https://orcid.org/0000-0002-7474-1915
Figures
Abstract
Microplastics (MPs) contaminate every conceivable terrestrial and aquatic environment including high peaks and deep marine trenches. Agricultural lands alone are expected to receive plastic up to 23 times more than ocean basins. In this study, soil samples were collected from peri-urban agricultural lands of Lahore on four sides including Kala Shah Kaku (KSK), Punjab University (PU), Dera Gujran (DG), and Sagian (SG). National Oceanic and Atmospheric Administration (NOAA) protocol was used for MPs extraction and analysis. Extracted MPs were analyzed under microscope at 40X magnification and their composition was analyzed using Fourier Transform Infrared (FTIR) spectroscopy. A considerable concentration of MPs was recorded at all sites. The highest contamination was found at SG with 876 ±194 MPs/kg of soil, and the lowest contamination was recorded at PU with 672 ±235 MPs/kg of soil. However, these differences among the sites were not statistically significant (p = 0.29). The overall predominant shape of MPs was fibers (613±71, 79.73%) followed by sheets (125±55, 16.28%), fragments (30±5, 3.9%) and foam particles (1±2, .09%). The differences in the distribution of MPs in various types were statistically significant (p = 0), while differences between sites were insignificant (p = 0.13). About 95% of MPs were less than 2 mm and 85% were less than 1 mm size. The distribution of MPs in various sizes (p = 0) and differences of this distribution between sites (p = 0.037) were both statistically significant. A good diversity of nine colored MPs was recorded, however majority of the MPs were transparent (89.57%). Six polymer including Polyethylene (PE), Polyethylene terephthalate (PET), Polypropylene (PP), Polystyrene (PS), Polycarbonate (PC), and Polyvinyl Chloride (PVC) were identified by FTIR. The current levels of MPs pollution are higher than in many other parts of the world. Composition of MPs (types, colors, sizes, and polymer types) indicates the diversity of their sources and their possible implications on agricultural ecosystem.
Citation: Luqman M, Shahid T, Awan MUF, Kashif SUR, Arooj F, Awan AR (2023) Quantification and characterization of microplastics (MPs) pollution in peri-uburban agricultural lands of Lahore, Pakistan. PLoS ONE 18(10): e0291760. https://doi.org/10.1371/journal.pone.0291760
Editor: Amitava Mukherjee, VIT University, INDIA
Received: April 26, 2023; Accepted: September 5, 2023; Published: October 3, 2023
Copyright: © 2023 Luqman et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: all the data is in the paper.
Funding: The authors received no specific funding for this work. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Plastic production, its indiscriminate use and mismanagement of plastic waste are the burning environmental issues. When this durable polymer is exposed to temperature fluctuations, weathering, erosion and UV rays, it disintegrates into smaller components of < 5 mm in size, called microplastics (MPs) [1]. Any synthetic or semi-synthetic polymer with thermoset and thermoplastic characteristics is defined as plastic [2]. Plastics can be synthesized from hydrocarbon chains or biomass raw materials. Production of plastic has seen an exponential growth since its arrival on the consumer stage. Owing to being cheaper, durable, versatile and portable, the plastic have become an integral part of human life [3]. Today, our globe without plastic and polymers seems unrealistic and unimaginable. Plastic was not used outside of the military sector until the end of World War II but now plastic production has surpassed every type of man-made material in the world [4]. Plastics are polymers that can be reshaped and reused after heating. Many types of plastic are produced synthetically including nylon, polyester, polystyrene (PS), polyethylene (PE), polyvinyl chloride (PVC), Teflon, low- and high- density polyethylene (LDPE and HDPE), etc. The global plastic production in 2018 was close to 360 million tons, for which China (30%), Europe (17%), and North America (18%) primarily manufactured the raw materials [5].
A large amount of plastic waste is released into the environment due to mismanagement and mishandling of solid waste [6]. It reaches oceans, beaches, agricultural lands and water reservoirs making them hotspots of plastic pollution. The released plastic waste disintegrates into smaller size debris by UV rays, weathering and other chemical, physical and biological drivers [7]. Polymers are also degraded by photochemical reactions which include thermal, photo and mechanical degradation while hydrolysis and oxidation contribute to chemical and biological pathways [8]. Once disintegrated, they become macroplastics (<25–5 mm), MPs (5 mm-1 μm), nanoplastics (<1 μm), and some large particles [9]. MPs are released into the environment as primary or secondary MPs. MPs that are originally manufactured in microscopic size particles are primary MPs [10]. Secondary MPs are produced due to degradation, disintegration and fragmentation of large size plastics [11]. MPs types include fibers, films, fragments, foams and pellets which come from numerous sources manufactured and used in the environment [12].
