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Superior absorption capacity of tremella like ferrocene based metal-organic framework in removal of organic dye from water
Journal of Hazardous Materials 392 (2020) 122274
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Superior absorption capacity of tremella like ferrocene based metal-organic framework in removal of organic dye from water
T
Jiyang Liu, Haojie Yu*, Li Wang* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Deyi Hou
Removal of organic dyes from water by porous materials is considered as an efficient and low-cost way. Herein for the first time novel tremella-like ferrocene based metal-orgainc framework (TMOF) nanosheets designated as TFMOF were synthesized through a traditional solvothermal method. This ferrocene based TFMOF exhibit outstanding removal efficiency towards organic dye Congo red (CR) from water. After optimizing the reaction conditions, the highest adsorption capacity of 252.25 mg g−1 could be achieved within 10 min. Furthermore, the investigation of adsorption kinetic indicated this adsorption process could be described as a pseudo-second order kinetic model with k2 and qe of 0.0488 g mg−1 min−1 and 241.5 mg g−1, respectively. The adsorption isotherm could also be described as the Sips isotherm model according to the fitting calculation. The removal efficiency could maintain around 50 % with adsorption capacity of 124.38 mg g−1 after 3 cycles, giving the TFMOF promising potential in the practical water treatment.
Keywords: Metal-organic framework Nanosheets Congo red Water treatment Adsorption
1. Introduction With the rapid expansion of modern industry, water contamination has been emerging as a severe problem due to the unrestrained emission of pollutant from factories (Yagub et al., 2014; Wang and Peng, 2010; Gupta, 2009; Ansari et al., 2011). Among all of these pollutants, removal of water-soluble dyes is of great challenge because of their
⁎
acute toxicity even in very trace amount. Congo red (CR) is a kind of artificial azobenzene dyes which is widely used in leather, fabrication and food industries (Olivera et al., 2016; Du et al., 2014). As CR is quite soluble and stable in water, during the production and dyeing process large amount of waste water containing CR may be poured into rivers or lakes, so the treatment of poisonous dyes containing water is of great significance for our health and environment. So far several approaches
Corresponding authors. E-mail addresses: [email protected] (H. Yu), [email protected] (L. Wang).
https://doi.org/10.1016/j.jhazmat.2020.122274 Received 14 August 2019; Received in revised form 29 November 2019; Accepted 10 February 2020 Available online 11 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 392 (2020) 122274
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sonicated for 30 min. Then the autoclave was put into an oven and heated at 120 ℃ for 9 h. After cooling down to room temperature, the black suspension was separated by centrifuge and the precipitation was washed with DMF for three times. Finally, the obtained FMOF was freeze-dried for 5 days. The synthesis of TFMOF-100 ℃ and TFMOF-150 ℃ was same as that of TFMOF except being beat at 100 ℃ and 150 ℃ for 9 h, respectively. The synthesis of TFMOF-Non, TFMOF-HCl and TFMOF-Formic was same as that of TFMOF except that the no acetic acid was added in the solution, or was replaced with 0.6 mmol of HCl and HCOOH, respectively.
have been developed for the removal of CR from water such as photodegradation, filtration, Fenton oxidation and adsorption (Yang et al., 2018; Lei et al., 2017; Vimonses et al., 2009). Among these employing porous materials as adsorbents like active carbon has been recognized as an ideal method for their low-cost as well as facile operation. But there remains some inevitable drawbacks of traditional adsorbents such as uneven dispersion in aqueous environment, low adsorption capacity, long time consuming and unsatisfactory reusability (Hema et al., 2007), so stable and highly efficient adsorbents are still in great demand. Recently, metal-organic framework (MOF) featuring regular spatial structure, ultrahigh surface areas and easy-modified ligands has attracted extensively interests and its applications in catalysis, membrane separation, chemical sensor and energy storage (Xu et al., 2016; Liu et al., 2018; Wu et al., 2013; Cheng et al., 2017; Lu et al., 2018). Given the intrinsic micro-mesopores and tunable organic ligands with various hybrid atom such as O, N or S, which may exhibit specific affinity to dye molecular, their performance in water treatment has been explored by scholars around the world (Howe et al., 2017). Qiu etc. prepared a partially positive charged UiO-66 applied as an energetic adsorbent for the selective removal of ionic dyes in water (Qiu et al., 2017). Esra etc. employing flexible MIL-53 for the removal of methyl red via the π-π interaction between the terephthalic acid ligands in MIL-53 and methyl red as well as the breathing effect of MIL-53 (Yılmaz et al., 2016). Besides, some MOF composites like MOF@graphene oxide or MOF@ polymer were also prepared as competent adsorbents combining the merits from the composed countparts (Zhao et al., 2017; Xin et al., 2014). However, since the channels or cavities inside MOF might be too narrow for the diffusion of dye molecular especially ones with large sizes, thus 2D MOF nanosheets are considered to be much more beneficial for the adsorption because of the larger surface areas and more exposed active sites compared to other MOF with bulky or particle morphology (Cao et al., 2016; Yuan et al., 2020; Bhadra and Jhung, 2018; Wang et al., 2016a; Guo et al., 2017). As of now few MOF nanosheets have been reported and applied in the water treatment, here for the first time, by utilizing 1,1′-ferrocene-dicarboxylic acid (FDC) and Zr(OAc)4 as ligands and metal nodes, a novel ferrocene based tremella-like MOF (TFMOF) nanosheet has been synthesized successfully though a traditional solvothermal method. Taking the advantages of 2D MOF nanosheets with large exposed surface and the uncoordinated Z6O8 clusters which act as potential active sites, the performance of TFMOF in the removal of CR in water was investigated and adsorption conditions like temperature, pH and initial dye concentrations were optimized. Moreover, the study of adsorption kinetic and adsorption isothermal was also examined by various models to explore the adsorption mechanism. Besides, the reusability of FMOF was also tested giving it promising value in practical water treatment.
2.3. Characterization Scanning electron microscope (SEM, Zeiss, Utral 55, beam energy =5 kV) and high resolution transmission electron microscope (HRTEM, JEOL, JEM 2100 F) was taken to examine the morphology of TFMOF. The precise crystal structure and construction of TFMOF was determined by X-ray diffraction (XRD, PANalytical B.V. X-pert Powder, anode material: Cu) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher, Escalab 250Xi). Besides, the thermal stability of TFMOF was evaluated by thermogravimetric analysis (TGA, TA instrument, TAQ500) and the surface area were analyzed through BET test. Zeta potential of the samples were measured by laser particle size analyzer (LPZA, Malvern, Zetasizer3000HSA). 2.4. Absorption study of Congo red Before the adsorption test, the as prepared TFMOF was activated by heat at 120℃ under vacuum for 5 h to remove the residual solvents. 10 mg of TFMOF was dispersed in 20 mL deionized water with various pH value (4–7) and ultrasonicated for 10 min. 500 μL of CR stock solution (5 mg/mL) was add into the above suspension and then the mixed suspension was stirring at 303−323 K for 10 min. During the adsorption process, 2 mL of the suspension was taken out and filtered through a 0.22 μm microporous filter head (the membrane was penetrated with 1 mL of the suspension to reach its adsorption-desorption equilibrium). The concentration of CR in the filtrate was examined by UV–vis spectrum (HITACHI, U2900) according to the abs at 495 nm. The conversion of CR was calculated from the following equation:
con(%) =
N−C×V × 100% N
in which N (mmol), C (mmol/mL) and V (mL) stand for the initial amount of CR (mg), concentration of CR in the filtrate and the total volume of the mix suspension. On the other hand, the adsorption capacity (qe) of TFMOF was calculated by equation:
2. Experimental
mg ⎞ N−C×V q e ⎛⎜ ⎟ = g m ⎝ ⎠
2.1. Materials
in which m (mg) represented the amount of TFMOF added in the suspension. For the reusability test, after each cycle of adsorption, the separated TFMOF was immersed into 30 mL ethanol for 1 h then washed with deionized water. After that TFMOF was re-dispersed in 20 mL deionized water for next cycle of adsorption.
All agents are commercially available and no extra purification were taken before reactions. Zirconium acetate (Zr(OAc)4, Zr content: 15 wt %), Zirconium chloride (ZrCl4, 99 %), 1,4- dicarboxybenzene (BDC, 98 %) and Congo red (CR, 98 %) were purchased from Aladdin; 1,1′-ferrocene-dicarboxylic acid (FDC; 98 %) was purchased from Energy; acetic acid (AA, 99.5 %), formic acid (HCOOH, 98 %), hydrochloride acid (HCl, 37 wt%), absolute ethanol (99.7 %) and N, NDimethylformamide (DMF; 99.5 %) were purchased from SCR.
3. Results and discussion 3.1. Characterization
2.2. Synthesis of TFMOF nanosheets Our early study of ferrocene based metal organic framework demonstrated that ultrathin MOF nanosheets (Zr-FMOF) could be obtained by employing ZrCl4 and FDC as ligands and metal nodes (Deng et al., 2019). In this work, when the metal salt was displaced by Zr (OAc)4, the as-prepared TFMOF exhibited a tremella-like morphology,
In a typical synthesis procedure, 0.6 mmol of Zr(OAc)4 and 1,1′ferrocene-dicarboxylic acid were dissolved in 20 mL of DMF with 30 mmol of acetic acid which was used as the acid modulator. The above mixed solution was sealed into a Teflon autoclave and ultra2
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Scheme 1. Illustration of the preparation and crystal structure of TFMOF.
