Microsoft Word - APV-pipiska FullText.doc Nova Biotechnologica VII-I (2007) 23 SORPTION OF COBALT AND ZINC FROM SINGLE AND BINARY METAL SOLUTIONS BY Evernia prunastri MARTIN PIPÍŠKA, MIROSLAV HORNÍK, ĽUBOŠ VRTOCH, SOŇA ŠNIRCOVÁ, JOZEF AUGUSTÍN Department of Biotechnology, University of SS. Cyril and Methodius, J. Herdu 2, Trnava, SK-917 01, Slovak Republic (pipiskam@ucm.sk) Non-living lichen Evernia prunastri was studied as biosorbent material for zinc and cobalt removal from single and binary metal solutions. Sorption equilibrium of Zn2+ and Co2+ ions was reached within 1 hour. Both cobalt and zinc biosorption was not pH dependent within the range pH 4-6 and negligible at pH 2. The experimental results were fitted to the Langmuir, Freundlich, Redlich-Peterson and Langmuir-Freundlich adsorption isotherms to obtain the characteristic parameters of each model. The Langmuir, Redlich-Peterson and Langmuir-Freundlich isotherms were found to well represent the measured sorption data. According to the evaluation using the Langmuir equation, the maximum sorption capacities of metal ions onto lichen biomass were 112 μmol/g Zn and 97.2 μmol/g Co from single metal solutions. E. prunastri exhibited preferential uptake of zinc from equimolar binary Zn2+ - Co2+ mixtures within the range 50 – 4000 μM. Even thought mutual interference was seen in all Co-Zn binary systems. To evaluate the two-metal sorption system, simple curves had to be replaced by three-dimensional sorption surface. These results can be used to elucidate the behavior of lichens as bioindicators of cobalt and zinc pollution in water and terrestrial ecosystems. Key words: zinc, cobalt, Evernia prunastri, sorption, equilibrium, isotherms 1. Introduction Sorption processes represent one of the possible mechanisms of interaction of toxic metals in contaminated aquatic systems. Bioremoval of single species of metal ions is affected by several factors such as the specific surface properties of biosorbent, temperature, pH, initial metal ion concentration and biomass concentration. When several metals are present, many other parameters affect the sorption process. Simple sorption isotherms are usually constructed as a result of studying equilibrium batch sorption behavior of different biosorbent materials. These curves enable quantitative evaluation of biosorption performance of these materials for only one metal (HAMMAINI et al., 2003; AKSU and DŐNMEZ, 2006) When more than one metal at a time is present in a sorption system, evaluation, interpretation, and representation of biosorption results become much more complicated. The aim of this study was to investigate the ability of the lichen E. prunastri growing in Slovak territory to sorb Co2+ and Zn2+ from single and binary metal solutions. Co-Zn system was chosen as representative of bivalent metals found in industrial effluents. A range of equilibrium adsorption isotherms were obtained to quantitatively describe cobalt and zinc uptake. 24 Pipíška, M. et al. 2. Materials and methods 2.1 Biomass Biomass of fruticose lichen E. prunastri were taken from the spruces (Picea abis) grown in the forest of High Tatras, Slovak Republic. The lichen was washed twice in deionized water and oven-dried at 45°C for 72 h. Cut thallus sections of diameter 3-4 mm were used in experiments. 2.2 Sorption studies Sorption experiments were carried out in 10 ml solutions ranged from 50 to 4000 μM of Zn2+, Co2+ and Zn2+-Co2+, spiked with 65Zn or 60Co. The pH value was adjusted to 4.0. Biomass (50 mg, d.w.) was added, and the content in flasks was agitated on a reciprocal shaker (120 rpm) for 4 h at 20°C. At the end of the experiments radioactivity of both lichen and liquid samples was measured. All experiments were carried out four times. If not otherwise stated, presented data are arithmetic mean values. To calculate the Qmax values and the corresponding parameters of adsorption isotherms (Tab. 1) non-linear regression analysis was performed by the