Biosorption of cadmium and zinc by activated sludge from single and binary solutions: Mechanism, equilibrium and experimental design study

Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 433–443

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Biosorption of cadmium and zinc by activated sludge from single and binary solutions: Mechanism, equilibrium and experimental design study Lucia Remena´rova´, Martin Pipı´sˇka *, Miroslav Hornı´k, Maria´n Rozlozˇnı´k, Jozef Augustı´n, Juraj Lesny´ Department of Ecochemistry and Radioecology, Faculty of Natural Sciences, University of SS. Cyril and Methodius, J. Herdu 2, Trnava, SK-917 01, Slovak Republic

A R T I C L E I N F O

A B S T R A C T

Article history: Received 1 March 2011 Received in revised form 6 December 2011 Accepted 10 December 2011 Available online 30 December 2011

Dried activated sludge (DAS) has been used for obtaining quantitative data of the cadmium and zinc biosorption from single and binary aqueous CdCl2 and ZnCl2 solutions in batch experiments using radiotracer technique. It was shown that the metal removal is a rapid process significantly influenced by solution pH. The mechanisms of biosorption of Cd and Zn by DAS were examined by FTIR, SEM–EDX analysis and chemical blocking of functional groups. Results revealed the dominant role of the carboxyl group in Cd and Zn ion-binding by DAS and participation of the ion-exchange mechanism in metal ion biosorption. The maximum uptake capacity Qmax at pH 6.0 calculated from Langmuir isotherm was 540  16 mmol/g for Zn2+ and 510  17 mmol/g for Cd2+ ions. The response surface methodology was used for investigation of interactions and competitive effects in binary metal system Cd2+–Zn2+. Interaction plots revealed strong interactions between the initial concentration of co-ion, solution pH and sorption capacity of the primary ion. Maximum sorption capacities of DAS in the binary system Cd2+–Zn2+ were 321 mmol Cd2+/g and 312 mmol Zn2+/g, indicating a higher affinity for Cd2+ compared with Zn2+ ions. ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Biosorption Cadmium Zinc Activated sludge Equilibrium Biosorption mechanism

1. Introduction Rapid industrialization and urbanization have resulted in the generation of large quantities of aqueous effluents, many of which contain high levels of toxic pollutants (heavy metals, organic compounds and radionuclides). Development of efficient clean-up technologies is of major interest. From current research activities is evident that various technologies based on interactions between pollutants and biological systems in a contaminated environment are investigated [1,2]. Biosorption has been investigated as one of the most promising methods for toxic metal removal. It appears that biosorption is an ideal candidate for replacing conventional methods for metals removal from wastewaters such as chemical precipitation, electrowinning, membrane separation, evaporation and ion-exchange, especially in cases when low concentrations of metals are present in wastewaters. Sewage sludge is regarded as the residue produced by the wastewater treatment process, during which liquids and solids are being separated. Liquids are being discharged to an aqueous environment while solids are removed for further treatment and final disposal. Sludges of wastewater treatment plants contain variable amounts of heavy metals; however, sorption capacity of sludge is generally much higher and thus can be considered as a

* Corresponding author. Tel.: +421 335921452; fax: +421 335565185. E-mail addresses: [email protected], [email protected] (M. Pipı´sˇka).

cheap sorbent. The use of activated sludge, both from municipal and industrial wastewater plants as biosorbent for metals removal was extensively studied [3,4]. Comparing with other types of biosorbents, dried activated sludge represents a low cost, easily available and well sedimenting material with a large specific surface area which is suitable for the removal of toxic metals. Activated sludge showed high equilibrium sorption capacities Qeq for As, Cd, Cr, Cu, Ni, Pb and Zn from both single and multi-component sorption systems [5–9]. Nevertheless, there is a lack of detailed information about metal–sludge interactions and mechanisms participating in metal binding. Laurent et al. [10] found that ion-exchange was obviously not the only mechanisms responsible for Cd removal by thermal treated sludge. In addition, Xu and Liu [11] identified, besides ionexchange, chemical precipitation as the mechanism responsible for Cd and Cu binding by aerobic granules. Also the contributions of functional groups [12] and extracellular polymeric substances [13] on metal biosorption by activated sludge were studied. This suggests that the sorption abilities of sludge should be attributed not only to the above mentioned mechanisms but also to some other factors or their combinations. Therefore this study adopts a systematic approach by studying mechanisms of Cd and Zn ions uptake by activated sludge. When the chemical or physiological reactions occurring during biosorption are known, the rate, quantity, and specificity of the metal uptake can be manipulated through the specification and control of process parameters [14].