Fig. 1. SEM images of (a) TFMOF, (b) TFMOF-Non, (c) TFMOF-HCl and (d) TFMOF-Formic.
Fig. 2. HRTEM images of TFMOF in different magnifications.
Considering that it was only the kind of metal salt changed from chloride to acetate and the acetic acid played a critical role in the formation of ultrathin MOF naosheets, the intrinsic bonded acetate in Zr nodes may cause the change in morphology due to steric effect during the formation of MOF crystals, and the unit of TFMOF was also
which was quite different from that of the abovementioned Zr-FMOF (Scheme 1, Figs. 1a and S1). Compared with the thin film like Zr-FMOF with lateral size of ca. 500 nm, enormous intercalation of nanosheets could be found in that of TFMOF and their thickness also showed slight increment according to the folded shadow in HRTEM images (Fig. 2). 3
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Fig. 3. SEM images of TFMOF prepared in different reaction time. (a) TFMOF-0.5 h, (b) TFMOF-1 h, (c) TFMOF-1.5 h, (d) TFMOF-2 h, (e) TFMOF-9 h, (f) TFMOF12 h.
characteristic peaks had emerged in the first 2 h. Consequently, 9 h was chosen as the optimum reaction time for the synthesis of TFMOF, for no more differences of FMOF in morphology could be observed. And longer reaction time (12 h) did not lead to a higher yield after 9 h. Noting that the XRD pattern of TFMOF was highly similar to that of the previous reported FMOF (Fig. 4b), implying they had same crystal structure and network while only diverse morphology could be observed when employing various Zr sources. On the other aspect, the reaction temperature was also investigated by preparing TFMOF at 100 ℃ and 150 ℃. As was depicted in the SEM images (Fig. 5a and b), lower reaction temperature resulted loose architectures while higher temperature might cause disintegration of the MOFs crystals, which could also be reflected in the XRD patterns that the diminution of crystallization led to weak peaks (Fig. 5c). Figs. 6a–b and S3 showed the XPS patterns of Fe and Zr in TFMOF. The binding energy peaks of Fe 2p at 707.78 eV and 720.38 eV were corresponding to the Fe2+ in ferrocene (Yuan et al., 2020), while in the pattern of Zr 3d, the binding energy at 182.08 eV and 184.48 eV were in good agreement with the Zr-O bond, suggesting the Zr6O8 cluster as the nodes in TFMOF (Decoste et al., 2013). Fig. 6c showed the TG curves of the as-prepared TFMOF. It was indicated a slight loss of weight before 200 ℃ was ascribed to the guest solvent molecular. And this TFMOF
supposed to be [(Zr6O4](OH)4(FDC)4]. As was found in the SEM images of TFMOF-Non, TFMOF-HCl and TFMOF-Formic, which were prepared without addition of acetic acid or with the addition of HCl and formic acid instead of exhibited amorphous morphology. It was assumed that the exchanges of bonded monocarboxylic acetic acid (Fig. 1b–d, Fig. S2), no regular MOF crystals could be observed while all of them acid on Zr clusters with ditopic ligands resulted in the formation of MOF crystals (Guillerm et al., 2010). So the addition of HCl or formic acid might handicap the coordination between Zr clusters and FDC ligands, leading to unsuccessful crystallization of TFMOF. To give a deep insight on the growth of TFMOF, the SEM images and XRD patterns of grown TFMOF was displayed in Figs. 3 and Fig. 4a. In the first 30 min, only amorphous agglomeration of MOF could be found and the broad peaks in its XRD patterns also implying no regular crystal structure was formed in this stage. When the reaction time increased to 1 h, few small tremella like MOF nanosheets begin to construct and overlapped with former amorphous agglomerate meanwhile sharp peaks in PXRD patterns appears, confirming the formation of MOF crystals. Further enhancing reaction time to 1.5 h and 2 h, the tremella like MOF nanosheets kept on growing with dissipation of agglomerates, which could be thought as the agglomerates was transferring to regular crystals with time passed by. It was also worth noting that all 4
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Fig. 4. (a) XRD patterns of TFMOF obtained after different reaction time and (b) XRD patterns of simulated TFMOF, TFMOF (12 h) and FMOF.
was quite stable till 400 ℃ after that a harsh collapse of structure happened caused a weight loss of about 20 %. Interestingly, the BET specific surface area of TFMOF was calculated to be 89.9 m²/g and the average pore size was ca. 11.7 nm according to the N2 adsorption/desorption isotherm (Fig. 6d). And the t-plot further confirming that there were no micropores but meso-macropores in this MOF (Fig. 6d, inset). As the previous prepared FMOF featured a BET surface areas of 296.7 m²/g as well as micropores inside, it was such a surprise their morphologies exhibited marvelous discrepancies, which may be explained by the overlapping of nanosheets.
3.2. Adsorption of CR Fig. 7 displayed the max adsorption capacity of TFMOF in water solution with various pH adjusted by diluted HCl. With an initial concentration of 150 mg/L, it could be found that the adsorption capacity enhanced from 194.00 mg g−1 to 249.18 mg g−1 with the acidity crept up. Since CR is a kind of ionic dyes, and the raise in adsorption capacity may be attribute to the promoted electrostatic interaction between CR and TFMOF. As was found in the zeta potential of TFMOF in different solution (Fig. 8), the surface of TFMOF was negative charged at the pH of 7, which was supposed to be attributed to the carboxyl groups on the ligands. While the surface of TFMOF was positive charged at the pH of 4–6, suggesting the zero point of charge of TFMOF in water was around
Fig. 5. SEM images of TFMOF prepared at different reaction temperature. (a): TFMOF-100 ℃ and (b): TFMOF-150 ℃. (c): XRD patterns of TFMOF prepared at different temperature. 5
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Fig. 6. XPS patterns of (a) Fe 2p and (b) Zr 3d in TFMOF. (c): TG curve of TFMOF and (d): N2 adsorption/desorption isotherm of TFMOF (N2 atmosphere, 77 K); inset: t-plot curve of TFMOF.
Fig. 7. (a) Adsorption capacity and removal percentage of TFMOF towards CR in water with different pH value (4-7) and (b) UV–vis absorption spectrum of CR after adsorbed in water with different pH value. (Adsorption conditions: C0 = 150 mg/L; amount of TFMOF = 0.5 mg/mL; T =303 K; t = 10 min.).
that under visible light, confirming the intrinsic adsorption capacity of TFMOF. The adsorption performance of TFMOF prepared at various temperature was explored as was exhibited in Fig. 9a. Although TFMOF prepared at 100 ℃ and 150 ℃ have larger surface areas (Fig. S4 and Table S1), the reduce of the adsorption capacity was possibly due to the insufficient crystallization of TFMOF prepared at lower temperature while the adsorbent prepared at higher temperature might make more ligands coordinate with the metal clusters, which gave less adsorb sites
6.5, and the increment of positivity might promote the affinity of TFMOF toward the negative charged CR molecular. It is also worth noting that the CR would self-degrade under visible irradiation when the pH went down to 3 or lower, so pH 4 was chosen as the optimized condition in followed sections. To excluding the possibility that the CR was removed via a Fenton-like photocatalytic degradation in the presence of Fe2+ and visible light, a control test was carried out by putting the mixture of TFMOF and CR suspension in the dark, and the result indicating that the removal efficiency in this test was just as good as 6
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252.25 mg g−1. The temperature also played a curial role in the adsorption process. Fig. 9c gave the time-depended adsorption capacity of TFMOF at different temperature (303 K–323 K). Under all three different temperatures more than 90 % of the CR in the suspension was removed within 3 min as the color faded rapidly from deep red to sight pink (Fig. S5). And the max adsorption capacity of CR on TFMOF were 240.80 mg g−1, 242.09 mg g−1 and 242.61 mg g−1 at 303 K, 313 K and 323 K respectively, suggesting the temperature contributed negligible effect on the adsorption capacity, and the energy of activations was calculated to be 37.27 kJ/mol from the Arrhenius equation (Fig. 9d). To give a deeper insight on the adsorption kinetic of TFMOF at various temperature, two kinetic models, pseudo-first order and pseudo-second order models were employed for the simulation (Febrianto et al., 2009). The pseudo-first-order kinetic model was described as the followed equation:
ln(q e − qt) = ln(q e) − k1t where qe (mg·g−1) represented the equilibrium adsorption capacity; qt (mg·g−1) was the adsorption capacity measured at the contact time of t; k1 (min−1) stood for the pseudo-first order kinetic rate constant, which could be obtained from the ln (qe – qt) vs t linear fitting. On the other hand, the pseudo-second order kinetic model could be expressed as:
Fig. 8. Zeta potential of TFMOF in water with different pH value.
for the CR molecular. The max adsorption capacity was then investigated by employing TFMOF as adsorbent with various initial concentration of CR, as was displayed in Fig. 9b. The TFMOF exhibited outstanding adsorption performance and over 96 % of the CR could be removed within 6 min with concentration of 100 mg/L and 125 mg/L. Under initial concentration of 150 mg/L, over 80 % of the dye could be removed within 10 min with a max adsorption capacity of
t 1 t = + qt qe k2q 2e in
which
the
pseudo-second
order
kinetic
rate
constant-k2
Fig. 9. (a): Adsorption of CR by TFMOF prepared at various temperature. (b): Adsorption od CR by TFMOF with different initial dye concentrations (TFMOF = 0.5 mg/mL, pH = 4, T =303 K.). (c): Adsorption of CR at different temperature. (C0 = 125 mg/L, TFMOF = 0.5 mg/mL, pH = 4.) and (d): linear plot of lnK vs 1/T for the adsorption of CR by TFMOF. 7
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Table 1 Kinetic study of CR adsorption on TFMOF at 303-323 K. Temperature/K
Pseudo-first order kinetic model k1/min
303 313 323
−1
qe/mg g
0.386 0.283 0.237
Pseudo-second order kinetic model −1
R
49.90 23.78 20.49
2
0.9436 0.7236 0.8841
k2/g mg−1 min−1
qe/mg g−1
R2
0.0196 0.0376 0.0488
244.50 245.09 241.54
0.9999 0.9999 0.9998
Adsorption conditions: C0 = 125 mg/L; amount of TFMOF = 0.5 mg/mL; t = 10 min; pH = 4.
