Cadmium Selective Accumulation And Speciation Transformation Mechanism By Biological Soil Crust In Typical Paddies From South China

Heavy metal pollution is a vital problem of paddy environment that China faces now.The most pernicious heavy metal pollution source in South China is Cadmium(Cd).This paper focuses on Biological Soil Crust(BSC)that collected from typical paddy field environment in South China.The BSC was characterized and tested on adsorptive property.The result showed BSC was consisted of biological components and mineral,a large number of functional groups on its surface.Adsorption capacity had a linear relation with equilibrium concentration,and the maximum adsorption capacity and remove rate was 8.48mg/g and 99.79%.BSC was showed excellent remove capacity of Cd when p H was 7.It showed the feature of preferential adsorption in high calcium ions system,and had good removal efficiency of cadmium complex that molecule weight less than 107.Through further research on the mechanism of cadmium speciation and distribution in BSC,the BSC were collected from four locations of typical paddy field environment in South China.The result showed that water is the main source of the accumulation of cadmium in BSC,the Cd of BSC was mainly distributed in Sorbed and no-EDTA-exchangeable fractions(almost 95%),less in Extracellular Polymeric Substances(EPS)fraction.The mechanism of Cd enrichment in Sorbed fraction was biological physical activity;in no-EDTA-exchangeable fraction,metal including Cd in the form of oxide,in this fraction,Cd accumulation was concerned with biological mineralization and Mn Ox.These results suggest that Cd enrichment by BSC depends mainly on biological activity of microorganism and adsorptive action of surface Mn Ox.According to the Bacterial diversity research,low level of Cd contamination had no obvious effects on bacterial community.But high levels of Cd contamination decrease biological diversity,the bacterium that was sensitive to Cd contamination were eliminated.Survivors such as Bacteroidetes and Cyanobacteria played a significant role in the accumulation of Cd biological activity by in BSCs.Hydrodictyon was one of eukaryote of BSC.Accumulation contents of Cd which loaded on colloidal Kaolin by Hydrodictyon were in inverse proportion to particle size of colloid,these contents were generally below the content of ionic Cd.After reaction24 and 48 h,enrichment mechanisms of Cd in all samples mostly were biologicalactivity and biomineralization,less in EPS.After complete reaction,sample D5 was different from before.In this sample,primary of Cd enrichment mechanism was biological activity,the second was combining with EPS loosely,accordingly Cd concentration of EPS fraction in D5 was highest.Cd accumulation process along with redistribution of Zn and Fe,distribution characteristics of Zn was similar to Cd,there existed competition between biological activity and biomineralization of Fe enrichment,the Al belongs to Kaolin was taken off and mineralized.In the process,oxygenic functional groups of Hydrodictyon were consumed,but no significant change of physiological index was observed.