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Crab shell for the removal of heavy metals from aqueous solution
PII: S0043-1354(01)00099-9
Wat. Res. Vol. 35, No. 15, pp. 3551–3556, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
CRAB SHELL FOR THE REMOVAL OF HEAVY METALS FROM AQUEOUS SOLUTION H. K. AN, B. Y. PARK and D. S. KIM* Department of Environmental Science, Catholic University of Daegu, Gyeongbuk, South Korea (First received 24 July 2000; accepted in revised form 6 February 2001) Abstract}The ability of crab shell to remove heavy metals from aqueous solution was evaluated by comparing with that of several sorbents (cation exchange resin, zeolite, granular activated carbon, powdered activated carbon). All experiments were conducted using several heavy metal ion solutions (Pb, Cd, Cu, Cr). The orders of heavy metal removal capacity and initial heavy metal removal rate were found as crab shell>cation exchange resin>zeolite>powdered activated carbongranular activated carbon. Therefore, crab shell is satisfactory as a good biosorbent for the heavy metal removal. The study indicates that the removal of these heavy metals is selective, with Pb and Cr being removed in preference to Cd and Cu. The sorption equilibrium of heavy metal ions on sorbents was modeled on the applications of Langmuir and Freundlich. # 2001 Elsevier Science Ltd. All rights reserved Key words}biosorbents, heavy metal, crab shell, cation exchange resin, zeolite, granular activated carbon, powdered activated carbon
INTRODUCTION
The presence of heavy metals in the environment can be detrimental to a variety of living species including human. Metals can be distinguished from other toxic pollutants, since they are non-biodegradable and can accumulate in living tissues, thus becoming concentrated throuhout the food chain. A variety of industries are responsible for the release of heavy metals into the environment through their wastewater. Many researchers have made efforts to remove these heavy metals from industrial wastewater using several processing method such as chemical precipitation, evaporation, ion exchange, cementation electrolysis and reverse osmosis (Janson et al., 1982; Grosse, 1986). However these conventional technologies appear to be inadequate and expensive. They often create secondary problems with metal-bearing sludge (Brady et al., 1994). Accordingly, the metal removal capacities of various biological materials have been focused to remove toxic metals from dilute wastewaters. The high efficiency of various bacteria, yeast, fungi and algae to uptake heavy metals has been observed over two decades (Kuyuck and Volesky, 1988). The phenomenon that metallic ions are concentrated by even dead cell through purely physico-chemical process is called ‘‘biosorption’’. Even denatured cells are able to sequester hazardous heavy metals and *Author to whom all correspondence should be addressed. E-mail: [email protected]
radioactive elements in wastewater without considering the viability of cells. Precious metals that sorbed on cell can be desorbed by proper treatment and biomass can be reused repeatedly (Brierley, 1990). Metal sequestration by different parts of the cell can occur via complexation, coordination, chelation of metallic ions, ion exchange, adsorption, and inorganic microprecipitation (Scott and Palmer, 1990; Peng and Koon, 1993). The major interesting features of newly developed biosorbent materials are high versatility, metal selectivity, no concentration dependence, high tolerance for organics and regeneration (Volesky, 1990). These properties coupled with high uptakes and rapid kinetics of the biosorption system allowed engineering of new and highly effective yet simple industrial biosorbent for heavy metal removal processes. Microorganisms have a problem about separation from solution if heavy metal removal process is operated continuously. Even though the problem of separation can be solved by the immobilization, the operation cost is increased and several problems caused by desorption will be occurred. Crab shell can be considered to be competent as a new biosorbent satisfying these conditions because of the granular state. Total crab landings have reported a steady increase over the 1990–1997 period from 8.87 108 tons in 1990 to almost 1.2 109 tons in 1997 in global (FAO, 2000). Most of crab products are used for food processing to be made into canned crabmeat or
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frozen goods. The crab shells can be obtained cheap from byproducts or wastes in the middle of the process. In the present work, the ability of raw crab shell to remove heavy metals (Pb, Cd, Cu, Cr) from aqueous solution was investigated by comparing with that of cation exchange resin, zeolite, granular activated carbon, and powdered activated carbon. The ability for the removal of heavy metal was assessed by the heavy metal removal capacity, removal rate and removal efficiency.
MATERIALS AND METHODS
Materials-sorbents Crab shell of Chinonecetes opilio used in this experiment was obtained as a waste from the food industry in a crabmeat processing plant around eastern coast of Korea. It was separated from the crabmeat by steaming or boiling and washed then dried without any special processes. After pulverization to a geometric mean particle size of 420– 841 mm (20–40 mesh), it was stored at room temperature. The specific surface area and average pore diameter of crab shell were 13.35 m2/g and 3685.8 nm by BET method, respectively. The chemical compositions of crab shell were known to protein (29.19%), ash (40.60%), lipid (1.35%), chitin (26.65%) as dry weight basis (No and Meyers, 1997). A Dowex 50W strongly acidic cation exchange resin (CER), gel type of 354–1000 mm (16–42 mesh), was used for the present work. It has high cation exchange capacity as 1.8 meq/ml. Powdered activated carbon (PAC) made from coconut shell char (Indine No. is 1050 mg/g) and zeolite was sized 44–74 mm (200–300 mesh) and 841–2830 mm (7–20 mesh), respectively. Granular activated carbon of which BET surface area and average pore diameter were 1032.78 m2/g and 246.8 nm, respectively, was sized 1000– 1410 mm (12–16 mesh).
