Comparison between Different Keratin-composed Biosorbents for the Removal of Heavy Metal Ions from Aqueous Solutions

Transcription

1 393 Comparison between Different Keratin-composed Biosorbents for the Removal of Heavy Metal Ions from Aqueous Solutions Fawzi Banat*, Sameer Al-Asheh and Dheaya Al-Rousan Department of Chemical Engineering, Jordan University of Science and Technology, Irbid-22110, Jordan. (Received 24 October 2001; accepted 26 January 2002) INTRODUCTION ABSTRACT: This study examined and compared the ability of chicken feathers, human hair and animal horns, as keratin-composed biosorbents, for the removal of Zn 2+ and Cu 2+ ions from single metal ion aqueous solutions under different operating conditions. The three biosorbents investigated in this study were all capable of adsorbing Zn 2+ and Cu 2+ ions from aqueous solutions. The biosorbent showing the highest uptake of Zn 2+ and Cu 2+ ions was animal horns. Chicken feathers showed a higher Cu 2+ ion uptake and a lower Zn 2+ ion compared to human hair. Increasing the initial concentration of Zn 2+ or Cu 2+ ions, or increasing the initial ph value, increased the metal ion uptake. Such uptake decreased when the temperature was raised from 25ºC to 50ºC for all adsorbent/metal ion combinations except for Zn 2+ ion/human hair where the uptake increased with temperature. It was demonstrated that the addition of NaCl salt to the metal ion solution depressed the metal ion uptake. The Freundlich isotherm model was found to be applicable to the adsorption data for Cu 2+ and Zn 2+ ions. Heavy metal pollution of water is an extremely serious environmental problem which has emerged in recent years due to the development of industries that transmit heavy metals to water. Metal processing, electroplating, photography and ceramic industries are typical examples (Dean et al. 1977). Studies conducted during the past few years have shown that low concentrations of many heavy metals are capable of causing acute lethal toxicity. Copper and zinc are among heavy metals found in municipal and industrial wastewaters. Copper is particularly toxic, complexing with enzymes and other metabolic agents connected with respiration and rendering them inactive. In addition, copper is an irritant to the skin causing itching and dermatitis, and may result in keritinization of the hand and soles of the feet (Sitting 1981). Because of the hazardous effects of heavy metals, wastewaters that contain heavy metal ions should be treated to reduce such contents to acceptable levels before being discharged to the environment. Many processes can be used for the removal of heavy metal ions from wastewaters, including chemical precipitation, coagulation, solvent extraction, membrane separation, ion exchange and adsorption. However, ion exchange and adsorption are the most effective methods of removal, especially when dealing with dilute metal ion concentrations (Brown et al. 2000). Adsorbents such as activated carbon have been used for a long time for the removal of heavy metal ions from wastewaters. However, the high capital and regeneration costs for such adsorbents have prompted researchers to find new cheap adsorbents, mostly of a biological origin. Use of *Author to whom all correspondence should be addressed. banatf@just.edu.jo.

2 394 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No these new adsorbents has led to a new term biosorption to describe the accumulation of metal ions from solution by using materials of biological origin, particularly micro-organisms and plant products. The uptake of metal ions by these materials has been attributed to their constituents which contain functional groups such as carboxy, hydroxy and amine groups that act as binders for these ions (Kuyucak and Volesky 1988). Many biosorbents have been used in the past few years for the removal of heavy metal ions from aqueous solutions. For example, Brown et al. (2000) used peanut hull pellets for the removal of Cu 2+, Cd 2+, Zn 2+ and Pb 2+ ions, Al-Asheh et al. (1999) used spent animal bones for the removal of Cu 2+ and Zn 2+ ions, Al-Asheh and Duvnjak (1997) used pine barks for the removal of Cd 2+ ions and Kappor and Viraraghavan (1998) used immobilized fungal biomass for the removal of various metal ions. Fibrous proteins contain intricate networks of stable and water-insoluble fibres with high surface areas and are abundant bioresources. Some researchers have examined the effectiveness of different types of fibrous protein for the removal of heavy metal ions from aqueous solution. For example, Ishikawa and Suyama (1998) examined the use of egg-shell membranes, chicken feathers, wool, silk and elastin for the removal of the gold-cyanide ion, Suyama et al. (1996) used chicken feathers for the recovery of precious metal ions such as gold and platinum, Tan et al. (1985) used human hair waste, activated and non-activated, for the removal of several heavy metal ions, Lechaveleir and Drobot (1981) used feathers, hair and powdered hoofs for the removal of noble metals such as Pt, Pd or Rh, while Friedman et al. (1973) used wool for the removal of Hg 2+ ions. Banat and Al- Asheh (2000) confirmed the suitability of chicken feathers for the removal of phenolic compounds from aqueous solutions. Human hair, chicken feathers and animal horns are composed of a fibrous proteinaceous material known as keratin. Keratin has a complicated structure that exhibits a large surface area. The use of these materials (which are widely and abundantly available all over the world) in lieu of activated carbon would reduce the cost of an adsorption system. The main objectives of the present work were: (1) to examine and compare the effectiveness of chicken feathers, human hair and animal horns, as keratinous materials, in removing heavy metal ions from aqueous solutions; and (2) to study the effect of different operating parameters, such as ph, temperature and salt addition, on the adsorption capacity of the above-mentioned adsorbents. These materials were chosen because of their keratin content which is extremely rich in functional groups, mainly carboxy and amine, which could assist the adsorption of metal ions. MATERIALS AND METHODS Adsorbents Animal horn Animal horns were collected from butchers shops, washed several times with tap water and a detergent before being rinsed with distilled water. They were then dried in an oven for 2 d at 105ºC. The dried horns were crushed, milled and sieved into different sizes with that in the range mm being subsequently used experimentally. Chicken feathers Chicken feathers collected from poultry shops were washed with a detergent, rinsed several times

