Role of surface-enhanced reduction during Pu(V) sorption to sediments

Full text of publication follows: Plutonium +4 and +5 sorption to four sediments was measured as a function of pH (pH 2 to 10) after 1 and 33 days. The four sediments represented Pu-sorbing end-members with respect to the possible sediments expected on the Savannah River Site, located in South Carolina, USA. After one day, Pu(V) sorption was appreciably less than that of Pu(IV), except in the sediment containing the highest organic matter concentrations (3.2 wt-%). In this high organic matter sediment, both Pu(IV) and Pu(V) had similarly very high percentages of sorbed Pu across the entire pH range investigated. After 33 days, the Pu(V) and Pu(IV) sorption data as a function of pH resembled each other. To determine whether this was caused by the sediments promoting Pu(V) reduction to Pu(IV), a kinetic study was conducted in which sediment-Pu oxidation state distributions were measured by wet-chemistry techniques as a function of time after introducing aqueous Pu(V) to one of the sediments. Within minutes of introducing the Pu(V), Pu(IV) was detected on the sediment. Within 48 hr, >90% of the sorbed Pu was Pu(IV). These studies support the findings that sediments may play an important role in not only sorbing Pu, but also promoting its reduction. (authors)