174 Acta Chim. Slov. 2005, 52, 174–181 Technical Paper Equilibrium Sorption Study of Al3+, Co2+ and Ag+ in Aqueous Solutions by Fluted Pumpkin {Telfairia Occidentalis HOOK f) Waste Biomass Michael Horsfall Jnr* and Ayebaemi I. Spiff Department of Pure and Industrial Chemistry, University of Port Harcourt, Uniport P. O. Box 402, Choba, Port Harcourt, Nigeria, E-mail: horsfalljnr@yahoo.com Received 08-12-2004 Abstract An ensemble of equilibrium sorption techniques was combined to study the influence of ionic radius on the sorption characteristics of Al3+, Co2+ and Ag+ by fluted pumpkin waste biomass. The experimental results were analyzed in terms of five two-parameter adsorption isotherm equations - the Langmuir, Freundlich, Temkin, Dubinin-Radushkevich and Flory-Huggins isotherms. According to the evaluation using Langmuir equation, the monolayer sorption capacity obtained was 16.98 mg/g, 10.34 mg/g and 8.03 mg/g for Al3+, Co2+ and Ag+ respec-tively. The data further showed that, the Freundlich and Langmuir isotherms described the data appropriable than Temkin, Dubinin-Radushkevich and Flory-Huggins isotherms. The result showed that fluted pumpkin waste could be used for the removal of Al3+, Co2+ and Ag+ from wastewater and ionic radius influences the rate of metal ion migration to the biomass surface and the adsorption intensity of the metal. Key words: equilibrium sorption, Flory-Huggins isotherm, fluted pumpkin, wastewater treatment, process biotechnology Introduction Fluted Pumpkin (Telfairia occidentalis) is a creep-ing vegetative shrub that spread low across the ground with large lobed leaves, and long rvvisting tendrils. The genus Telfairia (Cucurbitaceae) comprises two spe-cies, T. pedata and T. occidentalis. T pedata is grown in East Africa for its oil and protein-rich seeds while T. occidentalis is grown in some parts of West Africa for its nutritious leaves and seeds.1 After harvesting, the leaves are carefullv removed from the stem, which are discarded as waste. A single stem with leaves weighing 1.2 kg produces less than 200 g of leaves leaving over 1 kg stem as wastes. Fluted pumpkin is the largest consumed vegetable in the West Afričan sub-region and therefore creates one of the major agrowaste problems in Nigeria. Preliminary investigations showed that several tons of these wastes are produced daily in market places around the country but are scarcelv useful and therefore create environmental nuisance. For this reason, they would be tested as adsorbents for toxic and valuable metals from industrial wastewater. Due to the bioaccumulating tendencv and toxiciry of heavy metals in the environment, it has been consist-ently desired that their levels be considerabh/ reduced in industrial and municipal effluents to meet regulatory standards before final repository in the ecosvstem. Tech-niques presently in existence for removal of heavy met- als from wastewater are relatively expensive and non-environment friendly. It is therefore necessary to search for agricultural by-product that is relatively abundant in our environment and transform such material to an adsorbent. Agrowastes are currenth/ receiving attention as raw materials for water pollution control because of their low cost and availabiliry. A range of products has been examined clay,2 sago waste,3 cassava waste,4 banana pith,5 peanut skin,6 Medicago sativa (Alfalfa)7 and sphagnum moss peat8 just to mention a few. These work-ers have used mostly divalent metal ions. A literature search reveals that no work has been reported on the use of waste from fluted pumpkin (Telfairia occidentalis Hook f) as adsorbent for metal removal from aqueous systems. Thus, the purpose of our project is to test the influence of