Investigation of the adsorption–desorption behavior of antibiotics by polybutylene succinate and polypropylene aged in different water conditions

Microplastics (MPs) are widely present in aqueous environments and aged by natural components of complex water environments, such as salinity (SI) and dissolved organic matter (DOM). However, the effects of multicondition aging on the physicochemical properties and environmental behavior of MPs have not been completely investigated. In this study, the degradable MP polybutylene succinate (PBS) was used to investigate the environmental behavior of sulfamethoxazole (SMZ) and was compared with polypropylene (PP). The results showed that the single-factor conditions of DOM and SI, particularly DOM, promoted the aging process of MPs more significantly, especially for PBS. The degrees of MP aging under multiple conditions were lower than those under single-factor conditions. Compared with PP, PBS had greater specific surface area, crystallinity, and hydrophilicity and thus a stronger SMZ adsorption capacity. The adsorption behavior of MPs fitted well with the pseudo-second-order kinetic and Freundlich isotherm models, indicating multilayer adsorption. Compared with PP, PBS showed relatively a higher adsorption capacity, for example, for MPs aged under DOM conditions, the adsorption of SMZ by PBS was up to 5.74 mg/g, whereas that for PP was only 3.41 mg/g. The desorption experiments showed that the desorption amount of SMZ on MPs in the simulated intestinal fluid was greater than that in Milli-Q water. In addition, both the original PBS and the aged PBS had stronger desorption capacities than that of PP. The desorption quantity of PBS was 1.23–1.84 times greater than PP, whereas the desorption rates were not significantly different. This experiment provides a theoretical basis for assessing the ecological risks of degradable MPs in complex water conditions.


Introduction
Plastics are widely used in many fields such as agriculture, commerce, industry, and daily necessities because of their low price, easy processing, and stable performance (Chan et al. 2022). However, plastic products exposed to the environment are further broken down by heat, salt, and dissolved organic matter (DOM) into many different particle sizes (Liu et al. 2022a), and those smaller than 5 mm are called microplastics (MPs) (Bakir et al. 2014).
Because of their light weight and small dimensions, MPs can easily enter water environments through atmospheric deposition and effluent discharges (Perumal and Muthuramalingam 2022). In this regard, Nizzetto et al. (2016) showed that about 1.5-4.5% of the total plastics produced globally are released directly into the ocean. The amount of marine plastic waste is huge, difficult to degrade, and gradually accumulates, causing damage to the marine environment (Davis et al. 2022). In actual marine environments, plastics undergo a series of aging processes, including physical wear and tear, ultraviolet radiation, biodegradation, and chemical oxidation (Tian et al. 2023). During the aging process, MPs release additives and intermediates, further increasing their ecological risks Luo et al. 2020). Aged MPs have increased specific surface areas and greater hydrophilicity and fluidity, which increase their ability to adsorb pollutants and act as pollutant carriers in the marine environment. In addition, because of its small dimensions, they are easily ingested by marine life, posing health risks to the latter (Bhatt et al. 2021;Gola et al. 2021).
Many studies have already looked into the physicochemical properties and environmental behavior of aged MPs. However, most of these are limited to aging in pure water, ignoring the influence of the natural components of water on the aging process (Fan et al. 2021;Sun et al. 2022). These natural components in the water may change the MP's microstructure, surface morphology, and environmental behavior (Liu et al. 2022a).
Salinity (SI) and DOM are the main components of seawater (Ding et al. 2020;Schmidt et al. 2017). A previous study has shown that functional groups (e.g., -OH, C-H) of conventional plastics aged in seawater change significantly with an increase in oxygen-containing functional groups, resulting in the high hydrophilicity and fluidity of the aged MPs (Ding et al. 2020). Meanwhile, Cao et al. (2022) showed that DOM promoted the weathering of aliphatic polypropylene (PP). However, the current research on MPs ignores the influence of the co-existence of SI and DOM during the aging process . Therefore, the influence of environmental factors in water on the aging process of MPs must be explored.
Degradable plastics are widely used daily to reduce the pollution caused by traditional plastics (Sato et al. 2017). However, there have only been a few investigations on degradable MPs . In this respect, polybutylene succinate (PBS) is a petroleum-based degradable plastic widely used in food packaging films, plant protection films, bone tissue engineering, and other applications. Degradable plastics can be completely degraded by the action of microorganisms in water (Zhu and Wang 2020). During degradation, PBS releases millions of plastic fragments (Wei et al. 2022). Compared with conventional MPs, degradable MPs have larger specific surface areas and can absorb more pollutants (Fan et al. 2021;Yu et al. 2019). However, although there have been some studies on degradable MPs, there is still a lack of research on degradable MPs that age in seawater. Thus, the investigation of the environmental behavior of biodegradable plastics is a matter of great necessity.
In recent years, antibiotic compounds were widely detected in the aqueous phase and classified as a new type of pollutant because of their overuse (Liu et al. 2022b;Roy et al. 2021). For instance, sulfamethoxazole (SMZ) is a type of antibiotic that can be found in concentrations of up to 1390 ng/L in the marine environment . MPs in the ocean can be carriers of SMZ and spread through the marine environments. Because of their small dimensions, marine organisms confuse MPs with food, which leads to accidental ingestion, causing MPs to accumulate in the food chain (Liu et al. 2022d). Antibiotic-loaded MPs may migrate through the food chain and adversely affect individuals and different communities of organisms (Capolupo et al. 2020). For example, SMZ can desorb in the intestinal fluid and cause toxicity to marine life (De Liguoro et al. 2009). Therefore, the interaction mechanisms between MPs and antibiotics must be understood to determine the potential ecological risks of degradable MPs on pollutants.
In this study, PBS and PP were used as target MPs. Meanwhile, SMZ was used as the target pollutant. The main objectives of this research were to (1) investigate the effect of aging on the physical properties of MPs under different conditions (i.e., DOM, SI, and DOM/SI), (2) study the behavior of PBS and PP in the adsorption of SMZ after aging, and (3) explore the difference in MPs' desorption behaviors for SMZ in Milli-Q water and simulated intestinal fluid. This study broadens the scope of research on MPs and contributes to the more comprehensive assessment of the potential ecological risks of MPs.

