Adsorption mechanism of synthetic reactive dye wastewater by chitosan

Adsorption mechanism of synthetic reactive dye wastewater by chitosan

Journal of Colloid and Interface Science 286 (2005) 36–42 www.elsevier.com/locate/jcis Adsorption mechanism of synthetic reactive dye wastewater by c...

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Journal of Colloid and Interface Science 286 (2005) 36–42 www.elsevier.com/locate/jcis

Adsorption mechanism of synthetic reactive dye wastewater by chitosan Niramol Sakkayawong a , Paitip Thiravetyan a,∗ , Woranan Nakbanpote b a School of Bioresources and Technology, King Mongkut’s University of Technology, Thonburi, 83 Moo. 8 Thakham, Bangkhuntien,

Bangkok, 10150, Thailand b Pilot Plant Development and Training Institute, King Mongkut’s University of Technology, Thonburi, 83 Moo. 8 Thakham, Bangkhuntien,

Bangkok, 10150, Thailand Received 9 September 2004; accepted 13 January 2005 Available online 25 March 2005

Abstract Chitosan was able to remove the color from synthetic reactive dye wastewater (SRDW) under acidic and caustic conditions. The effect of the initial pH on SRDW indicated that electrostatic interaction occurred between the effective functional groups (amino groups) and the dye under acidic conditions. Moreover, SRDW adsorption under caustic conditions was also affected by the covalent bonding of dye and hydroxyl groups of chitosan. In addition, elution tests confirmed that chemical adsorption occurred under acidic conditions, while both physical and chemical adsorption appeared under caustic conditions. The spectra of attenuated total reflectance Fourier transform infrared spectrometry confirmed the functional groups of chitosan that affected the SRDW adsorption. However, the maximum adsorption capacities of chitosan increased when the temperature increased. The maximum adsorption capacity of chitosan obtained from the Langmuir model was 68, 110, and 156 mg g−1 under a system pH of 11.0 at 20, 40, and 60 ◦ C, respectively. The negative values of enthalpy change (H ), free energy change (G), and entropy change (S) indicated an exothermic, spontaneous process and decreasing disorder of the system, respectively. Therefore, the mechanism of SRDW adsorption by chitosan was probably by chemical adsorption for a wide range of pH’s and at high temperatures.  2005 Elsevier Inc. All rights reserved. Keywords: Adsorption; ATR-FTIR; Chitosan; Reactive dye wastewater; Electrostatic; Thermodynamic

1. Introduction The disposal of textile wastewater is currently a major problem in Thailand. Textile industries produce a lot of wastewater, which contains a number of contaminants, including acidic or caustic dissolved solids, toxic compounds, and dyes [1]. Among textile effluents, reactive dyes are hardly eliminated under aerobic conditions and are probably decomposed into carcinogenic aromatic amines under anaerobic conditions [2]. Furthermore, it is difficult to remove reactive dyes using chemical coagulation due to the dyes’ high solubility in water [3]. Adsorption with activated carbon appears to be the best prospect of eliminating this dye. Inspite of its good efficiency, this adsorbent is expen* Corresponding author. Fax: +66-2-452-3455.

E-mail address: [email protected] (P. Thiravetyan). 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.01.020

sive and difficult to regenerate after use. Therefore, many researches in recent years have focused on the use of various low-cost adsorbents instead of activated carbon [4]. Chitosan is usually obtained from waste materials from the seafood processing industry, mainly shells of crabs, shrimp, prawns, and krill [5]. Shrimp shell waste is a plentiful waste from Thai seafood processing industries (200,000 tons per year). Therefore, these wastes are low-cost, available in abundance, and suitable for producing chitosan polymer. Chitosan has a linear structure of 2-amino-2-deoxy-Dglucopyranose units joined by β(1 → 4) glucosidic bonds. This polymer is also known as an effective adsorbent of reactive dyes [6–9] due to the cationic nature of this polymer and forms a water-soluble complex with anionic polyelectrolyte. Accordingly, chitosan has potential as an inexpensive and effective adsorbent for reactive dye wastewater removal. So far, no studies have been reported on the mechanisms or thermodynamics of SRDW adsorption by chitosan.

