Avoiding Bias in Microplastic Transport: Development of an Improved Dispersant-Free Soil Column Experiment Protocol

Microplastics (MPs) have emerged as contaminants of global concern due to their ubiquity and potential ecological risks. Understanding MP transport, behavior, and fate in soils is crucial for assessing their interactions with soil organisms and for conducting environmental risk assessments. Most studies on MP transport are conducted in laboratory settings, often using soil column experiments. These experiments typically examine MP mobility by assessing the retention of MPs in different soil types under varying flow conditions. To ensure consistent MP application over time or volume, dispersants are frequently added to MP suspensions, particularly when targeting floating or highly hydrophobic MPs. However, dispersants may alter both the behavior of MPs and their interactions with soil particles, potentially introducing biases when attempting to understand natural MP transport—a critical aspect that remains underexplored.

Therefore, this study developed an improved soil column experiment protocol that excludes dispersants while maintaining consistent MP application through a low-liquid-level, continuously stirred suspension. The Coefficient of Variation (< 5%) for this improved experimental design is found to be statistically acceptable.

Based on this improved method, MP transport was investigated in soil column experiments with quartz and natural sandy soil as matrices. Rhodamine B-stained polystyrene (RhB-PS) particles (D90 < 10 µm) were intermittently pumped upward into the columns, with and without the dispersant (0.25% v/v Tween 20). Drainage samples were collected after each RhB-PS application and during intermittent flushing with artificial rainwater. Fluorescence microscopy was used to quantify RhB-PS concentrations in the drainage samples on haematocrit plates.

The analysis of drainage samples revealed that dispersants significantly enhanced MP mobility, allowing more MPs to bypass soil retention. The clay and organic matter in natural sandy soil, through their fine particles and surface charges, may potentially enhance the interaction between microplastics and soil, thereby reducing their movement within the natural sandy soil, regardless of whether dispersants were used. These results suggest that existing transport studies and related models, which are based on dispersant-assisted experiments, may not accurately reflect the natural behavior of MPs in soils.