Supercritical fluid extraction of organic and inorganic mercury from solid materials

0039-9MO/93s6.00+ 0.00

Tdmra,Vol. 40, No. 9, pp. 1325-1330,1993

Copyright 8 1993Pergamon Press Ltd

Printed in Great Britain. All rights reserved

SUPERCRITICAL FLUID EXTRACTION OF ORGANIC AND INORGANIC MERCURY FROM SOLID MATERIALS C. M. WA],* YUEHE LIN, RUSSELL BRAUER and SHAOFENWANG Department of Chemistry, University of Idaho, Moscow, ID 83843, U.S.A. WERNER F. BECKERT EPA, Environmental Monitoring Systems Laboratory, Las Vegas, NV 89119, U.S.A. (Received 21 April 1993. Accepted 23 April 1993)

Summary-Mercuric ions (Hg+) can be extracted from solid samples (cellulose matrix) using methanol modified supercritical CO, containing the fluorinated chelating agent lithium bis(tritluoroethyl)dithiocarbamate (LiFDDC). Methylmercuric chloride (CH,HgCl) and dimethylmercury [(CH,),Hg) can be extracted by supercritical CO, without chelating agent and modifier. The solubility of Hg(FDDC), in supercritical CO, has been determined to be 5 x 10e3M at 50°C and 150 atm, which is about 3 orders of magnitude greater than that of the non-fluorinated analogue Hg(DDC),. Use of methanol (5%)modified CO, further enhances the solubility of Hg(FDDC), by a factor of 2.4. A small amount of water added to the sample matrix tends to facilitate the extraction of Hg(FDDC), and CH,HgCI. Potential applications of this in situ chelation-supercritical fluid extraction method for the preconcentration of mercury species and treatment of mercury contaminated wastes are discussed.

Supercritical fluid extraction (SFE) has become an attractive alternative to conventional solvent extraction for the recovery of organic compounds from environmental and biological samples because of several advantages, including increased speed, better recovery, and the reduction in both solvent usage and solvent waste generation.14 To date, most of the published SFE work has focused on organic compounds, and few reports have dealt with SFE of metal ions and organometallic compounds. Wai and coworkers first reported the extraction of Cu2+ from liquid and solid materials using supercritical carbon dioxide containing lithium bis(trifluoroethyl)dithiocarbamate (LiFDDC) as an extractant.’ These authors also showed that the solubilities in supercritical CO, of some metal-FDDC complexes, including those of Cu*+, Ni*+, Co3+, and Bi3+, are 2-3 orders of magnitude greater than those of the non-fluorinated analogues.6 The use of fluorinated ligands for SFE of metal ions appears necessary in order to achieve significant recoveries of metals from liquid and solid materials. Addition of a modifier such as methanol to supercritical CO2 alters the polarity of the fluid phase which may also enhance the extraction of metal chelates. However, no quantitative information regard*Author for correspondence.

ing the effect of modifiers on the SFE of metal chelates is available in the literature. This paper reports an in situ chelation method for the extraction of Hg*+ from solid materials with supercritical CO2 and methanol-modified CO,, both containing LiFDDC as a chelating agent. The extraction of CH,HgCl and (CH3)*Hg from solid materials by supercritical CO2 is also described. EXPERIMENTAL

Reagents and materials

Lithium bis(trifluoroethyl)dithiocarbamate was synthesized according to a procedure outlined in the literature.’ The starting material, bis(trifluoroethyl)amine, was obtained from PCR Chemicals (Gainesville, FL). Other chemicals used in the synthesis, including n-butyllithium (2X4 in hexane), carbon disulfide, and isopentane were obtained from Aldrich Chemical Co. (Milwaukee, WI). Sodium diethyldithiocarbamate, NaDDC, was purchased from Fisher Scientific Company (Pittsburgh, PA). The Hg(FDDC), and Hg(DDC), were prepared by mixing LiFDDC or Na(DDC) with an excess amount of HgZ+ in a buffered aqueous solution at pH 3. The resulting precipitate was extracted with chloroform, and the organic phase was washed with deionized water after phase

