Synthesis of YolkâShell Magnetic Porous Organic Nanospheres for

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Synthesis of Yolk-Shell Magneticporous Organic Nanospheres for Efficient Removal of Methylene Blue from Water Minghong Zhou, Tianqi Wang, Zidong He, Yang Xu, Wei Yu, Buyin Shi, and Kun Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01807 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Sustainable Chemistry & Engineering

Synthesis of Yolk-Shell Magnetic Porous Organic Nanospheres for Efficient Removal of Methylene Blue from Water Minghong Zhou, Tianqi Wang, Zidong He, Yang Xu, Wei Yu, Buyin Shi and Kun Huang* School of Chemistry and Molecular Engineering, East China Normal University, 500 N, Dongchuan Road, Shanghai, 200241, P. R. China. E-mail: [email protected], Keywords: yolk-shell magnetic porous organic nanospheres, ship-in-bottle, hypercrosslinking mediated self-assembly, hierarchically porous structure, sorbent

Abstract：

Herein, we propose a synthesis of yolk-shell magnetic porous organic nanospheres (YSMPONs) by combining hyper-crosslinking mediated self-assembly method with ship-inbottle technology. Yolk-shell structure could be facilely produced by controlling the loading amount of single iron precursors (FeSO4) in the hollow cavity of nanospheres. The resulting YS-MPONs nanocomposites possess a micro- and mesoporous structure, high surface area,

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excellent magnetic response and good chemical inertness. Because lots of carboxylate anions as end-groups were formed in the cavity after degradation of polylactide block, the obtained YS-MPONs as a sorbent showed the good adsorption performance for cationic dyes in aqueous solution. Adsorption kinetics for removal of methylene blue (MB) revealed that the adsorption rate was suitable for the pseudo-second-order rate model, while the adsorption isotherm fitted the Freundlich model. Owing to the higher surface area and larger pore volume, the YS-MPONs showed a better adsorption capacity (134 mg/g Vs 104 mg/g of CSMPONs) for MB dyes. In addition, YS-MPONs adsorbents can be magnetically separated and show the excellent reusability, being effective after at five consecutive cycles.

Introduction Recently, core-shell and yolk-shell porous nanomaterials (CS-PNs and YS-PNs) have received much attention because of their remarkable properties and extensive applications.1-4 However, the YS-PNs can usually offer better performance over simple CS-PNs because they possess the large internal void, micro/mesoporous shell and movable core.5 In particular, magnetic YS-PNs6 as a significant class of multifunctional materials have wide applications on the following fields: catalysis,7-10 lithium batteries,11-16 supercapacitors,17 biomedicines,1821

sensing material,22-23 adsorbents24-25 and microwave absorbers.26-29 To date, by combining

with various porous materials such as carbon12, 15, 24, 30-32 and mesoporous silica10, 33, different


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kinds of strategies have been used for the synthesis of magnetic YS-PNs, including pyrolysis strategy,15, 32 hydrothermal method,18, 23, 27, 34 host-guest approach,12 etching and sacrificialtemplating process.12, 14, 20, 24, 28, 30, 35-36 Although most current researches were concentrated on the development of different synthesis strategies of magnetic YS-PNs with various shell materials, the exploration of a facile method for synthesizing YS-PNs with large surface area, specific shell structure, various functionality and tunable magnetic property still remains a challenge. Recently, porous organic polymers (POPs) have received a particular attention due to their large specific surface area, good physi-chemical stability, and diverse synthesis methods.37-39 In particular, magnetic functionalized POPs have also attracted peculiar interest owing to their easy separation, mutilfunctionalization and porous property. For example, magnetic nanoparticles can be successfully introduced into some novel porous polymeric networks such as porous carbonaceous polymers by a microwave-enhanced synthesis,40 POPs by emulsion polymerization and following Friedel-Crafts hypercrosslinking reaction,41-43 and conjugated microporous polymer (CMPs) by Sonogashira coupling reaction.44-46 These magnetic porous polymeric hybrid materials exhibited the potential applications in adsorption and catalysis. Water soluble organic dyes are one of the most dangerous contaminants, which are contaminating the environment and threatening our health and life. Recently, some magnetic POPs have been utilized for dye adsorption due to their attractive adsorption capacity and easy magnetic separation.47, 48 However, the adsorption performance affected


