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Sorption of chromium(VI) using modified forms of chitosan beads
International Journal of Biological Macromolecules 47 (2010) 308–315
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Sorption of chromium(VI) using modified forms of chitosan beads G.N. Kousalya a , Muniyappan Rajiv Gandhi b , S. Meenakshi b,∗ a b
Department of Chemistry, GTN Arts College, Dindigul 624 003, Tamilnadu, India Department of Chemistry, Gandhigram Rural University, Gandhigram 624 302, Dindigul, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 22 January 2010 Received in revised form 13 February 2010 Accepted 23 March 2010 Available online 31 March 2010 Keywords: Chitosan Beads Isotherm Chromium Sorption
a b s t r a c t Modified forms of chitosan beads were prepared and used for chromium removal from the aqueous solution. The prepared chitosan beads viz., protonated chitosan beads (PCB), carboxylated chitosan beads (CCB) and grafted chitosan beads (GCB) possess enhanced chromium sorption capacities (SCs) of 3239, 3647 and 4057 mg/kg respectively than the raw chitosan beads (CB) which possess the SC of 1298 mg/kg with a minimum contact time of 10 min. The sorption experiments were carried out in batch mode to optimize various influencing parameters viz., contact time, pH, common ions and temperature. The sorbents were characterized by FTIR and SEM with EDAX analysis. The modified chitosan beads remove chromium by means of electrostatic adsorption coupled reduction and complexation. The adsorption data was fitted with Freundlich and Langmuir isotherms. The calculated values of thermodynamic parameters indicate the nature of chromium sorption. A field trial was carried out with water collected from a nearby industrial area. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Contamination of aquatic media by heavy metals is a serious environmental problem, mainly due to the discharge of industrial waste [1]. Heavy metals are highly toxic at low concentrations and can accumulate in living organisms, causing several disorders and diseases [2]. Chromium is one of the major contaminants in the wastewater of dyes and pigments, film and photography, galvanometric, electroplating, leather and mining industries [3]. Chromium exists in several oxidation states out of which Cr(III) and Cr(VI) are the most stable forms. Because of its high toxicity and potential carcinogenicity, Cr(VI) is of special concern [4]. Hexavalent chromium exists in water as oxyanions such as chromate (HCrO4 − ) and dichromate (Cr2 O7 2− ). It causes diseases such as epigastric pain, nausea, vomiting, severe diarrhoea and hemorrhage by ingestion. The maximum allowed concentration of chromium content in drinking water is 0.05 mg/L [5]. Conventional methods applied for Cr(VI) removal are chemical precipitation, oxidation/reduction, filtration, ion exchange, membrane separation and adsorption [6]. Adsorption is the most frequently applied technique owing to its advantages such as variety of adsorbent materials and high efficiency at a relatively low cost [7]. Chitosan is a cationic aminopolysaccharide copolymer of glucosamine and N-acetyl glu-
∗ Corresponding author. Tel.: +91 451 2452371; fax: +91 451 2454466. E-mail addresses: [email protected] (G.N. Kousalya), [email protected] (M. Rajiv Gandhi), drs [email protected] (S. Meenakshi). 0141-8130/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ijbiomac.2010.03.010
cosamine, obtained by partial deacetylation of chitin, which originates from shells of crustaceans such as crabs and prawns [8]. Chitin and chitosan have varied potential applications in the areas of biotechnology, biomedicine and food ingredients [9–11]. In addition chitin and chitosan appear to be more useful biopolymers reported for the high potential of sorption of metal ions [12–14]. Chitosan is biodegradable, biocompatatible and non-toxic biopolymer, reported to be an efficient heavy metal scavenger due to the presence of amino group [15] than the chitin. Chitosan is soluble in dilute mineral acids, except in sulphuric acid and it is necessary to reinforce chemical stability using cross-linking agents like glutaraldehyde [16]. Derivatives of chitosan have been applied for the removal of various metal ions from aqueous solution. Sorption mechanism of mercury was studied on a chitosan derivative by Gavilan et al. [17] and equilibrium and kinetic studies of adsorption of Cu(II) on chitosan/PVA beads were carried out by Wan Nagh et al. [18]. Sorption performances can be enhanced by modification of chitosan. The selectivity of the sorption is improved through chemical modification techniques based on protonation, carboxylation and grafting. The main objective of this paper is a systematic evaluation of the performance of a novel, chemically modified chitosan beads for the uptake of Cr(VI) from water and establish the probable mechanism of chromium sorption. The studies have been conducted in the optimization of various experimental conditions like contact time, pH, temperature and effect of common ions. A comparative assessment of chitosan beads and their modified forms were also made.
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Scheme 1. Preparation of PCB, CCB and GCB.
