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Solubilities of some hydrous REE phosphates with implications for diagenesis and sea water concentrations
Solubilities of some hydrous REE phosphates with implications for diagenesis and sea water concentrations R. G. JONASSON, G. M. BANCROFT and H. W. N~ssrrr Departments of Chemistry and Geology, University of Western Ontario, London, Ontario N6A 5B8
(Received September 11, f984: accepted in revised.f’ormJuly 12, 1985) Abstract-Solubility product determinations suggest that the hydrous phosphates of the rare earths, REP04 *xHIO, are important in controlling the sea water REE concentrations. Two of these solids, rhabdophane. (P6222) and “hydrous xenotime”, (Irli/amd), have been synthesized at 100°C via the acid hydrolysis of the respective REE pyrophosphate. The solubility products at infinite dilution were determined to be pJ? = 24.5, (La at 25°C); 26.0, (Pr at IOO’C); 25.7, (Nd at 100°C): and 25.5, (Er at IOO”C). On the basis of calculations involving the reaction of RE’+ with apatite to form the hydrous phosphate, the lanthanum concentration in sea water is predicted to be about 140 pmol/L. Laboratory experiments support the hypothesis that apatite is a substrate for reactions with dissolved REE.
nodules the REE are bound initially by two sites: a phosphatic phase and a surface layer of phosphate which had previously been adsorbed onto hydrous iron oxides. Upon diagenesis, the REE may be incorporated into recrystallized biogenic apatite. Some insight into the reactions of REE in the oceans can also be obtained by examining the REE concentration patterns in various phases in the marine environment. PIPER (1974) showed, for example, that REE are fractionated to various extents, depending on the location in the ocean that one sampies for REE. In this paper, the reactions of the REE in the oceans wilt be discussed, in the light of solubility data for various hydrous REE phosphates, also obtained in this study. romanganese
THE RARE EARTH element (REE) concentrations in sea water are very low. For example, HOYLE et al. (1984)
measured the concentrations of La, Ce and Nd in coastal sea water to be 44,63 and 46 pmollkg, respectively. DE BAAR et al. (1983) found La concentrations from 16 pmol/kg near the surface of the ocean to 80 pmol/kg near the bottom (at 4.5 km). Other REE including Pr, were found to have concentrations of about an order of magnitude less. It is likely that one or more rare earth phosphate phases control the REE concentmtions, since these phases are so insoluble. For example, TANANAEV and VASIL’EVA (1963) measured the solubility product of freshly precipitated Lap@ {probably amorphous) to be 3.75 X 10sz3 at 25°C. In nature, the light rare earth
elements (LREE) commonly occur as the phosphate mineral monazite. The heavy rare earth elements plus yttrium (HREE f Y) form the phosphate mineral xenotime. The chemical differences among the REE are small. yet also large enough that the LREE and HREE form phosphates with differing cation coordination numbers. in any case, the ~1ubiIities of these minerals are low enough that they are common com~nents of heavy beach sands. It is perhaps interesting that these minerals have also been found in bauxites, (MAWMOW and PANTO, 1980). The association of REE with marine phospho~tes has been known for some time (GOLDBERG et al., 1963; SCHOFIELD and HASKIN, 1964). ELDERFIELD er al. ( 198 1) and LI ( 1982) have demonstrated an association between phosphate and REE in ferromanganese nodules and associated sediments. TURNER and WHITFIELD (1979) suggested that as a first step in the immobilization process, the REE are
adsorbed onto various mineral sufiaces. Then, as diagenetic reactions occur in the sediments, the REE are incorporated into various authigenic mineral lattices. ELDERRELD et al. (198 1) also concluded that initially the REE are immobilized by surface reactions. In fer-
In order to produce valid solubility products, the nature of the solid phase must be known. The problem with salts of low solubility, however, is that reactions forming well crystallized materials are much slower than those forming non-crystalline products. In order to reduce the rate of precipitation of the RE phosphates sufficiently to allow the formation of crystals, a synthesis was developed based on the oblation that REE p~ph~t~. (P,O:-), will slowly hydrolyse in acid solution to form the respective REE 0~~~~~~ (KIZILYA~LI,1976). The precipitation reactions were carried out at low pH in order to keep the REE concentrations in a tractable analytical range. to eliminate the possible interferences from carbonate or hydroxide, to accelerate the recrystallization reaction and to simplie the ~lculations by keeping the ph~phate primarily as H,PO,. Ion-pair formation by the small amounts of H,POi present in solution can be ignored, as can be seen from the formation constant of the RE”+ -HlPOi ion-pair. It has been measured by RAO el al. (1970) to be about 40 at I = 0.5 mol/L. All rare earth chloride salts were 99.9% pure, (Aldrich), and were used as received. REC13 solutions were titrated with EDTA using Arsenazo 1 indicator and a 6.5 pH-buffer. (The chloride salts of the REE will lose water of ~s~lli~tion on prolonged drying, but are also hygroscopic in the open air.) Phosphate and pyrophosphate solutions were titrated against standard acid or base. Phosphate precipitation reactions were carried out by refluxing a 400-500 ml aqueous solution containing 0.0050
2133
2134
R. G. Jonasson, G. M. Bancroft and H. W. Nesbitt
moles rare earth salt, 0.0026 moles Na,Pz07 and between 0. I and 0.5 moles HNOs, depending on the pH desired. A known excess of p~oph~phate was used, in order to make the rare earth ion the limiting reagent. The initial precipitate was seen to form within a day. A finer precipitate was seen to replace the former after several days. The transformation from REE pyrophosphate (coarse ppt) to REE orthophosphate (fine ppt) was followed by optical microscopy and powder x-my diffraction (Fig. 1). The initial precipitate consists of long, tabular crystals up to 0.2 mm lon8. The later precipitate consists of rounded grains about 0.005 mm in diameter. RefIux was stopped after there was no detectable pyrophosphate remaining, as determined by powder dilTraction. Conversion was usually complete after three weeks. The final solution was h&red hot tluou8h 0.45 rrn Mill&ore filters. The concentrations of the colored lanthanides were conveniently measured against standards on a Varian CARY 118 s~ophotom~. Soiutions which were vgr dilute were concentrated by evaporation to a known volume. The colored rare earth ions show very sharp absorption lines in the visible spectral region, that are quite insensitive to the chemical environment of the particular REE ion. Calibration curves were found to be linear between 0. I mol/L and the detection limit of 0.1 mmol/L. Evidence of nitric acid decomposition products were not observed in the spectra. The absorption lines selected for chemical analysis were 575, 442 and 379 nm for the elements Nd, Pr and Er, respectively. The lanthanum analyses were done by neutron activation analysis (NAA) on completely evaporated solutions in plastic vials. The lanthanum standards gave a straight calibration curve passing through zero.
