Chao
Zhang
a,
Xingyu
Liu
a,
Li
Zhang
b,
Qindong
Chen
a and
Qiyong
Xu
*a
aShenzhen Engineering Laboratory for Eco-efficient Recycled Materials, School of Environment and Energy, Peking University Shenzhen Graduate School, University Town, Xili, Nanshan District, Shenzhen, 518055, P. R. China. E-mail: qiyongxu@pkusz.edu.cn; Fax: +86-755-26035332; Tel: +86-755-26033226
bBureau of Agriculture and Livestock, Wongniute, Inner Mongolia 024500, P. R. China
First published on 21st May 2024
Polyethylene mulch films (MFs) are widely employed in agricultural land to enhance crop yield and quality, but the MF residue causes significant environmental concerns. To promote the sustainable application of MFs, it is essential to assess their fate throughout their service life and understand the underlying degradation mechanisms. In this study, surface-exposed and soil-buried MFs were separately collected from agricultural land in Inner Mongolia, China. The variations in aging performance and corresponding property alterations of MF were thoroughly examined. The results indicated that sunlight exposure considerably hastens MF degradation, whereas buried MFs experience a more moderate aging process due to the inhibitory effects of the dark and anaerobic environment on oxidation. Surface cracking was observed in MF-Light samples as a result of photodegradation, while chemical and moisture interactions with soil caused partial perforation in MF-Soil samples. Relative to the pristine MF, the oxidation, unsaturation, and hydroxylation levels of MF-Light increased to 0.88, 0.35, and 0.73, respectively, with corresponding values for MF-Soil at 0.44, 0.13, and 0.24. The generated oxygen-containing functional groups lead to a decrease in contact angles of MF-Light and MF-Soil, enhancing their hydrophilicity. The aging process of MFs led to a decline in mechanical properties, posing challenges for recycling. Moreover, nearly all phthalate esters (PAEs) were released from MFs, regardless of sunlight exposure or soil burial. The use of MFs also impacted the abundance of soil microbial communities. Specifically, the selected polyethylene MF enriched Actinobacteriota by 75%, while reducing Chloroflexi and Firmicutes by 27% and 45%, respectively.
Environmental significancePolyethylene mulch films (MFs) are widely employed in agricultural land, however, the MF residue causes significant environmental concerns. To promote the sustainable application of MFs, it is essential to evaluate their fate throughout their service life and understand the underlying degradation mechanisms. The aging process of MFs during their service life is a complex process. In this study, we aimed to: (1) explore the different aging properties of PE MFs under various conditions in a real agricultural environment; (2) reveal the interactions between the aging of MFs and their recycling properties, as well as their potential environmental pollution. This work is critical to understanding and controlling the plastic pollution caused by degradation of MFs and providing suggestions for their recycling. |
The utilization of MFs in China was not uniformly distributed, and some regions, such as Inner Mongolia, had a higher proportion of usage due to their cold and arid climatic conditions.7 The abundance of microplastics found in Inner Mongolia soil was 77.6 kg ha−1, which was considerably higher than that in other provinces, except Xinjiang.8 Additionally, Inner Mongolia experiences high levels of UV radiation,9 which is a critical factor that influences the aging process of MFs and affects their mechanical and physicochemical properties, making their recycling challenging.10 An in-depth comprehension of the mechanisms involved in the aging process and their impact on MF recycling can aid in the development of strategies to prolong the lifespan of MFs and reduce their environmental impact.
Current research on the aging and recycling of MFs showed mixed results, as the aging process of MFs during their service life is a complex process that involves several factors, such as UV radiation, temperature fluctuations, moisture, and chemical degradation. Aging conditions can have a significant impact on the quality and feasibility of MF recycling. Thermal oxidation reactions due to UV radiation and high-temperature conditions can cause polymer chain scission, resulting in a reduction in the film's tensile strength, elongation, and tear resistance.11 Exposure to chemicals, such as fertilizers, pesticides, and herbicides, can cause chemical degradation of MFs, leading to a decrease in their mechanical properties.12 In farmland, the edges of the MFs are secured with a generous amount of soil to fix the MF, and both abiotic and biotic factors affect the aging process of MFs. Photodegradation is the primary abiotic factor causing a loss of mechanical properties. In contrast, the MF buried in the soil undergoes a distinct aging process due to anaerobic conditions, absence of light radiation, and varying moisture content, temperature conditions, and microbiological environments. After harvesting, the MF should be removed from the land, while the aging and mechanical properties of the buried MF may be different from those of the sunlight-exposed MF, which should be considered during MF recycling. However, most studies on MF aging are conducted under laboratory conditions in simulated aging environments that may differ significantly from real-farmland conditions, and the buried MF, which has a significant contribution to plastic residue in the soil, has been ignored in previous studies.
