Microsoft Word - Pipiska NB 2010-2.doc Nova Biotechnologica 10-2 (2010) 117 CADMIUM AND ZINC UPTAKE BY DRIED ACTIVATED SLUDGE: EQUILIBRIUM AND EXPERIMENTAL DESIGN STUDY LUCIA REMENÁROVÁ1, MARTIN PIPÍŠKA1,2, MIROSLAV HORNÍK1,2, JANA MAREŠOVÁ1, JOZEF AUGUSTÍN1,2 1Department of Ecochemistry and Radioecology, University of SS. Cyril and Methodius, J. Herdu 2, Trnava, SK-917 01, Slovak Republic (pipiska@ucm.sk) 2Consortium for Environmental Biotechnology and Environmental Chemistry, Hlavná 418, Špačince, SK-919 51, Slovak Republic Abstract: Removal of Cd2+ and Zn2+ ions from single and binary solutions by dried activated sludge was studied in batch experiments. It was shown that the metal removal is a rapid process significantly influenced by solution pH. Maximum uptake of both Cd and Zn was reached at pH 6.0 and negligible uptake was observed at pH 2.0. The Langmuir isotherm was found to well represent the measured equilibrium sorption data in single metal systems and the maximum sorption capacities Qmax of the activated sludge (d.w.), calculated from Langmuir model were 540 ± 16 μmol/g for Zn2+ and 510 ± 17 μmol/g for Cd2+ ions. The Response surface methodology (RSM) was used for investigation of interaction and competitive effects in binary metal system. It was found that dried activated sludge in binary system Cd-Zn has slightly higher affinity for Cd2+ comparing with Zn2+ ions. Competitive effect of Cd on Zn uptake increased with increasing solution pH and Cd initial concentration. Maximum sorption capacities of the activated sludge were 321 μmol Cd2+/g and 312 μmol Zn2+/g. RSM appears to be a better tool for the evaluation of interaction and competitive effects in binary systems than both the simple extrapolation from single-component systems and experimentally difficult study of multi-component systems. Key words: cadmium, zinc, biosorption, activated sludge, isotherms, RSM 1. Introduction Industrial effluents contain both organic and inorganic pollutants. These pollutants upset natural balance in water ecosystems, interfere with organisms, accumulate in biota and enter into the food chain with human on the top. Contaminants removal by conventional treatment methods, such as chemical precipitation, membrane separation, evaporation and ion-exchange is often limited due to their low efficiency and economic viability (NAYAK and LAHIRI, 2006). Therefore, there is a need for an effective and economical treatment alternative. Biological processes such as biosorption and bioaccumulation represent possible interactions of toxic pollutants with biological systems in contaminated environment. Current research activity in the field attempts to evaluate whether biosorption may eventually provide such an effective and economical treatment process alternative (NAJA et al., 2010). Many researchers studied biosorption from single systems (GHODBANE et al., 2008; KANG et al., 2007), although the degree of removal of metal ions from wastewaters by biosorption depends mainly on the competitive interactions of co-ions when present in solution (MA and TOBIN, 2003). Results from single component systems can provide useful data about the uptake capacity of used biosorbent, but they 118 Remenárová, L. et al. do not exactly reflect the real situation in wastewaters (GÖNEN and AKSU, 2009). Therefore it is necessary to study sorption process as complex process, which consists of many mechanisms and can be affected by many parameters. Effect of various parameters (pH, temperature, sorbent particle size, initial concentration of sorbates) can be studied by the classical approach, when one variable is changed at a time. However this procedure represents a time consuming and less effective method. Multivariate optimization is faster and overcome circumstances not explained by the traditional methods such as interactions between the variables that influence the response e.g. sorption capacity (ZOLGHARNEIN et al., 2010). 