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Binary biosorption of iron(III) and iron(III)-cyanide complex ions on Rhizopus arrhizus: modelling of synergistic interaction
Process Biochemistry 38 (2002) 161 /173 www.elsevier.com/locate/procbio
Binary biosorption of iron(III) and iron(III)-cyanide complex ions on Rhizopus arrhizus: modelling of synergistic interaction Zu¨mriye Aksu *, Hanife Gu¨len Department of Chemical Engineering, Hacettepe University, 06532, Beytepe, Ankara, Turkey Received 27 November 2001; received in revised form 21 December 2001; accepted 19 February 2002
Abstract Many heavy metal-bearing wastewaters also contain their metal cyanide complex ions. Although the biosorption of single or multi-metal ions to various microorganisms has been extensively studied, very little attention has been given to the bioremoval and the expression of the adsorption equilibrium and kinetics of metal /metal cyanide complex ion systems. In this study, the simultaneous biosorption of iron(III) (ferric) cations and iron(III)-cyanide complex (ferricyanide) anions to Rhizopus arrhizus from binary mixtures was studied and compared with single metal and metal cyanide complex ion situation in a batch stirred system. The effects of initial pH and single and dual-component concentrations on the biosorption kinetics and equilibrium uptake of each component, both singly and in mixture were investigated. The working pH value for both species was determined as 2.0. Multicomponent biosorption studies were also performed at this pH value. The biosorption rates and equilibrium uptakes of iron(III) or iron(III)-cyanide complex ions increased by the presence of increasing concentrations of the other ion up to 200 mg l 1 for iron(III) and up to 1000 mg l 1 for iron(III)-cyanide complex ions. This situation showed a synergistic interaction between these ions. The Freundlich, Langmuir and Redlich /Peterson adsorption models were used to predict the mono-component equilibrium uptake and model parameters were estimated by the non-linear regression. It was seen that the mono-component adsorption equilibrium data fitted very well to the mono-component Langmuir and Redlich /Peterson models for both the components at moderate ranges of concentration. A modified synergistic Langmuir model was proposed for dual-component system and model parameters were also estimated by the non-linear regression. The pseudo second-order kinetic model was applied to single and multi-component experimental data assuming that the external mass transfer limitations in the system can be neglected and biosorption is sorption controlled. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Iron(III); Iron(III)-cyanide complex ions; Biosorption; Synergism; Isotherms; Second order kinetics; R. arrhizus
1. Introduction Cyanide is commonly found as a contaminant in wastewaters from various industries including metal cleaning, plating, electroplating, metal processing, automobile parts manufacture, steel tempering, mining, photography, pharmaceuticals, coal coking, ore leaching, plastics, etc. Consequently, the wastewaters generated by these industries often contain significant quantities of heavy metals such as nickel, copper, zinc and iron besides cyanides. Since cyanide is highly reactive, it will readily bind metals as a strong ligand
* Corresponding author. Tel.: /90-312-2977434; fax: /90-3122992124. E-mail address: [email protected] (Z. Aksu).
to form complexes of variable stability and toxicity. Examples include the well-known hexacyano complex of iron (both iron(II)- and iron(III)-cyanide complex ions) and the tetracyano complexes of divalent nickel, copper and zinc. Traditionally, biological treatment, activated carbon adsorption, solvent extraction, chemical oxidation with ozone and ultraviolet radiation are the most widely used methods for removing metal cyanide complex ions including iron-cyanide complexes which are anionic, and very stable complexes from wastewaters [1 /9]. The main techniques which have been utilized for treatment heavy metal-bearing waste streams also include precipitation, evaporation, adsorption, ion exchange, membrane processing, solvent extraction, etc. All these methods have been found to be limited, since they often involve high capital and operational costs and
0032-9592/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 2 ) 0 0 0 6 2 - 6
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may also be associated with the generation of secondary wastes which present treatment problems. The special surface properties of microorganisms enable them to adsorb different kinds of pollutants from solutions. This passive bioaccumulation process (biosorption) has distinct advantages over the conventional methods: the process does not produce chemical sludges (i.e. nonpolluting), it could be highly selective, more efficient, easy to operate and hence cost effective for the treatment of large volumes of wastewaters containing low pollutant concentrations [1,12 /23]. Biosorption is generally used for the treatment of heavy metal pollutants in wastewaters and may be also used for the treatment of wastewaters containing metal cyanide complex ions. Rhizopus arrhizus is a well-known fungus used extensively for heavy metal biosorption. Thousands of tons of residual R. arrhizus biomass coming from some industrial fermentation processes such as lipase, fumaric acid, lactic acid, steroids, etc. in each year could serve as a potential biosorbent. The cell wall of R. arrhizus essentially consists of various organic compounds including chitin, acidic polysaccharides, lipids, amino acids, glucans and other cellular components, and could provide passive uptake of metal and metal cyanide complex ions by surface adsorption, ion exchange, micro-precipitation, complexation, chelation, etc. [9,11,14,15,20]. Bioremoval of a single species of pollutants using microorganisms is affected by several factors. These factors include the chemical nature of the pollutant (species, size, ionic charge), the species of microorganism, the specific surface properties of the microorganism and environmental conditions (pH, temperature, ionic strength, existence of other components). Many other parameters affect the capacity of microorganisms to bind more than one species simultaneously. The combined effects of two or more components on microorganisms also depend on the number of pollutants competing for binding sites, pollutant combination, levels of pollutant concentration, order of pollutant addition and residence time. While the knowledge of the toxicity and general uptake of single and multi species of heavy metal ions by microorganisms is increasing, relatively little is known about the combined effects of two or more metal and metal cyanide complex ions and simultaneous removal of metal and metal cyanide complex ions from their mixture. It has also to be recognized that single toxic metallic species rarely exist in natural and wastewaters [18 /23]. 1.1. Single- and multi-component equilibrium modelling At the first stage of biosorption, a rapid equilibrium is established between sorbed component on the cell and unadsorbed component remaining in solution. This equilibrium can be represented by adsorption isotherms.
The single-component adsorption isotherm is the mathematical function describing quantitatively, at a constant temperature, the relationship between residual (equilibrium) component concentration left in solution after binding (Ceq) and amount of component bound to the biomass (qeq, usually determined by difference). The most widely used isotherm equation for modelling equilibrium is the Langmuir equation, based on the assumption that there is a finite number of binding sites which are homogeneously distributed over the adsorbent surface, these binding sites have the same affinity for adsorption of a single molecular layer and there is no interaction between adsorbed molecules. The mathematical description of this model for a single-component adsorption is qeq
Qo bCeq 1 bCeq
(1)
;
where Qo is the maximum possible amount of component per unit weight of adsorbent to form a complete monolayer on the surface bound at high Ceq, and b is a constant related to the affinity of the binding sites. Another way of expressing Qo is as the maximum value of qeq. The empirical Freundlich model also considers a monomolecular layer coverage of solute by the sorbent. However, it assumes that the sorbent has a heterogeneous surface suggesting (as expected) that binding sites are not equivalent and/or independent. This model takes the following form for a single-component adsorption: 1=n qeq KF Ceq ;
(2)
where KF and n are the Freundlich constants related to the adsorption capacity and intensity of the sorbent, respectively. The Freundlich model is more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model. The three-parameter Redlich /Peterson equation was proposed to improve the fit by the Langmuir or Freundlich equation and is given by Eq. (3). qeq
KRP Ceq b 1 aRP Ceq
;
(3)
where KRP, aRP and b are the Redlich /Peterson parameters. The exponent b lies between 0 and 1. For b /1 Eq. (3) converts to the Langmuir form [24]. The two parameters in the Langmuir and Freundlich equations and the three parameters in the Redlich /Peterson equation can be obtained using a least-squares fitting procedure which minimizes the deviation between calculated and measured data. In general, a mixture of components can produce three possible types of behaviour: synergism (the effect of the mixture is greater than that of each of the individual effects of the constituents in the mixture),
Z. Aksu, H. Gu¨len / Process Biochemistry 38 (2002) 161 /173
antagonism (the effect of the mixture is less than that of each of the individual effects of the constituents in the mixture) and non-interaction (the effect of the mixture is no more nor no less than that of each of the individual effects of the constituents in the mixture) [18,20]. The equilibrium data obtained in a multi-component system indicate that how these components affected each other’s biosorption equilibrium due to solution pH as compared with results from single-component adsorption situation. The prediction of multi-component equilibrium data has always been complicated due to the interactive and competitive effects involved. The behaviour of each species in a multi-component system depends strongly on the physical and chemical properties of both sorbent and sorbate. This determines the sorbate/sorbent chemical relation which affects the equilibrium behaviour hereafter. In addition, the number and kind of species present, concentration of each component, the pH of solution decide the shape and equilibrium constants of the isotherm. Nevertheless attempts are carried out to predict and correlate multicomponent data from single-component data. Several competitive multi-component adsorption models have been proposed to describe the antagonistic interaction between the adsorbed quantity of one component and the concentrations of all other components, either in solution or already adsorbed at equilibrium. These isotherms range from simple models related to the individual isotherm parameters only (non-modified adsorption models), to more complex models related to the individual isotherm parameters and to correction factors (modified adsorption models) [17,19/21,23,25]. However, a multi-component equilibrium model has not been proposed yet to predict the synergistic interaction between the adsorbed component and the concentrations of all other components. Here a synergistic equilibrium model was developed for multi-component systems from the mono-component Langmuir adsorption model using the single-component parameters and correction factors and has been proposed by Eq. (4).
qeqi
Qoi bi Ceqi [1Fi (C)]; 1 bi Ceqi
(4)
where Fi (C ) represents the positive fractional deviation of the multi-component adsorption isotherm from the single-component Langmuir isotherm for component i. Ceqi and qeqi are the unadsorbed concentration of i component in the mixture at equilibrium and the adsorbed quantity of i component per g of adsorbent at equilibrium, respectively. bi and Qoi are derived from the corresponding individual Langmuir isotherm equation. Fi (C ) assumed to be specific for each qi is expressed by Eq. (5)
163
PN
j i;j"i Kj Coj ; Fi (C) P N j i Kj Coj
(5)
where Coj represents the initial concentration of j component in the mixture, and Kj is a modification coefficient for component j. According to Eqs. (4) and (5) the binary form of the modified Langmuir model for the first and the second components can be expressed as: Qo1 b1 Ceq1 K2 Co2 qeq1 ; (6a) 1 1 b1 Ceq1 K1 Co1 K2 Co2 1 Qo b C K1 Co1 qeq2 2 2 eq2 1 : (6b) 1 b2 Ceq2 K1 Co1 K2 Co2 2 The parameters Qoi and bi are obtained from the singlecomponent adsorption isotherms, while Kj values are estimated from numerical simulations of two-component adsorption data [19]. 1.2. Single- and multi-component kinetic modelling In order to investigate the mechanism of biosorption and potential rate controlling step such as mass transport and chemical reaction processes, kinetic models have been used to test experimental data both in singleand multi-component systems. When the biomass is employed as a free cell suspension in a well-agitated batch system, all the cell wall binding sites are made readily available for uptake so the effect of external film diffusion on biosorption rate can be assumed not significant and ignored in any engineering analysis [18]. The pseudo second-order kinetic model can be used in this case assuming that measured concentrations are equal to cell surface concentrations. The pseudo second-order kinetic model is based on the sorption capacity of the solid phase. Contrary to the first-order kinetic model it predicts the behaviour over the whole range of adsorption and is in agreement with an adsorption mechanism being the rate controlling step. If the rate of sorption is a second-order mechanism, the pseudo second-order chemisorption kinetic rate equation for each component, both singly and in the mixture is expressed as: dqi k2;i (qeqi qi )2 ; dt
(7)
where k2,i is the rate constant of each component for second-order biosorption. For the boundary conditions t /0 to t /t and qi /0 to qi /qeqi ; the integrated and linear form of Eq. (7) becomes t qi
1 k2;i q2eqi
1 qeqi
t:
(8)
If second-order kinetics are applicable, the plot of t /q against t of Eq. (8) should give a linear relationship,
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from which qeqi and k2,i can be determined from the slope and intercept of the plot and there is no need to know any parameter beforehand [16,22]. Relatively little work appears to have been done on the ability of R. arrhizus biomass to adsorb metal and metal cyanide complexes together. In this study, the simultaneous biosorption of ferric and ferricyanide ions to R. arrhizus from binary mixtures was studied and compared with single-component situation in a batch stirred system. The binding capacity of biomass was shown as a function of pH, single and dual-component concentrations. The mono-component sorption phenomena were expressed by the Langmuir, Freundlich and Redlich /Peterson adsorption models and model parameters were determined by non-linear regression. A multi-component Langmuir model was developed to express the adsorption isotherms of ferric and ferricyanide ions mixtures by the biomass. The pseudo secondorder kinetic model was also applied to single and multicomponent experimental data. This is the first detailed study of the binding of these two ions simultaneously to R. arrhizus .
