808
Ind. Eng. Chem. Res. 1997, 36, 808-812
Sunflower Stalks as Adsorbents for Color Removal from Textile Wastewater Gang Sun* and Xiangjing Xu Division of Textiles and Clothing, University of California, Davis, California 95616
Sunflower stalks as adsorbents for two basic dyes (Methylene Blue and Basic Red 9) and two direct dyes (Congo Red and Direct Blue 71) in aqueous solutions were studied with equilibrium isotherms and kinetic adsorptions. The maximum adsorptions of two basic dyes on sunflower stalks are very high, i.e., 205 and 317 mg/g for Methylene Blue and Basic Red 9, respectively. The two direct dyes have relatively lower adsorption on sunflower stalks. The adsorptive behaviors of sunflower stalk components are different. The pith, which is the soft and porous material in the center of stalks, has twice the adsorptive capacity of the skin. Particle sizes of sunflower stalks also affect the adsorption of dyes. The adsorption rates of two basic dyestuffs are much higher than that of the direct dyes. Within 30 min about 80% basic dyes were removed from the solutions. Comprehensive studies on textile wastewater treatment with plant residues such as sugarcane bagasse and maize cob have been reported (McKay et al., 1987; El-Geundi, 1991; Laszlo, 1994). The utilization of plant residues for the wastewater treatment at least has the following advantages: (1) Plant residues are agricultural wastes available abundantly at no or low cost. (2) Plant residues are cellulosic materials which have an inherent ability to adsorb waste chemicals such as dyes and cations in water due to the coulombic interaction. (3) Disposal of the wastes is a serious environmental problem in the states which have extensive agricultural activities such as California and North Dakota. There are two major technologies available for wastewater treatment in textile industry, i.e., oxidation and adsorption. In oxidation methods, UV/ozone or UV/ H2O2 treatments are possibly the best technologies to totally eliminate organic carbons in wastewater (Huang and Shu, 1995; Ruppet et al., 1994), but they are only effective in wastewater with very low concentrations of organic compounds. Thus, significant dilution is necessary as a facility requirement. In adsorption methods, activated carbon and polymer resins are the best adsorbents which can remove waste chemicals from relatively concentrated wastewater (Blum et al., 1993), but the adsorbents are more expensive compared to UV/ H2O2 treatment and it is difficult to regenerate the adsorbents. The aim of the present research is to explore the feasibility of utilizing sunflower stalks as single-use and low-cost filters for color removal in textile wastewater. Sunflower stalks consist of cellulose, hemicellulose, lignin, and other compounds similar to most plant residues but have a high holocellulose content (71 wt %) (Bonilla et al., 1990). The polyol structure of cellulose-based materials has relatively strong chemical adsorption to cations such as metal ions and organic bases as well as physical adsorption to other materials such as acidic and anionic compounds. In this study, four dyestuffs, Congo Red (CR), Direct Blue 71 (DB), Methylene Blue (MB), and Basic Red 9 (BR), were used in adsorption isotherm and kinetic adsorption studies of sunflower stalks. The effects of the size and type of sunflower stalks were also evaluated. * Author to whom correspondence should be addressed. S0888-5885(96)00383-1 CCC: $14.00
Experimental Methods Materials. Sunflower stalks were supplied by North Dakota State University. Sunflower stalk pith was separated from the skin in order to test the different adsorptive properties of the materials. Samples were ground to pass through 25-45 and 60 mesh sieves and then washed with deionized water until the effluent was colorless. Congo Red (CR), Direct Blue 71 (DB), and Methylene Blue (MB) were obtained from Aldrich Chemical Co. (Milwaukee, WI), and Basic Red 9 (BR) was obtained from Eastman Kodak Co. (Rochester, NY). All dyestuffs were used without purification. Methods. The concentrations of dyestuffs were measured with a Hitachi U-2000 spectrophotometer and two 1-cm cells. The wavelength of the maximum absorbency for each dye was selected, and λmax values are listed in Table 1. Lambert-Beer calibration curves were prepared. The equilibrium isotherms were determined by mixing 0.20 g of sunflower stalks with 50.00 mL of a dye solution in a 125 mL Erlenmeyer flask at 21 °C and a relative humidity (RH) of 65%. Each isotherm consisted of 10 dye concentrations varied from 50 to 1000 ppm for direct dyes and 100 to 2000 ppm for basic dyes. The flasks containing dye