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Experimental and kinetic studies on methylene blue adsorption by coir pith carbon
Bioresource Technology 98 (2007) 14–21
Experimental and kinetic studies on methylene blue adsorption by coir pith carbon D. Kavitha a
a,*
, C. Namasivayam
b
Department of Environmental System Engineering, Hallym University, 1 Okchon-dong, Chuncheon, Gangwon-do 200-702, Republic of Korea b Environmental Chemistry Division, Department of Environmental Sciences, Bharathiar University, Coimbatore 641 046, India Received 22 August 2005; received in revised form 29 November 2005; accepted 2 December 2005 Available online 19 January 2006
Abstract Varying the parameters such as agitation time, dye concentration, adsorbent dose, pH and temperature carried out the potential feasibility of thermally activated coir pith carbon prepared from coconut husk for removal of methylene blue. Greater percentage of dye was removed with decrease in the initial concentration of dye and increase in amount of adsorbent used. Kinetic study showed that the adsorption of dye on coir pith carbon was a gradual process. Lagergren first-order, second-order, intra particle diffusion model and Bangham were used to fit the experimental data. Equilibrium isotherms were analysed by Langmuir, Freundlich, Dubnin–Radushkevich, and Tempkin isotherm. The adsorption capacity was found to be 5.87 mg/g by Langmuir isotherm for the particle size 250–500 lm. The equilibrium time was found to be 30 and 60 min for 10 and 20 mg/L and 100 min for 30, 40 mg/L dye concentrations, respectively. A maximum removal of 97% was obtained at natural pH 6.9 for an adsorbent dose of 100 mg/50 mL and 100% removal was obtained for an adsorbent dose of 600 mg/50 mL of 10 mg/L dye concentration. The pH effect and desorption studies suggest that chemisorption might be the major mode of the adsorption process. The change in entropy (DS0) and heat of adsorption (DH0) of coir pith carbon was estimated as 117.20 J/mol/K and 30.88 kJ/mol, respectively. The high negative value of change in Gibbs free energy indicates the feasible and spontaneous adsorption of methylene blue on coir pith carbon. 2005 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Coir pith carbon; Methylene blue; Kinetic study
1. Introduction Dye removal from wastewater has received considerable attention with several adsorbents and several classes of dye being investigated (Meyer et al., 1992; Allen et al., 1998). The successful prediction of adsorption isotherms of dyes on activated carbon has been reported (McKay et al., 1998). The conventional methods for removal of dyes using alum, ferric chloride, coconut shell based activated carbon etc., are not economical in the Indian context. Pollard et al. (1992) and Namasivayam (1995) have reviewed non-conventional adsorbents used for the removal of dyes and heavy metals. Research is therefore needed to develop new alternative environmental friendly applications that *
Corresponding author. Tel.: +82 33 248 2165; fax: +82 33 241 1536. E-mail address: [email protected] (D. Kavitha).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.12.008
can further exploit activated carbon. Recently, various kinds of activated carbon have been used as low-cost adsorbents for removal of heavy metals, organics and dyes from waters. Activated carbon has many applications, one of which is used as an efficient and versatile adsorbent for purification of water, air and many chemical and natural products (Hassler, 1963). This is possible due to the highly porous nature of the solid and its extremely large surface area to volume ratio. Much of this surface area is contained in micropores and mesopores. Currently, activated carbon has been an effective adsorbent for dye removal (Walker and Weatherley, 2000; Kannan and Sundaram, 2001). The adsorption capacity of a certain carbon is known to be a function of porous structure, chemical nature of the surface, and pH of the aqueous solution. In addition, the adsorption process is influenced by the nature of the adsor-
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21
bate and its substituent groups. The presence and concentration of surface functional groups plays an important role in the adsorption capacity and the removal mechanism of the adsorbates (Yenisoy et al., 2004). The recent studies on methylene blue adsorption onto activated carbon (Shaobin et al., 2005), rice husk (Vadivelan and Vasanth Kumar, 2005), peanut hull (Renmin et al., 2005) KOH-activated and steam-activated carbons (Wu et al., 2005), pumice powder (Feryal, 2005), glass fibers (Sampa and Binay, 2005) basic and acid dyes onto oxihumolite (oxidised young brown coal) (Pavel et al., 2005), cationic and anionic dyes on pyrophyllite (Aslıhan et al., 2005), have been reported. Coir pith is a byproduct of coconut coir industries in southern India. It is a soft biomass separated from the coconut husk during the preparation of coir fiber. Efforts have been made to carbonize the coir pith and to use it for wastewater treatment. Since the coir pith is abundantly available in Tamil Nadu and Kerala, coir pith carbon may prove to be not only as cost-effective and non-conventional adsorbent but also help reduce the solid waste disposal problems. The colour removal by coir pith carbon (94%) was comparable with commercial carbon (90%) at natural pH 8.6. coir pith carbon was also found to be effective for the removal of metals (Kadirvelu and Namasivayam, 2001), and chlorophenols (Namasivayam and Kavitha, 2004). The purpose of this work was to investigate the removal of methylene blue by coir pith carbon. 2. Methods 2.1. Physico-chemical analysis of adsorbent Coir pith was collected from nearby coconut coir industries, dried in sunlight for 5 h and ground. The dried coir pith powder was sieved to 250–500 lm size. It was subjected to carbonization at 700 C for 1 h using a muffle furnace under closed condition. The carbonized material was taken out, sieved to 250–500 lm size again and used for adsorption studies. The characteristics of coir pith carbon (APHA, 1980, 1998; ISI, 1977) reported in Table 1. The porous texture was further examined by scanning electron microscope (SEM) observation. The SEM analysis was carried out on the Hitachi 2300 scanning electron microscope. 2.2. Experimental procedures Adsorption experiments were carried out by agitating 300 mg of carbon with 50 mL of dye solution of desired concentration and pH at 200 rpm, 35 C in a thermostated rotary shaker (ORBITEK, Chennai, India). Methylene blue concentration was estimated spectrophotometrically by monitoring the absorbance at 660 nm using UV–VIS spectrophotometer (Hitachi, model U-3210, Tokyo). pH was measured using pH meter (Elico, model LI-107, Hyderabad, India). The dye solution was separated from the adsorbent by centrifugation at 20,000 rpm for 20 min and its
15
Table 1 Characteristics of coir pith carbon Physical parameters Specific surface area (m2/g) Bulk density (g/mL) Conductivity (1% solution) (mS/cm) Mechanical moisture content (%) Specific gravity Decolourising power (mg/g) Iodine number (mg/g) Chemical parameters (%) Sodium Calcium Iron
167 0.12 2.3
pHZPC pH (1% solution) Ash content (%)
8.0 10.1 79.87
5.88
Porosity (%)
93.11
1.742 21.0 101.52
Volatile matter (%) Fixed carbon (%) Ion exchange capacity
58.38 41.62 Nil
0.14 0.22 0.18
Potassium Phosphorous
0.18 0.01
absorbance was measured. Effect of adsorbent dosage was studied with different adsorbent doses (25–600 mg) and 50 mL of dye solutions and agitated for equilibrium time. Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich isotherms were employed to study the equilibrium adsorption. Effect of pH was studied by adjusting the pH of dye solutions using dilute HCl and NaOH solutions and the solutions were agitated with 300 mg/50 mL adsorbent dose at 40 and 60 min for 10 and 20 mg/L dye concentrations, respectively. Desorption studies of the adsorbent that was used for the adsorption of 10 or 20 mg/L of dye solution was separated from the solution by centrifugation. The dye-loaded adsorbent was filtered using Whatman filter paper and washed gently with water to remove any unadsorbed dye. Several such samples were prepared. Then the spent adsorbent was agitated for 40 or 60 min with 50 mL of distilled water and adjusted to different pH values. The desorbed dye was estimated as before. For temperature studies, adsorption of 10 mg/L of methylene blue by 200 mg of adsorbent was carried out at 35, 40, 50 and 60 C in the thermostated rotary shaker, respectively. 3. Results and discussion 3.1. Scanning electron micrograph analyses The textural structure examination of coir pith particles can be observed from the SEM photographs (Fig. 1). This photomicrograph shows fibrous structure of coir pith carbon reveals surface texture and porosity. SEM is widely used to study the morphological features and surface characteristics of the adsorbent materials (Nelly and Isacoff, 1982; Rook, 1983). 3.2. Adsorption experiment studies 3.2.1. Effects of agitation time and concentration of dye on adsorption The amount of dye adsorbed (mg/g) increased with increase in agitation time and reached equilibrium. The equilibrium time was 40 and 60 min for 10 and 20 mg/L
16
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21
Fig. 1. Scanning electron micrograph coir pith carbon.
