Exploring the dependence of bulk properties on surface chemistries and microstructures of commercially composite RO membranes by novel characterization approaches

Abstract

Two kinds of novel reverse-osmosis (RO) membrane characterization approaches were firstly proposed for observing the backside and cross-sectional morphologies of polyamide separation layers and their extents of cross-linking, by using scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques, respectively. The surface chemistries (streaming potential and functional groups) and microstructures (polyamide thickness, morphology, and roughness) of three kinds of new commercial RO membranes (LP21, ULP21, and FR11) were comprehensively investigated. In addition, their salt rejections, and permeate fluxes were investigated in 2000ppm NaCl solution under different operating pressures ranging from 125 to 250psi; and their fouling resistances to insoluble CaCO3, MgCO3, and SiO2 colloids were also detailedly investigated. Based on the above results, the dependence of bulk properties (salt rejection, permeate flux, fouling resistance, and hydrophilicity) on surface chemistries and microstructures was carefully analyzed. Most importantly, it was observed that both of the top surface and backside of polyamide separation layer exhibit loose structures. The inner of separation layer should play an important role for the salt-rejection property of RO membrane. These results also provide important pieces of evidence for some previous reports on the microstructures of polyamide separation layers and desalting mechanism of RO membranes.