J. Phys. Chem. 1995,99, 4205-4210
4205
Sequestration of the Tributyl Phosphate Complex of Europium Nitrate in the Clay Hectorite: A 31PNMR Study Cynthia J. Hartzell,* Szu-Wei Yang, and Roderic A. Parnellt Departments of Chemistry and Geology, Northern Arizona University, Flagstafi Arizona 8601 I
David E. Morris Chemical Sciences and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received: August 17, 1994; In Final Form: October 31, 1994@
The behaviour of organic complexes of actinides and lanthanides in clays plays an important role in environmental remediation. Application of 31PNMR to distinguish Eu(NO3)3-complexed tributyl phosphate from uncomplexed tributyl phosphate (TBP) facilitates an understanding of the fate of these species in the clay hectorite. Solution 31PNMR studies show dramatic shifts of -156.0 to -172.9 ppm for E U ( N O ~ ) ~ * ~ H ~ O dissolved in TBP at TBP:Eu ratios 1: 1 to 1:2.5. Pure TBP exhibits a 31Pchemical shift of -0.3 ppm. Mixtures with higher TBP:Eu ratios display lines progressively downfield of - 156.0 ppm, reflecting exchange of complexed and free TBP. The Eu(NO3)3-complexed TBP adsorbed into hectorite displays a 31Pchemical shift of -180.7 to -193.8 ppm. Clays adsorbed with solutions that are 3:1, 5:l and 7:l TBP:Eu display peaks due to complex as well as peaks in the range -4 to -6 ppm attributed to uncomplexed TBP. No evidence of exchange is observed. Spectra of the TBP-adsorbed clay give a TBP line at -0.7 ppm which changes slightly to -5.0 after exchange with Eu(NO&. The Eu(NO3)3-exchanged hectorite displays a line at -17.6 ppm after adsorption of TBP. Neither of the latter two samples show indication of complex formation. The hectorite samples exposed to TBP or the complex display (001) d spacings of 17.9-18.4 8, compared to 12.3-12.5 8, for hectorite which has not been exposed to TBP.
Introduction The fate of actinides and lanthanides as well as organic compounds in the subsurface environment has become a focus of attention since the recent mandate for cleanup of waste at nuclear fuel processing sites and United States Department of Energy weapons production facilities. The focus of this work is the interaction, within a clay, of the lanthanide europium and the neutral organic, tributyl phosphate. Common clays are known to have high sorption capacities for both organics and metal cations. However, recent studies have shown that nuclear waste products are being leached out of clays and into aquifers rather than remaining sequestered as previously proposed.' As a consequence, detailed studies of the interaction of the actinides and lanthanides with neutral organics within the clay interlayer or on the surface hydroxyl sites are necessary to probe the formation and behavior of complexes in the clay environment. Tributyl phosphate is one of the organic compounds which is used commonly as a ligating agent in the PUREX process for nuclear fuel processing and is a widespread contaminant in ground water2 and soils3 surrounding processing facilities. Controlled releases of TBP as well as the actinides and lanthanides have occurred over the past 40 years.4 The europium isotopes I5'Eu, 1 5 2 Eand ~ , 1 5 4 Eare ~ among the released contaminants although the activity (in Ci) is less than 10% that of the released actinides. Studies of Eu3+ complexation with TBP are expected to yield information on the fate of the actinide Am3+since the equilibrium constant for extraction of Eu3+ by TBP is similar to the extraction equilibrium constant for Am3+, 1.82 and 1.58, re~pectively.~ Clays are phyllosilicate minerals composed of layered aluminosilicate sheets. The structure of 2:l clays consists of an
' Department of Geology. @
Abstract published in Advance ACS Abstracts, March 1, 1995.
