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Environ Eng Res > Volume 26(5); 2021 > Article
Zafar, Park, and Kim: Surface modification of polyethylene microplastic particles during the aqueous-phase ozonation process

Abstract

Microplastics coexist with the chemical reactive oxygen species in natural waters, however, there is still a lack to elucidate the effect of these radicals on the microplastic surficial oxidation. In this study, the ozonation of polyethylene microplastics was carried out under varying ozone dosages ranging from 4 to 7 mg/min for 60, 120 and 180 min, where its ozone uptake was iodometrically compared and surficial modification was spectroscopically analyzed using FTIR and XPS. For that, the lowest ozone uptake was 16% at 4 mg/min ozone supplied for 60 min whereas the highest was observed of 44% at 7 mg/min ozone added for 180 min. Moreover, in the FTIR analysis, carbonyl (1,600–1,800 cm−1) and hydroxyl (3,200–3,600 cm−1) indices were improved more than 20% and 13% when they were ozonized at 7 mg/min for 180 min compared to 4 mg/min for 60 min, respectively. XPS also revealed that 7 mg/min of ozone supplied for 180 min provided the highest of oxygen functionalities, but while there was no significant change in C-C bond. It can be concluded that the surficial modification of PE including formation of oxygen functionalities could be more preferably influenced by the reaction time than ozone dosages.

1. Introduction

The prevalence of microplastics (MPs) has been recently recognized as a threat to the biosphere owing to their infinitesimal size responsible for highly stable nature towards natural degradation processes [1, 2], thereby related adverse effects have been reported across all over ecosystems from low to high trophic levels of food chain: microorganisms [35], plants [6, 7], animals [811], and even human beings [12]. These significant ecological impacts persuade the researchers to investigate the aging behavior of these tiny particles in an environment. Till date, several studies have been conducted to determine the changes in physicochemical properties occurred as a result of physical, chemical [13, 14] and biological processes [14], moreover the requirement for the detailed research is obligatory.
In line with this, the advanced oxidation processes (AOPs) have been widely implemented and proven to be effective for the oxidation of plastics owing to the presence of reactive oxygen species including hydroxyl radicals, superoxide, peroxide, and atomic oxygen formed under a variety of chemical processes including ozonation [15, 16], persulfates activation [17], UV/O3 [18], UV/H2O2 [19] and etc. Recently, the application of these technologies in the field MPs oxidation has gained attention for which several studies can be found in this regard. Briefly, Liu et al. studied the alteration properties of polystyrene (PS) and polyethylene (PE) MPs particles using chemical reagents such as potassium persulfate and Fenton reagent and observed the formation of oxygen functionalities on their surface [17]. Similarly, Mao et al. investigated the aging behavior of PS exposed to ultraviolet-A (UV-A) irradiation having an absorbance at 340 nm in the different environmental matrix (i.e., air, water, and sea water) [20], implying that the highest oxidation of the PS was detected in air matrix. In addition to, Tajkia et al. [21] observed an increase of 30% in carbonyl indices of low density polyethylene (LDPE) MPs treated under UV irradiation using zinc oxide (ZnO) nanorods as a photocatalyst. These studies provide a clear evidence of applicability and effectiveness of AOPs for the alterations of MPs. Moreover, it eventually contributed to similar resultant owing to the involvement of conspecific radicals, although there are differences in their oxidation initiators [22]. Hence, the ozonation can be also one of the potential candidates for simulating the effect of reactive oxygen species on the surficial oxidation of MPs in the natural waters. Molecular ozone is a powerful oxidant with the oxidation potential of 2.07 V in a gaseous phase [23], making it turning into the hydroxyl radicals and other reactive oxygen species in an aqueous phase [24] and its polymeric oxidation have been investigated a lot [23, 25, 26].
This is the first attempt to explore the effect of ozonation of PE MPs particles through an aqueous phase ozonation at 4 different ozone dosages (i.e., 4, 5, 6, and 7 mg/min) for three different time durations of 60, 120, and 180 min. Herein, PE MP particles used was consisting of 5 different particle size range distribution: (i) < 75 m constitute 3.4% of sampled MPs; (ii) 75–125 μm, 7.2%; (iii) 125–250 μm, 43.0%; (iv) 250–500 μm, 43.1%; and (v) > 500 μm, 3.3%, respectively, which were determined by multi-phase sieving. After that, the degree of oxidation of PE MPs was determined by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). This study should be helpful to elucidate the effect of the reactive oxygen species to MPs distributed in natural waters along with their surficial alteration.

