Prognosticating the humic complexation for redox sensitive actinides through analogy, using the charge neutralisation model

Abstract

The complexation of redox sensitive elements by humic acid (HA), described through the charge neutralisation model (CNM), has been reviewed in order to have a comprehensive scope. The data acquired in HUMICS program on thorium(IV), and data available in the literature, were reinterpreted according to the CNM, and adapted to uranium(IV), neptunium(IV) and plutonium(IV) through analogy in order to draw a boundary prediction. Otherwise, available data obtained in the framework of the CNM were used, or adapted if necessary, for other redox states when the analogy is justified, i.e. Am³⁺-Cm³⁺ for Pu³⁺, NpO₂⁺ for PuO₂⁺ and UO₂²⁺ for PuO₂²⁺.

The obtained speciation diagrams indicate that, when (HA)=100 mg/L, redox sensitive actinides should be reduced to their +IV state when E_h≤650 mV/SHE for plutonium, E_h≤100 mV/SHE for neptunium and E_h≤-20 mV/SHE for U. Plutonium could be present as mixtures of plutonium(III) and (IV) depending on the pH value in reducing ground waters -150 ≤ E_h (mV/SHE) ≤ 150. The known reduction of neptunium(V) to neptunium(IV) in Gorleben ground waters seems also well represented, so is the stability of uranium(VI) in humic solution when E_h≥ 100 mV/SHE. Conversely, the known association of plutonium(VI) in marine systems is not satisfactorily represented, so does is the uranium behaviour under reducing conditions. Experiments under well-controlled conditions are still needed to ascertain the plutonium and uranium comportment in the presence of humic acid.