hydroxides on sorption and speciation

Accepted Manuscript Np(V) uptake by bentonite clay: Effect of accessory Fe oxides/hydroxides on sorption and speciation Parveen K. Verma, Anna Yu Romanchuk, Irina E. Vlasova, Victoria V. Krupskaya, Sergey V. Zakusin, Alexey V. Sobolev, Alexander V. Egorov, Prasanta K. Mohapatra, Stepan N. Kalmykov PII:

S0883-2927(16)30273-6

DOI:

10.1016/j.apgeochem.2016.12.009

Reference:

AG 3774

To appear in:

Applied Geochemistry

Received Date: 31 August 2016 Revised Date:

7 December 2016

Accepted Date: 14 December 2016

Please cite this article as: Verma, P.K., Romanchuk, A.Y., Vlasova, I.E., Krupskaya, V.V., Zakusin, S.V., Sobolev, A.V., Egorov, A.V., Mohapatra, P.K., Kalmykov, S.N., Np(V) uptake by bentonite clay: Effect of accessory Fe oxides/hydroxides on sorption and speciation, Applied Geochemistry (2017), doi: 10.1016/ j.apgeochem.2016.12.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Np(V) uptake by bentonite clay: effect of accessory

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Fe oxides/hydroxides on sorption and speciation

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Parveen K. Verma1, Anna Yu. Romanchuk2*, Irina E. Vlasova2, Victoria V. Krupskaya2,3,

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Sergey V. Zakusin2,3, Alexey V. Sobolev2, Alexander V. Egorov2, Prasanta K. Mohapatra1,

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Stepan N. Kalmykov2

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Bhabha Atomic Research Centre, Mumbai – 400 085, Mumbai, India

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Lomonosov Moscow State University, Moscow, Russia

Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Science (IGEM RAS), Moscow, Russia

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KEYWORDS: Neptunium, interfacial behavior, sorption, speciation, bentonite clays, iron

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oxides, goethite nanoparticles, accessory minerals, thermodynamic modeling, surface

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complexation modeling

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ABSTRACT

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Batch sorption experiments on thoroughly characterized bentonites and thermodynamic

15

modeling studies were conducted to reveal the role of iron-containing accessory phases on the

16

interfacial behavior of Np(V). Bentonite clays from different industrial deposits with varying

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total iron contents were selected for the studies. The samples were characterized by XRD,

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Mossbauer spectroscopy, XRF, HRTEM, SEM-EDX and other techniques, and wherever

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possible, the accessory iron phases were identified and quantified. Thermodynamic modeling

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using available surface complexation data revealed the dominant role of the goethite accessory

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phase, which was present as nanoparticles, in Np(V) sorption at trace level concentrations (10-14

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M). This fact is independently supported by the combination of SEM-EDX and α-track

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radiography. These studies illustrate the important role that accessory minerals can play in

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radionuclides sorption data. This is important to the understanding and modeling of the

25

molecular-level speciation of radionuclides.

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INTRODUCTION

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The wide distribution of radionuclides in the subsurface environment from nuclear weapons

28

testing and/or from accidental events, such as Fukushima (2011) and Chernobyl (1986) is of

29

concern for humanity currently and in the future (Buesseler et al., 2011; Cuddihy et al.,

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1989;Kashkarov et al., 2001;Steinhauser et al., 2015). Neptunium (237Np) is a transuranic

31

element of particular concern because of its long half-life (2.1·106 years) and high geochemical

32

mobility under oxic conditions.

33

time in deep geological repositories (Nash, 2006). A large volume of R&D work is ongoing to

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understand the sorption, migration and speciation behavior of different radionuclides of several

35

contaminated sites, such as the Hanford Site (Cantrell and Felmy, 2012;Felmy et al., 2010),

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Nevada Test Site (Kersting et al., 1999;Kersting and Zavarin, 2011;Smith et al., 2003), Savannah

37

River Site (Xu et al., 2014) and Mayak Site (Batuk et al., 2015;Novikov et al., 2006). The

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understanding of the long-term behavior of the different radionuclides under various chemical

39

and physical conditions helps to better assess their migration and associated contamination or

40

radiation doses that the population might receive from the exposure to these radionuclides.

41

The long-term safety of geological repository sites for radioactive waste disposal is not only

42

related to the capacity to confine large volumes of radiotoxic waste but also to how well the

43

stored high-level radioactive waste can be isolated from the biosphere for thousands of years. For

44

this reason, deep geological repositories are conceptualized and engineered with buffer/backfill

45

materials to block radioactivity release from anthropogenic or environmental mishaps in the

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future. Bentonite, an aluminosilicate, was found to be a promising candidate for backfill

Np will be a significant radiotoxicity contributor for a long

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material in engineered barrier systems due to its favorable chemical and physical properties

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under deep geological conditions, with efficient radionuclide retention capacity (Akgun,

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2010; Holopainen, 1985; NEA, 2003;).

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Sorption reactions play an important role in the immobilization/retention of radionuclides (Davis

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et al., 2005;Geckeis and Rabung, 2008;Goldberg et al., 2007). The majority of the literature on

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SCM describes the adsorption of metal ions by different pure or synthetic mineral phases under

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well-controlled laboratory conditions. The complexity of natural soil systems makes it very

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difficult to apply the concept of SCM to soil and/or natural sediments. To simulate adsorption

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onto well-characterized natural samples, two basic modeling approaches were proposed: (a) the

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component additivity (CA) approach and (b) the generalized composite (GC) approach (Davis et

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al., 1998; Davis et al., 2005). The component additivity approach is based on summing the

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sorption of individual components in the complex mixture. To simplify the model, it is important

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to determine the mineral components that dominate sorption (Payne et al., 2013). For example,

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Payne et al. (2004) found that the presence of trace amounts of anatase determine sorption of

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U(VI) onto Georgia kaolinite. Kalmykov et al. (2015) demonstrated the dominating effect of

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Fe/Cr corrosion products (oxides) on plutonium (Pu) partitioning in sedimentary rocks.

