CYANOBACTERIAL TOXINS - Cyanobacteria - page 568

but only helps in their separation from water. The retentate has to be disposed off very carefully as
it might contaminate other sources of water.
v) Ultrasonication : Ultrasonic radiation inhibited growth of Microcystis and also did not contibute
to the increase of the extracellular MC concentration. This suggests that ultrasonic irradiation
shorter than 5 min did not lead to lysis of cells. Further, effective degradation of MCs occurred at
150 KHz and 30 W for 20 min sonication resulting in a removal rate of 70% (Ma et al ., 2005). However,
it appears for complete degradation higher frequencies of sonication are required as exemplifi ed
by the studies of Song et al . (2005). Rapid degradation of MC-LR by ultrasonic irradiation at 640
KHz resulted in degradation products that did not exhibit any biological activity as assessed by
PP1 inhibition assay. However, a concentration range of 0.03 to 3.0 µM and the presence of hydroxy
radicals appeared to be important factors for degradation besides other processes of hydrolysis and
pyrolysis (Song et al ., 2005).
(B) Chemical treatment methods for toxin removal: The utility of a number of oxidizing agents
have been tried for the removal of cyanobacterial toxins. The chemical oxidants can be
placed in increasing order of their oxidizing potential as: chlorine dioxide>
chlorine>hypochlorous acid>permanganate>perhydroxy radical>hydrogen peroxide>ozone>
hydroxy radical. Thus the hydroxy radical has the highest oxidizing potential of 2.80 Volts while
chlorine dioxide has the lowest of 1.28 Volts. Although treatment time, concentration, pH and
removal effi ciency of various oxidizing chemicals have been worked out, the lacuna lies in the lack
of characterization of decomposition products. The toxicity testing has also been limited to mouse
bioassay. Since most of the oxidizing agents react with unsaturated bonds present in Adda moiety of
MC, any change in its characteristics will lead to a lowering of its UV absorption at 238 nm leading
to a notion that the toxin has been decomposed. In the absence of characterization of degradation
products with reference to their quantitative determination and toxicity testing no oxidizing agent
seems to serve the purpose. However, a lot of knowledge has been generated on the possible removal
of MCs by chlorination, hydrogen peroxide, potassium permanganate and ozonation.
i) Chlorination : Chlorine readily dissolves in water by forming hypochlorous acid. Depending on
pH, the hypochlorous acid dissociates forming hypochlorite ions. This reaction is initiated at a pH
of 5.0 and 100% dissociation takes place above pH 10.0. The effi cacy of chlorination thus depends on
the formation of hypochlorous acid. It largely depends on the chloride compound and concentration
used. Water is exposed to varying doses of chlorine for 30 min at an elevated pH. More than 95%
of MCs and nodularin can be removed by concentrations greater than 1 mg L -1 of aqueous chlorine
and calcium hypochlorite whereas sodium hypochlorite and chloramines at the same dose can
remove 40-80% of these toxins (Nicholson et al ., 1993, 1994). It is effective in removing most of the
cyanobacterial toxins except anatoxin-a. On the other hand, CYN (at concentrations 20-24 µg L -1 ) was
effectively oxidized by 4 mg L -1 chlorine at pH 7.2-7.4 (Nicholson et al ., 1993). Though chlorination
is practised very widely in USA for removing dissoloved algal toxins, chlorination byproducts have
not been characterized which may also be toxic (Svreck and Smith, 2004). Chlorination can cause lysis
of cyanobacterial cells and there is a chance of the toxin content increasing if applied in the initial
stages. It is advisable that the cells are fi rst removed by coagulation or sedimentation or fl occulation.
Chlorination thus can be best applied as a terminal step of the treatment process and this will also
reduce the demand for higher levels of chlorine (Chorus and Bartram, 1999). Oxidation of MC-LR
into non-toxic dihydroxyisomers of MC-LR by treatment with chlorine dioxide (ClO 2 ) was monitored
by UV-spectroscopy and HPLC. The characterization of reaction products was performed with mass