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Study of Extraction of Metals from CCA-Treated Wood with Supercritical CO2 Containing Acetylacetone: Extraction of Cu by Continuous Addition of Acetylacetone Y. Takeshita,* Y. Sato, and S. Nishi NTT Lifestyle and Environmental Technology Laboratories, 3-1 Morinosato Wakamiya Atsugi-shi, Kanagawa 243-0198, Japan
Although waste CCA-treated wood containing toxic heavy metals (Cu, Cr, and As) can contaminate soil and be hazardous to human health, there is currently no reliable detoxification technique. To solve this problem requires a method for extracting the toxic metals from such wood. We have previously shown that supercritical CO2 with initially added acetylacetone (AA) as a chelating agent achieved a Cu extraction ratio of up to 71.0%. The purpose of this study was to explore conditions for improving the extraction efficiency of Cu: that is, to extract Cu by using supercritical CO2 with continuously added AA and to investigate the effect of the flow rate and AA/CO2 mixing ratio on the extraction behavior. The highest ratio we obtained for Cu was 89.4% at 14.7 MPa and 423.2 K with a flow rate of 0.40 mol/min and a mixing ratio of 4.8 mL of AA/mol of CO2. We found that the extraction process consisted of static and dynamic extraction stages and that a high flow rate produced a high extraction rate. After the extraction had proceeded for a while, the extraction rate decreased because of the decrease in the residual metal in the wood. 1. Introduction Wood containing preservatives has long been used worldwide in large quantities for telegraph poles, street lighting poles, and railway ties. Many of these preservatives are liquid-treatment agents containing chromium, copper, and arsenic compounds, so they are called CCAs. The hazardous heavy-metal components in waste CCAtreated wood have the potential to cause environmental problems in groundwater, soil, and air and damage human health. Our company has installed over 1 million poles throughout Japan. When this CCA-treated wood is burned in garbage incinerators, the heavy metals survive and concentrate in the ashes and incineration gases, which might cause pollution. Nishitani et al. reported that As becomes volatile at the beginning of incineration in their study on metal behavior during the incineration of CCA-treated wood.1 Even when this wood is not incinerated but dumped in landfills, the heavy metals can leach out. This may damage both soil and groundwater. Furthermore, when this wood is recycled, its toxic content can damage human health. However, there are no reliable techniques for detoxifying CCA-treated wood. Kanjo et al. reported a novel technique using sulfuric acid heated for 5 h to extract over 95% of metals from wood that had been cut into small pieces.2 Kajimoto et al. also reported the highspeed separation of preservative and wood components at a high temperature of over 1000 °C.3 Traditional solvent extraction requires the use of an organic liquid, which causes environmental problems for both solvent handling and disposal. Therefore, our goal is to develop a new technique for rendering waste CCAtreated wood harmless. Specifically, we want to find an * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: +81-46-240-3142. Fax: +8146-270-2320.
environmentally friendly way of extracting heavy metals from this wood. Supercritical CO2 has been widely investigated for metal extraction.4-6 It is very attractive from an environmental point of view because it is nontoxic and nonflammable and has the potential for recycling. We have tested the use of supercritical CO2 with initially added acetylacetone (AA) as a chelating agent for extracting metals from CCA-treated wood and reported that the Cu extraction ratio was 71.0%, which suggests that it has the potential to become a new detoxification method.7-9 The purpose of this study was to explore conditions for improving the Cu extraction efficiency. We extracted Cu by using supercritical CO2 with continuously added AA and investigated the effect of the CO2 flow rate and AA/CO2 mixing ratio on the concentration of Cu in supercritical CO2 during extraction. The extraction behavior is also discussed. 2. Experimental Section 2.1. Materials. CCA-treated wood (supplied by Kanematsu Nissan Norin Ltd., Tokyo, Japan) containing chromium, copper, and arsenic compounds was used as received. The wood had been immersed in a liquidtreatment agent containing chromium, copper, and arsenic compounds at 45-51%, 17-21%, and 30-38% in the form of CrO3, CuO, and As2O5, respectively. CCAtreated wood was obtained in pieces of about 10 cm × 10 cm × 20 cm from the supplier and was then cut into small cubes with a side of about 1 cm. One cube was selected at random for each run of the experiment. Because this wood was originally meant for industrial use, we could not guarantee the uniformity of the metal concentration in each piece. The weight of the wood sample and experimental conditions are shown in Table 1. High-purity carbon dioxide (more than 99.5% pure; Suzuki Shokan Co., Tokyo, Japan) was used as received.
