Impact of Chitosan Pretreatment to Reduce Microfibers Released from Synthetic Garments during Laundering

Abstract

Sewage treatment can remove more than 90% of microplastics, yet large amounts of microplastics are discharged into the ocean. Because microfibers (MFs), primarily generated from the washing of synthetic clothes, are the most abundant type of microplastics among various microplastics detected in the sewage treatment, reducing the amount of MFs entering these treatment plants is necessary. This study aimed to test whether the amount of MFs released from the washing process can be reduced by applying a chitosan pretreatment to the garments before washing. Before the chitosan pretreatment, the polyester clothes released 148 MFs/L, whereas 95% of MFs were reduced after the chitosan pretreatment with 0.7% of chitosan solution. The chitosan pretreatment was applied to other types of garments, such as polyamide and acrylic garments, by treating them with 0.7% of chitosan solution; subsequently, MFs reduced by 48% and 49%, respectively. A morphology analysis conducted after washing revealed that chitosan coating on the polyamide and acrylic were more damaged than on polyester, suggesting that the binding strength of polyamide and acrylic with chitosan was weaker than that of polyester garment. Thus, the results suggested that the chitosan pretreatment might be a promising solution for reducing the amount of MFs generated in the laundering process.

1. Introduction

Since its invention, plastic has been incorporated into many products due to its convenience, thereby increasing the use of plastic dramatically [1]. The generation of plastic waste has increased in recent years, leading to 4.8–12.7 million tons of plastic discharging into oceans every year because of waste mismanagement [2]. Microplastics are less than 5 mm in size and can be generated by plastic wastes introduced into the environment, which undergo pyrolysis and physical and chemical transformations, by which they are decomposed and fragmented into smaller particles [3,4]. Such microplastics have been detected in the form of films, flakes and fibers and, of these, microplastic fibers (MFs) are commonly detected in all aquatic environments, such as lakes, rivers, oceans and marine sediments [5,6,7,8,9,10].

MFs are typically released when synthetic textiles are laundered in washing machines [11,12]; therefore, ongoing research has focused on MFs released during the washing process. Considering the amount of MFs that can be generated (162 ± 52 MFs/g(garments), from a single washing), it is estimated that 6 million MFs can be released from a 5 kg load of polyester (PE) textiles [13]. It is estimated that about 1.4 trillion MFs occur in the ocean that are difficult to be removed, due to their settlement in the sediment layer and their small size [14,15,16]. Therefore, decreasing the amount of MFs, before their entry into waterbodies, is an optimal solution.

De Falco et al. [17] reported that pectin pretreatment of polyamide (PA) garments can reduce the amount of released MFs up to 90%, compared with untreated cloth. Despite a good efficiency on reducing MFs generated by applying the pectin pretreatment, an additional crosslinking agent (glycidyl methacrylate) is required to increase the bonding strength of pectin with garments. Koo et al. [18] showed that addition of chitosan film foam to polyester fabric increases the stiffness by approximately 63%. Considering that most MFs are released by physical damage during laundering [19,20], we hypothesized that the chitosan film-covered PE garments can strengthen the stiffness of PE fabric and reduce the amount of MFs generated by resisting physical damage. Therefore, in this study, we explored if the chitosan pretreatment applied to PE garments before laundering can reduce the amount of MFs generated during the washing process. The amount and length of MFs released from the chitosan pretreated PE and untreated PE garments were compared and their characteristics were also analyzed. In addition, the main cause of MF generation during the washing process was examined. Finally, to compare the applicability of chitosan solution to other polymer garments (acrylic, AC and PA), the characteristics of the pretreated garments and the reduction in MFs were analyzed.

2. Materials and Methods

2.1. Reagents and Polyester (PE) Garment Samples

Acetic acid (CH₃COOH, extra pure reagent) was obtained from Daejung (Seoul, Korea) and chitosan (C₆H₁₁NO₄) was purchased from Showa (Tokyo, Japan). The material used for the experiments consisted of 100% PE obtained from fitness vests (69 g, 55 × 57 cm) (Table S1). Further, to effectively discern MFs from garments, fluorescent-colored garments were used.

