Mélanie Davranche
email: melanie.davranche@univ-rennes1.fr

Olivier Pourret

Gérard Gruau

Aline Dia Caren

Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide

Keywords: Ce anomaly, REE, humic acid, hydrous ferric oxide, oxidation/scavenging, ternary surface complex

Adsorption experiments of rare-earth elements (REE) onto hydrous ferric oxide (HFO) were performed to evaluate the impact of organic complexation on both REE(III) adsorption and Ce(III) oxidation rate.

Scavenging experiments were performed at pH 5.2 with NaCl and NaNO 3 solutions containing either free REE (III) or REE(III)-humate complexes. The REE d K log patterns obtained from HFO suspensions exhibit a slight positive Ce anomaly and an M-type lanthanide tetrad effect, in contrast with the partitioning between REE(III)humate complexes and HFO, which yields completely flat distribution patterns. The 'organic' partitioning runs yield DOC d REE d K log K log organic ratios (DOC=Dissolved Organic Carbon) close to 1.0, implying that the REE(III) and humate remain bound to each other during the adsorption experiment. The lack of any positive Ce anomaly or M-type lanthanide tetrad effect in the organic experiments seems to reflect an anionic adsorption of the REE-humate complex. Adsorption onto HFO takes place via the humate side of the REE(III)-humate complexes. The oxidation of Ce(III) by Fe(III) and the proportion of surface hydroxyl groups coordinated to REE(III) at the HFO surface are the two most commonly invoked processes for explaining the development of positive Ce anomalies and the M-type tetrad lanthanide effect. However, such processes cannot proceed since the REE are not in direct contact with the HFO suspensions, the latter being shielded by PHA. The present results

further complicate the use of Ce anomalies as reliable paleoredox proxies in natural precipitates. They are also further demonstration that organic matter may inhibit the lanthanide tetrad effect in geological samples.

INTRODUCTION

Hydrous ferric oxides (HFO) are often thought to control the mobility of trace elements in aqueous systems [START_REF] Davis | Mineral Interface Geochemistry[END_REF]. Due to their large specific surface area, these mineral phases are indeed very effective scavengers of trace elements, and hence, they exert a key control on the concentration and migration of trace metals in aqueous systems [START_REF] Dzombak | Surface Complexation Modeling, Hydrous Ferric Oxide[END_REF]. Interactions between trace elements and Fe oxyhydroxides depend upon various processes such as adsorption, surface precipitation, etc. and, in the particular case of the REE (Rare earth elements), oxidation/scavenging [START_REF] Koppenkastrop | [END_REF](4)(5).

Understanding the REE distribution between Fe oxyhydroxides and aqueous solutions continues to be a major issue in REE studies on geochemical processes (6). Marine ferromanganese crusts are fairly enriched in REE and some of them exhibit large positive Ce anomalies (7)(8)(9)(10)(11). Deep-sea ferromanganese nodules, banded iron formations and Fe and Mn concretions in paleosols are geological deposits of major geochemical importance because they can record the REE signatures of ancient marine and fresh waters (8,(12)(13)(14)(15)(16).

Several authors have examined the adsorption processes of REE onto Fe oxyhydroxides and Mn oxides (3-5, 17, 18). Koeppenkastrop and De Carlo (3), presented the available data on sorption of REE from seawater, noting the close similarity between their experimental determined K d REE values and those estimated from REE analyses of marine Mn-Fe deposits and seawater samples. More recently, Bau (4) and Ohta and Kawabe (5) reported experimental investigations of the sorption of REE from low-salinity solutions. These authors showed that (i) Fe and Mn oxyhydroxides can both oxidize Ce(III) into Ce(IV), even though the resulting preferential enrichment of Ce as compared to its trivalent REE neighbours (the so-called positive Ce-anomaly) is much larger with Mn oxide than with Fe oxyhydroxide, and

(ii) at pH ≥ 5, the M-type lanthanide tetrad effect appears on patterns of apparent REE distribution coefficients between Fe and Mn oxyhydroxides and aqueous solutions.

However, very little is known about the influence of organic matter on the sorptive properties of REE onto Fe oxyhydroxides. In a recent paper, we reported experimental K d between Mn oxide suspensions and weakly acid (pH = 5.2) NaCl and NaNO 3 solutions containing REE as REE(III)-humate complexes [START_REF] Davranche | Revised version submitted to Geochim[END_REF]. Under such conditions, the M-type lanthanide tetrad effect is easily suppressed and the oxidation of Ce by Mn oxide is greatly inhibited. The speciation of the REE partitioning between Fe oxyhydroxide and solution governs the REE behaviour in natural waters, as well as the oxidative scavenging of Ce(III).

This behaviour has possible major implications on the use of Ce anomalies as paleo-redox proxies (20)(21)(22)(23)(24). The aim of this article is (i) to compare apparent K d REE between hydrous ferric oxide (HFO) suspensions and synthetic solutions in which the REE(III) occurs alternatively as free REE(III) and REE(III)-humate complexes, and (ii) to examine the effect of organic complexation on Ce(III) oxidation and tetrad lanthanide effects in the presence of HFO.

MATERIALS AND METHODS

All chemicals used in this study were of analytical grade, and the solutions were all prepared with doubly de-ionized water (Milli-Q system, Millipore The HFO was synthesized according to Schwertmann and Cornell [START_REF] Schwertmann | Iron Oxide in the Laboratory, Preparation and Characterization[END_REF]. 1.5 mole of NaOH (pellets) were added slowly to 1 L of a solution containing 0.5 M Fe(NO 3 ) 3 , The solution was continuously bubbled with nitrogen to avoid carbonate precipitation. The HFO precipitate was centrifuged and washed several times with Milli-Q water, then dried at 20 °C, sieved for homogenization and, finally, stored in a dessicator. The crystallographic structure of the HFO was checked by X-ray diffraction (XRD). The total number of [?negatively charged] surface sites of the solid phase was estimated using the solid cation exchange capacity (CEC) determined from the cobalthexammine method, ISO 11260 [START_REF]Qualité des Sols[END_REF]. Ions bound to the solid surface were exchanged with cobalthexammine ions, the CEC being taken as equal to the concentration of cobalthexammine ions removed from solution. Five grams of HFO were mixed with 10 mL of 0.017 M cobalthexammine solution for 3 h. The suspension was then centrifuged and the concentration of cobalthexammine ion remaining in solution was measured at 470 nm with a Shimadzu-160 A U.V.-spectrophotometer. The CEC of the HFO used in this study is 24.5 meq/100g.

The surface acidity constants were determined from potentiometric titrations of suspensions of 5 g.L -1 HFO with NaOH (0.1 M) or HNO 3 (0.1 M), with a 0.1 M NaCl solution as the supporting electrolyte [START_REF] Davranche | [END_REF]. Titrations were carried out with a Metrohm 794 DMS Titrino apparatus equipped with a Metrohm combined (3 M KCl) glass electrode. The HFO acidity constants so obtained were as follows: pK a1 = 6.77 and pK a2 = 8.56, while pH zpc was 7.7.

Humate, referred to below as PHA (purified humic acid), was obtained from the synthetic Aldrich humic acid (Aldrich, H1,675-2) according to the process described by Vermeer et al. (28). The PHA obtained in this way is ash free and in its protonated form, with The first set of experiments carried out with free REE(III) was essentially conducted to validate the experimental and analytical protocol used in this study. In the second and third series of experiments, the equilibration was made with the REE(III)-PHA complexes described above.

We can describe the adsorption behaviour of the REE using the apparent partition coefficient K d , expressed in Eq. [START_REF] Davis | Mineral Interface Geochemistry[END_REF] as

solution in REE µg.mL HFO g adsorbed REE µg K 1 d - = [1]
Suspension aliquots of about 10 ml were sampled at predetermined time intervals and filtered in-line through 0. Dissolved organic carbon concentrations were determined at Rennes University using a Shimadzu 5000 TOC analyzer. The precision on DOC contents and log K d DOC are better than ±1.5%.

RESULTS AND DISCUSSION

Description of log REE

Description of mechanisms

These data support the earlier findings by Davranche et al. ( 19) that organic complexation may inhibit Ce(III) oxidation and prevent the development of a M-type lanthanide tetrad effect during interaction between dissolved REE(III) and Fe-Mn oxyhydroxides. REE patterns from the organic runs do not exhibit any Ce anomaly or M-type lanthanide tetrad effect whatever the supporting electrolyte (Fig. 4). Such results are clearly in strong contrast with the inorganic runs, implying that the mechanisms involved under organic conditions are distinctly different. (ligand linked to the surface over the cation) [START_REF] Schindler | Mineral-Water Interface Geochemistry[END_REF][START_REF] Nowack | [END_REF]. The formation of both types of complexes depend on whether the surface groups of the oxide can participate in exchanging their OH -with the cation-ligand complex. These reactions strongly depend on pH. When pH is lower than the solid pH zpc (pH at which the solid surface charge is globally neutral: i.e., the positive charges are equal to the negative charges), adsorption is of anionic type ( LM S -≡

).