MPs are exposed to a range of chemicals after releasing into the environment and cause direct as well as indirect toxicity to human beings, terrestrial and marine biota [13]. Direct toxicity is caused by stabilizers, dyes and different toxic additives/chemicals [14]. Common additives of plastics like phenols, and esters continue to spread in surroundings, leading to lasting and severe impacts on all life forms and the environment [15]. Chemicals like polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) can attach to their surfaces causing combined toxicity [11]. Indirect toxicity is caused by the hydrophobic nature of microplastic and their surface area to volume ratio. Numerous chemicals are absorbed onto the surface of MPs and they react with the chemicals when exposed to sunlight, oxidation and other chemical processes, becoming an entirely new species and causing toxicity in a new way [16]. Common MPs like PE, PS, PP and also PVC can adsorb organic pollutants 3–6 times more in the presence of co-existing surfactants [17,18].
MPs pollution in agricultural soils is a recent challenge for environmental sciences and agriculturists as sources, release, transportation, fate and effects of MPs in terrestrial ecosystem are relatively little acknowledged compared to an aquatic environment [19]. MPs are likely to be present more abundantly in soils, where they are produced, compared to aquatic ecosystems. MPs in the soils may pose risks to food security, food quality and human health [20]. MPs can enter the agricultural soils through various means like use of plastic based biosolids, irrigation with wastewater and use of plastic mulches [21]. Complex nature of soil matrix can increase the aggregation level of MPs causing ambiguous threats to the crops [22]. Further, plastic tunnels for vegetables, plastic labels and protection covers of vegetables and fruits are noteworthy origins of input of MPs in agricultural soils [23]. Compost, produced by biodegradable waste material, is used as fertilizer in every part of the world. Inappropriate waste separation can lead to a large fraction of plastics and MPs (when disintegrated) into the soil [24]. Soils act as long-lasting sink to accumulate MPs leading to affecting the agroecosystems even more adversely. In addition, different farming practices like ploughing, irrigation, and fertilization lead to horizontal and vertical movement of MPs in soil [25]. These MPs can further reach to water channels via agricultural runoffs ending up in aquatic environments [26].
MPs in agricultural soils affect plant communities directly, as increasing fragmentation leads to a decrease in size of MPs, through accumulation of various sizes and shapes in root, stem, and leave [27]. Furthermore, MPs can reduce the uptake of water and affect the nutritional status by blocking seed pores. Indirect impacts of MPs include the varying physiochemical attributes of the soil and soil-dwelling microbes [28]. Introduction of MPs into soil leads to increased soil water evaporation and desiccation of surfaces which further affect vertical uptake of water [29]. Phytotoxicity, and significant reduction in maize biomass and chlorophyll content in leaves have been observed due to soil contamination with Polylactic Acid (PLA), which is thermoplastic polyester [30]. MPs with heavy metals can drive changes in plant performance [30]. Edible plants are directly subjected to MPs when manure, bio fertilizers and plastic mulches are applied. MPs can pose a serious threat to crops as they may block seed pores and can limit the uptake of nutrients and water from roots [28]. They can also impact soil organisms like earth worms and are vertically moved through their feces [28]. Presence of cadmium (Cd) with a high concentration of polylactic acid (PLA) may cause a strong phototoxicity [30]. Coexistence of MPs and Cd can jointly drive shifts in plant performance and root symbiosis, thereby posing additional risks for agro-ecosystems and soil biodiversity [30,31].
Lahore is a capital city of the Province of Punjab, Pakistan with about 14 million population [32]. Non availability of canal water, high costs of electricity and fertilizers, and lack of regulations convince the farmer community to use untreated wastewater (purely or mixed with irrigation water) for growing vegetables and crops in peri-urban agricultural lands of Lahore [33]. The use of wastewater (containing domestic sewage and effluents from hundreds of textile and other industries) has the potential to bring lots of MPs in such agricultural lands. The use of plastic tunnels to grow vegetables and dumping of domestic and construction waste near these lands further aggravate the situation. These practices are posing serious risk of microplastic (MPs) pollution in agricultural soils and threat to agricultural ecosystem and human health. The status of MPs pollution in these agricultural lands was direly needed. Under these circumstances, this study was conducted to assess the magnitude of MPs pollution in peri-urban agricultural lands of the Lahore, Pakistan.
