1. IntroductionPlastics are widely used by humans due to their low cost, lightweight, and strong durability; global plastics production is estimated to have reached 348 million tons in 2017 [1,2]. Plastics and their products are mostly disposable products, which can easily cause environmental pollution due to their persistence by discarding the used plastics indiscriminately. Plastic debris undergoes chemical, physical, and biological degradation to form tiny plastic fragments. When the particles are less than 5 mm in diameter, these are called microplastics (MPs) [3]. MPs are widely present in a variety of environments, and their potential impact on the environment is being extensively studied [4]. The fate of MPs in the environment has been previously investigated in the literature, such as spatial distribution [5], biotoxicity [6], and transport [7]. MPs are not only directly harmful, but when they adsorb pollutants, they transport them into the environment through migration and diffusion, causing more serious pollution [8]. More importantly, MPs’ adsorption of pollutants may affect the overall toxicity of the mixture due to synergistic or antagonistic effects [9], which can cause harm to human health through the food chain. Therefore, it is crucial to predict the impact of MPs on the environment by studying the interaction of MPs with pollutants.The properties of MPs can be altered by sunlight irradiation. Ultraviolet (UV) irradiation alters the polymer structure by increasing the oxygen-containing functional groups, resulting in chain breaks [10]. While the chemistry of MPs present in the environment differs from the plastic models commonly used in laboratory studies, which are typically pristine particles with uniform shape and size, the same aging effect as in the environment can also be achieved by UV aging experiments in the laboratory. Therefore, attention has been paid to the properties and pollutant adsorption of MPs in a UV aging simulation environment. Vroom et al. showed that the aged polystyrene MPs are more easily ingested by zooplankton [11]. Zhang et al. confirmed the improved adsorption of organic triclosan by aged polyethylene [12]. In contrast, Huffer et al. showed a decreasing trend in the adsorption properties of aged polystyrene MPs for different organic compounds [13]. The above results indicate that UV aging has a significant effect on the performance of MPs, especially on the adsorption performance of pollutants. Although some studies have focused on this field, more research is needed to summarize the impact rules and thus, to solve the environmental problems caused by MPs.The largest categories of total global non-fibrous plastics production are estimated to be polyethylene (PE) at 36% and polystyrene (PS) at 10% [14]. Both are the most common MPs in the aqueous environment [15]. In addition, biodegradable plastics such as poly (butylene adipate-co-terephthalate) (PBAT) are beginning to be widely used in agricultural mulch, cling film, and plastic bags. The annual production of biodegradable plastics is estimated to be about 2.1 million tons, which is expected to increase to 2.4 million tons per year by 2025 [16]. Therefore, it is important to investigate the effects of two common environmental MPs (PS and PBAT) before and after aging on the sorption of organic pollutants.Personal care products (PPCPs) and pharmaceuticals are frequently detected in the natural waters [17]. Adsorption of PPCPs in soil/sediment is a major process affecting their mobility and ultimate fate in the environment [18]. Among them, diclofenac (DCF) is widely used in PPCPs, which is an anti-inflammatory compound for humans and animals. Table S2 describes the structure and properties of DCF. The global annual consumption of DCF is estimated to be about 940 tons [19]. DCF has been detected in the groundwater and surface water in different regions such as Germany, Pakistan, Spain, Europe, China, and other regions [20]. The removal rate of DCF in wastewater treatment is only 21% to 40%, which poses serious threats to human health and aquatic ecology, such as gastrointestinal damage to humans, and hepatotoxicity and reproductive defects to marine animals [21,22]. DCF has been included in the surface water observation list of EU Resolution 2015/495, as it may cause more serious consequences when combined with other pollutants [23].

In this study, DCF was used as a model pollutant, and PS and PBAT were used as model MPs to compare the adsorption properties of different MPs on DCF before and after aging. The possible adsorption mechanism was proposed by combining the effect results of kinetics, isotherms, and environmental factors on the adsorption of DCF by MPs. The results show that surface aging can increase the adsorption capacity of MPs to organic pollutants. The adsorption process involves hydrophobic, electrostatic, and hydrogen bonding forces and other forces, which are also the main reasons for the adsorption differences before and after aging. These findings have important implications for the environmental fate of pollutants in the presence of MPs and the potential danger of MPs as carriers of micropollutants.

2. Materials and Methods 2.1. MaterialsPS and PBAT with particle sizes ranging within 75–150 μm were selected for the experiments and purchased from China Hengfa Plastic Technology Co. Ltd. The chemical structures and properties of PS and PBAT can be found in Table S1. The MPs were ultrasonically cleaned with deionized water for 5 min each time and repeated 3 times. The cleaned MPs were placed in a drying oven at 50 °C for 12 h and then stored. DCF was purchased from China Aladdin Industries, Inc, and its structure and properties are shown in Table S2. Humic acid (HA) was supplied by China Beijing Dometic Technology Co. Sodium hydroxide (NaOH), hydrochloric acid (HCl), and sodium chloride (NaCl) were purchased from China Aladdin Industries. Deionized water was used in all experiments, and the purity of the reagents not mentioned was in analytical purity. The appropriate concentrations of the solutions could be obtained by following these steps: accurately weigh 500.0 mg of DCF reagent into methanol, dissolve it completely, transfer it to a 25 mL volumetric flask for volume fixation to obtain a concentration of 20 g/L of DCF stock solution, and then place it under shade and set aside; weigh 4.0 g of NaOH and dissolve it in distilled water, then transfer it to a 1000 mL volumetric flask and fix the volume to 1000.0 mL to get 0.1 mol/L NaOH solution; and lastly, transfer 8.3 mL of 12 mol/L concentrated hydrochloric acid to distilled water, then transfer it to a 1000 mL volumetric flask and fix the volume to 1000.0 mL to get 0.1 mol/L HCl solution. 2.2. Preparation and Aging of MPs

