Adsorption of methyl orange from aqueous solution by composite magnetic microspheres of chitosan and quaternary ammonium chitosan derivative

Accepted Manuscript Adsorption of methyl orange from aqueous solution by composite magnetic microspheres of chitosan and quaternary ammonium chitosan derivative

Bin Zhao, Xiaojing Sun, Liang Wang, Lixiang Zhao, Zhaohui Zhang, Junjing Li PII: DOI: Reference:

S1004-9541(18)31121-2 https://doi.org/10.1016/j.cjche.2018.12.014 CJCHE 1352

To appear in:

Chinese Journal of Chemical Engineering

Received date: Revised date: Accepted date:

26 July 2018 25 November 2018 13 December 2018

Please cite this article as: Bin Zhao, Xiaojing Sun, Liang Wang, Lixiang Zhao, Zhaohui Zhang, Junjing Li , Adsorption of methyl orange from aqueous solution by composite magnetic microspheres of chitosan and quaternary ammonium chitosan derivative. Cjche (2018), https://doi.org/10.1016/j.cjche.2018.12.014

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ACCEPTED MANUSCRIPT Materials and Product Engineering Adsorption of methyl orange from aqueous solution by composite magnetic microspheres of chitosan and quaternary ammonium chitosan derivative☆ 1, 2

, Xiaojing Sun 2, Liang Wang

1, 2,

*, Lixiang Zhao 2, Zhaohui Zhang

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State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin

Polytechnic University, Tianjin 300387, China

School of Environmental and Chemical Engineering, Tianjin Polytechnic University,

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2

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Tianjin 300387, China

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* Corresponding author, Email: [email protected] (Liang Wang)

☆

,

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Junjing Li 1, 2 1

1, 2

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Bin Zhao

☆

Supported by the National Key Project for Research and Development (2016YFC0400503), the National Natural Science Foundation of China (51478314, 51678408, 51508385), and the Science and Technology Plans of Tianjin (17PTSYJC00050, 17ZYPTJC00060). 1

ACCEPTED MANUSCRIPT Abstract Novel composite magnetic microspheres containing chitosan and quaternary ammonium chitosan derivative (CHMMs) were prepared by inverse suspension method, and used for the methyl orange (MO) removal from aqueous solutions. The

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CHMMs were characterized by scanning electron microscope, transmission electron

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microscope, and Fourier transform infrared spectroscopy, respectively. Compared

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with the chitosan beads, the incorporation of quaternary ammonium chitosan derivative significantly reduced the particle size. The MO adsorption by CHMMs was

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investigated by batch adsorption experiments. The adsorption kinetics was conformed

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to the pseudo second-order kinetics equation. The adsorption isotherm followed the Langmuir model better than the Freundlich model and the calculated maximum MO

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adsorption capacity was 266.6 mg·g-1 at 293K. Thermodynamic studies indicated that

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the MO adsorption was endothermic in nature with the enthalpy change (∆H°) of 99.44 kJ·mol-1. The CHMMs had a stable performance for MO adsorption in the pH

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range of 4-10, but high ionic strength deteriorated the MO removal due to the

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shielding of the ion exchange interaction. A 1 mol·L-1 NaCl solution could be used to regenerate the exhausted CHMMs. The proposed CHMMs can be used as an effective adsorbent for dye removal or recovery from the dye wastewater. Keywords: chitosan; quaternary ammonium chitosan derivative; waste water; adsorption; ion exchange

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ACCEPTED MANUSCRIPT 1. Introduction Nowadays, dyes are widely used as coloring agents in various industries such as dyestuff, textiles, gasoline, plastic industries, leather, paper, rubber, paint, cosmetics, etc [1]. About 100,000 kinds of synthetic dyes and pigments are produced worldwide,

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which are more than 7×105 tons annually [2]. Nevertheless, around 10-15% of dyes

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are discharged into the industrial wastewater [3]. It is well known that most dyes are

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hazardous to organisms and pose risks to the eco-system. Moreover, they are stable and difficult to be completely decomposed by the conventional biological treatment

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processes. Even just 1.0 mg·L-1 dye in water produces significant color, making it

stringent

issue,

numerous

adsorption,

technologies

coagulation,

have

been

membrane

taken,

including

separation,

catalytic

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photodegradation,

and

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unsuitable for human use [2]. Therefore, the removal of dye from wastewater is a

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degradation, chemical oxidation, etc [4-9]. Among these technologies, adsorption is considered as one of the most promising methods owing to its high removal efficiency,

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simplicity of design, and ease of operation.

