ZnO/PES composite membranes were fabricated by phase inversion method using DMAc as a solvent. The structure of ZnO was investigated using TEM, SEM, XRD, and TGA. TEM images of ZnO nanoparticles were well-defined, small, and spherically shaped with agglomerated nanoparticles particles of 50 nm. The SEM and XRD results were an indication that ZnO nanoparticles were present in the prepared ZnO/PES composites membranes. Contact angle measurements were used to investigate surface structures of the composite membranes. The amount of ZnO nanoparticles on PES membranes was varied to obtain the optimal performance of the composite membranes in terms of pure water flux, flux recovery, and fouling resistance using the protein bovine serum albumin (BSA) as a model organic foulant. The results showed that addition of ZnO to PES membranes improved the hydrophilicity, permeation, and fouling resistance properties of the membranes. Pure water flux increased from a low of 250 L/m2h for the neat membrane to a high of 410 L/m2h for the composite membranes. A high flux recovery of 80–94% was obtained for the composite membranes. The optimal performance of the composite membranes was obtained at 1.5 wt% of ZnO.
The use of polyethersulfone (PES) in the preparation of membranes for water treatment is widespread [
Several approaches have been explored to alleviate the fouling problem of PES membranes. These are grafting of the membranes with surface functionalities to enhance hydrophilic surface characteristics [
Titanium dioxide (TiO2) is among the commonly used nanoparticles for enhancing surface hydrophilicity and water permeation [
On the other hand, ZnO received less attention compared to TiO2. However, it has similar or better desirable properties such as large surface area to volume ratio, mechanical and chemical properties, and photocatalysis (with a higher band gap than TiO2). It is more cost-effective than TiO2 [
The positive improvement of nanoparticles incorporated into membranes in water treatment depends on their ability to enhance surface hydrophilicity and water permeability. When ZnO nanoparticles were incorporated into PES, a low flux decline and better permeability were obtained [
In the present study, the objective was to prepare ZnO embedded PES membranes by phase inversion method using DMAc (as a solvent) and polyvinylpyrrolidone (PVP) (as pore former additive and to reduce aggregation of nanoparticles) to improve water permeation and fouling resistance. It has been indicated that DMAc enhances porosity far much better than NMP and that high porosity leads to high water fluxes [
Polyethersulfone (PES MW = 232.258 g/mol) was supplied by Solvay Plastics. Dimethylacetamide (DMAc), polyvinylpyrrolidone (PVP MW = 10,000 g/mol), zinc acetate dihydrate (Zn(CH3COO)2·2H2O), sodium hydroxide (NaOH), and bovine serum albumin (BSA, MW = 66000 Da) were obtained from Sigma-Aldrich. All reagents were used without any further purification. Freshly deionized water was used in all the preparations.
ZnO nanoparticles were synthesized using established protocols [
Neat PES membranes were prepared via phase inversion induced by immersion precipitation [
Composition of the casting solution used to prepare ZnO/PES membrane composites.
Membranes | PES wt.% | PVP wt.% | ZnO wt.% | DMAc wt.% | |
---|---|---|---|---|---|
Pure PES | A | 18 | 2 | 0 | 80 |
PES/ZnO | B | 18 | 2 | 0.5 | 80 |
PES/ZnO | C | 18 | 2 | 1 | 80 |
PES/ZnO | D | 18 | 2 | 1.5 | 80 |
PES/ZnO | E | 18 | 2 | 2 | 80 |
The surface morphology of zinc oxide nanoparticles was studied using the transmission electron microscopy (TEM). TEM is a technique used to produce images of a sample by passing a beam of electrons through a sample. TEM is based on transmitted electrons. The beam of electrons interacts with the sample forming images. TEM images are focused onto an imaging device. TEM has a high magnification that can go into the nanoscale range and therefore can be used to see what is inside the material, that is, beyond the surface of the material.
