Low-Pressure Membrane for Water Treatment Applications

Department of Physics, College of Science, King Faisal University, PO Box 400, Al-Ahsa 31982, Saudi Arabia Chemical Engineering and Pilot Plant Department, Engineering Research Division, National Research Centre, 33 El-Bohouth St. (Former El-Tahrir St.), Dokki, Giza, PO Box 12622, Egypt Physics Department, Faculty of Science, Cairo University, Giza, Egypt Faculty of Sciences of Tunis, Laboratory of Fluid Mechanics, Physics Department, University of Tunis El Manar, 2092 Tunis, Tunisia


Introduction
Fresh water is a vital resource for human life and ecosystem. The earth is covered with 72% water, while 97% of this water is not suitable for drinking because it is salty water [1][2][3].
Also, the population growth causes an increasing in demand on the quantity and quality of drinking water. Although there is groundwater and rivers in many places, they are affected by different levels of pollution. The modern industries and urbanization contribute to many of such pollution [1,2].
During the last few decades, the membrane technology is selected over other technologies for desalination and wastewater purification. This attributed to its high efficiency, saving energy, high capacity, low cost, and eases in operation for the production of clean water [4,5].
The most common membranes used for wastewater treatment are the polymeric membranes. Flexibility, mechan-ical properties (such as elastic strength), and chemical and thermal stability in addition to the high selectivity of perm are factors making polymeric materials the first choice in the membrane industry for water purification [6][7][8][9][10].
The most common membrane process for wastewater treatment is a pressure-driven processes [11,12]. Figure 1 illustrates the classification of pressure-driven membrane processes. Polymeric membrane can be classified according to its pore size and filtration process. According to this, membranes are classified into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). For MF, the pore size ranges from 1 to 0.1 μm. It is applied for the separation of colloids, macromolecules, bacteria, and other particulates. UF membranes have a pore size in the range of 0.1-0.01 μm to separate out viruses/macromolecules and solutes of high molecular weights as retentate, allowing water or low molecular weight solutes to pass through the pores. Nanofiltration (NF) membranes have a dense surface of membrane to remove divalent ions and trivalent ions with a pore size range of 0.1-0.001 μm, while monovalent ions are separated using reverse osmosis membranes [11][12][13][14][15].
Major problems of membrane are fouling, which lead to decline in the permeation flux, removal percentage, and reduction in membrane life during filtration operation. According to the membrane processes, the fouling can be categorized into inorganic fouling, colloidal fouling, organic fouling, and biofouling [5,6]. Thus, fabrication of antifouling membranes is a common strategy to overcome this problem because the fouling causes membrane pore clogging. Hence, antifouling membranes can help to reduce the demand for energy. These membranes are characterized according to the hydrophilicity, surface smoothness, and its biocidal properties which avoid the accumulation of foulants on membrane surface [7][8][9].
In this work, three ultrafiltration membranes were prepared by phase inversion technique. The prepared membranes were characterized using scan electron microscope, mechanical testing, porosity, and pore size determination. Membrane performance was tested using synthetic solutions of drinking water contaminated with humic acid to be as a contaminated drinking water by sewage wastewater.

Experimental Work
2.1. Materials. Polyethersulfone (PES) was purchased from BASF (Germany). Polyvinylidene difluoride (PVDF) and polyacrylonitrile (PAN) were purchased from the Sigma-Aldrich Company. N-Methyl pyrrolidone (NMP) was used as a solvent, polyvinylpyrrolidone K 90 (PVP K90) was used as a pore former, and they were purchased from Roth (India).

