Tungsten Speciation and Solubility in Munitions-Impacted Soils

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Tungsten Speciation and Solubility in Munitions-Impacted Soils Benjamin C. Bostick, Jing Sun, Joshua D. Landis, and Jay L. Clausen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05406 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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Environmental Science & Technology

Tungsten Speciation and Solubility in Munitions-Impacted Soils

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Benjamín C. Bostick1*, Jing Sun2*, Joshua D. Landis3 and Jay L. Clausen4

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1

Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA.

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2

University of Western Australia, School of Earth Sciences, 35 Stirling Highway, Perth, WA

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6009, Australia.

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3

Dartmouth College, Department of Earth Sciences, Hanover NH, 03755, USA.

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4

Research and Development Center, Cold Regions Research and Engineering Laboratory, 72

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Lyme Road, Hanover, NH 03755, USA.

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*Corresponding Authors.

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Bostick: Phone: (+1) 845 365 8659; Fax: (+1) 845 365 8155; Email: [email protected]

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Sun: Phone: (+61) 8 9333 6011; Fax: (+61) 8 9333 6499; Email: [email protected]

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Prepared for Submission to Environmental Science & Technology.

1

ACS Paragon Plus Environment


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ABSTRACT

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Considerable questions persist regarding tungsten geochemistry in natural systems, including

26

which forms of tungsten are found in soils and how adsorption regulates dissolved tungsten

27

concentrations. In this study, we examine tungsten speciation and solubility in a series of soils at

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firing ranges in which tungsten rounds were used. The metallic, mineral and adsorbed forms of

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tungsten were characterized using X-ray absorption spectroscopy and X-ray microprobe, and

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desorption isotherms for tungsten in these soils were used to characterize its solid-solution

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partitioning behavior. Data revealed the complete and rapid oxidation of tungsten metal to

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hexavalent tungsten(VI), and the prevalence of adsorbed polymeric tungstates in the soils rather

33

than discrete mineral phases. These polymeric complexes were only weakly retained in the soils,

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and porewaters in equilibrium with contaminated soils had 850 mg L-1 tungsten, considerably in

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excess of predicted solubility. We attribute the high solubility and limited adsorption of tungsten

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to the formation of polyoxometalates such as W12SiO404-, an α-Keggin cluster, in soil solutions.

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Although more research is needed to confirm which of such polyoxometalates are present in

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soils, their formation may not only increase the solubility of tungsten but also facilitate its

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transport and influence its toxicity.

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GRAPHICAL ABSTRACT

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INTRODUCTION

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The uses of tungsten (W) have been expanding for numerous reasons, including the general

44

assumption that tungsten is neither toxic nor particularly mobile in the environment.1 As a result,

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numerous environments have been impacted by tungsten mineral extraction and processing, and

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industrial uses including specialty alloys, electrodes, filaments, and others.2,

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environments have recently been the focus of research due to the correlation of groundwater

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tungsten contamination with leukemia in Fallon, NV, Untied States.4 Although more research is

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needed to determine what role tungsten plays, if any, in the occurrence of cancer in Fallon,5, 6

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tungsten does appear to be highly toxic to a variety of organisms.7-11

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These

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While most uses of tungsten involve its stability, high conductivity, and low vapor pressure,

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other applications depend on its exceptionally high density (19.1 g cm-3 at 20°C).1, 12, 13 Among

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the applications, tungsten is used in munitions as a substitute for lead or depleted uranium.3

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Since 1999, nearly 85 million rounds of tungsten core projectiles, which are made by

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compressing tungsten metal spheres in nylon, have been produced and used in military training.14

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The soils adjacent to numerous test firing ranges have been impacted by tungsten munitions, and

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this tungsten, if soluble, has the potential to be leached from the soils and contaminate

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groundwater. Indeed, tungsten in munitions-impacted soils appears to be soluble and mobile.3, 14-

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17

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with common tungsten minerals, suggests dissolved tungsten concentrations limited to ~1 mg L-1

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by equilibrium with scheelite (CaWO4) or tungsten oxides (including WO3·H2O). Moreover,

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tungstate (WO42-) adsorption to common soil minerals such as iron (Fe) oxides is fast, strong,

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and extensive under typical soil pHs, which should keep actual dissolved tungsten concentrations

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even lower.12,

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solubility in some soil environments. Few studies have examined tungsten speciation in soils,16,

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20

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adsorption regulates dissolved tungsten concentrations do not have a clear answer.

However, thermodynamic modeling of circumneutral soil solutions based on known equilibria

18, 19

Current data on tungsten is insufficient to explain its unexpectedly high

and simple questions such as which tungsten phases are found in soils, and the extent to which

69 70

In principle, the thermodynamics of tungsten are simple. Tungsten metal is unstable in the

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environment, and turns to hexavalent tungsten(VI).1, 21 The solution chemistry of tungsten(VI),

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however, is exceedingly complicated. Tungstate (WO42-) is similar to molybdate (MoO42-), 4


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which forms a variety of stable polyatomic anions, including H2W12O406-, HW6O203-, and

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W6O20(OH)5-.

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with WO42- stable in dilute and basic solutions, while H2W12O406- and related ions stable in more

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concentrated and acidic solutions.1,

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polyoxometalates (POMs) in solution.13, 26-30 POMs are cluster compounds that contain two or

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more metal atoms. Tungsten POMs are constructed of WO6 and a central XO4 tetrahedron.

