Occurrence And Release Characteristics Of Manganese In The Soil From An Ion-absorbed Rare Earth Mining Area In Southern Jiangxi

The use of high-concentration ammonium sulfate leaching agent for the mining of ion-absorbed rare earth ore will change the chemical properties of the raw ore soil environment,thereby inducing the release of associated heavy metals.After leaching,the residual heavy metal elements in the tailing soil with deteriorated properties are easily released again when encountering acid rain,which poses a great threat to the safety of ecological environment of the mining area and its surrounding areas.At present,serious manganese pollution has been found in the environment of ion-absorbed rare earth mining areas,but the environmental chemical process of manganese in the mine soil has not been fully recognized,and the corresponding prevention and control measures of manganese pollution are still lack of theoretical basis for reference.In this paper,an ion-absorbed rare earth mining area in southern Jiangxi was selected as the research area,firstly,the spatial distribution characteristics of manganese in the soil and water environment were investigated through field investigation experiments,and the level of manganese pollution in the soil was evaluated by the method of Index of Geo-accumulation;on this basis,the manganese in the mine soil was taken as the research object,the mineralogical characteristics and chemical form were analyzed by instrument characterization methods?XRF,XRD and SEM-EDS?and sequential extraction experiment,and the potential migration index was introduced to describe the potential mobility of manganese;then,release kinetics experiment,batch release experiments and simulated leaching experiment were carried out to study the release characteristics of manganese in the raw ore soil under ion-absorbed rare earth mining conditions;finally,the release characteristics of manganese in the tailing soil under simulated acid rain?different acidity?leaching were investigated by soil column leaching experiment.The main conclusions are as follows:?The manganese content of the raw ore soil was significantly higher than that of the tailing soil,and the manganese content of the tailing soil near the rare earth mining site was higher than that of the other tailing soil;the closer to the rare earth mining site,the greater the manganese concentration of the surface water.The raw ore soil and the tailing soil near the rare earth mining site were at the level of slight manganese pollution.?The principal manganese-bearing minerals in the mine soil were rhodonite and ramsdellite,and a small amount of manganese-bearing components were adsorbed on the surface of kaolinite,orthoclase and other minerals in dispersed forms.The chemical forms of manganese in the mine soil were mainly residual form and reducible form,followed by acid extractable form and oxidizable form.The manganese in the raw ore soil and the tailing soil near the rare earth mining site had a strong potential mobility.?Under the action of ammonium sulfate leaching agent,the release process of manganese in the raw ore soil included fast release?0-1 h?and slow release?after 1 h?,which could be optimally fitted with Elovich equation;after treatment,the proportion of reducible manganese decreased 4.24%,the proportion of residual manganese increased 4.19%,while the proportions of the other two forms remained basically unchanged.With the decrease of the initial pH value?5-2?of the leaching agent and the increase of the ambient temperature?10-40°C?,the release amount of manganese in the raw ore soil increased significantly;while with the increase of the initial concentration?1.5-4.5%?of the leaching agent,the release amount of manganese in the raw ore soil was almost unchanged firstly and then decreased significantly.When the leachate of the raw ore soil was in the NH₄⁺outflow stage and the pH value was low,the manganese concentration was high and changed rapidly,while when the leachate of the raw ore soil was in the NH₄⁺equilibrium stage and the pH value was large,the manganese concentration was low and the change was slow;with the increase of soil depth,the proportion of reducible manganese gradually increased,and the proportion of residual manganese gradually decreased.?During simulated acid rain leaching with different acidity?pH=5.6,4.4,2.5?,the cumulative release amount of manganese in the tailing soil showed an overall trend of increasing gradually with the increase of leaching days.For the simulated acid rain leaching treatment group with pH=5.6 and pH=4.4,the cumulative release process of manganese was basically the same,both belonged to the slow release,and Double-constant rate equation and Parabolic diffusion equation had a better fitting effect;when the pH value of simulated acid rain dropped to 2.5,the cumulative release process of manganese could be clearly divided into fast release?1-11 d?and slow release?11-23 d?,Double-constant rate equation had a good fitting effect on the fast release stage,and the slow release stage could be better described by Double-constant rate equation,Parabolic diffusion equation,and Elovich equation.As the pH value of the simulated acid rain decreased,the proportion of reducible manganese gradually decreased,the proportion of residual manganese gradually increased,the proportion of acid extractable manganese remained basically unchanged at first and then significantly decreased,while the proportion of oxidizable manganese was almost unaffected.