Study of the interaction of a soil fulvic acid with UO22+ by self-modelling mixture analysis of synchronous molecular fluorescence spectra

Abstract

The interactions between UO_2²⁺ and fulvic acids (FUA) were studied by a methodology that involves synchronous molecular fluorescence spectroscopy, to monitor the quenching of the intrinsic fluorescence of FUA by UO_2²⁺, and a self-modelling mixture analysis method (SIMPLISMA), to treat spectroscopic data. This methodology was applied to the analysis of the interaction of UO_2²⁺ with salicylic acid at pH 3.5 and with a soil FUA at pH 3.5 and 7.0, in this case in the presence of various concentrations of carbonate ion (10^–5, 10^–4 and 10^–3 mol l^–1). From the calculated quenching fluorescence intensity profiles, using either the Stern–Volmer relationship or a non-linear least-squares method, mean conditional stability constants (log values with standard deviations in parentheses) were estimated for salicylic acid [respectively 2.72(4) and 2.77(6)], for FUA at pH 3.5 [3.93(2) and 4.4(1)] and pH 7.0 [4.06 and 4.1 (average values for the various concentrations of carbonate ion)]. The non-linear least-squares method also allowed the estimation of the number of binding sites that exist in FUA (0.11 and 0.24 mol g^–1 at pH 3.5 and 7.0, respectively).