Impact Of Uranyl-Carbonate/Calcium-Uranyl-Carbonate Complexes On Uranium Adsorption To Red Soils And Nanoscale Zero-Valent Iron

Selecting appropriate adsorbents as remediation materials in the consideration of practical U(VI) species is very important in uranium contaminated soils remediation. U(VI)-CO3 and Ca-U(VI)-CO3 complexes are the frequent dominant species in groundwater, and the concentration is effected by p H values, dissolved carbonate and calcium concentration. In this thesis, the feasibility and performance of selective remediation materials-nanoscale zero-valent iron(NZVI) used for remedying uranium contaminated red soils was investigated by batch adsorption and fixed-bed column experiments. The mechanism of NZVI/red soils on U(VI) adsorption was investigated by XPS,XRD,ZETA potential, adsorption kinetics and isotherms. The research results can provide technical or theoretical data for uranium contaminated red soils remediation.The 50-100 nm NZVI particles were successfully synthesized through liquid-reduction process. Batch adsorption experiments indicated that the adsorption capacity(qe) and the distribution constant(Kd) of NZVI and red soils decreased with increasing p H values, dissolved carbonate and calcium concentration. U(VI) adsorption on red soils included both monolayer and heterogeneous adsorption, and U(VI) adsorption on NZVI mainly contained heterogeneous adsorption. The kinetics were well fitted to pseudo-second order kinetic model which meant that the adsorption process is chemisorptions. The adsorption capacity of NZVI which appeared less influence by p H values, calcium and carbonate concentration was 5 to 10 times higher than red soils. c(U-CO3)T can be used to predict uranium adsorption capacity of red soils and c(U-CO3)T has a nonlinear relationship with qe.Fixed-bed column experiments suggested that the breakthrough pore volume(PV) values increased with the decrease of p H values, dissolved carbonate and calcium concentration. The data of column studies were in accordance with Thomas model and Yoon and Nelson model, indicating the long-term performance of permeable-reactive barriers(PRB) columns. The breakthrough PV values of PRB column filled with 5% content of NZVI were 2.0 to 3.5 times higher than 100% red soils column under the same experimental conditions. The high performance of uranium adsorption can be achieved using NZVI as the reactive medium in PRB.The results of ZETA potential suggested the absorbed U(VI) species were mainly UO2(CO3)22-, UO2(CO3)34- and UO2CO3(aq). XPS and XRD analysises confirmed the reduction process from U(VI) to U(IV) and the formation of Fe OOH which could further react with U(VI)-CO3 complexes on NZVI surface. OH-contained surface sites played an important role in U(VI) adsorption.The positively charged materials can be the optimum adsorbents for adsorbing uranium in the consideration of practical U(VI) species. The experimental data investigated that the adsorption capacity of CTAB-bentonite which appeared less influence by carbonate concentration could be many times higher than bentonite under the same experimental conditions.