Removal of Safranin Basic Dye from Aqueous Solutions by Adsorption

Sep 21, 2006 - Chennai 600 025, India. Adsorption of a basic dye, safranin, from aqueous solutions onto activated carbon prepared from corncobs. (ACCC...
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Ind. Eng. Chem. Res. 2006, 45, 7627-7632

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Removal of Safranin Basic Dye from Aqueous Solutions by Adsorption onto Corncob Activated Carbon S. Preethi,‡ A. Sivasamy,*,† S. Sivanesan,‡ V. Ramamurthi,‡ and G. Swaminathan† Chemical Engineering DiVision, Central Leather Research Institute, Adyar, Chennai 600 020, India, and Department of Chemical Engineering, A. C. College of Technology, Anna UniVersity, Guindy, Chennai 600 025, India

Adsorption of a basic dye, safranin, from aqueous solutions onto activated carbon prepared from corncobs (ACCC) has been investigated. Various experiments have been carried out using batch adsorption technique to study the effects of the process variables, which include initial pH, adsorbent dosage, initial dye concentration, particle size, temperature, and agitation speed, on the adsorption process. The adsorption of safranin onto the adsorbent was found to improve with the increase in adsorbent dosage and finer mesh size. Maximum adsorption was observed at pH > pHzpc in the pH values ranging from 5 to 9. It was observed that the rate of adsorption improves with increasing temperature and the process is endothermic with an ∆H value of 35.698 kJ/mol. The kinetics followed is first order in nature. The results showed that both Langmuir and Freundlich isotherms fit the equilibrium data. Also, the results revealed that activated carbon from corncob, an agricultural waste biomass, proved to be an excellent low-cost sorbent. 1. Introduction Pigments and dyes are widely used in the textile and leather dyeing, paper, printing, pharmaceutical, and cosmetic industries. About 10 000 different dyes weighing approximately 0.7 million tons are produced annually for various industrial processes.1 A considerable percentage of these dyes go into the effluent during the dyeing process. Many of these have been identified2 as toxic or even carcinogenic. Discharge of these toxic substances into water bodies could pollute water and make it unfit for aquatic life. Further, the dyes could contaminate water and make penetration of sunlight to reach the lower layers impossible, thus affecting the possibility for aquatic plants to perform photosynthesis.3 Polluted water not only damages plants and animals but also is harmful to the environment. A majority of these dyes are stable to light and oxidation. They are immune to aerobic digestion. Biological treatment would be costeffective, but most dyes are resistant to bacterial degradation.3 This treatment may remove BOD, COD, and suspended solids but is ineffective in removing the color of dyes. Many physicochemical methods such as adsorption, coagulation, precipitation, filtration, and oxidation have been attempted for treatment of effluent containing dyes. The potential of various methods for the removal of chemical dyes from effluents have been explored, and the adsorption process has been found to be the most effective. Commercially available activated carbon as an adsorbent has yielded excellent results.4 However, taking into account the high costs involved in the preparation and regeneration process, the feasibility of alternate adsorbents has been studied recently by scientists. These include industrial as well as agricultural byproducts. Some of the industrial waste products are slurry from fertilizer plants and slag from blast furnaces. A variety of agricultural waste biomass has been used for adsorption processes, namely rice straw, coconut husk, rice husk, bagasse, and tree bark.5 In a * To whom correspondence should be addressed. Fax: +91-0442491 1589 or +91-044-2491 2150. E-mail: arumugamsivasamy@ yahoo.co.in. † Central Leather Research Institute. ‡ Anna University.