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However, the identification of mineral components that dominate sorption in natural sediments is

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a challenging task. Even if quantitative and/or qualitative data analysis was performed, further

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understanding of the combined behavior toward radionuclide sorption under the given physical

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or chemical conditions is complicated. Furthermore, the components can be distributed quite

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unevenly. For example, iron oxide/hydroxide minerals may coat clay minerals, (Stubbs et al.,

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2009) which can influence the sorption process. Prieve et al. (1978) showed that the nature of the

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electrical double layer was affected by the presence of other heterogeneous particle surfaces.

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Np(V) sorption has been previously studied on pure or pretreated clay minerals (Benedicto et al.,

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2014; Bertetti et al., 1998; Bradbury and Baeyens, 1997; Fröhlich, 2015; Kozai, et al., 1996; Li

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et al., 2015; Marsac, et al., 2015; Turner et al., 1998; Zavari et al., 2012). Observations of its

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sorption on natural raw bentonite clays (without treatment) are still lacking (Bradbury and

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Baeyens, 2011; Fernandes et al., 2015). The present paper is intended to provide insight in that

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direction. Three samples of raw bentonite clay collected from industrial deposits with varying

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mineralogical compositions, i.e., different accessory phase compositions and iron speciation, are

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used for sorption experiments with trace level Np(V). The experimental data are interpreted in

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terms of the dominating component additivity approach.

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MATERIALS AND METHODS

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Characterization of bentonite

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Bentonite samples from the Khakassia (Russia), Rajasthan and Kutch (India) industrial deposits

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were used for this study. The Rajasthan and Kutch bentonites were crushed in a ball mill, sieved

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to size fractions <75 µm and were used without chemical treatment. Khakassia bentonite was

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used in the Na-form after treatment with Na2CO3.

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Chemical analysis was performed using XRF in accordance with the standard procedure using an

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AxiosmAX spectrometer (PANalytical, The Netherlands). Samples were dried at 110°C and

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were prepared by fusion with lithium borate at 1200°C. The iron content in the samples was

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determined in the form of total Fe2O3, regardless of the actual valence state.

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XRD was used to investigate the mineral composition of clay samples, as well as colloidal clay

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fractions (<0.5 µm), that was selected from an water suspension according to Stokes' law,

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without using of chemical dispersants. X-ray diffraction patterns were obtained with an Ultima-

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IV X-ray diffractometer (Rigaku) acquired in a frame of the Moscow State University

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Development Program. The measurement conditions were as follows: Cu-Kαradiation, D/Tex-

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Ultra 1D-detector, scan range: 3.6-65° 2θ, scan speed: 5°/min, step: 0.02° 2θ, max intensity: ~

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25000 counts. Identification of the mineral composition was performed in the textural (oriented)

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samples prepared from a bentonite suspension in the air-dried and ethylene glycol solvated states

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(Moore and Reynolds, 1997). The quantitative mineral composition was calculated by the

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Rietveld method (Bish and Post, 1993) using the BGMN program.

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Mössbauer spectra at the

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Fe nuclei were obtained by a MS-1104Em-type spectrometer

operating in constant acceleration mode. Spectra were recorded using a JANIS closed-cycle

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cryostat and were processed using the SpectrRelax program (Matsnev and Rusakov, 2012). All

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isomer shifts are given relative to the Mössbauer spectra of α-Fe at room temperature.

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The specific surface areas of the samples were determined using the BET technique (ASAP

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2010N, Micrometrics). The HRTEM images were obtained with an aberration-corrected JEOL

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2100F operated at 200 kV.

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The original Kutch bentonite was also treated with 1 M NaClO4 for approximately 20 h to

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convert it to the Na form before centrifuging. The procedure was repeated thrice, and the

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resultant solid was washed thoroughly with Millipore water to obtain the modified bentonite,

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which is further referred to as Na-form Kutch (see “Sorption of Np(V) onto clay samples under

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the steady state” section for details).

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Sorption experiments

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The 239Np (4·10-14 M) used for the sorption experiments was collected from the parent 243Am; the

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extraction and stripping details can be found elsewhere (Sill, 1966). Its purity was ascertained by

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radiometry. The suspensions of bentonite (0.5 g/L) were prepared in 0.01 M or 1 M NaClO4 for

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all sorption experiments, which were performed under ambient atmospheric conditions at 25 ±

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3°C with continuous shaking. The suspensions were conditioned for 48 hours in the background

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electrolyte solution prior to the addition of the

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suspensions was adjusted by aliquots of dilute solutions of HClO4 (ultra-pure grade) or NaOH

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(ultra-pure grade). The proton concentration (molal scale) was measured using a pH electrode (In

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lab, Mettler Toledo) calibrated against standard pH. At high ionic strength (1M), the measured

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Np(V) radiotracer. The pH of the different

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pH was corrected by adding the correction factor (Altmaier et al., 2003). The correction factor

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was determined by measuring a series of pH samples of known concentration of H+ at both low

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(0.01 M) and high (1M) ionic strength, and the difference in the two measured pH values was

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taken as the correction factor for further experiments (0.65 pH units). Aliquots of the supernatant

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solutions were mixed with scintillation cocktail (Ultima gold) for the

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scintillation counting using a Quantulus-1220, Perkin Elmer) after centrifugation at 30,000 g for

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20 min (Allegra 64R, Beckman Coulter). The sorption was calculated based on the difference

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between the radioactivity of the

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remaining in the supernatant after equilibration (the measured activity was decay-corrected).

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A blank experiment (without bentonite) was also performed under the same experimental

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conditions to account for the sorption of 239Np onto the vial used for the sorption experiments.

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α-Track radiography and SEM-EDX

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α-Track radiography and SEM-EDX studies were conducted to identify the local distribution of

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Np on bentonite samples at the micrometer scale. α-Emitter

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with the bentonite samples at pH ~ 9. The concentration of the solid phase was 0.2 g/L. The solid

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phase was separated from the solution by one centrifugation (30,000 g for 20 min). As a result of

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centrifugation, two fractions with different colors were collected and named the “dark” and

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“light” fractions, depending upon their relative color intensities. Both fractions were placed on

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glass slides and covered with CR-39 type polycarbonate film (TASTRAK PADC) for α-

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radiography. The polycarbonate film was removed after 17 h and was etched in 6 M NaOH at

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75±1°C for 6 h. The α-track images along with the corresponding areas of the solid samples were

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observed using an Olympus BX-51 optical microscope. The elemental composition of the solid

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phase was determined by SEM-EDX (JEOL JSM-6380 LA with a JED 2300 analyzer).

