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Removal of Dyes from Wastewater Using Flyash, a Low-Cost Adsorbent† Dinesh Mohan,‡ Kunwar P. Singh,*,‡ Gurdeep Singh,§ and Kundan Kumar§ Environmental Chemistry Division, Industrial Toxicology Research Centre, Post Box 80, Mahatma Gandhi Marg, Lucknow 226 001, India, and Centre of Mining Environment, Indian School of Mines, Dhanbad-826001, India
The use of low-cost adsorbent has been investigated as a replacement for the current expensive methods of removing dyes from wastewater. As such, fly ash generated in National Thermal Power plant was collected and converted into a low-cost adsorbent. The prepared adsorbent was characterized and used for the removal of dyes from wastewater. Adsorption studies were carried out for different temperatures, particle sizes, pH’s, and adsorbent doses. The adsorption of each dye was found to increase with increasing temperature, thereby indicating that the process is endothermic in nature. The removal of each dye was found to be inversely proportional to the size of the fly ash particles, as expected. Both the linear and nonlinear forms of the Langmuir and Freundlich models fitted the adsorption data. The results indicate that the Freundlich adsorption isotherm fitted the data better than the Langmuir adsorption isotherm. Further, the data were better correlated with the nonlinear than the linear form of this equation. Thermodynamic parameters such as the free energies, enthalpies, and entropies of adsorption of the dye-fly ash systems were also evaluated. The negative values of free energy indicate the feasibility and spontaneous nature of the process, and the positive heats of enthalpy suggest the endothermic nature of the process. The adsorptions of crystal violet and basic fuschin follow first-order rate kinetics. In comparison to other low-cost adsorbents, the sorption capacity of the material under investigation is found to be comparable to that of other commercially available adsorbents used for the removal of cationic dyes from wastewater. 1. Introduction There is a considerable need for the removal of color from wastewater/effluents. The discharge of dye-bearing wastewater into natural streams and rivers from the textile, paper, carpet, leather, distillery, and printing industries poses severe problems, as dyes impart toxicity to the aquatic life and are damaging to the aesthetic nature of the environment. Many of the dyes used in these industries are stable to light and oxidation, as well as being resistant to aerobic digestion. Basic dyes are the brightest class of soluble dyes used by the textile industry.1 Their tincorial value is very high: less than 1 ppm of the dye produces an obvious coloration. Therefore, it is required that the color-bearing effluents be treated to remove the color/dye in an economical fashion to the prescribed concentration levels before they are discharged into bodies of water. Considering both discharge volume and effluent combustion, the wastewater from the textile industry is rated as the most polluting among all industrial sectors.2 Therefore, there is a definite need for a dye/color-removal technology that works suitably under the above circumstances and is cost-effective. Various methods of dye/color removal, including aerobic and anaerobic microbial degradation, coagulation, chemical oxidation, membrane separation, electrochemical treatment, dilution, filtration, flotation, softening, hydrogen peroxide catalysis, and reverse * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Industrial Toxicology Research Centre. § Indian School of Mines. † ITRC communication number 2195.
