3. 1. Characteristics
3. 1. 1. Degree of crystallinity, morphology and surface properties
Table 1 compared the changes in crystallinity of PBS and PP before and after ageing. The crystallinity of PBS increased from 24.82–26.31% (Pure water-aged), 29.24% (DOM-aged), 28.56% (SI-aged) and 26.68% (DOM/SI-aged), respectively. The crystallinity of PP increased from 14.14% to 19.48–25.98%. The crystallinity of the MPs increased during the ageing. The findings were consistent with previous studies (Qinke et al., 2022). The single conditions (i.e. DOM, SI) promoted the ageing of MPs. The increase of crystallinity is due to the destruction of the non-crystalline structures of the MPs by reactive oxygen species (ROS) during the ageing process, resulting in localized areas of secondary crystallization (Peipei et al., 2021). In the condition of severe photodegradation, the crystallinity of the MPs increased significantly (Olga et al., 2022). Therefore, an in-depth study of PBS was carried out. The fragmentation of the PBS surface was observed by SEM.
Table 1
The Degree of crystallinity and SBET(m2/g) of the original and aged PBS, PP.
Microplastics
|
Aging environment
|
Degree of crystallinity
|
SBET(m2/g)
|
PBS
|
Original
|
24.82%
|
0.35
|
Pure water-aged
|
26.31%
|
0.43
|
Dom-aged
|
29.24%
|
0.51
|
SI-aged
|
28.56%
|
0.47
|
DOM/SI-aged
|
26.68%
|
0.45
|
PP
|
Original
|
14.15%
|
0.37
|
Pure water-aged
|
19.48%
|
0.45
|
Dom-aged
|
25.98%
|
0.82
|
SI-aged
|
23.17%
|
0.67
|
DOM/SI-aged
|
21.91%
|
0.45
|
Fig. 1 showing the SEM images of original PBS and PBS aged in different water conditions (i.e. pure water, DOM, SI, DOM/SI). The surface of the original PBS is smooth (Fig. 1a). The PBS aged in pure water for 48h shows small cracks on the surface (Fig. 1b). According to Figs.1(c-e), the PBS aged in DOM, SI, or DOM/SI component exhibits visible holes and cracks on the surface. It can be clearly seen from the images that some of the plastic fragments have come off the whole. This is because MPs ageing generally has two modes of degradation i.e. cracking and flaking (Gao et al., 2021), and PBS is degraded mainly by cracking (Si et al., 2022). Fig. 1 shows the complexed water environments accelerates the formation of pores and cracks on the PBS surface, which increases the adsorption sites for pollutants, thus it increases the ability to adsorb them.
The specific surface area (SBET) figures for PBS and PP are illustrated in Table 1. The SBET of the original PBS and PP MPs were 0.35 m2/g and 0.37 m2/g, respectively, and increased to 0.43 m2/g and 0.45 m2/g after aging in pure water. As it can be seen in Table 1, natural components in the water column promote the ageing process of MPs. For example, the SBET of PBS increased to 0.51 m2/g, 0.47 m2/g, 0.45 m2/g in the DOM, SI, and DOM/SI respectively. The SBET of PP after ageing in three components are 0.82 m2/g, 0.67 m2/g, and 0.45 m2/g, under the conditions of DOM, SI, and DOM/SI, respectively. The SBET of the MPs surface increases due to photodegradation. Under single conditions, DOM and SI accelerated the ageing process of MPs, and after ageing, the SBET of the MPs increased, which was similar to the results of previous studies (Yinghua et al., 2022a). Under the coexistence of SI and DOM, SI will reduce the solubility of DOM and forms solidification or precipitation (Robert et al., 2018), which slows down the MPs photodegradation. This suggests that single conditions in water promote the ageing process of MPs. The results for the specific surface area are in agreement with the XRD and SEM results.
