Chen060319.qxd The Journal of Engineering Research Vol. 5, No.1 (2008) 20-29 1. Introduction In 1978, the United States environmental protection agency (USEPA) prepared a list of 129 organic and inor- ganic pollutants found in wastewater that constitute seri- ous health hazards. This list, known as the Priority Pollutants List, includes the following thirteen metals: antimony, arsenic, beryllium, cadmium, chromium, cop- per, lead, mercury, nickel, selenium, silver, thallium, and zinc. Unlike organic compounds, metals are non- _______________________________________ *Corresponding author’s e-mail: ashour@squ.edu.om biodegradable and, therefore, must be removed from wastewater. Zinc is present in the air, soil, water, and in almost all food. Zinc is naturally released into the environ- ment, although industrial activities are mostly responsible for zinc pollution. Elevated levels of zinc may come from a variety of sources like mining and foundry activities, zinc, lead, and cadmium refining, steel production, carbon combustion, and solid waste incineration. Zinc is com- monly used to coat iron and other metals for the preven- tion of oxidation. Various zinc salts are industrially used in wood preservatives, catalysts, photographic paper, and accelerators for rubber vulcanization, ceramics, textiles, Biosorption of Zinc on Immobilized Green Algae: Equilibrium and Dynamics Studies D. Sheikha1, I. Ashour*2 and F.A. Abu Al-Rub3 1Department of Chemical and Petroleum, UAE University, P.O. Box 17 555 Al-Ain, UAE 2Department of Petroleum and Chemical Engineering, Sultan Qaboos University, P.O. Box 33, Al-Khoud, Muscat 123, Sultanate of Oman 3Chemical Engineering Department, JUST, P.O. Box 30, Irbid, Jordan Received 29 March 2006; accepted 24 January 2007 Abstract: The efficacy of using blank alginate beads and immobilized dead algal cells for the removal of zinc ions from aqueous solutions was investigated. It was found that the sorption capacities were significantly affected by solution pH; with higher pH favoring higher zinc ion uptake. Dynamics and isotherm experiments were carried out at the optimal pH 5.0. Zinc uptake on either sorbent was found to be rapid where approximately 90% of the maximum zinc uptake occurred within the first 30 min in both cases of blank alginate and immobilized algal cells. The equilibrium data for the biosorption of zinc ions onto both sorbents were fitted to the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm equations. The pres- ence of copper and nickel in aqueous solutions was found to suppress the sorption process. The results of the dynamics stud- ies revealed that the biosorption of zinc on immobilized dead algal cells followed pseudo-second order kinetics with little intraparticle diffusion mechanism contribution. Keywords: Biosorption, Heavy metals, Immobilized green algae, Pseudo second order model, Alginate, Equilibrium isotherms á«cô◊Gh ¿GõJ’G á°SGQO :AGô° ÿG ÖdÉë£dG ≈∏Y ∂fõdG ¢UÉ°üeOEG áî«°T .O1Qƒ°TÉY ,2*ÜôdG ƒHG , áá°°UUÓÓÿÿGG.√É«ŸG ‘ áÑFGòdG Ú°UQÉÿG äÉfƒjCG øe ¢ü∏îàdG ‘ AGô° ÿG ÖdÉë£dG ÉjÓN É¡«∏Y âÑãe äÉÑ«ÑM h IOôØæe äÉÑ«ÑM ΩG~îà°SEG á«dÉ©a øe ≥≤ëà∏d á°SGQ~dG √òg ±~¡J : ~bh .»æ«LhQ~«¡dG ºbôdG IOÉjõH OGOõJ É¡à«dÉ©a h √õટG IOÉŸG á©°S ¿G å«M ,»æ«LhQ~«¡dG ºbôdÉH GÒãc ôKÉàJ ¢UÉ°üeO’G IQ~b ¿G âæ«H ¢UÉ°üeO’G ÜQÉéàd ájÈıG èFÉàædG á°ùaÉæe äÉfƒjG OƒLh ¿CG ÜQÉéàdG ∂∏J âæ«H ɪc .∂«d~fôa h Òª‚’ »LPƒ‰ πãe á°UÉÿG á«°VÉjôdG êPɪædG ΩG~îà°SÉH 5 »æ«LhQ~«¡dG ºbôdG ~æY ¿GõJ’G äÉ°SGQO èFÉàf π«∏– ” OGƒŸG ≈∏Y 5 »æ«LhQ~«¡dG ºbôdG ~æY ¢UÉ°üeO’G á«∏ªY ¿G á«cô◊G äÉ°SGQ~dG èFÉàf âæ«H ɪc .É¡àFÉØc øe π∏≤jh ¢UÉ°üeO’G á«∏ªY §«ÑãJ ¤G …ODƒj πµ«ædG h ¢SÉëædG πãe ¬«Ñ°ûH ¬Ø°U h øµÁ Ú°UQÉÿG ¢UÉ°üeOG ¿G èFÉàædG ∂∏J âæ«H ɪc ,¤h’G ¬YÉ°S ∞°üædG ∫ÓN â∏°üM ∫ƒ∏ÙG øe IõટG Ú°UQÉÿG ᫪c øe %90 ¿G å«M á©jô°S â°SQO »àdG .