The Inhibition of Microcystin Adsorption by Microplastics in the Presence of Algal Organic Matters

Microplastics (MPs) could act as vectors of synthetic chemicals; however, their influence on the adsorption of chemicals of natural origin (for example, MC-LR and intracellular organic matter (IOM), which could be concomitantly released by toxic Microcystis in water) is less understood. Here, we explored the adsorption of MC-LR by polyethylene (PE), polystyrene (PS), and polymethyl methacrylate (PMMA). The results showed that the MPs could adsorb both MC-LR and IOM, with the adsorption capability uniformly following the order of PS, PE, and PMMA. However, in the presence of IOM, the adsorption of MC-LR by PE, PS, and PMMA was reduced by 22.3%, 22.7% and 5.4%, respectively. This is because the benzene structure and the specific surface area of PS facilitate the adsorption of MC-LR and IOM, while the formation of Π-Π bonds favor its interaction with IOM. Consequently, the competition for binding sites between MC-LR and IOM hindered MC-LR adsorption. The C=O in PMMA benefits its conjunction with hydroxyl and carboxyl in the IOM through hydrogen bonding; thus, the adsorption of MC-LR is also inhibited. These findings highlight that the adsorption of chemicals of natural origin by MPs is likely overestimated in the presence of metabolites from the same biota.


Introduction
Microplastics (MPs), particles of sizes less than 5 mm [1], are widely found in the environment. Recently, it was found that the amount of MPs in Lake Chaohu reached 6250 particles/m 3 and could reach approximately 100,000 particles/m 3 in the water [2]. It is also found that nearly 80% of the MPs are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PUR), PE terephthalate (PET), and polystyrene (PS) [3], which are primarily associated with plastic production. Considering that MPs are pervasive in the environment, their potential impact on chemicals and biota (including human beings) has caught the public's attention. MPs can also alter the physical properties of ultraviolet radiation, mechanical damage, and biological degradation [4].
Apart from the intrinsic properties of the MPs, the most common concern from the public is the interaction between MPs and other pollutants. Research has found that persistent organic pollutants, pharmaceuticals, pesticides, and personal care product ingredients can be adsorbed by MPs [5,6], and that the MP types, particle size, and surface properties, as well as the crystallinity of the polymer, can influence the pollutants' adsorption [6,7]. For example, it was previously found that the surface carboxyl functional groups and H-bonding jointly regulated the adsorption of malachite green on nylon MPs [8], while the

Batch Adsorption Experiments
In the kinetics experiments, 2 mg of the PE, PS, and PMMA were individually added into the Erlenmeyer flask containing 100 mL Milli-Q water (with an MP concentration of 20 mg/L). Then, three independent adsorption tests were performed. In the first experiment, the equivalent amount of MC-LR standard was added into each flask (containing water and MPs) to reach the toxin concentration of 400 µg/L. The solution was shaken for 48 h at 150 rpm, and the water was collected after 0 min, 30 min, 1 h, 2 h, 6 h, 12 h, 24 h, and 48 h by a 0.45 µm filter (cellulose acetate filter) to remove the MPs, and the aqueous MC-LR was quantified using an HPLC-MS system (Agilent1100, Agilent, Wilmington, DE, USA) [25]. In the second experiment, the adsorption of IOM by PE, PS, and PMMA was evaluated, with the experimental system being the same as experiment 1, but with the addition of IOM (to reach the initial concentration of 5 mg/L) instead of MC-LR. The IOM concentration (as revealed by the concentrations of DOC) in the supernatant was determined using a TOC analyzer (Shimadzu, TOC-L) at the same intervals as experiment 1. In order to evaluate the influence of IOM on the adsorption of MC-LR by MPs, the third experiment was conducted in the presence of 5 mg/L IOM and MC-LR (with the same concentration of MC-LR as the experiment 1). The Milli-Q water that contained the same concentration of MC-LR and IOM, but without the addition of MPs, was used as the control. NaN 3 was added to eliminate the influence of microorganisms [19]. The initial pH of the solutions was adjusted using 0.1 M HNO 3 or 0.1 M KOH solution. All of the samples were shaken in a dark and constant temperature shaker (25 ± 1 • C, 150 rpm). All samples were performed in triplicate.
The adsorption isotherm experiments were undertaken in triplicate. For the first sorption isotherms, we selected different concentrations of MC-LR (50,100,150,200, 300, 400, and 500 µg/L). The same mass of PE, PS, and PMMA (20 mg/L) was transferred to each flask, respectively. The mixtures were shaken at 150 rpm for 24 h (the equilibration time was determined from the sorption kinetics experiments). Then, the aqueous MC-LR concentration was measured. The second sorption isotherm experiments were performed to assess the adsorption capability of PE, PS, and PMMA towards IOM. Specifically, 1, 2, 4, 6, 8, 10, and 12 mg/L of IOM was prepared in the flask containing the same mass of the studied MPs, respectively, and the IOM concentration in the water was detected after 24 h of incubation, shaken at 150 rpm. To identify the influence of IOM on the adsorption of MC-LR by PE, PS, and PMMA, the third experiment was conducted. The experimental condition was consistent with the first adsorption isotherm experiment, but with the addition of IOM (at a concentration of 5 mg/L and could be totally adsorbed by the MPs according to the adsorption capability of IOM by the studied MPs that were quantified from the second sorption isotherm experiment). The MC-LR concentration in the water was determined after 24 h with the same methods described in the kinetics experiments. All of the isotherm experimental conditions were consistent with the kinetics experiments. After adsorption, the PE, PS, and PMMA were collected and freeze-dried, and then underwent FTIR and XPS determination for further mechanism analysis.

