Influence of humic acid on plutonium sorption to gibbsite: Determination of Pu-humic acid complexation constants and ternary sorption studies

Abstract

In this work stability constants describing Pu(IV), Th(IV), and Np(V) binding to Leonardite humic acid (HA) were determined using a discrete pK model. A hybrid ultra-filtration/equilibrium dialysis, ligand exchange technique was used to generate the partitioning data. Ethylenediaminetetraacetic acid (EDTA) was used as a reference ligand to allow the aqueous chemistry of the Pu(IV)-HA system to be examined over a range of pH values, while minimizing the possibility of precipitation of Pu(IV). The conditional stability constant for Pu(IV) complexation with HA determined as part of this work is log 112 = 6.76 ± 0.14 based on the equation:resented by HL3 (a binding site on the HA with a pK value of 7). This value is three orders of magnitude higher than the Th(IV)-HA constant and between six and eight orders of magnitude higher than the Np(V)-HA complex. The magnitude of the stability constants and the general trend of increasing complexation strength with increasing pH is consistent with previous observations. The Pu(IV)-HA stability constants were used to model sorption of Pu(IV) to gibbsite in the presence of HA. Assuming only aqueous Pu-HA complexes and AlOH-Pu surface complexes, the model was unable to predict the observed data which exhibited greater sorption at pH 4 relative to pH 6; a phenomenon which does not occur in the absence of HA. Therefore, this study demonstrates that ternary Pu-HA-gibbsite complexes may form under low pH conditions and exhibit greater sorption than that observed in the absence of HA. Although the presence of HA may increase the solubility/aqueous concentrations of Pu in the absence of a solid phase, formation of ternary complexes may indeed retard the subsurface migration of Pu. The corollary to this finding is that increased mobility may occur if the ternary surface complex forms on a mobile colloid rather than part of the subsurface matrix

Plutonium(IV) sorption to montmorillonite in the presence of organic matter

“…Similar decreases in Pu(IV) sorption were also observed in ternary experiments of HA-Pu(IV)-gibbsite at pH 6 34 . In our experiments, the presence of HA strongly affected the behavior of Pu resulting in increased aqueous Pu concentrations.…”

Impact of Natural Organic Matter on Plutonium Vadose Zone Migration from an NH4Pu(V)O2CO3(s) Source

“…Several studies have shown that the fate of Pu in the environment may be controlled by transport on colloidal iron oxide and clay mineral particles. [16][17][18] Additional studies have demonstrated that association of Pu with organic colloids can enhance pore water concentrations [19][20][21][22][23][24][25][26][27] . Moreover, the presence of organic matter can affect the oxidation state as it is known that Pu is rapidly reduced from Pu(VI)/Pu(V) to Pu(IV)/Pu(III).…”

Experimental evidence for ternary colloid-facilitated transport of Th(IV) with hematite (α-Fe2O3) colloids and Suwannee River fulvic acid

“…Further, Th(IV) and Pu(IV) have been shown to complex similarly in the presence of humic substances. However, Zimmerman et al (2014) measured only slightly greater sorption for Th than Pu after 10 days in batch experiments with 10 mg C /L as humic acid. Further, complexation constants have been measured to be up to three orders of magnitude greater for Pu(IV) as compared to Th(IV) as measured in 0.1 M NaCl and 10 mg C /L of humic acid for pH 4-7 (Artinger et al, 2003;Zimmerman et al, 2014).…”

“…However, Zimmerman et al (2014) measured only slightly greater sorption for Th than Pu after 10 days in batch experiments with 10 mg C /L as humic acid. Further, complexation constants have been measured to be up to three orders of magnitude greater for Pu(IV) as compared to Th(IV) as measured in 0.1 M NaCl and 10 mg C /L of humic acid for pH 4-7 (Artinger et al, 2003;Zimmerman et al, 2014). It is clear that the interactions between tetravalent actinides and humic substances are very strong and are likely formed through carboxylate groups (Schild and Marquardt, 2000;Geckeis et al, 2002;Reiller et al, 2003;Sachs et al, 2005;Choppin, 2007;Reiller et al, 2008).…”

“…Therefore, it is necessary to examine previous experiments investigating sorption of actinides to binary systems in the presence of only organic matter and actinides. Zimmerman et al (2014) reported > 50% sorption of Pu and Th for pH 4.0 -7.…”