2. Materials and methods 2.1. Microorganism and growth conditions R. arrhizus , a filamentous fungus obtained from the U.S. Department of Agriculture Culture Collection was used in this study. Tie microorganism was grown at 25 8C in agitated liquid media containing malt extract (17 g l 1) and soya peptone (5.4 g l 1). The pH was adjusted to 5.4 /5.6 with concentrated and diluted H2SO4 solutions. 2.2. Preparation of the microorganism for biosorption After the growth period, R. arrhizus was washed twice with distilled water, inactivated using 1% formaldehyde and then dried at 110 8C for 24 h. For biosorption studies, a weight amount of dried biomass was suspended in 100 ml double-distilled water and homogenized in a homogenizer (Janke and Kunkel, IKALabortechnick, Ultra Turrax T25) at 8000 rpm for 20 min and then stored in the refrigerator.
Fisher, in 1 l of double-distilled water. Stock ferric iron solution was prepared from ferrous ammonium sulphate as follows: 7.02 g of crystallized ferrous ammonium sulphate was dissolved in 500 ml double-distilled and deionized water and 50 ml of 1:1 H2SO4 was added. The solution was warmed and oxidized with approximately 0.1% (w/v) potassium permanganate solution until the iron(III) solution remained faintly pink. Then, the solution was cooled and diluted to 1.0 l. For binary ferric and ferricyanide ions mixture studies, desired combinations of iron(III)-cyanide complex ions and iron(III) ions were obtained by diluting 1.0 g l 1 of stock solutions of components and mixing them in the test medium. Before mixing with the fungal suspension, the pH of each test solution was adjusted to the required value with concentrated H2SO4 solutions. The ranges of concentrations of iron(III) and iron(III)-cyanide complex ions prepared from stock solutions varied between 25 /200 mg l 1 and 25/1000 mg l 1, respectively. 2.4. Batch biosorption studies The factors that affect the adsorption rate and uptake capacity of the biosorbent were examined in Erlenmayer flasks. Aliquots of 15 ml of fungal suspension were contacted with 135 ml of ferric or ferricyanide ions or ferric ion and ferricyanide mixture-bearing solution at desired level of each component at the beginning of the adsorption at the desired temperature and pH. All the final solutions contained 1.0 g l 1 mass of biosorbent. The flasks were agitated on a shaker at a 150 rpm constant shaking rate for 24 h to ensure equilibrium was reached. Samples (5 ml) were taken before mixing the biosorbent solution and the metal ion or metal cyanide complex ion or metal /metal cyanide complex ion mixture-bearing solution, at 5 min intervals at the beginning of adsorption and 25 /30 min intervals after reaching equilibrium. The samples were centrifuged at 5000 rpm for 3 min and then the supernatant liquid was analysed for iron(III) and iron(III)-cyanide complex ions. Studies were performed at a constant temperature of 25 8C to be representative of environmentally relevant conditions. All the biosorption experiments were repeated twice to confirm the results.
2.3. Chemicals
2.5. Analysis of iron and iron(III)-cyanide complex ions
The test solutions containing single iron(III) or iron(III)-cyanide complex ions were prepared by diluting 1.0 g l 1 stock solutions of iron(III) or iron(III)cyanide complex ions to the desired concentrations. Stock solution of iron(III)-cyanide complex ions was prepared by dissolving 1.551 g potassium ferricyanide (K3Fe[CN]6) of analytical reagent grade obtained from
The concentration of the residual iron(III)-cyanide complex ions in the biosorption medium was determined iodometrically. In this method, iron(III)-cyanide complex ions were oxidized to iron(II)-cyanide complex ions by potassium iodide in the acidic medium and iodine was formed. The amount of iodine (or indirectly the amount of iron(III)-cyanide complex ions) was deter-
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mined by titration with sodium thiosulphate solution [26]. The concentration of free iron(III) ions in the biosorption medium was determined spectrophotometrically. The coloured complex of iron(III) ions with sodium salicylate was read at 530 nm. No interference of iron(III)-cyanide complex ions on the analysis of iron(III) ions was observed [27].
3. Results and discussion The kinetic and equilibrium results are given as the units of adsorbed iron(III) or iron(III)-cyanide complex ion quantity in single- or multi-component situation per g of biosorbent at any time (qi ; mg g 1) and at equilibrium (qeqi ; mg g 1) and residual iron(III) or iron(III)-cyanide complex ion concentration in single or multi-component situation at equilibrium (Ceqi ; mg l 1). The adsorption yield is defined as the ratio of sorbed concentration of iron(III) or iron(III)-cyanide complex ions at equilibrium (this value is also equal to qeqi value since the biomass concentration is 1.0 g l 1) to the initial iron(III) or iron(III)-cyanide complex ion concentration for single-component removal situation (Ad%). Individual and total adsorption yields in the simultaneous removal from the mixture of iron(III) and iron(III)-cyanide complex ions were also defined as the ratios of individual and total adsorbed concentrations of each component at equilibrium to individual and total initial component concentrations, respectively (Adi %, AdTot%). 3.1. Effect of initial pH on the biosorption of iron(III) and iron(III)-cyanide complex ions The most critical parameter in the treatment of monoand multi-component systems by the biomass is the pH of biosorption medium. Previous investigations concerning the single-component situation have shown that the optimum adsorption pH depended on both cell surface binding sites and chemistry of components in aqueous solution. The uptake of iron(III) and iron(III)cyanide complex ions by the biomass was a function of pH too. The effect of initial pH on iron(III) and iron(III)-cyanide complex ion uptake capacity of R. arrhizus was studied at 100 mg l 1 initial concentration and the variation of equilibrium uptake with initial pH was given in Fig. 1. It was shown that the removal of both the single species of iron(III) and iron(III)-cyanide complex ions from aqueous solution was more efficient at increasing pH values. Cell walls have binding sites that are considered as being part of ionizable groups such as carboxyl, hydroxyl, amino and imino. At highly acidic pH values, metal cations and protons compete for binding sites on
Fig. 1. Effect of initial pH on the equilibrium uptake of iron(III) and iron(III)-cyanide complex ions in the single-component situation (Co: 100 mg l 1, X : 1.0 g l 1, T : 25 8C).
cell wall, which results in lower uptake of metal. It has been suggested that at low pH values, cell wall ligands would be closely associated with H3O that restrict access to ligands by metal ions as a result of repulsive forces. It is to be expected that as pH values are increased, more ligands with a negative charge would be exposed with subsequent increase in attraction for positively charged iron(III) ions [12 /15,20]. However, biosorption studies could not be performed at pH values higher than 2.5 because of the reduced solubility and precipitation of iron(III) ions. Metals present in wastewaters as cyanide complexes, generating anions should be repelled by the cellular wall. However, precipitation and complexation may be the main mechanisms responsible for iron(III)-cyanide complex ions removal at higher pH values. Iron-cyanide complexes are very stable complexes which react with metals for the formation of metal complexes by organic compounds present in the cell wall [8 /11]. There is little literature on the interactions of iron(III)-cyanide complex ions with the biosorbent surface. Nevertheless the working adsorption pH value was chosen as 2.0 for both components since it was not possible to undertake studies at higher pH values for iron(III) ions. 3.2. Single and dual biosorption of iron(III) and iron(III)-cyanide complex ions The equilibrium uptakes and biosorption yields obtained at single iron(III) and iron(III)-cyanide complex ions situation at pH 2.0 are shown in Figs. 2 and 3 and are presented in Table 1. As seen from the figures and the table, the biosorption capacity of dried R. arrhizus biomass for iron(III) was generally less than that of the iron(III)-cyanide complexes and increasing the initial concentration up to 200 mg l 1 for iron(III) and up to 1000 mg l 1 for iron(III)-cyanide complex
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Fig. 2. Effect of initial iron(III) concentration on iron(III) equilibrium uptake with the iron(III) present as the single metal ion and in the presence of increasing concentrations of iron(III)-cyanide complex ions at pH 2.0 (X : 1.0 g l 1, T : 25 8C).
Fig. 3. Effect of initial iron(III)-cyanide complex ion concentration on iron(III)-cyanide complex ion equilibrium uptake with the iron(III)cyanide complex ion present as the single-component and in the presence of increasing concentrations of iron(III) ions at pH 2.0 (X : 1.0 g l 1, T : 25 8C).
