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Adsorption of Dyes onto Carbonaceous Materials Produced from Coffee Grounds by Microwave Treatment
Journal of Colloid and Interface Science 254, 17–22 (2002) doi:10.1006/jcis.2002.8570
Adsorption of Dyes onto Carbonaceous Materials Produced from Coffee Grounds by Microwave Treatment Mizuho Hirata,∗ Naohito Kawasaki,∗ Takeo Nakamura,∗ Kazuoki Matsumoto,† Mineaki Kabayama,‡ Takamichi Tamura,§ and Seiki Tanada∗,1 ∗ School of Pharmaceutical Sciences, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan; †Faculty of Humanities, Seitoku University, 550, Iwase, Matsudoshi, Chiba 271-8555, Japan; ‡Tomita Pharmaceutical Co., Ltd., 85-1, Maruyama, Seto-cho, Naruto, Tokushima 771-0360, Japan; and §School of Medicine, The University of Tokushima, 3, Kuramoto-cho, Tokushima 700, Japan Received November 19, 2001; accepted March 23, 2002
(4), and a technique for producing rubber from carbonaceous materials (5). To promote “zero emission” of organic wastes, it is necessary to build a carbon circulation process. By changing the difficult recycling of an organic by-product into carbonaceous materials without disposing of it as in the past, it is possible to reduce the quantity of carbon dioxide discharged. Activated carbons are widely used in the industry for separation, purification, and recovery processes. The demand for environmental protection has increased every year. Any inexpensive materials with a high carbon content can be used as a raw material for the production of activated carbon. The purpose of this study was to investigate the feasibility of manufacturing carbonaceous adsorbents from coffee grounds as an agricultural by-product and to apply this product for the removal of dyes. Coffee is produced all over the world. In the year 2000, the production of coffee amounted to 6,432,000 tons. A portion of the coffee grounds were recycled into soil remediation materials or adsorbents for odor, but most of the coffee grounds were disposed of by burning. The amount of carbon dioxide produced by burning 1000 g of coffee grounds is 538 g. When the coffee grounds are recycled into soil remediation materials or adsorbents, they have to be carbonized. Effluents from the textile industries are important sources of water pollution, because dyes in wastewater undergo chemical as well as biological changes, consume dissolved oxygen, and destroy aquatic life (6, 7). Therefore, it is necessary to treat textile effluents prior to their discharge into the receiving water. Several researchers have studied the removal of dyes from wastewater using various adsorbents (8–14). In this paper, the adsorption mechanisms of dyes onto the carbonaceous materials, which were produced from coffee grounds by microwave treatment, were investigated based on the results of the amount of dyes adsorbed and the properties of the carbonaceous materials.
Organic wastes have been burned for reclamation. However, they have to be recycled and reused for industrial sustainable development. Carbonaceous materials were produced from coffee grounds by microwave treatment. There are many phenolic hydroxyl and carboxyl groups on the surface of carbonaceous materials. The base consumption of the carbonaceous materials was larger than that of the commercially activated carbon. The carbonaceous materials produced from coffee grounds were applied to the adsorbates for the removal of basic dyes (methylene blue and gentian violet) in wastewater. This result indicated that the adsorption of dyes depended upon the surface polar groups on the carbonaceous materials. Moreover, the Freundlich constants of isotherms for the adsorption of methylene blue and gentian violet onto the carbonaceous materials produced from coffee grounds were greater than those for adsorption onto activated carbon or ceramic activated carbon. The interaction was greatest between the surface or porosity of the carbonaceous materials and methylene blue and gentian violet. The microwave treatment would be useful for the carbonization of organic wastes to save energy. C 2002 Elsevier Science (USA) Key Words: dye; adsorption; microwave treatment; coffee grounds; carbonaceous materials.
INTRODUCTION
Zero emission is desired to reduce the load on the earth close to zero, because it makes use of waste resources, suppressing the consumption of resources (1). The building of a recycling society can be realized by changing new industries, the economy, and lifestyles. There are many techniques for decreasing the by-products of individual production processes. New collection techniques for these by-products and techniques for giving additional value and reusing the by-products are needed (2). Various kinds of technology for sustainable development have been developed. For example, there is a technique for the efficient collection of foaming polystyrene or replay material, a technique for reusing the dye from used inkjet ribbons (3), a technique for decomposing a polymer into a monomer by supercritical water
MATERIALS AND METHODS
Materials The dyes used were orange II, methylene blue, and gentian violet. Their structures are shown in Fig. 1. Methylene
1 To whom correspondence should be addressed. Fax: +81-6-6730-1394. E-mail: [email protected].
17
0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
18
HIRATA ET AL.
FIG. 1.
Structure of the dyes.
blue and gentian violet are basic dyes, and orange II is an acidic dye. The activated carbon (AC) produced from nutshells, Shirasagi G, was commercially obtained from the Takeda Chemical Ind. Co., Ltd.. The ceramic activated carbon (CC) produced from organic waste, CARBO-TEC, was obtained from Bron Electric Co., Ltd.
methylene blue (665 nm), and gentian violet (582 nm). The equilibrium amount adsorbed was calculated using the equation X = (C0 − C)V /M, where X is the equilibrium amount adsorbed per gram of carbonaceous materials (mg/g), C0 and C are the initial and equilibrium concentration (mg/L), respectively, V is the volume of the solution (L), and M is the weight of the carbonaceous materials (g).