Materials and methods
Sampling sites
Lahore, the second largest city of Pakistan, is a business, residential and industrial hub of the country with an area of 1,772 km2. It receives a large amount of plastic waste. In this study, four peri-urban agricultural lands in the vicinity of Lahore were selected to quantify the level of MPs pollution in soil. One acre from each sampling site (northern, southern, eastern and western side of the city) was selected for sampling (Fig 1). Soil sampling for research from peri-urban agricultural lands of Lahore required no permit from any city or provincial authority, hence no permit was obtained. Agricultural lands including Punjab University (PU), Sagian (SG), Kala Shah Kaku (KSK), and Dera Gujran (DG) are neighbouring residential areas of Lahore city.
Punjab University (PU) site is situated in about 8 km2 pocket of agricultural land touching the western border of the Lahore city. This pocket is dedicated to the University of the Punjab’s new campus. Wheat and fodder are the main crops grown in this pocket of agricultural land. Vegetables are also grown on some acres. Use of plastic tunnels to grow vegetable has not been observed in this piece of land during winter season. There is no nearby slum to this land except University Faculty residential area and properly managed societies which are always much cleaner than slums. Impact of improper disposal of plastic bags and waste from nearby population is relatively less. This site is one kilometer away from Wahdat Road (Light traffic, with high intensity) and Maulana Shaukat Ali Road (Light traffic with high intensity), and 1.25 kilometer from Canal Road (Light traffic with high intensity).
Sagian site is a pocketed agricultural land in outer urban sprawl, about seven KM west of Lahore city’s outer perimeter. This site is 350 meter away from M2 Motorway (Heavy and light trafiic with high intensity) and about 100 meter from Lahore-Jaranwala road (light traffic with low intensity). Main crops include fodder and vegetables with continous use of low volume of domestic sewage and industrial wastewater. Use of plastic tunnels in winter to grow vegetable has also been seen frequently on this site. Compost ferlizers are also used at this site. Significant impact of improper dumping of domestic waste (including plastic waste) is evident on this site due to nearby low-income population in unplanned slums.
KSK site is seven kilometer in north of lahore city’s outer perimeter. Two major roads including M-11 motorway (light traffic, moderate intensity) and N5-GT road (light and heavy traffic, high intensity) are about 2.5 kilometer and 2.7 kilometer away from this site respectively. Two other roads also pass more than 2.5 KM away from this site i.e. Kalakhatai road (light traffic, low intensity) and Lahore bypass road (Light traffic, low intensity). Wheat, rice, and fodder are main crops on this site, while vegetable are also frequently grown. Tunnel farming is used to grow the non-seasonal vegetables in winters. Use of the compost ferlizers is a usual phenomenon at this site. Occasionally, to reduce input costs, farmers use untreated wastewater for growing vegetables. Impact of improper dumping of domestic waste is common from residents in nearby settlements.
Dera Gujran (Site) is 3.5 KM in the east of Lahore city’s outer perimeter. This site is one kilometer from the Lahore ring road, which has light traffic of low intensity. Rice and wheat are the main crops in these lands, however pockets of fodder are also frequently grown. Lakhoder landfill site is two kilometer east of this sampling site.
Sample collection
Samples were collected on clear sunny days of winter season in the month of December 2021. Five replicate soil samples were collected from each sampling site randomly (one sample from each corner of the acre and one from the centre). The total number of samples taken for the study was twenty (n = 20). Samples were collected using a quadrant of one square foot by placing it randomly at each sampling site and upper 10 cm of soil layer was taken as sample with the help of a spade. Soil samples were taken in aluminum bags and transported to the laboratory for further analysis.
