Fabrication of lightweight and reusable salicylaldehyde functionalized chitosan as adsorbent for dye removal and its mechanism

International Journal of Biological Macromolecules 141 (2019) 626–635

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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac

Fabrication of lightweight and reusable salicylaldehyde functionalized chitosan as adsorbent for dye removal and its mechanism Nira Parshi a,1, Dipika Pan a,1, Vishal Dhavle a,b, Biswajit Jana a, Santu Maity a, Jhuma Ganguly a,⁎ a b

Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, West-Bengal 711103, India Department of Chemistry, Sardar Vallabhbhai National Institute of Technology, Ichchhanath Surat- Dumas Road, Keval Chowk, Surat, Gujarat 395007, India

a r t i c l e

i n f o

Article history: Received 23 August 2019 Received in revised form 30 August 2019 Accepted 4 September 2019 Available online 05 September 2019 Keywords: Functionalized chitosan Dye adsorption Kinetics Mechanism Regeneration

a b s t r a c t Bio-resources have a very significant role in current research approach for the synthesis of benign functionalized biological macromolecules for their stable structural integrity and inherent nature-inspired potentialities. Here, chitosan is used as a core moiety for designing of a porous adsorbent after the attachment of salicylaldehyde to remove the toxic dyes. Salicylaldehyde linked chitosan, with excellent surface porosity, lightweight, nonglucose and low-cost feature, makes it as an efficient adsorbent. The dye loaded material is very easy to remove from the top of the water as it is suspended on water. The physico-chemical characterizations are done by FTIR, rheology, SEM and swelling study. The removal efficiency is 98% and 99% for Crystal Violet and Rose Bengal from water respectively. The thorough adsorption with mechanistic approach shows the Freundlich model as an appropriate one and follows closely pseudo-second-order kinetics model. Thermodynamic study reveals the endothermic nature of the process. Moreover, the reusability of Salicylaldehyde linked Chitosan shows its persistence with the same amount and concentration of dyes in water up to three consecutive cycles. So, the chitosan based macromolecules can be a sustainable candidate in the current scenario for the removal of dyes without the dislocation of the water container. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Natural biological macromolecules have been currently focused in pursuing the research to preserve the significant natural structural integrities into the new functionalized bio-materials. They would have inherent nature inspired potentialities for compatible applications without any hamper to environment. The sugar based materials e.g. cellulose, chitosan, glycan, etc. are popular naturally occurring Bio resources. They are biocompatible, biodegradable, protective, lightweight, and soluble in water and biological fluid, low cost and easily available natural resources. Dyes, coming out from industries and textile waste water, are main organic pollutants having highly destructive impact on aquatic lives and human health [1]. They are highly toxic, mutagenic, carcinogenic and allergenic on expose to animal organism [2]. Additionally, their non-biodegradable nature further helps to intensify their toxicity. Therefore, the eco-friendly research for the control and removal of these toxic dyes is really added a protective advancement in the context. Till now number of strategies are taken for the elimination of dyes from contaminated waters e.g. chemical precipitation [3], ion exchange [4], membrane filtration [5], coagulation [6], ⁎ Corresponding author. E-mail address: [email protected] (J. Ganguly). 1 Contributed equally to the work.

https://doi.org/10.1016/j.ijbiomac.2019.09.025 0141-8130/© 2019 Elsevier B.V. All rights reserved.

flotation and electrochemical treatment, physical adsorption [7], chemical oxidation/reduction [8] and biological treatment [9] etc. Among all, adsorption based purification of wastewater is an ideal strategy for its easy handling, high efficiency, simple and economic viability [10]. Designing of an environment-benign, cheaper and efficient adsorbents using polysaccharides, methylcellulose [11], chitosan [12], starch [13] and cyclodextrin [14] etc. are slowly explored. N-acetylation of Chitin moiety restricts the functionalities to modify it into a good adsorbent, whereas chitosan, for the presence of free amine group, could be a better alternative eco-friendly and bio-resources in current days. Chitosan moiety contains excessive number of hydroxyl groups and amino groups that help in modification for multi functionalities. The free amine groups of chitosan are easily functionalized with aromatic moiety. Hydrogel obtained from imine functionalized chitosan, with three dimensional network and water insolubility, is capable to adsorb and store various materials e.g. dyes, metals within it. In recent past, various modified Chitosan based hydrogels have been applied for heavy and radioactive metals removal from water [15] from our group. However, the fabrication of chitosan based Hydrogels for the removal of dyes from aqueous medium by adsorption strategy with thorough mechanism and reusability is not properly reported in the literature till date. In this work, a lightweight and porous salicylaldehyde linked chitosan (ChS), as a bio adsorbent, has been synthesized by Schiff base modification of chitosan and applied for the excellent performance

