Pu(V) reduction and enhancement of particle-water partitioning by exopolymeric substances

Abstract

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 fluorescens Biovar 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 0.1 M NaClO4 with an inert buffer (Tris) and 5 μm silica particles of high specific surface area (500 m2/g) and small (3 nm) 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 (≥1 kDa), 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 (K d, K dc), quickly (1 day) for Pu(IV) and slowly (1 month) 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) K d 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).

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

“…Many studies argued that Pu(V) aq has weak affinity for mineral surfaces (an implication of higher mobilization). An interpretation that slow reduction of Pu(V) aq to Pu(IV) aq , which has a higher tendency to complex with ≡X-OH sites (X = Al, Fe, and Mn), was usually given (Roberts et al, 2008;Begg et al, 2013;Powell et al, 2014). Instead, much lower mobilization of Pu(V) aq was observed in our transport experiments.…”

Effect of Fulvic Acid Surface Coatings on Plutonium Sorption and Desorption Kinetics on Goethite

“…For instance, Roberts et al 18 found that, at pH 8.2, Pu sorption to silica was decreased if Pu(IV) was pre-equilibrated with humic acid prior to any metal−mineral contact. Blinova et al 51 argued that Pu−FA complexation will result in more mobile Pu(IV) species at higher pH values (i.e., pH 6), where one would expect lower FA sorption and larger FA fractions remaining in solution.…”

“…This finding is in agreement with previous results suggesting that a fraction of Pu(IV) may become "irreversibly" sorbed to silica when Pu is complexed with humic acid prior to metal contact with the mineral phase. 18 This inverse relationship between FA concentration and Pu desorption suggests that, under the given pH conditions, ternary surface complexes are likely more stable than binary Pu−goethite complexes on the goethite surface. This is also in accordance with modeling results, which indicate higher Pu K dT values for Pu sorption in most systems containing FA.…”

Influence of organic matter on the adsorption of 210Pb, 210Po and 7Be and their fractionation on nanoparticles in seawater

“…For sorption experiments, 49 ml of the <1 kDa ultrafiltrate of seawater in a stirred-cell unit was added with 1 ml of noncomplexing Tris-HCl buffer solution to maintain a constant pH of 8.1 and to prevent formation of pseudo-colloids during the addition of spikes (Roberts et al, 2008(Roberts et al, , 2009. For the single-sorbent experiments, 2 mg of inorganic particles was added into the solution while stirring, corresponding to a particle concentration of about 40 mg/l, falling in the concentration range of particles in coastal seawater Yang et al, 2013), but somewhat higher than that in open seawater (Guo et al, 1997).…”