Sample preparation and density separations
MPs were extracted by following the standard protocol of National Oceanic and Atmospheric Administration (NOAA) for MPs extraction from sediments [34]. Samples were sun-dried through the windowpane in the laboratory in trays covered with aluminum foil, for 24 hours, to avoid plastic contamination and stored in glass jars. Samples were initially sieved with a sieve of 5 mm mesh size to remove all the particles greater than 5 mm. Soil of 200 g from each sample was added into a 1000 mL glass beaker and dried overnight in the oven at 80°C to dry completely. Samples were then subjected to density separation to extract floating materials by adding 500 mL of supersaturated NaCl (5 M) solution (density = 1.15 g/mL) to the beaker of dried sample. Mixture was vigorously shaken with the help of a metal spatula for three minutes to float out MPs and beaker was kept on rest for an hours. Then floating solids were sieved through 0.06 mm mesh size customized sieve. Walls of the beaker were rinsed carefully with 5 M NaCl solution and all the residual solids were poured into the sieve. The procedure was repeated five times to extract all the floating material on the sieve, while the settled solids were discarded in the end. Debris collected over the 0.06 mm sieve was put in the glass beaker and oven dried at 80° C for 24 hours or longer until dryness. Although, the supersaturated NaCl solution have some limitations to separate out few high density polymers, yet most commonly used chemical in majority of protocoals including NOAA.
Digestion by wet peroxide (WPO)
Digestion by WPO method was done following the standard protocol of digestion described by NOAA for MPs extraction from sediments [34]. Dried matter was subjected to the process of wet peroxide digestion by 0.05 M Fe (II) solution and 30% hydrogen peroxide (H2O2). Iron salt used in this process was FeSO4.7H2O. Twenty milliliters of 0.05 M Fe (II) solution was added to the beaker of dried solids, followed by 20 mL of 30% H2O2. Mixture was left in the laboratory at room temperature for five minutes and then heated on a hotplate with magnetic stirrer at 75°C until the bubbles appeared on the surface. Then beaker was placed in fume hood until “effervescence” was settled. Distilled water was added to slow down the reaction in case of overflow. Process of heating and addition of more H2O2 was done until all the organic solids were invisible and mixture became transparent. After completion of oxidation process, six grams of NaCl per 20 mL of sample was added to make the aqueous solution denser and mixture was heated to 75°C to dissolve the salt.
Density separation
For density separation, WPO solution was transferred to density separator made with funnel and clamped rubber tubing. Beaker was rinsed carefully with distilled water to transfer all remaining debris to the funnel, which was then loosely covered with aluminum foil for overnight stay. Settled solids were visually inspected to see if any visible plastic particle was present. In case of visible MPs, particles were separated with forceps and the remaining material was drained. Floating particles were collected in a custom 0.06 mm clean sieve and the density separator was rinsed several times with distilled water. The sieve was air dried for 24 hours while loosely covered with aluminium foil to avoid contamination.
Quality control
During sampling, transportation, drying, sieving, density separation and microscopic analysis, all the potential hotspots of cross contamination and particle loss were taken care to minimize the issue. No plastic container or equipment is used during the whole process. Distilled water used in the laboratory and all the reagents were filtered through 11 μm filter. All the glassware and apparatus used in MPs analysis was washed with filtered distilled water and kept covered with aluminium foil. A well cleaned part of the laoratory was dedicated to MPs research and no one was allowed in that area with any plastic and synthetic fiber bearing attire, cap, hair accessories, cosmetic use, googles, etc. Further, a procedural blank sample (wet filter paper) was parallel run with each sampling site to find the numbers of false positive particles from the environment. Number and types of MPs found in the blank were deducted from the total MPs count of each replicate. Total four procedural blanks (quadruplicate) were run along the samples. Blank sample (wet filter paper) sealed in aluminium bag were tranported to the sampling sites, exposed in the same environment in the field and put back into the aluminium and transported to the lab along-with samples. Representativeness of samples at each site was ensured by taking five replicates. Blank wet filter paper was kept with the samples in a beaker in the same laboratory environment till the digestion step. Then it was rinsed well in the same beaker and analysed for any MPs contents paralel with samples.
Microscopic examination and FTIR analysis
MPs analysis was done at 40X magnification under microscope to identify the shapes, colors, and sizes of MPs on the sieve. Larger MPs (>2.0 mm), in visible range with naked eye, were picked from sieve with the help of forceps and transferred to clean glass vial for composition analysis of polymer types by Fourier Transform Infrared Spectrometry (FTIR). The compopsition of polymer type was determined by acquiring spectra using Therrmo Scientific Nicolet 6700 FTIR within the wavelength range of 4000–500 cm-1 with 8 sample scans and 8 background scans per spectrum, and resolution of 4 cm-1. Smple spectra were analyzed with Thermo OMNIC software and compared with OMNIC polymer spectral library for identification of polymer type. Minimum 70% recognition limit was set for polymer type identification.