N. Parshi et al. / International Journal of Biological Macromolecules 141 (2019) 626–635

to remove dyes from aqueous solution. The functionalized Chitosan shows hydrogel characteristics. The physic-chemical nature of hydrogel has been elucidated by swelling study, FTIR spectra, rheological experiment, Scanning electron microscope (SEM) etc. Two wellknown organic dyes i.e. Crystal violet and Rose Bengal are chosen as the model toxic dyes for the adsorption by ChS. The adsorption mechanism, isotherms, kinetics and thermodynamic parameters are also calculated and explained. Eventually, the performance of recycle ability has been investigated and excellent reusability is obtained without any degradation of ChS and no wastages in nature. Easy recovery of the adsorbent is possible after treatment of dye contaminant water. The development of ChS for the removal of toxic dyes from water is highly effective and environ-benign. So, the sustainable, lightweight ChS can be a potential candidate in current scenario for the removal of dyes unsetting the water container. 2. Experimental section

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2.5. Adsorption studies 2.5.1. Dye adsorption study Two toxic organic dyes, cationic Crystal violet (CV) and neutral dye Rose Bengal (RB) were selected as the model dye molecules for the estimation of the adsorption efficiency of dried gel. UV–Vis absorbance spectra of ChS (Fig. S1(b)) is recorded and it shows no interference in absorbance for the current adsorption study of the dye molecules as the peaks arise for Crystal Violet (λmax: 553 nm) and Rose Bengal (λmax: 541 nm) is different from ChS (broad peak, 300-350 nm). Adsorption isotherms, kinetic and thermodynamics experiments by batch equilibrium have been thoroughly calculated with the change in pH, concentration of dye, hydrogel dosage and temperature during adsorption. Dye solution (2 mL) with dried gel (40 mg) is taken for the experiments at 160 rpm for required time range. The resultant solutions are analyzed by UV spectrophotometer. The equilibrium adsorption capacity (Qe, mg/g) for dye on gel is expressed (Eq. (2)) as:

2.1. Materials Low molecular weight chitosan (LMWC) (C6H11NO4)n viscosity (20 °C): 10–150 mPa.s, molecular weight: 50–190 kDa, deacetylated degree: min. 90% (SRL) is obtained from SRL chemicals. Salicylaldehyde, glacial acetic acid (purity 99%), Crystal Violet and Rose Bengal are purchased from Sigma-Aldrich. Absolute Ethanol is obtained from Merck for the synthesis. 2.2. Synthesis of hydrogel (ChS) 1 mL glacial acetic acid solution was added slowly to Chitosan solution (1 g in 50 mL distilled water). A transparent solution was obtained after stirring continuously for 15 min. Salicylaldehyde (6 mL, 2 M in EtOH) added quickly to 2 mL acidic Chitosan solution with stirring at 70 °C and a light yellow hydrogel gel formed within 1 min (Fig. 1). The resulted ChS hydrogel is washed further by distilled water and ethyl alcohol and dried in a freeze drier for further application. 2.3. Characterization Characterization of ChS has been done by several techniques. FTIR spectra are performed by a FTIR Spectrometer (model 1600, PerkinElmer Co., USA and JASCO, FT/IR-460 PLUS, KBr pellet) within 900 to 4000 cm−1. Rheological measurements are carried out by Anton Paar Physica MCR 301 rheometer, USA, using parallel plate PP50/P-PTD200 geometry (50 mm diameter; 0.1 mm gap) at 25 °C by a Viscotherm VT2 circulating water bath. The scanning electron microscopic (SEM) images have been taken with the use of Carl Zeiss EVO 18 (EDS 8100) microscope with Zeiss Inca Penta FETX 3 (Oxford instruments) attachment. UV–Vis spectra are recorded by a JASCO UV-V530 spectrophotometer.