octahedral sheet between two tetrahedral sheets. Cations are adsorbed into the interlayers between 2:l sheet packages and along the edges of the clays to balance negative charges resulting from substitutions of lower charged cations for more highly charged cations in these sheets. In hectorite, Li+ is substituted for Mg2+ in the octahedral sheet. In this study we exploit the ability of the lanthanide europium to induce dramatic changes in the NMR chemical shifts of nearby nuclei. The interaction of tributyl phosphate with europium nitrate is probed by observing the 31PNMR signal of TBP. Solution 31PNMR results are consistent with the 3:l T B P Eu stoichiometry determined for the complex Eu(N03)3*3TBP by Jackson.6 The 31P chemical shift of Ew3TBP is an unambiguous indicator of the presence of this complex in the clay hectorite. The purposes of this work are threefold: (1) determine the chemical conditions under which the complex forms in the clay; (2) ascertain whether exchange of TBP between the complexed and uncomplexed state occurs in the clay; and (3) ascertain whether the sorption complex is located in the fixed-charge (interlayer) sites or on the edge (surface hydroxyl) sites of the clay. The literature is sparse both on the use of NMR to study organic molecules adsorbed in clay^^-^ and on the application of NMR to study the nature of cation adsorption in clay^.'^-'^ In a recent paper, Weiss provides a detailed discussion of the study of cation adsorption in clays by NMR.'* This NMR study of the structural sites occupied by adsorbed 133Csin hectorite reveals distinctly different chemical environments. Of particular relevance to the present work is the report of motional averaging due to exchange between sites. Exchange behavior has also been reported for water molecules in ~ 1 a y s . l In ~ this case, exchange is shown to occur between free water and waters of hydration about a cation. However, NMR studies of the behavior in clays of organic molecules complexed to metal ions are noticeably lacking.
0022-3654/95/2099-4205~09.00/0 0 1995 American Chemical Society
Hartzell et al.
4206 J. Phys. Chem., Vol. 99, No. 12, 1995
We have previously reported on the use of I3C NMR to probe the incorporation of the Eu-3TBP complex in the smectite clay, SAzL8 The C-1 carbons of the TBP n-butyl groups were labeled with I3C. The small chemical shift changes in the presence of Eu rendered the data marginally informative. In contrast, 31P displays large chemical shifts in the presence of Eu. The spin I/* nucleus 31P also has the advantage of being naturally occurring at 100% abundance. The work presented in the first section of this paper is a determination of the solution 31Pchemical shift of TBP in the presence of varying molar ratios of Eu. The NMR results are consistent with a molecule undergoing rapid exchange between two environments. In the presence of rapid exchange, the observed peak is the population-weighted average of the peaks of each of the two sites.I5 Exchange behavior is observed between complexed TBP and uncomplexed TBP. The results of this study facilitate interpretation of the spectra obtained in the study of TBP and Eu(N03)3 in a clay. The second section probes the interaction of TBP and Eu(NO& in the clay hectorite. The hectorite samples are prepared using three different sequences and each is studied for evidence of complex formation. One clay is prepared by first exchanging the clay with Eu then adsorbing with TBP. The second clay is adsorbed with TBP then exchanged with Eu. In the third case, the preformed complex is exchanged directly into the hectorite. The 31P NMR MAS spectra are analyzed for evidence of Eu.3TBP. Powder X-ray analysis confirms the presence of TBP or complex in the interlayer by a characteristic increase in the d spacing.
