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Eu(III)–Fulvic Acid Complexation: Evidence of Fulvic Acid Concentration Dependent Interactions by Time-Resolved Luminescence Spectroscopy

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journal contribution
posted 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 substancesthat is, the increase of the asymmetry ratio between the 5D07F2 and 5D07F1 transitions up to a plateau, and the occurrence of a biexponential decaythe 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 interactionsintraparticulate complexation, there might be interactions at longer rangeinterparticulate repulsionbetween particles that are complexing Eu­(III) at high CSRFA. These interactions are not yet accounted by the different complexation models.