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Fig. 1. Structure of cellulose, chitosan, and chitin.

Therefore, the objective of this research is to study the feasibility of using chitosan as an adsorbent for the removal of reactive dye from SRDW. The SRDW, which contains C.I. Reactive Red 141, sodium sulfate, and sodium carbonate, substituted for real dyeing wastewater from the textile industry. The effect of pH, the adsorption isotherms, and thermodynamic study were investigated in order to achieve the optimum pH for dye removal, maximum adsorption capacity, and to find an explanation for the adsorption mechanism. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of chitosan (before and after SRDW adsorption under acidic and caustic condition) were investigated to confirm the functional groups of chitosan that affect SRDW adsorption.

2. Materials and methods Chitosan (90%DD) and chitin (48%DD) from shrimp shell waste were supplied by Seafresh Chitosan (Lab) Co. Ltd., Thailand. The average particle size of each was 850 µm–1 mm. Chitin was used to investigate the effect of the degree of deacetylation-enhanced SRDW adsorption. The chitosan and chitin structures are shown in Fig. 1. C.I. Reactive Red (RR141) was obtained from DyStar Thai. Ltd.; the structure is shown in Fig. 2. The wavelength of maximum absorbance (λmax ) for RR141 and synthetic reactive dye wastewater was 544 nm. RR141 had a molecular weight and solubility of 1774 and 50 g l−1 , respectively. Moreover,

Fig. 2. Chemical structure of diazo C.I. Reactive Red (RR141).

microcrystalline cellulose (Avicel, Fluka) and cotton wool were also used for testing the ability of hydroxyl groups to adsorb synthetic reactive dye wastewater. The cellulose structure is shown in Fig. 1. The dyeing process was obtained from DyStar Thai. Ltd. The laboratory dyeing process is shown in Fig. 3: 4 g l−1 of RR141, 90 g l−1 of sodium sulfate, and 20 g l−1 of sodium carbonate were added during the dyeing process. Sodium sulfate and sodium carbonate were added to increase the dye substantivity and improve fastness, respectively. Moreover, the ratio of fabric to dyeing solution was 1:10 (w/v). After the dyeing process, the dyeing wastewater contained 400 mg l−1 of RR141 and was used as the SRDW sample for this research. The experiment was performed in a 15-ml vial with 1% (w/v) of chitosan powder and 10 ml of SRDW and shaken at 150 rpm. The effect of the initial pH of SRDW was investigated at 30 ◦ C for 24 h by adjusting the system pH by 1.0 M HCl and 1.0 M NaOH. The control sample in this experiment was a synthetic reactive dye for which the system pH was adjusted by the same acid and base in order to compare the

Fig. 3. The dyeing process of reactive dye (from DyStar Laboratory manual).

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changing of color under the same conditions. The adsorption isotherm of dye removal was studied by varying the dosage of adsorbents in the range 0.10–1.75% (w/v) and varying the temperature to 20, 40, and 60 ◦ C. A sample was centrifuged at 4500 rpm for 10 min. The changing pH of the supernatant was detected by a pH meter (Mettler Delta 340, England) and then analyzed for color by a spectrophotometer (Unico2100, USA). A vacuum manifold was used in an elution test with a flow rate in the range of 0.29–0.31 ml min−1 . Dyeadsorbed chitosan was eluted by deionized water and 1 M NaOH. The SRDW adsorption by chitosan under acidic and caustic conditions was confirmed by ATR-FTIR spectrometry (Magna-IR 750 Series II, USA).

