Assessing the passage of small pesticides through reverse osmosis membranes

Attenuation of trace organic chemicals (TOrCs), including pesticides, by reverse osmosis (RO) membrane treatment is critical for ensuring public health protection in potable water reuse. This study aimed to elucidate the mechanisms underlying the poor rejection of small pesticides by polyamide-based RO membranes. Rejection of the selected TOrCs (four N-nitrosamines and 158 pesticides) was primarily governed by size exclusion, charge interactions, and dipolar interactions when evaluated at high water temperatures. Further investigation indicated that small and uncharged secondary amide pesticides showed low and highly variable rejections, compared to similarly sized counterparts with no amide functional groups. Remarkably, three secondary amide pesticides that have no other atoms holding a high partial negative charge showed very low rejections (34–65%), likely due to the cooperativity of hydrogen bonding which occurs between amide functional groups of the pesticides and RO membranes. In contrast, secondary amide pesticides that have an atom holding a high partial negative charge showed higher rejections (72–98%), likely due to the inducted electrostatic repulsion. This study proposed that secondary amide pesticides that have no other atoms holding a high partial negative charge can be poorly rejected. The findings are useful to predict the rejection level of unregulated TOrCs.