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Batch zinc removal from aqueous solution using dried animal bones
Separation and Purification Technology 21 (2000) 155 – 164 www.elsevier.com/locate/seppur
Batch zinc removal from aqueous solution using dried animal bones Fawzi Banat *, Sameer Al-Asheh, Fadhel Mohai Department of Chemical Engineering, Jordan Uni6ersity of Science and Technology, P.O Box 3030, Irbid 22110, Jordan Received 4 May 1999; received in revised form 15 June 2000; accepted 18 July 2000
Abstract The effectiveness of animal bones (AB) to adsorb zinc from aqueous solution was studied. Batch kinetics and isotherm studies were carried out to investigate the effect of contact time, initial concentration of the adsorbate, particle size, temperature, pH, and the addition of salt (NaCl) on this adsorption process. It was noted that an increase in the zinc concentration, temperature, and initial pH of the metal solution resulted in an increase in the metal uptake per unit weight of the sorbent. The decrease in the particle size of the sorbent resulted in an increase in the metal uptake per unit weight of the sorbent. The concentration of salt in the metal solution showed significant influence on the zinc ion sorption by the sorbent. Freundlich and Langmuir isotherm models were found to be applicable for the experimental data of Zn2 + sorption by AB. Desorption of metals from pre-loaded AB with zinc ions was carried out with different acid eluants and it was found that H2SO4 is the most effective desorbent. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Sorption; Zinc; Animal bones; Desorption
1. Introduction Zinc pollution is a very serious environmental problem. There are several sources of water pollution by zinc, such as petrochemicals, fertilizer, petroleum refining and steam generation power plant [1]. The harmful effects of zinc are due to the diseases resulted from it, for example an irritant to the skin causing itching and dermatitis, and it may also causes kertinization of the hands and soles of * Corresponding author. Tel.: +962-295111; fax: + 962295123. E-mail address: [email protected] (F. Banat).
the feet [2]. Having in mind the adverse effects of zinc, and other heavy metals, environmental agencies set permissible limits for their levels in drinking water and other types of waters. For example according to World Health Organization (WHO), the permissible limit of Zn2 + is 3 ppm [3]. Considering the harmful effects of heavy metals, it is necessary to remove them from liquid wastes at least to a limit accepted by national and international regulatory agencies before their discharge to the environment. Several methods have been suggested for the removal of toxic metals from wastewater. These include precipitation, complexation, ion exchange and adsorption.
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F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164
The use of adsorbents, e.g. activated carbon and ion exchange resins to remove trace metals from aqueous systems has been widely investigated [4–6]. Because of the high capital and regeneration costs of activated carbon and ion-exchange resins [7], researchers were encouraged to look for other types of adsorbents. Therefore, for the last decade, research has involved materials of biological origins and many forms of biomass have been shown to be effective for the removal of heavy metals [8 – 10]. The uptake of metals by these materials was attributed to their constituents of carbohydrates, proteins and lignin which contain functional groups, such as carboxyl, hydroxyl and amine groups, that are responsible for metal sorption [11]. Therefore, biosorption is a physio-chemical binding of a substance, sorbate, to a biological material. For example, Aksu and Kutsal [12] used the biomass Chlorella 6ulgaris for the removal of lead ions from wastewater, Baldry and Dean [13] used Escherichia coli to sorb copper ions from aqueous solutions. There are also investigators who used agricultural and animal waste materials as biological sorbents. Al-Asheh and Duvnjak [14] used Canola meal, a by-product of Canola oil production from Canola seeds, to remove various heavy metal ions from their aqueous solutions and Lee et al. [15] used Crab shell as animal sorbent for the removal of lead from aqueous solution. In this work, a new biosorbent from animal origin, bones, will be considered for the recovery of heavy metals. The effect of different operating parameters, such as concentration of heavy metals, initial pH, temperature, particle size, addition of salt (NaCl) on the sorption of zinc will be presented. Regeneration of the sorbent will be investigated using different types of eluants.
2. Materials and methods
2.1. Adsorbent Animal bones (AB) were collected from butchers’ shops. The bones were cleaned from meat and fat and then washed with tap water for several times. Then, they were transferred to the oven at
80°C to dry. The dried bones were crushed and milled into different particle sizes in the range of 0.71–2.0 mm and were later used in the sorption tests. It is worth mentioning that no storage problem of the dried ground bones such as bacterial contamination was observed during the coarse of experiments which lasted for about 3 months.
2.2. Batch sorption experiments Fifty ml of Zn2 + solution (in the form ZnSO4 · 7H2O) was transferred into bottles containing certain amount of sorbent to make sorbent concentration 4 mg/l. The metal concentrations were in the range of 10–60 ppm. A shaker (Kottermann, Germany) was used to agitate the mixture at room temperature (unless otherwise stated). Samples were taken at certain periods of time to study the kinetics of the sorption process. The bones were separated from the samples by centrifugation (3000× g, 10 min) and the upper layer was analyzed for the metal under consideration using atomic absorption spectrophotometer (Varian, Spectr AA.10). Neither precipitate nor metal ions adsorbed to the wall of the bottle with the tested metals under the experimental conditions. To study the effect of temperature, sorption tests were carried out in a temperature water bath controlled at 20, 30, 40 and 50°C. The effect of initial pH on the sorption of zinc ion was investigated by preparing zinc solutions adjusted at different pH values using either 0.1 M HCl or 0.01 M NaOH.
