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 FeNX, CaCXNy, 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.