Ecological risk assessment of microplastics in urban farmland soils. A case of Mabibo Bonde la Mchicha Farm, Dar es Salaam, Tanzania

Abstract

• This study investigated the ecological risks of microplastics in MBMF soils. • MPs mean concentration in soils: 0-5cm: 29.5, 5-10cm: 32.1, 10-15cm: 18.9 items/g. • All soil samples indicated a hazard level I category implying minor risk (PLI < 10). • Highly toxic polymers (PVC and PA) suggested a high degree of chemical risks (PHI) • PERI values indicated a minor to medium ecological risk to soil biota Microplastics (MPs), as a cocktail of contaminants, have raised significant health concerns for biota, including humans. In this study, we investigated the ecological risks of soil MPs using the Pollution Load Index (PLI), Polymer Hazard Index (PHI), and Potential Ecological Risk Index (PERI) indices across 13 sampling locations at Mabibo Bonde la Mchicha Farm (MBMF), located in the northwest of Dar es Salaam. A total of 39 soil samples were investigated using a stereomicroscope and Q Attenuated Total Reflectance Single Reflection Fourier Transform Infrared Spectroscopy (QATR-S FTIR). The mean concentration of MPs in the 0-5, 5-10, and 10-15 cm soil layers was 29.5, 32.1, and 18.9 items/g, respectively. Polypropylene (PP) and Polyethylene (PE) were the most dominant types of MPs found in the soils of MBMF, likely derived from the application of organic fertilizers and surface water irrigation. Although the PLI and PERI indicated a low degree of MPs contamination in the soil samples, the presence of highly toxic polymers such as Polyamide (PA) and Poly Vinyl Chloride (PVC) suggested a high PHI, indicating a high to extreme level of chemical risk, signifying adverse effects. The study emphasizes the need for further research on risk assessment and direct measurement of biological effects. Recommends policy formulation, the development of microplastic standards and improved management of plastic waste to ensure urban ecological sustainability.