The determination of the thermodynamic constants of MgUO2(CO3)32− complex in NaClO4 and NaCl media by time-resolved luminescence spectroscopy, and applications in different geochemical contexts

Abstract

The formation constants and specific ion interaction coefficients of MgUO₂(CO₃)₃²⁻ complex were determined in 0.1 to 1.0 mol kg_w⁻¹ NaCl and 0.10 to 2.21 mol kg_w⁻¹ NaClO₄ media in the framework of the specific ion interaction theory (SIT), by time-resolved laser-induced luminescence spectroscopy. The upper limits of ionic strength were chosen in order to limit luminescence quenching effects generated by high concentrations of Cl⁻ and ClO₄⁻ already observed during our earlier studies on Ca_nUO₂(CO₃)₃^(4−2n)− complexes (Shang & Reiller, Dalton Trans., 49, 466; Shang et al., Dalton Trans., 49, 15443). The cumulative formation constant determined is , and the specific ion interaction coefficients are ε(MgUO₂(CO₃)₃²⁻, Na⁺) = 0.19 ± 0.11 kg_w mol⁻¹ in NaClO₄ and ε(MgUO₂(CO₃)₃²⁻, Na⁺) = 0.09 ± 0.16 kg_w mol⁻¹ in NaCl. Two gratings of 300 and 1800 lines per mm were used to acquire MgUO₂(CO₃)₃²⁻ luminescence spectra, where the high-resolution 1800 lines per mm grating detected slight spectral shifts for the principal luminescent bands relative to Ca_nUO₂(CO₃)₃^(4−2n)−. The applications of the consistent set of thermodynamic constants and ε values for M_nUO₂(CO₃)₃^(4−2n)− (M = Mg and Ca) were examined in different geochemical contexts, where Mg over Ca concentration ratio varies to help defining the relative importance of these species.