3.1.2. Surface functional groups and contact angles
Figure 2 reveals the changes in the functional groups of MPs before and after ageing. For PBS (Fig. 2a), the peak near 3441 cm− 1 is the absorption peak caused by the hydroxyl group(-OH), and the peak at 2960 cm− 1 is the absorption peak caused by the anti-symmetric stretching vibration of the methyl group(-CH3). The absorption peak of the carbonyl group(C = O) is located near 1725 cm− 1 and the absorption peak of -CH groups is located near 1152 cm− 1 (Meng et al., 2022). According to Fig. 1a, the intensity of the carbonyl peak of PBS increased after ageing due to the oxidation of C-H. For PP, the main characteristic peaks are around 3426 cm− 1, 2964 cm− 1, 1720 cm− 1, corresponding to the functional groups -OH, -CH3 and C = O, respectively. The peak near 738 cm− 1 is produced by the -CH2 bending vibration (Jiang et al., 2022). The high oxygen content of original PP may be caused by plasticizers added during the production process (Yinghua et al., 2022b). The weakening of the absorption peak of -CH3 in aged PP is due to oxidation process during ageing.
After ageing, the oxygen-containing functional groups such as hydroxyl and carbonyl groups on the surface of the MPs increased, which is similar to the results of previous studies (Jianxin et al., 2022). The C = O stretching vibration is more pronounced in aged MPs. This is because the ageing conditions contain Cl− 1 (Mao et al., 2020), which is more susceptible to substitution reactions and therefore has a higher degree of oxidation. Qiu et al. (2022b) points out that DOM can release ROS and promotes the ageing 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 ageing condition of DOM (Hongwei et al., 2022a). However, the co-existence of DOM and SI conditions did not show a higher degree of oxidation, probably because the DOM and the SI are mutually constraining, thus reducing the ageing process of the MPs. The increase in the number of oxygen-containing functional groups increases the hydrophilicity of the aged MPs.
Figure S1 shows the changes in contact angles of the MPs before and after ageing. The contact angle of the original PBS was 81.11°. After ageing, the contact angle of the PBS is reduced to 55°-64°. The contact angle of the original PP was 125.83°, while all the contact angles were less than 80° after ageing. This phenomenon indicates that PP changes from hydrophobic to hydrophilic, and this is similar to the results of previous studies (You et al., 2021). These results suggested that ageing increases the oxygen-containing functional groups and therefore increased the hydrophilicity of the MPs. This is in agreement with the FTIR results. This study shows that UV radiations and water environment factors are conducive to enhance the hydrophilicity, and accelerates the ageing process of MPs.
3.1.3. The O/C ratio and CI index
The O/C ratio and carbonyl index (CI) are used to indicate the degree of ageing of MPs (Fan et al., 2021c). 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), respectively. 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 the PP, which means that PBS is more susceptible to ageing than PP.
The CI index is being used as a metric to indicate the degree of ageing of MPs. The CI index of PBS increased from the original 0.077 to 0.082 (pure water), 0.220 (DOM), 0.190 (SI), 0.162 (DOM/SI), respectively (Table 2). PP increased from 0.129 to 0.171–0.254. The pattern of change in the CI index is consistent with the O/C ratio. The MPs aged under DOM conditions have a higher degree of ageing. This may be attributed to the fact that ROS produced by DOM under UV irradiations promote the ageing process of MPs (Qiu et al., 2022a). The CI index of MPs aged SI conditions is lower than aged in DOM. This is probably due to the presence of Cl−could impeded the photo aging process of MPs (Xiaowei et al., 2021).
Table 2
Changes of O/C ratio and CI index in PBS and PP after UV radiation ageing.