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Chemical and physical properties of the adsorbents play vital roles in the adsorption performance. Up to date, various adsorbents have been used for the removal of dye from wastewater, such as activated carbon, clay, resin, metal oxide, and industrial waste [10-14]. Activated carbon is a well-known absorbing material, but it has several problems including high cost, poor recovery, and difficult regeneration, which impede its use [15]. Resin has a practical value in the advanced treatment of dye wastewater because of its high adsorption capacity, easy regeneration, and outstanding reusability. 3

ACCEPTED MANUSCRIPT However, its high cost is the constraint factor [12]. Clay and solid waste have the advantages of low cost; however, the secondary pollution is of concern [11]. Recently, adsorbents from natural polymers have gained an increasing attention due to the high efficiency and excellent environmental compatibility [16]. Thereof, chitosan (CS) and

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its derivatives have been well studied as the emerging adsorbents for the removal of

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dyes from wastewater [17].

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CS, one of the most abundant polysaccharide in nature, can be obtained by the deacetylation of chitin, and has the properties of non-toxicity, excellent adsorption

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capacity, antibacterial activity, biodegradability, and biocompatibility [17]. It

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possesses substantial amino and hydroxyl groups. Under acidic condition, the amino group in CS is protonated, and therefore the anionic dye can be adsorbed via the

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strong electrostatic attraction [18]. However, acidic condition can also lead to the

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dissolve of CS and damage its mechanical strength. Cross-linking has been used to solve these problems. Chiou et al. used epichlorohydrin as cross-linking reagent and

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the as-obtained CS beads had high adsorption capacities for several reactive dyes even

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in a acidic solution of pH 3 [19]. Modification is another effective strategy to improve the CS based adsorbents. Yu et al. prepared a chitosan grafted poly (trimethyl allyl ammonium chloride) by graft copolymerization, and coated it on the surface of the sodium citrate coated Fe3O4 nanoparticles. The as-prepared composite magnetic nanoparticles possessed abundant surface positive charges, and achieved a high adsorption of food yellow 3 at pH 3-7 [20]. Xie et al. prepared cross-linked quaternized CS microspheres to remove perchlorate from solution [21]. The 4

ACCEPTED MANUSCRIPT maximum adsorption capacity was 2.5 times higher than that of the protonated cross-linked CS, and the adsorption process was almost independent of pH in a range from 4.0 to 10.3. Bhulla et al synthesized CS-based composite hydrogel by microwave-assisted grafting of acrylic acid and the removal of rhodamine 6G was

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markedly improved [22].

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The synthesis of partially quaternized CS was complicated. Therefore, in this study,

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a facile procedure was proposed for the introduction of quaternary ammonium groups into the CS adsorbents by blending commercial CS and quaternized CS derivative

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(HTCC). The novel composite microspheres (CHMMs) were prepared by the inverse

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suspension method with Fe3O4 nanoparticles as the magnetic core to achieve the effective separation under magnetic field. In order to avoid the leakage of water

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soluable HTCC, a two-step cross-linking protocol was employed in the preparation

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process. Briefly, tripolyphosphate (TPP) was used as the pre-crosslinking reagent to achieve the preliminary cross-linking between quaternized CS and protonated CS

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through ionic bond, and then glutaraldehyde was used as the post-crosslinking reagent

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to chemically crosslink the residual primary amine in CS by the Schiff’s base reaction. The as-prepared CHMMs were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), and Fourier transform infrared spectroscopy (FT-IR), respectively. Equilibrium, kinetic and thermodynamic studies for methyl orange (MO) adsorption by CHMMs were conducted, and the reuse of CHMMs was also investigated. 2. Materials and methods 5

ACCEPTED MANUSCRIPT 2.1. Materials CS with an average molecular weight of 2.5×105 g·mol-1 and a deacetylation degree of 90% was purchased from Sinopharm Group Chemical Reagent Co. Ltd (Shanghai, China). HTCC with an average molecular weight of 4×105 g·mol-1 and a substitution

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degree of 1.03 was obtained from Shanghai Yiji Industrial Co. Ltd (Shanghai, China).

