Identification of microplastics from urban informal solid waste landfill soil; MP associations with COD and chloride

Microplastics (MPs) in soil, air and water are a multifarious group of plastics defined as particles <5 mm in size. The carbon footprint of plastic has doubled since 1995, reaching 2 Gt CO₂-equivalent accounting for 4.5% of the global greenhouse gas emissions as of 2015. Increased combustion of coal for plastic production, manufacturing of countless plastic products, resin production and related upstream activities have been the major driver for the increase in MPs in the environment. Plastic production represents ∼96% of the plastics' carbon footprint since 1995, while recycling, incineration, and landfills accounted for only 6% (Cabernard et al. 2022). Due to the electricity and heat derived from coal for resin production and plastic processing, coal-based emissions have contributed to nearly half the carbon footprint of global plastic production in 2015 with an addition of 6% global coal electricity used in plastic production (Cabernard et al. 2022). At present MPs are of major concern for all living forms because it is found in the soil matrices, marine and freshwaters and now in the human/animal food chain. MP particles are of more health concern because of its non-biodegradability, long-term persistence, heat-holding capacity, bio-accumulation, alteration in soil micro biota, and most importantly cumulative subsidence in the food chain and food webs. Long-term accumulation of plastic fragments severely affects soil quality; the disintegrated MP fibres and fragments slowly find their path into the groundwater and surface water through soil pores and runoff. In vitro studies have shown the presence of MPs and their potential effects on human-derived cells, human blood, and lungs (Hwang et al. 2019; Jenner et al. 2022; Leslie et al. 2022). Animals are shown to ingest MPs; and when human beings ingest animal-derived food, microplastics enter the human peristaltic system.

Landfilling is widely seen as a pseudo-cost-effective option to temporarily dispose of municipal solid waste (MSW) including plastic waste, throughout the world (Golwala et al. 2021). Alternately, plastic waste is incinerated with or without energy recovery (Thompson et al. 2009). In developing countries, informal/unorganized landfills are en masse and common, causing devastating effects (Yang et al. 2018) on all the three environmental attributes-air, water and soil. An estimated 36% of all plastics produced in developing countries are used in packaging, including food and beverage containers made of single-use plastic that are disposed into landfills or waste dump yards after recovering the biodegradable valuables. The inseparable organic waste with plastics is dumped at random locations without any EIA screening procedures. One of the reasons ascribed to low life expectancy of people (currently <73.33 years) in developing countries is the external unhygienic polluted environment; this situation is now linked with clairvoyance to microplastics. WHO reported a 40% increase in the use of disposable personal protective equipment (PPE) for healthcare professionals. Billions of potentially contaminated masks, gloves and antiseptic wipes are littered in the environment because of NIMBY syndrome from health-related professions. Statistical data on common plastic waste management practices showed that the United States incinerated 15.8% and landfilled 75.7% of the plastic wastes generated during COVID-19 (US EPA 2020). Majority of the plastic wastes dumped in landfills remain in the environment for several decades fragmenting into much smaller sizes by the vagaries of weather. In the rainy seasons most fragmented MP particles are carried away into water courses of which nano-plastic particles slowly percolate into groundwater. Recently, MPs were found in compost samples (Scopetani et al. 2022) and the transfer of hazardous contaminants from plastics to compost; and further into soil. This condition makes it worse, if the hazardous compounds enter the living food chain/web. Microplastics are detected in landfill soil and leachate from refuse previously (Afrin et al. 2020; Sun et al. 2021; Wan et al. 2022). In India, most MSW landfills do not follow any standard sanitary landfill design procedures. The determination of the contamination levels by MPs are very essential before choosing the most suitable mitigation methods. The term ‘microplastics’ is recoined to define with sizes <1 mm as the name suggests. The word micro refers to anything ≤ 1 mm (i.e. < 1,000 μm) in size (Hartmann et al. 2019). The novelty of this research is to investigate and technically report the current status of MPs in the urban landfill and establish environmental parameter interlinks. We have focused on plastic fragments of size <1 mm size as against the general definition of 5 mm mentioned in open literature. We report research findings on extraction and identification of various types of microplastics from contaminated urban informal MSW landfill site, with a special focus on MP associations with the water quality parameters: chemical oxygen demand (COD) and chloride.

