Spectroscopic characterisation and thermodynamics of the complexation of Np(V) with sulfate up to 200 °C

Martin M Maiwald; Andrej Skerencak-Frech; Petra J Panak

doi:10.1016/j.saa.2020.118579

Spectroscopic characterisation and thermodynamics of the complexation of Np(V) with sulfate up to 200 °C

Spectrochim Acta A Mol Biomol Spectrosc. 2020 Oct 15:240:118579. doi: 10.1016/j.saa.2020.118579. Epub 2020 Jun 10.

Authors

Martin M Maiwald¹, Andrej Skerencak-Frech², Petra J Panak³

Affiliations

¹ Ruprecht Karls Universität Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany. Electronic address: m.maiwald@pci.uni-heidelberg.de.
² Karlsruher Institut für Technologie (KIT), Institut für Nukleare Entsorgung (INE), D-76344 Eggenstein-Leopoldshafen, Germany.
³ Ruprecht Karls Universität Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany; Karlsruher Institut für Technologie (KIT), Institut für Nukleare Entsorgung (INE), D-76344 Eggenstein-Leopoldshafen, Germany.

PMID: 32645651
DOI: 10.1016/j.saa.2020.118579

Abstract

In the present work the complexation of Np(V) with sulfate in aqueous solution is studied in a temperature range up to 200 °C by absorption spectroscopy. For this purpose, a new spectroscopic setup is implemented and tested for its suitability for Vis/NIR absorption spectroscopy at elevated temperatures. The complexation of Np(V) with sulfate is studied as a function of the total ligand concentration at various temperatures (T = 25-200 °C) and ionic strengths (I_m(NaClO₄) = 1.0-4.0 mol kg^-1 NaClO₄). The exclusive formation of NpO₂(SO₄)^- up to 200 °C is confirmed by peak deconvolution and slope analyses. The thermodynamic stability constants log β⁰₁(T) are obtained from linear regressions according to the specific ion interaction theory (SIT). A systematic increase of the log β⁰₁(T) is observed with increasing temperature, resulting in a linear correlation of log β⁰₁(T) with T^-1. The magnitude of the increase is 1.9 logarithmic units at 200 °C in comparison to log β⁰₁(25 °C) = 1.05 ± 0.16. Thus, the standard reaction enthalpy and entropy (Δ_rH⁰_m, Δ_rS⁰_m) are determined with the integrated Van't Hoff equation revealing Δ_rH⁰_m = 31.0 ± 1.0 kJ mol^-1 and Δ_rS⁰_m = 123 ± 9 J mol^-1 K^-1. In addition, the stoichiometric sum of the specific binary ion-ion interaction coefficient (Δε₀₁(T)) is determined up to 200 °C showing an insignificant temperature dependence. Thus, a temperature-independent ε(Na⁺, NpO₂(SO₄)^-) = 0.07 ± 0.11 is calculated for the temperature range up to 200 °C. Comparison of the present results with literature data confirms the excellent applicability of the new high-temperature absorption spectroscopic setup for complexation studies up to 200 °C.

Keywords: Absorption spectroscopy; Complexation; High-temperature spectroscopy; Neptunium; Sulfate; Thermodynamics.