Studies On The Transport,transformation And Control Method Of Uranium In Red Soils

Jiangxi,Hunan and Guangdong provinces were the main producing areas of uranium(U)in China.There are many U tailing impoundments in these provinces.As time goes by,U in the U tailing impoundments have been migrating and diffusing to the surrounding environment,and contaminating the farmland soil and groundwater through various physical,chemical and biological processes.Red soil is the main soil type in Jiangxi,Hunan and northern Guangdong provinces.Therefore,studies of the transport,transformation and control method of U in red soils are very helpful to comprehend the specific chemical behaviors of U in environmental,and to predict the migration trend of U in red soils,as well as to provide theoretical bases for remediation of U-contaminated red soil.It is of great significance for U pollution control in soil and groundwater around U tailing impoundments.In this study,the red soil around a U tailing impoundment in southern China is selected as the research object.The process and mechanism of U transport and transformation in soil,as well as the migration activity control method in U-contaminated soil are investigated by a series of experiments(batch adsorption experiments,column experiments,transformation experiments,and remediation experiments)combined with model simulation and various analysis and detection methods.The main contents and conclusions are as follows:1.The vadose zone red soil was collected next to a large U tailing impoundment in southern China,and its physicochemical characteristics were analysed.It showed that the red soil is characterized by loamy clay texture,low soil organic matter and high acidity.Batch adsorption experiments were carried out to study the adsorption model and mechanism of U on red soil at different initial pH and carbonate concentrations with the sulfate background electrolyte solution.It was found that,(1)the adsorption kinetics of U on red soil can be well described by the quasi-second-order kinetics model under different initial pH values and different carbonate concentration,and their isothermal adsorption can be properly fitted by Freundlich isothermal adsorption model.(2)U is adsorbed mainly by interaction with hydroxyl(-OH)on the surface of red soil.The adsorption capacity of U on red soil is controlled by the equilibrium pH of solution in the absence of carbonate.However,it is controlled by the concentration of carbonate and initial pH in the presence of carbonate.2.The transport behaviors and models of U in red soil at different initial pH and carbonate concentrations with the sulfate background electrolyte solution were investigated by laboratory column experiments and theoretical simulation with Hydrus-1D software.The results showed that,(1)The lower the initial pH or the higher the concentration of carbonate,the higher the migration activity of U in red soil.(2)The U transport in the red soil column at different initial pH and different carbonate concentrations can be well simulated by the two-site chemical non-equilibrium model(CNEM).Furthermore,the fractions of equilibrium adsorption sites,f,seems to correlate with the fractions of positively charged complexes of U species in solution.In the experiment under varying carbonate concentration,f equal to0,so one-site CNEM is suitable for the simulation of the breakthrough curves(BTCs).(3)The sensitivity analysis results indicated that partition coefficients,k_d?,is the most critical parameter.And the sensitivity of the four model parameters in the two-site CNEM are in descending order as follows:partition coefficients,_dk?,first-order rate coefficient,?,fractions of equilibrium adsorption sites,f,and longitudinal dispersivity,?.Meanwhile,the sensitivity of the three model parameters in the one-site CNEM are in descending order as follows:partition coefficients,_dk?,first-order rate coefficient,?,and longitudinal dispersivity,?.3.Redox oscillations and reduction experiments were carried out to investigate the regularity and mechanism of valence and species transformation of U in red paddy soil in the redox dynamic environment.The conclusions could be drawn as follows,(1)In reduction experiment,there are upper limit values for percentage of stable U(IV)species(e.g.,uraninite and/or U(IV)-phosphate)in soil,and the labile U(IV)species can not transform to stable U(IV)species in a short period of time.(2)The concentration of water-soluble U in soil increase slightly during redox oscillations,which increase the risk of environmental pollution,however,the percentage of stable U(IV)species in soil increase significantly in redox oscillations experiment.In addition,the stable U(IV)species in solid phase is not re-oxidized during aerobic period.(3)The conversion of microbial community composition and the redox transition of Fe enriched on the surface of soil play a major role in speciation transformation of U under redox oscillations conditions.(4)Tessier sequential extraction revealed that the increase of stable U(IV)species content reflect the uranium speciation transition from acetate extract to more recalcitrant hydroxylamine extract.4.The remediation of the U-contaminated red soil by rice husk biochar was studied.The adsorption behaviors of U in solution by rice husk biochar,and the chemical stability and migration activity of U in the red soil after the remediation by rice husk biochar were conducted.The following conclusions were obtained,(1)Rice husk biochar have strong adsorption capacity for U in solution in neutral environment.(2)The pH values of U-contaminated red soil increase after remediation by rice husk biochar.(3)Synthetic Precipitation Leaching Procedure(SPLP)was employed to investigate the short-term U leachability of rehabilitated soils treated by rice husk biochar.It was found that about 26.53%of U in U-contaminated red soil can be extracted,while the leaching ratio reduce to about 2.58%and 1.40%in rehabilitated soils treated by 5%and 10%rice husk biochar,respectively,which indicating that the short-term release potential of U in soil after rice husk biochar remediation greatly reduce.(4)Tessier sequential extraction results showed that,as the proportion of rice husk biochar increase,the fractions of exchangeable U in soil decrease,the fractions of carbonate-bound U increase,the fractions of Mn-and Fe oxide-associated U decrease slightly,and the fractions of organic-bound and residual U remain almost unaltered.Moreover,the sum of the fractions of exchangeable and carbonate-bound U increase slightly,indicating that the chemical stability of U is still not high after rice husk biochar remediation.(5)Simulated three years of acid rain leaching experiments showed that 26.37%of U can be leached from U-contaminated red soil column,whereas,the leachability of U in rehabilitated soils column treated by 5%and 10%rice husk biochar reduce to 7.30%and 3.18%,respectively,indicating that the long-term release potential of U in soil after rice husk biochar remediation also greatly reduce.