Weiye Liu, Mingjie Sun, Binghua Liu, Peng Lin, Xinghong Liu, Yanping Wang, Fangchun Liu, Hailin Ma
Sodium ions are the main harmful ions in coastal saline–alkali soils, and they seriously affect crop growth and soil structure. A bentonite/humic acid composite hydrogel, synthesized via graft copolymerization as a new type of water-retaining agent, can adsorb excessive Na+ in soil, thereby slowing down its adverse effects. This study used batch adsorption experiments to systematically investigate the effects of contact time, initial concentration, pH, temperature, and repeated cyclic adsorption on Na+ adsorption performance of the hydrogel material. The results indicated that Na+ equilibrium was achieved in 25 min, and the maximum adsorption capacity was 91.29 mg/g. Optimal adsorption occurred at pH 6–8.5, particularly in neutral to weakly alkaline conditions. At 30–50 °C, the bentonite substrate maintained excellent adsorption performance despite structural damage to the grafted copolymer. Mechanistic analysis revealed that adsorption followed pseudo-second-order kinetics and the Langmuir isotherm model, indicating chemisorption-dominated monolayer adsorption controlled by both intra-particle and liquid film diffusion. These findings demonstrate the potential of bentonite-based hydrogels for remediating coastal saline–alkali soils by mitigating Na+ toxicity.