MICROCYSTIN LR AND LA, SIMILARITIES AND DIFFERENCES IN RESPONSE TO WATER TREATMENT PROCESSES

摘要

Over seventy variants of the blue-green algal toxin microcystin have been identified. Thetwo microcystin variants LR and LA vary in only one amino group ie.arginine for microcystin LR (m-LR) and alanine for microcystin LA (m-LA). In the literature to date, the general consensus has beenthat m-LR and m-LA should respond similarly to a range of water treatment processes. This is the casefor ozonation and biodegradation by organisms colonising granular activated carbon filters; there isnegligible difference in the response to these processes between the two variants. However, theadsorption of m-LR onto activated carbon is significantly higher than that of m-LA. This result issurprising as m-LA has a lower molecular weight, and is more hydrophobic, factors that would beexpected to favour the adsorption of this compound over m-LR. The effect is seen on both negativelyand positively charged carbons, indicating that the difference between the variants is not caused byelectrostatic interactions with the carbon surface. It is possible that electrostatic repulsion between theadsorbed m-LA molecules, with a net charge of -2, is responsible for the low adsorption. MicrocystinLR has a net charge of -1, and would therefore experience lower intermolecular repulsion in theadsorbed state.