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Surface complexation modeling

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Np assay (liquid

Np initially added to the suspension and the radioactivity

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Np (2·10-6 M) was equilibrated

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Sorption was modeled using geochemical speciation software PHREEQC (USGS, 2015).

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Thermodynamic constants for Np aqueous speciation were taken from the NEA thermodynamic

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database (Guillaumont et al., 2003). The all present modeling results are calculated at 0.01 M

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ionic strength.

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RESULTS AND DISCUSSION

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Characterization of bentonite clay from different origins

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Samples from three bentonite industrial deposits were characterized with respect to their

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elemental composition, mineral phase composition, including accessory minerals, and Fe-

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speciation. The elemental compositions of the studied samples are presented in Table 1. The high

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contents of Si and Al are due to the clay minerals present in the tetrahedral and octahedral sheets

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of smectites (montmorillonite), as well as admixture of kaolinite and chlorite. The higher amount

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of Si in Khakassia clay compared to the other samples is due to the higher quartz content, which

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was supported by the XRD results (Figure 1). The presence of relatively high concentration of

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Ca and Mg can be partly attributed to the presence of carbonate minerals (calcite and dolomite),

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as indicated by XRD.

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The Rajastan and Kutch clays had high contents of iron and titanium. Titanium is associated with

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the presence of anatase, which was identified by XRD. Iron can be included in the octahedral

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sheets of clay minerals, mostly smectites, and can also be found as separated accessory iron

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phases.

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Table 1. XRF data of the chemical compositions of the studied bentonite clays

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Sample

LOI

Rajastan 20.06 clay Kutch 21.43 clay Khakassia 12.74 clay

Na2O MgO Al2O3

Composition, % SiO2 K2O CaO TiO2 MnO Fe2O3 P2O5

1.15

4.64

16.28 35.12 0.30 6.67

3.09

0.16

12.27

0.11

0.72

2.02

12.33 42.52 0.11 2.77

1.42

0.19

15.58

0.27

2.98

2.48

15.19 58.36 0.97 3.05

0.63

0.09

3.42

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Figure 1. X-Ray diffraction patterns collected for the non-oriented natural clays of Rajastan,

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Kutch and Khakassia bentonite clays. M – montmorillonite, K – kaolinite, Q – quartz, C –

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calcite, D – dolomite, At – anatase, G – gypsum, Gt – goethite, H – hematite, F – feldspars.

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The X-Ray diffraction patterns for all the studied samples have a (001) reflection (Figure

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1) with d=12.8 Å for Rajastan, reported mainly as a mixture of Na+ and Ca,Mg2+ forms, 15.2 Å

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for Kutch, interpreted as a mixture of Na+ and Ca,Mg2+ forms, and d=12.6 Å for Khakassia clay,

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due to the predominant of Na+ form of smectite. The non-basal reflection has the same value for

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all studied clays: d=4.47 Å, 2.56 Å, 1.69 Å for Rajastan, Kutch and Khakassia, respectively, as

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well as some other small non-basal reflections. The values of the (060) reflection of the Kutch

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and Rajastan clays are higher than that of the Khakassia clay, which indicates the presence of Fe

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in the 2:1 layer. The strongest reflections of the admixed clay and non-clay minerals are shown

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in Figure 1.

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The compositions of colloidal fractions <0.5 µm for all clay samples are enriched in Na-

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montmorillonite (Fig. S1), with basal reflections at d(001) = 12.5 Å, d(002) = 6.11-6.23 Å, and d(004) 8

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= 3.12-3.13 Å, d(006) = 2.07 Å. The admixture of kaolinite in Rajastan clay was identified by a

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series of basal reflections at d(001) = 7.16 Å, d(002) = 3.57 Å, d(003) = 2.38 Å, and d(004) = 1.79 Å.

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The presence of goethite in the Rajastan and Kutch clay was identified by the small reflection at

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d = 4.19 Å, and the admixture of gypsum in the Kutch sample was identified based on the small

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reflection at d = 7.62 Å.

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In short, all the studied clays are enriched with smectite (montmorillonite), with some admixture

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of other clay minerals (chlorite, kaolinite), as well as carbonates, quartz, different iron oxides

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and hydroxides and other minerals in relatively small amounts. Due to the different position of

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the (001) reflection, the Na+ and Ca2+ forms were identified and estimated.

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Mössbauer spectroscopy was used for speciation of Fe in the clay samples. The Mössbauer

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spectra of the

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clay samples present a superposition of a broadened quadrupole doublet and broadened magnetic

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component, which can be attributed to a magnetically ordered phase. The minor components of

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the Khakassia clay sample are presented in the Figure 2. The best fit hyperfine parameters of the

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de-convoluted spectra at different temperatures are shown in Table S1. From these studies, one

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may conclude that all the bentonite clay samples consist of ferric atoms in the high-spin state

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within an octahedral oxygen environment, in accordance with the isomer shift values (Menil,

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1985), which can be attributed to iron atoms within nonmagnetic minerals. In addition, the

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Khakassia clay sample also consists of two nonmagnetic quadrupole doublets with large isomer

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shifts and quadrupole splitting (see Table S1), indicating the presence of Fe2+ sites. The

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hyperfine parameters are similar to those for Si and Al minerals (Murad, 1988) but cannot be

203

attributed to a specific source.

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The hyperfine parameter analysis shows non-significant changes in the sub-spectra areas versus

205

temperature. One may conclude that the ferric atoms within all clay samples are separated into

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two essentially different states. The first can be presented as a distribution of quadrupole

Fe nuclei (Figure 2, S2) measured at different temperatures for three bentonite

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doublets, but it is impossible to fit it correctly with several quadrupole doublets of natural line

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width (Figure 2). The average isomer shift of this distribution is illustrative of high-spin Fe3+

209

atoms within an octahedral oxygen environment. The wide distribution of quadrupole splitting

210

values (Fig. S3A, C, E) is related to different forms of ferric hydroxides, surface atoms, and solid

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solutions of nonmagnetic Al and Si phases (Murad, 1988). It can be observed that the profiles of

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the quadrupole splitting distribution are similar for all samples and do not change with

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temperature.