The chemical reagents required for determining MPs were prepared following standard procedures described (Marine Debris Program 2015). The environmental parameters were determined following the procedures described in Standard Methods (APHA 2017). The various chemicals used were standard 30% H₂O₂ solution, Fe²⁺ solution, standard buffer solution, ferric chloride, potassium dichromate (K₂Cr₂O₇), sulfuric acid (H₂SO₄), potassium chromate indicator, silver nitrate (KNO₃) solution, conditioning reagent and sodium chloride (NaCl). All the chemicals used were of analytical grade and purchased from HiMedia Laboratories, Mumbai, India. Glassware and instruments used were COD vials, hand auger, crucibles, conical flasks, aluminium containers, sieve plates (Nos. 18 and 30), beakers, nitrocellulose membranes, measuring cylinder, Petri plates, measuring cylinder, filter funnel equipment and vacuum filtration unit and separation funnel.

The pH of landfill runoff water was measured using a digital pH meter (ELICOL1 127). The electrical conductivity (EC) values were measured using MH Digital-EC meter. The morphology of MPs and elemental composition was checked using Carl Zeiss (Germany) EVOLS15 scanning electron microscope (SEM), and AMETEK-EDAX-Electron Dispersion Spectroscopy (EDS). Perkin-Elmer Spectrum Two Fourier Transform Infrared Spectroscopy was operated in the frequency range of 600–4,000 cm⁻¹, to identify the type of plastics which were extracted from the MP-contaminated landfill soil sample. The COD was determined using a COD digester HACH DRB 200.

A soil sample without any MP contamination (baseline) was collected, shown marked in Figure 1(a) as a small pink circle. Soil sampling locations were identified at random. Eight landfill cover soil samples (grab) were collected from landfill cover soil at the landfill and two samples at ground level. One kg of top cover soil up to 10 cm depth (Constant et al. 2021) was collected in aluminium containers and transported to the laboratory for analysis. All the soil samples were first weighed and then oven dried at 110 °C for 24 h to determine the MC. The dried samples were then sieved through 1 mm sieve mesh to obtain coarse samples and then through 0.6 mm sieve mesh to obtain fine samples for homogenization (Zhang et al. 2018). Each sample collected from the sampling locations was replicated into two – coarse and fine samples; and then evaluated separately for the presence of MPs.

The MC of the soil samples were analyzed gravimetrically as described (APHA 2017). The pH of freshly collected MP-contaminated soil samples was noted using a digital pH meter after calibration. The sample was prepared taking a soil-water ratio of 1: 2.5, and the soil-water mix was homogenized by stirring and allowing it to stay under quiescent conditions for ∼1 h. The soil to water (distilled water) ratio was 1:5 to determine the EC. The EC of the samples were determined following the procedures described (He et al. 2012).

Visible MPs collected on the membrane were visually sorted and counted. The dried membrane was further observed under a stereo zoom microscope to fetch a count of smaller MPs at 300x magnification. The membrane was then subject to SEM and EDS for analytics of MP particles on the membrane; and then the selected MP particles were taken to ATR-FTIR analysis to identify the type of MPs present in the soil sample.

To reduce MP contamination of sampled soil and to eliminate the risk of MP losses from soil samples, the steps recommended by (Piehl et al. 2018) was followed wherein the equipment was thoroughly cleaned with prefiltered (0.2 μm) deionized and 35% ethanol before analysis of each soil samples. Steel shovels and aluminium foil containers were used to collect all the soil samples. The processing apparatus was covered with aluminium foil to check airborne MP contamination.

In Figure 3, of the 10 sampling locations, S2, S4, S6 and S9 in the study area showed MC values matching with the baseline natural soil environment MC of 1.1–1.8%. High MC zones were obtained at lower areas of the open landfill; this is because the fragmented plastic debris in the top layers holds the morning dew below it on another plastic material and might slowly release it down the waste hill as it accumulates more dew.

