2.1. Materials
Polyacrylonitrile (PAN) ultrafiltration membranes developed at the Institute of Physical Organic Chemistry of the National Academy of Sciences of Belarus (Minsk, Belarus) were used as porous membrane-substrates for the formation of TFC membranes. Poly(acrylonitrile-co-methyl acrylate) copolymer (ratio of acrylonitrile and methyl acrylate monomer units: 93.6:6.4, Mw = 76,000 g/mol, Mw/Mn = 2.88; ηsp = 0.76, Haihang Industry Co., Ltd., Jinan, China) was used as the membrane material for the preparation of casting solutions for ultrafiltration PAN membranes (substrates) via the non-solvent induced phase inversion (NIPS). N,N-dimethylformamide (DMF, reagent grade, Khimmed, Moscow, Russia) was used as a solvent.
Chitosan (CS) (with medium molecular weight, “Sigma-Aldrich Co.”, St. Louis, MO, USA) was applied as a polymer material for the formation of a thin selective sublayer (the first dense layer) on ultrafiltration substrates. The maleic acid (MA, “Sigma-Aldrich Co.”, St. Louis, MO, USA) was used as cross-linking agent to stabilize the thin dense CS layer. MA was applied without additional treatment.
Triethylenetetramine (TETA, ≥96%, “BASF”, Ludwigshafen, Germany) as the amine component and trimesoyl chloride (TMC, ≥99%, “Sigma-Aldrich Co.”, St. Louis, MO, USA) as an acyl component were used for the formation of a second (upper) ultrathin selective layer on the TFC membrane by the interfacial polymerization technique. Distilled water was used to prepare TETA solutions, and «NEFRAS S2» (80/120, “Vershina” LLC, St. Petersburg, Russia) was used as an organic solvent for TMC. «NEFRAS S2» is a mixed type gasoline solvent, which contains not more than 50% hydrocarbons of each group; low boiling fraction of dearomatized gasoline of catalytic reforming.
Isopropanol (chemically pure, “Vecton”, St. Petersburg, Russia) and distilled water without additional treatment were used in pervaporation experiments.
Bovine serum albumin (BSA, M = 68,000 g/mol, “PanReac AppliChem”, Moscow, Russia) with a concentration of 0.5 g/L in phosphate buffer solution at pH = 7, aqueous solution of PVP K-30 (Mn = 40,000 g/mol, “Fluka”, Munich, Germany) and PVP K-15 (Mn = 10,000 g/mol, “Fluka”, Munich, Germany) with a concentration of 3 g/L were used as model solutions for rejection tests in ultrafiltration.
Poly (allylamine hydrochloride) (PAH, Mw = 50,000 g/mol, “Sigma-Aldrich Co.”, St. Louis, MO, USA) and chitosan (with medium molecular weight, “Sigma-Aldrich Co.”, St. Louis, MO, USA) were used as cationic polyelectrolytes, and poly (sodium 4-styrenesulfonate) (PSS, “Sigma-Aldrich Co.”, St. Louis, MO, USA) was used as an anionic polyelectrolyte in layer-by-layer assembly.
2.3. Membrane Investigation Methods
2.3.1. Scanning Electron Microscopy (SEM)
SEM micrographs of the cross-section and the selective surface structure of CS-based TFC membranes and PAN substrates were obtained using Zeiss Merlin SEM (Carl Zeiss SMT, Oberhochen, Germany). The cross-section of samples was obtained by submerging the sample in liquid nitrogen for 10–20 s and subsequent splitting perpendicular to the surface. SEM images were obtained at a voltage of 1 kV.
2.3.2. Atomic Force Microscopy (AFM)
The topography of the selective layer surface of CS-based TFC membranes and PAN porous anisotropic membrane-substrates were studied on an NT-MDT nTegra Maximus atomic force microscope with standard silicon cantilevers with a rigidity of 15 N∙m−1 (“NT-MDT”, Moscow, Russia) in tapping mode.
2.3.3. The Standard Porosimetry Method
The porosity of the porous PAN membrane-substrates was determined by the standard porosimetry method on a Porosimeter 3.1 instrument (Porotech Ltd., Ottawa, ON, Canada) at 30 °C. n-Octane was used as the reference liquid.
2.3.4. Ultrafiltration
PAN porous substrates (ultrafiltration membranes) used for the preparation of supported CS-based membranes were studied in ultrafiltration to evaluate their performance (pure water flux, flux of BSA or PVP solutions, rejection coefficients of BSA or PVP, flux recovery ratio). The measurements were carried out on a laboratory setup (stirred ultrafiltration cell) at room temperature (25 °C) with a transmembrane pressure of 1 bar and a stirring speed of 250–300 rpm (
Figure 2). BSA solution (0.5 g/L) prepared in a phosphate buffer solution (pH = 7) was used as a protein foulant to study antifouling performance.
