Improved Permeate Flux and Rejection of Ultraltration Membranes Prepared From Polyethylene Terephthalate (PET) Bottle Waste

Polyethylene terephthalate (PET) ultraltration membranes were prepared using two different sources of polymer material, namely PET bottle waste and PET resin. The membrane prepared from PET bottle waste and that prepared from PET resin showed similar membrane characteristics such as IR spectra, morphology, hydrophilicity and porosity, indicating that instead of using PET resin, PET bottle waste can be utilized as a source of the polymer material to fabricate low-cost membranes. The morphology, hydrophilicity and porosity of the membranes were strongly affected by the additive concentration. The analysis of the membrane morphology using Scanning Electron Microscopy (SEM) showed that the membranes had an asymmetric structure that consists of a macroporous cross section and a smooth active layer. Increasing the additive concentration of polyethylene glycol (PEG 400) resulted in a smaller pore size, however the hydrophilicity and the porosity of the membranes increased. As a result, the membranes showed an increase in both permeate ux and rejection with increasing concentration of PEG 400 as observed from the results of the ultraltration experiments. Using Bovine Serum Albumin as a solute model in the feed, high values of rejection of up to 93.9 % were achieved.


Introduction
Ultra ltration membranes are widely used for separation processes of aqueous solutions in various industries such as food, dairy, beverage, pharmaceutical, textile, electronics, and chemical industries.
They are mostly applied for water treatment such as the production of pure water to remove microorganisms, bacteria, virus, colloidal substances, and suspended micro particles from the water [1,2]. Other application of ultra ltration membranes is for the concentration of protein or enzyme [3,4].
Ultra ltration membranes have usually a porous asymmetric structure with a macroporous cross section and a smooth active layer that is able to reject high molecular weight solutes such as protein, virus, bacteria, etc., whereas water or low molecular weight solutes can permeate through the membrane. The separation using the ultra ltration membrane is a pressure-driven separation process, which can be simply operated using a pump without the use of heat. Therefore, the use of the ultra ltration membrane for separation processes has many advantages due to the lower energy consumption and the high selectivity.
Commercial ultra ltration membranes available in the market are usually made from cellulose acetate, polysulfone, polyethersulfone or polyvinylidene uoride. Many studies to develop ultra ltration membranes using other polymer materials such as polyetherimide, polyvinyl chloride, chitosan, and other materials have been reported [5,6,7]. Studies on the modi cation of the membranes to improve the permeate ux and the rejection have also been reported [1,8,9]. Recently, our previous study on the use of polyethylene terephthalate (PET) bottles to prepare PET ultra ltration membranes have been reported [10]. PET packaging is widely used by the food and beverage industries because of its excellent mechanical strength, good chemical resistance, good transparency, and excellent gas-barrier resistance.
PET lms are also suitable for many other applications due to their excellent mechanical properties [11,12], and good chemical resistance against acids and low concentration of alkalies [13]. The outstanding mechanical and chemical properties of PET open the opportunity to fabricate ultra ltration membranes from PET. The source of the polymer material even can be found in used PET bottles or other used PET packaging that are usually considered as waste. In our previous study on the development of ultra ltration membranes using PET bottle waste, it was observed that the permeate uxes increased by decreasing the polarity of the non-solvent, by increasing the molecular weight of the additive, or by increasing the additive concentration [10]. However, it was observed that the permeate ux enhancement was followed by a decline of the rejection rate, because of the enlargement of the membrane pore size.
The same phenomenon has been also reported in other studies [5,14]. Ultra ltration membranes with high permeate uxes are desired since the ultra ltration membranes have been known to have a drawback, namely fouling problem, that is the permeate ux decline with the operating time because of the concentration polarization on the surface of the membranes. In order to eliminate fouling, many studies have been done to develop membranes with improved permeate uxes [1,7,8,9]. However, the increase in the permeate ux is usually followed with the decrease in the rejection of the membranes.
Thus, it is very crucial to develop ultra ltration membranes with improved permeate ux without any decrease in the rejection.
The objective of this work is to develop PET ultra ltration membranes which exhibit improved permeate uxes with high rejection values. The membranes were developed using PET bottle waste as the polymer material using polyethylene glycol with molecular weight of 400 Da (PEG 400) as the additive. The aim of the utilization of PET bottle waste is also to give a contribution in the plastic recycling to reduce plastic waste. Since PET bottles are originally produced from PET resin, PET resin was also used in this work as the polymer material to prepare the membranes with the aim to compare the characteristics of the membrane developed from PET bottles and that from PET resin. The effect of the PEG 400 concentration on the microstructure, the hydrophilicity and the porosity of the membranes was studied by using Scanning Electron Microscopy (SEM), water contact angle measurement, and gravimetric method, respectively. The membranes were characterized using Fourier Transform Infrared (FTIR) spectroscopy to study the chemical properties. Furthermore, the membranes were characterized for their ultra ltration performances through ultra ltration experiments using pure water and a feed solution containing Bovine Serum Albumin (BSA) molecules (MW: 66,000 Da) as a feed model.

