Environ. Sci. Technol. 2005, 39, 2357-2363
Effective Recovery of Harmful Metal Ions from Squid Wastes Using Subcritical and Supercritical Water Treatments OMID TAVAKOLI AND HIROYUKI YOSHIDA* Department of Chemical Engineering, College of Engineering, Osaka Prefecture University, 1-1, Gakuen-Cho, Sakai 599-8531, Japan
The Japanese common squid wastes contained high concentration of metal ions such as 31.7 ppm Cd(II), 264.0 ppm Cu(II), and 140.0 ppm Zn(II). The use of sub- and supercritical water treatment has been investigated as a new method of recovering heavy metals from squid wastes. The reactions were carried out in the temperature range of 443-653 K, a pressure range of 0.792-30 MPa, and reaction times of 1-40 min. The wastes were decomposed into soluble proteins, organic acids, amino acids, and so on in the aqueous phase, and the fat and oil were extracted by sub- and supercritical water. The maximum yields on concentration of Cd(II), Cu(II), and Zn(II) in the solid, fat, and oil phases were found at 653, 573, and 513-573 K, respectively. The aqueous phase showed the lowest concentration of the metal ions (0.05-0.5 ppm). The distribution coefficient of metal ions in the fat, solid, and oil phases to aqueous phase were examined and found highest in the fat phase (max. 48 000). The solid phase (max. 39 000) and oil phase (max. 245) showed the second and third highest. Moreover, the fat and oil phases produced during this method act as chelating agents to catch metal ions with an order of recovery of Cu2+ > Zn2+ > Cd2+ and Zn2+ > Cu2+ > Cd2+, respectively.
representing an important field of environmental research as a result of their toxicological properties. These metal ions, when present in sufficient quantity, can be harmful to aquatic life and human health. A total of 517 960 tons of squid in 1997 and 329 590 tons in 1998, including imports, was on the market in Japan. In the same period, the catch of Todarodes pacificus squid in Japan’s domestic waters was 310 000 and 143 930 tons in 1997 and 1998, respectively. Since around 45% of the whole squid is disposed of, a large amount of harmful wastes accumulates and thus must be properly treated. This matter has become serious especially after the London treaty (November 1996), which mainly bans the disposal of organic wastes, resulting in incineration becoming the most popular approach for treatment of seafood waste. However, incineration is not only energy intensive but also produces harmful volatile out-streams of residues, such as bottom, fly ashes, and flue gas streams including cadmium and other metals (9). To overcome such environmental issues, a processing technology for squid wastes was recently established in Hokkaido, Japan, which employs an electrolysis method for removal of metal ions on a negative plate. This plant has been designed to process a capacity of 60 tons per day of squid waste (water content ≈55%). However, this method of waste treatment is economically more intense than incineration, not to mention the initial cost of construction, which is estimated to be around 905 million Japanese yen. In 1999 and 2001, Yoshida et al. (10, 11) proposed a subcritical water treatment method that is not only a energy efficient but also converts seafood waste to useful resources. Yoshida and Tavakoli (12) reported that squid wastes can be converted into oil, fat, and soluble proteins, organic acids, and amino acids. Within this scope, the present work examines the feasibility of using sub- and supercritical treatments for removal or recovery of harmful metal ions from squid wastes. Our investigation focus on the kinds of heavy metal ions present in the waste and the way in which they behave in aqueous, oil, fat, and solid phases after such treatments. In addition, we demonstrate that metal ions can be recovered from waste streams and recycled back to the related industries.
