Photocatalytic Degradation of Microcystin-LR and Off-Odor Compounds in Water under UV‑A and Solar Light with a Nanostructured Photocatalyst Based on Reduced Graphene Oxide–TiO2 Composite. Identification of Intermediate Products

Microcystin-LR (MC-LR) is the most common and toxic variant of the group of microcystins (MCs) produced during the formation of harmful cyanobacterial blooms. Geosmin (GSM) and 2-methylisoborneol (MIB) may also be produced during cyanobacterial blooms and can taint water causing undesirable taste and odor. The photocatalytic degradation of MC-LR, GSM, and MIB in water under both UV-A and solar light in the presence of reduced graphene oxide–TiO2 composite (GO–TiO2) was studied. Two commercially available TiO2 materials (Degussa P25 and Kronos) and a reference TiO2 material prepared in the laboratory (ref-TiO2) were used for comparison. Under UV-A irradiation, Degussa P25 was the most efficient photocatalyst for the degradation of all target analytes followed by GO–TiO2, ref-TiO2, and Kronos. Under solar light irradiation GO–TiO2 presented similar photocatalytic activity to Degussa P25, followed by Kronos and ref-TiO2 which were less efficient. Intermediate products formed during the photocatalytic process with GO–TiO2 under solar light were identified and were found to be almost identical to those observed by Degussa P25/UV-A. Assessment of the residual toxicity of MC-LR during the course of treatment with GO–TiO2 showed that toxicity is proportional only to the remaining MC-LR concentration. The photocatalytic performance of GO–TiO2 was also evaluated under solar light illumination in real surface water samples, and GO–TiO2 proved to be effective in the degradation of all target compounds.