Modelling and Kinetics of Cd(II) Biosorption onto Inactive Instant Dry Baker s Yeast

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1 Modelling and Kinetics of Cd(II) Biosorption onto Inactive Instant Dry Baker s Yeast ANA-MARIA STANESCU, LIGIA STOICA, CAROLINA CONSTANTIN*, GABRIELA BACIOIU University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, Dept. of Inorganic Chemistry, Physical Chemistry and Electrochemistry, 1 G. Polizu Str., , Bucharest, Romania This study presents the modeling and kinetics of Cd(II) biosorption from synthetic aqueous solutions onto inactive instant dry baker s yeast Saccharomyces cerevisiae. The biosorption equilibrium was studied by using Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherm models. Based on the isotherm parameters and correlation coefficients (R 2 ) values, we could estimate that Cd(II) biosorption equilibrium was accurately described by Lang-muir model. The maximum monolayer adsorption capacity (Q m ) determined from the intercept of the linearized Langmuir model was 50 mg g -1. Cd(II) biosorption onto the inactive instant dry baker s yeast biomass well fitted the pseudo second-order kinetic model. The value of the adsorption energy (E < 16 kj mol -1 ) and the kinetics results suggested that Cd(II) biosorption by the inactive instant dry baker s yeast biomass involved both, electrostatic and chemical interactions. Keywords: Cd(II); biosorption; isotherm models; kinetics; mechanism High concentrations of toxic heavy metals are being released into the environment from industry activities endangering natural ecosystems and human health worldwide. Moreover, the heavy metals have high mobility in aqueous systems and may present high toxicity [1-3]. Some heavy metals ions are very toxic even at low concentration levels ( mg/l), i.e. cadmium is such a case [4]. Cadmium contamination is generally being caused by a variety of industrial processes, such as: electroplating, paint pigments, alloy preparation, plastics, nickel-cadmium batteries, silver-cadmium batteries, coating operations, smelter operations, photography, television phosphors etc [2,5,6]. Cadmium tends to accumulate in the human body when exposed through air, water or food sources causing serious damage to the kidneys, liver and bones and it is probably best known for its association with Itai-Itai disease [2,6-8]. Compared with the classical methods used for heavy metals decontamination from wastewaters (i.e. chemical precipitation, ion exchange, solvent extraction, electrochemical technologies, membrane technologies, reverse osmosis, filtration, evaporation, etc) that have several drawbacks (i.e.: incomplete metal removal, high operating costs, high reagent and/or energy requirements, etc.), biosorption is considered a cost-effective, eco-friendly and easy to operate alternative technology [1,4,6-20]. Biosorption exploits the general property of living and/or non-living biomass (biosorbents) to rapidly bind/ adsorb and/ or concentrate heavy metals ions even from very diluted aqueous solutions (< 100 mg/l) by physicochemical mechanisms [9,13,16-18]. It is well known that biomaterials like, bacteria, fungi, yeast, algae, food industry/agricultural waste, plants, animal origin by-products etc. can remove toxic metals and radionuclides, but can also recover precious metals and light metals from aqueous solutions to various extents [1, 4, 8, 12, 15-18,21-29]. Although several research groups investigated heavy metal removal by diverse S. cerevisiae types (lab cultivated yeasts, waste from food industry, immobilized yeast, magnetically/ chemically/ thermally modified yeast, * carolinaconstantin@gmail.com; Tel.: commercial baker and brewer yeasts subsequently lab cultivated, fresh/ compressed baker yeast, dry baker yeast, etc.), the biosorption mechanism is not fully understood [4,7,10-13,15,18,22,29-33]. A number of references attributed the metal-binding interactions to chemisorption (via ion exchange, coordination or chelation), complexation, precipitation, physisorption, each mechanism functioning independently and/or simultaneously [1,12,29,34]. Due to the fact that heavy metals biosorption mechanism is complex and not fully understood, this field of the biosorption processes requires further investigation. The aim of this study was to investigate the modeling and kinetics of Cd(II) biosorption (from synthetic aqueous solutions) onto the inactive instant dry baker yeast biomass. The biosorption equilibrium and