Ind. Eng. Chem. Res. 2003, 42, 6619-6624
6619
SEPARATIONS Removal of Zinc from Aqueous Solutions Using Bagasse Fly Ash a Low Cost Adsorbent Vinod K. Gupta* and Saurabh Sharma Department of Chemistry, Indian Institute of Technology, Roorkee, Roorkee 247 667 (U.A.) India
Bagasse fly ash, a sugar industry waste, has been converted into an inexpensive and efficient adsorbent. The product obtained has been characterized and utilized for the removal of zinc from aqueous solutions over a wide range of initial metal ion concentration (3.06 × 10-4 to 3.06 × 10-3 M), contact time (24 h), adsorbent dose (5-20 g L-1), and pH (1.0-6.0). The removal of Zn2+ is 100% at low concentrations, whereas it is 60-65% at higher concentrations at an optimum pH of 4.0, using 10 g L-1 of adsorbent in 6-8 h of equilibration time. The uptake decreases with a rise in temperature indicating the process to be exothermic in nature. Kinetic studies have been performed to understand the mechanism of adsorption. The removal takes place through film diffusion mechanism at lower concentrations (e1.84 × 10-3 M) and by particle diffusion at higher concentrations (g 3.06 × 10-3 M). Introduction Environmental pollution by toxic metals is well recognized and can be detrimental to human health and the environment. Metals can be distinguished from other toxic pollutants, because these are nonbiodegradable, may undergo transformations, and can have a large environmental, public health, and economic impact.1 This has led to more stringent legislation and has prompted industry to seek more effective methods of pollution control as the permissible limits are reduced.2 Zinc is a toxic element that can be found in natural water, as well as in various industrial effluents.3 The average human body contains about 2 g of zinc, which is essential for the normal activity of DNA polymerization and for protein synthesis. However, the inhalation of zinc fumes may produce “Zinc Fever”, which is characterized by chills and fevers. Edema of the lungs from fumes of zinc chloride (ZnCl2 smoke) is sometimes fatal. Soluble and astringent acid salts, such as ZnSO4 in large doses (about 10 g), have caused internal organ damage and death. Zinc has a secondary drinking water standard of 5.0 mg L-1 principally because of its bitter metallic taste.3 Among the methods examined for zinc removal from water, adsorption on activated carbon has been shown to be very efficient.4,5 But the use of commercially available activated carbon is limited, especially in developing countries, because of its high cost and difficulties in its regeneration.6 Accordingly, current research is focused on the development of inexpensive materials having high affinity, selectivity, and capacity toward metals. The potential of various agricultural and industrial waste materials has received the most attention for this purpose, which results in 2-fold benefits, * To whom correspondence should be addressed. Tel.: 00911332-285801. Fax: 0091-1332-273560. E-mail: vinodfcy@ iitr.ernet.in.
i.e., the disposal of these wastes and their usage as a low cost adsorbents for the treatment of wastewater. Contributions in this regard have been made by many researchers who have utilized a number of materials such as fly ash,7 peat,8 bentonite,9 lignin,10 charcoal,11 straw,12 blast furnace slag,13,14 biomass,15,16 sludges,17-19 red mud,20,21 and so forth. The sugar industry is one of the most important agrobased industries in India. Bagasse fly ash, a waste material of sugar industries, causes a disposal problem. Currently this fly ash is being used as filler in building materials, but to date no other proper application of this waste has been found. Recently, in our laboratory, bagasse fly ash has been converted into an effective adsorbent and used for the removal of toxic substances.22,23 In continuation, the aim of the present research is to utilize the bagasse fly ash for the removal of zinc from aqueous solutions and the results are discussed in this communication. Materials and Methods All the reagents used were of A. R. grade. Stock solution of zinc (1 × 10-2 M) was prepared by dissolving Zn(NO3)2 in doubly distilled water. Equipment. Atomic absorption spectra were recorded using an atomic absorption/emission spectrophotometer (Perkin-Elmer model 3100) using light source Hollow Cathode; lamp current, 15.00 mA; wavelength, 213.9 nm; slit width, 0.7 nm; sensitivity, 1.0 mg L-1. X-ray measurements were done by a Phillips X-ray diffractometer employing nickel filtered Cu KR radiations. The surface area of the sample was measured by a surface area analyzer (model QS-7; Quantasorb surface area analyzer). pH measurements were made using a pH meter (model CT No. CL 46; Toshniwal, India). Scanning electron microscopy (SEM) was performed using a Phillips SEM 501 electron microscope. The
10.1021/ie0303146 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/12/2003
6620 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
Figure 1. X-ray diffraction pattern of bagasse fly ash.
