Microsoft Word - 164.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 56, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Jiří Jaromír Klemeš, Peng Yen Liew, Wai Shin Ho, Jeng Shiun Lim Copyright © 2017, AIDIC Servizi S.r.l., ISBN 978-88-95608-47-1; ISSN 2283-9216 Equilibrium Study of Cadmium Ions Adsorption on Sericin/Alginate Particles Jacqueline O. Limaa, Mariana F. Ragassia, Marcelino L. Gimenesb, Melissa G. A. Vieiraa, Meuris G. C. da Silvaa,* a School of Chemical Engineering, University of Campinas, UNICAMP, 13083-825, Campinas – SP, Brazil b Department of Chemical Engineering, State University of Maringá, UEM, 87020-900, Maringá – PR, Brazil meuris@feq.unicamp.br Cadmium is a pollutant widely found in industrial effluents and its toxicity affects the ecosystem and presents human health risk. The adsorption can be used to remove toxic metals in dilute solutions. Industrial waste, such as sericin, can efficiently replace conventional bioadsorbent materials. Sericin is a protein present in silkworm cocoon and is usually discarded in effluent of spinning processes. Alginate is a polysaccharide used to improve the characteristics of the blend. The objective of this study was to investigate the use of sericin/alginate particles as bioadsorbent to remove cadmium ions (Cd2+) in waste water through adsorption equilibrium study. Equilibrium experiments were conducted to evaluate the influence of adsorption temperature (10 °C, 20 °C, 40 °C and 60 °C), which indicated that the cadmium adsorption capacity increased with decreasing temperature. Adsorption isotherm models, Langmuir, Freundlich, Dubinin-Radushkevich and Temkin were used to analyse the equilibrium data. Cadmium adsorption equilibrium could be described by the Freundlich adsorption model at all the temperatures essayed. The values of the n parameter, which are greater than unity, indicate that Cd2+ ions are favourably adsorbed by sericin/alginate particles at the temperature range studied. 1. Introduction Environmental pollution due to the technological development is a critical issue, mainly in respect to toxic metals, potential pollutants widely found in industrial effluents. Among the toxic metal, cadmium has been classified as a human carcinogen and teratogenic. It has been released into the environment through electroplating and manufacturing of batteries and pigments (Boparai et al., 2011). Conventional methods for the removal of toxic metal ions from waste water include chemical reduction, co- precipitation, coagulation, complexation, electrochemical treatment and adsorption. Adsorption is considered very attractive considering its pollutant removal efficiency in dilute solutions. This technique generally involves the use of conventional adsorbents, such as activated carbon, which are effective in removing various pollutants. However, they are generally expensive and have a high cost of regeneration, thereby motivating the research for new low cost materials. Bioadsorption is based on the binding capacity of various metals with organic materials, including wastes of industrial operations, as in the case of sericin, disposed in the processing of silk. The sericin, a protein that constitutes 20 - 30 % of the total mass of cocoons of the Bombyx mori, has amino acids with strong polar groups such as hydroxyl, carboxyl and amino groups. These groups allow crosslinking, copolymerisation and formation of blends with other polymers such as alginate. This allows achieving better material characteristics, since the sericin has poor structural properties and high solubility in water (Zhang, 2002). Alginate is a polysaccharide easy to obtain, available from natural sources, renewable and abundant. It can be found in brown algae and bacteria and is biodegradable and have good biocompatibility (Volesky, 1990). Recently, Silva et al. (2015) studied the production of sericin/alginate particles by ionic and thermal reticulation processes. The potential of these particles as bioadsorbent of