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Chemical Geology 278 (2010) 1–14

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Research paper

Rare earth element release from phosphate minerals in the presence of organic acids

Keith W. Goyne a,b,⁎, Susan L. Brantley c, Jon Chorover a

a Department of Soil, Water and Environmental Science, University of Arizona, 429 Shantz Bldg., Tucson, AZ 85721, United Statesb Department of Soil, Environmental and Atmospheric Sciences, University of Missouri, 302 ABNR Bldg., Columbia, MO 65211, United Statesc Department of Geosciences, The Pennsylvania State University, 2217 Earth and Engineering Building, University Park, PA 16802, United States

⁎ Corresponding author. Department of Soil, EnvSciences, University of Missouri, 302 ABNR Bldg., ColumTel.: +1 573 882 0090; fax: +1 573 884 5070.

E-mail address: [email protected] (K.W. Goyne)

0009-2541/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.chemgeo.2010.03.011

a b s t r a c t
a r t i c l e i n f o
Article history:Received 24 November 2009Received in revised form 2 March 2010Accepted 9 March 2010

Editor: J.D. Blum

Keywords:ApatiteDissolutionMonaziteOrganic acidsRare earth elementsYttrium

The primary objective of this research was to investigate the effects of aliphatic and aromatic low molecularweight organic acids (LMWOAs) on rare earth element and yttrium (REY) release from the phosphateminerals apatite and monazite. Since prior studies have shown that redox status can affect REY partitioningduring incongruent dissolution, a secondary objective was to assess the influence of dissolved O2 con-centration. Increasing LMWOA concentrations from 0 to 10 mM resulted in enhanced REY release. In general,REY release increased in the order: no ligand≈salicylatebphthalate≈oxalatebcitrate. REY–ligand stabilityconstants were only useful for predicting REY release for oxalate reacted with apatite and phthalate reactedwith monazite. The role of dissolved oxygen in dissolution of the phosphate minerals was mixed andinconsistent. Mineral type was observed to significantly affect REY pattern development. REY releasepatterns for apatite range from nearly flat to those exhibiting the lanthanide contraction effect (radius-dependent fractionation); whereas, monazite REY release patterns are best described as exhibiting an M-type lanthanide tetrad effect (radius-independent fractionation). Weathering of apatite in the presence ofaliphatic LMWOAs resulted in development of the lanthanide contraction effect fractionation pattern, andthe aliphatic LMWOAs further developed MREE and radius-independent fractionation during monazitedissolution. Geochemical and mineral-specific REY signatures may, therefore, have utility for distinguishingthe impacts of biota on soil weathering processes on early Earth. The development of such signatures may bemitigated, in part, by accessory mineral composition, the types and concentration of LMWOAs present, andprecipitation of secondary minerals.

ironmental and Atmosphericbia, MO 65211, United States.

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l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Low-molecular weight organic acids (LMWOAs) in soil are mainlycomprised of aliphatic compounds with 1–3 carboxylic acid functionalgroups and methoxy, hydroxy, carboxylic acid substituted benzoic andcinnamic (aromatic) acids (Strobel, 2001). In soil solutions, thesecompounds normally exist in the µM to mM concentration range, withhalf-lives of ca. 1 to 6 h (Fox and Comerford, 1990; Stevenson, 1991;Baziramakenga et al., 1995; Krzyszowska et al., 1996; Jones, 1998;Strobel, 2001; vanHees et al., 2005). Because they form stable complexeswith lithogenic metals, LMWOAs play a significant role in pedogenicmineral transformations and the amelioration of Al toxicity to plants.Organic acids promotemineral dissolutionby (1) donatingH+ toproton-promoted dissolution processes (Furrer and Stumm, 1986; Tan, 1986),(2) forming inner-sphere surface complexes that dislodge structuralmetals from the mineral surface (Furrer and Stumm, 1986; Stumm,1997), and (3) the formation of aqueous metal–ligand complexes that

reduce the relative solution saturation with respect to mineralsundergoing dissolution (Drever and Stillings, 1997; Ganor et al., 2009).

The origin of LMWOAs in soil is primarily attributed to microbialprocessing of biomolecules and humic substances (Tan, 1986; Steven-son, 1994; Jones, 1998; Jones et al., 2003; Neaman et al., 2005a), as wellas plant root exudation and leaching of surficial plant detritus (Fox,1995; Jones, 1998; Jones et al., 2003). The concentration and aromaticityof LMWOAs in porewaters have likely increased over geologic timedue to the progressive colonization of weathering environments byprokaryotes and later by vascular plants (Neaman et al., 2005a). Giventhe variation in magnitude of metal–ligand stability constants charac-teristic of LMWOAs, we have suggested that these compounds mayleave biosignatures in geomedia (e.g., paleosols) that indicate thepresence/absence of biogenic ligands and/or molecular oxygen duringweathering (Neaman et al., 2005a,b, 2006; Goyne et al., 2006).

The rare earth elements or lanthanides include lanthanum (La),cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm),samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb),dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium(Yb), and lutetium (Lu). Yttrium (Y) is often included with these ele-ments because it has a valence and ionic radius similar to Ho (Tyler,2004a). Bau and Dulski (2003) described three types of rare earth

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element and yttrium (REY) fractionation patterns observed in rock–water systems, including (1) the lanthanide contraction effect, (2)redox-induced REY anomalies, and (3) electron-structure-induced REYanomalies. Smooth, radius-dependent fractionation, which reflects thefact that lanthanides possess progressively smaller ionic radii withincreasing atomic mass (i.e., the lanthanide contraction effect), couldpotentially result from LMWOA-promoted dissolution, since REYstability constants are observed in some well-known cases to increaseas a function of atomic mass. However, as shown in Fig. 1, metal–ligandstability constant trends are dependent on ligand type; smoothfractionation might be expected for citrate and oxalate (aliphaticligands) but not for phthalate and salicylate (aromatic ligands). Redox-induced fractionation may give rise to Ce and/or Eu anomalies becauseof Ce(III to IV) oxidation and/or Eu(III to II) reduction. The third REYfractionation type, resulting from differences in REY electronic struc-tures (radius-independent fractionation), maymanifest as Y, La, and Gdanomalies. Hence, fractionation patterns may enhance our understand-ing of geochemical systems because they result not only from inherentproperties of REY themselves, but also from how these properties aresuperimposed with geochemical conditions.

REY have potential utility as tracers in rock/water interactionprocesses, groundwater flow and mixing, soil genesis, and as proxiesfor evaluating paleoceanographic and paleoclimatic change (Braunet al., 1998; Land et al., 1999; Ingri et al., 2000; Dia, et al., 2000; Aubertet al., 2001; Aide and Pavich, 2002; Aide and Smith-Aide, 2003;Compton et al., 2003; Gruau et al., 2004; Tyler, 2004b; Haley et al.,2005; Andersson et al., 2006). A more thorough understanding oforganic compound—REE interactions is particularly important fordeveloping and evaluating these proxies based on previous studiesnoting strong positive correlations between REY and dissolved or-ganic carbon concentrations (Dupré et al., 1996; Land et al., 1999; Diaet al., 2000; Tyler, 2004a). Laboratory studies further elucidating theeffect of redox status on REY fractionation are also warranted.

In this study, we explored whether REY dissolution from phos-phate minerals is altered in the presence of LMWOAs. We postulatedthat fractionation of REY in the presence of these ligands would followtrends in metal–ligand stability (Fig. 1). Apatite and monazite werechosen because they are a principal source of REY in natural systems,and they are found as accessory minerals in a variety of rock types(Gromet and Silver, 1983; Grauch, 1989; McLennan, 1989). The workwas driven by an overarching interest in establishing the utility oftrace element signatures as possible “organomarkers”, i.e., as indi-

Fig. 1. Stability constants (log β1) for REY complexation with organic acids. Data wereobtained from: Byrne and Li (1995), I=0.10 M and T=20–25 °C; Schijf and Byrne (2001),I=0.05 M and T=25 °C; Wood (1993), I=0M and T=25 °C. (β1=[ML] [M3+]−1 [L]−1,where M is the metal, L is the ligand, and ML is the metal–ligand complex).

cators of weathering in the presence of biogenic organic molecules.In addition, since REY contain redox active species, we sought todetermine whether dissolution patterns might be diagnostic of weath-ering in the presence versus absence of molecular oxygen. In eithercase, REY could then prove useful to establishing the geochemicalconditions (presence/absence of biota and/or oxygen) dominant inweathering systems, as would be relevant to studies of paleosols.

