Anal. Chem. 1990, 62,622-625
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RECEIVEDfor review June 27,1989. Accepted December 15, 1989. This document is contribution number 4980 from Du Pont’s Central Research Department.
Extraction of Tervalent Lanthanides as Hydroxide Complexes with Tri-n -octylphosphine Oxide Ted Cecconie and Henry Freiser* Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, Arizona 85721
The extraction behavior of lanthanum( I I I), praseodymium( I I I ) , europkrm(III), terbium(III), hdmium(III), and ytterbium( I I I ) from dilute chloride solutions into chloroform solutions contalnlng trkn-octylphosphlne oxide (TOPO) was examined. The lanthanides were found to extract as hydroxide complexes of the form Ln(OH),.nTOPO. The extraction selectivity for thls system increases with increasing TOPO concentration, rlvallng those for common acidic organophosphorus extractants In the TOPO concentration range 0.025-0.100 M and surpassing them at higher concentratkns. When TOPO concentrations of 0.25 M or greater are employed, the extraction system is, overall, the most selective for lanthanlde separations to date. Thls Is the first report of the lanthanldes extracting as hydroxide complexes.
INTRODUCTION The use of monodentate neutral phosphorus compounds in metal ion extraction is widely known (I). These compounds generally function as adductants, auxiliary complexing agents, which enhance the extractability of coordinatively unsaturated chelates by replacing water molecules remaining coordinated to the complexed metal ion. The report of extraction of lanthanides and actinides by sulfur ligands in the presence of tri-n-octylphosphine oxide (TOPO) ( 2 , 3 )led us to examine systems such as quinolin-%thiol and dithizone in the presence of TOPO for selected Ln3+. On observing the surprising extent of extraction in the absence of the S-containing ligand, we decided to examine this unusual phenomenon further. Our hypothesis of the formation and extraction of a lanthanide hydroxide complex was so unexpected that we took particular care to examine and reject alternate explanations.
EXPERIMENTAL SECTION Apparatus. Infrared spectra were obtained with a PerkinElmer 1800 Fourier transform infrared spectrometer. 31PNMR spectra were obtained at 250 MHz with a Bruker WM-250 spectrometer. Phosphorus and lanthanide concentrations were determined with a Perkin-Elmer 6500 ICP spectrometer. Reagents. Stock solutions containing lanthanum(III), praseodymium(III), europium(III), terbium(III), holmium(III), and ytterbium(II1) were prepared from chloride salts (Alfa Inorganics, 99.9%). Chloroform (AR grade) was washed 3 times with deionized water prior to use. TOPO (courtesy of American Cyanamid, >99%) was purified to remove all traces of acidic impurities, such as di(n-octy1)phosphoric acid, which are found in commercial sources of TOPO. This purification involved equilibrating a concentrated (0.5 M) TOPO solution in chloroform 8 times with an equal volume of a 0.1 M acetate solution, pH = 5, to remove the acidic impurities (4). The phosphorus level of all except the first of the eight washes was below the inductively coupled plasma limit of detection for P ( 5 X lo4 M). The acetate-washed solution was equilibrated 12 times with an equal volume of deionized water to remove any acetic acid extracted by TOPO ( 5 , 6 ) . The final TOPO solution was placed in a Rotovap to remove the chloroform solvent, with the resulting oily material placed in a vacuum desiccator to (P=O); crystallize overnight: mp, 50-52 “C; IR (KBr), qll& 31PNMR (CDCI,), b 48.7 ppm. Distribution Studies. An unbuffered solution (p = 0.1, HCl/NaCl) containing the lanthanides at 10”’ M was equilibrated with an equal volume of a TOPO solution in chloroform for at least 15 min, a time found adequate for equilibrium to be reached. The equilibrium pH of the aqueous phase was measured after phase separation. Metal distribution results were obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES)analysis. The lanthanide concentration remaining in the aqueous phase was determined directly, employing manganese as an intemal standard. The concentration of lanthanide extracted into the organic phase was determined by back extraction into
0003-2700/90/0362-0622$02.50/00 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 6, MARCH 15, 1990
I
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./l
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-
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.3
I
-
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3.45
log [TOPOI
PH Figure 1. log D dependence on pH. [TOPO] = 0.0500 M in chloroform. Initial lanthankle concentrations were as follows: Yb(III), 12.5 ppm; Ho(III), 25.0 ppm; Tb(II1). 25.0 ppm; Eu(III), 25.0 ppm.
