One-step synthesis of chitosan-polyethyleneimine with calcium chloride as effective adsorbent for Acid Red 88 removal

Journal Pre-proof One-step synthesis of chitosan-polyethyleneimine with calcium chloride as effective adsorbent for Acid Red 88 removal

N.H. Yusof, K.Y. Foo, B.H. Hameed, M. Hazwan Hussin, H.K. Lee, S. Sabar PII:

S0141-8130(19)38460-0

DOI:

https://doi.org/10.1016/j.ijbiomac.2019.11.218

Reference:

BIOMAC 13999

To appear in:

International Journal of Biological Macromolecules

Received date:

18 October 2019

Revised date:

15 November 2019

Accepted date:

26 November 2019

Please cite this article as: N.H. Yusof, K.Y. Foo, B.H. Hameed, et al., One-step synthesis of chitosan-polyethyleneimine with calcium chloride as effective adsorbent for Acid Red 88 removal, International Journal of Biological Macromolecules(2019), https://doi.org/ 10.1016/j.ijbiomac.2019.11.218

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© 2019 Published by Elsevier.

Journal Pre-proof One-step synthesis of chitosan-polyethyleneimine with calcium chloride as effective adsorbent for Acid Red 88 removal N.H. Yusofa, K.Y. Foob, B.H. Hameedc, M. Hazwan Hussind, H.K. Leee,f,g, S. Sabara,e,h* a

Chemical Sciences Programme, School of Distance Education (SDE), Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia

b

River Engineering and Urban Drainage Research Centre (REDAC), Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

c

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f

Department of Chemical Engineering, College of Engineering, Qatar University, P.O. Box: 2713, Doha, Qatar

d

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Materials Technology Research Group (MaTReC), School of Chemical Sciences, Universiti

e-

Sains Malaysia, 11800 Minden, Penang, Malaysia e

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Department of Chemistry, National University of Singapore, 3 Science Drive, 3 Singapore 117543, Singapore

f

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National University of Singapore Environmental Research Institute, National University of

g

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Singapore, T-Lab Building #02-01 5A, Engineering Drive 1, Singapore 117411, Singapore

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Tropical Marine Science Institute, National University of Singapore, S2S Building, 18 Kent Ridge Road, Singapore 119227, Singapore

h

School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia

* Corresponding author. Tel.: +6046532284; Fax: +6046576000. E-mail address: [email protected] (S. Sabar)

Journal Pre-proof Abstract Chitosan-polyethyleneimine with calcium chloride as ionic cross-linker (CsPC) was synthesized as a new kind of adsorbent using a simple, green and cost-effective technique. The adsorption properties of the adsorbent for Acid Red 88 (AR88) dye, as a model analyte, were investigated in a batch system as the function of solution pH (pH 3 ‒ 12), initial AR88 concentration (50 ‒ 500 mg L-1), contact time (0 ‒ 24 h), and temperature (30 ‒ 50°C). Results showed that the adsorption process obeyed the pseudo-first order kinetic model and

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the adsorption rate was governed by both intra-particle and liquid-film mechanism. Equilibrium data were well correlated with the Freundlich isotherm model, with the

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calculated maximum adsorption capacity (qm) of 1000 mg g-1 at 30 °C. The findings

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underlined CsPC to be an effective and efficient adsorbent, which can be easily synthesized

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dyes from the aqueous solutions.

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via one-step process with promising prospects for the removal of AR88 or any other similar

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Keywords: Calcium Chloride; Chitosan; Polyethyleneimine

Introduction

Synthetic dyes are extensively used in the dyeing, textile, cosmetics and printing industries. In most cases, the dyes produced must be highly stable with the desired fastness. Consequently, the dye effluents discharged from the dyeing process become more difficult to be treated because the dyes contained high organic matter [1] and are more resistant to chemical and biological oxidation [2]. The dyes have a strong persistent color which is not only aesthetically unpleasant but it also decreases the penetration of sunlight and oxygen into the aquatic ecosystem [3]. Besides, many of these dyes are toxic, mutagenic and

Journal Pre-proof carcinogenic. Thus, they need to be carefully treated before being discharged, since it can cause environmental deterioration, and affect human health adversely [4]. Adsorption has emerged to be the most suitable treatment method for dye effluents due to its simple design, low operating costs and effectiveness in removing different types of pollutants [5]. Activated carbon is a common and efficient adsorbent used to remove dyes from wastewaters because of its large surface area, high adsorption capacity and diverse functional groups. Nevertheless, large-scale application of activated carbon is hindered

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because this material is non-renewable and requires expensive precursors to produce high quality activated carbon. This has encouraged researchers to find cheaper and more

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environmentally friendly sorbents [3]. Among the low-cost adsorbents studied, chitosan (Cs)

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biopolymer has shown promising results as a good adsorbent. Cs is obtained from the

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deacetylation of chitin, which can be obtained from the exoskeleton component in crustaceans, such as shrimps and crab shells. Its ease of modification together with other

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properties such as biodegradability, biocompatibility, non-toxicity and hydrophilicity make it

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a versatile polymer [6]. The amino (NH2) and hydroxyl (OH) functional groups on its

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backbone act as adsorption sites for the removal of various pollutants from wastewater, such as heavy metals [7], pesticides [8] and dyes [9]. Nevertheless, Cs has low adsorption capacity, weak mechanical strength, high solubility in acidic conditions and is prone to deformity after drying [10].