dye concentration, respectively and 120 min for both 30 and 40 mg/L dye concentration. The amount of dye removed at equilibrium increased from 1.6 to 5.4 mg/g with the increase in dye concentration from 10 to 40 mg/L. It is clear that the removal of dyes depends on the concentration of the dye. It also observed that for an initial dye concentration of 10 mg/L, the maximum amount (approximately 97.3% of total amount of dye removed) of dye was adsorbed within the first 30 min at an average adsorption rate of 0.054 mg/g min and thereafter the adsorption rate tends to decrease and proceeds at an average adsorption rate of 0.016 mg/g min. A similar trend was observed for the remaining range of initial dye concentrations (20– 40 mg/L) studied. The initial rapid phase may be due to increased the number of vacant sites available at the initial stage, as a result there exist increased concentration gradient between adsorbate in solution and adsorbate in the adsorbent. 3.2.2. Studies on pH effect Effect of pH on the removal of methylene blue is shown in Fig. 2. The percent removal was more than 90% in the pH range 2–11. At pH 2, though positively charged surface sites on the adsorbent do not favor the adsorption of dye cations due to the electrostatic repulsion, dye removal 102
20
100
18 16
98
% Removal
14
A
12
94
10
92 Dye conc. 10 mg/L 20 mg/L
90 88
8 6 B
86
4 2
84 82
% Desorption
96
2
3
4
5
6
7
8
9
10
11
0
Initial pH
Fig. 2. (A) Effect of pH on removal of methylene blue, adsorbent dose, 300 mg/50 mL; 10 mg/L, agitation time 40 min; 20 mg/L, agitation time 60 min. (B) Effect of pH on desorption of dye from dye-loaded adsorbent. Adsorbent dose, 300 mg/50 mL; 10 mg/L, agitation time 40 min; 20 mg/L, agitation time 60 min.
was still high (more than 90%). As the pH increased, the removal increased slightly. Several investigations have reported that methylene blue adsorption usually increases as the pH is increased (Gupta et al., 2004; Singh et al., 2003; Janos, 2003). Basically, methylene blue and other cationic dyes produce an intense molecular cation (C+) and reduced ions (CH+). At high pH, OH on the surface of adsorbent will favor the adsorption of cationic dye molecules. 3.2.3. Effect of temperature Increase of temperature increased the percent removal. The change in standard free energy, enthalpy and entropy of adsorption were calculated using the following equations: DG0 ¼ RT ln K c
ð1Þ
where R is gas constant and Kc is the equilibrium constant and T is the temperature in K. According to van’t Hoff equation, log10 K c ¼ DS 0 =2:303R DH 0 =2:303RT
ð2Þ
0
Positive values of DH (30.88 J/mol/K) show the endothermic nature of adsorption. The negative values of (5.27, 5.72, 6.50 and 7.64) DG0 indicate the spontaneous nature of adsorption for methylene blue at 35, 40, 50 and 60 C. The positive values of DS0 (117.2 kJ/mol) suggest the increased randomness at the solid/solution interface during the adsorption of dye on coir pith carbon. Kinetic models also applied for temperature studies and it shows perfect linear curves for methylene blue at different temperature studies and the equilibrium data obtained for all the kinetics are presented in Table 2. 3.2.4. Desorption studies Regeneration of exhausted carbon and recovery of dye contribute to the economy of wastewater treatment. Also desorption studies help elucidate the mechanism of adsorption. Desorption was less than 10% in the entire pH range 2–10 though there was slight increase from 0.5 to 9.0 (Fig. 2). Insignificant desorption at different pH values confirms that chemisorption is the major mode of adsorption process.
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21
17
Table 2 Kinetic parameters for the removal of methylene blue by coir pith carbon k1 (min1)
Concentration
qe (mg/g)
R2
Temperature (C)
k1 (min1)
qe (mg/g)
R2
0.3663 0.6108 0.7803 1.3259
0.9929 0.9864 0.9655 0.9780
35 40 50 60
0.117 0.124 0.103 0.081
2.314 1.992 1.314 0.586
0.967 0.989 0.971 0.927
k2 (g/mg min)
R2
Temperature (C)
h (mg/g min)
k2 (g/mg min)
R2
0.6049 0.2545 0.1112 0.0646
0.9998 0.9998 0.9999 0.9992
35 40 50 60
1.9331 2.4825 3.6178 7.813
0.09 0.11 0.16 0.34
0.999 0.999 0.999 1.000
k0 (mL/g/L)
a
R2
Temperature (C)
k0 (mL/g/L)
a
R2
6.5109 5.7179 5.5788 4.495
0.0737 0.0799 0.0464 0.086
0.9733 0.9852 0.9517 0.9926
35 40 50 60
5.2837 6.3173 6.885 7.1258
0.1246 0.0763 0.0559 0.0447
0.9826 0.9887 0.9882 0.9957
Pseudo-first-order constants 10 0.1089 20 0.0525 30 0.0173 40 0.0233 h (mg/g min) Pseudo-second-order 10 20 30 40
Bangham constants 10 20 30 40
constants 1.6913 2.3924 1.9534 1.845
kid (mg/g min) Intra-particle diffusion constants 10 0.060 20 0.1058 30 0.074 40 0.1447
R2
Temperature (C)
kid (mg/g min)
R2
0.9464 0.9379 0.9859 0.9558
35 40 50 60
0.0936 0.0616 0.0466 0.0383
0.9414 0.9593 0.9566 0.9922
3.3. Adsorption kinetic studies
70
logðqe qÞ ¼ log qe k 1 t=2:303
ð3Þ
where qe and q are the amounts of dye adsorbed (mg/g) at equilibrium and at time t (min), respectively, and k1 is the rate constant of adsorption (L/min). The values of k1 and qe at different concentrations are presented in Table 2. 3.3.2. The second-order kinetic model The second-order kinetic model (McKay and Ho, 1999) is expressed as t=q ¼ 1=k 2 q2e þ t=qe
ð4Þ
the initial adsorption rate, h (mg/g min), as t ! 0 can be defined s h ¼ k 2 q2e
ð5Þ
where the initial adsorption rate (h), the equilibrium adsorption capacity (qe), and the second-order constants k2 (g/mg min) can be determined experimentally from the slope and intercept of plot t/q versus t (Fig. 3). Calculated correlations are closer to unity for second-order kinetics model; therefore the adsorption kinetics could well be approximated more favorably by second-order kinetic model for methylene blue. The k2 (g/mg min) and h values are calculated from Fig. 3 are listed in Table 2.
Dye conc. 10 mg/L 20 mg/L 30 mg/L 40 mg/L
60
t/q (min.g/mg)
3.3.1. Adsorption dynamics The rate constant of adsorption is determined from the first-order rate expression given by Lagergren and Svenska (1898).
50 40
R2 0.9998 0.9998 0.999 0.9992
30 20 10 0 0
20
40
60
80
100
120
Time (min)
Fig. 3. Second-order kinetics plots for the removal of methylene blue, at different initial dye concentrations: adsorbent dose, 300 mg/50 mL; initial pH, 6.9, temperature 35 C.
