clay microspheres with hierarchical architecture for highly efficient removal of organic dyes

Accepted Manuscript Title: Chitin/clay microspheres with hierarchical architecture for highly efficient removal of organic dyes Authors: Rui Xu, Jie Mao, Na Peng, Xiaogang Luo, Chunyu Chang PII: DOI: Reference:

S0144-8617(18)30091-2 https://doi.org/10.1016/j.carbpol.2018.01.073 CARP 13223

To appear in: Received date: Revised date: Accepted date:

3-11-2017 9-1-2018 20-1-2018

Please cite this article as: Xu, Rui., Mao, Jie., Peng, Na., Luo, Xiaogang., & Chang, Chunyu., Chitin/clay microspheres with hierarchical architecture for highly efficient removal of organic dyes.Carbohydrate Polymers https://doi.org/10.1016/j.carbpol.2018.01.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Chitin/clay microspheres with hierarchical architecture for highly efficient removal of organic dyes

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072,

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a

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Rui Xu a,† , Jie Mao a,†, Na Peng c, Xiaogang Luo b,*, Chunyu Chang a,*

China

School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology,

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b

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School of Chemistry and Chemical Engineering, Wuhan University of Science and

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c

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Xiongchu Ave., Wuhan 430073, Hubei, China

Technology, Wuhan 430081, China

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R.X and J.M. contributed equally to this work

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†

Xiaogang Luo (Email: [email protected])

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Corresponding author: Chunyu Chang (Email: [email protected])

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Graphical abstract

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Highlights

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1. Novel chitin/clay microspheres were fabricated by incorporating clay nanosheets

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into chitin microsphere matrix during the thermally induced sol-gel transition.

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2. Clay nanosheets were uniformly embedded in the chitin matrix, leading to the hierarchical structure of chitin/clay microspheres.

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3. These microsphere exhibited high absorption capacity and removal efficiency for

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methylene blue dyes.

ABSTRACT

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Numerous adsorbents have been reported for efficient removal of dye from water, but the high cost raw materials and complicated fabrication process limit their practical applications. Herein, novel nanocomposite microspheres were fabricated from chitin and clay by a simple thermally induced sol-gel transition. Clay nanosheets were uniformly embedded in a nanofiber weaved chitin microsphere matrix, leading to their 2

hierarchical architecture. Benefiting from this unique structure, microspheres could efficiently remove methylene blue (MB) through a spontaneous physic-sorption process which fit well with pseudo-second-order and Langmuir isotherm models. The maximal values of adsorption capability obtained by calculation and experiment were 152.2 and 156.7 mg g-1, respectively. Chitin/clay microspheres (CCM2) could remove

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99.99% MB from its aqueous solution (10 mg g-1) within 20 min. These findings

provide insight into a new strategy for fabrication of dye adsorbents with hierarchical

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structure from low cost raw materials.

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KEYWORDS: chitin/clay microsphere, hierarchical architecture, highly efficient,

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dye removal

1. Introduction

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With the fast development of industry, water pollution has become a leading global risk factor for human health (Bolisetty & Mezzenga, 2016). Most organic dyes

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pollute both surface and groundwater because of their non-biodegradability, which

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not only disrupt the growth of living organisms but also cause illness, disease, and even human death. Methylene blue (MB) is a common basic dye used as a coloring agent in pharmaceuticals, pesticides, and rubbers, which may induce hemolytic

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anemia and skin desquamation of infants at a very low dose (2~4 mg kg-1) (Nursam et al., 2016). Several strategies such as catalyst oxidation, membrane separation, coagulation/flocculation, and adsorption have been well developed to remove organic dyes from wastewater, but all come with their advantages and disadvantages. For example, photo-catalytic oxidation could completely mineralize the organic dyes into 3

harmless products at room temperature with a relatively low cost, but the catalyst necessitates secondary pollutant recovery (Yao et al., 2016). Membrane separation is attractive for removal of dyes from wastewater because of its high separation efficiency. The major disadvantage of this technique is the high energy cost to improve water permeability (Yao et al., 2016). Coagulation/flocculation is also highly

of toxic sludge is a main disadvantage (Yagub et al. 2014). In the view of

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efficient for dye removal from wastewater. However, the generation of large amounts

