Transition Metal Ions as Ozonation Catalysts: An Alternative Process of Heterogeneous Catalytic Ozonation

The aim of this study is to elucidate the mechanism of micropollutants’ removal in drinking water by the application of catalytic ozonation, using transition metals as appropriate catalysts. For that purpose, the degradation of 500 μg/L of p-chlorobenzoic acid (p-CBA) and benzotriazole with the addition of 2 mg/L of ozone in the presence of 1 mg/L of Co(II) or Fe(II) and at pH 7.8 were examined. It was found that in distilled water experiments, both metal ions can be characterized as catalysts, enhancing the ozonation process; however, in the natural water matrix, only iron presented higher removal rates of examined organic pollutants, when compared to single ozonation. The metal ions present catalytic activity, when they can form precipitates, hence converting the initially homogeneous process of catalytic ozonation towards a heterogeneous one. However, when 2 mg/L of ozone was applied in natural water experiments, Co(II)—unlike Fe(II)—could not be oxidized into its trivalent form, hence it cannot precipitate as Co(OH)3. Therefore, under these experimental conditions, this metal was not found to present any catalytic activity. Nevertheless, the addition of phosphates (PO43−) in concentrations higher than 100 mg/L can increase the oxidation ability of the Co(II)/O3 system, due to the resulting sufficient formation of Co3(PO4)2 precipitates. Although cobalt can enhance the •OH production (and therefore, the ozonation procedure) under these conditions, the relatively highly added concentration of phosphate ions makes the treated water non-potable, resulting in the application of further treatment to remove the excess phosphates. Therefore, only Fe(II) can be considered as a sufficient catalyst to enhance the ozonation processes.