3 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 395 with distilled water and then left to dry at room temperature. An electrical cutting machine was used to cut the feathers used in subsequent experiments. Human hair Human hair waste was collected from various local barbershops and ladies hair salons. The collected hair was mixed together, washed with distilled water and detergent, and then left to dry at room temperature. An electrical cutting machine was then used to cut the hair into small sizes to be used in subsequent experiments. Batch adsorption experiments Copper ion solutions in the range mg/l and Zn 2+ ion solutions in the range mg/l were prepared from copper sulphate (CuSO 4 5H 2 O) and zinc sulphate (ZnSO 4 7H 2 O) salts, respectively. Batch sorption tests were conducted by placing a known quantity of one of the above-mentioned adsorbents in bottles containing 50 ml of an aqueous solution of Cu 2+ or Zn 2+ ions of a predetermined concentration. The final adsorbent concentration was 5 mg/ml unless stated otherwise. The suspension was agitated in a shaker and samples were taken at known time intervals for the purpose of studying the kinetics of the sorption process. Otherwise, the samples were allowed to reach equilibrium (24 h) and then filtered. The residual Cu 2+ or Zn 2+ ion content in the filtrate was analyzed using a Varian Spectro AA10 atomic absorption spectrophotometer. Two replicates per sample were used with the average results being presented in this work. A blank experiment was conducted in the absence of sorbent and no metal sorption by the bottle or by the filter paper employed was detected. The effect of solution ph on the sorption of Cu 2+ and Zn 2+ ions was examined at four ph levels, viz. 3.5, 4.0, 4.5 and 5.0. Adjustments to the ph were made using 0.1 M HCl solution. Experiments were conducted at 25ºC, 40ºC and 50ºC and other fixed operating conditions to determine the effect of temperature on the sorption of Cu 2+ and Zn 2+ ions by the aforementioned adsorbents. The effect of the presence of inorganic salts such as NaCl in solution on the adsorbability of Cu 2+ and Zn 2+ ions was also investigated. RESULTS AND DISCUSSION Kinetics of the adsorption process The adsorption rate is an important factor in designing adsorption systems. Consequently, it is necessary to study the variation of adsorbate uptake with contact time. The amounts of Cu 2+ and Zn 2+ ion removed by the three adsorbents were determined by varying the initial Cu 2+ and Zn 2+ ion concentration in the presence of fixed amounts of adsorbent. Thus, the initial metal ion concentration was varied over the range mg/l for Cu 2+ ions and mg/l for Zn 2+ ions. Batch kinetics studies showed that an equilibrium time of 8 h was needed for the adsorption of Cu 2+ or Zn 2+ ions by chicken feathers and animal horns while 3 h was sufficient to attain equilibrium when human hair was used (Figures 1 and 2). It was also shown that an increase in the initial metal ion concentration led to an increase in uptake by all adsorbents for both Zn 2+ and Cu 2+ ions. This trend can be explained by the progressive increase in electrostatic interaction relative to covalent interaction of sites having a lower affinity for Zn 2+ or Cu 2+ ions with increasing initial metal ion concentration (Al-Asheh and Duvnjak 1997). The higher uptake at higher initial concentrations is

4 396 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No Figure 1. Effect of Zn 2+ ion concentration on its uptake by (a) animal horns, (b) chicken feathers and (c) human hair. Adsorbent concentration = 5 mg/ml; initial concentration (mg/l):, 20;, 40; D, 60; Ñ, 80; {, 100.

