Rémi M Marsac
email: remi.marsac@univ-rennes1.fr

Nidhu L Banik

Johannes L Lützenkirchen

Charlotte Catrouillet

Christian M Marquardt

Karen H Johannesson

Modeling metal ion-humic substances complexation in highly saline conditions

Keywords: humic, radionuclide, complexation, brine, saline, Model VII, NICA-Donnan, specific ion interaction theory

published or not. The documents may come

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3 Abstract. Because highly saline groundwaters are found at potential repository sites for nuclear waste, geochemical models should predict the speciation of relevant radionuclides in brines, including their complexation with substances such as humic acids (HA). In this study, available experimental radionuclide-HA complexation data in high 1:1 background electrolyte solutions (0.01 < / / < 4 molal, m) are reviewed. Discrepancies in the amplitude of ionic strength effects on radionuclide-HA complexation are observed, which might depend on the nature of the interacting radionuclide or on the origin of HA. However, significant differences in the experimental conditions and calculations applied to determine conditional metal ion-HA complexation constants hamper direct comparison between these datasets. To clarify whether metal ion-HA binding in saline solutions can be described, two sophisticated humic-ion binding models (Model VII and NICA-Donnan) are presently used. This is the first time that Model VII and NICA-Donnan are applied to predict metal ion-HA binding at high ionic strength (I > 1 m).

The advantage of these models, compared to more simple ones (e.g., the polyelectrolyte or the charge neutralization models), is that both electrostatic and chemical contributions to the overall metal ion-HA binding are explicitly taken into account. Model VII and NICA-Donnan are shown to produce very similar results. Trends in conditional metal ion-HA binding constants and in the maximum metal ion uptake by HA (e.g., the loading capacity) with I agree with experiments. The present data evaluation suggests that most of the apparent discrepancies between various experimental datasets arise from differences in the experimental conditions. Both Model VII and NICA-Donnan predict that the specific ion interaction theory (SIT) parameters for metal ion-HA systems, which are required for high ionic strength with more simple models, vary with pH and metal loading. Overall, Model VII and NICA-Donnan are able to account for various mechanisms involved in metal ion-HA complexation, including the metal loading effects and cation

Introduction

Humic substances (HS) such as humic (HA) and fulvic (FA) acids are ubiquitous in natural waters and form complexes with dissolved metal ions. They play a crucial role for metal ion mobility and bioavailability in the environment. HS exhibit extreme complexity. The major HS cation-binding groups are the carboxylic and phenolic groups [START_REF] Ritchie | Proton-binding study of standard and reference fulvic acids, humic acids, and natural organic matter[END_REF], but less abundant softer Lewis bases (e.g., N-and S-containing groups) also contribute to cation-HS complexation [START_REF] Tipping | Humic ion-binding model VI: an improved description of the interactions of protons and metal ions with humic substances[END_REF][START_REF] Hesterberg | Bonding of Hg(II) to reduced organic, sulfur in humic acid as affected by S/Hg ratio[END_REF]. HS are macro-ions and electrostatic effects are relevant for their complexation properties. Moreover, several HS groups may bind a single cation, which either leads to a chelation effect [START_REF] Martell | Metal Complexes in Aqueous Solutions[END_REF] or to the formation of a cation bridge between different organic molecules (e.g., Kunhi [START_REF] Mouvenchery | Cation-mediated cross-linking in natural organic matter: a review[END_REF]. Hence, metal ions can form a large variety of complexes with HS, leading to apparent complexation constants that depend on pH, ionic strength and metal ion/HS concentration ratio (i.e., the metal loading) including the presence of competing cations like Ca and Mg. Substantial efforts have been made to determine thermodynamic metal ion-HS complexation constants and to develop predictive models for environmentally relevant conditions (e.g., [START_REF] Kim | Complexation of metal ions with humic acid: metal ion charge neutralisation model[END_REF][START_REF] Benedetti | Metal-ion binding to humic substances: application of the nonideal competitive adsorption model[END_REF][START_REF] Tipping | Humic ion-binding model VI: an improved description of the interactions of protons and metal ions with humic substances[END_REF][START_REF] Milne | Generic NICA-Donnan model parameters for proton binding by humic substances[END_REF]2003;[START_REF] Sasaki | Discrete fragment model for complex formation of europium(III) with humic acid[END_REF].

Because of the complexity of HS, very different approaches have been proposed to describe the reaction between HS and metal ions.

The resulting metal ion-HS complexation models were developed for, and mainly applied to describe metal ion speciation under freshwater conditions, but were shown to be applicable for the more saline conditions that occur in estuaries and seawaters (e.g., [START_REF] Hiemstra | Biogeochemical speciation of Fe in ocean water[END_REF]Turner et al., 2008;[START_REF] Stockdale | Trace metals in the open oceans: speciation modelling based on humic type ligands[END_REF]. Nonetheless, few studies have investigated metal ion-HS interactions under highly saline conditions, such as ionic strength (I) exceeding that of seawater (i.e., I > 0.7 m). The latter conditions are relevant with regard to the Germany may contain salt concentrations as high as about 4 M (Mühlenberg et al., 1997).

Sedimentary rocks currently investigated in Canada are in contact with brine solutions up to 6.5

M [START_REF] Fritz | Saline Groundwaters in the Canadian shield -a first overview[END_REF]. Although high ionic strength commonly leads to the coagulation of HS, this process has been shown to be incomplete in many cases and non-negligible amounts of dissolved HS were reported to persist (Wall and Choppin, 2003), which can react with dissolved metal ions.

Most of the radionuclide-HS complexation studies in saline solutions focused on HA.

Because few data for FA exist, only HA binding properties at high ionic strength are discussed herein. Generally, at constant pH, the apparent radionuclide-HA complexation constants in monovalent background electrolyte solutions (e.g., NaCl or NaClO 4 ) decrease with increasing ionic strength from very dilute aqueous solutions up to 1 molal (mol kg -1 , hereafter denoted m).

However, different binding behaviors are observed for I > 1 m. Specifically, UO 2 2+ -and Pu 4+ -HA complexation constants increase with increasing ionic strength [START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF][START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF], whereas Co 2+ -HA complexation does not vary substantially, and Ni 2+ -HA complexation slightly decreases [START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF] with increasing ionic strength.

Although data from [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF] and Wall et al. (2002) consistently show increasing Am 3+ /Cm 3+ -HA binding with increasing I, [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF] observed minor variation in complexation constant values with I. This led them to propose an average value (with standard variation of ± 0.14 log units) for the entire range of I investigated, which contrasts with Wall et al. (2002) where much larger variations were reported (about 3 log units). Furthermore, for 1 ≤ I ≤ 3.5 m, the maximum amount of radionuclide that is experimentally found to bind to HA (e.g., the so called loading capacity in the charge neutralization model) was shown to decrease with increasing I for trivalent actinides [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF], whereas it remained constant in the case of Pu(IV) [START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF]. All these discrepancies might be indicative of conformational changes in saline solutions, which would depend on the nature of the interacting radionuclide as well as on the origin of HA. However, significant differences exist in the experimental conditions and calculations applied to determine conditional metal ion-HA complexation constants between these different studies, which complicates data comparison. Some studies were conducted in noncomplexing background electrolyte solutions (NaClO 4 ; [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF][START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF]. High [Cl -] is environmentally relevant (as opposed to high [ClO 4 -]), but metal ions can bind Cl -, which may affect the determination of radionuclide-HA complexation constants.

Complexation data for UO 2 2+ -and Am 3+ -HA binding from [START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF]Wall et al. (2002) were obtained in acetate buffers under ambient (air) atmosphere. Like many ligands, acetate and carbonate form aqueous complexes with UO 2 2+ and Am 3+ and can compete with HA.

Experiments were conducted at different radionuclide-to-HA concentration ratios, which might also affect HA charge and conformation. Finally, pH measurement is non-trivial in saline solutions. More specifically, "constant pH values" can refer to constant proton activity (pH =log ; [START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF], proton molality (pH m = -log ; [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF][START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF]Wall et al., 2002), or constant experimental values, as read on the pHmeter (pH exp ; [START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF] where the pH exp -pH m relationship is provided). Deviation between pH, pH m and pH exp as affected by I may also hamper comparison between different datasets.

Available radionuclide-HA complexation data have been analyzed using relatively simple models such as the Polyelectrolyte Model (PM; [START_REF] Torres | Europium(III) and Americium(III) stability constants with Humic acid[END_REF] or the Charge Neutralization Model (CNM; [START_REF] Kim | Complexation of metal ions with humic acid: metal ion charge neutralisation model[END_REF]. These models can be conveniently included in the speciation codes used for performance safety assessment. Within these models,

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ACCEPTED MANUSCRIPT 8 binding parameters may vary with pH and I. High salt levels require an appropriate treatment of activity coefficients for aqueous species in geochemical models, such as application of specific ion interaction theory (SIT; [START_REF] Ciavatta | The specific interaction theory in the evaluating ionic equilibria[END_REF]. Metal-HA complexation constants can also be extrapolated to I = 0 using SIT, when considering HA as a solute. However, due to HA complexity, SIT parameters are no more than adjustable parameters and their values have no clear physical significance according to the original authors [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF][START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF]. Given the differences in the experimental conditions between previous radionuclide-HA complexation studies and the different calculations applied for the determination of metal-HA complexation constants (e.g., with the CNM or the PM), it is difficult to evaluate how SIT parameters would evolve with changing metal ion concentrations and physico-chemical conditions.