Adsorption Model
The Pseudo-first-order (Equation (1)) and Pseudo-second-order (Equation (2)) kinetic models were used to analyze the experimental data.
where q t is the adsorption capacity (mg/g) at time t (min), q e is the adsorption capacity (mg/g) at equilibrium time, k 1 is the first-order rate constant (mg/g·min) and k 2 is the second-order rate constant (mg/g·min). Freundlich and Langmuir adsorption models were applied to fit the adsorption isotherms of the MC-LR, IOM, and MC-LR adsorption in the presence of IOM.
where q e and q m are the equilibrium and maximum adsorption amount, respectively. C e is the concentration pollutants at the equilibrium. k a is the Langmuir adsorption constant. k f is the Freundlich adsorption coefficient, and n is the Freundlich isotherm exponent.

Instrumental Analyses
The characterization of the studied MPs was performed to illustrate their surface property. Specifically, the zeta potential of the MPs in the experimental medium under pH 2.5-9 was measured using a Malvern Zetasizer instrument (Nano ZS, Worcestershire, UK). Scanning electron microscopy (SEM, Nova Nano SEM 450, FEI Co., Hillsboro, OR, USA) was used observe the surface morphology. X-ray diffraction (XRD, D8 Advance, Bruker, Germany) was adopted to detect the change in crystallinity of the MPs. The surface wettability of the MPs was detected using the contact angle meter (JC2000D, Powereach Co., Shanghai, China). X-ray photoelectron spectroscopy (XPS) spectra of the MP samples before and after the adsorption experiments were collected with a PHI5000 XPS (ULVAC-PHI, Osaka, Japan). Fourier-transform infrared (FTIR) spectroscopy (Nicolet 5700, Thermo Electron Corp., Madison, WI, USA) was chosen to characterize the change in the surface functional groups of the studied MPs. Specifically, the measurement mode and parameters were as follows: scanning range, 400-4000 cm −1 ; resolution, 4 cm −1 ; and number of scans, 64. The resulting spectra of the MPs were calibrated with the OMNIC software package (Thermo, Nicolet 380, Madison, WI, USA). The baseline was obtained by scanning spectralgrade potassium bromide pellets. The IOM composition was analyzed by three-dimensional excitation emission matrix (3D-EEM) fluorescence spectroscopy (F7000, Hitachi, Tokyo, Japan). The specific surface area was determined by N 2 -BET analysis (Micromeritics).

Statistical Analysis
The adsorption kinetics and isotherms were fitted using Origin 2015 software, and an analysis of variance (ANOVA) was applied. Statistical differences were considered significant at p < 0.05.