Powder x-ray apron was done on a Rigaku x-my diffractometer. The water content of the solids was measured by weight loss on heatin to 800°C for 15 minutes. XPS spectra were run on a Surface Science Laboratories SSX-100 ESCA Spectrometer, using ALK, radiation. In the dissolution experiments, previously prepand and characterized material was allowed to react with fresh, dilute nitric acid for a period of four to five weeks. In order to check for equilibrium, the concentration of the REE ion was measured ~ph~orn~~y as a Function of time. The results presented later in the report suggest that ~u~lib~urn was indeed achieved in the dissolution, solubility studies. The pH meter was calibrated using an HCI solution whose concentration was measured by titration to be 0.100 mol/L. Solutions of comparable ionic strength measured on the pH meter would thus be characterized by their hydro8en ion concentrations. Su~uently, calculated single-ion activity coefficients would be used to obtain hydrogen-ion activities. Measured and calculated values of pH, (-lo8 H’ concentration) are listed in Table 1. The calculated values were obtained from a mass balance calculation of the starting concentrations of reagents in solution. Also listed in Table 1 are the pH’, (-log H” activity), values used in the solubility calculations.
CALCULATIONS
No commonly satisfactory method of obtainmg activlt). coefficients was found, since the ionic strengths of many at the solutions studied in this report were greater than 0.1 mol/ L. (a value commonly taken to be the limit of applicability of equations used to calculate activity coe@cients). Further, there exists evidence indicating ion-pairing between RE3+ ions and NO; ions (SPEDDING et a!.. 19791. The procedure adopted in this study was to calculate singleion activity coefficients for the singly-char8ed ions using the Davies equation,
1 4 Lw
-log 1 = .4?{(l)‘“/(
-- 0.31I,
ii:
where A = I .82 X 106(cT)-3’1,t is the dielectric consent, ,I 1s the charge on the ion, and f is the ionic strength. The Davies equation is considered by some workers, (STUMMand MOR” CAN, 198 I), to be valid for solutions of ionic strengths up to 0.5 mol/L. The merits and limitations of the Davies equation are also discussed by DAVIES( 1962). The single-ion activity coefficients calculated for the mtrate ion were then used in conjunction with measured mean activity coefficients of rare earth nitrate solutions, (RARD efal., 1979), to calculate single-ion activity coeffcients for the RI?+ ions, using the relation, &a(NO&)
= y( La” f-?(NO, ).
where the comments in brackets are labefs co~es~ndin8 the activity coefficients. Values for relevant activity coefficients are pven Table 1. Solubiliry
C?) to tn
produns
The equilibrium studies were camed out at pH values less than three, so that the dominant species in solution are HIPO,, H$O;. H+ and RE’+. Species of lesser importance include RE(H2p0,)*’ and RE(N4)*‘. If. as an approximation, one ignores these last two species. then the total phosphate concentration. P. is given by. P = [H,PO;] + [H,POe] which can also be expressed as.
+ 1-yK*/-yH3&K,&K~(H-!j~ I.
t4i
where (H’) is the hydrogen ion activity, -y, are single-ion activity coefficients and K, are association constants of phosphoric acid. Values for these K, are listed in Table 2. Theyare valid at zero ionic strength. In this paper, square brackets refer to concentrations, while round brackets refer to activities. lf the RE’+ ion is not complexed. then the RE ~n~ntm~n is equal to the total, analytical concentration of RE. so that. log K$ = log 7Rt t- log [RE] t log I’ .. log ~(&K#W,%H~~C*) +
Ih',li2,h.'rfH")"/y,,~~,,j,
(Ji
where #$ is the the~~ynamic solubility product of REPO, -xHIO. The value of faleo, was taken as unity. Measured concentrations are given in Table 1, in order to allow the calcuiations to be re-done. if desired. Dissolutron 50
L5
LO
35
30 28
25
20
kmetrcs
15
Idcgracsl
FIG. 1. X-my powder pattern of Lap04 *xHzO with rhabdophane structure (CuY, radiation).
In order to ensure that equilibnum had been reached in the solubility studies run at 25°C. a kinetics dissolution experiment was undertaken using previously prepared Ndrha~ophane (sample number “Nd03” in Table it The ex-
2135
Solubilities of some REE phosphates
3.29
E-C
2.72
E-‘
2.62
E-4
3.02
E-L
1.13
E-‘
1.22
E-L
0.65
E-L
0.59
E-L
0.1s
E-C
0.19
E-C
0.u
E-C
0.22
E-L
3.89
E-L
2.65
E-3
2.9
E-3
1.7‘
E-3
1.75
E-2
A.15
E-3
7.89
E-L
1.73
E-3
E-L 2.72 E-C 2.62 E-L 3.02 E-L
0.05
1.7
1.7
I.0
0.823
-2L.16
0.05
1.7
1.7
1.B
0.823
-24.32
1.13 1.22 0.65 0.59
E-L E-L E-L E-C
0.18 0.19 0.u 0.22
E-C E-L E-L E-6
3.29
0.03
1.7
1.7
1.8
0.851
~ZL.ZL
0.05
1.7
1.7
1.8
0.823
-24.23
0.w
2.0
2.0
2.1
0.835
-2L.26
25
-24.16 -24.71 -2.2.82 -21.0‘ -21.79 -25.07 -2L.67 -26.23
3.89 E-L 2.10 E-3 1.l. E-3
100
-26.8‘ -ZL.B, -25.20
l.OL E-3 7.99 E-3 4.15 E-3
-26.22 -25.91
25
-25.e
1.97 E-L 1.73 E-3
100
-2d.18
25
c =
total phosphate; RI+'+ = measured CREI. ml/-L; PH., = -1oeIH'J. measured; plc = -lor[n+], calculated; & = log(li+). calculated. yj = activxty coefficxent of Ion with charge 1.
periment was run at a pH of 0.5 (concentration units) and a temperature of 25°C. The error bars in Fig. 2 refer to analytical error limits. In analysing the data it was found that the dissolution reaction was first order in Nd concentration: dC/dt= k(C, - C),
= exp(--kt).
(7)
The rate constant can easily be obtained from a linear plot of In (C, - C)/C, time, as shown in Fig. 3. The value of C, was obtained by selecting some value for C, and iterating the relation for C, and k until a straight line was obtained of an acceptable correlation coefficient. In this case, for a correlation coefficient of 0.987, k was found to be 0. I3 days-‘. Note that there is a practical limit to how far the iteration procedure can be continued. The term In (C, - C)/C, becomes very sensitive to the inherent analytical error in C, as t goes to infinity. At t = 42 days, for example, the value (C, - C) rapidly becomes smaller than the analytical error in C, on iteration. For this reason, the system can be considered to be effectively at equilibrium after 42 days.
Table
2.
and solid/solution ratios. RESULTS
(6)
where C, is the concentration of Nd at equilibrium. Integration of this expression yields: (C, - C)/C,
It should be pointed out that the rate constant so obtained is conditional,since it will vary for different pII’s, temperatures
Association constants of tt+o~ at zero ionic strength and temperatures of 25O and loo"c -
25
2.152 io.001
7.206 m.003
12.38 to.1
loo
2.618 to.ow
7.332 %0.018
11.125 10.4
AND DI!SCUSSlON
The solid phase
The unit cell parameters determined by x-my powder diffraction are given in Table 3. A representative x-ray powder pattern is presented in Fig. 1, in order to demonstrate the crystallinity and purity of the material used subsequently in the solubility studies. The rare earth elements La, Nd and Pr were found to form phosphates with the rhabdophane structure. This is in agreement with the results of MOONEY ( 1950) and MOONEY-SLATER ( 1962). Erbium was found to form a phosphate with the xenotime structure. The unit cell parameters
L ; g
3 t
5
10
15
20
25
30
35
LO
,I,".Idays
Dataare
fral IozsnER arId BAES, (197f.j and (IARSHALL and PRANX, (1981).
