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Equilibrium study and kinetics of Cu2+ removal from water by zeolite prepared from oil shale ash
Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 261–266
Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Equilibrium study and kinetics of Cu2+ removal from water by zeolite prepared from oil shale ash Reyad Shawabkeh ∗ Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Box 5050, Dhahran, 31261, Eastern Province, Saudi Arabia
a b s t r a c t Engineered zeolite was produced from oil shale ash by reaction with sodium hydroxide in a closed vessel reactor. This adsorbent was used for removal of copper ions from aqueous solution. The maximum adsorption capacity was 504.6 mg Cu2+ /g zeolite. Kinetic studies showed that the rate of adsorption of copper is increased with increasing the solution pH and temperature, quantity of the zeolite and agitation speed. The kinetic data were fitted to homogeneous micropore model and found that the mass transfer coefficient and diffusivity of the Cu2+ are directly affected by the kinetic parameters. The increase in solution concentration will decrease the mass transfer coefficient while diffusivity is increased. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: A: Oil shale ash; B: Zeolite; C: Adsorption kinetics; D: Copper
1.
Introduction
Oil shale is considered one of the largest energy resources in the world. The oil equivalent of oil shale around the world is estimated to be around 30 times the reserve of the crude oil. The average oil yield for oil shale ranges between 10 and 20% depending on the particle sizes while the rest is ash (NERC, 2001). Although oil shale has been utilized as a source of liquid fuel, its future is still uncertain due to several factors such as high environmental impact, where a by-product of oil shale processing is ash, which is considered a serious environmental problem could face the country. Therefore, there is a need for a proper strategy for ash handling, disposal and utilization. Ash produced from municipal incinerators and power plants worldwide is usually used for production of cement and concrete, asphalt shingle, quarry-fill and sludge stabilization, soil treatment for agricultural purpose, while the large portion is dumped in landfills (Cangialosi et al., 2008; Kurama and Kaya, 2008; Singh et al., 2008). An alternative manner is the conversion of this by-product into a high-grade zeolite, which is considered an environmental friendly product. Several researchers were synthesized different types of zeolite from fly ash. Most of the research work reported in the
∗
literature has been oriented towards establishing the experimental conditions for optimum zeolite production (Reinik et al., 2008), the effect of alkali treatment on zeolite synthesis (Reinik et al., 2007), and application to metal removal from solutions and toxic gases from gas streams (Mouflih et al., 2005; Namasivayam and Sureshkumar, 2007; Ornek et al., 2007; Ozturk and Yildirim, 2008; Shawabkeh, 2006; Wang and Wu, 2006). In the present work, application of zeolite from oil shale ash is investigated for removal of heavy metals from water. Particularly for removal of copper ions from aqueous solution as a major contaminant that enters water by corrosion of household plumbing systems. The effects of adsorption kinetic parameters are tested in order to obtain the optimum kinetic parameter for minimizing the rate of adsorption of copper.
2.
Experimental
2.1.
Materials
Oil shale was brought from EL-Lajjun—Jordan, crushed and grinded to different particle sizes and stored in closed containers for further uses. Stock solution of 1000 mg Cu2+ /L was
Permanent address: Department of Chemical Engineering, Mutah University, AL-Karak, 61710, Jordan. Tel.: +962 3 2372380x3277. E-mail addresses: [email protected], [email protected]. Received 23 June 2008; Received in revised form 31 January 2009; Accepted 2 April 2009 0957-5820/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2009.04.001
262
Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 261–266
freshly prepared by mixing 249.68 g CuSO4 ·5H2 O in 1 L deionized water and diluted to different final concentrations for isotherm and kinetic studies. All chemicals were analytical grade from Scharlau-Spain, and all the glasses were Pyrex rinsed and washed several times with diluted nitric acid and then with distilled water to remove any adhered impurity on the surface of the glass.
ticles and their sizes do not change with time as a result of water swelling. Consequently, the transport of the adsorbate within the particles is described using the partial differential equation (Tscheliessnig et al., 2005):
2.2.
where c and q are the concentrations of adsorbate inside the micropores and on the inner surface, respectively. r is the radial particle coordination, εp is the porosity of the particle and Deff is the effective pore diffusion coefficient. The initial and boundary conditions are:
Zeolite synthesis
The synthesis procedure was described in our early work (Shawabkeh et al., 2004a,b), where samples of oil shale were washed at 950 ◦ C and reacted with 8 M NaOH in a closed reactor at different temperature values (25–180 ◦ C) and reaction periods (1–36 h). The sample that treated at 160 ◦ C for 24 h showed the formation of Na-PI type zeolite at 29.5◦ , 32.2◦ and 34.4◦ 2 degrees using Riyankn UY 10392 X-ray diffractometer.
2.3.
