Influence of solution and surface chemistry on yttrium and rare earth element sorption

Transcription

1 Unversty of South Florda Scholar Commons Graduate heses and Dssertatons Graduate School 2006 Influence of soluton and surface chemstry on yttrum and rare earth element sorpton Kelly Ann Qunn Unversty of South Florda Follow ths and addtonal works at: Part of the Amercan Studes Commons Scholar Commons Ctaton Qunn, Kelly Ann, "Influence of soluton and surface chemstry on yttrum and rare earth element sorpton" (2006). Graduate heses and Dssertatons. hs Dssertaton s brought to you for free and open access by the Graduate School at Scholar Commons. It has been accepted for ncluson n Graduate heses and Dssertatons by an authorzed admnstrator of Scholar Commons. For more nformaton, please contact scholarcommons@usf.edu.

2 Influence of Soluton and Surface Chemstry on Yttrum and Rare Earth Element Sorpton by Kelly Ann Qunn A dssertaton submtted n partal fulfllment of the requrements for the degree of Doctor of Phlosophy College of Marne Scence Unversty of South Florda Major Professor: Robert H. Byrne, Ph.D. Peter R. Betzer, Ph.D. Erc H. De Carlo, Ph.D. Lus Garca-Rubo, Ph.D. Edward S. Van Vleet, Ph.D. Date of Approval: July 7, 2006 Keywords: ron, ph, carbonate, onc strength, ICP-MS Copyrght 2006, Kelly Ann Qunn

3 Acknowledgments I wsh to thank my commttee members, Peter Betzer, Erc De Carlo, Lus Garca- Rubo, and Edward Van Vleet, for ther support and gudance throughout the dssertaton process. I want to especally thank my advsor, Robert Byrne, for acceptng me nto the graduate program as hs student and encouragng me to pursue a doctoral degree. I would also lke to thank Johan Schjf for teachng me how to operate the ICP-MS and for the numerous conversatons about my research and lfe n general. Fnally, I want to thank my famly and frends, especally my husband Erc and my parents, for all of ther love and support over the years. hs work was funded by a grant from the Natonal Scence Foundaton (OCE ). Addtonal fnancal assstance was provded by the Von Rosenstel, Rggs, and Gulf Oceanographc Chartable rust Endowed Fellowshps. I am extremely grateful to all of the donors who help support the College of Marne Scence.

4 able of Contents Lst of ables Lst of Fgures Lst of Acronyms and Symbols Abstract v x x 1. Introducton 1 2. Comparatve Scavengng of Yttrum and the Rare Earth Elements n Seawater: Compettve Influences of Soluton and Surface Chemstry Abstract Introducton Materals and Methods Results and Dscusson Comparatve log K S Results Lnear Free-energy Relatonshps Inter-element Patterns n YREE Soluton Complexaton and Surface Complexaton Oceanc log K S Patterns Comparatve log K Fe Data Obtaned for Freshly Precptated Fe(III) Hydroxdes Comparatve log K S Behavor of Yttrum and the Rare Earth Elements Crtcal Issues n YREE Surface Complexaton Behavor Sorpton of Yttrum and Rare Earth Elements by Amorphous Ferrc Hydroxde: Influence of ph and Ionc Strength Abstract Introducton Materals and Methods Materals and Preparaton of the Expermental Solutons ph Dependence of YREE Sorpton Ionc Strength Dependence of YREE Sorpton Samplng and Analyss Data Analyss 51

5 3.4.1 Modelng of ph and Ionc Strength Effects Surface Complexaton Model Results and Dscusson Emprcal Model of the log K Fe Dependence on ph and Ionc Strength Surface Complexaton Model Results Comparatve log K Fe Predctons usng SCM Results log K Fe Predctons for Seawater Conclusons Sorpton of Yttrum and Rare Earth Elements by Amorphous Ferrc Hydroxde: Influence of Soluton Complexaton wth Carbonate Abstract Introducton heory Materals and Methods Results and Dscusson Model Results Consderng Sorpton of Only Free YREEs Model Results Includng Sorpton of a YREE Carbonate Complex Examnaton of the Competng Influences of Surface and Soluton Complexaton on K Fe Summary Sorpton of Yttrum and Rare Earth Elements by Amorphous Ferrc Hydroxde: Influence of emperature Introducton Materals and Methods Results and Dscusson 107 References 116 Appendces 127 Appendx A: Data for Freshly Precptated Hydroxdes of rvalent Catons (Al 3+, Ga 3+, and In 3+ ) 128 Appendx B: ph Dependent Data for Amorphous Ferrc Hydroxde 131 Appendx C: Ionc Strength Dependent Data for Amorphous Ferrc Hydroxde 139 Appendx D: Data for Amorphous Ferrc Hydroxde Coverng a Range of 2 Carbonate Concentratons ([CO 3 ] ) 142 Appendx E: emperature Dependent Data for Amorphous Ferrc Hydroxde 155 About the Author End Page

6 Lst of ables able 1.1 able 1.2 able 2.1 Some basc YREE propertes, ncludng name, symbol, atomc number (Z), atomc weght, and trvalent onc radus for coordnaton number 6 (Shannon, 1976) 2 YREE abundances (ppm and µmol/kg) n mean shale (Haskn and Haskn, 1966), NASC (Haskn et al., 1968), and PAAS (McLennan, 1989) 5 Average log K S results for ron, alumnum, gallum and ndum (Fgures 2.2A, 2.3A, 2.3B, and 2.2B, respectvely) 31 able 3.1 Results for the coeffcents of equaton (3.2) 57 able 3.2 YREE surface complexaton constants ( S β n ) determned wth equaton (3.19) and the data n ables B.1 B.6 63 able 3.3 Estmated removal rates for YREEs va authgenc ron 75 able 4.1 YREE surface complexaton constants ( S β 1 and S β 2 ) determned usng equatons (4.12) and (4.13) wth [HCO 3] = 0 M, log S K 1 = 4.76 (Qunn et al., 2006a), and the expermental dstrbuton coeffcent results from carbonate-free solutons n the present work (ables B.1 B.6 and D.4 D.7) 90 CO3 able 4.2 YREE surface complexaton constants ( S β 1, S β 2, and Sβ 1) determned wth equatons (4.12) and (4.13), log S K 1 = 4.76 (Qunn et al., 2006a), and the expermental dstrbuton coeffcent results from the present work (ables B.1 B.6 and D.1 D.7) able 5.1 YREE surface complexaton constants ( Sβ 1 and 0 enthalpy values ( and ; kcal/mol) determned wth 0 H 1 H Sβ 2 ) and equatons (5.4) and (5.6), log S K 1 = 4.76 (Qunn et al., 2006a), and the dstrbuton coeffcent data n ables B.1 B.6, D.4 D.7, and E.1 E.4 112