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Tungsten-containing α-Keggin clusters, W12XO40y-, have been intensively studied due to their

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exceptional size, molecular mass, structure and chemical reactivity.28 Although there is some

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evidence that such tungsten POMs are important in environmental systems,31,

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thermodynamic data is available to predict their formation and stability in soils, nor have they

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been conclusively identified in soils.16

13, 21-23

These tungsten(VI) species have well-known thermodynamic properties, 13, 21, 24-26

Tungstate can also form a variety of

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little or no

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The objectives of this study were to understand the speciation and solubility of tungsten in

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tungsten-contaminated soils and subsequently to understand the mechanism(s) for the

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anomalously high mobility of tungsten in the environment. We examine munitions-impacted

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soils from three test firing ranges in the United States. We use synchrotron-based X-ray

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absorption spectroscopy (XAS) to determine the oxidation state of tungsten residues in these

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soils, and to evaluate the mineral and adsorbed forms of tungsten. We supplement our

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investigation of bulk speciation with µ-X-ray fluorescence (µ-XRF) and µ-X-ray absorption

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spectroscopy (µ-XAS) to more conclusively identify tungsten bearing phases. We then evaluate

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tungsten solubility from these soils using desorption experiments, and propose a mechanistic link

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between tungsten speciation and solubility. The information gleaned from this study is critical to

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evaluate the fate of tungsten in these environments and to make management decisions regarding

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the contaminated areas.

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MATERIALS AND METHODS

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Site Information and Sample Collection.

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Soil samples were collected from firing range sites at Camp Edwards (Buzzards Bay, MA) which

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is part of the Massachusetts Military Reservation (MMR), Fort Lewis (Tacoma, WA) and Fort

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Benning (Columbus, GA). Detailed sampling locations are in Supporting Information (SI) Figure

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SA1 and description of soil sampling procedures has been previously published.14, 33, 34 Tungsten 5



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munitions have been used at these ranges for as much as five years, and all were sampled within

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about one year of the last use of tungsten munitions. Surface (0-5 cm depth) soils were collected

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from a variety of locations within the ranges, including firing points, the range floor, and at or

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beneath the targets and collection trough. Additional samples outside of ranges were selected as

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control samples to examine background tungsten levels. The distribution of contaminants in each

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site is variable due to the heterogeneous nature of their application; thus each soil sample was an

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aggregate of several smaller samples, the total of which weighed ~5 kg. At Camp Edwards

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MMR, soil profiles were collected at two locations within the target zone, to examine the vertical

112

distribution of tungsten. At each location, four adjacent soil cores were collected, and identical

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depth intervals from four cores were aggregated to obtain representative samples. Once

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collected, soil samples were transported to the laboratory (Hanover, NH) and air-dried on

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aluminum foil-lined trays. After drying, each of the aggregated soil samples was sieved to < 2

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mm and homogenized using a ball mill or mill grinder prior to analysis.

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Bulk Soil Analysis.

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Soil samples was analyzed in the field using a portable X-ray fluorescence (XRF) spectrometer

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(Innov-X Alpha-4000s). Soil composition was also determined by digestion. The digestion

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procedure initially followed EPA method 3050B

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instead of HCl or HNO3 acid, the digestate was dissolved in NH4OH and then diluted with a

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mixture of NH4OH and EDTA. This modification was made because tungsten is relatively

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insoluble in HCl or HNO3 if no complexation occurs.13, 36, 37 The use of NH4OH and EDTA is

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effective to maintain tungsten in soluble forms.38 For digestion, 0.5 g aliquots of homogenized

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soil was heated in 5 mL concentrated HNO3 and 2 mL 30% H2O2 at 100°C, and this suspension

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was evaporated to dryness. Once dry, 2 mL of concentrated HNO3 was added, and the

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suspension was again evaporated to dryness. Tungsten was recovered by dissolving the digestate

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in 2 mL of 20% NH4OH, followed by sonicating and vortexing to ensure mixing. Once complete,

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20 mL of 2% NH4OH/1% EDTA was added. The mixture was centrifuged at 4,000 rpm for 10

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minutes, and the supernatant was diluted (x51) with 0.2% NH4OH/0.1% EDTA and analyzed by

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inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo IRIS Intrepid II

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XSP with radial optics). Quantification by ICP-OES followed previously published procedure.38

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Tungsten is spectrally rich. The 207.9 nm used for ICP-OES analysis had a detection limit of 6

35

to decompose the soil matrix. However,

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µg L-1, corresponding to 0.1 mg kg-1 in the soil in digestion. The accuracy of this modified

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digestion method was verified by comparison with XRF results obtained independently, and

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102% recovery of a NIST-2710 reference.

138 139

X-ray Absorption Spectroscopy.

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X-ray absorption spectroscopy (XAS) was performed at the Stanford Synchrotron Radiation

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Laboratory (SSRL) on beamline 11-2. A 30-element Ge detector in combination with 6 µx Cu

142

filter was used to collect spectra in fluorescence mode. Sample slits for measurement

143

configuration were 1 mm by 10 mm. The beam was detuned as needed to reject higher-order

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harmonic frequencies and prevent detector saturation. The monochromator crystal reflection

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used was Si(220) with phi angle of 90°. The X-ray energy of the acquired spectra was calibrated

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by the first inflection of a tungsten metal foil to 10,207.0 eV. XAS spectra were collected at

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around the tungsten L3-edge from -235 to 900 eV (corresponding k-range from 0 to 15.4 Å-1). In

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the X-ray absorption near edge structure (XANES) region, an energy interval of 0.3 eV was

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used; in the extended X-ray absorption fine structure (EXAFS) region, a k interval of 0.05 Å-1

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was used. At least three scans were obtained for each sample and then averaged. There was no

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noticeable change in spectral shape between parallel scans.

