Geochemical Conditions and Design Considerations Affecting Electrode-induced Removal of Uranium(VI) and Technetium(VII) from Acidic Groundwater.

摘要

Electrode-based remediation has emerged as an alternative approach for managing radionuclide contamination of groundwater and enabling permanent site restoration. However, little is known about the performance of electrodes under environmental conditions or how aqueous geochemical factors will impact remediation and recovery of radionuclides from the subsurface.;The overarching objective of this research was to provide an improved understanding of how aqueous geochemical conditions impact the removal of U and Tc from groundwater with polarized graphite electrodes and how engineering design may be utilized to optimize removal of these radionuclides. Experiments in this research were designed to address the unique conditions in Area 3 of the U.S. DOE Y-12 site in Oak Ridge, TN while also providing broader insight into other contamination conditions. The specific objectives of this work were: 1) to quantify the impact of common aqueous geochemical and operational conditions on the rate and extent of U and Tc removal and recovery from water with polarized graphite electrodes, 2) to examine U and Tc treatment from Area 3 groundwater with polarized electrodes, 3) to determine the capacity of a graphite electrode for U(VI) removal and 4) to develop a mathematical kinetic model for the removal of U(VI) from aqueous solution with polarized electrodes.;Experiments were conducted in batch reactors with graphite electrodes polarized by power supplies to quantify the impact of geochemical and operational factors on U and Tc removal and recovery. Treatment of U and Tc in synthetic wastewater were studied separately. Common geochemical and operational conditions studied for groundwater treatment included pH, applied potential, initial concentration of radionuclide, presence and concentration of other cations, ionic strength of the solution, and concentration of humic acid. Both U and Tc were removed with polarized electrodes, most likely through electrosorption and electroreduction, respectively. Although low pH had an adverse impact on U removal rate: it can be overcome by optimizing the applied potential. Tc was also removed slower at lower pH. The influence of Al3+, Mg2+, and Na+ was related to their aqueous chemical properties and condition around the electrode. Ionic strength and humic acid did not influence U and Tc removal within studied range. The removed radionuclides were readily recovered in solution after removing the applied potential. Recovery rate of U was higher at lower pH, while the rate of Tc recovery increased with pH.;Experiments with actual site water from Area 3 showed that simultaneous U and Tc removal can be achieved with polarized graphite electrodes. Despite low pH (∼3) and high concentration of nitrate (∼300 mM), both U and Tc were removed at the cathode within 3-7 days. The presence of nitrate did not prevent U or Tc removal from electrodes.;A semi-continuous study on U removal at pH 3 in batch reactors was conducted to determine the capacity of graphite electrodes for U removal at 2.0 V. Results suggested that graphite electrodes have a finite capacity for U electrosorption, which can be predicted by the Langmuir isotherm.;A kinetic, mathematical model was developed based on empirical first-order kinetics, to predict kobs for U(VI) removal under the influence of some major environmental and operational effectors. The S/m ratio, or electrode surface area (S) to molar mass of adsorbate (m) ratio term was created to stress the combined effect of electrode surface area, solution volume, and adsorbate concentration. Double layer capacity, Cd, was selected as a term to define the influence of applied potential. Ionic strength considerations were based on Gouy-Chapman-Stern (GCS) theory. The output model equation is kobs = 0.022(pH -- 1.6)ln(0.85Cd S m ). Verification of the model showed that it can accurately predict kobs, in over the range of ionic strength examined (10 -3 -- 0.24 M). ^ This dissertation describes investigations into the critical site considerations and design parameters for optimizing the removal of radionuclides from contaminated groundwater using an electrode-based approach. Results of this study make clear the importance of pH for the efficiency of electrode-based remediation of U and Tc in contaminated groundwater. The adverse impact of low pH can be overcome by optimizing the applied potential. These findings can provide valuable information for the design of pilot-scale testing of electrode-based remediation of U and Tc.