7 able A.1 able A.2 able A.3 able B.1 able B.2 able B.3 able B.4 able B.5 able B.6 able C.1 able C.2 Dstrbuton coeffcent (log K Al ) results from the experment performed at ph = 5.86 ± 0.18 wth an alumnum concentraton of 1.00 ± 0.05 mm 128 Dstrbuton coeffcent (log K Ga ) results from the experment performed at ph = 6.12 ± 0.34 wth a gallum concentraton of 1.11 ± 0.06 mm 129 Dstrbuton coeffcent (log K In ) results from the experment performed at ph = 6.08 ± 0.04 wth an ndum concentraton of 1.09 ± 0.05 mm 130 Dstrbuton coeffcent (log K Fe ) results from the experment performed at ph = 5.15 ± 0.02 wth an ron concentraton of ± mm 131 Dstrbuton coeffcent (log K Fe ) results from the experment performed at ph = 6.12 ± 0.05 wth an ron concentraton of ± mm 132 Dstrbuton coeffcent (log K Fe ) results from the experment performed at ph = 7.06 ± 0.05 wth an ron concentraton of ± mm 133 Dstrbuton coeffcent (log K Fe ) results from the experment performed over the ph range wth an ron concentraton of ± mm 134 Dstrbuton coeffcent (log K Fe ) results from the experment performed over the ph range wth an ron concentraton of ± mm 135 Dstrbuton coeffcent (log K Fe ) results from the experment performed over the ph range wth an ron concentraton of 10.0 ± 0.7 mm 136 Dstrbuton coeffcent (log K Fe ) results from the experment performed over the onc strength range M wth an ron concentraton of ± mm 139 Dstrbuton coeffcent (log K Fe ) results from the experment performed over the onc strength range M wth an ron concentraton of ± mm 141 v

8 able D.1 able D.2 able D.3 able D.4 able D.5 able D.6 able D.7 able E.1 able E.2 able E.3 able E.4 Dstrbuton coeffcent (log K Fe) results from the experment performed at ph = 5.38 ± 0.02 and 30% CO Dstrbuton coeffcent (log K Fe) results from the experment performed over the ph range at 3% CO Dstrbuton coeffcent (log K Fe) results from the experment performed over the ph range at 30% CO Dstrbuton coeffcent (log K Fe) results from the experment performed over the P range 0% 30% at ph = 6.52 ± CO 2 Dstrbuton coeffcent (log K Fe) results from the experment performed over the P range 0% 30% at ph = 6.68 ± CO 2 Dstrbuton coeffcent (log K Fe) results from the experment performed over the P range 0% 30% at ph = 7.06 ± CO 2 Dstrbuton coeffcent (log K Fe) results from the experment performed over the P range 0% 30% at ph = 7.10 ± CO 2 Dstrbuton coeffcent (log K Fe ) results from the experment performed at = 10.0 o C over the ph range wth an ron concentraton of 1.08 ± 0.08 mm 155 Dstrbuton coeffcent (log K Fe ) results from the experment performed at = 10.0 o C over the ph range wth an ron concentraton of 1.08 ± 0.08 mm 156 Dstrbuton coeffcent (log K Fe ) results from the experment performed at = 39.1 o C over the ph range wth an ron concentraton of ± mm 157 Dstrbuton coeffcent (log K Fe ) results from the experment performed at = 39.3 o C over the ph range wth an ron concentraton of ± mm 158 v

9 Lst of Fgures Fgure 1.1 (A) Dssolved YREE concentratons (pmol/kg) at a depth of 689 m n the Pacfc Ocean (Zhang and Nozak, 1996). (B) Shalenormalzed pattern of the seawater sample shown n panel A usng the PAAS values gven n able 1.2 (McLennan, 1989) 4 Fgure 1.2 YREE specaton n seawater (ph 8.2 and [HCO 3] = 2 mm) expressed as the rato of speces concentraton to the total YREE concentraton (log ([ML]/M )) 10 Fgure 2.1 Fgure 2.2 Fgure 2.3 Fgure 2.4 Fgure 2.5 Fgure 2.6 Comparson between (A) predcted log K S values (Byrne and Sholkovtz, 1996) and (B) drectly measured log K Fe values usng ferrc oxyhydroxdes (Bau, 1999) 24 log K S results from fltered samples. (A) 100 µm Fe(OH) 3 (able B.2). (B) 1 mm In(OH) 3 (able A.3). (C) 1 mm Al(OH) 3 (able A.1). (D) 1 mm Ga(OH) 3 (able A.2) 28 log K S results normalzed to ph (A) 1 mm Al(OH) 3. (B) 1 mm Ga(OH) 3 30 Stablty constants for YREE complexes wth hydroxde (Klungness and Byrne, 2000) and fluorde (Luo and Byrne, 2000), for the condtons of our experments ( = 25 o C; I = M) 32 Lnear free-energy relatonshps between the log K S results from ths work and log OH β 1 data from Klungness and Byrne (2000). (A) 100 µm Fe(OH) 3. (B) 1 mm In(OH) 3. (C) 1 mm Al(OH) 3. (D) 1 mm Ga(OH) 3 33 Rato of K S (ths work) and F β 1 (Luo and Byrne, 2000), shown as log ( K S / F β 1 ). (A) 100 µm Fe(OH) 3. (B) 1 mm In(OH) 3. (C) 1 mm Al(OH) 3. (D) 1 mm Ga(OH) 3 35 v

10 Fgure 2.7 Fgure 2.8 (A) Drectly measured YREE concentratons n seawater at three depths (Zhang and Nozak, 1996), normalzed to Post-Archean Australan Shale (PAAS) (McLennan, 1989). (B) he calculated soluton complexaton term from equaton (2.2) at ph 7.6, 7.9, and 8.2 wth a bcarbonate concentraton of M. (C) Predcted log K S values, calculated by subtractng each of the curves n Fgure 2.7A from the curve for ph 7.9 n Fgure 2.7B 38 Comparson between the log K Fe values from ths work and log K S values for the three seawater samples from Fgure 2.7C 39 Fgure 2.9 Comparson between the log K Fe values from ths work at ph 6.12, Bau (1999) at ph 5.97 (A), and Ohta and Kawabe (2000, 2001) at ph 6.01 (B) 40 Fgure 3.1 Fnal regressons of log K Fe (ables B.1 B.6; normalzed to I = 0 M) versus ph for La, Sm, Dy, and Lu 55 Fgure 3.2 Fnal regressons of log K Fe (ables C.1 and C.2; normalzed to ph 6.13) versus onc strength (I) for La, Sm, Dy, and Lu 56 Fgure 3.3 Coeffcents of equaton (3.2) 58 Fgure 3.4 log K Fe (pred)/log K Fe (meas) versus ph for La, Sm, Dy, and Lu, where log K Fe (pred) are dstrbuton coeffcents predcted from equaton (3.2) usng the coeffcents lsted n able 3.1, and log K Fe (meas) are expermentally observed dstrbuton coeffcents (ables B.1 B.6) 59 Fgure 3.5 log K Fe results for experments performed at (A) ph = 7.06 ± 0.05 and I = M (able B.3) and (B) ph = 6.10 ± 0.03 and I = M (able C.1) 60 Fgure 3.6 Fgure 3.7 log K Fe (pred)/log K Fe (meas) versus ph for Sm. (A) log K Fe (pred) are dstrbuton coeffcents predcted from equaton (3.19) wth the assumpton that S K 1 [H + ] 1. (B) log K Fe (pred) are dstrbuton coeffcents predcted from equaton (3.19) usng log S K 1 = 4.76 and the surface complexaton constants ( S β n ) lsted n able Surface stablty constants (equaton (3.19)) for YREE sorpton by amorphous ferrc hydroxde 64 v