Production of Carbonaceous Materials from Coffee Grounds Carbonization by muffle furnace. Raw coffee grounds (CGRs) were dried at 383 K for 24 h. CGRs were inserted into a metal container which was covered and inserted into an electric muffle furnace heated to 1073 K at a rate of 10 K/min in a nitrogen gaseous phase. The materials were maintained at this temperature for 1 h and then cooled to obtain the carbonaceous material (CGM). Carbonization by microwave. The wetted coffee grounds were carbonized in air by microwave treatment at a frequency of 2450 MHz and an output of 500 W; the treatment period was 7 min (CG7), 9 min (CG9), or 12 min (CG12). The carbonization temperature of CG increased to about 673 K. Amount of Dyes Adsorbed onto the Carbonaceous Materials The amount of dyes adsorbed onto the carbonaceous materials was calculated from the initial concentration of dyes and the equilibrium concentration. The equilibrium concentration was calculated using the absorbance of orange II (485 nm),
Physical and Chemical Properties of the Carbonaceous Materials The specific surface area of the carbonaceous material was measured using a FlowSorb II 2300 (Micromeritics Co., Ltd.). The base and acid consumption of the carbonaceous materials was measured using the acid–base titration method (15, 16). The carbonaceous materials were added to a 100-ml portion of hydrochloric acid (0.1 mol/L), and the suspension was shaken for 15 h. Base consumption was determined by titrating the aqueous sodium hydroxide (0.1 mol/L) into the filtrate. The carbonaceous materials were added to a 10-ml portion of sodium hydroxide (0.1 mol/L), and the suspension was shaken for 15 h. Acid consumption was determined by titrating the hydrochloric acid (0.1 mol/L) into the filtrate. These results are shown in Table 1. The yield of carbonaceous materials produced from coffee grounds decreased with increasing microwave treatment period because of the increasing carbonization, that is, a part of carbon in coffee grounds became carbon dioxide. Generally speaking, the porosity of the carbonaceous materials increases
ADSORPTION OF DYES
19
TABLE 1 Elemental Analysis of Carbon, Hydrogen, and Nitrogen Contents for Carbonaceous Materials Samples
Carbon (%)
Hydrogen (%)
Nitrogen (%)
CGR CGM CG9
53.8 87.5 60.1
6.6 1.9 6.6
2.3 3.1 2.6
with an increase specific surface area. The ACs has many pores, while CGs hardly have any. RESULTS AND DISCUSSION
Production of Carbonaceous Materials from Coffee Grounds The carbon, hydrogen, and nitrogen contents of CGR, CGM, and CG9 were measured by elemental analysis, as shown in Table 2. The amount of carbon contained in CG9 was greater than that contained in CGR, because of the carbonization of the coffee grounds. There was a greater amount of carbon contained in the carbonaceous material produced in the electric muffle furnace, because the carbonization temperature in the electric muffle furnace is higher than that in the microwave. The amount of electricity used to manufacture the carbonaceous materials by carbonization by the electric muffle furnace and the microwave apparatus was 3750 and 75 Wh, respectively. The Ministry of Environment reported in 1999 that the amount of carbon dioxide produced per kilowatt-hour was 0.550 kg. The amount of carbon dioxide produced was 1.44 and 0.03 kg, respectively. The results of the elemental analysis were used for an evaluation of the discharge quantity of carbon dioxide produced by the carbonization of the coffee grounds. The discharge quantity of carbon dioxide produced by burning the organic by-products is about 13 million tons. The annual coffee bean consumption in the world was about 6,432,000 tons in the year 2000. We calculated the amount of carbon dioxide produced based on results of elemental analysis. If all of the coffee grounds are completely burned, 3,460,000 tons of carbon dioxide are discharged. However, on the assumption that only carbon dioxide is produced, when the coffee grounds are carbonized using the electric muffle furnace and the microwave apparatus, the amount of produced carbon dioxide can TABLE 2 Physical and Chemical Properties of Carbonaceous Materials Consumption (mEq/g) Samples
Specific surface area (m2 /g)
Yield (%)
Acid
Base
CG7 CG9 CG12 AC CC
1< 1< 1< 1119 133
32.8 29.0 25.3 — —
0.082 0.037 0.040 0.040 0.072
3.16 3.07 3.10 1.44 1.21
FIG. 2. Removal percentages of dyes with carbonization time. , Methylene blue; , orange II; , gentian violet.
