EFFECT OF CATIONS AND ANIONS PRESENCE ON CADMIUM SORPTION KINETICS FROM AQUEOUS SOLUTIONS BY DRIED SUNFLOWER LEAVES

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1 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt EFFECT OF CATIONS AND ANIONS PRESENCE ON CADMIUM SORPTION KINETICS FROM AQUEOUS SOLUTIONS BY DRIED SUNFLOWER LEAVES H. Benaïssa and M-A. Elouchdi Laboratory of Sorbent Materials and Water Treatment Department of Chemistry, Faculty of Sciences, University of Tlemcen P.O. Box 9, 3 Tlemcen, Algeria yahoo.fr ABSTRACT The effect of ions presence such as: Na +, K +, Ca 2+, Cl -, SO 4 2- and CO 3 2-, at various initial concentrations, on the kinetics of cadmium sorption by dried sunflower leaves was studied at 25 C in batch conditions. The presence of these ions in solution was found to inhibit the uptake of cadmium by dried sunflower leaves at different degrees. Na + and K + ions have no significant effect. For Ca 2+, SO 4 2- and CO 3 2- ions, the effects ranged from an inhibition of cadmium by Ca 2+ and CO 3 2- to a weak inhibition by SO Cl - ion was found to enhance slightly cadmium uptake level. Simplified kinetics model: a pseudo second-order equation adequately described the kinetics of cadmium sorption by sunflower leaves: alone and in the presence of cations or anions. In the presence of cations, cadmium sorption kinetics by dried sunflower leaves was largely determined by external mass transfer diffusion steps. In the presence of anions, cadmium sorption kinetics was largely determined by intraparticle transfer diffusion steps. Keywords: sorption kinetics; cadmium; dried sunflower leaves; effect of cations and anions presence. INTRODUCTION Industrial wastewater effluents bearing heavy metals, pose a serious problem for the environment. Cadmium has been well recognized for its negative effect on the environment where it accumulates readily in living systems (Hutton & Symon [], Nriagu [2]). Because waste agricultural by-products are broadly available and relatively inexpensive, an investigation of their use as a sorbent material seems most appropriate. Various studies have shown that these low-cost materials have the aptitude to remove important quantities of metallic cations from simple solutions (Friedman & Waiss [3], Gaballah et al. [4], Gloaguen & Morvan [5], Marshall & Champagne [6]). However, most of the studies presented on this subject have been based on the sorption of single metal solutions in the absence of others ions. In wastewater streams, the metal of interest is usually found in a matrix containing

2 2 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt several ions (Fourest [7], Tobin et al. [8], Chong & Volesky [9], De Carvalho et al. [], Blanco et al. [], Diard [2]). These various components can interact with heavy metals and to modify thus their behaviour towards the sorbent material used. Our previous work has shown that dried sunflower leaves have the aptitude to remove important quantities of cadmium ions from simple solutions (Benaïssa & Elouchdi [3]). As a continuation, the present study describes the results of the experimental investigation and modelling of the influence of the different ions presence such as: sodium, potassium, calcium, sulphate, chloride and carbonate, at various initial concentrations, on cadmium sorption kinetics by dried sunflower leaves, in batch conditions at 25 C. The choice of these ions has been made because of their permanent presence in industrial waste waters. A simplified kinetic model: a pseudo second-order equation was selected to follow the sorption process. The kinetics controlling mechanisms of cadmium sorption by sunflower leaves: external mass transfer and intraparticle diffusion were also investigated. MATERIALS AND METHODS. Materials and metallic ion Dried sunflower leaves as an agricultural waste, collected in autumn from the region of Sidi Abdelli Tlemcen in Algeria in the form of large flakes, were used as a sorbent material after the following treatment. 5 g of dried sunflower leaves were contacted with L of distilled water in a beaker agitated vigorously by a magnetic stirrer at ambient temperature of 25 C during 4 hr, then continuously washed with distilled water to remove the surface adhered particles and water soluble materials, and ovendried overnight at 6-8 C for 24 hours after filtration. This material was crushed and sieved to have particles of size mm for further batch sorption experiments. Solutions of determined concentration in cations or anions have been prepared respectively from nitrate salts of: cadmium (Windor Laboratories Limited), sodium (Rhône-Poulenc), magnesium (Merck), and calcium (Merck); and salts of: chloride (Nentech), sulphate (Merck) and carbonate of sodium (Azochim) by dissolving the exact quantities of theses salts in distilled water. All chemicals were commercial products used without purification. 2. Sorption kinetics The initial solution metal concentration was mg/l for all experiments. Different concentrations in co-ions were tested for their effects on cadmium sorption by sunflower leaves. For metal removal kinetics studies,.6 g of dried sunflower leaves were contacted with 3 ml of metal solutions in a beaker agitated vigorously by a magnetic stirrer using a water bath maintained at a constant temperature of 25 C. In all cases, the working ph was that of the solution and was not controlled. The residual cadmium concentration in the aqueous solution at appropriate time intervals was