1876-1070/$ – see front matter ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2011.12.004

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The use of a variety of analytical techniques will allow deeper understanding of the processes involved in metal binding by biosorbents. These include Fourier transformed infrared spectroscopy (FTIR) for determination of functional groups present on the biosorbent; scanning electron microscopy connected with EDX (SEM–EDX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) for surface characterization and localization of metals on the biosorbent [15–17]. Limited knowledge is available regarding the use of the combination of SEM and EDX elemental mapping for precise topographical visualization of sorbed metals on biosorbent surface. It appears that the combination of various analytical procedures is necessary to explore mechanisms of metal biosorption. Biosorption is also influenced by many physico-chemical parameters such as solution pH, temperature, biosorbent particle size, initial sorbate concentration, as well as by the presence of a number of chemical species in solution [18]. Therefore it is necessary to study biosorption as a complex process, which consists of many mechanisms and can be affected by many parameters, especially in multi-component sorption systems. The effect of various factors can be studied by the classical approach, when one variable is changed at a time and the effects of variables on the sorption process are investigated. However in the case of multi metal systems this procedure represents a time consuming and expensive method. Response surface methodology (RSM) is a collection of mathematical and statistical techniques useful for designing experiments, building models and analyzing the effects of the several independent variables [19]. The main advantage of the statistical design of experiments is the reduced number of experiments to be performed as well as the consideration of interactions among the variables and its use for optimization of the operating parameters in multivariable systems [18]. To the best of our knowledge, the use of RSM methodology to analyze the competitive and interaction effects in Cd and Zn sorption by activated sludge from binary metal system has not been reported. Considering the above mentioned aspects, the key objectives of our research are as follows: (a) to determine the functional groups participating in metal biosorption, (b) to investigate the major mechanisms of cadmium and zinc ions biosorption, (c) to quantify biosorption capacity and equilibrium and (d) to determine mutual competitive and interaction effects of metals in multi-component systems. Therefore, in this study, scanning electron microscopy connected with EDX, FTIR analysis and blocking of functional groups were used for characterization of biosorbent prepared from dried activated sludge (DAS) and for the study of Cd2+ and Zn2+ biosorption mechanisms. Equilibrium isotherm models according to Langmuir and Freundlich were used for mathematical description of sorption equilibria in single systems. To obtain reliable experimental biosorption data within a broad concentration range, radiotracer technique with 109CdCl2 and 65ZnCl2 was used. RSM was used for analyzing competitive and interactions effects in the binary sorption system Cd2+–Zn2+. We expected that the employment of these sophisticated analytical and computational techniques could supplement the present knowledge of metal biosorption by DAS, which are necessary for prospective application of sludges in water treatment processes and also for the disposal of sludges generated in industrial wastewater treatments plants. 2. Materials and methods 2.1. Biosorbent preparation Activated sludge used in this study was obtained from the wastewater treatment plant in Enviral Inc. producing bioethanol

(Leopoldov, Slovak Republic). The biomass was washed twice in deionised water (Simplicity 185, Millipore, USA), oven-dried for 72 h at 90 8C. After drying activated sludge was ground to various particle sizes, from which the particle size <450 mm was used in biosorption experiments. 2.2. Biosorption experiments in single systems The experimental procedure and conditions were based on our previous study [20]. Biosorption kinetics: experiments were carried out by suspending of DAS (2.5 g/L, d.w.) in solutions with concentrations of 1000 mM of CdCl2 or ZnCl2 (analytical grade, Sigma Aldrich, Germany), spiked with 109CdCl2 or 65ZnCl2, and the pH was adjusted to 6.0. Flasks were agitated on a reciprocal shaker (Environmental shaker ES20, Biosan, Latvia) at 120 rpm and 20 8C. Aliquot samples were taken from individual flasks in intervals 1, 10, 20, 30, 60, 120, 240 and 1440 min and radioactivity was measured. Biosorption equilibrium: experiments were carried out by suspending of DAS (2.5 g/L, d.w.) in solutions containing CdCl2 or ZnCl2 in concentration range 100–4000 mM, spiked with 109 CdCl2 or 65ZnCl2 and pH was adjusted to 6.0. Flasks were agitated on a reciprocal shaker at 120 rpm for 4 h at 20 8C. At the end of the experiments the biosorbent was filtered out, washed twice in deionised water and the radioactivity of both the activated sludge and the liquid phase was measured. The metal uptake was calculated as: Q eq ¼ ðC 0  C eq Þ

V M

(1)

where Q is the uptake (mmol/g), C0 and Ceq are the initial and the final metal concentrations in solution (mmol/L), V is volume (L) and M is the amount of dried biosorbent (given in g). The Langmuir (Eq. (2)) and Freundlich (Eq. (3)) adsorption models were used for describing equilibrium data in single metal systems. The integral equations are as follows: Q eq ¼

bQ max C eq 1 þ bC eq

ð1=nÞ

Q eq ¼ KCeq

(2)

(3)

where Qmax represents the maximum sorption capacity upon complete saturation of the sorbent, b is a constant related to the energy of adsorption. K and 1/n values are the Freundlich constants referring to adsorption capacity and intensity of adsorption, respectively. 2.3. Biosorption experiments in binary system The Box–Behnken design under response surface methodology (RSM) was used to investigate interactions and competitive effects in the binary metal system Cd2+–Zn2+. Levels of factors (initial concentrations C0 of Cd2+ and Zn2+ ions and initial pH of solution) considered for biosorption in binary system are shown in Table 1. The ranges of selected parameters were determined by preliminary experiments, and sequentially the matrix of 16 experiments was designed using Statgraphics Centurion XV (StatPoint Inc., USA). Based on the matrix, experiments were performed in flasks, activated sludge (2.5 g/L, d.w.) was added and the content was agitated on a reciprocal shaker (120 rpm) for 4 h at 20 8C. At the end of the experiments the biosorbent was filtered out, washed twice in deionised water and the radioactivity of both the activated sludge and the liquid phase was measured. The metal uptake was calculated according to Eq. (1).

