ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2015, Vol. 89, No. 13, pp. 198–202. © Pleiades Publishing, Ltd., 2015.
PHYSICAL CHEMISTRY OF SURFACE PHENOMENA
Concurrent Co2+ and Sr2+ Sorption from Binary Mixtures using Aluminum Industry Waste: Kinetic Study1 A. Milenkovića, I. Smičiklasa, M. Šljivić-Ivanovića, and N. Vukelićb a
University of Belgrade, “Vinиa” Institute of Nuclear Sciences, Radiation and Environmental Protection Department, P.O. Box 522, 11001 Belgrade, Serbia b University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12-16, 11158 Belgrade, Serbia e-mail:
[email protected] Received November 27, 2015
Abstract—Multi-component sorption studies are essential to identify the applicability of red mud as a lowcost sorbent for the simultaneous removal of metal ions from wastewaters. Sorption kinetics of Co2+ and Sr2+ ions was investigated, at different total concentrations of mixtures and different molar ratios of two cations. Kinetics of metal sorption from binary systems was found to be well described by pseudo-second-order rate model. Equilibrium sorbed amounts and equilibrium times for Co2+ sorption increased with the increase of its total concentration in the mixture, whereas pseudo-second order rate constants exhibited the opposite trend. Sr2+ sorption was strongly suppressed in the presence of Co2+ ions, and the removal efficiency decreased with increasing concentration and mole fraction of Co2+. Red mud can be used for simultaneous Co2+ and Sr2+ removal from mixtures of lower initial concentration, otherwise Co2+ sorption is dominant. Keywords: competitive sorption, kinetics, red mud, Co2+, Sr2+. DOI: 10.1134/S0036024415130269
INTRODUCTION Sorption processes onto selective materials represent one of the methods for decontamination of liquid radioactive wastes [1]. Utilization of by-products or wastes from various industries for the invention of cost-effective sorbents is particularly beneficial in terms of economy and environment [2]. Red mud, an alkaline waste of alumina industrial production from bauxite ore, is an example of heterogeneous material interesting for environmental applications [3, 4]. It is mainly composed of Fe- and Al-oxides and hydroxides, and up to twenty other mineral components such as: rutile, anatase, sodalite, cancrinite, calcite, kaolinite, quartz, whewellite, etc. [5]. Such versatile mineral composition and alkaline reaction with water, particularly favors immobilization of heavy metal cations. In the previous study, red mud from Bosnia and Herzegovina was found to be a promising sorbent for 60Co and 90Sr ions from single metal solutions [6]. As in contaminated environments different pollutants frequently come together, it is necessary to investigate how the competition between radionuclides affects their sorption kinetics and capacity. Therefore, multicomponent sorption studies are necessary to identify the capabilities and limitations of red mud as a low1 The article is published in the original.
cost sorbent for the simultaneous removal of 60Co and from wastewater. In this paper, time dependant sorption of 60Co and 90Sr was examined using mixtures with different total concentrations and different molar ratios of selected cations. 90Sr
EXPERIMENTAL Sorbent Preparation and Surface Analyses The sample of red mud was collected from “Bira” Alumina Factory (Bosnia and Herzegovina). After settling, the alkaline liquid phase above red mud particles was decanted, whereas the solid residue was dried at 105°C and subsequently homogenized in the mortar. Sorbent was used in the powdered form, without further treatments. Chemical and mineral compositions of red mud were previously reported [7]. Surface properties of untreated red mud were evaluated using a Sorptomatic 1990 Thermo Electron instrument. The nitrogen adsorption-desorption isotherms were determined at –196°C. Prior to the sorption measurements, 7 g of sample was degassed under reduced pressure for 18h at 110°C. The obtained data were analyzed by various models and appropriate software-ADP Version 5.1 Thermo Electron. The specific surface area of the red mud (SBET) was calculated from
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RESULTS AND DISCUSSION
Vads, cm3 g−1 80
Surface Characteristics of Red Mud
70
According to the IUPAC classification [11], the sample exhibited type II adsorption isotherm (Fig. 1), characteristic for nonporous materials.
