Thermodynamic description of the plutonium – α-d-isosaccharinic acid system I: Solubility, complexation and redox behavior

Abstract

The impact of α-d-isosaccharinic acid (HISA), a degradation product of cellulose, on the solubility and redox behavior of hydrous Pu(IV) oxide was investigated from undersaturation conditions under Ar atmosphere in 0.1 m NaCl–NaOH–NaISA solutions. Redox conditions were buffered with hydroquinone (HQ) or Sn(II), resulting in (pe + pHm) ≈ 9.0 and 1.5, respectively. The influence of ISA on Pu solubility was investigated as function of pHm (8 ≤ pHm ≤ 13) and ligand concentration (10−6 m ≤ m(ISA)tot ≤ 0.1 m).In-situ X-ray diffraction and X-ray absorption spectroscopic measurements indicate that PuO2(ncr,hyd) controls the solubility of Pu in all investigated systems. Presence of ISA poses an increase in plutonium solubility by up to 2.5 log-units. In HQ systems, slope analysis of solubility data in combination with solid phase characterization and density functional theory calculations resulted in chemical and thermodynamic models including the predominance of Pu(IV)(OH)3ISA–H– and Pu(IV)(OH)3ISA–2H2– complexes below and above pHm ≈ 12, respectively. In Sn(II) systems with pHm < 11.5, a reductive dissolution occurs involving the formation of Pu(III)(OH)ISA–H(aq). Experimental observations suggest also that Pu–ISA colloids importantly contribute to the overall Pu concentration in solution, both in HQ and Sn(II) systems. Although this process is not included in the thermodynamic model derived, it cannot be neglected for estimating Pu solubility limits under these conditions. This work provides the most comprehensive thermodynamic dataset available to date for the system Pu3+–Pu4+–OH––Cl––ISA––H2O(l) valid under a wide range of conditions relevant for nuclear waste disposal.