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Its structure is shown in Fig. 1. TPP, glutaraldehyde, NH3·H2O (25 wt.%), acetic acid,

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and polyethylene glycol 2000 (PEG 2000) were supplied by Tianjin Fengchuan Chemical Reagent Co. Ltd (Tianjin, China). Liquid paraffin, Span 80, and ethanol

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were purchased from Kemiou Chemical Reagent Co. Ltd (Tianjin, China).

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FeCl3·6H2O and FeSO4·7H2O were purchased from Fuchen Chemical Reagent Co.

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Ltd (Tianjin, China).

Figure 1 The molecular structure of HTCC

2.2. Preparation of Fe3O4 nanoparticles 6

ACCEPTED MANUSCRIPT Superparamagnetic magnetite nanoparticles (Fe3O4 NPs) were prepared by the chemical co-precipitation of Fe2+ and Fe3+ salts with aqueous NH3 [23]. In the nitrogen atmosphere, 11.35 g FeCl3·6H2O and 5.84 g FeSO4·7H2O were dissolved in 120 mL deionized water, and then transferred into a three-necked flask. The

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temperature of the mixture was raised to 333 Km, and then 40 mL NH3·H2O (25 wt.%)

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was added under continuous stirring for 30 min. The temperature was increased to

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358 K, thereafter and kept for 30 min to obtain a brownish black suspension. The as-prepared Fe3O4 NPs were washed with deionized water until neutral, and dried

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under vacuum at 313 K.

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2.3. Preparation of CHMMs

1.2 g CS, 1.2 g HTCC, and 0.4 g PEG 2000 were dissolved in a 40 mL acetic acid

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solution (2 wt.%). 0.6 g Fe3O4 NPs were fully dispersed in the above solution under

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ultrasonication. The oil phase containing 100 mL liquid paraffin and 4 mL Span 80 was added into a three-necked flask, and stirred at 240 rpm and 313 K for 40 min. The

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CS/HTCC aqueous solution was added dropwise into the oil phase. The mixture was

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stirred at 360 rpm for 60 min to obtain a uniform emulsion. 0.4 g and 0.8 g TPP was added into the mixture for pre-crosslinking to prepare CHMMs with the TPP dosages of 17 wt.% and 33 wt.% (relative to the total mass of CS and HTCC), respectively. After stirred at 300 rpm for additional 30 min, a 2 mL glutaraldehyde solution (25% wt.%) was added for post cross-linking. The reaction was carried out at 313 K for 5 h. The as-obtained product was washed several times with hot ethanol and deionized water, and then stored in deionized water before use. CHMMs without TPP were also 7

ACCEPTED MANUSCRIPT prepared as control. 2.4. Batch adsorption experiments All experiments were carried out in a HNY-200B shaker (Tianjin Honour Instrument Co. Ltd, China) at 200 rpm for 2 h. 0.1 g of CHMMs was added in a 250

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mL flask, which contained a 100 mL MO solution. The initial MO concentration was

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50 mg·L-1 unless other mentioned. When the effects of pH and ionic strength were

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investigated, the adsorption was carried out 293 K. The initial pH of the solutions was adjusted between 2 and 10 using HCl and NaOH, and the ionic strength was adjusted

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over a range of 0-0.5 mol·L-1 NaCl. The adsorption isotherms at 293, 303, and 313 K

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were carried out with different initial MO concentrations (50-300 mg·L-1). The adsorption capacity at equilibrium were calculated using Eq. (1). C0  Ce V m

(1)

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qe 

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where C0 and Ce (mg·L-1) are the concentrations of MO in solution at initial and equilibrium, respectively; V (L) is the volume of the solution; m (g) is the mass of

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CHMMs.