3 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt 3 obtained by using a Cd 2+ ion - selective electrode technique. Based on the ready availability of this electrode in our laboratory, this method was chosen. The electrode used for measurement of cadmium was Orion Model 9448 and was used in conjunction with Orion Model reference electrode and an Orion Model 7A meter, which provided readings accurate to. mv. For the measurement of ph, an Orion Model 97 combination electrode, with the afore-mentioned meter, was used. ph readings were monitored to +. unit. For certain experiments, this cadmium concentration was also done using a Perkin Elmer Model 228 atomic absorption spectrophotometer. No differences in the results obtained by these two methods of analysis were observed. The metal uptake q t (mg ion metal/g sunflower leaves) was determined as follows: q t = ( C C t ) x V/m () where C and C t are the initial and final cadmium concentration (mg/l), respectively, V is the volume of solution (ml), and m is the sunflower leaves weight (g) in dry form. Preliminary experiments had shown that cadmium adsorption losses to the container walls and to the filter paper were negligible. RESULTS AND DISCUSSION - Effect of cations To study the effect of the presence of some cations frequently met in metallic solutions on the kinetics of cadmium sorption by dried sunflower leaves, we have chosen as ions: sodium, potassium and calcium whose the concentrations.2;. and 2. g/l have been chosen from the literature (Fourest [7], Diard [2], Matheickal et al. [4]). To facilitate the comparison of results, all data in this present study have been obtained with nitrate salts. Figures and 2 as examples, present the kinetics curves of cadmium sorption by dried sunflower leaves in the presence of competitor ions. Whatever the nature and the tested ion concentration, the curves present a same shape characterized by a strong increase of the capacity of cadmium removal during the first minutes of contact solution sorbent material, follow-up of a slow increase until to reach a state of equilibrium. The presence of cations has a weak influence on the time of equilibrium reached by cadmium alone in the absence of these ions. For sodium as a typical example, the equilibrium time was 4 h for cadmium alone and in presence of sodium at different concentrations, respectively. Although that sodium concentrations used were largely superior to that of cadmium, the curves of kinetics of cadmium sorption (see Fig. ), in the absence or in presence of sodium at different initial sodium concentrations, are practically superposed indicating that sodium does not disturb significantly the sorption of cadmium by dried sunflower leaves. In the presence of calcium (see Fig. 2), the curves of kinetics of cadmium sorption were slightly staggered to that corresponding to the cadmium sorption in their absence indicating