2. Materials and methods 2.1. Materials Chitosan with its deacetylation degree of 85% was supplied by Pelican Biotech and Chemicals Labs, Kerala (India). The viscosity of the chitosan solution was determined to be 700 mPa s by Brookfield Dial Reading Viscometer using electronic driveRVT model (USA make). The chitosan solution was maintained at a constant viscosity for beads preparation in order to maintain uniform molecular weight. NaOH, HCl, glacial acetic acid, glutaraldehyde, chloroacetic acid, ethylenediamine, potassium dichromate and all other chemicals and reagents were of analytical grade. 2.2. Preparation of cross-linked chitosan beads (CB) Chitosan (20 g) was dissolved in 2.0% glacial acetic acid solution (1000 ml). The chitosan solution was dropped into a 0.5 M aqueous NaOH solution to form uniform chitosan beads. After gelling for a minimum of 16 h in 0.5 M NaOH solution, the beads were washed with distilled water to a neutral pH. The wet beads were cross-linked with 2.5 wt.% glutaraldehyde solution and the ratio of glutaraldehyde to chitosan beads was approximately 1.5 ml/g of wet beads [19]. Cross-linking reaction occurred for 48 h and then cross-linked beads were washed with distilled water to remove any free glutaraldehyde. 2.3. Preparation of protonated cross-linked chitosan beads (PCB) PCB was prepared in order to effectively utilize the amino groups of cross-linked chitosan beads for chromium sorption. The crosslinked chitosan beads were treated with concentrated HCl for 30 min for protonation of beads [20] and the reaction is shown in Scheme 1. The PCB was washed with distilled water to neutral pH, dried at room temperature and used for sorption studies [21]. 2.4. Preparation of carboxylated cross-linked chitosan beads (CCB) CCB was prepared in order to effectively utilize the hydroxyl groups of cross-linked chitosan beads for metal sorption. The cross-
linked chitosan beads were treated with aqueous 0.5 M chloroacetic acid maintained at pH 8.0 using 0.1 M NaOH for 10 h at room temperature to convert the hydroxyl groups to carboxyl groups [20] as shown in Scheme 1. CCB was washed with distilled water to neutral pH, dried at room temperature and used for sorption studies [22]. 2.5. Preparation of grafted cross-linked chitosan beads (GCB) The aminated reaction of chitosan is composed of two steps as shown in Scheme 1. In the first step, chloroacetic acid is used to introduce carboxyl groups to the hydroxyl groups in chitosan [23]. The wet cross-linked chitosan beads were treated with aqueous 0.5 M chloroacetic acid maintained at pH 8.0 using 1.0 M NaOH for 10 h at room temperature and then the reacted chitosan beads were washed with distilled water several times to remove unreacted reagents. In the second step, the carboxylated chitosan beads obtained through first reaction were treated in 100 ml of 1.0 M ethylenediamine. Reaction was carried out for 5 h and the final chitosan beads were thoroughly washed using distilled water for several times and treated with with concentrated HCl for 30 min for protonation of GCB. The protonated GCB beads were washed with distilled water to neutral pH, dried at room temperature and used for sorption studies. 2.6. Sorption experiments The sorption experiments were performed by batch equilibration method. Stock solution of chromium containing 1000 mg/L was prepared and this was used for sorption experiments. Batch sorption experiments in duplicate were carried out by mixing 0.1 g of sorbent with 50 ml of 10 mg/L as initial chromium concentration. The contents were shaken thoroughly using a thermostated shaker rotating at a speed of 200 rpm. The solution was then filtered and the residual chromium concentration was measured using UV–visible spectrophotometer (Pharo 300 Merck) at 540 nm, according to the 1,5-diphenyl-carbazide method [24]. pH measurements were carried out with the expandable ion analyzer EA 940 with pH electrode. The thermodynamic parameters of the adsorption were established by conducting the experiments at 303, 313 and 323 K in a temperature controlled mechanical shaker. The SCs of the sorbents were studied at different conditions like contact time of the sorbent for maximum sorption, pH of the medium and
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Fig. 1. FTIR spectra of CB, PCB, CCB, and GCB.
Fig. 2. FTIR spectra of (a) PCB and (b) chromium sorbed PCB.
the effect of other common ions present in the water. All other water quality parameters were analyzed by using standard methods [24]. 2.7. Analysis The prepared sorbents were characterized by Fourier transform infrared spectroscopy (FTIR). FTIR spectra of the samples as solid by diluting in KBr pellets were recorded with JASCO-460 plus model. The results of FTIR spectrometer were used to confirm the functional groups present in the surface morphology of the modified chitosan beads before and after chromium sorption was studied with scanning electron microscope (SEM) with JOEL JSM 6390 LV model. Elemental spectra were obtained using an energy dispersive X-ray analyzer (EDAX) during SEM observations which allow a qualitative detection and localization of elements in the modified chitosan beads. Computations were made using Microcal Origin (Version 6.0) software. The goodness of fit was discussed using regression correlation coefficient (r) and chi-square analysis. Fig. 3. FTIR spectra of (a) CCB and (b) chromium sorbed CCB.
3. Results and discussion 3.1. Characterization of the sorbents FTIR spectrum of chitosan beads and their modified chitosan beads are shown in Fig. 1. The IR spectra of both chitin and chitosan are very similar to those reported in the literature [25,26]. The chitosan spectrum differs from that of chitin the band at 1555 cm−1 corresponding to the N–H deformation that is present only in a lower extent in chitosan samples. Although there is a possibility of overlapping between –NH2 and –OH stretching vibrations, the strong broad band at the wave number region of 3300–3500 cm−1 is the characteristic of –NH2 stretching vibration. The major bands for the chitosan bead can be assigned as follows: 3440 cm−1 (–OH and –NH2 stretching vibrations), 2921 cm−1 (–CH stretching vibration in –CH and –CH2 ), 1652 cm−1 (–NH2 bending vibration), 1379 cm−1 (–CH symmetric bending vibrations in –CHOH–), 1067 and 1028 cm−1 (–CO stretching vibration in –COH) [27]. After cross-linking the chitosan beads, the –OH and –NH2 stretching vibration around the wave number of 3440 cm−1 and the –NH2 bending vibration at the wave number 1652 cm−1 was shifted to lower frequencies.
Fig. 4. FTIR spectra of (a) GCB and (b) chromium sorbed GCB.
