Metal Ion Binding to Organic Matter: a Review on Concepts and Models

natural waters because of the breakdown of plant and animal residues by microbial activity. The various theoretical models have been described and the relative merits of the approaches compared. The discussion will focus on 1) the discrimination between chemical and electrostatic interactions, 2) the binding site heterogeneity, 3) the models for the electrostatic interactions, such as the Donnan model 4) the complications inherent to ion binding in multi-component systems, including a different stoichiometry (monodentate, bidentate and tridentate) for different ions and 5) the competition between specificallybound ions, especially protons and metal ions, and electrostatic effects which give rise to ionic strength effects and the nonspecific binding of counter ions. Important model parameters such as the site density and median binding constants (log K˜) are not dependent on pH, metal ion concentration, ionic strength, etc. Data are analyzed for H+, Ca2+, Cd2+, Cu2+, Pb2+ and Al3+ binding to a single purified peat humic acid. The NICCA– Donnan model captures the non-linearity of the observed isotherms even at very low free metal ion concentrations.

Keywords:

Organic matter , Metals , surface waters , groundwater , Non Ideal Competitive Adsorption Model , Electrostatic Interactions