When pH is higher than pH zpc , the cationic type of adsorption (

ML O S - - ≡
) is promoted.

The HFO used in the experiments exhibits a pH zpc value of 7.7, while the experiments were carried out at 5. ,4). These results are in good agreement with a recently completed study on the adsorption of REE-PHA onto MnO 2 [START_REF] Davranche | Revised version submitted to Geochim[END_REF]. Thus, the distribution of REE in the presence of organic complex is determined by the distribution of humate at the HFO surface. Takahashi et al. (33) obtained the same result in their study of the adsorption of Cm(III)-fulvate onto an acid-montmorillonite, even when using relatively low concentrations of organic matter.

suspensions exhibit slight positive Ce anomalies and an M-type lanthanide tetrad effect. This is in contrast with the distribution patterns obtained between REE(III)-humate complexes and HFO, which are completely flat. These modifications are believed to reflect an anionic adsorption of the organic complex. The ternary complexes formed at the surface of the HFO represent structures in which the binding of the REE(III)-PHA with the HFO surface takes place on the humate side. Consequently, the humate ligand physically separates Ce(III) ions from the HFO catalysis surface, thus preventing their oxidation by Fe(III) and stopping the development of an M-type tetrad effect. The distribution of REE at the HFO surface is then governed by the behaviour of humic substances.

These data add further limitations to the use of Ce anomalies in reconstructing redox conditions in ancient geological systems. The present results, as well as those recently reported by Davranche et al. [START_REF] Davranche | Revised version submitted to Geochim[END_REF] for the MnO 2 system, strongly suggest that use of this proxy could be rendered impossible in organic-rich systems such as paleosols.

DL: Detection Limits

The amplitude and efficiency of the Gd and Tb corrections can be evaluated from Table 4, where we present measured and corrected concentrations of Gd and Tb measured for the SLRS-4 water geostandard along with reference data (34). The correction amplitudes are moderate, being 6% for Gd and only 2% for Tb. The corrected values in both cases show relative discrepancies of ≤2.5%, thus lying within the range of the reference values published for the SLRS-4 water geostandard (34). These values should be regarded as maximum values for the results presented here, since the ratios of [Interfering Element]/[Interfered Element] are close to unity in our synthetic solutions, as against 10-65 in the SLRS-4 geostandard. Finally, we can check the overall validity of the results using Table 5, which reports duplicate analyses of the synthetic solutions. Table 6 reports the concentrations of the SLRS-4 standard compared with published reference values (34). From these two tables, and from an analysis of the efficiency of the correction procedure used to correct the REE results for oxide and hydroxide isobaric interferences, we estimate the total uncertainty on the REE results presented here at ±2%.

Comparison of the analytical accuracy of the Agilent Technology

  the following elemental composition (in weight percent): C = 55.8%, O = 38.9%, H = 4.6%, N = 0.6%. Purified humic acid has a mean molecular weight of 23 000 Daltons(28). Prior to use, the freeze-dried PHA was resuspended overnight in a NaOH solution (pH 10) to ensure complete dissolution of the sample.Rare-earth element-humate complexes were prepared as follows. Twenty mg of dissolved PHA were enclosed in a 250-mL sodium-acetate dialysis bag with a pore size of 12 000-14 000 Daltons. The bag was introduced into a 5 ppb REE solution (corresponding to a molar concentration range of 36 to 29 nM depending on the REE considered), the ionic strength being buffered at 10 -3 M with NaCl or NaNO 3, and the pH at 7 with HNO 3 . Both NaCl and NaNO 3 were used as neutral electrolytes (see below). The suspension was stirred for 48 h (equilibrium time determined from preliminary kinetic experiments), to allow equilibration and partitioning of the REE between the aqueous solution and the PHA suspension. The dialysis bag was then removed and the REE-PHA complexes recovered. The concentrations of REE in solution both inside and outside the dialysis bag were monitored vs. time in order to quantify the amount of REE complexed to PHA. Possible REE adsorption onto the dialysis bag was checked by analysing REE content of the membrane (dissolved by acidic digestion with HNO 3 14 N). The results show that around 95% (with NaCl) or 60% (with NaNO 3 ) of the REE initially present in the solution were complexed to the PHA. The remaining 5 or 40 % were left in solution outside the membrane or adsorbed onto the dialysis bag. Moreover, complexation rates are found to increase slightly but regularly from the heavy to the light REE, which is consistent with the stability constant order determined byTakahashi (29) for REE(III)-humate complexes.Adsorption procedureFour time-series experiments were carried out -two with the REE occurring as REE(III)-humate complexes and two with the REE occurring as free, aqueous species -to assess the effects of organic complexation during REE(III) adsorption onto HFO. All suspensions were made up from 100 mg.L -1 HFO left to equilibrate with a 0.001 M NaCl or NaNO 3 aqueous solution at pH 5.2. Experiments were carried out at room temperature. The pH was monitored periodically with a pH-meter and adjusted to 5.2 by addition of HNO 3 (4.6 N) or NaOH (4 N). The pH value of 5.2 was chosen because it was sufficiently basic to promote adsorption processes, but low enough to avoid total REE sorption that might mask any eventual development of Ce anomaly[START_REF] Davranche | Revised version submitted to Geochim[END_REF].

3 inorganic

 3 The time dependence of the sorption of free REE(III) and REE-PHA complexes onto HFO is presented in Figs1 and 2, respectively. Both the organic and inorganic experiments exhibit a faster adsorption in runs conducted with NaCl solutions as compared with those conducted with NaNO 3 . However, the composition of the neutral electrolyte has no effect on kinetics were slowed down by a factor of two in organic experiments compared with runs where REE(III) occurred as free species. The values 1.0, indicating that the REE(III) and humate remain bound to each other during interaction of the REE(III)-humate complexes with the HFO surface. inorganic REE d K Log equilibrium data between HFO and solutions containing free REE(III) are comparable to earlier values reported by Ohta and Kawabe (5) at similar pH conditions.All inorganic patterns exhibit slight positive Ce anomalies and a clear M-type lanthanide tetrad effect (Fig.3). The tetrad effect often corresponds to a very small discrete feature on the REE patterns of geological samples Mc Lennan(30). However, in the present study, the total uncertainty on the REE and Log K d REE is estimated at 2%. In NaNO relative error of 5.5 % is far higher than the uncertainty determined for analytical measures, confirming that the observed tetrad effect reflects a true feature rather than an analytical artifact.Positive Ce anomalies and a clear M-type lanthanide tetrad effect were already apparent in the REE d K log patterns obtained by Bau (4) and Ohta and Kawabe (5) under similar experimental conditions. Ce inorganic anomaly values (Ce/Ce*) as calculated by 11 to 0.15. However, organic patterns differ markedly from the former in showing no positive Ce anomaly (Ce/Ce*= 0.01) and no M-type lanthanide tetrad effect (Fig.4). The general lack of a conspicuous "M"-shaped lanthanide tetrad effect in the organic patterns is very interesting and probably highly significant concerning the mechanism(s) of adsorption of the REE(III)-PHA complexes onto HFO.The complexation of REE(III) by humic acid has a large effect on REE is modified, 60 h under organic conditions v.s. 40 h in inorganic experiments (Figs.1 and 2).

Bau ( 4 )

 4 and Ohta and Kawabe(5) suggested that the formation and growth of an M-type tetrad effect might reflect a change in the coordination state of the REE(III) adsorbed onto Fe and Mn oxyhydroxides. They proposed that REE(III) adsorbed onto MnO 2 are linked with hydroxyl ions, water molecules and oxygen that are bound to the Mn(IV). According to these authors, this coordination would explain the increase of a convex tetrad effect with pH, by increasing the proportion of hydroxyl ions bound with REE(III). Similarly, Ce is oxidized to Ce (IV) by an oxidation/scavenging process involving dissolved Ce(III) and solid Mn-Fe oxyhydroxides. Evidence for this oxidation process was given by Ohta and Kawabe (5), who determined the redox state of Mn in each of their experiments. However, REE and Ce must be in contact with the solid surface to allow both of these features to develop. . 1 and 2) show that REE and humate are strongly linked and that the bonding between these elements is maintained throughout the reaction. Cation ligand complexes, denoted as ML (with M and L representing the cation and the ligand, respectively), can adsorb onto a solid ( S ≡ ) to form ternary surface complexes either as LM S -≡ (cation linked to the mineral surface over the ligand) or as ML

2 .

