Alkali-Modified Zeolite: Adsorption Performance for Pb and Ammonia-Nitrogen and Its Remediation Effect on Soil from Rare Earth Mines

Co-contaminated soils in rare earth mining areas, particularly with Pb and ammonia-nitrogen, present a significant environmental challenge. These contaminants have the potential for lasting, irreversible effects on both ecosystems and human health. Therefore, developing efficient, sustainable, and cost-effective soil remediation techniques is critical. Remediation in these areas is not only vital for reducing environmental pollution and protecting ecosystems but also supports sustainable mining practices and resource utilization.　　Current research in this field, especially regarding Pb and ammonia-nitrogen co-contamination, is limited. Zeolite adsorption, a popular method globally, is effective in treating heavy metal contamination in soils, showing superior results over lime and phosphate treatments. However, enhancing the adsorption capacity of natural zeolite is necessary, for which various modification methods are being explored. Among these, wood vinegar, a product of biomass pyrolysis, shows promise in improving pollutant removal due to its antimicrobial properties. This study explores the potential of wood vinegar as an additive to alkali-modified zeolite to stabilize heavy metals and ammonia-nitrogen in soils.

In order to tackle the remediation of co-contaminated soil in rare earth mines.

Wood vinegar, sodium hydroxide, and wood vinegar-sodium hydroxide were employed for zeolite modification, and Pb and ammonia-nitrogen speciation were determined by a continuous extraction method. Dynamic adsorption experiments were conducted to preliminarily analyze distinct modified zeolites’ adsorption performance. Optimal mixing ratio of modified zeolites with soil samples was determined by column leaching experiments.　　Through indoor stabilization experiments, the stabilization effects of different modified zeolites on Pb and ammonia-nitrogen were compared, and chemical speciation changes and their environmental implications were discussed. Investigating the stabilizing impact of various modified zeolites on soil Pb and ammonia-nitrogen, including their influence on specific ammonia-nitrogen forms, was further explored. SEM, BET, and XRD analyses were employed to assess morphological changes and phase composition variations of zeolites before and after modification. Considering process and cost, alkali-modified zeolite was chosen for pilot-scale tests, verifying the stabilization efficacy of remediation materials in practical applications.

Adsorption performance in aqueous solutions. NaOH-MZ (modified zeolite by sodium hydroxide) and NaOH-2%WV-MZ (modified zeolite by sodium hydroxide combined with 2% wood vinegar) exhibited outstanding performance in Pb removal, achieving over 94% removal from a 200mg/L initial concentration. For ammonia-nitrogen removal, NaOH-2%WV-MZ and NaOH-MZ outperformed other zeolites, with removal rates of 66% and 65% for 30 mg/L, and 44% for 100mg/L. The adsorption efficiency of modified zeolite on ammonia-nitrogen varied with concentration, suggesting a correlation with zeolite dosage. Increasing modified zeolite dosage can enhance adsorption efficiency, as higher concentrations of ammonia-nitrogen reach saturation more rapidly.　　Column leaching experiments. At a 2% addition ratio of alkali-modified zeolite in soil, the most effective reduction in available Pb and ammonia-nitrogen forms was observed. Reduction trends continued over time, with a 50% decrease in Pb available form after 40 days and a 73% decrease in ammonia-nitrogen’s available form after 6 days.　　Stabilization effects on soil Pb and ammonia-nitrogen. NaOH-MZ and NaOH-2%WV-MZ exhibited stabilization effects on soil Pb and facilitated the transformation of ammonia-nitrogen. After 7 days, NaOH-MZ reduced available Pb content by 20%, and NaOH-2%WV-MZ by 26%, surpassing control and NaCl-MZ. The stabilization effect persisted over time, with optimal outcomes observed in the 6th week for 2% NaOH-MZ and NaOH-2%WV-MZ.　　Microscopic analysis. Alkali-modified zeolites showed structural changes favoring adsorption, with NaOH-2%WV modification leading to a looser structure with enhanced adsorption potential. The alkaline modification process involves cation exchange and leaching of silica components, transforming impurities like quartz into active silicates and amorphous silica.　　Pilot-scale tests. In an abandoned rare earth mining area, pilot-scale tests demonstrated the efficacy of 2% alkali-modified zeolite in reducing soil ammonia-nitrogen content. After 6 months, a remarkable reduction of 94.61% was achieved, highlighting the potential of alkali-modified zeolites for sustainable soil remediation.

Alkali and alkali-wood vinegar modifications enhanced zeolite structure, reducing impurities like quartz, and improving adsorption. Experiments show that alkali-modified zeolites, particularly at a 2% addition rate, effectively remove Pb and ammonia-nitrogen, achieving up to 50% Pb stabilization and a 94.61% reduction in ammonia-nitrogen in field trials. This research informs effective soil remediation technologies and sustainable development. While showing promise, further investigation is needed to assess long-term stability.