Floating sponge coated in charcoal-like powder destroys harmful algal cells

In the peak heat of summer, beachgoers don’t want their plans thwarted by harmful algal blooms (HABs). But current methods to remove or kill toxin-producing algae and cyanobacteria aren’t efficient or practical for direct applications in waterways.

Now, researchers reporting in ACS ES&T Water have coated a floating sponge in a charcoal-like powder. When paired with an oxidizing agent, the technique destroyed over 85% of algal cells from lake and river water samples.

Swaths of electric green and bright orange-red HABs, or the less brilliantly colored cyanobacteria Microcystis aeruginosa, can produce toxins that can sicken humans and animals. Researchers have shown that acids and strong oxidizing agents damage and destroy M. aeruginosa cells, but simultaneously generate unwanted, potentially harmful products.

More recently, Jiangfang Yu, Lin Tang and colleagues suggested that persulfate-based oxidants could control this species’ algal blooms, but these compounds require catalysts, such as powdered biochar—a charcoal-like substance made from carbon-containing waste—to be effective. So, the team wanted to develop a floating material containing biochar that would boost the destruction of harmful algal cells without damaging the environment with byproducts.

The researchers coated commercially available melamine sponges with powdered biochar to create a floating, porous and absorbent material. Then, they added two different persulfate-based oxidizing agents to the sponge and exposed the materials to M. aeruginosa cells taken from lake and river water samples. The sponge’s absorption of the cells improved the performance of the oxidants, which destroyed most of the algal cells. Then, the sponge could be removed from the water to prevent any harmful byproducts from forming.

The researchers also used the sponge to treat lake and river water samples laden with HABs and other types of cyanobacteria. In these experiments, the technique reduced cell densities by about 90% in one hour, and the sponge could be used multiple times.

Although persulfate-based oxidants can generate byproducts such as sulfate and hydroxyl radicals, the researchers found that these byproducts were not released into the water when the sponge was used. Additionally, the sponge materials did not leach any biochar or other substances that would harm the environment.

The researchers say that with the appropriate amount of oxidant and biochar loading, the sponge could be used to treat large volumes of water in lakes and rivers. They are working on adapting the sponge to remove other types of pollutants from water.

This innovative approach shows promise in effectively and efficiently controlling harmful algal blooms, protecting both the environment and public health during the summer months when these issues are most prevalent.

Pairing this floating sponge with an oxidizing agent could help destroy harmful algal blooms. Credit: ACS ES&T Water (2023). DOI: 10.1021/acsestwater.3c00202

The researchers began their work by using a porous sponge made of melamine and creating powdered biochar from shrimp shells. They cleverly combined these materials by adding a thin layer of polyvinyl alcohol between the sponge and biochar, linking them at a temperature of 572 degrees Fahrenheit. When they introduced a persulfate-based oxidizing agent, the floating sponge effectively damaged the membranes of around 90% of the M. aeruginosa cells in lab dishes within just five hours. As the membranes split, the cells released their contents, which rapidly broke down into smaller components.

The team didn’t stop at lab tests; they took their catalyst sponge and persulfate system to real-world lake and water samples. The results were promising, as it inactivated over 85% of the algal cells. These findings indicate that this new system could be a successful technique for dealing with algal blooms in affected environments.

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