Sorption of neptunium(V) on Opalinus Clay under aerobic/anaerobic conditions

Abstract

The interaction between neptunium(V) and a natural argillaceous rock (Opalinus Clay (OPA), Mont Terri, Switzerland) has been investigated in batch sorption experiments by varying pH (6–10), Np(V) concentration (10⁻¹²–10⁻⁴ M), solid-to-liquid ratio (2–20 g/L), and partial pressure of CO₂ (10^−3.5 and 10^−2.3 atm) under aerobic/anaerobic conditions in saturated calcite solution. All batch experiments were carried out using well characterized aerobic and anaerobic dry powders of OPA. The results show a great influence of pH on Np(V) sorption. Under aerobic conditions sorption increases with increasing pH until maximum sorption is reached between pH 8–9. At pH>9 sorption decreases due to the formation of negatively charged Np(V)-carbonate complexes. By increasing p_CO2 from 10^−3.5 to 10^−2.3 atm, the sorption edge is shifted ≈0.5 units to lower pH values. Under anaerobic conditions stronger sorption of 8×10⁻⁶ M Np(V) was found, possibly due to partial reduction of Np(V) to Np(IV). The sorption of 8×10⁻⁶ M Np(V) under aerobic conditions at pH 8.2 in saturated calcite solution increases continuously with increasing solid-to-liquid ratio of OPA in the range of 2–20 g/L with a constant K_d value of 126±13 L/kg. The sorption isotherm was measured over seven orders of magnitude in Np(V) concentration using ²³⁹Np as tracer. The sorption isotherm could be divided in a part of linear sorption behaviour between 10⁻¹³–10⁻⁹ M Np(V) and non-linear behaviour in the range of 10⁻⁹–10⁻⁴ M Np(V).