The NH2 and OH groups on Cs play a significant role in its adsorption performance. Thus, increasing the density of either NH2 and/or OH groups will enhance the adsorption capacity of Cs. Polyethyleneimine (PEI) is a branched cationic polymer with repeating unit composed of the amino group and the aliphatic CH 2CH2 spacer. It has been considered as the best choice of amino-functionalized reagent in developing a high performance adsorbent [11]. Previously, Wong et al. [12] have successfully developed PEI modified spent tea leaves for

Journal Pre-proof the removal of Reactive Black 5 and Methyl Orange dyes, with the adsorption capacities of 71.9 and 62.1 mg g-1, respectively. Chatterjee and co-worker [13] have also reported the capability of Cs beads to remove Reactive Black 5 from aqueous solution. The authors showed that the adsorption capacity of the beads increased by more than threefold once modified with PEI. However, increasing the density of amino groups on Cs backbone may cause partial dissolution of the grafted Cs in acidic solution and reduce its effectiveness. Cross-linking of

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Cs with suitable cross-linker would overcome this problem. Cross-linking is believed to improve the pore size distribution, chemical stability, mechanical resistance and

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adsorption/desorption properties [14]. Different types of cross-linkers have been adopted to

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produce more chemically-stable Cs derivatives. Among the cross-linkers used, an ionic cross-

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linker is considered as less expensive and less toxic [15]. Different charges between the cross-linker and the Cs in aqueous solution may cause electrostatic attraction which induces

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this ionic interaction [16]. These non-permanent networks does not only neutralize the acidic

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groups, but are also reversible [16]. Calcium chloride (CaCl2) has been applied as a cross-

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linker for a variety of materials, including alginate [17], carboxymethyl-chitosan (CMC) [18], and N,O-CMC [19]. The incorporation of Ca2+ onto the surface of the Cs could offer modular Lewis acid binding sites for the increment in the anion adsorption abilities over a broad range of pH conditions [20]. Although several studies have evaluated on the adsorption properties of Cs-PEI and Cs-CaCl2, the synthesis of Cs-PEI with an ionic cross-linker for the removal of water pollutants has not been documented. In this study, Cs-PEI with CaCl2 cross-linker (CsPC) was synthesized and employed for Acid Red 88 (AR88) dye adsorption from the aqueous solution. The surface characteristics and morphology of CsPC were correlated with its adsorption performance at different batch conditions. The adsorption isotherms, kinetics and thermodynamic were further elucidated.

Journal Pre-proof 2.

Materials and method

2.1

Chemicals Cs flakes (75 ‒ 85% degree of deacetylation with medium molecular weight of 310,

000 g mol-1), PEI (50% w/v aqueous solution) and AR88 (dye content: 75%, color index no.: 1658-56-6, chemical formula: HOC10H6N NC10H6SO3Na, molecular weight: 400.38 g mol-1 and λmax: 505 nm) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl) (37% w/v aqueous solution) and sodium hydroxide (NaOH) pellets were provided by R&M Chemicals.

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CaCl2 and acetic acid were supplied by Systerm and QReC, respectively. These chemicals were used without additional purification unless otherwise stated. Distilled water was used

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for the preparation of all chemical solutions. The molecular structure of AR88 is provided as

Preparation of the adsorbent

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2.2

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Supplementary material, Fig. S1.

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Cs solution was prepared by dissolving 0.6 g Cs powder in 30 mL of 5% v/v acetic acid

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and stirred at room temperature until the Cs powder was completely dissolved. Next, 5 mL of

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1% v/v of PEI and 5 mL of 1% v/v of CaCl2 were added dropwise into the Cs solution and stirred for 30 minutes to homogenize the solution. The sample was transferred into a Teflonlined stainless-steel autoclave and heated in an oven at a constant temperature of 60 °C for 1h. The autoclave was cooled to room temperature naturally after the reaction was completed. The produced precipitate was neutralized by stirring in 2 M of NaOH solution for 2 h. After washing with water, the sample was dried and milled to fine powder. The synthesis of the adsorbent was optimized at different PEI concentrations (1 – 5% v/v), CaCl2 concentrations (1 – 5% v/v), heating temperature (30 – 80 °C) and heating time (1 – 15 h). The produced adsorbent is hereafter known as CsPC.

Journal Pre-proof 2.3

Physico-chemical characterization The surface morphology and the functional groups of the adsorbents were observed and

verified via scanning electron microscope coupled with energy dispersive X-ray (SEM-EDX) (Model Quanta FEG 650 made by FEI, USA), and attenuated total reflectance Fourier transform infrared (ATR-FTIR) (Model 2000, Perkin Elmer, USA) within 4000 ‒ 500 cm-1, respectively. The Brunauer-Emmet-Teller (BET) surface area and micropore volume of the adsorbents were determined under N2 gas at –196 °C by using a physisorption analyzer

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(Model ASAP2020, Micromeritic, USA). X-ray diffraction (XRD) analysis was conducted on a Malvern PANalytical X’pert Pro MRD PW 603040/60, UK system with Cu-Kα (λ=1.5406

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Å) and scanning range of 10 ‒ 80° (2θ angle range). Thermogravimetric analyzer (TGA)

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(Model STA 6 000, Perkin-Elmer, USA) was performed under N2 atmosphere at the heating

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scanning rate of 10 °C min-1 within the temperature of 30 to 900 °C. The point of zero charge (pHpzc) was determined by varying the initial pH (pHi) of 100 mL distilled water from 2 to 12

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using 0.1 M HCl or NaOH. The final pH of the solutions (pH f) containing the adsorbent was

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measured after 48 h of shaking at 30 °C. A curve of pHf against pHi was plotted and the pHpzc

2.4

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was taken at the point where the curve intersected the line pHf =pHi.

