Ind. Eng. Chem. Res. 1997, 36, 2207-2218
2207
Equilibrium Uptake, Sorption Dynamics, Process Optimization, and Column Operations for the Removal and Recovery of Malachite Green from Wastewater Using Activated Carbon and Activated Slag Vinod K. Gupta,* Suresh K. Srivastava, and Dinesh Mohan Chemistry Department, University of Roorkee, Roorkee 247 667, U.P., India
The waste slurry generated in fertilizer plants and slag (blast furnace waste) have been converted into low-cost adsorbents, activated carbon and activated slag, respectively, and these are utilized for the removal of malachite green (a basic dye) from wastewater. In the batch experiments, parameters studied include the effect of pH, sorbent dosage, adsorbate concentration, temperature, and contact time. Kinetic studies have been performed to have an idea of the mechanistic aspects and to obtain the thermodynamic parameters of the process. The uptake of the dye is greater on carbonaceous material than on activated slag. Sorption data have been correlated with both Langmuir and Freundlich adsorption models. The presence of anionic surfactants does not affect the uptake of dye significantly. The mass transfer kinetic approach has been applied for the determination of various parameters necessary for the designing of fixed-bed contactors. Chemical regeneration has been achieved with acetone in order to recover the loaded dye and restore the column to its original capacity without dismantling the same. 1. Introduction Disposal of dyeing industry wastewater pose one of the major problems, because such effluents contain a number of contaminants including acid or base, dissolved solids, toxic compounds, and color. Out of these, color is the first contaminant to be recognized because it is visible to the human eye. Removal of many dyes by conventional waste treatment methods is difficult since these are stable to light and oxidizing agents and are resistant to aerobic digestion. Possible methods of color removal from textile effluents include chemical oxidation, froth flotation, adsorption, coagulation, etc. Among these, adsorption currently appears to offer the best potential for overall treatment, and it can be expected to be useful for a wide range of compounds, more so than any of the other listed processes. Recognizing the high cost of activated carbon, many investigators have studied the feasibility of cheap, commercially available materials as its possible replacements. Such materials range from industrial waste products such as waste rubber tyres, blast furnace slag, and lignin to agricultural products such as wool, rice straw, coconut husk, saw dust, and peat moss. Various workers have exploited substances such as peat (Allen et al., 1989; Mckay et al., 1981; Mckay and Allen, 1981), bagasse pitch (Mckay et al., 1991; Al Duri et al., 1990), fuller’s earth (Mckay et al., 1985), coal (Mittal and Venkobachar, 1993; Gupta et al., 1988), wool carbonizing waste (Perinea et al., 1983), maize cob, (ElGeundi, 1991), clays (Khare et al., 1988), activated carbon (Leitao et al., 1992), MgCO3 (Judkins and Homsky, 1978), zeolites (Handreck and Smith, 1988), fibers (Yoshuda et al., 1993), etc. for this purpose. Other adsorbents used for dye removal from wastewater are described in a review article by Lambert and Graham (1989). Fertilizer plants generate a waste slurry due to liquid fuel combustion, and this causes a disposal problem. The waste slurry is converted into a cheap carbonaceous * Corresponding author. Telephone: 0091-1332-74458 (residence); 0091-1332-65801 (office). FAX: 0091-1332-73560. E-mail:
[email protected]. S0888-5885(96)00442-3 CCC: $14.00
adsorbent and used for the removal of metal ions and phenols (Srivastava et al., 1987, 1989). Steel plants produce granular blast furnace slag as a byproduct, and this material also causes a disposal problem. Presently this is being used as a filler. Efforts have been made to convert this waste into a potential and low-cost adsorbent. Malachite green is a broadly used dye in the textile industry that has properties that make it difficult to remove from solutions. It is also used in leather industries and distilleries for coloring purposes (Gupta, 1996). In order to assess the ability of activated slag and carbon for dye removal, malachite green (MG) has been selected for the present study. 2. Material and Methods All reagents used were of AR grade. Stock solution of the dye was made using doubly distilled water. The structure of the dye used [basic green 4 or malachite green (C.I. No. 42000)] is
