Tomonari Fujita

Modeling of Neptunium(V) Sorption Behavior onto Iron-Containing Minerals

Sorption behaviors of neptunium (V) on naturally-occurring magnetite (Fe 3 O 4 ) and goethite (α-FeOOH) in 0.1M NaN0 3 electrolyte solution under aerobic conditions were interpreted using the surface complexation model (SCM). The surface properties of these materials were experimentally investigated by C0 2 -free potentiometric titration, and SCM parameters for the constant capacitance model, such as protonation/deprotonation constants of the surface hydroxyl group, were determined. The number of negatively charged sorption sites of goethite rapidly increased with the increase of the bulk solution pH compared with that of magnetite and this tendency was similar to the pH dependence of neptunium sorption. This implies that the neptunyl cation, NpO 2 + , plays a dominant role in possible sorption reactions. Assuming that the dominant surface complex is XO-NpO 2 , modeling by means of SCM was carried out, and the results were found to agree with experimental data.

Surface complexation modeling for description of actinide sorption at the buffer materials/water interface

Abstract As mechanistic modeling becomes more feasible, surface complexation modeling was applied to actinide sorption reactions with the edge-surface hydroxyl groups of montmorillinite contained in bentonite, which is a potential buffer material for radwaste disposal. The determined surface properties of Japanese bentonite samples suggested possible interactions between positively charged surface sites and anionic Np(V) species in alkaline pH region under high inorganic carbon concentration conditions. The interfacial capacitance density affected determination of the bentonite surface parameters and the Np(V) surface complexation constants. The valence of the aqueous sorbing species was clarified to be useful in determining surface complexation constants of actinide sorption.

Influence of carbonate ions on europium sorption onto iron-oxides

The sorption behavior of europium onto two iron-oxides, goethite and magnetite, focused on the effect of carbonate species concentration was investigated in 0.01 M NaClO{sub 4} solution by the batch method over a pH range of 4 to 11. It was found that sorption of europium was enhanced in the higher concentration solution of carbonate species for both materials. The zeta-potential of goethite in the solutions including sodium bicarbonate was also measured to estimate the effect of sorbed carbonate species. Experimental results and model calculation suggest that the production of surface carboxylic group FeOCOO{sup {minus}} by the sorption of carbonate species lowered the zeta-potential and enhanced europium sorption.

Modelling the Sorption of Actinides onto Cement: An Approach with the Surface Co-Precipitation Model

An approach, assuming that co-precipitation on the surface of the cement phase dominates ‘sorption’, is proposed and discussed to interpret the sorption behaviour of the cement-complex system. This model takes account of the mineralogical composition of Ordinary Portland Cement (OPC) and assumes that Calcium Silicate Hydrates (C-S-H) dominates the sorption (fixation) of actinide onto OPC and that a small amount of surface co-precipitation of actinides with calcium hydroxide in the structure of C-S-H is formed during the late period of the fixation process. Therefore, this will be referred to as the ‘Surface Co-Precipitation Model (SCPM)’. The modelling results show generally good agreement between the predicted and measured distribution ratios of plutonium, thorium and neptunium, of which the dominant species in high pH solution are neutral species, on OPC.