RESULTS AND DISCUSSION
Residual heavy metal ions The residual heavy metals (Pb, Cd, Cr, Cu) concentrations in the removal of heavy metals by crab shell and several chemical sorbents (CER, zeolite, GAC, PAC) were examined when the initial heavy metal and sorbent concentrations was 0.5 mmol/l and 1.0 g/l, respectively (Fig. 1). In all the cases of crab shell (Fig. 1(a)) and CER (Fig. 1(b)), the residual heavy metal concentration was decreased abruptly according to time and maintained below 0.01 mmol/l after 24 h, however, the time required to reach an equilibrium state were 6 and 12 h, respectively. In the case of zeolite (Fig. 1(c)), the order of intimacy to heavy metal were repealed definitely and was Pb>CdCu>Cr. The heavy metal removal rate was lower and the time required to reach an equilibrium state was shorter as the heavy metal could not be removed. The heavy metal removal by zeolite was not so great compared to those by crab shell and CER. In the results of GAC (Fig. 1(d)) and PAC (Fig. 1(e)), the definite difference between them was not found and the variation of residual heavy metal concentration according to time was not so great compared to the results of other sorbents. Therefore,
Sorption experiments Sorption experiments were carried out in batches as follow. Stock heavy metal solutions (Pb, Cd, Cu, Cr) were prepared as 1 mM by dissolving nitrate salt of analytical grade in distilled deionized water. Experiments were carried out in 300 ml Erlenmeyer flasks containing 250 ml with different heavy metal concentrations. The initial pH of the solution was adjusted to 5.0 by adding small amounts of dilute NaOH or HNO3 solution. To each flask was added the same amount of sorbents (0.25 g), and then all the flasks were sealed with silicon cap to minimize evaporation and shaken at 150 rpm in rotary shaking incubator (308C, 24 h). Samples of 1.8 ml were taken from each flask at the proper time period and centrifuged (7200 g) immediately for 10 min. The heavy metal concentration in the supernatant was analyzed by atomic absorption spectrophotometer (Unicam 929/1071). Removed heavy metal amounts per unit sorbent dry weight were calculated from heavy metal mass balance yield. The removed heavy metal amounts by sorbent was calculated as: Q ¼ VðC0 C1 Þ=1000 M, where Q is the specific metal removal (mmol/g sorbent), V is the volume of metal solution (l), C0 is the initial concentration of metal in solution (mmol/l), C1 is the concentration of metal in solution (mmol/l), and M is the dry weight of sorbent (g). The initial heavy metal removal rate was measured by calculating the slope from the plot of the removed heavy metal amounts per sorbent dry weight (mmol/g sorbent) vs. time (min) at t ¼ 0. The definition of percent removal is as follows, % removal={ðC0 C1 Þ=C0 g 100.
Fig. 1. Time courses of heavy metals removal by (a) crab shell, (b) CER, (c) zeolite, (d) GAC, (e) PAC for various heavy metals; (*) Pb, (*) Cd, (.) Cr, (5) Cu.
Crab shell for the removal of heavy metals
contrary to common sense, we cannot expect the heavy metal removal by activated carbons. Heavy metal removal capacity In order to compare the heavy metal removal capacity, the removed heavy metal amounts per unit sorbent dry weight according to the residual heavy metal concentration in equilibrium state was observed (Fig. 2). In our experimental ranges, the removed heavy metal amount was increased as the increase of residual heavy metal concentration. Above a certain residual heavy metal concentration, the removed heavy metal amounts were shown as gentle increase or constant value. In the case of Pb (Fig. 2(a)), the removal capacity of crab shell was higher than those of any other sorbents and it was followed by those of CER, zeolite, PAC and GAC. Especially, the heavy metal removal capacities of PAC and GAC were remarkably lower than those of any other sorbents. Also, in the cases of Cd (Fig. 2(b)) and Cr (Fig. 2(d)), the trend was similar to the case of Pb. In the case of Cd, the removal capacity of crab shell was considerably higher than those of any other sorbents. In Cr, the removal was not accomplished well comparing to the cases of other metals, especially, no removals by zeolite, PAC and GAC were occurred. In the case of Cu (Fig. 2(c)), the heavy metal removal capacity of CER was slightly higher than that of crab shell. The removal trend was similar to those of any other metals. The summary results of Fig. 2 were applied to Langmuir and Freundlich isotherm model
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(Table 1). In our experimental ranges, generally, the application of Langmuir isotherm model was more appropriate than that of Freundlich model and the applicability of PAC and GAC was not found. A similar result was reported by Kapoor et al. (1999) that the adsorption results of Pb, Cd and Cu on activated carbon could not applied well on both the Langmuir and Frendlich isotherm model. In order to investigate the heavy metal removal capacity quantitatively, q0:1 and q1:0 values were assessed where q0:1 and q1:0 values were the heavy metal removal capacities when the residual heavy metal concentrations were 0.1 and 1.0 mmol/l, respectively (Table 1). As shown in Table 1, the order of q0:1 and q1:0 values was crab shell>CER> zeolite>PACGAC in all heavy metals. The order of q0.1 and q1.0 values in crab shell according to heavy metals was Cd>Pb>CrCu. These results are in agreement with the findings of Wilson (1993), who reported that Cd was the most effectively removed by seaweed biomasses compared with a range of other metallic ions. Also, the order by four types of algae was Cd>Cu>Ni (Williams et al., 1998). However, the order of heavy metal removal by fungus, Rhizopus arrhizus, was Pb>Cu>Cd (Brady and Tobin, 1995), and the order by Phormidium laminosum was Pb>Cd>Cu>Zn>Cr>Ni (Sampedro et al., 1995). The order of heavy metal removal was correlated to the ionic radius (Tobin et al., 1984) and to the covalent index (Brady and Tobin, 1995) which is the function of electronegativity and ionic radius, therefore the order was shown as Pb>Cu>Cd, which was slightly different from ours. By the report of Leush
Fig. 2. The equilibrium isotherms of (a) Pb, (b) Cd, (c) Cu, (d) Cr removal by several sorbents; (*) crab shell, (*) CER, (&) zeolite, (m) GAC, (4) PAC.
