Plutonium Partitioning Behavior to Humic Acids from Widely Varying

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Plutonium partitioning behavior to humic acids from widely varying soils is related to carboxyl-containing organic compounds Peng Lin, Chen Xu, Saijin Zhang, Nobuhide Fujitake, Daniel I. Kaplan, Chris Yeager, Yuko Sugiyama, Kathleen A. Schwehr, and Peter H. Santschi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03409 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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Environmental Science & Technology

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Plutonium partitioning behavior to humic acids from widely varying soils is related to

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carboxyl-containing organic compounds

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Peng Lin1*, Chen Xu1, Saijin Zhang1, Nobuhide Fujitake2, Daniel I. Kaplan3, Chris M. Yeager4,

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Yuko Sugiyama5, Kathleen A. Schwehr1, Peter H. Santschi1

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77553, United States

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Department of Marine Science, Texas A & M University at Galveston, Galveston, Texas

Division of Agroenvironmental Biology, Graduate School of Agriculture Science, Kobe

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University, Kyoto, 606-8501, Japan

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Savannah River National Laboratory, Aiken, South Carolina 29808, United States

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Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

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School of Human Science and Environment, University of Hyogo, 1-1-12 Shinzaike-Honcho,

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Himeji, Hyogo, 670-0092, Japan

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*Corresponding Author: Peng Lin ([email protected])

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For resubmission to Environmental Science & Technology

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1

ACS Paragon Plus Environment


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Abstract:

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In order to examine the influence of the HA molecular composition on the partitioning of Pu,

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ten different kinds of humic acids (HAs) of contrasting chemical composition, collected and

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extracted from different soil types around the world were equilibrated with groundwater at

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low Pu concentrations (10-14 M). Under mildly acidic conditions (pH~5.5), 29±24% of the

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HAs were released as colloidal organic matter (>3kDa to 0.45 µm),

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colloidal (3 kDa to 0.45 µm) and truly dissolved (< 3 kDa) HA fractions. Additionally,

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correlations were evaluated between the Pu partitioning parameters and elemental and functional

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groups abundance of organic matter among these three phases. As the remobilization of soil OM

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(e.g., HAs) occurs during soil erosion of natural or man-induced episodic flooding, the intent of

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this work was to make inferances about the tendency for HAs to behave as a sink or a source for

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Pu during these events. That is, Pu bound to colloidal HA could potentially act as a source

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because it would presumably be more mobile than the larger particulate fraction. Conversely, Pu

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bound to the particulate fraction could potentially act as a sink because it would presumably be

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less mobile and remain strongly bound to soil. We present here novel results from controlled

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laboratory experiments conducted using environmentally relevant Pu concentrations to document

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a relationship between size-fractionated carboxyl-containing or organic nitrogen compounds and

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the Pu activity among particulate, colloidal, and dissolved phases.

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MATERIALS AND METHODS

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Sampling of humic acid substances

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Ten soils from around the world were carefully selected to represent a wide range of origins,

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with respect to soil order (i.e., soil genesis), land use, and geography (Table 1). Additionally, we

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chose these soils, because they contain a wide range of concentrations of N and different organic

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functional groups (e.g., N ranges from 1.18% to 6.59%, and Alkyl C ranges from 5.5% to 23.8%,

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and COO C varies from 13% to 20%, Table 1). Nitrogen-containing organic compounds have

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been reported to be an important component of NOM responsible for binding Pu.20-23 The six soil

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included Podzols, that are infertile acid soils; Chernozyoms, that are rich black soils and

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considered as excellent agricultural soils; Andisols found in volcanic areas; Histosols that are

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composed primarily of organic matter; Cambisols that are very young soils that have not yet

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developed well defined horizons; and Luvisols which are commonly found in forested regions.