Materials
The PBS and PP used in this experiment were purchased from the Shanghai Guanbu Electromechanical Technology Co. (Shanghai, China). The average particle size of these two MPs was 40 μm. Chemicals needed for simulating seawater, such as NaCl, MgCl 2 , Na 2 SO 4 , CaCl 2 , and fulvic acid (FA), were purchased from Aladdin Industrial Co. (USA), with purity ≥ 98%. The drugs used in the experiment, sodium taurocholate (ST) and bovine serum albumin (BSA) with purity ≥ 98%, were purchased from Aladdin Industrial Corporation (USA).

MPs aging experiment
Four different solutions were prepared to simulate the aging behavior of MPs in water: pure water, a 4% SI solution, a 6-mg/L DOM solution, and a mixture of the two solutions. The 4% SI solution was prepared using an inorganic salt concentration of 28 g/L NaCl, 4.67 g/L Na 2 SO 4 , 6.05 g/L MgCl 2 , and 1.32 g/L CaCl 2 . The PBS and PP in were then placed in quartz tubes containing the solution described above. During the aging process, the MPs were stirred using a magnetic stirrer so that they were evenly distributed in the solution and fully aged. We placed the quartz tubes in a radiation chamber with 30 W/m 2 irradiance to better simulate the aging process of MPs in nature. The MPs were aged under UV irradiation for 48 h. This avoids inadequate MP aging and prevents the complete degradation of degradable plastics.

Characterization
The surface morphology of the MPs was characterized using scanning electron microscopy (SEM, Hitachi S-4800). The specific surface areas of the samples were measured using an ASAP 2020 instrument (Micromeritics, USA). The functional groups of original and aged MPs were characterized using X-ray photoelectron spectroscopy (ULVAC-PHI Inc., Japan) and Fourier transform infrared spectrometer (FTIR,Tensor 27.,Bruker). Variations in the crystallinity of the reaction sample were performed using X-ray diffraction (XRD, X'Pert PRO MPD., Netherlands). By measuring the contact angle (Dataphysics OCA20, German) of the samples, the changes in hydrophilicity were investigated.

