International Journal of Biological Macromolecules 67 (2014) 180–188
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Comparative studies on the removal of heavy metals ions onto cross linked chitosan-g-acrylonitrile copolymer P. Shankar a , Thandapani Gomathi b,1 , K. Vijayalakshmi b , P.N. Sudha b,∗ a b
Department of Chemistry, Sathyabama University, Chennai, Tamilnadu, India Research Department of Chemistry, DKM College for Women, Sainathapuram, Vellore 632002, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 15 November 2013 Received in revised form 30 January 2014 Accepted 6 March 2014 Available online 25 March 2014 Keywords: Graft copolymer Chitosan Adsorption isotherms
a b s t r a c t The graft copolymerization of acrylonitrile onto cross linked chitosan was carried out using ceric ammonium nitrate as an initiator. The prepared cross linked chitosan-g-acrylonitrile copolymer was characterized using FT-IR and XRD studies. The adsorption behavior of chromium(VI), copper(II) and nickel(II) ions from aqueous solution onto cross linked chitosan graft acrylonitrile copolymer was investigated through batch method. The efficiency of the adsorbent was identified from the varying the contact time, adsorbent dose and pH. The results evident that the adsorption of metal ions increases with the increase of shaking time and metal ion concentration. An optimum pH was found to be 5.0 for both Cr(VI) and Cu(II), whereas the optimum pH is 5.5 for the adsorption of Ni(II) onto cross linked chitosan-g-acrylonitrile copolymer. The Langmuir and Freundlich adsorption models were applied to describe the isotherms and isotherm constants. Adsorption isothermal data could be well interpreted by the Freundlich model. The kinetic experimental data properly correlated with the second-order kinetic model. From the above results it was concluded that the cross linked chitosan graft acrylonitrile copolymer was found to be the efficient adsorbent for removing the heavy metals under optimum conditions. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The continuous increase of water pollution was mainly due to the progressive increase of industrial technology. For minimizing these hazardous pollutants a great effort has been done. The effort was mainly taken to avoid the dangerous effects caused by the pollutants on animals, plants and humans. Contamination by the heavy metal has become a main critical problem recently because the metals gets persisted and accumulated in the environment [1,2]. The toxic metal compounds contaminate underground water in trace amounts by leaking from the soil after rain and snow [3]. The effluents of industrial wastewaters often contain considerable amounts of toxic and polluting heavy metals such as chromium, mercury, lead, copper, etc [4]. Toxic metals ions in trace quantities are difficult to remove from aqueous solution. Adsorption is the promising alternatives for removing the toxic metal ions in trace quantities from aqueous solutions especially using low-cost adsorbents like clay material,
∗ Corresponding author. Tel.: +91 9842910157 (mob.); fax: +91 0416 2260550. E-mail addresses: [email protected] (T. Gomathi), [email protected] (P.N. Sudha). 1 Tel.: +91 9894212668. http://dx.doi.org/10.1016/j.ijbiomac.2014.03.010 0141-8130/© 2014 Elsevier B.V. All rights reserved.
agricultural wastes and seafood processing wastes [5]. The new technology searches have directed attention to biosorption for removing the metal ions from waste water, based on metal binding capacities of various biological materials. Recently there is an increasing interest in the application of materials having biological origin in the removal of heavy metal ions from aqueous solutions. Since the cost of these materials is much lower than the cost of commercial adsorbents, such as activated carbon or ion-exchange resins, the prepared biological materials might gain a special attention [6]. Chitin and chitosan have more applications as biopolymers reported for their high potential of adsorption of metal ions [7]. Most adsorbents developed nowadays for the removal of heavy metal ions was based on their interactions with the functional groups on the surfaces of the adsorbents. From these interactions it was concluded that the functional groups present in the adsorbents have important effects on the effectiveness, capacity and reusability for the removal of heavy metal ions [8,9]. Muzzarelli documented that because of the high content of nitrogen on chitosan which acts as an electron donor [10,11] the chitosan exhibited high adsorption capacity for harmful metal ions such as copper, lead, mercury, and uranium from wastewater. Chitosan and its derivates was reported to be an efficient heavy metal scavenger due to its chelating ability [12,13]. The adsorption of the metals on the polymeric backbone was mainly done by the
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secondary bonding interactions like hydrogen bonding, coordinate bonding involving the metal ions and the electron donating groups present at the polymer. The large number of OH groups plays the metal ion coordination sites in polysaccharides like starch, guargum, xanthan gum etc. In some other polysaccharides like sodium carboxylmethylated cellulose (CMC), sodium alginate and chitosan, apart from OH groups COO (CMC and sodium alginate), NH2 groups (Chitosan) are present. These functional groups play an important role for the binding of metal ions. Thus it is possible to develop efficient low cost sorbents for the heavy metals by combining the metal ions attraction capabilities of both the natural polysaccharide and the synthetic polymers. This can be done by grafting various synthetic polymeric chains of vinyl monomers into the various polysaccharides backbones. Because of instability of chitosan in acidic solutions, cross linking agents like glutaraldehyde [14–16], epichlorohydrin [17] and ethylene glycol diglycidyl ether [18] were used to improve chitosan’s durability. Studies on the graft copolymerization of chitosan with various vinyl monomers have been conducted with different initiating systems and different mechanisms [19]. In the present work, using the redox initiator ceric ammonium nitrate, the novel cross linked chitosan graft acrylonitrile copolymer was synthesised. The adsorption behavior of cross linked chitosan graft acrylonitrile copolymer toward Cr(VI), Cu(II) and Ni(II) ions from aqueous solutions was investigated using batch technique. The influence of experimental conditions such as pH, agitation period and adsorbent dose will be studied. The Langmuir and Freundlich equations were used to fit the equilibrium isotherm. The adsorption rates were determined quantitatively and compared by the first- and second-order kinetic model.
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Perkin–Elmer spectrophotometer with KBr pelletisation in a wide wavelength range between 400 cm−1 and 4000 cm−1 and X ray diffraction studies were performed using X ray powder diffractometer (XRD–SHIMADZU XD–D1) using a Nifiltered Cu K␣ X ray radiation.
2.4. Batch adsorption studies Batch adsorption studies were performed using different concentrations of potassium dichromate, copper sulphate and nickel chloride. The extent of metal ion removal was investigated separately by changing the adsorbent dose, pH of the solution and time of shaking of the adsorbent metal solution mixture. Stock solutions of copper(II) ion was prepared by dissolving CuSO4 , that of nickel(II) ion was prepared by dissolving NiCl2 and that of chromium(VI) ion was prepared by dissolving K2 Cr2 O7 in distilled water. The concentration of metal ion solution was 200 mg/l in all three cases. Batch adsorption experiments were conducted by treating 1 g of cross linked chitosan-g-acrylonitrile copolymer with 100 ml of potassium dichromate, copper sulphate and nickel chloride solutions taken in a 250 ml stoppered bottles separately. This solution was then agitated at 30 ◦ C using orbital shaker at fixed speed of 160 rpm. After attaining the equilibrium adsorbent was separated by filtration using Whattman filter paper and aqueous phase concentration of metal was determined with atomic adsorption spectrophotometer. A similar procedure was carried out at different time intervals, adsorbent doses and pH. The pH of each solution was adjusted to different values using either NaOH or HCl.