152 39

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Sample spectra were processed using SIXPack

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normalized with linear pre-edge and quadratic post-edge functions. Normalized XANES spectra

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and k3-weighted EXAFS chi functions were compared to a spectral library of commonly

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encountered tungsten references, including tungsten metal foil, intact fresh bullet cores,

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wolframite (FeWO4), scheelite (CaWO4), pH=10 sodium tungstate solution (Na2WO4),

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ammonium tungstate ((NH4)2WO4), sodium polytungstate (Na6H2W12O40), phosphotungstic acid

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(H3W12PO40), and tungsten oxide (WO3·H2O). Details and spectra of the references are in SI

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Section B. Once relevant references were selected, least-squares linear combination fitting was

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used to determine the relative fraction of each reference in each sample. The fractions can be

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converted to concentrations by multiplying by bulk soil tungsten concentrations from digestions.

unless mentioned otherwise. The spectra were

163 164

Local structural information around the tungsten atom in selected soils was obtained by

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performing theoretical shell fitting on EXAFS spectra using SIXPack. The element (Z), 7



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coordination number (CN), distance (R), and the Debye-Waller factor (σ2) for each shell were

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determined by fitting the experimental spectrum using phase and amplitude functions derived

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using FEFF 8.01. The accuracy of these phase and amplitude functions were confirmed by

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comparing fits of model compounds with known structures. Both single and multiple scattering

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paths were considered, although no multiple scattering paths significantly improved fitting. The

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E0 was constrained to the same value for all shells; all other parameters were varied during

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fitting. The accuracy of the fits was estimated using the χ2 statistical parameter, for which

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smaller values correspond to better fits. This procedure accurately determines interatomic

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distances (within 0.02 Å); however, it has considerable errors in coordination environment (30%

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for the first shell and less accurate for more distant shells) due to the high correlation of CN and

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σ2. Therefore, the coordination numbers were constrained based on crystallographic information

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if possible. The identification of different elements in the coordination shell relies on the

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measurement of interatomic distances, which is diagnostic of specific ion pairs, and phases and

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amplitude functions of those ion pairs.

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X-ray Microprobe.

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X-ray microprobe analysis were obtained at SSRL on beamline 2-3, to determine the distribution

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and association of tungsten with various mineral phases in soils. This beamline uses Kirkpatrick-

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Baez focusing optics to focus the incident beam to a 1-2 µm diameter. The samples were

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prepared by mounting a thin layer of soil grains on Kapton tape. µXRF spectra were collected at

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13,000 eV by continuously rastering the beam across the sample every 5 µm, and measuring for

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250 µs at each point. Regions of interest were defined for a number of elements, including W,

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Fe, Si, S, K, Ca, Mn, Ti, Cu, and Zn, and the integrated counts were used to estimate elemental

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abundance. While counts are proportional to concentrations, it is difficult to accurately determine

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concentrations from counts without standardization and accounting for variable sample

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thickness. Therefore, data were presented as fluorescence counts, IF, normalized to incident

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photon intensity I0 (IF/I0). Grain mounts are of variable thickness, which potentially influences

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elemental abundance and elemental correlations. The effect from variable thickness can be

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accounted for by normalizing counts using transmittance (I1/I0), which, however, did not affect

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overall elemental correlations in these soil samples and thus is not used in the presented data.

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Points of high and average tungsten and/or iron counts identified in µXRF maps were chosen for 8


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µXANES study, at which tungsten L3-edge XANES spectra and iron K-edge XANES spectra

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were obtained using a micro-focused X-ray beam. µXANES spectra were also processed using

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SIXPack.

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Desorption Isotherm Experiments.

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Desorption isotherms were conducted to examine dissolved tungsten concentrations in soil

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porewaters.40 A desorption isotherm for a representative soil sample MB036S3, from a bullet

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pocket at Camp Edwards MMR, was created by varying the solid-to-solution ratio between 0.2

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and 6.6 g mL-1 by adding different amounts of water. Filtered, sterilized water obtained from the

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Connecticut River

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showed undetectable tungsten (< 6 µg L-1). The pH of these suspensions was monitored using a

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calibrated pH electrode. After equilibrating at room temperature for 51 hours on an orbital shaker,

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the suspensions were centrifuged at 4,000 rpm for 10 minutes, and the supernatants were filtered

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to 0.2 µm with Nylon membrane filters (Whatman). Unfiltered supernatants were also sampled.

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Solutions were then divided into aliquots for major and trace element analysis, and tungsten

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analysis. Common major and trace elements were determined by ICP-OES using conventional

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method.42 Tungsten were preserved and analyzed as described above. Another desorption

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isotherm was created using 20 representative soil samples at a constant solid-solution ratio of 0.3

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g mL-1. Tungsten concentrations in these soils varied by two orders of magnitude, which could

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yield a wide range of solid and dissolved tungsten concentrations after equilibration.

41

was used for equilibration. The water was analyzed prior to use, which

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Solid-Solution Speciation Modeling

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Visual MINTEQ 43 was used to estimate solution speciation of soil solutions, and to calculate the

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stable thermodynamic phases in desorption isotherm experiments. The reactions and equilibrium

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constants were mainly from the default databases thermo.vdb and type6.vdb, and the ones for

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tungstate, calcium, carbonate, silicate etc. sorption were from the surface complexation database

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feo-dlm_2008.vdb, which was based on Dzombak and Morel.44 The databases in Visual

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MINTEQ contain recent data including those for tungstate complexation and adsorption (see

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Table SA1 for details),19 but it should be noted that only simple polytungstates are included.

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Numerous ion pairs and other polymeric tungstates, including POMs, probably existed in our

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experimental system. However, due to the lack of equilibrium constants, these complexes

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currently cannot be described using thermodynamic modeling approaches.

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RESULTS

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Soil Tungsten Concentrations and Distribution.

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Soil tungsten concentrations varied widely within the firing ranges (Table SB1), and were higher

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than background levels of ~1.5 mg kg-1.14 In impact berms, bullet pockets and other areas

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directly receiving munitions, soil tungsten concentrations were as high as 5,500 mg kg-1; in other

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portions of the firing surface, soil tungsten concentrations were generally lower. Soil tungsten

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concentrations also decreased rapidly with depth (Figure 1). Composite soil depth profiles

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showed consistent trends in tungsten concentrations, with high concentrations in the surface

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soils, and decreasing with depth to 20-150 mg kg-1 at 100 cm, still significantly above

239

background levels (Table SA2). Soil concentrations of lead, which is also derived from previous

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munition uses, were on the order of 100s of mg kg-1 in the samples collected from Camp

241

Edwards MMR and were not correlated with tungsten concentrations.

242 243

Tungsten Speciation and Structure.