Adsorption experiments
In the adsorption kinetics test, the time intervals were set to 30-2880 min, and experiments were conducted at 25 °C by mixing the 50-mg MP samples with 50 mL of the SMZ solution in the centrifuge tubes. The concentration of SMZ was set to 50 mg/L to highlight the adsorption quantity differences between the MPs and to reduce experimental errors. Centrifuge tubes were shaken at 150 rpm in a dark air bath thermostatic shaker. They were then removed at the set time intervals, and the sample solution was filtered through a 0.45-μm membrane filter. The concentration of SMZ in the filtrate was determined using high-performance liquid chromatography (HPLC).
Adsorption isothermal experiments were conducted in 100-mL centrifuge tubes. Referring to previous studies, the SMZ concentration was set to 1-20 mg/L . In this study, 50-mg samples were placed in the centrifuge tubes containing 50 mL of the solution for the experiment. The experiments were performed at 25 °C with a shaking speed of 150 rpm. The equilibrium time was set to 48 h. After the oscillation, the sample solution was filtered through a 0.45-μm membrane filter and then loaded into a brown injection bottle for testing.

Desorption experiments
In the desorption experiment, 500 mg samples were mixed with the 500-mL SMZ solution. The concentration of SMZ was set to 15 mg/L on the basis of the results of the isotherm experiments. After the adsorption saturation of the MPs, the samples were filtered and then dried at low temperatures. The desorption experiments were conducted in Milli-Q water and simulated intestinal fluid. The simulated intestinal fluid consisted of 5.0 g/L BSA and 10 mM ST in a 100-mM NaCl solution (Liu et al. 2020). Centrifuge tubes containing 50 mL of pure water and simulated intestinal fluid were taken and placed in an air bath constant temperature oscillator shaker (150 rpm) in the dark at 25 °C. The samples were shaken for 1-48 h. Thereafter, they were filtered through a 45-μm membrane filter and then measured using HPLC. On the basis of the experience of previous studies, we selected the 48-h desorption results for analysis (Cui et al. 2022b;Song et al. 2022).

Date analysis
The calculation formula of the carbonyl index (CI) is as follows (Prata et al., 2020): The calculation of the carbonyl index was determined on the basis of the absorbance at 1720-1725 cm −1 for carbonyl groups, and the absorbance reference peak depended on the type of MPs. The absorbance reference peaks of PBS were 3441, 2960, 1725, and 1152 cm −1 , whereas the peaks of PP were 3426, 2964, 1100, and 738 cm −1 .
For the adsorption kinetics studies, the pseudo-first-order and pseudo-second-order kinetic models were selected, and their equations are as follows : where Q t (mg/g) is the SMZ adsorption capacity of the MPs at time t(min). k 1 (min −1 ) and k 2 (g/mg·min) are the pseudo-first-order and pseudo-second-order rate constants, respectively. An intraparticle diffusion model was used to understand the rate at which SMZ was adsorbed by two different types of MPs.
where k p (mg/[g min 0.5 ]) is the internal diffusion rate and C is the mean boundary-layer thickness.
Adsorption isotherms can be fitted using the following models.
Langmuir isotherm: Freundlich isotherm: where Q max (mg/g) is the maximum value of the SMZ adsorption capacity of the MPs. k L (L/mg) and k F ([mg/g] [L/mg]) are the Langmuir and Freundlich distribution coefficients, respectively. Meanwhile, 1∕n F is the Freundlich model parameter that reflects the adsorption intensity and heterogeneity of the adsorbent. Table 1 compares the changes in crystallinity of PBS and PP before and after aging. The crystallinity of PBS increased from 24.82 to 26.31% (pure water-aged), 29.24% (DOMaged), 28.56% (SI-aged), and 26.68% (DOM/SI-aged). The crystallinity of PP increased from 14.14 to 19.48-25.98%. The crystallinity of the MPs increased during aging. These findings were consistent with those of a previous study (Cui et al. 2022a). The single-factor conditions (i.e., DOM and SI) promoted MP aging. The increase in crystallinity was due to the destruction of the noncrystalline structures of the MPs by reactive oxygen species (ROS) during the aging process, resulting in localized areas of secondary crystallization . Under severe photodegradation, the crystallinity of the MPs increased significantly (Arvaniti et al. 2022). Compared with PP, PBS was more crystalline and had a greater degree of surface breakage. Many studies have analyzed the surface morphology of PP, but there are fewer analyses of PBS (Luo et al. 2022). Therefore, the surface morphology of PP was not studied. The results of previous experiments show that degradable plastics generally degrade first (Fan et al. 2021). Therefore, an in-depth study of PBS was performed. The fragmentation of the PBS surface was observed using SEM. Figure 1 shows the SEM images of the original PBS and the PBS aged under different water conditions (i.e., pure water, DOM, SI, and DOM/SI). The surface of the original PBS is smooth (Fig. 1a). Meanwhile, the PBS aged in pure water for 48 h shows small cracks on the surface (Fig. 1b). According to Fig. 1c-e, the PBS aged in different water conditions had rough surface morphologies. The PBS aged in DOM had cracks and holes in the surface (Fig. 1c). In Fig. 1d, some of the plastic fragments came off the whole. This is because MP aging generally has two modes of degradation (i.e., cracking and flaking) , and PBS is degraded mainly by cracking (Liu et al. 2022c). In Fig. 1e, a small number of holes formed on the surface of the MPs. The change in surface roughness in Fig. 1c-e was not significant, probably because of the short aging time. The degradation rate is positively correlated with time . Figure 1 shows that complex water environments accelerate the formation of pores and cracks on the PBS surface, which increases the adsorption sites for pollutants and thus its pollutant adsorption ability.