ions increased the equilibrium uptake and decreased the adsorption yield of both components. When the initial iron(III)-cyanide complex ion concentration increased from 26.7 to 992.0 mg l 1, the loading capacity of dried
biomass increased from 13.3 to 94.3 mg g 1. The equilibrium uptakes obtained from experimental data also showed that when the initial iron(III) ion concentration increased from 27.0 to 210.0 mg l 1, the iron(III) loading capacity of dried biomass increased from 12.8 to 30.0 mg g 1. The simultaneous biosorption of iron(III) and iron(III)-cyanide complex ions from binary mixture was also investigated at pH 2.0. In the first stage of biosorption studies, while initial iron(III) concentration was changed from 0 to 200 mg l 1, initial iron(III)cyanide complex ion concentration was held constant at 0, 25, 50, 100, 200, 500 or 1000 mg l 1. The nonlinearised adsorption isotherms of ferric ions in the absence and presence of increasing concentrations of ferricyanide ions is shown in Fig. 2. Equilibrium iron(III) uptake increased with increasing initial iron(III) concentration up to 200 mg l 1 at all ferricyanide ion concentrations studied. The curvilinear relationship between the amount of iron(III) adsorbed per unit weight of microorganism and the residual iron(III) concentration at equilibrium suggests that saturation of cell-binding sites occurred at higher concentrations of this component. The equilibrium uptake of iron(III) increased regularly with increasing ferricyanide ion concentration. The individual and total biosorption equilibrium uptakes and yields of iron(III) and iron(III)-cyanide complex ions to R. arrhizus found at different iron(III) concentrations in the absence and presence of increasing concentrations of iron(III)-cyanide complex ions are also listed in Table 1. In general the increase in iron(III)cyanide complex ion concentration increased the individual adsorption yields of iron(III) and total adsorption yields for each experimental set. The results also showed that the equilibrium uptakes of iron(III) ions increased with increasing initial iron(III)-cyanide complex ion concentration. At 100 mg l 1 initial iron(III) concentration, in the absence of iron(III)-cyanide complex ions and in the presence of 100 mg l 1 iron(III)-cyanide complex ion concentration, adsorbed iron(III) quantities at equilibrium were 26.7 and as 41.7 mg g 1, respectively. The combined effects of two components seemed to be synergistic. To decide whether those metal/metal cyanide complex ion combinations were interacting in a synergistic manner, the adsorption yields of single- and multi-component systems were also compared. For instance, using Table 1, it was expected that total adsorption yield must be equal to 34.7% for total 195.6 mg l 1 metal /metal cyanide complex ion mixture containing 93.3 mg l 1 iron(III) and 102.3 mg l 1 iron(III)-cyanide complex ion together [AdTot% / 34.7 /100 /[(26.7 mg l 1 iron(III)/41.2 mg l 1 iron(III)-cyanide complex ion)/195.6 mg l 1 initial total concentration], but from Table 1, experimental total adsorption yield was 40.0% for total 222.4 mg l 1
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Table 1 Comparison of individual and total biosorption equilibrium uptakes and yields found at different iron(III) concentrations in the absence and presence of increasing concentrations of iron(III)-cyanide complex ions CoFe (mg l 1)
CoFeCn (mg l 1)
qeqFe (mg g 1)
qeqFeCn (mg g 1)
/
qeq(Fe FeCn) (mg g 1)
AdFe%
AdFeCn%
AdTot%
27.0 51.0 93.3 210.1
0.0 0.0 0.0 0.0
12.8 18.3 26.7 30.0
0.0 0.0 0.0 0.0
12.8 18.3 26.7 30.0
47.5 35.9 28.6 14.3
0.0 0.0 0.0 0.0
47.5 35.9 28.6 14.3
0.0 29.3 49.5 101.7 181.3
26.7 28.4 27.0 28.4 26.7
0.0 16.9 23.2 33.3 36.5
13.2 16.2 17.4 19.5 20.1
13.2 33.1 40.6 52.8 56.6
0.0 57.7 46.8 32.8 20.1
50.0 56.9 64.4 68.6 75.2
50.0 57.3 53.0 40.6 27.2
0.0 26.7 52.0 116.7 206.7
56.8 56.8 56.8 57.5 56.1
0.0 17.2 25.8 38.7 41.7
24.7 29.0 31.2 32.0 33.1
24.7 46.2 57.1 70.7 74.8
0.0 64.5 49.7 33.1 20.2
43.5 51.0 55.0 55.7 59.0
43.5 55.3 52.5 40.6 28.5
0.0 28.3 49.3 113.3 215.2
102.3 102.3 109.1 109.1 108.0
0.0 18.3 27.0 41.7 43.7
41.2 43.6 46.6 47.3 48.2
41.2 61.9 73.5 89.0 91.8
0.0 64.7 54.6 36.8 20.3
40.3 42.6 42.7 43.4 44.6
40.3 47.4 46.4 40.0 28.4
0.0 28.3 52.3 117.0 225.2
204.6 211.2 215.9 225.7 227.3
0.0 19.6 29.0 45.0 46.7
63.9 73.3 74.9 78.8 79.8
63.9 92.9 104.0 123.7 126.5
0.0 69.0 55.4 38.5 20.7
31.3 34.7 34.7 34.9 35.1
31.3 38.8 38.8 36.1 28.0
0.0 28.3 56.3 117.0 216.7
505.7 484.0 511.2 526.3 534.1
0.0 21.1 31.9 47.3 52.0
82.4 103.6 109.4 113.2 122.3
82.4 124.0 141.0 159.8 174.3
0.0 74.3 56.5 40.5 24.0
16.2 21.4 21.4 21.5 22.9
16.2 24.3 24.9 24.8 23.2
0.0 28.0 56.3 119.7 193.3
992.0 965.0 988.3 1025.3 1034.1
0.0 21.1 31.9 47.3 52.0
94.3 112.9 116.6 121.0 125.1
94.3 133.9 153.6 170.7 179.2
0.0 76.2 66.3 41.7 27.9
9.5 11.7 11.8 11.8 12.1
9.5 13.5 14.7 14.9 14.6
iron(III)/iron(III)-cyanide complex ion mixture consisting of 113.3 mg l 1 iron(III) and 109.1 mg l 1 iron(III)cyanide complex ion together [AdTot% /40.0 /100 / [(41.7 mg l 1 iron(III)/47.3 mg l 1 iron(III)-cyanide complex ion)/222.4 initial total concentration]. As a result, the effect of the mixture was more than that of each of the individual effects of the constituents in the mixture so the interaction between iron(III) and iron(III)-cyanide complex ion could be assumed to be synergistic. The uptake of iron(III)-cyanide complex ions in the absence and presence of increasing concentrations of iron(III) ions in the range of 25 /200 mg l 1 was also studied. Fig. 3 depicts the variations of iron(III)-cyanide complex ions uptakes at equilibrium with increasing initial iron(III)-cyanide complex ions concentrations (from 25 to 1000 mg l 1) at constant initial iron(III) concentrations at pH 2.0. Similar biosorption patterns
were obtained both in the single-iron(III)-cyanide complex ion and iron(III) /iron(III)-cyanide complex ion systems; iron(III)-cyanide complex ion equilibrium uptakes increased with increasing initial iron(III)-cyanide complex ion concentrations up to 1000 mg l 1. Increases in iron(III) concentration also increased slightly the equilibrium uptake quantities of ferricyanide. The data given in Table 1 also showed how the presence of ferric ions affected the equilibrium uptakes of ferricyanide ions, both singly and in mixture, individual ferricyanide, individual ferric and total adsorption yields at pH 2.0. The results in Table 1 indicate that the presence of iron(III) had also an encouraging effect on the equilibrium uptake of iron(III)-cyanide complex ions in this situation. In the absence of iron(III) ions, equilibrium iron(III)-cyanide complex ion uptake was determined as 41.2 mg g 1 at 102.3 mg l 1 initial iron(III)-cyanide complex ion concentration. When
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initial iron(III) concentration was kept at 113.3 mg l 1 at the same initial iron(III)-cyanide complex ion concentration, this value was increased to 47.3 mg g 1. The data obtained in the single and dual systems indicated that the adsorption capacity of R. arrhizus for iron(III) was generally less than that of iron(III)-cyanide complex ions. It was clear that a significant part of the adsorption capacity of the microorganism was used for iron(III)-cyanide complex ions adsorption. There are possible interaction effects between different species in solution and in particular potential interactions on the surface depending on the biosorption mechanism. Factors that affect the biosorption preference of a biosorbent for different kinds of adsorbates may be related to the characteristics of the binding sites (e.g. functional groups, structure, surface properties, etc.), the properties of the adsorbates (e.g. concentration, ionic size, ionic weight, ionic charge, molecular structure, ionic nature or standard reduction potential, etc.) and the solution chemistry (e.g. pH, ionic strength, etc.). It is difficult and complicated to find a common rule from the physical and chemical properties of iron(III) and iron(III)-cyanide complex ions to identify how these properties affect the selective sorption of biosorbent. This is because the observed behaviour may result from a combination of all the above factors [13 /16,18,20]. For simultaneous biosorption of iron(III) and iron(III)-cyanide complex ions it was seen that the adsorptive capacity of iron(III)-cyanide complex ions in the presence of iron(III) ions slightly increased whereas the uptake of iron(III) significantly increased by the addition of iron(III)-cyanide complex ions. Higher uptake in multi-component system as compared to single-component system for iron(III) and iron(III)cyanide complex ions may be due to chemistry of the components and chemical nature of the binding sites effects. A number of fungi has been shown to sequester iron(III) with the help of iron-chelating agents. Excess iron(III) can facilitate the precipitation of iron-cyanide complex ions on the surface [7,10,11,28]. Due to a large number of variables involved in the components by the biosorbent and the complexity of the surface and water chemistry, the exact mechanisms explaining the biosorption of ions from aqueous solutions are unclear. 3.3. Application of mono- and multi-component adsorption models for single and dual biosorption of iron(III) and iron(III)-cyanide complex ions Equilibrium data of components both singly or in mixture, commonly known as adsorption isotherms, are basic requirements for the design of adsorption systems. To obtain the isotherm data, initial concentrations of iron(III) or iron(III)-cyanide complex ions were varied while the amount of biosorbent in each sample was kept constant for each component. Classical mono-compo-
nent adsorption models (Langmuir, Freundlich and Redlich /Peterson) are used to describe the equilibrium between the iron(III) or iron(III)-cyanide complex ions in solution (Ceq) and iron(III) or iron(III)-cyanide complex ions adsorbed on the cell surface (qeq). The individual adsorption constants for each component obtained by evaluating the isotherms are listed in Table 2 with the average percentage errors. The average percentage errors between the experimental and predicted values are calculated using Eq. (9). In Eq. (9), the subscripts ‘exp’ and ‘calc’ show the experimental and calculated values and N the number of measurements. PN ½(qeq;i;exp qeq;i;calc )=qeq;i;exp ½ o% i 1 100: (9) N In view of the values of percentage errors in the table, the Langmuir and Redlich /Peterson models exhibited the best fit to the adsorption data of iron(III) and iron(III)-cyanide complex ions. However, the Freundlich model also seemed to agree well with the experimental data of two components considering that obtained percentage error values are lower than 15.2%. The experimental equilibrium data of iron and iron(III)cyanide complex ions are compared with the theoretical equilibrium data obtained from these adsorption models in Fig. 4. The figure also confirmed that the adsorption equilibrium data fitted very well to the Langmuir and Redlich /Peterson models in the studied concentration ranges for both components. Adsorption model constants, the values of which express the surface properties and affinity of the biosorbent, can be used to compare the adsorptive capacity of biosorbent for different components. From Table 2, the magnitude of KF and n; the Freundlich constants, showed easy uptake of iron(III) and iron(III)cyanide complex ions from wastewater with high adsorptive capacity of the dried biomass. Table 2 also indicated that n is greater than unity, indicating that both ions are favourably adsorbed by R. arrhizus . While the Freundlich model does not describe the saturation behaviour of the biosorbent, Qo; the monocomponent Langmuir constant is the monolayer saturation at equilibrium, b; the other mono-component Langmuir constant corresponds to the concentration at which an iron(III) or an iron(III)-cyanide complex ion amount of Qo/2 is bound and indicates the affinity for the binding of iron(III) or iron(III)-cyanide complex ions. The value of Qo tabulated in Table 2 appeared to be significantly higher for iron(III)-cyanide complex ions in comparison with the uptake of iron(III) at the same pH value of 2.0. The higher value of b implied the strong bonding of iron(III) to the cells. Relevant adsorption parameters have also been calculated according to the non-competitive three-parameter isotherm of Redlich /Peterson for each
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169
Table 2 Comparison of the Freundlich, Langmuir and Redlich /Peterson adsorption constants obtained from the mono-component Langmuir, Freundlich and Redlich /Peterson adsorption models for iron(III) and iron(III)-cyanide complex ions with the average percentage errors Iron(III) ions o
Q (mg g Langmuir model
1
b (l mg
1.45
o (%) Qo (mg g 1)
)
0.042 1
5.46 KRP (l g
Redlich /Peterson model
)
33.9 KF [(mg g
Freundlich model
Iron(III)-cyanide complex ions 1
)(mg l
1 n
) ]
3.7
n
)
aRP (l mg
98.0
o (%) KF [(mg g
2.90 1
7.2 1 b
)
0.043
component and are also tabulated in Table 2. The value of b is equal to 1.0 for iron(III)-cyanide complex ions and tends to unity for iron(III) ions, that is the isotherms are approaching the Langmuir form. The simultaneous biosorption phenomena of iron(III) and iron(III)-cyanide complex ions to R. arrhizus were expressed by the proposed multi-component Langmuir model. Using the mono-component Langmuir constants for each ion given in Table 2, the multi-component Langmuir constants were determined as K1 /0.68 and K2 /1.14. Using the individual and modified multicomponent Langmuir adsorption constants, qeq values were predicted from the related multi-component Langmuir adsorption formula. According to the theoretical base of multi-component Langmuir model, the adsorbed quantity of the first component increased with increasing the concentration of the second component, depending on the values of the individual Langmuir constants of both the components. The comparison of the experimental and calculated qeq values of iron(III) and iron(III)-cyanide complex ions in mixtures is presented in Fig. 5. Basically, if most of the data are distributed around the 458 line this indicates that the model represent well the experimental data of the system so as shown in Fig. 5. The multi-component Langmuir model fitted reasonably well the binary uptake data of iron(III) and iron(III)-cyanide complex ions in the studied concentration range, although slight deviations (The average percent magnitude of absolute deviation was changed from 0.0 to 20.0% in the model for the entire data set of iron(III) and iron(III)-cyanide complex ions.) were observed between the experimental and calculated results from the model at lower and higher component concentrations. These results can be attributed to the insensitivity of model to interactive effects existing in multi-component systems and the characteristics of Langmuir model, which is not valid for high concentrations assuming limited number of identical sites for sorption. It was concluded that the proposed
b 1.00
b (l mg 1)
5.07 o (%) 3.5
0.012 1
)(mg l
1 n
) ]
3.6
n
o (%)
2.17 KRP (l g 1.12
o (%)
15.2 1
) aRP (l mg 1)b 0.011
b
o (%)
0.99 2.6
multi-component Langmuir model provided a more realistic description of the biosorption process and can be used to predict the uptake of iron(III) or iron(III)cyanide complex ions in the binary system with more accuracy.
