The aim of this study consisted in the development of an alternative synthesis procedure for hybrid ultrafiltration membranes for water purification. The membranes were obtained by wet-phase inversion method based on aliquots of polysulfone (PSF) and graphene nanoplatelets modified with poly(styrene) (G-PST). The hybrid materials were modified by electrochemical deposition of zinc oxide (ZnO) on one side of the membranes in the presence of water soluble polymers. Raman, XPS, and TGA analyses were used to characterize the chemical and thermal characteristics of the PST-G. SEM analysis showed the formation of asymmetric porous configuration in all cases and the generation of ZnO with different shapes/structures on the bottom surface of the membrane or inside the porous channels. EDS analysis confirmed the formation of ZnO.
Water purification continues to be considered a major problem of the current century although tremendous efforts were made at industrial level to improve membrane technology.
Regardless of their classification microfiltration (MF, pores 0.1–1
Although at industrial level polymer membranes such as poly(vinylidene fluoride) (PVDF), polysulfone (PSF), and poly(ethersulfone) (PES) are the most widely used also for their thermal and chemical resistance [
For this reason, research studies were driven to increasing the hydrophilicity of the polymer membranes by physical blending with more hydrophilic polymers or inorganic compounds, by chemical and/or surface modification [
The filtration membranes were obtained using the classical immersion phase inversion method starting from aliquots of polymer solutions with previously synthesized inorganic particles like zinc oxide (ZnO), titanium dioxide (TiO2), alumina (Al2O3), silica (SiO2), silver, or carbon [
Polymer-inorganic nanocomposites have attracted significant scientific and technological interest. The incorporation of inorganic nanoparticles into the polymer matrix can provide high-performance novel materials that find applications in many industrial fields. As a result of the development in nanotechnology, inorganic nanostructured materials have been designed and fabricated with important cooperative physical phenomena such as superparamagnetism, size-dependent band-gap, ferromagnetism, and electron and phonon transport [
Carbon nanomaterials possess some unusual size-dependent properties (e.g., morphological, electrical, optical, and mechanical) useful in enhancing energy-conversion, storage performance, and mechanical properties [
Furthermore, incorporation of pristine carbon nanotubes (CNTs) or graphene oxide (GO) at certain concentrations into the polymer matrix proved to be an advantageous alternative to realize filtration membranes with improved hydrophilicity, thermal stability, and water flux permeability without damaging the mechanical properties of the hybrid membranes [
For complementary purposes composite filtration membranes were obtained using both inorganic nanoparticles and CNTs or GO in order to significantly increase the fouling resistance and in some cases to obtain additional photocatalytic or antimicrobial properties [
In our study graphene nanosheets were modified with polystyrene in order to favour a better dispersion of the sheets into the polysulfone matrix. The hybrid polysulfone-poly(styrene)-graphene (PSF-G-PST) membranes were synthesized by casting the solution of PSF and G-PST dissolved in N-methyl-pyrrolidone (NMP) on glass plates followed by coagulation in water bath. The electroconductive properties of the synthesized membranes were exploited for the electrochemical deposition of ZnO. In order to control the ZnO morphology, the electrochemical procedure was applied in the presence of water soluble compounds in order to form ZnO nanoparticles on one side of the composite membrane. The PSF membrane is hydrophobic. ZnO was generated in the presence of hydrophile polymers. Thus, ZnO was deposited on the surface of the PSF membrane in order to increase the membrane antifouling properties.
The chemical modification and thermal behaviour of modified graphenes were investigated by Raman spectroscopy, X-ray photon spectroscopy (XPS), and thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) was employed to investigate the pores structures and ZnO nanoparticles modifications depending on the water soluble compounds. Energy dispersive spectroscopy (EDS) evidenced the formation of ZnO on the surface of the hybrid membranes.
Graphene nanoplatelets (5
A black dispersion of 150 mg of G in 8 mL of ST was added to 10 mL of DMF followed by nitrogen purging for 20 min. The reaction was initiated with 100 mg of PL (lauroyl peroxide) and kept under continuous stirring overnight at 80°C. The resulting product (G-PST) was purified by precipitation in M. Finally, the gray-black powder was vacuum dried until constant mass.