More sophisticated models exist, such as the humic ion binding Model VII [START_REF] Tipping | Humic ion-binding Model VII: a revised parameterisation of cation-binding by humic substances[END_REF], or its previous versions (Models V/VI: [START_REF] Tipping | A unifying model of cation binding by humic substances[END_REF][START_REF] Tipping | Humic ion-binding model VI: an improved description of the interactions of protons and metal ions with humic substances[END_REF], and the NICA-Donnan model [START_REF] Kinniburgh | Metal ion binding by humic acid: application of the NICA-Donnan model[END_REF][START_REF] Koopal | Ion binding to natural organic matter: General considerations and the NICA-Donnan model[END_REF]. The description of HA properties relies on several assumptions, and the various models include a more or less detailed description of metal ion-HA interaction. In particular, the electrostatic and chemical contributions to the overall metal ion-HA binding are separated in the models. This is a major advantage for understanding metal ion-HA binding in saline solutions, because ionic strength is expected to more strongly affect electrostatic than chemical binding properties of HA. Unfortunately, as pointed out by [START_REF] Tipping | Humic ion-binding model VI: an improved description of the interactions of protons and metal ions with humic substances[END_REF], the electrostatic approach included in Models V/VI/VII is unlikely to be applicable for I > 1 m. In contrast, NICA-Donnan equations seem to be applicable in highly saline conditions (up to 2 M), as shown for HA proton titration data [START_REF] Benedetti | Humic substances considered as a heterogeneous Donnan gel phase[END_REF], but, to our knowledge, it has never been tested for metal ion-HA complexation data. Finally, the impact of the physico-chemical conditions on the experimental determination of SIT parameters for more simple cation-HA models is discussed.

Theoretical background

Aqueous speciation calculations and codes

Ionic strength (I) affects the activity of dissolved ions in solution, which must be accounted for, for instance, when extrapolating formation constants to hypothetical infinite dilution condition (i.e., for I = 0 m). In the present study, activity coefficients (γ) are calculated according to the specific ion interaction theory (SIT; [START_REF] Ciavatta | The specific interaction theory in the evaluating ionic equilibria[END_REF]. SIT is generally considered valid for ionic strengths up to 3 -4 m. At 25 °C, activity coefficients for an aqueous species i with a charge z i are calculated as follows:

log = -² 0.509 × √ 1 + 1.5√ + ! "#$, &' × ( ( = -²) + ! "#$, &' × ( ( #1'
where D is the Debye-Hückel term used in SIT, m k is the molality of the aqueous species k (mol kg -1 ), and ε(i,k) is the specific ion interaction coefficient between species i and k (kg mol -1 ).

In the present study, we use PHREEQC (version 2; [START_REF] Parkhurst | User's guide to PHREEQC (Version 2) -a computer program for speciation, batch reaction, one-dimensional transport and inverse geochemical calculation[END_REF] anion, is avoided in the models by defining perchlorate as a master species. The Pitzer approach [START_REF] Pitzer | Ion interaction approach: theory and data correlation[END_REF] can be applied to calculate activity coefficients for aqueous ions in even more concentrated media than appropriate for SIT, which would be more relevant for brine solutions.

However, in the present study, the SIT was chosen: (i) for the sake of simplicity, as it is approximately equivalent to a simplified Pitzer model (Grenthe et al., 1993); (ii) because most of the thermodynamic data are taken from the NEA database [START_REF] Guillaumont | Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium[END_REF]

Determination of empirical metal ion-HA complexation constants

Empirical models consider that the metal ion binds to one generic HA site. Non-specific metal ion-HA interactions are not considered explicitly. The general complexation reaction of a cation M with HA can be written as follows:

+ + ,-= +,- #2'
and the corresponding conditional stability constant is

/ 0 = [+,-] [+] 3 [,-] 3 #3'
Various approaches can be found in the literature to determine HA β values. Therefore, it is not possible to directly compare HA β values reported in separate M-HA complexation studies unless the same calculations were made or appropriate corrections are applied. The differences arise from the various possibilities for defining [M] f or [HA] f in eq. 3. For example, [M] f may refer to (i) the total dissolved metal ion concentration at equilibrium (noted [M] tot,f in the following), which includes complexes of M with any ligand except HA (e.g. OH -, Cl -, CO 3 2-or acetate when used as a pH buffer) or (ii) the "free" aquo-ion only, [M z+ ] f . In the remainder of the text, M-HA complexation constants referring to the total dissolved metal ion concentration and to the aquoion will be denoted HA β(M) and HA β(M z+ ), respectively. [M z+ ] f can be calculated by dividing the [M] tot,f value by the side reaction coefficient (5 6 ; [START_REF] Ringböm | Complexation in Analytical Chemistry[END_REF]: The definition of [HA] f (eq. 3) also depends on the humic-ion binding model considered.

5 6 = 1 + ! / *
In the present study, the notation of [START_REF] Marquardt | Complexation of Np(V) with fulvic acid[END_REF] is used for the different HA β (namely, HA K, HA β LC or HA β α , as defined below). HA site density, commonly corresponding to the proton exchange capacity of HA (PEC, in eq g(HA) -1 ), can be taken into account to determine the total HA site concentration ([HA] tot , in eq kg(H 2 O) -1 ). The corresponding M-HA complexation constant, denoted HA K, can be calculated considering that:

[,-] 3 = [,-] <=< -[+,-] #5'
where HA K varies with pH, ionic strength, metal loading, etc.

Competition between the metal ion and H + can be included via the degree of deprotonation (5 0 ) in order to suppress the dependence of the M-HA complexation constant on pH:

[,-] 3 = [,-] <=< × 5 0 -[+,-]. #6'
The concomitant complexation constant is noted HA β α and refers to the Polyelectrolyte Model (PM; [START_REF] Torres | Europium(III) and Americium(III) stability constants with Humic acid[END_REF]. The degree of deprotonation, 5 0 , is experimentally determined by proton titration and depends on both pH and ionic strength.

Another approach is to consider the effective amount of binding sites at a given pH and I.

Here, the aim is to suppress the dependence of the M-HA complexation constant on both pH and HA site saturation at high metal loading. In a Pu(IV)-HA study, Szabó et al. ( 2010) determined the maximum complexing capacity (B max , in mol g -1 ) of HA immobilized on silica gel (19.9 mg HA g -1 ), associated with a complexation constant denoted HA β(Pu 4+ ):

[,-] 3 = ? @AB -[CD,-] #7' M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 13
where B max and HA β(Pu 4+ ) are determined by analyzing SiO 2 -HA-Pu binding isotherms with a

Langmuir-type equation.

The effective amount of binding considered in the Charge Neutralization Model (CNM; [START_REF] Kim | Complexation of metal ions with humic acid: metal ion charge neutralisation model[END_REF] is the "loading capacity" (LC, generally in mol g -1 ). For this model, it is assumed that the total amount of HA sites that is available to neutralize the metal depends on the metal charge (z). In the CNM, the free HA site concentration is defined as:

[,-] 3 = [,-] <=< × FG/ -[+,-]. #8'
The corresponding M-HA complexation constant is noted HA β LC .

Humic-ion binding models NICA-Donnan and Model VII

Chemical part of the models

NICA-Donnan and Model VII have been described in several publications (e.g., [START_REF] Koopal | Ion binding to natural organic matter: General considerations and the NICA-Donnan model[END_REF][START_REF] Tipping | Humic ion-binding Model VII: a revised parameterisation of cation-binding by humic substances[END_REF]. Because the effect of the ionic strength is taken into account through the electrostatic part of these models, the chemical part is only briefly discussed here.

Cation-HA complexation (including H + ) in NICA-Donnan and Model VII follows identical reaction equations (eq. 2). Additional equations are used to account for HA heterogeneity. NICA-Donnan describes HA heterogeneity by a continuous affinity distribution for the interaction between a cation and HA, whereas Model VII considers a large number of binding sites with different but discrete affinities for the cation. The chemical cation-HA binding parts of these models aim at describing the overall cation-HA complexation (e.g., expressed as a log HA K value) as a function of pH, cation to HA concentration ratio (or "metal loading"), and take into account cation competition with a limited number of parameters. The latter parameters are "intrinsic" because they do not vary with the physico-chemical conditions. However, parameters usually vary with the type of HA, including origin or composition. Generic parameters for a wide range

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ACCEPTED MANUSCRIPT 14 of metal ions and HAs were determined for NICA-Donnan [START_REF] Milne | Generic NICA-Donnan model parameters for proton binding by humic substances[END_REF]2003) and Model VII [START_REF] Tipping | Humic ion-binding Model VII: a revised parameterisation of cation-binding by humic substances[END_REF] by fitting experimental datasets. These generic parameters capture "average" HA behavior, and will be used as such in this study. Consequently, the present work

does not aim at discussing the capability of the generic parameters to precisely simulate a given dataset (i.e., no parameter optimization is made). Rather the capability of NICA-Donnan and

Model VII to predict variations in apparent cation-HA binding constants with the ionic strength will be discussed (e.g., trends in log HA K versus I).