Isotherm and kinetic experiments
Aqueous solutions of copper sulfate of initial concentrations ranging from 10 to 500 mg/L were prepared. Batch adsorption isotherms were performed for fixed particle size of zeolite (<45 m). Using two sets of stopper bottles, equal weights of 0.1 g of the produced zeolite were added to 100 mL of Cu2+ solution. Then the zeolite/Cu2+ mixtures were placed in an isothermal shaker (23 ± 1 ◦ C) for 3 days to allow complete equilibration. Based on different nature of acidity of the produced zeolite, pH was adjusted by adding few drops of NaOH or HCl. Batch adsorption kinetic studies were performed in a 2-L beaker with Plexiglas cover. This cover is attached with 4 Plexiglas baffles spaced around the circumference at 90◦ to ensure smooth mixing without stagnant areas and vortex formation. Mixing was carried out using a two-blade stainless steel impeller driven by a variable speed motor. The experiments were established by adding a fixed weight of the produced zeolite into 1700 mL of known Cu2+ solution, and kept agitated for 120 min. All experiments were carried out at ambient temperature (23 ± 1 ◦ C) unless otherwise stated. Effect of initial copper concentration (50, 100 and 200 mg/L), solution pH (2 and 6) and temperature (0, 25 and 50 ◦ C), mass of the zeolite (0.5, 1.0 and 1.5 g) and stirring speed (200, 500 and 1000 rpm) were also considered in the kinetic experiments. Samples were withdrawn at time interval of 0.5, 1, 2, 3, 5, 10, 20, 30 and 60 min using a fritted glass tube then centrifuged to remove the suspended particles. The resulting samples were analyzed using ThermoElement Atomic Absorption Spectrophotometer.
3.
Theoretical
Mass transfer of the solutes from the aqueous phase to the solid phase is usually explained by passing the solute molecules through a set of resistances. These resistances include transport of solute molecules from the bulk stream to the liquid boundary surrounding the surface of the adsorbent, diffusion throughout the liquid boundary, diffusion within the macropore and micropore of the adsorbent particle, and adsorption onto the surface of the particle. The existence of these resistances depends on the type of the interface between the solute and the adsorbent, the solution condition and the type of the adsorbent. For the case of the produced zeolite particles it is assumed that the film and macropores diffusion are negligible where the particle size is <45 m. Moreover, it is assumed that the zeolite has highly porous spherical par-
∂q ∂c = Deff εp + (1 − εp ) ∂t ∂t
2 ∂c ∂2 c + 2 r ∂r ∂r
c(0, r) = 0 Deff
(1)
(2)
∂c(t, R) = kf (C − c) ∂r
(3)
∂c =0 ∂r t,r=0
(4)
where C is the bulk concentration of the adsorbate in solution and is related to the mass transfer coefficient, kf , the volume of solution, V, and zeolite particle, Vm , and the equilibrium solution concentration, ce , by −3kf Vm dC = (C − ce ) dt RV
(5)
C(0) = C0
(6)
The equilibrium concentration of adsorbate in the outer film of the particle is related with that on the surface by Langmuir and Freundlich adsorption isotherms accordingly Langmuir isotherm q =
Qbce 1 + bce
Freundlich isotherm q = Kce˛
(7) (8)
where Q is the saturation capacity to cover monolayer of adsorption, b, K and ˛ are the isotherm constants. The above system of equation was solved to obtain the values of kf , and Deff that fit the kinetic data.
4.
Results and discussion
Oil shale ash was cured at different reaction conditions in a closed vessel reactor. The reaction of alumina and silica in ash with 8 M NaOH at 160 ◦ C produced Na-PI zeolite. This material was further tested for its adsorption capacity against copper ions in solution. The equilibrium data of Cu2+ obtained at different initial concentrations of 25, 50, 75, 100, 150, 200, 250, 300 and 350 mg/L is shown in Fig. 1. It is cleared that a slightly favorable adsorption isotherm of type I is best describing these data, where the zeolite uptake is increased with increasing the initial copper concentration. These data were correlated to Langmuir and Freundlich models in order to predict the saturation capacity of the zeolite. Langmuir model provides fitting to the experimental data with correlation coefficient, R2 = 0.961 and sum of square errors of 4565 accordingly q=
15.704ce 1 + 3.110 × 10−2 ce
(9)
Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 261–266
Fig. 1 – Adsorption Isotherm of Cu2+ using the produced zeolite from oil shale ash. The saturation capacity was estimated by fitting Eq. (7) to the experimental data using PEAKFIT (version 2.01) and found 504.6 mg Cu2+ /g zeolite. This model is valid for monolayer sorption onto a surface with a finite number of identical sites where the energy of adsorption is constant and there is no transmigration of adsorbates in the plan of the surface of the adsorbent. However, the isotherm data did show a monolayer of adsorption but with an incremental change in this capacity by increasing the initial concentration. This may be attributed to the variation of the surface energy at the interface with the initial concentration. Hence Freundlich model may be applied to provide a better fit to the isotherm data with correlation coefficient, R2 = 0.978 and sum of square errors of 2592 as q = 30.552ce0.590
(10)
In order to obtain the optimum operating conditions for copper adsorption, some parameters were chosen in order to study the feasibility of the produced zeolite for removal of Cu2+ with short time. These parameters include initial copper concentration, mixing effect, solution temperature and pH, zeolite mass.
4.1.
Effect of initial concentration
Experiments for studying effect of initial solute concentration were performed at 50, 100, and 200 mg/L where the solution pH and temperature were fixed at 6 and 23 ± 1 ◦ C, respectively. Changing the initial concentration had a noticeable effect on the adsorption kinetic of copper (Fig. 2). It appears that as the initial concentration increase, the time required for copper ions to achieve maximum uptake on the zeolite surface will increase. When the initial concentration is 50 mg/L, only 15 min is required to diminish the amount of Cu2+ from solution. However, this copper removal could not be achieved for a solution containing 200 mg/L even though in 60 min. This is more likely attributed to the increase of internal diffusion resistance inside the pores with increasing the initial concentration while at lower values only external diffusion resistance is predominant. This behavior is supported by molecular kinetic theory, in which an increase in initial concentration of Cu2+ will increase the number of collisions between its molecules, resulting in a decrease in the mean free path for copper ions transfer into the surface (Schwitzer, 1997)
263
Fig. 2 – Effect of initial concentration on the adsorption kinetics of Cu2+ obtained at 25 ◦ C, 1.0 g of zeolite, pH 6, and 500 rpm.
