Trace-level plutonium(IV) polymer stability and its transport in coarse-grained granites

Colloid-associated actinides are widely studied in both lab-scale and field experiments. However, little is known about the fate and transport of Pu(IV) polymers (i.e., Pu intrinsic colloids) in environmental media. Different opinions on the mobilization of this Pu species were expressed in recent reviews. To fill this gap, we conducted studies on the stability of trace-level Pu(IV) polymers (~ 10− 10 mol/L 239Pu) and on their transport characteristics in saturated coarse-grained granites. The results show that the percentage of polymerized Pu (P>1.5nm, i.e., the fraction of Pu(IV) polymer in the Pu suspensions aged at room temperature) declined within 15 days and then slightly decreased to 21.0% at 135 days. This reveals slow oxidative solubilization of Pu(IV) polymers. In contrast, the P>1.5nm in the suspensions aged at 95 °C declined within 5 h and then gradually increased from 5 h to 50 h. The average particle size of Pu(IV) polymers aged at these two temperature conditions overall decreased, as indicated by the continuous decline in P>450nm/P>1.5nm. Such a decrease in the size of the polymers was further confirmed by the observed reduction in the transport velocity of the polymers with respect to 3H2O (UPu/UT) through the coarse-grained granites. According to the transport results, the fraction of mobile Pu (RPu, i.e., the percent recovery of Pu) in the suspensions aged at room temperature followed a linear relationship with P>1.5nm. For the suspensions aged at 95 °C, an exponential relationship between RPu and P>1.5nm existed. These transport results strongly demonstrate that the polymers transported almost unretarded through the granite systems. The fact that the fraction of colloidal Pu (P>1.5nm = ~ 98.5%) in the column effluents was significantly larger than that of Pu(V)aq (~ 1.6%) supported the finding of their unretarded transport characteristics. For the Pu suspensions aged at 95 °C and room temperature, the log[Pu(IV)aq]total (i.e., the solubility of Pu(IV) hydroxide under neutral to alkaline conditions) were experimentally determined as − 10.4 ± 0.2 and − 10.2 ± 0.3, which are consistent with the reported solubility constant of logKs(14)o = − 10.4 ± 0.5. The findings in this study are helpful in understanding the fate and transport of Pu(IV) polymers and accurately assessing the environmental risks of the long-lived toxic 239Pu in the geosphere.