It may be observed in Figure 3 that the EC values are high at locations devoid of plastic debris. The baseline natural soil without any MP contamination showed higher EC values of 673 μS/cm; while at other sampling locations the EC values were relatively low, except at S3. It is known that the EC/TDS values are linked to mineral content in soil. A higher value of EC was observed at S3 because EC indirectly represents TDS and chlorides; increased chloride values can interfere with the plant growth as noticed on the MSW landfill. The neighbouring land adjacent to the landfill site was considered a reference showing no/minimal MP contamination, except for airborne/rainfall–runoff conditions. The reference MP values showed an MC of 12–30%.

In Figure 4, the decrease in soil pH is surmised with two reasons: (i) acidic nature of hydrocarbons in plastic ultra-nano and micro fragments, and (ii) the formation of weak acids HNO₃, H₂SO₄, and H₂CO₃ by NO₂, SO₂ and CO₂ by the scavenging action of local rainwater droplets from short hydrological cycles (Zhao et al. 2021; Gharahi & Zamani-Ahmadmahmoodi 2022). The decomposition of organic matter in the landfill site can also contribute to the decrease in pH values. Further, the electrostatic attraction will decrease as the solution pH decrease (Fred-Ahmadu et al. 2020). Some hydrocarbons like benzene, pesticides and organic compounds in a complex soil matrix/water environment can be weakly acidic. The electronegativity factor can possibly make the hydrocarbon more acidic. Sampling locations S1, S2, S7 and S10 showed relatively lower pH values <7.0 compared to the pH values at other sampling locations.

The distribution of MP abundance with respect to its size is shown in Figure 5. From this plot, it is inferred that MP particles extracted from S7 were mostly smaller in size than those from other locations. This is because MP particles degrade into much small-sized nanoparticles as landfill ages over time, and the rate of abrasive physical fragmentation of plastics to MPs increases due to various external forces (Su et al. 2019).

In Figure 6, fragments were the predominant MP particles accounting for an average of 56.1% from S1 to 10. Similarly, fibres accounted for 26.4% average from S1 to 10. Films and foam MP particles were averaged to 13.7 and 3.8% respectively.

SEM images in Figure 7 show that the samples were non-conducting and so SEM images showed clear images with less noise. Plastic material commingled with other non-biodegradable waste are not picked out for recycling. The leftover plastic waste over time abrades into smaller sizes of decimeter, centimeter, millimeter and micrometer; which further breakdown into nano-sized plastic particles giving a gnawing effect. The weathered MP particles showed numerous abrasions, cracks, and fragmentation on the surface and all along the perimeter of MP fragments (Figure 7(a)–7(d)).

Thin fibre fragments of cross-sectional size <100–500 μm showed further fragmentation possibilities in the next level of abrasion. The changes in the surface roughness of the plastic fragments as a result of time abrasion are observed. The sequential state of MP particles by the combined effect of soil grains, water droplets and wind breeze causes the deterioration of MPs into much small sizes. Four-dimensional abrasion can occur at the edges of MP particles corner, top and bottom surfaces, and all along the peripheries. Most plastic materials crumble before their designated half-life timeline. Continuous modification in the surface roughness in all dimensions of the degrading MP particles, heat, UV radiation, and abrasion by airborne soil particles (Gniadek & Dąbrowska 2019) can disintegrate at a faster rate. Peripheral abrasion (Figure 7 (g)) and fibre separation (Figure 7 (h)) from the parent MP particle can result in thousands of such fibres (Figure 7(i)). These fibres further disintegrate into countless nano-plastic fibrous particles finally unseen to the naked eye.

The MP particles were subject to FTIR analysis for the MP particles between 1 mm and 600 μm. One of the most popular reliable, quick, and non-destructive chemical techniques to identify MPs are the use of ATR-FTIR spectroscopy by which particles > 500 μm can be observed. The methodology applied depends on the molecular structure and composition of the substance wherein the samples are exposed to a defined range of IR radiation by which the type of MP particle can be identified (Li et al. 2018).

Figure 11 shows MP associations with the salt content. The baseline soil without MPs showed Cl⁻ values averaged to 98 mg/L. The Cl⁻ values showed a decreasing trend with increase in the particle numbers of MPs. This observation is hypothecated surmising that the soil contaminated with MPs tends to lose the ability of salt retention in the top soil of the landfill site; and also, that the salts are absorbed onto the surface of MP particles. In effect, the cumulative surface runoff could contain larger levels of salts in the receiving waters such as ponds, lakes, rivers and possibly groundwater and possibly springs. It is known that TDS is associated with the EC values as also the mineral content. The chloride values may further fall down to lower levels if the MP particles degrade into many small sizes invisible to the naked eye. This condition can grossly affect most of the microbial living forms perturbing the soil ecosystem and the aquatic ecosystem particularly those landfills/open dumps in close vicinity to water courses/cultivable soil.