Experimental Section The spray-dried hectorite from Hector, CA, was used as supplied by NL Industries. Hectorite is a Li, Mg-smectite. The cation-exchange capacity of the clay is 100 mequid100 g. The tributyl phosphate and Eu(N03)3*6H20were used as supplied by Aldrich. Preparation of Europium-ExchangedHectorite, Sample XI. The basic procedure used in this laboratory for incorporating cations into clays has been described previously.16 Specifically, an aqueous solution that is 1.77 mM in Eu3+ is prepared by dissolving 0.416 g (0.933 "01) of Eu(N03)3*6H20into 527 mL of deionized water. To ensure a constant, low ionic strength of 0.014, 0.194 g (1.60 mmol) of NaC104 is dissolved in the solution and the pH adjusted to 5 with HC1. The europiumexchanged hectorite (Eu-hectorite) is prepared by stirring this solution for 24 h with 0.4 g hectorite, (2.8 mequiv Eu3+/0.40 mequiv hectorite). The clay, Eu-hectorite, is separated from the aqueous solution by gentle centrifugation then washed with deionized water. After drying under a stream of N2 for 24 h, the clay is ground to a powder, sample XI (Table 2). Adsorption of TBP into Eu-hectorite, Samples VIII-X. Adsorption of TBP into Eu-hectorite at a TBP:Eu ratio of 2:l is accomplished by stirring 0.70 g Eu-hectorite in a mixture of 55.0 p L TBP and 5 mL hexane for 24 h. The clay, TBPEuhectorite, is separated by centrifugation and allowed to air dry. A 0.3 g portion of this clay is withheld as sample VIII. The remaining 0.4 g clay is further stirred with 1 mL TBP and 5 mL hexane, collected by centrifugation, and dried. This yields Eu-hectorite adsorbed with excess TBP, samples IX and X. Preparation and Adsorption of Eu*3TBP Complex into Hectorite, Samples 111-VII. A solution of TBP-Eu is prepared by dissolving 0.457 g (1.02 m o l ) Eu(N03)3*6H20 into a solution of TBP and 10 mL hexane. The 3:l TBP:Eu solution is prepared using 0.83 mL (3.06 mmol) of TBP and a 7:l TBP:
Eu solution is prepared using 1.94 mL of TBP. One gram of hectorite, 1.00 mequiv based on exchange capacity, is stirred in this solution for 24 h and then centrifuged. The clay, TBPEdhectorite, is washed three times with hexane. The clay is air-dried then ground to a powder. Adsorption of TBP into Hectorite, Sample I. TBPadsorbed hectorite (TBP-hectorite) is prepared by stirring 0.8 g of hectorite with a mixture of 0.33 mL (1.21 "01) TBP and 5.0 mL hexane for 24 h. The clay is collected by centrifugation and dried. Exchange of Europium into TBP-hectorite, Sample 11. The exchange procedure is similar to that given above for Euhectorite. The quantities used are 0.6 g of TBP-hectorite,0.209 g of Eu(N0&-6H20, and 0.097 g of NaC104 in 263 mL of deionized water. Preparation of TBP/Eu Solutions for 31PNMR. 31PNMR studies were carried out on solutions prepared by directly dissolving Eu(N03)3*6H20(Aldrich) in 0.1 mL (0.37 "01) of TBP (Aldrich) in molar ratios ranging from 1:l to 20:l TBP: Eu. To facilitate dissolution, the solutions were heated at 40 'C for 1 h and then cooled for 1 h. The solution was transferred to a capillary tube for NMR studies. Eu Analysis. Determination of Eu3+ in the clay is carried out by ICP analysis of ammonium acetate extracts of the clay." A 0.5 g sample of clay is stirred in 5 mL of 1 N ammonium acetate for 24 h. Following vacuum filtration, the extract is diluted to 25 mL. Both sample extracts and standards are analyzed by ICP. The Eu3+ content is read from a standard curve covering the range 100-500 ppm. NMR. Solution 31PNMR spectra were acquired at 81 MHz on a Bruker CXP-200 wide-bore spectrometer, 4.7 T, using a standard Bloch decay experiment with a relaxation delay of 5 s. Spectra were acquired at a sweep width of 50 000 Hz and were accumulated as 8K complex data points. Chemical shifts are reported relative to an external reference of 85% orthophosphoric acid. 31PMAS spectra of TBP in hectorite were acquired on the CXP-200 using a Doty 5 mm MAS probe at spinning speeds of 2.2-2.5 W z . A Bloch decay pulse sequence is used with a 15 ps pulse at 220 W and a relaxation delay of 5 s. No H decoupling is applied. The MAS spectra required the collection of 14 000 transients. X-ray Analysis. The d spacing is determined by powder X-ray diffraction using a Cu tube. The < 2 pm samples are prepared by centrifugation of sonically dispersed material onto oriented porous plates. The data is collected over the 2 0 range 2-45' at a step size of 0.02" and counting time of 1.3 s. The instrument is a Siemens D-500 X-ray diffractometer interfaced with a PC-AT and a DACO-MP microprocessor. Five to seven (001) reflections were used to determine the (001) spacings reported in Table 2.
