Wettable magnetic hypercrosslinked microporous nanoparticle as an efficient adsorbent for water treatment

Accepted Manuscript Wettable magnetic hypercrosslinked microporous nanoparticle as an efficient adsorbent for water treatment Qingyin Li, Zhen Zhan, Shangbin Jin, Bien Tan PII: DOI: Reference:

S1385-8947(17)30798-2 http://dx.doi.org/10.1016/j.cej.2017.05.049 CEJ 16947

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

11 March 2017 7 May 2017 8 May 2017

Please cite this article as: Q. Li, Z. Zhan, S. Jin, B. Tan, Wettable magnetic hypercrosslinked microporous nanoparticle as an efficient adsorbent for water treatment, Chemical Engineering Journal (2017), doi: http:// dx.doi.org/10.1016/j.cej.2017.05.049

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Wettable magnetic hypercrosslinked microporous nanoparticle as an efficient adsorbent for water treatment Qingyin Li1, Zhen Zhan1, Shangbin Jin1*, and Bien Tan1* 1

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of

Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China * Correspondence and requests for materials should be addressed to B.T. and S. J. (E-mail: bien.tan@mail. hust.edu.cn, [email protected])

Highlight:


SA-MMNPs were synthesized combining microporous polymer with magnetic nanoparticles.




SA-MMNPs can be easily separated by magnet.




SA-MMNPs can be well dispersed in water.




SA-MMNPs have enhanced adsorption performance towards water-soluble pollutant in aqueous solution compared with hydrophobic magnetic microporous nanoparticle.

1

Abstract Microporous organic polymers (MOPs) are promising adsorbents for water treatment owing to their nanoporous structure with high surface areas. However, their hydrophobic nature, high cost and difficulty in recycling limited their practical application in water treatment. Here, we report the synthesis of new MOPs which combine sodium acrylate (SA) functionalized HCPs groups with magnetic Fe3O4 nanoparticles to form a hybrid material (SA-MMNPs). The magnetic hybrid material can be directly aggregated and separated by applying an external magnetic field because of Fe3O4. More importantly, after introducing carboxyl group on the skeleton of HCP, the hydrophilic nature of SA-MMNPs is improved, thus SA-MMNPs can disperse in water well after vigorous shaking. The adsorption property of SA-MMNPs toward water-soluble contaminants was studied by using Rhodamine B (RhB) as adsorbates. The maximum adsorption capacity for RhB of this polymer is up to 216 mg g-1 and show better adsorption capacity than hybrid materials without modification (MMNPs). The results constitute a new HCP paradigm with hydrophilic segments for removal of water soluble contaminants with high efficiency and good recyclability in water treatment.

Graphic abstract

Keywords Wettable hypercrosslinking polymer, Magnetic nanoparticle, Water treatment, Organic contaminants

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1. Introduction In the past decades, microporous organic polymers (MOPs) formed by organic building blocks, have attracted increasing interest for their well defined porosity, high surface area, lightweight and ease of function. Tremendous studies have been focused on the development of synthetic strategies for MOPs with well controlled porosity, shape and composition. Types of MOPs including hypercrosslinked polymers (HCPs) [1], polymers of intrinsic microporosity (PIMs) [2], conjugated microporous polymer (CMPs) [3] and covalent organic frameworks (COFs) [4] have been developed and used in many fields with promising performances, such as gas storage and separation [5], catalysis [6], adsorbent [7], energy storage [8], sensors [9] and so on. Because of their controllable porous features, high surface areas and prominent physical properties, MOPs have been provided as efficient absorbents for toxic chemicals such as metal ions [10, 11], dyes [11], amines [12, 13], iodine [14, 15], organic solvents [16-18] and oils [7, 11]. However, most of these MOPs are prepared using high cost starting materials or catalysts, which makes scale-up of the materials challenging and limit the real application. Among various MOPs, HCPs could be prepared by Friedel-Crafts alkylation reaction catalyzed by using inexpensive FeCl3 and can be facilely scaled up in relative low cost. [19-22] Till now, MOPs are found an attractive kind of organic polymer-based adsorbents for water treatment [10, 11, 16-18]. However, due to their oleophilic nature, organic polymer-based adsorbents can easily remove hydrophobic pollutants, such as organic solvents and oils, from wastewater driving by Van der Waals interaction between the adsorbates and adsorbents.[18] As far as the adsorption of hydrophilic or water-soluble contaminants, such as metal iron and watersoluble dye, the adsorption capability of MOPs will be reduced due to the strong solute-water interaction between the contaminants and water. [23] Therefore, the introduction of some

