Chemosphere 327 (2023) 138544
Available online 28 March 2023
0045-6535/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Exploring the mechanisms of humic acid mediated degradation of polystyrene microplastics under ultraviolet light conditions
Xiqing Wang
a, Atif Muhmood
b, Deqing Ren
a, Pengjiao Tian
a, Yuqi Li
a, Haizhong Yu
a, Shubiao Wu
b,*aCollege of Food Science Technology and Chemical Engineering, Hubei University of Arts and Science, Xiangyang, Hubei, 441053, China
bDepartment of Agroecology, Aarhus University, Blichers Alle 20, 8830, Tjele, Denmark
H I G H L I G H T S G R A P H I C A L A B S T R A C T
•Humic acid (HA) accelerated the pho- todegradation of polystyrene micro- plastics (PS-MPs).
•HA as reaction oxygen species generator involved in the photodegradation of PS- MPs.
•Lower average particles size was deter- mined in aged PS-MPs with HA participation.
•Higher degree of weight loss and oxygen-containing components was observed in aged PS-MPs.
A R T I C L E I N F O Handling Editor: Michael Bank Keywords:
Polystyrene microplastic Humic acid
Photodegradation Free radicals Degradation products
A B S T R A C T
Microplastics (MPs) are emerging pollutants that interact extensively with dissolved organic matter (DOM) and this influences the environmental behavior of MPs in aqueous ecosystems. However, the effect of DOM on the photodegradation of MPs in aqueous systems is still unclear. The photodegradation characteristics of polystyrene microplastics (PS-MPs) in an aqueous system in the presence of humic acid (HA, a signature compound of DOM) under ultraviolet light conditions were investigated in this study through Fourier transform infrared spectroscopy coupled with two-dimensional correlation analysis, electron paramagnetic resonance, and gas chromatography- mass spectrometry (GC/MS). HA was found to promote higher levels of reactive oxygen species (0.631 mM of
▪OH), which accelerated the photodegradation of PS-MPs, with a higher degree of weight loss (4.3%), higher level of oxygen-containing functional groups, and lower average particle size (89.5 μm). Likewise, GC/MS analysis showed that HA contributed to a higher content of oxygen-containing compounds (42.62%) in the photodegradation of PS-MPs. Moreover, the intermediates and final degradation products of PS-MPs with HA were significantly different in the absence of HA during 40 days of irradiation. These results provide an insight into the co-existing compounds on the degradation and migration processes of MP and also support further research toward the remediation of MPs pollution in aqueous ecosystems.
* Corresponding author.
E-mail address: [email protected] (S. Wu).
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Chemosphere
journal homepage: www.elsevier.com/locate/chemosphere
https://doi.org/10.1016/j.chemosphere.2023.138544
Received 1 March 2023; Received in revised form 26 March 2023; Accepted 28 March 2023
1. Introduction
Microplastics (MPs), plastic pollutants with particles less than 5 mm, have been observed in different aqueous systems, including lakes, oceans, and rivers (Thompson et al., 2004; Xiang et al., 2022). MPs undergo various in-situ environmental processes (e.g., photo- degradation and biodegradation) after entering an aquatic system (Browne et al., 2007; Andrady, 2011). Irradiation-mediated degradation of MPs is a major process, resulting in chain scission of MPs and the release of various degradation compounds (e.g., volatile organic com- pounds) and low molecular weight (MW) plastic additives (e.g., plasti- cizers and colorants) that threaten the stability of aquatic ecosystems and the safety of the food chain (Wu et al., 2022; Rani et al., 2017).
Generally, the irradiation-mediated degradation process of MPs is significantly related to their interfacial chemical behavior, which is closely influenced by environmental factors (e.g., ultraviolet radiation and oxygen content) and coexisting active compounds (Wu et al., 2021).