ionic charge on the removal of metal ions from aqueous systems by using metal ions of different charges. An additional goal is to establish the abiliry of five two-parameter equations - (the Langmuir, Freundlich, Temkin, Dubinin-Radushkevich and Flory-Huggins isotherms) to model the equilibrium sorption data. To achieve these goals a batch sorption tech-nique over a wide range of initial metal ion con-centrations on the ability of fluted pumpkin waste for the removal of Al3+, Co2+ and Ag+ from aque-ous systems were tested. This information will undoubtedh/ contribute to the sorption data bank. Horsfall and Spiff Equilibrium Sorption Study of Al3+, Co2+ and Ag Acta Chim. Slov. 2005, 52, 174–181 175 Results and discussion The percent removal of the metal ions from aqueous solution was found to decrease with in-crease in initial metal ion concentration (Figure 1). This may be due to the fact that at lower con-centrations almost ali the ions were adsorbed very quickly and further increases in initial metal ion concentrations led to saturation of biomass surface. 120 100 80 60 - 40 - 20 - ¦-¦-¦-¦¦-*- ¦ *..*> ¦ -Al3+ Co2+ "-Ag+ 0 20 40 60 80 100 120 Initial metal ion cone. (mg/dm3) Figure 1. Effect of initial metal ion concentration in the sorption of the three metal ions onto fluted pumpkin waste biomass. The analysis of experimental results by equilib-rium sorption isotherms are important in develop-ing accurate data that could be used for sorption design purposes. The sorption equation param-eters and the underlying thermodynamic assump-tions of these equilibrium models often provide some insight into both the sorption mechanism and the surface properties and affinity of the sorbent. Langmuir Isotherm The Langmuir isotherm model was chosen for the estimation of maximum adsorption capacity correspond-ing to complete monolayer coverage on the biomass surface. The plots of specific sorption (Ce/qe) against the equilibrium concentration (Ce) for Al3+, Co2+ and Ag+ are shown in Figure 2 and the linear isotherm param-eters, qm, KL and the coefficient of determinations are presented in Table 1. The sorption capacity, qm, which is a measure of the maximum sorption capacity corre-sponding to complete monolayer coverage showed that the fluted pumpkin waste had a mass capacity for Al3+ (16.98 mg g–1) than Co2+ (10.34 mg g–1) and Ag+ (8.03 mg g–1). The adsorption coefficient, KL that is related to the apparent energy of sorption for Ag+ (6.39×10–1) was greater than that of Co2+ (1.86×10–1 dm3g–1) and Al3+ (2.22×10–2 dm3g–1). This observation showed that the energy of adsorption is not very favourable to Ag+ prob-ably due to its large ionic radius; hence not all binding sites may be available to Ag+. The same capacity order have been reported9,10 for different metal ion sorption on modified coconut noir and cassava wastes biomass. 14 - 12 10 Ce 6 -4 -2 0 0 20 40 60 Ce 80 100 Figure 2. Langmuir equilibrium isoterm model for the sorption of the three metal ions onto fluted pumpkin waste biomass. The data in Table 1 further indicated that, the effectiveness of fluted pumpkin waste in the sorption of the three metals from aqueous system was Al3+ >Co2+ >Ag+. This preferential sorption behaviour could be explained in terms of ionic radii of the metal ions (Al3+= 0.52 A; Co2+ = 0.78 A; Ag+ = 1.26 A). The element with smaller ionic radius will compete faster for exchange sites than those of larger ionic radius. The larger the charge of an ion, the smaller, the ionic radius, hence the charge of an ion may influence its ability to sorb on biosorbents. Hydration energy is an important factor in sorption process accounting for the hydrolysis of metal ions, which oecurs by the replace-ment of water