á«fÉãdG ¬LQ~dG øe »cô◊G §ªædG áá««MMÉÉààØØŸŸGG ääGGOOôôØØŸŸGG.áàHÉK IQGôM äÉLQO ~æY ¿GõJ’G ,á«fÉãdG áLQ~dG øe á«cô◊G á°SGQ~dG §‰ ,äÉæ«÷’G ,áàÑãŸG AGô° ÿG ÖdÉë£dG ,á∏«≤ãdG ¿OÉ©ŸG ,…ƒ«◊G ¢UÉ°üeO’G : 21 The Journal of Engineering Research Vol. 5, No.1 (2008) 20-29 fertilizers, pigments, and batteries. Water reservoirs are contaminated by the run-off from these industries. Other sources of metallic zinc traces in drinking water are water treatment processes and pick-up of metallic ions during storage/distribution. These toxic metals cause accumula- tive poisoning, cancer, brain damage, etc., when they are found above the tolerance levels. According to few sur- veys from the public health services of different countries, significant number of people has been exposed to the haz- ards of excess metals in the municipal water supplies (Agrawal and Pandey, 2004). Traditional treatment methods for removing zinc from wastewater include chemical solvent extraction (Kongolo, et al. 2003; Preston and du Preez, 2000), chemical precip- itation (Veeken, et al. 2003), membrane filtration (Srisuwan and Thongchai, 2002), ion exchange (Kurama and Catalsarik, 2000), and adsorption (Ramos, et al. 2002; Mohan and Singh, 2002; Peric, et al. 2000; Galiatsatou, et al. 2002). For dilute metal concentration, ion exchange, reverse osmosis and adsorption can be applied. However ion exchange and reverse osmosis have high operating cost, which makes adsorption a better alternative for heavy metals removal (Abu Al-Rub, et al. 2004 ). One of the promising techniques for the removal of metals is the use of living or nonliving organisms and their derivatives. In general, biosorption can be defined as the passive sequestering of metal ions by metabolically inac- tive biomass (Volesky, 2003). Biosorbents, which are sor- bents of biological origin, have proved to be good sor- bents for many different pollutants. Research in the area of biosorption suggests it to be an ideal alternative for decontamination of metal containing effluents (Gupta, et al. 2000). Indeed, a wide variety of microorganisms (both living and nonviable) have been found to be capable of sequestering trace levels of metal ions from dilute aque- ous solutions. The nonviable forms have been proposed as potential sorbents, since these are essentially dead materi- als, which require no nutrition to maintain the biomass. Problems associated with metal toxicity in living biomass and the need to provide suitable growth condition also do not arise. Early studies have shown that nonliving bio- mass may be even more effective than living cells in sequestering metallic elements (Sheng, et al. 2004). Three major sources of biomass can be readily identi- fied: (1) Waste biomass (Aksu and Yener, 2001; Aksu and Gonen 2003; Aksu and Akpinar, 2001), (2) Microorganisms (Rao and Viraraghavan, 2002; Denzili, et al. 2004; Feng and Aldrich, 2004; Arica, et al. 2004; Abu Al-Rub, et al. 2004; Pagnanelli, et al. 2001; Ibanez and Umetsu, 2004). (3) Agricultural wastes bark (Mckay, et al. 1999), sugar industry mud (Magdy and Daifullah, 1998), peat (Ho, et al. 2002), tree fern (Ho et al. 2005), olive pomace ( Pagnanelli, et al. 2003), palm tree leaves (Abu Al-Rub, 2006). Cell immobilization is an attractive technique to fix and retain biomass on suitable natural or synthetic materials support for a range of physical and biochemical unit oper- ations (Abu Al. Rub, et al. 2004). The main advantages of this technique include: improvement of biomass perform- ance, biosorption capacity and facilitate separation of bio- mass from