acrylic acid cellulose hydrogels for the adsorption of heavy metal ions

Accepted Manuscript Title: Preparation of Acrylamide/Acrylic Acid Cellulose Hydrogels for the Adsorption of Heavy Metal Ions Authors: Binchan Zhao, Huabin Jiang, Zongkun Lin, Shaofan Xu, Jun Xie, Aiping Zhang PII: DOI: Article Number:

S0144-8617(19)30689-7 https://doi.org/10.1016/j.carbpol.2019.115022 115022

Reference:

CARP 115022

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

4 May 2019 21 June 2019 23 June 2019

Please cite this article as: Zhao B, Jiang H, Lin Z, Xu S, Xie J, Zhang A, Preparation of Acrylamide/Acrylic Acid Cellulose Hydrogels for the Adsorption of Heavy Metal Ions, Carbohydrate Polymers (2019), https://doi.org/10.1016/j.carbpol.2019.115022 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.

Preparation of Acrylamide/Acrylic Acid Cellulose Hydrogels for the Adsorption of Heavy Metal Ions

a

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Binchan Zhaoa, Huabin Jianga, Zongkun Lina, Shaofan Xua, Jun Xiea, Aiping Zhanga,b* College of Forestry and Landscape Architecture, South China Agricultural University,

b

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Guangzhou 510642, P. R. China

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and

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Development, Guangzhou 510640, China

*

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Corresponding authors: [email protected] (A.P. Zhang)

Hydrogels synthesized from microcrystalline cellulose (MCC) and acrylamide

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

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Highlights

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(AAm) and acrylic acid (AA). 

Synergy between AA, AAm and MCC enhanced adsorption of Cu (II), Pb (II)

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and Cd (II).



The metal ions attached to functional groups and formed chelate compounds.

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Abstract

Modified cellulose hydrogels were prepared by blending and cross-linking with acrylamide and acrylic acid. The structure of hydrogels was characterized and analyzed

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with fourier transform infrared spectroscopy (FTIR), X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA).

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Under the optimized conditions, the maximum absorption capacity in modified cellulose hydrogels of Cu (II), Pb (II) and Cd (II) ions were 157.51, 393.28 and 289.97

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mg/g, respectively. In addition, the metal ion adsorption process accorded with pseudo-

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second-order rate equation and Langmuir adsorption isotherm. Based on the

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microstructure analysis and adsorption kinetics, the adsorption mechanisms such as

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physical, chemical, and electrostatic interactions are discussed. The adsorption process

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was controlled by the ion-exchange mechanism.

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Keywords: Modified cellulose hydrogels; Heavy metal ions; Adsorption isotherm;

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Adsorption kinetics

1. Introduction

As a consequence of the rapid development of industrialization and urbanization, large amounts of heavy metal ions discharge into the environment such as water, air

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and soil, which poses a severe hazard to human health and living organisms (Huang et al., 2018; Du et al., 2014). Environmental and safety regulations which severely limit concentrations of these heavy metal ions that can be released into the environment to potentially impact drinking water criterion, control water pollution, and impact water

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purification have resulted in extensive attention to mechanisms to mitigate these

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environmental impacts (Elwakeel et al., 2018; Luo et al., 2009). At present, the main treatment technologies that have been used for metal ion reduction from wastewater, are processes such as ion exchange, chemical precipitation, flocculation, coagulation,

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membrane separation, reverse osmosis and adsorption (Fakhre et al., 2018). Compared

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with other methods, adsorption is one of the superior physicochemical methods for

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sewage disposal, which has several advantages of high removal efficiency, lower costs, no chemical sludge and easy accessibility (Kong et al., 2018). Developing an efficient

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bio-based material to eliminate the heavy metals from water is necessary.