Membrane
Preparation. PES/PVP blending membranes were prepared using the phase inversion technique. Figure 2 illustrates a summary for the steps of this technique.
Both of PES and PVP K90 were dissolved in N-methyl pyrrolidone (NMP). The mixture was stirred under 45°C for 10 h. The PVDF/PVP membrane was also prepared using the phase inversion process, and the mixing process (using magnetic stirrer) was carried out for 8 h. The PAN/PVP blending membrane was also prepared by phase inversion, and the mixing was carried for 6 h. The percentage of polymeric blend compositions was depicted in Table 1. The wet casted membrane thickness of 200 μm on the glass plate was immersed in water coagulation bath. After membrane formation, the prepared membranes were stored in distilled water until use.

Scanning Electron Microscopy.
To investigate the prepared blend membrane morphology, scanning electron microscopy (JEOL 5410) was used after coating the sample surface with gold. The examination of the sample was carried out under 20 kV and magnification of 6000x.

Mechanical
Properties. H5KS TINIUS OLSEN mechanical system was used to investigate the tensile strength for the prepared samples. The samples were tested at room temperature (25°C) in uniaxial tension at the rate of 0.5 mm/min.

Porosity Measurements.
Densometers represent one of the accepted standard methods for measuring the porosity and air permeability of membrane. The Gurley Standard Densometer was used to investigate our samples with an area of about 25 cm 2 .
The porosity was determined by knowing the weight of the wet sample and of the same sample after drying to reach its stable value. The following equation was used to calculate porosity where A represents the effective area (cm 2 ), L is the thickness (cm), and ρ the density of pure water(g cm -3 ). To

Membrane Performance.
A laboratory-scale dead-end cell consists of feeding tank, dead-end membrane cell, and peristaltic pump as shown in Figure 3. The effective area of the membrane under test was 13.86 cm 2 . As a simulation for the contaminated water, a synthetic solution of a humic acid with different concentrations (0.1, 0.3, and 0.5 g/l) mixed with salty water (500 ppm) was used to investigate the membrane performance.
The permeate flux and separation percentage for all membranes were calculated from equations (2) and (3) [16]: where Q is the permeate volume (l), A is the effective membrane area (m 2 ), and Δt is the permeation time (h).
where O p and O f are the concentrations of the solute in permeate and feed, respectively.  Figure 4 illustrates the surface morphology for the prepared samples. It is clear from the figure that the three samples exhibit a porous with different sizes at the membrane's top surface. The addition of PVP, as a pore former, makes a good pore distribution on the membrane surface that causes increasing in macrovoids in membranes and reduces the dense membrane at the top layer [17]. M2 represents PVDF membrane where the surface of the membrane displays the smallest pore size compared with PES and PAN, because the molecular weight of PVDF is the highest one, which was 390,000 g/mol, where PES was 62000 g/mol and PAN 53000 g/mol. M3 represents the PAN membrane which has a highly homogenous porous surface [18,19]. Although the polyacrylonitrile (PAN) membrane (M3) has the highest level of porosity compared to other membranes, it must be wet during testing as well as upon application. This is due to the formation of microcracks on its surface when the membrane is dried, and this in turn reduces       International Journal of Polymer Science its mechanical properties especially its tensile strength. This can interpret the reason of its low tensile strength. Polyvinylidene difluoride (PVDF) membrane (M2) represents one of the common membranes which has a high mechanical strength and thermal and chemical stability with high hydrophobic nature. The polymer chains in PVDF are in the polar zigzag confirmation that has a high dipolar moment and can exhibit interesting electroactive that leads to high mechanic properties of prepared membrane [16]. Polyethersulfone (PES) membrane has high hydrolytic strength over a large range of pH (from 2 to 12) and also has high thermal and chemical stability. Its mechanical properties represent the intermediate between PVDF and wet PAN membranes as shown in Figure 5. Table 2 illustrates the overall porosity percentage and average pore size of the prepared membranes. The results indicated that polyacrylonitrile (PAN) membrane (M3) has the highest porosity of 75.3%, while polyethersulfone (PES) membrane (M1) has a porosity of 66.8% and polyvinylidene difluoride (PVDF) membrane (M2) has a porosity of 65.1% [19,20]. The mean pore size indicates that the prepared membranes are in the range of ultrafiltration membranes, where the pore size of prepared membranes (18.2 to 33.6 nm) matched with the range of the ultrafiltration membranes pore size from 2 nm to 100 nm [13,16].