L. Remena´rova´ et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 433–443 Table 1 Levels of factors considered for biosorption of Cd2+ and Zn2+ ions by DAS from binary system Cd2+–Zn2+ using Box–Behnken design. Factor

Factor code

Coded levels 1

0

1

C0 Cd2+ (mmol/L) C0 Zn2+ (mmol/L) pH

A B C

1000 1000 3.0

2000 2000 4.5

3000 3000 6.0

The behavior of the binary sorption system is explained by the following empirical second-order polynomial model:

Q ¼ b0 þ

k X i¼1

bi xi þ

k X

bii x2i þ

i¼1

k X

bi j xi x j

(4)

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Activated sludge was firstly pretreated by 0.5 M solution of CaCl2 for 4 h. The surface structure analysis of Ca pretreated sludge before and after Cd2+ and Zn2+ sorption from single systems and EDX microanalysis were performed by scanning electron microscope VEGA 2 SEM (TESCAN s.r.o., Czech Republic) coupled with an EDX, QUANTAX QX2 detector (RONTEC, Germany) for electron dispersive X-ray analysis. Prior to the SEM and EDX analysis, the samples of activated sludge were dried for 72 h at 90 8C and stuck to an aluminium sample holder using conductive adhesive (Ag). Samples were then coated with Au using BP 343.7 Evaporator (TESLA ELMI a.s., Czech Republic). The analyses were performed at voltage 30 kV, vacuum pressure 9.0  103 Pa and magnification 250. 2.7. Blocking of functional groups

1i j

where Q is the predicted response (specific sorption Qeq of both Cd and Zn), xi, xj, . . .xk are the input variables (C0 Cd2+, C0 Zn2+ and solution pH) which affect the response Q, b0 is the intercept term, bi is the linear effect, bii is the quadratic effect and bij is the interaction effect [21]. 2.4. Influence of pH and metal speciation To analyze the influence of pH on metal ions biosorption, DAS (2.5 g/L, d.w.) was shaken in Cd2+ and Zn2+ solutions (C0 = 1000 mM) of desired pH spiked with 109Cd and 65Zn for 4 h on a reciprocal shaker at 120 rpm and 20 8C. In order to eliminate the interference of buffer components on biosorption, the nonbuffered solutions in deionised water were adjusted to the desired pH values by adding 0.5 M HCl or 0.1 M NaOH throughout the entire study. The Visual MINTEQ (version 3.0) program [22] was used to calculate the theoretical Cd and Zn speciation depending on total Cd and Zn concentrations, pH and concentrations of cations and anions in the solution. All data sets were calculated considering the carbonate system naturally in equilibrium with atmospheric CO2 (pCO2 = 38.5 Pa). 2.5. Radiometric analysis In chemical and physico-chemical reactions cadmium and zinc do not show isotopic effects and radiotracer techniques such as isotope dilution analysis is generally accepted for determination of metal concentrations in single and multi-component systems. The gamma spectrometric assembly using the well type scintillation detector 54BP54/2-X, NaI(Tl) (Scionix, The Netherlands) and the data processing software Scintivision 32 (ORTEC, USA) were used for 109Cd and 65Zn determination in DAS and supernatant fluids at the energy of g-photons: 109Cd–88.04 keV and 65Zn–1115.52 keV. Standardized 109CdCl2 solution (3.857 MBq/ml, CdCl2 50 mg/L in 3 g/L HCl) and 65ZnCl2 solution (0.8767 MBq/ml; 50 mg ZnCl2/L in 3 g/L HCl) were obtained from the Czech Institute of Metrology, Prague (Czech Republic). 2.6. FTIR and SEM EDX analysis The FTIR analysis was carried out to identify the chemical functional groups present on dried activated sludge and to explain the biosorption mechanism. FTIR spectra of the control sample and Cd and Zn loaded dried activated sludge were performed using Nicolet NEXUS 470 spectrometer (Thermo Scientific, USA). Samples were mixed with KBr at a ratio 1:100 for making pellets. The FTIR spectra were obtained within the range of 400– 4000 cm1.

Blocking of carboxylic groups of biosorbent prepared from activated sludge was performed in accordance with the method of Gardea-Torresdey et al. [23], which includes the following reaction with methanol: Hþ

RCOOH þ CH3 OH!RCOOCH3 þ H2 O

(5)

Blocking was realized by suspending DAS (250 mg) in a mixture of anhydrous methanol (13 ml) and concentrated HCl (120 mL), with exposure for 6 h on a reciprocal shaker (250 rpm). Hydroxyl groups of activated sludge were blocked in reaction with formaldehyde according to procedure of Chen and Yang [24]: 2R  OH þ HCHO ! ðR  OÞ2 CH2 þ H2 O

(6)