toxic metals was evaluated and the authors obtained concentration reductions of about 75 % and 65 % for copper and zinc ions, respectively. DOI: 10.3303/CET1756316 Please cite this article as: Lima J.O., Ragassi M.F., Gimenes M.L., Vieira M.G.A., Silva M.G.C., 2017, Equilibrium study of cadmium ions adsorption on sericin/alginate particles, Chemical Engineering Transactions, 56, 1891-1896 DOI:10.3303/CET1756316 1891 The objective of this study was to investigate the use of sericin/alginate particles as bioadsorbent to remove cadmium ions (Cd2+) through adsorption equilibrium study using Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherm models. 2. Materials and Methods 2.1 Preparation of the Particles The Bombyx mori silkworm´s cocoons were kindly provided by Bratac Silk Mills Company, located in the State of Paraná – Brazil. Methods for preparation of cocoons, extraction of sericin and higher molecular weight sericin fractionation were performed according to Silva et al., 2014. The concentration of sericin was determined and then adjusted to 25 g/L (2.5 % w/V). To obtain the blend, commercial sodium alginate (Sigma-Aldrich, UK) was incorporated at a ratio of 2 % w/V in the adjusted sericin solution with constant stirring. The particles were prepared by ionic gelation process, where the blend was dripped, with a peristaltic pump (Masterflex L/S, 77800-60, USA), in aqueous solution of CaCl2 (3 % w/V). At the end of this process, the particles formed were kept in constant agitation in the calcium chloride solution for 12 h to complete the crosslinking process. Subsequently, the particles were dried at room temperature and then kept in a continuous flow oven at 100 °C for 24 h for the thermal crosslinking to occur, in order to improve the mechanical properties of the particles. 2.2 Adsorption Equilibrium Adsorption isotherm assays were conducted to determine the influence of temperature on the adsorption of cadmium ions on sericin/alginate particles. In these experiments, 0.5 g of particle was added to 50 mL cadmium ions solutions, which concentration ranged from 0.02 - 7 mmol/L. The system was stirred at 250 rpm at temperatures of 10 °C, 20 °C, 40 °C and 60 °C using a shaker to ensure homogeneity and contact the solution with the particles for 24 h. After this time, mixture was centrifuged to separate the supernatant and adsorbent. The concentration of Cd 2+ ions in the supernatant was measured using an atomic absorption spectrophotometer (Shimadzu, AA-7000, Japan). The quantity of Cd2+ ions adsorbed at equilibrium was calculated by Eq(1): = ( − ). (1) where is the initial concentration of Cd2+ ions (mmol/L), is the equilibrium concentration of Cd2+ ions (mmol/L), is the volume of the solution (L) and is the mass of the particles (g). In the present study, Langmuir, Freundlich, Dubinin–Radushkevich and Temkin adsorption isotherm models were fitted to the adsorption equilibrium data. 2.2.1 Langmuir Isotherm Langmuir isotherm equation assumes monolayer adsorption on a uniform surface with finite number of identical adsorption sites (Langmuir, 1918). The form of the Langmuir isotherm model is described as in Eq(2): = ( )1 + (2) where is equilibrium capacity (mmol/g), is the equilibrium concentration (mmol/L), is the Langmuir constant related to the energy of adsorption (L/mmol) and ( ) is the maximum adsorption capacity (mmol/g). The essential characteristics of Langmuir isotherm can be explained in terms of a dimensionless constant separation factor, , defined by the Eq(3): = 11 + (3) Values of indicate the type of Langmuir isotherm: irreversible ( = 0), favorable (0 < < 1), linear ( = 1) or unfavorable ( > 1). 2.2.2 Freundlich Isotherm The empirical Freundlich equation, Eq(4) considers the existence of a multilayer structure (heterogeneous surfaces) and assumes that the concentration of adsorbate on the adsorbent surface increases with the concentration of the adsorbate (Freundlich, 1906): = ⁄ (4) 1892 where (L/mmol) and are Freundlich isotherm constants related to adsorption capacity and adsorption intensity. The adsorption is favorable when is in the range 1 - 10. 