11 Fgure 3.8 Comparson between measured dstrbuton coeffcents and log K Fe values predcted from equaton (3.19). (A) Result from Bau (1999) at ph (B) Result from Ohta and Kawabe (2000, 2001) at ph (C) Result from De Carlo et al. (1998) at ph 6.25 and I = 0.1 m 66 Fgure 3.9 Comparson between the average log K Fe result at ph = 3.96 ± 0.10 from the present work (able B.6) and the log K Fe result at ph 3.91 from Bau (1999) 68 Fgure 3.10 Regressons of log K Fe versus ph for La, Sm, Dy, and Lu 69 Fgure 3.11 Regressons of log K Fe versus ph for La, Sm, Dy, and Lu 70 Fgure 3.12 Fgure 4.1 (A) Dstrbuton coeffcents (log K Fe ) expressed n terms of free YREE concentratons ([M 3+ ]) usng equaton (3.19) and the surface complexaton constants ( S β n ) lsted n able 3.2. (B) Dstrbuton coeffcents ( log K Fe) expressed n terms of total YREE concentratons n seawater (M SW ) usng equatons (3.19) and (3.22) 3 and assumng [HCO 3] 2 10 M. (C) Comparson between the predcted log K Fe pattern for seawater at ph 7.8 (equatons (3.19) and (3.22)) and the measured log K Fe pattern at ph 7.8 from Koeppenkastrop and De Carlo (1992) 73 log K results at ph 7.06 and varous carbonate concentratons, [CO ], lsted n the legend. (A) log K (pred) are dstrbuton Fe 2 3 Fe coeffcents predcted from equaton (4.12) usng the S β 1 and S β 2 CO3 results lsted n able 4.1 and Sβ 1 = 0. (B) log K Fe(meas) are drectly measured dstrbuton coeffcents from an experment performed at constant ph (7.06) and ncreasng P (able D.6) 91 CO 2 Fgure 4.2 log K (meas) versus Fe log K (pred) for La, Sm, Dy, and Lu 93 Fe Fgure 4.3 Fgure 4.4 Fgure 4.5 Surface stablty constants (equaton (4.12)) for YREE sorpton by amorphous ferrc hydroxde 95 Regressons of log K Fe(meas) versus log K Fe(pred) for La, Sm, Dy, and Lu 97 log K patterns coverng a range of carbonate concentratons 98 Fe v

12 Fgure 4.6 Patterns of the surface complexaton term (log ([MS ] [M 3+ ] -1 [S ] -1 )) n equaton (4.26) calculated wth equaton (4.27) 100 Fgure 4.7 Fgure 5.1 Patterns of the soluton complexaton term (log (M /[M 3+ ])) n equaton (4.26) calculated wth equaton (4.8) 101 log K Fe results obtaned over a range of temperatures, ndcated n the legend, at ph 5.61 ± 0.05 (A) and ph 7.06 ± 0.03 (B) 108 Fgure 5.2 log K Fe versus ph for La, Sm, Dy, and Lu at 10, 25, and 40 o C 109 Fgure 5.3 Fgure 5.4 Fgure 5.5 Regressons of log K Fe (meas) versus log K Fe (pred) for La, Sm, Dy, and Lu 110 Enthalpy values (equaton (5.4)) for YREE sorpton by amorphous ferrc hydroxde from 10 to 40 o C 113 Regressons of log K Fe (meas) versus log K Fe (pred) for La, Sm, Dy, and Lu 115 x

13 Lst of Acronyms and Symbols HREE ICP-MS ID-IMS INAA LREE MREE NASC PAAS SCM YREE heavy rare earth elements nductvely-coupled plasma mass spectrometer sotope dluton thermal onzaton mass spectrometry nstrumental neutron actvaton analyss lght rare earth elements mddle rare earth elements North Amercan Shale Composte Post-Archean Average Australan Shale surface complexaton model yttrum and the rare earth elements K S dstrbuton coeffcent, defned as K S = [MS ] /([M 3+ ] [S ]), where the subscrpt S can be Fe, Al, Ga, or In [MS ] total concentraton of sorbed YREE [M 3+ ] concentraton of a free hydrated YREE on [S ] concentraton of sorptve sold substrate K Fe dstrbuton coeffcent expressed n terms of total YREE concentraton n soluton, defned as K Fe = [MS ] / (M [Fe 3+ ] S ) M total concentraton of YREE n soluton [Fe 3+ ] S concentraton of precptated ron K Fe dstrbuton coeffcent expressed n terms of total YREE concentraton n seawater, defned as K Fe = [MS ] / (M SW [Fe 3+ ] S ) M SW total concentraton of YREE n seawater I onc strength Q ph slope of the lnear regresson of log K Fe wth respect to ph Q I slope of the lnear regresson of log K Fe wth respect to onc strength log K Fe (ph 0, I = 0) ntercept of the lnear regresson of log K Fe Sβ n stablty constant for free YREE sorpton by surface hydroxyl groups, defned as S β n = ([S FeO n (OH) 3-n M 3-n ] [H + ] n )/ ([M 3+ ] [S Fe(OH) 3 ]) where S represents the bulk sold SK 1 ferrc hydroxde surface protonaton constant, defned as S K 1 = [S Fe(OH) ]/ [S Fe(OH) ] [H ] ( ) 2 3 x