liquids in the inner coordination sphere with hydroxo groups. Adsorption may be related to the loss of the entire hydration sphere that precedes hydrolysis. The observed order indicates that Al3+ may have greater accessibility to the surface of certain pores than Co2+ and Ag+ due to its small ionic radius. Furthermore favourability of adsorption of the three metal ions on the fluted pumpkin waste biomass was tested using the essential features of the Langmuir isotherm model, expressed in terms of a dimensionless constant called separation fac- 8 0 Horsfall and Spiff Equilibrium Sorption Study of Al3+, Co2+ and Ag 176 Acta Chim. Slov. 2005, 52, 174–181 tor Sp which is defined by the following relationship. sp = 1 1 + KLC0 (1) Where KL = Langmuir isotherm constant; C0 = initial metal ion concentration. The separation parameters for the three met-als are less than unity indicating that fluted pumpkin waste biomass is an excellent adsorbent for the three metal ions. However, SF value of Ag+>>A13+ and Co2+, indicating that in a mixed metal ion system, Al3+ and Co2+ will compete for binding sites faster than Ag+. This observed separation factor indicates that high concentration of Al3+, Co2+ and Ag+ in an effluent may not be a limiting factor in the abil-ity of fluted pumpkin waste to sorb these metal ions. Table 1. Linear Langmuir isotherm parameters. Metal ions qm mg g ' KL dm3g ' Sf Al3+ 16.98 2.22×10 2 0.009 Co2+ 10.34 1.86×10 ' 0.10 Ag+ 8.03 6.39×10 ' 0.03 The Langmuir capacities can also be used to compare the efficiency of fluted pumpkin waste biomass with other materials which have been tested as biosorbents for metal ions. Table 2 makes such a com-parison and, although several of the metals studied based on a survey of adsorption shows that there is very little or no information in the literature based on Al3+, Co2+ and Ag+ removal by sorption. According to Marulanda and Harcum,11 C. caldarium an algal biomaterial had a capacity of 1.40 mg g"1 for Al3+. This value is significantly lower than the capacity of fluted pumpkin waste biomass (16.98 mg g"1) towards Al3+. For Co2+, the effect is not as marked when compared with agro-waste of soybean but much better than cottonseed hulls. While it seems that Ag+ sorption study is very little. Medicago sativa (Alfalfa)7 has a limited capacity for Al3+, sphagnum moss peat,8 has been utilized for Co2+ and Ag+. The fluted pumpkin waste has greater capacity than many of the materials tested previously. Table 2. Comparative Langmuir maximum sorption capacities of Al3+, Co2+ and Ag+. Metals Sorbent Xm(mgg1) Reference Ag3+ Fluted pumpkin waste Cyanidium caldarium Medicago sativa (Alfalfa) 16.94 1.40 3.45 This study 11 7 Co2+ Fluted pumpkin waste Soybeanby-product Cottonseed hulls sphagnum moss peat 10.34 15.39 1.77 8.29 This study 12 12 8 Ag+ Fluted pumpkin waste sphagnum moss peat 8.03 7.24 This study 8 Freundlich Isotherm The Freundlich model was chosen to estimate the adsorption intensity of the sorbate on the sorbent surface. The experimental data from the batch sorption study of the three metal ions on fluted pumpkin waste biomass were plotted logarithmicalh/ (Figure 3) using the linear Freundlich isotherm equation. The linear Freundlich isotherm constants for Al3+, Co2+ and Ag+ on fluted pumpkin waste biomass are pre-sented in Table 3. The Freundlich isotherm parameter 1/ n measures the adsorption intensity of metal ions on the biomass. Examination of Table 2 showed that the values of 1/n were found to be greater than unity indicating that the isotherms can be characterized by a convex Freundlich isotherm. This implies that significant adsorption may take plače even at high metal ion concentration. The high 1/n value of Al3+ (1.64) in relation to Co2+ (1.01) and Ag+ (1.04), first indicate the preferential sorption of Al3+ than Co2+ and Ag+ probabh/ due to its smaller ionic radius and secondly shows the ability of the fluted pumpkin biomass to remove these three metal ions from solution even at high concentrations. 