Neutral and small molecules such as boric acid(H
3 BO
3 )are poorly rejected by state-of-the-art reverse osmosis(RO)and nanofiltration(NF)membranes. Therefore, the objective of this study was to investigate physico-chemical properties of active layers controlling the H
3 BO
3 rejection by polyamide composite RO/NF membranes. To achieve the this objective, we measured the concentration of deprotonated carboxy group(R-COO
-)of polyamide active layer of eight commercial RO/NF membranes by probing R-COO
- with the Ag
+ at pH 6.0 and 10.0 and quantifying the Ag
+ using a Rutherford backscattering spectrometer. We also characterized polyamide active layers using thermal gravimetric analysis. These physico-chemical properties of RO/NF membranes were then used to investigate the relationship with water/H
3 BO
3 transport parameters obtained by modeling experimental data using the solution-diffusion model allowing for the existence of unhindered advection through nano-scale imperfections. It was found that the H
3 BO
3 permeability and water selectivity of RO/NF membranes were most strongly related to the temperature at which 20 % weight loss occurs, an indicator of molecular weight of polyamide active layers. Positive correlation, although weak, was also observed with the concentration of R-COO
- at pH 6.0. These results indicate that aggregate pores in polyamide active layers play an important role in water and H
3 BO
3 permeation through RO/NF membranes.
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