pollutant bearing solution. The objective of this study is to investigate the effica- cy of using blank alginate beads and immobilized algal biomass for the removal of zinc ions from aqueous solu- tions. The effects of different parameters, such as solution pH, shaking time, and zinc ions concentrations on the sorption capacity were investigated. Comparison between sorption on blank alginate and immobilized dead algal biomass will also be investigated. Equilibrium modeling will be carried out using the Langmuir, Freundlich, and Dubinin-Radushkevich isotherm equations. 2. Experimental 2.1 Chemicals The stock solution of zinc used in this study was pre- pared using analytical reagent grade of ZnSO4.7H2O (BDH, UK) in deionized water. 8.8 g of ZnSO4.7H2O was weighed and added into 1000 ml volumetric cylinder and completed with deinionized water, then stored. 2.2 Preparation of Biosorbent Immobilized algal cells were prepared by entrapping powdered Chlorella Pyenoidosa (Watershed, USA) in an alginate matrix produced by ionic polymerization in calci- um chloride solution, according to the following proce- dures (Abu Al Rub, et al. 2004): the powdered algal cells were suspended in a 2% sodium alginate (BDH, UK) solu- tion kept at a temperature of 60oC. The mixture was then dropped into a 2% calcium chloride (BDH, UK) solution using a peristaltic pump. The drops of Na-alginate solu- tion gelled into 3.5±0.1 mm diameter beads upon contact with calcium chloride solution. The beads were washed well and then rinsed in deionized water and stored at 4oC. For blank alginate beads, similar procedures were fol- lowed, but without algae. 2.3 Determination of Functional Groups The functional acidic groups on the prepared algal cells were determined using Boehm's titration method (Abdulkarim and Abu Al-Rub, 2004; Strelko, et al. 2002): 1 g of the powdered algal cells was dispersed in 50 ml deionized water. The suspension was mixed with 0.1N solutions of sodium bicarbonate, sodium carbonate, and sodium hydroxide, and then shaken for 48 h at room tem- perature. After this time, the sample was left for 6 h so that particles can settle. The sample was then filtered and 10 ml of filtrate were titrated with 0.1 N volumetric HCl standards using a methyl red as the indicator. According to Boehm's titration method, sodium bicarbonate can neu- tralize carboxyl groups, sodium carbonate can neutralize carboxyl, lactones and lactols groups, and sodium hydrox- ide can neutralize carboxyl, lactones, lactols and phenols 22 The Journal of Engineering Research Vol. 5, No.1 (2008) 20-29 groups. Table 1 lists the different functional groups avail- able on algal cells. 3. Procedure 3.1 Equilibrium Adsorption Isotherm All experiments were conducted by adding a specific amount of beads into 100 ml reagent bottles containing 50 ml of the zinc solution. Different initial concentrations were used: for zinc (20-350 ppm). The mixtures in these bottles were agitated for 1 hour in a shaker at 25oC. The zinc solution, then, was separated from sorbent and the concentration of zinc ions was determined using a Varian atomic absorption spectrophotometer (Spectra AA, 880). The uptake, which represents the amount sorbate sorbed per units mass of sorbent is calculated using the following equation: (1) where Qe is the uptake (mg/g) at equilibrium, Co the initial concentration sorbate (mg/ml), Ce the concentration at equilibrium (mg/ml), V the initial volume of solutions, and W is the mass of sorbent (g). 3.2 Adsorption Dynamics The dynamics studies were carried out by conducting batch biosorption experiments with 100 ppm of zinc at pH 5.0. Samples were taken at different time periods and ana- lyzed for their zinc concentrations. 