EFFECTS OF OZONE ON THE COLLOIDAL STABILITY AND AGGREGATION OF PARTICLES COATED WITH NATURAL ORGANIC-MATTER

Adsorption of natural organic matter (NOM) onto particles contributes to their colloidal stability and inhibits coagulation in water treatme nt. Ozone has been shown to assist in the destabilization of particles during water treatment by several hypothesized mechanisms. The goal o f this research was to study the effects of ozone on NOM-coated partic les in model (synthetic) waters to determine prevailing mechanisms and to quantify the effects of different variables. The experimental syst em included particles, aqueous phase NOM in equilibrium with particle phase (sorbed) NOM, and a background solution chemistry reflecting pH, ionic strength, and potentially influential ions (e.g., calcium, bica rbonate). Particle stability was assessed through electrophoretic mobi lity measurements while particle aggregation was assessed through use of a collision efficiency factor (a) under fluid shear mixing conditio ns. Changes in molecular weight, acidity, and complexation capacity of NOM (aqueous and mineral-bound) with calcium was studied before and a fter ozonation. Oxalic acid was used as a model compound for ozonated NOM. It was found that ozone-induced particle destabilization occurred only in the presence of calcium. Based on the overall results, it is proposed that the ozone-induced particle destabilization observed in o ur system (with calcium present) is due to the following: (a) an incre ase in calcium association with ozonated NOM constituents thereby inhi biting adsorption of anionic species onto the alumina surface and (b) a production of more ligand sites by ozone on the surface-sorbed NOM l eading to increased NOM complexation with calcium. These effects lead to a reduction in particle stability through surface charge reduction.