4.2.
Effect of mass of adsorbent
Zeolite dosage was varied from 0.5 to 1.5 g/1.7 L at fixed values of initial concentration, solution pH, temperature and mixing speed (100 mg/L, 6, 23 ± 1 ◦ C and 500 rpm, respectively). Fig. 3 shows the varying of the mass of zeolite from 0.5 to 1.5 g where the time necessary for Cu2+ removal from the solution is decreased as a result of increasing the availability of the exchangeable sites and surface area for adsorption. The incremental Cu2+ removal becomes very low with increasing time as the surface of the zeolite reached the equilibrium condition with copper ion (Srivastava et al., 2006).
4.3.
Effect of agitation speed
Agitation speeds were carried out at 200, 500 and 1000 rpm (Fig. 4). It appears that the increase in speed of agitation decreased the time required for copper to equilibrate. This increase in agitation speed decreased the boundary layer resistance to mass transfer, increased the diffusion rate of Cu2+ from the bulk into zeolite particles and hence increased in the rate of copper uptake. Moreover, the number of collisions between Cu2+ ions inside the micropores will increase leading to the increase in diffusion coefficient which increase the rate of adsorption.
Fig. 3 – Effect of mass of zeolite on the adsorption kinetics of Cu2+ obtained at 100 mg Cu/L, 25 ◦ C, pH 6, and 500 rpm.
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Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 261–266
Fig. 4 – Effect of agitation speed on the adsorption kinetics of Cu2+ obtained at 100 mg Cu/L, 25 ◦ C, pH 6, and 1.0 g of zeolite.
Fig. 6 – Effect of solution temperature on the adsorption kinetics of Cu2+ obtained at 100 mg Cu/L, pH 6, 1.0 g of zeolite and 500 rpm.
4.4.
of the boundary layer surrounding the Cu2+ with temperature and hence increasing the movement of copper ions to the zeolite surface (Meena et al., 2005; Ozsoy and Kumbur, 2006).
Effect of pH
Effect of solution pH was studied at two different solution acidity (pH 2 and pH 6). The amount adsorbed copper ions were found to increase with increasing solution pH, and showed a 90% reduction of Cu2+ concentration from solution was reached with in 30 min at pH 6 (Fig. 5). Higher than this pH value, the copper ions will precipitate as Cu(OH)2 . On the other hand, at lower pH the zeolite surface becomes more protonated and competitive adsorption occurred between H+ and Cu2+ on the available surface area. Moreover, the ionic strength of the solution will increase and consequently the hydrogen ions will adsorb first due its high ratio of charge to ionic radium compared to that of Cu2+ .
4.5.
Effect of temperature
Effect of solution temperature was carried out at 0, 25, and 50 ◦ C (Fig. 6). It is shown that the increase in solution temperature has increased the rate of adsorption. The increase of adsorption of Cu2+ with temperature indicates that a high temperature favors the Cu2+ removal and the adsorption is controlled by endothermic process (Zou et al., 2006). Moreover, this increase in adsorption with temperature may be attributed to either increase in the number of active surface sites available for adsorption or the decrease in the thickness
Fig. 5 – Effect of pH on the adsorption kinetics of Cu2+ obtained at 100 mg Cu/L, 25 ◦ C, 1.0 g of zeolite and 500 rpm.
4.6.
Model simulation
Mathematical interpretation of adsorption kinetics of Cu2+ by the surface of zeolite particles is presented by intraparticle diffusion model. The Freundlich isotherm model was used with the kinetic model as a result of best fitting the isotherm data compared with the Langmuir one. The model was solved numerically by using MATHEMATICA software (version 5) to obtain the concentration distribution of copper ions inside the zeolite particle and the corresponding concentration in the bulk solution. Fig. 7 illustrates the theoretical output of Cu2+ concentration inside the pores as a function of both particle radius and time. It is apparent that the particle is initially free of Cu2+ then an increase in concentration distribution is occurred with time. This increase is cleared at the outer surface of the particle to reach a value of 45 mg/L within 60 min. Fig. 8 predicts the effect of particle porosity on the rate of
Fig. 7 – Concentration distribution of Cu2+ inside the zeolite particle.
265
Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 261–266
Fig. 8 – Estimation of average particle concentration with changing of porosity and time. Cu2+ uptake inside the particle. The increase in porosity will decrease the time required to reach the ultimate Cu2+ uptake. When the particle porosity is 0.9 the Cu2+ concentration inside the pore reached 20 mg/L within 50 s while 300 s are required to obtain the same concentration when the porosity is reduced to 0.1. The kinetic data were fitted to the model and the corresponding model parameters are shown in Table 1. The mass transfer coefficient was increased with decreasing the initial concentration. This is because the driving force for mass transfer is time dependent, thus the increase in initial concentration of copper ions reduces their mobility to transfer rapidly across the boundary layer. As a result, the mass transfer coefficient decreases (McKay et al., 1986). On the other hand, this increase in initial concentration would increase the diffusion coefficient inside the zeolite particles by affecting the surface coverage of the zeolite (Kapoor and Yang, 1991; Pengthamkeerati et al., 2008). On the other hand, the increase in zeolite concentration in solution provided more surface area for adsorption and hence decreased the diffusion coefficient and increased the mass transfer coefficient a result of the dependence of external mass transfer on the solute driving force per unit area. Mixing speed, solution temperature and pH showed similar effect on these constant by increasing the mass transfer coefficient and decreasing the diffusion coefficient with increasing the values of these parameters. The results obtained from
Fig. 9 – Effect of mass transfer coefficient on the adsorption of copper by zeolite particle.
