Effects of Microplastics on the Adsorption and Bioavailability of Three Strobilurin Fungicides.

Microplastics (MPs) and fungicides have been recognized as two main pollutants in ecological environments, especially in aqueous ecosystems. In this study, the adsorption behavior of three typical strobilurins (azoxystrobin, picoxystrobin, and pyraclostrobin) on polystyrene (PS) and polyethylene (PE) was investigated, and the effects of adsorption on the residual behavior and bioavailability of pyraclostrobin were evaluated. The results showed that MPs had strong adsorption capacity for the three kinds of strobilurins. Under similar conditions, the adsorption capacity was the highest for pyraclostrobin, followed by picoxystrobin and azoxystrobin, which was consistent with their octanol-water partition coefficients. Moreover, the adsorption capacity of PS was slightly higher than that of PE. The pH of aqueous solution had little effect on adsorption capacity, while an increase in ionic strength increased the adsorption capacity of azoxystrobin and picoxystrobin. The Fourier transform infrared spectra of MPs showed that no new chemical groups were formed during the adsorption process. Thus, it is speculated that hydrophobic interactions may be the driving force behind the adsorption of strobilurins on the MPs. Additionally, the adsorption of pyraclostrobin on MPs significantly reduced its residual amount in the aqueous solution, which reduced the adsorption and bioavailability of pyraclostrobin in black bean seedlings. The study provides effective information for environmental safety risk assessments with regard to the combined pollution risks of MPs and strobilurins.

Introduction

In recent years, microplastics (MPs) have become a global concern as an emerging contaminant.[1,2] MPs in the environment can be ingested and transferred by organisms through inhalation and swallowing and may enter cells through the cell membrane.[3,4] Increasing studies have shown that MPs are found in plankton, shellfish, seabirds, mammals, and even Arabidopsis thaliana.[5−8] The presence of MPs has adverse impacts on the organisms (e.g., reduction of food intake, lipid accumulation, inhibition of growth, development, and reproduction, and interference with the metabolism), which leads to energy imbalance and possibly death.[9−11]

In addition to environmental safety risks, MPs can change the mobility and bioavailability of organic and metal pollutants through adsorption.[12−14] It has been reported that MPs can adsorb polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), organochlorine pesticides, and heavy metals,[15−17] at an adsorption capacity reaching up to μg/g.[12] This is problematic because organic pollutants could transfer and accumulate along with MPs throughout the food chain.[18,19] PBDEs have been found to transfer with MPs to amphipod, fish, and seabirds.[16] Moreover, after the pollutants were adsorbed, the biodegradation rate of the pollutants decreased and environmental persistence was enhanced.[20] The microbial degradation rate of phenanthrene was reduced by six times when it was combined with polyethylene (PE).[21] Thus, the synergistic effect of MPs and pollutants may enhance the toxic effects of the pollutants, as was the case with the increase of chromium toxicity in the presence of MPs in Pomatoschistus microps juveniles.[22]

The ubiquitous use of plastic in consumer products has made MPs a great threat to agricultural ecosystems.[23,24] MPs can be found in almost all applications, such as PE in agricultural mulch and the use of polystyrene (PS) additives in pharmaceutical and personal care products.[25,26] PS and PE are abundant in wastewater and in fish.[4,7] Moreover, they have large densities, which is why PS and PE are found in the environment, where they come into contact with various pollutants.[9] These MPs then migrate via rainwater and surface runoffs and become one of the most commonly found MPs in aquatic environments.[27] Meanwhile, considering the importance of pesticides in agricultural production, their environmental safety risks cannot be ignored. As they are currently used, only a small fraction of the pesticide actually serves its purpose, while most of it enters the environment as runoff, leading to the coexistence of MPs and pesticides in agricultural ecological environments.[28−30] Currently, the combined ecotoxicology pollution related to MPs is mainly focused on heavy metals, persistent organic pollutants, new highly toxic organic pollutants, and so on.[31,32] However, there are few studies on the combined pollution of MPs and widely used pesticides.