Results and Discussion Solution 31PN M R Studies. The 31PNMR spectra of TBPEu(NO3)s solutions show systematic variation in the chemical shift as a function of the TBP:Eu ratio as shown in Figure 1. Chemical shift values are tabulated in Table 1. If rapid exchange occurs between the free and complexed TBP, then the observed resonance is expected to shift toward that of free TBP as the ratio increases. The chemical shift is found to be relatively constant for TBP:Eu ratios 1:l to 3:l. As the ratio increases, the observed peak shifts toward that of free TBP. This behavior is consistent with the formation of a 3:l TBP:Eu complex which undergoes rapid exchange of solvate molecules in the presence of excess TBP.
Tributyl Phosphate Complex of Europium Nitrate in Hectorite
J. Phys. Chem., Vol. 99, No. 12, 1995 4207 TBP:Eu Ratio 0 1
E
&
-60
2:l -100 -120
t
::::
+Experimental
-180
Figure 2. Calculated and experimental values of the 31Pchemical shift of neat TBP-Eu(NO3)3-6H20 plotted as a function of TBP:Eu ratio.
4:1
-v,
I
A
t
k
6:1
1O:l
15:l
Euo3TBP complex is consistent with the proposed formation of a coordinate bond between the phosphoryl oxygen and the metal in the salt solvate.'* There has been disagreement over the solvation numbers for the lanthanide and actinide salts. In the case of europium, the solvate number depends on the diluent. However, studies indicate that the trisolvate results when undilited TBP is used.6 The relative constancy of the chemical shift in the TBP:Eu range 1:l to 3:l corroborates the results obtained by dilution studies. The progression of the chemical shift with increasing TBP: Eu ratio is expected for a situation in which rapid exchange is occumng between the complexed and uncomplexed TBP. In an exchanging system, the observed chemical shift is the population-weightedaverage of the shifts of the bound and free species. The dependence of lanthanide-induced shifts on shift reagent equilibria has been analyzed mathemati~ally.'~-~' A general expression for treatment of NMR-exchange data is
+
a,,= PAOA P,o,
(1)
where P A and PB are the fraction of atoms A and B and OA and UB are the chemical shifts of sites A and B, respectively. This equation can be modified to relate the chemical shifts of two solutions of compositions n and i expressed as molar ratios TBP:Eu
I
I
,
200
I
100
,
1 0 ,
(1)+ a,,, ("i i1
a, = (Ti ,
,
I
-100
,
-200
PPm
Figure 1. 31PNMR spectra (81.O MHz) of solutions prepared by direct dissolution of Eu(N03)3 in TBP. The molar TBP:Eu ratios are indicated.
