Removal of heavy metal ions and anionic dyes from aqueous solutions using amide-functionalized cellulose-based adsorbents

Removal of heavy metal ions and anionic dyes from aqueous solutions using amide-functionalized cellulose-based adsorbents

Journal Pre-proof Removal of heavy metal ions and anionic dyes from aqueous solutions using amide-functionalized cellulose-based adsorbents Jing Liu, ...

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Journal Pre-proof Removal of heavy metal ions and anionic dyes from aqueous solutions using amide-functionalized cellulose-based adsorbents Jing Liu, Tai-Wen Chen, Ya-Li Yang, Zong-Chun Bai, Li-Ru Xia, Min Wang, Xiao-Lan Lv, Li Li

PII:

S0144-8617(19)31287-1

DOI:

https://doi.org/10.1016/j.carbpol.2019.115619

Reference:

CARP 115619

To appear in:

Carbohydrate Polymers

Received Date:

8 June 2019

Revised Date:

29 October 2019

Accepted Date:

12 November 2019

Please cite this article as: Liu J, Chen T-Wen, Yang Y-Li, Bai Z-Chun, Xia L-Ru, Wang M, Lv X-Lan, Li L, Removal of heavy metal ions and anionic dyes from aqueous solutions using amide-functionalized cellulose-based adsorbents, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115619

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Removal of heavy metal ions and anionic dyes from aqueous solutions using amide-functionalized cellulose-based adsorbents

Jing Liu c, Tai-Wen Chen c, Ya-Li Yang c, Zong-Chun Bai a, b, Li-Ru Xia a, b, Min Wang a

, Xiao-Lan Lv a,b,d* [email protected], Li Li, a, b, d*[email protected]

Institute of Agricultural Facilities and Equipment, Jiangsu Academy of Agricultural Sciences

b

Key Laboratory for Protected Agricultural Engineering in the Middle and Lower Reaches of Yangtze

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a

River, Ministry of Agriculture

Department of Pharmaceutical Analysis, China Pharmaceutical University

d

School of Materials Science & Engineering, Jiangsu University

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Corresponding author:Professor Li Li, Address: 50 Zhongling Stree, Nanjing 210014,

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*

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c

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China: (L. Li) Fax: +86-25-8439 1661Professor Xiao-Lan Lv, Address: 50 Zhongling

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Street, Nanjing 210014, China: (X. L. Lv) Fax: +86-25-84390082

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Graphical Abstract

Highlights 

Amide-functionalized cellulose-based adsorbent was synthesized.



The prepared adsorbent could efficiently remove Acid Black 1, Acid Red 18 and copper ions.



The adsorption experimental data was fit with the pseudo second order model.



The prepared adsorbent has excellent regeneration.

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ABSTRACT An efficient, ecofriendly, amide-functionalized cellulose-based porous adsorbent was synthesized by a cross-linking reaction between cellulose filament fibers and

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bisacrylamide at room temperature. This process is simple, fast and inexpensive, and has significant potential for industrial applications. The prepared material has numerous

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adsorption sites, resulting in the highly efficient removal of anionic dyes and copper ions

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from aqueous media. The maximum adsorption capacities of this cellulose-based adsorbent for the dyes Acid Black 1 and Acid Red 18 and for copper ions were 751.8,

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417.9, and 51.3 mg·g-1, respectively. Regeneration experiments showed that the removal efficiencies for all model pollutants remained above 92% after five consecutive recycling

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trials. These results indicate that amide-functionalized cellulose-based adsorbents could

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possibly be used to treat industrial wastewaters. Keywords:

Cellulose-based adsorbent; Amide-functionalized adsorbent; Chemical cross-linking; Adsorption.

1. Introduction Environmental pollution resulting from the release of industrial wastewaters has attracted increasing attention worldwide (Zhu et al., 2018). Such wastes can contain organic dyes, many of which (especially anionic dyes) are toxic, non-biodegradable, and possibly teratogenic, carcinogenic, and mutagenic (Shi, Lv, Wu, & Hou, 2017; Gao, Su, Li, & Cheng, 2018). As such, these compounds represent a serious threat to marine

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organism as well as humans (Mirzaei, Chen, Haghighat, & Yerushalmi, 2017; Ruan, Chen, Chen, Qian, & Frost, 2016). Heavy metal ions are also commonly found in wastewaters,

tend to persist in the environment and are highly toxic even at trace levels (Setyono, &

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Valiyaveettil, 2016; Zou, Wang, Tan, Song, & Cheng, 2017; Feng et al., 2017; Ji et al.,

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2016). Thus, an inexpensive, efficient, robust and ecofriendly method for the removal of anionic dyes and heavy metal ions from wastewater is urgently required.