2. Experimental Section 1. Introduction It is commonly known that the aquatic environment and the organism itself determine bioaccumulation of trace metals in seafood. The relationship between trace metal content in biota with the surrounding environment such as water, sediment, and food are environmental factors correlated with metal accumulation in internal organs of marine animals (1, 2). Some researchers have reported that cephalopods (squid, cuttlefish, octopus, etc.) accumulate high concentrations of metal ions such as Cu, Zn, and Cd in their digestive gland (3-8). The high levels of cadmium observed in several cephalopod species is thus an important reason to investigate how these amounts of toxic metals are tolerated (8). The removal of metal ions from waste streams has recently received considerable attention due to their bioaccumulation and responsibility for various disorders. Particularly seafood wastes such as squid entrails or scallops contain significant and highly variable concentrations of harmful metal ions, * Corresponding author phone and fax: 81-722-54-9298; e-mail:
[email protected]. 10.1021/es030713s CCC: $30.25 Published on Web 02/17/2005
2005 American Chemical Society
2.1. Materials, Reagents, and Equipment. An entrails of Japanese common squid (T. pacificus), which was supplied from Hakodate (Hokkaido, Japan), was used in this experimental study. All reagents were laboratory grade and deionized distilled water (17.2 MΩ grade) was used. Metal standard solutions for atomic absorption analysis were obtained from Kishida Chemicals Co. and Wako Pure Chemicals Industries Ltd. A stainless tube (SUS 316, 0.0168 m i.d. × 0.15 m length) with Swedgelok caps was used as a reactor (volume: 35.0 cm3) for sub- and supercritical water experiments. About 8.0 g of the squid entrails (water content, 64.5%) and 14-22 g distilled pure water were charged into the reactor tube. Dissolved oxygen in the sample was removed by purging with argon gas. The reactor was then sealed by Swedgelok caps and immersed in a preheated molten salt bath (Thomas Kagaku Co. Ltd.) as shown in Figure 1. The reactor was shaken in the salt bath during the reaction. The reactions were carried out in the temperature range between 443 and 653 K and the pressure range of 0.792-30.0 MPa. After the desired reaction time, the reactor was allowed to cool by soaking in a water bath. VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(max. (5.0%). With respect to this deviation and the small difference observed in our results, other experiments were conducted individually.
3. Results and Discussion
FIGURE 1. Schematic of the batch experimental setup and internal parts (1, isolate chamber; 2, inner salt bath; 3, heater (4000 W); 4, temperature sensor, PID control; 5, mixer; 6, stirring motor; 7, operation panel; 8, stainless reactor). 2.2. Sample Preparation. The reaction produced four phases (unreacted solid, aqueous, fat, and oil), which were separated. The aqueous phase was diluted to 200 cm3 with distilled water and then filtered with Millipore membranes (0.22 µm) to remove fat, oil, and other insoluble droplets or particles. The oil phase was extracted from the products with hexane followed by the organic solvent being purged with a vacuum pump. Thereafter, the fat phase was recovered, and the unreacted solid remained in the vial as the final product. The procedure for preparation of samples was as follows: Specific amounts of dried solid, fat, and oil were weighed and then 50 mL of 2 M HNO3 was added and left for 2 days to cold digest. Nitric acid is capable of decomposing and destroying complexes of metals with organic matter in order to solubilize the elements of interest. Finally after acid digestion, the aqueous solution was removed and filtered for analysis. 2.3. Analytical Method. Samples were analyzed for their concentration of metal ions by using a Seiko atomic absorption spectrophotometer (Model SAS 7500A) with D2 deuterium lamp background correction in air-acetylene flame absorption mode. The metal concentrations in aqueous phase also were analyzed by direct aspiration into the flame atomic absorption. Cadmium, copper, and zinc were respectively analyzed at wavelengths of 228.8, 324.75, and 213.86 nm. The ion concentrations were determined in relation to calibration standards. The detection limit for analyzing metal ions was measured as following: 0.05 ppm for Cd(II), 0.05 ppm for Cu(II), and 0.02 ppm for Zn(II). Mercury was determined by atomic absorption spectroscopy using the flameless cold vapor technique at a wavelength of 253.65 nm. To clearly identify the coexisting metal ions, it is known that Cu(II), Cd(II), and Zn(II) may affect their atomic absorption measurements, and thus specific amounts of Cu(II), Cd(II), and Zn(II) were added individually to a sample solution and the concentrations of the metals were measured by assuming a single component system. The added ions did not affect the concentrations of Cd(II), Cu(II), and Zn(II). On the other hand, to give better statistics, multiple experiments were carried out at three temperatures of 473, 513, and 573 K, and the results showed a small deviation 2358
