Anal. Chem. 1994,66, 1971-1975
Supercritical Fluid Extraction of Lanthanides with Fluorinated @-Diketones and Tributyl Phosphate Yuehe Lin and C. M. Wai' Department of Chemistry, University of Idaho, Moscow, Idaho 83843
Trivalent lanthanide ions (La3+,Eu3+, and Lu3+) in solid materials can be effectively extracted by methanol-modified carbon dioxide containing a suitable fluorinated &diketone (such as HFA, l T A , or FOD) at 60 OC and 150 atm. Addition of a small amount of water to the solid samplescan significantly increase the extraction efficiency. Tributyl phosphate (TBP) shows a strong positive synergistic effect with the fluorinated 8-diketonesfor the extractionof the lanthanides in supercritical C02 without methanol modifier. Quantitative extraction of the lanthanides (92-9%) from sand or a cellulose-based solid material can be achieved using a mixture of TBP and one of the fluorinated &diketones in neat COZ at 60 OC and 150 atm. The synergistic effect depends on the structure and fluorine substitutionin the B-diketone. In soil matrix, TBP+HFA are more effective than TBP+TTA or TBP+FOD for lanthanide extractionin supercritical C02. Without fluorinesubstitution, as in the case of acetylacetone, the positive synergistic extraction of lanthanides with TBP is negligible. With the mixed-ligand approach, high efficiencies of lanthanide extraction from aqueous solutions by neat C02 can also be accomplished. Supercritical fluid extraction (SFE) has become an attractive alternative to conventional solvent extraction for the recovery of organic compounds from environmental samples.' Carbon dioxide is the gas of choice in SFE because of its moderate critical constants (T, = 3 1 OC, Pc = 73 atm), inertness, and availability in purified form. Although SFE of organic compounds has been the subject of many studies, little information is available in the literature regarding SFE of metals. It is known that direct extraction of metal ions by supercritical COZis highly inefficient because of the charge neutralization requirement and the weak solute-solvent interactiom2 However, when metal ions are bound to organic ligands, they may become quite soluble in supercritical COZ. Wai and co-workers have recently demonstrated that Cu2+ and Hg2+in solid materials can be extracted by supercritical COZcontaining the chelatingagent lithium bis(trifluoroethy1)dithiocarbamate (LiFDDC).2.3 The choice of the fluorinated ligand was based on the fact that the solubilities of the metalFDDC complexes are significantly higher (2-3 orders of magnitude) than those of the non-fluorinated anal0gues.~*5 The success of this new metal extraction technique for (1) Hawthorne, S.B. A M / . Chem. 1990.62, 633A. (2) Laintz. K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Anal. Chem. 1992, 64, 2875. ( 3 ) Wai, C. M.; Lin, Y.; Brauer, R. D.; Wang, S.;Beckert, W. F. Talanta 1993, 40, 1325. (4) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. J. Supercrit. Fluids 1991, 4, 194. ( 5 ) Laintz, K. E.; Yu, J. J.; Wai, C. M. Anal. Chcm. 1992, 64, 311.
0003-27QQJ94JQ388-197 1SQ4.5QJQ 0 1994 American Chemical Society
analytical applications depends largely on the effectiveness and availability of the ligand. LiFDDC is an effective ligand for a large number of metal ions but is ineffective for complexation with certain groups of elements including the lanthanides. Furthermore, LiFDDC is not available commercially, and the starting material for its synthesis is expensive. Recently, Wai and co-workers reported that uranyl and trivalent lanthanide ions can be extracted by supercritical C02 containing a fluorinated fl-diketone, 2,2-dimethyl6,6,7,7,8,8,8-heptafluoro-3,5-octanedione (FOD)? This observation is significant because a number of fluorinated &diketones including FOD are commercially available and are known to form stable complexes with lanthanides and actinides. These fluorinated ligands are potential extractants for SFE of the f-block elements. Furthermore, lanthanide&diketone complexes are known to form adducts with neutral donors, often resulting in enhanced extractionof the complexes in conventional solvent extraction proces~es.~Synergistic extraction of metal ions using mixed ligands is another approach to increase the efficienciesof metal recovery in SFE. This paper describes the conditions of extracting lanthanide ions from solid and liquid materials by supercritical C02 containingdifferent fluorinated @-diketonesand their mixtures with a common neutral ligand, tributyl phosphate (TBP). Potential applications of this SFE method for separating the trivalent lanthanide ions from solid and liquid samples are discussed.
