Physiological ageing of microplastics in enzyme-containing gastric fluid alters kaolin-microplastic interaction and Pb adsorption behaviour

Microplastics (MPs) are synthetic polymer particles whose surface properties are susceptible to environmental and physiological ageing; however, how physiological ageing in enzyme-containing gastric fluid alters clayey soil-microplastic interactions and metal retention remains poorly understood. This study investigated how ageing of three representative MPs in simulated gastric fluid (SGF; containing pepsin as the enzymatic component) altered their interactions with kaolin (a simplified clay mineral model) and affected Pb adsorption behaviours. SGF exposure induced detectable modifications in MPs’ surface chemistry in part due to protein corona deposition, strong acid erosion, and oxidation of functional groups. These modifications influenced the MP vector effects for Pb (notably, the adsorption capacity of digested tyre-MPs decreased by ~35.6%), as well as kaolin-MP interactions and the Pb adsorption capacity of the mixtures. Aggregation between MPs and kaolin particles reduced Pb adsorption relative to the expected additive effect, likely due to the formation of heterogeneous aggregates through electrostatic interactions and physical entrapment that masked potential Pb-binding sites. Notably, zeta potential differences among kaolin-MP mixtures correlated strongly with their Langmuir adsorption capacities (correlation coefficient r = 0.933), highlighting the role of electric double layer in governing Pb adsorption. From a geo-environmental perspective, Pb adsorption in the clay-rich mixtures may be altered not simply by the addition of MPs as extra sites, but by the aggregation behaviour, interfacial charge, and mineral binding-site accessibility of the mixtures.