3.3.3. Intra-particle diffusion study An empirically found functional relationship, common to the most adsorption processes, is that the uptake varies almost proportionally with t1/2, the Weber–Morris plot, rather than with the contact time t (Weber and Morris, 1963). qt ¼ k id t1=2 þ C
ð6Þ
where kid is the intra-particle diffusion rate constant. According to Eq. (6), a plot of qt versus t1/2 should be a straight line with a slope kid and intercept C when adsorption mechanism follows the intra-particle diffusion process. Values of intercept give an idea about the thickness of boundary layer, i.e., larger the intercept with greater is
18
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21 6 2
Dye conc. R 10 mg/L 0.9464
5
20 mg/L 0.9379 30 mg/L 0.9859 40 mg/L 0.9558 3
1
q (mg/g)
4
2
3.3.5. Adsorption equilibrium study To optimize the design of an adsorption system for the adsorption of methylene blue, it is important to establish the most appropriate correlation for the equilibrium curves. Various isotherm equations have been used to describe the equilibrium nature of adsorption. Some of these equations are Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich (D–R).
1
0
0
2
4
6
8
10
3.3.6. Adsorption isotherms Langmuir isotherm (1918) is represented by the following equation:
12
Time1/2(min)1/2
Fig. 4. Weber and Morris intra-particle diffusion plots for removal of methylene blue, at different initial dye concentrations: adsorbent dose, 300 mg/50 mL; initial pH, 6.9, temperature 35 C.
the boundary layer effect (Kannan and Sundaram, 2001). In Fig. 4, plot of mass of dye adsorbed per unit mass of adsorbent, qt versus t1/2 is presented for methylene blue. The linear plots are attributed to the macro pore diffusion that is the accessible sites of adsorption. This is attributed to the instantaneous utilization of the most readily available adsorbing sites on the adsorbent surface. The values of kid as obtained from the slope of straight lines are listed in Table 2. 3.3.4. Bangham’s equation Kinetic data can further be used to check whether pore diffusion is the only rate-controlling step or not in the adsorption system using Bangham’s equation (Aharoni et al., 1979). log logðC 0 =C0 qt mÞ ¼ logðk 0 m=2:303V Þ þ logðtÞ
ð7Þ
where C0 is the initial concentration of adsorbate in solution (mg/L), V is the volume of solution (mL), m is the weight of adsorbent per liter of solution (g/L), qt (mg/g) is the amount of adsorbate retained at time t, and a (less than 1) and k0 are constants. The double logarithmic plot (Fig. 5) according to above equation yielded perfect linear curves for methylene blue removal by carbon, showing that the diffusion of adsorbate into pores of the adsorbent is not the only rate controlling step (Tutem et al., 1998). -1.6
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
loglog(C0/C0-qtm)
-1.75
where Ce is the concentration of dye solution (mg/L) at equilibrium. The constant Q0 signifies the adsorption capacity (mg/g) and b is related to the energy of adsorption (L/mg). Linear plot of Ce/qe versus Ce shows that adsorption follows Langmuir isotherm (figure not shown). Values of Q0 and b were calculated from the slope and intercept of the linear plot and are presented in Table 3. The adsorption capacity can be correlated with the variation of surface area and porosity of the adsorbent. Higher surface area and pore volume will result in higher adsorption capacity. Several investigators have been conducted using various low-cost adsorbent. Table 4 compares the adsorption capacity of different types of adsorbents using for methylene blue adsorption. It is seen low cost material from bark, rice husk, cotton waste and coal (McKay et al., 1999) reported the highest adsorption capacity such as 914, 312, 277, and 250 mg/g and activated carbon prepared from rice husk-H3PO4 impregnated (Singh and Srivastava, 2001) observed 333 mg/g revealing that low cost material can be employed as a promising adsorbent for dye adsorption. The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter RL, defined by Weber and Chakkravorti (1974): RL ¼ 1=ð1 þ bC 0 Þ
ð9Þ
where b is the Langmuir constant and C0 is the initial dye concentration (mg/L), RL values indicate the type of isotherm. Table 3 shows RL values between zero and one, which indicate favorable adsorption.
20 mg/L 0.9852 30 mg/L 0.9517 40 mg/L 0.9926
log10 ðx=mÞ ¼ log10 k f þ ð1=nÞlog10 C e
Dye conc. R 2 10 mg/L 0.9733
-1.8 -1.85 -1.9
ð8Þ
3.3.7. Freundlich isotherm Freundlich (1985) was also applied to plots of equilibrium adsorption data:
-1.65 -1.7
C e =qe ¼ l=Q0 b þ C e =Q0
log t
Fig. 5. Bangham’s plot for removal of methylene blue, at different initial dye concentrations: adsorbent dose, 300 mg/50 mL; initial pH, 6.9, temperature 35 C.
ð10Þ
where x is the amount of dye adsorbed (mg), m is the weight of the adsorbent used (g), Ce is the equilibrium concentration of dye in solution (mg/L), kf (mg11/n L1/n g1) and 1/n are Freundlich constants. The correlation coefficients were found to be less than 0.9 (Fig. 6) and values of kf and n were calculated from intercept and slope of plots and are presented in Table 3.
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21
19
Table 3 Isotherm parameters for removal of methylene blue by coir pith carbon Concentration Langmuir constants 10 20 30 40
Q0 (mg/g)
b (L/mg)
5.87
n
R2
0.705 0.731 0.785 0.860
0.8972 0.8675 0.8968 0.8840
E (kJ/mol)
R2
3.54
0.8421
KT (L/mg)
B1
R2
23.25
0.9618
0.952
1.192 0.845 0.726 0.778 qs (mg/g)
Dubnin–Radushkevich constants 10 20 30 40
Tempkin constants 10 20 30 40
0.766 0.621 0.522 0.451
0.93
kf (mg11/n L1/n g1) Freundlich constants 10 20 30 40
RL
4.266
Table 4 Langmuir and Freundlich constants for methylene blue adsorption by various adsorbents reported in literature kf (mg11/n L1/n g1)
n
References
0.017 0.043
D 57.54
D 2.86
McKay et al. (1999) Singh and Srivastava (2001)
323.68 312.26 277.78 204.00 194.3
0.004 0.017 0.009 16.34 0.066
D D D 17.04 6.083
D D D 0.047 5.579
McKay et al. (1999) McKay et al. (1999) McKay et al. (1999) Ghosh and Bhattacharyya (2001) Rozada et al. (2003)
158.23 133.33 122.01 96.2 91.87 56.31 49.0 32.26 27.49 13.44
0.019 0.290 20.49 6.67 · 104 15.55 8.88 0.05 0.38 13.99 7.59
D 52.48 23.05 D 16.39 9.01 D 14.58 14.85 6.57 2.22 21.38
D 4.0 0.098 D 0.070 0.061 D 0.26 0.151 0.075 0.90 1.46
McKay et al. (1999) Singh and Srivastava (1999) Ghosh and Bhattacharyya (2001) Jain et al. (2003) Ghosh and Bhattacharyya (2001) Ghosh and Bhattacharyya (2001) Miguel et al. (2002) De and Basu (1998) Ghosh and Bhattacharyya (2001) Ghosh and Bhattacharyya (2001) Vasanth Kumar and Bhagavanulu (2002) Namasivayam and Yamuna (1994)
Adsorbent
Q0 (mg/g)
Bark Activated carbon (rice husk-H3PO4 impregnated) Coal Rice husk Cotton waste NaOH-treated raw clay Activated carbon (sewage sludge-H2SO4 impregnated) Hair Tree leaves NaOH-treated pure clay Carbon slurry waste Pure clay Calcined pure clay Activated carbon from waste rubber P-700 Saw dust Raw clay Calcined raw clay Boiler bottom ash Biogas residual slurry
914.59 333.3
b (L/mg)
*
*
*
*
* Does not follow Langmuir isotherm/not reported. D Does not follow Freundlich isotherm/not reported.
3.3.8. Tempkin isotherm Tempkin isotherm contains a factor that explicitly takes into account adsorbing species–adsorbate interactions. This isotherm assumes that: (i) the heat of adsorption of all the molecules in the layer decreases linearly with cover-
age due to adsorbate–adsorbate interactions, and (ii) adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy (Tempkin and Pyzhev, 1940). Tempkin isotherm is represented by the following equation:
20
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21 1.6 1.4 1.2
log (x/m)
1 0.8 0.6
Dye conc.
