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Sep 17, 2015 - heavy metal copper from aqueous solution through batch adsorption ... dizziness, vomiting, diarrhea and an illness called metal fume fever.

Accepted Manuscript Title: REMOVAL OF COPPER (II) FROM AQUEOUS SOLUTION USING NANOCHITOSAN/SODIUM ALGINATE/MICROCRYSTALLINE CELLULOSE BEADS Author: K. Vijayalakshmi Thandapani Gomathi Srinivasan Latha T. Hajeeth P.N. Sudha PII: DOI: Reference:

S0141-8130(15)00685-6 http://dx.doi.org/doi:10.1016/j.ijbiomac.2015.09.070 BIOMAC 5406

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

10-7-2015 17-9-2015 29-9-2015

Please cite this article as: http://dx.doi.org/10.1016/j.ijbiomac.2015.09.070 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

REMOVAL OF COPPER (II) FROM AQUEOUS SOLUTION USING NANOCHITOSAN/SODIUM ALGINATE/MICROCRYSTALLINE CELLULOSE BEADS K. Vijayalakshmi, 1Thandapani Gomathi, 2Srinivasan Latha, 3T. Hajeeth and 1*P.N. Sudha 1,1*

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1

PG & Research Department of Chemistry, DKM College for Women, Vellore, Tamil Nadu, 2

Department of Chemistry, SRM University, Chennai, Tamil Nadu, India

Department of Chemistry, Sathyabama University, Chennai, Tamil Nadu, India

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3

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India

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(*Corresponding author: e-mail id: [email protected]; Mob.No: 9842910157)

Abstract

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The present study was aimed to prepare the novel ternary biopolymeric beads of nanochitosan (NCS)/sodium alginate (SA)/microcrystalline cellulose (MC) for the removal of

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heavy metal copper from aqueous solution through batch adsorption mode. The polymeric beads

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were characterized before and after adsorption using FTIR, XRD and EDX-SEM studies. The efficiency of the adsorbent was analyzed by varying the parameters such as initial metal ion

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concentration, contact time, adsorbent dose and pH. The experimental data obtained were fitted in the isotherm models such as Langmuir, Freundlich and Tempkin models and in pseudo first and second order kinetics studies. The isotherm and kinetics models revealed that the adsorption was found to fit well with Freundlich isotherm and follows pseudo second-order kinetics.

Keywords: Nanochitosan, sodium alginate, banana fiber, batch adsorption, Isotherm models.

1. Introduction

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Biopolymers generated from renewable natural sources (i.e. micro-organisms, plants and animals) are often biodegradable, non-toxic and are industrially attractive because of their wide range of applications especially in water treatment. Also the biopolymers have the capability of

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lowering transition metal-ion concentration to parts per billion concentrations [1] and leading to environmental safety.

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Investigation of heavy metals contaminated wastewater has become essential focus of

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environmental scientists in recent years, because the concentrations of heavy metals increase in the environment from year to year [2]. Heavy metals are the worst group of pollutants in the

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environment. The heavy metals are essential to living organisms at low concentrations, but many of them are toxic at elevated concentrations [3]. Heavy metals are elements such as Cu (Copper),

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Zn (Zinc), Ag (Silver), Cr (Chromium), Hg (Mercury), Cd (Cadmium), Fe (Iron), Co (Cobalt),

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As (Arsenic) which is usually associated with toxicity. Among the various heavy metals, copper

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is an important engineering material with wide industrial application and is an essential trace nutrient that is required in small amounts (1-1.5 mg per day in food) by humans, other mammals,

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fish and shell fish for the synthesis of haemoglobin, carbohydrate metabolism and the functioning of more than 30 enzymes. Although copper can be an essential trace element, it could be harmful when it exceeds the tolerance limit. The aesthetic objective for copper in drinking water is set at a maximum of 1.0 mg/L. Acute exposure to large doses could be harmful for humans and required control of exposure [4]. In humans, about 10-20 mg of orally ingested Cu2+ may cause intestinal discomfort, dizziness and headaches while ingestion of copper salts in excess of 500-100 mg have caused fatal acute poisoning such as vomiting, diarrhea with bleeding, circulatory collapse, failure of the liver and kidneys and severe haemolysis [5]. 2    Page 2 of 53

Excessive copper in the marine system has also been found to damage marine life and damage the gills, liver, kidneys, the nervous system and changing sexual life of fishes [6]. In humans, copper toxicity leads to neurotoxicity commonly known as “Wilson’s disease” which

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occurs due to deposition of copper in the lenticular nucleus of the brain. The long-term exposure of copper fumes causes irritation of nose, throat, eyes and causes headaches, stomachaches,

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dizziness, vomiting, diarrhea and an illness called metal fume fever. Thus the efficient removal

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of copper from industrial wastewater becomes essential.

The most common adsorbents for the removal of copper include clay minerals, activated

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carbon [7] and polymers [8 - 10]. In this present work, we concentrated on the preparation of novel, cheap and more effective sorbents using the biopolymers. The naturally occurring

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polysaccharides such as alginic acid, cellulose and chitosan are elected to prepare a novel

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adsorbent due to their extraordinary affinity to heavy metal ions.

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Chitosan, a deacetylated form of chitin composed of poly (β-1-4)-2-amino-2-deoxy-Dglucopyranose can be utilized as an adsorbent to remove heavy metals and dyes due to the

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presence of amino and hydroxyl groups which can serve as the active sites [11]. Among the various active sites, the amine group present in chitosan acts as the main active sites and comparatively the hydroxyl groups (especially in the C-3 position) also contribute to slight metal ion adsorption [12]. The binding mechanism of the transition metal ion onto chitosan can be explained through the chelation process in which the nitrogen present in amine groups gets interacts with the copper ions. Also the adsorption efficiency of the chitosan can be improved by converting it to nanochitosan, a superior environmental friendly material through ionic crosslinking method using sodium tripolyphosphate (TPP), the non-toxic polyanion [13]. Due to

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this modification the dissolution of chitosan in strong acids was prevented, the mechanical strength improved and amorphous nature was increased. Some of the unique advantages of nanoparticles are their small size and the large surface

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area which theoretically increases the adsorption capacity for nanoparticle adsorbents [14]. Many size-dependent phenomena such as chemical, electronic, magnetic and mechanical properties are

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introduced by the high surface-to-volume ratio together with size effects (quantum effects) of

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nanoparticles [15].

