Scientific paper Biosorption of Cadmium, Cobalt and Zinc by Moss Rhytidiadelphus squarrosus in the Single and Binary Component Systems Martin Pipiska,1'2'* Miroslav Hornik,12 Lucia Remenarova,1 Jozef Augustin12 and Juraj Lesny12 1 Department of Biotechnology, Faculty of Natural Sciences, University of SS. Cyril and Methodius in Trnava, J. Herdu 2, Trnava, SK-917 01, Slovak Republic. 2 Consortium for Environmental Biotechnology and Environmental Chemistry, Hlavna 418, Spacince, SK-919 51, Slovak Republic * Corresponding author: E-mail: pipiska@ucm.sk Received: 27-05-2009 Abstract Biomass of moss Rhytidiadelphus squarrosus was studied as a potential biosorbent for cadmium, cobalt and zinc removal from single and binary solutions. It was shown that solution pH significantly influenced Cd, Co and Zn biosorption. Maximum uptake was reached at pH 5.0-6.0 and negligible biosorption was observed at pH 2.0. Experimental equilibrium biosorption data for cadmium, cobalt and zinc were analysed by the Langmuir and Freundlich isotherm models. The Langmuir isotherm was found to well represent the measured sorption data in single metal systems. The maximum sorption capacities Qmax onto moss biomass were 186 |mol/g for Zn, 173 |mol/g for Cd and 123 |mol/g for Co. Results revealed that the presence of Cd more significantly decreased the sorption of Co in binary Cd-Co mixtures than vice versa. In Cd-Zn binary system, both cadmium and zinc were sorbed with equal efficiency. The competitive Langmuir equations were used to fit the experimental data from the Zn-Cd and Cd-Co binary systems and simple two-dimensional isotherm curves were replaced by three-dimensional sorption isotherm surfaces. Keywords: Metals, biosorption, competitive biosorption, Rhytidiadelphus squarrosus, isotherms 1. Introduction The removal of toxic metals from industrial wastewaters and aquatic environment is an important environmental issue to be solved today. Biosorption processes represent one of the possible interactions of toxic metals with biological systems in contaminated environment. 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 and biomass concentrations.1'2 Biosorption of metals is not based on only one mechanism. It consists of several mechanisms that quantitatively and qualitatively differ according to the type of biomass. Metal sequestration may involve the complex mechanisms, mainly ion exchange, chelation, adsorption by physical forces and ion entrapment in inter-and intrafibrilar capillaries and spaces of the structural polysaccharide cell wall network.3,4 While much research has been carried out on the uptake of single metal species, little attention seems to have been given to the study of multi-metal systems. Since industrial effluents can contain several metals, it is necessary to study the simultaneous sorption of two or more metal ions and also to quantify the mutual effect of one metal on the other. Therefore to ensure the applicability of biosorption technology, more works are still needed for the sorption of a mixture of metals at various operating conditions.5 To optimize the design of the sorption system (either single or multi-component) it is important to establish the most appropriate correlation for the equilibrium isotherms. Various simple isotherm models suitable for description of single component system are not suitable for prediction of ion equilibrium in multi-component system.6 They are unable to describe the sorption behavior of primary metal ion as a function of the concentration of both metal ions in the binary metal solution. For this purpose multi-component isotherm equations have been used. Recent reports on the sorption of multi-metal systems include that of Srivastava et al7 who studied the simultaneous sorption of Cd2+ and Zn2+ by rice husk ash using multi-component Langmuir and Freundlich adsorption isotherms. They found that the extended Freundlich model could be used to describe the sorption equilibrium in binary system. Romera et al8 used binary Langmuir type equations to describe the sorption of Cd2+, Zn2+, Ni2+, Cu2+ and Pb2+ in binary, ternary and multi-metallic systems by brown alga Fucus spiralis. Papageorgiou et al9 found that the extended predictive Langmuir