Adsorption Characteristics of Sodium Ions by Bentonite–Humic Acid Hydrogel: A Promising Water-Retaining Agent for Saline–Alkali Soil Improvement

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.