Humic acid (HA), a type of dissolved organic matter (DOM), is the most abundant and active organic compound in aqueous systems (Wang et al., 2021). It contains a stable structure and various reactive func- tional groups (e.g., quinones and acidic functional groups) that mediate the migration and transformation of inorganic (e.g., heavy metals) and organic pollutants (e.g., dibutyl phthalate in mollisol) via adsorption, complexation, and redox processes (Zhang et al., 2019a; Liu et al., 2018). Our previous research confirmed that HA with a higher electron transfer ability mediated the abiotic redox process of Cr (VI) under aerobic/anaerobic conditions, converting high-valent to low-valent chromium and reducing toxicity (Wang et al., 2022). Similarly, HA has recently been shown to act as a terminal electron acceptor and electron transfer carrier involved in microbial respiration, thereby contributing to the biological redox reactions of heavy metals and organic pollutants (Zhang et al., 2019b; Tao et al., 2019; Chen et al., 2011). For instance, Tao et al. (2019) in their study confirmed that the two major functional groups (aryl C–O and alkyl ester C––O) of HA act as the main electron shuttles, which contributes to the biodegradation of dibutyl phthalate in mollisol (Tao et al., 2019). Moreover, the quinone structure present in HA acts as an electron transfer carrier to promote electron transfer while catalyzing the reduction of O2 to generate reac- tive oxygen species (such as ▪O2− and ▪OH) (Xu et al., 2020). This is one of the important pathways for the degradation of heavy metals and organic pollutants in aquatic systems (Zhao et al., 2023).
The interfacial interaction between MPs and HA has attracted attention in recent years, and the effect of HA on the transport and aging behavior of MPs has generally been studied (Wang et al., 2022b; Luo et al., 2022). Previous studies have investigated the effects of HA on the transport capacity of MPs in saturated porous media with peanut shell biochar and MgO-modified peanut shell biochar. The results showed that the HA concentration played a dominant role in controlling MPs transport and the transport capacity of MPs in saturated porous media owing to physical attachment and chemical reactions (Wang et al., 2022b). Moreover, recent research has reported that HA, as a reactive oxygen species generator, is involved in the photoaging of poly- propylene microplastics (PP-MPs) in the aquatic system, resulting in an acceleration of the aging process (Luo et al., 2022). Likewise, Qiu et al.
(2022) investigated that the effect of different dissolved organic matter (humic acid and fulvic acid) on the structural evolution of PS-MPs under dark and UV condition via the FTIR analysis. Results showed that HA and FA were found to promote electron transfer to generate reactive oxygen species under dark conditions and the aging of PS-MPs, while the process of HA and FA generating reactive oxygen species under UV light was more susceptible to photoelectrons and accelerated the aging pro- cess of PS-MPs. The evaluation of the mechanism of HA’s action involved in the degradation behavior of MPs is a critical step in assessing the environmental behavior of MPs in aquatic environments. Nonethe- less, to date, information on the effect of HA on the photodegradation behavior of MPs in aquatic systems, particularly the mechanisms by
which HA mediates the redox process involved in the photodegradation of MPs, is insufficient.
To fill this knowledge gap, this study has primarily investigated the mechanism of interaction between MPs and HA under ultraviolet light conditions. Polystyrene microplastics (PS-MPs) were chosen as model plastics because of their wide applicability. The aims of this study were to (i) explore the effect of HA on the evolution of PS-MPs structure under UV light conditions via 2D COS Fourier-transform infrared spectroscopy (FTIR) analysis, (ii) clarify the release of degrading compounds from photoaging of PS-MPs under HA addition conditions via gas chromatography-mass spectrometry (GC/MS) analysis, and (iii) reveal the photodegradation transformation and decomposition pathway of PS- MPs under HA addition.
2. Materials and methods 2.1. Chemical and materials
Polystyrene plastic was purchased from Liming Plastics Co., Ltd.
(China), crushed using a grinder, and passed through a 100-mesh sieve.
Humic acid (BR; CAS 1415-93-6) was purchased from Shanghai Yuanye Bio-Technology Co., Ltd. (China). Other chemical reagents were pur- chased from Sigma-Aldrich Co. Ltd.
