The Input Mechanisms Of Exotic Uranium In Soils Around A Uranium Mill Tailings Pond In Hunan Province,China

The uranium mill tailings are a type of solid waste materials produced by grinding and hydrometallurgy of uranium ores which are mined by conventional mining method,and they still contain a certain amount of residual uranium.Being generally open-air stacked in the tailings ponds,they may lead to the uranium diffusion by weathering,leaching and even air-borne dust,and thus cause potential radioactive risk to their ambient environment,in which the soil is an important carrier.The uranium tailings pond has been a widely concerned uranium-polluted source generated by human in the surface environment.The current research work has focused on the assessment and remediation of uranium-contaminated soils,and there is a lack of specialized research on the input mechanism of exogenous uranium in soils,which has important indicative meaning for the proper assessment of the pathway and manner of uranium release from the pollution source into the environment,and is a crucial reference for effective uranium tailings reservoir management.In this paper,three agricultural soil profiles were took?i.e.,S1,S2 and S3?as the research objective,which are around a uranium mill tailings pond in Hunan Province,China,and are 10 m,30 m and 200 m away from the pond,respectively,and discussed the input mechanisms of exotic uranium in the soils by contrasting their geochemical characteristics with those of the background profiles?i.e.,B1 and B2?and uranium mill tailings sands in the study area and by using the sequential chemical extraction technique for uranium.The results show as follows:1.Uranium tailings have produced uranium pollution to ambient soils.The average uranium contents of the three soil profiles S1,S2 and S3 are84.82 mg·kg^-1,65.96 mg·kg^-1,and 6.03 mg·kg^-1,respectively,and are higher or much higher than those of the soils in Hunan Province and local background profiles,and display a increasing trend with decreasing distance from the pollution source.2.As an agricultural soil,profiles S1,S2,and S3 were imported by exotic uranium throughout the section due to human disturbance in the tillage process.3.The profiles S1 and S2 near the source received heavy uranium contamination and their uranium pollution index?Pi?characterized by the single factor index method was 18.98 and 14.76,respectively,and the profile S3 far away from the source was subjected to slight uranium contamination?i.e.,Pi=1.35?.4.The input of exotic uranium in soils,especially for that in the heavily uranium-polluted soils near the source,mainly included two mechanisms,namely,one was physical interfusion of uranium carried by tailings sands,the other was the entrance of water-soluble uranium ion carried by seepage fluid from the pond,and the former was dominant.However,for the exotic uranium of specific adsorption state?II?,which possessed absolute superiority in the amount among various speciations of exotic uranium in the soils,it was mainly contributed from the latter.For uranium carried by tailings sands,its speciations were inherited in the soils,and for uranium carried by seepage fluid,it was mainly specifically adsorbed onto the soil matrices.5.The management of uranium tailings pond in the study area should be further strengthened.On the one hand,uranium tailings releasing through the mechanical diffusion has a reality of pollution for the surrounding soil;on the other hand,water-soluble uranium of uranium tailings have been migrated to the surrounding environment.Therefore,for the management of uranium tailings pond,more effective engineering means should be carried out to solve the above questions.