solution and sunflower stalk particles were placed in a shaker and agitated for 5 days at constant temperature in a conditioning room (21 °C, 65% RH). The equilibrium concentrations of different combinations were measured by the spectrophotometer and referenced with the calibration curves. The kinetic measurement of adsorption properties of sunflower stalks was carried out with similar equipment and conditions. The sample mass was 1.00 g and the volume of the dye solution was 200 mL in this series of tests. Initial dye concentrations (C0) and liquid-phase dye concentrations at time t(Ct) were measured. Results and Discussion Adsorption Capacity. The adsorption capacity of sunflower stalks for the four dyes was determined by measuring equilibrium isotherms. Of the four dyes, Direct Blue 71 and Congo Red are anionic colorants and Basic Red 9 and Methylene Blue are cationic colorants. The sunflower stalks were used in different particle size ranges and in separated components, namely, skin and © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 809 Table 1. Dye Structures and Their λmax Values dye
structure
Congo Red (C.I. 22120) (CR)
λmax (nm)
NH2 N
N
N
N
SO3Na
Direct Blue 71 (C.I. 34140) (DB)
497
NH2
SO3Na
SO3Na
594
OH N
N
N
N
N
N
NaO3S
NH2
SO3Na SO3Na
Methylene Blue (C.I. 51015) (MB)
N H3C
N
N+
S
CH3
CH3
CH3
Basic Red 9 (C.I. 42500) (BR)
661
Cl–
3 H2O
544
N+H2
Cl– C
H2N
NH2
Figure 1. Adsorption isotherms of four dyes on sunflower stalks (total).
Figure 2. Langmuir adsorption plots of four dyes on sunflower stalks (total).
pith, as well as the whole stalks (total). Adsorption isotherms of sunflower stalks (total, 25-45 mesh) for the four dyes are shown in Figure 1. The adsorption capacities of cationic dyes are much higher than that of the anionic dyes, which is consistent with other cellulosic materials (McKay et al., 1987; El-Geundi, 1991). The greater affinities of basic dyes for sunflower stalks than that of anionic dyes can be attributed to the cellulosic structure of the materials. The coulombic forces between dye species and negatively charged cellulose in water are the major interactions which affect the adsorption of dyes on the materials. Utilizing the Langmuir isotherm (eq 1) to analyze the equilibrium isotherms of the dyes gave the linear plots over a broad concentration range which are shown in Figure 2. The values of constant KL/aL represent the maximum adsorption capacity (qmax) of the adsorbents to a particular dyestuff. By using the same method, the maximum adsorption capacities of all the adsorbents for different dye molecules were obtained.
Table 2. Surface Areas of Sunflower Stalks
Ce/Qe ) 1/KL + (aL/KL)Ce
(1)
aL ) Langmuir isotherm constant (L/mg), KL ) Langmuir equilibrium constant (L/g), Ce ) equilibrium
component
size (mesh)
BET surface area (m2/g)
Langmuir surface area (m2/g)
skin
25-45 >60 25-45 >60 25-45 >60
0.9516 1.1159 1.1027 1.2054 2.0225 2.3151
1.5867 1.8161 1.8323 1.9684 3.5544 3.8662
total pith
liquid-phase dye concentration (mg/L), and Qe ) equilibrium solid-phase dye concentration (mg/g). Sunflower stalks have two prominently different components, pith and skin. Pith is a soft and porous cellulosic material, while the skin has a cellulose-based and layered fibrous structure. The pith definitely should be a better adsorbent material due to its porosity and large surface area (Table 2). The Langmuir and BET surface areas of sunflower stalk pith are twice as much as that of sunflower stalk skin. Figures 3 and 4 show the adsorptive equilibrium isotherms of pith, skin, and the total of sunflower stalks to Congo Red and Methylene Blue. The Langmuir adsorption parameters of four dyes on different components and different sizes of sunflower stalks are listed in Table 3. The pith of
810 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 Table 3. Langmuir Adsorption Parameters of Sunflower Stalks dye Basic Red 9
component
size (mesh)
KL (L/g)
aL (L/mg)
qmax (mg/g)
correlation coefficient
skin
25-45 >60 25-45 >60 25-45 25-45 >60 25-45 >60 25-45 25-45 >60 25-45 >60 25-45 25-45 25-45 25-45
2.55 2.95 2.36 2.85 5.58 8.70 11.42 21.57 10.32 10.38 1.04 1.43 1.07 1.51 1.77 0.16 0.19 1.17
0.0138 0.0108 0.0096 0.0090 0.0109 0.0798 0.0691 0.1632 0.0502 0.0242 0.0492 0.0455 0.0339 0.0400 0.0262 0.010 0.0071 0.0154
184.59 272.22 245.61 317.34 510.50 108.97 165.33 132.19 205.41 428.13 21.12 31.45 31.53 37.78 67.59 15.91 26.84 75.82
0.98 0.97 0.99 0.98 0.95 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.98
total Methylene Blue
pith skin total
Congo Red
pith skin total
Direct Blue 79
pith skin total pith
Figure 3. Adsorption isotherms of CR on different components of sunflower stalks.