3. Results and discussion 2. Materials and methods 3.1. Characterization of the grafted copolymer 2.1. Materials Chitosan was kindly donated from Indian Sea food, Cochin, Kerala, India was analytical reagent grade. The acrylonitrile monomer and the cross linking agent glutaraldehyde were obtained from Central Drug House Private Ltd, Mumbai. The ceric ammonium nitrate Ce(NH4 )2 (NO3 )6 and nitric acid used was purchased from Thomas Bakers Chemical and Company. All the reagents used were of the analytical grade. 2.2. Preparation of cross linked chitosan copolymer Chitosan solution was prepared by dissolving 2 g of chitosan flakes in 100 ml of 2% aq. acetic acid solution with constant stirring. To the above prepared chitosan solution about 15 ml of cross linking agent glutaraldehyde was added. This mixture was then stirred well for 20 min using magnetic stirrer. To this cross linked chitosan solution, a known amount of acrylonitrile (1 g in 50 ml of water) was added drop by drop with continuous stirring. Then to initiate the polymerization process in the above mixture a solution of ceric ammonium nitrate (CAN) (0.5 g in 10 ml of 1 N nitric acid) was added. The temperature of reaction was maintained at 70 ◦ C for 45 min. After the completion of addition the product was precipitated by using excess of 2 N sodium hydroxide solution with vigorous stirring. The obtained precipitate was then washed with distilled water several times to remove homopolymer formed. Finally the graft copolymeric product was filtered and dried. 2.3. Polymer characterization The FT-IR studies of the prepared cross linked chitosan-gacrylonitrile copolymer in solid state was characterized using
3.1.1. FTIR Fig. 1 shows the FTIR spectrum of (a) cross linked chitosan and (b) cross linked chitosan-g-acrylonitrile copolymer. The broad and strong absorption peak at around 3454 cm−1 (O H and N H stretching), peak at 2923 cm−1 (C H stretching), the three peaks at range of 1000–1157 cm−1 (C O stretching) were common in both spectra (a) and (b) which may be due to the cross linked chitosan backbone. On comparing the FT-IR spectra of cross linked chitosan with the graft copolymer it was observed that the strong peak was obtained at around 2244 cm−1 in spectra (b) corresponding to the presence of CN stretching. The above obtained peak proved the successful graft copolymerization of cross linked chitosan with the acrylonitrile [20].
3.1.2. XRD The XRD pattern of cross linked chitosan (a) and its graft copolymer (b) was represented in Fig. 2. The cross linked chitosan shows two diffraction peaks at around 11◦ and 20◦ . These are characteristics of the hydrated crystalline structure of cross linked chitosan. The peak of (b) obtained at around 42◦ was due to the overlapped diffraction peaks from the AN’s crystal. While comparing the XRD spectrum of cross linked chitosan with cross linked chitosan-g-acrylonitrile, it was observed that diffraction intensity of the peak obtained at around 20◦ in spectra (b) was obviously weakened. This obtained result indicate that the crystallinity of the cross linked chitosan was found to be decreased after modification. This phenomenon was due to the strong interaction (formation of covalent bond) between cross linked chitosan and acrylonitrile. In other word, copolymerization improved the compatibility between cross linked chitosan and acrylonitrile.
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Fig. 1. FTIR spectra of (a) cross linked chitosan, and (b) cross linked CS-g-AN.
3.2. Factors influencing the adsorption of Cr(VI), Cu(II) and Ni(II) ions The influence of several operational parameters such as the effect of adsorbent dosage, pH and contact time were investigated.
decreases with increase in dose of adsorbent, which may be due to lower utilization of adsorption sites of the sorbent at higher dosage [21]. A Similar trend was reported by [22,23]. The maximum % removal of Cr(VI) was about 84% at the dosage of 6 g, while for Ni(II) and Cu(II) it was 81% and 86% at the dosage of 5 g and 6 g.
3.2.1. Effect of adsorbent dose The effect of adsorbent dose on the removal of Cr6+ , Ni2+ and Cu2+ ion was presented in Fig. 3. The dependence of various metal ion removal on adsorbent dose was investigated by varying the amount of cross linked chitosan graft acrylonitrile copolymer from 1 to 6 g separately, while keeping other parameters (pH, and time) as constant. Fig. 3 shows that the removal percentage increases with increasing adsorbent dose. The increase in metal ion removal percentage with increase in adsorbent dose may be due to greater availability of extra adsorption sites for metal ions. However, the uptake capacity of metal ion per unit mass of biosorbent (mg/g)
3.2.2. Effect of contact time Time taken for the adsorption process to attain thermodynamic equilibrium is very important in characterization and prediction of both the efficiency and the feasibility of an adsorbent for its use in water pollution control. The effect of contact time on the adsorption efficiency of Cr6+ , Ni2+ and Cu2+ ions onto the cross linked chitosan graft acrylonitrile copolymer was shown in Fig. 4, keeping the other parameters such as dose of adsorbent and pH of solution as constant. From the results presented in Fig. 4, it was observed that the removal efficiency increases with increase in time of contact,
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183
% Removal of Cu 2+ ,Cr 6+ and Ni
2+
100
Ni2+ 90
Cu2+ Cr6+
80 70 60 50 4
6
8
10
pH Fig. 5. Effect of pH on the removal of Cu2+ , Cr6+ and Ni2+ .
Fig. 2. XRD of (a) cross linked chitosan; (b) cross linked chitosan-g-acrylonitrile.
80
60
Ni2+
% Removal of Cu
2+
,Cr
6+
and Ni
2+
100
which was due to the availability of more time for metal ions to adsorb. The progressive increase in adsorption and consequently the attainment of equilibrium adsorption may be due to limited mass transfer of the adsorbate molecules from the bulk liquid to the external surface of adsorbent (cross linked chitosan-g-acrylonitrile) [24]. After some extent further increase in contact time shows a decrease in the uptake. This may be due to the decrease of the easily available active sites for adsorption till the equilibrium is reached. The obtained results indicated that Ni(II) removal was increased from 14 to 62.5% with the contact time variation from 30 to 300 min, respectively. From 300 to 360 min, the percentage removal of Ni(II) remains constant (63%), which showed that equilibrium was reached at 300 min itself. Similarly for the metal ion Cr(VI), the rate of removal was increased with the increase in the contact time up to 360 min and remained constant (78.5%) from 360 to 400 min. Thus the results illustrated that the optimum contact time for maximum removal (63%) of Ni(II) was 300 min, for maximum removal (78.5%) of Cr(VI) was 360 min and for maximum removal of Cu(II) (73%) was 360 min.
Cu2+
40
Cr6+ 20
0 0
2
4 6 Adsorbent dose (g)
8
Fig. 3. Effect of adsorbent dose on the removal of Cu2+ , Cr6+ and Ni2+ .