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 (MUNE et al., 2008). Main advantage of statistical design of experiments is the reduced number of experiments to be performed as well as it considers interactions among the variables and can be used for optimization of the operating parameters in multivariable systems (GÖNEN and AKSU, 2009). From the literature it was found that 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 (FEREIDOUNI et al., 2009; PAKSHIRAJAN and SWAMINATHAN, 2009). Within this context the objective of present study was firstly to quantify the ability of the biosorbent prepared from dried activated sludge to sorb Cd2+ and Zn2+ ions. Equilibrium isotherm models according to Langmuir and Freundlich were used for mathematical description of sorption equilibria in single systems. The second objective was to study the competitive and interaction effects of above mentioned ions in binary Cd2+-Zn2+ system at various solution pH values using Response surface methodology (RSM). 2. Material and methods 2.1 Biosorbent preparation Activated sludge was obtained from waste water treatment plant in Enviral a.s. (Leopoldov, Slovak Republic) producing fuel ethanol. The sludge was washed twice in deionised water, oven-dried for 72 h at 90°C. After drying activated sludge was ground to various particle sizes, from which particle size <450 μm was used in biosorption experiments. 2.2 Batch sorption experiments in single systems The biosorption kinetics was determined by suspending of activated sludge (2.5 g/L, d.w.) in metal solutions (pH 6.0) containing 1000 μmol/L CdCl2 or ZnCl2 spiked with 109CdCl2 or 65ZnCl2. The content was agitated on a reciprocal shaker (120 rpm) at 20°C and in time intervals liquid samples were taken and the radioactivity was measured. Nova Biotechnologica 10-2 (2010) 119 The metal sorption capacity of dried activated sludge was determined by suspending of activated sludge (2.5 g/L, d.w.) in metal solutions (pH 6.0) containing CdCl2 or ZnCl2 in concentration range 100-4000 μmol/L spiked with 109CdCl2 or 65ZnCl2 and exposing for 4h at 20°C on a reciprocal shaker (120 rpm). To analyze the influence of pH, activated sludge was shaken in metal solutions containing 1000 μmol/L CdCl2 or ZnCl2 spiked with 109CdCl2 or 65ZnCl2 for 4 h on a reciprocal shaker at 120 rpm and 20°C adjusted to different pH values (2.0 – 9.0) by adding 0.5 M HCl or 0.1 M NaOH. At the end biomass was filtered out, washed twice in deionised water and radioactivity of both activated sludge and liquid phase was measured. The metal uptake was calculated as mCCVQ eq /)( 0 −= (1) where Q is the uptake (μmol/g, d.w.), C0 and Ceq is the initial and the final metal concentrations in solution (μmol/L) and m is the amount of dried biosorbent (given in grams). All experiments were performed in duplicate. 2.3 Experimental design of binary Cd-Zn system The Box-Behnken design under Response surface methodology (RSM) was used to investigate interaction and competitive effects in binary metal system Cd2+-Zn2+. Levels of factors (initial concentrations C0 of Cd 2+ and Zn2+ ions and initial pH of solution) considered for sorption in binary system are shown in Table 1. The design matrix of 16 experiments is given in Table 3. Based on the matrix, experiments were performed in Erlenmeyer 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°C. Table 1. Levels of factors considered for sorption of Cd2+ and Zn2+ ions from binary system using Box- Behnken design. Coded levels Factor Unit Factor code -1 0 1 C0 Cd 2+ µmol/L A 1000 2000 3000 C0 Zn 2+ µmol/L B 1000 2000 3000 pH C 3.0 4.5 6.0 The behavior of the binary sorption system is explained by the following empirical second-order polynomial model ∑∑∑ ≤≤== +++= k ji jiij k i iiii k i i xxxxQ 11 2 10 ββββ (2) where Q is the predicted response (specific sorption Qeq of both Cd and Zn), xi, xj, .....xk are the input variables (C0 Cd 2+, C0 Zn 2+ and solution pH) which affect the response Q, β0 is the intercept term, βi is the linear effect, βii is the quadratic effect and βij is the interaction effect (FEREIDOUNI et al., 2009). 