2. Materials and methods

2.1. Specimen mineral preparation

Research grade apatite (Durango, Mexico) was purchased fromWard's Natural Science (Rochester, N.Y.) and monazite (New Mexico,U.S.) was obtained from Minerals Unlimited (Ridgecrest, CA). Min-erals were fractured using a zirconia ceramic vial/ ball set and ball mill(SPEX Certiprep, Metuchen, NJ), and sieved to obtain the 75–150 μm(100–200 mesh) particle size fraction. Fine particles were removedthrough repeated ultrasonication in Barnstead Nanopure water fol-lowed by repeated ultrasonication in HPLC-grade acetone until super-natant solutions were clear, and then samples were dried for 12 hat 60 °C. Powder X-ray diffraction (XRD) analyses were conductedon randomly-oriented, back-filled 15 mm×5 mm circular samplesmounted in spinning holders to confirm mineral composition. Allpatterns were collected using a Philips X'pert MPD diffractometerequipped with spinning stage and X'Celerator multiple strip detectorusing Ni-filtered CuKα radiation at 50 kV and 40 mA. A continuousscan mode was used to collect 2θ data from 3 to 80° with a step size ofapproximately 0.017°. The divergent slit size was 0.1250°.

Total elemental composition of the isolated particle size fractionwas determined using lithium metaborate/tetraborate fusion fol-lowed by aqueous phase analyses by inductively coupled plasma(ICP)–optical emission spectrometry (OES) and ICP–mass spectrom-etry (MS) for major and trace elements, respectively (ActivationLaboratories Ltd., Tucson, AZ). Analyses were performed in quadru-plicate. These data, normalized to 100% volatile free mass (Table 1),were used to calculate chemical formulae of Ca5(PO4)2.82(F,Cl,OH)1.54and (La0.12Ce0.32Pr0.04Nd0.16Sm0.05 Gd0.02Y0.08Th0.09)P0.81Si0.30O4

for apatite and monazite, respectively. Analyses also indicated thatZr concentrations (9.1 and 136 mg kg−1 for apatite and monazite,respectively) in fractured samples were generally lower than REYconcentrations. Thus, suggesting insignificant contamination ofsamples from the Zirconia vial and ball set.

2.2. Organic acids

Mineral dissolution was investigated in the absence and presenceof aliphatic (citrate and oxalate) and aromatic (phthalate and salic-ylate) organic acids. Neaman et al. (2005a, 2006) noted that theseorganic acids significantly enhance basalt and granite dissolutionduring batch reaction. Neaman et al. (2005a) observed that gallicacid enhanced rock dissolution to a greater extent than the aromaticcompounds used in the present study, but gallate was found to beunstable in the presence of Li+, which was used here as a backgroundelectrolyte cation. Kekulé structures and acid dissociation constants(I=0 M and 298 K) of the organic acids studied are shown in Fig. 2.

2.3. Oxic and anoxic dissolution experiments

Dissolution experiments were conducted in batch reaction underoxic and anoxic conditions using acid-washed, 50 mL Teflon centri-fuge tubes. A mineral mass of 0.3 g was added to each tube andmeasured to 0.001 g. Reaction vessels with and without mineral (i.e.,mineral-free controls)were autoclaved to eliminatemicrobial growth.Organic acid stock solutions were prepared 24 h prior to each ex-periment by dissolving individual organic acids (Fluka and Sigma,

Table 1Mean elemental composition of apatite and monazitea.

Element Apatite Monazite Element Apatite Monazite Element Apatite Monazite

Mean (g kg−1) Mean (mg kg−1) Mean (g kg−1) Mean (mg kg−1) Mean (g kg−1)

Si 3.2±0.3b 34±3 La 4200±110 66±2 Ho 30.3±0.6 0.7±0.1Al 0.30±0.05 22.2±0.4 Ce 5370±40 174±9 Er 95±3 2.5±0.3Fe 0.5±0.2 7.2±0.6 Pr 470±10 23±1 Tm 12.3±0.4 0.47±0.07Mn 0.101±0.01 0.02±0.01 Nd 1610±30 89±3 Yb 60±1 3.6±0.6Mg 0.1±0.1 b0.001 Sm 248±6 30±1 Lu 6.5±0.4 0.50±0.07Ca 395.2±0.9 b0.001 Eu 20.6±0.5 0.05±0.00 Th 370±34 78±2Na 1.7±0.3 1.6±0.2 Gd 225±4 15.3±0.6K 0.1±0.1 2.0±0.1 Tb 32.4±0.5 1.69±0.09Ti 0.01±0.02 0.04±0.08 Dy 164±2 5.4±0.4P 172±3 98.5±0.9 Y 1030±5 28±4

a Structural formulas: apatite, (Ca5)(PO4)2.82(F,Cl,OH)1.54; monazite, (La0.12Ce0.32Pr0.04Nd0.16Sm0.05 Gd0.02Y0.08Th0.09)P0.81Si0.30O4. Apatite data obtained from Goyne et al. (2006).b Error represents 95% confidence interval.

3K.W. Goyne et al. / Chemical Geology 278 (2010) 1–14

≥99% purity) in amber glass jars containing 0.01 M LiCl (Sigma,99.99+% purity), and adding 0.01 M HCl (EM Science, OmniTraceUltrapure) or 0.1 M LiOH (Sigma, ≥99% purity) as needed to achievepH 5. An ionic strength of 0.01 M and pH 5were chosen because theyare representative of natural soil solution conditions (Harter andNaidu, 2001). Reactions were performed in the absence of pH buffersto minimize elemental contamination and sorption artifacts. Amberglass jars and LiCl solutions were autoclaved prior to use, and Al foilwas wrapped around amber jars containing organic acid solutionsto prevent photodegradation. Organic acid stock solutions werefilter-sterilized (Corning, sterilized 0.2 µm polyethersulfone filtra-tion units) within a sterile laminar flow-hood immediately prior tostart of an experiment.

Dissolution experiments were initiated by adding to reactionvessels appropriate quantities of organic acid stock solutions and/or0.01 M LiCl to achieve initial organic acid concentrations of 0, 1, 5, and10 mM at pH 5 and a solid to solution mass ratio of 1:100 (0.3 g per

Fig. 2. Kekulé structures and acid dissociation constants (pKa) of aliphatic and aromaticorganic acids used in dissolution experiments (Martell and Smith, 2003).

30 mL). Solutions were added via pipette using sterilized pipette tips.Mineral-free blanks were prepared concurrently, and all experimentswere conducted in triplicate. Reaction vessels were wrapped in Alfoil and placed in end-over-end shakers (8 rpm) at 23±2°C for 28 d.At the end of the reaction period, samples were centrifuged at26,000 rcf for 15 min and supernatant solutions were removed bypipette. Mineral pellets were washed twice in ethanol and once inwater prior to freeze-drying. A Barnstead NANOpure ultrafiltration/ultraviolet water unit served as the water source throughout theexperiments.

As noted previously, experiments were conducted in the presenceand absence of O2. Although all experiments followed the generalprotocol outlined above, there were some differences between thetwo types of experiments. In the case of oxic experiments, reactionvessels were exposed to the ambient atmosphere (within a sterilelaminar flow-hood) three times per week to maintain equilibriumwith atmospheric O2 (g). For anoxic experiments, autoclaved reactionvessels and 0.01 M LiCl solution (purged 30 min with filter sterilizedN2 (g)) were placed in a Coy (Grass Lake, MI) oxygen-free (b1 ppm)anaerobic chamber one week prior to starting an experiment. Pre-liminary trials indicated that one week was sufficient time for oxygento diffuse from vessels and solutions into the chamber's atmospherewhere it was removed by reaction with a palladium catalyst. Organicacid stock solutions were prepared outside the anaerobic chamberusing deoxygenated 0.01 M LiCl, followed by a 30 min purge withN2 (g). Stock solutions (loosely capped) were placed on a stir platewithin the anaerobic chamber and allowed to mix for 24 h priorto filter-sterilization. All solution additions to reaction vessels wereconducted within the anaerobic chamber, and centrifuged sampleswere opened only after reintroduction to the oxygen-free atmosphere.