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H. K. An et al. Table 1. Comparison of the removed metal amounts at q0:1 and q1:0 between crab shell and chemical sorbents q0.1a (mmol/g)
q1.0a (mmol/g)
0.90 0.86 0.83 0.86 0.93
0.96 0.97 0.48 0.03 0.12
1.29 1.23 0.54 0.08 0.13
0.99 0.99 0.99 0.99 0.96
0.95 0.91 0.94 0.96 0.79
1.37 0.90 0.20 0.01 0.01
1.77 1.20 0.27 0.03 0.03
Crab shell CER Zeolite GAC PAC
0.82 0.98 0.99 0.89 0.90
0.71 0.87 0.95 0.95 0.95
0.88 0.94 0.20 0.04 0.05
0.98 1.18 0.23 0.08 0.07
Crab shell CER Zeolite GAC PAC
0.93 0.99 0.54 0.96 0.57
0.84 0.99 0.60 0.99 0.61
0.95 0.65 0.04 0.01 0.02
1.06 0.84 0.04 0.04 0.02
Model (r2 value)
Metals
Materials
Langmuir
Freundlich
Pb
Crab shell CER Zeolite GAC PAC
0.99 0.99 0.99 0.78 0.93
Cd
Crab shell CER Zeolite GAC PAC
Cu
Cr
a
q0:1 and q1:0 represent the removed metal amounts (mmol/g) at the equilibrium concentrations of 0.1 and 1.0 mmol/l, respectively.
et al. (1995), the order was Pb>Cd>Cu. Therefore, we can see that the order can be changed by the sort of sorbents since the heavy metal removal mechanism is variable according to the sort of sorbents and metals. Tsezos et al. (1996) have stated that the affinity of a metal ionic species for a ligand binding site on the biomass surface is likely to be affected by the chemical coordination characteristics of that metal ion. Even though the order of heavy metal removal was changed by the sort of sorbents and metals, generally, Pb and Cd could be easily removed by biosorbents. Heavy metal removal rate With the heavy metal removal capacity in equilibrium state, the initial heavy metal removal rate is very important factor for the reactor design and the process optimization in heavy metal removal process. The initial heavy metal removal rates of crab shell and several sorbents according to the increase of initial heavy metal concentration were shown in Fig. 3. The heavy metal removal rate was increased as the increase of initial heavy metal concentration and was increased slowly or maintained constant above a certain concentration. The heavy metal removal rate was in the order of crab shell>CER>zeolite> PACGAC in all heavy metals used in this experiment. Especially, in Pb removal by crab shell (Fig. 3(a)), the Pb removal rate was shown as 3.0 mmol/g-min when the initial Pb concentration was increased to 1.0 mmol/l, which was very high value compared to those of other sorbents. Therefore, we can see that crab shell is satisfactory as a
good biosorbent for the heavy metal removal because of the high heavy metal removal rate and removal capacity. The heavy metal removal rate was changed by the sort of heavy metals, and the order was shown as Pb>Cd>CuCr. By the above results, the order concerning the heavy metal removal capacity was not correlated perfectly with that about the heavy metal removal rate. Heavy metal removal efficiency When the heavy metal removal process is operated continuously, the long hydraulic retention time such as 24 h is ineffective in operation aspect. Therefore, the operation time for metal removal should be less than 12 h at most. Metal removal efficiency according to the increase of initial metal concentration was observed at the operation time of 12 h (Fig. 4). In Pb removal (Fig. 4(a)), from the results of crab shell and CER, both the removal efficiencies were over 97% up to the initial Pb concentration of 1.0 mmol/l and decreased abruptly above 1.0 mmol/l. The results of crab shell and CER were 45 and 38% at the initial concentration of 3.0 mmol/l, respectively. Comparing the results of crab shell and CER, the removal efficiency of crab shell was a little higher than that of CER at the high initial concentration of 3.0 mmol/l even though the result of crab shell was similar to that of CER at relatively low initial Pb concentration. In the case of zeolite, the removal efficiency was over 96% below the initial concentration of 0.25 mmol/l, and it was decreased to 18% at 3.0 mmol/l. The removal efficiencies of PAC and GAC decreased from 76 and 31% to 5 and 4% as the
Crab shell for the removal of heavy metals
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Fig. 3. Comparison of the initial (a) Pb, (b) Cd, (c) Cu, (d) Cr removal rate among several sorbents; (*) crab shell, (*) CER, (&) zeolite, (m) GAC, (4) PAC.