TABLE 1: 31PChemical Shift of Solutions Prepared at Different TBP:Eu Ratios TBP:Eu ratio
31Pchemical shift, ppm
1:l 2: 1 2.5: 1 3: 1 4: 1 6: 1 8: 1 1O:l 15:l 20: 1 TBP
-158.1 - 172.9 - 156.0 - 150.9 -132.8 -99.1 -61.1 -48.1 -34.6 -23.4 0.3
Solutions in the range 1:l to 3: 1 show dramatic upfield shifts relative to pure TBP. These values fall in the range -150.9 to -172.9 ppm and are indicative of the proximity of the phosphorus atom to europium. The 31Pchemical shift in the
The predicted shift of solution n, a,, can be calculated given the shift of solution i, ui, and the shift of pure TBP. A calculation of the predicted chemical shift at 3:l using the experimental chemical shift at 15:l yields a value of u3 = -172.2 ppm. This calculation employs the experimental values OTBP = 0.03 ppm and 015 = -34.6 ppm. The fractions of unbound and bound TBP are 15-3 and 3, respectively, since one Eu complexes three TBP molecules. The calculated value falls in the range of shifts observed for 1:l to 3:l solutions, - 150.9 to - 172.9 ppm. This agreement indicates that TBP is essentially fully complexed at these ratios. Calculated chemical shifts at all ratios are determined using eq 2. A plot comparing these calculated values to the experimental values is shown in Figure 2. The close agreement between experimental and calculated values indicates a rapid exchange of TBP between the free and complexed state. Hectorite Studies. The behavior of TBP and Eu(N03)3 in the clay hectorite is studied by 31PMAS of the solid samples. The observed 31Pchemical shift values (a) and the d spacing determined by powder X-ray diffraction are tabulated in Table 2. A comparison of the spectra in Figure 3 reveals that only the spectrum of hectorite adsorbed with 3:l TBP:Eu displays
Hartzell et al.
4208 J. Phys. Chem., Vol. 99, No. 12, 1995
I (TBP-Eu)/Hect
TBPMeCt 160.0
80.0
0.0
-80.0
-160.0
-240.0
I
offset spike is seen at -100 ppm.
TABLE 2: 31PChemical Shift, d-Spacing Values for Hectorite Exchanged with Europium, TBP or Eu-TBP sample chemical d spacing, no. samole shift. Dum A -0.3 TBP (solution) I -0.7 18.4 TBP-hectorite 18.1 I1 -5.0 TBP-hect + Eu (IS = 0.014) -2.3 111 hectorite 3:l TBP-Eu IV
+ hectorite + 3:1 TBPEu
V
duplicate of sample IV
VI
hectorite
IX X XI XI1
+ 5:1 TBP:Eu hectorite + 7: 1 TBP:Eu Eu-hectorite + TBP (TBP to Eu ratio was 2:1) Eu-hectorite + TBP (excess) duplicate of sample IX
Eu-hectorite Ca-hectorite
160.0
80'.0
O.'O
-8010
-160.0
-240.0
PPm
Figure 3. 3'P MAS spectra (81.0MHz) of TBP present in hectorite samples: (3:1 TBP-Eu)/hectorite,sample V; TBP/(Eu-hectorite),sample IX; Eu/(TBP-hectorite),sample 11; TBP-hectorite, sample I. The DC
VI11
v TB P/Hect
PPm
VI1
I
-180.7 -6.2 -193.8 -6.1 -189.1 -4.0 -189.3 -6.0 -194.8 -16.6 -17.6 -6.2 no signal -
18.2 17.9 18.0 16.7 12.4 18.4 17.9 12.3 12.5
an upfield peak at - 189 ppm. The other clay preparations with Eu and TBP show peaks downfield in the range -5 to -17.6 ppm. These values are very close to that of TBP alone adsorbed into hectorite, -0.7 ppm. Dramatic shift differences are seen in spectra of hectorite adsorbed directly with 3: 1,5:1 and 7: 1 TBPEu solutions, shown in Figure 4. These clays show peaks in the range -189.1 to - 194.8 ppm which are attributed to the Eua3TBP complex. Each of these spectra displays a second peak in the region -2.3 to -6.0 ppm which is attributed to uncomplexed TBP. Analysis of the spectra in Figure 4 also addresses the important question of whether exchange occurs between free and complexed TBP in the clay. The two separate peaks observed in the spectra of the 5:l and 7:l clays contrast strikingly with the single exchange-averaged peaks in the solution spectra in the region -61 to -132 ppm. No spectroscopic behavior consistent with the exchange of TBP is observed in the clay.