FIG. 2. Dissolution kinetics of Nd-rhabdophane HNO, at 25°C; experimental data points.
in dilute
2136
R. G. Jonasson, G. M. Bancroft and H. W. Nesbitt Table
L.
EBtlrated FZFO~~x&O
errors
for
solublllty
products
FIG.3. Integrated rate law for the dissolution of Nd-rhabdophane.
are slightly larger than those for anhydrous ErP04. probably reflecting the water content of the crystal structure, This is consistent with results of HEZEL and Ross ( 1967). Heating of Nd-rhabdophane to 800°C for fifteen minutes yielded a material with the diffraction pattern of monazite. Again, this is consistent with the results of MOONEY ( 1950). Heating of the ErPO,, exHzO did not change the crystal structure, but the unit cell parameter ‘a’ decreased from 6.88(2) to 6.85 A. The ‘c’ parameter stayed the same, within error. Other phosphate compounds of the REE are known, and might have been expected products of the syntheses. Evidence of the presence of material with either the P222 HoPO, . 2Hz0 structure, (DONALDSON et al., 1967), or the C2/c, churchite structure, (CLARINGBULLand HEY, 1953), was not found. Solubilitirs Calculated solubility products of the hydrous phosphates of La, Nd, Pr and Er are presented in Table 1. The determinations of the solubility product of LaPOh - xH20 vary from pKe = 24.4 to 25.1. The mean value for the twelve determinations is 24.5, with a standard deviation of 0.32. The estimated errors are listed in Table 4. Selection of different dissociation constants of phosphoric acid from the literature would give rise to a variation in solubility products of about 0.6 log units. All other sources of variation should be
considered in relation to this value. In other words, there is no point in worrying about the second significant figure in the values for the activity coefficients, for example. The p&,, value reported b> TANANAEV and VASIL'EVA(1963) for lanthanum phosphate is 22.4 at an ionic strength of 0.5 mol/L. Activity coefficient corrections would change this value to 25.1, compared with our value of 24.5. REACTION OF pr” WITH FLUORAPATITE ELDERFIELD rfal. (198 1) suggested the possible importance of apatite surfaces in adsorbing REE in marine sediments. A preliminary experiment was undertaken to test this possibility. Figure 4 shows a broadscan photoelectron spectrum of a sample of fluorapatite which had been soaked in a 0.05 mol/L solution of PrCl, for two weeks at 100°C. Photoelectron peaks corresponding to binding energies of electrons in various core orbitals of phosphorus, praseodymium, oxygen and fluorine are readily observed in the spectrum. Characteristic peaks of calcium are not present. This suggests that the reacted layer at the surface of the apatite is at least of the thickness corresponding to the sampling depth of the instrument, (5- 10 atomic layers]. A detailed study of the mechanism of surface reaction and Kr, values would constitute an entire investigation unto itself. The results reported here are included merely to lend weight to arguments developed later in the discussion.
IMPLICATIONS FOR MARINE AND DIAGENETIC ENVIRONMENTS
The total dissolved phosphate concentration in recent marine sediments ranges from I X IO6to 1 X 10’ pmol/L, (SAYLES et al., 1973; GIESKES. 19749. From these values one should be able to predict the REE concentrations in interstitial water in sediments. In calculating the La concentration. we have taken the
37
Solubilities of some REE phosphates
Binding Energy
(eV)
FIG. 4. X-ray photoelectron spectrum of the surface of fluorapatite soaked in for two weeks.. ionic strength of sea water as 0.7 mol/L, (STUMM and MORGAN, 1981). The activity coefficient of HPO- was calculated via the Davies equation to be 0.32. The activity coefficient of La”* was estimated via Eqn. 2 to be 0.024. Assuming, for the sake of argument that ionpair fo~ation between La3+ and HPOZ- is negligible, the predicted La3’ activity in solution with 1 X 10’ pmol/L HPO- is given by, log (La) = log KO, - log K3 - log [HPO:-] - log YHpo$.f log (H+).
(8)
Introducing the activity coefficient of La3+ allows calculation of the lanthanum concentration as follows,
+7-I-0.49-8+
1.62,
(9)
where su~titutions were made respectively, and - 1.62 is the value of tog yh. The lanthanum concentmtion in interstitial waters is then 9.0 pmol/L. The range in La concentrations corresponding to the range in phosphate concentrations is therefore 0.90 to 90 pmol/L. If ion-pair formation between Ca2+ and Mg2+ and HPO:- is considered in the calculation, the free phosphate con~ntmtion will be reduced and the calculated La con~ntration increased. Also, if complexation between La and other ligands as occur for example in dissolved organic matter is considered, the La concentration wili also be predicted to be higher. One way around the difficulty in estimating the free phosphate concentmtion is to write a reaction between La3+ and apatite, (JONASSON and NE.SBI~, 1983): 3La3’ + CaS(PO&OH = 3LaP01 + 5CaZ* + OH--. equilibrium
where log kfap= -58.5 is the log of the soIubility product of apatite, (MCDOWELL et al., 1977), K$, is the solubility product of rhabdophane, (LaPO, 8xH*O), the activity of Ca*+ is 0.0021 mol/L, (STUMM and MORGAN, 198 1) and the activity of hydrogen ion is I X IO-* mol/L. Taking the activity c~~cient of La3* as0.024, we calculated the La concentration to be 140 pmol/L. The range in La concentrations corresponding to the range of log KS values (plus or minus one standard deviation; see Table 4) is 68 to 300 pmol/L. Estimates of concentrations of several REE are compared with concentrations measured in open sea water in Table S. REE patterns in sea water
log [La] = -24.5 + 12.35
The corresponding
0.05 mol/L PrClj at 100°C
(10)
constant is then
Table
f 11)
5.
Predicted andapasured concentrations of La. Nd
and
ET
1. m?