2.2. Batch experiment setup
In this study, laboratory-accelerated batch aging experiments were conducted under static conditions. The experimental setup is shown in Fig. S1. The experimental vessels were 40 mL quartz tubes with silica caps and the UV box contained two UV lamps (365 nm, Xujiang Elec- tromechanical Inc., China) with an irradiation intensity of 60 mW/cm2, which was detected using a UV irradiation meter (Shenzheng Zhuoyue Yi Biao Co., Ltd., China). Before the aging experiments, the quartz tubes were treated in a muffle furnace at 500 ◦C for 4 h. 0.5 g microplastics and 0.5 g humic acid at a 1:1 (w: w) ratio were placed into in the quartz tubes. Similarly, 30 mL of ultrapure water was added to the quartz tubes containing the MP and HA mixture samples to simulate the environment within the aquatic system. The temperature of the chamber was main- tained at 25 ±3 ◦C. To investigate the evolution of the structure of PS- MPs and the release of degradation compounds, tube samples were taken at time points of 0, 10, 20, 30, and 40 days. In order to reduce the error of the test, each sample was taken in accordance with the principle of sampling the entire tube. After sampling, solid-liquid separation was first performed, in which the solid fraction was used for the determi- nation of the structure of MPs, while the liquid fraction was used for the analysis of degradation products. In the control groups including HA and PS-MPs alone degradation were subjected to the same conditions, respectively. The degradation treatment and control groups were con- ducted in triplicate.
2.3. Characterization of the microplastics
MPs were extracted from samples collected at each time point ac- cording to previous studies. Briefly, the samples were subjected to solid- liquid separation through a glass microfiber filter, and followed by an impurity removal operation for the solid phase fraction. The solid phase was first rinsed three times with methanol and filtered to remove degradation products that might have adhered to the surface. The solid phase fraction was then mixed with 30% hydrogen peroxide solution for 12 h and filtered to remove the HA absorbed on the surface of the MPs.
The HA residues was determined via the excitation-emission matrix spectra (EEMs) according to our previous studies (Wang et al., 2021).
Finally, the collected MPs were oven-dried (50 ◦C, 2 h) for surface morphology, chemical bonding, and functional group characterization.
In this study, the surface morphology variability of pristine and degraded MPs was analyzed using a scanning electron
microscropy-energy dispersive spectrometer (SEM-EDS, SU8020, Japan). The evolution of the PS-MP particle size was further analyzed using a laser particle size analyzer (LPSA). Similarly, X-ray photoelec- tron spectroscopy (XPS, Thermoescalab 250Xi, USA) was used to determine and analyze the changes in the chemical bonding of PS-MPs throughout the degradation process. High-resolution C 1s and O 1s re- gions were recorded in the XPS spectra. Fourier-transform infrared spectroscopy (FTIR) was performed in the range of 4000–400 cm−1 for the analysis the functional groups of PS-MPs at different time points.
Then, the data matrix was further analyzed using two-dimensional correlation spectroscopy (2D COS) through the 2D Shige software (2D Shige version 1.3, Kwansei-Gakuin University, Japan) to explore the evolution of functional groups.
2.4. Determination of the free radicals
The free radicals (▪OH) of the samples were determined using elec- tron paramagnetic resonance spectroscopy (EPR, A300-10/12, Bruker, Germany) according to the method recommended by Jia et al. (2020).
Briefly, approximately 9 mL samples collected at different irradiation times were mixed with 1 mL of 5,5 dimethyl-1-pyrroline-N-oxide (0.03 M) in the dark and stirred for 2 min. The mixed system was then filtered through a 0.45 μm filter membrane and analyzed by EPR spectrometry at room temperature. The operating parameters of the EPR instrument were as follows: microwave power, 10 mW; modulation amplitude is 2.071 G; sweep width, 100G; center field, 3480 G; sweep time, 41.943 s.
Furthermore, in order to analyze the content of ▪OH in the photo- degradation process of PS-MPs, sodium benzoate (0.01 M) was selected as the probe molecule for ▪OH, and the generation of ▪OH was quanti- fied by monitoring the formation of p-hydroxybenzoic benzoate content via high performance liquid chromatography (HPLC, Agilent 1200) (Qiu et al., 2022; Lian et al., 2021). Briefly, the samples (5 mL) collected at different irradiation times were mixed with sodium benzoate (0.01 M) in the dark and stirred for 20 min. Then, 2 mL mixture was filtered and determined using HPLC.