2.3. Desorption tests Once desorption equilibrium was achieved, metal-laden sorbents were separated from the solution and transferred into a bottle. Then, certain volume of desorbent, either HCl, or H2SO4 or HNO3, at a specified molarity was added into these bottles. The samples were agitated in the shaker until equilibrium. The solutions were then separated by centrifugation (3000× g, 10 min) and the upper layer was analyzed for the metal under consideration, as previously stated. The amount of metal desorbed is reported as the ratio
F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164
of the total metal released from the laden-bones to that taken up during its sorption.
3. Results and discussion In this research, the use of AB for the uptake of zinc was examined. The preliminary results indicated that after 96 h of adsorption time, the uptake of Zn2 + by bones was 0.1764 mmol/g, when the initial concentration of metal was 0.918 mmol/l and the sorbent concentration was 4 mg/ ml. This gives a removal of 77%. The effect of the initial metal concentration on its uptake, which would give an indication of the sorption capacity of the new sorbent, was tested in Zn2 + solutions with concentrations in the range 10–60 ppm. The results (Fig. 1) showed that an increase in the initial metal concentration resulted in an increase in the sorption of zinc. Similar results for the increase in the metal uptake with the increase in the initial metal concentration was obtained when using the sorbents sphagnum moss peat, moss and canola meal for the sorption of Ni2 + [16], Cu2 + [17] and Cr3 + [18], respectively.
157
To demonstrate the participation of intrapore diffusion in this sorption process, the results of Fig. 1 for the Zn2 + uptake were plotted against the square root of the adsorption time. These plots are shown in Fig. 2 for different Zn2 + concentration. It is seen that most of the data can be represented by straight lines, but these data do not pass through the origin. According to Weber and Morris [19], these linear correlations indicate that intrapore diffusion was involved in the sorption process, but it is not the controlling step, because the lines do not pass through the origin. To examine the effect of the initial pH of the solution on the uptake of Zn2 + by bones, zinc solutions at pH 2, 4 and 5 were prepared in the concentration range 10–60 ppm. Sorbent was added to these solutions to make its concentration 4 mg/ml, and adsorption was followed until equilibrium at room temperature (20°C). The results, Fig. 3, showed that the increase in the initial pH of the solution resulted in an increase of Zn2 + uptake by the bones. The trend for the effect of pH on the biosorption process was also observed by many researchers and was attributed to the increase of negatively charged groups at the surface of the biosorbent cells [19]. For example, the uptake of Cu2 + by the plant sorbent moss was
Fig. 1. Effect of zinc concentration on its uptake by bones. Bones concentration 4 mg/ml.
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Fig. 2. Zinc uptake by bones vs. square root of time at various zinc concentrations.
Fig. 3. Relationship between equilibrium zinc concentration and its uptake at various pH values using 4 mg/ml of bone suspension. Symbols are experimental data and lines are predicted data using Freundlich model.
increased as the initial pH of the metal solution increased from 3.5 to 5 [20], the uptake of Ni2 + by the decaying leaves was increased as the initial pH of the metal solution increased from 1.5 to 9 [21]. Ferro-Garcia et al. [22] investigated the uptake of Zn2 + , Cd2 + and Cu2 + by activated car-
bon obtained from agricultural by-products. They found that the uptake of these metals was increased at high pH levels (pH 5). Those authors attributed their results to the change of the carbon surface with the change in pH. The electrostatic repulsion between cations and positively
F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164 Table 1 Freundlich constants for the sorption of Zn2+ by bones at various pH values pH
k
1/n
R2
2 4 5
0.07 0.308 0.337
0.46 0.69 0.623
0.947 0.99 0.982
charged surface of activated carbon took place at low pH values, while with an increase in pH, metal ions replaced hydrogen ions from the carbon and, therefore, adsorption increased. These explanations can also be applied to the sorbents used in this work assuming that its constituents are similar to these biosorbent reported in the literature. The results of Fig. 3 were found to be well represented by the linearized Freundlich isotherm model 1 ln qe =ln k + ln Ce n
(1)
where qe is the uptake of metal (mmol/g) at the equilibrium concentration Ce (mmol/l), k and 1/n
159
are the Freundlich constants. The values of these constants can be found from the intercept and the slop, respectively, of the linear plot of ln qe versus ln Ce (Fig. 3). These constants at various pH values are displayed in Table 1. The constant k is an indication of the sorption capacity of the sorbent. Table 1 shows that the k values increase with the increase in the initial pH of the solution, which means that sorption capacity is increased with the increase in the initial pH of the solution. To examine the effect of particle size on the sorption process, the sorbent was screened through sieves of sizes less than 0.71, 0.71–1, 1–2, and greater than 2 mm. Each of these sizes was exposed to a series of Zn2 + solution in the concentration range 10–80 ppm and sorbent concentration 4 mg/ml. Experiments were carried out at room temperature (20°C) and uncontrolled pH (5–5.5). The results, Fig. 4, indicate that sorption is increasing with the decrease in the particle size of the sorbent. This increase in the sorption capacity is attributed to the increase in the external surface area of the sorbent available for adsorption. Similar results for the increase in metal uptake with the decrease of particle size was
Fig. 4. Relationship between zinc concentration and its uptake at various particle sizes using 4 mg/ml of bones suspension.
