Quantitative regulation of DOM on ferrihydrite colloids migration in saturated kaolinite and MgAl-LDH-coated sand columns

The adsorption of ferrihydrite colloids (Fh-NPs) on solid-phase media is the main factor determining their migration. However, most studies on colloids migration only provide qualitative descriptions. This study explored the impact and mechanisms of humic acids (HAs) and fulvic acids (FAs) on Fh-NPs migration in different saturated media via quantitative adsorption analysis. The coexistence of high HAs/FAs concentration promoted the migration of Fh-NPs in the negatively charged kaolinite-coated sand columns, but hindered migration in the positively charged MgAl-layered double hydroxides (LDH)-coated sand columns. The high HAs/FAs concentration combined with Fh-NPs reversed the surface positive charge of Fh-NPs, reducing adsorption capacity of kaolinite for Fh-NPs to 1.93/3.93 mg/g (from 20.8 mg/g without HAs/FAs) and increasing that of LDH for Fh-NPs to 31.1/30.6 mg/g (from nearly 0 mg/g without HAs/FAs). HAs/FAs could also combine with LDH to form a composite solid phase, thereby increasing the adsorption capacity of the solid phase for Fh-NPs. Furthermore, the influence of HAs on the Fh-NPs migration behavior was more significant than that of FAs due to the stronger binding interaction between HAs and Fh-NPs. Fluorescence spectroscopy combined with two-dimensional correlation spectroscopy revealed that Fh-NPs that preferentially combined with HAs/FAs caused a decrease in the ability of HAs/FAs to complex with kaolinite and LDH. The results of the present study provided a deep understanding of the migration mechanism of Fh-NPs and HAs/FAs in the soil environment, and provided scientific basis for predicting the geochemical behavior of pollutants or carbon in the soil environment.