2.5 2 1.5 ] 1 0.5 0 0 12 3 logCe Figure 3. Freundlich equilibrium isoterms model for the sorption of the three metal ions onto fluted pumpkin waste biomass. The observed differential sorption behaviour is useful in that high Al3+ concentrations in an effluent may not limit the ability of fluted pumpkin waste to sorb other metals. The ultimate adsorption capacity Kp of the adsorbent was calculated from the isother-mal linear regression equation. The KF value of Al3+ (4.27) is greater than that of Co2+ (3.43) and Ag+ (1.06), suggesting and confirming that Al3+ has greater adsorption tendency towards the fluted pumpkin waste biomass than the other two metals. Again, the ionic radius of the metal ions may be responsible for this observation. Previous studies81314 have shown that Horsfall and Spiff Equilibrium Sorption Study of Al3+, Co2+ and Ag Acta Chim. Slov. 2005, 52, 174–181 177 the smaller the ionic radius, the greater the affinity of cationic species towards binding sites on biomaterials. Table 3. Freundlich isotherm parameters. Metal ions 1/n KF Al3+ 1.64 4.27 Co2+ 1.01 3.43 Ag+ 1.04 1.06 Temkin Isotherm The Temkin adsorption isotherm model was chosen to evaluate the adsorption potentials of the adsorbent for adsorbates. The Temkin isotherm plot for the three metal ions are presented in Figure 4 and the isotherm parameters is given in Table 4. 20 18 -16 -14 -12 -qe10 -8 -6 -4 -2 0 0 2 4 6 8 In Ce Figure 4. Temkin equilibrium isoterm model for the sorption of the three metal ions onto fluted pumpkin waste biomass. The Temkin adsorption potential, KT of fluted pumpkin waste biomass for Al3+, Co2+ and Ag+ are 1.90, 1.74 and 1.47 respectively, indicating a lower biomass-metal ion potential for Ag+ probably due to its large ionic radius. The Temkin constant, bT related to heat of sorption for the three metal ions were 8.96 kJ mol"1, 5.16 kJ mol"1 and 5.10 kJ mol"1 for Al3+, Co2+ and Ag+ respec-tively. It has been reported (Ho et al. 1995) that the typi-cal range of bonding energy for ion-exchange mechanism is 8-16 kJ mol"1. The low values in this study indicates a weak interaction bervveen sorbate and sorbent, support-ing an ion-exchange mechanism for the present study. Table 4. Temkin isotherm parameters. Metal ions K_T bT kJ mol ' Al3+ 1.90 8.96 Co2+ 1.76 5.16 Ag+ 1.47 5.10 Dubinin – Radushkevich isotherm The Dubinin – Radushkevich model was chosen to estimate the characteristic porosity and the ap-parent free energy of adsorption. The linear regres-sion of the Dubinin-Radushkevich isotherm plot for the sorption of the three metal ions on fluted pumpkin waste biomass are presented in Figure 5, and the isotherm parameters are shown in Table 5. 7 6 5 -4 -3 2 5 10 e^lO^mor2 15 Figure 5. Dubinin-Radushkevich equilibrium isoterm model for the sorption of the three metal ions onto fluted pumpkin waste biomass. The sorption affinity of the biomass for Al3+, Co2+ and Ag+ are 7.55 mg g"1, 6.02 mg g"1 and 5.32 mg g"1 respectiveh/, indicating that the biomass had a greater affinity for Al3+. The porosity factors, KDR for the biomass towards the metal ions were 0.46 (Al3+), 0.95 (Co2+) and 1.54 (Ag+). The porosity factors were found to be less than unity, except that of Ag+, indicating that sorption of Ag+ by fluted pumpkin waste biomass may not be significant in a mixed metal ion system such as an industrial effluent probably due to its large size. This implies that the use of fluted pumpkin waste biomass