Preparation and adsorption properties of citrate-crosslinked chitosan salt microspheres by microwave assisted method

Journal Pre-proofs Preparation and Adsorption Properties of Citrate-Crosslinked Chitosan Salt Microspheres by Microwave Assisted Method Nan Zhang, Huanhuan Zhang, Rong Li, Yanjun Xing PII: DOI: Reference:

S0141-8130(19)36021-0 https://doi.org/10.1016/j.ijbiomac.2019.10.203 BIOMAC 13704

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International Journal of Biological Macromolecules

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4 August 2019 13 October 2019 23 October 2019

Please cite this article as: N. Zhang, H. Zhang, R. Li, Y. Xing, Preparation and Adsorption Properties of CitrateCrosslinked Chitosan Salt Microspheres by Microwave Assisted Method, International Journal of Biological Macromolecules (2019), doi: https://doi.org/10.1016/j.ijbiomac.2019.10.203

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Preparation and Adsorption Properties of Citrate-Crosslinked Chitosan Salt Microspheres by Microwave Assisted Method

Nan ZHANGa, Huanhuan ZHANGb, Rong LIc, Yanjun XINGa,*

a

Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of

Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China. b

c

Shanghai Institute of Quality Inspection and Technical Research, Shanghai, 200233, China National Engineering Research Center for Dyeing and Finishing of Textiles, Donghua

University, Shanghai, 201620, China *Corresponding author. E-mail address: [email protected] (Y. J. Xing)

Abstract: Citrate-crosslinked chitosan salt (CSC) microspheres were prepared using chitosan as raw material and citric acid as salifying agent by microwave assisted heating method. The product was characterized by XRD, SEM, NMR, TG, FT-IR, GPC and BET. The citric acid solution concentration, microwave reaction temperature and microwave reaction time showed influence on the morphology and size of chitosan citrate salt microspheres. The prepared chitosan citrate salt microsphere showed a high-efficiency adsorption ability to remove Cr(VI) in water. The best adsorbent CSC-2-1H had an ability to absorb Cr(VI) of 172 mgg-1 and reached adsorption equilibrium at 30 min. The adsorption equilibrium data of

CSC-2-1H were analyzed with Langmuir and Freundlich adsorption models. The adsorption kinetics and adsorption thermodynamics of Cr (VI) on CSC-2-1H was also analyzed using the pseudo-first-order model, pseudo-second-order model, van't Hoff equation and Arrhenius equation, respectively.

Key Words: Chitosan microsphere; ionic crosslinking; citrate; Cr(VI); isotherm; kinetics

1. Introduction Since chitosan can react with ions in aqueous solution via hydrogen bonds, coordination bonds or covalent bonds through free amino groups and hydroxyl groups, it has been used in anionic water pollution treatment, such as Cr(VI) [1,2]. However, the small specific surface area, low porosity and poor acid stability always reduced the adsorption ability of chitosan. Chitosan has been modified by physical or chemical method, such as crosslinking method [3,4], to improve not only the physicochemical properties but also the adsorption capacity of chitosan [5-7]. The adsorption capacity of chitosan undoubtedly depended on not only the crosslinker but also the crosslinking type of chitosan [8]. The organic acid-crosslinked chitosan has been also used to remove heavy metal, such as, Cr(VI), Cu(II) and Pb(II) in water pollution [9-14]. For organic acid crosslinker, the crosslinking process of chitosan could be processed via ionic or covalent bonding between acid and hydroxyl group or amino group [8]. The reported

crosslinked chitosan using acid as crosslinker always formed covalent amido group between amino and acid, such as succinic acid, malic acid, tartaric acid, citric acid and oxalic acid [4,11,12,15-17]. The crosslinked chitosan by amido bond formed between citric acid and amino group in chitosan had been also reported to show an adsorption capacity for Cr(VI) [4,18]. However, due to the steric hindrance caused by the covalent crosslinking formed between -NH2 groups of chitosan and acid, anionic Cr(VI) can not easily access to the amine groups in adsorbate. Although further chemical modification was used to form quaternary ammonium cation in covalent-crosslinked chitosan chain, the adsorption capacities were always not improved significant [8,19,20-22]. Moreover, since the amido group is a covalent bond, the formation of amido group always need a long time for reaction [4,15,16], catalyst [4] or high reaction temperature (~130 C) [16]. These also hindered the mass application of chitosan. The ionic crosslinked chitosan was also a typical adsorbent for Cr(VI) using the electrostatic attraction between the positive quaternary ammonium cation in chitosan and the negative chromate anion [2,3,23]. In order to avoid secondary pollution and reduce costs, it was proposed that the environmental friendly polycarboxylic acid, such as citric acid, could used as the crosslinking agent for chitosan and multidentate chelating agent for metal cations [11-14]. However, to the best of our knowledge, there is no quaternary ammonium salt microsphere based on crosslinked chitosan as adsorbent for Cr(VI), using citric acid as cross-linking agent [11-14]. In this paper, a simple, and convenient process to prepare a stable, porous and

spherical chitosan citrate salt microsphere as an ionic crosslinking agent under microwave heating condition was studied [3]. The adsorption capacity for Cr(VI) of chitosan citrate salt microsphere was studied from aqueous solution. The isotherms, kinetics and thermodynamics of adsorption of Cr(VI) by chitosan citrate salt microsphere was also investigated.