5 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 397 Figure 2. Effect of Cu 2+ ion concentration on its uptake by (a) animal horns, (b) chicken feathers and (c) human hair. Adsorbent concentration = 5 mg/ml; initial concentration (mg/l):, 10;, 20; D, 30; Ñ, 40; {, 50.

6 398 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No consistent with other studies of metal ion adsorption on to activated carbons and low-cost media (Suh and Kim 2000; Salim et al. 1994; Deshkar et al. 1990). As shown in Figures 1 and 2, the rate of adsorption was very fast during the first hour of the adsorption process but became slower after that time. The initial fast adsorption rate may be associated with rapid attachment of the Zn 2+ or Cu 2+ ions to the surface of the adsorbent. During the slow rate region, the surface was saturated by metal ions so that the latter started to diffuse through the pores in the adsorbent medium (i.e. the onset of an intrapore diffusion mechanism). It would appear that intrapore diffusion was more important when animal horns and chicken feathers were the adsorbents than when human hair was employed. To examine the importance of pore diffusion in the adsorption process, plots of Cu 2+ and Zn 2+ ion uptake versus the square root of time were constructed (Figures 3 and 4) as suggested by Weber and Morris (1963). According to these authors, if intraparticle diffusion was involved in the sorption process, then uptake of the adsorbate should vary linearly with the square root of time and if this line passes through the origin then intraparticle diffusion is the rate-controlling factor. As shown in Figures 3 and 4, the corresponding lines for the systems studied are linear but do not pass through the origin. Thus, intraparticle diffusion was involved in the adsorption processes but was not the rate-controlling step. However, the fact that the plots for animal horn and chicken feather systems have steeper slopes than those for human hair indicates that intraparticle diffusion was of greater importance for the former adsorbents than for the latter. Equilibrium isotherms The equilibrium isotherms for the sorption of Zn 2+ and Cu 2+ ions by animal horns, chicken feathers and human hair are presented in Figure 5 in terms of the linearized Freundlich isotherm: 1 ln q e = ln k F + ln C e (1) n where q e (mmol/g) is the adsorption capacity in equilibrium with the metal ion concentration in the solution, C e (mmol/l), and k F and n are the Freundlich coefficients. The value of k F is related to the adsorption capacity while the value of l/n is related to the adsorption intensity. The linearized Freundlich isotherm model fits the equilibrium experimental data for Zn 2+ and Cu 2+ ions quite well (Figure 5), the data indicating that the adsorption capacity of the tested adsorbents followed the order animal horns > chicken feathers > human hair over the entire range of concentrations investigated. An exception was noted for the Cu 2+ /human hair system, where the adsorption capacity was high at low Cu 2+ ion concentrations. Furthermore, the steep slopes exhibited by the isotherms of the three adsorbents indicate that their adsorptive capacity increased at higher equilibrium solute concentrations compared to that at lower concentrations. Adsorbents that exhibit such a trend are recognized to be more efficient in column operation than adsorbents with flat isotherms (Benefield et al. 1982). It is evident that both chicken feathers and human hair have the same adsorptive capacity at the point at which their isotherm lines intersect (Figure 5). However, at high equilibrium concentrations, human hair had a greater adsorptive capacity towards Zn 2+ ions while chicken feathers had a greater adsorptive capacity towards Cu 2+ ions. The opposite trend occurred at low concentrations (Figure 5). The Freundlich constants calculated from the data depicted are listed in Table 1. Since the value

7 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 399 Figure 3. Plots of Zn 2+ ion uptake versus the square root of time at different initial Zn 2+ ion concentrations: (a) animal horns, (b) chicken feathers and (c) human hair. Adsorbent concentration = 5 mg/ml; initial concentration (mg/l):, 20;, 40; D, 60; Ñ, 80; {, 100.

8 400 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No Figure 4. Plots of Cu 2+ ion uptake versus the square root of time at different initial Cu 2+ ion concentrations: (a) animal horns, (b) chicken feathers and (c) human hair. Adsorbent concentration = 5 mg/ml; initial concentration (mg/l):, 10;, 20; D, 30; Ñ, 40; {, 50.

