Spectral and temporal luminescent properties of Eu(III) in humic substances solutions from different origins
Résumé
Although a high heterogeneity of composition is awaited for humic substances, their complexation properties do not seem to greatly depend on their origins. The information on the difference in the structure of these complexes is scarce. To participate in the filling of this lack, a study of the spectral and temporal evolution of the Eu(III) luminescence implied in humic substance (HS) complexes is presented. Seven different extracts, namely Suwannee River fulvic acid (SRFA) and humic acid (SRHA), and Leonardite HA (LHA) from the International Humic Substances Society (USA), humic acid from Gorleben (GohyHA), and from the Kleiner Kranichsee bog (KFA, KHA) from Germany, and purified commercial Aldrich HA (PAHA), were made to contact with Eu(III). Eu(III)-HS time-resolved luminescence properties were compared with aqueous Eu$^{3+}$ at pH 5. Using an excitation wavelength of 394 nm, the typical bi-exponential luminescence decay for Eu(III)-HS complexes is common to all the samples. The components $\tau$1 and $\tau$τ2 are in the same order of magnitude for all the samples, i.e., 40 $\leq$ $\tau$1 ($\mu$s) $\leq$ 60, and 145 $\leq$ $\tau$2 ($\mu$s) $\leq$ 190, but significantly different. It is shown that different spectra are obtained from the different groups of samples. Terrestrial extract on the one hand, i.e. LHA/GohyHA, plus PAHA, and purely aquatic extracts on the other hand, i.e., SRFA/SRHA/KFA/KHA, induce inner coherent luminescent properties of Eu(III) within each group. The $^5$D0 $\rightarrow$ $^7$F$_2$ transition exhibits the most striking differences. A slight blue shift is observed compared to aqueous Eu$^{3+}$ ($\lambda_{max}$ = 615.4 nm), and the humic samples share almost the same $\lambda_{max}$ $\approx$ 614.5 nm. The main differences between the samples reside in a shoulder around $\lambda$$\approx$ 612.5 nm, modelled by a mixed Gaussian–Lorentzian band around $\lambda$$\approx$ 612 nm. SRFA shows the most intense shoulder with an intensity ratio of $I_{612.5}$/$I_{614.7}$ = 1.1, KFA/KHA/SRHA share almost the same ratio $I_{612.5}$/$I_{614.7}$ = 1.2–1.3, whilst the LHA/GohyHA/PAHA group has a $I_{612.5}$/$I_{614.5}$ = 1.5–1.6. This shows that for the two groups of complexes, despite comparable complexing properties, slightly different symmetries are awaited.
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