Transport of polystyrene microplastics in paddy soils with different chemical conditions: effects of anionic and cationic surfactants

Surfactants are extensively utilized in daily life, and the presence of surfactants in soil can alter the hydrophilicity of microplastics, further affecting the transport of soil colloidal particles. This study investigated polystyrene microplastics (PSMPs) transport in paddy soils under the influence of anionic (SDBS) and cationic (CTAB) surfactants, with consideration of chemical factors including ionic strength, cation type, and saturation conditions. The mixture of microplastics and surfactants was introduced into the soil column at a flow rate of 1 mL/min using a peristaltic pump. The effluent was collected by an automatic partial collector with a collection time of 3 min for each sample. The concentration of microplastics was determined using a fluorescence spectrophotometer. After injecting 60 mL of the mixture, it was replaced with 60 mL of surfactant solution for drenching. Transport experiments were performed under different conditions of ionic strength (0,1,5,10mM), cation type (Na+, Ca2+), water content (saturation, unsaturation) and surfactant concentration (50, 200 mg/L), and each condition was replicated three times. The results demonstrated that both CTAB and SDBS (50 mg/L) enhanced MPs transport in saturated media at IS = 0 mM. Under unsaturated conditions, MPs transport was primarily impeded by physical straining enhanced by water bridges and entrapped air. In contrast, under saturated conditions, capillary forces facilitated the detachment of MPs from adsorption sites, thereby promoting their transport. However, this promotive effect diminished with increasing IS and was further weakened under unsaturated conditions. Critically, at 50 mg/L SDBS and 5 mM IS, Ca2+ inhibited MP transport by 89% more than Na+, due to its stronger charge neutralization capability. At low IS (1 mM), surfactants in Ca2+ solutions slightly promoted transport, whereas inhibition was observed in Na+ solutions. At higher IS (5–10 mM), both surfactants consistently inhibited transport. The study quantitatively links solution chemistry to MPs mobility, highlighting the critical roles of ion type and valence in environmental risk assessment. These results indicate that cationic and anionic surfactants affect the transport of microplastics in saturated and unsaturated agricultural soils to varying degrees, and chemical factors such as ionic strength and cation type make their transport behavior more complex.