Effects of membrane degradation on the removal of pharmaceutically active compounds (PhACs) by NF/RO filtration processes

The impacts of membrane degradation due to chlorine attack on the rejection of pharmaceutically active compounds (PhACs) by nanofiltration and reverse osmosis membranes were investigated in this study. Membrane degradation was simulated by soaking the membranes in a sodium hypochlorite solution of various concentrations over 18 h. Changes in membrane surface properties were characterised by contact angle measurement, atomic force microscopy analysis, and streaming potential measurement. The impacts of hypochlorite exposure to the membrane separation processes were ascertained by comparing the rejection of PhACs by virgin and chlorine-exposed membranes. Overall, the reverse osmosis BW30 membrane and the tight nanofiltration NF90 membrane were much more resilient to chlorine exposure than the larger pore size TFC-SR2 and NF270 nanofiltration membranes. In fact, rejection of all three PhACs selected in this study by the BW30 remained largely unchanged after hypochlorite exposure and further characterisation did not reveal any evidence of compromised separation capability. In contrast, the effects of chlorine exposure to the two loose nanofiltration membranes were quite profound. While chlorine exposure generally resulted in reduced rejection of PhACs, a small increase in rejection was observed when a more dilute hypochlorite solution was used. Changes in the membrane surface morphology as well as observed rejection of inorganic salts and PhACs were found to be consistent with mechanisms of chlorine oxidation of polyamide membranes reported in the literature. Chlorine oxidation consistently resulted in a more negative zeta potential of all four membranes investigated in this study. Conformational alterations of the membrane polyamide active skin layer were also evident as reflected by changes in surface roughness before and after chlorine exposure. Such alterations can either loosen or tighten the effective membrane pore size, leading to either a decrease or an increase in rejection. Both of these phenomena were observed in this study, although the decrease in the rejection of PhACs was overwhelming from exposure to highly concentrated hypochlorite solution.