R2
10 mg/L
0.8972
20 mg/L
0.8675
30 mg/L
0.8968
40 mg/L
0.884
removal of dye follows pore diffusion process since the coefficient values are in the range of 10111013 cm2/s. values of Dp for methylene blue are 0.884 and 0.589 · 1012 cm2/s for 10 and 20 mg/L and 0.295 · 1012 cm2/s for 30 and 40 mg/L have been observed. 4. Conclusions
0.4 0.2 0 --1
-0.5
0
0.5
1
1.5
2
log ce
Fig. 6. Freundlich plots for the adsorption of methylene blue by coir pith carbon.
qe ¼ RT =b lnðK t C e Þ
ð11Þ
Eq. (11) can be expressed in its linear form as qe ¼ B1 ln K t þ B1 ln C e
ð12Þ
where B1 ¼ RT =b
ð13Þ
The adsorption data can be analysed according to Eq. (12). A plot of qe versus ln Ce enables the determination of the isotherm constants Kt and B1. Kt is the equilibrium binding constant (L/mol) corresponding to the maximum binding energy and constant B1 is related to the heat of adsorption. The values of the parameters are given in Table 3. 3.3.9. Dubinin and Radushkevich isotherm Another equation used in the analysis of isotherms was proposed by Dubinin and Radushkevich (1947): 2
qe ¼ qs expðBe Þ
ð14Þ
where qs is D–R constant and e can be correlated: e ¼ RT lnð1 þ 1=C e Þ E ¼ 1=ð2BÞ
1=2
ð15Þ ð16Þ
Calculated Dubinin–Radushkevich constants for the adsorption of methylene blue on coir pith carbon are shown in Table 3; the values of correlation coefficients are much lower than other three isotherms values. In this case, D–R equation represents the poorer fit of experimental data than the other isotherm equation. 3.4. Pore diffusion coefficient Assuming spherical geometry for the adsorbent, the time for half adsorption can be correlated to the pore diffusion coefficient (Michelson et al., 1975). t1=2 ¼ 0:03r20 =Dp
ð17Þ
where t1/2 is the time for half adsorption (s), r0 is the radius of the adsorbent particle (cm) and Dp is the diffusion coefficient (cm2/s). Values of Dp have been calculated for different temperatures and different concentrations of dye. The
The present study shows that the coir pith carbon is an effective adsorbent for the removal of methylene blue from aqueous solution. Higher methylene blue by coir pith carbon is possible provided that the initial adsorbate concentration was low in the solution. Equilibrium adsorption was achieved in about 40 min. Batch kinetic studies performed on the carbon–dye system indicated the adsorption capacity of coir pith carbon. Kinetics data tended to fit well in second-order kinetics, confirming the chemisorption of methylene blue onto coir pith carbon particles. By second-order model to study the mechanism of adsorption, calculated qe values, agreed well with the qe experimental values, supporting the chemisorption. The Langmuir and Tempkin isotherms best represent the equilibrium adsorption data. The effective diffusion coefficient of methylene blue is of the order of 1012 cm2/s. As the raw material, coir pith is discarded as waste in coir industries; the treatment method using coir pith carbon is expected to be economical. The cost and adsorption characteristics favor coir pith carbon to be used as an effective adsorbent for the removal of methylene blue from wastewater. References Aharoni, C., Sideman, S., Hoffer, E., 1979. Adsorption of phosphate ions by colloid ion-coated alumina. J. Chem. Technol. Biotechnol. 29, 404– 412. Allen, S.J., Mckay, G., Khader, K.Y.H., 1998. The adsorption of acid dye onto peat from aqueous solution–solid diffusion model. J. Colloid Interface Sci. 126, 517–524. APHA, 1980. Standard Methods for the Examination of Water and Wastewater, 15th ed. American Public Health Association, Washington, DC. APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC. Aslıhan, G., Savas, S., Sedat, B., Ali, M.M., 2005. Adsorption and kinetic studies of cationic and anionic dyes on pyrophyllite from aqueous solutions. J. Colloid Interface Sci. 286, 53–60. De, D.S., Basu, J.K., 1998. Adsorption of methylene blue on to a low cost adsorbent developed from saw dust. Indian J. Environ. Prot. 19, 416– 421. Dubinin, M.M., Radushkevich, L.V., 1947. Equation of the characteristic curve of activated charcoal. Chem. Zentr. 1, 875. Feryal, A., 2005. Adsorption of basic dyes from aqueous solution onto pumice powder. J. Colloid Interface Sci. 286, 455–458. Freundlich, H., 1985. Uber die adsorption in lunsungen. J. Phys. Chem. 57, 387–470. Ghosh, D., Bhattacharyya, K.G., 2001. Removing colour from aqueous medium by sorption on natural clay: a study with methylene blue. Indian J. Environ. Prot. 21, 903–910. Gupta, V.K., Suhas, A.I., Saini, V.K., 2004. Removal of rhodamine B, fast green, and methylene blue from wastewater using red mud, an aluminum industry waste. Ind. Eng. Chem. Res. 43, 1740–1747.
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21 Hassler, J.W., 1963. Activated Carbon. Chem. Publ., New York. ISI, 1977. Methods of sampling and tests for activated carbon used for decolourising vegetable oils and sugar solutions. Indian Standards Institution (ISI), 877. Jain, A.K., Gupta, V.K., Bhatngar, A., Jain, S., Suhas, A.I., 2003. A comparative assessment of adsorbents prepared from industrial wastes for the removal of cationic dye. J. Indian Chem. Soc. 80, 267–270. Janos, P., 2003. Sorption of basic dyes onto iron humate. Environ. Sci. Technol. 37, 5792–5798. Kadirvelu, K., Namasivayam, C., 2001. Adsorption of nickel (II) from aqueous solution onto activated carbon prepared from coir pith. Sep. Purif. Technol. 24, 497–505. Kannan, K., Sundaram, M.M., 2001. Kinetics and mechanism of removal of methylene blue by adsorption on various carbons—a comparative study. Dyes Pig. 51, 25–40. Lagergren, S., Svenska, B.K., 1898. Zur theorie der sogenannten adsorption geloester stoffe. Vetenskapsakad. Handl. 24, 1–39. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361. McKay, G., Al-Duri, B., Allen, S.J., Thomson, A., 1998. Adsorption for liquid process effluents. IchE Symp. Pap 17 (3), 1–17. McKay, G., Ho, Y.S., 1999. Pseudo-second order model for sorption processes. Process. Biochem. 34, 451–465. McKay, G., Porter, J.F., Prasad, G.R., 1999. The removal of dye colours from aqueous solutions by adsorption on low cost materials. Wat. Air. Soil. Pollut. 114, 423–438. Meyer, V., Carlsson, F.H.H., Oellermann, R.A., 1992. Decolourization of textile effluent using a low cost natural adsorbent material. Wat. Sci. Technol. 26, 1205–1211. Michelson, L.D., Gideon, P.G., Pace, E.G., Kutal, L.H., 1975. Removal of soluble mercury from wastewater by complexing technique, US Department Industry, Office of Water Research and Technology, Bull. No. 74. Miguel, G.S., Fowler, G.D., Sollars, C.J., 2002. Adsorption of organic compounds from solution by activated carbons produced from waste tyre rubber. Sep. Sci. Technol. 37, 663–676. Namasivayam, C., 1995. Adsorbents for the treatment of wastewaters. In: Trivedy, R.K. (Ed.), Encyclopedia of Environmental Pollution and Control, Vol. 1. Enviro-media, Karad, Maharastra, India, pp. 30–49. Namasivayam, C., Kavitha, D., 2004. Adsorptive removal of 2,4dichlorophenol from wastewater by low-cost carbon from an agricultural solid waste: coconut coir pith. Sep. Sci. Technol. 39, 1407–1425. Namasivayam, C., Yamuna, R.T., 1994. Utilizing biogas residual slurry for dye adsorption. Am. Dyestuff Rep. 83, 22–28. Nelly, J.W., Isacoff, E.G., 1982. Carbonaceous Adsorbents for the Treatment of Ground and Surface Water. Marcel Dekker, New York. Pavel, J., Pavel, S., Milena, R., Sylvie, G., 2005. Sorption of basic and acid dyes from aqueous solutions onto oxihumolite. Chemosphere 59, 881– 886.