F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164
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Fig. 5. Relationship between equilibrium zinc concentration and its uptake at different temperatures using 4 mg/ml bones suspension. Symbols are experimental data and lines are predicted data using Freundlich model.
obtained by Acacia arabica bark for removal of Cr6 + [22] and by rice-husk ash for removal of Hg2 + from aqueous solutions [23]. It is also known that sorption processes can be influenced by the temperature of the medium. This was studied in this work by conducting a series of isotherms at temperatures of 20, 30, 40 and 50°C. The results are shown in Fig. 5. The Freundlich isotherm constants at different temperature are given in Table 2. The results showed that the sorption process is enhanced by the increase in the temperature. This enhancement could be explained by the dissociation of some compounds available at the surface of the bones, which respond to metal adsorption, thus providing more sites for metal adsorption. Many other authors have reported an increase in metal sorption by biological materials with an increase in temperature, e.g. Singh et al. [24] noticed a higher lead sorption by tea leaves at 70°C than at 25°C. The results of Fig. 5 where also found to be well represented by the following linearized Langmuir isotherm model
1 1 1 1 = + qe KLb Ce KL
(2)
where KL and b are the Langmuir constants related to the capacity and energy of adsorption, respectively. These constants were evaluated from the intercept and the slope of the linear plot of 1/qe versus 1/Ce based on experimental data at various temperatures (plots are not shown). Table 3 shows the KL and b values at various temperatures, and as expected the KL value increases with an increase in temperature. The values of b at different temperatures can be used to determine the change in the apparent enthalpy (DH) and entropy (DS) of adsorption of metal by the adsorbent using the following thermodynamic relations ln b=ln b´−
DH RT
(3)
Table 2 Freundlich isotherm constants at different temperatures for the sorption of Zn2+ by bones Temperature (°C)
k
1/n
R2
20 30 40 50
0.187 0.198 0.205 0.247
0.417 0.195 0.10 0.111
0.956 0.976 0.941 0.94
F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164
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Table 3 Langmuir constants and thermodynamic parameters at different temperatures for the sorption of zinc by animal bones Temperature (°C)
KL (mmol/g)
b (mmol/l)
DG (kJ/mol)
DS (kJ/kmol K)
20 30 40 50
0.181 0.191 0.187 0.210
3.676 18.056 106.8 172.8
3.17 7.29 12.16 13.84
356.0 330.72 304.61 290.0
Fig. 6. Plot of ln b vs. 1/T for sorption of zinc by bones.
DG RT
(4)
DH − DG T
(5)
ln(1/b)= − DS =
where b% is the adsorption energy constant, DG is the free energy change for adsorption of metal by the adsorbent and T is the absolute temperature of the metal solution. DH is determined from the slope of ln b versus 1/T, as shown in Fig. 6, while DG and DS can be determined directly from Eqs. (4) and (5), respectively, for a given temperature. The calculated enthalpy was found to be 107.5 kJ/mol. The positive DH value confirms the endothermic nature of the sorption process and suggests the possibility of a strong bonding be-
tween sorbate and sorbent [7]. The values of DG and DS at different temperatures are listed in Table 3. The thermodynamic parameters, DH, DG and DS, obtained in this work are in the range of those obtained by other researchers for the sorption of lead by tea leaves [7] and for the sorption of copper by moss [20] It is known that salts have significant effect on the biosorption process. In this work sodium chloride (NaCl) was selected as an example to investigate its influence on the sorption of Zn2 + by the bones. Samples of bones were exposed to a series of sodium chloride (NaCl) solutions in the concentration range 0.01–0.05 M. Initial Zn2 + concentration of 40 and 60 ppm were tested in these systems and sorbent concentration was fixed
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at 4 mg/ml. These results are presented in Fig. 7. It is seen that the increase in the salt concentration resulted in a decrease of Zn2 + uptake by bones. This is true up to 0.1 M NaCl, while NaCl concentration in the range of 0.1 – 0.5 M have almost the same depression levels of Zn2 + uptake by bones. The results also show that the addition of salt has a more significant influence at low Zn2 + concentration than at higher Zn2 + concentration. This trend indicates that the binding efficiency decreased when sodium chloride increased in the metal solution, which can be attributed to the competitive effect between heavy metal ions and cations from the salt for the sites available for
the sorption process. Other researchers reported similar results for the decrease in metal uptake with the increase in salt concentration. For example Harris and Ramelow [25] reported that the sorption of Ag+, Cu2 + , Cd2 + , and Zn2 + ions by particulate biomass derived from C. 6ulgaris decreased with an increase in the salt concentration. The metal ions sorbed by biomaterial can be released using various kinds of desorbents. In this work, H2SO4, HNO3 and HCl were tested for the desorption of Zn2 + from bone. Batch desorption experiments were performed by transferring 10 ml of 10 mM of each eluants to a metal-laden bone having a specified metal uptake. The samples were
Fig. 7. Effect of NaCl concentration on the sorption of zinc by bones. Bones concentration: 4 mg/ml.