osmosis, have been proposed from time to time.3-6 However, all of these methods suffer from one or another limitations, and none of them were successful in completely removing the color from wastewater.7 Although biological treatment processes remove BOD, COD, and suspended solids to some extent, they are largely ineffective in removing color from wastewater, as most dyes are toxic to the organisms used in such processes. The coagulation process effectively decolorizes insoluble dyes, but it fails to work well with soluble dyes. Chemical destruction by oxidation is effective, but the oxidant requirements are very high and thus expensive. Photochemical degradation in aqueous solution is likely to progress slowly, as synthetic dyes are, in principle, designed to exhibit high stability to light. Accordingly, the removal of dyes from effluent in an economical fashion remains a major problem. Removal of color/dye by adsorption is a relatively new technology. Regarding the selection of adsorbents, a literature survey reveals that materials such as commercially available activated carbons8-12 and zeolites,12,13 among others, have been used in the past for the treatment of textile effluents. Despite the prolific use of activated carbon for wastewater treatment, carbon adsorption remains an expensive process, and this fact has prompted growing research interest into the production of low-cost alternatives to activated carbons in recent years. Various workers have exploited substances such as peat,14-17 bagasse pitch,18,19 Fuller’s Earth,20 lignite,21 activated carbon,8-12 coal,22 activated slag,23 activated carbon developed from fertilizer waste,23 bagasse fly ash,24 activated carbon fibers,25-26 wool carbonizing waste,27 maize cob,28 clays,29 zeolites,12,13 MgCO3,30 perlite,31 wool,32 silica,33,34 wood meal,35 shale oil ash,36 activated
10.1021/ie010667+ CCC: $22.00 © 2002 American Chemical Society Published on Web 06/28/2002
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carbon developed from bamboo,37 recycled alum sludge,38 biomaterial,39 and fly ash40 for this purpose. Other adsorbents and various anaerobic treatment processes are also mentioned in the review articles by Lambart and Graham41 and Delee et al.42 However, the problems associated with these adsorbents are regeneration and recovery of the useful materials, which make them unattractive for wider commercial applications. This calls for a research effort into the development of some suitable low-cost, efficient indigenous technology capable of removing various dyes/coloring materials from industrial effluents. Basic violet and basic fuschin are broadly used dyes in the textile industry and have properties that make them difficult to remove from solutions. These dyes are also used in the leather and carpet industries for coloring purposes.43,44 For quite some time, we have been involved in developing low-cost activated carbons/adsorbents for the removal and recovery of organics/metal ions from wastewater.23,24,48,50 Continuing our activities in this direction, we have successfully derived a new low-cost activated carbon from fly ash (a thermal power waste) for the removal of dyes from synthetic dye wastewater. Thermal power plants produce huge amounts of fly ash as a byproduct, and the safe disposal of this material is problematic. Presently, it is being used as a filler. 2. Materials and Methods All reagents were AR-grade chemicals. Stock solutions (10-4 M) of the test reagents were made by dissolving the dye in doubly distilled water of appropriate pH, i.e., 6.0 for rosaniline hydrochloride and 8.0 for crystal violet. The pH of the test solutions was adjusted using reagentgrade dilute sulfuric acid (0.1 N) and sodium hydroxide (0.1 N). Further solutions of different concentrations were made by using the same stock solutions. The dyes selected for the studies, along with their structures, are given below. Crystal Violet or Basic Violet (CI No. 42555). Formula: C25 H30Cl N3. Molecular weight: 407.99. Manufacturer: Loba Chemie, Bombay, India.
Rosaniline Hydrochloride or Basic Fuchsin (CI No. 42500). Formula: C20H20N3Cl. Molecular weight: 337.86. Manufacturer: Loba Chemie, Bombay, India.
2.1. Equipment. The pH measurements were made using a pH meter (model CT CL46, Toshniwal Instruments, Delhi, India). X-ray measurements were performed using a Phillips X-ray diffractometer employing nickel-filtered Cu KR radiation and Ni filters. The infrared spectrum of the adsorbent was recorded in potassium bromide and Nujol mull in the range of 5004000 cm-1 using a Perkin-Elmer spectrophotometer. The surface area was measured with a model QS-7 Quantasorb surface area analyzer. The porosity and density of the adsorbent were determined with a mercury porosimeter and with specific gravity bottles, respectively. The chemical constituents of fly ash were analyzed following the routine methods of chemical analysis.45,46 All color measurements were made on a Shimadzu UV-visible spectrophotometer (model UVVis 265) in the visible range in absorbance mode. The spectrophotometer response