Batch adsorption study

The batch adsorption studies were conducted in a batch system by adding 20 mg of the adsorbents into 50 mL of different AR88 concentrations at unadjusted pH in 250-mL conical flasks. The samples were agitated at 200 rpm in a Memmert water bath shaker at a temperature of 30 °C. The AR88 concentrations were determined at a specified time by using a Shimadzu UV-1800 spectrophotometer (Japan). The amount of AR88 dye adsorbed at equilibrium, qe (mg g-1) was calculated using the following equation:

Journal Pre-proof (1) where Co and Ce are the initial and equilibrium liquid phase concentrations of AR88 (mg L -1), respectively; V is the volume of dye solution (L); and m is the dry weight of the adsorbents (g). Each experiment was repeated for at least three times under identical conditions and the results were reported as an average.

Results and discussion

3.1

Characterizations

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3.

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3.1.1 SEM-EDX analysis

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The SEM micrographs of Cs and CsPC at the 6000× magnification are shown in Fig. 1. As can be seen in the figure, the surface of Cs (Fig.1(a)) is smooth and dense, while the

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surface of CsPC (Fig. 1(b)) is rough with visible pores of varied sizes and shapes. According

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to Yan et al. [21], the irregular surface structure of modified Cs does not only increase the

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amino groups, but also favor the mass transfer of dye molecules. After AR88 adsorption, the porosity of CsPC was reduced due to the accumulation of adsorbed dye, but its surface

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roughness was still visible as shown in Fig. 1(c). This indicated the abundance of available active sites on the surface of CsPC. The active sites of CsPC have the capability of attracting the anionic AR88 molecules onto their external surface which diffused through the open pores [22]. The EDX results demonstrated in Table 1 shows that the amount of N element in Cs increased from 7.30 to 13.3% after the modification. The N atoms of the amino groups played an important role as they acted as active sites to attract the AR88 dye. Therefore, it is predicted that CsPC would be a better adsorbent than Cs for the adsorption of anionic pollutants since it has more N atoms [13]. Besides, the presence of the ionic cross-linker in CsPC was confirmed by the detection of 9.62% of Ca. After AR88 adsorption, sulfur (S) was found. AR88 molecules have a sulfonic acid group which is responsible for the presence of S

Journal Pre-proof in the molecular structure. The appearance of S with weight percentage of 6.73% for CsPC confirmed that AR88 dye has successfully adsorbed onto the adsorbent surface.

3.1.2 FTIR analysis The FTIR spectra of Cs as well as CsPC before and after AR88 adsorption are shown in Fig. 2. Cs shows common peaks at 3292.4 cm-1 (‒OH overlapped with stretching of ‒NH bond), 2876.4 cm-1 (stretching of C‒H), 1632.3 cm-1 (C=O stretching in amide group, amide I

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vibration)), 1536.7 cm-1 (N–H bending in amide group, amide II vibration), 1370.7 cm-1 (bending vibrations of C‒N) and 1032.8 cm-1 (C‒O‒C bridge) [21].

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As for CsPC, the broad peak at 3292.4 cm-1 has been shifted to 3357.6 cm-1 with

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broader and lower intensity as compared to Cs indicating the increase of ‒NH density [13],

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and the possible involvement of Ca2+ with NH2 and OH groups to form a stable gel network [6]. The shifting of N‒H bending spectra of an amino group from 1632.3 to 1589.5 cm-1 in

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CsPC provided evidence for the formation of Ca2+ complexes [20]. The new bands observed

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at 879.9 cm-1 represented the NH3+ rocking vibrations which proved that more amino groups

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were protonated to form ‒NH3+ in CsPC [23]. After the modification, the coordination of Ca2+ between the amino group of Cs and PEI has disrupted the H-bonding between the adjacent polymer units, thereby creating favorable binding sites for the adsorbates. These findings proved that the formation of CsPC was through ionic interaction between the protonated amino groups (‒NH3+) of Cs and Ca2+ ions of CaCl2. Fig. 3 shows the schematic of the cross-linking reaction between Cs and PEI using CaCl2 as a cross-linking agent. After the adsorption of AR88 onto CsPC, the peak associated with the stretching vibration of hydroxyl and amino groups was shifted to 3381.9 cm-1. Besides, the intensity of the characteristic adsorption bands at around 1600.0 cm-1 responsible for N‒H bending vibration in the amide group in all the adsorbents were reduced [14]. This indicated that

Journal Pre-proof amino groups played a significant role in the adsorption of AR88 [21]. Furthermore, a peak at 1500.5 cm-1 also arose attributed to the stretching vibrations of the presence of an asymmetrically substituted ‒N=N‒ group in the AR88 molecular structure [4]. Other strong peaks were also observed at 1157.0 cm-1 which can be assigned to the stretching vibration bands of S=O groups of AR88 [1]. Besides, the peaks at 826.9, 750.4 and 678.3 cm-1 were observed after the AR88 adsorption due to the contribution of the aromatic C‒H bending vibrations of AR88 loaded on the adsorbent [4]. These spectra indicated that AR88 was

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successfully adsorbed by CsPC.

