Facile fabrication of polyacrylic acid functionalized carboxymethyl chitosan microspheres for selective and efficient removal of Ni(II) from multicomponent wastewater

Journal Pre-proof Facile fabrication of polyacrylic acid functionalized carboxymethyl chitosan microspheres for selective and efficient removal of Ni(II) from multicomponent wastewater Tao Tian (Writing - original draft) (Methodology), Zhishan Bai, Bingjie Wang (Supervision) (Writing - review and editing), Shenghao Zhao, Yong Zhang

PII:

S0927-7757(20)30269-7

DOI:

https://doi.org/10.1016/j.colsurfa.2020.124676

Reference:

COLSUA 124676

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

18 December 2019

Revised Date:

5 March 2020

Accepted Date:

7 March 2020

Please cite this article as: Tian T, Bai Z, Wang B, Zhao S, Zhang Y, Facile fabrication of polyacrylic acid functionalized carboxymethyl chitosan microspheres for selective and efficient removal of Ni(II) from multicomponent wastewater, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124676

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Facile fabrication of polyacrylic acid functionalized carboxymethyl chitosan microspheres for selective and efficient removal of Ni(II) from multicomponent wastewater

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Tao Tian, Zhishan Bai*, Bingjie Wang*, Shenghao Zhao, Yong Zhang

State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on

Chemical Process, School of Mechanical and Power Engineering, East China University of Science and

Corresponding Authors, Email address: [email protected] (Z. Bai); [email protected] (B.

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*

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Technology, Shanghai 200237, P. R. China

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Wang). Tel: +86 (021) 64253693; Fax: +86 (021) 64253693.

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Graphical abstract

Highlight: 

A novel PAA-CMCs adsorbent was synthesized by carboxyl modification to CMCs.



The maximum Ni(II) adsorbance of PAA-CMCs obtained from L-F model was up to 200.588 mg



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g-1.

Excellent selectivity for Ni(II) in binary solution was verified by electronegativity and electrophilic

The reabsorption rate was higher than 89.7% after six adsorption-desorption cycles.

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

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index.

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Abstract

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The unpurified discharge of highly toxic Ni(II)-contained wastewater represents a serious threat to ecological environment and human health. In this study, a novel polyacrylic acid (PAA) functionalized carboxymethyl

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chitosan microspheres (PAA-CMCs), synthesized by grafting copolymerization method, was used to removal Ni(II) from wastewater. The PAA-CMCs was characterized by SEM, elemental analyzer, FT-IR and XPS. The equilibrium data was better described by L-F model with high coefficients of determination (R2>0.991). Meanwhile, the maximum Ni(II) adsorption capacity of PAA-CMCs deduced from L-F model was as high as 200.588 mg g-1. Moreover, the selective capacity was investigated by batch adsorption 2

experiments and its mechanism was revealed by electronegativity and electrophilic index. Besides, the excellent regeneration performance of PAA-CMCs was verified after six adsorption-desorption cycles. Thus, the newly developed PAA-CMCs is a promising candidate for deep treatment of heavy metal-contained wastewater. Keywords: carboxymethyl chitosan; wastewater treatment; polyacrylic acid grafting; Ni(II) removal;

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selective adsorption

1. Introduction

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Ni(II) was widely applied in electroplating, storage battery, steel, zinc base casting and nickel-based alloy industries [1]. However, Ni(II)-contained wastewater produced in the above industrial processes poses a

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serious threat to ecological environment [2]. Moreover, Ni(II) was difficult to biodegrade in human system

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after ingestion, which leads to the accumulation of Ni(II) in human tissues [3]. High toxicity caused by excessive Ni(II) intake can result in serious diseases such as renal edema, lung cancer, gastritis, neurasthenia

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and genetic mutations [4]. Meanwhile, Ni(II) is a kind of high-value metals, recycling it from wastewater would cut the cost of industry [5]. Therefore, the removal of and recovery Ni(II) from wastewater is

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evaluated a matter of top priority. At present, traditional methods of separation and recovery of Ni(II) from

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wastewater included chemical precipitation, solvent extraction, ion exchange, reverse osmosis, electrodialysis and membrane separation [6]. Compared with the above methods, adsorption is considered as a simple, effective, economic and convenient method to remove Ni(II) from wastewater [7, 8]. In recent years, many natural materials have been applied to remove heavy metal ions from wastewater [9, 10]. Therein, chitosan, the only natural alkaline polysaccharide derived from chitin by deacetylation, was 3

the commonly used adsorbent for heavy metal removal with the advantages of low-cost, extensive-sourced and biodegradable. Particularly, chitosan has a substantial number of amino and hydroxyl groups which offers a plethora of active sites for heavy metal ions removal [11]. However, the adsorption capacity of chitosan has not reached a satisfactory level and its widespread application was severe limited by its poor water-solubility and high acid-solubility [12-14]. Alternatively, carboxymethylation is a simple and effective

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method to improve the adsorption capacity and water solubility of chitosan [15]. Carboxymethyl chitosan (CMC) dissolves in acidic, alkaline and neutral environment, which has many excellent physicochemical and biological properties such as eco-friendly, nontoxicity, biodegradable and metal-chelating ability [12].

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Nevertheless, raw CMC cannot be directly applied to adsorption because it is difficult to separate from aqueous solution after adsorption [16, 17]. Thus, to avoid secondary pollution and stabilize the CMC for its

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application in wastewater, microspheres or beads instead of flakes or powdered CMC have been prepared

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[18]. Moreover, the chemical cross-linking was necessary applied to improve the chemical stabilities of the microspheres or beads in acidic solution [18, 19]. But the adsorption capacities of cross-linked CMC may

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significantly decrease, since the amine groups are inevitable consumed in chemical reactions with crosslinking agent, such as glutaraldehyde, formaldehyde and epichlorohydrin [17, 19].