80
60
% Removal of Cu
2+
,Cr
6+
and Ni
2+
100
Ni
40
20
2+
Cu
2+
Cr
6+
0 0
100
200
300
400
Time in Minutes
Fig. 4. Effect of contact time on the removal of Cu2+ , Cr6+ and Ni2+ .
3.2.3. Effect of pH The pH of the solution strongly affects the adsorption capacity of cross linked chitosan graft acrylonitrile copolymer. The initial pH of the adsorbate solution was varied in the range of 4–8 to investigate the role of pH in metal ion removal efficiency. The dependence of amount of adsorption of Cr(VI), Ni(II) and Cu(II) ions on pH was represented in Fig. 5. The results presented in Fig. 3 indicate that the adsorption increases with an increase in pH of the metal ion solution at first and thereafter it shows a decrease. This may be due to the fact that at low pH, more protons will available to protonate amine groups to form groups NH3 + , reducing the number of binding sites for the adsorption of Cu(II). While, at higher pH adsorption of Cu(II) increases due to the inhibitory effect of H+ decreased with the increase in pH [5]. Decrease in adsorption at very high pH values was due to formation of soluble hydroxyl complexes [25,26]. In case of Cr(VI) ions and Cu(II), optimum adsorption was obtained at pH 5 and then it decreases, whereas in case of Ni(II) ions an optimum adsorption was observed at pH 5.5 and thereafter decreases. The maximum % removal for both Cr(VI) and Cu(II) was about 86% at pH 5 and the optimum % removal of Ni(II) was 85% at pH 5.5. 3.3. Adsorption isotherm A reliable, efficient and low-cost technique for metal ion removal from wastewater is the adsorption using biomaterials. Using the various steps, such as metal ion transfer from the bulk solution to the boundary film covering the adsorbent surface, metal ion transport from the boundary film to the surface of the adsorbent, (external diffusion), transfer of the metal ion from the surface
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(b)
150
(a) C ad s (mg/g)
C ad s (mg/g)
150
100
50
100
50
0
0 0
100
200
300
0
400
100
3
200
300
400
500
3
Ceq (mg/dm )
Ceq (mg/dm ) 150
(c)
100
50
0 0
100
200
300
400
500
3
Ceq (mg/dm ) Fig. 6. Adsorption isotherm for (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
(a)
(b)
3
C e q /C a d s (m g /d m )
5
3
C e q /C a d s (m g /d m )
3.5
3.0
2.5
4
3
2
1
0
2.0 0
100
200
300
0
400
100
3
200
300
400
500
3
Ceq(mg/dm )
Ceq(mg/dm )
3
C e q /C a d s (m g /d m )
5
(c)
4
3
2
1
0 0
100
200
300
400
500
3
Ceq(mg/dm ) Fig. 7. Langmuir plot for the adsorption of (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
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Table 1 Comparison of Langmuir and Freundlich constants for chromium(VI), nickel(II) and copper(II) ions. Langmuir constants
Metal ions
Cr(VI) Ni(II) Cu(II)
b (dm3 /mg)
Cmax (mg/g)
R2
P (dm3 /g)
n
R2
2.297 1.915 1.343
0.0187 0.005341 0.005819
122.83 358.54 230.79
0.9304 0.9363 0.9832
0.5640 1.1244 1.5492
1.091 1.3403 1.3848
0.9992 0.9988 0.9911
2.5
2.5
(a)
2.0
2.0
1.5
1.5
lo g q e
lo g q e
Freundlich constants
KL (dm3 /g)
1.0
(b)
1.0 0.5
0.5
0.0
0.0 1.0
1.5
2.0
2.5
1.0
3.0
1.5
2.0
2.5
3.0
log Ce
log Ce 2.5
(c)
log q e
2.0 1.5 1.0 0.5 0.0 0.5
1.0
1.5
2.0
2.5
3.0
log Ce Fig. 8. Freundlich plot for the adsorption of (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
to the intraparticular active sites, (internal diffusion), metal ion uptake on the active sites of adsorbent [27] the adsorption process was controlled. Based on a set of assumptions that are mainly related to the heterogeneity/homogeneity of adsorbents, the type of coverage and possibility of interaction between the adsorbate species, the distribution of the adsorbate species among liquid and adsorbent was described by the mathematical models (adsorption isotherms). Adsorption isotherm is important to describe how solutes interact with adsorbent. Several models had been used in literatures to describe the experimental data of adsorption isotherms. The Freundlich [28] and Langmuir [29] models are often used to describe the equilibrium sorption isotherms.
3.3.1. Langmuir adsorption isotherm The Langmuir equation has been frequently used to give the sorption equilibrium [30]. The prologue investigations were carried out in batches in different conditions of pH, concentration, time, amount of adsorbent, temperature, etc., to check the propensity of adsorption process. The most widely used Langmuir equation, which is valid for monolayer sorption on to a surface with a finite number of identical sites was given by the following equation
Cads =
KL Ceq
1 + bCeq
The linearised form of the Langmuir isotherm was given below bCeq Ceq 1 = + Cads KL KL
(1)
KL b
(2)
Cmax =
where Cads is the amount of metal ion adsorbed (mg/g); Ceq is the equilibrium concentration of metal ion in solution (mg/dm); KL is the Langmuir constant (dm3 /g); b is the Langmuir constant (dm3 /mg), and Cmax is the maximum metal ion adsorbed. The constant b in the Langmuir equation is related to the energy or the net enthalpy of the sorption process. The constant KL can be used to determine the enthalpy of adsorption [31]. The constants b and KL are the characteristics of the Langmuir equation and can be determined from the linearized form of the Langmuir equation (1). The Langmuir equation is found to satisfactorily describe the adsorption isotherms. A linearized plot of Ceq /Cads against Ceq gives “KL ” and “b”. In a sorbent and solution system, a 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 expressed by one or more isotherms [32]. Fig. 6(a) and (c) shows the isotherm of the sorption of chromium, nickel and copper ions by cross linked chitosan-g-acrylonitrile copolymer. The isotherm is characterized
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(a)
3
2.5
(b)
log (qe -qt)
log (q e -qt)
2.0 2
1
1.5 1.0 0.5
0 0
100
200
300
0.0
400
0
100
Time in minutes
200
300
400
Time in minutes
(c)
2.5
log (q e -q t)
2.0 1.5 1.0 0.5 0.0 0
100
200
300
400
Time in minutes Fig. 9. Pseudo-first-order kinetic plot for the adsorption of (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
by the initial region, which is represented as being concave to the concentration axis [33]. To describe the data derived from the adsorption of Cr(VI),Ni(II) and Cu(II) ions by cross linked chitosan-g-acrylonitrile copolymer adsorbent over the entire concentration range studies the
2.5
Langmuir equation was used. The Langmuir plot (Ceq /Cads vs. Ceq ) for the adsorption of (a) chromium(VI), (b) nickel(II) and (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer yielded a straight line (see Fig. 7(a–c)). This observed straight lines confirm the applicability of the Langmuir adsorption isotherm.