MPs Characterization
The average particle size (d (0.5) ) of the PE, PS, and PMMA, calculated from Figure S1a, was 43.06, 49.96, and 55.47 µm, respectively. The negative values of the three studied MPs (<0 mV at pH in the range between 4.5 and 9, Figure S1b) manifested that PE, PS, and PMMA existed stably in the suspensions because of electrostatic repulsion between the particles. The degree of crystallinity of the three studied MPs followed the order of: PE (35%), PS (3.7%), PMMA (6.1%) ( Figure S2). Figure S3 illustrates the SEM images of the studied MPs. It was observed that PE revealed irregular granular morphology and had relatively more dents, while PS and PMMA presented a smooth surface. It was previously found that with the increase in crystallinity, the plastics would be more fragile [26]. The maximum crystallinity of PE may lead to the fragmentation of the particles; thus, the particle surface may seem rough ( Figure S3). It can also be observed from Figure S3 that the contact angles of PE and PS were 108.8 • and 106.3 • , respectively, indicating that the surfaces of PE and PS are hydrophobic. In contrast, the contact angles of PMMA were identified to be 85.1 • , suggesting PMMA is hydrophilic. Table 1 presents the characterization of the studied MPs and clearly shows that the maximum specific surface area was recorded in PS, followed by PE and PMMA. found that with the increase in crystallinity, the plastics would be more fragile [26]. The maximum crystallinity of PE may lead to the fragmentation of the particles; thus, the particle surface may seem rough ( Figure S3). It can also be observed from Figure S3 that the contact angles of PE and PS were 108.8° and 106.3°, respectively, indicating that the surfaces of PE and PS are hydrophobic. In contrast, the contact angles of PMMA were identified to be 85.1°, suggesting PMMA is hydrophilic. Table 1 presents the characterization of the studied MPs and clearly shows that the maximum specific surface area was recorded in PS, followed by PE and PMMA.

Adsorption Kinetics
The performance of MC-LR, IOM, and MC-LR on adsorbents in the presence of IOM as a function of contact time is illustrated in Figures 1-3. The adsorption capacity of PE, PS, and PMMA for MC-LR uniformly increased rapidly in the first 6 h of the adsorption (Figure 1), and the absorbed MC-LR accounted for 88.89%, 92.27%, and 88.52% of the total MC-LR that PE, PS, and PMMA can absorb, respectively. However, as the adsorption time increased, the adsorption efficiency gradually slowed down. After 12 h incubation, the adsorption amount displayed an insignificant increase (p > 0.05). Based on the results presented above, a contact time of 24 h was selected for subsequent experiments. The Pseudofirst-order and Pseudo-second-order kinetic models were used to fit the adsorption data (Table S3). The data of the fitting parameters indicated that the MPs were well matched by both the Pseudo-first-order curve (R 2 : 0.992-0.997) and Pseudo-second-order model (R 2 : 0.998-0.999), and the difference between the R 2 values of the two models was small (Table S1). 43 found that with the increase in crystallinity, the plastics would be more fragile [26]. The maximum crystallinity of PE may lead to the fragmentation of the particles; thus, the particle surface may seem rough ( Figure S3). It can also be observed from Figure S3 that the contact angles of PE and PS were 108.8° and 106.3°, respectively, indicating that the surfaces of PE and PS are hydrophobic. In contrast, the contact angles of PMMA were identified to be 85.1°, suggesting PMMA is hydrophilic. Table 1 presents the characterization of the studied MPs and clearly shows that the maximum specific surface area was recorded in PS, followed by PE and PMMA.

Adsorption Kinetics
The performance of MC-LR, IOM, and MC-LR on adsorbents in the presence of IOM as a function of contact time is illustrated in Figures 1-3. The adsorption capacity of PE, PS, and PMMA for MC-LR uniformly increased rapidly in the first 6 h of the adsorption (Figure 1), and the absorbed MC-LR accounted for 88.89%, 92.27%, and 88.52% of the total MC-LR that PE, PS, and PMMA can absorb, respectively. However, as the adsorption time increased, the adsorption efficiency gradually slowed down. After 12 h incubation, the adsorption amount displayed an insignificant increase (p > 0.05). Based on the results presented above, a contact time of 24 h was selected for subsequent experiments. The Pseudofirst-order and Pseudo-second-order kinetic models were used to fit the adsorption data (Table S3). The data of the fitting parameters indicated that the MPs were well matched by both the Pseudo-first-order curve (R 2 : 0.992-0.997) and Pseudo-second-order model (R 2 : 0.998-0.999), and the difference between the R 2 values of the two models was small (Table S1). 49 found that with the increase in crystallinity, the plastics would be more fragile [26]. The maximum crystallinity of PE may lead to the fragmentation of the particles; thus, the particle surface may seem rough ( Figure S3). It can also be observed from Figure S3 that the contact angles of PE and PS were 108.8° and 106.3°, respectively, indicating that the surfaces of PE and PS are hydrophobic. In contrast, the contact angles of PMMA were identified to be 85.1°, suggesting PMMA is hydrophilic. Table 1 presents the characterization of the studied MPs and clearly shows that the maximum specific surface area was recorded in PS, followed by PE and PMMA.