For this purpose, DAS (250 mg) was suspended in HCHO (12 ml) and agitated for 6 h. In both cases, the biosorbent was then separated by centrifugation, washed twice in deionised water, oven-dried and used in biosorption experiments. 2.8. Data analysis To calculate the Qmax values and the corresponding parameters of adsorption isotherms non-linear regression analysis was performed by ORIGIN 7.0 Professional (OriginLab Corporation, Northampton, USA). The experimental design and regression analysis of the obtained data were performed by Statgraphics Centurion XV (StatPoint Inc., USA) and the Design Expert 7.0 (StatEase, Inc., USA). 3. Results and discussion 3.1. Cd2+ and Zn2+ biosorption by DAS The choice of cadmium and zinc for experimental study was made with regard to their industrial use and potential pollution impact. In preliminary experiments the kinetic studies were realized (data not shown) and, as expected, we found that the biosorption of cadmium and zinc ions from single systems by dried activated sludge is a rapid two-phase process. At the initial phase the driving force is higher and binding sites on DAS with higher affinity are occupied. Residual sites with lower affinity are occupied slowly during the next 2 h. The final equilibrium was reached within 180 min and after this time there was no considerable increase until the end of experiment. Such twophase sorption has been also reported by Yang et al. [25] and Yao et al. [26]. They found that sorption of Zn2+ ions by activated sludge and Pb2+ ions by aerobic granules markedly increased in the first 60 min of experiments and the equilibrium was reached after 180 min. This rapid biosorption phenomenon has importance for

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3.2.1. FTIR spectroscopy FTIR analysis was performed to identify the surface nature of DAS as well as to identify major functional groups and to confirm their possible participation in biosorption process. The FTIR spectrum (Fig. 1(A)) of unloaded DAS depicts a number of bands indicating the complex nature of the biosorbent. Bands in the FTIR spectrum were assigned to various groups according to wave numbers (Table 2) as reported in literature [5,25,27,28]. The absorption band at 1655 cm1 could be assigned to the strong vibration of C5 5O and C–N groups of the peptidic bond of proteins. The 1535 cm1 band represents the stretching vibration of C–N and the deformation vibration N–H of the peptidic bond of proteins. Bands at 1419 cm1 and 1234 cm1 could be assigned to the vibration of C5 5O group of carboxylates and carboxylic acids. These groups are characteristic of the bacterial cell wall of both G+ and G bacteria that generally coexist in biological wastewater treatment plants. The fingerprint region demonstrated the existence of sulfur or phosphate groups [29]. Changes in band intensity and frequency after metal binding can be used to identify the functionalities involved in metal binding [30]. The main differences between spectra of unloaded DAS and metal-loaded DAS are associated with the amide II (N–H bonding) vibrations. Evident shifts of the absorption band from 1535 cm1 to 1516 cm1, after both Cd2+ and Zn2+ biosorption, were recognized. The observed shifts are likely due to conformational changes associated with changes of H-bonds as well as the effect on both Cd and Zn bonding. As seen from Fig. 1(A), (B) and (C) also the intensity of the band at 1655 cm1 decreased considerably after Cd2+ and Zn2+ biosorption. The clear changes of relative absorption intensities of bands corresponding to carboxyl groups (1419 and 1234 cm1) were observed after metal biosorption. Based on the extensive infrared studies [31] and obtained results, we suggest that the carboxylate ions may coordinate to Cd2+ and Zn2+ ions as chelating (bidentate) complexes. The importance of carboxylic groups and amino groups in metal biosorption has been discussed in literature [4,32]. The contribution of the hydroxyl group in metal binding by DAS was not expressly confirmed by FTIR spectra. Fig. 1. FTIR spectra of unloaded (A) and Zn (B) and Cd (C) loaded DAS.

the scale-up of the process ensuring efficiency in practical applications. 3.2. Mechanism of Cd2+ and Zn2+ biosorption Many papers dealing with metal biosorption discuss the metal uptake mainly on the basis of biosorption kinetics, equilibrium isotherms, influence of temperature, pH, biosorbent dosage although the mechanism of metal ion binding is not consistently studied. Appropriate analytical techniques are needed to elucidate the mechanisms participating on the biosorption processes and to get insight into the localization and chemical nature of metals sorbed. Therefore, in our work, the mechanistic aspects of Cd2+ and Zn2+ biosorption by DAS were studied by different experimental approaches.

3.2.2. Blocking of functional groups To verify and quantify the participation of functional groups in biosorption of Cd2+ and Zn2+ ions, batch experiments using DAS with blocked carboxyl and hydroxyl groups were carried out. Comparison of equilibrium sorption capacities Qeq of unmodified DAS and DAS with esterified carboxyl and methylated hydroxyl groups is shown in Table 3. The amount of both Zn2+ and Cd2+ ions adsorbed decreased significantly when carboxyl groups were esterified. Decrease of approx. 43% and 34% in the sorption of Cd2+ and Zn2+ reflects important role of carboxyl groups and ionexchange in the sorption process. Kılıc¸ et al. [12] confirmed the carboxylic groups participate in Pb2+ biosorption by activated sludge and modification of the carboxylic group by esterification caused significant decrease of Pb2+ binding. On the contrary, blocking of hydroxyl groups caused no decrease in both metal ions biosorption indicating that hydroxyl groups do not participate in sorption of Cd and Zn by DAS. Also Pe´rez-Marı´n et al. [33] showed that hydroxyl groups have negligible effect on Cd, Cr and Zn

Table 2 Main functional groups of DAS with corresponding wave numbers obtained using FTIR analysis. Wave number (cm1)

Vibration type

Functional type

3290–3400 1655 1535 1419 1234

Stretching vibration of OH Strong vibration of C5 5O and C–N (primary amide) Stretching vibration of C–N and deformation vibration of N–H (secondary amide) Stretching vibration of C5 5O Deformation vibration of C5 5O

–OH of DAS polymeric substances Proteins (peptidic bond) Proteins (peptidic bond) Carboxylates Carboxylic acids

