Utilization of chitosan as an additive for enhancing the performance of polyethersulfone membranes for water treatment

This research investigates the impact of adding chitosan as an additive to improve membrane performance. Membrane fabrication was conducted using the non-solvent-induced phase separation method with polymer blending. Characterization was conducted by evaluating functional groups using FTIR, morphological structure using SEM, and water contact angle using a WCA meter. The research findings show that chitosan additives have a positive influence on the prepared PES membrane. There was a change in the morphological form on the membrane, as evidenced by SEM photos of the cross-section of the membrane. An increase in chitosan concentration resulted in improved hydrophilic properties of the membrane, indicated by a 63.2° reduction in water contact angle. The results show that the best membrane performance was achieved with 4% chitosan addition, resulting in a water flux of 38.2 L/m2.h as well as humic acid rejection of 67.9%.


Introduction
The current water quality, especially in urban areas, is a major concern.Most well water sources are contaminated with bacteria and chemicals, making them unsuitable for household use.Water contamination can come from seepage of seawater, herbicides or pesticides, or residual contaminants, causing surface water to exceed permissible limits.The increasing demand for drinking water has heightened interest in using membranes for microfiltration [1], ultrafiltration [2], and nanofiltration [3] in water treatment processes.Membranes are considered as alternatives to many current unit processes such as ion exchange, carbon adsorption, or sand filtration.One specific group of contaminants found in water sources that has raised concerns in the water industry is humic substances.
As a safer and environmentally friendly alternative process, the removal of humic acid can be achieved through membrane filtration.Ultrafiltration membrane process have rapidly developed compared to other processes because they require lower energy, are easily scaled up, selectively separate materials, and do not require large media.Thus, membrane-based filtration processes are important in water purification and wastewater treatment and can be a suitable alternative to replace related processes such as distillation, adsorption, extraction, and ion exchange [4].Polymers are one of the most widely used materials as the main raw material in membrane manufacturing.
Polyethersulfone (PES) is a polymer used as the material for making membranes.This polymer has good mechanical resistance, high-temperature resistance, excellent chemical stability, and can be used over a wide pH range [5].However, its long-term use has limitations because it is hydrophobic, causing membranes to easily adsorb hydrophobic and amphiphilic solutes, facilitating concentration polarization and fouling on the membrane surface.
Several additives have been used to enhance the hydrophilicity of PES membranes, including CuO nanoparticles [6], Mg(OH)2 [7], polyvinylpyrrolidone (PVP) [8], silica [9], and chitosan [10].Chitosan is a hydrophilic biopolymer generally used to improve hydrophobic membranes to enhance their hydrophilicity.Chitosan possesses good biocompatibility, is non-toxic, cost-effective, a natural biopolymer, and a renewable resource [11].Chitosan's advantageous characteristics have led to its widespread application in various fields, including pharmaceuticals, biomaterials, wastewater treatment, the removal of heavy metal ions from water, and the development of membrane materials.Moreover, chitosan has proven effective in improving the hydrophilic properties of hydrophobic membranes like polyvinylidene difluoride (PVDF), PES, and polyacrylonitrile (PAN) [12,13].
The objective of this study is to improve the hydrophilic properties of PES membranes by incorporating chitosan as a modifying agent in the membrane fabrication process, utilizing dimethyl acetamide (DMAc) as the solvent.This modification involves investigating the impact of varying chitosan compositions on PES membranes.The membranes are manufactured using the non-solventinduced phase separation (NIPS) method, which involves polymer blending.

Materials
In the fabrication of the membranes, we employed PES (Molecular weight: 6500, Ultrason E6020) from BASF Co as the primary polymer.As for the solvent, we utilized Dimethyl acetamide (DMAc) from Merck, Germany.Chitosan from Sigma Aldrich, Germany, was incorporated as an additive.Deionized water served as the non-solvent in the process.For simulating contaminated water, we used a solution containing humic acid.Additionally, our filtration setup involved the use of dead-end filtration equipment.

Preparation of chitosan sample
To prepare the chitosan solution, we began by weighing 1 gram of nanochitosan and dissolving it in 100 mL of a 1% acetic acid solution.The mixture was stirred continuously for 24 hours until a uniform and homogeneous solution was obtained [12].After homogenizing the dope sample, it was subjected to 30 minutes of sonication in a bath.Following this step, the resulting chitosan solution was stored at room temperature and reserved for later use as a chitosan additive.