Wan Ngah and Fathinathan [16] reported the adsorption properties of nanochitosan using

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Pb(II) and Cu(II) aqueous solutions in the form of beads. The experimental data were correlated with the Langmuir, Freundlich and Dubinin-Radushkevich isotherm models. The maximum

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adsorption capacities of Pb(II) and Cu(II) ions in a single metal system based on the Langmuir

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isotherm model were found to be 57.33 and 26.06 mg/g, respectively. In addition, Sivakami and

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her coworkers investigated the sorption mechanism of chromium ions onto chitosan nanoparticles. Based on the Langmuir, Freundlich and Temkin sorption isotherms, it was

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reported that the sorption behavior of nanochitosan was very high and favouring multilayer adsorption process due to the increased amorphous nature [17]. Hence from the literature survey, nanochitosan was proven as the excellent material for the removal of toxic heavy metals and from the reports suggested by various authors, the nanochitosan was chosen for the study. Similar to chitosan, alginate is also a natural polymer which can form salts with heavy metal ions. Sodium alginate is a linear polyuronate which contains variable amounts of Dmannuronic acid and L-guluronic acid and can be cross linked easily by using calcium ions [18]. The calcium ions cooperatively interact with G and M residues of sodium alginate to form cross links in the gelation process. Interactions of alginates with bivalent calcium cations are especially 4    Page 4 of 53

important as they lead to the chain-chain associations and employed in formation of strong, hydrophilic gels. Such gels play an important role in water treatment technologies. The uptake of heavy metal ion takes place by ion-exchange between Ca (II) of the beads and the heavy metal

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ions in the aqueous solution.

The natural polymers cellulosic natural fibers are envisioned as the most suitable ways to

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solve these problems especially environment related issues. The components which are present

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in natural fibers are cellulose (α-cellulose), hemicelluloses, lignin, pectin and waxes. Steam explosion pretreatments involving the successive chemical and mechanical treatments are one of

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the best methods carried out to reduce the noncellulosic compounds such as hemicelluloses, lignin, pectin that cement the fiber aggregates [19]. Generally the chemical or thermo chemical

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modifications have done with cellulosic raw materials which render them more effectively for

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the collection and binding of various metal ions.

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Many researchers reported that the biopolymers in the form of beads can effectively remove heavy metals from industrial wastewater [20]. Hence in the present work the ternary

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beads were prepared by mixing the nanochitosan, sodium alginate and microcrystalline cellulose (extracted from banana fiber). The main objective of this study was to examine the extent of removal of copper ions from the synthetic wastewater onto a novel alginate, nanochitosan and cellulose beads. The influence of contact time, pH, adsorbent dose and metal ion concentration on the removal of copper ions by NCS/SA/MC bead was investigated. Also the analyzed equilibrium data were fitted to Langmuir, Freundlich and Temkin isotherm equations to find the best fit model. In addition the kinetic studies were done and the results were investigated. 2. Materials and Methods 2.1 Materials 5    Page 5 of 53

The banana fiber was collected from local farms. The analytical grade reagents such as sodium hydroxide (commercial grade), sodium alginate, sodium hypochlorite and oxalic acid were purchased from Central Drug House Pvt. Ltd, Nice chemicals, Kerala, India. The

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crosslinking agent calcium chloride, sodium tripolyphosphate and the solvent glacial acetic acid were procured from Finar chemicals, Ahmedabad and Thomas Bakers chemicals Pvt. Ltd.,

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Mumbai.

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2.2 Extraction of cellulose from the banana fiber

The isolation of cellulose from the banana fiber was done using the steam explosion

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method as per the procedure reported by Bibin Mathew cherian and his coworkers. A combination of chemical (acidic) and mechanical treatments was done with the steam exploded

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fibers to extract the cellulose completely. Steam explosion of the chopped lignocellulosic banana

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fibers (30g) with 2% NaOH (fiber to liquor ratio 1:10) solution was carried out in an autoclave at

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a pressure of 20 lb for a period of 1 h. The steamed alkali treated fibers were followed by sodium hypochlorite bleaching, further treated with oxalic acid and stirred mechanically well to extract

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the cellulose completely with different degrees of crystallinity. 2.3 Preparation of nanochitosan The method of ionotropic gelation where chitosan is ionically cross-linked with sodium

tripolyphosphate was used to synthesize the nanoparticles in dilute solution. About 1 g of chitosan dissolved in 200 ml of 2% acetic acid solution was kept under magnetic stirring for about 20 minutes to create a homogeneous viscous chitosan gel. Then to the above prepared chitosan solution, 0.8 g of sodium tripolyphosphate dissolved in 107 ml of conductivity water was added drop wise. The above mixture was then stirred further for 30 min before sitting an additional 24 hours to reach equilibrium. A milky coloured emulsion like appearance of chitosan 6    Page 6 of 53

nanoparticles were formed upon mixing the TPP solution to the chitosan solution. After reaching equilibrium, the supersaturated solution was decanted. The thick emulsion which was obtained

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above was then poured into the petri plates and it is allowed to dry for few hours.

2.4 Preparation of NCS/SA/MC (2:8:1) bead

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Ternary biopolymeric beads were prepared by mixing the nanochitosan, sodium alginate

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and microcrystalline cellulose extracted from the banana fiber in certain ratio. Three solutions were prepared: (i) an aqueous 2.5% nanochitosan solution; (ii) aqueous 10 wt% alginate solution

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and (iii) aqueous 1.25 wt% microcrystalline cellulose solution. An emulsion was formed by blending above prepared solutions at 500 rpm for about 30 minutes. After this process is over the

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above prepared emulsion was then added drop wise into minimum amount of 0.2 M CaCl2

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solution with the help of a syringe under constant magnetic stirring conditions to perform the

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cross-linking. The beads so produced were allowed to harden by leaving them in CaCl2 solution for few hours, thereafter filtered, washed thrice with double distilled water and then dried.

