Thermodynamics of Np(IV) complexes with gluconic acid under alkaline conditions: sorption studies

Abstract

The complexation of Np(IV) with gluconic acid (GLU) under alkaline conditions was investigated in the absence of Ca by carrying out a series of sorption experiments. The decrease of Np(IV) sorption on the sorbing material at increasing concentrations of GLU was interpreted as the formation of Np(IV)-GLU aqueous complexes. The modelling of experimental data according to the Schubert method [1] confirmed the formation of a complex with a Np : GLU ratio 1 : 1. The stoichiometry of the complex Np(OH)4GLU− was proposed based on the experimental observation that no proton exchange occurred during the course of the complexation reaction and that Np(OH)4(aq) was the predominant hydrolysis product in the absence of GLU. A log * β 1,4,1 0 = −2.92 ± 0.30 for the formation reaction Np4+ + 4H2O + GLU−⇔Np(OH)4GLU− + 4H+ was calculated based on the conditional stability constants determined from sorption experiments and using the Np(IV) thermodynamic data selected in the NEA reviews [2]. Linear free energy relationships (LFER) confirmed that the stoichiometry and stability of the Np(IV)-GLU complex characterized in this work are consistent with data available for Th(IV)-, U(IV)- and Pu(IV)-GLU complexes.

Influence of the redox state on the neptunium sorption under alkaline conditions: Batch sorption studies on titanium dioxide and calcium silicate hydrates

“…In addition, the sorption behaviour of Np(IV) and Np(VI) on C-S-H phases observed in the present experiments (see section 3.3) is similar to the sorption behaviour of Th(IV) and U(VI) onto C-S-H phases reported in the literature [27,41]. i.e., the d values for Np(IV) and Th(IV) sorption on C-S-H phases are both independent of the C:S ratio whereas d values for Np(VI) and the U(VI) sorption both decrease with increasing C:S ratio.…”

“…Gaona et al [19,22] and Rojo et al [41] showed that Na-dithionite and Na-hypochlorite have no influence on the sorption behaviour of tetravalent and hexavalent actinides. XANES spectra of C-S-H pastes containing much higher…”

Crystal and solution structures of calcium complexes relevant to problematic waste disposal: calcium gluconate and calcium isosaccharinate

“…In being structurally similar, Gluc À has been widely used as a readily available analogue for -Isa À , for which extraction and purification has been challenging and time consuming (Glaus et al, 2006;Giroux et al, 2002;Tits et al, 2005;Zhang et al, 2007;Zhang et al, 2009;Colà s et al, 2013a,b;Rojo et al, 2013;Dudá s et al, 2017). It has been recently shown though, that there are some fundamental differences between the complexing properties of the two ligands in alkaline medium in the presence of calcium (Dudá s et al, 2017).…”

Complex formation between calcium and hydroxy/oxocarboxylates in neutral and highly alkaline aqueous solutions

“…[70] The concentration of Glucin cement pore waters may range from 10 -5 to 10 -2 M, [101] which is several orders of magnitude lower than that of α-Isa -, which could reach as high as 0.1 M. [85] As the preparation and purification of Isais rather time consuming, Gluc -, being structurally similar, has been widely used as a readily available analogue for Isa -. [102][103][104][105][106][107][108][109] Birjkumar et al, however, scrutinized the similarities between Isaand Glucin U(VI) chemistry and suggested that caution should be exercised when using Glucas a thermodynamic model for Isain such systems. [110] Both Isaand Gluchave been shown to be effective complexing agents in the presence of actinides and lanthanides.…”

“…[107] In highly alkaline medium (pH = 13.3), the sorption of Np(IV) was also shown to decrease at Glucconcentrations higher than 10 -6 M due to the formation of Np(IV)-Gluccomplexes. [109] For the reaction Np 4+ + Gluc -+ 4 H2O ⇌ Np(OH)4Gluc -+ 4 H + , logβ = -4.5(3) was deduced.…”