CYANOBACTERIAL TOXINS - Cyanobacteria - page 569

spectrometry analysis where toxicity was determined by PP1A inhibition assay. But the overall rate
constant (k) for the reaction suggested that ClO 2 is not a suitable oxidant for the degradation of
MCs in drinking water treatment processes (Kull et al ., 2004). Subsequently, they have suggested
that the oxidation of MCs resulted in the permitted concentrations as per guidelines of WHO and
also chloride at its normal concentration could be effectively used for the detoxifi cation of MCs.
Thus, the oxidation of MC-LR (at 10 µg L -1 ) by ClO 2 (1 mg L -1 ) led to the formation of non-toxic
oxidation products, dihydroxyisomers of MC-LR. The presence of organic matter enabled ClO 2 to
be consumed more rapidly by fulvic and humic acids, leaving a residual ClO 2 that could oxidize
MC-LR. PP1 inhibition was observed by intact MC-LR while the oxidation products did not inhibit
the activity of PP1 (Kull et al ., 2006). Electrogenerated active chlorine effi ciently decomposed MC-RR
and MC-LR within 15 min electrolysis under a current density of 8.89 mA cm -2 at 20ºC and pH 7.00.
The removal rates of MCs increased with increasing concentrations of chloride. The inactivation
kinetics of MC-LR by free chlorine revealed that MC-LR could be inactivated more readily at pH
6.0 which decreased with increase of pH to 9.0 (Xagoraraki et al ., 2006).
ii) Hydrogen peroxid e : Though hydrogen peroxide has been applied for the oxidation of phenolic
wastewater and for the treatment of organic wastewater, its utility for treatment of toxin removal
has been very restricted. Irradiation with UV light increases the effi cacy of hydrogen peroxide since
it dissociates to form two hydroxy radicals. However, there are no reports on treatment of hydrogen
peroxide in combination with UV light for the removal of MCs from drinking water supplies.
Treatment with hydrogen peroxide is not very effective as 17% of MC-LR was decomposed in 60
minutes with a 20 mg dm -3 solution of hydrogen peroxide (Drikas, 1994). Rositano et al . (1998) used 2
mg dm -3 hydrogen peroxide to treat 1 mg dm -3 solution of MC-LR. Though no toxin decomposition
was noted within the fi rst 10 min, combination with ozone, however, abolished the toxicity
within 30 s.
iii) Permanganate : It generally attacks functional groups with multiple bonds. So it can be effective
in decomposing MCs by attacking the unsaturated bonds in the Adda moiety. Rositano et al . (1998)
reported that 1 mg dm -3 of potassium permanganate removed 95% of of 200 µg dm -3 of MC-LR in
30 min. It compared very well with the oxidation potential of chlorine but due to its higher oxidation
potential it removed MCs much more rapidly than chlorine. Much more needs to be done with regard
to its decomposition products, their toxicity and optimization parameters for MC destruction.
iv) Ozonation : A process that has generally been used for disinfecting purposes or to remove colour
and/or odour of the waters in Europe and North America, ozonation has occupied a signifi cant place
in water treatment plants for the removal of cyanobacterial toxins. A two-stage ozonation water
treatment is suggested that can be either with pre- and inter-ozonation, inter- and post-ozonation,
or pre- and post-ozonation (Langlais et al ., 1991). Though early studies suggested the removal of
MCs by a 2 s ozonation of waters, details regarding concentrations of MCs or ozone employed
are not available (James and Fawell, 1991). Subsequent studies revealed that 800 µg L -1 of MC-LR
could be oxidized to below the detectable limits of HPLC by concentrations of ozone lower than 0.2
mg L -1 within seconds to minutes whereas 88 µg L -1 nodularin was completely inactivated by 0.05
mg L -1 of ozone within 15 s (Nicholson et al ., 1993; Rositano and Nicholson, 1994; Rositano et al .,
1998). These studies indicate that the removal of MCs is directly proportional to the concentration
of ozone and there exists a residual concentration of ozone after its demand has been met. Further,
the effi cacy of ozone is reduced by the presence of organic matter and alkalinity. Hitzfeld et al .
(2000) have summarized the observations of various workers. In these studies the concentration of