Modeling ion binding to humic substances: Elastic polyelectrolyte network model

SILVIA ORSETTI; ESTELA M. ANDRADE; FERNANDO V. MOLINA

LANGMUIR

AMER CHEMICAL SOC

Año: 2010 vol. 26 p. 3134 - 3144

0743-7463

A new model for non-specific ion binding to humic substances is proposed, and applied to published data for proton binding to fulvic and humic acids. The Elastic Polyelectrolyte Network model treats humic substance particles as composed by two parts, an external one directly in contact with the solution, and an internal part or gel fraction which is considered, from a statistical point of view, as a charged polymer network swelled by the electrolyte solution, in the framework of the Flory theory of polymer networks. The electrostatic effect is given by a Donnan potential, which can be regarded as an average value over the gel fraction of the humic particle. The gel fraction expands as the pH and humic charge are increased, determining the Donnan potential and consequently the ion activity inside the gel. The model was fitted to published experimental data with good agreement. The model predictions are discussed, and the electrostatic behavior suggests, for some cases, the presence of intramolecular hydrogen bonds which are removed as the carboxylic sites are deprotonated.