<p>The adsorption of Sr²⁺ and Cs⁺ onto Gaomiaozi bentonite colloids (GMZC) was investigated to evaluate their role in radionuclide migration. The results show that within a concentration range of 20–100&#xa0;mg·L⁻¹ for Sr²⁺ and Cs⁺, the distribution coefficient decreases with increasing ion concentration. The mechanistic study indicates that the adsorption of Sr²⁺ is predominantly governed by surface complexation, whereas that of Cs⁺ is mainly driven by ion exchange. Furthermore, Sr²⁺ induces structural ordering of GMZC layers and promotes particle aggregation. However, in Na-Cl-SO₄ groundwater, the bentonite suspension showed poor stability and a significantly weakened adsorption capacity for both ⁹⁰Sr and ¹³⁷Cs, demonstrating the decisive impact of water chemistry on its barrier function.</p>