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We also attempted to include soils from varying land uses, including forest, arable land, and

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grassland. Finally, the soils originated from four continants, South America, Europe, Africa, and

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Asia. The HAs from these soils were isolated and further purified according to the alkaline

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extraction method from the International Humic Substance Society (IHSS).25,26 It should be

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noted that 1) chemical and physical alteration of the soil organic matter likely occurred during

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the alkaline extraction; 2) the alkaline extractant (i.e., the humic acids as defined) is likely not

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capable of representing the whole soil organic matter.27 However, IHSS recommended these HA

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extraction procedures that were used in our study because they are standardized, making

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comparisons between different scientific studies possible, and they have been widely accepted by

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the scientific community as represented by the IHSS. In addition, the results and the correlations

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obtained from this study (see below) should be independent from the “artifacts” of the extraction

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method and still imply the one of the responsible functional groups in the soil for Pu binding.

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Briefly, the dried soil was pre-treated with a 1 M HCl solution to separate fulvic acid

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(supernatant) from the soil. The resulting soil pellet was dissolved by adding 0.1 M KOH under

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N2 purging and 0.3 M K+, as KCl. The supernatant was acidified (pH 1.0) using 6 M HCl to

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precipitate HAs, which were pelleted by centrifugation and then suspended in a 0.1 M HCl/0.3 M

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HF solution overnight for five times to minimize ash content. After HF digestion, the HA was

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washed repeatedly with Milli-Q water to minimize ions. After the purification, the abundance of

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organic C and N in HAs was determined, and their molecular composition was chemically

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characterized by solid state 13C nuclear magnetic resonance (NMR) spectroscopy (Table 1); this

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HA characterization data was previously reported elsewhere.24, and the purified HAs were

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provided from Division of Agroenvironmental Biology, Graduate School of Agriculture Science,

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Kobe University. All HA samples for the HA-groundwater resuspension experiments described

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below were stored frozen at -5 ºC.

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HA-groundwater resuspension experiment Artificial groundwater27 was used as the medium for the HA-groundwater suspension

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experiments. An average groundwater composition was chosen in these experiments rather than

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varying groundwaters representative of the soil’s origin, in order to simplify interpretation of the

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results. The implications of this simplification are that they may not represent site-specific

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conditions, but they ease comparisons of results between the 10 soils. To simplify the

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experimental approach, we used an averaged groundwater composition at a constant pH, which

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is the most important variable for the metal partitioning to soils, although the groundwater

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composition can change in different HA-source areas. This artificial groundwater has low salt

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concentrations (10-2 M), no organic matter, and low carbonate concentrations because of its

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mildly acidic pH of 5.5, the average pH of natural rainwater28 and also the background

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groundwater pH at the Savannah River Site (SRS), USA.29 The HA-groundwater suspension

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batch experiments were conducted at room temperature (~20 ºC) under oxic condition, basically

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similar to previously reported procedure.30 In brief, prior to adding a Pu isotopic tracer, 5-6 mg

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of the purified HAs were pre-equilibrated in artificial groundwater in a 15-mL polypropylene

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centrifuge tube for 48 h. 238Pu (as Pu(V) oxidation state) was amended to noncomplexing PIPBS

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(piperazine-N,N”-bis(4-butane sulfonic acid); GFS chemicals, Cat 2360) buffer (pH of 5.5) in

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order to neutralize the acidic Pu tracer and maintain this pH during the incubation period. The

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use of noncomplexing buffers avoids the formation of polymeric or colloidal Pu oxide

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particles.30-32 The 238Pu-amended PIPBS was then added to the HA-groundwater mixture to a

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final volume of 4 mL and Pu concentration of 8.4 × 10-14 M.8,33-35 The 238Pu-HA-groundwater

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slurry was then mixed continuously for 7 days in the dark with an end-over-end mixer to ensure

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that quasi-equilibrium was attained.30 It should be noted that some recent studies36 have

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suggested that viewing soil organic matter as a continuum (soil continuum

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model, SCM) does not allow one to consider concentration as varying parameter in the

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interpretation of data. Rather, known concentrations of HA or known HA:groundwater (w/v

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ratios) are needed in laboratory experiments to observe comparable and accurate data in metal

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partitioning experiments. High concentrations of HAs were also needed to mimic the soil

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erosion, since during the soil erosion and runoff events, pond discharge or rainwater passes

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through surface soil environments, where organic-rich soils predominate and the HA

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concentration is thus high in the resulting slurry system.