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Figure 2. Mössbauer spectra measured at 77 K together for the (A) – Rajastan, (B) – Kutch and (C) – Khakassia clays.

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The second is a broadened Zeeman component that is present in all samples, which could appear

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due to the size effects of nanocrystalline particles (Morup and Knudsen, 1985). This hypothesis

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is logical because the magnetic component becomes more “classic” when the temperature

217

decreases (Figure 2, S2). These components were fitted (Figure S3B, D, F) as a distribution of

218

Zeeman sextets with different frequencies of slow superparamagnetic relaxation (Morup and

219

Knudsen, 1985), which is a manifestation of the particle size effect. The hyperfine parameters of

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these distributions (Table S1) correspond to major goethite nanocrystalline phase, for which the

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absolute value of the quadrupole shift ε is slightly higher than for the α-Fe2O3 hematite phase

222

(Vandenberghe et al., 1986). Thus, the Kutch and Rajastan samples contain only a small amount

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(~3-6%) of “bulk” α-Fe2O3. The weak changes in the quadrupole shift average values (Table S1)

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are related to the Morin transition (TM ≈ 260 K) of the hematite phase (Nininger and Schroeer,

225

1978).

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The presence of iron oxide/hydroxide nanoparticles was confirmed by HRTEM. Figure 3 shows

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typical images of goethite nanoparticles in the Kutch clay samples. The particle size varies in the

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range from 2 to 10 nm.

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Figure 3. (A), (B) HRTEM images of goethite nanoparticles from Kutch clay samples. Selected area electron diffraction are presented on the insert, white lines correspond to the position of the goethite reflexes.

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The X-ray diffraction patterns of the Rajastan and Kutch clay samples showed the presence of

230

amorphous phases. Based on the detailed analysis of the Mossbauer spectra, we assume this

231

amorphous phase can be attributed to goethite nanoparticles. Therefore, the concentrations of

232

various accessory iron-containing phases were calculated using the Mossbauer and XRF data.

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The amounts of other mineral phases were based on the XRD data. All data for the clay samples

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studied in the present work are listed in Table 2.

Table 2. Composition and related properties of the studied bentonite samples

Na+-smectite 2+

23.6 66.4

3.5

1.6

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Khakassia clay

32.8

65

14.4

4.5 1.3 2.2 15.9 6.3 3.8 0.5 0.5 -

26.4 1.9

3.2 11.9 1.3 1.1 2.9 3.0 4.5 0.7 0.7 2 Surface area, m /g 115 50

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Ca,Mg -smectite Illite Kaolinite Chlorite Quartz Albite Calcite Dolomite Gypsum Anatase Goethite Hematite

Kutch clay Rajastan clay Composition, mass-%

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Kinetics of Np(V) sorption onto bentonite

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The Np(V) uptake studies were performed as described above. The kinetics of Np(V) sorption

239

onto bentonite of different origins was measured from 0.01 M and 1 M NaClO4 at pH ~8.5

240

(Figure 4). Various theoretical kinetic models exist in the literature for metal ion absorption at

241

mineral-water interfaces (Azizian, 2004;Ho and McKay, 1999;Ho, 2006;Plazinski et al., 2009).

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For the present system, different kinetic models (i.e., pseudo-first-order and pseudo-second-

243

order) along with the diffusion model were tested for Np(V) sorption onto different bentonite

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samples at different ionic strengths. The different kinetic equations and their linear forms used to

245

model the sorption data are presented in Table S2.

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The best fit was obtained by the pseudo-second-order based on the linear fitting statistical

247

analysis (R2 value) for all the studied systems. The derived kinetics parameters for different

248

bentonite clay samples are listed in Table S3. The steady-state conditions for Np(V) sorption

249

were reached during the first 24 hours for all bentonite samples at different ionic strengths

250

(Figure 4). The sorption of Np(V) onto Khakassia clay was faster but reached a lower value.

251

Although there were mineralogical differences in the bentonite from Rajasthan and Kutch, the

252

sorption and kinetics of Np(V) on these samples were similar. The difference in the rate of

253

Np(V) sorption onto Khakassia bentonite is attributed to differences in their mineralogy and/or

254

mechanism of sorption and will be discussed below. 100

B 100 80

60 40

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Sorption, %

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Kutch clay Rajasthan clay Khakassia clay

20 0 10

20

30

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0

40

Sorption, %

A

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60 40

Kutch clay Rajasthan clay Khakassia clay

20 0

50

0

Time, h.

10

20

30

40

50

Time, h.

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Figure 4. Kinetics of Np(V) sorption onto bentonite of different origins at (A) 0.01 M and (B) 1 M ionic strength (pHinitial = 8.5, [solid phase] = 0.5 g/L, [Np] = 4·10-14 M).

255

Sorption of Np(V) onto clay samples under the steady state

256

The sorption of

257

pH in 0.01 M and 1 M NaClO4 (Figure 5). In all cases, sorption increased with increasing pH at a

258

given ionic strength. The shape and position of the pH sorption edges is different for the studied

259

bentonite samples — Khakassia bentonite has the lowest sorption. The conditions of our

260

experiments correspond to a high excess of the solid phase compared to the Np(V) concentration,

239

Np(V) (4·10-14 M) onto different bentonite samples was studied with varying

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with a constant solid to liquid ratio of 0.5 g/L. However the values of specific surface area of the

262

studied samples are different. Therefore the changes in pH-edges from sample to sample can be

263

effected by the difference in surface area (see Fig.S5) and mineral composition (Table 2).

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Figure 5. Thermodynamic modeling and experimental data of the pH dependence of Np(V) sorption onto (A) Kutch, (B) Rajastan and (C) Khakassia clay at different ionic strengths. Black 15

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and red lines – sorption of Np(V) onto Na+-form of montmorillonite using the constants from Bradbury and Baeyens (1997), blue line – sorption of Np(V) onto goethite using the constants from Kohler et al. (1999), magneta line – sorption of Np(V) onto Na+-form of montmorillonite using the constants from Tachi et al. (2014). The values of concentration of each proposed

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phases in the legend correspond to the values given in Table 2. Since in 1M NaClO4 smectite is completely converted to the Na+-form (see discussion below), two modeling curves for Np(V) sorption onto montmorillonite are presented for Indian clays – for initial sample and after complete conversion to Na+-form.

