Complexation of Eu and Tb with fulvic acids as studied by time-resolved laser-induced fluorescence

Complexation of Eu and Tb with fulvic acids (FA) has been studied by time-resolved laser-induced fluorescence. The lanthanide species are excited by a pulsed Xe/Cl excimer laser and the short-lived luminescence of the fulvic acids is eliminated by time gating. The conditional binding constants (K') and the total metal-binding capacities of the fulvic acids have been determined from the equilibrium titration curves. No significant variations of the log K' values (about 6) have been found in the pH range investigated (2.7, 5.5, 6.5) at ionic strengths of 0.1 and 1M (NaClO(4)). An inverse relationship of K' to total metal concentration is observed. Substitution of Eu for Al has been found to depress the fluorescence signal of the Eu-FA complex. The binding constants of Al(3+) and Eu(3+), derived from the competition experiments, are of the same order of magnitude. The effect of competition on metal binding and trace-metal transport is discussed.