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3.1. Effect of Reaction Temperature on Metal Ion Recovery. The concentrations of Cd(II), Cu(II), and Zn(II) found in the digestive gland of squid reported by other researchers are shown in Table 1. The high level of metals reported from different areas of the world suggests that the internal organisms of squid have efficient metal-binding mechanisms. Table 1 also presents the experimental metal ion content in raw squid waste entrails used in this study. The order of metal enrichment was Cu(II) > Zn(II) > Cd(II) . Hg(II), which was similar to those reported by other authors for T. pacificus, with the mercury ion concentration being the lowest. The amounts of Co(II), Cr(II), and Ni(II) were also analyzed, and nothing was observed. These results suggested that the accumulation of metal ions is mainly a function of the metal’s affinity for proteins, fat, or oil in the entrails. The metalbinding protein, metallothionein, has been found to be involved in the binding of cadmium and other trace metals in many marine molluscs (13). Moreover, dissolution of metal ions from the reactor wall in sub- and supercritical water (with only water as reactant) was analyzed and a small amount of Fe(II), Ni(II), and Mn(II) was found in the water. Since the metal ion content in squid waste studied here does not included those metals, it could be concluded that metal dissolution from the reactor has no effect on the material balance of the original metals present in the squid wastes. The elemental content in dry squid wastes is shown in Table 2. This table provides information on squid waste structure and describes the many organic molecules inside waste, which contain carboxylic group (-COOH), amino group (-NH2), sulfur bond (SdS), and so on. To develop an economical method to convert the squid wastes including those metal ions to useful resources, we show the possibility of using subcritical water to recover metal ions as a pretreatment step. Figure 2 shows the effect of reaction temperature on the residual solid and the yields of fat and oil [kg/kg of dry entrails] at 10 min, which at this time the subcritical reaction is almost completed (Figures 8 and 10). With increasing reaction temperature the residual solid and yield of fat phase decreased and the yield of oil phase increased. The amount of solid became very small at temperatures higher than 553 K. This means that 99% of the solid has been converted to other compounds; proteins in the solid were decomposed to soluble proteins, peptides, amino acids, and organic acids in the aqueous phase, and fat and oil were extracted by subcritical water as previously reported (12). Figure 2 also provides the pressure changes under reaction condition in the experiments. In addition, gases such as CO2, CH4, N2, and O2 were produced during the reaction, and depending on the reaction conditions, the amounts of these gases were different and increased by increasing temperature and pressure. Under subcritical conditions, only small amounts of these gases were produced, while under supercritical conditions, due to decomposition of organic compounds, an increase in the rate of gaseous production was observed. Figure 3 illustrates the effect of reaction temperature on the concentration of Cd(II), Cu(II), and Zn(II) in the residual solid at a reaction time of 10 min. The concentrations of these metals, which were based on the dry amount of solid after reaction at each temperature, increased with increasing temperature and were observed to be 20 000, 9500, and 1560 mg/kg of dry solid for Cu(II), Zn(II), and Cd(II), respectively. These results suggest that metal ions are concentrated in the solid phase with increasing temperature. These metal ions may be adsorbed on the -NH2 and -COOH groups of
TABLE 1. Reported Concentrations of Copper, Cadmium, and Zinc in Squid concentration, ppm, wet wt squid
organ
Cu2+
Zn2+
Cd2+
Hg2+
ref
T. pacificus Nototodarus gouldi Loligo opalescent Illex argentinus Symplecteuthis oual. Loligo forbesi T. pacificus
liver liver liver liver liver liver entrails
111-267 66.7 1550 233.4 318.5 66 264.0a
31-89 152.6 22.4 149.3 95 96.7 140.0a
15-33 6.07 83.1 485 144.8 4.77 31.69a
0.0275a
3 4 5 6 5 7 this workb
a
ppm, dry weight.
b
Cr(II), Ni(II), and Co(II) not determined.
TABLE 2. Composition of Dry Raw Squid Entrails
CHNSO
wt %
ratio of no. of atoms to carbon
carbon hydrogen nitrogen
58.86 9.625 8.84
1 2.1 0.13
CHNSO
wt %
ratio of no. of atoms to carbon
sulfur oxygen ash
1.53 17.15 ∼4.0
0.01 0.22 -
FIGURE 4. Effect of reaction temperature on recovery of metal ions in the fat phase at a reaction time of 10 min.
FIGURE 2. Effect of reaction temperature and pressure on yield of solid, fat, and oil phases for a reaction time of 10 min.
