Effect of ionic strength on complexation of Pu(IV) with humic acid

Abstract

International audienceSuccessful geochemical modelling of the migration of radioactive materials, such as the transuranic elements, from nuclear waste repositories is dependent upon an understanding of their interaction with biogeopolymers such as humic acids, the most likely complexing agents in groundwaters. An established silica/humic acid composite has been evaluated as a model substrate for naturally occurring humate-coated minerals that are likely to be present in the vicinity of the repositories. The binding of Pu(IV), the highly likely oxidation station, by the silica/humic substrate was examined at pH 4 in the range 0.02 to 3.00 M NaClO4 by the titration method. Pu(IV)-humate conditional stability constants have been evaluated from data obtained from these experiments by using non-linear regression of binding isotherms. The results have been interpreted in terms of complexes of 1:1 stoichiometry. Analysis of the complex formation dependency with ionic strength shows that the effect of ionic strength on humate complexation of Pu(IV) is not dramatically pronounced. The complexation constants are evaluated for the humate interaction with Pu4+ and Pu(OH)3+ at pH 4. The complexation constants are found, respectively, to be logHAβ0(Pu4+)=16.6±0.3 and logβ01.3.1=46.6±2.3. The estimations through analogy from previous results are in agreement with these new experimental data

Stabilization of polynuclear plutonium(IV) species by humic acid

“…Reiller et al (2008) reviewed all the available tetravalent actinide (An(IV)) complexation data with HA and found a high consistency between them. More recent Pu(IV)-HA binding studies are in relatively good agreement with the latter observation (Szabó et al, 2010;Sasaki et al, 2012). In these references, Pu(IV)-HA interaction is described in terms of complexation of mononuclear Pu with humic acid.…”

Apparent formation constants of Pu(IV) and Th(IV) with humic acids determined by solvent extraction method

“…In a recent solubility study, Kobayashi et al suggested the presence of ternary Th(OH) 2 (L) 0 as the solubility-limiting solid phase, where L represented the dicarboxylic acids, while the aqueous species was assumed to be the binary complex Th(L) p , instead of Th(OH) n (L) p , for better convergence of the solubility fitting analysis [1,12]. In the humic acid system, Szabó et al discussed the formation constant of Pu(IV) and the contribution of hydrolysis [13]. On the basis of a strong hydrolysis effect, they believed that Pu(OH) + 3 was the predominant species, and then surmised that the ternary complex was Pu IV (OH) 3 (L).…”

“…The E h value of the aqueous phase in the Pu system was measured using a couple consisting of Pt and Ag/AgCl electrodes after calibration with a redox standard solution containing the redox couple ferricyanide/ferrocyanide [11]. [11], and the state in the presence of humic acid has been suggested in the literature [13]. Since the mass balance of Th at pH c > 3 was more than 90% in back-extraction, no major unexpected reactions such as adsorption of colloids on the polypropylene vessel wall or precipitation at the liquidliquid interface were observed.…”

“…Recently, the formation constant was evaluated by a liquid-solid equilibrium reaction, in which the solid phase was humic acid immobilized on silica gel and the liquid phase contained Pu(IV) and citrate at pH 4 [13]. The log β app value for the PuL complex at I = 0.101 was reported as 15.61 ± 0.77, with q = 0 in Eq.…”

Discrete fragment model for apparent formation constants of actinide ions with humic substances

“…In their solubility study, Kobayashi et al suggested the formation of binary Th-L and/or ternary Th-OH-L complexes as the solubility-limiting solid phase, where the dicarboxylic acids (L 2− ) were oxalic, glutaric, and adipic acids for the binary system and succinic and glutaric acids for the ternary system [25,26]. For Pu 4+ , a ternary 1 : 1 : 1 complex of Pu 4+ -OH-L was assumed to form in a complexation study of Pu 4+ with L 2− at lower pH values [27], and Szabó et al selected Pu 4+ -(OH) 3 -HA in their study of an HA system [28]. Also, Marsac et al discussed the complexation of Pu 4+ and proposed a ternary complex such as Pu 4+ -(OH) 2 -ALHA and its polynuclear species under weakly acidic conditions [29].…”