Study of barium and radium complex formation with EDTA in alkaline sodium chloride media

Abstract

The complex formation of Ra2+ and Ba2+ with ethylenediaminetetraacetic acid (EDTA) was studied at two pH regions where the EDTA4- and HEDTA3- species are dominant. The complex formation was investigated at 25°C in NaCl ionic media at ionic strengths 0.2, 0.5, 1.0, 2.0 and 2.5 mol∙l-1, using an ion exchange method and batch technique. Radium-226 and barium-133 were used as radiotracers and their activities in the aqueous phases were measured using gamma spectrometry and liquid scintillation counting, respectively. The specific ion interaction theory, SIT, was used to account for the complex formation between the ionic media and the ligand, [NaEDTA]3- and [NaHEDTA]2- complex formations, as well as for extrapolating determined apparent stability constants to zero ionic strength (BaEDTA2-: log10β=9.88±0.11 and RaEDTA2-: log10β=9.11±0.09). SIT was also used in order to obtain the Ba2+ and Ra2+ ion interaction coefficient terms (Δε(BaEDTA2-)=-(0.44±0.07); Δε(RaEDTA2-)=-(0.54±0.06)). From these terms, the ion interaction coefficients of Ba2+ and Ra2+ with various ions were calculated as: ε(Na+,BaEDTA2-)=-(0.03±0.11) and ε(Na+, RaEDTA2-) =-(0.10±0.11). It was found that in the HEDTA3- dominant pH region, the reaction of Ba2+ and Ra2+ with the HEDTA3- results in the emission of a proton and formation of the BaEDTA2- and RaEDTA2- complexes. The comparison of ion interaction parameters of Ba2+ and Ra2+ strongly indicate that both metal ions and their EDTA complexes have similar activity coefficients and undergo similar short-range interactions in aqueous NaCl media.