(g·mg−1 min−1) could be calculated through the linear fitting of t/qt vs t. As was showed in Table 1 and Fig. S6, the qe calculated from pseudo-first-order kinetic model at different temperature was far lower compared to the max adsorption capacity determined in the experiments. Besides, the rate constant k1 seemed to decrease as the temperature increased, which was contrary to the former observed phenomenon. According to the parameters obtained from the pseudo-firstorder kinetic model, the theoretical max adsorption capacities were close to the reality situation, and k2 also rose gradually as the temperature increased. Furthermore, the correlation coefficients of pseudosecond order kinetic model were all exactly close to 1, indicating the adsorption of CR on TFMOF could be described well as a pseudo-second order kinetic model than pseudo-first order kinetic model. The adsorption thermodynamics was also investigated to give thermodynamic parameters like Gibbs free energy (ΔG), standard enthalpy change (ΔH) and standard entropy change (ΔS). And the parameters could be calculated as the followed Van 't Hoff equation:
lnK=
where qe (mg·g−1) represented the CR adsorbed on TFMOF with different initial concentration of CR (50−400 mg/L); Ce (mg/L) represented the equilibrium concentration of CR in the suspension; qmax (mg·g−1) represented the theoretical max adsorption capacity of CR on TFMOF; and KL (L/mg) was the Langmuir equilibrium constant. The Freundlich isotherm model on the other hand, predicted a nonuniform adsorption on heterogeneous adsorbents (Günay et al., 2007), which was expressed as the followed equation:
q e = KF CneF The KF (mg1−n·Ln/g) and nF were the Freundlich equilibrium constants. The Sips isothermal could be recognized as a combination of both Langmuir and Freundlich that the equation could be recognized as the Freundlich equation at low initial concentration of guest molecular while it would predict a typical monolayer adsorption as Langmuir isothermal at high initial concentration of guest molecular (Vargas et al., 2011). It could be expressed as:
−ΔH ΔS + RT R
q e = qmax
(KSCe)nS 1 + (KSCe)nS
where the KS (L/mg) and nS were the Sips equilibrium constants. According to the data calculated from the nonlinear fitting (Table 3 and Fig. S7), the Sips isothermal model could be considered as the most suitable models to describe this adsorption behavior with a reliable correlation coefficient compared with that of the other two models. The adsorption behavior followed Freundlich adsorption isothermal at low dye concentration that the adsorption capacity increased with the initial dye concentration, while at higher dye concentration it was supposed to predict a monolayer adsorption as Langmuir isothermal model. Moreover, the theoretical max adsorption capacity obtained from Sips isothermal model showed slight decline at high temperature, implying this adsorption was probably an exothermic process.
ΔG = -RTlnK where the K was the adsorption equilibrium constant that could be calculated from qe/Ce, ΔH and ΔS could be obtained from the slope and intercept in the linear fitting of K vs 1/T (Lai et al., 2019). The calculated parameters were exhibited in Table 2. The positive ΔH value indicating the adsorption of CR on TFMOF was an endothermic process, which could also explain the slight decrease of adsorption capacity at higher temperature. Meanwhile the negative ΔG implying the adsorption was carried out spontaneously at various temperature, and the adsorption was more likely to happen at higher temperature according to the decline of ΔG value.
3.4. Adsorption mechanism 3.3. Adsorption isotherm Based on the experimental data, the excellent adsorption capacity of TFMOF towards CR could be explained by the following two reasons: (i) It has been reported “missing linker” phenomenon was frequently happened in UiO-66, which was composed of Zr6O8 cluster and 1,4dicarboxybenzene, as monoacid was used in the formation of MOF structure, resulting in unsaturated metal nodes (Gutov et al., 2015). As the same kind of Zr6O8 clusters existed in TFMOF and each Z6O8 cluster was supposed to be only coordinated with 4 FDC ligands, the Z6O8 clusters would exhibit strong affinity to the electron-rich O atoms in sulfoacid group (Scheme 2). (ii) The large exposed surface areas of TFMOF would be benefit for the contact of TFMFO and target molecular, and the electrostatic interaction between anionic CR and positive charged TFMOF, which comes from the uncoordinated Z6O8 cluster. Also noting that the XPS pattern of Fe showed no obvious changes after the adsorption, suggesting that there was no interaction between CR and Fe sites in the ligands (Fig. S8a). Generally, such a guest molecular with large size like CR was not able to get access to the nodes in bulk MOF, which featured intrinsic narrow channels or cavities. As a contrast, the adsorption performance of UiO-66 which was composed of the
To get a better understanding of the adsorption mechanism, the adsorption isotherms of CR on TFMOF were investigated in this study and two models, Langmuir, Freundlich and Sips isotherms were applied for the simulation. The Langmuir isotherm model is generally employed when the guest was adsorbed on the adsorbents uniformly and in monolayer coverage (Chatterjee et al., 2009), which was expressed as the equation:
qe =
KL qmax Ce 1 + KL Ce
Table 2 Thermodynamic parameters of TFMOF in the adsorption of CR. T/K
ΔG/KJ mol−1
ΔH/KJ mol−1
ΔS/J mol−1
303 313 323
−9.39 −13.28 −14.02
75.24
280.93
8
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Table 3 Adsorption isotherm parameters calculated in different isothermal models. Temperature (K)
Langmuir qmax (mg g
303 313 323
254.14 251.62 247.14
Freundlich −1
)
KL (L mg 1.59 1.49 1.56
−1
)
R
2
0.9109 0.9712 0.9829
KF (mg
1−n
Sips L
1−n
-1
g )
156.24 165.24 155.55
2
nF
R
0.1225 0.0947 0.1125
0.7561 0.5461 0.7542
qmax (mg g−1)
KS (L/mg)
nS
R2
274.14 256.31 255.86
1.24 1.44 1.47
0.66 1.31 0.77
0.9494 0.9917 0.9996
(Fig. 10b), demonstrating the good stability of TFMOF. On the other hand, the tremella-like nanosheets structure of TFMOF maintained well after the adsorption process (Fig. 10c–d), indicating the excellent stability of TFMOF in aqueous environment. Noting that though only a max adsorption of 124.4 mg g−1 could be realized after three cycles, the performance of TFMOF was still better than most previous reported adsorbents especially for the superior removal time, including other kinds of porous MOF or MOF composites. As was listed in Table 4, the fast adsorption rate, remarkable adsorption capacity and facile preparation approach made TFMOF a superior adsorbent in the application of water treatment.
same Zr6O8 cluster and planar ligand 1,4- dicarboxybenzene was investigated (Fig. S9). Only less than 30 % of the Congo red could be removed under the same conditions compared to that of TFMOF (> 80 %). Taking the advantages of TFMOF nanosheets, the exposed Z6O8 clusters on the surface would be much accessible for CR to contact. So it can be interpreted that the unsaturated Z6O8 clusters in TFMOF and 2D nanosheets structure was of great benefit for the efficient adsorption of CR. 3.5. Reusability test To certify the stability and reusability of TFMOF as a promising adsorbent, the adsorption capacity of TFMOF was tested for several cycles. As was found in Fig. 10a, the max adsorption capacity suffered a sharp decline of nearly 50 % since the second cycle and only a max capacity of 124.4 mg g−1 could be achieved when it turned to the third cycle. Such a loss in capacity was not unanticipated cause as there was strong affinity between TFMOF and CR molecular, reflecting in high adsorption capacity in the first cycle, thus desorption of guest CR that coordinated with the Zr6O8 clusters in TFMOF would be an arduous process, and binding energy peak of S 2p at 167.38 eV in the XPS pattern of TFMOF after adsorption, which exhibited slight shift compared with that of pure CR (168.48 eV), indicating the existence of undesorbed CR on TFMOF (Figs. S8b and S7c). And the BET specific surface areas of TFMOF declined from 89.9 m2/g to 55.4 m2/g after the adsorption of CR, suggesting the coverage of adsorbed CR on the surface of TFMOF (Fig. S10). So we proposed that during the desorption process, around half of CR in the first adsorption were attached on the surface of TFMOF though weak electrostatic interaction and would be quite easy to be removed when suitable solvents were utilized. Meanwhile the other part of CR could not be desorbed due to the strong coordination bond. Besides, since the adsorbent was recycled by centrifuge during cycles, the loss of adsorbent may also be ascribed to the decline in adsorption capacity. No obvious differences could be observed according to the XRD pattern of TFMOF after the adsorption
4. Conclusion In summary, a novel ferrocene-based TFMOF nanaosheets was firstly prepared with Zr(OAc)4 and 1,1′-ferrocene-dicarboxylic acid. The obtained TFMOF exhibited a unique tremella-like morphology. The formation stages of TFMOF during the crystallization was investigated and demonstrating the in-situ growth of TFMOF nanosheets. Subsequently, the as-prepared TFMOF was utilized as an efficient porous adsorbent in the removal of organic dye CR from water. A highestadsorption capacity of 252.25 mg g−1 could be achieved at pH of 4. A pseudo-second order kinetic model was recognized as the most precise model to describe the adsorption behavior of CR on TFMOF with a k2 of 0.0488 g·mg-1 min-1 at 323 K. On the other hand, the CR was considered to be adsorbed on the surface of TFMOF in monolayer as was induced from a Langmuir isotherm model. The unprecedented adsorption capacity of TFMOF towards CR might be attributed to the exposed Zr6O8 cluster which could coordinate with negative charged CR molecular via the electrostatic interaction. Finally, the reusability of TFMOF was tested and the max adsorption could maintain as higher as 124.38 mg g−1 after three cycles. Thus the tremella-like TFMOF nanosheets could be thought as a promising adsorbent in the water treatment with ideal removal efficient and favorable reusability.