for the removal of Ag+ in wastewater may require several numbers of cycles to reduce the concentra-tion of this metal ion to below regulatory levels. The apparent free energies from the Dubinin - Radush-kevich model for the sorption process are -0.96 kJ mol"1 (Al3+), -1.38 kJ mol"1 (Co2+) and -1.52 kJ mol"1 (Ag+) respectively. Physisorption processes have adsorption energies less than -40 kJ mol"1 and the energy values for the three metal ions sorption on the fluted pumpkin waste biomass indicates that the sorption process is physisorption. The negative values of Es indicate 1 0 Horsfall and Spiff Eguilibriiim Sorption Study of Al3+, Co2+ and Ag 178 Acta Chim. Slov. 2005, 52, 174–181 that the sorption process is exothermic and that lower solution temperature will favour the sorption process. Table 5. Dubinin-Radushkevich isotherm parameters. Metal ions xm Kdr Es Al3+ 7.95 0.46 -0.96 Co2+ 6.02 0.95 -1.38 Ag+ 5.32 1.15 -1.52 Flory-Huggins Isotherm The Flory-Huggins model was chosen in order to account for the degree of surface coverage charac-teristics of the adsorbate on the adsorbent. The plot of log(9/C0) versus log(l-B) for the three metal ions was made (Figure 6) and regression lines obtained. The isotherm data (Table 6) showed that, the apparent number, n, of metal ions occupying sorption sites is greater for Al3+ (0.671) than that of Co2+ (0.571) and Ag+ (0.528). The overall coverage processes indicate that over 50% of binding sites on the biomass surface were covered by metal ions during sorption process. The equilibrium constants, KFH, as obtained by the Flory-Huggins isotherm showed that KFH for Al3+, Co2+ and Ag+ are 1.25, 2.13 and 1.38 respectiveh/. Furthermore, the equilibrium constants, KFH, obtained from the Flory-Huggins isotherm model were used to compute the Gibbs free energy of spontaneity. The Gibbs free energy of spontane-ity is related to equilibrium constant as follows AG° = -RTLnKPH (2) where R is universal gas constant 8.324 J/mol, T is absolute temperature (K) and KFH is equilib-rium constant from Flory-Huggins isotherm equation. log^ -3 -2 Čo -1 -1 -2 -3 -4 -5 -6 -7 Figure 6. Flory-Huggins equilibrium model for the sorption of the three metal onto fluted pumpkin waste. The negative values of AG° (Table 6) indicate that the sorption process is spontaneous in nature and supports an exothermic reaction. The low energy values obtained using the Flory-Huggins model supports the values from the Dubinin-Raduskevich model. Table 6. Dubinin-Radushkevich isotherm parameters. Metal ions n Kfh AG° Al3+ 0.67 2.13 -0.56 Co2+ 0.57 1.38 -1.91 Ag+ 0.53 1.25 -0.81 Coefficients of determination The regression coefficients of determination, r2, from the linearization of the five two-parameter isotherm models are listed in Table 7. The r2 values suggest that the Langmuir and Dubinin-Radushkevich isotherms provide a good model for the sorption of Al3+ than Co2+ and Ag+. While the Freundlich, Temkin and Flory-Huggins isotherms produce a reasonable fit to the experimental data for Co2+ than Al3+ and Ag+. Due to the bias resulting from linearization, the internal structure not accessible at first glance of the r2 values in Table 7 were determined by two-way analysis of variance (ANOVA) without replication (P<0.5). This method provides explanation to the relationships (1) betvveen the five two-parameter isotherm models in describing the sorption system and (2) betvveen the three metal ions for binding sites on the fluted pumpkin waste biomass. The summary of the statistical analysis is presented in Table 8a and b. Consideration of the comparative magnitudes of the r2 values (Table 8a) suggest that the Langmuir isotherm model does provide a better model for the sorption systems