Degree of crystallinity, morphology and surface properties
The specific surface area (S BET ) figures for PBS and PP are presented in Table 1. The S BET of the original PBS and PP MPs were 0.35 and 0.37 m 2 /g, respectively, which increased to 0.43 and 0.45 m 2 /g after aging in pure water. As can be seen in Table 1, natural components in the water column promoted the aging process of MPs. For example, the S BET of PBS increased to 0.51, 0.47, and 0.45 m 2 /g in the DOM, SI, and DOM/SI, respectively. Meanwhile, the S BET values of PP after aging were 0.82, 0.67, and 0.45 m 2 /g in DOM, SI, and DOM/SI, respectively. The S BET of the MP surface increases due to photodegradation. MPs aged under single-factor conditions had a high degree of aging. The S BET of the aged MPs increased, which was similar to the results of previous studies .
The degree of MP aging varied in different water environments. According to Fig. 1 and Table 1, the single-factor conditions promoted MP aging. However, multiple aquatic environmental factors slowed down the aging process of MPs, which may be related to the interaction between DOM and salt. In the coexistence of DOM and salt, inorganic salt ions such as Ca 2+ and Mg 2+ in the solution reduce the solubility of DOM in water (Strehse et al. 2018), which in turn enables solidification or precipitation. Because of the small concentration of DOM in the solution (only 6 mg/L), the observation of solidification and precipitation with the naked eye was difficult. The concentration of environmental components involved in the aging process and the concentrations of dissolved substances in the water were reduced. As such, the aging process of MPs was slowed down. This suggests that single-factor conditions in water promote the aging process of MPs. The results for the specific surface area agree with the XRD and SEM results.