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Conventional treatment methods include chemical precipitation, electrochemical

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degradation, photo-degradation, ion exchange, membrane filtration, bio-removal, and adsorption (Peng, Li, Liu, & Song, 2017; Salleh, Mahmoud, Karim, & Idris, 2011; Zhang,

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et al., 2017; Chakraborty, Bhattacharyya, Hazra, Ghosh, & Maji, 2016). Among these, adsorption is considered optimal for the treatment of industrial wastewaters because of

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the high uptake capacity, ease of use and economic viability of this process (Yang, et al., 2018; Fakhre, & Ibrahim, 2017). Various adsorbents with high capacities, such as activated carbon, alumina nanoparticles, modified metal oxides, silica mesoporous materials, and cellulose adsorbents, have been investigated in recent years (Islam, Choi, Nam, Yoon, & Lee, 2017; Bai et al., 2018; Xu, Lv, Zeng, & Cao, 2017). Cellulose-based

adsorbents have attracted increasing attention because they are biodegradable, renewable, inexpensive, biocompatible, ecofriendly and highly stable in the presence of most organic solvents (Martins, Toledo, & Petri, 2017; Suhas et al., 2016; Maatar, Alila, & Boufi, 2013). Many modifications to such materials intended to increase the capacities of cellulose-based adsorbents have been assessed, and amide crosslinking has been recognized as one of the most efficient ways to remove various anions from solutions

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(Hokkanen, Bhatnagar, & Sillanpää, 2016; Khalfaoui et al., 2015). Some studies have

focused on the synthesis of amide-functionalized cellulose adsorbents via modifications of cellulose using different methods (Dong, Liu, Yuan, Yi, & Zhao, 2016; Liu, Chen,

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Huang, & Lai, 2016). Other researches have examined the direct fabrication of amide-

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functionalized cellulose adsorbents, based on the copolymerization of cellulose with cross-linkers and functional monomers (Liu et al., 2015; Liu, Xie, Li, Zhang, & Yao,

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2016). However, these modifications involve tedious preparation processes, require large

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quantities of chemical reagents and are time-consuming. In the present work, cellulose filament fibers were cross-linked with bisacrylamide,

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in the absence of heating, to produce adsorbents based on amide-functionalized cellulose. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) 13

C solid-state NMR spectroscopy analyses confirmed the presence of amide

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and

functional groups on these materials. Subsequently, the adsorption characteristics of amide-functionalized cellulose-based adsorbents were investigated, using the dyes Acid Black 1 (AB1) and Acid Red 18 (AR18) and copper ions (Cu2+) as model pollutants. The results show that these materials have higher adsorption capacities for each of the model

pollutants and thus confirm that amide-functionalized cellulose-based adsorbents have potential with regard to the treatment of industrial wastewaters.

2. Experimental 2.1. Materials Cellulose filament fibers were purchased from the Hubei Chemical Filament Co. (Xiangfan, China). Its average molecular weight was determined by laser light scattering

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in an aqueous solution containing 4.6 wt% lithium hydroxide and 15 wt% urea to be 9.77 ×104 (Shi et al., 2014). The cellulose filament fibers were dried under vacuum at 50 °C

for 24 h to remove any moisture before use. Sodium hydroxide, urea, bisacrylamide,

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copper sulphate, acetic acid, hydrous ethanol and surfactant-free cellulose acetate

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membrane filters (pore size 0.45 μm) were purchased from Aladdin Reagent Co. (Shanghai, China). AB1 (purity >95%) and AR18 (purity >95%) were purchased from

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the Sinopharm Chemical Co. (Shanghai, China). All other chemicals were analytical

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reagent grade and were purchased from Nanjing Reagent Co. (Nanjing, China). Water used in experimental work was purified using an MZY-U10 ultra-pure water system

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(Miaozhiyi Electronic Technology Co. Ltd., Nanjing, China). 2.2. Preparation of the amide-functionalized cellulose-based adsorbents

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The amide-functionalized cellulose-based adsorbents were prepared according to the

procedures used by Geng (Geng, 2018), with several modifications. Briefly, specific quantities cellulose filament fibers and bisacrylamide were added into 100 mL of aqueous solution containing 7 wt% sodium hydroxide and 13 wt% urea. After stirring at 1000 r·min-1 for 5 min, the mixture was placed in a freezer held at -12 °C for 30 min, then

removed and stirred at 1000 r·min-1 for a further 30 min at room temperature. The mixture was subsequently stored under dark condition for 2 h at room temperature. This procedure produced the amide-functionalized cellulose-based materials. The products were soaked in a large volume of deionized water to remove residual sodium hydroxide, urea and uncross-linked bisacrylamide. The water was replaced as necessary until the pH reached 7. In all trials, the cellulose fibers were added to the reaction solution at a concentration

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of 60 g·L-1, while the mass ratio of bisacrylamide to the fibers was 0.4:1, 0.6:1, 0.8:1, or 1:1. The resulting materials are referred to herein as Adsorbents 4, 6, 8 and 10, respectively. A pure cellulose adsorbent was treated in the same manner but without

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bisacrylamide and gelling for 48 h at 50 °C. The pure cellulose adsorbent is referred to as

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Adsorbent 0 herein.

