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Modified chitosan gel incorporated with magnetic nanoparticle for removal of Cu(II) and Cr(VI) from aqueous solution
Accepted Manuscript Modified chitosan gel incorporated with magnetic nanoparticle for removal of Cu(II) and Cr(VI) from aqueous solution
S.M. Anush, B. Vishalakshi PII: DOI: Reference:
S0141-8130(19)30599-9 https://doi.org/10.1016/j.ijbiomac.2019.04.179 BIOMAC 12255
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
24 January 2019 16 April 2019 26 April 2019
Please cite this article as: S.M. Anush and B. Vishalakshi, Modified chitosan gel incorporated with magnetic nanoparticle for removal of Cu(II) and Cr(VI) from aqueous solution, International Journal of Biological Macromolecules, https://doi.org/10.1016/ j.ijbiomac.2019.04.179
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ACCEPTED MANUSCRIPT Modified chitosan gel incorporated with magnetic nanoparticle for removal of Cu(II) and Cr(VI) from aqueous solution
Anush S. M. and *Vishalakshi B.
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Department of Post-Graduate Studies & Research in Chemistry, Mangalore University,
fax: +91 824 2287367/2287424
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Mangalagangothri, 574199 (DK), Karnataka, IndiaTel.: +91 824-2287262;
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e-mail; * [email protected]
Abstract
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A novel adsorbent material for removal of metal ion from aqueous solution was made by modification of chitosan.Schiff base prepared from reaction of chitosan with 3-methyl-1-phenyl-
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5-(piperidin-1-yl)-1H- pyrazole-4-carbaldehyde was crosslinked with epichlorohydrin to form a crosslinked gel. Fe3O4nanoparticles were incorporated into the modified chitosan gel to obtain a
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magnetic adsorbent material. The magnetic nanocomposite thus obtained was characterized using FTIR, TGA, SEM, EDS and XRD techniques and evaluated for adsorptive removal of Cu
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(II) and Cr (VI) ions from aqueous solutions. The maximum adsorption capacity of the adsorbent for Cu (II) and Cr (VI) ions was found to be 90.90 and 83.33 mgg-1respectively. The adsorption
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data fitted well with Langmuir isotherm model and the pseudo second order kinetic model. Thermodynamic parameters indicated the adsorption to be spontaneous and endothermic. The
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desorption studies revealed the efficient recovery of adsorbate species and possible reusability of the adsorbent material.
Keywords; Chitosan, Schiff base, Epichlorohydrin, Magnetic nanoparticle, Adsorption, Cu (II) and Cr (VI) removal.
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ACCEPTED MANUSCRIPT 1.0 Introduction Contamination of the aquatic media by heavy metal ions isa serious environmental issue arising mainly due to the release of effluents from industries[1].The heavy metal ions are highly toxic even at very low concentrations and are non biodegradable[2]. Copper and chromium are two metals widely used in various industries such as paper, leather, petroleum refining, plating,
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mining, smelting, brass manufacture, electroplating and coating[3-4]. Consumption of water
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contaminated with Cu(II)and Cr(VI) salts may cause serious health issues such as mucosal
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irritation, renal and central nervous system damage [5]. The maximum acceptable concentration of Cu(II) in drinking water is 2.0mg/L and of Cr (VI) is 0.02 mg/L [6]. Therefore, these heavy
possible level so that the ill effects can be reduced.
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metal ions are of concern and care should be taken to keep their concentrations at a minimum
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Several methods have been employed to remove heavy metal ions from the contaminated water such as dialysis, electroplating, filtration, flotation, chemical coagulation and precipitation
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[7].Some of these methods are having disadvantages such as low efficiency, cost effectiveness and sludge formation [8]. However, adsorption is one of the most efficient methods because of
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its cost effectiveness and simplicity. A wide range of different organic and inorganic adsorbents such as activated carbon [9], clays [10], carbon nanotubes [11] and polymeric materials [12&13]
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have been prepared and used for metal adsorption. Chelating resins are one of the promising materials for adsorption due to their high selectivity, studies have been reported on the use of
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magnetic nanoparticle containing resins for the efficient removal of heavy metal ions [1422].The magnetic nanoparticle in the resin structure facilitates easy separation from the aqueous
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medium using an external magnetic field and displayed higher uptake capacity compared to the magnetic particles-free resin [23-24]. These methods are also highly efficient, cheap and scalable.
Attention has been focused on the use of the biopolymer, namely, chitosan as adsorbent. It is a hetero polysaccharide obtained from deacetylation of chitin, a polysaccharide of natural origin and second most abundant to cellulose, Although the ß(1,4)-anhydroglycosidic bond of chitin is also present in cellulose, the characteristic properties of chitin/chitosan are not shared by cellulose [25].Considerable interest lies in the use of chitosan as an adsorbent due to its interesting
and
attractive
properties
such 2
as
biocompatibility,
biodegradability,
ACCEPTED MANUSCRIPT cytocompatibility, hemocompatibility etc[26-27]. Chitosan is known to exhibit good adsorption capacity due to the presence of functional groups and its macromolecular structure [28].Chitosan is particularly interesting in metal nanoparticle synthesis due to its interaction with metal ions and metal oxide nanoparticles. During chelation it evenly disperses nanoparticles throughout the Chitosan structure and hence is considered as a good dispersant [29]. Several modifications have been done on chitosan by incorporating new functional groups which eventually helps in
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improving the adsorption capacity.
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In this work chitosan has been modified by Schiff base formation and crosslinking. Also
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considering the advantages of magnetic nanoparticles, a novel adsorbent is made by incorporating magnetic nanoparticle into the modified chitosan.
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The Schiff base has been made using 3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole4- carboxaldehyde as a precursor. The crosslinking has been achieved using epichlorohydrin.
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Langmuir and Freundlich isotherms were used to validate the equilibrium data for the adsorption of Cu (II) ions. Pseudo first order and pseudo second order kinetic models were used to obtain the kinetic parameters. Thermodynamic parameters such as change in standard Gibbs free energy
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(ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were also studied for the removal of metal ions. The
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regeneration studies were carried out to assess the reusability of the adsorbent.
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2. Experimental.
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2.1 Materials and methods.
Chitosan (CS) (75–85% deacetylated, molecular weight: 310 kDa based on viscosity)was
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purchased from Sigma Aldrich (India). Acetic acid, acetone, ethanol, dimethyl formamide (DMF),ferric
chloride,
ferrous
chloride,
ammonium
hydroxide,
phenyl
hydrazine,
ethylacetoacatate, phosphorous oxychloride, potassium hydroxide, hydrochloric acid, ethanoland copper sulphate were purchased from Spectrochem (India)and used as received. 2.2 A Synthesis of 3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde(Pippyrazole). Compound, 3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde was synthesized according to method reported in our previous work [30]. The mixture of phenyl hydrazine and ethylacetoacetate was refluxed in ethanol for 4 h to obtain 5-methyl-2-phenyl-2,43
ACCEPTED MANUSCRIPT dihydro-3H-pyrazol-3-one. The obtained product on vilsmier-Haack formylation using anhydrous DMF and POCl3at a temperature of 90ºC for 6 h yields solid 5-chloro-3-methyl-1phenyl-1H-pyrazole-4- carbaldehyde. The product was then refluxed with piperidine in presence of potassium carbonate at a temperature of 120°C in DMF for 8 h to yield 3-methyl-1-phenyl-5(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde (M.P: 73-75 ºC). 2.2 B Synthesis of epichlorohydrin crosslinked chitosan Schiff base (ECSB).
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1.0 g of CS was dissolved in 100mL of 5% aqueous aqueous acetic acid by continuous
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stirring. 1mL of epichlorohydrin was added to the above solution followed by addition of0.9 g of
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3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde. The resulting solution was maintained under mild stirring at 60ºC for 6 h and then stirred vigorously for 24 h. The reaction
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mass was precipitated using acetone. The product,coded as ECSB, was filtered, washed and used in further studies.