Adsorption Kinetics
The performance of MC-LR, IOM, and MC-LR on adsorbents in the presence of IOM as a function of contact time is illustrated in Figures 1-3. The adsorption capacity of PE, PS, and PMMA for MC-LR uniformly increased rapidly in the first 6 h of the adsorption (Figure 1), and the absorbed MC-LR accounted for 88.89%, 92.27%, and 88.52% of the total MC-LR that PE, PS, and PMMA can absorb, respectively. However, as the adsorption time increased, the adsorption efficiency gradually slowed down. After 12 h incubation, the adsorption amount displayed an insignificant increase (p > 0.05). Based on the results presented above, a contact time of 24 h was selected for subsequent experiments. The Pseudofirst-order and Pseudo-second-order kinetic models were used to fit the adsorption data (Table S3). The data of the fitting parameters indicated that the MPs were well matched by both the Pseudo-first-order curve (R 2 : 0.992-0.997) and Pseudo-second-order model (R 2 : 0.998-0.999), and the difference between the R 2 values of the two models was small (Table S1). 55.47 0.118 85.1 ± 1.9 6.1

Adsorption Kinetics
The performance of MC-LR, IOM, and MC-LR on adsorbents in the presence of IOM as a function of contact time is illustrated in Figures 1-3. The adsorption capacity of PE, PS, and PMMA for MC-LR uniformly increased rapidly in the first 6 h of the adsorption (Figure 1), and the absorbed MC-LR accounted for 88.89%, 92.27%, and 88.52% of the total MC-LR that PE, PS, and PMMA can absorb, respectively. However, as the adsorption time increased, the adsorption efficiency gradually slowed down. After 12 h incubation, the adsorption amount displayed an insignificant increase (p > 0.05). Based on the results presented above, a contact time of 24 h was selected for subsequent experiments. The Pseudo-first-order and Pseudo-second-order kinetic models were used to fit the adsorption data (Table S3). The data of the fitting parameters indicated that the MPs were well matched by both the Pseudo-first-order curve (R 2 : 0.992-0.997) and Pseudo-second-order model (R 2 : 0.998-0.999), and the difference between the R 2 values of the two models was small (Table S1).
The adsorption of IOM on the studied MPs seemed slower than MC-LR. As a function of contact time, the adsorption of IOM on PS occurred quickly within the first 12 h, and the adsorption equilibrium was sufficiently reached after 24 h. The PS seemed to exhibit the most efficient adsorption capability, followed by PE and PMMA. According to the R 2 values (Table S2), the Pseudo-second-order model fit better than the Pseudo-first-order model.
In the presence of IOM, the adsorption profile of MC-LR on PE, PS, and PMMA was not obviously changed in comparison to that without IOM. However, in addition to the slower adsorption rates (as the equilibrium state was reached within 24 h, but at 12 h for MC-LR adsorption by the MPs with and without the presence of IOM, respectively), the maximum amount of MC-LR absorbed by the three kinds of MPs was substantially reduced at the initial concentration of 400 µg/L MC-LR. These results were in disagreement with previous studies, which found that montmorillonite (clay minerals) adsorbed a moderate amount of fulvic acid, thereby enhancing MC-LR adsorption to Na-montmorillonite [27]. It should be noted that the adsorption of organic matter to an interface could decrease the sorption of organic pollutants due to competition for adsorption sites, or enhance it through increasing the surface hydrophobicity of the particles [28]. Considering the IOM and MC-LR shared the same adsorption process, it was likely that competition for active sites occurred when the MC-LR and IOM coexisted in the solution.