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by different structures is rarely discussed. Herein, magnetic porous organic nanospheres with different structures were successfully prepared by hyper-crosslinking mediated self-assembly and ship-in-bottle technology. The obtained YS-MPONs were comprehensively characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and N2 sorption et. al. The influence of different contact time, solution pH value and initial MB concentration were studied to test the adsorption performance of CS-MPONs and YS-MPONs. Furthermore, the resulting YS-MPONs demonstrated the better performance for removal of the watersoluble organic dyes compared to CS-MPONs.

Scheme 1. Synthesis of core-shell and yolk-shell magnetic porous organic nanospheres (CSMPONs and YS-MPONs).

Experimental Materials


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All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. 2, 2Azobisisobutyronitrile (AIBN) and D, L-lactide (LA) were recrystallized from methanol and ethyl acetate, respectively. Styrene was purified with basic alumina. Characterizations 1H

NMR characterization was taken on a Bruker AVANCEIIITM 500 spectrometer (500

MHz). Solvent used for the NMR measurement was CDCl3. TEM measurement was made using a JEM-2100F instrument. Nitrogen adsorption-desorption isotherm was collected on a Quantachrome Autosorb IQ adsorption analyzer. The pore size distribution was calculated by density functional theory (DFT). The infrared (IR) spectra were taken on a Thermo NICOLET is50 instrument. The vibrating sample magnetometer (VSM) was used to measure the magnetic properties. The powder X-ray diffraction (XRD) pattern was collected using a D8 Advance X-ray diffractometer (Bruker AXS, Germany). UV-Vis spectra were recorded using the SOPTOP UV2400 spectrophotometer. Thermalgravimetric analysis (TGA) was conducted on a Mettler Toledo thermobalance model TGA/SDTA851e instrument with 10 °C /min.

Synthesis of magnetic porous organic nanospheres. Synthesis of hollow porous organic nanospheres (HPONs) The polylactide-b-polystyrene (PLA-b-PS) diblock copolymers were prepared according to our previous work.49 And then, the HPONs were synthesized as following steps: 700 mg of PLA170-b-PS120 precursors were dissolved in CCl4 (70 mL). Subsequently, 2.5 g of anhydrous


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ferric chloride were added to the above reaction mixture and allowed to react at 90 °C for 24 hours. After that, the obtained crude product was washed with water and then further purified by a Soxhlet with ethanol. Finally, the product was dried under vacuum at 80 °C for 24 h. Yield=500 mg. Synthesis of core-shell magnetic porous organic nanospheres (CS-MPONs). 540 mg of hollow porous organic nanospheres and 2.5 g of FeSO4·7H2O were mixed in 25 mL DMF. After stirring 12 hours, the solid product was washed by DMF (20 mL). Next, 20 mL of ammonia water was added to the solution to react at 80 °C for 1 hour. Finally, the product was washed by water and dried under vacuum at 80 °C for 24 h. Yield=740 mg. Synthesis of yolk-shell magnetic porous organic nanospheres (YS-MPONs). The YS-MPONs were synthesized following the similar procedure as CS-MPONs except for treating with more DMF (50 mL). Adsorption and regeneration experiments. The effect of pH was tested by adding CS-MPONs or YS-MPONs (10 mg) into MB solution (10 mL, 500 mg/L) with initial pH values (2.0 ~ 9.0). The pH value of MB solution was adjusted using 0.1 M HCl or NaOH solution. Next, the mixture solution were continuously shaken for 24 h at room temperature to reach equilibrium under a 250 rpm speed. After adsorption, the samples were collected under an external magnetic field. The concentration of MB dye in the supernatant part was determined using an UV-visible spectrophotometry at 664 nm. For the kinetics study, at a pH value of 7.0, CS-MPONs or YS-MPONs (10 mg) were dispersed in MB solution (10 mL, 500 mg/L) and shaken from 1 h to 25 h. The concentration of MB was