14 SK 2 CO 3 Sβ 1 β n H CO3 β n 0 H n enthalpy ferrc hydroxde surface deprotonaton constant, defned as + [S Fe(OH) ] [H ] /[S Fe(OH) ] SK 2 = ( ) stablty constant for sorpton of YREE-carbonate complexes, defned as 3 ( Sβ 1 = ) ([MCO 3] [S Fe(OH) 3] ) [S FeO(OH) MCO ] [H ] / stablty constant of the n th complex of metal M wth soluton lgand L, defned as β n = [M(L ) n ]/([M 3+ ] [L ] n ) where L can 2 be CO 3, OH -, F -, HCO 3, etc. YREE complexaton constant wth carbonate expressed n terms of H total bcarbonate concentratons, defned as β = ( 3 2n + n ) ( 3 + [M(CO n 3) n ] [H ] / [M ] [HCO 3] ) CO3 n x

15 Influence of Soluton and Surface Chemstry on Yttrum and Rare Earth Element Sorpton Kelly Ann Qunn ABSRAC he sorpton behavor of yttrum and the rare earth elements (YREEs) was nvestgated usng a varety of hydroxde precptates over a range of soluton condtons. Experments wth amorphous hydroxdes of Al, Ga, and In were conducted at constant ph (~6.0) and constant onc strength (I = 0.01 M), whle YREE sorpton by amorphous ferrc hydroxde was examned over a range of onc strength (0.01 M I 0.09 M), ph (3.9 ph 7.1), carbonate concentraton (0 M [CO ] amorphous ferrc hydroxde, and (v) enthalpes for M 3+ sorpton. hs quanttatve model x µm), and temperature (10 o C 40 o C). Sorpton results were quantfed va dstrbuton coeffcents, expressed as ratos of YREE concentratons between the sold and the soluton, and normalzed to concentratons of the sorptve sold substrate. Dstrbuton coeffcent patterns for Al, Ga, and In hydroxdes were well correlated wth the pattern for YREE hydrolyss. In contrast, amorphous ferrc hydroxde developed a dstnct pattern that was dfferent than those for Al, Ga, and In precptates but smlar to the pattern predcted for natural marne partcles. YREE sorpton was shown to be strongly dependent on ph and carbonate concentraton, sgnfcantly dependent on temperature, and weakly dependent on onc strength. Dstrbuton coeffcents for amorphous ferrc hydroxde ( K Fe ) were used to develop a surface complexaton model that contaned () two equlbrum constants for sorpton of free YREE ons (M 3+ ) by surface hydroxyl groups, () one equlbrum constant for sorpton of YREE carbonate complexes ( MCO + 3 ), () soluton complexaton constants for YREE carbonates and bcarbonates, (v) a surface protonaton constant for

16 accurately descrbed () an ncrease n K Fe wth ncreasng ph, () an ntal ncrease n K Fe wth ncreasng carbonate concentraton due to sorpton of MCO + 3, n addton to M 3+, () a subsequent decrease n K Fe due to ncreasng YREE complexaton by carbonate ons (especally extensve for the heavy REEs), and (v) an ncrease n K Fe wth ncreasng temperature. x

17 1. Introducton Yttrum and the fourteen stable rare earth elements (YREEs) are extensvely used to study geochemcal processes n all types of natural water (e.g., rvers, estuares, and the ocean). he YREEs are deal tools to explore fundamental aqueous reactons because they form a coherent seres of elements whose chemcal propertes dsplay small but systematc changes wth ncreasng atomc number. hs chemcal coherence s due to the gradual fllng of ther 4f electron shell. Because outer electrons (n = 5, 6) sheld ths nner shell, there are only mnor dfferences n the chemcal reactvty along the seres. he empty (La 3+ ), half-flled (Gd 3+ ), and completely flled (Lu 3+ ) 4f electron shells have ncreased stablty and therefore may dsplay anomalous behavor relatve to the rest of the YREE seres (e.g., de Baar et al., 1991; McLennan, 1994). he domnant systematc change that s observed n YREE chemcal propertes, such as soluton complexaton, s caused by the decrease n onc radus wth ncreasng atomc number, whch s known as the lanthande contracton. able 1.1 lsts the onc rad of the YREEs, along wth ther atomc numbers and atomc weghts. As can be seen n able 1.1, Y has an onc radus almost equal to that of Ho and, therefore, the two elements are expected to dsplay smlar geochemcal behavors. It has been shown though that Y resembles a varety of REEs n ts complexaton characterstcs (e.g., Moeller, 1963, 1972; Moeller et al., 1965; Byrne and Lee, 1993; Lu and Byrne, 1995). Because the 4f orbtals of the REEs nfluence bondng, the REEs dsplay enhanced covalency compared to Y (Sekersk, 1981; Borkowsk and Sekersk, 1992). hs delocalzaton of f orbtals n the REEs causes Y to behave as a lght pseudolanthande when complexng wth soft lgands (e.g., organcs). On the other hand, Y approaches the chemcal behavor of Ho when partcpatng n onc nteractons wth hard lgands (e.g., carbonate). 1

18 able 1.1. Some basc YREE propertes, ncludng name, symbol, atomc number (Z), atomc weght, and trvalent onc radus for coordnaton number 6 (Shannon, 1976). Element Symbol Z Atomc weght (g/mole) Ionc radus (Å) yttrum Y lanthanum La cerum Ce praseodymum Pr neodymum Nd promethum Pm 61 (145) - samarum Sm europum Eu gadolnum Gd terbum b dysprosum Dy holmum Ho erbum Er thulum m ytterbum Yb lutetum Lu Another property that makes the YREEs a good probe of geochemcal processes s oxdaton state. All YREEs are present as trvalent ons n natural waters, wth Ce and Eu also exstng n the tetravalent and dvalent states, respectvely. Oxdaton of Ce, whch results n the formaton of an nsoluble oxde (CeO 2 ), rapdly occurs n the upper water column of the ocean (e.g., Goldberg et al., 1963; de Baar et al., 1983; German et al., 1995; Albo and Nozak, 1999). Processes that convert dssolved Ce 3+ to partculate Ce 4+ nclude bologcally medated oxdaton (Moffett, 1990, 1994a,b) and abotc oxdaton on the surfaces of Mn oxdes (Koeppenkastrop and De Carlo, 1992; Sholkovtz et al., 1994). Reducton of Eu, on the other hand, generally occurs at hgh temperatures and pressures, such as those found n hydrothermal fluds (e.g., Mchard et al., 1983; German et al., 1990; Klnkhammer et al., 1994). he anomalous behavors of Ce and Eu caused by redox reactons are quantfed va Ce and Eu anomales, defned as: Ce anomaly = 3Ce n /(2La n + Nd n ) or 2Ce n /(La n + Pr n ) (1.1) and 2