Scheme 2. Proposed adsorption mechanism of CR on TFMOF by the clusters. 9
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Fig. 10. (a) Max adsorption capacity of TFMOF after three cycles, (b) XRD patterns of TFMOF before and after the adsorption of CR, (c) SEM and TEM (d) images of TFMOF after the adsorption process.
acknowledged.
Table 4 Time-consuming and adsorption capacity of reported adsorbents. Adsorbent
Time/min
qmax/mg g−1
Ref
Ni/Mg/Al LDO UiO-66 Fe3O4@SiO2 MoS2/Ag2S/Ag Co3O4/TiO2/GO Co-MOF ZIF-9@SMM DS-Zn/Al LDHs ZIF-8DMF-M TFMOF
300 120 360 12 90 180 700 180 60 10
1250.00 97.70 45.46 36.37 36.40 27.57 146.00 149.30 394.00 252.25
(Lei et al., 2017) (Qiu et al., 2017) (Wang et al., 2016b) (Zeng et al., 2019) (Jo et al., 2017) (Rajak et al., 2017) (Dai et al., 2017) (Grover et al., 2019) (Liu et al., 2019) This work
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2020.122274. References Ansari, R., Keivani, M.B., Delavar, A.F., 2011. Application of polyaniline nanolayer composite for removal of tartrazine dye from aqueous solutions. J. Polym. Res. 18, 1931–1939. Bhadra, B.N., Jhung, S.H., 2018. Adsorptive removal of wide range of pharmaceuticals and personal care products from water using bio-MOF-1 derived porous carbon. Microporous Mesoporous Mater., S1387181118302439. Cao, F., Zhao, M., Yu, Y., Chen, B., Huang, Y., Yang, J., Cao, X., Lu, Q., Zhang, X., Zhang, Z., Tan, C., Zhang, H., 2016. Synthesis of two-dimensional CoS1.097/nitrogen-doped carbon nanocomposites using metal–Organic framework nanosheets as precursors for supercapacitor application. J. Am. Chem. Soc. 138, 6924–6927. Chatterjee, S., Lee, D.S., Lee, M.W., Woo, S.H., 2009. Enhanced adsorption of congo red from aqueous solutions by chitosan hydrogel beads impregnated with cetyl trimethyl ammonium bromide. Bioresour. Technol. 100, 2803–2809. Cheng, Y., Wu, J., Guo, C., Li, X.-G., Ding, B., Li, Y., 2017. A facile water-stable MOFbased “off–on” fluorescent switch for label-free detection of dopamine in biological fluid. J. Mater. Chem. B 5, 2524–2535. Dai, J., Xiao, S., Liu, J., He, J., Lei, J., Wang, L., 2017. Fabrication of ZIF-9@supermacroporous microsphere for adsorptive removal of Congo red from water. RSC Adv. 7, 6288–6296. Decoste, J.B., Peterson, G.W., Jasuja, H., Glover, T.G., Huang, Y.G., Walton, K.S., 2013. Stability and degradation mechanisms of metal-organic frameworks containing the Zr6O4(OH)4 secondary building unit. J. Mater. Chem. A 1, 5642–5650. Deng, Z., Yu, H., Wang, L., Liu, J., Shea, K.J., 2019. Ferrocene-based metal–organic framework nanosheets loaded with palladium as a super-high active hydrogenation catalyst. J. Mater. Chem. A 7, 15975–15980. Du, Q., Sun, J., Li, Y., Yang, X., Wang, X., Wang, Z., Xia, L., 2014. Highly enhanced adsorption of congo red onto graphene oxide/chitosan fibers by wet-chemical etching off silica nanoparticles. Chem. Eng. J. 245, 99–106.
Author contributions 1 Jiyang Liu: Author of this manuscript. 2 Haojie Yu: Professor Yu checked the origin data and provided his suggestions during the revision. 3 Li Wang: Professor Wang provided his suggestions about the contents and format, and gave the final approval of the manuscript to be submitted. Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgements Financial supports from National Natural Science Foundation of China (51873189, 51673170, and 51811530097) are gratefully 10
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highly selective adsorption of anionic dyes: adsorption performance and mechanisms. J. Colloid Interface Sci. 499, 151. Rajak, R., Saraf, M., Mohammad, A., Mobin, S.M., 2017. Design and construction of a ferrocene based inclined polycatenated Co-MOF for supercapacitor and dye adsorption applications. J. Mater. Chem. A 5, 17998–18011. Vargas, A.M.M., Cazetta, A.L., Kunita, M.H., Silva, T.L., Almeida, V.C., 2011. Adsorption of methylene blue on activated carbon produced from flamboyant pods (Delonix regia): study of adsorption isotherms and kinetic models. Chem. Eng. J. 168, 722–730. Vimonses, V., Lei, S., Jin, B., Chow, C.W.K., Saint, C., 2009. Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials. Chem. Eng. J. 148, 354–364. Wang, S., Peng, Y., 2010. Natural zeolites as effective adsorbents in water and wastewater treatment. Chem. Eng. J. 156, 11–24. Wang, J.-S., Jin, F.-Z., Ma, H.-C., Li, X.-B., Liu, M.-Y., Kan, J.-L., Chen, G.-J., Dong, Y.-B., 2016a. Au@Cu(II)-MOF: highly efficient bifunctional heterogeneous catalyst for successive oxidation–condensation reactions. Inorg. Chem. 55, 6685–6691. Wang, P., Wang, X., Yu, S., Zou, Y., Jian, W., Chen, Z., Alharbi, N.S., Alsaedi, A., Hayat, T., Chen, Y., 2016b. Silica coated Fe 3 O 4 magnetic nanospheres for high removal of organic pollutants from wastewater. Chem. Eng. J. 306, 280–288. Wu, H., Yong, S.C., Krungleviciute, V., Tyagi, M., Chen, P., Yildirim, T., Zhou, W., 2013. Unusual and highly tunable missing-linker defects in zirconium metal–organic framework UiO-66 and their important effects on gas adsorption. J. Am. Chem. Soc. 135, 10525–10532. Xin, C., Lin, D., Jing, L., Wang, L., Jing, H., Deng, L., Lei, J., 2014. Fabrication of ZIF-8@ super-macroporous poly(glycidyl methacrylate) microspheres. Inorg. Chem. Commun. 50, 65–69. Xu, H., Gao, J., Qian, X., Wang, J., He, H., Cui, Y., Yang, Y., Wang, Z., Qian, G., 2016. Metal–organic framework nanosheets for fast-response and highly sensitive luminescent sensing of Fe3+. J. Mater. Chem. A 4, 10900–10905. Yagub, M.T., Sen, T.K., Afroze, S., Ang, H.M., 2014. Dye and its removal from aqueous solution by adsorption: a review. Adv. Colloid Interface Sci. 209, 172–184. Yang, Q., Wang, Y., Wang, J., Liu, F., Hu, N., Pei, H., Yang, W., Li, Z., Suo, Y., Wang, J., 2018. High effective adsorption/removal of illegal food dyes from contaminated aqueous solution by Zr-MOFs (UiO-67). Food Chem. 254, 241–248. Yılmaz, E., Sert, E., Atalay, F.S., 2016. Synthesis, characterization of a metal organic framework: MIL-53 (Fe) and adsorption mechanisms of methyl red onto MIL-53 (Fe). J. Taiwan Inst. Chem. Eng. 65, 323–330. Yuan, S., Bo, X., Guo, L., 2020. In-situ Insertion of Multi-walled Carbon Nanotubes in the Fe3O4/N/C Composite Derived From Iron-based Metal-organic Frameworks As a Catalyst for Effective Sensing Acetaminophen and Metronidazole, Talanta. Zeng, Y., Guo, N., Li, H., Wang, Q., Xu, X., Yu, Y., Han, X., Yu, H., 2019. Construction of flower-like MoS2/Ag2S/Ag Z-scheme photocatalysts with enhanced visible-light photocatalytic activity for water purification. Sci. Total Environ. 659, 20–32. Zhao, S., Chen, D., Wei, F., Chen, N., Liang, Z., Luo, Y., 2017. Removal of Congo red dye from aqueous solution with nickel-based metal-organic framework/graphene oxide composites prepared by ultrasonic wave-assisted ball milling. Ultrason. Sonochem. 39, 845–852.