Strobilurins are a class of fungal fungicides that have good field prevention, treatment, and eradication effects.[33] They are used in common crops such as soybeans, peas, corn, tobacco, and rice.[34] Azoxystrobin, picoxystrobin, and pyraclostrobin are typical strobilurins whose sales are at the forefront.[35] They are widely used for their long efficacy and good control but have been shown to have adverse effects on aquatic life and human health and safety. Studies have found that low concentrations of azoxystrobin residues in the environment can be toxic to daphnia magna straus, fish, and invertebrates.[34,36] Pyraclostrobin is potentially genotoxic and cytotoxic to human peripheral blood lymphocytes, neurotoxic to primary cultured cutaneous neurons, and is speculated to be potentially associated with autism and other diseases in humans.[37] Strobilurins and MPs often coexist in the aquatic ecosystem.[38,39] However, it is still unclear whether the MPs can adsorb strobilurins and affect their effective behavior.

In this study, the adsorption behaviors of azoxystrobin, picoxystrobin, and pyraclostrobin on PS and PE were studied, and the influence of environmental factors on adsorption capacity was subsequently analyzed. Finally, the effects of PS and PE on the bioavailability of pyraclostrobin were further evaluated. These results will provide effective information for a comprehensive evaluation of the environmental safety assessment of MPs and strobilurins.

Materials and Methods

Materials and Instruments

Azoxystrobin, picoxystrobin, and pyraclostrobin (purity > 99%) were purchased from Aladdin Reagent Co., Ltd. (China), and the standard stock solution was prepared with methanol. Pristine PS and PE particles (100 μm) were purchased from Shanghai Guanbu Co., Ltd. (China). Methanol and acetonitrile were bought from Quanchang Reagent Co. (China). Black beans were purchased from Aijia Germplasm Co. (China). Ultrapure water was prepared using a Milli-Q ultrapure water system. Attenuated total reflectance in conjunction with Fourier transform infrared spectroscopy (ATR-FTIR IS50, Thermo Fisher) was utilized to verify the characteristic functional groups of MPs before and after adsorption of strobilurins. An ultraperformance liquid chromatography-tandem mass spectrometer (UPLC-MSMS, 4500 Orbitrap, AB SCIEX) was used to determine the content of the strobilurins (Tables S1 and S2).

Adsorption and Desorption Experiments

The batch equilibrium method was used to carry out the adsorption and desorption experiments. Considering the residual concentrations of strobilurins and MPs in the environment and making an appropriate estimate, the initial concentrations of strobilurins and MPs in the adsorption system were set to 100 μg/L and 1 g/L, respectively. It has been reported that the concentrations of strobilurins and MPs in the aquatic environment are closest to 1.582 μg/L and 5.51 ± 9.09 mg/L, respectively.[37,40] Considering the possible scenario that pesticides and MPs are highly enriched in wastewater, 100 μg/L of strobilurins and 1 g/L of MPs are selected for the experiment.

Specifically, 100 mL of strobilurin solution prepared with ultrapure water was added into a 150 mL brown glass bottle for adsorption kinetics. The concentrations for the adsorption isotherm test were set to 10, 20, 50, 100, and 200 μg/L, and the sampling time was 24 h. Then, 100 mg of MPs were added and mixed. The adsorption solution was shaken at 150 rpm at 25 °C. In the specified sampling time, after the glass bottle was allowed to stand for 5 min, 1 mL of the supernatant was taken and filtered through a 0.22 μm filter membrane for analysis. After completion of the adsorption experiments, the desorption experiments were carried out. Aqueous solution (80 mL) from the adsorption equilibrium system was accurately removed for filtration using a filter membrane and then the MPs on the filter membrane were washed into the bottle with the same amount of ultrapure water. The concentration of desorbed strobilurins was measured after shaking for 24 h, and the desorption process was repeated twice. The methanol content in all experiments was controlled at 1%, and each treatment was repeated three times.

