FLUORESCENCE QUENCHING AND ALUMINUM COMPLEXATION BY A CHESTNUT LEAF LITTER EXTRACT

Fluorescence quenching (FQ) on addition of Al (III) to aqueous solutions of chestnut (Castanea sativa L.) leaf litter extract (LLE) has been interpreted previously as a parameter directly proportional to the mole fraction of Al-complexed organic ligands in the LLE. To provide independent confirmation of this interpretation, FQ and labile Al (defined by a 15-s reaction with 8-hydroxyquinoline) were measured simultaneously in mixtures of Al(ClO4)3 with the LLE. Solutions containing about 0.3 kg m-3 organic matter with total Al concentrations in the range 0 to 60 mmol m-3 were investigated at pH 4.0, 4.5, and 5.0. The total concentration of inorganic Al in the solutions (Alin) at each pH value was calculated as the sum of labile Al and the concentrations of the species Al(OH)2+ and Al(OH)4- as estimated with thermodynamic hydrolysis constants. The independently-measured parameters. FQ and Alin, then were used to calculate an overall stability coefficient for Al complexation by the leaf litter extract at each pH value. The values obtained were in good agreement with previous determinations of the stability coefficient based on fluorescence quenching alone. The conditional stability constant c.beta.0/0 for the quasiparticle species AIL (L = organic ligands) was found to have the value 108.7 .+-. 0.5 in agreement with a previous estimate of 108.6 .+-. 0.1 derived from FQ data only. These results confirmed the hypothesis that fluorescence quenching is proportional to Al complexation by the ligands in the LLE.