practical water treatment, which will motivate more extensive investigations on using ZIF-8 for water purification.

Acknowledgement We

gratefully

acknowledge

the

financial

support

from

the

Programs

(2012DFG91870) on Application Study and Demonstration of Advanced Papermaking Wastewater treatment technology by the Ministry of Science and Technology of China (MOST) and the National Science Foundation of China (51203117). We also acknowledge the support from the Africa-China Cooperation Programs (2010DFA92820 and 2010DFA92800) on Environment which is supported by the Ministry of Science and Technology of China (MOST) and coordinated by the United Nations Environment Programme (UNEP).

Supporting Information

Details about the experimental procedure details; Characterization of the synthesized ZIF-8 before and after adsorption; Isotherms and kinetics parameters obtained from corresponding models for As(V) adsorption onto ZIF-8; This information is available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents Imag

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Adsorption isotherms of As(V) onto ZIF-8. The inset shows the high-resolution isotherm at extremely low equilibrium concentrations. 57x40mm (300 x 300 DPI)



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FTIR spectra of ZIF-8 before and after As(V) adsorption. 57x40mm (300 x 300 DPI)


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XPS spectra of ZIF-8 before and after As(V) adsorption. (a) Wide scan spectra; (b) O1s spectra with high resolution; (c and d) Deconvoluted O1s spectra of ZIF-8 before and after As(V) adsorption. Full and dashed lines represent the raw intensity data and the deconvolution fitting intensities, respectively. 115x80mm (300 x 300 DPI)



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Proposed adsorption mechanism and simulated locations of As(V) molecules onto ZIF-8. Colour scheme: Zn, blue; O, yellow; N, gray; C, dark; H, red; and As, brown. All metal ions and arsenate are present as the polyhedral. 177x66mm (300 x 300 DPI)


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Adsorption properties of arsenate onto ZIF-8: (a) Adsorption kinetics. (b) pH effects on adsorption capacity and surface charge of ZIF-8. (c) Effects of competition anions on adsorption capacity. (d) Adsorption capacity ratios in three recycle times. 115x80mm (300 x 300 DPI)



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35x22mm (300 x 300 DPI)


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Zeolitic Imidazolate Framework-8 with High Efficiency in Trace

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