A green method to treat exhausted fire-extinguishing powders for reuse as agricultural fertilizer

The most common powdered fire-extinguishing formulations (FEF) are finely dispersed mineral salts composed of mono-ammonium phosphate and ammonium sulphate as main ingredients, together with functional additives so as to improve the rheological properties of the FEF. After its whole service life has expired (36 months, under current legislation), the presence of coating agents such as water-proofing and organosilicon-based liquids makes problematic the environmentally friendly disposal of exhausted FEF, with little chance to reuse the phosphate-ammonium salts of the particles. This work aims at developing a green biotechnology-based method alternative to the chemical extraction with highly impacting organic solvents (acetone, CH2Cl2, n-hexane, etc.) so as to lyse the particle external coating and recover the phosphate-containing salts for potential use as inorganic fertilizer in agriculture. To this aim, two yeast strains were inoculated in a nutrient culture medium at 1% and 5% and grown overnight before adding the exhausted fire-extinguishing powder. After 24 and 48 h of incubation, the suspension was filtered to collect the treated particles, which were oven-dried (60 °C) and then documented under scanning electron microscopy (SEM). Untreated powder was taken as control. Whereas, the clean filtrate was analysed by ion chromatography. Results show that whatever the yeast strain, there was an increased release of ammonium, phosphate and sulphate ions in the medium. However, the two strains showed different performance according to the inoculum concentration and time of contact with the powders. Even if the treatment with bi-distilled water showed an ionic release, SEM images highlighted a more severe rupture pattern of the external coating of the particle compared to the water treatment. The external coating of the particle granule is being analysed by IR spectroscopy, TGA, and GC-MS so as to deepen the knowledge on the different mode of action of yeasts and water on the outer part of the exhausted fire-extinguishing powder. The ultimate aim is to develop an environmentally friendly strategy to recover the ammonium-phosphate content of the granules and make it suitable for use in agriculture as a chemical fertilizer directly or as an additive during the composting process.