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

Abstract

In this study, a representative redox mediator, 2,2,6,6-tetramethylpiperidine-<i>N</i>-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 (<i>k</i>_obs, min^-1) of TrOC degradation by Mn(VII)/TEMPOs were also affected by the p<i>K</i>_a of the TrOCs. Generally, the highest <i>k</i>_obs values for individual TrOCs were observed at pH near the p<i>K</i>_a even for TEMPOs⁺ with relatively pH-invariant reduction potentials. Overall, TrOC abatement kinetics were related to a combination of reactive species (Mn(VII), <i>in situ</i> formed MnO₂, and TEMPOs⁺). For BPA, the relative contributions (<i>R</i>) of reactive species ranked as R(TEMPOs⁺) > R(Mn(VII)) > R(<i>in situ</i> formed MnO₂) 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<i>K</i>_a1 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.