Membrane structure-dependent limiting flux behavior and membrane selectivity loss during gypsum scaling: Implications for pressure-retarded osmosis operation and membrane design

Herein, we systematically investigated the influence of membrane structural properties on limiting flux behavior and selectivity loss during gypsum scaling in osmotically driven membrane processes. We selected two typical osmotic membranes, thin-film composite (TFC) polyamide (PA) membrane and integrally asymmetric cellulose triacetate (CTA) membrane with different structures, for gypsum scaling tests in active-layer-facing-draw-solution orientation (an operating mode preferred for pressure-retarded osmosis). Compared to the CTA membrane, the TFC membrane suffered severer internal scaling and achieved a lower limiting flux primarily due to its greater structural parameter that induced severer internal concentration polarization (ICP)-enhanced scaling. The limiting flux is inversely proportional to the membrane structural parameter. For the first time we observed that the TFC membrane suffered a drastic loss of integrity and selectivity after gypsum scaling in PRO. We confirmed that the thin PA layer of TFC membrane is more prone to being damaged by the growth of gypsum crystals inside the confined and unstirred support layer, whereas the integrally asymmetric membrane with a thicker active layer could better maintain its integrity. While TFC membrane is the mainstream for PRO in osmotic power harvesting, our study suggests that the integrally asymmetric membrane may be more suitable under severe scaling conditions.