2.5. Analysis of release of the degradation compounds
The degradation compounds from the MP aging process were determined by gas chromatography-mass spectrometry (GC/MS). The extraction of degradation compounds was performed according to the standard method described by Shi et al. (2021). Briefly, the liquid phase was first obtained using the above separation process and pretreated using the solid phase microextraction (SPME) technique. Around 10 mL sample was transferred to a 20 mL headspace vial and subjected to solid phase microextraction (DVB/CAR/PDMS fiber, LabTech) at 80 ◦C. The adsorption time was 40 min, and the test was performed after desorption at 250 ◦C for 3 min. Finally, the collected degradation compounds were concentrated to 1 mL for GC/MS analysis. GC-MS analysis was carried out using GC-MS QP2010 ultra (Shimadzu, Kyoto, Japan) with the DB-5MS Column (Agilent, J&W Scientific, 30 m ×0.25 mm ×0.25 μm).
Helium was used as carrier gas with a constant flow rate of 1 mL/min.
The temperature program started at 40 ◦C for 2 min, rose to 200 ◦C with a rate of 6 ◦C/min, and then to 300 ◦C with a rate of 15 ◦C (3 min hold time). The temperature of inlet, transfer line, and ion source was set to 250, 280, and 220 ◦C, respectively. MS acquisition was performed in the range of 33–500 mass units.
3. Results and discussion
3.1. Characteristics of the physical properties
Previous studies have confirmed that MPs undergo significant changes in their physical properties, including surface characteristics, weight loss, and particle size, during the aging/degradation process.
Thus, to investigate the effect of HA on the photodegradation of PS-MPs,
the surface characteristics of PS-MPs were first determined using a scanning electron microscropy-energy dispersive spectrometer (SEM- EDS). A significant difference in the surface morphology between the initial and aged PS-MPs was observed in this study. The results showed that the surface morphology of the initial PS-MPs was relatively smooth and shiny (Fig. 1a). However, the surface properties of PS-MPs without interaction with HA became relatively rough after 40 d of irradiation (Fig. 1b). In contrast, the surface properties of the PS-MPs deteriorated, and some cracks and holes were formed with the involvement of HA in the aging process (Fig. 1c). The significant changes in the surface properties of the PS-MPs may be due to the shrinkage and reorganization of the structure due to the removal of photodegradation by-products, thus leading to cracks (Endo et al., 2005; Song et al., 2022). Likewise, the presence of cracks increases the degree of degradation because it provides a pathway for oxygen to penetrate deeper into the PS-MPs and enhance photo-oxidation, resulting in the appearance of holes on the surface of the PS-MPs (Endo et al., 2005).
Moreover, the weight loss of the PS-MPs is also commonly used to characterize the degradation efficiency under irradiation conditions, although the degradation of PS-MPs is slower under natural conditions (Song et al., 2020; Chen et al., 2021). The weight loss of PS-MPs in the absence and presence of HA was determined (Fig. 1d). An increase in the weight loss of the PS-MPs was observed in this study with a prolonged irradiation time, which is consistent with the results of a previous study.
Previous studies have demonstrated that the weight loss of PS-MPs caused by biodegradation is only in the range of 0.005%–1.5% within 60 days (Yang et al., 2015). Likewise, the weight loss of PS-MPs in the presence of HA (4.3%) was higher than that in the absence of HA (1.6%) when irradiated for 40 days, indicating that HA might promote the photodegradation process of PS-MPs. Previous studies found that HA can serve as an electron shuttle carrier, mediate electron transfer during redox reactions, thereby promoting the generation of ROS and degrad- ing the pollutants.
Furthermore, the evolution of the PS-MP particle size was deter- mined using a laser particle size analyzer (Fig. 2). A decreasing trend in the particle size distribution of PS-MPs was observed during photo- degradation. This result can be attributed to the enhanced chain break and degradation of PS-MPs during irradiation for 40 days. During the degradation process, various volatile organic compounds and plastics additives released from PS-MPs, resulting in the high degree of weight loss and low particle size (Lomonaco et al., 2020). Moreover, the frag- mentation below the filtration threshold formed from photodegradation of PS-MPs may also have resulted in apparent weight loss and decreased particle size. In this study, the average diameter of PS-MPs decreased to 89.5 μm, which was 39.5% and 17.9% lower than that in the original (148 μm) and without the addition of HA (109 μm), respectively. These results indicate that HA may promote the photodegradation of PS-MPs, resulting in fracture and degradation of the chain and surface structure.
