posted on 2021-05-19, 16:35authored byHonglong Zhang, Zhenyu Shi, Ruopeng Bai, Dingxiang Wang, Fuyi Cui, Jing Zhang, Timothy J. Strathmann
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 self-coupling 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.