4 q q 4 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt well a phenomenon of competition between Cd 2+ and Ca 2+ ions for the sites of binding on sunflower leaves. These observations are in agreement with those observed by many authors (Tobin et al. [8], Diard [2], Muzarelli & Tubertini [5]) using different sorbent materials Pure cadmium mg/l [Na + ] =.2 g/l [Na + ] = g/l [Na + ] = 2 g/l Time (min) Figure : Effect of Na + ion on the kinetics of cadmium sorption by dried sunflower leaves Pure cadmium mg/l [Ca 2+ ] =.2 g/l [Ca 2+ ] = g/l [Ca 2+ ] = 2 g/l Time (min) Figure 2: Effect of Ca 2+ ion on the kinetics of cadmium sorption by dried sunflower leaves. During the course of cadmium removal by dried sunflower leaves, whatever the nature and the tested ion concentration, we have noticed that the presence of cations does not affect the evolution of the initial ph of solution (see Figure 3 as a typical example). We have always an increase in the initial value of ph analogous to that observed in the case of the sorption of cadmium alone reaching the limit value of its precipitation in many cases. This can be interpreted by a possible competition between cadmium ions and H 3 O + for binding sites.

5 tq t (min.g/mg) ph Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt 5 8, 7,8 7,6 7,4 7,2 7, 6,8 6,6 6,4 6,2 Pure cadmium mg/l [Ca 2+ ] =.2 g/l [Ca 2+ ] = g/l [Ca 2+ ] = 2 g/l 6, Time (min) Figure 3: ph profile of cadmium sorption by sunflower leaves in presence of calcium ions The kinetics of cadmium sorption by dried sunflower leaves were modelled using a pseudo- second order rate equation (Ho [6]; Ho & McKay, [7]). The choice of this model has been done from a previous study in which it has been shown its suitability to describe the data of cadmium sorption kinetics by sunflower leaves (Benaïssa & Elouchdi [3]). The kinetic rate equation is: t / q t = / kq e 2 + t / q e (2) where k (g.mg -.min - ) is the rate constant of sorption, q e and q t are the amounts of metal ion sorbed (mg.g - ) at equilibrium and at time t, respectively. As shown in Figure 4, as a typical example, the pseudo second-order reaction rate model adequately described the kinetics of cadmium sorption with high correlation coefficients (.9999). The k values from the slopes and intercepts are summarized in the Table Pure cadmium mg/l [Ca 2+ ] =.2 g/l [Ca 2+ ] = g/l [Ca 2+ ] = 2 g/l Time (min) Figure 4: Linearization of cadmium sorption kinetics by dried sunflower leaves in presence of calcium cation using a pseudo-second order rate model

6 6 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt Table : Pseudo second-order rate constants for cadmium sorption kinetics by dried sunflower leaves in presence of cations Na + Initial Na + concentration (g/l) q e exp q e cal k. 3 (min -.g / mg) K + Initial K + concentration (g/l) q e exp q e cal k. 3 (min -.g / mg) Ca 2+ Initial Ca 2+ concentration (g/l) q e exp q e cal k. 3 (min -.g / mg) Effect of anions The nature of the counter-ions, destined to stabilize heavy metals in the cationic form, can also influence their sorption by sorbent materials. Some anions can have an affinity towards the metal that they form an insoluble or soluble complex, displaced with difficulty in the presence of the sorbent material (Fourest [7]). For these reasons, we have respectively studied the effect of the presence of three classic anions: chloride, sulphate and carbonate on the kinetics of cadmium sorption by sunflower leaves in the same operative conditions. Studied anions were in the form of sodium salts since sodium did not disturb the sorption of cadmium by dried sunflower leaves already confirmed previously. The chosen concentrations were:.2;. and 2. g/l respectively for chloride and sulphate ions; and.5;. and.2 g/l for carbonate ion, chosen from bibliographical data (Fourest [7], Diard [2]). Figures 5 and 6 as examples, present the kinetics curves of cadmium sorption by dried sunflower leaves in the presence of anions at different initial concentrations. Here also, whatever the