Table 1 Effect of the degree of deacetylation in chitin and chitosan on synthetic reactive dye wastewater removal at 0.5% (w/v) and 1.0% (w/v) dosage under acidic conditions

3. Results and discussion 3.1. Effect of pH The effects of the initial pH of SRDW (pH 2.0–11.0) using chitosan are shown in Fig. 4. The system pH changed from 8.0 to 8.5 during SRDW adsorption by chitosan at an initial solution pH of 2.0–5.0. The explanation for this is that under acidic conditions hydrogen atoms (H+ ) in the solution could protonate the amine groups (–NH2 ) of chitosan and thus cause the increased pH [9,10]: R –NH

2

+

H+

 R –NH+ 3.

(1)

In aqueous solutions, the synthetic reactive dye was dissolved and the sulfonate group of the reactive dye was dissociated and converted to anionic dye ions [11]: H2 O

DSO3 Na →

DSO− 3

+

Na+ .

(2)

The adsorption process then proceeded due to the electrostatic interaction between these two counterions: − +−  O3 SD. R –NH+ 3 + DSO3  R –NH3

Fig. 4. Effect of pH on color removal of synthetic reactive dye wastewater by chitosan.

(3)

Therefore, the SRDW was 99% removed by chitosan at the initial pH of 2.0–5.0 (Fig. 4). Moreover, the effects of the degree of deacetylation in chitin and chitosan on SRDW adsorption at 0.5% (w/v) and 1.0% (w/v) dosage under acidic conditions are shown in Table 1. Chitin (48%DD) and chitosan (90%DD), at a dosage of 0.5% (w/v), removed 64 and 82% of the SRDW, while the system pH of chitin and chitosan was 1.90 and 2.47, respectively. Furthermore, the SRDW was removed by 82 and 85% chitin (48%DD) and chitosan (90%DD), at a dosage of 1.0% (w/v), respectively, while the system pH of chitin and chitosan was 2.06 and 6.43, respectively. In addition, chitin had only 48% amine groups, while chitosan had 90%. Therefore, chitosan at a dosage of 1.0% could adsorb more SRDW than chitin and the initial pH changed from 2.11 to 6.43, whereas chitosan at a dosage of 0.5% slightly changed the initial pH from 2.11 to 2.47. This result indicated that the increase of SRDW adsorption under acidic conditions was followed by the increase of amine groups in the chitosan polymer.

Adsorbent

Dosage of adsorbent % (w/v)

Initial pH

pH of system

Dye removal (%)

Chitin Chitosan Chitin Chitosan

0.5

2.11

1.90 2.47 2.06 6.43

63.89 81.91 81.66 84.57

1.0

However, the SRDW was still adsorbed by chitosan to 96–98% at an initial system pH of 6.0–11.0, as shown in Fig. 4. This phenomena may be explained by the accessibility of hydroxyl groups in chitosan. The deprotonation of the hydroxyl group [10] occurred under caustic conditions: CH2 OH + OH−  CH2 O− + H2 O.

(4)

The hydroxyl group of the chitosan polymer could adsorb SRDW by covalent bonding, which is the same as the adsorption mechanism of cellulose polymers with reactive dyes in dyeing processes (Fig. 5). In dyeing processes, it is known that the chloride group (–Cl) of reactive dyes can react with fabric molecules at the hydroxyl group of cellulose (HO–cellulose) by ionization after the higher pH is adjusted with Na2 CO3 . The HO–cellulose was deprotonated under caustic conditions and transformed to a cellulose ion (− O– cellulose): Cell–OH + OH−  Cell–O− .