Figure 4. Adsorption isotherms of MB on different components of sunflower stalks.
sunflower stalks has maximum adsorptive capacities of 67.59 and 428.13 mg/g (Table 3) for Congo Red and Methylene Blue, respectively, which are at least double or triple the capacity of the skin of sunflower stalks. The total which is the unseparated sunflower stalk has adsorption capacities closer to the skin because the mass percentage of skin is higher than that of pith. The adsorption capability of more than 200 mg of dyes/g of the adsorbent materials makes sunflower stalks a promising material as adsorbent materials for color removal of basic dyes from wastewater. Different sizes of the material also have effects on the maximum
adsorptions of dyestuffs. Smaller sizes of particles have substantially higher adsorption than the larger particles do. The differences of the surface areas of sunflower stalks shown in Table 2 primarily attribute to the differences of maximum adsorptions on varied components and particle sizes of the sunflower stalks. Another important phenomenon is that compared to other tested cellulosic materials, sunflower stalks (total) have higher equilibrium adsorptions for multivalent anionic dyes than wood and other materials (Table 4). The higher binding ability of sunflower stalks to anionic species may increase their potentials as adsorbents for anionic dye containing wastewater treatment. Kinetic Adsorption. The mechanism of dye adsorption on the plant residues in the color removal process is similar to dyeing textile materials, which may involve the following three steps: (i) diffusion of dye molecules through the solution to the surface of adsorbents; (ii) adsortion of dye molecules on the surface of the materials through the molecular interactions; (iii) diffusion of dye molecules from the surface into the interior of the adsorbent materials. The first step of adsorption may be affected by the dye concentration and agitation. Thus, an increase of the dye concentration may accelerate the diffusion of dyes from the dye solution onto the adsorbents (Figure 5). But the maximum adsorption of the adsorbents to the dyestuffs is constant. Therefore, the equilibrium concentrations of initially concentrated dye solutions are higher as the equilibrium is reached. Figure 5 shows the effects of increased concentrations of Methylene Blue on Ct/C0. The concentration of Methylene Blue varies from 20 to 100 ppm. In the first 10 min, 100 ppm of concentration was taken up more quickly than 20 and 50 ppm. A similar phenomenon was observed with other dyes. The second step of the adsorption of dyes on the materials is dependent on the nature of the dye molecules such as anionic or cationic structures. Due to the negatively charged characteristics of cellulosic materials in aqueous media, the cationic dyes should be adsorbed more rapidly than anionic dyes. Figure 6 shows adsorption rates of different dye molecules on sunflower stalks (total, 25-45 mesh). The 50 ppm of concentration was adopted in the tests. Basic dyes have quick adsorption on the sunflower stalks. In about 30 min, over 80% basic dyes can be removed from the effluents, while for anionic species, only about 10% of them were removed by the adsorbents. The results obtained here indicate the effect of coulombic interac-
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 811 Table 4. Adsorptive Capacities of Some Plant Residues material
basic dye (mg/g)
acid dye (mg/g)
ref
maize cob bagasse pith wood shavings sunflower stalks
160 (Basic blue 69) 158 (Basic blue 59)
41.4 (Acid Blue 25) 20-25 (Acid Blue 25) 0.7 (Congo Red) 31.5-37 (Congo Red)
El-Geundi, 1991 McKay et al., 1987 Abo-Elila and El-Dib, 1987
245 (Basic red 9)
Figure 5. Concentration effect of MB on adsorption rate of sunflower stalks (total).
Figure 7. Effect of different components of sunflower stalks on adsorption rate of CR.
pH’s, and the salts contained in the dyebath. Although the maximum adsorption and rate of adsorption of anionic dyes by sunflower stalks are lower than those of cationic dyes, the abundant availability and low cost for anionic dyes may still make them economically feasible for use as adsorbents. Chemical modification of sunflower stalks with cationic groups will dramatically increase their adsorption to anionic dyestuffs. This type of research has been reported in several papers (Laszlo, 1996; Hwang and Chen, 1993). Chemical modification may increase the cost of the products, but the enhanced capabilities of the adsorbents may compensate.
