Rapid destruction of triclosan by Iron(III)-Tetraamidomacrocyclic ligand/hydrogen peroxide system
Graphical abstract
Introduction
Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is primarily employed as the antibacterial, antimicrobial and preservative agent in consumer products and also as an active ingredient in personal care products, e.g., soaps, toothpaste, skin creams and deodorants (Sanchez-Prado et al., 2006; Aranami and Readman, 2007). However, due to the inability of conventional wastewater treatment processes (McAvoy et al., 2002; Sabaliunas et al., 2003; Winkler et al., 2007; Ying et al., 2009), triclosan cannot be removed efficiently, thus leading to the detection of its residue and derivatives in various environmental matrices, such as surface waters, sludge and sediments (Singer et al., 2002; Stackelberg et al., 2004; Weigel et al., 2004; Hua et al., 2005). As indicated by the results of a reconnaissance study by the United States Geological Survey, triclosan was confirmed to be one of the most frequently detected organic wastewater contaminants in 139 streams located in the USA, with the maximum concentration of 2.3 μg L−1 (Kolpin et al., 2002). Triclosan has exhibited acute and chronic toxicity to various aquatic organisms, including algae, photobacterium and fish, with the median effect concentration in the range of μg L−1 to mg L−1 (Adolfsson-Erici et al., 2002; Orvos et al., 2002). Moreover, Oliveira et al. (2009) found that triclosan could induce endocrine disruption for embryos and larvae of zebrafish, as indicated by the observations of hatching delay, teratogenic responses and mortality. Triclosan was also reported to bioaccumulate in kinds of aqueous species, e.g., algae, snails, fish and marine mammals (Lindstrom et al., 2002; Fernandes et al., 2011; Zhang et al., 2015), which might exhibit severe health effects along the food chain (Houtman et al., 2004; Coogan and La Point, 2008; Fair et al., 2009). In addition to the hazard of parent molecule, products of triclosan degradation were reported more toxic, which could be attributed to the generation of carcinogenic byproducts. Photodegradation of triclosan produces 2,8-dichlorodibenzo-p-dioxin, which is well recognized as a kind of carcinogen (Latch et al., 2005; Sanchez-Prado et al., 2006; Aranami and Readman, 2007; Wong-Wah-Chung et al., 2007). Biological methylation of triclosan generates more lipophilic and bioaccumulative methyl triclosan (Poiger et al., 2003). Moreover, triclosan reacting with free chlorine in wastewater and drinking water treatment would produce chloroform and chlorinated phenols, such as 2,4-dichlorophenol and 2,4,6-trichlorophenol (Rule et al., 2005), which are ranked by the US EPA as probable human carcinogens and listed on the Contaminant Candidate List (Song et al., 2012).
Considering the persistence and potential risk of triclosan in aquatic environment, it is necessary to develop efficient techniques to degrade it. It was reported that triclosan could not be effectively eliminated by traditional techniques, such as biofilm metabolism, chlorine disinfection, MnO2 oxidation and UV irradiation, and the toxic and carcinogenic byproducts, e.g., chloroform, chlorinated phenols and even dioxin-type intermediates were generated during the degradation process (Latch et al., 2003; Zhang and Huang, 2003; Rule et al., 2005; Ricart et al., 2010; Song et al., 2012). Recently, some advanced strategies, such as electrochemical (Martín de Vidales et al., 2013), sonochemical (Sanchez-Prado et al., 2008) and sonoelectrochemical (Ren et al., 2014) processes, have been developed to degrade triclosan, whereas high energy cost inevitably limits their factual application. In addition, Fenton-like techniques have been confirmed to efficiently deplete triclosan, while formation of ferric sludge and carcinogenic byproducts (e.g., 2,4-dichlorophenol) would still be problematic issues (Munoz et al., 2012; Song et al., 2012).