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Pollard, S.J.T., Fowler, G.D., Sollars, C.J., Perry, R., 1992. Low cost adsorbents for water and wastewater treatment; a review. Sci. Tot. Environ. 116, 31–52. Renmin, G., Yingzhi, S., Jian, C., Huijun, L., Chao, Y., 2005. Effect of chemical modification on dye adsorption capacity of peanut hull. Dyes Pig. 67, 175–181. Rook, J.J., 1983. In: McGuire, M.J., Suffet, I.H. (Eds.), Treatment of Water by Granular Activated Carbon. American Chemical Society, Washington, DC. Rozada, F., Calvo, L.F., Garcia, A.I., Villacorta, M., Otero, M., 2003. Dye adsorption by sewage sludge-based activated carbons in batch and fixed-bed systems. Biores. Technol. 87, 221–230. Sampa, C., Binay, K.D., 2005. On the adsorption and diffusion of methylene blue in glass fibers. J. Colloid Interface Sci. 286, 807–811. Shaobin, W., Zhua, Z.H., Anthony, C., Haghseresht, F., Luc, G.Q., 2005. The physical and surface chemical characteristics of activated carbons and the adsorption of methylene blue from wastewater. J. Colloid Interface Sci. 284, 440–446. Singh, K.P., Mohan, D., Sinha, S., Tondon, G.S., Gosh, D., 2003. Color removal from wastewater using low-cost activated carbon derived from agricultural waste material. Ind. Eng. Chem. Res. 42, 1965–1976. Singh, D.K., Srivastava, B., 1999. Removal of basic dyes from aqueous solutions by chemically treated Psidium guava leaves. Indian J. Environ. Hlth. 41, 333–345. Singh, D.K., Srivastava, B., 2001. Basic dyes removal from wastewater by adsorption on rice husk carbon. Indian J. Chem. Technol. 8, 133– 139. Tempkin, M.J., Pyzhev, V., 1940. Acta Physiochim. URSS 12, 217–222. Tutem, E., Apak, R., Unal, C.F., 1998. Adsorptive removal of chlorophenols from water by bituminous shale. Wat. Res. 32, 2315–2324. Vadivelan, V., Vasanth Kumar, K., 2005. Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk. J. Colloid Interface Sci. 286, 90–100. Vasanth Kumar, K., Bhagavanulu, D.V.S., 2002. Boiler bottom ash, for removal of methylene blue from synthetic dye effluents. Dyes Chem., Textile Mag., 65–66. Walker, G.M., Weatherley, L.R., 2000. Textile wastewater treatment using granular activated carbon adsorption in fixed beds. Sep. Sci. Technol. 35, 1329–1341. Weber, T.W., Chakkravorti, P., 1974. Pore and solid diffusion models for fixed-bed adsorbers. AIChEJ 20, 228. Weber Jr., W.J., Morris, J.C., 1963. Kinetics of adsorption on carbon from solution. J. Sanit. Eng. Div. ASCE 89 (SA2), 31–59. Wu, F.C., Tseng, R.-L., Hu, C.-C., 2005. Comparisons of pore properties and adsorption performance of KOH-activated and steam-activated carbons. Micropor. Mesopor. Mater. 80, 95–106. Yenisoy, K.S., Aygun, A., Gunes, M., Tahtasakal, E., 2004. Physical and chemical characteristics of polymer-based spherical activated carbon and its ability to adsorb organics. Carbon 42, 477–484.
Experimental and kinetic studies on methylene blue adsorption by coir pith carbon D. Kavitha a
a,*
, C. Namasivayam
b
Department of Environmental System Engineering, Hallym University, 1 Okchon-dong, Chuncheon, Gangwon-do 200-702, Republic of Korea b Environmental Chemistry Division, Department of Environmental Sciences, Bharathiar University, Coimbatore 641 046, India Received 22 August 2005; received in revised form 29 November 2005; accepted 2 December 2005 Available online 19 January 2006
Abstract Varying the parameters such as agitation time, dye concentration, adsorbent dose, pH and temperature carried out the potential feasibility of thermally activated coir pith carbon prepared from coconut husk for removal of methylene blue. Greater percentage of dye was removed with decrease in the initial concentration of dye and increase in amount of adsorbent used. Kinetic study showed that the adsorption of dye on coir pith carbon was a gradual process. Lagergren first-order, second-order, intra particle diffusion model and Bangham were used to fit the experimental data. Equilibrium isotherms were analysed by Langmuir, Freundlich, Dubnin–Radushkevich, and Tempkin isotherm. The adsorption capacity was found to be 5.87 mg/g by Langmuir isotherm for the particle size 250–500 lm. The equilibrium time was found to be 30 and 60 min for 10 and 20 mg/L and 100 min for 30, 40 mg/L dye concentrations, respectively. A maximum removal of 97% was obtained at natural pH 6.9 for an adsorbent dose of 100 mg/50 mL and 100% removal was obtained for an adsorbent dose of 600 mg/50 mL of 10 mg/L dye concentration. The pH effect and desorption studies suggest that chemisorption might be the major mode of the adsorption process. The change in entropy (DS0) and heat of adsorption (DH0) of coir pith carbon was estimated as 117.20 J/mol/K and 30.88 kJ/mol, respectively. The high negative value of change in Gibbs free energy indicates the feasible and spontaneous adsorption of methylene blue on coir pith carbon. 2005 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Coir pith carbon; Methylene blue; Kinetic study
1. Introduction Dye removal from wastewater has received considerable attention with several adsorbents and several classes of dye being investigated (Meyer et al., 1992; Allen et al., 1998). The successful prediction of adsorption isotherms of dyes on activated carbon has been reported (McKay et al., 1998). The conventional methods for removal of dyes using alum, ferric chloride, coconut shell based activated carbon etc., are not economical in the Indian context. Pollard et al. (1992) and Namasivayam (1995) have reviewed non-conventional adsorbents used for the removal of dyes and heavy metals. Research is therefore needed to develop new alternative environmental friendly applications that *
Corresponding author. Tel.: +82 33 248 2165; fax: +82 33 241 1536. E-mail address: [email protected] (D. Kavitha).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.12.008
can further exploit activated carbon. Recently, various kinds of activated carbon have been used as low-cost adsorbents for removal of heavy metals, organics and dyes from waters. Activated carbon has many applications, one of which is used as an efficient and versatile adsorbent for purification of water, air and many chemical and natural products (Hassler, 1963). This is possible due to the highly porous nature of the solid and its extremely large surface area to volume ratio. Much of this surface area is contained in micropores and mesopores. Currently, activated carbon has been an effective adsorbent for dye removal (Walker and Weatherley, 2000; Kannan and Sundaram, 2001). The adsorption capacity of a certain carbon is known to be a function of porous structure, chemical nature of the surface, and pH of the aqueous solution. In addition, the adsorption process is influenced by the nature of the adsor-
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21