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XANES Spectroscopy. Tungsten XANES was used to identify metallic and oxidized tungsten in

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the soils. The XANES spectra of the tested soil samples were remarkably similar: each spectrum

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contained an inflection edge at 10,210.9 eV followed by a strong white line feature at 10,213.5

247

eV which are characteristic of tungsten(VI) (Figure SB1). There were no observable spectral

248

features at lower energy that are characteristic of metallic tungsten which has an edge at 10,207.1

249

eV and a considerably lower white line intensity. The fraction of tungsten metal and tungstate in

250

each sample was estimated using linear combination fitting. All sample spectra were adequately

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fit with only a single reference spectrum of monomeric tungstate (WO42-), with fits indicating

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98% of tungsten was present as tungsten(VI) and little if any metallic tungsten was present.

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Different tungsten(VI) species have similar XANES spectra. However, differences in the white

254

line intensity can be used to differentiate between monomeric and polymeric tungstates.45 The

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soil XANES spectra did not contain post-edge shoulder at around 10,225 eV that exists in the

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spectrum of monomeric tungstate. Instead, the soil XANES spectra resembled that of polymeric

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tungstates, and also showed obvious splitting (i.e., two peaks overlapping instead of one single

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peak) often observed in polymeric tungstates (Figure SB1).

259 260

EXAFS Spectroscopy. Tungsten speciation was also quantified by fitting soil EXAFS spectra

261

with linear combinations of known reference spectra (Figure SB2). The effective implementation

262

of linear combination fitting often depends on the unique characteristics of each reference

263

spectra. In case of tungsten, spectral similarities within specific classes of tungsten(VI) phases

264

required the aggregation of model compounds into spectrally distinct components (see discussion

265

in SI Section B). Additionally, some polymeric tungstates, such as W6O20(OH)5-, may exist in

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soils,1, 13, 21 but no spectra are readily available for such materials. To address these limitations,

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linear combination fitting was performed using four references: tungsten metal, (NH4)2WO4 that

268

is representative of adsorbed monomeric tungstate, H3W12PO40 representative of adsorbed

269

polytungstates and POMs, and wolframite representative of mineral tungstates. These four

270

references were also evaluated using target transform analysis,46 which testified the reliability of

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the choice of the references (Figure SB3). (Additional fits with tungsten oxides or scheelite were

272

not necessary or their inclusion resulted in unstable fits due to the spectral similarity with

273

wolframite.) The (NH4)2WO4, H3W12PO40 and wolframite references should be regarded as

274

representative model compounds rather than discrete species. For example, fitting with

275

H3W12PO40 does not imply that phosphotungstic acid is present in the soils (although it may be),

276

rather it represents polytungstates/POMs more generally. This does not imply that it is

277

impossible to more conclusively identify POMs using EXAFS, but such determinations depend

278

on tungsten-tungsten distances and coordination numbers obtained from shell fitting.

279 280

EXAFS linear combination fits (Figure 1 and Table SB3) showed that these soil samples,

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regardless of depth or location, contained little metallic tungsten (< 9 mol% of the total

282

tungsten), in agreement with XANES data. Crystalline mineral tungstates (modeled with

283

wolframite) were detected but never represented a majority of tungsten in the soils (< 31%). In

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most samples, adsorbed polytungstates and/or POMs appeared to be the dominant fraction,

285

particularly at depths < 20 cm (fraction > 40%, higher than any of the other three components).

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The fraction of adsorbed monomeric tungstates was also significant, particularly in deeper soils

287

(as bulk tungsten concentration decreased, Figure 1).

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Shell fitting was done on two surface soils in an attempt to differentiate between polytungstates

290

and POMs (Table 1 and Figure 2). These soils were chosen based on their high tungsten

291

concentrations, which improved data quality, and their linear combination fitting results, which

292

implied that 50-60% of the tungsten was present as polytungstates and/or POMs. In each case,

293

the structural parameters of the fits matched the tungsten-oxygen and tungsten-tungsten shells of

294

well-characterized POMs, and were different from those of available conventional polytungstates

295

(Table 1 and Table SB2), but it is possible that conventional polytungstates were present at some

296

levels as part of a mixture.

297 298

X-ray Microprobe. Three soils collected from a munitions trough from Camp Edwards MMR

299

were chosen for X-ray microprobe analysis to further identify metallic tungsten and mineral

300

tungstates if any, and the minerals on which monomeric and polymeric tungstates were sorbed.

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The µXRF map of contaminated surface soils appeared more heterogeneous than soils at depth

302

(Figure 3). The surface soil, 0-5 cm, contained regions of high iron, tungsten and calcium counts

303

(hotspots) that were separated from the bulk matrix. By 5-10 cm, tungsten hotspots were no

304

longer prominent, and tungsten appeared to be relatively evenly distributed throughout the soil.

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This well distributed tungsten was also found at 15-20 cm depth. Tungsten counts in these µXRF

306

maps were not correlated to calcium or manganese counts (R2 < 0.10). Instead, in each µXRF

307

map, tungsten counts were correlated to iron counts (R2 = 0.64 for the 5-10 cm depth interval),

308

although this correlation was not strong for the surface sample (R2 = 0.21) because it had a

309

relatively narrow range of tungsten counts. The correlation between iron and tungsten counts

310

implies that much of the tungsten in the samples was from iron tungsten containing minerals

311

(e.g., wolframite) or was adsorbed on iron minerals. μXANES spectroscopy confirmed adsorbed

312

tungsten(VI) on iron(III) oxides. Tungsten and iron hotspots in the 0-5 cm depth interval had

313

tungsten μXANES consistent with monomeric or polymeric tungstates (Figure 4A) and iron

314

μXANES indicative of ferrihydrite and, in one case, hematite (Figure 4B).

315 316

Apparent Solubility of Soil Tungsten.