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Chelation ion exchange means that chelating polymers and heavy metal ions are

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bonding with coordination bond, which has the advantage of making harmless ions entering the environment, while only removes toxic metal ion (Mohamed et al., 2017). Therefore, chelation ion exchange method in wastewater treatment has attached

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increasing attention. Cellulose, as natural macromolecular compound and one of the most plentiful

renewable resources in nature, is the main source of wood, bamboo, cotton, reed, straw and other higher plants fibers. It is a natural polysaccharide that consists of a linear 3

polymer of β-D-glucose linked by β-1,4 glycosidic linkages, which is used in many fields to deal with various kinds of pollution (Candido et al., 2017; Hokkanen et al., 2016). Raw cellulosic material derived from plants has low adsorption capacity. Therefore, chemical or physical modification of cellulose must occur to generate

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efficient adsorption capability. In addition, many properties of cellulose such as its

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hydrophilic or hydrophobic character, elasticity, thermostability, resistance to microorganisms and hygroscopicity can be changed by chemical or physical

modification of the basic cellulosic polymer (O'Connell et al., 2008). In recent years,

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chemical modification mainly includes the following three aspects. The first approach

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is focused on cellulose-based composites with organic polymers or hybrid materials

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(Sun et al., 2018). Another consists in the direct chemical functionalization with the hydroxyl groups on the cellulose (Zhou et al., 2014). The last strategy involves grafting

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copolymerization adsorption groups onto the cellulose backbone (Nongbe et al., 2018).

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The performance of modified cellulose is also greatly dependent on the adsorption

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groups. In accordance with graft copolymerization, adsorption groups such as carboxy, amino, phosphate, sulfonic and amide (Li et al., 2018), are favored by adsorption of metal ions because of complexation and electrostatic interaction (Ding et al., 2018).

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Fakhre et al. (2018) modified cellulose with ethers to remove Cd(II), Zn(II), Ni(II), Pb(II) and Cu(II) ions from aqueous solutions. Jilal et al. (2018) was grafting of EDTA on hydroxyethyl cellulose (HEC) to remove Pb (II) and Cu (II) from aqueous solutions. The purpose of this work was to develop cellulose-graft-poly (acrylic acid (AA)4

co-acrylamide (AM)) cellulose composite hydrogels and study its adsorption characteristics as a bioadsorbant for the removal of Cu (II), Pb (II) and Cd (II) from aqueous solutions. The structure and morphology of cellulose composite hydrogels were characterized with fourier transform infrared spectroscopy (FTIR), scanning

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electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetric analysis

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(TGA). Also, adsorption conditions such as ratios of reactants, solution pH and the effects of ionic strength were optimized. Through the uptake kinetics, equilibrium

adsorption isotherms, the sorption properties of the sorbent were evaluated, and the

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sorbent recycling and metal desorption were discussed.

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2. Experimental section

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2.1. Materials

Microcrystalline cellulose (MCC) was obtained from Aladdin Industrial

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Corporation (Shanghai, China). AM, AA, ammonium persulfate (APS) and N,N’-

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methylene bisacrylamide (MBA) were purchased from Shantou Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Copper nitrate hydrate (Cu(NO3) 2·3H2O), lead

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nitrate (Pb(NO3)2), and cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O), were acquired from Macklin Inc. (Shanghai, China). All other chemical reagents were of analytical grade and used directly without further purification.

2.2. Preparation of MCC -g-poly (AA-co-AM) hydrogels

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MCC (1.5 g) was dispersed in 30 mL distilled water by magnetic stirring (200 rmp), in a 250 mL three-necked flask equipped with the nitrogen line, a reflux condenser, and a thermometer, and the prepared solution were purged with nitrogen to remove dissolved oxygen. With stirring at 50 ℃ for 15 min, APS (0.3 g) was added into the

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solution to generate hydroxyl radicals. Then, the mixed solution (the neutralization

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degree of AA (9 mL) was 80%, AM (0.3 g), and MBA (0.1 g)) was slowly added into three-necked flask with continuous stirring for 15 min. Thereafter, the mixture was

stirred at 50 ℃ for 2 h to complete the polymerization. The entire process was shown in

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Fig. 1. After the reaction, the samples were washed excessively with anhydrous ethanol

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to remove unreacted reagents and byproducts and then dried in an oven under the

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reduced pressure at 60 ℃. The dried samples were milled and screened to collect the

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2.3. Characterization

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desired portions pass 30 mesh for further analysis.

The functional groups of MCC and MCC-g-(AA-co-AM) were characterized with

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FTIR (VERTEX 70, Bruker). By using a KBr disk containing 1% finely ground samples, the spectrum was collected from 4000 cm-1 to 400 cm-1.