2.2. Grafting of Chitosan Solution on PE Garments and Washing Process

A predetermined amount of chitosan powder was added to 1 L of 1% (v/v) aqueous acetic acid (Daejung, Korea) and its final concentrations were 0.1%, 0.4%, 0.7% and 1% (w/v) [21]. Each solution was stirred for 24 h until the powder was completely dissolved. Whole PE garments were immersed in the chitosan solution for 1 h at 24 °C for each experiment and were placed in a drying oven at 120 °C for 2 h to ensure complete drying.

To measure the amount of MFs generated during domestic washing, a top-loading household washing machine (TR138K, LG, Korea) was used. The top-loading machine had a built-in sieve of 330 μm pore size to remove dust generated during the washing process. In terms of washing condition, 40 mL of detergent was used for each washing (40 L of 21 °C water used per washing). The wash cycle was performed at 135 rpm for 10 min and the washing test was repeated three times. To avoid any external MFs entering the washing machine, the washing machine was cleaned twice with distilled water before and after washing in each experiment. The collected washing drainage (40 L) was filtered using a 100 μm sieve and the sieved MFs were transferred to a beaker using distilled water. The sieved MFs were re-filtered using a track-etched polycarbonate filter (20 µm pore size and 47 mm diameter) (GVS, Sanford, ME, USA); then, they were dried in a desiccator prior to analysis to minimize air contamination. To prevent additional contamination, all glassware was rinsed at least three times with distilled water and the researchers wore black gowns to distinguish the MFs released from the washing experiments.

2.3. Application of Chitosan Pretreatment on Other Polymer Types

For the applicability of the chitosan pretreatment method, we used two different polymer garments, such as PA and AC. PA garments weighed 150 g and their dimensions were 50 × 55 cm, whereas AC garments weighed 156 g and their dimensions were 50 × 52 cm. Similar to the PE garments, both garments were pretreated with chitosan solution and washed. During washing, a new garment (unwashed) was put in each time and the washing experiment was repeated three times.

2.4. Instrumental Analyses of MFs

To easily recognize MF generation, MFs were stained with Nile red (NR) following the NR plate method proposed by Kang et al. [22] and observed under a stereoscopic microscope (Discovery V8, Carl Zeiss, Jena, Germany). UV light with a wavelength of 365 nm was applied and luminescent MFs were counted. Lengths of MFs were recorded by an IMT cam 6.3 camera (IMT i-Solution Inc., Burnaby, BC, Canada) and their length was measured using the i-Solution software. We randomly selected 100 MFs of PEs after washing to sort size distribution.

The morphologies of PE garments before and after the chitosan pretreatment were analyzed by scanning electron microscopy (SEM, S-4800, Hitachi, Ltd., Japan) with 3.0 kV and 1000–3000 times magnification. PE samples were cut into 1 × 1 cm pieces and coated by platinum. To measure the coating degree of untreated and chitosan-treated garments, a stiffness test was performed using a drape tester. The garment samples were cut into 5 × 5 cm pieces and each sample was measured five times using a drape tester (James H. Heal, UK).

3. Results and Discussion

3.1. Characteristics of Untreated MFs Generated during the Washing Process

The amount of untreated MFs per one garment released in the washing drainage was 148 ± 21 MFs/L, which was equal to 85 ± 12 MFs/g on a weight basis. This value was similar to that observed in other, previous studies (82.6 and 78.1 MFs/g) [11,13]. We expected that the length of MFs contained in the drainage would be less than 300 μm owing to the 300 μm built-in sieve in the washing machine that filtered the drainage. However, 97% of the detected MFs were over 300 μm, indicating that flexible MFs can easily pass the 300 μm sieve screen. The most frequently detected size of MFs was in the range of 500–1000 μm in length with a median value of 957 μm and a maximum MF length of 2114 μm (Figure 1). Thus, this indicates that the MFs generated in the process of laundering in a real environment may have various lengths, regardless of the built-in sieve size in the washing machine.

