Scientists in the US have created an economically viable, sustainable flame retardant by combining a waste product from water treatment plants with a viscoelastic hydrogel.

Forest fires are a serious problem in the US, threatening homes, life and health with increasing prevalence. The US Forest Service deploys thousands of cubic meters of fire retardant from the air every year. This fire retardant typically consists of ammonium polyphosphate in water thickened with clay and polysaccharides.

In 2016, Stanford University’s Craig Criddle and Eric Appel reported a method for making a hydrogel that could be used to provide flame retardants. It involved mixing together cellulose derivatives (commonly used as viscosity modifiers) and colloidal silica nanoparticles in water. “The main goal in developing these materials was to improve the adhesion of the sprayed protective agent to the vegetation of interest and to improve its retention at this point in order to create a permanent barrier that would last for the duration of the fire season,” explains Appel.

While investigating other phosphorus-based chemicals, they discovered an unclear evidence that magnesium ammonium phosphate is used as a flame retardant for textiles. Magnesium ammonium phosphate, or struvite, is known to be an inconvenience in the water industry. “Usually it’s a pest,” says Criddle. “It tends to block pipelines and fail in places we don’t want it. So people will put a lot of energy and expense into removing it.”

The group wondered if struvite would work in a wildfire environment with their hydrogel, as it has similar chemical properties to ammonium polyphosphate. They grind struvite pellets into a fine powder and mix it into the hydrogel by shaking it in a bottle. Subsequent experiments suggested that struvite-suspended hydrogels fight fires just as well as commercially available ammonium polyphosphate-based flame retardants and adhere even better to surfaces.

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Currently, struvite obtained from waste streams is more of a slow release fertilizer, but this is not the most cost effective use of the crystals. Investigations into the value of struvite extracted for fire fighting purposes showed that it was the second most valuable resource after water itself, which was extracted from the sewage treatment plant. Further analysis showed that there is enough struvite as waste in the United States to meet forest fire program requirements. Hence, using it as a flame retardant could be both economical and environmentally friendly. “Struvite as a green source of fire retardants recycled from wastewater in wildlife areas has great potential to reduce the need for degraded phosphorus and promote cleaner waters,” added Appel.

Andrea Hicks, sustainability expert at the University of Wisconsin-Madison in the USA, comments: “The economic discussion is particularly interesting because the extinguishing agent would and has a higher value than what is currently found in the sale of struvite as fertilizer” Potential to create incentives for more struvite production from sewage treatment plants. ‘

Appel founded a start-up called LaderaTECH to commercialize forest fire prevention technology. In May 2020, LaderaTECH was acquired by Perimeter Solutions, which manufactures all of the flame retardants used by the US Forest Service.

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