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Interaction of trivalent lanthanides and Cm(III) with humic acid (HA) (10 mg/L) is studied as a function of metal ion concentration (1×10 -8  to 5×10 -5  mol/L) and pH (2.5 to 7.0) in 0.01 mol/L NaClO 4 . Time resolved laser fluorescence spectroscopy (TRLFS) reveals a slight blue-shift/broadening of the fluorescence peak position for the Cm(III)-HA complex combined with an enhanced contribution of a short-lived fluorescence component with increasing metal ion loading and decreasing pH. Fluorescence decay for Cm(III)-HA is non-monoexponential under all investigated conditions and lifetimes can be described by assuming a short lived (τ=61 μs) and a longer lived contribution (τ=142 μs). These findings are explained by the existence of a certain heterogeneity in binding sites inducing slightly variable ligand-field splitting in the Cm(III)-HA complex. Batch experiments with a Eu(III)-HA solution contacted with a chelating resin reveal a decrease of the solid/liquid distribution coefficient by decreasing the occupancy of HA ligand sites from 4% to 0.2%. This observation indicates that complexation constants for the Eu(III)-HA complex vary with metal loading. Differences in related constants are, however, smaller than one order of magnitude. Variations in fluorescence lifetimes are interpreted as a consequence of increasing HA agglomeration at increasing metal loading and decreasing pH, thus leading to an enhanced local density of chromophoric groups close to the Cm(III) ion favouring non-radiative energy transition. HA agglomeration at low pH and increasing metal ion loading is well known from previous studies using flow-field flow fractionation (FFF) coupled to ICP-mass spectrometry and UV-Vis spectrophotometry. Our experimental results demonstrate the dynamic properties of metal-HA complexes depending on chemical conditions.

Influence of pH and metal ion loading on the Cm(III) humate complexation: a time resolved laser fluorescence spectroscopy study