Effect of Nanofiltration Operating Parameters on Phytochemical Separation Using Polybenzimidazole Mixed Matrix Membrane via Organic Solvent Nanofiltration

Nanofiltration (NF) offers a sustainable, low-temperature strategy for the recovery of thermolabile bioactive compounds from plant-derived matrices; however, the coupled effects of operating parameters during NF of ethanol-rich food extracts remain insufficiently explored. This study investigated the combined influence of temperature (from 20 to 30 °C) and transmembrane pressure (TMP, 10–30 bar) on flavonoid rejection, total antioxidant activity (TAA) rejection, and permeate flux during the NF of a 75% (v/v) ethanolic citrus peel extract using a phosphoric acid-crosslinked polybenzimidazole (PBI) mixed matrix membrane. A face-centered central composite design within a response surface methodology framework produced statistically robust quadratic models ( R 2 > 0.92). Results revealed that TMP predominantly enhances rejection through increased convective transport, and interfacial accumulation, while temperature exhibited a nonlinear effect governed primarily by a reduction in hydraulic resistance, accompanied by modest increases in solute diffusivity and temperature-dependent changes in phytochemical intermolecular interactions. The observed deviation between flavonoid and TAA rejection trends reflects compositional heterogeneity and size-dependent transport of structurally diverse antioxidant species. Multi-response optimization identified an optimum at 25.3 °C and 30 bar, achieving 91.5% flavonoid rejection, 77% TAA retention and a permeate flux of 11.4 L m −2 h −1 . Fouling analysis revealed intermediate pore blocking, coupled with concentration polarization, as the dominant fouling mechanism. Under the optimized operating conditions, the optimized PBI-O membrane exhibited superior antifouling performance and cleanability, achieving a lower fouling index (FI, 40.2% vs 76.5%) and a higher flux recovery ratio (FRR, 90.5% vs 85.1%) compared with the unmodified PBI membrane. These findings provide mechanistic insight for designing sustainable NF processes for valorizing citrus waste into functional food ingredients from complex ethanolic feeds.