One-step growth of porous cellulose beads directly on bamboo fibers

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One-step growth of porous cellulose beads directly on bamboo fibers via oxidation-derived method in aqueous phase and their potential for heavy metal ions adsorption Kaifeng Du, Shikai Li, Liangshen Zhao, Liangzhi Qiao, Hao Ai, and Xiaohong Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04433 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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ACS Sustainable Chemistry & Engineering

One-step growth of porous cellulose beads directly on bamboo fibers via oxidation-derived method in aqueous phase and their potential for heavy metal ions adsorption

Kaifeng Du*, Shikai Li, Liangshen Zhao, Liangzhi Qiao, Hao Ai, Xiaohong Liu

Department of Pharmaceutical & Biological Engineering, School of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China

Corresponding author: Dr. Kaifeng Du Department of Pharmaceutical & Biological Engineering, School of Chemical Engineering, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu 610065, China Tel.: +86-28-85405221; Fax: +86-28-85405221 Email: [email protected]

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ABSTRACT: Cellulose beads consist of natural polysaccharide fibers and are widely used in various industrial materials and applications. Nevertheless, the key challenge is how to effectively avoid the tedious preparation process and environmentally-harmful chemicals that exist in traditional fabrication routes. This study proposes a green and facile route to fabricate porous cellulose beads by selecting bamboo fibers as raw material. Without tedious multistep process (dissolution, emulsification and regeneration) and environmental harmful solvents, porous cellulose beads were formed directly on bamboo fibers under aqueous phase with sodium periodate. Varied methods of SEM, XRD were applied to characterize porous cellulose beads at different fabrication stages. By this investigation, the feasible mechanism toward spherical cellulose formation was raised. After being modified with glycine, the cellulose beads were evaluated by means of adsorption isotherm and kinetics for investigating its adsorption potential by using Co2+ and Cu2+ as probes. All of results illustrated that the porous cellulose beads by this strategy showed excellent adsorption efficiency for the removal of metal ions from wastewater. Keywords: porous cellulose beads; bamboo fibers; green chemistry; adsorption; adsorbent.

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INTRODUCTION Cellulose is one of most common natural raw materials on earth with biosynthesis production of almost 1011-1012 dons per year.1 In general, the cellulose material possesses excellent mechanical property together with the features of natural resource, such as low cost, abundance, environmentally friendly, biodegradability, and renewability. Furthermore, the structure of cellulose network is composed of β-1,4 linked D-glucose unites with lots of hydroxyl groups, which provides a accessible path to convert its surface properties from hydrophilic to hydrophobic and/or from non-charged to anionic or cationic.2-6 With these advantages, cellulose based material serves as an effective substitute to synthetic polymer materials based on petroleum oil resource and has been used widely in varied applications.7 For better utilization, cellulose materials are often constructed into well-defined and varied objects such as fibers, films, sponges and beads with different size and pore structure.5 Among these structure geometries, spherical cellulose is a versatile geometry for numerous applications, especially when it is chemically modified to increase its functionality. For example,

surface-functionalized

cellulose

beads

serve

as

excellent

adsorbents by filling them into column for purifying human bloods and/or separating varied substances such as proteins, endotoxins, organic dyes, and heavy metal ions, and so on.8-10 The popularity lies in that constructing adsorbent into spherical geometry facilitates both high bed permeability and 3



large binding capacity for high adsorption efficiency.11-14 Despite that, pharmaceutical applications have been reported with the use of porous cellulose beads. Typically, varied drugs are loaded into these porous cellulose beads, and afterwards released gradually for diseases treatment. Given these widely applications, there is great requirement for porous cellulose beads with suitable porous structure, which promotes the development of fabrication techniques for producing spherical cellulose. Until now, several strategies have been applied to prepare porous cellulose beads, including water/oil emulsification, enzymatic hydrolysis of cellulose fibers, spray-drying method, and micro-fluid technique.15 Although these preparation routes have been brought to commercial applications for production of cellulose beads, they showed several disadvantages and hampered the utilization of cellulose beads to some extent. For example, spray-drying and micro-fluid techniques not only need special machines together with more heating energy consumption but also often lead to a broad size distribution of cellulose beads.11-13 While the water/oil emulsification requires lots of organic solvent and surfactants for constructing spherical cellulose materials, which would produce more additional waste and also conduce to high production cost.14-17 Therefore, it is necessary to explore novel way to prepare high-performance cellulose beads. Recently, a periodate oxidation method has been developed for preparing porous cellulose beads, in which algae Cladophora was used as cellulose 4