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3.1.3 Pore structural analysis

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The BET surface area and average pore diameter analyses for Cs and CsPC were

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performed and the obtained results are given in Table 1. From the presented table, the BET surface area of Cs increased from 4.47 to 8.45 m2 g-1 after the modification. The results were

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consistent with the roughness of CsPC surface as indicated in the SEM analysis. The increase

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in the BET surface area and surface roughness of CsPC was due to the surface

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functionalization with PEI through the CaCl2 cross-linker onto the Cs backbone. The pores were uniformly distributed, and were helpful not only for the rising amino groups but also to provide more active sites for AR88 adsorption [21]. The relative adsorption performance of the adsorbent was also dependent on the internal pore structure of each material. According to the International Union of Pure and Applied Chemistry (IUPAC) classification [24], the textural characteristics of CsPC is classified as a mesoporous material with an average pore diameter of 2.39 nm. It has been previously reported that mesoporous adsorbents have shown favorable adsorption towards medium-sized dye molecules, such as Rhodamine B [25]. Therefore, CsPC is expected to facilitate the adsorption of AR88 dye due to its high surface area and mesopores texture.

Journal Pre-proof 3.1.4 XRD analysis The XRD data of Cs and CsPC are depicted in Fig. 4. From Fig. 4, it could be clearly revealed that that Cs (Fig. 4(a)) has two diffraction peaks at 2θ = 10° and 2θ = 20° which are assigned to the crystalline regions formed by the intermolecular hydrogen bonds between the amino and hydroxyl groups [26]. For CsPC (Fig. 4(b)), the intensity of the peaks significantly decreased due to the disruption of the intermolecular hydrogen bonds caused by the crosslinking reaction. In addition, the XRD pattern of CsPC showed an additional peak at 2θ = 30o

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which indicated the presence of Ca2+ as an ionic cross-linker [27]. These results indicated that the modification caused the destruction of the ordered crystal structure of Cs which may

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result in the improvement of adsorbent performance [28].

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3.1.5 TGA analysis

Fig. 5 shows the TGA and DTG analysis, to outline the relative thermal stability

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information about their relative thermal stability [8]. For CS (Fig. 5(a)), the first stage of

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decomposition occurs between 29 ‒ 168 °C with a weight loss of 10.6 %. This is attributed to

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the loss of adsorbed and bound water from CS [29]. The second decomposition stage occurs between 168 – 439 °C with a weight loss of 48.3% (DTG peak at 293 °C). This stage is associated with the degradation of CS, including dehydration of the saccharide rings, depolymerization and decomposition of the acetylated and deacetylated units [30]. In contrast, CsPC (Fig. 5(b)) undergoes three main decompositions with lower weight loss of hydration by 9.1% at 30 to 182 °C as compared to CS. It was further followed by a 40 % weight loss within 182 to 454 °C (DTG peak at 298 °C) due to the complete decomposition of CS polymer. At temperatures higher than 454 °C, another DTG peak appeared at 739 °C which contributes to the decomposition PEI polymer [30]. There is a slight shift in the temperature

Journal Pre-proof after PEI was grafted onto CS through Ca2+ cross-linker. Higher decomposition temperature of CsPC as compared to CS indicated that CsPC is more thermally stable.

3.2

Adsorbent synthesis The following parameters were optimized: (a) different PEI concentrations; (b) CaCl 2

concentrations; (c) heating temperature; and (d) heating time. The respective data and figures are provided in the Supplementary material, Fig. S2. The optimal conditions for the synthesis

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of the adsorbent were found to be at 4% v/v of PEI, 3% v/v of CaCl2, heating temperature of 30 °C and heating time of 1 h. Based on the preliminary study (Supplementary material, Fig.

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S3), CsPC has much higher adsorption efficiencies than Cs, polyethyleneimine-chitosan (CsP)

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and chitosan-CaCl2 (CsC). The results indicated that both PEI and CaCl2 were needed for a

3.3

Batch adsorption study

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3.3.1 Effect of pH

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successful cross-linking reaction and effective AR88 dye adsorption.

