Characterization into environmentally persistent free radicals formed in incineration fly ash and pyrolysis biochar of sewage sludge and biomass

Abstract

Environmentally persistent free radicals (EPFRs) are emerging contaminants formed during thermochemical processes. However, the formation and control strategy of EPFRs during the thermochemical process of traditional organic solid waste sludge and biomass is not well understood. This work investigated the EPFRs formations of fly ash and biochar via sampling in running factories and theoretical study on a laboratory scale. The oxygen-centered and carbon-centered EPFRs were detected in the pyrolysis biochar sampled on-site. EPFRs are rarely detected in fresh fly ash from incinerators, which is due to paramagnetic metal interference detection signals and sulfate hindering the potential formation sites of EPFRs. The formation of EPFRs was further detected in the simulated combustion particles, and the characteristics of EPFRs were closely related to the precursor and temperature. Sludge-based biochar (SBC) and rice husk-based biochar (RBC) prepared at different temperatures and residence times were further investigated. The pyrolysis temperature significantly impacted the EPFRs types and spin concentration in the resulting biochar. The pyrolysis temperature of 500 °C resulted in the highest spin concentration of 1.07✕1018 spins/g in RBC. Appropriate residence time could limit EPFRs formation. The minimum spin concentration of EPFRs in RBC and SBC was obtained at 2 h. In addition, adding biomass into sludge pyrolysis could lead to more EPFRs. Controlling the pyrolysis temperature above 500 °C and the residence time of around 2 h can impose restrictions on forming EPFRs. The findings improved the fundamental understanding of EPFRs generation in solid products derived from organic solid waste thermochemical processing, providing an opportunity to control EPFRs production.