Over sixty variants of the blue-green algal toxin microcystin have been identified. The two microcystin variants LR and LA vary in only one amino group ie. arginine for microcystin LR and alanine for microcystin LA. In the literature to date, the general consensus has been that m-LR and m-LA should respond similarly to a range of water treatment processes. This is the case for ozonation and biodegradation by organisms colonising granular activated carbon filters; there is negligible difference in the response to these processes between the two variants. However, the adsorption of m-LR onto activated carbon is significantly higher than that of m-LA. This result is surprising as m-LA has a lower molecular weight, and is more hydrophobic, factors that would be expected to favour the adsorption of this compound over m-LR. This trend is also seen for the variants RR and YR. The effect is seen on both negatively and positively charged carbons, indicating that the difference between the variants is not caused by electrostatic interactions with the carbon surface. Electrostatic shielding experiments suggest that electrostatic repulsion between the adsorbed m-LA molecules, with a net charge of -2, may be responsible for the low adsorption. The other variants tested have a lower net charge and therefor experience lower intermolecular repulsion in the adsorbed state.
The cyanobacterium Anabaena circinalis has the ability to co-produce geosmin and saxitoxins, compounds which can compromise the quality of drinking water. This study provides pertinent information in optimising water treatment practices for the removal of geosmin and saxitoxins.In particular, it demonstrates that pre-oxidation using potassium permanganate could be applied at the head of water treatment plants without releasing intracellular geosmin and saxitoxins from A. circinalis. Furthermore, powdered activated carbon (PAC) was shown to be an effective treatment barrier for the removal of extracellular (dissolved) geosmin and saxitoxins, with similar adsorption trends of both compounds. The relative removal of the saxitoxins compared with geosmin was determined to be 0.84^0.27, which implies that saxitoxin removal with PAC can be estimated to be approximately 60 to 100% of the removal of geosmin under equivalent conditions. Chlorine was shown to be effective for the oxidation of the saxitoxins with CT values of approximately 30 mg min l 21 required for greater than 90% destruction of the saxitoxins.
Cylindrospermopsin (CYN) is an emerging cyanobacterial toxin whose detection frequency is increasing in water bodies worldwide. Because it has a high propensity to exist in extracellular form, effective water treatment practices are required to ensure removal. In this study, powdered activated carbon (PAC) and chlorination were evaluated for CYN removal under practical water treatment plant conditions. The homogenous surface diffusion model (HSDM) was used to predict CYN adsorption using two different PACs. The HSDM also aided in the derivation of a PAC dose table that could be used to optimize PAC application for CYN removal. Chlorine was shown to be an effective oxidant for CYN; the chlorine concentration times contact time value of 3 mg × min/L was shown to be sufficient for complete CYN oxidation in two drinking waters. This study provides pertinent information for the application of PAC and chlorination practices for optimal CYN removal.
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