Figure 4. 31PMAS spectra (81.0MHz) of TBP in hectorite samples prepared by adsorbing TBP-Eu(NO& solutions with TBP:Eu ratios 3:1, 5:1, 7:1,and pure TBP. The DC offset spike is seen at -100 ppm. Line widths of 1.0-2.4 kHz are observed for the 31Psignal in most of these clay samples. Even TBP adsorbed to a Caexchanged hectorite (data not shown) displays a line width of 1800 Hz. In contrast, the line width of TBP in hectorite is 432 Hz. These line widths most likely reflect the inhomogeneity of the clay environment. Since the spectra were acquired without proton decoupling, differences in hydration could also affect the line shape. The conclusions of this study are based primarily on the variation of chemical shift values. Line shape will be of importance in future studies of hydration effects. The d spacings of the clays adsorbed either with TBP or with the TBP-Eu solutions range from 17.9 to 18.2 8,. This is a marked increase over the 12.3 A spacing of Eu-hectorite. In the 3:1, 5:l and 7:l TBP:Eu hectorite this large d spacing is a strong indication of the presence of the complex Ew3TBP within the interlayer region of the clay. The spectrum of the (3: 1 TBP: Eu)/hectorite strongly argues in favor of this point since there is very little uncomplexed TBP present to cause this expansion. TBP that is adsorbed into Eu-exchanged hectorite, samples VIII-X, shows downfield signal in the range -6.2 to -17.6 ppm. Although the shift to -17.6 ppm reflects the presence of Eu in the clay, a peak indicative of the Ew3TBP complex is absent from these spectra. Relative P-Eu distances can be estimated using the McConnell-Robertson equationzzdescribing the dipolar shifts for lanthanides
A; = K(3 cos2 t?;-l)/r;
(3)
where Ai is the dipolar shift, K is a group of constants, rj is the distance between nucleus i and the paramagnetic ion, and 0 is the angle between the vector rj and the magnetic symmetry axis of the paramagnetic ion. The ratio of shifts Aa/Ab of 31Pnuclei at two distances from Eu, r, and rt,, is proportional to the inverse cube ratio An estimation of the relative P-Eu distances for TBP displaying the chemical shifts D = -17.6 ppm (sample IX) and cr = -189.1 ppm (complex) yields rc = 0.45rrx. If a distance in the 3-4 A range is assumed for the complex, then the estimated distance of TBP exchanged into the Eu-hectorite is 7-9 8, from the sites of the exchanged Eu3+. This distance is close in value to the size of the interlayer spacing.
Tributyl Phosphate Complex of Europium Nitrate in Hectorite
TABLE 3: Europium Content of Exchanged Hectorite Samples sample no. sample mmol of Eu/g of hectorite V IX XI
+
hectorite 3: 1 TBP-Eu Eu-Hectorite TBP Eu-Hectotite
+
J. Phys. Chem., Vol. 99, No. 12, 1995 4209
(3:l TBP-Eu)/Hect
I
0.245 0.236 0.253
The Eu-hectorite was prepared under conditions of low ionic strength at pH = 5. Under similar conditions, Zachara and McKinley23 found that sorption of divalent cations (Cd2+, U02*+) on smectite clay minerals is dominated by exchange in the fixed-charge sites. This would argue for the location of Eu3+ within the interlayer. The d spacings of 18.17 and 17.94 8, found in samples IX and X indicate that TBP is adsorbed into the interlayer. That the expansion of the interlayer is sensitive to the amount of TBP is demonstrated by sample VIII. The low TBP levels resulted in a d spacing of 12.36 A. These studies support the coexistence of TBP and Eu3+in the interlayer with no complex formation. Analysis by ICP of the Eu3+ content of Eu-hectorite yields a value of 0.24 mmol(0.72 mequiv) Eu3+per gram of clay (Table 3). Given that the cation-exchange capacity of the clay is 1 mequivlg, the exchange of Eu3+ in the hectorite was 72% of the possible value. Similar values were obtained for Euhectorite