Speciation of REEs in ionic REE mine tailings under reclamation: Experimental and modelling study
Résumé
Specific to Southern China, ion-adsorption REEs deposits represented 30% of worldwide production in 2017. REEs were traditionally mined by heap leaching using ammonium sulfate, leading to significant environmental damage. Reclamation of the resulting mine tailings is limited by the poor physical conditions of the tailings, their low organic matter and nutrient levels as well as their high residual REEs content. Reclamation practices of such sites can include natural attenuation or phytoremediation, using soil amendment or construction, and planting of various species. These practices induce drastic changes in the biogeochemical conditions of the soils and thus potentially modify REEs and nutrients availability, affecting the sustainability of the reclamation.
To gain understanding on the impact of reclamation practices on REEs dynamics, we study a mine tailing site in Ganzhou city, Jiangxi province (116 km²), that has been abandoned for 10y. Experimental plots (2x2m) were installed on the top of the tailings to monitor plants and soil development over time and evaluate different phytoremediation techniques: phytostabilization using perennial grasses or fiber plants, and phytoextraction using REEs hyperaccumulators.
Understanding REEs fate and bioavailability requires knowledge of the speciation of REEs in the tailings. Size fractionation and elemental analysis showed that REEs are concentrated in the fine fraction of the material. SEM and X-ray microprobe analyses showed that Ce is mostly present as Ce4+, but Ce3+ was evidenced in co-localization with other REEs on several occurences in topsoil sample.
To evaluate the different practices, we developed a geochemical model describing REEs speciation in these materials. A process-based multi-surface model was parametrized for REEs surface complexation in soils in the platform ORCHESTRA. Based on measured contents of REEs, clays and Al/Fe-(hydr)oxides and solution pH and DOC concentration, we predicted dissolved REEs concentrations in pore water and CaCl2-extract. The model is evaluated by comparing modelled vs. measured REEs solid:solution partitioning in different plots. Results show the primordial influence of both pH and organic content in speciation and partitioning of REEs.