2.4. Aqueous phase analyses

After centrifugation, supernatant solutions were partitioned intofour separate vials for aqueous phase analyses. Aliquots taken forsubsequent elemental analysis were added to acid-washed polypro-pylene tubes, acidified to bpH 2 with 6 M HCl (OmniTrace Ultrapure),and stored at 4 °C. Aqueous phase element concentrations weredetermined by a Perkin-Elmer Elan DRC II ICP-MS. Precautions weretaken to investigate multiple isotopes to ensure that polyatomicinterferences did not compromise accuracy of analyte measurements.Non-acidified samples taken for subsequent organic acid and anionanalysis by ion chromatography (IC; Dionex DX-600) were stored in4 mL glass amber vials, wrapped in aluminum foil, and analyzedwithin 48 h. Measurement of pH (Orion, Ross semi-micro combina-tion electrode), Eh (Orion, Redox/ORP combination electrode), anddissolved oxygen (D.O.; VWR Model 4000) was conducted immedi-ately following the end of the experiment using samples stored in40 mL glass amber vials (Table 2). Anoxic samples used for ICP-MS

Table 2Mean pH, Eh, and dissolved oxygen (D.O.) measured at end of apatite and monazite dissolution experiments.

Apatitea

pH Eh (mV) D.O. (mg L−1)

Ligand concentration (mM)

Oxygen status Ligand 0 1 5 10 0 1 5 10 0 1 5 10

Oxic No Ligand 6.84 537 3.29Citrate 5.46 5.25 5.18 552 538 528 3.84 3.76 3.74Oxalate 6.72 6.67 6.61 471 459 455 3.76 3.88 4.03Phthalate 5.31 5.07 4.98 589 577 567 3.83 3.67 3.73Salicylate 6.80 6.69 6.42 578 554 544 4.11 4.19 4.12

Anoxic No Ligand 7.34 −355 0.00Citrate 5.45 5.25 5.18 −269 −258 −254 0.00 0.00 0.00Oxalate 6.89 6.78 6.64 −359 −351 −343 0.00 0.00 0.00Phthalate 5.28 5.06 5.01 −264 −250 −247 0.00 0.00 0.00Salicylate 7.24 6.91 6.67 −359 −352 −341 0.00 0.00 0.00

Monazite

pH Eh (mV) D.O. (mg L−1)

Ligand concentration (mM)

Oxygen status Ligand 0 1 5 10 0 1 5 10 0 1 5 10

Oxic No ligand 5.91 518.9 3.33Citrate 5.44 5.23 5.16 513.5 527.2 528.9 3.98 3.84 3.74Oxalate 6.48 6.48 6.39 440.4 438.9 439.1 3.93 3.86 3.93Phthalate 5.14 5.05 4.99 464.0 477.5 496.3 3.32 3.15 3.27Salicylate 6.02 6.01 5.75 529.0 520.3 523.8 3.79 3.70 3.71

Anoxic No ligand 6.15 −309.2 0.00Citrate 5.44 5.24 5.17 −260.2 −247.0 −239.2 0.00 0.00 0.00Oxalate 6.63 6.58 6.45 −336.5 −332.7 −322.3 0.00 0.00 0.00Phthalate 5.16 5.05 5.01 −255.6 −247.2 −246.0 0.00 0.00 0.00Salicylate 5.94 6.00 5.76 −308.7 −317.4 −296.9 0.00 0.00 0.00

a Apatite data obtained from Goyne et al. (2006).


analyseswere acidified in the anaerobic chamber prior to removal andstorage at 4 °C, and all other anoxic samples were stored and analyzed(except IC analysis) within the chamber.

2.5. Post-reaction solid phase analysis

After centrifugation and removal of supernatant solution, mineralpellets were washed twice with ethanol and once with BarnsteadNanopure water. Pellets were stored in a −20 °C freezer and freeze-dried prior analysis. Diffuse reflectance infrared Fourier transform(DRIFT) spectroscopy and an HCl selective extraction technique wereemployed to investigate formation of secondary mineral phases.Spectra of minerals were collected on neat samples (no KBr diluent)by averaging 400 scans at 4 cm−1 resolution collected with a NicoletMagna 560 spectrometer. The HCl selective extraction was performedby reacting 0.1 g of mineral in 2 mL of 0.5 M HCl for 2 h on an end-over-end shaker (8 rpm). Following reaction, samples were centri-fuged at 10,300 g and supernatant solutions removed by pipette priorto ICP-MS analysis for element concentrations.

3. Results and discussion

3.1. REY release from apatite

3.1.1. Effects of ligandsThe release of REY and major elements from apatite in the pres-

ence and absence of LMWOAs is impacted by both ligand type andconcentration (Figs. 3 and 4 and Table 3). Sum of the REY release(Fig. 3) increases linearly from 0 to 10 mM concentrations of oxa-late and phthalate. Citrate induces a dramatic increase in ∑REY insolution between 0 and 1 mM concentrations, followed by a linearincrease as the concentration of citrate in solution increases from 1to 10 mM. In contrast, the impact of salicylate on ∑REY release

is minimal and similar to ligand-free experiments in most instances.Additionally, the ∑REY release is greater for apatite reacted withaliphatic relative to aromatic LMWOAs. This trend is consistent withREY–LMWOA stability constants (Fig. 1).

The general order of ligand-promoted REY release from apatiteis citrateNoxalateNphthalateNsalicylate≈no ligand, and this orderfollows that for P release from apatite observed in our previous work(Goyne et al., 2006). The weaker acidity of aromatic acids is expectedto diminish surface cation complexation (and hence ligand-promoteddissolution) at the experimental pH (Furrer and Stumm, 1986). Con-sideration of pH is also warranted when evaluating the general ligandorder (Drever, 1994), because we intentionally did not use pH buffersin our experiments as discussed in Materials and methods. Finalsuspension pH values for ligand-free, oxalate, and salicylate (Table 2)are comparable (pH 6.5–7.5) and fall within a range where apatitedissolution is relatively invariant (Welch et al., 2002; Guidry andMackenzie, 2003; Harouiya et al., 2007). Solution pH at the end ofcitrate and phthalate experiments was likewise similar (pH 5.0–5.5).

As shown in Fig. 4, the patterns of REY release from apatite in thepresence of organic ligands, irrespective of ligand type or concentration,appear to be similar to those in ligand-free controls. Changes in REYpatterns were investigated by comparing light, middle and heavy rareearth element release (LREE, MREE, and HREE, respectively), and dis-solution behavior of REEs that sometimes exhibit anomalous fraction-ation (La, Ce, Eu, Gd, and Y). Ratios of LREE/MREE, LREE/HREE, andMREE/HREE in the presence and absence of organic ligands are shownin Table 4. Because of low release rates in the ligand-free controls,and the resulting relatively high coefficients of variation for REY ratiosfor those samples, the data indicate few significant differences betweenthe release ratios in presence or absence of LMWOAs.

It is interesting to note, however, the large decreases in LREE/MREE, LREE/HREE and MREE/HREE for apatite reacted with oxalateunder oxic conditions. These ratios suggest that oxalate preferentially

Fig. 3. Effect of (a) oxic and (b) anoxic condition and ligand concentration on the sum ofrare earth element and yttrium release (∑REY) from apatite. Values are expressed asgrams of elements released (ER) per kilogram of element in the structure (ES) ofunweathered mineral. Error bars represent one standard deviation from triplicateaqueous phase sample analyses (when not visible, they are smaller than symbol size).


releases MREE and HREE (consistent with Fig. 1) and/or LREE—andMREE—oxalate complexes are precipitating under oxic conditions.REY release in the presence of 5 mM and 10 mM oxalate (oxic con-ditions) is evidently correlated with REY–ligand stability constantvalues (Schijf and Byrne, 2001) (Fig. 5). Slope and intercept valuesfor the predictive equations at oxalate concentrations of 5 mM (REYrelease=−0.477+0.109 log β1; r2=0.76; p=2.09×10−4) and10 mM (REY release=−0.663+0.155 log β1; r2=0.76; p=2.04×10−4) are not significantly different, based on 95% CI, when apparentoutlying data (i.e., La and Eu) are removed.