Q e ¼ ½ðC0 −Ce Þ  V=W

ð2Þ

where, C0: dye concentration at initial time; Ce: dye concentration at equilibrium; V: volume of dye solutions; and W: weight of dried gel. 2.5.2. pH sensitivity in adsorption pH depended adsorption experiment of Crystal Violet and Rose Bengal on gel, is carried out with pH 3 to pH 10. pH of the dye solutions are adjusted with HCl (pH 1 and 2), acetate (pH 3 to 6) and phosphate buffer (pH 7 to 10). 2 mL Dye solutions having concentration of 10−4 M with dried xerogel (40 mg) are taken at 303 K for experiment with various pH. 2.5.3. Adsorption isotherms The initial dye concentration (from 8 × 10−4 to 5 × 10−6 M) for Crystal violet at pH 6 and for Rose Bengal at pH 4 is used for isotherm mechanism. The adsorption isotherm is also studied by using Langmuir and Freundlich models. The Langmuir adsorption isotherm model (Eq. (3)) is taken for optimization, where, Qmax (mg/g): maximum adsorption at monolayer coverage: and b (L/mmol): Langmuir adsorption equilibrium constant used for the energy of adsorption. 1 1 1 1 ¼ þ Q e Q max Q max bC e

ln Q e ¼

ð3Þ

1 ln C e þ ln k F n

ð4Þ

The adsorption on heterogeneous surfaces is considered by the Freundlich isotherm model (Eq. (4)). KF and 1/n: the Freundlich characteristic constants.

2.4. Swelling measurement The determination of the swelling efficiency in deionized water with change of pH was performed by gravimetric method at room temperature. Briefly, weighed dry materials (20 mg) were suspended in excess water phase for 6 h. HCl used for the preparation of the solutions for pH 1 and pH 2 and HEPES buffer used for higher pH solutions. The fully swollen and enlarged materials are weighed after the socking of excessive surface water with filter paper. Repetition measurements were performed to standardise the constant mass for swollen ChS. Eq. (1) is used for the calculation of the swelling ratio. Swelling ratio ð%Þ ¼ ½ðWs –Wd Þ=Wd   100

ð1Þ

where, Ws: Weight of sample at equilibrium swollen state and Wd: weight of dry sample at experimental temperature.

Fig. 1. Gelation of ChS.

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Fig. 2. ChS hydrogel preparation.

2.5.4. Adsorption kinetics The adsorption kinetics is established with the help of pseudo-first and pseudo-second order kinetic models at 303 K for the time ranging from 1 min to 24 h. Here, Qt (mg of dye/g of gel): adsorption uptake of the hydrogel at various contact times, and R%: removal percentages are determined using Eqs. (5) and (6) respectively. Where, Ct is dye concentration at time t (Eq. (5)),   ðC 0 −C t ÞV Qt ¼ W R% ¼

  C 0 −C e  100… Ci

t 1 t ¼ þ Q t Q 2e K 2 Q e ln ðQ e −Q t Þ ¼ ln Q e −k1 t

ð5Þ

 Qe … Ce

ΔGo ¼ ð−RTlnkc Þ:   ΔH o ΔSo þ : ln kc ¼ − RT R

3.1. The formation of ChS hydrogels

ð7Þ ð8Þ

3.2. Compositional studies

ð6Þ

2.5.5. Adsorption thermodynamics To optimize the effect of temperature for dye adsorption is performed by varying the temperatures at 25 °C, 30 °C and 40 °C and 50 °C. The thermodynamic parameters, standard free energy (G°), enthalpy change (H°), and entropy change (S°) are taken into consideration for the evaluation of the viability [16]. The thermodynamic parameters and distribution coefficient (kc) at four different temperatures, are determined using the following equations: 

3. Results and discussions

The formation of ChS hydrogel is fabricated with Schiff base mechanism. The free -NH2 groups of chitosan chains with carbonyl groups of salicylaldehyde are stabilised by the formation of imine bonds (Fig. 2). The presence of enormous no of H-bodings from free hydroxyl groups in chitosan and salicylaldehyde moieties, the as prepared material is highly cross linked and gelation is appeared within a min. The hydrogel matrix formed is thoroughly washed with ethanol and water and dried.

