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 treatment. Ozone has been shown to assist in the destabilization of particles during water treatment by several hypothesized mechanisms. The goal of this research was to study the effects of ozone on NOM-coated particles in model (synthetic) waters to determine prevailing mechanisms and to quantify the effects of different variables. The experimental system 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, bicarbonate). Particle stability was assessed through electrophoretic mobility measurements while particle aggregation was assessed through use of a collision efficiency factor (a) under fluid shear mixing conditions. Changes in molecular weight, acidity, and complexation capacity of NOM (aqueous and mineral-bound) with calcium was studied before and after 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 our system (with calcium present) is due to the following: (a) an increase in calcium association with ozonated NOM constituents thereby inhibiting adsorption of anionic species onto the alumina surface and (b) a production of more ligand sites by ozone on the surface-sorbed NOM leading to increased NOM complexation with calcium. These effects lead to a reduction in particle stability through surface charge reduction.