and that Co2+ experimental data exhibits a better fitting to the five isotherm models. However, the two-way ANOVA results (Table 8b) indicate no significant difference betvveen the five two-parameter isotherm models in describing the sorption process of the three metal ions on fluted pumpkin waste biomass. This indicates that the five two-parameter isotherm models are appropriate in their own merits in describing the potential of fluted pumpkin waste biomass for the removal of Al3+, Co2+ and Ag+ from aqueous solution. The ANOVA data fur-ther showed that metal ion sorption on the biomass may not be too restricted to differences in their ionic radius. Other physical parameters such as hydration energy, ionic mobility, electronegativity and so on may also be a contributing factor. These observations are further con-firmed by the overall minimal error (4.44xl0~4) from the source of variation from ali the r2 sets for the isotherms. 0 0 Horsfall and Spiff Equilibrium Sorption Study of Al3+, Co2+ and Ag Acta Chim. Slov. 2005, 52, 174–181 179 Table 7. Linear isotherm coefficients of determination (r2). Linear isotherm Metal ions Al3+ Co2+ Ag+ Langmuir 0.9967 0.9903 0.9961 Freundlich 0.9930 0.9970 0.9830 Temkin 0.9898 0.9925 0.9857 Dubini-Radushkevich 0.9949 0.9899 0.9736 Flory-Huggins 0.9712 0.9907 0.9835 Table 8. Two-way analysis of variance (ANOVA) without replica-tion at oc = 0.05. A. Summarv Count Sum Average Variance Langmuir 3 2.9831 0.9944 1.25x10 5 Freundlich 3 2.973 0.9910 5.20x10 5 Temkin 3 2.968 0.9893 1.17x10 5 D-R 3 2.9584 0.9861 1.24x10 4 F-H 3 2.9454 0.982 9.72x10 5 Al3+ 5 4.9456 0.989 1.07x10 ' Co2+ 5 4.9604 0.992 8.55x10 " Ag+ 5 4.9219 0.984 6.45x10 5 B. ANOVA Sourceof Sums of Degrees of Mean sums Calculated StatisticalF variation square freedom ofsquare F ratio ratio value value Isotherms 2.76x10 4 4 Metals 1.51x10 4 2 Error 4.44x10 4 8 Total 8.7x10 4 14 6.89x10 5 1.24 3.83 7.54x10 5 1.36 4.46 5.55x10 5 for the removal of Al3+, Co2+ and Ag+ from wastewater. The fluted pumpkin waste is abundantly available but is scarceh/ useful. A single stem with leaves weighing 1.2 kg produces over 1 kg stem as wastes, which in-turn produced over 320 g of biomass. For this reason, they could be regarded as ecomical adsorbents for toxic and valuable metals from industrial wastewater. Experimental Adsorbent. The experiments were conducted with fluted pumpkin waste (T. occidentalis) biomass sourced from Nigeria. The waste was washed and then dried at a temperature of 55±5 °C to constant weight and finalh/ screened to particle size of 100-/xm before use. Adsorbent Characterization. The surface charac-teristics of the fluted pumpkin waste biomass has earlier been characterized for surface area, particle densitv, pore volume, porositv and surface charge densitv10 and found to exhibit characteristics which are favourable for the sorption of divalent metal ions. In order to find out the inherent metal ions of natural origin, metal concentrations in the fluted pumpkin waste biomass was determined by digesting 1.0 mg sample by heating in nitric acid and filtering. The metal ion concentrations were determined by flame atomic absorption spectrometry (FASS). The results are given in Table 9. Table 9. Mean metal concentrations (/ig g-1) in fluted pumpkin waste biomass (results are given as mean of triplicate analvses). Conclusions A detailed isotherm analysis of experimental data was carried out to determine the best isotherm models for the sets of equilibrium data for three metal ions: Al3+, Co2+ and Ag+ on fluted pumpkin waste. It was noted that ionic radius has an influence in the magni-tude of metal loading on the adsorbent. The experi-mental results were analyzed using five two-parameter