FIGURE 5. Effect of reaction temperature on recovery of metal ions in the oil phase at a reaction time of 10 min.
FIGURE 3. Effect of reaction temperature on metal ions concentration in the solid phase at a reaction time of 10 min. proteins of the squid entrails and solid residues:
(R-NH2)n + M2+ a (R-NH2)nM2+
(1)
2R-COOH + M2+ a (R-COO-)2M2+ + 2H+
(2)
Since the affinity of metal-amine and metal-carboxylic acid group complexes have been reported as Cu(II) > Cd(II) > Zn(II) and Cu(II) . Zn(II) > Cd(II) (14), respectively, both eqs 1 and 2 affect the adsorption of the metal ions. With increasing reaction temperature, higher amounts of solid decompose, as represented by Figure 2. However, at higher temperatures, the oil and fat phases decompose, releasing metal ions. Thus, it is thought that the metal ions discharged from the decomposed oil, fat, and some parts of solids were heavily adsorbed on to the residual solid, resulting in the formation of a complex as represented by eqs 1 and
2 utilizing the free active sites. Therefore, an increase in the metal ion concentration is observed in the residual solid. The effect of temperature on the concentration of metals in the fat and oil phases at a reaction time of 10 min is shown in Figures 4 and 5, respectively. Fat and oil produced by the subcritical water acted as chelating agents to entrap almost all metal cations from squid entrails. The concentrations of metal ions in the fat phase were about 100-300 times higher than those in the oil phase and close to those found in the solid phase. To understand the mechanism of metal binding to organic compounds, in the postreaction stage, the triglycerides and free fatty acids in the fat and oil phases would be a first choice in the investigation. Since in the fat phase, triglycerides are the main component, the binding between the fat and metal ion may be given by the following chelating reaction:
In addition, free fatty acids may also have strong affinity for metal ions. The reaction mechanism between metal ion and free fatty acids in these phases thus is given by VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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2R-COOH + M2+ a (2R-COO-)M2+ + 2H+
(4)
The content of free fatty acids in the fat phase is lower than that in the oil phase. Using commercial fatty acids, triglycerides, and fresh squid oil, the above-proposed mechanism was confirmed under sub- and supercritical water conditions. In the fat phase, triglycerides and free fatty acids were able to bind divalent heavy metal ions with the order of affinity Cu2+ > Zn2+ > Cd2+, whereas in the oil phase the order was Zn2+ > Cu2+ > Cd2+, respectively. This finding suggests that the binding constants for zinc to fatty acids in these two phases are different. In the fat phase, the concentration of metal ions increased with increasing temperature, reaching a maximum value at 573 K, and exhibited a minor decrease beyond this temperature. Among the metal ions, copper showed a maximum concentration of 23 000 mg/kg, zinc was second highest at 10 380 mg/kg, and cadmium at 2270 mg/kg of fat represents the third highest. These results suggest that by increasing the temperature the amount of fat phase decreased and functional groups may be saturated by the metal ions. The oil phase also showed a maximum concentration of Zn(II) at 86 mg/kg of oil at 513 K and for Cu(II) at 573 K a maximum value of 68 mg/kg of oil was obtained. Cd(II) showed an almost constant recovery of about 10 mg/kg of oil. With increasing reaction temperature, Zn(II) and Cu(II) complex, which were decomposed in the solid and fat, entered the oil phase and the concentration of these metals increased. In the oil phase free fatty acids are the main components for reaction with metal ions, and thus, variation of the acids is an important factor to understand with respect to differences in metal complexes. The results obtained from analyzing fatty acids indicated that the ones containing double bonds decomposed and decreased along with an increase in temperature particularly at temperatures higher than 500 K. This suggests that at higher temperature the metal complex with the fatty acids decomposes, showing a decrease in concentration. To explain this phenomenon, Figure 6 shows the effect of reaction temperature on the trends of docosahexaenoic acid (DHA), oleic acid, and 13docosenoic (erucic acid) as examples of some fatty acids in the oil phase. As illustrated, polyunsaturated DHA and monounsaturated oleic and erucic acids yields increased with temperature and due to thermal decomposition appeared to decrease at 473, 513, and 533 K, respectively. DHA with six double bonds showed rapid decomposition at temperatures above 533 K. The reason may be due to the fact that at high temperature the double bonds are broken, leading to a saturation of the triglyceride molecules (15). The behaviors of the acids are similar to the phenomena observed for Zn(II) and Cu(II) as represented by Figure 5. Figure 7 demonstrates the results in the aqueous phase at a constant reaction time of 10 min. The concentrations of Cd(II), Cu(II), and Zn(II) decreased with increasing temperature and reached a final value of 0.07, 0.5, and 0.25 ppm at 653 K, respectively. At high temperatures the interaction between metal ions and water occurs via hydration and dehydration steps. Increasing the temperature with simultaneous decrease on water polarity and hydrogen bonding resulted in a decrease in solubility and thus release of metal ions from the aqueous phase. An increase in temperature and pressure result in a decrease in metal ion concentration in the aqueous phase due to adsorption of free ions by the complex formation reaction in other phases and low ion solubility in the aqueous phase. With respect to recovery of useful organic materials (e.g., organic acids, amino acids and proteins) the high-temperature process is also best suited due to the low concentration of metal ions in the aqueous phase. 2360
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FIGURE 6. Effect of reaction temperature on the amount of three fatty acids in the oil phase at a reaction time of 10 min.