(a)
3
(b)
2
1.5
t/qt
t/qt
2.0
1.0
1
0.5 0.0
0
0
100
200 300 Time in minutes 3
400
0
100
200
300
400
Time in minutes
(c)
t/qt
2
1
0 0
100
200 300 Time in minutes
400
Fig. 10. Pseudo second order kinetic plot for the adsorption of (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
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Table 2 Pseudo-first-order (k1 ) and pseudo-second-order (k2 ) kinetics rate constants. Metal ion
Cr(VI) Ni(II) Cu(II)
Pseudo-first-order kinetic model
qe (mg/g)
k1 (min−1 )
R2
432.04 456.75 423.45
0.00596 0.005202 0.005774
0.8749 0.8046 0.9402
qe (mg/g) 138 150 160
Table 1 shows the calculated results of the Langmuir isotherm constants and Cmax . From the results it is evident that the adsorption of Cr(VI), Cu(II) and Ni(II) onto cross linked chitosan-gacrylonitrile copolymer correlates well with the Langmuir equation under the concentration studies. The degree of suitability of the cross linked chitosan copolymer toward chromium(VI),copper(II) and nickel(II) was estimated from the values of the separation factor constant RL which can be calculated from the equation: RL =
1 1 + bCf
(3)
where Cf is the final Cr(VI), Ni(II) and Cu(II) concentration (mg/dm3 ) and b is the Langmuir adsorption equilibrium constant (dm3 /mg). The value of RL was calculated for different initial Cr(VI), Ni(II) and Cu(II) concentration. The constant separation factor RL is used to predict whether an adsorption system is “favorable” or “unfavorable” [34]. RL > 1.0 unsuitable; RL = 1 linear; 0 < RL < 1 suitable; RL = 0 irreversible [35]. In the present case the results of RL values were observed in the range of 0 < RL < 1. This indicate that the adsorption of Cr(VI), Ni(II) and Cu(II) onto cross linked chitosan-g-acrylonitrile copolymer was found to be favorable. Also the Cmax value is greater for nickel ion compared with others. 3.3.2. Freundlich adsorption isotherm The Freundlich equation is mainly used to describe heterogeneous surface energies which were presented by Herbert Freundlich in 1906. The Freundlich isotherm is the most widely used non-linear sorption model and is given by the general form: 1/n
qe = P Ceq
(4)
Linearised form of the Freundlich equation. log qe = log P +
1 n log Ceq
Pseudo-second-order kinetic model
Experimental value qe (mg/g)
(5)
where qe is the amount of metal ion adsorbed (mg/g); Ceq is the quilibrium concentration in solution (mg/dm); 1/n is the Freundlich constant, and P is the Freundlich constant (g/dm). The Freundlich plot (log qe vs. log Ceq ) for the adsorption of (a) chromium(VI), (b) nickel(II) and (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer (Fig. 8(a–c) yielded a straight line which gives the value of P and n. The linearity of the plot supports the applicability of the Freundlich adsorption isotherm in this study. The values of the Langmuir constants and Freundlich constants of chromium(VI), copper(II) and nickel(II) was represented in Table 1. A comparison between Langmuir and Freundlich isotherm models was made. From the comparison of the obtained R2 values it was concluded that the Freundlich model better describes the adsorption process very effectively when compared to the Langmuir model. 3.4. Kinetics studies Adsorption kinetics depends on the adsorbate–adsorbent interaction and system condition which has been investigated for their
362.58 337.22 259.00
k2 (g mg−1 min−1 ) 0.0031 0.004027 0.003832
R2 0.9379 0.8375 0.9624
suitability for application in water pollution control. Lagergren kinetics equation has been most widely used for the adsorption of an adsorbate from an aqueous solution. He suggested a pseudofirst-order equation for the sorption of liquid/solid system based on solid capacity in the year 1898 [36]. 3.4.1. Pseudo-first- and second-order equation The Lagergren equation is the most widely used rate equation in liquid phase sorption. The linearized form of pseudo-first-order equation is given as follows log (qe − qt ) =
log qe − k1 t 2.303
The pseudo-second-order rate equation can be represented as follows t 1 t = + qt qe k2 qe 2 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 second order adsorption rate, respectively. The linear plots of log (qe − qt ) versus t and (t/qt ) versus t are drawn for the pseudo-first-order and the pseudo-second-order models, respectively (Fig. 9(a–c) and Fig. 10(a–c)). The rate constants k1 and k2 can be obtained from the plot of experimental data. 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 values of k1 , k2 , qe and the correlation coefficient (R2 ) obtained from the linear plots was shown in Table 2. From Table 2, it was observed that 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 when compared to the pseudo-first-order model. 4. Conclusion The graft copolymerization of acrylonitrile onto the cross linked chitosan was carried out in the presence of CAN as redox initiator. Investigations were done to evaluate the capacity of cross linked chitosan-g-acrylonitrile copolymer to adsorb Cr(VI), Ni(II) and Cu(II) ions. The observed results indicate that the change of adsorbent dose, pH and the contact time had a pronounced effect on the removal of Cr(VI), Ni(II) and Cu(II) ions from aqueous solution. The adsorption isotherms could be well fitted by the Langmuir equation, the adsorption process could be best described by the second order equation. From the above results it can be concluded that cross linked chitosan-g-acrylonitrile copolymer is an effective and low cost adsorbent for the collection of metal ions. Hence the cross linked chitosan-g-acrylonitrile can be used for waste water treatment at industrial level.
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[20] T. Sun, P. Xu, Q. Liu, J. Xue, W. Xie, Eur. Polym. J. 39 (2003) 189–192. [21] M. Rafatullah, O. Sulaiman, R. Hashim, A. Ahmad, J. Hazard. Mater. 170 (2009) 969–977. [22] M. Dakiky, M. Khamis, A. Manassra, M. Mereb, Adv. Environ. Res. 6 (2002) 533–540. [23] J. Acharya, J.N. Sahu, C.R. Mohanty, B.C. Meikap, Chem. Eng. J. 149 (2009) 249–262. [24] Y.B. Onundi, A.A. Mamun, M.F. Al Khatib, Y.M. Ahmed, Int. J. Environ. Sci. Technol. 7 (2010) 751–758. [25] A.K. Meena, G.K. Mishra, S. Kumar, C. Rajagopal, P.N. Nagar, Adsorption of Ni(II) and Zn(II) from aqueous solution by chemically treated activated carbon, in: National Conference on Carbon, DMSRDE, Kanpur, 2003, pp. 31–140. [26] C. Basualto, M. Poblete, J. Marchese, A. Ochoa, A. Acosta, J. Sapag, F. Valenzuela, J. Braz. Chem. Soc. 17 (2006) 1347–1354. [27] Iulia Nina, Monica Iorgulescu, Manuela Florea Spiroiu, M. Ghiurea, C. Petcu, ´ din Bucuresti Chimie Anul XVI (serie nou˘a) Otilia Cinteza, Analele Universit Nii 1 (2007) 59–67. [28] H. Freundlich, H. Stafford Hatfield, Colloid and Capillary Chemistry, Methuen and Co. Ltd., London, 1926, J. Chem. Technol. Biot. 45, 797–798. [29] I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361–1403. [30] B. Koumanova, P. Peeva, S.J. Allen, K.A. Gallagher, M.G. Healy, J. Chem. Technol. Biotechnol. 77 (2002) 539–545. [31] R. Schmuhl, H.M. Krieg, K. Keizer, Water SA 27 (2001) 1–7. [32] A. Findon, G. Mckay, H.S. Blair, J. Environ. Sci. Health. 28 (1993) 173–185. [33] G.D. Parfitt, C.H. Rochester, Adsorption from Solution at Solid/Liquid interface, Academic Press, London, 1983. [34] W.S.W. Ngah, A. Musa, J. Appl. Polym. Sci. 69 (1998) 2305–2310. [35] G. Mckay, H.S. Blair, J.R. Gardener, J. Appl. Polym. Sci. 27 (1982) 3043–3057. [36] S. Lagergren, K. Sven. Vet. Akad. Handl. 24 (1898) 1–39.