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measured at various times. For adsorption isotherm experiments, CS-MPONs or YS-MPONs (10 mg) were added into a series of MB solution (10 mL, pH = 7) with various initial concentration (50 ~ 800 mg/L). Then, the mixture solution was shaken at 25 °C for 24 h. Furthermore, the adsorption capability of CS-MPONs or YS-MPONs for various water-soluble dyes was conducted. The exact structure of dyes are illustrated in Figure S8. CS-MPONs or YS-MPONs (10 mg) were dispersed in 10 mL of dye solution (500 mg/L, pH = 7) and shaken at room temperature for 24 h. Adsorption capacity of CS-MPONs or YS-MPONs for MB dye at different time t (Qt, mg/g) and under different equilibrium (Qe, mg/g) were calculated according to the following Eq. (1) and (2):. Qt =

(C0 - Ct)

Qe =

m

(C0 - Ce) m

(1)

V

(2)

V

Where C0 (mg/L) is the initial concentration of MB dye, Ct and Ce (mg/L) represent the time t and equilibrium concentration of MB dye, respectively; V is the volume of the solution (L), and m is the adsorbent weight (g). The pseudo-first-order (Eq 3) and pseudo-second-order (Eq 4) models are respectively used to evaluate the adsorption kinetic data according to the following nonlinear form: Qt = Qe(1 ― e - k1t) 𝑘2𝑄2𝑒 𝑡

𝑄𝑡 = 1 + 𝑘2𝑄𝑒𝑡

(3) (4)


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where Qe and Qt (mg/g) represent the adsorption amounts of MB dye at equilibrium and at contact time t (min), respectively; K1 and K2 are the pseudo-first-order rate constant and the pseudo-second-order rate constant, respectively. The adsorption isotherm models can be represented by the following nonlinear form (Langmuir (Eq 5) and Freundlich (Eq 6)): 𝑄𝑒 =

𝐾𝐿𝑄𝑚𝑎𝑥𝐶𝑒 1 + 𝐾𝐿𝐶𝑒

(5)

1

𝑄𝑒 = 𝐾𝐹𝐶𝑒𝑛

(6)

where Qe (mg/g) represents the adsorption amount at equilibrium; Qmax (mg/g) and Ce (mg/L) represent the maximum adsorption capacity and equilibrium concentration, respectively; KL and KF are the Langmuir and Freundlich adsorption equilibrium parameters, respectively; 1/n represents the adsorption intensity. For the regeneration studies of CS-MPONs or YS-MPONs, the MB-loaded adsorbent was desorbed by washing with acetic acid/methyl alcohol (3% Vol) solution several times. After that, the adsorbent was treated with water and dried. The CS-MPONs or YS-MPONs adsorbent was reused for MB adsorption experiment (20 mg adsorbent, 20 mL MB solution with initial concentration of 500 mg/L). The process was repeated 5 times. Point of zero charge (pHPZC) refers to pH value at which the electrical charge density on the adsorbent surface is zero. The measurement of pHPZC was determined by a drift method.50 The typical procedures are as following: (1) a series of 50 mL NaCl solutions (0.01 mol/L, with pH values from 4 to 12) were obtained by using 0.1 mol/L HCl or NaOH solution. (2) About 0.05 g of CS-MPONs or YS-MPONs were transferred into each solution, and then the mixtures were


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shaken at room temperature for 0.5 h. The final pH values (pHf) of the mixture solutions were measured after 48 h. (3) The difference between the final and initial pH values (ΔpH = pHf - pHi) was plotted against the pHi. The pH value of solution at which the curve crosses the line of (pHf pHi = 0) was taken as the pHPZC of sample. To test the stability of YS-MOPNs, the leaching of Fe ions from YS-MOPNs at various pH levels was studied. 10 mg of YS-MOPNs was added into 10 mL aqueous solution (pH = 2.0 ~ 9.0) and shaken at room temperature for 24 h. The concentration of leached Fe was measured by an atomic absorption spectroscopy.