FIGURE 7. Effect of reaction temperature on recovery of metal ions in the aqueous phase at reaction time of 10 min.
FIGURE 8. Time course of yield of solid for three reaction temperatures. 3.2. Effect of Reaction Time on Metal Ion Concentration in the Solid Phase. The effect of reaction time on unreacted solid for three different reaction temperatures is shown in Figure 8. At 553 K at a reaction time of 5 min, since the amount of residual solid is almost zero, the producibility of valuable materials in this temperature is much better than others. The time courses of the concentration of Cu(II), Zn(II), and Cd(II) are shown in part a and b of Figure 9 for two reaction temperatures of 513 and 553 K, respectively. In case of 513 K (Figure 9a), the maximum concentration was observed at 20 min (Cu) and 30 min (Cd and Zn). This may be due to a decrease in adsorption of the metal ions to the solid phase and the rate of complex decomposition affecting the transfer of metal ions to the fat phase. In Figure 9b (553 K), the metal concentrations increased with a reaction time of 40 min, and a maximum value of 13 500, 7500, and 1500 mg/kg of dry solid was observed for Cu(II), Zn(II), and Cd(II), respectively. This suggests that at 553 K increasing the reaction time causes an increase in the adsorption rate of discharged ions into the solid phase due to the decomposition of complexes with higher molecular weight compounds. Among metal ions, copper has maximum yield (Table 1). 3.3. Effect of Reaction Time on Metal Ion Concentration in the Fat and Oil Phases. The effect of reaction time on the metal ion concentration at two different temperatures, 513
FIGURE 9. Time course of metal ion concentration in the solid phase at reaction temperatures of (a) 513 K (3.35 MPa) and (b) 553 K (6.42 MPa).
FIGURE 11. Time course of concentration of metal ions in the fat phase at reaction temperatures of (a) 513 K (3.35 MPa) and (b) 553 K (6.42 MPa).