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The experiments mentioned above were conducted in duplicate. Two controls were included,

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1) a Pu tracer control (i.e., Pu-groundwater suspension without the HAs) to monitor Pu loss

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during the incubation due to adsorption to labware including centrifuge tubes and filters, and 2) a

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HA control (i.e., HA-groundwater suspension without Pu) to monitor organic matter released

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from the HA over the course of the experiment to ease the elemental analysis of different soil

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fractions without Pu tracer.

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After a 7-day incubation period, particulate (> 0.45 µm), colloidal (3 kDa to 0.45 µm), and

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truly dissolved (< 3kDa) phases were separated through 0.45 µm centrifugal filter tubes followed

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by ultrafiltration with 3 kDa Microsep centrifugal filter tubes (Millipore). Each fraction was

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analyzed for Pu activity and organic carbon and nitrogen concentrations, as described below. On

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the other hand, it should be mentioned that potential experimental artifacts, such as the

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aggregation of colloidal into particulate fractions or re-partitioning, may happen during a one-

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week resuspension experiment. Nevertheless, our observation showing the predominance of

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colloidal Pu and strong binding of Pu with organic matter (see discussion below) would suggest

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negligible influence from aggregation and re-partitioning during such an experiment.

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Determination of plutonium activity concentrations 238

Pu activities were determined by alpha-spectroscopy as previously described.16,21 Briefly,

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after resuspension experiment and separation as mentioned above, 242Pu was added into each

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fraction, as the yield tracer of Pu purification. The samples were oven-dried, then heated at 600

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ºC overnight in a ceramic crucible. The resulting ash was then digested in Teflon tubes overnight

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in concentrated HNO3 and HCl (1:1) at 85 ºC. The remaining solid residue was collected by

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centrifugation and disadarded, and the supernatant was further evaporated to incipient dryness. to

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convert all Pu to Pu(IV), a FeSO4•7H2O (0.2 g/mL) solution, followedby the 0.25 g NaNO2,

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were added to each sample, and the final volume of sample was 3 mL. Samples were then run

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through an UTEVA column (Cat. #: UT-C50-A, Eichrom, USA) to separate Pu from other alpha-

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emitting radionuclides (e.g., 238U, 241Am). After washing the column with a 8 M HNO3 solution,

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the 238,242Pu was eluted using freshly-prepared 0.02 M NH2OH HCl/0.02M ascorbic acid in 2M

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HNO3. The Pu-containing eluent was evaporated and re-constituted in 0.4 M (NH4)2SO4 (pH~2.6)

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for electroplating onto a stainless steel planchet at 0.6 Amps current for 2 h. Sample-bearing

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planchets were then analyzed via alpha spectroscopy for at least one week to obtain counting

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errors (1 sigma) lower than 5%. The detection limit for 238Pu and 242Pu is 6 × 10-5 Bq and 2.5 ×

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10-4 Bq, respectively.

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Measurement of organic carbon and nitrogen Concentrations of organic carbon and nitrogen in the colloidal and truly dissolved fractions

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were determined using a Shimadzu TOC-L analyzer and a high temperature combustion

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method.37 The organic carbon and nitrogen concentrations in the particulate phase of the HA-

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groundwater suspension were calculated as the difference between the total carbon/nitrogen

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contents of the added HAs and the sum of colloidal and truly dissolved phases.

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Partitioning coefficient values of Pu

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The traditional partitioning coefficient (Kd value) was used to describe the partitioning of Pu

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between particulate (>0.45 µm) and dissolved phases (0.45 µm), Ac and Ad

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represent the activity concentration of Pu in the colloidal (3 kDa to 0.45 µm) and truly dissolved

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phases (0.45 µm), colloidal (3 kDa to 0.45 µm) and truly

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dissolved phases (3 kDa; Equation 2) were three orders of magnitude greater, on average, than the logKd

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values (>0.45 µm; Equation 1) (logKdc = 5.05 ± 0.17 vs. logKd = 1.83 ± 0.29; Figure 1b).

