Unrecognized role of humic acid as a reductant in accelerating fluoroquinolones oxidation by aqueous permanganate

Abstract

A great concern has been raised regarding the issue of fluoroquinolones (FQs) in the environment. In this work, the transformation of FQs by commonly used oxidant permanganate (Mn(VII)) in the absence and presence of humic acid (HA), ubiquitously existing in aquatic environments, was systematically investigated. Here, the catalytic role of in-situ formed MnO2 on Mn(VII) oxidation of FQs depending on solution pH and co-existing substrates was firstly reported. It was interestingly found that HA could appreciably accelerate FQs degradation by Mn(VII) at environmentally relevant pH. HA as a reductant in accelerating FQs by Mn(VII) oxidation was distinctly elucidated for the first time, where MnO2in situ formed from the reduction of Mn(VII) by HA served as a catalyst. Similar products were observed in the presence versus absence of HA. Considering that the accelerating role of HA was related to its reducing ability, an activation method based on Mn(VII) and reductant (i.e., Fe(II), Mn(II) and (bi)sulfite) was proposed, which exhibited considerable potential for application in the treatment of FQs contaminated water.