The linear forms of the pseudo first order and pseudo second order kinetic models are expressed in Eqs. (7) and (8) respectively. The rate constant of the pseudo-first-order model: K1 (min−1) (Eq. (7)); and the rate constant of the pseudo-second-order model: K2 (g/mg/min) is (Eq. (8)) are calculated following the above equation.

kc ¼

of DMSO (2 mL) at stirring for 60 min at 303 K. The regenerated, renewed and unchanged matrix have been used for the another same experiments for adsorption cycles.

ð9Þ

3.2.1. FT-IR study The formation of hydrogel is established by the help of FT-IR spectra of ChS (Fig. S1). In contrast of Chitosan [17], the strong characteristic peaks appear at 1625 cm−1(C_N bonds of imine), 3100–3750 cm−1 (stretching vibration of O\\H and N\\H bond) and 1029 cm−1 (bridged C–O–C bond) in ChS due to formation of hydrogel by Schiff base condensation reaction [18]. 3.2.2. Morphological study by scanning electron microscope (SEM) SEM images describe the cross-sectional morphology for Surface of the dried ChS is determined. ChS exhibits a macro-porous surface (Fig. 3) with interconnected cross-linked channels and void space. Hence, the improved and tuned morphology are much suitable for the adsorption rate and capacity and the gel have been suspended on water [19].

ð10Þ ð11Þ

The change of free energy ΔGo obtained from kc (Eq. (9)). ΔHo and ΔS calculated by kc in the van't Hoff equation (Eq. (11)), Where, R is the gas constant (8.314 J/mol/K) and T is the absolute temperature (K). o

2.5.6. Reusability experiment The recovery of matrix is studied with ChS (40 mg) and dye solution (3 mL, 10−4 M) at 303 K for 60 min incubation. The desorption and regeneration of matrix from trapped dyes has been done by suspending in

Fig. 3. SEM image of ChS hydrogel.

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Fig. 4. Rheology for ChS (a) Strain sweep and (b) Frequency sweep mode.

3.2.3. Mechanical properties of the hydrogels The mechanical strength of ChS is evaluated by rheological study. The two main factor G′ (storage modulus) and G″ (loss modulus) is considered for strain sweep and frequency sweep experiment in this regards. G′ explore the elastic behavior of the hydrogel, and G″ signifies the viscous nature of the gel. At first, G′ and G″ vs. % strain are performed in strain sweep experiment to elucidate the linear viscoelastic regime, (Fig. 4(a)). The increased rate of G′ is quite higher than G′′ and thus the elasticity of hydrogel dominates [20]. The linear viscoelastic edge for ChS is nearly up to 45% strain. Next, frequency sweep experiment is done with a constant strain of 2% that is much less than the deformation limit of the hydrogel. The frequency sweep experiment (Fig. 4(b)) confirms that the elastic modulus (G′) is practically independent of frequency and its magnitude is nearly two orders greater than that of viscous modulus (G′′). These features characterize ChS as a “Strong” gel in presence of water [21].

