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Abstract

This study assessed the interactive changes to the endocrine disruptor 17 alpha-ethynylestradiol (EE2) triggered by photoaging onto fibrous microplastics frequently found in the environment. The physicochemical property change of the polyethylene (PE) microfiber according to irradiation (i.e. 14 d UV-C (254 nm)) was studied through Fourier transform infrared spectroscopy, scanning electron microscope, and contact angle analysis. Additionally, the EE2 adsorption kinetics experiment was performed for the PE microfiber before and after UV irradiation to assess the change in adsorption characteristics. After UV irradiation, the PE microfiber surface roughness increased, the oxygen-containing functional group (e.g. carbonyl group) increased, and the contact angle (virgin PE: 80.02°, aged PE: 65.13°) decreased. A decrease in the surface hydrophobicity led to a decrease in the adsorption rate of EE2 (virgin PE: k = 0.0105 h⁻¹, aged PE: not calculated). The hydrophobic interaction significantly affects the adsorption behavior of hydrophobic organic pollutants such as EE2 onto MPs, and continuous photo-aging of MPs may cause a new pattern of ecological risk. Therefore, there is a greater necessity for additional research relevant to this issue.

Keywords

Microplastics polyethylene UV irradiation synthetic estrogens endocrine disrupting chemicals

Introduction

Chemical resistance, corrosion resistance, and cost-effectiveness are some advantages of plastics that have driven their continuous production and usage. As plastics are not readily degradable, they accumulate in the environment,¹ and microplastics (MPs, i.e. plastic particles below 5 mm) have received increasing scientific and societal attention.² MPs often have irregular shapes and occur as fibers, granules, pellets, or fragments³ and are divided into primary and secondary types according to their source. Primary MPs generally come from industrial preproduction pellets, which are primarily used as an ingredient in beauty care products such as facial cleansers, cosmetics, toothpaste, hand cleaners, and exfoliation scrubs, as well as in drilling fluids and clothing. Wave action, UV radiation, and other physical, chemical, and/or biological processes in the environment generate secondary MPs from large plastic debris.⁴ Approximately 92% of MPs in southwest China were fibers (4% were spheres), according to Zhang and Liu,⁵ which is a more environmentally significant form than spheres or pellets.

By continuously exposing polymer composites to ultraviolet (UV) light, Song et al.⁶ reported surface weakening, and the resulting mechanical wear and thermal degradation accelerated MP formation. Long-term exposure to sunlight enriched by UV light can change MP surface properties by causing free radical formation, oxygen addition, hydrogen abstraction, chain scission, and crosslinking.⁷ According to Halle et al.,⁸ plastics become more hydrophilic as they age due to surface oxidation. Liu et al.⁹ reported that UV light can cause polymers to degrade, change their colors (yellowing), crack, chalk, and lose their mechanical properties.

17α-ethynylestradiol (EE2) is a synthetic hormone found in most modern oral contraceptive pill formulations and has relatively poor water solubility (4.8 mg/L at 20°C) compared to the natural estrogenic steroid. EE2's strong degradation resistance and increased oral bioavailability are beneficial for contraceptive function; however, this property permits EE2 to enter the environment via home wastewater, resulting in high EE2 concentrations in wastewater effluent and river ecosystems. It is commonly acknowledged that EE2 can alter an organism's endocrine system at low concentration levels of sub-nanograms per liter (ng/L) and can influence organisms directly and indirectly. Increased plasma vitellogenin in male and female fish, enhanced proportions of intersex fish, decreased egg and sperm production, reduced gamete quality, complete feminization of male fish, reduced fertility and fecundity, and behavioral changes are all effects of exposure to these chemicals, according to both field and laboratory studies.^10,11

