Anal. Chem. 1998, 70, 400-404
Technical Notes
A Comparison of Liquid and Supercritical Carbon Dioxide as an Extraction Solvent for Plating Bath Treatment Kenneth E. Laintz,*,† Christopher D. Hale,‡ Peter Stark, Connie L. Rouquette,§ and Jack Wilkinson§
Chemical Science and Technology Division, Los Alamos National Laboratory, CST-12, Los Alamos, New Mexico 87545
Because of increasing federal regulations, alternative metal recovery methods for spent electroplating baths are needed. Supercritical extraction of waste metals using CO2 is potential candidate for this process. However, because of the high-cost of supercritical fluid processing equipment, an extraction system modeled on the use of liquid CO2 rather than supercritical CO2 may be more economical. For this reason, the use of liquid CO2 containing β-diketones as an extraction solvent for the removal of Ni2+ and Zn2+ from an electroplating solution was investigated and compared to results obtained with supercritical CO2. Acetylacetone (ACAC) and its fluorinated analogues were used as the ligands for the extraction of Ni2+ and Zn2+ from the electroplating bath. While the fluorinated ACAC showed a potential for a higher extraction efficiency, over 70% of both Ni2+ and Zn2+ was extracted using both liquid and supercritical CO2 using a 100-fold molar excess of ACAC. With isopropyl alcohol as a modifier, extraction efficiency was increased to over 80% for both metals using both phases of CO2 as the carrier fluid. The advantages of using a supercritical fluid as an extraction solvent are well documented, and many books have been written regarding supercritical fluid technology. However, industrial interest in supercritical fluid applications tends to fluctuate. There will be a flurry of activity in supercritical fluid research for a several years and then interest will wane followed by a period of renewed interest. One possible cause for this cyclic interest could be the high-cost of capital equipment for large- or industrial-scale supercritical fluid applications. While prices have certainly dropped over the years and have become quite competitive with alternative technologies, the perceived high-cost barrier of high-pressure equipment still exists. In many cases, liquid CO2 could be used † Current address: Isotag, LLC, Isotag Research, 600 6th St., Los Alamos, NM 87544. ‡Current address: Department of Chemistry, University of Massachusetts, Lowell, MA 01854. § Current address: Environmental Technology, Nunez Community College, Chalmette, LA 70043.
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in place of supercritical CO2, thus reducing the overall capital equipment expense due to lower operating pressures of such a system. This has been shown to be true in the use of CO2 as a replacement solvent in cleaning applications, especially in the use of liquid CO2 as a drycleaning solvent.1 Therefore, it is possible that an extraction system modeled on the use of liquid CO2 rather than supercritical CO2 may be more economically attractive to industry, thus keeping interest in dense-phase fluid processing from cycling. Electroplating is an area where economic alternative processing technologies are needed. Traditionally, the electroplating industry has relied upon precipitation reactions for the treatment of spent plating bath solutions. The resulting sludge from such reactions was then subsequently disposed. This treatment scheme is no longer feasible as the disposal of the unstabilized sludge is no longer allowed. In addition, many current metal removal techniques are inadequate because they do not comply with current metal discharge limits.2 For this reason, alternative treatment methods for spent plating bath solutions are needed. Solvent extraction processes have often been considered for the replacement of precipitation techniques; however, these methods often use hazardous organic solvents. Furthermore, the use of such solvents in extraction processes generates large volumes of hazardous waste. Since goals of environmentally conscious manufacturing are the reduction of waste and elimination of hazardous solvents, an environmentally benign plating bath treatment method needs to be implemented. A potential candidate for this process uses extraction with carbon dioxide. As