Unveiling the transformation of dissolved organic matter during ozonation of municipal secondary effluent based on FT-ICR-MS and spectral analysis
Graphical abstract
Introduction
The utilization of reclaimed municipal wastewater has gained wide attention and application due to the shortage of water resources globally (Becerra-Castro et al., 2015; Jin et al., 2016; Michael-Kordatou et al., 2015; Pisani and Menge, 2013). In viewpoint of chemical composition, low amount of highly complex and heterogeneous dissolved effluent organic matter (dEfOM) is still residual in secondary effluent of municipal wastewater, mainly including dissolved natural organic matter (NOM), soluble microbial products, and micropollutants of concern (e.g., pharmaceuticals, personal care products, endocrine disrupting compounds, surfactants, etc.) (Chen et al., 2017; Fatta-Kassinos et al., 2011; Jin et al., 2016; Li et al., 2017; Michael-Kordatou et al., 2015). These compounds, especially for micropollutants, are usually refractory to traditional biological treatment and can possibly bring adverse effects (e.g., genotoxicity and estrogenic activity) (Tang et al., 2013; Zhang et al., 2019a) to ecosystem and human health. Therefore, advanced treatment of municipal wastewater is usually a requisite step to minimize the environmental risks before water reuse (Chen et al., 2017; Knopp et al., 2016; Michael-Kordatou et al., 2015).
In the past decades, ozonation has been extensively utilized as an advanced treatment option for secondary effluent to realize decoloration, disinfection, and elimination of micropollutants (Hollender et al., 2009; Huber et al., 2005; Knopp et al., 2016; Lee et al., 2013; Peter and Von Gunten, 2007). The reactions between ozone and organic compounds include their direct reaction as well as the indirect reactions associated with various reactive species (mainly hydroxyl radical (•OH)) produced from the decomposition of ozone in aqueous solution (Staehelin and Hoigne, 1982; von Gunten, 2003). In general, ozone reacts preferentially with electron-rich moieties (e.g., C = C double bonds and aromatic rings), while •OH reacts with most organic molecules (Bourgin et al., 2017; Li et al., 2017). For a given pollutant of concern, the oxidation kinetics, mechanisms, and products have been extensively investigated during ozonation (Lee et al., 2017; Sui et al., 2017; Tekle-Röttering et al., 2016a, 2016b; Willach et al., 2017). Comparatively, very limited studies are available focusing on the transformation of complex dEfOM during ozonation of secondary effluent, where spectroscopic and chromatographic methods (e.g., UV–vis and fluorescence spectroscopy, and size exclusion chromatography) are usually applied to characterize their overall structural variation (Chen et al., 2017; Jin et al., 2016). Although the changes of aromaticity, fluorescence, and molecular weight (MW) during ozonation could be qualitatively/quantitatively reflected based on the above methods, it still remains a challenge to understand the transformation of dEfOM during ozonation at molecular level.
In the past years, several studies investigated the transformation of NOM (These and Reemtsma, 2005) and dissolved organic matter (DOM) (Phungsai et al., 2018, 2019), including dEfOM (Phungsai et al., 2016), during ozonation on the basis of high-resolution mass spectrometry (MS) (e.g., time-of-flight MS and Orbitrap MS), indicating that the highly unsaturated compounds in NOM/DOM were preferentially decomposed during ozonation, and the newly formed compounds usually featured with higher O/C. Schollée et al. (2018) employed liquid chromatography coupled with Orbitrap MS and statistical analysis to reveal the linkages between potential parents and products during ozonation of micropollutants in wastewater. However, the hitherto acquired information has been basically focused on the transformation of overall DOM formulas or features (definition by m/z and retention time) without formulas. The complex reaction processes for different categories of compounds based on elemental composition (e.g., CHO, CHON, CHOS, and CHONS) in a complicated DOM system (e.g., secondary effluent) during ozonation are still not fully understood. Compared with the above MS techniques, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is a more powerful tool in analysis of complex DOM at molecular level (Marshall and Chen, 2015; Ohno et al., 2010). The increasing interest has been gained to use FT-ICR-MS to characterize the molecular composition of various environmental samples (Li et al., 2018; Maizel et al., 2017; Maizel and Remucal, 2017; Zhou et al., 2019), mostly focusing on deciphering the DOM composition of natural water bodies (Bai et al., 2017; Kellerman et al., 2018; Wagner et al., 2015) instead of industrial and municipal wastewater (Maizel and Remucal, 2017; Zhang et al., 2019b). In a previous study, it was reported that the number of molecular formulae identified by FT-ICR-MS was about three times than those identified by Orbitrap MS for the same sample (Hawkes et al., 2016). Therefore, it can be anticipated that more abundant molecular information of DOM in secondary effluents and ozonated effluents could be obtained using FT-ICR-MS analysis. Furthermore, FT-ICR-MS combined with statistical analysis should be a promising tool to comprehensively investigate the transformation processes during ozonation of dEfOM.
Herein, we collected secondary effluent samples from two municipal wastewater treatment plants (WWTPs) and carried out ozonation treatments at four ozone dosage levels. The main objective of this study was to reveal the transformation of dEfOM during ozonation based on FT-ICR-MS analysis. Several spectroscopic tools were also used to characterize the changes of dEfOM and to establish the relationships between the spectral properties and the molecular-level parameters based on correlation analyses. Further, identification of the number and elemental composition of possible precursor-product pairs during ozonation was conducted based on the mass difference analysis between precursors and products to reveal the dominancy rank of common reactions of different compound categories in such a complex system. We believe this study will provide new molecular insights into the transformation of dEfOM during ozonation of the municipal secondary effluent.
Section snippets
Sampling and ozonation experiments
Wastewater samples were collected from two secondary clarifiers of Jiangxinzhou and Tiebei municipal WWTPs respectively, both of which are located in Nanjing, China, on the basis of the same main scheme (primary clarifier-anaerobic/anoxic/oxic (A2/O)-secondary clarifier-UV disinfection). The samples were filtered through 0.45 μm mixed cellulose membranes (Jinteng, China) and stored in brown glass bottles at 4 °C for experiments. Ozonation experiments were conducted in a 1-L glass cylindrical
Overall changes in dEfOM during ozonation
As shown in Table 1, DOC concentrations reduced slightly (8.9% for WWTP-A and 9.1% for WWTP-B) even at the highest ozone dosage of 1.21 mg O3/mg DOC, implying the challenge of dEfOM mineralization during ozonation (Knopp et al., 2016; Michael-Kordatou et al., 2015; Zimmermann et al., 2011). SUVA254 as a surrogate measurement for aromaticity (Weishaar et al., 2003) was decreased for both WWTP samples with the increasing ozone dosage (Table 1), suggesting the DOM chromophores (e.g., electron-rich
Conclusions
In general, this study systemically unveiled the transformation of complex dEfOM during ozonation of municipal secondary effluent based on FT-ICR-MS and spectral analysis. The fluorescent compounds with removal efficiencies of 45.6–100% were more readily decomposed than the aromatic ones with reduction of 35.5–63.2%. The relative abundance of CHO class was gradually increased with the increasing ozone dosage, whereas an opposite trend was observed for CHOS class. The unsaturated and reduced
Declaration of Competing Interest
The authors declare that they have no known competing financial interestss or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was financially supported from Natural Science Foundation of China (Grant No. 21925602/51761165011) and National Key Research and Development Program of China (Grant No. 2016YFA0203104).
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