Natural organic matter and colloidal stability: models and measurements

Abstract

Laboratory and field observations by several investigators indicate that natural organic matter (NOM) affects and probably controls the colloidal stability of particles in aquatic systems. The enhanced stability of particles in aquatic systems containing NOM is a consistent observation without a clear cause. In this work, the potential importance of the macromolecular nature of NOM was investigated using model systems. A mathematical model for the adsorption of linear, flexible polyelectrolytes was used to examine the effects of molecular weight, pH, and ionic strength on the conformations of these surrogates for NOM at interfaces in natural waters. Laboratory experiments involving submicron hematite particles, two anionic polyelectrolytes, and an aquatic NOM were used to examine the effects of solution composition on colloidal stability. Together, the results of the mathematical simulations and the laboratory experiments indicate that electrostatic effects dominate particle-particle interactions, but that the macromolecular nature of NOM can have direct influence under certain conditions. At low ionic strength, anionic polyelectrolytes affect the coagulation of positively charged particles by altering net surface charge in a way similar to specifically adsorbing, multivalent, monomeric anions. At high ionic strength (I ≥ 0.1), the conformational characteristics of adsorbed polyelectrolytes at the solid/water interface directly affect coagulation by expanding the effective distance of electrostatic repulsion between approaching particles, as well as by altering net surface charge. Non-electrostatic steric repulsion plays little or no role in the stabilization of hematite particles by the organic macromolecules used in this work. Calcium acts to destabilize hematite particles in the presence of the organic macromolecules, perhaps through a combination of specific chemical and charge effects.