(5)

The cellulose ion formed a covalent bond on the reactive dye, after which the chloride groups of the reactive dye were released into the solution (Fig. 5). In this dyeing process, Na2 SO4 was added in the first step of dyeing to prevent dye hydrolysis and increase the physical adsorption between the dye and fabric. The synthetic reactive dyeing wastewater also consisted of reactive dye, Na2 SO4 , and Na2 CO3 . Therefore, the hydroxyl groups of chitosan might be attached to the reactive dye by covalent bonding under caustic pH, as in the case of cellulose in dyeing processes. Table 2 shows the effect of the hydroxyl group on chitosan and cellulose polymers (cotton wool and microcrys-

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Fig. 5. Mechanism of reactive dye adsorption in dyeing process by cellulose. Table 2 Comparison of dye removal from 400 mg l−1 synthetic reactive dye wastewater by chitosan, cotton wool, and microcrystalline cellulose at system pH 10 at equilibrium time of 24 h Adsorbent 1% (w/v)

Dye removal (%)

pH of system

Chitosan Cotton wool Microcrystalline cellulose

95.96 67.05 67.73

10.12 10.08 10.09

talline cellulose) that adsorbed SRDW under caustic conditions. Chitosan, cotton wool, and microcrystalline cellulose were able to remove SRDW to 96, 67, and 68% of SRDW, respectively, at a system pH of 10 at an equilibrium time of 24 h. This indicated that hydroxyl groups were involved in SRDW adsorption under caustic conditions. 3.2. Elution test 3.2.1. Elution test of dye-adsorbed chitosan under acidic conditions Table 3 shows that water could not elute the SRDW from chitosan polymer, whereas NaOH could elute only 32%. In addition, the synthetic reactive dye had many sulfate anionic groups in the dye polymer, which could attach to many amine groups on the chitosan polymer, and electrostatic interaction occurred. However, the amine groups are hydrogen-bonded to the sulfate anions and the water molecules form infinite (H2 O)n chains of dye and, through the sulfate anions, link the amine group in the three-dimensional

Table 3 Elution test of dye-adsorbed chitosan under acidic conditions using water and 1 M NaOH as eluents Eluent Water 1 M NaOH Total of elution (%)

pH Initial

System

Elution (%)

7.96 13.17

7.41 12.82

0 31.96 31.96

hydrogen bond network [12]. Therefore, the interaction between sulfate anions and amine groups difficulty in elution to 100% using 1 M NaOH. In addition, this result also confirmed that the adsorption mechanism was chemical adsorption. 3.2.2. Elution study of dye-adsorbed chitosan under caustic conditions In addition, the hydroxyl group of chitosan might be attached with the reactive dye by covalent bonding under caustic pH, as in the case of cellulose in dyeing processes (Fig. 5). Therefore, water should not be able to elute SRDW from chitosan polymer. However, Table 4 shows that water could elute SRDW from chitosan, after SRDW adsorption under caustic conditions, to 33%, while 1 M NaOH could elute only 14%. Not only were covalent linkages involved in the adsorption mechanism, but also van der Waals forces occurred. This explanation follows the dyeing theory, which mentions the ionic forces, hydrogen bonds, van der Waals forces, and covalent linkages involved in the dyeing process [13]. Van der Waals forces are weak bond-

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Table 4 Elution test of dye-adsorbed chitosan under caustic conditions by using water and 1 M NaOH as eluents Eluent Water 1 M NaOH

pH Initial

System

Elution (%)

7.96 13.17

7.78 12.81

32.98 14.14

Total of elution (%)

47.12

ings of physical adsorption. They occur between azo groups (–N=N–) of reactive dye and hydroxyl groups (–OH) of chitosan. Therefore, water and NaOH could destroy some of the bonding and elute the dye out. However, covalent linkages are actual chemical bonds between dyes and hydroxyl groups of chitosan, resulting in the total elution of only 47%. The results imply that the mechanism of SRDW adsorption by chitosan under caustic conditions involved both chemical and physical adsorption. 3.3. ATR-FTIR analysis 3.3.1. Mechanism of adsorption of SRDW by chitosan under acidic conditions The ATR-FTIR spectra of chitosan before and after adsorption of SRDW are shown in Figs. 6a and 6b, respectively. The peaks of 1596 cm−1 (–NH2 ), 1657 cm−1 (amide I band), 3360 cm−1 (N–H), 1033 cm−1 (C–N), and 1142 cm−1 (C–N) represent the chitosan peak (Fig. 6a), which involved the functional group of amine on chitosan polymer. Accordingly, under acidic conditions, electrostatic interaction occurred between the protonated amine and the anionic dye ion, as shown in Fig. 6b. The 1596 cm−1 (–NH2 ) and 1657 cm−1 (amide I) were shifted to the peaks at 1533 and 1628 cm−1 , respectively (Fig. 6b). Furthermore, the absence of sharp peaks at 1142 (C–N) and 1033 cm−1 (C–N) confirmed that the amine group of chitosan changed to be-