9 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 401 Figure 5. Freundlich isotherms for the sorption of (a) Zn 2+ ions and (b) Cu 2+ ions by, animal horns,, chicken feathers and D, human hair, using an adsorbent concentration of 5 mg/ml for each adsorbent studied.

10 402 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No TABLE 1. Freundlich Constants for Zn 2+ and Cu 2+ Ion Adsorption by Animal Horns, Chicken Feathers and Human Hair a Systems k F l/n R 2 Animal horns Zn Cu Chicken feathers Zn Cu Human hair Zn Cu a Adsorbent concentration = 5 mg/ml. of k F provides an indication of the adsorption capacity of the adsorbent, it is clear that the affinity of animal horns and chicken feathers towards Cu 2+ ion adsorption was better than that towards Zn 2+ ions. However, the behaviour of human hair was different since it exhibited a greater affinity towards the removal of Zn 2+ ions than towards the removal of Cu 2+ ions. The greater uptake of a particular metal ion could be attributed to a preference of certain active sites on the adsorbent surface towards that ion. The higher uptake of Cu 2+ ions by animal horns and chicken feathers relative to Zn 2+ ions may be attributed to the smaller size of the Cu 2+ ion (ionic radius = nm) compared with Zn 2+ ions (ionic radius = nm) (Chen and Lin 2000). Because Cu 2+ ions have a smaller radius, they can easily penetrate into smaller pores in animal horns and chicken feathers and thus have greater access to the adsorbent surfaces in the interiors of the materials. This result is in agreement with the above discussion concerning the importance of intrapore diffusion when using these adsorbents. Figure 6 shows a comparison between the uptake of the three adsorbents towards Zn 2+ or Cu 2+ ions. The figure shows that the adsorption capacities of human hair and chicken feathers towards both metal ions were comparable to each other but remarkably less than that of animal horns. It is believed that the higher adsorption capacity of animal horns is due to their inner layer which is part of the scull and capable of incorporating large amounts of exchangeable light cations such as Ca 2+ ions. The other constant mentioned in Table 1 is l/n which is a measure of the adsorption intensity. It has been reported that systems having low l/n values exhibit a sharp increase in uptake at low concentrations (Fumiaki et al. 2000). Thus, in terms of the l/n values in Table 1, it would appear that the adsorption intensity of animal horns and chicken feathers towards Cu 2+ ions was higher than that towards Zn 2+ ions. In contrast, the adsorption intensity of human hair toward Zn 2+ ions was higher than towards Cu 2+ ions. Effect of temperature Temperature is an important parameter in adsorption processes and is often considered by many researchers when studying new metal/sorbent systems. Such studies have been undertaken, for example, in considerations of the adsorption of Hg 2+, Cd 2+ and Pb 2+ ions on lignite (Eligwe et al. 1999), the adsorption of Cu 2+ ions by moss (Al-Asheh and Duvnjak 1997) and the adsorption of Cu 2+ ions by sawdust (Ajmal et al. 1998). In the present work, the adsorption of Cu 2+ and Zn 2+ ions by the three adsorbents considered

11 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 403 Figure 6. Comparison between animal horns, chicken feathers and human hair for the removal of (a) Zn 2+ ions and (b) Cu 2+ ions using 5 mg/ml adsorbent concentrations.

12 404 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No Figure 7. Freundlich isotherms at different temperatures for the removal of Zn 2+ ions by (a) animal horns, (b) chicken feathers and (c) human hair. Temperature (ºC): 25,, ; 40,, -----; 50, D, ; adsorbent concentration = 5 mg/ml.

13 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 405 Figure 8. Freundlich isotherms at different temperatures for the removal of Cu 2+ ions by (a) animal horns, (b) chicken feathers and (c) human hair. Temperature (ºC): 25,, ; 40,, -----; 50, D, ; adsorbent concentration = 5 mg/ml.