2. Experiment 2.1 Materials Low molecular weight chitosan (Mr 70 000, deacetylation grade 90%, Sinopharm Chemical Reagent Co., Ltd, China), citric acid and sodium hydroxide (≥99.5%, Pinghu

Chemical

Reagent

Company,

China),

potassium

dichromate

and

diphenylcarbazide (≥99.5%, Sinopharm Chemical Reagent Company, China), acetone (≥99.5%, Shanghai Yunli Economic and Trade Co., Ltd, China) were all analytical grade and used directly without further purification.

2.2 Preparation of chitosan citrate microspheres 1.0 g chitosan was added to a solution with certain concentration (1-3mol·L-1) of citric acid and the mixture was maintained at 70–90℃ at a certain time (1–3h) under microwave-assisted heating (MAS-3 type universal microwave synthesis instrument, Xinyi Microwave Chemical Technology Co., Ltd. input power 1360 W). After the solution was allowed to cool, the pH was adjust to 5 with 4mol·L-1 sodium hydroxide. The mixture was allowed to stand for 1 hour, and centrifuged

(TG1650-WS Centrifuge, Shanghai Lu Xiangyi Centrifuge Instrument Co., Ltd) at 8000 r/min for 5 min. The obtained sediments were washed by suspending in water. These two purification steps were repeated until pH=7. The products was freeze-dried for 12 hours. Chitosan citrate salt microspheres was named as CSC. According to the concentration of citric acid solution and reaction time, the product was named as CSC-x-yH, where x (x=1, 1.5, 2, 2.5, 3) means the concentration of citric acid, y (y=1, 1.5, 2, 2.5, 3) means the reaction time, and H means hour. Under the heating condition of oil bath at 90 ℃, 1g chitosan reacted with 100 mL 2mol·L-1 citric acid solution for 1h. After the pH of the solution was adjusted to 5, the product was filtered, washed and freeze-dried. The product was named as CSC-2-1H-O.

2.3 Characterization The XRD analysis of chitosan and CSC were performed using an X-ray diffractometer (Rigaku Max-2250PC, Japan). X-ray diffraction patterns of the samples were measured over the diffraction angle (2θ) range from 5° to 90° with a Cu Kα target at 40 kV and 200 mA. Solid state

13

C NMR spectra of the samples were carried out on an Avance 400

NMR spectrometer (Bruker, Switzerland). The samples were measured at room temperature. The recycle time was 4 s, and the spinning rate was 5 kHz. The XPS spectra were recorded for the samples and using an Escalab 250XI

spectrometer (Thermo Fischer, USA) equipped with a monochromated Al Kα X-ray source at 12.5 kV and under the current of 16 mA as operating conditions. The modified and unmodified samples were placed at the angle of 90° under the ultralow pressure of 8×10-10 Pa. XPS data were calibrated using the binding energy of C1s (284.6 eV) as the standard. The scanning electron microscopy analysis of chitosan and CSC is to analysis the shape and size. And the samples were mounted on aluminum stubs, sputter coated with gold-palladium and observed at 15 kV in a TM- 1000 SEM instrument (Hitachi, Japan), and the particle sizes were measured and calculated using the Nano measurer software. The thermal stability of CSC was tested using TG-DTG (Netzsch, Germany). The test was carried out in N2 atmosphere at a heating rate of 20°C/min and from 30°C to 900 °C. The special surface area and pore structure were calculated using the Brunauer–Emmett–Teller

(BET)

method

with

Autosorb-iQ

(Quantachrome,

America). The weight-average molecular weight (Mw) of samples were calculated using the Gel-Permeation-Chromatography (GPC) method with Waters GPC 1515 (Waters, America). The carboxyl content value was tested using conductivity meter (DDSJ-308A, Instrument Electric Scientific Instrument Co., Ltd, China). Record conductivity meter readings per 1 mL of sodium hydroxide standard solution [24].