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hydrophilic groups onto the surface of the hydrophobic adsorbent will be a good way to improve its adsorption behaviors [23-25]. For example, Li et al. [10] reported a sulfonic acid-modified HCPs (SAM-HCPs) synthesized by sulfonation of HCPs, which were found to have good adsorption capacity for metal ions because of the improved hydrophilic nature, microporous structure and active sites. However, the difficulty in collecting them from dispersing media causes much inconvenience in their practical application. Fe3O4 nanoparticles are important magnetic materials and have been widely applied in fabricating functionalized magnetic materials because of their prominent superparamagnetism with a low coercive force and remanent magnetization.[26] To date, many types of magnetic porous materials have been used as the adsorbents for water treatment, such as metal-organic framework (MOF) [27] and porous carbon [28] etc. All these results indicated that magnetically functionalized adsorbents porous structures are attractive for their adsorption capacity, easy separation and operation under an external magnetic field. In this work, we adopted a new strategy by modifying the precursor of magnetic micropours nanoparticles (MMNPs) with sodium acrylate (SA) and prepared an iron oxide hybrid polymer, namely Fe3O4@poly(styrene-co-sodium acrylate) (Fe3O4@PSSA). After hypercrosslinking catalyzed by iron (III) chloride, carboxyl group modified MMNPs (SA-MMNPs) were successfully achieved. We found that SA-MMNPs demonstrated its better performance towards removing water-soluble pollutants rhodamine B (RhB) from aqueous solution than that doing by MMNPs. SA-MMNPs also show good recyclability which can be reused for 5 cycles. The results demonstrate that the materials prepared by this method are promising adsorbents in practical wastewater treatments.

2. Experimental

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2.1 Preparation of SA-MMNPs SA-MMNPs were synthesized using a reported method with some modification (Scheme 1).[29] Briefly, 5.0 g Fe3O4 NPs, 20 ml oleic acid and 20 ml toluene were mixed by vigorous mechanical agitation with ultrasonic vibration and the oil-Fe3O4 NPs were obtained from the mixture via magnetic separation. Then the mixture of 5 g oil-Fe3O4 NP, 10 g St, 0.25 g DVB, 2.4 g n-hexane, 1.6 g n-cetane and 0.5 g AIBN was added dropwise in the water phase (0.4 g SDS dissolving in 100 ml H2O) with ultrasonic emulsification. The miniemulsion polymerization was carried out at 80 ºC for 4 h under nitrogen atmosphere and then 10 ml 10v% SA aqueous solution was added dropwise and reacted for another 1 h to obtain Fe3O4@poly(styrene-co-sodium acrylate) (Fe3O4@PSSA). Thirdly, 10.0 g Fe3O4@PSSA were swelled in 100 ml DCE under nitrogen atmosphere, then added 10.5 g FDA mechanically agitated for 5 min, followed by addition of 11.2 g FeCl3 at 80 ºC for 6 h under nitrogen atmosphere to obtain SA-MMNPs. Poly(styrene-co-sodium acrylate) (PSSA) and SA modified microporous nanoparticle (SAMNPs) was made by the same method without adding oil-Fe3O4 NPs. Fe3O4@PS and MMNPs was made by the same method without adding SA. (Scheme 1) The content of –COOH are calculated by the method reported by Kawaguchi et al. [30] Briefly, 0.01 g SA-MMNPs were dispersed in 6mL 0.1mol L-1 NaCl aqueous solution, and then potentiometric titrated by 0.01 mol L-1 NaOH aqueous solution. The blank titration curves were also made without adding SA-MMNPs under the same condition. The amount of –COOH was calculated by the following equation. n-COOH =

CNaOH ×(VNaOH -VNaOH, blank ) mSA-MMNPs



"""""""""""""""c

""""""""""" (1)

