Development and Performance Evaluation of Self-Healing PVA-PAA-Coated PES Membrane for Water Pollution Mitigation ^†

Abstract

Water pollution is a major environmental issue caused by the discharge of untreated or partially treated wastewater into rivers and oceans. Self-healing materials, which can repair localized damage, have become a promising approach to counter membrane performance decline from mechanical wear. However, ensuring stability and effectiveness in self-healing membranes remains a challenge. Polyvinyl alcohol (PVA) has been widely studied for its self-healing properties, while polyacrylic acid (PAA) is often used as a crosslinking agent due to its compatibility with PVA, especially in biomedical and filtration applications. In this study, a self-healing PVA-PAA coating was applied to a PES membrane. The PVA solution (5 wt%) was prepared by dissolving beads in distilled water and stirring at 80 °C for 6 h, while the PAA solution was diluted to match this concentration. The two solutions were mixed in a 3:1 molar ratio and heated to form a homogenous mixture, then coated onto PES membranes and crosslinked at 140 °C. Scanning electron microscopy (SEM) revealed a uniform, crack-free coating on the membrane surface. The mechanical properties of the membrane show a tensile strength of 4.85 MPa and elongation of 71.9%. Filtration tests showed that the PVA-PAA-coated PES membrane achieved a water flux of 36.16 L/m²h. The performance of the PVA-PAA coated PES membrane remained stable in terms of water flux and dye rejection after it healed, and the water flux was recorded at the range of 34.24 to 36.02 L/m²h after the seal healing. This self-healing PVA-PAA coated PES membrane demonstrates the practical potential for sustainable water treatment, offering reduced maintenance and extended lifespan for filtration systems.

1. Introduction

Water covers up to 71% of the surface of the globe, making it one of the essential needs to sustain the ecological processes, human activities, and the natural processes. However, with the rapid development of technology and the escalating demands of modern life, water pollution has become a widespread and persistent issue around the world [1]. Recent studies revealed that over two-thirds of the world’s population is experiencing severe water scarcity, which leads to illness, desertification, environmental degradation, and economic impact [2]. The major contributors to water pollution and scarcity are the expansion of population growth, climate change, and industrial activities. It was found that 50% of wastewater produced by humans is dumped untreated into rivers or oceans, endangering both the environment and human health [3]. The sustainable development goal (SDG 6) aims to ensure the accessibility of clean water and adequate sanitary facilities through the infrastructure improvement, protection, and restoration of water–ecosystem and hygiene education among the public. Water reclamation is the process of recovering the useable water from municipal wastewater and this is crucial to ensure water security. Although the treated water, known as effluent, is not suitable for drinking purposes, it can be widely reused in industries such as power plants, the milling industry, agricultural irrigation, and the restoration of coastal aquifers [4].

With the long-term filtration application, damage to the membrane inevitably persists and results in declined performance. This is attributed to the challenge of accurately locating the damaged area, compounded by the fact that prior attempts to develop smart sensors were unsuccessful and incurred high costs. Therefore, there is a burgeoning interest in crafting self-healing materials as a promising solution to address issues related to material degradation and structural damage [5]. Despite advancements in material science, the development of self-healing materials with robust and reliable healing capabilities remains a significant challenge. A previous study conducted by Lim with PVA-PAA as self-healing materials showed excellent self-healing ability [6]. However, from our observation, the composite is unstable as it tends to dissolve in water. As a result, the performance of the membrane in wastewater applications over the long term results in a shorter lifespan of the membrane [7,8].

Self-healing material has been studied due to its various applications in the membrane filtration process such as in the textile industry and wastewater treatment. Self-healing technology autonomously identifies material damage and initiates repair processes under specific conditions. Such membranes have the potential to revolutionize various industries by offering a longer lifespan, ultimately reducing maintenance costs, and environmental impact, and minimizing downtime. Therefore, this study has assessed the role and performance of the PVA-PAA-coated PES membrane on its self-healing properties.

2. Materials and Methods

2.1. Materials

The pristine membrane used in this study is a commercial polyethersulfone (PES) membrane obtained from Rising Sun Membrane Technology Co., Ltd., Beijing, China. The commercial membrane was used without any modification. The PVA-PAA self-healing coating is made from polyvinyl alcohol (PVA) in bead form with a purity of more than 90% and polyacrylic acid (PAA) in powder form with a purity of more than 90%. Both PVA and PAA were obtained from Sigma Aldrich, Germany. The dye used in the dye removal study is tartrazine dye with more than 85% purity derived from Sigma Aldrich, Germany.

2.2. Preparation of the Self-Healing Membrane

The PVA beads were first dissolved in distilled water and stirred magnetically at 300 rpm and 80 °C for 6 h using a hot plate to obtain 5 wt% of PVA solution. An equal weight percentage of PAA solution was prepared by diluting the 35 wt% PAA powder to 5 wt% using distilled water. The PVA-PAA mixture with a molar ratio of 3:1 was prepared with 75 mL of PVA mixed with 25 mL of PAA at 60 °C for 1 h to form a homogenous solution. The commercial PES membrane was cut into a 5.1 cm diameter circle and cleaned using distilled water followed by drying at room temperature. The homogenous casting solution was then slowly poured onto the pre-cleaned and dried PES membrane. Next, the coated membrane was dried in a vacuum oven at 35 °C, 0.4 bar for 30 min to remove excess ethanol and pure water. Lastly, the coated membrane was heated to catalyze crosslinking between PVA-PAA at 140 °C for 5 min in the thermosetting oven.

