Molecular weight-dependent adsorption heterogeneities of humic acid on microplastics in aquatic environments: Further insights from fluorescence spectra combined with two-dimensional correlation spectroscopy and site energy distribution analysis

Microplastics as emerging environmental contaminants have attracted global attention. Ubiquitous dissolved organic matter (DOM), a heterogeneous mixture with various organic components and continuous molecular weight (MW) distribution, can significantly influence the fate, bioavailability and toxicity of microplastics in aquatic environments. However, to date, understanding of the MW-dependent adsorption heterogeneities of DOM on microplastics in aquatic environments has remained unknown. In this study, the soil humic acid (HA), a representative DOM, was fractionated into >100 kDa HA, 30-100 kDa HA, 10-30 kDa HA, 3-10 kDa HA and <3 kDa HA, whose adsorption behaviors on polystyrene microplastics (PSMPs) under different electrolytes at pH6.0 investigated by multispectral techniques, as well as two-dimensional correlation spectroscopy and site energy distribution analysis. High molecular weight MW-fractionated HAs dominated the soil HA. Compared to the lower molecular weight MW-fractionated HAs, the higher molecular weight counterparts exhibited higher aromaticity, showing higher adsorption affinities onto PSMPs in aquatic environments. Adsorption isotherms and kinetics of the MW-fractionated HAs onto PSMPS can be reasonably explained by Langmuir models and the linear driving force (LDF) model, respectively. Enhanced aromaticity in residual solutions was observed, suggesting that the preferable adsorption of the higher aromatic moieties in the pristine and MW-fractionated HAs on PSMPs could be subsequently replaced by the lower aromatic moieties for the limited binding sites. Combined, the humic-like materials in MW-fractionated HAs could be significantly impacted the complexation of PSMPs in aquatic environments. The heterogeneous/complicated distributions of active adsorption sites in the humic-like materials of the MW-fractionated HAs, and the subsequent subtle changes of these sites to PSMPs were characterized by synchronous fluorescence spectroscopy coupled with two-dimensional correlation spectroscopy. High molecular weight MW-fractionated HAs provided more active adsorption sites than those in the low molecular weight counterparts, which also possessed stronger adsorption affinities and higher complexation capacities to PSMPs. The site energy distributions for the adsorption of the MW-fractionated HAs on PSMPs were calculated based on Langmuir model. The adsorption heterogeneities of PSMPs at the experimental conditions were close. This study provides deep insights into the MW-dependent adsorption heterogeneities of HA on PSMPs, and the results are of significance to understand the biogeochemical behaviors of DOM and MPs in aquatic environments. • Enhancing aromaticity in the residual solutions of the MW-fractionated HAs-PSMPs complexes were observed. • Molecular weight-dependent adsorption heterogeneities of the MW-fractionated HAs to PSMPs were characterized by SF-2D-COS. • High molecular weight MW-fractionated HAs offered more heterogeneous adsorption sites and stronger adsorption affinities to PSMPs. • Adsorption heterogeneity of PSMPs at the experimental conditions were close.