Similarly, LREE/MREE and LREE/HREE release ratios for citratesolutions (oxic conditions) are lower than ligand-free values, suggestingpreferential release of elements with smaller ionic radii (higher atom-icmass), consistentwith Fig. 1. However, the citrate data exhibit greaterMREE/HREE ratios at 5 and 10 mM citrate concentrations, relative toligand-free samples, counter to prediction from REY–ligand stabilityconstant values. The correlation between REY release in the presenceof citrate and log β1 stability constants was weak even at a 10 mMcitrate concentration (r2b0.15). Phthalate data appear to be generallyconsistent with stability constants for REE–phthalate complexes (Fig. 1;Wood, 1993). However, the REY release patterns in the presence ofphthalate are not a mirror image of REE-phthalate stability complexes,as reflected by poor correlation (r2b0.13). Similarly, poor correlation(r2b0.20) was observed for REY release and stability constants forsalicylate.

Overall, we observe that increasing aliphatic ligand concentrationstransforms REY patterns for apatite into those that exhibit the lan-thanide contraction effect. The development of this REY pattern is inagreement with bond-valence sums for REE in apatite. Hughes et al.(1991) showed that Gd to Lu are underbondedwithin apatite in eitherseven- or nine-fold coordination, which leads to a concentration oflight REE during mineral formation. We presume that poor compat-ibility of heavy REE with apatite polyhedra also contributes to theirpreferential release during aliphatic ligand-promoted dissolution.Thus, development of a REY release pattern that exhibits the lan-thanide contraction effect is likely the result of superimposed in-fluences of mineral bond-strength and REY complex stability of thealiphatic ligands.

Investigation of element release ratios commonly used tocharacterize REY partitioning-La/Pr, La/Yb, Ce/Pr, Eu/Eu* (Eu*=0.67(Sm)+0.33(Tb)), Gd/Gd* (Gd*=0.33(Sm)+0.67(Tb)), and Y/Ho(Bau et al., 1997)—for apatite reacted in presence of the four LMWOAsshow no significant differences from ligand-free solutions (Table 5).However, the elemental ratios La/Pr and La/Yb do show lower meanvalues for citrate and oxalate reaction relative to phthalate; thus,indicating a preferential release of Pr and Yb relative to La in presenceof aliphatic ligands. Blank corrected concentrations of Ce in solutionafter oxalate reaction with apatite were approximately zero, andexamination of Ce/Pr release ratios further accentuates this observa-tion. Cerium is known to form aqueous complexes with PO4

3− andoxalate that may precipitate from solution as rhabdophane (CePO4 •

H2O (s)) and cerium oxalate (Ce2(C2O4)3 (s)), respectively (Banfieldand Eggleton, 1989; Cervini-Silva et al., 2005). Jonasson et al. (1985)and Cervini-Silva et al. (2008) report solubility product constants(Ksp) for REE-phosphate minerals, including rhabdophane, on theorder of 10−25.

Diffuse reflectance infrared Fourier transformed (DRIFT) spectros-copy confirms the presence of oxalate precipitates in powder samplesafter reaction (Fig. 6b and d). Bands associated with precipitatesare clearly observed in Fig. 6b at 3330–3500 cm−1 (OH stretchingassociated with water of crystallization), 1644 cm−1 (HOH bending),1620 cm−1 (asymmetric stretch of COO−), and 1315 cm−1 (symmet-ric stretch of COO−). However, not all of these vibrations are presentin 6d and peak intensity is greatly diminished. These spectral featuresare consistent with the formation of calcium oxalate hydrate (CaC2O4 •

nH2O (s)) and REE-oxalate crystals (John and Ittyachen, 2001;Bouropoulos et al., 2004), thus confirming our previous geochemicalmodeling results that predicted formation of CaC2O4 •nH2O (s)(Goyne et al., 2006). The lack of soluble Ce and the detection ofoxalate precipitates suggests the formation of Ce2(C2O4)3 (s) andperhaps other REE-oxalate complexes that may be co-precipitatedwith CaC2O4 •nH2O (s).

3.1.2. Effects of oxic/anoxic conditionsPatterns of REY release from apatite are similar under oxic and

anoxic conditions (Fig. 4). However, the sum of REY released issignificantly greater in the presence of O2(g), irrespective of citrateconcentration (0 to 10 mM) (Fig. 3) . In contrast, REY release in thepresence of oxalate is significantly greater under anoxic conditions,irrespective of ligand concentration. Although this latter result seemsto suggest REY scavenging during precipitation of Fe and/orMn oxidesin oxic apatite experiments, REY concentrations in HCl extracts arenot significantly different (data not shown). Presence or absence ofO2(g) had no significant effect on the sum of REY released in 1 mMphthalate or salicylate, or at a 5 mM salicylate. However, at phthalateconcentrations of 5 and 10 mM, oxic conditions promoted greaterREY release. The opposite is true at 10 mM salicylate concentration.Dissolved O2(g) concentration had no consistent influence on releaseof Ce or Eu, the redox active REEs (Table 5). The overall lack ofconsistent dissolved O2(g) effect on REY release from apatite is in

Fig. 4. Effect of ligand concentration and dissolved oxygen on individual rare earth element and yttrium release from apatite: (a) oxic—0 and 10 mM ligand, (b) anoxic—0 and 10 mMligand, (c) oxic—5 mM ligand, (d) anoxic—5 mM ligand, (e) oxic—1 mM ligand, and (f) anoxic—1 mM ligand. The release of each element is expressed as milligrams of elementreleased (ER) per kilogram of element in the structure (ES) of unweathered mineral. Error bars represent one standard deviation from triplicate aqueous phase sample analyses(when not visible, they are smaller than symbol size).


agreement with our previous study investigating Ca and P release(Goyne et al., 2006).

3.2. REY release from monazite

3.2.1. Effects of ligandsRare earth element, yttrium, and other major element release

frommonazite is enhanced by citrate, oxalate, and phthalate, and REY

concentrations in solution generally increase as a function of ligandconcentration (Figs. 7 and 8 and Table 6). Similar to the apatiteexperiments, ligand presence in solution increases REY release one totwo orders of magnitude, with the exception of salicylate whichreduced monazite dissolution relative to ligand-free controls. Theorder of REY release, based on the ∑REY, is citrateNoxalate≈phthalateNno ligandNsalicylate. This order deviates slightly fromthe ligand order observed in the apatite experiments. Final solution

Table 3Major element release from apatite.

Ligand Ligand conc. Caa Oxic Anoxic P Oxic Anoxic

mM g ER kg ES−1 b

No ligand 0 0.26±0.02c 0.20±0.02 0.43±0.02 0.26±0.01Citrate 1 0.993±0.008 1.47±0.02 2.26±0.02 2.29±0.03

5 2.6±0.1 3.69±0.09 6.22±0.08 6.3±0.210 3.78±0.05 5.44±0.07 9.89±0.03 10.2±0.1

Oxalate 1 0.072±0.02 0.13±0.02 1.15±0.01 1.27±0.075 0.09±0.03 0.16±0.01 4.58±0.05 4.83±0.09

10 0.10±0.01 0.2±0.1 8.63±0.02 9.13±0.09Phthalate 1 1.060±0.004 0.95±0.04 1.17±0.01 1.23±0.04

5 1.18±0.02 1.08±0.04 1.240±0.004 1.34±0.0210 1.22±0.06 1.19±0.07 1.33±0.02 1.38±0.037

Salicylate 1 0.26±0.07 0.12±0.08 0.38±0.02 1.08±0.055 0.42±0.02 0.06±0.05 0.55±0.02 0.7±0.6

10 0.50±0.01 0.26±0.04 0.58±0.02 1.4±0.1

a Data obtained from Goyne et al. (2006).b g ER kg ES

−1, milligrams of elements released (ER) per kilogram of elements in themineral structure (ES).

c Error represents 95% confidence interval.Fig. 5. Relationship between REY–ligand stability constants (log β1; Schijf and Byrne,2001) and REY release from apatite in the presence of 5 and 10 mM oxalate under oxicconditions. The apparent outliers La and Eu were excluded from linear regressionanalysis.


pH values (Table 2) were comparable for citrate and phthalate(pH 5.0–5.5), although slightly higher for salicylate (pH 5.8–6.0),ligand-free (pH 5.9–6.2), and oxalate (pH 6.4–6.6). These pH values allfall within a range wheremonazite dissolution is relatively insensitiveto pH (Oelkers and Poitrasson, 2002), indicating that ligand-promoteddissolution enhances REY release. It should also be noted that largevariability associated with REY release in presence of oxalate (Fig. 8)may be attributed to precipitation of REE-oxalate complexes based on