19 Eu anomaly = 2Eu n /(Sm n + Gd n ), (1.2) where the subscrpt n represents shale-normalzed concentratons. Depletons or enrchments of ether element relatve to neghborng elements yeld values less than 1 (negatve anomales) or greater than 1 (postve anomales). Dfferences n the onc rad and the oxdaton states of the YREEs lead to mass fractonaton, defned by Byrne and Sholkovtz (1996) as the varaton n the relatve lanthande concentratons through bogeochemcal reactons. hs fractonaton n natural samples can be masked by the effect of the Oddo-Harkns Rule, whch holds that elements wth an even atomc number are more cosmogencally abundant than those wth an odd atomc number. As an example of ths odd-even pattern, Fgure 1.1 shows the YREE abundances n a seawater sample (Zhang and Nozak, 1996). o remove ths sawtooth dstrbuton, samples are normalzed to a chosen reference materal, whch s typcally shale for samples obtaned n the marne envronment (Fgure 1.1). As dscussed by Pper (1974), REE patterns of marne samples more closely resemble the pattern of shale than that of chondrte, the preferred reference materal for normalzaton of terrestral rocks and mnerals. Shale also represents the upper contnental crust (aylor and McLennan, 1985), whch s consdered to be the prncpal source materal for REEs to the ocean. Several dfferent shale values n the lterature have been used for normalzaton, ncludng () mean shale, an average of the North Amercan, European, and Russan shale composte (Haskn and Haskn, 1966; Pper, 1974; de Baar et al., 1985a), () North Amercan Shale Composte (NASC), an average of 40 shales manly from North Amerca (Haskn et al., 1968; Gromet et al., 1984; Goldsten and Jacobsen, 1988), and () Post- Archean Average Australan Shale (PAAS), an average of 23 shales from Australa (Nance and aylor, 1976; McLennan, 1989). able 1.2 lsts the YREE abundances (ppm and µmol/kg) n these three shales. By calculatng anomales n a manner smlar to that shown n equatons (1.1) and (1.2), Albo and Nozak (1999) showed for several seawater samples that the magntude and drecton (postve or negatve) of the anomaly for all REEs, except La and Ce, vared dependng on the shale values used n the normalzaton. 3

20 dssolved [YREE] (pmol/kg) (logarthmc scale) Y La Ce Pr Nd Pm Sm Eu Gd b Dy Ho Er m Yb Lu A shale-normalzed rato (x10 8 ) (logarthmc scale) Y La Ce Pr Nd Pm Sm Eu Gd b Dy Ho Er m Yb Lu B Fgure 1.1. (A) Dssolved YREE concentratons (pmol/kg) at a depth of 689 m n the Pacfc Ocean (Zhang and Nozak, 1996). (B) Shale-normalzed pattern of the seawater sample shown n panel A usng the PAAS values gven n able 1.2 (McLennan, 1989). hs ndcates that nterpretatons of anomales must be made cautously. Despte the varaton n specfc anomales, the major features of shale-normalzed patterns descrbed below are mantaned wth dfferent reference shales. As can be seen n Fgure 1.1, the YREEs dsplay a shale-normalzed pattern n seawater that s enrched n heavy REEs (HREEs) relatve to lght REEs (LREEs). Addtonally, there s a pronounced Ce depleton relatve to La and Pr creatng a large 4

21 able 1.2. YREE abundances (ppm and µmol/kg) n mean shale (Haskn and Haskn, 1966), NASC (Haskn et al., 1968), and PAAS (McLennan, 1989). Mean shale NASC PAAS ppm µmol/kg ppm µmol/kg ppm µmol/kg Y La Ce Pr Nd Pm Sm Eu Gd b Dy Ho Er m Yb Lu negatve Ce anomaly. he HREE enrchment and Ce depleton ncrease wth depth n the ocean and also ncrease from the Atlantc Ocean to the Pacfc Ocean (Byrne and Sholkovtz, 1996, and references theren). As descrbed above, Ce exhbts anomales due to ts actve redox chemstry. For elements not nfluenced by redox transformatons, several descrptons of the processes that control the fractonaton between HREEs and LREEs n seawater have been consdered over the past 40 years. Followng one of the frst measurements of REE concentratons n seawater, Goldberg et al. (1963) suggested that the HREE enrchment may be due ether to ncreasng stablty of soluton complexes across the REE seres or dfferental sorpton by solds. Høgdahl et al. (1968) proposed that REE fractonaton may be caused ether by redox reactons along wth dfferental mneral uptake or by dfferental solublty due to varatons n onc radus. However, t was antcpated (Moeller et al., 1965; Høgdahl et al., 1968) that nether process would be effectve for the REEs n seawater. Basng ther 5

22 nterpretaton on the proposton that REE dstrbutons are determned by the suspended matter n natural waters, Kolesov et al. (1975) stated that the relatve abundances of the REE depend on the composton of the suspended matter, the formaton of REE soluton complexes wth organc and norganc ons, and REE solublty. Despte the fact that these ntal studes were unable to establsh the cause of fractonaton, they were able to provde relatvely accurate descrptons of the overall REE fractonaton pattern (.e., Fgure 1.1). In addton to measurng REE concentratons n seawater, Goldberg et al. (1963) measured concentratons n a manganese nodule and suggested that ncorporated REE were precptated drectly from seawater. Several studes were subsequently performed wth oceanc ferromanganese nodules to determne the mechansm of REE ncorporaton nto nodules (Elderfeld et al., 1981, and references theren), but some researchers also used these studes to look at REE fractonaton n seawater. Based on observatons that REE concentratons n ferromanganese nodules from depths greater than approxmately 3500 m dsplay a mrror-mage pattern relatve to seawater, Pper (1974) suggested that formaton of deep-water nodules fractonates the REE and therefore could be responsble for the observed seawater fractonaton pattern. Pper (1974) concluded that the mechansm controllng the fractonaton mght nvolve REEs beng released from dssolvng bogenc tests and then beng coprecptated wth Fe and Mn phases. Elderfeld et al. (1981) agreed wth Pper (1974) that REE fractonaton may be controlled by the formaton of nodules but suggested that the pattern reflects the relatve proportons of Ferch and P-rch carrer phases n the nodule, wth the Fe-rch phase producng the nverted seawater pattern. REE concentraton determnatons n seawater experenced a hatus of several years as methods more precse than nstrumental neutron actvaton analyss (INAA) were developed to measure pcomolar YREE concentratons. Compared to the REE chemcal yeld averagng about 90% n the work of Goldberg et al. (1963), de Baar mproved the analyss of seawater by INAA, demonstratng a chemcal yeld of 100% and precson of 2 5% (1σ) (de Baar et al., 1983, 1985a,b). A major advancement came wth the use of sotope dluton thermal onzaton mass spectrometry (ID-IMS) by Elderfeld and 6

