Comparative Coprecipitation of Phosphate and Arsenate with Yttrium ...

Journal of Solution Chemistry, Vol. 26, No. 12, 1997

Comparative Coprecipitation of Phosphate and Arsenate with Yttrium and the Rare Earths: The Influence of Solution Complexation Xuewu Liu, Robert H. Byrne,* and Johan Schijf Received August 20, 1997; revised September 23, 1997 Coprecipitation of yttrium (Y) and rare earth elements (REEs) with phosphate and arsenate removes these elements from solution in variable proportions. During both phosphate and arsenate Coprecipitation, middle REEs (Sm and Eu) are progressively depleted in solution relative to heavier and lighter elements. Solution complexation by oxalate (Ox 2 - ) influences Y and REE removal patterns by strongly enhancing the retention of Y and the heaviest REEs in solution. The extent of this enhancement is well described by a quantitative account of the comparative solution complexation of Y and REEs as M(Ox)+ and M(Ox)2. The comparative behavior of phosphate and arsenate Coprecipitation exhibits both similarities and differences. During arsenate Coprecipitation the light REEs are retained in solution, relative to the heavy REEs, to a greater extent than is the case for phosphate Coprecipitation. Notable irregularities are observed in the comparative Coprecipitation behavior of nearest-neighbor elements (e.g., EuGd-Tb and Tm-Yb-Lu). Such irregularities are very similar for phosphate and arsenate Coprecipitation in the absence and in the presence of solution complexation. KEY WORDS: Coprecipitation; rare earths; lanthanides; yttrium; phosphate; arsenate.

1. INTRODUCTION Observations of yttrium (Y) and rare earth element (REE) phosphate solubility products(1,2,3) indicate that formation of Y and REE coprecipitates may determine the upper-bound concentrations of these elements in the ocean and may also strongly influence the relative abundance patterns of Y and

Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701.

1187 0095-9782/97/1200-1187$12.50/0 C 1997 Plenum Publishing Corporation

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REEs in seawater.(2,4) Little work has been previously devoted to examination of coprecipitation on comparative REE abundances in solution. The work of Byrne and Kim(2) investigated the influence of phosphate coprecipitation on the removal of Ce, Eu, Gd, Tb, and Yb from solution. Their work showed substantial differences in the extent of REE removal from solution during coprecipitation and demonstrated that removal patterns (fractionations) are strongly influenced by REE solution complexation. Byrne et al.(4) investigated the influence of phosphate coprecipitation on fourteen REEs and Y in the absence of solution complexation. Their work demonstrated that removal of Y and REEs from solution as phosphate coprecipitates depletes Sm in solution relative to heavier and lighter elements, and produces a variety of nearestneighbor concentration anomalies consistent with concentration anomalies reported in high precision analyses of open-ocean water. In the present work we have extended the work of Byrne et al.(4) to include the influence of solution complexation on comparative Y and REE phosphate coprecipitation. In the natural environment, carbonate ion is the dominant inorganic ligand which influences the solution complexation behavior of Y and REEs. Since carbonate and oxalate (Ox 2- ) have similar Y and REE complexation properties, and the use of oxalate in open systems obviates considerations of gas exchange equilibria, Ox2- was used as a convenient proxy for CO2- in our work. Additionally, in view of the high concentrations of dissolved arsenate in some environmental systems,(5,6) we have investigated the comparative behaviors of Y and REEs during coprecipitation with arsenate. This investigation allows direct examination of comparative phosphate and arsenate coprecipitation with Y and REEs, as well as a comparative examination of the influence of solution complexation on Y and REE coprecipitation with both phosphate and arsenate. 2. EXPERIMENTAL PROCEDURES The experimental procedures in this work follow those described in Byrne et al.(4) with minor modifications. Coprecipitation experiments were performed using Y and all REEs, except Pm. Yttrium and REE solutions were made up from individual ICP standards (SPEX Chemical). Sodium phosphate dibasic (ultra-pure grade), sodium oxalate, and sodium borate decahydrate were obtained from J. T. Baker. Sodium arsenate heptahydrate was from Sigma Chemicals. All labware was cleaned by washing with Micro solution (Cole-Parmer), followed by immersion in 4 M HC1 for at least one week and subsequent rinsing with Milli-Q water. Coprecipitation was carried out in 1000 cm3-capacity Teflon bottles. For experiments not involving oxalate, 500 cm3 of REE plus Y solution was mixed with an equal volume of phosphate or arsenate solution. The initial