3.4. Comparison of biosorption kinetics of single and dual biosorption of iron(III) and iron(III)-cyanide complex ions to R. arrhizus Fig. 6 showed the biosorption kinetics of iron(III) and iron(III)-cyanide complex ions by plotting the uptake capacity, q , versus time for 100 mg l 1 initial ion concentration in single and binary systems. Results clearly indicated that the single sorption of iron(III) and iron(III)-cyanide complex ions by the biomass increased rapidly up to about 60 min with 70.4 and 74.2% sorption, respectively, showing physical adsorption at the cell surface and thereafter rose slowly before attaining a saturation value (i.e. biosorption equilibrium). Biosorption reached equilibrium in 6 h at this initial concentration of both components. The equilibrium sorption capacities of iron(III) and iron(III)cyanide complex ions were determined as 26.7 and 41.2 mg g 1, respectively. The equilibrium time of 6 h was not affected by the binary sorption of iron(III) and iron(III)-cyanide complex ions as observed in singlecomponent system. However, the sorption capacity of iron(III) and iron(III)-cyanide complex ions was appreciably increased in binary combination. A 35.3% increase in iron(III) sorption and a 14.2% increase in iron(III)-cyanide complex ion sorption were observed in 100 mg l 1 iron(III) and 100 mg l 1 iron(III)-cyanide complex ion containing mixtures. This figure demonstrates that the presence of iron(III)-cyanide complex ions has a marked impact on the sorption of the iron(III) ions.
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Fig. 5. Comparison of the experimental and predicted qeq values of iron and iron(III)-cyanide complex ions in binary mixtures.
Fig. 6. Biosorption kinetics of iron(III) and iron(III)-cyanide complex ions to R. arrhizus at 100 mg l 1 initial ion concentration in single and binary systems.
Fig. 4. Comparison of the experimental equilibrium data of iron and iron(III)-cyanide complex ions at pH 2.0 with the theoretical equilibrium data obtained from the Freundlich, Langmuir and Redlich / Peterson adsorption models.
3.5. Application of single and multi-component kinetic modelling for single and dual biosorption of iron(III) and iron(III)-cyanide complex ions In order to analyse the biosorption kinetics of iron(III) and iron(III)-cyanide complex ions in single and binary systems, the second-order kinetic model was applied to data. Fig. 7 shows the plots of linearised form of the second-order kinetic model for 100 mg l 1 initial iron(III) or iron(III)-cyanide complex ion concentration in single and binary systems. The values of the
Fig. 7. Comparison of t /q vs. t plots for 100 mg l 1 initial ion concentration in single and binary systems.
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171
Table 3 Comparison of the second-order adsorption rate constants and calculated and experimental equilibrium uptake values obtained at different initial ion concentrations CoFe (mg l 1)
CoFeCn (mg l 1)
qeqFe,exp (mg g 1)
qeqFeCn,exp (mg g 1)
k2Fe 103 (g mg 1 min 1)
k2FeCn 103 (g mg 1 min 1)
qeqFe,teo (mg g 1)
qeqFeCn,teo (mg g 1)
27.0 51.0 93.3 210.0
0.0 0.0 0.0 0.0
12.8 18.3 26.7 30.0
0.0 0.0 0.0 0.0
3.01 2.50 1.69 1.31
0.00 0.00 0.00 0.00
13.8 18.6 27.0 30.5
0.0 0.0 0.0 0.0
0.0 29.3 49.5 101.7 181.3
26.7 28.4 27.0 28.4 26.7
0.0 16.9 23.2 33.3 36.5
13.2 16.2 17.4 19.5 20.1
0.00 3.11 3.08 1.71 1.49
1.87 2.20 2.64 5.27 8.18
0.0 17.2 23.4 33.8 37.2
14.0 16.5 17.7 19.7 20.2
0.0 26.7 52.0 116.7 206.7
56.8 56.8 56.8 57.5 56.1
0.0 17.2 25.8 38.7 41.7
24.7 29.0 31.2 32.0 33.1
0.00 3.52 3.14 1.73 1.50
1.41 1.68 1.84 3.37 4.34
0.0 17.5 26.3 39.4 42.2
26.0 29.9 31.7 32.4 33.4
0.0 28.3 49.3 113.3 215.2
102.3 102.3 109.1 109.1 108.0
0.0 18.3 27.0 41.7 43.7
41.2 43.6 46.6 47.3 48.2
0.00 3.80 3.34 1.78 1.52
1.01 1.14 1.32 2.03 2.65
0.0 18.6 27.3 42.2 44.4
43.1 45.1 47.2 48.1 48.8
0.0 28.3 52.3 117.0 225.2
204.6 211.2 215.9 225.7 227.3
0.0 19.6 29.0 45.0 46.7
63.9 73.3 74.9 78.8 79.8
0.00 4.59 3.47 1.97 1.75
0.73 0.83 1.05 1.35 1.84
0.0 19.7 29.2 45.7 47.6
65.2 74.6 75.8 80.0 80.7
0.0 28.3 56.3 117.0 216.7
505.7 484.0 511.2 526.3 534.1
0.0 21.1 31.9 47.3 52.0
82.4 103.6 109.4 113.2 122.3
0.00 5.38 3.77 2.08 1.78
0.70 0.71 0.94 1.30 1.56
0.0 21.3 32.1 48.1 52.4
83.3 105.3 109.9 113.6 123.4
0.0 28.0 56.3 119.7 193.3
992.0 965.0 988.3 1025.3 1034.1
0.0 21.1 31.9 47.3 52.0
94.3 112.9 116.6 121.0 125.3
0.00 6.01 3.80 2.10 1.84
0.66 0.68 0.82 1.01 1.46
0.0 21.7 37.7 51.0 54.6
95.2 113.6 117.7 123.5 126.6
parameters k2,ad and qeq and of correlation coefficients are also presented in Table 3 for all combinations. The correlation coefficients for the second-order kinetic model are obtained 1.000 for all mixtures. The secondorder adsorption rate constants of iron(III) and iron(III)-cyanide complex ions in single and multi-component systems decreased with increasing concentrations. The theoretical qeq values also agreed very well with the experimental qeq values in the case of pseudo secondorder kinetics. There was a very minor deviation between expected and observed qeq values. These data suggest the applicability of second-order kinetic model based on the assumption that the rate limiting step may be the biosorption in explaining the kinetics of biosorption.
4. Conclusions In the mixtures of two or more species in a solution, the synergistic or antagonistic interaction occurring between the species might affect the individual uptake by the microorganism. In the study, the capability of the fungi R. arrhizus to bind iron(III) and iron(III)-cyanide complex ions individually and simultaneously from aqueous solutions was examined, including equilibrium and dynamic studies. Experiments were performed as a function of initial pH, initial iron(III) and iron(III)cyanide complex ion concentration. In conventional biological treatment systems where insufficient contact time is available for biodegradation to occur, biosorption results in the removal of toxic compounds from the
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aqueous waste stream and in the accumulation of hazardous pollutants in the microbial sludge in a short time. The experimental results showed that R. arrhizus has a considerable potential for the rapid uptake of iron(III) and iron(III)-cyanide complex ions over a wide range of concentration. Since the sequence of second ion addition affected the adsorption rate and equilibrium uptake positively, the combined effect of iron(III) and iron(III)-cyanide complex ions on the adsorption of R. arrhizus was found to be synergistic. For the single ion situation, the Freundlich, Langmuir and Redlich /Peterson adsorption models were used for the mathematical description of the biosorption of iron(III) and iron(III)-cyanide complex ions to dried R. arrhizus and the isotherm constants were evaluated to compare the biosorptive capacity of the dried biomass for the iron(III) and iron(III)-cyanide complex ions. It was seen that the adsorption equilibrium data fitted well to all the models. The individual Langmuir constants evaluated from the Langmuir isotherm were used to find the correction factors in the multi-component Langmuir model describing binary adsorption equilibrium. It was concluded that the proposed Langmuir model agreed well with the results found experimentally. As a result, it could be said that iron(III) / iron(III)-cyanide complex ions multi-ion system could be defined with Langmuir adsorption isotherm and it could be used to model the adsorption of binary systems from aqueous solutions. Assuming the batch biosorption as a single-staged equilibrium operation, the separation process for the single and binary sorption of iron(III) and iron(III)-cyanide complex ions can be mathematically defined using these isotherm constants to estimate the residual concentration of components or amount of biosorbent for desired purification. The suitability of second-order kinetic model for the single and binary sorption of iron(III) and iron(III)cyanide complex ions onto biomass was also discussed assuming no effect of mass transfer on the biosorption rate. The kinetic parameters obtained can be used for a bioreactor design. It may be suitable to apply such simple kinetic models to a well-agitated batch biosorption system consisting of free cell suspension neglecting external film diffusion. It may be concluded that dried R. arrhizus may be used successfully for the removal of iron(III) and iron(III)-cyanide complex ions. Biosorption by R. arrhizus can be proposed as an alternative to more costly methods such as biological treatment, activated carbon adsorption, solvent extraction, chemical oxidation for removal of iron(III) and iron(III)-cyanide complex ions from waste streams. This work could enable to extrapolate the prediction of adsorption equilibria of the single and binary system if experimental data are not available for a certain level of bisolute concentrations.
Acknowledgements The authors wish to thank to Hacettepe University Research Fond, Turkey, Project No.YDABC ¸ AG198Y097, for the partial financial support.
Appendix A: Nomenclature Redlich /Peterson adsorption constant (l mg 1)b Langmuir adsorption constant (l mg 1) unadsorbed concentration of the single-component at equilibrium (mg l 1) Ceqi unadsorbed concentration of each component in the binary mixture at equilibrium (mg l 1) Coi initial concentration of each component (mg l 1) q adsorbed quantity of the single-component per g of dried biomass at any time (mg g 1) qi adsorbed quantity of each component in the binary mixture per g of dried biomass at any time (mg g 1) qeq adsorbed quantity of the single-component per g of dried biomass at equilibrium (mg g 1) qeqi adsorbed quantity of each component in the binary mixture per g of dried biomass at equilibrium (mg g 1) KF Freundlich adsorption constant [(mg g 1)(mg l 1)n ] KRP Redlich /Peterson adsorption constant (l g 1) Kj multi-component Langmuir adsorption constant k2,ad second-order adsorption rate constant (g mg 1 min 1) n Freundlich adsorption constant Qo Langmuir adsorption constant (mg g 1) X biosorbent (dried R. arrhizus ) concentration (g l 1) b Redlich /Peterson adsorption constant o (%) average percentage error aRP b Ceq
References [1] Patterson JF. Industrial wastewater treatment technology. London: Butterworths, 1985. [2] Palmer SAK, Breton MA, Nunno TJ, Sullivan DM, Surprenant NF. Metal-cyanide containing wastes-treatment technologies. Pollution technology reviews, vol. 158. New Jersey: Noyes Data Corporation, 1988. [3] Bodek H, Ehnfholt DJ, Glazer AE, Loretti E, Lyman W. Environmental inorganic chemistry: properties, processes and estimation methods. New York: Pergamon Press, 1995. [4] Sharp AG. The chemistry of cyano complexes of the transition metals. London: Academic Press, 1976. [5] Dursun AY, .C ¸ alık A, Aksu Z. Degradation of ferrous(II) cyanide complex ions by Pseudomonas fluorescens . Process Biochem 1999;34:901 /8.
Z. Aksu, H. Gu¨len / Process Biochemistry 38 (2002) 161 /173 [6] Silva-Avalos J, Richmond MG, Nagappan OJ. Degradation of the metal cyano-complex of tetracyano-nickelate(II) by cyanide utilising bacterial isolates. Appl Environ Microbiol 1990;56:3664 /70. [7] Aronstein BN, Maka A, Srivastava VJ. Chemical and biological removal of cyanides from aqueous and soil-containing systems. Appl Microbiol Biotechnol 1994;41:700 /7. [8] Patil YB, Paknikar KM. Removal and recovery of metal cyanides using a combination of biosorption and biodegradation processes. Biotechnol Lett 1999;21:913 /9. [9] Aksu Z, .C ¸ alık A, Dursun AY, Demircan Z. Biosorption of iron(III)-cyanide complex anions to Rhizopus arrhizus : application of the adsorption isotherms. Process Biochem 1999;34:483 / 91. [10] Gonc¸alves MMM, Pinto AF, Granato M. Bioegradation of free cyanide, thiocyanate and metal complexed cyanides in solutions with different compositions. Environ Technol 1998;19:133 /42. [11] Gomes NCM, Figueira MM, Camargos ERS, Mendonc¸a-Hagler LCS, Dias JCT, Linardi VR. Cyano-metal complexes uptake by Aspergillus niger . Biotech Lett 1999;21:487 /90. [12] Wilde EW, Benemann JR. Bioremoval of heavy metals by the use of microalgae. Biotech Adv 1993;11:781. [13] Veglio F, Beolchini F. Removal of metals by biosorption: a review. Hydrometallurgy 1997;44:301 /16. [14] Volesky B. Biosorption by fungal biomass. In: Volesky B, editor. Biosorption of heavy metals. Boca Raton, FL: CRC Press, 1990:7 /43. [15] Kapoor A, Viraraghavan T. Fungal biosorption-an alternative treatment option of heavy metal bearing wastewater: a review. Biores Tech 1995;53:196 /205. [16] Aksu Z. Equilibrium and kinetic modelling of cadmium(II) biosorption by C. vulgaris in a batch system: effect of temperature. Sep Pur Tech 2001;21:285 /94.