3.2.4. Swelling measurement The hydroxyl group rich ChS is very sensitive towards the influence of pH. Using gravimetric method, the swelling study with variation of pH (Fig. S2) is done which illustrates the higher swelling ratio at lower pH. The cross-linked hydrogel show a higher swelling ratio of 70–75% at pH 2–5 and then a drop in swelling ratio occur at higher pH. The mechanism follows the swelling involving protonation of the hydroxyl, imine. These protonated internal functional groups boost the positive charge ratio within the three dimensional hydrogel

networks. The electrostatic repulsion causes the increased osmotic pressure among the gel particles. This repulsive forces neutralised and swollen the gel by the entering of counter-ions of water with gel networks [22]. 3.3. Adsorption study To study the performance efficiency of ChS for the adsorption for CV and RB dyes, were first conducted a batch experimental setup. The influences of various parameters such pH in solution, dose of ChS and dyes, contact time on adsorption patterns and were captured. 3.3.1. Effect of pH in solution The hydrogel is very sensitive towards pH in solution and initially the maximum interaction of dye molecules with matrix is to be found out. The measurement of optimized pH for the adsorption on ChS, dye concentration at 10−4(M) and adsorption time, 4 h is considered (Fig. 5). The highest adsorption of Crystal Violet dye on ChS is observed at pH 6 and it remain almost constant at pH ˃ 6. Whereas, at pH b3, the removal efficiency of CV from water is very small. At this acidic pH, H+ has faced competitive participation with dye ions for adsorption on ChS surface [23,24] thereby inhibits the adsorption of dye molecules only. The best adsorption of rose Bengal takes place at the pH 3 to pH 4 for on ChS matrix. However, there is a decrease in adsorption capacity from 0.36 to 0.026 mg/g with further increase in pH from 4 to 10. The higher adsorption takes place probably because of protolytic equilibria involving the formation of a less reactive RB species. In the acidic region, the

Fig. 5. Adsorption study for (a) CV and (b) RB on ChS at 30 °C K in pH 1 to pH 10.

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Fig. 6. Hydrogel dosage dependent removal efficiency of (a) CV and (b) RB at 303 K.

main two types of interactions are taking place one is Hydrogen bonding between the nitrogen of quaternary ammonium groups and hydroxyl group of RB and another is π-π interaction between the aromatic rings present in the gel network and RB which favor the adsorption process at lower pH [25]. 3.3.2. Effect of ChS dosages Batch experiments were investigated with change of hydrogel dosages (50 mg, 40 mg, 30 g, 20 mg, 10 mg, 5 mg) for the Crystal Violet (pH 6) and Rose Bengal (pH 4) dyes (3 mL, 10−4 M) adsorption at 30 °C for 2 h. The efficiency of adsorption (Fig. 6) increase with the increase amount of ChS primarily but then, it becomes saturated at 30 °C. The % adsorption is increased on variation dosage from 5 mg to 40 mg of ChS and the results show an increase in removal percentage from 91.5

to 98.2% for CV and from 11.5 to 92% for RB. These results arise due to the quick accumulation of both the dye molecules in the active porous sites of ChS initially but after certain dosage of ChS the available active sites get saturated by dye molecules of the solution at that moment and therefore the adsorption % capacity remain unaltered. Hence, the 3 mL dye solution (10−4 M) has been optimized for the optimum ChS dosages (40 mg). Eq. (6) is used for the measurement of the % removal of dyes. 3.3.3. Effect of initial dye concentration and adsorption isotherm The influence of initial dye concentration on adsorption has been investigated with the dyes (3 mL solution) in the range of 8 × 10−4–5 × 10−6 (M) with ChS (40 mg,) at pH 4 and pH 6 for RB and CV respectively (Fig. S3). An increase in initial dye concentration (C0) enhances

Fig. 7. (a) Langmuir and (b) Freundlich adsorption isotherm at 303 K.

N. Parshi et al. / International Journal of Biological Macromolecules 141 (2019) 626–635 Table 2 Adsorption Kinetics study.