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Fig. 6. SEM pictures of (a) CCB and (b) chromium sorbed CCB, EDAX spectra of (c) chromium treated CCB. Fig. 5. SEM images of (a) PCB and (b) chromium sorbed PCB, EDAX spectra of (c) chromium treated PCB.
Fig. 2 shows the FTIR spectra of (a) PCB and (b) chromium sorbed PCB. Fig. 3 shows the FTIR spectra of (a) CCB and (b) chromium sorbed CCB. Fig. 4 shows the FTIR spectra of (a) GCB and (b) chromium sorbed GCB. The FTIR spectra of Cr(VI) loaded sorbents having a new band at 530–540 cm−1 confirm the formation of Cr(OH)3 in the chromium sorbed PCB, CCB and GCB [28,29]. The other bands are the characteristics of the sorbents. SEM pictures of PCB and the chromium sorbed PCB are shown in Fig. 5a and b respectively, SEM images of CCB and the chromium sorbed CCB are shown in Fig. 6a and b respectively. SEM micrographs of GCB and the chromium sorbed GCB are shown in Fig. 7a and b respectively. The surface change in the SEM micrographs of the modified chitosan beads before and after chromium treatment indicates the structural changes in the sorbents. This is further supported by EDAX analysis which provides the direct evidence for the sorption of chromium onto the beads. The EDAX spectra of chromium sorbed modified chitosan beads show the presence of chromium peaks which confirms that the chromium sorption has occurred onto the modified chitosan beads (cf. Figs. 5–7c).
3.2. Effect of contact time The SCs of CB, PCB, CCB and GCB were determined by varying the contact time in the range of 5–60 min. About 0.1 g of the sorbent was placed into 50 ml of the 10 mg/L initial chromium solution. The iodine flasks with their contents taken for the study were shaken thoroughly using a mechanical shaker at 200 rpm. All the six iodine flasks were removed from the shaker and the contents were filtered and analyzed for Cr(VI). As it is evident from Fig. 8, the SC of all the sorbents reached saturation after 10 min. Hence, 10 min is fixed as the contact time for the sorbents for further studies. Maximum SCs of CB, PCB, CCB, GCB were found to be 1298, 3239, 3647 and 4057 mg/kg respectively. Among the sorbents, GCB experienced higher SC than CB, PCB and CCB.
3.3. Effect of pH The removal of ion from aqueous solution was highly dependent on the solution pH in many cases as it altered the surface charge on the sorbent. Therefore, the SC of all the three sorbents was determined at five different pH levels such as 3, 5, 7, 9 and 11. The results are shown in Fig. 9. It is obvious from the figure that the pH has a slight influence on the SC of sorbents. A maximum SC was observed at pH 5–7 and a slight decline in SC was observed in alkaline medium. At acidic pH, the predominant species of chromium are HCrO4 − and Cr2 O7 2− in aqueous solutions. On the other hand, under acidic conditions, the surfaces of all the sorbents were highly protonated which favours the uptake of Cr(VI) in the anionic form. With the increase in pH, the degree of protonation of surface reduces gradually [30]. In addition, the active binding cites of the sorbents were occupied by the OH− ions and hence sorption is decreased. Further, experiments were carried out at pH 4 without adjusting the solution pH as the capacity of the sorbents is not reduced significantly at this pH level. 3.4. Effect of common ions in the medium Thorough investigation was made to find out the SCs of the sorbents in the presence of other common ions which are normally present in water viz., Cl− , NO3 − , SO4 2− and HCO3 − ions with a fixed initial concentration of 200 mg/L of these ions by keeping 10 mg/L as initial chromium concentration at 303 K. Fig. 10 explains the effect of common ions on the SC of GCB and show that these ions do not have much significant effect on SC of GCB. This is because of the fact that the GCB selectively removes chromium even in the presence of other common ions. Similar results were obtained for PCB and CCB. 3.5. Sorption isotherms To quantify the sorption capacity of the sorbents studied for the chromium removal, the two most commonly used isotherms, namely Freundlich and Langmuir isotherms have been adopted.
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Fig. 7. SEM micrographs of (a) GCB and (b) chromium sorbed GCB, EDAX spectra of (c) chromium treated GCB.
Freundlich isotherm constants for PCB, CCB and GCB were calculated from the linear plot of log qe vs. log Ce and the values are presented in Table 1. The conditions are found to be favourable for adsorption as the values of 1/n lie between 0 and 1 and the values of n greater than 1. The kF values of all the sorbents were found to increase with the increase in temperature. The increase in kF values confirms the endothermic nature of sorption. Higher r values were obtained for all the sorbents. These values indicate the applicability of Freundlich isotherm. Langmuir isotherm constants Qo and b for PCB, CCB and GCB were determined from the respective slope and intercept of the
linear plot of Ce /qe vs. Ce . These values are presented in Table 1. The higher values of r indicate its applicability of Langmuir isotherm. The values of Qo for all the sorbents increased with the increase in temperature. This confirms the endothermic nature and temperature dependence of the sorption process. The RL values were studied and calculated at different temperatures and are listed in Table 1. The RL values lying between 0 and 1 indicate that the conditions were favourable for adsorption [31]. The low chi-square [32] values of Langmuir over Freundlich isotherm indicate the suitability of Langmuir isotherm than the Freundlich isotherm.