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Based on the data shown in Figure 1, different HAs can have different influence on the

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distribution of Pu in particulate and colloidal fractions, resulting in a considerable range for

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particulate and colloidal Pu activity percentages, e.g., 70 ± 1% in the HA-1 (originating from

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Valence Lake, Hungary) to 92 ± 1.7% in the HA-4 (Karcag, Hungary) for the colloidal phase and

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4 ± 0.7% in the HA-7 (Gyosei, Japan) to 14 ± 1.3% in the HA-8 (Ichijima, Japan) for the

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particulate phase (ANOVA, p < 0.01). Moreover, if normalized by the concentrations of

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particulate or colloidal organic carbon, specific activity concentrations of Pu on POC and COC

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(Par-Pu/POC and Col-Pu/COC) can range from 9 Bq/g-POC to 27 Bq/g-POC and from 110

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Bq/g-COC to 1384 Bq/g-COC, respectively (Table S1), further indicating the preference of Pu

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for colloidal HAs. In contrast, with such widely contrasting soil origins from different soils on

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different continents, the results exhibit a relatively narrow range and similar magnitude of

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partitioning coefficient values for Pu, especially the logKdc values, if minimum and maximum

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points were not considered (4.90 ± 0.13 to 5.22 ± 0.07, Table 1, Figure 1b, ANOVA, p >0.05).

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This may be partially attributed to how we operationally defined the colloidal fraction and logKdc

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values (Eq. 2), but most importantly, it was basically due to the the fact that, the vast majority of

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Pu parititioned to the colloidal phase at pH of 5.5, a potentially more mobile Pu fraction,

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regardless of the type of the soil from which the HA was extracted. Only a minor portion of Pu

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was associated with the particulate phase, suggesting that soil erosion caused by natural or

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anthropogenic processes can trigger the release of Pu isotopes. By virtue of its size, it is expected

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that the colloidal fraction would be more mobile than the particulate fraction.

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Such Pu mobilization would be largely associated with the release and remobilization of

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organic macromolecules (i.e., colloidal organic matter, COM) from the HAs. The results indicate

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that our HAs samples behaved differently, with different organic macromolecules being released

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differently when they were resuspended in the groundwater. For example, the released COC and

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CON concentrations varied from 45 to 532 mg/L and from 0 to 54 mg/L, respectively,

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corresponding to colloidal percentages in the bulk organic C and N pool ranging from 5% to 74%

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for COC and from 0% to 74% for CON (Table S1). With such a wide range in both Pu activity

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percentages and organic matter concentrations, the possible correlations can be interpreted as

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indicating the extent of Pu binding to organic matter. As shown in Figure 2, the concentrations of

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colloidal organic nitrogen (CON in terms of mg/L, Figure 2b) and carbon (COC, Figure 2e)

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strongly and positively correlated with the colloidal Pu fraction. Similarly, the activity

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percentage of particulate Pu was also positively correlated with the contents of particulate

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organic-nitrogen (Figure 2a) and -carbon (Figure 2d), suggesting that the organic matter still

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associated with particulate HAs after one week of equilibration in artificial groundwater, also

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exhibited a significant influence on the fixation and immobilization of Pu. Furthermore, the

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adsorption behavior of Pu on particulate HAs (i.e., logKd) is strongly regulated by the

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partitioning between potentially-immobile and mobile organic compounds during the HA-

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groundwater mixing period, as indicated by the positive relationship between Pu logKd and

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organic nitrogen or carbon between particulate and dissolved phases (i.e., logKd-ON in Figure 2c

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and logKd-OC in Figure 2f. POC or PN concentrations were calculated by the ratio of the sum of

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colloidal and truly dissolved organic carbon or nitrogen concentrations and the total mass

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concentration of the added HA concentration). Although the relationship of colloidal partitioning

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between Pu and organic nitrogen/carbon cannot be tested due to the unknown concentrations of

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total colloidal matter, we propose that the partitioning between colloidal and truly dissolved Pu

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during the HA-groundwater interactions (e.g., logKc) can be also controlled by the relative

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distribution of colloidal and truly dissolved organic nitrogen (i.e., logKc-ON).