pal/L
La Nd
58-300 5.6 300
ET
log K$ - 3 log e,, = 5 log (Cal + log (OH) - 3 log (La),
It is common practice in geochemical discussions to normalize the REE patterns obtained for a given geochemical system to either chondriie or average shale values. The chondrite values are taken to represent cosmic abundances of the rare earths, while shale values are taken to represent crustal abundances, (GROMET et al., 1984). Any chemical differentiation that has taken place in the system of interest can thus be separated from global abundance variations due to nucleosynthesis effects. One such effect is the Oddo-Har-
in sea
z PoL'kg
‘1 9.9
I. This work. 2. iKHu!T et al., 198.4 3. DE MAR et al., 1983
water 3
Wl/kS
M-112 3.0-10.7 ._
2138
R. G. Jonasson. G. M. Bancrofi and H. W. Nesbitt
dissolvino detritus iind. adsorbed WEE ! kins rule, which basically states that even numbered elements have a greater cosmic abundance than odd numbered elements. This zig-zag pattern. for which the normalization is intended to correct. is not expected to be connected in any way with the crystal-chemical or thermodynamic properties of the REE. In other words. if the REE are saturated in sea water, and if they are in chemical equilibrium with the sediments. then the un-normalized REE pattern in sea water is 3 not expected to show the familiar zig-zag pattern. Sea water concentra~on data from HOYLE efal.( 1984). DE BAAR etaf. (i 983). and ELDERFIELD~J~ al.(1981), all suggest a zig-zag pattern in sea water REE conceninonazite rhabdophaoe grains QPllM trations. We therefore suggest that the sea water REE concentrations are not solely controlled by equilibrium thermodynamics. For the sake of argument, one can FIG. 5. Diagram outlining the proposed reactmns 01’the LREE in marine environments. Pathway 1refers to adsorption consider two sorts of REE phases in ocean environon apatite followedby reaction with the surfaceto form REPOT ments: metastable phases of relatively high solubility. grains. Pathways 2 and 3 refer to direct precipitation of rhabincluding surface adsorbed RE-phases. and more stable, dophane and monazite, respectiveI\. low solubility phases, such as the RE phosphates. In such a system, the sea water would tend to be underrha~ophane would be the analogous quickl~formed saturated with respect to the metastable phases and meta-stable phase. The reason for considering monazite supersaturated with respect to the stable solid phases. to be the stable phase is its much more common ocTo the processes of rne~~ble-phi di~iution, and currence in sediments. The reason for doing the REstable-phase precipitation, rate-constants can be atconcentration calculations with rhabdophane is its tributed. In a steady-state system the rules of these two greater ease of formation and dissolution: monazite is processes will tend to become equal. The concentramuch more difficult to precipitate from solution below tions of the relevant solution species will then depend lOO”C, and also dissolves more slowly than rhabdoon the values of the rate-constants of the two processes. phane. The analogy here is the relationship between Now let us assume that the rates of dissolution of opal and quartz, in which the opal both precipitates the various rare earths depend on their concentrations and dissolves more quickly than the quartz. in the metastable solid phases. The solution concenReferring again to Figure 5. pathways 2 and 3 are trations of the rare earths will then reflect the solid distinguished from pathway I. in that 2 and 3 involve phase concentrations of the RE. In this way, the order direct precipitation of the RE-phosphate. This is exof magnitude of the REE sea water concentmt~ons is determined by the sotubility products of the REE. pected to occur in sediments poor in apatite and rich in organic matter. Sediments with high organic conwhereas the fine detaif of the REE patterns is still contents are expected to release considerable quantities of trolled by the patterns in the source materials. reactive phosphate into the interstitial solution phase The premise that the REE are not in thermodynamic upon oxidation of the organic matter, (BERNER, 1980). equlibrium with ocean sediments is supported by the fact that the residence times of the REE are very short The total phosphate concentration may increase by by geological standards: 200 years for the elements La more than an order of magnitude under these condi1982). to Gd. and 400 years for the elements Ho to Lu, tions (FROELJCHital..
(WILDEMAN and HASKIN.1965). Though the gross picture may be explained in terms of two types of RE-bearing phases, the actual situation is almost certainly more complicated. Figure 5 is a summary of RE reactions in the marine environment. as they appear to us at the moment. “LREE3’” refers to the solution phase. The previous discussion dealt with the reactions connecting “dissolving detritus”. “LREE3+” and “rhabdophane grains” in Fig. 5, for example. The relationship between rhabdophane and monazite is not clear. but there is reason to suspect the relationship is similar to that between gypsum and anhydrite. where the anhydrous phases are favoured by higher temperatures. The relationship may also be similar to that between Mg-caicite and dolomite, where
CONCLUSIONS Evidence has been presented in this paper to support the hypothesis that rha~ophane and hydrous xenotime are important phases in contro~ljng REE concentrations in the oceans. Light rare earths have been demonstrated to react with fluorapatite surfaces to form phosphate phases, at least under conditions of high REE concentration. Calculations predicting REE concentrations in sea water are in fair agreement with measured concentrations of REE in sea water. considering the uncertainties involved. would like to thank MSYuch~Chenl: of the department of Geology for doing the powder diffraction patterns, and MSMargaret Hyland and Surface Science West-
A~knowl~d~~mmts-We
Solubilities of some REE phosphates ern for their help with the ESCA instrument. NAA was done by Nuclear Activation Services Limited, Hamilton. Ontario. Thanks also goes to the various anonymous reviewers for their suggestions regarding the presentation of calculations and for pointing out useful references. This work was funded in part by Atomic Energy of Canada Limited and the Natural Sciences and Engineering Council of Canada {NSERC).
‘139