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separation. Purification of Hg(FDDC), and Hg(DDC), was achieved by recrystallization from chloroform/ethanol (1: 1 v/v) at 60°C. After evaporation of the chloroform, the crystals of mercury chelates were collected by filtration. An aqueous Hg*+ solution was prepared by dissolving a known amount of solid Hg(NOj)* in deionized water. Methylmercuric chloride standard solution and dimethylmercury (Alfa Chemical Co., Ward Hill, MA) were used as received. For the extraction of mercury compounds from the solid matrix, a known amount (usually 10 pg) of Hg*+, CH,HgCl or (CH,),Hg was spiked onto a cellulose-based filter paper (Whatman 42, 0.5 x 2 cm in size). For the preparation of standard samples of mercury for neutron activation analysis, we found it necessary to add a small amount of LiFDDC solution to the sample spiked with the Hg*+ compound in order to stabilize the Hg*+. Otherwise, loss of mercury might occur during neutron irradiation and analysis. Apparatus

All experiments were performed with a laboratory-built supercritical fluid extraction apparatus. SFC-grade CO, or CO, with 5% methanol (Scott Specialty Gases, Plumsteadville, PA) was delivered to the system using a microprocessor-controlled high-pressure pump (Haskel Inc., Burbank, CA). The pressure of the system was monitored to &5 psi using a Setra Systems (Acton, MA) pressure transducer. The extractor consisted of an inlet valve (SUPELCO, Bellefonte, PA) and an outlet valve connected to a 3.5 mL commercial extraction cell (Dionex, Sunnyvale, CA). The extraction cell was placed in an oven that was temperature controlled by a thermostat. Fused-silica tubing (Dionex, 50 pm i.d. and 20 cm in length) was used as the pressure restrictor for the exit gas. The SFE system allows static and dynamic extractions to be performed by use of the outlet and inlet valves. Extraction procedures

A glass tube plugged at the forward end with a piece of glass wool was used as a sample holder. Into the rear end of the glass tube, a filter paper spiked with 10 fig of Hg*+ was inserted, and 10 mg of solid LiFDDC was added. The rear end was plugged with glass wool and the sample tube was placed in the extraction cell. The extraction cell was immedi-

ately installed in the oven, heated to 50°C and pressurized to 100 atm. After 20 min of static extraction, the exit valve was opened and the sample was extracted dynamically for 10 min. The tube was removed from the extraction cell and the filter paper was placed into a polyethylene vial which was then heat sealed for neutron irradiation. A standard consisting of a filter paper spiked with 10 pg of Hg2+ was also sealed in a polyethylene vial and irradiated together with the samples under identical conditions. The extraction efficiencies, as reported here, are the activities of ‘97Hg found in the extracted samples times 100, divided by the activity of ‘97Hgfound in the standard. The SFE conditions for CH,HgCl and (CH,)*Hg were the same as in the Hg*+ experiments, except no LiFDDC was added to the sample. Solubility measurements

A weighed amount (around 100 mg) of Hg(FDDC)* or Hg(DDC), was placed in a glass sample tube. The sample tube was then plugged with glass wool at both ends and inserted into an extraction cell of known volume (0.57 mL). The sample was extracted at 50°C under 150 atm of CO2 for 30 min. After this static extraction, the fluid phase was vented into a collection vial containing 4 mL of chloroform. The sample tube was removed from the cell and the empty cell was reinstalled into the oven. The system was then flushed with CO, for 20 min to collect any mercury complex that had precipitated within the system during depressurization. The total amount of the mercury complex collected in the chloroform solution was back-extracted with 2 mL of 50% Ultrex HNO,. Aliquots of 0.5 mL of the acid solution were sealed in polyethylene vials for neutron activation analysis. The solubility was calculated from the amount of mercury collected in the chloroform divided by the volume of the extraction cell. Neutron activation analysis (NAA)

All samples and standards were irradiated for 1 h in a 1 MW TRIGA nuclear reactor at a steady flux of 6 x lOI* n cm-* s-‘. After irradiation, the samples were cooled for 24 h before counting. Each sample was counted for 200 s in a large-volume ORTEC Ge(Li) detector with a resolution (FWHM) of about 2.3 keV at the 1332 keV ‘jOCopeak. The 77.6 keV gamma peak from ‘97Hg (tj = 65 h) was used for the detection of mercury. The detector output was fed to a Nuclear Data 4096-channel pulse-height

Mercuric ion extraction analyser. The details of the NAA procedures are given elsewhere.’ RESULTS

