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Removal of reactive dyes from wastewater by adsorption on coir pith activated carbon
Bioresource Technology 97 (2006) 1329–1336
Removal of reactive dyes from wastewater by adsorption on coir pith activated carbon K. Santhy *, P. Selvapathy Center for Environmental Studies, Anna University, Chennai 600 025, India Received 16 August 2003; received in revised form 5 May 2005; accepted 13 May 2005 Available online 22 July 2005
Abstract The removal efficiency of activated carbon prepared from coir pith towards three highly used reactive dyes in textile industry was investigated. Batch experiments showed that the adsorption of dyes increased with an increase in contact time and carbon dose. Maximum decolourisation of all the dyes was observed at acidic pH. Adsorption of dyes was found to follow the Freundlich model. Kinetic studies indicated that the adsorption followed first order and the values of the Lagergren rate constants of the dyes were in the range of 1.77 · 102–2.69 · 102 min1. The column experiments using granular form of the carbon (obtained by agglomeration with polyvinyl acetate) showed that adsorption efficiency increased with an increase in bed depth and decrease of flow rate. The bed depth service time (BDST) analysis carried out for the dyes indicated a linear relationship between bed depth and service time. The exhausted carbon could be completely regenerated and put to repeated use by elution with 1.0 M NaOH. The coir pith activated carbon was not only effective in removal of colour but also significantly reduced COD levels of the textile wastewater. 2005 Elsevier Ltd. All rights reserved. Keywords: Coir pith activated carbon; Adsorption; Reactive dyes; BDST; Textile effluent
1. Introduction The textile finishing industry generates a large amount of wastewater. Wastewaters from dyeing and subsequent rinsing steps form one of the largest contributions to wastewater generation in the textile industry. Because dyes are almost invariably toxic, their removal from effluent stream is ecologically necessary. Reactive dyes pose the greatest problem in terms of colour, which is exacerbated by the dominance of cotton in todayÕs fashion industry. The human eye can detect concentrations of 0.005 mg/L of reactive dye in water, and therefore, presence of dye exceeding this limit would not be permitted on aesthetic grounds (Pierce, 1994). After the reactive dyeing process is complete, upto 800 mg/L
*
Corresponding author. E-mail address: [email protected] (K. Santhy).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.05.016
of hydrolysed dye remains in the bath (Steankenrichter and Kermer, 1992). Fixation rates for reactive dyes tend to be in the range of 60–70% although the values tend to be higher in dyes containing two reactive groups (Carr, 1995). Therefore, upto 40% of the colour is discharged in the effluent from reactive dyeing operation resulting in a highly coloured effluent. An additional problem is that the reactive dyes in both ordinary and hydrolysed forms are not easily biodegradable, and thus, even after extensive treatment, colour may still remain in the effluent. The conventional processes such as coagulation, flocculation and biological methods adopted for decolourising effluents containing reactive dyes are no longer able to achieve an adequate colour removal. Adsorption methods have been invariably successful to decolourise textile effluents, but this application is limited by the high cost of adsorbents. The carbon derived from agricultural wastes is gaining importance as it inexpensive and are perfectly suitable for the removal
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of organic and inorganic contaminants from wastewater. Some of the materials used with varying success include: saw dust, rice husk (Malik, 2003), rice shells, peanut shells, cotton seed shell (Kim et al., 2003), myrobalan, rubber seed coat, cashewnut sheath, palm seed coat, palm tree flower, pongam seed coat (Rengaraj et al., 1999), cornelian cherry, apricot stone, almond shell (Demirbas et al., 2004), oak wood waste, corn hulls, corn stover (Zhang et al., 2004) and cotton stalks (Attia et al., 2004). The coir industry is one of the most important agricultural industries in India. Coir pith, a waste material from coir industries, causes a disposal problem. Being resistant to biodegradation it is heaped along road sides (Krishnamoorthy and Rao, 1998). Since the material is rich in lignocellulosic content, an effective solution to the problem may lie in the use of the material for the preparation of activated carbon. Activated carbon prepared from coir pith was successfully used for removal of dyes (Namasivayam and Kavitha, 2002), 2-chlorophenol (Namasivayam and Kavitha, 2003) and heavy metals (Namasivayam and Kadirvelu, 2001). In our previous study activated carbon from coir pith showed remarkable efficiency for the removal of heavy metals from metal polishing industrial effluent (Santhy and Selvapathy, 2004). Hence a further attempt of the feasibility of applying activated carbon from coir pith towards the removal of reactive dyes from aqueous and industrial effluents was approached. In this work, the removal efficiency of chemically treated coir pith activated carbon towards three highly used reactive dyes in the textile industry was investigated. The adsorption capacities of such an activated carbon for the anionic reactive dyes, namely; Procion Reactive Orange 12, Reactive Red 2, and Reactive Blue 4 were determined. The procion dyes and the effluent were obtained from Nova Industries, Kancheepuram district, Tamil Nadu, India.
2. Methods 2.1. Preparation of activated carbon by acid process Activated carbon was prepared by treating 50 g of dried coir pith with 50 mL of concentrated sulphuric acid and keeping this in an air oven maintained at 105 ± 5 C for 24 h. The resulting char was washed with water followed by a 2% solution of sodium bicarbonate until effervescence had ceased and then kept in a 2% solution of sodium bicarbonate overnight. The char was then separated and washed with water until free from bicarbonate and dried, and then activated at a temperature of 900 C in an atmosphere of carbon dioxide for 30 min. The activated material was repeatedly washed with water and soaked in 10% HCl to remove
calcium oxide. The acid washed material was repeatedly washed with water to remove the free acid and then dried at 110 C. Activated carbon with particle sizes in the range of 0.2–0.5 mm (400–500 lm) (retained to a maximum of 85%) was used for further studies. 2.2. Agglomeration of carbon with polyvinyl acetate Powdered activated carbons (PAC) are preferred over granular activated carbon (GAC) in liquid phase batch application because of their high adsorption capacity. The dusty nature of PACs poses handling problems and makes regeneration ineffective. In recent years the PACs are converted to granular forms by various techniques. In order to use this material for continuous treatment of wastewater, the PAC was converted to granular form by agglomerating with polyvinyl acetate emulsion. One part by weight of polyvinyl acetate (PVAC) emulsion and three parts by weight of powdered coir pith carbon were mixed thoroughly to produce a semisolid mass. The agglomerated product was separated and pressed between porcelain tiles into a thin sheet of 2–3 mm thickness. The sheet was washed with water and dried. The dried material was then cut to 2 mm size range. The granules obtained were washed thoroughly with water, dried at 110 C and used for further investigations. 2.3. Adsorption studies Batch experiments were carried out by shaking 100 mL of dye solution with 0.2 g of the adsorbent in glass stoppered conical flasks at a temperature of 20 C at the rate of 150 rpm. The effect of various parameters such as initial concentration, contact time, pH and adsorbent dose on the adsorption process was investigated. The progress of adsorption during the experiments was determined by removing flasks after desired contact time, centrifuging and analysing the supernatant spectrophotometrically for dye concentration. Column studies were conducted using a downflow technique. The granular carbon was packed in a column of 60 cm height and internal diameter of 3 cm. A solution containing an initial dye concentration of 20 mg/L adjusted to pH 3 was allowed to percolate through carbon. Adsorption systems are easily designed using breakthrough curves obtained from pilot plant studies. The breakthrough curve was obtained by plotting the ratio of effluent concentration to influent concentration against time (or volume of water treated, V). Carbon particles were wetted for 24 h in distilled water. This was then fed into the test column (which was also filled with water) as a slurry and then allowed to settle for an additional 24 h. Uptake of dye by the adsorbent (as percent) was calculated as ([C0 Ct]/C0) · 100 where C0 and Ct are the
K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336
C.I. Reactive Orange 12 (Yellow H3R)—415 nm. C.I. Reactive Red 2 (Red M5B)—540 nm. C.I. Reactive Blue 4 (Blue MR)—610 nm. In the case of textile effluents containing a mixture of dyestuffs, measurement of absorbance was done at the standard wavelengths of 436 nm, 525 nm and 620 nm according to the German standard method (DIN, 1991; Gregor, 1991; Frank et al., 1994). The colour intensity of the sample was expressed as absorption coefficient calculated from the equation 1
Colour ðabsorbance; m Þ ¼
absorbance dilution factor pathlength ðmÞ
ð1Þ
Table 1 Physicochemical characteristics of coir pith activated carbon S. no.
