Reversible CO ₂ Capture via Fly Ash-Encapsulated Eggshell Sorbents: A Dual-Waste Valorization Approach toward Sustainable Carbon Management

The increasing demand for efficient in-process carbon capture technologies necessitates the development of sustainable, high-performance sorbents capable of withstanding repeated adsorption–desorption cycles. In this work, we have reported a sustainable dual-waste valorization strategy for developing a structurally stabilized calcium oxide (CaO)-based sorbent using waste eggshells as a CaO precursor and fly ash (FA) from wastewater treatment plant incinerator as the dopant to reinforce the structural stability of eggshell for improved CO2 capture. Eggshell-derived CaO was stabilized with 5 wt % FA through mechanical mixing followed by the thermal treatment, producing a fly ash-modified eggshell sorbent which enhanced the cyclic CO2 capture stability. The raw and FA-modified eggshell samples were evaluated over 20 successive carbonation-calcination cycles with and without in situ hydration during carbonation. The pristine eggshell sorbent exhibited an initial CO2 uptake of 28.7 mmol g–1 that declined due to sintering. In contrast, the fly ash-modified eggshell combined with in situ hydration (FAESCC&H) achieved an initial uptake of 35.8 mmol g–1 and retained 28.9 mmol g–1 after 20 cycles, corresponding to approximately 25–30% higher cumulative CO2 capture. Comprehensive elemental, morphological, surface, and structural analyses, X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, carbon dioxide temperature-programmed desorption (CO2-TPD), and X-ray photoelectron spectroscopy, of raw and modified eggshells were performed before and after carbonation/calcination. The results revealed enhanced structural integrity in the modified sorbents due to the formation of a thermally stable calcium silicate and aluminate framework, effectively suppressing CaO grain growth and pore collapse. Concurrently, steam hydration improved the porosity and reactivity through the formation of calcium hydroxide and the development of surface cracks. The work demonstrates a low-cost, scalable pathway for producing long-life CaO sorbents from industrial and biogenic waste streams, offering a promising solution for sustainable high-temperature CO2 capture and circular carbon management.