Fig. 4. Heavy metal removal efficiencies of (a) Pb, (b) Cd, (c) Cu, (d) Cr in several sorbents; (*) crab shell, (*) CER, (&) zeolite, (m) GAC, (4) PAC.
initial concentration increased from 1.0 to 3.0 mmol/ l, respectively. Generally, the removal efficiencies of Cd were lower than those of Pb (Fig. 4(b)). The removal efficiency of crab shell was the highest among the sorbents. While the removal efficiency of CER was from 38% as low to 86% as high, that of crab shell was over 90% below 1.0 mmol/l and fell down rapidly over 1.0 mol/l. The removal efficiency of zeolite was over 60% below 0.25 mmol/l but
decreased to 15% at 3.0 mmol/l. In the cases of GAC and PAC, the removal efficiency was very low as below 6% even at low concentration. From the results of Cr (Fig. 4(c)) and Cu (Fig. 4 (d)) removal, the removal efficiencies of crab shell were higher than those of CER below the low concentration of 1.0 mmol/l. However, the results were reversed above 3.0 mmol/l. The Cr removal efficiencies of crab shell and CER were 28 and 38%, respectively, at the high concentration of 3.0 mmol/l.
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H. K. An et al.
Also, the Cu removal efficiency of crab shell (35%) was lower than that of CER (43%) at 3.0 mmol/l. Especially the removal efficiencies of zeolite was lower compared to the cases of other metals. Undoubtedly, the removal efficiencies of GAC and PAC were the lowest. By the report of Wilkins and Yang (1996), the removal efficiencies in Cu, Cd and Zn by GAC at pH 4.5 during 1 h operation were very low as 42–53%, 15–40% and 6–10%, respectively. Huang and Huang (1996) observed that Aspergillus oryzae was superior to activated carbon for adsorption of Cu. Kapoor et al. (1999) reported that Pb, Cd and Cu removals were higher for Aspergillus niger biomass than for activated carbon. Suh et al. (2000) also stated that the removed Pb amounts by Aureobasidium pullulans, Saccharomyces cerevisiae and activated sludge were much higher than those by GAC and PAC. CONCLUSIONS
In order to examine the availability and effectiveness of crab shell for the removal of heavy metals from aqueous solutions, the heavy metal removal capacity and removal rate of crab shell were observed in comparison with those of CER, zeolite, GAC and PAC using four heavy metals (Pb, Cd, Cr, Cu). In the case of crab shell using all heavy metals, the equilibrium state reached within 12 h in our experimental ranges. The heavy metal removal capacity and removal rate were affected by the sorts of sorbents and metals. In the cases of Pb, Cd and Cr, the removal capacity of crab shell was higher than those of any other sorbents and it was followed by those of CER, zeolite, PAC and GAC. However, in the case of Cu, the removal capacity of crab shell was slightly lower than that of CER. In heavy metal removal by crab shell, the application of Langmuir isotherm model was more appropriate than that of Freundlich model. The order of heavy metal removal capacity in crab shell was Cd>Pb>CrCu. Pb and Cd can be easily removed by crab shell even though the order is slightly differed by the sort of sorbents. Like the case of removal capacity, the heavy metal removal rate was in the order of crab shell>CER>zeolite>PACGAC. These properties in conjunction with the low cost of crab shell could provide an economic effluent treatment system. Under the heavy metal concentration of 1.0 mmol/l, the heavy metal removal efficiency of crab shell was maintained as 93–100%, which was much higher than those of any other sorbents. Acknowledgements}This work was supported by Korea Research Foundation Grant. (KRF-2000-041-E00459).
REFERENCES
Brady D., Stoll A. and Duncan F. R. (1994) Biosorption of heavy metal cations by nonviable yeast biomass. Environ. Technol. 15, 428–429. Brady J. M. and Tobin J. M. (1995) Binding of hard and soft metal ions to Rhizopus arrhizus biomass. Enz. Microbial Technol. 17, 791–796. Brierley J. A. (1990) Production and application of a Bacillus-based product for use in metals biosorption. In Biosorption of Heavy Metals, ed. B. Volesky, pp. 305–310. CRC Press, Boston. Food and Agriculture Oranization of the United Nations (2000) http://www.fao.org/. Grosse D. W. (1986) A review of alternative treatment processes for metal bearing hazardous waste streams. J. Air Pollut. Contr. Assoc. 36, 603–614. Huang C. and Huang C. P. (1996) Application of Aspergillus oryzae and Rhizopus oryzae for Cu(II) removal. Water Res. 30, 1985–1990. Janson C. E., Kenson R. E. and Tucker L. H. (1982) Treatment of heavy metals in wastewaters. Environ. Prog. 1, 212–216. Kapoor A., Viraraghavan T. and Cullimore D. R. (1999) Removal of heavy metals using the fungus Aspergillus niger. Bioresour. Technol. 70, 95–104. Kuyuck N. and Volesky B. (1988) Biosorbents for recovery of metals from industrial solutions. Biotechnol. Lett. 10, 137–142. Leush A., Holan Z. R. and Volesky B. (1995) Biosorption of heavy metals (Cd, Cu, Ni, Pb, Zn) by chemicallyreinforced biomass of marine algae. J. Chem. Technol. Biotechnol. 62, 279–288. No H. K. and Meyers S. P. (1995) Preparation and characterization of chitin and chitosan. J. Aquat. Food Prod. Technol. 4, 27–51. Peng T. Y. and Koon T. W. (1993) Biosorption of cadmium and copper by Saccharomyces cerevisiae. Microbial Utilis. Renew. Resour. 8, 494–504. Sampedro M. A., Blanco A., Llama M. J. and Serra J. L. (1995) Sorption of heavy metals to Phormidium laminosum biomass. Biotechnol. Appl. Biochem. 22, 355–366. Scott J. A. and Palmer S. J. (1990) Sites of cadmium uptake in bacteria used for biosorption. Appl. Microbiol. Biotechnol. 33, 221–225. Suh J. H. and Kim D. S. (2000) Comparison of different sorbents (inorganic and biological) for the removal of Pb2+ from aqueous solutions. J. Chem. Technol. Biotechnol. 