Mechanisms underlying degradation pathways of microcystin-LR with doped TiO2 photocatalysis

The presence of cyanotoxins in different water sources, originating from cyanobacterial algal blooms, is a ubiquitous, global concern. Microcystins (MCs) are the most commonly occurring class of cyanbacterial toxin, and are characterized as cyclic heptapeptides containing five D-amino acids and two variable L-amino acids. Over 100 MC variants exist, and microcystin-LR (MC-LR) is one of the most hazardous and prevalent. Removing MC-LR from aqueous solutions by titanium dioxide (TiO2) photocatalysis has been proposed as a possible solution for this. Both metal and non-metal doped TiO2 photocatalysts have been investigated over recent decades for their MC-LR photocatalytic degradation properties under visible light. This review provides an overview of visible light-activated TiO2 synthesis technologies, and structural and electronical properties. The photocatalytic behavior of visible light-activated TiO2 for the photodegradation of MC-LR is discussed. The structural elucidation of reaction intermediates of the degradation pathways, and the mechanisms of MC-LR degradation by visible-light activated TiO2 in comparison with conventional UV-activated TiO2, is given particular emphasis. We also aimed to identify research gaps and uncertainties that exist for the use of visible light-activated TiO2 in photocatalysis. Additional research requirements for visible light-activated TiO2 photocatalysts and potential areas for future applications of doped TiO2 photocatalysts under visible light are also proposed.