adsorption isotherm models - the Langmuir, Freundlich, Temkin, Dubinin-Radushkevich and Flory-Huggins isotherms. Evaluating the correlation coefficients from the five isotherm equations using two-way ANOVA at p<0.05 for fitting the analytical data showed that the Freundlich and Langmuir isotherms described the data appropriable than Temkin, Dubinin-Radushkevich and Flory-Huggins isotherms. Sorption capacity increases with increase in smaller ionic radius metal ion. The result showed that fluted pumpkin waste could be used Metal Conc.Cugg1) Metal (ngg1) Iron 743.12 ±1.8 Potassium <1.0 Aluminium 197.82 ±1.21 Copper <1.0 Lead 24.69 ±0.91 Chromium <1.0 Cadmium 1.28 ±0.04 Manganese 26.76 ±0.01 Nickel 0.89 ±0.02 Magnesium 107.12 ±0.97 Zine 35.33 ±0.11 Mercurv <1.0 Cobalt 0.87 ±0.041 Silver <1.0 Activation and Purification of the biomass. Since the native fluted pumpkin waste contain some metals (Table 9), it is necessary to purify the biomass to completeh/ remove ali metals bioaccumulated while growing in the field. 500 g of finely divided biomass was activated and at the same time purified by soak-ing in excess 0.3M HN03 for 24h, after which it was vvashed thoroughly with deionized water until a pH of 7.1±0.1 was attained and then air-dried. The air-dried activated biomass was then washed with deionized water and re-suspended in 1.0 M hydroxylamine to remove ali 0-acetyl groups. To remove ali other solu-ble materials, the biomass was washed with deionized water and centrifuged at 3000 x g for five minutes using a Portable Refrigerated test tube centrifuge model Horsfall and Spiff Eguilibriiim Sorption Study of Al3+, Co2+ and Ag 180 Acta Chim. Slov. 2005, 52, 174–181 PR - 2 with 20” diameter stainless solid basket 3/4HP 1/60/115 volt motor with temperature indicator, timer and speed controls. The supernatants obtained were discarded and the purified biomass cake obtained dried at room temperature. 1.0 mg of the purified biomass was further digested and analvsed for the same metal ions and found to contain less than 1.0 /xg g"1 in ali cases. Sorption Equilibrium studies. The sorption experiments for the metal svstems were carried out as follows. Several standard solutions with concentra-tions of 10, 20, 30, 40, and 50 mg/L were made from spectroscopic grade standards of Al3+, Co2+ and Ag+. The metal solutions made separatelv were adjusted to pH 5.0 with cone. HC1. Fifty milliliters of each metal ion solution was added to accuratelv weighed (250 ± 0.01 mg) activated/purified biomass in different flasks and agitated for two hours to ensure that equilibrium was achieved. At the end of the time, the suspension was filtered through Whatman No 45 filter paper and centrifuged at 2800 x g. The supernatants were analvzed for metal ions by flame atomic absorption spectroscopv. Analysis of Metal Content. The Al3+, Co2+ and Ag+ content in each experiment were determined with a Bučk Scientific Flame Atomic Absorption Spec-trometer (FAAS) model 300A. Spectroscopic grade standards were used to calibrate the instrument, which was checked periodicallv throughout the analysis for instrumenfs response. The bateh experiments were performed in triplicates and the means were computed for each set of values to maintain quality assurance. Data Evaluation Calculation of Al3+,Co2+ and Ag+ Removed By Biomass The amount of Al3+, Co2+ and Ag+ removed by the biomass during the series of bateh investigations were determined using a mass balance equation expressed as in equation 3. v(C0-Ce) (3) m where qe = metal concentration on the biomass (mg/g biomass) at equilibrium, Ce = metal concentration in solution (mg/L) at equilibrium, C0 = initial metal concentration in solution (mg/L), v = volume of initial metal