Abatement of Organic Contaminants by Mn(VII)/TEMPOs: Effects of TEMPOs Structure, Organic Contaminant Speciation, and Active Oxidizing Species.

In this study, a representative redox mediator, 2,2,6,6-tetramethylpiperidine--oxyl (TEMPO), and its para-substituted derivatives (TEMPOs: 4-hydroxyl-TEMPO, 4-acetylamino-TEMPO, and 4-amino-TEMPO) significantly accelerated the abatement of trace organic contaminants (TrOCs, i.e., bisphenol-A (BPA), phenol, amines, and phenylbutazone) by Mn(VII) over a wide pH range of 4.0-9.0. The addition of substituents at para to the > N-O moiety significantly influenced the degradation kinetics of TrOCs by changing the reduction potentials of TEMPOs and the corresponding oxoammonium cations (TEMPOs); a linear relationship was observed between the substituents' para Hammett sigma constants and the reduction potentials of TEMPOs and TEMPOs. Pseudo-first-order reaction rate constants (, min) of TrOC degradation by Mn(VII)/TEMPOs were also affected by the p of the TrOCs. Generally, the highest values for individual TrOCs were observed at pH near the p even for TEMPOs with relatively pH-invariant reduction potentials. Overall, TrOC abatement kinetics were related to a combination of reactive species (Mn(VII), formed MnO, and TEMPOs). For BPA, the relative contributions () of reactive species ranked as R(TEMPOs) > R(Mn(VII)) > R( formed MnO) at pH 4.0-8.0, whereas R(Mn(VII)) > R(TEMPOs) at pH 9.0 mainly owing to a change in BPA speciation as the pH approached the p value for BPA. The results of this study are useful for the development of heterogeneous TEMPO-based redox mediators and future applications of TEMPO-mediated oxidation systems for accelerated abatement of TrOCs in water.