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The crystal structure of samples was measured with XRD (D8-Advance, Bruker).

With Cu Kα radiation (λ = 0.15406 nm) generated at 40 kV and 30 mA, XRD spectra were obtained at a rate of 5 °C/min from 10° to 80°. The surface morphology and structure of the samples were observed by using

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SEM-EDS (XL-30-ESEM, Netherlands FEI) with an accelerating voltage of 5 kV. The samples were mounted on an aluminum stub by using double sided tape and coated with a thin gold layer. The thermal stability of the powered samples was investigated by TGA on the STA

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409 PC thermal analyzer (NETZSCH, Germany). Approximately 10 mg samples were

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heated from 20 ℃ to 600 ℃ at a heating rate of 15 ℃/min under N2 atmosphere.

2.4. Adsorption experiments

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Batch sorption experiments of Cu (II), Pb (II), and Cd (II) on the modified

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cellulose hydrogels were studied at ambient temperature (about 27 ℃) by adding 50 mg

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dry samples to 100 mL conical flasks containing 50 mL heavy metal aqueous solution. The conical flasks were sealed and agitated in the constant-temperature oscillator

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(agitation speed 200 rmp) and solution pH was adjusted by adding appropriate

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NH3·H2O and HCl solutions. The influence of pH on metal adsorption varied from 1.0 to 6.0 was studied. For the adsorption kinetics experiment, the contact time changed

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from 10 s to 40 min. For the sorption isotherms experiment, the metal ion concentration ranged from 10 to 800 mg/L. After adsorption, the residual concentration of heavy

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metal ion in the bottle was measured by Atomic Absorption Spectroscopy (AAS) (AA7000, Shimadzu). The adsorption capacity of the hydrogels at equilibrium (qe, mg/g) and the amount of Cu (II), Pb (II), Cd (II) ions adsorbed at time t (qt, mg/g) were determined by the

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following equations (Dai et al., 2018): 𝑞𝑒 =

(𝐶0 −𝐶𝑒 )𝑉

𝑞𝑡 =

(𝐶0 −𝐶𝑡 )𝑉

(1)

𝑊

(2)

𝑊

where qe (mg/g) is the amount adsorbed at equilibrium, qt (mg/g) is the amount

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adsorbed at specific time, C0 (mg/L) is the initial concentration, Ce (mg/L) is the

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equilibrium concentration, Ct (mg/L) is the concentrations of heavy metal solution at

the time t (h), V (mL) is the volume of heavy metal ion solution, W (mg) is the dosage

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of dried hydrogels.

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2.5. Desorption and regeneration

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The metal ions-loaded adsorbent was added 30 mL of 0.1 mol/L HCl and stirred (200 rpm) for 1 h. The hydrogels were washed thoroughly with deionized water and

regeneration

performances

were

examined

for

five

times

by

the

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the

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reused for the next adsorption processes. According to the method as described in above,

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adsorption/desorption cycles, each experiment with the same heavy metal ions solution.

3. Results and discussion

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3.1. Structural characterization and analysis of MCC-g-(AA-co-AM)

3.1.1.

FTIR analysis

The various functional groups in the modified cellulose hydrogels were measured with FTIR, as shown in Fig. 2(a). The FTIR spectrum of MCC showed the typical 8

characteristic peaks of cellulose. The absorption bands around at 3354 cm-1 were related to O-H groups, and a sharp peak at 2903 cm-1 was attributed to C-H stretching vibrations (Li et al., 2018; Peng et al., 2014). Additional peaks at 1430 cm-1 and 1050 cm-1 correspond to the bending vibrations of -CH2 groups and C-O-C stretching

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vibration in the glucopyranose (Huang et al., 2017). For MCC-g-(AA-co-AM), the

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characteristic peaks of cellulose still existed in the hydrogels. Additionally, a peak at

1564 cm-1 was attributed stretching of C-N bonds (Lalita et al., 2017), and the strong peak at 1706 cm-1 was attributed to the stretching of C=O groups from polyacrylic acid

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and polyacrylamide (Nourelhoda et al., 2015). Moreover, the peak corresponding to O-

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H groups at 3354 cm-1 shifted to 3433 cm-1, which overlapped with N-H groups

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(Gharekhani et al., 2017), and the band at of C-O groups was weakened and shifted after reaction (Ren et al., 2017). From the above-mentioned results, AA, and AM had

X-ray diffraction analysis

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3.1.2.

