Adsorption Of Typical Nuclides On Red Soil Colloids

With the decommissioning of nuclear facilities and the sudden occurrence of nuclear leakage accidents,the diffusion and migration of radionuclides in environmental media has become a research hotspot in various countries.As the smallest and the most active component in the soil,colloids are crucial for the adsorption and transport of radionuclides,and tiny colloidal particles are easily transferred from the soil to the air,surface runoff and deep soil,resulting in secondary pollution.In this study,the topsoil of Changzhutan area in Hunan Province was selected,and the redistribution and enrichment of typical heavy metals as well as typical nuclear species of Sr and Cs was studied.The different forms of Sr and Cs were studied enriched in different particle size soil particles and soil colloids to get the distribution characteristics.Through the characterization of soil colloids,the adsorption behaviors of Sr and Cs on soil colloids and environmental influencing factors were emphatically studied,and the adsorption mechanism was preliminary inferred.(1)Heavy metal background analysis of three soil samples from LF,SM and YJ,soil particle size and soil colloids was carried out.In LF and YJ,the concentration of heavy mental is Zn>Cr>Cu>Pb>Ni>As>Cd;Sr>Cs.And in SM,the concentration level of heavy mental is Zn>Pb>Cr>Cu>Ni>As>Cd>;Sr>Cs.At the same time,it was found that with the decrease of the particle size of the soil particle group,the enrichment of elements increased.The soil colloids had the most abundant metals.In order to avoid matrix effects,the laboratory static adsorption experiments were conducted on LF soils that were relatively lightly contaminated.(2)Morphological analysis of radionuclides of Sr,Cs in the bulk soil,different particle size and soil colloids in LF showed that the distributions patterns were similar.Sr in more than 40%of the particle size are in the residual chemical state.Sr in each particle size is basically:residual state>exchangeablestate>carbonate boundstate>F e-Mn oxide state>organic combination state>water soluble;Cs in more than 95%of the particle size is the residual state.Cs in each particle size is basically:residual state>organic combination state>exchangeablestate>Fe-Mn oxidestate>carbonate-boundstate>watersoluble.Considering the bioavailability,in addition to the residue state,the remaining several forms can be considered to have direct or potential biological validity.Therefore,the content of active Sr in different particle sizes is about 50%-60%,and mainly in exchangeable Sr,indicating that Sr is highly active in soil and can be easily utilized by plants.Cs is relatively active state accounted for less than 5%of the lower,but the water-soluble Sr,Cs belong to strong variation(variation coefficient of about 50%),so having greater harm to animals and plants and human beings.(3)To explore the effect of different environmental factors on the adsorption of Sr and Cs on soil colloids,it was found that with the increase of soil colloidal amount,pH and temperature,the adsorption rate of Sr increased gradually.With the increase of strontium nitrate concentration and ionic strength,the adsorption rate of Sr decreased gradually.It was found that the adsorption rate of Cs increased with the increasing of the amount of colloids and pH,and the adsorption rate of Cs decreased with the increase of strontium nitrate concentration and ionic strength.Temperature has no obvious effect on the adsorption rate of Cs.The results showed that the response of organic carbon was different.After organic matter removal,the adsorption rate of Sr was decreased,but the adsorption rate of Cs was not change significantly.The effects of temperature,soil organic matter Colloid adsorption of Cs has no obvious effect.(4)The kinetics,isotherms and thermodynamics of Sr and Cs adsorption on soil colloids were fitted.It was found that the adsorption processes of Sr and Cs all conformed to the second-order kinetics and Langmuir adsorption isotherms.According to the thermodynamic parameters,?H>0,indicating that the process of colloidal adsorption of Sr and Cs is an endothermic reaction,and ?H<30 KJ·mol-1,indicating that the adsorption reaction under this condition is mostly physical adsorption;?S>0,indicating The adsorption process is an entropy increasing reaction.And the degree of disturbance of the whole system of the soil colloidal adsorption Sr2+,Cs+ process increases;?G<0,indicating that the adsorption process of the soil colloids on Sr,Cs can proceed spontaneously.For soil colloids adsorbed on Cs,the smaller the ?S,the smaller the effect of temperature on it,and the influence of temperature on the adsorption process was not obvious.(5)The comparison of the spectra before and after the adsorption of Sr and Cs by soil colloids showed that the functional groups of soil colloids did not change after the adsorption reaction.The same electron microscopy showed that the morphology of soil colloids adsorbed Sr and Cs did not change obviously.