Fig. 10 – Effect of diffusion coefficient on the adsorption of copper by zeolite particle.
the model support these finding (Fig. 9), where the increase in mass transfer coefficient per unit time would increase the concentration of the Cu2+ in the macro and micropores of the zeolite particles and hence more adsorption takes place on the inner sites of these pores. Similarly, the increase in diffusion coefficient inside these pore would also increase enhance the copper uptake inside the particles (Fig. 10).
Table 1 – Adsorption kinetic parameters for removal of Cu2+ from aqueous solution. Kinetic parameter
Common parameters
Values
kf (cm/s)
Deff (cm2 /s)
25 ◦ C, 1.0 g, pH 6, and 500 rpm
50 mg/L 100 mg/L 200 mg/L
0.0075 0.0062 0.0022
8.02 × 10−6 1.01 × 10−5 1.49 × 10−5
100 mg/L, 1.0 g, pH 6, and 500 rpm
0 ◦C 25 ◦ C 50 ◦ C
0.0015 0.0062 0.0070
9.00 × 10−7 1.12 × 10−5 1.0 × 10−5
100 mg/L, 25 ◦ C, pH 6, and 500 rpm
0.5 g 1.0 g 1.5 g
0.0050 0.0062 0.0065
1.81 × 10−4 1.01 × 10−5 6.04 × 10−6
100 mg/L, 25 ◦ C, 1.0 g, and 500 rpm
2 6
0.0003 0.0061
1.04 × 10−6 1.02 × 10−5
100 mg/L, 25 ◦ C, 1.0 g, and pH 6
200 rpm 500 rpm 1000 rpm
0.0010 0.0062 0.0063
5.01 × 10−6 1.01 × 10−5 5.30 × 10−5
Initial concentration
Temperature
Zeolite mass
pH
Mixing speed
266
5.
Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 261–266
Conclusion
The adsorption isotherm and kinetics data obtained for the removal of copper ions from aqueous solution would be perfectly attained by the zeolite produced from oil shale ash. The adsorption is affected by the concentration of Cu2+ where an increase in adsorption capacity is cleared with the increase in initial concentration. The kinetic parameters showed that the rate of adsorption is well enhanced with agitation speed, solution pH and temperature and the mass of zeolite. The provided model best fitted to the experimental data and predicted the variation of particles porosity on the removal efficiency. The produced material is not expensive, easily produced and solve and environmental issue.
References Cangialosi, F., Notarnicola, M., Liberti, L. and Stencel, J.M., 2008, The effects of particle concentration and charge exchange on fly ash beneficiation with pneumatic triboelectrostatic separation. Separation and Purification Technology, 62(1): 240. Kapoor, A. and Yang, R.T., 1991, Contribution of concentration-dependent surface diffusion to rate of adsorption. Chemical Engineering Science, 46(8): 1995. Kurama, H. and Kaya, M., 2008, Usage of coal combustion bottom ash in concrete mixture. Construction and Building Materials, 22(9): 1922. McKay, G., Bino, M.J. and Altememi, A., 1986, External mass transfer during the adsorption of various pollutants onto activated carbon. Water Research, 20(4): 435. Meena, A.K., Mishra, G.K., Rai, P.K., Rajagopal, C. and Nagar, P.N., 2005, Removal of heavy metal ions from aqueous solutions using carbon aerogel as an adsorbent. Journal of Hazardous Materials, 122(1–2): 161. Mouflih, M., Aklil, A. and Sebti, S., 2005, Removal of lead from aqueous solutions by activated phosphate. Journal of Hazardous Materials, 119(1–3): 183. Namasivayam, C. and Sureshkumar, M.V., 2007, Modelling thiocyanate adsorption onto surfactant-modified coir pith, an agricultural solid ‘waste’. Process Safety and Environmental Protection, 85(6 B): 521. NERC., (2001). Extraction of Crude Oil From Oil Shale Using Organic Solvents in Soxhlet Extractor. (N.E.R. Center, Royal Scientific Society, Amman, Jordan), pp. 1–35 Ornek, A., Ozacar, M. and Sengil, I.A., 2007, Adsorption of lead onto formaldehyde or sulphuric acid treated acorn waste:
equilibrium and kinetic studies. Biochemical Engineering Journal, 37(2): 192. Ozsoy, H.D. and Kumbur, H., 2006, Adsorption of Cu(II) ions on cotton boll. Journal of Hazardous Materials, 136(3): 911. Ozturk, B. and Yildirim, Y., 2008, Investigation of sorption capacity of pumice for SO2 capture. Process Safety and Environmental Protection, 86(1): 31. Pengthamkeerati, P., Satapanajaru, T. and Chularuengoaksorn, P., 2008, Chemical modification of coal fly ash for the removal of phosphate from aqueous solution. Fuel, 87(12): 2469. Reinik, J., Heinmaa, I., Mikkola, J.P. and Kirso, U., 2007, Hydrothermal alkaline treatment of oil shale ash for synthesis of tobermorites. Fuel, 86(5–6): 669. Reinik, J., Heinmaa, I., Mikkola, J.P. and Kirso, U., 2008, Synthesis and characterization of calcium-alumino-silicate hydrates from oil shale ash—towards industrial applications. Fuel, 87(10–11): 1998. Schwitzer, P., (1997). Handbook of Separation Techniques for Chemical Engineers (3rd ed.). (McGraw-Hill Company, New York), pp. 3–4 Shawabkeh, R., Al-Harahsheh, A. and Al-Otoom, A., 2004, Production of zeolite from Jordanian