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Photograph of the prepared NCS/SA/MC bead was represented in Figure 1.

2.5 Characterization

The FT-IR spectrum of chitosan, nanochitosan and NCS/SA/MC bead was recorded

using Perkin Elmer 200 FTIR spectrophotometer. The FT-IR spectrum was obtained in the wave number range from 4000 cm-1 to 450 cm-1 during 64 scans with 2 cm-1 resolution. The particle size and size distribution of the nanoparticles were measured by dynamic light scattering method (DLS, Zetasizer Nano-S, Malvern, England). A suitable amount of the dried nanochitosan was suspended in deionised water and was sonicated for a suitable time period before the 7    Page 7 of 53

measurement. The volume, mean diameter, size distribution and polydispersity of the resulting homogeneous suspension were determined using DLS technique. The X-ray diffraction patterns of the above prepared samples were tested by an X-ray scattering Shimaduz XD-Diffractometer

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using Ni filter Cu Kα radiation source ( λ=0.154nm) set at scan rate = 10˚/min using a voltage of 40kV and a current of 30 mA. The scanning electron micrograph (SEM) analysis of the samples

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was recorded with the help of WD14mmSS25 instrument set at 20kV range.

2.6 Batch Adsorption Studies

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Batch adsorption experiment was performed at room temperature for the removal of Cu (II) using NCS/SA/MC (2:8:1) bead as a function of various parameters such as by changing

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the adsorbent dose, pH of the solution and time of shaking of the adsorbent - metal solution mixture. Initially a standard solution (200mg/L) of copper (II) ion was prepared by dissolving

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0.7859 g of copper sulphate in 1000 mL double distilled water (DDW). For each experimental

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run, about 1 g of NCS/SA/MC bead (adsorbent) was added to 100 ml of Cu (II) solution

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(200mg/L). The mixture was agitated well using orbit shaker at fixed speed of 160 rpm for 1 hr at room temperature. After attaining the equilibrium the adsorbent was separated by filtration using Whattman filter paper and aqueous phase concentration of metal was analyzed by using atomic absorption spectrophotometer (AAS Analysis). The % removal of Cu (II) ion was determined by following equation % Removal = {(Co - Ce) /Co } x 100.

where Co = initial concentration of copper (mg/L). Ce = Cu (II) concentration remaining in solution (mg/L). 3

Results and Discussion 8 

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3.1. Particle size measurements of nanochitosan The mean size and size distribution of prepared nanochitosan was analysed using the particle size analyser (DLS, Zetasizer Nano-S, Malvern, England). The observed findings showed

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that the particle size of nanochitosan was around 100.02 nm and its size distribution was in the range of 90–110 nm.

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3.2. FT-IR studies

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The FTIR spectral details of chitosan and nanochitosan are represented in Figure 2a –2b. The FT-IR spectral details of chitosan (Figure 2a) showed the prominent peaks at various

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wavenumbers such as 3454.75 cm-1, 2923.08 cm-1, 1628.87 cm-1 and so on. For the chitosan molecule, the hydroxyl (OH) peaks can be assigned at 3454.75 cm-1 and alkyl C–H stretching

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vibration were identified as doublets at 2923.08/2830.05 cm-1 respectively [21,22]. Strong peaks

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observed at1628.87 cm-1, 1540.02 cm-1 and 1421.52 cm-1 indicate the presence of C=O stretching

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(amide-I band) [23], N-H bending and C-H deformation. The absorption bands observed at 1384.01 cm-1, 1322.23 cm-1, 1151.84 cm-1, 1098.72 cm-1 and 1021.37 cm-1 were due to the

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presence of OH in plane bending in alcohols, twisting and wagging in CH2 group, C-O stretching in secondary alcohols, C-O-C linkage and C-C stretching respectively. Figure 2b represents the FT-IR spectral details of nanochitosan prepared from the chitosan using ionotropic gelation method. The absorption band obtained at 3385.92 cm-1 attributed to –NH group in chitosan was broadened by the physical interactions with TPP, while a shoulder appeared at 1635.20 cm-1 due to chitosan amide at same position after crosslinking with TPP indicated the interaction of chitosan amide with added polyions [24]. Strong peaks obtained at 2920.57 cm-1, 2908.57 cm-1, 1510.05 cm-1, 1376.80 cm-1 and 1219.00 cm-1 indicate the presence of asymmetrical and symmetrical stretching in CH2 group, 9    Page 9 of 53

NH3+ stretching, OH in plane bending in alcohols and P=O stretching [25]. The peak obtained at 1163.38 cm−1 indicated the overlapping peak of C–O stretching in polysaccharide and formation of chitosan nanoparticles due to the interaction of ammonium ion and phosphate ion in chitosan

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nanoparticle molecules [26]. Absorption bands obtained at 1113.24 cm-1, 1038.87 cm-1 and 897.95 cm-1 were assigned to C-O-C linkage, P-O stretching and C-C stretching respectively.

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On comparing the FT-IR spectral details of chitosan with nanochitosan, it was observed

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that the nanochitosan which was prepared from chitosan by ionotropic gelation method showed some new bands when compared to pure chitosan. The shift of the peak observed at 3454.75 cm-1

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corresponding to the presence of NH, OH stretching in chitosan to lower wavenumber (3385.92 cm-1) in nanochitosan sample indicate that crosslinking had taken place effectively

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between the sodium tripolyphosphate and the chitosan. The above observed results showed that the amide group and NH2 group of chitosan are both slightly crosslinked with a TPP molecule.

