Alkaline Soil Disintegration and Organic Nanocolloidal-dissolved U Release under IPCC-scenario Elevated CO2 with Global Environmental Risk Implication

The alkaline soils sustain billions of people worldwide. These soils serve as vast uranium (U) reservoir by containing ppm levels of U. Global elevated CO2 (eCO2) may lead to massive U release from alkaline soils and pose a great environmental risk. However, the mechanistic responses of U release in alkaline soils to eCO2 are largely unknown. In this study, we investigated structural stability, U mobility, and phase partitioning in two alkaline soils under IPCC scenario-based eCO2 conditions, using single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) elemental mapping, in conjunction with U speciation analysis. Exposure to eCO2 caused substantial U release from both soils dominantly in the dissolved phase (51.07%–78.10%) and nanocolloidal phase (10.24%–45.85%). Within nanocolloids, U was predominantly enriched in organic phases derived from disaggregated calcareous mineral-organic complexes. Both the pH drop and DIC rise induced by eCO2 are crucial for U release. Reduced pH caused dissolution of cementing calcareous minerals and breakdown of mineral-OM complexes and thus release of colloids, DOM and their associated U. Enhanced DIC under eCO2, resulted in an increase of UO2(CO3)34- but a decrease of UO2(CO3)22-, which promoted release of dissolved U. These novel findings are helpful for deep insight into U behavior and risk in soils in context of future climate change, and also imply that future global climate change may drastically weaken soil health and amplify environmental risks of U, which is a global environmental and health concern that needs enough attention.