75, 279–284. Tsezos M., Remoudaki E. and Angelatou V. (1996) A study of the efffects of competing ions on the biosorption of metals. Int. Biodeterior. Biodegrad. 38, 19–29. Tobin J. M., Copper D. G. and Neufeld R. J. (1984) Uptake of metal ions by Rhizopus arrhizus biomass. Appl. Environ. Microbiol. 47, 821–824. Volesky B. (1990) Removal and recovery of heavy metals by biosorption. In Biosorption of Heavy Metals, ed. B. Volesky, pp. 7–43. CRC Press, Boston. Wilkins E. and Yang Q. (1996) Comparison of the heavy metal removal efficiency of biosorbents and granular activated carbons. J. Environ. Sci. Health A31, 2111–2128. Williams C. J., Aderhold D. and Edyvean R. G. J. (1998) Comparison between biosorbents for the removal of metal ions from aqueous solutions. Water Res. 32, 216– 224. Wilson M. W. (1993) Biosorption for the treatment of industrial wastewaters. Ph.D. Thesis, The University of Leeds, UK.
Wat. Res. Vol. 35, No. 15, pp. 3551–3556, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
CRAB SHELL FOR THE REMOVAL OF HEAVY METALS FROM AQUEOUS SOLUTION H. K. AN, B. Y. PARK and D. S. KIM* Department of Environmental Science, Catholic University of Daegu, Gyeongbuk, South Korea (First received 24 July 2000; accepted in revised form 6 February 2001) Abstract}The ability of crab shell to remove heavy metals from aqueous solution was evaluated by comparing with that of several sorbents (cation exchange resin, zeolite, granular activated carbon, powdered activated carbon). All experiments were conducted using several heavy metal ion solutions (Pb, Cd, Cu, Cr). The orders of heavy metal removal capacity and initial heavy metal removal rate were found as crab shell>cation exchange resin>zeolite>powdered activated carbongranular activated carbon. Therefore, crab shell is satisfactory as a good biosorbent for the heavy metal removal. The study indicates that the removal of these heavy metals is selective, with Pb and Cr being removed in preference to Cd and Cu. The sorption equilibrium of heavy metal ions on sorbents was modeled on the applications of Langmuir and Freundlich. # 2001 Elsevier Science Ltd. All rights reserved Key words}biosorbents, heavy metal, crab shell, cation exchange resin, zeolite, granular activated carbon, powdered activated carbon
INTRODUCTION
The presence of heavy metals in the environment can be detrimental to a variety of living species including human. Metals can be distinguished from other toxic pollutants, since they are non-biodegradable and can accumulate in living tissues, thus becoming concentrated throuhout the food chain. A variety of industries are responsible for the release of heavy metals into the environment through their wastewater. Many researchers have made efforts to remove these heavy metals from industrial wastewater using several processing method such as chemical precipitation, evaporation, ion exchange, cementation electrolysis and reverse osmosis (Janson et al., 1982; Grosse, 1986). However these conventional technologies appear to be inadequate and expensive. They often create secondary problems with metal-bearing sludge (Brady et al., 1994). Accordingly, the metal removal capacities of various biological materials have been focused to remove toxic metals from dilute wastewaters. The high efficiency of various bacteria, yeast, fungi and algae to uptake heavy metals has been observed over two decades (Kuyuck and Volesky, 1988). The phenomenon that metallic ions are concentrated by even dead cell through purely physico-chemical process is called ‘‘biosorption’’. Even denatured cells are able to sequester hazardous heavy metals and *Author to whom all correspondence should be addressed. E-mail: [email protected]
radioactive elements in wastewater without considering the viability of cells. Precious metals that sorbed on cell can be desorbed by proper treatment and biomass can be reused repeatedly (Brierley, 1990). Metal sequestration by different parts of the cell can occur via complexation, coordination, chelation of metallic ions, ion exchange, adsorption, and inorganic microprecipitation (Scott and Palmer, 1990; Peng and Koon, 1993). The major interesting features of newly developed biosorbent materials are high versatility, metal selectivity, no concentration dependence, high tolerance for organics and regeneration (Volesky, 1990). These properties coupled with high uptakes and rapid kinetics of the biosorption system allowed engineering of new and highly effective yet simple industrial biosorbent for heavy metal removal processes. Microorganisms have a problem about separation from solution if heavy metal removal process is operated continuously. Even though the problem of separation can be solved by the immobilization, the operation cost is increased and several problems caused by desorption will be occurred. Crab shell can be considered to be competent as a new biosorbent satisfying these conditions because of the granular state. Total crab landings have reported a steady increase over the 1990–1997 period from 8.87 108 tons in 1990 to almost 1.2 109 tons in 1997 in global (FAO, 2000). Most of crab products are used for food processing to be made into canned crabmeat or
3551
3552
H. K. An et al.
frozen goods. The crab shells can be obtained cheap from byproducts or wastes in the middle of the process. In the present work, the ability of raw crab shell to remove heavy metals (Pb, Cd, Cu, Cr) from aqueous solution was investigated by comparing with that of cation exchange resin, zeolite, granular activated carbon, and powdered activated carbon. The ability for the removal of heavy metal was assessed by the heavy metal removal capacity, removal rate and removal efficiency.