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To explore the adsorption kinetics at 293 K, 0.3 g CHMMs were added into a 300 mL solution with a MO concentration of 50 mg·L-1. The MO uptake by CHMMs (qt, mg·g-1) was calculated by Eq. (2). qt 

C0  C t V m

(2)

where Ct (mg·L-1) is the concentration of MO in solution at t time. The reuse of CHMMs was investigated by the cycles of MO adsorption and desorption. The adsorption was carried out at 293 K, and the initial MO concentration 8

ACCEPTED MANUSCRIPT was 50 mg·L-1 and the CHMMs dosage was 1 g·L-1. After the adsorption, the MO-loaded CHMMs were collected by a permanent magnet, and regenerated using a 1 mol·L-1 NaCl solution at 293 K. After washing with distilled water several times, the regenerated CHMMs were used again for MO adsorption.

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2.5. Characterization of CHMMs

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The Fourier transformation infrared (FT-IR) spectra in the region of 500-4000 cm-1

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were recorded using a Nicolet FT-IR 6700 spectrophotometer (Thermo, USA). The magnetization curve of CHMMs was determined by a vibration sample magnetometer

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(VSM, 9600-1 LDJ, USA) with an applied field between -10,000 and 10,000 Oe.

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Scanning electron microscopy (SEM, ProX, PHENOM, Netherlands) and transmission electron microscopy (TEM, H7650, Hitachi, Japan) were used to

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examine the morphological structures of the prepared adsorbents. The particle size

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distribution was analyzed with a Mastersizer 2000 instrument (Malvern, Britain). 3. Results and discussion

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3.1. Effect of TPP dosage

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TPP was used as the pre-crosslinker in this study to achieve the preliminary cross-linking between HTCC and protonated CS through ionic bonds. Its dosage played an important role in the morphology and the formation of CHMMs. Spherical microspheres with smooth edge were formed without TPP (Fig. 2(a)); however, these particles were large and had a broad size distribution from 21 μm to 97 μm in diameter. When the TPP dosage was 17 wt.%, the particle size was significantly reduced compared to that without TPP (Fig. 2(b)). Particles with a diameter less than 9

ACCEPTED MANUSCRIPT 10 μm accounted for the majority. Since TPP occupied partial protonated primary amine groups in CS, the cross-linking sites available for glutaraldehyde markedly decreased. As a result, the effective combination between the CS molecules was negatively affected, and thereby the particle size was reduced. However, when the

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TPP dosage was 33 wt.%, the average particle size increased instead (Fig. 2(c)). The

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SEM image also indicated that the integrality of the as-obtained CHMMs was poor

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and some beads were even broken. This was probably because TPP occupied too many protonated primary amine groups and the chemical cross-linking by

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glutaraldehyde was severely restricted. Therefore, the mechanical strength of CHMMs

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dramatically decreased at high TPP dosage.

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Figure 2 Effect of TPP dosage on the morphology of CHMMs: (a) without TPP; (b) 17 wt.%; (c) 33 wt.%.

The particle size distribution was also measured by laser light scattering method (Fig. 3). However, the results were not identical to those obtained by SEM. The particle size value obtained by laser light scattering was actually the dynamic diameter of swelling CHMMs in solution, instead of the dry samples for SEM 10

ACCEPTED MANUSCRIPT observation. Therefore, the value was much larger. When the TPP dosage was 17 wt.%, the average particle size reached 120.2 μm, and some small particles also appeared at 11.5 μm. However, the effect of TPP dosage on the CHMMs size

8

without TPP 17 wt.% 33 wt.%

363.1

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6

120.2

5 4 3

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Volume (%)

7

239.9

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9

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distribution showed similar trend as that observed in SEM.

2

11.5

1 0.1

1

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0

10 100 Particle size (m)

1000

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Figure 3 Effect of TPP dosage on the particle size distribution of CHMMs

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The water content and ion exchange capacity of CHMMs barely changed at the TPP dosage of 17wt.% compared to the control (Fig. 4). As the TPP dosage further

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increased to 33 wt.%, the water content slightly increased from 75.6% to 79.3% while the ion exchange capacity decreased from 0.80 meq·g-1 to 0.74 meq·g-1. Increasing the TPP dosage restricted the strong chemical cross-linking by glutaraldehyde, thereby reducing the mechanical strength of CHMMs. As a result, the water content increased. In addition, TPP occupied partial quaternary ammonium groups in HTCC, leading to the slightly loss of the ion exchange capacity.