Unlike biological tissues, plastics are chemically stable. Due to their hydrophobicity and large specific surface area, MPs demonstrate strong sorption affinities to many organic contaminants,¹² thereby posing a threat to ecosystems and humans because of their easy ingestion by organisms. Lu et al.¹³ evaluated the adsorption characteristics of EE2 on MPs in seawater. Different MP material types were evaluated for 4–5-mm size particles with smooth surfaces. They confirmed that the adsorption of EE2 to MPs increased with increasing salinity due to the surface charge changes. Humic acid (HA) can also increase EE2 adsorption on the MP surfaces through soluble EE2–HA complex formation. As mentioned above, EE2, an endocrine disruptor, can potentially cause serious environmental and human health problems by interacting (i.e. sorption) with MPs. Hence, there is a need to understand the interaction between EE2 and MPs in the environment. In particular, the surface properties of MPs (e.g. surface functional groups, surface roughness, hydrophobicity) can be changed due to physicochemical weathering processes, which can cause a change in the relationship between MPs and contaminants in the environment.

This study aims to assess the adsorption characteristics of EE2 on fibrous MPs. Specifically, the change in PE microfiber surface properties with UV irradiation and its effect on EE2 adsorption characteristics were evaluated to understand the effect of photo-weathering. The interaction between weathered fibrous MPs and EE2, which causes endocrine-disrupting effects even in trace amounts, was observed for the first time.

Materials and methods

Preparation of PE microfiber

The PE pellets without adding additives (e.g. plasticizer or flame retardant) purchased at LG Chemical Seetec Ltd, Korea were prepared into fiber form using a laboratory mixing extruder take-up system (Dynisco, Germany) at 145°C (melting point of PE: 135 to 140°C). MP samples were prepared by cutting PE fibers with scissors into <5 mm lengths.

Preparation of aged PE microfibers and analysis of their surface characteristics

A stainless-steel chamber (length: 550 mm, width: 250 mm, height: 180 mm) containing four 15-W UV-C lamps (254 nm) was constructed to accelerate and simulate photo-oxidation and photolysis. The photo-aging was performed with PE microfibers of <5 mm in length placed inside the chamber for 14 d.

An attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR; IRTracer-100, Shimadzu, Japan) was used to assess the changes in the surface functional groups of the PE microfiber with photo-aging. The transmittance and absorbance over a wavenumber range of 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹ without any sample pretreatment were observed. Furthermore, a high-resolution scanning electron microscope (HR-SEM; SU8220, Hitachi, Japan) was used to observe the surface morphology of the PE microfiber (coated with Pt) at a voltage of 5 kV and at 250, 3500, and 4000 times magnification. To confirm the change in surface hydrophobicity of the PE microfiber before and after photo-aging, the angle of water droplets formed on a specimen surface, which was prepared in an electric furnace by melting to form a flat surface, was measured using a contact angle analyzer (Phenix 300, S-EO Co., Korea). A drop of 2.6 μL deionized water was applied to the specimen through a 0.4064-mm diameter needle, and the angle between the specimen surface and the water drop was measured.

Analysis of EE2

A high-performance liquid chromatography triple quadrupole mass spectrometer (HPLC-QMS) (Finnigan TSQ Quantum Ultra EMR, Thermo Scientific), equipped with electrospray ionization (ESI), was used to analyze EE2. A Roc C18 column with 3 mm × 150 mm, and 5 μm pore size (Restek, Bellefonte, PA, USA) was used at 25°C for the separation. The gradient elution program was used in all HPLC-QMS analyses by using 0.2 mM ammonium fluoride and acetonitrile with a volume ratio change of 7:3 to 0:10. The eluent flow rate and sample injection volume was set to 300 μL/min, and 10 μL, respectively. Mass spectra obtained from ESI–MS ES⁻ (negative mode) scan of EE2 were about to m/z 145.06 and 143.05 for product ions. For MS analysis, spray voltage, capillary temperature, and collision energy were set to 3000 V, 300°C, and 50 V, respectively.

Sorption kinetics between virgin/aged PE microfiber and EE2

Using an end-over-end shaker, 0.05 g of PE microfiber (virgin and aged) and 10 mL of EE2 solution (initial EE2 concentration measured = 293 ng/mL) were placed in a 50 mL glass vial and reacted for 0.5, 2, 6, 12, and 24 h at 25°C and at 160 rpm. After each reaction time was completed, the solution was filtered through 0.3 μm glass fiber membrane filters (GF7547MM, Advantec, Japan), and the filtrate was analyzed using HPLC-QMS. The analysis results were fitted with a first-order reaction equation (1) to derive rate constants.