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aforementioned approaches, adsorption is one of the most effective and popular

techniques for dye removal from wastewater. To date, numerous adsorbents, such as

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metal-organic framework (MOF) (Zhao et al., 2015), magnetic nanoparticles (Wang

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et al., 2015), carbon nanotubes (Yao et al. 2016), graphene oxide (Robati et al. 2016),

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porous carbon materials (Alatalo et al. 2016), and their composites, have been

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reported for dye removal with high adsorption capacity and efficiency. However, the high cost of raw materials and complicated preparation process considerably limit

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their scale-up production and practical applications. Therefore, the developing tendency for dye absorbents is to use cheap raw materials such as natural polymers, or

capacity.

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biomass, to master scalable preparation methods, and to obtain excellent adsorption

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Chitin, poly--(1→4)-N-acetyl-D-glucosamine, is the most abundant natural

amino polysaccharide distributed throughout nature in marine invertebrates, insects,

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fungi, and yeast. Although chitin is insoluble in common organic solvents, we have previously reported that it could be dissolved in NaOH/urea aqueous solution through a freeze-thaw cycle process (Chang et al. 2011), and nanofibrous microspheres with highly specific surface area could be prepared by a thermally induced sol-gel transition method (Duan et al. 2015). Clay (montmorillonite) with layered structure 4

has a large specific surface and high cationic exchange capacity, whose current market price is 20 times cheaper than that of activated carbon (Zhu et al. 2016). Due to the presence of negative charges on the surface of clay nanosheets, they exhibit strong sorption capacity for basic dyes through electrostatic attraction (Bentahar et al.

homogeneous incorporation into cellulose-based hydrogels. Despite these

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2017). In our previous work, we found clay was robust and adsorbent after

nanocomposite hydrogels exhibited high adsorption capacity for MB, it took dozens

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of hours to reach adsorption equilibrium, indicating low adsorption rate (Peng et al. 2016).

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Keeping in mind that high specific surface area of materials increases the

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adsorption rate for dyes (Pal et al. 2015), we herein developed a series of novel

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chitin/clay microspheres with hierarchical structures as environmental pollutant

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scavengers, which could rapidly and effectively remove MB from wastewater. We investigated the structure, morphology, adsorption capacity, and cycling performance

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of these new microsphere-based adsorbents, as well as the influence of clay content on the properties of microspheres. The effects of time, temperature, and pH on the

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adsorption capacity of microspheres for MB were systemically evaluated and the corresponding adsorption mechanisms were extensively explored. Experimental

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results showed that our developed microspheres could efficiently remove MB through a spontaneous physic-sorption process which fit well with pseudo-second-order and

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Langmuir isotherm models.

2. EXPERIMENTAL SECTION 2.1. Materials. Chitin powder with a degree of acetylation (DA) of 0.94 was purchased from Golden-Shell Biochemical Co. Ltd (Zhejiang, China). The weight5

average molecular weight (Mw) of the chitin was determined to be 5.34 × 106 in a 5% (w/v) mixture of lithium chloride and N,N-dimethylacetamide (LiCl/ DMAc) by dynamic light scattering analysis (DLS, ALV/GGS-8F, Germany). The chitin powder was purified according to our previous method before use (Duan et al. 2014). Inorganic clay (Montmorillonite) (cation exchange capacity = 100 mequiv/100 g) was

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supplied by Hongyu Clay Company (Zhejiang, China). Methylene blue trihydrate

(C16H18ClN3S•3H2O) was purchased from Sinopharm Chemical Reagent Co. Ltd.