5

where n-COOH (mmol g-1) is the molar ratio of –COOH; VNaOH and VNaOH, blank (L)are inflection points of the titration curves of SA-MMNPS and blank; CNaOH (mol L-1) is the concentration of NaOH; mSA-MMNPs (g) is the weight of SA-MMNPs added for titration. Swelling ratio was measured by dispersing materials in excess of aqueous solutions (5 ml) and stayed at room temperature for 24 h to reach swelling equilibrium. Swelling ratios of these particles were determined by the weight ratio of wet gels to dried gels. [31]

Scheme 1 Illustration of synthetic procedures for SA-MMNPs. 2.2 Adsorption study The adsorption capability of SA-MMNPs towards water-soluble pollutant was evaluated based on the removal of the RhB from aqueous solutions. Batch equilibrium experiments were carried out to study the contact time and equilibrium isotherms. 15 ml conical flasks were filled with 5 ml of RhB solution with various initial concentrations (80–400 ppm). 5 mg SA-MMNPs were added to the RhB solutions and each sample was placed on a shaker at 200 rpm at room temperature for 24 h to reach equilibrium of the adsorption at 25 ºC. The contact time optimization was performed using 5 mL of 120 ppm RhB aqueous solution and 5 mg of SAMMNPs stirred with 200 rpm at 25 ºC. SA-MMNPs were removed magnetically and the remaining concentration of RhB solution was analyzed by a UV/VIS spectrometer at a wavelength of 554 nm (maximum absorbance of RhB).

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The regeneration studies of SA-MMNPs were carried out in the batch process. Typically, the used SA-MMNPs were washed thoroughly with ultrapure water and ethanol. After magnetic separation and drying at 60 ºC overnight, the regenerative adsorbent was used again. The amount of organic dye removed from per unit mass of the prepared material (qt) was calculated using the equation below: qt =(C0 -Ct )V/m

2)

where C0 and Ct (mg L-1) are the concentrations of RhB at initial stage and after time t, respectively, m (g) is the dry weight of the added adsorbent in the adsorption test and V (L) is the volume of the aqueous solution. The adsorption rates and potential rate-controlling steps were studied by the kinetic data given by pseudo-first-order rate equation and pseudo-second-order rate equation. The pseudo-firstorder and the pseudo-second-order rate equations are expressed in the following equations [32, 33]: qt =qe 1-e-k1t 

3)

t⁄qt =1⁄k2 qe 2 +t⁄qe 

4)

where qe and qt (mg g-1) indicate the amount of adsorbed RhB at the equilibrium and at the adsorption time t (min), respectively. k1 and k2 are the rate constant of the pseudo-first-order model, respectively. Langmuir and Freundlich isotherm equations were applied to study the sorption isotherm. Generally, Langmuir model represents uniform energies of adsorption onto the material surface (monolayer) and there is no transmigration of adsorbate in the plane of the surface[34], whereas the Freundlich model is based on the adsorption mode on heterogeneous surfaces[35]. The linearized Langmuir and Freundlich adsorption isotherm equations are as follows:

7

1 1 1 1 = + qe KL qmax Ce qmax

(5)

1 lnqe =lnKF + lnCe n

(6)

where qe (mg g-1) is the equilibrium amount of RhB adsorption; qmax (mg g-1) is the maximum adsorption capacity; Ce (mg L-1) is the equilibrium solute concentration; KL is the Langmuir constant representing the affinity of binding sites; KF and 1/n (unitless) are the Freundlich parameters, standing for the adsorption capacity and the adsorption intensity, respectively.