10.1021/ie000180l CCC: $19.00 © 2000 American Chemical Society Published on Web 11/02/2000
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Figure 1. Experimental apparatus. Table 1. Wood Samples and Experimental Conditions Used in This Study
run
wt of wood w [g]
CO2 flow rate v [mol/min]
AA/CO2 m [mL of AA/ mol of CO2]
1 2 3 4
0.788 1.529 0.634 0.909
0.40 0.18 0.18 0.18
4.8 4.2 9.7 2.1
Reagent-grade acetylacetone (more than 99.0% pure; Wako Pure Chem. Ind. Ltd., Osaka, Japan) was used as a chelating agent. 2.2. Equipment and Procedure. A flow-type apparatus, shown schematically in Figure 1, was used in this study. From a cylinder (1) with a siphon attachment, liquid CO2 was supplied through a cooling unit (2) and sent by a high-pressure pump (3) to the extraction cell (4), which was heated to the desired temperature by a cartridge heater (5), while the content in the cell was stirred with a stirrer (6). The stirrer was 25 mm in diameter and 10 mm in width; the stirring rate was 350 rpm. The cell was made of SUS316, and its inside volume was 92 cm3. The pipe, made of SUS316, was also heated to the same temperature as the extraction cell by a ribbon heater (7). When CO2 passed through the extraction cell and the pipe, it became a supercritical fluid. One piece of CCA-treated wood was put into the cell in each experimental run. A total of 7 mL of AA was poured into the cell at the beginning of the extraction procedure; this amount was more than enough to react with all of the metal in the wood. The temperature in the cell was monitored with a thermocouple (8) and kept within (1.0 K of the desired temperature using a PID controller. The inside pressure was controlled by a back-pressure regulator (11) to an accuracy of (0.1 MPa. The pressure was measured with a Bourdon gauge (9). When the temperature and pressure reached the desired values, the contents of the cell were stirred for 15 min (static extraction). Then CO2 was continuously supplied by the high-pressure pump (3). At the same time, AA was continuously supplied by the high-pressure pump for solvent (15). This moment was defined as the beginning of extraction. After passing through the back-pressure regulator (11), decompressed gaseous CO2 and the extracts were separated in the flask (12). The acid solution in the flask was renewed every 30 min. The flow rates at which liquid CO2 and AA were introduced were adjusted using the high-pressure pumps (3 and 15). The volume of CO2 was measured with a wet gas meter (13). The volume of AA introduced was determined from the reduced volume of AA measured in the vessel (14). The amounts of Cu collected in the flask and those remaining in the wood were determined by inductively coupled plasma (ICP) measurement (SPS1700HVR,
Figure 2. (a) Extraction curves for runs 1 and 2, concentration of Cu vs extraction time. (b) Extraction curves for runs 1 and 2, concentration of Cu vs CO2.
Seiko Instruments Co. Ltd., Chiba, Japan). The wood sample was decomposed using concentrated H2SO4 and H2O2 and then diluted to a standard volume and measured. These procedures followed the guidelines laid down in JIS-A9107. Two pieces weighing 0.1-0.2 g were cut from the wood at random, the concentrations of Cu were measured, and the average was determined. The wavelength for measurement was 324.8 nm for Cu. We also used Mn as a standard element (wavelength for measurement: 257.6 nm). We selected the condition of 423.2 K and 14.7 MPa in this study because it gave a relatively high extraction ratio in our previous work.7-9 2.3. Evaluation. We evaluated the extraction behavior from the change in concentration of Cu in the solvent (supercritical CO2), which was determined from the metal collected in the flask using the following equation:
concentration of Cu in the solvent [mol/mol of CO2] ) metal collected CO2 participating during extraction To evaluate how much metal was recovered in this procedure, we defined the extraction ratio of Cu as
extraction ratio [%] ) metal collected × metal in wood after extraction + metal collected 100 3. Results and Discussion 3.1. Extraction Behavior and Effect of the CO2 Flow Rate. The extraction behavior was investigated by observing the change in concentration of Cu with extraction time (Figure 2a). The CO2 flow rates ranged
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Figure 4. Extraction curves for runs 2-4, concentration of Cu vs extraction time. Figure 3. Extraction ratios for runs 1 and 2.
