Effects of Fulvic Acid on Uranium(VI) Sorption Kinetics - Environmental

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Effects of Fulvic Acid on Uranium(VI) Sorption Kinetics Ruth Maria Tinnacher, Peter Silvio Nico, James A. Davis, and Bruce D. Honeyman Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es304677c • Publication Date (Web): 03 Apr 2013 Downloaded from http://pubs.acs.org on April 22, 2013

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Environmental Science & Technology

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Effects of Fulvic Acid on Uranium(VI)

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Sorption Kinetics

3 Ruth M. Tinnacher1,2,*, Peter S. Nico2, James A. Davis2, Bruce D. Honeyman1

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Department of Civil and Environmental Engineering, Colorado School of Mines,

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Golden, CO 80401, USA 2

Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

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*)

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Corresponding author: Current contact information: Earth Sciences Division, Lawrence Berkeley National Laboratory 1 Cyclotron Rd., MS 74R0120 Berkeley, CA 94720 Phone: (510) 495 8231 Fax: (510) 486 5686 Email: [email protected]

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KEYWORDS

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Uranium, fulvic acid, natural organic matter, sorption, adsorption, kinetics

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ACS Paragon Plus Environment

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ABSTRACT

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This study focuses on the effects of fulvic acid (FA) on uranium(VI) sorption kinetics to a silica

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sand. Using a tritium-labeled FA in batch experiments made it possible to investigate sorption

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rates over a wide range of environmentally-relevant FA concentrations (0.37-23 mg l-1 TOC).

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Equilibrium speciation calculations were coupled with an evaluation of U(VI) and FA sorption

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rates based on characteristic times. This allowed us to suggest plausible sorption mechanisms as

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a function of solution conditions (e.g., pH, U(VI)/FA/surface site ratios). Our results indicate

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that U(VI) sorption onto silica sand can be either slower or faster in the presence of FA

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compared to a ligand-free system. This suggests a shift in the underlying mechanisms of FA

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effects on U(VI) sorption, from competitive sorption to influences of U(VI)-FA complexes, in

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the same system. Changes in metal sorption rates depend on the relative concentrations of

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metals, organic ligands and mineral surface sites. Hence, these results elucidate the sometimes

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conflicting information in the literature about the influence of organic matter on metal sorption

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rates. Furthermore, they provide guidance for the selection of appropriate sorption equilibration

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times for experiments that are designed to determine metal distribution coefficients (Kd values)

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under equilibrium conditions. Characteristic time for U(VI) sorption [hr]

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Slower uranium sorption kinetics

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Faster uranium sorption kinetics

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Data-based Model-based

Fulvic acid concentration [M]

TOC Art

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INTRODUCTION

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Numerous studies have demonstrated the influence of Natural Organic Matter (NOM) on metal

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sorption behavior onto minerals. In comparison, little is known about NOM effects on metal

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sorption kinetics, despite the fact that these effects can be relevant for the following reasons.

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First, lab-studies are often designed with the objective of determining metal sorption

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parameters in the presence and absence of NOM under equilibrium conditions.

If NOM

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influences metal sorption rates, then equilibrium time-frames selected for binary metal-mineral

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systems may not be sufficiently long to attain sorption equilibria in ternary systems. Besides

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other scale-dependent differences, this may further lead to errors in contaminant transport

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models, e.g. if lab-based metal distribution coefficients (Kd values) are used to evaluate NOM

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effects on contaminant mobility.

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Second, under advective flow conditions, e.g., in laboratory-scale advective column studies1-3

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and dynamic flow-systems in the field4, local contact times between metal contaminants and bulk

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mineral phases may often be too short to attain full sorption equilibria. Hence, in the presence of

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NOM, kinetic metal transport models may also need to include potential organic matter effects

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on metal sorption kinetics. In fact, in kinetically-limited systems apparent NOM effects on metal

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sorption may be due to a combination of different (equilibrium) metal sorption affinities and

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changing metal sorption rates.

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Third, an understanding of the underlying mechanisms of sorption processes is important for

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the development of defensible, kinetically-based transport models, especially in systems where

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metal/NOM concentration ratios are changing over time and space. It is well known that overall

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reaction rates change if the series of elemental reactions comprising a reaction pathway has been


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altered5. Hence, kinetic studies provide a simple, experimental tool to identify potential shifts in

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metal sorption mechanisms based on the observation of varying sorption rates, e.g., as a function

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of organic ligand concentrations.

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Overall, NOM effects on metal sorption behavior have been ascribed to a variety of underlying

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processes, including (1) metal-ligand solution complexation combined with different sorption

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affinities of the resulting complexes compared to ‘free’ metals in solution6, 7, and (2) NOM

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sorption onto minerals leading to competitive sorption behavior8,

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surface characteristics10. Hence, organic matter may change both aqueous and sorbed metal

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speciation, and thus affect the underlying pathways and rates of metal sorption reactions in the

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following ways.

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and/or changes in mineral

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First, metal-ligand dissociation reactions occurring prior to the sorption of ‘free’ metal ions

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may slow down overall metal sorption kinetics11, 12. Second, NOM sorption onto the mineral

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surface may lead to a competition between metals and organic ligands for the same reactive

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surface sites, potentially decreasing metal sorption rates based on mass action principles. Last,

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the sorption kinetics of metal-NOM solution complexes may be regulated by rates of NOM

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surface reactions. In this case, possibly rate-limiting steps during NOM sorption reactions could

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include NOM diffusion to mineral surface sites, steric rearrangements on the surface13, and

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different NOM sorption rates for various ligand sizes14, 15. In contrast, faster sorption rates may

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be possible in case of multi-layer sorption of organics.

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With respect to uranium(VI), natural organic matter, such as fulvic acid, may complex U(VI)

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in solution16,

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natural waters18, which affects U(VI) sorption19, 20 and mobility both on the lab and field scale21,

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, and dominate U(VI) speciation in the acidic to circum-neutral pH range in

. At this point, systematic studies on organic matter effects on radionuclide sorption kinetics


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are still quite limited. Results from previous studies focusing on U(VI) sorption kinetics onto

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Savannah River Site sediments23 and montmorillonite24 suggest faster U(VI) sorption rates in the

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presence of humic acid, while europium(III) sorption onto quartz sand has been reported to be

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slower25. Based on these ‘apparent discrepancies’, we believe that changes in U(VI) sorption

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rates may be highly dependent on chemical solution conditions, metal and organic ligand

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speciation, mineral surface characteristics, and the relative concentrations of metals, organic

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ligands and mineral surface sites. Hence, further research is needed to systematically evaluate

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NOM effects on U(VI) sorption kinetics.

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In this study, we investigate the effects of fulvic acid (FA) on uranium(VI) sorption kinetics at

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various ratios of metal/organic ligand concentrations with the goal to qualitatively assess the

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mechanisms of U(VI) sorption in ternary systems.

