Micro-to nano-scale characterisation of polyamide structures of the SW30HR RO membrane using advanced electron microscopy and stain tracers

Abstract

The development of new reverse osmosis (RO) membranes with enhanced performance would benefit from a detailed knowledge of the membrane structures which participate in the filtration process. Here, we examined the hierarchical structures of the polyamide (PA) active layer of the SW30HR RO membrane. Scanning electron microscopy combined with focused ion beam milling (FIB-SEM) was used to obtain the 3-D reconstructions of membrane morphology with 5 nm cross-sectional resolution (comparable with the resolution of low magnification TEM imaging in 2D) and 30 nm slice thickness. The complex folding of the PA layer was examined in 3 dimensions, enabling the quantification of key structural properties of the PA layer, including the local thickness, volume, surface area and their derivatives. The PA layer was found to exhibit a much higher and convoluted surface area than that estimated via atomic force microscopy (AFM). Cross-sectional scanning transmission electron microscopy (STEM) was used to observe the distribution of a tracer stain under various conditions. The behaviour of stain in dry and wet PA indicated that the permeation pathways have a dynamic nature and are activated by water. High resolution STEM imaging of the stained PA nano-films revealed the presence of <1 nm pore-like structures with a size compatible with free volume estimations by positron annihilation lifetime spectroscopy (PALS). This study presents a comprehensive map of the active PA layer across different length scales (from micro- to sub-nanometre) and mechanistic insight into their role in the permeation process

Morphology of Thin Film Composite Membranes Explored by Small-Angle Neutron Scattering and Positron-Annihilation Lifetime Spectroscopy

“…These large lifetimes are beyond the Tao-Eldrup model. However, a mean pore radius in the order of 40-90 Å can be estimated considering the o-Ps mean free path and measurements at room temperature [33,35], which is consistent with the observation of individual voids of about 75 Å in the PA surface layer [36]. The near surface region (<600 Å) of the sample exhibits small free volume, which is indicated by the low intensity of both τ 3 , and τ 4 (Figure 9a) and the decline of τ 3.…”

“…Taking into account that PALS "sees" the shortest distance within a pore, and SANS averages over all distances in a pore, both methods give very similar pore radii. A further aspect to be considered is the network structure of R < 10 Å pores predicted in [36] and observed with SANS/PALS in [4], which appears relevant for the transport of water.…”

Neutron Reflectivity and Performance of Polyamide Nanofilms for Water Desalination

“…[13][14][15] Despite considerable research in RO membrane structure and transport, [10] our fundamental understanding and thus ability to achieve high selectivity (in addition to high permeability) and, in turn, reduce the energy consumption of separation processes and improve performance [16,17] remains limited and progress has been largely empirical. [21,22] The resulting cross-linked polyamide (PA) film has an overall "apparent" film thickness of, typically, a few hundred nm, and is rough and crumpled, with individual PA film thickness of the order of 10 nm, as revealed by recent high-resolution imaging reports, [23][24][25][26] supported by a porous polysulfone layer with a heterogeneous nanoscale interface layer. [10,[18][19][20] The separating or active layer is usually manufactured by interfacial polymerization (IP) at the organic/aqueous interface between an aromatic diamine (m-phenylenediamine, MPD) and trimesoyl chloride (TMC).…”

“…Formally, the organic-soluble component (acid chloride) is insoluble into the polymer, whereas the water-soluble component (diamine) diffuses through the film to react with the acid chloride at the film/organic phase interface, where the film grows. [25,30,31] Toward this goal, considerable effort has been dedicated to the structural characterization of the skin layer of commercially available RO membranes, [26,[28][29][30][31][32][33][34][35][36][37] generally carried out under vacuum (away from operating conditions), molecular modeling, [38][39][40][41][42][43] or by developing more controlled synthetic pathways. [27] Despite being widely used on an industrial scale, membranes obtained via IP are generally inhomogeneous in terms of spatial variation, chemistry, and porosity at the micrometer to nanoscales.…”

Electron tomography reveals details of the internal microstructure of desalination membranes

“…Nevertheless, resolving the interplay between the 3D nanostructure and transport properties remains challenging, despite notable advances (1). Developing approaches to resolve and analyze the 3D structure of membrane materials at scales approaching molecular dimensions will likely be crucial in applications such as ultrafiltration (2), nanofiltration (3), virus filtration membranes (4), gas separations (5), filtration for pharmaceuticals (6, 7), and reverse osmosis (RO) (1,8).Of the various available technologies, RO has become the most widely used approach for desalination and wastewater reuse due to its economic benefits, high permeability, and selectivity (9-14). RO systems rely on a semipermeable membrane to enable water passage while rejecting ions.…”