Transformation of 17β-estradiol in humic acid solution by ε-MnO2 nanorods as probed by high-resolution mass spectrometry combined with 13C labeling☆
Graphical abstract
Introduction
Over the past decades, the occurrence of steroidal estrogens (SEs) in environments has raised a wide public concern for their disruptions to endocrine systems of organisms (Auriol et al., 2007, Lloret et al., 2013, Miller et al., 2001, Sumpter and Johnson, 2005). SEs enter aquatic environment mainly through discharges of municipal and industrial wastewater (Fan et al., 2013). Although wastewater treatment plants are able to remove high levels of SEs, considerable concentrations of residuals still remained in the effluents (D'ascenzo et al., 2003, Kuch and Ballschmiter, 2001, Le Noir et al., 2007). Recently, continuous increase in the level of SEs has been detected in rivers, lakes, drinking water, and groundwater, which results in serious physiological consequences of some aquatic lives (Benotti et al., 2008, Diamanti-Kandarakis et al., 2009, Snyder et al., 2003). Hence an effective method is in need to remove SEs in aquatic systems.
Advanced oxidation processes (AOPs) by chlorine, ozone and ferrate, etc. are widely applied for the removal of SEs from aquatic systems (Deborde and von Gunten, 2008, Irmak et al., 2005, Okamoto et al., 2006). The effectiveness of these oxidation processes are however limited when SEs are present at extremely low concentrations. Recent works have focused on the development of enzymatic catalysis for SEs removal (Auriol et al., 2007 and Mao et al., 2008). Enzyme-catalyzed oxidative coupling reactions (ECOCRs) can serve as an innovative approach for eliminating SEs via a covalent binding mechanism, resulting in a rapid decrease of its estrogenic activity (Mao et al., 2010). ECOCRs may however suffer some limitations, such as the instability of enzyme-SEs complex and the high cost, which limits their practical applications (Mai et al., 2000).
Manganese oxides have been considered as promising materials for the removal of phenolic contaminants because of their stability, high reactivity, environmental compatibility, and cost effectiveness (Forrez et al., 2009, Lin et al., 2009, Saputra et al., 2013). For instance, Xu et al. (2008) demonstrated that δ-MnO2 acted as a promising oxidizing agent for removal of SEs from water under some optimized conditions. Recently, MnO2 nanomaterials have been used to enhance oxidation and/or catalysis (Dong et al., 2009 and Han et al., 2015). Those nanomaterials exist in different crystalline structures, such as α-, β-, γ-, δ- and ε-MnO2, and show varying efficiencies in the removal of phenolic contaminants (Wang et al., 2012). For instance, the α-MnO2 nanowires, β-MnO2 nanowires and γ-MnO2 nanofibers differed in phenol removal efficiency, which is depended on their surface areas and crystalline structures (Saputra et al., 2013). Han et al. (2015) demonstrated that the effectiveness of the stabilized MnO2 nanoparticles (using carboxymethyl celluloses as a stabilizer) for removing aqueous and soil-sorbed estradiol. However, little information is available on the performance of ε-MnO2 nanomaterials in transforming SEs in the aqueous solution.
In natural water, the dissolved natural organic matter (NOM) may interfere with the transformation of phenolic contaminants (Dec et al., 2003 and Sun et al., 2013). The NOM may suppress the removal of phenolic contaminants by acting as an antioxidant and binding agent to phenolic residues (Bialk et al., 2005 and Li et al., 2012). The NOM-bound phenolic residues might still preserve the aromatic rings that could undergo the cross-coupling reactions with NOM via the formation of free phenoxyl radicals (Li et al., 2012 and Mao et al., 2010). Currently, there is a lack of a comprehensive understanding of the mechanistic role of NOM in the transformation of phenolic contaminants by ε-MnO2 nanomaterials.
The primary objective of this study is to explore the role of NOM in the removal and transformation of SEs with ε-MnO2 nanomaterials. To achieve the goal, a series of experiments were conducted using ε-MnO2 nanorods as the oxidizing agent, with 17β-estradiol (E2) as the model SEs, and humic acid (HA) as the model NOM. In particular, a mixture of un-labeled E2 with 13C3-labeled E2 at a 1: 1 set ratio (w/w) was used to probe both known and unknown products by high-resolution mass spectrometry (HRMS). The high m/z accuracy of HRMS enabled the use of the isotope ratio as a tracer to identify possible cross-coupling products between E2 and HA. The product identification with the use of HRMS and 13C3-labeling is crucial for understanding the role of HA in the transformation of SEs.
Section snippets
Chemicals and materials
17β-estradiol (E2, CAS 50-28-2) and 17β-estradiol-2,3,4-13C3 (13C3-E2, CAS 1261254-48-1) with high purities (≥98%) were purchased from Sigma-Aldrich Chemical Co. The molecular structure of E2 and the location of 13C3 labeling are shown in Fig. S1 (Supplementary Information). A commercial humic acid (HA, CAS 1415-93-6) with technical grade was also supplied by Sigma-Aldrich Chemical Co. Manganese (IV) oxide nanorods (product ID 8005NJ, diameters 5–30 nm, lengths 80–100 nm, and epsilon order were
Performance of ε-MnO2 nanorods in removing E2
The pH value may play an important role in removal of SEs by ε-MnO2 nanorods. As shown in Fig. 1a, E2 removal ratio after 3 h incubation significantly increased from 0.4 to 97.0% with a decreasing pH over the tested range of pH 10.0–3.0, likely due to stronger redox potential of ε-MnO2 at acidic conditions. As noted earlier, H+ ions could participate in the oxidation reaction at acidic conditions (ε-MnO2 + 4H+ + 2e− → Mn2+ + 2H2O), which dramatically enhanced the oxidation potential of ε-MnO2 (
Conclusions
Many SEs originating from municipal wastewater and industrial discharge are present in the aquatic environment. The rapid removal of E2 by ε-MnO2 nanomaterials demonstrated a high potential of the process for dissipation or remediation of SEs in contaminated wastewater or polluted environmental matrices, and the E2 removal efficiency depended strongly on the pH of the reaction solution and availability of oxide surface sites for reaction. Notably, the presence of HA hindered the removal of E2
Acknowledgements
We thank Dr. Chau-wen Chou from Proteomic and Mass Spectrometry Core Facility, University of Georgia, for helping with the HRMS analysis and Mr. Yang Yue for his involvement during the initial phase of this study. Kai Sun acknowledges China Scholar Council for supporting his work at UGA. YG thanks the support from Jiangsu Provincial Funds for Distinguished Young Scientists, China (BK20130030), and the Special Fund for Agro-scientific Research in Public Interest, China (201503107).
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This paper has been recommended for acceptance by B. Nowack.