Results and discussion Synthesis and characterization of yolk-shell MPONs. Based on the hyper-crosslinking mediated self-assembly and ship-in-bottle method, the distinct yolk-shell magnetic porous organic nanospheres (YS-MPONs) were successfully prepared (Scheme 1). First, well-defined hollow porous organic nanospheres (HPONs)49 were synthesized by using PLA170-b-PS120 as precursors, anhydrous FeCl3 as catalysts and CCl4 as both cross-linker and solvent at 90 °C for 24 h. FT-IR spectra confirmed the complete degradation of PLA due to the disappearance of its characteristic peak at 1758 cm-1 (Figure S1). The hollow nanosphere morphology with the diameter of about 35 nm was also proven by TEM. (Figure S2). Next, yolk-shell structure of Fe3O4@MPONs could be synthesized by finely tuning the loading amount of single iron precursors Fe2+ in the hollow cavity of HPONs based on the ship-in-bottle strategy. Fe3O4 nanoparticles were formed by adding ammonia water


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and the subsequent oxidation of the suspension at higher temperatures in air for one hour. However, we found that only little Fe3O4 nanoparticles can be formed at the low temperature of 40 and 60 oC, which is evidenced by XRD measurement (Figure S4). The Fe-O stretching peak (580 cm-1) appeared in FTIR spectrum (Figure S1) and TEM image (Figure 1) mean that the Fe3O4 core was successfully formed in the hollow porous organic nanospheres. The selected area electron diffraction (SAED) image confirms the polycrystalline structure of Fe3O4. (Figure 1(B)) The high resolution TEM images also proved the yolk-shell structure, which consists of about 8 nm Fe3O4 core, an apparent chamber and the outer microporous hyper-crosslinked layer. (Figure 1C and 1D) The inter-fringe distances corresponded to the lattice spacing of the (220) and (311) planes are measured to be 0.30 and 0.25 nm, respectively. As a comparison, core-shell structure of Fe3O4@HPONs (Figure S3) was obtained according to our reported method.49


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Figure 1. TEM (A), SAED (B) and high resolution TEM (C and D) images of YS-MPONs. Figure 2(A) shows the XRD patterns of the CS-MPONs and YS-MPONs nanocomposites. The broad peak at about 20° can be ascribed to the amorphous structure of polymer matrix.42 Especially, the characteristic peaks of the magnetite nanoparticles in CS-MPONs and YSMPONs were clearly observed in the XRD patterns. A vibrating sample magnetometer (VSM) is further used to investigate the magnetic characters of CS-MPONs and YS-MPONs nanocomposites (Figure 2(B)). The saturation magnetization value of the CS-MPONs and YSMPONs nanocomposites was measured to be 24 and 13 emu/g respectively, which are higher than some other reported magnetic nanocomposites (Table S1). The higher saturation magnetization of CS-MPONs can mainly be attributed to a higher iron content and larger


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particle size. Despite the decrement, the magnetization of YS-MPONs is enough for fast separation. Additionally, no remanence was detected for both structure of Fe3O4@HPONs, indicating a super-paramagnetic property.

Figure 2. (A) XRD patterns of CS-MPONs (a), YS-MPONs (b) and the relative reference JCPDS no. 88-0315, and (B) Magnetization hysteresis loops of CS-MPONs (a) and YS-MPONs (b).


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Figure 3. N2 sorption isotherms of HPONs, CS-MPONs and YS-MPONs. Inset: pore size distributions from DFT method. The N2 sorption isotherms of HPONs, CS-MPONs and YS-MPONs exhibit type-IV isotherms with a clear hysteresis loop, showing the presence of mesoporous characteristics. (Figure 3) Herein, YS-MPONs was chosen as a typical example to describe in detail. The DFT pore size distribution (PSD) for YS-MPONs (Figure 3 inset) shows that the micropore size distribution has a maximum at 1.7 nm. Herein, the peak at 3.8 nm may be an artefact from tensile strength effect (TSE),51-52 However, based on the adsorption branch of BJH model, the the PSD exhibits a broad mesopore size distribution at around 8 nm. Because the adsorption branch is hardly influenced by any TSE, it refers to exact pore size calculations (Figure S5). This is also consistent with the


