Controlling the cyanobacterium microcystis aeruginosa and the cyanotoxin microcystin by (advanced) oxidation processes

Abstract

Reservoirs are used in much of northeastern Brazil for human supply, but they are commonly eutrophic. Eutrophication degrades the quality of fresh water and supports the flowering of cyanobacteria potentially producing cyanotoxins that can be released from cell lysis. In this context, alternative water treatment processes that produce reactive oxygen species and that can be applied in-situ were used to evaluate the removal of the cyanobacterium Microcystis aeruginosa PCC7813 and four microcystin variants (MC-LR, MC-LY, MC -LY and MC-LF). In addition, different methods of detecting stress in cyanobacterial cells were compared. Initially, hydrogen peroxide was applied to remove Microcystis aeruginosa PCC7813 and study cell stress. In addition, cell density, photosynthetic activity, chlorophyll concentration and microcystin concentration were used as different methodologies for detecting cell stress. In that study, in addition to the effective removal of Microcystis aeruginosa PCC7813 and microcystin cells, photosynthetic activity indicated cyanobacterial cell stress in just 6 hours, which was considerably faster than the other methods. In a second study, photolysis and heterogeneous photocatalysis were compared using small reusable spheres made of recycled glass coated with thin layers of titanium dioxide and light emitting diode (LED) lighting containing UV light (365 nm) in a pilot scale reactor for removal of Microcystis aeruginosa PCC7813 and four microcystin variants (MC-LR, MC-LY, MC-LY and MC-LF). During photolysis, cell density decreased significantly over 5 days until few cells remained in the sample from an initial concentration of 5.8 x 10 6 cells mL-1. Both intra- and extracellular microcystin concentrations were significantly reduced by 100 and 92%, respectively, on day 5 of UV treatment for all microcystin variants. In addition, UV-A photolysis has been shown to have long-term effects on M. aeruginosa PCC7813 cells, inhibiting regrowth for at least six days after treatment. During heterogeneous photocatalysis, there was great variability between repetitions, making it difficult to predict the behavior of cells and toxins.