Materials
Plastic bottles made from polyethylene terephthalate (PET) were used. The PET bottles were previously used as packaging for mineral water and was obtained from the local supermarket in Indonesia. PET resin was also used as the polymer material, and was manufactured and supplied by PT Indorama Ventures Indonesia. Phenol (≥ 99%) was used as the solvent, and was supplied by Merck, Germany. Lowmolecular weight of polyethylene glycol (PEG 400) was used the additive, and was supplied by Merck, Germany. Bovine serum albumin (BSA, molecular weight: 66,000 Da) was also supplied by Merck, Germany. Technical grade ethanol (96%), monosodium phosphate (NaH2PO4, ≥ 99%), disodium phosphate (Na2HPO4, ≥ 99%), potassium chloride (KCl, ≥ 99%), and sodium chloride (NaCl, ≥ 99%) were all supplied by Merck, Germany. All chemicals were used as received. Distilled deionized water was used.

Preparation of PET membranes
After removing the labels and bottle caps, the bottles were thoroughly washed. The clean and dry bottles were then cut to obtain small PET shards. PET resin was also used as the membrane material with the aim to compare the characteristics of the membranes with that prepared from the used PET bottles. To prepare the casting solution, phenol was heated at 40°C to liquify it as phenol is a solid at room temperature. Then, the PET bottle shards or the PET resins were added into the phenol under continuous stirring and heating at 100°C using a hot plate (Barnstead Thermolyne) equipped with a magnetic stirrer. Meanwhile, a solution of PEG 400 in phenol was prepared separately by dissolving PEG 400 in phenol at the same condition as above. Both polymer solutions were then mixed at 100°C for one hour under continuous stirring to obtain a homogeneous polymer solution. The composition of the PET, the PEG 400 and the solvent in the casting solutions can be seen in Table 1. The membranes were then prepared from the casting solutions by the phase-inversion technique. The polymer solution was cast onto a glass plate and then submerged in a non-solvent bath containing solution of water-ethanol (1:12 v/v) at room temperature. As a result, a white solid at membrane was obtained. After rinsing several times using distilled deionized water, the membranes were stored in plastic containers containing distilled deionized water for further use.

Characterization of PET membranes
These at-sheet membranes were characterized for their average thicknesses using a micrometer (Tricle, China) from the measurement of ve different locations of the membrane. Analysis using Fourier Transform Infrared spectroscopy was conducted to study the chemical structure of the membranes using FTIR spectrometer (Shimadzu IR Prestige-21, Japan). Analysis using Scanning Electron Microscopy (SEM, Quanta 650) was conducted to study the microstructures of the membranes. Gravimetric method was used to determine the membrane porosity using the following equation [15,16]: where w 1 and w 2 are the weight of the wet membrane and that of the dry membrane, respectively, whereas d w and d p are the density of the water and that of the polymer, respectively. The wet membrane was obtained by immersing the membrane in distilled deionized water at room temperature for 24 h, while the dry membrane was obtained by drying the membrane in an oven at 110°C for 3 h. Five membrane samples were used to obtain the average value of the porosity. The membranes were characterized for their hydrophilicity by measuring the water contact angle using a water contact angle meter (Face CA-D, Kyowa Kaimengaku, Japan). The measurement was conducted using distilled deionized water at room temperature, and repeated six times to obtain the average value of the contact angle.