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resource. With this oxidation technique, the porous cellulose beads were spontaneously formed in one step and without the need for complex procedures, e.g., dissolution of raw cellulose fibers, emulsification by organic solvents, and regeneration. Obviously, the oxidization method seems like novel and interesting for producing porous cellulose beads. But it is hard to extend to industrial production since the algae, as a special cellulose resource, is limited in output. Despite that, the algae-derived cellulose beads exhibited weak mechanical stability and poor porosity during the drying process, which compromised its practice value for adsorption operation. In this way, it is necessary to make more efforts to develop this oxidized process and suitable cellulose raw materials toward high-performance porous cellulose beads. For this purpose, bamboo fabrics were studied as cellulose resource in this study. The choice of bamboo is ascribed to two points. Firstly, different from algae, bamboo has unique shape of long and straight fibrils and high-degree crystallinity, which endow it more suitable for uncovering the bead formation mechanism during the oxidization process. Secondly, the bamboo is rich and common in nature, utilization of bamboo is more competent and favorable to extend

the

production

bamboo-based

cellulose

industrialization.18,19 beads,

the

After

physical

the

fabrication

characterizations

of

were

performed in details to explore the external morphology, pore structure and surface chemistry of cellulose beads. And, after being modified with glycine,

5



the cationic porous cellulose beads were evaluated for its adsorption efficiency by using Co2+ and Cu2+ aqueous solutions as model waste water.

MATERIALS AND METHODS Chemicals. Bamboo cane collected from local bamboo trees. Sodium metaperiodate (NaIO4), was purchased from Aladdin (Shanghai, China). Formic acid, acetic acid, hydrogen peroxide, glycine and other chemicals used were received from Kelong Chemical (Chengdu, Sichuan, China). All chemical agents used were analytical grade. Pretreatment of bamboo fibers. The bamboo cane was firstly cut into chips in length of 2-3 mm by mechanical method. Furthermore, 20 g dried bamboo chips were put into 100 mL of formic solution (88% of mass fraction) with 2.4 g hydrogen peroxide. The reaction mixture was stirred intensely at 90℃ for 2 h. Subsequently, the bamboo fibers were collected from the mixture by filtration and further treated with 12% NaOH for 12h at room temperature. With the repeated reactions of three times, the lignin and semi-cellulose were removed completely from bamboo fibers. The treated cellulose fibers were washed with deionized water and kept as the starting material. Preparation of porous cellulose beads. The porous cellulose beads were prepared by an improved oxidation process with sodium periodate.20 Briefly, 10 g bamboo fibers were dispersed in 800 mL of acetic acid buffer (pH 4.0) containing 70 g of sodium periodate (about 5 mol per mol of anhydrous 6


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glucose unites). The reaction mixture was stirred with a magnetic stirrer at room temperature under the dark by the aluminum sheets protection for several hours. During the oxidation process, the porous cellulose beads were generated along with bamboo fibers. Finally, the reaction was quenched by dropping ethylene glycol into the reaction system and the prepared cellulose beads were washed with pure water. Preparation of Schiff-Base chelating modified porous cellulose beads. The glycine-modified porous cellulose beads were prepared by Schiff-base reaction. The chemical route was described as follows.21 At first, 5 mmol glycine and 5 mmol NaOH were dissolved into 50 mL DMSO/water solvent in volume ratio of 1:1. Then the mixture was poured into a 250 mL three-necked flask, and followed by the addition of 50 mL DMSO and 400 mg cellulose beads. The reaction system was heated at 60 ℃ and kept for 12 h under continuous stirring, which led to the generation of glycine-modified cellulose beads. Finally, the modified cellulose beads were washed with deionized water and dried in ambient air at room temperature. Characterization. The microscopic morphology of porous cellulose beads was investigated by scanning electron microscopy (SEM; Philips XL30, Netherlands), and the information on chemical bonds of samples was analyzed by Fourier transform infrared spectroscopy (FTIR; Bruker Optics Tensor 27, Germany). X-ray diffraction (XRD) patterns of the samples were analyzed on an X-ray diffractometer (Rigaku D/max-2550pc, Japan) with a 7