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TEM result. The above results show that YS-MPONs have a hierarchically porous structure. The micropores was produced from the hyper-crosslinking polystyrene. Whereas, the inner cavity and loose aggregation in YS-MPONs may form the meso/macroporous structure. The HPONs and CS-MPONs also presented the similar porous properties as YS-MPONs. The porous data of HPONs, CS-MPONs and YS-MPONs were summarized in Table 1. The BET surface area and pore volume of HPONs are 806 m2/g and 1.28 cm3/g, respectively. When Fe3O4 nanoparticles were successfully encapsulated into HPONs, the BET surface area and pore volume accordingly reduced because Fe3O4 nanoparticles not only increased the weight of non-porous part but also occupied the whole inner cavity of HPONs. However, the BET surface area and pore volume of YS-MPONs are 538 m2/g and 1.18 cm3/g, which are higher than those of CS-MPONs nanocomposites (329 m2/g and 0.77 cm3/g, respectively). The higher surface area may be related to their yolk-shell structure because of the large inner cavity volume between the yolk and the shell and the lesser sample’s density. Such the high surface area and hierarchical porosity will be important for the improvement of mass diffusion and adsorptive capacity. Therefore, it is expected that the YS-MPONs with a higher surface area are beneficial to the better adsorption for organic contaminants.

Table 1. Porous characterization of HPONs, CS-MPONs and YS-MPONs nanocomposites.

Samples HPONs CS-MPONs YS-MPONs

SBET [a] (m2/g) 806 329 538

Pore parameters Smico[b] (m2/g) Smeso[c] (m2/g) 133 673 100 229 97 441

Vtotal[d] (cm3/g) 1.28 0.77 1.18

BET surface area calculated from N2 adsorption isotherm; [b] Microporous surface area obtained from t-plots method; [c] Mesoporous surface area; [d] Total pore volume (P/P0=0.995). [a]


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Stability of adsorbent The thermal stability of YS-MPONs is evaluated by thermogravimetric analysis (TGA). The result is shown in Figure S6. The initial decomposition temperature of YS-MPONs is at about 250 °C and the total mass loss is 47.1wt%, which confirmed that the obtained YS-MPONs have a good themal stability. Considering that leaching of Fe ions into the treated water will affect magnetic separation during the adsorption process. Therefore, a leaching test is further performed to evaluate the stability of YS-MPONs at different pH levels (Figure S7). From the results, we can see that as the pH increased from 2.0 to 5.6, the percentage of the leached Fe ions decreased from 2.8% to 0.06%. The leaching of Fe ions can be negligible at pH ˃ 5.6. Besides, YS-MPONs still possess sufficient magnetic response under magnetic fields after treated by pH=2.0 acidic aqueous solution. Above results imply that YS-MPONs possess excellent stability in weak acidic, neutral and basic aqueous solution.

Effect of Solution pH on Dye Adsorption


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Figure 4. (A) Point of zero charge of CS-MPONs and YS-MPONs; (B) Effect of solution pH on the MB adsorption of the CS-MPONs and YS-MPONs. The solution pH is an important parameter for dye adsorption because it not only changes the surface charge of the adsorbent, but also affects the molecule structure of dye. Herein, the pH dependent adsorption of methylene blue (MB) on CS-MPONs and YS-MPONs was investigated and the results were shown in Figure 4. Firstly, the pHPZC of CS-MPONs and YS-MPONs were measured to be 6.3 and 6.8 respectively as shown in Figure 4(A), which means that CSMPONs or YS-MPONs adsorbent acquires a negative charge once the solution pH > pHpzc (6.3 or 6.8), Conversely, YS-MPONs or CS-MPONs possesses positively charged surfaces if solution pH < 6.8 or 6.3. As MB is cationic dye, it can easily form positively charged species over a wide pH range. So, as can be seen in Figure 4(B), YS-MPONs was more favorable to adsorb cationic dyes when solution pH value is over 6.8. The adsorption capacity of YS-MPONs toward MB will reduce with a decrease in solution pH, which can be related to the unfavorable electrostatic interaction between the positively charged surfaces of YS-MPONs at lower pH and the positively charged MB dyes. CS-MPONs also exhibited the similar pH dependent adsorption for MB dyes. Although electrostatic interactions play a key role on the adsorption, other interactions such as hydrogen bonding, π-effects and hydrophobic effects may also devote some influence on the adsorption behavior between MPONs and MB dyes. As comparison, eight water-soluble organic dyes with different charge were further used to study the adsorption behavior of YSMPONs or CS-MPONs. The structure of these dyes are provided in Figure S8, including the anionic dyes (Eosin B (EB), Alizarin Red S (ARS), Eosin Y (EY), and Amido black 10B (AB10B)) and the cationic dyes (Rhodamine 6G (R6G), Butyl Rhodamine B (BRB), Methylene