It is important to note that, in NICA-Donnan and Model VII, Na + is not considered to bind specifically to HA unlike Ca 2+ or Mg 2+ , which complex with HA in solution. Therefore, for high 1:1 Na-containing background electrolyte solution (e.g. NaCl/NO 3 /ClO 4 ), Na + is considered to control the ionic strength and to affect other metal ion complexation by HA only via electrostatic effects.

Electrostatic models

HAs are large and negatively charged polyelectrolytes. This leads to an accumulation of cations in the vicinity of HA binding sites. Electrostatic models aim at converting the dissolved cation concentration in the bulk solution (

[C] i ) to a local dissolved cation concentration ([C] loc,i )
that occurs adjacent to the HA site. To accomplish this, the electrostatic potential of HA particles (Ψ, in V) is computed using a Boltzmann factor:

[G] 9=I, = [G] × exp #-MN/OP' (9)
Here, z i is the charge of the cation, T is absolute temperature, F is the Faraday constant and R is the gas constant.

Within the NICA-Donnan framework, HAs are considered as permeable spheres.

Counter-ions are accumulated in a Donnan phase. The electrostatic potential (Ψ D ) is constant

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15 inside the Donnan volume and equals zero outside (i.e., in the bulk solution). The Donnan volume (V D , in L kg(HA) -1 ) is calculated as follows:

log Q R = S#1 -log ' -1 (10)
The parameter b is adjusted by fitting acid-base titration experiments at varying I and depends on the type of HA (origin, composition, etc). In the Donnan volume, the negative charge of HA (Q) is neutralized by counterions:

T U V + ∑ X[G] R, -[G] Y = 0 (11)
where C D,i and C i refer to the concentration of the ion i with a charge z i in the Donnan phase and in the bulk solution, respectively. The activity of the ion in the Donnan phase is required for calculation of the specific binding.

Within the Model VII framework, HA molecules are considered as impermeable spheres.

Since the conceptualization of HA particles and the numerical treatment of the electrostatic effects resemble a surface complexation model (e.g., see the present implementation of Model VII in PHREEQC in the following section), we will denote the electrostatic potential Ψ 0 , similarly to the surface potential of minerals. The electrostatic correction is an empirical equation that mimics the Boltzmann factor:

exp #-MN Z /#OP'' = exp #-2C[\]^# '' ( 12
)
where I is the ionic strength (mol L -1 ), P is an adjustable parameter (generally -400 < P < -100 for HA) and Q is the net humic acid charge (eq g -1 ). In NaCl/NO 3 /ClO 4 background electrolytes, the molality and molarity scales do not significantly differ for I ≤ 1 M (or m). The molality scale is used to extrapolate the model to high I. A Donnan model is also used in the original version of Model VII, but it only accounts for counter-ion accumulation (i.e., to calculate the amount of ion bound to HA in a non-specific manner) and has no effect on electrostatics, in contrast to the NICA-Donnan model. Because in Model VII, HA is considered to be an impermeable sphere of

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16 radius r, the Donnan volume is a layer at the surface of the sphere, with a thickness equal to the Debye length (κ -1 = (3.29×10 9 ×I 1/2 ) -1 ; in meters, I in mol L -1 and at 25°C). With the molar mass and the radius of HA (15000 g mol -1 ; 1.72 nm) inherent to Model VII, the surface area of HA (A HA ) equals 1500 m² g -1 . Hence, the Donnan volume in Model VII equals A HA ×κ -1 (in m 3 g -1 ).

Unfortunately, the "-donnan" keyword cannot be used with SIT and Pitzer options in PHREEQC.

It can only be used with other databases, which do not involve SIT or Pitzer models (e.g. which use Davies or Debye-Hückel equations to calculate activity coefficients). However, during preliminary tests, we found very little effect of these calculations on the overall cation-HA binding (see Fig. S2), especially at high I where the Donnan volume drastically shrinks.

Therefore, this option is not used in the present study, that is, counter-ion accumulation is neglected.

Implementation of Model VII in PHREEQC

The complete Model VII chemical reaction database, described in [START_REF] Tipping | Humic ion-binding Model VII: a revised parameterisation of cation-binding by humic substances[END_REF], was previously included in PHREEQC by [START_REF] Marsac | Stabilization of polynuclear plutonium(IV) species by humic acid[END_REF] and is used in this study. In the supporting information, a file that can be used to modify Model VII chemical reaction database (e.g., to include other cations or to change binding parameters) and an example of PHREEC input file are given.

Previous studies, where Models V, VI or VII were coupled with PHREEQC, attempted to convert this empirical electrostatic humic ion-binding model into the diffuse layer model (DLM)

formalism [START_REF] Appelo | Geochemistry, groundwater and pollution[END_REF][START_REF] Liu | The influence of natural organic matter on the speciation and solubility of Eu in Boom Clay porewater[END_REF][START_REF] Marsac | An improved description of the interactions between rare earth elements and humic acids by modeling: PHREEQC-Model VI coupling[END_REF][START_REF] Marsac | Stabilization of polynuclear plutonium(IV) species by humic acid[END_REF][START_REF] Catrouillet | Geochemical modeling of Fe(II) binding to humic and fulvic acids[END_REF]. Such a conversion requires the calculation of a surface area (A HA ) that depends on the ionic strength. Similar computations have been performed for polyelectrolytes such as polyacrylic acid [START_REF] Lützenkirchen | Comparison of various models to describe the charge-pH dependence of poly(acrylic acid)[END_REF]. Such approaches result in physically unreasonable

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17 surface areas (above 10 4 m² g -1 ) [START_REF] Appelo | Geochemistry, groundwater and pollution[END_REF][START_REF] Liu | The influence of natural organic matter on the speciation and solubility of Eu in Boom Clay porewater[END_REF][START_REF] Lützenkirchen | Comparison of various models to describe the charge-pH dependence of poly(acrylic acid)[END_REF][START_REF] Marsac | An improved description of the interactions between rare earth elements and humic acids by modeling: PHREEQC-Model VI coupling[END_REF][START_REF] Marsac | Stabilization of polynuclear plutonium(IV) species by humic acid[END_REF][START_REF] Catrouillet | Geochemical modeling of Fe(II) binding to humic and fulvic acids[END_REF]. Hence, we suggest the use of the constant capacitance model (CCM) might be a better choice [START_REF] Catrouillet | Thiol groups controls on arsenite binding by organic matter: New experimental and modeling evidence[END_REF]. For the CCM, the capacitance (C 1 , in F m -2 ) evolves with log I in the case of minerals [START_REF] Lützenkirchen | The Constant Capacitance Model and Variable Ionic Strength: An Evaluation of Possible Applications and Applicability[END_REF], and a log I-term is also found in the empirical electrostatic model in Model VII (eq. 12).

Specifically, the CCM employs a linear relationship between the charge density at the surface (σ 0 , in C m -2 ) and the surface potential (Ψ 0 , in V):

_ Z = G `× N Z . ( 13
)
Combination of equations 12 and 13 gives:

G `= M a × #2OPC-0 log# '' b`. (14)
Therefore, using the CCM leads to an expression for C 1 that does not depend on the pH. Given the surface area of HA in Model VII (A HA = 1500 m² g -1 ), the range of C 1 values found for I < 1 m corresponds to that commonly reported for minerals (0.5 < C 1 < 10 F m -2 ; [START_REF] Lützenkirchen | The Constant Capacitance Model and Variable Ionic Strength: An Evaluation of Possible Applications and Applicability[END_REF]. Although the CCM is not implemented in PHREEQC, it can be used by applying appropriate corrections to the three plane model (TPM) in PHREEQC. Details are given in supporting information.

Results and Discussion

Proton titration in saline solutions

Although proton titrations of HA are rarely carried out for I > 1 m, a number of critical studies do exist. [START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF] and [START_REF] Laszak | Interaction study between Ca 2+ and humic acids in brine media[END_REF] performed proton titrations of HA at I = 0.1, 0.3, 1, 3 and 5 m (NaCl), and calculated apparent dissociation constants (pK a ) for HA considering two acidic groups. They found that pK a variation was insignificant (±0.1) within this range of ionic strengths. [START_REF] Marinsky | The interaction of Cu(II) ion with humic acid[END_REF] reported similar

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18 observations for ionic strength between 0.2 and 2 M NaNO 3 . Furthermore, the charging curves for HA versus the pH m reported by [START_REF] Maes | Measurement of the potentially available charge and the dissociation behaviour of humic acid from cobaltihexammine adsorption[END_REF] exhibited only slight differences between 1 and 3 M NaClO 4 . Van Dijk (1959) observed a shift in the titration curves to lower "pH" for I increasing from 0.02 to 2 M NaCl but mentioned no pH-correction. This effect is qualitatively consistent with the increased deviation between pH exp -pH m with increasing I (pH exp < pH m ) that is commonly observed (e.g., [START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF][START_REF] Altmaier | Solid-liquid equilibria of Mg(OH) 2(cr) and Mg 2 (OH) 3 Cl•4H 2 O (cr) in the system Mg-Na-H-OH-Cl-H 2 O at 25 °C[END_REF]. The surface of biological cells, such as the seaweed Ulva lattuca, can be considered as a polyelectrolyte, and the cation sorption properties onto U. lattuca can be treated with the models applied to HA (e.g. Turner et al., 2008). Surface acid-base properties of such biosorbents show very little influence of I above 1 m (e.g. [START_REF] Rey-Castro | Acid-base properties of brown seaweed biomass considered as a Donnan gel. A model reflecting electrostatic effects and chemical heterogeneity[END_REF][START_REF] Schijf | Investigation of the ionic strength dependence of Ulva lactuca acid functional group pKas by manual alkalimetric titrations[END_REF].