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scan range between 5° and 40° of 2-theta. For the determination of aldehyde groups, the oxidized cellulose beads were reacted with hydroxylamine by Schiff base reaction and followed by being analyzed through atomic absorption spectroscopy.22 The N2 adsorption measurement was used to evaluate the specific surface area of porous cellulose beads by BET method (NOVA 2000 porosimeter, Quantanchrome, USA). Adsorption

experiments.

The

adsorption

of

Co2+and

Cu2+

on

glycine-modified porous cellulose beads was tested by batch experiments at different pH values (3.0−8.0). Adsorption equilibrium isotherms were carried out with the initial concentrations in the range of 20−200 mg L−1; adsorption kinetics was analyzed in different time intervals from 0 to 150 min. In the typical adsorption operations, the experiments were carried out in flasks containing 0.1 g of cellulose beads and 100 mL of metal ion solution at room temperature with shaking for defined time intervals. The initial pH values were adjusted using 0.1 M HNO3 and 0.1 M NaOH solutions. After the completion of adsorption, the cellulose beads were separated from solution by centrifugation at 5000 rpm for 5 min. Meanwhile, the residual heavy metal concentrations in solutions were determined by the atomic adsorption spectrophotometer (AAS). The adsorption capacity of cellulose beads toward heavy metal ions was calculated by the following equation (1): q 

C 2

 C1   V M

(1)

8


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where q is equilibrium adsorption capacity of metal ions (mg g−1); C1 and C2 (mg L−1) are initial and equilibrium metal ions concentrations in solutions, respectively. V (L) is the volume of the adsorbed solution, and M (g) is the weight of dried modified cellulose beads.

RESULTS AND DISCUSSIONS Characterization of porous cellulose beads. A schematic diagram for the self-growth of porous cellulose beads on bamboo fibers was present in Figure 1. First, bamboo chips were prepared by mechanical process. Subsequently, removing of hemi-cellulose and lignin from bamboo chips by formic acid and H2O2 gave the production of pure cellulose fibers with high degree crystallinity. For bead formation, high degree crystallinity of cellulose sample was considered as the key point for self-growth of cellulose beads. Subsequently, the perodate oxidation was performed on cellulose fibers, leading to the conversion of anhydroglucose into noncyclic 2,3-dialdehyde structure. With a curing process, the oxidized cellulose long chains swelled partly and reassembled into porous spherical cellulose. The advantage of this proposed strategy lies in that the cellulose beads were prepared in one step and in pure aqueous phase without the utilization of organic solvents. Hence, the proposed synthesis route was regarded as a green technique For practical application, these cellulose beads were modified with glycine to form cationic adsorbent and then evaluated by the metal ions 9



adsorption from effluent.

Figure 1 Schematic strategy for fabrication of glycine-modified porous cellulose beads.

The SEM images in Figure 2 showed the self-grown process of cellulose beads along the bamboo fibers in different stages, indicating the drastic morphological changes induced by periodate oxidation. It found in Figure 2A that the bamboo–derived cellulose fibers, serving as the starting materials, were straight rods with smooth surface in diameter size of about 4 μm. After a period of oxidation reaction, the surface of cellulose fibers became more rough and flexible (Figure 2B), indicating that part of surface cellulose was converted into loosen oxidized cellulose long chains and then dissolved into solvent. As the degree of oxidation increased, more oxidized cellulose long chains swelled on cellulose fibers in the solvent. With the reaction continued further, some of 10


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these cellulose long chains were reassembled into spherical states by hydrogen bonding interactions. This can be proved by that a cluster of porous cellulose beads grew gradually on the surface of bamboo fibers (Figure 3C). At highest degree of oxidation, the porous cellulose beads grew into a relatively uniform and more compact spherical morphology and dropped finally from the bamboo fibers, as the observation of Figure 2D. Meanwhile, the stiffness of oxidized cellulose fibers reduced gradually since the cellulose beads production reduced to some extent the fiber dimension.