23 Greaves (1982), whch ncreased the precson and senstvty of measurements to typcally ± 1% (2σ). Over the next couple of years, addtonal measurements of REE concentratons n the ocean were nterpreted as beng ndcatve of fractonaton durng YREE scavengng by marne partculate matter (Elderfeld and Greaves, 1982; de Baar et al., 1983; Klnkhammer et al., 1983). It was also proposed that snce HREEs form more stable soluton complexes n seawater, LREEs would be preferentally removed by the scavengng process (Elderfeld and Greaves, 1982; de Baar et al., 1983; Klnkhammer et al., 1983). Referrng to the steady state scavengng model of Schndler (1975) and Balstrer et al. (1981), de Baar et al. (1985b) stated that REE fractonaton occurs durng the equlbraton of REE(III) between norganc complexes n soluton and surface stes on small suspended partcles. he work of de Baar et al. (1985a,b) may have been the frst to explctly attrbute YREE fractonaton n seawater to the competton between soluton chemstry and surface chemstry. At the tme of the de Baar et al. publcatons, YREE surface complexaton characterstcs were unknown and lttle was known about YREE soluton complexaton. Despte the fact that equlbrum constants had not yet been measured for REE soluton complexes, urner et al. (1981) assessed the specaton of trvalent REEs n seawater based on lnear free-energy relatonshps for dvalent metals. Usng the observed correlaton between the stablty constants of carbonate and oxalate complexes for dvalent metals, urner et al. (1981) calculated that at a free carbonate on concentraton of M, the fracton of free on n seawater decreased along the REE seres from 38% for La to 5% for Lu, whle the fracton of carbonate complexes ( MCO only) generally ncreased from 22% for La to 81% for Yb. he remanng fracton of REEs was attrbutable to a combnaton of hydroxde, chlorde, and sulfate complexes wth fluorde complexes only accountng for approxmately 1% of the total for each ndvdual REE (urner et al., 1981). Based on the fact that carbonate domnated the specaton of REEs n seawater, most of the ntal studes nvolvng YREE soluton complexaton focused on the carbonate on. Intally, carbonate complexaton constants were measured for ndvdual REE, ncludng La (Cavatta et al., 1981), Eu (Lundqvst, 1982), Ce (Ferr et al., 1983), and Y

24 (Spahu, 1985). Snce these four studes were nterested n groundwater specaton, experments were performed at onc strengths between 0.3 M and 3 M. In order to determne constants approprate to seawater and also to mprove the correlaton used by urner et al. (1981), Cantrell and Byrne (1987a) measured carbonate and oxalate stablty constants for Ce, Eu, and Yb at an onc strength of 0.68 m. It was shown that M(CO 3) 2, along wth MCO + 3, are the domnant speces for REEs n seawater (Cantrell and Byrne, 1987a), whch sgnfcantly altered the specaton scheme of urner et al. (1981). Usng the measured carbonate stablty constants for Ce, Eu, and Yb, along wth those estmated for the remanng REEs va a quadratc functon n atomc number, Cantrell and Byrne (1987a) calculated that at a total carbonate on concentraton of m (free concentraton = m, usng the on parng model of Mllero and Schreber, 1982), the fracton of carbonate complexes ( MCO + 3 and M(CO 3) 2) ncreased along the YREE seres from 86% for La to 98% for Lu, whle the fracton of free on decreased from 7% for La to 0.3% for Lu. Over the next several years, addtonal measurements of carbonate complexaton were carred out to further mprove ths specaton scheme, although the majorty of studes nvolved only Eu (hompson and Byrne, 1988; Chatt and Rao, 1989; Rao and Chatt, 1991). Snce shale-normalzed REE patterns n seawater exhbt an anomaly at Gd (de Baar et al., 1985b, 1991), Lee and Byrne (1993) determned carbonate complexaton constants for Gd and ts neghbors, Eu and b, as well as representatve lght and heavy REEs (Ce and Yb). In addton to demonstratng the Gd-break n the seres, they also estmated carbonate stablty constants for the other REEs usng the measured constants plus multple lnear regresson analyses. By measurng carbonate stablty constants for Y and Gd and then comparng the resultng Y and REE soluton chemstres, Lu and Byrne (1995) showed that the soluton complexaton behavor of Y closely resembles that of b. he ntroducton of nductvely-coupled plasma mass spectrometry (ICP-MS) allowed the entre YREE seres to be studed smultaneously, greatly mprovng the understandng of comparatve YREE aquatc geochemstry relatve to perspectves that are ganed by measurements of ndvdual elements. Lu and Byrne (1998) determned carbonate stablty constants for the entre YREE seres by solvent exchange and ICP-MS. Stablty 8

25 constants for elements such as Ce, Eu, Gd, b, and Yb, whch had been examned prevously, were n good agreement wth pror results. A plot of YREE carbonate complexaton constants versus atomc number showed () a general ncrease from La to Lu, () a negatve anomaly at Gd, and () a close relatonshp between Y and Eu (Lu and Byrne, 1998). Usng adsorptve exchange analyss, Kawabe et al. (1999a) and Ohta and Kawabe (2000) also determned carbonate complexaton constants for the entre YREE seres. However, ther results were log unts hgher than essentally all prevously determned values obtaned usng a varety of technques. In order to better constran the results, Luo and Byrne (2004) utlzed potentometry to measure YREE carbonate stablty constants. hs work showed excellent agreement wth the solvent exchange results of Lu and Byrne (1998). YREE behavor n natural waters has become ncreasngly well characterzed through ncluson of YREE stablty constant results for a varety of soluton lgands, ncludng bcarbonate (Luo and Byrne, 2004), fluorde (Schjf and Byrne, 1999; Luo and Byrne, 2000; Luo and Mllero, 2004), hydroxde (Klungness and Byrne, 2000), chlorde (Luo and Byrne, 2001), and sulfate (Schjf and Byrne, 2004). Usng the above-referenced soluton complexaton constants, YREE specaton for seawater can be calculated at ph 8.2 and a total bcarbonate concentraton of 2 mm. Fgure 1.2 shows the fracton of each speces as log ([ML]/M ), where M represents a YREE on and L represents a soluton lgand. It can be seen that the results do not dffer much from those of Cantrell and Byrne (1987a). In contrast to the extensve studes on norganc complexaton, lttle s known about YREE soluton complexaton wth natural organc matter. Byrne and L (1995) summarzed YREE complexaton constants wth 101 organc lgands, whch were obtaned from the complatons of Crtcal Stablty Constants by Martell and Smth (1974, 1977, 1982) and Smth and Martell (1975, 1989). More specfc studes have determned YREE complexaton constants wth acetate (e.g., Kolat and Powell, 1962; Wood et al., 2000) and oxalate (e.g., Schjf and Byrne, 2001). hese nvestgatons have 9