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(China) and used directly. All other chemical reagents of analytical-grade were

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purchased from commercial sources in China and used without further purification.

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2.2. Preparation of Chitin/Clay Microspheres (CCMs). To prepare the CCMs,

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a certain amount of clay was added into the chitin solution prepared according to our

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previous method (Chang et al. 2011), and homogenized by continuous agitation at 200 rpm for 20 min. Then, the chitin/clay solution (20 g) was added dropwise to a flask

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containing a well-mixed suspension of isooctane (50 g) and Span 85 (3.3 g) in an ice bath. After stirring at 1000 rpm for 50 min, Tween 85 (1.8 g) was injected into the

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flask and mixture was left to stir for another 50 min. Subsequently, the reaction temperature was raised to 80 °C and the mixture was stirred for 15 min. Thereafter, 10

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wt% hydrochloric acid aqueous solution was added to adjust the solution pH to 7. To purify CCM suspension, the mixture was washed three times with distilled water and

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ethanol successively to remove the residual urea, sodium chloride, isooctane, Span 85, and Tween 85. CCMs containing 33 wt% and 70 wt% clay were named as CCM1 and CCM2, respectively, and the pure chitin microspheres were coded as CCM0.

2.3. Characterization. Fourier transform infrared (FT-IR) spectroscopy was 6

carried out on a Nicolet 5700 FT-IR spectrometer (PerkinElmer Spectrum one, USA) using KBr pellets in the region of 500−4000 cm−1. The thermal properties of the CCMs were studied using Q500 (TA Instruments, USA) at a heating rate of 10 °C min–1 from 30 to 600 °C under air atmosphere. The wide-angle X-ray diffraction (WAXD) measurements were performed using a XRD-6000 diffraction micrometer

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(Shimadzu, Japan) with Cu-Kα radiation (λ= 0.15406 nm) in a 2θ scanning

configuration in the range of 5°–50° at a scanning rate of 1° min−1. The Brunauer–

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Emmet–Teller (BET) surface areas were calculated by nitrogen adsorption–desorption isotherms using autosorbiQ2 (Quantachrome, USA). TEM observation of the CCMs

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was carried out on a JEM-2010 FEF (UHR) transmission electron microscope (JEOL

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TEM, Japan). Before the observation, samples were embedded into epoxy resin and

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sliced for ultrathin sections. The morphology of the CCMs was observed on an EX20

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optical microscope (Shunyu, China) and field emission scanning electron microscopy (Zeiss, Germany), respectively. Ultraviolet–visible (UV–vis) spectroscopy was

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performed using a Shimadzu UV-2450 spectrometer (Shimadzu, Japan).

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2.4. Dye Adsorption. The effect of time, temperature, and pH on the adsorption efficiency of the CCMs for removal of MB was studied by batch experiment.

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Adsorption kinetics was measured with an initial MB concentration of 100 mg L−1 at room temperature by adding 27 mg of CCM2 to 75 mL of MB solution. At

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predetermined time intervals, 0.5 mL of solution was withdrawn, diluted, and measured by UV spectrophotometer. The MB concentration was calculated according to the calibration curve (Fig. S1). The calibration curve was reproducible and linearly covered the concentration range used in this work. For evaluation of the thermodynamic properties, adsorption isotherm experiments were conducted by 7

adding 1.8 mg of CCM2 to 5 mL of MB solution with the concentration ranging from 10 to 100 mg L−1 at 303, 313, and 323 K, respectively. After absorption equilibrium was achieved (24 h), the MB concentration was subsequently determined by UV spectrophotometer. To study the effect of pH, 1.8 mg of CCM2 was added into 5 mL of MB solution (100 mg L−1) with pH ranging from 1 to 11. The various pH values

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with 0.1 M ionic strength were adjusted by using NaOH, HCl, and NaCl, and

determined by a pH meter (BEBCH/PHS-25, ±0.01). Twenty-four hours later, the

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final MB concentration was measured by UV spectrophotometer.

