Phytoremediation strategies for the reclamation of tailings and mining soils in an active open-pit site

This study evaluated two phytoremediation strategies at an active open-pit mining site within a dunite deposit, an ultramafic igneous rock rich in nickel. The strategies included phytostabilization for tailings, which are fine fractions resulting from rock processing, and phytoextraction for mining soil, composed of natural soil aggregates mixed with fine dunite fragments. Tailings had an alkaline pH (8.8), low organic matter content (9.4 g kg^-1), and high available Ni concentrations, requiring immobilization using compost, vermicompost, biochar, and vermichar. These amendments were tested in 1-kg pots over 180 days, assessing soil properties, plant growth, and Ni accumulation in Lolium perenne L. In contrast, mining soil showing a more neutral pH (7.6), higher organic matter (106 g kg^-1), and also high available Ni contents, underwent phytoextraction supported by EDTA and citric acid in a parallel experiment. Phytostabilization with compost, vermicompost, and vermichar significantly reduced available Ni (<5 mg kg^-1), improved plant growth, and increased labile carbon, whereas biochar was less effective. Vermicompost proved particularly beneficial, enhancing nutrient and carbon availability, minimizing Ni leaching, and preventing excessive accumulation in plant aerial parts. Meanwhile, phytoextraction was successfully implemented without altering soil properties, with EDTA boosting Ni accumulation in aerial biomass up to 400 mg kg^-1, outperforming citric acid. These findings highlight the adaptability of phytoremediation, demonstrating the successful application of both strategies in the same environment, with Lolium perenne L. functioning either as a metal accumulator or a phytostabilizing plant depending on the amendments used.