The Experimental Study On In Situ Remediation Of U-contaminated Soils With Attapulgite

Soil by radionuclide uranium pollution is becoming more and moreserious, many scholars had put forward many methods about themanagement of U-contamination soil, but there are advantages anddisadvantages respectively, really can be applied methods remains to bestudied. Passivating agent added in the U-contaminated soil is to preventthe migration and transformation of uranium in the soil environment, andto reduce the absorption of the plant, may be a simple and economic ideaabout the management of radionuclide contaminated soil, that is to say insitu passivation restoring technology. The method is to add a passivationagent to chang the effective concentration or oxidation reduction state ofheavy metals in the soil, thereby reducing the effective concentration ofheavy metals in the soil and the toxicity of the organism. In situimmobilization remediation technology is a cost-effective, non-pointsource pollution control technology, due to rapid effect and convenientoperation, so that in the remediation of contaminated soil plays a role cannot be replaced, it is in line with China’s sustainable agriculturaldevelopment needs, the technology is increasingly widespread concern bysoil and environmental scientists.Attapulgite （also called palygorskite） belongs to the sepiolite groupin mineralogical classification, is a crystalline hydrated magnesiumaluminum silicate minerals, its basic structure unit is2:1layer, that is tosay two layers of silicon-oxygen tetrahedron clip a layer of magnesiumoxygen octagonal to form a cell layer. As a result of attapulgite containsSi⁴⁺can be Al³⁺replacement, so the surface have certain residual negativecharge which can adsorb the heavy metal ions and prevent the migrationof heavy metal ions. These studies by using static test method analyze theadsorption characteristics and adsorption mechanism of uranium on attapulgite in aqueous solution, at the same time of studying attapulgitepassivation restoring U-contaminated soil and the passivation effect.Details are as follows:1. The adsorption characteristics of uranium on attapulgite wereinvestigated by conducting a series of batch adsorption experiments inthis study. The influence of solution pH value, initial uraniumconcentrations and contact time was investigated. Scanning electronmicroscope （SEM） and X-ray diffraction （XRD） were used tocharacterize the surface structure of the attapulgite, Fourier transforminfrared spectrometer （FTIR） were used to characterize the surfaceproperties of the attapulgite which adsorbed uranium before and after, andto analyze the adsorption mechanism and adsorption kinetics of uraniumon attapulgite. The experimental results showed that sorption of uraniumon attapulgite was strongly dependent on pH values, and adsorptionreached most at pH=5. The adsorption quantity increased with time,adsorption could achieve balance in2h. The adsorption isotherm equationconformed to the Langmuir isothermal adsorption model and adsorptionprocess could be described by the two-order kinetics model. According toFTIR spectral, the absorbance of attapulgite decreased, which may resultfrom R-OUO+2or （R-O）2UO2formed by the bond between uranium andR-OH of attapulgite in the high frequency area3700-3000cm^-1，andwhich uranium ion and magnesium ions may produced ions exchangeaction in the intermediate frequency area1700-800cm^-1. Adsorptionmechanism of uranium on attapulgite was mainly ion exchange andcomplexation.2. The passivation effect of U-contaminated soil with attapulgite wasinvestigated by laboratory and training. The different dosage ofattapulgite on the influence of pH of contaminated soil in differentconcentration was investigated, as well as on uranium in soil watersoluble, exchangeable, carbonate-bound state, iron and manganese binding state effect. The results showed that the pH value of differentconcentrations of uranium in contaminated soil rised with the attapulgitedosage increased, and the growth trend was more obvious, probablybecause of the attapulgite is alkaline, the soil is acidic, attapulgite is therole of adjusting the pH. Water soluble uranium and exchangeableuranium in the soil as attapulgite dosage increased, and their contentsshowed a downward trend, so the application amount was the greater canreduce the concentrations of water soluble and exchangeable. Soilcarbonate bound, as attapulgite dosage increased, its contents increasedfirst and then decreased, but compared to the control group, the content ofcarbonate bound uranium applying different percentages of attapulgitegroup was higher than that of control group, so the application ofattapulgite increased the carbonate combined form of uranium in soil.With attapulgite dosage increased, the content of iron and manganesebinding state of uranium decreased first and then increased. In the sameattapulgite dosage experiment, the content of iron and manganese wasthe lowest when carbonate combined state was the highest, indicated thatiron and manganese binding state and carbonate combined statecomplemented each other, the total amount was same. Attapulgiteconversed water soluble and exchangeable into organic conjugated orresidual. In summary, attapulgite can remediated the U-contaminatedsoils.