Ind. Eng. Chem. Res. 2005, 44, 3655-3664
3655
Removal of Dyes from Wastewater Using Bottom Ash V. K. Gupta,*,† Imran Ali,‡ V. K. Saini,† Tom Van Gerven,§ Bart Van der Bruggen,§ and Carlo Vandecasteele§ Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India, National Institute of Hydrology, Roorkee 247 667, India, and Department of Chemical Engineering, Katholieke Universiteit Leuven, Leuven, Belgium
Municipal solid waste incinerator (MSWI) bottom ash was converted into a low-cost adsorbent, characterized and used for the removal of alizarin yellow, fast green, and methyl violet from wastewater. The percentages of removal of alizarin yellow (1.0 × 10-4 M), fast green (1.0 × 10-4 M), and methyl violet (1.0 × 10-4 M) on this adsorbent were 87.5, 97.0, and 73.0, respectively. In batch experiments, parameters studied include the effect of the adsorbate concentration, pH, sorbent dosage, temperature, and contact time. The optimum contact time for all of the three dyes has been found to be 4 h, while the maximum adsorption was observed at pH 5.5, 5.0, and 8.0 for alizarin yellow, fast green, and methyl violet, respectively. The adsorbent dose has been optimized as 10.0 g/L for the three dyes. Kinetic studies showed a rate of adsorption of first order with respect to the dye solution concentration. Adsorption processes were found to be film-diffusion-controlled for all of the three dyes. Thermodynamic parameters such as free energy, enthalpy, and entropy of adsorption of the dye-bottom ash systems were also evaluated. 1. Introduction Many industries release dyes into wastewater, thus contaminating water resources. Relevant industries are textile companies, dye manufacturing industries, paper and pulp mills, tanneries, electroplating factories, distilleries, food companies, etc. Some of the dyes are stable to light and oxidation and resistant to aerobic digestion. Basic dyes are the brightest class of soluble dyes used by the textile industries. The total dye consumption of the textile industry worldwide is more than 10 000 tonnes/year, with an estimated 90% in fabrics industries. It is reported that approximately 100 tonnes of dyes are discharged into waste streams by the textile industry per year.1 Dye producers and users are interested in the stability and fastness of dyes and consequently produce dyes that are more difficult to remove from wastewater after use.1 A range of conventional treatment technologies for dye removal have been investigated extensively,2-9 and these are activated sludge, chemical coagulation, carbon adsorption, electrochemical treatment, reverse osmosis, hydrogen peroxide catalysis, etc. However, most of the above methods suffer from one or another limitation, and none of them were successful at economical levels.10 Adsorption can, in principle, handle fairly large flow rates and produces a high-quality effluent that does not result in the formation of harmful substances, such as ozone and free radicals that are formed during the photodegradation process using UV radiation. Moreover, adsorption is universal, inexpensive, and fast in nature. A literature survey reveals that materials such as commercially available activated carbon11-14 and zeo* To whom correspondence should be addressed. Tel.: 00911332-274458. Fax: 0091-1332-273560. E-mail: vinodfcy@ iitr.ernet.in. † Indian Institute of Technology Roorkee. ‡ National Institute of Hydrology. § Katholieke Universiteit Leuven.
lites15 have been used in the past for the treatment of textile effluents. In addition, a number of nonconventional sorbents have been used for the removal of chemical pollutants, such as peat,16,17 chitin,18 apple promace, wheat straw,19 sulfonated coal,20 organomontmorillonite,21 coir pith,22 slag from the manufacture of steel,23 fly ash,22,23 and activated slag from fertilizer plants.24 Recently, Gupta and Ali25 have reviewed the utility of low-cost adsorbents for the removal of various pollutants including dyes. Earlier publications report the development of lowcost activated carbon/adsorbent for the removal and recovery of organic and inorganic pollutants including dyes from wastewater.25-28 In this paper, the ability of MSWI-generated bottom ash to remove commercial dyes, viz., Alizarin Yellow R. (CI 14030) [Chrome Orange; Mordant Orange 1], Fast Green FCF (CI 42053) [Food Green 3], and Methyl Violet 2B (CI 42535) [Gentian Violet], is presented. The equilibrium sorption capacities of dyes on MSWI bottom ash have been studied using the adsorption isotherm technique. The results of this research are presented herein.
