Introduction
Agricultural plastic waste (APW) has been collected by the Korea Environment Corporation (K-eco) and the private sector. APW that has failed to be collected is either buried or incinerated, which has a negative impact on the environment. Various plastics involved in APW were reported to have the highest average annual incineration amount of 68.6 kg/yr, which accounted for 37.9% of the total agricultural waste incineration amount of 181.8 kg/yr (Ministry of Agriculture, Food and Rural Affairs (MAFRA; 2020). In particular, among domestic APWs, over 50,000 tons of Greenhouses LDPE film waste (GLFW) was generated annually from 2017 to 2020, but the amount collected in each of the same years was estimated to be less than 6 tons, with the exception of 2017 (K-eco, 2021).
The environmental effects of open-air incineration of APW result from the emission of particulate and gaseous pollutants. Emission factors for TSP, CO, and NOx to predict such effects were estimated in lab units, but were not applied to the Clean Air Policy Support System (CAPSS; Kim et al., 2012). On the other hand, the United States intensively manages polycyclic aromatic hydrocarbons (PAHs) emitted during open-air incineration of APW; they are classified as used/unused, and are reflected in the emission factor and open-air incineration category under the condition of blocking or artificial supply of outside air during combustion (US EPA, 2015). The EU also classifies APW open-air incineration into crop residues, pruning wastes, plastics, and general wastes, and manages them based on integrated emission factors; emission factors of average pollutant emissions generated from forest by-products and orchard crops are reported by waste type (EEA, 2019).
Particulate matter (PM) with an aerodynamic diameter of 10 μm or less has a practical effect on the human body (Bae, 2014), and a systematic analysis that reflects its physical characteristics (Feng et al., 2009) and chemical composition (Son, 2012) is required. This involves various analysis methods. For an analysis of the particle shape and chemical composition of particulate pollutants (PPs), the SEM-EDS is mainly used, which combines a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). It enables us to observe the surface shape of specimens at high magnification and analyze the chemical properties of specific particles without physical decomposition. In an SEM-EDS analysis reflecting back-scattered electrons (BSE) of APW collected in Huelva and Almeria, Spain, chlorine stemmed from potassium chloride (KCl) and sodium chloride (NaCl) was found abundantly in addition to single elements including Zn and Fe, or oxides (Picuno et al., 2012). In addition, X-ray fluorescence (XRF) also allows us to conduct such an analysis, which analyzes the emitted X-ray fluorescence radiation without damaging the global chemical properties of specimens. It facilitates quantitative analyses on the chemical species of large-area specimens. An analysis of the chemical homogeneity of PM10 and total suspended particulates (TSP) generated in the nonferrous metal industry through WD-XRF spectrometry found that the TSP of larger particle size collected by membrane filters was non-homogeneous (Vanhoof et al., 2003).
PPs have different effects on the human body depending on their chemical composition and particle shape. Based on previous studies, among the fine dust generated by coal fuel combustion in Hangzhou, China, PM10 mainly contained Na, Al, Si, S, K, Ca and Fe; S mainly had the shape of SO4 2−; and SO4 2−, NO3− , and NH4+ were analyzed as main ions accounting for about 20% of the mass of PM10 (Cao et al., 2009). As such, where heavy metals including Cr, Cd, Pb, Fe, and Zn are included in PPs such as TSP and PM10 or less, a serious health risk was reported to be posed (Leili et al., 2008). In addition, long-term exposure to PPs increases cardiovascular and infant mortality, and affects the respiratory and immune systems, exacerbating respiratory diseases including asthma, chronic obstructive pulmonary disease (COPD), and pneumonia, as well as cardiovascular diseases and diabetes (Kappos et al., 2004).
Air pollution from aerosolized particulates is attributable to fine droplets of liquid or solid particulates suspended in the atmosphere. Emission sources of PPs can be classified as natural or man-made, and the morphological characteristics based on the type classification can also be used to identify the source (Ramirez-Leal et al., 2014). PPs have various particle shapes depending on their chemical composition, including polyhedral, spherical, and irregular shapes. This was confirmed through analyses of various PPs: a study found that particulates reacted with various metals take the shape of irregular aggregates through an SEM-EDS analysis of chemical species of PM10 collected from three types of urban spaces (Ramirez-Leal et al., 2014); another study reported the same trend in mineral aggregates (Slezakova et al., 2008).
