Interactions between anionic complex species of actinides and negatively charged mineral surfaces

Abstract

Sorption of actinides onto negatively charged mineral surfaces was investigated under conditions that actinides were predominantly present as anionic complex species. U(VI), Np(V) and Sn(IV) were prepared as 1 mol kg −1 Na-ClO 4 /OH solutions and equilibrated with γ -Al 2 O 3 at pcH 11-13.6 where these elements form UO 2 (OH) 4 2− , NpO 2 (OH) 2 − and Sn(OH) 5 − species, respectively. Sorption of Np(IV), Th(IV) and Am(III) on amorphous SiO 2 and on γ -Al 2 O 3 was investigated in 0.03-0.3 mol kg −1 Na-HCO 3 /CO 3 solutions where these actinides are expected to be stable as anionic carbonate complexes. Distribution coefficients of U(VI), Np(V), Sn(IV), Th(IV) and Am(III) decreased with the increasing pH or with the increasing carbonate concentrations. The monotonous decrease in the distribution coefficients in the investigated pcH range suggests that anionic complex species of actinides were not sorbed on negatively charged mineral surfaces. A proportional relation was found between the experimentally obtained K d values and calculated mole fraction of electrically neutral species in equilibrated solutions. Sorption occurs under the condition that both actinides and mineral surfaces are negatively charged because minor neutral species are sorbed on mineral surfaces.

The immobilization of U(vi) on iron oxyhydroxides under various physicochemical conditions

“…), together with the negative charged surface, become dominant under alkaline conditions. 30 The inuence of ionic strength on U(VI) sorption on the iron oxyhydroxides is illustrated in Fig. 5.…”

Sorption of U(VI) at the TiO2–water interface: An in situ vibrational spectroscopic study

“…Under acidic conditions, U(VI) adsorption is limited by electrostatic repulsion between positively charged uranyl units (UO 2þ 2 ) and the protonated oxide surfaces, whereas at alkaline pH values, the sorption is hampered because of the formation of negatively charged U(VI) carbonate complexes in solution and the negative surface charges which become predominant above the zero point of charge (PZC) of the solid phases (Yamaguchi et al, 2004). Under near neutral conditions, where several uranyl hydroxo species dominate the speciation, deprotonation of hydroxyl groups of oxide surfaces potentially provides an increased number of adsorption sites.…”

Speciation and interactions of plutonium with humic substances and kaolinite in aquifer systems

“…The sorption of actinide ions onto kaolinite and other mineral surfaces has been investigated in the last decades [6][7][8]. The results show that the sorption depends significantly on the oxidation state, pH, ionic strength, temperature, aerobic or anaerobic conditions, and the type of mineral surface.…”