2. Materials and Methods

2.1. Ozonation of PE MP Particles

The ozone treatment system was composed of ozone generation system and reaction chamber as presented in Fig. 1. Reaction chamber was equipped with two 300 mL Erlenmeyer flasks, which were served as the main ozonation chamber followed by ozone trapping equipment, respectively. Prior to the ozonation, the reaction mixture (pH 6.59 at 25°C) was made of 1% (w/v) PE MP particles (medium density of 0.94 g/mL; melting point of 109–111°C, Aldrich Chemical Co., USA) in distilled water and then infused into the main flask. It was continuously agitated on a magnetic stirrer set with a stirring speed of 100 rpm under the ambient temperature to uniformly disperse PE MP particles during the treatment. Secondary flask was prepared by adding 100 mL of 1.2 M potassium iodide (KI, Samchun Chemicals, Korea) solution to capture off-gas ozone which was released to be unreacted from the ozonation chamber. These two flasks were thoroughly capped by using a silicon stoppers and connected each other with the Pyrex tube (6.34 mm diameter, Koreel, Korea) to prevent ozone undesirably escaping. And thereafter, ozone was generated through dielectric barrier discharge (DBD) plasma [27] from the dry oxygen (Purity; 99.5–99.9%) fed at a pressure of 1 MPa into the ozone generator (Ozonetech, Korea). The ozone dosage was then adjusted to variably keep at 4, 5, 6 and 7 mg/min referring to the readings of ozone gas analyzer (H1 Series Ozone Analyzers, INUSATM, USA). They were immediately entered into the reaction chamber where PE MP particles were oxidized for varying times of 60, 120 and 180 min. After the termination of each experiment, the treated PE MPs were separated using 0.45 μm cellulose acetate membrane filter (ADVANTEC®, Tokyo Roshi Kaisha Ltd., Japan) to analyze the possible surficial modification followed by being dried for 24 h in a drying oven (50°C, Vision Scientific Co., Ltd., Korea), which were then stored in vacuum desiccator at room temperature for further surface analysis of FTIR and XPS.

2.2. Measurement of Ozone Uptake by PE MP Particles

The off-gas of ozone released during the oxidation of PE MP particles was captured by KI solution in the off-gas ozone trap, which were used to measure the extent of the flow rates of ozone gassed out and ozone uptake by PE MP particles. First, the flow rate was determined by modified iodometric titration method [17, 28], where the residual ozone reacted with iodide ion (I) to form iodine (I2). Briefly, 10 mL of KI solution captured off-gas ozone was mixed with 5 mL of 2% (w/v) of sulfuric acid (95%, Samchun Chemicals, Korea) solution and the mixture was then titrated with standardized sodium thiosulfate solution (0.001 N, Na2S2O3.5H2O, Daejung Chemical, Korea) till pale yellow color appeared. After that, 1 mL of 1% (w/v) of starch ((C6H10O5)n, Duksan Chemicals, Korea) indicator was added, which were then secondary titrated till color changed from blue to colorless. From that, the total volume of sodium thiosulfate solution consumed for titration was recorded, from which the flow rate (O3, off-gas) was calculated by using the Equation (1), presented by Chtourou et al. [15]:
(1)
O3,   off-gas(mg/min)=(A×N×B×24)(t×V)
Where, A is the volume of sodium thiosulfate solution used for titration (mL), B is the total volume of KI solution used to capture ozone (100 mL), V is the volume of KI used for titration (10 mL), N is the normality of sodium thiosulfate solution (0.001 N), and t is the ozonation time (min). It was converted to the flow rate of off-gas ozone by multiplying the conversion factor of 24. After that, the percentage ozone uptake by PE MP particles was also determined by Equation (2), with the off-gas ozone flow rate obtained under the presence (O3, PE) over absence (O3, Blank) of PE particles in the reaction chamber, respectively.
(2)
Ozone uptake (%)=[O3,Blank]-[O3,PE]O3,Blank×100