decrease to about 1,370,000 and 1,112,000 tons, respectively. Therefore, the carbonization of coffee grounds is an excellent way to process organic wastes and contribute to decreased global warming. Percentage of Dyes Removed by the Carbonization Materials The percentage of dyes removed during the increasing microwave treatment period is shown in Fig. 2. Methylene blue and gentian violet were completely removed in all samples, while orange II required a longer microwave treatment period to be removed. This result indicated that methylene blue and gentian violet, which are basic dyes, are easily removed because the surface polarity of CG9 is a large base consumption. Moreover, the functional groups in the surface of CG9 can relate the CG9/orange II interaction to the adsorption capacity because the specific surface area of CG9 is less than 1 m2 /g. The removal percentage of orange II by CG is lower than that of methylene blue and gentian violet, because orange II is an acidic dye. Adsorption of Dyes onto the Carbonaceous Materials The isotherms of methylene blue adsorption onto AC, CG9, and CC at 293 K are shown in Fig. 3a. The amount of methylene blue adsorbed increased in the order CC, CG9, and AC. The adsorption isotherms of the dyes were approximated by the Freundlich equation, log X = log k + (1/n) log C, where X is the amount of dyes adsorbed, C is the equilibrium concentration, and k and 1/n are the adsorption constants. The reciprocal of the slope (1/n) of the Freundlich plot measured the affinity between the surface of the carbonaceous materials and dyes. The Freundlich plots of the isotherms for adsorption of methylene blue onto the carbonaceous materials were linear (Fig. 3b). However, the Freundlich plots for adsorption of methylene blue onto AC can be shown with two lines. Boki et al. reported that when the adsorbates at low concentration are adsorbed into the pores of adsorbents, which have micropores, the Freundlich plots become two lines (17). This result indicates that in the case of the
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HIRATA ET AL.
the structure of gentian violet is like that of a propeller. The Freundlich plots of the isotherms of gentian violet adsorption onto the carbonaceous materials are shown in Fig. 4b. These plots became linear. The mechanism of gentian violet adsorption may be monolayer adsorption onto the heterogeneous surface of the adsorbents. The isotherms of orange II adsorption onto AC, CG9, and CC are shown in Fig. 5a. The amount of orange II adsorbed onto AC was the greatest, and that onto CG9 was similar to that onto CC. The mechanism of orange II adsorption onto the activated carbon was reported in the electrostatic interaction between the dye molecules and carbon surface (18). The orange II molecules can hardly ever be adsorbed onto the surface of CG9, because orange II is an acidic dye. Moreover, the specific surface area of CG9 is very small. The amount of orange II adsorbed onto CC is very small, because the specific surface area of CC is small. These results indicated that the adsorption site of orange II would be the pores of the carbonaceous materials, which have an acidic surface. Generally speaking, the specific surface area of the activated carbon is corrected to the pore volume and the micropore volume is greater. Orange II would only be adsorbed
FIG. 3. (a) Isotherms of methylene blue adsorption onto carbonaceous materials. , AC; , CG9; , CC. (b) Freundlich plots of isotherms for methylene blue adsorption onto carbonaceous materials. , AC; , CG9; , CC.
adsorption of methylene blue onto AC, methylene blue is adsorbed into the pores, but in the case of the adsorption of methylene blue onto CG9, it is adsorbed onto the surface of CG9. The affinity between the surfaces of carbonaceous materials and methylene blue increases in the order of CC, AC, and CG9. The constant 1/n of CG9 is greatest, because the base consumption of CG9 is greatest and methylene blue is a basic dye. The isotherms of gentian violet adsorption onto AC, CG9, and CC are shown in Fig. 4a. The amount of gentian violet adsorbed onto the carbonaceous materials was larger in the order of CG9, AC, and CC. Even if the specific surface area of CG9 is less than 1 m2 /g, gentian violet was adsorbed onto CG9. This result indicates that the gentian violet is not adsorbed in the pores of CG9. The adsorption mechanism of gentian violet on CG9 will be a chemical interaction, because gentian violet is a basic dye, while the adsorption of gentian violet onto AC or CC will be physical adsorption, because the specific surface areas of AC and CC are 1119 and 133 m2 /g, respectively. The amount of gentian violet adsorbed onto CC is greater than that adsorbed onto AC. It is assumed that the pore radius of CC is larger than that of AC. The mechanism of gentian violet adsorption onto the carbonaceous materials depended on the porosity, because
FIG. 4. (a) Isotherms of gentian violet adsorption onto carbonaceous materials. , AC; , CG9; , CC. (b) Freundlich plots of isotherms for gentian violet adsorption onto carbonaceous materials. , AC; , CG9; , CC.
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ADSORPTION OF DYES
FIG. 6. Isotherms of adsorption of dyes onto CG9. , Methylene blue; , orange II; , gentian violet.