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sorption experiments in order to characterize uranium(VI) and FA sorption onto a pretreated

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silica sand over a range of pH conditions and FA concentrations, either at specific sorption

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equilibration times (batch sorption envelope experiments) or as a function of time (batch sorption

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kinetic experiments). Potential changes in U(VI) or FA sorption kinetics were determined based

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on the calculation of characteristic times for overall sorption reactions. The relevance of U(VI)-

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FA solution complexes under various experimental conditions was evaluated by simulating

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U(VI) speciation based on existing thermodynamic data.

For this purpose, we performed batch

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We specifically selected a natural organic matter fraction (fulvic acid) and a mineral phase

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(pretreated silica sand) that would allow us to create experimental systems with tractable

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complexity. This approach provided us with the best chance to achieve our goal of improving

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conceptual models of NOM effects on U(VI) sorption kinetics. However, more complex and

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natural systems should be investigated in the future, with a particular focus on the role of


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competing ions, such as calcium, magnesium and sulfate, which may limit the impacts of NOM

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on radionuclide mobility26-28.

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MATERIALS AND METHODS

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Uranium, Fulvic Acid and Silica Sand

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Uranium(VI) solutions contained natural uranium (99.28 % U-238, 1 mg ml-1 U ICP-Standard,

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Anderson Laboratories) and a uranium-233 tracer (4 µCi ml-1 UO2(NO3)2, Isotope Products

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Laboratory) , which was

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TR 2500/TR 1600 Liquid Scintillation Analyzers).

quantified by liquid scintillation counting (Packard Instruments

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A standardized form of fulvic acid, Suwannee River fulvic acid (FA) reference (International

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Humic Substances Society, Cat. No. 1R101F-1), was used in order to ensure the reproducibility

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and comparability of experimental results (for FA characteristics, see Table S1, Supporting

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Information, and the literature29).

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scintillation counting of a tritium-labeled fulvic acid tracer (1.9 mCi mg-1 FA, detection limit: 0.3

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µg l-1 FA)30. No significant differences in FA sorption behavior between the original and

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radiolabeled form of FA were found in previous testings with hematite30.

Fulvic acid concentrations were determined by liquid

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Commercially available silica sand (Q-ROK #1 Ground Silica, U.S. Silica) was selected as the

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mineral phase and pretreated31, 32 (for details, see Supporting Information) to produce a uniform,

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well-mixed mineral phase of known particle size range, low organic carbon content, and with a

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minimum potential for the release of mineral colloids in later experiments (average particle size:

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418 ± 40 µm; geometrically-estimated specific surface area: 54 cm2 g-1; total reactive surface site

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concentration (assuming spherical particles and 5 sites nm-2)33: 4.5 × 10-8 mol g-1; pHpzc = 6.3).


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A characterization of mineral abundances on the surface based on QEMSCAN/EDX analysis

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indicated the presence of a total of 0.2% of surface impurities, including kaolinite, muscovite,

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Fe-oxides, K-feldspar, and rutile/anatase (Table S2, Supporting Information).

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Batch Sorption Envelope Experiments

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All experiments were performed at room temperature and open to the atmosphere. In batch

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sorption envelope experiments, pretreated silica sand and aliquots of UV-water (Barnstead

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EASYpure UV compact ultrapure water system) and 1 M NaCl solution were transferred into 20

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ml polyethylene liquid scintillation vials to give 200 or 400 g l-1 solid in 10 ml final sample

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volume with a total ionic strength of 0.01 M NaCl (or NaCl/NaHCO3 for target pH > 7.0).

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Solution pH values were adjusted with small volumes of 0.1, 0.01, 0.001 M HCl or NaOH

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solutions, and samples pre-equilibrated under shaking over 12 to 24 hours (TITER shaking

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table). Then, uranium and/or fulvic acid solutions were added individually, without any prior

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metal-NOM pre-complexation. After pH re-adjustments, sorption equilibration was allowed

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during shaking in the dark over 48 (binary U(VI)-silica sand systems) or 72 hours (any system

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containing FA) to allow for a comparison of results with literature data19, 34. Afterwards, final

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solution pH values were recorded and fractions of supernatant solutions collected, without any

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further filtration or centrifugation, for the analysis of remaining solute concentrations.

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Uranium(VI) and/or FA wall sorption effects were corrected by washing sample vials with acid

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or base solutions (0.12 M HCl for any U(VI) systems; 0.1 M NaOH for binary FA-sand

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systems); experimental standards were included to represent 100 % of U(VI) and/or FA

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concentrations in mineral-free solutions.

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Batch Sorption Kinetic Experiments

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In kinetic experiments, silica sand and aliquots of UV-water, 1 M NaCl solution, and NaHCO3

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buffer (for pH >7.0), were transferred into autoclaved Nalgene bottles to give the desired sample

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composition in the final sample volume (ITot=0.01 M, VTot=250 ml, 200 g l-1 silica sand).

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Solution pH values were adjusted to pH=7 or pH=8, and solutions allowed to equilibrate with

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mineral surfaces and atmospheric CO2 overnight. After pH re-adjustments on the next day, an

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aliquot of each electrolyte solution in contact with the mineral phase (20 ml) was transferred into

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a 50 ml polycarbonate vial. Solute(s) of interest was/were added to this small vial individually,

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and the pH value re-adjusted. Then, the content of each 50 ml polycarbonate vial was rapidly

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transferred to the corresponding Nalgene bottle. Immediately afterwards, solution fractions were

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sampled, without further filtration or centrifugation, in order to determine exact initial solute

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concentrations at time zero. Closed sample vials were then set up on the shaking table to allow

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for sorption equilibration while minimizing evaporation losses.

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At time-points shown in Fig. 5, shaking was interrupted for scheduled sampling events

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(without filtration/centrifugation) and pH control/re-adjustments. Both inherently lead to slight

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changes in solid-liquid ratios over time. In order to minimize this effect to ≤10%, the largest

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feasible sample volume (250 ml) was combined with the smallest possible supernatant fractions

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(1 ml). No wall sorption correction was performed, since the mineral surface area largely

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exceeds the container surface area in this setup.

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Calculation of Characteristic Times

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For the interpretation of experimental kinetic data, our goal was to determine differences (or

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similarities) between sorption rates in systems with varying FA concentrations based on an


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objective mathematical parameter, namely the characteristic time for overall sorption reactions.

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It was not our goal to develop a model with predictive capabilities or to find the best model fits.

