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

The humic complexation of redox sensitive elements, described through the charge neutralisation model (CNM) has been reviewed in order to have a comprehensive scope. The data which were 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. Otherwise, available data which were obtained in the framework of the CNM were used, or adapted when necessary, for other redox state when the analogy is justified, i.e. Am3+-Cm3+ for Pu3+, NpO2+ and UO22+ for PuO22+. The resulting speciation diagrams indicate that, when (HA) = 100 mg/L, redox sensitive actinides should be reduced to their +IV state when EH ≤ 650 mV for plutonium, EH ≤ 150 mV for neptunium and EH ≤ 20 mV for U. Plutonium could be present as mixtures of plutonium (III) and (IV) depending on the pH value in reducing groundwater -150 ≤ EH (mV) ≤ 150, but the known association of plutonium (VI) in marine systems are not satisfactorily represented. The known reduction of neptunium in Gorleben groundwater seems to be also well represented. The stability of uranium (VI) in humic solution when EH ≥ 100 mV is also represented, but these values do not represent the data obtained under reducing conditions. Experiments under well-controlled conditions are still needed to ascertain the comportment of uranium in the presence of humic substances. (orig.)