country like India where agriculture is the primary occupation, agricultural waste biomass such as corncobs are abundantly available. The area under maize cultivation in India is 6.5 million hectares with a production of 11.8 million tons.6 The dissection of five maize plants of the same variety shows that 38% of the biomass is comprised of grains, 16% is cobs and ear husks, and the remaining 46% is leaves, sheaths, and stalks6 (dry weight basis). Therefore, it would be worthwhile to develop a costeffective low-cost adsorbent from waste biomass, which would in turn assist in environmental decontamination processes. Presently, corncobs are used in papermaking, in light packing material, in fuel, in smoking products, and as a source of charcoal.7 The present study is undertaken for the better application and management of such a valuable agricultural biomass for a useful purpose. In this direction, this paper reports the preparation of low-cost, active adsorbent from corncobs, a waste agricultural biomass, and its utilization in the removal of color (safranin as a model dye) from simulated dye bath effluents. 2. Materials and Methods 2.1. Adsorbate. Safranin (Basic Red 2, C.I. 50240), a cationic dye, was supplied by CDH, New Delhi, India. Sodium hydroxide, hydrochloric acid, and sulfuric acid were obtained from E. Merck Limited, Mumbai, India. The structure of safranin is given in Figure 1. 2.2. Preparation of Activated Carbon from Corncob Biomass. Corncobs, which had been used as the sorbent in this study, were obtained from a local agricultural field (after the grains were removed). The waste cobs alone were cut into small pieces and soaked in concentrated sulfuric acid (98%), 1:2 (w/ v), for 48 h. These were then separated, washed several times with distilled water to eliminate the excess acid, and soaked in distilled water for 24 h until the wash water attained neutral pH. The water was then drained off, and the material was dried in a hot air oven at a temperature of 110 °C until the material became dry. The dried material was then ground using a mortar and pestle. The activated corncob thus obtained was then sieved into three different sizes, viz., 106, 250, and 500 µm. These

10.1021/ie0604122 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2006

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Figure 1. Chemical structure of safranin.

samples were stored in separate airtight containers free from moisture until further use. 2.3. Characterization of the Adsorbent. Surface chemistry of the corncob activated carbon such as specific surface area, pore volume distribution, and pore diameter were measured using the nitrogen gas adsorption technique using an ASAP 2010 micropore analyzer with liquid nitrogen at 77 K. The surface area was calculated using the BET method.8,9 Pore volume was determined by the BJH method.10,11 The acidity and basicity of the adsorbent were determined by the Boehm titration method12-16 as reported elsewhere. The zero point charge of the activated carbon prepared from corncobs (ACCC) was determined by the solid addition technique.17 2.4. Analysis of Dye. The concentrations of safranin dye before and after the adsorption processes were monitored using a UV-visible spectrophotometer (Shimazdu 2101 PC) at λmax ) 516 nm. 2.5. Batch Adsorption Experiments. A stock solution of 5000 ppm was prepared by dissolving the required amount of safranin in distilled water, and further dilutions were made to get particular concentrations of dye in the batch experiments. Adsorption experiments were conducted in a batch reactor system. An orbital shaker, Orbitek, Scigenics (Biotech, Pvt Ltd., Chennai, India), was used for the experiments at 30 °C. Experiments were conducted to study the adsorbent capacity of ACCC by varying the pH of the dye solution, the adsorbent dosage, and the adsorbate concentration. The above experiments were carried out for 106, 250, and 500 µm particle sizes. Solutions at different pHs were prepared, ranging from pH 1 to 12, by adjusting with 0.1 N HCl and NaOH solutions. A known amount of the adsorbent of the appropriate particle size was added to the batch reactor system and kept in the shaker for 24 h to attain equilibrium. The adsorbent dosage varied from 0.001 to 1 g. After a period of 24 h, the aqueous phase was analyzed for the residual concentration of dye using the UVvisible spectrophotometer. The absorbance measured was then converted to concentration, and the percentage removal was assessed using the equation

(Ci - Co)/Ci × 100

(1)

where Ci ) initial dye concentration and Co ) final dye concentration after adsorption. The optimum adsorbent dosage in each mesh size was then standardized. To understand the maximum uptake of the adsorbent and its behavior at very high dye concentrations, solutions of various concentrations ranging from 50 to 5000 ppm were prepared in separate 100 mL conical flasks. The standardized adsorbent dosage was transferred to each of the conical flasks. These flasks were placed in the orbital shaker for 24 h, and the residual concentration of dye was measured after centrifuging. Duplicate experiments were performed in parallel to get concordant results. The results showed that the error was within 5%.

