Membrane distillation (MD) is a rapidly developing field of research and finds applications in desalination of water, purification from nonvolatile substances, and concentration of various solutions. This review presents data from recent studies on the MD process, MD configuration, the type of membranes and membrane hydrophobization. Particular importance has been placed on the methods of hydrophobization and the use of track-etched membranes (TeMs) in the MD process. Hydrophobic TeMs based on poly(ethylene terephthalate) (PET), poly(vinylidene fluoride) (PVDF) and polycarbonate (PC) have been applied in the purification of water from salts and pesticides, as well as in the concentration of low-level liquid radioactive waste (LLLRW). Such membranes are characterized by a narrow pore size distribution, precise values of the number of pores per unit area and narrow thickness. These properties of membranes allow them to be used for more accurate water purification and as model membranes used to test theoretical models (for instance LEP prediction).
In this paper, we present the results of liquid low-level radioactive wastes (LLLRW) treatment by direct contact membrane distillation (DCMD) using polyethylene terephthalate (PET) track-etched membranes (TeMs). PET TeMs were modified by styrene and triethoxyvinylsilane (TEVS) using UV-induced grafting. Modification led to increase in the contact angle to 99°of PET TeMs (pore size from 150 to 300 nm). Hydrophobic PET TeMs were investigated by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), goniometric analysis, gas permeability test, liquid entry presser (LEP) analysis and scanning electron microscope (SEM). Prepared membranes were tested in treatment of LLLRW by DCMD. The influence of pore size on water flux and rejection degree was studied. Rejection degree was evaluated by conductometry and atomic emission methods. Decontamination factors (evaluated by gamma-ray spectroscopy) for 60 Co, 137 Cs, and 241 Am were found to be 85, 1900 and 5 respectively. In most cases degree of rejection of Cs, Mo, Sr, Sb, Al, Ca, Fe, K, Mg and Na ions were more than 90% and close to 100%. The use of TeMs with a narrow pores size distribution and without tortuous channels allowed us to achieve better purification from radioactive wastes in comparison with hollow-fiber membranes.
Phenols are one of the most common surface water pollution. The discharge of phenolic waters into water bodies and streams sharply degrade their general sanitary condition, since these compounds have a toxic effect, and phenols can intensively absorb oxygen dissolved in water, which negatively affects the life of organisms in water bodies. Therefore, water treatment of phenols is an important environmental problem. In this study, the hydrophobic polyethylene terephthalate track-etched membranes (PET TeMs) were tested in water treatment from phenol by direct contact membrane distillation (DCMD). Hydrophobic PET TeMs were obtained by UV-graft polymerization of styrene, triethoxyvinylsilane with the addition of vinylimidazole (VIM), as well as by coating with fluorine-containing silanes. Scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy and liquid entry pressure (LEP) analysis were used for membrane characterization. The contact angle after modification of PET TeMs was reached more than 130°. The efficiency of water purification from phenol was evaluated by water-flux measurements and fluorimetric method. The phenols solution was used at a concentration of 0.5, 1 and 2 g/l. The largest permeate flux of hydrophobized membranes was 1.1 kg/ m2•h.
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