L. Remena´rova´ et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 433–443 Table 3 Biosorption of Cd2+ (1000 mmol/L CdCl2, 109CdCl2 91 kBq/L) and Zn2+ (1000 mmol/L ZnCl2, 65ZnCl2 82 kBq/L) from single solutions using DAS and DAS with chemically blocked carboxyl and hydroxyl groups (2.5 g/L, d.w.) at pH 6.0 and 20 8C. Metal

Cd2+ Zn2+

Metal biosorption (mmol/g) Unmodified DAS

DAS with blocked carboxyl groups

DAS with blocked hydroxyl groups

297  11 268  2

160  4 177  1

296  1 271  1

sorption onto orange waste. It is not surprising because hydroxyl groups become negatively charged only at pH >10 and therefore binding of divalent cations at lower pH is implausible. Nevertheless, there was approximately 57% uptake of cadmium and 66% uptake of zinc ions after blocking of carboxyl groups indicating the presence of another active sites (e.g. phosphoric groups associated with phospholipids, sulfonate groups involved in the sulfated polysaccharides and some amino acids) or mechanisms of metal uptake (e.g. intraparticle diffusion). Also Cd and Zn precipitation cannot be neglected (see metal speciation below). This is consistent with the fact that metal biosorption is a complex phenomenon. 3.2.3. SEM–EDX analysis Scanning electron microscopy (SEM) connected with EDX represents another useful tool for studying mechanistic aspects of metal biosorption and for the qualitative visualization of metalbiosorbent interactions [15]. In our work SEM–EDX analysis was carried out to characterize the DAS surface before and after Cd2+ and Zn2+ ions uptake in single systems. Comparison of unloaded sorbent SEM images and images of DAS after biosorption of Cd2+ and Zn2+ ions (magnification 2500) shows that there are no morphological changes on the biomass surface and there did not appear to be any specific localization of both metals on the biosorbent surface (data not shown). This technique also enables us to trace the distribution of metal ions on the surface of the biosorbent particles. To confirm the participation of ion-exchange in Cd and Zn biosorption, sorbent was treated with CaCl2 solution. Elemental mapping of Ca-treated DAS clearly demonstrated distribution of Ca on the biosorbent surface (Fig. 2(A) and (B)). Ca distribution seems to be non-uniform

437

with the exception of areas with intense red color on DAS surface indicating the precipitation of Ca in the form of CaCO3. SEM pictures of DAS after Zn uptake are presented in Fig. 3(A)– (D). Elemental mapping of the sludge particles after Zn biosorption showed uniform distribution on DAS surface (Fig. 3(C)), with the exception of areas where Ca2+ ions precipitated (Fig. 3(C), areas 1 and 2). This suggests that negatively charged functional groups have exchanged Ca2+ ions with Zn2+ ions from aqueous solution and therefore ion-exchange participates in zinc biosorption. No ion-exchange was observed in places where Ca precipitated. This phenomenon is clearly visible from Fig. 3(D) (areas 1 and 2) where distribution of Ca and Zn on DAS is depicted together. Davis et al. [34] pointed out that the term ion-exchange does not explicitly identify the binding mechanism; rather it is used as a wider term to describe results of experiments. We observed similar behavior in the case of Cd2+ biosorption by DAS. SEM–EDX mapping images confirmed that Cd2+ ions were sorbed on the DAS surface, replacing Ca2+ ions from binding sites (Fig. 4(A)–(D)). Additionally, very marked decrease of Ca peaks on the EDX spectra of DAS was observed after the sorption of Cd2+ or Zn2+ ions (not shown). 3.2.4. Influence of pH and metal speciation For biosorption of heavy metals ions, pH is one of the most important environmental factors. The pH value of a solution strongly influences not only the site dissociation of the biomass surface, but also the solution chemistry of the heavy metals: hydrolysis, complexation by organic and/or inorganic ligands, redox reactions, precipitation and the biosorption availability of the heavy metals [35]. To establish the influence of pH on the sorption of Cd2+ and Zn2+ ions from single metal solutions onto DAS, batch equilibrium studies at different pH values were carried out, and the data, necessary for an accurate evaluation of biosorption mechanism, are shown in Figs. 5 and 6. It can be seen that maximum biosorption of both Cd and Zn occurred at pH 6.0. Slightly lower biosorption was observed at pH 4.0 and negligible at pH 2.0. This type of curve is characteristic also for Cd, Cu, Ni, Pb and Zn biosorption by activated sludge [26,36,37]. However, the pH also influences the ionization and speciation of metals in aqueous solution. Calculation by Visual MINTEQ speciation program showed that cadmium predominantly exists

Fig. 2. SEM pictures of Ca-treated DAS before Cd2+ and Zn2+ ions biosorption; magnification 250 (A). EDX elemental mapping of Ca (B).

