Mechanism of uranium release from uranium mill tailings under long-term exposure to simulated acid rain: Geochemical evidence and environmental implication
2019
Yin, Meiling | Sun, Jing | Chen, Yongheng | Wang, Jin | Shang, Jianying | Belshaw, Nick | Shen, ChuanChou | Liu, Juan | Li, Huosheng | Linghu, Wensheng | Xiao, Tangfu | Dong, Xinjiao | Song, Gang | Xiao, Enzong | Chen, Diyun
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.
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