120 Remenárová, L. et al. 2.4 Radiometric analysis 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 activated sludge and supernatant fluids at the energy of γ- photons: 109Cd – 88.04 keV and 65Zn – 1115.52 keV. Standardized 109CdCl2 solution (3.937 MBq/ml; 50 mg CdCl2/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). 2.5 Speciation modeling Prediction of the speciation of Cd and Zn in the aqueous systems as a function of total salt concentration and solution pH was performed using the Visual MINTEQ (version 2.52) program. Visual MINTEQ 2.52 is a chemical equilibrium computer program that has an extensive thermodynamic database for the calculation of metal speciation, solubility and equilibria (GUSTAFSON, 2004). 2.6 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) and GraphPad Prism 5.0 (GraphPad Software, USA). Response surface graphs and regression analysis of the obtained data were performed by Statgraphics Centurion XV (StatPoint Inc., USA) and the Design Expert 7.0 (Stat-Ease, Inc., USA). 3. Results and discussion 3.1 Cd2+ and Zn2+ uptake by activated sludge In order to determine the minimum necessary time to reach the sorption equilibrium, the time-course studies on the biosorption of cadmium and zinc ions from single metal systems by biosorbent prepared from dried activated sludge were performed. Fig. 1 shows that biosorption of Cd2+ and Zn2+ ions is a rapid process where equilibrium is reached within several tens minutes. At initial phase driving force is higher and binding sites on activated sludge with higher affinity are occupied. After that time concentration of Cd2+ and Zn2+ ions in solution decreases and the residual binding sites with lower affinity toward metal ions are occupied slowly. The final equilibrium was reached within 4 hours. Such two-phase sorption has been also reported by YANG et al. (2010) in the case of Zn sorption by activated sludge. They found that zinc adsorption capacity increased obviously during the first 60 min and the final equilibrium was reached within 180 min. Nova Biotechnologica 10-2 (2010) 121 From Fig. 2 it can be seen that maximum biosorption of both Cd2+ and Zn2+ occurred at pH 6.0. This curve is characteristic also for Cd and Zn biosorption by the moss Rhytidiadelphus squarrosus (PIPÍŠKA et al., 2010) and biosorption of other metal ions on activated sludge, algae and other biosorbents (WANG et al., 2010; LIU et al., 2009; GUNDOGDU et al., 2009). Observed slightly lower biosorption at pH 4.0 and negligible at pH 2.0 is closely related to protonation of binding sites, resulting in competition between H+ and Cd2+ or Zn2+ ions for occupancy of the active sites. In the case of Zn2+ at pH 8.0 sharp decrease of biosorption was observed. 0 200 400 600 800 1000 1200 1400 1600 0 50 100 150 200 250 300 350 400 Q eq [μ m ol /g ] t [min] Cd2+ Zn2+ Fig. 1. Effect of contact time on Cd2+ (C0 = 1000 μmol/L, 100 kBq 109CdCl2) and Zn2+ (C0 = 1000 μmol/L, 65 kBq 65ZnCl2) ions biosorption by dried activated sludge (2.5 g/L d.w.) at 20°C and pH 6.0. It must be pointed out, that the pH influenced also the level of ionization and speciation of metals in aqueous solution. As can be calculated by Visual MINTEQ speciation program (data not shown) at pH 8.0 besides the divalent cation Zn2+, zinc occurs in solution also in insoluble forms such as [Zn(OH)]+. Therefore the decrease of Zn2+ sorption at higher pH values (> pH 8.0) can be caused also by precipitation. According to calculations insoluble cadmium species occurred at