Fig. 7. ATR-FTIR spectra of chitosan (a) before and (b) after adsorption of synthetic reactive dye wastewater under caustic conditions.

come a protonated amine (Eq. (1)). In addition, new peaks appeared at 1210 and 1459 cm−1 . These peaks are SO3 of dye and a ring of dye, respectively, which confirmed the attachment of dye on the chitosan polymer. 3.3.2. Mechanism of adsorption of SRDW by chitosan under caustic conditions Figs. 7a and 7b show the ATR-FTIR spectra of chitosan before and after SRDW adsorption under caustic conditions, respectively. Under caustic conditions, the peaks at 1596 (–NH2 ), 1657 (amide I band), 3360 (N–H), 1033 (C–N), and 1142 cm−1 (C–N) imply that the hydroxyl group of chitosan was the main functional group for SRDW adsorption by covalent bonding. The peak at 1073 cm−1 (C–O of primary alcohol and secondary alcohol) was decreased in sharpness. In addition, a small peak of OH appeared at 2922 cm−1 . These data confirmed that the hydroxyl group of chitosan polymer changed to become a deprotonated hydroxyl group (Eq. (4)). Moreover, the two new peaks at 1210 and 1459 cm−1 indicated SO3 and the ring of dye, respectively. These peaks confirmed the attachment of synthetic reactive dye on chitosan polymer under caustic conditions. 3.4. Adsorption isotherms Experimental equilibrium data for SRDW adsorption by chitosan were investigated at temperatures of 20, 40, and 60 ◦ C. However, the adsorption of chitosan was followed with the Langmuir adsorption model [14], qe =

Fig. 6. ATR-FTIR spectra of chitosan (a) before and (b) after adsorption of synthetic reactive dye wastewater under acidic conditions.

qmax Ce , (1/b + Ce )

(6)

where b = Langmuir constant related to energy (l mg−1 ), Ce = concentration of dye at equilibrium concentration (mg l−1 ), C0 = initial dye concentration (mg l−1 ), qe = amount of dye adsorbed at equilibrium (mg g−1 ), and qmax = maximum adsorption capacity (mg g−1 ).

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Table 5 Langmuir constant, separation factor, and thermodynamic parameters for the adsorption of synthetic reactive dye wastewater by chitosan at temperatures of 20, 40, and 60 ◦ C Adsorbent

Temperature (◦ C)

qmax (mg g−1 )

b × 10−3 (l mg−1 )

RL

Chitosan

20 40 60

67.93 110.11 155.93

18.5 13.0 7.5

0.12 0.16 0.25

G (kJ mol−1 ) −7.10 −6.67 −5.58

S (J mol−1 K−1 )

H (kJ mol−1 )

−37.88 −36.88 −37.89

−18.20

(S) are calculated [16–18] by −G = 2.303RT log b, H + constant, log b = − 2.303RT G = H − T S,

Fig. 8. Isotherm for synthetic reactive dye wastewater adsorption by chitosan at different temperatures: (1) 20; (P) 40; and (!) 60 ◦ C.