solution used (L), m = mass of biomass used (g). Equilibrium Sorption Five two-parameter equations - the Lang-muir, Freundlich, Temkin, Dubinin-Radushkevich and Flory-Huggins isotherms were examined for their ability to model the equilibrium sorption data. The linear form of the Langmuir equation is usu-ally expressed by le -------+ —~ 9mKL le (4) where KL = Langmuir isotherm constant (dm3 g"1); qm= Langmuir monolayer sorption capacity (mg g"1). A plot of Ce/qe against Ce was made to confirm the Langmuir isotherm. The Freudlich model is represented in equation 4: logqe=logKp+-lonCe (5) where qe = sorption density (mg/g); Ce = cone. Of metal ion in solution at equilibrium mg/dm3); KL and 1/n are the Freundlich constants. The value of n indicates the affinity of the sorbent towards the biomass. Plotting log qe against log Ce was used to test the Freundlich model. The linear form of the Temkin isotherm model as shown in equation 5 was plotted as qe against In Ce. Ve RT RT, lnK +----In C K K (6) where the l/bT indicates the adsorption potential of the adsorbent and KT is the Temkin isotherm constant (dm3 g"1). The linear form of the Dubinin-Radushkevich equation is represented by equation 6 lnq e=\nXm Kme2 (7) where K is related to the free energy of sorption and Xm is the Dubinin-Radushkevich isotherm constants related to the degree of sorbate sorption by the biomass surface. The parameter s is expressed by (equation 7) e = RT In C, (8) A plot of Inqe against s2 yielding a straight line was made to confirm the model. The mean free energy of adsorption (Es) from the Dubinin-Radushkevich equation can be computed using the following relationship (equation 8) E (-2K) -1/2 (9) The linear form of the Flory-Huggins equation is represented by log— = logKPH + nlog(l-0) (10) where 6 = (1-CJC0) is the degree of surface coverage, n is the number of metal ions occupying sorption sites, Ka is the equilibrium constant and C is the equilibrium n q = Horsfall and Spiff Equilibrium Sorption Study of Al3+, Co2+ and Ag Acta Chim. Slov. 2005, 52, 174–181 181 concentration. A plot of log(6>/C) against log(l- 6) yield-ing a straight line was made to confirm this model. Acknowledgements The International Foundation for Science, Stockholm, Sweden supported this research, through grant number W/3624 - 1 to Dr M. Horsfall Jnr. References 1. B. E. Okoli, B. L. Nyanayo. 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Removal of copper and aluminum ions from solution by Cyanidium caldarium. Technical completion report for WERC project, New Mejrico, USA 1999. 12. E. W. Marshall, E. T. Champagne, /. Environ. Sci. Health Part A. 1995, 30, 241-261. 13. F. E. Okiemen, A. O Maya, C.O Oriakhi, Inter. J. Environ. Anal. Chem. 1987, 32, 23-27. 14. G. Mckay, M. E. I. Geundi, M. M Nassar, Wat. Res 1987, 21, 1513-1520. Povzetek Z različnimi metodami smo raziskovali vpliv ionskih radijev na sorpcijo Al3+, Co2+ in Ag+ ionov na odpadni biomasi iz buč. Eksperimentalne rezultate smo analizirali s pomočjo petih dvo parametrskih enačb: Langmuir-jeve, Freundlichove, Temkinove, Dubinin-Radushkevicheve in Flory-Hugginsove izoterme. Dobljena kapaciteta monoplastne adsorpcije znaša po Langmuirjevi adsorpcijski izotermi 16.98 mg/g, 10, 34 mg/g in 8.03 mg/g za Al3+, Co2+ in Ag+ ion. Ugotovili smo, da Freundlichova in Langmuirjeva izoterma process adsorpcije opišeta bolje kot Temkinova, Dubinin-Radushkevicheva in Flory-Hugginsova izoterma. Rezultati kažejo, da ionski radij vpliva na hitrost migracije kovinskega iona k površini biomase ter s tem na intenziteto adsosrpcije ter, da so bučni odpadki uporabni za odstranjevanje Al3+, Co2+ in Ag+ ionov iz odpadnih vod. Horsfall and Spiff Equilibrium Sorption Study of Al3+, Co2+ and Ag