FIG. 5. (a) Isotherms of orange II adsorption onto carbonaceous materials. , AC; , CG9; , CC. (b) Freundlich plots of isotherms for orange II adsorption onto carbonaceous materials. , AC; , CG9; , CC.
onto the pores of AC. The Freundlich plots of isotherms for adsorption of orange II onto the carbonaceous materials are shown in Fig. 5b. The Freundlich plots became linear. The adsorption mechanism of orange II may be monolayer adsorption onto the carbonaceous materials, which have a heterogeneous surface. The isotherms of adsorption of dyes onto CG9 are shown in Fig. 6. The amount adsorbed onto CG9 is larger in the order of orange II, gentian violet, and methylene blue. Methylene blue and gentian violet are basic dyes, and orange II is an acidic dye. The mechanism of adsorption of dyes onto the activated carbon was reported in the molecular sieve effect (18). If there are no pores in the adsorbents, the adsorption would depend upon the surface polarity of the adsorbents. The correlation coefficient and the constants 1/n of the Freundlich plot of the adsorption of dyes were estimated by linear regression analysis and are shown in Table 3. The constant 1/n expresses an affinity between the adsorbate and adsorbent. The Freundlich plots of methylene blue onto CG9 curved at 1.5 mg/L of equilibrium concentration. Although AC does not have a greater base consumption, the constant 1/n was greatest at an equilibrium concentration less than 1.5 mg/L. On equilib-
rium concentration of more than 1.5 mg/L, the constant 1/n of AC was similar for CG9. Methylene blue was adsorbed in the AC pores, because the specific surface area was greatest. Adsorption onto an activated carbon whose surface is hydrophobic is based mainly on the London dispersion force, which is part of the van der Waals force causing a decrease in the constant 1/n. The more closely the adsorbate molecules in the pores are located on the surrounding pore walls, the higher the adsorption force will be (19). Adsorption at a very low concentration occurs mainly in the pores with a high adsorption force. The methylene blue and gentian violet were adsorbed onto the surface of CG9, because methylene blue and gentian violet are basic dyes, and the surface of CG9 is acidic. The constant 1/n of methylene blue and gentian violet increased in the order of CC, AC, and CG9. An increase in affinity between methylene blue or gentian violet and the carbonization materials depended upon the increase in acidity on the surface of the carbonization materials. As AC has a large pore volume, the constant 1/n of orange II was greatest onto AC. Therefore, it is thought that the interaction was greatest between the carbonization and the orange II. The interaction between CG9 and methylene blue or gentian violet was greater than the interaction between CG9 and orange II, because the surface of CG9 was the most basic. TABLE 3 Freundlich Constants of Adsorption Isotherms for Dyes Methylene blue
Orange II
Gentian violet
Samples
1/n
r
1/n
r
1/n
r
CG9 AC
3.00 43.0a 1.12b 0.21
0.961 0.930a 0.945b 0.903
0.27 119
0.928 0.896
0.95 0.63
0.953 0.959
0.034
0.912
0.38
0.908
CC
Note. r = correlation coefficient. The equilibrium concentration is 0.57 to 1.17 mg/g. b The equilibrium concentration is 1.17 to 11.6 mg/g.
a
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HIRATA ET AL.
In conclusion, (i) the adsorption of dyes depended upon the base and acid consumption of the carbonaceous materials, (ii) methylene blue and gentian violet are adsorbed in micropores or on the acidity sites of the carbonaceous materials, (iii) orange II is adsorbed in the pores of carbonaceous materials, (iv) the carbonaceous material produced from the coffee grounds has a larger base consumption, and (v) the microwave treatment of coffee grounds can be utilized for the production of carbonaceous materials for saving energy and decreasing carbon dioxide in the atmosphere. REFERENCES 1. 2. 3. 4. 5.
Takebayashi, M., Eng. Jpn. 63(2), 76 (1999). Suzuki, M., Chem. Eng. Jpn. 63(2), 69 (1999). Watanabe, H., Chem. Eng. Jpn. 63(2), 79 (1999). Adschiri, T., J. Chem. Eng. Jpn. 26, 676 (1993). Abe, I., Kawasaki, N., Nakamura, T., Kondo, T., and Tanada, S., Environ. Sci. 13, 235 (2000).
6. Ajmal, M., and Khan, A. U., Environ. Pollut. Ser. A 37, 131 (1985). 7. Gupta, G. S., and Bhattacharya, P. K., J. Chem. Tech. Biotechnol. B 35, 23 (1985). 8. Gupta, G. S., Prasad, G., and Singh, V. N., Res. Ind. 33, 132 (1988). 9. Allen, S. J., Fuel 66, 1175 (1987). 10. Allen, S. J., McKay, G., and Khader, K. Y. H., Environ. Pollut. 52, 39 (1988). 11. Allen, S. J., McKay, G., and Khader, K. Y. H., J. Colloid Interface Sci. 126, 517 (1988). 12. Allen, S. J., Prasad, G., and Singh, V. N., Environ. Technol. Lett. 9, 153 (1988). 13. Gupta, G. S., Prasad, G., and Singh, V. N., J. Environ. Sci. Health A 23, 205 (1988). 14. Nawar, S. S., and Doma, H. S., Sci. Total Environ. 79, 271 (1989). 15. Akiyoshi, M., and Shirasaki, M., Nippon Kagaku Kaishi 5, 1181 (1976). 16. Okazaki, S., and Toyoda, S., Kogyo Kagaku Zasshi 73, 478 (1970). 17. Boki, K., Mori, H., and Kawasaki, N., J. Am. Oil Chem. Soc. 71, 596 (1994). 18. Kasaoka, S., Sakata, Y., Tanaka, E., and Naotoh, R., Nippon Kagaku Kaishi 12, 2266 (1987). 19. Kondo, S., Ishikawa, T., and Abe, I., in “Science of Adsorption,” Maruzen, Tokyo.
