New insight into deciphering the component heterogeneity of humic acid in governing carbon nanotubes dispersion

The elucidation of mechanisms underlying the dispersion of carbon nanotubes (CNTs) by natural dissolved organic matter (DOM) is still controversial, with disparities in findings likely due to oversimplified DOM that overlook its intrinsic compositional heterogeneity. In this study, we comprehensively analyzed the physicochemical properties of sequentially extracted humic acids (HAs) from three typical soils, and introduced a novel HA diversity index to quantify the effect of compositional heterogeneity on the dispersion of hydroxylated multi-walled CNTs (MWCNTs-OH). The later-extracted HAs, enriched in aliphatic macromolecules, exhibited a higher adsorption affinity on MWCNTs-OH due to pronounced hydrophobic interaction and chain entanglement. While MWCNTs-OH dispersion showed a positive correlation with HAs aromaticity within individual soils, this relationship did not hold across different soils. Notably, a robust and generalized positive correlation was observed between MWCNTs-OH dispersion and the HA diversity index irrespective of soil origin. HAs with rich components and varying properties could more effectively disperse MWCNTs-OH through a synergistic effect of steric hindrance and hydrophilicity. This study provides an effective way to link the heterogeneity of DOM with the dispersion behavior of CNTs, offering crucial insights for the environmental risk assessment of carbon-based nanomaterials within intricate natural ecosystems.