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Preparation and characterization of nano chitosan for treatment wastewaters
International Journal of Biological Macromolecules 57 (2013) 204–212
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Preparation and characterization of nano chitosan for treatment wastewaters M.S. Sivakami a , Thandapani Gomathi b , Jayachandran Venkatesan c , Hee-Seok Jeong c , Se-Kwon Kim c,∗ , P.N. Sudha a,b,∗∗ a
Department of Chemistry, Government Thirumagal Mills College, Gudiyattam, Tamil Nadu, India PG and Research Department of Chemistry, DKM College for Women, Vellore, Tamil Nadu, India c Department of Chemistry and Marine Bioprocess Research Center, Pukyong National University, Busan 608-737, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 27 December 2012 Received in revised form 22 January 2013 Accepted 2 March 2013 Available online 13 March 2013 Keywords: Chitosan Marine polymers Chromium (VI) Sorption isotherms Industrial waste
a b s t r a c t Chitosan nanorod with minimum particle size of <100 nm was prepared by crosslinking low molecular weight chitosan with polyanion sodium tripolyphosphate and physicochemically characterized (FT-IR, XRD, SEM, AFM, TGA and DSC) for waste water treatment. Its sorption capacity and sorption isotherms for chromium (Cr) were studied. The effect of initial concentration of Cr ions, sorbent amount, agitation period and pH of solution that influence sorption capacity were also investigated. It was found that nanochitosan in the solid state was rod shaped which could sorb Cr (VI) to Cr (III) ions effectively. Based on the Langmuir, the Freundlich and the Temkin sorption isotherms, the sorption capacity of chitosan nanoparticles is very high and the adsorbent favors multilayer adsorption. The kinetics studies show that the adsorption follows the pseudo-second-order kinetics, which infers the transformation of Cr (VI) to Cr (III). From the results it was concluded that nanochitosan is an excellent material as a biosorbent for Cr removal from water. © 2013 Elsevier B.V. All rights reserved.
1. Introduction 71% of the total surface of earth is water. Most of the water bodies as ponds, lakes, streams, rivers, sea and oceans, have become polluted due to population expansion, haphazard rapid urbanization, industrial and technological expansion, energy utilization and waste generation from domestic and industrial sources. The presence of heavy metals in the environment has been of great concern due to their increased discharge, toxic nature and other adverse effects on receiving waters. Chromium (Cr) and its compounds are present in the wastewater from electroplating, leather tanning, film and photography, dyes and pigments, textiles, steel fabrication and canning industries [1]. In environmental waters, Cr exists predominantly in two stable oxidation states Cr (III) and Cr (VI) and have contrasting toxicities mobility and availability [2]. Cr (III) is an essential
∗ Corresponding author at: Department of Chemistry and Marine Bioprocess Research Center, Pukyong National University, Busan 608-737, Republic of Korea. Tel.: +82 51 629 7094; fax: +82 51 629 7099. ∗∗ Corresponding author at: PG and Research Department of Chemistry, DKM College for Women, Vellore, Tamil Nadu, India. Tel.: +91 9842910157. E-mail addresses: [email protected] (M.S. Sivakami), [email protected] (S.-K. Kim), [email protected], [email protected] (P.N. Sudha). 0141-8130/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2013.03.005
component having an important role in the glucose, lipid and protein metabolism, while Cr (VI) has a definite adverse impact on living organisms. Cr (VI) is known to have 100-fold higher toxicity than Cr (III) because of its high water solubility, mobility and easy reduction [3]. The United States Environmental protection Agency has laid down the maximum contaminant level for Cr (VI) into inland surface waters as 0.1 mg/L and in domestic water supplies to be 0.05 mg/L. The toxicological effect of Cr (VI) originates from the action of this form itself as an oxidizing agent, as well as formation of free radicals during the reduction of Cr (VI) to Cr (III) occurring in the cell [4]. Research on carcinogenicity of Cr (VI) has focussed on the fact that the free radicals produced inside the cell strongly bind to DNA. Breathing and holding of Cr (VI) containing material can cause perforation of the nasal septum, asthma, bronchitis, pneumonitis, inflammation of larynx and liver and increased incidence of bronchogenic carcinoma [5]. The removal of toxic heavy metal ions from industrial effluents, water supplies and mine waters has received much attention in recent years. Several methods adopted to remove Cr from the industrial wastewater are chemical precipitation, ion exchange, reduction, electrochemical precipitation, solvent extraction, membrane separation, cementation, electrodialysis and adsorption. Among the cited methods, each method has its own limits and in recent years biosorption is recognized as an emerging technique for the treatment of wastewater containing heavy metals
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[6]. The major advantages of biosorption over conventional treatment methods include: low price, high effectiveness, minimization of chemical and/or biological mud, restoration of biosorbent and possibility of metal recovery [7–11]. Some of the biosorbents include chitosan and modified chitosan. Chitosan is a biopolymer of polyaminosaccharide, synthesized from the deacetylation of chitin, consisting of unbranched chains of -(1, 4)2-acetoamido-2-deoxy-d-glucose. Chitosan is more useful than chitin since it possesses more number of chelating amino groups and can be chemically modified [12]. Chitosan is an antibacterial, biocompatible, environment friendly, biodegradable material and has great potential for sorption of metal ions due to amino and hydroxyl groups in its chemical structure [13–17]. Compared to traditional micro-sized supports used in separation process, nano-sized adsorbents possess good performance due to high specific surface area, small size and quantum size effect that could make it exhibit higher capacities for metal ions [18]. Investigations reveal that chitin and chitosan are easily processed into gels, membranes, nanofibers, beads, microparticles, nanoparticles, scaffolds and sponges [19]. The present investigation aims to synthesize chitosan nanoparticles by ionic gelation of chitosan and tripolyphosphate and evaluates its sorption capacity for Cr (VI) by varying the operational factors (pH of solution, agitation time, ion concentration and adsorbent concentration) that are responsible for adsorption. This is the first report on sorption property of chitosan nanorod in the conversion of Cr (IV) to Cr (III). Fro this, the equilibrium data were fit to the Langmuir, the Freundlich and the Temkin sorption isotherms. 2. Materials and methods 2.1. Chemicals and reagents Chitosan (deacetylation 92% and MW 120,000) was procured from India Sea Foods, Cochin, Kerala, India. Sodium tripolyphosphate, potassium dichromate and acetic acid of AR grade were used without any further purification. 2.2. Preparation of chitosan nanoparticles
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out on a SDT Q600 V8.0 Build 95 instrument at a heating rate of 10 ◦ C/min in nitrogen atmosphere. The weight losses at different stages were analyzed. DSC curves of the samples were obtained using a NETZSCH DSC 200PC in a Al pan, pierced lid in the N2 atmosphere at a heating rate of 10 ◦ C/min. The results were recorded and analyzed. 2.3.4. Size and morphology of chitosan nanoparticles Chitosan nanoparticles were analyzed for its particle size and distribution using Nanotrac analyzer 6Hx4Wx15D, ModelNanotrac 150 with a measuring range of 0.8–6500 nm (10−9 m). Chitosan nanoparticles were cut into pieces of various sizes and wiped with a thin gold–palladium layer by a sputter coater unit (UG-microtech, UCK field, UK) and the morphology of nanoparticles was analyzed with a Cambridge stereoscan 440 scanning electron microscope (SEM, Leica, Cambridge, UK). AFM was used for visualization of chitosan nanoparticles rough nature to facilitate adsorption of heavy metals. AFM imaging was performed using CONTR, EZ-2-AFM analysis. 2.4. Sorption experiments Sorption experiments were conducted in 250 ml conical flasks containing 100 ml of various concentrations of Cr (VI) solution using accurately weighed chitosan nanoparticles. The flasks were agitated in an orbit shaker at room temperature. The effect of contact time on the sorption capacity of sorbent was studied in the range 1–5 h at an initial Cr concentration of 200 mg/L. Effect of initial pH on the sorption capacity of sorbent for Cr (VI) was studied by varying solution pH from 3 to 9 at the sorbent dosage of 1 g/100 ml for 1 h contact time using 200 mg/L initial Cr concentration. The solution pH was adjusted with dilute HCl or NaOH solution. The effect of sorbent dosage was studied from 1 g to 5 g for 1 h contact time. Adsorption isotherms were studied at different initial Cr concentrations viz., 75, 125, 250, 500, 750, 1000 mg/L at room temperature. The data were fitted into the Langmuir, the Freundlich and the Temkin isotherm models. 2.5. Analysis of Cr (VI) ions
The method adopted is as reported by Tang et al. [35]. 20 mg chitosan was dissolved in 40 ml of 2.0% (v/v) acetic acid. 20 ml of 0.75 mg/ml sodium tripolyphosphate was dropped slowly with stirring. Chitosan nanoparticles as a suspension were collected and stored in deionised water. Supernatant was discarded and nanochitosan was air dried for further use and analysis.