Effect of Environmental Factors

According to the above experimental steps, the influence of different factors in the system on the adsorption capacity was evaluated. HCl and NaOH were used to adjust the pH to 5.5, 7.0, and 8.5, and NaCl was used to adjust the ionic strength to 0, 1, 10, and 50 mmol/L. The adsorption capacity under different conditions was determined after the adsorption process reached equilibrium.

Residual Behavior and Bioavailability

Beans are considered to be an excellent model organism for biological and toxicological research.[41] The effect of MPs on the residual behavior and bioavailability of pyraclostrobin with the largest adsorption capacity was analyzed. MPs (1 g) and 1 L of 100 μg/L pyraclostrobin aqueous solution were added to a stainless-steel seedling tray. The seedling tray was slightly vibrated to evenly disperse the MPs. Germinated black bean seedlings were placed in the seedling tray so that the roots made contact with the aqueous solution and then cultured under natural conditions. The black bean seedlings and the aqueous solution were sampled to determine the concentration of pyraclostrobin.

Statistical Analysis

Method of detection of strobilurins by UPLC-MSMS and the precision and reproducibility of the analysis can meet the testing requirements.[40] Adsorption kinetics were fitted with pseudo-first-order, pseudo-second-order, and internal diffusion models. The adsorption isotherms were fitted using the Linear, Freundlich, and Langmuir models (Table S3). The differences of adsorption capacity were evaluated by a single-factor analysis of variance combined with post-test multiple tests (p < 0.05). SPSS software (V 20.0) was used for data analysis, and the graph was drawn using Origin (V 8.0725). Deviations within the triplicated experiments are reflected by the error bars in each figure.

Results and Discussion

Adsorption Kinetics

PS and PE had different degrees of adsorption to the three strobilurins. The adsorption rate was initially fast, and the adsorption capacity increased rapidly in the first 4 h. Thereafter, the adsorption rate slowed down, and the adsorption equilibrium was reached after 24 h (Figure A). The fitting results of the kinetic models showed that the adsorption process correlated well with the pseudo-first-order (R2 > 0.80) and pseudo-second-order (R2 > 0.99) kinetic equations (Figure B,C). However, the experimental adsorption capacity was basically consistent with that calculated using the pseudo-second-order kinetic equation, while it showed obvious deviation from that calculated using the pseudo-first-order kinetic equation (Table ). Therefore, the pseudo-second-order kinetic model most accurately described the adsorption process of three strobilurins on PS and PE. The fitting results of the internal diffusion models showed that the adsorption process may be divided into several stages (Figure D). Strobilurins may first be transferred from the aqueous phase to the surface of the MPs, followed by diffusion into the interior of MPs through the pore filling.[42]

Figure 1

Adsorption capacity of strobilurins on PS and PE (A), and linear plots for adsorption kinetics based on the pseudo-first-order (B), pseudo-second-order (C), and intraparticle diffusion models (D).

Table 1

Adsorption Kinetics Parameters of Strobilurins on PS and PE

		qe exp (μg/g)	pseudo-first-order			pseudo-second-order
MPs	strobilurins		qe (μg/g)	K1	R2	qe (μg/g)	K2	R2
PS	azoxystrobin	21.25	11.52	0.1801	0.9434	20.88	0.0023	0.9990
	picoxystrobin	25.76	9.55	0.1262	0.8872	25.51	0.0015	0.9998
	pyraclostrobin	91.23	30.26	0.1428	0.8496	92.59	0.0001	0.9995
PE	azoxystrobin	17.07	6.82	0.1386	0.9253	16.69	0.0036	0.9991
	picoxystrobin	22.89	7.73	0.1145	0.8771	22.73	0.0019	0.9997
	pyraclostrobin	81.02	26.69	0.1589	0.8651	81.97	0.0001	0.9998