Table 1 Langmuir and Freundlich adsorption isotherm. Dyes

CV RB

Langmuir isotherm

631

Dye used

Freundlich isotherm

b(L/mg)

Qmax(mg/g)

R2

KF(L/mg)

n

R2

0.3532 0.0030

6.85 15.26

0.98365 0.97426

4.6399 0.0319

0.7298 0.711

0.9926 0.9875

the equilibrium dye adsorption capacity of ChS. The stronger interactions i.e. H-bonding, electrostatic interaction π-π interaction between the hydrogel and the functional groups of the dye is responsible for the excellent adsorption of dyes at higher C0. The free functional sites of ChS surface instantaneously bind with free dye molecules. At higher initial concentration of dye the rate of adsorption is higher but over time the adsorption is inhibited by paucity of free sites of ChS by dye molecules and saturation occurs. A relationship at constant temperature between the mass of adsorbed dye per unit mass of the adsorbent (ChS) and the dye concentration at liquid phase has been described by Adsorption isotherm. To investigate the adsorption performance of ChS for the dye, two well-established models are considered i.e. Langmuir adsorption isotherm and Freundlich Adsorption isotherm models. The Langmuir model is mainly applicable for monolayer coverage by adsorption on a homogeneous surface whereas, the Freundlich adsorption isotherm model is mainly valid by the assumption of adsorption on heterogeneous surfaces. The linear fitted experimental data to Langmuir and Freundlich isotherm models are shown in Fig. 7 for CV and RB. The related parameters evaluated from the Langmuir and the Freundlich models are listed in Table 1. In Langmuir model, the Qmax values give the estimation about the adsorbed dye. The calculated Qmax, estimation amount of adsorbed dye is 6.85 mg/g (CV) and 15.26 mg/g (RB). Langmuir adsorption equilibrium constants (b, L/

CV RB

Pseudo-first order kinetics

Pseudo-second order kinetics

k1 (/min)

Qe (mg/g)

R2

k2(g/mg/min)

Qe(mg/g)

R2

0.036 0.0314

0.2528 0.00098

0.9733 0.87992

0.795 168.55

2.004 0.044

0.996 0.999

mmol) are 0.3532 (CV) and 0.0030 (RB) which are within the range from 0 to 1. The Freundlich constant (1/n) is associated with the adsorption intensity of the adsorbent and signify about the spontaneity of the process. For excellent adsorption: 0.1 b 1/n ≤ 0.5; moderate adsorption: 0.5 b 1/n ≤ 1; difficult adsorption: 1/n N 1 [26]. The calculated 1/n (CV) is 0.7298 for and 1/n (RB) is 0.711 at 30 °C. Here, for both the experiments, 1/n value is nearly 0.7 demonstrating moderate adsorption on the surface of ChS. The adsorption of both the dyes on ChS more closely follows the Freundlich model equation for the experimental range of concentration as the R2 value is nearly unity. It also depicted that CV and RB adsorption on ChS is mainly take place via multilayer adsorption on heterogeneous surface [27]. 3.3.4. Effect of contact time and adsorption kinetics The time dependent adsorption on hydrogel is performed. The adsorption rates are faster within first 30 min for both CV (at pH 6) and RB (at pH 4), with 10−4(M) dye concentration at 30 °C and 40 mg ChS at shaking condition. Then, the rates of adsorption become slower and the equilibrium is reached in further 15 min. The initial quick adsorption rate is due to the presence of a huge number of active binding sites or definite cavities on the ChS. Relation between uptake of dye (Qt) in the contact time (t) from 0 to 60 min of adsorption for dye on ChS is

Fig. 8. (a) Pseudo-first order and (b) Pseudo-second order kinetics model at 303 K.

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Fig. 9. (a) Temperature dependent adsorption capacity and (b) van't-Hoff's plots.

studied (Fig. S4). The time dependent dye adsorption takes place in two stages which includes a fast adsorption within a short time followed by a slow adsorption process. The equilibrium dye adsorption capacity of ChS is calculated for CV is 2.00 mg/g and RB is0.0039 mg/g (Eq. (3)). Nearly, 95% adsorption has occurred by ChS in 50 min for Crystal Violet and 30 min (Fig. S5) for Rose Bengal dyes and after that the adsorption becomes slow down. These dye adsorption process are further studied with the help of pseudo-first-order and pseudo-second-order kinetics model. Adsorption kinetic usually provides knowledge about the rate of adsorption, investigate the mechanism of adsorption and help in determination of the possible rate-controlling steps. Here, kinetic of the adsorption is established using Crystal Violet and Rose Bengal as model dye on ChS. The pseudo-first-order kinetic and the pseudo-second-order kinetic (Eqs. (7) and (8) respectively) models are two well established model in determination of the rate constant and to investigate the mechanistic background of the adsorption process. The obtained experimental data are fitted linearly (Fig. 8) for pseudo-first order and pseudo second order model using Eqs. (7) and (8) respectively for the dyes. The calculated kinetic parameters are listed in Table 2. The adsorption of both dyes steadily follows the