2.3. Evaluation of the interaction between cellulose and bisacrylamide

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FTIR, XPS and 13C solid-state NMR were used to evaluate the extent of the reaction

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between the filament fibers and bisacrylamide. In preparation for these analyses, adsorbent specimens were first freeze-dried at -35 °C for 20 h with a chamber pressure of

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0.111 kPa using a VirTis AdVantage Plus freeze dryer (SP Scientific, New York, United States), then ground to a powder. FTIR spectra were acquired using a Nicolet iS50 FTIR

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instrument (Thermo Scientific, Massachusetts, United States) with a scan range from 400 to 4000 cm-1. The XPS data were obtained with a PHI Quantera II XPS instrument (UlvacPhi Co., Chigasaki, Japan). The 13C cross-polarization magnetic angle spinning (CP/MAS) solid-state NMR spectra were recorded on a Bruker Advance Ⅲ 400MHz spectrometer (13C frequency = 100 MHz) with a CP/MAS unit at ambient temperature. The spinning

rate and the contact time were 10.0 kHz and 2.0 ms, respectively. Pulse width was 2.10 μs, spectra width was 50.000 kHz, acquisition time was 34.45 ms and the spectrum was accumulated 2000 times. The

13

C solid-state NMR quantitative method and the elemental analysis method

have been used as the reference methods to determine the crosslinking degree of copolymers (Geng, 2018). 13

C solid-state NMR quantitative, the crosslinking degrees of cellulose based

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adsorbents were calculated according to the equation 1 (Capitani, Nobile, Mensitieri,

Sannino, & Segre, 2000; Lenzi, Borriello, Porro, Capitani, & Mensitieri, 2003; Geng,

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2018)

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𝐂𝐫𝐨𝐬𝐬𝐥𝐢𝐧𝐤𝐢𝐧𝐠 𝐃𝐞𝐠𝐫𝐞𝐞 (%) =

𝑺𝑴⁄ 𝟐 𝑺𝑪

1

Where, SM is the one-half of the area of methylene resonance peak at around 37 ppm,

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and SC is the area of cellulose C1 resonance peak at around 103 ppm.

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For the elemental analysis method, the crosslinking degrees of cellulose based adsorbents were calculated according to the equation 2 (Hebeish, Farag, Sharaf, &

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Shaheen, 2014; Wang et al., 2016)

𝐂𝐫𝐨𝐬𝐬𝐥𝐢𝐧𝐤𝐢𝐧𝐠 𝐃𝐞𝐠𝐫𝐞𝐞 (%) =

𝟏𝟎𝟎×𝑵%×𝑴𝑨𝑴 𝑾𝒈 ×𝟏𝟒

2

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Where, N% is the percentage content of nitrogen, MAM is the molecular weight of bisacrylamide, and Wg is the weight of cellulose filament fibers (g). 2.4. Morphology of the adsorbents The surface morphologies of the adsorbents were observed by scanning electron microscopy (SEM) using an EVO-LS10 instrument (ZEISS, Oberkochen, German).

Freeze-dried adsorbents were mounted on an aluminum sample holder and subsequently coated with a 6 nm thick gold film before obtaining the SEM images. The images were acquired at magnifications from 20× to 5000× under high vacuum using an accelerating voltage of 10 kV. 2.5. Zeta potentials of the adsorbents The zeta potentials of these adsorbents were determined by dynamic light scattering

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at room temperature using a Malvern Zetasizer Nano ZS90 instrument (Malvern Instruments Ltd., Worcestershire, UK). 2.6. Adsorption experiments

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As noted, Cu2+, AB1 and AR18 were used as model pollutants to investigate the

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adsorption characteristics of the amide-functionalized cellulose-based adsorbents. Firstly, 0.1 g of the freeze-dried adsorbents were kept in excess water for 3 days to ensure the

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equilibrium of water uptake. Then, the swollen adsorbents were centrifuged at 3000

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r·min-1 for 10 min and collected for adsorption experiments. After that, the solutions of model pollutants were filtered via surfactant-free cellulose acetate membrane filters (pore

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size 0.45 μm). Except for the study of contact time, all adsorption experiments were carried out by adding above swollen adsorbents to 100 mL of model pollutant solution,

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followed by stirring at 100 r·min-1 for 2 h. After adsorption, the adsorbent was removed by centrifugation at 1000 r·min-1 for 5 min. The residual Cu2+ concentration in the supernatant was determined by atomic adsorption spectroscopy using a PinAAcle 900T instrument (Perkin Elmer, Massachusetts, United States) (Han, Du, Zou, Li, & Zhang, 2015), while the residual concentrations of the anionic dyes were determined by UV-

visible adsorption spectroscopy using a Nano Drop 2000 instrument (Thermo Scientific, Massachusetts, United States) (Ruan, Strømme, & Lindh, 2018). The adsorption capacities for the various model pollutants were calculated according to the equation 3 𝒒𝒕 =

(𝒄𝟎 − 𝒄𝒕 )𝑽⁄ 𝑾

3

where, qt is the adsorption capacity after time t (mg·g-1), V is the solution volume (L), c0

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and ct are the initial concentration of the model pollutant and the concentration at time t (mg·L-1), and W is the adsorbent mass (g) (Santos et al., 2018).