EXPERI MENTAL SECTION The fluorinated @-diketoneswere purchased from the Aldrich Chemical Co. (Milwaukee, WI) and used without further purification. Solutionsof La3+,Eu3+,and Lu3+were prepared from their nitrates, also obtained from Aldrich. All other chemicals used were analytical-reagent grade. Filter papers used as the solid sample matrix were obtained from Whatman Ltd. (Maidstone, England). Solid samples were prepared by spiking 10 r g each of a mixture of La3+, Eu3+, and Lu3+on prewashed filter papers (Whatman 42,0.5 cm X 2 cm in size, washed with Ultrex HNO3 and rinsed with deionized water) or on prewashed sand. The spiked solid samples were allowed to air dry at a room temperature of 23 "C. The weight of the sand samples used in our SFE experiments was usually 300 mg. Water samples were 0.05 M HAc/LiAc buffer solution (pH 4.0) containing 2.5 pg/mL each of La3+, Eu3+, and Lu3+. All experiments were performed with a lab-built SFE apparatus. SFC-grade C02 or C02 with 5 mol 8 methanol (6) Lin. Yuebe; Brauer, R. D.; Laintz, K. E.; Wai, C. M. Anal. Chem. 1993,65,
2549.
(7) Sekine, T.; Dynsen, D. J. Inorg. Nucl. Chem. 1967, 29, 1481.
chemism, Vd. 66, No. 13, Ju& 1, 1994 * O H
modifier (Scott Specialty Gases, Plumsteadville, PA) was delivered to the SFE system using a Haskel pump (Haskel Inc., Burbank, CA). The system pressure was monitored to f5 psi using a Setra System (Acton, MA) pressure transducer. The extractor consisted of an inlet valve (Supelco, Bellefonte, PA) and an outlet valve connected to an extraction vessel. For the extraction of solid samples, a commercial extraction vessel (Dionex, Sunnyvale, CA) having a volume of 3.5 mL was used. The main body of the liquid extraction vessel was made from a stainless steel column (0.75 cm i.d. and 14cm in length, Alltech), and the infittings were obtained from Swagelok (Seattle, WA). The vessel was modified for use with liquid samples in the same manner as reported earlier by Hendrick et a1.8 A 3.5-mL stainless steel high pressure cylinder containing TTA as chelating agent was connected upstream of the liquid extraction vessel. The ligand cylinder and the extraction vessel were placed in an oven with the temperature controlled to *O. 1 OC by an Omega (Stamford, CT) CN9000A temperature controller. A fused-silica tubing (Dionex, 50 pm i.d. and 20 cm in length) was used as the pressure restrictor for the exit gas. The flow rate of the exit gas was measured to be around 650 mL/min, corresponding to about 2 mL/min supercritical C02 at 60 OC and 150 atm. The SFE system allows static and dynamic extraction steps to be carried out by closing and opening of the inlet and outlet valves. For solid sample extraction, a glass tube (0.5 cm i.d. and 3 cm in length) was plugged at one end with a piece of glass wool, previously cleaned with Ultrex nitric acid. To the open end of the glass tube, a spiked solid sample was inserted. Water (10 pL) and about 80 pmol of a 8-diketone ligand were introduced to the sample in that order, and the open end was plugged with a piece of clean glass wool. The sample tube was placed immediately into the extractionvessel and installed in the SFE oven. The temperature of the oven was set at 60 OC, and the cell was pressurized to 150 atm. The chelation and the extraction processes were allowed to occur under a static SFE condition for 10 min. After that, the exit valve was opened and the sample was extracted and flushed under dynamic condition for 20 min. The specified extraction conditions were adopted on the basis of a previous study regarding the SFEof lanthanides using FOD as an extractante6 When the dynamic extraction step was completed, the sample was removed from the SFE system and the filter paper was analyzed by a nondestructive neutron activation analysis (NAA). A standard solid samplecontaining the same amount of the lanthanide ions was irradiated and counted with the sample under identical conditions. The extraction efficiencies were calculated on the basis of the amount of the lanthanide found in the solid sample before and after the extraction. The solutes trapped in a chloroform solution were determined by back-extraction with 50% HNO3, followed by NAA of the acid solution. Direct collection of the extracted complexes by 50% H N 0 3 was found less effective and usually resulted in about 60% of the recovery efficiency relative to chloroform. For the extraction of the lanthanides from water, 4 mL of the spiked water sample was placed in the liquid extraction vessel. The pH of the solution was controlled by an acetate (8) Hedrick, J.
