Upcycling eggshell waste into CaO-enhanced PVDF mixed matrix membranes for optimized CO2/CH4 separation

The valorization of agricultural waste into functional nanomaterials is rapidly emerging, yet their integration into high-performance gas separation membranes is still relatively unexplored. In this study, calcium oxide (CaO) derived from eggshell waste is employed for the first time as a CO2-philic active filler in poly(vinylidene fluoride) (PVDF) mixed matrix membranes (MMMs) for CO2/CH4 separation. Unlike conventional CaO fillers, the waste-derived CaO produced here possesses an exceptionally high surface area and a nanostructured morphology, which promotes stronger polymer-filler interfacial interactions. Its structure–property relationships were systematically probed using BET, FTIR, SEM, TGA, XRD and particle size analyses. A response surface methodology (RSM) central composite design (CCD) was applied to optimize CaO and polyethylene glycol (PEG) loadings, enabling systematic evaluation of their synergistic effects on gas transport properties. The optimized membrane exhibited a CO2 permeability of 12,108.97 Barrer with a CO2/CH4 selectivity of 12.87 under single gas conditions, and 9242.13 Barrer with a selectivity of 9.06 for a 40:60 CO2/CH4 mixture, surpassing the performance of MMMs incorporating conventional fillers reported previously. These results show that eggshell-derived CaO acts as an active CO2-affinitive phase rather than a simple inert additive, providing a low-cost, sustainable and performance boosting alternative to traditional nanoparticles. Overall, the work introduces a new route toward next-generation waste-derived MMMs for efficient and environmentally sustainable gas separation.