2.3. Fourier Transform Infrared (FTIR) Spectroscopy

For the first time, a pellet was made by grinding a mixture of 100 mg of potassium bromide (KBr, FTIR grade, Merck, Germany) and 5 mg of oxidized PE MP particles in an agate mortar followed by pelletizing with AltasTM manual hydraulic press (SPECAC Inc., USA). The surface modification of those pellets of PE MP particles after ozonation at 4 different ozone dosages for three different time reactions was analyzed by FTIR (VERTEX 80V, Bruker, Germany) in the wavenumber ranging from 4,000 cm−1 to 400 cm−1 with a resolution of 4 cm−1. The obtained numbers of FTIR spectra were corrected by baseline method [29] and then normalized to the band at the wavenumber of 1,472 cm−1 (C=C, double bond stretching) because it was to be unchangeably sustained as of that of virgin PE peak even though PE particle has been oxidized, otherwise rest of those peaks having been changed after the oxidation. From these normalized spectra, the degree of carbonyl and hydroxyl indices were determined by integrating the peak areas in between 1,600–1,800 cm−1 and 3,200–3,600 cm−1, respectively, using a linear end-to-end baseline approach.
In the meantime, the crystallinity of PE MPs was estimated as suggested by Zerbi et al. [30]. Namely, comparing peak height at 730 cm−1 (corresponding to crystalline content) versus 719 cm−1 (amorphous content) can obtain the percentage of crystalline phase content as calculated by the following Eq. (3):
(3)
Crystalline phase content (%)=[1-{1-(I730/I719)/1.233)}(1+I730/I719)]×100
Where, I730 and I719 are the respective absorbance intensity of peak at 730 cm−1 and 719 cm−1 in the normalized FTIR spectra, and 1.233 is the theoretical intensity ratio of at the settling angle of 42°.

2.4. X-ray Potoelectron Spectroscopy (XPS)

X-ray photoelectron spectra of PE MP samples were determined using XPS (K-Alpha, Thermo scientific, USA) equipped with monochromatic AI-Kα source. From this, low resolution spectrum (0 to 1,200 eV of binding energy) was used to monitor possible changes of elemental concentrations such as sodium (Na), silicone (Si), and nitrogen (N), along with the determination of ratio of oxygen to carbon (O/C), while high-resolution spectra for C1s region (280 to 293 eV) and O1s region (526–538 eV) were deconvoluted into subpeaks by Origin software (OriginPro 9.0, Electronic Arts, USA) to distinct the type of carbon-oxygen functionalities newly appeared on the PE surface since after the ozonation.