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Fig. 3. SEM pictures of Ca-treated DAS after Zn2+ ions biosorption (4000 mM ZnCl2) at pH 6.0; magnification 250 (A). EDX elemental mapping of Ca (B), elemental mapping of Zn (C) and elemental mapping of Ca and Zn on DAS surface (D). 1, 2 – areas indicate Ca precipitation.

as Cd2+ (88%) and CdCl+ (10%) cations within pH ranging from 2.0 to 7.5 (Fig. 5). A number of other ionic forms such as CdOH+, Cd2OH3+, Cd(OH)3 and Cd(OH)42 are present in solution between pH 8.0 and 12.0. The concentration of Cd2+ starts to decrease at pH >8.0 and the precipitation of Cd started at pH >9.0. Maximum Cd biosorption (74.9%) was observed at pH 6.0 when Cd2+ cations represent 87.6% of the total ionic forms of cadmium. From the difference between these values it is evident that: (1) as pH level increases, more negatively charged functional groups would be exposed with the subsequent increase in attraction sites to positively charged cadmium ions, (2) cadmium binding is closely related to protonation of binding sites, resulting in competition between H+ and Cd2+ ions for occupancy of the active sites. A different pattern was observed in the case of zinc biosorption by DAS. Zinc occurs in solution almost exclusively as divalent cation Zn2+ at pH values below 5.0. Other ionic forms such as ZnCl+, ZnOH+ and Zn2(OH)3+ exist only at higher pH values. The precipitation of zinc (formation of Zn(OH)2 form) started at pH 7.5 and reached maximum at pH 10.0 (Fig. 6). Maximum Zn biosorption (74.6%) was observed at pH 6.0 when Zn2+ ions represent 99.5% of the total ionic forms of zinc. The substantial

decrease in Zn uptake observed at pH 8.0 may be explained by the decrease in the concentration of Zn2+ ions in solution. From results presented in Fig. 6 it is reasonable to suppose that: (1) zinc uptake at higher pH is closely related to the metal speciation in solution and (2) the contribution of zinc precipitation cannot be neglected. It must be pointed out that the dependence of cadmium and zinc uptake on pH is also closely related to the surface functional groups. Moreover, extreme pH values can damage the structure of DAS and therefore decrease metal uptake. 3.3. Biosorption equilibrium in single metal systems Generally known Langmuir (Eq. (2)) and Freundlich (Eq. (3)) isotherms were fitted to the equilibrium data for Cd2+ and Zn2+ biosorption from single systems by DAS. Isotherm curves and parameters of the models determined from the experimental data using non-linear regression analysis are reported in Fig. 7 and Table 4. In many cases the suitability of Langmuir and Freundlich isotherms to describe the experimental equilibrium data was

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Fig. 4. SEM pictures of Ca-treated DAS after Cd2+ ions biosorption (4000 mM CdCl2) at pH 6.0; magnification 250 (A). EDX elemental mapping of Ca (B), elemental mapping of Cd (C) and elemental mapping of Ca and Cd on DAS surface (D). 3, 4 – areas indicate Ca precipitation.

Fig. 5. Effect of pH on the biosorption of Cd (1000 mmol/L CdCl2, 109CdCl2 91 kBq/L) by DAS (2.5 g/L, d.w.) from single metal solution and theoretical Cd speciation in solution. Error bars represent standard deviation (SD) of the mean (n = 2). Cd speciation was calculated using Visual MINTEQ version 3.0 with initial conditions: 1000 mmol/L CdCl2, T = 20 8C, pCO2 = 38.5 Pa.

Fig. 6. Effect of pH on the biosorption of Zn (1000 mmol/L ZnCl2, 65ZnCl2 82 kBq/L) by DAS (2.5 g/L, d.w.) from single metal solution and theoretical Zn speciation in solution. Error bars represent standard deviation (SD) of the mean (n = 2). Zn speciation was calculated using Visual MINTEQ version 3.0 with initial conditions: 1000 mmol/L ZnCl2, T = 20 8C, pCO2 = 38.5 Pa.

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demonstrated by higher values of the coefficient of determination (R2), by the more homogeneous standard deviation of each observed parameter and by the lower of the sum of squares (RSS) and AICc values obtained (Table 5). Also other authors found that the sorption of Cd2+, Cr3+ and Zn2+ ions by activated sludge was well described using the Langmuir isotherm [6,25,36]. Parameters of Langmuir and Freundlich adsorption isotherms provide an insight into the sorption process, reflect the nature of the sorbent and its surface properties, as well as the degree of the affinity of the sorbents, and also can be used to compare biosorption performance [40]. The maximum sorption capacity of DAS Qmax obtained from the Langmuir isotherm for Cd2+ was 510  17 mmol/g at pH 6.0. A slightly higher value of Qmax was observed in the case of Zn2+ sorption, i.e. 540  16 mmol/g at pH 6.0. The affinity constant b of the isotherms corresponds to the initial gradient, which indicates the biosorbent affinity at low concentrations of metal ions. A higher initial gradient corresponds to a higher affinity constant b [41]. From Fig. 7(A) and (B) it is evident that the cadmium and zinc isotherms have similar behavior at lower equilibrium concentrations. The difference in the b values for cadmium and zinc, 0.005  0.001 L/mmol and 0.006  0.001 L/mmol respectively, indicates slightly higher affinity of DAS for zinc ions. It should be realized that despite the fact that the Langmuir isotherm offers no insights into the mechanism of biosorption [42], it remains a convenient tool for comparing equilibrium data on a quantitative basis (determination of maximum sorption capacity Qmax and affinity parameters b) and providing information on biosorption potential. 3.4. Biosorption of Cd2+ and Zn2+ ions in binary system Cd2+–Zn2+ Response surface methodology (RSM) has been widely used for optimization of biosorption processes mainly in single systems. Only a few studies utilized RSM methodology for statistical analysis of individual and interaction effects of parameters in binary and ternary sorption systems [18,43,44]. Box–Behnken design under the RSM was used in our study for investigation of interactive and competitive effects between variables in the binary system Cd2+–Zn2+. According to preliminary experiments and our previous studies [20,39] the sorption capacity of biosorbent in a multi-component system mainly depends on the initial concentration of primary ion and co-ions in solution and on the solution pH. Therefore, the initial concentration of Cd and Zn and the pH of the solution were used as process variables in experimental design (Table 3), and two responses – equilibrium sorption capacities Qeq(Cd) and Qeq(Zn) were studied simultaneously. The behavior of the binary sorption system Cd2+–Zn2+ is explained by the following quadratic models determined by multiple regression analysis according to Eq. (4):