Figure 2. Effect of pH on adsorption of safranin with different mesh sizes of activated carbon. Dye concentration ) 10 ppm; adsorbent dosage ) 50 g L-1; temperature ) 30 °C. Table 1. Physicochemical Properties of Corncob Activated Carbon parameter surface area (m2 g-1) bulk density (g mL-1) pore volume (cm3 g-1) pore size distribution (nm) zero point charge (pHpzc) total surface acidity (mmol g-1) total surface basicity (mmol g-1)

15.45 0.606 0.0465 200-1200 2.5 0.735 0.1273

2.6. Kinetics of Adsorption. The kinetics of adsorption studies were followed using 106 µm particle size adsorbent with 250 mL of safranin dye solution, and the concentration was varied from 50 to 300 ppm. The samples were withdrawn at regular intervals and the residual concentration of dye in the aqueous phase was analyzed after centrifuging. 3. Results and Discussion 3.1. Characterization of the Adsorbent. Physicochemical properties of the ACCC, such as BET surface area, bulk density, pore volume, pore size distribution, zero point charge, total surface acidity, and total surface basicity, are shown in Table 1. The majority of functional groups present on the surface of the ACCC are acidic in nature, which includes carboxylic, lactonic, and phenolic functional groups. 3.2. Effect of pH. Adsorption of dye onto ACCC is influenced by the pH of the aqueous phase and zero point charge of the adsorbent (∆pHzpc). The effect of pH on the adsorption of dye by ACCC of three different particle sizes was studied by varying the pH of the dye solution from 1.0 to 12.0 for an initial concentration of 10 ppm (Figure 2). The zero charge of ACCC (∆pHzpc) has been found to occur at pH 2.5, as shown in Figure 3. The adsorption of dye was higher at aqueous phase pH > pHzpc; this could be due to more negative charges at the ACCC surface and in the aqueous phase pH < pHzpc did not influence much the adsorption of dye onto ACCC. Therefore, it could be noted that the zero point charge of the adsorbent played an important role in the specific adsorption of dye when the aqueous phase pH > pHzpc.18 It was evident that the adsorbent showed better adsorption capacity in the pH range from 5 to 9 (Figure 2). This may be due to the higher adsorption of the dye molecules by the negatively charged surface of the adsorbent. Similar adsorption results were observed on the

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Figure 3. Determination of zero point charge of ACCC by solid addition method. Figure 5. Effect of initial dye concentration on adsorption onto corncob activated carbon. Adsorbent dosage ) 0.2 g/10 mL; temperature ) 30 °C.

Figure 4. Effect of adsorbent dosage on adsorption of safranin. Dye concentration ) 10 ppm; temperature ) 30 °C.

removal of methylene blue by activated carbon prepared from wood waste.19 The percentage of adsorption decreases with increasing pH from 9.0 to 12.0 of the aqueous phase.20 Equilibrium pH of the aqueous phase was measured after the adsorption process; it is observed that it ranges from pH 3.8 to 6.4. For example, the initial pH of the aqueous phase was 6.0; after the adsorption by ACCC the equilibrium pH was found to be 5.6. This is 0.4 unit less than the initial pH and clearly shows that not much change occurred in the treated wastewater. Moreover, under this experimental condition, it is observed that the dye adsorbed onto ACCC is not degraded. Since dye molecules after adsorption do not undergo structural changes, these can be recovered and reused by suitable desorption techniques24 as reported elsewhere. 3.3. Variation in Adsorbent Dosage. As the adsorbent dosage increases, the adsorbent sites available for the dye molecules also increase and consequently better adsorption takes place. In the present study, the adsorbent dosages were varied from 1 to 100 g L-1 in 10 ppm dye solution. As shown in Figure 4, the maximum adsorption was observed for all the three mesh sizes at 40 g L-1 in 10 ppm dye solution. However, it was observed that the efficiency did not increase linearly with the increase in the adsorbent dosage. As reported,21 with the increase in adsorbent dosage the percentage adsorption also increases up to 99.8% at 40 g L-1 for the particle sizes of 250 and 106 µm for 10 ppm dye solution. After that, even though the adsorbent dosage increases in the adsorption system, because of the unavailability of the adsorbate, the percentage adsorption remains constant.