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[17] Ho YS, McKay G. Correlative biosorption equilibria model for a binary batch system. Chem Eng Sci 2000;55:817 /25. [18] Ting YP, Lavson F, Prince IG. Uptake of cadmium and zinc by the algae C. vulgaris : Part II: Multi-ion species. Biotech Bioeng 1991;37:445 /52. [19] Chang J-S, Chen C-C. Quantitative analysis and equilibrium models of selective adsorption in multimetal systems using a bacterial biosorbent. Sep Sci Tech 1998;33:611 /32. ¨ , Aksu Z, Kutsal T. A comparative study for the [20] Sag˘ Y, Ac¸ıkel U simultaneous biosorption of iron(III) and chromium(VI) on C. vulgaris , and R. arrhizus : application of the competitive adsorption models. Process Biochem 1998;33:273 /81. [21] Al-Asheh S, Banat F, Al-Omari R, Duvnjak Z. Predictions of binary sorption isotherms for the sorption of heavy metals by pine bark using single isotherm data. Chemosphere 2000;41:659 /65. [22] Singh S, Rai BN, Rai LC. Ni(II) and Cr(VI) sorption kinetics by Microcystis in single and multimetallic system. Process Biochem 2001;36:1205 /15. [23] Pagnanelli F, Trifoni M, Beolchini F, Esposito A, Toro L, Veglio F. Equilibrium biosorption studies in single and multi-metal systems. Process Biochem 2001;37:115 /24. [24] Redlich OJ, Peterson DL. A useful adsorption isotherm. J Phys Chem 1959;63:1024. [25] Bellot JC, Condoret JS. Modeling of liquid chromatography equilibria. Process Biochem 1993;28:365 /76. [26] Vogel AI. A text book of quantitative inorganic analysis, theory and practice. London: Lowe and Brydone Printers Ltd, 1948. [27] Snell FD, Snell CT. Colorimetric methods of analysis, 3rd ed.. New York: Van Nostrand Company, 1959. [28] Emelyanova EV. Relationship between magnesium and iron uptake by the yeast Candida ethanolica . Process Biochem 2001;28:517 /25.
Binary biosorption of iron(III) and iron(III)-cyanide complex ions on Rhizopus arrhizus: modelling of synergistic interaction Zu¨mriye Aksu *, Hanife Gu¨len Department of Chemical Engineering, Hacettepe University, 06532, Beytepe, Ankara, Turkey Received 27 November 2001; received in revised form 21 December 2001; accepted 19 February 2002
Abstract Many heavy metal-bearing wastewaters also contain their metal cyanide complex ions. Although the biosorption of single or multi-metal ions to various microorganisms has been extensively studied, very little attention has been given to the bioremoval and the expression of the adsorption equilibrium and kinetics of metal /metal cyanide complex ion systems. In this study, the simultaneous biosorption of iron(III) (ferric) cations and iron(III)-cyanide complex (ferricyanide) anions to Rhizopus arrhizus from binary mixtures was studied and compared with single metal and metal cyanide complex ion situation in a batch stirred system. The effects of initial pH and single and dual-component concentrations on the biosorption kinetics and equilibrium uptake of each component, both singly and in mixture were investigated. The working pH value for both species was determined as 2.0. Multicomponent biosorption studies were also performed at this pH value. The biosorption rates and equilibrium uptakes of iron(III) or iron(III)-cyanide complex ions increased by the presence of increasing concentrations of the other ion up to 200 mg l 1 for iron(III) and up to 1000 mg l 1 for iron(III)-cyanide complex ions. This situation showed a synergistic interaction between these ions. The Freundlich, Langmuir and Redlich /Peterson adsorption models were used to predict the mono-component equilibrium uptake and model parameters were estimated by the non-linear regression. It was seen that the mono-component adsorption equilibrium data fitted very well to the mono-component Langmuir and Redlich /Peterson models for both the components at moderate ranges of concentration. A modified synergistic Langmuir model was proposed for dual-component system and model parameters were also estimated by the non-linear regression. The pseudo second-order kinetic model was applied to single and multi-component experimental data assuming that the external mass transfer limitations in the system can be neglected and biosorption is sorption controlled. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Iron(III); Iron(III)-cyanide complex ions; Biosorption; Synergism; Isotherms; Second order kinetics; R. arrhizus
1. Introduction Cyanide is commonly found as a contaminant in wastewaters from various industries including metal cleaning, plating, electroplating, metal processing, automobile parts manufacture, steel tempering, mining, photography, pharmaceuticals, coal coking, ore leaching, plastics, etc. Consequently, the wastewaters generated by these industries often contain significant quantities of heavy metals such as nickel, copper, zinc and iron besides cyanides. Since cyanide is highly reactive, it will readily bind metals as a strong ligand
* Corresponding author. Tel.: /90-312-2977434; fax: /90-3122992124. E-mail address: [email protected] (Z. Aksu).
to form complexes of variable stability and toxicity. Examples include the well-known hexacyano complex of iron (both iron(II)- and iron(III)-cyanide complex ions) and the tetracyano complexes of divalent nickel, copper and zinc. Traditionally, biological treatment, activated carbon adsorption, solvent extraction, chemical oxidation with ozone and ultraviolet radiation are the most widely used methods for removing metal cyanide complex ions including iron-cyanide complexes which are anionic, and very stable complexes from wastewaters [1 /9]. The main techniques which have been utilized for treatment heavy metal-bearing waste streams also include precipitation, evaporation, adsorption, ion exchange, membrane processing, solvent extraction, etc. All these methods have been found to be limited, since they often involve high capital and operational costs and
0032-9592/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 2 ) 0 0 0 6 2 - 6
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may also be associated with the generation of secondary wastes which present treatment problems. The special surface properties of microorganisms enable them to adsorb different kinds of pollutants from solutions. This passive bioaccumulation process (biosorption) has distinct advantages over the conventional methods: the process does not produce chemical sludges (i.e. nonpolluting), it could be highly selective, more efficient, easy to operate and hence cost effective for the treatment of large volumes of wastewaters containing low pollutant concentrations [1,12 /23]. Biosorption is generally used for the treatment of heavy metal pollutants in wastewaters and may be also used for the treatment of wastewaters containing metal cyanide complex ions. Rhizopus arrhizus is a well-known fungus used extensively for heavy metal biosorption. Thousands of tons of residual R. arrhizus biomass coming from some industrial fermentation processes such as lipase, fumaric acid, lactic acid, steroids, etc. in each year could serve as a potential biosorbent. The cell wall of R. arrhizus essentially consists of various organic compounds including chitin, acidic polysaccharides, lipids, amino acids, glucans and other cellular components, and could provide passive uptake of metal and metal cyanide complex ions by surface adsorption, ion exchange, micro-precipitation, complexation, chelation, etc. [9,11,14,15,20]. Bioremoval of a single species of pollutants using microorganisms is affected by several factors. These factors include the chemical nature of the pollutant (species, size, ionic charge), the species of microorganism, the specific surface properties of the microorganism and environmental conditions (pH, temperature, ionic strength, existence of other components). Many other parameters affect the capacity of microorganisms to bind more than one species simultaneously. The combined effects of two or more components on microorganisms also depend on the number of pollutants competing for binding sites, pollutant combination, levels of pollutant concentration, order of pollutant addition and residence time. While the knowledge of the toxicity and general uptake of single and multi species of heavy metal ions by microorganisms is increasing, relatively little is known about the combined effects of two or more metal and metal cyanide complex ions and simultaneous removal of metal and metal cyanide complex ions from their mixture. It has also to be recognized that single toxic metallic species rarely exist in natural and wastewaters [18 /23]. 1.1. Single- and multi-component equilibrium modelling At the first stage of biosorption, a rapid equilibrium is established between sorbed component on the cell and unadsorbed component remaining in solution. This equilibrium can be represented by adsorption isotherms.
The single-component adsorption isotherm is the mathematical function describing quantitatively, at a constant temperature, the relationship between residual (equilibrium) component concentration left in solution after binding (Ceq) and amount of component bound to the biomass (qeq, usually determined by difference). The most widely used isotherm equation for modelling equilibrium is the Langmuir equation, based on the assumption that there is a finite number of binding sites which are homogeneously distributed over the adsorbent surface, these binding sites have the same affinity for adsorption of a single molecular layer and there is no interaction between adsorbed molecules. The mathematical description of this model for a single-component adsorption is qeq
Qo bCeq 1 bCeq
(1)
;
where Qo is the maximum possible amount of component per unit weight of adsorbent to form a complete monolayer on the surface bound at high Ceq, and b is a constant related to the affinity of the binding sites. Another way of expressing Qo is as the maximum value of qeq. The empirical Freundlich model also considers a monomolecular layer coverage of solute by the sorbent. However, it assumes that the sorbent has a heterogeneous surface suggesting (as expected) that binding sites are not equivalent and/or independent. This model takes the following form for a single-component adsorption: 1=n qeq KF Ceq ;
(2)
where KF and n are the Freundlich constants related to the adsorption capacity and intensity of the sorbent, respectively. The Freundlich model is more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model. The three-parameter Redlich /Peterson equation was proposed to improve the fit by the Langmuir or Freundlich equation and is given by Eq. (3). qeq
KRP Ceq b 1 aRP Ceq
;
(3)
where KRP, aRP and b are the Redlich /Peterson parameters. The exponent b lies between 0 and 1. For b /1 Eq. (3) converts to the Langmuir form [24]. The two parameters in the Langmuir and Freundlich equations and the three parameters in the Redlich /Peterson equation can be obtained using a least-squares fitting procedure which minimizes the deviation between calculated and measured data. In general, a mixture of components can produce three possible types of behaviour: synergism (the effect of the mixture is greater than that of each of the individual effects of the constituents in the mixture),
Z. Aksu, H. Gu¨len / Process Biochemistry 38 (2002) 161 /173
antagonism (the effect of the mixture is less than that of each of the individual effects of the constituents in the mixture) and non-interaction (the effect of the mixture is no more nor no less than that of each of the individual effects of the constituents in the mixture) [18,20]. The equilibrium data obtained in a multi-component system indicate that how these components affected each other’s biosorption equilibrium due to solution pH as compared with results from single-component adsorption situation. The prediction of multi-component equilibrium data has always been complicated due to the interactive and competitive effects involved. The behaviour of each species in a multi-component system depends strongly on the physical and chemical properties of both sorbent and sorbate. This determines the sorbate/sorbent chemical relation which affects the equilibrium behaviour hereafter. In addition, the number and kind of species present, concentration of each component, the pH of solution decide the shape and equilibrium constants of the isotherm. Nevertheless attempts are carried out to predict and correlate multicomponent data from single-component data. Several competitive multi-component adsorption models have been proposed to describe the antagonistic interaction between the adsorbed quantity of one component and the concentrations of all other components, either in solution or already adsorbed at equilibrium. These isotherms range from simple models related to the individual isotherm parameters only (non-modified adsorption models), to more complex models related to the individual isotherm parameters and to correction factors (modified adsorption models) [17,19/21,23,25]. However, a multi-component equilibrium model has not been proposed yet to predict the synergistic interaction between the adsorbed component and the concentrations of all other components. Here a synergistic equilibrium model was developed for multi-component systems from the mono-component Langmuir adsorption model using the single-component parameters and correction factors and has been proposed by Eq. (4).