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2.2 C Synthesis of Fe3O4 nano particles
Fe3O4 nanoparticles were synthesized using the precipitation method[31]. FeCl3. 6H2O
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(2.9g) and FeCl2. 4H2O (1.05g) were dissolved in 100 mL distilled water by stirring for 1 h at room temperature. NH4OH (3.73 mL of NH3 solution in 100 mL water) was added dropwise to
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the resulting mixture and the mixture was stirred for another 10 h at room temperature. The obtained black colored precipitate was separated by filtration, washed with distilled water 3 to 4
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times. The obtained Fe3O4nanoparticle was further dried in an oven at 48° C for 24 h. 2.2 D Preparation of epichlorohydrin crosslinked chitosan schiff base-Fe3O4 nanocomposite
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(ECSBNC)
To the reaction mixture containing ECSB, described in section 2.2A above, 0.5g of
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Fe3O4 nanoparticle was added and made to disperse uniformly by continuously stirring for 6 h. The product was precipitated in acetone. It was filtered and dried in an oven at 60°C for 24 h. The compound was coded as ECSBNC. 3.0 Characterization The FTIR spectra of the compounds CS, Pip-pyrazole and ECSBNC were recorded on an IRprestige-21, FTIR spectrometer (Shimadzu, Japan) in the range 4000-500 cm-1. Thermal degradation behaviour of CS, ECSB and ECSBNC were studied using Shimadzu DTG60 thermogravimetric analyzer. Samples were heated from 0 to 600°C with a heating rate of 10°C/min. 4
ACCEPTED MANUSCRIPT Diffraction patterns of CS,Fe3O4 and ECSBNC were analyzed using X-ray diffractometer (Rigaku Miniflex 600-XRD Instrument, USA) using CU Kα radiation generated at 40 kV and 35mA in the differential angle (2θ). Surface morphology of CS, Fe3O4and ECSBNC were studied using JOEL-JSM5800LV (Japan) Scanning electron microscope under a voltage of 10 kV with 10k magnification. Adsorption studies
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The adsorption capacity of CS for Cu (II) and that of ECSBNC for Cu (II) and Cr (VI)
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ions from aqueous solutions were determined in duplicate using CuSO4 and K2Cr2O7 solutions.
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The adsorption capacity of CS for Cr (VI) could not be determined due to the solubility of CS (in the film form) at pH 3.The metal ion concentration in solutions before and after adsorption was
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determined in mgL-1 using GB 932 plus Atomic Absorption spectrophotometer (Australia) and the instrument was calibrated using standard metal ion solutions. A mass of 30 mg of the CS or
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ECSBNC was suspended in metal ion solutions with varying metal ion concentration from 20 to 100 mg L-1for 3 h with continuous stirring. The amount of metal ions (mg/g) adsorbed by
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ECSBNC and CS was calculated using the following equation: qe = (C0- Ce) ×
(1)
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where qe is the equilibrium capacity of the adsorbent for the metal ions (mg g-1). Co and Ce indicate the initial and equilibrium concentrations of the metal ions in solution. W is the study.
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Desorption studies
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amount of adsorbent taken (mg). V is the volume of aqueous solution (mL) used for adsorption
To check the possibility of reusing the adsorbent, desorption studies were performed.
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30mg of ECSBNC was added to 25 ml of individual solutions of Cu (II) and Cr (VI) and maintained for 2 hours under continuous stirring. Later the metal ion loaded ECSBNC was taken out washed, dried and used for desorption studies. The sample containing the metal ions was suspended in a 25ml of HCl solution of pH 1.2 for desorption of Cu (II) and in 25 mL of 0.5N NaOH solution for the desorption of Cr (VI) ions and maintained for 2h under stirring. The supernatant solution was diluted volumetrically and the concentration of desorbed metal ion was analyzed using AAS. The percentage of desorption was calculated by the following equation
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ACCEPTED MANUSCRIPT Desorption (%) =
×100
(2)
Thermodynamic parameters were evaluated by carrying out the adsorption studies at different temperatures, viz. 30, 40, 45 and 50°C.
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4.0 Results and discussion
Chitosan
on
reacting
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4.1 Synthesis of ECSB
with3-methyl-1-phenyl-5-(piperidin-1-yl)-1H-
pyrazole-4-
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carbaldehyde yields the Schiff base through the -NH2groups. This on treatment with epichlorohydrin results in a crosslinked structure, where the chains of chitosan schiff base get
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covalently linked by epichlorohydrin by the involvement of primary hydroxyl groups present at
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C6 position of chitosan. The various steps of formation of ECSB are shown in Scheme 1.
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4.2 Infrared spectral characterization
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Scheme 1:
Fig 1
The IR spectrum of Pip- pyrazole (Fig.1a) showed absorption bands at 2991 and 3065
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cm-1 corresponding to aliphatic and aromatic C-H stretching respectively. The carbonyl stretching band is observed at 1670 cm-1, C=N stretching is observed at 1558cm-1 and C-O-C
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stretching is observed at 1219cm-1.
FIG 2
The infrared spectrum of CS Fig 2(a) shows a broad band at 3486 cm-1 attributed to –NH and –OH stretching and a weak band at 2927 cm-1 attributed to –CH2 stretching. The weak band at 1642 cm-1 indicates the presence of free amine. A band at 1060 cm-1is attributed to the C-O-C of carbohydrate structure.
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ACCEPTED MANUSCRIPT As seen from Fig 2(b), the intensity of the broad band due to-NH2 and -OH is lower in ECSBNC due to the involvement of hydroxyl group of CS in crosslinking. The band at 1041 cm1
is assigned to the –CO stretching vibration of –C–OH. The
band at 1650 cm-1(Fig 2a)
characteristic of the –NH2 group of the CS has disappeared due to the reaction of CS with –CHO group of 3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole-4- carboxaldehyde to form the Schiff base (C=N). The band at 570 cm−1is attributed to Fe-O stretching vibration implying that
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the Fe3O4 nanoparticles have been successfully introduced into the epichlorohydrin crosslinked
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chitosan Schiff base (ECSBNC).
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4.3 Thermogravimetric analysis
The thermogram of CS shows three steps degradation (Fig 3a)in the temperature range of
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30-600°C. The first step takes place in the range of 30-100°C accounting for a weight loss of around 12% which occurs due to the loss of moisture contained in CS. The second step of
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degradation due to breakdown of polysaccharide structure occurring in the range of 225°C to 340°C results in the weight loss of around 55% and the final loss of 33% gradually takes place in
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the range from 300-600°C.Fig 3b depicts the degradation pattern of ECSBwhich is very similar to that of CS. In the first step degradation about 10% of weight loss takes place in a temperature
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zone of 30-80°C and is due to the loss of water molecules. In the second step, the weight loss is observed in the temperature zone of 100-300°C and is due to the decomposition of the chitosan
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backbone followed by decomposition of pyrazole moiety in the temperature range of 300-600°C. About 20% of the compoundremained as residual matter.The thermal degradation behavior of
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ECSBNC shown in Fig 3c is found to be a continuous process. First step degradation occurs in the range of 30-170°C, with a weight loss of 20%, due to the loss of water molecules. The
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second step degradation observed in the temperature zone of 170-300°C with a weight loss of 30% due to the degradation of polymeric chain and the third significant loss occurred in the range 300-600°C with a weight loss of 20% due to the decomposition of pyrazole moiety present in the Schiff base. Even after 650°C, about50% of the mass remained as a residual matter, due to the presence of Fe3O4 particles in the sample along with the residue of ECSB.
FIG 3 4.4 XRD analysis
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ACCEPTED MANUSCRIPT The XRD pattern of CS, Fe3O4 and ECSBNC are shown in Fig 4. The CS (Fig 4a) is found to be semi crystalline in nature with predominant peaks at 2ϴ= 9°, 21.09° and 30.0°.The diffraction pattern of Fe3O4shown in Fig 4b indicates peaks at 2ϴ= 30.1°, 35.5°, 43.1°, 53.5°,57.0°and 62.6°. The peaks appear to be sharp and clear indicating the highly crystalline nature of Fe3O4 particles. The XRD pattern of ECSBNC in Fig 4c shows the broad bands due to chitosan as well as sharp peaks of Fe3O4 structure, indicating the incorporation of Fe3O4particles
FIG 4 5.0 Adsorption
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into the crosslinked CS structure.
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The binding capacity of the adsorbent materials is attributed to the presence of functional groups containing nitrogen, sulphur and oxygen atoms, which helps in the removal of metal ions
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from the aqueous solutions [32]. Chemical modification of CS with 3-methyl-1-phenyl-5(piperidin-1-yl)-1H- pyrazole-4- carboxaldehyde and epichlorohydrin increases the number of
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binding sites which is expected to enhance the binding ability of the material. The presence of Fe3O4particlesis expected to further enhance the binding capacity due to adsorption on the
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surface of the nanoparticle. In order to check this prediction, adsorption studies were carried out for CS and ECSBNC from aqueous solutions containing Cu (II) and Cr (VI) ions in separate
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experiments which were carried out in duplicate. Adsorption process of Cu (II) ions was carried out at pH 7. At low pH the adsorption decreases due to the protonation of basic nitrogen centers
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on the polymeric chains. Adsorption of Cr (VI) was studied at pH 3. It is evident that under acidic conditions HCrO4- is the active species and the adsorption process is expected to take
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place due to the electrostatic interactions between the ECSB and the adsorbate molecules. The results are discussed below.
