Dissolution of Technetium(IV) Oxide by Natural and Synthetic Organic Ligands under both Reducing and Oxidizing Conditions

• Published in: Environmental science & technology Vol. 45; no. 11; pp. 4771 - 4777
• Main Authors: Gu, Baohua, Dong, Wenming, Liang, Liyuan, Wall, Nathalie A
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.06.2011
• Subjects: Anaerobiosis; Applied sciences; CHELATING AGENTS; Chemical elements; Chemical precipitation; Contamination; DISSOLUTION; Earth sciences; Earth, ocean, space; EDTA; ELECTRON TRANSFER; Engineering and environment geology. Geothermics; Environmental Processes; Exact sciences and technology; FUNCTIONALS; HALF-LIFE; HUMIC ACIDS; KINETICS; Ligands; MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; mobilization; Organic Chemicals - chemistry; organic matter; OXIDATION; Oxidation-Reduction; OXIDES; OXIDIZERS; Pertechnetate; Pollution; Pollution, environment geology; PRECIPITATION; Radioactive Pollutants - chemistry; RADIOACTIVE WASTES; reduction; SOLUBILITY; STABILITY; TECHNETIUM 99; Technetium Compounds - chemistry; Wastes; United States; North America; Tennessee; America; Organic matter; Radioactive pollution; Pollutant behavior; Pollution prevention; Organic ligand; Immobilization; Mobility; Environmental factor; Hazardous waste; Radioisotope; EDTA; Dissolution; Decontamination; Carboxylic acid; Aminoacid; Chelating agent; Radioactive waste storage; Water pollution; Technetium 99; Ground water
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/es200110y

Technetium-99 (Tc) in nuclear waste is a significant environmental concern due to its long half-life and high mobility in the subsurface. Reductive precipitation of technetium(IV) oxides [TcO2(s)] is an effective means of immobilizing Tc, thereby impeding its migration in groundwater. However, technetium(IV) oxides are subject to dissolution by oxidants and/or complexing agents. In this study we ascertain the effects of a synthetic organic ligand, ethylenediaminetetraacetate (EDTA), and two natural humic isolates on the dissolution and solubility of technetium(IV) oxides. Pure synthetic technetium(IV) oxide (0.23 mM) was used in batch experiments to determine dissolution kinetics at pH ∼6 under both reducing and oxidizing conditions. All organic ligands were found to enhance the dissolution of technetium(IV) oxides, increasing their solubility from ∼10–8 M (without ligands) to 4 × 10–7 M under strictly anoxic conditions. Reduced Tc(IV) was also found to reoxidize rapidly under oxic conditions, with an observed oxidative dissolution rate approximately an order of magnitude higher than that of ligand-promoted dissolution under reducing conditions. Significantly, oxidative dissolution was inhibited by EDTA but enhanced by humic acid compared to experiments without any complexing agents. The redox functional properties of humics, capable of facilitating intramolecular electron transfer, may account for this increased oxidation rate under oxic conditions. Our results highlight the importance of complex interactions for the stability and mobility of Tc and thus for the long-term fate of Tc in contaminated environments.