qeqi
Qoi bi Ceqi [1Fi (C)]; 1 bi Ceqi
(4)
where Fi (C ) represents the positive fractional deviation of the multi-component adsorption isotherm from the single-component Langmuir isotherm for component i. Ceqi and qeqi are the unadsorbed concentration of i component in the mixture at equilibrium and the adsorbed quantity of i component per g of adsorbent at equilibrium, respectively. bi and Qoi are derived from the corresponding individual Langmuir isotherm equation. Fi (C ) assumed to be specific for each qi is expressed by Eq. (5)
163
PN
j i;j"i Kj Coj ; Fi (C) P N j i Kj Coj
(5)
where Coj represents the initial concentration of j component in the mixture, and Kj is a modification coefficient for component j. According to Eqs. (4) and (5) the binary form of the modified Langmuir model for the first and the second components can be expressed as: Qo1 b1 Ceq1 K2 Co2 qeq1 ; (6a) 1 1 b1 Ceq1 K1 Co1 K2 Co2 1 Qo b C K1 Co1 qeq2 2 2 eq2 1 : (6b) 1 b2 Ceq2 K1 Co1 K2 Co2 2 The parameters Qoi and bi are obtained from the singlecomponent adsorption isotherms, while Kj values are estimated from numerical simulations of two-component adsorption data [19]. 1.2. Single- and multi-component kinetic modelling In order to investigate the mechanism of biosorption and potential rate controlling step such as mass transport and chemical reaction processes, kinetic models have been used to test experimental data both in singleand multi-component systems. When the biomass is employed as a free cell suspension in a well-agitated batch system, all the cell wall binding sites are made readily available for uptake so the effect of external film diffusion on biosorption rate can be assumed not significant and ignored in any engineering analysis [18]. The pseudo second-order kinetic model can be used in this case assuming that measured concentrations are equal to cell surface concentrations. The pseudo second-order kinetic model is based on the sorption capacity of the solid phase. Contrary to the first-order kinetic model it predicts the behaviour over the whole range of adsorption and is in agreement with an adsorption mechanism being the rate controlling step. If the rate of sorption is a second-order mechanism, the pseudo second-order chemisorption kinetic rate equation for each component, both singly and in the mixture is expressed as: dqi k2;i (qeqi qi )2 ; dt
(7)
where k2,i is the rate constant of each component for second-order biosorption. For the boundary conditions t /0 to t /t and qi /0 to qi /qeqi ; the integrated and linear form of Eq. (7) becomes t qi
1 k2;i q2eqi
1 qeqi
t:
(8)
If second-order kinetics are applicable, the plot of t /q against t of Eq. (8) should give a linear relationship,
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from which qeqi and k2,i can be determined from the slope and intercept of the plot and there is no need to know any parameter beforehand [16,22]. Relatively little work appears to have been done on the ability of R. arrhizus biomass to adsorb metal and metal cyanide complexes together. In this study, the simultaneous biosorption of ferric and ferricyanide ions to R. arrhizus from binary mixtures was studied and compared with single-component situation in a batch stirred system. The binding capacity of biomass was shown as a function of pH, single and dual-component concentrations. The mono-component sorption phenomena were expressed by the Langmuir, Freundlich and Redlich /Peterson adsorption models and model parameters were determined by non-linear regression. A multi-component Langmuir model was developed to express the adsorption isotherms of ferric and ferricyanide ions mixtures by the biomass. The pseudo secondorder kinetic model was also applied to single and multicomponent experimental data. This is the first detailed study of the binding of these two ions simultaneously to R. arrhizus .
2. Materials and methods 2.1. Microorganism and growth conditions R. arrhizus , a filamentous fungus obtained from the U.S. Department of Agriculture Culture Collection was used in this study. Tie microorganism was grown at 25 8C in agitated liquid media containing malt extract (17 g l 1) and soya peptone (5.4 g l 1). The pH was adjusted to 5.4 /5.6 with concentrated and diluted H2SO4 solutions. 2.2. Preparation of the microorganism for biosorption After the growth period, R. arrhizus was washed twice with distilled water, inactivated using 1% formaldehyde and then dried at 110 8C for 24 h. For biosorption studies, a weight amount of dried biomass was suspended in 100 ml double-distilled water and homogenized in a homogenizer (Janke and Kunkel, IKALabortechnick, Ultra Turrax T25) at 8000 rpm for 20 min and then stored in the refrigerator.
Fisher, in 1 l of double-distilled water. Stock ferric iron solution was prepared from ferrous ammonium sulphate as follows: 7.02 g of crystallized ferrous ammonium sulphate was dissolved in 500 ml double-distilled and deionized water and 50 ml of 1:1 H2SO4 was added. The solution was warmed and oxidized with approximately 0.1% (w/v) potassium permanganate solution until the iron(III) solution remained faintly pink. Then, the solution was cooled and diluted to 1.0 l. For binary ferric and ferricyanide ions mixture studies, desired combinations of iron(III)-cyanide complex ions and iron(III) ions were obtained by diluting 1.0 g l 1 of stock solutions of components and mixing them in the test medium. Before mixing with the fungal suspension, the pH of each test solution was adjusted to the required value with concentrated H2SO4 solutions. The ranges of concentrations of iron(III) and iron(III)-cyanide complex ions prepared from stock solutions varied between 25 /200 mg l 1 and 25/1000 mg l 1, respectively. 2.4. Batch biosorption studies The factors that affect the adsorption rate and uptake capacity of the biosorbent were examined in Erlenmayer flasks. Aliquots of 15 ml of fungal suspension were contacted with 135 ml of ferric or ferricyanide ions or ferric ion and ferricyanide mixture-bearing solution at desired level of each component at the beginning of the adsorption at the desired temperature and pH. All the final solutions contained 1.0 g l 1 mass of biosorbent. The flasks were agitated on a shaker at a 150 rpm constant shaking rate for 24 h to ensure equilibrium was reached. Samples (5 ml) were taken before mixing the biosorbent solution and the metal ion or metal cyanide complex ion or metal /metal cyanide complex ion mixture-bearing solution, at 5 min intervals at the beginning of adsorption and 25 /30 min intervals after reaching equilibrium. The samples were centrifuged at 5000 rpm for 3 min and then the supernatant liquid was analysed for iron(III) and iron(III)-cyanide complex ions. Studies were performed at a constant temperature of 25 8C to be representative of environmentally relevant conditions. All the biosorption experiments were repeated twice to confirm the results.
2.3. Chemicals
2.5. Analysis of iron and iron(III)-cyanide complex ions
The test solutions containing single iron(III) or iron(III)-cyanide complex ions were prepared by diluting 1.0 g l 1 stock solutions of iron(III) or iron(III)cyanide complex ions to the desired concentrations. Stock solution of iron(III)-cyanide complex ions was prepared by dissolving 1.551 g potassium ferricyanide (K3Fe[CN]6) of analytical reagent grade obtained from
The concentration of the residual iron(III)-cyanide complex ions in the biosorption medium was determined iodometrically. In this method, iron(III)-cyanide complex ions were oxidized to iron(II)-cyanide complex ions by potassium iodide in the acidic medium and iodine was formed. The amount of iodine (or indirectly the amount of iron(III)-cyanide complex ions) was deter-
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165
mined by titration with sodium thiosulphate solution [26]. The concentration of free iron(III) ions in the biosorption medium was determined spectrophotometrically. The coloured complex of iron(III) ions with sodium salicylate was read at 530 nm. No interference of iron(III)-cyanide complex ions on the analysis of iron(III) ions was observed [27].
3. Results and discussion The kinetic and equilibrium results are given as the units of adsorbed iron(III) or iron(III)-cyanide complex ion quantity in single- or multi-component situation per g of biosorbent at any time (qi ; mg g 1) and at equilibrium (qeqi ; mg g 1) and residual iron(III) or iron(III)-cyanide complex ion concentration in single or multi-component situation at equilibrium (Ceqi ; mg l 1). The adsorption yield is defined as the ratio of sorbed concentration of iron(III) or iron(III)-cyanide complex ions at equilibrium (this value is also equal to qeqi value since the biomass concentration is 1.0 g l 1) to the initial iron(III) or iron(III)-cyanide complex ion concentration for single-component removal situation (Ad%). Individual and total adsorption yields in the simultaneous removal from the mixture of iron(III) and iron(III)-cyanide complex ions were also defined as the ratios of individual and total adsorbed concentrations of each component at equilibrium to individual and total initial component concentrations, respectively (Adi %, AdTot%). 3.1. Effect of initial pH on the biosorption of iron(III) and iron(III)-cyanide complex ions The most critical parameter in the treatment of monoand multi-component systems by the biomass is the pH of biosorption medium. Previous investigations concerning the single-component situation have shown that the optimum adsorption pH depended on both cell surface binding sites and chemistry of components in aqueous solution. The uptake of iron(III) and iron(III)cyanide complex ions by the biomass was a function of pH too. The effect of initial pH on iron(III) and iron(III)-cyanide complex ion uptake capacity of R. arrhizus was studied at 100 mg l 1 initial concentration and the variation of equilibrium uptake with initial pH was given in Fig. 1. It was shown that the removal of both the single species of iron(III) and iron(III)-cyanide complex ions from aqueous solution was more efficient at increasing pH values. Cell walls have binding sites that are considered as being part of ionizable groups such as carboxyl, hydroxyl, amino and imino. At highly acidic pH values, metal cations and protons compete for binding sites on
Fig. 1. Effect of initial pH on the equilibrium uptake of iron(III) and iron(III)-cyanide complex ions in the single-component situation (Co: 100 mg l 1, X : 1.0 g l 1, T : 25 8C).
cell wall, which results in lower uptake of metal. It has been suggested that at low pH values, cell wall ligands would be closely associated with H3O that restrict access to ligands by metal ions as a result of repulsive forces. It is to be expected that as pH values are increased, more ligands with a negative charge would be exposed with subsequent increase in attraction for positively charged iron(III) ions [12 /15,20]. However, biosorption studies could not be performed at pH values higher than 2.5 because of the reduced solubility and precipitation of iron(III) ions. Metals present in wastewaters as cyanide complexes, generating anions should be repelled by the cellular wall. However, precipitation and complexation may be the main mechanisms responsible for iron(III)-cyanide complex ions removal at higher pH values. Iron-cyanide complexes are very stable complexes which react with metals for the formation of metal complexes by organic compounds present in the cell wall [8 /11]. There is little literature on the interactions of iron(III)-cyanide complex ions with the biosorbent surface. Nevertheless the working adsorption pH value was chosen as 2.0 for both components since it was not possible to undertake studies at higher pH values for iron(III) ions. 3.2. Single and dual biosorption of iron(III) and iron(III)-cyanide complex ions The equilibrium uptakes and biosorption yields obtained at single iron(III) and iron(III)-cyanide complex ions situation at pH 2.0 are shown in Figs. 2 and 3 and are presented in Table 1. As seen from the figures and the table, the biosorption capacity of dried R. arrhizus biomass for iron(III) was generally less than that of the iron(III)-cyanide complexes and increasing the initial concentration up to 200 mg l 1 for iron(III) and up to 1000 mg l 1 for iron(III)-cyanide complex
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Fig. 2. Effect of initial iron(III) concentration on iron(III) equilibrium uptake with the iron(III) present as the single metal ion and in the presence of increasing concentrations of iron(III)-cyanide complex ions at pH 2.0 (X : 1.0 g l 1, T : 25 8C).
