Bio-weathering of a uranium-bearing rhyolitic rock from Xiangshan uranium deposit, Southeast China

Uranium mining and processing has left a serious problem in terms of waste disposal. Microbially mediated U release from crystalline rocks has made the problem even more challenging, but the specific mechanisms for such U release have remained elusive. In this work, bio-weathering experiments of a U-bearing rhyolitic rock were conducted to investigate the rates and mechanisms of microbially-induced U mobilization. Four bacterial isolates from Xiangshan uranium deposit, one of the largest volcanic rock-hosted U deposits worldwide, were able to grow in an oligotrophic medium (OM) in the presence of a U-bearing rhyolitic rock. These bacteria liberated U from the rock through a combination of acidolysis, metabolite-promoted complexation, and oxidative dissolution, among which U mobilization via complexation with citrate was one of the most significant mechanisms. Metabolite secretion was strain-specific, which accounted for the differences in U mobilization efficiency among different bacterial isolates. In particular, Microbacterium sp. 6-1 selectively colonized the surface of U-bearing minerals and mobilized a substantially higher amount of U than other isolates, suggesting a potentially active, mineral-specific attachment and bio-weathering mechanism. Our results demonstrated a high potential and capacity of native microorganisms to mobilize U from U-bearing rock, posing a great challenge for long-term U waste disposal, but also providing an opportunity for U recovery from mine tailings.