Characteristics
CPC
GCPC
1 2 3 4 5 6 7 8 9 10 11
Moisture content (%) Ash content (%) Matter soluble in water (%) Matter soluble in acid (%) pH Decolourising power (mg/g) Ion exchange capacity (meq/g) Iron content (%) Phenol number Apparent density (g/cm3) Surface area (m2/g)
8.7 2.93 0.95 1.85 6.5 218 0.75 0.03 43 0.19 598
8.0 2.97 1.17 2.26 6.5 201 0.59 BDL 57 0.45 557
100 90
Colour removal (%)
initial and dye concentrations at time t, respectively. The absorbance of the samples was measured at wavelengths characteristic of dyes using 20 Genysis spectrophotometer. The three reactive dyes viz. Reactive Orange 12, Reactive Red 2 and Reactive Blue 4 were chosen for the study based on the fact that they cover the range of visible spectrum. Their commercial names and kmax values are given below:
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80 70 60 50 40
20mg/L 40mg/L 60mg/L 80mg/L 100mg/L
30 20 10 0
2.4. Regeneration studies The exhausted carbon must be regenerated and reused. To render the use of activated carbon economically feasible in wastewater treatment, the carbon bed was washed with 0.5 M NaOH percolating through the bed at a flow rate of 5 mL/min. The carbon was washed several times with water and again used in the next cycle. The regeneration cycle was repeated several times until the adsorption capacity of carbon was exhausted.
3. Results and discussion The characteristics of the powdered coir pith activated carbon (CPC) and granular coir pith carbon (GCPC) obtained by agglomeration are presented in Table 1. Except for a decrease in the surface area, all characteristics of the granular coir pith carbon was similar to that of the powdered carbon. The agglomerated granular carbon GCPC with a density of 0.45 g/cm3, can be suitably employed in a continuous treatment process. 3.1. Effect of initial concentration and contact time The effect of initial dye concentration and contact time on the removal of Reactive Orange 12 is shown in Fig. 1. From the figure, it was evident that the removal of dye increased with decreasing dye concentra-
0
1
2
3
4
5
6
7
Contact time (hour)
Fig. 1. Effect of initial concentration and contact time on the adsorption of Reactive Orange 12 by CPC.
tion. Adsorption increased with an increase in contact time. It was found that equilibrium was attained at 4 h. Similar results were obtained in the removal of Reactive Red 2 and Reactive Blue 4 also. Removals of Reactive Orange 12, Reactive Red 2 and Reactive Blue 4 were found to be 82%, 77% and 72%, respectively, at 4 h equilibration time and at 40 mg/L initial concentration. 3.2. Effect of pH The removal of dyes as a function of pH is shown in Fig. 2. The removal of dyes by CPC was found to be a maximum in the acidic pH range of 1–3. The pH value of the dye solution plays an important role in the whole adsorption process and particularly on the adsorption capacity. Similar observations have been reported by other workers for adsorption of reactive dyes indicating that the carbon has a net positive charge on its surface (Bousher et al., 1997). As the pH of the adsorption solution was lowered, the positive charges on the surface increased. This would attract the negatively charged functional groups located on the reactive dyes. In subsequent studies, it was decided to maintain a pH of 3.0, at which the removal was maximum.
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K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336 Table 2 Intercept and slope values of Freundlich isotherm for CPC
Colour removal (%)
100 90
S. no.
Dye
Slope (1/n)
Intercept (KF)
80
1 2 3
Reactive Orange 12 Reactive Red 2 Reactive Blue 4
0.262 0.243 0.279
11.45 12.88 9.16
70 60 Reactive Orange 12 Reactive Red 2
50
Reactive Blue 4
40 0
1
2
3
4
5
6
7
8
9
10
pH
from the intercept and the slope of the straight line are furnished in Table 2. The values of 1/n lie between 0.2 and 0.3 which represents good sorption potential of the sorbent.
Fig. 2. Effect of pH on the adsorption of reactive dyes by CPC.
4. Kinetics of adsorption 3.3. Effect of carbon dose The effect of carbon dose on the removal of dyes is shown in Fig. 3. It was observed that in all cases the dye uptake increased with increasing dose of carbon. A complete removal (100%) of Reactive Orange 12 and Reactive Red 2 of 50 mg/L concentration could be achieved with the dose of carbon at 3.0 g/L. For Reactive Blue 4 of 50 mg/L concentration, the carbon dose required to achieve 100% removal of the dye was 4.0 g/L. 3.4. The Freundlich adsorption isotherm The equilibrium data for adsorption of reactive dyes on CPC were fitted with the Freundlich adsorption isotherm according to the equation Qeq ¼ K F C e1=n
ð2Þ
where Qeq is the amount adsorbed in mg/g, KF and n are Freundlich constants, and Ce is the equilibrium concentration of the dye solution. Straight line plots of ln Qeq vs. ln Ce confirmed the adherence to Freundlich isotherm. The empirical constants KF and 1/n determined
Over 25 models are reported in the literature, all attempting to describe quantitatively the kinetic behaviour during the adsorption process. Each adsorption kinetic model has its own limitation and is derived according to certain initial conditions based on certain experimental and theoretical assumptions. A number of these models assume linear equilibrium isotherms, while fewer models assume the equilibrium isotherms to be nonlinear. Among many mathematical models, the first order reaction rate model known as the Lagergren kinetic equation is widely employed (Trivedi et al., 1973). The Lagergren kinetic equation takes the form (Lagergren, 1898): logðQeq QÞ ¼ log Qeq ðK ad =2.303Þt
ð3Þ
where Q and Qeq are the amounts of dye adsorbed (mg/g) of carbon at time, t (min) and at equilibrium time, respectively, and Kad is the rate constant of adsorption (time1). Fig. 4 shows the plot of log (Qeq Q) against reaction time. The straight line nature of the plot confirmed adherence to the Lagergren equation and showed that the removal of reactive dyes by adsorption followed the first order kinetics. The rate constants Kad calculated from the slope of the linear plot for the three reactive dyes studied are presented in Table 3. These values are
90 2.5 80
Reactive Orange 12
2
70
log (Qeq-Q)
Colour removal (%)
100
Reactive Orange 12 Reactive Red 2 Reactive Blue 4
60
Reactive Red 2 Reactive Blue 4
1.5 1 0.5
50 0
0.2
0.4
0.6
0.8
Carbon dose (g/L)
0 0
10
20
30
40
50
60
70
80
90
Contact time (Minute)
Fig. 3. Effect of carbon dose on the adsorption of reactive dyes by CPC.