317

The desorption isotherm of a typical contaminated soil MB036S3 with moderate soil tungsten

318

concentration exhibited significant desorption, with dissolved tungsten concentrations reaching > 12


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850 mg L-1 (Figure 5A). At such high concentrations, the effective partition coefficient (Kd) was

320

1.7 L kg-1. The desorption isotherm of soil MB036S3 was fit effectively with both Langmuir and

321

Freundlich equations, although Freundlich isotherm had slightly better fit.

322 323

Desorption experiment was also done on the soils with variable compositions and properties

324

(Figure 5B). All the soil samples from Camp Edwards MMR roughly obeyed a single desorption

325

isotherm. This isotherm seemed to have an adsorption maximum and was approximately linear at

326

low tungsten concentrations. The samples high in iron were also high in sorbed tungsten, and

327

normalizing the sorbed tungsten concentrations to soil iron concentrations improved the isotherm

328

(Figure 5C). Compared to Camp Edwards MMR, the soils from Forts Benning and Lewis

329

showed lower tungsten solubility, as evidenced by their lower amounts of tungsten release during

330

desorption experiments (Figure 5B and 5C). Dissolved tungsten concentrations in the filtered

331

(0.2 µm) and unfiltered supernatant samples from the experiment were identical.

332 333

DISCUSSION

334

Much of the tungsten in munition-impacted soils was found as polytungstates and/or POMs.

335

These polymeric tungstates appear to be soluble and mobile in soils. The following discussion

336

examines the distribution of different tungsten species, the potential factors that allow

337

polytungstates and POMs to form and persist in the contaminated soils, and how their formation

338

affects tungsten solid-solution partitioning in the environment.

339 340

Prevalence of Polymeric Tungstates in Munitions-Impacted Soils.

341

X-ray absorption spectroscopy was used in this study to identify and quantify the presence of

342

various tungsten species and mineral forms in intact soils. The XANES spectra of all soil

343

samples overlapped and did not vary with depth or sampling location, and were indicative of the

344

fully oxidized tungsten(VI) (Figure SB1). The tungsten munitions are metallic, and have had

345

only a few years to oxidize once deposited on the soil surface. Given that limited time, it is

346

surprising that the tungsten has oxidized to tungsten(VI), but is consistent with the facile

347

oxidation of tungsten metal from bullet cores reacted with water and rapid release of dissolved

348

tungsten.13, 14 This rapid oxidation indicates that tungsten solubility is not limited by the rate of

13



Page 14 of 31

349

oxidation, and that the speciation of tungsten(VI), as either adsorbed or mineral tungstates, is of

350

critical importance to accurately assess the fate and transport of tungsten in the soil systems.

351 352

EXAFS linear combination fitting suggested that polytungstates/POMs dominated the soil

353

tungsten at shallow depths (Figure 1 and Table SB3). The prevalence of polytungstates/POMs at

354

the soil surface might result from their high tungsten concentrations that favor polymerization.

355

The preponderance at the soil surface might also result from their lower pH (Table SA2).

356

Monomeric tungstate is more stable at the neutral and slightly alkaline pH of deeper soil

357

horizons, while polymeric tungstates are more stable in acidic to slightly acidic conditions of

358

surface soils.1, 13, 21, 24-26

359 360

Numerous tungsten POMs are known, including silicotungstate and phosphotungstate.26,

28

361

Despite the inferred stability, little is known about the existence of tungsten POMs in

362

environmental systems. In many EXAFS experiments, it is possible to identify the phase

363

responsible for ion retention, and the adsorption mechanism, by examining the distance and

364

identity of the second shell metal.47-49 While such approaches may be possible for monomeric

365

tungstates, it is difficult for adsorbed polytungstates or POMs because second-shell atoms

366

indicative of adsorbent are masked by large tungsten-tungsten shells from the ion itself.

367

Nevertheless, EXAFS shell fitting suggests that such POMs may indeed be important adsorbed

368

species (Table 1 and Figure 2), although more work is required to conclusively identify which

369

specific species are present, and to decipher how they are retained in the soils.

370 371

A number of studies have shown that POMs or polytungstates can adsorb on mineral surfaces.37,

372

38, 50-53

373

monomeric tungstate, and that polymeric tungstates adsorbing on the surface may in some cases

374

decompose over time due to either their instability under the experimental conditions or the

375

conversion to more stable monomeric surface complexes.50 However, the POMs observed in the

376

soils in this study appears to be thermodynamically stable (the tungsten in the soils is well

377

equilibrated), although more data would be required to confirm this assertion. In general, POMs

378

containing heteroatoms are more stable than their corresponding conventional polytungstate

These studies show that polymeric tungstates have lower affinities for the surfaces than

14


Page 15 of 31


379

analogues.13, 22 POMs thus may be stabilized by the presence of a variety of other ions (e.g.,

380

phosphate and silicate) in soil solutions that can be incorporated into the POM structure.

381 382

The presence of high-tungsten regions in the µXRF maps, particularly at the soil surface (Figure

383

3), might suggest the presence of tungsten rich minerals, including wolframite ((Fe,Mn)WO4),

384

scheelite (CaWO4), or tungsten oxide (WO3·H2O). While small quantities of these minerals

385

might be present at the shallow depth interval (Figure 1), tungsten-rich regions were not overly

386

enriched in calcium, manganese or iron, and iron µXANES spectra were inconsistent with

387

wolframite (Figure 4B). Instead, these tungsten-rich regions resulted from adsorbed tungsten(VI)

388

on aggregates of iron(III) oxides, primarily ferrihydrite. Tungstate indeed adsorbs strongly to

389

ferrihydrite surface under circumneutral conditions.19 The well-defined positive correlation

390

between sorbed tungsten concentrations and soil iron concentrations in the desorption

391

experiments (Figure 5B and 5C) further establishes that iron(III) oxides were the primary phases

392

responsible for tungsten retention. Some tungsten(VI) could adsorb on other minerals. It was not

393

possible to accurately measure aluminum or silicon using µXRF in this study due to detector

394

limitations. Therefore, the role of aluminum (oxyhydr)oxides and aluminosilicates such as

395

allophane in tungsten retention was not determined. However, they can potentially be important

396

in these soils and worth investigation in future experiments.24, 54

397 398

Mobility of Munitions-Impacted Soil Tungsten.