The carboxyl content formula for CSC samples is as follows: 𝐷𝑆 = 203𝐶(𝑉2 − 𝑉1 )⁄[𝑚(1 − 𝑤) − 80𝐶(𝑉2 − 𝑉1 ) + 22𝐶(𝑉3 − 𝑉0 ) + 42𝐶(𝑉3 − 𝑉2 )] × 100%

where DS is carboxyl content value of samples. C is the concentration (mol·L-1) of sodium hydroxide standard solution, V1 (mL) is the volume of sodium hydroxide solution consumed by titration of excess hydrochloric acid, V2 (mL) is the total volume of sodium hydroxide solution consumed at the end of titration -CH2COOH, V3 (mL) is the total volume of sodium hydroxide solution consumed at the end point for titration -NH3+ and -NH2+CH2COO-, V0 (mL) is the volume of the sodium hydroxide solution consumed by the blank test hydrochloric acid, m is the mass (mg) of the sample, w is the water content (%) of the sample. 203 is the relative molecular mass of the chitin residue, 42 and 80 are the relative molecular masses of the acetyl group and the -CH2COONa group, respectively, and 22 is the difference in the relative molecular mass of the -CH2COONa and -CH2COOH groups [24].

2.4 Adsorption experiment The concentration of the Cr(VI) solution was tested at 540 nm using an ultraviolet spectrophotometer (UV-1800, Shimadzu Corporation, China). 0.01g CSC adsorbent was added to 50mL of 100mg·L-1 Cr(VI) solution at pH 5, and after a certain time, the supernatant was filtered. According to GB/T7467-1987, the absorbance of the CSC adsorption residue was tested by diphenylcarbazide spectrophotometry. The following formula is the calculation of the adsorption amount of Cr(VI) by CSC at different times:

𝑄𝑒 = [(𝐶0 − 𝐶𝑡 )⁄𝑊] × 𝑉

(1)

where Qe (mg·g-1) is the amount of adsorption of CSC on the Cr(VI) solution after adsorption for a certain period of time. C0 (mg·L-1) is the initial concentration of Cr(VI), and Ct(mg·L-1) is the concentration of Cr(VI) after CSC adsorbed Cr(VI) for a certain period of time, and W(g) is the mass of input CSC adsorbent, V (mL) is the volume of Cr (VI) solution.

2.5 Equilibrium and kinetic of adsorption In order to evaluate equilibrium adsorption data for adsorbing Cr (VI) from aqueous solution, Langmuir and Freundlich nonlinear isotherm models were applied. The Langmuir adsorption equation is a widely used monolayer adsorption model [25].The amount of adsorption is calculated according to the linearized Langmuir adsorption equation by the following formula: 𝑐𝑒 ⁄𝑞𝑒 = 1⁄𝑞0 𝑏 + 𝑐𝑒 ⁄𝑞0

(2)

where q0 is the theoretical maximum adsorption amount and b is the adsorption energy. The theoretical maximum adsorption amount and adsorption energy were obtained by plotting ce/qe on ce. Freundlich isotherm model describes adsorption on heterogeneous systems and the Freundlich linear isotherm adsorption equation is as follow: 𝑙𝑜𝑔𝑞𝑒 = 𝑙𝑜𝑔𝑘𝑓 + (1⁄𝑛) 𝑙𝑜𝑔𝑐𝑒

(3)

where kf and n are Freundlich constants, which represent theoretical adsorption capacity and adsorption strength, respectively. The value of theoretical adsorption

capacity and adsorption strength was obtained by plotting logqe on logce. In

this

experiment,

the

pseudo-first-order

kinetic

equation

and

the

pseudo-second-order kinetic equation were selected to fit the adsorption data of all CSC

adsorbents.

The

pseudo-first-order

kinetic

equation

and

the

pseudo-second-order kinetic equation can calculate the rate constant and evaluate the theoretical maximum adsorption amount. The most important point is to evaluate the adsorption mechanism of CSC samples. The pseudo-first-order kinetic equation is as follow: 𝑙𝑜𝑔(𝑞𝑒 − 𝑞𝑡 ) = 𝑙𝑜𝑔𝑞𝑒 − 𝑘1 𝑡⁄2.303

(4)

where qe and qt respectively represent the adsorption amount at the adsorption equilibrium and the adsorption amount at the time of adsorption, and k1 is a pseudo-first-order kinetic rate constant. k1 is obtained by plotting log(qe-qt) against t of adsorption curve. The pseudo-second-order kinetic equation is as follow: 𝑡⁄𝑞𝑡 = 1⁄𝑘2 𝑞𝑒 2 + 𝑡⁄𝑞𝑒

(5)

where k2 is the pseudo-second-order kinetic rate constant. qe and qt respectively represent the adsorption amount at the adsorption equilibrium and the adsorption amount at the time of adsorption t. The adsorption performance of CSC-2-1H was tested by using 20 mg·L-1, 40 mg·L-1, 60 mg·L-1, 80 mg·L-1, and 100 mg·L-1 Cr(VI) solution. The theoretical maximum adsorption amount and pseudo-second-order kinetic rate constant were obtained by plotting the t/qt on t of the adsorption curve.