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Characteristic time ( t1/ 2 ) is defined as the time needed to reach a specific fraction of the final

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equilibrium concentration, e.g., 50% for pseudo-first order reversible sorption kinetics5,

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Generally, long characteristic times (large t1/ 2 values) indicate slow sorption kinetics; short

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characteristic times (small t1/ 2 values) represent fast kinetics. Systems with the same sorption

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kinetics show the same fraction of the equilibrium surface concentration sorbed at any given

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point in time, independent of the individual equilibrium values approached. Hence, comparable

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values of t1/ 2 indicate similar sorption kinetics.

.

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In this study, characteristic times for U(VI) and FA sorption reactions were determined in two

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ways, a data-based estimation method and a model-based calculation using fitted rate constants.

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The data-based approach has the primary advantage that, unlike any kinetic model, it does not

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require any a priori assumptions regarding the order or mechanism of sorption reactions. Invalid

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modeling assumptions may potentially lead to misleading conclusions. However, since only a

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few data points are used in this estimation, experimental errors may negatively affect the

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comparison of data-based t1/ 2 values, especially if kinetic differences are small. Therefore, we

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also applied a model-based calculation, which involves kinetic rate constants determined by

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fitting all experimental time-points of a kinetic experiment.

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First, in the data-based estimation, we calculated the fractions of U(VI) and FA equilibrium

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surface concentrations reached over time while assuming that the last experimental time-points

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represent equilibrium surface concentrations.

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concentration of 3.64 × 10-10 mol U(VI) gSolid-1 (10-7 M U(VI)Total, 200 g/l silica sand, pH=7),

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1.99 × 10-10 mol U(VI) gSolid-1 represent 54.6% of the equilibrium surface concentration, and the

For example, for an equilibrium surface


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time needed to reach this concentration is slightly longer than t1/ 2 . After plotting these fractions

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as a function of time (Fig. 6), the system-specific values of t1/ 2 were determined based on a

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linear regression between the two data points ‘bracketing’ the 50% fraction of the equilibrium

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surface concentration, e.g., 34.2% and 54.6%.

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For the model-based calculations (for details, see Supporting Information), we tested various

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rate expressions36, and selected pseudo-first order reversible sorption kinetics, as total surface

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site coverages were expected to be low in kinetic sorption experiments under most chemical

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conditions. Furthermore, the relative simplicity of their mathematical expressions37 provides a

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direct and objective approach to compute characteristic times, which is not equally possible for

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higher order models. In addition, this rate law allows us to include the reversibility of surface

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reactions, which is necessary for a characterization of long-term sorption behavior with time-

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points close to or at apparent equilibrium.

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Using U(VI) as example, and assuming a large excess of reactive surface sites, the change in U(VI) solution concentration over time (t) due to U(VI) sorption reactions is described by

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d [U ] = −k 'f [U ] + kr [≡ SU ] dt

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where [U] and [≡ SU] represent concentrations of U(VI) in solution and on the surface

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respectively (all in mol l-1), and the variables kf’ (hr-1) and kr (hr-1) are the forward and reverse

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rate constants for pseudo-first order sorption kinetics. Fitted rate constants (Mathematica 7.0)

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are then used to calculate t1/ 2 for overall sorption reactions under various chemical conditions

t1/ 2 = 219

ln 2 k + kr ' f


(1)

(2)

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whereas the expression (k 'f + k r ) represents the ‘natural’ rate at which sorption equilibrium is

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approached35.

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Chemical Speciation Modeling

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Speciation models were set up in HYDRAQL38. Uranium(VI) complexation with inorganic

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ligands was based on thermodynamic data from the NEA database39 (Table S3, Supporting

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Information). Fulvic acid was simulated as a suite of discrete, monoprotic ligands with set acid

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dissociation constants (pKa=2, 4, 6, 8, 10) following the approach by Westall et al.40. Individual

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concentrations of monoprotic ligands as well as U(VI)-fulvic acid complexation reactions and

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constants (Table S3, Supporting Information) were adopted from a previous study using the same

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type of fulvic acid17, 41. Effects of different ionic strengths used in complexation and sorption

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experiments on complexation constants were corrected mathematically. However, as U(VI)-FA

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complexation constants were determined under fairly acidic conditions (pH=4 and 5), and pH

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conditions in kinetic experiments were slightly higher (pH=7 and 8), the presented speciation

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diagrams should be regarded as qualitative rather than quantitative results.

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RESULTS

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Batch Sorption Envelope Experiments

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U(VI) Batch Sorption Envelopes

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Uranium(VI) sorption onto silica sand is characterized by low sorption at low and high pH

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conditions, and a maximum at around pH 6-6.5 (Fig. 1). At low pH, U(VI) sorption is limited


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due to competition with protons for surface sites15. At high pH, low uranium sorption is

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attributed to increasing carbonate concentrations leading to competing carbonato species and/or

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weakly or non-sorbing uranium-carbonato complexes42, 43 (Fig. S1, Supporting Information).

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In this study, decreasing solid-liquid ratios cause a ‘shrinking’ of uranium sorption envelopes

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indicating surface site limitations and/or a distribution of surface site types. All experimental

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systems except one (10-5 M U(VI)Tot, 200 g l-1 sand) are estimated to provide an excess of total

249

surface site concentrations.

250

Information) suggest 0.2% of surface impurities, which could result in a limited number of high-

251

affinity sites on other minerals besides a large excess of weakly-sorbing silanol sites.

However, QEMSCAN/EDX data (Table S2, Supporting

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Fulvic Acid Batch Sorption Envelopes

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Results from binary fulvic acid sorption experiments indicate a ligand-like sorption behavior

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with lower FA sorption at increasing pH (Fig. 2). Larger fractions of FA are sorbed at lower

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total organic ligand concentrations, indicating a fractionation of the FA mixture during

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sorption14, 44, a limitation in surface sites and/or a distribution of surface site types.

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Despite the fairly small FA fractions sorbed, a large number of surface sites may still become

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occupied with organic ligands over the tested concentration range (6.15 × 10-8-6.15 × 10-5 M FA,

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0.02-23 mg l-1 TOC). Under acidic conditions, FA sorption isotherms suggest a saturation of

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total sites at 23 mg l-1 TOC (Figs. S2 and S3, Supporting Information), while strong surface sites

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may become saturated far below this concentration. Assuming FA sorption in the form of

263

monoprotic ligands (6.22 mM ligands g-1 FA19,

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comprised of surface impurities these sites could be fully occupied by FA at most conditions

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tested (exceptions: 6.15 × 10-5 M and 6.15 × 10-8 M FA at pH~9). Hence, FA sorption onto silica

41

) and a fraction of 0.2% of strong sites


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sand may limit the availability of strong sites for U(VI) sorption reactions, even at very low FA

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concentrations (6.15 × 10-8 M FA; 23 µg l-1 TOC).

