NATURAL ORGANIC-MATTER AND COLLOIDAL STABILITY - MODELS AND MEASUREMENTS

Laboratory and field observations by several investigators indicate th at natural organic matter (NOM) affects and probably controls the coll oidal stability of particles in aquatic systems. The enhanced stabilit y of particles in aquatic systems containing NOM is a consistent obser vation without a clear cause. In this work, the potential importance o f the macromolecular nature of NOM was investigated using model system s. A mathematical model for the adsorption of linear, flexible polyele ctrolytes was used to examine the effects of molecular weight, pH, and ionic strength on the conformations of these surrogates for NOM at in terfaces 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 t he 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 positivel y charged particles by altering net surface charge in a way similar to specifically adsorbing, multivalent, monomeric anions. At high ionic strength (I greater-than-or-equal-to 0.1), the conformational characte ristics of adsorbed polyelectrolytes at the solid/water interface dire ctly affect coagulation by expanding the effective distance of electro static repulsion between approaching particles, as well as by altering net surface charge. Non-electrostatic steric repulsion plays little o r no role in the stabilization of hematite particles by the organic ma cromolecules used in this work. Calcium acts to destabilize hematite p articles in the presence of the organic macromolecules, perhaps throug h a combination of specific chemical and charge effects.