14 406 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No TABLE 2. Freundlich Constants for Zn 2+ and Cu 2+ Ion Adsorption by Animal Horns, Chicken Feathers and Human Hair at Different Temperatures a Adsorbent Temp. Zn 2+ ions Cu 2+ ions (ºC) k F l/n R 2 k F l/n R 2 Animal horns Chicken feathers Human hair a Adsorbent concentration = 5 mg/ml. was studied at 25ºC, 40ºC and 50ºC, respectively. The results obtained are shown in Figures 7 and 8 again in the form of the linearized Freundlich isotherms. The corresponding Freundlich constants are listed in Table 2. With the exclusion of the Zn 2+ /human hair system, the results showed that a decrease in the temperature of the system increased the metal ion uptake. For the removal of Cu 2+ ions by chicken feathers, an increase in temperature led to an increase in Cu 2+ ion uptake at low equilibrium concentrations, but this trend was reversed at high equilibrium concentrations where the Cu 2+ ion uptake decreased with increasing temperature. These different behaviours could probably be attributed to the mixed effects of (i) keratin denaturation with temperature, (ii) dissociation of some compounds available at the surface of the adsorbent which could be responsible for metal adsorption and (iii) the nature of possible temperature-dependent interactions between metal ions and active sites on the adsorbent surface. Denaturation of the keratin structure, for example, may lead to distortion of some sites available for metal sorption on the adsorbent surface and consequently decrease the uptake of metal ions. In general, the results of this work indicate that the effect of temperature on Cu 2+ ion sorption was greater than that on Zn 2+ ion sorption. This is demonstrated by the variation of k F with temperature. The decrease in the k F value with temperature was more noticeable for the sorption of Cu 2+ ions than for Zn 2+ ions. Thermodynamic parameters which characterize the system equilibrium, viz. the enthalpy change (DH), the Gibbs free energy change (DG) and the entropy change (DS), were calculated employing the method reported by Lopez-Delgado et al. (1996) which depends on the apparent equilibrium constant. Values of the apparent equilibrium constants (K c ) for the adsorption process may be calculated from the following equation: % adsorption K c = (2) 1 % adsorption The values of the thermodynamic quantities DG, DH and DS for the metal ion adsorption process can then be calculated using the K c values with the following equations:

15 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 407 DG = RT ln K c (3) DH DS ln K c = + (4) RT R Accordingly, DH and DS can be obtained from the slope and intercept of the linear plots of ln K c versus l/t, respectively. Negative values of DG, DH and DS obtained for the adsorption of Zn 2+ and Cu 2+ ions by animal horns indicate that this adsorption process was spontaneous, exothermic and favourable. The average DG, DH and DS values for the adsorption of Zn 2+ ions by animal horns were 4.4 kj/mol, 15.8 kj/ mol and 0.04 (kj K)/mol, respectively. The average DH values were negative for all the systems studied except for the Zn 2+ /human hair system (DH = 13.4 kj/mol). This positive value of DH indicates that the adsorption of Zn 2+ ions by human hair was endothermic in nature. Other researchers have reported similar behaviours. Thus, Viraraghavan and Dronamraju (1993), for example, reported that Cu 2+, Ni 2+ and Zn 2+ ion adsorption on fly ash was exothermic in nature. Hasany et al. (2000) found that cobalt ion adsorption on to polyurethane foam was exothermic while Khalid et al. (1999) found that Hg 2+ ion adsorption on to rice husks was endothermic. Ferro-Garcia et al. (1988) reported that while Zn 2+ and Cd 2+ ion adsorption by activated carbons prepared from agricultural by-products was exothermic, Cu 2+ ion adsorption by the same materials was endothermic. Effect of initial ph It has been mentioned by many researchers that solution ph plays an important role in the sorption of heavy metal ions. For example, Kumar and Dara (1980) found that the sorption of Cu 2+ and Zn 2+ ions increased with an increase in solution ph, while Periasamy and Namasivayam (1994) found that the removal of Cd 2+ ions by peanut hull carbon and coconut activated carbon increased with increasing solution ph. In the present investigation, the influence of the initial ph value on Zn 2+ and Cu 2+ ion uptakes by the three adsorbents considered was studied at four ph levels, viz. 3.5, 4.0, 4.5 and 5.0. This range of ph was selected to avoid the precipitation of metal hydroxides that could occur at high ph levels. The results of studies of the effect of ph are presented in the form of the linearized Freundlich isotherms depicted in Figures 9 and 10 for Zn 2+ and Cu 2+ ion adsorption, respectively. The Freundlich constants at the four ph levels are listed in Table 3. The results obtained indicate that metal ion uptake increased with increasing initial ph. Such an increase could be related to surface charges; thus, at lower ph values, hydrogen ions could compete strongly with metallic ions in the adsorption process. Luef et al. (1991) and Ferro-Garcia et al. (1988) have stated that electrostatic repulsion between cations and the positively charged surface of an adsorbent takes place at low ph. However, at higher ph values, metal ions replace hydrogen ions on the adsorbent surface and consequently adsorption increases. In addition to H + ion competition, the ph has a strong effect on the ionization of the proteins themselves. As a proteinaceous material, keratin is a large polyelectrolyte whose outer surfaces are studded with weak acid ( COOH) and basic ( NH 2 ) groups. The degree of ionization of these groups depends on the ph, and so does the charge on the keratin molecule. In sufficiently acidic solutions, keratin is strongly positive with a possible charge number between +10 and +20 (Garcia et al. 1997). As the ph increases, the charge decreases and becomes zero at the isoelectric point where the positive and negative charges in the system cancel each other out. In basic solutions, the protein becomes