Table 1 Freundlich and Langmuir isotherms of the modified chitosan beads on chromium sorption. Sorbents
Temp. (K)
Freundlich isotherm
Langmuir isotherm
1/n
n
kF (mg/g)(L/mg)
r
1/n
2
Qo (mg/g)
b (L/g)
RL
r
2
PCB
303 313 323
0.498 0.585 0.627
2.000 1.709 1.594
2.084 2.152 2.192
0.999 0.963 0.999
5.9E−03 7.6E−03 9.2E−05
7.518 7.812 8.064
0.215 0.174 0.156
0.571 0.533 0.500
0.998 0.947 0.999
1.8E−04 2.7E−03 8.4E−05
CCB
303 313 323
0.391 0.386 0.375
2.557 2.590 2.666
2.917 3.341 3.899
0.999 0.987 0.966
8.3E−04 1.8E−03 3.9E−03
8.264 8.620 9.174
0.394 0.481 0.630
0.116 0.101 0.090
0.998 0.991 0.992
2.6E−04 1.7E−03 2.1E−03
GCB
303 313 323
0.266 0.261 0.249
3.759 3.831 4.016
4.149 4.508 4.988
0.997 0.987 0.996
2.0E−04 1.1E−03 3.0E−04
9.340 11.490 13.690
0.970 1.174 1.476
0.079 0.068 0.060
0.998 0.999 0.998
6.7E−04 3.7E−04 1.3E−04
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Table 2 Thermodynamic parameters of the modified chitosan beads on the removal of chromium. Thermodynamic parameters Go (kJ mol−1 )
303 K 313 K 323 K
Ho (kJ mol−1 ) So (kJ mol−1 K−1 )
PCB
CCB
GCB
−5.60 −6.05 −6.23 3.99 0.03
−4.69 −4.36 −3.97 15.69 0.03
−3.44 −3.21 −2.64 15.52 0.03
Table 3 Field trial results of PCB, CCB, and GCB. Water quality parameters
Chromium (mg/L) pH Cl− (mg/L) Total hardness (mg/L) Total dissolved solids (mg/L)
Before treatment
0.12 7.65 1704 1160 4650
After treatment PCB
CCB
GCB
Nil 7.07 951 1080 2350
Nil 7.14 923 1000 2250
Nil 7.10 937 900 2000
3.6. Thermodynamic treatment of the sorption process For identifying the nature of sorption process, the values of thermodynamic parameters for PCB, CCB and GCB were calculated. They are tabulated and presented in Table 2. The thermodynamic treatment of the sorption data indicates that Go values were negative at all temperatures investigated [33,34]. The negative values of Go confirm the spontaneous nature of chromium sorption by the sorbents. The positive values of So indicates that the freedom of Cr(VI) ions is not too restricted in the sorbents. The positive values of Ho for chromium removal proves that the nature of sorption process by all the modified chitosan beads is endothermic. 3.7. Mechanism of chromium sorption All the three modified chitosan beads remove chromium by electrostatic adsorption coupled with reduction and complexation mechanism [35]. The modified forms gets positively charged at the experimental pH 4, due to the protonation of amino groups which removes hydrogen chromate ion by means of electrostatic attraction/complexation [29,36].
Fig. 8. Effect of contact time on the SC of the sorbents at 303 K.
Fig. 9. Influence of pH on the SC of the sorbents at 303 K.
The possible mechanism of chromium removal by PCB, CCB and GCB is given in Scheme 2. The presence of chromium peak in the EDAX spectra of chromium sorbed modified chitosan beads confirms the occurrence of chromium sorption onto the beads (cf. Figs. 6c, 7c and 8c). The FTIR spectra of Cr(VI) loaded sorbent have a new band at 540 cm−1 . This confirms the formation of Cr(OH)3 in the chromium sorbed beads [28]. Cr(VI) in acidic solution demonstrates a very high positive redox potential (+1.33 V) which denotes that it is strongly oxidizing and unstable in the presence of electron donors having lower reduction potential values than that of Cr(VI). This results in increased chromium sorption at low pH ranges. The reactive –OH and –NH2 electron donor groups present in the modified chitosan beads reduce the toxic Cr(VI) to less toxic Cr(III) compounds confirms the adsorption coupled reduction [29,36,37]. This information was supported by the greenish colour of chromium sorbed chitosan beads which is the typical colour of Cr(III) complexes. In addition, Cr(VI) is also reduced to Cr(III) by glutaraldehyde molecules (cross-linker) [35]. The enhancement in SCs of modified chitosan beads over chitosan beads may be due to the modification of chitosan beads.
Fig. 10. Effect of common ions on the SC of GCB at 303 K.
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Scheme 2. Mechanism of chromium removal by modified chitosan beads.
3.8. Field study The suitability of PCB, CCB and GCB is tested with a field sample taken from a nearby industrial area. About 0.1 g of sorbent was added to 50 ml of water sample and the contents were shaken with constant time at room temperature. The results are presented in Table 3. There is a significant reduction in the levels of other water quality parameters in addition to chromium. It is evident from the result that all the sorbents can be effectively employed for removing the chromium from water. 4. Conclusions The SCs of the modified chitosan beads PCB, CCB and GCB are found to be 3239, 3647 and 4057 mg/kg respectively while the CB showed only 1298 mg/kg. Sorption of chromium on modified chitosan beads was influenced by the pH of the medium. All the sorbents remove Cr(VI) selectively in the presence of common ions.
Chromium sorption follows Langmuir isotherm. The nature of sorption process is spontaneous and endothermic. The mechanism of chromium sorption on all the modified forms of chitosan beads is governed by electrostatic adsorption coupled reduction and complexation. These modified chitosan beads are stable and selective for chromium sorption and could be used for field applications. Acknowledgement The authors are grateful to Defence Research and Development Organization (DRDO) (No. ERIP/ER/0703670/M/01/1066), New Delhi, India for the provision of financial support to carry out this research work. References [1] C. Redded, C. Gerente, Y. Andres, P.L. Cloiree, Environ. Sci. Technol. 36 (2002) 2067–2073.