The apparent pK a of one HA site at a given ionic strength (pK a (I)), which is obtained experimentally, can be written as follows:

cd A # ' = -\]^[ , e ][,-b ] [,,-] = cd A # = 0' + \]^ # ' -MN/#OP\f#10'' #15'
According to eq. 15, the observed small dependence of conditional HA proton dissociation constants on I for highly saline conditions (I > 1 m) might occur (i) due to small dependencies of both Ψ and on I or (ii) compensation of the effects of I on Ψ and . Figure 1a compares the effect of I on Ψ 0 (for Model VII) and Ψ D (for NICA-Donnan) at constant proton activity (pH = 5.5). As stated by [START_REF] Tipping | Humic ion-binding model VI: an improved description of the interactions of protons and metal ions with humic substances[END_REF], Model VI is unlikely to find application at ionic strengths higher than I = 1 m, because of the electrostatic model (the same is true for Model VII). When the electrostatic term is translated to the CCM formalism (e.g., to be used in PHREEQC; eq. 14), and for the case when I increases to 1 m, C 1 tends to infinity and the model becomes essentially non-

electrostatic (Ψ 0 = 0). When I > 1 m, C 1 is negative, which is physically unrealistic, M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
19 demonstrating that Model VII cannot be used with PHREEQC when I > 1 m (except by suppressing the electrostatic term), consistent with the conclusion of [START_REF] Tipping | Humic ion-binding model VI: an improved description of the interactions of protons and metal ions with humic substances[END_REF]. [START_REF] Benedetti | Humic substances considered as a heterogeneous Donnan gel phase[END_REF] previously demonstrated the capability of NICA-Donnan to simulate HA charging curves versus pH in highly saline conditions (up to 2 M). In this case Ψ D increases with I and only small variation in Ψ D can be seen for I > 1 m (Fig. 1a). The NICA-Donnan model's inherent electrostatic contribution in cation-HA binding is approximately constant (-60 mV ≤ Ψ D ≤ -40 mV) for I > 1 m, and only a small dependence of apparent pK a values (i.e., corrected for the activity coefficient effect) on Ψ D is predicted. This is because the Donnan volume is small for I > 1 m and only slightly shrinks when I further increases.

Interestingly, the amplitude of the variation in Ψ is similar for both models when using the generic parameters: Ψ D ≈ Ψ 0 -60 mV for I < 1 m and pH = 5.5, as highlighted in Figure 1a.

Therefore, by suppressing the electrostatics in Model VII for I ≥ 1 m, we produce a comparable effect of I on cation-HA binding constants as in NICA-Donnan. This leads to a simplification of Model VII equations compared to lower I. The slight variation in both Ψ and for 1 < I < 4 m is consistent with the experimental observations showing little dependence of apparent pK a with I under highly saline conditions [START_REF] Marinsky | The interaction of Cu(II) ion with humic acid[END_REF][START_REF] Maes | Measurement of the potentially available charge and the dissociation behaviour of humic acid from cobaltihexammine adsorption[END_REF][START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF]. Note that the value of has no direct impact on NICA-Donnan results, because concentrations are involved in the Charge (eq kgHA -1

)

pH m 0.1 M (exp) 1 M (exp) 3 M (exp) 0.1 M (Model VII) 1 M (Model VII) 3 M (Model VII) (c

Anticipated effects of high ionic strengths on metal ion-HA binding

As for the proton, apparent metal ion complexation constants with one HA site at a given ionic strength (log K(I); data that can be obtained experimentally) can be calculated as follows:

log / 0 # ' = \]^[ +,-g h bB ] [+ g h e ][,-Bb ] = log / 0 # = 0' + \]^ # 6 i h ' -MN/#OP\f#10'' #16'
The term MN/#OP\f#10'' leads to substantial log HA β(I) variations for I < 1 m. The amplitude of log HA β(I) variations should increase with increasing metal ion charge z i . It should also vary with Ψ, and hence with the HA charge, Q. Therefore, we can expect that larger log HA β(I) variations would be observed at high pH and low metal loading compared to low pH and high metal loading. These three anticipated effects are purely related to HA physico-chemical behavior, which are predicted by Model VII and NICA-Donnan equations and parameters.

Other effects can be anticipated, which are not related to HA behavior but to physicochemical phenomena in solution. As for the proton, 6 i h varies with the ionic strength and the nature of background electrolytes (eq.1), which would directly affect log HA β(I) in eq.16. In the presence of complexing ligands other than HA (e.g., OH -, CO 3 2-, Cl -, acetate, etc), conditional metal-ligand complexation constants will also evolve with ionic strength, which would affect 5 6 (eq.4). Therefore, we can expect that (i) the presence of ligands will affect trends in log HA β versus I whatever the mathematical expression used for log HA β (see section 2.2) is, and (ii) M-HA complexation constants referring to the total dissolved metal ion concentration ( HA β(M)) and in NaClO 4 are initially discussed below (i.e., section 3.3), followed by data collected in NaCl background electrolyte solutions (section 3.4).

Cation-HA complexation in NaClO 4 solutions

Am(III)/Cm(III). [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF] investigated Am 3+ and Cm 3+ complexation with HA at pH m = 6 and in various I (NaClO 4 electrolyte solution). Because (i) most of the data are available for Am 3+ and (ii) Am 3+ and Cm 3+ are generally considered as chemical analogues, they will not be distinguished and we will only refer to Am 3+ for both datasets in this section. The . However, a linear decrease of the LC was observed with √ . For I < 1 m, log HA β LC was shown to decrease, whereas it increased for I > 1 m, although in all cases the maximum variation in log HA β LC is relatively small. Accordingly, the original authors reported an average value of log HA β LC = 6.24 ± 0.14. Below, we test the capabilities of Model VII and NICA-Donnan to predict the effect of the ionic strength on log HA β LC and LC for data from [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF]. To do so, simulations are made with Model VII and NICA-Donnan under the same conditions as those studied by [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF]. The model results are then treated using the equations of the CNM.

The results of the simulations with Model VII are shown on Figure 2a, where the measured and simulated Am-HA binding isotherms for various I are compared. Some discrepancies (either underestimation or overestimation of the model) are observed, which we attribute to the use of generic Model VII parameters and will not be further discussed. As observed experimentally, Model VII predicts decreasing Am-HA complexation with increasing I from 0.01 to 1 m. However, unlike the experimental results, Am-HA complexation is predicted to increase as ionic strength increases from 1 m to 3.5 m NaClO 4 . Because Model VII is used as a non-electrostatic model (Ψ 0 = 0) for I ≥ 1 m, the discrepancy between the experimental data and the model results can best be explained by changes in the activity coefficient of Am 3+ , which increases between 1 and 3.5 m NaClO 4 according to SIT (ε(Am 3+ ,ClO 4 -) = 0.49 kg mol -1 ; [START_REF] Guillaumont | Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium[END_REF], as anticipated in eq.1 and eq.16. cannot compensate for the increase in Ψ D with increasing I, according to NICA-Donnan equations (i.e. based on Am 3+ concentrations).

Pu(IV).

A further example that can be analyzed in the present context is the study by Here, the same results are obtained when using the CCM to account for electrostatic effects.

A modeling approach similar to that used for Am is applied to Pu (Fig. 3). Simulations were performed with Model VII for conditions comparable to those studied by [START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF]. The simulated Pu-HA binding isotherms for each ionic strength are treated according to the equations given in the latter study to determine the maximal binding capacity of the HA grafted silica gel for Pu (B max ) and log HA β(Pu 4+ ). The calculations include the side reaction coefficient for Pu(IV) (i.e., eq. 4), and the Pu(IV) hydrolysis constants and SIT parameters employed by Szabò et al.

(2010), which were originally obtained from [START_REF] Guillaumont | Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium[END_REF]. It is important to note that Pu(IV) exhibits strong hydrolysis and that, at pH = 4, 5 jk varies with I because of ionic strength effects on conditional Pu(IV) hydrolysis constants.

Simulations with Model VII are compared to the experimental results of Szabò et al.

