INVESTIGATION OF METAL-BINDING SITES ON SOIL FULVIC-ACID USING EU(III) LUMINESCENCE SPECTROSCOPY

The F-7(0) --> D-5(0) excitation spectra of Eu(III) complexed with soil fulvic acid (FA) were acquired over a range of solution pH (2.9-7.8) and FA concentrations (800-3200 mg L(-1)) using a pulsed tunable dye laser system. The broad asymmetric excitation spectra were well-fitted to a sum of two conventional Lorentzian-shaped curves, revealing the existence of two types of carboxylate moieties for the binding of metal ions on FA which formed 1:1 (EuL(2+); L = carboxylate) and 1:2 complexes (EuL(2)(+)). The weaker binding species, EuL(2+), seemed to be quite abundant and showed a rapid increase as the pH was raised from 2.9 to 6.3, but it was susceptible to hydrolysis at pH higher than 7 while the stronger binding species, EuL(2)(+), showed only a modest growth with an increase in pH. By contrast, on a more flexible synthetic linear polymer, poly(acrylic acid) (PAA) and poly(vinylbenzoic acid) (PVBA) as model polymers, EuL(2)(+) was seen as the dominant species except in acidic media.