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([Np(V)] = 4·10-14 M, [solid phase] = 0.5 g/L).

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The differences and similarities in radionuclide sorption at different ionic strengths are indicative

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of their speciation on the surface (Goldberg et al., 2007). In general, sorption remains the same

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for inner-sphere complexation with increasing ionic strength, whereas it is decreased at higher

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ionic strengths for outer-sphere complexation. In the present case, the sorption of Np(V) onto

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Rajastan and Khakassia bentonite clays at 0.01 M and 1 M ionic strengths is the same,

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suggesting inner-sphere complexation. Previously it was shown that the sorption mechanism of

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Np(V) onto clay minerals depends on the pH. At low pH, ion-exchange is dominant (Benedicto

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et al., 2014;Zavarin et al., 2012), whereas at higher pH, surface complexation is the major

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mechanism of uptake. In our experiments, sorption at pH < 6 was low and barely changed with

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increasing ionic strength. This effect may be partially due to the hindered exchange of Np(V)

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with Ca2+ and Mg2+, especially in the case of Kutch and Rajastan clays, as well as the

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competition with these cations for sorption sites. These cations are found in solution due to the

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partial dissolution of accessory minerals (calcite, dolomite and others) and the exchange of

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interlayer cations in the clay minerals. The presence of Ca2+ and Mg2+ in solution may affect the

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ion exchange of Np(V) (see Table S4).

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In the case of Kutch clay, the sorption differs at varying ionic strengths, but the trend is

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unexpected. The experimental results demonstrate higher sorption of Np(V) on Kutch clay at

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higher ionic strength. Kutch clay mostly contains Mg2+- and Ca2+-substituted smectite (Table 2),

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which may influence the sorption of Np(V), consistent with previously published data — Np(V)

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sorption is higher onto Na+-smectite than Ca2+-smectite (Benedicto et al., 2014;Kozai et al.,

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1996), which may explain the lower Np(V) sorption onto Kutch clay at lower ionic strength. To

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determine the impact, an additional experimental approach was taken. The original Kutch

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bentonite was treated with 1 M NaClO4 to convert the Ca/Mg2+-smectite form of Kutch to the

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Na+ form. The treated Kutch bentonite (Na+-smectite form), now called Na-form Kutch clay, was

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further characterized by XRD (Figure S4). The XRD diffraction pattern confirm the smectite

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modification — Na+-Kutch bentonite has two visible basal reflections with d(001)=12.6 Å and

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d(002)=6.3 Å, which indicate interlayer cation exchange to Na+.

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The Np(V) sorption kinetics and pH-edge on Kutch and Na-form Kutch clays were compared

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(Figure 6). The rate of Np(V) sorption onto Na-form Kutch clay was faster than onto the original

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Kutch samples, whereas the equilibrium sorption value was similar for both samples (Table S3,

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Figure 6A). When comparing the pH edges of Np(V) sorption (Figure 6B), it is clear that the

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sorption of Np(V) onto Na-form Kutch is similar at different ionic strengths and similar to the

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sorption data for the original Kutch clay at higher ionic strength (1 M NaClO4).

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Figure 6. (A) Kinetics of Np(V) sorption onto Kutch and Na-form Kutch clays; (B) pH

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dependence of Np(V) sorption onto Kutch and Na-form Kutch clays at different ionic strengths. Modeling of Np(V) sorption onto clays of different origin

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Among the papers devoted to Np(V) sorption onto bentonites accompanied with thermodynamic

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modeling, the papers by Bradbury and Baeyens (1997) and Tachi et al.(2014) are highlighted. In

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both manuscripts, surface complexation modeling of Np(V) sorption onto purified

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montmorillonite was performed, and the findings were fitted using a linear free-energy

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relationship (LFER). The notable differences in the above papers were that Bradbury and

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Baeyens studied the sorption of Np(V) at trace concentrations (<10-13 M) and used two-site

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protolysis non-electrostatic surface complexation (2SPNE SC/CE), whereas Tachi et al.

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performed their experiment at higher Np(V) concentrations and used one-site protolysis non-

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electrostatic surface complexation modeling (1SPNE SC/CE).

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These two models and corresponding surface complexation constants (Table S5) were applied to

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our experimental data using the concentration of smectite in different bentonites determined

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from the characterization experiments (Figure 5). Because of the absence of the equilibrium

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constants for Np(V) sorption onto Ca2+/Mg2+-smectite, modeling was done only for Np(V)

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sorption onto Na+-smectite. Based on the assumption that in 1M NaClO4 all smectite forms are

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converted to the Na+-form, two modeling curves for Np(V) sorption onto montmorillonite are

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presented for Indian clays. Namely, in case of Kutch clay black and red lines in fig.5A calculated

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for 23% of mass fraction of montmorillonite in the sample (correspond to 0.01 M NaClO4) and

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90% of mass fraction for montmorillonite in the sample (correspond to 1 M NaClO4). Increasing

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of Np(V) sorption with mass fraction of montmorillonite that was found by modeling correlates

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with experimental data on Np(V) sorption onto Kutch clay with increasing of ionic strength.

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However modeling of Np(V) sorption onto pure montmorillonite did not result in sufficient

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agreement with experimental data in case of Kutch clay. Np(V) sorption Rajasthan clay does not

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converge well with the models of Np(V) sorption onto montmorillonite as well. Np(V) sorption

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onto Khakassia bentonite is satisfactorily fitted by the model of Bradbury and Baeyens. Whereas

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modeling of sorption with equilibrium constants from Tachi et al. does not give adequate

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modeling in all cases (shown only for Khakassia clay).

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We proposed that admixture phases present in the studied clay may influence on Np(V) sorption.

