Sorption of Np(V), Pu(V), and Pu(IV) on colloids of Fe(III) oxides and hydrous oxides and MnO2

AbstractSorption of Np(V), Pu(V), and Pu(IV) on colloids of synthetic goethite (α-FeOOH), hematite (α-Fe2O3), maghemite (γ-Fe2O3), and amorphous MnO2 was studied over wide ranges of solution pH and ionic strength by solvent extraction and X-ray photoelectron spectroscopy (XPS). Plutonium(V) is reduced upon sorption on α-FeOOH, but not on α-Fe2O3 and γ-Fe2O3. On the MnO2 surface, Pu occurs as Pu(VI). From the pH dependences of the actinide sorption, the equilibrium constants of the reactions of Np(V)O2+ and Pu(V)O2+ with the surface hydroxy groups of the investigated colloid particles and a set of the equilibrium constants of the reactions of Pu(IV) hydroxo complexes with α-FeOOH were obtained. If no redox reactions occur on the surface of the colloid particles, these constants decrease in the order $$ K_{MnO_2 } > K_{\alpha - FeOOH} > K_{\alpha - Fe_2 O_3 } \sim K_{\gamma - Fe_2 O_3 } $$ .