pH > 9.0. It is reasonable to suppose that the dependence of metal uptake on pH is related to both the surface functional groups on the biosorbent and the metal speciation in solution. During sorption experiments pH was not regulated. From Fig. 2 it is evident that the equilibrium pH increased after biosorption experiments what indicates that functional groups on the surface accumulate besides Cd2+ and Zn2+ ions simultaneously H+ ions. Similar behaviour observed also CHEN et al. (2002) in Cu sorption by activated sludge. Several mechanisms of heavy metals uptake by activated sludge have been proposed (WANG et al., 2010; LAURENT et al., 2010). Short-term cation uptake is generally regarded as an abiotic process governed by: surface complexation of cations with exposed functional groups (carboxyl-, sulfhydryl-, amine- etc.) on the biosorbent; ion exchange; coordination and chelation of metals; adsorption or by precipitation of solid phases on the cell walls. 122 Remenárová, L. et al. 2 4 6 8 0 50 100 150 200 250 300 350 Q eq Cd2+ pH 0 Q eq [μ m ol /g ] 2 4 6 8 pH eq 2 4 6 8 0 50 100 150 200 250 300 350 Q eq Zn2+ pH 0 Q eq [μ m ol /g ] 2 4 6 8 pH eq Fig. 2. Effect of initial pH on Cd2+ and Zn2+ ions (c0 = 1000 µmol/L) sorption by dried activated sludge (2.5 g/L, d.w.) at 20°C; (- -) represents shifts in pH after biosorption. 3.2 Sorption equilibrium in single systems Two well known adsorption isotherm models - Langmuir (Eq. 3) and Freundlich (Eq. 4) were applied for the analysis of the experimental data in single sorption systems. eq eq eq bC CbQ Q + = 1 max (3) )/1( n eqeq KCQ = (4) Parameters of these models provide an insight into the sorption process, reflect the nature of the sorbent, surface properties as well as the degree of the affinity of the Nova Biotechnologica 10-2 (2010) 123 sorbents and can be used to compare biosorption performance (VOLESKY, 2003). The Langmuir and Freundlich isotherms were fitted to the equilibrium data for Cd2+ and Zn2+ sorption on dried activated sludge. Isotherm curves and parameters of the models determined from the experimental data using non-linear regression analysis are reported in Fig. 3, 4 and Table 2. 0 1000 2000 3000 0 200 400 600 Langmuir A Ceq [μmol/L] Q eq [ μ m ol /g ] 0 1000 2000 3000 0 200 400 600 800 Freundlich B Ceq [μmol/L] Q eq [ μ m ol /g ] Fig. 3. Isotherms for the sorption of Zn2+ ions by dried activated sludge at 20°C and pH 6.0 according to Langmuir (A) and Freundlich (B). Data points represent experimental results, curves represent the calculated values from isotherm models, dotted lines represent the 95 % confidence interval. The Langmuir isotherm fits the data of both Cd2+ and Zn2+ ions sorption by dried activated sludge better than the Freundlich isotherm, as is demonstrated by higher values of coefficient of determination (R2), the more homogeneous standard deviation of each observed parameter and by the lower the sum of squares (RSS) values obtained as well as standard deviation of the residuals (Sy,x ) (Table 2). Also HAMMAINI et al. (2007) found that the sorption of Cd2+ and Zn2+ ions by activated sludge was well described using the Langmuir isotherm. The maximum sorption capacity Qmax obtained from Langmuir isotherm for Cd 2+ was 510 ± 17 μmol/g at pH 6.0. Slightly higher value of Qmax was observed in the case of Zn2+ sorption, 540 ± 16 μmol/g at pH 6.0. The affinity constant b of the isotherms corresponds to the initial gradient, which indicates the biosorbent affinity at low 124 Remenárová, L. et al. 0 1000 2000 3000 0 200 400 600 Langmuir A Ceq [μmol/L] Q eq [ μ m ol /g ] 0 1000 2000 3000 0 200 400 600 800 Freundlich B Ceq [μmol/L] Q eq [ μ m ol /g ] Fig. 4. Isotherms for the sorption of Cd2+ ions by dried activated sludge at 20°C and pH 6.0 according to Langmuir (A) and Freundlich (B). Data points represent experimental results, curves