The effects of varying pH on substrate removal, nitrate formation, and dissolved organic carbon (DOC) removal during the ozonization of aqueous solutions of 2,4-dinitrotoluene (DNT) and 4-nitroaniline (NA) were investigated in semibatch systems under partially mass-transfer–limited
conditions. If ozonization was performed outside the region of strong mass-transfer limitation, a strong pH influence was observed for the treatment of DNT. At pH 2, the rate of DNT removal was small and DNT was poorly degraded. Both nitro groups were split off and immediately oxidized to
nitrate at pH 2 and pH 7. An increase in the pH, to a minimum value of 7, during the oxidation of DNT and NA caused radical reactions and, thus, a higher degree of simultaneous DOC removal. Furthermore, nitrogen-containing organic metabolites were produced from DNT because of these radical
reactions when the ozonization was carried out at pH 11. Sequential semibatch ozonization (pH 7) and biodegradation resulted in complete removal of the nitroaromatics and a minimum overall DOC removal of 75% with minimum specific-ozone doses of 4 g/g and 3 g/g for DNT and NA, respectively.
Compared to this, no ozone savings were achieved with a continuous-flow chemical–biological treatment system. This effect was caused by the unfavorable reaction conditions in the totally mixed ozonization reactor.
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