Degradation Mechanism of Cyanobacterial Toxin Cylindrospermopsin by Hydroxyl Radicals in Homogeneous UV/H 2 O 2 Process Xuexiang He, †,∥ Geshan Zhang, † Armah A. de la Cruz, ‡ Kevin E. O’Shea, § and Dionysios D. Dionysiou* ,†,∥ † Environmental Engineering and Science Program, University of Cincinnati, Cincinnati, Ohio 45221-0012, United States ‡ Office of Research and Development, U.S. Environmental Protection Agency, Cincinnati, Ohio 45268, United States § Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, United States ∥ NIREAS-International Water Research Centre, University of Cyprus, Nicosia, 1678, Cyprus * S Supporting Information ABSTRACT: The degradation of cylindrospermopsin (CYN), a widely distributed and highly toxic cyanobacterial toxin (cyanotoxin), remains poorly elucidated. In this study, the mechanism of CYN destruction by UV-254 nm/H 2 O 2 advanced oxidation process (AOP) was investigated by mass spectrometry. Various byproducts identified indicated three common reaction pathways: hydroxyl addition (+16 Da), alcoholic oxidation or dehydrogenation (−2 Da), and elimination of sulfate (−80 Da). The initiation of the degradation was observed at the hydroxymethyl uracil and tricyclic guanidine groups; uracil moiety cleavage/fragmentation and further ring-opening of the alkaloid were also noted at an extended reaction time or higher UV fluence. The degradation rates of CYN decreased and less byproducts (species) were detected using natural water matrices; however, CYN was effectively eliminated under extended UV irradiation. This study demonstrates the efficiency of CYN degradation and provides a better understanding of the mechanism of CYN degradation by hydroxyl radical, a reactive oxygen species that can be generated by most AOPs and is present in natural water environment. ■ INTRODUCTION The increasing prevalence of severe cyanobacterial harmful algal blooms in sources of drinking water supply and reservoirs has become a worldwide concern. 1−4 Along with microcystins (MCs), the most frequently detected and studied cyanobacte- rial toxins (cyanotoxins), cylindrospermopsin (CYN), has emerged as a significant threat. 1,4,5 The high cytotoxicity and genotoxicity of CYN pose a major concern to human and ecosystem health. 4,6−9 No regulations have been established against this toxin, although a guideline value of 1 μg/L has been proposed. 10 MC-producing algae form densely colored blooms, while common CYN-producing cyanobacteria seldom produce visible scums preventing detection that a harmful cyanobacterial bloom is occurring. 3,5,8 CYN-producing cyanobacteria are highly adaptable and invasive and have been detected in tropical, subtropical as well as in temperate areas. 5 Conventional water treatment processes (e.g., coagulation, flocculation and filtration) may not effectively remove and degrade cyanotoxins, especially during a bloom event. Several oxidizing agents have also been evaluated to inactivate CYN. Common oxidants/disinfectants (e.g., Cl 2 , ClO 2 , and NH 2 Cl) show varying degrees of toxin degradation; long contact times and high doses are required potentially generating disinfection byproducts that are suspected carcinogens. 4,11 Ozone has been shown to be highly effective in CYN removal. 12 Ozone, however, is highly unstable, produces formaldehyde and can convert bromide in source waters into bromate; both compounds are known carcinogens. 13 Physical removal of CYN by powdered activated carbon (PAC) shows modest removal and toxin residuals in PAC remain a concern. The presence of natural organic matter (NOM) in the source water typically reduces the effectiveness of the PAC process and results in higher treatment costs. 14 Efficient and practical processes to remove and degrade CYN in water need to be evaluated. A number of technologies have been investigated to remove and/or degrade cyanotoxins with advanced oxidation processes (AOPs) being among the most promising and effective for water detoxification. 4,15 AOPs are chemical treatments involving the generation of a very powerful oxidant, hydroxyl radical, that can oxidize a wide range of chemicals in water, wastewater, air, and soil. In this study, hydroxyl radical species were generated by UV- 254 nm irradiation of H 2 O 2 (UV/H 2 O 2 ) to degrade CYN in Received: August 21, 2013 Revised: March 3, 2014 Accepted: March 13, 2014 Published: March 13, 2014 Article pubs.acs.org/est © 2014 American Chemical Society 4495 dx.doi.org/10.1021/es403732s | Environ. Sci. Technol. 2014, 48, 4495−4504