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Y and REE concentrations after mixing were 100 ppb (w/w) for each metal, and the total concentration of REEs and Y (S[oM3+]T) was 1.0X 10-5 molal. The concentration of phosphate or arsenate was 2X 10-4 molal. For experiments involving oxalate, sodium oxalate salt was first dissolved in phosphate or arsenate solution, and subsequent to mixing with the REE plus Y solution the final oxalate concentration was 1.0X10-5 molal. After mixing, Teflon bottles were thermostated at 25.0 ± 0.1°C. The total ionic strength of these mixed solutions was on the order of 1X10-3 molal. The initial solutions were sufficiently acidic (pH 3.3) to keep Y and REEs from precipitating. Solution pH was measured with a Ross combination electrode calibrated on the free hydrogen ion concentration scale.(7,8,9) After taking samples for initial concentrations of Y and REEs, the pH was slowly raised by adding 0.2 M borate solution while stirring vigorously. Borate is a convenient titrant for such work because B(OH)4 is a weakly complexing ligand with Y and the rare earths and, under the conditions of our experiments (pH < 6.1), less than 0.1% of the total boron in solution (B(OH)3 and B(OH)4) is present as B(OH)4. After each pH adjustment, a 5 cm3 sample was taken to check for onset of precipitation. If precipitation was observed from measurement of dissolved Y and REE concentrations, samples were taken at regular time intervals. Dissolved concentrations of Y and REEs were measured with a Fisons PQS ICP-mass spectrometer. No sample processing other than filtration was required prior to analysis. Subsequent to filtration with 0.22 um acetate cartridge filters (Corning), 5 cm3 of sample were combined with 5 cm3 of 100 ppb indium solution used as an internal standard. The internal standard corrected for matrix effects, which were very minor even in the presence of oxalate. Samples were analyzed immediately after filtration, thereby preventing further chemical fractionation prior to analysis. Sample concentrations were measured against a mixed Y and REE standard solution (SPEX Chemical). 3. THEORY

The coprecipitation phenomena observed in this work were quantitatively described using the Doerner and Hoskins equation(10) originally developed to examine the coprecipitation of Ba and Ra sulfate. The results of Doerner and Hoskins(10) support the assumption that relative metal concentrations on a coprecipitate surface are directly proportional to relative metal concentrations in solution:

where (Mi)s and (Mj)s are the surface concentrations of metal ions Mi and

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MJ, [M3+] and [M3+] are the concentrations of free (uncomplexed) metal ions in solution and Xij is a proportionality or fractionation factor for metals Mi and MJ. If the concentrations of metals MI and MJ on the coprecipitate surface are directly proportional to free metal ion concentrations in solution as shown in Eq. (1) then, in the presence of solution complexation, Eq. (1) can be expressed in terms of total metal ion solution concentrations ([M3+]T) as follows

where

Under our experimental conditions, where metal ions are complexed by oxalate, Eq. (3) can be written as

where OxB1(M) = [MOx+]/[M3+][Ox2-] and oxB2(M) = [M(Ox)2]/ [M3+][Ox2-]2 and brackets [] denote the free concentrations of each indicated species. Beginning with Eqs. (1, 2), following the derivations of Doerner and Hoskins(10) it can be shown that the solution concentrations of metals M; and MJ in a coprecipitation process can be written, in the presence and absence of solution complexation, in the following forms

and

where [M3+]T, [M3+]T, [M3+] and [M3+] represent solution metal concentrations at any point in time subsequent to initiation of coprecipitation and [0M3+]T, [oMj3+]T, [0M3+] and [oM3+] represent corresponding concentrations at any prior point in time. Equations (5,6) were used to quantitatively describe the coprecipitation of Y and REE elements during the formation of phosphate and arsenate coprecipitates in the presence and absence of oxalate.

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4. RESULTS AND DISCUSSION

The extent of metal removal from solution during coprecipitation is shown in Fig. 1 as a function of time for three rare earths. REE concentrations decreased steadily through time, although at variable rates in each experiment. The pH required to initiate coprecipitation in arsenate experiments was higher than that for phosphate due to the greater solubility of REE arsenates. Metal

Fig. 1. Selected rare earth concentrations (La, Sm, Lu) as functions of time: (a) Rare earth phosphate coprecipitation, (b) rare earth phosphate coprecipitation in the presence of oxalate, (c) rare earth arsenate coprecipitation, and (d) rare earth arsenate coprecipitation in the presence of oxalate.