AND DISCUSSION

Solubilities of Hg(FDDCh with and without modifier

in supercritical

CO2

Supercritical fluid extraction of metal chelates in CO2 has received little attention in the literature. This is believed to be caused by the low solubilities of these compounds in supercritical fluids. Several studies have been reported on the separation of metal chelates by supercritical fluid chromatography using CO, as a mobile phase, but no solubility data were given.g*‘OIn a recent study, we observed that a number of metaldiethyldithiocarbamates (DDC) exhibited limited solubilities in supercritical C02.“*‘2 It was also found that if fluorine was substituted for hydrogen in the ligand, as in the case of bis(trifluoroethyl)dithiocarbamate (FDDC), the solubilities of the fluorinated metal chelates of Cu, Co, Ni, and Bi in supercritical CO, at 50°C and 100 atm were increased by 2-3 orders of magnitude. The solubilities of these metal-FDDC complexes were measured by their absorption in the UV-Vis region using a high-pressure view-cell described previously.6 The spectroscopic analysis method can not be applied to Hg(FDDC), because it does not have characteristic absorption peaks in the UV-Vis region. Therefore, we measured in this study the solubility of Hg(FDDC), by NAA as the amount of the metal chelate dissolved in a known volume of CO, in an extractor at fixed T and P. The procedure described in the experimental section for the solubility measurement was tested with Cu(FDDC), whose solubility in supercritical CO2 had previously been determined by the spectroscopic method.6 The chemical analysis method gave a solubility value for Cu(FDDC), within 5% of that determined by the UV-Vis spectroscopic method, thus proving the reliability of the method used in this study. The solubilities of Hg(DDC), and Hg(FDDC), determined at 50°C and 150 atm are (8.2 + 0.6) x 10m6M and (5.0 + 0.4) x 10-3M, respectively. Thus, the solubility of the fluorinated complex Hg(FDDC), in supercritical CO, is about three orders of magnitude greater than that of the nonfluorinated analogue Hg(DDC), under the conditions used. Another factor that affects the solubility of metal chelates in supercritical CO2 is the presence of a polar modifier such as methanol. The

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solubilities of Hg(FDDC), and Hg(DDC)* increased by a factor of 2.4 and 3.6, respectively, when 5% methanol was added to CO* (Table 1). The critical temperature and pressure for the methanol (5%) modified CO2 are known in the literature (T, = 41.8”C and P, = 73.6 atm).13 Our experimental T and P were kept significantly higher than the T, and P, of the modified fluid phase to ensure that the extraction occurred in the supercritical fluid region. The strong solubility increase when methanol is added as a modifier is a useful feature for the SFE of metal-chelates. Efects of methanol modifier on the extraction mercury

of

The efficiency of the SFE of solutes from solid matrices generally depends on at least the following three factors: (1) the analyte solubility in the supercritical fluid, (2) the interactions between the substrate and the analytes, and (3) the kinetics of the fluid to transport analytes from active sites on the matrix. In this study, a cellulose-based filter paper was used as a solid support where both strong chemical and physical interactions between solute and matrix can lead to slow extraction of ionic analytes. In our initial SFE experiments with mercury compounds, we used 10 pg Hg*+ (about 5 x lo-* moles) spikes on 2 x 0.5 cm filter paper and 10 mg LiFDDC (3.8 x lO-6 moles) as the chelating agent. The ligand was in large excess relative to the mercury ions in these experiments. A static extraction time of 20 min at 50°C and 100 atm, followed by a dynamic extraction for 10 min at the same T and P was chosen for extraction efficiency studies. The results of the SFE of Hg*+ with CO2 under these conditions are summarized in Table 2. The extraction efficiency of Hg2+ from filter paper with pure CO, in the absence of LiFDDC was insignificant (~2%). With the addition of LiFDDC, some Hg2+ was extracted by C02, but the extraction efficiency was low (8-12%) for dry samples. This could

Table 1. Solubility of Hg(FDDC), and Hg(DDC)2 in pure and modified CO, at 150 atm and 50°C Metal chelate Hg(FDDC), Hg(DDC), Hg(FDDC), Hg(DDC),

Solubility (mol/L)

Ratio (FDDCIDDC)

Carbon dioxide (5.0 + 0.4) x IO-” (8.2 + 0.6) x 1O-6

610

Carbon dioxide with 5% methanol (1.2 f 0.4) x 10-Z 400 (3.0 f 0.5) x 1o-5

C. M. WAI et al.

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Table 2. Extraction efficiencies of H$+ from Whatman 42 filter paper by pure and modified CO, at 100 atm and 50°C Fluid phase CO, CO, CO* CO, + S%MeOH CO, + S%MeOH

Ligand amount Li(FDDC)