the nature of the adsorption process (as chemical and/or physical) were evaluated by means of Langmuir, Freundlich, Dubinin-Radushkevich (D- R) and Temkin isotherm models. Experimental part Biosorbent preparation Commercial instant dry baker yeast (S. cerevisiae) purchased from local commercial company was prepared as inactive biomass by oven drying at 105 o C for 24 h [35]. Subsequently, the inactive instant dry baker yeast biomass was stored in desiccators till further use [29]. Reagents Cadmium stock solution of 1000 mg L -1 was prepared by dissolving 3CdSO 4 8H 2 O of analytical reagent grade (Merck, Germany) into distilled water. Cadmium test solutions of different concentrations (10, 25, 50, 100, 150, 200 and 250 mg L -1 ) were obtained by diluting the stock solution. The ph of the solutions was adjusted by adding 0.1 M H 2 SO 4 or 0.1M NaOH solutions. All reagents were of analytical reagent grade. Equipment The ph values were measured with an Orion 290 A phmeter. The batch biosorption experiments were conducted by using a Heildorph Vibramax 100 shaker. Cd(II) equilibrium concentrations in liquid phase were REV. CHIM. (Bucharest) 66 No

2 determined by using an Unicam Pay SP9 atomic absorption spectrophotometer. Equilibrium biosorption experiments Biosorption studies were performed under batch conditions with continuous stirring (200 rpm), at room temperature (20 C), ph 5.5 by adding a constant dose of inactive instant dry baker yeast biomass of 0.5g/100 ml sample of different initial metal concentrations, for 30 min contact time. The biosorption studies were carried out at the optimal operating conditions/ parameters previously investigated and reported [6]. In order to evaluate Cd(II) biosorption kinetics, the contact time was varied from 0 to 360 min at the optimal operating parameters. The Cd(II) loaded inactive instant dry baker yeast biomass was separated from the metal solutions by decantation [29]. The equilibrium adsorption capacity was calculated according to the following equation [35]: (1) where Q e is the equilibrium adsorption capacity (mg g -1 ); C 0 and C e are the initial and final/equilibrium metal concentration in solution (mg L -1 ); V is the volume of the metal solution (L) and m represents the weight of the dry biosorbent (g). Results and discussions Modelling The equilibrium adsorption data was modeled using Langmuir, Freundlich, Dubinin-Radushkevich (figs. 1-3) and Temkin (figure not shown) isotherms and the quality of the fit was assessed by the value/ magnitude of the correlation coefficients (R 2 ). Data regarding the Langmuir, Freundlich, D-R and Temkin isotherm constants, obtained in this study are listed in table 1. The linearized form of the Langmuir isotherm model for monolayer adsorption onto a surface with a finite number of identical sites can be expressed as [25]: (2) where Q m is maximum monolayer adsorption capacity (mg g -1 ) and K L is Langmuir isotherm constant (L mg -1 ) related to the energy of sorption that quantitatively reflects the affinity between the biosorbent and the metal ions. The essential features of the Langmuir isotherm can also be evaluated by using the separation factor (R L ), defined by equation (3) [35]: 1/Qe R 2 = /Ce Fig. 1. Linearized Langmuir isotherm model obtained for Cd(II) biosorption onto the inactive instant dry baker yeast biomass (Agitation rate: 200 rpm; Temperature: 20 C; ph: 5.5; Biosorbent dose: 0.5g; Time: 30 min) where C 0 is initial metal concentration (mg L -1 ). The value of dimensionless parameter, R L indicates the type of the adsorption process (R L = 0, irreversible; 0 < R L < 1, favorable; R L = 1, linear; R L > 1, unfavorable) [35]. Figure 1 illustrates the linearized Langmuir isotherm model obtained for Cd(II) biosorption onto the inactive instant dry baker yeast biomass. Based on the correlation coefficients (table 1), we could estimate that Cd(II) biosorption equilibrium was accurately described by Lang-muir isotherm model. Tonk et al. [7] also found that cadmium biosorption on immobilized brewery waste biomass followed a Langmuir isotherm. The value of the maximum monolayer adsorption capacity, Q m (50 mg g -1 ) determined from the intercept of the liniarized Langmuir model (1/Q e vs. 1/C e plot) was almost equal to the maximum experimental adsorption capacity (49.2 mg g -1 ) [6], suggesting a good adsorption of Cd(II) ions by the inactive instant dry baker s yeast biomass [25,29]. The maximum experimental adsorption capacity was 49.2 mg g -1 and was obtained after 30 min contact time, at ph 5.5, for a biosorbent dose of 0.5 g and an initial metal concentration of 250 mg L -1 [6]. The value of the separation factor, R L was and therefore indicates that the Langmuir isotherm was favourable. The linearized form of the Freundlich isotherm model for heterogeneous surfaces of multilayer adsorption is given in equation (4) [25]: (3) Table 1 LANGMUIR, FREUNDLICH, D-R AND TEMKIN ISOTHERM PARAMETERS OBTAINED FOR Cd(II) BIOSORPTION ONTO THE INACTIVE INSTANT DRY BAKER YEAST REV. CHIM. (Bucharest) 66 No

