The impact of preozonation on the coagulation of cellular organic matter produced by Microcystis aeruginosa and its toxin degradation
Graphical abstract
Introduction
Due to the anthropogenically intensified eutrophication of water sources, many drinking water treatment utilities worldwide currently face the development of seasonal algal blooms. While high removal efficiencies (> 90%) of algal and cyanobacterial cells can be achieved through conventional treatment processes based on coagulation, the removal of associated dissolved algal organic matter (AOM) is more challenging (Baresova et al., 2017; Henderson et al., 2010; Pivokonsky et al., 2016; Zhang et al., 2012). In addition to the extracellular organic matter (EOM) produced during the algal proliferation period and affecting cell separation itself (Henderson et al., 2010; Paralkar and Edzwald, 1996; Pranowo et al., 2013), considerable amounts of cellular organic matter (COM) are released and significantly affect water quality in the entire water column when algal bloom decay occurs accompanied by a breakdown of cells (Pivokonsky et al., 2016). The overall removal of dissolved AOM (either EOM or COM) by optimized coagulation ranges up to only 50% (Baresova et al., 2017; Henderson et al., 2010; Pivokonsky et al., 2009; Widrig et al., 1996). Coagulation is generally considered to have limited effectiveness, particularly for the removal of low-molecular-weight (MW) compounds, including harmful cyanobacterial toxins (Al Momani et al., 2008; Himberg et al., 1989; Onstad et al., 2007; Shang et al., 2018; Sharma et al., 2012; Westrick et al., 2010); hence, the application of additional treatment processes is often required. Chemical oxidation has previously been reported to be a promising treatment for cyanotoxin elimination (Sharma et al., 2012; Westrick et al., 2010), and among the commonly used oxidants, ozone was found to be most effective (Newcombe and Nicholson, 2004; Rodríguez et al., 2007; Sharma et al., 2012).
Ozonation has also been proposed to enhance the coagulation of algae-laden waters. Most studies have focused on the impact of ozonation on the removal of algal and cyanobacterial cells, changes in cell integrity, the release of intracellular matter including cyanotoxins and their possible degradation (Coral et al., 2013; Miao and Tao, 2009; Plummer and Edzwald, 2002; Pranowo et al., 2013; von Gunten, 2003; Wen et al., 2017). The improvements in cell coagulation resulting from preozonation were usually ascribed to cell inactivation and destabilization, changes in external cell architecture and the release of high-MW AOM that contributes to cell coagulation (Plummer and Edzwald, 2002; Pranowo et al., 2013; von Gunten, 2003). On the other hand, excessive preozonation may alter cell permeability and induce toxin and internal cellular content release preceding actual cell lysis (Coral et al., 2013; Miao and Tao, 2009; Plummer and Edzwald, 2002; Xie et al., 2013). For instance, Coral et al. (2013) and Xie et al. (2013) observed an immediate loss of cell integrity of M. aeruginosa and Anabaena flos-aquae after exposure to an ozone dose as low as 0.4–0.5 mg/L. In contrast, studies on the ozonation of dissolved AOM in the absence of algal cells are very scarce and have mostly dealt with the ozonation of EOM released during the growth of algae (Paralkar and Edzwald, 1996; Wei et al., 2016; Widrig et al., 1996). Previous results showed that ozone induces the degradation of high-MW compounds to medium- and low-MW components (Paralkar and Edzwald, 1996; Wei et al., 2016), which is likely to be detrimental to subsequent coagulation (Pivokonsky et al., 2016). On the other hand, Widrig et al. (1996) observed a slight increase in the coagulation efficiency of EOM, up to 5%–15%, depending on the phytoplankton species and reaction conditions. This effect was ascribed to the chemical transformation of EOM components. However, the study reported only limited optimization of reaction conditions: the application of very high coagulant doses (up to 0.5 mmol Al/Fe per mg DOC) was considered, and only one ozone dose (0.8 mg O3/mg DOC) and only two ozonation and coagulation pH values (both pH 5 and 8) were tested.
As already mentioned, ozonation is capable of efficiently destroying dissolved cyanotoxins (Al Momani et al., 2008; Rodríguez et al., 2007; Rositano et al., 1998). However, some investigations revealed an inhibition of toxin degradation in the presence of other organic compounds (Miao and Tao, 2009; Rodríguez et al., 2007; Rositano et al., 1998, 2001). For example, at a fixed initial concentration of microcystins (MCs), specifically MC-RR congener, the residual MC-RR increased from 10% to nearly 30% with increasing DOC of humic acids from 1 to 4 mg/L (Miao and Tao, 2009). In the same study, a decrease in MC-RR decomposition was also observed in samples containing algal supernatant (20% residual MC-RR, in contrast to complete toxin degradation in the absence of algal material). Rositano et al. (1998) reported similar findings in the effective ozonation of MC-LR and also indicated the role of competing ozone reactions between toxins and organic matter, in this case, an extract of M. aeruginosa. However, toxin degradation in the presence of a natural AOM matrix, as well as AOM/COM ozonation itself, has not been systematically evaluated.
The current study thus aimed to provide insights into the effects of ozone oxidation on dissolved COM derived from cyanobacteria M. aeruginosa, its subsequent coagulation, and the degradation of toxins naturally contained in the COM. To achieve this, a detailed optimization of reaction conditions was undertaken by testing various ozone and coagulant doses and ozonation and coagulation pH values. The specific objectives were (1) to evaluate changes in the COM concentration and MW distribution after ozonation, (2) to assess the influence of preozonation on the coagulation of COM by Al- and Fe-based hydrolysing coagulants, and (3) to evaluate the microcystin content after ozonation, coagulation, and preozonation-coagulation processes. The findings improve our understanding of ozonation mechanisms and the impact of preozonation on the characteristics and coagulation of COM, which has implications for algae-laden water treatment.
Section snippets
Cultivation of Microcystis aeruginosa and COM preparation
M. aeruginosa (strain Zap. 2006/2), a toxin-producing cyanobacterium that commonly occupies water reservoirs worldwide (Pivokonsky et al., 2016), was obtained from the Department of Culture Collection of the Algal Laboratory, Institute of Botany CAS (Czech Republic). M. aeruginosa was cultivated as described previously (Pivokonsky et al., 2009) and harvested at the late stationary growth phase (20th day of cultivation) when cell counts reached 107 cells/mL. Cells were separated from the culture
Ozonation of M. aeruginosa COM
COM rapidly consumed significant amounts of ozone and accelerated ozone decomposition, especially under alkaline pH conditions, as derived from the ozone decay curves in the presence and absence of COM (Appendix A Section S1, Fig. S1). However, only a slight decrease (up to 7%) in COM from the initial DOC concentration of 10 mg/L was observed for applied ozone doses (0.1–1.0 mg O3/mg DOC). As shown in Fig. 1, the DOC reduction was more significant at higher ozone doses and higher pH values.
Conclusion
Investigating the effects of ozonation on dissolved COM from M. aeruginosa and its coagulation has shown the following: (1) A slight DOC decrease was observed after COM ozonation; however, ozonation led to the decomposition of high-MW to low-MW fractions, and the effect was most pronounced at higher ozone doses and acidic pH. (2) Preozonation decreased the coagulation efficiency, and the effect was more noticeable with increasing ozone dosage and decreasing ozonation pH; Fe-based coagulation
Acknowledgments
This work was supported by the Czech Science Foundation (No. GA18-14445S) and by the institutional support of the Czech Academy of Sciences (RVO: 67985874).
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