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Recent researches have been paid a growing attention in chemical modification of CMC to enhance their

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adsorption properties and expand their potential applications, including new functional groups grafting and polymer pre-embedding [12]. The stronger chelation between carboxyl groups and metal ions have been verified [7, 20, 21]. Therefore, further carboxylation modification of CMC microspheres or beads is an effective method to improve the adsorption performance. However, up to now, there are few published literatures research on this aspect, and the studies on Ni(II) adsorption mechanism are even less. 4

To fill this gap, in this study, a novel PAA-CMCs synthesized by grafting copolymerization method was used to removal Ni(II) from wastewater. The SEM, elemental analysis, FT-IR and XPS were used to characterize PAA-CMCs. The effects of acrylic acid dosage, adsorbent dosage, solution pH, contact time and temperature on Ni(II) adsorption were studied. Furthermore, the models of kinetics and multivariate isothermal were applied to fit adsorption experiment data and to illuminate the adsorption mechanism.

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Meanwhile, the selective adsorption was investigated in binary solution. The regeneration properties and methods were also studied. 2. Materials and methods

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2.1 Chemicals and reagents

Carboxymethyl chitosan (degree of substitution>80%, and a viscosity of 20 cps) was purchased from Yuanye

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Biotechnology Technology Co., Ltd, Shanghai, China. Span-80, N, N '-Methylene-Bis-Acrylamide (MBA,

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99% purity), N-octane (C.P., 95%), glutaraldehyde (25%) and acrylic acid (AA, 99.5%) and potassium persulfate (K2S2O8, 99% purity) were supplied by Titan Technology Co., Ltd, Shanghai, China. NiCl2·6H2O,

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CoCl2·6H2O, MnCl2·4H2O, CrCl2, NaCl and AlCl3·6H2O were obtained from Adamas Reagent Co., Ltd, Shanghai, China. All the stock solutions of metal ions were prepared in deionized water and then diluting to

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the required initial concentrations. HCl, NaOH, CH3COOH and other reagents (except n-octane) utilized in

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this study were A.R. and used as received. Deionized water was used in all experiments. 2.2 Preparation of adsorbent 2.2.1 Preparation of CMCs As shown in Fig.1, CMCs were prepared by inverse crosslinking-emulsion with glutaraldehyde as the crosslinking agent. First of all, the continuous phase was mixed by n-octane (100 mL) and span-80 (4 mL), 5

while the dispersed phase was prepared by dissolving CMC (1 g) in deionized water (20 mL). Then the dispersed phase was added dropwise into the continuous phases on a water bath (HDK-4, DeKe instrument manufacturing Co., Ltd, China) at 328 K, 800 rpm. After stirring for 30 min, emulsion droplets were formed in the solution. Subsequently, glutaraldehyde (0.2 mL) was added to the solution, where Schiff’s base reaction occurred between -CHO of glutaraldehyde and -NH2 of CMC. Finally, the CMCs were thoroughly

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washed with acetone, ethanol and deionized water before drying at 303 K in an oven (PCD-C3000, Heheng instrument equipment Co., Ltd, China) for 4 h. 2.2.2 Preparation of PAA-CMCs

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As shown in Fig.2, PAA-CMCs was prepared by grafting PAA onto CMCs with K2S2O8 as initiator and MBA as cross-linking agent. The AA solution was prepared by adding AA (10 g) to deionized water (100

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mL) in advance. After that, the as-prepared CMCs and 0.15 g K2S2O8 (as the initiator) were added to AA

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solution placed on the water bath at 323 K. The graft copolymerization reaction took place between C=C of AA and -NH2 of CMCs. After 2 h, MBA (0.15 g) as the crosslinking agent was added to the above solution

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and crosslinked for 30 min at 333 K. Finally, the microspheres were washed several times with NaOH solution (0.01 mol L-1), ethanol and deionized water before drying in the oven (303 K) for 4 h to obtain

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PAA-CMCs.

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2.3 Analysis and characterization of adsorbent The details of methods and instruments used for analysis and characterization were described in supplementary materials (SI). 2.4 Adsorption experiments Batches of adsorption experiments were carried out by adding 0.05 g adsorbents to 50 mL Ni(II) solution 6

(150 mg L-1, a certain amount of NiCl2·6H2O was dissolved in deionized water, and the pH value was adjusted by 0.1mol L-1 HCl and NaOH) placed in the water bath at 298 K. The Ni(II) concentrations of solution were detected by the inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent ICP-725ES). The same procedure was applied to study the effect of the initial Ni(II) concentration (50-500 mg L-1), contact temperature (298 K, 308 K and 318 K), adsorbent dosage (0.05-0.30 g), AA dosage (2-12 g) and solution pH values (2-7) on adsorption performance. Besides, the value of solution pH was measured

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by a pH meter (3110-SET2, WTW, Germany). The adsorption amount at time t (qt, mg g-1) and the equilibrium adsorption capacity (qe, mg g-1) of adsorbents were calculated by the following formula: 𝑞𝑡 =

(𝐶0 −𝐶𝑡 )𝑉0

𝑞𝑒 =

(𝐶0 −𝐶𝑒 )𝑉0

(1)

𝑚

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𝑚

(2)

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Where C0 (mg L-1) is the initial Ni(II) concentration and Ct (mg L-1) is the Ni(II) concentration at time t. Ce

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(mg L-1) is the equilibrium concentration of Ni (II) solution. V0 (L) is the initial volume of the solution. m (mg) is the weight of adsorbents used in the experimental.

