Colloidal interactions and fouling of NF and RO membranes: a review

Colloids are fine particles whose characteristic size falls within the rough size range of 1-1000 nm. In pressure-driven membrane systems, these fine particles have a strong tendency to foul the membranes, causing a significant loss in water permeability and often a deteriorated product water quality. There have been a large number of systematic studies on colloidal fouling of reverse osmosis (RO) and nanofiltration (NF) membranes in the last three decades, and the understanding of colloidal fouling has been significantly advanced. The current paper reviews the mechanisms and factors controlling colloidal fouling of both RO and NF membranes. Major colloidal foulants (including both rigid inorganic colloids and organic macromolecules) and their properties are summarized. The deposition of such colloidal particles on an RO or NF membrane forms a cake layer, which can adversely affect the membrane flux due to 1) the cake layer hydraulic resistance and/or 2) the cake-enhanced osmotic pressure. The effects of feedwater compositions, membrane properties, and hydrodynamic conditions are discussed in detail for inorganic colloids, natural organic matter, polysaccharides, and proteins. In general, these effects can be readily explained by considering the mass transfer near the membrane surface and the colloid-membrane (or colloid-colloid) interaction. The critical flux and limiting flux concepts, originally developed for colloidal fouling of porous membranes, are also applicable to RO and NF membranes. For small colloids (diameter?100 nm), the limiting flux can result from two different mechanisms: 1) the diffusion-solubility (gel formation) controlled mechanism and 2) the surface interaction controlled mechanism. The former mechanism probably dominates for concentrated solutions, while the latter mechanism may be more important for dilute solutions. Future research needs on RO and NF colloidal fouling are also identified in the current paper.     

Properties of electrospun PVDF/PMMA/CA membrane as lithium based battery separator

Poly vinylidene fluoride:poly methyl methacrylate:cellulose acetate (CA) at ratios of 100:0:0, 90:10:0, 90:5:5 and 90:0:10 respectively, were successfully electrospun. These membranes were mixed to form a 12 wt% solution prepared with volume ratio 7:3 of DMAc:acetone solvents. These membranes were then analyzed using differential scanning calorimetry, scanning electron microscopy, FTIR, WAXD, pore size, porosity% and electrolyte uptake (EU)%. It was observed that the best absorption results were obtained in the presence of CA. The electrospun membrane at ratio of 90:0:10 was observed with the highest porosity of 99.1 % and EU at 323 %. It also had a 43.6 % crystallinity and a 162 °C melting temperature. It was then concluded that addition of CA improved the separator properties.     

Phase diagrams,mechanisms and unique characteristics of alternating-structured polymer self-assembly via simulations

Alternating-structured polymers(ASPs), like alternating copolymers, regular multiblock copolymers and polycondensates, are very important polymer structures with broad applications in photoelectric materials. However, their self-assembly behaviors,especially the self-assembly of alternating copolymers, have not been clearly studied up to now. Meanwhile, the unique characteristics therein have not been systematically disclosed yet by both experiments and theories. Herein, we have performed a systematic simulation study on the self-assembly of ASPs with two coil alternating segments in solution through dissipative particle dynamics(DPD) simulations. Several morphological phase diagrams were constructed as functions of different impact parameters. Diverse self-assemblies were observed, including spherical micelles, micelle networks, worm-like micelles, disklike micelles, multimicelle aggregates, bicontinuous micelles, vesicles, nanotubes and channelized micelles. Furthermore, a morphological evolutionary roadmap for all these self-assemblies was constructed, along with which the detailed molecular packing models and self-assembly mechanisms for each aggregate were disclosed. The ASPs were found to adopt a folded-chain mechanism in the self-assemblies. Finally, the unique characteristics for the self-assembly of alternating copolymers were revealed especially, including(1) ultra-fine and uniform feature sizes of the aggregates;(2) independence of self-assembled structures from molecular weight and molecular weight distribution;(3) ultra-small unimolecular aggregates. We believe the current work is beneficial for understanding the self-assembly of alternating structured polymers in solution and can serve as a guide for the further experiments.     

Asararenes—A Family of Large Aromatic Macrocycles

We announce the establishment of a new family of macrocycles—the asararenes, which are based on para‐methylene linked “asarol methyl ether” (1,2,4,5‐tetramethoxybenzene) units. Macrocycles with 6–12 aromatic units have been synthesized and isolated in a single step from asarol methyl ether and paraformaldehyde. Even larger rings, with up to 15 asarol methyl ether units, have been observed by high‐resolution mass spectrometry. Single‐crystal X‐ray structures of asar[6]‐, asar[7]‐, asar[8]‐, asar[9]‐, asar[10]‐ and asar[11]arene highlight the diverse structural features of this family of macrocycles. While the cavities of the asar[6–8]arene macrocycles are mostly filled with methoxyl groups, the asar[9]‐ and asar[10]arene rings contain accessible cavities and self‐assemble into infinite channels filled with solvent molecules in the solid state. These solid‐state structures highlight the potential of this family of macrocycles for a wide range of potential applications.     

The role of foulant–foulant electrostatic interaction on limiting flux for RO and NF membranes during humic acid fouling—Theoretical basis,experimental evidence,and AFM interaction force measurement

A limiting flux model has been recently developed for predicting the fouling behavior of reverse osmosis and nanofiltration membranes by organic macromolecules [C.Y. Tang, J.O. Leckie, Membrane independent limiting flux for RO and NF membranes fouled by humic acid, Environmental Science and Technology 41 (2007) 4767–4773]. Several interesting results have been observed: (a) there was a maximum pseudo-stable flux (the limiting flux) beyond which further increase in applied pressure did not translate to a greater stable flux; (b) all membrane samples attained the limiting flux under constant pressure conditions as long as their initial flux was greater than the limiting flux; (c) the limiting flux did not depend on the properties of membranes; (d) the limiting flux had strong dependence on the feedwater composition, such as pH, ionic strength, and divalent ion concentration. The current study investigates the dependence of limiting flux on intermolecular interaction between foulant molecules. It was observed that the limiting flux was directly proportional to the intermolecular electrostatic repulsive force and that conditions enhancing foulant-deposited-foulant repulsion resulted in greater limiting flux values. Such observations agree well with a theoretical model capturing both hydrodynamic and DLVO interactions. Interaction force measurements by atomic force microscopy (AFM) were also performed. The limiting flux correlated reasonably well with AFM interaction force between the model foulant and the fouled membrane surface.