268 269

U(VI)-Fulvic Acid Batch Sorption Envelopes

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For all FA concentrations tested, the presence of organic ligands causes a decrease in U(VI)

271

sorption within a pH range from 5.5-7.0 (Fig. 3). However, results from speciation modeling

272

suggest that the presence of U(VI)-FA complexes is fairly limited in these systems, e.g., with

273

≤4.74% of U(VI) complexed with organic ligands at pH=7 (Fig. 4). Hence, FA effects on U(VI)

274

sorption are probably due to a competition between ‘free’ metals and organic ligands for mineral

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surface sites. This hypothesis is further supported by the strong decrease in U(VI) sorption

276

observed at 10-5 M FA compared to lower FA concentrations, and by calculations of total surface

277

site coverages. Using a specific site concentration of 4.5 × 10-8 mol g-1, U(VI) site coverage

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decreases from 0.9% to 0.7% and 0.2% at zero, 10-6 M and 10-5 M FA, respectively. In contrast,

279

FA site coverage increases from 5.1% (10-6 M FA) to 53.4% (10-5 M FA) based on binary FA

280

sorption data. Hence, the surface site limitations indicated for U(VI) sorption in binary systems

281

(Fig. 1) are further enhanced in the presence of FA. Overall, in mineral systems, which are

282

characterized by a distribution of surface site types and/or surface site limitations, FA has the

283

potential to decrease U(VI) sorption and increase U(VI) mobility even at very low FA

284

concentrations (6.15 × 10-8 M FA, 23 µg l-1 TOC).

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Batch Sorption Kinetic Experiments and Modeling

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U(VI) and FA Sorption Kinetics and Solution Speciation at pH=7

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In the absence of FA, U(VI) sorption to silica sand is characterized by a fast initial uptake

291

followed by slower sorption processes (Fig. 5), a behavior that is typical for many metal-mineral

292

systems45.

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complexation reactions, which can be affected by metal-ligand dissociation reactions12, 46, the

294

breakage of surface chemical bonds and the removal of products from the near-surface

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environment47. The slower sorption step has been ascribed to a distribution of reactive surface

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sites, surface precipitation and/or other processes controlled by mass transfer limitations37, 47-49.

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Considering the results from binary batch sorption envelope experiments, we believe that a

298

distribution of strong and weak surface sites is likely the reason for the observed kinetics.

The fast initial uptake is often attributed to the chemical kinetics of surface

299

In ternary systems at pH=7 (Figs. 5a and 5b), experimental data suggest an influence of FA on

300

U(VI) sorption kinetics. For example, U(VI) sorption reactions equilibrate more slowly in the

301

ligand-free system than in the presence of 6.15 × 10-5 M FA (23 mg l-1 TOC). The U(VI)

302

fraction sorbed increases from 79.6% to 90.2% between day 4 and 29 in the binary system, while

303

it remains stable at 6.15 × 10-5 M FA (7.6% and 7.2%). Fulvic acid sorption kinetics in binary

304

and ternary systems appear concentration-dependent, with higher organic ligand concentrations

305

leading to faster sorption equilibration (Figs. 5c and 5d). The presence of U(VI) does not have

306

any apparent effects on FA sorption kinetics in ternary systems.

307

Based on a comparison (Fig. 5a versus 5b, and Fig. 5c versus 5d), sorption data from short-

308

term experiments, performed over a few days, cannot capture slow U(VI) or FA kinetics, which

309

become apparent only over extended time-frames. Hence, the interpretation of short-term kinetic


14

Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


310

data may potentially cause an underestimation of time-frames required for complete sorption

311

equilibration.

312

Results from characteristic time calculations combined with chemical speciation modeling

313

allow for additional data interpretation (Figs. 4, 6, 7; Table S4, Supporting Information). For t1/ 2

314

computations, absolute values of characteristic times differ only slightly between the data-based

315

and model-based calculation methods (Fig. 7). The trends observed for characteristic time

316

changes as a function of FA concentrations are the same. Hence, the two methods of data

317

interpretation confirm each other, and the underlying assumptions of the kinetic model have no

318

effect on the interpretation of kinetic differences between these systems. For all model-fitted

319

parameters, the statistical analyses of differences in kinetic parameters are based on standard t-

320

tests with a 90% significance level.

321

For FA sorption to silica, characteristic time calculations clearly confirm a concentration-

322

dependence of FA sorption kinetics (Fig. 7); a significant decrease in FA characteristic times is

323

observed with increasing FA concentrations.

324

fractionation of the FA mixture due to FA surface reactions over time14, 44, 50, 51. Furthermore,

325

these calculations also confirm that U(VI) has no effect on FA sorption kinetics in ternary

326

systems. Model-based characteristic times for FA sorption (Fig. 7) are the same at any tested FA

327

concentration in the presence and absence of U(VI) (90% significance level). In these systems,

328

U-FA solution complexes represent only very small fractions of total FA solution species. For

329

instance, at 10-6 M FA, U(VI)-ligand complexes contribute ~0.008% to all FA species; at higher

330

FA concentrations, this fraction will be even smaller (Fig. S4, Supporting Information).

This could indicate multi-layer sorption or a

331

For U(VI) sorption to silica sand, characteristic time calculations indicate varying effects of

332

FA on U(VI) sorption kinetics. First, in the presence of the two lower, total FA concentrations,


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

333

characteristic times for U(VI) sorption to silica are significantly increased relative to the

334

corresponding U(VI)-silica system (Fig. 7). The model-based characteristic time for U(VI)

335

sorption is 23.42 hours in the absence of FA, but 34.47 and 46.84 hours at 10-6 and 10-5 M FA

336

(0.37 and 3.7 mg l-1 TOC), respectively. Therefore, U(VI) sorption is slowed down in the

337

presence of low organic ligand concentrations. In these ternary systems, uranium(VI) speciation

338

is primarily controlled by (mixed) carbonato and hydroxide species while U(VI)-FA complexes

339

can be neglected (0.32 and 4.74% of total U(VI) species, Fig. 4).

340

In contrast, at the highest FA concentration tested (6.15 × 10-5 M FA, 23 mg l-1 TOC) a

341

significant decrease in the characteristic time for U(VI) sorption (4.74 hours) is observed relative

342

to the binary system (Fig. 7). Hence, at this high organic ligand concentration, U(VI) sorption

343

kinetics are significantly faster, and a substantial fraction of U(VI) solution species is found in

344

the form of metal-ligand complexes (53.94%; Fig. 4). Furthermore, in this system, characteristic

345

times, and hence the kinetics, for U(VI) and FA sorption reactions, are the same (90%

346

significance level).

347

In summary, at pH=7 the presence of fulvic acid can either accelerate or slow down U(VI)

348

sorption reactions to the same, heterogeneous silica surface compared to a ligand-free system.