Results
During the current study, average abundance of MPs recorded was 672 (±235), 876 (±194), 831 (±220), and 699 (±54) MPs/kg of soils at Punjab University, Sagian, Kala Shah Kaku, and Dera Gujran sites respectively (Table 1). Highest contaminated site was Sagian (876 ±194), while the lowest was Punjab University (672 ±235). Most contaminated individual replicates were belonging to Sagian and Kala Shah Kaku (Sagian R-5, KSK-R-2), while least contaminated replicate were Punjab University R-1 and R-2. The observed differences in abundance of MPs among the four sites were not statistically significant (p-value 0.29).
Various types recorded during the current study included fibers, sheets, fragments and foam (Fig 2). Fibers were dominant at all sites ranging from 76–90% of the recorded types (Fig 3). Second most frequent type of MPs was sheets (7–20%), while third dominant type was fragments (3–9%). Foam particles were found very few in number (0.34%) and only at Sagian site. Collectively, including all the MPs at all replicates from four sampling sites, 79.56% were fibers, 16.42% were sheets, 3.93% were fragments, and only 0.09% of them were foam particles (Fig 4). Highest numbers of fiber were found at Sagian, R-5 with 945 fiber/kg of soil, while minimum numbers were at Punjab University R-2 with 315 fiber plastic per kilogram of soil. Sheets were next to fibers, and then fragments, while foam particles were recorded only in one replicate at Sagian R-2 with 15 numbers per kilogram of soil. Mean values of MPs types varied across all the four sites (Table 2, Fig 5). The differences in the distribution of MPs in various types at the four sites were statistically significant (p-value of 0.0) (Table 2). However, the difference in distribution of various types across the four sites is not statistically significant (p-value 0.13). Also the difference between the sample averages of all groups is not big enough to be statistically significant (p-value 0.19).
Images of some MPs in peri-urban agricultural lands of Lahore, Pakistan; A) fiber, B) film, C) fiber, D) fragment, E) fragment, F) fiber, G) sheet, H) sheet, I) fragment, J) fiber, K) fragment, L) Foam palette, M) sheet, N) fragment.
Majority of the MPs found in peri-urban lands of Lahore were smaller in size. About 95% of MPs were less than 2 mm size and 85% were less than 1 mm (Table 3). About 41% of MPs were in the size range of 0.25–0.50 mm. Only 2.8% of MPs were found in the size range of 2–5 mm. Smallest size in the current study was 0.0625–0.125 mm and only 1.9% of the MPs were found in this range. Larger sized MPs (≥ 4 mm) uniquely were not recorded at Sagian and Kala Shah Kaku sites at all (Fig 6). Fibers were generally present in all size fractions, maximum being in size range of 0.25–0.5 mm (Fig 7). Next most occurring shape in all the size ranges was sheet, maximum being in size range of 0.25–0.5 mm. Foam particles were only observed in size range of 2–4 mm and were only 15 in number (Fig 7). The differences in the distribution of MPs in various sizes were statistically significant (p-value of 0.0) (Table 4). The differences in size distribution of MPs across the four sites were also statistically significant (p-value 0.036).
In current study, MPs were observed in nine different colors including red, green, white, purple, yellow, blue, black, brown and transparent (Table 3, Fig 8). Majority of them (about 90%) were transparent consisting of fibers, sheets and fragments. The largest fraction of transparent MPs was reported in Sagian soil samples. Further, the black color MPs were only observed at Sagian site.
MPs in the size range of 1–2 mm have shown highest color diversity with green, red, white, blue, purple, black and yellow MPs (Table 2). Only transparent MPs were found in the smallest size range fraction of 0.0625–0.125 mm, however, next size range from 0.125–0.25 mm, in addition to transparent particles, green, red, blue and white MPs were recorded. Other colors including blue, purple, brown, black, and yellow were absent altogether in size from 0.0625–0.25. Larger size fraction has shown greater color diversity compared to lower size.
Regression equations show that there was a relationship between numbers of transparent, green, brown, red, and blue color MPs with their size (Fig 9). Their number increases with decrease in size and vice versa. However, such relationship was not observed for white, black, purple and yellow.