3. Results and discussion 3.1 Characterization of SA-MMNPs PSSA is an ionomer with PS that has carboxylic acid (–COOH) and carboxylate (–COO−Na+) groups.[36] Fourier transform infrared (FT−IR) spectra of Fe3O4@PSSA and unmodified Fe3O4@PS are shown in Figure 1a. Compared with Fe3O4@PS, FT−IR spectra of synthesized Fe3O4@PSSA shows new peaks at 3199–3657 cm-1 which may due to O−H stretching of –COOH. And new bond at 1560 cm−1 is owing to symmetric and asymmetric resonance of the COO− group of the (sodium acrylate) units. [37-39] The energy dispersive X-ray analysis (EDX) spectra of PSSA without Fe3O4 (Figure 1b) showed the new elements O and Na present on PS molecular skeleton. The results from FT−IR and EDX could confirm –COO–Na+ was successfully added on the PS skeleton by adding in the emulsion dropwise. The content of –COOH were measured by potentiometric titration. [30] The titration curves are shown in Figure S1. Inflection points VNaOH and VNaOH,

blank

are found 0.34 mL and 0.26mL. 0.8 mmol g-1 –COOH is calculated by equation

(1).

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Figure 1 (a) FT-IR spectra of Fe3O4@PS and Fe3O4@PSSA; (b) EDX spectra of spectra of PSSA. X-Ray diffraction (XRD) pattern of SA-MMNPs (Figure 2a) shows a broad diffraction peak at 20º assigned to amorphous structure of polymer matrix which is associated with SA-MNPs without Fe3O4. The characteristic peaks associated with cubic Fe3O4 in SA-MMNPs is observed and is in good agreement with JCPDS No. 19-0629, which indicates that there is no phase transformation during Friedel-Crafts hypercrosslinking process. The thermal stability of SAMMNPs was also studied and is shown in Figure 2b. SA-MMNPs starts to decompose at 350 ºC and exhibits a steep slope, indicating about 90% weight loss due to decomposition of the porous PSSA. Thermogravimetric analyses (TGA) indicated that SA-MMNPs containing 11 wt% Fe3O4 NPs.

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Figure 2 (a) XRD pattern of SA-MNPs and SA-MMNPs; (b) TGA curves for SA-MMNPs. With the help of carboxylic group, SA-MMNPs can disperse well in water after vigorous shaking and can be separated by placing a magnet near the bottle (Figure 3a), which provides an easy way to separate SA-MMNPs from a suspension under an external magnetic field. On the other hand, most of MMNPs without carboxylic group floated on the water because of hydrophobic property (Figure S2). Figure 3b shows the magnetization curve of SA-MMNPs at room temperature. A magnetic hysteresis loop implies a strong magnetic response to a varying magnetic field. The saturation magnetization value of SA-MMNPs is 9.5 emu g-1 which is higher than some other reported magnetic organic particles[40], magnetic carbon[41] and MOF[42].

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Figure 3 (a) digital graphs of SA-MMNPs dispersed in water (left) and separation of SAMMNPs by a magnet (right); (b) magnetization curves of SA-MMNPs. Figure 4a shows a scanning electron microscopy (SEM) image of SA-MMNPs with a spherical morphology with the particle size less than 200 nm, which are determined by the size of miniemulsion droplet size. The morphology of MMNPs shows no big difference from SA-MMNPs (Figure S3). TEM image of SA-MMNPs shows Fe3O4 NPs less than 10 nm were wrapped in the polymer spheres (Figure S4). Dynamic light scattering (DLS) curve (Figure 4b) shows that the hydrodynamic diameter of SA-MMNPs particles is 2 µm in water which is due to the swelling and aggregation of SA-MMNPs in aqueous media.

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Figure 4 (a) SEM image of SA-MMNPs; (b) DLS of SA-MMNPs in water. Table 1 summarizes the porous characterization data for SA-MMNPs and MMNPs. The surface areas are calculated using Brunauer-Emmett-Teller (BET) and Langmuir methods respectively. SA-MMNPs possesses a high BET surface area as high as 485 m2 g-1 and a pore volume as 0.64 cm3 g-1, which are only a bit smaller than those of MMNPs. The less surface area may be attributed to the introduction of carboxylic group, which only increases the weight of non-porous part and thus decrease the entire surface area. Despite the decrement, the surface area of SA-MMNPs is at the same level to MMNPs, which will be beneficial for the high performance adsorption of organic contaminant.