7 q q Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt 7 nature and the tested anion concentration, these curves obtained have the same shape characterized by a strong increase of the amount of cadmium sorbed by sunflower leaves during the first minutes of contact solution - sorbent, follow-up of a slow increase until to reach a state of equilibrium. The presence of anions has a weak influence on the time of equilibrium reached by cadmium alone in the absence of these anions. The influence of the presence of sulphate and carbonate ions on the sorption kinetics of cadmium by sunflower leaves was more marked than that observed in the presence of chloride ions. For carbonate anion, the kinetic curve of cadmium sorption is strongly reduced compared to that observed in presence of sulphate anions Pure cadmium mg/l [SO -2] =.2 g/l 4 [SO -2] = g/l 4 [SO -2] = 2 g/l Time (min) Figure 5: Effect of SO 4 2- ion on the kinetics of cadmium sorption by sunflower leaves Pure cadmium mg/l [CO -2] =.5 g/l 3 [CO -2] =. g/l 3 [CO -2] =.2 g/l Time (min) Figure 6: Effect of CO 3 2- ion on the kinetics of cadmium sorption by sunflower leaves

8 ph 8 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt During the experiments of cadmium sorption, we have also observed an evolution in the initial ph value of solutions, similar to that observed previously for cations, for only chloride and sulphate anions ( Figures not shown here). Concerning the presence of carbonate anion, we have observed a slight diminution in initial ph value of the solution for the initial concentration in carbonate.2 g/l provoking the training of a precipitate (see Fig. 7) Pure cadmium mg/l [CO -2] =.5 g/l 3 [CO -2] =. g/l 3 [CO -2] =.2 g/l Time (min) Figure 7: ph profile of cadmium sorption by sunflower leaves in presence of carbonate anions When these data were also fitted to the pseudo-second order rate equation, straight lines were obtained (Figures not shown here) indicating that the process follows a pseudo second-order kinetics. The rate constants calculated from their slopes and intercepts are shown in Table 2.

9 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt 9 Table 2: Pseudo second-order rate constants for cadmium sorption kinetics by sunflower leaves in presence of anions Cl - Initial Cl - concentration (g/l) q e exp q e cal k. 3 (min -.g / mg) SO Initial SO 4 concentration (g/l) q e exp q e cal k. 3 (min -.g / mg) CO 3 2- Initial Ca 2+ concentration (g/l).5..2 q e exp q e cal k. 3 (min -.g / mg) C- Rate determining steps The sorption of solute on solid particles has been extensively studied. It is generally agreed that there are four consecutive steps which describe the overall sorption process of solute from a solution by a sorbent particle (Furusawa & Smith [8]). These steps, as adapted to apply to the sorption of metal ions by a sorbent particle, are as follows: - External mass transfer of the metal ions from the solution bulk to the boundary film; 2- Metal ions transport from the boundary film to the surface of the sorbent particle; 3- Diffusion of the metal ions within the sorbent particle to the sorption sites: internal diffusion of metal ions; 4- Final uptake of metal ions at the sorption sites, via complexation, sorption, or precipitation, which is fast.