The composite membranes were obtained by the wet-phase inversion method [
The ZnO nanostructures were generated on one side of the hybrid membranes using a modified electrochemical method described elsewhere [
Electrochemical deposition conditions for ZnO generation on the composite membrane.
Sample code | Polymer | Concentration of polymer in solution |
Deposition time |
---|---|---|---|
PSF-G-PST-PVA 2.5 | PVA | 2.5 | 1 |
PSF-G-PST-PVA 10 | PVA | 10 | 1 |
PSF-G-PST-PAA 2.5 | PAA | 2.5 | 1 |
PSF-G-PST-HEC 2.5 | HEC | 2.5 | 1 |
Graphene chemical functionalization was studied by Raman and XPS spectroscopy. The Raman spectra have been registered on a DXR Raman Microscope from Thermo Scientific with a 633 nm laser. The laser beam has been focused with a 10x objective. The XPS (X-ray photoelectron microscopy) analysis has been performed on a K-Alpha instrument from Thermo Scientific, using a monochromated Al K
The thermal properties of the material were ascertained using thermogravimetric analysis (TGA). The TGA analysis has been performed on Q500 TA Instruments equipment, under oxygen atmosphere, using a heating rate of 20°C/min from room temperature to 900°C.
The morphologies of the membranes have been investigated by SEM (scanning electron microscopy) using FEG-SEM-Nova NanoSEM 630 (FEI). The presence of ZnO nanostructures on the surface of the hybrid materials was confirmed by energy dispersive spectroscopy (EDS).
In this research study, the porous membranes were designed using an alternative synthesis procedure in order to meet various requirements necessary for membranes used in wastewater treatment applications. For this reason, the materials were modified to ensure good mechanical and thermal resistance and hydrophilic and antifouling properties simultaneously.
Although high amounts of graphene or graphene oxide can decrease mechanical properties of hybrid membranes, these fillers have been used in polymer membranes synthesis in order to increase thermal, hydrophilic, antifouling, and antibacterial properties of different polymer membranes. Due to its poor solubility in common organic solvents, graphene has been chemically modified in order to enhance the compatibility with other materials [
In our study, to favour a better dispersion of graphene sheets in polysulfone matrix, the polymer composite G-PST was obtained based on a solution polymerization reaction of styrene.
Raman and XPS analyses were performed in order to put into evidence the chemical modification of graphene with polystyrene, while TGA analysis was employed to establish the thermal properties of the G-PST composite.
The Raman spectrum (Figure
Raman analysis of G (black line) and modified G-PST (red line).
The XPS analysis from Figure
XPS analysis, C1s deconvolution spectra for G (a) and G-PST (b).
Pristine G
G-PST
As exhibited in Figure
TGA analysis of the G and G-PST nanocomposite.
The blank membrane was obtained according to Section
SEM image of polysulfone-graphene-polystyrene (PSF-G-PST) membrane: (a) top; (b) cross-section; (c) bottom.
Figure
Aiming also at the enhancement of antimicrobial properties of our composite membranes, the next step of this study consisted in the electrochemical generation of ZnO. In order to insert a high content of ZnO in the structure of the composite membrane, flat sheets were cut (1 × 1 cm) and positioned with the bottom pores facing the opposite direction of the electrical field. This strategy allows the growth and aggregation of ZnO structures inside the larger pores. Filling the pores with ZnO will eventually create a compact structure that will favour a better contact between contaminated water and antimicrobial compound.
The electrochemical deposition of ZnO on the composite membrane intended also the attachment of the inorganic particles inside the macrovoids, deeper in the structure of the pores. However, the cross-section image (Figure
SEM images of PSF-G-PST-ZnO membrane: (a) cross-section; (b) bottom; (c, d) details of macrovoids from the bottom surface.
According to previous studies, the morphology of inorganic particles generated in electrical field can be severely influenced by the presence of water soluble polymers [
In Figure
SEM images of ZnO deposition on PSF-G-PST in the presence of PVA (2.5% wt.): (a) cross-section; (b) bottom; (c) detailed image of the bottom surface.
For the next experiment, the generation of ZnO was conducted in the presence of higher concentration of PVA, knowing the fact that polymer solutions with higher viscosity will considerably reduce the size of the particles, affecting the crystal nucleation and growth process [
In the case of using 10% wt PVA the bottom surface of the composite membrane PSF-G-PST-PVA10 is characterized by large pores (around 20
SEM analysis of ZnO deposited on PSF-G-PST membrane in the presence of PVA (10% wt.): (a) cross-section; (b) bottom; (c) detailed image of the bottom surface.
