Freshwater Harmful Algal Blooms (HABs), caused by overgrowth of potentially toxin-producing cyanobacteria, are plaguing U.S. Army Corps of Engineers projects and freshwater resources across the nation. Researchers at the U.S. Army Engineer Research and Development Center (ERDC) Environmental Laboratory are testing a new, ultraviolet light-based treatment technology to treat HABs without introducing chemicals that may have undesirable secondary effects.
By utilizing 3D printed Photocatalytic Oxidation Devices (PODs) for light-based HAB toxin mitigation, the program utilizes a more self-contained system to reduce its effects on the environment and more specifically target the microcystin toxin within HABs. The Environmental Protection Agency classifies HABs bloom and decay cycles and microcystin to be a potent liver toxin and possible human carcinogen, further, as the HAB biomass decays, hypoxia can cause fish kills. This program is part of the USACE Harmful Algal Bloom Research & Development Initiative, which focuses on delivering scalable freshwater HAB prevention, detection and management technologies through collaboration, partnership and cutting-edge science. The work is funded by the Aquatic Nuisance Species Research Program (ANSRP), and the program manager is Dr. Mandy Michalsen.
“This is a very targeted approach to try to treat the toxins, without affecting anything else within the ecosystem,” said Dr. Alan Kennedy, a research biologist with the ERDC Environmental Laboratory. “We mix the polylactic acid filament with titanium dioxide nanoparticles,” said Kennedy, “And what occurs is UV light hits the surface of the titanium dioxide nanoparticles and then free radicals come out, breaking apart the water to make hydroxyl ions, which is the OH part of water. The reaction also makes super oxygen. Oxygen is normally O2, but now it's got a free radical. The free radicals can attack a chemical and break up those bonds. And that's how we degrade microcystin.”
One primary concern for HABs treatment is not introducing any new chemicals with adverse side effects into the ecosystem. However, mechanical management practices cannot always account for the sheer amount of biomass nor toxins that cyanobacteria releases. Kennedy and his team combined the two methods by introducing titanium dioxide (TiO2) into the polylactic acid filament for a 3D print and activating it through ultraviolet light. For the design, segmented corkscrews, or static mixers, were created to agitate the water column as it passed through, allowing the cyanobacteria-produced toxins to have prolonged exposure compared to other methods, increasing the destruction of the microcystin toxins. Further, keeping the TiO2 infused PLA within the device itself makes it easily recoverable and reusable. Lastly, because the effects take place almost entirely within the system, far less free radicals are introduced into the environment.
“The attractive feature of PODs is they are activated by UV light,” said Kennedy. “They only live for fractions of a second and then they’re gone. So, for any nearby fish, the treatment will be fine, not only because the radicals live for an extremely short period of time, but they don't go very far from the surface. We’re trying to reduce use of some of the chemicals that have been used before in surface treatment, such as copper or peroxides.”
These authorized algaecides such as hydrogen peroxide, copper sulfate and copper citrate, and lime formulations may lead to increased resistances from cyanobacteria to this treatment, limit the growth of other organisms in the ecosystem and can even break down the cyanobacteria’s cell walls before collection, releasing microcystin toxins into the ecosystem. PODs, by comparison, has far less ecological impact hurdles to overcome. Further, a key advantage of the PODs is the lack of geometric constraints. Almost any design could be printed with the material to fit the application required. The team has also experimented with segmented mats with small weaves that the water column can pass through, which is a method that could prove useful for HABs near recreational swimming and boating sites.
“So, it's the same material, we just print it in a different geometry,” said Kennedy. “That’s the attractiveness of this. If they want a different design, they could have the feedstock material and just 3D print it into whatever size or shape that is, custom fit for whatever they want to do.”
Solutions in the lab are one thing, to utilize these technologies in the field our scientists will enlist researchers specializing in scalability. Jose Mattei-Sosa, a chemical engineer in the Environmental Laboratory, is one such scalability expert.
“Little by little, we have been able to make advancements by working with researchers from across multiple fields and applying our expertise in scaling up their research,” said Mattei-Sosa. “We account for the various environmental factors that impact upscaling success because sometimes methods that work well in a controlled lab setting need to be adjusted to work well in the field.”
With the greatest concerns being free radicals leaving the system and effectiveness of treatment, Mattei-Sosa focused on increasing the contact time within the environment and using pumps to create a treatment zone for free radicals to dissipate before the water enters back into the ecosystem.
“That's why we did these corkscrew shapes, they are basically a static mixer,” said Mattei-Sosa. “So, you're having lots of turbulence in there which allows all the water to touch the corkscrews that are in there, increasing contact time with the corkscrews which are activated with the UV light that's hitting them from the outside.”
As the system matures, Kennedy hopes to have a more ecologically friendly option for treating HABs that doesn’t have the adverse reactions that require algacidal regulations from the EPA.