All absorption tests were conducted in a 10 mL polyethylene tube containing 1.8

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mg of dry CCM sample and 5 mL of MB solution. The tubes were fixed in a

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temperature controlled shaker at 80 rpm (except kinetic experiments). At

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predetermined time intervals, 0.5 mL of MB solution was withdrawn and the MB

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concentration was calculated by UV spectrophotometer. The amount of MB adsorbed on the sample after equilibrium was described as the adsorption capacity (Qe) and

𝑄𝑒 =

𝐶0 −𝐶𝑒 𝑊

𝑉

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calculated according to eq 1.

(1)

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where C0 (mg L-1) and Ce (mg L-1) are the initial and equilibrium MB concentrations

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respectively, W (g) is the weight of dried CCM sample, and V (L) is the volume of MB solution. Parallel studies of adsorption were carried out three times, and the average value of three independent experiments was used for calculation. For the

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recycling performance of the CCMs, desorption of MB from CCM2 was conducted in 1M NaOH aqueous solution. The adsorption/desorption was repeated 5 times, and the Qe of CCM2 was calculated in each cycle. To obtain the maximal removal efficiency of the microsphere, CCM2 (9 mg) was added into MB solution (5 mL, 10 mg L-1) and the UV spectra were measured at 8

different time intervals. The removal efficiency of the microsphere was calculated according to eq 2. 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 % =

𝑊0 −𝑊𝑒 𝑊0

× 100%

(2)

where W0 (g) and We (g) are the initial weight of MB in the solution and the weight of

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MB adsorbed into CCM2, respectively.

3. RESULTS AND DISCUSSION

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3.1. Fabrication and Characterizations of the CCMs. The schematic illustration for preparation of CCMs is displayed in Fig. 1. Firstly, chitin derived from crab was

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dissolved in NaOH/urea aqueous solution at low temperature. Then, clay nanosheets

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were mixed with chitin solution through vigorous agitation to obtain a

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homogeneous suspension. Finally, the mixture was emulsified into liquid

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microspheres in isooctane with the aid of surfactants under rigorous stirring. The nanocomposite microspheres (CCMs) could thus be obtained via a thermally induced

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sol-gel transition (Duan et al. 2015). We adjusted the preparation conditions, including oil/water ratio, surfactant concentration and stirring rate, to optimize the

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physiochemical properties of the CCMs. Our CCMs had chitin matrices with nano-

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fibrous weaved porous morphology that could provide effective channels for the entrance of dye molecules. Additionally, the intercalated clay nanosheets had a large

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number of active sites that could be used for dye adsorption.

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Fig. 1. Schematic illustration for the preparation of the CCMs.

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FT-IR was employed to examine the structure of the CCMs. As shown in Fig.

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2a, the absorption peaks at 2923 and 2847 cm-1 in the spectrum of the chitin microsphere (CCM0) could be attributed to the symmetric and antisymmetric

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stretching vibration of C-H in methyl and methylene groups of chitin, while the shoulder at 1745 cm-1 corresponded to the stretching vibration of C=O in acetamide

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groups, which almost disappeared in the spectra of CCMs due to the incorporation of

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clay (Singh et al. 2017). In addition, the typical absorption band at 1034 cm-1 corresponding to the stretching vibration of Si-O-Si bonds in the clay could be also found in the spectra of the CCMs, strongly demonstrating that the clay nanosheets

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were successfully incorporated into the chitin matrix. Thermal stability of the CCMs was investigated from the TGA curve recorded in the range of 30-600 °C under air flow conditions (Fig. 2b). The sample weight decreased sharply from 250 to 350 °C, which could be ascribed to chitin degradation. With an increase of clay content in the CCMs, the decomposition temperature increased from 319 to 331 °C (Fig. S2), 10

suggesting that the incorporation of clay nanosheets slightly improved the thermal stability of the CCMs. At 600 °C, the remnant content of CCM0 was about 6 wt%, while the remnant content of CCM1 and CCM2 were ~28 wt% and ~66 wt%, respectively. Compared to the clay content during the preparation of the CCMs (33 wt% for the CCM1 and 70% for the CCM2), the decline of clay content in the final

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CCMs might suggest that the clay nanosheets were not completely incorporated into

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chitin matrix during the preparation process.

