RESEARCH ARTICLE
Nano-MnO
2
-mediated transformation of triclosan with humic molecules
present: kinetics, products, and pathways
Kai Sun
1,2
&
Shunyao Li
3
&
Michael Gatheru Waigi
3
&
Qingguo Huang
2
Received: 7 November 2017 /Accepted: 28 February 2018 /Published online: 10 March 2018
#
Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract
It has been shown that manganese dioxide (MnO
2
) can mediate transformation of phenolic contaminants to form phenoxyl
radical intermediates, and subsequently, these intermediates intercouple to form oligomers via covalent binding. However, the
reaction kinetics and transformation mechanisms of phenolic contaminants with humic molecules present in nano-MnO
2
-medi-
ated systems were still unclear. In this study, it was proven that nano-MnO
2
were effective in transforming triclosan under acidic
conditions (pH 3.5–5.0) during manganese reduction, and the apparent pseudo first-order kinetics rate constants (k =0.0599–
1.5314 h
−1
) increased as the pH decreased. In particular, the transformation of triclosan by nano-MnO
2
was enhanced in the
presence of low-concentration humic acid (1–10 mg L
−1
). The variation in the absorption of humic molecules at 275 nm
supported possible covalent binding between humic molecules and triclosan in the nano-MnO
2
-mediated systems. A total of
four main intermediate products were identified by high-resolution mass spectrometry (HRMS), regardless of humic molecules
present in the systems or not. These products correspond to a suite of radical intercoupling reactions (dimers and trimers), ether
cleavage (2,4-dichlorophenol), and oxidation to quinone-like products, triggered by electron transfer from triclosan molecules to
nano-MnO
2
. A possible reaction pathway in humic acid solutions, including homo-coupling, decomposition, oxidation, and
cross-coupling, was proposed. Our findings provide valuable information regarding the environmental fate and transformation
mechanism of triclosan by nano-MnO
2
in complex water matrices.
Keywords Nano-MnO
2
.
Triclosan
.
Humic acid
.
Product identification
.
Transformation mechanism
Introduction
Triclosan is a widespread environmental pollutant used in the
manufacture of pharmaceutical and personal care products as
a broad-spectrum antimicrobial and preservative agent
(Dhillon et al. 2015;Westerhoffetal.2005; Yazdankhah
et al. 2006). It is potentially destructive to the ecological
environment and human health, and thus has been designated
priority contaminants by the US Environmental Protection
Agency (EPA) (Halden and Paull 2005;LiuandWong
2013;Kolpinetal.2002). For instance, triclosan and its trans-
formation products were continuously released into the natu-
ral aquatic environments through the municipal and industrial
wastewater discharges (Luo et al. 2014). This has been report-
ed in several studies to have a negative influence on wild
species (i.e., suppress bacterial growth, enhance bacterial re-
sistance, and cause death of fish) and have an endocrine-
disrupting effect on humans (Dhillon et al. 2015; Gee et al.
2008;Geeretal.2017;Levyetal.1999; Mcmurry et al. 1998;
Singer et al. 2002). Therefore, the fate and transformation of
triclosan in the aquatic environments have drawn significant
attention in recent years (Gao et al. 2014).
Multiple chemical oxidants (e.g., Fenton reagent, free chlo-
rine, sulfate radicals, ozone, permanganate, and manganese
dioxide) have been shown to transform triclosan in an aqueous
solution (Chen et al. 2012; Jiang et al. 2009a; Nfodzo and
Choi 2011;Pengetal.2016;Ruleetal.2005;Shaikhetal.
Responsible editor: Philippe Garrigues
* Qingguo Huang
qhuang@uga.edu
1
Anhui Province Key Laboratory of Farmland Ecological
Conservation and Pollution Prevention, School of Resources and
Environment, Anhui Agricultural University, 130 Changjiang West
Road, Hefei, Anhui 230036, China
2
Department of Crop and Soil Sciences, University of Georgia,
Griffin, GA 30223, USA
3
College of Resources and Environmental Sciences, Nanjing
Agricultural University, Nanjing 210095, China
Environmental Science and Pollution Research (2018) 25:14416–14425
https://doi.org/10.1007/s11356-018-1637-7
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