3.4. Variation in Initial Dye Concentration. Adsorption studies have been conducted with different initial concentrations of dye solutions using three different particle sizes of the absorbent, viz., 500, 250, and 106 µm. The percentage of adsorption decreases with increasing initial concentrations of dye for all three particle sizes as shown in Figure 5. Moreover, the adsorbent had the capacity to remove up to 90% color even for a concentrated solution of 3000 ppm using an adsorbent size of 106 µm. Almost 50% color removal was observed for 3000 ppm with adsorbent particle sizes of 250 and 500 µm. 3.5. Effect of Particle Size. The finer the particle size, the larger the surface area is, which would in turn provide better contact between the adsorbate and adsorbent. Hence the adsorption capacity would be better. Smaller particles take a shorter time to equilibrate.22-24 Particles of 106 µm size when compared to 150 and 200 µm sizes showed better adsorption characteristics (Figures 2, 4, and 5). 3.6. Adsorption Isotherms. Adsorption isotherm data have been described by the Langmiur adsorption isotherm.25 The equilibrium concentration of dye in the aqueous phase is denoted by c, and the amount of dye adsorbed per unit weight of the adsorbent is denoted by x. The values of Langmuir adsorption parameters are presented in Table 2. If we consider the linear form of the Langmuir adsorption equation

(x/m)-1 or (m/x) ) (K/Xm)(1/c) + 1/Xm

(2)

the plot of m/x versus 1/c would be linear and the reciprocal of the intercept yields a (a ) 1/Xm), the Langmuir constant. The slope corresponds to K/Xm, from which the Langmuir constant K can be evaluated. Xm represents the adsorption capacity of the adsorbent and is the maximum surface coverage representing the monomolecular layer on the surface of the adsorbent. K is related to the intensity of adsorption coefficient b (K ) 1/b). The plot of the Langmuir adsorption isotherm is shown in Figure 6. The performance of the ACCC in dye removal from the aqueous solution has also been studied using the Freundlich isotherm. It is employed to describe heterogeneous systems; the Freundlich model is given by

X ) kfc1/n

(3)

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Table 2. Freundlich and Langmuir Isotherm Parameters for Safranin Adsorption onto Corncob Activated Carbona Freundlich isotherm kf

(mg/g)/(dm3/g)n 5.06

a

Langmuir isotherm

n

1/n

Xm (mg/g)

Kl (L/g)

R2

1.01

0.9900

1428.57

3.2857

0.999 75

Neutral pH; adsorbent dosage ) 0.2 g/10 mL; temperature ) 30 °C.

Figure 6. Langmuir isotherm for safranin onto corncob activated carbon. Adsorbent dosage ) 0.2 g/10 mL; temperature ) 30 °C. Figure 8. Lagergren plots for the adsorption of safranin with different initial dye concnetrations at 30 °C. Adsorbent dosage ) 5 g; volume ) 250 mL; temperature ) 30 °C. Table 3. Adsorption Rate Constants for Safranin-Corncob Activated Carbon System safranin concn (mg L-1) adsorption rate const × 10-2 (min-1) 50 100 300

5.23 3.69 1.87

R2 0.995 42 0.980 48 0.991 17

3.7. Kinetics of Adsorption. The kinetic study of adsorption processes provides useful data regarding the efficiency of the adsorption and the feasibility for scale-up operations. The kinetic data of adsorption can be evaluated using different types of mathematical models,26,27 of which the one most widely used is Lagergren’s rate equation. The kinetics of the adsorption process was analyzed using the first-order rate equation23 given by Figure 7. Freundlich adsorption isotherm of safranin onto corncob activated carbon. Adsorbent dosage ) 0.2 g/10 mL; temperature ) 30 °C.