Fig. 2. Characterizations of the CCMs: (a) FTIR spectra; (b) TGA curves; (c) XRD

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patterns; and (d) Nitrogen adsorption-desorption isotherms.

To further understand the structure of clay in chitin matrix, XRD patterns of the

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CCMs were examined and the corresponding results are shown in Fig. 2c. Four typical peaks at 9.3, 12.8, 19.2, and 26.4° were observed in XRD pattern of the CCM0, reflecting the (020), (021), (110), and (013) of -chitin (Chang et al. 2011). In general, the diffraction peak of clay corresponding to the d-spacing usually emerges at 2 = ~7° (Chen et al. 2014; Peng et al. 2016), which slightly decreased to 5.6° when 11

incorporated into chitin microspheres, suggesting the presence of intercalated clay layers throughout the polymer matrix. The structure of clay in the chitin matrix was also investigated with TEM. As shown in Fig. 3a, clay nanosheets were uniformly distributed in chitin microspheres. More importantly, the intercalated clay layers (Fig. 3b) could benefit the enhancement of their specific surface areas, which was a

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potential advantage for the adsorption of dye molecules. The porous properties of the CCMs were studied by measuring nitrogen adsorption isotherms (Fig. 2d). The

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Brunauer-Emmett-Teller (BET) surface area of CCM0 and CCM2 were determined to be 115.7 and 55.2 m2 g-1, respectively. Although there was a decrease in the specific

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surface area after incorporating the clay nanosheets into chitin matrix, the formed

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CCMs could still provide accessible channels for MB immigration according to our

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following experimental results.

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Fig. 3. TEM images of the CCM2 under different magnification.

The presence of clay nanosheets in the CCMs was also confirmed by optical

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microscope. As shown in Fig. 4a, the CCM0 composed of chitin and water was translucent, while the CCM2 showed a black appearance, as clay nanosheets in the chitin microspheres decreased their transmittance (Fig. 4d). Both the CCM0 and CCM2 exhibited uniform spherical morphology with similar size; their diameters were 58.7 ± 6.8 m and 58.3 ± 7.6 m respectively (Fig. S3). The CCM0 displayed a 12

porous, weaved nanofiber structure (Fig. 4b), which was consistent with previous result (Duan et al. 2015). The nanofibers, with diameter of ~60 nm, formed via the assembly of chitin chains during the preparation of chitin/clay microspheres (Fig. 4c). Due to the incorporation of clay into the chitin matrices, the CCM2 showed much denser structure in comparison with CCM0 (Fig. 4e). Additionally, clay nanosheets

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embedded in the chitin matrix could be observed under SEM at higher magnification (Fig. 4f). The hierarchical structure of chitin/clay microsphere would benefit the

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adsorption of the MB, where the dye molecules could be adsorbed onto the surface of microsphere, then transferred to the chitin channels, and finally interact with the clay

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nanosheets as well as the chitin matrix.

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Fig. 4. Morphology of chitin/clay microspheres: Optical microscope images (a, d) and SEM images (b, c, e, f) of the CCM0 (a-c) and CCM2 (d-f) under different

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magnification.