The Cr sorbet adsorbent was dried and preserved. For desorption, the Cr-sorbed adsorbent was shaken with dilute hydrochloric acid for one hour and filtered. The filtrate was analyzed for the oxidation state of Cr using UV–visible spectrophotometer, Model. Hitachi U-2800 in the wavelength ranges 190–1100 nm.
2.3. Characterization of chitosan nanoparticles
3. Results and discussion
2.3.1. Fourier transform infrared spectroscopy (FT-IR) analysis FT-IR analysis of chitosan and chitosan nanoparticles were carried out in the range between 4000 and 375 cm−1 using Thermo Nicolet AVATAR 300 FTIR spectrometer using KBr pellet method.
3.1. General discussion
2.3.2. X-ray diffraction (XRD) analysis In X-ray diffraction technique (XRD), X-ray diffraction profiles of chitosan and chitosan nanoparticles were recorded by Bruker, Germany powder X-ray diffractometer, model D8 Advance, source 2.2 kW Cu anode, Ceramic X-ray tube. The relative intensities were recorded within the range of 10–90◦ (2) at a scanning rate of 5◦ min−1 . 2.3.3. Thermogravimetirc (TGA) and differential thermo gravimetric (DTG) analysis Thermogravimetric (TG) analysis and differential thermogravimetry of chitosan and chitosan nanoparticles were carried
Chitosan, a biopolymer is biodegradable, biocompatible, nontoxic, economic and having great potential over wide range of application, which was due to the presence of OH and NH2 groups. Being a good adsorbent for the removal of heavy metals from wastewater, it also has some disadvantages. Poor mechanical properties and solubility in low pH limit its application. To overcome these limitations chitosan needs modification. We modified chitosan into chitosan nanoparticles with rod shape by ionic gelation method, which have more advantages over chitosan. Due to this modification to nano size the amorphous nature increases and acts as a better adsorbent. (A) Chitosan solution, (B) chitosan film, (C and D) chromium sorbed nanochitosan wet condition and dried condition have been shown in Fig. 1(I). It was found that chitosan films are present in the form of off white color, whereas, the treated chitosan are found in
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Fig. 1. (I) (A) Chitosan solution, (B) chitosan film, (C and D) chromium sorbed nanochitosan wet condition and dried condition. (II) (A) Chitosan flakes, (B) nanochitosan emulsion, (C) nanochitosan suspension after 1 day and (D) nanochitosan film.
the form of yellow color. It implies that chitosan nanorods significantly removed the Cr from the wastewater. It is not only removed the Cr from wastewater and it is also made the conversion of Cr (IV) into Cr (III). It is already explained about toxicity issue of Cr valancy states. In addition, (A) chitosan flakes, (B) nanochitosan emulsion, (C) nanochitosan suspension after 1 day and (D) nanochitosan film are depicted in Fig. 1(II). 3.2. FT-IR spectroscopy analysis For chitosan and chitosan nanorod, the spectrum showed the typical characteristic absorption bands closer of chitosan as reported earlier [20]; which are carbonyl group (C O) at 1740 cm−1 , C H stretching of rock at 1421 cm−1 The peaks at 3454 cm−1 in chitosan and 3455 cm−1 in chitosan nanoparticles are due to the N H and OH stretching vibrations. The FT-IR spectrum of (A) chitosan and (B) chitosan nanorods are shown in Fig. 2. Peak at 1540 cm−1 is assigned to N-H bending vibration of amide II and the peak at 1322 cm−1 is characteristic of absorption of amide III, The peak at 1384 cm−1 is assigned to the–CH3 symmetrical deformation mode (scissoring) in amide group. The bands assigned
to the stretching vibrations of C O C linkages in the polysaccharide structure appear at 1151 cm−1 , 1098 cm−1 and 1021 cm−1 and 1151 cm−1 is the antisymmetric stretching of C O C bridge, 1098 cm−1 and 1021 cm−1 are the skeletal stretching vibrations of C O, characteristic peaks of polysaccharide structure in chitosan. In Fig. 2(B), the sharp peak at 1628 cm−1 in chitosan nanoparticles is assigned to amide I since chitosan used was not 100% deacetylated. The appearance of a new peak at 1268 cm−1 indicating P O stretching which confirms the crosslinking of negatively charged tripolyphosphate and positively charged chitosan. Cr ions are sorbed on chitosan nanoparticles. As can be seen from the IR spectrum of Cr sorbed chitosan nanoparticles, the peak indicating P O stretching at 1268 cm−1 disappeared due to the ionic cross linking between Cr ions and phosphoric groups. The peaks at 1639 cm−1 and 1540 cm−1 assigned to CONH2 and N H respectively are sharper in the spectrum of Cr-sorbed nanoparticles. Thus chitosan nanoparticles provide more sorption sites for Cr ions along with the amino and hydroxyl groups by introducing phosphoric groups due to the cross linking with tripolyphosphate [21].
3.3. Thermal properties of chitosan nanoparticles
Fig. 2. FT-IR spectra of (A) chitosan and (B) chitosan nano particles.
Fig. 3(A, B) and (C, D) shows the TGA and DTG curves of chitosan and chitosan nanoparticles respectively which reveals the weight loss of the material as it is heated. From TGA analysis of chitosan (Fig. 3(A)), two weight losses are observed as reported [22]. The first stage of weight loss starts between 50 and 150 ◦ C which is attributed to the removal of water absorbed and the second stage of weight loss between 200 and 450 ◦ C is due to the thermal degradation of chitosan. The TGA curve of chitosan nanoparticles (Fig. 3(B)) also shows two significant weight losses, one weight loss at 50–150 ◦ C is due to the moisture vaporization and the other at 200–350 ◦ C is due to the thermal degradation of chitosan nanoparticles. Differential thermogravimetry (DTG) curves of chitosan and its nanoparticles have been shown in Fig. 3(C) and (D), have a peak at 312 ◦ C for chitosan corresponding to its decomposition and at 218 ◦ C for chitosan nanoparticles indicating its crosslinking breakage. Chitosan has a mass loss of 38.35% between 200 and 600 ◦ C and a maximum weight loss of 70% at 840 ◦ C [23]. Chitosan nanoparticles have a mass loss of 33.65% between 209 and 250 ◦ C and a maximum weight loss of only 45% at 788 ◦ C indicating the stability of the polysaccharide structure. This reveals that chitosan nanoparticles are thermally stable over a temperature up to 800 ◦ C. This may be due to high crosslinking of the chitosan nanoparticles.
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Fig. 3. TGA curves of (A) chitosan, (B) chitosan nanoparticles and DSC curves of (C) chitosan and (D) chitosan nanoparticles.
Fig. 3(C) shows a wide endothermic peak at 89 ◦ C which is attributed to the elimination of absorbed water and a sharp exothermic peak at 312 ◦ C which is due to the decomposition of chitosan chains. The DSC curve of chitosan nanoparticles (Fig. 3(D)) has a wide enothermic peak below 80 ◦ C which is due to the removal of absorbed water and a sharp endothermic peak at 217 ◦ C associated with the breakage of chitosan phosphoric acid cross linkage. The decomposition of chitosan nanoparticles is expected to happen well above 300 ◦ C as evidenced from TGA studies. Decreased crystallinity indicates change in solid state structure of chitosan due to crosslinking [24].
3.4. Crystallinity of chitosan nanoparticles Fig. 4(B) shows the X-ray diffraction patterns of chitosan nanoparticles. Chitosan gives two characteristic peaks at 2 = 10◦ and 20◦ (DATA NOT SHOWN). XRD of chitosan nanoparticles showed two peaks at 2 = 17◦ and 25◦ indicating a shift from the normal chitosan peaks a broad peak around 20◦ showing its increased amorphous nature, thus decreasing the crystal structure of chitosan after crosslinking with sodium tripolyphosphate. The crystallinity of chitosan is a key-parameter in the accessibility to internal sites for both water and metal ions. Studies show that decreasing the crystallinity results in an improvement in metal ion sorption properties [25].