Table 1 Surface area and porosity of SA-MMNPs and MMNPs

SBET[a]

SL[b]

M.A.[c]

PV[d]

M.P.V.[e]

m2 g-1

m2 g-1

m2 g-1

cm3 g-1

cm3 g-1

SA-MMNPs

485

654

207

0.64

0.09

MMNPs

503

678

220

0.75

0.10

Sample

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[a]

Surface area calculated from nitrogen adsorption isotherms at 77.3 K using BET equation.

[b]

[c]

t-

[e]

t-

Surface area calculated from nitrogen adsorption isotherms at 77.3 K using Langmuir equation. Plot micropore area.

[d]

Pore volume calculated from nitrogen isotherm at P/P0 =0.995, 77.3 K.

Plot micropore volume.

The analysis of N2 sorption isotherm of SA-MMNPs shows type I character according to the IUPAC classification which is consistent with that of a substantially microporous material (Figure 5a). A slight hysteresis loop reflects the presence of a spot of the mesopores. And a sharp rise at medium and high pressure region (P/P0=0.8–1.0) indicates the presence of macropores in these materials due to the particles packing. [22] The pore size distributions also confirm the presence of such microporous, mesoporous and macroporous structures (Figure 5b). The MMNPs has the same tendency in N2 sorption and pore size distributions isotherm with SAMMNPs. The heterogeneous porous structures are assumed to be advantageous for the guest molecule diffusion and adsorption.

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Figure 5 (a) N2 sorption isotherms of MMNPs and SA-MMNPs at 77 K (the isotherms of SAMMNPs were shifted vertically by 100 cm3 g-1); (b) pore distributions of MMNPs and SAMMNPs calculated using DFT methods (the isotherms of SA-MMNPs were shifted vertically by 1 cm3 g-1). Because of its hydrophilic property, swelling behavior of SA-MMNPs was determined, as well as MMNPs, Fe3O4@PSSA and Fe3O4@PS. Table S1 compares the swelling ratios of SAMMNPs, MMNPs, Fe3O4@PSSA and Fe3O4@PS. It is found that MMNPs without hydrophilic group also shows increasing swelling ratio compared with un-hypercrosslinked Fe3O4@PS, which indicated that porous structure also plays important role in the swelling of materials in water, and the similar tendency can be observed in the samples of SA-MMNPs and Fe3O4@PSSA. Among them, SA-MMNPs shows highest swelling ratio because of the both effects of hydrophilic group and porous structure. 3.2 Adsorption of RhB Aqueous dye pollutants are one type of most dangerous water-soluble contaminants, which are threatening the living life on the earth. RhB is one of the typical water-soluble dye pollutants which are harmful to health. [43] We thus studied the adsorption property of SA-MMNPs toward water-soluble contaminants by using RhB as adsorbates (Figure S5). Adsorption of RhB solution at 25 ºC with an initial concentration of 120 mg L-1 is shown in Figure 6a. The adsorption capacity (qe) of SA-MMNPs is calculated to be 102 mg g-1. The contrast experiment was conducted by adsorption RhB towards MMNPs under the same conditions and the result showed that MMNPs only have an adsorption capacity (qe) of 71 mg g-1 (Figure 6a). It was also found that the adsorption could reach the equilibrium within a less time interval, which takes about 8 h for SA-MMNPs. In contrast, it takes about 18 h for MMNPs in the adsorption test. Therefore, the introduction of carboxylic group in MMNPs could dramatically increase the

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adsorption performance. The result indicated that although highly porous structures of SAMMNPs and MMNPs make their adsorb RhB easily, the better dispersity of SA-MMNPs in water lead to better adsorption performance. This may attribute to the merit of the better dispersibility of the materials make the contact of the organic dye with adsorbent more efficiently and improve the adsorption performance further. [25] The reusability of adsorbent is considered to have a great cost benefit for practical applications. SA-MMNPs can be easily regenerated by desorption of RhB in ethanol and separated by a magnet. 5 cycles were carried out by using the same SA-MMNPs. The results in Figure 6b shows that the adsorption capacity stays almost the same and decreases only 4% over 5 cycles. These results indicate that the SA-MMNPs have very good recyclability.

Figure 6 (a) time-dependent adsorption capacity of SA-MMNPs and MMNPs; (b) regeneration cycles of SA-MMNPS.