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In addition the appearance of a new peak at 1210.00 cm−1and 1038.87 cm-1 attributed to the presence of P=O stretching and P-O stretching in case of nanochitosan also confirms the

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crosslinking of negatively charged tripolyphosphate and positively charged chitosan. Hence from the obtained results it was concluded that the nanochitosan was formed from the chitosan. The FTIR spectrum of NCS/SA/MC (2:8:1) bead before and after Cu(II) adsorption was shown in Figure 2c and 2d. The FTIR spectrum of NCS/SA/MC (2:8:1) bead before and after Cu(II) adsorption were obtained to identify the possible sites in the beads for the bonding of Cu(II) ions. The FTIR spectrum of NCS/SA/MC (2:8:1) bead before adsorption shows the peaks at various wave numbers such as 3424.96 cm-1, 2892.97 cm-1, 2376.82 cm-1, 1630.49 cm-1, 1512.57 cm-1, 1458.77 cm-1, 1124.55 cm-1 and so on. A broad peak obtained at 3424.96 cm-1 was indicative of the intermolecular hydrogen bonded OH stretching and NH stretching vibration 10    Page 10 of 53

[27]. The peak observed at 2892.97 cm-1 was attributed to the presence of aliphatic CH stretching in CH2. The combination band arising at 2376.82 cm-1 confirms the presence of NH3+ group. A strong peak observed at 1630.49 cm-1 , 1512.57 cm-1, 1458.77 cm-1, 1338.23 cm-1 shows the

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presence of carbonyl stretching in carboxylate ion, N-H bending, C-H bending, C-N stretching respectively.

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Similar type of FT-IR results were obtained for the NCS/SA/MC (2:8:1) bead after

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Cu(II) adsorption with small changes in peak positions. On comparing the FTIR spectrum of NCS/SA/MC bead after adsorption with the FTIR spectrum of NCS/SA/MC bead before

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adsorption, it was observed that the peak at 3424.96 cm-1 of –OH and –NH groups was shifted to 3417.86 cm-1 (lower wave number) after adsorption. The peak at 1338.21 cm-1 which can be

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assigned to –C-N stretching vibration in amines was also shifted to lower wave number. This

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reduction in peak position suggested that an interaction was formed between Cu(II) ions and

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nitrogen atoms. The Cu(II) ion adsorption was found to affect the bonds related to nitrogen atoms. This confirms that the nitrogen atoms could be the main adsorption sites for the metal

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ions attachment. Apart from this after the adsorption of copper (II) ions the intensity of P-O-H (around 1100 cm-1) was reduced. Also this observed result confirms the participation of oxygen atom present in the hydroxyl groups attached to phosphorous atom in the adsorption process [28, 29].

The chelation of divalent metal ions with the alginate functional groups takes place via interor intramolecular association mechanism. The metal ions chelate with the carboxylate and hydroxyl groups of alginate macromolecular chains through partially ionic and partially coordinate bonds respectively [30,31]. The comparison of FT-IR spectral details of prepared NCS/SA/MC (2:8:1) bead before Cu(II) adsorption with that of NCS/SA/MC bead after Cu(II) 11    Page 11 of 53

revealed that the symmetric vibrations due to the carboxylate ion group (1630.49 cm-1) is shifted to lower wavenumber in case of NCS/SA/MC bead with Cu(II) loading respectively [32,33]. This obtained result concludes the complexation between the metal ions with the carboxylate ion

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groups of alginate macrostructure. The proof of metal ion attachement onto NCS/SA/MC beads was also evidenced from the appearance of new peak at 449.12 cm-1 corresponding to M-L

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stretching vibrations.

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Thus these observations suggested that the adsorption of Cu(II) ions was probably

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through the formation of complexes with the nitrogen atom of NH2 group (which were not involved in the ionic-cross-linking process) and oxygen atom (from hydroxyl groups of cellulose

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and nanochitosan, from P-O-H groups and from carboxylate ion of sodium alginate).

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3.2 XRD diffraction studies

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The XRD pattern of the NCS/SA/MC bead prepared in 2:8:1 ratio (Figure 3) shows peaks at various 2θ values such as 20.63˚, 42.25˚, 56.11˚ and 75.04˚. Also in addition the XRD pattern of NCS/SA/MC bead prepared in 2:8:1 ratio shows a broad peak at around 2θ= 40˚. From the observed broad nature of the diffractogram and the calculated lower percentage degree of crystallinity values it was concluded that the prepared bead has highly amorphous nature which will be suitable for adsorption process. 3.3 SEM-EDX studies

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The surface morphology and the percentage of elements present in the prepared bead were identified from the SEM and EDX study. Figure 4a and 4b shows the SEM micrograph and EDX spectra of NCS/SA/MC bead (2:8:1) before and after adsorption of Cu(II) ions. The SEM

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micrograph of NCS/SA/MC bead (before adsorption) exhibited the good spherical rough texture. The observed surface morphology shows that the cellulose and nanochitosan were coated and

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dispersed uniformly with alginate matrix effectively. After Cu(II) adsorption onto the

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NCS/SA/MC bead the surface morphology drastically changed whereby the surface was more in irregular form showing agglomerate-like surface [34] and the presence of white particles of

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Cu(II) ions on the surface confirms the adsorption process had taken place.

The EDX spectrum showed that the bead exhibits the major component of nanochitosan,

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sodium alginate and microcrystalline cellulose. The observations of peaks due to phosphorous,

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calcium and chlorine in EDX spectra of bead conclude that the cross linking processes had taken

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place effectively between the added polymeric components. Furthermore, the copper peak present in EDX spectrum of NCS/SA/MC (2:8:1) bead taken after adsorption process confirms

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that adsorption of Cu(II) ions had taken place effectively onto the bead. These observed results showed that the prepared NCS/SA/MC bead was a very good adsorbent for practical applicability for the removal of copper (II) from aqueous solution.

Based on the above observed results the following mechanism is proposed for the adsorption of Copper (II) onto NCS/SA/MC bead (Figure 5). 3.4.

Optimization of Adsorption Parameters

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The adsorption parameters such as contact time, adsorbent dosage, initial metal ion concentration and pH have immense effect on the adsorption efficiency of an adsorbent.

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Therefore these parameters were optimized to study its effects on adsorption of Cu(II).