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2.5 Selective adsorption experiments

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Batches of selective adsorption experiments were carried out between Ni(II) and competitive metal ions, which include Na(Ⅰ), Mn(II), Cr(II), Co(II) and Al(Ⅲ). 0.05 g adsorbents were added respectively into the

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binary solution with same initial concentration (150 mg L-1) at 298 K. The selectivity coefficients can be obtained from the following equations [22]: Distribution coefficient: 𝐾𝑑 = Selectivity coefficient: 7

𝑞𝑒 𝐶𝑒

(3)

𝐾𝑠 =

𝐾𝑑 (𝑁𝑖)

(4)

𝐾𝑑 (𝑀)

Relative selection coefficient: 𝐾𝑟 =

𝐾𝑠𝑝

(5)

𝐾𝑠𝑐

Where Kd (Ni) is the distribution coefficient of Ni(II) and Kd (M) is the distribution coefficient of the competitive metal ions. Ksp and Ksc were the selectivity coefficients for Ni(II) of PAA-CMCs and CMCs,

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respectively. 2.6 Regeneration experiments

In regeneration experiments, the saturated adsorption of PAA-CMCs was eluted with different agents of HCl,

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CH3COOH and NaOH (0.1 mol L-1). Besides, the adsorption-desorption cycle was carried out six times to

can be calculated as following [23]:

𝑚𝑑𝑒𝑠

𝑚𝑎𝑑𝑠

×100%

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𝐷𝐸 =

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validate the reusability of PAA-CMCs. The desorption rate (DE) and reabsorption rate (RE) of the adsorbent

𝑅𝐸 =

𝑞𝑟−𝑚

𝑞𝑎−𝑚

× 100%

(6) (7)

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where mdes (mg) was the amount of Ni(II) desorbed to the desorption agent solution, mads (mg) was the amount of Ni(II) adsorbed onto the PAA-CMCs. qa-m (mg g-1) and qr-m (mg g-1) were the maximum

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adsorption capacity in adsorption and reabsorption experiment, respectively.

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3. Results and Discussion

3.1 Characterization of PAA-CMCs 3.1.1 SEM analysis

Fig.3 presented the wrinkled adsorbent surfaces with the crisscross ravines and gullies distributed. Compared to CMCs, the distribution of gullies on PAA-CMCs surface were more densely, which can attribute to the 8

PAA surficial grafting. Moreover, the specific surface area of PAA-CMCs measured by BET method was 32.581 m2 g-1, which was outclass that of CMCs (23.635 m2 g-1). This result indicates that PAA-CMCs with the higher specific surface area are favorable for rapid adsorption[10].

3.1.2 Elemental analysis As shown in Table 1, the mass ratios of C, H and N elements in PAA-CMCs and CMC were close to each

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other. It is worth noting that the mass ratio of O element in PAA-CMCs (40.83%, Wt) was much higher than that in CMCs (32.51%, Wt), which indicated the success of the PAA modification.

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3.1.3 FT-IR analysis

FT-IR analysis was carried out to verify the conversions of functional groups in CMC powders, CMCs and

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PAA-CMCs. As shown in Fig.4a, the peaks at 3430 cm-1 could be assigned to the stretching vibration of N-

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H and O-H. Peaks at 2929 cm-1 and 2854 cm-1 are C-H asymmetric and symmetric stretching vibration, respectively[7]. The bending vibration peak of N-H and symmetric stretching vibration peak of -COO- are

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located at 1601 cm-1 and 1402 cm-1, respectively[24]. The peaks at 1054 cm-1 is ascribed to the stretching vibration of C-OH. Compared with CMC, a new characteristic peak (C=N) appeared in the infrared spectrum

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of CMCs (Fig.4b) at 1656 cm-1, indicating that the successful process of Schiff’s base reaction [25]. In

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addition, as shown in Fig.4c, the new stretch peak appeared at 1723 cm-1 is attributed to C=O in -COOgroup of PAA [26, 27], which confirmed the PAA has been successfully grafted onto CMCs. Moreover, the disappearance of N-H bending vibration peak at 1601 cm-1 indicates the reaction of -NH2 during grafting. The peak at 1634 cm-1 is related to the stretching vibration of C=O in the amide group of MBA, and the stretching vibration peak observed at 1166 cm-1 is attributed to C-N in MBA [28, 29]. This demonstrates that 9

the chemical crosslinking reaction occurs between the PAA and MBA. 3.2 Adsorption experiment 3.2.1 Effect of the dosage of AA and PAA-CMCs As shown in Fig.5a, in the AA dosage of 2-10 g, the adsorption capacity of PAA-CMCs to Ni (II) increases with the increase of AA consumption and reaches the maximum at 10 g. The improvement of adsorption

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capacity may be attributed to the increasing grafting of PAA onto carboxymethyl chitosan molecules. More active carboxyl groups were introduced to CMCs which significantly improve the adsorption capacity of PAA-CMCs. However, the adsorption capacity decreased as the dosage of AA continues to increase. This

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phenomenon may be due to: (i) the preferential homopolymerization over graft copolymerization or (ii) an increased chance of chain transfer to monomer molecules [30]. Moreover, the effect of dosage of PAA-

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CMCs on adsorption capacity also was investigated in this study. As can be seen from Fig.5b, the adsorption

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capacity increased with the increase of adsorbent dosage, and the maximum adsorption capacity appeared at dosage of 0.05g. Subsequently, the adsorption capacity of microspheres decreased with increasing excess

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respectively.

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adsorbents. Therefore, the optimal dosages of AA and PAA-CMCs in experiments were 10 g and 0.05 g,

3.2.2 Effect of solution pH

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As shown in Fig.6a, the maximum adsorption capacity of the PAA-CMCs and CMCs were located at pH=5 and 6, respectively. The adsorption capacity declined rapidly with further decreasing pH (pH<5, PAA-CMCs; pH<6, CMCs), which can be explained by a competition between H+ and Ni(II) on the surface of adsorbent. Moreover, the protonation of -NH2 and -COO- occurs in excessive acidic environments, resulting in a reduction of the binding sites for Ni(II) chelation [31]. While at high pH (pH>5, PAA-CMCs; pH>6, CMCs), 10

a decrease in adsorption was presumably owed to the formation of the Ni(OH)2 precipitation on the microspheres surface, which will block interstices (pores) and prevent further adsorption [32]. Moreover, the pHpzc of the microspheres which refers to the pH value corresponding to the zero charge of solid surface, were determined by the drift method reported in published literature [33]. When the pH of solution is lower than the pHpzc of adsorbent, the functional groups on the microspheres are protonated and positively charged.