Febrianto, J., Kosasih, A.N., Sunarso, J., Ju, Y.-H., Indraswati, N., Ismadji, S., 2009. Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: a summary of recent studies. J. Hazard. Mater. 162, 616–645. Grover, A., Mohiuddin, I., Malik, A.K., Aulakh, J.S., Kim, K.H., 2019. Zn-Al layered double hydroxides intercalated with surfactant: synthesis and applications for efficient removal of organic dyes. J. Clean. Prod. 240, 10. Guillerm, V., Gross, S., Serre, C., Devic, T., Bauer, M., Férey, G., 2010. A zirconium methacrylate oxocluster as precursor for the low-temperature synthesis of porous zirconium(iv) dicarboxylates. Chem. Commun. 46, 767–769. Günay, A., Arslankaya, E., Tosun, İ., 2007. Lead removal from aqueous solution by natural and pretreated clinoptilolite: adsorption equilibrium and kinetics. J. Hazard. Mater. 146, 362–371. Guo, W., Sun, W., Lv, L.P., Kong, S., Wang, Y., 2017. Microwave-assisted morphology evolution of Fe-based metal-organic frameworks and their derived Fe2O3 nanostructures for Li-Ion storage. ACS Nano 11, 4198. Gupta, V.K., 2009. Application of low-cost adsorbents for dye removal–a review. J. Environ. Manage. 90, 2313–2342. Gutov, O.V., Miguel González, H., Escudero-Adán, E.C., Alexandr, S., 2015. Metal-organic framework (MOF) defects under control: insights into the missing linker sites and their implication in the reactivity of zirconium-based frameworks. Inorg. Chem. 54, 8396–8400. Hema, M., Arivoli, S., Hema, M., 2007. Comparative study on the adsorption kinetics and thermodynamics of dyes onto acid activated low cost carbon. Int. J. Phys. 2, 10–17. Howe, J.D., Liu, Y., Flores, L., Dixon, D.A., Sholl, D.S., 2017. Acid gas adsorption on metal-organic framework nanosheets as a model of an "All-Surface" material. J. Chem. Theory Comput. 13, 1341–1350. Jo, W.K., Kumar, S., Isaacs, M.A., Lee, A.F., Karthikeyan, S., 2017. Cobalt promoted TiO 2 /GO for the photocatalytic degradation of oxytetracycline and Congo Red. Appl. Catal. B Environ. 201, 159–168. Lai, Y., Wang, F., Zhang, Y., Ou, P., Wu, P., Fang, Q., Chen, Z., Li, S., 2019. UiO-66 derived N-doped carbon nanoparticles coated by PANI for simultaneous adsorption and reduction of hexavalent chromium from waste water. Chem. Eng. J. 378, 122069. Lei, C., Zhu, X., Zhu, B., Jiang, C., Le, Y., Yu, J., 2017. Superb adsorption capacity of hierarchical calcined Ni/Mg/Al layered double hydroxides for Congo red and Cr(VI) ions. J. Hazard. Mater. 321, 801–811. Liu, J., Yu, H., Wang, L., Deng, Z., Naveed, K.-u.-R., Nazir, A., Haq, F., 2018. Two-dimensional metal-organic frameworks nanosheets: synthesis strategies and applications. Inorganica Chim. Acta 483, 550–564. Liu, J., Li, J., Wang, G., Yang, W., Yang, J., Liu, Y., 2019. Bioinspired zeolitic imidazolate framework (ZIF-8) magnetic micromotors for highly efficient removal of organic pollutants from water. J. Colloid Interface Sci. 555, 234–244. Lu, B.B., Yang, J., Che, G.B., Pei, W.Y., Ma, J.F., 2018. Highly stable copper(I)-based metal-organic framework assembled with resorcin[4]arene and polyoxometalate for efficient heterogeneous catalysis of azide-alkyne "Click" reaction. ACS Appl. Mater. Interfaces 10, 2628–2636. Olivera, S., Muralidhara, H.B., Venkatesh, K., Guna, V.K., Gopalakrishna, K., Kumar K, Y., 2016. Potential applications of cellulose and chitosan nanoparticles/composites in wastewater treatment: a review. Carbohydr. Polym. 153, 600–618. Qiu, J., Feng, Y., Zhang, X., Jia, M., Yao, J., 2017. Acid-promoted synthesis of UiO-66 for
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Superior absorption capacity of tremella like ferrocene based metal-organic framework in removal of organic dye from water
T
Jiyang Liu, Haojie Yu*, Li Wang* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Deyi Hou
Removal of organic dyes from water by porous materials is considered as an efficient and low-cost way. Herein for the first time novel tremella-like ferrocene based metal-orgainc framework (TMOF) nanosheets designated as TFMOF were synthesized through a traditional solvothermal method. This ferrocene based TFMOF exhibit outstanding removal efficiency towards organic dye Congo red (CR) from water. After optimizing the reaction conditions, the highest adsorption capacity of 252.25 mg g−1 could be achieved within 10 min. Furthermore, the investigation of adsorption kinetic indicated this adsorption process could be described as a pseudo-second order kinetic model with k2 and qe of 0.0488 g mg−1 min−1 and 241.5 mg g−1, respectively. The adsorption isotherm could also be described as the Sips isotherm model according to the fitting calculation. The removal efficiency could maintain around 50 % with adsorption capacity of 124.38 mg g−1 after 3 cycles, giving the TFMOF promising potential in the practical water treatment.
Keywords: Metal-organic framework Nanosheets Congo red Water treatment Adsorption
1. Introduction With the rapid expansion of modern industry, water contamination has been emerging as a severe problem due to the unrestrained emission of pollutant from factories (Yagub et al., 2014; Wang and Peng, 2010; Gupta, 2009; Ansari et al., 2011). Among all of these pollutants, removal of water-soluble dyes is of great challenge because of their
⁎
acute toxicity even in very trace amount. Congo red (CR) is a kind of artificial azobenzene dyes which is widely used in leather, fabrication and food industries (Olivera et al., 2016; Du et al., 2014). As CR is quite soluble and stable in water, during the production and dyeing process large amount of waste water containing CR may be poured into rivers or lakes, so the treatment of poisonous dyes containing water is of great significance for our health and environment. So far several approaches
Corresponding authors. E-mail addresses: [email protected] (H. Yu), [email protected] (L. Wang).
https://doi.org/10.1016/j.jhazmat.2020.122274 Received 14 August 2019; Received in revised form 29 November 2019; Accepted 10 February 2020 Available online 11 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
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sonicated for 30 min. Then the autoclave was put into an oven and heated at 120 ℃ for 9 h. After cooling down to room temperature, the black suspension was separated by centrifuge and the precipitation was washed with DMF for three times. Finally, the obtained FMOF was freeze-dried for 5 days. The synthesis of TFMOF-100 ℃ and TFMOF-150 ℃ was same as that of TFMOF except being beat at 100 ℃ and 150 ℃ for 9 h, respectively. The synthesis of TFMOF-Non, TFMOF-HCl and TFMOF-Formic was same as that of TFMOF except that the no acetic acid was added in the solution, or was replaced with 0.6 mmol of HCl and HCOOH, respectively.
have been developed for the removal of CR from water such as photodegradation, filtration, Fenton oxidation and adsorption (Yang et al., 2018; Lei et al., 2017; Vimonses et al., 2009). Among these employing porous materials as adsorbents like active carbon has been recognized as an ideal method for their low-cost as well as facile operation. But there remains some inevitable drawbacks of traditional adsorbents such as uneven dispersion in aqueous environment, low adsorption capacity, long time consuming and unsatisfactory reusability (Hema et al., 2007), so stable and highly efficient adsorbents are still in great demand. Recently, metal-organic framework (MOF) featuring regular spatial structure, ultrahigh surface areas and easy-modified ligands has attracted extensively interests and its applications in catalysis, membrane separation, chemical sensor and energy storage (Xu et al., 2016; Liu et al., 2018; Wu et al., 2013; Cheng et al., 2017; Lu et al., 2018). Given the intrinsic micro-mesopores and tunable organic ligands with various hybrid atom such as O, N or S, which may exhibit specific affinity to dye molecular, their performance in water treatment has been explored by scholars around the world (Howe et al., 2017). Qiu etc. prepared a partially positive charged UiO-66 applied as an energetic adsorbent for the selective removal of ionic dyes in water (Qiu et al., 2017). Esra etc. employing flexible MIL-53 for the removal of methyl red via the π-π interaction between the terephthalic acid ligands in MIL-53 and methyl red as well as the breathing effect of MIL-53 (Yılmaz et al., 2016). Besides, some MOF composites like MOF@graphene oxide or MOF@ polymer were also prepared as competent adsorbents combining the merits from the composed countparts (Zhao et al., 2017; Xin et al., 2014). However, since the channels or cavities inside MOF might be too narrow for the diffusion of dye molecular especially ones with large sizes, thus 2D MOF nanosheets are considered to be much more beneficial for the adsorption because of the larger surface areas and more exposed active sites compared to other MOF with bulky or particle morphology (Cao et al., 2016; Yuan et al., 2020; Bhadra and Jhung, 2018; Wang et al., 2016a; Guo et al., 2017). As of now few MOF nanosheets have been reported and applied in the water treatment, here for the first time, by utilizing 1,1′-ferrocene-dicarboxylic acid (FDC) and Zr(OAc)4 as ligands and metal nodes, a novel ferrocene based tremella-like MOF (TFMOF) nanosheet has been synthesized successfully though a traditional solvothermal method. Taking the advantages of 2D MOF nanosheets with large exposed surface and the uncoordinated Z6O8 clusters which act as potential active sites, the performance of TFMOF in the removal of CR in water was investigated and adsorption conditions like temperature, pH and initial dye concentrations were optimized. Moreover, the study of adsorption kinetic and adsorption isothermal was also examined by various models to explore the adsorption mechanism. Besides, the reusability of FMOF was also tested giving it promising value in practical water treatment.