a replacement solvent, supercritical CO2 is generally less expensive than conventional organic solvents and is easily recyclable and nontoxic.3 In addition, extractions using supercritical CO2 are generally more efficient in terms of speed as compared to conventional solvent extraction because the more favorable mass transport properties of a supercritical fluid.4 Recently, supercritical CO2 has been used as an effective replace(1) Laintz, K. E.; Williams, S. B.; Spall, W. D.; Bustos, L. Los Alamos Natl. Lab. Rep. 1996, LA-UR-96-822. (2) Sauer, N. N.; Smith, B. F. Los Alamos Natl. Lab. Rep. 1993, LA-12532MS, UC-367. (3) Spall, W. D. Int. J. Environ. Conscious Des. Manuf. 1993, 2 (1), 81. (4) Brennecke, J. F.; Eckert, C. A. AIChE J. 1989, 35, 1409. S0003-2700(97)00285-0 CCC: $15.00
© 1998 American Chemical Society Published on Web 01/15/1998
ment solvent in industrial cleaning processes.5-7 In addition, it has also been used effectively in conjunction with ligands or chelating agents in laboratory experiments for the extraction of various metals from aqueous matrixes.8-10 Liquid CO2 can often substitute for supercritical CO2 since solvent strength tends to be comparable. In fact, the synthesis of ligands with enhanced CO2 solubilities, especially in liquid CO2, for the recovery of heavy metals has been suggested.11 While most extraction studies have looked at environmentally significant samples and used supercritical CO2 as the extractant, industrially relevant samples have yet to be explored. For this reason, the use of liquid CO2 containing β-diketones as an extraction solvent for the removal of zinc and nickel from an electroplating solution was investigated and compared to results obtained using supercritical CO2. The results of this preliminary investigation are presented in this paper.
EXPERIMENTAL SECTION A Ni/Zn plating bath solution was prepared according to previously reported instructions.2 It should be noted that the exact composition of the plating solution that was used in this particular study is proprietary. Since the high ionic strength plating solution contains approximately 1.8% Ni2+ and 0.9% Zn2+, the plating solution was diluted 100-fold with distilled deionized water prior to use in extraction experiments to conserve ligand use. The diluted plating solution was adjusted to a pH of 6.5 by dropwise addition of HCl or NH4OH to maintain the pH of the parent solution. Freshly diluted plating solution was used for each experimental trial. A 100-mL aliquot of the diluted plating bath stock solution was poured into a 150-mL stainless steel aqueous sample extraction vessel (CF Technologies, Hyde Park, MA) for each extraction experiment. This vessel was designed similarly to that used previously.9 A Teflon-coated magnetic stir bar was placed into the vessel which was placed on a Corning hot plate/magnetic stirrer. The solution was stirred to facilitate metal chelation and aqueous CO2 phase mixing. The hot plate was used to heat the vessel for those experiments conducted using supercritical CO2. In these experiments, the extraction vessel and plating solution were preheated to the desired temperature prior to supercritical fluid extraction. For ionic species such as metal ions to be extracted using CO2, a complexing agent must be added to the system. An appropriate amount of ligand was added to the extraction vessel, the solution was stirred, and the lid was placed on the vessel sealing it. The ligands were 2,4-pentanedione or acetylacetone (ACAC), 1,1,1trifluoroacetylacetone (TFA), and hexafluoroacetylacetone (HFA), (5) Williams, S. B.; Laintz, K. E.; Barton, J. C.; Spall, W. D. In Emerging Technologies in Hazardous Waste Management VI; Tedder, D. W., Pohland, F. G., Eds.; American Academy of Environmental Engineers: Annapolis, MD, 1996; pp 13-28. (6) Spall, W. D.; Laintz, K. E. Los Alamos Natl. Lab. Rep. 1995, LA-UR-951445. (7) Spall, W. D.; Williams, S. B.; Barton, J. C.; Laintz, K. E. Los Alamos Natl. Lab. Rep. 1994, LA-UR-94-3313. (8) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Anal. Chem. 1992, 64, 2875. (9) Laintz, K. E.; Tachikawa, E. Anal. Chem. 1994, 66, 2190. (10) Wang, J.; Marshall, W. D. Anal. Chem. 1994, 66, 1658. (11) Yazdi, A.; Beckman, E. J. Mater. Res. Soc. Symp. Proc. 1994, 344, 211.
