Development of Eco-Friendly Soap-Based Firefighting Foam for Forest Fire

Abstract

In recent years, large-scale forest fires have been occurring frequently worldwide and 60,000 to 140,000 km of forests have been vanishing yearly. The forest fires and the damage from the smoke associated with them have caused lost of human life and health hazards as well as loss of forest vegetation, biomass, and ecosystems in massive areas. The damage in the Europe’s Mediterranean coast, North America’s west coast, Russia, and Australia are especially serious. There were over 170 large-scale forest fires concentrated throughout Greece from June to October in 2007. Additionally, in 2007 the forest fires that broke out from October 20 to 23 in Southern California of North America over an area almost equivalent to the size of Tokyo (2,187 km) destroyed approximately 2,000 km of forests and about 1,700 homes in 7 d time until October 27 (Hayasaka, 2010). In these forest fires, the use of firefighting foam has greatly improved fire extinguishing effects. The surfactant in firefighting foam significantly lowers surface tension in water and allows water to be efficiently adherent to flammable materials and additionally allows water to penetrate into the flammable materials. Because of this, it is highly effective in firefighting efforts by the increased cooling effect, suppression of oxygen supply, suppression of chemical reactions in the flammable material, and blockage of radiated heat, etc. However, for forest fires, the use of water alone is common in extinguishing activities. This is because the mountainous areas are also the source of water for urban residents and the pollution caused by the run-off from the firefighting foam into rivers and the surrounding environment would be especially concerning. A cautious stance in the use of firefighting foam is being taken and even in the United States, therefore, there was a case where the concern regarding secondary environmental damage from firefighting foam led to allowing fires to extinguish naturally and resulted in a large area of new and old forests to be destroyed. We have been developing firefighting foam involving naturally derived fatty acid salts (soap) as the main component. The soap is much lower in toxicity to aquatic organisms compared to synthetic detergents with environmental water in river, lake and sea (Lin et al., 2006; Kawano et al., 2007; Goto et al., 2007; 2008; Kawano et al., 2014). The reason why the toxicity of the soap is lower to aquatic microorganisms and fish is that it reacts with metal ions such as calcium ions that exist in the environment to form metal soap. The metal soap does not have the interfacial activity and its toxicity is significantly low. It is desirable to use soap that has very low toxicity to aquatic organisms since the spreading fire extinguishing agents could have a large impact on the natural environment. The soap-based firefighting foam is diluted by 1 wt% with environmental water in use. The metal ions such as calcium ions in the environmental water may cause the forming of metal soap, and the interfacial activity may be lowered. Therefore, it is necessary to add a chelating agent in the firefighting foam to prevent soap and metal ions from forming metal soap. Ethylenediaminetetraacetic acid (EDTA), commonly used chelating agent, is very effective in masking metal ions. However, EDTA is much less biodegradable and its long-term residual in the environment, particularly as heavy metal complex, raises concerns about ecotoxicities (Oviedo et al., 2003). For this reason, the highly biodegradable chelating agent, L-glutamic acid N,Ndiacetic acid (GLDA), was used in firefighting foam for general buildings (Mizuki et al., 2007). However, the firefighting foam with GLDA suffered from the disadvan-