 2 Consequently, this pH condition -much lower than the solid pH zpcpromotes anionic-type adsorption REE -PHA S -≡ , where REE are present as REE/PHA complexes. The complexes are bound to the HFO surface on their humate side (Eq. [3]): organic experiment as compared with the inorganic runs (Figs. 1, 2 and Figs. 3

Fig. 4

 4 Fig.4 Mélanie Davranche, Olivier Pourret, Gérard Gruau, Aline Dia

  Fig. 1. Log inorganic REE d K

Fig. 2 Fig. 3 .

 23 Fig. 2. Log organic REE d K

Fig. 4 .

 4 Fig. 4. Patterns of equilibrium log organic REE d K

  in organic experiments were all immediately digested with sub-boiled nitric acid (HNO 3 14 N) at 100°C, then resolubilized in HNO 3 0.4 N after complete evaporation in order to avoid interferences with organic matter during mass analysis. REE concentrations were determined by ICP-MS at Rennes University. Precisions on individual REE concentrations and log K d values are estimated at ±2% (See Appendix ).

	The adsorption behaviour of the REE(III)-humate complexes was also monitored by
	measuring the dissolved organic carbon (DOC) content. DOC measurements can be converted
	into apparent partition coefficients using Eq. [2]:
	K	d	=	solution HFO g adsorbed in DOC 1 DOC µg.mL µg -	[2]

2 µm cellulose acetate membranes (Sartorius). The REE concentrations were then measured to determine the K d REE variations. Sample aliquots used for REE determination

TABLE 2 Detection limits and chemical blanks (in pM) measured during the course of the study

 2

	Isotope	DL	Chemical Blanks
	La 139	1.15	4.37
	Ce 140	3.21	5.71
	Pr 141	0.64	2.35
	Nd 146	3.54	7.28
	Sm 147	5.52	0.64
	Eu 153	0.92	0.76
	Gd 158	2.67	2.07
	Tb 159	0.38	1.50
	Dy 163	1.29	2.76
	Ho 165	0.49	1.08
	Er 166	1.26	0.88
	Tm 169	0.59	1.27
	Yb 174	1.44	1.14
	Lu 175	0.51	0.85

TABLE 4 Amplitude and efficiency of oxide and hydroxide interference correction on Gd and Tb concentrations of the SLRS-4 water geostandard. Concentrations are expressed in nM.

 4

	Isotope	Reference Values	Measured Values	Corrected Values	Correction (%)	(%)
	Gd 158	0.210	0.228	0.213	6.4	-1.5
	Tb 159	0.026	0.026	0.026	2.4	2.4

: Relative discrepancy between corrected and reference values

Reference concentrations are from

Yeghicheyan et al. (34)

MS set up at Rennes University against published reference values of the SLRS-4 water geostandard. Concentrations are expressed in nM.

	    HP4500 ICP-Isotope Average Reference Concentrations (n=13) Concentrations *
	La 139	2.069	2.066
	Ce 140	2.548	2.569
	Pr 141	0.493	0.492
	Nd 146	1.842	1.865
	Sm 147	0.371	0.382
	Eu 153	0.055	0.053
	Gd 158	0.219	0.217
	Tb 159	0.027	0.027
	Dy 163	0.143	0.149
	Ho 165	0.028	0.028
	Er 166	0.081	0.080
	Tm 169	0.011	0.010
	Yb 174	0.068	0.069
	Lu 175	0.011	0.011

* From Yeghicheyan et al.

(34)

ACKNOWLEDGEMENTS

We wish to thank the technical staff in Rennes for their assistance during the experimental and analytical work. This research was supported by the INSU-CNRS through Programme PNSE.

Consequences on using the Ce anomaly as a paleo-redox proxy

Taken together with the results obtained by Davranche et al. [START_REF] Davranche | Revised version submitted to Geochim[END_REF], the present study shows that the Ce anomaly can hardly be used as a paleo-redox proxy in organic-rich environments. This particularly applies to paleosols, where Ce anomalies have often been reported (13,16,22) and authors have explored the possibility of using Ce depletions to reconstruct the redox state of paleosols at the time of their formation (e.g., 22). From a paleoclimatic point of view, we may consider that wet periods lead to the dominance of oxidizing conditions in paleosols -due to the continuous recharge of the soil solutions by oxygen-rich rainwater -a process that would stabilize both Mn and Fe oxydroxide phases in the weathering front. Just considering the inorganic experimental results of Bau (4) and Ohta and Kawabe (5), we would expect the development of either negative or positive Ce anomalies in paleosols formed under such conditions. However, these experimental results lead us to suppose that organic complexation can severely inhibit Ce(III) oxidation. This calls into question the validity of using Ce anomalies as a reliable paleo-redox proxy in paleosols. The interplay between variable redox conditions and the REE(III) organic speciation is expected to lead to complex Ce anomaly patterns in paleosols. The behaviour of Ce might not unambiguously reflect the redox conditions at time of the soil genesis.

SUMMARY

Appendix -REE analyses

Rare earth concentrations were determined by ICP-MS (Agilent Technologies  HP4500) at the Rennes University (France). Normal plasma conditions were used, with the instrumental parameters reported in Table 1.

TABLE 1

Instrumental and data acquisition parameters mg.L -1 , USA). Indium was added to all samples as an internal standard at a concentration of 0.87 µM (100 ppb) to correct for instrumental drift and possible matrix effects. Indium was also added to the external standard solutions. Calibration curves were calculated from measured REE/Indium intensity ratios.

Chemical blanks of individual REE were all lower than 10 pM, which is a negligible value (Table 2) three to four orders of magnitude lower than the concentrations measured in the synthetic solutions used in the adsorption experiments.

Isobaric interferences due to the formation and ionisation of oxides and/or hydroxides in the ICP-MS can modify the determination of Sm, Eu, Gd and Tb concentrations. The interference list is given in Table 3, along with the equations used at Rennes to correct REE concentrations from this potential source of error. Note that Ce and its neighbours La and Prand thus the calculated value of the Ce anomaly as well -are unaffected by isobaric interference problems. There was no need to correct Eu and Sm for BaO and/or BaOH interference because our synthetic solutions did not contain any Ba.


Mélanie Davranche
email: melanie.davranche@univ-rennes1.fr

Olivier Pourret

Gérard Gruau

Aline N Dia

Dan Jin

Dominique Gaertner

COMPETITIVE BINDING OF REE TO HUMIC ACID AND MANGANESE OXIDE: IMPACT OF REACTION KINETICS ON DEVELOPMENT OF CERIUM ANOMALY AND REE ADSORPTION

Keywords: rare earth elements, humic acid, MnO 2, kinetics, competition, complexation, dissociation

HAL is

A C C E P T E D

M A N U S C R I P T

Introduction

Organic matter has recently been shown to be one of the key factors controlling rare earth element (REE) speciation in aquatic environments such as shallow groundwaters, surface waters and soil solutions. This fundamental role has been studied by direct techniques including ultrafiltration, electrochemical measurements of organic-rich waters (Tanizaki et al., 1992;Viers et al., 1997;[START_REF] Dupré | Major and trace elements associated with colloids in organic-rich river waters: ultrafiltration of natural and spiked solutions[END_REF][START_REF] Dia | The distribution of rare-earths in groundwater: Assessing the role of source-rock composition, redox changes and colloidal particles[END_REF][START_REF] Ingri | Temporal variations in the fractionation of the rare earth elements in a boreal river; the role of colloidal particles[END_REF][START_REF] Johannesson | Rare earth elements concentrations and speciation in organic-rich blackwaters of the Great Dismal Swamp, Virginia, USA[END_REF] and laboratory studies and model calculations (Tang and Johanesson (2003); Sonke and Salters, 2006;Yamamoto et al., 2005;Pourret et al, 2007a). The dominantly organic speciation of REE has several consequences on the distribution and behaviour of these elements in aquatic environments. REE organic complexation suppresses the development of a Ce anomaly and limits the REE adsorption onto Fe-and Mn-oxides. Recently, [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF]2005) designed laboratory experiments to study the impact of REE organic complexation during REE adsorption onto potential oxidative surfaces of HFO (hydrous ferric oxide) as well as MnO 2 . These authors introduced REE in the form of REE-humate complex into a suspension of oxidative solid. Their studies demonstrated (i) a strong decrease of REE adsorption onto MnO 2 , (ii) a suppression of the tetrad effect and (iii) a suppression of the Ce anomaly, when REE occurs as REE-humate complexes. These authors (op. cit.) attributed such phenomena to an anionic adsorption process mediated by organic matter (adsorption by the humate side of the REE-humate complex) as well as the fact that REE cannot behave independently from each other since they are collectively bound to the humate molecule.

