Experimental and theoretical investigation of the mobilization, immobilization and sequestration of subsurface metals through manipulation of carbonate equilibria
This dissertation investigates the forced mobilization, immobilization and sequestration of heavy metals and radionuclides in the ground water environment. Metal mobility/immobility can be effectively controlled by the manipulation of carbonate equilibria. Three different aspects of its potential application as an in situ remediation technology to clean up contaminated aquifers are investigated. ^ A laboratory experiment was conducted to determine the effectiveness of CO2 partial pressure manipulation, acting as the driver of the carbonate equilibria system, to modify metal mobility in calcite-bearing soils. In a first step, mobilization occurs when CO2 is injected into the ground water lowering its pH; this forces the release of the metals bound to the solid phase. Down gradient, immobilization occurs when the excess CO 2 is removed from the ground water during air sparging, reversing the previous process by increasing the pH of the ground water and causing the metals to partition onto the solid phase. The results show that CO2 charging/air sparging effectively mobilized/immobilized Mn2+, Zn2+, Sr2+ and Ba2+. ^ Predictions of mobilized and immobilized concentrations were made. These were based on the equilibrium exchange reaction of cations between the solid solution and the aqueous phase. The predicted results were in close agreement with the observed data. ^ A second laboratory experiment was conducted to assess the effectiveness of solid solution formation through carbonate precipitation for the in situ remediation of ground water contaminated with heavy metals and radionuclides. The experiment simulated the flow of ground water through an inert isotropic porous medium. Three different treatments (calcium carbonate, metal carbonate, and pH) were compared to a control baseline run. Plumes of contamination containing Pb2+, Cd2+, Cu 2+, Mn2+, Zn2+, Co2+, Sr2+, and Ba2+ were intercepted by remediation zones generated by the injection of different treatment solutions. ^ Metal carbonate precipitation was the most effective treatment in terms of aqueous metal removal, whereas calcium carbonate (calcite) treatment was the most effective in terms of pH stability. Based on these results a sequence of treatments is proposed where a major portion of contamination is precipitated as carbonates, followed by a calcite treatment to coat the other precipitates, to keep the system carbonate saturated and to further decrease the aqueous metals concentrations by coprecipitation. ^ In addition to the two laboratory experiments, a computer model was developed to obtain information about both the substituting and non-substituting ions during solid solution formation and how their proportions affect the coprecipitation of the foreign ion. The governing equations of this model are based on a mass balance done in differential form of the main species present in the system, which is conceptually represented by a mixed flow reactor. An experimental reaction rate equation from the literature is used to approximate the reaction kinetics existent during the precipitation of calcite. ^ Results indicate that coprecipitation increases when CO3 > Ca. This occurs even for trace elements that preferentially partition into the aqueous phase. ^
Clague Romero, Juan W. Santiago, "Experimental and theoretical investigation of the mobilization, immobilization and sequestration of subsurface metals through manipulation of carbonate equilibria" (2001). ETD Collection for University of Texas, El Paso. AAI3023412.