11

90

0.8

80

0.6

70

0.4

60

0.2

0

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50 17

33

0.0

-1

1.0

Ion exchange capacity (meqg )

100

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Water content (%)

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TPP dosage (wt.%)

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Figure 4 Effect of TPP dosage on the water content and ion exchange capacity of

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CHMMs.

Based on the above results, 17 wt.% TPP was proposed for pre-crosslinking during

adsorption study.

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the preparation of CHMMs, and these beads were subjected to the following tests and

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3.2. FT-IR analysis of CHMMs

The FT-IR spectra of CS, HTCC, and CHMMs are shown in Fig. 5. In the

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spectrum of CS powder, the bands at 3358 and 3284 cm-1 were assigned to the N-H and O-H stretching vibration, respectively [24]. The absorptions at 1593, and 1320 cm-1 were attributed to the bending vibration of N-H, and the stretching vibration of C-N, respectively, which were the characteristic peaks of amide, confirming the residual acetyl amino present in CS [25]. The peaks located at 1062 and 1023 cm-1 were assigned to the C-O stretching vibration in primary and secondary alcohol in CS [26]. 12

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3358 3284 1593

1023

1150 1320

CS

HTCC

1645 CHMMs

3000

2000

1000

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4000

1563

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1478

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Absorbance

1062

-1

Wavenumber (cm )

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Figure 5 FT-IR spectra of CS, HTCC, and CHMMs.

In the spectrum of HTCC, the strong absorption at 1478 cm-1 was assigned to the

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asymmetric angular bending of the -CH3 groups in -N+(CH3)3Cl-, which was the

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convinced evidence for the existence of quaternary ammonium groups in HTCC [20]. In addition, the absorption of at 1593 cm-1, attributed to the -NH2 groups in CS,

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disappeared in the HTCC spectrum due to the substitution on the -NH2 groups [27].

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The FT-IR spectrum of CHMMs contained all the characteristic peaks of HTCC, indicating that HTCC was successfully incorporated into CHMMs. The peak at 1645 cm-1 was attributed to the imine bonds formed by the Schiff’s base reaction due to the cross-linking by glutaraldehyde [21]. In addition, a new band appeared at 1563 cm-1, which was assigned to the ethylenic bonds. This band was reported to result from the disappearance of the band at 1593 cm-1, corresponding to the amine group, after the the cross-linking of CS with glutaraldehyde [28]. 13

ACCEPTED MANUSCRIPT 3.3. Morphology of CHMMs A large number of pores with diameters around 0.4 μm were evenly distributed on the surface, indicating the as-prepared CHMMs beads were macroporous (Fig. 6(a)). The porous structure of CHMMs was supposed to result from the addition of the

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porogen of PEG-2000 and the good solubility of HTCC. Fig. 6(b) is the TEM image

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of CHMMs. It can be seen clearly that the CHMMs had a core-shell structure. The

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-NH2 and -OH groups in CS and HTCC can be coordinated with Fe3O4 NPs [29]. The subsequent cross-linking reaction formed a stable network structure, which

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successfully wrapped Fe3O4 NPs inside. This structure was favorable for the sufficient

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exposure of the active functional groups in CS and HTCC and beneficial to the

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adsorption.

Figure 6 Morphology of CHMMs: (a) SEM; (b) TEM

3.4. Magnetic properties of CHMMs The magnetic hysteresis loop of CHMMs is shown in Fig. 7. The coercivity and remanence were close to zero, implying the as-prepared CHMMs were 14

ACCEPTED MANUSCRIPT superparamagnetic [30]. The saturation magnetization (Ms, emu·g-1), which was a critical factor for the successfully magnetic separation of the magnetic particles from water, was 3.51 emu·g-1, enough for the magnetic separation [31]. The insets of Fig. 7 are the photographs of the CHMMs separation without and with a magnetic field. It

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can be concluded that CHMMs can be easily separated and recovered from the treated

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effluent with a magnetic field.

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Figure 7 Magnetic hysteresis loop of CHMMs (insets: the photographs of the

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CHMMs separation without (a) and with (b) a magnetic field).