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solubilization due to oxalate complexation (Figs. 1b and 1d) also increased the pH requisite for coprecipitation. The comparative behaviors of Y and REEs during coprecipitation are shown in Fig. 2. Phosphate and arsenate coprecipitation patterns show both substantial similarities and significant differences. Maxima in the extent of REE coprecipitation with phosphate and arsenate were observed at Sm and Eu (Figs. 2a and 2c). Solution complexation shifts these coprecipitation maxima toward lighter elements because heavy REEs are more strongly complexed than light REEs, favoring their retention in solution. For example, Cantrell and Byrne(11) observed that the formation constant of YbOx+ is larger

Fig. 2. Fractionation patterns observed in the coprecipitation processes. Fractionation is depicted as log[M3+]/[0Mi3+] or log[M3+]T/[0M3+]T where [0M3+] and [oM3+]T are the initial concentrations of each element. Sample collection times (hours) are shown for each fractionation pattern, (a) Rare earth phosphate coprecipitation, (b) rare earth phosphate coprecipitation in the presence of oxalate, (c) rare earth arsenate coprecipitation, and (d) rare earth arsenate coprecipitation in the presence of oxalate.

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than that of CeOx+ by approximately a factor of five, and the Yb(Ox)2 formation constant is larger than that of Ce(Ox)2 by more than a factor of thirty. In the presence of oxalate the minimum in the phosphate coprecipitation pattern is shifted to Ce (Fig. 2b) and the arsenate minimum is shifted to Sm (Fig. 2d). Inspection of the Fig. 2 results for each of the four coprecipitation experiments shows that coprecipitation patterns developed early in each experiment are generally retained throughout the experiment. For example, higher Er concentrations relative to its nearest neighbors (Ho and Tm) are seen throughout the experimental patterns in Figs. 2a and 2c, as are elevated Gd concentrations relative to Eu and Tb. The most appropriate means of comparison of the metal fractionation patterns shown in Fig. 2 is examination of fractionation factors (Xy and Xy) calculated via Eqs. (5, 6). Figs. 3a and 3b show the phosphate Xy and Xy results and Figs. 3c and 3d show arsenate Xjj and Xy results obtained (Table I) using the final four metal fractionation patterns (log[Mi+]/[oM3+]) in each experiment. Sm is used as the reference element for all calculations ([Sm3+] = [M3+] and [Sm3+]r = [M3+]T) because its relatively large extent of coprecipitation allowed more sensitive determinations of log[Sm3+]/[0Sm3+] than is possible for most other elements. The error bars given in Table I, and shown graphically in Fig. 3, represent the standard errors of the calculated Aij and Xy results for the four measurements. Comparison of Figs. 3a and 3c shows that, while Xij results for phosphate and arsenate are similar in general appearance, Xij results for the lightest REEs (La, Ce, Pr) relative to the heaviest REEs (Ho-Lu) differ distinctly for phosphate and arsenate coprecipitation. These differences are also seen in comparisons of Figs. 3b and 3d. Comparisons of Xij results with Xij results (Figs. 3b vs. 3a and 3d vs. 3c) show that the increase in Xij for elements heavier than Sm (Figs. 3a and 3c) is enhanced in the presence of oxalate (Figs. 3b and 3d), as reflected in corresponding increases in Xij values. None of the curves shown in Fig. 3 are smooth. All of the patterns (Xij and Xij) in Fig. 3 show irregularity with increasing atomic number. The most consistent and obvious instances of this irregularity are seen in the patterns of Eu-Gd-Tb and Tm-Yb-Lu. As a means of directly comparing the fractionation patterns obtained for phosphate and arsenate, Fig. 4 shows X(j results for phosphate normalized to Xij results for arsenate (X ij (PO 4 )/X ij (AsO 4 - )) as well as Xij results similarly compared ( X i j P O 4 ) X i j A s O 4 ) ) . Figure 4 shows that arsenate normalization of phosphate fractionation factors produces curves which are very similar in appearance and are largely devoid of element-to-element irregularities. The smooth trend observed for normalized Xij data (phosphate/arsenate) shows that observed irregularities in the coprecipitation behavior of adjacent elements are nearly identical in phosphate and arsenate coprecipitation. The relationships shown in Fig. 4 indicate that

