<p>Specific ion effects have been proven to play an important role in the aggregation process of soil or clay mineral colloids. Previous studies indicated that goethite promoted the aggregation of montmorillonite colloids through electrostatic attraction; however, the underlying mechanism of specific ion effects remains unclear. This study investigated the hydrodynamic diameter of montmorillonite colloids that changed with the content of goethite. Moreover, at a certain content of goethite, the aggregation kinetics of the mixture of montmorillonite and goethite (MG) colloids were studied under NaNO₃ and KNO₃ solutions. Total average aggregation (TAA) rate, critical coagulation concentration (CCC), and activation energy were employed to compare specific ion effects between K⁺ and Na⁺. The result revealed that (i) Electrostatic repulsive force between colloidal particles governed montmorillonite aggregation, with hydrodynamic diameter reaching a maximum at a goethite relative content of 0.78, and (ii) the TAA rate was higher in K⁺ solutions than in Na⁺ solutions, while CCC values and activation energies followed the order of Na⁺ &gt; K⁺. Based on zeta potential measurements and differences in activation energy between K⁺-systems and Na⁺-systems, we proposed that non-classical polarization of monovalent cations might explain the specific ion effects. Atomic force microscopy (AFM) analysis demonstrated that goethite acted as a bridging agent to enhance montmorillonite coagulation. This study provides mechanistic insights into iron oxide-mediated clay mineral aggregation.</p>