Furthermore, in Figure
The presence of well-dispersed graphene inside the composite membrane assisted by the high voltage applied to the water soluble polymer solution, the size of the macrovoids, and the higher concentration of PVA allowed the formation of ZnO nucleus deeper into the membrane structure favouring the growth of inorganic spherical particles (with size in the range of 0.5–1
Synthesis of inorganic particles in the presence of water soluble polymers is strongly influenced by the molecular weight of the polymer and also by the functional groups [
Thus, in the next experiment, the ZnO nanostructures were generated in the presence of PAA at low concentration (2.5% wt.)
Similar to PSF-G-PST-PVA 2.5 membrane, the cross-section did not show the presence of any zinc oxide particles inside the porous structure of the membrane. Thus, at this concentration, the amount of PAA inhibited the crystal growth process of the ZnO nanoparticles. Figures
SEM analysis of ZnO deposited on PSF-G-PST membrane in the presence of PAA (2.5% wt): (a) cross-section; (b) bottom; (c) detailed image of the bottom surface.
Hydroxyethyl cellulose (HEC) is a water soluble polymer derived from cellulose and usually used as thickening agent in cosmetics, cleaning products, and drug or oilfield chemical products. Recent studies confirmed that HEC has an enhancing inhibitory effect in crystal growth processes in aliquots with poly(N-vinyl pyrrolidone) (PNVP) compared to polyethylene glycol (PEG) and polyacrylamide (PAM) [
Based on these aspects, our experiment was carried out in the presence of HEC dissolved together with the ZnO precursors. After 1 h deposition time the samples were removed from the electrochemical bath and analysed by SEM (Figure
SEM image of ZnO deposited on PSF-G-PST in the presence of HEC: (a) cross-section; (b) bottom; (c) detailed image of the bottom surface.
The SEM analysis shows a clear evidence of the ZnO generated inside the structure of the pores (black circles marked in Figure
Compared to PSF-G-PST-PVA10 membrane, where ZnO was also generated inside the porous structure (see Figure
In conclusion, the use of HEC improved the deposition rate of ZnO nanostructures and favoured the electrochemical generation of the inorganic spheres in the whole structure of the composite membrane compared to the other water soluble polymer presented/used in this study.
In order to confirm the formation of ZnO on the membranes surface EDS analysis was performed on the bottom side (Figure
SEM EDS and SEM analyses for ZnO deposited on the PSF-G-PST-membranes.
The EDS spectrum demonstrates the presence of zinc (Zn) and oxygen (O) peaks together with a strong signal from carbon (C) peak attributed to the composite membrane. Thus, this result validates the deposition of ZnO on the bottom surface of the hybrid membrane.
The aim of this work was to develop an alternative synthesis procedure for hybrid polysulfone membranes endowed combined properties (i.e., mechanical and thermal resistance, antifouling and antimicrobial characteristics) in one single membrane designed for water purification.
In this work, graphene nanoplatelets were modified with PST in order to obtain polysulfone composite membranes relevant for electrochemical deposition of ZnO. The deposition procedure was performed on one side of the composite membrane, at high voltage, in the presence of water soluble polymers.
At low concentrations (2.5% wt) of water soluble polymers, such as PVA and PAA, the generation of ZnO nanostructures was influenced by the functional groups. In this case, the inorganic structures were generated mainly at the edges of the bottom macropores hardly protruding inside the pore channels.
The experiments confirmed that ZnO nanostructures can be embedded inside the porous structure of the composite membrane at high concentrations of PVA (around 10% wt.).
The use of water soluble polymers with strong inhibitory effect on the crystal growth process, such as HEC, improved the deposition rate, decreased the size of inorganic structures, and increased the amount of ZnO deposited in the whole structure of the composite PSF-G-PST-membrane. These results are promising and further research will be performed to reach the goal of obtaining new composite materials for improved efficiency membranes for water purification systems.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors would like to acknowledge the financial support provided by the National Authority for Scientific Research from the Ministry of Education, Research and Youth of Romania through the