Plutonium - Humic Acid Stability Constant Determination and Subsequent Studies Examining Sorption in the Ternary Pu(IV) - Humic Acid - Gibbsite System

Plutonium has been released to the environment through a variety of intentional and unintentional mechanisms, including atmospheric testing, disposition from weapons manufacturing processes, and subsurface disposal. Therefore, a thorough understanding of the chemical, physical, and biological processes affecting plutonium transport is imperative. It has been shown that humic acid (HA) (a refractory component of natural organic matter (NOM)) can effectively solubilize plutonium (Santschi et al., 2002). Increased solubility may result in enhanced subsurface transport, due to the higher concentration of Pu in the aqueous phase. In contrast, the formation of ternary surface complexes may hinder actinide transport. Solution pH is likely to affect the dominance of one species over another. For these reasons, a better understanding of binary Pu-HA and Pu-mineral and ternary Pu-HA-mineral systems is essential for accurately predicting plutonium fate and transport. The primary objective of this research was to determine the conditional stability constant for Pu-HA complexes using a hybrid ultra-filtration/equilibrium dialysis ligand exchange (EDLE) technique from pH 4 to 6.5. Ethylenediaminetetraacetic acid (EDTA) was used as a reference ligand to allow the aqueous chemistry of the Pu-HA system to be probed at increased pH, without appreciable metal hydrolysis. The conditional stability constant for Pu(IV) complexation with Leonardite HA determined as part of this work is logβ112=6.76 ± 0.14. This stability constant is valid over the pH range 4 - 6.5 and I = 0.1 M NaCl, for the equation shown below: Pu4+ + HL1 + 2H2O → Pu(OH)2L1+ + 3H+ where HA is represented by HL3 (a binding site on the HA with a pKa value of 7). The conditional stability constant for Th(IV) complexation with Leonardite HA was also determined. Over the pH range 4 - 6.5 the value logβ112=4.63 ± 0.04 was calculated (I = 0.1 M NaCl) for the reaction shown below: Th4+ + HL1 + 2H2O → Th(OH)2L1+ + 3H+ where HL1 represents a pKa 3 binding site on the HA. Preliminary sorption studies using gibbsite were also conducted to evaluate the effects of Pu-HA complex formation on Pu sorption behavior. Enhanced Pu sorption to gibbsite was observed in the presence of HA. Notably, enhanced sorption was observed at low pH (pH 4) which is indicative of ligand promoted sorption. Therefore, despite observations of increased solubility of Pu in the presence of HA, the formation of ternary surface complexes may prevent enhanced subsurface transport. The data from these studies will aid in modeling the fate and transport of Pu in the environment and inform the development of conceptual models describing the influence of ternary surface complex formation on Pu sorption