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On the contrary, when the pH of solution is higher than the pHpzc of adsorbent, the surface of microspheres is negatively charged, which is conducive to the adsorption of positively charged Ni(II). As shown in Fig.6b, the pHpzc value of PAA-CMCs was 4.85, which was lower than that of CMCs (pHpzc=5.74). This may be

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due to the fact that the grafting of PAA introduced a large number of negatively charged carboxyl groups, which reduced the electropositivity of microspheres. At the same time, the different optimal adsorption pH

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values of the two adsorbents could be explained by the different pHpzc values. Therefore, the different optimal

3.2.3 Effect of contact time

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pH values were used in following adsorption experiments for PAA-CMCs (pH=5) and CMCs (pH=6).

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Fig.7 shows the variation on the adsorption amount (qt) of the two adsorbents with contact time t. The changing trends of the two adsorption processes were similar, both of which show rapid adsorption at the

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initial stage and finally slowly reach adsorption equilibrium. However, there were significant differences in

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adsorption capacity and adsorption rate. The adsorption capacity of PAA-CMCS to Ni(Ⅱ) was obviously improved. Meanwhile, PAA-CMCs reached the adsorption equilibrium at about 960 min, which was only half of that of the CMCs (1920 min). The faster adsorption rate of PAA-CMCs can be attributed to the improvement of specific surface area and increasing number of -COO- groups on surface.

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3.3 Adsorption kinetics Adsorption kinetics can be expressed by solute removal rate which controls the retention time of adsorbate at the liquid-solid interface [19]. To better understand the adsorption behavior of Ni(Ⅱ), pseudo-first-order model and pseudo-second-order model were used to fit the data. The pseudo-first-order model (Eq.8) and pseudo-second-order model (Eq.9) equations of linear expression were given as follows [34]: 𝑙𝑛(𝑞𝑒 − 𝑞𝑡 ) = 𝑙𝑛 𝑞𝑒 − 𝑘1 𝑡

𝑡 𝑞𝑡

=

1 𝑘2 𝑞𝑒2

+

1 𝑞𝑒

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and

(8)

𝑡

(9)

Where k1 (g mg-1 min-1) and k2 (g mg-1 min-1) are the pseudo-first-order model and the pseudo-second-

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order model adsorption rate constant, respectively.

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As shown in Fig.8a and Table 2, it can be seen that the coefficients of determination (R2>0.992) of the pseudo-second-order model for PAA-CMCs and CMCs were closer to 1 than that of the pseudo-first-order

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kinetic model (R2>0.862). The above results indicated that the adsorption process of Ni(II) was consistent with the assumptions of the pseudo-second-order model, which is that the adsorption of Ni(II) onto PAA-

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CMCs and CMCs was a chemical adsorption by the exchange of valence forces or sharing of electrons

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between Ni(II) and functional groups (-COO-, -NH2, -OH) . However, since neither the pseudo-first-order kinetic nor pseudo-second-order model could provide specific

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information about the diffusion mechanism of Ni(II) onto adsorbent, the intraparticle diffusion model was adopted to analyze the experimental data. The equation of intraparticle diffusion model was generally described by Weber-Morris [35]: 𝑞𝑡 = 𝐾𝑝 𝑡 0.5 + 𝐶

(10)

Where Kp (g mg-1 min-1) is the parameter of intraparticle diffusion model, C is the intercept which is 12

proportional to the thickness of the microsphere boundary layer. As shown in Fig.8b, the intraparticle diffusion model represents the multiple stages of adsorption process. The first stage is attributed to the rapid adsorption of Ni(II) on the external surface or boundary diffusion layer. The second stage corresponds to gradual adsorbed by CMC, which reflects the intraparticle diffusion as the rate limiting step [36]. In final stage, with the concentration of Ni(II) decreases and increase of

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diffusion resistance in microsphere, the diffusion rate begin to slow down and adsorption gradually reach an equilibrium [37]. However, from Fig.8b, it also shows that the fitting line does not pass through the origin, indicating that intra-particle diffusion is not the only rate control step in the process and other diffusion

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mechanisms, such as film diffusion or boundary layer diffusion, may be operating simultaneously [38]. See

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the SI for more information.

3.4 Adsorption isotherms

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The equilibrium isotherm represents the adsorption capacity of the adsorbent under different experimental conditions [39]. In this study, to reveal the adsorption mechanism of Ni(Ⅱ) of microspheres, two-parameter

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models isotherms including Langmuir, Freundlich and Dubinin-Radushkevich (D-R), as well as three-

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parameter isotherms including Langmuir-Freundlich (L-F) and Redlich-Peterson(R-P) models were employed. All parameters of the above-mentioned models calculated by nonlinear regression method were

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presented in Table 3-4. The fitting curves obtained by plotting qe and Ce are shown in the Fig.9. 3.4.1 Two-parameter models Langmuir isothermal is widely used to describe adsorption of solute from aqueous solution. In its assumptions, all binding sites on the adsorbent surface are evenly distributed and have the same affinity for the adsorbate [40]. The Freundlich isothermal model is an empirical equation used to explain multimolecular 13

layer adsorption on heterogeneous surfaces. It is assumed that the adsorption binding sites and energy are exponentially distributed [41]. The D-R isothermal model indicates that the physical and chemical properties of the adsorption process [3]. The equations of three models were expressed as follows: 𝑞𝑒 = 𝑞𝑚

𝑏𝐿 𝐶𝑒

(11)

1+𝑏𝐿 𝐶𝑒 1

𝑞𝑒 = 𝐾𝐹 𝐶𝑒𝑛

(12)

𝑞𝑒 = 𝑞𝑚 𝑒 −𝐾𝐷 𝜀

2

(13)

𝐶𝑒

(14)

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1

𝜀 = 𝑅𝑇 𝑙𝑛 (1 + )

Meanwhile, the free energy E (kJ mol-1) of adsorption determined the type of adsorption is calculated by

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using the parameter KD in the D-R model [42]. When 1.0 kJ mol-1 < E < 8.0 kJ mol-1, the adsorption process was a physical adsorption. When E >8.0 kJ mol-1, it is a chemical adsorption with ion exchange. The