F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164 Table 4 Adsorption/desorption of zinc ions by bone using various desorbentsa Desorbent
Amount desorbed (%)
H2SO4 HCl HNO3
93.88 67.57 43.32
a
Average uptake of zinc: 0.095 mmol/g.
agitated in a shaker for 24 h. The solutions were then separated by centrifugation, and the upper layer was analyzed for the metal under consideration. Table 4 showed that H2SO4 is a better eluant for desorption than HCl and HNO3. Similar results were also obtained by Al-Asheh and Duvnjak [20] for the desorption of copper from moss. Tests were also carried out to determine the relationship between the volume and concentration of H2SO4, as a desorbent, and the amount of zinc desorbed. The results in Fig. 8 indicate that the desorption of Zn2 + ions from bones is influenced by both the volume of the desorbent and its strength. A higher level of desorption can be attained with higher volume of the desorbent or higher concentration levels of the desorbent. If the concentration of the desorbent was to be in-
163
creased, the volume for the desorption process could be decreased to attain the level of zinc desorption. The same thing can also be achieved with a higher volume of lower morality desorbent. Nonetheless, increasing the volume of desorbent above 10 ml was not beneficial.
4. Conclusions This study showed that animals’ bones could be used as a sorbent for the removal of Zn2 + ions from aqueous solutions. It was noticed that an increase in the metal concentration or an increase in the initial pH of the metal solution or an increase in the temperature of metal solution resulted in an increase of zinc sorption. It was noticed that the decrease in the particle size of the sorbent resulted in an increase of zinc sorption. Addition of salt showed inhibition effect on this sorption process. Freundlich and Langmuir isotherm type models showed reasonably good representation of the equilibrium experimental data. Sulfuric acid can be considered as a good desorbent for the release of metals from the spent AB. The desorption process was affected by both the volume and the concentration of the desorbent.
Fig. 8. Effect of volume and concentration of sulfuric acid on the desorption of zinc from the bone. Bones concentration: 4 mg/ml.
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References [1] J.G. Dean, F.L. Bosqui, K.L. Lanouette, Removing heavy metals from wastewater, Environ. Sci. Technol. 6 (1977) 518 – 524. [2] M. Sitting, Handbook of Toxic and Hazardous Chemicals, Noyes, Park Ridge, New Jersey, 1981. [3] M.C. Van der Leeden, F.L. Troise, D.K. Todd, The Water Encyclopedia, Second Edition, Lewis, Michigan, 1990. [4] C.P. Huang, F.B. Ostovic, Removal of Cadmium by activated carbon adsorption, J. Environ. Eng. 104 (1978) 863 – 878. [5] E. Maliou, M. Malamis, P.O. Sakellarides, Lead and cadmium removal by ion exchange, Wat. Sci. 25 (1992) 133 – 138. [6] M. Sengupta, T.B. Paul, Multicomponent ion exchanges equilibria. I. Zn2 + –Cd2 + –H+ and Cu2 + –Ag+ –H+ on Amberlite IR 120, Reac. Pol. 3 (1985) 217–229. [7] D.K. Singh, D.P. Tiwari, D.N. Saksena, Removal of lead from aqueous solutions by chemically treated tea leaves, Indian J. Environ. Health 35 (1993) 169–177. [8] B. Volesky, Biosorption for metal recovery, Tibtech 5 (1987) 96 – 101. [9] S.V. Avery, J.M. Tobin, Mechanism of adsorption of hard and soft metal ions to Saccharomyces cere6isia and influence of hard and soft anions, Appl. Environ. Microbiol. 9 (1993) 2851 –2856. [10] R.H. Crist, J.R. Martin, P.W. Guptill, J.M. Eslinger, D.R. Crist, Interaction of metals and protons with algae. 2. Ion exchange in adsorption and metal displacement by protons, Environ. Sci. Technol. 24 (1990) 337–342. [11] N. Kuyucak, B. Volesky, A method of metal removal, Wat. Pollut. Res. J. 23 (1988) 424–433. [12] Z. Aksu, T. Kutsal, A biosorption process for removing lead ions from wastewater by using C. Vulgaris, J. Chem. Tech. Biotechnol. 52 (1991) 109–118.