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The most likely candidates are goethite, hematite and anatase. In this work, sorption onto

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goethite was taken into account because of its higher concentration than other accessory phases

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in the samples and its higher Np(V) sorption constant (Kohler et al., 1999) than hematite (Muller

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et al., 2015;Romanchuk and Kalmykov, 2014) and anatase (Gracheva et al.,2014). The sorption

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of Np(V) onto goethite was modeled using its total mass fraction from the Table 2 and by

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assuming that the specific surface area of goethite, i.e. the surface area per mass of goethite, is

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identical with the specific surface area of the sample as a whole, namely 115 m2/g for Kutch

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clay, 50 m2/g for Rajastan clay, 15 m2/g for Khakassia clay (Figure 5). The surface area of each

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mineral phase within the sample is then proportional to its mass fraction. An experimental

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determination of the surface area of individual phases in a complex mineral mixture is difficult

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and remains an open task. The sorption of Np(V) onto Kutch and Rajastan bentonites fit well

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with Np(V) sorption onto goethite (blue line in Figure 4), indicating that the presence of even a

338

relatively small amount of iron hydroxide/oxide in natural clays may have influence on Np(V)

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uptake.

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These findings can explain the difference in the kinetics of Np sorption—the rate of sorption of

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Np(V) onto the Khakassia clay is faster than Rajastan and Kutch clays (Figure 4). In the sample

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from Khakassia, Np(V) sorption occurred on the major minerals phases (montmorillonite),

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whereas for both Rajastan and Kutch clays, Np(V) is presumably sorbed onto both

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montmorillonite and the accessory mineral goethite, which composes approximately 3% of the

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mass. Therefore, the diffusion process can explain the slower kinetics of sorption for the

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Rajastan and Kutch clays.

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Local distribution of Np(V) on the clays

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The local distribution of Np(V) on the clays was studied at higher surface loading compared to

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the batch experiments. The samples were divided into “light” and “dark” fractions by

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centrifugation.

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Figure 7 shows typical optical microscope and corresponding α-track analysis images for Np(V)

352

sorbed onto the Rajastan clay. The “light” fraction of Np(V) is evenly distributed, whereas the

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“dark” fraction shows hot-spots where

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fraction contains mainly Fe- and Fe/Ti-containing oxides and hydroxides (Figure S6), whereas

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the “light” fraction predominantly contains clay minerals. Thus, even under high surface loading

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conditions, the presence of accessory minerals, such as iron oxides/hydroxides, in clays can

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greatly influence Np(V) sorption.

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Np is concentrated. SEM-EDS showed that the “dark”

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CONCLUSIONS

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Within the scope of this work we study the uptake mechanisms of tracer amount of Np(V) on the

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actual clays varying in mineralogical composition and accessory mineral content. We showed

361

that Fe oxides/hydroxide, goethite in this case, present even at low concentrations (few percent)

362

play the important role in Np(V) sorption and speciation. This has influence both on the kinetics

363

of sorption, and especially on pH sorption edges at the steady state and possibly on the

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reversibility of sorption. It is important to conclude that the surface complexation data are highly

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important to the modeling of such complex heterogenic systems as soils or bottom sediments.

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Figure 7. Optical microscope images and corresponding α-track analysis images for Np(V) sorbed onto Rajastan clay. 367

ACKNOWLEDGEMENT

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The Np(V) sorption studies and thermodynamic modeling were supported by the Russian

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Scientific Foundation (project 16-13-00049). The XRD mineralogical studies were supported by

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the Russian Scientific Foundation (project 16-17-10270).

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REFERENCES Akgun, H., 2010. Geotechnical characterization and performance assessment of bentonite/sand mixtures for underground waste repository sealing. Appl. Clay Sci. 49, 394-399.

375 376 377

Altmaier, M., Metz,V., Neck,V., Muller, R., Fanghanel, Th., 2003. Solid-liquid equilibria of Mg(OH)2(cr) and Mg2(OH)3Cl·4H2O(cr) in the system Mg-Na-H-OH-Cl-H2O at 25°C. Geochim Cosmochim. Acta 67, 3595-3601.

378 379

Azizian, S., 2004. Kinetic models of sorption: a theoretical analysis. J. Colloid Interface Sci. 276, 47-52.

380 381 382 383 384

Batuk, O.N., Conradson, S.D., Aleksandrova, O.N., Boukhalfa, H., Burakov, B.E., Clark, D.L., Czerwinski, K.R., Felmy, A.R., Lezama-Pacheco, J.S., Kalmykov, S.N., Moore, D.A., Myasoedov, B.F., Reed, D.T., Reilly, D.D., Roback, R.C., Vlasova, I.E., Webb, S.M., Wilkerson, M.P., 2015. Multiscale Speciation of U and Pu at Chernobyl, Hanford, Los Alamos, McGuire AFB, Mayak, and Rocky Flats. Environ. Sci. Technol. 49, 6474-6484.

385 386 387

Benedicto, A., Begg, J.D., Zhao, P., Kersting, A.B., Missana, T., Zavarin, M., 2014. Effect of major cation water composition on the ion exchange of Np(V) on montmorillonite: NpO2+ -Na+K+-Ca2+-Mg2+ selectivity coefficients. Appl. Geochem. 47, 177-185.

388 389 390

Bertetti, F.P., Pabalan, R.T., Almendarez, M.G., 1998. Studies of Neptunium Sorption on Quartz, Clinoptilolite, Montmorillonite, and a-Alumina. In: Jenne, E.A. (Ed.), Adsorption of Metals by Geomedia Academic Press: San Diego, pp. 131-148.

391 392

Bish, D.L., Post, J.E., 1993. Quantitative mineralogical analysis using the Rietveld full-pattern fitting method. American Mineralogist 78, 932-940.

393 394

Bradbury, M.H., Baeyens, B., 1997. A mechanistic description of Ni and Zn sorption on Namontmorillonite Part II: modelling. J. Cont. Hydrol. 27, 223-248.

395 396 397

Bradbury, M.H., Baeyens, B., 2011. Predictive sorption modelling of Ni(II), Co(II), Eu(IIII), Th(IV) and U(VI) on MX-80 bentonite and Opalinus Clay: A “bottom-up” approach. Appl. Clay Sci. 52, 27–33.

398 399

Buesseler, K., Aoyama, M., Fukasawa, M., 2011. Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity. Environ. Sci. Technol. 45, 9931-9935.

400 401

Cantrell, K.J., Felmy, A.R. Plutonium and Americium Geochemistry at Hanford: A Site-Wide Review . PNNL-21651. 2012. Pacific Northwest National Laboratory .