3.5. Size and morphology of chitosan nanoparticles The ionotropic gelation method is commonly used to prepare chitosan nanoparticles. During the process the NH2 group of the chitosan molecule is protonated to form NH3 + . We prepared chitosan nanoparticles based on the ionic gelation interactions between positively charged amino groups on chitosan and negative tripolyphosphate ions at room temperature through either intermolecular or intramolecular linkages of the anionic counter ions [26]. Fig. 4(C) shows the SEM images of chitosan nanoparticles in the nanorod form. Chitosan nanoparticles were analyzed for its particle size and distribution using Nanotrac analyzer. Findings show that the minimum particles size of chitosan nanoparticles was around 100 nm and distribution in the range of 100–400 nm. Fig. 4(D) shows the AFM topography of chitosan nanoparticles indicating the rough nature of the surface, due to the rod shaped nanochitosan. 3.6. Sorption studies of Cr (VI) 3.6.1. Influence of nanoparticles amount The amount of biosorbent was one of the important parameter that strongly affects the biosorption capacity. The dependence of Cr (VI) sorption on nanochitosan was studied by varying the amount of the adsorbent from 1 g to 5 g while keeping the other parameters such as pH, metal solution volume (100 ml), concentration (200 mg/L), and contact time (60 min) constant. Fig. 5(A) shows
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Fig. 4. (A) UV–vis spectrum of nanochitosan, (B) X-ray diffraction spectrum of nanochitosan and surface topography of chitosan nanoparticles, (C) scanning electron microscopy and (D) atomic force microscopy.
that the removal percentage increases with increasing adsorbent dose from 61% to 75%. However, the uptake capacity of metal ion per unit mass of biosorbent (mg/g) decreases with increase in dose of adsorbent, which may be due to lower utilization of adsorption sites of the sorbent at higher dosage [27].
3.6.2. Influence of pH The effect of pH on the adsorption of Cr is presented in Fig. 5(B). The pH of the aqueous solution is an important operational parameter in the adsorption process because it affects the solubility of the metal ions, concentration of the counter ions on the functional groups of the adsorbent and the degree of ionization of the adsorbate during reaction [1]. The active sites on an adsorbent can either be protonated or deprotonated depending on the pH while at the same time the adsorbate speciation in solution depends on the pH too. At lower pH, low metal ion uptake is observed due to the competitive adsorption of the H+ and Cr (VI) ions on the nanochitosan surface. At low pH values, the adsorbent is positively charged with higher H+ ion concentration, reducing the number of binding sites for metal ion. In addition, the protonation of amino groups in acidic solution induces an electrostatic repulsion of metal cations that reduces the number of binding sites available for metallic ions [28]. However, Cr (VI) uptake increases as the pH increases, while most active sites on the adsorbent are de-protonated resulting to more net attractive force which is responsible for high chromium removal from aqueous solution. The optimum adsorption takes place at pH 5. Further increase in pH leads to the precipitation of chromium hydroxide complexes which inhibits the adsorption process.
3.6.3. Influence of contact time Contact time is one of the effective factors in batch adsorption process. Keeping all of the parameters except contact time, including temperature (25 ◦ C), adsorbent dose (1 g/100 ml), pH (5), initial chromium concentration (200 mg/L) and agitation speed (250 rpm), were kept constant, the adsorption of chromium on nanochitosan was studied. The effect of contact time on chromium adsorption efficiency is shown in Fig. 5(C). As it is shown, adsorption rate initially increased rapidly, and the optimal removal efficiency was reached within about 120 min to 69%. There was no significant change in equilibrium concentration after 180–300 min, the adsorption phase reached equilibrium. Faster initial removal percent rate is possibly due to the availability of sufficient vacant adsorbing sites in presence of higher chromium ion concentration. Afterwards, the removal percent rate decreased significantly due to availability of limited vacant adsorption sites. 3.6.4. Influence of initial concentration The experimental results of the effect of initial chromium concentration on removal efficiency were presented in Fig. 5(D). The experiment was conducted using the volumes of solutions as 100 ml, initial concentration of metal as 75, 125, 250, 500, 750 and 1000 mg/L solution of Cr (VI) in conical flasks, were gently shaken with 1 g of nanochitosan for 60 min with 250 rpm in the orbit mechanical shaker, with initial pH of the solution 5. Fig. 5(D) shows the chromium removal efficiency decreased with the increase in initial chromium concentration. In case of low chromium concentrations, the ratio of the initial number of moles of chromium ions to the available surface area of adsorbent is large and subsequently the fractional adsorption becomes independent of initial concentration. However, at higher concentrations, the available sites of adsorption become fewer, and hence the
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Fig. 5. Absorption of chromium (A) effect of adsorbent dosage, (B) effect of pH, (C) effect of contact time and (D) effect of initial concentration (P < 0.05).
percentage removal of metal ions which depends upon the initial concentration, decreases [29]. The removal percentage decreases from 82 to 41% as the concentration increases. 3.7. Sorption isotherm models Adsorption is a separation process in which some materials (adsorbate) are concentrated from a bulk vapor or liquid phase onto the surface of a porous solid (adsorbent). Several isotherm models such as Langmuir, Freundlich and Temkin model can describe the distribution of metal ions between the liquid phase and the solid phase. Absorption of chromium (I) (A) effect of initial concentration, (B), Langmuir plot for the adsorption of Cr (VI) ions onto nanochitosan, (C) Freundlich plot for the adsorption of Cr onto nanochitosan, and (D) Tempkin plot for the adsorption of Cr onto nanochitosan have been shown in Fig. 6(I). 3.7.1. Langmuir isotherm Langmuir isotherm is based on the assumption that point of valence exists on the surface of the adsorbent and that each of these sites is capable of adsorbing one molecule. Thus, the adsorbed layer will be one molecule thick. Furthermore, it is assumed that all the adsorption sites have equal affinities for molecules of the adsorbate and that the presence of adsorbed molecules at one site will not affect the adsorption of molecules at an adjacent site. The Langmuir equation is commonly written as qe =
Q0 bCe 1 + bCe
where qe is the amount adsorbed (mg/g) and Ce is the equilibrium concentration of adsorbate (mg/L), Q0 and b are the Langmuir constants related to capacity and energy of adsorption, respectively. The linear form of the Langmuir isotherm can be expressed as, 1 1 1 = + Q0 qe bQ0 Ce When 1/qe is plotted against 1/Ce , a straight line with slope 1/bQ0 is obtained which shows that the adsorption follows the Langmuir isotherm as shown in Fig. 6(II)(B). The Langmuir constants b and Q0 are calculated from the slope and intercept, which are shown in Table 1(A). The essential features of Langmuir adsorption isotherm can be expressed in terms of a dimensionless constant called separation factor or equilibrium parameter (RL ), which is defined by the following relationship [30] RL =
1 1 + bC0
where C0 is the initial Cr (VI) concentration (mg/L). The RL value indicates the shape of the isotherm to be irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 0) or unfavorable (RL > 1) [24,36]. Through the above-mentioned equation, RL value is investigated for Cr–adsorbent system. From the value of RL , it is confirmed that the adsorption of chromium from aqueous solution under the conditions was favorable.
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Fig. 6. Absorption of chromium. (I) (A) Effect of initial concentration, (B) Langmuir plot for the adsorption of Cr (VI) ions onto nanochitosan, (C) Freundlich plot for the adsorption of chromium onto nanochitosan, (D) Tempkin plot for the adsorption of chromium onto nanochitosan. (II) (A) Langmuir isotherm, (B) Freundlich isotherm, (C) Temkin isotherm and (D) pseudo-first-order kinetic model.
3.7.2. Freundlich isotherm The Freundlich model, which is indicative of surface heterogeneity of the sorbent, is given by the following linearized equation
log qe = log Kf +
1 log Ce n
where qe = amount adsorbed per unit weight of adsorbent at equilibrium (mg/g), Ce = equilibrium concentration of adsorbate in solution after adsorption (mg dm3 ), Kf = empirical Freundlich constant or capacity factor (mg/g), 1/n = Freundlich exponent. Freundlich isotherm constants of the adsorbent were calculated from the slope and intercept of the linear plot log qe versus log Ce . The values of 1/n lying between 0 and 1 confirm the favorable
conditions for adsorption. Linear plots of log Ce versus log qe confirm the applicability of Freundlich models as shown in Fig. 6(B). For Freundlich isotherm, n is equal to 1.07158. The situation n > 1 is most common and may be due to a distribution of surface sites or any factor that cause a decrease in adsorbent–adsorbate interaction with increasing surface density [31] and the values of n within the range of 2–10 represent good adsorption [32]. The regression values indicate that the adsorption data for chromium removal fitted well within the Langmuir and the Freundlich isothermal plots for the concentration studied. However, the higher correlation coefficient obtained from Freundlich model (R2 = 0.9991) compared to Langmuir plot (R2 = 0.9768) suggests the adsorption was taken place on the heterogeneous surface of the adsorbent much better than the monolayer adsorption.
