Co-transport of U(VI) and gibbsite colloid in saturated granite particle column: Role of pH, U(VI) concentration and humic acid

Abstract

Understanding the in-situ transport behavior of U(VI) in granitic formations is of considerable interest for geological disposal of high-level radioactive wastes (HLW). In this context, the co-transport of U(VI) and representative naturally-occurring colloids, i.e., humic acid (HA) and gibbsite colloid (GC), was studied in granite column as a function of pH, U(VI) concentration and HA amount. It was found that, in addition to pH, co-transport of U(VI) and GC was also controlled by U(VI) concentration, the effect of which can be transport-facilitating and transport-impeding for U(VI) at relatively low concentration (2.0 × 10−6 mol/L) and for U(VI) at high concentration (5.0 × 10−5 mol/L), respectively. HA can present opposite effects on GC transport depending on HA amount. The transport-impeding effect by small amount of HA (5 mg/L) is due to strong aggregation between GC and HA from electrostatic attraction and complexation, whereas the transport-facilitating effect by big amount of HA (20 mg/L) is because of the complete HA coating which stabilizes associated colloids and alters surface charge from positive to negative. In ternary co-transport systems, a similar HA-dependent effect was also observed for both U(VI) and GC regardless of presence of high concentration U(VI). Besides the application of the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, the mechanisms behind binary and ternary co-transport of U(VI), GC and HA were also analyzed by assessing the evolutions of zeta potential and particle size in the column effluents. Finally, a two-site non-equilibrium model and a two-site kinetic attachment/detachment model were applied to describe the breakthrough curves of U(VI) and individual/combined colloids, respectively. The findings of this study indicated that combined effects of GC and HA on radionuclides transport is dominated by the amount of HA, and a facilitating transport of radionuclide can be expected in the underground environment rich in humic acid.