Solution chemistry of the rare earth elements in ...

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Solution chemistry of the rare earth elements in seawater ARTICLE in EUROPEAN JOURNAL OF SOLID STATE AND INORGANIC CHEMISTRY · MAY 1991

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Eur. J. Solid Stale lnorg. Chern. t. 28, 1991, p. 357-373

Solution chemistry of the rare earth elements in seawater H. J. W. DE BAAR Netherlands Institute for Sea Research, P.O. Box 59, 1790 Den Burg, Texel, Netherlands

J. SCHIJF Laboratory for Isotope Geology, Free University, de Boelelaan 1085, 1081 HV Amsterdam, Netherlands

R.H.BYRNE University of South Florida, 140 7th Avenue South, St. Petersburg. FL 33701, U.S.A.

ABSTRACT. - In seawater the REE(IIl) series exhibits gradual enrichment with increasing atomic number from 57La to nLu. The exception is 5SCe which is strongly depleted in seawater due to its unique oxidation to highly insoluble Ce(1V) slate. Next to these major trends the elements 57La and 64Gd appear to be modestly enhanced relative to their neighbours in the REE series. Complexation in seawater is dominated by carbonates, increasingly so with atomic number and strongly dependent on pH shifts around the typical pH 8.2 in the ocean. Shifts in hydration at or about 64Gd might yield some discontinuity in the series. The actual removal of REE from seawater is through adsorption on particles. Organic ligands tend to have lower formation constants for 57La and 64Gd. Carboxylic surface functional groups on particles would remove La and Gd less effectively, thus promoting the observed anomalous enhancements of La and Gd in seawater. The proposed interaction with biological particles is also supported by the vertical and lateral distributions of the REE, whose similarity to nutrients suggests an involvement in the organic geochemical cycle. The effect of dissolved organic ligands on solution chemistry is yet to be investigated.

INTRODUCTION The Rare Earth Elements or Lanthanides comprise a remarkably coherent group of 15 transition elements. Nucleosynthesis during stellar evolution generally favors even numbered over odd numbered atoms. Within the REE series the even/odd predominance with atomic number Z convincingly illustrates that this general rule of Oddo-Harkins applied when our solar system was formed. The extreme case is odd-

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La Ce Pr

Nd Pm Sm Eu Gd Tb

Dy Ha Er

Tm Yb Lu

f'ig. 1. Th~ ionization potential for th~ electrons (i.e. for the 3 + cation) along the REE Note the lower overall ionization potential at La, Gd. Lu (tak~n from Rd. 7).

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numbered element 61 Pm without natural occurence other than negligible amounts of short lived radioisotopes produced by rarely occurring spontaneous fission of 238U. The radioisotope 147Pm (t 1l2 = 2.62 a) has also been re-introduced on earth as fallout of nuclear bomb testing (1) and the Chemobyl nuclear accident (2,3,4,5). The common valency of the REE is the (III) oxidation state, but Ce can be oxidized to the REE-EDTA very insoluble Ce(IV) state whereas reduced Eu(II) may occur under high temperature conditions (6). In the laboratory some other 19 REE may exists in other valencies (e.g. Pr 6 0 11 , Sm 2+, Yb 2+) in solid, dissolved and organic forms, but these valencies are irrelevant for geochemical purposes. 18 With increasing atomic number (S7La to 71 Lu) the stepwise filling of fourteen positions in the inner 4f electron shell leads to gradual changes in chemical properties. The La3+, Gd 3+ and Lu 3+ cations with exactly empty, half-filled and completely filled 4f 16 electron shell are particularly stable (Fig. 1). This has been shown to lead to excursions from these elements from the gradual trend with atomic number. Examples of such 15 excursions are notably well documented for (bl stabilities of REE-organic bonds, for example the discontinuity at Gd for REE - EDTA (Fig. 2). Sometimes a full fledged tetrad effect has been suggested, where the term is meant to Fill;. 2. Th~ stability constant of ~thylene suggest also weaker excursions at exactly 1/4 and 3/4 filling of the 4f shell, that is between diamine tetraacetic acid (EDTA) with the Nd-Pm as well as between Ho-Errespectively REE =i~s (After Ref. II). (8,9). Evidence for such weaker excursions at 1/4 and 3/4 filling is generally less convincing. In the case of seawater the 1/4 filling excursion cannot be found as Pm does not exist in nature. The presumed 3/4 filling deviation suffers from the fact that the two analytical methods currently used can TOME

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La Ce Pr

Nd Pm Sm Eu Gd Tb

Dy He Er

359

Tm Yb Lu

Fig. l. The ionization potential for three electrons (i.e. for the 3+ cation) along the REE series. Note the lower overall ionization potential at La, Gd, Lu (taken from Ref. 7).