Table 1 (A) RL values based on Langmuir adsorption, (B) the Langmuir isotherm and Freundlich isotherm parameters of nanochitosan, (C) Temkin parameters and (D) parameters and correlation coefficients of pseudo-first- and pseudo-second-order kinetics model. (A) Metal ion
Initial concentration, Cf
Cr (VI)
RL values
1000 750 500 250 125 75
0.2619 0.3212 0.4151 0.5867 0.7395 0.8255
(B) Metal ions
Langmuir constants 3
Cr (VI)
Freundlich constants 3
KL (dm /g) 3.932
b (dm /mg) 0.002818
Cmax (mg/g) 1395.32
2
R 0.9768
Kf 1.8247
n 1.07158
R2 0.9991
(C) Metal ion
Temkin parameters
Cr (VI)
a
b
R2
−69.55
22.15
0.9270
(D) Metal ion
Cr (VI)
Pseudo-first-order kinetic model
Experimental value
Pseudo-second-order kinetic model
qe (mg/g)
k1 (min−1 )
R2
qe (mg/g)
qe (mg/g)
k2 (g mg−1 min−1 )
R2
323.55
0.005619
0.9581
134
18.58
0.006318
0.9997
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3.7.3. Temkin isotherm Temkin and Pyzhev considered the effects of some indirect adsorbate/adsorbent interaction on adsorption isotherms and suggested that because of these interactions the heat of adsorption of all the molecules in the layer would decrease linearly with coverage [33]. The Temkin isotherm has been generally applied in the following form: qe =
RT b ln(ACe)
and can be linearized as: qe = B ln A + B ln Ce where B = RT/b, b is the Temkin constant related to heat of sorption (J/mol), A is the Temkin isotherm constant (L/g), R is the gas constant (8.314 J/mol K), and T is the absolute temperature (K). Fig. 6(II)(C) shows the linear plot of Temkin isotherm of Cr (VI) sorption on nanochitosan at room temperature. The constants A and B are calculated from the intercept and slope of the plot and are listed in Table 1(C). 3.7.4. Adsorption dynamics In order to define the adsorption kinetics of solute uptake on to the adsorbent, the kinetics parameters for the adsorption process were studied for contact times ranging from 60 to 360 min by monitoring the removal percentage of the Cr (VI). The data were then regressed against the Lagergren equation, which represents a pseudo-first-order kinetics equation [34] and against a pseudosecond-order kinetics equation log(qe − qt ) = log qe −
k1 t 2.303
1 t t = + qt qe k2 q2e where qe and qt are the amounts of metal adsorbed (mg/g) at equilibrium and at time t (min), k1 (min−1 ) and k2 (g mg−1 min−1 ) are the adsorption rate constant of pseudo-first-order, pseudo-secondorder adsorption kinetics, respectively. The values of k1 can be determined from the slope of the linear plot of log(qe − qt ) versus t, and k2 can be calculated from the slope of the linear plot t/qt versus t. The linear plots of two kinetic models are presented in Fig. 6(II)(D). The values of k1 , k2 , qe and the correlation coefficient (R2 ) from the linear plots are shown in Table 1(D). The pseudo-second-order linear plots resulted in higher R2 values than the pseudo-first-order. The values of qe (cal) from the pseudo-second-order were close to qe (exp) than that from the pseudo-first-order. These indicated the better applicability of the pseudo-second-order model. 3.8. Mechanism of chromium sorption and desorption The proposed mechanism of removal of Cr (VI) by the modified chitosan nanoparticles was adsorption which proceeds due to the electrostatic attraction between two oppositely charged ions [37], since the chromium(VI) ions in the solutions are present in the form of dichromate ions which are negatively charged [38] and chitosan nanoparticles having an overall positive surface charge [39]. The chromium (VI) ions adsorbed on to chitosan nanoparticles were desorbed using 0.5 M HCl as a desorption agent at room temperature for 1 h. The resulting chromium solution was analyzed for its oxidation state using UV spectrometer. The appearance of three peaks indicates the presence of chromium in the oxidation state (III). It was detected from the results (Fig. 1(I)) that chitosan nanoparticles reduced Cr (VI) to Cr (III) thus favoring non-toxicity.
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4. Conclusion Chitosan nanoparticles have been prepared by ionic gelation of chitosan and tripolyphosphate. FT-IR spectra revealed the functional groups of chitosan nanoparticles provide sorption, sites for Cr ions. Chitosan nanoparticles possess lower crystallinity than chitosan as it is evident from the XRD patterns. The new functional groups and low crystallinity of chitosan nanoparticles are responsible for its sorption efficiency. Thermal analysis (TGA, DTG, DSC) of chitosan nanoparticles reveal that they are thermally less stable compared to chitosan but on the other hand they are stable over a long range of temperature up to 800 ◦ C. SEM images show that chitosan nanoparticles are in the nanorod form not reported till recently. AFM topography of chitosan nanoparticles suggests roughness on the surface. The small particle size and greater surface area facilitate adsorption process. Based on the Langmuir, the Freundlich and the Temkin sorption isotherms, the sorption capacity of chitosan nanoparticles is very high and the adsorbent favors multilayer adsorption. The kinetics studies show that the adsorption follows the pseudo-second-order kinetics. The choice of chitosan nanoparticles as a biosorbent for heavy metal remediation is quite promising. Acknowledgements We would like to thank University Grants Commission (SERO), Hyderabad, India (No. F.1-2/2010-2011 (RO/SERO/MRP)) for financial assistance. This work was also supported by a grant from Marine Bioprocess Research Centre of the Marine Bio 21 Center funded by the Ministry of Land, Transport and Maritime, Republic of Korea. References [1] O. Amuda, F. Adelowo, M. Ologunde, Colloids Surface B: Biointerfaces 68 (2009) 184–192. [2] N. Saifuddin, M.P. Kumaran, Electronic Journal of Biotechnology 8 (2005) 43–53. [3] V. Gómez, M. Callao, TrAC Trends in Analytical Chemistry 25 (2006) 1006–1015. [4] A.K. Das, Coordination Chemistry Reviews 248 (2004) 81–99. [5] D. Nityanandi, C. Subbhuraam, Journal of Hazardous Materials 170 (2009) 876–882. [6] W. Ngah, S. Ab Ghani, A. Kamari, Bioresource Technology 96 (2005) 443–450. [7] B. Saha, C. Orvig, Coordination Chemistry Reviews 254 (2010) 2959–2972. [8] V. Gupta, P. Carrott, M.M.L.R. Carrott, Suhas, Critical Reviews in Environmental Science and Technology 39 (2009) 783–842. [9] V. Gupta, A. Rastogi, M. Dwivedi, D. Mohan, Separation Science and Technology 32 (1997) 2883–2912. [10] I. Ali, V. Gupta, Nature Protocols 1 (2007) 2661–2667. [11] I.S. Chang, P.K. Shin, B.H. Kim, Water Research 34 (2000) 1269–1277. [12] R. Jayakumar, D. Menon, K. Manzoor, S.V. Nair, H. Tamura, Carbohydrate Polymers 82 (2010) 227–232. [13] Y. Gad, Radiation Physics and Chemistry 77 (2008) 1101–1107. [14] R. Jayakumar, M. Prabaharan, S. Nair, S. Tokura, H. Tamura, N. Selvamurugan, Progress in Materials Science 55 (2010) 675–709. [15] R. Jayakumar, N. Nwe, S. Tokura, H. Tamura, International Journal of Biological Macromolecules 40 (2007) 175–181. [16] S. Maya, S. Indulekha, V. Sukhithasree, K. Smitha, S.V. Nair, R. Jayakumar, R. Biswas, International Journal of Biological Macromolecules 51 (2012) 392–399. [17] D. Sankar, K. Chennazhi, S.V. Nair, R. Jayakumar, Carbohydrate Polymers 90 (2012) 725–729. [18] L. Qi, Z. Xu, Colloids and Surfaces A: Physicochemical and Engineering Aspects 251 (2004) 183–190. [19] Sudha, Chitin, Chitosan, Oligosaccharides and their Derivatives, CRC Press, New York, 2010. [20] J. Venkatesan, Z.-J. Qian, B. Ryu, N. Ashok Kumar, S.-K. Kim, Carbohydrate Polymers 83 (2011) 569–577. [21] L. Qi, Z. Xu, X. Jiang, C. Hu, X. Zou, Carbohydrate Research 339 (2004) 2693–2700. [22] J. Venkatesan, Z.J. Qian, B.M. Ryu, N. Ashok Kumar, S.K. Kim, Carbohydrate Polymers 83 (2011) 569–577. [23] E. Günister, D. Pestreli, C.H. Ünlü, O. Atıcı, N. Güngör, Carbohydrate Polymers 67 (2007) 358–365. [24] H. Zhang, M. Oh, C. Allen, E. Kumacheva, Biomacromolecules 5 (2004) 2461–2468. [25] M. Jaworska, K. Kula, P. Chassary, E. Guibal, Polymer International 52 (2003) 206–212.