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Fig. 3. Fractionation factors (Xij and Xij) from Table I. (a) Rare earth phosphate coprecipitation, (b) rare earth phosphate coprecipitation in the presence of oxalate, (c) rare earth arsenate coprecipitation, and (d) rare earth arsenate coprecipitation in the presence of oxalate.

there are striking similarities underlying the irregular patterns of Y and REE phosphate coprecipitation and arsenate coprecipitation. The comparative behavior of Eu-Gd-Tb and Tm-Yb-Lu is, for example, very similar for phosphate and arsenate coprecipitation. It is difficult to rationalize the irregular coprecipitation behavior of Eu-Gd-Tb and Tm-Yb-Lu, observed in all experiments, directly in terms of crystal radii since the ionic radii of the REE elements decrease monotonically with increasing atomic number.(12) More appealing explanations for the observed comparative behaviors will likely involve accounts of the irregular extent of REE hydration, both in solution(13,14) and in a hydrated coprecipitate at the solution-solid interface. It is plausible to suggest that REEs are removed from solution during coprecipitation via incorporation, initially, in a hydrated solid. Whereas REEs in solution have

Effect of Complexation on Coprecipitation Table I.

REE Y

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

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Fractionation Factors Xij and Xij for Yttrium and Rare Earth Coprecipitation Calculated Using Eqs. (5, 6) Xij (phosphate) (no oxalate)

Xij (phosphate) (with oxalate)

Xij (arsenate) (no oxalate)

Xij (arsenate) (with oxalate)

0.352±0.037 0.455±0.019 0.687±0.013 0.769±0.009 0.770±0.006

0.216±0.014 0.794±0.016 1.056±0.014 1.054±0.009 0.961 ±0.015

0.310±0.015 0.171±0.006 0.363±0.006 0.503 ±0.006 0.558±0.008

0.216±0.021 0.393 ±0.020 0.636±0.010 0.713±0.020 0.715 ±0.022

1

1

1

1

0.956±0.015 0.706±0.014 0.742±0.011 0.663 ±0.01 8 0.530±0.019 0.486±0.011 0.500±0.009 0.529±0.018 0.473±0.023

0.848±0.010 0.609 ±0.006 0.529 ±0.006 0.420±0.007 0.303 ±0.007 0.250±0.005 0.234±0.004 0.238 ±0.007 0.203 ±0.007

1.017±0.007 0.738 ±0.009 0.868±0.009 0.819±0.012 0.637 ±0.015 0.586±0.012 0.600±0.013 0.681+0.008 0.581 ±0.010

0.940±0.008 0.686±0.011 0.650±0.020 0.563±0.028 0.398±0.013 0.344±0.014 0.293 ±0.020 0.322±0.015 0.265 ±0.017

inner-sphere hydration numbers on the order of nine for the light rare earths and eight for the heavy rare earths (with a break between Sm and Tb), coprecipitated REEs at the solution-solid interface should have much reduced hydration spheres. It is possible that the hydration-dehydration behaviors of yttrium and REEs during solid phase formation (MiPO4.nH2O) are substantially more complex than is the case for elements in solution, thereby accounting for the irregular Xij and Xij patterns shown in Fig. 3.

Fig, 4. Comparison of phosphate and arsenate Coprecipitation patterns (Xij(phosphate)/Xij (arsenate).

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An assessment of the influence of oxalate complexation on phosphate and arsenate coprecipitation is seen in Fig. 5. The calculated quotients, Xij/ Xij, shown is Fig. 5 provide assessments of Y and REE oxalate complexation in our experiments, relative to Sm

Experimentally derived values ofAij\Aij,obtained using the Aij results given in Table I, are in good agreement with calculated R values [right-hand side of Eq. (7)] obtained using the oxalate stability constant data given in Table II. Calculated R values shown in Fig. 5 are appropriate to the beginning (initial [Ox2-]) and end (final [Ox2-]) of each experiment. Free oxalate

Fig. 5. Effect of solution complexation on yttrium and rare earth fractionation factors, (a) Phosphate coprecipitation, (b) arsenate coprecipitation.

Effect of Complexation on Coprecipitation Table II.