from 0.18 to 0.40, and the mixing ratio of AA and CO2 was constant at about 4-5 mL of AA/mol of CO2. The concentration of Cu initially increased steeply with extraction time and then gradually decreased for run 2, similar to typical curves for extracts from solids such as coffee beans and oil seeds reported by Brunner.10 We think that the extraction process consisted of static and dynamic extraction stages. The initial part of the curve was due to the static batch process. In the second part of the extraction proceeding, the concentration of Cu decreases with ongoing extraction, because of increasing mass-transfer resistances and depletion of metal in the wood. When the flow rate increased from 0.18 to 0.40 mol/ min, the maximum concentration appeared at an earlier time. We also examined this behavior in Figure 2b, where the horizontal axis is moles of CO2. In Figure 2b, the two curves are almost overlapped. This shows that even when the CO2 flow rate increased, the concentration of Cu in the solvent was maintained, which indicates that the extraction rate (amount of Cu extracted per unit time) was also enhanced by increasing the CO2 flow rate. The solubility of cupric acetylacetonate in supercritical CO2 extrapolated from Lagalante’s work11 is also shown in Figure 2. The Cu concentrations in our study were lower than that extrapolated value. This may be caused by the fact that, in the extraction process, the effects of CO2 and AA diffusion into the wood chip, the formation of the metal chelating compound of Cu and AA, and additional interaction between Cu and chip are related. Figure 3 shows the extraction ratios for runs 1 and 2. They initially increased steeply and then increased gradually. We found that the extraction reaction proceeded steadily during the entire extraction time. This may be caused by the fact that the continuous addition of AA efficiently contributed to the extraction process. We also found that, after extraction had proceeded for a while, the extraction rate decreased because of the decrease in residual metal. 3.2. Effect of the Mixing Ratio of AA and CO2. Figure 4 shows the effect of the mixing ratio of AA and CO2 on the extraction behavior. The mixing ratios were 2.1, 4.2, and 9.7 mL of AA/mol of CO2, and the CO2 flow rate was constant at 0.18 mol/min. The initial slopes for runs 2 and 3 were almost the same. This may be because they shared the same flow rate. The curve for run 4 fell earlier than those of runs 2 and 3. This may
Figure 5. Extraction ratios for runs 3 and 4.
Figure 6. Highest extraction ratios in various conditions.
be because the extraction of run 4 had already proceeded more by the first data point (30 min). This was also supported by Figure 5, in which the first data point of run 4 showed an extraction ratio 45.4% higher than that of run 3 (1.7%). We think that the AA/CO2 mixing ratio of run 4 was lower than those of runs 2 and 3, so the small amount of AA did not prevent the diffusion of the chelating compound like AA-Cu from the interior of the wood chip to its exterior. The values in this study were lower than the extrapolated solubility, as in Figure 2. Figure 5 shows the extraction ratios for runs 3 and 4. They lie between 60 and 80%. It seems that run 4, in which AA/CO2 was lower, had a better extraction condition than run 3 because of the rapid progress of extraction. Finally, Figure 6 shows the highest extraction ratios in this study along with that without AA at 14.7 MPa and 423.2 K. When AA was continuously added, the extraction ratio was improved and reached 89.4%. This may be caused by the fact that the added AA played an effective role in the extraction reaction throughout the extraction time.