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The results of our laboratory HA-groundwater mixing experiments demonstrated a direct

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relationship of the organic carbon, and more importantly the organic nitrogen with the Pu

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activity concentration, which are consistent with and confirm previous field observations.20-23

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More importantly, with our size fractionation among particulate, colloidal and dissolved

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fractions, the results provide novel and direct evidence supporting the essential role of particulate

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organic compounds in immobilizing Pu in contaminated areas. Although these correlation data

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can not provide direct evidence for the complete cause of Pu remobilization in the real soil

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environments, our results still imply that colloidal organic compounds can enhance Pu

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remobilization in contaminated soils. This implies that a duel influence from both POM and

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COM on Pu behavior may occur during storm runoff and discharge events in natural

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environments. In comparison, the colloidal organic fraction provides more surface availability of

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complexing ligands or higher specific surface areas for Pu mobilization than the particulate

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organic fraction did for Pu binding, as indicated by a greater amount of Pu existing in a colloidal

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form. This was the case even though the abundance of COM relative to POM was lower in terms

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of carbon (e.g., 29 ± 24% on average for COC in the bulk carbon pool, Figure S1 and Table S1).

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It was also possible that at weak acidic pH (5.5) condition, smaller molecules containing higher

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aromaticity can be better solubilized into colloidal fraction than those larger molecules

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containing less aromaticity. Furthermore, this characteristic of colloidal organic compounds can

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directly reduce the Pu binding ability of residual particulate HAs, as is evident from the negative

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correlation between logKd values and the proportion of released CON or COC from the bulk

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organic matter pool (Figure 3). In natural environments the soil-erosion derived remobilization

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of organic macromolecules from sedimentary organic matter can vary with soil type and/or

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composition.40,41 Accordingly, in the present study the proportion of released organic matter

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from the initial HAs also varied among different HAs (Figure S1), consequently causing the

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variation in the resultant logKd values of Pu among different HAs. For instance, more organic

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matter was released from Chernozems-type HA (greater aromaticity) into the colloidal fraction,

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while the opposite was observed for Cambisols-type HA (less aromaticity) (Table 1 and Figure

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S1). Together, our observations not only confirm the role of natural organic matter (NOM) to

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convert Pu from a sink to a source during soil erosion, but also suggest that the desorption

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behavior of Pu from sedimentary organic matter is largely dependent on how much organic

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matter would be released into the colloidal fraction during a given soil erosion event.

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Supporting evidence of carboxylate groups responsible for Pu partitioning behavior Given the essential role of NOM in controlling the partitioning of Pu, as mentioned above,

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we further explore and characterize the organic functional moieties of the two organic fractions

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and attempt to correlate them to Pu partitioning coefficients. Through solid state 13C nuclear

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magnetic resonance (NMR) spectroscopy measurements,26 the main carbon functionalities of

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HAs had been identified and quantified, as consisting of alkyl, O-alkyl, aryl, O-aryl, COO and

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CO associated compounds, and composing 16 ± 7%, 20 ± 7%, 35 ± 13%, 8 ± 2%, 17 ± 2% and 3

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± 1% of the total organic carbon pool, respectively (Table 1).

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Among these organic ligands, only the abundance of the major COO group moiety (i.e., the

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carboxylate group) was found to significantly and positively correlate with the Pu partitioning

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between surface-bound (>3 kDa) and truly dissolved fractions (i.e., logKdc values, Figure 4a,

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Table 1), suggesting that the carboxylate group can be a specific functional group responsible for

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the relatively constant specific activity concentrations of Pu in particulate and colloidal phases as

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mentioned above. To futher evaluate this correlation in particulate and colloidal phases, we

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assumed that the carboxyl-containing molecules in the initial HA samples have the same

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releasing ability to the colloidal fraction as the bulk organic carbon pool does (i.e., 29 ± 24% of