3.2. Adsorption Kinetics. Prior to investigating the adsorption kinetic of the

CCMs, we first examined the adsorption capacity of the CCMs. Experimental results showed that the CCM2 with high clay content had a much stronger adsorption of MB than that of CCM1 or CCM0 (Fig. S4). Therefore, we chose the CCM2 to 13

systemically investigate its adsorption kinetic and isotherms. It is known that adsorption kinetics is important in the analysis of the adsorption process, and can depict the adsorption rate and control the contact time. Fig. 5a shows the adsorption amount of the MB (100 mg mL-1) on microspheres as a function of time at 30 °C. The MB adsorption onto the CCM2 increased rapidly in the first 30 min, and then the

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adsorption proceeded at a slow rate, and finally reached equilibrium 3 h later. The maximal adsorption capacity (Qe) was determined to be 130.5 mg g-1 after 20 h

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treatment. The adsorption kinetic of the MB onto the CCM2 was investigated with pseudo-first-order kinetic model:

(3)

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𝑄t = 𝑄e − 𝑄e 𝑒 −𝑘1 𝑡

where Qe (mg g-1) and Qt (mg g-1) are the adsorption capacities at equilibrium and at

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time t, respectively. k1 (h-1) is the rate constant of pseudo-first order adsorption. As

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shown in Fig. 5b, Qe and k1 were calculated to be 121.5 mg g-1 and 3.419 h-1, respectively, according the nonlinear curve fit of experimental data with pseudo-first

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order model (R2 = 0.9814).

The kinetic results were also analyzed by the pseudo-second-order kinetic model: 𝑄t

=

1 k2 𝑄e2

+

𝑡

(4)

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𝑡

𝑄e

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where t (h) is the adsorption time, Qt (mg g-1) and Qe (mg g-1) are the adsorption capacity at time t and equilibrium, respectively, and k2 (g mg-1 h-1) is the pseudosecond-order rate constant, where Qe (131.8 mg g-1) and k2 (0.033 g mg-1 h-1) were

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calculated by the plot of t/Qt versus t as shown in Fig. 5c. The calculated Qe was much close to the experimental Qe (130.5 mg g-1), and the correlation coefficients (R2) for the pseudo-second-order model was 0.9999 (Table S1), demonstrating that the pseudo-second-order adsorption model was superior to depict the adsorption process.

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Fig. 5. Adsorption kinetics of the MB onto CCM2. (a) Effect of contact time on the

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MB adsorption at room temperature (MB concentration = 100 mg mL-1). (b) Pseudo-

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first-order kinetic plot for the MB adsorption. (c) Pseudo-second-order kinetic plot for

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the MB adsorption. (d) Ultraviolet spectra of the MB solution during the adsorption

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process (Inset: appearances of MB solution (10 mg mL-1) before and after the CCM2

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treatment).

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UV-vis spectrum of the MB solution (10 mg g-1) was employed to monitor the adsorption process (Fig. 5d). The characteristic absorption band of the MB appeared

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at ~664 nm, and this peak intensity decreased dramatically when incubating the MB solution with the CCM2. After 20 min incubation, it was difficult to observe the absorption peak of the MB in the UV-vis spectrum, and the removal efficiency was

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calculated to be 99.99%. These results indicated that the CCM2 could efficiently remove MB even at a very low MB concentration of 10 mg g-1.

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Fig. 6. Adsorption isotherms of the MB onto the CCM2. (a) Adsorption isotherms of

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the MB onto the CCM2 at 303, 313, and 323K. (b) Langmuir adsorption isotherm

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plots for the MB adsorption. (c) Freundlich adsorption isotherm plots for the MB

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adsorption. (d) Van’t Hoff plot for the MB adsorption onto the CCM2.