In this study, MFs that were sunlight-exposed and soil-buried were separately collected from the farmland in Inner Mongolia after completing their service life. The morphology, chemical, thermal and mechanical properties of MFs have been studied extensively. We aimed to: (1) understand the aging mechanism of MFs exposed to sunlight and buried in soil in a real agricultural environment; (2) explore the property change of corresponding MFs under different aging conditions; (3) reveal the interactions between the aging of MFs and their recycling properties, as well as their potential environmental pollution.
Besides the MF, the soil with and without MF cover was collected in this study to investigate the effect of the MF on the soil environment. The cultivation, irrigation, and fertilization conditions, as well as the climatic conditions for all the soil samples, are the same except for the laying of the MF. The cultivated soil layer (0–20 cm) has the ability to supply nutrients and affect crop productivity, and the cultivated layer soil quality is directly affected by crop roots and farming conditions.13 In addition, Ma et al. (2018) found that mulching promoted soil moisture and nitrate concentrations in topsoil (0–20 cm), but had little effect in the deeper soil layer.14 So, the soil samples were collected from the cultivated soil layer with a depth of 0–15 cm in this study. Three soil samples with MF cover were randomly collected from different sites, and labeled as PT1, PT2, and PT3. The soil without MF cover was labeled as NONE. Soil samples were sealed and stored at −18 °C after receiving until further analysis.
(1) |
(2) |
(3) |
In addition, the chemical structure of the MF was determined using a Raman microscope (287 Q00, Renishaw, UK) that coupled a Raman spectrometer to an optical microscope (Leica DM 2500 M). The hydrophily of the MF was characterized by using a contact angle measurement instrument (ThetaLite, Biolin Scientific, Sweden).
Thermogravimetric analysis (TGA) was conducted using a TGA-50 analyzer (Shimadzu, Japan) to determine the thermal stability of the different MF samples. Samples of 5–10 mg were placed in aluminum oxide crucibles and heated from room temperature to 800 °C at a rate of 10 °C min−1 in the presence of nitrogen (10 mL min−1). Thermal analysis of the MF was performed using a differential scanning calorimeter (DSC-60 Plus, Shimadzu, Japan). Approximately 6 mg of the MF sample was sealed in an aluminum pan, then heated from room temperature to 150 °C with a heating rate of 10 °C min−1, and held at this temperature for 2 min to erase any thermal history. After that, the sample was cooled down to 35 °C with a cooling rate of 10 °C min−1, and then heated again to 150 °C again with the same heating rate. The melting temperature was measured during the second scan. The crystallinity of MF-Raw, Light and Soil was calculated from the DSC test according to eqn (4).
(4) |
FTIR-ATR was employed to examine the functional groups of the MF under different aging conditions (Fig. 2). The characteristic bands of PE were observed in all spectra of MF samples: the two representative peaks at 2914 and 2847 cm−1 are attributed to asymmetric and symmetric absorption of CH2 groups; the band located at 1462 cm−1 corresponds to bending and stretching of CH2 groups; the band at 719 cm−1 is due to the occurrence of C–H bending. The intensity of these bands decreased in both MF-Light and MF-Soil, with a more pronounced decrease observed in MF-Light. The same phenomenon was observed by Xiong et al., (2023),21 who attributed the decrease in peak intensity to the aging process and the oxidation reaction of the films. Compared to MF-Raw, an extra band at 1027 cm−1 corresponding to CO groups was shown for MF-Light and MF-Soil, indicating the generation of carbonyl groups. The bands between 1610–1800 cm−1 are attributed to –CC–, –CO of acids, esters, and ketones. The peak intensity at this region was stronger for MF-Light. The border bands between 1620–1660 cm−1 for MF-Light illustrated the development of unsaturation.11 This result was confirmed by the stronger peaks of CC wagging vibrations at 874 and 911 cm−1 for MF-Light.22
For MF-Light, the absorbed UV radiation led to the formation of free radicals due to the breakage of chemical bonds in the MF. The generated free radicals then react with oxygen in the air to form peroxyl radicals, generating –OH, CO, COOH, and COO– groups on the main chain of the MF matrix. In addition, the formed reactive intermediates such as free radicals and carbonyl groups can react with each other to form double bonds, leading to the development of unsaturation in the polymer. Hydroxylation could be caused by hydroxy and peroxyl radicals. All these reactions resulted in the chain scission of the MF. In this study, C.I., V.I. and H.H.I. as measurements of the oxidation level, unsaturation level, and hydroxylation level were calculated based on the FTIR spectra (Table 1). MF-Light had the highest C.I. and H.H.I. value because the free radical generation and oxidation degradation rate of the polymer were determined by the UV irradiation and the content of oxygen. MF-Soil was buried under the soil, where the dark and anaerobic environments inhibited the oxidation process, resulting in a much lower intensity of the peak at 1027 cm−1 compared to that of MF-Light. Soil inhibits the photoaging, while chemicals, metals, enzymes and microorganisms can cause gradual oxidation and degradation.23 PE is highly hydrophobic because of the presence of the linear backbone of carbon atoms, its degree of crystallinity and its high molecular weight.23 The generation of oxygen-containing functional groups and hydroxylation altered the hydrophobic nature of the MF surface. As shown in Fig. 2B, the contact angle of MF-Raw was 95.07°, while after use in the farmland, the contact angles of MF-Light and MF-Soil decreased to 73.31° and 86.82°, respectively. This finding is consistent with other research.24 In addition, the generation of oxygen-containing functional groups could make the MF adsorb organic pollutants and nutrients more easily,25 increasing the risk of environmental pollution. UV irradiation also resulted in the highest V.I. value of MF-Light. It was reported that the unsaturated MF is more susceptible to oxidative degradation than the saturated one. In addition, the unsatured MF is more brittle and exhibits a decrease in mechanical strength.