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The adsorption ability of CsPC towards AR88 dye as a function of pH in the range of 3 to 12 at a fixed dye concentration of 100 mg L-1 and temperature of 30 °C is illustrated in Fig. 6. From the graph, the optimum pH was achieved at pH 3. The adsorption performance at different pH can be attributed to the acid dissociation constant (pKa) of the dye molecules and pHpzc of the adsorbent. In aqueous solution, AR88 dissociates to form sodium ion (Na +) and sulfonate anions (SO3‒). The pKa of the acid dyes is usually lower than 1, indicating that the dye is in its anionic forms at pH higher than 1 [31]. On the other hand, the pHpzc of CsPC was determined to be 9.5 (Supplementary material, Fig. S4), suggesting that the adsorbent surface was positively charged when the pH is lower than the corresponding pH pzc value (pH < pHpzc). Consequently, the adsorption was favored at low to neutral pH due to the strengthening of

Journal Pre-proof electrostatic attraction between the protonated amino groups on the adsorbent and the dye anions [9]. The adsorption significantly dropped at alkaline pH due to the electrostatic repulsion between the negatively charged surface CsPC and negatively charged sulfonate groups (SO3‒) of AR88. Besides, the presence of large quantities of hydroxyl ions (OH ‒) in basic conditions competing with AR88 anions for the adsorptive sites caused a significant

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3.3.2 Effect of initial dye concentration and contact time

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drop in dye removal [32].

The effect of initial dye concentration and contact time on the adsorption of AR88 dye

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by CsPC is shown in Fig. 7. It can be seen that the amount of AR88 adsorbed increased

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significantly at the beginning of the adsorption process. After 90 minutes, the adsorption

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started to slow down before reaching the equilibrium. The time required to reach equilibrium at all concentrations was within 480 minutes. A relatively longer time was required to

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achieve equilibrium, since the dye molecules tended to diffuse into the internal pores of the

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adsorbent and utilize more active sites [33]. As the initial AR88 concentration was increased

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from 50 to 500 mg L-1, the equilibrium adsorption capacity of CsPC increased significantly from 118.2 to 980.4 mg g-1. At higher concentration, less driving force was needed to overcome the mass transfer resistance between the aqueous and the solid phases [31].

3.4

Adsorption isotherms Three linear isotherm models, namely the Langmuir, Freundlich and Temkin, were

further applied to provide an understanding of the adsorption mechanism, surface properties and insight on the nature of the adsorbent‒adsorbate interactions [23]. The Langmuir model is represented as follows [34]:

Journal Pre-proof (2) where qm is the maximum adsorption capacity of the adsorbents (mg g -1) and KL (L mg-1) is the Langmuir model constant related to the energy of adsorption. The expression of the Freundlich model is given by Eq. (3) [35]: (3)

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where KF is the Freundlich constants that relate to the bonding energy (L g -1) and n is the

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adsorption intensity and/or surface heterogeneity of the adsorption process. The Temkin

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isotherm in the non-linear form is described by Eq. (4) [36]:

(4)

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The term, B = RT/ΔHT, is Temkin isotherm constant associated with adsorbent‒adsorbate

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interaction, R is the gas constant (8.31 J mol-1 K-1), T is the absolute temperature (K), ΔHT corresponds to the enthalpy of adsorption (kJ mol-1), and A is the binding constant at

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equilibrium which corresponds to the maximum binding energy (L g -1).

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The calculated values of the parameters for all the isotherm models are presented in

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Table 2. Based on the coefficients of determination (R2), the adsorption of AR88 by CsPC best fitted the Freundlich isotherm model due to its higher R2 (R2 ≥ 0.98) value as compared to the Langmuir and Temkin models. The applicability of the model suggested multilayer adsorption of AR88 onto heterogeneous adsorption sites of the adsorbent. Besides, the n value was found to be 3.00 indicating favorable adsorption process [31]. Additionally, CsPC showed an outstanding adsorption performance with high qm value of 1000 mg g-1 towards AR88 dye. The qm value of CsPC found in this work was relatively higher to other adsorbents as reported in the literature (Table 3). Therefore, CsPC is believed to be an effective and suitable adsorbent for the removal of AR88 from the aqueous solution.

Journal Pre-proof 3.5

Adsorption kinetics Investigation of sorption kinetics would assist in elucidating the rate and mechanism

that control the adsorption process. The interpretation of sorption kinetic data at different initial dye concentrations ranging from 25 to 500 mg g -1 was conducted using pseudo-first order (PFO) and pseudo-second order (PSO) models. Lagergren and Svenska proposed the PFO kinetic model and the linearized equation is as described in Eq. (5) [40]:

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(5)

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where qe (mg g-1) and qt (mg g-1) are the amounts of dye adsorbed on the adsorbent at

(6)

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PSO kinetic model is defined by Eq. (6) [41]:

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equilibrium and at time t, respectively; and k1 (min-1) is the PFO rate constant. While, the

where k2 (g mg-1 min-1) is the PSO rate constant.

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The experimental kinetic data of CsPC were fitted to PFO and PSO models and the

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kinetic parameters are summarized in Table 4. The results showed that the R2 value of the

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PFO kinetic model (R2 ≥ 0.98) were higher than that of the PSO kinetic model. Moreover, the values of calculated adsorption capacity (qe, cal) fitted well with the values of experimental adsorption capacity (qe, exp). These findings suggested that the PFO model was more suitable for describing the adsorption process and that the physisorption was the possible rate-limiting step [13]. Similar finding was reported by Chatterjee et al. [13] for Reactive Black 5 adsorption onto PEI grafted Cs hydrogel beads generated by alkali and sodium dodecyl sulfate gelation.

