Regulation of SiO2 Nanoparticles on the Adsorptive Fractionation of Dissolved Organic Matter by Goethite

SiO₂ nanoparticles (SiO₂NPs) are most widely available and coexisting with DOM at the mineral-water interface; however, the role of SiO₂NPs in DOM fractionation and the underlying mechanisms have not been fully understood. Using Fourier transform ion cyclotron resonance mass spectrometry, combined with Fourier transform infrared spectroscopy and X-ray adsorption fine structure spectroscopy, was employed to investigate the adsorptive fractionation of litter layer-derived DOM on goethite coexisting with SiO₂NPs under different pH conditions. Results indicated that the inhibitory effect of the coexisting SiO₂NPs on OM sorbed by goethite was waning as environmental pH increased due to the reduced steric interactions and the concurrent elevated hydrogen bonding/hydrophobic partitioning interactions on the goethite surface. We observed the coexisting SiO₂NPs inhibited the adsorption of high carboxylic-containing condensed aromatic/aromatics compounds on goethite under different pH conditions while improving the adsorption of highly unsaturated aliphatic/phenolic and carbohydrate-like compounds in an alkaline and/or circumneutral environment. More nitrogen-containing structures may favor the adsorption of phenolic and nonaromatic compounds to goethite by counteracting the negative effect of SiO₂NPs. These findings suggest that DOM sequestration may be significantly regulated by the coexisting SiO₂NPs at the mineral-water interface, which may further influence the carbon-nitrogen cycling and contaminant fate in natural environments.