isotherm accurately predicted the experimental data from Cu2+-Cd2+, Pb2+-Cu2+ and Pb2+-Cd2+ binary systems. The extended Sips, Sheindorf-Rebuhn-Sheintuch (SRS) and Redlich -Peterson multi-component models as well as Ideal Adsorbed Solution Theory (IAST) were also employed to evaluate multi-component data.9-12 In general, although the binary sorption could be described using various isotherm models, the extended competitive Langmuir model was more commonly used than the others.8,13,14 In the case of multi-component systems, evaluation and interpretation in 2-D geometry is rather complicated. In such cases, 3-D biosorption isotherm surfaces are more appropriate and correct way of representing the sorption equilibrium of two metal systems.15 This approach was successfully used by Ma and Tobin16, Hammaini et al17 and Fraile et al.18 Within this context, the objective of our study was firstly to quantify the ability of the moss R. squarrosus to sorb Co2+, Cd2+ and Zn2+ ions from single and binary solutions using radiometric analysis. The second objective was to compare the affinity of biosorbent for the above mentioned ions in binary metal systems Cd-Co and Cd-Zn. Finally, binary competitive Langmuir equations were chosen for describing mutual competitive effect of Co2+, Cd2+ and Zn2+ ions in binary systems and three-dimensional sorption isotherm surfaces for each binary system were generated. The choice of metals was made with regard to their industrial use and potential pollution impact. 2. Experimental 2. 1. Biosorbent Preparation Biomass of moss R. squarrosus was collected in June 2008 from the forests of High Tatras Mountains, Slovak Republic. The biomass was washed twice in deioni-sed water, oven-dried for 72 h at a maximum of 45 °C to avoid the degradation of binding sites. After drying, the biomass was milled and sieved. The 300 - 600 pm particle size was used in biosorption experiments. 2. 2. Batch Experiments for Single-metal Systems Batch biosorption experiments in single-metal systems were carried out in solutions ranged from 100 to 4000 pM of CdCl2, CoCl2 or ZnCl2 in deionised water, spiked with 109CdCl2, 65ZnCl2 or 60CoCl2 and adjusted to pH 4.0 and 6.0. Biomass (2.5 g/L, d.w.) was added to 8 ml of solution, and the content in Erlenmeyer flasks was agitated on a reciprocal shaker (Multi-shaker PSU 20, Biosan) at 120 rpm for 4 h at 20 °C. Contact time 4 h was sufficient to reach equilibrium which was shown in preliminary experiments. At the end of each experiment biomass was filtered out, washed twice with deionised water and radioactivity of both moss biomass and liquid phase was measured. This approach was also successfully used in research published in our previous papers.19,20 The metal uptake was calculated as (1) where Q is the uptake (pmol/g), C0 and Ceq are the initial and the final metal concentrations in solution (pmol/L) and m is the amount of dried biosorbent (given in grams). 2. 3. Batch Experiments for Binary-metal Systems Batch biosorption experiments in binary-metal systems were carried out in series of solutions (Cd-Zn or Cd-Co) containing each metal in concentrations varying from 100 to 4000 pM in various molar ratios 2:1, 1:1, 1:2 spiked with 109CdCl2, 60CoCl2 or 65ZnCl2 and adjusted to pH 6.0 with 0.1 M NaOH. Metal solutions were prepared by dissolving CdCl2, CoCl2 and ZnCl2 in deionised water to the desired initial concentrations. Biomass (2.5 g/L, d.w.) was added, and the content in Erlenmeyer flasks was agitated on a reciprocal shaker (120 rpm) for 4 h at 20 °C. At the end of each experiment biomass was filtered out, washed twice with deionised water and radioactivity of both moss biomass and liquid phase was measured. All experiments were performed in duplicate. If not otherwise stated, presented data are arithmetic mean values. 2. 4. Effects of pH The moss biomass sample was shaken in Cd2+, Co2+ or Zn2+ solutions of desired pH spiked with 109CdCl2, 65ZnCl2 and 60CoCl2 for 4 h on a reciprocal shaker at 120 rpm and 20 °C. In order to eliminate interference of buffer components on biosorption, the non-buffered solutions in deionised water were adjusted to the desired pH values by adding 0.5 M HCl or 0.1 M NaOH throughout the entire study. 2. 