Journal Pre-proof 3.6

Adsorption mechanism During the adsorption process, the adsorbates will move to the boundary layer of the

adsorbent, diffuse into the adsorbent surface and finally adsorbed into the interior pores on the adsorbent surface. The diffusion of adsorbates is generally controlled by either the external mass and/or intra-particle diffusion. The experimental data were evaluated by the Weber-Morris intra-particle and Boyd diffusion models in order to determine the diffusion

(7)

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mechanism. The intra-particle diffusion model is described as [42]:

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where kid (mg g-1 min-1/2) is the intra-particle diffusion rate constant and C (mg g-1) is related to the boundary layer effect. The values of kid and C are obtained from the gradient and

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intercept of the linear plots of qt versus t1/2 (Fig. 8). It could be observed that the plots

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exhibited three linear regions with different slopes. The first region represents external surface adsorption or an instantaneous adsorption, the second is a gradual adsorption stage

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where intra-particle diffusion is the controlling factor, and the final region is the equilibrium

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stage where intra-particle diffusion starts to decelerate. The second region does not pass

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through the origin indicating that intra-particle diffusion was not the only rate-limiting step [43]. Therefore, other processes may affect the adsorption process. The kinetic parameters calculated by the intra-particle diffusion model (Table 5) showed that the intercept C values are high (43.9 ‒ 228 mg g-1) suggesting greater boundary layer effects and thus more time was required to achieve equilibrium [43]. The kinetic data were further analyzed via the Boyd model to determine if the liquidfilm diffusion is the sole controlling mechanism. The model is generally expressed by the following equation [44]: (

)

(8)

Journal Pre-proof where kfd (min-1) is the liquid-film diffusion constant determined from the gradient of the linear plots between ln (1-qt/qe) and t (Fig. 9 and Table 6). The figure shows the plots were linear, but do not pass through the origin. The findings suggested that the rate was controlled by the liquid-film diffusion [43]. However, this is not the only factor. The relatively high BET surface area and mesopores texture of CsPC may have also promoted the intra-particle diffusion of AR88 dye. Moreover, the structure of CsPC exhibited more ‒NH2 groups which increased the binding of AR88 molecules to the surface of the CsPC. The possible

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interactions between AR88 dye and CsPC is illustrated in Fig. 10. It is expected that the major interactions involved in the interaction of AR88 dye with CsPC includes: (1)

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electrostatic interactions between the negatively charged sulfonate (‒SO3–) group of AR88

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and the positively charged amino (‒NH3+) groups on CsPC; (2) hydrogen bonding between

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the available hydrogen on CsPC with O and N atoms available in the AR88 molecules [45]; (3) Lewis acid-base interactions between the acidic Ca2+ complexes and basic components of

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AR88 dye [20]; and (4) n–π interaction between the lone pair electrons of oxygen and

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nitrogen atoms in CsPC with the π orbital of aromatic ring in AR88 dye [46].

3.7

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Thermodynamic study

The thermodynamic parameters such as the changes in standard enthalpy (ΔHo), standard entropy (ΔSo) and standard free energy (ΔGo) were calculated using the following equations [47]: (9) (10) where R, T and kd is the universal gas constant (8.314 J mol-1 K-1), temperature (K) and the distribution adsorption coefficient, respectively. The kd values can be calculated from the following equation:

Journal Pre-proof (11) where Co, Ce, V and m is the initial concentration (mg L-1), the equilibrium concentration (mg L-1), volume of the solution (L) and dose of the adsorbent (g), respectively. The values of ΔHo and ΔSo were acquired from the slope and intercept of van’t Hoff plot of ln kd versus 1/T (Supplementary material, Fig. S5). The calculated thermodynamic parameters are listed in Table 7. The rising values of ΔG° at the higher temperature regium

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suggested that the adsorption process was more favorable at low temperature and was less spontaneous at higher temperature [31]. The negative ΔH° value indicated that the adsorption

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of AR88 onto CsPC was an exothermic process. The increase of temperature promoted the

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mobility of dye molecules and caused the escape of the AR88 molecules from the solid phase to the liquid phase [31]. Therefore, more dye molecules would be in the bulk solution rather

Pr

than being adsorbed as the equilibrium shifted to favor desorption. The negative value of ΔSo

process [31].

Conclusion

Jo u

4.

rn

al

referred to the decreased of the randomness at the solid-interface during the adsorption

In the current work, the chitosan-polyethyleneimine with calcium chloride as an ionic cross-linker (CsPC) has been successfully synthesized. The CsPC has been proposed as a new adsorbent for dye removal using Acid Red 88 (AR88) as the model pollutant. Both FTIR and EDX analyses showed an increase in the number of amino groups in the adsorbents which resulted in an increased adsorption capacity for AR88 dye. Furthermore, the modified adsorbent also presented higher surface area and porosity with reduced crystallinity and better thermal stability as compared to unmodified Cs. The adsorption equilibrium was best described by the Freundlich isotherm model suggesting heterogenous adsorption with maximum adsorption capacity (qm) of 1000 mg g-1. The adsorption kinetics favored the

Journal Pre-proof pseudo-first order kinetic (PFO) model indicating that the overall rate of the adsorption process is controlled by physisorption. The thermodynamic study showed that the adsorption process of AR88 by CsPC was exothermic, enthalpy-driven and spontaneous in nature. The findings of this study indicated that CsPC can be easily synthesized using a one-step process with promising prospects as an effective and efficient adsorbent with high adsorption capability for the removal of AR88 or any other similar dyes from aqueous solutions. The

oo

f

fact that Cs can be easily harvested from a sustainable resource is also an advantage.