Sample | A 1650–1850 | A 1420–1500 | A 3150–3650 | A 1620–1660 | C.I. | V.I. | H.H.I. |
---|---|---|---|---|---|---|---|
MF-Raw | 0.26 | 2.49 | 0.45 | 0.08 | 0.10 | 0.03 | 0.18 |
MF-Light | 1.83 | 2.08 | 1.52 | 0.73 | 0.88 | 0.35 | 0.73 |
MF-Soil | 1.06 | 2.37 | 0.58 | 0.33 | 0.44 | 0.13 | 0.24 |
FTIR-ATR probes the near-surface region of a sample only, and Raman spectra are not influenced by the thickness of samples. It could provide additional information about contained pigments.26 As shown in Fig. 3, besides the typical PE bands at 1061, 1127 and 1300 cm−1, the band at 1530 cm−1 which corresponds to the pigment26 was clearly present in the Raman spectra of MF-Raw. After the aging process, the peak intensity at 1530 cm−1 decreased, indicating the release of the additives. The two Raman peaks at 2846 and 2881 cm−1 were the identification peaks of PE. Different from FTIR results, the aged MF, including MF-Light and MF-Soil had a stronger intensity at 2846 and 2881 cm−1 which corresponds to CH2 groups. This may be caused by the loss of additives. In addition, Raman spectra provided the crystallization structure of different MF samples. The bands at 1440 and 1460 cm−1 are assigned to the bending modes of the amorphous trans chains and amorphous chains, respectively.16 Compared to MF-Raw, MF-Light had a stronger intensity at 1440 cm−1, while a weaker intensity at 1460 cm−1 (Fig. 3C), suggesting the formation of trans chains in the amorphous phase, and the recrystallization occurred during the MF aging under light irradiation. This result could be confirmed from the DSC results in Fig. 4(B).
Fig. 3 The whole Raman spectra of different MF samples (A) and the Raman spectra in the wavelength ranges of 3000–2750 cm−1 (B) and 1800–900 cm−1 (C). |
DSC provided information about the melting and crystallization behavior of MF samples (Fig. 4B). Upon heating, MF-Raw showed melting peaks at 125.45 °C. After recycling from the farmland, the melting temperatures of MF-Light and MF-Soil were slightly increased to 128.05 and 127.79 °C. The crystallinity of various MF samples was determined using eqn (4). Studies about the PE aging process performed in the lab showed that the crystallinity decreases under UV irradiation, while our results showed that the aged MF in natural environments had a higher crystallinity than the pristine one, no matter under light or soil conditions (Table 2). It was probably caused by the loss of small-sized microplastics during the aging process. Chain scission during the aging process promoted the release of the entangled polymer chains in the amorphous regions, leading to an increase in crystallinity.10
Sample | T m (°C) | ΔHm (J g−1) | T c (°C) | X c (%) |
---|---|---|---|---|
MF-Raw | 125.45 | 55.95 | 107.07 | 19.47 |
MF-Light | 128.05 | 64.96 | 108.25 | 22.61 |
MF-Soil | 127.79 | 61.83 | 109.03 | 21.52 |
Compared to the crystallization temperature (Tc) of MF-Raw, the value of MF-Light and MF-Soil increased to 108.25 and 109.03 °C, respectively, during the DSC cooling process. The chain scission of MF amorphous during the aging process allowed crystalline phases to form at higher temperatures.10 These results suggested that the MF underwent chain scission, but the aging extent was not yet serious enough to cause crystallinity loss.