4

(𝒄𝟎 − 𝒄𝒕 ) × 𝟏𝟎𝟎⁄ 𝒄𝟎

4

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% 𝐑𝐞𝐦𝐨𝐯𝐚𝐥 =

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The Cu2+, AB1 and AR18 removal percentages were calculated using the equation

where, c0 and ct are the initial concentration of the model pollutant and the concentration

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at time t (mg·L-1) (Liu et al., 2015). All of the adsorption experiments were carried out in

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triplicate.

3. Results and discussion

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3.1. Synthesis and Characterization of the adsorbents The possible mechanism for the reaction between the cellulose and bisacrylamide is

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shown in Figure 1. After cellulose filament fibers and bisacrylamide were dissolved in the aqueous sodium hydroxide/urea solution, the cellulose hydroxyl groups cross-linked with the bisacrylamide double bond generating the amide-functionalized cellulose-based adsorbents.

The functional groups on these materials were examined by acquiring FTIR spectra of the adsorbents (Figure 2A). Compare with Adsorbent 0, the functionalized adsorbents generated a new adsorption peak at 1545 cm-1. A peak was also observed at 1657 cm-1, and this peak increased in intensity as the amount of bisacrylamide in the original reaction mixture was increased. These two peaks are assigned to -NH flexural vibrations (1545 cm-1) and –C=O stretching vibrations (1657 cm-1), and are both also observed in the

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bisacrylamide spectrum (Geng, 2018; Saber-Samandari, Saber-Samandari, Joneidi-Yekta, & Mohseni, 2017; Liu et al., 2015). These results indicate that the bisacrylamide was successfully cross-linked with the filament fibers to form the desired product.

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The cross-linking reaction between cellulose fibers and bisacrylamide was further

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clarified using 13C solid-state NMR and XPS. The 13C solid-state NMR has been widely used in identification and characterization of the structures of solid-state materials.

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Compared with bisacrylamide, the chemical shifts of carbon atom of –CH= and CH2= at

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125-135 ppm were disappeared on the functionalized adsorbents (Figure 2B). These results further supported the chemical reaction between the cellulose and bisacrylamide

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(Figure 1). The double bond of bisacrylamide cross-linked with the hydroxyl groups of cellulose to form a new chemical shift of carbon atom of cellulose–O–CH2–CH2– at

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around 37 ppm (Geng, 2018).

The XPS overview spectra of the amide-functionalized cellulose-based adsorbents show an N1s peak at the binding energy of 400 eV (Figure 3A) in addition to C1s and O1s peaks. According to the cross-linking reaction (Figure 1), there will be carbonyl and

amide groups on the obtained adsorbents, depending on the bisacrylamide. Thus, the convoluted O1s and N1s core-level spectra of the obtained adsorbents were used to clarify the cross-linking reaction between cellulose and bisacrylamide. The O1s core-level spectrum of Adsorbent 0 (Figure 3B) shows the presence of one peak at 531.2 eV, which is assigned to C-O-C. While, the O1s core-level spectrum of Adsorbent 6 (Figure 3C) shows the presence of two peaks at 529.4 and 531.2 eV, which are assigned to carbonyl

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groups and C-O-C, respectively (Sivaranjini, Mangaiyarkarasi, Ganesh, & Umadevi, 2018). These results supported that the presence of carbonyl groups on Adsorbent 6. The

amide groups are confirmed by the N1s core-level spectrum of Adsorbent 6 (Figure 3D),

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where the peak at 398.1 eV is attributed to the amino/amide groups (N-H) (Mitra et al.,

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2018). These results further confirmed that the bisacrylamide was successfully crosslinked with the filament fibers to form the amide-functionalized cellulose-based

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adsorbents.