349

The change in metal sorption rates depends on metal solution speciation and the relative

350

concentrations of metals, organic ligands and mineral surface sites. This suggests a change in

351

the pathways of metal sorption reactions, as well as in the underlying mechanisms of FA effects

352

on metal sorption behavior.

353 354


16

Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

355


U(VI) and FA Sorption Kinetics and Solution Speciation at pH=8

356

Effects of FA on U(VI) sorption kinetics at pH=8 were interpreted using the same approach as

357

for pH=7 systems. At pH=8, the presence of low FA concentrations (10-6 and 10-5 M FA, 0.37

358

and 3.7 mg l-1 TOC) results in slower U(VI) sorption kinetics, while the formation of U(VI)-FA

359

solution complexes can be neglected (Tables S5 and S6 and Figs. S5-S8, Supporting

360

Information). (A FA concentration of 6.15 × 10-5 M was not tested at pH=8.) Hence, these

361

results confirm the trends observed for pH-7-systems at the two lower FA concentrations.

362 363

DISCUSSION

364 365

At this point, multiple interpretations of FA effects on U(VI) sorption kinetics are possible,

366

including: 1) rate-limiting dissociation reactions of metal-ligand solution complexes prior to the

367

sorption of ‘free’ metals, 2) competitive sorption reactions leading to a lower ‘effective’ surface

368

site concentration available for metal reactions, and 3) NOM sorption kinetics driving metal

369

sorption rates in the form of metal-ligand solution complexes. The first two mechanisms are

370

expected to lead to slower metal sorption kinetics; the last one to either faster or slower kinetics

371

depending on NOM sorption rates.

372

The observed decrease in U(VI) sorption rates at low FA concentrations may be attributed to

373

rate-limiting metal-ligand dissociation reactions or competitive sorption behavior.

374

strong evidence from previous experimental and modeling studies for the relevance of rate-

375

limiting dissociation reactions1, 2, 11, 52-55. However, these studies typically involve higher organic

376

ligand concentrations and a pre-equilibration of mineral phases with organic matter, which could

377

limit competitive sorption reactions later in the experiment. In addition, the largest rate-limiting


There is

17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

378

effects of dissociation reactions would be expected at the highest FA concentration (6.15 × 10-5

379

M, pH=7), with a high concentration of U(VI)-FA solution complexes in the form of ‘non-labile’

380

complexes55, characterized by slow dissociation reactions. However, in this case, U(VI) sorption

381

rates are faster than in the U(VI)-silica system, and mathematically the same as for FA.

382

Hence, at this point, a competition of metals and organic ligands for a limited number of strong

383

sites, present in the form of surface impurities, seems a more likely hypothesis for slower U(VI)

384

sorption rates at low concentrations of FA and U(VI)-FA solution complexes. FA binding to

385

these sites reduces the effective site concentration available for U(VI) surface reactions, which

386

results in slower, overall U(VI) sorption based on mass action principles. In contrast, at the

387

highest FA concentration, U(VI) sorption kinetics appear to be directly affected by FA sorption

388

rates. This could further indicate the formation of ternary U(VI)-FA surface complexes, either

389

by direct sorption of U(VI)-FA solution complexes, or by U(VI) binding to a rapidly-forming

390

organic surface coating.

391

Independent of the underlying processes, the selection of appropriate time-frames for

392

‘equilibrium’ sorption experiments is important, especially if distribution coefficients (Kd values)

393

are used to quantify relative differences in metal sorption due to the presence of organic

394

ligands56. For example, the apparent U(VI) Kd value can either decrease by a factor of 44 or a

395

factor of 94 in the presence of 6.15 × 10-5 M FA, depending on a 3-day or a 34-day experiment.

396 397

ACKNOWLEDGEMENTS

398

The authors thank Manfred Geier for help with kinetic modeling, Emily Lesher for facilitating

399

QEMSCAN/EDX surface analysis, and LLNL for providing Mathematica 7.0. Funding provided

400

by the National Science Foundation, the Austrian Academy of Sciences, the U.S. DOE NABIR


18

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


401

Program, and in part by the U.S. DOE Subsurface Biogeochemical Research program’s

402

Sustainable Systems Science Focus Area at Lawrence Berkeley National Laboratory (Contract

403

No. DE-AC02-05CH11231).

404 405

Supporting Information Available. This information is available free of charge via the Internet

406

at http://pubs.acs.org.

407


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

408

REFERENCES

409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451

1.

2.

3. 4. 5.

6. 7. 8. 9. 10. 11.

12.

13.

14. 15.

16. 17.

Page 20 of 30

Schussler, W.; Artinger, R.; Kim, J. I.; Bryan, N. D.; Griffin, D., Numerical modeling of humic colloid borne Americium(III) migration in column experiments using the transport/speciation code K1D and the KICAM model. J. Contam. Hydrol. 2001, 47, (2-4), 311-322. Artinger, R.; Rabung, T.; Kim, J. I.; Sachs, S.; Schmeide, K.; Heise, K. H.; Bernhard, G.; Nitsche, H., Humic colloid-borne migration of uranium in sand columns. J. Contam. Hydrol. 2002, 58, (1-2), 1-12. Gabriel, U.; Gaudet, J. P.; Spadini, L.; Charlet, L., Reactive transport of uranyl in a goethite column: an experimental and modelling study. Chem. Geol. 1998, 151, (1-4), 107-128. Steefel, C. I., Geochemical kinetics and transport. In Kinetics of Water–Rock Interaction, Brantley, S. L.; Kubicki, J. D.; White, A. F., Eds. Springer: New York, 2008. Stone, A. T.; Morgan, J. J., Kinetics of chemical transformations in the environment. In Aquatic chemical kinetics: reaction rates of processes in natural waters, Stumm, W., Ed. John Wiley & Sons, Inc.: New York, 1990; pp 1-41. Christl, I.; Kretzschmar, R., Interaction of copper and fulvic acid at the hematite-water interface. Geochim. Cosmochim. Acta 2001, 65, (20), 3435-3442. Schmeide, K.; Bernhard, G., Sorption of Np(V) and Np(IV) onto kaolinite: Effects of pH, ionic strength, carbonate and humic acid. Appl. Geochem. 2010, 25, (8), 1238-1247. Davis, J. A.; Leckie, J. O., Effect of Adsorbed Complexing Ligands on Trace-Metal Uptake by Hydrous Oxides. Environ. Sci. Technol. 1978, 12, (12), 1309-1315. Davis, J. A., Complexation of trace metals by adsorbed natural organic matter. Geochim. Cosmochim. Acta 1984, 48, (4), 679-691. Neihof, R.; Loeb, G., Dissolved Organic-Matter in Seawater and Electric Charge of Immersed Surfaces. J. Mar. Res. 1974, 32, (1), 5-12. Bryan, N. D.; Jones, D. L. M.; Keepax, R. E.; Farrelly, D. H.; Abrahamsen, L. G.; Pitois, A.; Ivanov, P.; Warwick, P.; Evans, N., The role of humic non-exchangeable binding in the promotion of metal ion transport in groundwaters in the environment. J. Environ. Monit. 2007, 9, (4), 329-347. Schmitt, D.; Saravia, F.; Frimmel, F. H.; Schuessler, W., NOM-facilitated transport of metal ions in aquifers: importance of complex-dissociation kinetics and colloid formation. Water Res. 2003, 37, (15), 3541-3550. Ochs, M.; Cosovic, B.; Stumm, W., Coordinative and Hydrophobic Interaction of Humic Substances with Hydrophilic Al2o3 and Hydrophobic Mercury Surfaces. Geochim. Cosmochim. Acta 1994, 58, (2), 639-650. Davis, J. A.; Gloor, R., Adsorption of Dissolved Organics in Lake Water by AluminumOxide - Effect of Molecular-Weight. Environ. Sci. Technol. 1981, 15, (10), 1223-1229. Stumm, W., Chemistry of the solid-water interface: processes at the mineral-water and particle-water interface in natural systems. John Wiley & Sons, Inc.: New York, 1992; p 428. Czerwinski, K. R.; Buckau, G.; Scherbaum, F.; Kim, J. I., Complexation of the Uranyl-Ion with Aquatic Humic-Acid. Radiochim. Acta 1994, 65, (2), 111-119. Lenhart, J. J.; Cabaniss, S. E.; MacCarthy, P.; Honeyman, B. D., Uranium(VI) complexation with citric, humic and fulvic acids. Radiochim. Acta 2000, 88, (6), 345-353.