Adsorption Isotherms
The adsorption isotherms results showed that the adsorption amount of MC-LR ( Figure 2a) and IOM ( Figure 2b) increased with the initial concentrations. The adsorption rate of MC-LR was fast when the toxin concentration was in the range of 50 to 300 µg/ L. When the concentration of MC-LR increased, the adsorption seemed slow down. Figure 2c also revealed that the isotherm curve of MC-LR by MPs in the presence of IOM tended to stabilize after an initial marked increase. A general comparison of the results indicates that in the presence of IOM, the adsorption capacity of MPs for MC-LR is substantially reduced.
The adsorption data were fitted by the Langmuir and Freundlich models and the parameters are presented in Table 2. The results show that MC-LR can be better fitted by the Langmuir model based on the R 2 values (0.933-0.951), which suggests that the MC-LR sorption onto PE, PS, and PMMA involved monolayer adsorption processes occurring at homogeneous adsorption sites. Similarly, the Langmuir model is more suitable for description of the IOM adsorption by PE, PS, and PMMA, as the correlation coefficient R 2 was in the range of 0.982 to 0.992. The presence of IOM did not influence the isotherm model of MPs towards MC-LR (better fitting for the Langmuir model, as the R 2 values were in the range of 0.923 to 0.979, which was higher that the R 2 of the Freundlich models). However, the adsorption capacity of MC-LR by PE, PS, and PMMA in the presence of IOM decreased by 22.3%, 22.7%, and 5.4%, respectively, in comparison to that without. In addition, of the studied MPs, PS exhibited the greatest adsorption capability toward both MC-LR and IOM, with the adsorption capability reaching 843 and 1470 µg/g, respectively.
Recently, it was demonstrated that the hydrophilic MC-LR [29] had lower adsorption onto PE and PS compared to the more hydrophobic Microcystins variant MC-LF [16]. However, the current study revealed that the adsorption capability of MC-LR to hydrophilic PMMA accounted for 61.8% of that on PS. This may be due to the minimum specific surface area of PMMA (0.118 m 2 /g) between the studied MPs (5.136 and 1.281 m 2 /g for PE and PS, respectively). Additionally, the degree of crystallinity of the MPs can also affect the adsorption of MPs. Generally, the degree of crystallinity displayed a negative relationship with the permeability of the polymer. The higher crystallinity of PMMA (Table 1) than PS suggested that PMMA should have a lower adsorption capability towards organic pollutants. It should be noted that the crystallinity of hydrophobic PE (35%) ranked the maximum between the studied MPs, while its adsorption of MC-LR and IOM was greater than for PMMA. This may be linked to the fact that the PE displayed an approximately 10.67-fold greater specific surface area than that of PMMA.