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4. Conclusions We extracted the metals in CCA-treated wood chips, determined the extraction ratios, and investigated the effect of the CO2 flow rate and mixing ratio on the extraction behavior. The following conclusions were obtained: (1) The extraction process consisted of static and dynamic extraction stages. The extraction amount initially increased and then gradually decreased, similar to the typical extraction of substances with supercritical fluids from solid substrate. (2) When the CO2 flow rate increased, the maximum concentration appeared earlier, and a large amount of metal could be extracted in a short time. (3) The highest extraction ratio was 89.4% with a flow rate of 0.40 mol/min and a mixing ratio of 4.8 mL of AA/mol of CO2. To ensure that the extraction efficiency is high enough for industrial use, a more suitable agent for each metal should be investigated, and the extraction conditions should be studied in detail in future work. Acknowledgment We are grateful to Professor Motonobu Goto of Kumamoto University for his invaluable suggestions and discussions and to Dr. Nakahachiro Honma of NTT Advanced Technology Corp. for his enthusiastic support in the ICP analysis. We also thank Kanematsu Nissan Norin Ltd. for providing the CCA-treated wood. Nomenclature m ) mixing ratio of AA and CO2, mL of AA/mol of CO2 v ) CO2 flow rate, mol/min w ) weight of the wood sample, g
Literature Cited (1) Nishitani, T.; Fukunaga, I.; Ito, H. Osaka Public Environ. Sci. Res. Rep., Invest. Res. Ann. Rep. 1994, 56, 46-52.
(2) Kanjo, Y.; Kimoto, A.; Honda, A. Extraction of Chrome, Copper, and Arsenic Compounds from Waste Preservative-Treated Wood. Waste Manage. Res. 1994, 5, 185-192. (3) Kajimoto, T.; Takagaki, M.; Hata, T.; Imamura, H. Separation of Components of CCA-treated Wood by High-Speed Thermal Decomposition; Japan Wood Preserving Association, 15th Annual Meeting, Tokyo, May 1999; Ceec Corporation: Tokyo, pp 43-47. (4) Sasaki, T.; Meguro, Y.; Yoshida, Z. Spectrophotometric Measurement of Uranium (VI)-Tributyl phosphate Complex in Supercritical Carbon Dioxide. Talanta 1998, 46, 689-695. (5) Cross, W.; Akgerman, A., Jr.; Erkey, C. Determination of Metal-Chelate Complex Solubilities in Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1996, 35, 1765-1770. (6) Ohashi, K.; Tatenuma, K. Extraction Behavior of Gallium(III) with 2-Methyl-5-hexyloxymethyl-8-quinolinol and 5-Hexyloxymethyl-8-quinolinol from Weakly Acidic Solution into Supercritical CO2 and Selective Separation of Gallium(III) from Aluminum(III). Chem. Lett. 1997, 11, 1135-1136. (7) Takeshita, Y.; Sato, Y.; Nishi, S. Supercritical Fluid Extraction of Toxic Metals From Woods Containing Preservatives. In Proceedings of the First International Symposium on Environmentally Conscious Design, Tokyo, Japan, Feb 1999; Yoshikawa, H., Yamamoto, R., Kimura, F., Suga, T., Umeda, Y., Chairmen; IEEE Computer Society: Piscataway, NJ, 1999; pp 906-910. (8) Takeshita, Y.; Sato, Y.; Nishi, S. A Study of Feasibility of Extracting Metals from CCA-treated Woods by Using Supercritical Carbon Dioxide. J. Jpn. Soc. Waste Manage. Experts 2000, 11, 94100. (9) Takeshita, Y.; Sato, Y.; Nishi, S. Extraction of Metals from CCA-treated Woods by Supercritical Carbon DioxidesExtraction of Copper and Chromium. Wood Preserv. 2000, 26, 17-25. (10) Brunner, G. Gas Extraction; Steinkopff Darmstadt Springer: New York, 1994; pp 179-192. (11) Lagalante, A. F.; Hansen, B. N.; Bruno, T. J.; Sievers, R. E. Solubilities of Copper(II) and Chromium(III) β-Diketonates in Supercritical Carbon Dioxide. Inorg. Chem. 1995, 34, 5781-5785.
Received for review February 7, 2000 Revised manuscript received June 12, 2000 Accepted June 13, 2000 IE000180L
