Green remediation of REEs-contaminated soil by biodegradable chelators with optimization and risk assessment

The omnipresence of rare earth elements (REEs) in the environment, driven by their extensive industrial applications and common occurrence in some weathered crusts, has raised significant concerns as emerging pollutants. Soil washing has been recognized as an effective approach to remediate REEs-contaminated soils. However, traditional washing agents frequently lead to soil acidification and salinization, adversely affecting soil microbial communities and plant growth. This study explored the use of biodegradable chelators (BCs)-specifically N,N-bis(carboxymethyl)-L-glutamic acid (GLDA), iminodisuccinic acid (ISA), and polyaspartic acid (PASP)-as alternatives for removing REEs from contaminated agricultural soils. Our findings demonstrated that the removal efficiency of REEs positively correlates with BCs concentration, liquid-to-solid ratio, and washing time, while showing a negative correlation with solution pH. Through response surface analysis, we determined the optimal parameters for the washing process, revealing that GLDA, ISA, and PASP achieved total REEs removal efficiencies of 50.8%, 40.5%, and 23.2%, respectively. Statistical analysis confirmed that the concentration of BCs was the primary factor influencing washing effectiveness. Moreover, washing with BCs significantly removed reducible REEs-those bound to iron/manganese oxides-thereby decreasing the mobility and bioavailability of REEs in soil. The reduction in bioavailable REEs significantly lowered the environmental risk associated with contaminated soil. Notably, the activity of soil enzymes improved post-washing with BCs, indicating a positive impact on soil health. This study provides valuable insights into the remediation of REEs-contaminated soils using BCs, with GLDA emerging as a particularly effective agent.