Effects of Humic Substances on Metal Speciation

Abstract

This chapter addresses some of the problems that must be understood and solved before the effects of metal-humic substance complexation on water-treatment processes can be quantitatively addressed. The heterogeneity of ligands in a humic substance not only complicates the mathematical description of equilibrium data, but also makes the complexation capacity of a humic substance almost impossible to determine accurately. Complexation capacities (meq/g) of humic substances are widely reported to vary with pH, ionic strength, concentration of the humic substance used in the measurement, and nature of the metal being studied. By analogy with the behavior of a simple ligand (citrate), this chapter demonstrates that the reported effects of humic-substance concentration on complexation capacity is probably an artifact and that other experimental parameters affect conditional concentration quotients for metal complexation reactions. These effects create the illusion that complexation capacity is a function of pH, ionic strength, and nature of the added metal ion.