Behavior and mechanism of different fraction lead leach with several typical sulfate lixiviants in the weathered crust elution-deposited rare earth ore.

Recently, some new leaching agents without ammonium, such as magnesium sulfate (MgSO4) and aluminum sulfate [Al2(SO4)3], have been developed to eliminate ammonia nitrogen pollution in in situ mining process of the weathered crust elution-deposited rare earth ore (WCED-REO), but they might cause heavy metal contamination. In this study, characteristics and mechanisms of different fractions of lead (Pb) released by (NH4)2SO4, MgSO4 and Al2(SO4)3 leaching agents were investigated using batch experiments and column leaching tests. The experimental results showed that the amounts of Pb released by the different leaching agents followed the trend of Al2(SO4)3 > (NH4)2SO4 > MgSO4 under the same total cationic charge, and both the acid extractable and reducible fractions of Pb were released. The release of acid extractable fraction Pb was related to the cation hydration radius of NH4+, Mg2+, and Al3+, whereas the release of reducible fraction Pb was mainly influenced by the concentration of H+, especially at pH < 4.0. Furthermore, column leaching tests indicated that pH has little effect on the Pb contents of different fractions released by (NH4)2SO4 and MgSO4 in leaching the WCED-REO. Although Al2(SO4)3 released the largest contents of rare earth and Pb in leachate, the content of residual acid extractable fraction Pb in soil was the most after water injection (simulating the cleaning process after mining). This work can provide a scientific method and theoretical basis for comprehensively assessing the environmental impact of new leaching agents on WCED-REO mining.