| The weathering soil of carbonate rocks is widely distributed in Karst reigns, Environmental problems in karst regions attract more and more attention of many scholars, especially soil environment in karst reigns where there is radon anomalies, which bring threats to the environment in recent years, Studies have shown that radon anomaly in soil was related with radionuclide such as uranium and thorium. However, uranium and thorium both appear the phenomenon of significant enrichment in the weathering soil of carbonate rocks, whether uranium and thorium with enrichment will be released to the environment to be pollution sources is still unknown. From this point, we selected the five weathering profiles of carbonate rocks from different times in Guizhou Pronvince. Correlation analysis and mass balance calculation methods were used. We studied the distribution characteristics and geochemical behavior in the process of evolution, discussed the relevance of U, Th and p H, the organic matte and major element, We analyzed the factors which influenced the distribution of uranium and thorium. Continuous chemical extraction technology which is widely used in speciation studies of elements,was used to explore the speciation and environmental activity of U and Th in the weathering soil of carbonate rocks. Our object is to promote the understanding of the environmental geochemistry circulation of U and Th, as well as to provide a theoretical basis for carring out the karst reigns of radioactive environmental quality assessment and soil remediation through this research. This study obtained the following understanding :(1)Different bedrocks develop into weathering profiles of carbonate rocks, the weathering degree of limestone higher than the weathering degree of dolomite.(2)Different bedrock develop into weathering profiles of carbonate rocks living with different environment, causing that there is difference with the correlation between uranium, thorium content and p H, the content of major elements, organic matter content. Effects factors of the distribution of uranium, thorium in the profile may be more than one factor, but the combined effects of several factors.(3)There is no exchangeable U in the weathering profiles of carbonate rocks developed from different times, there are three phases with more uranium, associated with residue phases > associated with carbonates phases > coprecipitated with amorphous ferromanganese oxyhydroxides phases. Plenty of thorium is distributed in three phases:residuephases, co-precipitated with amorphous ferromanganese oxyhydroxides phases and associated with crystalline ferromanganese oxyhydroxides phases,which shows that thorium is distributed mainly in primary and secondary of the residue.(4)In a further weathering process of weathering profiles of carbonate rocks, U and Th both have transfer or redistribution between phases, As for U, with the process of evolution, residue phases of U tends to decompose,associated with carbonates phases of U tends to increase, the other phases of U will be decreasing or increasing trend because of differencet environment. Each phase of Th are likely to occur decreas or increase in the process of evolution,and it could be transformed in different forms at different stages of weathering process. Co-precipitated with amorphous ferromanganese oxyhydroxides phases and associated with crystalline ferromanganese oxyhydroxides phases of U and Th could be transformed into each other.(5)In the five profiles, as for Dafang profile,Tianbaqiao profile,Tongren profile,Houchang profile, the proportion of the inert phases > the proportion of active phases > the proportion of potential active phases. In the Fenggang profile, the proportion of the inert phases(the average proportion is 64.07%)> the proportion of potential active phases(the average proportion is 20.29%) >the proportion of active phases(the average proportion is 15.64%). And as the five profiles, the proportion of the inert phases of thorium> the proportion of potential active phases of thorium >the proportion of active phases of thorium.(6)As for the proportion of active uranium phases and the proportion of potential active uranium phases, Tongren profile(an average of 44.52%)> Tianbaqiao profile(an average of 40.46%)> Houchang profile(an average of 38.20%)> Fenggang profile(an average of 35.93%)> Dafang profile(an average of 30.05%). While the proportion of active thorium phases and the proportion of potential active thorium phases, Fenggang profile(an average of 55.08%)> Tongren profile(an average of 51.83%)> Houchang profile(an average of 49.32%)> Tianbaqiao profile(an average of 28.23%)> Dafang profile(an average of 15.68%). The proportion of active uranium phases and the proportion of potential active uranium phases of Tongren profile and Tianbaqiao profile is more than 40%, The proportion of active thorium phases and the proportion of potential active thorium phases of Fenggang,Tongren and Houchang profiles is more than 50%, The environment effect of these profiles should be given high priority. |
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