Adsorption of Dyes onto Carbonaceous Materials Produced from Coffee Grounds by Microwave Treatment Mizuho Hirata,∗ Naohito Kawasaki,∗ Takeo Nakamura,∗ Kazuoki Matsumoto,† Mineaki Kabayama,‡ Takamichi Tamura,§ and Seiki Tanada∗,1 ∗ School of Pharmaceutical Sciences, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan; †Faculty of Humanities, Seitoku University, 550, Iwase, Matsudoshi, Chiba 271-8555, Japan; ‡Tomita Pharmaceutical Co., Ltd., 85-1, Maruyama, Seto-cho, Naruto, Tokushima 771-0360, Japan; and §School of Medicine, The University of Tokushima, 3, Kuramoto-cho, Tokushima 700, Japan Received November 19, 2001; accepted March 23, 2002
(4), and a technique for producing rubber from carbonaceous materials (5). To promote “zero emission” of organic wastes, it is necessary to build a carbon circulation process. By changing the difficult recycling of an organic by-product into carbonaceous materials without disposing of it as in the past, it is possible to reduce the quantity of carbon dioxide discharged. Activated carbons are widely used in the industry for separation, purification, and recovery processes. The demand for environmental protection has increased every year. Any inexpensive materials with a high carbon content can be used as a raw material for the production of activated carbon. The purpose of this study was to investigate the feasibility of manufacturing carbonaceous adsorbents from coffee grounds as an agricultural by-product and to apply this product for the removal of dyes. Coffee is produced all over the world. In the year 2000, the production of coffee amounted to 6,432,000 tons. A portion of the coffee grounds were recycled into soil remediation materials or adsorbents for odor, but most of the coffee grounds were disposed of by burning. The amount of carbon dioxide produced by burning 1000 g of coffee grounds is 538 g. When the coffee grounds are recycled into soil remediation materials or adsorbents, they have to be carbonized. Effluents from the textile industries are important sources of water pollution, because dyes in wastewater undergo chemical as well as biological changes, consume dissolved oxygen, and destroy aquatic life (6, 7). Therefore, it is necessary to treat textile effluents prior to their discharge into the receiving water. Several researchers have studied the removal of dyes from wastewater using various adsorbents (8–14). In this paper, the adsorption mechanisms of dyes onto the carbonaceous materials, which were produced from coffee grounds by microwave treatment, were investigated based on the results of the amount of dyes adsorbed and the properties of the carbonaceous materials.
Organic wastes have been burned for reclamation. However, they have to be recycled and reused for industrial sustainable development. Carbonaceous materials were produced from coffee grounds by microwave treatment. There are many phenolic hydroxyl and carboxyl groups on the surface of carbonaceous materials. The base consumption of the carbonaceous materials was larger than that of the commercially activated carbon. The carbonaceous materials produced from coffee grounds were applied to the adsorbates for the removal of basic dyes (methylene blue and gentian violet) in wastewater. This result indicated that the adsorption of dyes depended upon the surface polar groups on the carbonaceous materials. Moreover, the Freundlich constants of isotherms for the adsorption of methylene blue and gentian violet onto the carbonaceous materials produced from coffee grounds were greater than those for adsorption onto activated carbon or ceramic activated carbon. The interaction was greatest between the surface or porosity of the carbonaceous materials and methylene blue and gentian violet. The microwave treatment would be useful for the carbonization of organic wastes to save energy. C 2002 Elsevier Science (USA) Key Words: dye; adsorption; microwave treatment; coffee grounds; carbonaceous materials.
INTRODUCTION
Zero emission is desired to reduce the load on the earth close to zero, because it makes use of waste resources, suppressing the consumption of resources (1). The building of a recycling society can be realized by changing new industries, the economy, and lifestyles. There are many techniques for decreasing the by-products of individual production processes. New collection techniques for these by-products and techniques for giving additional value and reusing the by-products are needed (2). Various kinds of technology for sustainable development have been developed. For example, there is a technique for the efficient collection of foaming polystyrene or replay material, a technique for reusing the dye from used inkjet ribbons (3), a technique for decomposing a polymer into a monomer by supercritical water
MATERIALS AND METHODS
Materials The dyes used were orange II, methylene blue, and gentian violet. Their structures are shown in Fig. 1. Methylene
1 To whom correspondence should be addressed. Fax: +81-6-6730-1394. E-mail: [email protected].
17
0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
18
HIRATA ET AL.
FIG. 1.
Structure of the dyes.
blue and gentian violet are basic dyes, and orange II is an acidic dye. The activated carbon (AC) produced from nutshells, Shirasagi G, was commercially obtained from the Takeda Chemical Ind. Co., Ltd.. The ceramic activated carbon (CC) produced from organic waste, CARBO-TEC, was obtained from Bron Electric Co., Ltd.
methylene blue (665 nm), and gentian violet (582 nm). The equilibrium amount adsorbed was calculated using the equation X = (C0 − C)V /M, where X is the equilibrium amount adsorbed per gram of carbonaceous materials (mg/g), C0 and C are the initial and equilibrium concentration (mg/L), respectively, V is the volume of the solution (L), and M is the weight of the carbonaceous materials (g).