3. Results and Discussion

3.1. Ozone Uptake by PE MPs

The flow rate of off-gas ozone released during ozonation was determined using Eq. (1) and thereafter, compared in the presence (the test) over the absence (i.e., the control) of PE MP particles under the given experimental conditions of 4 different ozone dosages and 3 different reaction times, as presented in Fig. 2. It showed that the flow rate was continuously declined in both control and test samples as the inlet ozone dosages was increased from 4 to 7 mg/min for each reaction time: for the control samples, it was decreased from 3.5 to 1.2 mg/min just since after 60 min, from 2.9 to 1.0 mg/min immediately after 120 min, and from 2.5 to 0.8 mg/min after 180 min, respectively. It was also confirmed that the prolonged treatment duration made off-gas ozone flow rate much furtherly decreased. It can be attributed to ozone decomposition by OH, HO2, O2 radical (O2) and ozone itself ( pH 6.59 at 25°C), as suggested in Eq. (4)(7) [25, 26, 28]. These reactions depicted that ozone decomposition could be accelerated as the ozone dosage and the reaction time increased, more promoting the generation of superoxide anion (O2) with a much higher reaction rate of 1.6 × 109 M−1s−1 (Eq. (6)) involved in ozone decomposition, consequently leading to decrease of the off-gas ozone flow rate.
(4)
O3+OH-HO2-+O2         k1=70M1s-1
(5)
O3+HO2-O3-+HO2         k2=2.8×106M-1s-1
(6)
O3+O2-O3-+O2         k3=1.6×109M1s-1
(7)
O3-+H2OOH+O-H+O2         k4=20-30M1s-1
In the meantime, Fig. 3 showed that as higher as ozone dosage and as long as the reaction time could make the higher percentage of ozone uptake to be evident by comparing the control over the test of off-gas ozone flow rate: the ozone uptake by PE MP particles has been increased at the lowest to be 16% (injected ozone of 4 mg/min for 60 min), while at the highest to be 44% (7 mg/min; 180 min). Moreover, it was presumed that the MP oxidation could be triggered as of a result of ozone uptake by PE MP particles. In other words, relatively lower dosage for shorter reaction time could be served as initiating oxidation of PE MP particles, which could be slightly enhanced by increasing the amount of ozone dosage at the same reaction time (i.e., 60 min). On the contrary, extending the reaction time longer than 60 min at the same dosage could more increase the extent of oxidation of those of particles. It therefore implied that the reaction time can be considered as the key factor in the uptake of ozone by PE MP particles, whereas the ozone dosage might be served as a potential factor contributing in ozonation of them.

3.2. FTIR Analysis of Ozonized PE MP Particles

3.2.1. FTIR Spectra of treated PE MP particles

FTIR spectra (Fig. S1) showed that ozonation has generated new functionalities on the surface of PE MP particles at the wavenumber of 1,030 cm−1 (C-O), 1,652 cm−1 (R-CHO with intermolecular H-bond), 1,683 cm−1 (R-CO-OR′), 1,718 and 1,720 cm−1 (RC(=O)R′) irrespective of varying dosages and reaction times, whereas the peaks for vinyl group (900–1,000 cm−1) was not observed. In particular, the absorption band at 1,720 cm−1 (>C=O, stretching ketonic group) reflects to the strong probability of aliphatic aldehydes, carboxylic acid and aliphatic ketones being occurred [31] as well as smaller amount of carboxyl groups as of intermediates [32]. In addition to, Kefeli et al. [33] reported that the ozonation of such saturated hydrocarbon chains forms peroxy-radicals, which is then converted into peroxides, ketones, and aldehydes [3436]. In other words, the appearance of these functionalities suggested by the fact that peroxy radicals formed on the polymeric chain of PE MP particles due to the attack of ozone and its derivates (i.e., reactive oxygen species) [28], furtherly generates hydroperoxides, ketones and alcohols [37], as presented in following Eq. (8)-(10):
(8)
RH+O3ROO+OH
(9)
2ROO+RHROOH+R
(10)
2ROORC=O+RC-OH+O2
Besides, a broad absorption band between 1,100 to 1,200 cm−1 could be resulted from the overlapping of different absorption groups including such as alcohols, esters, ethers and acids [38], which were also attributed to the attack of OH, HO2 and O2 on the surface of the PE MP particles. The generated OH played a role of initiator in the oxidation process by abstracting hydrogen atoms from the polymeric chain, leading to the formation of alkyl radicals (Equation (11)) which can eventually result in the generation of oxygenated functionalities. In addition to, they are responsible for the formation of O2 under the presence of promoters (e.g., ozone and primary alcohol) inducing the surficial oxidation of PE MP particles [26].
(11)
RH+OHH2O+R
In line with, the appearance of these carbon-oxygen functionalities is served as the major indicator of changes taken place from oxidatively deformed polymers [39] which can in turn provide the evident clue for their possible oxidative modifications occurred referring to the Norrish type I and II reactions. The Norrish type I reaction (Fig. S2 (a)) produces acyl and alkyl radicals. The acyl radicals further react with oxygen to produce polymer peracid radicals and followed by polymer peracids abstracting hydrogen. These polymer peracids can breakdown into hydroxyl radicals and polymer carboxyl radicals eventually converting into carbonyls together with other functional groups. This reaction may be terminated via crosslinking. Whereas, in Norrish type II reaction (Fig. S2 (b)), the hydrogen is abstracted from a ϒ carbon or polymeric chain-end is decomposed to generate carbonyl and vinyl groups [29].
In the meantime, the broad absorption peaks observed between 3,400–3,600 cm−1 or at 1,377cm−1 commonly assigned to the O-H bond of the alcohol groups. They might be arisen due to the reaction with another alkyl radicals which can produce double bond/cross-link, or reaction with the other reactive oxygen species leading to the formation of OH-containing functionalities such as alcohols and hydroperoxide on the surface of PE [38]. In particular, the peak height corresponded at the wavenumber of 3,555 cm−1 was appeared to be formed by the presence of hydroperoxides [40], ultimately contributing to the generation of various functional groups such as ketones [41] and concomitantly providing the active sites for the further oxidation process of PE [34].