Fig. 3. Effect of initial iron(III)-cyanide complex ion concentration on iron(III)-cyanide complex ion equilibrium uptake with the iron(III)cyanide complex ion present as the single-component and in the presence of increasing concentrations of iron(III) ions at pH 2.0 (X : 1.0 g l 1, T : 25 8C).
ions increased the equilibrium uptake and decreased the adsorption yield of both components. When the initial iron(III)-cyanide complex ion concentration increased from 26.7 to 992.0 mg l 1, the loading capacity of dried
biomass increased from 13.3 to 94.3 mg g 1. The equilibrium uptakes obtained from experimental data also showed that when the initial iron(III) ion concentration increased from 27.0 to 210.0 mg l 1, the iron(III) loading capacity of dried biomass increased from 12.8 to 30.0 mg g 1. The simultaneous biosorption of iron(III) and iron(III)-cyanide complex ions from binary mixture was also investigated at pH 2.0. In the first stage of biosorption studies, while initial iron(III) concentration was changed from 0 to 200 mg l 1, initial iron(III)cyanide complex ion concentration was held constant at 0, 25, 50, 100, 200, 500 or 1000 mg l 1. The nonlinearised adsorption isotherms of ferric ions in the absence and presence of increasing concentrations of ferricyanide ions is shown in Fig. 2. Equilibrium iron(III) uptake increased with increasing initial iron(III) concentration up to 200 mg l 1 at all ferricyanide ion concentrations studied. The curvilinear relationship between the amount of iron(III) adsorbed per unit weight of microorganism and the residual iron(III) concentration at equilibrium suggests that saturation of cell-binding sites occurred at higher concentrations of this component. The equilibrium uptake of iron(III) increased regularly with increasing ferricyanide ion concentration. The individual and total biosorption equilibrium uptakes and yields of iron(III) and iron(III)-cyanide complex ions to R. arrhizus found at different iron(III) concentrations in the absence and presence of increasing concentrations of iron(III)-cyanide complex ions are also listed in Table 1. In general the increase in iron(III)cyanide complex ion concentration increased the individual adsorption yields of iron(III) and total adsorption yields for each experimental set. The results also showed that the equilibrium uptakes of iron(III) ions increased with increasing initial iron(III)-cyanide complex ion concentration. At 100 mg l 1 initial iron(III) concentration, in the absence of iron(III)-cyanide complex ions and in the presence of 100 mg l 1 iron(III)-cyanide complex ion concentration, adsorbed iron(III) quantities at equilibrium were 26.7 and as 41.7 mg g 1, respectively. The combined effects of two components seemed to be synergistic. To decide whether those metal/metal cyanide complex ion combinations were interacting in a synergistic manner, the adsorption yields of single- and multi-component systems were also compared. For instance, using Table 1, it was expected that total adsorption yield must be equal to 34.7% for total 195.6 mg l 1 metal /metal cyanide complex ion mixture containing 93.3 mg l 1 iron(III) and 102.3 mg l 1 iron(III)-cyanide complex ion together [AdTot% / 34.7 /100 /[(26.7 mg l 1 iron(III)/41.2 mg l 1 iron(III)-cyanide complex ion)/195.6 mg l 1 initial total concentration], but from Table 1, experimental total adsorption yield was 40.0% for total 222.4 mg l 1
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167
Table 1 Comparison of individual and total biosorption equilibrium uptakes and yields found at different iron(III) concentrations in the absence and presence of increasing concentrations of iron(III)-cyanide complex ions CoFe (mg l 1)
CoFeCn (mg l 1)
qeqFe (mg g 1)
qeqFeCn (mg g 1)
/
qeq(Fe FeCn) (mg g 1)
AdFe%
AdFeCn%
AdTot%
27.0 51.0 93.3 210.1
0.0 0.0 0.0 0.0
12.8 18.3 26.7 30.0
0.0 0.0 0.0 0.0
12.8 18.3 26.7 30.0
47.5 35.9 28.6 14.3
0.0 0.0 0.0 0.0
47.5 35.9 28.6 14.3
0.0 29.3 49.5 101.7 181.3
26.7 28.4 27.0 28.4 26.7
0.0 16.9 23.2 33.3 36.5
13.2 16.2 17.4 19.5 20.1
13.2 33.1 40.6 52.8 56.6
0.0 57.7 46.8 32.8 20.1
50.0 56.9 64.4 68.6 75.2
50.0 57.3 53.0 40.6 27.2
0.0 26.7 52.0 116.7 206.7
56.8 56.8 56.8 57.5 56.1
0.0 17.2 25.8 38.7 41.7
24.7 29.0 31.2 32.0 33.1
24.7 46.2 57.1 70.7 74.8
0.0 64.5 49.7 33.1 20.2
43.5 51.0 55.0 55.7 59.0
43.5 55.3 52.5 40.6 28.5
0.0 28.3 49.3 113.3 215.2
102.3 102.3 109.1 109.1 108.0
0.0 18.3 27.0 41.7 43.7
41.2 43.6 46.6 47.3 48.2
41.2 61.9 73.5 89.0 91.8
0.0 64.7 54.6 36.8 20.3
40.3 42.6 42.7 43.4 44.6
40.3 47.4 46.4 40.0 28.4
0.0 28.3 52.3 117.0 225.2
204.6 211.2 215.9 225.7 227.3
0.0 19.6 29.0 45.0 46.7
63.9 73.3 74.9 78.8 79.8
63.9 92.9 104.0 123.7 126.5
0.0 69.0 55.4 38.5 20.7
31.3 34.7 34.7 34.9 35.1
31.3 38.8 38.8 36.1 28.0
0.0 28.3 56.3 117.0 216.7
505.7 484.0 511.2 526.3 534.1
0.0 21.1 31.9 47.3 52.0
82.4 103.6 109.4 113.2 122.3
82.4 124.0 141.0 159.8 174.3
0.0 74.3 56.5 40.5 24.0
16.2 21.4 21.4 21.5 22.9
16.2 24.3 24.9 24.8 23.2
0.0 28.0 56.3 119.7 193.3
992.0 965.0 988.3 1025.3 1034.1
0.0 21.1 31.9 47.3 52.0
94.3 112.9 116.6 121.0 125.1
94.3 133.9 153.6 170.7 179.2
0.0 76.2 66.3 41.7 27.9
9.5 11.7 11.8 11.8 12.1
9.5 13.5 14.7 14.9 14.6
iron(III)/iron(III)-cyanide complex ion mixture consisting of 113.3 mg l 1 iron(III) and 109.1 mg l 1 iron(III)cyanide complex ion together [AdTot% /40.0 /100 / [(41.7 mg l 1 iron(III)/47.3 mg l 1 iron(III)-cyanide complex ion)/222.4 initial total concentration]. As a result, the effect of the mixture was more than that of each of the individual effects of the constituents in the mixture so the interaction between iron(III) and iron(III)-cyanide complex ion could be assumed to be synergistic. The uptake of iron(III)-cyanide complex ions in the absence and presence of increasing concentrations of iron(III) ions in the range of 25 /200 mg l 1 was also studied. Fig. 3 depicts the variations of iron(III)-cyanide complex ions uptakes at equilibrium with increasing initial iron(III)-cyanide complex ions concentrations (from 25 to 1000 mg l 1) at constant initial iron(III) concentrations at pH 2.0. Similar biosorption patterns
were obtained both in the single-iron(III)-cyanide complex ion and iron(III) /iron(III)-cyanide complex ion systems; iron(III)-cyanide complex ion equilibrium uptakes increased with increasing initial iron(III)-cyanide complex ion concentrations up to 1000 mg l 1. Increases in iron(III) concentration also increased slightly the equilibrium uptake quantities of ferricyanide. The data given in Table 1 also showed how the presence of ferric ions affected the equilibrium uptakes of ferricyanide ions, both singly and in mixture, individual ferricyanide, individual ferric and total adsorption yields at pH 2.0. The results in Table 1 indicate that the presence of iron(III) had also an encouraging effect on the equilibrium uptake of iron(III)-cyanide complex ions in this situation. In the absence of iron(III) ions, equilibrium iron(III)-cyanide complex ion uptake was determined as 41.2 mg g 1 at 102.3 mg l 1 initial iron(III)-cyanide complex ion concentration. When
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initial iron(III) concentration was kept at 113.3 mg l 1 at the same initial iron(III)-cyanide complex ion concentration, this value was increased to 47.3 mg g 1. The data obtained in the single and dual systems indicated that the adsorption capacity of R. arrhizus for iron(III) was generally less than that of iron(III)-cyanide complex ions. It was clear that a significant part of the adsorption capacity of the microorganism was used for iron(III)-cyanide complex ions adsorption. There are possible interaction effects between different species in solution and in particular potential interactions on the surface depending on the biosorption mechanism. Factors that affect the biosorption preference of a biosorbent for different kinds of adsorbates may be related to the characteristics of the binding sites (e.g. functional groups, structure, surface properties, etc.), the properties of the adsorbates (e.g. concentration, ionic size, ionic weight, ionic charge, molecular structure, ionic nature or standard reduction potential, etc.) and the solution chemistry (e.g. pH, ionic strength, etc.). It is difficult and complicated to find a common rule from the physical and chemical properties of iron(III) and iron(III)-cyanide complex ions to identify how these properties affect the selective sorption of biosorbent. This is because the observed behaviour may result from a combination of all the above factors [13 /16,18,20]. For simultaneous biosorption of iron(III) and iron(III)-cyanide complex ions it was seen that the adsorptive capacity of iron(III)-cyanide complex ions in the presence of iron(III) ions slightly increased whereas the uptake of iron(III) significantly increased by the addition of iron(III)-cyanide complex ions. Higher uptake in multi-component system as compared to single-component system for iron(III) and iron(III)cyanide complex ions may be due to chemistry of the components and chemical nature of the binding sites effects. A number of fungi has been shown to sequester iron(III) with the help of iron-chelating agents. Excess iron(III) can facilitate the precipitation of iron-cyanide complex ions on the surface [7,10,11,28]. Due to a large number of variables involved in the components by the biosorbent and the complexity of the surface and water chemistry, the exact mechanisms explaining the biosorption of ions from aqueous solutions are unclear. 3.3. Application of mono- and multi-component adsorption models for single and dual biosorption of iron(III) and iron(III)-cyanide complex ions Equilibrium data of components both singly or in mixture, commonly known as adsorption isotherms, are basic requirements for the design of adsorption systems. To obtain the isotherm data, initial concentrations of iron(III) or iron(III)-cyanide complex ions were varied while the amount of biosorbent in each sample was kept constant for each component. Classical mono-compo-
nent adsorption models (Langmuir, Freundlich and Redlich /Peterson) are used to describe the equilibrium between the iron(III) or iron(III)-cyanide complex ions in solution (Ceq) and iron(III) or iron(III)-cyanide complex ions adsorbed on the cell surface (qeq). The individual adsorption constants for each component obtained by evaluating the isotherms are listed in Table 2 with the average percentage errors. The average percentage errors between the experimental and predicted values are calculated using Eq. (9). In Eq. (9), the subscripts ‘exp’ and ‘calc’ show the experimental and calculated values and N the number of measurements. PN ½(qeq;i;exp qeq;i;calc )=qeq;i;exp ½ o% i 1 100: (9) N In view of the values of percentage errors in the table, the Langmuir and Redlich /Peterson models exhibited the best fit to the adsorption data of iron(III) and iron(III)-cyanide complex ions. However, the Freundlich model also seemed to agree well with the experimental data of two components considering that obtained percentage error values are lower than 15.2%. The experimental equilibrium data of iron and iron(III)cyanide complex ions are compared with the theoretical equilibrium data obtained from these adsorption models in Fig. 4. The figure also confirmed that the adsorption equilibrium data fitted very well to the Langmuir and Redlich /Peterson models in the studied concentration ranges for both components. Adsorption model constants, the values of which express the surface properties and affinity of the biosorbent, can be used to compare the adsorptive capacity of biosorbent for different components. From Table 2, the magnitude of KF and n; the Freundlich constants, showed easy uptake of iron(III) and iron(III)cyanide complex ions from wastewater with high adsorptive capacity of the dried biomass. Table 2 also indicated that n is greater than unity, indicating that both ions are favourably adsorbed by R. arrhizus . While the Freundlich model does not describe the saturation behaviour of the biosorbent, Qo; the monocomponent Langmuir constant is the monolayer saturation at equilibrium, b; the other mono-component Langmuir constant corresponds to the concentration at which an iron(III) or an iron(III)-cyanide complex ion amount of Qo/2 is bound and indicates the affinity for the binding of iron(III) or iron(III)-cyanide complex ions. The value of Qo tabulated in Table 2 appeared to be significantly higher for iron(III)-cyanide complex ions in comparison with the uptake of iron(III) at the same pH value of 2.0. The higher value of b implied the strong bonding of iron(III) to the cells. Relevant adsorption parameters have also been calculated according to the non-competitive three-parameter isotherm of Redlich /Peterson for each
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169
Table 2 Comparison of the Freundlich, Langmuir and Redlich /Peterson adsorption constants obtained from the mono-component Langmuir, Freundlich and Redlich /Peterson adsorption models for iron(III) and iron(III)-cyanide complex ions with the average percentage errors Iron(III) ions o
Q (mg g Langmuir model
1
b (l mg
1.45
o (%) Qo (mg g 1)
)
0.042 1
5.46 KRP (l g
Redlich /Peterson model
)
33.9 KF [(mg g
Freundlich model
Iron(III)-cyanide complex ions 1
)(mg l
1 n
) ]
3.7
n
)
aRP (l mg
98.0
o (%) KF [(mg g
2.90 1
7.2 1 b
)
0.043
component and are also tabulated in Table 2. The value of b is equal to 1.0 for iron(III)-cyanide complex ions and tends to unity for iron(III) ions, that is the isotherms are approaching the Langmuir form. The simultaneous biosorption phenomena of iron(III) and iron(III)-cyanide complex ions to R. arrhizus were expressed by the proposed multi-component Langmuir model. Using the mono-component Langmuir constants for each ion given in Table 2, the multi-component Langmuir constants were determined as K1 /0.68 and K2 /1.14. Using the individual and modified multicomponent Langmuir adsorption constants, qeq values were predicted from the related multi-component Langmuir adsorption formula. According to the theoretical base of multi-component Langmuir model, the adsorbed quantity of the first component increased with increasing the concentration of the second component, depending on the values of the individual Langmuir constants of both the components. The comparison of the experimental and calculated qeq values of iron(III) and iron(III)-cyanide complex ions in mixtures is presented in Fig. 5. Basically, if most of the data are distributed around the 458 line this indicates that the model represent well the experimental data of the system so as shown in Fig. 5. The multi-component Langmuir model fitted reasonably well the binary uptake data of iron(III) and iron(III)-cyanide complex ions in the studied concentration range, although slight deviations (The average percent magnitude of absolute deviation was changed from 0.0 to 20.0% in the model for the entire data set of iron(III) and iron(III)-cyanide complex ions.) were observed between the experimental and calculated results from the model at lower and higher component concentrations. These results can be attributed to the insensitivity of model to interactive effects existing in multi-component systems and the characteristics of Langmuir model, which is not valid for high concentrations assuming limited number of identical sites for sorption. It was concluded that the proposed
b 1.00
b (l mg 1)
5.07 o (%) 3.5
0.012 1
)(mg l
1 n
) ]
3.6
n
o (%)
2.17 KRP (l g 1.12
o (%)
15.2 1
) aRP (l mg 1)b 0.011
b
o (%)
0.99 2.6
multi-component Langmuir model provided a more realistic description of the biosorption process and can be used to predict the uptake of iron(III) or iron(III)cyanide complex ions in the binary system with more accuracy.