3 R 2 = Log Qe Log Ce Fig. 2. Linearized Freundlich isotherm model obtained for Cd(II) biosorption onto the inactive instant dry baker yeast biomass (Agitation rate: 200 rpm; Temperature: 20 C; ph: 5.5; Biosorbent dose: 0.5g; Time: 30 min) where K F (mg g -1 ) is the relative adsorption capacity and n the intensity of adsorption. The linearized Freundlich isotherm model obtained for biomass is depicted in figure 2. The values of the Freundlich constants, K F and n (determined from the intercept and the slope of the logqe vs. logce plot) were mg g 1 and , respectively. The Dubinin-Radushkevich isotherm model was used to distinguish the nature of the adsorption process as physical or chemical and its linearized form can be described by the following equation [25]: where K (mol 2 kj -2 ) is a constant related to mean adsorption energy and ε is the Polanyi potential, defined by equation (6): (6) Dubinin-Radushkevich constants, K and Q m were determined from the slope and the intercept, respectively of the lnqe vs.ε 2 plot. The adsorption energy, E (kj mol -1 ) was determined by means of the subsequent equation [25]: (7) The value of the adsorption energy, E indicates the nature of the adsorption mechanism (E=1-16 kj mol -1, physisorption; E > 16 kj mol -1, chemisorption) [25]. The linearized D-R isotherm model obtained for Cd(II) biosorption onto the inactive instant dry baker yeast biomass is presented in figure 3. The estimated value of E ( kj mol -1 ) implied that Cd(II) adsorption mechanism onto the inactive instant dry baker yeast biomass might involve physical interactions [25]. The positive value of E indicated that Cd(II) biosorption process was endothermic [25]. Temkin isotherm model was used to asses the adsorption potential of the inactive dry baker yeast biomass for Cd(II) ions. The linearized form of Temkin model is given by the following equation [25]: (4) (5) Fig. 3. Linearized Dubinin Radushkevich isotherm model obtained for biomass (Agitation rate: 200 rpm; Temperature: 20 C; ph: 5.5; Biosorbent dose: 0.5g; Time: 30 min) where B T = RT/b T (kj mol -1 ), T is the temperature in K and R is the universal gas constant (8.314 J molk -1 ). The constant b T is related to the heat of adsorption and A T is the equilibrium binding constant corresponding to the maximum binding energy. B T and A T were determined from the slope and respectively, intercept of the Qe vs. lnce plot. Temkin isotherm constants and the corresponding correlation coefficient (R 2 ) are listed in table 1. The value of the constant A T was 24.07, implying a good adsorption potential of the inactive instant dry baker yeast for Cd(II) ions [29]. However, the magnitude of the correlation coefficient obtained for Temkin model was the lowest when compared to the other three isotherm models. This suggests that Temkin isotherm model was not suitable for Cd(II) adsorption onto the inactive instant dry baker s yeast biomass. Kinetics In order to evaluate the kinetics of Cd(II) biosorption process, the pseudo first-order and pseudo second-order models were tested to correlate the experimental data. The linear form of Lagergren pseudo first-order equation is given by the following expression [25,36,37]: where Q e and Q t are the adsorption capacities (mg g -1 ) at equilibrium and at time t (min), respectively, and k 1 is the rate constant of the first-order adsorption (min -1 ). The correlation coefficients, R 2 obtained from the plot log(q e - Q t ) vs. t of the pseudo first-order model were very low (fig. 4), suggesting that Cd(II) biosorption did not followed pseudo first-order kinetics. Figure 4 suggests that application of equation (9) is inappropriate for Cd(II) biosorption onto the inactive instant dry baker yeast biomass, as the experimental points are non-linear [37]. Therefore, the constants for the pseudo first-order model were not considered. Similar observations were reported by other researchers [36-38] for Cu(II), Ni(II), Cr(VI) and respectively, Cd(II) adsorption from aqueous solution by diverse adsorbents. The linear form of the pseudo second-order model developed by Ho [39,40] can be expressed as equation (10): (9) (10) (8) REV. CHIM. (Bucharest) 66 No

