pH-Dependent Binding of Aluminum by a Fulvic Acid

In this study, we use the Suwannee fulvic acid to investigate the chemistry of organic aluminum complexes in natural waters. We show that formation of organically bound Al (AlORG) is systematically pH-dependent and can be modeled using a macroscopic binding parameter, K_org, comprised of three concentration quantities that can be measured in natural water (AlORG, Al³⁺, and dissolved organic carbon). We demonstrate that AlORG behaves effectively as a polyprotic acid, and we provide an analysis framework from which the H⁺-dissociation constants of AlORG can readily be determined. A diprotic model of AlORG acidity is presented for the Suwannee fulvic acid [formula omitted]. Our results suggest that aluminum organic complexes may contribute significantly to the acid-base chemistry of natural waters. We also show that fulvic acid combines with and/or facilitates the formation of polynuclear hydroxy-Al ions, depending on the pH and total concentration of Al.