Concerning these data, [START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF] state that Ce anomalies may not be a reliable proxy of redox conditions in organic-rich waters or in precipitates formed in equilibrium with waters rich in humic substances. However, REE in soils, soil solutions and aquatic systems draw their source from rocks and rock-forming minerals subjected to weathering/alteration processes (e.g., Sholkovitz, 1995;[START_REF] Gaillardet | Trace elements in rivers[END_REF]. The chemical reactions involved in

A C C E P T E D M A N U S C R I P T

ACCEPTED MANUSCRIPT 4 these processes are mainly hydrolysis inducing the release of REE as free REE(III) species. In this context, released REE are likely to be complexed by the ligands occurring in solution, including carbonate, sulphate, soluble organic matter, colloids and the solid surfaces. The distribution of REE between all these different ligands is therefore controlled by complexation stability constants, reactions kinetics, element concentrations and physicochemical parameters (pH, Eh and T°). Furthermore, natural environments are often non steady-state systems, requiring kinetic approaches to determine more realistic chemical speciation of elements [START_REF] Sekaly | Effect of metal/fulvic acid mole ratios on the[END_REF]Xue et al., 2001;[START_REF] Guthrie | Kinetic studies of nickel speciation in model solutions of a well-characterized humic acid using the competing ligand exchange method[END_REF]Wang et al., 2005). Therefore, there are two main drawbacks in [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF]2005) conclusion that REE organic complexation suppresses the oxidative scavenging of Ce and limits REE adsorption onto MnO 2 . Firstly, these authors do not take into account the respective kinetics of REE complexation reaction by organic matter and MnO 2 and, secondly, the REE-humate complex is completely stabilized before its interaction with MnO 2 . [START_REF] Kawabe | Distribution coefficients of REE between Fe oxyhydroxide precipitates and NaCl solutions affected by REE-carbonate complexation[END_REF] and [START_REF] Ohta | Rare earth element partitioning between Fe oxyhydroxide precipitates and aqueous NaCl solutions doped with NaHCO 3 : Determinations of rare earth element complexation constants with carbonate ions[END_REF] noted similar suppression of Ce anomalies with both REE and REE(III)-carbonate complexes during adsorption onto FeOOH. They suggested that the Ce anomaly did not develop because Ce(III) complexation with carbonate was faster than Ce(III) oxidation kinetics. Thus, the key question concerning Ce anomaly suppression and, more generally, the REE partitioning between surface solid and organic solution is the rate at which REE are complexed by MnO 2 and organic matter, respectively. If oxidation and adsorption kinetics were faster than organic complexation, a Ce anomaly could nevertheless develop, despite the presence of organic matter, and a high amount of REE could be adsorbed onto MnO 2 . Another concern is the stability of the REE-humate complex. [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF] indicated that reactions necessary to stabilize cation-humate complex take place at least after 24 h. Several authors have also provided evidence that the time-dissociation of cation-humate complexes in the presence of a ligand competitor depends on the prior contact time between cations and organic molecules [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF][START_REF] Geckeis | Humic colloid-borne natural polyvalent metal ions: Dissociation experiment[END_REF]Wang et al., 2005).

A C C E P T E D M A N U S C R I P T

Under the experimental conditions used by [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF]2005), organic REE complexes were completely stabilized before contact with MnO 2 . Thus, MnO 2 could not destabilize the REE-humate complex. Clearly, the natural environment could be different when REE occurs in contact with organic matter and solid surface at the same time.

Therefore, the conditions prevailing in nature need to be experimentally explored.

In this study, competitive kinetic experiments were carried out to assess the impact of the competitive kinetic reaction between MnO 2 and humic acid for REE complexation on Ce anomaly development and REE adsorption. The experiments were performed with MnO 2 and purified humic acid in the presence of all 14 REE occurring simultaneously. To allow kinetic competition, MnO 2 , humic acid and REE solutions were prepared separately and mixed together simultaneously. The REE phases included organic and/or inorganic REE bound to MnO 2 surface, REE complexed to aqueous humic acid and soluble REE in the inorganic solution. Each of these phases was separated using a combination of filtration/ultrafiltration techniques. Two series of experiments were designed to allow the determination of the respective roles of humic acid/MnO 2 ratio and pH.

Materials and methods

All chemicals used were of analytical grade, and all the experimental solutions were prepared with doubly deionised water (MilliQ system, Millipore™) REE concentrations REE in HA were below the detection limit of ICP-MS measurement (i.e.

below 1 ppt). HA has a mean molecular weight of 23 kDa (Vermeer et al., 1998). Prior to use, the freeze-dried humate was resuspended overnight in an 0.001 mol L -1 NaCl electrolyte solution at pH = 10, to ensure complete dissolution of the sample [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF]2005).

Manganese oxide

Synthetic

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7 Surface acidity constants were determined from potentiometric titrations of 5g.L -1 of solid with NaOH (0.1 M) and HNO 3 (0.1 M), in 0.1 M NaCl solution as the supporting electrolyte (Davranche et al., 2003). [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF][START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF]Pourret et al., 2007b).

Experiments on REE complexation competition between humate and MnO 2 . All reactants were prepared separately and mixed together simultaneously to allow a detailed simulation of [START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF]. The three solutions were then mixed together to provide a suspension containing 100 mg.L -1 of MnO 2 , 5 ppb of each REE (e.g., 36 nmol.L -1 La to 28.6 nmol.L -1 Lu) and 2.5, 10, 20 and 40 mg.L -1 of HA. All competition-experiments were performed in duplicate or triplicate. The pH was monitored periodically with a pH-meter and re-adjusted to 5, when necessary, by addition of 0.1 N HNO 3 or 0.1 N NaOH. Suspension aliquots of 10 mL were sampled immediately to determine the initial concentration of REE and HA in suspension. In addition, 15 mL of aliquot suspension were regularly sampled and filtered at 0.2 µm, yielding a retentate representing the inorganic or organic (as REE-HA complexes) REE fraction complexed with MnO 2. 10 mL of this sample were immediately digested for REE analysis, while the remaining 5 mL were stored for DOC analysis. An additional 10 mL volume was sampled and ultra-filtered at 10 kDa to separate the REE complexed with HA or MnO 2 from the remaining inorganic REE. Ultrafiltrations were carried out by centrifuging the 10 mL solution samples through 15 mL centrifugal tubes equipped with permeable membranes of 10 kDa pore size (Millipore Amicon Ultra-15). All centrifugal filter devices were washed and rinsed with 0.1 mol L -1 HCl and Milli-Q water two times before use to minimize contamination.

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Centrifugations were performed using a Jouan G4.12 centrifuge with swinging bucket rotor at 3000 g for 30 min. This allowed the MnO 2 -and HA-REE complexes to be quantitatively separated from uncomplexed inorganic REE species. The selectivity of the 10 kDa membrane with regard to the REE-HA complexes was checked by monitoring the DOC contents of the ultrafiltrates. Results show that the latter were systematically lower or equal to blank values (below 0.1 ppm). Possible adsorption of inorganic REE species onto the membrane or onto cell walls was also monitored. Inorganic REE solutions of given REE concentration were

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9 ultrafiltered several times. Results showed that 100% of the REE present in solution were recovered in the ultrafiltrates, demonstrating that none of the REE was adsorbed, either on the membranes or on the walls of the cell devices. Moreover, to verify that no MnO 2 colloids were present in the 0.2µm filtrate. Mn concentration was systematically analysed in each fractions (<0.2 µm and < 10 kDa). The concentration obtained was systematically lower than 1% of the total Mn and equal is each fraction, indicating that Mn is not under colloidal form but under free inorganic species.

The amount of (organic and/or inorganic) REE complexed with MnO 2 corresponds to the difference between the initial REE concentration and the remaining REE concentration in the <0.2 µm filtrate, while the amount complexed with aqueous HA corresponds to the difference between the REE concentration in the < 0.2 µm filtrate and the <10 kDa ultrafiltrate.

Finally, to simulate the role of pH in the competition between HA and MnO 2 for REE binding, the same protocol as previously described was performed from pH 3 to 8 for a HA/MnO 2 ratio=0.1. Suspension aliquots were sampled after 48 h for comparison with the inorganic reference experiments (section 2.3).