Figure 2 SEM images of bamboo fibers at different stages: raw materials (A); early oxidation reaction (B); porous cellulose beads at initial stage (C) and final porous cellulose beads (D).

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For better explanation, the comparison between raw material, cellulose in the oxidized process, and cellulose beads were tested by FTIR analysis and the results were shown in Figure 3A. Comparing with raw cellulose material, a characteristic peak at 1731 cm-1 was generated and observed in the oxidized cellulose samples, which was ascribed to the conversion of -OH (3375 cm-1) on C2, C3 positions into -CHO groups by the oxidation reaction.22 In addition, a new peaks at 885 cm-1 and a obviously decrease peak at 1731 cm-1 (-CHO) were found on the oxidized porous cellulose beads, which was ascribed to the further reaction between the -CHO and –OH terminals.20 The crystallographic structure of varied cellulose samples was analyzed by X-ray diffraction (Figure 3B). As showed here, the width of diffraction peaks of cellulose fiber seemed nearly constant and only the peak heights diminished along with the oxidation reaction. It demonstrated that the oxidation reaction occurred only on the external surface of cellulose fibers, which was also consistent to the observation in Figure 2. As for the samples of oxidized cellulose beads, the characteristic peaks of cellulose crystal form reduced obviously, indicating that the crystallographic structure was destructed absolutely. Accordingly, the prepared cellulose beads were presented in completely amorphous and disordered state. The loss of crystalline was ascribed to the ring-opening reaction of glucose molecules together with the destruction of their ordered packing.23 Furthermore, the prepared porous cellulose beads were analyzed by BET method and their specific surface area was determined to be about 58.7 m2 g-1, which was larger than other 12


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similar porous cellulose adsorbents. Obviously, the larger specific surface area is ascribed to both the conversion of crystalline to amorphous state and the more porous texture of cellulose network.

Figure 3 FT-IR spectra (A) and XRD patterns (B) of raw bamboo fibers, fibers in the reaction process, and cellulose beads.

Combining with the analysis of SEM, FT-IR, and XRD, the mechanism of one-step growth of porous cellulose beads by the oxidation reaction of NaIO4 in aqueous phase was proposed as follows. First, sodium periodate mediated oxidation on cellulose fibers is a highly selective reaction, which oxidized the OH terminals of

C2 and C3 positions on cellulose fibers to the corresponding

aldehydes.23 Further oxidation facilitated the swelling of oxidized cellulose fibers in the solution. This was ascribed to the following reaction. Under the oxidization process, the cellulose crystalline was converted into amorphous oxidized cellulose long chains together with the formation of dialdehyde terminals. Such surprising finding was contrast with the previous report by 13



Ung-Jin Kim, in which the oxidation often crushed the cellulose fibers into short fragments.23 With the curing process, the dissolute oxidized cellulose long chains were reassembled with hydrogen bonding interaction and then caused the swelled part being gathered spherically layer by layer. Finally, with the excessive oxidation of the terminal reductive glucose, which is called “peeling reaction”, the cellulose beads “grew” big enough to drop from the fiber bone. Thus, the porous cellulose beads with lots of active groups of -CHO and -OH could be obtained. By elemental analysis, the amount of aldehyde groups on oxidized cellulose beads were determined to be about 14.7 mmol per gram of cellulose sample, which provided the sufficient active sites for the immobilization of adsorption ligands. The interesting finding lies in reducing obviously the oxidation time for fabricating the cellulose beads. By this way, the production cost reduced effectively and facilitated the following industrial production. Adsorption evaluation of glycine-modified porous cellulose beads. In general, the adsorbent is composed of both support and adsorption ligands. In terms of adsorbent, the adsorption of special substances was determined by the immobilized ligands on supports. In this context, glycine was choose as the adsorption ligands because of its simple chemical structure, non-toxicity, and containing both N and O donor atoms.24 Based on Schiff-base reaction, a novel cationic adsorbent was prepared by modifying glycine onto the prepared porous cellulose beads. And, the glycine modified adsorbent was evaluated 14


