Degradation Mechanisms of Microcystin-LR during UV-B Photolysis and UV-B/H2O2 Processes: By-products and Pathways

Abstract

In recent years, the increasing intensity and frequency of algal blooms in surface water has become a serious problem. The most common toxins produced by cyanobacteria are microcystins (MCs), which are reported to be cyclic heptapeptides. In particular, microcystin-LR (MC-LR) has been found to be one of the most abundant and toxic compounds. Therefore, the presence of MC-LR in water sources is of concern due to its direct threats to human health in finished water. In order to remove the MC-LR, the removals of MC-LR by chlorination, ozonation, and UV/AOPs have been widely reported, but only a little were investigated on the formation of their by-products during water treatment techniques. Therefore, the investigation of MC-LR by-products formed during treatment processes needed. In this study, removal and degradation pathways of MC-LR (m/z 995.6) were examined during UV-B photolysis and UV-B/H2O2 processes based on the identification of by-products using liquid chromatography–tandem mass spectrometry (LC-MS/MS). The UV-B/H2O2 process was more efficient than UV-B photolysis for MC-LR removal. While eight by-products were newly identified during UV-B photolysis (m/z 414.3, 417.3, 709.6, 428.9, 608.6, 847.5, 807.4, and 823.6), eleven by-products were newly confirmed during the UV-B/H2O2 process (m/z 707.4, 414.7, 429.3, 445.3, 608.6, 1052.0, 313.4, 823.6, 357.3, 245.2, and 805.7). Most MC-LR by-products had lower m/z than MC-LR during two processes. Based on the identified by-products and their peak area patterns, potential degradation mechanisms during the two processes were proposed. While bond cleavage and free electron pair transfer reactions containing nitrogen atoms were the major reactions during UV-B photolysis, the reaction with an OH radical and addition reaction with other by-products were identified as additional reaction pathways during the UV-B/H2O2 process.