Samples
|
Wide-Scan elemental content
|
CI index
|
C (%)
|
O (%)
|
O/C
|
Original PBS
|
70.70
|
29.19
|
0.412
|
0.077
|
Pure water-aged PBS
|
70.51
|
29.40
|
0.416
|
0.082
|
Dom-aged PBS
|
70.53
|
30.51
|
0.432
|
0.229
|
SI-aged PBS
|
70.52
|
29.67
|
0.420
|
0.190
|
DOM/SI-aged PBS
|
70.50
|
29.40
|
0.417
|
0.162
|
Original PP
|
77.95
|
21.93
|
0.281
|
0.129
|
Pure water-aged PP
|
75.57
|
26.28
|
0.348
|
0.171
|
Dom-aged PP
|
72.11
|
27.62
|
0.383
|
0.254
|
SI-aged PP
|
72.62
|
26.69
|
0.368
|
0.231
|
DOM/SI-aged PP
|
74.11
|
27.62
|
0.372
|
0.183
|
3.2. Adsorption kinetics
In order to better analyze the adsorption process and elucidate the adsorption mechanism, kinetic equations were used to fit the experimental data, and the resulting parameters are summarized in Table S1. The pseudo-second-order model (R2 > 0.99) was better fitted to the SMZ adsorption data for PBS and PP. This suggested that the adsorption processes of MPs on SMZ are mainly chemisorption (Bao et al., 2020).
The \({k}_{2}\) values of aged MPs decreased during the ageing process. The low values of \({k}_{2}\) indicated that the adsorption rate decreased with time, and absorption rates are proportional to the number of unoccupied sites (Gupta et al., 2010). As shown in Fig. S2, the adsorption quantity of MPs increases gradually during the first 1500 min, and after 1500 min, the adsorption quantity hardly varies with time, so saturation is reached at this point.
According to Fig. S2, the adsorption quantity of SMZ by the aged MPs increased with an increase in time. MPs aged under DOM condition shows a strong carrier capacity for pollutants, especially for PBS (Fig. S2). For example, the adsorption quantity of PBS increased from 4.56 mg/g to 5.74 mg/g, and PP increased from 2.80 mg/g to 3.41 mg/g. The adsorption capacity of MPs is related to their physicochemical properties. Compared to PP, FTIR results showed that PBS had more oxygen-containing functional groups on its surface, which facilitates the adsorption of SMZ on PBS. At the same time, PBS has larger specific surface area (Fig. 1, Table 1), thus increasing the adsorption sites on the surface of the MPs, and enhancing its ability to adsorb pollutants. Previous studies have also confirmed these results (Huimin et al., 2021). In addition, the crystallinity of the MPs increased during ageing, which facilitated the enhanced adsorption to the SMZ (Shi et al., 2021a). Thus, compared to PP, PBS can adsorb more antibiotics, has a greater carrier capacity, and poses a greater ecological risk to water environment.
In order to clarify the adsorption mechanism of SMZ on original and aged MPs, an intraparticle diffusion model was used to fit the adsorption kinetic data. The results are summarized in Fig. 3, Table S2. It is clear from Fig. 3 that the internal particle diffusion model is a linear relationship, where kp1>kp2>kp3. The slope determines the rate of adsorption, and the fitting parameter(\(C\)) reflects the causes that influence the rate of adsorption (Inyang et al., 2016). Therefore, the adsorption process of SMZ on MPs is divided into three main stages (Li et al., 2021). Stage Ⅰ: external mass transfer; Stage Ⅱ: Interfacial diffusion; Stage Ⅲ: Intraparticle diffusion.
At the Stage Ⅲ, the adsorption of MPs has reached an equilibrium. The kpi values of the aged MPs are higher than that of the original MPs. This may be due to the increased number of MPs adsorption sites after ageing, which facilitates faster access of SMZ to the adsorption sites. Meanwhile, the kpi values for aged PBS were higher than that of aged PP, indicating that the antibiotics diffused more rapidly in aged PBS. PBS has developed large cracks and higher crystallinity during the ageing process, which makes it easier for antibiotics to be adsorbed (Pin et al., 2022; Shi et al., 2021b).