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Comparative studies on the removal of heavy metals ions onto cross linked chitosan-g-acrylonitrile copolymer P. Shankar a , Thandapani Gomathi b,1 , K. Vijayalakshmi b , P.N. Sudha b,∗ a b
Department of Chemistry, Sathyabama University, Chennai, Tamilnadu, India Research Department of Chemistry, DKM College for Women, Sainathapuram, Vellore 632002, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 15 November 2013 Received in revised form 30 January 2014 Accepted 6 March 2014 Available online 25 March 2014 Keywords: Graft copolymer Chitosan Adsorption isotherms
a b s t r a c t The graft copolymerization of acrylonitrile onto cross linked chitosan was carried out using ceric ammonium nitrate as an initiator. The prepared cross linked chitosan-g-acrylonitrile copolymer was characterized using FT-IR and XRD studies. The adsorption behavior of chromium(VI), copper(II) and nickel(II) ions from aqueous solution onto cross linked chitosan graft acrylonitrile copolymer was investigated through batch method. The efficiency of the adsorbent was identified from the varying the contact time, adsorbent dose and pH. The results evident that the adsorption of metal ions increases with the increase of shaking time and metal ion concentration. An optimum pH was found to be 5.0 for both Cr(VI) and Cu(II), whereas the optimum pH is 5.5 for the adsorption of Ni(II) onto cross linked chitosan-g-acrylonitrile copolymer. The Langmuir and Freundlich adsorption models were applied to describe the isotherms and isotherm constants. Adsorption isothermal data could be well interpreted by the Freundlich model. The kinetic experimental data properly correlated with the second-order kinetic model. From the above results it was concluded that the cross linked chitosan graft acrylonitrile copolymer was found to be the efficient adsorbent for removing the heavy metals under optimum conditions. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The continuous increase of water pollution was mainly due to the progressive increase of industrial technology. For minimizing these hazardous pollutants a great effort has been done. The effort was mainly taken to avoid the dangerous effects caused by the pollutants on animals, plants and humans. Contamination by the heavy metal has become a main critical problem recently because the metals gets persisted and accumulated in the environment [1,2]. The toxic metal compounds contaminate underground water in trace amounts by leaking from the soil after rain and snow [3]. The effluents of industrial wastewaters often contain considerable amounts of toxic and polluting heavy metals such as chromium, mercury, lead, copper, etc [4]. Toxic metals ions in trace quantities are difficult to remove from aqueous solution. Adsorption is the promising alternatives for removing the toxic metal ions in trace quantities from aqueous solutions especially using low-cost adsorbents like clay material,
∗ Corresponding author. Tel.: +91 9842910157 (mob.); fax: +91 0416 2260550. E-mail addresses: [email protected] (T. Gomathi), [email protected] (P.N. Sudha). 1 Tel.: +91 9894212668. http://dx.doi.org/10.1016/j.ijbiomac.2014.03.010 0141-8130/© 2014 Elsevier B.V. All rights reserved.
agricultural wastes and seafood processing wastes [5]. The new technology searches have directed attention to biosorption for removing the metal ions from waste water, based on metal binding capacities of various biological materials. Recently there is an increasing interest in the application of materials having biological origin in the removal of heavy metal ions from aqueous solutions. Since the cost of these materials is much lower than the cost of commercial adsorbents, such as activated carbon or ion-exchange resins, the prepared biological materials might gain a special attention [6]. Chitin and chitosan have more applications as biopolymers reported for their high potential of adsorption of metal ions [7]. Most adsorbents developed nowadays for the removal of heavy metal ions was based on their interactions with the functional groups on the surfaces of the adsorbents. From these interactions it was concluded that the functional groups present in the adsorbents have important effects on the effectiveness, capacity and reusability for the removal of heavy metal ions [8,9]. Muzzarelli documented that because of the high content of nitrogen on chitosan which acts as an electron donor [10,11] the chitosan exhibited high adsorption capacity for harmful metal ions such as copper, lead, mercury, and uranium from wastewater. Chitosan and its derivates was reported to be an efficient heavy metal scavenger due to its chelating ability [12,13]. The adsorption of the metals on the polymeric backbone was mainly done by the
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secondary bonding interactions like hydrogen bonding, coordinate bonding involving the metal ions and the electron donating groups present at the polymer. The large number of OH groups plays the metal ion coordination sites in polysaccharides like starch, guargum, xanthan gum etc. In some other polysaccharides like sodium carboxylmethylated cellulose (CMC), sodium alginate and chitosan, apart from OH groups COO (CMC and sodium alginate), NH2 groups (Chitosan) are present. These functional groups play an important role for the binding of metal ions. Thus it is possible to develop efficient low cost sorbents for the heavy metals by combining the metal ions attraction capabilities of both the natural polysaccharide and the synthetic polymers. This can be done by grafting various synthetic polymeric chains of vinyl monomers into the various polysaccharides backbones. Because of instability of chitosan in acidic solutions, cross linking agents like glutaraldehyde [14–16], epichlorohydrin [17] and ethylene glycol diglycidyl ether [18] were used to improve chitosan’s durability. Studies on the graft copolymerization of chitosan with various vinyl monomers have been conducted with different initiating systems and different mechanisms [19]. In the present work, using the redox initiator ceric ammonium nitrate, the novel cross linked chitosan graft acrylonitrile copolymer was synthesised. The adsorption behavior of cross linked chitosan graft acrylonitrile copolymer toward Cr(VI), Cu(II) and Ni(II) ions from aqueous solutions was investigated using batch technique. The influence of experimental conditions such as pH, agitation period and adsorbent dose will be studied. The Langmuir and Freundlich equations were used to fit the equilibrium isotherm. The adsorption rates were determined quantitatively and compared by the first- and second-order kinetic model.