REE and dissolved organic matter analysis

REE concentrations were determined with an Agilent Technologies TM HP4500 ICP-MS instrument. Quantitative analyses were performed using a conventional external calibration procedure. Three external standard solutions with REE concentrations similar to the analysed samples were prepared from a multi-REE standard solution (Accu TraceTM Reference, 10 mg.L -1 , USA). Indium was added to all samples as an internal standard at a concentration of 0.87 µmol.L -1 (100 ppb) to correct for instrumental drift and possible matrix effects. Indium was also added to the external standard solutions. Calibration curves were

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10 calculated from measured REE/indium intensity ratios. As established from repeated analyses of multi-REE standard solution (Accu TraceTM Reference, USA) and the SLRS-4 water standard, the instrumental error on REE analysis in our laboratory is below 3 %. Organic samples used for REE determination were all immediately digested with sub-boiled nitric acid (HNO 3 14 N) at 100°C, then resolubilized in HNO 3 0.4 N after complete evaporation, to avoid interferences with organic matter during mass analysis by ICP-MS for the organic experiment samples.

Dissolved organic carbon concentrations were determined at Rennes University using a Shimadzu 5000 TOC analyzer. The accuracy of DOC concentration measurements is estimated at ±5% as determined by repeated analysis of freshly prepared standard solutions (potassium biphtalate).

(ICPMS REE detection limits and chemical blank reproducibility can be found in the supplementary file).

Results

(All experimental data can be found in the supplementary file)

The complexation behaviour of REE onto the MnO 2 surface and/or HA is described using the apparent partition coefficient K d , expressed as follow: Considering this method of calculation, REE adsorbed on the MnO 2 surface could occur either, as inorganic REE or, as REE-HA complexed species.

( ) ( ) a 1 a REE/MnO d REE Aqueous µg.mL oxide Mn g REE Adsorbed µg K 2 - = (1) ( ) ( ) b 1 b REE/HA d REE aqueous Inorganic µg.mL HA Aqueous g REE Adsorbed µg K - = (
Ce anomalies on log REE d K patterns are determined as follows:

( ) ( ) ( ) [ ] Pr d La d Ce d K log K log 2 1 K log * / + = Ce Ce (3)
Finally, DOC partitioning between MnO 2 and solution are converted into apparent partition coefficients using: reveal that a pseudo-equilibrium was reached before 10 h (Fig. 1). Previous studies by [START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF] provide evidence that, when REE are initially present as REE-humate complexes, a pseudo-equilibrium is attained more slowly after about 10 days (see Fig. 6 in [START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF]. Therefore, a pseudo-steady state is reached more rapidly when REE are introduced as free species in solution.

For a given HA/MnO 2 ratio, REE partition coefficients are higher for aqueous HA than on MnO 2 . This result suggests that REE are more complexed to HA than MnO 2 , whatever the initial concentration of HA in the solution. However, log

Discussion

As previously suggested by [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF]2005), the present data provide evidence that REE organic complexation results in a decrease of the REE adsorption onto

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MnO 2 and a strong limitation of the development of a Ce anomaly at the mineral surface.

However, we point out here two significant differences compared with previous studies: (i) the Ce anomaly develops through time and with decreasing HA/MnO 2 ratio, (ii) some of the REE-HA complexes are unstable, and are dissociated through time in the presence of MnO 2 . To interpret their data, they proposed two hypotheses. Firstly, under their experimental conditions, the REE-HA complex was adsorbed at the MnO 2 or HFO surface by an anionic adsorption mechanism. REE-HA was then absorbed by the humate side of the complex.

Slowdown and reduction in the development of Ce anomalies

Therefore, Ce was not in direct contact with the potential oxidative MnO 2 or HFO surface, so Ce(III) was unlikely to be oxidized into Ce(IV). Secondly, different REE could not behave independently from each other in the organic experiments since they were collectively bound to humate molecules. In this way, the Ce anomaly is unable to develop, simply due to the binding of the entire REE pool to humate. [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF]2005) based their two hypotheses on the fact that log The inorganic Ce required for the development of a Ce anomaly on MnO 2 likely draws its source from dissociation of the organic complex. Moreover, Figs. 2a and 3a indicate a dissociation of the REE-HA complex that increases with increasing HA/MnO 2 ratio. The amount adsorbed from the dissociation of the REE-HA complex is easily accessible if we consider the time when the Ce anomaly appears as the reference time after which readsorption occurs (readsorption of inorganic REE). However, it is impossible to assess the amount of REE bound as inorganic and/or REE-HA complex in the first stage of the experiment.

Consequently, calculations can only yield the proportion of inorganic REE that is adsorbed onto MnO 2 from the dissociation of inorganic REE-HA complexes (Tables 1 and3). For the experiment at HA/MnO 2 = 0.1 and pH 5, Table 1 reports the proportion of Ce bound to each phase of the suspension and the proportion of inorganic Ce supplied by Ce-HA complex dissociation and readsorbed onto MnO 2 . The data show a time-dependent increase of the inorganic Ce bound onto MnO 2 , supplied by dissociation of the organic complex, which represents from 16.2 % to 32.3 % of all the Ce adsorbed onto MnO 2 at 8 h and 72 h, cations and humate contact time, pH of solution and humate initial concentration on the dissociation time. [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF] and [START_REF] Geckeis | Humic colloid-borne natural polyvalent metal ions: Dissociation experiment[END_REF] showed that a longer duration of cation-humate pre-equilibration decreases the complex dissociation. They suggested a complex ripening process where, after an initial sorption step, the cations migrate to stronger binding sites or move to inner sites of the macromolecular structure of the humic molecule. [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF] further showed that long-term reaction between cation and humate may induce molecular conformation changes that can influence the dissociation reaction rate.

Concerning the influence of pH, [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF] demonstrated that complex dissociation falls sharply between pH 5 and 7, in contrast to [START_REF] Monsallier | Kinetic investigation of Eu(III)-humate interactions by ion exchange resins[END_REF] and Wang et al. (2005), who showed an enhancement of the dissociation of Eu-humate and Am-humate complex between pH 3 and 6. [START_REF] Monsallier | Kinetic investigation of Eu(III)-humate interactions by ion exchange resins[END_REF] attributed their result to different humic colloid structures for the different pH values. Finally, [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF] and [START_REF] Monsallier | Kinetic investigation of Eu(III)-humate interactions by ion exchange resins[END_REF] provided evidence that increasing the humic concentration decreases the cationhumate complex dissociation rate. Regarding the influence of these parameters on dissociation, and hence the stability of the cation-humate complex, [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF] inferred that thermodynamic complex stability is more determinant in complex dissociation than nondiffusive processes.

The present experimental set-up may be regarded as similar to that of previous cited studies. HA represents the humic molecule, REE the cation and MnO 2 can be considered as the competitive ligand. The present experiments were designed to study the kinetics of REE binding to HA and MnO 2 , and therefore all reactants were added simultaneously. While a major proportion of the REE appears to be immediately complexed to HA, the complex is not immediately stabilized thermodynamically due to the competitive presence of MnO 2 . REE could be dissociated from the earlier-formed REE-humate complex, and then readsorbed partly onto MnO 2 . Experiments conducted as a function of HA/MnO 2 ratio show that the dissociation-redistribution process increases with decreasing HA/MnO 2 ratio, i.e. with

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ACCEPTED MANUSCRIPT 20 decreasing HA concentration, as demonstrated by [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF] and [START_REF] Monsallier | Kinetic investigation of Eu(III)-humate interactions by ion exchange resins[END_REF]. The log REE/HA d K pattern for the competitive experiment (Fig. 6) and the variation of Gd/Yb ratio (Fig. 7), both represented as function of pH, show that HREE log REE/HA d K decrease is not developed for pH >6. Such decrease is characteristic of HREE dissociation from the REE-HA complex, implying that the dissociation increases up to pH 6 and decreases thereafter. Therefore, this new dataset agrees with the results of [START_REF] Monsallier | Kinetic investigation of Eu(III)-humate interactions by ion exchange resins[END_REF] and Wang et al (2005). These latter authors demonstrated an increase in complex dissociation between pH 3 and 6, while [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF] obtained a dissociation decrease between pH 5 and 7.

