Fulvic acid-iron colloid promotes the migration of As(III) in porous media.

Fulvic acid (FA), a highly reactive and soluble fraction of dissolved organic matter in cultivated soils, facilitates the formation of stable colloids through complexation with iron (Fe), thereby significantly modulating the environmental mobility of arsenic (As). However, the migration behavior of As associated with FA-Fe colloids in porous media remains insufficiently characterized, particularly regarding the integration of coupled migration processes with quantitative modeling. This study investigated colloid-mediated As(III) migration in saturated porous media using column experiments and a time-fractional advection-dispersion equation (fADE). Increasing FA concentration enhanced As mobility, as evidenced by elevated breakthrough ratios and an increase in the fractional order α from 0.475 to 0.881, signifying the attenuation of memory effects and a transition toward Fickian migration. Conversely, elevated Fe concentrations promoted colloidal aggregation and suppressed As migration, with α decreasing to 0.437, capturing non-Fickian behavior associated with particle retention and deposition. Mechanistically, FA stabilizes FA-Fe colloids through electrostatic repulsion and steric hinderance while competing for adsorption sites, whereas Fe induces aggregation and enhances pore-scale interception, leading to As sequestration via inner-sphere complexation with Fe-OH groups. Under alkaline conditions, surface charge effects strengthened electrostatic repulsion and promoted migration, while elevated ionic strength compressed the electrical double layer, facilitated deposition. These results demonstrate that As migration is governed by the coupling between colloidal stability and interfacial interactions, which is effectively quantified by fADE. These findings provide a theoretical framework for understanding As mobility in subsurface environments and offer critical insights for groundwater remediation strategies involving colloid-facilitated migration.