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Contents lists available at SciVerse ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Preparation and characterization of nano chitosan for treatment wastewaters M.S. Sivakami a , Thandapani Gomathi b , Jayachandran Venkatesan c , Hee-Seok Jeong c , Se-Kwon Kim c,∗ , P.N. Sudha a,b,∗∗ a
Department of Chemistry, Government Thirumagal Mills College, Gudiyattam, Tamil Nadu, India PG and Research Department of Chemistry, DKM College for Women, Vellore, Tamil Nadu, India c Department of Chemistry and Marine Bioprocess Research Center, Pukyong National University, Busan 608-737, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 27 December 2012 Received in revised form 22 January 2013 Accepted 2 March 2013 Available online 13 March 2013 Keywords: Chitosan Marine polymers Chromium (VI) Sorption isotherms Industrial waste
a b s t r a c t Chitosan nanorod with minimum particle size of <100 nm was prepared by crosslinking low molecular weight chitosan with polyanion sodium tripolyphosphate and physicochemically characterized (FT-IR, XRD, SEM, AFM, TGA and DSC) for waste water treatment. Its sorption capacity and sorption isotherms for chromium (Cr) were studied. The effect of initial concentration of Cr ions, sorbent amount, agitation period and pH of solution that influence sorption capacity were also investigated. It was found that nanochitosan in the solid state was rod shaped which could sorb Cr (VI) to Cr (III) ions effectively. Based on the Langmuir, the Freundlich and the Temkin sorption isotherms, the sorption capacity of chitosan nanoparticles is very high and the adsorbent favors multilayer adsorption. The kinetics studies show that the adsorption follows the pseudo-second-order kinetics, which infers the transformation of Cr (VI) to Cr (III). From the results it was concluded that nanochitosan is an excellent material as a biosorbent for Cr removal from water. © 2013 Elsevier B.V. All rights reserved.
1. Introduction 71% of the total surface of earth is water. Most of the water bodies as ponds, lakes, streams, rivers, sea and oceans, have become polluted due to population expansion, haphazard rapid urbanization, industrial and technological expansion, energy utilization and waste generation from domestic and industrial sources. The presence of heavy metals in the environment has been of great concern due to their increased discharge, toxic nature and other adverse effects on receiving waters. Chromium (Cr) and its compounds are present in the wastewater from electroplating, leather tanning, film and photography, dyes and pigments, textiles, steel fabrication and canning industries [1]. In environmental waters, Cr exists predominantly in two stable oxidation states Cr (III) and Cr (VI) and have contrasting toxicities mobility and availability [2]. Cr (III) is an essential
∗ Corresponding author at: Department of Chemistry and Marine Bioprocess Research Center, Pukyong National University, Busan 608-737, Republic of Korea. Tel.: +82 51 629 7094; fax: +82 51 629 7099. ∗∗ Corresponding author at: PG and Research Department of Chemistry, DKM College for Women, Vellore, Tamil Nadu, India. Tel.: +91 9842910157. E-mail addresses: [email protected] (M.S. Sivakami), [email protected] (S.-K. Kim), [email protected], [email protected] (P.N. Sudha). 0141-8130/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2013.03.005
component having an important role in the glucose, lipid and protein metabolism, while Cr (VI) has a definite adverse impact on living organisms. Cr (VI) is known to have 100-fold higher toxicity than Cr (III) because of its high water solubility, mobility and easy reduction [3]. The United States Environmental protection Agency has laid down the maximum contaminant level for Cr (VI) into inland surface waters as 0.1 mg/L and in domestic water supplies to be 0.05 mg/L. The toxicological effect of Cr (VI) originates from the action of this form itself as an oxidizing agent, as well as formation of free radicals during the reduction of Cr (VI) to Cr (III) occurring in the cell [4]. Research on carcinogenicity of Cr (VI) has focussed on the fact that the free radicals produced inside the cell strongly bind to DNA. Breathing and holding of Cr (VI) containing material can cause perforation of the nasal septum, asthma, bronchitis, pneumonitis, inflammation of larynx and liver and increased incidence of bronchogenic carcinoma [5]. The removal of toxic heavy metal ions from industrial effluents, water supplies and mine waters has received much attention in recent years. Several methods adopted to remove Cr from the industrial wastewater are chemical precipitation, ion exchange, reduction, electrochemical precipitation, solvent extraction, membrane separation, cementation, electrodialysis and adsorption. Among the cited methods, each method has its own limits and in recent years biosorption is recognized as an emerging technique for the treatment of wastewater containing heavy metals
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[6]. The major advantages of biosorption over conventional treatment methods include: low price, high effectiveness, minimization of chemical and/or biological mud, restoration of biosorbent and possibility of metal recovery [7–11]. Some of the biosorbents include chitosan and modified chitosan. Chitosan is a biopolymer of polyaminosaccharide, synthesized from the deacetylation of chitin, consisting of unbranched chains of -(1, 4)2-acetoamido-2-deoxy-d-glucose. Chitosan is more useful than chitin since it possesses more number of chelating amino groups and can be chemically modified [12]. Chitosan is an antibacterial, biocompatible, environment friendly, biodegradable material and has great potential for sorption of metal ions due to amino and hydroxyl groups in its chemical structure [13–17]. Compared to traditional micro-sized supports used in separation process, nano-sized adsorbents possess good performance due to high specific surface area, small size and quantum size effect that could make it exhibit higher capacities for metal ions [18]. Investigations reveal that chitin and chitosan are easily processed into gels, membranes, nanofibers, beads, microparticles, nanoparticles, scaffolds and sponges [19]. The present investigation aims to synthesize chitosan nanoparticles by ionic gelation of chitosan and tripolyphosphate and evaluates its sorption capacity for Cr (VI) by varying the operational factors (pH of solution, agitation time, ion concentration and adsorbent concentration) that are responsible for adsorption. This is the first report on sorption property of chitosan nanorod in the conversion of Cr (IV) to Cr (III). Fro this, the equilibrium data were fit to the Langmuir, the Freundlich and the Temkin sorption isotherms. 2. Materials and methods 2.1. Chemicals and reagents Chitosan (deacetylation 92% and MW 120,000) was procured from India Sea Foods, Cochin, Kerala, India. Sodium tripolyphosphate, potassium dichromate and acetic acid of AR grade were used without any further purification. 2.2. Preparation of chitosan nanoparticles
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out on a SDT Q600 V8.0 Build 95 instrument at a heating rate of 10 ◦ C/min in nitrogen atmosphere. The weight losses at different stages were analyzed. DSC curves of the samples were obtained using a NETZSCH DSC 200PC in a Al pan, pierced lid in the N2 atmosphere at a heating rate of 10 ◦ C/min. The results were recorded and analyzed. 2.3.4. Size and morphology of chitosan nanoparticles Chitosan nanoparticles were analyzed for its particle size and distribution using Nanotrac analyzer 6Hx4Wx15D, ModelNanotrac 150 with a measuring range of 0.8–6500 nm (10−9 m). Chitosan nanoparticles were cut into pieces of various sizes and wiped with a thin gold–palladium layer by a sputter coater unit (UG-microtech, UCK field, UK) and the morphology of nanoparticles was analyzed with a Cambridge stereoscan 440 scanning electron microscope (SEM, Leica, Cambridge, UK). AFM was used for visualization of chitosan nanoparticles rough nature to facilitate adsorption of heavy metals. AFM imaging was performed using CONTR, EZ-2-AFM analysis. 2.4. Sorption experiments Sorption experiments were conducted in 250 ml conical flasks containing 100 ml of various concentrations of Cr (VI) solution using accurately weighed chitosan nanoparticles. The flasks were agitated in an orbit shaker at room temperature. The effect of contact time on the sorption capacity of sorbent was studied in the range 1–5 h at an initial Cr concentration of 200 mg/L. Effect of initial pH on the sorption capacity of sorbent for Cr (VI) was studied by varying solution pH from 3 to 9 at the sorbent dosage of 1 g/100 ml for 1 h contact time using 200 mg/L initial Cr concentration. The solution pH was adjusted with dilute HCl or NaOH solution. The effect of sorbent dosage was studied from 1 g to 5 g for 1 h contact time. Adsorption isotherms were studied at different initial Cr concentrations viz., 75, 125, 250, 500, 750, 1000 mg/L at room temperature. The data were fitted into the Langmuir, the Freundlich and the Temkin isotherm models. 2.5. Analysis of Cr (VI) ions
The method adopted is as reported by Tang et al. [35]. 20 mg chitosan was dissolved in 40 ml of 2.0% (v/v) acetic acid. 20 ml of 0.75 mg/ml sodium tripolyphosphate was dropped slowly with stirring. Chitosan nanoparticles as a suspension were collected and stored in deionised water. Supernatant was discarded and nanochitosan was air dried for further use and analysis.
