Valorization of agricultural waste and red mud through CO₂-mediated pyrolysis system

This study presents a strategic method to improve carbon utilization in agricultural waste, specifically spent mushroom substrate (SMS), while valorizing metal-rich waste, red mud, through pyrolysis platform. To offer sustainability, CO₂ was utilized as reaction medium. The utilization of CO₂ in SMS pyrolysis promotes syngas production, particularly CO, through homogeneous reactions (HRs) with volatile compounds (VCs). Indeed, the amount of syngas in N₂ and CO₂ were 2.42 and 4.04 mmol g_sms^-1, respectively. The inclusion of red mud in CO₂-mediated pyrolysis of SMS accelerated the reaction kinetics of HRs lined to CO₂, resulting in enhanced syngas production. The iron-functionalized biochars (Fe-FBs) produced at 700 °C in N₂ and CO₂ were evaluated for Cr(VI) removal. Faster kinetics for Cr(VI) removal was observed in Fe-FB in N₂ compared with that in CO₂. This is likely due to the higher capability of elemental iron (Fe⁰) in reduction of Cr(VI). However, Fe-FBs in CO₂ exhibited superior adsorption capacities relative to that in N₂, indicating that Fe₃O₄ is a primary contributor to Cr(VI) removal through adsorption. Notably, removal of Fe-FBs in N₂ and CO₂ via adsorption reached up to 17.2 and 82.8 %, respectively. Considering toxic nature of Cr species (Cr(VI) and Cr(III)), their immobilization through adsorption on Fe-FBs in CO₂ may offer a more environmentally benign strategy. These findings suggest that the pyrolysis of SMS with red mud using CO₂ as a reactant provides two benefits: enhanced syngas production and the fabrication of Fe-FBs that can serve as environmentally benign materials for Cr(VI) removal.