Iron(III)-tetraamidomacrocyclic ligand (Fe(III)-TAML) activators are regarded as functional analog of peroxidase enzyme (Sen Gupta et al., 2002). This kind of activators can catalyze hydrogen peroxide (H2O2) to oxidize substrates efficiently (Su et al., 2018). When Fe(III)-TAML reacts with H2O2, a high valence iron-oxo complex is formed and restores itself to Fe(III)-TAML while oxidizing substrates (Su et al., 2018). The reaction of Fe(III)-TAML/H2O2 system and substrates is well-known to be pH-dependent (Popescu et al., 2008; Wang et al., 2017a). Because the proportion of ionized Fe(III)-TAML species (i.e., [Fe(III)-TAML(OH)]2-) increases as pH increases. The [Fe(III)-TAML(OH)]2- form was reported to exhibit greater potential to be oxidized by H2O2 than [Fe(III)-TAML(OH2)]-, indicating that active Fe-TAML (e.g., Fe(IV)-TAML or Fe(V)-TAML) was more facile to be obtained at higher pH levels (Popescu et al., 2008; Wang et al., 2017a). Therefore, the reactivity of Fe(III)-TAML is enhanced dramatically with increasing pH. Fe(III)-TAML/H2O2 system can degrade varieties of contaminants such as phenols (Sen Gupta et al., 2002; Kundu et al., 2015; Wang et al., 2017a), dyes (Chahbane et al., 2007; Ghosh et al., 2008; Ellis et al., 2009), estrogens (Shappell et al., 2008; Chen et al., 2012; Onundi et al., 2017), organophosphorus pesticides (Chanda et al., 2006), molluscicides (Tang et al., 2016), pharmaceutical ingredients (Shen et al., 2011) and explosives (Kundu et al., 2013). The complete removal of pollutants can be obtained within few seconds at optimal conditions, in which trace concentration of Fe(III)-TAML is utilized (Wang et al., 2017a). Moreover, the previous study showed that the toxicity of contaminated water dramatically decreased after Fe(III)-TAML/H2O2 treatment, meanwhile no adverse effects on fish or microorganism were observed with the residual Fe(III)-TAML (Ellis et al., 2010). Therefore, Fe(III)-TAML/H2O2 exhibits great potential to accomplish high degradation efficiency for triclosan.
Natural organic matter (NOM) exists widely in water environment (Matilainen and Sillanpaa, 2010). When applying techniques to remove pollutants in water, we should take NOM into consideration, as it has tendency to interfere the process. Lado Ribeiro et al. (2019) have summarized the influence of organic matter in some different advanced oxidation technologies. For photolysis, photocatalysis, H2O2- and Fenton-based treatment, organic matter can cause both promotion and inhibition; while for O3- based water treatment methods, organic matter can only inhibit the procedure. However, for Fe(III)-TAML/H2O2 system, there has been no research about the influence of NOM. The objective of this study is to investigate the degradation efficiency of triclosan by Fe(III)-TAML/H2O2 system and the effects of pH and NOM on the reaction rates. Moreover, the underlying reaction mechanism and degradation pathway are elucidated based on the results of mass spectroscopy and theoretical calculation. Furthermore, the acute toxicity is also evaluated during degradation process.
Section snippets
Materials
Triclosan, sodium hydroxide (NaOH), perchloric acid (HClO4), sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), sodium sulfite (Na2SO3), sodium chloride (NaCl), HPLC grade methanol and acetonitrile purchased from Sigma-Aldrich. H2O2 (30%, w/w), prototype Fe(III)-TAML (Fig. 1) and NOM (i.e., humic acid (HA) and fulvic acid (FA)) were purchased from Fisher Scientific, GreenOx Catalysts (Mellon Institute,
Degradation of triclosan by Fe(III)-TAML/H2O2 system at different pHs
As shown in Fig. S1, when only adding Fe(III)-TAML or H2O2 to triclosan solution, the concentration of triclosan almost did not decrease in 10 min. That means if triclosan could decrease with both Fe(III)-TAML and H2O2, it is because of the cooperation between the two substances. As shown in Fig. 2, the degradation of triclosan by Fe(III)-TAML/H2O2 system exhibited strong pH-dependence, and the reaction rate increased with the increase in pH level. Almost complete degradation of triclosan by
Conclusions
In this study, we investigated the degradation of triclosan by Fe(III)-TAML/H2O2 system and the effects of pH and NOM. Our results showed that the degradation of triclosan by Fe(III)-TAML/H2O2 was highly pH dependent with the highest reaction rate at pH 10.0. In addition, the presence of NOM showed inhibition effects for the degradation of triclosan, and HA exhibited greater inhibition than FA at pH 8.0 and 9.0. However, the inhibition effect became insignificant when pH increased to 10.0,
Author statement
Sijia Liang: Methodology, Data curation, Writing-Reviewing and Editing, Zeyu Xian: Investigation, Haotian Yang: Investigation, Ziyu Wang: Investigation, Chao Wang: Conceptualization, Methodology, Data curation, Writing-Reviewing and Editing, Xiaoxia Shi: Investigation, Haoting Tian: Conceptualization, Writing-Reviewing
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Key Research and Development Plans of Special Project for Site Soil (No. 2018YFC1802003), the National Science Foundation of China (No. 21906079), the Collaborative Innovation Center for Regional Environmental Quality, and International Institute for Environmental Studies. We thank the Analytical Center of Nanjing University for the characterization of samples and computational study.
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