Surface functional groups and contact angles
Figure 2 reveals the changes in the functional groups of MPs before and after aging. For PBS (Fig. 2a), the peak near 3441 cm −1 was the absorption peak caused by the hydroxyl group (− OH), whereas the peak at 2960 cm −1 was the absorption peak caused by the antisymmetric stretching vibration of the methyl group (-CH 3 ). The absorption peak of the carbonyl group (C = O) was located near 1725 cm −1 , whereas the absorption peak of − CH groups was located near 1152 cm −1 ). According to Fig. 2a, the intensity of the carbonyl peak of PBS increased after aging because of the oxidation of C-H. For PP, the main characteristic peaks were around 3426, 2964, and 1720 cm −1 , corresponding to the functional groups -OH, -CH 3 , and C = O, respectively. The peak near 738 cm −1 was produced by the -CH 2 bending vibration . As PP is a nonbiodegradable plastic and is more difficult to degrade than PBS, the change in its functional groups before and after aging was slight . During the aging process, PP releases plasticizers and the carbonyl peak signal is therefore weakened Yan et al. 2021). After aging, the oxygen-containing functional groups of PP increased . The increase in oxygen-containing functional groups in MPs after aging was probably due to the C-H bond breaking in the presence of UV light and reacting with oxygen to form oxygen-containing groups. These groups then combine with hydrogen from the surrounding environment to form hydrogen peroxide groups (-COOH), which then further decompose into other products (C = O, C-OH, and O-C = O) Qiu et al. 2022).
The C = O stretching vibration was more pronounced in aged PBS but was hardly observed in PP. This is because conventional plastics are more difficult to degrade (Tong et al. 2022). At the same time, the aging time of PP is short, and it does not age sufficiently; thus, changes in C = O are difficult to observe ). The aging conditions contain Cl −1 , which is more susceptible to substitution reactions and therefore has a higher degree of oxidation . Qiu et al. (2022) point out that DOM can release ROS and promotes the aging process of MPs. The interaction of PP with the aromatic structure in FA results in a more pronounced change in the characteristic peak under the aging condition of DOM (Cao et al. 2022). This change is mainly reflected in the carbonyl peaks (1720 cm −1 ). FA usually contains more carbonyl and fatty functional groups, interacting with PP through π-π bonds (Abdurahman et al. 2020). However, the co-existence of DOM and SI conditions did not show higher degrees of oxidation, probably because DOM and SI were mutually constraining, which reduced the aging process of the MPs. Meanwhile, the increase in the number of oxygen-containing functional groups increased the hydrophilicity of the aged MPs. Figure S1 shows the changes in contact angles of the MPs before and after aging. The contact angle of the original PBS was 81.11. After aging, the contact angle of the PBS was reduced to 55-64°. The contact angle of the original PP was 125.83°, whereas all the contact angles were less than 80° after aging. This phenomenon indicates that PP changes from hydrophobic to hydrophilic, similar to the results of previous studies (You et al. 2021). These results suggest that aging increases the oxygen-containing functional groups and therefore increases the hydrophilicity of MPs. This agrees with the FTIR results. This study shows that UV radiation and water environment factors are conducive to hydrophilicity enhancement and accelerate the aging process of MPs.

The O/C ratio and CI index
The O/C ratio and CI are used to indicate the degree of MP aging (Fan et al. 2021). As can be seen from Table 2, the O/C ratio of PBS increased from 0.412 to 0.416 (pure water), 0.432 (DOM), 0.420 (SI), and 0.417 (DOM/SI). Meanwhile, the O/C ratio of PP increased from 0.281 to 0.348-0.383. Under the same conditions, the O/C ratio of PBS was higher than that of PP, which means that PBS is more susceptible to aging than PP.
The CI index is used as a metric to indicate the degree of MP aging. The CI index of PBS increased from the original 0.077 to 0.082 (pure water), 0.220 (DOM), 0.190 (SI), and 0.162 (DOM/SI) (Table 2). Meanwhile, that of PP increased from 0.129 to 0.171-0.254. The pattern of change in the CI index was consistent with the O/C ratio. The MPs aged under DOM conditions had higher degrees of aging. This may be attributed to the fact that ROS produced by DOM under UV irradiation promotes the aging process of MPs (Qiu et al. 2022). The aging of MPs was more influenced by DOM than by SI. This was probably because the presence of Cl − could impede the photo-aging process of MPs . However, the level of MPs aged in SI remained large compared with those aged in DOM/SI and pure water, possessing a large CI index. The CI index and O/C ratio can reflect the degree of MP aging (Fan et al. 2021). Generally, the water environment factor promotes MP aging, and the degree of MP aging depends mostly on the aging conditions ).