Measurement of permeate ux and rejection through ultra ltration experiment
Ultra ltration experiments were performed to measure the pure water permeate ux using distilled deionized water as the feed that was pumped through a membrane cell. The membrane cell had an effective area of 51.8 cm2. The experiment was conducted in a cross-ow mode at a trans-membrane pressure of 1 bar at room temperature. The permeate ux F was determined from the weight of the collected permeate m p divided by the membrane area A and the time interval Δt using the equation below: (2) The membranes were then characterized for their ability to reject macromolecules through ultra ltration experiments using an aqueous phosphate buffered-saline solution containing 1000 ppm BSA. The method to prepare the phosphate buffered-saline solution can be found elsewhere [10]. To determine the rejection R, the following equation was used: ( where C F and C P are the concentration of BSA in the feed solution and that in the permeate, respectively. A UV-vis spectrophotometer (PG instrument T-60, UK) was used to measure the BSA concentration at a wavelength of 280 nm.

Comparison of membranes prepared from used PET bottles and PET resin
It has been known that the morphology of a membrane prepared by using the phase-inversion technique is strongly affected by the polymer, the solvent, the non-solvent and the additive. In this study, the polymer used to prepare the membranes was polyethylene terephthalate (PET). Two different sources of the polymer material were used, namely used PET bottles and PET resin which is the raw material to produce the PET bottles. Since used PET bottles are considered as waste, it is important to compare the characteristics of the membrane developed from the PET bottles and that prepared from the PET resin. The FTIR spectra of the PET membrane developed from the PET bottles are shown in Fig. 1, whereas that of the membrane from PET resin are shown in Fig. 2. Both membranes were prepared without additive. As can be seen, both membranes showed similar IR spectra, indicating that there is no difference in the chemical structure of the membrane from the used PET bottles and that from the PET resin. The FTIR spectra of both membranes are similar to the spectra of PET lms that have been analyzed by other studies [17,18]. Figure 2 shows the chemical structure of PET, whereas the interpretation of the FTIR spectra of both PET membranes is listed in Table 2.  Furthermore, the microstructure of the membrane was analyzed by using Scanning Electron Microscopy (SEM). Figure 4 and Fig. 5 show the SEM images of the cross section and surface of the membrane from used PET bottles and that from PET resin, respectively. No additive was used to prepare both membranes. Both membranes had an asymmetric structure that consists of a macroporous cross section and a smooth surface as the active layer of the membrane. There was almost no difference between the morphology of the membrane from used PET bottles and that from PET resin. This nding is in accordance with the FTIR analysis of both membranes that showed no difference of the FTIR spectra as described previously.
The membranes had an average thickness of 149 ± 13 µm as measured using a micrometer. Furthermore, the porosity and the water contact angle of both membranes are listed in Table 3. There was no signi cant difference between the porosity of both membranes. Both membranes also showed almost the same hydrophilicity as measured using the water contact angle method. Again, these results are in agreement with the results of FTIR analysis and SEM analysis as described previously.
Since both membranes prepared from used PET bottle and PET resin exhibited the same properties such as the chemical structure, the microstructure, the porosity and the hydrophilicity as described above, it can be concluded that instead of using PET resin, used PET bottles that are usually considered as waste can be utilized as the source of the polymer material to prepare the PET ultra ltration membranes. The utilization of used PET bottles is advantageous since it will not only reduce the cost of the membrane material, but also will contribute in the efforts of plastic recycling process for a sustainable environment.