5.1 SEM and EDS analysis SEM images of Fe3O4,ECSBNC and; Cu(II) and Cr (VI) loaded ECSBNC are presented in Fig 5. Surface image of Fe3O4 showed agglomeration of spherical shaped nano particles with 8
ACCEPTED MANUSCRIPT varying size (Fig 5a). Fig 5b showed irregular flake like structure of Schiff base of CS containing Fe3O4nanoparticles. Fig 5c and 5d showed the adsorption of Cu (II) and Cr (VI) respectively on the surface of the adsorbent. The adsorption appears to be uniform covering the whole surface.
FIG 5
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The electron dispersive spectrum of Fig 6a shows the presence of carbon, nitrogen,
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oxygen, copper and iron in the Cu (II) adsorbed ECSBNC material. Similarly, Fig 6bshows the
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presence of chromium in addition to carbon, nitrogen, oxygen, and iron in the structure in the Cr
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(VI) adsorbed ECSBNC material.
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FIG 6
5.2 Effect of initial metal ion concentration on adsorption
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Adsorption studies were carried out with different initial Cu (II) and Cr (VI) ion concentrations ranging from 20 to 100 mg/L, keeping all parameters fixed and the results are
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displayed in Fig 7. At low metal ion concentration the availability of metal ions will be less compared to the available sites on the adsorbent resulting in the effective removal of metal ions..
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Increase in metal concentration results in increased sorption of metal ions by ECSBNC. Higher the initial metal ion concentration, higher will be the concentration gradient which results in the
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transfer of metal ions from the solution to the adsorbent favoring adsorption process. At any given time, the amount of Cu (II) adsorbed on ECSBNC is found to be higher than Cr (VI). Fig 7
5.3Adsorption kinetics In order to describe the kinetics and mechanism that controls the adsorption process, the experimental data were interpreted by using Lagergren pseudo-first-order model [33] and Ho ‘pseudo-second order model [34].
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ACCEPTED MANUSCRIPT The change in the uptake of Cu (II) and Cr (VI) ions as a function of time is presented in Fig 8. It can be seen that the adsorption process was rapid at the beginning and attained equilibrium gradually and remained constant later on till the completion of the experiment. . The Pseudo first order model is given by the equation
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(3)
)
log
(4)
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log (
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The above equation on integration gives
where qe is the adsorption capacity of the adsorbent at equilibrium (mg/g), qt is the amount of adsorbate adsorbed (mg/g) at time ‘t’(min-1), k1 is the first order rate constant (min-1). The data
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obtained in the present study, on fitting with the above equation, did not show linearity as indicated by the low value of R2 ( for Cu(II) 0.537 and for Cr(VI) 0.678) which indicates that
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adsorption is not a first order kinetic process. Hence the fit of data with second order kinetic model was checked.
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The pseudo second order model is represented by the following equation
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(5)
The integration of this equation with condition that when t=0 and qt=0 gives the equation
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(6)
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Where k2 is the second order rate constant (g mg-1 min-1). The plot of t/qt vst for the adsorption data obtained is shown in Fig 9. The obtained linear plots were validated to check the fitness of
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the straight line. The kinetic parameters such as rate constants (k2), equilibrium adsorption capacity (qe) and the regression coefficient (R2) values are shown in table 1. The data reveals that pseudo second order model was best fitted as the R2valueswereclose to 1 compared to pseudo first order model. Further, the experimental qe value was also in agreement with the theoretically obtained qe values showing correctness of pseudo second order fit. The adsorption process is influenced by the concentration of both adsorbate ions and the metal binding sites which tentatively leads to the deposition of metal ions on the adsorbent. Table 1 Fig 8 10
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Fig 9 5.4 Adsorption isotherms The equilibrium adsorption isotherm is the fundamental for defining the interactive behavior between the solid phase and the metal solution phase. To study the nature of adsorption
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several isotherm models have been employed, the most commonly used models being of
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Langmuir [35] and Freundlich [36]. Fig 10 shows the adsorption isotherms of Cu (II) and Cr (VI)
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on ECSBNC. Fig 10
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To interpret the adsorption data, Langmuir and Freundlich isotherm models were fitted as described below.
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Freundlich isotherm model is based on the assumption that adsorption surface is heterogeneous and adsorption is reversible. The heterogeneity arises due to the presence of
possibility to multilayer adsorption.
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different functional groups. It is not restricted to monolayer adsorption and can consider the
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The mathematical expression for the Freundlich model is given in the following equation.
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(7)
which is expressed in the logarithmic form as
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(8)
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where qe is the amount adsorbed (mg/g) and Ce corresponds to concentration of adsorbate solution at equilibrium (mg L-1). KF and n are constants which give an idea of the extent of adsorption and the non linearity between the solution concentration and the adsorption. If the value of n is greater than 1 it is considered as favorable adsorption [10].
The Langmuir model assumes the formation of a saturated monolayer of solute molecules on the adsorbent. . The linearised form of Langmuir equation is given as
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ACCEPTED MANUSCRIPT (9) where qe and Ce have their meaning as defined earlier; qm is the maximum adsorption capacity corresponding to the monolayer formation on the adsorbent surface. KL is the Langmuir constant related to the energy of adsorption. The Langmuir constants qm and KLare obtained from the linear plots of Ce/qe vs Ce. The essential characteristics of the Langmuir adsorption model is
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given in the terms of a constant separation factor RL. It is given as
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(10)
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where Co (mg/L) corresponds to initial concentration of metal ion.
The parameter RL indicates the nature of adsorption which is given below. Unfavorable adsorption
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Favorable adsorption
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RL>1
The Freundlich and Langmuir isotherms plots for the adsorption data obtained in the present
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study are displayed in Fig 11 & 12 respectively. The isotherm parameters KF, n,qm, KL and RL, for
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the adsorption of metal ions on CS and ECSBNC are presented in Table 2 along with the regression coefficient R2 for the fit of two models. The R2 value for the Langmuir fit in all cases
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was close to 1.0 when compared to the Freundlich model fit indicating the best fit of adsorption data with Langmuir model. This shows the adsorption of Cu (II) and Cr (VI) ions onCS and
Table 2
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CE
ECSBNC as a monolayer coverage process.
Fig11 Fig 12 5.5 Comparison studies The adsorption capacity of the presently studied ECSBNC system for the metal ions is higher than reported in literature for other modified chitosan systems as discussed below. The Cu 12
ACCEPTED MANUSCRIPT (II) ions uptake by epichlorohydrin crosslinked chitosan was found to be 35.50 mg/g [37]. The adsorption of xanthate modified magnetic chitosan was found to be 34.50 mg/g for Cu (II) ions [38]. Ethylenediamine modified chitosan was found to exhibit an adsorption capacity of 38.0 mg/g which was attributed to the presence of amine group in the modified structure [32]. In chitosan cellulose composites, the hydroxyl and amine groups were believed to be the metal ion binding sites. The capacity of adsorption Cu (II) was found to be 26.50 mg/g [39]. Chitosan
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blended with poly (vinyl alcohol) was evaluated for Cu (II) binding ability and was found to be
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47.85 mg/g [40]. The adsorption of chromate and cupric ions on chitosan-coated cotton
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gauze showed the maximum adsorption capacity for chromate ion to be 12.4 mg/g at pH 3 and 14.1 mg/g for Cu (II) at pH 5 [41]. The Ca (II)–Chitosan in microspheres form was reported as a
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green and effective absorbent for the treatment of hazardous wastewater containing heavy metal ions. The adsorption capacity for Cu (II) was found to 41.5 mg/g [42]. The adsorption of
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chemically modified chitosan for the adsorption of Cu (II) ions was found to be 43.47mg/g [43]. Ethylenediamine modified cross-linked magnetic chitosan resin showed maximum adsorption of
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51.0 mg/g of hydrochromate ion at pH 3 through electrostatic force of interaction [44]. The adsorption capacity of magnetic chitosan 2 for the recovery of Cr (VI) metal ions was 137.27
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mg/g; the increased adsorption capacity may be due to the presence of magnetic nano particles in the chitosan structure [45]. Chitosan-biochar/g-Fe2O3 composite was found to exhibit maximum
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adsorption capacity of 167.31 mg/g [46]. Heterocyclically modified chitosan exhibited adsorption capacities of 83.73 and 85.0 mg/g for Cu (II) and Cr (VI) ions [47]. Graphene oxide
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incorporated chitosan Schiff base showed a maximum adsorption capacity of 111.11 and 76.92 mg/g for Cu(II) and Cr(VI) respectively [48]. The adsorption capacity of the Spirulina platensis
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biosorbent for the adsorptive removal of Cu(II) was found to be 817.7 mg/g [49]. Nano-scale zero-valent iron impregnated cashew nut shell showed a maximum adsorption capacity of 48.05 mg/g [50]. Sulfuric acid modified eucalyptus seeds were found to exhibit a maximum monolayer adsorption capacity of 76.94 mg/g for Cu(II) ions [51]. Ultrasonic assisted Spirulina platensis showed high adsorption process with an adsorption capacity of 577.9 mg/g for Cr(VI) ions [52]. Active carbon derived from coffee waste showed an uptake capacity of 156.7 mg/g for Cr(VI) [53]. Biosorbent derived from custard apple and aspergillus niger showed a percentage removal of 95.1% for Cr(VI) [54].
