Mechanism of uranium release from uranium mill tailings under long-term exposure to simulated acid rain: Geochemical evidence and environmental implication.

To date, there is not sufficient knowledge to fully understand the occurrence, transport and fate of residual uranium (U) from uranium mill tailings (UMT). Herein this study investigated different U release behaviors from natural UMT (without grinding) under four simulated acid rain (pH = 2.0-5.0) compared with controlled scenario (pH = 6.0) for 25 weeks. The results showed that the most notable U release was observed from UMT, followed by UMT whereas a nonlinear relationship between pH and U release was observed from UMT. The divergence of U release behaviors was attributed to the presence of minerals such as calcite and clinochlore. Autunite, a secondary mineral formed after leaching, might regulate U release in UMT. Fick theory model revealed the shift of U release mechanism from surface dissolution to diffusion transport for UMT, UMT and UMT at varied stage, whereas UMT and UMT displayed univocal dissolution and diffusion mechanism, respectively. This study highlights the necessity of performing long-term leaching tests to detect the "shift event" of leaching kinetics and to better understand the mechanism of U release influenced by mineralogy of the natural UMT under simulated acid rain conditions, which is conducive to developing UMT management strategies to minimize the risk of U release and exposure.