10 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt The first and the second step are external mass transfer resistance steps, depending on various parameters such as agitation and homogeneity of solution. In this study, the agitation given here to the solution (4 rpm) is considered as sufficient to avoid steps and 2 being controlling steps. In a well agitated batch system, the boundary layer surrounding the particle is much reduced, reducing the external mass transfer coefficient; hence, the third intraparticle diffusion resistance step is more likely to be the rate controlling step (Sag & Aktay, [9]). In the process of establishing the rate limiting step, the fourth step is assumed to be very rapid and is therefore not considered in any kinetic analysis (Findon et al. [2): sorption is a quasi-instantaneous process, as well as complexation mechanism, precipitation seems to occur with a lower rate (Tsezos & Volesky [2]). The sorption rate will be controlled by the rate of diffusion (Peniche-Covas et al. [22]). Consequently, the two rate limiting steps investigated are external film mass transfer and intraparticle diffusion, either singly or in combination. Models were established to determine the two coefficients initially based on single resistance mass transport analysis (McKay et al. [23]). 2.- External mass transfer resistance model: This model assumes that the surface concentration of solute, C s, on the sorbent is negligible at t =, and that intraparticle diffusion is also negligible; it is applied to calculate the initial rate of metal sorption (McKay & Poots [24]). The initial rate of sorption can be determined using the classic mass transfer equation (3) which describes the evolution of metal ion concentration C t in solution: dc t / dt = L S ( C t C s ) (3) where L is the external mass transfer coefficient, C t the liquid phase solid concentration at a time t, C s the liquid phase solute concentration at the particle surface and S the specific surface area for mass transfer. This equation can be simplified, by substituting the following boundary conditions: C t C o and C s when t ; C o = initial metal ion concentration (McKay et al. [24], McKay & Poots [24], Weber & Morris [25]) to: d(c t / C o ) / dt = L S (4) So the external mass transfer rate, L S, is approximated by the initial slope of the C t / C o vs. time graph and can be calculated either by assuming a polynomial relation between C t /C and time or based on the assumption that the relation-ship was linear for the first initial rapid phase. The first technique was used here Intraparticle diffusion resistance model: Weber and Morris [25] and McKay et al. [23] demonstrated that in intraparticle diffusion studies, rate processes are usually expressed in terms of square root of time. So q t or fraction metal sorbed is plotted against t.5 as follow:

11 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt q t = k i x t.5 (5) where: q t is the solute concentration in the solid and k i the slope of the plot defined as an intraparticle diffusion rate parameter. If particle diffusion is rate controlling, the plots q t versus t.5 are linear and the slope of the plots is defined as an intraparticle diffusion rate parameter, k i (mg metal g - sorbent time -.5 ) (McKay et al. [23]). In theory, the plot between q t and t.5 is given by four regions representing the external mass transfer followed by intraparticle diffusion in macro, meso and micropore (Ho & McKay [26]. From a mechanistic viewpoint, to interpret the experimental data, it is necessary to identify the steps involved during cadmium sorption in the presence of cations and anions, described by external mass transfer (boundary layer diffusion) and intraparticle diffusion. Table 3 summarises the values of mass transfer coefficients according to the different kinds of resistance models tested here. The different plots, q t vs. t.5 presented three linear portions (Figures not shown here): a first linear portion followed by two other linear portions before equilibrium. The double nature of the curve reflects two stages: external mass transfer followed by intraparticle diffusion of cadmium onto sunflower leaves particles. The slope of the third linear portion characterizes the rate parameter corresponding to the intraparticle diffusion, whereas the intercept, t, is proportional to the boundary layer thickness: the larger intercept the greater is the boundary layer effect (Kumar et al. [27]). In general, the linearization of q t versus t /2 gave a positive and significant ordinate intercept, indicating the influence of external rate control (Sag & Aktay [9]. These observations indicate that the cadmium sorption by dried sunflower leaves is a complex process. Thus in order to characterize what the actual rate-controlling step involved in copper sorption process is, the sorption data were further analysed by the kinetic expression given by Boyd et al. [28]. F= (6/ 2 ) exp( Bt) (6) where F is the fraction of solute sorbed at different times t and Bt is a mathematical function of F and given by: F = q t /q (7) where, q t and q represents the amount sorbed at any time t and at infinite time. Substituting Eq. (6) into Eq. (7), the kinetic expression becomes: Bt=.4967 ln( q t /q ) (8)

12 2 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt Table 3: Effect of different experimental parameters on diffusion coefficients for cadmium sorption by dried sunflower leaves Effect of cations Parameter External mass transfer model L S x 2 (min - ) Intraparticle diffusion model k i (mg g - min -.5 ) t [Na + ] (mg/l) Effect of Na [K + ] (mg/l) Effect of K [Ca 2+ ] (mg/l) Effect of Ca Boyd model B. 2 (min - ) Thus the value of Bt can be calculated for each value of F using Eq. (8). The calculated Bt values were plotted against time (Figures not shown here). The linearity of this plot will provide useful information to distinguish between external transport and intraparticle transport controlled rates of sorption (Kumar et al. [27]). For all experimental parameters studied, it was observed that the plots were linear but did not pass through the origin, indicating that external mass transport mainly governs the rate-limiting process (Boyd et al. [28]).