399

Although tungsten is concentrated at the soil surface, measurable tungsten contamination is

400

present at soil depths in excess of 50 cm (Figure 1 and Table SA2). Tungsten munitions have

401

been used for relatively short times at these ranges, and detection of tungsten at depth implies

402

that tungsten in these soil systems has been transported. The solubility and transport properties of

403

tungsten in soils depend strongly on its adsorption affinity for common soil minerals, and was

404

measured using desorption isotherms (Figure 5). The desorption isotherms confirm that much of

405

the tungsten in these soils is labile. The desorption isotherms from Camp Edwards MMR soils

406

had shapes characteristic of nonlinear adsorption phenomena, with an adsorption maximum (or

407

nearly so) and no apparent limits on dissolved tungsten concentrations. In contrast, mineral

408

dissolution would result in relatively constant dissolved concentrations controlled by mineral

409

quantity or solubility.55 While the adsorption maxima were reasonably high, the partition 15



Page 16 of 31

410

coefficient, Langmuir and Freundlich constants calculated from the desorption isotherms were

411

consistent with weak tungsten(VI) adsorption (Figure 5A). Similarly weak adsorption has been

412

observed in other contaminated sites, particularly in those containing tungsten munitions.12, 14, 16

413 414

Compared to Camp Edwards MMR, the munitions-impacted soils from Forts Benning and Lewis

415

showed lower tungsten solubility (Figure 5B). The lower solubility may result from their higher

416

soil iron concentrations (Figure 5C). There is considerably more Fe(III) (oxyhydr)oxides in these

417

soils on which tungsten can adsorb. Thus, tungsten in these Forts Benning and Lewis soils would

418

be less susceptible to leaching losses.

419 420

Possible Explanation for the High Solubility of Tungsten.

421

Tungsten released extensively from soils to water in desorption experiments, most of which

422

showed 10 to 500 mg L-1 dissolved tungsten (Figure 5). The pH of desorption experiments was

423

~6.5, and they contained ~2 mM Ca2+ (Table SA3). Under these conditions, scheelite (CaWO4)

424

should be highly supersaturated (saturation is achieved at ~1 mg L-1 tungsten), and its

425

precipitation would limit dissolved tungsten concentrations. It is possible that the precipitations

426

of scheelite and/or other tungsten-containing compounds existed in the desorption experiments

427

as suspended colloids, some of which might have escaped centrifugation (4,000 rpm) and

428

filtration (0.2 µm).56, 57 Metal cation Ca2+ and Mg2+ also form ion pairs with tungstate (i.e.,

429

CaWO4(aq) and MgWO4(aq)). Although these ion pairs may be important in some systems, they

430

are not sufficiently stable (log K = 2.57 and 3.03, respectively, for the formation of CaWO4(aq)

431

and MgWO4(aq) from WO42-) to create tungsten solubility observed in these soils, because they

432

only impact dissolved tungsten concentration in equilibrium calculations by a factor of ~2.

433 434

The extensive tungsten release also differs considerably from the strong adsorption behavior

435

observed in model systems with iron (oxyhydr)oxides.18, 19 In fact, tungstate adsorption is so

436

sufficiently strong that it is capable of displacing phosphate from adsorption sites, which is

437

attributed to the formation of stable tungstate surface complexes (Equations 1 and 2):19

438

≡ FeOH + + 2 + ↔ ≡ − ( ) ,

439

≡ FeOH + + ↔ ≡ − + ,

log K = 19.31 log K = 6.4

(1) (2)

440 16


Page 17 of 31


441

The high tungsten solubility observed in desorption isotherm experiments cannot be explained by

442

disequilibrium (i.e., kinetic limitation). If these isotherms were adsorption isotherms, we might

443

be tempted to explain the high solubility with disequilibrium processes such as a lack of mineral

444

precipitation or slow adsorption. The dissolved tungsten concentrations in desorption

445

experiments, however, only represent minimum equilibrium concentrations in that additional

446

reaction time would result in additional desorption and even higher dissolved tungsten

447

concentrations.

448 449

The observed high tungsten solubility also cannot be solely explained by competitive desorption.

450

Soil solutions undoubtedly contain a number of possible ions that could compete with tungstate

451

for adsorption sites, such as silicate, phosphate, molybdate, and carbonate. Such competition

452

affect should inhibit tungsten retention to some extent. However, the concentrations of all

453

competing ions present in desorption experiments would result in only a 2% change in

454

tungsten(VI) partitioning based on calculated adsorption equilibria (Table SA3).18, 19

455 456

In the desorption experiments, the desorption of tungsten was much more pronounced than

457

desorption of other elements that are often present as adsorbed species in soils. For example, in

458

the samples where dissolved tungsten concentrations were high, dissolved concentrations of

459

arsenic and phosphorus were relatively low, and sulfur (indicating sulfate) did not desorb but

460

instead sorbed to the soils. This is consistent with surface sites being available but not being used

461

by tungsten because the soil tungsten existed in chemical form that is distinct from strongly

462

adsorbing monomeric tungstate.