Interaction Mechanism
As can be seen from Figure 3a, the obvious peaks at 2912.5 and 2854.65 cm −1 were assigned to CH stretching in -CH 2 -groups [30,31]. The peak at 1465.9 cm −1 represented the bending vibrations of the CH 2 and CH 3 aliphatic groups [32], while the peak at 721.38 cm −1 was ascribed to the -(CH 2 )n-in-plane oscillating vibration [33]. A comparison of the bands between the pristine PE and that of adsorbed MC-LR or IOM showed that the band intensity at 2912.5 and 1465.9 cm −1 were stronger when PE adsorbed MC-LR or IOM than the pristine one, indicating that MC-LR and IOM was adsorbed onto PE. However, no new chemical bonds were formed during the adsorption; this suggested that adsorption did not produce a new substance. The peaks of PS can be described at 3024 cm −1 , which represented the bond-stretching vibrations between the carbon of the aromatic ring and hydrogen, and the peak at 1600 and 1451 cm −1 represented the deformation vibration of the C=C, and at 2919 cm −1 was attributed to the asymmetric stretching vibration of aliphatic C-H, respectively [34]. The peak at 698 and 752 cm −1 was ascribed to the bending C-H of mono-substituted benzene derivatives [35]. In the PMMA spectra (Figure 3c), a relatively broad peak at wavelengths of 2830-2940 cm −1 , a sharp and intense peak at 1730 cm −1 , a moderate peak at 1440 cm −1 , and a peak at 1148 cm −1 represented -CH 2 , C=O, O-CH 3 , and C-O bonds, respectively [36]. It can be observed from Figure 3a-c that the peaks were basically similar, with only some adsorption peaks differing slightly, and no new functional groups were detected after the adsorption of MC-LR or IOM by the three MPs, which suggested that physical interaction was the dominant process [37].
In the IOM (Figure 4a), peak T at the Ex/Em of 270-275/278-356 nm was identified as soluble microbial metabolites. Peak B (Ex/Em of 210-230/310-344 nm) was considered to be protein-like substances. Peak A (Ex/Em 250-300/400-500 nm) represented fulvic acid-like substances, and peak C (Ex/Em 350-400/400-500 nm) represents humic-like substances [38]. With respect to the adsorption of IOM by PE, the peak in fluorescence from humic-like substances decreased dramatically (Figure 4b). The most obvious alternation in the fluorescence peaks was recorded when PS was used as adsorbent, in which both the fulvic acid-like substances and the humic-like substances were substantially reduced (Figure 4c). Despite PMMA showing affinity to the substance at both peak A and C (Figure 4d), these peaks did not decrease significantly in comparison to PE and PS. The peaks detected in this study disclosed that a considerable amount of IOM was adsorbed by PS, while the humic-like substances might be more easily adsorbed by PE.
The full survey XPS spectra revealed that the studied MPs contained the elements C, N, and O ( Figure S4), and the peak of N1s displayed insignificant changes. C1s peaks can be fitted into two components at 284.7 and 286.0 eV, respectively, representing C-C/C-H and C-Cl [39]. According to the XPS spectrum ( Figure 5), the peaks of C1s at 286 eV in PE that absorbed IOM were relatively lower, but at 284.8 eV, were stronger than the pristine PE and absorbed MC-LR. The O1s of MPs can be divided into two parts at 532.8 and 533.8 eV, corresponding to -OH and C-O-C/-O-C=O groups, respectively [40], which was substantially enhanced in PE that adsorbed MC-LR in the presence of IOM. A similar pattern was also observed for PS and PMMA. The difference in oxygen content in the three MPs may be caused by the different additives contained in the MPs as well as the adsorbed substance. Table S4 shows the oxygen and carbon content obtained from the wide-scan XPS analysis. The results show that the carbon-to-oxygen (C/O) ratio for PE increased after adsorption, while a dramatic reduction was recorded for PS and PMMA. It should be noted that the oxygen measured in the pristine PE and PS may be due to polymer damage during the blending process, or additives [41]. However, the oxygen content exhibited a dramatic increase after adsorption, indicating the role of oxygen-containing groups in the adsorption process, which contributed to the adsorption [42]. Nevertheless, PMMA had the maximum oxygen content, while displaying the minimum adsorption capability in the studied MPs ( Table 2); this may be due to the minimum specific surface area (Table 1). humic-like substances decreased dramatically (Figure 4b). The most obvious alternation in the fluorescence peaks was recorded when PS was used as adsorbent, in which both the fulvic acid-like substances and the humic-like substances were substantially reduced (Figure 4c). Despite PMMA showing affinity to the substance at both peak A and C ( Figure  4d), these peaks did not decrease significantly in comparison to PE and PS. The peaks detected in this study disclosed that a considerable amount of IOM was adsorbed by PS, while the humic-like substances might be more easily adsorbed by PE.  