all obtained from the Aldrich Chemical Co. (Milwaukee, WI). In each case, the ligand was allowed to react 5 min with stirring prior to pressurization with CO2. This reaction time was chosen arbitrarily, and it is quite possible that a longer reaction time could produce more efficient extractions. Off-line liquid and supercritical CO2 extractions of the plating bath solutions were performed using an Isco Model 260D syringe pump and controller (Isco, Lincoln, NE). Both liquid and supercritical CO2 extractions were conducted under static conditions (extraction vessel pressurized with solvent having no flow through the cell) for 15 min followed by dynamic extraction (solvent flow through the cell) for 50 min at a flow rate of 3 mL/min. This flow rate was liquid flow measured at the pump head. The dynamic extraction corresponded to three volume changes of the fluid phase totaling ∼150 mL of CO2. Liquid CO2 extractions were performed at 1500 psi and 25 °C, corresponding to a fluid density of ∼0.82 g/cm3. A comparable density of ∼0.82 g/cm3 for extractions using supercritical CO2 was achieved at 42 °C and 2800 psi. Since overall organic content in the fluid phase was not a primary concern because the analytes were transition metals, and because typical industrial conditions were to be simulated, food-grade CO2 (TriGas, Irving, TX) was used for both liquid and supercritical fluid extractions. It should be noted that food-grade CO2 was found to be pure enough for precision cleaning applications,5-7 and in cases where ultratrace analysis is not being performed, is an acceptable fluid for extraction purposes. In those experiments where isopropyl alcohol (IPA) was used as a modifier, SFE-grade CO2 containing 5% IPA was used (Scott Specialty Gases, Inc., Longmont, CO). This cylinder was rolled on the floor prior to use to ensure adequate mixing of the liquid CO2 and IPA. The cylinder cap must be left in place on the valve head when the cylinder is rolled to ensure safety. All extraction experiments were run in triplicate. The extraction pressure and flow rate were measured at the pump and maintained with a 25 cm × 50 µm i.d. fused-silica tubing restrictor. The extracts were collected by inserting the restrictor into a 50:50 (v/v) solution of methanol and water contained in a 100-mL beaker. The collection solvent was also warmed on a hot plate to prevent restrictor plugging in a manner analogous to previous studies9 since plugging commonly occurs when extraction is from aqueous matrixes from entrained water freezing during fluid expansion. Since the goal of this study was to compare liquid versus supercritical CO2 extraction of Ni and Zn from an aqueous matrix and to determine the extraction efficiency of such a process, the overall collection efficiency was not studied and the extract solution in the collection vessel was not analyzed. The extraction efficiency was then determined by the amount of metal remaining in the sample solution after extraction multiplied by 100 divided by the amount of metal present prior to extraction. Nickel and zinc concentration results were obtained through the use of a Varian Liberty 220 sequential ICP-OES system arranged and operated according to manufacturer’s recommended procedures. The argon plasma was operated at 1.0 kW power, with a plasma flow rate of 15.0 L/min and a nebulizer flow rate of 1.5 L/min. Nickel emission was monitored at 231.604 nm and zinc was monitored at 213.856 nm. The vertical and horizontal slits were set at 300 and 50 µm, respectively. The viewing height above the load coil was set at 15 mm for both species. Data from the Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
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spectrometer were collected by a Griff MFP+ MS-DOS-based data acquisition station. Three replicates were performed for each sample with the mean concentration reported. RESULTS AND DISCUSSION With more economically attractive alternative solvent processing in mind, the use of liquid CO2 to extract spent plating solutions was investigated. While ligand-modified supercritical CO2 has been shown to quantitatively extract metals from aqueous matrixes,8-10 the extraction efficiency of ligand-modified liquid CO2 for metals has been largely untested. For liquid CO2 at a room temperature of 25 °C, a pressure ∼1000 psi is needed. In order to ensure operation in the liquid phase, liquid CO2 extractions were performed at 1500 psi and 25 °C, which corresponds to a fluid density of ∼0.82 g/cm3. For comparative