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-N-oxyl (TEMPO), and its paramino-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 center dot 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 (kobs, min-1) of TrOC degradation by Mn(VII)/TEMPOs were also affected by the pKa of the TrOCs. Generally, the highest kobs values for individual TrOCs were observed at pH near the pKa even for TEMPOs+ with relatively pH-invariant reduction potentials. Overall, TrOC abatement kinetics were related to a combination of reactive species (Mn(VII), in situ formed MnO2, and TEMPOs+). For BPA, the relative contributions (R) of reactive species ranked as R(TEMPOs+) > R(Mn(VII)) > R(in situ 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 pKa1 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.

Automated summary

In this study, TEMPO-based redox mediators significantly accelerated the degradation of trace organic contaminants by Mn(VII), with the addition of substituents influencing the degradation kinetics. The degradation rate constants were affected by the pKa of the contaminants, with the highest values observed near the pKa value. The relative contributions of reactive species in the degradation process varied with pH and BPA speciation.