3.5. Methyl orange adsorption by CHMMs 3.5.1. Effect of pH The effect of pH on MO removal by CHMMs is shown in Fig. 8. The equilibrium MO uptake was rather stable at around 48.5 mg·g-1 in the pH range of 4-10. It was reported that the best adsorption of MO using CS microspheres was usually achieved in a narrow pH range of 3-4, and higher pH completely deteriorated the adsorption 15

ACCEPTED MANUSCRIPT performance [19]. The point of zero charge (PZC) of CS was reported to be around 6.3 [32]. At pH below PZC, the primary amine groups were protonated and the CS molecules were positively charged. As the pH value increased, less primary amine groups were protonated. Therefore, the electrostatic attractions between the MO

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anions and the positively charged CS were weakened and the MO uptake by CS was

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decreased. As a result, the effective working pH of CS was generally narrow. CHMMs

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prepared in this study markedly extended the working pH range. The quaternary ammonium groups in HTCC were strong base groups. It can immobilize MO anions

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by ion exchange. Its presence effectively compensated the decrease in MO adsorption

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by CS at base pH. At pH 11, the electrostatic interaction was shielded by the strong ionic strength and the MO uptake declined sharply. Compared with the pristine CS

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beads, the as-prepared CHMMs were more applicable and effective for dye

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50

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-1

Equilib. MO uptake (mgg )

wastewater treatment over a wide pH range.

40 30 20 10 0

4

5

6

7

8

9

10

11

pH

Figure 8 Effect of pH on MO uptake by CHMMs

16

ACCEPTED MANUSCRIPT 3.5.2. Effect of ionic strength Fig. 9 shows the effect of NaCl concentration on the MO adsorption by CHMMs. At neutral pH, the MO uptake strongly depended on the ionic strength of the solution, implying that ion exchange was one of the important mechanisms for MO removal by

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CHMMs. When the NaCl concentration was 0.10 mol·L-1, the equilibrium MO uptake

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was only 17% of that without NaCl. Further increase in the NaCl concentration to

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0.50 mol·L-1 barely changed the equilibrium uptake of MO, and the remaining interaction between MO and CHMMs was mainly based on the hydrogen bonds and

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50

-1

Equilib. MO uptake (mgg )

van der Waals force [33].

40

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30 20 10

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0

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0.0

0.1

0.2

0.3

0.4

0.5

-1

NaCl conc. (molL )

Figure 9 Effect of ionic strength on MO uptake by CHMMs

3.5.3. Adsorption kinetics Fig. 10 shows the kinetic profile for MO adsorption by CHMMs. The uptake of MO onto CHMMs was fast. It cost only 30 min to approach the equilibrium. Compared with other CS-based adsorbents [34], the size of CHMMs prepared in this 17

ACCEPTED MANUSCRIPT study was much smaller. Therefore, it had larger surface area, which was beneficial to the sufficient contact between the functional groups and the adsorbates in solution. Furthermore, the large pores present on the CHMMs surface (Fig. 6) were also favorable for reducing the MO diffusion resistance. The high water content of

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CHMMs indicated their internal structure was loose and open, ensuring the high

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internal diffusion rate in the adsorption kinetics. Moreover, the rate of the ion

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exchange process was also assumed to be faster compared to the conventional

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adsorption process.

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40 30

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20

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MO uptake (mgg-1)

50

Pseudo-first order model Pseudo-second order model

10

0

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0

10

20

30

40

50

60

Time (min)

Figure 10 Kinetics of MO uptake by CHMMs.

The time profile of the adsorption was fitted by the kinetic models of pseudo-first order (Eq. (3)) and pseudo-second order (Eq. (4)), respectively. dqt  k1  qe  qt  dt dqt 2  k 2  qe  qt  dt

(3) (4)

where k1 (min-1) and k2 (g·mg-1·min-1) are rate constants for the pseudo-first order and 18

ACCEPTED MANUSCRIPT the pseudo-second order kinetic models, respectively; qe (mg·g-1) and qt (mg·g-1) are the amount of MO adsorbed per unit mass of CHMMs at equilibrium and at time t (min), respectively. Table 1 lists the fitting results. The calculated qe values and the correlation

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coefficients (R2) obtained by the pseudo-first order kinetic model were close to those

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by the pseudo-second order kinetic model. However, according to Fig. 10, the fitting

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results of pseudo-second order kinetic model were more consistent with the experimental data, especially in the rapid adsorption stage (0-20 min). Previous works

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also demonstrated that the pseudo-second order kinetic model can be used to depict

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the kinetics of dye adsorption by CS beads [19, 35].