5. Speciation Modeling Prediction of the Cd, Co and Zn speciation in the aqueous systems as a function of total salt concentration and solution pH was performed using the Visual Minteq version 2.53.21 The programme has an extensive ther-modynamic database for the calculation of metal specia-tion, solubility and equilibrium. 2. 6. Radiometric Analysis For radiometric determination of 109Cd, 60Co and 65Zn in liquid samples and biomass, gamma spectrometric scintillation detector 54BP54/2-X and 76BP76/3 with well type crystal NaI(Tl) (Scionix, Netherlands) and data processing software Scintivision32 (Ortec, USA) were used. Standardized 109CdCl2 (3.857 MBq/mL, CdCl2 50 mg/L in 3 g/ L HCl), 60CoCl2 (5.181 MBq/mL, CoCl2 20 mg/L in 3 g/L HCl) and 65ZnCl2 (0.8767 MBq/mL, ZnCl2 50 mg/L in 3 g/L HCl) solutions werewere obtained from the Czech Institute of Metrology (Prague, Czech Republic). 2. 7. Data Analysis To calculate the maximum sorption capacities Qmax values and the corresponding parameters of adsorption isotherms non-linear regression analysis was performed by the ORIGIN 7.0 Professional (OriginLab Corporation, Northampton, USA). The 3-D sorption surfaces for each binary system were obtained by plotting the experimental metal equilibrium concentrations Ceq on the X and Y axes, against the metal uptake Qeq on the Z axis. The TableCur-ve 3D 4.0 (Systat Software, Inc., Chicago, USA) software was used for this purpose. The corrected Akaike's information criterion (AlCc), residual sum of squares (RSS), coefficients of determination between theoretical and experimental values (R2) and root mean squared errors (RMSE), were used to assess the goodness-of-fit. 3. Results and Discussion 3. 1. Metal Uptake Metals speciation in solution is important in biosorption processes since the metal uptake depends on the solution pH. According to the Visual Minteq speciation results (data not shown), cobalt and zinc in the single and binary systems Co-Cd and Zn-Cd at pH 6.0 occur practically as free cations (>99.4 % Co2+, >97.8% Zn2+) in the concentration range studied. Cadmium in single and Co-Cd or Zn-Cd binary systems occurs as free divalent cation Cd2+ and CdCl+ cation. At pH 6.0, free Cd2+ form ranged between 98% and 55.4% of the total cadmium within concentration range 100-4000 pM, while CdCl+ increased from 3% to 43%. CdCl2 (aq) form represent max. 1.9% in concentration range studied. The speciation of cadmium is very similar in both Cd-Co and Cd-Zn binary systems. Data sets were calculated considering the carbonate system naturally in equilibrium with atmospheric CO2 (p-CO2 = 38.5 Pa). The time-course studies of the biosorption of cadmium, cobalt and zinc ions from single systems showed that sorption of metal ions by moss R. squarrosus is a rapid process. Similar kinetic behaviours were observed for all experiments (data not shown) and equilibrium was reached within one hour. The mechanism of short-term metal cations uptake by moss is generally regarded as an abiotic process22 governed by surface complexation of cations with exposed functional groups (such as carboxyl-, sulfhydryl- and amino- group) on the moss surface, coordination and chelation of metals, ion exchange, adsorption or by the precipitation of solid phases on the cell walls. Sari et al23 using FTIR analysis confirmed that carboxyl and hydroxyl groups participate in Pd2+ biosorption by moss Racomitrium lanuginosum. It was reported in another study that the same functional groups of terrestrial moss Pleurozium schreberi was responsible for metal-binding capacity.24 Due to the complexity of biomaterials, it is probable that at least some of the above-mentioned mechanisms are acting simultaneously, to varying degrees, depending on the biosorbent and the solution chemistry. The same conclusions were also postulated by Sheng et al.25 3. 2. Effect of Initial pH on Metal Sorption To establish the effect of pH on the Cd2+, Co2+ or Zn2+ sorption onto R. squarrosus biomass from single metal systems, batch equilibrium studies at different pH values were carried out, and the results are shown in Fig. 1. Analysis of the experimental data shows that maximum biosorption of Cd2+, Co2+ and Zn2+ occurred at pH of 5.0 and 6.0 after 4 h incubation. Observed lower biosorption at pH 3.0 and negligible at pH 2.0 can be explained by protonation of active sites, resulting in competition between H+ and Cd2+, H+ and Co2+ or H+ and Zn2+ for occupancy of the binding sites.26 The pH value can change the state of the binding sites, which are usually acidic. Their protonation and consequently their availability can chan- 3 -b .......