The last point concerns the quantitative redistribution of the REE between MnO 2 and inorganic solution. As expected by the dissociation process, the major part (about 98%) of the REE dissociated from REE-HA is readsorbed onto the MnO 2 surface. However, a difference in behaviour appears between LREE and HREE, the latter being more efficiently removed to inorganic solution (Table 2). A possible explanation could be that REE-MnO 2 adsorption constants are higher for LREE than HREE, thus inducing a higher concentration of HREE in solution. In the literature, there are no REE-MnO 2 adsorption constants available for the whole REE series. However, experiments of REE adsorption or coprecipitation with MnO 2 have provided some evidence that HREE are more effectively adsorbed than LREE onto the MnO 2 surface [START_REF] Koeppenkastrop | Sorption of rare-earth elements from seawater to mineral particles. 1. Specific and electrostatic Interactions[END_REF][START_REF] De Carlo | The influence of redox reactions on the uptake of dissolved Ce by suspended Fe and Mn oxide particles[END_REF][START_REF] Bau | Scavenging of dissolved yttrium and rare earths by precipitating iron oxyhydroxide: Experimental evidence for Ce oxidation, Y -Ho fractionation, and lanthanide tetrad effect[END_REF][START_REF] Ohta | REE(III) adsorption onto Mn dioxide ( β-MnO 2 ) and Fe oxyhydroxide: Ce(III) oxidation by ß-MnO 2[END_REF][START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF]. Moreover, [START_REF] Quinn | Sorption of yttrium and rare earth elements by amorphous ferric hydroxide: influence of pH and ionic strength[END_REF] complexant (Wood et al., 1990). In the literature, HREE enrichment in solution has often been interpreted in terms of a preferential carbonate complexation [START_REF] Millero | Stability constants for the formation of rare earth inorganic complexes as a function of ionic strength[END_REF][START_REF] Lee | Complexation of trivalent rare earth elements (Ce, Eu, Gd, TbYb) by carbonate ions[END_REF]Johanesson et al., 1996). However, in the present study, even if each experiment was performed under atmospheric conditions, the pH value of 5 rules out a significant complexation of HREE by carbonate. Wood et al. (1990) calculated the distribution of Eu among the various carbonate and bicarbonate complexes as a function of carbonate activity, proposing that Eu speciation is dominated by the free Eu(III) form at pH 5. Consequently, further investigations are required to explain the distribution of HREE between MnO 2 surface and inorganic solution.

Implications regarding Ce and REE adsorption when organic matter occurs

The results demonstrate that, as previously observed by [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF]2005), the REE adsorption onto MnO 2 decreases with respect to inorganic experiments.

Nevertheless, the adsorption rate is not totally controlled by the REE-HA adsorption.

Experiments performed as a function of pH provide evidence that log 3). At 72 h, a non-negligible proportion of REE is bound in an inorganic form onto MnO 2, at the expense of REE-HA dissociation, i.e. between 13.5 % for La and 32.5 % for Ce, representing 1.9 to 5.7 % of the total REE in suspension, respectively.

Experiments conducted as a function of pH reveal that competition between HA and MnO 2 increases with the pH value. However, even if MnO 2 appears to compete with HA for REE complexation, the major part of the REE remains as a soluble REE-HA complex, i.e. 82.5% at 72h for La, in the experiment conducted at pH= 5 with HA/MnO 2 ratio = 0.01. In the suspension, REE speciation is thus dominated by organic complexed species. It is important to note that the role of soluble REE organic complexation could become less dominant with increasing pH (Fig. 5 b).

However, in the context of ongoing solid alteration processes (in soils, or aquifers)

where REE are released in a soluble inorganic form, the present work provides evidence that REE speciation will be dominated by organic complexation and also partially by solid adsorption. As a function of pH and organic matter/solid surface ratio, an increasing proportion of REE might be sorbed, particularly as ionic species. This latter point is of major importance. The present study implies that the development of a Ce anomaly can be limited but not entirely suppressed in organic-rich solution as suggested previously, according to the pH conditions and organic matter/oxidative surface ratio. Therefore, we could raise the question of whether the HA/MnO 2 ratios used in the experiments are comparable with natural conditions. To address this question, DOC/Mn ratios were calculated from 16 natural soil samples (Alfisols, Aridisols, Inceptisols, Mollisols and Ultisols), using the estimated Mn concentrations (mg.kg -1 ) in Mn oxide form and the DOC concentrations (mg.kg -1 ) provided in [START_REF] Jiang | Adsorption of arsenate on soils. Part 2: Modeling the relationship between adsorption capacity and soil physiochemical properties using 16 Chinese soils[END_REF]. DOC/Mn ratio varies from 0.3 to 11.9. The HA/MnO 2 ratios used in the present experiments are expressed in terms of DOC/Mn ratio, with values varying between 3.6 and 0.2. The mechanism described here could therefore potentially occur in natural

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environments. However, the competition between HA and MnO 2 for REE complexation will be more markedly expressed on the REE patterns for low DOC/Mn ratio and pH from 3 to 6.

A decrease of REE adsorption onto MnO 2 is observed in the presence of organic matter, while the development of a Ce anomaly is effectively limited but not suppressed.

Therefore, the present study leads to the same restriction on the use of the Ce anomaly as a redox proxy as previously stated by [START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF]. However, the new experimental dataset sheds more light on these surface processes, by allowing us to assess the impact of pH Moreover, the increase of log 2 MnO / REE d K with increasing HA/MnO 2 ratio also indicates that REE are partially bound to HA, which is itself adsorbed onto the solid surface. Consequently, the REE are adsorbed onto MnO 2 surfaces as REE(III) and REE-HA complex. The processes described in this study are pH-dependent. At alkaline pH, the REE-HA complex becomes less dominant and dissociation does not occur since no modification is observed in the shape of the REE-HA pattern. The competition for REE complexation between HA and MnO 2 is more intensive at low DOC/Mn ratio and pH from 3 to 6. The present study corroborates [START_REF] Davranche | Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide[END_REF]2005) conclusions regarding the use of Ce anomalies as a redox proxy in paleoenvironments. However, this new dataset highlights the importance of (i) competition between ligands for REE binding and (ii) pH control on REE distribution. When the REE-HA complex is not stabilized, the presence of competitive surface could induce dissociation of the organic complex and a redistribution of the REE between the solid surface and the inorganic solution. This would not only lead to modifications of the solution composition but also the REE pattern in solution, which itself

Tables and figure captions

Table 1 Ce distribution in each phase of the suspension, estimate of C e involved in anomaly development and amount of inorganic Ce adsorbed onto MnO 2 , for the experiment with HA/MnO 2 ratio = 0.1 and pH=5.

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  Purified humate, referred to below as HA (humic acid), was obtained from synthetic Aldrich™ humic acid (Aldrich™, H1, 675-2) following the protocol described byVermeer et al. (1998). The HA sample was freeze-dried and stored in a glass container at room temperature. HA obtained was ash free and in its protonated form, with the following elemental composition (in weight percent): C = 55.8 %, O = 38.9 %, H = 4.6 %, N = 0.6 %.

  MnO 2 (Aldrich) was used in the adsorption experiments. The solid structure was analysed by X-Ray diffraction (XRD) on a Siemens D5000 diffractometer. The principal d spacing indicated a pyrolusite (MnO 2 ) structure. The total surface site number of MnO 2 was estimated using the solid CEC (Cation Exchange Capacity) and determined following the cobaltihexammine method, ISO 11260[START_REF] Afnor | Qualité des Sols[END_REF]. Ions bound with the solid surface are exchanged with cobaltihexammine ions, and the CEC is the concentration of cobaltihexammine ions eliminated from the solution. Five g of solid MnO 2 were mixed with 10 mL of 0.017 M cobaltihexammine solution for 3 h. The suspension was then centrifuged and the concentration of cobaltihexammine ion remaining in solution was measured at 470 nm with an Uvikon Biotek Instruments UV spectrophotometer. The analyses indicated that the MnO 2 used in this study has a CEC of 70 meq/100g.

  2) (Adsorbed REE) a represents the concentration of REE adsorbed onto MnO 2 obtained by substracting REE concentrations left in solution after filtration at 0.2 µm from the initial REE concentration. (Aqueous REE) a corresponds to the REE concentration left in solution after filtration at 0.2 µm. (Adsorbed REE ) b represents the concentration of REE complexed with aqueous HA, and is calculated by substracting the REE concentrations left in solution after A C C E P T E D M A N U S C R I P T filtration at 0.2 µm and the inorganic REE concentration in the 10-kDa ultrafiltrate. (Inorganic aqueous REE) b corresponds to the inorganic REE concentration in the 10-kD ultrafiltrate.