time was 0.1 s, and the instrument had a resolution of 0.1 nm. Absorbance values were recorded at the wavelength of maximum absorbance (λmax) for each dye, and each dye solution was initially calibrated for concentration in terms of absorbance units. The concentration of each dye was measured with a 1-cm-path-length cell, with an absorbance accuracy of (0.004 at λmax of the dye (589 nm for crystal violet and 545 nm for basic fuchsin). Absorbance was found to vary linearly with concentration, and dilutions were performed when the absorbance exceeded 0.8. Biological degradation of the dyes was also taken into account by running blanks. The UV spectra of the dyes were also analyzed at different pH’s, i.e., 2, 4, 6, 7, 8, and 10 (data not included), and it was found that the λmax of the dyes did not shift with the change in pH. Crystal violet changes color in the pH range 0.0-1.8, thereby suggesting a pKa value between 0 and 1.8, whereas rosaniline hydrochloride changes color in the pH range 1.0-3.1, thereby indicating a pKa value between 1 and 3.1. Standard plots were made as a function of concentration. The plots were made in the concentration range of 10-5-10-6 M. 2.2. Fly Ash Collection and Adsorbent Development. In modern thermal power stations, pulverized coal is used, and fly ash is obtained as a waste product in large quantities. A representative sample of the raw material (fly ash) was collected from Chandrapura Thermal Power Plant in Bakaro, Jharkhand, India. The fly ash was in the form of small, spherical grayish-black particles. The collected fly ash was sieved to the desired particle size ranges, including -16 to +16, -16 to +30, and -30 to +72 mesh. Each of these particle size lots was washed a number of times with distilled water to remove the adhering organic material and then dried at 100 ( 5 °C for 24 h and kept in a separate plastic bag in a vacuum desiccator until required in the adsorption studies. 2.3. Sorption Procedure. Sorption studies were performed by the batch technique to obtain rate and equilibrium data. The batch technique was selected because of its simplicity. Batch pH studies were conducted to determine the optimum pH at which maximum color removal could be achieved with fly ash for each of the two dyes. Batch sorption studies were performed for different temperatures, particle sizes, and adsorbent doses to obtain the equilibrium isotherms and the data required in the design and operation of column reactors for the treatment of dye-bearing wastewater. For isotherm studies, a series of 100-mL conical flasks
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Figure 1. Effect of contact time on the rates of adsorption of crystal violet and rosaniline hydrochloride by fly ash at 25 °C and an adsorbate concentration of 5 × 10-3 M.
was employed. Each conical flask was filled with 50 mL of dye solution of varying concentration (10-5 -10-4 M) and adjusted to the desired pH using dilute sulfuric acid and sodium hydroxide. A known amount of adsorbent was added to each conical flask, and the flasks were agitated intermittently for the desired time periods. The contact time and other conditions were selected on the basis of preliminary experiments, which demonstrated that equilibrium was established in 18-20 h (Figure 1). Equilibration for longer times, that is, between 20 and 48 h, gave practically the same uptake results. Therefore, a contact period of 20 h was finally selected for all of the equilibrium tests. After this period, the solutions were filtered using Whatman no. 42 filter paper and spectroscopically analyzed at the corresponding λmax for the concentration of dye remaining in solution. The effect of pH was observed by studying the adsorption of dye over a pH range of 2-10. For these experiments, a series of 100-mL conical flasks was again employed. Each conical flask was filled with 50 mL of dye solution having a concentration of 5 × 10-5 M at varying pH and 25 °C. Adsorbent (10 g/L) was added to each conical flask, and the flasks were agitated intermittently for 20 h. After this period, the solutions were filtered using Whatman no. 42 filter paper and spectroscopically analyzed for the concentration of dye remaining in solution. The dye concentration retained in the adsorbent phase was calculated according to
qe )
(Co - Ce)V W
(1)
where Co and Ce are the initial and equilibrium concentrations (M), respectively, of dye in solution; V is the volume (L); and W is the weight (g) of the adsorbent. 3. Result and Discussions 3.1. Characterization. The fly ash sample (1.0 g) was stirred with deionized water (100 mL, pH 6.8) for 2 h, and the mixture was left for 24 h in an airtight stoppered conical flask. An increase in the pH of the deionized water was observed (pH 7.89). None of the metal ions Al, Si, or Fe were eluted from the fly ash with cold water, hot water, dilute HCl, or dilute NaOH, but they were eluted with concentrated acid. The
Figure 2. Effect of pH on the adsorptions of crystal violet and rosaniline hydrochloride by fly ash at 25 °C and an adsorbate concentration of 5 × 10-3 M.