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ACCEPTED MANUSCRIPT The present work when compared with the reports mentioned above indicated that the adsorption capacities of Epichlorohydrin cross linked chitosan containing magnetic particles for Cu (II) and Cr (VI) ions are 83.33 and 90.90 mg/g respectively, which is significantly higher compared to many other reported chitosan adsorbents. The adsorption capacity may be due to presence of nitrogen containing functional group which acts as an excellent chelating agent for
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binding with metal ions.
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5.6Thermodynamic parameters
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The thermodynamic parameters of adsorption, namely the changes in standard free energy, enthalpy and entropy were evaluated by carrying out adsorption at 4 different temperatures (30, 40, 45 and 50°C) and the parameters were calculated using the following equations.
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ΔG°= -RTlnKc (11)
Where Kc is the equilibrium constant and T and R have their usual meaning. The equillbrium constant is expressed as the concentration of metal ion on adsorbent to the residual
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concentration of the metal ion in solution at equilibrium. (12)
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Kc=
The relation between ΔG°, ΔH° and ΔS° is given by equation (13)
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ΔG°= ΔH°-TΔS°
(13)
lnKc =
CE
Combining Equation (11) and (13) gives the van’t Hoff equation, (14)
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The change in enthalpy (ΔH°)and entropy (ΔS°) were calculated from the slope and intercept of linear plot of lnKC versus 1/T (Fig 13). The values of thermodynamic parameters obtained are displayed in Table 3.The negative ΔG ° value indicates the spontaneous nature of adsorption for both the metal ions at the temperatures studied. The positive (ΔH°)value suggested that the adsorption is endothermic in nature. The positive (ΔS°) reveals the increased randomness at the solid-solution interface during the adsorption of the metal ions on the adsorbent. Fig 13 Table 3
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ACCEPTED MANUSCRIPT 5.7 Desorption The desorption studies were carried out to check the recovery of metal ions adsorbed and the subsequent reuse of the adsorbent. HCl solution of pH 1.2 was used as stripping solution for Cu (II). At pH 1.2 desorption takes place due to the protonation of acid sites on the adsorbent which subsequently lose their affinity for Cu (II) ions. Desorption of Cu (II) was found to be 82%.An amount of 85% of Cr (VI) were found to be desorbed in the stripping solution which
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was 0.5 N NaOH, as at this pH, the adsorption sites get deprotonated and lose their affinity for
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Fig 14
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Cr(VI) ions. The results obtained are displayed in Fig 14.
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6.0 Conclusion
Modification of CS was achieved by Schiff base formation with 3-methyl-1-phenyl-5-
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(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde, then by crosslinking with epichlorohydrin followed by the incorporation of Fe3O4 nanoparticles. The material thus modified was found to
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have higher adsorption capacity for Cu (II) and Cr(VI) compared to the unmodified CS. Adsorption process is observed to follow Langmuir isotherm model with monolayer adsorption
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on the adsorbent surface and the maximum adsorption capacity observed was 90.90 and 83.33 mgg-1 for Cu (II) and Cr (VI) respectively. The adsorption was found to be pseudo second order
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kinetic process. The negative values of ΔGº and the positive values of ΔHº indicated the adsorption process to be spontaneous and endothermic. The positive values of ΔS° indicated the
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increase in randomness at the solid/ liquid interface during the adsorption. The desorption studies revealed about 80% desorption of Cu(II) and Cr(VI) in appropriate stripping solutions confirming the possibility of recovery of adsorbate species and reuse of the adsorbent material effectively.
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ACCEPTED MANUSCRIPT
Figures and captions 1. Scheme 1: Synthesis of ECSB
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2. Fig 1 FTIR spectra of Pip- pyrazole
4. Fig 3 TGA curves of a) CS, b) ECSB and c) ECSBNC
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5. Fig 4 XRD pattern of a) CS b) Fe3O4 and c) ECSBNC
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3. Fig 2 FTIR spectra of a) CS and b) ECSBNC.
6. Fig 5 SEM Micrographs of a) Fe3O4, b) ECSBNC c) Cu (II) adsorbed ECSBNC and d)
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Cr (VI) adsorbed ECSBNC
7. Fig 6 EDS spectra of a) Cu (II) and b) Cr (VI) adsorbed ECSBNC.
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8. Fig 7 Effect of initial metal ion concentration on adsorption of Cu (II) and Cr (VI) on ECSBNC
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9. Fig 8 Uptake of Cu (II) and Cr (VI) on ECSBNC 10. Fig 9 Second order kinetic plot for adsorption of Cu (II) and Cr (VI) on ECSBNC
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11. Fig 10 Adsorption isotherm of a) Cu (II), b) Cr (VI) on ECSBNC and c) Cu (II) on CS 12. Fig 11 Freundlich isotherm model fit for the adsorption of a) Cu (II) on CS; b) Cu (II)
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and c) Cr (VI) on ECSBNC
13. Fig 12 Langmuir isotherm model fit for the adsorption of a) Cu (II) on CS; b) Cu(II) and
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c) Cr (VI) on ECSBNC.
14. Fig 13 Plot of ln Kc vs 1/T for the uptake of Cu (II) and Cr (VI) on ECSBNC
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15. Fig 14 Desorption of Cu (II) and Cr (VI) ions from Cu (II) and Cr (VI) adsorbed ECSBNC.
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Table 1 Kinetic parameters for the adsorption of Cu (II) and Cr (VI) on ECSBNC. Pseudo second order k2 (g mg−1 min−1)
qe, cal (mgg-1)
Qe, exp
100
71.42
67.33
Cr (VI)
100
51.75
55.55
0.0085
0.999
0.0049
0.999
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Cu (II)
R2
T
CO (mg/L)
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Metal
(I) CS Adsorbate
Langmuir model
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Table 2 Isotherm parameters for adsorption of Cu(II) and Cr(VI) on CS and ECSBNC
qmax
RL
20-100
9.70
2.06
0.010
R2 0.998
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Cu(II)
KL
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Co(mgL-1)
Freundlich model
(II) ECSBNC
Langmuir model
Co(mgL-1) 20-100
Cr(VI)
20-100
qmax
RL
1.76
R2
n 4.065
0.954
Freundlich model KL
R2
KF
R2
n
90.90
1.0
0.00012
0.993
65.46
0.55
0.980
83.33
0.9
0.00014
0.995
38.63
0.63
0.972
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Cu(II)
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Adsorbate
KF
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Table (3) Thermodynamic parameters for adsorption on ECSBNC
-3.68
318
1.73
-3.95
323
1.78
-4.18
303
1.55
-0.81
313
1.59
-0.89
318
1.62
-1.01
323
1.66
-1.10
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1.67
43.78
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313
ΔS°(J/mol/K)
9.96
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-3.30
3.5
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1.56
ΔH°(kJ/mol)
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M
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303
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Cr (VI)
ln Kc
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Cu (II)
ΔG°(kJ/mol)
T(K)
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Adsorbate
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14.25
ACCEPTED MANUSCRIPT
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CR
US AN M ED PT
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Heterocyclic modification of Chitosan has been achieved for the removal of metal ions from aqueous solutions. The extent of adsorption was further enhanced by the incorporation of magnetic nanoparticles. The desorption studies revealed the efficient recovery of adsorbate and possible reusability of the adsorbent material.