2. Materials and Experimental Methods 2.1. Materials. 2.1.1. Adsorbent. The adsorbent used in this study was obtained from a municipal solid waste incinerator (MSWI), where solid waste is incinerated at elevated temperature and bottom ash is obtained as a waste product in large quantities. A representative sample of the bottom ash was collected from a local MSWI from Belgium. The bottom ash already underwent an onsite washing and sieving treatment. The fraction with particle sizes of 0.1-2 mm is used in this study. The bottom ash was in the form of small, spherical, grayish-black, uneven-sized particles. The raw bottom ash was dried at 105 °C for 24 h and was further activated at 500 °C in a furnace for 6 h. Heating
10.1021/ie0500220 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/09/2005
3656
Ind. Eng. Chem. Res., Vol. 44, No. 10, 2005
of the bottom ash to 500 °C had an additional advantage besides activation because leaching of heavy metals from the adsorbent decreases to very low concentrations.29 To see the affect of heating above 500 °C on the adsorption capacity of the adsorbent, it was heated at more than 500 °C, and no improvement was observed in the adsorption capacity. The activated bottom ash was then crushed into smaller particles and sieved to obtain particles of sizes 100-150, 150-200, 200-250, and >250 BSS mesh. Finally, the product was stored in vacuum desiccators until required for use. The cationexchange capacity (CEC) was measured to evaluate the adsorption capacity, and it was determined by the ammonium acetate method.30 A total of 4 g of air-dried ash was saturated with 33 mL of 1.0 M sodium acetate (pH 8.2) four times separately and three times with 33 mL of 99% ethanol separately and individually. The adsorbed sodium (Na+) was extracted with three aliquots of 33 mL of 1.0 M ammonium acetate (pH 7.0) separately. These three aliquots were mixed together to get 99 mL, which was brought to 100 mL by adding 1.0 M ammonium acetate. This extracted mixtrure was analyzed by atomic absorption spectroscopy to calculate the concentration of Na
CEC (mequiv/100 g) ) mequiv/L of Na (mmol/L) × A 100 Wt 1000 where A is the total volume of the extract and Wt is the weight of air-dried bottom ash. 2.1.2. Adsorbates. Alizarin yellow, fast green, and methyl violet were obtained from Aldrich Chemical Co. (Milwaukee, WI). All other reagents were of analytical reagent grade and were also obtained from Aldrich Chemical Co. and used without any further purification. A calculated amount of each dye was dissolved separately in 1 L of deionized water to prepare stock solutions, which were kept in dark colored glass bottles. For batch study, aqueous solutions of these dyes (1.2 × 10-4-1.0 × 10-5 M) were prepared from stock solutions in deionized water. The λmax values of alizarin yellow, fast green, and methyl violet were measured at 385, 628, and 584 nm, respectively. 2.2. Equipment for Concentration Measurement and Calibration. pH measurements were made using a pH meter (Hach Co., Loveland, CO). An atomic absorption spectrophotometer (model 3100; PerkinElmer, Wellesley, MA) was used to determine sodium (Na+) in an ammonium acetate method for CEC calculation. Deionized water was prepared using a Millipore Milli-Q (Bedford, MA) water purification system. X-ray measurement was performed using a Philips X-ray diffractometer employing Ni-filtered Cu KR radiation and Ni filters. A Leo 435 VP model was used for scanning electron microscopy (SEM). The surface area of the adsorbent was measured with a Quantasorb surface area analyzer (model QS-7; Quantachrome, Ontario, Canada). The porosity of the adsorbent was determined with a mercury porosimeter (AutoPore II 9220; Micromeritics, Norcross, GA), and the density was determined with a specific gravity bottle. The chemical constituents of MSWI bottom ash were analyzed using routine methods.31,32 All color measurements were made using a Specord 200 UV-visible spectrophotometer (model UV-vis 1601) in the visible range in the absorbance mode. Absorbance values were recorded at the wavelength of maximum absorbance (λmax) for each dye.
The concentration of each dye was measured in a 1-cmpath-length cell, with an absorbance accuracy of (0.005 at λmax of the dye. Absorbance was found to vary linearly with concentration, and dilutions were carried out when the absorbance exceeded 0.8. Biological degradation of the dyes was also taken into account by storing different concentration solutions of used dyes at different time intervals and different temperatures. It was observed that there was no change in the concentration and λmax values even after 24 h, illustrating the stability of these dyes. Standard plots were made as a function of the concentration. 2.3. Equilibrium Sorption Studies. Sorption studies were performed in batch experiments to obtain rate and equilibrium data. The batch technique was selected because of its simplicity. Batch sorption studies were performed at different temperatures, particle sizes, adsorbent doses, and pHs to obtain the equilibrium isotherm. For isotherm studies, a series of solutions in 100-mL conical flasks were used. Each conical flask was filled with 50 mL of a dye solution at varying concentrations (1.2 × 10-4-1.0 × 10-5 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 are discussed in the Results and Discussion section of the work using the batch approach. The slurries were centrifuged at 2200rpm speed for 15 min to remove the adsorbent from the solution. The solutions were then spectroscopically analyzed at the corresponding λmax for the concentration of the dye remaining in the solution. The dye concentration retained in the adsorbent phase was calculated using
qe ) [(C0 - Ce)V]/W where C0 and Ce are the initial and equilibrium concentrations (M), respectively, of the dye in solution, V is the volume (L), and W is the weight (g) of the adsorbent. The developed method was applied for the removal of these dyes from wastewater. Wastewater was collected from municipal effluent of Roorkee, India. Wastewater was filtered through Whatman 24 filter paper. The physicochemical analyses of wastewater carried out involved pH, calcium, potassium, chloride, sulfate, nitrates, etc. The reported dyes (1.0 × 10-4 M for each) were spiked with the filtered wastewater for batch studies. 2.4. Adsorption Models. To determine the mechanistic parameters associated with adsorption, the results of the adsorption experiments were analyzed using the models of Langmuir and Freundlich. 2.5. Method Validation. Validation of the developed method was carried out using three sets (n ) 3) of experiments under identical conditions. The regression analysis was carried out using Microsoft Excel. The correlation coefficient for different experiments has been calculated to find out the relationship between two involved variables. The linearity range was from 1 × 10-5 to 9 × 10-5 M. The correlation coefficient (R2) and confidence level for the experimental data were found to be 0.975 ( 0.022 and 98.7%. Similarly, the standard deviations for the adsorption method and experiments were found (0.14 and (0.10, respectively.
