Aluminium complexation by an aquatic humic fraction under acidic conditions

Equilibrium dialysis and acid-base titration were used to investigate the interactions between Al and a fraction of aquatic humic substances (HS), in the pH range 3–5. Binding of Al by the HS increased with Al³⁺ activity and with pH. Under conditions relevant to natural waters, υ (mol Al bound per gHS) varied from 0 to 1.5 × 10⁻³. The data were modelled with an emprical linear logarithmic expression (Model I) and on the basis of the polyelectrolyte nature of the HS, incorporating competitive binding of H⁺ and Al³⁺ (Model II). Both models gave tolerable fits (r = 0.93). Model I is simpler to apply, while Model II allows the calculation of proton release accompanying Al binding, and provides information on the net charge of the Al---HS complexes. The results were used to calculate the distribution of Al between organic and inorganic forms under conditions prevailing in acidic natural waters.