C_{t} = C_{0} \cdot e^{- k t}

(1)where C_t is the EE2 concentration at time t and C₀ is the initial EE2 concentration, and k is the first-order reaction rate constant.

Results and discussion

Calibration of EE2 and analytical performance

In a reversed-phase C18 column, the retention time of the compound is determined by the hydrophobicity of the target compound,¹⁴ and it is used to separate hydrophobic compounds.¹⁵ This study also used the C18 column to separate and analyze EE2, and its retention time was 10.28 min (Figure 1(a)). This was similar to the results of the HPLC study (retention time = 12.53 ± 0.2 min) for EE2 in a water sample in a previous study.¹⁶

Figure 1.

(a) Chromatogram of 200 μg/L EE2 standard solution in the HPLC analysis (retention time of EE2 = 10.28 min) and (b) calibration curve drawn with the area integrated with the measured intensity (y-axis) and the EE2 concentration of standard solution (x-axis).

The calibration curve for EE2 exhibited strong linearity (R² = 0.9999) (Figure 1(b)). To assess the detection capabilities of this analytical method, the limit of quantification (LOQ) was determined. The LOQ is defined as 10 times the standard deviation of the blank (σ, n = 7) divided by the slope of the calibration curve (p) and was found to be 21.3 μg/L.

Changes in the surface properties of the PE microfiber after aging

Changes in the surface properties of the PE microfiber with UV irradiation were assessed using ATR-FTIR (Figure 2). A slight increase in the carbonyl group (C=O) stretching vibration ranged between 1740 and 1690 cm⁻¹ as a probable indicator of photo-oxidation.^17,18 The HR-SEM analysis results corroborated that UV light source was used to age the microfiber samples and caused the increase in PE surface roughness (Figure 3). The contact angles of virgin PE microfiber and aged PE microfiber were measured as 80.02° and 65.13°, respectively, showing less hydrophobicity with the change in surface physicochemical properties (Figure 4). It is believed that the weakening of the intermolecular bonds with UV irradiation led to changes in the physicochemical properties of the MP surface^19–21 Overall, an increase in the C=O bond and a decrease in hydrophobicity of the PE microfiber surface were observed, and the surface morphology was also changed. Huang et al.¹⁷ irradiated 254 nm UV-C onto powder type (<5 μm) polystyrene (PS) MPs for 240 h, and observed microfractures formed on the aged MP surface, and the increased specific surface area (from 5.47 to 6.92 m²/g). Additionally, an increase in carbonyl groups (wavenumber 1700–1801 cm⁻¹) was observed through FTIR analysis. Cai et al.²² continuously irradiated UV onto pellet-type MP_S for 30, 60, and 90 d to study the changes in surface properties of three MPs (PE, PP, and PS) according to UV-A (340 nm) irradiation. After 90 d, local oxidation (i.e. formation of carbonyl and hydroxyl functional groups) was identified on the surface of MPs. Similarly, changes in MPs surface properties owing to UV irradiation, which have been previously studied by many researchers (Table 1), were also observed in this study. As reported in the literature,²³ the increase of oxygen-containing functional groups such as ketones, esters, and carboxylates on the surface of MPs can decrease the sorption of hydrophobic organic contaminants along with the decrease in the hydrophobicity of MPs. Hence, the investigation of the change in surface properties (e.g. increase in MPs surface roughness, and decrease in MPs hydrophobicity) after the photo-oxidation (UV irradiation) can be helpful to understand the interaction of MPs with hydrophobic organic contaminants in the environment.²⁴

Figure 2.

Infrared transmittance of PE microfiber before and after aging as was determined by attenuated total reflection Fourier-transform infrared spectroscopy between 4000 and 400 cm⁻¹. The enlarged part shows the carbonyl group (C=O) stretching vibration formed in the range of 1740–1690 cm⁻¹ after the photo-oxidation.

Figure 3.