463 464

We thus hypothesize that the high apparent tungsten solubility in the tungsten-contaminated soils

465

results from the presence of some ions which significantly alters tungsten(VI) solution

466

speciation. The soil solutions contain a number of species including phosphate and silicate that

467

are suitable for incorporating free tungstates into POMs such as Keggin clusters (Equations 3 and

468

4):26, 28, 29

469

12 + ( ) + 20 ↔ + 12

(3)

470

12 + + 24 ↔ + 12

(4)

17



Page 18 of 31

471

Although it is difficult to quantitatively evaluate the stabilities of POMs in soil solutions due to a

472

paucity of thermodynamic data for even the most common clusters, POMs often have increased

473

stability than conventional polytungstates.13, 22 Although the available spectroscopic data cannot

474

conclusively differentiate between conventional polytungstates and POMs, EXAFS spectra of

475

the soils contain features attributed to POMs rather than polytungstates (Figure 2). The formation

476

of POMs can increase tungsten solubility and mobility. In fact, aluminum POMs have been

477

identified in soils and appear to facilitate aluminum transport.32

478 479

The hypothesis above is further supported by our previous work, Sun and Bostick (2015), which

480

used model systems to evaluate the effect of POM formation on tungsten(VI) adsorption at

481

circumneutral pHs.38 Sun and Bostick (2015) showed that adsorption of monomeric tungstate on

482

ferrihydrite was similar to others in the literature.18 Adding silicates to the system, 10 mg L-1 Si,

483

significantly suppressed tungstate adsorption. This small concentration of dissolved Si, which

484

was similar to dissolved Si concentrations in desorption experiments in this study, was

485

insufficient to affect tungsten retention by competitive desorption, but was sufficient to convert

486

all the monomeric tungstate to silicotungstate W12SiO404-. The weak retention of W12SiO404-

487

and/or other POMs would diminish adsorption by decreasing the concentration of free tungstate

488

in solution. Sun and Bostick (2015) also examined the adsorption of a model POM,

489

phosphotungstate, to determine if such species would adsorb less than tungstate.38 When reaction

490

time was short, phosphotungstate was metastable, and its adsorption to ferrihydrite was indeed

491

limited. When the phosphotungstate reacted for longer, this Keggin anion decomposed to

492

monomeric tungstate and phosphate, which increased adsorption. The decomposition of such

493

POMs, however, was very slow and could affect tungsten adsorption for months.38

494 495

IMPLICATIONS

496

In the munitions-impacted soils, tungsten metal was rapidly oxidized to a variety of tungsten(VI)

497

species, with adsorbed polymeric tungstates collectively representing the largest fraction. The

498

tungsten in these soils was quite soluble, with desorption experiments producing hundreds of mg

499

L-1 dissolved tungsten. This high solubility is probably due to weak retention of tungsten POMs

500

in soils. The weak retention of POMs implies that the tungsten may be susceptible to long-range

18


Page 19 of 31


501

transport into deeper soils, groundwater or further beyond. Indeed, tungsten has been detected in

502

the groundwaters in the vicinity of Camp Edwards.14

503 504

A number of ions including silicate, which is ubiquitous in nature, could form soluble POMs

505

with (poly)tungstate and create high levels of tungsten in solutions. The toxicity of tungsten also

506

could be influenced by the formation of POMs. More work is needed to confirm the presence

507

and prevalence of POMs in natural systems, especially contaminated systems where POM

508

formation should be encouraged, and to evaluate their toxicity. Also, basic thermodynamic and

509

kinetic data are needed to better ascertain the conditions in which tungsten POMs form and are

510

stable in the environment. As an example of the dearth of data, we are unaware of few

511

equilibrium constants for tungstate solution complexes (except hydrolysis products) including

512

POMs or ion pair reported in the literature. Among these, only MgWO4(aq) and CaWO4(aq)

513

have known equilibrium constants. To fully understand tungsten geochemistry and to establish

514

remediation strategies for the tungsten contaminated areas, continued research effort is clearly

515

required.

516 517

ACKNOWLEDGMENTS

518

This study was funded by National Science Foundation - Environmental Chemistry (NSF-CHE-

519

1010368), the Department of Defense BAA program, and the Department of Agriculture NIFA

520

program. Portions of this study were conducted at the Stanford Synchrotron Radiation

521

Lightsource, a national user facility operated by Stanford University on behalf of the Department

522

of Energy. This is LDEO Contribution Number XXXX (provided by LDEO upon acceptance).

523 524

ASSOCIATED CONTENT

525

Supporting Information: Section A contains additional information and data on the soils,

526

experiments and modeling; and Section B contains additional XAS spectra, fits, and discussion

527

of the tungsten references. The Supporting Information is available free of charge on the ACS

528

Publications website at DOI: 10.1021/acs.est.xxxxx.

529 530

NOTES: The authors declare no competing financial interest.

19



531

Page 20 of 31

TABLES.

532 533

Table 1. EXAFS shell fitting parameters for selected soil samples high in polytungstates/POMs.

534

The coordination number (CN) is typically accurate to within ±1, interatomic distance (R) within

535

± 0.02 Å; σ2 represents the variance in R (in Å2).

536 537 538

FIGURES.

539 540

Figure 1. The changes in tungsten speciation with depth for soil profile in berm (A) and trough

541

(B) collected from Camp Edwards MMR. Errors reported reflect the propagation of errors from

542

least-squares linear combination fitting and bulk soil composition analysis. The EXAFS spectra

543

and linear combination fits are in Figure SB4 and Table SB3.

544 545

Figure 2. (A) Tungsten L3-edge EXAFS spectra and fits of soil sample Test 05 from Camp

546

Edwards MMR. (B) Detailed theoretical shell fits for soil sample Test 05. This soil sample

547

contains ~50% POMs or polytungstates based on least-squares linear combination fitting. The

548

theoretical shell fitting results are consistent with the prevalence of POMs in the sample.

549 550

Figure 3. Microprobe XRF images of normalized tungsten (green), iron (red), and calcium (blue)

551

fluorescence intensities for soil samples Test 05, Test 510 and Test 1520, collected from a

552

munitions trough from Camp Edwards MMR. Tungsten, iron, and calcium intensities in the

553

surface contain numerous hotspots, but are not obviously collocated in the same grains. The

554

correlation of log normalized tungsten and iron counts (upper right panel) is apparent in all

555

samples, although it is stronger for the deeper soil samples with overall lower tungsten

556

concentrations.

557 558

Figure 4. Tungsten (A) and iron µXANES spectra (B) for representative tungsten and/or iron-

559

rich spots in the µXRF image from soil sample Test 05 (0-5 cm depth) in Figure 3. Selected

560

tungsten and iron reference spectra are offset from the soils spectra for clarity. These spectra

20


Page 21 of 31


561

indicate that the iron is present as ferrihydrite and hematite, and the tungsten is monomeric

562

tungstate or polytungstate/POM.

