VARIATION OF THE STABILITY OF COMPLEXES OF AL(III) WITH A FULVIC-ACIDEXTRACTED FROM A HUMIC CAMBISOL SOIL IN THE PH RANGE 3 TO 5

The interaction of the aluminum ion [AI(III)I with the major constitue nt of soil organic matter, fulvic acids (FA), was investigated in the acid pH range. The complexation of a FA extracted from a humic cambiso l soil (at a concentration of 83 mg/L) when titrated with Al(III) were monitored directly by synchronous fluorescence (SyF) spectroscopy at five pH values (3.0, 3.5, 4.0, 4.5, and 5.0), at 25 degrees C and with 0.1 N KNO3. Inverted second derivative SyF data were preprocessed by a self-modeling mixture analysis method to obtain fluorescence enhance ment profiles as a function of the increasing AI(III) concentration. A ssuming that the concentration of complexes formed between Al(VI) and FA is proportional to the fluorescence intensity, these profiles were adjusted to a 1:1 complexation model and the following conditional sta bility constants (log K-c) were calculated (standard deviation in pare nthesis), pH = 3.0, 4.3 (1); pH = 3.5, 4.3 (1); pH = 4.0, 5.0 (1); pH = 4.5, 5.1 (3); end pH = 5.0, 5.3 (2). These constants show that the s tability of the complexes decreases markedly at pH < 4. The environmen tal implication of this result is that an increase in the extent of th e dissociation of the Al(III) + FA complexes is expected at pH < 4, in creasing the free Al(III) concentration and probably its ecotoxicity.