NLREG® software (Statistical analysis program, version 6.3 created by P.H. Sherrod) and ORIGIN 7.0 Professional (OriginLab Corporation, Northampton, USA). The 3-D sorption surfaces were obtained by plotting the experimental Zn and Co equilibrium concentrations Ceq on the X and Y axes, against the Co, Zn and total metal uptake Qeq on the Z axis. The STATISTICA 7.0 (StatSoft, Inc., Tulsa, USA) and NLREG® software were used for this purpose. Table 1. Adsorption isotherm models used in this work. Isotherm Equation Equation No. Adjustable parameters Langmuir eq eq eq bC CbQ Q + = 1 max 1 Qmax, b Freundlich Qeq= K Ceq (1/n) 2 b, n Redlich – Peterson g eq eq eq CB AC Q )(1+ = 3 A, B, g Langmuir – Freundlich ( ) n eq n eq eq bC bCQ Q /1 /1 max )(1+ = 4 Qmax, b, n Binary Langmuir 2211 1max1 1 1 eqeq eq eq CbCb CQb Q ++ = 5 Qmax, b1,b2 2.3 Speciation modeling Prediction of the speciation of Co and Zn in the aqueous systems was performed using the VisualMINTEQ (version 2.52) program. This speciation model allows the calculation of the composition of solutions for specified conditions. Nova Biotechnologica VII-I (2007) 25 2.4 Radiometric analysis For radiometric determination of 60Co and 65Zn in liquid samples and lichen biomass, gamma spectrometric scintillation detector 54BP54/2-X with well type crystal NaI(Tl) (Scionix, Netherlands) and data processing software Scintivision32 (Ortec, USA) were used. Standardized 60CoCl2 solution (5.181 MBq.ml -1, CoCl2 20 mg.l-1 in 3 g.l-1 HCl) and 65ZnCl2 (0.8767 MBq.ml -1, ZnCl2 50 mg.l -1 in 3 g.l-1 HCl) obtained from Alldeco Inc., Slovakia were used in all experiments. 3. Results and discussion 3.1 Metal uptake Metals speciation in solution is important in sorption studies since the metal uptake depends on the solution pH. As can be calculated by VisualMINTEQ speciation program, cobalt and zinc in the single and binary solutions at pH 4 occur practically as free cations (>99,5 % Co2+, >98 % Zn2+) (data not shown). The time-course studies on the biosorption of cobalt and zinc ions were performed by contacting Co and Zn solutions with lichen biomass at pH 4.0 and 20 °C. Biosorption of Co2+ and Zn2+ ions by E. prunastri shown in Fig. 1 is a rapid process. Maximum uptakes (approximately 90%) were reached within 1 hour and practically did not change during the next 24 hours. 0 5 10 15 20 25 0 20 40 60 80 100 B io so rp tio n [% ] t [h] Zn2+ Co2+ Fig. 1. Biosorption kinetics of Co2+ (100 µmol/L, 60CoCl2 89 kBq/L) and Zn2+ (100 µmol/L, 65ZnCl2 70 kBq/L) by E. prunastri (5 g /L, d.w.) from single metal solutions at 20°C. Initial pH 4.0; pH 4.2 after 24 hours. Error bars represent standard deviation of the mean (n = 4). Similar adsorption kinetics of Cd2+, Zn2+, Pb2+ and Cu2+ by lichen E. prunastri was observed by ANTONELLI et al., (1998), where equilibrium was reached within a few minutes. The above mentioned results indicate that biosorption of the aforementioned cations was not dependent on metabolic activity. The biosorption process can be attributed to interactions of cations with anionic functional groups on the thallus surface. The thallus surface was supposed to contain ionogenic groups that generate a negative net charge (GARTY, 2001). 26 Pipíška, M. et al. 3.2 Sorption equilibrium in single metal solution Analysis of equilibrium data on a specific mathematical equation is of significance for comparing different sorbents under different experimental conditions. Four well known adsorption isotherm models: Langmuir, Freundlich, Redlich-Peterson and Langmuir-Freundlich (see equations in Table 1) were applied for the analysis of the experimental data. These models use parameters that reflect the nature of the sorbent and can be used to compare biosorption performance. Qmax represents the maximum sorption capacity, b is a constant related to the energy of adsorption. k and 1/n values are the Freundlich constants referring to adsorption capacity and intensity of adsorption, respectively. A, B and g (0