different chemical constituents of fly ash are SiO2 (56.70%), Al2O3 (23.80%), Fe2O3 (4.0%), CaO (2.10%), MgO (1.40%), and LIO (7.4%); the surface area is 1490 m2 g-1; and the specific gravity is 1.37 g/cm3 as characterized previously.45 Fly ash predominantly consists of trace metals, with the chief constituents being oxides, silicates, and aluminosilicates of calcium, iron, and other bases. These oxides have a tendency to form metal hydroxide complexes in the solution, and the subsequent acidic or basic dissociation of these complexes at the solid-solution interface leads to the development of a positive or negative charge on the surface. The infrared spectrum of fly ash includes broad and weak peaks in the region 4000-500 cm-1. Absorption bands in the region 3700-3500 cm-1 are assigned to free hydroxyl. The peaks at 1080 and 915 cm-1 are assigned to R-quartz, those in the range 662-580 cm-1 to γ-Al2O3, and that at 953 cm-1 to Brunite. Although some inference can be made about surface functional groups from IR spectra, the weak and broad bands do not provide any definitive information about the nature of the surface oxides. The data, however, indicate the presence of some surface groups on the adsorbent material. X-ray diffraction analysis of the fly ash samples indicated the presence of R-quartz (d ) 4.25 Å), kaolinite (d ) 4.25 Å), hematite (d ) 2.947 Å), Illite (d ) 3.382 Å), tridymite (d ) 3.292 Å), and rutile (d ) 4.12 Å). 3.2. Sorption Studies. Adsorption isotherms were determined for various dye-adsorbent systems. The distribution of dye between the adsorbent and the dye solution at equilibrium is important in establishing the capacity of the adsorbent for the dye. 3.2.2. Effect of pH. The pH is the most important factor affecting the adsorption process. The studies in this report were carried out at an initial concentration of 5 × 10-5 M. The changes in adsorption of crystal violet and basic fuchsin over a broad pH range of 2-10 are depicted in Figure 2. The adsorption of each of the cationic dyes increases with increasing pH of the medium. This can be explained by considering the zero point of charge of the fly ash. The pH at the zero point of charge (pHZPC) is reported to be 8.2 and 2.3 for alumina and silica, respectively. The composite pHZPC of the fly ash is 2.8. Thus, it seems likely that, for pH values above 2.8, the negative charge density on the
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surface of the fly ash increases, and the charge developed in the acid medium (pH < 2.8) does not favor the association of the cationic adsorbate species. However, for pH values above 2.8, as the adsorbent surface gradually becomes negatively charged, it offers suitable sites for the adsorption of dyes. Also, the crystal violet and basic fuchsin molecules become protonated in the acidic medium, with deprotonation likely taking place at higher pH’s because their pKa values lies between 0 and 3.1. Consequently, the positive charge density would be located more on the dye molecules at low pH, and this accounts for the higher dye uptake on the negatively charged surface. Thus, it seems likely that, above pH 2.8, the negative charge density on the surface will increase and will be associated with H+ or Na+ ions according to the pH of the solution. These positively charged ions in the presence of dye solution can then be exchanged with dye cations as follows
At solution pH < 2.8, the surface becomes positively charged and is associated with negatively charged Cl-
Obviously, no exchangeable cationic species will be on the adsorbent surface in acidic medium, resulting in unfavorable conditions for the adsorption of dye. The metal oxides present in the adsorbent form aqua complexes in the presence of water dipoles and develop positively or negatively charged surfaces through amphoteric dissociation as follows
Therefore, all of the adsorption studies were performed at pH 6 for rosaniline hydrochloride and at pH 8.0 for crystal violet to correlate the dye removal process with adsorption and to avoid any precipitation. 3.2.3. Effect of Temperature. The removal of each crystal violet and rosaline hydrochloride (basic fuchsin) was studied at 10, 25, and 40 °C to determine the
Figure 3. Adsorption of crystal violet on fly ash at different temperatures. The solid lines represent fits to the data by the (a) Freundlich and (b) Langmuir isotherms.