bate and its substituent groups. The presence and concentration of surface functional groups plays an important role in the adsorption capacity and the removal mechanism of the adsorbates (Yenisoy et al., 2004). The recent studies on methylene blue adsorption onto activated carbon (Shaobin et al., 2005), rice husk (Vadivelan and Vasanth Kumar, 2005), peanut hull (Renmin et al., 2005) KOH-activated and steam-activated carbons (Wu et al., 2005), pumice powder (Feryal, 2005), glass fibers (Sampa and Binay, 2005) basic and acid dyes onto oxihumolite (oxidised young brown coal) (Pavel et al., 2005), cationic and anionic dyes on pyrophyllite (Aslıhan et al., 2005), have been reported. Coir pith is a byproduct of coconut coir industries in southern India. It is a soft biomass separated from the coconut husk during the preparation of coir fiber. Efforts have been made to carbonize the coir pith and to use it for wastewater treatment. Since the coir pith is abundantly available in Tamil Nadu and Kerala, coir pith carbon may prove to be not only as cost-effective and non-conventional adsorbent but also help reduce the solid waste disposal problems. The colour removal by coir pith carbon (94%) was comparable with commercial carbon (90%) at natural pH 8.6. coir pith carbon was also found to be effective for the removal of metals (Kadirvelu and Namasivayam, 2001), and chlorophenols (Namasivayam and Kavitha, 2004). The purpose of this work was to investigate the removal of methylene blue by coir pith carbon. 2. Methods 2.1. Physico-chemical analysis of adsorbent Coir pith was collected from nearby coconut coir industries, dried in sunlight for 5 h and ground. The dried coir pith powder was sieved to 250–500 lm size. It was subjected to carbonization at 700 C for 1 h using a muffle furnace under closed condition. The carbonized material was taken out, sieved to 250–500 lm size again and used for adsorption studies. The characteristics of coir pith carbon (APHA, 1980, 1998; ISI, 1977) reported in Table 1. The porous texture was further examined by scanning electron microscope (SEM) observation. The SEM analysis was carried out on the Hitachi 2300 scanning electron microscope. 2.2. Experimental procedures Adsorption experiments were carried out by agitating 300 mg of carbon with 50 mL of dye solution of desired concentration and pH at 200 rpm, 35 C in a thermostated rotary shaker (ORBITEK, Chennai, India). Methylene blue concentration was estimated spectrophotometrically by monitoring the absorbance at 660 nm using UV–VIS spectrophotometer (Hitachi, model U-3210, Tokyo). pH was measured using pH meter (Elico, model LI-107, Hyderabad, India). The dye solution was separated from the adsorbent by centrifugation at 20,000 rpm for 20 min and its
15
Table 1 Characteristics of coir pith carbon Physical parameters Specific surface area (m2/g) Bulk density (g/mL) Conductivity (1% solution) (mS/cm) Mechanical moisture content (%) Specific gravity Decolourising power (mg/g) Iodine number (mg/g) Chemical parameters (%) Sodium Calcium Iron
167 0.12 2.3
pHZPC pH (1% solution) Ash content (%)
8.0 10.1 79.87
5.88
Porosity (%)
93.11
1.742 21.0 101.52
Volatile matter (%) Fixed carbon (%) Ion exchange capacity
58.38 41.62 Nil
0.14 0.22 0.18
Potassium Phosphorous
0.18 0.01
absorbance was measured. Effect of adsorbent dosage was studied with different adsorbent doses (25–600 mg) and 50 mL of dye solutions and agitated for equilibrium time. Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich isotherms were employed to study the equilibrium adsorption. Effect of pH was studied by adjusting the pH of dye solutions using dilute HCl and NaOH solutions and the solutions were agitated with 300 mg/50 mL adsorbent dose at 40 and 60 min for 10 and 20 mg/L dye concentrations, respectively. Desorption studies of the adsorbent that was used for the adsorption of 10 or 20 mg/L of dye solution was separated from the solution by centrifugation. The dye-loaded adsorbent was filtered using Whatman filter paper and washed gently with water to remove any unadsorbed dye. Several such samples were prepared. Then the spent adsorbent was agitated for 40 or 60 min with 50 mL of distilled water and adjusted to different pH values. The desorbed dye was estimated as before. For temperature studies, adsorption of 10 mg/L of methylene blue by 200 mg of adsorbent was carried out at 35, 40, 50 and 60 C in the thermostated rotary shaker, respectively. 3. Results and discussion 3.1. Scanning electron micrograph analyses The textural structure examination of coir pith particles can be observed from the SEM photographs (Fig. 1). This photomicrograph shows fibrous structure of coir pith carbon reveals surface texture and porosity. SEM is widely used to study the morphological features and surface characteristics of the adsorbent materials (Nelly and Isacoff, 1982; Rook, 1983). 3.2. Adsorption experiment studies 3.2.1. Effects of agitation time and concentration of dye on adsorption The amount of dye adsorbed (mg/g) increased with increase in agitation time and reached equilibrium. The equilibrium time was 40 and 60 min for 10 and 20 mg/L
16
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21
Fig. 1. Scanning electron micrograph coir pith carbon.
dye concentration, respectively and 120 min for both 30 and 40 mg/L dye concentration. The amount of dye removed at equilibrium increased from 1.6 to 5.4 mg/g with the increase in dye concentration from 10 to 40 mg/L. It is clear that the removal of dyes depends on the concentration of the dye. It also observed that for an initial dye concentration of 10 mg/L, the maximum amount (approximately 97.3% of total amount of dye removed) of dye was adsorbed within the first 30 min at an average adsorption rate of 0.054 mg/g min and thereafter the adsorption rate tends to decrease and proceeds at an average adsorption rate of 0.016 mg/g min. A similar trend was observed for the remaining range of initial dye concentrations (20– 40 mg/L) studied. The initial rapid phase may be due to increased the number of vacant sites available at the initial stage, as a result there exist increased concentration gradient between adsorbate in solution and adsorbate in the adsorbent. 3.2.2. Studies on pH effect Effect of pH on the removal of methylene blue is shown in Fig. 2. The percent removal was more than 90% in the pH range 2–11. At pH 2, though positively charged surface sites on the adsorbent do not favor the adsorption of dye cations due to the electrostatic repulsion, dye removal 102
20
100
18 16
98
% Removal
14
A
12
94
10
92 Dye conc. 10 mg/L 20 mg/L
90 88
8 6 B
86
4 2
84 82
% Desorption
96
2
3
4
5
6
7
8
9
10
11
0
Initial pH
Fig. 2. (A) Effect of pH on removal of methylene blue, adsorbent dose, 300 mg/50 mL; 10 mg/L, agitation time 40 min; 20 mg/L, agitation time 60 min. (B) Effect of pH on desorption of dye from dye-loaded adsorbent. Adsorbent dose, 300 mg/50 mL; 10 mg/L, agitation time 40 min; 20 mg/L, agitation time 60 min.
was still high (more than 90%). As the pH increased, the removal increased slightly. Several investigations have reported that methylene blue adsorption usually increases as the pH is increased (Gupta et al., 2004; Singh et al., 2003; Janos, 2003). Basically, methylene blue and other cationic dyes produce an intense molecular cation (C+) and reduced ions (CH+). At high pH, OH on the surface of adsorbent will favor the adsorption of cationic dye molecules. 3.2.3. Effect of temperature Increase of temperature increased the percent removal. The change in standard free energy, enthalpy and entropy of adsorption were calculated using the following equations: DG0 ¼ RT ln K c
ð1Þ
where R is gas constant and Kc is the equilibrium constant and T is the temperature in K. According to van’t Hoff equation, log10 K c ¼ DS 0 =2:303R DH 0 =2:303RT
ð2Þ
0
Positive values of DH (30.88 J/mol/K) show the endothermic nature of adsorption. The negative values of (5.27, 5.72, 6.50 and 7.64) DG0 indicate the spontaneous nature of adsorption for methylene blue at 35, 40, 50 and 60 C. The positive values of DS0 (117.2 kJ/mol) suggest the increased randomness at the solid/solution interface during the adsorption of dye on coir pith carbon. Kinetic models also applied for temperature studies and it shows perfect linear curves for methylene blue at different temperature studies and the equilibrium data obtained for all the kinetics are presented in Table 2. 3.2.4. Desorption studies Regeneration of exhausted carbon and recovery of dye contribute to the economy of wastewater treatment. Also desorption studies help elucidate the mechanism of adsorption. Desorption was less than 10% in the entire pH range 2–10 though there was slight increase from 0.5 to 9.0 (Fig. 2). Insignificant desorption at different pH values confirms that chemisorption is the major mode of adsorption process.
