Synthesis of chemically resistant polybenzimidazole membrane via green phosphoric acid crosslinking for efficient separation of selective flavonoids from citrus waste

This study introduces a chemically resilient Polybenzimidazole (PBI) thin-film composite membrane for organic solvent nanofiltration (OSN) based flavonoid fractionation of citrus waste, employing Phosphoric Acid (PA) as a mild, versatile, and low cost green crosslinker. The influence of crosslinking PA concentration (6, 10, 20 wt%), and time (12, 24, 48 h) on flavonoid rejection was investigated. Optimal molecular sieving occurred at 10 wt% PA, 24 h, and 25°C, yielding 80.8% rejection and correlating with significant bound PA (BPA) and negligible unbound acid. Structural characterization confirmed stable PBI-PA bonding by Fourier Transform Infrared (FTIR) spectroscopy analysis. The newly developed PBI-PA membrane exhibited remarkable chemical resistance, retaining stability against ethanolic citrus extract, reactive compounds such as limonene, and harsh organic solvents including N,N dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO). After crosslinking with PA, the fragile PBI membrane was transformed into a robust, flexible structure with a tensile strength of 7.91 ± 0.26 MPa and a permeate flux of the ethanolic citrus extract of 18.06 Lm−2h−1. These results demonstrate the potential of PBI-PA membranes to efficiently fractionate and concentrate antioxidant-rich flavonoids from citrus waste via OSN, with broader applicability for separating phytochemicals from diverse natural sources.