16 408 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No Figure 9. Freundlich isotherms at different ph values for the removal of Zn 2+ ions by (a) animal horns, (b) chicken feathers and (c) human hair. ph value: 3.5,, ; 4.0,, -----; 4.5, D, ; 5.0, Ñ, ; adsorbent concentration = 5 mg/ml.

17 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 409 Figure 10. Freundlich isotherms at different ph values for the removal of Cu 2+ ions by (a) animal horns, (b) chicken feathers and (c) human hair. ph value: 3.5,, ; 4.0,, -----; 4.5, D, ; 5.0, Ñ, ; adsorbent concentration = 5 mg/ml.

18 410 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No TABLE 3. Freundlich Constants for Zn 2+ and Cu 2+ Ion Adsorption by Animal Horns, Chicken Feathers and Human Hair at Different ph Values a Adsorbent ph Zn 2+ ions Cu 2+ ions k F l/n R 2 k F l/n R 2 Animal horns Chicken feathers Human hair a Adsorbent concentration = 5 mg/ml. negative with the charge number diminishing to 10 or 20 (Garcia et al. 1997). Since the isoelectric point (pi) of a protein is similar to that of the predominant amino acid residue, and because cystiene, glycine and alanine, the main amino acid residues of the keratinous materials used in this study, have an isoelectric point greater than 5.0, the net charge of keratinous materials in the selected ph range is positive. This net positive charge decreases with increasing solution ph, leading to a decrease in the repulsion between the sorbent surface and the metal ions and thus improving the adsorption capacity. The results could also be explained in terms of an ion-exchange mechanism. It is possible that at higher ph levels soft ions such as K + and Ca 2+ were released from the surface of the adsorbents, leaving their sites to be occupied by Cu 2+ and Zn 2+ ions. This will be elaborated further below. Effect of adsorbent concentration The effect of adsorbent concentration on the removal of Zn 2+ and Cu 2+ ions was examined by varying the concentration of chicken feathers and human hair from 1 to 20 mg/ml and of animal horns from 1 to 30 mg/ml at two levels of metal ion concentration. As shown in Figures 11 and 12, increasing the adsorbent concentration from 1 to 5 mg/ml led to a sharp increase in the amount of Zn 2+ or Cu 2+ ion removed from the system. However, varying the adsorbent concentration beyond 5 mg/ml seemed to have little effect on the removal of Zn 2+ or Cu 2+ ions. This is not unexpected since an increase in the adsorbent concentration would lead to more exchangeable/adsorption sites becoming available. In other words, increasing the sorbent concentration by a certain extent increases the number of vacant sites attractive towards metal ions. These findings are compatible with others reported in the literature concerning the effect of adsorbent concentration on metal ion uptake. Thus, Khan et al. (1995) found that an increase in bentonite concentration increased the removal of Cu 2+ and Ag + ions, while Dronnet et al. (1997) found that the adsorption of Cu 2+ ions by sugar-beet pulp increased with an increase in the adsorbent concentration.

19 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 411 Figure 11. Effect of the concentration of animal horns ( ), chicken feathers ( ) and human hair (D) on the removal of Zn 2+ ions: (a) initial Zn 2+ ion concentration = 80 mg/l; (b) initial Zn 2+ ion concentration = 40 mg/l.

20 412 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No Figure 12. Effect of the concentration of animal horns ( ), chicken feathers ( ) and human hair (D) on the removal of Cu 2+ ions: (a) initial Cu 2+ ion concentration = 40 mg/l; (b) initial Cu 2+ ion concentration = 20 mg/l.

