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

Abstract

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 (k_obs,MilliQ = 0.117 cm²/mJ; k_{obs,50ppmAlk.} = 0.0497 cm²/mJ; k_obs,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 (k_obs,50ppmCl = 0.0939 cm²/mJ). In natural water, MC-LR could be degraded from a typical concentration (˜15 μg/L) to below detection (<0.5 μg/L) with a UV₂₅₄ fluence of 200 mJ/cm² using UV/VUV. The presence of cyanobacterial cells impeded MC-LR degradation; however, 90% MC-LR degradation could still be achieved. UV/VUV is a promising chemical-free technology capable of MC-LR degradation in a variety of water conditions, and a potentially suitable treatment option for small, remote communities.

Keywords: Advanced oxidation; Cyanotoxin; Microcystin-LR; Photolysis; Ultraviolet.