2.6 Thermodynamic of adsorption In order to investigate the thermodynamic process (298K, 308K, 318K) of CSC samples in Cr(VI) solution, Gibbs free energy formula, van’t Hoff equation and the Arrhenius equation were used to investigate the entire adsorption process. According to reference [26], the Gibbs free energy formula is as follow: G=H-TS

(6)

van’t Hoff equation is as follow: 𝑙𝑛𝑘1 = ∆𝑆°⁄𝑅 − ∆𝐻°⁄𝑅𝑇

(7)

where k1 is the distribution coefficient. R is gas constant (8.314J/(K·mol)). ΔH° and ΔS° were obtained when lnk1 is plotted against 1/T at different temperatures. k1 can be obtained from the following equation: 𝑘1 = [(𝐶0 − 𝐶𝑒 )⁄𝐶𝑒 ] × 𝑉 ⁄𝑀

(8)

where C0 (mg·L-1) is initial concentration of Cr(VI) solution and Ce is equilibrium concentration after adsorption of CSC samples. V (L) is the volume of Cr(VI) solution and M(g) is the mass of CSC sample. The activation energy A of adsorption was obtained by the following Arrhenius equation: 𝑙𝑛𝑘2 = 𝑙𝑛𝐴 − 𝐸𝑎 ⁄𝑅𝑇

(9)

where k2 is the rate constant of pseudo-second-order kinetic(g·mg-1·min-1). Ea is the Arrhenius activation energy of adsorption and A is the Arrhenius equation constant.

3. Results and Discussion

3.1 Microwave degradation efficiency Chitosan citrate microspheres (CSC) were synthesized using chitosan and citric acid as raw materials by microwave-assisted heating and conventional heating method (oil bath). The microspheres of crosslinked chitosan citrate were formed via the intermolecular or intramolecular ionic bond between the protonated chitosan with multivalent counterions (citrate) [3]. The related reactions are shown as Fig. 1. Figure 1 The influence of microwave heating method was investigated by comparing with carboxyl content value and weight-average molecular weight (Mw) of samples prepared by microwave heating method and conventional heating method (oil bath), respectively. Although the carboxyl content value of CSC-2-1H (43.5%) was close to that of CSC-2-1H-O (42.2%), the carboxyl content value of CSC-2-2H (62.0%) was higher than to that of conventional heating product CSC-2-3H-O (48.2%). This indicated that the efficiency of microwave heating was significantly higher than the conventional heating. The weight-average molecular weight (Mw) of CSC-2-1H (Mw=18350 Da, PDI=1.03) by GPC was very lower than that of CSC-2-1H-O (Mw=54720 Da, PDI=1.25). This indicated that the microwave-assisted heating was also very effective for depolymerization of chitosan [27]. The efficiency could be due to the degradation mechanism and mechanical shear caused by molecular vibrations in microwave field [27].

3.2 XRD of CSC samples The XRD of CSC under different citric acid concentration and time condition was summarized in Fig. 2. The CSC microspheres showed different XRD spectra with chitosan. All the typical crystallization peak of unmodified chitosan at 10.7° and 20.9° couldn’t be found in the XRD spectra of CSC microspheres. Three new crystallization peak at 11.7°, 18.1° and 24.7° appeared in all CSC samples. This indicated the formation of crosslinking network in CSC microspheres [3]. The intensity of the main peaks (11.7°, 18.1°, 24.7°) increased with increasing of reaction time and concentration of citric acid. Samples CSC-2-1H-O, CSC-1-2H, CSC-1.5-2H, CSC-2-2H, CSC-2.5-2H, CSC-2-1H, CSC-2-1.5H and CSC-2-2.5H had a d-spacing value of 7.72, 7.06, 7.43, 7.60, 7.78, 7.41, 7.53 and 7.85Å at 2θ=10-11°, respectively. As compared with chitosan (d-spacing 8.05Å), the decrease of d-spacing in CSC could be due to the crosslinking of chitosan caused by citric acid [3]. Figure 2

3.3 13C NMR solid-state magnetic analysis of CSC samples. The solid-state

13

C NMR of chitosan and CSC-2-2H was shown in the Fig. 3.