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Highlights
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
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S.M. Anush, B. Vishalakshi PII: DOI: Reference:
S0141-8130(19)30599-9 https://doi.org/10.1016/j.ijbiomac.2019.04.179 BIOMAC 12255
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
24 January 2019 16 April 2019 26 April 2019
Please cite this article as: S.M. Anush and B. Vishalakshi, Modified chitosan gel incorporated with magnetic nanoparticle for removal of Cu(II) and Cr(VI) from aqueous solution, International Journal of Biological Macromolecules, https://doi.org/10.1016/ j.ijbiomac.2019.04.179
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ACCEPTED MANUSCRIPT Modified chitosan gel incorporated with magnetic nanoparticle for removal of Cu(II) and Cr(VI) from aqueous solution
Anush S. M. and *Vishalakshi B.
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Department of Post-Graduate Studies & Research in Chemistry, Mangalore University,
fax: +91 824 2287367/2287424
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Mangalagangothri, 574199 (DK), Karnataka, IndiaTel.: +91 824-2287262;
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e-mail; * [email protected]
Abstract
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A novel adsorbent material for removal of metal ion from aqueous solution was made by modification of chitosan.Schiff base prepared from reaction of chitosan with 3-methyl-1-phenyl-
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5-(piperidin-1-yl)-1H- pyrazole-4-carbaldehyde was crosslinked with epichlorohydrin to form a crosslinked gel. Fe3O4nanoparticles were incorporated into the modified chitosan gel to obtain a
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magnetic adsorbent material. The magnetic nanocomposite thus obtained was characterized using FTIR, TGA, SEM, EDS and XRD techniques and evaluated for adsorptive removal of Cu
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(II) and Cr (VI) ions from aqueous solutions. The maximum adsorption capacity of the adsorbent for Cu (II) and Cr (VI) ions was found to be 90.90 and 83.33 mgg-1respectively. The adsorption
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data fitted well with Langmuir isotherm model and the pseudo second order kinetic model. Thermodynamic parameters indicated the adsorption to be spontaneous and endothermic. The
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desorption studies revealed the efficient recovery of adsorbate species and possible reusability of the adsorbent material.
Keywords; Chitosan, Schiff base, Epichlorohydrin, Magnetic nanoparticle, Adsorption, Cu (II) and Cr (VI) removal.
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ACCEPTED MANUSCRIPT 1.0 Introduction Contamination of the aquatic media by heavy metal ions isa serious environmental issue arising mainly due to the release of effluents from industries[1].The heavy metal ions are highly toxic even at very low concentrations and are non biodegradable[2]. Copper and chromium are two metals widely used in various industries such as paper, leather, petroleum refining, plating,
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mining, smelting, brass manufacture, electroplating and coating[3-4]. Consumption of water
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contaminated with Cu(II)and Cr(VI) salts may cause serious health issues such as mucosal
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irritation, renal and central nervous system damage [5]. The maximum acceptable concentration of Cu(II) in drinking water is 2.0mg/L and of Cr (VI) is 0.02 mg/L [6]. Therefore, these heavy
possible level so that the ill effects can be reduced.
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metal ions are of concern and care should be taken to keep their concentrations at a minimum
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Several methods have been employed to remove heavy metal ions from the contaminated water such as dialysis, electroplating, filtration, flotation, chemical coagulation and precipitation
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[7].Some of these methods are having disadvantages such as low efficiency, cost effectiveness and sludge formation [8]. However, adsorption is one of the most efficient methods because of
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its cost effectiveness and simplicity. A wide range of different organic and inorganic adsorbents such as activated carbon [9], clays [10], carbon nanotubes [11] and polymeric materials [12&13]
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have been prepared and used for metal adsorption. Chelating resins are one of the promising materials for adsorption due to their high selectivity, studies have been reported on the use of
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magnetic nanoparticle containing resins for the efficient removal of heavy metal ions [1422].The magnetic nanoparticle in the resin structure facilitates easy separation from the aqueous
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medium using an external magnetic field and displayed higher uptake capacity compared to the magnetic particles-free resin [23-24]. These methods are also highly efficient, cheap and scalable.
Attention has been focused on the use of the biopolymer, namely, chitosan as adsorbent. It is a hetero polysaccharide obtained from deacetylation of chitin, a polysaccharide of natural origin and second most abundant to cellulose, Although the ß(1,4)-anhydroglycosidic bond of chitin is also present in cellulose, the characteristic properties of chitin/chitosan are not shared by cellulose [25].Considerable interest lies in the use of chitosan as an adsorbent due to its interesting
and
attractive
properties
such 2
as
biocompatibility,
biodegradability,
ACCEPTED MANUSCRIPT cytocompatibility, hemocompatibility etc[26-27]. Chitosan is known to exhibit good adsorption capacity due to the presence of functional groups and its macromolecular structure [28].Chitosan is particularly interesting in metal nanoparticle synthesis due to its interaction with metal ions and metal oxide nanoparticles. During chelation it evenly disperses nanoparticles throughout the Chitosan structure and hence is considered as a good dispersant [29]. Several modifications have been done on chitosan by incorporating new functional groups which eventually helps in
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improving the adsorption capacity.
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In this work chitosan has been modified by Schiff base formation and crosslinking. Also
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considering the advantages of magnetic nanoparticles, a novel adsorbent is made by incorporating magnetic nanoparticle into the modified chitosan.
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The Schiff base has been made using 3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole4- carboxaldehyde as a precursor. The crosslinking has been achieved using epichlorohydrin.
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Langmuir and Freundlich isotherms were used to validate the equilibrium data for the adsorption of Cu (II) ions. Pseudo first order and pseudo second order kinetic models were used to obtain the kinetic parameters. Thermodynamic parameters such as change in standard Gibbs free energy
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(ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were also studied for the removal of metal ions. The
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regeneration studies were carried out to assess the reusability of the adsorbent.
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2. Experimental.
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2.1 Materials and methods.
Chitosan (CS) (75–85% deacetylated, molecular weight: 310 kDa based on viscosity)was
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purchased from Sigma Aldrich (India). Acetic acid, acetone, ethanol, dimethyl formamide (DMF),ferric
chloride,
ferrous
chloride,
ammonium
hydroxide,
phenyl
hydrazine,
ethylacetoacatate, phosphorous oxychloride, potassium hydroxide, hydrochloric acid, ethanoland copper sulphate were purchased from Spectrochem (India)and used as received. 2.2 A Synthesis of 3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde(Pippyrazole). Compound, 3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde was synthesized according to method reported in our previous work [30]. The mixture of phenyl hydrazine and ethylacetoacetate was refluxed in ethanol for 4 h to obtain 5-methyl-2-phenyl-2,43
ACCEPTED MANUSCRIPT dihydro-3H-pyrazol-3-one. The obtained product on vilsmier-Haack formylation using anhydrous DMF and POCl3at a temperature of 90ºC for 6 h yields solid 5-chloro-3-methyl-1phenyl-1H-pyrazole-4- carbaldehyde. The product was then refluxed with piperidine in presence of potassium carbonate at a temperature of 120°C in DMF for 8 h to yield 3-methyl-1-phenyl-5(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde (M.P: 73-75 ºC). 2.2 B Synthesis of epichlorohydrin crosslinked chitosan Schiff base (ECSB).
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1.0 g of CS was dissolved in 100mL of 5% aqueous aqueous acetic acid by continuous
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stirring. 1mL of epichlorohydrin was added to the above solution followed by addition of0.9 g of
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3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde. The resulting solution was maintained under mild stirring at 60ºC for 6 h and then stirred vigorously for 24 h. The reaction
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mass was precipitated using acetone. The product,coded as ECSB, was filtered, washed and used in further studies.