Fig. 4. Lagergren plot for adsorption of dyes.
100 110
K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336 Table 3 Lagergren rate constants for the adsorption of reactive dyes
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S. no.
Dye
Adsorption rate constant (Kad, min1)
1 2 3
Reactive Orange 12 Reactive Red 2 Reactive Blue 4
2.69 · 102 2.39 · 102 1.78 · 102
Colour removal (%)
74
comparable with those reported in the literature (Jain et al., 2003; Kannan and Meenakshisundaram, 2002).
72 70 68 66 64 62
Reactive Orange 12
0
1000
2000
Reactive Red 2
3000
Reactive Blue 4
4000
5000
6000
Concentration of sulphate (mg/L)
4.1. Effect of anions
Fig. 6. Effect of sulphate on dye adsorption.
The influence of chloride and sulphate which are generally present in high concentrations in a textile effluent was evaluated on the adsorption capacity of CPC for the removal of reactive dyes. The sodium salt of these ions in the concentration range of upto 5000 mg/L was introduced to the dye solution and the influence of each anion was separately evaluated. The influence of chloride ion on the removal of reactive dyes is shown in Fig. 5. The adsorption capacity was not significantly affected by the presence of chloride ion. But, increase in sulphate ion concentration beyond 2000 mg/L enhanced the adsorption capacity of carbon (Fig. 6). Similar results have been reported in the adsorption of Direct Orange on commercial granular activated carbon (Perineau et al., 1982).
5. Column studies
5.2. Effect of bed depth
5.1. Effect of flow rate
Colour removal (%)
74 72 70 68 66
Reactive Orange 12
Reactive Red 2
Reactive Blue 4
62 0
1000
2000
3000
4000
5000
The breakthrough plot for Reactive Orange 12 at different bed depths is presented in Fig. 8. From the figure it was noticed that as the bed depth increased the volume of water treated also increased indicating the availability of more carbon surface for adsorption. Similar plots were obtained for the removal of Reactive Red 2 Residual dye concentration (mg/L)
Dye solutions of 20 mg/L concentration at pH 3.0 were used as the influent. In a packed column of uniform cross-sectional area, the volumetric flow rate was directly proportional to the overall linear flow through the bed. The column was packed to a depth of 15 cm with GCPC which amounts to 51.9 g. To study the influ-
64
ence of flow rate, the dye solutions were allowed to flow through the carbon bed at different flow rates such as 15, 20 and 25 mL/min. The eluents from the column were collected at 10 min intervals and analysed for the respective residual dye concentration. Percolation of the solution through the column was stopped as soon as colour was noticed in the effluent. The breakthrough plot of Reactive Orange 12 obtained at different flow rates is shown in Fig. 7. It is evident from the figures that as the flow rate increased, the service times were shortened and therefore the volume treated. This was due to decreased contact time between the dye and carbon at higher flow rates; as the adsorption was diffusion controlled, premature breakthrough occurred. Similar results were obtained for the removal of Reactive Red 2 and Reactive Blue 4 also.
6000
2 1.8 1.6 1.4 1.2 1 0.8 15mL/min
0.6
20mL/min
0.4
25mL/min
0.2 0 0
50
100
150
200
250
300
350
400
450
Service time (Minute)
Concentration of chloride (mg/L)
Fig. 5. Effect of chloride on dye adsorption.
Fig. 7. Breakthrough curves at different flow rates and constant bed depth for Reactive Orange 12.
K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336
1.8
t = 0, N = N0 and at Z = 0, C = C0 (influent concentration). Eq. (4) can be expressed as
1.6
t ¼ aZ þ c
2
1.4 1 15cm
0.8
20cm
0.6
25cm
0.4
c¼
0.2 0 0
1
2
3
4
5
6
7
8
9
10
1 ln½ðC 0 =C b Þ 1 C0K
ð6Þ
11
Volume treated (Litre) Fig. 8. Breakthrough curves at different bed depths and constant flow rate for Reactive Orange 12.
and Reactive Blue 4 also. However, most research carried out on the adsorption of dyes showed breakthrough curves similar in trend to the profiles reported here. The variation in concentration profile was due to the relatively large adsorption zone. The rate at which the adsorption zone travels through the bed decreases with bed depth, suggesting that beds of an increased height may be required for dye adsorption. This phenomenon occurs in the adsorption of dyestuffs because of their large molecular structure, resistance to internal diffusion was much higher than smaller molecules such as phenol. This resulted in dye molecules not having enough contact time to diffuse from the surface of the particle to the adsorption sites and therefore were being pumped further up the column to fresh activated carbon where the rate of dye uptake was higher. 5.3. BDST analysis The bed depth service time (BDST) model proposed by Bohart and Adams and later linearised by Hutchins (1974) has been reported as offering the simplest approach and most rapid prediction of adsorber performance (McKay and Bino, 1990). The BDST model is well established for dye adsorption in fixed bed systems (Walker and Weatherly, 1997), therefore only a brief discussion is included. According to the BDST model described by Hutchins (1974) ln½ðC 0 C b Þ 1 ¼ ðK ad N 0 Z=V Þ K ad C 0 t
ð5Þ
where t is the service time in minutes upto breakthrough concentration, a is the slope of BDST graph given by a = N0/C0V, Z is the bed depth (cm) and c is the ordinate intercept.
1.2
ð4Þ
where t is the service time (h), i.e. the time required for the effluent to reach the specific breakthrough concentration Cb, Kad is the adsorption rate constant (m3/kg h), Z is the depth of adsorbent (m), V is the hydraulic loading rate (or) linear flow velocity of wastewater passing through the adsorbent (m/h), C is the concentration of adsorbate in wastewater, N is the residual adsorbing capacity per unit volume of bed (kg/m3). At
The BDST curves for Reactive Orange 12 is shown in Fig. 9. Similar linear plots were obtained for Reactive Red 2 and Reactive Blue 4 also. The linear plots showed that, when all the other parameters were maintained constant, the volume of effluent treated upto breakthrough was a direct function of bed depth (depth of carbon). 5.4. Recycling of carbon bed In order to arrive at the number of cycles over which the adsorption capacity of GCPC would remain unaffected, experiments were carried out for the desorption of reactive dyes using 1.0 M NaOH as the eluent. The influent was allowed to percolate through the saturated carbon bed of 20 cm at a flow rate of 10 mL/min. After the initial desorption, the carbon bed was washed with water repeatedly to remove the free alkali and again washed with 50 mL of 1.0 M HCl at a flow rate of 5 mL/min to neutralize any residual alkalinity. The carbon bed was finally washed with water and the next cycle of operation of removal of dye from solution was performed. Similar results were obtained for the removal of Reactive Red 2 and Reactive Blue 4 also. The process of adsorption of dye and regeneration of carbon beds were repeated over 4 cycles of operation. The results of the recycling studies are shown in Fig. 10. The capacity of CPC remained unaffected upto 3 cycles of operation.
