Use of sulfide-rich tailings recovered with Tecnosols for the cultivation of species of energetic interest

Decarbonization and various energy improvement strategies are threatening the sustainability of food production as productive agricultural and forest land is increasingly occupied. Furthermore, mine soils and tailings, both in active and abandoned mines, represent a challenge in environmental rehabilitation, as well as an opportunity to use and valorize these spaces for several socioeconomic activities. The use of these areas for the production of energy crops or the occupation of energy production structures can be an option with great socio-economic interest. The present study evaluated the recovery of mine soils rich in sulfides through the application of a Technosol designed with alkaline properties and the development of a crop of energy interest, fodder maize. For this purpose, a mesocosm assay was carried out under controlled greenhouse conditions for three months with (n=4/treatment): mixture of the Tecnosol + Mine soil, in a 1:5 ratio (V:V), and a control with Mine soil only. Three maize seeds were sown in each pot. For the evaluation of effectiveness of the environmental rehabilitation, the physicochemical changes at the edaphic level were evaluated, as well as the percentage of germination, biomass produced and metal contents in the biomass.

The mine soil showed an acid reaction (pH 2.5), low fertility and high content of potentially toxic elements (PTE) both in the total (g/kg – S: 60.1, Fe: 54.1, Cu: 0.16, Zn: 1.3) and available fraction in the pore water (mg/L -Fe: 342; Cu: 38.5; Zn: 506). The Technosol application to the mine soil allowed an increase in the acid neutralization capacity, reaching reaction values around 7.7, and the fertility (e.g. organic C from <2 to 12 g/kg; available P from 0.4 to 22 mg/kg). Another important improvement of the mine soil was the decrease of PTE availability in the soil solution by ≈99%, reaching very low concentrations (e.g. <0.5 mg/L for Fe and Cu; <0.15 mg/L for Zn). This decrease in PTE concentrations was associated with an increase in pH and a decrease in the oxide-reduction potential. In the mine soil (control), the maize did not germinate, while with the Technosol application, the germination percentage reached 67%. After three months of growth, the maize reached an average height of 25.58 ± 3.89 cm and a fresh biomass of 14.24 ± 3.76 g per pot, corresponding to a production of 1.2 t/ha. The elemental composition of the maize biomass was analyzed, and the metal contents (e.g. Cu: 0.03 mg/kg; Zn: 0.18 mg/kg) were below the levels of quality feed for livestock consumption, thus posing no risk of toxicity for either livestock consumption or energy production.

The results of the study showed the effectiveness of the designed Technosols in the recovery of sulfide-rich mine soils as well as the feasibility of implementing energy crops in these recovered areas with a socio-economic use.

Acknowledgment: This research was supported by Fundação para a Ciência e Tecnologia (UID/AGR/04129/2020, Non-foodCropMine Project). The authors thank Minera Los Frailes for technical cooperation and providing access to the study area and field samples.