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organic carbon was released from initial HAs into colloidal fraction, Figure S1) although this

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assumption might possibly bias the correlation. However, because of their pKa values below the

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pH of the water, the carboxyl groups are mostly dissociated and thus, impart a negative charge to

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the HAs, and giving them a solubilizing effect. The results showed that the particulate or

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colloidal Pu activity concentrations can positively correlate with the concentrations of particulate

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or colloidal carboxyl-containing molecules, respectively (Figures 4b and 4c). In addition, in

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terms of partitioning coefficient, the positive relationship between Pu and carboxyl-containing

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molecules in the particulate-dissolved partitioning is also evident (Figure 4d), similar to those

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observed for organic carbon and nitrogen contents (Figures 2c and 2f). These tight correlations

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between Pu and carboxyl-containing molecules, regardless of being in the particulate or colloidal

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phase, further suggest that the bulk organic carboxylate functionality is related to the main Pu

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complexing agent for particulate and colloid organic matter, even though carboxylates were not

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the predominant carbon functionality in our HAs samples (Table 1, which may also explain why

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the correlation between organic carbon/nitrogen contents and logKdc values was weak).

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Clustered carboxylate groups can have strong binding properties for metal ions in natural

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waters, like the binding of actinide metals (e.g., thorium) with carboxyl-containing acid

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polysaccharides.42-46 Previous studies using electrospray ionization combined with Fourier-

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transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) and multi-NMR

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demonstrated that the predominant Pu carrying macromolecules in soil contained more N than

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the bulk soil colloidal organic matter and were also abundant in carboxylic chelating functional

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groups.20,22,23 Therefore, by combining our observations on the significant relationship between

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Pu partitioning and organic nitrogen or carboxyl-containing compounds (Figures 2 and 4), our

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results not only support the observed relation between organic nitrogen compounds and Pu

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binding from natural soils, but also provides additional evidence supporting the conclusion that

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carboxylate containing compounds, in both particulate and colloidal phases, appear to be

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important moieties related to Pu complexation by NOM in natural environments. Importantly,

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these relationships were derived from HA compounds extracted from NOM-containing soils

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collected from different world-wide regions. Thus, the interaction between HAs with other

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components could happen in natural soils, which can potentially affect the HAs’ nature and their

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interaction with Pu and warrants further studies.

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The strongest known naturally occurring binding agents for Pu containing carboxyl groups

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have previsouly been reported as hydroxamates,20,22,47-49 which are siderophore compounds with

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–NOH and COO- moieties, whereby Pu is directly bound to the O of the –NO- group. Because

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hydroxamate compounds chelate Pu, this results in the greatest association constants for Pu

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known in nature.50 Although it is likely that hydroxamate siderophores contributed to the positive

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relation between the partitioning of Pu and organic nitrogen or carboxyl-containing organic

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compounds observed in our study, the present study can not provide any direct evidence. This

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was due to the fact that the hydroxamate contents in the present study could not be quantified

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due to interferences from the high concentrations of HAs and the universally low abundance of

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hydroxamates in the bulk organic matter pool (e.g., 0.019– 13.671 mg-C/g-C for a contaminated

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soil in Savannah River Site20). Further studies are required to identify other potentially abundant

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organic compounds that can be responsible for the binding and remobilization of Pu in soils.

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Implications on Pu environmental behavior

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Our laboratory equilibration experiments were carried out under slightly acidic conditions

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(pH ~5.5), which resulted in 76 ± 12% of total Pu and 29 ± 24% of organic matter partitioning

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into the colloidal phase (Figure 1 and Figure S1). Considering that increases in pH promote

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COM release during extreme soil saturation events and strong Pu binding with COM,30,51-53

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enhanced COM-derived remobilization of potentially mobile Pu would be expected to be more

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pronounced under less acidic conditions during natural or anthropogenic events such as base

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injection remediation, intense rainstorms or flooding. Under these conditions, the Pu bound to

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OM would potentially behave as a contaminant source to the surrounding environments30.