Although the NMR peak of C1 at 105.3 ppm didn’t show a legible bimodal splitting, the position of C1 peak indicated that the reaction of chitosan and citric acid formed a typical type-II chitosan salt [3,28]. This also indicated microwave has no influence on the structure of chitosan chain in CSC. The change of positions of the peaks of

C2, C3, C4, C5, and C6 of the glucose unit of chitosan were 56.8 ppm, 74.6 ppm, 82.7 ppm, 75 ppm and 60.9 ppm also indicated the formation of salt [3,28]. The characteristic peak of the secondary carbon atom (C1’) in citrate was also observed at 44.3 ppm (at 46 ppm in Ref. [29]). However, the tertiary carbon atom (C2’) in citric acid at 75.0 ppm was enveloped by the strong peak of chitosan. A broad peak between 170–187 ppm with peak maximum at 179.42 ppm from the carboxyl carbon (C3’-C4’) of citric acid (in different chemical environments) also indicated the presence of citrate in CSC sample. The peak maximum at 179.42 ppm showed that the carboxyl carbon was main in the form of –C(=O)O- anion [29,30]. The presence of –C(=O)O- anion in NMR spectra indicated that the crosslinking network formed between amino group and citrate was the main driving force for the formation of microspheres. Figure 3

3.4 XPS of CSC samples The XPS analysis of CSC sample was shown in Fig. 4 and Table 1. The C1s of the CSC sample and chitosan resolved from XPS was shown in Fig. 4a. As compared with chitosan, the intensity of peak of contaminated C or C-C or C-H bond (285.0 eV) and peak of O-C-O bond (288.1 eV) in CSC-2-2H increased while the intensity of peak of C-O, C-N or C-OH bond (286.5 eV) decreased (Table 1). In the N 1s XPS spectra of chitosan and CSC-2-2H (Fig. 4b), the peak intensity of free amino group in CSC-2-2H (399.5 eV) showed a large decrease after reaction. A new

peak at 401.6 eV assigned to the ammonium form (-NH3+) was found in the deconvolution XPS spectrum of CSC-2-2H. The deconvolution XPS spectrum of O 1S (Fig. 4c) of CSC-2-2H showed that both the intensity of –C=O bond (531.0 eV) and C-OH bond (532.5 eV) increased, while the intensity of peak of C-O-C (532.9 eV) decreased. All the results indicated the presence of ionic bonding between ammonium group of chitosan and citrate [3]. Figure 4 Table 1

3.5 Analysis of thermal stability of CSC samples TG-DTG curve was given in Fig. 5. All investigated samples showed the loss of water from 30 °C. The high water loss of CSC samples (~10.5%, more than that of chitosan) indicated that CSC samples were more hydrophilic than chitosan, which was beneficial to the adsorption of Cr (VI). Different from the decomposition of chitosan, the polymer decomposition of CSC samples involving two degradation stages was observed over a wide temperature range, from 150 to 400 °C. The first stage occurred from 150 °C and the weight loss rate is ~18%. At this stage, the citrate anion in CSC samples began to degrade, accompanied by the melting and burning [31]. The maximum rate of weight loss occurred at 216 °C (Fig. 5b). The second stage of CSC samples began from ~274 °C and reached a maximum at ~310 °C with a weight loss percentage of ∼40%, while chitosan decomposed from ~275 °C and reached a maximum at ∼318 °C with a weight loss percentage of about

46.6%. According to the decomposition of chitosan, this stage in CSC samples could be attributed to the polymer decomposition of chitosan chain. However, the polymer decomposition of CSC samples in this stage didn’t show a strong exothermic process at the highest weight loss rate (Fig. 5b). This could be due to the incorporation of citric acid in chitosan [26]. Although the correspondent derivative curve of CSC-2-2H and CSC-2-2.5H after 400 °C both exhibited a long tail, CSC-2-2H showed a different decomposition process from that of CSC-2-2.5H. CSC-2-2H reached the zero value at 900 °C, while CSC-2-2.5H still remained 10.7%. The different residual rate could be due to the different size and surface area of CSC-2-2H and CSC-2-2.5H, which caused a different thermal decomposition [8]. Figure 5

3.6 Surface morphology of CSC samples The morphology of CSC samples prepared under different concentration of citric acid were investigated by SEM (Fig. 6). The morphologies of all CSC samples were different from that of chitosan [3]. It was shown that the concentration of citric acid had a great influence on the morphology of CSC microspheres. The results showed that all CSC samples showed a regular and uniform spherical shape and extremely narrow particle size distribution, except CSC-2.5-2H. However, a slight reunion phenomenon was observed in all CSC samples. The particle sizes of CSC-1-2H, CSC-1.5-2H, CSC-2-2H, CSC-2.5-2H and CSC-3-2H were 1.18 μm, 1.24 μm, 0.74 μm, 1.72 μm and 0.87 μm (particle size distribution was shown in Fig. S2) with the

relative uniform size distribution, respectively. The BET surface area of CSC-2-1H and CSC-2-2H were 9.92 m2·g-1 and 17.48 m2·g-1, respectively, whereas that of chitosan was 4.35 m2·g-1 and CSC-2-1H-O was 3.72 m2·g-1. Although a significant adsorptive specific surface area was not observed, the larged the adsorptive specific surface area of CSC samples, which could further Figure 6

The influence of microwave reaction temperature on the morphology of CSC microspheres was shown in Fig. 7. It can be seen that the formed CSC particles were irregular microsphere when the temperature were lower than 90°C. Figure 7