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3.4.1. Effect of adsorbent dose

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The dependence of the metal ion removal on adsorbent dose was investigated by varying the amount of NCS/SA/MC bead (2:8:1) from 1 to 6 g, while keeping other parameters as

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constant. The effect of adsorbent dose on the adsorption of Cu2+ ions was shown in Figure 6. From Figure 6, it was evident that the removal percentage of copper (II) ion increases with

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increasing the adsorbent dose from 1g to 4g. The initial increase in the metal ion removal

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percentage was due to the greater availability of the exchangeable sites or surface area at higher

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concentration provided by the adsorbent necessary for the adsorption to occur [35]. Any further addition of the adsorbent beyond this 4 g does not cause that much

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significant change in the adsorption. This may be due to the overlapping/saturation of adsorption sites as a result of overcrowding of adsorbent particles [36]. Another reason may be the interparticle interaction, such as aggregation, resulting from high adsorbent dose which will lead to a decrease in the total surface area of the adsorbent and also an increase in diffusional path length [37]. The maximum % removal of Cu (II) was about 82 % at the dosage of 4g. From the above observed results it was identified that the optimum adsorbent dosage for removing Cu2+ ion was found to be 4g. 3.4.2. Effect of contact time 14    Page 14 of 53

Contact time is one of the effective factors in batch adsorption process. The effect of contact time on the adsorption of Cu 2+ ions onto the synthesized NCS/SA/MC bead (2:8:1) was represented in Figure 7. Figure 7 shows a rapid initial adsorption rate of copper at the beginning

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until 240 mins of contact time, thereafter, the adsorption rate became practically constant.          The fast adsorption of copper (II) ion at the initial stage with increase in contact time was due to

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the increase in the number of vacant active sites available on the beads surface.

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Further increase in contact time does not show that much increase in metal ion uptake.  The progressive increase in adsorption and consequently the attainment of equilibrium

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adsorption might be due to limited mass transfer of the adsorbate molecules from the bulk liquid to the external surface of synthesized NCS/SA/MC beads (2:8:1) [38]. The obtained results

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clearly indicate that Cu(II) removal was increased from 80.02% to 90.96% with the contact time

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found to be 240 minutes.

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variation from 60 to 360 mins, respectively. The optimum contact time for removal of Cu(II) was

< Please Insert Figure 7>

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3.4.3. Effect of pH

The adsorption characteristics of the adsorbents are highly pH dependent.  The pH of aqueous solution strongly affects the uptake and percentage removal of metals from the aqueous solution [39, 40]. The effect of pH on adsorption process was investigated by varying the pH of the metal solution from 4 to 8 while keeping other parameters constant. Figure 8 shows the influence of solution pH on the adsorption of NCS/SA/MC bead (2:8:1) for Cu2+ ion from aqueous solution. The results revealed that the adsorption increases with an increase in pH of the metal ion solution initially (from pH: 4 to 5) but thereafter it declines (pH>5). The optimum pH for removal of Cu(II) ion was found to be 5. The minimum adsorption observed at low pH could 15    Page 15 of 53

be due to the fact that the presence of higher concentration and higher mobility of H+ ions favoured H+ adsorption compared to M(II) ions and as a result of this H+ ion adsorption, metal ions is prevented from approaching the binding sites of the adsorbents. The drastic decrease in

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adsorption at higher pH (pH >5) was attributed to the precipitation of copper hydroxide



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3.4.4. Effect of initial metal ion concentration

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complexes and so this will ultimately lead to the reduction in sorption capacity of metal ions.

Copper adsorption is significantly influenced by the initial concentration of Cu 2+ ions in

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aqueous solution. In the present study, the initial Cu2+ concentration is varied from 62.5 mg/L to 1000 mg/L while maintaining the other parameters as constant. Figure 9 shows the effect of

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initial metal ion concentration on percentage removal of Cu2+ ions. The percentage removal decreases from 86.92% (for 62.5 mg/L) to 42.736 % (for 1000 mg/L) with an increase in initial

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Cu2+ ion concentration. At low metal ion concentration the number of metal ions available in the solution is less as compared to the available sites on the adsorbent and hence the metals are

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adsorbed by specific active sites very effectively leading to the higher percentage removal of Cu2+ ion whereas at higher metal ion concentration due to the saturation of adsorption sites the lower % removal was observed [41, 42].

3.5.

Adsorption isotherm

The distribution of metal ions between solution and biomass is a measure of the position of equilibrium and can be expressed by one or more isotherms. The distribution of metal ions between liquid and solid phases is generally described by Lanmguir model, Freundlich model,

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Tempkin model and Dubinin– Radushkevich (D–R) models. Among these Langmuir, Freundlich

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and Tempkin adsorption models are commonly used to analyze and fit experimental data.

3.5.1. Langmuir isotherm model

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Linear regressions were used to determine the best fit model and least squares has been

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widely used for obtaining the isotherm constant [43]. Langmuir isotherm model is based on the assumption that there is a finite number of binding sites which are homogeneously distributed

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over the adsorbent surface having the same affinity for adsorption of a single molecular layer and there is no interaction between adsorbed molecules [44]. All adsorption sites involved are

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energetically identical and the intermolecular force decreases as the distance from the adsorption

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surface increases [45, 46]. The linearised form of Langmuir isotherm was given below

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Ceq/Cads = bCeq/KL + 1/KL

where

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Cmax = KL/b

Cads = amount of metal ion adsorbed (mg/g) Ceq = equilibrium concentration of metal ion in solution (mg/dm3) KL = Langmuir constant (dm3/g) b = Langmuir constant (dm3/mg) Cmax = maximum metal ion adsorbed The constants “b” and “KL” can be obtained from linearized form of Langmuir equation. Figure 10 and 11 shows the linear isotherm plot and Langmuir adsorption isotherm plot of the sorption of copper (II) ions by NCS/SA/MC beads (2:8:1). In a sorbent and solution system, a 17    Page 17 of 53

graph of the solute concentration in the solid phase Cads (mg/g) can be plotted as a function of the solute concentration in the liquid phase Ceq (mg/dm3) at equilibrium. At equilibrium there is a defined distribution of the solute between the liquid and the solid phases which can generally be

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expressed by one or more isotherms [47].