3.3 Competitive Biosorption The competitive biosorption of zinc with nickel and copper were investigated. The studies involved experi- ments with different metals concentrations. All the exper- iments were conducted at 25oC using the same procedures used in the single adsorption experiments, and were car- ried out in triplicate. 4. Results and Discussion 4.1 Effect of pH on Zinc Biosorption Heavy metals biosorption is highly pH dependent (Abu Al Rub, et al. 2004). The effect of pH on zinc sorption capacity of the immobilized dead algal cells was studied at 100 ppm zinc initial concentration and 0.15 g of algal cell. Figures 1 and 2 demonstrate the variation of the uptake of zinc with equilibrium pH. The uptake of zinc increases from 1.83 to 7.26 mg/g over a narrow pH range (3.0-5.0). No significant increase in zinc uptake at pH values above 5.0 was observed. Similar trend was obtained in previous studies (Martins, et al. 2004; Mohapatra and Gupta, 2005; Ramos, et al. 2002). Solution pH affects both cell surface metal binding sites and metal chemistry in water (Abu Al-Rub, et al. 2006). At low pH values, ion exchange reaction involving metals are in competition with the high concentrations of H+ in the solution. With increasing pH, more ligands, such as amino and carboxyl groups, on sorbent are exposed and thus negative charges result and attraction between these negative charges and the metals increases the biosorption capacity on the cell surface. Another reason for increasing zinc removal is that the isoelectric point for algal biomass is at pH 3.0 (Abu Al-Rub et al. 2006). At pH values above zero-point charge, the algal cells would have a negative net charge Functional Group Meq H+/g algae Carboxyl 0.02 Carboxyls, Lactones and Lactols 0.01 Carboxyls, Lactones, Lactols and Phenols 0.035 Table 1. The functional groups on the Algal cells W V)CC( Q eoe − = pH 1 2 3 4 5 6 7 e 0 2 4 6 8 Im m obilized dead algal cell Figure 1. Effect of pH on zinc uptake (initial zinc ions concentration = 100 ppm mass of algal cell = 1.15 g) 0 2 4 6 8 1 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 1 2 3 4 5 6 Q e (m g /g ) C e (m g/l) p H Figure 2. Effect of pH on zinc removal (mass of algal cells = 0.30 g) Q e (m g/ g) pH Q e (m g/ g) 23 The Journal of Engineering Research Vol. 5, No.1 (2008) 20-29 and the ionic state of ligands on algal cell surface is such as to endorse reaction with zinc ions. Thus, electrostatic attraction between zinc and the negatively charged algal cells surface occurs which enhances the biosorption above pH 3.0 (Abu Al-Rub, et al. 2006). 4.2 Dynamics of Zinc Biosorption The variation in uptake of zinc with shaking time was studied using solutions of zinc with initial concentration of 100 ppm at pH 5.0 and 0.3 g of algal cell. The shaking time was varied from 3 to 180 min. Figure 3 reveals the variation of the uptake of zinc versus time for immobi- lized inactive algae and blank alginate. The maximum zinc uptake with either sorbent was reached after 60 min. Figure 3 also indicates that the sorption of zinc on these sorbents involves two stages: in the first stage, sorption is rapid where approximately 90% of the maximum zinc uptake occurs within the first 30 min in both cases of blank alginate and immobilized algal cells. This rapid sorption indicates that passive surface sorption occurs on the algal cells or beads surface. The second stage is slow and may involve other adsorption mechanisms such as intraparticle diffusion. The advantage of such rapid sorp- tion in practical applications is that smaller reactor vol- umes can be used (Abu al Rub, et al. 2004). The