Aluminium-organic complexation: formation constants and a speciation model for the soil solution

Stability constants describing Al³⁺ combination with soil polycarboxylates were measured using the methods of Gregor et al. (1955) and Tanford (1961). It is suggested that the high stability of such complexes is derived largely from the electrostatic energy associated with the high surface charge of soluble humus acids. Chemically, the affinity of fulvic acids for Al³⁺ appeared to be no greater than that of a weak complexant such as adipic or acetic acid. A species distribution model was constructed showing the proportion of soluble Al that is bound by organic complexes. The model uses certain approximations regarding the degree of dissociation of the humus acids, but from measurements of soluble humus and Al in soil solutions it was suggested that these approximations were reasonably valid. The range of the model falls within conditions likely to occur in agricultural topsoils and demonstrates the importance of organic complexants in limiting Al³⁺ concentrations in natural aqueous systems.