Release Characteristics of Manganese in Soil under Ion-absorbed Rare Earth Mining Conditions

Abstract

ABSTRACT The use of high-concentration ammonium sulfate leaching agent for the mining of ion-absorbed rare earth ore will change the soil environment properties, thereby inducing the release of associated heavy metals. Serious manganese pollution has been observed in the environment of rare earth mining areas, but the environmental chemical process of manganese in the raw ore soil has not been fully recognized. In this study, a sequential extraction experiment showed that the manganese in the soil included acid extractable form, reducible form, oxidizable form, and residual form, among which residual form and reducible form were the main forms. According to the release kinetics experiment, the release process of manganese included the fast release and slow release, which was controlled by multiple diffusion mechanisms and could be optimally fitted with the Elovich equation. The treatment of ammonium sulfate leaching agent could reduce the proportion of reducible form and increase the proportion of residual form, but there was no significant change in other forms. In addition, batch release experiments indicated that leaching factors (the initial concentration and initial pH of leaching agent, and temperature) could affect the release of manganese. Lower pH and higher temperature were not conducive to the stability of manganese and significantly aggravated the release amount. Excessive initial concentration had a certain inhibitory effect on the release amount, but the effect was weak. Considering the significant effect of initial pH and temperature on the release of manganese, during the mining process of ion-absorbed rare earth, controlling the initial pH of leaching agent and reducing the mining scale in the hot summer are the key measures to manage the release risk of manganese.