MATERIALS AND METHODS
Materials-sorbents Crab shell of Chinonecetes opilio used in this experiment was obtained as a waste from the food industry in a crabmeat processing plant around eastern coast of Korea. It was separated from the crabmeat by steaming or boiling and washed then dried without any special processes. After pulverization to a geometric mean particle size of 420– 841 mm (20–40 mesh), it was stored at room temperature. The specific surface area and average pore diameter of crab shell were 13.35 m2/g and 3685.8 nm by BET method, respectively. The chemical compositions of crab shell were known to protein (29.19%), ash (40.60%), lipid (1.35%), chitin (26.65%) as dry weight basis (No and Meyers, 1997). A Dowex 50W strongly acidic cation exchange resin (CER), gel type of 354–1000 mm (16–42 mesh), was used for the present work. It has high cation exchange capacity as 1.8 meq/ml. Powdered activated carbon (PAC) made from coconut shell char (Indine No. is 1050 mg/g) and zeolite was sized 44–74 mm (200–300 mesh) and 841–2830 mm (7–20 mesh), respectively. Granular activated carbon of which BET surface area and average pore diameter were 1032.78 m2/g and 246.8 nm, respectively, was sized 1000– 1410 mm (12–16 mesh).
RESULTS AND DISCUSSION
Residual heavy metal ions The residual heavy metals (Pb, Cd, Cr, Cu) concentrations in the removal of heavy metals by crab shell and several chemical sorbents (CER, zeolite, GAC, PAC) were examined when the initial heavy metal and sorbent concentrations was 0.5 mmol/l and 1.0 g/l, respectively (Fig. 1). In all the cases of crab shell (Fig. 1(a)) and CER (Fig. 1(b)), the residual heavy metal concentration was decreased abruptly according to time and maintained below 0.01 mmol/l after 24 h, however, the time required to reach an equilibrium state were 6 and 12 h, respectively. In the case of zeolite (Fig. 1(c)), the order of intimacy to heavy metal were repealed definitely and was Pb>CdCu>Cr. The heavy metal removal rate was lower and the time required to reach an equilibrium state was shorter as the heavy metal could not be removed. The heavy metal removal by zeolite was not so great compared to those by crab shell and CER. In the results of GAC (Fig. 1(d)) and PAC (Fig. 1(e)), the definite difference between them was not found and the variation of residual heavy metal concentration according to time was not so great compared to the results of other sorbents. Therefore,
Sorption experiments Sorption experiments were carried out in batches as follow. Stock heavy metal solutions (Pb, Cd, Cu, Cr) were prepared as 1 mM by dissolving nitrate salt of analytical grade in distilled deionized water. Experiments were carried out in 300 ml Erlenmeyer flasks containing 250 ml with different heavy metal concentrations. The initial pH of the solution was adjusted to 5.0 by adding small amounts of dilute NaOH or HNO3 solution. To each flask was added the same amount of sorbents (0.25 g), and then all the flasks were sealed with silicon cap to minimize evaporation and shaken at 150 rpm in rotary shaking incubator (308C, 24 h). Samples of 1.8 ml were taken from each flask at the proper time period and centrifuged (7200 g) immediately for 10 min. The heavy metal concentration in the supernatant was analyzed by atomic absorption spectrophotometer (Unicam 929/1071). Removed heavy metal amounts per unit sorbent dry weight were calculated from heavy metal mass balance yield. The removed heavy metal amounts by sorbent was calculated as: Q ¼ VðC0 C1 Þ=1000 M, where Q is the specific metal removal (mmol/g sorbent), V is the volume of metal solution (l), C0 is the initial concentration of metal in solution (mmol/l), C1 is the concentration of metal in solution (mmol/l), and M is the dry weight of sorbent (g). The initial heavy metal removal rate was measured by calculating the slope from the plot of the removed heavy metal amounts per sorbent dry weight (mmol/g sorbent) vs. time (min) at t ¼ 0. The definition of percent removal is as follows, % removal={ðC0 C1 Þ=C0 g 100.
Fig. 1. Time courses of heavy metals removal by (a) crab shell, (b) CER, (c) zeolite, (d) GAC, (e) PAC for various heavy metals; (*) Pb, (*) Cd, (.) Cr, (5) Cu.