Adsorption Isotherms

The adsorption isotherm models can predict the relationship between adsorption capacity and solution concentration at a certain temperature.[43,44] The fitting results of the three models of strobilurins on MPs are listed in Table . The equation fitted by the Freundlich model had good linearity (R2 > 0.90), and the fitting adsorption capacity was well fitted with the data at adsorption equilibrium (Figure B). Although the results of the Linear model fitting were also good (R2 > 0.80), the fitting adsorption capacities of azoxystrobin and picoxystrobin deviated significantly from the experimental values (Figure A). Besides, the fitting results of the Langmuir model for other adsorption processes were poor, except for the adsorption of azoxystrobin on PE (Figure S1). Therefore, the Freundlich model was best suited to describe the relationship between the adsorption capacity of MPs and solution concentrations. The KF and n values of pyraclostrobin were significantly higher than those of the other two strobilurins, indicating that MPs had the highest adsorption rate and adsorption capacity for pyraclostrobin.[31] There existed obvious heterogeneity in the adsorption sites of MPs.[44] High-energy sites were first occupied by strobilurins, followed by low-energy sites. Similarly, the adsorption isotherms of ciprofloxacin, trimethoprim, and tetracycline on PS and PE obviously fit well with the Freundlich model.[46] However, some other studies suggested that the Linear or Langmuir model was more suitable to simulate the adsorption isotherms of some organic compounds on MPs.[31,45] It was speculated that the adsorption isotherms may vary with the chemical properties of organic compounds and MPs.[46]

Table 2

Isothermal Adsorption Parameters of Strobilurins on PS and PEa

			Linear	Freundlich				Langmuir
MPs	strobilurins	LogKow	Kd	R2	KF	n	R2	KL	Qmax (μg/g)	R2
PS	azoxystrobin	3.09	0.2040	0.9324	0.3634	0.9737	0.9344	0.0101	45.05	0.9061
	picoxystrobin	3.83	0.1628	0.8053	0.3078	0.9224	0.9496	n.a	n.a	<0.7000
	pyraclostrobin	4.23	14.779	0.9904	0.5142	2.2152	0.9450	n.a	n.a	<0.7000
PE	azoxystrobin	3.09	0.1278	0.9557	0.4355	0.7876	0.9881	n.a	n.a	<0.7000
	picoxystrobin	3.83	0.1294	0.8084	0.2761	0.9578	0.9083	n.a	n.a	<0.7000
	pyraclostrobin	4.23	9.9060	0.9731	0.5494	1.8840	0.9506	n.a	n.a	<0.7000

n.a: not applicable.

Figure 2

Linear plots for adsorption isotherms based on the Linear (A) and Freundlich (B) models.

Adsorption and Desorption of Strobilurins on MPs

The adsorption capacities of azoxystrobin, picoxystrobin, and pyraclostrobin were 0.021, 0.026, and 0.091 mg/g on PS and 0.017, 0.023, and 0.081 mg/g on PE, respectively (Figure A). The order of adsorption capacity was as follows: pyraclostrobin > picoxystrobin > azoxystrobin, which was consistent with their octanol–water partition coefficients (LogKow). LogKow is an important parameter for the environmental behavior of chemical contaminants in water, which reflects the hydrophobicity of chemical contaminants.[47] The adsorption capacity of the three strobilurins increased with increasing hydrophobicity, which may be the main driving force behind adsorption. For each strobilurin, the adsorption capacity on PS was slightly higher than that on PE (Figure A). These results agree with those of PS that exhibited a higher sorption affinity to tetracycline than PE.[48] Physicochemical properties of MPs such as specific surface area and polarity may affect their adsorption capacities. The larger specific surface area and complex pore structure of PS result in more adsorption sites (Figure S2; Table S5). In addition, both PS and strobilurins have benzene ring functional groups, which can lead to nonspecific van der Waals interaction and π–π interaction, while PE can only undergo the van der Waals interactions.[45−48] The adsorption capacities were similar to pyrene and tetracycline to MPs but different from other contaminants to MPs in aquatic environments (Table S4). This suggested that the diameter and molecular properties of MPs and hydrophobicity of pollutants may be the main reasons for the difference.[49,50]

FTIR analysis indicated that the intensity of the peaks increased, even when no new functional groups appeared after adsorption (Figure S3A,B). This proved that the strobilurins were present on the surface of the MPs, which may be facilitated by surface adsorption and the hydrophobic effect.[47]

After two desorption cycles, azoxystrobin and picoxystrobin had mostly desorbed from the MPs, with desorption rates of more than 90%. On the other hand, pyraclostrobin had the highest adsorption capacity and the lowest desorption capacity of the three, with desorption capacity rates less than 10% for both MPs (Figure ). The larger adsorption capacity and lower desorption rates may enable MPs to become an important medium for the migration and transmission of pyraclostrobin.[51]

Figure 3

Adsorption and desorption capacities of strobilurins on PS and PE.