Table 4Release from apatite of light, middle, and heavy REE and associated release ratios for oxic a

Ligand Ligand conc. LREEa Oxic Anoxic

mM g ER kg ES−1 d

No ligand 0 0.05±0.01e 0.019±0.004Citrate 1 7.90±0.08 7.3±0.1

5 26.8±0.1 23.8±0.310 42±1 37.7±0.1

Oxalate 1 0.11±0.05 0.23±0.035 0.33±0.04 0.508±0.010

10 0.53±0.03 1.1±0.4Phthalate 1 0.101±0.003 0.1±0.02

5 0.56±0.02 0.37±0.0110 1.15±0.03 0.84±0.02

Salicylate 1 0.011±0.010 0.012±0.0025 0.007±0.003 0.03±0.02

10 0.009±0.003 0.13±0.07

Ligand Ligand Conc. LREE/MREE Oxic Anoxic

mM

No ligand 0 1.4±0.6 0.4±0.4Citrate 1 0.92±0.04 0.82±0.03

5 0.94±0.01 0.95±0.0210 0.95±0.02 0.96±0.01

Oxalate 1 1.0±0.7 0.6±0.25 0.9±0.5 0.7±0.3

10 0.94±0.1 0.7±0.3Phthalate 1 1.5±0.1 0.5±0.5

5 1.0±0.2 0.94±0.0810 0.95±0.05 0.91±0.08

Salicylate 1 2±2 0.3±0.25 0.1±0.2 0.6±0.5

10 3±4 0.6±0.5

a Sum of the light rare earth elements (LREE; La, Ce, Pr, and Nd).b Sum of the middle rare earth elements (MREE; Sm, Eu, Gd, and Tb).c Sum of the heavy rare earth elements (HREE; Er, Tm, Yb, and Lu).d g ER kg ES

−1, milligrams of elements released (ER) per kilogram of elements in the minere Error represents 95% confidence interval.

the generally inverse relationship between REY release and oxalateconcentration (Fig. 7).

Presence of organic ligands during monazite dissolution notonly increases REY (aq) concentrations, but also changes REY releasepatterns (Fig. 8). Observed REY release patterns also differ sub-stantially from those of stability constant values (Fig. 1), with the

nd anoxic conditions.

MREEb Oxic Anoxic HREEc Oxic Anoxic

0.03±0.01 0.04±0.04 0.04±0.02 0.005±0.0038.6±0.4 8.9±0.2 10.4±0.2 9.5±0.1

28.4±0.3 24.9±0.4 29.5±0.4 25.8±0.444.6±0.4 39.2±0.5 45.2±0.5 39.5±0.30.11±0.05 0.4±0.1 0.17±0.06 0.31±0.030.4±0.2 0.8±0.4 0.7±0.3 1.0±0.5

0.57±0.05 1.6±0.5 1.05±0.04 1.9±0.50.068±0.005 0.2±0.2 0.044±0.007 0.06±0.02

0.6±0.1 0.4±0.03 0.46±0.04 0.33±0.031.21±0.06 0.92±0.07 1.13±0.04 0.81±0.04

0.007±0.008 0.04±0.03 0.015±0.009 0.012±0.0070.1±0.2 0.05±0.03 0.004±0.002 0.03±0.02

0.003±0.004 0.2±0.1 0.009±0.005 0.07±0.02

LREE/HREE Oxic Anoxic MREE/HREE Oxic Anoxic

1.2±0.67 4±2 0.8±0.5 9±90.76±0.02 0.77±0.02 0.82±0.04 0.93±0.030.91±0.01 0.92±0.02 0.96±0.02 0.97±0.020.93±0.02 0.956±0.007 0.99±0.01 0.99±0.020.7±0.4 0.8±0.1 0.6±0.4 1.2±0.40.5±0.2 0.5±0.2 0.5±0.4 0.8±0.6

0.51±0.034 0.6±0.3 0.54±0.05 0.8±0.42.3±0.4 1.7±0.6 1.6±0.3 3±31.2±0.1 1.1±0.1 1.2±0.3 1.2±0.2

1.02±0.04 1.03±0.05 1.07±0.06 1.1±0.10.8±0.8 0.9±0.6 0.5±0.6 3±31±1 0.9±0.8 20±50 1±1

1.0±0.6 2±1 0.3±0.4 3±2

al structure (ES).

Table 5Mean aqueous REY ratios following oxic and anoxic dissolution of apatite.

Ligand Ligand conc. La/Pr Oxic Anoxic La/Yb Oxic Anoxic Ce/Pr Oxic Anoxic

mM

No ligand 0 1±0.8a 0.9±0.6 1±2 4±5 1.0±0.8 1.0±0.7Citrate 1 0.83±0.02 0.85±0.04 0.67±0.02 0.7±0.03 0.9±0.03 0.92±0.04

5 0.88±0.01 0.94±0.03 0.85±0.02 0.89±0.03 0.95±0.01 1.00±0.0310 0.91±0.01 0.931±0.009 0.89±0.02 0.92±0.01 0.96±0.02 1.019±0.009

Oxalate 1 1±1 0.9±0.2 0.8±0.8 0.8±0.3 – 0.2±0.25 0.9±0.3 0.88±0.04 0.6±0.1 0.64±0.05 – –

10 1.0±0.1 0.9±0.8 0.69±0.06 0.7±0.6 – –

Phthalate 1 1.5±0.1 1.3±0.7 1.8±0.6 1.9±1.0 1.0±0.1 1.0±0.65 1.5±0.1 1.6±0.1 1.8±0.2 1.5±0.2 1.00±0.09 1.0±0.1

10 1.36±0.09 1.4±0.08 1.4±0.09 1.4±0.2 0.98±0.06 1.00±0.06Salicylate 1 1±2 1.2±0.4 1±2 0.7±0.3 1±2 0.6±0.2

5 1±2 1±1 1±1 1±2 1±1 1±210 0.9±0.9 0.5±0.6 1±1 1.0±0.6 0.7±0.9 1±1

Ligand Ligand Conc. Eu/Eu* b Oxic Anoxic Gd/Gd* c Oxic Anoxic Y/Ho Oxic Anoxic

mM

No ligand 0 – 6±11 2±2 1±2 1±1 0.9±0.6Citrate 1 0.5±0.2 0.95±0.10 1.13±0.08 1.1±0.1 0.82±0.04 0.97±0.03

5 0.78±0.03 0.90±0.05 1.09±0.03 1.11±0.05 0.81±0.03 0.96±0.0510 0.84±0.02 0.90±0.04 1.10±0.03 1.10±0.04 0.82±0.02 0.96±0.02

Oxalate 1 – 2±2 1±2 1.2±0.5 0.9±0.9 1.0±0.35 2±1 0.9±0.1 1.2±0.4 1.0±0.1 1±0.2 1.04±0.07

10 0±0.2 1±1 1.2±0.2 1±1 1.0±0.1 1.0±0.8Phthalate 1 – 6±10 1.2±0.2 1±1 1.6±0.3 1.3±0.8

5 0.8 ± 1.0 1.0±0.4 1.4±0.2 1.3±0.2 1.2±0.1 1.3±0.110 0.7±0.2 1.1±0.4 1.3±0.1 1.3±0.2 1.15±0.08 1.22±0.07

Salicylate 1 – – 1±4 3±3 2±6 0.7±0.45 50±190 3±7 1±2 1±2 2±2 1±1

10 – 1±2 5±39 1±2 2±4 0.9±0.5

a Error represents 95% confidence interval.b Eu*0.67(Sm)+0.33(Tb).c Gd*0.33(Sm)+0.67(Tb).


exception of phthalate. The release pattern reflects the M-type lan-thanide tetrad effect (Masuda et al., 1987; Bau, 1999) or radius-independent fractionation. The convex nature of the first three seg-

Fig. 6. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of apatite reactedwith (a) 10 mM citrate, (b) 10 mM oxalate, (c) 1 mM citrate, (d) 1 mM oxalate, and(e) 0.01 M LiCl (ligand-free) under oxic conditions. Spectrum (f) is unreacted apatite.

ments (I: La–Nd; II Nd–Gd; III: Gd–Ho) generally fits the first, secondand third tetrad effects; however, the fourth segment (IV: Ho–Lu)exhibits a concave shape. Table 7 shows a comparison of LREE/MREE,LREE/HREE, and MREE/HREE release ratios in the absence and pres-ence of LMWOAs. In most instances, significant treatment effects areobserved. In contrast to the pattern for apatite, monazite dissolutiondata clearly demonstrate an increase in MREE relative to LREE andHREE release. The unusual behavior of LREE and increased errorassociated with these particular REE in the presence of oxalate alsosuggests precipitation of LREE-oxalate complexes. Indeed, DRIFTcharacterization of the reacted solids (Fig. 9b and d) indicates pre-cipitation of REE-oxalate complexes based on changes in vibrationalmodes between 1260 and 1650 cm−1, particularly the greater inten-sity of peaks at 1514 and 1641 cm−1.