Production of Carbonaceous Materials from Coffee Grounds Carbonization by muffle furnace. Raw coffee grounds (CGRs) were dried at 383 K for 24 h. CGRs were inserted into a metal container which was covered and inserted into an electric muffle furnace heated to 1073 K at a rate of 10 K/min in a nitrogen gaseous phase. The materials were maintained at this temperature for 1 h and then cooled to obtain the carbonaceous material (CGM). Carbonization by microwave. The wetted coffee grounds were carbonized in air by microwave treatment at a frequency of 2450 MHz and an output of 500 W; the treatment period was 7 min (CG7), 9 min (CG9), or 12 min (CG12). The carbonization temperature of CG increased to about 673 K. Amount of Dyes Adsorbed onto the Carbonaceous Materials The amount of dyes adsorbed onto the carbonaceous materials was calculated from the initial concentration of dyes and the equilibrium concentration. The equilibrium concentration was calculated using the absorbance of orange II (485 nm),
Physical and Chemical Properties of the Carbonaceous Materials The specific surface area of the carbonaceous material was measured using a FlowSorb II 2300 (Micromeritics Co., Ltd.). The base and acid consumption of the carbonaceous materials was measured using the acid–base titration method (15, 16). The carbonaceous materials were added to a 100-ml portion of hydrochloric acid (0.1 mol/L), and the suspension was shaken for 15 h. Base consumption was determined by titrating the aqueous sodium hydroxide (0.1 mol/L) into the filtrate. The carbonaceous materials were added to a 10-ml portion of sodium hydroxide (0.1 mol/L), and the suspension was shaken for 15 h. Acid consumption was determined by titrating the hydrochloric acid (0.1 mol/L) into the filtrate. These results are shown in Table 1. The yield of carbonaceous materials produced from coffee grounds decreased with increasing microwave treatment period because of the increasing carbonization, that is, a part of carbon in coffee grounds became carbon dioxide. Generally speaking, the porosity of the carbonaceous materials increases
ADSORPTION OF DYES
19
TABLE 1 Elemental Analysis of Carbon, Hydrogen, and Nitrogen Contents for Carbonaceous Materials Samples
Carbon (%)
Hydrogen (%)
Nitrogen (%)
CGR CGM CG9
53.8 87.5 60.1
6.6 1.9 6.6
2.3 3.1 2.6
with an increase specific surface area. The ACs has many pores, while CGs hardly have any. RESULTS AND DISCUSSION
Production of Carbonaceous Materials from Coffee Grounds The carbon, hydrogen, and nitrogen contents of CGR, CGM, and CG9 were measured by elemental analysis, as shown in Table 2. The amount of carbon contained in CG9 was greater than that contained in CGR, because of the carbonization of the coffee grounds. There was a greater amount of carbon contained in the carbonaceous material produced in the electric muffle furnace, because the carbonization temperature in the electric muffle furnace is higher than that in the microwave. The amount of electricity used to manufacture the carbonaceous materials by carbonization by the electric muffle furnace and the microwave apparatus was 3750 and 75 Wh, respectively. The Ministry of Environment reported in 1999 that the amount of carbon dioxide produced per kilowatt-hour was 0.550 kg. The amount of carbon dioxide produced was 1.44 and 0.03 kg, respectively. The results of the elemental analysis were used for an evaluation of the discharge quantity of carbon dioxide produced by the carbonization of the coffee grounds. The discharge quantity of carbon dioxide produced by burning the organic by-products is about 13 million tons. The annual coffee bean consumption in the world was about 6,432,000 tons in the year 2000. We calculated the amount of carbon dioxide produced based on results of elemental analysis. If all of the coffee grounds are completely burned, 3,460,000 tons of carbon dioxide are discharged. However, on the assumption that only carbon dioxide is produced, when the coffee grounds are carbonized using the electric muffle furnace and the microwave apparatus, the amount of produced carbon dioxide can TABLE 2 Physical and Chemical Properties of Carbonaceous Materials Consumption (mEq/g) Samples
Specific surface area (m2 /g)
Yield (%)
Acid
Base
CG7 CG9 CG12 AC CC
1< 1< 1< 1119 133
32.8 29.0 25.3 — —
0.082 0.037 0.040 0.040 0.072
3.16 3.07 3.10 1.44 1.21
FIG. 2. Removal percentages of dyes with carbonization time. , Methylene blue; , orange II; , gentian violet.
