Quantifying the influence of solute-membrane interactions on adsorption and rejection of pharmaceuticals by NF/RO membranes

Abstract

The solute-membrane interactions between trace organic compounds (TrOCs) and nanofiltration (NF) or reverse osmosis (RO) membranes result in the adsorption of TrOCs onto membranes and in turn affect the rejection of TrOCs. This study investigated the adsorption and rejection of four positively charged, two neutral and one negatively charged pharmaceuticals (PhACs) by four commercial NF/RO membranes to correlate the adsorbed amount and the steady-state rejection, and to quantitatively evaluate the role of various solute-membrane interactions in adsorption and rejection. The adsorbed amounts of PhACs were determined in static adsorption tests using the isolated polyamide (PA) layers when the RO and tight NF membranes were used, and were calculated from the decline of rejection during filtration when the two loose NF membranes were used. The impacts of electrostatic and non-electrostatic interactions (including hydrophobic interaction and hydrogen bonding) were quantified for positively charged PhACs by comparing the respective adsorption and rejection at neutral pH with that at the isoelectric point (IEP) of each membrane. Results showed that at neutral pH, the adverse effect of adsorption on the steady-state rejection was < 6% for the tight ESPA1 and NF90 membranes and 7–36% for the loose NF270 and HL. A higher adsorbed amount generally corresponded to a larger relative decrease of rejection for all the four membranes. The adsorbed amounts of the positively charged PhACs onto the isolated PA layers of ESPA1 and NF90 at neutral pH were primarily attributed to electrostatic attraction (generally > 65%). Electrostatic attraction was also found to cause the rejection of the tight membranes for the positively charged PhACs to decrease by 0.8–4.3%, that of the loose membranes to decrease by 13.4–28.3%, while the impact of non-electrostatic interactions on the rejection was 1.1–2.3% for the tight membranes and 4.0–9.8% for the loose membranes.