Ind. Eng. Chem. Res., Vol. 44, No. 10, 2005 3657
Figure 1. XRD spectrum of MSWI bottom ash. Table 1. Chemical Composition of Bottom Ash (Units in wt %) composition particle size (mesh)
SiO2
Al2O3
TiO2
Fe2O3
MgO
CaO
Na2O
K2O
MnO
P 2 O5
250
32.8 27.9 27.8 18.8 16.0
7.7 11.3 9.9 9.4 8.5
1.1 1.5 2.0 2.4 2.2
11.9 7.3 4.0 4.6 3.2
2.1 2.6 3.3 3.3 4.6
21.4 23.5 25.9 27.3 30.4
4.2 3.3 3.3 1.5 2.7
1.6 1.8 1.8 1.7 1.6
0.1 0.1 0.2 0.2 0.4
6.4 6.0 6.9 6.7 6.1
3. Results and Discussion 3.1. Characterization of the Adsorbent Material. The different chemical constituents of bottom ash are given in Table 1. The surface area, density, and porosity of 150-200 BSS mesh size are 14.10 m2/g, 1.22 g/cm3, and 0.3%, respectively. The X-ray diffraction (XRD) spectrum of bottom ash (Figure 1) shows that it consists of oxides, silicates, and aluminosilicates of calcium, iron, and other bases. Oxides have a tendency to form metal hydroxide complexes in solution and subsequently dissociate at the solid solution interface, giving an alkaline pH. The concentrations of CaO, P2O5, MgO, and TiO2 decreased with increasing grain size, while the concentrations of Fe2O3 and Al2O3 increased with decreasing grain size33 (Table 1). SEM images of the activated bottom ash (Figure 2) at four different magnifications helped in recognizing the texture of aluminosilicates, distributed with the heavy constituent of iron. SEM photomicrographs clearly reveal the porous texture of the product. It is clear that bottom ash particles are in the form of indefinite shape and of a wide range of sizes. As shown in the images, many small holes are seen on the surface of bottom ash, which have the largest volume of foam. The weight fraction, CEC, and leachate pH of the different particle size fractions of bottom ash are summarized in Table 2. CEC is the total amount of exchangeable cations. It is expressed in units of milliequivalents per 100 g of dry material. The CEC increases with decreasing particle size. Leaching con-
Table 2. CEC, pH, and Weight Fraction (%) Distribution with the Particle Size of Bottom Ash particle size (mesh)
weight fraction (%)
CECa (mequiv/100 g)
pH
100-150 150-200 200-250 >250
52.0 29.8 6.7 3.9
8.0 15.3 21.1 23.0
12.3 12.3 12.5 12.6
a
CEC ) cation-exchange capacity.
centrations of heavy metals decreased in the order of Cr (0.56 mg/kg), Zn (0.14 mg/kg), Cu (0.03 mg/kg), and Pb (0.02 mg/kg) in bottom ash.29 The final pH value of the leachate was between 12.3 and 12.6 in all ranges of particle sizes (Table 2). The high pH value is attributed to the leaching of alkalis, such as Ca(OH)2. 3.2. Sorption Studies. 3.2.1. Effect of the Contact Time. The effect of the contact time on the adsorption of alizarin yellow, fast green, and methyl violet was studied at given conditions (Figure 3). The results indicate that equilibrium was achieved within 4 h for all three dyestuffs. Further, it showed that the rates of uptake of all three dyes were fast in the beginning, with about 50% of the ultimate adsorption within 1.5 h of contact time. This optimum contact time, i.e., 4 h, has been selected for all further studies. 3.2.2. Effect of the Concentration of Adsorbates. The effect of the amount of adsorbate on the rate of uptake of dyes was studied at various concentrations in given conditions (Figure 4). The amounts of dyes
3658
Ind. Eng. Chem. Res., Vol. 44, No. 10, 2005
Figure 2. SEM images of activated MSWI bottom ash at different magnifications: (a) 4000×; (b) 2280×; (c) 500×; (d) 90×.
Figure 3. Effect of the contact times on the rates of adsorption of alizarin yellow, fast green, and methyl violet on MSWI bottom ash.
Figure 4. Effect of the concentrations of alizarin yellow, fast green, and methyl violet on the adsorption on MSWI bottom ash.
adsorbed at equilibrium were plotted as a function of the initial concentration, showing that the adsorption of alizarin yellow reaches 98% at lower concentrations (
