Uncovering the Synergistic Role of Endogenous Minerals and Microwaves in Limiting the Formation of Gaseous Nitrogen Pollutants during Sludge Pyrolysis

The ongoing emission of gaseous pollutants from sewage sludge microwave pyrolysis poses formidable threats. This study specially elucidates the evolutional pathways between reactive and stable nitrogen-containing compounds in a novel sludge endogenous mineral-mediated microwave pyrolysis system, primarily responsible for curtailing NH₃ and HCN production. NH₃ and HCN yields were reduced by 33.2 and 22.0%, respectively, which are derived from the comprehensively oxidative catalysis of Al₂O₃, CaO, Fe₂(SO₄)₃, FeCl₃, and Fe₂O₃. Minerals further accelerated the conversion of 40.7% NH₃ and 74.0% HCN to N₂ by inducing the formation of FeN_X, CaC_XN_y, and Ca₂Fe₂O₅. The lowest release levels of NH₃ and HCN were achieved with 20% Fe₂O₃ and 20% CaO at 500 °C, respectively. Additionally, 600 °C was identified as a critical threshold for minerals and microwave inhibition of NH₃ and HCN formation, as higher temperatures weaken the inhibitory effect. Crucially, minerals enhanced engagement and cross-linking of nitrogenous species by attaching with proteins and occupying their redox-active sites, which directly promoted the formation of stable nitrogenous structures and ultimately hindered the formation and transformation of heterocyclic-N and nitrile-N. Minerals also imposed the transfer of reactive heterocyclic-N to stable heterocyclic-N and reactive alkyl nitrile-N to stable nitrile-N, thereby hampering the production of NH₃ and HCN. This study provides a novel strategy for the in situ control of gaseous nitrogen pollutant formation using minerals and microwave technology in practical applications.