A langmuir adsorption isotherm plot of Ceq/Cads Vs Ceq yielded a straight line confirming

cr

the applicability of the Langmuir adsorption isotherm. With the help of the slope and intercept of

us

linear plot of Ceq/Cads against Ceq, the Langmuir constants KL and b can be calculated. The calculated values of Langmuir constants and correlation coefficient were represented in Table 1.

an





M



d

The shape of the isotherm can be used to predict whether adsorption system is favorable

te

or unfavorable in a batch adsorption system. The parameter RL indicates the shape of the isotherm and nature of the sorption process. The essential features of a Langmuir isotherm can be

Ac ce p

expressed in terms of a dimensionless constant separation factor or equilibrium parameter RL. The separation factor RL is defined by: RL = 1 / 1 + b Cf

where Cf is the final Cu (II) concentration (mg/dm3 ) and b is the Langmuir adsorption equilibrium constant (dm3 /mg). If RL > 1 then unfavorable isotherm, RL = 1 then linear isotherm RL = 0 then irreversible isotherm and 0 < RL < 1 then favorable isotherm [48]. The values of RL were calculated for different initial Cu(II) concentrations and the calculated RL values were represented in Table 2. 18    Page 18 of 53

From Table 2 it was evident that the observed RL values are in the range of 0 < RL < 1. Since the calculated RL values are in the range of 0 to 1, it was concluded that the NCS/SA/MC

3.5.2.

ip t

beads (2:8:1) was found to be the favourable adsorbent.

Freundlich isotherm model

cr

Freundlich presented the earliest known sorption isotherm equation in the year 1906 [49].

us

This empirical model can be applied to non ideal sorption on heterogeneous surfaces as well as multilayer sorption [50]. The widely used empirical Freundlich equation based on a

an

heterogeneous surface was given by

qe = Kf Ce 1/n

M

The linearised form of Freundlich equation is expressed as:

d

log qe = log KF + 1/n log Ce

te

where KF and n are Freundlich isotherm constants related to adsorption capacity and adsorption intensity, qe is the amount of metal ion adsorbed (mg/g) and Ce is the equilibrium

Ac ce p

concentration (mg L−1) [51]. The linear regression plot of Freundlich isotherm for Cu(II) uptake by NCS/SA/MC bead (2:8:1) was shown in the Figure 12. A plot of log qe versus log Ce gives a straight line of slope (1/n) and an intercept of log KF. The calculated values of freundlich constants KF and n was represented in Table 3. 3.5.3. Tempkin isotherm model The Tempkin adsorption isotherm model was mainly chosen to evaluate the adsorption potentials of the adsorbent for adsorbates [52]. This model assumes that the heat of adsorption of 19    Page 19 of 53

all the molecules in layer decreases linearly due to adsorbent-adsorbate interactions rather than logarithmic with coverage [53]. The Tempkin isotherm is characterized by a uniform distribution

following equations qe = RT/bT ln (AT Ce)

us

qe = BT ln AT + BT ln Ce

cr

qe = (RT/bT) lnAT + (RT/bT) ln Ce

ip t

of the bonding energies up to some maximum binding energy [54] which is represented by the

where BT = RT/bT , R = universal gas constant (8.314J/mol/K), T= absolute temperature

an

(K), bT = variation of adsorption energy (kJ/mol), R = universal gas constant (8.314 J/mol/K), AT = Temkin isotherm equilibrium binding constant (L/mg), BT = Temkin constant related to

M

heat of adsorption (kJ/mol).From the slope and intercept of plot of qe versus ln Ce (Figure 13) the

te

values was represented in Table 4

d

Temkin constants can be calculated. The calculated values of Temkin isotherm constants and R2



Ac ce p



From the comparison of observed R2 values (presented in the Tables – 1, 3, 4), it was

concluded that the synthesized NCS/SA/MC (2:8:1) bead followed the Freundlich model when compared to the Langmuir and Temkin model. 3.6.

Kinetics studies

The kinetics of adsorption was determined by analyzing adsorptive uptake of heavy metals from the aqueous solution at different time intervals. The pseudo-first-order and pseudosecond-order model equations are fitted to model the kinetics of Cu (II) uptake by NCS/SA/MC

20    Page 20 of 53

bead (2:8:1). A good correlation of the kinetic data explains the adsorption mechanism of the metal ions on the solid phase [55]. 3.6.1. Pseudo first order kinetics

log (qe-qt) = log qe - k1t / 2.303

ip t

A linear form of pseudo-first-order model was described by Lagergren (1898) in the form:

cr

where qe and qt are the amounts of metal adsorbed (mg/g) at equilibrium and at time t

us

(min) and k1 (min-1) is the adsorption rate constant of pseudo-first-order adsorption respectively. A linear plot of log (qe - qt) against time allows one to obtain the rate constant. The pseudo first

an

order kinetic plot for the sorption of Cu (II) ion from aqueous solution onto NCS/SA/MC bead was represented in Fig-14. From the slope and intercept of the linear plot of log (qe - qt) against

M

time, the pseudo first order rate constant k1 and the equilibrium adsorption capacity qe was

d

calculated.

 

te



Ac ce p

3.6.2. Pseudo second order kinetics

The pseudo-second-order rate equation can be represented as follows t 1 t ---- = ------- + ----k2qe2 qe qt

where qe and qt are the amounts of metal adsorbed (mg/g) at equilibrium and at time t

(min), and k2 (g mg-1 min-1) is the adsorption rate constant of pseudo second order adsorption rate respectively. A linear plot of (t/qt) versus t drawn for the pseudo-second-order model was represented in Figure 15. The slope and intercept of the linear plot of t/qt versus time gives the values of KF and n respectively. 21    Page 21 of 53

3.6.3. Intraparticle diffusion Intraparticle diffusion is a transport process involving the diffusion of the adsorbate

ip t

(particle) from the outer surface into the pores of the sample (movement of species from the

qt = Kd t1/2 + I

-----(3)

us

where Kd : is the particle diffusion rate constant (mg/g/min1/2)

cr

bulk). The intraparticle diffusion also known as Weber and Morris equation was given as follows

I : characterizes the extent of diffusion i.e. intercept of the line which is directly

an

proportional to the boundary layer thickness.