The flux was determined as follows: ultrafiltration of pure water at 1 bar and room temperature was carried out for 30 min to reach the stationary ultrafiltration mode of the membrane and to wash it from glycerol. Thereafter, pure water flux was determined. Then a solution of BSA or PVP was placed into the cell and filtered for 30 min under the same conditions. Thereafter the BSA or PVP solution flux was measured. The flux
J (L/m
2h) was calculated according to the equation:
where
V (L) is the volume of liquid passing through the membrane (permeate),
A (m
2) is the membrane area and
t (h) is the time of ultrafiltration.
The BSA concentration in the feed solution and permeate was determined using SF-102 spectrophotometer at a wavelength of 280 nm. The PVP concentration was determined using LIR-2 interferometer («Zagorsk optical and mechanical plant», Sergiev Posad, Russia). The rejection coefficient
R (%) was calculated according to the equation:
where
(g/L) is the BSA or PVP concentration in permeate,
(g/L) is the BSA or PVP concentration in the feed solution.
The pure water flux was measured 30 min after ultrafiltration to assess the antifouling properties of the membranes in ultrafiltration of BSA solution. Then the BSA solution was filtered for 1 h. Thereafter water was again passed through the membrane for 30 min and pure water flux was measured again. The flux recovery ratio
FRR (%) was calculated according to the equation:
where
J (L/(m
2h)) is the pure water flux after BSA solution ultrafiltration and
J0 is the initial pure water flux.
2.3.5. Fourier-Transform Infrared Spectroscopy (IR Spectroscopy)
IR spectra of the selective layer surface of TFC membranes were obtained on a BRUKER-TENSOR 27 IR Fourier spectrometer (Bruker Optik GmbH, Billerica, MA, USA) in the range of 700–4000 cm−1 at 25 °C to confirm the formation of polyamide upper layer by IP method.
2.3.6. Pervaporation
The transport characteristics of the developed membranes were studied by vacuum pervaporation in a steady-state laboratory setup (
Figure 3) for the separation of the isopropanol/water mixture in a wide concentration range (12–100 wt.% water in the feed) and at various temperatures (28, 35 and 50 °C). The pervaporation condition was the pressure < 0.01 mmHg in the submembrane space.
The composition of the feed and the permeate was determined using SHIMADZU GC-2010 gas chromatograph. The permeation flux
J (kg/(m
2h) was calculated according to the equation:
where
W (kg) is the permeate weight passing through the membrane,
A (m
2) is the membrane area and
t (h) is the measurement time.
The permeance (
P/
l) (component flux normalized for the driving force) was calculated according to the equation:
where
ji is the partial flux of the
i component,
and
are the component vapor pressures of the feed and the permeate, respectively,
l is the membrane thickness.
The selectivity (
β) which is the ratio of component permeances was calculated according to the equation:
where
Pi/
l (GPU) is the water permeance and
Pj/
l (GPU) is the isopropanol permeance.
The membrane flash index (
MFLI) was calculated according to the equation [
35]:
where
is the component concentration in the permeate (wt.%),
is the component vapor concentration (the equilibrium distillation value) (wt.%).
2.3.7. Contact Angle Determination
Water contact angles for the selective layer surface of PAN ultrafiltration membranes (substrates) were determined by the attached bubble technique using the LK-1 goniometer (“Otkrytaya nauka”, Krasnogorsk, Russia) in the “membrane surface-water-air bubble” system to study the changes of surface properties. Water contact angles for CS-based TFC membranes were determined by sessile drop method using LK-1 goniometer. The measurements were carried out only from the side of the selective layer to study the surface characteristics of the membranes.
2.3.8. Layer-by-Layer Assembly
Membranes with polyelectrolyte nanolayers were prepared using the ND Multi Axis Dip ND-3D 11/5 robot (Nadetech, Navarra, Spain), which had a wide range of membrane immersion rates in solutions (1–2000 mm/min) providing high reproducibility of the deposited layer thickness. The deposition of polyelectrolyte (PEL) layer was carried out as follows: a membrane fixed on special substrate was immersed in a polyanion solution of PSS (10−2 mol/L) for 10 min, then in water to wash and thereafter in a polycation solution of PAH (10−2 mol/L, pH = 4) or CS (4.7 wt.%) for 10 min, then membrane was washed again. Thus, one bilayer of PEL was created on the surface. In this work the optimal number of cycles (depositions) is five bilayers.