The role of humic substances’ hydrophobicity in heterogeneous adsorption onto microplastics: Insights from two-dimensional correlation hydrophilic interaction chromatography

Abstract

The heterogeneous composition of humic substances (HS) complicates understanding their interactions with microplastics (MPs), which is critical for assessing the environmental roles of MPs in aquatic systems. This study investigated the adsorption behavior of HS onto pristine and aged polyethylene (PE) MPs, with a focus on how hydrophobic properties influence these interactions. Hydrophilic interaction chromatography (HILIC) revealed preferential adsorption of HS molecules with higher log K_ow values onto MPs compared to those with lower values. This trend was more pronounced for aged MPs than pristine MPs and was stronger under acidic conditions. Two-dimensional correlation HILIC (2D-Co-HILIC) provided further insights, revealing sequential adsorption of HS subfractions based on hydrophobicity. For pristine MPs, Freundlich adsorption affinity (K_F) values followed the log K_ow order of 3.2 > 3.4 > 2.5, with corresponding K_F values of 4.64 ± 0.12 > 0.71 ± 0.05 > 0.70 ± 0.08 (Abs/g)(Abs)^1/n. For aged MPs, the order was 3.2 > 3.4 > 3.0 > 2.5, with K_F values of 5.44 ± 0.07 > 5.02 ± 0.06 > 0.77 ± 0.07 > 0.23 ± 0.05. Hydrophobic interactions played a dominant role, particularly for aged MPs, where greater competition among HS molecules for adsorption sites occurred. Changes in the molecular weight of residual HS, inferred from fluorescence component ratios, indicated molecular size as a secondary factor in adsorption. This study highlights the critical role of hydrophobicity in HS-MP interactions and calls for further research into how these interactions impact the environmental behavior of HS and MPs in aquatic systems.