numbered element 61 Pm without natural occurence other than negligible amounts of short lived radioisotopes produced by rarely occurring spontaneous fission of 238U. The radioisotope 147Pm (t1/2 = 2.62 a) has also been re-introduced on earth as fallout of nuclear bomb testing (I) and the Chemobyl nuclear accident (2,3,4,5). The common valency of the REE is the (III) oxidation state, but Ce can be oxidized to the REE-EDTA very insoluble Ce(lV) state whereas reduced Eu(ll) may occur under high temperature conditions (6). In the laboratory some other 19 REE may exists in other valencies (e.g. Pr6 0 11 , Sm 2+, Yb 2+) in solid. dissolved and organic forms, but these valencies are 18 irrelevant for geochemical purposes. With increasing atomic number (S7La to >o w 71 Lu) the stepwise filling of fourteen positions in the inner 4f electron shell leads to gradual changes in chemical properties. The 0' o La3+, Gd 3+ and Lu 3+ cations with exactly empty, half-filled and completely filled 4f 16 electron shell are particularly stable (Fig. I). This has been shown to lead to excursions from these elements from the gradual trend with atomic number. Examples of such 15 excursions are notably well documented for (bJ stabilities of REB-organic bonds, for example the discontinuity at Gd for REE - EDTA (Fig. 2). Sometimes a full fledged tetrad effect has been suggested, where the term is meant to Fig. 2. The stability constant of ethylene suggest also weaker excursions at exactly 1/4 diamine tetraacetic acid (EDTA) with the and 3/4 fIlling of the 4f shell, that is between REE series (After Ref. 11). Nd-Pm as well as between Ho-Er respectively (8,9). Evidence for such weaker excursions at 1/4 and 3/4 filling is generally less convincing. In the case of seawater the 1/4 filling excursion cannot be found as Pm does not exist in nature. The presumed 3/4 filling deviation suffers from the fact that the two analytical methods currently used can

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determine either Ho (INAA-V) or Er (IDMS) but not both. For such reasons Masuda and Ikeuchi (10) were not able to make a convincing case for a full fledged tetrad effect in seawater in an otherwise acceptable dataset of REB in one seawater sample. Any changes in chemical properties can only be visualized after filtering out the overriding even/odd predominance through normalization versus REB abundance in either chondritic meteorites (12) or shales (13,14). Chondrites or shales are thought to represent the primordial bulk earth or the earth's crust respectively. Latter crustal abundance (i.e. shales) is commonly taken as a good approximation of the relative abundance of the REE input into the oceans upon continental weathering. Deviations from the shale pattern are then ascribed to chemical fractionations within the marine environment. In seawater all dissolved REE exhibit the (Ill) state largely in various complexes with abundant anions. Goldberg and co-workers (15) first determined the REB concentrations in one seawater sample. Upon shale-normalization they ascribed the ensuing gradual enrichment of the heavy REB (HREE, towards nLu) to their more stable complexes in solution. ANALYTICALMEmODS The pioneers (15,16) had to rely on Instrumental Neutron Activation Analysis, laborious separation of individual REB-radionuclides and f)-counting. Renewed interest for the REB in seawater led to the simultaneous development of Isotope Dilution Mass Spectrometry (IDMS) and Instrumental Neutron Activation Analyis with v-detection (17,18,19). These methods provide assessment of to or 12 out of the 14 REB in seawater. IDMS needs at minimum two stable isotopes for a ratio, the mono- isotopic elements Pr,Th,Ho and Tm are not suitable. For INAA-V short lived radioisotope 16SmDy escapes detection whereas a weak signal of 171 Er is masked by overlap from 169Yb. In the published datasets occasionally not all to or 12 REE were obtained with IDMS or INAA-V.

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Fig. 7. (a) The ratio MTolaJ/(MFreel calculated at pH 8.2 from the inorganic solution speciation model with carbonate constants derived from interpolation between measured constants of 58Ce, 63Eu and 10Yb (30). Note that the overall complexation of Go is actually somewhat lower due to the lower carbonate formation constants (48). In itself the complementary higher percentage of free Gd(llI) ion would indicate higher overall removal of Gd from seawater, leading to a depletion rather than the observed Gd enrichment in seawater. (b) The average of formation constants of 15 monocarboxylic acids as a function of REE atomic number (from ref. 53). Note the discontinuities at 51La and 64Gd. (c) The numerical difference of 4(a) and 4(b) as an example of the outcome of a combined surface/solution model of seawater. Note the similarity to (Fig. 4), indicating that there is a molecular basis not only for the gradual REE enrichment with atomic number but also for the positive deviations of 51La and 64Gd from that trend. EUR. J. SOLID STATE INORG. CHEM.