13 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt 3 Table 4: Effect of different experimental parameters on diffusion coefficients for cadmium sorption by dried sunflower leaves Effect of anions Parameter External mass transfer model L S x 2 (min - ) Intraparticle diffusion model k i (mg g - min -.5 ) t [Cl - ] (mg/l) Effect of Cl [SO 4 2- ] (mg/l) Effect of SO [CO 3 2- ] (mg/l) Effect of CO Boyd model B. 2 (min - ) The results of Table 3, except certain experimental points concerning the presence of Na + ion in solution where no clear tendency was obtained, in general shows that the external mass transfer rate, L S, decreases with increasing of initial cation concentration, while the intraparticle mass-transfer coefficient k i increases. These observations suggest that the cadmium sorption kinetics by dried sunflower leaves is largely determined by external mass transfer diffusion steps. The results of Table 4, except certain experimental points concerning the presence of Cl - ion in solution where no clear tendency was obtained, in general show that the external mass transfer rate, L S, increases with increasing of initial anion concentration, while the intraparticle mass-transfer coefficient k i decreases. These observations suggest that the cadmium sorption kinetics by dried sunflower leaves is largely determined by intraparticle transfer diffusion steps. CONCLUSION Results obtained from this study show that the presence of some ions can inhibit or exert between them a competitive action in the presence of a sorbent material. This means that they bind to sites of identical sorption, and the competition plays in favour

14 4 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt of the element that possesses the best affinity for these groups. It appears that cations: sodium and potassium are not sorbed in quantity relatively important at the same time that cadmium. Concerning the effect of the presence of anions on the kinetics of cadmium sorption by dried sunflower leaves, except for the slight positive influence of chloride ion, sulphate and particularly carbonate anions inhibit this sorption. The results also showed that the kinetics of cadmium sorption were well described by a pseudo-second order rate model. In the presence of cations, cadmium sorption kinetics by dried sunflower leaves was largely determined by external mass transfer diffusion steps. In the presence of anions, cadmium sorption kinetics by dried sunflower leaves was largely determined by intraparticle transfer diffusion steps. REFERENCES [] Hutton, M. and Symon, C., Quantities of cadmium, lead, mercury, and arsenic entering the environment from human activities, Science Total Environment, Vol. 57, pp. 29-5, 986. [2] Nriagu, J.O., A silent epidemic of environmental metal poisoning?, Environmental Pollution, Vol. 5, pp. 39-6, 988. [3] Friedman, M. and Waiss, A.C., Journal of Environment Science Technology, Vol. 6(5), pp , 972. [4] Gaballah, I., Goy, D., Allain, E., Kilbertus, G. and Thauront, J., Metallurgy Material Transaction, Vol. 28B, pp. 3-23, 997. [5] Gloaguen, V. and Morvan, H., Journal of Environment Science and Health, Vol. A32 (4), pp. 9-92, 997. [6] Marshall, W. E., Champagne, E. T., Journal of Environment Science and Health, Vol. A3 (2), pp , 995. [7] Fourest, E., Etude des mécanismes de biosorption des métaux lourds par des biomasses fongiques industrielles en vue d un procédé d épuration des effluents aqueux contaminés, Thèse de Doctorat, USTM. Grenoble, France, 993. [8] Tobin, J.M., Cooper, D.G. and Neufeld, R.J., Uptake of metal ions by Rhizopus arrhizus biomass, Applied Environment Microbiology, Vol. 47, pp , 984. [9] Chong, K. H. and Volesky, B., Metal biosorption equilibria in a ternary system, Biotechnology Bioengineering, Vol. 49, pp , 996. [] De Carvalho, R.P., Chong, K.H. and Volesky, B., Evaluation of the Cd, Cu and Zn biosorption in two-metal systems using an algal biosorbent, Biotechnology Progress, Vol., pp , 995. [] Blanco, A., Sanz, B., Llama, M.J. and Serra, J.L., Reutilisation of non viable biomass of phormidium laminosum for metal biosorption, Biotechnology Applied Biochemistry, Vol. 27, pp , 998. [2] Diard, P., Etude de la biosorption du plomb et du cuivre par des boues de stations d épuration. Mise en œuvre d un procédé de biosorption à contrecourant, Thèse de Doctorat, INSA. Lyon, France, 996. [3] Benaïssa, H., Elouchdi, M-A., Kinetics of cadmium removal from aqueous solutions by sunflower leaves: experimental study and modelling, 4 th European

