Sorption kinetics of plutonium (V)/(VI) to three montmorillonite clays

Pu(V) adsorbed to the surface of montmorillonite clay can be reduced to Pu(IV). In this work, the role of clay structural Fe on this surface mediated reduction reaction was investigated by quantifying Pu(V)/(VI) sorption rates under atmospheric (oxic) conditions to three montmorillonite clays with variable Fe content (SWy-1: 2.6 wt % Fe; STx-1: 0.6 wt % Fe; Barasym synthetic mica-montmorillonite: 0.01 wt % Fe) at pH 4, pH 6, and pH 8. The experiments were performed over a period of one year, a timescale at which the sorption rates were expected to capture both adsorption and surface mediated reduction processes. At pH 6 and pH 8, the rate of sorption was positively correlated with structural Fe content. However, by 360 d, the extent of sorption was independent of Fe. Moreover, in the case of the synthetic montmorillonite it was not apparent that sorption equilibrium had been achieved by the end of the experiment. There was minimal difference in sorption rates between the clays at pH 4, suggesting that structural Fe content may be less important in Pu(V)/(VI) reduction at low pH. The results indicate that at circumneutral pH, structural Fe will affect the kinetics of Pu surface mediated reduction on montmorillonite clays but not necessarily the equilibrium Pu sorption affinity at environmentally relevant timescales. The differences in rates on the three clays emphasize the need to perform long-term sorption experiments (>1 year) to adequately capture the equilibrium processes controlling the uptake of Pu under atmospheric conditions. Comparison to other minerals indicates that the sorption rates can vary by as much as five orders of magnitude depending on the structure and composition of the surface.