The crosslinking degrees (CD) of the amide-functionalized cellulose-based

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adsorbents were determined using the elemental analysis method. For the

13

13

C solid-state NMR quantitative method and

C solid-state NMR quantitative method, the CD

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expressed as the number of cross-linkers per monomer, which calculated from the ratio between one half of the area of resonance at 37 ppm (the chemical shift of carbon atom of cellulose–O–CH2–CH2–) and the area of resonance at 103 ppm (the chemical shift of C1 of cellulose). The convoluted 13C solid-state NMR spectra of the obtained adsorbents and related fitted results were shown in Supplementary Figure 1. According to the

equation 1, the CDs of the amide-functionalized cellulose-based adsorbents were calculated. The CD of Adsorbent 4 was 11.76%. With the increase of bisacrylamide, the CDs were increased to 16.41% (Adsorbent 6), 16.72% (Adsorbent 8) and 15.14% (Adsorbent 10). For the elemental analysis method, the concentrations of N element on the adsorbents were obtained by XPS, which used to determine the content of bisacrylamide. The concentrations of N element on the adsorbents were found to increase

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from 3.5% to 4.6% as the bisacrylamide: cellulose mass ratio was increased from 0.4:1

to 0.6:1, above which the N element percentage plateaued (Supplementary Table 1). The CDs of the amide-functionalized cellulose-based adsorbents were calculated according to

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the equation 2. The CD of Adsorbent 4 was 7.71%. With the increase of bisacrylamide,

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the CDs were increased to 10.13% (Adsorbent 6), 10.35% (Adsorbent 8) and 10.13%

quantitative analysis results.

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C solid-sate NMR

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(Adsorbent 10). These results were comparable to those of the

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Thus, the optimum bisacrylamide: cellulose mass ratio was evidently 0.6:1. For this reason, Adsorbent 6 was used in the subsequent trials, together with Adsorbent 0 for

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comparison purpose.

3.2. Inner structure of the adsorbents

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It is important for an adsorbent to have a porous structure, and Figure 4 presents

SEM micrographs showing the internal structures of the adsorbents produced in this work. Adsorbent 0 had a well-distributed porous structure (Figure 4A), while some larger pores are evident on the surface of Adsorbent 6 (Figure 4B). Adsorbent 6 was also viewed from the side (Figure 4C) and the resulting image confirms a well-ordered skeleton and the

homogeneous distribution of large pores. In addition, even at a magnification of 5000×, pores are not visible in the skeleton of Adsorbent 6 (Figure 4D). These results demonstrate that Adsorbent 6 had a compact skeleton that would be expected to provide good mechanical properties, together with continuously distributed large pores.

3.3. Adsorption of model pollutants

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3.3.1. Effect of pH

The pH of the aqueous solution is an important factor that can greatly affect the adsorption of model pollutants. The adsorption of a positively charged species has been

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shown to improve in the case that the pH of the solution is greater than the pH of zero

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point charge (pHzpc) of the adsorbent. Conversely, the adsorption of negatively charged species is favored at pH levels less than pHzpc (Song, Xu, Xu, Xie, & Yang, 2017; Jiang

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et al., 2018; Zhang et al., 2018). Therefore, the adsorption of anionic dyes should be

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favored in solutions with pH values below the pHzpc of the adsorbent, while the adsorption of metal cations should be improved at pH values higher than pHzpc.

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In this work, Adsorbent 0 and Adsorbent 6 were used to remove AB1, AR18 and Cu2+ from aqueous solutions at various pH values (Figure 5). The initial concentrations

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of AB1, AR18, and Cu2+ were 1000 mg·L-1, 500 mg·L-1 and 60 mg·L-1, respectively. The UV-visible adsorption spectra of the anionic dyes show that, over the pH range of 2 to10, the pH had a negligible effect on the maximum adsorption wavelengths of the AB1 and AR18, suggesting that there were no significant changes in the molecular structures of the dyes (Supplementary Figure 2). Thus, the effects of pH on AB1 and AR18 were

studied over this pH range. Considering that Cu2+ would precipitate under alkaline condition, the effect of pH on Cu2+ adsorption were examined in the range of 2-7 (Darmayanti, Kadja, Notodarmojo, Damanhuri, & Mukti, 2019; Deng et al., 2019). Compared with Adsorbent 0, Adsorbent 6 exhibited higher uptakes of the anionic dyes. The maximum capacities of Adsorbent 6 at pH 2 were 751.8 mg·g-1 for AB1 and 417.9 mg·g-1 for AR18. The adsorption capacities for AB1 and AR18 were also found to

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decrease greatly on going from pH 2 and 4 while, between pH 4 and 10, the uptakes of

both dyes were essentially constant (Figure 5A and 5B). In the case of Cu2+, Adsorbent 6 and Adsorbent 0 showed similar adsorption properties, and both exhibited increased

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adsorption with the increases of pH. At pH 7, the maximum adsorption capacities of the

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Adsorbent 6 and Adsorbent 0 were 51.3 and 50.6 mg·g-1, respectively (Figure 5C).

