Degradation of atrazine by manganese-catalysed ozonation—influence of radical scavengers

Abstract

The effect of radical scavengers such as bicarbonate and tert-butanol on the Mn II catalysed ozonation of atrazine, an important herbicide and well-established radical probe substance, was studied using a conventional gas bubble-contacting column. It was found that the presence of a small amount of Mn II (0.3–1.2 mg/l) greatly increased the degradation rate of atrazine, with the formation of by-product compounds of a lower molecular weight and a greater polarity (as indicated by shorter retention times in HPLC chromatography). However, the presence of either bicarbonate or tert-butanol had a negative effect on the degradation of atrazine. With an increase of bicarbonate concentration, the oxidation rate of atrazine by Mn II-catalysed ozone was substantially reduced and a correspondingly higher residual ozone was observed. In the presence of tert-butanol, greater reductions of the degradation rate of atrazine were observed and the decomposition of ozone was greatly retarded, resulting in a higher residual ozone; this was the case for either ozone oxidation alone or Mn II-catalysed ozone oxidation. These results appear to confirm that the degradation of atrazine by ozone in the presence of Mn II follows a radical mechanism. It is believed that Mn II catalyses the decomposition of ozone through the formation of intermediate manganese species (such as Mn IV), leading to the generation of hydroxyl radicals.