The fate of plastics over a time frame can have long-term irreversible devastating effects by the MPs on the environment. The grinding effect of the natural soil, wind, water impacts, and the sun's heat on the littered plastic fragments reduces its size and shred from large particle fragments into smaller particle fragments to <10⁻⁹m. These particles because of extreme negative charge and invisibility to the naked eye can alter the natural baseline pristine qualities of the water/soil and air. These particles are surmised to have the ability to diffuse through cell membranes (Järvenpää et al. 2022) and penetrate into functional attributes destructing the infinite capacity of the living cell. Water treatment facilities, wastewater treatment facilities must be more robust for removing sub-nano-plastic entities from water. It is believed that even the best membranes in the household/commercial RO units may not be able to remove the invisible/camouflaging plastic particles from water. In pre-modern times, water (Earth's blood) was called as an excellent ‘Good’; and today it has become a ‘Rival Bad’, because of negative frequencies by humans with mother nature.

Informal landfills are loaded with plastics in poorly managed urban areas of developing countries. Plastic waste can be as small as a thin dental floss thread or a large discarded bath tub. Given a certain time, these wide ranges of plastic waste disintegrate into smaller and much smaller fragments by the weathering action. The abundance of MPs in the open landfill site in Mysore city, in Karnataka state was evaluated ranging from 180 to 1,120 MP particles/ kg of soil. The MC, EC, and pH in 10 soil sampling locations averaged out to 2.96%, 187–441 μS/cm and 6.94, less than the normal soil pH. SEM images of MPs extracted from MP-contaminated urban landfill soil showed a wide variety of MPs – fragments, films, fibres and foam respectively. The fibre content accounted for 26.4% average for sampling locations S1–10; whereas films and foam were 13.7 and 3.8% respectively. The morphology of MPs in SEM showed four-dimensional abrasion of all MPs at the landfill site. SEM images showed continuous fragmentation of MPs into sub-micron sizes. EDS results showed carbon 71.8% and 24.5% oxygen with traces of Na, Al, Si and Cl⁻. The results of FTIR spectroscopy for three random MP particles extracted from contaminated landfill soil identified the type of MPs to be an unidentified polymer, polyethylene, and polypropylene. The association of MP particles with COD and chloride parameters were established. Up to 20 MP particles of 1 mm size, there was no effect on the intrinsic COD values. As the MP particle spikes were increased in number, the COD values increased, and for 800 MPs the COD value was 1,100 mg/L. Similarly, chloride values decreased by ∼ 31% from the baseline chloride value of 97.97 mg/L. It was concluded that the increased abrading affect at the landfill site loaded with MPs can escalate the fragmentation of MP particles into much smaller nano sizes. These nano-sized plastic bits, light in density can sway off into the nook and niche of the natural ecosystems, causing devastating effects on the environment with high levels of vulnerability.

We gratefully thank Institution of Engineers India (IEI) Kolkata, India for sponsoring this novel research work. We acknowledge JSS Mahavidyapeetha, Suttur Sri Kshetra in Karnataka State, for providing research space in the electrochemical laboratory (first author) of the Department of Environmental Engineering, Sri Jayachamarajendra College of Engineering, JSS Science and Technology University, JSSTI Campus, Mysuru.

Ethics approval was not required for this research.

All authors have agreed to this study.

Mahesh Shivaswamy: Conceptualization, funding proposal and acquisition, design ideas, manuscript writing and editing, graphics, overall supervision. Nisarga K Gowda: funding acquisition, laboratory investigations, draft paper preparation, data plots and cites, and manuscript correction. Sahana Mahesh: Literature search, data analysis and interpretations.

This research was financially supported by the Institution of Engineers India (IEI) in the scope of funding undergraduate (UG) research program: Project ID UG2022006, Ref: R.4/2/UG/2021-22.

All the data is included in the main manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.