The influence of microwave reaction time on the morphology of CSC microspheres was also investigated by SEM (Fig. 8). When the reaction time is 1 h, a hollow bulky material was observed. With the time increased, the spherical shape was more clear and smooth in surface. However, the reunion phenomenon was also more severe after the reaction time reached 3h. The particle sizes of CSC-2-1H, CSC-2-1.5H, CSC-2-2H, CSC-2-2.5H and CSC-2-3H are 1.31 μm, 0.81 μm, 0.74 μm, 2.56 μm and 2.67 μm (particle size distribution was shown in Fig. S3), respectively. Since the depolymerization and salinification of chitosan under microwave took place at the same time, the particle size of CSC samples decreased

first and increased subsequently with the increase of reaction time. The smallest particle size of CSC-2-2H also indicated that the complete depolymerization of chitosan could be reached in 2 hrs. The SEM image of CSC-2-1H-O (Fig. 8f) was similar to that of unmodified chitosan with compact structure in bulky form (Fig. 6f) except the smooth surface of CSC-2-1H-O. However, no particle was found in the SEM image of CSC-2-1H-O. This indicated that the depolymerization of chitosan didn't took place (or in a very slow rate) and the salinification with citric acid was main reaction under conventional heating method. Figure 8

3.7 Adsorption of Cr (VI) by CSC Samples Since the pH of the solution affected the surface charge of the adsorbent, the degree of ionization and the speciation of the adsorbate species, the influence of pH on adsorption capacity of Cr (VI) of CSC-2-1H was first investigated at pH 3~5 (Fig. 9). The adsorption capacity of CSC-2-1H at pH 3, 4 and 5 was 134 mg·g-1, 145 mg·g-1 and 172 mg·g-1, respectively. As shown in Fig. 9, the adsorption capacity of CSC-2-1H increased with the increase of pH value. The results also showed that the adsorption of Cr (VI) of CSC-2-1H was more favorable and efficient at pH 5, which was benefit for the formation of the negatively charged Cr(VI) stable complexes (HCrO4−, etc) ions [32] and their electrostatic interaction with positive –NH3+ in CSC-2-1H. Since the maximum adsorption of Cr (VI) of CSC-2-1H occurred at pH

5, and no CSC product was obtained at pH>5 under microwave heating method, the following adsorption capacity of Cr (VI) by CSC samples was investigated at pH 5. Figure 9 All CSC samples were used to adsorb Cr (VI) with uniform concentration. The adsorption capacity of CSC samples can be seen from the time-lapse curve of 0-40 min in Fig. 10a. When the concentration of Cr(VI) solution was 100 mg·L-1, CSC-2-1H, CSC-2-1.5H, CSC-2-2H, CSC-2-2.5H and CSC-2-3H were used as adsorbent in an amount of 0.2 g·L-1. It can be seen from the adsorption curve that all CSC samples were highly efficient adsorbent, and reached adsorption equilibrium in 30 min. The adsorption capacity of CSC-2-1H, CSC-2-1.5H, CSC-2-2H, CSC-2-2.5H and CSC-2-3H was 172 mg·g-1, 161 mg·g-1, 146 mg·g-1, 138 mg·g-1 and 122 mg·g-1, respectively. The adsorption capacity of CSC sample was higher than that of unmodified chitosan (55 mg·g-1 after 30 min and 81 mg·g-1 after 150 min, Fig. 10b). The adsorption capacity of CSC samples decreased with the increase of crosslinking degree. CSC-2-1H showed the best adsorption capacity. The result also indicated that the crosslinking degree of CSC largely affected Cr(VI) adsorption behavior of CSC samples. Figure 10 As can be seen from Fig. 11, the maximum adsorption amount of CSC-2-1H increased with the increase of initial concentration of Cr(VI) solution at pH 5 and at the adsorption equilibrium time (30 min). When the initial concentration of Cr(VI) increased from 20 mg·L-1 to 220 mg·L-1, the maximum adsorption amount of

CSC-2-1H also changed from 87 mg·g-1 to 176 mg·g-1. When the initial concentration was more than 100 mg·g-1, the change of maximum adsorption amount (Qe) of CSC-2-1H was negligible. Figure 11 The conventional heating product CSC-2-1H-O also showed an efficient adsorption and reached adsorption equilibrium in 30 min (Fig. S8). The adsorption capacity of CSC-2-1H-O was 115 mg·g-1, which was lower than that of CSC-2-1H. The result indicated that microwave assisted heating has a significant influence on the adsorption capacity of CSC samples. This could be due to the effective depolymerization and salinification of microwave on chitosan. However, the adsorption capacity of CSC-2-1H-O was still higher than that of unmodified chitosan. This also indicated the introduction of citric acid could improve the adsorption capacity of chitosan.