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calculation equation is as follows:

1

√2𝐾𝐷

(15)

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𝐸=

Where qm (mg g-1) is the maximum adsorption capacity and bL (L mg-1) represents the affinity between

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adsorbent and adsorbate. KF (L mg-1) and n are Freundlich constants whose values are related to the nature of the adsorbent surface. Parameter of n can indicate the difficulty of adsorption, when n>1, it indicates that

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the adsorption process is easy to proceed. when n<1, the adsorption process is difficult to carry on. KD (mol2

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J-2) is the parameter dependent on the adsorption energy, and ε (kJ mol-1) is the Polanyi potential. R is the molar constant of the gas (8.314 J mol-1 K-1). T (K) is the thermodynamic temperature. As illustrated in Table 3, it can be seen that qm increase with the rise of temperature, indicating that the adsorption process is an endothermic process. The value of n at three temperatures is larger than 1, indicating that the adsorption process of Ni(II) by the two adsorbents is in favorable conditions. The free energy (E) of

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Ni(II) adsorption by PAA-CMCs and CMCs were between 12.633 kJ mol-1 and 9.793 kJ mol-1, indicating that the Ni(II) adsorption process is chemical adsorption dominated by ion exchange. According to Table 3, among two-parameter models, the Langmuir model is more suitable to describe the adsorption behavior of Ni(II) by PAA-CMCs and CMCs since the coefficients of determination (R2> 0.982) were closer to 1. Therefore, in combination with the assumptions of Langmuir, it indicates that the active binding sites on the

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adsorbent surface are uniformly distributed, and the adsorption process of Ni(II) onto the adsorbent is monomolecular chemical adsorption.

However, Langmuir model is a model that assumes uniformity, whereas the SEM results presented an

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anisotropic surface of PAA-CMCs. Besides, the coefficients of determination of PAA-CMCs obtained from

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Langmuir model are lower than 0.99, which is unconvincing to accurately describe the adsorption process.

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Meanwhile, although Freundlich model is a model with heterogeneous hypothesis, it is incapable of describing saturated behavior. Therefore, due to the limitations of Langmuir and Freundlich models, the

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three-parameter models described this equilibrium data more accurately was introduced to this study.

3.4.2 Three-parameter models

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The R-P model is an empirical equation that can express adsorption equilibrium over a wide range of

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concentrations [43]. The models can be expressed as: 𝑞𝑒 =

𝐾𝑅𝑃 𝐶𝑒 𝛽

1+𝛼𝑅𝑃 𝐶𝑒 𝛽

(16)

Where KRP and αRP (L mg-1) are the R-P model constants. β is the R-P model exponent, whose value is usually around 0 or 1. As β approaches 0, the R-P model approximates to Henry's law, and it behaves similar to Langmuir model when β in the vicinity of 1.

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L-F model, also known as the Sips model, is the combination of Langmuir and Freundlich model [25]: 𝑞𝑒 = 𝑞𝑚

(𝐾𝐿𝐹 𝐶𝑒 )𝛾

(17)

1+(𝐾𝐿𝐹 𝐶𝑒 )𝛾

Where KLF (L mg-1) is the L-F model equilibrium constants, γ is L-F model exponent which reflects heterogeneity of the adsorbent surface. When γ<1, the surface of adsorbent is heterogeneous; when γ=1, it is homogeneous. At low adsorbate concentration, the L-F model reduces to the Freundlich model. At high

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adsorbate concentration, it predicts the monomolecular adsorption capacity of Langmuir model. As shown in Table 4, the exponent β values (1.002-1.077) of PAA-CMCs in R-P model were close to 1, which indicated that the equilibrium data was preferably fitted with Langmuir isotherm. The γ values of

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PAA-CMCs and CMCs were lower than 1, indicating that the active binding sites on the adsorbent surface are heterogeneously distributed. According to Fig.9 and Table 3-4, among isotherm models, the L-F

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isotherm model is more suitable to describe the adsorption behavior of Ni(II) by PAA-CMCs and CMCs

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since the coefficients of determination (R2> 0.991) were closer to 1 at all study temperatures. Besides, the similar results have been found in other literatures such as Co(II)-Valentiae algae [39], Ni(II)-polyrhodanine

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chitosan [44]. From Table 4, the qm of PAA-CMCs obtained by L-F model fitting is 200.588 mg g-1, which

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about 1.6 times higher than that of CMCs (122.014 mg g-1). It was revealed that the introduction of -COOsignificantly improved the adsorption performance of CMCs. Moreover, it worth noting that the value of qm

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(PAA-CMCs) is much higher than that of chitosan-based or CMC-based adsorbents for Ni(II) reported in previous literatures (Table 5). The fitting curves of the isothermal model at 298 K and 308 K can be found in SI (Fig.S2). Besides, the thermodynamic analysis of PAA-CMCs and CMCs in Ni(Ⅱ) adsorption were investigated in SI.

16

3.5 Adsorption mechanisms To verify the conversion of functional groups in PAA-CMCs for Ni(II) adsorption, the FTIR spectra of PAA-

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CMCs before and after adsorption of Ni(II) were obtained and shown in Fig.10. After the adsorption (Fig.10b), the new peak appears at 1560 cm-1 is assigned to asymmetric -COO- stretching vibration. Meanwhile, the stretching vibration peak of -COO- group (PAA) has shift from 1723 cm-1 to 1718 cm-1,

-p

while the symmetric vibration peak of -COO- group is shift from 1403 cm-1 to 1405 cm-1. The above results

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are attributed to the -COO- groups in PAA and CMCs were involved in the chelating interaction of Ni(II) [48]. Besides, the stretching vibration peak of C-OH is shift from 1056 cm-1 to 1058 cm-1, indicating that -

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OH groups were also involved in the adsorption process.

