Association of tungsten with aluminosilicate mineral colloids and silicotungstates in soil porewaters: Insights into the unexpectedly high tungsten mobility in soil

Abstract

As an emerging contaminant, tungsten (W) displays unexpectedly high mobility in soil despite its extremely low solubility, challenging current scientific understanding. This paradox underscores the limited knowledge regarding the specific W species responsible for its high mobility. In this study, a series of field and incubation experiments were conducted across multiple soil types to investigate the distribution and speciation of W in soil porewater, widely known as ″the most mobile fraction″. Ultrafiltration analysis revealed that W in soil porewater predominantly existed in colloidal-size (5 kDa–0.45 μm) phases rather than the ″truly-dissolved″ phase (<5 kDa). Especially in deeper soil layers approaching shallow groundwater, colloidal W content exceeded 93 %. XANES spectra showed that colloidal W was mainly in the hexavalent state (WVI), and insoluble W metal (W0) entering the soil could rapidly oxidize into WVI through biotic or abiotic processes. Advanced characterizations, including STEM-EDS-SAED, SEM-EDS, and SR-XRF, identified aluminosilicate mineral colloids as the primary carrier for W in soil porewater. Within these W-bearing aluminosilicate mineral colloids, W was primarily present as Al2(WO4)3 precipitates with a W–Al distance of ∼ 3.64 Å, as confirmed by EXAFS. Additionally, a minor fraction of silicotungstates was also detected in the colloidal fraction using XAS and STEM-EDS-SAED. These two species were further substantiated through geochemical modeling and density functional theory (DFT) analysis. Importantly, this study hypothesizes that the associations of W with aluminosilicate mineral colloids and silicotungstates are widespread across different soil types. The finding suggests that colloid-associated W mobility is a dominant yet previously overlooked process, helping to explain why W, despite its low solubility, exhibits exceptionally high mobility in soils.