Plasma-treated polystyrene microplastics for enhanced transport studies in porous media: A surfactant-free approach

Microplastics (MPs) are increasingly recognized as persistent contaminants in soils and groundwater systems worldwide. Polystyrene MPs are highly hydrophobic and show strong homoaggregation, which affects their mobility in porous media. Most laboratory transport studies therefore use chemical dispersants, such as Tween 20, to achieve a uniform distribution of particles during transport experiments. However, these additives coat plastic surfaces and alter plastic–soil interactions, making it difficult to assess environmentally realistic transport behavior. Here, we introduce a surfactant-free approach based on controlled radio-frequency oxygen plasma treatment to modify MP surface properties. Oxygen plasma-treated polystyrene particles (<10 μm) became fully hydrophilic, with water contact angles decreasing from 137° to 0°. This surface modification enabled the formation of stable, well-dispersed particle suspensions at loadings of 200 mg L⁻¹ without any surfactants, overcoming the strong aggregation typically observed for pristine particles. Importantly, plasma treatment did not cause bulk polymer degradation, and the particles remained physically intact without melting or fragmentation. We hypothesize that plasma-treated MPs will exhibit transport behavior distinct from both pristine hydrophobic MPs and chemically dispersed MPs. Column experiments using quartz sand will compare the mobility of untreated, surfactant-assisted, and plasma-treated polystyrene particles to evaluate whether dispersants artificially enhance MP transport by suppressing soil–particle interactions. Overall, this surfactant-free method offers a step toward supporting more accurate environmental risk assessments without bias from additional surfactants.