pseudo-second-order kinetic model the in respect to pseudo-firstorder kinetic model as the calculated correlation coefficients (R2) are nearly unity for the pseudo-second-order kinetic model. The calculated theoretical value of Qe(Eq. (8)) is closer to the experimental equilibrium amount of absorbed dyes Qecal for pseudo-second-order kinetic model (Table 2). Thus, the dye adsorption on ChS satisfies the pseudosecond- order kinetic model more efficiently signifying that the ratecontrolling stage in adsorption process is proceeding via chemisorption process where exchanging or sharing of electron pairs are occurred between cationic groups and the oxygen present in adsorbent and adsorbate [28,29]. 3.3.5. Effect of temperature and thermodynamics study The effect of temperature (T) on adsorption process has been studied and the results are depicted in Fig. 9. The removal percentages for

Table 3 Thermodynamic Study for adsorption. Dye used

Temperature (K)

283

303

323

343

CV

ΔG° (J/mol) ΔH° (kJ/mol) ΔS° (J/mol/K) ΔG° (J/mol) ΔH° (kJ/mol) ΔS° (J/mol/K)

−4.03 × 103 34.013 132.57 −44.289 43.112 119.164

−4.40 × 103

−6.10 × 103

−1.17 × 104

−128.247

−687.46

−1086.64

RB

Fig. 10. Reusability study of ChS with Crystal Violet and Rose Bengal.

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633

Scheme 1. Overview of Interaction of (a) CV and (b) RB with ChS gel.

the dyes from aqueous medium using matrix are enhanced with the rise of temperature. In the endothermic reaction, temperature dependence on adsorption has been investigated using ChS (40 mg) with dye solution (2 mL, 10−4 M) at 10 °C, 30 °C, 50 °C and 70 °C temperature. The adsorption efficiency of ChS for CV and RB is trends to increase from 1.52 to 1.62 mg/g and 0.126 to 0.369 mg/g respectively (Fig. 9a), with the increase in temperature range from 10 °C to 70 °C suggesting that the higher temperature assists the appreciable adsorption of both the dyes on ChS. The change in enthalpy and entropy are two key factors [30] in any kind of spontaneous process. So, it is important to evaluate the change in thermodynamic parameters during any process like standard free energy (G°), enthalpy change (H°), and entropy change (S°) etc. to understand the feasibility of the adsorption process by applying the thermodynamic equations. The values of ΔG° for adsorption of CV and RB are listed in the Table 3. Here, the negative ΔG° value of adsorption confirmed a spontaneous and favorable reaction during adsorption at experimental temperatures and from tabulated data, more energetically appropriate adsorption occurred at 70 °C. Other two main thermodynamic parameters i.e. ΔH° and ΔS° have been also calculated from the slope and intercept of van't Hoff plots (Fig. 9b) of lnKc versus 1/T. ΔH° and ΔS° values are placed in Table 3.

The positive H° suggested the endothermic adsorption process. The positive S° indicated a good affinity of both CV and RB towards the gel surface and enhanced randomness at the solid-solution interface [31,32]. 3.3.6. Reusability experiment The regeneration study for the matrix is very important to prove its eco-friendliness behavior. Three consecutive adsorption cycles have been performed separately for the adsorption of both dyes with same ChS in each case. The removal efficiency of ChS after one and three cycles are 97.94% and 94.54% respectively for Crystal Violet and 98.24% and 93.88% respectively for Rose Bengal (Fig. 10). This result show that the adsorption performance of the bio adsorbent (ChS) is almost unchanged during the whole reusability process. This also concludes that the prepared ChS hydrogel is a potential reusable and recycle candidate for absorption in the removal of Crystal Violet and Rose Bengal dye. 3.3.7. Mechanism of adsorption The Mechanism of adsorption depends primarily on functional behaviors and surface porosities of both adsorbent and adsorbate molecules, Here, the interactions during adsorption process has been established by using FT-IR, SEM and spectroscopic study. Three

Fig. 11. SEM images of (a) ChS: before adsorption (b) ChS-RB after adsorption and (c) ChS-CV after adsorption.

