Dual-functional remediation of ammonia nitrogen contamination in ionic rare earth mines: Efficient washing and soil stabilization using lignosulfonates

Ammonia nitrogen (NH₄⁺-N) pollution from in-situ leaching of ionic rare earth mines poses severe ecological risks, including soil acidification and groundwater contamination. This study proposes a novel dual-functional strategy using lignosulfonates—calcium lignosulfonate (CLS) and sodium lignosulfonate (SLS)—to simultaneously remove NH₄⁺-N and stabilize soil structure. Batch and column experiments revealed that CLS washed 297.70 mg NH₄⁺-N (compared to 170.23 mg by water washing) from simulated contaminated soil, while SLS removed 306.79 mg. Kinetic modeling using the Elovich and Two-constant equations (R² = 0.910–0.941) indicated that NH₄⁺-N removal was primarily governed by heterogeneous diffusion and cation exchange processes. CLS outperformed SLS in soil stabilization, increasing shear strength by 114 % and reducing soil porosity by 49.40 % through particle aggregation. Field-collected soil experiments validated CLS's robust performance, achieving > 91 % removal of exchangeable NH₄⁺-N while maintaining a neutral pH (7.54 compared to SLS-induced pH 10.08), avoiding secondary alkalization risks. Mechanistic analysis highlighted Ca²⁺-mediated ion exchange, electrostatic interactions with sulfonic groups, and hydrogen bonding as key pathways. This work provides a sustainable solution leveraging industrial byproducts (lignosulfonates) for eco-friendly mine remediation, aligning with circular economy principles.