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Hydrated zirconium molybdate (ZM_h) is known to precipitate from solutions of dissolved spent nuclear fuel, particularly from the waste fission product solution after the uranium and plutonium have been extracted during reprocessing. Its precipitation can cause major problems during waste treatment, and therefore a complete understanding of its chemical behaviour, especially with regard to its role in the nuclear fuel cycle, is desirable. We have used a number of complementary analytical techniques to elucidate the hitherto incompletely understood chemistry of formation of ZM_h in synthetic fuel reprocessing solutions. We have demonstrated that ZM_h formation was governed by multi-step surface reactions and does not involve the formation of colloids or particulates in solution. The first step in the deposition of ZM_h onto surfaces is the formation of an amorphous film with a Zr ∶ Mo ratio close to unity. It is followed by the formation, growth and nucleation of ZM_h particles of varying degrees of crystallinity with a Zr ∶ Mo ratio close to 0.5. The X-ray diffraction pattern of deposited ZM_h particles is in agreement with the reported crystallographic data. The structural features of the film and ZM_h were also examined at the nanometer scale.

The dissociation of Eu(iii)–HS (HS=fulvic or humic acid) complexes has been investigated using cation exchange resins to separate ‘free’ from ‘complexed’ Eu(iii). With relatively small amounts of resin, the amount of complexed Eu(iii) remaining after any resin contact time is calculated using four components to describe the Eu(iii) binding viz.: an easily exchangeable fraction of Eu (calculated using a single equilibrium constant) and three kinetically hindered fractions (calculated from three consecutive pseudo-first order rate expressions). With larger amounts of resin, the easily exchangeable Eu(iii) becomes negligible. The slowest dissociation step, which probably is the most important with regard to radionuclide transport, has a half life of ≈200 h. The fraction of Eu(iii) in the kinetically hindered sites increased with metal–HS contact time, pH, FA concentration and the degree of humification (HA>FA) but competing cations (Ca, Mg, Al or Fe) had little effect. Information on the effect of temperature on the rate constant for the slowest dissociation step was used to calculate the activation energy barrier.

A new physicochemical model of metal complexation by humic substances is described. The model takes into account the effects of double layer relaxation, chemical bond formation, and the increase in entropy associated with metal dehydration. In the case of the double layer thermodynamics, mathematical models, which may be used to calculate the enthalpic, entropic and total free energy contributions to the complexation reaction are formulated. The contribution of chemical bond formation to the enthalpy of reaction is investigated by comparison with simple ligand reaction enthalpies. In the case of the entropic contribution of metal dehydration, partial molar entropies were found to give the best measure. Binding enthalpies and entropies for Eu³⁺ and UO₂²⁺ with Aldrich humic acid have been determined: ΔH(Eu)=+36 kJ mol^-1; ΔS(Eu)=+276 J K^-1 mol^-1; ΔH(UO₂²⁺)=+62 kJ mol^-1; ΔS(UO₂²⁺)=+62 J K^-1 mol^-1. In addition, the effect of ionic strength on the binding of Cu and Cd has been studied electrochemically. Using these data, combined with data from the literature, the model has been tested, both qualitatively and quantitatively. The suitability of the model for the prediction of unknown humic/metal complexation enthalpies, entropies and total binding strengths is discussed.