Membrane Performance Test.
Humic acid solutions of 0.1, 0.3, and 0.5 g/l were mixed with tap water that was used for the membrane performance test. Figures 6 and 7 show the performance of prepared membranes in terms of humic acid removal percentage and permeate flux, respectively.  Figure 7, while M1 and M2 permeate fluxes are comparable to each other. Polyethersulfone polymer has two groups, the ether group and sulfone chain, so the membrane surface has a hydrophobic/hydrophilic nature. Accordingly, using PVP improves the hydrophilic nature of PES membrane surface, and this will lead to the improvement of the permeate flux and removal percentage for the membrane. The PVDF membrane has a high hydrophobic nature, so using PVP provides a slight improvement in the hydrophilic nature of the surface; therefore, PES (M1) will provide less fouling as compared with PVDF (M2) due to the polymorphs of PVDF which can cause various interaction between PVDF surface and the feeding solution leading to surface fouling. PAN (M3) provides the lowest removal and high permeate flux due to high porosity of the membrane as shown in Table 2. Also, PAN is a highhydrophilic membrane, but the membrane is brittle and the aggregation of it can occur after drying which can oppose its application [17,[20][21][22]. Dang et al. studied the performance of ultrafiltration polyethersulfone membranes after the addition of hydrophilic modifier during membrane preparation; the experiments were carried out for long term using polyethylene glycol 600 and ethylene oxide as a feeding solution. The results indicated a high hydraulic resistance for PES, which is inversely proportional to the flux, so after 50 h, the flux declined. Applying continuous stages from feeding and washing indicated that the flux reduced at the first 6 h and reached to the steady state after 80 h; the decline in flux is related to the mean pore diameter and the pore structure. Dang et al. also studied the PES-LSMM membrane flux using real river water feeding where the results indicated that flux has not been

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International Journal of Polymer Science declined significantly; it was held rather steady during the Ottawa River water filtration test [23].
Narbaitz et al. studied the modification of cellulose acetate membrane by incorporating charged surface to increase their surface charge. The results indicated that increasing the operating pressure led to increase in permeate flux. However, commercial thin film composite composed of polysulfone or polyether sulfone support provided higher fluxes than the modified CA membrane [24].
Total tap water analysis was performed to indicate the effect of membranes in the drinking water treatment or purification. Table 3 indicates all prepared membranes provide good separation of turbidity, COD, and BOD, while there was no separation for di-or trivalent ions, because ultrafiltration membranes separate only organics, high molecular weight substances, bacteria, and viruses, but divalent and trivalent ions need nanofiltration membrane. The high turbidity, COD, and BOD in tap water are related to the addition of humic acid in the water samples to simulate the contaminated water by sewage water. Prepared membranes exhibit good separation especially PVDF membrane for COD and BOD. This can be attributed to the fact that it has the lowest porosity and pore size compared with other membranes [22].

Conclusions
Ultrafiltration membranes were prepared using phase separation techniques. The polyvinylidene difluoride (M2) has the highest tensile strength of 33.2 MPa with elongation of 52.3%, while polyacrylonitrile (M3) has the lowest mechanical properties, tensile strength of 16.4 MPa with elongation of 42.7%.
The membranes mean pore size indicates that the prepared membranes are in the range of ultrafiltration membranes, where the pore size of prepared membranes (18.2 to 33.6 nm) matched with the range of the ultrafiltration membranes pore size from 2 nm to 100 nm.
Analysis of water samples indicates that the prepared membranes can be used to treat the contaminated drinking water which produced the high quality of drinking water after treatment, where the turbidity, COD, and BOD after treatment by membranes reached the World Health Organization standard for drinking water.

Data Availability
Data are available with the corresponding author under request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.