Separation of organic pollutants by reverse osmosis and nanofiltration membranes: Mathematical models and experimental verification

Predictive reverse osmosis (RO) models have been well-developed for many sy stems. However, the applications to dilute organic-water systems require th e modification of transport models and the understanding of solute-polymer interactions. Studies with various substituted, nonionized phenolic compoun ds showed that these could cause substantial membrane water flux drop, even in dilute solutions with negligible osmotic pressure. Further, the organic s could significantly adsorb on the cross-linked aromatic polyamide active layer. In some cases, even concentrations as low as 0.2 mM, 2,4-dinitrophen ol (solution in particle-free, double-distilled water) can cause as much as a 70% flux drop with an aromatic polyamide membrane. Two models are presen ted in this paper: a modified steady-state solution diffusion model and an unsteady-state diffusion adsorption model which are able to predict flux an d permeate concentrations from a single RO experiment. Further, the develop ment of these models allows for the understanding of the mechanisms of orga nic-membrane interactions; For instance, it has been proposed that increase d adsorption inherently leads to an increase in flux drop. However, we have found, on one hand, that due to specific interactions with membrane water transport groups, chloro- and nitro-substituted phenols cause significant f lux drops. On the other hand, benzene had a high physical adsorption but ca used negligible flux drop. The results were further extended to nanofiltrat ion experiments with an aromatic pollutant containing two types of charge g roups. The adsorption and separation results are explained according to an ionization model.