(2010) in Figure 3. The experimental B max decreases with increasing I up to I = 0.5 m and thereafter remains nearly constant up to I = 3.5 m. Model VII predicts little variation in B max with ionic strength (i.e., predicted B max ranges between 0.52-0.59 µmol g -1 ), in contrast to variation of the experimental results (0.16-1.57 µmol g -1 ), and to the modeled LC Am(III) variations. Differences between modeling results for Am 3+ and Pu 4+ likely arise either from the different metal loadings investigated ([Am(III)-HA] ≤ 1 and [Pu(IV)-HA] ≤ 3×10 -2 mol kgHA -1 ), which has an impact on HA charge, because Pu 4+ and Am 3+ show different hydrolysis behavior, or because HA grafted silica gel behaves differently than dissolved HA. Presently, we cannot explain the discrepancies between experimental and model results for Pu 4+ . Nevertheless, the predicted variation in B max falls within the experimental range reported by [START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF] so that the impact on the prediction of overall Pu 4+ -HA binding is limited. Experimental and simulated log HA β(Pu 4+ ) values are also shown in Figure 3. Generally good agreement is found between the experimental

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28 and predicted log HA β(Pu 4+ ) values (Fig. 3), i.e., a decrease up to I = 1 m followed by an increase up to 3.5 m. The differences are about ±0.5 log units for the stability constant. For Ψ 0 = 0 (i.e., for I > 1 m), the increase in log HA β(Pu 4+ ) between I = 1 and 3.5 m is due to 5 jk , which increases by 0.84 log units over this range of I at pH = 4. Indeed, according to the SIT model, [Pu 4+ ] decreases with increasing I because of the formation of its hydrolysis products (i.e. the ∑ l * m @ n m 8 term increases in eq. 4). This phenomenon is not observed with Am 3+ because its hydrolysis can be neglected at pH = 6 and 0 < I < 4 m.

Cation-HA complexation in NaCl solutions

In a number of laboratory studies, Choppin and co-workers [START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF][START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF][START_REF] Laszak | Interaction study between Ca 2+ and humic acids in brine media[END_REF]Wall et al., 2002) , Cl -is a complexing anion, albeit, a weak one. In addition, the Am-and U(VI)-HA experiments were carried out in the presence of 0.01 M acetate buffer. Acetate is known to bind to metal ions and must therefore be taken into account in the calculations [START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF]Wall et al., 2002). Finally, unlike the other cations, for pH m ≈ 5 under ambient (air) atmosphere, UO 2 2+ hydrolysis and complexation by carbonate anions cannot be neglected [START_REF] Langmuir | Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits[END_REF][START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF]. Therefore, M-HA complexation datasets obtained for high Ao9 are less suitable for testing the applicability of NICA-Donnan and Model VII than experiments conducted in a more inert background electrolyte such as NaClO 4 . Indeed, it was found that the modeling results discussed below strongly depend on the thermodynamic databases in solution and their respective capabilities to accurately handle the speciation of Am 1×10 -8 0 -500 0.1 -3 4.5 -9.5 (pH m ) No

The reported M-HA complexation constants pertain to the PM [START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF][START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF][START_REF] Laszak | Interaction study between Ca 2+ and humic acids in brine media[END_REF]Wall et al., 2002). For the studies listed in Table 1, 5 0 values were determined for each pH and I from HA proton titrations for the specific metal ion-HA complexation studies. Because NICA-Donnan and Model VII are able to describe M-HA complexation as a function of the pH, it is more convenient to compare experimental constants that have not been corrected for 5 0 . This can be easily accomplished because all of these studies focused on low metal loadings, where [MHA] can be neglected in eq. 6 (i.e., [HA] f ≈ [HA] tot × 5 0 ). Note that the carboxylic groups of HA were considered responsible for M-HA complexation and only these groups were considered in the calculation of

[HA] tot (eq. 5-6) [START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF][START_REF] Laszak | Interaction study between Ca 2+ and humic acids in brine media[END_REF]Wall et al., 2002). [START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF] and Wall et al. (2002) reported 1:1 and 1:2 UO 2 2+ /Am 3+ -HA complexation constants. Because, the constants show similar variation with I, either for UO 2 2+ or Am 3+ , the 1:2 complexes are not discussed here and the simulations are made for the lowest [HA] investigated experimentally (i.e., where the 1:1 complex prevails).

Am(III)

. Wall et al. (2002) investigated the effect of NaCl on Am-HA complexation for pH m = 5.1 in 0.01 M acetate buffer. Although constants were corrected for effects of side

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
31 reactions, such as the formation of Am 3+ -acetate complexes (eq. 4), Am-Cl complexation was not taken into account by the original authors. Side reaction corrections for the formation of Am 3+acetate complexes employed stability constants reported by [START_REF] Moore | Thermodynamic modeling of actinide complexation with acetate and lactate at high ionic strength[END_REF].

Our simulations using Model VII and NICA-Donnan are for the lowest HA concentration ([HA] = 1 mg L -1 ). Preliminary calculations showed that Am-Cl and Am-acetate complexation have no more than a minor impact on the trend in HA K(Am 3+ ) versus I. Experimental and simulated results (now accounting for Am-Cl complexation) are compared on Figure S5. Good prediction of Am 3+ -HA binding is obtained by both models using generic parameters for I = 0.1 m, where experimental uncertainty is relatively large, whereas data for all other I are overestimated (see Fig. S5). To better compare experimental and simulated effects of I on HA K(Am 3+ ), model results were decreased by 1.5 log unit on Figure 4a. In fact, the adjustment of Am-HA binding parameters would produce the same results. When using the generic Am-HA binding parameters, both models produce similar results, especially for the evolution of log K with I, as would be expected from the similar variation of Ψ 0 and Ψ D with I (Fig. 1a). Although model log HA K(Am 3+ ) values variations with I are not as pronounced as experimental ones, the trend is consistent. Nearly constant log HA K(Am 3+ ) values are predicted for I > 1 m by both approaches, which agrees relatively well with the experimental results. Unlike Model VII (where Ψ 0 = 0 for I > 1 m), NICA-Donnan predicts a slight decrease of HA K(Am 3+ ) for I > 1 m. This is related to the evolution of Ψ D with I, and the fact that NICA-Donnan equations do not account for 0@ . Overall, the deviation between both models is small, as expected (Fig. 1a), showing that, by accounting for electrostatic effects, results from Model VII and NICA-Donnan can be extrapolated to highly saline conditions, provided that the specific binding parameters are calibrated for the respective type of HA. ) and the generic U(VI)-HA binding parameters. As for Am, preliminary tests showed that inclusion or omission of UO 2 2+ -Cl complexation did not impact the trend in HA K(UO 2 2+ ) versus I.

Experimental and simulated results are compared on Figure 4a. The generic U(VI)-HA parameters produce accurate predictions of the HA K(UO 2 2+ ) measured by Labonne-Wall et al.

(1999) as well as the evolution of HA K(UO 2 2+ ) with I up to 2 m. At I = 3 m, a higher HA K(UO 2 2+ ) value than at I = 2 m was measured, which is also predicted by Model VII. As in the case of Pu 4+ , where the apparent hydrolysis constants increase with above 1 m, this increase in HA K(UO 2 2+ ) is driven by 5 p#Uq' . Similar conclusions can be made with NICA-Donnan, except a smaller re-increase in HA K(UO 2 2+ ) that was observed with increasing I, as seen and explained for Am 3+ . Interestingly, in agreement with the experimental data, Model VII and NICA-Donnan predict a more pronounced decrease in HA K(Am 3+ ) than for HA K(UO 2 2+ ) when I increases from 0 to 1 m. This feature is driven by the Boltzman factor (eq. 9), which involves the net charge of the cation (i.e. +3 versus +2, respectively) for Am(III) and U(VI). (dashed lines) using the generic parameters (but shifted down for the case of Am-HA, see text for details). Experimental error bars are commonly smaller than the symbols.

Co(II)/Ni(II)

. [START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF] investigated the effect of I on Co-and Ni-HA complexation. Unlike Am and U(VI), with data obtained for constant pH m , Co and Ni data are reported for constant pH exp = 6 by [START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF], noting that the deviation between pH exp and pH m increases with increasing I. For example, for pH exp = 6 and I = 4 m, and with the calibration provided by the original authors, the corresponding pH m value is 6.95. The originally reported constants were not corrected for side reactions. Again, our simulations using Model VII ), all without SIT parameters (i.e. ε(i;k) = 0). In the SIT database provided with Visual MINTEQ, Co-and Ni-Cl complexation are described similarly and NICA-Donnan predicts the same trend in HA K(Ni) and HA K(Co) versus I.

The reliability of the thermodynamic aqueous databases is beyond the scope of the present paper.

Nonetheless, despite the potential uncertainties in the databases, overall, the predicted effect of I is relatively small above 1 m for both Ni and Co, in agreement with the experimental results. 35 Donnan do a relatively good job at predicting these trends, although the measured effect of I is weaker between 0.1 and 1 m, and log HA K(Ca 2+ ) are not predicted to evolve substantially for I between 1 and 3 m with Model VII. Interestingly, the measured effect of I appears more pronounced for pH m ≈ 9 than for pH m ≈ 6.5, which is indeed predicted by both models as anticipated (eq.16). According to the models, the larger negative charge of HA (Q being proportional to Ψ 0 or Ψ D ) at high pH is responsible for the larger ionic strength dependence of HA K(Ca 2+ ).