The elements La, Eu, and Y appear to exhibit anomalous behavior,particularly in the presence of citrate and phthalate (Table 8). Lan-thanum release is diminished relative to Pr in the presence of theseligands, but not in ligand-free experiments. In the presence of oxygen,citrate and phthalate give Eu/Eu* values greater than one, whereas amean Eu/Eu* release ratio of one is observed in the absence of ligands.Lastly, citrate and phthalate strongly fractionate the “geochemicaltwins” Y and Ho through greater release of Ho into aqueous solution. Asdiscussed by Bau (1999), the specific electron configurations of Y3+

([Kr]4d0) and La3+ ([Xe] 4f0) result in greater covalency upon com-plexation relative to 4f transition elements that exhibit greater ioniccharacter. This difference arises due to less effective nuclear chargeshielding by 4f electrons as compared to electrons residing in otherorbitals. Although the degree of covalent binding will be specified bythe complex formed (Bau, 1999), we postulate that Y and La stronglycomplexed with phosphate within the crystal structure of monazitemay diminish the ability of organic ligands to break crystal lattice bondsresulting, subsequently, in negative anomalies for these elements.

Fig. 7. Effect of (a) oxic and (b) anoxic conditions and ligand concentrations on the sumof rare earth element and yttrium release frommonazite (∑ REY). Values are expressedas grams of elements released (ER) per kilogram of elements in the structure (ES) ofunweathered mineral. Error bars represent one standard deviation from triplicateaqueous phase sample analyses (when not visible, they are smaller than symbol size).


The fact that citrate and phthalate generate a positive Eu anomalyunder oxic conditions is difficult to explain. Our observation of en-hanced Eu release is in agreement with several samples studied byHannigan and Sholkovitz (2001) where it was demonstrated that theweathering of phosphate minerals leads to MREE enrichment insolution. Shibata et al. (2006) demonstrated the importance of Eucompatibility to Eu anomalies within the host mineral by comparisonof plagioclase and pyroxene dissolution. However, monazite crystalstructure results in enrichment of REEs with larger ionic radius (Lato Gd) (Ni et al., 1995; Williams et al., 2007). Europium is the leastconcentrated REE in the monazite sample studied perhaps lendingsome credibility to the incompatibility hypothesis, but fluid compo-sition at the time of mineral formation also controls stoichiometry.

With the exception of phthalate, REY release from monazite wasnot well predicted by REY–ligand stability constants. Linear regressionanalysis of REY release against stability constant values resulted inpoor correlation for citrate, oxalate and salicylate even at the highestligand concentrations (r2b0.26). REE-ligand stability constants (logβ1) were, however, a moderate to strong predictor of REE release forphthalate (stability constant for Y-phthalate complex was excluded asconstant was not present in data compiled by Wood, 1993). Analysesperformed using the entire suite of REE release data at phthalateconcentrations of 1, 5 and 10 mM under oxic conditions yielded r2

values of 0.45, 0.51, and 0.54, respectively. Removal of the apparentoutliers Nd and Sm from the linear regression analyses improved thedata fits (Fig. 10). Listed below are the predictive equations developed

upon removal of the apparent outlying data (units associated withREE release are g released per kg of element in the mineral structure):

1 mM phthalate-oxic

REE release = −1:71 + 0:393 logβ1; r2 = 0:75;p = 2:69 × 10−4

5 mM phthalate-oxic

REE release = − 5:66 + 1:27 logβ1; r2 = 0:92;p = b 0:0001

10 mM phthalate-oxic

REE release = −7:43 + 1:67 logβ1; r2 = 0:92;p = b 0:0001

Thus, in the case of monazite and apatite, REY–ligand stabilityconstants for only one of the four ligands studied was a useful pre-dictor of REY release.

Although the REE–ligand stability constants aid in explaining REErelease in the presence of phthalate and specific electron configura-tions assist in understanding the anomalous behavior of Y and La,the overall enhanced release of MREE from monazite remains to beexplained. Monazite preferentially concentrates lighter REE (La–Gd),similar to apatite. Based on this information and the REY-stabilityconstants in Fig. 1, one might predict the development of a lanthanidecontraction effect pattern for monazite dissolution. The fact that thispattern does not develop suggests a difference in energy associatedwith ligand-promoted REY release from monazite as compared toapatite, which is likely due to differences in mineral structure. Inapatite, lighter REEs are thought to favor seven-fold coordination sites,thus leaving the heavier REEs to occupy nine-fold coordination sites(Hughes et al., 1991). In contrast, lighter REE in monazite are foundwithin irregular nine-fold coordination (Ni et al., 1995;Williams et al.,2007). Heavier REE (Tb–Lu) in the monazite sample studied are likelycontained within xenotime crystals or xenotime-like structures in thelarger sample matrix (Franz et al., 1996). Xenotime preferentiallyincorporates heavier REE into eight-fold coordination, and Tb isthe largest REE that can be incorporated into this polyhedron (Niet al., 1995). If the MREE are approaching the lower limit for REEO9

polyhedra and the upper limit for xenotime REEO8 polyhedra, thiscould explain greater MREE release in our dissolution experiments.

3.2.2. Effects of oxic/anoxic conditionsAs for apatite, the patterns of REY release from monazite are

essentially independent of dissolved O2(g) (Figs. 7 and 8). The ex-ceptions are positive anomalous behavior of redox active Ce and Euunder anoxic conditions in organic ligand-free solutions. The Ce/Prrelease ratio is significantly greater under anoxic relative to oxicconditions (Table 8), the latter being closer to the unit value expectedfor stoichiometric release. These data suggest that Ce remains in the+3 valence state under anoxic conditions and, therefore, remainsmore soluble than under oxic conditions. Oxidation of Ce3+ (aq) toCe4+ (aq) in oxic solutions is not possible because the emf for the halfcell (1.74 V) is well above the upper stability limit for water (Brookins,1989; Pourret et al., 2008). However, under oxic conditions, it is quitepossible that Ce3+ (aq) is released and oxidized to form of cerianite,CeO2 (s) (Brookins, 1989; Pourret et al., 2008), or the element is beingoxidized at the mineral surface while remaining within the monazitecrystal structure. The lack of a negative Ce anomaly in the oxic ex-periments and the fact that Cervini-Silva et al. (2008) did not observeCe oxidation at the surface of CePO4 •H2O (s) in oxic solutions inabsence of organic molecules seems to suggest that an alternative, butas of yet unknown, mechanism of release may be occurring. Thoughthe mean Eu/Eu* ratio appears greater for anoxic conditions, thiseffect is not statistically significant; the ratio for both anoxic and oxic

Fig. 8. Effect of ligand concentration and dissolved oxygen on rare earth element and yttrium release frommonazite: (a) oxic—0 and 10 mM ligand, (b) anoxic—0 and 10 mM ligand,(c) oxic—5 mM ligand, (d) anoxic—5 mM ligand, (e) oxic—1 mM ligand, and (f) anoxic—1 mM ligand. The release of each element is expressed asmilligrams of element released (ER)per kilogram of element in the structure (ES) of unweathered mineral. Error bars represent one standard deviation from triplicate aqueous phase sample analyses (when not visible,they are smaller than symbol size).


conditions is not significantly different from unity. Enhanced releaseof Eu from the mineral surface via a reductive dissolution mechanismin anoxic experiments may explain the elevated release of Eu relativeto neighboring REE. However, the pe-pH region where Eu2+ (aq)predominates is rather narrow (Brookins, 1989) and others havesuggested that the Eu3+/Eu2+ redox boundary is below the lowerstability field of water at earth surface conditions (Elderfield, 1988).The role of organic ligand as reductant also remains unclear, althoughprevious studies have demonstrated that organic acids serve as re-ductants in various abiotic systems (Krumpolc and Roček, 1985;

Pohlman and McColl, 1989; Deng and Stone, 1996). More detailedstudies are required to elucidate the geochemical reactions resultingin the anomalous behavior of Ce and Eu as they are released frommonazite.