Based on the above Weber Morris equation, a plot was made between the qt and t1/2

M

(Figure 16). From the obtained slope and intercept values of plot of the amount of sorbate

te

calculated [56].

d

adsorbed, qt (mgg-1) versus the square root of the time, the rate constant (Kd) and I value was



Ac ce p

The intra particle diffusion is considered to be the rate determining step if it gives a

linear line which passes through the origin . But from Fig-15, it was evident that the line in the initial stage does not pass through the origin instead it depicts two linear portions; the first part of the curve is due to boundary layer diffusion whereas the last part of the curve is due to the diffusion of Pb2+ ions [57,58]. Similar conclusions have been reported in the earlier studies of adsorption of Zn (II) ions onto chitosan. Along with the correlation coefficient values (R2), the calculated equilibrium adsorption capacity (qe), the pseudo first order rate constant (k1), pseudo second order rate constant (k2)

22    Page 22 of 53

values and the intraparticle diffusion rate parameters for Cu(II) sorption by NCS/SA/MC bead (2:8:1) was represented in Table 5.

ip t

If the plot was found to be linear with good correlation coefficient means it indicates that the particular kinetic model is appropriate to copper sorption onto NCS/SA/MC bead synthesized

cr

in the ratio 2:8:1 respectively. It can be seen from the Table 5, that there is an excellent

us

agreement between the experimental qe value and the calculated qe value for the pseudo second order kinetic model when compared to the pseudo first order kinetic model and intraparticle

an

diffusion model. Hence from the obtained results it was concluded that the pseudo second order kinetic model better describes the adsorption kinetic process than the other models. Conclusion

M

4

d

This study investigated the feasibility of nanochitosan/sodium alginate/microcrystalline

te

cellulose bead prepared in 2:8:1 ratio as low cost adsorbent for the removal of copper (II) ion from aqueous solution. FTIR analysis revealed that the nitrogen and oxygen atoms found in the

Ac ce p

beads were the binding sites for the metal ions. The XRD studies reveal the amorphous nature of the NCS/SA/MC bead which will be suitable for adsorption process and the elemental analysis from EDX-SEM results concludes the adsorption of Cu (II) ion onto the bead respectively. This study demonstrated that the variation of adsorbent dose, contact time, metal ion concentration and pH had a marked influence on the removal of Cu (II) ions from aqueous solution. The experimental results showed that the optimum adsorbent dose, contact time and pH were found to be 4g, 240 minutes and pH-5. The experimental results were analyzed using adsorption isotherm models. The Freundlich isotherm showed a better fit than the langmuir isotherm and tempkin isotherm, thus indicating the applicability of multilayer coverage of copper (II) ion on 23    Page 23 of 53

NCS/SA/MC bead surface. The kinetic studies showed that the adsorption adhered to the pseudo second order model since theoretical and experimental sorption capacities were in excellent agreement with R2=0.999. The findings of this study suggest that NCS/SA/MC bead prepared in

ip t

2:8:1 ratio can be used in simple water treatment units to remove copper (II) ions to enhance the quality of water and/or wastewater since it is of low-cost, abundant and a locally available

cr

adsorbent.

[1]

us

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cr

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ip t

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[58] K.

Bakiya

lakshmi,

P.N.

Sudha,

Adsorption

of

Copper

(II)

ion

onto

chitosan/sisal/banana fiber hybridcomposite, Int. J. Environ. Sci, 3 (2012) 453-470.

        30    Page 30 of 53

     

ip t

   

cr

 

us

 

Figure captions

Figure 1: Photography of nanochitosan (NCS)/sodium alginate (SA)/microcrystalline

an

cellulose (MC) (2:8:1) bead Figure 2a: FT-IR spectrum of chitosan

M

Figure 2b: FT-IR spectrum of nanochitosan

Figure 2c: FTIR spectra of nanochitosan (NCS)/sodium alginate (SA)/microcrystalline cellulose(MC) (2:8:1) bead before Cu2+ adsorption

d

Figure 2d: FTIR spectra of nanochitosan (NCS)/sodium alginate (SA)/ microcrystalline

te

cellulose (MC) (2:8:1) bead after Cu 2+ adsorption Figure 3: XRD spectra of nanochitosan (NCS)/sodium alginate (SA)/microcrystalline

Ac ce p

cellulose (MC) (2:8:1) bead

Figure 4: EDX-SEM micrograph of Nanochitosan/sodium alginate/microcrystalline cellulose (2:8:1) bead (a) before Copper (II) adsorption; (b) after Copper (II) adsorption Figure 5: Proposed binding mechanism of metal ion onto NCS/SA/MC bead Figure 6: Effect of adsorbent dose Figure 7: Effect of contact time Figure 8: Effect of pH

Figure 9: Effect of initial metal ion concentration Figure 10: Linear isotherm plot for the removal of Cu(II) ions Figure 11: Langmuir adsorption isotherm plot for the removal of Cu(II) ions Figure 12: Freundlich adsorption isotherm plot for the removal of Cu(II) ions Figure 13: Tempkin adsorption isotherm plot for the removal of Cu(II) ions 31    Page 31 of 53

Figure 14: Pseudo first order kinetic plot for the removal of Cu(II) ions Figure 15: Pseudo second order kinetic plot for the removal of Cu(II) ions

Ac ce p

te

d

M

an

us

cr

ip t

Figure 16: Intraparticle diffusion plot for the removal of Cu(II) ions

32    Page 32 of 53

ip t cr us

 

Ac ce p

te

d

M

an

Figure 1: Photography of nanochitosan (NCS)/sodium alginate (SA)/microcrystalline cellulose (MC) (2:8:1) bead

33    Page 33 of 53

ip t cr us an M

Ac ce p

te

d

Figure 2a: FT-IR spectrum of chitosan

34    Page 34 of 53

ip t cr us an M

Ac ce p

te

d

Figure 2b: FT-IR spectrum of nanochitosan

35    Page 35 of 53

ip t cr us an M

Ac ce p

te

d

  Figure 2c: FTIR spectra of nanochitosan (NCS)/sodium alginate (SA)/microcrystalline cellulose(MC) (2:8:1) bead before Cu2+ adsorption

36    Page 36 of 53

ip t cr us an M

 