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been successfully introduced on MCC by grafting copolymerization.

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The crystalline structure of MCC and modified cellulose hydrogels was confirmed

by XRD analysis in Fig. 2(b). The diffraction peaks at 22.5°, 14.5° and 34.6°

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corresponded to the 101, 002 and 040 crystal planes of cellulose I (Chen et al., 2018), respectively. However, the disappearance of two diffraction peaks proved that the graft polymerization took place for MCC-g-(AA-co-AM), and the XRD patterns of hydrogels only displayed a weak diffraction peak at 22.5°, which showed that graft

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polymerization increased the amorphous nature of the polymer (Lalita et al., 2017). These results indicated that cellulose occurred of grafting with AA and AM, which were in consistent with the FTIR results described earlier.

Thermogravimetric analysis

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3.1.3.

The thermal characteristics and applicable temperature of modified cellulose

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hydrogels were analyzed by TGA in Fig. 2(c). The weight loss under 100 ℃ was owing to water evaporation from the samples (Wei et al., 2017; Peng et al., 2015). The MCC

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exhibited a sharp decomposition rate was in the temperature range of 240 ℃ - 360 ℃,

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which was the decomposition of cellulose, and a small amount of residua carbide

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remained (Xing et al., 2018). For the MCC-g-(AA-co-AM), the main decomposition stage was evidently extended and the rate was reduced. Also, residua carbide increased

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as compared to that of MCC. Furthermore, the carboxyl from AA groups and amide

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groups from AM decomposed in the temperature range of 340 ℃ - 500 ℃ (Gharekhani et al., 2017). The correlation analysis indicated that the hydrogel samples had good

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thermal stability.

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3.1.4.

Surface morphology analysis

The surface morphologies and the properties of modified cellulose hydrogels were

subjected to SEM analysis in Fig. 3(a). The sample surface was coarse, irregular and had a decidedly porous structure, with an average pore size of about 5 μm, which could

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provide more adsorption sites to heavy metal ions and improve overall adsorption performance. The morphology of copper-loaded hydrogels was represented in Fig. 3(b). The copper-loaded hydrogels were relatively rough, which we interpreted to possibly be due to significant levels of copper ion binding. The EDS analysis (Figs. 3(c)(d)) was

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used to detect the element composition. The copper on signals were not observed in

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untreated samples in Fig. 3(c), while presence of copper was detected in copper-loaded hydrogels (Fig. 3(d)), which clearly the enrichment of copper in the treated samples.

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Effect of pH on Adsorption

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3.2.1.

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3.2. Adsorption of MCC-g-(AA-co-AM)

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The pH effect on adsorption capacity of Cu (II), Pb (II), Cd (II) was investigated.

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When initial pH > 6, metal hydroxides would be precipitated from the solution, which could affect the accuracy of results. It was conspicuous in Fig. 4(a) that adsorption

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capacity increased with the increasing solution pH and remained basically invariable at

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pH 3. This could be attributed to the fact that the metal ion binding competed with high concentration of hydrogen ions at lower pH level (Kong et al., 2018). Meanwhile, the major adsorption sites were protonated under low pH, and the adsorbent surface was

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positively charged, which could also reduce the stability and number of electrostatic interactions between hydrogels and heavy metal ions (Ren et al., 2016; Lalita et al., 2017). At the same time, the low pH was reduced swelling capacity of adsorbent, which led to less heavy metal ions going into the interior of the hydrogels (Li et al., 2018). By 11

comprehensive consideration, pH 5 was the suitable choice.

3.2.2.