800
Service time (Minute)
Residual dye concentration (mg/L)
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700 600 500 400 300 200 100
10%
0
20%
-100
30%
-200
0
5
10
15
20
Bed depth (cm) Fig. 9. BDST plot of Reactive Orange 12.
25
30
2.5 2 1.5 I cycle
1
II cycle III cycle IV cycle
0.5 0 0
2
1
4
3
5
7
6
8
Residual colour (Absorbance, m-1)
Residual dye concentration (mg/L)
K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336
5 4.5 4 3.5 3 2.5 2
I cycle
1.5
II cycle III cycle
1
IV cycle
0.5 0 0
1
3
2
4
6
5
7
8
Volume treated (Litre)
Volume treated (Litre)
Fig. 12. Decolourisation of effluent by GCPC (kmax = 525 nm).
Residual colour (Absorbance, m-1)
Fig. 10. Regeneration of GCPC.
Table 4 Characteristics of textile mill effluent S. no.
Parameter
Value
1 2
pH Colour (absorbance, m1) 436 nm 525 nm 620 nm Alkalinity Total hardness Total dissolved solids Chemical oxygen demand Chloride Sulphate
6.96
3 4 5 6 7 8
1335
165.3 146.0 129.0 1075 280 3752 528 2086 1091
3 2.5 2 1.5 I cycle
1
II cycle III cycle
0.5
IV cycle
0 0
1
2
3
4
5
6
Volume treated (Litre) Fig. 13. Decolourisation of effluent by GCPC (kmax = 620 nm).
All values are in mg/L, except pH and colour.
5.5. Removal of colour from textile mill effluent Table 5 Colour and COD removal from textile mill effluent by batch process S. no.
Residual colour (Absorbance, m-1)
1 2
Carbon type
Colour removal (%) 436 nm
525 nm
620 nm
CPC GCPC
90 86.5
92.2 88
94 88.4
COD removal (%) 46.5 35
7 6 5 4 3 I cycle II cycle
2
III cycle IV cycle
1 0 0
1
2
3
4
5
6
7
8
Volume treated (Litre) Fig. 11. Decolourisation of effluent by GCPC (kmax = 436 nm).
9
The suitability of CPC and GCPC for the removal of colour from textile mill effluent was examined. The effluent was characterised using standard methods (APHA, 1995) and the results are summarised in Table 4. Batch experiments were conducted with 100 mL of textile mill effluent along with 0.5 g of carbon adjusted to pH 3.0 by equilibrating for a period of 4 h. The efficiency of various carbons on colour and COD removal is shown in Table 5. It was observed that besides removal of colour, there was concurrent reduction of COD of the effluent. The results of the decolourisation studies with GCPC are presented in Figs. 11–13, at the three standard wavelengths of 436, 525 and 620 nm, respectively. The effluent was treated until the acceptable limit of 7, 5 and 3 m1 absorbance (Allen et al., 1995) was reached at the standard wavelengths of 436, 525 and 620 nm, respectively. It was noted that the capacity of the carbon for decolourisation was effective upto 3 cycles similar to single dye solutions. GCPC could treat 8, 7.6 and 5.2 L of effluent at the standard wavelengths of 436, 525 and 620 nm, respectively. The removal of colour from the effluent was lower than sorption from a single component system. The reduced capacity can be attributed to
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a number of factors combined together. These include: (1) interaction between the dyes and other components in the effluent; (2) change of the adsorbent surface charge due to adsorption; (3) competition of other components of the effluent for active sites on the carbon surface where displacement effects replace the other components from the adsorption sites.
6. Conclusions The experimental results showed that activated carbon prepared from coir pith was a suitable adsorbent for removal of reactive dyes from both synthetic and textile effluent. In batch studies, the adsorption increased with an increase of contact time and carbon dose and decreased with an increase in solute concentration. Removal of dyes was higher at the acidic pH range. The results showed that the carbon affinity was higher for Reactive Orange followed by Reactive Red and was lowest for Reactive Blue. The increase in bed height and decrease in flow rate increased the adsorption capacity of the column. The BDST analysis of sorption of dyes showed linear relationship between bed depth and service time, which could be used successfully for scale-up purposes. Regeneration of the carbon was found to be effective with 1.0 M NaOH and the regenerated carbon was found to be effective upto 3 cycles of operation. Application of the carbon to textile effluent decolourisation studies showed significant removal of colour and COD.
References Allen, S.J., Khadher, K.Y.H., Bino, M., 1995. Oxidation of dyestuffs in wastewaters. Journal of Chemical Technology and Biotechnology 62, 111–117. APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association, Washington. Attia, A.A., El-Hendawy, A.A., Khedr, S.A., El-Nabarawy, Th., 2004. Textural properties and adsorption of dyes onto carbons derived from cotton stalks. Adsorption Science and Technology 22, 411– 426. Bousher, A., Shen, X., Edyvean, R.G.J., 1997. Removal of coloured organic matter by adsorption onto low cost waste materials. Water Research 31, 2084–2092. Carr, K., 1995. Reactive dyes, especially bi-reactive molecules: structure and synthesis. In: Peters, A.T., Freeman, H.S. (Eds.), Modern Colourants: Synthesis and Structure. Blackie Academic and Professional, London, pp. 87–122. Demirbas, E., Kobya, M., Senturk, E., Ozkan, T., 2004. Adsorption kinetics for the removal of chromium (VI) from aqueous solutions on the activated carbons prepared from agricultural wastes. Water SA 30, 533–539. DIN 38 404, 1991. German Methods for the Examination of Water, Wastewater and Sludge, Physical and Physical–Chemical Para-
meters (group C), Determination of Colour, Deutsches Institut fur Normung e.v., Berlin, pp. 1–7. Frank, G., Frank, H., Wilhelm, O., 1994. Ozonation—an important technique to comply with new German laws for textile wastewater treatment. Water Science and Technology 30, 255–263. Gregor, K.H., 1991. Oxidative Decolourisation of Textile Wastewater with Advanced Oxidation Processes. Chemical Oxidation—Technologies for the Nineties. Technomic Publishing Co., Lanchester, pp. 161–193. Hutchins, R.A., 1974. New method simplifies design of activated carbon systems. Chemical Engineering 80, 133–138. Jain, A.K., Gupta, V.K., Bhatnagar, A., Suhas, 2003. Utilization of industrial waste products as adsorbents for the removal of dyes. Journal of Hazardous Materials 101, 31–42. Kannan, N., Meenakshisundaram, M., 2002. Adsorption of Congo Red on various activated carbons. A comparative study. Water, Air, and Soil Pollution 138, 289–305. Kim, T.Y., Baek, I.H., Jeoung, Y.D., Park, S.C., 2003. Manufacturing activated carbon using various agricultural wastes. Journal of Industrial and Engineering Chemistry (Seoul, Republic of Korea) 9, 254–260. Krishnamoorthy, V., Rao, K.B., 1998. Nutritive status of coir pith of varying age and from different sources. Coir News 27, 17–19. Lagergren, S., 1898. Zur theorie der sogenannten adsorption gelo¨ster stoffe. Kungliga Svenska Vetenskapsakademiens. Handlingar, Band 24 (4), 1–39. Malik, P.K., 2003. Activated carbon from saw dust and rice husk use of activated carbons prepared from sawdust and rice-husk for adsorption of acid dyes: a case study of Acid Yellow 36. Dyes and Pigments 56, 239–249. McKay, G., Bino, M.J., 1990. Fixed bed adsorption for the removal of pollutants from water. Water Environment pollution 66, 33–53. Namasivayam, C., Kadirvelu, K., 