Effect of PEG 400 concentration on the microstructure, hydrophilicity and porosity
The use of additives for the preparation of membrane by using the phase inversion technique has been known to be effective to achieve the desired membrane characteristics such as microstructure, hydrophilicity, porosity, and exibility [19]. In this study, polyethylene glycol with a molecular weight of 400 Da (PEG 400) was used as the additive. PEG has been known as a pore forming agent, a pore reducer, and a plasticizer for various polymers [19,20,21]. In this work, low-molecular weight PEG such as PEG 400 was chosen as the additive for the PET membranes since our previous study revealed that the use of high molecular weights of PEG such as PEG 4000 resulted in the membranes with too large pore size that decreased the rejection rate of the membranes [10]. Figure 6 and Fig. 7 show the SEM images of the cross section and surface of the membranes with various concentrations of PEG 400. All of the membranes were prepared using PET bottles as the polymer material. All membranes showed an asymmetric structure that consists of a macroporous cross section and a smooth surface as the active layer. Interestingly, the morphology of the membranes changed as the formation of the pores was in uenced by the PEG 400 concentration. It can be observed from the SEM images that the increment of the PEG 400 concentration resulted in a smaller pore size of the membrane cross section. It has been known that low-molecular weight PEG such as PEG 400 acts as a pore reducer for various polymer membranes [5,19]. The formation of pores occurred when the casted polymer solution consisting of PET, phenol (the solvent) and PEG 400, was immersed in the water-ethanol as the non-solvent. Due to the solvent and non-solvent exchange, precipitation took place, and PEG 400 acted as a pore reducer for the membrane. The growth of the pore formation was hindered when the membrane contained a high concentration of PEG 400. As a result, the pore size of the of the membrane cross section decreased with increasing concentration of PEG 400.
Furthermore, Fig. 8 shows the effect of the PEG 400 concentration on the membrane porosity. It can be obviously seen that the porosity of the PET membrane increased by the addition of PEG 400 as the additive. The porosity of the PET membrane without additive was 69.7% ± 0.5 %, and the porosity increased sharply to 79.4% ± 0.3% through the addition of 4.95 wt% of PEG 400. A further increase in the porosity with increasing PEG 400 concentration was observed, and then the value of the porosity became stable at high concentrations of PEG 400. High values of porosity of 82.4 % ± 0.4 % and 82.2 ± 0.2 % were achieved by adding 7.69 wt% and 11.11 wt% of PEG 400, respectively. This phenomenon occurred since PEG 400 acted as pore former that increased the membrane porosity as described above. Figure 9 shows the effect PEG 400 concentration on the water contact angle of the membranes. It can be seen obviously that the water contact angle decreased signi cantly with increasing concentration of PEG 400. This indicated that the hydrophilicity of the membranes increased. The hydrophilic characteristic of PEG was effective to increase the hydrophilicity of the membranes. Other studies have reported a similar phenomenon for polysulfone and polyethersulfone membranes that showed an increase in the hydrophilicity by the addition of polyethylene glycol as the additive [19]. The increase in the porosity and the hydrophilicity is desired as the ultra ltration membranes are mostly applied for water treatment.
3.3. Results of ultra ltration experiment using PET membranes 3.3.1. Comparison of ultra ltration performance of the membrane from PET bottle and that from PET resin The membranes prepared from used PET bottle and that from PET resin were then tested through ultra ltration experiments to measure the permeate ux of pure water. The membranes prepared from PET bottle and that from PET resin with the addition of PEG 400 showed a good exibility, since PEG acted as a plasticizer for the membranes [20,21]. However, the membrane developed from used PET bottle and that from PET resin without PEG 400 were so stiff that they could not be tted in the membrane cell for the ultra ltration experiment. Figure 10 shows the permeate ux of pure water for the membrane developed from PET bottle in comparison with that from PET resin using PEG 400 as the additive. Both membranes were prepared using the same PEG 400 concentration of 11.1 wt%. It can be seen that both membranes exhibited almost the same values and pro les of water permeate uxes as a function of the permeation time. In the beginning, the water permeate uxes decreased with time, then they became stable after around 2 hours. The decline of the permeate ux with time was caused by the physical compaction of the newly prepared membranes. The phenomenon of the physical compaction of polymer membranes has been also found in many other membranes [22,23,24,25]. As described previously, both membranes prepared from PET bottle and PET resin showed similar membrane properties such as morphology, porosity and hydrophilicity. The similar membrane properties of both membranes resulted in similar permeate ux during the ultra ltration experiment. This result revealed that instead of using PET resin as the source of the polymer, the PET ultra ltration membranes could be prepared using PET bottle waste. Since PET bottle waste needs to be recycled, the conversion of used PET bottles into PET ultra ltration membranes has great potential in the contribution for the environment conservation.