402 403 404

Cuddihy, R.G., Finch, G.L., Newton, G.J., Hahn, F.F., Mewhinney, J.A., Rothenberg, S.J., Powers, D.A., 1989. Characteristics of radioactive particles released from the Chernobyl nuclear reactor. Environ. Sci. Technol. 23, 89-95.

405 406 407

Davis, J.A., Coston, J.A., Kent, D.B., Fuller, C.C., 1998. Application of the Surface Complexation Concept to Complex Mineral Assemblages. Environ. Sci. Technol. 32, 28202828.

AC C

EP

TE D

M AN U

SC

RI PT

368

22

ACCEPTED MANUSCRIPT

Davis, J.A., Ochs, M., Olin, M., Payne, T.E., Tweed, C.J. NEA Sorption Project Phase II. Interpretation and Prediction of Radionuclide Sorption onto Substrates Relevant for Radioactive Waste Disposal Using Thermodynamic Sorption Models. 5992. 2005. OECD.

411 412 413

Felmy, A.R., Cantrell, K.J., Conradson, S.D., 2010. Plutonium contamination issues in Hanford soils and sediments: Discharges from the Z-Plant (PFP) complex. Phys. Chem. Earth, Parts A/B/C 35, 292-297.

414 415

Fernandes, M.M., Ver, N., Baeyens, B., 2015. Predicting the uptake of Cs, Co, Ni, Eu, Th and U on argillaceous rocks using sorption models for illite. Appl. Geochem. 59, 189–199.

416 417

Fröhlich, D.R., 2015. Sorption od neptunium on clays and clay minerals – a review. Clays Clay Mineral. 63 (4), 262-276.

418 419

Geckeis, H., Rabung, T., 2008. Actinide geochemistry: From the molecular level to the real system. J. Contam. Hydrol. 102, 187-195.

420 421

Goldberg, S., Criscenti, L.J., Turner, D.R., Davis, J.A., Cantrell, K.J., 2007. AdsorptionDesorption Processes in Subsurface Reactive Transport Modeling. Vadose Zone J. 6, 407-435.

422 423 424

Gracheva, N.N., Romanchuk, A.Y., Smirnov, E.A., Meledina, M.A., Garshev, A.V., Shirshin, E.A., Fadeev, V.V., Kalmykov, S.N., 2014. Am(III) sorption onto TiO2 samples with different crystallinity and varying pore size distributions. Appl. Geochem. 42, 69-76.

425 426 427 428

Guillaumont, R., Fanghanel, T., Neck, V., Fuger, J., Palmer, D.A., Grenthe, I., Rand, M.H. Update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technecium. 2003. Amsterdam, The Netherlands, OECD Nuclear Energy Agency. Chemical Thermodynamics.

429 430

Ho, Y.S., McKay, G., 1999. Pseudo-second order model for sorption processes. Proc. Biochem. 34, 451-465.

431 432

Ho, Y.S., 2006. Review of second-order models for adsorption systems. J. Hazardous Materials 136, 681-689.

433 434

Holopainen, P., 1985. Crushed aggregate-bentonite mixtures as backfill material for repositories of low- and intermediate level radioactive wastes. Engineering Geology 21, 239-245.

435 436 437

Kalmykov, S.N., Vlasova, I.E., Romanchuk, A.Yu., Zakharova, E.V., Volkova, A.G., Presnyakov, I.A., 2015. Partitioning and speciation of Pu in the sedimentary rocks aquifer from the deep liquid nuclear waste disposal. Radiochim. Acta 103, 175-185.

438 439 440 441

Kashkarov, L.L., Tcherkezian, V.O., Kalinina, G.V., Ivliev, A.I., Shalaeva, T.V., Korovaikov, P.A., 2001. Chernobyl "Hot Particles": Radionuclide Composition and Contribution in the Total Soil Radioactivity. In: Frontasyeva, M., Perelygin, V., Vater, P. (Eds.), Radionuclides and Heavy Metals in Environment Springer Netherlands, pp. 43-48.

442 443

Kersting, A.B., Efurd, D.W., Finnegan, D.L., Rokop, D.L., Smith, D.K., Thompson, J.L., 1999. Migration of plutonium in groundwater at the Nevada Test Site. Nature 397, 56-59.

444 445 446

Kersting, A.B., Zavarin, M., 2011. Colloid-facilitated transport of plutonium at the Nevada Test Site, NV, USA. In: Kalmykov, S.N., Denecke, M.A. (Eds.), Actinide Nanoparticle Research Springer-Verlag, Berlin.

447 448

Kohler, M., Honeyman, B.D., Leckie, J.O., 1999. Neptunium(V) sorption on hematite (a-Fe2O3) in aqueous suspension: The effect of CO2. Radiochim. Acta 85, 33-48.

449 450

Kozai, N., Ohnuko, T., Matsumoto, J., Banba, T., Ito, Y., 1996. A study of the specific sorption of neptunium(V) on smectite in low pH solution. Radiochim. Acta 75, 149-158.

AC C

EP

TE D

M AN U

SC

RI PT

408 409 410

23

ACCEPTED MANUSCRIPT

Li, P., Liu, Z., Ma, F., Shi, Q., Guo, Z., Wu, W., 2015. Effects of pH, ionic strength and humic acid on the sorption of neptunium(V) to Na-bentonite. J. Mol. Liq. 206, 285-292.

453 454 455

Marsac, R., Banik, N.l., Lützenkirchen, J., Marquardt, C.M., Dardenne, K., Schild, D., Rothe, J., Diascorn, A., Kupcik, T., Schäfer, T., Geckeis, H., 2015. Neptunium redox speciation at the illite surface. Geochim. Cosmochim. Acta 152, 39-51.

456 457

Matsnev, M.E., Rusakov, V.S., 2012. SpectrRelax: An application for Mossbauer spectra modeling and fitting. P Conf. Proc. 1489, 178-185.

458 459 460 461

Menil, F., 1985. Systematic trends of the 57Fe Mossbauer isomer shifts in (FeOn) and (FeFn) polyhedra. Evidence of a new correlation between the isomer shift and the inductive effect of the competing bond T-X (-Fe) (where X is O or F and T any element with a formal positive charge). J. Phys. Chem. Solids 46, 763-789.