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The adsorption kinetics was also studied. The pseudo-first-order and pseudo-second-order rate equations were employed to fit the adsorption kinetic data (calculated by eq. 3 and eq. 4) (Figure S6). Table 2 lists the fitted results of SA-MMNPs and shows that the adsorption obeys the pseudo-first-order rate model (Figure S6). Results shown in Table S2 suggest MMNPs also obey the pseudo-first-order rate model better.

Table 2 kinetic parameters for RhB adsorption on to SA-MMNPs at 25 ºC.

Sample

Model

K

Qe cal. mg g

2

-1

R

Qe exp. -1

mg g

Pseudo-first-order model

0.616

102

0.9853

102

Pseudo-second-order model

0.00312

136

0.9234

102

SA-MMNPs

The equilibrium data of RhB was analyzed by Langmuir and Freundlich isotherms (eq. 5 and eq. 6). The Langmuir isotherm postulates a monolayer adsorption which takes place at binding sites and no interactions between the molecules adsorbed neither is transmigration on the surface of adsorbent.[34] While the Freundlich model is used to describe the sorption characteristics on heterogeneous surfaces taking into account the interactions between the adsorbed molecules.[35] As shown in Table 3 and Figure S7, the experimental data of RhB fits Langmuir adsorption isotherm well with correlation coefficients of 0.9820 (0.9456 for Freundlich isotherm). The value of the maximum adsorption capacity (qmax) for Langmuir isotherm is 216 mg g-1. And this result is found to be comparable with many magnetic and/or porous adsorbents even with higher BET surface areas reported in the literatures given in Table S3. Table 3 Adsorption isotherm parameters of the adsorption of RhB onto SA-MMNPs at 25 ºC. Langmuir

Freundlich

16

qmax

KL -1

-3

2

R

-1

mg g

10 L g

215.98

6.81

KF -1

-1 n

2

n

R

2.16

0.9456

mg g (L mg )

0.9820

10.48

Meanwhile, other hydrophobic HCPs (benzene-HCPs, thiophene-HCPs, methylbenzene-HCPs, phenol-HCPs) with different BET surface areas and structures were also used as adsorbents for Rh B removal. The results in Table S4 show that only benzene-HCPs with highest BET surface area (1263 m2 g-1) has high adsorption capacity (119 mg g-1), while methylbenzene-HCPs with 2

-1

-1

high BET surface area (808 m g ) has least adsorption capacity (22 mg g ) due to the hydrophobic property of its methyl group. The results indicate that hydrophilic property is important for water-soluble pollutant.

4. Conclusion In conclusion, sodium acrylate has been successfully integrated into the skeleton of magnetic microporous nanoparticles (SA-MMNPs). The SA-MMNPs can be obtained with high surface area and good magnetic property. The high BET surface area ensured the excellent adsorption capacity and the magnetic property allow the materials to be easily separated in the water treatment. More importantly, the hydrophilic functional groups in the polymer remarkably improved the dispersibility in aqueous solution and thus showing the enhanced adsorption performance than the unmodified counterpart. These results constitute a new paradigm for the development of water compatible porous polymers for efficient water treatment.

Supporting information Materials and Characterizations, Potentiometric titration curves of SA-MMNPs and blank sample and their first derivative results, SEM image of MMNPs, TEM image of SA-MMNPs,

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photographs of the RhB solution mixing with SA-MMNPs, the solution after removal of RhB dyes by SA-MMNPs, pseudo-first-order rate and pseudo-second-order adsorption kinetics fitting of SA-MMNPs and MMNPs, Kinetic parameters for RhB adsorption on to MMNPs, Swelling ratio of SA-MMNPs, MMNPs, Fe3O4@PSSA and Fe3O4@PS, adsorption capacities of RhB on various adsorbents reported, adsorption capacities of RhB on various HCPs adsorbents. Author information Corresponding Authors E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest. Acknowledgements We thank Analysis and Testing Center, Huazhong University of Science and Technology for their assistance in characterization of materials. This work was financially supported by Program for National Natural Science Foundation of China (21474033), the International S&T Cooperation Program of China (2016YFE0124400), and the Program for HUST Interdisciplinary Innovation Team (2016JCTD104).

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