oil shale ash and application for zinc removal from wastewater. Oil Shale, 21(2): 125. Shawabkeh, R., Al-Harahsheh, A., Hami, M. and Khlaifat, A., 2004, Conversion of oil shale ash into zeolite for cadmium and lead removal from wastewater. Fuel, 83(7–8): 981. Shawabkeh, R.A., 2006, Adsorption of chromium ions from aqueous solution by using activated carbo-aluminosilicate material from oil shale. Journal of Colloid and Interface Science, 299(2): 530. Singh, A., Sharma, R.K. and Agrawal, S.B., 2008, Effects of fly ash incorporation on heavy metal accumulation, growth and yield responses of Beta vulgaris plants. Bioresource Technology, 99(15): 7200. Srivastava, V.C., Mall, I.D. and Mishra, I.M., 2006, Characterization of mesoporous rice husk ash (RHA) and adsorption kinetics of metal ions from aqueous solution onto RHA. Journal of Hazardous Materials, 134(1–3): 257. Tscheliessnig, A., Hahn, R. and Jungbauer, A., 2005, In situ determination of adsorption kinetics of proteins in a finite bath. Journal of Chromatography A, 1069(1): 23. Wang, S. and Wu, H., 2006, Environmental-benign utilisation of fly ash as low-cost adsorbents. Journal of Hazardous Materials, 136(3): 482. Zou, W., Han, R., Chen, Z., Jinghua, Z. and Shi, J., 2006, Kinetic study of adsorption of Cu(II) and Pb(II) from aqueous solutions using manganese oxide coated zeolite in batch mode. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 279(1–3): 238.
Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Equilibrium study and kinetics of Cu2+ removal from water by zeolite prepared from oil shale ash Reyad Shawabkeh ∗ Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Box 5050, Dhahran, 31261, Eastern Province, Saudi Arabia
a b s t r a c t Engineered zeolite was produced from oil shale ash by reaction with sodium hydroxide in a closed vessel reactor. This adsorbent was used for removal of copper ions from aqueous solution. The maximum adsorption capacity was 504.6 mg Cu2+ /g zeolite. Kinetic studies showed that the rate of adsorption of copper is increased with increasing the solution pH and temperature, quantity of the zeolite and agitation speed. The kinetic data were fitted to homogeneous micropore model and found that the mass transfer coefficient and diffusivity of the Cu2+ are directly affected by the kinetic parameters. The increase in solution concentration will decrease the mass transfer coefficient while diffusivity is increased. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: A: Oil shale ash; B: Zeolite; C: Adsorption kinetics; D: Copper
1.
Introduction
Oil shale is considered one of the largest energy resources in the world. The oil equivalent of oil shale around the world is estimated to be around 30 times the reserve of the crude oil. The average oil yield for oil shale ranges between 10 and 20% depending on the particle sizes while the rest is ash (NERC, 2001). Although oil shale has been utilized as a source of liquid fuel, its future is still uncertain due to several factors such as high environmental impact, where a by-product of oil shale processing is ash, which is considered a serious environmental problem could face the country. Therefore, there is a need for a proper strategy for ash handling, disposal and utilization. Ash produced from municipal incinerators and power plants worldwide is usually used for production of cement and concrete, asphalt shingle, quarry-fill and sludge stabilization, soil treatment for agricultural purpose, while the large portion is dumped in landfills (Cangialosi et al., 2008; Kurama and Kaya, 2008; Singh et al., 2008). An alternative manner is the conversion of this by-product into a high-grade zeolite, which is considered an environmental friendly product. Several researchers were synthesized different types of zeolite from fly ash. Most of the research work reported in the
∗
literature has been oriented towards establishing the experimental conditions for optimum zeolite production (Reinik et al., 2008), the effect of alkali treatment on zeolite synthesis (Reinik et al., 2007), and application to metal removal from solutions and toxic gases from gas streams (Mouflih et al., 2005; Namasivayam and Sureshkumar, 2007; Ornek et al., 2007; Ozturk and Yildirim, 2008; Shawabkeh, 2006; Wang and Wu, 2006). In the present work, application of zeolite from oil shale ash is investigated for removal of heavy metals from water. Particularly for removal of copper ions from aqueous solution as a major contaminant that enters water by corrosion of household plumbing systems. The effects of adsorption kinetic parameters are tested in order to obtain the optimum kinetic parameter for minimizing the rate of adsorption of copper.
2.
Experimental
2.1.