2.1. Equipment. pH measurements were made on a pH meter, Model CT No. CL46, Toshniwal, India. IR spectra were recorded on an infrared spectrophotometer, (FTIR) Perkin Elmer Model 1600. X-ray measurements were made by a Phillips X-ray diffractophotometer employing nickel filtered Cu KR radiation. Surface area of the samples was recorded by a Quantasorb Model QS-7 surface area analyzer. The porosity and density of the adsorbents were determined by a mercury poro© 1997 American Chemical Society
2208 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
simeter and specific gravity bottles, respectively. The chemical constituents of slag and carbon were obtained by the chemical analysis of samples (Vogel, 1989). Scanning electron microscopy was performed using a Phillips SEM 501 electromicroscope. Absorbance measurements were made on a DU 6 spectrophotometer. 2.2. Material Developments. 2.2.a. Preparation of Activated Carbon. The waste material from National Fertilizer Limited (NFL) Bhatinda, India, was in the form of small, spherical, black, greasy granules. As reported earlier (Srivastava et al., 1987), this waste was treated with hydrogen peroxide to oxidize the adhering organic material and then heated to 200 °C until the emission of black soot stopped. The product was cooled and activated in air in an ordinary furnace at 450 °C for 1 h. These conditions were optimized by activating the raw material for different intervals of time at varying temperatures and by observing the surface properties of the product. The product obtained at a temperature higher or lower than 450 °C exhibited poor adsorption capacity probably (at higher temperature) due to collapse of surface functional groups. This material was treated with 1.0 M HCl solution to remove the ash content, and finally dried at 100 °C. The adsorbent thus obtained was broken into smaller particles and sieved to desired particle size such as 100150, 150-200, and 200-250 mesh and were stored in a vacuum desiccator until required. 2.2.b. Preparation of Activated Slag. The waste obtained from Tata Iron and Steel Company, Limited, Jamshedpur (India), was in the form of small, spherical, soft granules. This was washed with distilled water to remove the adhering impurities and dried at 200 °C. The heated product was cooled and activated in air in a muffle furnace at 600 °C for 1 h. The temperature and time were optimized by observing the surface properties of the activated product obtained by treating the raw material for different intervals of time at varying temperatures. Treatment at a temperature higher or lower than 600 °C provided material having poor adsorption capacity. The product so obtained was sieved to desired particle size such as 100-150, 150200, and 200-250 mesh. Finally, products were stored in a vacuum desiccator until required. 2.3. Adsorption Studies. Sorption studies were performed by the batch technique to obtain rate and equilibrium data. For isotherm studies a series of 50 mL test tubes were employed. Each test tube was filled with 10 mL of dye solution of varying concentrations and maintained at the desired pH and temperature. A known amount of adsorbent was added into each tube and agitated intermittently for a maximum period of 24 h. A 6-8 h reaction period was found to be quite sufficient for equilibrium attainment for the dye. After this period the supernatant solution was centrifuged and the uptake of the dye was monitored spectrophotometrically at 425 nm. Observations were recorded by taking 1.0 × 10-4 to 1.0 × 10-3 M concentration. These concentrations were however decided after a good deal of preliminary investigation wherein the adsorbent was found to remove the dye to different extent. Sorption studies were carried out at 30, 40, and 50 °C to find out the effect of temperature. The effect of pH was observed by studying the adsorption of dye over a pH range of 2-10. The effect of an anionic detergent (Manoxol-1B) on the adsorption of dye was studied as a function of concentration of detergent at optimum pH. Stoppered glass tubes containing adsorbate solution along with the
Table 1. Chemical Constituents and Characteristics of Adsorbents constituents
percentage by weight
constituents
C Al Fe silica ash