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Blue (MB), Safranine T (ST)). The saturated adsorption capacity (Qeq) for ARS, AB10B, EY, EB, R6G, MB, BRB and ST adsorbed by CS-MPONs are 52, 42, 20, 40, 88, 113, 130, 860 mg/g and by YS-MPONs are 21, 30, 36, 82, 138, 140, 183, 1080 mg/g, respectively (Figure 5). Because YS-MPONs or CS-MPONs possess the negatively charged surfaces at neutral aqueous solution, they exhibit the higher adsorption capacity for the cationic dyes compared with the anionic dyes. Meanwhile, owing to the higher surface area and larger cavity volume, YSMPONs show the higher saturated adsorption capacity for cationic dyes compared with the CS-MPONs. In particular, by using an external magnet around solution, the YS-MPONs nanomaterials can be quickly separated. (Figure 5, inset)


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Figure 5. The saturated adsorption capacities of YS-MPONs and CS-MPONs for ARS, AB10B, EY, EB, R6G, MB, BRB and ST dyes water solution at 25 oC. The insets are photographs of solutions containing MB before and after adsorption by the YS-MPONs under the magnet (10 mg/L of initial MB concentration). Adsorption Kinetics The adsorption kinetics is extremely important for the adsorption process as it is related to the adsorption rate, which can control the residual time of the adsorbate uptake at the interface of solid-solution.53 Herein, MB dye is selected as a typical sample to investigate the adsorption kinetics of CS-MPONs and YS-MPONs. As showed in Figure 6(A), the adsorption equilibrium was reached within 5 h for CS-MPONs and 10 h for YS-MPONs, respectively. We can note that although CS-MPONs possess a lower surface area than the YS-MPONs, the adsorption rate of CS-MPONs for MB dye is faster, which may be because the CS-MPONs with higher specific gravity make them better dispersity and more efficient contact with dyes.42 However, when the adsorption equilibrium was reached, the adsorption capacity of YS-MPONs is higher because of the higher surface area and more nanoviod space between the yolk and shell. Adsorption kinetic data were analyzed by pseudo-first-order and pseudosecond-order kinetic equations using a non-linear fitting. The kinetic parameters and experimental adsorption capacities are presented in Table 2. The results show that the correlation coefficients (R2) from pseudo-second-order kinetic for CS-MPONs and YSMPONs are obvious higher than those from pseudo-first-order kinetic

54,55

(Table 2 and

Figure S9). Besides, the calculated equilibrium adsorption capacities (Q2,cal.) from pseudosecond-order kinetic equation were consistent with the experimental adsorption capacities


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(Qexp), suggesting that the adsorption kinetics of MB on the CS-MPONs and YS-MPONs follows the pseudo-second-order kinetics model.

Table 2. Kinetic parameters and experimental adsorption capacities for the adsorption of MB onto the CS-MPONs and YS-MPONs.

Adsorbent

Qexp. (mg/g)

CS-MPONs YS-MPONs

Pseudo-first-order model

Pseudo-second-order model

Q1,cal. (mg/g)

k1

R2

Q2,cal. (mg/g)

k2

R2

113

108

1.251

0.9847

115

0.02057

0.9991

143

138

0.345

0.9699

161

0.00276

0.9836

Adsorption Isotherms As shown in Figure 6(B), the equilibrium adsorption capacities of MB onto the CSMPONs and YS-MPONs increased remarkably when the initial concentration increased. However, the growth of the adsorption capacity was retarded at higher initial concentration of MB due to the increasing driving force from concentration gradient. In addition, YSMPONs exhibited the higher equilibrium adsorption capacities for MB compared to CSMPONs owing to their unique yolk-shell structure. Adsorption isotherm and relevant parameters are applied to interpret the adsorption behavior. The Langmuir isotherm model postulates a homogeneous monolayer adsorption onto the adsorbent surface.56 The Freundlich isotherm model assumes a heterogeneous multilayer adsorption onto the