Acknowledgements

pr

The authors are grateful to the Ministry of Education - Higher Education (MOE),

e-

Malaysia for supporting this project under the Fundamental Research Grant Scheme (FRGS)

Pr

(203/PJJAUH/6711445). Additionally, S. Sabar would like to thank Universiti Sains

al

Malaysia (USM) and MOE for the SLAB Scholarship under the Post-Doctoral program.

S. Dong, Y. Wang, Removal of Acid Red 88 by a magnetic graphene oxide/cationic

Jo u

[1]

rn

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Journal Pre-proof Author Statement N.H. Yusof: Formal analysis, Investigation, Data Curation, Writing - Original Draft, Visualization K.Y. Foo: Conceptualization, Methodology, Validation, Resources, Writing - Review & Editing, Supervision B.H. Hameed: Validation, Writing - Review & Editing, Funding acquisition, Supervision

oo

H.K. Lee: Validation, Writing - Review & Editing

f

M. Hazwan Hussin: Validation, Resources, Writing - Review & Editing, Supervision

pr

S. Sabar: Conceptualization, Methodology, Validation, Resources, Writing - Review &

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al

Pr

e-

Editing, Visualization, Supervision, Project administration, Funding acquisition

Fig. 1.

Jo u

rn

al

Pr

e-

pr

oo

f

Journal Pre-proof

SEM micrographs of (a) Cs, (b) CsPC before and (c) after AR88 adsorption at 6 000× magnification.

Pr

e-

pr

oo

f

Journal Pre-proof

Fig. 2.

Jo u

rn

al

FTIR spectra of (a) Cs, (b) CsPC before and (c) after AR88 adsorption.

Journal Pre-proof

f o

l a

Fig. 3.

o r p

r P

e

o J

n r u

Schematic of the cross-linking reaction between Cs and PEI using CaCl2 as a cross-linking agent.

Pr

e-

pr

oo

f

Journal Pre-proof

Fig. 4.

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al

X-ray diffractograms of (a) Cs and (b) CsPC.

e-

pr

oo

f

Journal Pre-proof

Jo u

rn

al

Pr

(a)

(b) Fig. 5 (a) TGA and (b) DTG curves of CS and CsPC.

Journal Pre-proof pHpzc= 9.5

300 250 200 150

f

100 50

oo

Adsorption capacity, qe (mg g-1)

350

0 4

5

6

7

8

9

pr

3

10

11

12

e-

pH

Pr

Fig. 6.

Effect of initial pH on the adsorption of AR88 by CsPC with an initial dye concentration of

Jo u

rn

al

100 mg L-1, temperature of 30 °C and contact time of 24 h.

Journal Pre-proof 1200

50 mg L⁻¹

100 mg L⁻¹

200 mg L⁻¹

300 mg L⁻¹

400 mg L⁻¹

500 mg L⁻¹

800

600

f

400

oo

Adsorption capacity, qt

1000

0 250

500

750 Time, min

1000

1250

1500

Pr

0

e-

pr

200

al

Fig. 7.

Jo u

CsPC at 30 °C.

rn

The effect of initial dye concentrations and contact time on the amount of AR88 adsorbed by

Journal Pre-proof 1200

50 mg L⁻¹

100 mg L⁻¹

200 mg L⁻¹

300 mg L⁻¹

400 mg L⁻¹

500 mg L⁻¹

1000

qt (mg g-1)

800

600

oo

f

400

0 0

5

10

15

e-

pr

200

20

25

30

35

40

Pr

t1/2 (min1/2)

al

Fig. 8.

rn

The intra-particle diffusion model for the adsorption of AR88 by CsPC at different initial dye

Jo u

concentrations and temperature of 30 °C.

Journal Pre-proof 3.5 50 mg L⁻¹ 300 mg L⁻¹

100 mg L⁻¹ 400 mg L⁻¹

200 mg L⁻¹ 500 mg L⁻¹

3.0

2.5

Bt

2.0

oo

f

1.5

pr

1.0

0.0 20

30 40 Time (min)

50

60

al

Fig. 9.

10

rn

0

Pr

e-

0.5

The Boyd diffusion model for the adsorption of AR88 by CsPC at different initial dye

Jo u

concentrations and temperature of 30 °C.

Pr

e-

pr

oo

f

Journal Pre-proof

al

Fig. 10.

Jo u

rn

Possible interactions of AR88 dye with different sites on the surface of CsPC.