3.3. Adsorption Isotherms. The adsorption behaviors of CCM2 were also

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evaluated by the adsorption isotherms and relevant parameters. Fig. 6a shows the

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adsorption isotherms of the MB onto the CCM2 at different temperatures (303, 313, and 323K). Qe values increased dramatically with the increase of the equilibrium

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concentration (Ce) of the dye solution. To clarify the effect of temperature on the sorption of the MB onto the CCM2, both the Langmuir and Freundlich isotherm models were adopted to describe the relationship between adsorption capacity of the

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CCM2 and the equilibrium concentration at 303, 313, and 323 K, respectively. The Langmuir isotherm adsorption model was expressed as eq. 5, 𝐶e 𝑄e

=

𝐶e 𝑄max

+

1

(5)

𝑄max 𝑏

where Qe (mg g-1) is the amount of adsorption at equilibrium, Ce (mg L-1) is the 16

equilibrium MB concentration, Qmax (mg g-1) is the maximum adsorption at monolayer coverage, and b (L mg-1) is the Langmuir adsorption equilibrium constant related to the free energy of adsorption, respectively. Qmax and b were calculated from the slope and intercept of Langmuir isotherm (Fig. 6b), and their values are listed in

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Table S2.

experiment and calculation.

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Table 1. Maximum adsorption capacity of composite microspheres obtained by

Qe （mg gDiameter 1

Dye m

Refs

Exp./Cal.

~0.55

Methylene blue

50.7/64.24

(Fu et al. 2015)

Chitosan/activated carbon

200-400

Methyl orange

35.4/--

(Chen & He, 2017)

Fe3O4/Bi2S3

1.3-4.5

Congo red

92.24/90.58

(Zhu et al. 2017)

FHPC

2-20

Methyl orange

41.6/--

(Wang et al. 2008)

Fe3O4@MIL-100(Fe)

~0.36

Methylene blue

34.5/73.8

(Shao et al. 2016)

Activated carbon/CoFe2O4

--

81.94/89.29

(Ai et al. 2010)

Magnetic porous carbon

~0.07

Methylene blue

56.44 /61.65

(Zhang et al. 2016)

HAM@-AlOOH

~0.45

Methylene blue

87.80/--

NF/CF-BYs

--

Methylene blue

141.75/153.7

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PCPP

Malachite green

(Zhang, Ye, et al. 2016) (Du et al. 2017)

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）

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Microsphere a

a

PCPP: poly(cyclotriphosphazene-co-phloroglucinol); FHPC: Ferromagnetic

hierarchical porous carbon; HAM: hollow aluminosilica; NF: Nano-Fe3O4; CF-BYs:

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carboxyl-functionalized baker’s yeasts.

The Freundlich isotherm model is usually employed to depict the adsorption for dye from a liquid to a solid surface with heterogeneous multilayer adsorption mechanism, as shown in eq. 6, 17

1

ln𝑄e = ln 𝐾F + ln𝐶e

(6)

𝑛

where KF (mg g-1) is the Freundlich constant and n is the heterogeneity factor, which was determined by the intercept and slope of the lnQe versus lnCe linear plot. The MB adsorption on the CCM2 was fitted well by the Langmuir isotherm model with larger R2 values (>0.99) in comparison with the Freundlich isotherm model (Table S2),

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indicating that the MB was adsorbed onto the CCM2 via a monolayer adsorption. The Qmax values obtained by calculation and experiment were 152.2 and 156.7 mg g-1 at

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40 °C, respectively. The results reported in literature of other microspheres for MB

removal are summarized in Table 1. The Qmax value of the CCM2 developed in our

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work was higher than that of most reported microspheres, and comparable with that of

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NF/CF-BY composites, revealing that the hierarchical nanostructure of the CCMs led

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to their high adsorption capacity for MB in aqueous solution.