2.2.3 Dubinin-Radushkevich Isotherm The Dubinin–Radushkevich (D–R) isotherm (Dubinin and Radushkevich, 1947) is more general than the Langmuir isotherm because it does not assume a homogeneous surface or constant sorption potential. D–R’s equation as shown in Eq(5) is: = ( ) − (1 + 1⁄ ) (5) where R is the gas constant (8.314 J/mol K), T is the absolute temperature (K), is the D–R constant (mol2/kJ2) and ( ) is the maximum adsorption capacity (mmol/g). The mean free energy of adsorption per molecule of the Cd2+ ions can be evaluated from Eq(6): = 12 (6) This parameter gives information about chemical or physical adsorption. With the magnitude of E between 8 and 16 kJ.mol−1, the bioadsorption process follows chemical ion-exchange, while for the values of E < 8 kJ.mol−1, the bioadsorption process is of a physical nature. 2.2.4 Temkin Isotherm The Temkin isotherm model assumes that the adsorption energy decreases linearly with the surface coverage due to adsorbent–adsorbate interactions (Temkin and Pyzhev, 1940). The linear form of Temkin isotherm model is given by the Eq(7): = + (7) where b is the Temkin constant related to the heat of sorption (kJ/mol) and K is the Temkin isotherm constant (L/mmol). 3. Results and Discussion Figure 1 shows the experimental data of adsorption isotherms Cd2+ ions by the sericin/alginate particles. It can be seen that the adsorption capacity of Cd2+ ions by adsorption onto particles is favourable at low temperatures. This may be due to a tendency of the Cd2+ ions to escape from the solid phase to the bulk phase with an increase in the temperature of the solutions. This effect suggests that an explanation of the adsorption mechanism associated with the removal of Cd2+ ions involves a physical process, which is usually associated with low adsorption heat (Azouaou et al., 2010). Isotherms exhibit the same behaviour for all temperatures, which indicates the adsorption is favourable according to classification of McCabe (1993) as shown in Figure 1. Figure 1: Effect of temperature on the Cd2+ adsorption capacity on sericin/alginate particles 1893 To evaluate the isotherms models and its capabilities to fit to the experimental data, the graphs for each isotherm were plotted for four different temperatures evaluated: 10 °C (a), 20 °C (b), 40 °C (c) and 60 °C (d), which are shown in Figure 2. Figure 2: Experimental and adjusted isotherms for adsorption of cadmium on sericin/alginate particles at different temperatures: (a) T = 10 °C; (b) T = 20 °C; (c) T = 40 °C and (d) T = 60 °C The best fit among the isotherm models is assessed by the linear coefficient of determination (R2) and average relative error (ARE). The average relative error test measures the difference between the experimental and model data. If data from the model are similar to experimental data, ARE numbers will be small. The adsorption constants for each model with correlation coefficients and average relative error are represented in Table 1. By comparing the coefficient of determination (R2), it was found that Langmuir and Freundlich isotherms represent good models for the adsorption system. However, the Freundlich model, which predicts the heterogeneity of the adsorption sites, is a better fit to the experimental data at all temperatures evaluated because it presented lower values of average relative error (ARE). According to Figure 2, the Dubinin- Radushkevich and Temkin models provide a good fit to the experimental data concerning low equilibrium concentrations. Regarding to the parameters, the values of the dimensionless parameter RL of the Langmuir model lay between 0 and 1, indicating a favourable adsorption. The RL values increased with increasing temperature, wherein the highest value was obtained for temperature of 40 °C. The Freundlich model is usually applied to adsorption on heterogeneous surfaces strongly. The values of n are