  Impact of HA/MnO 2 ratio on competition between HA and MnO 2 for REE binding. in the supplementary file.)Kinetics. Figure1presents the time-series variation of log adsorbed at solid surface) for La, Gd and Lu as well as log DOC/MnO2 d K for HA/MnO 2 ratios of 0.4, 0.1 and 0.025. Time-series variations of log REE/MnO2 d K

  for the occurrence of a Ce anomaly for HA/MnO 2 ratios of 0.1 and 0.025.[START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF] reported the same type of pattern when REE was initially present as REE-humate complex. They interpreted this pattern shape as reflecting the anionic adsorption of the REE-humate complex onto the MnO 2 surface. As illustrated in Fig.2aand Fig.3, where Ce anomaly variations are expressed as Ce/Ce*, the Ce anomaly increases with decreasing HA/MnO 2 ratio and increasing time. Moreover, the Ce anomaly develops rapidly in the first hour, for experiments conducted with a HA/MnO 2 ratio of 0.025, and after 10h for experiments conducted with ratios of 0.1 and 0.2. Furthermore, at these three HA/MnO 2 ratios, the amplitude of the Ce anomaly is time-dependent. No steady state was reached at 72h. Although the experiment at ratio = 0.2 continued until 288 h (data not shown here), there was no observed stabilization of the Ce-anomaly value (Ce/Ce* = 1.033 at 288 h). Such a result differs markedly from the study by[START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF], who report no development of Ce anomaly, even after 20 days of experiment run-time (Fig.5, in[START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF]). The log REE/HA d K patterns presented in Fig. 2b show a slight increase in LREE log REE/HA d K (light rare earth element) and decrease in HREE log REE/HA d K on each pattern. However, the pattern obtained for HA/MnO 2 = 0.4 is flatter. This type of pattern is different from the patterns for the REE-humate complex obtained by Davranche et al. (2005) and Pourret et al. (2007b). The present patterns exhibit downward-concave and convex shapes in the MREE for the HA fraction and the solution, respectively. The decrease observed for HREE is counterbalanced by the HREE concentration increase in the dissolved inorganic REE pattern. The discrepancy in HREE slopes between the patterns displayed in Figs. 2b and 2c is due to the fact that the REE pattern in HA fraction is expressed in log K d , while the REE pattern in solution is expressed in ppb. In Fig. 4, Gd/Yb ratio is plotted versus time for aHA/MnO 2 ratio of 0.1. The Gd/Yb ratio calculated from log REE/= 1.03 and 1.08 at t = 1 h and 168 h, respectively. On the contrary, the Gd/Yb ratio calculated from the inorganic REE concentration in solution decreases with time; Gd/Yb = 0.6 and 0.27 at t = 1 h and 168 h, respectively. The variation of Gd/Yb ratio with time suggests that the HREE log REE/inorganic solution of part of the HREE complexed to HA .3.2. Impact of pH on competition between HA and MnO 2 for REE adsorption (HA/MnO 2 ratio = 0.1 and t = 48 h)Experimental data are illustrated in Fig.5for four REE (La, Ce, Gd and Lu). of pH, without HA in solution. REE adsorption ontoMnO 2 is strongly dependent on pH with, for example, when pH increases from 3 to 8. De Carlo et al. (1998) and[START_REF] Ohta | REE(III) adsorption onto Mn dioxide ( β-MnO 2 ) and Fe oxyhydroxide: Ce(III) oxidation by ß-MnO 2[END_REF] observed the same pH dependence. As can be seen, Ce is more strongly adsorbed than the other REE. Figure5billustrates the 5.88 and 3.67 at pH 8 without and with HA, respectively.Whatever the pH is, a major part of each REE is complexed with HA; e.g-dependent, with REE being more easily complexed to HA at alkaline than at acidic pH. However, we can observe a slight decrease in the log REE/respectively. This decrease develops along with an increase in the adsorption of REE onto MnO 2 (Fig.6b). Several studies on the adsorption of organic matter onto solid surface have shown that the value of log 2 DOC/MnO d K decreases with increasing pH[START_REF] Gu | Adsorption and desorption of the natural organic matter on iron oxide: Mechanisms and models[END_REF][START_REF] Langmuir | Aqueous Environmental Geochemistry[END_REF][START_REF] Avena | Kinetics of Humic Acids Adsorption at Solid-Water Interfaces[END_REF]. As a result, and in contrast to the experiment performed at pH 5, the ratio of the pH range concerned indicating that REE and HA are not entirely bound to each other when adsorbed onto the MnO 2 surface. Figure6presents the patterns of anomaly that decreases with increasing pH (Fig.6a). Log REE/HA d K patterns exhibit two types of shape, (i) a relatively flat shape and (ii) a shape characterized by HREE Log REE/HA d K value decrease, such as developed in the log REE/HA d K pattern obtained at pH 5 for the various HA/MnO 2 ratios (Figs. 2b and 6b).Fig. 7 illustrates the evolution of the Gd/Yb ratio relative to pH for the competitive complexation experiment. The evolution of Gd/Yb ratio on HA reflects the HREE Log REE/HA d K value decrease. At low pH, when low amounts of REE are complexed by HA, the ratio is equal to ~1, namely, no HREE Log REE/HA d K decrease occurs. From pH 4 to 5, the ratio increases to an average value of 1.045, and decrease takes place. At higher pH, the value of Gd/Yb ratio decreases, while the shape of the log REE/HA d K pattern becomes flat once again. These results suggest that HREE Log

Figures 2a and 3

 3 Figures 2a and 3 provide evidence that the development of a Ce anomaly at the MnO 2

  suggesting that REE and HA remain bound to each other during adsorption of the REE-HA complexes onto the MnO 2 and HFO surfaces. However, the new experimental dataset presented here provides a distinct interpretive framework. Even if the Ce anomaly is greatly A C C E P T E D M A N U S C R I P T attenuated compared with inorganic experiments (Davranche et al., 2005), a slight Ce anomaly is developed whose amplitude increases with time and decreasing HA/MnO 2 ratio (Fig. 3). Moreover, competitive experiments conducted as a function of pH demonstrate different relative to the pH and for low HA/MnO 2 ratios as well. Therefore, REE and HA are not entirely bound to each other when adsorbed onto the MnO 2 surface (Fig. 6b). Linked with the time-dependent development of the Ce anomaly (Fig. 3), this observation suggests that (i) part of Ce adsorbed onto MnO 2 is directly in contact with the surface and is therefore oxidized by MnO 2 and (ii) there is a time-dependent 'source' of Ce in the suspension, which becomes oxidized in Ce(IV). As shown above in Fig.1, log REE/HA d K decreases with time and decreasing HA/MnO 2 ratio. This decrease indicates a dissociation of the REE-HA complex with time.

A

  recently provided a data-set of REE-HFO complexation constants, showing that log REE HFO ß increases with the REE atomic number. In view of the above considerations, some other explanations are required. For instance, HREE could be complexed with an inorganic ligand that allows its stabilisation in the inorganic solution. The experimental conditions in our study involve very few possible ligands, in fact only Cl -and CO 3 2-. The stability constants of REE-Cl 2+ values with increasing atomic number of the REE, thus excluding the role of Cl -as a stronger HREE

  of large Ce anomalies with increasing HA/MnO 2 ratio also implies that part of the REE occurs as REE-HA complexes themselves adsorbed onto the solid surface. Thus, REE are not only sorbed onto MnO 2 as inorganic species but also as REE-HA complexes. We performed calculations to estimate the amount of REE bound onto MnO 2 as REE-HA and as inorganic species in the experiment at pH 5 with

  and competition between complexation reactions. The formation and dissociation of REE-HA complexes are indeed both pH-dependent. Increasing the pH decreases the concentration of REE-HA complexes in solution and favours the sorption reaction onto MnO 2 . Indeed, the dissociation of REE-HA complexes is at a maximum in the 3 to 6 pH range, implying that solid surfaces behave as a competitive ligand especially over this pH range. Furthermore, MnO 2 and HA competition for REE binding not only influences the REE distribution between the solution and the relevant solid but also the kinetics of the adsorption and dissociation reactions, and hence the development of a Ce anomaly. As a result, in the experiment carried out for 288 h (results not shown here), the amplitude of the Ce anomaly never reached a steady state, meaning that REE-HA dissociation and inorganic REE readsorption onto MnO 2 were still occurring at that time. This last point raises questions on the impact of such mechanisms in the context of a long-term rock or mineral alteration. What will be the amplitude of the Ce anomaly? Will the amount of adsorbed REE at the solid surface always remain negligible compared to the amount of REE complexed with organic matter? Finally, the present dataset provides evidence that complexation mechanisms are involved depending on the initial REE chemical species present. the kinetic competition between HA and MnO 2 for REE complexation were carried out using a combination of filtration/ultrafiltration techniques to study the impact on Ce anomaly development and REE fractionation. REE speciation is dominated by the REE-HA complex. Accordingly, in contrast with inorganic conditions, there is a decrease in the amount of REE adsorbed by MnO 2 and the development of a Ce anomaly is strongly limited as previously shown by[START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 : Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF]. Nevertheless, for the tested HA/MnO 2 ratios below 0.4, the present data show that MnO 2 acts as a competitor able to induce a slight dissociation of the REE-HA complex. Most of the dissociated REE is subsequently readsorbed onto the MnO 2 surface, and this readsorption is recorded by an increase in development of the Ce anomaly, despite the overall REE organic speciation.