The full survey XPS spectra revealed that the studied MPs contained the elements C, N, and O ( Figure S4), and the peak of N1s displayed insignificant changes. C1s peaks can be fitted into two components at 284.7 and 286.0 eV, respectively, representing C-C/C-H and C-Cl [39]. According to the XPS spectrum ( Figure 5), the peaks of C1s at 286 eV in PE that absorbed IOM were relatively lower, but at 284.8 eV, were stronger than the pristine PE and absorbed MC-LR. The O1s of MPs can be divided into two parts at 532.8 and 533.8 eV, corresponding to -OH and C-O-C/-O-C=O groups, respectively [40], which was substantially enhanced in PE that adsorbed MC-LR in the presence of IOM. A similar pattern was also observed for PS and PMMA. The difference in oxygen content in the three MPs may be caused by the different additives contained in the MPs as well as the adsorbed  Table 3 presents the available adsorption mechanism that PE and PS involved (little information was available for PMMA) in the adsorption of organic pollutants. It can be seen from the table that the physical interactions, including partitioning, van der Waals force, electrostatic interactions, and intermolecular hydrogen bonding, were the dominant mechanisms. It can also be concluded from Table 3 that PS exhibited greater adsorption capability towards the same organic chemicals than PE, which was in line with the current study concerning the adsorption of MC-LR and IOM. dramatic increase after adsorption, indicating the role of oxygen-containing groups in the adsorption process, which contributed to the adsorption [42]. Nevertheless, PMMA had the maximum oxygen content, while displaying the minimum adsorption capability in the studied MPs (Table 2); this may be due to the minimum specific surface area (Table 1).  Table 3 presents the available adsorption mechanism that PE and PS involved (little information was available for PMMA) in the adsorption of organic pollutants. It can be seen from the table that the physical interactions, including partitioning, van der Waals force, electrostatic interactions, and intermolecular hydrogen bonding, were the dominant mechanisms. It can also be concluded from Table 3 that PS exhibited greater adsorption capability towards the same organic chemicals than PE, which was in line with the current study concerning the adsorption of MC-LR and IOM.     In the current study, the FTIR results showed that no new chemical bonds were formed during the adsorption of MC-LR by the three MPs, suggesting that physical adsorption may play a crucial role in the adsorption process. According to the zeta potential of the three kinds of MPs ( Figure S1), PE, PS, and PMMA had negative inherent charges (<0 mV at pH in the range between 4.5 and 9), indicating that electrostatic repulsion may inhibit the adsorption of MC-LR, since the toxin was negatively charged at most pH values (3 < pH < 12) [37]. Apart from that, the van der Waals force and pore filling effect may be involved in the physical adsorption of MC-LR by the three kinds of MPs. In this study, the crystallinity of PE reached 35%, which may hinder the mass transfer of MC-LR due to its low permeability. The IOM displayed higher affinity towards PE, as revealed by the increased relative intensity in C1s (284 eV) (Figure 5a) and the substantial reduction of humic-like substances during the adsorption of IOM (Figure 5b). Therefore, it was highly possible that the co-existence of IOM competed the active sites between MC-LR ( Figure 6a); thus, the MC-LR adsorption capability was reduced upon the presence of IOM (Table 2). In terms of PS, the benzene structure and the specific surface area may contribute to the maximum adsorption capability towards both MC-LR and IOM. In addition, it is possible that Π-Π bonds strengthened the interactions between PS and the chemicals; this agreed well with a recent study that investigated the adsorption of hydrophilic organic chemicals by PS [37]. This structure could also increase the polarity of PS, which prompted its interaction with IOM, as revealed by the substantial reduction of fulvic acid-like substances and humic-like substances that originated from IOM (Figure 4c). Consequently, the increased competition for binding sites between MC-LR and IOM reduced its adsorption capability once the IOM was prevalent in the surrounding environment (Figure 6b). PMMA displayed hydrophobicity and was anticipated to adsorb more hydrophilic MC-LR than PE and PS, while its C=O facilitated its conjunction with the hydroxyl and carboxyl in the IOM through hydrogen bonding; thus, the adsorption of MC-LR can be depressed (Figure 6c). This was supported by the XPS and 3D-EEM analysis. The multiple processes of the adsorption of MC-LR and IOM by PE, PS, and PMMA, as well as the influence of IOM on MC-LR adsorption, are illustrated in Figure 6.
LR than PE and PS, while its C=O facilitated its conjunction with the hydroxyl and carboxyl in the IOM through hydrogen bonding; thus, the adsorption of MC-LR can be depressed (Figure 6c). This was supported by the XPS and 3D-EEM analysis. The multiple processes of the adsorption of MC-LR and IOM by PE, PS, and PMMA, as well as the influence of IOM on MC-LR adsorption, are illustrated in Figure 6.

