Effect of EDTA Complexation on the Kinetics and Thermodynamics of Uranium Redox Reactions Catalyzed by Pyrite: A Combined Electrochemical and Quantum-Mechanical Approach

Abstract

Ethylenediaminetetraacetic acid (EDTA), a strong complexing agent, is a common constituent of liquid nuclear waste streams. The interaction of EDTA or other organic or inorganic ligands with uranyl enhances the mobilization of uranium and therefore significantly influences the disposal and remediation of uranium waste. Although numerous studies have focused on the effect of EDTA on the mobilization of environmental uranium, including the adsorption and redox reactions of uranium on mineral surfaces, few studies have differentiated the reaction-controlling steps (diffusion, adsorption, and chemical reaction/electron transfer) occurring at heterogeneous interfaces to assess the effect of EDTA complexation on each of these processes. Here, a combination of in situ electrochemical methods and quantum-mechanical calculations was used to study the effect of EDTA complexation on the kinetics and thermodynamics of redox reactions of uranium at the pyrite-solution interface. Experiments indicate a potential shift of U­(VI)/U­(V) on the pyrite surface of ∼0.04 V due to surface adsorption and −0.21 V due to EDTA complexation, with similar results from the calculations. The oxidation of the U­(IV) reaction is more sensitive to EDTA complexation in solution than that of U­(V). Disproportionation, affecting 2/3 of the intermediate pentavalent state, is the rate-limiting step of the overall redox cycle of uranyl. The charge transfer was controlled by diffusion and adsorption processes. EDTA can promote the delivery of free uranyl from the bulk solution to the pyrite–water interface where the reaction takes place, making the electron-transfer process less diffusion-limited. Adsorption of uranyl onto pyrite surfaces in the gradient electric field is multimolecular layer adsorption, through which electron transfer can occur.

Effect of Organic Compounds and Copper on Chromium(VI) Reduction: Electrochemical Investigation of Electron Transfer Rates

“…Similar electrochemical studies may be applied to other redox-active elements, such as copper, arsenic, selenium, and uranium. Recent research has demonstrated the usefulness of electrochemistry in studying the fluctuation of oxidation states, effect of reducing mineral surfaces, and influence of organics on the uranium and selenium systems. − Electrochemical measurements can provide better quantitative data on adsorption and redox reactions during redox changes than other geochemical tools.…”

Leaching Mechanism and Health Risk Assessment of As and Sb in Tailings of Typical Antimony Mines: A Case Study in Yunnan and Guizhou Province, Southwest China

“…In addition, combined with the XRD in Figure 2, pyrite was identified in the tailings of the Zuoxiguo antimony mine. Under natural conditions, the pyrite in the tailings undergoes a redox reaction (2FeS 2 + 7O 2 + 2H 2 O→2Fe 2 + 4SO 4 2− + 4H + ), thereby releasing a large number of hydrogen ions into the tailings [37], which lower the pH. Heavy metals associated with pyrite are released into the environment along with heavy metal carbonates [38], which may be another reason for the high Sb and As content in the tailings.…”

Investigations of the Redox Reactions of Potassium Trisoxalatoferrate(III) with Perchlorate Ion in Acidic Medium: A Kinetic Approach

“…"Conversely, under the right circumstances, perchlorate can also act as a goitrogen in rodents and prevent thyroid hormone formation by interfering with iodide uptake" [7,9]. "The redox reactions of perchlorate with metal cations is receiving attention" [10,11,12,13]. It is in pursuance of more knowledge into the redox reactions of potassium trisoxalatoferrate (III) with oxoanions that we report the results of our investigations of its reactions with perchlorate ion.…”