Dqt/dt ) k1(qe - qt)

For fitting the experimental data, the Freundlich model is linearized as follows:

where qe and qt are the concentrations of the dye (mg g-1) at equilibrium and at time t, respectively, after the adsorption processes. k1 is the adsorption rate constant for dye adsorption. On integration with limits from t ) 0 to t and q1 ) 0 to q1, we have

log x/m ) log kf + (1/n) log c

(4)

The plot of the Freundlich adsorption isotherm (Figure 7) is based on the assumption that the sites on the surface of the adsorbent have different binding energies. In the above equation, x is the amount of adsorbate, m is the weight of adsorbent, c is the equilibrium concentration of the adsorbate in solution, and kf [(mg/g)(dm3/mg)n] and n are empirical constants indicative of adsorption capacity and intensity, respectively. Since this is an equation of a straight line, the linearity of the plot of log x/m versus log c indicates the applicability of the Freundlich isotherm with 1/n as the slope. The values of log kf and 1/n correspond to the approximate measures of the adsorption capacity, and the intensity has been calculated by a least-squares method. The Freundlich adsorption constants are shown in Table 2.

log(qe - qt) ) log qe - ((k1/2.303)t)

(5)

(6)

Plots of log(qe - qt) versus time are shown in Figure 8. This is found to be linear with intercepts on the ordinate and negative slope equal to -k/2.303. The adsorption rate constants thus evaluated making use of the Lagergren equation are given in Table 3. The rate constants decrease with increase in concentrations of the adsorbate. 3.8. Effect of Agitation. Increase in the speed of agitation enhances the rate of adsorption. In the present study, the adsorption process of safranin onto ACCC is studied for varying speeds of agitation from 50 to 200 rpm. The results are presented in Figure 9. By increasing the speed of agitation, the randomness

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Figure 11. Plot of ln k vs 1/T for safranin adsorption by ACCC. Figure 9. Effect of agitation speed on adsorption of safranin by corncob activated carbon. Dye concentration ) 300 ppm; volume ) 250 mL; adsorbent dosage ) 5 g; temperature ) 30 °C.

Table 4. Thermodynamic Parameters for Safranin Adsorption onto Corncob Activated Carbon temp (°C)

-∆G° (kJ mol-1)

∆H° (kJ mol-1)

∆S° (kJ mol-1 K-1)

20 40

22.97 25.41

35.698

0.043 0.0328

spontaneous nature of the adsorbent on the dye. The positive values of ∆H° confirm the endothermic nature of adsorption.5

Figure 10. Effect of temperature on adsorption of safranin. Dye concentration ) 300 ppm; adsorbent dosage ) 5 g; volume ) 250 mL.

increases during the adsorption process, resulting in better contact between the adsorbate and adsorbent in the system and hence enhancing the rate of adsorption.27 It was observed that equilibrium was attained faster at 200 rpm than at 50 rpm. 3.9. Effect of Temperature on Adsorption. The degree of adsorption depends on the temperature of the solid-liquid interface. The rates of adsorption were studied in the temperature range of 18-45 °C by withdrawing samples at specific time intervals. Interestingly, it was observed that, at higher temperatures, the rate of adsorption was faster. The process was hence concluded to be endothermic and spontaneous. The plot of the effect of temperature on the adsorption process is shown in Figure 10. 3.10. Thermodynamics of Adsorption. The kinetics of the rate of adsorption was assessed at 20 and 40 °C. The value of ∆H determines whether a process is endothermic or exothermic. Here, the value of ∆H is found to be positive and hence the reaction involved is endothermic. Also, the negative value of ∆G indicates the spontaneous nature of adsorption of safranin onto ACCC. Thermodynamic parameters ∆G, ∆S, and ∆H were calculated using the following equations, and the values are shown in Table 4. The negative values of ∆G° indicate the