Membrane fabrication
A total of 18% by weight of PES was dissolved in 82% by weight of DMAc.PES polymer and chitosan with predetermined concentrations were placed in a vial bottle and homogenized using a magnetic stirrer for 24 hours until homogeneous.Membranes were fabricated using the NIPS method.The casting process was done by pouring the dope solution onto a support medium (glass plate) with a thickness of 300 µm.Solidification was then performed, and the membranes obtained were preserved in deionized water to undergo filtration and characterization.Table 1 presents the content of each dope sample.

Membrane analysis
Membrane analysis was conducted by observing the morphological structure of the membrane's crosssection with Scanning Electron Microscope (SEM), detecting the functional group composition with Fourier Transform Infrared (FTIR), and measuring membrane hydrophilicity using a water contact angle meter (WCA).

Pure water flux performance and humic acid rejection
We assessed membrane performance parameters during the filtration process, focusing on pure water flux and humic acid rejection using a dead-end filtration module.To determine the membrane's pure water flux, a dead-end ultrafiltration module operating at 1 bar of pressure was employed.Equation 1was applied to compute the pure water flux of the membrane [14].
Where V was the volume of water passing through the membrane (L), A was the surface area of the membrane, and t was time (seconds).Selectivity membrane testing was conducted with humic acid rejection using a 50 ppm humic acid solution.Equation 2 was used to calculate the humic acid rejection coefficient (R) [14].
Here, the rejection coefficient (R) was determined using the humic acid concentrations in the permeate (Cp) and feed (Cf).

Morphological structure of membranes
Figure 1 shows the cross-sectional morphological form of the P-Ch0 and P-Ch4 membranes.The images indicate that all membranes have an asymmetric structure, containing an upper (dense layer) and lower (sublayer) layer.Macrovoid-like pore structures are visible in the sublayer of the P-Ch0 and P-Ch4 membranes.The asymmetric structure is typical of membranes formed using the NIPS method.Differences in the morphological structure between the P-Ch0 and P-Ch4 membranes can be observed.The dense layer of the P-Ch4 membrane is thinner than that of the P-Ch0 membrane (membrane without modification).Furthermore, a notable distinction can be observed in the macrovoid structure within the second sublayer of the membranes.In the P-Ch4 membrane, there is a discernibly higher presence of macrovoid-like protrusions when compared to the P-Ch0 membrane.This heightened occurrence of macrovoids is attributed to the likelihood of some additive escaping from the polymer solution during the coagulation process within the coagulant bath, which consists of a non-solvent, specifically, distilled water.This occurs due to irregularities in the bonding between the polymer solvent and additive during the membrane formation process [15].In this case, the addition of the chitosan solution is believed to lead a reduction in the amount of non-solvent required while the phase separation process.This could be a plausible explanation for the heightened formation of macrovoids observed in the membrane incorporating the chitosan additive (P-Ch4).

Functional groups
Figure 2 presents the results of the FTIR analysis, which revealed the presence of various functional groups in all the analyzed membranes.These included aromatic C-H groups at 837 cm -1 , vibrations associated with aromatic rings (C=C) at 1485 and 1575 cm -1 , sulfone groups (O=S=O) at 1147 and 1155 cm -1 , and aromatic ether groups (C-O-C) at 1232 and 1253 cm -1 .These detected wavelength peaks correspond to vibrations generated by the constituent materials of PES.

Figure 2. FTIR spectra of various tested membranes
According to previous studies [16], chitosan typically exhibits vibrations stretching of hydroxyl groups (O-H) and amine groups (N-H) (asymmetric stretching) at a wavelength of 3362 cm -1 .Furthermore, the appearance of a peak at a wavelength of 1640 cm -1 indicated the presence of N-H groups stemming from secondary amine groups, specifically, symmetric stretching.Nevertheless, the IR analysis in this study did not reveal any observable chitosan functional groups within the modified membranes.Based on these findings, it can be concluded that there might not be a substantial physicochemical interaction between PES and chitosan.Additionally, the absence of chitosan in the IR analysis may be attributed to the likelihood of most chitosan particles leaching out during the phase inversion process.This is a common phenomenon due to the additive's hydrophilic nature, rendering it more reactive towards water than the polymer membrane itself.This ultimately influences the formation of macrovoids, as previously demonstrated in the SEM analysis.