L.: Mulcahey, L. J.; Taylor L. T. In Supercritical Fluid
Technology-Theoretical and Applied Approaches to Analyticai Chemistry; Bright, F. V., McNally, M. E., as. ACS ; Symposium Series 488; American Chemical Society: Washington, DC, 1991; pp 206-220.
1972 Ana&ticalChemistty, Vol. 86, No. 13, July 1, 1994
buffer. The water samples usually contained 2.5 pg/mL each of La3+, Eu3+, and Lu3+. A certain amount of TTA (approximately 50 mg) was loaded in theligand cylinder placed upstream from the liquid extraction vessel. The samples were extracted dynamically at 60 OC and 150 atm for 20 min. When the extraction was complete, the sample was removed from the extraction vessel and analyzed by NAA. A standard solution containing 2.5 pg/mL each of the lanthanide ions was irradiated and counted with the sample under identical conditions. The extraction efficiencies were calculated on the basis of the amounts of the lanthanides found in the aqueous solution before and after the extraction. The blank runs with deionized water and the ligands showed no detectable amounts of lanthanides in the collection solution after extraction. However, traces of Cr, Fe, and Ni (on the order of 1 pg) were found in the collection solution by ICPMS, indicating some interactions between the ligands and the stainless steel extractor. Because the amounts of the metals removed from the extractor are so minute, the safety risk is minimal. The mechanical wear of the extractor tread will be a limiting factor in the lifetime of the extractor, according to our experience. All samples and standards were irradiated for 2 h in a 1 MW Trigar nuclear reactor at a steady flux of 6 X 1OI2n cm-2 s-l. After irradiation, the samples were cooled for 2-3 days before being counted. Each sample was counted for a fixed time (usually 300 s) on a large-volume Ortec Ge(Li) detector with a resolution (fwhm) of about 2.3 keV at the 1332-keV ~ Cpeak. O The followingradioisotopesand their characteristic y energies were used for the identification and quantification of the lanthanides: 140La (tlp = 40.2 h, 487 keV), Is2"'Eu (tip = 9.3 h, 121 keV), and 177Lu(tl/2 = 6.7 d, 208 keV). The detailed procedures of NAA are described el~ewhere.~
RESULTS AND DISCUSSION SFE of Lanthanidesfrom Solid Materials with &Diketones The &diketones ( R I C O C H ~ C O Rand ~ ) their abbreviations used in this study are given as follows: acetylacetone (AA, RI = R2 = CHs), trifluoroacetylacetone (TAA, R1= CH3, R2 = CF3), hexafluoroacetylacetone (HFA, R1 = R2 = CF3), thenoyltrifluoroacetone (TTA, R1 = thienyl, R2 = CF3), and FOD (R1 = tert-butyl, R2 = n-C3F7). The properties of these @-diketonesare given in the literature.1° Except for TTA, the other @-diketoneslisted above are all liquid at room temperature under atmospheric pressure. In a previous SFEstudy of lanthanides using FOD as an extractant, it was noted that the presence of a small amount of water would significantly enhance the extraction efficiency of lanthanides from a cellulose-based solid material.6 Water molecules probably compete with the active sites of the cellulose matrix to release the lanthanide-FOD complexes as water adducts. The lanthanide-FOD complexes are known to form adducts with water and other Lewis bases, a property which makes them useful as lanthanide shifting agents.ll In addition, the presence of water may also facilitate the enolate anion formation and thus accelerates the metal chelate formation as illustrated by the following equilibria: (9) Tang, J.; Wai, C. M. Anal. Chem. 1986.58, 3233. (10) Chen,K. L.;Ueno,K.;tmamura,T.HandbookofOrganicAnalyricalReagmts; CRC Press: Boca Raton, FL, 1982. ( 1 1) Morrill, T. C. In Lanthanide Shift Reagents in Stereochemical Analysis; Morrill, T. C., Ed.; VCH Publishers, Inc.: New York, 1986.