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These results can be attributed to the change of adsorption mechanism between the

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adsorbents and adsorbate (Figure 6). In this work, the pHzpc of Adsorbent 6 was approximately 4.62 (Supplementary Figure 3) and this material had high concentrations

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of surface amide groups, hydroxyl groups and carbonyl groups. Acidic pH values (<4.62) resulted in the protonation of the amide groups, such that the cellulose surfaces were

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positively charged. Consequently, the adsorption of anionic dyes was enhanced because of the strong electrostatic interaction between the protonated amide groups on the adsorbent and the negatively charged SO32- groups on the AB1 and AR18. In contrast, the removal of Cu2+ was inhibited because of strong electrostatic repulsion effect. As the pH was increased, the amide groups were deprotonated, which in turn reduced the

electrostatic interaction such that Adsorbent 6 showed a decrease in anionic dye adsorption and an increase in Cu2+ adsorption. Moreover, Adsorbent 6 exhibited higher uptakes of the anionic dyes than Adsorbent 0 even over pH ranging from 5-10. These results might be due to hydrogen bonding interaction between the amide groups of Adsorbent 6 and hydroxyl groups of anionic dyes (Harings et al., 2009; Aljohani, Pallopurath, McArdle, & Erxleben, 2017). For removal of Cu2+, Adsorbent 6 and

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Adsorbent 0 have the similar adsorption capacities in the range of pH 2-7. The results can be possibly attributed to physical adsorption of porous materials and coordination of active groups, such as hydroxyl.

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These results show that Adsorbent 6 could adsorb at least 357.7 mg·g-1 AB1 and

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235.2 mg·g-1 of AR18 between pH 6-8, and 36.8mg Cu2+ between pH 6-7. This pH range

industrial wastewaters.

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3.3.2. Adsorption isotherms

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is that of natural waters. Thus, this adsorbent has significant potential for the treatment of

Adsorption isotherm models can be used to describe the interaction between an

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adsorbate and adsorbent, and are often employed to investigate the mechanism of adsorption. In this work, we use the Langmuir and Freundlich models to obtain isotherm

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parameters for adsorption of AB1, AR18 and Cu2+ by Adsorbent 6. The equations for the Langmuir (Langmuir, 1918) and Freundlich model (Freundlich, 1906) are 𝑪𝒆⁄ 𝟏 𝒒𝒆 = ⁄𝑲𝑳 𝒒𝒎𝒂𝒙 + 𝑪𝒆 /𝒒𝒎𝒂𝒙

5

𝑳𝐧𝒒𝒆 = 𝑳𝒏𝑲𝑭 + 𝑳𝒏𝑪𝒆 /𝒏

6

and

respectively, where qe and qmax (mg·g-1) are the adsorption capacity at equilibrium and the maximum adsorption capacity according to Langmuir monolayer adsorption. Ce (mg·L-1) is the equilibrium concentration of dye. KL (L·mg-1) is the Langmuir constant (related to free energy and affinity of the adsorbate for the binding sites), and KF (g·mg1

) and n are Freundlich constants related to the adsorption capacity and adsorption

intensity of the adsorbent, respectively.

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The adsorption isotherms of anionic dyes at pH 2 and 8 and Cu2+ at pH 2 and 7 are presented in Figure 7. The parameters calculated from these data are provided in Table 1. In the Langmuir isotherm model, qmax, is a measure of monolayer adsorption capacity of

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the model pollutant. In this study, we calculate qmax values of 769.23 mg·g-1 for AB1,

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476.19 mg·g-1g for AR18 and 26.31 mg·g-1 for Cu2+ at pH 2. Even at pH 8, the qmax values are 454.51 mg·g-1 for AB1 and 312.52 mg·g-1 for AR18. Meanwhile, it is 65.36 mg·g-1

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for Cu2+ at pH 7. The KL values were in the range of 0< KL< 1, indicating that Adsorbent

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6 is suitable for the adsorption of AB1, AR18 and Cu2+. In the case of the Freundlich isotherm model, the values of KF also show that Adsorbent 6 readily adsorbed AB1, AR18

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and Cu2+ and the adsorption capacities increase with increases in the initial concentrations. The values of n were determined in the range of 1 < n < 10, indicating a favorable

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adsorption process. A comparison of the results obtained from the two isotherm models show that the Langmuir isotherm model gave the higher R2 values and thus the better fits. This outcome indicates that Adsorbent 6 had a homogeneous surface and that AB1, AR18 and Cu2+ were adsorbed as monolayers. These results can be explained based on the internal structure of Adsorbent 6. That is, the pores in this material were larger than 200

μm (Figure 4C) and thus far larger than AB1, AR18 and Cu2+. 3.3.3. Adsorption kinetics Adsorption kinetics can reflect the process of adsorbate migration from a solution onto an adsorbent and thus provide fundamental information related to the adsorption process and the rate-limiting step (Chen, Lin, Ho, Zhou, & Ren, 2018). The constants of all kinetic models were calculated using equations

and 𝒕⁄ = 𝒕⁄ + 𝟏⁄ 𝒒𝒕 𝒒𝒆 𝒌𝟐 𝒒𝟐𝒆

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𝑳𝐧(𝒒𝒆 − 𝒒𝒕 ) = 𝑳𝒏𝒒𝒆 − 𝒌𝟏 𝒕

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where, qe and qt (mg·g-1) are the adsorption capacities at equilibrium and at time t,

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and pseudo second order, respectively.