High-resolution scanning electron microscopy image of the PE microfibers before (a) and after (b) UV irradiation (i.e. aging) for 14 d. Through the comparison of SEM images, it was visually/indirectly confirmed that the roughness of MP surface increased.

Figure 4.

Contact angle between the specimen and water drop: (a) virgin PE microfiber (contact angle = 80.02°), (b) aged PE microfiber (contact angle = 65.13°).

Table 1.

Changes in surface properties of MPs after UV irradiation reported in previous studies.

Light source	MPs	Type	Exposure duration	Key observations	Reference
UV-C lamp (254 nm)	PE	Fiber	14 d	Increased carbonyl group (C=O)	This study
				Observed wrinkles
				Decrease in the hydrophobicity (Contact angle analysis result: Virgin MPs (V): 80.02°, Aged MPs (A): 65.13°)
	PS	Powder	240 h	Observed micro-cracks	Huang et al.¹⁷
				Increase in the specific surface area (from 5.468 to 6.920 m²/g)
				Increase in the carbonyl group (C=O)
	PS	–	90 d	Observed crumpled surface and large number of granular bumps and dents	Wu et al.²⁵
	PS	–	90 d	increase in the carbonyl band (C=O) and hydroxyl band	Wu et al.²⁵
	PS	–	96 h	Observed small wrinkles	Liu et al.²⁰
	PVC	–	96 h	PS: increase in the bending vibration of −CH₂; PVC: increase in the carbonyl group	Liu et al.²⁰
	PE	–	600 h	Observed cracks and color changes (from the white to yellow)	Guan et al.²⁶
	PE			increase in the oxygen absorption band and carbonyl groups
	Polylactic acid (PLA)			Increased surface area (PE: from 0.58 to 1.03 m²/g; PLA: from 0.44 to 0.46 m²/g)
	Polylactic acid (PLA)			Decreased zeta potential (PE: from −5.3 to −16.8 mV; PLA: from −28.2 to −34 mV)
UV-A lamp (340 nm)	PE	Pellets	90 d	Increase in the hydroxyl group and carbonyl group	Cai et al.²²
	PP			Observed cracks
	PS			Observed cracks
	PS	Spheres	3 months	Increase in the oxygen containing functional groups	Mao et al.²⁷
				Observed generates pores, observed cracks
				Increase in the O/C ratio (from 0.044 to 0.322)
UV-A lamp (313 nm)	PVC	Beads	600 h	Increase in the specific surface area (from 0.49 to 1.02 m²/g)	Wang et al.²⁸
	PVC	Beads	600 h	Increase in the carbonyl group (C=O)	Wang et al.²⁸
	Polyethylene terephthalate (PET)	Slices	500 h	Observed cracks	Wang et al.²¹
				Observed cracks and color changes (from the white to yellow)
				Increase in the oxygen containing functional groups

Changes in sorption kinetics after aging

Adsorption kinetics experiments were performed for PE microfiber before and after 0.5, 6, 12, and 24 h of UV-C irradiation. The EE2 adsorption reaction constant (k) value determined for virgin PE using the first-order kinetic model was 0.0105 h⁻¹, and the k value could not be determined for aged PE as no significant changes in concentration were observed (Figure 5). Such results may signify that the PE microfiber adsorption capability for EE2 can be reduced via irradiation from UV-C exposure. Wu et al.²⁵ performed adsorption kinetics experiments to investigate the adsorption capability of PS MPs for the hydrophobic pollutant 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) before and after UV irradiation and found that the equilibrium adsorption times were equal at 8 h, but the equilibrium adsorption capacity had significantly changed (reduced from 6.16 to 3.53 ng/g after UV irradiation). This change was considered to have originated from the decrease in PS MPs hydrophobicity due to the formation of the carboxyl groups on the PS MP surface. Liu et al.²⁹ performed isothermal adsorption experiments to study the adsorption behavior of various MPs (i.e. low-density polyethylene [LDPE], linear low-density polyethylene [LLDPE], high-density polyethylene [HDPE], PP, PS, polycarbonate [PC], PVC, and polyamide [PA]) for 17β-estradiol (E2). The E2 adsorption capability varied according to the hydrophobicity of each MP, from which a positive correlation (p < 0.01, r = 0.834) was found for the hydrophobicity of MPs and E2 adsorption capacity of various MPs. The adsorption of hydrophobic organic pollutants onto MPs has been reported to occur mainly from hydrophobic interactions.^29–32 The hydrophobicity of MPs can be affected by the generated or increased oxygen-containing functional groups on the surface of MPs. Similarly, this research found a decrease in the hydrophobicity of the MPs along with an increase in oxygen-containing functional groups on the PE microfiber surface according to UV irradiation. Meanwhile, an increase in the roughness of the MP surface leads to an increase in the specific surface area, which can increase the sorption of hydrophobic organic contaminants to the MP surface.^24,33 In this study, the increase in surface roughness of MPs was indirectly confirmed through HR-SEM analysis, but the sorption amount of EE2 on weathered MPs was lower than that of virgin MPs. Overall, it demonstrated that the sorption of EE2 on weathered MPs was predominantly affected by hydrophobic interaction similar to other hydrophobic organic contaminants.³⁴

