Palladium-doped and undoped polystyrene nanoplastics in a chronic toxicity test for higher plants: Impact on soil, plants and ammonium oxidizing bacteria

Abstract

There is a lack of knowledge about the fate and impact of microplastics (MPs) and nanoplastics (NPs), as well as their potential uptake and impact on plants and microorganisms. The predicted environmental concentrations (PEC) of frequent polymers in soils are low, and therefore, difficult to detect with the available techniques, which explains the knowledge gaps. Therefore, model particles (polystyrene particles (PS-P), 343 nm) and palladium (Pd) nanoparticle-doped polystyrene particles (PS-Pd-PS-P, 442 nm) were synthesized, characterized, and subsequently applied to agricultural soils (Cambisol, Podzol, PS target contents: 25 mg kg^-1, 75 mg kg^-1, 225 mg kg^-1). A combination of different techniques, such as inductively coupled plasma-mass spectrometry (ICP-MS), pyrolysis-gas chromatography-mass spectrometry (Pyr-GC-MS), dynamic light scattering (DLS), and scanning electron microscopy (SEM), were used to characterize the particles in the dispersions, soils and plants. The spiked soils were applied to a chronical plant toxicity test with oat (Avena sativa). The applied particle contents could be recovered from both soils by ICP-MS (Pd, 89% - 99%), and Pyr-GC-MS (PS, 73% - 120%). Moreover, non-aggregated particles in soils and on oat roots were visualized through SEM. The ratio obtained for the Pd contents in oat roots to that in the Cambisol (2.2-2.7) and the Podzol (2.3-2.6) implied that particles accumulated on the root surface or in the roots. No Pd was detected in the oat shoots, which indicated that no translocation occurred from the roots to the shoots. Despite particle accumulation at or in the roots, no clear effects on plant growth were observed. Furthermore, the soil microorganisms (Podzol) and the soil water repellency (Cambisol, Podzol) showed no clear monotone concentration-response relationship after exposure to PS-P and PS-Pd-PS-P. The findings are complex and illustrate the urgent need for further sophisticated experimental studies to elucidate the impacts of NPs on physicochemical soil function, plants, and soil organisms. The model PS-P doped with Pd nanoparticles significantly enhanced the development and validation of methods for investigating MPs and NPs in environmental matrices, highlighting their considerable potential for further studies.

Keywords: Ecotoxicity; Microplastics; Nanoplastics; Oat (Avena sativa); Polystyrene particles.