Figure 6. Effect of different dyes on adsorption rate of sunflower stalks (total).
tions between the adsorbents and dyestuffs. It is obvious that higher adsorption capacities and higher adsorption rates for anionic dye removal are needed; the sunflower stalks should be chemically modified with cationic groups. The third step of the adsorption process is usually considered as a rate-determining stage in dyeing processes, which certainly should affect the adsorption of dyes on the substrates. However, it has the same effect on the kinetic adsorption as the second step of the process does. The structurally different adsorbents also demonstrated varied adsorption rates to the same dye molecules (Figure 7). The uptake rate of Congo Red on the sunflower stalk pith is much higher than that on the skin. It can be interpreted as due to the enlarged surface area of the porous sunflower stalk pith. For the same reason, smaller particle sizes will increase the uptake rate of dyes in the effluent. The adsorptive characteristics of the sunflower stalks (total) are in between those of the pith and skin but are closer to the skin, which is similar to the results obtained in the study of the equilibrium isotherms and the surface areas of the components. The rate of adsorption of dyes on sunflower stalks is affected by many other factors such as temperatures,
Conclusion Sunflower stalks were studied as adsorbents for removal of different dyestuffs in dyeing effluents with equilibrium isotherms and kinetic adsorptions. The results indicated that sunflower stalks have higher maximum adsorption capacities to basic dyestuffs, namely, 317 mg of Basic Red 9 dye and 205 mg of Methylene Blue per gram of sunflower stalks, respectively. The maximum adsorption capacities of anionic dyes on sunflower stalks are lower with 37.8 mg (>60 mesh) of Congo red dye and 26.8 mg (25-45 mesh) of Direct Blue dye per gram of the adsorbents. The higher adsorption rates of cationic dyes on the adsorbents were obtained with over 80% removal of dyestuffs in the effluents. Literature Cited Abo-Elila, S. I.; El-Dib, M. A. Color Removal Via Adsorption on Wood Shaving. Sci. Total Environ. 1987, 66, 269-273. Blum, D. J. W.; Suffet, I. H.; Duguet, J. P. Estimating the Activated Carbon Adsorption of Organic Chemicals in Water. Crit. Rev. Environ. Sci. Technol. 1993, 23, 121-136. Bonilla, J. L.; Chica, A.; Ferrer, J. L.; Jimenez, L.; Martin, A. Sunflower Stalks as a Possible Fuel Source. Fuel 1990, 69, 792-794. El-Geundi, M. S. Color Removal From Textile Effluents by Adsorption Techniques. Water Res. 1991, 25, 271-273.
812 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 Huang, C. R.; Shu, H. Y. The Reaction Kinetics, Decomposition Pathways and Intermediate Formations of Phenol in Ozonation, UV/O3 and UV/H2O2 Processes. J. Hazard. Mater. 1995, 41, 47-64. Hwang, M. C.; Chen, K. M. The Removal of Color From Effluents Using Polyamide-Epichlorohydrin-Cellulose Polymer. I. Preparation and Use in Direct Dye Removal. J. Appl. Polym. Sci. 1993, 48, 299-311. Laszlo, J. A. Removing Acid Dyes From Textile Wastewater Using Biomass For Decolorization. Am. Dyestuff Rep. 1994, 17-21. Laszlo, J. A. Preparing an Ion Exchange Resin From Sugarcane Bagasse To Remove Reactive Dye From Wastewater. Text. Chem. Color. 1996, 13-17. McKay, G.; Geundi, M. El.; Nassar, M. M. Equilibrium Studies During The Removal of Dyestuffs From Aqueous Solutions Using Bagasse Pith. Water Res. 1987, 21, 1513-1520.
Ruppet, G.; Bauer, R.; Heisler, G. UV-O3, UV-H2O2, UV-TiO2 and The Photo-Fenton ReactionsComparison of Advanced Oxidation Processes for Wastewater Treatment. Chemosphere 1994, 28, 1447-1454.
Received for review July 5, 1996 Revised manuscript received December 3, 1996 Accepted December 6, 1996X IE9603833
X Abstract published in Advance ACS Abstracts, January 15, 1997.
