LAUSR

To investigate co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions, NH 4 + was selected as the target contaminant, and silicon and humic acid (HA) were selected as typical organic and inorganic colloids in groundwater. Column experiments were then conducted to investigate the transport of NH 4 + colloids under various hydrochemical conditions. The results showed that because of the different properties of colloidal silicon and HA after combining with NH 4 + , the co-transport mechanism became significantly different. During transport by the NH 4 + –colloid system, colloidal silicon occupied the adsorption sites on the medium surface to promote the transport of NH 4 + , while humic acid (HA) increased the number of adsorption sites of the medium to hinder the transport of NH 4 + . The co-transport of NH 4 + and colloids is closely related to hydrochemical conditions. In the presence of HA, competitive adsorption and morphological changes of HA caused NH 4 + to be more likely to be transported at a higher ionic strength (IS = 0.05 m, CaCl 2 ) and alkalinity (pH = 9.3). In the presence of colloidal silicon, blocking action caused the facilitated transport to be dependent on higher ionic strength and acidity (pH = 4.5), causing the recovery of NH 4 + to improve by 7.99%, 222.25% (stage 1), and 8.63%, respectively. Moreover, transport increases with the colloidal silicon concentrations of 20 mg/L then declines at 40 mg/L, demonstrating that increased concentrations will lead to blocking and particle aggregation, resulting in delayed release in the leaching stage. Graphical abstract