563 564

Figure 5. Desorption isotherms for soil sample MB036S3 (A) at a range of solid-to-solution

565

ratios, and for all soils (B) at a fixed solid-to-solution ratio. The equilibrium pH for the soil

566

MB036S3 is pH ~6.4, and variable but is pH 6-7 for soils from Camp Edwards MMR, and pH 5-

567

6 for Fort Lewis and Fort Benning soils. Langmuir and Freundlich isotherms were derived for

568

soil sample MB036S3. Accounting for variable soil iron concentrations (C) suggests a single

569

isotherm, presumably on iron(III) oxides, controls adsorption in Camp Edwards MMR soils.

21



570 571 572 573

Table 1. EXAFS shell fitting parameters for selected soil samples high in polytungstates/POMs. The coordination number (CN) is typically accurate to within ±1, interatomic distance (R) within ± 0.02 Å; σ2 represents the variance in R (in Å2). Sample / Fit quality

Shell

CN

R (Å)

σ2 (Å2)

Test 05

W-O W-O W-W W-W

2a 3a 2a 2a

1.75 1.92 3.39 3.70

0.002 0.007 0.007 0.008

W-O W-O W-W W-W

2a 3a 2a 2a

1.74 1.93 3.42 3.71

0.003 0.008 0.006 0.009

MMR Range B, Trough χred2 = 6.72b

B023S2 MMR Range B, Bullet pocket χred2 = 8.1b

574 575 576 577 578

Page 22 of 31

(a): Fixed during fitting based on crystallographic data. &'

(* +)'

(b): Reduced chi-squared of the fit. !"#$ = ( = ( ∑ ,' , where ν is the degrees of freedom, O is the observed value at a point, E is the modeled (expected) value, and σ2 is the variance.

22


Page 23 of 31

579 580 581 582 583


Figure 1. The changes in tungsten speciation with depth for soil profile in berm (A) and trough (B) collected from Camp Edwards MMR. Errors reported reflect the propagation of errors from least-squares linear combination fitting and bulk soil composition analysis. The EXAFS spectra and linear combination fits are in Figure SB4 and Table SB3.

Concentrations of Different Tungsten Species (mg kg-1 soil)

Depth (cm)

0

100 200 300 400 500 600

0

0

0

20

20

40

40

60

60 MMR Range B, Berm

(A)

500 1000 1500 2000 2500 3000

Tungsten metal Mineral tungstates Monomeric tungstates Polytungstates/POMs

MMR Range B, Trough

(B)

584

23



585 586 587 588 589

Page 24 of 31

Figure 2. (A) Tungsten L3-edge EXAFS spectra and fits of soil sample Test 05 from Camp Edwards MMR. (B) Detailed theoretical shell fits for soil sample Test 05. This soil sample contains ~50% POMs or polytungstates based on least-squares linear combination fitting. The theoretical shell fitting results are consistent with the prevalence of POMs in the sample.

590

24


Page 25 of 31

591 592 593 594 595 596 597 598


Figure 3. Microprobe XRF images of normalized tungsten (green), iron (red), and calcium (blue) fluorescence intensities for soil samples Test 05, Test 510 and Test 1520, collected from a munitions trough from Camp Edwards MMR. Tungsten, iron, and calcium intensities in the surface contain numerous hotspots, but are not obviously collocated in the same grains. The correlation of log normalized tungsten and iron counts (upper right panel) is apparent in all samples, although it is stronger for the deeper soil samples with overall lower tungsten concentrations.

599

25



600 601 602 603 604 605

Figure 4. Tungsten (A) and iron µXANES spectra (B) for representative tungsten and/or ironrich spots in the µXRF image from soil sample Test 05 (0-5 cm depth) in Figure 3. Selected tungsten and iron reference spectra are offset from the soils spectra for clarity. These spectra indicate that the iron is present as ferrihydrite and hematite, and the tungsten is monomeric tungstate or polytungstate/POM.

2.0

Normalized Absorbance

5

606

Page 26 of 31

4 3 2

Background Fe hotspot W hotspot

1.5 WO42- (aq) H3W12PO40

Background W hotspot Fe hotspot

1.0 Ferrihydrite 0.5

1 0

Hematite 0.0 W metal

(A)

10180 10200 10220 10240 10260 eV

(B) 7075 7100 7125 7150 7175 7200 eV

26


Page 27 of 31

607 608 609 610 611 612 613


Figure 5. Desorption isotherms for soil sample MB036S3 (A) at a range of solid-to-solution ratios, and for all soils (B) at a fixed solid-to-solution ratio. The equilibrium pH for the soil MB036S3 is pH ~6.4, and variable but is pH 6-7 for soils from Camp Edwards MMR, and pH 56 for Fort Lewis and Fort Benning soils. Langmuir and Freundlich isotherms were derived for soil sample MB036S3. Accounting for variable soil iron concentrations (C) suggests a single isotherm, presumably on iron(III) oxides, controls adsorption in Camp Edwards MMR soils. 1600

(A)

Sorbed Tungsten (mg kg-1 soil)

1400

Soil MB036S3

1200 1000

Langmuir Equation

800

Γ=

600 400

Γmax K L C 1466 × 0.22C = 1 + KL C 1 + 0.22C

Freundlich Equation

200

Γ = K F C1/n = 1140C1/24.40

0 0

Sorbed Tungsten (mg kg-1 soil)

5000

200

400

600

800

1000

(B)

4000 3000 2000 1000 0 0

Sorbed Tungsten (mg g-1 iron)

700

20

40

60

80

100

120

(C)

600 500 400 300 200

Camp Edwards MMR Fort Lewis Fort Benning

100 0 0

20

40

60

80

100

120

Dissolved Tungsten (mg L-1) 614 27



615

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21. 22.

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26.

27.

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Tungsten Speciation and Solubility in Munitions-Impacted Soils

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