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adsorbent surface.57 Herein, Langmuir and Freundlich isotherm models are employed to investigate the adsorption processes at initial concentrations of MB from 50-800 mg/L (Table 3 and Figure 6B). Another important parameter named the separation factor (RL) is calculated by the following relation: RL =

1 1 + KLC0

Where RL represents the type of the isotherm to be either unfavorable (RL >1), linear (RL =1), favorable (0< RL 0.99). The Freundlich model indicated the multilayer adsorption of YS-MPONs and CS-MPONs for MB. The maximum monolayer capacity calculated by the Langmuir equation is 134 mg/g for MB on YS-MPONs, which is higher than that of CS-MPONs.59 Compared with majority of the magnetic adsorbents in the literature, YS-MPONs show good performance with satisfied magnetic and surface area (Table S1). The RL value of YS-MPONs for MB is 0.008, indicating that the adsorption of dye onto YS-MPONs is favorable. The good adsorption capacity of YS-MPONs mainly benefits from the yolk-shell nanostructure and large surface area, suggesting that the obtained YSMPONs can be used for the removal of water-soluble organic dyes.


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Figure 6. (A) Effect of contact time on the adsorption by CS-MPONs and YS-MPONs for MB; (B) Adsorption isotherms of MB for CS-MPONs and YS-MPONs. Table 3. Langmuir and Freundlich parameters for adsorption of MB by CS-MPONs and YSMPONs using the nonlinear method. Adsorbent CS-MPONs YS-MPONs

Qmax (mg/g) 104 134

Langmuir KL (L/g) 0.1208 0.1504

R2 0.768 0.689

Freundlich KF (L/mg) n 58.59 10.86 85.72 13.49

R2 0.996 0.998

Regeneration and repeated use. The reusability of the adsorbents is decisive for their practical application. Regeneration of CS-MPONs and YS-MPONs can be achieved by desorption of MB in acidic methanol and subsequently magnetic separation. 6 cycles were performed by using the same adsorbent. Figure 7 showed that the adsorption capacities of regenerated CS-MPONs decrease sharply and only 60% adsorption capacity was retained after five regenerations. While the YS-


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MPONs were found to be decreased slightly and retained 90% after four regenerations, which proved that YS-MPONs is more sustainable adsorbent than the core-shell ones due to their higher surface area and larger pore volume.

Figure 7. Adsorption capacity of regenerated CS-MPONs and YS-MPONs for MB dye in different cycles.

Conclusion In conclusion, magnetic core-shell and yolk-shell porous organic nanospheres were facile prepared by the hyper-crosslinking mediated self-assembly and ship-in-bottle two-step strategy, in which Fe3O4 nanoparticles were encapsulated in hyper-crosslinking hollow


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porous organic nanospheres. The adsorption property of CS-MPONs and YS-MPONs were studied. The results show that both of adsorbents exhibit the excellent adsorption performance and magnetic separation after adsorption process due to their large surface area and excellent magnetic property. The studies of adsorption kinetics and isotherms showed that both adsorption processes obey the pseudo-second-order kinetics and Freundlich isotherm. More importantly, the yolk-shell structure remarkably improves the adsorption capacity and reusability, which further demonstrates that the obtained YS-MPONs can be used as the promising adsorbents in the practical treatment of wastewater contaminated with dyes.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: FTIR spectra of YS-MPONs; TEM image of HPONs; TEM and HRTEM image of CS-MPONs; XRD patterns of different preparation temperature; pore size distribution of YS-MPONs; TGA curve of YS-MPONs; Fe leaching at different pH; structure and abbreviation of organic dyes; table of sorption capacities of MB comparison with other reported sorbents. (PDF)

AUTHOR INFORMATION


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Corresponding Author *[email protected] ORCID Kun Huang: 0000-0003-2737-1189 Notes The authors declare no competing financial interest.

Acknowledgements This work is supported by National Natural Science Foundation of China grant 21574042, 51273066.

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Synopsis.

Facile synthesis of different morphology magnetic porous organic nanospheres for wastewater treatment and fast magnetic separation.


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Synthesis of YolkâShell Magnetic Porous Organic Nanospheres for

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