Effect of contact time and initial ion concentration

Fig. 4(b) illustrated the influence of contact time on heavy metal ions adsorption

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by hydrogels. The adsorption increased dramatically within 2 min for Cu (II) ions, 3 min for Pb (II) ions and 5 min for Cd (II) ions. And the contact time reached the

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adsorption equilibrium within 15 min, which was shorter than the reported absorbent

based on other cellulosic materials (Jilal et al., 2018). It was related that repulsive forces

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between adsorbate present in solid and bulk phases. The remaining vacant sites were

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not enough to bind to heavy metal ions (Essawy et al., 2017), the binding site of the

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adsorbent rendered saturated.

Fig. 4(c) indicated the influence of initial metal ion concentration on adsorption

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heavy metal ions by hydrogels. The amount adsorbed increased dramatically with

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increasing the initial metal ion concentration, and the maximum adsorption ability was 177.02, 556.69 and 306.22 mg/g for Cu (II), Pb (II) and Cd (II), respectively. The metal

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ions across the formation of the liquid film on the adsorbent increased because of increase in initial metal ion concentration. Meanwhile the increased rate of mass

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transfer result from increased driving force of ions, which led to a high adsorption capacity (Garg et al., 2008). Thereafter, with further rise in initial concentration, the adsorption capacity remained unchanged or decreased, the hydrogels shrank for the high concentration of metal ions solution, which was decreased the specific surface area

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and adsorption sites.

3.2.3.

Adsorption kinetics

In order to investigate the effect of MCC-g-(AA-co-AM) on the adsorption rate,

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the dynamical model was used to fit the experimental data. By using for solid-liquid interaction the most common of kinetic models were the pseudo-first-order (Eq.(3)) and

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pseudo-second-order models (Eq.(4)) (Chen et al., 2018;). The pseudo-first-order kinetic model assumed that the rate of adsorption was controlled by diffusion and mass

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transfer, whereas the pseudo-second-order model assumed that chemisorption was the

=

1 𝑘2 𝑞𝑒2

+

𝑡

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𝑡 𝑞𝑡

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𝑘

1 log （𝑞𝑒 − 𝑞𝑡 ) = log 𝑞𝑒 − 2.303 𝑡

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rate-controlling step (Kara et al., 2004).

𝑞𝑒

(3) (4)

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where qe (mg/g) is the amount adsorbed at equilibrium, qt (mg/g) is the amount

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adsorbed at specific time, k1 (min-1) and k2 (g/(mg*min)) is pseudo-first-order and

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pseudo-second-order rate constants, respectively.

Figs. 5(a) (b). and Table 1 indicated fitting curves and fitting parameters of pseudo-

first-order and pseudo-second-order models. Table 1 shown that in pseudo-first-order,

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the calculated values (qe) did not match those of the experimental values, but that qe of the pseudo-second-order much more closely matched the experimental data. Consistent with this, the pseudo-second-order correlation coefficient (R2) was higher. According to the results, a pseudo-second-order kinetic equation better described metal ion 13

adsorption by the hydrogels, which meant chemisorption dominates the adsorption process (Chen et al., 2004; Li et al., 2017). Moreover, the pseudo-second-order kinetic was also indicated that the chemisorption rate of reaction was proportional to the square number of unoccupied adsorption sites (Nourelhoda et al., 2015).

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To understand the diffusion mechanisms during the uptake process on absorbent, it investigated the intra-particle diffusion model (Eq.(5)). This model assumed that the

influence factor to the sorption kinetics (Dai et al., 2018). 1

(5)

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𝑞𝑡 = 𝑘𝑖 𝑡 2 + 𝐶

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availability of adsorption sites and the mass transfer process was the most important

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where qt (mg/g) is the amount adsorbed at specific time, ki (mg/(g*min0.5)) is the

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boundary layer thickness.

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intra-particle diffusion constant which concerns to the rate of uptake and C is the

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Fig. 5(c) and Table 1 illustrated that the adsorption process was divided into two stages. The first stage described the metal ion solution diffusion mechanism on the

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adsorbent surface (0-10 min). The linear segment didn’t pass through the origin, which

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indicated intraparticle diffusion was not the main mechanism, and that extra-particle diffusion (liquid film diffusion and surface adsorption) also affected the adsorption process (Elwakeel et al., 2018). In the second phase, intraparticle diffusion controlled

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the adsorption process (10-40 min) until the adsorption process reached the equilibrium.

