The main objective of this study was to investigate the key water and arsenious acid (H
3AsO
3) transport process that is responsible for the difference in the H
3AsO
3 rejection by commercial polyamide reverse osmosis (RO) and nanofiltration (NF) membranes. Based on the partition/diffusion theory combined with the advective transport theory, H
3AsO
3 transport across membranes was characterized by the H
3AsO
3 partition coefficients (
KPA-w), the H
3AsO
3 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 (δ
PA).
The comparison of these water and H
3AsO
3 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×δ
PA between membranes were more pronounced, showing the H
3AsO
3 diffusivity and water permeability are the key transport processes differentiating the H
3AsO
3 removal efficiencies between membranes. The ESPAB RO membrane, which provided the highest H
3AsO
3 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 H
3AsO
3 removal efficiency due to the high
AD×δ
PA value, probably resulting from the thin dense polyamide layer effective to water transport restriction.
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