LI Y. (1982) Interelement relationship in abyssal Pacific ferromanganese nodules and associated sediments. Geockim. Cosmochim. Acta 46, 1053-1060. MAKSIMOVICZ. and PANTOGY. (1980) Bastnasite (La) and monazite (Nd) a new variety of monazite. form the Marmara bauxite deposit, (Greece). Bull. Acud. Serbe Sci. Art. 72, 35-42. MARSHALLW. L. and FRANK E. U. ( 198 1) Ion product of water substance 0-1ooO”C. l-10 000 Bars. New internaEdi~~)riajhnndling: H. Elderlield tiona1 formulation and its backaround. J. Phlr. Ckem. Ref Daia IO. 295-304. MCDOWELLH.. GREGORYT. M. and BROWNW. E. t 1977) REFERENCES Solubility of Car(PG&OH in the system Ca(OH)$,PO,: fir0 at 5, 15.25and 37°C. J. Res. i”iB.S.--ii. Physits and BERNERR. A. (1980) Ear& D~ugenesis. A Tkeoreri~ai ApCkemistrySIA, 273-201. proach. Princeton University Press. MESMERR. E. and BAES, JR. C. F. (1974) Phosphoric acid CL.AR~NGBULL G. F. and HEY M. H. (1953) A reexamination dissociation equilibria in aqueous solutions to 300°C. J. of churchite. Mineral. Maa. 30. 2 1l-2 II. Solutmn Chem. 3, 307-322. DAVIESC. W. (1962) Ion Association. Butterworths. DE BAARH. J. W.. BACONM. P. and BREWERP. G. ( 1983) MITCHELLR. S. (1965) Rhabdophane from the Champion Pegmatite. Amelia County, Virginia. Amer. Mineral. 50, Rare-earth distributions with a positive Ce anomaly in the 23 l-235. Western North Atlantic Ocean. Nature 301, 324-327. MOONEYR. C. L. (1950) X-my diffraction study of cerous DONALDSON J. D., HEZEL A. and Ross S. D. (1967) The phosphate and related crystals. I Hexagonal m~i~cation. hydrated phosphate of dysprosium and holmium. J. Inorg. Acta Crysf. 3, 337-340. Nucl. Ckem. 29, 1239-1242. MOONEY-SLATER R. C. L. (1962) Polymorphic forms of bisELDER~ELDH., HAWKESWORTH C. J., GREAVE~M. J. and muth phosphate. Z. Krismfloogr.117, 37 I-385. CALVERTS. E. ( 198 1) Rare earth element geochemistry of PIPER D. Z. (1974) Rare earth elements in the sedimentary oceanic ferromanganese nodules and associated sediments. cycle: A summary. Chem. Geol. b&285-304. i&o&n. ~asm~him. Acta 45,5 13-528. FROELICHP. N., BENDER,M. L., LUEDTKEN. A., HEATH RAo V. K., SHAHANIC. J. and RAO C. L. (I 970) Studies on G. R. and DE VRIE~ T. (1982) The marine phosphorus the phosphate complexes of actinium and lanthanum. Racycle. Amer. J. Sci. 282.474-5 11. diochim. Ada 14, 3 l-34. GIESKESJ. M. (1974) Interstitial water studies, Leg 2.5. In RARD J. A.. MILLERD. G. and SPEDDINGF. H. (1979) Isopiestic determinations of the activity coefficients of some Inirial Reports of the Deep Sea Drilling Project, 25. U.S. aqueous rare earth electrolyte solutions at 25°C. 4. Government Printing Office, Washington, D.C., 36 l-394. GOLDBERGE. D., KOIDE M., SCHMITT R. A. and SMITH La(NO&. Pr(NOr):, and Nd(NO&. J. Chem. Eng. Data R. H. (1963) Rare earth distributions in the marine envi24,348-354. ronment. J. Geophys. Res. 68,4209-42 17. SAYLESF. L.. MAN~~IM F. T. and WATERMANL. S. (1973) GROMETL. P.. DYMEK R. F., HASKINL. A. and KOROTEV lnte~titiai water studies on small core samples, Leg 15.In R. L. ( 1984) The “North American shale composite”: Its initial Reparts qf the Deep Sea Drilling Pr~~e~i*20, U.S. composition, major and trace element characteristics. GeeGovernment Printing Of&e, Washington, D.C., 783-804. chim. Cosmochim. Acta 48,2469-2482. SCHOFIELDA. and HASKINL. (1964) Rare earth distribution HEZELA. and ROSSS. D. ( 1967) X-my powder data and cell patterns in eight terrestrial materials. Geackim. Cosmorkim. dimensions of some rare earth orthophospbates. J. fnorg. iicfa Z&437-446. Nucf. Ckem. 29,2085-2089. SPEDDINCF. H.. BAKERJ. L. and WALTERSJ. P. (1979) ApHOYLE J., ELDERF~ELD H., GLEDHIU A. and GREAVESM. parent and partial molal heat capacities of aqueous rare ( 1984) The behaviour of rare earth elements during mixing earth nitrate solutions at 25°C. J. Ckem. Eng. Data 24, of river and sea waters. Geockim. Cosmockim. Acta 48, 298-305. 143-149. STUMMW. and MORGANJ. J. (198 I) Aquatic Chemistry 2nd JONASSONR. G. and NESBITT H. W. (1983) Lanthanide and ed. John Wiley and Sons. actinide solubilities in natural systems. Atomic Energy of TANANAEV1. V. and VASIL’EVAV. P. (1963) Lanthanum Canada Limited, #TR-216, 215-220. (This is an unrephosphates. Russ. J. Inorg. Ckem. 8, 555-558. stricted, unpublish~ report available from SDDO, CRNLTURNERD. R. and WHITFIELDM. (1979) Control of sea water AECL.) composition. Nature B&468-469. KIZILYALWM. ( 1976) Preparation and structure of ~dolinium WILDEMANT. R. and HASKIN L. (3965) Rare-earth elements pyrophosphate. J. Inorg. Nucf. Ckem. 38,483-486. in ocean sediments. J: Geaphys. Res. 70,2905-29 10.
(Received September 11, f984: accepted in revised.f’ormJuly 12, 1985) Abstract-Solubility product determinations suggest that the hydrous phosphates of the rare earths, REP04 *xHIO, are important in controlling the sea water REE concentrations. Two of these solids, rhabdophane. (P6222) and “hydrous xenotime”, (Irli/amd), have been synthesized at 100°C via the acid hydrolysis of the respective REE pyrophosphate. The solubility products at infinite dilution were determined to be pJ? = 24.5, (La at 25°C); 26.0, (Pr at IOO’C); 25.7, (Nd at 100°C): and 25.5, (Er at IOO”C). On the basis of calculations involving the reaction of RE’+ with apatite to form the hydrous phosphate, the lanthanum concentration in sea water is predicted to be about 140 pmol/L. Laboratory experiments support the hypothesis that apatite is a substrate for reactions with dissolved REE.
nodules the REE are bound initially by two sites: a phosphatic phase and a surface layer of phosphate which had previously been adsorbed onto hydrous iron oxides. Upon diagenesis, the REE may be incorporated into recrystallized biogenic apatite. Some insight into the reactions of REE in the oceans can also be obtained by examining the REE concentration patterns in various phases in the marine environment. PIPER (1974) showed, for example, that REE are fractionated to various extents, depending on the location in the ocean that one sampies for REE. In this paper, the reactions of the REE in the oceans wilt be discussed, in the light of solubility data for various hydrous REE phosphates, also obtained in this study. romanganese
THE RARE EARTH element (REE) concentrations in sea water are very low. For example, HOYLE et al. (1984)
measured the concentrations of La, Ce and Nd in coastal sea water to be 44,63 and 46 pmollkg, respectively. DE BAAR et al. (1983) found La concentrations from 16 pmol/kg near the surface of the ocean to 80 pmol/kg near the bottom (at 4.5 km). Other REE including Pr, were found to have concentrations of about an order of magnitude less. It is likely that one or more rare earth phosphate phases control the REE concentmtions, since these phases are so insoluble. For example, TANANAEV and VASIL’EVA (1963) measured the solubility product of freshly precipitated Lap@ {probably amorphous) to be 3.75 X 10sz3 at 25°C. In nature, the light rare earth
elements (LREE) commonly occur as the phosphate mineral monazite. The heavy rare earth elements plus yttrium (HREE f Y) form the phosphate mineral xenotime. The chemical differences among the REE are small. yet also large enough that the LREE and HREE form phosphates with differing cation coordination numbers. in any case, the ~1ubiIities of these minerals are low enough that they are common com~nents of heavy beach sands. It is perhaps interesting that these minerals have also been found in bauxites, (MAWMOW and PANTO, 1980). The association of REE with marine phospho~tes has been known for some time (GOLDBERG et al., 1963; SCHOFIELD and HASKIN, 1964). ELDERFIELD er al. ( 198 1) and LI ( 1982) have demonstrated an association between phosphate and REE in ferromanganese nodules and associated sediments. TURNER and WHITFIELD (1979) suggested that as a first step in the immobilization process, the REE are
adsorbed onto various mineral sufiaces. Then, as diagenetic reactions occur in the sediments, the REE are incorporated into various authigenic mineral lattices. ELDERRELD et al. (198 1) also concluded that initially the REE are immobilized by surface reactions. In fer-