Matrix condition

0 10 mg 10 mg 10 mg IO mg

Wet Dry Wet Dry Wet

Extraction efficiency (%)’ 1.2 + 10.3 + 84.5 f 92.5 + 99.5 f

0.5 2.0 3.0 2.5 2.0

*Average value of three runs.

not have been caused by a solubility limitation because the Hg2+ spike amounted to less than 1% of the solubility limit of Hg(FDDC)2 in the fluid phase. This low extraction efficiency is probably due to strong interactions between Hg*+ and the cellulose matrix. The extraction efficiency for Hg*+ increased from 10 to 84% when a small amount of water (10 pL) was added to the filter paper. The presence of water probably facilitates the chelation and transport of Hg(FDDC)* from the cellulose matrix to the fluid phase. The water may also serve as a matrix modifier by blocking active sites of the matrix and thus reducing sorption of the solute on the active sites of the polar matrix. Similar observations were reported in the literature for the extraction of polar compounds from polar matrices by supercritical C02.14 The efficiency of extracting Hg*+ can be further enhanced when methanol is added to the fluid phase. The results of extracting Hg*+ using a commercially available SFC grade CO2 containing 5% methanol are also summarized in Table 2. The methanol-modified CO2 increases

the extraction efficiency of Hg*+ from 10% to 92% under dry conditions and from 84 to > 99% under wet conditions. The large increase in SFE efficiency is most likely due to enhanced interactions between the solute and the modified solvent. Thus, quantitative extraction of Hg*+ directly by supercritical CO2 from a cellulose matrix becomes possible with methanol as a solvent modifier and water as a matrix modifier. The rate of extraction of H&+ from the filter paper using this in situ chelation/SFE approach was measured using CO2 modified with 5% methanol and 10 PL water deposited into the matrix. The extraction process consisted of a 20 min static extraction step, followed by a dynamic extraction step of varying time lengths. The experimental results show that quantitative extraction of Hg*+ was virtually achieved after 5 min of dynamic extraction. Variations of the static extraction time as shown in Fig. 1 (with a constant dynamic extraction time of 5 min) indicate that the efficiency increases rapidly in the first 10 min of extraction, with over 80% of Hg*+ removed after 5 min and about 90% of Hg*+ removed at the end of 10 min. After 20 min of static extraction and 5 min of dynamic extraction, over 97% of the spiked Hg*+ was removed. The standard extraction times we decided to use were therefore 20 min of static extraction followed by 10 min of dynamic extraction. Extraction of methylmercury and dimethylmercury with supercritical CO2 Interest in the speciation of mercury in environmental analysis stems from the marked

Fig. 1.Rate of extraction of Hg*+ from Whatman 42 filter paper using 5% methanol-modified supercritical CO2 containing LiFDDC at 50°C and 100 atm. Extraction conditions: 10 pg H$+, 10 mg LiFDDC, 10 PL H,O, and 5 min of dynamic flushing following each of the specified static extraction times.

Mercuric

ion extraction

difference in toxicity between Hg*+ and organomercurial compounds.‘5-‘7 The methylmercury compounds have been implicated in a number of mercury poisoning episodes.‘* Supercritical fluid extraction of CH,HgCl and (CHj)*Hg was investigated in this study by spiking each methylmercury compound separately on the cellulose based filter papers. The extraction efficiencies for CH3HgCl and (CH,)*Hg from the filter paper by supercritical CO2 at 100 atm and 50°C are given in Table 3. The samples were prepared by spiking 10 pg of CH,HgCl or (CH,),Hg on the filter paper similar to that described for the Hg’+ experiments. Methylmercuric chloride was spiked with the standard CH,HgCl solution obtained from Alfa Chemical Company. Dimethylmercury (boiling point 93°C at 1 atm) is a liquid at room temperature, therefore it was introduced directly onto the filter paper in its pure liquid form. The samples were extracted under the same conditions (20 min static followed by 10 min dynamic extraction) as the Hg*+ experiments except no LiFDDC was added. The results indicate that methylmercury can be quantitatively extracted from the filter paper by pure CO, when 10 PL of water is used as a matrix modifier. Apparently, CH,HgCl is soluble in supercritical CO, and no ligand is needed for its extraction from the filter paper. For dimethylmercury, quantitative extraction from the filter paper by supercritical CO2 can be achieved even without the presence of water. In this study, the extraction efficiencies were determined by comparing the extracted sample with a standard filter paper treated by the same analytical procedures. The filter papers were sealed separately in polyethylene vials during irradiation and counted without opening the container to avoid volatilization of the mercury compounds. Since monomethylmercury and dimethylmercury can be extracted directly by

