Degradation of cyanotoxin microcystin-LR in synthetic and natural waters by chemical-free UV/VUV radiation

[Display omitted] •MC-LR can be substantially degraded solely by 254 nm direct photolysis.•Combining 185 nm and 254 nm can achieve even greater degradation rates.•Despite complex photochemical reactions, chloride has low impact on degradation.•Alkalinity has low impact on MC-LR degradation rates in the presence of organics.•90% degradation could be achieved in unfiltered bloom water at severe cell densities. This study investigated the capability of ultraviolet radiation at 254 nm and 185 nm (UV/VUV) to degrade cyanotoxin microcystin-LR (MC-LR). Results showed 70% toxin reduction solely by 254 nm direct photolysis (ε254 = 13,225 ± 814 M−1cm−1; Φ254 = 0.29 ± 0.03 mol/Einstein). The addition of 185 nm increased MC-LR degradation through advanced oxidation by •OH (k•OH,MC-LR = 2.25 ± 0.39 × 1010 M−1s−1). Alkalinity and organics (DOC) reduced MC-LR degradation by scavenging •OH (kobs,MilliQ = 0.117 cm2/mJ; kobs,50ppmAlk. = 0.0497 cm2/mJ; kobs,6ppmDOC = 0.019 cm2/mJ). Chloride absorbed 185 nm, impacting •OH formation and generating Cl•, while also scavenging •OH. However, Cl• is reactive and •OH scavenging is reversible, resulting in relatively low impact on MC-LR degradation (kobs,50ppmCl = 0.0939 cm2/mJ). In natural water, MC-LR could be degraded from a typical concentration (˜15 μg/L) to below detection (