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Furthermore, the standard enthalpy (Ho), standard entropy (So), and Gibbs free

7 and 8. ∆𝑆 o 𝑅

−

∆𝐻 o 𝑅𝑇

(7)

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ln 𝐾d =

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energy (Go) for adsorption of the MB onto microspheres were also calculated by eqs

∆𝐺 o = ∆𝐻 o − T∆𝑆 o

(8)

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where Kd (L g-1), the distribution coefficient of absorbent at temperature T (K), is equal to Qe/Ce, and R (8.314 J mol-1 K-1) is the universal gas constant. Ho and So

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were determined by the slope and intercept of the linear plot of lnKd versus 1/T (Fig. 6d). The thermodynamic parameters for the adsorption of the MB onto the CCM2 are given in Table S3. The negative value of Ho (-26.92 kJ mol-1) indicated the exothermic nature of the adsorption process, while the negative value of So (-46.19 J mol-1) suggested a decrease in the degrees of freedom during the adsorption process. 18

Moreover, the value of Ho was less than 40 kJ mol-1, indicating that the adsorption was a physical process, which was also confirmed by the FT-IR results of the CCM2 before and after MB adsorption (Fig. S5). Furthermore, the value of the Gibbs energy changes (Go was -12.93, -12.47, -12.00 kJ mol-1 at 303, 313, and 323 K, respectively), suggesting that the MB adsorption process was spontaneous and

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thermodynamically favorable.

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3.4. pH Effects and Reusability. It is known that the adsorption efficiency of ionic adsorbents may be affected by the pH of dye solution (Yan et al. 2013).

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Therefore, the effect of initial solution pH on dye adsorption by the CCM2 was

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investigated at a pH range from 1.0 to 11.0 (Fig. 7a). Though the surface of clay

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nanosheets had negative charges, the Qe of microspheres changed slightly under

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removal was pH independent.

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various pH values, indicating that the adsorption behavior of the CCM2 for the MB

Fig. 7. (a) Effect of solution pH on the MB adsorption onto the CCM2 at room

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temperature, and (b) Qe values of the CCM2 after five consecutive adsorption/desorption cycles.

The reusability of adsorbents was performed by measuring the Qe values of the CCM2 during consecutive adsorption/desorption cycles (Fig. 7b). The Qe of original 19

CCM2 was 130.4 ± 2.5 mg g-1 and decreased to 105.6 ± 2.3 mg g-1 in the 2nd adsorption/desorption cycle. A possible reason was that a small amount of the MB molecules could not completely desorb from adsorbent (Fig. S6). Although the residual dye partially occupied the adsorption sites of the CCM2, its Qe maintained around 105 mg g-1 in the 2nd to 4th cycles. In the 5th cycle, there was a slight decrease

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in the Qe (98.6 ± 2.6 mg g-1). However, its removal efficiency for the MB solution (10

and good reusability of the CCM2 prepared in this work.

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4. CONCLUSIONS

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mg g-1) was still as high as 99%. These results clearly demonstrated the high stability

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We have successfully developed chitin/clay microspheres (CCMs) with

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hierarchical structure by a thermally induced sol-gel transition. Morphology

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observation showed that the clay nanosheets were embedded into the chitin weaved matrix and thus endowed high specific surface area, which provided efficient channels

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for dye adsorption. These microspheres could rapidly and completely remove the MB from aqueous solution at a low concentration (10 mg g-1). The adsorption of the MB

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onto microspheres was a spontaneous monolayer adsorption through a physical process, which fit the pseudo-second-order and Langmuir isotherm models.

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Furthermore, these chitin/clay microspheres exhibited high stability during 5 adsorption/desorption cycles. Along the design concept demonstrated herein, many

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other adsorbents with hierarchical structure could be developed from low cost raw materials for highly efficient removal of organic dyes.

Acknowledgements This work was supported by the National Natural Science Foundation of China 20

(21304021, 51703174), Hubei Province Science Foundation for Youths (2015CFB499, 2017CFB154), Jiangsu Province Science Foundation for Youths (BK20150382), Pearl River S&T Nova Program of Guangzhou (201506010101), Guangzhou Science and Technology Project (201510010221), the Fundamental Research Funds for the Central Universities (2042015kf0028), and Open Foundation

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of Hubei Key Laboratory of Novel Reactor and Green Chemical Technology (Wuhan

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Institute of Technology) (201713)..

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