3.2.4.

Adsorption isotherm

Adsorption isotherms were used to describe interface adsorption, which was a 14

physico-chemical adsorption phenomenon by the interactions between metal ions and adsorbent surface (Jilal et al., 2018). It studied the Langmuir, Freundlich Temkin isotherm model and Dubinin–Radushkevich, the Langmuir model was appropriate for monolayer adsorption systems, which illustrated that a limited number of adsorption

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sites was isolated each other and energetically equivalents, without chemical interaction (Nongbe et al., 2018). While the Freundlich model described the heterogeneous surface

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in adsorption surface, which was suitable for multi-layer adsorption or high adsorbate

concentration system (Chen et al., 2018; Hamdaoui et al., 2007), and Temkin model

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assumes the variation of adsorption energy was linear due to the interactions between

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heavy metal ion and adsorbent (Luo et al., 2018). By mean adsorption energy, Dubinin–

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Radushkevich adsorption isotherm supposed the rate limiting (chemical and physical

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absorption) (Kong et al., 2014). The linear form of the four models was expressed by

𝐶𝑒 𝑞𝑒

=𝐾

1

𝐶

𝐿 𝑞𝑚

+ 𝑞𝑒

𝑚

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the following equations.

1

{𝑅𝐿 = 1+𝐾

𝐿 𝐶0

}

(6)

1

ln 𝑞𝑒 = ln 𝐾𝐹 + 𝑛 ln 𝐶𝑒

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𝑅𝑇 𝑏

ln 𝐾𝑡 +

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𝑞𝑒 =

(7)

𝑅𝑇 𝑏

ln 𝐶𝑒

ln𝑞𝑒 = ln𝑞𝑚 − 𝛽𝜀 2

(8) 1

1

{𝜀 = 𝑅𝑇ln(1 + 𝐶 )} {𝐸 = (2𝛽)2 } 𝑒

(9)

where qe (mg/g) is the amount adsorbed at equilibrium, Ce (mg/L) is the

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equilibrium concentration, KL is Langmuir constant, qm (mg/g) is the maximum amount adsorbed overlaying the entire surface, RL is the separation factor or equilibrium parameter, C0 (mg/L) is the initial concentration of heavy metal ions, KF and n is Freundlich constant, Kt is Temkin constant, R (J/(mol*k)) is universal gas constant, T 15

(K) is the thermodynamic temperature, b is constant dependent on adsorption energy, qm (mg/g) is the theoretical saturation amount adsorbed, β is the adsorption constant in D-R,  is the Polanyi potential, E is parameter with determine the adsorption

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mechanism.

Fig. 6 and Table 2 described the fitting parameters and fitting curves from the four

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models, respectively. Based on correlation coefficients (R2), Langmuir was the best fit

for describing the adsorption process, which illustrated monolayer adsorption dominant

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adsorption process. In addition, the value of RL less than 1 and n > 1 both reflected that

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adsorbent had favorably adsorbed capacity, and the value of E > 8 kJ/mol in D-R model

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indicated that ion-exchange mechanism governs adsorption (Lin et al., 2011). The

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amide and carboxylic acid groups bounded on cellulose by grafting, while by

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electrostatic attraction and sharing or exchange of electrons the metal ions attached itself to functional groups, and forming chelate compounds (Fig. 7) (Abdel-Halim et al.,

Reusability of the prepared hydrogels

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3.2.5.

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2012; Li et al., 2018).

In general, good reproducibility of binding characteristics is also an important

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indicator to meet the requirements of low-cost in adsorbent (Dai et al., 2018). The desorption-adsorption process of different cycle times in different ions solution was shown in Fig. 8. After five cycles, the prepared hydrogels still had relatively high adsorption capacity, which illustrated that the MCC-g-(AA-co-AM) had a good 16

reusability.