4 0-0.5 Log(Qe-Qt) R 2 = R 2 = R 2 = t (min) 25 mg/l 50 mg/l 150 mg/l Fig. 4. Linearized pseudo first-order kinetic model obtained for biomass for different initial concentrations (Agitation rate: 200 rpm; Temperature: 20 C; ph: 5.5; Biosorbent dose: 0.5g) Fig. 5. Linearized pseudo second-order kinetic model obtained for biomass for different initial concentrations (Agitation rate: 200 rpm; Temperature: 20 C; ph: 5.5; Biosorbent dose: 0.5g) Table 2 KINETIC PSEUDO SECOND-ORDER PARAMETERS FOR Cd(II) BIOSORPTION ONTO INACTIVE INSTANT DRY BAKER YEAST AT 293 K Q e; (Q e cal ) and k 2, the pseudo second-order rate constant were determined from slope and intercept of the t/q t vs. t plot (fig. 5). Table 2 lists the pseudo second-order rate parameters for Cd(II) biosorption onto the inactive instant dry baker s yeast. The correlation coefficients, R 2 derived from the pseudo second-order kinetic model (fig. 5) were much higher than those obtained from the pseudo first-order kinetic model (fig. 4). Furthermore, the calculated equilibrium adsorption capacities, Q e,cal, determined from the pseudo second-order model fitted well with the experimental data (table 2). These results suggested that biomass appears to follow the pseudo second-order kinetic model and the adsorption mechanism might also involve chemical interactions [37-40]. Conclusions This study was focused on the modeling and kinetics of Cd(II) biosorption (from aqueous solutions) onto inactive instant dry baker yeast biomass. The equilibrium data obtained for Cd(II) biosorption was analyzed using various adsorption isotherm models (Langmuir, Freundlich, Dubinin Radushkevich and Temkin). The examination of the isotherm parameters showed that Langmuir model provided a good fit to the experimental equilibrium adsorption data. The maximum monolayer adsorption capacity, Q m determined from the linearized Langmuir model was 50 mg. g -1 and was almost equal to the maximum e xperimental adsorption capacity (49.2 mg g -1 ). Cd(II) biosorption equilibrium was also well described by D-R isotherm model. The value of the adsorption energy, E ( kj mol -1 ) indicated that physisorption was involved in Cd(II) bisorption onto the inactive instant dry baker yeast biomass and that the process was endothermic. Cd(II) biosorption onto the inactive instant dry baker yeast followed the pseudo second-order kinetic model. 176 Based on the isotherm data and the kinetics results we estimate that Cd(II) biosorption mechanism is complex, and it was simultaneously accomplished through chemical and electrostatic interactions. A similar trend was observed for Cu(II) biosorption by heat pretreated instant dry baker yeast [29]. Acknowledgments. This work has been funded by the Sectoral Operational Programme Human Resources Development of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreement POSDRU/107/1.5/S/ References 1.NAJA, G., MURPHY, V, VOLESKY, B., Biosorption, Metals. Encyclopedia of Industrial Biotechnology: Bioprocess, Bioseparation, and Cell Technology, John Wiley & Sons, New York, 2010, p NAJA, G., VOLESKY, B., Handbook on Heavy Metals in the Environment, Taylor & Francis & CRC Press, Wang, L.K., Chen, J.P., Hung, Y.T., Shammas, N.K., Boca Raton, 2009, p.13 3.VASQUEZ, T.G.P., BOTERO, A.E.C., de MESQUITA, L.M.S., TOREM, M.L., Miner. Eng., 20, 2007, p WANG, J., CHEN, C., Biotechnol. Adv., 24, 2006, p VOLESKY, B., MAY, H., HOLAN, Z.R., Biotechnol. Bioeng., 41, 1993, p STOICA, L., STÃNESCU, A.-M, CONSTANTIN, C., BÃCIOIU, G., Rev. Chim. (Bucharest), 65, no. 7, 2014, p TONK, S., MÃICÃNEANU, A., INDOLEAN, C., BURCA, S., MAJDIK, C., J. Serb. Chem. Soc., 76, 2011, p WASE, J., FORSTER, C., Biosorbents for Metal Ions, Taylor & Francis e-library, London, 2003, p. 2,11 9.CHOJNACKA, K., Biosorption and Bioaccumulation of Toxic Metals, Nova Science Publishers Inc., New York, 2009, p STOICA, L., CONSTANTIN, C., Depoluarea Sistemelor Apoase, I, Politehnica Press, Bucuresti, 2010, p MAJDIK, C., BURCÃ, S., INDOLEAN, C., MÃICÃNEANU, A., STANCA, M., TONK, S., MEZEY, P., Rev. Roum. Chim., 55, 2010, p ZAN, F., HUO, S., XI, B., ZHAO, X., Front. Environ. Sci. Engin., 6, 2012, p REV. CHIM. (Bucharest) 66 No

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