2001. Removal of heavy metals from industrial wastewaters by adsorption onto activated carbon prepared from an agricultural solid waste. Bioresource Technology 76, 63–69. Namasivayam, C., Kavitha, D., 2002. Removal of Congo Red from water by adsorption onto activated carbon prepared from coir pith, an agricultural solid waste. Dyes and Pigments 54, 47–48. Namasivayam, C., Kavitha, D., 2003. Adsorptive removal of 2-chlorophenol by low-cost coir pith carbon. Journal of Hazardous Materials 98, 257–274. Perineau, F., Molinier, J., Gaset, A., 1982. Adsorption of ionic dyes on charred plant material. Journal of Chemical Technology and Biotechnology 32, 749–758. Pierce, J., 1994. Colour in textile effluents—the origins of the problem. Journal of the Society of Dyers and Colourists 110, 131–133. Rengaraj, S., Arabindoo, B., Murugesan, V., 1999. Preparation and characterisation of activated carbon from agricultural wastes. Indian Journal of Chemical Technology 6, 1–4. Santhy, K., Selvapathy, P., 2004. Removal of heavy metals from wastewater by adsorption on coir pith activated carbon. Separation Science and Technology 39, 3331–3351. Steankenrichter, I., Kermer, W.D., 1992. Decolourising textile effluents. Journal of the Society of Dyers and Colourists 108, 182–186. Trivedi, H.C., Patel, V.M., Patel, R.D., 1973. Adsorption of Cellulose triacetate on calcium silicate. European Polymer Journal 9, 523– 531. Walker, G.M., Weatherly, L.R., 1997. Adsorption of acid dyes on to granular activated carbon in fixed beds. Water Research 8, 2093– 2101. Zhang, T., Walawender, W.P., Fan, L., Fan, M., Daugaard, D., Brown, R.C., 2004. Preparation of activated carbon from forest and agricultural residues through CO2 activation. Chemical Engineering Journal (Amsterdam, Netherlands) 105, 53–59.
Removal of reactive dyes from wastewater by adsorption on coir pith activated carbon K. Santhy *, P. Selvapathy Center for Environmental Studies, Anna University, Chennai 600 025, India Received 16 August 2003; received in revised form 5 May 2005; accepted 13 May 2005 Available online 22 July 2005
Abstract The removal efficiency of activated carbon prepared from coir pith towards three highly used reactive dyes in textile industry was investigated. Batch experiments showed that the adsorption of dyes increased with an increase in contact time and carbon dose. Maximum decolourisation of all the dyes was observed at acidic pH. Adsorption of dyes was found to follow the Freundlich model. Kinetic studies indicated that the adsorption followed first order and the values of the Lagergren rate constants of the dyes were in the range of 1.77 · 102–2.69 · 102 min1. The column experiments using granular form of the carbon (obtained by agglomeration with polyvinyl acetate) showed that adsorption efficiency increased with an increase in bed depth and decrease of flow rate. The bed depth service time (BDST) analysis carried out for the dyes indicated a linear relationship between bed depth and service time. The exhausted carbon could be completely regenerated and put to repeated use by elution with 1.0 M NaOH. The coir pith activated carbon was not only effective in removal of colour but also significantly reduced COD levels of the textile wastewater. 2005 Elsevier Ltd. All rights reserved. Keywords: Coir pith activated carbon; Adsorption; Reactive dyes; BDST; Textile effluent
1. Introduction The textile finishing industry generates a large amount of wastewater. Wastewaters from dyeing and subsequent rinsing steps form one of the largest contributions to wastewater generation in the textile industry. Because dyes are almost invariably toxic, their removal from effluent stream is ecologically necessary. Reactive dyes pose the greatest problem in terms of colour, which is exacerbated by the dominance of cotton in todayÕs fashion industry. The human eye can detect concentrations of 0.005 mg/L of reactive dye in water, and therefore, presence of dye exceeding this limit would not be permitted on aesthetic grounds (Pierce, 1994). After the reactive dyeing process is complete, upto 800 mg/L
*
Corresponding author. E-mail address: [email protected] (K. Santhy).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.05.016
of hydrolysed dye remains in the bath (Steankenrichter and Kermer, 1992). Fixation rates for reactive dyes tend to be in the range of 60–70% although the values tend to be higher in dyes containing two reactive groups (Carr, 1995). Therefore, upto 40% of the colour is discharged in the effluent from reactive dyeing operation resulting in a highly coloured effluent. An additional problem is that the reactive dyes in both ordinary and hydrolysed forms are not easily biodegradable, and thus, even after extensive treatment, colour may still remain in the effluent. The conventional processes such as coagulation, flocculation and biological methods adopted for decolourising effluents containing reactive dyes are no longer able to achieve an adequate colour removal. Adsorption methods have been invariably successful to decolourise textile effluents, but this application is limited by the high cost of adsorbents. The carbon derived from agricultural wastes is gaining importance as it inexpensive and are perfectly suitable for the removal
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of organic and inorganic contaminants from wastewater. Some of the materials used with varying success include: saw dust, rice husk (Malik, 2003), rice shells, peanut shells, cotton seed shell (Kim et al., 2003), myrobalan, rubber seed coat, cashewnut sheath, palm seed coat, palm tree flower, pongam seed coat (Rengaraj et al., 1999), cornelian cherry, apricot stone, almond shell (Demirbas et al., 2004), oak wood waste, corn hulls, corn stover (Zhang et al., 2004) and cotton stalks (Attia et al., 2004). The coir industry is one of the most important agricultural industries in India. Coir pith, a waste material from coir industries, causes a disposal problem. Being resistant to biodegradation it is heaped along road sides (Krishnamoorthy and Rao, 1998). Since the material is rich in lignocellulosic content, an effective solution to the problem may lie in the use of the material for the preparation of activated carbon. Activated carbon prepared from coir pith was successfully used for removal of dyes (Namasivayam and Kavitha, 2002), 2-chlorophenol (Namasivayam and Kavitha, 2003) and heavy metals (Namasivayam and Kadirvelu, 2001). In our previous study activated carbon from coir pith showed remarkable efficiency for the removal of heavy metals from metal polishing industrial effluent (Santhy and Selvapathy, 2004). Hence a further attempt of the feasibility of applying activated carbon from coir pith towards the removal of reactive dyes from aqueous and industrial effluents was approached. In this work, the removal efficiency of chemically treated coir pith activated carbon towards three highly used reactive dyes in the textile industry was investigated. The adsorption capacities of such an activated carbon for the anionic reactive dyes, namely; Procion Reactive Orange 12, Reactive Red 2, and Reactive Blue 4 were determined. The procion dyes and the effluent were obtained from Nova Industries, Kancheepuram district, Tamil Nadu, India.