2.3. Characterization Scanning electron microscope (SEM, Zeiss, Utral 55, beam energy =5 kV) and high resolution transmission electron microscope (HRTEM, JEOL, JEM 2100 F) was taken to examine the morphology of TFMOF. The precise crystal structure and construction of TFMOF was determined by X-ray diffraction (XRD, PANalytical B.V. X-pert Powder, anode material: Cu) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher, Escalab 250Xi). Besides, the thermal stability of TFMOF was evaluated by thermogravimetric analysis (TGA, TA instrument, TAQ500) and the surface area were analyzed through BET test. Zeta potential of the samples were measured by laser particle size analyzer (LPZA, Malvern, Zetasizer3000HSA). 2.4. Absorption study of Congo red Before the adsorption test, the as prepared TFMOF was activated by heat at 120℃ under vacuum for 5 h to remove the residual solvents. 10 mg of TFMOF was dispersed in 20 mL deionized water with various pH value (4–7) and ultrasonicated for 10 min. 500 μL of CR stock solution (5 mg/mL) was add into the above suspension and then the mixed suspension was stirring at 303−323 K for 10 min. During the adsorption process, 2 mL of the suspension was taken out and filtered through a 0.22 μm microporous filter head (the membrane was penetrated with 1 mL of the suspension to reach its adsorption-desorption equilibrium). The concentration of CR in the filtrate was examined by UV–vis spectrum (HITACHI, U2900) according to the abs at 495 nm. The conversion of CR was calculated from the following equation:
con(%) =
N−C×V × 100% N
in which N (mmol), C (mmol/mL) and V (mL) stand for the initial amount of CR (mg), concentration of CR in the filtrate and the total volume of the mix suspension. On the other hand, the adsorption capacity (qe) of TFMOF was calculated by equation:
2. Experimental
mg ⎞ N−C×V q e ⎛⎜ ⎟ = g m ⎝ ⎠
2.1. Materials
in which m (mg) represented the amount of TFMOF added in the suspension. For the reusability test, after each cycle of adsorption, the separated TFMOF was immersed into 30 mL ethanol for 1 h then washed with deionized water. After that TFMOF was re-dispersed in 20 mL deionized water for next cycle of adsorption.
All agents are commercially available and no extra purification were taken before reactions. Zirconium acetate (Zr(OAc)4, Zr content: 15 wt %), Zirconium chloride (ZrCl4, 99 %), 1,4- dicarboxybenzene (BDC, 98 %) and Congo red (CR, 98 %) were purchased from Aladdin; 1,1′-ferrocene-dicarboxylic acid (FDC; 98 %) was purchased from Energy; acetic acid (AA, 99.5 %), formic acid (HCOOH, 98 %), hydrochloride acid (HCl, 37 wt%), absolute ethanol (99.7 %) and N, NDimethylformamide (DMF; 99.5 %) were purchased from SCR.
3. Results and discussion 3.1. Characterization
2.2. Synthesis of TFMOF nanosheets Our early study of ferrocene based metal organic framework demonstrated that ultrathin MOF nanosheets (Zr-FMOF) could be obtained by employing ZrCl4 and FDC as ligands and metal nodes (Deng et al., 2019). In this work, when the metal salt was displaced by Zr (OAc)4, the as-prepared TFMOF exhibited a tremella-like morphology,
In a typical synthesis procedure, 0.6 mmol of Zr(OAc)4 and 1,1′ferrocene-dicarboxylic acid were dissolved in 20 mL of DMF with 30 mmol of acetic acid which was used as the acid modulator. The above mixed solution was sealed into a Teflon autoclave and ultra2
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Scheme 1. Illustration of the preparation and crystal structure of TFMOF.
Fig. 1. SEM images of (a) TFMOF, (b) TFMOF-Non, (c) TFMOF-HCl and (d) TFMOF-Formic.
Fig. 2. HRTEM images of TFMOF in different magnifications.
Considering that it was only the kind of metal salt changed from chloride to acetate and the acetic acid played a critical role in the formation of ultrathin MOF naosheets, the intrinsic bonded acetate in Zr nodes may cause the change in morphology due to steric effect during the formation of MOF crystals, and the unit of TFMOF was also
which was quite different from that of the abovementioned Zr-FMOF (Scheme 1, Figs. 1a and S1). Compared with the thin film like Zr-FMOF with lateral size of ca. 500 nm, enormous intercalation of nanosheets could be found in that of TFMOF and their thickness also showed slight increment according to the folded shadow in HRTEM images (Fig. 2). 3
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Fig. 3. SEM images of TFMOF prepared in different reaction time. (a) TFMOF-0.5 h, (b) TFMOF-1 h, (c) TFMOF-1.5 h, (d) TFMOF-2 h, (e) TFMOF-9 h, (f) TFMOF12 h.
characteristic peaks had emerged in the first 2 h. Consequently, 9 h was chosen as the optimum reaction time for the synthesis of TFMOF, for no more differences of FMOF in morphology could be observed. And longer reaction time (12 h) did not lead to a higher yield after 9 h. Noting that the XRD pattern of TFMOF was highly similar to that of the previous reported FMOF (Fig. 4b), implying they had same crystal structure and network while only diverse morphology could be observed when employing various Zr sources. On the other aspect, the reaction temperature was also investigated by preparing TFMOF at 100 ℃ and 150 ℃. As was depicted in the SEM images (Fig. 5a and b), lower reaction temperature resulted loose architectures while higher temperature might cause disintegration of the MOFs crystals, which could also be reflected in the XRD patterns that the diminution of crystallization led to weak peaks (Fig. 5c). Figs. 6a–b and S3 showed the XPS patterns of Fe and Zr in TFMOF. The binding energy peaks of Fe 2p at 707.78 eV and 720.38 eV were corresponding to the Fe2+ in ferrocene (Yuan et al., 2020), while in the pattern of Zr 3d, the binding energy at 182.08 eV and 184.48 eV were in good agreement with the Zr-O bond, suggesting the Zr6O8 cluster as the nodes in TFMOF (Decoste et al., 2013). Fig. 6c showed the TG curves of the as-prepared TFMOF. It was indicated a slight loss of weight before 200 ℃ was ascribed to the guest solvent molecular. And this TFMOF
supposed to be [(Zr6O4](OH)4(FDC)4]. As was found in the SEM images of TFMOF-Non, TFMOF-HCl and TFMOF-Formic, which were prepared without addition of acetic acid or with the addition of HCl and formic acid instead of exhibited amorphous morphology. It was assumed that the exchanges of bonded monocarboxylic acetic acid (Fig. 1b–d, Fig. S2), no regular MOF crystals could be observed while all of them acid on Zr clusters with ditopic ligands resulted in the formation of MOF crystals (Guillerm et al., 2010). So the addition of HCl or formic acid might handicap the coordination between Zr clusters and FDC ligands, leading to unsuccessful crystallization of TFMOF. To give a deep insight on the growth of TFMOF, the SEM images and XRD patterns of grown TFMOF was displayed in Figs. 3 and Fig. 4a. In the first 30 min, only amorphous agglomeration of MOF could be found and the broad peaks in its XRD patterns also implying no regular crystal structure was formed in this stage. When the reaction time increased to 1 h, few small tremella like MOF nanosheets begin to construct and overlapped with former amorphous agglomerate meanwhile sharp peaks in PXRD patterns appears, confirming the formation of MOF crystals. Further enhancing reaction time to 1.5 h and 2 h, the tremella like MOF nanosheets kept on growing with dissipation of agglomerates, which could be thought as the agglomerates was transferring to regular crystals with time passed by. It was also worth noting that all 4
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Fig. 4. (a) XRD patterns of TFMOF obtained after different reaction time and (b) XRD patterns of simulated TFMOF, TFMOF (12 h) and FMOF.
was quite stable till 400 ℃ after that a harsh collapse of structure happened caused a weight loss of about 20 %. Interestingly, the BET specific surface area of TFMOF was calculated to be 89.9 m²/g and the average pore size was ca. 11.7 nm according to the N2 adsorption/desorption isotherm (Fig. 6d). And the t-plot further confirming that there were no micropores but meso-macropores in this MOF (Fig. 6d, inset). As the previous prepared FMOF featured a BET surface areas of 296.7 m²/g as well as micropores inside, it was such a surprise their morphologies exhibited marvelous discrepancies, which may be explained by the overlapping of nanosheets.
