Elucidating the role of salinity in regulating gypsum scaling in reverse osmosis and nanofiltration

Mineral scaling remains a major limitation in reverse osmosis (RO) and nanofiltration (NF). Despite a wide range of salinities have been reported for feedwater treated by RO and NF, the role of salinity in regulating mineral scaling remains poorly understood. Here, we investigated how feedwater salinity, along with membrane transport property, governs gypsum scaling in dynamic crossflow RO/NF systems using membranes with varied perm-selectivity, complemented by static nucleation experiments. Static experiments showed that increasing NaCl concentration prolonged the induction time of gypsum nucleation by lowering ion activity and consequently the saturation index, whereas the energy barrier and kinetic pre-factor of nucleation remained unchanged. Dynamic RO/NF experiments further revealed that an elevated salinity alleviated gypsum scaling by decreasing the saturation level at the membrane surface (SIm). Although membranes with different water and salt permeances exhibited varying extents of gypsum scaling, SIm displayed a linear relationship with scaling severity across all conditions, indicating that feedwater salinity and membrane transport property jointly regulate gypsum scaling through their combined influence on SIm. These findings elucidate the importance of feedwater salinity in governing gypsum scaling via controlling the saturation level at the water-membrane interface, providing mechanistic insights into mineral scaling during RO and NF.