Determination of Partition and Diffusion Coefficients of Arsenious Acid into （in） the Active Layer of Thin-film Composite Reverse Osmosis and Nanofiltration Membranes

Abstract

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 （K_PA-w）, the H₃AsO₃ diffusion coefficients （D_PA）, 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 （A_D） normalized by the active layer thickness （δ_PA）. The comparison of these water and H₃AsO₃ transport parameters between membranes showed that K_PA-w were within a narrow range for all RO/NF membranes investigated （2.4-3.8）. On the other hand, the differences in D_PA and A_D×δ_PA 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 D_PA, 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 A_D×δ_PA value, probably resulting from the thin dense polyamide layer effective to water transport restriction.