Mechanical properties are critical to predicting the service life of the MF and providing suggestions for MF recycling. Tensile testing is the dominant measurement employed to assess the ability of films to withstand external forces resulting from film recycling and weed growth. The tensile strength tests were performed to study the mechanical properties of the MF samples. As shown in Fig. 5, all the MF samples displayed a linear elastic behavior at first, after yield initiation, a highly none nonlinear elastic behavior started. The yield point is defined as the point where the strain starts to increase significantly for a small increase in tensile strength.27 The aged MF showed a lower yield than the pristine one. MF-Soil lost the mechanical strength most rapidly and had the minimum elongation. The higher moisture content condition in the soil environment may accelerate the dissolution of additives, leading to a deterioration of the mechanical strength. For MF-Light, photodegradation caused the polymer chains to break down, leading to film embrittlement and losses in mechanical integrity with aging. The MF needs to be recycled after harvesting to avoid the accumulation of plastics in soils. MF collection is difficult and time-consuming because the aging process decreases mechanical integrity. Mechanical collection may break the MF and result in a low recycling rate. Additionally, the MF that is partially buried in the soil is more difficult to recycle because it has lower mechanical integrity, as shown in Fig. 5. This should be considered when optimizing the recycling methods.
Fig. 5 The dynamic mechanical experimental system and the mechanical properties of different MF samples. |
In this study, DEHP was the main PAE detected in MF-Raw (peak observed at the retention time of 45.058 min, as shown in Fig. 6), while little DBP was detected. The difference between our result and Zhang's data7 may be due to the different origins of the MF materials. DEHP was not detected in MF-Light and MF-Soil, suggesting that almost all PAEs were released during the MF's service life. The PAEs in MF-Soil would finally be released into the soil, while most of the PAEs in MF-Light would be released into the air.29 It has been reported that the release of PAEs can reach 84% after 150 days of exposure under air conditions. The PAE content in film-covered soils was reported to be up to 5 times higher than that in soil without film cover.30 The release rate of PAEs may be related to their composition, concentration and the MF aging process. Understanding their release performance is important to control the plasticizer pollution. In future studies, the plasticizer release rate under varied conditions should be considered. It should be noted that a peak at a retention time of 43.089 min was shown for all the samples, including the blank, the MF-Soil, MF-Light and MF-Raw. This peak may be attributed to the interferences introduced during the extraction process. We did not discuss it in this study.
In this study, the microbial community composition and abundance at the phylum levels of bacteria were analyzed to investigate the effect of the MF on soil microbial communities. The MF covering has little effect on the composition of soil microorganisms. Actinobacteriota, Chloroflexi, Firmicutes, Proteobacteria, Acidobacteria and Genmmatimonadetes are the most abundant communities at the phylum level for all the soil samples, accounting for more than 90% of the microorganism sequences (Fig. 7A). This finding is consistent with the other research.33 However, the PCoA revealed that soil samples PT1, PT2, and PT3 grouped tightly in PC1 (64.77%) but had a big distance with sample NONE, indicating that all the MF-covered soil samples share a high similarity bacterial structure, and show a distinct clustering from the soil without MF covering (Fig. 7B). After MF covering, the abundance of Actinobacteriota in soil was increased by 75%; this result was consistent with current findings, which found that the accumulation of microplastics, especially those derived from PE, in soils could increase the abundance of Actinobacteriota.34,35 On the other hand, the abundance of Chloroflexi and Firmicutes dropped by 27% and 45%, respectively for the MF-covered soil. Chloroflexi tends to dominate in oligotrophic environments where N availability is low. The decrease of Chloroflexi suggested that the MF has altered the N cycling in the soil environment. Firmicutes are generally associated with natural organic polymers, and they would be also involved in the degradation of synthetic polymers such as PE.36
Fig. 7 Effect of MFs on the soil microbial community at the phylum level (A) and the PCoA on the operational taxonomic unit (OTU) level (B). |
Currently, most studies focus on the microplastic pollution of terrestrial soil in mulch film research. The results indicated that microplastics may affect the soil properties, as well as the soil microbial community.31 While PE can remain relatively stable in soils, and the field environment was open and had a relatively strong buffering capacity, the microbial community change caused by the microplastic derived from MF aging needs a long time to be observed. Wang et al. (2023) indicated that, compared to concentration, the shape of the MF residue displays a more pronounced effect on the soil bacterial community.33 The larger-sized MF, including the soil-buried MF during service life and MF residues after harvesting, may increase the possibility of damaging the structure of soil aggregates,37 resulting in the nutrient release. The bacterial community structure may be affected accordingly.
The microbial community results indicated that usage of MFs changes the soil bacterial communities, thus affecting the biogeochemical processes of the soil ecosystem. The results in Fig. 7 are only a rough estimate of the actual soil microbial diversity. The enzyme, fungi of the soil, and the microbial community on the MF residue surface were not detected. A long-time examination of the MF effect on the soil properties, as well as the microbial environment, should be conducted in further study.
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