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To our knowledge, this is the first controlled laboratory experiment conducted using

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environmentally relevant Pu concentrations to document a relationship between size-fractionated

400

carboxyl-containing or organic nitrogen compounds and the Pu among particulate, colloidal, and

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dissolved phases. HAs with different molecular compositions and varying organic matter

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abundance (Table 1) that were collected from widely contrasting soil origins exhibited

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remarkably similar Kd (Equation 1) or Kdc (Equation 2). Furthermore, Pu partitioned to the HA

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extremely strongly, showing significant correlations with organic carbon or nitrogen (Figures 2

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and 3), supporting the regulating role of NOM in complexing and binding of Pu. In detail, the

406

present study further suggests that Pu binding to soil OM is directly or indirectly (e.g.,

407

carboxylate groups can be considered as proxies for other functional groups that occur in

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relatively fixed proportions) controlled by carboxyl-containing compounds (Figure 4). These

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latter results support field observations from natural soil environments in different continents,

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including the Savannah River Site, USA,20 Fukushima Prefecture area, Japan21 and Rocky Flats

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Environmental Technology Site, USA22-23 where N-containing compounds such as strongly Pu

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complexing N-containing hydroxamates were present in a Pu enriched fraction. In addition, the

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tendency of Pu to adsorb to colloidal HAs was much greater than to particulate HAs. Because

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colloidal HA is typically more mobile than particulate HA, these results have implications on

415

whether NOM acts as a Pu source or sink during natural or man-induced episodic flooding

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events. Most importantly, strong Pu binding to soil OM compounds such as HAs also reverses

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the pH dependency of Pu sorption to soils.30,49 Nevertheless, the limitation of the present study,

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such as not considering the existence of inorganic materials in the natural soil mixture, and

419

possible experimental artifacts that alkaline extractions of HA from SOM (e.g., harsh extractants)

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could potentially bias the conformation of the organic matter, might require more comprehensive

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studies in the future.

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Author Information

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Corresponding Author

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*Phone: +1-409-740-4530; fax: 1-409-740-4787; email: [email protected].

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Acknowledgments

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This work was funded by the U.S. Department of Energy (DOE) Office of Science Subsurface

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Biogeochemistry Research program (ER65222-1038426-0017532), DOE SBR grant #DE-

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ER64567- 1031562-0014364, DOE SBR award # DEeSC0014152, and DOE contract DE-AD09-

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96SR18500. Work was also supported by DOE’s contract DE-AC09-08SR22470 with Savannah

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River Nuclear Solutions, LLC. Yuko Sugiyama was supported by JSPS Bilateral Joint Research

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Projects with USA and Grants-in-Aid for Scientific Research #24248027. Nobuhito Ohte was

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partly supported by a grant for River Management Research (FY2014, 2015) from the River

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Foundation (Kasen Zaidan, Japan).

436 437

Associated Content:

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The Supporting Information is available:

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Partitioning and concentration of organic carbon and nitrogen in the particulate and colloidal

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phases after HA-groundwater resuspension

441 442

17



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References:

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1. Bunzl, K.; Kracke, W.; Schimmack, W. Migration of fallout 239+240Pu, 241Am and 137Cs in

445

the various horizons of a forest soil under pine. J. Environ. Radioact. 1995, 28, 17-34.

446

2. Zheng, J.; Tagami, K.; Watanabe, Y.; Uchida, S.; Aono, T.; Ishii, N.; Yoshida, S.; Kubota,

447

Y.; Fuma, S.; Ihara, S. Isotopic evidence of plutonium release into the environment from

448

the Fukushima DNPP accident. Sci. Rep. 2012, 2, 304.

449

3. Schneider, S.; Walther, C.; Bister, S.; Schauer, V.; Christl, M.; Synal, H. A.; Shozugawa,

450

K.; Steinhauser, G. Plutonium release from Fukushima Daiichi fosters the need for more

451

detailed investigations. Sci. Rep. 2013, 3, 2988; DOI:10.1038/srep02988.