Moreover, XPS was further used to analyze the binding energy transition of nitrogen and oxygen atoms in

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PAA-CMCs before and after Ni(II) adsorption. As shown in Fig.11, a new peak corresponding to Ni2p

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appeared at 856 eV after adsorption, which confirmed that the Ni(II) was adsorbed by PAA-CMCs. Besides, in the spectra of N1s (Fig.12a), it was observed that three binding energy peaks corresponding to C-NH2 or

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-NH- (399.4 eV), C=N-C or O=C-N (399.6 eV), C-N-C or C-NH3+ (402.1 eV), respectively. As shown in Fig.12b, the binding energy of the above groups were increased to 399.8 eV, 400.44 eV and 402.35 eV after Ni(II) adsorption. The transition of binding energy indicates that Ni(II) bind onto the N atoms in the formation of covalent bonds during Ni(II) adsorption [49]. The XPS spectra of O1s in PAA-CMCs before and after Ni(II) are compared in Fig.12c-d. The binding energy peaks at 531.0 eV, 533.1 eV and 532.3 eV, 17

which were assigned to C=O(C-O-C), -COO- and C-OH groups in PAA-CMCs, respectively. After adsorption, the binding energy of oxygen functional groups were moved to 531.6 eV, 532.5 eV and 533.5 eV, respectively. The changes of binding energy peak value in the XPS spectra of O1s was attributed to the chelating-coordination reaction between Ni(II) and oxygen atoms in COO- and OH- [50]. Therefore, the XPS analysis results confirmed that the -COO-, -NH2, -OH groups as the adsorption functional group of PAA-

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CMCs were participated in the Ni(II) adsorption. According to the above findings and results, the adsorption mechanism of PAA-CMCs and Ni(II) can be described in Fig.13. The surface grafted PAA and internal CMC in the PAA-CMCs provide a large number

-p

of binding sites for Ni(II) (Fig.13a). Initially, the Ni(II) in the solution were rapidly trapped by the active functional groups on the surface of the microspheres (Fig.13b). In this process, the dominant -COO- in PAA

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and few of functional groups (-COO-, -NH2 and -OH) in CMC adsorbed Ni(II) through chelation and

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coordination (Fig.13c). Subsequently, Ni(II) began to pass through external (boundary layer) and migrate to the binding sites inside the microsphere, which is affected by intraparticle diffusion (Fig.13d). Ni(II) are

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mainly adsorbed by functional groups (-COO-, -OH and -NH2) of CMC (Fig.13e). Finally, when both the surface and the internal binding sites of microsphere were occupied, the adsorption of Ni(II) by PAA-CMCs

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reached an equilibrium (Fig.13f). Moreover, the above results revealed that the -COO- play a dominant role

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in Ni(II) adsorption of PAA-CMCs since a large number of -COO- was introduced in PAA grafting and NH2 was consumed in crosslinking and grafting progress.

3.6 Selective adsorption study The selective adsorption of PAA-CMCs and CMCs was studied in binary solutions containing Ni(Ⅱ) and other competitive ions with the same concentration. Ni(Ⅱ), Na(Ⅰ) and Al(Ⅲ) were classified as the metal 18

ions with different valence state. Meanwhile, Ni(Ⅱ), Cr(II) and Mn(II) were categorized as the metal ions with same valence states. Co(II) as the congener of Ni(Ⅱ) was also taken into account. Although the adsorption performance of PAA-CMCs for above six metal ions in binary solution were improve to different degrees, its Ni(Ⅱ) adsorption capacity decreased with presence of coexisted competitive ions (Fig.14). Besides, the Kd, Ks and Kr of different metal ions are summarized in Table 6. The selective adsorption

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efficiency (Kd) of PAA-CMCs on Ni(II) was significantly higher than that of other metal ions, and the priority of adsorption according to the order of Ks was: Ni(II) > Co(II) > Cr(II) > Mn(II) >Al(III) > Na(I). The Kr value of PAA-CMCs is higher than CMCs, indicating that PAA-CMCs has a better affinity for Ni(II)

-p

than CMCs. Unfortunately, both adsorbents have poor adsorption properties for Na(Ⅰ) and Al(Ⅲ). This may be attributed to the fact that the Na(Ⅰ) with large ion radius are difficult to be trapped by -COO-, -NH2

re

and -OH groups in the adsorption process to form stable ionic bonds [22]. Moreover, more binding sites

lP

were consumed to adsorb Al(Ⅲ) due to the valence states of Al(Ⅲ) was higher than Ni(Ⅱ), leading to poor

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Al(Ⅲ) adsorption [35]. The radii of different metal ions were listed in SI.

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3.7 Analysis of selectivity mechanism

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Ligand binding phenomenon exists widely in the process of ion selective binding and has been the focus of researchers [51]. Most binding process take the form of covalent, coordination, or hydrogen bonds, which are accompanied by electron transfer [52]. Ionization energy (I) is the minimum energy required for an atom to lose electrons. Besides, the electron affinity (E) reflects the ability of an atom to gain electrons. The adsorption process is a process of gaining and losing electrons between functional groups and metal ions, so 19

it is not accurate to only use I or E to describe it. Therefore, the symbols such as electronegativity (symbol χ) and electrophilic index (symbol ω) that describe chemical reactions more accurately and completely should also be considered. All the parameters were listed in Table 7. 3.7.1 Electronegativity As an inherent property of elements, electronegativity takes both the I and E into consideration. Pauling

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described in a broadly qualitative manner as "the ability of atoms to attract electrons in the molecules of compounds" [53]. Gordy further proposed that the electronegativity of an element can be expressed as the effective potential resulting from the unscreened nuclear charge of the bonded atom on a bonding [54].

𝜒 = 0.31

𝑛+1±𝑐 𝑟

-p

Thereby, the electronegativity can be expressed as follow: + 0.50

(18)

re

Where n represents the number of electrons in the valence shell, r is the covalent radius, c is a formal charge

lP

of electron unit on the atom.

