The algal toxin saxitoxin was recently found in Ohio waters. Because saxitoxin is primarily associated with marine environments, Ohio utilities lack information they can use when treating freshwater sources that contain this toxin.
Building on previous HABRI research that addressed the toxin microcystin in drinking water, researchers are developing guidelines for use of powdered activated carbon to remove saxitoxin during water treatment. Activated carbon is commonly used for this purpose, but studies on this approach have produced conflicting results, meaning water plants can’t be sure the treatment will be successful.
The researchers are working on laboratory studies with different sources of powdered activated carbon (wood, coconut, coal blend and bituminous coal) to determine which are most effective at removing saxitoxin from a standardized solution that mimics Ohio surface waters.
Results indicate that the adsorption of saxitoxin by powdered activated carbon occurred to a lesser extent than adsorption of microcystin. The rate of saxitoxin adsorption to the activated carbon was also slower than the rate of microcystin adsorption to the same carbon types. Competition from natural organic matter with saxitoxin for adsorption by activated carbon was limited. This was counter to what was observed for microcystin and indicates that saxitoxin interactions with activated carbon were different than those involving microcystin.
One potential difference could reflect the fact that saxitoxin is positively charged under most ambient water pH values while microcystins are neutrally to negatively charged, depending on the specific variant. The extent of competition by natural organic matter did appear to depend on the source as the removal of saxitoxin in the presence of natural organic matter isolated from an algal source was lower than that in the presence of natural organic matter isolated from a terrestrial source.
Consistent with our prior observations with microcystin, the wood-based powdered activated carbon performed better than the other carbons tested. However, the degree of outperformance was more muted than what was observed for microcystin and for some systems the different carbon types all performed similarly. This could potentially indicate that because saxitoxin is a smaller molecule than microcystin, it can access a larger proportion of the interior pore spaces.
As results are finalized, the scientists involved will work to communicate outcomes of the study to local water treatment plants to keep them aware of research progress and to share findings and suggested guidelines.
The Bottom Line
Researchers are developing guidelines for the use of powdered activated carbon in drinking water treatment to remove saxitoxin, an emerging toxin of concern in Ohio waters.