greater than unity, indicating that the Cd 2+ ions are favourably adsorbed by sericin/alginate particles at all temperatures studied. The activation energy (E) results estimated by the Dubinin-Radushkevich model were low (< 8 kJ/mol) for all temperatures measured, indicating that the adsorption process is physical. This indicates the possibility of desorption of metal ion adsorbed on the particles sericin/alginate. The heat of adsorption values (b) of Temkin isotherm for the systems studied were found to be less than 8 kJ/mol, which also indicates that a weak interaction between the Cd2+ ions and the adsorbent. 1894 The cadmium adsorption capacity on sericin/alginate particles obtained by the Langmuir model at about room temperature (20 °C) was 0.8312 mmol Cd2+/g particle (or 93.44 mg Cd2+/g particle). This is higher than the adsorption capacity of other alternative adsorbents reported in the literature: coffee grounds (15.65 mg/g) (Azouaou et al., 2010); chestnut shell (4.07 mg/g) (Vásquez et al., 2009) and Eucalyptus seeds (71.15 mg/g) (Kiruba et al., 2014). It shows the potential use of this adsorbent for the removal cadmium ions at low concentrations. Table 1: Langmuir, Freundlich, D–R and Temkin isotherm model parameters for adsorption of cadmium on sericin/alginate particles Isotherm Parameters Temperature (ºC) 10 20 30 40 Langmuir qmax(L) (mmol/g) 0.8926 0.8312 0.5944 0.6446 KL (L/mmol) 0.9195 0.3872 0.2991 0.3809 RL 0.1384 0.2760 0.3305 0.2793 R2 0.9832 0.9976 0.9921 0.9984 ARE (%) 34.5296 12.2885 22.4210 8.0539 Freundlich KF (L/mmol) 0.5381 0.2643 0.1707 0.1548 n 1.1050 1.0642 1.5707 1.0529 R2 0.9801 0.9935 0.9877 0.9866 ARE (%) 32.5488 11.8199 13.1275 5.1130 Dubinin- Radushkevich qmax(D-R) (mmol/g) 0.3065 0.2207 0.1756 0.1625 KD (mol 2/kJ2) 0.0331 0.0362 0.0269 0.0306 E (kJ/mol) 3.8859 3.7174 4.3105 4.0435 R2 0.9036 0.8955 0.9118 0.8733 ARE (%) 39.0353 34.2106 45.6939 30.7564 Temkin KT (L/mmol) 6.5039 4.0336 5.8942 2.7092 b (kJ/mol) 7.7164 6.9807 6.8725 5.8541 R2 0.8729 0.9098 0.9227 0.9081 ARE (%) 51.9633 32.8637 35.2278 34.6883 4. Conclusions Sericin/alginate particles were investigated as adsorbents for the removal of cadmium ions from aqueous solutions through equilibrium study. It was found that temperature is an important factor influencing this adsorption process, wherein adsorption capacity increases with decreasing temperature, which demonstrates that low temperatures are more favourable. Equilibrium isotherms were evaluated using Langmuir, Freundlich, Dubinin-Radushkevich and Temkin models. Freundlich model, which predicts heterogeneity of adsorption sites, provided the best fit to experimental data for all temperatures (10 °C, 20 °C, 40 °C and 60 °C). The values of n are greater than unity, indicating that the Cd2+ ions are favourably adsorbed by sericin/alginate particles. Results showed that the sericin/alginate particles have a high adsorption capacity and are comparable to some bioadsorbents. They also suggest that these particles can be used as effective bioadsorbents for the removal of Cd2+ in wastewater. Acknowledgments The authors thank the BRATAC Company for providing the silkworm cocoons, CNPq (Proc. 470615/2013-3 and 300986/2013-0), CAPES and FAPESP (Proc. 2015/13505-9) for financial support. References Azouaou N., Sadaoui Z., Djaafri A., Mokaddem H., 2010, Adsorption of cadmium from aqueous solution onto untreated coffee grounds: Equilibrium, kinetics and thermodynamics, Journal of Hazardous Materials, 184, 126-134. Boparai H.K., Joseph M., O’Carroll D.M., 2011, Kinetics and thermodynamics of cadmium ion removal by adsorption onto nano zerovalent iron particles, Journal of Hazardous Materials, 186, 458-465. Dubinin M.M., Radushkevich L.V., 1947, Equation of the characteristic curve of activated charcoal, Proceedings of the Academy of Sciences, Physical Chemistry Section USSR, Moscow, 55, 331-333. Freundlich H., 1906. Over the adsorption in solution (in German). 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