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Pseudo-first order model

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Table 1 The fitting results of MO uptake by CHMMs using kinetic models. Pseudo-second order model

k1 (min-1) qe (mg·g-1)

R2

k2 (g·mg·min-1)

qe (mg·g-1)

R2

0.1567

0.9508

0.01056

50.3

0.9567

AC

CE

48.4

3.5.3. Adsorption thermodynamics According to the adsorption kinetic results above, the adsorption test lasted one hour for isotherm investigation. Fig. 11 shows the isotherms of MO uptake by CHMMs at 293, 303, and 313 K, respectively. The MO uptake increased with the equilibrium MO concentration, and high temperature favored the MO removal from solution. Therefore, it can be inferred that the MO adsorption onto CHMMs was an 19

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400 350 300 250

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200 293K 303K 313K line Langmuir dash Freundlich

150

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100 50 0 0

50

100

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Equilib. MO uptake (mgg-1)

endothermic process.

150

200

250

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Equilib. MO conc. (mgL-1)

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Figure 11 Isotherms of MO uptake by CHMMs.

Solid lines and dash lines in Fig. 11 represent the fitting results of the experimental

qe 

kL qmCe 1  kLCe

(5) (6)

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qe  kFCe1 /n

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data using Langmuir (Eq. (5)) and Freundlich (Eq. (6)) isotherms, respectively.

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where qm (mg·g-1) is the maximum adsorption capacity; qe (mg·g-1) and Ce (mg·L-1) are the amount of adsorbed MO and aqueous MO concentration at equilibrium, respectively; kL (L·mg-1) is the Langmuir adsorption constant; kF and n are empirical constants for Freundlich isotherm. The fitting results are listed in Table 2. Compared with the Freundlich isotherm, the experimental data at different temperatures were better fitted by the Langmuir isotherm with the correlation coefficients (R2) around 0.99. Langmuir isotherm was 20

ACCEPTED MANUSCRIPT derived based on the monolayer adsorption. Therefore, the uptake of MO by CHMMs was mainly through the adsorbate-adsorbent interactions. The calculated maximum adsorption capacity (qm) was 266.6 mg·g-1 at 293 K, and increased with temperature.

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Table 2 Fitting results of MO uptake by CHMMs using isotherm models. Langmuir

Freundlich

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Temp. (K) kL (L·mg-1)

qm (mg·g-1)

R2

1/n

R2

293

0.0442

266.6

0.9890

41.94

0.3344

0.9921

303

0.3882

303.6

0.9990

110.86

0.2212

0.8606

313

0.5019

331.8

0.9986

125.84

0.2213

0.8482

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kF

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The qm values for MO adsorption by other adsorbents are concluded in Table 3.

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CHMMs prepared in this study had a high adsorption capacity compared with other CS-based adsorbents. The reason may be ascribed to the incorporation of HTCC,

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which not only achieved the MO uptake through the ion exchange mechanism, but

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also developed fine structure for adsorption.

Table 3 Overall comparison of MO adsorption by various adsorbents. Adsorbent

Maximum

Temperature

adsorption

(K)

capacity (mg·g-1) 21

Reference

ACCEPTED MANUSCRIPT 44.1

298

[11]

CS microspheres

207.0

298

[34]

Poly (ethylenimine) modified CS beads

400.0

303

[35]

Cross-linked CS/bentonite composite

89.3

293

[36]

Goethite impregnated CS beads

84.0

298

[37]

Wheat straw

278.7

313

[38]

Lead doped zinc-aluminum oxide nanoparticles

200.0

Alkali-activated carbon nanotubes

149.0

298

[40]

298

[41]

16.8

298

[42]

266.6

293

this study

hollow

34.7

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microtubes Graphene oxide

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D

CHMMs

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[39]

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kapok

298

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Polyacrylonitrile-coated

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Zirconium immobilized bentonite

The thermodynamic parameters, such as free energy change (∆G°, kJ·mol-1),

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enthalpy change (∆H°, kJ·mol-1), and entropy change (∆S°, kJ·mol-1·K-1), was

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determined according to Eq. (7) and Eq. (8).