3.2.2. Carbonyl index (CI)

As aforementioned, carbonyl groups, in this study, were likely to be occurred on the surface of PE MP particles through Norrish Type I reaction, since the newly appearance of vinyl groups was not found in the FTIR spectra. They were significantly increased once ozone injected in comparison with those of control, clearly implying that these oxidized functionalities (i.e., ketones and esters) were generated by the attack of radicals. For this reason, the carbonyl indices (CIs) of each treated PE MP particles were compared by calculating based on the suggested methodology in section of 2.3. As can be seen in Fig. 4, CI value for 180th min was tended to be remarkably increased as the ozonation has been initiated, which was then furtherly escalated to the highest value of 20% at the ozone dosed at 7 mg/min, even though their degrees of increment were relatively small in the comparison with those of other literatures to be observed [32, 42]. Nevertheless, it was comparably implied that shorter reaction time may not effectively facilitate in deriving that much of increase in the CIs comparing to that of the longer reaction time. It might be ascribed to the fact that tight molecular arrangement still firmly resisted against those of given shorter time of reaction, which in turn could make the longer treatment duration to be needed to furtherly oxidize.

3.2.3. Hydroxyl index (HI)

Fig. 5 presented the changes in hydroxyl index (HI) of PE MP particles after the ozonation treatment under 4 different dosages and three different reaction times, and it showed an overall increasing trend of HIs (approx. 13%) for a relatively longer treatment duration (i.e., 120 and 180 min), while shorter reaction for 60 min rather led to decreasing the HI values, even though ozone has been increasingly injected as high as 7 mg/min into the reaction. It simply meant that the higher increase of those of index for longer reaction time could be obviously justified by the fact that there have been more extent of generation of hydroperoxides resulting from such escalating ozone dosages and longer reaction time, which can be consequently subject to more conversion of carbonyl groups, as previously described. However, in the case of 60 min, HI had a decreasing trend despite of the increment of ozone dosages, because there might be not given enough reaction time to be consumed to form the amount of hydroperoxides demanded in modifying that of PE surfaces, even at 7 mg/min of inlet ozone dosage. Therefore, longer time duration could be more predominant controlling factor rather than that of ozone dosage in more significantly breaking down the surficial chemical structure of PE MP particles.