26 0.0 log ([ML]/M ) M(CO 3 ) 2 - MCO 3 + MOH 2+ M 3+ MSO 4 + MCl 2+ MF 2+ MHCO Y La Ce Pr Nd Pm Sm Eu Gd b Dy Ho Er m Yb Lu Fgure 1.2. YREE specaton n seawater (ph 8.2 and [HCO 3] = 2 mm) expressed as the rato of speces concentraton to the total YREE concentraton (log ([ML]/M )) (see text for detals). utlzed a varety of well-characterzed organc lgands that may not accurately represent the complex propertes of natural organc matter, whch conssts of humc acds, fulvc acds, and other bologcally-derved materal. Relatvely few YREE bndng constants wth natural humc substances have been measured, and studes usually were conducted only for ndvdual elements (see electronc annex of Sonke and Salters, 2006). ang and Johannesson (2003) used the publshed data for Eu, b, and Dy plus lnear free-energy relatonshps wth the stablty constants of lactc and acetc acds to estmate equlbrum constants for the entre YREE seres. Combnng these estmated constants wth the Humc Ion-Bndng Model V (ppng, 1994), ang and Johannesson (2003) predcted that organc matter domnates YREE specaton n crcumneutral-ph rver waters and organc-rch groundwaters (dssolved organc carbon > 0.7 mg/l). o mprove future calculatons of YREE specaton n natural waters, Sonke and Salters (2006) determned humc substance bndng constants for the entre YREE seres. 10

27 In addton to complexaton by organc and norganc soluton lgands, the other domnant process controllng YREE dstrbutons n natural waters s scavengng, or the sorptve removal of dssolved elements from soluton by partcles (Goldberg, 1954; Goldberg et al., 1963; Balstrer et al., 1981; Bruland, 1983; Fowler and Knauer, 1986). YREEs are scavenged n estuares, where freshwater and seawater mx, by the formaton of collods (Sholkovtz, 1976, 1992; Sholkovtz and Elderfeld, 1988). In the upper ocean, YREEs are predomnantly scavenged by partcles and partcle coatngs, whch are composed of ether organc matter or Fe oxdes (Balstrer et al., 1981; Hunter, 1983). Below approxmately 300 m, concentratons of YREEs n seawater may also be controlled by YREE phosphate coprecptaton (Jonasson et al., 1985; Byrne and Km, 1993; Lu and Byrne, 1997; Lu et al., 1997). Before drect measurements were possble, the sorptve behavor of marne partculates was modeled based on the resdence tme of an element (Schndler, 1975). wo dfferent methods were used for ths modelng: () relatve concentratons of dssolved YREEs were estmated usng surface and soluton complexaton constants, and then compared to actual measurements, and () surface complexaton constants were estmated from shale-normalzed YREE concentratons and soluton complexaton constants that had been drectly measured. For the frst method, the relevant surface groups and assocated complexaton constants were not well known, so ether YREE hydrolyss constants (Balstrer et al., 1981; Erel and Morgan, 1991; Erel and Stolper, 1993) or YREE complexaton constants wth dssolved monocarboxylc acds (Byrne and Km, 1990) were used n the calculaton. For the second method, Lee and Byrne (1993) and Byrne and Sholkovtz (1996) used the most accurate YREE ocean concentratons and soluton complexaton constants that were avalable at the tme to calculate surface stablty constants. hese modelng efforts are descrbed n more detal n Secton 2.2. here have been relatvely few measurements of REE concentratons n marne partculates, ether snkng (Murphy and Dymond, 1984; Masuzawa and Koyama, 1989; Fowler et al., 1992; achkawa et al., 1997; Lerche and Nozak, 1998) or suspended (Bertram and Elderfeld, 1993; Sholkovtz et al., 1994; Albo and Nozak, 1999; achkawa et al., 1999; Kuss et al., 2001). he shale-normalzed REE patterns obtaned 11

28 n these studes show varous degrees of enrchment n ether HREEs (Murphy and Dymond, 1984; Kuss et al., 2001) or LREEs (Fowler et al., 1992; achkawa et al., 1997, 1999), whle some show an enrchment n mddle REEs (MREEs) (Lerche and Nozak, 1998; Albo and Nozak, 1999). In addton, Ce anomales are both negatve (Murphy and Dymond, 1984; Kuss et al., 2001) and postve (e.g., Sholkovtz et al., 1994; achkawa et al., 1997), and change wth depth (achkawa et al., 1999) or partcle flux (Fowler et al., 1992). he observed pattern varatons may be caused by dfferences n () the shale values used for normalzaton, () the chemcal methods used for leachng and/or dgeston, and () partcle propertes, such as composton, sze, and snkng velocty. Because multple parameters can nfluence compostonal results, comparsons between dfferent studes are dffcult. In one of the frst measurements of REE concentratons n snkng partcles, Murphy and Dymond (1984) observed shale-normalzed REE patterns smlar to seawater (.e., HREE enrchment and negatve Ce anomaly). In sharp contrast, the shale-normalzed REE patterns of Masuzawa and Koyama (1989) were flat wth postve Ce anomales. Masuzawa and Koyama (1989) attrbuted ther observatons to the preferental removal of Ce, along wth Mn, from seawater by settlng partcles. Although ther patterns were qute dfferent, Murphy and Dymond (1984) and Masuzawa and Koyama (1989) both determned that the total REE flux ncreases wth depth, possbly due to ongong sorpton of REEs by the snkng partcles. Usng regressons between measured REE concentratons and percentages of end member components, Murphy and Dymond (1984) calculated that 30 60% of the REE flux comes from the bogenc component, whch s domnant n the upper water column and decreases wth depth. he remander of the total flux was consdered to be detrtal, gradually ncreasng wth depth and becomng domnant n deeper water (Murphy and Dymond, 1984). Both of the above studes were performed durng a sngle tme perod and could not be used to draw any conclusons about temporal varatons n partcle fluxes. By usng automated tme-seres sedment traps, Fowler et al. (1992) were able to observe changes n shale-normalzed patterns of snkng partcles due to rapd varatons n partcle flux assocated wth the crash of a phytoplankton bloom. In the course of the Fowler et al. 12

