Behavior Of Element Cd In Weathering Pedogenesis Of Carbonate Rocks In Karst Area Of Longzhou Guangxi

The weathering pedogenesis of in weathering of carbonate rocks is the most common and basic geological and geochemical process,and it is an important source in the process of soil formation in Karst areas.During the process of the weathering pedogeneswas of carbonate rocks,the soil was enriched with the heavy metals cadmium（Cd）compared with the soil mother rocks.This paper takes Guangxi Longzhou Karst area as the study area,takes four different stages products of the weathering pedogeneswas as the main research objects,they are carbonate rocks,soil,bauxite and secondary carbonate rocks,through field study and sampling,totally collects 92samples of different kinds of rock,secondary carbonate rocks,bauxite and so on,94 soil samples and 19 water samples,analyzes the cadmium content and pH,distribution of soil granularity,XRD pattern and some other geochemical parameters of the samples indoor,according to the content change of the Cd at different stages of weathering pedogeneswas,enrichment and loss degree,by this,to understand the characterwastics of Cd content dwastribution and behavior changes in different stages of weathering pedogeneswas.By designing a leaching experiment to simulate the leaching effects caused by soil and carbonate rocks,we learn the effects of weathering pedogeneswas on Cd behavior.In this way,the causes of high Cd in soil in karst areas can be understood,and the scientific basis for soil heavy metals repair can be provided.The main research results and conclusions are as follows:1.Through the analysis of the 26 carbonate rock samples totally collected in the study area,the mean value of Cd content in limestone was 0.43mg/kg,the median was 0.29 mg/kg,and the content range was 0.05⁰1.63 mg/kg;the mean value of Cd content in dolomite was 0.52mg/kg,the median was 0.44 mg/kg,and the range was from 0.15 to 1.19 mg/kg.Relative to carbonate rocks,Cd content of all samples was enriched,and there was a phenomenon of high Cd in carbonate rocks.The mean value of Cd content in carboniferous and Permian was 0.47mg/kg and0.42mg/kg respectively,the median value in the two are both 0.31mg/kg.The content of Cd in carboniferous carbonate rocks was higher than that in Permian.The variable coefficient analysis shows that there was a significant difference in the content of Cd between carboniferous and Permian carbonate rocks.2.The mean pH values of water samples and soil samples in the study area are 7.92 and7.72 respectively,and both soil and water samples were alkaline.The average content of element Cd in water samples was 0.06 PPB,the median was 0.07 PPb,and the range was 0.0061⁰.1665PPB.With 279 ug/L,the theoretical calculation value of the Cd element content in the cadmium carbonate（CdCO₃）saturated solution at 25℃as the standard,the Cd elements in water samples was highly unsaturated.3.The mean Cd content of soil in the study area was 12.31mg/kg,the median value was7.08 mg/kg,and the content range was 1.41 to 31.70 mg/kg.The mean content of Zn was560.45mg/kg,the median value was 676 mg/kg,and the range was 30.50 to 864.00 mg/kg.With soil element background value in Guangxi as reference,the excess rate of Cd was 100%and of Zn in all was 95.6%.4.The average grain size of the soil profile from top to bottom became smaller;clay mineral content increases,calcite mineral etc.content decreases.The content of Al₂O₃ and Fe₂O₃increase gradually,the content of CaO and MgO decrease gradually,and the CIA index increases gradually.The content of Cd in soil profile was the highest on the surface,and the content of Cd in soil profile was higher than the average value of Cd content in surrounding rocks.In the process of soil formation,Cd in rocks was enriched in soil.Correlation analysis showed that there was no significant correlation between soil grain size,pH value and the level of Cd content in the soil profile,while CIA index and Cd content in the soil profile were associated with the moderate intensity of.5.The mean Cd contents of stalactites,chalk,travertine and calcite were 1.90mg/kg,7.43mg/kg,9.63mg/kg and 0.44mg/k respectively.the median values are 0.681mg/kg,4.56mg/kg,6.86 mg/kg and 0.41mg/kg,respectively.The average and median Cd contents of different secondary carbonate rocks are:calcite>chalk>stalactite>calcite.The variation coefficient analyswas shows that there are significant differences in the Cd content in different secondary carbonate rocks.6.The mean Cd content in bauxite samples was 3.51mg/kg,and the median was 2.89 mg/kg,range was 0.26 to 16.9 mg/kg.All of them were higher than the Cd abundance in carbonate rocks and soil element background value in Guangxi.Bauxite was the residual formed in weathered soil,and the Cd content in 52.38%of bauxite exceeded that in surrounding soil.7.In the leaching experiment,in the leaching solution,CaCO₃ material was formed by carbonate supersaturated and sediment,With 279 ug/L,the theoretical calculation value of the Cd element content in the cadmium carbonate（CdCO₃）saturated solution at 25℃as the standard,Cd elements in the leaching filtrate was unsaturated,adsorptive precipitation of Cd element caused by CaCO₃ made Cd enrichment in theCaCO₃.The leaching increased CIA index,clay mineral content and calcite,minerals such as calcite reduced.Al₂O₃ and Fe₂O₃ contents increased slightly,while CaO and MgO contents decreased gradually,which was consistent with the results of soil profile study.