Microplastics as Modulators of Xenobiotic Bioavailability and Phytotoxicity in Soil

The co-occurrence of pharmaceuticals and microplastics (MPs) in agricultural soils raises questions about their interactive effects on contaminant mobility and plant exposure. This study evaluated how selected microplastics influence the bioavailability and phytotoxicity of sulfamethoxazole (SMX) in soil using sorghum as a model plant. Polyethylene (PE), polystyrene (PS), and acrylonitrile–butadiene–styrene (ABS) were tested in pristine and artificially aged forms (modified Fenton protocol). Dose-response analysis demonstrated a biphasic effect of SMX: low concentrations (1 - 5 mg/kg) slightly stimulated shoot and root growth, whereas ≥ 10 mg/kg significantly inhibited development, with nearly complete suppression of root elongation at 50 mg/kg. Microplastics alone (0.1 - 1%) did not negatively affect germination or early growth. However, when 1% aged MPs were introduced into SMX-contaminated soil (EC25 and EC50 levels), phytotoxic effects were mitigated. Plants grown in MP-amended systems exhibited significantly improved germination and biomass compared to SMX-only treatments.
Analysis indicated that total SMX sorption in soil was not markedly altered by MPs, yet polymer type and ageing state influenced the water- and acid-extractable fractions. Primary PE and ABS increased SMX concentrations in eluates, while aged MPs generally showed enhanced retention capacity. Among the tested polymers, PS exerted the strongest effect on modifying SMX bioavailability and plant response.
These findings demonstrate that MPs act as dynamic regulators of antibiotic mobility rather than passive soil contaminants. By modifying sorption–desorption equilibria and the plant-accessible fraction of SMX, MPs can attenuate or reshape phytotoxic outcomes. The results highlight the necessity of incorporating MP–pharmaceutical interactions into environmental risk assessment frameworks, particularly in intensively managed agricultural systems where both contaminant groups co-occur.