Materials
Oil shale was brought from EL-Lajjun—Jordan, crushed and grinded to different particle sizes and stored in closed containers for further uses. Stock solution of 1000 mg Cu2+ /L was
Permanent address: Department of Chemical Engineering, Mutah University, AL-Karak, 61710, Jordan. Tel.: +962 3 2372380x3277. E-mail addresses: [email protected], [email protected]. Received 23 June 2008; Received in revised form 31 January 2009; Accepted 2 April 2009 0957-5820/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2009.04.001
262
Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 261–266
freshly prepared by mixing 249.68 g CuSO4 ·5H2 O in 1 L deionized water and diluted to different final concentrations for isotherm and kinetic studies. All chemicals were analytical grade from Scharlau-Spain, and all the glasses were Pyrex rinsed and washed several times with diluted nitric acid and then with distilled water to remove any adhered impurity on the surface of the glass.
ticles and their sizes do not change with time as a result of water swelling. Consequently, the transport of the adsorbate within the particles is described using the partial differential equation (Tscheliessnig et al., 2005):
2.2.
where c and q are the concentrations of adsorbate inside the micropores and on the inner surface, respectively. r is the radial particle coordination, εp is the porosity of the particle and Deff is the effective pore diffusion coefficient. The initial and boundary conditions are:
Zeolite synthesis
The synthesis procedure was described in our early work (Shawabkeh et al., 2004a,b), where samples of oil shale were washed at 950 ◦ C and reacted with 8 M NaOH in a closed reactor at different temperature values (25–180 ◦ C) and reaction periods (1–36 h). The sample that treated at 160 ◦ C for 24 h showed the formation of Na-PI type zeolite at 29.5◦ , 32.2◦ and 34.4◦ 2 degrees using Riyankn UY 10392 X-ray diffractometer.
2.3.
Isotherm and kinetic experiments
Aqueous solutions of copper sulfate of initial concentrations ranging from 10 to 500 mg/L were prepared. Batch adsorption isotherms were performed for fixed particle size of zeolite (<45 m). Using two sets of stopper bottles, equal weights of 0.1 g of the produced zeolite were added to 100 mL of Cu2+ solution. Then the zeolite/Cu2+ mixtures were placed in an isothermal shaker (23 ± 1 ◦ C) for 3 days to allow complete equilibration. Based on different nature of acidity of the produced zeolite, pH was adjusted by adding few drops of NaOH or HCl. Batch adsorption kinetic studies were performed in a 2-L beaker with Plexiglas cover. This cover is attached with 4 Plexiglas baffles spaced around the circumference at 90◦ to ensure smooth mixing without stagnant areas and vortex formation. Mixing was carried out using a two-blade stainless steel impeller driven by a variable speed motor. The experiments were established by adding a fixed weight of the produced zeolite into 1700 mL of known Cu2+ solution, and kept agitated for 120 min. All experiments were carried out at ambient temperature (23 ± 1 ◦ C) unless otherwise stated. Effect of initial copper concentration (50, 100 and 200 mg/L), solution pH (2 and 6) and temperature (0, 25 and 50 ◦ C), mass of the zeolite (0.5, 1.0 and 1.5 g) and stirring speed (200, 500 and 1000 rpm) were also considered in the kinetic experiments. Samples were withdrawn at time interval of 0.5, 1, 2, 3, 5, 10, 20, 30 and 60 min using a fritted glass tube then centrifuged to remove the suspended particles. The resulting samples were analyzed using ThermoElement Atomic Absorption Spectrophotometer.
3.
Theoretical
Mass transfer of the solutes from the aqueous phase to the solid phase is usually explained by passing the solute molecules through a set of resistances. These resistances include transport of solute molecules from the bulk stream to the liquid boundary surrounding the surface of the adsorbent, diffusion throughout the liquid boundary, diffusion within the macropore and micropore of the adsorbent particle, and adsorption onto the surface of the particle. The existence of these resistances depends on the type of the interface between the solute and the adsorbent, the solution condition and the type of the adsorbent. For the case of the produced zeolite particles it is assumed that the film and macropores diffusion are negligible where the particle size is <45 m. Moreover, it is assumed that the zeolite has highly porous spherical par-
∂q ∂c = Deff εp + (1 − εp ) ∂t ∂t
2 ∂c ∂2 c + 2 r ∂r ∂r
c(0, r) = 0 Deff
(1)
(2)
∂c(t, R) = kf (C − c) ∂r
(3)
∂c =0 ∂r t,r=0
(4)
where C is the bulk concentration of the adsorbate in solution and is related to the mass transfer coefficient, kf , the volume of solution, V, and zeolite particle, Vm , and the equilibrium solution concentration, ce , by −3kf Vm dC = (C − ce ) dt RV
(5)
C(0) = C0
(6)
The equilibrium concentration of adsorbate in the outer film of the particle is related with that on the surface by Langmuir and Freundlich adsorption isotherms accordingly Langmuir isotherm q =
Qbce 1 + bce
Freundlich isotherm q = Kce˛
(7) (8)
where Q is the saturation capacity to cover monolayer of adsorption, b, K and ˛ are the isotherm constants. The above system of equation was solved to obtain the values of kf , and Deff that fit the kinetic data.
4.
