Nanofiltration

Gold (Au) nanosols stabilized with poly(vinylalcohol) can be used as homogeneous oxidation catalysts, both in water and in a variety of alcoholic solvents. Under 0.5 MPa O2 pressure, 1,2-diols with varying chain length, such as 1,2-propanediol or 1,2-octanediol, are oxidized to the corresponding α-hydroxy-carboxylates with total chemoselectivity. The activity of the sol is of the same order as previously reported for stabilized nanoparticles deposited on carbon supports. The stabilized sols retain their activity over extended periods. The Au metal colloids can efficiently be recycled by means of a nanofiltration, either from an aqueous reaction medium, or from an organic solvent. The most suitable membranes for the Au sol recuperation are cellulose acetate membranes for the aqueous filtration, and poly(dimethyl)siloxane membranes for the solvent resistant filtration. Recycling tests after nanofiltration show that catalytic activity is largely preserved in consecutive runs.

This study investigated the removal efficiency and mechanisms of water contaminants (mainly N-nitrosamines) during municipal wastewater reclamation by a membrane bioreactor (MBR) and nanofiltration (NF) hybrid system. The removal of bulk water contaminants was governed by the microbial activities in the MBR and molecular weight cut-off (MWCO) of the NF membranes. The removal of N-nitrosamines by the MBR was primarily attributed to biodegradation by aerobic bacteria, which can be determined by the reactivity of the amine functional groups with the catabolic enzymes (removal efficiency=45-84%). Adsorption and formation of membrane fouling can enhance the removal of N-nitrosamines by the NF membranes. However, size-exclusion is found to play a major role in the removal of N-nitrosamines by the NF membranes since the removal efficiencies of N-nitrosamines varied significantly depending on molecular weight of the N-nitrosamines and MWCO of the NF membranes (removal efficiency: NE90>NE...

In this study the selectivity of two nanofiltration (NF) membranes was tested with the aim of increasing the sugar content of grape must used for wine production. The composition of grape must, concentrate, permeate and the rejection values for selected physicochemical parameters is presented. In particular, the NF membranes provided a high rejection for sugars (range 77–97%), whereas the malic acid was retained to a low extent (range 2–14%). A two-stage NF process of grape must remove a permeate volume of 14%, thus allowed concentration of the sugars, i.e. the potential alcohol, to ca. 16%. The wine obtained from a grape must treated by NF membrane was high in total plyphenols and DO (optical density) at 420 nm.

Due to their excellent resistance to a range of solvents, integrally skinned polyimide membranes have been used to achieve selective separations in a range of solvent-based industrial and lab-scale chemical operations. These include: homogeneous catalyst recycle, petrochemical dewaxing, solvent exchange and chiral resolutions. However, despite the widening scope of use of these membranes, there is still little understanding of how the microstructure defines their separation performance. As a first step towards determining this, integrally skinned nanofiltration membranes were fabricated by phase inversion using Lenzing P84 polyimide. The microstructures of these membranes, dry and wetted in solvent, were investigated by SEM, TEM and ESEM (where appropriate).SEM and TEM imaging of dry membranes revealed that this type of polyimide membrane has three microstructurally distinct polyimide layers, not the two indicated in prior literature. Furthermore, TEM images reveal nano-sized pore-like features in the polyimide structure, which indicate that the transport mechanism is probably neither only solution-diffusion nor only pore flow. ESEM imaging showed that when saturated in ethanol at the working pressure of the ESEM (5.50 Torr), the microstructure of the membranes changes; it is wispy and thus quite different to the more solid polymer nodules and interconnected polymer network observed in the dry membranes. Thus, transport and separation mechanisms based on the structure of the dry membranes may not be accurate. Overall, these results indicate that the current theory used to describe polyimide membrane mass transfer and separation performance (where it is based on dry membrane microstructures), most likely needs to be rethought.

A rapid, reliable and cheap method of characterizing the molecular weight cut-off (MWCO) of commercial and in-house fabricated membranes used in aqueous applications has been developed. MWCO can be determined by performing a single run of pressure filtrations with a mixture of polyethylene glycols (PEGs) in aqueous solution using one of two PEG oligomer mixtures: PEG 200 to PEG 1000 for nanofiltration membranes or PEG 1000 to PEG 6000 for low MWCO ultrafiltration membranes. Analysis is via a repeatable and accurate reverse phase high performance liquid chromatography (HPLC) method through a cheap chemical-bonded silica-based C8 column which finely resolves each of the PEG oligomers. Detection is via a low temperature evaporative light scanning detection (ELSD) method that resolves peaks to a molecular weight difference of just 44 g mol−1 (corresponding to the CH2–O–CH2 structural unit difference between the oligomers), allowing the most precise ever one-filtration determination of MWCO. MWCO testing of commercially available membranes (from Koch, Filmtec™ and Hydranautics) confirmed the method gives MWCO in the expected range. Consequently, this new and refined method can effectively replace the previously required time consuming and costly multiple filtrations of individual PEG oligomers and other compounds for the determination of membrane MWCO for aqueous applications.► One-filtration PEG based MWCO quantification for aqueous applications developed. ► Based on a high resolution reverse phase HPLC-ELSD analysis for PEG mixtures. ► Resolves MWCO within 44 g mol−1 giving the most precise ever one-filtration method. ► Produced accurate MWCO compared to manufacturers values for five commercial NF membranes.

Lactose retention was predicted by means of the Donnan Steric Partioning model (DSPM) and the Kedem–Spiegler model (KSM). Retention and permeate flux were measured using single lactose solutions and ultrafiltered whey (UF-whey) as feed solutions. The experiments were performed at constant pH with NF200 and desal 5 DL nanofiltration membranes supplied by Dow-Chemical (USA) and GE-Osmonics (USA), respectively. The two models were able to predict reasonably well the pattern and tendencies of lactose retention as a function of permeate flux when either a lactose solution or UF-whey was used as feed. The best predictions were obtained with the KSM. This model was observed to be able to predict lactose retention in the presence or absence of ions without providing any information about the membrane structure. However, the DSPM provides information about the charge, the thickness–porosity ratio and the pore size of the membranes. This contributes to a better understanding of the transport mechanism in nanofiltration processes. The presence of salts increased the values of the fitted mean pore radius and the fitted thickness/porosity ratio of the membranes. For the two membranes tested lactose retentions were lower in the case of UF-whey. This was explained taking into account the effect of salts on membrane performance.