Central to understanding the environmental behavior of Pu in vadose- and saturated-zones, as well as waste streams, is the contribution of colloidal natural organic mater (NOM),
e.g., biopolymeric exopolymeric substances (EPS) from Pseudomonas fluorescensBiovar II and geopolymeric humic acids (HA), to Pu speciation. Because Pu(V) and Pu(IV) are present in aquatic systems at sub-pM concentrations, many established techniques for chemical speciation and oxidation state determination are not available. However, the two redox states of Pu can be distinguished through their tendency to form colloidal species. Controlled laboratory experiments with low-colloid background solutions of 0.1M NaClO4 with an inert buffer (Tris) and 5m silica particles of high specific surface area (500m2/g) and small (3nm) pores investigated the ability of NOM compounds for their binding and redox qualities showed that: 1) Pu(V) and Pu(IV) tracers at pM concentrations could be distinguished by their tendency to bind to colloidal species (1kDa), with binding extent being high for Pu(IV), and low for Pu(V); 2) EPS can significantly enhance particle- (or colloid-) water partition coefficients to silica particles ( Kd, Kdc), quickly (1day) for Pu(IV) and slowly (1month) for Pu(V); 3) Pu(V), in the presence of EPS and silica particles can become associated with particles and colloids over periods of months, likely indicating reduction; 4) Kd values can be affected by the reaction sequence of tracer addition to the solution containing silica particles and NOM compounds ( e.g., EPS or HA).