60 50 40 30 20 10 0
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0.2
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Fig. 1. Nitrogen adsorption and desorption (open symbols) isotherms for red mud.
the linear part of the nitrogen adsorption isotherm according to the Brunauer, Emmett, Teller (BET) method [8]. The total pore volume (Vtot) was taken at p/p0 = 0.998. The mesopores volume and pore size 1 distribution were analysed according to the Barrett, Joyner and Halenda method from the desorption branch of the isotherm [9], whereas the micropores were analyzed using the Dubinin-Radushkevich method [10]. Sorption Kinetic Study Stock solutions of Co2+ and Sr2+ ions (2 × 10–3 and 5 × 10–3 mol/dm3) were prepared by dissolving Co(NO3)2 · 6H2O and Sr(NO3)2 salts, in deionized water. Simulated liquid wastes with different molar ratios of two cations (Co2+ : Sr2+ = 1 : 2, 1 : 1 and 2 : 1) were prepared from stock solutions, keeping the overall cation concentration constant either at 2 × 10–3 mol/ dm3 or at 5 × 10–3 mol/L. The initial pH of each mixture was adjusted to 5, with a few drops of 0.01 mol/dm3 HNO3. The effect of contact time on the amount of cations removed was determined by equilibrating 0.1000 g of red mud with 20.00 mL of mixture solutions, on the laboratory shaker. At different time intervals ranging from 15 min to 48 h, one of the batches was taken for solid/liquid separation by centrifugation and filtration. Cation concentrations before and after the sorption were determined by Perkin Elmer 3100 Atomic Absorption Spectrometer, and the differences were considered as sorbed amounts of Co2+ and Sr2+. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
The presence of barely noticeable hysteresis in the region of higher relative pressures is most likely the result of condensed liquid adsorbate present in spaces between non-porous particles, rather than the characteristic of material pore system. Moreover in mesoporous region, no characteristic maximum was detected nor do any indications of plateau exist at high relative pressure. Absence of these two crucial evidences of mesoporosity of materials leads to the conclusion that investigated material has no mesoporous structure. The obtained total pore volume 0.119 cm3/g, small values of micropore volume 0.005 cm3/g and the value of specific surface area of 17.0 m2/g confirms that this is a nonporous material. The results are in good correlation with the literature, as specific surface areas (BET) of red mud samples from different locations were found to be in the range of 7.3–34.5 m2/g [12]. Kinetics of Co2+ and Sr2+ Sorption from Binary Mixtures The amounts of Co2+ and Sr2+ sorbed at various time intervals from various mixed solutions are presented in the Fig. 2 and 3. It can be observed that the cation removal by red mud was rapid during first hours of contact, followed by a phase of slower sorption. Similar kinetic curves were obtained for Co2+ and Sr2+ sorption from single metal solutions [6]. Large difference between sorbate concentration in liquid and solid phases results in high mass transfer driving force at the beginning of the process. Conversely, metal concentration at longer contact times increases in solid and decreases in liquid phase, which leads to a decrease in driving force and lower process rates [13]. Rapid sorption was connected with film diffusion and chemical reactions, while slower phase can been attributed to different mechanisms such as surface or pore diffusion, surface polymerization and co-precipitation [14]. As Bosnian red mud was found to be non-porous, intraparticle diffusion mechanism can be neglected. On the other hand, various surface related mechanisms may be operating taking into consideration complex chemical and mineralogical composition of the applied sorbent [7]. With the increase of overall initial concentration of the mixture, and the ratio of each cation in the mixture, equilibrium times increased as well. In this manner, shaking time required to reach Co2+ and Sr2+ sorption equilibrium ranged from 6 to 48 h, depending on the experimental conditions.
