Degradation of atrazine by manganese-catalysed ozonation: Influence of humic substances

A bench-scale, laboratory study has been undertaken on the ozonation of atrazine in the presence of manganese using a conventional gas bubble-contacting column. Manganese was present in solution either as Mn(II) or as in situ formed Mn(IV). The experimental results indicate that the presence of humic substances has a substantial influence on the Mn-catalysed ozonation of atrazine. In the presence of a small amount of humic substances (e.g. 1 mg/l as DOC), the Mn-catalysed ozonation is obviously enhanced. However, with the further increase of humic substance concentration, a negative effect of humic substances on the catalytic oxidation of atrazine is observed for both the cases of Mn(II) and Mn(IV). It is proposed that Mn-catalysed ozonation of atrazine follows a radical-type mechanism. At low concentrations of humic substances, both the manganese species and the humic substances may initiate and promote the formation of hydroxyl radicals during ozonation which enhance the destruction of atrazine. However, at higher concentrations, humic substances may scavenge these highly reactive hydroxyl radicals, thus decreasing the oxidation efficiency of atrazine.