Transport of trace organic contaminants (TrOCs) in pressure-assisted forward osmosis (PAFO)

Abstract

Pressure-assisted forward osmosis (PAFO) tests were conducted in lab-scale by applying hydraulic pressure to the feed side to evaluate the rejection of trace organic contaminants (TrOCs) using a cellulose triacetate (CTA) forward osmosis membrane. The test results showed a clear difference in transport mechanisms of TrOCs depending on their physicochemical properties; neutral compounds were rejected predominantly by steric hindrance mechanism while charged compounds showed unique rejection patterns depending on the charge of the compounds implying the change of their rejection mechanisms by hydraulic pressure. To statistically find the mechanistic effects of hydraulic pressure on the rejection of TrOC, improvement ratio, IR (Jw,PAFO/Jw,FO), reverse solute diffusion (Jsw = Js/Jw)and solute diffusion ratio, SDR (Jsw,PAFO/Jsw,FO) were computed for Pearson correlation analysis with solute rejection ratio, SRR (RPAFO/RFO). The Pearson correlation analysis for the rejection of neutral compounds showed the strongest correlation with SDR suggesting that the transport of neutral compounds with sizes bigger than the pore size of the membrane were significantly affected by the combined effect of both permeate water flux and reverse solute flux in association with steric hindrance. Negatively charged compounds showed a distinguishing rejection pattern suggesting the critical pressure point of which the rejection is the highest. Unlike the rejections of neutral and negatively charged compounds, positively charged compounds showed a clear decreasing rejection pattern in high correlation with IR and SDR indicating the facilitated diffusion of positively charged compounds. To sum up, additional hydraulic pressure in FO (i.e. PAFO) does not alter the transport mechanism for neutral compounds but significantly affect the TrOC rejection mechanisms of charged compounds depending on physicochemical properties of TrOCs.