29 (1992) observatons, REE patterns of partcles remaned flat wth negatve Ce anomales throughout the water column. In comparson, when partcle fluxes were lower, REE patterns of partcles were flat n surface waters and developed LREE enrchments and postve Ce anomales wth depth. Fowler et al. (1992) concluded that slowly snkng partcles are more effectve at scavengng REEs, especally LREEs and Ce, compared to fast snkng partcles. hs same effect of vertcal flux on REE sorpton was observed by achkawa et al. (1997) n ther comparson between a mesotrophc ste wth large dust fluxes and an olgotrophc ste wth lower fluxes and greater LREE enrchments. Measurng REE concentratons n suspended partcles, Bertram and Elderfeld (1993) determned that < 5% of the total REE n seawater s n partculate form, except for Ce, for whch 20% of the total s partculate. Snce the REE patterns of dssolved/ partculate ratos resembled the shale-normalzed pattern of seawater, Bertram and Elderfeld (1993) suggested that partculate REEs are composed manly of detrtal matter. Sholkovtz et al. (1994) ncreased the understandng of marne partculate behavor by performng a seres of three chemcal dgestons on suspended partcles. he frst dgeston used acetc acd to remove the surface coatngs, whch conssted of organc matter and Mn oxdes. he acetc acd dgest, contanng 50 70% of the REEs and the man fracton of Mn, had a shale-normalzed REE pattern that was the mrror-mage of seawater (.e., LREE enrchment and postve Ce anomaly). he second dgeston, whch nvolved strong acds, and the thrd dgeston, whch was carred out n a dgeston bomb, both contaned the major fracton of Al and Fe, ndcatng a detrtal composton. he shale-normalzed patterns for the strong acd and bomb dgests were relatvely flat, whch s consstent wth a detrtal source. Based on these three dgestons, Sholkovtz et al. (1994) concluded that REEs are removed from seawater and fractonated by surface coatngs, wth Mn oxyhydroxdes possbly controllng Ce oxdaton and the preferental removal of LREEs compared to HREEs. Despte usng a sequental dgeston method smlar to Sholkovtz et al. (1994), Lerche and Nozak (1998) observed shale-normalzed patterns for snkng partcles that had a MREE enrchment n all three dgests. In addton, the Ce anomaly changed from negatve n the acetc acd dgest to unty n the acd leach to postve n the resdual 13

30 fracton. Lerche and Nozak (1998) suggested that the adsorbed REEs are altered nto a more refractory sold phase pror to or durng partcle snkng. he dfferences between the results of Sholkovtz et al. (1994) and Lerche and Nozak (1998) ndcate that the sorptve behavor of marne partculates s not yet fully understood. One of the major challenges n partculate REE studes s determnng the assocaton between the REEs and a specfc carrer phase snce partcle composton can vary throughout the ocean. Several studes used observatons of partculate Mn, Al, and Fe concentratons, whch represent norganc carrer phases, to nfer relatonshps wth the REEs (Masuzawa and Koyama, 1989; Sholkovtz et al., 1994; achkawa et al., 1997, 1999). Kuss et al. (2001) extended ths by also measurng concentratons of partculate organc carbon (POC), CaCO 3, and opal, whch are consdered to be bogenc YREE carrer phases n suspended matter. hrough lnear regresson analyses of YREE concentratons versus carrer phase values, Kuss et al. (2001) demonstrated strong fractonaton between HREEs and LREEs for each partculate fracton. Y and the HREEs were strongly correlated wth POC and Mn, ndcatng that organc lgands form stronger complexes wth ncreasng REE atomc number and that Mn may be assocated wth bogenc matter rather than oxyhydroxdes. On the other hand, the LREEs were strongly correlated wth Al and to a lesser extent Fe, whch ndcates a crustal or clay orgn and some sorpton onto an Fe-oxde phase. Snce marne partcles are composed of several dfferent phases (e.g., Fe-Mn oxdes, alumnoslcates, organc matter, etc.), surface complexaton constants for dscrete phases, whch are smlar to soluton complexaton constants, cannot be determned from the above-mentoned studes. In order to accurately model YREE dstrbutons n natural waters, a better understandng of YREE nteractons wth partcle surfaces must be ganed through observatons of YREE assocatons wth pure phases. o supplement the extensve database on YREE soluton complexaton, an ncreasng number of studes have been devoted to examnaton of YREE surface chemstry. Investgatons of YREE sorpton have utlzed a varety of pure substrates, such as alumna (e.g., Farhurst et al., 1995; Marmer et al., 1997; Rabung et al., 2000), slca (e.g., Kosmulsk, 1997; akahash et al., 1998; Marmer et al., 1999), rutle (e.g., Rdley et al., 2005), sand (e.g., ang and 14

31 Johannesson, 2005), organcs (e.g., Bngler et al., 1989; Stanley and Byrne, 1990), clays (e.g., Bradbury and Baeyens, 2002; Coppn et al., 2002; Rabung et al., 2005), manganese oxdes (e.g., Bdoglo et al., 1992; De Carlo et al., 1998; Davranche et al., 2005), and ron oxdes (e.g., Koeppenkastrop and De Carlo, 1992; Bau, 1999; Ohta and Kawabe, 2001). Because ron s ubqutous n natural waters and s the man substrate used n the present research, the followng dscusson wll prncpally focus on YREE sorpton by a varety of forms of partculate ron. Some of the frst studes nvolvng REE sorpton by ron(iii) hydroxde and oxde were nterested n preconcentraton and decontamnaton of radonucldes. Musć et al. (1979) and Musć and Rstć (1988) showed that REE sorpton n low onc strength solutons ( 0.15 M) ncreased as the ph ncreased from 4 to 7. he ph-adsorpton edges for Ce, Gd, and Yb were very smlar (Musć and Rstć, 1988), suggestng a lack of fractonaton. Although they descrbed the reactons nvolved n REE sorpton, nether nvestgaton modeled the process or related the results to seawater. Early nvestgatons of REE sorpton n seawater showed that LREEs are preferentally removed by most solds compared to HREEs, whch leads to fractonaton (Byrne and Km, 1990; Koeppenkastrop et al., 1991; Koeppenkastrop and De Carlo, 1992). Despte the fact that surface complexaton appeared to be stronger for LREEs, dfferences among varous solds were observed. Slca phases were unque n ther sorpton behavor because they dsplayed a greater affnty for HREEs, except when covered by a thn organc flm, whch enhanced LREE sorpton (Byrne and Km, 1990). Koeppenkastrop and De Carlo (1992) observed that REE sorpton by crystallne goethte was weaker than sorpton by amorphous ron oxyhydroxde, but that goethte created stronger fractonaton between the LREEs and the HREEs. In addton, the resdual seawater pattern from these experments resembled the shale-normalzed REE pattern n the ocean (Koeppenkastrop and De Carlo, 1992). As was dscussed earler, seawater contans numerous soluton lgands that compete wth surfaces for free YREE ons. o determne YREE surface complexaton constants, any effects from ths strong soluton complexaton must be removed. YREE sorpton onto amorphous ferrc hydroxde n smple synthetc solutons wthout complexng lgands has 15