Results and discussion
Oil shale ash was cured at different reaction conditions in a closed vessel reactor. The reaction of alumina and silica in ash with 8 M NaOH at 160 ◦ C produced Na-PI zeolite. This material was further tested for its adsorption capacity against copper ions in solution. The equilibrium data of Cu2+ obtained at different initial concentrations of 25, 50, 75, 100, 150, 200, 250, 300 and 350 mg/L is shown in Fig. 1. It is cleared that a slightly favorable adsorption isotherm of type I is best describing these data, where the zeolite uptake is increased with increasing the initial copper concentration. These data were correlated to Langmuir and Freundlich models in order to predict the saturation capacity of the zeolite. Langmuir model provides fitting to the experimental data with correlation coefficient, R2 = 0.961 and sum of square errors of 4565 accordingly q=
15.704ce 1 + 3.110 × 10−2 ce
(9)
Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 261–266
Fig. 1 – Adsorption Isotherm of Cu2+ using the produced zeolite from oil shale ash. The saturation capacity was estimated by fitting Eq. (7) to the experimental data using PEAKFIT (version 2.01) and found 504.6 mg Cu2+ /g zeolite. This model is valid for monolayer sorption onto a surface with a finite number of identical sites where the energy of adsorption is constant and there is no transmigration of adsorbates in the plan of the surface of the adsorbent. However, the isotherm data did show a monolayer of adsorption but with an incremental change in this capacity by increasing the initial concentration. This may be attributed to the variation of the surface energy at the interface with the initial concentration. Hence Freundlich model may be applied to provide a better fit to the isotherm data with correlation coefficient, R2 = 0.978 and sum of square errors of 2592 as q = 30.552ce0.590
(10)
In order to obtain the optimum operating conditions for copper adsorption, some parameters were chosen in order to study the feasibility of the produced zeolite for removal of Cu2+ with short time. These parameters include initial copper concentration, mixing effect, solution temperature and pH, zeolite mass.
4.1.
Effect of initial concentration
Experiments for studying effect of initial solute concentration were performed at 50, 100, and 200 mg/L where the solution pH and temperature were fixed at 6 and 23 ± 1 ◦ C, respectively. Changing the initial concentration had a noticeable effect on the adsorption kinetic of copper (Fig. 2). It appears that as the initial concentration increase, the time required for copper ions to achieve maximum uptake on the zeolite surface will increase. When the initial concentration is 50 mg/L, only 15 min is required to diminish the amount of Cu2+ from solution. However, this copper removal could not be achieved for a solution containing 200 mg/L even though in 60 min. This is more likely attributed to the increase of internal diffusion resistance inside the pores with increasing the initial concentration while at lower values only external diffusion resistance is predominant. This behavior is supported by molecular kinetic theory, in which an increase in initial concentration of Cu2+ will increase the number of collisions between its molecules, resulting in a decrease in the mean free path for copper ions transfer into the surface (Schwitzer, 1997)
263
Fig. 2 – Effect of initial concentration on the adsorption kinetics of Cu2+ obtained at 25 ◦ C, 1.0 g of zeolite, pH 6, and 500 rpm.
4.2.
Effect of mass of adsorbent
Zeolite dosage was varied from 0.5 to 1.5 g/1.7 L at fixed values of initial concentration, solution pH, temperature and mixing speed (100 mg/L, 6, 23 ± 1 ◦ C and 500 rpm, respectively). Fig. 3 shows the varying of the mass of zeolite from 0.5 to 1.5 g where the time necessary for Cu2+ removal from the solution is decreased as a result of increasing the availability of the exchangeable sites and surface area for adsorption. The incremental Cu2+ removal becomes very low with increasing time as the surface of the zeolite reached the equilibrium condition with copper ion (Srivastava et al., 2006).
4.3.
Effect of agitation speed
Agitation speeds were carried out at 200, 500 and 1000 rpm (Fig. 4). It appears that the increase in speed of agitation decreased the time required for copper to equilibrate. This increase in agitation speed decreased the boundary layer resistance to mass transfer, increased the diffusion rate of Cu2+ from the bulk into zeolite particles and hence increased in the rate of copper uptake. Moreover, the number of collisions between Cu2+ ions inside the micropores will increase leading to the increase in diffusion coefficient which increase the rate of adsorption.
Fig. 3 – Effect of mass of zeolite on the adsorption kinetics of Cu2+ obtained at 100 mg Cu/L, 25 ◦ C, pH 6, and 500 rpm.
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Fig. 4 – Effect of agitation speed on the adsorption kinetics of Cu2+ obtained at 100 mg Cu/L, 25 ◦ C, pH 6, and 1.0 g of zeolite.
Fig. 6 – Effect of solution temperature on the adsorption kinetics of Cu2+ obtained at 100 mg Cu/L, pH 6, 1.0 g of zeolite and 500 rpm.
4.4.
of the boundary layer surrounding the Cu2+ with temperature and hence increasing the movement of copper ions to the zeolite surface (Meena et al., 2005; Ozsoy and Kumbur, 2006).
Effect of pH
Effect of solution pH was studied at two different solution acidity (pH 2 and pH 6). The amount adsorbed copper ions were found to increase with increasing solution pH, and showed a 90% reduction of Cu2+ concentration from solution was reached with in 30 min at pH 6 (Fig. 5). Higher than this pH value, the copper ions will precipitate as Cu(OH)2 . On the other hand, at lower pH the zeolite surface becomes more protonated and competitive adsorption occurred between H+ and Cu2+ on the available surface area. Moreover, the ionic strength of the solution will increase and consequently the hydrogen ions will adsorb first due its high ratio of charge to ionic radium compared to that of Cu2+ .