3.2. Adsorption of CR Fig. 7 displayed the max adsorption capacity of TFMOF in water solution with various pH adjusted by diluted HCl. With an initial concentration of 150 mg/L, it could be found that the adsorption capacity enhanced from 194.00 mg g−1 to 249.18 mg g−1 with the acidity crept up. Since CR is a kind of ionic dyes, and the raise in adsorption capacity may be attribute to the promoted electrostatic interaction between CR and TFMOF. As was found in the zeta potential of TFMOF in different solution (Fig. 8), the surface of TFMOF was negative charged at the pH of 7, which was supposed to be attributed to the carboxyl groups on the ligands. While the surface of TFMOF was positive charged at the pH of 4–6, suggesting the zero point of charge of TFMOF in water was around
Fig. 5. SEM images of TFMOF prepared at different reaction temperature. (a): TFMOF-100 ℃ and (b): TFMOF-150 ℃. (c): XRD patterns of TFMOF prepared at different temperature. 5
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Fig. 6. XPS patterns of (a) Fe 2p and (b) Zr 3d in TFMOF. (c): TG curve of TFMOF and (d): N2 adsorption/desorption isotherm of TFMOF (N2 atmosphere, 77 K); inset: t-plot curve of TFMOF.
Fig. 7. (a) Adsorption capacity and removal percentage of TFMOF towards CR in water with different pH value (4-7) and (b) UV–vis absorption spectrum of CR after adsorbed in water with different pH value. (Adsorption conditions: C0 = 150 mg/L; amount of TFMOF = 0.5 mg/mL; T =303 K; t = 10 min.).
that under visible light, confirming the intrinsic adsorption capacity of TFMOF. The adsorption performance of TFMOF prepared at various temperature was explored as was exhibited in Fig. 9a. Although TFMOF prepared at 100 ℃ and 150 ℃ have larger surface areas (Fig. S4 and Table S1), the reduce of the adsorption capacity was possibly due to the insufficient crystallization of TFMOF prepared at lower temperature while the adsorbent prepared at higher temperature might make more ligands coordinate with the metal clusters, which gave less adsorb sites
6.5, and the increment of positivity might promote the affinity of TFMOF toward the negative charged CR molecular. It is also worth noting that the CR would self-degrade under visible irradiation when the pH went down to 3 or lower, so pH 4 was chosen as the optimized condition in followed sections. To excluding the possibility that the CR was removed via a Fenton-like photocatalytic degradation in the presence of Fe2+ and visible light, a control test was carried out by putting the mixture of TFMOF and CR suspension in the dark, and the result indicating that the removal efficiency in this test was just as good as 6
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252.25 mg g−1. The temperature also played a curial role in the adsorption process. Fig. 9c gave the time-depended adsorption capacity of TFMOF at different temperature (303 K–323 K). Under all three different temperatures more than 90 % of the CR in the suspension was removed within 3 min as the color faded rapidly from deep red to sight pink (Fig. S5). And the max adsorption capacity of CR on TFMOF were 240.80 mg g−1, 242.09 mg g−1 and 242.61 mg g−1 at 303 K, 313 K and 323 K respectively, suggesting the temperature contributed negligible effect on the adsorption capacity, and the energy of activations was calculated to be 37.27 kJ/mol from the Arrhenius equation (Fig. 9d). To give a deeper insight on the adsorption kinetic of TFMOF at various temperature, two kinetic models, pseudo-first order and pseudo-second order models were employed for the simulation (Febrianto et al., 2009). The pseudo-first-order kinetic model was described as the followed equation:
ln(q e − qt) = ln(q e) − k1t where qe (mg·g−1) represented the equilibrium adsorption capacity; qt (mg·g−1) was the adsorption capacity measured at the contact time of t; k1 (min−1) stood for the pseudo-first order kinetic rate constant, which could be obtained from the ln (qe – qt) vs t linear fitting. On the other hand, the pseudo-second order kinetic model could be expressed as:
Fig. 8. Zeta potential of TFMOF in water with different pH value.
for the CR molecular. The max adsorption capacity was then investigated by employing TFMOF as adsorbent with various initial concentration of CR, as was displayed in Fig. 9b. The TFMOF exhibited outstanding adsorption performance and over 96 % of the CR could be removed within 6 min with concentration of 100 mg/L and 125 mg/L. Under initial concentration of 150 mg/L, over 80 % of the dye could be removed within 10 min with a max adsorption capacity of
t 1 t = + qt qe k2q 2e in
which
the
pseudo-second
order
kinetic
rate
constant-k2
Fig. 9. (a): Adsorption of CR by TFMOF prepared at various temperature. (b): Adsorption od CR by TFMOF with different initial dye concentrations (TFMOF = 0.5 mg/mL, pH = 4, T =303 K.). (c): Adsorption of CR at different temperature. (C0 = 125 mg/L, TFMOF = 0.5 mg/mL, pH = 4.) and (d): linear plot of lnK vs 1/T for the adsorption of CR by TFMOF. 7
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Table 1 Kinetic study of CR adsorption on TFMOF at 303-323 K. Temperature/K
Pseudo-first order kinetic model k1/min
303 313 323
−1
qe/mg g
0.386 0.283 0.237
Pseudo-second order kinetic model −1
R
49.90 23.78 20.49
2
0.9436 0.7236 0.8841
k2/g mg−1 min−1
qe/mg g−1
R2
0.0196 0.0376 0.0488
244.50 245.09 241.54
0.9999 0.9999 0.9998
Adsorption conditions: C0 = 125 mg/L; amount of TFMOF = 0.5 mg/mL; t = 10 min; pH = 4.
(g·mg−1 min−1) could be calculated through the linear fitting of t/qt vs t. As was showed in Table 1 and Fig. S6, the qe calculated from pseudo-first-order kinetic model at different temperature was far lower compared to the max adsorption capacity determined in the experiments. Besides, the rate constant k1 seemed to decrease as the temperature increased, which was contrary to the former observed phenomenon. According to the parameters obtained from the pseudo-firstorder kinetic model, the theoretical max adsorption capacities were close to the reality situation, and k2 also rose gradually as the temperature increased. Furthermore, the correlation coefficients of pseudosecond order kinetic model were all exactly close to 1, indicating the adsorption of CR on TFMOF could be described well as a pseudo-second order kinetic model than pseudo-first order kinetic model. The adsorption thermodynamics was also investigated to give thermodynamic parameters like Gibbs free energy (ΔG), standard enthalpy change (ΔH) and standard entropy change (ΔS). And the parameters could be calculated as the followed Van 't Hoff equation:
lnK=
where qe (mg·g−1) represented the CR adsorbed on TFMOF with different initial concentration of CR (50−400 mg/L); Ce (mg/L) represented the equilibrium concentration of CR in the suspension; qmax (mg·g−1) represented the theoretical max adsorption capacity of CR on TFMOF; and KL (L/mg) was the Langmuir equilibrium constant. The Freundlich isotherm model on the other hand, predicted a nonuniform adsorption on heterogeneous adsorbents (Günay et al., 2007), which was expressed as the followed equation:
q e = KF CneF The KF (mg1−n·Ln/g) and nF were the Freundlich equilibrium constants. The Sips isothermal could be recognized as a combination of both Langmuir and Freundlich that the equation could be recognized as the Freundlich equation at low initial concentration of guest molecular while it would predict a typical monolayer adsorption as Langmuir isothermal at high initial concentration of guest molecular (Vargas et al., 2011). It could be expressed as:
−ΔH ΔS + RT R
q e = qmax
(KSCe)nS 1 + (KSCe)nS
where the KS (L/mg) and nS were the Sips equilibrium constants. According to the data calculated from the nonlinear fitting (Table 3 and Fig. S7), the Sips isothermal model could be considered as the most suitable models to describe this adsorption behavior with a reliable correlation coefficient compared with that of the other two models. The adsorption behavior followed Freundlich adsorption isothermal at low dye concentration that the adsorption capacity increased with the initial dye concentration, while at higher dye concentration it was supposed to predict a monolayer adsorption as Langmuir isothermal model. Moreover, the theoretical max adsorption capacity obtained from Sips isothermal model showed slight decline at high temperature, implying this adsorption was probably an exothermic process.
ΔG = -RTlnK where the K was the adsorption equilibrium constant that could be calculated from qe/Ce, ΔH and ΔS could be obtained from the slope and intercept in the linear fitting of K vs 1/T (Lai et al., 2019). The calculated parameters were exhibited in Table 2. The positive ΔH value indicating the adsorption of CR on TFMOF was an endothermic process, which could also explain the slight decrease of adsorption capacity at higher temperature. Meanwhile the negative ΔG implying the adsorption was carried out spontaneously at various temperature, and the adsorption was more likely to happen at higher temperature according to the decline of ΔG value.
