Eu(III)–Fulvic Acid Complexation: Evidence of Fulvic Acid Concentration Dependent Interactions by Time-Resolved Luminescence Spectroscopy
journal contributionposted on 29.02.2016, 00:00 by Yasmine Z. Kouhail, Marc F. Benedetti, Pascal E. Reiller
Europium speciation is investigated by time-resolved luminescence spectroscopy (TRLS) in the presence of Suwannee River fulvic acid (SRFA). From complexation isotherms built at different total Eu(III) concentrations, pH values, ionic strength, and SRFA concentrations, it appears that two luminescence behaviors of Eu(III) are occurring. The first part, at the lowest CSRFA values, is showing the typical luminescence evolution of Eu(III) complexed by humic substancesthat is, the increase of the asymmetry ratio between the 5D0 → 7F2 and 5D0 → 7F1 transitions up to a plateau, and the occurrence of a biexponential decaythe first decay being faster than free Eu3+. At higher CSRFA, a second luminescence mode is detected as the asymmetry ratio is increasing again after the previous plateau, and could correspond to the formation of another type of complex, and/or it can reflect a different spatial organization of complexed europium within the SRFA structure. The luminescence decay keeps on evolving but link to hydration number is not straightforward due to quenching mechanisms. The Eu(III) chemical environment evolution with CSRFA is also ionic strength dependent. These observations suggest that in addition to short-range interactionsintraparticulate complexation, there might be interactions at longer rangeinterparticulate repulsionbetween particles that are complexing Eu(III) at high CSRFA. These interactions are not yet accounted by the different complexation models.