15 Twelfth International Water Technology Conference, IWTC2 28 Alexandria, Egypt 5 Congress of Chemical Engineering "Chemical Engineering, a tool for progress", Granada (ESPAGNE), 2-25 September, 23. [4] Matheickal, J.T., Yu, Q. and Feltham, J., Cu(II) binding by Radiata biomaterial, Environmental Technology, Vol. 8, pp , 997. [5] Muzzarelli, R.A.A. and Tubertini, O., Chitin and chitosan as chromatography supports and adsorbents for collection of metal ions from organic and aqueous solutions and sea-water, Talanta, Vol. 6, pp , 969. [6] Ho, Y. S., Adsorption of heavy metals from waste streams by peat, Ph.D. Thesis, University of Birmingham, Birmingham, U.K., 995. [7] Ho, Y. S. and McKay, G., The kinetics of sorption of divalent metal ions onto sphagnum moss peat, Water Research, Vol. 34(3), pp , 2. [8] Furusawa, T. and Smith, J.M., Fluid-particle and intraparticle mass transport rates in slurries, Industrial Engineering Chemical Fundamental, Vol. 2, pp , 973. [9] Sag, Y. and Aktay, Y., Mass transfer and equilibrium studies for the sorption of chromium ions onto chitin, Process Biochemistry, Vol. 36, pp , 2. [2] Findon, A., McKay, G. and Blair, H.S., Transport Studies for the sorption of copper ions by chitosan, Journal of Environment and Science of Health, Vol. 28, pp , 993. [2] Tsezos, M. and Volesky, B., The mechanism of uranium biosorption by rhizopus arrhizus, Biotechnology and Bioengineering, Vol. 24, pp , 982. [22] Peniche-Covas, C., Alvarez L.W. and Arguelles-Mona, W., The adsorption of mercuric ions by chitosan, Journal of Applied Polymer and Science, Vol. 46, pp. 47-5, 992. [23] McKay, G., Blair, H.S. and Findon, A., Sorption of metal ions by chitosan. In: Heccles, Hunt, S. (Eds.), Immobilization of Ions by Biosorption. Ellis Horwood Ltd., Chichester, pp , 986. [24] McKay, G. and Poots, V.J.P., Kinetics and diffusion processes in colour removal from effluent using wood as an adsorbent, Journal of Chemical Technology and. Biotechnology, Vol. 3, pp , 984. [25] Weber, W.J. and Morris, G.C., Removal of biologically-resistant pollutants from waste waters by adsorption. In: Advances in Water Pollution Research. Proc. st Int. Conf. on Water Pollution Res., Pergamon Press, New York, pp , 962. [26] Ho, Y.S. and McKay, G., Canadian Journal of Chemical Engineering, 76, 822-, 998. [27] Kumar, K.V., Ramamurthi, V. and Sivanesan, S., Modeling the mechanism involved during the sorption of methylene blue onto fly ash, Journal of Colloid and Interface Science, Vol. 284, pp. 4-2, 25. [28] Boyd, G.E., Adamson, A.W. and Myers, L.S., The exchange adsorption of ions from aqueous solutions by organic zeolites. II. Kinetics, Journal of American Chemical Society, Vol. 69 (), pp , 947.