Np(V) and Pu(V) Ion Exchange and Surface-Mediated Reduction Mechanisms on Montmorillonite

Abstract

Due to their ubiquity and chemical reactivity, aluminosilicate clays play an important role in actinide retardation and colloid-facilitated transport in the environment. In this work, Pu(V) and Np(V) sorption to Na-montmorillonite was examined as a function of ionic strength, pH, and time. Np(V) sorption equilibrium was reached within 2 h. Sorption was relatively weak and showed a pH and ionic strength dependence. An approximate NpO(2)(+) → Na(+) Vanselow ion exchange coefficient (Kv) was determined on the basis of Np(V) sorption in 0.01 and 1.0 M NaCl solutions at pH < 5 (Kv ~ 0.3). In contrast to Np(V), Pu(V) sorption equilibrium was not achieved on the time-scale of weeks. Pu(V) sorption was much stronger than Np(V), and sorption rates exhibited both a pH and ionic strength dependence. Differences in Np(V) and Pu(V) sorption behavior are indicative of surface-mediated transformation of Pu(V) to Pu(IV) which has been reported for a number of redox-active and redox-inactive minerals. A model of the pH and ionic strength dependence of Pu(V) sorption rates suggests that H(+) exchangeable cations facilitate Pu(V) reduction. While surface complexation may play a dominant role in Pu sorption and colloid-facilitated transport under alkaline conditions, results from this study suggest that Pu(V) ion exchange and surface-mediated reduction to Pu(IV) can immobilize Pu or enhance its colloid-facilitated transport in the environment at neutral to mildly acidic pHs.

Neptunium redox speciation at the illite surface

“…By contrast, reactions with redox active solid phases are usually found to proceed fast (Stumm, 1992;Buerge and Hug, 1999;Liger et al, 1999;Nakata et al, 2002;Charlet et al, 2007;Bach et al, 2014). Notably for reactions with Pu(V) with mineral surfaces rapid reduction to Pu(IV) is stated (Powell et al, 2004(Powell et al, , 2005Hixon et al, 2010;Kirsch et al, 2011;Zaravin et al, 2012). The time necessary for a complete Pu(V) reduction varies from few hours to few days depending on the mineral.…”

“…Reduction is, however, also found at mineral surfaces where a specific reducing partner is absent (Hixon et al, 2013), suggesting that Pu(IV) under given conditions simply is the thermodynamically favored species. Zaravin et al (2012) studied Np(V) sorption onto montmorillonite under the same conditions as for Pu(V), but did not observe Np(V) reduction. This observation can be explained by the fact that the standard redox potential of the Np(V)/Np(IV) couple (E 0 NpO þ 2 =Np 4þ ¼ 0:604 V) is lower than that of the Pu(V)/Pu(IV) pair E PuO þ 2 =Pu 4þ ¼ 1:031 V (Guillaumont et al, 2003).…”

Np(V) Sorption and Diffusion on Montmorillonite Clay Progress Report M4FT-13LL08060327

“…Under aerobic conditions and at circumneutral pH values, Np(V) sorption to montmorillonite is likely to be dominated by surface complexation at the variably charged sorption sites (Bradbury and Baeyens, 2006;Turner et al, 1998b). However, at pH values below pH 6, cation exchange is expected to be the dominant sorption mechanism (Bradbury and Baeyens, 2006;Kozai et al, 1996;Nagasaki and Tanaka, 2000;Sabodina et al, 2006;Sakamoto et al, 1990;Turner et al, 1998b;Zavarin et al, 2012). Sorption by ion exchange is dependent on the composition of the clay exchange sites (Jensen, 1973), commonly occupied by Na + , K + , Mg 2+ and Ca 2+ , which is, in turn, strongly dependent on the aqueous conditions and electrolyte composition.…”

Distribution coefficients for the sorption of Th, U, Np, Pu, and Am on Opalinus Clay

“…However, up to now no sorption data for americium and plutonium on OPA have been published. More sorption studies have been reported for the uptake of actinides by pure clay minerals, that are the main constituents of OPA (e. g., montmorillonite [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], illite [32][33][34][35][36][37][38][39][40], kaolinite [31,[41][42][43][44][45][46][47][48][49][50][51] , the cation exchange capacity was equal to 10 ± 4 meq/100 g for both powders [9,11]. The mineral composition and the content of the trace elements of the used OPA measured by X-ray fluorescence analysis are given elsewhere [11].…”