chemical composition of bagasse fly ash was determined by using a quantitative method of chemical analysis.24 The density of the adsorbent was determined by use of specific gravity bottles. Material Development. Bagasse fly ash, a solid waste, was obtained from a sugar industry establishment at Bijnor (U. P.) India. This material in its untreated form showed poor adsorption properties. Therefore, this material was first treated with hydrogen peroxide 30% w/v (100 volume) at 60 °C for 24 h, till the evolution of bubbles stopped. The pretreatment with hydrogen peroxide perhaps removes heterogeneously distributed organic matter from the bagasse fly ash. Preliminary adsorption studies revealed that treating the waste material with hydrogen peroxide (30% w/v) imparts maximum adsorption characteristics, and, therefore, all investigations were carried out on samples after providing this treatment. The resulting product was then washed with distilled water, dried at 100 °C for 24 h, and sieved to desired particle sizes such as 100150, 150-200, and 200-250 BSS mesh. The finished product exhibited the best adsorption capacity and high surface area. The product obtained at temperatures higher than 100 °C exhibits poor adsorption capacity, probably due to collapse of surface functional groups on adsorbent. Finally, the product was stored in vacuum desiccators until used. Adsorption Studies. Adsorption studies were carried out in a routine manner by batch technique.23 Isotherms were run with zinc solutions of varying concentration (3.06 × 10-4 to 3.06 × 10-3 M), maintained at the desired pH and temperature. Stoppered glass tubes containing metal ion solutions (10 mL) and known amounts of bagasse fly ash (0.1 g; particle size 150-200 mesh) were stirred intermittently for a maximum period of 24 h. The contact time and other conditions were selected on the basis of preliminary studies that showed that the equilibrium was attained in 6-8 h, and beyond this time the adsorption of zinc ions on the adsorbent material remained almost constant. Equilibration for longer times, that is between 8 and 24 h, gave practically the same uptake. After this period the solutions were separated from the adsorbent (centrifuged at 4000 rpm for 15 min) and immediately analyzed to determine the uptake of zinc by the adsorbent. Adsorption studies were carried out at 30, 40, and 50 °C to establish the effect of temperature. The effect of pH was determined by studying the adsorption at a
fixed concentration over a pH range of 1.0-6.0. pH adjustment was made by dilute NaOH and dilute HNO3. The pH was monitored at the beginning and at the end of the run and no noticeable change in pH was observed. Kinetic Studies. The batch technique was selected because of its simplicity and reliability. A number of 50mL stoppered glass tubes containing 10 mL of solution of metal ion of known concentration (1.84 × 10-3 M) were placed in a thermostat cum shaking assembly. At the desired temperature, 0.1 g of bagasse fly ash was added into each tube and the solutions were agitated intermittently. At predecided intervals of time (30 min for the first two tubes and 60 min for all subsequent tubes) the test solutions in the specified tubes were centrifuged at 4000 rpm for 15 min and separated from the adsorbent material. The supernatant was analyzed for aqueous metal. The kinetic studies were also performed at different adsorbate (1.22 × 10-3 to 3.06 × 10-3 M) and adsorbent (5 g L-1 to 20 g L-1) concentrations. Results and Discussion Characterization of the Adsorbent Material. The adsorbent developed from bagasse fly ash was found to be of “L” type in nature as classified earlier,23 i.e., it lowers the pH when kept in deionized water. It is quite stable in water, salt solutions, acid, and bases. Chemical composition of the bagasse fly ash as determined by chemical analysis24 is SiO2, 61.44; Al2O3, 14.50; CaO, 2.82; Fe2O3, 4.86; MgO, 0.71, and loss on ignition, 17.12 wt %. It is necessary to mention that no leaching measurements were done in this work to estimate contamination of solution by solubilized oxides from bagasse fly ash. Leaching studies are out of the scope of the present paper and are now a part of our ongoing and future research. The X-ray diffraction pattern (Figure 1) of the product provided d spacing values which reflect the presence of different minerals, such as goethite, mullite, hematite, kaolinite, γ-alumina, and R-quartz, indicating high chemical stability of the bagasse fly ash.25 Bands observed at 1172, 1157, and 680 cm-1 in the IR spectrum (figure not shown) of bagasse fly ash were attributed to Si-O stretching vibrations, a characteristic of quartz.26 The adsorption bands at 3696 and 3670 cm-1 indicated the presence of kaolinite. A very weak band at 358 cm-1 was due to the presence of hematite. A strong band at 470 cm-1 suggested the presence of calcium silicate. The surface area of the
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Figure 3. Effect of pH on the adsorption of Zn2+ on bagasse fly ash.
