Interfacial engineering of thin-film composite membranes enabling high-efficiency reactive extraction of medium-chain fatty acids

Coupling membrane extraction with chain elongation enables in-situ recovery of medium-chain fatty acids (MCFAs) from waste activated sludge, but interfacial instability and phase breakthrough remain major obstacles to practical application. To address this challenge, a thin-film composite (TFC) membrane was fabricated on a hydrophobic polytetrafluoroethylene (PTFE) porous membrane substrate via a polydopamine/polyethyleneimine (PDA/PEI) co-deposition strategy. TFC membrane significantly increased breakthrough pressure from 13 kPa (pristine PTFE) to 164–288 kPa, effectively preventing phase breakthrough. Notably, during the reactive extraction of MCFAs, the dense films in the TFC membrane did not hinder mass transfer; instead, enhanced both extraction efficiency (from 71.0% to 95.9%) and selectivity. These improvements arise from the enlarged interfacial reaction area provided by the rough film surface and the stronger binding of short-chain fatty acids to hydrophilic functional groups of the film, as confirmed by density functional theory (DFT) calculations. When integrated into a real chain elongation system, the adoption of the TFC membrane increased caproate production by 50%, accompanied by clear environmental and economic benefits (indicating promising economic prospects, with a net profit of 0.795 USD m −3 ). To our knowledge, this represents the first demonstration of TFC membranes in membrane extraction, revealing their unique ability to couple stability, efficiency, and selectivity, and opening new opportunities for sustainable MCFAs bioconversion.