Potential Environmental Impact
MPs are receiving increasing attention since they can act as a vector for a variety of pollutants, which can enhance risk through the transfer of pollutants along the food chain. In this study, we evaluated the adsorption of pollutants of natural origin (MC-LR), which

Potential Environmental Impact
MPs are receiving increasing attention since they can act as a vector for a variety of pollutants, which can enhance risk through the transfer of pollutants along the food chain. In this study, we evaluated the adsorption of pollutants of natural origin (MC-LR), which may be accompanied by other chemicals that likely influence the adsorption, by three prevalent MPs of uniform size but differences in hydrophobicity. The results documented that PE, PS, and PMMA adsorbed MC-LR, with the maximum adsorption capability recorded in PS. This observation manifested the potential risk of MPs as vector for the cyanotoxin. However, since MC-LR is released into the surrounding water as a consequence of cell rupture or the self-dependence of toxin-producing Microcystis, the toxin is always detected in the natural water associated with the substantial increase of dissolved organic matter. Apart from the affinity of MC-LR towards the three kinds of studied MPs, IOM was also adsorbed onto the MPs, which increased competition for available binding sites between MC-LR, during which multiple physical processes were involved. This suggested that the batch experiment used to assess the role of MPs as pollutants of natural origin may be magnified. In addition, in our recent study, we found that PS absorbed MC-LR and favored the colonization of microcystin-degrading bacteria; thus, the biodegradation of the toxin was identified [25]. As a result, our previous and current study indicates that the potential risk and environmental impact of MPs should be evaluated under conditions that could more closely resemble the real scenario, which requires further multidisciplinary investigations.

Conclusions
In this study, we evaluated the adsorption of three kinds of microplastics towards toxic chemicals of natural origin (MC-LR), and investigated the influence of intracellular organic matter (IOM), which was released into the surrounding water, accompanied by MC-LR, by toxin-producing Microcystis, on the adsorption of MC-LR by MPs. We then identified the potential mechanism involved in the adsorption. We found that PE, PS, and PMMA, with the uniform size of 50 µm in diameter, displayed high affinity toward MC-LR, with adsorption capabilities of 722, 843, and 521 µg/g, respectively. This was caused by physical adsorption processes including van der Waals force, electrostatic interaction, and pore filling. In addition, the maximum adsorption of MC-LR was also attributed to the formation of Π-Π bonds that strengthened the interactions between PS and MC-LR, as well as the maximum specific surface area displayed by PS. Apart from that, the PE, PS, and PMMA could also adsorb IOM, with the adsorption capability of 1400 µg/g for PE, 1470 µg/g for PS, and 1300 µg/g for PMMA. This indicated that a higher affinity was shown towards IOM by the PE, PS, and PMMA in comparison to MC-LR. However, the increased competition for binding sites between MC-LR and IOM reduced the adsorption of MC-LR by PE and PS. Despite the PMMA displaying hydrophobicity, its C=O facilitated its conjunction with the hydroxyl and carboxyl in the IOM through hydrogen bonding; thus, the adsorption of MC-LR was depressed by 5.4% when the MC-LR and IOM co-existed in the solution. Given the potential role of microplastics as vectors of contaminants in the environment, the results of this study provide new insights to assess its ability to carry chemicals of natural origin.
Supplementary Materials: The supplementary materials include the characterization of the three kinds of MPs in this study, including particle size distribution, zeta potential, XRD patterns, and microscopic images of contact angles and SEM micrographs; the XPS spectra of a survey scan of the MPs before and after adsorption; and the adsorption kinetics fitting parameters and the carbon and oxygen content in the MPs before and after adsorption, obtained through wide-scan XPS analyses. The following supporting information can be downloaded at: https://www.mdpi.com/article/10 .3390/toxics10060339/s1, Figure S1: (a) Particle size distribution; (b) zeta potential of the studied microplastics; Figure S2: XRD patterns of (a) PE, (b) PS, and (c) PMMA; Figure S3: Microscopic images of contact angles (a) and SEM micrographs (b) of PE, PS, and PMMA 19 (magnification of 500×, 2000×); Figure S4: XPS spectra of survey scan of (a) PE, (b) PS, and (c) PMMA before and after adsorption; Table S1: Adsorption kinetics fitting parameters of MCs on microplastics; Table S2: Adsorption kinetics fitting parameters of IOM on microplastics; Table S3: Adsorption kinetics fitting parameters of MCs on microplastics in the presence of IOM; Table S4: Carbon and oxygen content in the MPs before and after adsorption, obtained through wide-scan XPS analyses.