pseudo-second order kinetic, suggested by (Ho and McKay, 1999 and Ho, 2004), is based on the sorption capacity of the sorbent and is given by: (2) An integrated pseudo-second order rate law can be obtained from Eq. (2) for the boundary conditions t = 0 to t = t and Qt = 0 to Qt = Qt, and is given by: (3) Eq. (3) can be rearranged to obtain a linear form: (4) Where Qe is the amount of sorbate sorbed at equilib- rium (mg g-1);); Qt is the amount of sorbate sorbed at time t (mg g-1); and k is the equilibrium rate constant of pseu- do-second order sorption (g mg-1 min-1). The linear form see Eq. (4) is obtained by plotting t/Qt versus t, with the slope of 1/Qe and intercept of 1/kQe2, as shown in Fig. 4. The values of sorption rate constant and the equilibrium uptake with the correlation coefficient are listed in Table 2. Figure 4 and the results listed in Table 2 indicate that the kinetics for the sorption of zinc on blank alginate beads and immobilized inactive algae follow pseudo-sec- ond order kinetics. The Weber and Morris equation, 1963, given by Eq. (5), can be used to test for the contribution of intraparticle diffusion. tim e (m in) 0 20 40 60 80 100 120 140 160 180 200 t 0 2 4 6 8 Im m obilized dead algal cell B lank alginate beads Figure 3. Effect of shaking time (initial zinc ions concentration = 100 ppm, pH = 5.0, mass of algal cell = 0.3 g) 2 te t )QQ(k dt dQ −= kt Q 1 )QQ( 1 ete += − kQ 1 Q t Q t 2 eet += Pseudo-second order Sorbents k1,ads (l/min) k2,ads (g/mg.min) Qe (mg/g) R 2 Immobilized dead algal cells 9*10-5 0.012 11.02 0.9985 Blank alginate beads 0.0031 0.011 10.24 0.9981 Table 2. Kinetic parameters for the biosorption of zinc ions on immobilized dead algal cells and blank alginate beads time (min) 0 5 10 15 20 25 30 35 t/Q t (m in g m g 1 ) 1 2 3 4 5 6 Immobilized dead algal cell Blank alginate beads Figure 4. Kinetics of biosorption of zinc: pseudo- second order kinetics (mass of algal cell = 0.3 g, initial zinc concentration = 100 ppm, T = 25oC, pH = 5.0) time (min) t/Q t( m in .g .m g- 1) time (min) Q t( m g/ g) 24 The Journal of Engineering Research Vol. 5, No.1 (2008) 20-29 (5) where kd is the rate constant of intraparticle diffusion. Intraparticle diffusion mechanism would be involved if plotting Qt vs. t0.5 resulted in a straight line. Figure 5 shows the variation of Qt versus t0.5. For immobilized inactive algae and blank alginate beads the relationship is a straight line but doesn't pass through the origin which indicates that the intraparticle diffusion is not the only mechanism involved in the biosorption of zinc on immo- bilized dead algal cell. 4.3 Biosorption Isotherm Zinc ions biosorption capacities of blank alginate beads and immobilized dead algal cells are presented as a func- tion of the equilibrium concentration of zinc ions and the results are shown in Fig. 6. Figure 6 displays that the amount of Zn+2 ions sorbed per unit mass of the biosorbent increased with increasing equilibrium concentration of zinc ions in the biosorption medium. Higher equilibrium concentration enhances the mass transfer driving force, thus increasing the uptake. In addition, increasing equilib- rium metal ion concentrations increases the number of collisions between metal ions and sorbent, which enhances the sorption process (Abu Al Rub, et al. 2004). From this Figure, the maximum biosorption capacity on the blank alginate beads was 8.58 mg/g and on immobi- lized dead algal cells was 9.38 mg/g. Table 3 compares the maximum sorption capacities obtained in this study with some other values reported in the literature. The sorption capacity of zinc using the immobilized dead algal cells Chlorella vulgaris