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for heavy metals ions adsorption from effluents. The adsorption process is illustrated in Figure 1. Adsorption effect of pH. The pH value in solution plays an important role on the adsorption behavior of an adsorbent because it affects not only the protonation degree of adsorption ligands but also the heavy metal ion forms in the solution.25,

26

For clear elucidate this, a series of batch equilibrium

experiments were carried out to testify the effect of pH on the adsorption of heavy metal ions on the adsorbent by using Co2+ and Cu2+ solution of initial concentration of 30 mg L−1, respectively. In addition, the pH in solution was adjusted in the range of 3.0-8.0 by using dilute HNO3 or NaOH solution, and the results are indicated in Figure 4.

15



Figure 4 Distribution of heavy metal ionic species as a function of initial pH and the effect of initial pH on adsorption capacity on adsorbent (inset in Figure 4).

As seen in Figure 4, the adsorption capacities of Co2+ and Cu2+ increased dramatically along with increasing the initial pH value of solution from pH 3.0 to 5.0. It was ascribed to the reduced competition between positively H3O+ and metal ions on the adsorption sites of adsorbent. While being in the lower pH value, there were more H3O+ ions in metal ions solution, which resulted in a fierce competition in the ion-exchange sites on the adsorbent. Due to the presence of repulsive force, the metal ions would be hindered from approaching the adsorbent sites and then compromised the adsorption capacity. 16


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Obviously, the adsorption capacity of metals ions on the glycine-modified adsorbent depends upon the pH value of the solution. Figure 4 presents the relationship between the adsorption capacities of metals ions and the initial pH values of the solution. As for the metal ions species in effluent, the cobalt in effluent served mainly as Co2+ when pH was lower than 7.0. At pH 6, the hydrolytic product of cobalt appeared in form of CoOH+. When pH value exceeded 8.0, the cobalt ions were converted into Co(OH)2 and precipitated from the effluent. The inset of Figure 4A showed that the adsorption capacities of Co2+ increased with increasing pH values from 3.0 to 7.0, and decreased obviously when exceeding pH 8.0. This phenomenon was similar for the Cu2+ adsorption. In theory, there should be no significant decrease for both metal ions adsorption on adsorbent at pH 8, because the divalent metal ions existed still dominantly in the effluent. Based on the adsorption study, this reduction of adsorption capacity was ascribed to the drop-off of effective adsorption sites. At pH 8.0, with the amount of OH- ions increased gradually, the structure of cross-linked semi-acetal decomposed and then compromised the adsorption capacity. In view of the role of initial pH of solution on the adsorption efficiency, all these results indicated that the adsorption ability of adsorbent was strong in nearly neutral solution and poor in acidic or alkaline solutions. Adsorption isotherms. The adsorption isotherm analysis is of fundamental importance in determining the adsorption capacity of the adsorbent and it diagnoses the nature of adsorption. In this part, Langmuir and Freundlich isotherm models (Equations 2 and 3) were applied to interpret the equilibrium adsorption data. The 17



Langmuir isotherm is based upon an assumption of monolayer adsorption onto a surface containing a finite amount of adsorption sites with no transmigration of adsorbate in the plane of the surface. The Langmuir isotherm model is described as follows: Langmuir:

Ce Ce 1   Qe Q m Q m K L

(2)

The Freundlich isotherm model assumes heterogeneous surface energy on the surface of adsorbent. The Freundlich isotherm model is described in the following equation: 1

Freundlich: Qe  K F C e n

(3)

where Ce is the equilibrium concentration (mol L−1), Qe the amount of adsorbed material at equilibrium (mol g−1), KL the “affinity” parameter (L.mg−1) and Qm the “capacity” parameter(mg g−1) as Langmuir constants. KF and n are Freundlich parameters. From Figure 5, it can be seen that the static adsorption capacities of glycine-modified cellulose beads increased gradually and approached the maximum values of about 115 mg g−1 and 92 mg g−1 for Co2+ and Cu2+ adsorption, respectively, when the concentrations of Co2+ and Cu2+ increased from 20 to 200 mg L−1. The results demonstrated that the initial metal ions concentration provided the large driving force to overcome the mass transfer resistance of metal ions between the aqueous phase and the solid phase. The validity of used isotherm model was tested by comparing the experimental and calculated data (Figure 5). As the fitted curves by isotherm models, the Langmuir gave a better fit over the entire range of concentration 18