∆G° ) -RT ln K

(7)

∆H° ) -R((T2T1)/(T2 - T1)) ln(K2/K1)

(8)

∆S° ) (∆H - ∆G)/T

(9)

where K1 and K2 are the Langmuir constants corresponding to the temperatures 20 and 40 °C. 3.11. Determination of Activation Energy for the Adsorption Process. The rate of a reaction depends on the temperature at which it is carried out. As the temperature increases, the randomness increases and hence the molecules tend to collide more frequently. The molecules also carry more kinetic energy. The relationship between the temperature and the rate constant for a reaction obeys the following equation.28

k ) Ze-Ea/RT

(10)

where k is the rate constant for the reaction; Z is the proportionality constant, which depends on the type of reaction. Ea is the activation energy for the reaction, R is the universal gas constant in J/(mol K), and T is the temperature in kelvin. The above equation has been used to determine the activation energy for the adsorption process. Applying the natural logarithm on both sides, we get

ln k )ln Z - Ea/RT

(11)

ln k ) (Ea/R)(1/T) + ln Z

(12)

or

A plot of ln k vs 1/T should give a straight line with a slope of -Ea/R as shown in Figure 11. The value of Ea hence calculated was found to be 114.62 kJ/mol, which shows that ACCC has strong binding sites. 4. Conclusions The present study revealed that activated carbon from corncobs can be used as a good low-cost adsorbent for the

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removal of color from wastewaters. The adsorbent worked best in pH > pHzpc and in the pH range from 5 to 9. As the adsorbent dosage increased, better color removal was observed. The adsorbent proved to be effective in removing color of very high initial concentrations. The efficiency of the dye adsorption decreased with the increase in particle size from 106, to 250, to 500 µM of the adsorbent. The experimental data were evaluated with the Langmuir and Freundlich isotherms, and the data fitted well in both model equations. The dye uptake capacity of the adsorbent was found to be 1428.57 mg/g. The kinetics of adsorption followed first-order kinetics. The rate of adsorption decreased with the increase in the initial dye concentration. The adsorption rate was found to increase with the increase in temperature and faster agitation of 200 rpm. The thermodynamic studies showed that the process was endothermic and spontaneous in nature. The value of ∆H was 35.698 kJ/mol, and Ea was calculated to be 114.62 kJ/mol. Literature Cited (1) Zollinger, H. Color ChemistrysSyntheses, Properties, and Applications of Organic Dyes and Pigments; VCH Publishers: NewYork, 1987. (2) Gong, R.; Sun, Y.; Chen, J.; Liu, H.; Yang, C. Effect of chemical modification on dye adsorption capacity of peanut hull. Dyes Pigm. 2005, 67, 175. (3) Reife, A.; Fermann, H. S. EnVironmental Chemistry of Dyes and Pigments; Wiley: New York, 1996. (4) Walker, G. M.; Weatherly, L. R. Fixed bed adsorption of acid dyes onto activated carbon. EnViron. Pollut. 1998, 99, 133. (5) Gupta, V. K.; Ali, I.; Mohan, D. Equilibrium uptake and sorption dynamics for the removal of a basic dye (basic red) using low cost adsorbents. J. Colloid Interface Sci. 2003, 265, 257. (6) http://agricoop.nic.in/statatglance2004/atglance. (7) Solpan, D.; Lge, Z.; Torun, M. Adsorption Properties of Poly(Nvinyl pyrrolidone co methacrylic acid) hydogels. J. Macromol. Sci., Part A: Pure Appl. Chem. 2006, 43, 129. (8) Brunauer, S.; Emmett, H. P.; Teller, E. Adsorption of gas in multimolecular layers. J. Am. Chem. Soc. 1938, 60, 309. (9) Barrett, E. P.; Joyner, L. S.; Halenda, P. P. The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherm. J. Am. Chem. Soc. 1951, 73, 373. (10) Lippens, B. C.; de Boer, J. H. Studies on pore system in catalysts V. the t method. J. Catal. 1965, 4, 319. (11) Harkins, W. D.; Jura, G. Surfaces of solids XIII: A vapor adsorption method for the determination of the area of a solid without the assumption of a molecular area and the areas occupied by nitrogen and other molecules on the surface of a solid. J. Chem. Phys. 1944, 66, 1366.