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formation constants of both 57La and 64Gd are systematically smaller, i.e. distinctly below the overall trend of the REE series. Combination of the averaged formation constants of monocarboxylic acids (Fig. 7b) with the most recent solution speciation model (30) yields a simulation of the REB abundances in seawater (Fig. 7c). Most obvious are the enhancements of 57La and 64Gd. where the pH 8.2 case strikingly approximates the REE pattern as observed in the real ocean.

REE Atomic Number Z 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

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Fig. 7. (a) The ratio MTotal/[MFreeJ calculated at pH 8.2 from the inorganic solution speciation model with carbonate constants derived from interpolation between measured constants of 58Ce, 63Eu and 70Yb (30). Note that the overall complexation of Gd is actually somewhat lower due to the lower carbonate formation constants (48). In itself the complementary higher percentage of free Gd(III) ion would indicate higher overall removal of Gd from seawater, leading to a depletion rather than the observed Gd enrichment in seawater. (b) The average of formation constants of 15 monocarboxylic acids as a function of REE atomic number (from ref. 53). Note the discontinuities at 57La and 64Gd. (e) The numerical difference of 4(a) and 4(b) as an example of the outcome of a combined surface/solution model of seawater. Note the similarity to (Fig. 4), indicating that there is a molecular basis not only for the gradual REE enrichment with atomic number but also for the positive deviations of 57La and 64Gd from that trend. TOME

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DISCUSSION The combined simulation of surface and solution chemistry of the REE in seawater (Fig. 7c) clearly demonstrates that observed anomalies of La (21) and Gd (31) are conceivable and might have been expected after all. This underlines the need for direct determinations of individual stability constants (e.g. Gd vs. Eu, Th) rather than interpolations for all REE. For the suite La-Ce-Pr-Nd the direct assessment would help resolve the perceived high LalNd ratio; also the exact positioning of dissolved Ce3+ between La3+ and Pr3+ (or Nd 3+) would be needed for refining the Ce anomaly concept. At the high end of the series the solution chemistry of 71 Lu versus 70Yb deserves attention. One is aware now that formation constants of Gd may quite often deviate below the value expected from interpolation between Eu and Th. This would also be in keeping with expectations based on the low ionization potential of the Gd 3+ ion (Fig. I). Presumably the effect of the deviation is strong for the organic surface ligands, thus Gd has lower affinity for particles and remains preferentially in seawater solution. The overall REE fractionation in seawater results from the subtle interaction between many factors: pH largely governing C03z- concentration hence REE-carbonate complexation, the strength of formation constants with carboxylic ligands versus their concentration on particles, as well as abundance (55) and residence time of particles (56). Through variation analysis of such factors in the surface/solution simulation model (Fig. 3) one would be able to conclude whether or not the (positive) anomalies of 57La and 64Gd are truly consistent features. Secondly the independent estimates of these factors (e.g.formation constants and concentrations of organic ligands collected in '1e real ocean) would constrain the simulation model. In analogy with the above surface organic ligands of distinct particles, it is expected that dissolved organic ligands as well as finely dispersed organic colloids will also playa role in fractionation of REE in seawater. When fractionation of REE is indeed dominated by organic particles then it is expected that most such fractionation would occur in surface waters with the highest standing stock of biological particles, i.e. plankton. Major inputs of REE from rivers are also in these surface waters. With most rivers draining into the Atlantic Ocean one indeed finds Atlantic surface waters to be less fractionated (Heavy REE enriched) than Pacific surface waters (21). Upon scavenging from surface waters the settling particles are known to remineralize again in the deep waters, releasing the adsorbed REE without any further fractionation. Hence with increasing age the deep water would contain more REE but these would not necessarily be more fractionated. This is exactly what is observed when comparing deep (young) Atlantic versus deep (old) Pacific waters (21). The one exception is Ce because throughout the deep water column its oxidation continues with time. The unique oxidation and reduction chemistry of Ce il' seawater has been described quite adequately with respect to solid CeOz at 25 °C and 1 atm pressure (26). Effects of temperature and pressure need further assessment within the oceanic range (-2 to 25 °C when disregarding hydrothermal systems; 1-500 atm), whereas other solid phases (amorphous states, solid solutions) may also playa role. ACKNOWLEDGEMENTS-. The authors are most honoured and grateful for the kind invitation by Prof. C Gorrler-Walrand and Prof. j, Hertogen to participate in the First International Conference on f-Elements, We are greatly indebted to Wim Helder, Mario Hoppema, Pielemel


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Montijn, Wim Mook, Rob Nolting and Paul Saager for valuable comments on the manuscript. This research will be part of the Ph.D. thesis of Johan Schijf and was supported by the home institutes of the authors as well as the Nederlandse Organisatie voor Wetenschappelijk Ondcrzoek (N.W.O.) and the U.S. NationaJ Science Foundation. REFERENCES

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