Crab shell for the removal of heavy metals
contrary to common sense, we cannot expect the heavy metal removal by activated carbons. Heavy metal removal capacity In order to compare the heavy metal removal capacity, the removed heavy metal amounts per unit sorbent dry weight according to the residual heavy metal concentration in equilibrium state was observed (Fig. 2). In our experimental ranges, the removed heavy metal amount was increased as the increase of residual heavy metal concentration. Above a certain residual heavy metal concentration, the removed heavy metal amounts were shown as gentle increase or constant value. In the case of Pb (Fig. 2(a)), the removal capacity of crab shell was higher than those of any other sorbents and it was followed by those of CER, zeolite, PAC and GAC. Especially, the heavy metal removal capacities of PAC and GAC were remarkably lower than those of any other sorbents. Also, in the cases of Cd (Fig. 2(b)) and Cr (Fig. 2(d)), the trend was similar to the case of Pb. In the case of Cd, the removal capacity of crab shell was considerably higher than those of any other sorbents. In Cr, the removal was not accomplished well comparing to the cases of other metals, especially, no removals by zeolite, PAC and GAC were occurred. In the case of Cu (Fig. 2(c)), the heavy metal removal capacity of CER was slightly higher than that of crab shell. The removal trend was similar to those of any other metals. The summary results of Fig. 2 were applied to Langmuir and Freundlich isotherm model
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(Table 1). In our experimental ranges, generally, the application of Langmuir isotherm model was more appropriate than that of Freundlich model and the applicability of PAC and GAC was not found. A similar result was reported by Kapoor et al. (1999) that the adsorption results of Pb, Cd and Cu on activated carbon could not applied well on both the Langmuir and Frendlich isotherm model. In order to investigate the heavy metal removal capacity quantitatively, q0:1 and q1:0 values were assessed where q0:1 and q1:0 values were the heavy metal removal capacities when the residual heavy metal concentrations were 0.1 and 1.0 mmol/l, respectively (Table 1). As shown in Table 1, the order of q0:1 and q1:0 values was crab shell>CER> zeolite>PACGAC in all heavy metals. The order of q0.1 and q1.0 values in crab shell according to heavy metals was Cd>Pb>CrCu. These results are in agreement with the findings of Wilson (1993), who reported that Cd was the most effectively removed by seaweed biomasses compared with a range of other metallic ions. Also, the order by four types of algae was Cd>Cu>Ni (Williams et al., 1998). However, the order of heavy metal removal by fungus, Rhizopus arrhizus, was Pb>Cu>Cd (Brady and Tobin, 1995), and the order by Phormidium laminosum was Pb>Cd>Cu>Zn>Cr>Ni (Sampedro et al., 1995). The order of heavy metal removal was correlated to the ionic radius (Tobin et al., 1984) and to the covalent index (Brady and Tobin, 1995) which is the function of electronegativity and ionic radius, therefore the order was shown as Pb>Cu>Cd, which was slightly different from ours. By the report of Leush
Fig. 2. The equilibrium isotherms of (a) Pb, (b) Cd, (c) Cu, (d) Cr removal by several sorbents; (*) crab shell, (*) CER, (&) zeolite, (m) GAC, (4) PAC.
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H. K. An et al. Table 1. Comparison of the removed metal amounts at q0:1 and q1:0 between crab shell and chemical sorbents q0.1a (mmol/g)
q1.0a (mmol/g)
0.90 0.86 0.83 0.86 0.93
0.96 0.97 0.48 0.03 0.12
1.29 1.23 0.54 0.08 0.13
0.99 0.99 0.99 0.99 0.96
0.95 0.91 0.94 0.96 0.79
1.37 0.90 0.20 0.01 0.01
1.77 1.20 0.27 0.03 0.03
Crab shell CER Zeolite GAC PAC
0.82 0.98 0.99 0.89 0.90
0.71 0.87 0.95 0.95 0.95
0.88 0.94 0.20 0.04 0.05
0.98 1.18 0.23 0.08 0.07
Crab shell CER Zeolite GAC PAC
0.93 0.99 0.54 0.96 0.57
0.84 0.99 0.60 0.99 0.61
0.95 0.65 0.04 0.01 0.02
1.06 0.84 0.04 0.04 0.02
Model (r2 value)
Metals
Materials
Langmuir
Freundlich
Pb
Crab shell CER Zeolite GAC PAC
0.99 0.99 0.99 0.78 0.93
Cd
Crab shell CER Zeolite GAC PAC
Cu
Cr
a
q0:1 and q1:0 represent the removed metal amounts (mmol/g) at the equilibrium concentrations of 0.1 and 1.0 mmol/l, respectively.
