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 UMTpH2.0, followed by UMTpH3.0 whereas a nonlinear relationship between pH and U release was observed from UMTpH4.0-6.0. 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 UMTpH3.0-6.0. Fick theory model revealed the shift of U release mechanism from surface dissolution to diffusion transport for UMTpH2.0, UMTpH3.0 and UMTpH5.0 at varied stage, whereas UMTpH4.0 and UMTpH6.0 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.