(12) Boehm, H. P. Chemical identification of surface groups. In AdVances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press: New York, 1966; Vol. 16, p 179. (13) Fabish, T.; Schleifer, D. E. Surface chemistry and carbon black work function. Carbon 1984, 22, 19. (14) Puri, B. R.; Bansal. R. C. Studies in surface chemistry of carbon blackssII: Surface acidity in relation to chemisorbed oxygen. Carbon 1964, 1, 457. (15) Barton, S. S.; Gillespie, D.; Harrison, B. H. Surface studies of carbon: acidic oxides on Speron 6. Carbon 1973, 11, 649. (16) Arico, A. S.; Antonucci, V.; Minutoli, M.; Giordano, N. The influence of functional groups on the surface acid-base characteristics of carbon blacks. Carbon 1989, 27, 337. (17) Balistrieri, L. S.; Murray, J. W. The surface chemistry of goethite (-FeOOH) in major ion sea water. Am. J. Sci. 1981, 281 (6), 788. (18) Al Degs, Y. S.; Kharaisheh, M. A. M.; Allen, S. T.; Ahmed, M. N. Adsorption of Ramazol reactive Black B on different types activated carbons: Adsorption on H and L carbon. AdV. EnViron. Res. 1999, 3, 132. (19) Garg, V. K.; Moirangthem, A.; Kumar, R.; Gupta, R. Basic dye (Methylene Blue) removal from simulated wastewater by adsorption using Indian Rosewood sawdust: a timber industry waste. Dyes Pigm. 2004, 63, 243. (20) Nigam, P.; Armour, G.; Banat, I. M.; Marchant, R.; Physical removal of textile dyes from effluents and soild-state fermentation of dyeadsorbed agricultural residues. Bioresour. Technol. 2000, 72, 219. (21) Demirbas, E.; Kobya, M.; Sentrurk, E.; Ozkan, T. Adsorption kinetics for the removal of chromium VI from aqueous solutions on the activated carbons prepared from agricultural wastes. Water SA 2004, 30, 4. (22) Reyad, A.; Shawabkeh; Maha; Tutunji, F. Experimental study and modeling of basic dye sorption by diatomaceous clay. Appl. Clay Sci. 2003, 24, 111. (23) Vasanth Kumar, K.; Ramamurthy, V.; Sivanesan, S. Modelling the mechanism involved during the sorption of methylene blue onto flyash. J. Colloid Interface Sci. 2005, 284, 14. (24) Namasivayam, C.; Dinesh Kumar, M.; Selvi, K.; Ashruffunissa Begum, R.; Vanathi, T.; Yamuna, R. T. Waste Coir Pithsa potential biomass for the treatment of dyeing waters. Biomass Bioenergy 2001, 21, 477. (25) Langmuir, I. The constitution and fundamental properties of solids and liquids. Part I. solids. J. Am. Chem. Soc. 1916, 38, 2221. (26) Lagergren, S. SVen. Vetenskapsakad. Handl. 1898, 24, 1. (27) Panday, K. K.; Prasad, G. V.; Singh, N. Copper (II) removal from aqueous solutions by fly ash. Water Res. 1985, 19, 869. (28) Namasivayam, C.; Ranganathan, K. Waste Fe(III)/Cr(III) hydroxide as adsorbent for the removal of Cr(VI) from aqueous solution and chromium plating industry wastewater. EnViron. Pollut. 1993, 82, 255.

ReceiVed for reView April 1, 2006 ReVised manuscript receiVed August 18, 2006 Accepted August 24, 2006 IE0604122