21 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 413 Effect of salt addition Light metal ions such as Na + often occur in industrial wastewaters. In the present study, the effect of Na + ions on the adsorption of Zn 2+ and Cu 2+ ions has been studied by adding different amounts of NaCl salt in the concentration range M to the various systems studied. The results presented in Figure 13 indicate that increasing the NaCl concentration up to 0.1 M caused a significant decrease in the uptake capacity of all the various sorbents towards Zn 2+ and Cu 2+ ions. However, no appreciable change in the uptake was noticed beyond a salt concentration of 0.1 M. Similar results have been reported by Harris and Ramelow (1990) for the sorption of Ag 2+, Cu 2+, Cd 2+ and Zn 2+ ions by a particulate biomass derived from Chlorella vulgaries and by Deshkar et al. (1990) for the sorption of Hg 2+ ions by modified hardwickia binata bark. The reduction in uptake in the presence of light metal ions could be attributed to a competitive ion effect between the light metal ions concerned and heavy metal ions for the available adsorption sites on the adsorbent surfaces. Investigation of an ion-exchange mechanism Several mechanisms have been proposed for the removal of heavy metal ions by biosorption, such as ion exchange, cation chelation, complexation and surface sorption. The possible existence of an ion-exchange mechanism was examined in this work by measuring the release of Mg 2+, Ca 2+, K +, and H + ions at the end of the sorption process. The concentration of soft ions released was compared with those in the control solution which consisted only of the adsorbent suspension. From the results presented in Table 4, it is obvious that an ion-exchange mechanism existed in all the metal/ sorbent systems studied, but that its significance varied from one system to another. This is demonstrated by the release of some cations such as Mg 2+, Ca 2+ and K + to the solutions. The release of such soft ions is attributed to their exchange with Cu 2+ and Zn 2+ ions on the adsorbent surface. A number of workers have reported that the displacement of the H + ion indicates covalent bonding of the metals (Crist et al. 1990) while the displacement of Ca 2+ and K + ions indicates ionic bonding with the metal (Avery and Tobin 1993). On this basis and with reference to the amount released of each of these light cations, it is possible to state that Cu 2+ and Zn 2+ ion adsorption by animal horns mainly involved ionic bonding whilst with human hair the bonding was mainly covalent. The equivalent ratio, i.e. the ratio of metal bound to metal released, provides an indication of the presence and significance of ion-exchange mechanisms. When this ratio approaches unity, the ionexchange mechanism becomes dominant. The data listed in Table 4 indicate that most of the systems studied exhibited a high equivalent ratio. Hence, ion exchange is not the only mechanism possible and others may be involved. CONCLUSIONS Three keratinous materials, i.e. chicken feathers, human hair and animal horns, were examined for their ability to function as biosorbents for metal ions such as Cu 2+ and Zn 2+. The results of this study have confirmed the effectiveness of the three keratinous materials in the removal of Cu 2+ and Zn 2+ ions from aqueous solutions, with animal horns being the most effective sorbent for such removal. The adsorptive capacity of chicken feathers was comparable to that of human hair. The adsorptive capacities of the biosorbents tested followed the order animal horns > chicken feathers > human hair for Cu 2+ ion adsorption, and animal horns > human hair > chicken feathers for Zn 2+ ion adsorption. The nature of the metal ion concerned, the initial concentration of metal ions, the

22 414 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No Figure 13. Effect of NaCl concentration on the sorption of (a) Zn 2+ ions and (b) Cu 2+ ions by animal horns ( feathers ( ) and human hair (D): adsorbent concentration = 5 mg/ml. ), chicken

23 Heavy Metal Ion Removal from Aqueous Solution by Different Keratin-containing Biosorbents 415 TABLE 4. Concentration of Ions in Solution at End of Adsorption Process a System Metal ion bound Amount of cation released b R c b/r (mequiv/l) (mequiv/l) Cu 2+ Zn 2+ Mg 2+ Ca 2+ K + H + Animal horns Control Cu Zn Chicken feathers Control Cu Zn Human hair Control Cu Zn Inactivated Control palm stones Cu Zn Activated Control palm stones Cu Zn a Initial concentration of Cu 2+ and Zn 2+ = 100 mg/l. Adsorbent concentration = 5 mg/ml. b Difference between metal ion released by the system and the control (sorbent + water). c R b/r = equivalent ratio of metal ion bound to metal ion released. ph of the solution, the temperature, the contact time, and the adsorbent identity and concentration basically affected the rate and extent of adsorption. The absolute amount of Cu 2+ ions adsorbed as well as of Zn 2+ ions increased with an increase in the initial metal ion concentration and solution ph. Measured thermodynamic parameters suggest that the adsorption process was exothermic except for Zn 2+ ion adsorption by human hair where it was endothermic. The Freundlich isotherm model was found to be applicable to the adsorption of Cu 2+ and Zn 2+ ions by the three adsorbents. The presence of light metal ions such as Na + decreased the uptake capacity towards Cu 2+ and Zn 2+ ions. Although an ion-exchange mechanism was found to be involved in the adsorption process, it was not dominant. Finally, even though chicken feathers were less effective in the removal of Cu 2+ and Zn 2+ ions than animal horns, their huge abundance worldwide makes such adsorbents the most practical for the removal of heavy metal ions from aqueous solutions. REFERENCES Ajmal, M., Khan, A., Ahmad, S. and Ahmad, A. (1998) Water Res. 32, Al-Asheh, S., Banat, F. and Mohai, F. (1999) Chemosphere 39, Al-Asheh, S. and Duvnjak, Z. (1997) J. Hazard. Mater. 56, 35. Avery, S. and Tobin, J. (1993) Appl. Environ. Microbiol. 9, Banat, F. and Al-Asheh, S. (2000) Environ. Eng. Policy 2, 85. Benefield, L., Joseph, F. and Barron, L. (1982) Process Chemistry for Water and Wastewater Treatment, Prentice- Hall, Inc., Englefield Cliffs, NJ.