  Ce-anomaly. Moreover, the competition between HA and MnO 2 involves a drastic slowdown of the reaction kinetics, inducing at long-term modification of the REE distribution as well as an increase in the amplitude of the Ce-anomaly. The process described above should play a major role when REE are released in the form of inorganic REE(III) species, notably during rock alteration/weathering. When present in free ionic form, REE are available for complexation with each ligand occurring in solution. As shown in this study, the kinetics of the complexation reactions and the stability of the complexes so formed will impose the REE speciation at equilibrium.

Fig

  Fig. 1. Time variation of log REE d K on MnO 2 and aqueous HA (not adsorbed on MnO 2 ) and

Fig. 4 .

 4 Fig. 4. Gd/Yb ratio versus time calculated from log REE/HA d K and soluble inorganic REE

  Fig.1

Fig. 2 A

 2 Fig.2

  Titrations were carried out with a Metrohm 794 DMS

	Titrino apparatus equipped with a Metrohm combined (3 M KCl) glass electrode. Measured
	acidity constants (pK a1 = 7.89 and pK a2 = 3.65) yielded a pH zpc equal to 5.8, a value that falls
	within the reported pH zpc range for MnO 2 (i.e. 4.5 to 7.8; Langmuir, 1997).
	2.3. Adsorption/Complexation procedures
	Inorganic REE adsorption onto MnO 2 . These series of experiments were carried out
	to provide the inorganic reference baseline necessary to interpret the data obtained during the
	experiments with organic matter. REE adsorption onto MnO 2 was carried out in batch reactor,
	in which 100 mL of 10 -3 M NaCl solution were prepared with 5 ppb of each REE (e.g., 36
	nmol.L -1 La to 28.6 nmol.L -1 Lu) and 100 mg.L -1 of MnO 2 . The pH was adjusted to the
	experimental value used, in the pH range from 3 to 8, by adding 0.1N HNO 3 or 0.1N NaOH.
	Experimental solutions were stirred for 48 h (the equilibrium time was determined from
	preliminary kinetic experiments) to allow equilibration and partitioning of REE between the
	aqueous solution and MnO 2 . Suspension aliquots of about 10 mL were sampled at steady state
	and filtered through 0.2 µm cellulose acetate filters (Sartorius). The REE concentration of the
	filtrate was analysed to determine the concentration of REE in solution. The amount of REE
	adsorbed onto MnO 2 was obtained by subtracting the REE concentration left in solution from
	the initial REE concentration taking into account sampling-induced volume variations as
	previously established elsewhere

Table 2

 2 Proportions of (i) REE dissociated from REE-HA complex, (ii) REE reabsorbed onto MnO 2 and (iii) REE removed to t he inorganic solution. Percentages are calculated for the competition experiment with HA/MnO 2 ratio = 0.025 between 1 and 72 h.

Table 3

 3 Proportions of REE adsorbed onto MnO 2 irrespective of their chemical form, as well as proportions of inorganic REE adsorbed onto MnO 2 and inorganic REE readsorbed onto

	MnO 2 , with respect to the total amount of REE resulting from dissociation of the REE-HA
	complex. Calculations are presented for the competition experiment with HA/MnO 2 ratio =
	0.1, pH = 5 and t = 72 h.

  Ce involved in development of the Ce anomaly, calculated with respect to % Ce bound to MnO 2 at 3 h, just prior to development of the Ce anomaly and Ce-HA inorganic REE adsorbed onto MnO 2, calculated with respect to percent of REE bound to MnO 2 at 3 h, just prior to development of Ce anomaly and REE-HA dissociation,

			ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT
	Time (h) 1 3 8 24 48 72 dissociation.( Ce adsorbed onto MnO 2 * (%) 8.1 REE dissociated from Ce bound to aqueous HA (%) 91.6 REE-HA complex/ REE total concentration dissociated from REE-HA Inorganic Ce in solution (%) readsorbed onto MnO 2 inorganic Ce from Ce-HA dissociation/total adsorbed Ce** (%) 0.3 -REE readsorbed onto MnO 2 / REE REE removed Ce involved in Ce anomaly development/total Ce concentration** (%) to inorganic solution/ REE dissociated from REE-HA REE bound to MnO 2 (%) Inorganic REE readsorbed Inorganic REE adsorbed onto from the REE-HA complex MnO 2 from the REE-HA complex dissociation/ REE total dissociation/ total adsorbed REE* concentration (%) (%) -11.8 88.0 0.2 --14.1 85.9 0.5 16.2 2.3 14.3 85.6 0.1 17.4 2.5 16.5 83.4 0.1 28.6 4.7 17.4 82.5 0.1 32.3 5.6 [ ] [ ] [ ] [ ] 100 * Ce or Ce Ce Ce % total MnO onto adorbed MnO onto adsorbed 3 t MnO onto adsorbed x t 2 2 2 = = -= ) Table 1 A C C E P T E D M A N U S C R I P T (%) (%) (%) La 7.9 98.9 1.1 Ce 11.3 99.1 0.9 Pr 8.4 99.0 1.0 Nd 7.4 98.1 1.9 Sm 8.6 99.1 0.9 Eu 8.6 99.0 1.0 Gd 6.8 98.9 1.1 Tb 7.5 98.8 1.2 Dy 7.0 98.2 1.8 Ho 6.8 98.6 1.3 Er 6.8 97.5 2.4 Tm 6.8 96.9 3.0 Yb 6.9 96.2 3.8 Lu 7.0 95.8 4.2 La 13.9 13.5 1.9 Ce 17.7 32.3 5.7 Pr 14.6 22.4 3.3 Nd 14.7 17.5 2.6 Sm 13.1 17.4 2.3 Eu 12.8 25.5 3.3 Gd 13.5 25.9 3.5 Tb 12.0 20.5 2.5 M A N U S C R I P T Dy 10.7 19.3 2.1 Ho 11.7 21.1 2.5 Er 11.8 16.8 2.0 Tm 11.9 17.0 2.0 Yb 12.0 13.8 1.7 A C C E P T E D Lu 12.1 13.6 1.6
	Table 2
	(	%	[ ] adsorbed [ ] adorbed REE MnO onto onto REE x t	[ ] or MnO REE adsorbed [ ] REE total onto 3 t	MnO	*	100
					34 35

* : Ce bound as inorganic species and/or Ce-HA complexes ** : % * : percent of

Acknowledgements. We thank the technical staff at Rennes (M. Le Coz-Bouhnik, O. Hénin and P. Petitjean) for their assistance during the experimental and analytical work Dr Michael Carpenter is acknowledged post-edited the English style. This research was supported by the CPER programmes "Développement de la Recherche sur la Maîtrise de la Qualité de l'Eau en Bretagne" funded by the Council of Rennes Métropole.

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M A N U S C R I P T

ACCEPTED MANUSCRIPT

18 respectively. At 72 h, this proportion corresponds to 5.6 % of the total Ce present in the suspension.

REE-HA complex dissociation

Another major result of the present study is the evidence of a dissociation of the REE-HA complex with time and with decreasing HA/MnO 2 ratio. Table 2 presents the proportion of REE dissociated from REE-HA complex, the proportion of REE dissociated from REE-HA complex and reabsorbed onto MnO 2 , as well as the proportion of REE dissociated from REE-HA complex and removed to the inorganic solution during the experiment with HA/MnO 2 = 0.025. These data imply that most of the dissociated REE is readsorbed onto the MnO 2 surface. LREE exhibit higher readsorption than HREE: 98.9% for La as against 95.8% for Lu.

By contrast, in the inorganic solution, the LREE make up a lower proportion than HREE: 1.1 % for La as against 4.2% for Lu. This contrasted behaviour of HREE and LREE is recorded in the shapes of the log REE d K and dissolved inorganic REE patterns (Fig. 1).

Several authors have proposed that a cation-humate (natural or synthetic) complex could be dissociated in the presence of a competitive ligand [START_REF] Rate | Response in copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation[END_REF][START_REF] Geckeis | Humic colloid-borne natural polyvalent metal ions: Dissociation experiment[END_REF][START_REF] Artinger | A kinetic sudy of Am(III)/humic colloid interactions[END_REF][START_REF] Monsallier | Kinetic investigation of Eu(III)-humate interactions by ion exchange resins[END_REF]Wang et al., 2005). Generally, the competitors used in these studies were chelating resins (Chelex 100 Biorad), ligands (metallochromic ligands, PAR) or cation exchanger resin (Dowex 50, HYPHAN). All these authors have established that humate complex dissociation is time-dependent, showing that dissociation of the cation-humate complex decreases with increasing reaction time. By studying the dissociation of an Eu-humate and Am-humate complex in the presence of a resin,