represent the calculated values from isotherm models, dotted lines represent the 95 % confidence interval. Table 2. Langmuir and Freundlich parameters for the sorption of Cd2+ and Zn2+ ions by dried activated sludge obtained by non-linear regression analysis. Model Metal ion Qmax [μmol/g] b [L/μmol] K [L/g] 1/n R 2 RSS Sy,x Zn2+ 540 ± 16 0.006 ± 0.001 - - 0.995 723 15.5 L an gm ui r Cd2+ 510 ± 17 0.005 ± 0.001 - - 0.994 769 16.0 Zn2+ - - 50.3 ± 29.4 0.30 ± 0.08 0.877 18817 79.2 Fr eu nd lic h Cd2+ - - 49.9 ± 28.9 0.29 ± 0.08 0.878 16347 73.8 concentrations of metal ions. A greater initial gradient corresponds to a higher affinity constant (SHENG et al., 2007). From Fig. 3 and 4 it is evident that the cadmium and Nova Biotechnologica 10-2 (2010) 125 zinc isotherms have similar behavior at lower equilibrium concentrations. In the case of b, cadmium recorded 0.005 ± 0.001 L/μmol compared to 0.006 ± 0.001 L/μmol for zinc indicating slightly higher affinity of activated sludge for zinc ions. It should be realized that despite the fact that Langmuir isotherm offers no insights into the mechanism of biosorption (LIU and LIU, 2008) 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. 3.3 Experimental design of binary Cd-Zn system Box-Behnken design under the Response surface methodology (RSM) was used for investigation of interaction and competitive effects between variables in binary system Cd-Zn. According to preliminary experiments and our previous studies (PIPÍŠKA et al., 2010; PIPÍŠKA et al., 2008) the sorption capacity of biosorbent in multi-component systems mainly depends on the initial concentration of primary ion and co-ions in sorption system and on the solution pH. Therefore, initial concentration of Cd and Zn and solution pH were used as process variables in experimental design (Table 3) and two responses – Qeq(Cd) and Qeq(Zn) were studied simultaneously. Table 3. Box-Behnken experimental design matrix and experimental and predicted values of sorption capacity (Qeq) of Cd2+ and Zn2+ ions by dried activated sludge from binary system Cd-Zn. A – C0 Cd (µmol/L), B – C0 Zn (µmol/L), C – pH. Factor Qeq Cd Qeq Zn Run order A B C Qeq (experimental) (µmol/g) Qeq (predicted) (µmol/g) Qeq (experimental) (µmol/g) Qeq (predicted) (µmol/g) 1 1000 1000 4.5 205.8 206.5 178.0 179.2 2 3000 1000 4.5 321.0 321.7 113.2 122.8 3 1000 3000 4.5 130.7 130.0 312.0 302.4 4 3000 3000 4.5 261.4 260.7 226.2 226.9 5 1000 2000 3 93.77 90.64 138.1 145.5 6 3000 2000 3 211.2 208.0 118.3 115.3 7 1000 2000 6 170.8 173.9 284.9 287.8 8 3000 2000 6 299.3 302.4 193.3 185.9 9 2000 1000 3 180.9 183.4 75.86 69.18 10 2000 3000 3 132.7 136.5 154.0 156.1 11 2000 1000 6 297.8 294.0 151.1 148.9 12 2000 3000 6 206.0 203.5 282.6 289.3 13 2000 2000 4.5 242.5 245.0 213.3 214.6 14 2000 2000 4.5 242.5 245.0 213.3 214.6 15 2000 2000 4.5 254.5 245.0 227.8 214.6 16 2000 2000 4.5 241.1 245.0 198.4 214.6 126 Remenárová, L. et al. The behavior of the binary sorption system Cd-Zn is explained by the following quadratic models determined by multiple regression analysis: ( ) pHZncpHcZncCdcpHZnc CdcpHZncCdcCdQeq ××−××+××⋅+×− ×⋅−×⋅−×+×⋅−×+−= − −−− 0007.000019.000109.303.170 103.20103.11940101.20097.0382 622 6255 (5) ( ) pHZncpHcZncCdcpH ZncCdcpHZncCdcZnQeq ××+××−××⋅−×− ×⋅−×⋅+×+×+×+−= − −− 0009.00012.000108.42.16 0102.10105.51870075.00008.0394 62 2526 (6) 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 experimental and predicted values of sorption capacity Qeq (Table 3) confirmed high values of coefficient of determination (R2), 0.997 (for Cd2+) and 0.987 (for Zn2+). Equations 5 and 6 represent the quantitative effect of the variables (initial concentration of Cd and Zn, solution pH) and their interactive effects on the Qeq(Cd) and Qeq(Zn) in binary system Cd-Zn. A positive sign in the equation implies a synergistic effect of the variables, while a negative sign indicates an antagonistic effect. 