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2.2 C Synthesis of Fe3O4 nano particles
Fe3O4 nanoparticles were synthesized using the precipitation method[31]. FeCl3. 6H2O
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(2.9g) and FeCl2. 4H2O (1.05g) were dissolved in 100 mL distilled water by stirring for 1 h at room temperature. NH4OH (3.73 mL of NH3 solution in 100 mL water) was added dropwise to
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the resulting mixture and the mixture was stirred for another 10 h at room temperature. The obtained black colored precipitate was separated by filtration, washed with distilled water 3 to 4
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times. The obtained Fe3O4nanoparticle was further dried in an oven at 48° C for 24 h. 2.2 D Preparation of epichlorohydrin crosslinked chitosan schiff base-Fe3O4 nanocomposite
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(ECSBNC)
To the reaction mixture containing ECSB, described in section 2.2A above, 0.5g of
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Fe3O4 nanoparticle was added and made to disperse uniformly by continuously stirring for 6 h. The product was precipitated in acetone. It was filtered and dried in an oven at 60°C for 24 h. The compound was coded as ECSBNC. 3.0 Characterization The FTIR spectra of the compounds CS, Pip-pyrazole and ECSBNC were recorded on an IRprestige-21, FTIR spectrometer (Shimadzu, Japan) in the range 4000-500 cm-1. Thermal degradation behaviour of CS, ECSB and ECSBNC were studied using Shimadzu DTG60 thermogravimetric analyzer. Samples were heated from 0 to 600°C with a heating rate of 10°C/min. 4
ACCEPTED MANUSCRIPT Diffraction patterns of CS,Fe3O4 and ECSBNC were analyzed using X-ray diffractometer (Rigaku Miniflex 600-XRD Instrument, USA) using CU Kα radiation generated at 40 kV and 35mA in the differential angle (2θ). Surface morphology of CS, Fe3O4and ECSBNC were studied using JOEL-JSM5800LV (Japan) Scanning electron microscope under a voltage of 10 kV with 10k magnification. Adsorption studies
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The adsorption capacity of CS for Cu (II) and that of ECSBNC for Cu (II) and Cr (VI)
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ions from aqueous solutions were determined in duplicate using CuSO4 and K2Cr2O7 solutions.
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The adsorption capacity of CS for Cr (VI) could not be determined due to the solubility of CS (in the film form) at pH 3.The metal ion concentration in solutions before and after adsorption was
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determined in mgL-1 using GB 932 plus Atomic Absorption spectrophotometer (Australia) and the instrument was calibrated using standard metal ion solutions. A mass of 30 mg of the CS or
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ECSBNC was suspended in metal ion solutions with varying metal ion concentration from 20 to 100 mg L-1for 3 h with continuous stirring. The amount of metal ions (mg/g) adsorbed by
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ECSBNC and CS was calculated using the following equation: qe = (C0- Ce) ×
(1)
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where qe is the equilibrium capacity of the adsorbent for the metal ions (mg g-1). Co and Ce indicate the initial and equilibrium concentrations of the metal ions in solution. W is the study.
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Desorption studies
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amount of adsorbent taken (mg). V is the volume of aqueous solution (mL) used for adsorption
To check the possibility of reusing the adsorbent, desorption studies were performed.
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30mg of ECSBNC was added to 25 ml of individual solutions of Cu (II) and Cr (VI) and maintained for 2 hours under continuous stirring. Later the metal ion loaded ECSBNC was taken out washed, dried and used for desorption studies. The sample containing the metal ions was suspended in a 25ml of HCl solution of pH 1.2 for desorption of Cu (II) and in 25 mL of 0.5N NaOH solution for the desorption of Cr (VI) ions and maintained for 2h under stirring. The supernatant solution was diluted volumetrically and the concentration of desorbed metal ion was analyzed using AAS. The percentage of desorption was calculated by the following equation
5
ACCEPTED MANUSCRIPT Desorption (%) =
×100
(2)
Thermodynamic parameters were evaluated by carrying out the adsorption studies at different temperatures, viz. 30, 40, 45 and 50°C.
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4.0 Results and discussion
Chitosan
on
reacting
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4.1 Synthesis of ECSB
with3-methyl-1-phenyl-5-(piperidin-1-yl)-1H-
pyrazole-4-
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carbaldehyde yields the Schiff base through the -NH2groups. This on treatment with epichlorohydrin results in a crosslinked structure, where the chains of chitosan schiff base get
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covalently linked by epichlorohydrin by the involvement of primary hydroxyl groups present at
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C6 position of chitosan. The various steps of formation of ECSB are shown in Scheme 1.
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4.2 Infrared spectral characterization
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Scheme 1:
Fig 1
The IR spectrum of Pip- pyrazole (Fig.1a) showed absorption bands at 2991 and 3065
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cm-1 corresponding to aliphatic and aromatic C-H stretching respectively. The carbonyl stretching band is observed at 1670 cm-1, C=N stretching is observed at 1558cm-1 and C-O-C
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stretching is observed at 1219cm-1.
FIG 2
The infrared spectrum of CS Fig 2(a) shows a broad band at 3486 cm-1 attributed to –NH and –OH stretching and a weak band at 2927 cm-1 attributed to –CH2 stretching. The weak band at 1642 cm-1 indicates the presence of free amine. A band at 1060 cm-1is attributed to the C-O-C of carbohydrate structure.
6
ACCEPTED MANUSCRIPT As seen from Fig 2(b), the intensity of the broad band due to-NH2 and -OH is lower in ECSBNC due to the involvement of hydroxyl group of CS in crosslinking. The band at 1041 cm1
is assigned to the –CO stretching vibration of –C–OH. The
band at 1650 cm-1(Fig 2a)
characteristic of the –NH2 group of the CS has disappeared due to the reaction of CS with –CHO group of 3-methyl-1-phenyl-5-(piperidin-1-yl)-1H- pyrazole-4- carboxaldehyde to form the Schiff base (C=N). The band at 570 cm−1is attributed to Fe-O stretching vibration implying that
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the Fe3O4 nanoparticles have been successfully introduced into the epichlorohydrin crosslinked
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chitosan Schiff base (ECSBNC).
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4.3 Thermogravimetric analysis
The thermogram of CS shows three steps degradation (Fig 3a)in the temperature range of
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30-600°C. The first step takes place in the range of 30-100°C accounting for a weight loss of around 12% which occurs due to the loss of moisture contained in CS. The second step of
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degradation due to breakdown of polysaccharide structure occurring in the range of 225°C to 340°C results in the weight loss of around 55% and the final loss of 33% gradually takes place in
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the range from 300-600°C.Fig 3b depicts the degradation pattern of ECSBwhich is very similar to that of CS. In the first step degradation about 10% of weight loss takes place in a temperature
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zone of 30-80°C and is due to the loss of water molecules. In the second step, the weight loss is observed in the temperature zone of 100-300°C and is due to the decomposition of the chitosan
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backbone followed by decomposition of pyrazole moiety in the temperature range of 300-600°C. About 20% of the compoundremained as residual matter.The thermal degradation behavior of
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ECSBNC shown in Fig 3c is found to be a continuous process. First step degradation occurs in the range of 30-170°C, with a weight loss of 20%, due to the loss of water molecules. The
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second step degradation observed in the temperature zone of 170-300°C with a weight loss of 30% due to the degradation of polymeric chain and the third significant loss occurred in the range 300-600°C with a weight loss of 20% due to the decomposition of pyrazole moiety present in the Schiff base. Even after 650°C, about50% of the mass remained as a residual matter, due to the presence of Fe3O4 particles in the sample along with the residue of ECSB.
FIG 3 4.4 XRD analysis
7
ACCEPTED MANUSCRIPT The XRD pattern of CS, Fe3O4 and ECSBNC are shown in Fig 4. The CS (Fig 4a) is found to be semi crystalline in nature with predominant peaks at 2ϴ= 9°, 21.09° and 30.0°.The diffraction pattern of Fe3O4shown in Fig 4b indicates peaks at 2ϴ= 30.1°, 35.5°, 43.1°, 53.5°,57.0°and 62.6°. The peaks appear to be sharp and clear indicating the highly crystalline nature of Fe3O4 particles. The XRD pattern of ECSBNC in Fig 4c shows the broad bands due to chitosan as well as sharp peaks of Fe3O4 structure, indicating the incorporation of Fe3O4particles
FIG 4 5.0 Adsorption
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into the crosslinked CS structure.
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The binding capacity of the adsorbent materials is attributed to the presence of functional groups containing nitrogen, sulphur and oxygen atoms, which helps in the removal of metal ions
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from the aqueous solutions [32]. Chemical modification of CS with 3-methyl-1-phenyl-5(piperidin-1-yl)-1H- pyrazole-4- carboxaldehyde and epichlorohydrin increases the number of
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binding sites which is expected to enhance the binding ability of the material. The presence of Fe3O4particlesis expected to further enhance the binding capacity due to adsorption on the
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surface of the nanoparticle. In order to check this prediction, adsorption studies were carried out for CS and ECSBNC from aqueous solutions containing Cu (II) and Cr (VI) ions in separate
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experiments which were carried out in duplicate. Adsorption process of Cu (II) ions was carried out at pH 7. At low pH the adsorption decreases due to the protonation of basic nitrogen centers
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on the polymeric chains. Adsorption of Cr (VI) was studied at pH 3. It is evident that under acidic conditions HCrO4- is the active species and the adsorption process is expected to take
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place due to the electrostatic interactions between the ECSB and the adsorbate molecules. The results are discussed below.