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responsible interactive forces i.e. van-der-Waals force and H-bonding and electrostatic interactions (Scheme 1) help to explain the adsorption mechanism. Scanning Electron Microscope images are very useful support for the investigation of surface and cross-section of any materials. SEM images of both adsorbed and non adsorbed matrix of ChS networks (Fig. 11a and b) are captured. The SEM image (Fig. 11a) show a clear, organized, three-dimensional porous network with a high number of microspores for matrix and these pores have a very thin wall before adsorption. The dye molecules have been trapped within these pore walls in the gel network by electrostatic and Hydrogen bonding interaction from free -OH, -NH2 and -NH=CH- imine groups. The porous feature of ChS has been changed after adsorption. The dye loaded ChS has been suspended on the top of water because of its lightweight (Fig. S6). It implies that both the CV and RB molecules get adhered within pore via an intermolecular interaction during adsorption [33,34]. The adsorption mechanism is established by FTIR spectrum (Fig. S1). The participation of H-bonding between native ChS and dye loaded ChS are the main factor. It has been already discussed above, the position of important peaks of ChS. These positions are shifted for CV and RB loaded ChS during adsorption process. For CV loaded ChS, the –OH peak shifted from 3450 cm−1 to 3458 cm−1 and the imine peak appeared at 1635 cm−1 (instead of 1625 cm−1 in ChS). In RB loaded ChS, the imine peak appeared at 1635 cm−1 and hydroxyl changes from 3450 cm−1 to 3459 cm−1. The strong bonding interaction of both the dye molecules with imines and -OH groups in gel caused the shifting of peaks. Another significant changes have been found for the presence of van-der-Waals interaction between the aromatic rings of dyes and ChS in -CH stretching vibration at 2942 cm−1 [from 2879 cm−1 to 2976 cm−1 (CV) and 2975 cm−1 (RB).] 4. Conclusions Here, we have been succeeded to prepare a lightweight, porous and reusable salicylaldehyde linked Chitosan material as hydrogel. The spectroscopic analysis, FT-IR, is used for the successful fabrication of the hydrogels from Chitosan and salicylaldehyde. The enhanced porous morphology and strong elastic character are confirmed by SEM and Rheology, respectively. Being the porous surface and lightweight, ChS shows excellent performance for the removal of RB and CV dyes from water without the movement of water container. Rose Bengal in acidic aqueous solution (pH 4) and crystal violet at neutral solution (pH 6) can be removed in the range of 98% and 99% by ChS respectively at room temperature. Mechanistic approach is investigated as supportive entities for the adsorption process. The multilayer coverage on a heterogeneous surface is observed by Freundlich adsorption isotherm and kinetics study justifies the pseudo second order model. Spontaneous and endothermic nature in gel networks are established by thermodynamic results. ChS having excellent surface porosity, lightweight, reusability, glucose free and low cost features make the hydrogel efficient for the removal of dyes. As prepared ChS from natural biological macromolecules has been composed with a remarkable contribution of natural structural integrities from Chitosan. It considers the environmental prospective by offering an effective way to remove harmful dyes from water and will be dealt with sustainability due to having impressive environ-benign reusable character. Acknowledgement We would like to thank the West Bengal DST (Memo No: 66(Sanc.)/ ST/P/S&T/15G-12/2018) for financial support. DP acknowledges IIEST, Shibpur for sponsoring Institute Fellowship. Associated content FT-IR spectra of ChS, swelling study diagram, effect of feed concentration curve, effect of contact time on adsorption curve (PDF).

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