Figure 4. Effect of temperature on the adsorption of Zn2+ on bagasse fly ash.
Figure 2. SEM photographs of bagasse fly ash at different magnifications: (a) 1 × 5000; (b) 1 × 2500; and (c) 1 × 121.
adsorbent as calculated by BET method was found to be 470 m2 g-1. The surface area was also worked out theoretically, by methylene blue dye adsorption, taking methylene blue dye molecule area as 197 Å2 and found to be 440 m2 g-1. This value is slightly less than that determined by N2 gas adsorption, and indicates that some pores of bagasse fly ash are not accessible to methylene blue. Scanning electron microscopic (SEM) photographs of bagasse fly ash (Figure 2 (a-c)) clearly reveal the surface texture and porosity of the sample. The material was so porous that it could be crushed easily by hand. The point of zero charge pHzpc (pH of the suspension at which surface charge density σ ) zero) and cation exchange capacity were determined by using potentiometric titration method.27,28 The characteristics
of the adsorbent are pHzpc, 3.8; density, 1.02 g cm3; porosity, 0.36 (fraction); and ion exchange capacity, 1.43 meq g-1. Adsorption Studies. The effect of pH on the uptake of zinc (Figure 3) clearly indicates that the percentage adsorption increases with pH to attain maximum at pH 4.0, and thereafter it decreases with any further increase in pH. The increase in the removal could be related to the surface charges that are strongly dependent on the pH of the solution. The ZPC of the adsorbent (pHzpc) is 3.8. Thus, below a pH of 3.8 the surface will have high positive charge density and under these conditions the uptake of zinc ions would be quite low due to electrostatic repulsion. With increasing pH, i.e., beyond ZPC, the negative charge density on the surface of adsorbent increases, thereby resulting in a sudden enhancement in metal adsorption. A similar behavior has been reported by Sen and De29 for Hg (II) and Gupta et al.23 for lead adsorption on fly ash and bagasse fly ash, respectively. All the adsorption studies were carried out at a pH of 4.0. The adsorption isotherms (Figure 4) are regular, positive, and concave to the concentration axis. The uptake is almost 100% at low concentrations (e 6.12 × 10-4 M), and the adsorption decreases with concentration at 1.84 × 10-3 M and ultimately becomes constant. This indicates the efficiency of the adsorbent for the removal of zinc from wastewater in a wide range of concentrations. Further, it is observed that the adsorption decreased with increase in temperature indicating that the process is exothermic in nature. The adsorption
6622 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 Table 1. Freundlich, Langmuir, and Thermodynamic Parameters for Zn2+ Removal by Bagasse Fly Ash Freundlich constant temp.