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Sorption of chromium(VI) using modified forms of chitosan beads G.N. Kousalya a , Muniyappan Rajiv Gandhi b , S. Meenakshi b,∗ a b
Department of Chemistry, GTN Arts College, Dindigul 624 003, Tamilnadu, India Department of Chemistry, Gandhigram Rural University, Gandhigram 624 302, Dindigul, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 22 January 2010 Received in revised form 13 February 2010 Accepted 23 March 2010 Available online 31 March 2010 Keywords: Chitosan Beads Isotherm Chromium Sorption
a b s t r a c t Modified forms of chitosan beads were prepared and used for chromium removal from the aqueous solution. The prepared chitosan beads viz., protonated chitosan beads (PCB), carboxylated chitosan beads (CCB) and grafted chitosan beads (GCB) possess enhanced chromium sorption capacities (SCs) of 3239, 3647 and 4057 mg/kg respectively than the raw chitosan beads (CB) which possess the SC of 1298 mg/kg with a minimum contact time of 10 min. The sorption experiments were carried out in batch mode to optimize various influencing parameters viz., contact time, pH, common ions and temperature. The sorbents were characterized by FTIR and SEM with EDAX analysis. The modified chitosan beads remove chromium by means of electrostatic adsorption coupled reduction and complexation. The adsorption data was fitted with Freundlich and Langmuir isotherms. The calculated values of thermodynamic parameters indicate the nature of chromium sorption. A field trial was carried out with water collected from a nearby industrial area. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Contamination of aquatic media by heavy metals is a serious environmental problem, mainly due to the discharge of industrial waste [1]. Heavy metals are highly toxic at low concentrations and can accumulate in living organisms, causing several disorders and diseases [2]. Chromium is one of the major contaminants in the wastewater of dyes and pigments, film and photography, galvanometric, electroplating, leather and mining industries [3]. Chromium exists in several oxidation states out of which Cr(III) and Cr(VI) are the most stable forms. Because of its high toxicity and potential carcinogenicity, Cr(VI) is of special concern [4]. Hexavalent chromium exists in water as oxyanions such as chromate (HCrO4 − ) and dichromate (Cr2 O7 2− ). It causes diseases such as epigastric pain, nausea, vomiting, severe diarrhoea and hemorrhage by ingestion. The maximum allowed concentration of chromium content in drinking water is 0.05 mg/L [5]. Conventional methods applied for Cr(VI) removal are chemical precipitation, oxidation/reduction, filtration, ion exchange, membrane separation and adsorption [6]. Adsorption is the most frequently applied technique owing to its advantages such as variety of adsorbent materials and high efficiency at a relatively low cost [7]. Chitosan is a cationic aminopolysaccharide copolymer of glucosamine and N-acetyl glu-
∗ Corresponding author. Tel.: +91 451 2452371; fax: +91 451 2454466. E-mail addresses: [email protected] (G.N. Kousalya), [email protected] (M. Rajiv Gandhi), drs [email protected] (S. Meenakshi). 0141-8130/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ijbiomac.2010.03.010
cosamine, obtained by partial deacetylation of chitin, which originates from shells of crustaceans such as crabs and prawns [8]. Chitin and chitosan have varied potential applications in the areas of biotechnology, biomedicine and food ingredients [9–11]. In addition chitin and chitosan appear to be more useful biopolymers reported for the high potential of sorption of metal ions [12–14]. Chitosan is biodegradable, biocompatatible and non-toxic biopolymer, reported to be an efficient heavy metal scavenger due to the presence of amino group [15] than the chitin. Chitosan is soluble in dilute mineral acids, except in sulphuric acid and it is necessary to reinforce chemical stability using cross-linking agents like glutaraldehyde [16]. Derivatives of chitosan have been applied for the removal of various metal ions from aqueous solution. Sorption mechanism of mercury was studied on a chitosan derivative by Gavilan et al. [17] and equilibrium and kinetic studies of adsorption of Cu(II) on chitosan/PVA beads were carried out by Wan Nagh et al. [18]. Sorption performances can be enhanced by modification of chitosan. The selectivity of the sorption is improved through chemical modification techniques based on protonation, carboxylation and grafting. The main objective of this paper is a systematic evaluation of the performance of a novel, chemically modified chitosan beads for the uptake of Cr(VI) from water and establish the probable mechanism of chromium sorption. The studies have been conducted in the optimization of various experimental conditions like contact time, pH, temperature and effect of common ions. A comparative assessment of chitosan beads and their modified forms were also made.
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Scheme 1. Preparation of PCB, CCB and GCB.
