A low-energy solution for the removal of cyanobacteria and cyanotoxins: UV-A LED photolysis

Climate change and eutrophication contribute to proliferation of cyanobacteria and cyanotoxins in aquatic environments. Conventional treatments can be ineffective to remove high cell densities of cyanobacteria and dissolved toxins. Therefore, alternative mitigation strategies are required to effectively remove these contaminants at source. In a previous photocatalytic study (Menezes et al., 2021a), UV-A irradiation was used as a light control system, however, UV-A irradiation was shown to be effective in the removal of the cyanobacterium Microcystis aeruginosa PCC 7813 and four microcystins. In this study, a chemical-free water treatment approach using photoinduced photolysis was explored by using UV-A 365 nm LED irradiation, a low-energy wavelength near the visible light spectrum, for the effective removal of cyanobacteria and toxins. Six Microcystis aeruginosa strains (SCIENTO, NIES1099, B2666, PCC7820, 7813 and 7806) were exposed to UV-A 365 nm irradiation for seven days. Photosynthetic activity significantly decreased after 24 h, with samples showing little to no photosynthetic activity by the end of the investigation. Total microcystin concentrations (intra- and extracellular) decreased in UV-A treated samples with 86 % removal of combined microcystins. Notably, microcystins removal varied by analogue (94 % of MC-RR was removed compared to 9 % of MC-YR). Furthermore, nutrients in the cyanobacteria growth medium used in lab-scale experiments, such as nitrate and iron, enhanced the UV-A photolytic degradation of microcystins, reflecting the potential for environmental conditions to amplify treatment effects. This study represents the first proposal of a UV-A LED-based photolytic method for cyanobacterial and cyanotoxin removal, offering a transformative, low-energy and chemical-free water treatment.