Fig. 7. Isotherms for the biosorption of Cd2+ (A) and Zn2+ (B) ions by DAS at 20 8C and pH 6.0 according to Langmuir and Freundlich. Data points represent experimental results; curves represent the calculated values from isotherm models.

determined only on the basis of the marginal differences between coefficients of determination (R2) [28,37]. From our point of view, the appropriate way to select a model which is best supported by the data, is through the application of model-selection criteria. Therefore the adequacy of the two models in our work was compared by using the Akaike’s information criterion (AIC). The AIC tool should be readily adopted for model discrimination in the field of biosorption modeling [38] because AICc (corrected AIC) is able to answer the question: which isotherm model is better for mathematical description of Cd2+ and Zn2+ biosorption by DAS? The isotherm model with the lower AICc value is considered most likely to be correct. This approach was also successfully used in our previous papers [20,39]. The Langmuir isotherm fits the data of both Cd2+ and Zn2+ ions sorption by DAS better than the Freundlich isotherm, as is

Q eq ðCdÞ ¼ 382 þ 0:097  C 0 Cd  2:1  105  C 0 Zn þ 194  pH  1:3  105  C02 Cd  2:3  106  C02 Zn  17:03  pH2 þ 3:9  106  C 0 Cd  C 0 Zn þ 0:0019  C 0 Cd  pH  0:007  C 0 Zn  pH

(7)

Table 4 Langmuir and Freundlich parameters for the biosorption of Cd2+ and Zn2+ ions by DAS obtained by non-linear regression analysis. Model

Metal

Langmuir

Zn2+ Cd2+ Zn2+ Cd2+

Freundlich

Qmax [mmol/g] 540  16 510  17 – –

b [L/mmol] 0.006  0.001 0.005  0.001 – –

K [L/g]

1/n

R2

– – 50.3  29.4 49.9  28.9

– – 0.30  0.08 0.29  0.08

0.995 0.994 0.877 0.878

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Table 5 Comparison of corrected Akaike’s information criterion (AICc) and residual sum of squares (RSS) values of Langmuir and Freundlich isotherms for Cd2+ and Zn2+ biosorption by DAS. Langmuir

Metal

Cd2+ Zn2+

Freundlich

RSS

AICc

Akaike’s weight

RSS

AICc

Akaike’s weight

769 723

55.18 54.87

0.9995 0.9997

16 347 18 817

70.46 71.17

0.0005 0.0003

Table 6 Box–Behnken experimental design matrix and experimental and predicted values of sorption capacity of DAS (Qeq) for Cd2+ and Zn2+ ions from binary system Cd2+–Zn2+. A – C0 Cd (mmol/L), B – C0 Zn (mmol/L), C – solution pH. Run order

Factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Qeq Cd

Qeq Zn

A

B

C

Qeq (experimental) (mmol/g)

Qeq (predicted) (mmol/g)

Qeq (experimental) (mmol/g)

Qeq (predicted) (mmol/g)

1000 3000 1000 3000 1000 3000 1000 3000 2000 2000 2000 2000 2000 2000 2000 2000

1000 1000 3000 3000 2000 2000 2000 2000 1000 3000 1000 3000 2000 2000 2000 2000

4.5 4.5 4.5 4.5 3 3 6 6 3 3 6 6 4.5 4.5 4.5 4.5

205.8 321.0 130.7 261.4 93.77 211.2 170.8 299.3 180.9 132.7 297.8 206.0 242.5 242.5 254.5 241.1

206.5 321.7 130.0 260.7 90.64 208.0 173.9 302.4 183.4 136.5 294.0 203.5 245.0 245.0 245.0 245.0

178.0 113.2 312.0 226.2 138.1 118.3 284.9 193.3 75.86 154.0 151.1 282.6 213.3 213.3 227.8 198.4

179.2 122.8 302.4 226.9 145.5 115.3 287.8 185.9 69.18 156.1 148.9 289.3 214.6 214.6 214.6 214.6

Eqs. (7) and (8) represent the quantitative effect of the variables (initial concentration of Cd and Zn, the pH of the solution) and their interactive effects on the Qeq(Cd) and Qeq(Zn) in the binary system Cd2+–Zn2+. A positive sign in the equation implies a synergistic effect of the variables, while a negative sign indicates an antagonistic effect. The adequacy of the models was further justified through ANOVA (data not shown). The model F-values of 246 (for Cd2+ sorption) and 58.3 (for Zn2+ sorption) and values of P < 0.0001 indicate that both models are significant. Good agreement between the experimental and predicted values of sorption capacity Qeq (Table 6) confirmed high values of coefficient of determination (R2), 0.997 (for Cd2+) and 0.987 (for Zn2+).