2. Methods 2.1. Preparation of activated carbon by acid process Activated carbon was prepared by treating 50 g of dried coir pith with 50 mL of concentrated sulphuric acid and keeping this in an air oven maintained at 105 ± 5 C for 24 h. The resulting char was washed with water followed by a 2% solution of sodium bicarbonate until effervescence had ceased and then kept in a 2% solution of sodium bicarbonate overnight. The char was then separated and washed with water until free from bicarbonate and dried, and then activated at a temperature of 900 C in an atmosphere of carbon dioxide for 30 min. The activated material was repeatedly washed with water and soaked in 10% HCl to remove
calcium oxide. The acid washed material was repeatedly washed with water to remove the free acid and then dried at 110 C. Activated carbon with particle sizes in the range of 0.2–0.5 mm (400–500 lm) (retained to a maximum of 85%) was used for further studies. 2.2. Agglomeration of carbon with polyvinyl acetate Powdered activated carbons (PAC) are preferred over granular activated carbon (GAC) in liquid phase batch application because of their high adsorption capacity. The dusty nature of PACs poses handling problems and makes regeneration ineffective. In recent years the PACs are converted to granular forms by various techniques. In order to use this material for continuous treatment of wastewater, the PAC was converted to granular form by agglomerating with polyvinyl acetate emulsion. One part by weight of polyvinyl acetate (PVAC) emulsion and three parts by weight of powdered coir pith carbon were mixed thoroughly to produce a semisolid mass. The agglomerated product was separated and pressed between porcelain tiles into a thin sheet of 2–3 mm thickness. The sheet was washed with water and dried. The dried material was then cut to 2 mm size range. The granules obtained were washed thoroughly with water, dried at 110 C and used for further investigations. 2.3. Adsorption studies Batch experiments were carried out by shaking 100 mL of dye solution with 0.2 g of the adsorbent in glass stoppered conical flasks at a temperature of 20 C at the rate of 150 rpm. The effect of various parameters such as initial concentration, contact time, pH and adsorbent dose on the adsorption process was investigated. The progress of adsorption during the experiments was determined by removing flasks after desired contact time, centrifuging and analysing the supernatant spectrophotometrically for dye concentration. Column studies were conducted using a downflow technique. The granular carbon was packed in a column of 60 cm height and internal diameter of 3 cm. A solution containing an initial dye concentration of 20 mg/L adjusted to pH 3 was allowed to percolate through carbon. Adsorption systems are easily designed using breakthrough curves obtained from pilot plant studies. The breakthrough curve was obtained by plotting the ratio of effluent concentration to influent concentration against time (or volume of water treated, V). Carbon particles were wetted for 24 h in distilled water. This was then fed into the test column (which was also filled with water) as a slurry and then allowed to settle for an additional 24 h. Uptake of dye by the adsorbent (as percent) was calculated as ([C0 Ct]/C0) · 100 where C0 and Ct are the
K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336
C.I. Reactive Orange 12 (Yellow H3R)—415 nm. C.I. Reactive Red 2 (Red M5B)—540 nm. C.I. Reactive Blue 4 (Blue MR)—610 nm. In the case of textile effluents containing a mixture of dyestuffs, measurement of absorbance was done at the standard wavelengths of 436 nm, 525 nm and 620 nm according to the German standard method (DIN, 1991; Gregor, 1991; Frank et al., 1994). The colour intensity of the sample was expressed as absorption coefficient calculated from the equation 1
Colour ðabsorbance; m Þ ¼
absorbance dilution factor pathlength ðmÞ
ð1Þ
Table 1 Physicochemical characteristics of coir pith activated carbon S. no.
Characteristics
CPC
GCPC
1 2 3 4 5 6 7 8 9 10 11
Moisture content (%) Ash content (%) Matter soluble in water (%) Matter soluble in acid (%) pH Decolourising power (mg/g) Ion exchange capacity (meq/g) Iron content (%) Phenol number Apparent density (g/cm3) Surface area (m2/g)
8.7 2.93 0.95 1.85 6.5 218 0.75 0.03 43 0.19 598
8.0 2.97 1.17 2.26 6.5 201 0.59 BDL 57 0.45 557
100 90
Colour removal (%)
initial and dye concentrations at time t, respectively. The absorbance of the samples was measured at wavelengths characteristic of dyes using 20 Genysis spectrophotometer. The three reactive dyes viz. Reactive Orange 12, Reactive Red 2 and Reactive Blue 4 were chosen for the study based on the fact that they cover the range of visible spectrum. Their commercial names and kmax values are given below:
1331
80 70 60 50 40
20mg/L 40mg/L 60mg/L 80mg/L 100mg/L
30 20 10 0
2.4. Regeneration studies The exhausted carbon must be regenerated and reused. To render the use of activated carbon economically feasible in wastewater treatment, the carbon bed was washed with 0.5 M NaOH percolating through the bed at a flow rate of 5 mL/min. The carbon was washed several times with water and again used in the next cycle. The regeneration cycle was repeated several times until the adsorption capacity of carbon was exhausted.
3. Results and discussion The characteristics of the powdered coir pith activated carbon (CPC) and granular coir pith carbon (GCPC) obtained by agglomeration are presented in Table 1. Except for a decrease in the surface area, all characteristics of the granular coir pith carbon was similar to that of the powdered carbon. The agglomerated granular carbon GCPC with a density of 0.45 g/cm3, can be suitably employed in a continuous treatment process. 3.1. Effect of initial concentration and contact time The effect of initial dye concentration and contact time on the removal of Reactive Orange 12 is shown in Fig. 1. From the figure, it was evident that the removal of dye increased with decreasing dye concentra-
0
1
2
3
4
5
6
7
Contact time (hour)
Fig. 1. Effect of initial concentration and contact time on the adsorption of Reactive Orange 12 by CPC.
tion. Adsorption increased with an increase in contact time. It was found that equilibrium was attained at 4 h. Similar results were obtained in the removal of Reactive Red 2 and Reactive Blue 4 also. Removals of Reactive Orange 12, Reactive Red 2 and Reactive Blue 4 were found to be 82%, 77% and 72%, respectively, at 4 h equilibration time and at 40 mg/L initial concentration. 3.2. Effect of pH The removal of dyes as a function of pH is shown in Fig. 2. The removal of dyes by CPC was found to be a maximum in the acidic pH range of 1–3. The pH value of the dye solution plays an important role in the whole adsorption process and particularly on the adsorption capacity. Similar observations have been reported by other workers for adsorption of reactive dyes indicating that the carbon has a net positive charge on its surface (Bousher et al., 1997). As the pH of the adsorption solution was lowered, the positive charges on the surface increased. This would attract the negatively charged functional groups located on the reactive dyes. In subsequent studies, it was decided to maintain a pH of 3.0, at which the removal was maximum.