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[13] M.G.C. Baldry, A.C.R. Dean, Copper accumulation by Escherichia Coli strain FE 12/5. 1 uptake during batch culture, Microb. Lett. 15 (1980) 83 – 87. [14] S. Al-Asheh, Z. Duvnjak, Biosorption of chromium by canola meal, Wat. Qual. Res. J. Canada 31 (1996) 319 – 328. [15] M.Y. Lee, J.M. Park, J.W. Yang, Micro precipitation of lead on the surface of crab shell particles, Proc. Biochem. 32 (1997) 671 – 677. [16] Y.S. Ho, D.A. Johnwase, C.F. Forster, Batch nickel removal from aqueous solution by sphagnum moss peat, Wat. Res. 29 (5) (1995) 1327 – 1332. [17] C.K. Lee, K.S. Low, Removal of copper from solution using moss, Environ. Technol. Lett. 10 (1989) 395 – 404. [18] W.J. Weber, J.C. Morris, Kinetics of adsorption on carbon from solution, J. San. Eng. Div. ASCE S A2 (1963) 31. [19] E. Luef, T. Pray, C.P. Kubicek, Biosorption of zinc by fungal mycelial wastes, Appl. Microbiol. Biotechnol. 34 (1991) 688 – 692. [20] S. Al-Asheh, Z. Duvnjak, Adsorption of metal ions by moss, Adv. Environ. Res. 1 (1997) 194 – 212. [21] R. Salim, Removal of nickel from water using decaying leaves — effects of pH and type of leaves, J. Environ. Sci. Health A23 (3) (1988) 183 – 197. [22] M.A. Ferro-Garcia, J. Rivera-Utrilla, J. RodriguezGordillo, I. Bautista-Toledo, Adsorption of zinc, cadmium and copper on activated carbon obtained from agricultural by-products, Carbon 26 (1988) 363 – 373. [23] D.P. Tiwari, D.K. Singh, D.N. Saksena, Hg (II) adsorption from aqueous solutions using rice-husk ash, J. Environ. Eng. 121 (1995) 479 – 481. [24] D.K. Singh, D.N. Saksena, D.P. Tiwari, Removal of chromium (VI) from aqueous solutions, Indian J. Environ. Health 36 (1994) 272 – 277. [25] P.O. Harris, G.J. Ramelow, Binding of metal ions by particulate biomass derived from Chlorella 6ulgaris and Scenedesmus quadricauda, Environ. Sci. Technol. 24 (1990) 220 – 228.
Batch zinc removal from aqueous solution using dried animal bones Fawzi Banat *, Sameer Al-Asheh, Fadhel Mohai Department of Chemical Engineering, Jordan Uni6ersity of Science and Technology, P.O Box 3030, Irbid 22110, Jordan Received 4 May 1999; received in revised form 15 June 2000; accepted 18 July 2000
Abstract The effectiveness of animal bones (AB) to adsorb zinc from aqueous solution was studied. Batch kinetics and isotherm studies were carried out to investigate the effect of contact time, initial concentration of the adsorbate, particle size, temperature, pH, and the addition of salt (NaCl) on this adsorption process. It was noted that an increase in the zinc concentration, temperature, and initial pH of the metal solution resulted in an increase in the metal uptake per unit weight of the sorbent. The decrease in the particle size of the sorbent resulted in an increase in the metal uptake per unit weight of the sorbent. The concentration of salt in the metal solution showed significant influence on the zinc ion sorption by the sorbent. Freundlich and Langmuir isotherm models were found to be applicable for the experimental data of Zn2 + sorption by AB. Desorption of metals from pre-loaded AB with zinc ions was carried out with different acid eluants and it was found that H2SO4 is the most effective desorbent. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Sorption; Zinc; Animal bones; Desorption
1. Introduction Zinc pollution is a very serious environmental problem. There are several sources of water pollution by zinc, such as petrochemicals, fertilizer, petroleum refining and steam generation power plant [1]. The harmful effects of zinc are due to the diseases resulted from it, for example an irritant to the skin causing itching and dermatitis, and it may also causes kertinization of the hands and soles of * Corresponding author. Tel.: +962-295111; fax: + 962295123. E-mail address: [email protected] (F. Banat).
the feet [2]. Having in mind the adverse effects of zinc, and other heavy metals, environmental agencies set permissible limits for their levels in drinking water and other types of waters. For example according to World Health Organization (WHO), the permissible limit of Zn2 + is 3 ppm [3]. Considering the harmful effects of heavy metals, it is necessary to remove them from liquid wastes at least to a limit accepted by national and international regulatory agencies before their discharge to the environment. Several methods have been suggested for the removal of toxic metals from wastewater. These include precipitation, complexation, ion exchange and adsorption.
