Ecological risk assessment of heavy metal-microplastic interactions in agricultural soils irrigated with secondary treated wastewater

The increased reuse of treated wastewater (TWW) for irrigation in arid and semiarid regions raises environmental concerns regarding microplastics (MPs), specifically their capacity to adsorb and transport heavy metals. This study assesses the quantity of MPs and associated metals (Cd, Cr, Cu, Pb, Ni, and Co) in agricultural soils irrigated with TWW in the Konya region of Türkiye. To this end, a total of 202 soil samples were collected from TWW-irrigated (180) and nonirrigated control fields (22) at two depths (0–10 and 10–20 cm). MPs were extracted from the samples using a density separation technique, identified microscopically, and analyzed for metal adsorption using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Contamination and risk were assessed using the geoaccumulation index (Igeo), contamination factor (CF), enrichment factor (EF), potential ecological risk index (ER), pollution index (PI), and Nemerow pollution index (NPI). MP abundance was observed to be significantly higher in the TWW-irrigated soil samples (100–840 MP/kg at 0–10 cm; 80–660 MP/kg at 10–20 cm) than in the controls (100–220 and 80–240 MP/kg, respectively) (p < 0.05). Cd (0.41 mg/kg) and Cr (12.46 mg/kg) were the most abundant metals associated with MPs. Igeo and CF indicated moderate to heavy contamination, while the ER values revealed Cd to be the dominant ecological risk factor. EF and PI suggested minor to moderate enrichment with localized Cd and Pb hotspots. NPI values were indicative of high to very high overall heavy-metal pollution. Overall, the results reveal MPs to be both persistent pollutants and vectors, facilitating the vertical migration of metals and posing long-term ecological risks in TWW-irrigated agroecosystems.