Takafumi Miyamoto

Potassium monopersulfate oxidation of 2,4,6-tribromophenol catalyzed by a SiO2-supported iron(III)-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin

Iron(III)-porphyrin complexes are generally regarded as green catalysts, since they mimic the catalytic center of cytochrome-P450 and widely used as green catalysts for degrading halogenated phenols in wastewater, such as landfill leachates. However, iron(III)-porphyrins are deactivated by self-oxidation in the presence of an oxygen donor, such as KHSO5. In the present study, to enhance the reusability of an iron(III)-porphyrin catalyst, iron(III)-5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (FeTCPP) was immobilized on a functionalized silica gel. The oxidative degradation of 2,4,6-tribromophenol (TrBP), a widely used brominated flame retardant that is found in landfill leachates, was examined using the prepared catalyst. In addition, the influence of humic substances (HSs), major components of leachates, on the TrBP oxidation was investigated. Concerning the effect of pH, more than 90% of the TrBP was degraded in the pH range of 3–8 in the absence of HS, while the optimal pH for the reaction was in the range of pH 5-7 in the presence of HS. Although the oxidation of TrBP was inhibited in the presence of HSs, more than 90% of the TrBP was degraded in the presence of 50 mg L−1 of HS. Thus, the prepared catalyst, SiO2-FeTCPP, showed a high catalytic activity and could be reused up to 10 times even in the presence of HS.

The oxidation of tetrabromobisphenol A by potassium monopersulfate with an iron(III)-phthalocyanine-tetrasulfonic acid catalyst in the presence of humic acid

Tetrabromobisphenol A (TBBPA), a type of brominated flame retardant that shows endocrine disruption effects, has been identified in leachates from landfills. Iron(III)-porphyrins that mimic the active site of peroxidases have been shown to be effective in oxidizing halogenated phenols, such as TBBPA. In the present study, TBBPA was subjected to oxidation with potassium monopersulfate (KHSO5) using an iron(III)-phthalocyanine-tetrasulfonic acid (FePcTS), structural analogue of iron(III)-porphyrin, in the presence of humic acid (HA), a major component in landfill leachates. When TBBPA was oxidized using the above system, the levels of degradation and debromination increased with increasing pH in the presence of HA. Because of landfill leachates are weakly alkaline (around pH 8), oxidation products derived from TBBPA were investigated at pH 8. Approximately 48% of the bromine in the degraded TBBPA was incorporated into HA, and hydroxy-tribromobisphenol A was determined to be the major brominated intermediate in the HA fraction. In the iron(III)-porphyrin catalytic systems, the brominated intermediate incorporated into HA is mainly TBBPA, and no hydroxy-substituted bromophenols are found. Thus, the catalytic power of FePcTS is higher than that of iron(III)-porphyrin catalysts.

Potential risk of coupling products between tetrahalobisphenol A and humic acid prepared via oxidation with a biomimetic catalyst

Tetrahalobisphenol A (TXPBAs, X = Br or Cl), TBBPA and TCBPA, which are widely used as flame retardants, ultimately disposed of in landfills. In landfills, enzymatically oxidized TXBPAs can be covalently incorporated into humic acids (HAs) to form coupling products (HA-TXBPAs). In the present study, HA-TXBPAs were prepared by catalytic oxidation with iron(III)-phthalocyanine-tetrasulfate as a model of oxidative enzymes. The stability of HA-TXBPAs was evaluated by incubating them under physicochemical conditions of landfills (pH 9 and 50 °C). For HA-TBBPA, 18-26% of TBBPA was released from HA-TBBPA, due to the acid dissociation of the loosely bound TBBPA. However, no additional release was observed, even after 30 days, indicating that 74-82% of the TBBPA was incorporated into the HA. For HA-TCBPA, 3-4% of TCBPA and a major byproduct, 4-(2-hydroxyisopropyl)-2,6-dichlorophenol, was found to be loosely incorporated into HA. For both TBBPA and TCBPA, covalently bound organo-halogens were not released during the 30 days of incubation. Inhibition of the growth of Chlamydomonas reinhardtii was indicated when trace levels of TXBPAs (approximately 0.1 μM) were present. These results suggest that HA-TXBPAs contain not only covalently incorporated TXBPAs but also loosely bound TXBPAs and halophenols. The latter in HA-TXBPAs have the potential to leach from landfills and affect aquatic ecosystems.