181
Perkin–Elmer spectrophotometer with KBr pelletisation in a wide wavelength range between 400 cm−1 and 4000 cm−1 and X ray diffraction studies were performed using X ray powder diffractometer (XRD–SHIMADZU XD–D1) using a Nifiltered Cu K␣ X ray radiation.
2.4. Batch adsorption studies Batch adsorption studies were performed using different concentrations of potassium dichromate, copper sulphate and nickel chloride. The extent of metal ion removal was investigated separately by changing the adsorbent dose, pH of the solution and time of shaking of the adsorbent metal solution mixture. Stock solutions of copper(II) ion was prepared by dissolving CuSO4 , that of nickel(II) ion was prepared by dissolving NiCl2 and that of chromium(VI) ion was prepared by dissolving K2 Cr2 O7 in distilled water. The concentration of metal ion solution was 200 mg/l in all three cases. Batch adsorption experiments were conducted by treating 1 g of cross linked chitosan-g-acrylonitrile copolymer with 100 ml of potassium dichromate, copper sulphate and nickel chloride solutions taken in a 250 ml stoppered bottles separately. This solution was then agitated at 30 ◦ C using orbital shaker at fixed speed of 160 rpm. After attaining the equilibrium adsorbent was separated by filtration using Whattman filter paper and aqueous phase concentration of metal was determined with atomic adsorption spectrophotometer. A similar procedure was carried out at different time intervals, adsorbent doses and pH. The pH of each solution was adjusted to different values using either NaOH or HCl.
3. Results and discussion 2. Materials and methods 3.1. Characterization of the grafted copolymer 2.1. Materials Chitosan was kindly donated from Indian Sea food, Cochin, Kerala, India was analytical reagent grade. The acrylonitrile monomer and the cross linking agent glutaraldehyde were obtained from Central Drug House Private Ltd, Mumbai. The ceric ammonium nitrate Ce(NH4 )2 (NO3 )6 and nitric acid used was purchased from Thomas Bakers Chemical and Company. All the reagents used were of the analytical grade. 2.2. Preparation of cross linked chitosan copolymer Chitosan solution was prepared by dissolving 2 g of chitosan flakes in 100 ml of 2% aq. acetic acid solution with constant stirring. To the above prepared chitosan solution about 15 ml of cross linking agent glutaraldehyde was added. This mixture was then stirred well for 20 min using magnetic stirrer. To this cross linked chitosan solution, a known amount of acrylonitrile (1 g in 50 ml of water) was added drop by drop with continuous stirring. Then to initiate the polymerization process in the above mixture a solution of ceric ammonium nitrate (CAN) (0.5 g in 10 ml of 1 N nitric acid) was added. The temperature of reaction was maintained at 70 ◦ C for 45 min. After the completion of addition the product was precipitated by using excess of 2 N sodium hydroxide solution with vigorous stirring. The obtained precipitate was then washed with distilled water several times to remove homopolymer formed. Finally the graft copolymeric product was filtered and dried. 2.3. Polymer characterization The FT-IR studies of the prepared cross linked chitosan-gacrylonitrile copolymer in solid state was characterized using
3.1.1. FTIR Fig. 1 shows the FTIR spectrum of (a) cross linked chitosan and (b) cross linked chitosan-g-acrylonitrile copolymer. The broad and strong absorption peak at around 3454 cm−1 (O H and N H stretching), peak at 2923 cm−1 (C H stretching), the three peaks at range of 1000–1157 cm−1 (C O stretching) were common in both spectra (a) and (b) which may be due to the cross linked chitosan backbone. On comparing the FT-IR spectra of cross linked chitosan with the graft copolymer it was observed that the strong peak was obtained at around 2244 cm−1 in spectra (b) corresponding to the presence of CN stretching. The above obtained peak proved the successful graft copolymerization of cross linked chitosan with the acrylonitrile [20].
3.1.2. XRD The XRD pattern of cross linked chitosan (a) and its graft copolymer (b) was represented in Fig. 2. The cross linked chitosan shows two diffraction peaks at around 11◦ and 20◦ . These are characteristics of the hydrated crystalline structure of cross linked chitosan. The peak of (b) obtained at around 42◦ was due to the overlapped diffraction peaks from the AN’s crystal. While comparing the XRD spectrum of cross linked chitosan with cross linked chitosan-g-acrylonitrile, it was observed that diffraction intensity of the peak obtained at around 20◦ in spectra (b) was obviously weakened. This obtained result indicate that the crystallinity of the cross linked chitosan was found to be decreased after modification. This phenomenon was due to the strong interaction (formation of covalent bond) between cross linked chitosan and acrylonitrile. In other word, copolymerization improved the compatibility between cross linked chitosan and acrylonitrile.
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Fig. 1. FTIR spectra of (a) cross linked chitosan, and (b) cross linked CS-g-AN.
3.2. Factors influencing the adsorption of Cr(VI), Cu(II) and Ni(II) ions The influence of several operational parameters such as the effect of adsorbent dosage, pH and contact time were investigated.
decreases with increase in dose of adsorbent, which may be due to lower utilization of adsorption sites of the sorbent at higher dosage [21]. A Similar trend was reported by [22,23]. The maximum % removal of Cr(VI) was about 84% at the dosage of 6 g, while for Ni(II) and Cu(II) it was 81% and 86% at the dosage of 5 g and 6 g.
3.2.1. Effect of adsorbent dose The effect of adsorbent dose on the removal of Cr6+ , Ni2+ and Cu2+ ion was presented in Fig. 3. The dependence of various metal ion removal on adsorbent dose was investigated by varying the amount of cross linked chitosan graft acrylonitrile copolymer from 1 to 6 g separately, while keeping other parameters (pH, and time) as constant. Fig. 3 shows that the removal percentage increases with increasing adsorbent dose. The increase in metal ion removal percentage with increase in adsorbent dose may be due to greater availability of extra adsorption sites for metal ions. However, the uptake capacity of metal ion per unit mass of biosorbent (mg/g)
3.2.2. Effect of contact time Time taken for the adsorption process to attain thermodynamic equilibrium is very important in characterization and prediction of both the efficiency and the feasibility of an adsorbent for its use in water pollution control. The effect of contact time on the adsorption efficiency of Cr6+ , Ni2+ and Cu2+ ions onto the cross linked chitosan graft acrylonitrile copolymer was shown in Fig. 4, keeping the other parameters such as dose of adsorbent and pH of solution as constant. From the results presented in Fig. 4, it was observed that the removal efficiency increases with increase in time of contact,
P. Shankar et al. / International Journal of Biological Macromolecules 67 (2014) 180–188
183
% Removal of Cu 2+ ,Cr 6+ and Ni
2+
100
Ni2+ 90
Cu2+ Cr6+
80 70 60 50 4
6
8
10
pH Fig. 5. Effect of pH on the removal of Cu2+ , Cr6+ and Ni2+ .
Fig. 2. XRD of (a) cross linked chitosan; (b) cross linked chitosan-g-acrylonitrile.