Journal Pre-proof Table 1 Elemental composition and pore structural analysis of Cs and CsPC. Cs

CsPC after AR88

adsorption

adsorption

45.5

33.6

54.9

N

7.30

13.3

11.5

O

47.2

43.6

31.6

Ca

-

9.62

5.29

S

-

-

6.73

BET surface area (m2 g-1)

4.47

8.45

Volume of pores (cm3 g-1)

0.014

0.005

-

Average pore diameter (nm)

2.11

2.39

-

oo

pr ePr

al rn

f

C

Jo u

Elements (%)

CsPC before AR88

-

Journal Pre-proof Table 2 The isotherm parameters of Langmuir, Freundlich and Temkin isotherm models for the adsorption of AR88 onto CsPC. Langmuir

Freundlich

Temkin

KL (L mg-1)

aL

qm (mg g-1)

R2

KF (L g-1)

n

R2

A (L g-1)

0.0381

3.82×10-5

1000

0.937

143

3.00

0.981

2.75

l a

o J

n r u

r P

e

o r p

f o

B

ΔHT (kJ mol-1)

R2

120

20.9

0.857

Journal Pre-proof Table 3 Comparison of the maximum adsorption capacities, qm of AR88 onto various adsorbents. Adsorbents

Methods

Bio-silica/chitosan

Chitosan mixed with bio-silica in acetic

nanocomposite

acid and dropwise addition into a

Temperature

Adsorbent dosage

Co

qm

(°C)

(g L-1)

(mg L-1)

(mg g-1)

25

3.00

10 – 400

25.8

[32]

0.20

5 – 40

41.8

[37]

30

0.20

10 – 56

54.4

[31]

o r p

mixture of NaOH and ethanol. Magnetic Fe@graphite

Chemical vapor deposition (CVD)

nanocomposite

l a

Magnetic multi-walled

Chemical vapor deposition (CVD)

carbon nanotubes-Fe3C

n r u

nanocomposite

o J

e

30

r P

f o

Ref

Magnetic ZnFe2O4

Microwave assisted hydrothermal

30

0.15

10 – 56

111

[38]

Quaternized

A combination process of freeze-drying

25

0.20

5 – 100

474

[39]

nanofibrillated cellulose and cross-linking with epichlorohydrin

a

Calcined alunite

Calcination process

25

0.80

NRa

833

[4]

CsPC

One-step synthesis

30

0.40

50-500

1000

This work

Not reported

Journal Pre-proof Table 4 Kinetic parameters of the PFO and PSO kinetic models for the adsorption of AR88 by CsPC at different initial dye concentrations. Co (mg L-1)

qe, exp (mg g-1)

50

Pseudo-first-order

Pseudo-second-order

qe, cal (mg g-1)

k1 (min-1)

R2

118

125

2.80×10-3

0.991

100

256

285

6.30×10-3

0.993

200

402

444

1.10×10-2

0.999

300

627

639

7.90×10-3

0.996

400

835

788

5.90×10-3

500

980

900

5.20×10-3

o J

n r u

l a

qe, mod (mg g-1)

0.999

R2

139

1.86×10-4

0.891

435

2.52×10-5

0.939

1000

7.72×10-6

0.945

1111

8.78×10-6

0.966

1000

1.54×10-6

0.999

1000

1.90×10-6

0.989

f o

ro

-p

e r P 0.999

k2 (g mg-1 min-1)

Journal Pre-proof Table 5 Kinetic parameters of the intra-particle diffusion models for the adsorption of AR88 by CsPC at different initial dye concentrations. Co

qe, exp

(mg L-1)

(mg g-1)

50

ki1

(mg

(R1)2

(R2)2

(R3)2

-

0.999

0.994

-

120

-

0.999

0.896

-

228

-

0.999

0.889

-

218

582

0.995

0.965

0.999

0

169

761

0.995

0.985

0.999

0

109

908

0.995

0.991

0.999

ki2

ki3

C1

C2

g-1 min-1)

(mg g-1 min-1)

(mg g-1 min-1)

(mg g-1)

(mg g-1)

118

9.56

3.86

-

0

43.9

100

256

22.9

8.92

-

0

200

402

39.5

11.7

-

0

300

627

52.9

27.4

1.15

400

835

61.5

43.1

1.94

500

980

68.7

55.6

1.90

o J

n r u

l a

ro

e r P

-p 0

f o

C3

(mg g-1)

Journal Pre-proof Table 6 Kinetic parameters of the liquid-film diffusion models for the adsorption of AR88 by CsPC at

R2

50

2.80×10-2

0.996

100

2.60×10-2

0.990

200

1.89×10

-2

0.992

300

1.50×10

-2

0.986

400

1.18×10-2

0.989

500

9.70×10-3

0.988

oo

kfd

Jo u

rn

al

Pr

e-

pr

Co (mg L-1)

f

different initial dye concentrations.

Journal Pre-proof Table 7 The thermodynamic parameters for the adsorption of AR88 by CsPC.

Temperature, (K)

Thermodynamic parameters ΔGo (kJ mol-1)

303

41.3

-9.38

313

3.61

-3.34

323

1.67

-1.38

ΔHo (kJ mol-1)

ΔSo (J mol-1 K-1)

-131

-404

Jo u

rn

al

Pr

e-

pr

oo

f

kd

Journal Pre-proof Highlights



Chitosan-polyethyleneimine with calcium chloride (CsPC) was prepared as adsorbent.



CsPC presented higher surface area and porosity with reduced crystallinity. Acid Red 88 was adsorbed effectively on CsPC with qm of 1000 mg g-1 at 30 °C.



Pseudo-first order kinetic and Freundlich isotherm models best fitted the adsorption.



The high adsorption uptakes were due to the different adsorption mechanisms.

Jo u

rn

al

Pr

e-

pr

oo

f



Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10