Radioactive contaminants in the subsurface: the influence of complexing ligands on trace metal speciation

The modeling of the behavior of hazardous materials under environmental conditions is among the most important applications of natural and technical sciences for the protection of the environment. In the field of radioactive waste management, the hazardous material consists to a large extent of actinides and fission products from nuclear power plants, in addition to lesser amounts from other sources such as waste from medicine, industry and research facilities. Equilibrium thermodynamics is one of the pillars which support safety analyses of repositories for radioactive waste. Thermodynamic constants are used in modeling reference pore waters, calculating radionuclide solubility limits, deriving case specific sorption coefficients, and last but not least in analyzing experimental results. It is important to use the same database throughout these modeling activities to guarantee internally consistent results. The research presented here consists of approaches to resolve the problems related to thermodynamic equilibrium constants and solubility of solid phases in the field of radioactive waste management. The thesis is composed of a series of manuscripts published between 1999 and 2005. Three different levels of problem solving strategies are presented. (i) Critical and comprehensive reviews of the available literature, which are necessary in order to establish a reliable thermo chemical database that fulfils the requirements for rigorous modeling of the behavior of the actinides and fission products in the environment, are shown. The first series of manuscripts, entitled 'Thermodynamic data: Facts and fiction', is devoted to aqueous carbonate complexes of ferric iron and nickel. Our experimental and review work allowed to close some important gaps, especially concerning nickel carbonate complexes. These topics and other reviews finally were summarized in the Nagra/PSI Chemical Thermodynamic Data Base (TDB), which is the official chemical thermodynamic database used in Swiss radioactive waste disposal projects. Within the scope of this TDB project I reviewed extensively thermodynamic data for Th, Pd, Al, and solubility and metal complexation of silicates, the review considering not only U, Np, Pu, Am, Tc, Ni, Se and Zr, but also the major constituents of ground and surface waters, i.e. H, Na, K, Mg and Ca. The decision to evaluate the organic ligands oxalate, citrate, ethylenediaminetetraacetate (edta) and α-isosaccharinate (isa) was based on two aspects, namely the importance of the ligands in radioactive waste problems, and the availability of experimental data. (ii) In many case studies involving inorganic and simple organic ligands a serious lack of reliable thermodynamic data is encountered. There, a new modeling approach to estimate the effects of these missing data was applied. This so called 'backdoor approach' begins with the question: 'What total concentration of a ligand is necessary to significantly influence the speciation, and hence the solubility, of a given trace metal?' Radioactive waste contains substantial amounts of ion-exchange resins from decontamination procedures. Degradation of these organic waste forms by radiolysis in a repository is a source of concern in radioactive waste management. Radiolytic degradation experiments with strong acidic ion exchange resins resulted in the formation of the complexing ligands oxalate and ligand X, whose structure could not be identified. In the case of anion exchange resins, ammonia and methylamines were detected. I assessed the influence of these ligands on radionuclide speciation in groundwater and cement pore water of a repository using the 'backdoor approach'. Prussian Blue, FeIII4 [FeII(CN)6]3, and structurally related transition metal compounds like Ni2[Fe(CN)6] are used as cesium ion exchangers in decontamination procedures of liquid radioactive waste. The used ion exchangers are conditioned as cementitious waste form for interim storage and finally will become part of the radioactive waste in geological repositories. The worst-case scenario is the instantaneous and complete dissolution and decomposition of the ion exchangers in the cementitious environment and the release of free cyanide. I assessed the effects of cyanide complexation on the speciation and solubility of radionuclides by 'backdoor' calculations. In the course of the review process for the Nagra/PSI TDB, two important gaps in the database were identified: a) experimental data for the system ThO2 -H2O cannot be interpreted by a unique set of thermodynamic constants; b) potentially important thermodynamic constants for mixed carbonatohydroxo complexes of tetravalent actinides are missing because of insufficient experimental data. Both problems have been solved using the 'backdoor approach'. (iii) In the field of natural organics, mainly humic and fulvic acids, we face an ill-defined problem concerning the molecular structure of the ligands. There, I proposed a pragmatic approach for performance assessment purposes, the 'conservative roof' approach, which does not aim to accurately model all experimental data, but allows estimates of maximum effects on metal complexation by humic substances to be calculated. The major effects influencing radionuclide-humate interactions are explored in detail: the metal concentration effect; the pH effect; competition effects. The first two effects are included in the 'conservative roof' approach in a simplified form suitable for performance assessment. The competition effects were explored by the 'backdoor approach' using the 'conservative roof' model and additional thermodynamic information. A worked out case study for a safety analysis of a Swiss geological repository project elucidated all important effects in quantitative terms. This case study may serve as a roadmap for the design of more sophisticated approaches to deal with radionuclide-humate interactions in future performance assessments