Anoxia increases the sum of REY released from monazite in thepresence of all concentrations of citrate. Anoxic conditions also resultin significantly greater REY sums for some experiments conducted inthe presence of phthalate (1 mM) and salicylate (5 and 10 mM). Oxicconditions induced significantly greater REY release in ligand-free,5 mM phthalate, all three oxalate concentrations. Overall, mixed and

Table 6Major element release from monazite (excluding rare earth elements and yttrium).

Ligand Ligand conc. P Oxic Anoxic Si Oxic Anoxic

mM g ER kg ES−1 a

No ligand 0 0.01±0.01b 0.008±0.005 2.9±0.7 2.3±0.4Citrate 1 3.5±0.7 2.4±0.4 6.2±0.2 8.2±0.8

5 4.8±0.2 3.73±0.09 8.6±0.3 12.1±0.510 5.7±0.3 4.75±0.07 10.0±0.7 13±1

Oxalate 1 0.16±0.02 0.14±0.07 3.3±0.2 2.8±0.15 0.65±0.02 0.63±0.08 8.13±0.06 7.4±0.2

10 2.00±0.01 1.83±0.06 13.0±0.35 12.5±0.7Phthalate 1 0.007±0.003 0.023±0.001 2.3±0.1 2.54±0.04

5 0.07±0.02 0.08±0.01 2.17±0.02 2.28±0.0110 0.12±0.03 0.12±0.02 2.26±0.02 2.66±0.08

Salicylate 1 0.048±0.007 0.00±0.01 2.0±0.2 3±15 0.044±0.007 0.02±0.02 2.7±0.2 3.9±0.5

10 0.01±0.01 0.008±0.008 2.7±0.2 4±1

a g ER kg ES−1, grams of elements released (ER) per kilogram of elements in the mineral

structure (ES).b Error represents 95% confidence interval.

Fig. 9. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of monazitereacted with (a) 10 mM citrate, (b) 10 mM oxalate, (c) 1 mM citrate, (d) 1 mM oxalate,and (e) 0.01 M LiCl (ligand-free) under oxic conditions. Spectrum (f) is unreactedmonazite.


inconsistent effects suggest a weak role of O2(g) in monazite dis-solution, similar to apatite experiments discussed previously.

3.3. Stoichiometry of element release and other potential factorsmitigating REY patterns

We reported previously that Ca and P release from apatite wasnonstoichiometric for all experiments as evidenced by mineral nor-malized molar Ca/P release ratios less than 1 (Goyne et al., 2006).Using data from the present work, we can likewise calculate aqueous

Table 7Release from monazite of light, middle, and heavy REE and associated release ratios for oxic and anoxic conditions.

Ligand Ligand conc. LREEa Oxic Anoxic MREEb Oxic Anoxic HREEc Oxic Anoxic

mM g ER kg ES−1d

No ligand 0 0.20±0.02e 0.24±0.06 0.19±0.02 0.19±0.03 0.14±0.01 0.07±0.01Citrate 1 6.99±0.07 7.6±0.2 12.2±0.2 13.7±0.4 4.1±0.1 4.7±0.2

5 11.8±0.2 13.6±0.24 18.9±0.3 22.3±0.4 5.2±0.2 6.3±0.110 13.7±0.2 15.6±0.1 21.5±0.2 25.2±0.3 5.4±0.1 6.5±0.2

Oxalate 1 0.93±0.04 0.6±0.1 1.66±0.04 1.6±0.1 1.96±0.06 1.35±0.045 0.33±0.09 0.20±0.06 0.6±0.4 0.5±0.2 0.6±0.3 0.3±0.2

10 1.0±0.2 0.2±0.1 6.4±0.7 0.6±0.1 3.2±0.3 0.30±0.05Phthalate 1 0.50±0.04 0.64±0.02 0.65±0.05 1.01±0.02 0.47±0.03 0.47±0.01

5 1.24±0.03 1.4±0.2 2.22±0.04 2.0±0.1 1.04±0.02 0.82±0.0610 1.73±0.06 2.07±0.08 3.0±0.1 2.87±0.04 1.42±0.06 1.15±0.01

Salicylate 1 0.06±0.01 0.06±0.01 0.050±0.008 0.07±0.01 0.028±0.003 0.030±0.0045 0.054±0.002 0.066±0.010 0.089±0.005 0.11±0.02 0.025±0.002 0.037±0.004

10 0.102±0.002 0.123±0.005 0.104±0.001 0.20±0.03 0.048±0.001 0.08±0.003

Ligand Ligand conc. LREE/MREE Oxic Anoxic LREE/HREE Oxic Anoxic MREE/HREE Oxic Anoxic

mM

No ligand 0 1.0±0.1 1.3±0.4 1.4±0.2 3.2±0.9 1.4±0.2 2.5±0.5Citrate 1 0.57±0.01 0.55±0.02 1.70±0.05 1.62±0.08 2.97±0.10 2.9±0.1

5 0.62±0.01 0.61±0.01 2.2±0.1 2.18±0.05 3.6±0.2 3.56±0.0810 0.64±0.01 0.620±0.008 2.52±0.06 2.4±0.08 3.96±0.10 3.9±0.1

Oxalate 1 0.56±0.03 0.41±0.07 0.47±0.02 0.48±0.08 0.85±0.03 1.2±0.15 0.5±0.3 0.4±0.2 0.5±0.3 0.6±0.4 1.0±0.8 1±1

10 0.15±0.03 0.4±0.2 0.29±0.06 0.8±0.4 2.0±0.3 1.9±0.5Phthalate 1 0.77±0.08 0.64±0.02 1.1±0.1 1.38±0.06 1.4±0.1 2.17±0.08

5 0.56±0.02 0.69±0.09 1.19±0.04 1.6±0.2 2.13±0.07 2.4±0.210 0.58±0.03 0.72±0.03 1.22±0.06 1.81±0.08 2.1±0.1 2.50±0.05

Salicylate 1 1.2±0.3 0.9±0.2 2.2±0.4 2.1±0.5 1.8±0.4 2.2±0.55 0.61±0.04 0.6±0.2 2.2±0.2 1.8±0.3 3.6±0.4 2.9±0.7

10 0.98±0.02 0.6±0.1 2.12±0.07 1.58±0.09 2.17±0.06 2.6±0.4

a Sum of the light rare earth elements (LREE; La, Ce, Pr, and Nd).b Sum of the middle rare earth elements (MREE; Sm, Eu, Gd, and Tb).c Sum of the heavy rare earth elements (HREE; Er, Tm, Yb, and Lu).d g ER kg ES

−1, milligrams of elements released (ER) per kilogram of elements in the mineral structure (ES).e Error represents 95% confidence interval.

Table 8Mean aqueous REY ratios following oxic and anoxic dissolution of monazite.