-"»........ —i .---i'...... ! Í ♦ j n Zn o Co » / ,3 * Cd pH Figure 1. Effect of initial pH on Cd' Bq/L ^ Zn (1000 |imol/L CdCl2, 60r 90 k- CdCl2), Co2+ (1000 |imol/L CoCl2, 75 kBq/L 60CoCl2) and 2+ (1000 |imol/L ZnCl2, 63 kBq/L 65ZnCl2) biosorption by moss R. squarrosus (2.5 g/L; d.w.) after 4 h incubation at 20 °C. Error bars represent standard deviation (SD) of the mean (n = 3). ge dramatically if the pH varied by 1 or 2 units.27 Our recent research showed similar effects of pH on Co2+ and Zn2+ sorption by lichen Evernia prunastri.20 Also Martins et al28 observed maximum biosorption of Cd2+ and Zn2+ at pH 5.0 and 6.0 using aquatic moss Fontinalis antipyretica. At higher pH values (pH > 8.0) cobalt exists mainly as [Co(OH)]+ form and therefore it becomes difficult to distinguish between sorption and precipitation. Similarly, insoluble cadmium and zinc species occurred at pH > 9.0. It is reasonable to suppose that the dependence of metal uptake on pH is related to both the surface functional groups on the biomass cell walls and the metal speciation in solution. Moreover, extreme pH values can damage the structure of biosorbent and therefore decrease metal uptake.29 3. 3 Equilibrium Modeling in Single Systems Analysis of equilibrium data on a specific mathematical equation is of significance for comparing different sorbents under different experimental conditions. The two well known adsorption isotherm models Langmuir (2) and Freundlich (3) were applied for the analysis of the experimental data in single sorption systems. Q„=KCt Cd >> Co (Table 1). This indicates higher affinity of R. squarrosus for Zn2+ than Cd2+ and Co2+ sorption from single metal solutions. Moreover, this is consistent with the idea that the difference in sorption capacity under similar environmental conditions could be attributed to different ionic characteristics of metal ions.33,34 Chen and Wang35 demonstrated that metal uptake capacities Qeq of divalent cations by Saccharomyces cerevisiae were significantly influenced by atomic number, covalent index, electronegativity, ionic radius and atomic weight and increased in the order Ni2+ < Sr2+ < Co2+ < Cd2+ < Zn2+ < Cu2+ < Pb2+. Comparison of Qmax values obtained in our work with those of other authors indicates that sorption of cadmium, zinc and cobalt ions by moss R. squarrosus is comparable with sorption of these metals by other algae, fungi, lichens and mosses (Table 3). Table 3. Biosorption of Cd , Zn and Co ions using different biosorbents. Biosorbent Qmax Dlmo^g] Cd2+ Zn2+ Co2+ Reference Chlorella minutissima (marine green alga) 99 Fontinalis antipyretica (aquatic moss) 249 Hylocomium splendens (moss) 289 Bifurcaria bifurcate (marine brown alga) 543 Penicillium chrysogenum (fungus) 191 Ulva fasciata sp. (marine green alga) -Hypogymnia physodes (foliose lichen) -Parmotrema tinctorum (foliose lichen) -Oscillatoria angustissima (blue-green alga) -Pilayella littoralis (marine brown alga) -Sargassum wightii (marine brown alga) - Rhytidiadelphus squarrosus (moss) 173 225 199 207 186 168 375 260 560 350 123 Roy et al 199336 Martins et al 200428 Sari et al 200837 Lodeiro et al 200538 Skowronski et al 200139 Kumar et al 200740 Pipfska et al 200719 Ohnuki et al 200341 Mohapatra and Gupta 200542 Carrilho and Gilbert 200043 Vijayaraghavan et al 200544 this study 3. 