In order to produce valid solubility products, the nature of the solid phase must be known. The problem with salts of low solubility, however, is that reactions forming well crystallized materials are much slower than those forming non-crystalline products. In order to reduce the rate of precipitation of the RE phosphates sufficiently to allow the formation of crystals, a synthesis was developed based on the oblation that REE p~ph~t~. (P,O:-), will slowly hydrolyse in acid solution to form the respective REE 0~~~~~~ (KIZILYA~LI,1976). The precipitation reactions were carried out at low pH in order to keep the REE concentrations in a tractable analytical range. to eliminate the possible interferences from carbonate or hydroxide, to accelerate the recrystallization reaction and to simplie the ~lculations by keeping the ph~phate primarily as H,PO,. Ion-pair formation by the small amounts of H,POi present in solution can be ignored, as can be seen from the formation constant of the RE”+ -HlPOi ion-pair. It has been measured by RAO el al. (1970) to be about 40 at I = 0.5 mol/L. All rare earth chloride salts were 99.9% pure, (Aldrich), and were used as received. REC13 solutions were titrated with EDTA using Arsenazo 1 indicator and a 6.5 pH-buffer. (The chloride salts of the REE will lose water of ~s~lli~tion on prolonged drying, but are also hygroscopic in the open air.) Phosphate and pyrophosphate solutions were titrated against standard acid or base. Phosphate precipitation reactions were carried out by refluxing a 400-500 ml aqueous solution containing 0.0050
2133
2134
R. G. Jonasson, G. M. Bancroft and H. W. Nesbitt
moles rare earth salt, 0.0026 moles Na,Pz07 and between 0. I and 0.5 moles HNOs, depending on the pH desired. A known excess of p~oph~phate was used, in order to make the rare earth ion the limiting reagent. The initial precipitate was seen to form within a day. A finer precipitate was seen to replace the former after several days. The transformation from REE pyrophosphate (coarse ppt) to REE orthophosphate (fine ppt) was followed by optical microscopy and powder x-my diffraction (Fig. 1). The initial precipitate consists of long, tabular crystals up to 0.2 mm lon8. The later precipitate consists of rounded grains about 0.005 mm in diameter. RefIux was stopped after there was no detectable pyrophosphate remaining, as determined by powder dilTraction. Conversion was usually complete after three weeks. The final solution was h&red hot tluou8h 0.45 rrn Mill&ore filters. The concentrations of the colored lanthanides were conveniently measured against standards on a Varian CARY 118 s~ophotom~. Soiutions which were vgr dilute were concentrated by evaporation to a known volume. The colored rare earth ions show very sharp absorption lines in the visible spectral region, that are quite insensitive to the chemical environment of the particular REE ion. Calibration curves were found to be linear between 0. I mol/L and the detection limit of 0.1 mmol/L. Evidence of nitric acid decomposition products were not observed in the spectra. The absorption lines selected for chemical analysis were 575, 442 and 379 nm for the elements Nd, Pr and Er, respectively. The lanthanum analyses were done by neutron activation analysis (NAA) on completely evaporated solutions in plastic vials. The lanthanum standards gave a straight calibration curve passing through zero.
Powder x-ray apron was done on a Rigaku x-my diffractometer. The water content of the solids was measured by weight loss on heatin to 800°C for 15 minutes. XPS spectra were run on a Surface Science Laboratories SSX-100 ESCA Spectrometer, using ALK, radiation. In the dissolution experiments, previously prepand and characterized material was allowed to react with fresh, dilute nitric acid for a period of four to five weeks. In order to check for equilibrium, the concentration of the REE ion was measured ~ph~orn~~y as a Function of time. The results presented later in the report suggest that ~u~lib~urn was indeed achieved in the dissolution, solubility studies. The pH meter was calibrated using an HCI solution whose concentration was measured by titration to be 0.100 mol/L. Solutions of comparable ionic strength measured on the pH meter would thus be characterized by their hydro8en ion concentrations. Su~uently, calculated single-ion activity coefficients would be used to obtain hydrogen-ion activities. Measured and calculated values of pH, (-lo8 H’ concentration) are listed in Table 1. The calculated values were obtained from a mass balance calculation of the starting concentrations of reagents in solution. Also listed in Table 1 are the pH’, (-log H” activity), values used in the solubility calculations.
CALCULATIONS
No commonly satisfactory method of obtainmg activlt). coefficients was found, since the ionic strengths of many at the solutions studied in this report were greater than 0.1 mol/ L. (a value commonly taken to be the limit of applicability of equations used to calculate activity coe@cients). Further, there exists evidence indicating ion-pairing between RE3+ ions and NO; ions (SPEDDING et a!.. 19791. The procedure adopted in this study was to calculate singleion activity coefficients for the singly-char8ed ions using the Davies equation,
1 4 Lw
-log 1 = .4?{(l)‘“/(
-- 0.31I,
ii:
where A = I .82 X 106(cT)-3’1,t is the dielectric consent, ,I 1s the charge on the ion, and f is the ionic strength. The Davies equation is considered by some workers, (STUMMand MOR” CAN, 198 I), to be valid for solutions of ionic strengths up to 0.5 mol/L. The merits and limitations of the Davies equation are also discussed by DAVIES( 1962). The single-ion activity coefficients calculated for the mtrate ion were then used in conjunction with measured mean activity coefficients of rare earth nitrate solutions, (RARD efal., 1979), to calculate single-ion activity coeffcients for the RI?+ ions, using the relation, &a(NO&)
= y( La” f-?(NO, ).
where the comments in brackets are labefs co~es~ndin8 the activity coefficients. Values for relevant activity coefficients are pven Table 1. Solubiliry
C?) to tn
produns
The equilibrium studies were camed out at pH values less than three, so that the dominant species in solution are HIPO,, H$O;. H+ and RE’+. Species of lesser importance include RE(H2p0,)*’ and RE(N4)*‘. If. as an approximation, one ignores these last two species. then the total phosphate concentration. P. is given by. P = [H,PO;] + [H,POe] which can also be expressed as.
+ 1-yK*/-yH3&K,&K~(H-!j~ I.
t4i
where (H’) is the hydrogen ion activity, -y, are single-ion activity coefficients and K, are association constants of phosphoric acid. Values for these K, are listed in Table 2. Theyare valid at zero ionic strength. In this paper, square brackets refer to concentrations, while round brackets refer to activities. lf the RE’+ ion is not complexed. then the RE ~n~ntm~n is equal to the total, analytical concentration of RE. so that. log K$ = log 7Rt t- log [RE] t log I’ .. log ~(&K#W,%H~~C*) +
Ih',li2,h.'rfH")"/y,,~~,,j,
(Ji
where #$ is the the~~ynamic solubility product of REPO, -xHIO. The value of faleo, was taken as unity. Measured concentrations are given in Table 1, in order to allow the calcuiations to be re-done. if desired. Dissolutron 50
L5
LO
35
30 28
25
20
kmetrcs
15
Idcgracsl
FIG. 1. X-my powder pattern of Lap04 *xHzO with rhabdophane structure (CuY, radiation).