Six types of plastic polymers are recorded in the current study including Polyethylene (PE), Polyethylene terephthalate (PET), Polypropylene (PP), Polystyrene (PS), Polycarbonate (PC), and Polyvinyl Chloride (PVC) (Fig 10). Total 75 fragments of more than 2 mm size (>2–4 mm = 30 MPs, ≥ 4 mm = 45 MPs) were picked up for FTIR analysis. Among these, 16, 22, 24, and 13 particles were from PU, SG, KSK, and DG sites respectively. Of the 75 particles analysed by FTIR, 39 (52%) were PE, 14 (18.6%) PET, 11 (14.6%) PP, 3 (4%) PS, 2 PC (2.6%), and 6 (8%) PVC (Fig 11).
Discussion
Abundance of MPs in agricultural soils recorded during the current study was higher than reported in many countries including Mauritius, Hungary, Chile, Germany, Japan, and Thailand, yet lower than many others (Table 5). Although the observed differences in abundance of MPs among the four sites were not statistically significant, PU and DG sites were apparently relatively cleaner with less MPs concentration in the soil compared to SG and KSK. Factors including use of plastic tunnels, plastic mulches, compost fertilizer, little or no wastewater were lacking at these two sites compared to SG and KSK where these factors were conspicuous. Also at PU and DG sites, no vegetables were grown in contrast to SG and KSK. For vegetables, the use of wastewater, tunnels and mulches is common in Pakistan compared to major crops like wheat, rice and fodder. Further near PU site no slums and dumping sites were spotted. Although DG site was only 2km from a city’s dumping site, yet relatively low MPs were recorded probably due the lack of earlier mentioned factors for SG and KSK. At SG and KSK sites there were slums and dumping sites for garbage including plastic. SG and KSK have nearby roads with heavy traffic compared to PU and DG that have light traffic roads near them. SG, which is most contaminated site has motorway with intense heavy and light traffic just 350 meters away.
In current study, four types of MPs including fibers, sheets, fragments and foam were recorded in the soil samples of peri-urban agricultural lands of Lahore. Fiber was the most abundant type of MPs found following sheets, fragments and foams. Fibers were 79.73%, while sheets, fragments and foams were 16.28, 3.9 and 0.09% respectively. This relative abundance of MPs types is similar to the results reported in the shallow soils of Shanghai, China, where fibers were found in highest percentage (53.33%) followed by fragments (37.58%), films (6.67%) and pelettes (2.12%) [52]. Similar results were recorded by Zhang and Liu in 2018 [53] in agricultural lands of south-western China, where fibers were recorded up to 92.1% of all MPs found while little concentration of fragments (4.1%) and films (3.7%). Supporting results were also recorded by several other studies while exploring MPs concentration in agricultural lands [54–56]. Greater concentration of MP fibers in the current study is suspected to be contributed by city dust from Lahore. Further reason could be the use of sewage water mixed with industrial effluents especially from textile industries especially at SG and KSK sites. Similarly MP fibers could be significantly contributed from domestic sewage where laundries are producing lots of fibers. Similarly polyester, acrylic, nylon, and several other plastic fibers used in winter clothes do also contribute to city dust and ultimately to this fraction of MPs in peri-urban agricultural lands of Lahore. According to Lahore Chamber of Commerce and Industry (LCCI), there are about 984 textile industries registered with LCCI [57]. Smaller hosiery and yarn setups are apart from these registered large textile setups. Textile industry seems to be one of the major contributors to the fiber pollution and they can reach agro-ecosystems through air, water, waste and other technologies using on the site. The second abundant type of MPs was sheets or film, which might have come from the agricultural practices like plastic mulching, plastic tunnels, and dumping of waste including plastic especially at SG, KSK sites. Possible sources of fragments could also be the waste material consisting of a range of plastic utensils, personal products, pet bottles, etc. in the dumping sites. The distribution of MP types across the four sites was not statistically significant despite the apparent presence of all of the types at SG and KSK sites in greater numbers. However, distribution of MPs in various types was significant at all the four sites. The possible factors for fibers, sheets, and fragments could be the use of plastic mulches, plastic tunnels, compost fertilizer, wastewater, dumping sites, and nearby slums.
