Gradient elution method for treating the residual ammonium from the weathered crust elution-deposited rare earth tailings

In-situ leaching of weathered crust elution-deposited rare earth ores generates substantial ammonium salt residues, which might cause environmental contamination. To address this issue, this study developed a novel gradient elution method combining sequential deionized water and 0.02 mol/L MgCl₂ elution, targeting differently bound ammonium fractions for efficient removal. Experimental results demonstrated that gradient elution method achieved a remarkable NH₄⁺ removal efficiency of 1290.0 mg/kg, significantly outperforming single deionized water elution (163.3 mg/kg) or 0.02 MgCl₂ elution (1146.8 mg/kg). Compared with single Mg²⁺ elution, the gradient elution method also reduced MgCl₂ consumption by 244.0 g per kg of tailings at equilibrium. Furthermore, it simultaneously removed 253.9 mg/kg of calcium ions, 135.8 mg/kg of nitrate, and 125.5 mg/kg of sulfate, mitigating secondary pollution risks. Mechanistic analysis revealed that deionized water preferentially extracts weakly bound water-soluble ammonium from the diffuse layer, while Mg²⁺ subsequently displaces ion-exchangeable ammonium in the Helmholtz plane via ion exchange reaction. This dual-stage strategy overcomes the inefficiency of single-step elution, optimizing reagent use while maximizing contaminant removal. The proposed method offers an economically viable and environmentally sustainable solution for rare earth tailings remediation, with significant implications for industrial-scale applications in minimizing post-leaching pollution.