This study aimed to examine the harmful substances that could be emitted by the open-air incineration of GLFW with a low collection rate among APW. To this end, the chemical composition and particle shape of the PPs emitted were analyzed; and the physicochemical characteristics that affect whether they are harmful to the human body and the characteristics of chemical species according to the morphological type were determined. It is expected to provide basic data that can be referenced in establishing APW management policies.
Research Methods
Status of APW collected
The status of APW can be obtained from the data of the Statistical Survey of Agricultural Waste (MAFRA, 2020) by K-eco. National statistics have been aggregated every year since 2004 by dividing APW into LDPE for green-houses, LDPE for mulching, HDPE, and others. The population for the survey was set based on the result of the 2015 Agriculture, Forestry and Fishery Census as of 2021, and the statistical data were collected with a target error of 3.4%. The survey targets generated amount of plastic waste for greenhouses and field mulching, and pesticide container waste (PCW); collected and recycled amount of APW and PCW, which does not reflect the performance by the private sector.
Greenhouse plastic film waste is classified as low-density polyethylene (LDPE), and the amount generated in 2020 is 53,589 tons, which is estimated to be around 50,000–70,000 tons per year. LDPE for greenhouses was generated in approximately 50% the amount of LDPE for mulching and high density polyethylene (HDPE), but a significantly lower amount has been collected compared to other plastic waste since 2018 (Table 1). The Survey on Agricultural By-products and Waste Burning in Rural Areas conducted by MAFRA in 2020 presented the status of farm waste incineration. Of the 536 farmers who responded to the survey, a total of 79 had burned plastic waste, which arouses concerns about the effects of open-air incineration for LDPE for greenhouses, which have a remarkably low collection rate.
Experimental cosnditions
According to K-eco (2021), APW generation in 2020 totaled 307,159 tons, with LDPE for mulching the most at 156,422 tons by plastic type, followed by HDPE at 81,517 tons, LDPE for greenhouses at 53,589 tons, and others at 15,631 tons. Of those, LDPE for greenhouses, which had the lowest collection rate, was selected as samples for an incineration experiment in this study. The samples used for the experiment were 15 kg obtained from the Dangjin office of K-eco. U.S. EPA test method, Method 5G was applied with necessary modifications for the experimental infrastructure, as no domestic experimental methods related to APW open-air incineration had been reported. PPs generated during the burning of experimental specimens were collected; a filter box, damper, and blower were installed at the end of the flue so that PPs generated from the incinerator (stove) could be forcibly sucked through the hood.
Experimental method
Using an experimental infrastructure consisting of an incinerator and flue to which EPA Method 5G was applied, samples were put into the incinerator by 150 g of green-house LDPE film waste (GLFW) per experimental session. A flue gas analysis system (Testo 350 K, Testo GmbH, Germany) was used to monitor the behavioral characteristics of flue gas passing through the flue in real time after the incineration experiment was started (Fig. 1 ). Glass fiber filters (∅︀ = 47 mm) were inserted into the filter box to collect PPs. In an atmospheric monitoring study conducted in Hermosillo, Mexico, chemical species of PM10 collected using 8x10 inch quartz filters were analyzed with SEM-EDS (Ramirez-Leal et al., 2014). In this study, a chemical species analysis of PPs emitted from a total of 3 sessions of repeated incineration experiments was conducted before and after incineration. To compare the chemical properties of the samples before and after the incineration experiment, the samples weighing 5 to 10 g before the experiment and those of PPs collected from the glass fiber filters (∅︀ = 47 mm) inserted in the filter box were analyzed using XRF (S4 PIONEER, Bruker). A qualitative analysis of the samples was performed using the SEM/EDS system (Bruker) to analyze the weight ratio and particle shape of the collected PPs after incineration; the samples were pretreated with an ion sputter coater (G20, GSEM, Korea), and then PPs were examined for each sample. The X-ray spectra were set at an accelerating voltage of 20 kV under the control of the ESPRIT Compact software (Bruker), and the working distance (WD) and magnification were set at 11–13 mm and 100x, respectively. The average weight ratio of the elements to be repeatedly detected for 300 seconds for each sample was estimated. The weight ratio was repeatedly analyzed three times for each of the detected particulates.