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K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336 Table 2 Intercept and slope values of Freundlich isotherm for CPC
Colour removal (%)
100 90
S. no.
Dye
Slope (1/n)
Intercept (KF)
80
1 2 3
Reactive Orange 12 Reactive Red 2 Reactive Blue 4
0.262 0.243 0.279
11.45 12.88 9.16
70 60 Reactive Orange 12 Reactive Red 2
50
Reactive Blue 4
40 0
1
2
3
4
5
6
7
8
9
10
pH
from the intercept and the slope of the straight line are furnished in Table 2. The values of 1/n lie between 0.2 and 0.3 which represents good sorption potential of the sorbent.
Fig. 2. Effect of pH on the adsorption of reactive dyes by CPC.
4. Kinetics of adsorption 3.3. Effect of carbon dose The effect of carbon dose on the removal of dyes is shown in Fig. 3. It was observed that in all cases the dye uptake increased with increasing dose of carbon. A complete removal (100%) of Reactive Orange 12 and Reactive Red 2 of 50 mg/L concentration could be achieved with the dose of carbon at 3.0 g/L. For Reactive Blue 4 of 50 mg/L concentration, the carbon dose required to achieve 100% removal of the dye was 4.0 g/L. 3.4. The Freundlich adsorption isotherm The equilibrium data for adsorption of reactive dyes on CPC were fitted with the Freundlich adsorption isotherm according to the equation Qeq ¼ K F C e1=n
ð2Þ
where Qeq is the amount adsorbed in mg/g, KF and n are Freundlich constants, and Ce is the equilibrium concentration of the dye solution. Straight line plots of ln Qeq vs. ln Ce confirmed the adherence to Freundlich isotherm. The empirical constants KF and 1/n determined
Over 25 models are reported in the literature, all attempting to describe quantitatively the kinetic behaviour during the adsorption process. Each adsorption kinetic model has its own limitation and is derived according to certain initial conditions based on certain experimental and theoretical assumptions. A number of these models assume linear equilibrium isotherms, while fewer models assume the equilibrium isotherms to be nonlinear. Among many mathematical models, the first order reaction rate model known as the Lagergren kinetic equation is widely employed (Trivedi et al., 1973). The Lagergren kinetic equation takes the form (Lagergren, 1898): logðQeq QÞ ¼ log Qeq ðK ad =2.303Þt
ð3Þ
where Q and Qeq are the amounts of dye adsorbed (mg/g) of carbon at time, t (min) and at equilibrium time, respectively, and Kad is the rate constant of adsorption (time1). Fig. 4 shows the plot of log (Qeq Q) against reaction time. The straight line nature of the plot confirmed adherence to the Lagergren equation and showed that the removal of reactive dyes by adsorption followed the first order kinetics. The rate constants Kad calculated from the slope of the linear plot for the three reactive dyes studied are presented in Table 3. These values are
90 2.5 80
Reactive Orange 12
2
70
log (Qeq-Q)
Colour removal (%)
100
Reactive Orange 12 Reactive Red 2 Reactive Blue 4
60
Reactive Red 2 Reactive Blue 4
1.5 1 0.5
50 0
0.2
0.4
0.6
0.8
Carbon dose (g/L)
0 0
10
20
30
40
50
60
70
80
90
Contact time (Minute)
Fig. 3. Effect of carbon dose on the adsorption of reactive dyes by CPC.
Fig. 4. Lagergren plot for adsorption of dyes.
100 110
K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336 Table 3 Lagergren rate constants for the adsorption of reactive dyes
1333
S. no.
Dye
Adsorption rate constant (Kad, min1)
1 2 3
Reactive Orange 12 Reactive Red 2 Reactive Blue 4
2.69 · 102 2.39 · 102 1.78 · 102
Colour removal (%)
74
comparable with those reported in the literature (Jain et al., 2003; Kannan and Meenakshisundaram, 2002).
72 70 68 66 64 62
Reactive Orange 12
0
1000
2000
Reactive Red 2
3000
Reactive Blue 4
4000
5000
6000
Concentration of sulphate (mg/L)
4.1. Effect of anions
Fig. 6. Effect of sulphate on dye adsorption.
The influence of chloride and sulphate which are generally present in high concentrations in a textile effluent was evaluated on the adsorption capacity of CPC for the removal of reactive dyes. The sodium salt of these ions in the concentration range of upto 5000 mg/L was introduced to the dye solution and the influence of each anion was separately evaluated. The influence of chloride ion on the removal of reactive dyes is shown in Fig. 5. The adsorption capacity was not significantly affected by the presence of chloride ion. But, increase in sulphate ion concentration beyond 2000 mg/L enhanced the adsorption capacity of carbon (Fig. 6). Similar results have been reported in the adsorption of Direct Orange on commercial granular activated carbon (Perineau et al., 1982).
5. Column studies
5.2. Effect of bed depth
5.1. Effect of flow rate
Colour removal (%)
74 72 70 68 66
Reactive Orange 12
Reactive Red 2
Reactive Blue 4
62 0
1000
2000
3000
4000
5000
The breakthrough plot for Reactive Orange 12 at different bed depths is presented in Fig. 8. From the figure it was noticed that as the bed depth increased the volume of water treated also increased indicating the availability of more carbon surface for adsorption. Similar plots were obtained for the removal of Reactive Red 2 Residual dye concentration (mg/L)
Dye solutions of 20 mg/L concentration at pH 3.0 were used as the influent. In a packed column of uniform cross-sectional area, the volumetric flow rate was directly proportional to the overall linear flow through the bed. The column was packed to a depth of 15 cm with GCPC which amounts to 51.9 g. To study the influ-
64
ence of flow rate, the dye solutions were allowed to flow through the carbon bed at different flow rates such as 15, 20 and 25 mL/min. The eluents from the column were collected at 10 min intervals and analysed for the respective residual dye concentration. Percolation of the solution through the column was stopped as soon as colour was noticed in the effluent. The breakthrough plot of Reactive Orange 12 obtained at different flow rates is shown in Fig. 7. It is evident from the figures that as the flow rate increased, the service times were shortened and therefore the volume treated. This was due to decreased contact time between the dye and carbon at higher flow rates; as the adsorption was diffusion controlled, premature breakthrough occurred. Similar results were obtained for the removal of Reactive Red 2 and Reactive Blue 4 also.
6000
2 1.8 1.6 1.4 1.2 1 0.8 15mL/min
0.6
20mL/min
0.4
25mL/min
0.2 0 0
50
100
150
200
250
300
350
400
450
Service time (Minute)
Concentration of chloride (mg/L)
Fig. 5. Effect of chloride on dye adsorption.
Fig. 7. Breakthrough curves at different flow rates and constant bed depth for Reactive Orange 12.