1383-5866/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 0 ) 0 0 1 9 9 - 4
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F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164
The use of adsorbents, e.g. activated carbon and ion exchange resins to remove trace metals from aqueous systems has been widely investigated [4–6]. Because of the high capital and regeneration costs of activated carbon and ion-exchange resins [7], researchers were encouraged to look for other types of adsorbents. Therefore, for the last decade, research has involved materials of biological origins and many forms of biomass have been shown to be effective for the removal of heavy metals [8 – 10]. The uptake of metals by these materials was attributed to their constituents of carbohydrates, proteins and lignin which contain functional groups, such as carboxyl, hydroxyl and amine groups, that are responsible for metal sorption [11]. Therefore, biosorption is a physio-chemical binding of a substance, sorbate, to a biological material. For example, Aksu and Kutsal [12] used the biomass Chlorella 6ulgaris for the removal of lead ions from wastewater, Baldry and Dean [13] used Escherichia coli to sorb copper ions from aqueous solutions. There are also investigators who used agricultural and animal waste materials as biological sorbents. Al-Asheh and Duvnjak [14] used Canola meal, a by-product of Canola oil production from Canola seeds, to remove various heavy metal ions from their aqueous solutions and Lee et al. [15] used Crab shell as animal sorbent for the removal of lead from aqueous solution. In this work, a new biosorbent from animal origin, bones, will be considered for the recovery of heavy metals. The effect of different operating parameters, such as concentration of heavy metals, initial pH, temperature, particle size, addition of salt (NaCl) on the sorption of zinc will be presented. Regeneration of the sorbent will be investigated using different types of eluants.
2. Materials and methods
2.1. Adsorbent Animal bones (AB) were collected from butchers’ shops. The bones were cleaned from meat and fat and then washed with tap water for several times. Then, they were transferred to the oven at
80°C to dry. The dried bones were crushed and milled into different particle sizes in the range of 0.71–2.0 mm and were later used in the sorption tests. It is worth mentioning that no storage problem of the dried ground bones such as bacterial contamination was observed during the coarse of experiments which lasted for about 3 months.
2.2. Batch sorption experiments Fifty ml of Zn2 + solution (in the form ZnSO4 · 7H2O) was transferred into bottles containing certain amount of sorbent to make sorbent concentration 4 mg/l. The metal concentrations were in the range of 10–60 ppm. A shaker (Kottermann, Germany) was used to agitate the mixture at room temperature (unless otherwise stated). Samples were taken at certain periods of time to study the kinetics of the sorption process. The bones were separated from the samples by centrifugation (3000× g, 10 min) and the upper layer was analyzed for the metal under consideration using atomic absorption spectrophotometer (Varian, Spectr AA.10). Neither precipitate nor metal ions adsorbed to the wall of the bottle with the tested metals under the experimental conditions. To study the effect of temperature, sorption tests were carried out in a temperature water bath controlled at 20, 30, 40 and 50°C. The effect of initial pH on the sorption of zinc ion was investigated by preparing zinc solutions adjusted at different pH values using either 0.1 M HCl or 0.01 M NaOH.
2.3. Desorption tests Once desorption equilibrium was achieved, metal-laden sorbents were separated from the solution and transferred into a bottle. Then, certain volume of desorbent, either HCl, or H2SO4 or HNO3, at a specified molarity was added into these bottles. The samples were agitated in the shaker until equilibrium. The solutions were then separated by centrifugation (3000× g, 10 min) and the upper layer was analyzed for the metal under consideration, as previously stated. The amount of metal desorbed is reported as the ratio
F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164
of the total metal released from the laden-bones to that taken up during its sorption.
3. Results and discussion In this research, the use of AB for the uptake of zinc was examined. The preliminary results indicated that after 96 h of adsorption time, the uptake of Zn2 + by bones was 0.1764 mmol/g, when the initial concentration of metal was 0.918 mmol/l and the sorbent concentration was 4 mg/ ml. This gives a removal of 77%. The effect of the initial metal concentration on its uptake, which would give an indication of the sorption capacity of the new sorbent, was tested in Zn2 + solutions with concentrations in the range 10–60 ppm. The results (Fig. 1) showed that an increase in the initial metal concentration resulted in an increase in the sorption of zinc. Similar results for the increase in the metal uptake with the increase in the initial metal concentration was obtained when using the sorbents sphagnum moss peat, moss and canola meal for the sorption of Ni2 + [16], Cu2 + [17] and Cr3 + [18], respectively.
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To demonstrate the participation of intrapore diffusion in this sorption process, the results of Fig. 1 for the Zn2 + uptake were plotted against the square root of the adsorption time. These plots are shown in Fig. 2 for different Zn2 + concentration. It is seen that most of the data can be represented by straight lines, but these data do not pass through the origin. According to Weber and Morris [19], these linear correlations indicate that intrapore diffusion was involved in the sorption process, but it is not the controlling step, because the lines do not pass through the origin. To examine the effect of the initial pH of the solution on the uptake of Zn2 + by bones, zinc solutions at pH 2, 4 and 5 were prepared in the concentration range 10–60 ppm. Sorbent was added to these solutions to make its concentration 4 mg/ml, and adsorption was followed until equilibrium at room temperature (20°C). The results, Fig. 3, showed that the increase in the initial pH of the solution resulted in an increase of Zn2 + uptake by the bones. The trend for the effect of pH on the biosorption process was also observed by many researchers and was attributed to the increase of negatively charged groups at the surface of the biosorbent cells [19]. For example, the uptake of Cu2 + by the plant sorbent moss was
Fig. 1. Effect of zinc concentration on its uptake by bones. Bones concentration 4 mg/ml.