20

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497


18. 19. 20. 21.

22.

23. 24.

25.

26. 27. 28.

29.

30.

31.

32.

33. 34.

35.

Moulin, V.; Tits, J.; Ouzounian, G., Actinide Speciation in the Presence of Humic Substances in Natural-Water Conditions. Radiochim. Acta 1992, 58-9, 179-190. Lenhart, J. J.; Honeyman, B. D., Uranium(VI) sorption to hematite in the presence of humic acid. Geochim. Cosmochim. Acta 1999, 63, (19-20), 2891-2901. Payne, T. E.; Davis, J. A.; Waite, T. D., Uranium adsorption on ferrihydrite - Effects of phosphate and humic acid. Radiochim. Acta 1996, 74, 239-243. Mibus, J.; Sachs, S.; Pfingsten, W.; Nebelung, C.; Bernhard, G., Migration of uranium(IV)/(VI)in the presence of humic acids in quartz sand: A laboratory column study. J. Contam. Hydrol. 2007, 89, (3-4), 199-217. McCarthy, J. F.; Czerwinski, K. R.; Sanford, W. E.; Jardine, P. M.; Marsh, J. D., Mobilization of transuranic radionuclides from disposal trenches by natural organic matter. J. Contam. Hydrol. 1998, 30, (1-2), 49-77. Wan, J.; Dong, W.; Tokunaga, T. K., Method to Attenuate U(VI) Mobility in Acidic Waste Plumes Using Humic Acids. Environ. Sci. Technol. 2011, 45, (6), 2331-2337. Nagasaki, S., Sorption of uranium(VI) on Na-montmorillonite colloids - effect of humic acid and its migration. In Stud. Surf. Sci. Catal., Iwasawa, Y.; Oyama, N.; Kunieda, H., Eds. Elsevier: 2001; Vol. 132, pp 829-832. Pitois, A.; Abrahamsen, L. G.; Ivanov, P. I.; Bryan, N. D., Humic acid sorption onto a quartz sand surface: A kinetic study and insight into fractionation. J. Colloid Interface Sci. 2008, 325, (1), 93-100. Choppin, G. R.; Shanbhag, P. M., Binding of calcium by humic acid. J. Inorg. Nucl. Chem. 1981, 43, (5), 921-922. Higgo, J. J. W.; Kinniburgh, D.; Smith, B.; Tipping, E., Complexation of Co2+, Ni2+, UO22+ and Ca2+ by humic substances in groundwaters. Radiochim. Acta 1993, 61, (2), 91-103. Joseph, C.; Schmeide, K.; Sachs, S.; Brendler, V.; Geipel, G.; Bernhard, G., Sorption of uranium(VI) onto Opalinus Clay in the absence and presence of humic acid in Opalinus Clay pore water. Chem. Geol. 2011, 284, (3-4), 240-250. Averett, R. C.; Leenheer, J. A.; McKnight, D. M.; Thorn, K. A. Humic substances in the Suwannee River, Georgia: interactions, properties, and proposed structures; Water-Supply Paper 2373; U.S. Geological Survey: Denver, 1994. Tinnacher, R. M.; Honeyman, B. D., A new method to radiolabel natural organic matter by chemical reduction with tritiated sodium borohydride. Environ. Sci. Technol. 2007, 41, (19), 6776-6782. Kantar, C. The role of citric acid in the transport of U(VI) through saturated porous media: the application of surface chemical models to transport simulations of bench-scale experiments. Colorado School of Mines, Golden, 2001. Sanpawanitchakit, C. The application of surface complexation modeling to the adsorption of uranium(VI) on natural composite materials. Ph.D., Colorado School of Mines, Golden, 2001. Armistead, C. G.; Tyler, A. J.; Hambleton, F. H.; Mitchell, S. A.; Hockey, J. A., Surface hydroxylation of silica. J. Phys. Chem. 1969, 73, (11), 3947-3953. Murphy, R. J.; Lenhart, J. J.; Honeyman, B. D., The sorption of thorium (IV) and uranium (VI) to hematite in the presence of natural organic matter. Colloids Surf., A 1999, 157, (13), 47-62. Espenson, J. H., Chemical kinetics and reaction mechanisms. 2nd ed.; McGraw-Hill, Inc.: New York, 1995; p 281.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543

36.

37. 38.

39.

40.

41.

42.

43.

44. 45. 46.

47. 48. 49. 50.

51.

52.