3.4. Adsorption mechanism 3.3. Adsorption isotherm Based on the experimental data, the excellent adsorption capacity of TFMOF towards CR could be explained by the following two reasons: (i) It has been reported “missing linker” phenomenon was frequently happened in UiO-66, which was composed of Zr6O8 cluster and 1,4dicarboxybenzene, as monoacid was used in the formation of MOF structure, resulting in unsaturated metal nodes (Gutov et al., 2015). As the same kind of Zr6O8 clusters existed in TFMOF and each Z6O8 cluster was supposed to be only coordinated with 4 FDC ligands, the Z6O8 clusters would exhibit strong affinity to the electron-rich O atoms in sulfoacid group (Scheme 2). (ii) The large exposed surface areas of TFMOF would be benefit for the contact of TFMFO and target molecular, and the electrostatic interaction between anionic CR and positive charged TFMOF, which comes from the uncoordinated Z6O8 cluster. Also noting that the XPS pattern of Fe showed no obvious changes after the adsorption, suggesting that there was no interaction between CR and Fe sites in the ligands (Fig. S8a). Generally, such a guest molecular with large size like CR was not able to get access to the nodes in bulk MOF, which featured intrinsic narrow channels or cavities. As a contrast, the adsorption performance of UiO-66 which was composed of the
To get a better understanding of the adsorption mechanism, the adsorption isotherms of CR on TFMOF were investigated in this study and two models, Langmuir, Freundlich and Sips isotherms were applied for the simulation. The Langmuir isotherm model is generally employed when the guest was adsorbed on the adsorbents uniformly and in monolayer coverage (Chatterjee et al., 2009), which was expressed as the equation:
qe =
KL qmax Ce 1 + KL Ce
Table 2 Thermodynamic parameters of TFMOF in the adsorption of CR. T/K
ΔG/KJ mol−1
ΔH/KJ mol−1
ΔS/J mol−1
303 313 323
−9.39 −13.28 −14.02
75.24
280.93
8
Journal of Hazardous Materials 392 (2020) 122274
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Table 3 Adsorption isotherm parameters calculated in different isothermal models. Temperature (K)
Langmuir qmax (mg g
303 313 323
254.14 251.62 247.14
Freundlich −1
)
KL (L mg 1.59 1.49 1.56
−1
)
R
2
0.9109 0.9712 0.9829
KF (mg
1−n
Sips L
1−n
-1
g )
156.24 165.24 155.55
2
nF
R
0.1225 0.0947 0.1125
0.7561 0.5461 0.7542
qmax (mg g−1)
KS (L/mg)
nS
R2
274.14 256.31 255.86
1.24 1.44 1.47
0.66 1.31 0.77
0.9494 0.9917 0.9996
(Fig. 10b), demonstrating the good stability of TFMOF. On the other hand, the tremella-like nanosheets structure of TFMOF maintained well after the adsorption process (Fig. 10c–d), indicating the excellent stability of TFMOF in aqueous environment. Noting that though only a max adsorption of 124.4 mg g−1 could be realized after three cycles, the performance of TFMOF was still better than most previous reported adsorbents especially for the superior removal time, including other kinds of porous MOF or MOF composites. As was listed in Table 4, the fast adsorption rate, remarkable adsorption capacity and facile preparation approach made TFMOF a superior adsorbent in the application of water treatment.
same Zr6O8 cluster and planar ligand 1,4- dicarboxybenzene was investigated (Fig. S9). Only less than 30 % of the Congo red could be removed under the same conditions compared to that of TFMOF (> 80 %). Taking the advantages of TFMOF nanosheets, the exposed Z6O8 clusters on the surface would be much accessible for CR to contact. So it can be interpreted that the unsaturated Z6O8 clusters in TFMOF and 2D nanosheets structure was of great benefit for the efficient adsorption of CR. 3.5. Reusability test To certify the stability and reusability of TFMOF as a promising adsorbent, the adsorption capacity of TFMOF was tested for several cycles. As was found in Fig. 10a, the max adsorption capacity suffered a sharp decline of nearly 50 % since the second cycle and only a max capacity of 124.4 mg g−1 could be achieved when it turned to the third cycle. Such a loss in capacity was not unanticipated cause as there was strong affinity between TFMOF and CR molecular, reflecting in high adsorption capacity in the first cycle, thus desorption of guest CR that coordinated with the Zr6O8 clusters in TFMOF would be an arduous process, and binding energy peak of S 2p at 167.38 eV in the XPS pattern of TFMOF after adsorption, which exhibited slight shift compared with that of pure CR (168.48 eV), indicating the existence of undesorbed CR on TFMOF (Figs. S8b and S7c). And the BET specific surface areas of TFMOF declined from 89.9 m2/g to 55.4 m2/g after the adsorption of CR, suggesting the coverage of adsorbed CR on the surface of TFMOF (Fig. S10). So we proposed that during the desorption process, around half of CR in the first adsorption were attached on the surface of TFMOF though weak electrostatic interaction and would be quite easy to be removed when suitable solvents were utilized. Meanwhile the other part of CR could not be desorbed due to the strong coordination bond. Besides, since the adsorbent was recycled by centrifuge during cycles, the loss of adsorbent may also be ascribed to the decline in adsorption capacity. No obvious differences could be observed according to the XRD pattern of TFMOF after the adsorption
4. Conclusion In summary, a novel ferrocene-based TFMOF nanaosheets was firstly prepared with Zr(OAc)4 and 1,1′-ferrocene-dicarboxylic acid. The obtained TFMOF exhibited a unique tremella-like morphology. The formation stages of TFMOF during the crystallization was investigated and demonstrating the in-situ growth of TFMOF nanosheets. Subsequently, the as-prepared TFMOF was utilized as an efficient porous adsorbent in the removal of organic dye CR from water. A highestadsorption capacity of 252.25 mg g−1 could be achieved at pH of 4. A pseudo-second order kinetic model was recognized as the most precise model to describe the adsorption behavior of CR on TFMOF with a k2 of 0.0488 g·mg-1 min-1 at 323 K. On the other hand, the CR was considered to be adsorbed on the surface of TFMOF in monolayer as was induced from a Langmuir isotherm model. The unprecedented adsorption capacity of TFMOF towards CR might be attributed to the exposed Zr6O8 cluster which could coordinate with negative charged CR molecular via the electrostatic interaction. Finally, the reusability of TFMOF was tested and the max adsorption could maintain as higher as 124.38 mg g−1 after three cycles. Thus the tremella-like TFMOF nanosheets could be thought as a promising adsorbent in the water treatment with ideal removal efficient and favorable reusability.
Scheme 2. Proposed adsorption mechanism of CR on TFMOF by the clusters. 9
Journal of Hazardous Materials 392 (2020) 122274
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Fig. 10. (a) Max adsorption capacity of TFMOF after three cycles, (b) XRD patterns of TFMOF before and after the adsorption of CR, (c) SEM and TEM (d) images of TFMOF after the adsorption process.
acknowledged.
Table 4 Time-consuming and adsorption capacity of reported adsorbents. Adsorbent
Time/min
qmax/mg g−1
Ref
Ni/Mg/Al LDO UiO-66 Fe3O4@SiO2 MoS2/Ag2S/Ag Co3O4/TiO2/GO Co-MOF ZIF-9@SMM DS-Zn/Al LDHs ZIF-8DMF-M TFMOF
300 120 360 12 90 180 700 180 60 10
1250.00 97.70 45.46 36.37 36.40 27.57 146.00 149.30 394.00 252.25
(Lei et al., 2017) (Qiu et al., 2017) (Wang et al., 2016b) (Zeng et al., 2019) (Jo et al., 2017) (Rajak et al., 2017) (Dai et al., 2017) (Grover et al., 2019) (Liu et al., 2019) This work
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2020.122274. References Ansari, R., Keivani, M.B., Delavar, A.F., 2011. Application of polyaniline nanolayer composite for removal of tartrazine dye from aqueous solutions. J. Polym. Res. 18, 1931–1939. Bhadra, B.N., Jhung, S.H., 2018. Adsorptive removal of wide range of pharmaceuticals and personal care products from water using bio-MOF-1 derived porous carbon. Microporous Mesoporous Mater., S1387181118302439. Cao, F., Zhao, M., Yu, Y., Chen, B., Huang, Y., Yang, J., Cao, X., Lu, Q., Zhang, X., Zhang, Z., Tan, C., Zhang, H., 2016. Synthesis of two-dimensional CoS1.097/nitrogen-doped carbon nanocomposites using metal–Organic framework nanosheets as precursors for supercapacitor application. J. Am. Chem. Soc. 138, 6924–6927. Chatterjee, S., Lee, D.S., Lee, M.W., Woo, S.H., 2009. Enhanced adsorption of congo red from aqueous solutions by chitosan hydrogel beads impregnated with cetyl trimethyl ammonium bromide. Bioresour. Technol. 100, 2803–2809. Cheng, Y., Wu, J., Guo, C., Li, X.-G., Ding, B., Li, Y., 2017. A facile water-stable MOFbased “off–on” fluorescent switch for label-free detection of dopamine in biological fluid. J. Mater. Chem. B 5, 2524–2535. Dai, J., Xiao, S., Liu, J., He, J., Lei, J., Wang, L., 2017. Fabrication of ZIF-9@supermacroporous microsphere for adsorptive removal of Congo red from water. RSC Adv. 7, 6288–6296. Decoste, J.B., Peterson, G.W., Jasuja, H., Glover, T.G., Huang, Y.G., Walton, K.S., 2013. Stability and degradation mechanisms of metal-organic frameworks containing the Zr6O4(OH)4 secondary building unit. J. Mater. Chem. A 1, 5642–5650. Deng, Z., Yu, H., Wang, L., Liu, J., Shea, K.J., 2019. Ferrocene-based metal–organic framework nanosheets loaded with palladium as a super-high active hydrogenation catalyst. J. Mater. Chem. A 7, 15975–15980. Du, Q., Sun, J., Li, Y., Yang, X., Wang, X., Wang, Z., Xia, L., 2014. Highly enhanced adsorption of congo red onto graphene oxide/chitosan fibers by wet-chemical etching off silica nanoparticles. Chem. Eng. J. 245, 99–106.
Author contributions 1 Jiyang Liu: Author of this manuscript. 2 Haojie Yu: Professor Yu checked the origin data and provided his suggestions during the revision. 3 Li Wang: Professor Wang provided his suggestions about the contents and format, and gave the final approval of the manuscript to be submitted. Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgements Financial supports from National Natural Science Foundation of China (51873189, 51673170, and 51811530097) are gratefully 10
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