is greater than that has been found using similar biosorbents. It is known that the constituents of the cells wall of algae provide an array of ligands with a mosaic of func- tional groups capable of binding various metallic ions. Indeed, it has been shown that many metal-binding mech- anisms are involved in the biosorption process; these include ion exchange, complexation, coordination, and microprecipitation (Sheng, et al. 2004). In addition to these, immobilization enhances the contribution of physi- cal sorption. The sorption data were analyzed according to the lin- ear form of the Langmuir isotherm (6) 5.0d tkQ = t0 .5 ( m in ) 0 .5 1 2 3 4 5 6 t 0 1 2 3 4 5 6 7 Im m o b iliz e d d e a d a lg a l c e ll B la n k a lg in a te b e a d s Figure 5. Variation of Qt versus time (Weber-Morris equation 5.6) Ce (mg/l) 0 50 100 150 200 250 300 350 0 2 4 6 8 10 Immobilized dead algal cell Blank alginate beads Figure 6. Experimental isotherms of zinc ions sorbed on different sorbents (pH = 5.0 mass of algal cell = 0.3 g) Adsorbent Adsorption Capacity (mg/g) of Zn+2 Reference Waste tea leaves 11.77 Tee and Khan, 1988 Moss (mixture) 9.87 Al-Asheh et al. 1997 Hazelnut shells 1.78 Cimino and Toscano, 2000 Peat 9.3 McKay et al. 1998 Fungi 9.81 Puranik et al. 1999 Aquatic moss Fontinalis antipyretica 15 Martins et al. 2004 Immobilized dead algal cells Chlorella vulgaris 9.38 This study Table 3. Adsorption capacity for Zn+2 using different low cost adsorbents e e max bC1 bC QQ + = t0.5 (min)0.5 Q t( m g. g- 1 ) Ce (mg/l) Q e (m g/ g) 25 The Journal of Engineering Research Vol. 5, No.1 (2008) 20-29 where Qmax is the maximum sorbate uptake under the given conditions, b is a coefficient related to the affinity between the sorbent and sorbate. The plots of specific sorption (Ce/Qe) against the equilibrium concentration (Ce) for zinc are shown in Fig. 7, and the isotherm con- stants Qmax, b, along with the correlation coefficient, R2, are presented in Table 4. The R2 values suggest that the Langmuir isotherm provides a good model of the sorption system. The sorption equilibrium constant of the Langmuir model b provides a measure for the adsorption efficiency since it indicates the sorbent affinity at low con- centrations, and hence it measures the initial gradient of the adsorption isotherm. Higher values of b indicate high- er affinity and thus higher sorption efficiency. The Sum of the Squares of the Errors (ERRSQ) was used as a non-linear error function to determine the Langmuir parameters as used by (Ho, et al. 2002): (7) where p is the number of experimental data. The results of using the non-linear error analysis are shown in Table 5, where they show that the Langmuir isotherm constants b, and Qmax are very consistent among all methods and they were very close to those obtained using linear analysis. The Freundlich equation is one of the earliest empiri- cal equations used to describe sorption equilibrium data. It does not establish a finite uptake capacity of the sorbent and can thus be reasonably applied in the low to interme- diate concentrations ranges. The Freundlich isotherm model is given by the equation (Ho, et al. 2002): (8) where K and n are the Freundlich constants which repre- sent sorption capacity and sorption intensity, respectively. The linear Freundlich isotherm plots for the sorption of the zinc onto both blank alginate beads and immobilized dead algal cells are presented in Fig. 8. Table 4 shows the linear Freundlich sorption isotherm constants and the coefficients of determination; R2. Examination of the plot and the R2 values suggests that the Freundlich isotherm could describe the sorption of zinc on both