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Applicability of pseudo-first and pseudo-second order kinetic models was tested for experimental data fitting. Higher correlation coefficients (R2) between the model and the experimental results were obtained using the pseudo-second-order kinetic equation [15]:
pose of data comparison and for the prediction of sorption capacity if equilibrium was not reached during the course of the experiment. Calculated kinetic parameters are presented in table.
where k2 (g/(mmol min)) is the pseudo-second-order rate constant, qe (mmol/g) the amount of cation sorbed at equilibrium, and qt is the amount of cation sorbed at any time t. Although such simple kinetic models cannot provide a deeper insight into the mechanism of cation sorption, they are useful for the pur-
From binary solutions, Co2+ was sorbed more selectively on red mud surface, showing higher qe values. This is in agreement with the results obtained for single-metal solutions, where sorption of Co2+ was higher in respect to Sr2+ [6]. Calculated Co2+ sorption capacities increased with its initial concentration in the mixture, whereas the rate constants decreased. Values of k2 typically decrease with increasing initial sorbate concentration, i.e. by increasing equilibrium sorbed amounts and longer equilibrium times [16].
Co2+ sorbed amount, mmol/g 0.5 Molar ratio Co2+: Sr2+ 1:1 0.4 1:2 2:1 0.3
Co2+ sorbed amount, mmol/g 0.35 Molar ratio 2+ Co : Sr2+ 0.30 1:1 2:1 0.25 1:2 0.20
t = 1 + t , qt k 2q e2 q e
(1)
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(a)
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0.08 0.07 0.06
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Sr2+ sorbed amount, mmol/g
Sr2+ sorbed amount, mmol/g 0.09
0
3000 t, min
(b)
(b)
0.20 Molar ratio Co2+: Sr2+ 1:1 2:1 0.15 1:2
Molar ratio Co2+: Sr2+ 1:1 1:2 2:1
0.05
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0.04 0.03 0.05
0.02 0.01
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Fig. 2. Effect of contact time on competitive Co2+ (A) and Sr2+ (B) sorption by red mud. Total cation concentration 2 × 10–3 mol/L.
0
500
1000
1500
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2500
3000 t, min
Fig. 3. Effect of contact time on competitive Co2+ (A) and Sr2+ (B) sorption by red mud. Total cation concentration 5 × 10–3 mol/L.
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The parameters of pseudo-second-order kinetic model for competitive Co2+ and Sr2+ sorption onto red mud Co2+
Molar ratio Co : Sr
qe, mmol/g
1:2 1:1 2:1 1:2 1:1 2:1
k2, g/(mmolmin)
0.98 0.98 0.99
0.314 0.405 0.471
Total concentration 5 × 10–3 mol/L 0.030 0.99 0.085 0.019 0.99 0.043 0.018 0.99 0.041
0.053 0.729 0.329
0.96 0.98 0.98
Cation sorbed, mmol/g Co2+ Sr2+
Cinitial = 5 × 10−3 mol/l 11.9% 71.2%
0.4 Cinitial = 2 × 10−3 mol/l
8.0% 81.2%
55.7% 100%
71.0% 100%
0.1 0 2:1
other hand, Sr2+ sorption was inhibited by Co2+ and the process effectiveness varied from 33.5 to 71%. The total sorption capacity remained relatively constant (Σqe = 0.326 ± 0.006 mmol/g). Sr2+ ions obviously occupied some surface sites that remained vacant after sorption of Co2+, thus, it can be concluded that there are some sorption centers that are common to both Co2+ and Sr2+. Using more concentrated mixture (5 × 10–3 mol/L), competitive effect of Co2+ was much more pronounced. With the increased Co2+ ratio in the binary solutions, sorbed amounts increased from 0.314 to 0.471 mmol/g, whereas percentage of removal decreased from 94.9 to 71.2%. Sorption of Sr was minimal, i.e. 8–14% was removed from the binary solution. Equilibrium capacities of 0.505 mmol Co2+/g and 0.326 mmol Sr2+/g were achieved with 5 × 10–3 mol/L single metal solutions. With the same initial concentration of the mixture, total amount of sorbed ions were close to, or slightly lower compared to the capacity observed for single Co2+ solution. At the same time, these quantities were higher compared to Sr2+ sorption capacity from monometal solution. From Fig. 4, it can be concluded that some sorption centers are available only for Co2+ ions.