The aging of the MPs was performed in the UV aging chamber, and the aging process was performed using a high temperature and UV aging simultaneously. During the irradiation process, the MPs were mixed every 6 h to ensure uniform exposure. The temperature was set to 70 °C and the wavelength to 254 nm. The UV intensity was 30 mW/cm2 and the distance was 14 cm. The pristine PS and PBAT plastics were evenly laid flat in the Petri dishes and the aging lasted for 15 d. After aging, the MPs were washed three times with deionized water, dried at room temperature, and reserved for use.

2.3. Characterization of MPs

To characterize the MPs, the samples were vacuum-dried for at least 3 days before use. The surface morphology of MPs was characterized using scanning electron microscopy (SEM) from Japan. Fourier transform infrared (FTIR) from Germany spectroscopy was used to study the changes in surface structure and the functional groups of MPs by aging and by adsorption in the range of 400–4000 cm−1. The oxygen content of the MPs’ particles by UV aging was studied using an energy dispersive spectrometer (EDS) from Japan. The contact angle tested the change in hydrophobicity before and after aging. The differences in crystallinity between MPs were examined by X-ray diffraction (XRD, XRD-7000 s/L) from Germany with a scan range 2θ of 10~70°. The zeta potential can determine the trend of the surface potential of MPs with pH.

2.4. Adsorption Experiments

The intermittent equilibrium method was used to evaluate the adsorption capacity of MPs on DCF. All adsorption experiments were performed using 60 mL brown glass bottles with 50 mL solution volume, pH adjusted to 7.0, and shaken at 25 ± 1 °C and 160 rpm. The methanol concentration was kept below 0.1% (v/v) to avoid co-solvent interactions. The 20 g/L DCF stock solution was pipetted (50 μL) and fixed to 50 mL so that the DCF concentration was 20 ppm. The mass of the MPs was accurately weighed to 10.0 mg. In the process of the kinetic experiments, nine time points (0, 0.5, 1, 3, 6, 9, 12, 24, 36 h) were selected as the detection points. The adsorption equilibrium time of DCF on MPs is about 12 h, indicating that 24 h is enough to reach the equilibrium. Therefore, 24 h was selected as the reaction time in the subsequent experiments. In the adsorption isotherm experiments, different initial concentrations of DCF solution were prepared so that the initial concentration of DCF was 15–35 mg/L and 10 mg of PS as well as PBAT were added to 50 mL of DCF solution of different concentrations.

The effects of HA, pH, and salinity on the adsorption capacity of MPs were investigated in the experiment. The pH was adjusted with 0.1 mol/L of HCl and NaOH to control the pH from 3 to 9. The adsorption capacity of DCF on MPs was measured in different pH solutions, different concentrations of HA solution (0–20 mg/L), and different salinity (NaCl concentration of 0–0.6 mol/L). At the same time, blank controls without plastic and with plastic but without contaminants were also measured. After standing for half an hour, the supernatant was collected using a 0.22 μm filter and analyzed on a UV-visible spectrophotometer. The detection wavelength of DCF that was detected on the UV spectrophotometer was 275 nm; and the blank control of spectrophotometry was 0.1% methanol blank solution.

4. Conclusions

The adsorption of DCF on MPs before and after aging and its influencing factors were investigated. The adsorption order of PS and PBAT to DCF before and after aging is Q(A-PBAT) (27.65 mg/g) > Q (A-PS) (23.91 mg/g) > Q (PBAT) (9.30 mg/g) > Q (PS) (9.21 mg/g). The adsorption capacity of aged MPs on DCF increased, and the adsorption equilibrium time of PS and PBAT before and after aging was about 12 h. The pseudosecondary kinetic model could better describe the adsorption kinetics compared with the pseudo-first-order kinetic model. The adsorption isotherms of different types of MPs and MPs before and after aging are different, and the adsorption model of MPs on DCF was more suitable for the Freundlich adsorption model than the Langmuir adsorption model. The adsorption process of MPs on DCF is highly dependent on the pH, ionic strength, and humic acid strength of the solution. The adsorption performance of MPs on DCF was better under acidic conditions, and both salinity and humic acid strength could inhibit the adsorption of DCF on MPs. In conclusion, during the adsorption of DCF by MPs, the adsorption performance decreases with the increase in pH, and the increase in salinity and humic acid concentration can inhibit the adsorption of DCF by MPs. The results show that the adsorption process is mainly controlled by hydrophobic, electrostatic, and π-π interactions, in which electrostatic and hydrogen-halogen bonds occupy the main position.