The Cr sorbet adsorbent was dried and preserved. For desorption, the Cr-sorbed adsorbent was shaken with dilute hydrochloric acid for one hour and filtered. The filtrate was analyzed for the oxidation state of Cr using UV–visible spectrophotometer, Model. Hitachi U-2800 in the wavelength ranges 190–1100 nm.
2.3. Characterization of chitosan nanoparticles
3. Results and discussion
2.3.1. Fourier transform infrared spectroscopy (FT-IR) analysis FT-IR analysis of chitosan and chitosan nanoparticles were carried out in the range between 4000 and 375 cm−1 using Thermo Nicolet AVATAR 300 FTIR spectrometer using KBr pellet method.
3.1. General discussion
2.3.2. X-ray diffraction (XRD) analysis In X-ray diffraction technique (XRD), X-ray diffraction profiles of chitosan and chitosan nanoparticles were recorded by Bruker, Germany powder X-ray diffractometer, model D8 Advance, source 2.2 kW Cu anode, Ceramic X-ray tube. The relative intensities were recorded within the range of 10–90◦ (2) at a scanning rate of 5◦ min−1 . 2.3.3. Thermogravimetirc (TGA) and differential thermo gravimetric (DTG) analysis Thermogravimetric (TG) analysis and differential thermogravimetry of chitosan and chitosan nanoparticles were carried
Chitosan, a biopolymer is biodegradable, biocompatible, nontoxic, economic and having great potential over wide range of application, which was due to the presence of OH and NH2 groups. Being a good adsorbent for the removal of heavy metals from wastewater, it also has some disadvantages. Poor mechanical properties and solubility in low pH limit its application. To overcome these limitations chitosan needs modification. We modified chitosan into chitosan nanoparticles with rod shape by ionic gelation method, which have more advantages over chitosan. Due to this modification to nano size the amorphous nature increases and acts as a better adsorbent. (A) Chitosan solution, (B) chitosan film, (C and D) chromium sorbed nanochitosan wet condition and dried condition have been shown in Fig. 1(I). It was found that chitosan films are present in the form of off white color, whereas, the treated chitosan are found in
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Fig. 1. (I) (A) Chitosan solution, (B) chitosan film, (C and D) chromium sorbed nanochitosan wet condition and dried condition. (II) (A) Chitosan flakes, (B) nanochitosan emulsion, (C) nanochitosan suspension after 1 day and (D) nanochitosan film.
the form of yellow color. It implies that chitosan nanorods significantly removed the Cr from the wastewater. It is not only removed the Cr from wastewater and it is also made the conversion of Cr (IV) into Cr (III). It is already explained about toxicity issue of Cr valancy states. In addition, (A) chitosan flakes, (B) nanochitosan emulsion, (C) nanochitosan suspension after 1 day and (D) nanochitosan film are depicted in Fig. 1(II). 3.2. FT-IR spectroscopy analysis For chitosan and chitosan nanorod, the spectrum showed the typical characteristic absorption bands closer of chitosan as reported earlier [20]; which are carbonyl group (C O) at 1740 cm−1 , C H stretching of rock at 1421 cm−1 The peaks at 3454 cm−1 in chitosan and 3455 cm−1 in chitosan nanoparticles are due to the N H and OH stretching vibrations. The FT-IR spectrum of (A) chitosan and (B) chitosan nanorods are shown in Fig. 2. Peak at 1540 cm−1 is assigned to N-H bending vibration of amide II and the peak at 1322 cm−1 is characteristic of absorption of amide III, The peak at 1384 cm−1 is assigned to the–CH3 symmetrical deformation mode (scissoring) in amide group. The bands assigned
to the stretching vibrations of C O C linkages in the polysaccharide structure appear at 1151 cm−1 , 1098 cm−1 and 1021 cm−1 and 1151 cm−1 is the antisymmetric stretching of C O C bridge, 1098 cm−1 and 1021 cm−1 are the skeletal stretching vibrations of C O, characteristic peaks of polysaccharide structure in chitosan. In Fig. 2(B), the sharp peak at 1628 cm−1 in chitosan nanoparticles is assigned to amide I since chitosan used was not 100% deacetylated. The appearance of a new peak at 1268 cm−1 indicating P O stretching which confirms the crosslinking of negatively charged tripolyphosphate and positively charged chitosan. Cr ions are sorbed on chitosan nanoparticles. As can be seen from the IR spectrum of Cr sorbed chitosan nanoparticles, the peak indicating P O stretching at 1268 cm−1 disappeared due to the ionic cross linking between Cr ions and phosphoric groups. The peaks at 1639 cm−1 and 1540 cm−1 assigned to CONH2 and N H respectively are sharper in the spectrum of Cr-sorbed nanoparticles. Thus chitosan nanoparticles provide more sorption sites for Cr ions along with the amino and hydroxyl groups by introducing phosphoric groups due to the cross linking with tripolyphosphate [21].
3.3. Thermal properties of chitosan nanoparticles
Fig. 2. FT-IR spectra of (A) chitosan and (B) chitosan nano particles.
Fig. 3(A, B) and (C, D) shows the TGA and DTG curves of chitosan and chitosan nanoparticles respectively which reveals the weight loss of the material as it is heated. From TGA analysis of chitosan (Fig. 3(A)), two weight losses are observed as reported [22]. The first stage of weight loss starts between 50 and 150 ◦ C which is attributed to the removal of water absorbed and the second stage of weight loss between 200 and 450 ◦ C is due to the thermal degradation of chitosan. The TGA curve of chitosan nanoparticles (Fig. 3(B)) also shows two significant weight losses, one weight loss at 50–150 ◦ C is due to the moisture vaporization and the other at 200–350 ◦ C is due to the thermal degradation of chitosan nanoparticles. Differential thermogravimetry (DTG) curves of chitosan and its nanoparticles have been shown in Fig. 3(C) and (D), have a peak at 312 ◦ C for chitosan corresponding to its decomposition and at 218 ◦ C for chitosan nanoparticles indicating its crosslinking breakage. Chitosan has a mass loss of 38.35% between 200 and 600 ◦ C and a maximum weight loss of 70% at 840 ◦ C [23]. Chitosan nanoparticles have a mass loss of 33.65% between 209 and 250 ◦ C and a maximum weight loss of only 45% at 788 ◦ C indicating the stability of the polysaccharide structure. This reveals that chitosan nanoparticles are thermally stable over a temperature up to 800 ◦ C. This may be due to high crosslinking of the chitosan nanoparticles.
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Fig. 3. TGA curves of (A) chitosan, (B) chitosan nanoparticles and DSC curves of (C) chitosan and (D) chitosan nanoparticles.
Fig. 3(C) shows a wide endothermic peak at 89 ◦ C which is attributed to the elimination of absorbed water and a sharp exothermic peak at 312 ◦ C which is due to the decomposition of chitosan chains. The DSC curve of chitosan nanoparticles (Fig. 3(D)) has a wide enothermic peak below 80 ◦ C which is due to the removal of absorbed water and a sharp endothermic peak at 217 ◦ C associated with the breakage of chitosan phosphoric acid cross linkage. The decomposition of chitosan nanoparticles is expected to happen well above 300 ◦ C as evidenced from TGA studies. Decreased crystallinity indicates change in solid state structure of chitosan due to crosslinking [24].
3.4. Crystallinity of chitosan nanoparticles Fig. 4(B) shows the X-ray diffraction patterns of chitosan nanoparticles. Chitosan gives two characteristic peaks at 2 = 10◦ and 20◦ (DATA NOT SHOWN). XRD of chitosan nanoparticles showed two peaks at 2 = 17◦ and 25◦ indicating a shift from the normal chitosan peaks a broad peak around 20◦ showing its increased amorphous nature, thus decreasing the crystal structure of chitosan after crosslinking with sodium tripolyphosphate. The crystallinity of chitosan is a key-parameter in the accessibility to internal sites for both water and metal ions. Studies show that decreasing the crystallinity results in an improvement in metal ion sorption properties [25].
