pH effect on OH radical production in photo/ferrioxalate system

In wastewater treatment using the Fenton and photofenton processes, pH is one of the critical operating parameters, due to the fact that the Fenton reaction can work only under acidic pH conditions. It is hoped that Ferric iron complexed with oxalate (Fe(III)-oxalate; ferrioxalate) will provide an alternative to the traditional Fenton process with its limited range of pH conditions, since its high solubility in aqueous media can broaden the available pH range of the Fenton reaction up to the near neutral pH regime. In this study, we investigated the pH dependency of OH production in the photo/ferrioxalate system, in the presence and absence of externally supplied H2O2, where 2,4-D was used as the probe compound for OH production at a wide range of pH values (1.2–7.4). In the absence of externally supplied H2O2, the 2,4-D degradation was considerably enhanced with increasing pH, whereas it was reduced with increasing pH in the presence of an excess amount of H2O2. These variations in the degradation of 2,4-D were thus found to be precisely related to the formation of H2O2, a factor to which little attention was paid in previous studies. In the absence of H2O2 addition, the in situ formation of H2O2 is facilitated with increasing pH by the reaction of Fe(II) with O2-, which increases with pH, augmenting the production of OH and thereby leading to the faster degradation of 2,4-D. This same reaction can also provide an explanation for the opposite pH dependence of 2,4-D degradation in the presence of H2O2.