Fabrication of a robust high-performance FO membrane by optimizing substrate structure and incorporating aquaporin into selective layer

In present study, we explored the fabrication of a high performance thin-film composite (TFC) forward osmosis (FO) membrane by minimizing the structural parameter of hollow fiber substrates and enhancing the water permeability of selective layer. A theoretical analysis specified that a minimized structural parameter of substrate combined with an enhanced water permeability of selective layer could generate a significantly high water flux in FO process. The experimental results showed that the addition of LiCl into polyetherimide (PEI) polymer dope made the membrane less tortuous and thereby significantly reduced the structural parameter. The elevated take-up speed reduced the substrate thickness but also made the substrate more tortuous. Further study showed that a robust hollow fiber with low structural parameter can be obtained by reducing the substrate wall thickness and dimension simultaneously. After optimization, the structural parameter of the substrate declined from 308µm to 172µm. After interfacial polymerization, the thin-film composite membrane constructed on the optimized substrate showed an improved water permeability (from 2.85Lm⁻²h⁻¹bar⁻¹ to 3.66Lm⁻²h⁻¹bar⁻¹) and a comparably low salt permeability (0.36Lm⁻²h⁻¹ versus 0.31Lm⁻²h⁻¹⁾. The low structural parameter, in combination with the high water permeability and low salt permeability, contributed to significantly increased water flux (Jv) from 25.4Lm⁻²h⁻¹ to 38.5Lm⁻²h⁻¹ when 1M NaCl was used as draw and deionised water as feed in a configuration of active layer facing feed solution (AL-FS). Furthermore, aquaporins were incorporated into the polyamide selective layer to enhance the water permeability, which remarkably reached up to 7.6Lm⁻²h⁻¹bar⁻¹. The aquaporin-incorporated FO membrane demonstrated a Jv of 49.1Lm⁻²h⁻¹ and a Js/Jv (Js indicates the salt flux) of 0.10g/L with 1M NaCl as draw and deionised water as feed in the AL-FS configuration, which is favourable to mitigate membrane fouling in practical application. The AQP-FO membrane reported in this study outperforms most of other reported FO membranes.