K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336
1.8
t = 0, N = N0 and at Z = 0, C = C0 (influent concentration). Eq. (4) can be expressed as
1.6
t ¼ aZ þ c
2
1.4 1 15cm
0.8
20cm
0.6
25cm
0.4
c¼
0.2 0 0
1
2
3
4
5
6
7
8
9
10
1 ln½ðC 0 =C b Þ 1 C0K
ð6Þ
11
Volume treated (Litre) Fig. 8. Breakthrough curves at different bed depths and constant flow rate for Reactive Orange 12.
and Reactive Blue 4 also. However, most research carried out on the adsorption of dyes showed breakthrough curves similar in trend to the profiles reported here. The variation in concentration profile was due to the relatively large adsorption zone. The rate at which the adsorption zone travels through the bed decreases with bed depth, suggesting that beds of an increased height may be required for dye adsorption. This phenomenon occurs in the adsorption of dyestuffs because of their large molecular structure, resistance to internal diffusion was much higher than smaller molecules such as phenol. This resulted in dye molecules not having enough contact time to diffuse from the surface of the particle to the adsorption sites and therefore were being pumped further up the column to fresh activated carbon where the rate of dye uptake was higher. 5.3. BDST analysis The bed depth service time (BDST) model proposed by Bohart and Adams and later linearised by Hutchins (1974) has been reported as offering the simplest approach and most rapid prediction of adsorber performance (McKay and Bino, 1990). The BDST model is well established for dye adsorption in fixed bed systems (Walker and Weatherly, 1997), therefore only a brief discussion is included. According to the BDST model described by Hutchins (1974) ln½ðC 0 C b Þ 1 ¼ ðK ad N 0 Z=V Þ K ad C 0 t
ð5Þ
where t is the service time in minutes upto breakthrough concentration, a is the slope of BDST graph given by a = N0/C0V, Z is the bed depth (cm) and c is the ordinate intercept.
1.2
ð4Þ
where t is the service time (h), i.e. the time required for the effluent to reach the specific breakthrough concentration Cb, Kad is the adsorption rate constant (m3/kg h), Z is the depth of adsorbent (m), V is the hydraulic loading rate (or) linear flow velocity of wastewater passing through the adsorbent (m/h), C is the concentration of adsorbate in wastewater, N is the residual adsorbing capacity per unit volume of bed (kg/m3). At
The BDST curves for Reactive Orange 12 is shown in Fig. 9. Similar linear plots were obtained for Reactive Red 2 and Reactive Blue 4 also. The linear plots showed that, when all the other parameters were maintained constant, the volume of effluent treated upto breakthrough was a direct function of bed depth (depth of carbon). 5.4. Recycling of carbon bed In order to arrive at the number of cycles over which the adsorption capacity of GCPC would remain unaffected, experiments were carried out for the desorption of reactive dyes using 1.0 M NaOH as the eluent. The influent was allowed to percolate through the saturated carbon bed of 20 cm at a flow rate of 10 mL/min. After the initial desorption, the carbon bed was washed with water repeatedly to remove the free alkali and again washed with 50 mL of 1.0 M HCl at a flow rate of 5 mL/min to neutralize any residual alkalinity. The carbon bed was finally washed with water and the next cycle of operation of removal of dye from solution was performed. Similar results were obtained for the removal of Reactive Red 2 and Reactive Blue 4 also. The process of adsorption of dye and regeneration of carbon beds were repeated over 4 cycles of operation. The results of the recycling studies are shown in Fig. 10. The capacity of CPC remained unaffected upto 3 cycles of operation.
800
Service time (Minute)
Residual dye concentration (mg/L)
1334
700 600 500 400 300 200 100
10%
0
20%
-100
30%
-200
0
5
10
15
20
Bed depth (cm) Fig. 9. BDST plot of Reactive Orange 12.
25
30
2.5 2 1.5 I cycle
1
II cycle III cycle IV cycle
0.5 0 0
2
1
4
3
5
7
6
8
Residual colour (Absorbance, m-1)
Residual dye concentration (mg/L)
K. Santhy, P. Selvapathy / Bioresource Technology 97 (2006) 1329–1336
5 4.5 4 3.5 3 2.5 2
I cycle
1.5
II cycle III cycle
1
IV cycle
0.5 0 0
1
3
2
4
6
5
7
8
Volume treated (Litre)
Volume treated (Litre)
Fig. 12. Decolourisation of effluent by GCPC (kmax = 525 nm).
Residual colour (Absorbance, m-1)
Fig. 10. Regeneration of GCPC.
Table 4 Characteristics of textile mill effluent S. no.
Parameter
Value
1 2
pH Colour (absorbance, m1) 436 nm 525 nm 620 nm Alkalinity Total hardness Total dissolved solids Chemical oxygen demand Chloride Sulphate
6.96
3 4 5 6 7 8
1335
165.3 146.0 129.0 1075 280 3752 528 2086 1091
3 2.5 2 1.5 I cycle
1
II cycle III cycle
0.5
IV cycle
0 0
1
2
3
4
5
6
Volume treated (Litre) Fig. 13. Decolourisation of effluent by GCPC (kmax = 620 nm).
All values are in mg/L, except pH and colour.
5.5. Removal of colour from textile mill effluent Table 5 Colour and COD removal from textile mill effluent by batch process S. no.
Residual colour (Absorbance, m-1)
1 2
Carbon type
Colour removal (%) 436 nm
525 nm
620 nm
CPC GCPC
90 86.5
92.2 88
94 88.4
COD removal (%) 46.5 35
7 6 5 4 3 I cycle II cycle
2
III cycle IV cycle
1 0 0
1
2
3
4
5
6
7
8
Volume treated (Litre) Fig. 11. Decolourisation of effluent by GCPC (kmax = 436 nm).
9
The suitability of CPC and GCPC for the removal of colour from textile mill effluent was examined. The effluent was characterised using standard methods (APHA, 1995) and the results are summarised in Table 4. Batch experiments were conducted with 100 mL of textile mill effluent along with 0.5 g of carbon adjusted to pH 3.0 by equilibrating for a period of 4 h. The efficiency of various carbons on colour and COD removal is shown in Table 5. It was observed that besides removal of colour, there was concurrent reduction of COD of the effluent. The results of the decolourisation studies with GCPC are presented in Figs. 11–13, at the three standard wavelengths of 436, 525 and 620 nm, respectively. The effluent was treated until the acceptable limit of 7, 5 and 3 m1 absorbance (Allen et al., 1995) was reached at the standard wavelengths of 436, 525 and 620 nm, respectively. It was noted that the capacity of the carbon for decolourisation was effective upto 3 cycles similar to single dye solutions. GCPC could treat 8, 7.6 and 5.2 L of effluent at the standard wavelengths of 436, 525 and 620 nm, respectively. The removal of colour from the effluent was lower than sorption from a single component system. The reduced capacity can be attributed to
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a number of factors combined together. These include: (1) interaction between the dyes and other components in the effluent; (2) change of the adsorbent surface charge due to adsorption; (3) competition of other components of the effluent for active sites on the carbon surface where displacement effects replace the other components from the adsorption sites.
6. Conclusions The experimental results showed that activated carbon prepared from coir pith was a suitable adsorbent for removal of reactive dyes from both synthetic and textile effluent. In batch studies, the adsorption increased with an increase of contact time and carbon dose and decreased with an increase in solute concentration. Removal of dyes was higher at the acidic pH range. The results showed that the carbon affinity was higher for Reactive Orange followed by Reactive Red and was lowest for Reactive Blue. The increase in bed height and decrease in flow rate increased the adsorption capacity of the column. The BDST analysis of sorption of dyes showed linear relationship between bed depth and service time, which could be used successfully for scale-up purposes. Regeneration of the carbon was found to be effective with 1.0 M NaOH and the regenerated carbon was found to be effective upto 3 cycles of operation. Application of the carbon to textile effluent decolourisation studies showed significant removal of colour and COD.
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