purposes, supercritical CO2 was also used in the extraction study. For solvation characteristics to behave similarly between liquid and supercritical CO2, the same density fluids were used. To achieve a comparable density of 0.82 g/cm3 for extractions using supercritical CO2, the extraction apparatus was heated to 42 °C and pressurized to 2800 psi. Because CO2 cannot extract charged species such as metal ions directly, a complexing agent must be added to the fluid phase. For a ligand-modified CO2 extraction system to be economically viable on an industrial-scale, the complexing agent needs to be readily available commercially. Accordingly, commonly available β-diketones were used for this study. One commonly used β-diketone for transition metals which is readily available is acetylacetone. Since fluorinated compounds, specifically fluorinated ligands and the corresponding metal chelates, have been shown to be more soluble in supercritical CO2 than their nonfluorinated analogues,8,11,12 acetylacetone analogues with varying degrees of fluorine substitution were chosen for this comparative study. This ligands were 1,1,1-trifluoroacetylacetone and hexafluoroacetylacetone. On the basis of this previous work, HFA and the corresponding Ni and Zn chelates are expected to be more soluble in CO2 than TFA and ACAC. Metal chelate coordination chemistry is an important factor in determining complex solubility. Zinc β-diketonates have a tetrahedral conformation while those of Ni2+ are square planar. The square-planar conformation is more polar because of the lone pairs of electrons of the central metal atom situated perpendicular to the ligand plane and therefore less likely to be extracted into nonpolar solvent.12 In a limiting situation where insufficient complex solubilities are present for complete complex formation and extraction, Zn2+ would be expected to have a higher extraction efficiency than Ni2+ because the tetrahedral zinc complex would be more soluble in the fluid phase. In any event, it was expected that the fluorinated ligands would have higher extraction efficiencies due to their higher overall solubilities. For both the liquid and supercritical extraction experiments, a 10-fold stoichiometric molar excess of each ligand was added for each metal. It was thought that this would be a sufficient quantity of ligand for quantitative extraction of both Ni2+ and Zn2+ from the plating bath solution based on previous work.8 Apparently, however, a 10-fold molar excess presented a limiting (12) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. J. Supercrit. Fluids 1991, 4 (3), 194.
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Table 1. Comparison of the Extraction Efficiency of Ligand-Modified Liquid and Supercritical CO2 extraction (%) liquid CO2 metal
ACAC
TFA
supercritical CO2 HFA
ACAC
TFA
HFA
4.2 ( 0.9 0 0 2.1 ( 0.1 1.6 ( 0.1 0 Zn2+ 19.2 ( 1.1 8.0 ( 0.2 37.3 ( 2.6 15.6 ( 0.6 16.9 ( 0.8 19.6 ( 0.4
Ni2+
situation, and as seen from the results summarized in Table 1, Zn2+ was extracted more successfully than Ni2+. As indicated in the table, Zn2+ had a much higher extraction efficiency than Ni2+ by ∼20% for both liquid and supercritical CO2. The best results were achieved using HFA in liquid CO2, with an extraction efficiency approaching 40%. In supercritical CO2, ACAC, TFA, and HFA all had nearly the same extraction efficiencies for Zn2+, approaching 20%. To a first approximation, these data would indicate that the Zn complexes were more soluble in CO2 than the Ni complexes as was expected. As also seen from Table 1, Ni2+ had a very low extraction efficiency from the plating bath solution. No detectable quantity of Ni2+ was extracted using liquid CO2 with TFA or HFA despite the fact that a 10-fold stoichiometric molar excess of each ligand had been added for each metal. However, ACAC, especially with liquid CO2, seemed to deliver the best extraction results for Ni2+ at 4%. Still, in all cases, the extraction efficiency of Ni2+ was minimal. Now, it is known that HFA undergoes a reverse Claisen condensation reaction (base-catalyzed hydrolysis) that involves β-diketone cleavage by nucleophilic attack when heated in the presence of water to form 1,1,1-trifluoroacetone and trifluoroacetic acid.13 It is possible that some of the HFA degraded in this manner under both liquid and supercritical conditions, especially considering the high concentrations of NH4+ and BO43- in the plating bath solution,2 resulting in the low extraction efficiencies of both Ni2+ and