2. Equilibrium Modeling in Binary Systems Sorption in binary systems is complicated, because of the possible interactions among the metals. Experimental data describing sorption of metal ions by R. squarrosus from binary system Zn-Cd and Co-Cd at pH 6.0 are shown in Fig. 3 and Fig. 4. Sorption of metal ions increased with the increasing solution concentration until the saturation level was obtained. In all cases, the addition of co-ions caused the decrease of sorption of primary ion. The presence of Cd2+ in different molar [Cd]:[Co] ratios caused significant decrease in Co2+ sorption from 130 to 48 pmol/g moss biomass (Fig. 3B). On the contrary the presence of cobalt caused less pronounced decrease in cadmium sorption from 172 to 87 pmol/g (Fig. 3A). The competing effect of cadmium is proportional to Cd2+ concentration. Sorption of Zn and Cd ions in binary Zn-Cd system is depicted in Fig. 4A, B. It is evident that both metals ha- ve very similar maximum sorption of 173 pmol/g for Cd and 189 pmol/g for Zn from single metal systems (calculated from Langmuir isotherm). Also, competition effects exhibited by each ion on the uptake of other ion were very similar. When Cd and Zn ions are present in equimolar ratio 1:1 maximum uptake was 95.8 pmol/g for Cd2+ and 96.2 pmol/g for Zn2+. It can be pointed out that at the highest initial concentration of Cd2+, a decrease of approx. 50% could be noted in the Zn2+ uptake and vice versa. In the next steps the collected equilibrium data from the binary systems Cd-Zn and Co-Cd were analysed using binary isotherm models. Romera et al8 pointed out that the most appropriate form to describe sorption equilibrium in binary systems is to adjust the experimental data to a mathematical model from which number of parameters can be obtained for quantitative interpretation of sorption equilibrium uptake. Since, practically in all cases the biosorption of cadmium, cobalt and zinc ions in single metal systems by R. squarrosus was well described by Lang- Molar ratio A Co: Cd — 1:1 -■-2:1 —o— 1:2 -□-1:0 _______ --- / . '-* f 500 1000 1500 2000 2500 3000 3500 4000 cyco) [nmol/L] Figure 3. Isotherms of the cobalt (A) and cadmium (B) biosorption by R. squarrosus (2.5 g/L, d.w.) at 20 °C and pH 6.0 from binary system at different initial molar [Co]:[Cd] ratios. C (Zn) [(imol/L] ce,(cd> t^mol/Lj Figure 4. Isotherms of the zinc (A) and cadmium (B) sorption from by R. squarrosus (2.5 g/L, d.w.) at 20 °C and pH 6.0 from binary system at different initial molar [Zn]:[Cd] ratios. muir isotherm (Table 1), we used in our study the competitive Langmuir model developed under the concept of original Langmuir isotherm for single systems where one binding site was only available for one sorbate.45 When equilibrium is established: (5) (6) where Me1 and Me2 are metal ions in solution, B represents the free binding site, KMe1 a KMe2 represent equilibrium constants for binding sites occupied with metals Me1 and Me2, respectively. The final expression of competitive Langmuir model is as follows: (7) (8) QjMe2]= ^K^jMe^b^CjMe,] - K. and b.. - -— ft.., The total metal uptake in binary systems can be expressed as follows: (9) where Qeq[Me1] and Qeq[Me2] represent equilibrium sorption capacities of metals Me1 and Me2, Qeq[Me1 + Me2] is the sum of uptakes of the two metals, Ceq[Me1] and Ceq[Me2] represent equilibrium concentration of metals remaining in solution and Qmax is the maximum sorption capacity for the binary component systems. bMe1 and bMe2 represent affinity constants of Langmuir model for the first and second metal ions.46 The competitive Langmuir model equations can be represented by 3-D sorption isotherm surfaces. Binary Langmuir type equations (7, 8, and 9) were used to fit the experimental data and parameters obtained by the application of these models are presented in Table 4. Because Langmuir constant bMei is related to the energy of adsorption through the Arrhenius equation, the higher bMei represents the higher affinity of the sorbent for the sorbate. The values of bMe1 and bMe2 in binary system Cd-Zn are 0.012 L/pmol for Cd2+ and 0.009 L/pmol for Zn2+. The negligible difference between values indicates approximately equal affinity of R. squarrosus biomass to Cd and Zn ions. The total metal uptake Qeq Cd + Zn (pmol/g) as a function of equilibrium concentration Ceq (pmol/L) of cadmium and zinc is presented in Figure 5A. Continuous surface represents total metal uptake as predicted by the equation (9). Experimental values of the a) Figure 5. 