Can rare earth elements be recovered from abandoned mine tailings by means of electrokinetic-assisted phytoextraction?

Given the high impact of traditional mining, the recovery of rare earth elements (REEs) from hazardous waste materials could become an option for the future in accordance with the principles of the circular economy. In this work, the technical feasibility of REEs recovery from metal mine tailings has been explored using electrokinetic-assisted phytoremediation with ryegrass (Lolium perenne L.). Phytoextraction combined with both AC current and DC current with reversal polarity was applied (1 V cm-1, 8 h day-1) to real mine tailings containing a total concentration of REEs (Sc, Y, La, Ce, Pr, and Nd) of around 146 mg kg-1. Changes in REEs geochemical fractionation and their concentrations in the soil pore water showed the mobilization of REEs caused by plants and electric current; REE availability was increased to a higher extent for combined electrokinetic-assisted phytoextraction treatments showing the relevant role of plants in the process. Our results demonstrated the initial hypothesis that it is feasible to recover REEs from real metal mining waste by phytoextraction and that the performance of this technology can be significantly improved by applying electric current, especially of the AC type, which increased REE accumulation in ryegrass in the range 57-68% as compared to that of the treatment without electric field application.

A new hyperaccumulator plant (Spergularia rubra) for the decontamination of mine tailings through electrokinetic-assisted phytoextraction.

The screening of new effective metal hyperaccumulators is essential for the development of profitable phytoremediation projects in highly degraded environments such as mining areas. The goal of this research was to analyze the phytoextraction potential of the native plant Spergularia rubra to decontaminate and eventually recover metals (phytomining) from the mine tailings (belonging to an abandoned Pb/Zn Spanish mine) in which it grows spontaneously. To do so, the ability of this plant species to accumulate metals was evaluated both under natural conditions and through simple and electrokinetically assisted phytoextraction tests using alternating current and different combinations of voltage gradient (1/2 V cm-1) and application time (6/12 h per day). The complete duration of the greenhouse trial was 64 days, although alternating current was applied only during the last 14 days. The results obtained demonstrated the exceptional effectiveness of S. rubra for metal hyperaccumulation and growth without affecting toxicity in highly contaminated mining waste. Zn was the metal accumulated to a higher extent in the shoots, reaching concentrations up to 17,800 mg kg-1; Pb was mainly accumulated in the roots reaching a maximum concentration of 8709 mg kg-1. Cu and Cd were accumulated to a lesser extent but the bioconcentration factors were much >1. It has been proved that S. rubra is a hyperaccumulator species for Zn and Cd both in natural and greenhouse conditions and, very probably, Pb in wild conditions. The application of AC current did not significantly increase metal concentrations in plant tissues but it was able to increase the aerial biomass of S. rubra by 49.8 %. As a result, the phytoextraction yields of all metals were significantly improved as compared to wild conditions (up to 86 % for Zn). It could open new expectations about the economic viability of recovering high-value metals from mine tailings.

A classical modelling of abandoned mine tailings' bioleaching by an autochthonous microbial culture.

The aim of this study was to study and model the bioleaching of abandoned mine tailings at different pulp densities 1-20% w/v by using an autochthonous mesophilic microbial culture. Because of the importance of the ferrous-iron oxidation as sub-process on the bioleaching of sulphide mineral ores, the ferrous-iron oxidation process by the autochthonous microbial culture was studied at different ferrous-iron concentrations. A mathematical model fitted to the experimental results and the main kinetic and stoichiometric parameters were determined, being the most relevant the maximum ferrous-iron oxidation rate 5.1 (mmol Fe2+/mmol C·h) and the biomass yield, 0.01 mmol C/mmol Fe2+, values very similar to that of mixed cultured dominated by Leptospirillum strains. This autochthonous culture was used in the bioleaching experiment carried out at different pulp densities, obtaining a maximum metal recovery in the tests carried out at 1% w/v, recovering a 90% of Cd, 60% of Zn, 30% of Cu, 25% Fe and 6% of Pb. Finally, the different leaching mechanisms were modelled by using the pyrite as ore model obtaining a bioleaching rate of 0.316 mmol Fe2+/(L·h) for the direct mechanisms and a bioleaching rate for the indirect and cooperative leaching mechanisms of 0.055 Fe2+/(L·h).