Water contact angle
Figure 3 displays the results of water contact angle analysis conducted on all the produced membranes.A consistent trend is evident across the modified membranes, where the water contact angle decreases, signifying an enhancement in their hydrophilic characteristics.Membrane P-Ch0, P-Ch2, and P-Ch4 have water contact angle values of 80.0°, 67.4°, 63.2°, respectively.This suggest that membrane P-Ch0 is hydrophobic, whereas membranes P-Ch2 and P-Ch4 can be considered more hydrophilic than membrane P-Ch0.Chitosan addition as an additive to the membrane enhances the hydrophilicity of the membrane.The increase in hydrophilicity is attributed to the presence of hydroxyl groups (-OH), which are hydrophilic, in chitosan, transforming the originally hydrophobic membrane (P-Ch0) into an incresaed hydrophilic one.As observed in the modified membranes, there is a notable reduction in the water contact angle on the membrane's surface, specifically in the cases of P-Ch2 and P-Ch4.These findings align with previous research [17], which also highlighted that elevating the concentration of the chitosan solution can effectively enhance membrane hydrophilicity, resulting in decreased water contact angle values.The findings indicate that the P-Ch0 membrane exhibits a pure water flux of 2.3 L/m²•h.However, when PES membranes are supplemented with 2% (P-Ch2) and 4% (P-Ch4) chitosan, the pure water flux notably increases to 19.2 L/m²•h and 38.2 L/m²•h, respectively.This improvement is attributed to disparities in the morphological structure of the membranes, as illustrated in Figure 1.Specifically, the denser upper layer of the P-Ch0 membrane (Figure 1a) is thicker compared to that of the P-Ch4 membrane (Figure 1b).This greater thickness in the upper layer of P-Ch0 hinders the separation of particles present in the sample solution, resulting in a lower pure water flux in contrast to the PES membranes modified with the addition of 2% chitosan (P-Ch2) and 4% chitosan (P-Ch4).
The water flux through the membranes is also influenced by their water contact angle values.The observed increase in water flux for membranes P-Ch2 and P-Ch4 can be elucidated by examining the water contact angle data presented in Figure 4.These measurements indicate that membranes P-Ch2 and P-Ch4 exhibit higher levels of hydrophilicity compared to membrane P-Ch0.This heightened hydrophilicity significantly impacts the water flux values of the membranes.Consequently, it can be inferred that a decrease in the water contact angle, indicative of increased hydrophilicity, has a positive effect on enhancing water flux across the membrane.

Humic acid rejection
To evaluate the membrane's capability to filter solute, a humic acid solution is employed as the feed.This test aims to assess the membrane's effectiveness in eliminating natural organic constituents from water.The evaluation is conducted using an ultrafiltration module, wherein a 50ppm humic acid solution is circulated for 60 minutes under an operating pressure of 1 bar.Figure 5 illustrates the humic acid rejection performance of the membranes.

Figure 5. Rejection of humic acid sample in the tested membranes
Among the membranes, P-Ch0 demonstrates the highest rejection rate at 79.8%.This superiority is believed to be linked to the smaller pore sizes in P-Ch0 (Figure 1a) when compared to P-Ch4 (Figure 1b).Smaller pores render the P-Ch0 membrane more selective in allowing only specific constituents to pass through.In contrast, membrane modification through the addition of chitosan leads to a reduction in rejection rates, with membranes P-Ch2 and P-Ch4 exhibiting rates of 69.9% and 67.9%, respectively.This can be attributed to the presence of larger pore sizes in the modified membranes, resulting in fewer chitosan particles being retained on the membrane surface.

Conclusions
Modification of PES membranes using chitosan as an additive with DMAc as the solvent was successfully performed.The addition of the additive increased pore size, influenced the change in membrane structure, and decreased water contact angle (increased hydrophilicity).The best performance was demonstrated in the modified membrane with 4% chitosan, which resulted in a pure water flux of 38.2 L/m 2 h with a humic acid rejection of 67.9%.These findings indicate that chitosan addition plays a role as a modifying agent that affects the increase in flux and humic acid rejection, which is not too low.

Figure 3 .
Figure 3. Water contact angle on various types of tested membranes Figure 4 illustrates the obtained results for pure water flux across the membranes.

Figure 4 .
Figure 4. Pure water flux across various types of tested membranes

Table 1 .
The composition of membrane dope solution