Humic acid binding of trivalent Tl and Cr studied by synchronous and time-resolved fluorescence

The complexation of Ti(III) and Cr(III) with groundwater humic acids (HA) was investigated at pH 4.5 and pH 5.5 using different fluorescence methods. These comprised both quenching measurements of the continuous and synchronous fluorescence of the HA ligand and and ion competition method based on the time-resolved laser-induced fluorescence of HA-bound Eu(III). Modeling of the experimental data using an electrostatic discrete site approach (Model V), yielded comparable values of intrinsic binding constants for each of the studied metal, independently of sample conditions (pH, metal:ligand ration) and fluorescence method used. The results confirmed the validity of fluorescence techniques in the analysis of low concentration metal ion-HA interactions and indicated that beside the aqua ion and the first hydroxc complex, other species may also bind to humic acids.