3.5. Size and morphology of chitosan nanoparticles The ionotropic gelation method is commonly used to prepare chitosan nanoparticles. During the process the NH2 group of the chitosan molecule is protonated to form NH3 + . We prepared chitosan nanoparticles based on the ionic gelation interactions between positively charged amino groups on chitosan and negative tripolyphosphate ions at room temperature through either intermolecular or intramolecular linkages of the anionic counter ions [26]. Fig. 4(C) shows the SEM images of chitosan nanoparticles in the nanorod form. Chitosan nanoparticles were analyzed for its particle size and distribution using Nanotrac analyzer. Findings show that the minimum particles size of chitosan nanoparticles was around 100 nm and distribution in the range of 100–400 nm. Fig. 4(D) shows the AFM topography of chitosan nanoparticles indicating the rough nature of the surface, due to the rod shaped nanochitosan. 3.6. Sorption studies of Cr (VI) 3.6.1. Influence of nanoparticles amount The amount of biosorbent was one of the important parameter that strongly affects the biosorption capacity. The dependence of Cr (VI) sorption on nanochitosan was studied by varying the amount of the adsorbent from 1 g to 5 g while keeping the other parameters such as pH, metal solution volume (100 ml), concentration (200 mg/L), and contact time (60 min) constant. Fig. 5(A) shows
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Fig. 4. (A) UV–vis spectrum of nanochitosan, (B) X-ray diffraction spectrum of nanochitosan and surface topography of chitosan nanoparticles, (C) scanning electron microscopy and (D) atomic force microscopy.
that the removal percentage increases with increasing adsorbent dose from 61% to 75%. However, the uptake capacity of metal ion per unit mass of biosorbent (mg/g) decreases with increase in dose of adsorbent, which may be due to lower utilization of adsorption sites of the sorbent at higher dosage [27].
3.6.2. Influence of pH The effect of pH on the adsorption of Cr is presented in Fig. 5(B). The pH of the aqueous solution is an important operational parameter in the adsorption process because it affects the solubility of the metal ions, concentration of the counter ions on the functional groups of the adsorbent and the degree of ionization of the adsorbate during reaction [1]. The active sites on an adsorbent can either be protonated or deprotonated depending on the pH while at the same time the adsorbate speciation in solution depends on the pH too. At lower pH, low metal ion uptake is observed due to the competitive adsorption of the H+ and Cr (VI) ions on the nanochitosan surface. At low pH values, the adsorbent is positively charged with higher H+ ion concentration, reducing the number of binding sites for metal ion. In addition, the protonation of amino groups in acidic solution induces an electrostatic repulsion of metal cations that reduces the number of binding sites available for metallic ions [28]. However, Cr (VI) uptake increases as the pH increases, while most active sites on the adsorbent are de-protonated resulting to more net attractive force which is responsible for high chromium removal from aqueous solution. The optimum adsorption takes place at pH 5. Further increase in pH leads to the precipitation of chromium hydroxide complexes which inhibits the adsorption process.
3.6.3. Influence of contact time Contact time is one of the effective factors in batch adsorption process. Keeping all of the parameters except contact time, including temperature (25 ◦ C), adsorbent dose (1 g/100 ml), pH (5), initial chromium concentration (200 mg/L) and agitation speed (250 rpm), were kept constant, the adsorption of chromium on nanochitosan was studied. The effect of contact time on chromium adsorption efficiency is shown in Fig. 5(C). As it is shown, adsorption rate initially increased rapidly, and the optimal removal efficiency was reached within about 120 min to 69%. There was no significant change in equilibrium concentration after 180–300 min, the adsorption phase reached equilibrium. Faster initial removal percent rate is possibly due to the availability of sufficient vacant adsorbing sites in presence of higher chromium ion concentration. Afterwards, the removal percent rate decreased significantly due to availability of limited vacant adsorption sites. 3.6.4. Influence of initial concentration The experimental results of the effect of initial chromium concentration on removal efficiency were presented in Fig. 5(D). The experiment was conducted using the volumes of solutions as 100 ml, initial concentration of metal as 75, 125, 250, 500, 750 and 1000 mg/L solution of Cr (VI) in conical flasks, were gently shaken with 1 g of nanochitosan for 60 min with 250 rpm in the orbit mechanical shaker, with initial pH of the solution 5. Fig. 5(D) shows the chromium removal efficiency decreased with the increase in initial chromium concentration. In case of low chromium concentrations, the ratio of the initial number of moles of chromium ions to the available surface area of adsorbent is large and subsequently the fractional adsorption becomes independent of initial concentration. However, at higher concentrations, the available sites of adsorption become fewer, and hence the
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Fig. 5. Absorption of chromium (A) effect of adsorbent dosage, (B) effect of pH, (C) effect of contact time and (D) effect of initial concentration (P < 0.05).
percentage removal of metal ions which depends upon the initial concentration, decreases [29]. The removal percentage decreases from 82 to 41% as the concentration increases. 3.7. Sorption isotherm models Adsorption is a separation process in which some materials (adsorbate) are concentrated from a bulk vapor or liquid phase onto the surface of a porous solid (adsorbent). Several isotherm models such as Langmuir, Freundlich and Temkin model can describe the distribution of metal ions between the liquid phase and the solid phase. Absorption of chromium (I) (A) effect of initial concentration, (B), Langmuir plot for the adsorption of Cr (VI) ions onto nanochitosan, (C) Freundlich plot for the adsorption of Cr onto nanochitosan, and (D) Tempkin plot for the adsorption of Cr onto nanochitosan have been shown in Fig. 6(I). 3.7.1. Langmuir isotherm Langmuir isotherm is based on the assumption that point of valence exists on the surface of the adsorbent and that each of these sites is capable of adsorbing one molecule. Thus, the adsorbed layer will be one molecule thick. Furthermore, it is assumed that all the adsorption sites have equal affinities for molecules of the adsorbate and that the presence of adsorbed molecules at one site will not affect the adsorption of molecules at an adjacent site. The Langmuir equation is commonly written as qe =
Q0 bCe 1 + bCe
where qe is the amount adsorbed (mg/g) and Ce is the equilibrium concentration of adsorbate (mg/L), Q0 and b are the Langmuir constants related to capacity and energy of adsorption, respectively. The linear form of the Langmuir isotherm can be expressed as, 1 1 1 = + Q0 qe bQ0 Ce When 1/qe is plotted against 1/Ce , a straight line with slope 1/bQ0 is obtained which shows that the adsorption follows the Langmuir isotherm as shown in Fig. 6(II)(B). The Langmuir constants b and Q0 are calculated from the slope and intercept, which are shown in Table 1(A). The essential features of Langmuir adsorption isotherm can be expressed in terms of a dimensionless constant called separation factor or equilibrium parameter (RL ), which is defined by the following relationship [30] RL =
1 1 + bC0
where C0 is the initial Cr (VI) concentration (mg/L). The RL value indicates the shape of the isotherm to be irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 0) or unfavorable (RL > 1) [24,36]. Through the above-mentioned equation, RL value is investigated for Cr–adsorbent system. From the value of RL , it is confirmed that the adsorption of chromium from aqueous solution under the conditions was favorable.
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Fig. 6. Absorption of chromium. (I) (A) Effect of initial concentration, (B) Langmuir plot for the adsorption of Cr (VI) ions onto nanochitosan, (C) Freundlich plot for the adsorption of chromium onto nanochitosan, (D) Tempkin plot for the adsorption of chromium onto nanochitosan. (II) (A) Langmuir isotherm, (B) Freundlich isotherm, (C) Temkin isotherm and (D) pseudo-first-order kinetic model.
3.7.2. Freundlich isotherm The Freundlich model, which is indicative of surface heterogeneity of the sorbent, is given by the following linearized equation
log qe = log Kf +
1 log Ce n
where qe = amount adsorbed per unit weight of adsorbent at equilibrium (mg/g), Ce = equilibrium concentration of adsorbate in solution after adsorption (mg dm3 ), Kf = empirical Freundlich constant or capacity factor (mg/g), 1/n = Freundlich exponent. Freundlich isotherm constants of the adsorbent were calculated from the slope and intercept of the linear plot log qe versus log Ce . The values of 1/n lying between 0 and 1 confirm the favorable
conditions for adsorption. Linear plots of log Ce versus log qe confirm the applicability of Freundlich models as shown in Fig. 6(B). For Freundlich isotherm, n is equal to 1.07158. The situation n > 1 is most common and may be due to a distribution of surface sites or any factor that cause a decrease in adsorbent–adsorbate interaction with increasing surface density [31] and the values of n within the range of 2–10 represent good adsorption [32]. The regression values indicate that the adsorption data for chromium removal fitted well within the Langmuir and the Freundlich isothermal plots for the concentration studied. However, the higher correlation coefficient obtained from Freundlich model (R2 = 0.9991) compared to Langmuir plot (R2 = 0.9768) suggests the adsorption was taken place on the heterogeneous surface of the adsorbent much better than the monolayer adsorption.