3.2.4. Crystallinity

Owing to their semi-crystalline structure [43], the crystallinity of PE MP particles has been changed during the surficial modification to be progressed. Chain scission resulting from Norrish I or II reactions reduces the entanglement density in amorphous phase, which allows for the shorter chained molecules to be more freely loosened and to recrystallize each of the loosened again to be attracted [39], consequently contributing to increase of crystallinity. On the contrary, the crosslinking mainly occurred by Norrish type I reaction decrease the crystallinity of polymer materials. In this study, the crystallinity has been declined all experimental conditions, in terms of 4 different dosages and three different reaction times, compared to that of unmodified (Fig. 6), implying that the Norrish type I reaction accompanied by cross-linking has been involved in the surface modification of PE MP particles, as aforementioned. Moreover, the relatively more prolonged reaction time could induce to the more inconsistent decrease of crystallinity in comparison with the shorter of 60 min. It is presumably supposed that the crosslinking between the PE molecules has been correspondingly broken down as a result of ozone uptake being increased to agreeably attack PE MP particles for relatively longer time.

3.3. X-ray Photoelectron Spectroscopy

In order to get the clear information for variation of the carbon-oxygen functionalities on PE MP surface after the ozone treatment for 180 min under the given 4 different ozone dosages, XPS spectra of them were comparatively analyzed with low- (Fig. S3) and high-resolution scan modes (Fig. 7), respectively. First, low-resolution scan modes for the control as of the unmodified and the treated PE particles were compared to each other with respect to their elemental composition and O/C ratio, which could distinguish to examine their possible surface oxidation to be occurred (Table 1). From this, carbon (C) and oxygen (O) were majorly detected along with a relatively small quantities of sodium (Na), nitrogen (N), and silicon (Si), which can be possibly derived from the plastic additives used in the manufacturing process. The O/C ratio of ozonated PE MP particles was gradually increased as higher as ozone dosage was applied, evidently indicating that the surface oxidation of PE has been associated into instigating more hydrophilic nature.
Second, to further explain the changes occurring on the PE surface, high resolution scans of the O1s and C1s region were obtained under four different ozone dosages as aforementioned, which were then deconvoluted into three sub-peaks of at the binding energy of 531 eV (O-H), 533 eV (C-O/C=O) and 534.5 eV (O-C=O) in the case of O1s region (Fig. 7 (a)–(e)), and 285 eV (C-C/C-H), 286.5 eV (C-OH), and 288 eV (C=O) for C1s region (Fig. 7 (f)–(j)), respectively. In regard to the O1s region, O-H and C-O bonds were initially formed in all PE MP particles ozonated, readily changed into C=O, which were further converted into O-C=O groups, depending on the inlet ozone dosages. For instance, it was observed that O-H groups are more dominantly formed on the surface of PE particles along with little amount of C=O/C-O even at 4 mg/min (Fig. 7 (b)). Further increasing ozone dosage initiated to convert O-H groups into carbonyl groups (Fig. 7 (c)) and consequently generated O-C=O groups at 7 mg/min (Fig. 7 (e)), being in agreement with the result of ozone uptake and FTIR analysis. Likewise, the deconvoluted C1s region (Fig. 7 (g)–(j) showed the increase in the degree of intensity of C-OH groups, whereas that of C-C bond was not significantly changed during ozonation process. It implied that surficial change occurs only in the amorphous region, not crystalline region where ozone hardly to penetrate, which has been verified from previous scientific publications [32].
On the other hand, the overall experimental data delineates that as greater as ozone dosage and as longer as treatment time can more favor the extent of oxidation process to be more taken place on the MP particles. However, in XPS result, it was observed that the relatively lower ozone dosage supplied to the reaction chamber, 4 and 5 mg/min, clearly favors the more increased formation of hydroxyl radicals [23], from which more than those of dosages further added were still kept in increasing to the level of the radicals although the ozone uptake was much higher increased. It was also demonstrated that ozonation of PE has a slow initiation stage due to tight packing of its polymer molecules, which require a longer treatment time (at least 6 to 8 h) for the penetrating into the crystalline region to break down the polymeric chain. In this aspect, the treatment duration of 180 min could be also still insufficient to effectively breaking down the polymeric chains or innovatively change the surface morphology of those.