3.2. Evolution of the chemical properties
The photodegradation of MPs is usually accompanied the change in the chemical bonds. Thus, to evaluate the influence of HA on the pho- todegradation of the PS-MPs, XPS analysis was used to characterize the evolution of the chemical bonds of PS-MPs (Figs. 3 and 2S). In the original PS-MPs (Fig. 2S), a small number of oxygen atoms were observed, suggesting that a low degree of oxidation had occurred on the surface of the PS-MPs during the production, crushing, and storage processes. When compared to the original PS-MPs, higher levels of ox- ygen atoms were determined in the aged PS-MPs, indicating a higher degree of oxidation of the PS-MPs under light irradiation conditions (Al Abdulal et al., 2015). Moreover, the XPS spectra of aged PS-MPs exhibited significant differences in the presence and absence of HA.
The O 1s peak value of the photodegradation of PS-MPs with HA was higher than that without HA.
Furthermore, based on the peak fitting of the high-resolution XPS
spectra of the O 1s regions, it can be concluded that the two peaks at 533.1 eV and 534.2 eV of the aged PS-MPs were significantly higher than those of the original PS-MPs in the O 1s spectra. These peaks were assigned to C–O and C––O, respectively, indicating that the surface chemical bonds of the PS-MPs were oxidized to carboxylic type com- pounds (Yang et al., 2022). Likewise, when compared with the original PS-MPs, the peak areas at 286.1 eV (C–H bonds) and 284.8 eV (C–C bonds) of C 1s XPS spectra decreased significantly for the aged PS-MPs, indicating that long-chain structure breakage occurred during the pho- todegradation process (Chen et al., 2020). Moreover, compared to the aged PS-MPs in the absence of HA, the effect of HA on the photo- degradation process of PS-MPs revealed a higher content of C––O and a lower content of C–C in the aged PS-MPs with HA, indicating that HA can contribute to the photooxidation of the PS-MPs. Furthermore, the carbonyl index (CI) and O/C ratio are important indicators that have been widely used to quantitatively characterize the degree of oxidation of MPs (SongHongJangHanJungShim, 2017; Satoto et al., 1997).
Generally, the CI value and O/C ratio are significantly positively correlated with the oxidation degree of MPs. In this study, the CI value of the PS-MPs in the presence of HA after irradiation for 40 days (0.53) was higher than that of the original PS-MPs (0.03) and aged PS-MPs in the absence of HA (0.21). Similarly, the O/C ratio showed a trend similar to that of the CI value (Fig. S3). These results suggest that HA, as an electron transfer carrier, may mediate the photooxidation of PS-MPs in the photodegradation process (Qiu et al., 2022).
3.3. Evolution of the functional groups
To investigate the evolution of the PS-MPs structure with and without HA under irradiation conditions, FTIR spectra combined with 2D COS technology were used to monitor the variation of the functional groups of the PS-MPs in this study. As shown in Fig. 4 a and b, six auto- peaks at 3100, 2800, 1700, 1600, 1360, and 1030 cm−1, corresponding to the C–H, O–C––O, C––C, C–O–H, and C–O functional groups, respectively, were observed in the synchronous correlation spectra of PS-MPs with and without HA. This finding indicated that increased of oxygen-containing functional groups during photodegradation of PS- MPs may be caused by the long-chain structure (C–H) breaking, which reacts with oxygen and hydrogen to form the oxygen-containing groups (O–C––O, C––C, C–O–H, and C–O) (Mao et al., 2020). Moreover, the intensities of these peaks in aged PS-MPs with HA was higher than without HA, indicating that HA contributed to the oxidation process of PS-MPs during photodegradation.
When compared with the synchronous spectrum, seven negative cross-peaks at (2800, 3300), (1700, 3300), (1600, 3300), (1030, 3300), (1030, 1360), (1030, 1700), (1600, 1700), and one positive cross-peak at (1700, 2800) were observed in the asynchronous correlation spec- trum of PS-MPs without HA (Fig. 4 c). While, three additional positive cross-peaks at (1600, 1700), (1030, 1600), and (1360, and 1600) were observed in the aged PS-MPs with HA. These cross-peaks appeared in the asynchronous map, indicating that the surface functional groups of PS- Fig. 1.The change in the physical properties of the PS-MPs after 40 d of irradiation. (a) The SEM image of the initial PS-MPs; (b) the SEM image of the aged PS-MPs with humic acid; (c) the SEM image of the aged PS-MPs without humic acid; (d) the dry weight loss of the aged PS-MPs.