Ca(II)

To summarize, beside the deviations between experimental and simulated results that are directly related to the parameterization of Am 3+ /UO 2 2+ /Co 2+ /Ni 2+ /Ca 2+ -HA complexation for a specific type of HA, the generally observed log HA K(M z+ ) dependence with I is relatively well predicted at low metal ion concentration by simply suppressing the electrostatic term in Model VII for I > 1 m. The NICA-Donnan model shows very similar results without the need to modify the model. More experimental results are required, however, to parameterize these models at high I and to test the relevance of additional corrections at high I to explain the noted discrepancies, which would make the models more complex. More specifically, complexation studies in noncomplexing background electrolyte (e.g., NaClO 4 ) in the absence of pH-buffer (e.g., without acetate) and under inert atmosphere (i.e., in the absence of carbonate) are recommended.

Metal ion-HA complexation using SIT

To apply simple models such as the PM or the CNM to various ionic strength solutions, they must include the activities of the aqueous species. According to equations 1 and 3, when all physico-chemical conditions are kept constant except I (pH, T, total metal ion concentration), the value of log β (i.e. log HA K, log HA β α , or log HA β LC ) can be extrapolated to I = 0 (log β 0 ) using:

log / = log / Z -r a × ) -r" × . #17' For 1:1 complexes between a metal ion (M z ) and a simple ligand (L y ), ∆ε = ε(ML y+z ,k) -ε(M z ,k)ε(L y ,k), and ∆z² = (z+y) 2 -z 2 -y 2 . Due to the complexity of HA, ∆z² and ∆ε become adjustable parameters of unclear physical meaning [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF][START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF].

Comparing Fig. 2b and Fig. 4a, the effect of I on Am-HA complexation appears to differ between the studies of Wall et al. (2002) and [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF], but it is difficult to directly compare the original datasets because: (i) the thermodynamic constants refer to different models;

(ii) pH m differs by one unit; and (iii) the metal loading differs by 3 orders of magnitude. As in Figure 4a for the log HA K values reported by Wall et al. (2002), log HA β LC values determined by [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF] are recalculated to log HA K values using the LC values and imposing negligible [AmHA] in eq. 8. The results are shown in Figure 6a. Although Wall et al. (2002) studied Am-HA complexation at one pH m unit lower than [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF], their log HA K values are higher. This can be attributed to the effect of the metal loading. Specifically, at low loading, Am binds to low abundance, strong HA sites, whereas, at high loading, these sites are saturated and Am mainly binds to the more abundant, weaker HA sites (e.g., see [START_REF] Marsac | Metal loading effect on rare earth element binding to humic acid: Experimental and modeling evidence[END_REF]. The effect of I below 1 m is more pronounced for the dataset of Wall et al. (2002), which would lead to different SIT parameters (∆z² and ∆ε) in eq. 17. Therefore, it is difficult to confidently apply an ionic strength correction for cation-HA binding constants using simple metal ion-HA binding models.

The different ionic strength effects observed by Wall et al. (2002) and [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF] might also arise from differences in metal loading. As pointed out by [START_REF] Hummel | Trace metal-humate interactions. II. The "conservative roof" model and its application[END_REF], ionic strength effects tend to vanish at high loadings. To illustrate this, log HA K values for Am-HA complexation are calculated using both Model VII and NICA-Donnan for pH = 5.5, 10 -3 < I < 4 m (NaCl), 1 mg L -1 HA, and [Am] tot = 10 -9 or 10 -6 m. The results are normalized to the log to the aggregation of HA [START_REF] Lippold | Influence of trivalent electrolytes on the humic colloid-borne transport of contaminant metals: competition and flocculation effects[END_REF]. Hence, the different evolution of log HA K with I observed by Wall et al. (2002) and [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF] may be partially attributed to the aggregation state (or the conformation) of HA in response to different [Am] to [HA] ratios. In Model VII, conformational changes of HA in response to variations of I are not explicitly treated, except via a change in Donnan volume, which we find to have a negligible effect on M-HA complexation in saline solutions. Because the charging behavior of HA is related to its conformation, such behavior can implicitly be taken into account within the electrostatic term. In NICA-Donnan, the ionic strength directly affects the Donnan volume. However, none of these models includes effects of conformational changes of HA when [Am] tot increases (e.g., in the case of NICA-Donnan, 10 -6 m of Am would not affect the Donnan volume), by contrast with the more recent Elastic Polyelectrolyte Network electrostatic model [START_REF] Montenegro | Modelling proton and metal binding to humic substances with the NICA-EPN model[END_REF].

Instead, NICA-Donnan and Model VII explain the less pronounced effect of I at increased loading via the charging behavior of HA. In the experiments of [START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF], the observed LC ranges between 50 and 70% of the PEC. The charge of HA is almost neutralized by Am at the highest [Am] tot investigated, which decreases electrostatic effects and flattens the log HA K versus I curve. Therefore, metal loading is an important parameter not only for the determination of apparent metal ion-HA complexation constants for given pH and I conditions, but also for their extrapolation to various ionic strengths (e.g., with SIT).

Unlike Na, with non-specific HA interaction, Ca and Mg bind more strongly to HA, and consequently may affect the charge of HA in brines. Furthermore, other metal ions (e.g. Fe(III), Al(III), divalent transition metals) strongly bind to HA in natural conditions [START_REF] Kinniburgh | Ion binding to natural organic matter: competition, heterogeneity, stoichiometry and thermodynamic consistency[END_REF][START_REF] Pinheiro | Effect of aluminum competition on lead and cadmium binding to humic acids at variable ionic strength[END_REF][START_REF] Tipping | Al(III) and Fe(III) binding by humic substances in freshwaters and implications for trace metal speciation[END_REF][START_REF] Gustafsson | Binding of iron(III) to organic soils: EXAFS spectroscopy and chemical equilibrium modeling[END_REF][START_REF] Marsac | Aluminium competitive effect on rare earth elements binding to humic acid[END_REF]2013). Hence, the metal loading must be defined on the basis of all cations bound to HA, [Ca] on Am-HA complexation than does NICA-Donnan. For pH = 5, 1 m NaCl, [Am] = 10 -9 m and 1 mg L -1 HA, between 0 and 1 m CaCl 2 , Model VII predicts a decrease of HA K(Am) by 1.5 log units against 0.9 using NICA-Donnan. These results highlight another source of variation and uncertainty for ∆z² and ∆ε at high [Ca]. Because it is also suggested that Ca 2+ -HA interaction is purely electrostatic (van Leeuwen and Town, 2016), Ca 2+ -metal ion competition experiments at high I are required to unravel the role of Ca on metal ion-HA complexation.

As shown above, the ionic strength dependence of HA K(Ca 2+ ) becomes more pronounced at high pH because of the higher charge of HA. It appears that ∆z² and ∆ε, when applied to simple models for M-HA complexation, remain conditional parameters, which depend on the pH, the ionic strength, and the composition of the solution, which in turn affects the loading of HA.

Therefore, empirical determination of ∆z² and ∆ε for various conditions requires a large experimental dataset. Because most metal ion-HA binding data at I > 1 m were obtained at pH m ≤ 6, data at higher pH would be necessary to further test the reliability of NICA-Donnan and Model VII.

More generally, the use of non-electrostatic models was recently shown to be particularly suitable for the prediction of metal ion sorption to various types of surfaces in brines, including marine microalgae (Schjif and Herbling, 2010;Zoll and Schjif, 2012), bacteria [START_REF] Ams | The effect of high ionic strength on neptunium (V) adsorption to a halophilic bacterium[END_REF], illite and smectite [START_REF] Schnurr | Sorption of Cm(III) and Eu(III) onto clay minerals under saline conditions: Batch adsorption, Laser-fluorescence spectroscopy and modeling[END_REF]. The present data evaluation suggests that this approach can be extended to humic substances using Model VII. Although NICA-Donnan

  waste disposal in rock salt formationsor in specific clay formations. For example, deep waters in the Jurassic and lower Cretaceaous clay rock formations in Northern

  , we evaluate the applicability of Model VII and the NICA-Donnan model for saline aqueous solutions, focusing on the concomitant electrostatic approaches. The following analysis involves close inspection of the model equations in conjunction with comparisons of simulated and experimental HA proton titration and radionuclide complexation data. The mechanisms responsible for the effect of ionic strength on cation-HA complexation are also discussed within the context of the assumptions inherent to Model VII and NICA-Donnan.

  to model cation-HA binding with Model VII. The NICA-Donnan model is not implemented in PHREEQC yet, which will require future modification of PHREEQC code. Therefore, Visual MINTEQ Gustafsson, 2012) is used to model cation-HA binding with NICA-Donnan. The SIT database provided with each code is used (which corresponds to ThermoChimie v.7.b in PHREEQC). Unless mentioned in this study, the same thermodynamic constants and SIT parameters were selected throughout. The metastability of ClO 4 -, sometimes used as background

Figure

  Figure 1b shows the change of log versus I between 0.1 and 4 m for different background electrolyte solutions (i.e., NaNO 3 , NaCl, and NaClO 4 ) computed according to the SIT equation, where ε(H + ,NO 3 -) = 0.07 ± 0.07 for NaNO 3 , ε(H + ,Cl -) = 0.12 ± 0.01 for NaCl, and

Figure 1 .