Page 22 of 30

Tinnacher, R. M. Effects of fulvic acid and extracellular polymeric substances on the mobility of uranium and plutonium in saturated groundwater systems. Colorado School of Mines, Golden, 2008. Sparks, D. L., Kinetics and mechanisms of soil chemical reactions. In Handbook of soil science, Summer, M. E., Ed. CRC Press: Boca Raton, 2000. Papelis, C.; Hayes, K. F.; Leckie, J. O. HYDRAQL: A program for the computation of chemical equilibrium composition of aqueous batch systems including surfacecomplexation modeling of ion adsorption at the oxide/solution interface; Stanford University: CA, 1988; p 130. Guillaumont, R.; Fanghanel, T.; Fuger, J.; Grenthe, I.; Neck, V.; Palmer, D. A.; Rand, M. H., Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium. Elsevier: Amsterdam, 2003; Vol. 5. Westall, J. C.; Jones, J. D.; Turner, G. D.; Zachara, J. M., Models for Association of MetalIons with Heterogeneous Environmental Sorbents.1. Complexation of Co(II) by Leonardite Humic-Acid as a Function of pH and NaClO4 Concentration. Environ. Sci. Technol. 1995, 29, (4), 951-959. Lenhart, J. J. The application of surface complexation modeling to the adsorption of uranium(VI) onto hematite in the presence of humic and fulvic acids. Colorado School of Mines, Golden, 1997. Davis, J. A.; Meece, D. E.; Kohler, M.; Curtis, G. P., Approaches to surface complexation modeling of uranium(VI) adsorption on aquifer sediments. Geochim. Cosmochim. Acta 2004, 68, (18), 3621-3641. Hsi, C. K. D.; Langmuir, D., Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model. Geochim. Cosmochim. Acta 1985, 49, (9), 1931-1941. Davis, J. A., Adsorption of natural dissolved organic matter at the oxide/water interface. Geochim. Cosmochim. Acta 1982, 46, (11), 2381-2393. Dzombak, D. A.; Morel, F. M. M., Surface complexation modeling: hydrous ferric oxide. John Wiley & Sons: New York, 1990. Nedobukh, T. A.; Kaftailov, V. V.; Betenekov, N. D., Radiochemical study of hydroxide films. V. Effect of carbonate ion on statics and kinetics of sorption of microamounts of uranium by thin-layer titanium hydroxide. Radiokhimiya 1987, 29, (6), 787-794. Stumm, W., Aquatic chemical kinetics. John Wiley & Sons: New York, 1990. Selim, H. M.; Amacher, M. C., Reactivity and transport of heavy metals in soils. CRC Lewis Publishers: Boca Raton, 1996; p 201. Skopp, J., Analysis of Time-Dependent Chemical Processes in Soils. J. Environ. Qual. 1986, 15, (3), 205-213. Van de Weerd, H.; Van Riemsdijk, W. H.; Leijnse, A., Modeling the dynamic adsorption desorption of a NOM mixture: Effects of physical and chemical heterogeneity. Environ. Sci. Technol. 1999, 33, (10), 1675-1681. van de Weerd, H.; van Riemsdijk, W. H.; Leijnse, A., Modeling transport of a mixture of natural organic molecules: Effects of dynamic competitive sorption from particle to aquifer scale. Water Resour. Res. 2002, 38, (8). Artinger, R.; Kienzler, B.; Schüßler, W.; Kim, J. I., Effects of humic substances on the 241Am migration in a sandy aquifer: column experiments with Gorleben groundwater/sediment systems. J. Contam. Hydrol. 1998, 35, (1–3), 261-275.


22

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

544 545 546 547 548 549 550 551 552 553 554


53. 54.

55.

56.

Artinger, R.; Schuessler, W.; Schaefer, T.; Kim, J.-I., A Kinetic Study of Am(III)/Humic Colloid Interactions. Environ. Sci. Technol. 2002, 36, (20), 4358-4363. Geckeis, H.; Rabung, T.; Manh, T. N.; Kim, J. I.; Beck, H. P., Humic colloid-borne natural polyvalent metal ions: Dissociation experiment. Environ. Sci. Technol. 2002, 36, (13), 2946-2952. Zhao, J.; Fasfous, I. I.; Murimboh, J. D.; Yapici, T.; Chakraborty, P.; Boca, S.; Chakrabarti, C. L., Kinetic study of uranium speciation in model solutions and in natural waters using Competitive Ligand Exchange Method. Talanta 2009, 77, (3), 1015-1020. Tinnacher, R. M.; Honeyman, B. D., Theoretical analysis of kinetic effects on the quantitative comparison of K(d) values and contaminant retardation factors. J. Contam. Hydrol. 2010, 118, (1-2), 1-12.

555 556 557


23


558

FIGURES AND FIGURE CAPTIONS

559 U=1.00E-8 M, Si=400 g/l

U=1.00E-7 M, Si=200 g/l

U=1.00E-7 M, Si=400 g/l

U=1.00E-6 M, Si=200 g/l

Series6

U=1.00E-5 M, Si=200 g/l

100 90

U(VI) sorbed [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

80 70 60 50 40 30 20 10 0 2

3

4

5

6

7

8

9

10

pH

560 561 562

Figure 1. Results from batch sorption envelope experiments for the sorption of uranium(VI) to

563

pretreated silica sand (Si) at I = 0.01 M NaCl/NaHCO3 and various solid-liquid ratios over 48

564

hours. Si concentrations of 200 g l-1 and 400 g l-1 correspond to 9 × 10-6 and 1.8 × 10-5 mol l-1

565

total surface sites. At around pH=7, U(VI) sorption densities range from 2.30 × 10-11 mol g-1 (at

566

U(VI)=10-8 M, Si=400 g l-1, pH=6.93) to 2.91 × 10-8 mol g-1 (at U(VI)=10-5 M, Si=200 g l-1,

567

pH=7.41) with corresponding total U(VI) surface site coverages of 0.05% and 63.63%. (Error

568

bars represent estimated 68% confidence intervals based on duplicate experiments performed at

569

10-7

M

U(VI),

200

g

l-1

silica

sand,

and

48


hours

equilibration

time).

24

Page 25 of 30

570 571

60

1.6E-10 mol FA/g=0.06 µg TOC/g

FA=6.15E-8 M FA=1.00E-6 M

50

FA=1.00E-5 M FA=6.15E-5 M

FA sorbed [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 1.2E-9 mol FA/g=0.46 µg TOC/g

30

7.8E-9 mol FA/g=2.92 µg TOC/g 7.2E-9 mol FA/g=2.68 µg TOC/g

20

10

0 2 572

3

4

5

6

7

8

9

10

pH

573 574

Figure 2. Results from batch sorption envelope experiments for the sorption of tritiated fulvic

575

acid (FA) to 200 g l-1 silica sand at I = 0.01 M NaCl/NaHCO3 over 72 hours. FA concentrations

576

of 6.15 x 10-8 M, 10-6 M, 10-5 M and 6.15 x 10-5 M FA correspond to approximately 0.02, 0.37,

577

3.7, and 23 mg l-1 TOC. Total surface site coverages are highly dependent on FA concentrations,

578

e.g. ranging from 1.5% (FA=6.15 × 10-8 M) to 75.5% (FA=10-5 M) at around pH=4.5. (Error

579

bars represent 90% confidence intervals for duplicate analysis of supernatant samples.)