REE Y La Ce Pr Nd Pm Sm Eu a

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Complexation Constants for Yttrium and Rare Earths with Oxalate at 25°C and Zero Ionic Strengtha logoxB1(M)

logoxB2(M)

REE

logoxB1(M)

logoxB2(M)

6.662 5.882 6.072 6.208 6.303

10.865 9.405 9.835 10.086 10.245

6.479 6.532

10.555 10.6S5

Gd Tb Dy Ho Er Tm Yb Lu

6.500 6.622 6.663 6.685 6.726 6.772 6.852 6.865

10.550 10.825 10.924 10.974 11.119 11.095 11.375 11.437

Complexation constants for Y, Ce, Eu, Tb, and Tm are obtained from Martell and Smith (Ref. 15). Yb results are from Cantrell and Byrne (Ref. 11) Other constants are assessed based on REE linear free energy relationships as characterized by Lee and Byrne (Refs. 16, 17)

concentrations were determined through iterative calculations using stability constant data in Table II, total oxalate concentrations equal to 1X 10-5 molal, and direct measurements of total dissolved metal concentrations through time (Fig. 2). Due to the similar initial concentrations of total metals and oxalate in our experiments (Fig. 2), free oxalate concentration varied from 1.7X10 -6 molal at the inception of coprecipitation and increased to 6.3 X 10-6 molal at the end of each phosphate and arsenate experiment as total metal concentrations decreased. The calculated R values shown in Fig. 5 exhibit only minor variation with extent of coprecipitation and are consistent with Aij and Aij results obtained from direct observations ([M3+]/[oM3+]) of metal removal from solution during coprecipitation. These results suggest that Y and REE coprecipitation behavior in complexing media can be reasonably predicted from Xij characterizations in the absence of solution Complexation (X ij , Table I) plus metal speciation calculations based on the solution concentrations of complexing ligands. 5. CONCLUSIONS Solution chemistry is seen to exert strong controls on the coprecipitation behavior of REE phosphates and arsenates. Strong solution Complexation enhances the retention of heavy REEs in solution. The extent of this enhancement, for Y and REE Complexation by oxalate, can be quantitatively described in terms of thermodynamic characterizations of Y and REE oxalate stability constants. The comparative behaviors of Y and REE coprecipitation by phosphate and arsenate evidence strong similarities in the fine structure of REE fractionation patterns. Solution chemistry may also play a strong role in this

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instance through the influence of irregular REE hydration behavior on relative metal affinities for solution and solids. ACKNOWLEDGMENT

This work was supported by contract OCE-95-22878 from the National Science Foundation. REFERENCES 1. X. Liu and R. H. Byrne, Geochim. Cosmochim. Acta 61, 1625 (1997). 2. R. H. Byrne and K.-H. Kim, Geochim. Cosmochim. Acta 57, 519 (1993). 3. R. G. Jonasson, G. M. Bancroft, and H. W. Nesbitt, Geochim. Cosmochim. Acta 49, 2133 (1985). 4. R. H. Byrne, X. Liu, and J. Schijf, Geochim. Cosmochim. Acta 60, 3341 (1996). 5. A. S. Maest, S. P. Pasilis, G. M. Laurence, and D. K. Nordstrom, in Water-Rock Interaction, Y. K. Kharaka and A. S. Maest, eds., (Balkema, Rotterdam, 1992). 6. L. C. D. Anderson and K. W. Bruland, Environ. Sci. Technol. 25, 420 (1991). 7. W. A. E. McBryde, Analyst 94, 337 (1969). 8. W. A. E. McBryde, Analyst 96, 739 (1971). 9. R. H. Byrne and D. R. Kester, J. Solution Chem. 7, 373 (1978). 10. H. A. Doerner and W. M. Hoskins, J. Am. Chem. Soc. 47, 662 (1925). 11. K. J. Cantrell and R. H. Byrne, Geochim. Cosmochim. Acta 51, 597 (1987). 12. R. D. Shannon, Acta Cryst. A32, 751 (1976). 13. E. N. Rizkalla and G. R. Choppin, in Handbook on the Physics and Chemistry of Rare Earths, Vol. 18, K. A. Gschneidner, L. Eyring, G. R. Choppin, and G. H. Lander, eds., (Elsevier, 1994). 14. E. N. Rizkalla and G. R. Choppin, in Handbook on the Physics and Chemistry of Rare Earths Vol. 15, K. A. Gschneidner and L. Eyring, eds., (Elsevier, 1991). 15. A. E. Martell and R. M. Smith, Critical Stability Constants, Vol. 3: Other Organic Ligands, (Plenum Press, New York, 1977). 16. J. H. Lee and R. H. Byrne, Geochim. Cosmochim. Acta 57, 295 (1993).

17. J. H. Lee and R. H. Byrne, Geochim. Cosmochim. Acta 56, 1127 (1992).