MPs changed at different rates with the photoaging process, resulting to the change in order of these functional groups (Wang et al., 2021). Ac- cording to Noda’s rules (Noda and Ozaki, 2004), the functional groups of the aged PS-MPs without HA changed in the following order: C–H (3300 cm−1 and 2800 cm−1)> C–O (1030 cm−1) >C–O–H (1360 cm−1)>
O–C––O (1700 cm−1)>C––C (1600 cm−1). The sequence of functional group variation in the aged PS-MPs with HA followed the order C–H (3300 cm−1) >C–O–H (1360 cm−1) >C––C (1600 cm−1) >C–O (1030 cm−1)> O–C––O (1700 cm−1). These results demonstrate that the presence of HA significantly influences the evolution of functional groups during the photodegradation of PS-MPs. Recent studies confirmed that HA and FA increased C–O, C–O–H, and C––O during the photodegradation process of PS-MPs and found that the C–H bond was broken and further oxidized into these oxygen-containing functional groups (Qiu et al., 2022). In this study, we further found that the pres- ence of HA caused simultaneous changes in the C––C, C–H, and oxygen-containing functional groups during the photodegradation pro- cess, which accelerated the photooxidation process of the MPs and contributed to increasing the content of oxygen-containing groups.
3.4. Characteristics of the degradation products
Previous studies have demonstrated that various low-molecular- weight products, short-chain compounds, and volatile organic com- pounds are produced during the photodegradation process (Lomonaco et al., 2020; Rabek, 1995). To further investigate the effect of HA on PS-MP degradation, GC/MS analysis was performed to determine and identify the degradation products (Fig. S4). Many products, including alkanes, olefins, ketones, alcohols, carboxylic acids, and esters, were identified in the aged PS-MPs (Table 1). Likewise, a significant differ- ence between being in the presence of HA and in the absence of HA was observed for the PS-MPs photodegradation process. In the absence of HA, higher relative contents of oxidation intermediates (e.g., alcohols, aldehydes, and ketones) and lower relative content of oxygen-containing products (e.g., carboxylic acids and esters) were identified in the aged PS-MPs (55.98% and 26.66%, respectively).
Unlike in the absence of HA, a higher content of oxygen-containing products (e.g., carboxylic acids and esters) was found in the aged PS-MPs in the presence of HA (42.62%). These results indicate that HA mediates a higher degree of oxidation in the PS-MPs photodegradation process, which is consistent with the XPS and FTIR analysis results.
Moreover, there was found to be a significant difference in the products during the photodegradation of PS-MPs in the presence and absence of HA through further analysis of the products at different times (Fig. 5a). In the early stages of photodegradation in the absence of HA, we found that the degradation products of PS-MPs were mainly hydro- carbons and small amounts of alcohols. The alcohol compounds showed an increasing trend with the irradiation time. After 40 days of photo- degradation, the main degradation products of the PS-MPs were ketones and aldehydes. These results indicate that the initial stage of photo- degradation is mainly chain breakage. Meanwhile, alcohols, as inter- mediate products in the initial reaction stage, were finally transformed into other oxidation products. When compared to the photodegradation in the absence of HA, a similar reaction was observed in the early stage of photodegradation of PS-MPs in the presence of HA (Fig. 5b). How- ever, in the subsequent reactions, small amounts of alcohols were found, whereas high values of oxygen-containing compounds (e.g., ketones and aldehydes) were identified in the presence of HA. Finally, esters and carboxylic acids were found to be the main degradation products. Thus, we speculate that the degradation pathway of the PS-MPs may be pro- moted in the presence of HA, especially in the later stages of the pho- todegradation pathway.