Table 1 (A) RL values based on Langmuir adsorption, (B) the Langmuir isotherm and Freundlich isotherm parameters of nanochitosan, (C) Temkin parameters and (D) parameters and correlation coefficients of pseudo-first- and pseudo-second-order kinetics model. (A) Metal ion
Initial concentration, Cf
Cr (VI)
RL values
1000 750 500 250 125 75
0.2619 0.3212 0.4151 0.5867 0.7395 0.8255
(B) Metal ions
Langmuir constants 3
Cr (VI)
Freundlich constants 3
KL (dm /g) 3.932
b (dm /mg) 0.002818
Cmax (mg/g) 1395.32
2
R 0.9768
Kf 1.8247
n 1.07158
R2 0.9991
(C) Metal ion
Temkin parameters
Cr (VI)
a
b
R2
−69.55
22.15
0.9270
(D) Metal ion
Cr (VI)
Pseudo-first-order kinetic model
Experimental value
Pseudo-second-order kinetic model
qe (mg/g)
k1 (min−1 )
R2
qe (mg/g)
qe (mg/g)
k2 (g mg−1 min−1 )
R2
323.55
0.005619
0.9581
134
18.58
0.006318
0.9997
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3.7.3. Temkin isotherm Temkin and Pyzhev considered the effects of some indirect adsorbate/adsorbent interaction on adsorption isotherms and suggested that because of these interactions the heat of adsorption of all the molecules in the layer would decrease linearly with coverage [33]. The Temkin isotherm has been generally applied in the following form: qe =
RT b ln(ACe)
and can be linearized as: qe = B ln A + B ln Ce where B = RT/b, b is the Temkin constant related to heat of sorption (J/mol), A is the Temkin isotherm constant (L/g), R is the gas constant (8.314 J/mol K), and T is the absolute temperature (K). Fig. 6(II)(C) shows the linear plot of Temkin isotherm of Cr (VI) sorption on nanochitosan at room temperature. The constants A and B are calculated from the intercept and slope of the plot and are listed in Table 1(C). 3.7.4. Adsorption dynamics In order to define the adsorption kinetics of solute uptake on to the adsorbent, the kinetics parameters for the adsorption process were studied for contact times ranging from 60 to 360 min by monitoring the removal percentage of the Cr (VI). The data were then regressed against the Lagergren equation, which represents a pseudo-first-order kinetics equation [34] and against a pseudosecond-order kinetics equation log(qe − qt ) = log qe −
k1 t 2.303
1 t t = + qt qe k2 q2e where qe and qt are the amounts of metal adsorbed (mg/g) at equilibrium and at time t (min), k1 (min−1 ) and k2 (g mg−1 min−1 ) are the adsorption rate constant of pseudo-first-order, pseudo-secondorder adsorption kinetics, respectively. The values of k1 can be determined from the slope of the linear plot of log(qe − qt ) versus t, and k2 can be calculated from the slope of the linear plot t/qt versus t. The linear plots of two kinetic models are presented in Fig. 6(II)(D). The values of k1 , k2 , qe and the correlation coefficient (R2 ) from the linear plots are shown in Table 1(D). The pseudo-second-order linear plots resulted in higher R2 values than the pseudo-first-order. The values of qe (cal) from the pseudo-second-order were close to qe (exp) than that from the pseudo-first-order. These indicated the better applicability of the pseudo-second-order model. 3.8. Mechanism of chromium sorption and desorption The proposed mechanism of removal of Cr (VI) by the modified chitosan nanoparticles was adsorption which proceeds due to the electrostatic attraction between two oppositely charged ions [37], since the chromium(VI) ions in the solutions are present in the form of dichromate ions which are negatively charged [38] and chitosan nanoparticles having an overall positive surface charge [39]. The chromium (VI) ions adsorbed on to chitosan nanoparticles were desorbed using 0.5 M HCl as a desorption agent at room temperature for 1 h. The resulting chromium solution was analyzed for its oxidation state using UV spectrometer. The appearance of three peaks indicates the presence of chromium in the oxidation state (III). It was detected from the results (Fig. 1(I)) that chitosan nanoparticles reduced Cr (VI) to Cr (III) thus favoring non-toxicity.
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4. Conclusion Chitosan nanoparticles have been prepared by ionic gelation of chitosan and tripolyphosphate. FT-IR spectra revealed the functional groups of chitosan nanoparticles provide sorption, sites for Cr ions. Chitosan nanoparticles possess lower crystallinity than chitosan as it is evident from the XRD patterns. The new functional groups and low crystallinity of chitosan nanoparticles are responsible for its sorption efficiency. Thermal analysis (TGA, DTG, DSC) of chitosan nanoparticles reveal that they are thermally less stable compared to chitosan but on the other hand they are stable over a long range of temperature up to 800 ◦ C. SEM images show that chitosan nanoparticles are in the nanorod form not reported till recently. AFM topography of chitosan nanoparticles suggests roughness on the surface. The small particle size and greater surface area facilitate adsorption process. Based on the Langmuir, the Freundlich and the Temkin sorption isotherms, the sorption capacity of chitosan nanoparticles is very high and the adsorbent favors multilayer adsorption. The kinetics studies show that the adsorption follows the pseudo-second-order kinetics. The choice of chitosan nanoparticles as a biosorbent for heavy metal remediation is quite promising. Acknowledgements We would like to thank University Grants Commission (SERO), Hyderabad, India (No. F.1-2/2010-2011 (RO/SERO/MRP)) for financial assistance. This work was also supported by a grant from Marine Bioprocess Research Centre of the Marine Bio 21 Center funded by the Ministry of Land, Transport and Maritime, Republic of Korea. References [1] O. Amuda, F. Adelowo, M. Ologunde, Colloids Surface B: Biointerfaces 68 (2009) 184–192. [2] N. Saifuddin, M.P. Kumaran, Electronic Journal of Biotechnology 8 (2005) 43–53. [3] V. Gómez, M. Callao, TrAC Trends in Analytical Chemistry 25 (2006) 1006–1015. [4] A.K. Das, Coordination Chemistry Reviews 248 (2004) 81–99. [5] D. Nityanandi, C. Subbhuraam, Journal of Hazardous Materials 170 (2009) 876–882. [6] W. Ngah, S. Ab Ghani, A. Kamari, Bioresource Technology 96 (2005) 443–450. [7] B. Saha, C. Orvig, Coordination Chemistry Reviews 254 (2010) 2959–2972. [8] V. Gupta, P. Carrott, M.M.L.R. Carrott, Suhas, Critical Reviews in Environmental Science and Technology 39 (2009) 783–842. [9] V. Gupta, A. Rastogi, M. Dwivedi, D. Mohan, Separation Science and Technology 32 (1997) 2883–2912. [10] I. Ali, V. Gupta, Nature Protocols 1 (2007) 2661–2667. [11] I.S. Chang, P.K. Shin, B.H. Kim, Water Research 34 (2000) 1269–1277. [12] R. Jayakumar, D. Menon, K. Manzoor, S.V. Nair, H. Tamura, Carbohydrate Polymers 82 (2010) 227–232. [13] Y. Gad, Radiation Physics and Chemistry 77 (2008) 1101–1107. [14] R. Jayakumar, M. Prabaharan, S. Nair, S. Tokura, H. Tamura, N. Selvamurugan, Progress in Materials Science 55 (2010) 675–709. [15] R. Jayakumar, N. Nwe, S. Tokura, H. Tamura, International Journal of Biological Macromolecules 40 (2007) 175–181. [16] S. Maya, S. Indulekha, V. Sukhithasree, K. Smitha, S.V. Nair, R. Jayakumar, R. Biswas, International Journal of Biological Macromolecules 51 (2012) 392–399. [17] D. Sankar, K. Chennazhi, S.V. Nair, R. Jayakumar, Carbohydrate Polymers 90 (2012) 725–729. [18] L. Qi, Z. Xu, Colloids and Surfaces A: Physicochemical and Engineering Aspects 251 (2004) 183–190. [19] Sudha, Chitin, Chitosan, Oligosaccharides and their Derivatives, CRC Press, New York, 2010. [20] J. Venkatesan, Z.-J. Qian, B. Ryu, N. Ashok Kumar, S.-K. Kim, Carbohydrate Polymers 83 (2011) 569–577. [21] L. Qi, Z. Xu, X. Jiang, C. Hu, X. Zou, Carbohydrate Research 339 (2004) 2693–2700. [22] J. Venkatesan, Z.J. Qian, B.M. Ryu, N. Ashok Kumar, S.K. Kim, Carbohydrate Polymers 83 (2011) 569–577. [23] E. Günister, D. Pestreli, C.H. Ünlü, O. Atıcı, N. Güngör, Carbohydrate Polymers 67 (2007) 358–365. [24] H. Zhang, M. Oh, C. Allen, E. Kumacheva, Biomacromolecules 5 (2004) 2461–2468. [25] M. Jaworska, K. Kula, P. Chassary, E. Guibal, Polymer International 52 (2003) 206–212.
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