Influence of adipic acid on the speciation of Eu(III): Review of thermodynamic data in NaCl and NaClO4 media, and a new determination of Eu-adipate complexation constant in 0.5 mol•kgw-1 NaClO4 medium by time-resolved luminescence spectroscopy

The complexation of Eu(III) by adipic acid - AdipH(2), hexane-1,6-dioic acid -, one of the major hydro-soluble degradation products (HDPs) from radio-oxidized polyesterurethane, is investigated by time-resolved luminescence spectroscopy (TRLS) in NaClO4 ionic medium (0.5 mol center dot kg(w)(-1)). The solubility and deprotonation constants of adipic acid, as well as specific interaction theory (SIT) coefficients of aqueous AdipH(2)(aq) and adipate ions (AdipH(-) and Adip(2-)) were estimated using available literature data in NaCl and NaClO4 ionic media. A 1:1 (EuAdip(+)) complex is evidenced in TRLS with log(10)beta(0.5 m) = 2.09 +/- 0.16. Using reaction constant at another NaClO4 ionic strength from literature, the logarithm of reaction constant log(10)beta degrees = 3.88 +/- 0.40 at 0 ionic strength and specific ion parameter epsilon(EuAdip(+), ClO4-) = (0.10 +/- 0.40) kg(w)center dot mol(-1) is estimated. This implies that adipic acid can control the Eu(III) speciation from mildly acidic to mildly basic conditions, but not in highly basic solutions characteristic of cementitious interstitial waters.