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Fig. 2. Zinc uptake by bones vs. square root of time at various zinc concentrations.
Fig. 3. Relationship between equilibrium zinc concentration and its uptake at various pH values using 4 mg/ml of bone suspension. Symbols are experimental data and lines are predicted data using Freundlich model.
increased as the initial pH of the metal solution increased from 3.5 to 5 [20], the uptake of Ni2 + by the decaying leaves was increased as the initial pH of the metal solution increased from 1.5 to 9 [21]. Ferro-Garcia et al. [22] investigated the uptake of Zn2 + , Cd2 + and Cu2 + by activated car-
bon obtained from agricultural by-products. They found that the uptake of these metals was increased at high pH levels (pH 5). Those authors attributed their results to the change of the carbon surface with the change in pH. The electrostatic repulsion between cations and positively
F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164 Table 1 Freundlich constants for the sorption of Zn2+ by bones at various pH values pH
k
1/n
R2
2 4 5
0.07 0.308 0.337
0.46 0.69 0.623
0.947 0.99 0.982
charged surface of activated carbon took place at low pH values, while with an increase in pH, metal ions replaced hydrogen ions from the carbon and, therefore, adsorption increased. These explanations can also be applied to the sorbents used in this work assuming that its constituents are similar to these biosorbent reported in the literature. The results of Fig. 3 were found to be well represented by the linearized Freundlich isotherm model 1 ln qe =ln k + ln Ce n
(1)
where qe is the uptake of metal (mmol/g) at the equilibrium concentration Ce (mmol/l), k and 1/n
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are the Freundlich constants. The values of these constants can be found from the intercept and the slop, respectively, of the linear plot of ln qe versus ln Ce (Fig. 3). These constants at various pH values are displayed in Table 1. The constant k is an indication of the sorption capacity of the sorbent. Table 1 shows that the k values increase with the increase in the initial pH of the solution, which means that sorption capacity is increased with the increase in the initial pH of the solution. To examine the effect of particle size on the sorption process, the sorbent was screened through sieves of sizes less than 0.71, 0.71–1, 1–2, and greater than 2 mm. Each of these sizes was exposed to a series of Zn2 + solution in the concentration range 10–80 ppm and sorbent concentration 4 mg/ml. Experiments were carried out at room temperature (20°C) and uncontrolled pH (5–5.5). The results, Fig. 4, indicate that sorption is increasing with the decrease in the particle size of the sorbent. This increase in the sorption capacity is attributed to the increase in the external surface area of the sorbent available for adsorption. Similar results for the increase in metal uptake with the decrease of particle size was
Fig. 4. Relationship between zinc concentration and its uptake at various particle sizes using 4 mg/ml of bones suspension.
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Fig. 5. Relationship between equilibrium zinc concentration and its uptake at different temperatures using 4 mg/ml bones suspension. Symbols are experimental data and lines are predicted data using Freundlich model.
obtained by Acacia arabica bark for removal of Cr6 + [22] and by rice-husk ash for removal of Hg2 + from aqueous solutions [23]. It is also known that sorption processes can be influenced by the temperature of the medium. This was studied in this work by conducting a series of isotherms at temperatures of 20, 30, 40 and 50°C. The results are shown in Fig. 5. The Freundlich isotherm constants at different temperature are given in Table 2. The results showed that the sorption process is enhanced by the increase in the temperature. This enhancement could be explained by the dissociation of some compounds available at the surface of the bones, which respond to metal adsorption, thus providing more sites for metal adsorption. Many other authors have reported an increase in metal sorption by biological materials with an increase in temperature, e.g. Singh et al. [24] noticed a higher lead sorption by tea leaves at 70°C than at 25°C. The results of Fig. 5 where also found to be well represented by the following linearized Langmuir isotherm model
1 1 1 1 = + qe KLb Ce KL
(2)
where KL and b are the Langmuir constants related to the capacity and energy of adsorption, respectively. These constants were evaluated from the intercept and the slope of the linear plot of 1/qe versus 1/Ce based on experimental data at various temperatures (plots are not shown). Table 3 shows the KL and b values at various temperatures, and as expected the KL value increases with an increase in temperature. The values of b at different temperatures can be used to determine the change in the apparent enthalpy (DH) and entropy (DS) of adsorption of metal by the adsorbent using the following thermodynamic relations ln b=ln b´−
DH RT
(3)
Table 2 Freundlich isotherm constants at different temperatures for the sorption of Zn2+ by bones Temperature (°C)
k
1/n
R2
20 30 40 50
0.187 0.198 0.205 0.247
0.417 0.195 0.10 0.111
0.956 0.976 0.941 0.94
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Table 3 Langmuir constants and thermodynamic parameters at different temperatures for the sorption of zinc by animal bones Temperature (°C)
KL (mmol/g)
b (mmol/l)
DG (kJ/mol)
DS (kJ/kmol K)
20 30 40 50
0.181 0.191 0.187 0.210
3.676 18.056 106.8 172.8
3.17 7.29 12.16 13.84
356.0 330.72 304.61 290.0
Fig. 6. Plot of ln b vs. 1/T for sorption of zinc by bones.