Adsorption kinetics
Kinetic equations were used to fit the experimental data to better analyze the adsorption process and elucidate the adsorption mechanism, and the resulting parameters are summarized in Table S1. The pseudo-second-order model (R 2 > 0.99) was better fitted to the SMZ adsorption data for PBS and PP. This suggests that the adsorption process of MPs on SMZ was mainly chemisorption (Bao et al. 2021).
The k 2 values of the aged MPs decreased during the aging process. The low values of k 2 indicated that the adsorption rate decreased with time, and the absorption rates were proportional to the number of unoccupied sites (Gupta et al. 2010). As shown in Fig. S2, the adsorption quantity of MPs increased gradually during the first 1500 min; after 1500 min, the adsorption quantity hardly varied with time, so saturation was reached at this point.
According to Fig. S2, the adsorption quantity of SMZ by the aged MPs increased with the increase in time. The MPs aged under DOM condition showed a strong carrier capacity for pollutants, especially for PBS (Fig. S2). For example, the adsorption quantity of PBS increased from 4.56 to 5.74 mg/g, whereas that of PP increased from 2.80 to 3.41 mg/g. The adsorption capacity of MPs was related to their physicochemical properties. Compared with that of PP, FTIR results showed that PBS had more oxygencontaining functional groups on its surface, which facilitated the adsorption of SMZ on PBS. At the same time, PBS had a larger specific surface area (Fig. 1, Table 1), which increased the adsorption sites on the surface of the MPs and enhanced their ability to adsorb pollutants. Previous studies have also confirmed these results (You et al. 2021). In addition, the crystallinity of the MPs increased during aging, which facilitated the enhanced SMZ adsorption . Thus, compared with PP, PBS can adsorb more antibiotics, has a greater carrier capacity, and poses a greater ecological risk to water environments. An intraparticle diffusion model was used to fit the adsorption kinetic data to clarify the adsorption mechanism of SMZ on the original and aged MPs. The results are summarized in Fig. 3 and Table S2. Figure 3 clearly shows that the internal particle diffusion model has a linear relationship, where k p1 > k p2 > k p3 . The slope determines the rate of adsorption, and the fitting parameter ( C ) reflects the causes that influence the rate of adsorption. Therefore, the adsorption process of SMZ on MPs is divided into three main stages : Stage I, external mass transfer; Stage II, interfacial diffusion; Stage III, intraparticle diffusion.
At Stage III, the adsorption of MPs reached equilibrium. The k pi values of the aged MPs were higher than that of the original MPs. This may be due to the increased number of MP adsorption sites after aging, which facilitated faster access of SMZ to the adsorption sites. Meanwhile, the k pi values for aged PBS were higher than that of aged PP, indicating that the antibiotics diffused more rapidly in aged PBS. PBS developed large cracks and higher crystallinity during the aging process, which enhanced antibiotic adsorption Yao et al. 2021).

Adsorption isotherms
The results of the Langmuir and Freundlich isothermal sorption models fitted to the sorption isotherm data are shown in Table 3. In comparing the R 2 of the two models, the Freundlich isotherm model (R 2 > 0.93) better describes the adsorption behavior of SMZ on MPs than Langmuir (R 2 < 0.89). This indicated that the adsorption of SMZ on For PBS, k F and 1∕n F increased after aging. Of these changes, the most pronounced effect was seen for the PBS aged in DOM, where k F increased from 0.5390 to 1.722 mg/g(L/mg) 1/n , whereas 1∕n F increased from 0.4285 to 0.7125 mg/g(L/mg) 1/n . This suggests that MPs aged in DOM have a strong adsorption capacity. This was because DOM accelerated the aging process of the MPs, creating cracks and pits on their surfaces and increasing their adsorption sites. Compared with PBS, PP had a weaker adsorption capacity. The k F values increased from 0.1959 to 0.3910-0.9108 mg/g(L/mg) 1/n . However, the values of 1∕n F did not differ much, which may have been due to the short aging time.
PBS had a stronger adsorption capacity for SMZ than PP, which may have been due to the different physicochemical properties of PBS and PP. The reasons for the different physicochemical properties are as follows: (1) According to Fig. 2, the aged MPs contained more oxygen functional groups. The oxygen-containing functional groups increased the hydrophilicity of MPs as they make hydrogen bonds with water (Fig. S1), which enhanced the adsorption of antibiotics (Shi et al. 2022); (2) the aged MPs had larger specific surface areas and more adsorption sites, which increase the ability of degradable MPs to adsorb antibiotics . Consequently, PBS is more likely to carry higher levels of contaminants such as antibiotics in water environments.