sorbents. This is supported by the fact that algal cells have active sites with different energies, a key assumption in Freundlich isotherm model. The values of the Freundlich model parameters can be used to predict the affinity between the C e (m g .l -1 ) 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 C e /Q e ( g .l - 1 ) 0 5 1 0 1 5 2 0 2 5 3 0 3 5 Im m o b iliz e d d e a d a lg a l c e ll B la n k a lg in a te b e a d s C e/ Q e (g .l- 1 ) Ce (mg.l -1) Figure 7. Langmuir isotherms of zinc ions sorbed on different sorbents of algal cell = 0.3 g, pH = 5.0) n/1ee KCQ = Model Parameter Blank alginate beads Immobilized algal cells K (l/mg)1/n (mg/g) 2.85 3.26 n 5.01 5.15 Freundlich R2 0.95 0.96 Qmax (mg/g) 9.25 9.67 b (l/mg) 0.04 0.06 Langmuir R2 0.99 0.99 QD (mmol/g) 8.83 9.44 BD (l/J2.mol2) 2.01*10- 9 2*10-9 E (kJ/mole) 15.772 15.81 D-R R2 0.97 0.98 Table 4. Adsorption linear isotherms parameters for the sorption of zinc ions by blank alginate beads and immobilized algal cells Model Blank Alginate Beads Immobilized Algal Cells Freundlich: K (l/mg)1/n (mg/g) 2.94 3.58 Freundlich n 5.27 5.86 Marquadt's PSD 0.031 0.12 Langmuir Qmax (mg/g) 8.37 8.83 Langmuir b (l/mg) 0.087 0.18 ERRSQ 0.96 0.95 Table 5. Adsorption non-linear isotherms parameters for the sorption of zinc ions by blank lagi- nate beads and immobilized algal cells ERRSQ = ( )∑ = − 2 1 p e,exp e,cal i i Q Q 26 The Journal of Engineering Research Vol. 5, No.1 (2008) 20-29 sorbate and sorbent. High values of these parameters indi- cate high adsorptive capacity. From Table 4 the magni- tude of K and n for both sorbents suggests easy uptake with high sorptive capacity. The Freundlich isotherm constants were also deter- mined by the non-linear regression of Marquardt's Percent Standard Deviation (MPSD) (Marquardt, 1963): (9) and the results are tabulated in Table 5. These results demonstrate that the values of the K and n obtained by non linear regression are remarkably consistent and quite sim- ilar to the linear transform values shown in Table 4. Another less commonly used model to describe sorp- tion of zinc on immobilized dead algal cells is the Dubinin-Radushkevich (D-R) isotherm. This isotherm is generally expressed as follows (Dubinin, 1960): (10) Radushkevich (1949) and Dubinin (1965) have report- ed that the characteristic sorption curve is related to the porous structure of the sorbent. The constant, BD, is relat- ed to the mean free energy of sorption per mole of the sor- bate as it is transferred to the surface of the solid from infi- nite distance in the solution and this energy can be com- puted using the following relationship (Ho, et al. 2002): (11) Figure 9 is a plot of the linear form of Eq. (10), and the parameters found from the slope and intercept are list- ed in Table 4. The values of E calculated are 15.77 kJ/mole and 15.81 kJ/mole for blank alginate beads and immobilized dead algal, respectively. These values are within the range of ion-exchange mechanisms (8-16 kJ/mole) (Abu Al-Rub, 2004), confirming that the ion exchange mechanism plays a significant rule in the biosorption mechanism. This finding of the ion exchange mechanism contribution agrees with the suggested mech- anism of biosorption proposed by (Volesky, 2003). The QD values are consistent with the linear Qmax values pre- viously determined for the Langmuir isotherm. 