CYANOBACTERIAL TOXINS - Cyanobacteria - page 566

of virgin GAC. PAC is quite costly and additional cost requirements for its removal and disposal
add to its cost. The benefi cial aspects of GAC are its long life, higher adsorptive capacity, the ease of
operation, more effi cient use of carbon, and the ability to regenerate the carbon for reuse.
PAC and GAC from different starting materials viz ., wood, coal, peat, coconut have been tested for
toxin removal studies. PAC is useful in targeted treatments whereas GAC can be used continuously
(Chorus and Bartram, 1999; Griffi ths and Saker, 2003; Svreck and Smith, 2004). Of a number of carbon
sources tested, wood-based GAC and PAC were more effective as MC adsorbents. Extracts from M .
aeruginosa and O . tenuis , containing 2 mg L -1 MC were allowed to react with activated carbon source
(100-500 mg L -1 ) for seven days at 22 ± 1ºC. PAC was more effi cient than GAC in removing MCs.
MC variants MC-RR, MC-YR and MC-WR from M . aeruginosa extracts adsorbed more signifi cantly
than MC-LR and MC-LHar from O . tenuis (Mohamed et al ., 1998). The surface characteristics of the
carbon source appear to be more important in sorption performance rather than the source from
which the PAC and GAC are derived. The removal of MCs is limited to a small percentage by many
of the PACs used at a dosing rate of 1 kg m -3 with at least 50 kg m -3 required for effective removal
of the toxins. Depending on the physical characteristics of PAC, the adsorption levels of MC-LR
is reported to be between 20 to 280 µg mg -1 (Falconer et al ., 1983). The dominant factor infl uencing
adsorption appears to be the mesopore range. Adsorption of MC-LR was found to be greatest at
estimated diameter of 1.2 to 2.6 nm. Available surface area and presence of organic matter in the
waters to be treated are the two important factors governing toxin removal. Toxin removal from
pure water and water loaded with organic matter very much differed due to competition between
binding sites.
The use of GAC fi ltration beds is increasingly becoming more common in modern treatment
methods. However, the forbidding cost and the efforts to recharge are becoming impediments for the
extensive use of GAC beds. GAC columns with 70 g of GAC (from different sources) could remove
toxins to the extent of 50% only after passing 10 dm 3 water. The adsorption capacity of used GAC
was considerably reduced when compared to unused GAC of the same type.
Another important aspect of cleaning of drinking waters is the treatment for removal of taste and
odour problems created by the presence of compounds 2-methylisoborneol (2-MIB) and geosmin that
are prevalent in HABs. The removal of these substances either by oxidation (treatment with chlorine
or ozone) or by adsorption (PAC fi lter) has been attempted. 2-MIB has an effective molecular diameter
of 6 Å and micropores of activated carbon are the best sites for effective adsorption as the compound
is able to create more permanent adsorption sites (Pendleton et al ., 1997; Newcombe et al ., 1997). Of
the two substances, the adsorption effi ciency of geosmin to PAC was superior to 2-MIB. The removal
effi ciencies by PAC fi lter/adsorber varied from 25.7% to 88.4%. The remaining concentrations of
these substances in the fi nished waters were below 30 ng L -1 . However, removal by oxidation was
very weak but higher doses of ozone (3.8 mg L -1 ) showed higher removal effi ciency (84.8% for 2-MIB)
with a contact time of 6.4 min (Jung et al ., 2005).
ii) Electrochemical degradation : The treatment of various industrial wastewaters by electrochemical
process for cleaning up has generated lot of interst. The electrochemical approach has been applied
to dyeing wastewater (Vlyssides and Israilides, 1998), wastewater containing phenol (Chin and
Cheng, 1985; Smith and Watkinson, 1981) or benzene derivatives (Comninellis and Plattnet, 1995).
The effective removal of algal cells from pond water by electrochemical method (Feng et al ., 2003),
prompted Feng et al . (2005) to apply this method for the degradation of MC-LR. The oxidation rate of
MC-LR increased with the current by the hydroxyl radicals produced and the results are comparable
to photocatalytic oxidation with no byproducts detected by HPLC. MC-LR (1.0 mg L -1 ) was oxidized