2.3. Characterization of the Self-Healing Membrane

Scanning electron microscopy (SEM) (S-3400, Hitachi, Tokyo, Japan) was used to study the surface morphology of the PVA-PAA-coated PES coated membrane from the high-resolution images. Prior to the SEM analysis, a smaller size of the specimen was obtained from the membrane and coated with gold in a sputter coating machine. This was to make sure that the samples were sufficiently conductive for the SEM to analyze the membranes. The SEM was supplied with 15.0 kV and the magnification power was set at 50.0 k magnification.

The tensile test was carried out to determine the tensile strength and elongation at break using the universal tensile tester with a load cell rated for a maximum load of 50 N and a precision of 0.001 N. Tensile strength refers to the maximum stress that a material can endure when subjected to pull apart with a specific load. Elongation at break refers to the ability of the material to resist deformation before rupture. The membrane samples were cut into 9 cm in length and 1 cm in width using a blade. The thickness of the membrane was measured using the micrometer screw gauge. The samples were measured with a gauge length of 15 mm while the tensile rate was maintained at 20 mm/min.

A dead-end filtration cell unit was used to determine the water permeation flux and rejection performance of the coated membrane. The membrane was placed inside the filtration unit containing the feed solution. Next, the feed solution was forced through the membrane using nitrogen gas as an inert gas at ambient temperature with constant pressure. The water flux and dye rejection test were operated at 2 bars and ran for 20 min. Lastly, the permeate passed through the membrane will be collected for further calculation.

3. Results and Discussion

3.1. Membrane Characteristics

The surface morphologies on the top surface of the membrane were observed through the SEM images. The pure PVA-PAA-coated PES membrane in Figure 1 exhibits a uniform surface morphology and the PVA-PAA coating was evenly distributed on the PES membrane [9]. The uniform distribution of the coating materials on the membrane surface was beneficial for the consistence performance in the separation. The morphology illustrates that the membrane surface is relatively smooth, suggesting that the membrane possesses a dense pore structure [10]. Tan and coworkers reported that membranes with dense pore structures yield good mechanical strength and permeate flux performance [11]. Additionally, the membrane yields a tensile strength of 4.85 MPa and an elongation of 71.9% (Figure 2), suggesting its relatively good mechanical strength under the applied dead-end filtration cell and its ability to withstand an operating pressure of 2 bars.

3.2. Membrane Flux Performance and Self-Healing Test

The PVA-PAA-coated PES membrane achieved a pure water flux of 36.16 L/m²h under the dead-end filtration operated at 2 bars. Subsequently, the membrane was used in the dye rejection testing using tartrazine dye, which is commonly used in food and cosmetic products. The performance of the PVA-PAA-coated PES membrane remained stable in terms of permeate flux recorded at 36.16 ± 2.73 L/m²h with a good dye rejection (

Table 1. Permeate flux performance (L/m²h) of the PVA-PAA-coated PES membrane.

Pristine Membrane	Damaged Membrane	Healed Membrane
36.16 ± 2.73	65.21 ± 1.64	35.13 ± 0.89

). The performance of the membrane deteriorated after prolonged usage in the experimental studies, where the permeate flux was increased to 65.21 ± 1.64 L/m²h for the damaged PVA-PAA-coated PES membrane. The membrane was then immersed in the water for 24 h to facilitate the process of self-healing. Subsequently, the performance of the membrane was re-tested after 24 h of the self-healing process. The water flux was recorded at the range of 34.24 to 36.02 L/m²h after the self-healing. This phenomenon demonstrates the healing performance of the water-responsive membrane. The healing mechanism of the PVA-PAA-coated PES membrane was due to the reversible hydrogen bond between the free hydroxyl group and the water molecule [12,13]. Moreover, it can be noticed that the pure water flux of the membrane after healing will be relatively higher than the initial reading. This might be due to the filling of the damaged area with the OH hydrophilic group during the self-healing process [14]. When the membrane experiences physical damage, the self-healing mechanism occurs through the swelling and molecular diffusion of the functional group. Consequently, the membrane, after healing, exhibited higher hydrophilicity.

4. Conclusions

To encapsulate the work, the PVA-PAA-coated PES membrane was successfully fabricated with good membrane surface morphology and mechanical strength. Filtration tests showed that the PVA-PAA-coated PES membrane achieved a water flux of 36.16 L/m²h. The performance of the PVA-PAA-coated PES membrane remained stable in terms of water flux and dye rejection after it healed, and the water flux was recorded at the range of 34.24 to 36.02 L/m²h after the seal healing. This self-healing PVA-PAA-coated PES membrane demonstrates the practical potential for sustainable water treatment, offering reduced maintenance and an extended lifespan for filtration systems.