3.8 Adsorption isotherms In order to study the isothermal adsorption effect of CSC samples, Langmiur adsorption model and Freundlich adsorption model were used for data fitting (Fig. S5). The fitted experimental parameters of the adsorption model of CSC-2-1H were listed in Table 2. The result showed that the regression fitting value of Langmuir adsorption model (0.978) was bigger than that of Freundlich model (0.864). This indicated that Langmuir adsorption model was more suitable for the adsorption process of CSC samples [33]. The theoretical maximum monolayer adsorption value

(q0) obtained from Langmuir adsorption model was 200 mgg-1, which was close to the experimental data 172 mgg-1. Table 2 The adsorption model of CSC-2-1H-O was also fitted by Langmuir adsorption model and Freundlich model (Fig. S9 and Table S2). The results indicated that Langmuir adsorption model was also more suitable for the adsorption process of CSC-2-1H-O. This indicated that the microwave assisted heating method could only influence the adsorption capacity instead of the adsorption isotherm. The monolayer adsorption q0 (161 mgg-1) of CSC-2-1H-O from Langmuir model was lower than that of CSC-2-1H. Since high specific surface area would provide more active site for Cr(VI) adsorption, CSC-2-1H (BET surface area 9.92 m2·g-1) showed more adsorption efficiency for Cr(VI) than CSC-2-1H-O (BET surface area 3.72 m2·g-1).

3.9 Adsorption kinetics Adsorption kinetics can well evaluate the adsorption process of adsorbents and was always required for selecting the optimum operating conditions for a pilot-scale process. The adsorption kinetics could also reflect the type and surface concentration of the active sites and the surface area of absorbent [34]. In order to study the adsorption mechanism of CSC samples and the adsorption process of CSC-2-1H, the pseudo-first-order kinetic equation (Fig. S6a) and the pseudo-second-order kinetic equation (Fig. S6b) were used to express the entire adsorption process of CSC-2-1H [35]. The rate constant (k1 or k2) and the linear

correlation coefficient (R2) under different Cr (VI) concentrations were all listed in Table 3. The fact that small lineal regression coefficients from the pseudo-first-order kinetic model and the significant difference between the calculated and experimental qe values all indicated the first order model didn’t fit the experimental data of Cr (VI) by CSC samples. The high linear regression coefficients of pseudo-second-order kinetic model (R2 > 0.99) at all initial Cr (VI) concentrations indicated that the pseudo-second-order kinetic model was more suitable for the adsorption kinetics of CSC samples for Cr (VI). The results indicated that the adsorption site occupancy of the surface of all samples are proportional to the square of the unoccupied active sites and the adsorbents are bound at different sites and no mass loss of adsorbent during adsorption particularly. Table 3

3.10 Adsorption thermodynamics In order to evaluate the thermodynamic process of adsorption of Cr(VI) by CSC samples, the experiment used van't Hoff equation and Arrhenius equation to explore the whole adsorption process of CSC-2-1H (Fig. S7). The thermodynamic parameters ΔH, ΔG and ΔS at different temperature (298 to 318 K) in Table 4 indicated that the adsorption capacity of CSC-2-1H decreased with increasing temperature. The negative ΔH implied that the adsorption process of CSC-2-1H was an exothermic chemical adsorption [4]. The negative ΔS suggested that the

adsorption process the solid-liquid surface of CSC-2-1H in Cr(VI) solution is enthalpy driven. The formation of an activated complex between Cr(VI) anion and CSC-2-1H through electrostatic interaction decreased the disorder at the solid/liquid interface. The negative ΔS also reflected that the internal structure of CSC-2-1H has no significant change during the adsorption process. ΔG were found to be negative below 298 K for adsorption of Cr(VI) onto CSC-2-1H, which indicated the feasibility and spontaneity of the adsorption processed at temperatures below 298 K. The increase of ΔG with an increase in temperature implied that lower temperature made the adsorption of Cr(VI) easier [4]. The regression coefficient of van’t Hoff equation and the Arrhenius equation were 0.993 and 0.997, respectively. As shown in Table 4, the adsorption activation energy value of CSC-2-1H was 42.03 kJmol-1, which indicated that the adsorption process of Cr(VI) onto CSC-2-1H mainly followed the chemical adsorption mechanism. Table 4

4. Conclusion Under the microwave condition, an uniform shape and size multi-layer chitosan citric salt microsphere was prepared. This experiment demonstrated that the morphology and size of chitosan citrate salt (CSC) microsphere changed with the temperature, time of microwave heating and the concentration of citric acid. The adsorption process of CSC-2-1H, CSC-2-1.5H, CSC-2-2H, CSC-2-2.5H and CSC-2-3H samples in Cr(VI) showed that all CSC samples can absorb Cr(VI) in aqueous solution. The best adsorption capacity of CSC-2-1H can reach 172 mgg-1.