3.5. Possible mechanisms of the MP photodegradation pathway
Generally, the degradation pathway of MPs includes photo- degradation and photooxidation stages, which are significantly posi- tively correlated with the value of reactive oxygen species (ROS) in the reaction system (Qiu et al., 2022; Lomonaco et al., 2020; Scoponi et al., 1995). Previous studies have reported that chemical bonds (e.g., C–C and C–H) in MPs are broken under the stimulation of light irradiation, resulting in the generation of ROS from MPs (Saron et al., 2006). The Fig. 2.The size distributions and averages sizes of the aged PS-MPs with HA and without HA under irradiation conditions.
generation of ROS is further involved in the photooxidation of MPs, contributing to the breakage and oxidation of long-chain or benzene ring structures in MPs (Lomonaco et al., 2020). Moreover, some studies have indicated that ROS can also be formed from polymers with conjugated benzene ring structures, including dissolved organic matter and lignin (CaiWangPengWuTan, 2018; Zhu et al., 2019). A recent study demon- strated that ROS derived from dissolved organic matter in water pro- motes the degradation process of MPs under dark conditions (Qiu et al.,
2022). Thus, to verify the mechanisms of the effect of HA on the degradation pathway of PS-MPs, the changes in ROS during the photo- degradation process were determined via EPR analysis (Fig. 6). The results showed that four characteristic peak signals were observed in the aged PS-MPs in the absence and presence of HA, indicating that ROS were produced in the system. Thus, we can infer that ROS may play an important role in the photodegradation of PS-MPs. Furthermore, to characterize the effect of ROS generated during the photodegradation of Fig. 3.X-ray photoelectron spectroscopy (XPS) spectra of the aged PS-MPs without HA (a) and with HA (b). High-resolution of the XPS spectra O 1s (c and d) and C 1s (e and f) of the PS-MPs in the absence and presence of HA, respectively.
the PS-MPs, the free radicals (▪OH) within the system at different irra- diation times were quantitatively analyzed by high-performance liquid chromatography (Fig. 6b). An increasing trend in ▪OH generation was
Fig. 4. 2D-COS FTIR spectra of the aged PS-MPs in the absence and presence of HA under irradiation conditions. Synchronous spectra of the aged PS-MPs in the absence (a) and presence of HA (b), respectively; Asynchronous spectra of the aged PS-MPs in the absence (c) and presence of HA (d), respectively.
Table 1
The degradation products of the aged PS-MPs in the absence and presence of HA.
Aged PS-MPs without HA Aged PS-MPs with HA
Compound Molecular
formula Compound Molecular
formula
Pentadecane C15H32 Tridecane C13H28
Hexadecane C16H34 Dodecane C12H26
Undecane, 4,7-dimethyl- C13H28 3-Hexadecene C16H32
Phenol, 2,5-bis(1,1-
dimethylethyl) C14H22O 1-Octanol, 2-butyl- C12H26O Dibutyl phthalate C16H22O4 4-Pentadecanone C15H30O Oxalic acid, butyl 6-eth-
yloct-3-yl ester C16H30O4 Benzaldehyde, 3,4-
dimethyl- C9H10O
Butyric acid, 3-penta-
decyl ester C19H38O2 Isopentyl hexanoate C11H22O2
Toluene C7H8 2-methyl-
Isobutyraldehyde C4H8O 1-(3-ethoxyphenyl)
ethanone C10H12O2 2-Methoxy-4-
vinylphenol C9H10O2
2-propenyl-benzene C9H10 2,3-Dimethyl-3-
heptene C9H18
benzyl alcohol C7H8O 2-nonyl ester C17H24O3
1-phenylethanol C8H10O 9-Octadecenoic acid, 12-hydroxy-, methyl ester,
C19H36O3
1,3-imethy benzene C8H10 Pentadecanoic acid C15H30O2
Styrene C8H8 Hexadecanoic acid C16H32O2
benzeneacetaldehyde C7H6O / /
Benzaldehyde C8H8O / /
Tetramethylhexanal C10H20O /
Fig. 5. Evolution of the degradation products of the PS-MPs in the absence of HA (a) and in the presence of HA (b) through GC/MS analysis.
Fig. 6. EPR spectra of ▪OH generation in the photodegradation of the PS-MPs in the absence of HA (a) and presence of HA (b), and the ▪OH content generation by photodegradation of the PS-MPs in the absence of HA (c) and presence of HA (d).
