Optimization of a detection and identification methodology for microplastics in tropical soil

Microplastics are plastic particles that are less than 5 mm in length. Microplastics pollution is an increasing global environmental health hazard. Still, identification of microplastics occurrence in tropical soil is inadequate compared to soils from temperate environment. The density separation method which uses chemical separation solutions of various densities for the extraction of soil microplastics, is unable to efficiently detect high-density plastic polymers. Furthermore, the chemical solutions have influenced the physical and chemical characteristics of microplastics through surface corrosion and degradation. This study aimed to develop and optimize a methodology based on density separation for the detection and identification of microplastics in tropical soil by determining the characteristics of surface soil, selecting the best density separation solution, investigating the effects of soil organic matter removal and duration of orbital shaking, testing the reusability of the density separation solution and the applicability of the optimized methodology. Spiking experiments were carried out using six plastic polymers; polystyrene, polypropylene, low density polyethylene, high density polyethylene, polyethylene terephthalate and polyvinyl chloride. Soil organic matter content was determined through Fenton’s oxidation. Extraction of spiked soil microplastics was investigated based on density separation using five different solutions, namely, deionized water, sodium sulphate, sodium carbonate, sucrose and coconut oil. Effects of soil organic matter removal and duration of orbital shaking were investigated based on recovery of spiked soil microplastics. Reusability of the density separation solution as a sustainable solution was tested. The optimized methodology was validated using agricultural field surface soil from two farms located at Klang Valley, Malaysia. Spiking experiments revealed that the best density separation solution was a saturated mixture of sucrose - sodium sulphate (1.8 g cm-3 ) ratio 1:1 (v/v), with a 2-hour orbital shaking. The separation solution was reusable up till three times for extraction of soil microplastics. The optimized method had retrieved 91.7% of spiked soil microplastics. The method application on agricultural field surface soils had presented the mean particle size of soil microplastics with 3260.76 ± 254.55 μm in farm A and 2822.31 ± 416.52 μm in farm B. Significant types of soil microplastics were fragments and films with major colours of white (59%) and transparent (28%) in farm A, while black (52%) and white (37.6%) in farm B. Plastic polymers detected were of polyvinyl chloride, high density polyethylene, polycarbonate and polystyrene. Sources of plastic products were plastic pipes, plastic films, fertilizer bottles and polystyrene storage boxes and the breakdown processes contributed to the microplastic pollution in these farms. The optimized methodology is effective and reliable for the detection of microplastics in tropical surface soil. Findings in this study provide an understanding on the vulnerability of soil microplastics from agricultural sectors. Monitoring and assessment of soil microplastics pollution will establish potential hazards to human and environmental health, which lead to effective mitigation measures. This study would be beneficial as supportive reference for the attempt to diminish microplastics pollution in soil.

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