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21
17
Table 2 Kinetic parameters for the removal of methylene blue by coir pith carbon k1 (min1)
Concentration
qe (mg/g)
R2
Temperature (C)
k1 (min1)
qe (mg/g)
R2
0.3663 0.6108 0.7803 1.3259
0.9929 0.9864 0.9655 0.9780
35 40 50 60
0.117 0.124 0.103 0.081
2.314 1.992 1.314 0.586
0.967 0.989 0.971 0.927
k2 (g/mg min)
R2
Temperature (C)
h (mg/g min)
k2 (g/mg min)
R2
0.6049 0.2545 0.1112 0.0646
0.9998 0.9998 0.9999 0.9992
35 40 50 60
1.9331 2.4825 3.6178 7.813
0.09 0.11 0.16 0.34
0.999 0.999 0.999 1.000
k0 (mL/g/L)
a
R2
Temperature (C)
k0 (mL/g/L)
a
R2
6.5109 5.7179 5.5788 4.495
0.0737 0.0799 0.0464 0.086
0.9733 0.9852 0.9517 0.9926
35 40 50 60
5.2837 6.3173 6.885 7.1258
0.1246 0.0763 0.0559 0.0447
0.9826 0.9887 0.9882 0.9957
Pseudo-first-order constants 10 0.1089 20 0.0525 30 0.0173 40 0.0233 h (mg/g min) Pseudo-second-order 10 20 30 40
Bangham constants 10 20 30 40
constants 1.6913 2.3924 1.9534 1.845
kid (mg/g min) Intra-particle diffusion constants 10 0.060 20 0.1058 30 0.074 40 0.1447
R2
Temperature (C)
kid (mg/g min)
R2
0.9464 0.9379 0.9859 0.9558
35 40 50 60
0.0936 0.0616 0.0466 0.0383
0.9414 0.9593 0.9566 0.9922
3.3. Adsorption kinetic studies
70
logðqe qÞ ¼ log qe k 1 t=2:303
ð3Þ
where qe and q are the amounts of dye adsorbed (mg/g) at equilibrium and at time t (min), respectively, and k1 is the rate constant of adsorption (L/min). The values of k1 and qe at different concentrations are presented in Table 2. 3.3.2. The second-order kinetic model The second-order kinetic model (McKay and Ho, 1999) is expressed as t=q ¼ 1=k 2 q2e þ t=qe
ð4Þ
the initial adsorption rate, h (mg/g min), as t ! 0 can be defined s h ¼ k 2 q2e
ð5Þ
where the initial adsorption rate (h), the equilibrium adsorption capacity (qe), and the second-order constants k2 (g/mg min) can be determined experimentally from the slope and intercept of plot t/q versus t (Fig. 3). Calculated correlations are closer to unity for second-order kinetics model; therefore the adsorption kinetics could well be approximated more favorably by second-order kinetic model for methylene blue. The k2 (g/mg min) and h values are calculated from Fig. 3 are listed in Table 2.
Dye conc. 10 mg/L 20 mg/L 30 mg/L 40 mg/L
60
t/q (min.g/mg)
3.3.1. Adsorption dynamics The rate constant of adsorption is determined from the first-order rate expression given by Lagergren and Svenska (1898).
50 40
R2 0.9998 0.9998 0.999 0.9992
30 20 10 0 0
20
40
60
80
100
120
Time (min)
Fig. 3. Second-order kinetics plots for the removal of methylene blue, at different initial dye concentrations: adsorbent dose, 300 mg/50 mL; initial pH, 6.9, temperature 35 C.
3.3.3. Intra-particle diffusion study An empirically found functional relationship, common to the most adsorption processes, is that the uptake varies almost proportionally with t1/2, the Weber–Morris plot, rather than with the contact time t (Weber and Morris, 1963). qt ¼ k id t1=2 þ C
ð6Þ
where kid is the intra-particle diffusion rate constant. According to Eq. (6), a plot of qt versus t1/2 should be a straight line with a slope kid and intercept C when adsorption mechanism follows the intra-particle diffusion process. Values of intercept give an idea about the thickness of boundary layer, i.e., larger the intercept with greater is
18
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21 6 2
Dye conc. R 10 mg/L 0.9464
5
20 mg/L 0.9379 30 mg/L 0.9859 40 mg/L 0.9558 3
1
q (mg/g)
4
2
3.3.5. Adsorption equilibrium study To optimize the design of an adsorption system for the adsorption of methylene blue, it is important to establish the most appropriate correlation for the equilibrium curves. Various isotherm equations have been used to describe the equilibrium nature of adsorption. Some of these equations are Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich (D–R).
1
0
0
2
4
6
8
10
3.3.6. Adsorption isotherms Langmuir isotherm (1918) is represented by the following equation:
12
Time1/2(min)1/2
Fig. 4. Weber and Morris intra-particle diffusion plots for removal of methylene blue, at different initial dye concentrations: adsorbent dose, 300 mg/50 mL; initial pH, 6.9, temperature 35 C.
the boundary layer effect (Kannan and Sundaram, 2001). In Fig. 4, plot of mass of dye adsorbed per unit mass of adsorbent, qt versus t1/2 is presented for methylene blue. The linear plots are attributed to the macro pore diffusion that is the accessible sites of adsorption. This is attributed to the instantaneous utilization of the most readily available adsorbing sites on the adsorbent surface. The values of kid as obtained from the slope of straight lines are listed in Table 2. 3.3.4. Bangham’s equation Kinetic data can further be used to check whether pore diffusion is the only rate-controlling step or not in the adsorption system using Bangham’s equation (Aharoni et al., 1979). log logðC 0 =C0 qt mÞ ¼ logðk 0 m=2:303V Þ þ logðtÞ
ð7Þ
where C0 is the initial concentration of adsorbate in solution (mg/L), V is the volume of solution (mL), m is the weight of adsorbent per liter of solution (g/L), qt (mg/g) is the amount of adsorbate retained at time t, and a (less than 1) and k0 are constants. The double logarithmic plot (Fig. 5) according to above equation yielded perfect linear curves for methylene blue removal by carbon, showing that the diffusion of adsorbate into pores of the adsorbent is not the only rate controlling step (Tutem et al., 1998). -1.6
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
loglog(C0/C0-qtm)
-1.75
where Ce is the concentration of dye solution (mg/L) at equilibrium. The constant Q0 signifies the adsorption capacity (mg/g) and b is related to the energy of adsorption (L/mg). Linear plot of Ce/qe versus Ce shows that adsorption follows Langmuir isotherm (figure not shown). Values of Q0 and b were calculated from the slope and intercept of the linear plot and are presented in Table 3. The adsorption capacity can be correlated with the variation of surface area and porosity of the adsorbent. Higher surface area and pore volume will result in higher adsorption capacity. Several investigators have been conducted using various low-cost adsorbent. Table 4 compares the adsorption capacity of different types of adsorbents using for methylene blue adsorption. It is seen low cost material from bark, rice husk, cotton waste and coal (McKay et al., 1999) reported the highest adsorption capacity such as 914, 312, 277, and 250 mg/g and activated carbon prepared from rice husk-H3PO4 impregnated (Singh and Srivastava, 2001) observed 333 mg/g revealing that low cost material can be employed as a promising adsorbent for dye adsorption. The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter RL, defined by Weber and Chakkravorti (1974): RL ¼ 1=ð1 þ bC 0 Þ
ð9Þ
where b is the Langmuir constant and C0 is the initial dye concentration (mg/L), RL values indicate the type of isotherm. Table 3 shows RL values between zero and one, which indicate favorable adsorption.
20 mg/L 0.9852 30 mg/L 0.9517 40 mg/L 0.9926
log10 ðx=mÞ ¼ log10 k f þ ð1=nÞlog10 C e
Dye conc. R 2 10 mg/L 0.9733
-1.8 -1.85 -1.9
ð8Þ
3.3.7. Freundlich isotherm Freundlich (1985) was also applied to plots of equilibrium adsorption data:
-1.65 -1.7
C e =qe ¼ l=Q0 b þ C e =Q0
log t
Fig. 5. Bangham’s plot for removal of methylene blue, at different initial dye concentrations: adsorbent dose, 300 mg/50 mL; initial pH, 6.9, temperature 35 C.
ð10Þ
where x is the amount of dye adsorbed (mg), m is the weight of the adsorbent used (g), Ce is the equilibrium concentration of dye in solution (mg/L), kf (mg11/n L1/n g1) and 1/n are Freundlich constants. The correlation coefficients were found to be less than 0.9 (Fig. 6) and values of kf and n were calculated from intercept and slope of plots and are presented in Table 3.