According to Gordy's work, the electronegativity values of metal elements in this study are shown in Table

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7 [54]. The high electronegativity of metal element revealed the strong electron attraction ability, resulting in the high stability of metal-complex [55]. It can be seen from the Table 7 that Ni(II) have the highest

ur

electronegativity (χ=1.91), which indicates that Ni(II) have the strongest binding ability with adsorbent

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molecules. This is consistent with the results of in the previous literatures [56-59] 3.7.2 Electrophilic index The electrophilic index was a special property of a chemical species which can be defined by the square of electronegativity divided by chemical hardness. Although both ω and E were measured the ability of ligand to get electrons, E reflects the ability to accept only one electron from the environment and while the 20

electrophilic index ω measures the energy lowering of a ligand due to maximal electron flow between donor and acceptor [60]. Therefore, the electrophilic index can be expressed as follow: 𝜔=

(𝐼+𝐸)2

(19)

8(𝐼−𝐸)

As shown in Table 7, the electrophilic index of metal ions was arranged in the following order: Ni(II) > Co(II) > Cr(II) > Mn(II) > Al(Ⅲ) > Na(Ⅰ), which again demonstrates that the preferential adsorption of Ni(II)

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by PAA-CMCs and CMCs.

3.8 Regeneration and reusability studies

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3.8.1 Desorption

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Fig.15a shows the desorption rates of PAA-CMCs microspheres with different desorption agents. It can be seen that 0.1mol L-1 HCL has the highest desorption rate, up to 97.5%. Under the effect of H+ in CH3COOH

lP

or HCl, the -NH2 and -COO- groups in PAA-CMCs were protonated (Eq.24 and Eq.25), resulting in the deprivation of Ni(II) from the binding site. Moreover, the low value of DE for CH3COOH can be explained

na

by the incomplete ionization of CH3COOH in aqueous solution. The NaOH through forming chemical

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precipitates to removes Ni(II) from PAA-CMCs (Eq.26), which shows an inefficient desorption ability. Compared with the other agents, it suggested that HCl proved to be quite efficient in eluting Ni(II) from the

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PAA-CMCs.

𝑝𝑟𝑜𝑡𝑜𝑛𝑎𝑡𝑒𝑑

𝑅 − 𝐶𝑂𝑂 − 𝑀𝑛+ + 𝐻 + →

𝑝𝑟𝑜𝑡𝑜𝑛𝑎𝑡𝑒𝑑

𝑅 − 𝑁𝐻2 − 𝑀𝑛+ + 𝐻 + →

𝑅 − 𝐶𝑂𝑂𝐻 + 𝑀𝑛+

(20)

𝑅 − 𝑁𝐻3+ + 𝑀𝑛+

(21)

𝑀𝑛+ + 𝑛(𝑂𝐻− ) → 𝑀(𝑂𝐻)𝑛 ↓

21

(22)

3.8.2 Reusability Fig.15b shows the adsorption capacity of PAA-CMCs for continuous six adsorption–desorption cycles with 0.1mol L-1 HCl (as the desorption agent). Although the adsorption capacity of PAA-CMCs decreased slightly with the increase of cycle times, the RE is still higher than 89.7% after six adsorption-desorption cycles. The decrease in RE can be attributed to the fact that the incomplete desorption of PAA-CMCs and the irreversible

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protonation of the functional groups. 4. Conclusion

The PAA-CMCs was prepared by grafting PAA onto CMCs with K2S2O8 as initiator and MBA as cross-

-p

linking agent. The L-F model fits the equilibrium data best among the isotherm models and the maximum Ni(II) adsorption capacity of PAA-CMCs was up to 200.588 mg g-1 at the optimal condition (the AA dosage

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of 10 g, adsorbent dosage of 0.05 g and pH=5). Moreover, the results of adsorption experiment and

lP

characterization analysis revealed that the -COO- play a dominant role in adsorption process. Although the coexisted cation in binary solution show a certain influence on targeted ions adsorption, PAA-CMCs always

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show priority for Ni(II) adsorption which can be illustrated by electronegativity and electrophilic index. Besides, the RE of PAA-CMCs was still higher than 89.7% after six cycles of adsorption-desorption.

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Therefore, PAA-CMCs is expected to be a promising high-performance adsorption material for

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multicomponent wastewater deep treatment.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

22

Acknowledgements This work is supported by National Natural Science Foundation of China (51578239, 21878099, 51608203), and China Postdoctoral Science Foundation (2019TQ0094).

CRediT authorship contribution statement

Zhishan Bai: Ideas; Resources; Funding acquisition Bingjie Wang: Supervision; Writing- Reviewing and Editing Shenghao Zhao: Experiment

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Yong Zhang: Software; Experiment

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Tao Tian: Writing-Original Draft; Methodology

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[60] R.G. Parr, L. Szentpaly, S. Liu, L. Von Szentpaly, L. Szentpály, T. Kigawa, R. Parr, Electrophilicity

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Figure captions

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Fig.1 Schematic diagram of CMCs preparation

29

Fig.2 Schematic diagram of PAA-CMCs preparation

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Fig.3 SEM micrographs of (a) CMCs and (b) PAA-CMCs

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Fig.4 FT-IR spectra: (a) CMC powder; (b) CMCs; (c) PAA-CMCs

Fig.5 Effect of the (a) dosage of AA and (b) dosage of PAA-CMCs on Ni(Ⅱ) adsorption (Ni(II) solution=50 mL, Ni(II) concentration=150 mg L-1, T=298 K)

30

Fig.6 Effect of (a) the pH of solution on Ni(Ⅱ) adsorption (Ni(II) solution=50 mL, Ni(II)

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concentration=150 mg L-1, T=298 K) and (b) the determination pHpzc of adsorbent by drift method

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Fig.7 The variation on the adsorption amount (qt) of two adsorbents with contact time (Ni(II) solution=50

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na

mL, Ni(II) concentrations=150 mg L-1, T=298 K, adsorbents dosage=0.05 g)

Fig.8 the linear fitting curves of (a) pseudo-second-order model; (b) intraparticle diffusion model (Ni(II) solution=50 mL, Ni(II) concentrations=150 mg L-1, T=298 K, adsorbents dosage=0.05 g )

