Mechanistic study of micropollutants rejection by nanofiltration of a natural water

A natural water sampled after a sand filtration step and spiked with four organic micropollutants (metolachlor ESA, metolachlor NOA, desethylatrazine and metaldehyde) was treated by a loose nanofiltration membrane. The Steric, Electric, and Dielectric model (SEDE model) was then used to predict the separation performance of the membrane towards the various ions and micropollutants in the water matrix in order to study the transport mechanism of ions and micropollutants through the membrane. The SEDE model was found to satisfactorily predict the rejection sequences of inorganic anions and cations, as well as neutral (desethylatrazine and metaldehyde) and charged (metolachlor ESA and metolachlor NOA) micropollutants. The dielectric exclusion mechanism was found to be negligible, most likely due to the loose structure of the membrane. The complex behaviour of cations (counterions) was explained by the interplay between the Donnan exclusion, electromigration and steric hindrance effects. The model was found to overestimate the rejection of charged micropollutants, such as metolachlor NOA and metolachlor ESA. It was suggested that it may be attributed to the adsorption of micropollutants on some weakly rejected fractions of natural organic matter (NOM) such as humic substances, which was supported by higher rejection rates observed in a model solution replicating the ionic composition of the natural water matrix but lacking NOM.