n ((0.02)
KF × ((0.005)
30 °C 40 °C 50 °C
0.36 0.44 0.46
3.10 1.23 0.132
105
Langmuir constant Q° ×
104
(mol ((0.01) 2.02 1.97 1.85
g-1)
b×
10-2
thermodynamic parameters mol-1)
(L ((0.02) 9.45 2.19 1.41
data fitted well to Freundlich and Langmuir adsorption isotherms, and the values of Freundlich and Langmuir constants as calculated from these isotherms are listed in Table 1. The adsorption capacity KF decreases as the temperature increases. The slope 1/n, which reflects the intensity of adsorption, presents the same trend. The value of Q° (i.e., maximum uptake) decreases with increase in temperature, thereby indicating exothermic nature of the process. The adsorption capacity of bagasse fly ash for zinc at pH 4.0 and 30 °C was found to be 13.21 mg g-1. The adsorption capacity obtained for bagasse fly ash is found to be comparable to, and in some cases better than, the other adsorbents and activated carbons as reported by Vishwanadham et al.3 for chitin (1.183 mg g-1); Weng and Huang7 for fly ash (0.27-4.64 mg g-1), Nuchar-SN carbon (66 mg g-1), and Filtrasorb-400 carbon (4.0 mg g-1); Gosset et al.8 for peat (11.2 mg g-1); Larsen and Schierup for activated carbon (6.2 mg g-1) and straw (5.3 mg g-1); Lopez et al.20 and Gupta et al.13 for red mud (12.59 mg g-1) and blast furnace slag (37.98 mg g-1), respectively. The dimensionless factor RL as calculated from RL ) 1/1+bCo was found to be 0.37. This indicates a highly favorable adsorption (RL , 1).30 The thermodynamic parameters obtained for this system using equations as described earlier14 are also given in Table 1. The negative free energy values indicate the feasibility of the process and its spontaneous nature without any induction period. The negative value of enthalpy change (∆H°) for the processes further confirms the exothermic nature of the process. Negative entropy of adsorption (∆S°) reflects the affinity of the adsorbent material toward zinc. Kinetic Studies. Preliminary studies on the rate of removal of Zn2+ by bagasse fly ash (at optimum pH and adsorbent concentration) indicated the process to be quite rapid. Nearly 50 to 60% of the adsorption capacity was attained within the first hour of contact. The initial rapid adsorption gives way to a very slow approach to equilibrium, and saturation is reached in 6-8 h (Figure 5). It is also evident from Figure 5 that at low concentration (1.22 × 10-3 M), 96.72% zinc is removed, whereas 92.93% removal was ascertained with the concentration of 1.84 × 10-3 M using 10 g L-1 of the adsorbent dose in 6-8 h. At higher concentration (3.06 × 10-3 M) nearly 62.75% of zinc is removed from the solution under identical conditions of adsorbent dose and contact time. It was also found that at fixed concentration the rate of removal of zinc increases with an increase in the amount of bagasse fly ash. There was a significant increase in the adsorption when the adsorbent amount was increased from 5 g L-1 to 10 g L-1. Any additional amount of bagasse fly ash did not cause any significant change. Thus, 10 g L-1 of bagasse fly ash has been used in all subsequent studies. The above observations can be explained on the basis of the following three consecutive steps, which may be involved in the adsorption of an organic/inorganic species by a porous adsorbent: (a) transport of the adsor-
mol-1)
-∆G° (kJ ((0.05) 17.27 14.03 13.30
-∆H° (kJ mol-1) ((0.05) mean
-∆S° (J mol-1 K-1) ((0.1) mean
76.83
197.95
Figure 5. Effect of adsorbate concentration on the rate of uptake of Zn2+ on bagasse fly ash.
bate to the external surface of the adsorbent (film diffusion); (b) transport of the adsorbate within the pores of the adsorbent except for a small amount of adsorption that occurs on the external surface (particle diffusion); and (c) adsorption of the adsorbate on the exterior surface of the adsorbent. It is generally accepted that process (a) is very rapid and does not represent the rate-determining step in the uptake of adsorbate.23 For the remaining two steps in the overall transport, three distinct cases may occur: case I, external transport > internal transport; case II, external transport < internal transport; and case III, external transport ≈ internal transport. In cases I and II, the transport of adsorbate to the adsorbent is quite rapid and is governed by film diffusion, whereas in case III, the transport of ions to the adsorbent surface does not occur at a significant rate, thereby leading to the formation of a liquid film with a concentration gradient surrounding the sorbent particles. Usually, external transport is the rate-limiting step in systems which have poor mixing, dilute concentration of adsorbate, small particle size, etc. In contrast, the intraparticle transport step (case III) governs the overall transfer for those systems having high concentration of adsorbate, good mixing, larger particle size, etc. Hence the low removal of zinc at higher concentration at fixed adsorbent concentration reflects the intraparticle diffusion, whereas high removal of zinc at low concentration might be due to the rapid external transport mechanism to be the rate controlling step in the overall removal of zinc at varying concentration range. Adsorption Dynamics. Lagergren’s rate equation as cited by Ho and Mckay31 was employed for studying the rate constant for the systems. The plot between log(qe - q) and t was found to be linear, which shows the first-order nature of the processes. The value of the rate constant, Kad, for the system was 8.73 × 10-3 min-1.