The physical characteristics of the membranes were assessed using a combination of techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle (CA), X-ray diffraction (XRD), and thermal analysis (TA). In order to visualise the membrane surface structure and morphology, membranes were assessed using scanning electron microscope (SEM, FEI NOVA-nanoSEM 200). The samples were coated by sputtering with gold and the micrographs obtained at operating voltage of 30 keV and the current of 10 mA Contact angles of the prepared membranes were measured using a DSA 10 Mk2 (Krüss, Germany) equipment. A water drop was lowered onto the membrane’s surface from a needle tip and a magnified image of the droplet was recorded by a digital camera until no further changes were observed. Static contact angles were determined from these images with calculation software. To minimize the experimental error, the contact angle measurement was taken as the mean value of 5 different points on each membrane sample. To confirm the presence of nanoparticles in the membranes, the X-ray diffraction analysis was done using a Bruker equipment (D8 ADVANCE axes) with a monochromatic Cu K
The performance of the membrane was measured through permeation flux, pure water permeability (membrane hydraulic resistance), and flux recovery after passing protein bovine serum albumin (BSA) as the model foulant. The pure water flux was obtained from a filtration system that contained a Sterlitech HP4750 dead-end stirred filtration cell with an effective area and volume of 14.6 cm2 and 250 mL, respectively. The nitrogen gas compressed in a cylinder gas was controlled by a regulator. The procedure involved first compacting the membranes at 150 KPa for 30 min, followed by reducing the operating pressure to 100 KPa to obtain the membrane performance at ambient temperature.
Pure water flux (
In obtaining the flux recovery (FR), the membranes used for BSA rejection were rinsed with deionized water by filling and shaking the cell for 1 min. three times. Thereafter, the deionized water flux (
The morphology of zinc oxide was studied using the TEM. The micrograph was taken at 100 nm as shown in Figure
TEM images of ZnO nanoparticles.
TEM studies were carried out to find out the structure and exact particle size of the synthesized ZnO. The structure of ZnO nanoparticles mainly adopted a hexagonal configuration. The number of alternating planes composed of tetrahedral coordinated O−2 and Zn+2 ions is stacked alternatively along
The surfaces of the fabricated membranes were observed using SEM. Figure
SEM images of (a) pure PES; (b) PES/ZnO (0.5%); (c) PES/ZnO (1.0%); (d) PES/ZnO (1.5%); (e) PES/ZnO (2.0%).
The contact angle (CA) is the angle at which the liquid meets the solid surface. The surface can be classified as hydrophobic or hydrophilic, if the CA is 90° <
Contact angle measurements of composite ZnO/PES membranes at different amounts of ZnO nanoparticles from (0, 0.5, 1, and 1.5 to 2 wt%).
A sudden increase in CA was observed at 2.0 wt% ZnO. The CA of 64° was recorded at this amount. A similar trend was observed by Damodar et al. (2009) [
The presence of ZnO nanoparticles in the membrane matrix was confirmed by XRD analysis. Figure
XRD patterns of PES membrane: (a) ZnO/PES composite, (b) pure PES, and (c) ZnO nanoparticles.
Thermal analysis (TGA) provides information on the decomposition temperature (Td) of a substance. It is defined as the temperature at 3% weight loss [
TGA results for increasing decomposition temperature for (0.0, 0.5, 1.0, 1.5, and 2.0 wt%) of ZnO on PES membranes.
The dead-end cell was used to measure the filtration properties of the membranes at 25°C. It was observed that the pure water flux of the composite membranes was higher than that of the neat membrane (Figure
Pure water flux and flux recovery, FR% (after filtration of BSA) changes with the addition of ZnO nanoparticles.
Figure
ZnO/PES composite membranes of different amounts of ZnO nanoparticles (0, 0.5, 1.0, 1.5, and 2.0 wt%) were prepared by phase inversion using DMAc as the solvent. TEM images of ZnO nanoparticles were well-defined, small, and spherically shaped with agglomerated nanoparticles particles of 50 nm. The SEM and XRD results were an indication that ZnO nanoparticles were present in the prepared ZnO/PES composites membranes. Addition of TiO2 nanoparticles improved the thermal stability of the PES membrane showed by TGA. It was demonstrated that the ZnO/PES composite membranes surface hydrophilicity was greatly improved. The lowest value of ZnO/PES contact angle that corresponded to the highest hydrophilicity was 53°, obtained at 1.5 wt% ZnO nanoparticles. The performance of the membranes showed that pure water flux on ZnO/PES membrane was relatively higher when compared to the neat membrane. Finally, the flux recovery of within 80–94% was obtained for the composite membranes when compared to 60% of the neat membrane and the optimal performance of the composite membranes was obtained at 1.5 wt% ZnO.
The authors declare that they have no competing interests.
The authors appreciate North-West University Chemistry Department, the Advanced Materials Division DST/Mintek, and Vaal University of Technology for providing conducive research platforms to successfully carry out this study. Financial assistance was provided from the National Research Foundation (NRF) and Sasol Inzalo, South Africa.