Zn2+. Similarly, TFA and ACAC could have undergone similar reactions, also resulting in the ligand-limited low-efficiency extractions. Rather than attempting to optimize extraction conditions since the goal of the experiment was to maintain liquid CO2 conditions, increased concentrations of ligand were added to the experimental system to increase extraction efficiency. Increasing temperature or pressure would have resulted in supercritical conditions which were already being investigated for comparative purposes. When excess ligand is added with the intention of modeling an industrialscale operation, ligand availability and cost must be considered. For this reason, excess ACAC was used because it is readily available rather inexpensively from commercial sources and thus most economically viable from an industrial standpoint. While the fluorinated ligands would probably form more soluble metal chelates, ACAC is much cheaper, thus allowing a much larger amount to be used. In order to improve on the results that were summarized in Table 1, a 100-fold molar excess of ACAC was added to the plating bath solution prior to extraction, thus increasing the previous ligand concentration by an order of magnitude. As shown in Table 2, increasing the ligand concentration substantially increased the extraction efficiency of both (13) Chiang, C.-M.; Miller, T. M.; Dubois, L. H. J. Phys. Chem. 1993, 97, 11781.
Table 2. Comparison of the Extraction Efficiency of Liquid and Supercritical CO2 Modified with a 100-Fold Molar Excess of ACAC
Table 4. Comparison of the Extraction Efficiency Based on Plating Bath Age extraction (%) using HFA with old plating bath solution
extraction (%) metal
liquid CO2
supercritical CO2
metal
liquid CO2
supercritical CO2
69.7 ( 11.5 71.7 ( 7.2
Ni2+
66.7 ( 6.6 62.9 ( 6.3
45.2 ( 2.7 47.3 ( 2.7
73.6 ( 7.5 76.8 ( 6.6
Ni2+ Zn2+
Table 3. Comparison of the Extraction Efficiency of Excess Ligand-Modified Liquid and Supercritical CO2 Containing 5% Isopropyl Alcohol extraction (%) liquid CO2
supercritical CO2
metal
ACAC
HFA
ACAC
HFA
Ni2+ Zn2+
83.2 ( 6.3 83.1 ( 0.5
83.3 ( 1.4 81.7 ( 0.5
85.2 ( 8.5 84.4 ( 8.1
82.6 ( 3.4 78.9 ( 0.8
metals. Unlike the previous results, this case posed no extractionlimiting situation with one metal beingpreferentially extracted over the other. This is evidenced by statistically equal concentrations of both Ni2+ and Zn2+ being extracted using both liquid and supercritical CO2. As seen from the results summarized in Table 2, ∼75% of both metals was extracted using liquid CO2 and ∼70% was extracted using supercritical CO2. While this increase in extraction efficiency is significant, it is still somewhat low for quantitative purposes. Therefore, another approach at optimizing extraction conditions was needed. In order to further increase the extraction efficiency of Ni2+ and Zn2+ from the plating solution, alcohol-modified CO2 containing excess ligand was used. Methanol-modified supercritical CO2 containing a dissolved ligand has been used successfully in the past to extract metals.14,15 In this case, the 100-fold excess of ACAC was used in conjunction with 5% IPA. Isopropyl alcoholmodified CO2 was used simply because it was on hand. Methanolmodified CO2 fluid systems probably would have worked equally as well.14,15 With IPA-modified liquid CO2, over 80% of both the Ni2+ and Zn2+ was extracted from the plating solution. As before with excess ACAC, equivalent amounts of both metals were extracted, and these results are summarized in Table 3. As seen from the table, IPA-modified supercritical CO2 also extracted over 80% of both metal ions. While still not totally quantitative, this overall level of extraction would be useful on an industrial-scale. For comparative purposes, an excess of fluorinated ligand with IPA-modified CO2 was also tested for effectiveness in the overall extraction efficiency of the metal ions from the plating solution. In this instance, a 20-fold molar excess of HFA was used with IPA-modified CO2, and these results are also summarized in Table 3. These results are the same as those achieved using a 100-fold molar excess of ACAC. Using both liquid and supercritical CO2, over 80% of both metals was extracted. This would suggest that the higher solubility of the fluorinated chelates leads to higher extraction efficiencies. Based on this assumption, using an even (14) Wai, C. M.; Lin, Y.; Brauer, R.; Wang, S.; Beckert, W. F. Talanta 1993, 40, 1325. (15) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M. Anal. Chem. 1993, 65, 2549.