FIGURE 10. Time course of fat and oil yields for three reaction temperatures of 473 K (1.55 MPa), 513 K (3.35 MPa), and 553 K (6.42 MPa). and 553 K, was examined. Figure 10 represents a decrease in fat and an increase in oil yields with increasing temperature and time, which indicate a fat to oil conversion. After 10-20 min the yield of fat reached a plateau at 0.02-0.07 kg/kg of dry entrails in the temperature range of 473-553 K. At these conditions the amount of oil increased with reaction time and remained constant after 10 min for a reaction temperature of 513 and 553 K. For reactions taking place at 473 K, the amount of oil remained constant after 30 min. Parts a and b of Figure 11 show the time course of the concentration of metal ions in the fat phase for temperatures 513 and 553 K, respectively. For reactions occurring at 513 K, the concentrations of metal ions increased with time, indicating that both triglyceride and fatty acids are present in sufficient amounts to make complexes with metal ions as shown by eqs 3 and 4. For reactions at 553 K, a maximum at 20 min was observed, indicating a higher rate of fat decomposition to the oil phase; thus, more triglycerides are broken down to fatty acids, causing the metal complexes to be broken. The maximum ion concentrations for Cu(II), Zn(II), and Cd(II) found in the fat phase at 553 K and a reaction time of 20 min were determined to be 30 000, 11 400, and 2200 mg/kg of dry fat, respectively. Figure 12a,b illustrates the concentration of metal ions in the oil phase at 513 and 553 K. The concentration of metal cations at 513 K increased with increasing time, showing a maximum value of 89 mg/kg of oil for Zn(II) at 20 min. The concentration of Cu(II) after 10 min reached a plateau at a
FIGURE 12. Time course of concentration of metal ions in the oil phase at reaction temperatures of (a) 513 K (3.35 MPa) and (b) 553 K (6.42 MPa). value of 23 mg/kg of oil with Cd(II) remaining constant at 10 mg/kg of oil (Figure 12a). At temperature of 553 K (Figure 12b) the maximum concentrations of 60, 54, and 16 mg/kg of oil for Cu(II), Zn(II), and Cd(II), respectively, was obtained for a reaction time of 20 and 10 min. To explain the maximum metal ion concentration, the time course of free fatty acids was analyzed. The results showed that at 513 and 553 K some of the fatty acids decomposed (especially fatty acids with some double bonds such as EPA and DHA), which suggested that with increasing reaction time the complex of metal and fatty acids in oil phase is decreased. 3.4. Effect of Reaction Time on Metal Ion Concentration in the Aqueous Phase. Figure 13a,b shows the time course of reaction on the metal ions in the aqueous phase. The concentrations sharply decreased from the initial values at 298 K, meaning that prior to the reaction the metal ions were dissolved in the aqueous phase. Upon the reaction being completed, over time the ions released from the aqueous VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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553 K (Figure 13b) illustrated much lower metal ion concentrations in the aqueous phase at 553 K due to the decrease of metal ion solubility with an increase in the temperature. 3.5. Distribution of the Metal Ions. To further understand the behavior of produced/extracted metal ions in the four phases after sub- and supercritical reaction, the distribution coefficients KD for Cd(II), Cu(II), and Zn(II) were determined as defined by eq 5:
KD )
FIGURE 13. Time course of concentration of reaction time on recovery of metal ions in the aqueous phase at reaction temperatures of (a) 513 K (3.35 MPa) and (b) 553 K (6.42 MPa).
Cimetal Caqmetal
(5)
where C represents the concentration of metal ion and i denotes fat, solid, and oil phases. Figure 14a-c represents the distribution coefficient as a function of reaction temperature at reaction time of 10 min with data being larger than unity. An increase in the distribution coefficient was observed with an increase in temperature above 500 K. The results thus indicated the priority in distribution for metal ions to be fat phase > solid phase . oil phase. This may be due to the fact that binding sites in fat (triglycerides with double bonds) and solid phases are much larger than in the oil phase. Further Zn(II) showed a maximum distribution coefficient among the three metal ions. The distribution coefficient of Cd(II) in fat and oil phases demonstrates peaks around 38 116 and 184, respectively, at 573 K (Figure 14a). These maxima were found in accordance with the maximum concentration of Cd(II) in fat and oil phases. The distribution in the solid phase increased to a value of 24 200 at 653 K. Figure 14b demonstrates no peak for the Cu(II) distribution coefficient, and for all phases the coefficient increased with an increase in temperature to 653 K. On the other hand, the presented distribution coefficient of Zn(II) (Figure 14c) showed peaks at 573, 613, and 553 K for fat, solid, and oil phases, respectively.
Acknowledgments A part of this research funds was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan in the form of 21st century COE Program (24403, E-1).
Literature Cited
FIGURE 14. (a-c) Effect of reaction temperature on metal ion distribution coefficient between fat, solid, and oil to aqueous phase for a reaction time of 10 min. phase can adsorb to the organic compounds in the other phases, especially fat and oil. As shown, at 513 K (Figure 13a) Cu(II) and Cd(II) concentrations reached a constant value of 2.5 and 0.06 g/m3, respectively after 10 min. The concentration of Zn(II) was found to decrease to a value of 0.5 g/m3 after 40 min. A comparison between the results at 513 and 2362
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Received for review December 8, 2003. Accepted June 6, 2004. ES030713S
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