In order to ensure that equilibnum had been reached in the solubility studies run at 25°C. a kinetics dissolution experiment was undertaken using previously prepared Ndrha~ophane (sample number “Nd03” in Table it The ex-
2135
Solubilities of some REE phosphates
3.29
E-C
2.72
E-‘
2.62
E-4
3.02
E-L
1.13
E-‘
1.22
E-L
0.65
E-L
0.59
E-L
0.1s
E-C
0.19
E-C
0.u
E-C
0.22
E-L
3.89
E-L
2.65
E-3
2.9
E-3
1.7‘
E-3
1.75
E-2
A.15
E-3
7.89
E-L
1.73
E-3
E-L 2.72 E-C 2.62 E-L 3.02 E-L
0.05
1.7
1.7
I.0
0.823
-2L.16
0.05
1.7
1.7
1.B
0.823
-24.32
1.13 1.22 0.65 0.59
E-L E-L E-L E-C
0.18 0.19 0.u 0.22
E-C E-L E-L E-6
3.29
0.03
1.7
1.7
1.8
0.851
~ZL.ZL
0.05
1.7
1.7
1.8
0.823
-24.23
0.w
2.0
2.0
2.1
0.835
-2L.26
25
-24.16 -24.71 -2.2.82 -21.0‘ -21.79 -25.07 -2L.67 -26.23
3.89 E-L 2.10 E-3 1.l. E-3
100
-26.8‘ -ZL.B, -25.20
l.OL E-3 7.99 E-3 4.15 E-3
-26.22 -25.91
25
-25.e
1.97 E-L 1.73 E-3
100
-2d.18
25
c =
total phosphate; RI+'+ = measured CREI. ml/-L; PH., = -1oeIH'J. measured; plc = -lor[n+], calculated; & = log(li+). calculated. yj = activxty coefficxent of Ion with charge 1.
periment was run at a pH of 0.5 (concentration units) and a temperature of 25°C. The error bars in Fig. 2 refer to analytical error limits. In analysing the data it was found that the dissolution reaction was first order in Nd concentration: dC/dt= k(C, - C),
= exp(--kt).
(7)
The rate constant can easily be obtained from a linear plot of In (C, - C)/C, time, as shown in Fig. 3. The value of C, was obtained by selecting some value for C, and iterating the relation for C, and k until a straight line was obtained of an acceptable correlation coefficient. In this case, for a correlation coefficient of 0.987, k was found to be 0. I3 days-‘. Note that there is a practical limit to how far the iteration procedure can be continued. The term In (C, - C)/C, becomes very sensitive to the inherent analytical error in C, as t goes to infinity. At t = 42 days, for example, the value (C, - C) rapidly becomes smaller than the analytical error in C, on iteration. For this reason, the system can be considered to be effectively at equilibrium after 42 days.
Table
2.
and solid/solution ratios. RESULTS
(6)
where C, is the concentration of Nd at equilibrium. Integration of this expression yields: (C, - C)/C,
It should be pointed out that the rate constant so obtained is conditional,since it will vary for different pII’s, temperatures
Association constants of tt+o~ at zero ionic strength and temperatures of 25O and loo"c -
25
2.152 io.001
7.206 m.003
12.38 to.1
loo
2.618 to.ow
7.332 %0.018
11.125 10.4
AND DI!SCUSSlON
The solid phase
The unit cell parameters determined by x-my powder diffraction are given in Table 3. A representative x-ray powder pattern is presented in Fig. 1, in order to demonstrate the crystallinity and purity of the material used subsequently in the solubility studies. The rare earth elements La, Nd and Pr were found to form phosphates with the rhabdophane structure. This is in agreement with the results of MOONEY ( 1950) and MOONEY-SLATER ( 1962). Erbium was found to form a phosphate with the xenotime structure. The unit cell parameters
L ; g
3 t
5
10
15
20
25
30
35
LO
,I,".Idays
Dataare
fral IozsnER arId BAES, (197f.j and (IARSHALL and PRANX, (1981).
FIG. 2. Dissolution kinetics of Nd-rhabdophane HNO, at 25°C; experimental data points.
in dilute
2136
R. G. Jonasson, G. M. Bancroft and H. W. Nesbitt Table
L.
EBtlrated FZFO~~x&O
errors
for
solublllty
products
FIG.3. Integrated rate law for the dissolution of Nd-rhabdophane.
are slightly larger than those for anhydrous ErP04. probably reflecting the water content of the crystal structure, This is consistent with results of HEZEL and Ross ( 1967). Heating of Nd-rhabdophane to 800°C for fifteen minutes yielded a material with the diffraction pattern of monazite. Again, this is consistent with the results of MOONEY ( 1950). Heating of the ErPO,, exHzO did not change the crystal structure, but the unit cell parameter ‘a’ decreased from 6.88(2) to 6.85 A. The ‘c’ parameter stayed the same, within error. Other phosphate compounds of the REE are known, and might have been expected products of the syntheses. Evidence of the presence of material with either the P222 HoPO, . 2Hz0 structure, (DONALDSON et al., 1967), or the C2/c, churchite structure, (CLARINGBULLand HEY, 1953), was not found. Solubilitirs Calculated solubility products of the hydrous phosphates of La, Nd, Pr and Er are presented in Table 1. The determinations of the solubility product of LaPOh - xH20 vary from pKe = 24.4 to 25.1. The mean value for the twelve determinations is 24.5, with a standard deviation of 0.32. The estimated errors are listed in Table 4. Selection of different dissociation constants of phosphoric acid from the literature would give rise to a variation in solubility products of about 0.6 log units. All other sources of variation should be
considered in relation to this value. In other words, there is no point in worrying about the second significant figure in the values for the activity coefficients, for example. The p&,, value reported b> TANANAEV and VASIL'EVA(1963) for lanthanum phosphate is 22.4 at an ionic strength of 0.5 mol/L. Activity coefficient corrections would change this value to 25.1, compared with our value of 24.5. REACTION OF pr” WITH FLUORAPATITE ELDERFIELD rfal. (198 1) suggested the possible importance of apatite surfaces in adsorbing REE in marine sediments. A preliminary experiment was undertaken to test this possibility. Figure 4 shows a broadscan photoelectron spectrum of a sample of fluorapatite which had been soaked in a 0.05 mol/L solution of PrCl, for two weeks at 100°C. Photoelectron peaks corresponding to binding energies of electrons in various core orbitals of phosphorus, praseodymium, oxygen and fluorine are readily observed in the spectrum. Characteristic peaks of calcium are not present. This suggests that the reacted layer at the surface of the apatite is at least of the thickness corresponding to the sampling depth of the instrument, (5- 10 atomic layers]. A detailed study of the mechanism of surface reaction and Kr, values would constitute an entire investigation unto itself. The results reported here are included merely to lend weight to arguments developed later in the discussion.
IMPLICATIONS FOR MARINE AND DIAGENETIC ENVIRONMENTS
The total dissolved phosphate concentration in recent marine sediments ranges from I X IO6to 1 X 10’ pmol/L, (SAYLES et al., 1973; GIESKES. 19749. From these values one should be able to predict the REE concentrations in interstitial water in sediments. In calculating the La concentration. we have taken the
37
Solubilities of some REE phosphates
Binding Energy
(eV)
FIG. 4. X-ray photoelectron spectrum of the surface of fluorapatite soaked in for two weeks.. ionic strength of sea water as 0.7 mol/L, (STUMM and MORGAN, 1981). The activity coefficient of HPO- was calculated via the Davies equation to be 0.32. The activity coefficient of La”* was estimated via Eqn. 2 to be 0.024. Assuming, for the sake of argument that ionpair fo~ation between La3+ and HPOZ- is negligible, the predicted La3’ activity in solution with 1 X 10’ pmol/L HPO- is given by, log (La) = log KO, - log K3 - log [HPO:-] - log YHpo$.f log (H+).