(a) Activated Carbon 90-92 loss on ignition 0.4-0.6 porosity 0.6-0.8 surface area (m2 g-1) nil density (g/cm3) nil
CaO SiO2 S MgO MnO Al2O3
30.47 30.77 0.85 9.95 0.59 23.30
(b) Activated Slag FeO loss on ignition porosity surface area (m2 g-1) density (g/cm3)
percentage by weight 12.07 78.0 629 1.30
0.54 6.23 67.50 107 2.36
detergent and fixed amount of adsorbent (10 g L-1) were equilibrated for 24 h. The supernatant liquid was centrifuged and analyzed for the dye uptake. 2.4. Kinetic Studies. For kinetic investigations the bath technique was selected because of its simplicity. A number of stoppered Pyrex glass tubes (50 mL capacity), containing 10 mL solution of malachite green of known concentration, were placed in a thermostat cum shaking assembly. When the desired temperature was reached, 0.1 g of adsorbent was added into each tube and the solutions were agitated by mechanical shaking. At predecided intervals of time, the solutions of the specified tubes were separated from the sorbent material and analyzed for the uptake of dye. Equilibrium was attained in about 6-8 h. 2.5. Column Studies. Adsorption isotherms have traditionally been used for preliminary investigations and fixing the operational parameters. But in practice the final technical systems normally use column type operations. Moreover, the isotherms cannot give accurate scaleup data in a fixed bed system, so the practical applicability of products in column operations has also been investigated to obtain a factual design model. A glass column (40 × 0.5 cm) was filled with activated slag and activated carbon (mesh size 200-250) on a glass wool support. A weighed quantity of adsorbent was made into a slurry with hot water and fed slowly into the column, displacing a heel of water as outlined by Fornwalt and Hutchins (1966). The technique avoids air entrapment. The column was loaded with the appropriate solution which percolated downward, under gravity at a flow rate of 0.4 mL min-1. The operation on the column was stopped when 90% of the capacity was used up. 2.6. Column Regeneration. Recovery of the adsorbate material as well as the regeneration of adsorbent is an important process in wastewater treatment. Consequently, experiments have been carried out in which adsorbents were loaded with malachite green and subjected to elution of dye with acetone. Simultaneous regeneration of adsorbents was also thought of. 3. Results and Discussion 3.1. Characterization. The different chemical constituents of activated carbon and activated slag are given in Table 1 along with some other characteristics. Surface area of the samples activated in air is 629 and 107 m2 g-1 for activated carbon and activated slag, respectively. X-ray spectra of both adsorbents do not show any peak indicating the amorphous nature of activated carbon and activated slag.
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2209
Figure 1. SEM photographs of activated carbon at different magnifications: (a) 1 × 640; (b) 1 × 320; (c) 1 × 160.
Scanning electron microscopic (SEM) photographs of activated carbon (Figure 1) clearly reveal the surface texture and porosity of the sample activated in air. The photomicrographs also show that the particles can be roughly approximated as spheres if the roughness factor is included to account for the irregularities. Similarly SEM photographs of activated slag (Figure 2) at different magnifications clearly reveal the porous texture of the product. As shown in photographs, many small holes are seen on the surface of slag which has largest volume of foam. The slag was so porous that it could be crushed easily by hand.
Figure 2. SEM photographs of activated slag at different magnifications: (a) 1 × 640; (b) 1 × 320; (c) 1 × 160.
IR spectra (Figure 3) of activated carbon indicated weak and broad peaks in the region of 1800-1600 cm-1. The band at 1700 cm-1 corresponds to a normal carbonyl group (Alpert et al., 1970) while the one at 1605 cm-1 may be due to conjugated hydrocarbon bonded carbonyl groups as suggested by Hallum and Drushal (1958) and Freidal and Queiser (1956). Although some inference can be drawn about the surface functional groups from IR spectra, the weak and broad bands do not provide any authentic information about the nature of surface oxides. The data, however, indicate the presence of some surface groups on the adsorbent material activated in air.
