Solid Phase Speciation of Tungsten in Contaminated Soils.

Tungsten composites in heavy metal alloys (WHAs) are being investigated actively by the United States and European Union military organizations for use in armor piercing weaponry.  Tungsten is viewed by the military community with great promise as a nonradioactive alternative to depleted uranium, yet uncertainty about this element’s environmental fate has been an issue of concern.  Recent leaching experiments reported by the U.S. Army Engineer Research & Development Center (ERDC) showed that W readily oxidizes upon addition to soil to highly soluble, tungstate monomers, polymeric phosphotungstate and polytungstate species, which may be  highly toxic to organisms.  In this work, we focused on in-situ characterization of the contaminant using soils collected from various military bases spiked with metallic W powder and aged under ambient conditions.  Soil particles mapped by SEM-EDX showed W domains overlapped by Al, Si, Ca, and transition metals Fe and Mn, suggestive of the broad range of complexes available.  XANES analysis confirmed W was oxidized to W(VI).  Mortlet wavelet transform maps of the W-XAFS data pointed to second shell association of soil metal oxyhydroxides – a finding not observed in our previous leaching studies.  Given the very limited information on W minerals, XAFS data was modeled using schleeite (CaWO₄) and scattering pathways calculated from crystallographic files doped with 5% W.  Results showed that W atoms were surrounded by 4 to 6 O atoms in the first shell – with P observed in the first shell for only one soil.  Second shell atoms consisted of Fe, Ca (from the highly insoluble Ca-polytungstate mineral), and in one case, another W atom.  This work demonstrates the complex interactions and pathways controlling the environmental fate of W in soils.