decrease to about 1,370,000 and 1,112,000 tons, respectively. Therefore, the carbonization of coffee grounds is an excellent way to process organic wastes and contribute to decreased global warming. Percentage of Dyes Removed by the Carbonization Materials The percentage of dyes removed during the increasing microwave treatment period is shown in Fig. 2. Methylene blue and gentian violet were completely removed in all samples, while orange II required a longer microwave treatment period to be removed. This result indicated that methylene blue and gentian violet, which are basic dyes, are easily removed because the surface polarity of CG9 is a large base consumption. Moreover, the functional groups in the surface of CG9 can relate the CG9/orange II interaction to the adsorption capacity because the specific surface area of CG9 is less than 1 m2 /g. The removal percentage of orange II by CG is lower than that of methylene blue and gentian violet, because orange II is an acidic dye. Adsorption of Dyes onto the Carbonaceous Materials The isotherms of methylene blue adsorption onto AC, CG9, and CC at 293 K are shown in Fig. 3a. The amount of methylene blue adsorbed increased in the order CC, CG9, and AC. The adsorption isotherms of the dyes were approximated by the Freundlich equation, log X = log k + (1/n) log C, where X is the amount of dyes adsorbed, C is the equilibrium concentration, and k and 1/n are the adsorption constants. The reciprocal of the slope (1/n) of the Freundlich plot measured the affinity between the surface of the carbonaceous materials and dyes. The Freundlich plots of the isotherms for adsorption of methylene blue onto the carbonaceous materials were linear (Fig. 3b). However, the Freundlich plots for adsorption of methylene blue onto AC can be shown with two lines. Boki et al. reported that when the adsorbates at low concentration are adsorbed into the pores of adsorbents, which have micropores, the Freundlich plots become two lines (17). This result indicates that in the case of the
20
HIRATA ET AL.
the structure of gentian violet is like that of a propeller. The Freundlich plots of the isotherms of gentian violet adsorption onto the carbonaceous materials are shown in Fig. 4b. These plots became linear. The mechanism of gentian violet adsorption may be monolayer adsorption onto the heterogeneous surface of the adsorbents. The isotherms of orange II adsorption onto AC, CG9, and CC are shown in Fig. 5a. The amount of orange II adsorbed onto AC was the greatest, and that onto CG9 was similar to that onto CC. The mechanism of orange II adsorption onto the activated carbon was reported in the electrostatic interaction between the dye molecules and carbon surface (18). The orange II molecules can hardly ever be adsorbed onto the surface of CG9, because orange II is an acidic dye. Moreover, the specific surface area of CG9 is very small. The amount of orange II adsorbed onto CC is very small, because the specific surface area of CC is small. These results indicated that the adsorption site of orange II would be the pores of the carbonaceous materials, which have an acidic surface. Generally speaking, the specific surface area of the activated carbon is corrected to the pore volume and the micropore volume is greater. Orange II would only be adsorbed
FIG. 3. (a) Isotherms of methylene blue adsorption onto carbonaceous materials. , AC; , CG9; , CC. (b) Freundlich plots of isotherms for methylene blue adsorption onto carbonaceous materials. , AC; , CG9; , CC.
adsorption of methylene blue onto AC, methylene blue is adsorbed into the pores, but in the case of the adsorption of methylene blue onto CG9, it is adsorbed onto the surface of CG9. The affinity between the surfaces of carbonaceous materials and methylene blue increases in the order of CC, AC, and CG9. The constant 1/n of CG9 is greatest, because the base consumption of CG9 is greatest and methylene blue is a basic dye. The isotherms of gentian violet adsorption onto AC, CG9, and CC are shown in Fig. 4a. The amount of gentian violet adsorbed onto the carbonaceous materials was larger in the order of CG9, AC, and CC. Even if the specific surface area of CG9 is less than 1 m2 /g, gentian violet was adsorbed onto CG9. This result indicates that the gentian violet is not adsorbed in the pores of CG9. The adsorption mechanism of gentian violet on CG9 will be a chemical interaction, because gentian violet is a basic dye, while the adsorption of gentian violet onto AC or CC will be physical adsorption, because the specific surface areas of AC and CC are 1119 and 133 m2 /g, respectively. The amount of gentian violet adsorbed onto CC is greater than that adsorbed onto AC. It is assumed that the pore radius of CC is larger than that of AC. The mechanism of gentian violet adsorption onto the carbonaceous materials depended on the porosity, because
FIG. 4. (a) Isotherms of gentian violet adsorption onto carbonaceous materials. , AC; , CG9; , CC. (b) Freundlich plots of isotherms for gentian violet adsorption onto carbonaceous materials. , AC; , CG9; , CC.
21
ADSORPTION OF DYES
FIG. 6. Isotherms of adsorption of dyes onto CG9. , Methylene blue; , orange II; , gentian violet.
FIG. 5. (a) Isotherms of orange II adsorption onto carbonaceous materials. , AC; , CG9; , CC. (b) Freundlich plots of isotherms for orange II adsorption onto carbonaceous materials. , AC; , CG9; , CC.
