Valorization of municipal sludge protein into amine-functionalized composite aerogels for CO2 capture

Municipal sludge contains abundant protein resources but remains underutilized. In this study, purified sludge protein (PN) was used as a nitrogen-containing component, while sodium alginate (SA) served as a structural component to prepare PN/SA composite aerogels by freeze-drying. The effects of the PN/SA ratio and 3-aminopropyltriethoxysilane (APTES) loading on the pore structure, surface chemical environment, and CO₂ adsorption performance were investigated. The results showed that PN contributed amino-, amide-, and carboxyl-containing groups, whereas SA facilitated the formation and stabilization of the porous framework. Among the PN/SA aerogels, PN/SA-1 exhibited the most favorable pore structure, with a specific surface area of 28.31 m² g⁻¹ and a CO₂ uptake of approximately 0.23 mmol g⁻¹ at the highest relative pressure under the tested conditions. APTES functionalization introduced additional Si- and N-containing components and enhanced the CO₂ adsorption performance. Among the functionalized samples, 1AP-GEL exhibited the highest CO₂ uptake, reaching 0.82 mmol g⁻¹ at approximately P/P₀ = 1. The Freundlich model provided a better empirical description of the adsorption data than the Langmuir model. In addition, 1AP-GEL retained 89.61% of its initial CO₂ uptake after four adsorption–desorption cycles, indicating preliminary short-term regeneration performance. These results demonstrate the feasibility of using recovered municipal sludge protein as a nitrogen-containing component in biomass-based aerogels and provide a potential strategy for the value-added utilization of sludge.