Table 3. Extraction efficiency of CH,HgCI and (CH,),Hg from the filter paper with supercritical CO, at 100 atm and 50°C Sample No. #1

#2 #3 #4 #l #2

Matrix condition CH,HgCI Dry

Dry

Wet Wet CH,HgCH,

Extraction efficiency (%) 16.3 + 14.5 + 99.2 + 100.0 +

2.0 2.0 1.0 1.0

99.2 k 1.0 99.0 * 1.0

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supercritical CO2 without the use of a ligand, this method may be used to separate the methylmercury compounds from Hg*+ in environmental samples. CONCLUSIONS

Methylmercuric chloride and dimethylmercury can be extracted directly from cellulose based filter paper with neat supercritical CO*. Mercuric ions (Hg*+) adsorbed on the filter paper can be effectively extracted by an in situ chelation/SFE technique using LiFDDC as a chelating agent. Water and methanol can significantly improve the extraction efficiency of Hg*+ from the polar matrix. The solubility of Hg(FDDC), in supercritical CO, at 50°C and 150 atm is about three orders of magnitude greater than the non-fluorinated analogue Hg(DDC),. The choice of ligand is important for this in situ chelation/SFE technique. LiFDDC happens to be moderately soluble in supercritical CO2 and complexes effectively with Hg*+ to form a stable chelate which also has a sufficient solubility in the fluid phase. The polar cellulose matrix used in this study should resemble other polar matrices in biological samples such as plant and animal tissue. The SFE technique described in this paper thus offers a potential new approach for the preconcentration of mercury ions (Hg*+) and methylmercury compounds from environmental samples for analytical purposes and for treatment of mercury contaminated wastes. Acknowledgements-This material is based upon work supported by the Center for Hazardous Waste Remediation Research of the University of Idaho and by the U.S. Environmental Protection Agency, Environmental Monitoring Systems Laboratory, Las Vegas, Nevada. Neutron irradiation was performed at the Nuclear Radiation Center, Washington State University, under a Reactor Sharing Program supported by DOE. The information contained in this paper does not necessarily reflect the views of the funding agencies.

REFERENCES

1. S. B. Hawthorne, Anal. Chem., 1990, 62, 633A. 2. L. J. Mulcahey and L. T. Taylor, Anal. Chem., 1992.64, 981. 3. J. W. Hills and H. H. Hill Jr., Anal. Chem., 1991, 63, 2152. 4. S. B. Hawthorne, D. J. Miller, D. E. Nivens and D. C. White, Anal. Chem., 1992, 64, 405. 5. K. E. Laintz, C. M. Wai, C. R. Yonker and R. D. Smith, Anal. Chem., 1992, 64, 2875. 6. K. E. Lain@ C. M. Wai, C. R. Yonker and R. D. Smith, J. Supercritical Fluids, 1991, 4, 194.

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C. M. WAI et al.

I. A. Tavlaridis and R. Neeb, Fresenius Z.Anal. Chem., 1978, 242, 135. 8. W. M. Mok and C. M. Wai, Anal. Chem., 1987,59,233. 9. M. Ashraf-Khorassani, J. W. Hellgeth and L. T. Taylor, Anal. Chem., 1987, 59, 2017. 10. F. Brickmann and B. Wenclawial, Fresenius Z. Anal. Chem., 1984, 319, 305. 11. K. E. Lain@ G. M. Shieh and C. M. Wai, J. Chromatogr. Sci., 1992, 30, 120. 12. K. E. Lain& J. J. Yu and C. M. Wai, Anal. Chem., 1992, 64, 311. 13. B. I. Lee and M. G. Kessler, AIChE J., 1975, 21, 510.

14. C. R. Knipe, R. D. Gere and M. E. P. McNally, in Supercritical Fluid Technology-Theoretical and Applied Approaches to Analytical Chemistry, F. V. Bright and M. E. McNally (Ed@; ACS Symposium Series 488; Amer. Chem. Sot., Washington D. C., 1991, pp. 251-265. 15. K. Irukayama, Adv. Water Poll. Res., 1967, 3, 153. 16. K. Burg, H. Wanntorp, K. Erne and E. Hanko, J. Appl. Ecol., 1971, 3, 171. 17. J. M. Wood, Adv. Environ. Sci. Technol., 1971, 2, 39. 18. L. J. Goldwater, Methyl Mercury in Fish. Nordisk Hygienisk Tidskrift, Supplementurn 4, Stockholm, 1976.