adsorption capacity at different temperatures and the thermodynamic parameters. The extents of adsorption of both dyes were found to increase with temperature (Figures 3 and 4), indicating the endothermic nature of the process. All of the isotherms are positive, regular, and concave with respect to the concentration axis. The uptake of each of the dyes by fly ash is almost 100% at low adsorbate concentration, and it decreases as the adsorbate concentration increases. The increase in dye uptake with increasing temperature might also be due to the enhanced rate of intraparticle diffusion of the adsorbate, as diffusion is an endothermic process. The increase in adsorption behavior also suggests that the number of active surface centers available for adsorption increase with increasing temperature. In surfaceadsorption studies, the relationship between the solution concentration and the species uptake can be described in terms of either a Freundlich-type or a Langmuir-type isotherm; therefore, the data were evaluated using linear and nonlinear Langmuir and Freundlich isotherms.47,48 Figures 3 and 4 show the applicability of both models over a wide range of concentrations. A detailed analysis of the regression coefficients indicated that both the Langmuir and Freundlich models adequately described the adsorption data at different temperatures, but that the data were slightly better fitted by the Freundlich adsorption isotherms. Further, the nonlinear form of the Freundlich adsorption isotherm fitted the data better than the linear form. The Freundlich and Langmuir parameters as calculated
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Figure 4. Adsorption of rosaniline hydrochloride on fly ash at different temperatures. The solid lines represent fits to the data by the (a) Freundlich and (b) Langmuir isotherms. Table 1. Nonlinear Freundlich Isotherm Constants for Crystal Violet and Rosaniline Hydrochloride Adsorption on Fly Ash at Different Temperatures crystal violet temp (°C)
KF × 103 (mol/g)
1/n
10 25 40
0.048 5.00 5.40
1.05 0.37 0.50
rosaniline hydrochloride R2
KF × 103 (mol/g)
1/n
R2
0.9799 0.9649 0.9760
3.10 3.30 3.43
0.54 0.47 0.65
0.4905 0.8930 0.8930
Table 2. Nonlinear Langmuir Isotherm Constants for Crystal Violet and Rosaniline Hydrochloride Adsorption on Fly Ash at Different Temperatures crystal violet temp Q0 × 106 b × 10-5 (°C) (mol/g) (L/mol) 10 25 40
9.70 9.76 9.81
0.62 2.85 2.88
rosaniline hydrochloride R2 0.8355 0.9409 0.9787
Q0 × 106 b × 10-5 (mol/g) (L/mol) 1.30 1.35 1.41
4.18 4.29 9.70
R2 0.4243 0.8879 0.8621
using the nonlinear forms of the equations at different temperatures are given in Tables 1 and 2, respectively. It can be seen from Table 2 that the values of Q0 are found to be higher in the case of the fly ash-rosaniline hydrochloride system. The values of the Freundlich constant, KF, follow the same pattern as Q0 because KF also indicates the relative adsorption capacity. 3.2.4. Effect of Particle Size. The effects of the adsorbent particle size on the uptake rates of crystal violet and rosaniline hydrochloride (basic fuchsin) are
Figure 5. Adsorption of crystal violet on fly ash at different particle sizes of adsorbent. The solid lines represent fits to the data by the (a) Freundlich and (b) Langmuir isotherms.