0 I1
::
0 II
PH
Therefore, the solid samples in this SFE study were all extracted under a wet condition by depositing 10 pL of water on each filter paper, sand, or soil sample immediately before the experiment. The results of extracting La3+, Eu3+,and Lu3+ from the cellulose-based filter paper by supercritical COZcontaining different @-diketonesare given in Table 1. In the absence of Table 1. Percenl Edractbn ol Laa+, E$+, and LIP+ hom a cdkdougaod Fmer P a w wlth Supmmkrl C02 Cordalnlng B-Dlketoner at 150 atm and Bo O C % extraction
fi-diketone
ligand amt (pmol)
FOD TTA HFA TAA AA
80 80 80 80 80
FOD TTA HFA TAA AA
80 80 80 80 80
La3+
coz
10*2 1412 7h2 5*2 5*2
CO2
Eu3+
Lu3+
13+2 16&3 13&2 10*2 4*2
19*2 20 3 15+2 11*2 6*2
96&2 90*2 93*4 80*3 25*2
99* 1 94i3 95 3 84+3 33*3
*
+ 5% MeOH 91k2 14*3 70*3 54*2 11*2
*
E Each filter aper sample (1 cm2 in area) contains 10 pg each of La3+, Eu3+,and Lu3 .
P
a chelating agent, extraction of the lanthanide ions from the filter paper by supercritical C02 is negligible ( HFA > TAA > AA. Acetylacetone (AA), which has the lowest molecular weight and no fluorine substitution, is the least effective ligand for the SFE of lanthanides. The efficiencies for the extraction of the lanthanides from the cellulose based solid material are 6% or less when AA is used as an extractant in supercritical C02 at 60 OC and 150 atm. Substitution of CF3 for CH3 in AA increases the efficiency of lanthanide extraction, as shown in Table 1, for TAA and HFA. The extraction efficiencies of TAA and HFA for Lu3+are 1 1%and 15% respectively, under the same conditions. TTA, having a bulky thienyl group in RI,is more effective than HFA for the extraction of the lanthanides from the solid material in supercritical COZ.The extraction efficiencies of TTA for La3+, Eu3+, and L 3 + in supercritical C02 are 14%, 16%. and 208,respectively. The extraction efficiencies of FOD for the lanthanides are close to those observed for TTA. All of the fluorinated @-diketones investigated in this SFE study show a preference for the heavy lanthanides over the light ones. The relative extraction
efficiency of Lu3+ over La3+, the two end members of the lanthanide series, is about a factor of 2 using the fluorinated &diketones as an extractant in supercritical C02. The observed extraction efficiencies for the trivalent lanthanides with fluorinated b-diketones in neat CO2 are not high enough for practical applications. The most effective ligands appear to be TTA and FOD, which are capable of extracting not more than 20% of the spiked lanthanides in neat COz at 60 OC and 150 atm. Increasing the pressure (or density) of the C02 did not show significant improvements in the extraction efficiencies of the lanthanides. For example, the extraction efficiencies of La3+, Eu3+, and Lu3+ by TTA in neat COz increase from 14% 16%, and 20%, respectively, at 150 atm to 17%,20%, and 238,respectively, at 300 atm at a constant temperature of 60 OC. The addition of methanol to COz modifies the polarity of the fluid phase, which should enhance the extraction efficiencies for me talc he late^.