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 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
(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.