3.4. Comparison of biosorption kinetics of single and dual biosorption of iron(III) and iron(III)-cyanide complex ions to R. arrhizus Fig. 6 showed the biosorption kinetics of iron(III) and iron(III)-cyanide complex ions by plotting the uptake capacity, q , versus time for 100 mg l 1 initial ion concentration in single and binary systems. Results clearly indicated that the single sorption of iron(III) and iron(III)-cyanide complex ions by the biomass increased rapidly up to about 60 min with 70.4 and 74.2% sorption, respectively, showing physical adsorption at the cell surface and thereafter rose slowly before attaining a saturation value (i.e. biosorption equilibrium). Biosorption reached equilibrium in 6 h at this initial concentration of both components. The equilibrium sorption capacities of iron(III) and iron(III)cyanide complex ions were determined as 26.7 and 41.2 mg g 1, respectively. The equilibrium time of 6 h was not affected by the binary sorption of iron(III) and iron(III)-cyanide complex ions as observed in singlecomponent system. However, the sorption capacity of iron(III) and iron(III)-cyanide complex ions was appreciably increased in binary combination. A 35.3% increase in iron(III) sorption and a 14.2% increase in iron(III)-cyanide complex ion sorption were observed in 100 mg l 1 iron(III) and 100 mg l 1 iron(III)-cyanide complex ion containing mixtures. This figure demonstrates that the presence of iron(III)-cyanide complex ions has a marked impact on the sorption of the iron(III) ions.
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Fig. 5. Comparison of the experimental and predicted qeq values of iron and iron(III)-cyanide complex ions in binary mixtures.
Fig. 6. Biosorption kinetics of iron(III) and iron(III)-cyanide complex ions to R. arrhizus at 100 mg l 1 initial ion concentration in single and binary systems.
Fig. 4. Comparison of the experimental equilibrium data of iron and iron(III)-cyanide complex ions at pH 2.0 with the theoretical equilibrium data obtained from the Freundlich, Langmuir and Redlich / Peterson adsorption models.
3.5. Application of single and multi-component kinetic modelling for single and dual biosorption of iron(III) and iron(III)-cyanide complex ions In order to analyse the biosorption kinetics of iron(III) and iron(III)-cyanide complex ions in single and binary systems, the second-order kinetic model was applied to data. Fig. 7 shows the plots of linearised form of the second-order kinetic model for 100 mg l 1 initial iron(III) or iron(III)-cyanide complex ion concentration in single and binary systems. The values of the
Fig. 7. Comparison of t /q vs. t plots for 100 mg l 1 initial ion concentration in single and binary systems.
Z. Aksu, H. Gu¨len / Process Biochemistry 38 (2002) 161 /173
171
Table 3 Comparison of the second-order adsorption rate constants and calculated and experimental equilibrium uptake values obtained at different initial ion concentrations CoFe (mg l 1)
CoFeCn (mg l 1)
qeqFe,exp (mg g 1)
qeqFeCn,exp (mg g 1)
k2Fe 103 (g mg 1 min 1)
k2FeCn 103 (g mg 1 min 1)
qeqFe,teo (mg g 1)
qeqFeCn,teo (mg g 1)
27.0 51.0 93.3 210.0
0.0 0.0 0.0 0.0
12.8 18.3 26.7 30.0
0.0 0.0 0.0 0.0
3.01 2.50 1.69 1.31
0.00 0.00 0.00 0.00
13.8 18.6 27.0 30.5
0.0 0.0 0.0 0.0
0.0 29.3 49.5 101.7 181.3
26.7 28.4 27.0 28.4 26.7
0.0 16.9 23.2 33.3 36.5
13.2 16.2 17.4 19.5 20.1
0.00 3.11 3.08 1.71 1.49
1.87 2.20 2.64 5.27 8.18
0.0 17.2 23.4 33.8 37.2
14.0 16.5 17.7 19.7 20.2
0.0 26.7 52.0 116.7 206.7
56.8 56.8 56.8 57.5 56.1
0.0 17.2 25.8 38.7 41.7
24.7 29.0 31.2 32.0 33.1
0.00 3.52 3.14 1.73 1.50
1.41 1.68 1.84 3.37 4.34
0.0 17.5 26.3 39.4 42.2
26.0 29.9 31.7 32.4 33.4
0.0 28.3 49.3 113.3 215.2
102.3 102.3 109.1 109.1 108.0
0.0 18.3 27.0 41.7 43.7
41.2 43.6 46.6 47.3 48.2
0.00 3.80 3.34 1.78 1.52
1.01 1.14 1.32 2.03 2.65
0.0 18.6 27.3 42.2 44.4
43.1 45.1 47.2 48.1 48.8
0.0 28.3 52.3 117.0 225.2
204.6 211.2 215.9 225.7 227.3
0.0 19.6 29.0 45.0 46.7
63.9 73.3 74.9 78.8 79.8
0.00 4.59 3.47 1.97 1.75
0.73 0.83 1.05 1.35 1.84
0.0 19.7 29.2 45.7 47.6
65.2 74.6 75.8 80.0 80.7
0.0 28.3 56.3 117.0 216.7
505.7 484.0 511.2 526.3 534.1
0.0 21.1 31.9 47.3 52.0
82.4 103.6 109.4 113.2 122.3
0.00 5.38 3.77 2.08 1.78
0.70 0.71 0.94 1.30 1.56
0.0 21.3 32.1 48.1 52.4
83.3 105.3 109.9 113.6 123.4
0.0 28.0 56.3 119.7 193.3
992.0 965.0 988.3 1025.3 1034.1
0.0 21.1 31.9 47.3 52.0
94.3 112.9 116.6 121.0 125.3
0.00 6.01 3.80 2.10 1.84
0.66 0.68 0.82 1.01 1.46
0.0 21.7 37.7 51.0 54.6
95.2 113.6 117.7 123.5 126.6
parameters k2,ad and qeq and of correlation coefficients are also presented in Table 3 for all combinations. The correlation coefficients for the second-order kinetic model are obtained 1.000 for all mixtures. The secondorder adsorption rate constants of iron(III) and iron(III)-cyanide complex ions in single and multi-component systems decreased with increasing concentrations. The theoretical qeq values also agreed very well with the experimental qeq values in the case of pseudo secondorder kinetics. There was a very minor deviation between expected and observed qeq values. These data suggest the applicability of second-order kinetic model based on the assumption that the rate limiting step may be the biosorption in explaining the kinetics of biosorption.
4. Conclusions In the mixtures of two or more species in a solution, the synergistic or antagonistic interaction occurring between the species might affect the individual uptake by the microorganism. In the study, the capability of the fungi R. arrhizus to bind iron(III) and iron(III)-cyanide complex ions individually and simultaneously from aqueous solutions was examined, including equilibrium and dynamic studies. Experiments were performed as a function of initial pH, initial iron(III) and iron(III)cyanide complex ion concentration. In conventional biological treatment systems where insufficient contact time is available for biodegradation to occur, biosorption results in the removal of toxic compounds from the
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aqueous waste stream and in the accumulation of hazardous pollutants in the microbial sludge in a short time. The experimental results showed that R. arrhizus has a considerable potential for the rapid uptake of iron(III) and iron(III)-cyanide complex ions over a wide range of concentration. Since the sequence of second ion addition affected the adsorption rate and equilibrium uptake positively, the combined effect of iron(III) and iron(III)-cyanide complex ions on the adsorption of R. arrhizus was found to be synergistic. For the single ion situation, the Freundlich, Langmuir and Redlich /Peterson adsorption models were used for the mathematical description of the biosorption of iron(III) and iron(III)-cyanide complex ions to dried R. arrhizus and the isotherm constants were evaluated to compare the biosorptive capacity of the dried biomass for the iron(III) and iron(III)-cyanide complex ions. It was seen that the adsorption equilibrium data fitted well to all the models. The individual Langmuir constants evaluated from the Langmuir isotherm were used to find the correction factors in the multi-component Langmuir model describing binary adsorption equilibrium. It was concluded that the proposed Langmuir model agreed well with the results found experimentally. As a result, it could be said that iron(III) / iron(III)-cyanide complex ions multi-ion system could be defined with Langmuir adsorption isotherm and it could be used to model the adsorption of binary systems from aqueous solutions. Assuming the batch biosorption as a single-staged equilibrium operation, the separation process for the single and binary sorption of iron(III) and iron(III)-cyanide complex ions can be mathematically defined using these isotherm constants to estimate the residual concentration of components or amount of biosorbent for desired purification. The suitability of second-order kinetic model for the single and binary sorption of iron(III) and iron(III)cyanide complex ions onto biomass was also discussed assuming no effect of mass transfer on the biosorption rate. The kinetic parameters obtained can be used for a bioreactor design. It may be suitable to apply such simple kinetic models to a well-agitated batch biosorption system consisting of free cell suspension neglecting external film diffusion. It may be concluded that dried R. arrhizus may be used successfully for the removal of iron(III) and iron(III)-cyanide complex ions. Biosorption by R. arrhizus can be proposed as an alternative to more costly methods such as biological treatment, activated carbon adsorption, solvent extraction, chemical oxidation for removal of iron(III) and iron(III)-cyanide complex ions from waste streams. This work could enable to extrapolate the prediction of adsorption equilibria of the single and binary system if experimental data are not available for a certain level of bisolute concentrations.
Acknowledgements The authors wish to thank to Hacettepe University Research Fond, Turkey, Project No.YDABC ¸ AG198Y097, for the partial financial support.
Appendix A: Nomenclature Redlich /Peterson adsorption constant (l mg 1)b Langmuir adsorption constant (l mg 1) unadsorbed concentration of the single-component at equilibrium (mg l 1) Ceqi unadsorbed concentration of each component in the binary mixture at equilibrium (mg l 1) Coi initial concentration of each component (mg l 1) q adsorbed quantity of the single-component per g of dried biomass at any time (mg g 1) qi adsorbed quantity of each component in the binary mixture per g of dried biomass at any time (mg g 1) qeq adsorbed quantity of the single-component per g of dried biomass at equilibrium (mg g 1) qeqi adsorbed quantity of each component in the binary mixture per g of dried biomass at equilibrium (mg g 1) KF Freundlich adsorption constant [(mg g 1)(mg l 1)n ] KRP Redlich /Peterson adsorption constant (l g 1) Kj multi-component Langmuir adsorption constant k2,ad second-order adsorption rate constant (g mg 1 min 1) n Freundlich adsorption constant Qo Langmuir adsorption constant (mg g 1) X biosorbent (dried R. arrhizus ) concentration (g l 1) b Redlich /Peterson adsorption constant o (%) average percentage error aRP b Ceq
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