Oxidation of Bisphenol A, Triclosan and 4-Nonylphenol by Fe-B* activated peroxide

Abstract

The widely used Bisphenol A (BPA), triclosan (TCS) and nonylphenol (NP) are commonly encountered as contaminants in wastewater and surface water as a result of their incomplete treatment in traditional water treatment facilities. The presence of these compounds in our water resources poses direct threat to aquatic species and potential health risk to humans. This thesis describes a systematic investigation of BPA, TCS and NP oxidation by a green catalyst system. The catalyst system used is the FeIII-TAML/H2O2 (TAML = tetra-amido-macrocyclic ligand) catalytic oxidation system. The commercialized Fe-B* type of the FeIII-TAML is utilized throughout this study. A three-stage investigation process was designed to eliminate these compounds and build on the understanding of the likely reactions that would take place in real life: 1) develop an understanding of the fate of each BPA, TCS and NP during treatment by FeIIITAML/ H2O2 under varying pH and catalyst concentration; 2) study competition reactions among the three substrates for available oxidant, monitoring substrate preferential selectivity by the oxidant; 3) monitor the oxidation of BPA, TCS and NP by oxidant in the presence of natural organic matters (NOM). The FeIII-TAML/H2O2 (4 nM/ 4 mM) oxidation of each BPA (43.8 μM), TCS (34.5 μM) and NP (34.5 μM) were observed to be pH dependent within the range investigated (pH 6.0-10.5). The maximum rate of oxidative loss of BPA, TCS and NP occurred at pH 9.5, 9 and 10, respectively. The oxidant system is proposed to mediate one-electron oxidation of each compound, producing phenolic radicals which then couple into dimeric products of each compound; increased FeIII-TAML concentration above 4 nM result in oligomers, forming dimer, trimer and tetramer (as detected by the LC-Tof-MS under negative ion mode). The 1H NMR analysis of BPA-dimer product suggest BPA phenolic radicals coupled via orthoposition C-C bonding, while the GC-MS analysis identified the C-C and C-O bonded BPA-dimers, at a ratio approximately of 14:1. Suggesting relatively low C-O coupled dimer BPA as compared to C-O coupled one. Density functional theory (DFT) predicted ortho position bonding of BPA phenolic radicals, this was consistent with structure of BPA dimer analyzed by the 1H NMR and GC-MS analysis. Similar coupling positions as BPA were predicted for NP dimers by the DFT calculations, while ortho and para bonding positions were predicted for TCS coupled products. The Oxidation energies for the transformation pathways obtained using the DFT calculations suggest that anions (phenolates) of BPA, TCS and NP are more susceptible to oxidation than their phenolic counterparts. DFT calculations show that BPA and NP require about similar amount of oxidation energy, while TCS is the most difficult to oxidise. Energy calculations were observed to be consistent with the selective nature of FeIII-TAML/H2O2, which suggest that easier to oxidize substrate was selected over harder to oxidize ones. In the presence of natural organic matters (NOM) sourced from Suwannee River water. Results suggest that at low concentration of NOM (1 mg/L DOC), enhanced removal of the 10 mg/L BPA (43.8 μM), TCS (34.5 μM) and 5 mg/L NP (34.5 μM) by FeIII-TAML/H2O2 (8 nM/ 4 mM) were observed at pH 8.5. In the presence of excess amount of NOM (> 10 mg/L DOC), competition by NOM reduced substrates removal rates. It was proposed that presence of humic acid and cross-coupling between substrate and phenolic moieties in NOM molecules caused the enhanced removals observed at low NOM concentration. In the presence of excess NOM, excess competition from NOM molecules for available oxidant and shielding by NOM molecules could have prevented substrates from assessing oxidant, resulting in the reduced removals observed.