5.1 SEM and EDS analysis SEM images of Fe3O4,ECSBNC and; Cu(II) and Cr (VI) loaded ECSBNC are presented in Fig 5. Surface image of Fe3O4 showed agglomeration of spherical shaped nano particles with 8
ACCEPTED MANUSCRIPT varying size (Fig 5a). Fig 5b showed irregular flake like structure of Schiff base of CS containing Fe3O4nanoparticles. Fig 5c and 5d showed the adsorption of Cu (II) and Cr (VI) respectively on the surface of the adsorbent. The adsorption appears to be uniform covering the whole surface.
FIG 5
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The electron dispersive spectrum of Fig 6a shows the presence of carbon, nitrogen,
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oxygen, copper and iron in the Cu (II) adsorbed ECSBNC material. Similarly, Fig 6bshows the
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presence of chromium in addition to carbon, nitrogen, oxygen, and iron in the structure in the Cr
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(VI) adsorbed ECSBNC material.
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FIG 6
5.2 Effect of initial metal ion concentration on adsorption
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Adsorption studies were carried out with different initial Cu (II) and Cr (VI) ion concentrations ranging from 20 to 100 mg/L, keeping all parameters fixed and the results are
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displayed in Fig 7. At low metal ion concentration the availability of metal ions will be less compared to the available sites on the adsorbent resulting in the effective removal of metal ions..
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Increase in metal concentration results in increased sorption of metal ions by ECSBNC. Higher the initial metal ion concentration, higher will be the concentration gradient which results in the
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transfer of metal ions from the solution to the adsorbent favoring adsorption process. At any given time, the amount of Cu (II) adsorbed on ECSBNC is found to be higher than Cr (VI). Fig 7
5.3Adsorption kinetics In order to describe the kinetics and mechanism that controls the adsorption process, the experimental data were interpreted by using Lagergren pseudo-first-order model [33] and Ho ‘pseudo-second order model [34].
9
ACCEPTED MANUSCRIPT The change in the uptake of Cu (II) and Cr (VI) ions as a function of time is presented in Fig 8. It can be seen that the adsorption process was rapid at the beginning and attained equilibrium gradually and remained constant later on till the completion of the experiment. . The Pseudo first order model is given by the equation
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(3)
)
log
(4)
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log (
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The above equation on integration gives
where qe is the adsorption capacity of the adsorbent at equilibrium (mg/g), qt is the amount of adsorbate adsorbed (mg/g) at time ‘t’(min-1), k1 is the first order rate constant (min-1). The data
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obtained in the present study, on fitting with the above equation, did not show linearity as indicated by the low value of R2 ( for Cu(II) 0.537 and for Cr(VI) 0.678) which indicates that
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adsorption is not a first order kinetic process. Hence the fit of data with second order kinetic model was checked.
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The pseudo second order model is represented by the following equation
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(5)
The integration of this equation with condition that when t=0 and qt=0 gives the equation
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(6)
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Where k2 is the second order rate constant (g mg-1 min-1). The plot of t/qt vst for the adsorption data obtained is shown in Fig 9. The obtained linear plots were validated to check the fitness of
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the straight line. The kinetic parameters such as rate constants (k2), equilibrium adsorption capacity (qe) and the regression coefficient (R2) values are shown in table 1. The data reveals that pseudo second order model was best fitted as the R2valueswereclose to 1 compared to pseudo first order model. Further, the experimental qe value was also in agreement with the theoretically obtained qe values showing correctness of pseudo second order fit. The adsorption process is influenced by the concentration of both adsorbate ions and the metal binding sites which tentatively leads to the deposition of metal ions on the adsorbent. Table 1 Fig 8 10
ACCEPTED MANUSCRIPT
Fig 9 5.4 Adsorption isotherms The equilibrium adsorption isotherm is the fundamental for defining the interactive behavior between the solid phase and the metal solution phase. To study the nature of adsorption
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several isotherm models have been employed, the most commonly used models being of
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Langmuir [35] and Freundlich [36]. Fig 10 shows the adsorption isotherms of Cu (II) and Cr (VI)
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on ECSBNC. Fig 10
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To interpret the adsorption data, Langmuir and Freundlich isotherm models were fitted as described below.
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Freundlich isotherm model is based on the assumption that adsorption surface is heterogeneous and adsorption is reversible. The heterogeneity arises due to the presence of
possibility to multilayer adsorption.
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different functional groups. It is not restricted to monolayer adsorption and can consider the
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The mathematical expression for the Freundlich model is given in the following equation.
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(7)
which is expressed in the logarithmic form as
CE
(8)
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where qe is the amount adsorbed (mg/g) and Ce corresponds to concentration of adsorbate solution at equilibrium (mg L-1). KF and n are constants which give an idea of the extent of adsorption and the non linearity between the solution concentration and the adsorption. If the value of n is greater than 1 it is considered as favorable adsorption [10].
The Langmuir model assumes the formation of a saturated monolayer of solute molecules on the adsorbent. . The linearised form of Langmuir equation is given as
11
ACCEPTED MANUSCRIPT (9) where qe and Ce have their meaning as defined earlier; qm is the maximum adsorption capacity corresponding to the monolayer formation on the adsorbent surface. KL is the Langmuir constant related to the energy of adsorption. The Langmuir constants qm and KLare obtained from the linear plots of Ce/qe vs Ce. The essential characteristics of the Langmuir adsorption model is
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given in the terms of a constant separation factor RL. It is given as
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(10)
US
where Co (mg/L) corresponds to initial concentration of metal ion.
The parameter RL indicates the nature of adsorption which is given below. Unfavorable adsorption
0
Favorable adsorption
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RL>1
The Freundlich and Langmuir isotherms plots for the adsorption data obtained in the present
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study are displayed in Fig 11 & 12 respectively. The isotherm parameters KF, n,qm, KL and RL, for
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the adsorption of metal ions on CS and ECSBNC are presented in Table 2 along with the regression coefficient R2 for the fit of two models. The R2 value for the Langmuir fit in all cases
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was close to 1.0 when compared to the Freundlich model fit indicating the best fit of adsorption data with Langmuir model. This shows the adsorption of Cu (II) and Cr (VI) ions onCS and
Table 2
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CE
ECSBNC as a monolayer coverage process.
Fig11 Fig 12 5.5 Comparison studies The adsorption capacity of the presently studied ECSBNC system for the metal ions is higher than reported in literature for other modified chitosan systems as discussed below. The Cu 12
ACCEPTED MANUSCRIPT (II) ions uptake by epichlorohydrin crosslinked chitosan was found to be 35.50 mg/g [37]. The adsorption of xanthate modified magnetic chitosan was found to be 34.50 mg/g for Cu (II) ions [38]. Ethylenediamine modified chitosan was found to exhibit an adsorption capacity of 38.0 mg/g which was attributed to the presence of amine group in the modified structure [32]. In chitosan cellulose composites, the hydroxyl and amine groups were believed to be the metal ion binding sites. The capacity of adsorption Cu (II) was found to be 26.50 mg/g [39]. Chitosan
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blended with poly (vinyl alcohol) was evaluated for Cu (II) binding ability and was found to be
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47.85 mg/g [40]. The adsorption of chromate and cupric ions on chitosan-coated cotton
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gauze showed the maximum adsorption capacity for chromate ion to be 12.4 mg/g at pH 3 and 14.1 mg/g for Cu (II) at pH 5 [41]. The Ca (II)–Chitosan in microspheres form was reported as a
US
green and effective absorbent for the treatment of hazardous wastewater containing heavy metal ions. The adsorption capacity for Cu (II) was found to 41.5 mg/g [42]. The adsorption of
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chemically modified chitosan for the adsorption of Cu (II) ions was found to be 43.47mg/g [43]. Ethylenediamine modified cross-linked magnetic chitosan resin showed maximum adsorption of
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51.0 mg/g of hydrochromate ion at pH 3 through electrostatic force of interaction [44]. The adsorption capacity of magnetic chitosan 2 for the recovery of Cr (VI) metal ions was 137.27
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mg/g; the increased adsorption capacity may be due to the presence of magnetic nano particles in the chitosan structure [45]. Chitosan-biochar/g-Fe2O3 composite was found to exhibit maximum
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adsorption capacity of 167.31 mg/g [46]. Heterocyclically modified chitosan exhibited adsorption capacities of 83.73 and 85.0 mg/g for Cu (II) and Cr (VI) ions [47]. Graphene oxide
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incorporated chitosan Schiff base showed a maximum adsorption capacity of 111.11 and 76.92 mg/g for Cu(II) and Cr(VI) respectively [48]. The adsorption capacity of the Spirulina platensis
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biosorbent for the adsorptive removal of Cu(II) was found to be 817.7 mg/g [49]. Nano-scale zero-valent iron impregnated cashew nut shell showed a maximum adsorption capacity of 48.05 mg/g [50]. Sulfuric acid modified eucalyptus seeds were found to exhibit a maximum monolayer adsorption capacity of 76.94 mg/g for Cu(II) ions [51]. Ultrasonic assisted Spirulina platensis showed high adsorption process with an adsorption capacity of 577.9 mg/g for Cr(VI) ions [52]. Active carbon derived from coffee waste showed an uptake capacity of 156.7 mg/g for Cr(VI) [53]. Biosorbent derived from custard apple and aspergillus niger showed a percentage removal of 95.1% for Cr(VI) [54].