580


25


581 582 U=1.00E-7 M, no FA

U=1.00E-7 M, FA=1.00E-6 M

U=1.00E-7 M, FA=6.15E-8 M

U=1.00E-7 M, FA=1.00E-5 M

U=1.00E-7 M, FA=6.15E-7 M

100

5E-10

80

4E-10

70 60

3E-10

50 40

2E-10

30 20

1E-10

U(VI) sorbed [mol/gSolid]

90

U(VI) sorbed [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

10 0

0E+00 2

583

3

4

5

6

7

8

9

10

pH

584 585

Figure 3. Results from batch sorption envelope experiments for the sorption of uranium(VI) to

586

200 g l-1 silica sand at I = 0.01 M NaCl/NaHCO3 and in the presence of various fulvic acid (FA)

587

concentrations over 72 hours. FA concentrations of 6.15 x 10-8 M, 6.15 x 10-7 M, 10-6 M and

588

10-5 M FA correspond to approximately 0.02, 0.23, 0.37, and 3.7 mg l-1 TOC. At around pH=7,

589

U(VI) surface site coverages range from 0.2% (FA=10-5 M) to 0.7% (FA=10-6 M) and 0.9% (no

590

FA). (Error bars represent estimated 68% confidence intervals based on duplicate experiments

591

performed at 10-7 M U(VI), 200 g l-1 silica sand, and 48 hours equilibration time.)


26

Page 27 of 30

592 593

7.0 UO2(L2)(L3) (UO2)2CO3(OH)3-

7.5

UO2CO3

pC U(VI) [mol/L]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8.0

8.5

UO2(OH)2 UO2(OH)+

UO2L2+

UO2(CO3)22-

UO2L3+

9.0

9.5

FA conc.

U-FA total

6.15E-8 M 6.15E-7 M 1.00E-6 M 1.00E-5 M 6.15E-5 M

0.02% 0.19% 0.32% 4.74% 53.94%

UO2L1

UO22+

10.0 8.0 594

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

pC FA [mol/L]

595 596

Figure 4. Speciation of 10-7 M U(VI) at pH=7 and pCO2=10-3.5 atm in 0.01 M NaCl/NaHCO3 as

597

a function of fulvic acid (FA) concentration. Speciation was simulated in HYDRAQL (without

598

ionic strength corrections) using existing thermodynamic data39,

599

speciation models, run at various FA concentrations; lines show trends between these models.

600

Tabulated values give the total fractions of U(VI) found in form of various U(VI)-FA solution

601

complexes at different, total FA concentrations.

41

.

Data points represent

602 603


27


a)

U only U, FA=1.00E-6 M

b)

U, FA=1.00E-5 M U, FA=6.15E-5 M

100

U only U, FA=1.00E-6 M

5E-10

50 40

2E-10

20

4E-10

70 60

3E-10

50 40

2E-10

30

1E-10

20

10


3E-10

80

U(VI) sorbed [%]

60


U(VI) sorbed [%]

4E-10

70

30

1E-10

10

0

0E+00 0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

3.5

0E+00 0

5

10

15

Time [d]

604

No U, FA=1.00E-6 M No U, FA=1.00E-5 M No U, FA=6.15E-5 M

30

20

25

30

35

Time [d]

U, FA=1.00E-6 M U, FA=1.00E-5 M U, FA=6.15E-5 M

d)

20

20

FA sorbed [%]

25

15

No U, FA=1.00E-6 M No U, FA=1.00E-5 M No U, FA=6.15E-5 M

30

25

U, FA=1.00E-6 M U, FA=1.00E-5 M U, FA=6.15E-5 M

1.0E-9 mol FA/g=0.39 µg TOC/g

15 3.4E-9 mol FA/g=1.27 µg TOC/g 1.2E-8 mol FA/g=4.42 µg TOC/g

10

10

5

5

0

0 0.0

605

5E-10

90

80

c)

U, FA=1.00E-5 M U, FA=6.15E-5 M

100

90

FA sorbed [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

0.5

1.0

1.5

2.0

2.5

3.0

0

5

10

Time [d]

15

20

25

30

35

Time [d]

606 607

Figure 5. a) and b): Sorption of 10-7 M U(VI)Tot to 200 g l-1 silica sand as a function of time at

608

pH=7 and in the presence of various fulvic acid (FA) concentrations. FA concentrations of 10-6

609

M, 10-5 M and 6.15 × 10-5 M FA correspond to approx. 0.37, 3.7, and 23 mg l-1 TOC. c) and d):

610

Kinetics of fulvic acid (FA) sorption to 200 g l-1 silica sand in the presence (full symbols) and

611

absence (open symbols) of 10-7 M U(VI)Tot at pH=7 and I=0.01 M NaCl/NaHCO3. Lines

612

represent model fits for pseudo-first order sorption kinetics.

613

confidence intervals.)


(Error bars represent 68%

28

Page 29 of 30

614 615 616 617

a)

b)

100 90 80 70 60 50 40 30 U only U, FA=1.00E-6 M U, FA=1.00E-5 M U, FA=6.15E-5 M

20 10

90 80 70 60 50 40 U, FA=1.00E-6 M No U, FA=1.00E-6 M U, FA=1.00E-5 M No U, FA=1.00E-5 M U, FA=6.15E-5 M No U, FA=6.15E-5 M

30 20 10

0

0 0

618

100 Fraction of FA equilibr. surf. conc. [%]

Fraction of U(VI) equilibr. surf. conc. [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20

30 40 Time [hrs]

50

60

70

0

10

20

30 40 Time [hrs]

50

60

70

619 620

Figure 6. Fractions of (a) U(VI) and (b) FA equilibrium surface concentrations reached over the

621

course of kinetic experiments at pH=7. For the data-based estimation method, linear regression

622

calculations were used to estimate t1/2 values, the time-points when 50% of individual

623

equilibrium surface concentrations have been reached. Values of t1/2 (dashed circles) represent

624

the intercepts between individual regression lines and the lines showing 50% of the equilibrium

625

surface concentration. (Error bars omitted for simplification.)

626 627


29


628 629

Data-based t_1/2

60

t1/2 for U(VI) or FA sorption [hr]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

U(VI) sorpt.: 1.00E-7 M U

Model-based t_1/2

FA sorption: 1.00E-7 M U

No U

50

40

30

20

10

0

630

FA concentration [M]

631 632

Figure 7. Characteristic times (t1/2) for the sorption of 10-7 M U(VI)Tot or various fulvic acid

633

concentrations to 200 g l-1 silica sand at pH=7 using a data-based estimation method (‘Data-

634

based t1/2’) or a calculation based on fitted modeling parameters (‘Model-based t1/2’, Table S4).

635

Error bars represent 68% confidence intervals for model-based values.

636


30

Effects of Fulvic Acid on Uranium(VI) Sorption Kinetics - Environmental

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