2. Materials and methods 2.1. Materials Chitosan with its deacetylation degree of 85% was supplied by Pelican Biotech and Chemicals Labs, Kerala (India). The viscosity of the chitosan solution was determined to be 700 mPa s by Brookfield Dial Reading Viscometer using electronic driveRVT model (USA make). The chitosan solution was maintained at a constant viscosity for beads preparation in order to maintain uniform molecular weight. NaOH, HCl, glacial acetic acid, glutaraldehyde, chloroacetic acid, ethylenediamine, potassium dichromate and all other chemicals and reagents were of analytical grade. 2.2. Preparation of cross-linked chitosan beads (CB) Chitosan (20 g) was dissolved in 2.0% glacial acetic acid solution (1000 ml). The chitosan solution was dropped into a 0.5 M aqueous NaOH solution to form uniform chitosan beads. After gelling for a minimum of 16 h in 0.5 M NaOH solution, the beads were washed with distilled water to a neutral pH. The wet beads were cross-linked with 2.5 wt.% glutaraldehyde solution and the ratio of glutaraldehyde to chitosan beads was approximately 1.5 ml/g of wet beads [19]. Cross-linking reaction occurred for 48 h and then cross-linked beads were washed with distilled water to remove any free glutaraldehyde. 2.3. Preparation of protonated cross-linked chitosan beads (PCB) PCB was prepared in order to effectively utilize the amino groups of cross-linked chitosan beads for chromium sorption. The crosslinked chitosan beads were treated with concentrated HCl for 30 min for protonation of beads [20] and the reaction is shown in Scheme 1. The PCB was washed with distilled water to neutral pH, dried at room temperature and used for sorption studies [21]. 2.4. Preparation of carboxylated cross-linked chitosan beads (CCB) CCB was prepared in order to effectively utilize the hydroxyl groups of cross-linked chitosan beads for metal sorption. The cross-
linked chitosan beads were treated with aqueous 0.5 M chloroacetic acid maintained at pH 8.0 using 0.1 M NaOH for 10 h at room temperature to convert the hydroxyl groups to carboxyl groups [20] as shown in Scheme 1. CCB was washed with distilled water to neutral pH, dried at room temperature and used for sorption studies [22]. 2.5. Preparation of grafted cross-linked chitosan beads (GCB) The aminated reaction of chitosan is composed of two steps as shown in Scheme 1. In the first step, chloroacetic acid is used to introduce carboxyl groups to the hydroxyl groups in chitosan [23]. The wet cross-linked chitosan beads were treated with aqueous 0.5 M chloroacetic acid maintained at pH 8.0 using 1.0 M NaOH for 10 h at room temperature and then the reacted chitosan beads were washed with distilled water several times to remove unreacted reagents. In the second step, the carboxylated chitosan beads obtained through first reaction were treated in 100 ml of 1.0 M ethylenediamine. Reaction was carried out for 5 h and the final chitosan beads were thoroughly washed using distilled water for several times and treated with with concentrated HCl for 30 min for protonation of GCB. The protonated GCB beads were washed with distilled water to neutral pH, dried at room temperature and used for sorption studies. 2.6. Sorption experiments The sorption experiments were performed by batch equilibration method. Stock solution of chromium containing 1000 mg/L was prepared and this was used for sorption experiments. Batch sorption experiments in duplicate were carried out by mixing 0.1 g of sorbent with 50 ml of 10 mg/L as initial chromium concentration. The contents were shaken thoroughly using a thermostated shaker rotating at a speed of 200 rpm. The solution was then filtered and the residual chromium concentration was measured using UV–visible spectrophotometer (Pharo 300 Merck) at 540 nm, according to the 1,5-diphenyl-carbazide method [24]. pH measurements were carried out with the expandable ion analyzer EA 940 with pH electrode. The thermodynamic parameters of the adsorption were established by conducting the experiments at 303, 313 and 323 K in a temperature controlled mechanical shaker. The SCs of the sorbents were studied at different conditions like contact time of the sorbent for maximum sorption, pH of the medium and
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Fig. 1. FTIR spectra of CB, PCB, CCB, and GCB.
Fig. 2. FTIR spectra of (a) PCB and (b) chromium sorbed PCB.
the effect of other common ions present in the water. All other water quality parameters were analyzed by using standard methods [24]. 2.7. Analysis The prepared sorbents were characterized by Fourier transform infrared spectroscopy (FTIR). FTIR spectra of the samples as solid by diluting in KBr pellets were recorded with JASCO-460 plus model. The results of FTIR spectrometer were used to confirm the functional groups present in the surface morphology of the modified chitosan beads before and after chromium sorption was studied with scanning electron microscope (SEM) with JOEL JSM 6390 LV model. Elemental spectra were obtained using an energy dispersive X-ray analyzer (EDAX) during SEM observations which allow a qualitative detection and localization of elements in the modified chitosan beads. Computations were made using Microcal Origin (Version 6.0) software. The goodness of fit was discussed using regression correlation coefficient (r) and chi-square analysis. Fig. 3. FTIR spectra of (a) CCB and (b) chromium sorbed CCB.
3. Results and discussion 3.1. Characterization of the sorbents FTIR spectrum of chitosan beads and their modified chitosan beads are shown in Fig. 1. The IR spectra of both chitin and chitosan are very similar to those reported in the literature [25,26]. The chitosan spectrum differs from that of chitin the band at 1555 cm−1 corresponding to the N–H deformation that is present only in a lower extent in chitosan samples. Although there is a possibility of overlapping between –NH2 and –OH stretching vibrations, the strong broad band at the wave number region of 3300–3500 cm−1 is the characteristic of –NH2 stretching vibration. The major bands for the chitosan bead can be assigned as follows: 3440 cm−1 (–OH and –NH2 stretching vibrations), 2921 cm−1 (–CH stretching vibration in –CH and –CH2 ), 1652 cm−1 (–NH2 bending vibration), 1379 cm−1 (–CH symmetric bending vibrations in –CHOH–), 1067 and 1028 cm−1 (–CO stretching vibration in –COH) [27]. After cross-linking the chitosan beads, the –OH and –NH2 stretching vibration around the wave number of 3440 cm−1 and the –NH2 bending vibration at the wave number 1652 cm−1 was shifted to lower frequencies.
Fig. 4. FTIR spectra of (a) GCB and (b) chromium sorbed GCB.
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Fig. 6. SEM pictures of (a) CCB and (b) chromium sorbed CCB, EDAX spectra of (c) chromium treated CCB. Fig. 5. SEM images of (a) PCB and (b) chromium sorbed PCB, EDAX spectra of (c) chromium treated PCB.