3-D sorption isotherm surfaces of Cd-Zn binary system: (A) total metal sorption (^mol/g); (B) Cd sorption (|jmol/g); (C) Zn sorption (nmol/g). The surfaces are predicted by the competitive Langmuir model (eq. 7, 8 and 9) and the symbols are experimental data obtained at pH 6.0 and 20 °C. total metal uptake are shown as individual data points. At high total metal concentrations sorbent easily reaches the saturation level demonstrated by the plateau of the sorption surface (Figure 5A). The overall total metal ions uptake by R. squarrosus calculated from equation (9) is 181 ± 3 pmol/g. This value is very close to maximum sorption capacities (Qmax) of Cd and Zn ions from single systems, which indicates that cadmium and zinc ions in binary Cd-Zn system compete for the same binding sites.47 Figures 5B and 5C show sorption of Zn and Cd in the presence of co-ion. Continuous surfaces represent Zn and Cd uptake as predicted from equations (7, 8). The amount of cadmium or zinc ions sorbed decreased with increasing concentration of the co-ion. The high value of coefficient of determination (R2) and low root mean squared error (RMSE) shown in Table 4 suggested that the competitive Langmuir model well describes the experimental biosorp- Figure 6. 3-D sorption isotherm surfaces of Cd-Co binary system: (A) total metal sorption (pmol/g); (B) Cd sorption (pmol/g); (C) Co sorption (pmol/g). The surfaces are predicted by the competitive Langmuir model (eq. 7, 8 and 9) and the symbols are experimental data obtained at pH 6.0 and 20 °C. tion data. Similarly, Pérez-Marín et al in the case of the Cd-Zn binary system observed that the removal of cadmium and zinc by orange wastes was moderately affected by the presence of another metal and the competitive Langmuir model adequately represented the experimental data for the Cd-Zn binary system. The values of bMe1 and bMe2 in binary system Cd-Co are 0.032 L/pmol for Cd2+ and 0.003 L/pmol for Co2+ (Table 4). The significant difference between these values indicates higher affinity of R. squarrosus biomass to Cd ions in comparison with Co ions in multi-component system. The total metal uptake Qeq Cd + Co (pmol/g) as a function of equilibrium concentration Ceq (pmol/L) of cadmium and cobalt is presented in Figure 6A. Sorption surfaces (Fig. 6B, 6C) show sorption of Co2+ and Cd2+ in the presence of co-ion as predicted from equations (7, 8). Despite the fact that slightly lower value of R2 and slightly Table 4. Equilibrium parameters for Cd2+, Co2+ and Zn2+ biosorption from the binary mixtures Cd-Zn and Cd-Co by moss R. squarrosus calculated from competitive Langmuir model by non-linear regression analysis. Binary system Q [pmol/g] b * bMe1 [L/pmol] bMe2 [L/pmol] R2 RMSE*** Cd-Zn Cd-Co 181 ± 3 172 ± 4 0.012 ± 0.002 0.032 ± 0.009 0.009 ± 0.002 0.003 ± 0.001 0.938 0.901 12.28 16.13 * Mel - Cd in binary systems Cd-Zn and Cd-Co ** Me2 - Zn in binary system Cd-Zn, Co in binary system Cd-Co *** Root mean squared error (fit standard error) higher value of RMSE were observed for Cd-Co than for Cd-Zn binary system, the competitive Langmuir model well describes the experimental biosorption data in Cd-Co binary system. 4. Conclusions Investigation of the metal ions removal from aqueous solution by dried moss R. squarrosus showed that Cd2+, Co2+ and Zn2+ biosorption is a rapid, pH dependent process. Maximum uptake of metals was found to occur at pH 5.0 to 6.0. The experimental equilibrium data of the single-component systems for Cd2+, Co2+ and Zn2+ ions were well described by the Langmuir isotherm. The maximum sorption capacity Qmax increased up to pH 6.0 and followed the order Zn > Cd >> Co. 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Povzetek Raziskovali smo adsorpcijo Cd, Co in Zn iz vodnih raztopin na biomaso mahu Rhytidiadelphus squarrosus (biosorpci-ja). Izkazalo se je, da na biosorpcijo bistveno vpliva pH sistema: maksimalna je bila dosežena pri pH = 5.0-6.0, medtem ko je pri pH = 2.0 skoraj zanemarljiva. Izoterme smo opisali z Langrnuirjevim modelom ter Freundlichov izotermo. Ugotovili smo, da Langmuirjev model dobro opiši biosorpcijo preiskovanih sistemov ter da je maksimalna sorpcijska kapaciteta za posamezne kovine različna 186 |mol/g za Zn, 173 |mol/g za Cd in123 pmol/g za Co. Prisotnost Cd bolj zmanjša sorpcijo Co iz binarne mešanice Cd-Co kot obratno. Iz mešanic Cd-Zn je učinkovitost sorpcije enaka za obe kovini. Za mešanice smo uporabili model kompetitivne Langmuirjeve izoterme.