Vol 14, No 5 (2024)

The effect of three techniques of modification of polyacrylonitrile (PAN) ultrafiltration membranes with polyelectrolytes was studied: (1) bulk modification by addition of polyacrylic acid (PAA) into the casting solution (CS), (2) surface modification by using aqueous solutions of polyethyleneimine (PEI) as a coagulation bath (CB), (3) combined modification by addition of PAA into the CS and using PEI solutions as CBs – on their structure and performance. In all three cases, modification with polyelectrolytes led to an effective hydrophilization of the surface of ultrafiltration membranes (the water contact angle decreased from 41 to 15–25°). It was found that bulk modification of PAN membranes with 0.2 wt. % PAA yielded the decrease of the water flux from 110 to 96 L/m² h. However, surface modification of PAN membranes using aqueous solutions of PEI as CBs resulted in the increase in water flux more than 2 times from 110 to 294 L/m² h. It was shown that the combined modification technique reduced the water flux of PAN membranes down to 44 L/m² h due to structure compaction, confirmed by scanning electron microscopy studies. It was revealed that the combined modification technique allowed to obtain ultrafiltration PAN membranes with a high degree of flux recovery ratio after filtration of model solutions of polyvinylpyrrolidone (73–100% compared to 65% for the reference PAN membrane) and humic acids (80% compared to 73% for the reference PAN membrane).

A method for calculating the rate constants of the limiting stages of the water molecules dissociation reaction in generating contacts of heterogeneous bipolar membranes (BPM) containing catalytic additive particles is developed. The method is based on using the equation of the current-voltage characteristic of the bipolar region of a heterogeneous BPM containing generating contacts of two types. For the case when the catalytic additive is a cation exchanger (CE), one of the contacts is formed by CE particles and anion exchanger (AE) particles contained in the BPM layers, and the other by catalytic additive particles and AE particles contained in the BPM layers. The series of the rate constants of the limiting stages of the water molecule dissociation reactions in the studied membranes is consistent with the catalytic activity series, the constants of which are calculated based on the proton transfer reactions between water molecules and ionogenic groups contained in the BPM layers.

The stability of the hydrophobic properties of coatings with a morphologically developed (textured) surface prepared from polytetrafluoroethylene and ultra-high molecular weight polyethylene during storage, as well as during prolonged contact with water and aqueous solutions of sodium chloride with concentrations from 5 to 15 g/L has been studied. These coatings were deposited on the surface of a poly(ethylene terephthalate) track-etched membrane by electron-beam dispersion of the pristine polymers in vacuum. It is found that coatings from polytetrafluoroethylene under the influence of real environmental conditions tend to age and gradually lose their hydrophobic properties. The water contact angle of the coatings decreases by an average of 30° during storage samples of composite membranes for 5 years. This is 23% of the pristine value. The decrease in the contact angle of coatings of this type is due to the transition from a heterogeneous wetting mode to a homogeneous one, the reason for which is the formation of an adsorption layer of water on their surface. In contrast, the water contact angle for coatings from ultra-high molecular weight polyethylene practically does not change during storage of membrane samples. A study of the polymer coatings stability during prolonged contact of composite membranes with water and aqueous solutions of sodium chloride showed that if coatings from ultra-high molecular weight polyethylene are stable in both water and aqueous solutions of sodium chloride, then coatings from polytetrafluoroethylene are more stable to the action of aqueous salt solutions than water.

Approximately 90 million barrels of crude oil are processed daily worldwide, with separation processes such as distillation accounting for 10−15% of global energy consumption. In this regard, the scientific community is faced with the ambitious task of finding alternative fractionation technologies that are not based on the volatility of individual components of complex liquid mixtures. The driving force of ultrafiltration is the pressure differential across the membrane, enabling separation without phase transitions and with significantly lower energy consumption compared to distillation. In recent years, there has been a growing interest in the development of membrane technologies for the purification and reuse of used lubricating oil. One of the key challenges in membrane filtration of oil and lubricants is their high viscosity. This review examines two approaches to reducing the viscosity of such systems: filtration at elevated temperatures and pre-dilution of the feedstock followed by filtration. A literature analysis revealed that in most cases, ultrafiltration with ceramic membranes is employed in the former approach, while the latter uses more cost-effective polymer membranes. Special attention in the review is given to the issues of membrane fouling and regeneration.