Complexation of NpO2+ with (2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) in aqueous solutions: thermodynamic studies and structural analysis

Abstract

Complexation of Np(V) with N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) was studied in aqueous solution (I = 1.0 mol L⁻¹ NaClO₄, t = 25 °C) by spectrophotometry, microcalorimetry and Extended X-ray absorption fine structure (EXAFS) spectroscopy. Equilibrium constants for the formation of three complexes, NpO₂L²⁻, NpO₂(HL)⁻, and (NpO₂)₂(OH)₂L₂⁶⁻, were determined to be (6.91 ± 0.06), (4.28 ± 0.03) and −(4.93 ± 0.03), respectively. The enthalpies of complexation were determined to be −(8.0 ± 2.0) kJ mol⁻¹ for NpO₂L²⁻ and −(2.2 ± 2.0) kJ mol⁻¹ for NpO₂(HL)⁻. Thermodynamic data of the complexation of Np(V) with HEDTA were compared to those of Np(V) with other aminopolycarboxylic acids, gaining insight into the possible coordination modes of the complexes. The EXAFS studies provided further structural information on those modes. In both NpO₂L²⁻ and NpO₂(HL)⁻ complexes, HEDTA coordinates to Np(V) in a tridentate mode through two oxygens of two carboxylic groups and one nitrogen of the amine group. In the (NpO₂)₂(OH)₂L₂⁶⁻ complex, two Np(V) atoms are bridged by two hydroxides and each HEDTA maintains the tridentate coordination mode.