24 416 Fawzi Banat et al./adsorption Science & Technology Vol. 20 No Brown, P., Jefocat, I., Parrish, D., Gill, S. and Graham, E. (2000) Adv. Environ. Res. 4, 19. Chen, J. and Lin, M. (2000) Sep. Sci. Technol. 35, Crist, R., Martin, J., Guptil, P., Eslinger, J. and Crist, D. (1990) Environ. Sci. Technol. 24, 337. Dean, J., Bosqu, F. and Lannouette, K. (1977) Environ. Sci. Technol. 6, 518. Deshkar, A., Bokade, S. and Dara, S. (1990) Water Res. 24, Dronnet, V., Renard, C., Axelos, M. and Thibault, J. (1997) Carbohydr. Polym. 34, 73. Eligwe, A., Okolue, B., Nwambu, O. and Nwoko, I. (1999) Chem. Eng. Technol. 22, 45. Ferro-Garcia, M., Rivera-Utrilla, J., Rodriguez-Grodillo, J. and Bautista-Toledo, I. (1988) Carbon 26, 363. Friedman, H., Harrison, C., Ward, W. and Ludgren, H. (1973) Appl. Polym. Sci. 17, 377. Fumiaki, K., Abe, I., Kamaya, H. and Ueda, I. (2000) Surf. Sci. 67, 131. Garcia, A., Bonen, M., Vick, K., Sadaka, M. and Vuppu, K. (1997) Biosorption Process Science, Blackwell Science, New York. Harris, P. and Ramelow, G. (1990) Environ. Sci. Technol. 24, 220. Hasany, S., Saeed, M. and Ahmed, M. (2000) Sep. Sci. Technol. 35, 379. Ishikawa, S. and Suyama, K. (1998) Appl. Biochem. Biotechnol. 72, 719. Kapoor, A. and Viraraghavan, T. (1998) Water Res. 32, Khalid, N., Ahmed, S., Kiani, S. and Ahmed, J. (1999) Sep. Sci. Technol. 34, Khan, A., Rehman, R. and Khan, A. (1995) Waste Manage. 15, 271. Kumar, P. and Dara, S. (1980) Indian J. Environ. Health 22, 196. Kuyucak, N. and Volesky, B. (1988) Water Pollut. Res., J. 23, 424. Lechavelier, H. and Drobot, W. (1981) Fr. Pat A Lopez-Delgado, A., Perez, C. and Lopez, F. (1996) Carbon 34, 423. Luef, E., Prey, T. and Kubicek, C. (1991) Appl. Microbiol. Biotechnol. 34, 688. Periasamy, K. and Namasivayam, C. (1994) Ind. Eng. Chem., Res. 33, 317. Salim, R., Al-Subu, M. and Qashoa, S. (1994) J. Environ. Sci. Health A 29, Sitting, M. (1981) Handbook of Toxic and Hazardous Chemicals, Noyes Publications, Park Ridge, NJ. Suh, J. and Kim, D. (2000) J. Chem. Technol. Biotechnol. 75, 279. Suyama, K., Fukazawa, Y. and Suzumura, H. (1996) Appl. Biochem. Biotechnol. 57, 67. Tan, T., Chia, C. and Teo, C. (1985) Water Res. 19, 157. Viraraghavan, T. and Dronamraju, M. (1993) Water Res. J. Canada 28, 369. Weber, Jr., W.J. and Morris, J. (1963) J. Sanit. Eng. Div., Am. Soc. Civil Eng. 89, 31.