4.4 Sorption/Desorption of Zinc Sorption and desorption of zinc on immobilized dead algal cells and blank alginate beads were investigated by conducting three repeated cycles sorption/desorption experiments. Desorption experiments were carried out by shaking the sorbents with 20 ml 0.1 M HCl for two hours. The sorbents were then rinsed with deionized water to remove any residual acidity. The regenerated sorbents were then used in the removal of zinc ions and the results are demonstrated in Fig. 10. Figure 10 indicates that zinc uptake was improved after the first cycle. This improve- ment may be attributed to the fact that the acid used in desorption could remove some contaminants that might have been bound previously by the algal cells. ln (C e) 0 1 2 3 4 5 6 e 0.0 0.5 1.0 1.5 2.0 2.5 Im m obilized dead algal cell B lank alginate beads ln (Ce) ln ( Q e) Figure 8. Freundlich isotherms of zinc ions sorbed on different sorbents (mass of algal cell = 0.3 g, pH = 5.0) ⎟⎟ ⎟ ⎠ ⎞ ⎜⎜ ⎜ ⎝ ⎛ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ +−= 2 e DDe )C 1 1ln(RTBexpQQ DB2 1 E = (ln (1+1/Ce)) 2 20 40 60 80 100 120 (Q e ) 0.0 0.5 1.0 1.5 2.0 2.5 Immobilized dead algal cell ln (1+1/Ce), blank vs ln (qe), blank (ln (1+1/Ce)) 2 ln ( Q e) Figure 9. Dubinin-Radushkevich equation isotherm of zinc ion sorbed on different sorbents (mass of algal cell = 0.3 g, pH = 5.0) MPSD = ∑ = ⎛ ⎞− ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜⎝ ⎠ 2 1 P e,exp e,cal i e,exp i Q Q Q 27 The Journal of Engineering Research Vol. 5, No.1 (2008) 20-29 4.5 Effect of Impurities on Biosorption of Zinc The biosorption of Zn+2 + Cu+2 and Zn+2 + Ni+2 at equi- librium pH 5.0 on immobilized dead algal cells are shown in Fig. 11. These figures show the equilibrium uptake of zinc decreased regularly with increasing equilibrium met- als concentrations. The decrease in the uptake of zinc in the presence of Ni+2 ions and Cu+2 ions is simple to explain. The presence of the other component develops a competition for the adsorption sites on the surface and some sites are occupied by the second component. As a consequence, the first component has a smaller "parking space" and its uptake is decreased (Aksu and Akpinar, 2001). These results were in agreement with other studies using different adsorbent and/or different heavy metals (Abu Al Rub, 2004; Juang, et al. 1999). The electronegativity values of Zn+2, Cu+2, and Ni+2 metal ions are 1.65, 1.75, and 1.8 respectively. The more electronegative metal ions will be more strongly attracted to the immobilized dead algal cells surface (Arica, et al. 2004). Cu+2 has the highest affinity for immobilized dead algal sites and it has a greater negativity than both Zn+2 and Ni+2. This can explain the significant suppression of Zn+2 uptake in the presence of Cu+2. 5. Conclusions This study proved the practical feasibility of using blank alginate beads and immobilized dead algal cells for the removal of zinc from wastewaters. The solution pH plays a very important role for zinc adsorption onto both sorbents. At pH < 2, zinc did not adsorb, the adsorption increased drastically as the pH was increased from 2.0 to 6.0, and the maximum adsorption uptake was achieved at pH 5.0. In addition, the uptake of zinc increased as the equilibrium zinc ion concentration increased. Sorption of zinc on these sorbents was found to follow pseudo-second order kinetics. Moreover, alginate beads and immobilized Chlorella vulgaris cells could be used in successive sorp- tion/desorption cycles to remove zinc ions from aqueous solutions, which suggest that immobilization can provide an efficient and convenient method for repetitive use of algal cells. Sorption of zinc on alginate beads and immo- bilized Chlorella vulgaris has been found to follow Langmuir, Freundlich, or D-R isotherm models. The pres- ence of Cu+2and Ni+2 in the system led to a decrease in the removal of zinc ions by immobilized dead algal cells. 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