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Figure 3. IR spectra of activated carbon and activated slag.
Figure 4. Effect of pH on adsorption of malachite green on activated carbon and activated slag.
Infrared spectra of activated slag (Figure 3) indicated broad and weak peaks in the region 4000-500 cm-1. The adsorption bands in the region 3700-3500 cm-1 are assigned to a free hydroxyl group while the band at 3622 cm-1 indicates the presence of interlayer hydrogen bonding. The peaks at 3427, 3290, 3095-3050, and 2985-2883 cm-1 indicate the presence of nordstranite, brucite, bochmite, and diaspore. The band at 2857 cm-1 confirms the presence of γ-FeOOH while those at 937 and 733 cm-1 suggest the presence of braumite and goethite in the sample (Gadsen, 1975). 3.2. Sorption Studies. The change in adsorption of the dye over a pH range of 2-10 is depicted in Figure 4. Malachite green (pKa ) 10.3), becomes protonated in the acidic medium and deprotonation takes place at higher pH. Consequently, the positive charge density would be more on the dye molecule at lower pH; this accounts for the higher uptake on the negatively charged surface of carbonaceous adsorbent. A fall in adsorption with increasing pH (1 r)1 05 × 10-5). It has also been possible to regenerate the adsorbents and to quantitatively recover the dye with acetone. The developed adsorbents are quite cheaper than commercially available carbons while their performance is comparable. Acknowledgment D.M. is highly thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, for awarding a Research Associateship to undertake this work.
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Nomenclature B ) time constant b ) Langmuir constant (L mol-1) C ) mass concentration of solute in the effluent Ct ) concentration of adsorbate (mg L-1) after time t C0 ) initial concentration of adsorbate (mg L-1) Ci ) effluent concentration D0 ) preexponential factor Di ) effective diffusion coefficient d ) average distance between two successive sites of adsorbent dp ) particle diameter (cm) Ea ) energy of activation Fm ) mass rate of flow to the adsorber F ) fractional attainment f ) fractional capacity ∆G° ) Gibb’s free energy (kJ mol-1) ∆H° ) enthalpy change of the process (kJ mol-1) h ) Planck’s constant k ) Boltzmann constant Kad ) rate constant (min-1) K, K1, K2 ) Langmuir constants at 30, 40, and 50 °C KF ) Freundlich constant of solute (mol g-1) qe ) amount of adsorbate adsorbed per gram of adsorbent (mol g-1) at equilibrium qt ) amount of adsorbate adsorbed per gram of adsorbent (mol g-1) at any time t Q° ) Langmuir monolayer capacity (mol g-1) R ) gas constant r ) radius of the adsorbent particle assumed to be spherical ∆S# ) entropy of activation Ss ) outer surface of the adsorbent per unit volume of particle free slurry (cm-1) ∆S° ) entropy of adsorption (J K-1 mol-1) Ms ) amount of adsorbate adsorbed in PAZ from breakpoint to exhaustion m ) mass of adsorbent per unit volume of particle free adsorbate solution (g L-1) n ) Freundlich constant of solute (dimensionless) tx ) total time involved for the establishment of primary adsorption zone tδ ) time for primary adsorption zone to move down its length tf ) time of initial formation of PAZ V ) volume of particle free adsorbate solution (L) Ve ) total quantity of solute free water W ) weight of adsorbent (g) ZPC ) zero point charge βL ) mass transfer coefficient (cm s-1) δ ) length of the primary adsorption zone p ) porosity of adsorbent particles (%) Fp ) density of adsorbent (g cm-1) BSS ) British standard size
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Received for review July 25, 1996 Revised manuscript received January 23, 1997 Accepted January 27, 1997X IE960442C Abstract published in Advance ACS Abstracts, April 1, 1997. X