1000,0 c0 Zn (µmol/L) 3000,0 6,0 Q eq C d (µ m ol /g ) 160 190 220 250 280 310 c0 Cd (µmol/L) 3000,0 1000,0 pH 3,0 Fig. 5. Main effects plot for Cd2+ biosorption by dried activated sludge from binary system Cd-Zn. Fig. 7A, B, C illustrate the three-dimensional response surface plots of the quadratic model (Eq. 5) for Cd2+ sorption from binary system Cd-Zn. The combined effect of initial Cd2+ concentration and pH on Cd2+ sorption by dried activated sludge at various concentrations of zinc as co-ion is shown. It is evident that Cd2+ uptake increased with increasing of initial solution pH as well as with initial Cd2+ concentration. On the contrary, increasing Zn2+ concentration from 1000 to 3000 μmol/L diminished Cd2+ sorption as a result of competition between metal ions (Fig. 5). Increase in solution pH and initial Zn2+ sorption caused increase in Zn2+ sorption as can be seen from main effects plot (Fig. 6). Similarly, increase in Cd2+ concentration decreased Zn2+ sorption from binary system Cd-Zn. Nova Biotechnologica 10-2 (2010) 127 1000,0 c0 Zn (µmol/L) 3000,0 3,0 6,0 Q eq Z n (µ m ol /g ) 120 150 180 210 240 270 c0 Cd (µmol/L) 3000,0 1000,0 pH Fig. 6. Main effects plot for Zn2+ biosorption by dried activated sludge from binary system Cd-Zn. 3.00 3.75 4.50 5.25 6.00 1000.00 1500.00 2000.00 2500.00 3000.00 0 87.5 175 262.5 350 Q eq C d µm ol /g C: pH C0 Cd µmol/L 3.00 3.75 4.50 5.25 6.00 1000.00 1500.00 2000.00 2500.00 3000.00 0 87.5 175 262.5 350 Q eq C d µm ol /g C: pH C0 Cd µmol/L B A 128 Remenárová, L. et al. 3.00 3.75 4.50 5.25 6.00 1000.00 1500.00 2000.00 2500.00 3000.00 0 87.5 175 262.5 350 Q eq C d µm ol /g C: pH C0 Cd µmol/L Fig. 7. Response surface plots showing the effect of pH and initial Cd2+ concentration on Cd2+ biosorption by dried activated sludge (2.5 g/L d.w.) at 20°C and different concentrations of Zn2+ as competing ion (A) 1000 µmol/L, (B) 2000 µmol/L, (C) 3000 µmol/L. Maximum sorption capacities of the activated sludge were 321 μmol Cd2+/g and 312 μmol Zn2+/g. It was found that dried activated sludge in binary system Cd-Zn has higher affinity for Cd2+ ions when Cd2+ and Zn2+ are present in equimolar ratio 1:1 (Table 3) comparing with higher affinity to Zn2+ in single systems. This is consistent with the hypothesis that variance in affinity in multi-component systems could be attributed to different ionic characteristics of metal ions (PAKSHIRAJAN and SWAMINATHAN, 2009). 4. Conclusions Biosorption of Cd2+ and Zn2+ from aqueous solution by dried activated sludge is a rapid and pH dependent process. Maximum uptake of metals was found to occur at pH 6. The experimental equilibrium data of the single-component systems for Cd2+ and Zn2+ ions have been well described by the Langmuir isotherm and the maximum sorption capacity Qmax were 540 ± 16 μmol/g for Zn 2+ and 510 ± 17 μmol/g for Cd2+ ions. The use of RSM revealed the existence of interaction and competitive effects between variables in binary system Cd-Zn. Cd2+ uptake increased with increasing of initial solution pH as well as with initial Cd2+ concentration. On the contrary, increasing Zn2+ concentration from 1000 to 3000 μmol/L diminished Cd2+ sorption as a result of competition between metal ions. Activated sludge in binary system Cd-Zn has higher affinity for Cd2+ comparing with Zn2+ ions when Cd2+ and Zn2+ are present in equimolar ratio 1:1. RSM appears to be a useful tool for obtaining interaction and competitive effects in binary systems since it requires less reagents and experimentation time. References CHEN, J.P., LIE, D., WANG, L., WU, S., ZHANG, B.: Dried waste activated sludge as biosorbents for metal removal: adsorptive characterization and prevention of organic leaching. J. Chem. Technol. Biotechnol., 77, 2002, 657-662. 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