14.0% 94.9%
33.5% 100%
1:1
R2
0.015 0.033 0.227
Results demonstrate that presence of Sr2+ ions did not interfere with Co2+ sorption when 2 × 10–3 mol/L mixtures were used. At this particular cation concentration and under the applied experimental conditions (temperature, solid/liquid ratio, pH, mixing rate, etc.), surface of red mud is not saturated to its full sorption capacity [6]. Therefore, the entire amount (100%) of Co2+ was sorbed by red mud, regardless of its share in the mixture Co2+ sorption was superior in terms of both the capacity and the rate (table). On the
0.2
k2, g/(mmolmin)
0.134 0.203 0.275
For better visualization of the results, individual and overall sorption capacities reached at equilibrium under different experimental conditions are presented in Fig. 4.
0.3
qe, mmol/g
Total concentration 2 × 10–3 mol/L 0.412 0.99 0.205 0.128 0.99 0.117 0.043 0.99 0.045
Sr2+ sorption was radically reduced with the increase of mixture concentration from 2 × 10–3 to 5 × 10–3 mol/L. Sorption capacity reduction was accompanied by a rise in corresponding k2 values.
0.5
Sr2+
R2
2:1 1:2 1:1 Molar ratio Co2+: Sr2+
1:2
Fig. 4. Individual and overall equilibrium amounts of Co2+ and Sr2+ sorbed from different binary mixtures. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
CONCLUSION The effectiveness of red mud for the simultaneous removal of Co2+ and Sr2+ ions is highly dependent on the overall concentration of the mixture, as well as on the molar ratio of two cations. Sorption processes were well described using pseudo-second order kinetic model. With the increase of initial concentration of a particular cation in the mixture, equilibrium time increased. Sorption capacity of red mud for Co2+ ions increased in the same manner, whereas more Sr2+ was sorbed from mixtures of lower total concentrations. Coexisting Co2+ exerted strong inhibitory effect on Sr2+ removal. Consequently, the best performance was Vol. 89
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achieved at lower total concentration of cations in the starting solution, particularly when the Sr2 + was in excess in relation to the Co2+. On the other hand, Sr2+ removal was suppressed at higher total mixture concentrations, regardless of its ratio in the mixture. One of the potential solutions to this problem may be multi-step sorption process. In the first step Co2+ would be dominantly sorbed, thus, removal of Sr2+ would become more efficient in subsequent steps. ACKNOWLEDGMENT This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project III43009). REFERENCES 1. IAEA-TECDOC-1336, Final Report of a Coordinated Research Project, 1997–2001 (Vienna, 2003). 2. S. Bailey, T. R. Olin, and M. A. Dean, Water Res. 33, 2469 (1999). 3. S. Wang, H. M. Ang, and M. O. Tadé, Chemosphere 72, 1621 (2008).
4. W. Liu, J. Yang, and B. Xiao, Int. J. Miner. Process. 93, 220 (2009). 5. M. Gräfe, G. Power, and C. Klauber, Hydrometallurgy 108, 60 (2011). 6. A. Milenković, I. Smičiklas, J. Marković, et al., Nucl. Technol. Rad. 29, 79 (2014). 7. I. Smičiklas, S. Smiljanić, A. Perić-Grujić, et al., Chem. Eng. J. 214, 327 (2013). 8. F. Rouquerol, J. Rouquerol, and K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications (Academic Press, London, 1999). 9. E. P. Barrett, L. G. Joyner, and P. P. Halenda, J. Am. Chem. Soc. 73, 373(1951) 10. M. M. Dubinin, Progress in Surface and Membrane Science (Academic Press, New York, 1975) 11. K. S. W. Sing, D. H. Everett, R. A. W. Haul, et al., Pure Appl. Chem. 57, 603 (1985). 12. K. Snars and R. J. Gilkes, Appl. Clay Sci. 46, 13 (2009). 13. K. V. Kumar and S. Sivanesan, Pol. J. Environ. Stud. 13, 443 (2004). 14. J. Chorover and M. L. Brusseau, Kinetics of Water-Rock Interaction (Springer, New York, 2008) 15. Y. S. Ho and G. McKay, Process Biochem. 34, 451 (1999). 16. W. Plazinski, W. Rudzinski, and A. Plazinska, Adv. Colloid Interface 152, 2 (2009)
SPELL: 1. analysed
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