Towards Sustainable Remediation of Ionic Rare Earth Mining Areas in China: Enhancing Phytoremediation Efficiency of Dicranopteris pedata with Exogenous Organic Acids

Achieving sustainable land restoration in southern Chinese ionic rare earth mining areas remains a significant challenge due to the extended duration and low efficiency of conventional remediation approaches. Although the hyperaccumulator Dicranopteris pedata possesses a remarkable capacity for rare earth element (REE) enrichment, a significant knowledge gap exists regarding how to effectively combine exogenous organic acids with agronomic practices like clipping to enhance its remediation efficiency in an environmentally sustainable manner. Crucially, the potential environmental risks associated with such synergistic strategies have not been systematically evaluated, hindering their practical application. To address this, our study focused on Dicranopteris pedata and employed integrated pot and soil column leaching experiments to systematically analyze the effects of different concentrations of citric acid and tartaric acid on REE migration and transformation within the soil–plant system. The results demonstrated that exogenous organic acids significantly reduced soil pH and promoted the conversion of REEs from the residual to the exchangeable fraction. Specifically, the 20 mmol·kg−1 citric acid treatment increased the proportion of exchangeable REEs by 43.46%. Furthermore, organic acid treatments significantly altered the REE uptake patterns in Dicranopteris pedata, inhibiting the translocation and accumulation of REEs in the aboveground tissues. Soil column leaching experiments revealed that citric acid drove the migration of REEs to deeper soil layers, with the concentration peaking at 288.33 mg·kg−1 at a depth of 6–8 cm; concomitantly, the REE content in the leachate reached its maximum on the 5th day. This study demonstrates that the combined application of 20 mmol·kg−1 citric acid and 100% clipping management increased the annual REE accumulation in Dicranopteris pedata to 4.85 g·m−2, thereby significantly shortening the theoretical remediation period from 25.0 years in the control to 12.1 years. Soil column leaching experiments indicated no significant secondary pollution risk associated with this strategy. These findings provide a feasible, low-risk, and sustainable technical strategy for the synergistically enhanced remediation of REE-contaminated soils, offering a promising path for ecological restoration and sustainable land management in degraded mining ecosystems.