452

4. Riley, R. G.; Zachara, J. M.; Wobber, F. J. Chemical contaminants on DOE lands and

453

selection of contaminant mixtures for subsurface science research. US-DOE, Energy

454

Resource Subsurface Science Program, 1992, Washington, DC, USA;

455

DOI:10/2172/10147081.

456

5. Oughton, D. H.; Fifield, L. K.; Day, J. P.; Cresswell, R. C.; Skepperud, L.; Di Tada, M.

457

L.; Salbu, B.; Strand, P.; Drozcho, E.; Mokrov, Y. Plutonium from Mayak: measurement

458

of isotope ratios and activities using accelerator mass spectrometry. Environ. Sci. Technol.

459

2000, 34, 1938-1945.

460

6. Buesseler, K. O.; Kaplan, D. I.; Dai, M.; Pike, S. Source-dependent and source-

461

independent controls on plutonium oxidation state and colloid associations in

462

groundwater. Environ. Sci. Technol. 2009, 43, 1322-1328.

463

7. Kersting, A. B.; Efurd, D. W.; Finnegan, D. L.; Rokop, D. J.; Smith, D. K.; Thompson, J.

464

L. Migration of plutonium in ground water at the Nevada Test Site. Nature 1999, 397

465

(6714), 56-59.

466

8. Novikov, A. P; Kalmykov, S. N.; Utsunomiya, S.; Ewing, R. C. Colloid transport of

467

plutonium in the Far-Field of the Mayak production. Science 2006, 314, 638-641.

468

9. Demirkanli, D. I.; Molz, F. J.; Kaplan, D. I. Fjeld, R. A. A fully transient model for long-

469

term plutonium transport in the Savannah River site vadose zone: root water uptake.

470

Vadose Zone J. 2008, 7, 1099-1109.

471

10. Ovsiannikova, S.; Papenia, M.; Voinikava, K. Brown, J. Kipperud, L.; Sokolik, G.;

472

Svirschevsky, S. Migration ability of plutonium and americium in the soils of Polessie

473

State Rediation-Ecological Reserve. J. Radioanal. Nucl. Chem. 2010, 286, 409-415.

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Page 19 of 29

474


11. Schwantes, J. M.; Santschi, P. H. Mechanisms of plutonium sorption to mineral oxide

475

surfaces: new insights with implications for colloid-enhanced migration. Radiochimica

476

Acta 2010, 98, 737-742.

477 478 479 480 481

12. Bu, W.; Zheng, J.; Guo, Q.; Uchida, S. Vertical distribution and migration of global fallout Pu in forest soils in southwestern China. J. Environ. Radioact. 2014, 136, 174-180. 13. Orzel, J.; Komosa, A. Study on the rate of plutonium vertical migration in various soil type of Lublin region (Eastern Poland). J. Radioanal. Nucl. Chem. 2014, 299, 643-649. 14. Asbury, S. M. L.; Lamont, S. P.; Clark, S. B. Plutonium partitioning to colloidal and

482

particulate matter in an acidic, sandy sediment: implications for remediation alternatives

483

and plutonium migration. Environ. Sci. Technol. 2001, 35, 2295-2300.

484

15. Kalmykov, S. N.; Batuk, O. N.; Bouby, M.; Denecke, M. A.; Novikov, A. P.; Perminova,

485

I. V.; Shcherbina, N. S. Plutonium speciation and formation of nanoparticles with natural

486

organic matter in contaminated environment. Geochim. Cosmochim. Acta 2010, 74 (12),

487

A490−A490.

488 489

16. Santschi, P. H.; Roberts, K. A.; Guo, L. Organic nature of colloidal actinides transported in surface water environments. Environ. Sci. Technol. 2002, 36 (17), 3711−3719.

490

17. Eyrolle, F.; Charmasson, S. Importance of colloids in the transport within the dissolved

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phase (0.45 µm), colloidal (3 kDa to 0.45 µm), and truly

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dissolved phases (3 kDa); b) Partitioning

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coefficient values of Pu between particulate (>0.45 µm) and dissolved phases (3 kDa) and truly

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dissolved phase (

Plutonium Partitioning Behavior to Humic Acids from Widely Varying

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