The Ro Membrane With Or Without Pores Debate

The microstructure and filtration mechanism of reverse osmosis (RO) membranes, the most efficient filter membranes for contaminant retention, has been a hot topic of research. So, is an RO membrane a porous or non-porous membrane? The knowledge and answer to this question will have a direct impact on the understanding of the filtration mechanism of RO membranes. The answer to this question is changing with the advancement of technology and the deeper analysis of the microstructure of RO membranes.

Research into the mechanism of RO membrane filtration and its microstructure first began in the 1950s. As early as 1958, Kedem and other researchers proposed an apparent phenomenological model to describe the RO filtration process. This model only analysed the membrane surface at the apparent phenomenological level, dividing it into two parts: clean and clogged. The model assumptions were too ideal and did not address the structure of the RO membrane, which was far from the actual complexity.

Lonsdale et al. considered the functional layer of the RO membrane to be a dense, non-porous structure and proposed the first mechanistic model of RO membrane filtration based on this: the solution-diffusion model (SDM), in which solutes and solvents are driven by chemical potential differences to diffuse through the RO membrane. The SDM model was developed by Sherwood. This model was subsequently modified by Sherwood, who concluded that the surface of the RO membrane was not absolutely dense and non-porous, but had some 'pore' structures. However, this revision only considered these 'pores' as defects on the membrane surface and still ignored the effect of the membrane's fine structure on membrane performance, and did not consider the ideal RO membrane to be porous.

In 1970, RO membranes were first recognised as porous, and Sourirajan proposed the Preferention sorption-capillary flow model (PSCFM) based on the assumption of porous membranes. This model assumes that the pore size of the RO membrane surface is constantly changing and that the membrane pores can selectively permeate different components of the feed water, thus achieving separation. This provides a new way of thinking about RO membranes, which can be combined with Darcy's Law, a law of filtration for porous membranes, to describe the RO membrane filtration process.

Although most subsequent studies on the filtration mechanism of RO membranes have been based on the PSCFM hypothesis (i.e. RO membranes are porous), the porous structure of RO membranes has not been directly confirmed. It was not until 2011 that Chen et al. first demonstrated that RO membranes were porous using the novel Positron Annihilation Techniques. This was followed in 2014 by Jonathan et al. who used a new instrument, the nanopermporometry (NPP), to determine the pore size of RO membranes and compared the results to the normalised Knudsen-based permeance (NKP) and cationic attenuation (CEA). (NKP) and positron annihilation lifetime spectroscopy (PALS). Using positron annihilation lifetime spectroscopy, Hideaki et al. also calculated the pore size of RO membranes using positron annihilation lifetime spectroscopy. The pore size of the RO membrane was calculated to be in the range of 0.6-0.7 nm and is constantly changing, and the operating pressure was found to have an effect on the membrane pore size.

Due to the development of new instruments and techniques, there is an emerging consensus on the pore structure of RO membranes. This also means that the study of the RO membrane filtration process has entered a new phase.