Revealing the degradation intermediates and pathways of visible light-induced NF-TiO2 photocatalysis of microcystin-LR

• Nine products identified during visible light-activated NF-TiO2 photocatalysis.
• The observed intermediates suggest a significant role of hydroxyl radical.
• The proposed degradation pathways remove the toxic portion of MC-LR.

The effect of using only visible light to induce nitrogen- and fluorine-doped titanium dioxide (NF-TiO2) photocatalysis of degradation products of microcystin-LR (MC-LR), the most common problematic cyanotoxin, was explored by looking at the intermediate degradation products. Although the degradation mechanisms and products of conventional UV-based TiO2 photocatalysis of MC-LR have been well elucidated, the same is not true for visible light-based TiO2 photocatalysis. The results of LC/MS2 (and in one case LC/MS3) indicated that the intermediates are not drastically altered in comparison to traditional TiO2 photocatalysis using UV light. The data hint that the degradation is driven by hydroxyl radicals, as is UV-based TiO2 photocatalysis, although the mechanism for producing hydroxyl radicals is unclear since studies indicate drastically slower kinetics for visible light-based photocatalysis of MC-LR. Notably, the data indicate that visible light-induced NF-TiO2 photocatalysis degraded the portion of MC-LR that is responsible for biological toxicity. As a result of this, it was concluded that doping TiO2 with nitrogen and fluorine is an effective method for increasing utilization of visible light while degrading MC-LR in water, although it should still be noted that degradation kinetics are still slower than UV-based TiO2 photocatalysis.

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