The adsorption kinetics process of chitosan citrate salt microspheres can be completely expressed by Langmuir adsorption model with a correlation coefficient of 0.978 and pseudo-second-order kinetics with a correlation coefficient of 0.992~0.999. According to van’t Hoff equation and Arrhenius equation, the adsorption thermodynamics process of chitosan citrate salt microspheres mainly followed chemical adsorption mechanism and the adsorption capacity of CSC samples decreased with increasing temperature.

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Figure 1

Figure 1 Reaction equation diagram of chitosan and citric acid

Figure 2

Figure 2 X-ray diffractograms of CSC samples

Figure 3

Figure 3 CP-MAS 13C NMR spectra of chitosan and CSC-2-2H sample

Figure 4

Figure 4 XPS of chitosan and CSC-2-2H sample

Figure 5

Figure 5 Thermogravimetric analysis diagram of chitosan, CSC-2-2H and CSC-2-2.5H

Figure 6

Figure 6 SEM images of CSC prepared under different concentration of citric acid. a) CSC-1-2H, b) CSC-1.5-2H, c) CSC-2-2H, d) CSC-2.5-2H, e) CSC-3-2H, f) chitosan

Figure 7

Figure 7 SEM images of CSC-2-2H prepared at a) 70℃, b) 80℃ and c) 90℃.

Figure 8

Figure 8 SEM of CSC prepared with different time under microwave heating and oil at 90℃. a) CSC-2-1H, b) CSC-2-1.5H, c) CSC-2-2H, d) CSC-2-2.5H, e) CSC-2-3H, f) CSC-2-1H-O

Figure 9

Figure 9 Adsorption capacity of CSC-2-1H at pH=3, 4 and 5

Figure 10

Figure 10 Cr(VI) adsorption of a) CSC samples and b) chitosan.

Figure 11

Figure 11 Adsorption of CSC-2-1H for Cr(VI) at different initial concentrations of Cr(VI) and time of adsorption is 30 min

Table 1 XPS analysis of chitosan and CSC samples: surface concentration (mole fraction with respect to all elements except hydrogen, %) of elements and of species defined by the binding energy of peak components C

N

intensity total

Chitosan CSC-2-2H

BE (eV) Atomic (%)

59.87

BE (eV) Atomic (%)

61.85

O

intensity

O–C–O

C–O/C–N /C–OH

C–C/C–H

287.8

286.3

284.8

9.80

33.96

16.11

288.1

286.5

285.0

12.43

24.43

24.99

total

7.95 5.80

intensity

-NH2

amide

-NH3+

399.2

400.3

-

6.59

1.36

-

399.5

-

401.6

3.05

-

2.75

total

31.89 32.34

C–O–C

C–O–H

C=O

532.8

532.5

531.3

20.37

9.22

2.30

532.9

532.5

531.0

14.33

14.60

3.41

Table 2 Experimental parameters of the Langmuir adsorption model and Freundlich adsorption model of CSC-2-1H. q0 (mg·g-1)

B (L·mg-1)

R2

200

0.032

0.978

kf (mg·g-1)(L·mg-1)1/n

n

R2

3.96

2.4

0.864

Langmuir

Freundlich

Table 3 Experimental parameters of pseudo-first-order and pseudo-second-order kinetics model of CSC-2-1H Concentration (mg·L-1)

qeexp (mg·g-1)

20

Pseudo-first-order

Pseudo-second-order

qe (mg·g-1)

k1 (min-1)

R2

qe (mg·g-1)

k2 (g·mg-1·min-1)

R2

90

111

1.7×10-1

0.8562

97

2.8×10-3

0.9921

40

112

59

1.1×10-1

0.9228

103

2.6×10-3

0.9987

60

125

63

1.4×10-1

0.8608

121

3.3×10-3

0.9995

80

146

122

1.2×10-1

0.8801

139

1.3×10-3

0.9958

100

178

224

1.1×10-1

0.8179

169

5.6×10-4

0.9939

Table 4 Experimental parameters of CSC-2-1H in van’t Hoff equation and Arrhenius equation van’t Hoff Equation T(K)

K1(Lg-1)

G (kJmol-1)

298

2.57

-2.46

308

0.74

0.65

318

0.26

3.77

Arrhenius Equation

H (kJmol-1)

S (Jmol-1K-1)

R2

Ea(kJmol-1)

R2

-95.23

-311.31

0.993

42.03

0.997

Highlights 1. A simple preparation of chitosan citrate salt microspheres by microwave method was proposed. 2. CSC spheres with uniform spherical shape and narrow particle size distribution was obtained. 3. Adsorption isotherm, kinetic, thermodynamic model of Cr(VI) adsorption on CSC was studied.