onto the pores of AC. The Freundlich plots of isotherms for adsorption of orange II onto the carbonaceous materials are shown in Fig. 5b. The Freundlich plots became linear. The adsorption mechanism of orange II may be monolayer adsorption onto the carbonaceous materials, which have a heterogeneous surface. The isotherms of adsorption of dyes onto CG9 are shown in Fig. 6. The amount adsorbed onto CG9 is larger in the order of orange II, gentian violet, and methylene blue. Methylene blue and gentian violet are basic dyes, and orange II is an acidic dye. The mechanism of adsorption of dyes onto the activated carbon was reported in the molecular sieve effect (18). If there are no pores in the adsorbents, the adsorption would depend upon the surface polarity of the adsorbents. The correlation coefficient and the constants 1/n of the Freundlich plot of the adsorption of dyes were estimated by linear regression analysis and are shown in Table 3. The constant 1/n expresses an affinity between the adsorbate and adsorbent. The Freundlich plots of methylene blue onto CG9 curved at 1.5 mg/L of equilibrium concentration. Although AC does not have a greater base consumption, the constant 1/n was greatest at an equilibrium concentration less than 1.5 mg/L. On equilib-
rium concentration of more than 1.5 mg/L, the constant 1/n of AC was similar for CG9. Methylene blue was adsorbed in the AC pores, because the specific surface area was greatest. Adsorption onto an activated carbon whose surface is hydrophobic is based mainly on the London dispersion force, which is part of the van der Waals force causing a decrease in the constant 1/n. The more closely the adsorbate molecules in the pores are located on the surrounding pore walls, the higher the adsorption force will be (19). Adsorption at a very low concentration occurs mainly in the pores with a high adsorption force. The methylene blue and gentian violet were adsorbed onto the surface of CG9, because methylene blue and gentian violet are basic dyes, and the surface of CG9 is acidic. The constant 1/n of methylene blue and gentian violet increased in the order of CC, AC, and CG9. An increase in affinity between methylene blue or gentian violet and the carbonization materials depended upon the increase in acidity on the surface of the carbonization materials. As AC has a large pore volume, the constant 1/n of orange II was greatest onto AC. Therefore, it is thought that the interaction was greatest between the carbonization and the orange II. The interaction between CG9 and methylene blue or gentian violet was greater than the interaction between CG9 and orange II, because the surface of CG9 was the most basic. TABLE 3 Freundlich Constants of Adsorption Isotherms for Dyes Methylene blue
Orange II
Gentian violet
Samples
1/n
r
1/n
r
1/n
r
CG9 AC
3.00 43.0a 1.12b 0.21
0.961 0.930a 0.945b 0.903
0.27 119
0.928 0.896
0.95 0.63
0.953 0.959
0.034
0.912
0.38
0.908
CC
Note. r = correlation coefficient. The equilibrium concentration is 0.57 to 1.17 mg/g. b The equilibrium concentration is 1.17 to 11.6 mg/g.
a
22
HIRATA ET AL.
In conclusion, (i) the adsorption of dyes depended upon the base and acid consumption of the carbonaceous materials, (ii) methylene blue and gentian violet are adsorbed in micropores or on the acidity sites of the carbonaceous materials, (iii) orange II is adsorbed in the pores of carbonaceous materials, (iv) the carbonaceous material produced from the coffee grounds has a larger base consumption, and (v) the microwave treatment of coffee grounds can be utilized for the production of carbonaceous materials for saving energy and decreasing carbon dioxide in the atmosphere. REFERENCES 1. 2. 3. 4. 5.
Takebayashi, M., Eng. Jpn. 63(2), 76 (1999). Suzuki, M., Chem. Eng. Jpn. 63(2), 69 (1999). Watanabe, H., Chem. Eng. Jpn. 63(2), 79 (1999). Adschiri, T., J. Chem. Eng. Jpn. 26, 676 (1993). Abe, I., Kawasaki, N., Nakamura, T., Kondo, T., and Tanada, S., Environ. Sci. 13, 235 (2000).
6. Ajmal, M., and Khan, A. U., Environ. Pollut. Ser. A 37, 131 (1985). 7. Gupta, G. S., and Bhattacharya, P. K., J. Chem. Tech. Biotechnol. B 35, 23 (1985). 8. Gupta, G. S., Prasad, G., and Singh, V. N., Res. Ind. 33, 132 (1988). 9. Allen, S. J., Fuel 66, 1175 (1987). 10. Allen, S. J., McKay, G., and Khader, K. Y. H., Environ. Pollut. 52, 39 (1988). 11. Allen, S. J., McKay, G., and Khader, K. Y. H., J. Colloid Interface Sci. 126, 517 (1988). 12. Allen, S. J., Prasad, G., and Singh, V. N., Environ. Technol. Lett. 9, 153 (1988). 13. Gupta, G. S., Prasad, G., and Singh, V. N., J. Environ. Sci. Health A 23, 205 (1988). 14. Nawar, S. S., and Doma, H. S., Sci. Total Environ. 79, 271 (1989). 15. Akiyoshi, M., and Shirasaki, M., Nippon Kagaku Kaishi 5, 1181 (1976). 16. Okazaki, S., and Toyoda, S., Kogyo Kagaku Zasshi 73, 478 (1970). 17. Boki, K., Mori, H., and Kawasaki, N., J. Am. Oil Chem. Soc. 71, 596 (1994). 18. Kasaoka, S., Sakata, Y., Tanaka, E., and Naotoh, R., Nippon Kagaku Kaishi 12, 2266 (1987). 19. Kondo, S., Ishikawa, T., and Abe, I., in “Science of Adsorption,” Maruzen, Tokyo.