Flgwe 3. log D - 3 pH dependence on log [TOPO] in the range -1.60 to -1.00. Initial lanthanide concentrations were as follows: Yb(III), 12.5 ppm; Ho(III), 25.0 ppm; Tb(III), 25.0 ppm; Eu(III), 25.0 ppm.
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Flgure 4. log D - 3 pH dependence on log [TOPO]. Initial lanthanide
PH Figure 2. log Ddependenceon pH. [TOPO]
,
= 0.500 M in chlorofrxm;
initial lanthanide concentrations were as follows: Eu(III), 12.5 ppm; Pr(III), 40.0 ppm.
0.1 M hydrochloric acid and determining the lanthanide concentration in the acid solution. The ICP-AES instrumental conditions are listed elsewhere (7).The concentration results were wed to calculate the distributionratio, D, defined as D = C,,,,JCb
RESULTS AND DISCUSSION The extraction behavior of the lanthanides was evaluated from the relationships of log D on pH, log [TOPO], and log [Ln]. As shown in Figures 1and 2 for Yb(III), Ho(III), Tb(III), and Eu(II1) ([TOPO] = 0.0500 M in CHC13),log D was found to have a slope 3 dependence on pH under conditions of constant TOPO and initial lanthanide concentrations. The log D relationships on log [TOPO] and log [Ln] were subsequently viewed in terms of log D - 3 p H versus log [TOPO] and log [Ln] since, in the absence of a buffer, it was found difficult to control the equilibrium pH to a constant value while the TOPO or lanthanide concentration was varied. Plots of log D - 3 pH versus log [TOPO] (Figures 3 and 4), under conditions of constant initial lanthanide concentrations, indicated linear relationships with slopes intermediate between 2 and 3. Plots of log D - 3 pH versus -log [Ln] (Figure 5), under conditions of constant TOPO concentration, indicate no dependence on lanthanide concentration.
concentrations were as follows: Yb(III), 12.5 ppm; Ho(III), 25.0 ppm; Tb(III), 25.0 ppm; Eu(III), 12.5 ppm; Pr(III), 40.0 ppm; La(III), 25.0 PPm.
1
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Figure 5. log D - 3 pH dependence on
log [Ln]. [TOPO] = 0.0500
M in chloroform. When TOPO concentrations of higher than 0.100 M are employed in extraction, although the least-squares slope re-
624
ANALYTICAL CHEMISTRY, VOL. 62, NO. 6, MARCH 15, 1990
sults for log D - 3 pH versus log [TOPO] are between 2.5 and 2.8 in the TOPO concentration 0.025-0.100 M, the extracted complex is treated as though it contains three TOPO molecules. On the basis of these results, the following extraction equilibrium holds: LnA3.nTOP0, 3H+ (1) Ln3+ + 3HA nTOPO,
+
+
where n can be 2 or 3, HA represents a weak acid, LnA3. nTOPO represents the extracted metal complex, and the subscript o refers to the organic phase. Unlike the systems based on TBP, where the lanthanides extract as metallic salt complexes (8-10), the distribution results indicate the lanthanides extract as hydroxide complexes, since water should be the only weak acid present. Since this is both highly unexpected and unprecedented for the lanthanides, other possibilities for the weak acid, HA, were considered. The presence of acidic impurities such as di(n-octy1)phosphoric acid (DOPA) in commercial sources of TOPO makes them candidates for the weak acid. Chloroform solutions of TOPO were treated with 0.1 M acetate buffer washes, pH = 5, prior to use for the removal of acidic impurities ( 4 ) . The amount of phosphorus-containing species in each acetate wash was monitored by ICP-AES using the 213.618 p I Mission line. The phosphorus content was found to decrease with increasing wash number, typically from = 170 mg/L in the first wash to no detectable amount in the eighth wash (C0.15 mg/L). Since it was observed that each successive acetate wash from the second to seventh had 40% the phosphorus content of the preceeding wash, one can estimate the amount of acidic impurity in purified TOPO to be