462 463

Moore, D.M., Reynolds, R.C., 1997. X-Ray Diffraction and the Identification and Analysis of Clay Minerals. Oxford University Press.

464 465

Morup, S., Knudsen, J.E., 1985. Molecular dynamics and spin-lattice relaxation of Fe3+ in a supercooled liquid. J. Phys. C 18, 2943.

466 467 468

Muller, K., Groschel, A., Rossberg, A., Bok, F., Franzen, C., Brendler, V., Foerstendorf, H., 2015. In situ Spectroscopic Identification of Neptunium(V) Inner-Sphere Complexes on the HematiteGAOWater Interface. Environ. Sci. Technol. 49, 2560-2567.

469 470 471

Murad, E., 1988. Properties and behavior of iron oxides as determined by Mossbauer spectroscopy. In: Stucki, J.W., Goodman, B.A., Schwertmann, U. (Eds.), Iron in Soils and Clay Minerals pp. 309-350.

472 473 474

Nash, K.L., 2006. Actinide Solution Chemistry And Chemical Separations: Structure-Function Relationships In The Grand Scheme Of Actinide Separations Science. In: May, I., Bryan, N.D., Alvares, R. (Eds.), Recent Advances In Actinide Science RSC Publishing, Cambridge, pp. 427.

475 476 477

NEA, 2003. Engineered Barrier Systems and the Safety of Deep Geological Repositories: stateof-the-art . Nuclear Energy Agency Organisation for Economic Co-Operation and Development, OECD .

478 479

Nininger, J., Schroeer, D., 1978. Mossbauer studies of the morin transition in bulk and microcrystalline a-Fe2O3. J. Phys. Chem. Solids 39, 137-144.

480 481 482

Novikov, A.P., Kalmykov, S.N., Utsunomiya, S., Ewing, R.C., Horreard, F., Merkulov, A., Clark, S.B., Tkachev, V.V., Myasoedov, B.F., 2006. Colloid Transport of Plutonium in the FarField of the Mayak Production Association, Russia. Science 314, 638-641.

483 484 485 486

Payne, T.E., Brendler, V., Ochs, M., Baeyens, B., Brown, P.L., Davis, J.A., Ekberg, C., Kulik, D.A., Lutzenkirchen, J., Missana, T., Tachi, Y., Van Loon, L.R., Altmann, S., 2013. Guidelines for thermodynamic sorption modelling in the context of radioactive waste disposal. Environ. Model. Software 42, 143-156.

487 488

Payne, T.E., Davis, J.A., Lumpkin, G.R., Chisari, R., Waite, T.D., 2004. Surface complexation model of uranyl sorption on Georgia kaolinite. Appl. Clay Sci. 26, 151-162.

489 490

Plazinski, W., Rudzinski, W., Plazinska, A., 2009. Theoretical models of sorption kinetics including a surface reaction mechanism: A review. Adv. Colloid Interface Sci. 152, 2-13.

491 492

Prieve, D.C., Ruckenstein, E., 1978. The double-layer interaction between dissimilar ionizable surfaces and its effect on the rate of deposition. J. Colloid Interface Sci. 63, 317-329.

493 494 495

Romanchuk, A.Yu., Kalmykov, S.N., 2014. Actinides sorption onto hematite: experimental data, surface complexation modeling and linear free energy relationship. Radiochim. Acta 102, 303310.

AC C

EP

TE D

M AN U

SC

RI PT

451 452

24

ACCEPTED MANUSCRIPT

Sill, C.W., 1966. Preparation of Neptunium-239 Tracer. Anal. Chem. 38, 802-804.

497 498 499

Smith, D.K., Finnegan, D.L., Bowen, S.M., 2003. An inventory of long-lived radionuclides residual from underground nuclear testing at the Nevada test site, 1951GAO1992. J. Environ. Radioactivity 67, 35-51.

500 501 502 503

Steinhauser, G., Niisoe, T., Harada, K.H., Shozugawa, K., Schneider, S., Synal, H.A., Walther, C., Christl, M., Nanba, K., Ishikawa, H., Koizumi, A., 2015. Post-Accident Sporadic Releases of Airborne Radionuclides from the Fukushima Daiichi Nuclear Power Plant Site. Environ. Sci. Technol. 49, 14028-14035.

504 505 506

Stubbs, J.E., Veblen, L.A., Elbert, D.C., Zachara, J.M., Davis, J.A., Veblen, D.R., 2009. Newly recognized hosts for uranium in the Hanford Site vadose zone. Geochim. Cosmochim. Acta 73, 1563-1576.

507 508 509

Tachi, Y., Ochs, M., Suyama, T., 2014. Integrated sorption and diffusion model for bentonite. Part 1: clay-water interaction and sorption modeling in dispersed systems. J. Nucl. Sci. Technol. 51, 1177-1190.

510 511

Turner, D.R., Pabalan, R.T., Bertetti, F.P., 1998. Neptunium(V) sorption on montmorillonite; an experimental and surface complexation modeling study. Clays and Clay Minerals 46, 256-269.

512

USGS, 2015. PHREEQC interactive. [3.2.0.9820].

513 514 515

Vandenberghe, R.E., De Grave, E., De Geyter, G., Landuydt, C., 1986. Characterization of goethite and hematite in a Tunesian soil profile by Moessbauer spectroscopy. Clays and Clay Minerals 34, 275-280.

516 517 518 519

Xu, C., Athon, M., Ho, Y.F., Chang, H.S., Zhang, S., Kaplan, D.I., Schwehr, K.A., DiDonato, N., Hatcher, P.G., Santschi, P.H., 2014. Plutonium Immobilization and Remobilization by Soil Mineral and Organic Matter in the Far-Field of the Savannah River Site, U.S. Environ. Sci. Technol. 48, 3186-3195.

520 521 522

Zavarin, M., Powell, B.A., Bourbin, M., Zhao, P., Kersting, A.B., 2012. Np(V) and Pu(V) Ion Exchange and Surface-Mediated Reduction Mechanisms on Montmorillonite. Environ. Sci. Technol. 46, 2692-2698.

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Accessory minerals in clay effect on pH edge and kinetics of Np(V) sorption

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SCM was applied to describe sorption of Np(V) onto raw clay samples

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