Influence of pH and Ionic Strength

Changes in pH values did not significantly affect the adsorption capacity of the three strobilurins on the MPs (Figure A). Previous studies have shown that a change in pH may lead to the dissociation of ionic compounds and affect their interaction with the surface charge of MPs.[52] However, these three strobilurins are neutral organic compounds, so pH has little effect on their dissociation. Similarly, it was found that the adsorption of perfluorooctanesulfonamide (FOSA) on PS, PE, and poly(vinyl chloride) (PVC) did not change significantly when the pH decreased from 7 to 3.[53]

Figure 4

Influence of pH (A) and ionic strength (B) on adsorption capacity.

On the other hand, an increase in ionic strength led to an increase in the adsorption capacity of azoxystrobin and picoxystrobin to the MPs, while there was no significant effect on the adsorption capacity of pyraclostrobin (Figure B). As stated previously, hydrophobicity was usually the main driving factor behind the adsorption of hydrophobic pollutants on MPs. The increase in ionic strength decreased the solubility of the strobilurins in the aqueous solution, which may have promoted the hydrophobic adsorption of MPs to azoxystrobin and picoxystrobin.[54,55] However, the adsorption capacity of pyraclostrobin on the MPs was nearly saturated in the absence of salt ions, so the change in salt ion concentration did not have an effect on its adsorption capacity or hydrophobicity. Similarly, the adsorption processes of some nonpolar organic compounds on PS and PE were mainly driven by hydrophobic interaction.[42,46]

Effect of Residual Behavior and Bioavailability

The presence of MPs significantly reduced the concentration of strobilurins in the aqueous solution and changed their residual behavior. In particular, the concentration of pyraclostrobin in the aqueous solution decreased significantly because most of it had adsorbed on the MPs (Figure A). The corresponding process is displayed in Figure S4. The bioavailability test showed that the presence of pyraclostrobin in black bean seedlings was significantly affected by its propensity to adsorb onto MPs (Figure B). In the absence of MPs, there was no significant change in the concentration of pyraclostrobin in black bean seedlings throughout the duration of the culture. However, in the presence of MPs, the concentration of pyraclostrobin in black bean seedlings showed an upward trend, but it was also significantly lower than that of the control group.

Figure 5

Residual amounts of pyraclostrobin on MPs (A) and effect of MPs on its bioavailability after adsorption (B).

Therefore, MPs can reduce the availability of pyraclostrobin and increase the residual time of pyraclostrobin in the environment. In the beginning, the concentration of pyraclostrobin in the aqueous solution decreased significantly, which affected the root adsorption and utilization (Figure S5). However, due to the strong adhesion of the MPs, they can easily be captured by the polysaccharide mucus secreted by the roots (Figure S6), which may lead to the increase of pyraclostrobin concentration around the rhizosphere.[56,57]

Conclusions

This study showed that three kinds of strobilurins can be adsorbed onto PS and PE suspended in an aqueous solution and subsequently affect their residual behavior. It was also shown that pH had little effect on their adsorption, while an increase in ionic strength increased the adsorption capacity to a certain extent. Thus, it can be concluded that hydrophobicity was the main mechanism for adsorption. Moreover, the bioavailability of pyraclostrobin in black bean seedlings decreased due to its adsorption on the MPs. Given the complex relationships between the three strobilurins and two MPs that were explored in this work, further studies elucidating the combined pollution risk of MPs and common pesticides would be valuable in assessing the safety of their use.