31

Fig.9 Isotherms of (a) two-parameter and (b) three-parameter models obtained using the non-linear method

ro of

for the adsorption of Ni(II) on CMCs and PAA-CMCs at 318 K (Ni(II) solution=50 mL, Ni(II)

lP

re

-p

concentrations=50-500 mg L-1, adsorbents dosage=0.05 g)

Jo

ur

na

Fig.10 FT-IR spectra of PAA-CMCs (a) before and (b) after adsorption

32

-p

ro of

Fig.11 XPS spectra of PAA-CMCs before and after adsorption

re

Fig.12 N1s spectra of PAA-CMCs (a) before and (b) after adsorption; O1s spectra of PAA-CMCs (c)

Jo

ur

na

lP

before and (d) after adsorption

33

ro of

Fig.13 Schematic diagram of adsorption mechanism

lP

re

-p

Fig.14 Selective adsorption capacity of (a) CMCs and (b) PAA-CMCs in binary solution system

na

Fig.15 (a) The desorption rates of PAA-CMCs with different desorption agents; (b) the adsorption capacity

Jo

ur

of PAA-CMCs for continuous six adsorption–desorption cycles

34

Table

Table 1 Weight Ratio of C, N, H and O Elements C (Wt %)

N (Wt %)

H (Wt %)

O (Wt %)

CMCs

31.77

3.95

4.70

32.51

PAA-CMCs

30.87

2.47

5.01

40.83

Table 2 Parameters of three kinetic models CMCs

Pseudo-first-order

PAA-CMCs

qe-cal

k1 (×10-2)

(mg g-1)

(g mg-1 min-1)

45.014

0.121

qe

k2 (×10-5)

(mg g-1)

(g mg-1 min-1)

62.112

5.252

C

Kp

ro of

Kinetic models

qe-cal

k1 (×10-2)

(mg g-1)

(g mg-1 min-1)

42.731

0.101

qe

k2 (×10-5)

(mg g-1)

(g mg-1 min-1)

83.963

9.275

C

Kp

(mg g-1)

(g mg-1 min-1)

R2

model

Intraparticle (mg

g-1)

(g

mg-1

R2

min-1)

diffusion model 1.261

0.978

lP

3.019

0.992

-p

order model

R2

re

Pseudo-second-

0.931

35.328

1.357

R2

0.862 R2

0.999 R2 0.945

Table 3 Fitting parameters of two-parameter isothermal models CMCs

Models

Parameters

308 K

318 K

298 K

308 K

318 K

108.316

109.941

111.832

137.306

189.913

194.241

0.014

0.019

0.023

0.026

0.026

0.043

R2

0.994

0.991

0.989

0.980

0.988

0.982

KF (L mg-1)

9.485

12.954

16.241

20.607

24.152

36.684

n

2.502

2.818

3.072

3.083

2.774

3.311

R2

0.988

0.977

0.974

0.901

0.940

0.921

qm (×10-3) (mol g-1)

3.829

3.598

3.556

4.435

6.655

6.256

Kd (×10-9) (mol2 J-2)

5.213

4.267

3.642

4.073

4.128

3.133

R2

0.991

0.988

0.987

0.929

0.964

0.949

9.793

10.824

11.717

11.079

11.006

12.633

na

qm (mg g-1) bL (L

mg-1)

ur

Langmuir

Jo

Freundlich

D-R

298 K

PAA-CMCs

E (kJ

mol-1)

Table 4 Fitting parameters of three-parameter isothermal models

35

Models

KLF (L

PAA-CMCs

298 K

308 K

318 K

298 K

308 K

318 K

g-1)

118.049

120.896

122.014

127.991

190.732

200.588

mg-1)

qm (mg L-F model

CMCs

Parameters

0.006

0.007

0.008

0.021

0.025

0.026

γ

0.699

0.619

0.574

0.754

0.967

0.749

R2

0.997

0.998

0.999

0.991

0.994

0.992

KRP (L mg-1)

2.075

3.010

3.918

3.190

4.918

8.334

αRP (L

0.044

0.064

0.078

0.015

0.025

0.042

β

0.857

0.856

0.863

1.077

1.006

1.002

R2

0.996

0.996

0.995

0.982

0.989

0.985

R-P model

mg-1)

Table 5 Adsorption capacity of the reported chitosan-based or CMC-based adsorbent for Ni(II)

Ni (II) ion imprinted CMC

6

82.78

Cross-linked CMC Resin

6

112.09

Xanthated CMC Beads

7

128.4

4-7

161.8

PAA-g-CS hydrogel

6

PAA-CMCs

5

[45] [46] [47] [21]

122.014

This work

200.588

This work

re

CMCs

Reference

ro of

qm (mg g-1)

-p

pH

Adsorbents

Table 6 Distribution coefficients (Kd), selectivity coefficients (Ks) and relative selectivity coefficients (Kr)

lP

of different metal ions CMCs Kd

Ks-c

0.569 0.013

Ni(II) Mn(II) Ni(II)

0.382 0.101

3.755

0.115

Al(Ⅲ)

7.612

Co(II)

0.161

Ni(II)

0.391

Jo

0.306

0.051

Kr

1.102 0.018 0.141

61.222

1.431

4.879

1.299

3.901

1.232

2.305

1.207

8.894

1.168

0.636

3.165 1.909

Ni(II)

Ks-p

0.688

0.366

ur

Cr(II)

42.783

na

Ni(II) Na(Ⅰ)

PAA-CMCs Kd

0.163 0.544 0.236 0.756 0.085

Table 7 Electronegativity (χ), Ionization Potential (I), Electron Affinity (E), Electrophilic Index (ω) (Unit: eV)

Na(Ⅰ)

χ

I

E

ω

0.93

5.14

0.55

0.88

36

1.55

7.43

0.00

0.93

Cr(II)

1.66

6.77

0.67

1.13

Co(II)

1.88

7.88

0.66

1.26

Ni(II)

1.91

7.64

1.16

1.49

Al(Ⅲ)

1.61

5.99

0.93

0.93

Jo

ur

na

lP

re

-p

ro of

Mn(II)

37