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than that of Freundlich, indicating that the adsorption behavior obeyed the Langmuir adsorption mechanism. The isotherm parameters from isotherm model are useful for predicting the adsorption capacity and also for incorporating into mass transfer relationship to guide the design of batch reactor. In this way, the values of all the isotherm parameters were calculated from the slope and intercept of the plots using regression analysis and were tabulated in Table 1. By Fitting Langmuir model, the maximum capacities (Qe) and the association constant (KL) for Co2+ on adsorbent were determined to be about 141.1 mg g−1 and 0.079 L mg-1, respectively, which were larger than those for Cu2+ adsorption. Obviously, higher value of KL was related to stronger interaction between metal ions and adsorption sites, and then contributed to larger adsorption capacity.

Figure 5 Adsorption isotherms of Co2+ and Cu2+ on glycine-modified porous cellulose beads.

Table 1 Adsorption

isotherm constants for Co2+ and Cu2+ adsorption on

glycine-modified porous cellulose beads. 19



isotherm equations

Langmuir

Freundlich

Page 20 of 32

parameters

Co2+

Cu2+

Qm(mg g-1)

141.1

110.7

KL(L mg-1)

0.079

0.062

R2

0.988

0.990

KF (mg1-1/nL1/ng-1)

26.628

18.591

n

2.756

2.728

R2

0.899

0.900

The proposed glycined-modified cellulose beads exhibited excellent adsorption performance, as described above. To further highlight it, the prepared cellulose beads were compared with other similar adsorbents for the same heavy metal adsorption. The static adsorption capacities of Co2+ and Cu2+ on varied adsorbents from different origins were summarized in Table 2. As seen here, the prepared glycined modified cellulose beads possessed larger adsorption capacities of 141.1 and 110.7 mg g-1 for Co2+ and Cu2+ adsorption (Langmuir model) in comparison with those reported literatures. The excellent adsorption capacity of metal ions on the adsorbent was ascribed to the amorphous and porous texture of cellulose network, which provided more adsorption sites for the metal ions adsorption.

Table 2 Adsorption comparison of Co2+ and Cu2+ between varied adsorbents 27-32

20


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static adsorption capacities (mg g-1) adsorbents Co2+

Cu2+

Pyridine-pyrazole chelating polymer

14.14

94.72

Schiff-based chelating polymer

71.29

81.72

Ion exchange resin

16.53

19.92

Titanium silicate zeolitic materials

58.93

106.88

Magnetic HAP/Aga hybrid beads

105.1

71.6

Carboxylated cellulose ad34sorbent

51.71

107.7

Porous cellulose beads (our study)

141.1

110.7

Adsorption kinetics. The effect of metal ions concentration on adsorption rate is very important in understanding adsorption mechanism. In order to quantify the variation of adsorption capacity with time and also evaluate the kinetic parameters for different metal ions concentrations, two kinetic models (Equations 4 and 5) were used to fit the kinetic adsorption data, and the results were summarized in Figure 6 and Table 3.33，34 Pseudo-first-order kinetic model: log(q e  qt )  log q e  Pseudo-second-order kinetic model:

t 1 t   2 q t k 2 qe qe

k1 t 2.303

(4)

(5)

where qt is the adsorption capacity in time t (mg g−1), andk1, k2 are the adsorption rate constants of pseudo-first-order (min−1), pseudo-second-order (g mg−1 min−1), respectively. Figure 6 showed the experimental and fitted adsorption capacities for 21