DG RT
(4)
DH − DG T
(5)
ln(1/b)= − DS =
where b% is the adsorption energy constant, DG is the free energy change for adsorption of metal by the adsorbent and T is the absolute temperature of the metal solution. DH is determined from the slope of ln b versus 1/T, as shown in Fig. 6, while DG and DS can be determined directly from Eqs. (4) and (5), respectively, for a given temperature. The calculated enthalpy was found to be 107.5 kJ/mol. The positive DH value confirms the endothermic nature of the sorption process and suggests the possibility of a strong bonding be-
tween sorbate and sorbent [7]. The values of DG and DS at different temperatures are listed in Table 3. The thermodynamic parameters, DH, DG and DS, obtained in this work are in the range of those obtained by other researchers for the sorption of lead by tea leaves [7] and for the sorption of copper by moss [20] It is known that salts have significant effect on the biosorption process. In this work sodium chloride (NaCl) was selected as an example to investigate its influence on the sorption of Zn2 + by the bones. Samples of bones were exposed to a series of sodium chloride (NaCl) solutions in the concentration range 0.01–0.05 M. Initial Zn2 + concentration of 40 and 60 ppm were tested in these systems and sorbent concentration was fixed
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at 4 mg/ml. These results are presented in Fig. 7. It is seen that the increase in the salt concentration resulted in a decrease of Zn2 + uptake by bones. This is true up to 0.1 M NaCl, while NaCl concentration in the range of 0.1 – 0.5 M have almost the same depression levels of Zn2 + uptake by bones. The results also show that the addition of salt has a more significant influence at low Zn2 + concentration than at higher Zn2 + concentration. This trend indicates that the binding efficiency decreased when sodium chloride increased in the metal solution, which can be attributed to the competitive effect between heavy metal ions and cations from the salt for the sites available for
the sorption process. Other researchers reported similar results for the decrease in metal uptake with the increase in salt concentration. For example Harris and Ramelow [25] reported that the sorption of Ag+, Cu2 + , Cd2 + , and Zn2 + ions by particulate biomass derived from C. 6ulgaris decreased with an increase in the salt concentration. The metal ions sorbed by biomaterial can be released using various kinds of desorbents. In this work, H2SO4, HNO3 and HCl were tested for the desorption of Zn2 + from bone. Batch desorption experiments were performed by transferring 10 ml of 10 mM of each eluants to a metal-laden bone having a specified metal uptake. The samples were
Fig. 7. Effect of NaCl concentration on the sorption of zinc by bones. Bones concentration: 4 mg/ml.
F. Banat et al. / Separation/Purification Technology 21 (2000) 155–164 Table 4 Adsorption/desorption of zinc ions by bone using various desorbentsa Desorbent
Amount desorbed (%)
H2SO4 HCl HNO3
93.88 67.57 43.32
a
Average uptake of zinc: 0.095 mmol/g.
agitated in a shaker for 24 h. The solutions were then separated by centrifugation, and the upper layer was analyzed for the metal under consideration. Table 4 showed that H2SO4 is a better eluant for desorption than HCl and HNO3. Similar results were also obtained by Al-Asheh and Duvnjak [20] for the desorption of copper from moss. Tests were also carried out to determine the relationship between the volume and concentration of H2SO4, as a desorbent, and the amount of zinc desorbed. The results in Fig. 8 indicate that the desorption of Zn2 + ions from bones is influenced by both the volume of the desorbent and its strength. A higher level of desorption can be attained with higher volume of the desorbent or higher concentration levels of the desorbent. If the concentration of the desorbent was to be in-
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creased, the volume for the desorption process could be decreased to attain the level of zinc desorption. The same thing can also be achieved with a higher volume of lower morality desorbent. Nonetheless, increasing the volume of desorbent above 10 ml was not beneficial.
4. Conclusions This study showed that animals’ bones could be used as a sorbent for the removal of Zn2 + ions from aqueous solutions. It was noticed that an increase in the metal concentration or an increase in the initial pH of the metal solution or an increase in the temperature of metal solution resulted in an increase of zinc sorption. It was noticed that the decrease in the particle size of the sorbent resulted in an increase of zinc sorption. Addition of salt showed inhibition effect on this sorption process. Freundlich and Langmuir isotherm type models showed reasonably good representation of the equilibrium experimental data. Sulfuric acid can be considered as a good desorbent for the release of metals from the spent AB. The desorption process was affected by both the volume and the concentration of the desorbent.
Fig. 8. Effect of volume and concentration of sulfuric acid on the desorption of zinc from the bone. Bones concentration: 4 mg/ml.
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