Uranium Speciation and Mobilization in Thawing Permafrost.

Area of Science:

• Environmental Science
• Geochemistry
• Soil Science

Background:

• Uranium is a toxic geogenic contaminant, often found with organic matter.
• Its behavior in organic-rich permafrost soils is poorly understood, hindering risk assessment for permafrost thaw.
• Previous studies indicate elevated uranium in bedrock and groundwater in the Yukon, Canada study area.

Purpose of the Study:

• To investigate uranium abundance and speciation in permafrost soils and thaw-liberated porewater.
• To assess the risks of uranium mobilization from thawing permafrost.
• To understand the biogeochemical factors controlling uranium fate during permafrost thaw.

Main Methods:

• Analysis of active-layer and permafrost soil samples from Yukon, Canada.
• Utilized X-ray absorption spectroscopy, micro-X-ray fluorescence, and electron microscopy for solid-phase uranium speciation.
• Employed Extended X-ray absorption fine structure (EXAFS) for detailed structural analysis.
• Analyzed porewater chemistry and performed geochemical modeling.

Main Results:

• Permafrost soils exhibited high organic carbon (1.1-28 wt%) and elevated uranium concentrations (median 25 μg g^-1), correlating positively with organic carbon.
• Solid-phase uranium was predominantly uranium(VI) complexed with soil organic matter, likely as bidentate-mononuclear uranyl complexes.
• Thawed porewater showed circumneutral pH (6.2-7.5) and elevated dissolved uranium (0.5-203 μg L^-1), dominated by calcium-uranyl-carbonate complexes.

Conclusions:

• Permafrost soils act as a significant source of uranium mobilization upon thaw.
• Uranium speciation in thaw porewater is controlled by calcium-uranyl-carbonate complexation.
• The fate of mobilized uranium is intricately linked to organic carbon content and groundwater geochemistry during permafrost thaw.