Ac ce p

te

d

Figure 2d: FTIR spectra of nanochitosan (NCS)/sodium alginate (SA)/ microcrystalline cellulose (MC) (2:8:1) bead after Cu 2+ adsorption 

37    Page 37 of 53

ip t cr us an

Ac ce p

te

d

M

  Figure 3: XRD spectra of nanochitosan (NCS)/sodium alginate (SA)/microcrystalline cellulose (MC) (2:8:1) bead  

38    Page 38 of 53

 

(a) 

an

us

cr

ip t

(b)

 

 

Ac ce p

te

d

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Figure 4: EDX-SEM micrograph of Nanochitosan/sodium alginate/microcrystalline cellulose (2:8:1) bead (a) before Copper (II) adsorption; (b) after Copper (II) adsorption

39    Page 39 of 53

O O

HO O

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O

O

OH

OH O

O

O

O

H2N

+

O

HO OH

M 2+

O

O-

H2N

O-

OH

-O

OOH

M 2+

O-

HO

O

P

O

H

-

O

HO

an

O

M 2+

O

O

HO

O

O

O

P

O

O

M

HO

OH

O

P

O HO O

O

NH2

n

OO

O

HO

NH3+

NH2

OH

O

O

O

O OH

-

OH

O

cr

O

O O

us

OH

H

O

-O

O

O OH

M 2+

O

OH

-O

O-

O

O

OH

O

O

O

OH O

OH

OH O

-

O

O

 

Figure 5: Proposed binding mechanism of metal ion onto NCS/SA/MC bead

 

40    Page 40 of 53

90

ip t

80 70 60 2

4

6

8

us

0

cr

% Removal of Cu2+

100

an

Adsorbent dose(g)

Ac ce p

te

d

M

Figure 6: Effect of adsorbent dose

41    Page 41 of 53

 

ip t

90

cr

80

70 0

100

200

us

% Removal of Cu2+

100

300

400

an

Time in minutes

 

Ac ce p

te

d

M

Figure 7: Effect of contact time

42    Page 42 of 53

 

ip t

80

cr

75

70 4

6

us

% Removal of Cu2+

85

8

an

pH

10

 

Ac ce p

te

d

M

Figure 8: Effect of pH

43    Page 43 of 53

ip t

80 60

cr

40 20

us

% Removal of Cu2+

100

0 0

500

1000

1500

an

Metal ion concentration (ppm)

Ac ce p

te

d

M

Figure 9: Effect of initial metal ion concentration

44    Page 44 of 53

50

ip t

30 20

cr

qe (mg/g)

40

0 0

200

400

600

an

Ceq(mg/dm3)

us

10

800

 

Ac ce p

te

d

M

Figure 10: Linear isotherm plot for the removal of Cu(II) ions

45    Page 45 of 53

 

ip t

15

cr

10 5 0 0

200

400

us

Ce q/Cads(g/dm3)

20

600

3

an

Ce q(mg/dm )

800

Ac ce p

te

d

M

Figure 11: Langmuir adsorption isotherm plot for the removal of Cu(II) ions 

46    Page 46 of 53

 

2.0

ip t

1.0

cr

log Ye

1.5

0.0 0.5

1.0

1.5

2.0

2.5

3.0

an

log Ce

us

0.5

Ac ce p

te

d

M

Figure 12: Freundlich adsorption isotherm plot for the removal of Cu(II) ions

47    Page 47 of 53

  50

ip t

30 20

cr

qe (mg/g)

40

0 2

4

us

10

6

an

ln Ce

8

 

Ac ce p

te

d

M

Figure 13: Tempkin adsorption isotherm plot for the removal of Cu(II) ions 

48    Page 48 of 53

  1.5

ip t

0.5 0.0 200

300

400

us

100

cr

log (qe-qt)

1.0

-0.5

Time in minutes

an

 

Ac ce p

te

d

M

Figure 14: Pseudo first order kinetic plot for the removal of Cu(II) ions 

49    Page 49 of 53

 

ip t

2.0 1.5

cr

1.0 0.5 0.0 0

100

200

us

t/qt (min. g. mg-1)

2.5

300

400

an

Time in minutes

 

Ac ce p

te

d

M

Figure 15: Pseudo second order kinetic plot for the removal of Cu(II) ions 

50    Page 50 of 53

 

200

ip t

190

qt

180

cr

170

150 10

15

t

1/2

an

5

us

160

20

 

M

Figure 16: Intraparticle diffusion plot for the removal of Cu(II) ions 

Ac ce p

te

d

 

51    Page 51 of 53

Table 1: Langmuir adsorption isotherm parameters Langmuir constants Metal ion 3 b (dm3/mg) Cmax (mg/g) KL (dm /g) 0.5211

0.01203

43.32

0.9263

ip t

Cu(II)

R2

cr

Table -2 : Calculated RL values Initial concentration Metal ion (Ci) (mg/L) 1000 750 500 Cu (II) 250 125 62.5

RL value

0.126761 0.166817 0.245573 0.612914 0.826137 0.91048

M

an

us

Final concentration (Cf) (mg/L) 572.641 415.178 255.37 52.498 17.494 8.173

2.8288

2.381

Ac ce p

Cu (II)

te

d

Table 3: Freundlich adsorption isotherm parameters Freundlich constants 3 Metal ion n (dm3/mg) KF (dm /g)

R2 0.9373

Table 4: Temkin adsorption isotherm parameters Metal ion Cu (II)

Temkin constants

AT (L/mg) 0.2302

BT 7.577

R2 0.9212

52    Page 52 of 53

an

us

cr

ip t

Table-5: Comparison of pseudo first order, pseudo second order and intra particle diffusion kinetic parameters Pseudo-first-order Experimental Pseudo-second-order Intra particle diffusion kinetic model value kinetic model model Metal kd k2 ion qe k1 (minqe 2 -1 2 R q (mg/g) (g mg R (mg/g/ I R2 1 e (mg/g) (mg/g) ) -1 1/2 min ) min ) Cu 0.771 36.22 0.0127 0.9619 173.8464 186.32 0.00064 0.9999 1.734 152.0 (II) 4

Ac ce p

te

d

M

 

53    Page 53 of 53

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