Zn2+
larger quantity of HFA for the extraction might lead to quantitative removal of both Ni2+ and Zn2+ from the plating solution. However, the cost of HFA would most likely preclude using a large quantity on an industrial-scale. It should be noted that a freshly prepared plating solution had much different extraction efficiencies compared to an old solution. In the old solution, one that had been sitting around for a couple of months in a cool location, much of the interfering ionic salts present precipitated out, leaving largely Ni2+ and Zn2+ in the solution. As shown in Table 1 with HFA, nearly 40 and 20% of the Zn2+ were extracted using liquid and supercritical CO2, respectively, from the fresh plating solution. Nickel was not extracted in either case. In contrast, Table 4 shows that a 10-fold molar excess of HFA extracted over 60% of both the Ni2+ and Zn2+ from the old solution using liquid CO2 as the carrier fluid. This was significantly more than was extracted using supercritical CO2, with only ∼45% of both metals being extracted. The difference in extraction efficiency is probably due to plating bath solution mixing prior to extraction, thus dissolving more of the inorganic salts, rather than due to solvent characteristics. However, the greater extraction efficiencies achieved with the old solution would suggest a spent plating bath pretreatment strategy if CO2 extraction was to be used as a metal recovery treatment method. CONCLUSION As seen in all investigated conditions, these preliminary data show that liquid CO2 containing a dissolved β-diketone ligand performed equally as well as supercritical CO2 containing the same ligand. Fluorinated HFA appeared to work slightly better than ACAC; however, more ACAC was able to be used to achieve the same end result. In addition, liquid CO2 has a lower solubility in aqueous solutions than supercritical CO2. This results in less dissolution of the carrier fluid in the sample matrix, thus reducing solution effervescence after extraction and resulting in quicker sample turnaround. Based on these results, dense-phase extraction with CO2 as the carrier solvent could be a potential candidate for an alternative spent plating bath treatment process. The importance of liquid CO2 extracting as well as supercritical CO2 is that, in some processes, liquid CO2 could be substituted for supercritical CO2. This would reduce the overall operating pressure of such systems. A lower operating pressure is potentially safer and would result in an overall reduced capital equipment expense. Because of this, an extraction system using liquid CO2 rather than supercritical CO2 is conceivably more economically viable from an industrial standpoint. Therefore, research efforts intended for eventual large-scale applications may benefit from investigation of the use of liquid CO2 in addition to supercritical CO2 for such processes. Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
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ACKNOWLEDGMENT The authors thank Drs. Tom Robison and Barb Smith of Los Alamos National Laboratory for helpful discussions and for the plating bath solutions used in this study. This work was funded by the University Outreach Program of the Science and Technology Base Programs Office of Los Alamos National Laboratory and
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by the Department of Energy. Los Alamos National Laboratory Report No. LAUR-97-757. Received for review March 14, 1997. Accepted October 16, 1997.X AC9702857 X
Abstract published in Advance ACS Abstracts, December 15, 1997.