Ligand Ligand conc. La/Pr Oxic Anoxic La/Yb Oxic Anoxic Ce/Pr Oxic Anoxic

mM

No ligand 0 1.0±0.2 a 1.0±0.5 1.8±0.4 2±1 0.8±0.2 4±2Citrate 1 0.70±0.02 0.70±0.05 1.58±0.05 1.5±0.1 0.84±0.02 0.82±0.06

5 0.73±0.04 0.73±0.04 2.0±0.4 2.1±0.2 0.86±0.04 0.84±0.0410 0.72±0.03 0.75±0.01 2.5±0.2 2.5±0.1 0.85±0.03 0.87±0.02

Oxalate 1 0.66±0.07 0.7±0.3 0.56±0.03 0.4±0.1 0.52±0.06 0.8±0.35 0.6±0.4 0.8±0.7 0.5±0.3 0.7±0.5 0.4±0.2 0.8±0.8

10 0.6±0.2 0.8±0.2 0.4±0.1 0.8±0.3 0.2±0.1 0.7±0.4Phthalate 1 0.7±0.1 0.92±0.09 0.9±0.2 1.31±0.09 0.7±0.1 0.96±0.10

5 0.72±0.05 0.8±0.3 1.06±0.07 1.6±0.6 0.76±0.04 0.8±0.210 0.73±0.08 0.82±0.08 1.1±0.1 1.6±0.1 0.77±0.07 0.9±0.1

Salicylate 1 1.0±0.4 0.9±0.5 2.3±0.9 2.4±0.9 0.9±0.4 0.8±0.55 0.9±0.1 1.0±0.4 1.6±0.3 2.2±0.7 0.88±0.08 0.8±0.4

10 0.88±0.03 0.9±0.1 1.56±0.08 1.7±0.3 0.86±0.06 0.82±0.09

Ligand Ligand conc. Eu/Eu* b Oxic Anoxic Gd/Gd* c Oxic Anoxic Y/Ho Oxic Anoxic

mM

No ligand 0 1.0±0.6 2±1 1.0±0.3 1.0±0.7 1.0±0.2 0.9±0.4Citrate 1 1.38±0.09 1.4±0.1 1.02±0.07 1.01±0.09 0.7±0.07 0.66±0.04

5 1.28±0.08 1.34±0.08 1.01±0.06 1.01±0.06 0.7±0.03 0.66±0.0210 1.29±0.05 1.31±0.05 1.00±0.04 1.00±0.04 0.69±0.05 0.67±0.02

Oxalate 1 1.2±0.1 1.3±0.4 0.89±0.07 0.9±0.2 0.8±0.1 0.93±0.105 1±1 1±1 0.8±0.9 0.9±0.9 0.9±0.7 1.0±0.7

10 0.9±0.4 0.9±0.5 0.8±0.3 0.8±0.5 0.7±0.2 0.8±0.4Phthalate 1 1.0±0.3 1.3±0.1 1.0±0.2 0.94±0.06 0.7±0.1 0.91±0.08

5 1.44±0.09 1.2±0.3 0.98±0.05 1.2±0.3 0.67±0.04 0.9±0.210 1.4±0.2 1.19±0.06 1.01±0.09 1.17±0.05 0.66±0.06 0.9±0.1

Salicylate 1 – 2±1 0.9±0.4 1.0±0.6 1.8±0.6 1.1±0.45 2.1±0.4 2±1 0.9±0.2 1.0±0.5 1.7±0.4 1.1±0.4

10 0.4±1.0 2.1±0.8 0.92±0.04 1.0±0.1 1.6±0.1 1.1±0.2

a Error represents 95% confidence interval.b Eu*=0.67(Sm)+0.33(Tb).c Gd*=0.33(Sm)+0.67(Tb).


phase molar REY/P ratios and normalize them to the same ratiosin mineral solids to assess the extent to which REY release wasstoichiometric. Resulting values indicate that REY release for apatitewas nonstoichiometric (ratiosb1) in all cases except for citrate ex-periments where REY/P ratios approach unity. Nonstoichiometricrelease of REY/P was generally observed for monazite although un-like the case for apatite, P release did not consistently equal or ex-

Fig. 10. Relationship between REE-ligand stability constants (log β1; Wood, 1993) andREE release from monazite in the presence of 1, 5, and 10 mM phthalate under oxicconditions. The apparent outliers Nd and Sm were excluded from linear regressionanalysis.

ceed that of the cations, rather the data indicate variation dependingon the ratio among the ligands and ligand concentrations. Despitethis variation, REY/P ratios were consistent between oxic and anoxicexperiments for both apatite and monazite. Of course, the REY trendsin preferential dissolution are also reflected directly in Figs. 3, 4 and7, 8, since these data have been normalized to REY content ofthe mineral solids.

The mechanisms of nonstoichiometric dissolution of apatite andmonazite are all not readily determined by the data collected inthis series of experiments. Potential explanations for nonstoichiome-teric element release include: (1) preferential element release frommineral surfaces due to differences in ligand-stability constants andREY coordination environments within the minerals; (2) re-adsorptionof REY tomineral surfaces; (3) precipitation and co-precipitation of REYin newly formed secondary minerals; and (4) precipitation of REY–organic ligand complexes. In addition to information obtained fromprevious REY studies, we used spectroscopic and extraction analyses toidentify particular instances where certain mechanisms influencingnonstoichiometric dissolution were clearly evident or hypothesized tobe significant. However, we acknowledge that the techniques employedmay not have identified all potential explanations for the REY patternsobserved, and factors other than organic ligand type and concentrationmay also be exerting an influence on the REY patterns developed in ourstudies.

3.4. Implications for using REY as organomarkers

Apatite and monazite data clearly demonstrate that mineral dis-solution was enhanced by three out of the four LMWOAs studied—salicylate being the exception. Thus, the presence of LMWOAs in soilsolutions should result in zones of eluviation and illuviation that canbe observed by studying REY content (Land et al., 1999; Tyler, 2004b).


REY release results indicate that aliphatic organic acid weatheringof apatite exhibits a lanthanide contraction effect (Bau-Dulski Type 1REY fractionation pattern) and monazite dissolution shows greaterMREE release and radius-independent (Bau-Dulski Type 3) REYfractionation. Therefore, ratios of La/Pr, Eu/Eu*, Y/Ho ratio and MREEfractionation may be particularly useful for identifying biotic effectsin paleosol formation when monazite is the dominant REE-bearingmineral present in soil parent material. In cases where soil is weath-ering from parent materials containing apatite, it may be possible todenote organic ligand effects through investigation of LREE/MREE,LREE/HREE, and MREE/HREE ratios.

Distinctive REY release patterns of the two phosphate mineralsis particularly noteworthy, as they indicate that REY patterns maymore strongly reflect the source mineral undergoing dissolution thanthe type of organic ligand present. Thus, our data suggest that REYpatterns in organic-rich natural waters may also facilitate identifica-tion of REY source minerals (e.g., apatite versus monazite). Develop-ment of larger data sets containing a greater variety of REY-hostminerals reacted with LMWOAs and humic substances, particularlythe more mobile fulvic acid fraction, would permit further evaluationof using REY patterns as a tool to identify phosphate mineral weath-ering in both contemporaneous and paleo environments. In addition,future studies should address other factors that could also influenceREY fractionation including adsorption/desorption processes, massmovement, solute migration, and secondary mineral formation. Nev-ertheless, REY patterns are clearly sensitive to weathering environ-ment and, therefore, represent a potential to complement measures ofmajor lithogenic element behavior to better understand weatheringphenomenon in contemporary soils or paleosols.

4. Conclusions

The presence of LMWOAs citrate, oxalate and phthalate enhancedREY release from apatite and monazite relative to inorganic weath-ering under comparable conditions. Aliphatic ligands promote REYrelease to a greater extent than aromatic ligands and increasing li-gand concentrations enhances mineral dissolution in nearly all cases.Within the group of organic ligands investigated, citrate enhancesmineral dissolution to the greatest extent (40-fold or greater increaserelative to ligand-free samples) and salicylate reduced or had a neg-ligible effect on dissolution, consistent with trends in stability con-stant data. When aliphatic ligands were reacted with apatite, REYrelease patterns exhibited the lanthanide contraction effect, where-as aliphatic and aromatic ligands reacted with monazite induceddevelopment of radius-independent REY fractionation patterns. REYrelease was poorly predicted by REY–ligand stability constants withthe exceptions of oxalate and phthalate in experiments containingapatite and monazite, respectively. This suggests that REY releasepatterns from apatite and monazite are controlled to a large extent bythe specific crystal structure of the mineral undergoing dissolution.The presence/absence of dissolved O2(g) did not result in consistenteffects on element release from the phosphate minerals. We concludethat REY may have utility as organomarkers, e.g., for distinguishingthe presence of terrestrial organisms during soil weathering processeson early earth, and REY fractionation patterns induced by organicligands may also permit determination of REY source mineralogy.

Acknowledgements

The authors wish to thank Mary Kay Amistadi for her assistancewith ICP-MS and IC analyses, Aaron Thompson for his thoughtfulcomments and suggestions, Sanjai Parikh, and John Villinski for theirassistance in the laboratory, and Sunkyung Choi and Michael Carduccifor assisting with analysis and interpretation of XRD data. This studywas funded by NASA grant NAG5-12330.

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