^^^ With TAA present in 5% methanolmodified COz, the extraction efficienciesfor La3+,E d + , and Lu3+become 54%, 808,and 84%, respectively,at 60 OC and 150atm. These extractionefficienciesare significantlyhigher than those obsgrved in neat COZ(5%, 1096,and 11% for La3+, Eu3+, and Lu3+, respectively) under the same experimental conditions. HFA, with two CF3 groups substituted for the methyl groups in AA, shows higher extraction efficienciesfor the lanthanides than TAA in methanol-modified COz. The percent extraction of La3+,Eu3+, and Lu3+with HFA reaches 70%, 938,and 9596,respectively,with 5% of methanol in the fluid phase. The extractionefficienciesof TTA for the trivalent lanthanides in methanol-modified COZ are close to those observed with HFA. Among the four fluorinated B-diketones investigated, FOD shows the highest extraction efficiencies for the lanthanides (91%,96%, and 99% for La3+,E d + , and Lu3+, respectively) in methanol-modified C02. Without fluorine substitution in &diketone, as in the case of AA, the extraction efficiencies for the trivalent lanthanide ions in methanol-modified COZ are much lower relative to the fluorinated analogues described above. At 60 OC and 150 atm, only 11%, 252,and 33% of the spiked La3+,E d + , and Lu3+,respectively,can be extracted with AA in 5% methanolmodified C02. The fluorinated lanthanide-dikettone complexes are solids with relatively high melting points. For instance, the melting points of La3+ complexes with TAA, HFA, TTA, and FOD are 169, 120-125, 135, and 215-230 OC, respectively, according to the 1iterature.l2J3 The solubilities of these fluorinated 0-diketone-lanthanide complexes in supercritical C02 are actually quite high. For example, the solubilities of La(FOD)3 and Eu(F0D)s in supercritical COzare 5.5 X 10-2 and 7.9 X le2 M,respectively, at 60 OC and 150 atm.6 In our SFE experiments, an excess amount of the ligand is used and the total amount of each lanthanides spiked in the solid material is much lower than the solubility limit. The extraction efficiencies of the lanthanidesobserved in the SFEexperiments with the solid materials are probably not determined by solubility alone. Other factors such as matrix and solvation effects are probably also important for the SFE of metal chelates. According to our experiments, water molecules ~~
~~
~~
(12) Springer, C. S.; Meek D. W.; Sievers, R. E. lmrg. Chcm. 1%7,6, 1105. (13) Purushotham, D.; Rao, V. R.; Reo, S.V. R. AMI. Chim. Acra 1965,33,182.
Amlj.t-1 C b d W , Vd. 66, No. 13, Ju3/ 1, 1994
1978
Tabk 2. Porcont Extrcrotlonol La*+, E@, and Lu)+ from the Fntr Papor’ with Noat COScont.inlng Mtxd Uganda at 150 atm and 80 “Cb % extraction
ligands
amt (pmol)
La’+
Eu3+
LU’+
TBP TBP+TTA TBP+FOD TBP+HFA TBP+TAA TBP+AA
80 40+40 40+40 40+40 40+40 40+40
2f 1 92i3 9013 9613 20t2 311
311 94t4 93f3 9814 26f3 5f2
411 95f4 9513 9813 2113 4f1
Table 3. P r c o n t Extraction and R.covory of Laa+, EtP, and Lua+ from Sad. with Neat Cor Containhg TTA, TBP, and Mbcd TTA+TBP at 60 “C and 150 a t d % extraction % recovery
ligand
amt (pmol)
La
Eu
Lu
La
Eu
Lu
TTA 80 40+3 5 1 1 3 65f4 29f3 4 0 1 3 6 0 1 4 TBP 80 4 1 2 3 f l 5 1 2 2 1 1 2 1 1 3fl TTA+TBP 40+40 9 1 1 3 9 2 1 4 9 5 1 4 9 1 1 3 89f4 9 1 1 4 a Each sand sample (300mg by weight) contains 10p g each of La)+, Eu3+,and Lu3+. 10min of static extractionfollowed by 20min of dynamic extraction.
a Each filter aper sample (1 cmz in area) contains 10 p g of La’++, Eu3+,and Lu’+. glOminof static extraction followed by 20minof dynamic extraction.
appear to facilitate the release of the lanthanide-&diketone complexes from the matrix by forming adducts with the complexes. Thus, H20 may be consideredas a matrix modifier in this case. Methanol modifies the solvation of the metal chelates and probably enhances their mobility in the fluid phase, leading to higher extraction efficiencies for the lanthanides. SynergisticExtraction ofLanthanides with Mixed Ligands. Tributyl phosphate (TBP) in supercritical C02 shows limited extraction of the lanthanides (