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the problem of its disposal and on other hand provide an effective adsorbent for the treatment of metalbearing wastewaters. Acknowledgment We are very thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India for awarding a Research Associateship to undertake this work. Nomenclature
Figure 6. McKay plots of Zn2+ on bagasse fly ash.
Mckay and co-workers32 model (eq 1) has been employed for the determination of the surface masstransfer coefficient βL.
ln
[
]
Ct 1 mk 1 + mk ) ln βL Ss t Co 1 + mk 1 + mk mk
(1)
The straight line plot between ln [Ct/Co - 1/(1 + mk)] and t proved the validity of the above model in the present study. The value of βL for the system as determined from the plot was 2.41 × 10-7 cm s-1. To know the rate controlling step of the process, McKay plots (Figure 6) at different adsorbate concentrations have been drawn. The plots between log(1 -F) and t, where F is the fractional attainment of equilibrium at time t and is obtained by F ) Qt/Q∞ where Qt is amount of uptake at time t and Q∞ is maximum equilibrium uptake for Zn2+ at lower concentrations (e 1.84 × 10-3 M) were found to be linear indicating a purely film diffusion process. However, nonlinear plot at g 3.06 × 10-3 M concentration supports the assumption that particle diffusion becomes the rate-limiting step at this concentration. Thus, it is evident that the rate of adsorption of zinc is limited by external transport at low concentrations, whereas at concentrations higher than 3.06 × 10-3 M the intraparticle diffusion mechanism is of greater importance in determining the overall rate of removal. Cost Estimation. In India, the cheapest variety of commercially available carbon costs (U.S. dollars) approximately $285 ton-1. Waste baggase fly ash is available for $2 ton-1, and considering the cost of transport, chemicals, electrical energy, etc., used in the process, the finished product would cost approximately $15 ton-1. Hence, the developed adsorbent would be a good replacement for commercially available carbon based on its comparatively low cost and good efficiency. Conclusions Bagasse fly ash, a waste residue generated in a substantial amount in the sugar refining industry, was converted into a suitable adsorbent. Bagasse fly ash exhibits promising adsorption characteristics for the removal of zinc and can be used for the treatment of metal bearing wastewater. The material under consideration is not only economical, but is a waste product. Hence, its use as an adsorbent, would on one hand solve
BSS)British Standard Sieve Ct)concentration of adsorbate at time t (mg g-1) C0)initial concentration of adsorbate (mg g-1) F)fractional attainment of equilibrium ∆G0)Gibbs free energy (kJ mol-1) ∆H0)enthalpy change of the process (kJ mol-1) KF)Freundlich constant of solute (mol g-1) Kad)Rate constant (min-1) Q∞)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 time t Q0)Langmuir monolayer capacity (mol g-1) RL)separation factor (dimensionless) Ss)outer surface of the adsorbent per unit volume of particle-free slurry (cm-1) ∆S0)entropy of adsorption (kJ K-1.mol-1) ZPC)zero point charge b, b1, b2)Langmuir constants at 30, 40, and 50 °C (L/mol) k)constant obtained by multiplying the Langmuir constants Q° and b (L g-1) m)mass of adsorbent per unit volume of particle free adsorbate solution (g L-1) n)Freundlich constant of solute (dimensionless) qe)amount adsorbed at equilibrium (mg g-1) q)amount adsorbed at time t (mg g-1) t)predecided time interval at which adsorbent is separated from the adsorbate solution (h) βL)mass transfer coefficient (cm s-1)
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Received for review April 14, 2003 Revised manuscript received July 28, 2003 Accepted August 11, 2003 IE0303146