80
60
Ni2+
% Removal of Cu
2+
,Cr
6+
and Ni
2+
100
which was due to the availability of more time for metal ions to adsorb. The progressive increase in adsorption and consequently the attainment of equilibrium adsorption may be due to limited mass transfer of the adsorbate molecules from the bulk liquid to the external surface of adsorbent (cross linked chitosan-g-acrylonitrile) [24]. After some extent further increase in contact time shows a decrease in the uptake. This may be due to the decrease of the easily available active sites for adsorption till the equilibrium is reached. The obtained results indicated that Ni(II) removal was increased from 14 to 62.5% with the contact time variation from 30 to 300 min, respectively. From 300 to 360 min, the percentage removal of Ni(II) remains constant (63%), which showed that equilibrium was reached at 300 min itself. Similarly for the metal ion Cr(VI), the rate of removal was increased with the increase in the contact time up to 360 min and remained constant (78.5%) from 360 to 400 min. Thus the results illustrated that the optimum contact time for maximum removal (63%) of Ni(II) was 300 min, for maximum removal (78.5%) of Cr(VI) was 360 min and for maximum removal of Cu(II) (73%) was 360 min.
Cu2+
40
Cr6+ 20
0 0
2
4 6 Adsorbent dose (g)
8
Fig. 3. Effect of adsorbent dose on the removal of Cu2+ , Cr6+ and Ni2+ .
80
60
% Removal of Cu
2+
,Cr
6+
and Ni
2+
100
Ni
40
20
2+
Cu
2+
Cr
6+
0 0
100
200
300
400
Time in Minutes
Fig. 4. Effect of contact time on the removal of Cu2+ , Cr6+ and Ni2+ .
3.2.3. Effect of pH The pH of the solution strongly affects the adsorption capacity of cross linked chitosan graft acrylonitrile copolymer. The initial pH of the adsorbate solution was varied in the range of 4–8 to investigate the role of pH in metal ion removal efficiency. The dependence of amount of adsorption of Cr(VI), Ni(II) and Cu(II) ions on pH was represented in Fig. 5. The results presented in Fig. 3 indicate that the adsorption increases with an increase in pH of the metal ion solution at first and thereafter it shows a decrease. This may be due to the fact that at low pH, more protons will available to protonate amine groups to form groups NH3 + , reducing the number of binding sites for the adsorption of Cu(II). While, at higher pH adsorption of Cu(II) increases due to the inhibitory effect of H+ decreased with the increase in pH [5]. Decrease in adsorption at very high pH values was due to formation of soluble hydroxyl complexes [25,26]. In case of Cr(VI) ions and Cu(II), optimum adsorption was obtained at pH 5 and then it decreases, whereas in case of Ni(II) ions an optimum adsorption was observed at pH 5.5 and thereafter decreases. The maximum % removal for both Cr(VI) and Cu(II) was about 86% at pH 5 and the optimum % removal of Ni(II) was 85% at pH 5.5. 3.3. Adsorption isotherm A reliable, efficient and low-cost technique for metal ion removal from wastewater is the adsorption using biomaterials. Using the various steps, such as metal ion transfer from the bulk solution to the boundary film covering the adsorbent surface, metal ion transport from the boundary film to the surface of the adsorbent, (external diffusion), transfer of the metal ion from the surface
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(b)
150
(a) C ad s (mg/g)
C ad s (mg/g)
150
100
50
100
50
0
0 0
100
200
300
0
400
100
3
200
300
400
500
3
Ceq (mg/dm )
Ceq (mg/dm ) 150
(c)
100
50
0 0
100
200
300
400
500
3
Ceq (mg/dm ) Fig. 6. Adsorption isotherm for (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
(a)
(b)
3
C e q /C a d s (m g /d m )
5
3
C e q /C a d s (m g /d m )
3.5
3.0
2.5
4
3
2
1
0
2.0 0
100
200
300
0
400
100
3
200
300
400
500
3
Ceq(mg/dm )
Ceq(mg/dm )
3
C e q /C a d s (m g /d m )
5
(c)
4
3
2
1
0 0
100
200
300
400
500
3
Ceq(mg/dm ) Fig. 7. Langmuir plot for the adsorption of (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
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185
Table 1 Comparison of Langmuir and Freundlich constants for chromium(VI), nickel(II) and copper(II) ions. Langmuir constants
Metal ions
Cr(VI) Ni(II) Cu(II)
b (dm3 /mg)
Cmax (mg/g)
R2
P (dm3 /g)
n
R2
2.297 1.915 1.343
0.0187 0.005341 0.005819
122.83 358.54 230.79
0.9304 0.9363 0.9832
0.5640 1.1244 1.5492
1.091 1.3403 1.3848
0.9992 0.9988 0.9911
2.5
2.5
(a)
2.0
2.0
1.5
1.5
lo g q e
lo g q e
Freundlich constants
KL (dm3 /g)
1.0
(b)
1.0 0.5
0.5
0.0
0.0 1.0
1.5
2.0
2.5
1.0
3.0
1.5
2.0
2.5
3.0
log Ce
log Ce 2.5
(c)
log q e
2.0 1.5 1.0 0.5 0.0 0.5
1.0
1.5
2.0
2.5
3.0
log Ce Fig. 8. Freundlich plot for the adsorption of (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
to the intraparticular active sites, (internal diffusion), metal ion uptake on the active sites of adsorbent [27] the adsorption process was controlled. Based on a set of assumptions that are mainly related to the heterogeneity/homogeneity of adsorbents, the type of coverage and possibility of interaction between the adsorbate species, the distribution of the adsorbate species among liquid and adsorbent was described by the mathematical models (adsorption isotherms). Adsorption isotherm is important to describe how solutes interact with adsorbent. Several models had been used in literatures to describe the experimental data of adsorption isotherms. The Freundlich [28] and Langmuir [29] models are often used to describe the equilibrium sorption isotherms.