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respectively, k1 (min-1) and k2 (g·mg-1·min-1) are the rate constants for pseudo first order

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Both pseudo first order and pseudo second order models were used to investigate the adsorption process (Figure 8, Table 2). The latter gave high R2 values for AB1, AR18 and

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Cu2+ (Table 2) and provided a better description of the adsorption kinetics. Moreover, the qe values calculated from the pseudo second order model were in good agreement with

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the experimental values for all model pollutants, which further confirms the validity of this model. These results indicate that the adsorption process is dependent on the amount of model pollutant adsorbed on the surface of Adsorbent 6 and the amount adsorbed at equilibrium (Song, Xu, Xu, Xie, & Yang, 2017; Chen, Lin, Ho, Zhou, & Ren, 2018). It is therefore evident that the adsorption of model pollutants onto Adsorbent 6 might occur

via chemisorption based on the sharing or exchange of electrons between the adsorbent and adsorbates. At pH 2, the qe values of Adsorbent 6 were 689.05 mg·g-1 for AB1, 396.60 mg·g-1 for AR18, and 14.46 mg·g-1 for Cu2+ (Table 2). Even at pH 8, the qe values were 326.46 mg·g-1 for AB1 and 250.21 mg·g-1 for AR18. Meanwhile, it is 54.60 mg·g-1 for Cu2+ at pH 7 (Table 2). These results show that Adsorbent 6 provided good adsorption, and it could possibly be applied to the removal of metal ions and anionic dyes from

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aqueous solutions. 3.3.4. Adsorption from complex system

Industrial wastewaters generally contain various different pollutants that may

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compete for adsorption sites on the adsorbent surface and consequently decrease the dye

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removal efficiency. Hence, we evaluated the adsorption capacities of Adsorbent 6 in the presence of competing species, using a mixture of AB1, AR18 and Cu2+ prepared in ultra-

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pure water. The compositions of the mixtures employed in these trials are provided in

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Supplementary Table 2, while Figure 9 shows the resulting adsorption efficiencies. A 0.1 g quantity of Adsorbent 6 exhibited initial adsorption efficiencies of 92.6% for

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AB1 (S AB1, 500 mg·L-1, 60 mL), and 87.8% for AR18 (S AR18, 500 mg·L-1, 60 mL). In the mixed system, the adsorption efficiencies for single anionic dye were found to decrease.

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In the absence of Cu2+, the adsorption of AB1 decreased from 81.3% (SAB1-1) in the presence of 100 mg·L-1 AR18 to 52.9% (SAB1-3) in the presence of 1000 mg·L-1 AR18. However, after adding Cu2+ to mixtures with the same dye concentrations, the AB1 adsorption efficiencies only decreased from 89.6% (SAB1-C-1) to 58.5% (SAB1-C-3). Similar results were observed for the adsorption of AR18 from various mixtures.

Without Cu2+, the AR18 adsorption decreased from 61.5% (SAR18-1) in the presence of 100 mg·L-1 AB1 to 37.3% (SAR18-3) in the presence of 1000 mg·L-1 AB1. In the presence of Cu2+ and with the same dye concentrations, the AR18 adsorption only decreased from 68.4% (SAR18-C-1) to 43.9% (S AR18-C-3). These results demonstrate that Adsorbent 6 was able to remove anionic dyes from complex systems. Moreover, the presence of Cu2+ in such systems effectively improved the anionic dye adsorption efficiency (Han et al., 2015).

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3.3.5. Regeneration and reusability

The ability to regenerate and reuse an adsorbent are important in pollution control, and hydrochloric acid solutions are widely used to desorb metal ions from adsorbents.

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Adsorption results obtained at different pH values (Figure 5) showed that Cu2+ could be

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desorbed using 0.1 M hydrochloric acid in ethanol (60/40 v/v). After washing to neutral pH, 0.1 M sodium hydroxide in ethanol (60/40 v/v) was used to desorb the anionic dyes.

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The reusability and stability of Adsorbent 6 were examined by performing multiple

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adsorption-desorption cycles in an aqueous buffer solution. After five cycles, the adsorption capacities were decreased only slightly, from 48.1 to 44.7mg·g-1 (Cu2+), 343.8

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to 320.2 mg·g-1 (AB1), and 237.3 to 219.4 mg·g-1 (AR18). These results confirm that Adsorbent 6 was stable and readily reused.