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ACCEPTED MANUSCRIPT The present work when compared with the reports mentioned above indicated that the adsorption capacities of Epichlorohydrin cross linked chitosan containing magnetic particles for Cu (II) and Cr (VI) ions are 83.33 and 90.90 mg/g respectively, which is significantly higher compared to many other reported chitosan adsorbents. The adsorption capacity may be due to presence of nitrogen containing functional group which acts as an excellent chelating agent for
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binding with metal ions.
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5.6Thermodynamic parameters
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The thermodynamic parameters of adsorption, namely the changes in standard free energy, enthalpy and entropy were evaluated by carrying out adsorption at 4 different temperatures (30, 40, 45 and 50°C) and the parameters were calculated using the following equations.
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ΔG°= -RTlnKc (11)
Where Kc is the equilibrium constant and T and R have their usual meaning. The equillbrium constant is expressed as the concentration of metal ion on adsorbent to the residual
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concentration of the metal ion in solution at equilibrium. (12)
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Kc=
The relation between ΔG°, ΔH° and ΔS° is given by equation (13)
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ΔG°= ΔH°-TΔS°
(13)
lnKc =
CE
Combining Equation (11) and (13) gives the van’t Hoff equation, (14)
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The change in enthalpy (ΔH°)and entropy (ΔS°) were calculated from the slope and intercept of linear plot of lnKC versus 1/T (Fig 13). The values of thermodynamic parameters obtained are displayed in Table 3.The negative ΔG ° value indicates the spontaneous nature of adsorption for both the metal ions at the temperatures studied. The positive (ΔH°)value suggested that the adsorption is endothermic in nature. The positive (ΔS°) reveals the increased randomness at the solid-solution interface during the adsorption of the metal ions on the adsorbent. Fig 13 Table 3
14
ACCEPTED MANUSCRIPT 5.7 Desorption The desorption studies were carried out to check the recovery of metal ions adsorbed and the subsequent reuse of the adsorbent. HCl solution of pH 1.2 was used as stripping solution for Cu (II). At pH 1.2 desorption takes place due to the protonation of acid sites on the adsorbent which subsequently lose their affinity for Cu (II) ions. Desorption of Cu (II) was found to be 82%.An amount of 85% of Cr (VI) were found to be desorbed in the stripping solution which
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was 0.5 N NaOH, as at this pH, the adsorption sites get deprotonated and lose their affinity for
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Fig 14
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Cr(VI) ions. The results obtained are displayed in Fig 14.
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6.0 Conclusion
Modification of CS was achieved by Schiff base formation with 3-methyl-1-phenyl-5-
M
(piperidin-1-yl)-1H- pyrazole-4- carbaldehyde, then by crosslinking with epichlorohydrin followed by the incorporation of Fe3O4 nanoparticles. The material thus modified was found to
ED
have higher adsorption capacity for Cu (II) and Cr(VI) compared to the unmodified CS. Adsorption process is observed to follow Langmuir isotherm model with monolayer adsorption
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on the adsorbent surface and the maximum adsorption capacity observed was 90.90 and 83.33 mgg-1 for Cu (II) and Cr (VI) respectively. The adsorption was found to be pseudo second order
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kinetic process. The negative values of ΔGº and the positive values of ΔHº indicated the adsorption process to be spontaneous and endothermic. The positive values of ΔS° indicated the
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increase in randomness at the solid/ liquid interface during the adsorption. The desorption studies revealed about 80% desorption of Cu(II) and Cr(VI) in appropriate stripping solutions confirming the possibility of recovery of adsorbate species and reuse of the adsorbent material effectively.
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Figures and captions 1. Scheme 1: Synthesis of ECSB
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2. Fig 1 FTIR spectra of Pip- pyrazole
4. Fig 3 TGA curves of a) CS, b) ECSB and c) ECSBNC
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5. Fig 4 XRD pattern of a) CS b) Fe3O4 and c) ECSBNC
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3. Fig 2 FTIR spectra of a) CS and b) ECSBNC.
6. Fig 5 SEM Micrographs of a) Fe3O4, b) ECSBNC c) Cu (II) adsorbed ECSBNC and d)
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Cr (VI) adsorbed ECSBNC
7. Fig 6 EDS spectra of a) Cu (II) and b) Cr (VI) adsorbed ECSBNC.
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8. Fig 7 Effect of initial metal ion concentration on adsorption of Cu (II) and Cr (VI) on ECSBNC
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9. Fig 8 Uptake of Cu (II) and Cr (VI) on ECSBNC 10. Fig 9 Second order kinetic plot for adsorption of Cu (II) and Cr (VI) on ECSBNC
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11. Fig 10 Adsorption isotherm of a) Cu (II), b) Cr (VI) on ECSBNC and c) Cu (II) on CS 12. Fig 11 Freundlich isotherm model fit for the adsorption of a) Cu (II) on CS; b) Cu (II)
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and c) Cr (VI) on ECSBNC
13. Fig 12 Langmuir isotherm model fit for the adsorption of a) Cu (II) on CS; b) Cu(II) and
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c) Cr (VI) on ECSBNC.
14. Fig 13 Plot of ln Kc vs 1/T for the uptake of Cu (II) and Cr (VI) on ECSBNC
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15. Fig 14 Desorption of Cu (II) and Cr (VI) ions from Cu (II) and Cr (VI) adsorbed ECSBNC.
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Table 1 Kinetic parameters for the adsorption of Cu (II) and Cr (VI) on ECSBNC. Pseudo second order k2 (g mg−1 min−1)
qe, cal (mgg-1)
Qe, exp
100
71.42
67.33
Cr (VI)
100
51.75
55.55
0.0085
0.999
0.0049
0.999
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Cu (II)
R2
T
CO (mg/L)
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Metal
(I) CS Adsorbate
Langmuir model
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Table 2 Isotherm parameters for adsorption of Cu(II) and Cr(VI) on CS and ECSBNC
qmax
RL
20-100
9.70
2.06
0.010
R2 0.998
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Cu(II)
KL
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Co(mgL-1)
Freundlich model
(II) ECSBNC
Langmuir model
Co(mgL-1) 20-100
Cr(VI)
20-100
qmax
RL
1.76
R2
n 4.065
0.954
Freundlich model KL
R2
KF
R2
n
90.90
1.0
0.00012
0.993
65.46
0.55
0.980
83.33
0.9
0.00014
0.995
38.63
0.63
0.972
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Cu(II)
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Adsorbate
KF
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Table (3) Thermodynamic parameters for adsorption on ECSBNC
-3.68
318
1.73
-3.95
323
1.78
-4.18
303
1.55
-0.81
313
1.59
-0.89
318
1.62
-1.01
323
1.66
-1.10
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1.67
43.78
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313
ΔS°(J/mol/K)
9.96
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-3.30
3.5
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1.56
ΔH°(kJ/mol)
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303
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Cr (VI)
ln Kc
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Cu (II)
ΔG°(kJ/mol)
T(K)
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Adsorbate
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14.25
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Heterocyclic modification of Chitosan has been achieved for the removal of metal ions from aqueous solutions. The extent of adsorption was further enhanced by the incorporation of magnetic nanoparticles. The desorption studies revealed the efficient recovery of adsorbate and possible reusability of the adsorbent material.
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Highlights
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