In
this study, a permanganate/redox mediator system for enhanced
transformation of a series of emerging contaminants was evaluated.
The presence of various redox mediators (i.e., 1-hydroxybenzotriazole, N-hydroxyphthalimide, violuric acid, syringaldehyde, vanillin,
4-hydroxycoumarin, and p-coumaric acid) accelerated
the degradation of bisphenol A (BPA) by Mn(VII). Since 1-hydroxybenzotriazole
(HBT) exhibited the highest reactive ability, it was selected to further
investigate the reaction mechanisms and quantify the effects of important
reaction parameters on Mn(VII)/redox-mediator reactions with BPA and
bisphenol AF (BPAF). Interestingly, not only HBT accelerated the degradation
of BPA, but also BPA enhanced the decay of HBT. Evidence for the in
situ formation of HBT· radicals as the active oxidant
responsible for accelerated BPA and BPAF degradation was obtained
by radical scavenging experiments and 31P NMR spin trapping
techniques. The routes for HBT· radical formation
involving Mn(VII) and the electron-transfer pathway from BPA/BPAF
to HBT· radicals demonstrate that the Mn(VII)/HBT
system was driven by the electron-transfer mechanism. Compared to
Mn(VII) alone, the presence of HBT totally inhibited self-coupling
of BPA and BPAF and promoted β-scission, hydroxylation, ring
opening, and decarboxylation reactions. Moreover, Mn(VII)/HBT is also
effective in real waters with the order of river water > wastewater
treatment plant (WWTP) effluent > deionized water.
Permanganate (Mn(VII)) has been widely applied as an oxidant in water treatment plants. However, compared with ozone, Fenton, and other advanced oxidation processes, the reaction rates of some trace organic contaminants (TrOCs) with Mn(VII) are relatively low. Therefore, further studies on the strategies for enhancing the oxidation of organic contaminants by Mn(VII) are valuable. In this work, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), as an electron shuttle, enhanced Mn(VII) oxidation toward various TrOCs (i.e., bisphenol A (BPA), phenol, estrone, sulfisoxazole, etc.). TEMPO sped up the oxidative kinetics of BPA by Mn(VII) greatly, and this enhancement was observed at a wide pH range of 4.0−11.0. The exact mechanism of TEMPO in Mn(VII) oxidation was described briefly as follows: (i) TEMPO was oxidized by Mn(VII) to its oxoammonium cation (TEMPO + ) by electron transfer, which was the reactive species responsible for the accelerated degradation of TrOCs and (ii) TEMPO + could decompose TrOCs rapidly with itself back to TEMPO or TEMPOH (TEMPO hydroxylamine). To further illustrate the interaction between TEMPO and target TrOCs, we explored the transformation pathways of BPA in Mn(VII)/ TEMPO oxidation. Compared to Mn(VII) alone, adding TEMPO into the Mn(VII) solution significantly suppressed BPA's selfcoupling and promoted hydroxylation, ring-opening, and decarboxylation. Moreover, the Mn(VII)/TEMPO system was promising for the abatement of TrOCs in real waters for humic acid, and ubiquitous cations/anions had no adverse or even beneficial impact on the Mn(VII)/TEMPO system.
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