Ultra ltration performances of PET membranes with different PEG 400 concentrations
To study the in uence of additive concentration on the ultra ltration performance, the membranes that were prepared from PET bottles with the addition of various concentrations of PEG 400 were tested through ultra ltration experiments. Figure 11 shows the permeate uxes of pure water for the membranes prepared from PET bottle with various PEG 400 concentrations. The membranes showed a decline of the permeate ux in the beginning of the permeation time, and then the permeate ux became stable after around 2 h, because of the physical compaction as described above. Interestingly, the membranes showed an increment of the permeate ux when the PEG 400 concentration was increased. It can be seen that the membrane with a low PEG 400 concentration of 4.95 wt% exhibited the lowest water permeate ux. The permeate ux increased sharply as the PEG 400 concentration was increased to 5.88 wt% and 7.69 wt%. A further increase in the PEG concentration of 11.11 wt% resulted in the highest permeate ux, however in the steady state condition the permeate ux values became almost the same with that of the membrane with PEG 400 concentration of 7.69 wt%. This result is in agreement with the results of the membrane characterization as explained previously. The increment of the permeate ux with increasing concentration of PEG 400 was caused by the increase in the hydrophilicity and the porosity of the membranes. As described previously, the PET membrane without PEG 400 showed a low hydrophilicity. When PEG 400 was introduced into the PET, the membrane became more hydrophilic, and water was attracted stronger onto the membrane, resulting in a higher water permeate ux. At the same time, the increment of the PEG 400 concentration also increased the membrane porosity, resulting in an increment of the permeate ux.
Furthermore, the membranes were tested through ultra ltration experiments using an aqueous feed solution containing 1000 ppm BSA. The permeate samples were collected after attaining a steady state condition, and the rejection was determined from the BSA concentration in the permeate and that in the feed using the Eq. (3). Table 4 depicts the results of the experiments, showing the BSA rejection of the PET membranes prepared with different PEG 400 concentrations. The membrane with a PEG 400 concentration of 4.95 wt% showed a low rejection value of 61 %, and the rejection increased by increasing the PEG 400 concentration. A high value of rejection rate of 93.9 % was achieved by the membrane having PEG 400 concentration of 11.11 wt%. This result is very interesting since the increment of the permeate ux did not decrease the rejection as usually observed in the development of ultra ltration membranes as reported in many studies [5,14]. Here, the PET membranes exhibited an increase in both permeate ux and rejection when more PEG 400 was added into the membranes. A similar phenomenon has also been observed by Eren et al., 2015 [24], who reported an improvement of both permeate ux and BSA rejection of polysulfone membrane containing hydrophilic modi ers. In this work, the increment of the rejection rate of the PET membranes with increasing PEG 400 concentration was caused by the decrease in the pore size of the membranes as revealed by the results of the SEM analysis, whereas the increment of the permeate ux was caused by the increase in the hydrophilicity and the porosity of the membranes. A high hydrophilicity resulted in a strong sorption of water to the membranes, whereas a high porosity increased the diffusivity of water through the membrane.

Conclusions
The ultra ltration membrane prepared from used PET bottles showed the same chemical property, morphology, porosity and hydrophilicity with that prepared from PET resin, indicating that instead of using PET resin, used PET bottles that are considered as waste can be utilized as a polymer source to prepare the PET membranes. The use of additive PEG 400 for the PET membranes increased the porosity and the hydrophilicity of the membranes, but decreased the membrane pore size as observed by the SEM analysis. As a result, both permeate ux and rejection of the PET membranes were improved by increasing the PEG 400 concentration. A high value of BSA rejection of 93.9 % was attained using the membrane prepared from PET bottles with the PEG 400 concentration of 11.11 wt%. The result of this study revealed that the low cost ultra ltration membranes with improved permeate ux and rejection could be prepared from used PET bottles as the polymer material with the addition of PEG 400 as the FT-IR spectra of PET membrane using PET bottles as the polymer source FT-IR spectra of PET membrane using PET resin as the polymer source  Water contact angle of the PET membranes as a function of PEG 400 concentration Figure 10 Comparison of permeate uxes of the membranes prepared from PET bottle and PET resin with PEG 400 as the additive