Fig. 2 shows the FTIR spectra of (a) PCB and (b) chromium sorbed PCB. Fig. 3 shows the FTIR spectra of (a) CCB and (b) chromium sorbed CCB. Fig. 4 shows the FTIR spectra of (a) GCB and (b) chromium sorbed GCB. The FTIR spectra of Cr(VI) loaded sorbents having a new band at 530–540 cm−1 confirm the formation of Cr(OH)3 in the chromium sorbed PCB, CCB and GCB [28,29]. The other bands are the characteristics of the sorbents. SEM pictures of PCB and the chromium sorbed PCB are shown in Fig. 5a and b respectively, SEM images of CCB and the chromium sorbed CCB are shown in Fig. 6a and b respectively. SEM micrographs of GCB and the chromium sorbed GCB are shown in Fig. 7a and b respectively. The surface change in the SEM micrographs of the modified chitosan beads before and after chromium treatment indicates the structural changes in the sorbents. This is further supported by EDAX analysis which provides the direct evidence for the sorption of chromium onto the beads. The EDAX spectra of chromium sorbed modified chitosan beads show the presence of chromium peaks which confirms that the chromium sorption has occurred onto the modified chitosan beads (cf. Figs. 5–7c).
3.2. Effect of contact time The SCs of CB, PCB, CCB and GCB were determined by varying the contact time in the range of 5–60 min. About 0.1 g of the sorbent was placed into 50 ml of the 10 mg/L initial chromium solution. The iodine flasks with their contents taken for the study were shaken thoroughly using a mechanical shaker at 200 rpm. All the six iodine flasks were removed from the shaker and the contents were filtered and analyzed for Cr(VI). As it is evident from Fig. 8, the SC of all the sorbents reached saturation after 10 min. Hence, 10 min is fixed as the contact time for the sorbents for further studies. Maximum SCs of CB, PCB, CCB, GCB were found to be 1298, 3239, 3647 and 4057 mg/kg respectively. Among the sorbents, GCB experienced higher SC than CB, PCB and CCB.
3.3. Effect of pH The removal of ion from aqueous solution was highly dependent on the solution pH in many cases as it altered the surface charge on the sorbent. Therefore, the SC of all the three sorbents was determined at five different pH levels such as 3, 5, 7, 9 and 11. The results are shown in Fig. 9. It is obvious from the figure that the pH has a slight influence on the SC of sorbents. A maximum SC was observed at pH 5–7 and a slight decline in SC was observed in alkaline medium. At acidic pH, the predominant species of chromium are HCrO4 − and Cr2 O7 2− in aqueous solutions. On the other hand, under acidic conditions, the surfaces of all the sorbents were highly protonated which favours the uptake of Cr(VI) in the anionic form. With the increase in pH, the degree of protonation of surface reduces gradually [30]. In addition, the active binding cites of the sorbents were occupied by the OH− ions and hence sorption is decreased. Further, experiments were carried out at pH 4 without adjusting the solution pH as the capacity of the sorbents is not reduced significantly at this pH level. 3.4. Effect of common ions in the medium Thorough investigation was made to find out the SCs of the sorbents in the presence of other common ions which are normally present in water viz., Cl− , NO3 − , SO4 2− and HCO3 − ions with a fixed initial concentration of 200 mg/L of these ions by keeping 10 mg/L as initial chromium concentration at 303 K. Fig. 10 explains the effect of common ions on the SC of GCB and show that these ions do not have much significant effect on SC of GCB. This is because of the fact that the GCB selectively removes chromium even in the presence of other common ions. Similar results were obtained for PCB and CCB. 3.5. Sorption isotherms To quantify the sorption capacity of the sorbents studied for the chromium removal, the two most commonly used isotherms, namely Freundlich and Langmuir isotherms have been adopted.
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Fig. 7. SEM micrographs of (a) GCB and (b) chromium sorbed GCB, EDAX spectra of (c) chromium treated GCB.
Freundlich isotherm constants for PCB, CCB and GCB were calculated from the linear plot of log qe vs. log Ce and the values are presented in Table 1. The conditions are found to be favourable for adsorption as the values of 1/n lie between 0 and 1 and the values of n greater than 1. The kF values of all the sorbents were found to increase with the increase in temperature. The increase in kF values confirms the endothermic nature of sorption. Higher r values were obtained for all the sorbents. These values indicate the applicability of Freundlich isotherm. Langmuir isotherm constants Qo and b for PCB, CCB and GCB were determined from the respective slope and intercept of the
linear plot of Ce /qe vs. Ce . These values are presented in Table 1. The higher values of r indicate its applicability of Langmuir isotherm. The values of Qo for all the sorbents increased with the increase in temperature. This confirms the endothermic nature and temperature dependence of the sorption process. The RL values were studied and calculated at different temperatures and are listed in Table 1. The RL values lying between 0 and 1 indicate that the conditions were favourable for adsorption [31]. The low chi-square [32] values of Langmuir over Freundlich isotherm indicate the suitability of Langmuir isotherm than the Freundlich isotherm.