4. Conclusion

In this study, PE MP particles were ozonated under the given ozone injection conditions in terms of varying ozone dosages and reaction time along with taking consideration for the quantification of ozone uptake to investigate any possible surface chemical modifications on PE MP particles as examined by the FTIR and XPS analyses. To sum up, it was concluded that shorter treatment duration was insufficient to support the higher ozone uptake by PE MPs, while increasing reaction time can enhance the level of ozone uptake rate and the degree of oxidation as having been verified with the CI, HI and crystallinity together with the variation in carbon-oxygen functionalities. These parameters overall indicated that the surface oxidation in the aqueous phase was mainly attributed to the attack of reactive oxygen species generated from ozone decomposition, which can make various functional groups on PE MP particles via Norrish type I reaction accompanied by crosslinking. In line with this, the optimum dosage and duration time were determined with 7 mg/min and 180 min, having a highest value of CI and HI while a lowest crystallinity after the surface modification of PE MP particles. Nevertheless, further research will be necessarily conducted to enhance the level of oxidation performance which can lead to more increasingly breaking down MP in natural waters.

Supplementary Information

Acknowledgment

This research was a part of project titled “Development of a water treatment system to remove harmful substances from ecological disturbances emitted from quarantine stations such as imported fishery products (No. 20180341)” supported by Korea Institute of Marine Science and Technology Promotion and funded by the Ministry of Oceans and Fisheries (MOF).

Notes

Author Contributions

R.Z. (M.S. student) designed and conducted the experiment along with data analysis and writing of the manuscript. S.Y.P (Ph.D. student) edited the manuscript and provided the critical feedback and contributed to shape the research study. C.G.K (Professor) approved all the experimental results and modified the manuscript.

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Fig. 1
Schematic diagram of ozone oxidation process on PE MP particles constituting of ozone generation system and reaction chamber.
/upload/thumbnails/eer-2020-412f1.gif
Fig. 2
The comparison of off-gas ozone flow rate in the presence/absence of PE MP particles under the given inlet ozone dosages of 4, 5, 6, and 7 mg/min for (a) 60 min, (b) 120 min, and (c) 180 min, respectively.
/upload/thumbnails/eer-2020-412f2.gif
Fig. 3
Percentage ozone uptake by PE MP particles during the ozonation at the ozone dosage of 4, 5, 6, and 7 mg/min for 60, 120, and 180 min.
/upload/thumbnails/eer-2020-412f3.gif
Fig. 4
The variation of carbonyl index (CI, 1,600–1,800 cm−1) estimated from the FTIR spectra for various ozone dosages and different treatment times in terms of 4, 5, 6, and 7 mg/min for 60, 120, and 180 min normalized to the reference absorption band at 1,472 cm−1.
/upload/thumbnails/eer-2020-412f4.gif
Fig. 5
The variation of hydroxyl index (HI, 3,200–3,600 cm−1) for various ozone dosages and different treatment times in terms of 4, 5, 6, and 7 mg/min for 60, 120, and 180 min estimated from normalization with reference band of 1,472 cm−1.
/upload/thumbnails/eer-2020-412f5.gif
Fig. 6
The variation of crystallinity for various ozone dosages and different treatment times in terms of 4, 5, 6, and 7 mg/min for 60, 120, and 180 min comparing to that of unmodified PE MP particles.
/upload/thumbnails/eer-2020-412f6.gif
Fig. 7
The comparative study of deconvoluted peaks for the control and the tested by high resolution spectra of O1s into O-H, C-O/C=O (a–e) and C1s into C-C, C-OH and C=O (f–j), respectively.
/upload/thumbnails/eer-2020-412f7.gif
Table 1
The Percentage of Element Composition of PE Particles and Their O/C Ratio from the Low-resolution Spectra of XPS after the Ozonation for 180 min
Element Unmodified PE Ozone Dosages
4 mg/min 5 mg/min 6 mg/min 7 mg/min
Na 0.12 0.18 0.13 0.11 0.22
N 0.11 0.23 0.62 0.82 1.88
Si 0.10 0.22 0.39 1.09 1.67
O 2.5 2.9 3.3 3.6 3.9
C 96.7 96.28 94.68 94.66 92.21
O/C 0.025 0.030 0.034 0.038 0.042
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