Desorption kinetics
The data for the desorption of MPs in pure water and intestinal fluid are shown in Figs. 4 and S3. The amount of desorption may be related to several factors: (1) the environment of desorption (Ho and Leung 2019), (2) the types of MPs, and (3) the aging conditions of the MPs. As can be seen in Fig. 4, the amount of desorption in the intestinal fluid is significantly greater than that in the water.
According to Fig. 4a, the desorption of the original PBS in the intestinal fluid was 1.14 mg/g, which increased after aging to 1.75 mg/g (pure water-aged), 3.15 mg/g (DOMaged), 2.94 mg/g (SI-aged), and 2.46 mg/g (DOM/SI-aged). From Fig. S3a, the desorption rate increased from 29.00 to 36.44%, 54.83%, 53.90%, and 46.51%, respectively (Fig. S3a). The amount of original PP desorbed in the simulated intestinal fluid increased from 1.10 to 1.71 mg/g, and the desorption increased from 39.36 to 50.59%. The quantity  and rate of desorption of antibiotics in Milli-Q water were much smaller than those in the intestinal fluid (Fig. S3b).
The quantity of MPs desorbed in the simulated intestinal fluid was large compared with that in the desorption in Milli-Q water. A previous study has shown that ST in the intestinal fluid can facilitate the desorption of some pharmaceuticals (McDougall et al. 2022). It increased the desorption quantity of MPs by increasing the solubility of the SMZ (Liu et al. 2020). The active agent on the intestinal surface can increase the desorption of the polymer by increasing the diffusion rate of the pores within the particles. In addition, hydrophobic organic pollutants can be easily desorbed from the MPs after adsorption onto the surface of the MPs (Hartmann et al. 2017;Song et al. 2022).
The adsorption of pollution by MPs is a reversible physical process . The quantity of MPs desorbed in the simulated intestinal fluid depends on the adsorption ability of the MPs (Ito et al. 2022). Consequently, PBS had a stronger desorption capacity for SMZ than that of PP. Thus, PBS may release more antibiotics in organisms than PP, causing more serious adverse effects in organisms.

Conclusions
In this study, the changes in the physicochemical properties and environmental behavior of PBS aged under different water environments were systematically investigated. PBS and PP MPs were used in this research, and the key findings are as follows: (1) The patterns of variation in crystallinity and O/C ratio indicated that PBS was more susceptible to aging and degradation than PP. The hydrophilicity and specific surface area of MPs increased during the aging process. The aging process of the MPs was encouraged under single-factor conditions (i.e., DOM and SI). The coexistence of multiple aqueous environmental factors (i.e., DOM and SI) did not have a synergistic action on the aging process of MPs, which may have been mutually constrained, and the degree of aging was lower than those under single-factor conditions.  (2) The adsorption kinetics tests showed that the adsorption of SMZ by MPs occurs mainly through surface adsorption and intraparticle diffusion. The results of the adsorption isotherms indicated that the adsorption of SMZ by MPs was multilayer on nonuniform surfaces. Compared with PP aged under the same conditions, PBS showed a better adsorption capacity. The aging process increased the adsorption capacity and strength of MPs, as well as their ecotoxicity. (3) The desorption experiments showed that the quantity of MPs desorbed on the simulated intestinal fluid was approximately 10 times higher than that in Milli-Q water. Aging enhanced the desorption ability of SMZ by MPs. Compared with PP, PBS showed a higher desorption capacity. This research provides a theoretical basis for assessing the ecological risks of degradable plastics in complex water conditions.