The main objective of this study was to investigate the key water and arsenious acid （H3AsO3） transport process that is responsible for the difference in the H3AsO3 rejection by commercial polyamide reverse osmosis （RO） and nanofiltration （NF） membranes. Based on the partition/diffusion theory combined with the advective transport theory, H3AsO3 transport across membranes was characterized by the H3AsO3 partition coefficients （KPA-w）, the H3AsO3 diffusion coefficients （DPA）, and the fraction of water passing through membranes by advection （α）. The transport parameter used to characterize the water transport was the partition/diffusion water permeability （AD） normalized by the active layer thickness （δPA）.
The comparison of these water and H3AsO3 transport parameters between membranes showed that KPA-w were within a narrow range for all RO/NF membranes investigated （2.4-3.8）. On the other hand, the differences in DPA and AD×δPA between membranes were more pronounced, showing the H3AsO3 diffusivity and water permeability are the key transport processes differentiating the H3AsO3 removal efficiencies between membranes. The ESPAB RO membrane, which provided the highest H3AsO3 removal efficiency, was characterized by the lowest DPA, resulting from the highest crosslinking degree. In contrast, the ESPA3 and NF90 RO/NF membranes, which were characterized by the relatively low crosslinking degree, also achieved high H3AsO3 removal efficiency due to the high AD×δPA value, probably resulting from the thin dense polyamide layer effective to water transport restriction.