after exchange of TBP, indicating that adsorption of TBP does not displace exchanged Eu3+. The Eu3+ level in the hectorite adsorbed with 3:l TBP:Eu is close to these values. This indicates a distribution of complex that is similar to the density of cation-exchange sites. Nonspinning spectra were acquired on the 3: 1 and 5: 1 TBP: Eu adsorbed clays. These spectra are compared with the MAS spectra in Figure 5. In the absence of a rigid-limit, 31Pchemical shift tensor for TBP, it is not possible to say whether the static spectrum represents a motionally averaged powder spectrum. Relevant chemical shift anisotropies for 31Prange from 36 ppm for KH2P0424to approximately 70 ppm for triethylph~sphine.~~ The width of the TBP lines change from 13.9 and 17.4 ppm under MAS to 52 and 55.5 ppm, respectively, in the static spectra. For the complex, the change is from 24.3 and 34.7 ppm (MAS) to 41.6 and 45.1 ppm static. The observed widths of the static spectra fall within the range of known CSA values; thus the spectra could be attributed to nonmobile species. This behavior will be pursued in future studies. As a control for inner- andlor outer-sphere edge-site adsorption of the complex on a surface hydroxyl group of the clay, adsorption of Eu.3TBP onto the surface of gibbsite was attempted using the same procedure as that described for hectorite. No signal was visible by NMR after accumulation of 30 000 transients, indicating any absorption of complex is less than 5% the amount adsorbed by hectorite based on the sensitivity of the instrument. This is in agreement with results indicating that the complex is sequestered in the interlayer of the clay.
Conclusions The NMR data presented in this study strongly support results from other methodologies on the mode of complexation and the solvation number for the solvate Ew3TBP. The NMR results also clarify the interactions of the species Eu3+, TBP, and Eu.3TBP in the clay hectorite. Both Eu-3TBP complex and free TBP are adsorbed into the clay. However, no exchange of TBP between these environments is observed. TBP and Eu3+ adsorbed sequentially into the clay do not undergo complexation. Based on these results, it is apparent that Eu3+ released to the subsurface environment can enter a clay in two forms: as
(5:l TBP-Eu)/Hect
160.0
80'.0
O.'O
-8O:O -160.0 PP"
-240.0
Figure 5. 31PMAS and nonspinning (static) spectra (81.0 MHz) of 3:l and 5:l TBP:Eu adsorbed into hectorite. The DC offset spike is seen at -100 ppm.
a cation, presumably from a highly aqueous environment, or as an organic complex formed prior to adsorption into a clay. Nonaqueous phase liquids (NAPL) form significant subsurface contaminants.26 Organo-actinide or organo-lanthanide complexes could form within these nonaqueous phase liquids prior to adsorption into a clay. Complexes present in the initial releases and stabilized by nonaqueous phase liquids could also adsorb into clays. Leaching studies must address the behavior of not only free cations but also organic complexes within the clay. Future studies in this laboratory will address the questions of leaching by studying NMR line shape and chemical shift changes of the complex in response to hydration and exposure of the adsorbed clay to organic liquids.
Acknowledgment. This work was initiated under the sponsorship of the U.S. Department of Energy, Environmental Restoration and Waste Management Young Faculty Award Program administered by Oak Ridge Associated Universities. Recent work was funded in part by the Subsurface Science Program of the Office of Health and Environmental Research, US. Department of Energy under Los Alamos National Laboratory subcontract 9-XA3-037KK-1. S.Z.Y. and C.J.H. acknowledge support from Organized Research, Northem Arizona University.