In both cases, from interaction plots (Figs. 8 and 9) it can be seen that increasing the concentration of co-ion in solution had a significant inhibitory effect on the biosorption of primary ion. Decrease in primary ion uptake was expected due to (1) competition between Cd2+ and Zn2+ ions for binding sites on the biosorbent surface; (2) the physico-chemical characteristics of metal ions (see below). Interaction plots revealed that zinc and cadmium biosorption was enhanced with increasing solution pH from 3.0 to 6.0. This fact is closely related to the competition between H+ and Cd2+ and Zn2+ ions for occupancy of the active sites. Moreover, interaction plots clearly indicate strong interactions between the initial concentration of co-ion, the pH of the solution and sorption capacity of primary ion by DAS. Maximum sorption capacities of DAS in a binary system determined experimentally were 321 mmol Cd2+/g and 312 mmol Zn2+/g. In comparison with Qmax values from a single system (Table 4) an evident decrease in individual metal uptake can be seen. However, the total metal uptake Qeq(Cd) + Qeq(Zn) was higher than the equilibrium sorption capacities from single metal solutions

Fig. 8. Interaction plot for Cd2+ biosorption by DAS from binary system Cd2+–Zn2+. A: C0 Cd (+ 3000 mmol/L;  1000 mmol/L); B: C0 Zn (+ 3000 mmol/L;  1000 mmol/L); C: solution pH (+ pH 6.0;  pH 3.0).

Fig. 9. Interaction plot for Zn2+ biosorption by DAS from binary system Cd2+–Zn2+. A: C0 Cd (+ 3000 mmol/L;  1000 mmol/L); B: C0 Zn (+ 3000 mmol/L;  1000 mmol/L); C: solution pH (+ pH 6.0;  pH 3.0).

Q eq ðZnÞ ¼ 394 þ 0:008  C 0 Cd þ 0:075  C 0 Zn þ 187  pH þ 5:5  10 2

6



C02 Cd

 pH  4:8  10

6

 1:2  10

5



C02 Zn

 16:2

 C 0 Cd  C 0 Zn  0:012  C 0 Cd

 pH þ 0:009  C 0 Zn  pH

(8)

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Table 7 Some physico-chemical properties of cadmium and zinc ions. Properties

Cd2+

Zn2+

Atomic weight Ionic radius r (pm) Electronegativity (pauling) Xm 2 Covalent index (Xm r)

112.41 97 1.69 2.71

65.39 74 1.65 2.04

indicating that in addition to competition between metal ions, a some of the cadmium and zinc is taken up without mutual competition, and some active sites may be specific to individual metals. The same conclusions were also postulated by Sag et al. [45]. It was found that DAS in the binary system Cd2+–Zn2+ has significantly higher affinity for Cd2+ ions when Cd2+ and Zn2+ are present in solution in equimolar ratio 1:1 (Table 6). This is consistent with the hypothesis that the variance in affinity in multi-component systems could be attributed to different ionic characteristics of the metal ions [43,46]. Brady and Tobin [47] showed that biosorption of Cd2+, Cu2+, Sr2+, Mn2+, Pb2+ and Zn2+ ions from single systems by Rhizopus arrhizus is related to the 2 covalent index (Xm r). The greater the covalent index value of the metal ion, the greater is the potential to form covalent bonds with biological ligands. It is reasonable to suppose that higher affinity for Cd in the binary system Cd2+–Zn2+ is also closely related to the 2 covalent index (Xm r), the electronegativity and the ionic radius (Table 7). According to the classification based on the covalent index [48] that reflects different ligand affinities, both cadmium and zinc belong to borderline metal ions. At cation binding sites, 2 borderline ions with higher Xm r displace borderline ions with 2 lower Xm r which confirms our results from the binary system Cd2+–Zn2+. Also the solution chemistry (pH and metal speciation) significantly influenced the preference of the metals for DAS. Similar conclusions were postulated by Mahamadi and Nharingo [49]. 4. Conclusions Increasing amounts of sludges produced by waste water treatment plants temporarily stored or disposed in landfills represent one of the main problems of sustainable development. A possible solution is to consider waste sludges as renewable biomaterials potentially to be used as sorbents of liquid toxic wastes. Data presented in our paper confirmed high sorption capacity of dried activated sludge (DAS) for Cd and Zn ions both in single and binary systems. Experimental equilibrium data of the metal sorption in single-component systems for Cd2+ and Zn2+ ions have been well described by the Langmuir isotherm and the maximum sorption capacities Qmax 540  16 mmol/g for Zn2+ and 510  17 mmol/g for Cd2+ ions were found. Qmax in the binary Cd–Zn system determined experimentally was 321 mmol Cd2+/g and 312 mmol Zn2+/g. FTIR analysis and chemical blocking of functional groups confirmed decisive participation of carboxyl groups in Cd2+ and Zn2+ biosorption and the existence of ion exchange of Cd and Zn by Ca confirmed by SEM–EDX analysis. Response surface method (RSM) appears to be a better tool for the evaluation of interactions and competitive effects in binary systems and for experimentally more difficult studies of multi-component systems than the simple extrapolation from single-component systems. The understanding of sorption mechanisms of metals from multicomponent solutions is necessary for the development of appropriate management strategies for non-agricultural applications of wastewater sludges. References [1] Chojnacka K. Biosorption and bioaccumulation – the prospects for practical applications. Environ Int 2010;36:299.

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