Emerging investigators series: a steric pore-flow model to predict the transport of small and uncharged solutes through a reverse osmosis membrane

Abstract

This study proposed a new approach to apply the steric pore-flow model to predict the rejection of eight N-nitrosamines and seven VOCs that are of great concern in potable water reuse through an RO membrane. In this approach, solute rejection is predicted by estimating the free-volume hole-size. The free-volume hole-radius was determined with pure water permeability of a membrane and a single reference compound – N-nitrosodimethylamine (NDMA) – by minimizing the variance between the experimentally obtained and calculated NDMA rejection values at the permeate flux of 20 L m⁻² h⁻¹. The obtained free-volume hole-radius of the ESPA2 RO membrane was 0.348 nm, which was larger than the value previously determined by positron annihilation lifetime spectroscopy (PALS) analysis (0.289 nm). The model incorporated with the estimated free-volume hole-radius could accurately predict the rejection of eight N-nitrosamines under a range of permeate flux (2.6–20 L m⁻² h⁻¹). The model was also validated using experimentally obtained VOC rejection values. The predicted VOC rejections at the permeate flux of 20 L m⁻² h⁻¹ were almost identical to their experimentally obtained rejections. However, VOC rejection prediction at a lower permeate flux was less accurate. Further improvement and validation of the model with a variety of trace organic chemicals is required to allow for a more accurate prediction. The model was also validated using the membrane free-volume hole-radius value previously obtained from PALS analysis. Using PALS data resulted in some over-prediction. The results suggest that additional adjustment is necessary when using data from PALS analysis for predicting the rejection of small and uncharged solutes.