4210 J. Phys. Chem., Vol. 99, No. 12, 1995
References and Notes (1) McCarthy, J. F.; Zachara, J. M. Environ. Sci. Technol. 1989, 3, 496. (2) Francis, A. J.; Iden, C. R.; Nine, B. J.; Chang, C. K. Nucl. Technol. 1980, 50, 158. (3) Riley, R. G.; Zachara, J. M. Chemical contaminants on DOE lands and selection of contaminant mixturesfor subsurface science research; DOE/ ER-05477. U.S. Department of Energy, Office of Energy Research, Washington, DC, 1992. (4) Stenner, R. D.; Cramer, K. H.; Higley, K. A,; Jette, S. J.; Lamar, D. A,; McLaughlin, T. J.; Shenvood, D. R.; Van Houten, N. C. Hazard Ranking System Evaluation of CERCLA Interactive Waste Sites at Hanford; PNL: Richland, WA, 1980; Vol 2, p 210. (51 Kalina. D. G.: Mason. G. W.: Horwitz. E. P. J . Inorn. Nucl. Chem. 1981, 43, 579. (6) Scareill, D.: Alcock, K.: Fletcher, J. M.; Hesford, E.; McKay, H. A. C. J. InoG. Nucl. Chem. 1957, 4, 304. (7) Tennakoon, D. T. B.; Thomas, J. M.; Jones, W.; Carpenter, T. A,; Ramdas, S. J. Chem. SOC.,Faraday Trans. 1 1986, 82, 545. (8) Hartzell, C. J.; Hsu, M. L.; Buscher, C. T.; Moms, D. E.; Eller, P. G. Mol. Cryst. Liq. Cryst. 1992, 21 1, 227. (9) Pratum, T. K. J . Phys. Chem. 1992, 96, 4567. (10) Bank, S.; Bank, J. F.; Ellis, P. D. J . Phys. Chem. 1989, 93, 4847. (11) Luca, V.; Cardile, C. M.; Meinhold, R. H. Clay Miner. 1989, 24, 115. (12) Weiss, C. A.; Kirkpatrick, R. J.; Altaner, S. P. Geochim. Cosmochim. Acta 1990, 54, 1655. (13) Laperche, V.; Lambert, J. F.; Prost, R.; Fripiat, J. J. J. Phys. Chem. 1990, 94, 8821.
Hartzell et al. (14) Fripiat, J. J.; Letellier, M.; Levitz, P. Philos. Trans. R. SOC. London 1984, A31 1 , 287.
(15) Raber, D. M. In Lanthanide Shqt Reagents in Stereochemical Analysis: Momll, T. C., Ed.; VCH Publishing: Weinheim, Germany, 1986. (16) Benally, N.; Hartzell, C. J. Proceedings of rhe Sixth National Conference on Undergraduate Research 3; Yearout, R. D., Ed.; The University of North Carolina at Asheville Press: Ashville, NC, 1992. (17) White, C.; Hartzell, C. J. Analysis of Ca2+ and Eu3+ in competitively exchanged clays. SACNAS, Albuqueque, poster, 1993. (18) Orth, D. A.; Wallace, R. M.; Karraker, D. G. In Science and Technology of Tributyl Phosphate; Schulz, W. W., Navratil J. D., Talbot A. E. Eds.; CRC, Boca Raton, FL, 1984. (19) Shapiro, B. L.; Johnston, M. D., Jr. J . Am. Chem. SOC.1972, 94, 5325. (20) Reuben, J. J. Am. Chem. Soc. 1973, 95, 3534. (21) Johnston, M. D. Jr.; Shapiro, B. L.; Shapiro, M. J.; Proulx, T. W.; Godwin, A. D.; Pearce H. L. J . Am. Chem. SOC. 1975, 97, 542. (22) McConnell, H. M.; Robertson, R. E. J . Chem. Phys. 1958,29, 1361. (23) Zachara, J. M.; McKinley, J. P. Aquatic Chem. 1993, 55, 250. (24) Burghoff, U.; Rosenberger, H.; Zeisds, R.; Muller, R.; Rashkovich, L. N. Phys. Status Solidi A 1974, 26, K171. (25) Baltusis, L.; Frye, J. S.; Maciel, G. E. J . Am. Chem. SOC. 1987, 109, 40. (26) “Western Region Hazardous Substance Research Center”; 1993 Annual Report, Department of Civil Engineering, Stanford University: Stanford, CA, 1993; p 8. Jp9422147
