Abundance and Characterization of Microplastics in Compost Produced in Mazandaran, Iran

One of the source of plastics and microplastics (MPs) entering the environment and the food chain is waste from various sources. Most plastic resins are utilized in packaging with relatively short lifespans; after consumption, these materials contribute to municipal solid waste. The composting process struggles to completely separate plastics, making microplastics a significant contaminant during composting. MPs are defined as plastic particles smaller than 5 mm. This study aims to examine the frequency, distribution, and characteristics of microplastics found in compost produced by Mazandaran compost factories. MPs in the samples were extracted and separated based on density differences and digestion. Subsequently, a stereomicroscope was used to observe the MPs. This cross-sectional study found an average microplastic content of 16,981 items/kg across all compost samples. Behshahr compost had the highest average at 1,818,622, while Babol had the lowest at 15,744 items/kg. The Kruskal-Wallis test indicated no statistically significant differences between the sampling stations (p > 0.05). The most common shapes were fibers (75%), with the predominant color being transparent-white (56%), and the most frequently observed size range was 500–1000 µm (40%). Additionally, polymers identified included polyurethane, nylon, low-density polypropylene, and polycarbonate. This study reveals the presence of microplastics in compost from Mazandaran's fertilizer and compost factories. To address this issue, it is essential to develop strategies for reducing plastic waste entering composting facilities and to establish effective monitoring programs to detect and identify microplastic hotspots.