Influence of hexamethyl phosphoramide on polyamide composite reverse osmosis membrane performance

Abstract

In order to improve the performance of polyamide thin-film-composite (TFC) reverse osmosis membranes, hexamethyl phosphoramide (HMPA) was used as an additive in the aqueous polyamine solution during the interfacial polymerization. The addition of HMPA facilitated the diffusion rate of MPD from the aqueous phase to the organic phase resulting in a thicker reaction zone. HMPA could also increase the reaction rate between m-phenylenediamine (MPD) and trimesoyl chloride (TMC). X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed that both the thickness and the cross-linking extent of the polyamide skin layer increased when more HMPA was added. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed that membrane surface morphology changed with increasing HMPA concentration. The contact angle measurement results revealed that the membrane surface became more hydrophilic due to the addition of HMPA. The flux and salt rejection of MH-3 membrane (3 wt% HMPA addition) reached 51.67 L/m 2 h and 98.27%, respectively, for 2000 mg/L NaCl under 1.55 MPa. The flux increased by 73% and the salt rejection loss was less than 0.21% compared with the non-additive membrane. Colloidal silica fouling experiments suggested that the average peak-valley distance and average peak spacing width on the membrane surface could be used as important parameters to explain fouling behavior.