Biology Reference
In-Depth Information
completely at an oxidation rate of 0.219 min
-1
at an applied current of 100 mA with a half-life period
of 2.5 min. Thus electrochemical method appears to be a promising approach.
iii) Photolysis
:
MCs are very stable under natural sunlight (Tsuji
et al
., 1994a). On the other hand,
Mazur and Plinski (2001) observed that the concentration of MC-LR, MC-RR and nodularin
decreased by 30% over a period of 21 days when incubated at 17 ± 1ºC in light (40 µmol photon
m
-2
s
-1
). The decomposition or degradation of hepatotoxins by treatment with UV has been studied.
Upon UV-irradiation, the decomposition of MCs proceeded by isomerization and the products
tended to be non-toxic (Tsuji
et al
., 1995). Photolytic and photocatalytic processes are recognized.
Photolytic processes involve treatment with ozone and/or hydrogen peroxide in presence of UV light
during which slow and reactive hydroxyl radicals are generated. Photocatalytic processes comprise
oxidation in presence of oxides with semi-conductive properties such as TiO
2
, ZnO, Fe
2
O
3
, WO
3
and CdSe (Fox and Dulay, 1993; Legrini
et al
., 1993). Of these, TiO
2
has been widely used because
it is chemically passive and stable over wide range of pH values. Radiations within range of 300 to
400 nm are absorbed by TiO
2
leading to the generation of hydroxyl radicals that have the highest
oxidation potential than ozone or chlorine. This has opened up another area known as Advanced
Oxidation Technologies (AOTs) and Membrane Separation Technologies (MSTs) that received great
attention for environmental remediation. AOTs are highly effective both functionally and cost-wise
(Fujishima
et al
., 1999). The illumination (330-450 nm from a xenon UV lamp) of MC-LR (in 50 to 200
µM range) in presence of a catalyst like TiO
2
(1% m/v solution) caused photocatalytic destruction
within a period of 10-40 minutes depending on the initial concentration. This method seems to
be quite useful for rapid elimination of hepatotoxins from potable waters (Robertson
et al
., 1997).
A thin layer of TiO
2
deposited on glass not only caused the destruction of extracellular toxins but
also proved to be lethal to toxic bacterial cells (
E
.
coli
within 2 h) and inhibited the growth of green
algal cells (to the extent of 60% of growth of
Oedogonium
) (Makowski and Wardas, 2001). Although
photocatalytic process is quite effective in eliminating extremely high concentrations of MC-LR, a
variety of byproducts are generated. Thus Liu
et al
. (2002) suggested that TiO
2
photocatalysis has
to be combined with treatment with H
2
O
2
. Since TiO
2
particles in suspension are diffi cult to be
removed after their application, attempts have been made to prepare thin fi lms and membranes
by immobilizing TiO
2
(Balasubramanian
et al
., 2003). Kinetic modelling studies of Feitz and Waite
(2003), on TiO
2
-catalyzed photodegradation, revealed that the strong adsorption of MC-LR to
semiconductor surface sites is followed by a reaction between surface located organic radicals
leading to the degradation of the toxin. The formation of organic radicals is explained on the basis
of interaction of the desorbed surface bound superoxide with organic substances generating organic
peroxy radicals. Further, the inhibition of the formation of organic radicals at alkaline pH due to the
formation and deposition of carbonate on the surface prevents degradation of MC-LR. Choi
et al
.
(2006) fabricated nanostructured TiO
2
fi lms and membranes that have multiple applications such
as decomposition of recalcitrant organic pollutants, destruction of biological toxins, inactivation
and killing of pathogenic microorganisms, physical separation of contaminants and intermediate
products and self-antibiofouling action.
iv) Reverse osmosis
:
A process that is generally used in desalination of brackish or estuarine waters,
reverse osmosis has also been put to use for the removal of toxins. Reverse osmosis membranes gave
a retention rate of 95% for MC-LR and MC-RR. These were also effective in retaining the nodularins
The retention of both salt and the toxins from waters by reverse osmosis renders almost clean water
for drinking purposes. Reverse osmosis does not help in the destruction or decomposition of toxins