Figure 5.

Changes in EE2 concentrations with the contact time (i.e. 0.5, 6, 12, and 24 h) of PE microfiber before and after photo-aging (UV irradiation) for 14 d. The error bars indicate the standard deviations from triplicates experiments. The solid line means the fitting with the first-order reaction kinetic model (k = 0.0105 h⁻¹).

Environmental implication

MPs that are discharged into the environment have hydrophobic surfaces that easily accumulate hydrophobic organic pollutants due to their high affinity to hydrophobic organic pollutants such as the endocrine system disruptor EE2. The intake of MPs particles that have accumulated EE2 is potentially harmful to an organism's endocrine system. On the other hand, a decrease in dissolved EE2 concentration due to MPs particles could reduce the potential harm to organisms affected by contact with water (as a contaminant sink). Meanwhile, weathering including photo-aging of MPs surface causes a decrease in hydrophobicity as observed in this research. This decrease indicates that EE2 desorption could be caused or the adsorption of newly released EE2 could be deterred. A new pattern of potential risk could be caused to the ecosystem, for which the interaction of hydrophobic organic pollutants including MPs and EE2 needs to be closely studied.

Conclusions

This study assessed the change in physicochemical characteristics of the PE microfiber surface according to UV-C irradiation and the change in adsorption properties for EE2, which is one endocrine system disruptor. An increase in the PE microfiber surface roughness due to UV irradiation, an increase in oxygen-containing functional groups (i.e. carbonyl group), and a decrease in contact angle (i.e. decrease in hydrophobicity) was observed. Additionally, the adsorption capability of aged PE for EE2 was lower than that of virgin PE. Overall, additional research on the interaction between aged MPs and hydrophobic organic pollutants is highly necessary.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Korea Environment Industry & Technology Institute (KEITI) through Measurement and Risk Assessment Program for Management of Microplastics Project, funded by Korea Ministry of Environment (MOE) (2020003110010).

ORCID iDs

Jin-Yong Lee

Jinsung An

Author biographies

Maria Gabriela Galarza Zambrano is an undergraduate student in the Department of Biological and Environmental Engineering.

Sang-Gyu Yoon is a doctoral course student in the Department of Smart City Engineering. He has been conducting research on the assessment of the behavior of contaminants in the environment.

Jin-Yong Lee is a professor in Hydrology. His area of research focuses on microplastics in soil and groundwater.

Jinsung An is an assistant professor in the Department of Civil and Environmental Engineering. He focuses on soil and groundwater risk assessment as his main research area.

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Effect of aging on polyethylene microfiber surface properties and its consequence on adsorption characteristics of 17alpha-ethynylestradiol

Abstract

Keywords

Introduction

Materials and methods

Preparation of PE microfiber

Preparation of aged PE microfibers and analysis of their surface characteristics

Analysis of EE2

Sorption kinetics between virgin/aged PE microfiber and EE2

Results and discussion

Calibration of EE2 and analytical performance

Changes in the surface properties of the PE microfiber after aging

Changes in sorption kinetics after aging

Environmental implication

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

Author biographies

References