DISSOCIATION PATHWAYS AND SPECIES DISTRIBUTION OF ALUMINUM BOUND TO AN AQUATIC FULVIC-ACID

A ligand-exchange reaction between Al-fulvic acid (FA) complexes and lumogallion (Lum), Al-FA + Lum --> Al-Lum + FA, was investigated by following the appearance of the fluorescent complex AlLum. Rate data fit a model of simultaneous first-order reactions and indicated that the concentration of the slowest reacting component increased with increasing fulvic acid concentration. Small changes in pH (5.0-5.5) and in ionic strength (I = 0.02-0.11) had no effect on the observed rates. The data were also analyzed in terms of two parallel mechanisms, Al-FA dissociation (disjunctive mechanism) and direct attack by Lum (adjunctive mechanism). Direct attack was the more important pathway when the incoming ligand, Lum, was in great excess of the Al-FA concentration or when free FA concentrations were much larger than bound FA concentrations. It was concluded that in natural freshwaters, where the incoming ligand is likely not to be in large excess, most Al-FA complexes undergo ligand exchange via the disjunctive pathway. Conditional binding constants were also estimated, and an Al species distribution was calculated.