Study On The Speciation Of Plutonium In Soil And Remediation Technology Of Radioactive Contaminated Soil

Due to the remarkable ecological hazard and high technical challenge in reduction of volume and weight,plutonium contaminated soil has been an important environmental issue in the context of nuclear waste disposal.In this present dissertation,a decommission technology containing chemical leaching of plutonium from soil and reclamation of plutonium from leachate was developed for remediation of plutonium contaminated soil.Both microwave digestion of soil samples,characterization of plutonium in soil,screening of leaching reagents,effect of leaching operation parameters and volume reduction methods of leachate were investigated in detail with the focous on the removal efficiency of plutonium from soil and weight lost of soil.The matrix interference,the peak tailing effect of ²³⁸U on ²³⁹Pu and the formation of the polyatomic ion ²³⁸U¹H⁺forms in the plasma are the vital limiting factors for the quantitative analysis of plutonium by inductively coupled plasma mass spectrometry?ICP-MS?.Base on the theory of isotope dilution technique,microwave digestion,co-precipitation and anionic exchange were introduced into the analysis of ²³⁹Pu in soil with a low detection limit of 8.4×10^-2 Bq/kg for 1g soil determined by ICP-MS.Plutonium in soil was characterized by using particle size sieve analysis and sequential extraction technique.An extreme uneven distribution of plutonim in different soil particle size fractions has been clarified,which conclude that size separation is not an appropriate technique for weight and volume reduction of the contaminated soil.Water soluable and exchangeable plutonium only occupied about 2%,which illuminated the low bioavailability of plutonium in soil.Meanwhile,plutonim bound to Fe/Me oxide,organic matter and residual predominated in different fraction and occupied about 95%of total plutonium in the soil.The fraction of plutonim bound to Fe-Mn oxide and organic matter were related to the contents of mineral constituents and organic matter in soil respectively.The concentration and speciation distribution of plutonium can be implied that the total dissolution of residual plutonium was essentially requied for decontamination of soil.The screened leaching reagents,hydrochloric acid,sulfuric acid and citric acid,were further investigated for their dependence of the decontamination efficiency with batch wise leaching conditions and the existence or not of oxidants.It was clarified that the decontamination efficiency depended on the speciation of plutonium in soil,content of hydrogen ion in leaching solution and complexation behavior of plutonium with anions originated from the acids or complex reagents.The decontamination efficiencies were improved with the augment of liquid-solid ratio,temperature,duration,and rinse or extracting time,also controlled by leaching mechanism and reaction dynamic between plutonium and leaching reagents.The decontamination efficiency from column leaching tests is compared with the value from batchwise leaching tests.Although the plutonium content varies dramatically for the experimental aliquots,plutonium remained in the leaching residue is generally less than1 ng/g,which fufilled the general remediation criterion of 4000 Bq/kg.Both of the flocculation precipitation and microbial biosorption are capable to remove plutonium from leachate completely and satisfy the requirement for directly discharge of radioactive liquid waste.The flocculation precipitation was found to be suited for treatment of leachate produced either by hydrochloric acid or sulfuric acid and biosorption was suited for all leachate in this present work.Over 80%of plutonium in contaminated soils would be reclaimed by chemical leaching combining flocculation precipitation or microbial biosorption.