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21
19
Table 3 Isotherm parameters for removal of methylene blue by coir pith carbon Concentration Langmuir constants 10 20 30 40
Q0 (mg/g)
b (L/mg)
5.87
n
R2
0.705 0.731 0.785 0.860
0.8972 0.8675 0.8968 0.8840
E (kJ/mol)
R2
3.54
0.8421
KT (L/mg)
B1
R2
23.25
0.9618
0.952
1.192 0.845 0.726 0.778 qs (mg/g)
Dubnin–Radushkevich constants 10 20 30 40
Tempkin constants 10 20 30 40
0.766 0.621 0.522 0.451
0.93
kf (mg11/n L1/n g1) Freundlich constants 10 20 30 40
RL
4.266
Table 4 Langmuir and Freundlich constants for methylene blue adsorption by various adsorbents reported in literature kf (mg11/n L1/n g1)
n
References
0.017 0.043
D 57.54
D 2.86
McKay et al. (1999) Singh and Srivastava (2001)
323.68 312.26 277.78 204.00 194.3
0.004 0.017 0.009 16.34 0.066
D D D 17.04 6.083
D D D 0.047 5.579
McKay et al. (1999) McKay et al. (1999) McKay et al. (1999) Ghosh and Bhattacharyya (2001) Rozada et al. (2003)
158.23 133.33 122.01 96.2 91.87 56.31 49.0 32.26 27.49 13.44
0.019 0.290 20.49 6.67 · 104 15.55 8.88 0.05 0.38 13.99 7.59
D 52.48 23.05 D 16.39 9.01 D 14.58 14.85 6.57 2.22 21.38
D 4.0 0.098 D 0.070 0.061 D 0.26 0.151 0.075 0.90 1.46
McKay et al. (1999) Singh and Srivastava (1999) Ghosh and Bhattacharyya (2001) Jain et al. (2003) Ghosh and Bhattacharyya (2001) Ghosh and Bhattacharyya (2001) Miguel et al. (2002) De and Basu (1998) Ghosh and Bhattacharyya (2001) Ghosh and Bhattacharyya (2001) Vasanth Kumar and Bhagavanulu (2002) Namasivayam and Yamuna (1994)
Adsorbent
Q0 (mg/g)
Bark Activated carbon (rice husk-H3PO4 impregnated) Coal Rice husk Cotton waste NaOH-treated raw clay Activated carbon (sewage sludge-H2SO4 impregnated) Hair Tree leaves NaOH-treated pure clay Carbon slurry waste Pure clay Calcined pure clay Activated carbon from waste rubber P-700 Saw dust Raw clay Calcined raw clay Boiler bottom ash Biogas residual slurry
914.59 333.3
b (L/mg)
*
*
*
*
* Does not follow Langmuir isotherm/not reported. D Does not follow Freundlich isotherm/not reported.
3.3.8. Tempkin isotherm Tempkin isotherm contains a factor that explicitly takes into account adsorbing species–adsorbate interactions. This isotherm assumes that: (i) the heat of adsorption of all the molecules in the layer decreases linearly with cover-
age due to adsorbate–adsorbate interactions, and (ii) adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy (Tempkin and Pyzhev, 1940). Tempkin isotherm is represented by the following equation:
20
D. Kavitha, C. Namasivayam / Bioresource Technology 98 (2007) 14–21 1.6 1.4 1.2
log (x/m)
1 0.8 0.6
Dye conc.
R2
10 mg/L
0.8972
20 mg/L
0.8675
30 mg/L
0.8968
40 mg/L
0.884
removal of dye follows pore diffusion process since the coefficient values are in the range of 10111013 cm2/s. values of Dp for methylene blue are 0.884 and 0.589 · 1012 cm2/s for 10 and 20 mg/L and 0.295 · 1012 cm2/s for 30 and 40 mg/L have been observed. 4. Conclusions
0.4 0.2 0 --1
-0.5
0
0.5
1
1.5
2
log ce
Fig. 6. Freundlich plots for the adsorption of methylene blue by coir pith carbon.
qe ¼ RT =b lnðK t C e Þ
ð11Þ
Eq. (11) can be expressed in its linear form as qe ¼ B1 ln K t þ B1 ln C e
ð12Þ
where B1 ¼ RT =b
ð13Þ
The adsorption data can be analysed according to Eq. (12). A plot of qe versus ln Ce enables the determination of the isotherm constants Kt and B1. Kt is the equilibrium binding constant (L/mol) corresponding to the maximum binding energy and constant B1 is related to the heat of adsorption. The values of the parameters are given in Table 3. 3.3.9. Dubinin and Radushkevich isotherm Another equation used in the analysis of isotherms was proposed by Dubinin and Radushkevich (1947): 2
qe ¼ qs expðBe Þ
ð14Þ
where qs is D–R constant and e can be correlated: e ¼ RT lnð1 þ 1=C e Þ E ¼ 1=ð2BÞ
1=2
ð15Þ ð16Þ
Calculated Dubinin–Radushkevich constants for the adsorption of methylene blue on coir pith carbon are shown in Table 3; the values of correlation coefficients are much lower than other three isotherms values. In this case, D–R equation represents the poorer fit of experimental data than the other isotherm equation. 3.4. Pore diffusion coefficient Assuming spherical geometry for the adsorbent, the time for half adsorption can be correlated to the pore diffusion coefficient (Michelson et al., 1975). t1=2 ¼ 0:03r20 =Dp
ð17Þ
where t1/2 is the time for half adsorption (s), r0 is the radius of the adsorbent particle (cm) and Dp is the diffusion coefficient (cm2/s). Values of Dp have been calculated for different temperatures and different concentrations of dye. The
The present study shows that the coir pith carbon is an effective adsorbent for the removal of methylene blue from aqueous solution. Higher methylene blue by coir pith carbon is possible provided that the initial adsorbate concentration was low in the solution. Equilibrium adsorption was achieved in about 40 min. Batch kinetic studies performed on the carbon–dye system indicated the adsorption capacity of coir pith carbon. Kinetics data tended to fit well in second-order kinetics, confirming the chemisorption of methylene blue onto coir pith carbon particles. By second-order model to study the mechanism of adsorption, calculated qe values, agreed well with the qe experimental values, supporting the chemisorption. The Langmuir and Tempkin isotherms best represent the equilibrium adsorption data. The effective diffusion coefficient of methylene blue is of the order of 1012 cm2/s. As the raw material, coir pith is discarded as waste in coir industries; the treatment method using coir pith carbon is expected to be economical. The cost and adsorption characteristics favor coir pith carbon to be used as an effective adsorbent for the removal of methylene blue from wastewater. References Aharoni, C., Sideman, S., Hoffer, E., 1979. Adsorption of phosphate ions by colloid ion-coated alumina. J. Chem. Technol. Biotechnol. 29, 404– 412. Allen, S.J., Mckay, G., Khader, K.Y.H., 1998. The adsorption of acid dye onto peat from aqueous solution–solid diffusion model. J. Colloid Interface Sci. 126, 517–524. APHA, 1980. Standard Methods for the Examination of Water and Wastewater, 15th ed. American Public Health Association, Washington, DC. APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC. Aslıhan, G., Savas, S., Sedat, B., Ali, M.M., 2005. Adsorption and kinetic studies of cationic and anionic dyes on pyrophyllite from aqueous solutions. J. Colloid Interface Sci. 286, 53–60. De, D.S., Basu, J.K., 1998. Adsorption of methylene blue on to a low cost adsorbent developed from saw dust. Indian J. Environ. Prot. 19, 416– 421. Dubinin, M.M., Radushkevich, L.V., 1947. Equation of the characteristic curve of activated charcoal. Chem. Zentr. 1, 875. Feryal, A., 2005. Adsorption of basic dyes from aqueous solution onto pumice powder. J. Colloid Interface Sci. 286, 455–458. Freundlich, H., 1985. Uber die adsorption in lunsungen. J. Phys. Chem. 57, 387–470. Ghosh, D., Bhattacharyya, K.G., 2001. Removing colour from aqueous medium by sorption on natural clay: a study with methylene blue. Indian J. Environ. Prot. 21, 903–910. Gupta, V.K., Suhas, A.I., Saini, V.K., 2004. Removal of rhodamine B, fast green, and methylene blue from wastewater using red mud, an aluminum industry waste. Ind. Eng. Chem. Res. 43, 1740–1747.
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