 1 Figure1cshows HA charge versus pH m in 0.1, 1 and 3 M (0.1, 1.051, 3.503 m) NaClO 4

  . (c) HA charge versus pH m in 0.1 M (black curve), 1 M (blue curve), and 3 M (red curve) [0.1, 1.051, 3.503 m, respectively] NaClO 4 . Points are experimental results of Maes et al. (1992) and lines are results from Model VII. (For interpretation to references to color, the reader is referred to the web version of this article.)

  -ion ( HA β(M z+ )) might diverge when varying I. For instance, if all conditional metalligand complexation constants increase with increasing I, HA β(M) will decrease, whereas HA β(M z+ ) will increase because 5 6 increases with increasing I. Prediction of ionic strength effects on 6 i h and metal-ligand complexation constants pertain to the thermodynamic database used for aqueous solution (thermodynamic constants and SIT parameters). Therefore, in the presence of ligands other than HA, it is more difficult to test the applicability of Model VII and NICA-Donnan in saline solutions because ionic strength effects on metal-ligand complexation must be discussed in parallel. In particular, experiments conducted in NaCl solutions are affected by M-Cl complexation, which increases with increasing [Cl -], and consequently, the conditional M-Cl complexation constants also evolve with I. For this reason, metal ion-HA binding data measured

  authors interpreted the data according to the CNM. Only those datasets that allow the determination of the LC (i.e., for I = 0.01, 0.1, 1.05 and 3.5 m) and that are within the applicability of SIT (I < 4 m) are considered in the present study. The original authors provided the complete raw dataset, which are not reproduced herein. The data are plotted in the form of a binding isotherm, [AmHA] (in mol kg HA -1 ) versus [Am] tot,f (in µm), for each I on Figure2a. Note that the experimental data for [Am] tot,f > 8 µm, which are only available for 0.1 and 3.5 m, are not . In addition, the two separate series of experiments in I = 0.01, 0.1 and 1 m cannot be visually distinguished on Figure2a. All the isotherms exhibit a plateau at ~1 mol kg HA -1

  Figure 2. (a) Experimental Am-HA binding isotherms of Czerwinski et al. (1996) for pH m = 6 and = 0.01, 0.1, 1 and 3.5 m (symbols) compared with simulations using Model VII (lines). Experimental (b) log HA β LC and (c) loading capacity (LC) values for Am compared with Model VII and NICA-Donnan predictions versus I (NaClO 4 ) in the experimental conditions of[START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF]. Experimental error bars are generally smaller than the symbols.

  [START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF] on Pu(IV) complexation to a HA grafted silica gel (HA = 20 mg g -1 ) at pH = 4 and 0.02 < I < 3.5 m (NaClO 4 ). To our knowledge, no Pu(IV)-HA binding parameters are available for NICA-Donnan, and hence, only Model VII can be discussed. The Pu(IV)-HA binding parameters for Model VII are taken from[START_REF] Marsac | Stabilization of polynuclear plutonium(IV) species by humic acid[END_REF]. Preliminary calculations showed that the formation of polynuclear Pu(IV) species in the presence of HA[START_REF] Marsac | Stabilization of polynuclear plutonium(IV) species by humic acid[END_REF] is not expected for the experimental conditions studied by[START_REF] Szabò | Effect of ionic strength on complexation of Pu(IV) with humic acid[END_REF].[START_REF] Marsac | Stabilization of polynuclear plutonium(IV) species by humic acid[END_REF] used a DLM to account for electrostatic effects when coupling PHREEQC and Model VII, area of HA was adjusted to obtain results similar to the original version of Model VII.

Figure 3 .

 3 Figure 3. Experimental binding capacity (B max ) and log HA β(Pu 4+ ) values of Szabò et al. (2010) versus I (NaClO 4 ) compared with Model VII predictions. Arrows refer to the y-axis corresponding to the data. Experimental error bars for B max are generally smaller than the symbols.

  [START_REF] Labonne-Wall | Actinide speciation in high ionic strength media: experimental and modeling approaches to predicting actinide speciation and migration in the subsurface[END_REF] investigated the effect of I on U(VI)-HA complexation for pH m = 4.9 and 0.01 M acetate. UO 2 2+ -acetate complexation constants were taken from Moore et al. (1999) and are also presently used. We simulated the experimental data of Labonne-Wall et al. (1999) using Model VII and NICA-Donnan for the lowest HA concentration ([HA] = 2 mg L - 1

Figure 4 .

 4 Figure 4. (a) Apparent Am-HA and U(VI)-HA complexation constants (Wall et al., 2002; Labonne-Wall et al., 1999) versus I (NaCl) for pH m = 5.1 and 4.9, respectively. (b) Apparent M-HA complexation constants (M = Ni 2+ or Co 2+ , Kurk and Choppin, 2000) versus I (NaCl) for pH exp = 6. For both figures, lines are predictions by Model VII (full line) and NICA-Donnan

  Donnan were conducted using the lowest HA concentrations reported byKurk and Choppin (i.e., [HA] = 1.3×10 -2 mg L -1 for Co; [HA] = 2×10 -3 mg L -1 for Ni). Experimental and simulated results are compared on Figure 4b. With the generic Co/Ni-HA parameters, Model VII overestimates Co/Ni-HA complexation by 1 log unit in the worst case (i.e., for HA K(Ni) at I = 3 m). Model VII consistently predicts a decrease in HA K with I increasing from 0.3 to 1 m, whereas for I > 1 m, the predicted HA K for Co and Ni diverge. Although pH m increases by 0.6 units (for constant pH exp ) between I = 1 and 3 m, HA K(Ni) remains almost constant, whereas HA K(Co) decreases by 0.2 log units. These variations reflect the complexation of both of these transition metals by Cl -in solution. In the SIT database provided with PHREEQC, only the NiCl + species is included, with ε(NiCl + ;Cl -) = 0.1, whereas four Co-Cl complexes are considered (i.e., from CoCl + to CoCl 4 2-

  Figure5shows HA K(Ca 2+ ) measured by[START_REF] Laszak | Interaction study between Ca 2+ and humic acids in brine media[END_REF] for Ao9 = 0.1, 1 and 3 m at various pH m under ambient atmosphere. The originally reported constants were corrected for side reactions (including Ca complexation to chloride and carbonate). Simulations are made with Model VII and NICA-Donnan for 100 mg L -1 HA using the generic Ca-HA binding parameters and compared to experimental log HA K(Ca 2+ ) versus pH m for I = 0.1 and 3 m (NaCl) in Figure5. Simulations with Model VII for I = 1 m do not significantly differ from I = 3 m and consequently are not shown. Experimentally, HA K(Ca 2+ ) increases with pH m and decreases with increasing I, as for the other cations investigated. Both Model VII (Fig.5a) and NICA-

Figure 5 .

 5 Figure 5. Experimental apparent Ca 2+ -HA complexation constants versus pH m for I = 0.1, 1 and 3 m NaCl (Laszak and Choppin, 2001) compared with (a) Model VII (simulated curves for I = 1 and 3 m NaCl overlap) and (b) NICA-Donnan predictions. Experimental error bars are commonly smaller than the symbols on all three figures.

  for I = 1 m (i.e. log HA K(I) -log HA K(I = 1 m)) and plotted versus I in Figure6b. Both models indeed predict a more pronounced effect of I on log HA K at low loading.

Figure 6 .

 6 Figure 6. (a) Experimentally observed effect of the ionic strength on Am-HA complexation at low(Wall et al., 2002; pH m = 5.1; NaCl) and high metal loading[START_REF] Czerwinski | Complexation of trivalent actinide ions (Am 3+ , Cm 3+ ) with humic acid: the effect of ionic strength[END_REF]; pH m = 6; NaClO 4 ). (b) Simulated effect of the ionic strength on Am-HA complexation at low ([Am] tot = 10 -9 m) and high metal loading ([Am] tot = 10 -6 m) with Model VII (black curves) and NICA-Donnan (blue curves) in NaCl for a HA concentration of 1 mg L -1 and pH = 5.5. (For interpretation to references to color, the reader is referred to the web version of this article.)