3.3. Adsorption isotherms
The results of the Langmuir and Freundlich isothermal sorption models fitted to the sorption isotherm data are shown in Table 3. By comparing the R2 of the two models, it was found that the Freundlich isotherm model (R2 > 0.93) better describes the adsorption behavior of SMZ on MPs than Langmuir (R2 < 0.89). This indicated that the adsorption of SMZ on PBS and PP was multi-layer adsorption. In Freundlich isotherm, \({k}_{F}\) (mg/g(L/mg)1/n) and \(1/{n}_{F}\) are the parameters reflecting adsorption capacity of the adsorbents and the parameter reflecting adsorption strength, respectively.
Table 3
Isotherm model parameters of SMZ adsorption by Original and aged PBS and PP.
Adsorbent
|
Langmuir
|
Freundlich
|
Qmax(mg/g)
|
\({k}_{L}\)
|
R2
|
\(1/{n}_{F}\)
|
\({k}_{F}\)
|
R2
|
Original PBS
|
9.3806
|
0.0650
|
0.6972
|
0.4285
|
0.5390
|
0.9924
|
Pure water aged-PBS
|
8.1633
|
0.0928
|
0.6476
|
0.7509
|
0.6834
|
0.9918
|
Dom-aged PBS
|
14.26534
|
0.1438
|
0.8470
|
0.7125
|
1.7220
|
0.9889
|
SI-aged PBS
|
5.9988
|
0.3527
|
0.7859
|
0.4507
|
1.5952
|
0.9960
|
DOM/SI-aged PBS
|
20.9205
|
0.0289
|
0.5769
|
0.5035
|
0.5995
|
0.9853
|
Original PP
|
5.6883
|
0.0083
|
0.1830
|
0.9899
|
0.1959
|
0.9866
|
Pure water aged-PP
|
4.6067
|
0.0102
|
0.1234
|
0.9975
|
0.7900
|
0.9716
|
Dom-aged PP
|
11.1732
|
0.1127
|
0.4743
|
0.8094
|
0.9108
|
0.9368
|
SI-aged PP
|
9.7561
|
0.0201
|
0.1628
|
0.9751
|
0.4078
|
0.9303
|
DOM/SI-aged PP
|
5.5096
|
0.0251
|
0.2647
|
0.9608
|
0.3901
|
0.9415
|
For PBS, the \({k}_{F}\) and \(1/{n}_{F}\) increased after ageing. Of the changes, the most pronounced effect was seen in PBS aged in DOM, where \({k}_{F}\) increased from 0.5390 mg/g(L/mg)1/n to 1.722 mg/g(L/mg)1/n, and \(1/{n}_{F}\) increased from 0.4285 mg/g(L/mg)1/n to 0.7125 mg/g(L/mg)1/n. This suggests that MPs aged in DOM have a strong adsorption capacity. This is because DOM accelerates the ageing process of the MPs, creating cracks and pits on his surface and increasing his adsorption sites. Compared to PBS, PP has a weaker adsorption capacity. The \({k}_{F}\) values increased from 0.1959 mg/g(L/mg)1/n to 0.3910–0.9108 mg/g(L/mg)1/n. However, the values of \(1/{n}_{F}\) do not differ much, which may be due to the short ageing time.
PBS has a stronger adsorption capacity for SMZ than PP, which may be due to the different physicochemical properties of PBS and PP. The reasons for the different physicochemical properties are following; (1) According to Fig. 2, aged MPs contain more oxygen functional groups. The oxygen-containing functional groups increased the hydrophilicity of MPs as they make hydrogen bonds with water (Fig. S1), thus enhancing the adsorption of antibiotics (Ke et al., 2022);(2) Aged MPs have a larger specific surface area and more adsorption sites, which will increase the ability of degradable MPs to adsorb antibiotics (Yanji et al., 2022), and as a result, PBS is more likely to carry higher levels of contaminants such as antibiotics in water environments.