G   RT ln K e

ln Ke  

(7)

H   1  S    R T  R

(8)

where Ke is the adsorption equilibrium constant, L·mol-1; R is the gas constant, J·mol-1·K-1; T is the adsorption temperature, K. Ke was calculated based on kL in the Langmuir isotherm (Table 2). The results of these thermodynamic parameters are

22

ACCEPTED MANUSCRIPT listed in Table 4. The values of ∆G° were negative for all temperatures investigated. Therefore, the MO adsorption on CHMMs was spontaneous. More negative ∆G° with the increase in temperature indicated a high temperature favored the adsorption process. It was reported that the ∆G° value for physical adsorption ranged from -20 to

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0 kJ·mol-1 while that for chemisorption was much higher in terms of absolute value,

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generally in the range of -80 to -400 kJ·mol-1 [43]. Based on the calculated ∆G°

SC

values in this study, it can be concluded that the adsorption reaction between MO and CHMMs combined both physical and chemical processes. The positive value of ∆H°

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(99.44 kJ·mol-1) demonstrated the adsorption process was endothermic, which was

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consistent with the enhancement in the equilibrium adsorption capacity as temperature rose (Fig. 11). The positive value of ∆S° (0.40 kJ·mol-1·K-1) implied the increase in

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CHMMs with MO [44].

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randomness at the solid-liquid interface due to the adsorption as well as the affinity of

293 303 313

Ke (L·mol-1)

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Temperature (K)

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Table 4 Thermodynamic parameters of MO adsorption by CHMMs. ∆G° (kJ·mol-1)

1.4452×104

-23.33

1.2707×105

-29.61

1.6429×105

-31.25

∆H° (kJ·mol-1)

∆S° (kJ·mol-1·K-1)

99.44

0.40

3.5.4. Reusability of CHMMs The regeneration of adsorbent is a key factor for its practical use. Since the main 23

ACCEPTED MANUSCRIPT mechanism for MO uptake by CHMMs was assumed to be ion exchange due to the presence of the quaternary ammonium groups, a 1 mol·L-1 NaCl solution was used to regenerate the used CHMMs. As shown in Fig. 12, when CHMMs were reused, the equilibrium MO uptake decreased by 12% after the first regeneration but remained

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stable afterwards. The equilibrium MO uptake remaining after 6 regenerations was

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34.5 mg·g-1, accounting for 71.2% of the initial value. The high ionic strength of the 1

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mol·L-1 NaCl solution was only able to release the MO immobilized through the ion exchange mechanism. The unrecovered adsorption capacity was mainly attributed to

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the MO adsorbed via the hydrogen bonds and van der Waals force.

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-1

Equillib.MO uptake (mg·g )

50 40

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20 10

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D

30

0

0

1

2

3

4

5

6

Cycle number

Figure 12 The reusability of CHMMs for MO adsorption.

4. Conclusions Composite CS/HTCC magnetic microspheres (CHMMs) were prepared by inverse suspension method. A two-step cross-linking protocol was proposed using TPP as the pre-crosslinking reagent and glutaraldehyde as the post-crosslinking reagent. TPP 24

ACCEPTED MANUSCRIPT dosage was crucial in terms of the morphology of the prepared microspheres. 17 wt.% TPP was proposed when the mass proportion of CS and HTCC was 1:1. The incorporation of Fe3O4 NPs generated core-shell structure and the microspheres can be easily recovered from solution under a magnet field. The adsorption of MO on

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CHMMs was dependent on ionic strength, but it was consistently effective over a

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wide pH range from 4-10. The adsorption well fitted the pseudo-second order kinetic

SC

model and the Langmuir isotherm model, and the calculated maximum adsorption capacity was 266.6 mg·g-1 at 293 K. Since the successful introduction of HTCC gave

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the CS-based adsorbent the capability of ion exchange, the exhausted CHMMs can be

MA

easily regenerated and effectively reused. CHMMs proposed in this study could also

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be extended to poisonous anion removal from water.

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