Co2+ and Cu2+ along with increased adsorption time at different concentrations. From the experimental adsorption data, it found that the adsorption was very rapid and up to the maximum uptake taking place within 30 min and reached adsorption equilibrium at 60 min. The result demonstrated the rapid surface adsorption of metal ions on adsorbent, which was mainly ascribed to the large amount and accessible adsorption sites on adsorbent. Initially, the adsorption sites were accessible easily for the metal complex and hence a higher rate could be observed at the beginning of adsorption process. With further adsorption, the adsorption sites entrapped in the external surface of adsorbent consumed gradually, which then compromised the adsorption rate. This adsorption process can be described quantitatively by kinetic model. Fitting these adsorption data into two kinetic models gave the kinetic plots and the kinetic parameters (Figure 6 and Table 3). Comparing the correction coefficients (R2) of Table 3 proved that the pseudo-second-order model fitted better the experimental adsorption data than the pseudo-first-order modelat different concentrations. Meanwhile, the values of k1 and k2 for both metals ions adsorption decreased regularly with an increase of initial metal ions concentrations. And, the value of qe for Co2+was larger than that for Cu2+under the same adsorption conditions. These results can be attributed to the smaller size of Co2+ over Cu2+, which Co2+ ions possess higher charge density due to their smaller size and favors a strong attraction into the lone pair of electrons in the nitrogen and oxygen atoms for the more stable complexes.

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Figure 6 Adsorption kinetics of Co2+ and Cu2+on glycine-modified porous cellulose beads.

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Table 3 Kinetic parameters for the adsorption of Co2+ and Cu2+ on glycine-modified porous cellulose beads

Co2+ isotherm equations

Cu2+

parameters 50(mgL-1)

120(mgL-1)

50(mgL-1)

200 (mgL-1)

qt,exp(mgg-1)

40.7

95.8

38.9

94.3

qt,cal(mgg-1)

39.4

92.4

37.9

85.3

k1(min-1)

0.186

0.129

0.131

0.117

R2

0.929

0.961

0.964

0.967

qt,cal(mgg-1)

46.3

101.8

42.2

95.7

0.006

0.002

0.004

0.002

0.989

0.988

0.994

0.994

pseudo-first-order

pseudo-second-order K2(gmg-1min-1) R2

24


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CONCLUSIONS Porous cellulose beads, as the ideal natural organic material, are critical for industrial applications. Here we successfully fabricated bamboo-derived porous cellulose beads without additional dissolution and droplet formation procedure atfirst time. In this study, we investigated the typical selected oxidation reaction on cellulose fibers with sodium periodate just under aqueous condition. Interestingly, porous cellulose beads were observed to grow automatically along the fiber rods in this convenient one-step method. Combining with obvious morphological change during the sectional swelling process, we raised the feasible mechanism of this novel method. Subsequently, porous cellulose beads were modified with glycine to form a novel cationic adsorbent for the adsorption of Co2+, Cu2+. Among these attempts, several parameters were investigated, including the influence of pH, adsorption isotherm and kinetics. All of results illustrated that the modified one-step growth cellulose beads could be used as ideal adsorbents for heavy metal removal from waste water. Notably, this work provided a novel fabrication technique of bamboo-based cellulose beads, especially under the increasing demand from regulatory authorities to move to more green chemistry in production.

AUTHOR INFORMATION 25



Corresponding Author *Kaifeng Du: E-mail: [email protected]. Tel.:0086 028 85405221. Fax: 0086 028 85405221. Notes The authors declare no completing financial interest.

ACKNOWLEDGMENTS The work was funded by National Natural Science Foundation of China (21676170 and 21476144). We would like to thank the Analytical & Testing Center of Sichuan University for analyzing the external morphology of the prepared samples and we would be grateful to Shuping Zheng for her help of SEM images. In addition, we would like to thank the “ceshigo, www.ceshigo.com” for providing the XRD analysis for cellulose samples.

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Table of Contents Only

Synopsis The work relates to the green preparation of cellulose-based adsorbent with the application for heavy metal ions removal from waterwaste, which contributes to the circular economy and sustainability.

31



Schematic strategy for fabrication of glycine-modified porous cellulose beads. 167x95mm (144 x 144 DPI)


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One-step growth of porous cellulose beads directly on bamboo fibers

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