The US Environmental Protection Agency (EPA) has awarded nearly US$ 700 000 to researchers at the University of Cincinnati in order to establish a baseline understanding of how toxins produced by cyanobacteria (blue-green algae) can be changed by exposure to ultraviolet (UV) light, a process used to disinfect drinking water.
The team, led by chemical engineer Dionysious Dionysiou, is to study processes used to purify drinking water. The research will be critical to developing cost-efficient UV technologies to treat water contaminated by such toxins. The team is also investigating treatment of algae-contaminated water specifically using sunlight and an environmentally friendly catalyst (Fig. 1).
“Some of the cyanobacterial toxins are even more toxic than the venom produced by many poisonous snakes,” says Dionysiou, associate professor of environmental engineering. “These toxins have even been included in the list of chemical or biological warfare agents.” He explains that the toxins produced by cyanobacteria include hepatotoxins, neurotoxins and dermatotoxins, which affect the liver, nervous system and skin, respectively. Among the most commonly found cyanobacterial toxins is a group called microcystins. Microcystin-LR, for example, is a potent hepatotoxin.
The problem is not confined to the US, where it is found in the Great Lakes region and Florida, for example. Cyanotoxins are also found in Northern European countries such as Scandinavia, as well as France, the UK, Turkey, and Australia.
Blue-green algae can grow in freshwater lakes, ponds and wetlands. They thrive in stagnant water under certain environmental conditions and eutrophication.
Eutrophication refers to the ‘enriching’ of a lake with nutrients such as phosphorous and nitrogen. This enrichment occurs frequently as a result of human activity, whether from domestic or industrial sewage, leaching of pesticides or draining of farm fertiliser runoff, as well other sources. For example, Lake Erie is a eutrophic lake that receives more than 65 billion gallons of domestic and industrial wastes each year, not even counting agricultural run-off. Some blue-green algae also produce their own food through photosynthesis.
Large growths of algae, known as harmful algal blooms, and their released toxins can be extremely toxic if swallowed by wildlife, livestock or people who drink untreated water. Because of such high toxicity, the World Health Organisation assigned a provisional concentration limit of one microgram per litre of microcystin-LR and other cyanobacterial toxins in water. In January 2007, an EPA panel suggested lowering the provisional level to 100nanograms per litre, or 100 parts per trillion.
While the problem associated with cyanobacterial toxins was known from early studies in 1870s in Australia, new developments of analytical methods helped determine the chemical structure of such toxins and identify new toxins. In addition, advances in instrumentation and chemical analysis helped detect such toxins in many other countries and at much smaller concentrations.
“The water in the Valle de Bravo dam, close to Mexico City, has very large concentrations of cyanotoxins,” Dionysiou notes. He has collaborated with Erick Bandala and his group of the Mexican Institute of Water Technology for the treatment of water from the Valle de Bravo dam which was contaminated with large concentrations of microcystin-LR. The team used a chemical oxidation system that was very effective in destroying the toxin.
UC’s programme is an umbrella with many spokes, including Kevin O’Shea (Florida International University), Judy Westrick (Lake Superior State University), Don Deis and Cheryl Miller (PBS&J, a company in Florida). One objective of the programme is to monitor for cyanobacteria in the Great Lakes, in the St Johns River in Florida, as well as several other locations in the country. Another objective is to conduct photochemical studies to see what happens to the toxins in natural aquatic systems.
O’Shea’s group will focus more on the photochemical aspects while the role of Westrick’s group is mainly on the monitoring, identification and quantification of toxins in the Great Lakes.
“Westrick samples water from the Great Lakes, especially at locations close to the water intake of drinking water treatment plants, screens this water for toxins and then coordinates with the other project collaborators to determine the proper method for destroying the toxins in such water.” Dionysiou explains. The role of the investigators from PBS&J is monitoring for cyanobacterial toxins in certain aquatic systems in Florida. “You develop methodologies but eventually you have to test the system in real water.”
He emphasises that algal cells must be removed early on in the process in a drinking water treatment plant “because they foul the equipment of the process train.”
He continues: “However, proper technologies need to be applied for cyanobacteria cell removal since some types of treatment methods for removing the cells, such as those that apply mechanical force, make the situation worse because they break the cells and release the intracellular toxins in water.”
For this reason, his lab is conducting research on appropriate physical-based technologies to first remove the cyanobacteria cells from water and subsequently apply chemical and UV light-based technologies to destroy the soluble cyanobacterial toxins in water. UV light techniques are gaining a lot of interest because of their effectiveness of disinfection, especially for some microorganism that are resistant to disinfection by chlorination.
“In general, UV takes only a few seconds. The DNA of the microorganisms is affected so they cannot reproduce and their concentration stays at low levels.” Dionysiou proposes certain modifications of UV technologies so they can be used for both disinfection of water as well as for the destruction of toxins in water.
He explains that also with longer study time, the researchers can now document what happens at different UV doses over time. “What happens to these toxins?” he asks rhetorically. “And what happens with the chemical intermediates generated.”
The researchers are now able to study the types of intermediate products formed, the reaction pathway, the toxicity of the intermediates and how long it takes to break them down. In addition, he is now studying the cost of using ultraviolet light. For example, catalysts – materials that are used to initiate or speed up a reaction without being affected by the reaction itself – can be combined with solar light to develop environmentally friendly, green, sustainable processes.
The value to using a technique that relies on solar light is that many Third World countries have solar light in abundance, due to their proximity to the equator. “In several African countries, millions of people do not have access to clean and safe drinking water at all and millions of people, especially children, around the world die every year due to waterborne diseases.”
Titanium dioxide, a ceramic material frequently used in paints and powders, is being investigated by Dionysiou's group as a catalyst to generate photochemical and chemical reactions that destroy the toxins in water. They use this catalyst fixed on a support to lengthen the effect of the catalyst and to keep the catalyst itself out of the water. The catalyst is made with very high surface area using nanotechnology methods and containing very small quantities of non-metals that make the catalyst operate using visible light, taking advantage of the sun’s light.