4.5.
Effect of temperature
Effect of solution temperature was carried out at 0, 25, and 50 ◦ C (Fig. 6). It is shown that the increase in solution temperature has increased the rate of adsorption. The increase of adsorption of Cu2+ with temperature indicates that a high temperature favors the Cu2+ removal and the adsorption is controlled by endothermic process (Zou et al., 2006). Moreover, this increase in adsorption with temperature may be attributed to either increase in the number of active surface sites available for adsorption or the decrease in the thickness
Fig. 5 – Effect of pH on the adsorption kinetics of Cu2+ obtained at 100 mg Cu/L, 25 ◦ C, 1.0 g of zeolite and 500 rpm.
4.6.
Model simulation
Mathematical interpretation of adsorption kinetics of Cu2+ by the surface of zeolite particles is presented by intraparticle diffusion model. The Freundlich isotherm model was used with the kinetic model as a result of best fitting the isotherm data compared with the Langmuir one. The model was solved numerically by using MATHEMATICA software (version 5) to obtain the concentration distribution of copper ions inside the zeolite particle and the corresponding concentration in the bulk solution. Fig. 7 illustrates the theoretical output of Cu2+ concentration inside the pores as a function of both particle radius and time. It is apparent that the particle is initially free of Cu2+ then an increase in concentration distribution is occurred with time. This increase is cleared at the outer surface of the particle to reach a value of 45 mg/L within 60 min. Fig. 8 predicts the effect of particle porosity on the rate of
Fig. 7 – Concentration distribution of Cu2+ inside the zeolite particle.
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Fig. 8 – Estimation of average particle concentration with changing of porosity and time. Cu2+ uptake inside the particle. The increase in porosity will decrease the time required to reach the ultimate Cu2+ uptake. When the particle porosity is 0.9 the Cu2+ concentration inside the pore reached 20 mg/L within 50 s while 300 s are required to obtain the same concentration when the porosity is reduced to 0.1. The kinetic data were fitted to the model and the corresponding model parameters are shown in Table 1. The mass transfer coefficient was increased with decreasing the initial concentration. This is because the driving force for mass transfer is time dependent, thus the increase in initial concentration of copper ions reduces their mobility to transfer rapidly across the boundary layer. As a result, the mass transfer coefficient decreases (McKay et al., 1986). On the other hand, this increase in initial concentration would increase the diffusion coefficient inside the zeolite particles by affecting the surface coverage of the zeolite (Kapoor and Yang, 1991; Pengthamkeerati et al., 2008). On the other hand, the increase in zeolite concentration in solution provided more surface area for adsorption and hence decreased the diffusion coefficient and increased the mass transfer coefficient a result of the dependence of external mass transfer on the solute driving force per unit area. Mixing speed, solution temperature and pH showed similar effect on these constant by increasing the mass transfer coefficient and decreasing the diffusion coefficient with increasing the values of these parameters. The results obtained from
Fig. 9 – Effect of mass transfer coefficient on the adsorption of copper by zeolite particle.
Fig. 10 – Effect of diffusion coefficient on the adsorption of copper by zeolite particle.
the model support these finding (Fig. 9), where the increase in mass transfer coefficient per unit time would increase the concentration of the Cu2+ in the macro and micropores of the zeolite particles and hence more adsorption takes place on the inner sites of these pores. Similarly, the increase in diffusion coefficient inside these pore would also increase enhance the copper uptake inside the particles (Fig. 10).
Table 1 – Adsorption kinetic parameters for removal of Cu2+ from aqueous solution. Kinetic parameter
Common parameters
Values
kf (cm/s)
Deff (cm2 /s)
25 ◦ C, 1.0 g, pH 6, and 500 rpm
50 mg/L 100 mg/L 200 mg/L
0.0075 0.0062 0.0022
8.02 × 10−6 1.01 × 10−5 1.49 × 10−5
100 mg/L, 1.0 g, pH 6, and 500 rpm
0 ◦C 25 ◦ C 50 ◦ C
0.0015 0.0062 0.0070
9.00 × 10−7 1.12 × 10−5 1.0 × 10−5
100 mg/L, 25 ◦ C, pH 6, and 500 rpm
0.5 g 1.0 g 1.5 g
0.0050 0.0062 0.0065
1.81 × 10−4 1.01 × 10−5 6.04 × 10−6
100 mg/L, 25 ◦ C, 1.0 g, and 500 rpm
2 6
0.0003 0.0061
1.04 × 10−6 1.02 × 10−5
100 mg/L, 25 ◦ C, 1.0 g, and pH 6
200 rpm 500 rpm 1000 rpm
0.0010 0.0062 0.0063
5.01 × 10−6 1.01 × 10−5 5.30 × 10−5
Initial concentration
Temperature
Zeolite mass
pH
Mixing speed
266
5.
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Conclusion
The adsorption isotherm and kinetics data obtained for the removal of copper ions from aqueous solution would be perfectly attained by the zeolite produced from oil shale ash. The adsorption is affected by the concentration of Cu2+ where an increase in adsorption capacity is cleared with the increase in initial concentration. The kinetic parameters showed that the rate of adsorption is well enhanced with agitation speed, solution pH and temperature and the mass of zeolite. The provided model best fitted to the experimental data and predicted the variation of particles porosity on the removal efficiency. The produced material is not expensive, easily produced and solve and environmental issue.
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