Fig. 7. The proposed possible degradation pathway of the PS-MPs with HA and without HA under irradiation conditions.
observed in the photodegradation process of PS-MPs in the absence and presence of HA. In the absence of HA, the average ▪ OH value derived from the PS-MPs reached 0.204 mM after irradiation for 40 days. When compared to photodegradation in the absence of HA, the ▪OH yield produced by the PS-MPs with the HA system reached 0.631 mM after 40 days of accumulation, which was significantly higher than that in the absence of HA. These results indicate that HA promoted ▪OH generation during the photodegradation of the PS-MPs. According to previous studies, the generation of ▪OH from HA is divided into two main path- ways. First, a small amount of ROS is generated spontaneously, mainly from the stabilized reactive radical functional groups on the surface of the HA structure. Moreover, HA, as an electron shuttle carrier, mediates electron transfer during redox reactions within the system, thereby promoting the generation of ROS. Thus, the higher ROS content in the presence of HA found under UV conditions may be due to the photo- electron stimulation and accelerated rate electron transfer. A previous study confirmed that ROS are an important driving force involved in the transformation and degradation of MPs (Saron et al., 2006). A stronger intensity of ROS formation from the PS-MPs with the HA system may accelerate the photooxidation of PS-MPs under 40 days of irradiation.
Based on the aforementioned results, the potential mechanism of the PS-MP photodegradation pathway in the absence and presence of HA was determined (Fig. 7). In the photodegradation of PS-MPs without HA, the long-chain structure with a benzene ring was first broken under the action of UV light and oxygen, mainly producing short-chain hydro- carbons and benzene ring compounds. Similarly, a certain amount of
▪OH is generated during this process. These compounds then undergo oxidation reactions in the presence of ▪OH radicals to produce alcohols, esters, and small amounts of ketones and aldehydes. Finally, the oxidation of alcohols, ketones, and aldehydes further intensified, forming carboxylic acid components. In the presence of HA, the pre- liminary stages of the photodegradation of PS-MPs exhibited a similar process to that in the absence of HA. However, the stronger ▪OH radicals derived from HA led to a deeper oxidation process of the subsequent short-chain hydrocarbons and benzene ring compounds, allowing alco- hols and esters to undergo additional oxidation processes as temporary oxidation intermediates; thus, some oxygenated compounds (ketones and aldehydes) were mainly generated in this stage. Subsequently, a portion of the C–O and C––O bonds within the structure of oxygen- containing compounds (ketones and aldehydes) underwent further ox- ygen addition reactions in the presence of ▪OH radicals, eventually producing carboxylic acids.
4. Conclusions
MPs and dissolved organic matter are ubiquitous in aquatic systems.
To better understand the effects of dissolved organic matter on the photodegradation of MPs. PS-MPs and HA were selected as representa- tive examples in order to investigate the effect of HA on the evolution of the structure and degradation products during the photodegradation of PS-MPs. The results showed that HA promoted weight loss and the generation of the aged PS-MPs with smaller particle sizes. Likewise, a higher content of oxygen-containing functional groups was observed in aged PS-MPs with HA through spectroscopic analysis. Moreover, owing to the higher content of free radicals formed from HA, the presence of HA led to significant differences in the functional group changes and degradation products within the structure of the aged PS-MPs when compared to those in the absence of HA. These finding indicated that HA as the co-existing compounds in the aquatic system, accelerated the degree of the photodegradation of MPs.
Credit author statement
Conceptualization: Xiqing Wang; Data curation: Xiqing Wang, Deq- ing Ren and Pengjiao Tian; Formal analysis: Xiqing Wang; Funding acquisition: Xiqing Wang and Pengjiao Tian; Investigation: Xiqing
Wang; Methodology: Xiqing Wang and Atif Muhmood; Project admin- istration: Shubiao Wu; Resources: Yuqi Li; Software: Haizhong Yu and Deqing Ren; Supervision: Shubiao Wu; Validation: Haizhong Yu; Roles/
Writing – original draft: Xiqing Wang; Writing – review & editing:
Shubiao Wu.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgements
This work was supported by the National Natural Science Foundation of China Cultivation Program (No.2022pygpzk01), Foundation of Educational Commission of Hubei University of Arts and Sciences (QDF2021011 and QDF2022007) and EU-funded Marie Sklodowska- Curie Postdoctoral Fellowships program (101062861).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.chemosphere.2023.138544.
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