Table 1 Freundlich and Langmuir isotherms of the modified chitosan beads on chromium sorption. Sorbents
Temp. (K)
Freundlich isotherm
Langmuir isotherm
1/n
n
kF (mg/g)(L/mg)
r
1/n
2
Qo (mg/g)
b (L/g)
RL
r
2
PCB
303 313 323
0.498 0.585 0.627
2.000 1.709 1.594
2.084 2.152 2.192
0.999 0.963 0.999
5.9E−03 7.6E−03 9.2E−05
7.518 7.812 8.064
0.215 0.174 0.156
0.571 0.533 0.500
0.998 0.947 0.999
1.8E−04 2.7E−03 8.4E−05
CCB
303 313 323
0.391 0.386 0.375
2.557 2.590 2.666
2.917 3.341 3.899
0.999 0.987 0.966
8.3E−04 1.8E−03 3.9E−03
8.264 8.620 9.174
0.394 0.481 0.630
0.116 0.101 0.090
0.998 0.991 0.992
2.6E−04 1.7E−03 2.1E−03
GCB
303 313 323
0.266 0.261 0.249
3.759 3.831 4.016
4.149 4.508 4.988
0.997 0.987 0.996
2.0E−04 1.1E−03 3.0E−04
9.340 11.490 13.690
0.970 1.174 1.476
0.079 0.068 0.060
0.998 0.999 0.998
6.7E−04 3.7E−04 1.3E−04
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Table 2 Thermodynamic parameters of the modified chitosan beads on the removal of chromium. Thermodynamic parameters Go (kJ mol−1 )
303 K 313 K 323 K
Ho (kJ mol−1 ) So (kJ mol−1 K−1 )
PCB
CCB
GCB
−5.60 −6.05 −6.23 3.99 0.03
−4.69 −4.36 −3.97 15.69 0.03
−3.44 −3.21 −2.64 15.52 0.03
Table 3 Field trial results of PCB, CCB, and GCB. Water quality parameters
Chromium (mg/L) pH Cl− (mg/L) Total hardness (mg/L) Total dissolved solids (mg/L)
Before treatment
0.12 7.65 1704 1160 4650
After treatment PCB
CCB
GCB
Nil 7.07 951 1080 2350
Nil 7.14 923 1000 2250
Nil 7.10 937 900 2000
3.6. Thermodynamic treatment of the sorption process For identifying the nature of sorption process, the values of thermodynamic parameters for PCB, CCB and GCB were calculated. They are tabulated and presented in Table 2. The thermodynamic treatment of the sorption data indicates that Go values were negative at all temperatures investigated [33,34]. The negative values of Go confirm the spontaneous nature of chromium sorption by the sorbents. The positive values of So indicates that the freedom of Cr(VI) ions is not too restricted in the sorbents. The positive values of Ho for chromium removal proves that the nature of sorption process by all the modified chitosan beads is endothermic. 3.7. Mechanism of chromium sorption All the three modified chitosan beads remove chromium by electrostatic adsorption coupled with reduction and complexation mechanism [35]. The modified forms gets positively charged at the experimental pH 4, due to the protonation of amino groups which removes hydrogen chromate ion by means of electrostatic attraction/complexation [29,36].
Fig. 8. Effect of contact time on the SC of the sorbents at 303 K.
Fig. 9. Influence of pH on the SC of the sorbents at 303 K.
The possible mechanism of chromium removal by PCB, CCB and GCB is given in Scheme 2. The presence of chromium peak in the EDAX spectra of chromium sorbed modified chitosan beads confirms the occurrence of chromium sorption onto the beads (cf. Figs. 6c, 7c and 8c). The FTIR spectra of Cr(VI) loaded sorbent have a new band at 540 cm−1 . This confirms the formation of Cr(OH)3 in the chromium sorbed beads [28]. Cr(VI) in acidic solution demonstrates a very high positive redox potential (+1.33 V) which denotes that it is strongly oxidizing and unstable in the presence of electron donors having lower reduction potential values than that of Cr(VI). This results in increased chromium sorption at low pH ranges. The reactive –OH and –NH2 electron donor groups present in the modified chitosan beads reduce the toxic Cr(VI) to less toxic Cr(III) compounds confirms the adsorption coupled reduction [29,36,37]. This information was supported by the greenish colour of chromium sorbed chitosan beads which is the typical colour of Cr(III) complexes. In addition, Cr(VI) is also reduced to Cr(III) by glutaraldehyde molecules (cross-linker) [35]. The enhancement in SCs of modified chitosan beads over chitosan beads may be due to the modification of chitosan beads.
Fig. 10. Effect of common ions on the SC of GCB at 303 K.
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Scheme 2. Mechanism of chromium removal by modified chitosan beads.
3.8. Field study The suitability of PCB, CCB and GCB is tested with a field sample taken from a nearby industrial area. About 0.1 g of sorbent was added to 50 ml of water sample and the contents were shaken with constant time at room temperature. The results are presented in Table 3. There is a significant reduction in the levels of other water quality parameters in addition to chromium. It is evident from the result that all the sorbents can be effectively employed for removing the chromium from water. 4. Conclusions The SCs of the modified chitosan beads PCB, CCB and GCB are found to be 3239, 3647 and 4057 mg/kg respectively while the CB showed only 1298 mg/kg. Sorption of chromium on modified chitosan beads was influenced by the pH of the medium. All the sorbents remove Cr(VI) selectively in the presence of common ions.
Chromium sorption follows Langmuir isotherm. The nature of sorption process is spontaneous and endothermic. The mechanism of chromium sorption on all the modified forms of chitosan beads is governed by electrostatic adsorption coupled reduction and complexation. These modified chitosan beads are stable and selective for chromium sorption and could be used for field applications. Acknowledgement The authors are grateful to Defence Research and Development Organization (DRDO) (No. ERIP/ER/0703670/M/01/1066), New Delhi, India for the provision of financial support to carry out this research work. References [1] C. Redded, C. Gerente, Y. Andres, P.L. Cloiree, Environ. Sci. Technol. 36 (2002) 2067–2073.
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