depicted in Figures 5 and 6, respectively, and the data are presented in Tables 3 and 4, respectively. The uptakes of both dyes on the fly ash increase with decreasing adsorbent particle size. This relationship clearly demonstrates the advantage of using powdered adsorbent rather than the granular form from a kinetic viewpoint, as it indicates that external transport limits the rate of adsorption in these cases. A detailed analysis of the regression coefficients showed that both the Langmuir and Freundlich models adequately described the adsorption data, but that the data were better modeled by the Freundlich isotherm. Further, the nonlinear forms correlated the adsorption data better than the linear forms. 3.2.5. Effect of Adsorbent Concentration. The effects of the amount of adsorbent on the rates of uptake of crystal violet and rosaniline hydrochloride (basic fuchsin) were also studied (figures omitted for brevity). The uptake of the dyes increases with increasing amount of adsorbent material (Tables 5 and 6). In the case of crystal violet, the adsorption capacity increases slightly when the dose of fly ash increases from 5 to 10 g/L, but there is a significant increase upon the further addition of 5 g/L. In the case of rosaniline hydrochloride, there is a substantial increase in adsorption when the fly ash dose is doubled, whereas the increase upon the introduction of additional fly ash is insignificant. With this result in mind, the amount of fly ash was kept at 10 g L-1 in all of the adsorption studies. Nonlinear and linear forms of both the Freundlich and Langmuir
Ind. Eng. Chem. Res., Vol. 41, No. 15, 2002 3693 Table 5. Nonlinear Langmuir Isotherm Constants for Crystal Violet and Rosaniline Hydrochloride Adsorption on Fly Ash at Different Adsorbent Doses crystal violet adsorbent Q0 × 106 b × 10-5 dose (g/L) (mol) (L/mol) 5 10 15
8.02 8.97 11.59
1.37 2.86 2.15
rosaniline hydrochloride R2
Q0 × 106 b × 10-5 (mol) (L/mol)
0.9749 0.9385 0.8933
10.84 13.47 14.25
7.75 4.29 9.61
R2 0.9234 0.9347 0.9405
Table 6. Nonlinear Freundlich Isotherm Constants for Crystal Violet and Rosaniline Hydrochloride Adsorption on Fly Ash at Different Adsorbent Doses crystal violet adsorbent dose KF × 103 (g/L) (mol) 5 10 15
5.10 5.40 17.5
1/n
rosaniline hydrochloride R2
KF × 103 (mol)
1/n
R2
3.70 6.81 6.94
0.49 0.54 0.48
0.9568 0.8895 0.9601
0.42 0.9927 0.37 0.9613 0.47 0.9613
Table 7. Thermodynamic Parameters for the Adsorption of Crystal Violet and Rosaniline Hydrochloride on Fly Ash dye crystal violet rosaniline hydrochloride
Figure 6. Adsorption of rosaniline hydrochloride on fly ash at different particle sizes of adsorbent. The solid lines represent fits to the data by the (a) Freundlich and (b) Langmuir isotherms. Table 3. Nonlinear Freundlich Parameters for Crystal Violet and Rosaniline Hydrochloride Adsorption on Fly Ash at Different Particle Sizes crystal violet particle size KF × 103 (BBS mesh) (mol/g) >16 16-30 30-72
9.00 5.00 7.30
rosaniline hydrochloride R2
1/n
0.48 0.9503 0.37 0.9649 0.56 0.7102
KF × 103 (mol/g)
1/n
R2
39.0 5.4 2.9
0.68 0.52 0.47
0.9720 0.9425 0.9304
Table 4. Nonlinear Langmuir Isotherm Constants for Crystal Violet and Rosaniline Chloride Adsorption on Fly Ash at Different Particle Sizes crystal violet Q0 ×
particle size b× (BBS mesh) (mol/g) (L/mol) >16 16-30 30-72
106
7.14 9.04 9.52
10-5
1.17 2.85 10.90
rosaniline hydrochloride R2
0.9342 0.9409 0.9409
Q0 × 106 b × 10-5 (mol/g) (L/mol) 1.20 1.35 1.44
5.05 4.29 5.71
R2 0.9267 0.8878 0.8904
isotherms also fitted the adsorption data quite well over the broad range of concentrations. As for the effects of temperature and particle size, the nonlinear Freundlich adsorption isotherm modeled the data better than the Langmuir adsorption isotherm. Overall, it can be concluded that the nonlinear Freundlich adsorption isotherm fitted the data better in all cases as compared to the Langmuir adsorption isotherm. The essential characteristic of the Langmuir isotherm can be expressed in terms of a dimensionless constant
-∆G (kJ mol-1) 10 °C 25 °C 40 °C 25.96 30.45
31.18 32.13
32.71 35.87
∆H (kJ mol-1)
∆S (J mol-1 K-1)
36.51 21.35
209.64 173.83
separation factor, RL, as defined by Weber and Chakravorti.40 This constant was found to be