4. Conclusion

The adsorbent hydrogels MCC-g-(AA-co-AM) were successfully prepared by

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simultaneous cross-linking and grafting with acrylic acid and acrylamide, and adsorbed Cu (II), Pb (II) and Cd (II) metal ions from solutions under a variety of different

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conditions. Through physico-chemical characterization of these hydrogels, we confirmed that the grafting of the acrylate and acrylamide monomers had occurred

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efficiently with the initial cellulosic material. SEM indicated the newly synthesized

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hydrogels surface was coarse, irregular, and displayed porous features, and that these

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hydrogels displayed good thermal stability. The maximum amount adsorbed of Cu (II),

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Pb (II) and Cd (II) ions solution was 157.51, 393.28 and 289.97 mg/g, respectively. The

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isotherm and kinetics of adsorption indicate that the amide and carboxylic acid groups were attached the metal ions and formed chelate compounds. In addition, the acidic

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solutions were efficient for metal desorption and adsorbent recycling, after five cycles

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the adsorption capacity was high. It can be used to dispose of the high metal ions concentration sewage by fixed-bed cartridge.

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Acknowledgements

This works was supported by National Nature Science Foundation of China (No.

31770617),

Natural

Science

Foundation

of

Guangdong

Province

(No.

2017A030310147) and the National Program for Support of Top-notch Young

17

Professionals.

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Cellulose,21, 2797-2809.

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Figure captions

A

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Fig. 1. Reaction scheme for synthesis of the bioadsorbent (MCC-g-(AA-co-AM))

25

IP T SC R U N

A

Fig. 2. FTIR spectra of MCC and MCC-g-(AA-co-AM) (a); XRD curves of MCC and

A

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PT

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M

MCC-g-(AA-co-AM) (b); TGA curves of MCC and MCC-g-(AA-co-AM) (c).

26

Fig. 3. SEM images of MCC-g-(AA-co-AM) (a); SEM images of copper-loaded MCC-g-(AA-co-AM) (b); EDS curves of MCC-g-(AA-co-AM) (c); EDS curves of

M

A

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copper-loaded MCC-g-(AA-co-AM) (d).

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Fig. 4. pH effect on sorption efficiency (a); Contact time effect on sorption efficiency

A

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(b); Initial metal ion concentration effect on sorption efficiency (c).

27

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N

Fig. 5. pseudo-first-order (a), pseudo-second-order (b) and intraparticle diffusion (c)

A

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M

A

kinetic model on Cu (II), Pb (II) and Cd (II) ions adsorption

28

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A

Fig. 6. Langmuir (a), Freundlich (b), Temkin (c) and Dubinin–Radushkevich (d)

A

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isotherm model on Cu (II), Pb (II), Cd (II) ions adsorption

Fig. 7. ion exchange mechanism

29

A

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Fig. 8. The reusability of MCC-g-(AA-co-AM)

30

I N U SC R

Table 1

A

Kinetic parameters for the adsorption of Cu (II), Pb (II) and Cd (II) pseudo-second-order

k1

qe1

R2

Cu (II)

0.1294

84.61

0.9147

Pb (II)

0.4958

201.18

0.9511

Cd (II)

0.2149

250.29

M

pseudo-first-order

qe2

R2

ki1

C1

R2

ki2

C2

R2

0.0046

175.13

0.9989

38.50

40.99

0.9532

2.86

152.28

0.9778

0.0058

400

0.9997

210.97

9.99

0.9764

1.50

386.46

0.4032

0.0015

333.33

0.9967

109.52

-0.95

0.9705

7.68

269.68

0.8144

PT

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k2

A

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0.9755

intra-particle diffusion model

31

I N U SC R A

Table 2

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Adsorption isotherms for the adsorption of Cu (II), Pb (II) and Cd (II) Freundlich

2

n

KF

R2

KT

b/RT

R2

qm

β

E

R

0.9971

3.88

46.48

0.6984

0.5720

38.48

0.8057

226.81

0.0061

9.08

0.7805

0.9511

0.9928

1.63

25.16

0.4348

0.1756

168.25

0.7186

483.01

0.0091

7.42

0.6465

0.0037

0.9755

0.9921

2.29

42.13

0.7984

3.9263

47.68

0.7153

208.63

0.0008

25.00

0.8473

RL

Cu (II)

170.94

0.0059

0.9147

Pb (II)

581.40

0.0017

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KL

A

273.97

Dubinin Radushkevich

R2

qm

Cd (II)

Temkin

PT

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Langmuir

32