The morphologies of most generated MFs were curved in shape due to their weaving in the garment manufacturing process (Figure 2). Further, no tangled MFs were observed; thus, washing released individual MFs from the weaving fibers.

To determine the main cause of MF generation during the washing process, the morphology of MFs produced in the drainage and intentionally cut MPs were compared using SEM analysis. According to the SEM analysis, the morphology of MFs in the drainage was different from that of the MFs that were cut intentionally with scissors (Figure 3c,d). The cross-section cut with scissors was extremely clear with no fragment generation. However, the cut edge of the generated MFs during laundering was not distinct, indicating that possibility of generation of nano- and/or micro-sized fragmented fibers (Figure 3a). It is speculated that the MFs generated during the washing process may be broken by physical damage by repeated twisting motion [23,24]. In addition, several nano-sized fragments were observed in the cross-section of MFs, where the fiber was broken (Figure 3b), indicating a possible risk of secondary pollution, such as that caused by secondary MPs generation.

3.2. Reduction in the Amount of MFs Generated after the Chitosan Pretreatment

The amount of untreated MFs released during the process of laundering was 85 ± 12 MFs/g. Despite pretreatment with 0.1% chitosan solution, reduction in the amount of released MFs was not observed (Figure 4). This indicates that the 0.1% chitosan concentration may be not strong enough to form a coating on whole garments due to its low dosage and may not prevent physical damage during washing.

In the case of pretreatment with 0.4%, 0.7% and 1% chitosan solutions, 36 (±21), 4 (±1) and 41 (±14) MFs/g, were released from the PE garments, respectively. Thus, the highest percent reduction was achieved with the 0.7% chitosan solution pretreatment, with a 95% reduction in the number of MFs released after washing.

Compared with the control, the fibers in the chitosan pretreated samples were well connected with each fiber by the chitosan solution (Figure 5). Thus, this indicated that the application of chitosan solution may link each fiber and reduce the generation of MFs. To be more specific, chitosan particles adhere to the fabric surface through linear bonding with the carboxyl group of chitosan and the terminal group of PE fabric, thereby increasing the tensile strength between each fiber [25]. When the samples were pretreated with a 0.7% chitosan solution, the coating was mostly uniformly formed on the MFs (Figure 5b), whereas the films on the MFs of the sample treated with 1% chitosan solution were not uniform because of film sagging (Figure 5c). Thus, it appeared as a non-uniform and thick-layered coating due to excessive chitosan concentration used. Therefore, it was confirmed that the chitosan coating was more uniform at a moderate concentration (0.7%) than at the highest concentration (1.0%); further, it was observed that the chitosan coating formed evenly with the 0.7% chitosan solution. Thus, the uniformity of the chitosan coating may be a key factor to reduce MF emissions from the laundering process. Optimal chitosan concentration may highly rely on the area/weight of garments; thus, further research on the optimal conditions for various areas and weights of garments is required.

To examine the changes in physical properties of MFs before and after chitosan pretreatment, the tensile strength was determined by measuring the drape coefficient of the garment pretreated with 0.7% chitosan and the untreated MFs (

Table 1. Result of drape index before and after chitosan pretreatment.

	Control	0.7% Chitosan Solution	% Increase
Drape coefficient	0.337 (±0.014)	0.572 (±0.009)	58 (±1)

). The results showed that the application of 0.7% chitosan pretreatment increased garment stiffness up to 58%. The increase in stiffness was due to the penetration of chitosan into the PE fabric, which promoted film formation on the fabric’s surface and reduced the degree of freedom of the MFs [25].