et al. (1995), the order was Pb>Cd>Cu. Therefore, we can see that the order can be changed by the sort of sorbents since the heavy metal removal mechanism is variable according to the sort of sorbents and metals. Tsezos et al. (1996) have stated that the affinity of a metal ionic species for a ligand binding site on the biomass surface is likely to be affected by the chemical coordination characteristics of that metal ion. Even though the order of heavy metal removal was changed by the sort of sorbents and metals, generally, Pb and Cd could be easily removed by biosorbents. Heavy metal removal rate With the heavy metal removal capacity in equilibrium state, the initial heavy metal removal rate is very important factor for the reactor design and the process optimization in heavy metal removal process. The initial heavy metal removal rates of crab shell and several sorbents according to the increase of initial heavy metal concentration were shown in Fig. 3. The heavy metal removal rate was increased as the increase of initial heavy metal concentration and was increased slowly or maintained constant above a certain concentration. The heavy metal removal rate was in the order of crab shell>CER>zeolite> PACGAC in all heavy metals used in this experiment. Especially, in Pb removal by crab shell (Fig. 3(a)), the Pb removal rate was shown as 3.0 mmol/g-min when the initial Pb concentration was increased to 1.0 mmol/l, which was very high value compared to those of other sorbents. Therefore, we can see that crab shell is satisfactory as a
good biosorbent for the heavy metal removal because of the high heavy metal removal rate and removal capacity. The heavy metal removal rate was changed by the sort of heavy metals, and the order was shown as Pb>Cd>CuCr. By the above results, the order concerning the heavy metal removal capacity was not correlated perfectly with that about the heavy metal removal rate. Heavy metal removal efficiency When the heavy metal removal process is operated continuously, the long hydraulic retention time such as 24 h is ineffective in operation aspect. Therefore, the operation time for metal removal should be less than 12 h at most. Metal removal efficiency according to the increase of initial metal concentration was observed at the operation time of 12 h (Fig. 4). In Pb removal (Fig. 4(a)), from the results of crab shell and CER, both the removal efficiencies were over 97% up to the initial Pb concentration of 1.0 mmol/l and decreased abruptly above 1.0 mmol/l. The results of crab shell and CER were 45 and 38% at the initial concentration of 3.0 mmol/l, respectively. Comparing the results of crab shell and CER, the removal efficiency of crab shell was a little higher than that of CER at the high initial concentration of 3.0 mmol/l even though the result of crab shell was similar to that of CER at relatively low initial Pb concentration. In the case of zeolite, the removal efficiency was over 96% below the initial concentration of 0.25 mmol/l, and it was decreased to 18% at 3.0 mmol/l. The removal efficiencies of PAC and GAC decreased from 76 and 31% to 5 and 4% as the
Crab shell for the removal of heavy metals
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Fig. 3. Comparison of the initial (a) Pb, (b) Cd, (c) Cu, (d) Cr removal rate among several sorbents; (*) crab shell, (*) CER, (&) zeolite, (m) GAC, (4) PAC.
Fig. 4. Heavy metal removal efficiencies of (a) Pb, (b) Cd, (c) Cu, (d) Cr in several sorbents; (*) crab shell, (*) CER, (&) zeolite, (m) GAC, (4) PAC.
initial concentration increased from 1.0 to 3.0 mmol/ l, respectively. Generally, the removal efficiencies of Cd were lower than those of Pb (Fig. 4(b)). The removal efficiency of crab shell was the highest among the sorbents. While the removal efficiency of CER was from 38% as low to 86% as high, that of crab shell was over 90% below 1.0 mmol/l and fell down rapidly over 1.0 mol/l. The removal efficiency of zeolite was over 60% below 0.25 mmol/l but
decreased to 15% at 3.0 mmol/l. In the cases of GAC and PAC, the removal efficiency was very low as below 6% even at low concentration. From the results of Cr (Fig. 4(c)) and Cu (Fig. 4 (d)) removal, the removal efficiencies of crab shell were higher than those of CER below the low concentration of 1.0 mmol/l. However, the results were reversed above 3.0 mmol/l. The Cr removal efficiencies of crab shell and CER were 28 and 38%, respectively, at the high concentration of 3.0 mmol/l.
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H. K. An et al.
Also, the Cu removal efficiency of crab shell (35%) was lower than that of CER (43%) at 3.0 mmol/l. Especially the removal efficiencies of zeolite was lower compared to the cases of other metals. Undoubtedly, the removal efficiencies of GAC and PAC were the lowest. By the report of Wilkins and Yang (1996), the removal efficiencies in Cu, Cd and Zn by GAC at pH 4.5 during 1 h operation were very low as 42–53%, 15–40% and 6–10%, respectively. Huang and Huang (1996) observed that Aspergillus oryzae was superior to activated carbon for adsorption of Cu. Kapoor et al. (1999) reported that Pb, Cd and Cu removals were higher for Aspergillus niger biomass than for activated carbon. Suh et al. (2000) also stated that the removed Pb amounts by Aureobasidium pullulans, Saccharomyces cerevisiae and activated sludge were much higher than those by GAC and PAC. CONCLUSIONS
In order to examine the availability and effectiveness of crab shell for the removal of heavy metals from aqueous solutions, the heavy metal removal capacity and removal rate of crab shell were observed in comparison with those of CER, zeolite, GAC and PAC using four heavy metals (Pb, Cd, Cr, Cu). In the case of crab shell using all heavy metals, the equilibrium state reached within 12 h in our experimental ranges. The heavy metal removal capacity and removal rate were affected by the sorts of sorbents and metals. In the cases of Pb, Cd and Cr, the removal capacity of crab shell was higher than those of any other sorbents and it was followed by those of CER, zeolite, PAC and GAC. However, in the case of Cu, the removal capacity of crab shell was slightly lower than that of CER. In heavy metal removal by crab shell, the application of Langmuir isotherm model was more appropriate than that of Freundlich model. The order of heavy metal removal capacity in crab shell was Cd>Pb>CrCu. Pb and Cd can be easily removed by crab shell even though the order is slightly differed by the sort of sorbents. Like the case of removal capacity, the heavy metal removal rate was in the order of crab shell>CER>zeolite>PACGAC. These properties in conjunction with the low cost of crab shell could provide an economic effluent treatment system. Under the heavy metal concentration of 1.0 mmol/l, the heavy metal removal efficiency of crab shell was maintained as 93–100%, which was much higher than those of any other sorbents. Acknowledgements}This work was supported by Korea Research Foundation Grant. (KRF-2000-041-E00459).
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