  . Additional cation competition studies should be conducted at high I to improve and homogenize metal ion-HA binding models. As an example, in NICA-Donnan Ca 2+ mainly interacts electrostatically with HA at high [Ca 2+ ][START_REF] Christl | Ionic strength-and pH-dependence of calcium binding by terrestrial humic acids[END_REF] whereas it chiefly binds specifically to HA in Model VII. As shown in FigureS6, Model VII predicts larger effects from

  model in highly saline solution, the almost invariant Donnan potential 788 produces similar ionic strength effects to those observed using Model VII.The applicability of Model VII and NICA-Donnan was tested at high 1:1 background electrolyte concentration (NaCl/ClO 4 ) in combination with SIT (I < 4 m). The empirical electrostatic term used in Model VII is related to the constant capacitance model (CCM). A method is proposed to use the CCM in the speciation code PHREEQC, for easier and more consistent implementation of Model VII in this code. The electrostatic term used in this model tends towards zero for I = 1 m, and thus, further metal ion accumulation in the vicinity of HA molecules can be neglected. Consequently, a non-electrostatic model in combination with the binding site definition in Model VII was tested for I > 1 m. The approach simplifies the model under these high ionic strength conditions, whereas NICA-Donnan can be used without modification. Both models do a relatively good job in predicting proton dissociation for HA groups where the apparent pK a variations at high I are mainly controlled by the activity coefficient of the proton. The trend in apparent metal ion-HA complexation constants with I is consistent with experimental results for Am 3+ , UO 2 2+ , Co 2+ , Ni 2+ , Ca 2+ and Pu 4+ , both at low and high metal loading. The maximum metal ion uptake by HA (e.g., the loading capacity) for various conditions is relatively well predicted. Because of the simple approaches used here and due to the limited number of datasets that are available for highly saline solutions, no attempt was made to improve the models in order to eliminate the discrepancies observed in the evolution of apparent complexation constants with I. Most of the discrepancies between experiments and predictions with Model VII or NICA-Donnan are related to the specific cation-HA binding parameters used. With appropriately calibrated specific binding parameters for a given type of HA (e.g. a given composition or origin), both models are expected to reliably predict cation-HA binding for a wide range of ionic strengths. the physico-chemical conditions on the experimental determination of SIT parameters for metal ion-HA complexation was also discussed. When HA is treated as a simple dissolved ligand, the obtained SIT parameters values are difficult to interpret with regards to their specific physical meaning owing to the complexity of HA molecules. It is shown here that the experimentally investigated pH and metal loading variations have a strong impact on SIT parameters via their effect on the charge of HA. Some effects of pH and metal loading on cation-HA complexation are well known, but the present study shows that they imply additional effects related to HA charge, which must be taken into account when extrapolating constants at various ionic strengths. Unlike Na, which only interacts electrostatically with HA, Ca or Mg can bind more strongly to HA. Because, highly saline waters commonly have substantial Ca or Mg concentrations, as well as other metal ions, relatively high overall loadings are to be expected.Although Model VII and NICA-Donnan can account for both the metal loading effects and cation competition at I < 1 m, additional cation competition experiments at high ionic strength are required to further validate or improve these models. Nevertheless, we suggest that both models might be used as helpful predictive tools in performance safety assessment even under highly saline conditions.AknowledgementsThis work was financed by the Federal Ministry of Economic Affairs and Energy (Germany) under contracts No. 02E10206 and 02E10961. K. H. Johannesson thanks Michael and MathildaCochran for establishing the Cochran Family Professorship in Earth and Environmental Sciences, which provided her financial assistance. We thank three anonymous reviewers and the associate editor (J. P. Gustafsfson) for interesting comments that substantially improved this manuscript.

Figure 1 .Figure 2 .

 12 Figure 1. a) Surface potential (Ψ 0 ) and Donnan potential (Ψ D ) calculated for Model VII and NICA-Donnan, respectively, for pH (= -log ) = 5.5 versus the ionic strength. The y-axis for Ψ D is shifted by 60 mV compared with the one of Ψ 0 to highlight their similar evolution with I. (b) Activity coefficient of the proton (log ) versus I in NaCl, NaClO 4 and NaNO 3 solutions, calculated with SIT. (c) HA charge versus pH m in 0.1 M (black curve), 1 M (blue curve), and 3 M (red curve) [0.1, 1.051, 3.503 m, respectively] NaClO 4 . Points are experimental results of Maes et al. (1992) and lines are results from Model VII. (For interpretation to references to color, the reader is referred to the web version of this article.)

Figure 3 .

 3 Figure 3. Experimental binding capacity (B max ) and log HA β(Pu 4+ ) values of Szabò et al. (2010) versus I (NaClO 4 ) compared with Model VII predictions. Arrows refer to the y-axis corresponding to the data. Experimental error bars for B max are generally smaller than the symbols.

Figure 4 .Figure 5 .

 45 Figure 4. (a) Apparent Am-HA and U(VI)-HA complexation constants (Wall et al., 2002; Labonne-Wall et al., 1999) versus I (NaCl) for pH m = 5.1 and 4.9, respectively. (b) Apparent M-HA complexation constants (M = Ni 2+ or Co 2+ ,[START_REF] Kurk | Determination of Co(II) and Ni(II)-humate stability constants at high ionic strength NaCl solutions[END_REF] versus I (NaCl) for pH exp = 6. For both figures, lines are predictions by Model VII (full line) and NICA-Donnan (dashed lines) using the generic parameters (but shifted down for the case of Am-HA, see text for details). Experimental error bars are commonly smaller than the symbols.

Figure 6 .

 6 Figure 6. (a) Experimentally observed effect of the ionic strength on Am-HA complexation at low (Wall et al., 2002; pH m = 5.1; NaCl) and high metal loading (Czerwinski et al., 1996; pH m = 6; NaClO 4 ). (b) Simulated effect of the ionic strength on Am-HA complexation at low ([Am] tot = 10 -9 m) and high metal loading ([Am] tot = 10 -6 m) with Model VII (black curves) and NICA-Donnan (blue curves) in NaCl for a HA concentration of 1 mg L -1 and pH = 5.5. (For interpretation to references to color, the reader is referred to the web version of this article.)

Table 1 .

 1 Only data within the applicability of SIT (I < 4 m) are considered in the present study.

	investigated U VI O 2	2+ ,

Table 1 .

 1 Summary of the experimental conditions for the M-HA complexation experiments of

	3+ ,

Table and figure captions:Table 1 .

 1 Summary of the experimental conditions for the M-HA complexation experiments of Wall et al. (2002) (Am 3+ ), Labonne-Wall et al. (1999) (UO 2 2+ ), Kurk and Choppin (2000) (Co 2+ and Ni 2+ ) and Laszak and Choppin (2001) (Ca 2+ ).


Lise Charuaud

Emilie Jardé

Anne Jaffrézic

Thierry Panaget

; Jarde

; Jaffrezic

; Panaget

; Le Bot

Prioritisation of veterinary pharmaceuticals prior to a monitoring

come

Contact: lise.charuaud@ehesp.fr 02 99 02 27 05

Background and objectives:

Pharmaceuticals residues are nowadays of growing concern, especially in aquatic environments. Numerous and detailed researches have been conducted on human pharmaceutical residues at national and european scale, while ocurrence of veterinary pharmaceutical residues remains still largely unknown. Brittany is a region subjected to high animal husbandry pressures. Veterinary pharmaceuticals residues can enter the environment directly or indirectly during pastures or spreading of animal manure on soils. Thus, aquatic environments in Brittany are sensitive spots, potentially contaminated by a lot of veterinary residues, such as antibiotics or antiparasitic drugs. The project aims at realizing an overview of the contamination (types molecules and levels of concentrations) of raw water resources subjected to a strong agricultural pressure, and into drinking water obtained from those resources. In order to realize a monitoring study during one year, the first step was to perform a prioritization of veterinary pharmaceuticals, to select the veterinary pharmaceuticals which are the most susceptible to reach and to be detected in the aquatic environment.

Method and Results:

A preliminary list composed of 73 veterinary pharmaceuticals based on the REMEDES project (ONEMA 2013(ONEMA -2015) ) and on the studies of ARS and DREAL in 2013 was used. Four criteria were considered to realize this prioritization. First, the potential to reach the environment which was decomposed into the animal target (aquaculture or livestock or pets), the administration route (topical application versus other administration routes), the usage and veterinary practices in Brittany and the behavior during storage of manure and slurry prior to spreading into land. Secondly, the metabolism was investigated. According to the metabolic rates obtained from ANSES data, it was decided to consider the parent compound or the main metabolite or both. The third element to be investigated was the fate in the environment and ability of the molecules to run off or leach from soil to water, and their behavior (biodegradation, photolysis, sorption to sediments…) once in water resources. The last criterion corresponds to the analytical feasibility. This prioritization allows a classification in groups of different levels of criticity toward the contamination of water resources. The veterinary pharmaceuticals in the first three groups were selected for the analytical development and to be monitored during a sampling campaign from September 2016 to September 2017.

Discussion and conclusion:

Prioritization was a complex task when concerning veterinary pharmaceuticals, indeed there is a lack of information concerning the environmental fate of the veterinary pharmaceuticals residues. Some families were extensively studied such as the sulfonamide antibiotics or tetracycline antibiotics, but information is still scarce for antiparasitic drugs or anticoccidians for example, while those veterinary medicines are widely used in intensive livestock areas like Brittany. Then, this work highlights the fact that the choice of veterinary molecules regarding their potential to reach the aquatic environment has to be performed at a regional or local scale. Indeed, the usage of veterinary medicines can differ deeply between several regions and even between veterinarians.