3.3. Application of Chitosan Pretreatment on Other Polymer Types

To apply the chitosan pretreatment on the PA and AC garments (Figure S1, Figure S2 and Figure S3), synthetic garments were pretreated with 0.7% chitosan solution, which had the highest efficiency on polyester. For the untreated samples, 85 ± 12, 160 ± 45 and 239 ± 37 of MFs particles/g were released from the PEs, AC and the PA garments, respectively (Figure 6). Considering that the densities of PE, AC and PA were 1.38 g/cm³, 1.2 g/cm³ and 1.14 g/cm³, respectively, the garments having high density released lower amounts of MFs than low density AC and PA garments. Similarly, Napper and Thompson [11] showed that the lowest amount of MFs were generated from the PE, compared to AC garments, during washing, with 82.6 MFs/g from PEs and 121.4 MFs/g from AC. Yang et al. [26] also reported that PE garment released 2012 MFs/m² and PA generated 50,686 MFs/m². Thus, these observations can support our hypothesis that the high density of garments reduced the amount of generated MFs.

In the pretreatment of other polymer garments, 0.7% chitosan solution, which had the best reduction efficiency in PEs, was used. After chitosan treatment, the amounts of MFs released from PE, PA and AC garments were reduced approximately by 95% ± 1.9%, 48 ± 10.45% and 49 ± 14.2%, respectively. To explore the possible reason for the different amounts of the generated MFs for different garments, the morphology of coating on each garment was analyzed using SEM. As shown in Figure 7, while PEs and ACs showed a good coating with the chitosan solution before washing (Figure 7a,b), PAs were not coated completely between each fiber (Figure 7c). After the washing process, PEs garments showed that the chitosan coating was tightly bound, whereas a part of the chitosan coating peeled off in AC (Figure 7d,e). Thus, this indicates that the degree of chitosan coating was affected by the type of polymer in garments. Therefore, optimal conditions for each type of clothes, such as chitosan concentration, soaking time and appropriate solutions, can be studied further.

4. Conclusions

This study demonstrates that chitosan pretreatment could efficiently reduce the amount of MFs released during the process of laundering. An optimal concentration of chitosan solution (0.7%) was detected for the pretreatment of PEs garments and the amounts of MFs generated before and after the chitosan pretreatment were compared. The amount of MFs released from the untreated PEs garments during the washing process was 148 ± 21 MFs/L with 957 μm of medium length. After the chitosan pretreatment, the number of MFs released was reduced by 95 ± 1.9%, compared with the untreated garments. To confirm the applicability of the chitosan pretreatment, other synthetic garments (PA and AC) were pretreated and, subsequently, 48 ± 10.45% and 49 ± 14.2% of MFs were reduced, respectively. The lower reduction rate in PAs and ACs than that in PEs could be because the chitosan concentration was not optimal or possibly because of poor binding strength of chitosan treatment with PAs and ACs.

Laundry is a major source of MF emissions and it is estimated that 6 million MFs can be released from 5 kg of synthetic textiles that flow into the water environment through sewage treatment plants [11,13,27]. Therefore, ongoing research has focused on methods for reducing microplastics generated during the washing process. The study of reducing the generation of MFs during laundry can be categorized into in-drum and external device approaches [14,28]. In-drum devices are placed inside the washing machine along with the garments to capture MFs and reduce friction and external devices use a membrane to reduce MFs contained in the washing drainage. MFs were reduced by 21–78% in in-drum and external device studies and the highest reduction rate was 87% when using the membrane.

Our study presented a method to reduce the generation of MFs from garments and not to reduce the generated MFs. However, the coating process in the chitosan pretreatment used in this study requires a relatively long time to be completed, resulting in unpractical efficiency. Therefore, further research is needed to develop a pretreatment method that improves the coating process time. In addition, further investigation into the reduced amount of MFs from several types of synthetic fabrics has been planned using the same pretreatment method.