Humic acids on fire? Physico-chemical, thermal, flammability features and extraction process of different humic acids in support of their possible applications

Humic acids (HA) consist in a multitude of heterogeneous organic molecules surviving the biological and chemical degradation of both vegetal and animal biomasses. The great abundance and chemical richness of these residues make their valorisation one of the most promising approaches to move towards a circular economy. However, the heterogeneity of the biomass from which HA are extracted, as well as the production process, significantly affects the nature and the relative content of functional groups (i.e. quinones, phenols and carboxylic and hydroxyl moieties), eventually changing HA reactivity and ultimately determining their application field. Indeed, depending on their properties, these substances can be used as flame retardants in the case of pronounced resilience degree (i.e., absent or low reactivity), or as antioxidant or antimicrobial agents in the case of pronounced reactivity, thanks to their redox behaviour. In this work we investigated the flammable, the thermal and the physico-chemical features of HA extracted from different composted biomasses to identify the reactivity or the resiliency of these moieties. Several techniques, including flammability characterization (LIT and MIE), laser diffraction granulometry, TG, XRD analyses, FTIR spectroscopy on both solid and gaseous phases, and Raman spectroscopy were integrated to investigate the correlation among the safety parameters, the distributions of particle sizes, as well as the thermal, the chemical properties of HA powders and the influence of post-extraction processes on HA final properties.