Results and Discussion
Experimental environment
The environment of the incineration experiment was measured by classifying it into combustion time and weight, and average airflow volume and gas temperature (Table 2). The combustion time was found to vary depending on the sample. It seemed to be affected by soil or foreign substances non-homogeneously attached to the surface of collected GLFW. The actual combustion weight of the samples used in the experiment was measured to be 108.0–142.6 g. The average airflow volume was maintained in the same conditions in the minimum operating range of the blower installed in the experimental infrastructure (input power/power consumption: 260 W/432 W, maximum static pressure: 34 mmAq). The average temperature of the flue gas measured by the gas probe, a gaseous pollutant sampler, was measured to be 71.40–73.86°C, less than 100°C.
XRF analysis results
Transition metal Fe (26.87%, SD = 20.67) was the most common element detected through an XRF analysis of the surface of the specimens before the incineration experiment, followed by metalloid Si (25.91%, SD = 11.29) and other metal Al (16.43%, SD = 7.23). Metalloid Si (44.51%, SD = 0 .25) was the most detected element in the samples after the experiment, followed by transition metal Zn (16.53%, SD = 0.03) and alkaline earth metal Ba (15.73%, SD = 0.05). All of Fe, Si, and Al, which were detected at high frequency on the surface of the samples before incineration, had high standard deviations (SD = 7.23–20.67) due to natural deterioration and different degrees of adhesion of foreign substances to the surface; while, for other detected chemical species, homogeneous analysis results with small SDs were obtained for each sample. The element that decreased the most after burning the samples was found to be Fe (22.0 %p), followed by Al (11.4 %p) and Ca (6.9 %p), while the element that increased the most was Si (18.6 %p), followed by Ba (15.7 %p) and Zn (12.3 %p) (Table 3, Fig. 2). Ba was barely detected before burning the samples, but was detected at a high rate after burning. Since the element was reported to be emitted into the atmosphere during combustion of coal, fossil fuels and waste (Koukoulakis et al., 2019), it seemed to be detected at a higher frequency after burning. Previous studies using the same experimental infrastructure reported that chemical species corresponding to transition metals and metalloids were detected from the open burning of agricultural HDPE film waste through an SEM-EDS analysis (Kim et al., 2021).
The changes in chemical species during combustion can be inferred from a previous study that pointed out the aging and degradation of polymers as a cause of promoting interaction with metals on the surface of microplastics (Binda et al., 2022). Such degradation of polymers results in an increase in oxidized functional groups and surface charge, as well as in surface area and reactive sites. The oxidized functional groups generated by the degradation not only can hydrophilize the surface of polymers, but also form coordination bonds with metals at the same time (Kim et al., 2022). Therefore, for GLFW degraded by sunlight for a long period of time, it is considered that homogeneous changes were not observed before and after the incineration experiment, as the coordination bonds with and electrostatic attraction to metallic chemical species were different for each of the samples. After burning, Cl, Cr, Mo, Nb, Ni, and Sr were additionally detected (Table 3). Of those, Cl and Sr seemed to be derived from chlorine-based fertilizers and soil (White and Broadley, 2001), and Cr, Mo, Nb, and Ni from alloys for agricultural equipment (Kirchgaßner et al., 2008). A study also confirmed this, which examined chemical species remaining in the soil due to agricultural machinery work (Kostencki et al., 2021); they reported that agricultural machinery, which was made using alloys, low-alloy steels, and ultra-light alloys as main materials, may leave residues of those materials in the soil when worn out during soil cultivation.
SEM-EDS analysis results
Based on an SEM-EDS analysis, PPs were classified into solid and cotton-shaped particles according to the degree of detection and physical shape of metallic chemical species. The characteristics of such elements were determined through micrographs of fine particles as follows (Figs. 3 and 4). It was found that rock-shaped particles had a lower weight ratio of non-metal C (8.94 wt%, SD = 1.90) compared to cotton-shaped particles, but a relatively high weight ratio of other metal Al (11.77wt%, SD = 3.08) and transition metal Fe ( 9.58 wt%, SD = 7 .14). For cotton-shaped particles, the weight ratio of non-metal C (29.44 wt%, SD = 3.97) was high compared to that of rock-shaped particles, but that of other metals Al (2.34 wt%, SD = 0.19) and transition metal Fe (ND) was found to be relatively low, or too small to be counted. A significant weight ratio of transition metal Zn was found in metal particles (1.82 wt%, SD = 0 .19) and metalloids (2.59 wt%, SD = 0 .76) (Tables 4, 5, Fig. 5). As in the previous study (Kim et al., 2021), an SEM-EDS analysis of those chemical species found that the weight ratio of transition and other metals was relatively low in the fine particles with equal content of non-metals C and O compared to those with a low content of non-metal C.