The SRDW adsorption by chitosan increased with the rise of temperature from 20 to 60 ◦ C at a system pH of 11.00. The values of qmax and b at different temperatures were determined from the slope and intercept of Langmuir plots, respectively. All constant values obtained by this model are represented in Table 5. Therefore, the b values also indicate that chitosan had a maximum affinity and higher strength of synthetic reactive dye wastewater adsorption at lower temperatures. But the qmax values of chitosan at lower temperatures were lower than at higher temperatures at equilibrium conditions of Langmuir plots (Fig. 8). This might indicate that chitosan could adsorb synthetic reactive dye at high temperatures, but it had a high affinity and strength for synthetic reactive dye wastewater adsorption at low concentrations at low temperatures. For practical application, the concentration of dye in the wastewater and the values of qmax and b were also considered for the determination of the most suitable temperature for wastewater treatment. The influence of the adsorption isotherm shape is discussed to determine whether adsorption is favorable in terms of RL [15], a dimensionless constant referred to as the separation factor. RL is calculated using the equation [16] RL =

1 . 1 + bC0

(7)

C0 is the initial synthetic reactive dye wastewater concentration (mg l−1 ). The values of RL calculated in the above equation are incorporated into Table 5. The results show that increasing the temperature from 20 to 60 ◦ C induced a higher maximum adsorption capacity. All of the RL values were between 0 and 1. This indicated that chitosan was a favorable adsorbent for SRDW removal [15,16]. The free energy change (G), enthalpy change (H ), and entropy change

(8) (9) (10)

where R is the universals gas constant (8.314 J K−1 mol−1 ) and T is temperature (K). The thermodynamic parameters for the adsorption of synthetic reactive dye wastewater on chitosan are summarized in Table 5. The enthalpy change (H ) is obtained from the slope of plotting between log b and 1/T . The enthalpy change (H ) of dye adsorption by chitosan was −18.20 kJ mol−1 . This indicates that the adsorption followed an exothermic process. Negative (G) values indicate that the synthetic reactive dye wastewater adsorption by this polymer was spontaneous and a favorable process. Negative S values also indicate that the disorder of the system decreased at the solid–solution interface during adsorption of synthetic reactive dye wastewater on chitosan [5]. The changing of the thermodynamic parameters also indicates that the synthetic reactive dye adsorption onto chitosan was mainly chemical.

4. Conclusions Chitosan has potential as an adsorbent for removal of reactive dyes from textile wastewater because it can adsorb reactive dyes over a wide pH range and at high temperatures. The effect of initial pH, elution studies, and the thermodynamic parameters demonstrated that the reactive dye was probably adsorbed onto chitosan by both physical and chemical adsorption. In addition, the adsorption mechanism under acidic conditions was chemical adsorption, while under caustic conditions was both physical and chemical adsorption. However, the ATR-FTIR spectra confirmed that the amines on chitosan polymer tend to be effective functional groups for dye adsorption under acidic conditions, while the hydroxyl group tended to be the effective functional group for dye adsorption under caustic conditions.

Acknowledgments The authors thank the University Development Scholarship (U.D.C) and the Royal Golden Jubilee Scholarship for

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financial support. We thank Associate Professor Dr. Sanong Ekgasit and Adchara Padermshoke (Ph.D. student) from the Sensor Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University for running samples using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectrometry. References [1] G. McKay, G.A. Sweeney, Water Air Soil Pollut. 4 (1980) 3–11. [2] D. Peterson, in: J. Shore (Ed.), Colorants and Auxiliaries: Organic Chemistry and Application Properties, vol. 1, BTTG-Shirley, Manchester, 1990, pp. 32–72. [3] L.C. Morais, O.M. Freitas, E.P. Goncalves, L.T. Vasconcelos, G.C.G. Beca, Water Res. 33 (1999) 979–998. [4] M.M. Nassar, G.M.S. El, J. Chem. Technol. Biotechnol. 50 (1999) 257–264. [5] S.C.D.A. Fernanda, F.S.V. Eunice, R.C. Antonio, J. Colloid Interface Sci. 253 (2002) 243–246.

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