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4. Conclusion

Bisacrylamide was cross-linked with cellulose filament fibers to form porous amide-

functionalized cellulose-based adsorbents at room temperature. These materials contained numerous adsorption sites, resulting in excellent anionic dyes and Cu2+ removal efficiencies. The maximum adsorption capacities for AB1, AR18 and Cu2+ were 751.8,

417.9 and 51.3 mg·g-1, respectively. This adsorbent was readily regenerated for reuse. In comparison with other cellulose-based adsorbents, the amide-functionalized cellulosebased adsorbents have higher adsorption efficiency for anionic dyes and Cu2+. Moreover, the simple fabrication process and the excellent reusability of this material suggest that amide-functionalized cellulose-based adsorbents have the potential to assist in the treatment of industrial wastewaters.

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Funding Sources

This work was financially supported by the Fundamental Research Funds for Jiangsu

Academy of Agriculture Science (Grant No. ZX (19)7002), Jiangsu Agricultural Science

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and Technology Innovation Fund (Grant No. CX (19)3082), the Six Talent Peaks Project

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of Jiangsu Province, China (Grant No. NY-034) and the Fundamental Research Funds for

Acknowledgements

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the Central Universities (Grant No. 2632018 PY11).

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We thank Ms. Rong Xu (Jiangsu Academy of Agricultural Sciences) for detection of FT-IR spectra, and Mr. Cunfa Xu (Jiangsu Academy of Agricultural Sciences) for

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detection of SEM image.

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pollutants under visible light. Journal of Hazardous Materials, 338, 276-286.

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Figure 1. The cross-linking reaction between the cellulose filament fiber and bisacrylamide.

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Figure 2. A) FTIR spectra of the obtained adsorbents. B) 13C solid-state NMR spectra of the obtained adsorbents. Figure 3. A) XPS overview spectra of the obtained adsorbents; B) The O1s core-level spectrum of Adsorbent 0; C) The O1s core-level spectrum of Adsorbent 6; D) the N1s core-level spectrum of Adsorbent 6. Figure 4. SEM micrographs of the adsorbents. A) Top view of Adsorbent 0; B) Top view of Adsorbent 6; C) Side view of Adsorbent 6; D) Side view of Adsorbent 6 with 5000X magnifications. Figure 5. Effects of aqueous solutions at various pH values on the adsorption of model pollutants. A) AB1; B) AR18; C) Cu2+. Figure 6. The adsorption mechanism between the adsorbents and adsorbate. Figure 7. A) Adsorption isotherm for AB1 at pH 2; B) Adsorption isotherm for AB1 at pH 8; C) Adsorption isotherm for AR18 at pH 2; D) Adsorption isotherm for AR18 at pH 8; E) Adsorption isotherm for Cu2+ at pH 2; F) Adsorption isotherm for Cu2+ at pH 7. Figure 8. A) Adsorption kinetics for AB1 at pH 2; B) Adsorption kinetics for AB1 at pH 8; C) Adsorption kinetics for AR18 at pH 2; D) Adsorption kinetics for AR18 at pH 8; E) Adsorption kinetics for Cu2+ at pH 2; F) Adsorption kinetics for Cu2+ at pH 7. Figure 9. Adsorption efficiencies of Adsorbent 6 in the mixtures. A) Adsorption of AB1; B) Adsorption of AR18.

Table 1. Isotherm parameters for the adsorption of AB1, AR18 and Cu2+ by Adsorbent 6. Langmuir isotherm model

AB1 AR18

pH

KL (L·mg-1)

qmax (mg·g-1)

R2

KF (g·mg-1)

n

R2

2

2.59×10-2

769.23

0.99

121.51

3.12

0.89

8

1.16×10-2

454.51

0.99

21.33

1.96

0.94

2

1.31×10-2

476.19

0.98

27.39

2.17

0.91

8

2.91×10-2

312.52

0.99

31. 81

2.32

0.91

2

1.87×10-2

26.31

0.98

1.13

1.61

0.94

7

7.32×10-2

65.36

0.97

10.66

2.45

0.92

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Cu2+

Freundlich isotherm model

Table 2. Kinetic parameters of pseudo first order and pseudo second order models for the adsorption of AB1, AR18 and Cu2+ by Adsorbent 6. Pseudo first order pH

AB1

AR18

qe

k2

qe

(min-1)

(mg·g-1)

(g·mg-1·min-1)

(mg·g-1)

2

3.01×10-2

689.05

0.94

5.63×10-5

763.36

0.98

8

3.07×10-2

326.46

0.94

1.22×10-4

357.14

0.98

2

2.91×10-2

396.60

0.97

7.79×10-5

458.71

0.99

8

3.21×10-2

250.21

0.90

1.24×10-4

293.25

0.99

2

1.86×10-2

14.46

0.98

1.21×10-3

17.19

0.99

7

1.82×10-2

0.98

4.17×10-4

59.88

0.97

54.60

R2

R2

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Cu2+

k1

Pseudo second order