In addition, the two types of particulate matter with such chemical properties were found to have morphologically different characteristics. The rock-shaped particles were observed to have hardened physical shapes with various sizes in the range of 10 μm to 20 μm, and mainly identified in particles with a size of about 20 μm. A study analyzing the physicochemical characteristics of PM for each urban space (Ramirez-Leal et al., 2014), reported that PM containing Si and composed of Al, Si, and Ca, or Al, Si and K are mainly in the form of angular polyhedrons, while PM with a high metal content including Fe, Zn, Ti, Cu, Mn, Pb, and Cr are in an atypical shape.
This was considered to be similar to the irregular shapes of rock-shaped particles found in this study. Compared to the rock-shaped particles, cotton-shaped particles with various sizes ranging from 5 μm to 20 μm were observed in the form of fluffy bundles, and were mainly small, in a size of 10 μm. The fine particles that were reacted with various metals maintained the same shape as an aggregate of small particles, which was similar to the PM reported by a previous study that analyzed mineral aggregates through SEM (Slezakova et al., 2008). An SEM analysis of the fly ash particles emitted from nine PC power plants found that mineral particles including Fe and Al-Si, or Ca-rich non-silicate mineral particles were mainly 1 to 100 μm in size and spherical solids (Kutchko and Kim, 2006). The cotton-shaped particles detected in this study were also found in soot aggregates from burned wood in another study (Feng et al., 2009). They reported that the collected PM2.5 was mainly composed of chemical species including C, Al, Si, and S, and had the form of cotton-like aggregates.
Since cotton-shaped particles have a shape that is relatively easy to scatter in the air compared to rock-shaped particles, a wide-ranging impact is predicted during open-air incineration. Of the transition metals, Zn was detected at a significant level in the cotton-shaped particles (Fig. 5). If Zn is absorbed into an organism, it may cause the generation of reactive oxygen species (ROS). ROS are involved in the intricate balance of control mechanisms that contribute to cell survival or death through intracellular signaling and chemical communication between cells (Habtemariam, 2019). Transition metals such as Cu, Ni, and Cr, which are detected in trace amounts in an XRF analysis of samples after incineration, also can cause oxidative stress in cells and tissues (Lodovici and Bigagli, 2011). According to a study on the effect of metal ions on lung cell damage and inflammation using soluble sample EHC-93 treated with atmospheric dust, of several metal ions, only a pure zinc salt induced an increase in inflammatory cells and proteins (Adamson et al., 2000). In addition, when zinc was inhaled during commercial production, complications developed, including fatigue, chills, fever, muscle pain, cough, dyspnea, leukocytosis, and thirst; and inhalation of large amounts of zinc chloride (ZnCl2) resulted in respiratory diseases such as edema in the alveolar surface (Cooper, 2008). Therefore, when GLFW that failed to be collected is open-air incinerated, the effects of certain transition metals such as Zn, which have been confirmed to be harmful to humans, may occur widely, so it is considered that appropriate measures are required.
Conclusion
Although agricultural waste has been systematically collected since 2017, a high percentage of GLFW has failed to be continuously collected since 2019. As APW that is not collected is often buried or burned at agricultural sites, it is necessary to consider the environmental impact it causes. Therefore, this study conducted an open-air incineration simulation of GLFW with a remarkably low collection rate.
An analysis of chemical species constituting PPs emitted during incineration of GLFW belonging to APW was performed using XRF before and after incineration; and the physicochemical and morphological characteristics of PM were analyzed with SEM-EDS. An XRF analysis found that elements such as Ba, which were not detected before incineration, were emitted in large amounts in the combustion process after incineration. In addition, trace amounts of Cl, Cr, Mo, Nb, Ni, and Sr were also detected, which are presumed to be derived from fertilizers, soil, or alloys for agricultural equipment. Based on an SEM-EDS analysis, PPs were classified into solid and cotton-shaped particles. It was found that cotton-shaped particles had a high weight ratio of non-metals, and low weight ratios of transition and other metals. Although Zn is a transition metal, it was detected from both types of particles, suggesting that it may be harmful for the human body when incinerated. In terms of morphology, it is expected that cotton-shaped particles, which are easier to scatter in the atmosphere than rock-shaped particles, will have a wide-ranging effect during open-air incineration. Therefore, it is considered that further studies on their specific behavioral characteristics are required.
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