(8)
Introducing the activity coefficient of La3+ allows calculation of the lanthanum concentration as follows,
+7-I-0.49-8+
1.62,
(9)
where su~titutions were made respectively, and - 1.62 is the value of tog yh. The lanthanum concentmtion in interstitial waters is then 9.0 pmol/L. The range in La concentrations corresponding to the range in phosphate concentrations is therefore 0.90 to 90 pmol/L. If ion-pair formation between Ca2+ and Mg2+ and HPO:- is considered in the calculation, the free phosphate con~ntmtion will be reduced and the calculated La con~ntration increased. Also, if complexation between La and other ligands as occur for example in dissolved organic matter is considered, the La concentration wili also be predicted to be higher. One way around the difficulty in estimating the free phosphate concentmtion is to write a reaction between La3+ and apatite, (JONASSON and NE.SBI~, 1983): 3La3’ + CaS(PO&OH = 3LaP01 + 5CaZ* + OH--. equilibrium
where log kfap= -58.5 is the log of the soIubility product of apatite, (MCDOWELL et al., 1977), K$, is the solubility product of rhabdophane, (LaPO, 8xH*O), the activity of Ca*+ is 0.0021 mol/L, (STUMM and MORGAN, 198 1) and the activity of hydrogen ion is I X IO-* mol/L. Taking the activity c~~cient of La3* as0.024, we calculated the La concentration to be 140 pmol/L. The range in La concentrations corresponding to the range of log KS values (plus or minus one standard deviation; see Table 4) is 68 to 300 pmol/L. Estimates of concentrations of several REE are compared with concentrations measured in open sea water in Table S. REE patterns in sea water
log [La] = -24.5 + 12.35
The corresponding
0.05 mol/L PrClj at 100°C
(10)
constant is then
Table
f 11)
5.
Predicted andapasured concentrations of La. Nd
and
ET
1. m?
pal/L
La Nd
58-300 5.6 300
ET
log K$ - 3 log e,, = 5 log (Cal + log (OH) - 3 log (La),
It is common practice in geochemical discussions to normalize the REE patterns obtained for a given geochemical system to either chondriie or average shale values. The chondrite values are taken to represent cosmic abundances of the rare earths, while shale values are taken to represent crustal abundances, (GROMET et al., 1984). Any chemical differentiation that has taken place in the system of interest can thus be separated from global abundance variations due to nucleosynthesis effects. One such effect is the Oddo-Har-
in sea
z PoL'kg
‘1 9.9
I. This work. 2. iKHu!T et al., 198.4 3. DE MAR et al., 1983
water 3
Wl/kS
M-112 3.0-10.7 ._
2138
R. G. Jonasson. G. M. Bancrofi and H. W. Nesbitt
dissolvino detritus iind. adsorbed WEE ! kins rule, which basically states that even numbered elements have a greater cosmic abundance than odd numbered elements. This zig-zag pattern. for which the normalization is intended to correct. is not expected to be connected in any way with the crystal-chemical or thermodynamic properties of the REE. In other words. if the REE are saturated in sea water, and if they are in chemical equilibrium with the sediments. then the un-normalized REE pattern in sea water is 3 not expected to show the familiar zig-zag pattern. Sea water concentra~on data from HOYLE efal.( 1984). DE BAAR etaf. (i 983). and ELDERFIELD~J~ al.(1981), all suggest a zig-zag pattern in sea water REE conceninonazite rhabdophaoe grains QPllM trations. We therefore suggest that the sea water REE concentrations are not solely controlled by equilibrium thermodynamics. For the sake of argument, one can FIG. 5. Diagram outlining the proposed reactmns 01’the LREE in marine environments. Pathway 1refers to adsorption consider two sorts of REE phases in ocean environon apatite followedby reaction with the surfaceto form REPOT ments: metastable phases of relatively high solubility. grains. Pathways 2 and 3 refer to direct precipitation of rhabincluding surface adsorbed RE-phases. and more stable, dophane and monazite, respectiveI\. low solubility phases, such as the RE phosphates. In such a system, the sea water would tend to be underrha~ophane would be the analogous quickl~formed saturated with respect to the metastable phases and meta-stable phase. The reason for considering monazite supersaturated with respect to the stable solid phases. to be the stable phase is its much more common ocTo the processes of rne~~ble-phi di~iution, and currence in sediments. The reason for doing the REstable-phase precipitation, rate-constants can be atconcentration calculations with rhabdophane is its tributed. In a steady-state system the rules of these two greater ease of formation and dissolution: monazite is processes will tend to become equal. The concentramuch more difficult to precipitate from solution below tions of the relevant solution species will then depend lOO”C, and also dissolves more slowly than rhabdoon the values of the rate-constants of the two processes. phane. The analogy here is the relationship between Now let us assume that the rates of dissolution of opal and quartz, in which the opal both precipitates the various rare earths depend on their concentrations and dissolves more quickly than the quartz. in the metastable solid phases. The solution concenReferring again to Figure 5. pathways 2 and 3 are trations of the rare earths will then reflect the solid distinguished from pathway I. in that 2 and 3 involve phase concentrations of the RE. In this way, the order direct precipitation of the RE-phosphate. This is exof magnitude of the REE sea water concentmt~ons is determined by the sotubility products of the REE. pected to occur in sediments poor in apatite and rich in organic matter. Sediments with high organic conwhereas the fine detaif of the REE patterns is still contents are expected to release considerable quantities of trolled by the patterns in the source materials. reactive phosphate into the interstitial solution phase The premise that the REE are not in thermodynamic upon oxidation of the organic matter, (BERNER, 1980). equlibrium with ocean sediments is supported by the fact that the residence times of the REE are very short The total phosphate concentration may increase by by geological standards: 200 years for the elements La more than an order of magnitude under these condi1982). to Gd. and 400 years for the elements Ho to Lu, tions (FROELJCHital..
(WILDEMAN and HASKIN.1965). Though the gross picture may be explained in terms of two types of RE-bearing phases, the actual situation is almost certainly more complicated. Figure 5 is a summary of RE reactions in the marine environment. as they appear to us at the moment. “LREE3’” refers to the solution phase. The previous discussion dealt with the reactions connecting “dissolving detritus”. “LREE3+” and “rhabdophane grains” in Fig. 5, for example. The relationship between rhabdophane and monazite is not clear. but there is reason to suspect the relationship is similar to that between gypsum and anhydrite. where the anhydrous phases are favoured by higher temperatures. The relationship may also be similar to that between Mg-caicite and dolomite, where
CONCLUSIONS Evidence has been presented in this paper to support the hypothesis that rha~ophane and hydrous xenotime are important phases in contro~ljng REE concentrations in the oceans. Light rare earths have been demonstrated to react with fluorapatite surfaces to form phosphate phases, at least under conditions of high REE concentration. Calculations predicting REE concentrations in sea water are in fair agreement with measured concentrations of REE in sea water. considering the uncertainties involved. would like to thank MSYuch~Chenl: of the department of Geology for doing the powder diffraction patterns, and MSMargaret Hyland and Surface Science West-
A~knowl~d~~mmts-We
Solubilities of some REE phosphates ern for their help with the ESCA instrument. NAA was done by Nuclear Activation Services Limited, Hamilton. Ontario. Thanks also goes to the various anonymous reviewers for their suggestions regarding the presentation of calculations and for pointing out useful references. This work was funded in part by Atomic Energy of Canada Limited and the Natural Sciences and Engineering Council of Canada {NSERC).
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