A Dual-Functional CO₂-Selective Membrane for Biogas Upgrading in a Microalgae Membrane Bioreactor

Upgrading biogas to pipeline-quality methane requires the efficient removal of CO₂, yet conventional physicochemical routes remain energy-intensive. Coupling a CO₂-selective membrane with microalgal photosynthetic fixation offers a green alternative, but is constrained by the low CO₂/CH₄ selectivity of common membranes and the poor adhesion of microalgae to hydrophobic membrane surfaces. Here, a dual-functional composite membrane was developed that simultaneously provides CO₂/CH₄ sieving and a biocompatible interface for microalgal attachment, and was integrated into a microalgae membrane bioreactor (MMBR). A cellulose acetate mixed-matrix membrane incorporating polyethyleneimine-grafted ZIF-8 (CA/PZIF-8(15)) achieved a mixed-gas CO₂ permeability of 122.3 Barrer and a CO₂/CH₄ selectivity of 41.17. An ionic-liquid-modified chitosan (CS/IL) coating, first optimized on a commercial flat-sheet polyethersulfone (PES) membrane used as a model surface for the adhesion study, reversed the surface charge from -30.8 to +3.75 mV, lowered the water contact angle to 51.2°, and increased the day-7 adhesion of Scenedesmus obliquus by ~108%. Transferring the coating onto CA/PZIF-8(15) further raised the permeability to 138 Barrer and the selectivity to 57.31, placing the composite above the 2008 Robeson upper bound. In the MMBR, CH₄ purity reached 95.13% after 48 h; a mass balance on the recirculating gas volume indicated that essentially all of the CO₂ removed from the gas phase permeated the membrane, of which an estimated 2% was fixed into microalgal biomass while the remainder was retained in the liquid phase. This work offers a membrane-design strategy that bridges gas-separation functionality and microalgal carbon fixation for sustainable biogas upgrading.