Majority of the MPs (95%) found in the current study were less than 2 mm size and 85% were less than 1 mm in size. The distribution of MPs in various sizes and the differences in size distribution of MPs across the four sites were statistically significant. The current pattern of size fraction of MPs is similar to a pattern fractions of MPs recorded in agricultural lands in China, where 84.4% of total MPs were ≥ 1 mm and only 5.9% of MPs were 2–5 mm in size [56]. In terrestrial environment like agricultural lands, sporadic exposure to sunlight, humidity and fluctuating temperature round the year in Pakistan seems to have a role in further disintegration of MPs into smaller sized fraction. Such situation are lacking in marine environment. Implications of abundance of lower sized MPs in agricultural soils are extensive on agricultural ecosystem. Many studies have reported the altered normal flora and reduced microbial diversity and biomass in the presence of MPs in the rhizosphere [58–61].
A variety of nine different colors seems to have originated from a range of sources and diverse types of plastic materials in the soil or nearby areas. The variety also suggests the multiple pathways of contamination in addition to agricultural practices like wind transport, direct disposal, runoff water etc. Predominance of transparent and smaller sized MPs also indicate that MPs are present in the agricultural soils for a significant period of time. Intermittent exposure to sunlight, temperature and humidity in agricultural fields may cause bleaching of the colors and disintegration of the MPs into smaller sized particles. This is further supported by the fact that in the current study only transparent MPs were found in the smallest size range fraction of 0.0625–0.125 mm. Diversity of pathways and sources of MPs pollution were not only indicated by the color diversity, but also complimented by diversity of composition types of MPs in the current study. The inverse relationship between number of some color MPs and size during regression analysis indicates interesting dynamics of color and size distribution of MPs in the current study. It reflects the possible relative fragmentation pattern among various colored MPs. Some colors including transparent, green, brown, red, and blue MPs have more inclination to disintegrate into smaller particles than others. The underlying variables that render some colored MPs to disintegrate more than others need to be investigated.
In the current study about 97.2% of MPs were less than 2 mm and 85.6% were less than 1.0 mm. MPs with lower sizes has higher surface area for the adsorption of toxic chemicals like heavy metals (Al, As, Cd, Cr, Sr, Co, Cu, Fe, Mn, Ni, Pb and Zn), POPs, hydrocarbons (DDTs and PCBs), pharmaceutical products, halogens, PAHs and others [62]. These adsorbed chemicals and other desorbed additives that are added during plastic production, can cause severe damages in the ecosystem, they are present.
Despite composition analysis of fragments of more than 2mm size, a good diversity of six polymer types including Polyethylene (PE), Polyethylene terephthalate (PET), Polypropylene (PP), Polystyrene (PS), Polycarbonate (PC), and Polyvinyl Chloride (PVC) during the current study indicate diversity of sources of contamination. Possible sources of these polymers in the present study are given in Table 6. PE (52%), PET (18.7%) and PP (14.7%) are widely used and produced plastic types, hence most dominant type of polymer in the current case also. PET has widespread use in as packaging material and containers, while PET can be linked to single use plastic products. PP is commonly used in automotive parts, packaging and textile. These polymers have a range of additives and chemical that contaminate the environment throughout their persistent life. Each of the identified polymer type is reported to be absorbent of various contaminants including heavy metals [63–67]. These recorded types are also reported to adsorb a wide range of pesticides and may contaminate the agricultural ecosystems synergistically [59,68–71].
Although there are no threshold values available to compare current contamination levels of MPs in peri-urban soil of Lahore, their diversity, types, and lower size range in ample quantities may result in further deterioration of agricultural soils. Agricultural soils around Lahore are using sewage water and organic fertilizers in routine and are already contaminated with heavy metals and pesticides. Being near to a large city with 14 million of population, these soils are also increasingly receiving MPs. It will only aggravate the already deteriorating situation, and there is a need to investigate the combined effects of all these contaminations on agricultural soils and crop production.
Conclusions
A significant level of MPs contamination was present in the peri-urban agricultural lands of Lahore, which is greater than in many parts of the world. Peri-urban agricultural lands of Lahore are nearly uniformly polluted by microplastics. Diversity of sizes, colors, types and chemical composition of MPs indicates the diversity of contamination pathways. Current agricultural practices (use of plastic materials, wastewater irrigation, use of compost and organic fertilizers) and dumping of solid waste near agricultural lands seem possible sources of MPs. Further research is needed to focus on the relative contribution of point sources and input pathways of MPs in agricultural soils.
Acknowledgments
Authors are cordially thankful to staff in post graduate laboratory of Department of Environmental Sciences, UVAS, Lahore for their co-operation during this investigation.
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