3.3.1. Langmuir adsorption isotherm The Langmuir equation has been frequently used to give the sorption equilibrium [30]. The prologue investigations were carried out in batches in different conditions of pH, concentration, time, amount of adsorbent, temperature, etc., to check the propensity of adsorption process. The most widely used Langmuir equation, which is valid for monolayer sorption on to a surface with a finite number of identical sites was given by the following equation
Cads =
KL Ceq
1 + bCeq
The linearised form of the Langmuir isotherm was given below bCeq Ceq 1 = + Cads KL KL
(1)
KL b
(2)
Cmax =
where Cads is the amount of metal ion adsorbed (mg/g); Ceq is the equilibrium concentration of metal ion in solution (mg/dm); KL is the Langmuir constant (dm3 /g); b is the Langmuir constant (dm3 /mg), and Cmax is the maximum metal ion adsorbed. The constant b in the Langmuir equation is related to the energy or the net enthalpy of the sorption process. The constant KL can be used to determine the enthalpy of adsorption [31]. The constants b and KL are the characteristics of the Langmuir equation and can be determined from the linearized form of the Langmuir equation (1). The Langmuir equation is found to satisfactorily describe the adsorption isotherms. A linearized plot of Ceq /Cads against Ceq gives “KL ” and “b”. In a sorbent and solution system, a 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 expressed by one or more isotherms [32]. Fig. 6(a) and (c) shows the isotherm of the sorption of chromium, nickel and copper ions by cross linked chitosan-g-acrylonitrile copolymer. The isotherm is characterized
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(a)
3
2.5
(b)
log (qe -qt)
log (q e -qt)
2.0 2
1
1.5 1.0 0.5
0 0
100
200
300
0.0
400
0
100
Time in minutes
200
300
400
Time in minutes
(c)
2.5
log (q e -q t)
2.0 1.5 1.0 0.5 0.0 0
100
200
300
400
Time in minutes Fig. 9. Pseudo-first-order kinetic plot for the adsorption of (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
by the initial region, which is represented as being concave to the concentration axis [33]. To describe the data derived from the adsorption of Cr(VI),Ni(II) and Cu(II) ions by cross linked chitosan-g-acrylonitrile copolymer adsorbent over the entire concentration range studies the
2.5
Langmuir equation was used. The Langmuir plot (Ceq /Cads vs. Ceq ) for the adsorption of (a) chromium(VI), (b) nickel(II) and (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer yielded a straight line (see Fig. 7(a–c)). This observed straight lines confirm the applicability of the Langmuir adsorption isotherm.
(a)
3
(b)
2
1.5
t/qt
t/qt
2.0
1.0
1
0.5 0.0
0
0
100
200 300 Time in minutes 3
400
0
100
200
300
400
Time in minutes
(c)
t/qt
2
1
0 0
100
200 300 Time in minutes
400
Fig. 10. Pseudo second order kinetic plot for the adsorption of (a) chromium(VI) ions; (b) nickel(II) ions; (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer.
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187
Table 2 Pseudo-first-order (k1 ) and pseudo-second-order (k2 ) kinetics rate constants. Metal ion
Cr(VI) Ni(II) Cu(II)
Pseudo-first-order kinetic model
qe (mg/g)
k1 (min−1 )
R2
432.04 456.75 423.45
0.00596 0.005202 0.005774
0.8749 0.8046 0.9402
qe (mg/g) 138 150 160
Table 1 shows the calculated results of the Langmuir isotherm constants and Cmax . From the results it is evident that the adsorption of Cr(VI), Cu(II) and Ni(II) onto cross linked chitosan-gacrylonitrile copolymer correlates well with the Langmuir equation under the concentration studies. The degree of suitability of the cross linked chitosan copolymer toward chromium(VI),copper(II) and nickel(II) was estimated from the values of the separation factor constant RL which can be calculated from the equation: RL =
1 1 + bCf
(3)
where Cf is the final Cr(VI), Ni(II) and Cu(II) concentration (mg/dm3 ) and b is the Langmuir adsorption equilibrium constant (dm3 /mg). The value of RL was calculated for different initial Cr(VI), Ni(II) and Cu(II) concentration. The constant separation factor RL is used to predict whether an adsorption system is “favorable” or “unfavorable” [34]. RL > 1.0 unsuitable; RL = 1 linear; 0 < RL < 1 suitable; RL = 0 irreversible [35]. In the present case the results of RL values were observed in the range of 0 < RL < 1. This indicate that the adsorption of Cr(VI), Ni(II) and Cu(II) onto cross linked chitosan-g-acrylonitrile copolymer was found to be favorable. Also the Cmax value is greater for nickel ion compared with others. 3.3.2. Freundlich adsorption isotherm The Freundlich equation is mainly used to describe heterogeneous surface energies which were presented by Herbert Freundlich in 1906. The Freundlich isotherm is the most widely used non-linear sorption model and is given by the general form: 1/n
qe = P Ceq
(4)
Linearised form of the Freundlich equation. log qe = log P +
1 n log Ceq
Pseudo-second-order kinetic model
Experimental value qe (mg/g)
(5)
where qe is the amount of metal ion adsorbed (mg/g); Ceq is the quilibrium concentration in solution (mg/dm); 1/n is the Freundlich constant, and P is the Freundlich constant (g/dm). The Freundlich plot (log qe vs. log Ceq ) for the adsorption of (a) chromium(VI), (b) nickel(II) and (c) copper(II) ions onto cross linked chitosan-g-acrylonitrile copolymer (Fig. 8(a–c) yielded a straight line which gives the value of P and n. The linearity of the plot supports the applicability of the Freundlich adsorption isotherm in this study. The values of the Langmuir constants and Freundlich constants of chromium(VI), copper(II) and nickel(II) was represented in Table 1. A comparison between Langmuir and Freundlich isotherm models was made. From the comparison of the obtained R2 values it was concluded that the Freundlich model better describes the adsorption process very effectively when compared to the Langmuir model. 3.4. Kinetics studies Adsorption kinetics depends on the adsorbate–adsorbent interaction and system condition which has been investigated for their
362.58 337.22 259.00
k2 (g mg−1 min−1 ) 0.0031 0.004027 0.003832
R2 0.9379 0.8375 0.9624
suitability for application in water pollution control. Lagergren kinetics equation has been most widely used for the adsorption of an adsorbate from an aqueous solution. He suggested a pseudofirst-order equation for the sorption of liquid/solid system based on solid capacity in the year 1898 [36]. 3.4.1. Pseudo-first- and second-order equation The Lagergren equation is the most widely used rate equation in liquid phase sorption. The linearized form of pseudo-first-order equation is given as follows log (qe − qt ) =
log qe − k1 t 2.303
The pseudo-second-order rate equation can be represented as follows t 1 t = + qt qe k2 qe 2 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 second order adsorption rate, respectively. The linear plots of log (qe − qt ) versus t and (t/qt ) versus t are drawn for the pseudo-first-order and the pseudo-second-order models, respectively (Fig. 9(a–c) and Fig. 10(a–c)). The rate constants k1 and k2 can be obtained from the plot of experimental data. 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 values of k1 , k2 , qe and the correlation coefficient (R2 ) obtained from the linear plots was shown in Table 2. From Table 2, it was observed that 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 when compared to the pseudo-first-order model. 4. Conclusion The graft copolymerization of acrylonitrile onto the cross linked chitosan was carried out in the presence of CAN as redox initiator. Investigations were done to evaluate the capacity of cross linked chitosan-g-acrylonitrile copolymer to adsorb Cr(VI), Ni(II) and Cu(II) ions. The observed results indicate that the change of adsorbent dose, pH and the contact time had a pronounced effect on the removal of Cr(VI), Ni(II) and Cu(II) ions from aqueous solution. The adsorption isotherms could be well fitted by the Langmuir equation, the adsorption process could be best described by the second order equation. From the above results it can be concluded that cross linked chitosan-g-acrylonitrile copolymer is an effective and low cost adsorbent for the collection of metal ions. Hence the cross linked chitosan-g-acrylonitrile can be used for waste water treatment at industrial level.
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