Yield and toxicity risks of a mixed pasture in sulfide-rich tailings recovered with a Technosol

The combination of designed Technosols and revegetation is one of the favored remediation at medium-long term and valorization strategies for sulfide-rich tailings. The plant-Technosol interactions improve several physic-chemical characteristics of mine tailing material and reduce its environmental risk. However, the transfer of potentially hazardous elements (PHE) for the aerial parts of plants can pose toxicity risks for fauna and limitations on the commercial valorization of the crop (e.g., animal feed). The current study assessed, in sulfide-rich tailings recovered with a Technosol designed with eutrophic and alkaline properties: i)Yield and toxicity risk of a mixed pasture; ii)Environmental risk of the PHE leaching. Sulfide-rich tailing without Technosol was used as control.

The current experiment was carried out in microcosm and greenhouse-controlled conditions for two consecutive growth cycles. Each plant pot had available for plant growth a surface of 200.96 c, 14 cm depth and ≈3 kg of Technosol+Sulfide tailing (1:2.5 m:m). A total of ≈13 g of seeds/pot was sown, one month after incubation of the materials (t0). Plant shoots dry weight was determined at what was considered the peak of each growth cycle, to estimate the crop yield. The PHE contents were determined in the dried plant shoots from each of sampling (t1 and t2: five and eight months after sowing, respectively) and in simulated soil pore water in t0 and t2.

Sulfide-rich tailings presented pH≈2.7, very low fertility and high PHE concentrations in pore water. These conditions did not allow any germination of pasture. The designed Technosol improved several physic-chemical characteristics of the rich-sulfide tailings (e.g. increase of pH to ≈8, available nutrients and organic C, and decrease of PHE in pore water), allowing a significant pasture cover.

The average yield of pasture was of 1.08 g and 1.05 g per pot, for t1 and t2, which correspond to an average yield of 0.530 t/ha. In general, PHE concentrations in plant shoots were within the range considered as sufficient/normal for plants and met the tolerable plant contents for agronomic crops (e.g., mg/kg- Cu: 14.76 and 8.95; Zn: 100.63 and 75.12; Fe: 305.26 and 417.79 for t1 and t2 respectively). By contrast, Mn concentration in plant shoots (mg/kg- t1:432.39; t2: 351.34) were above normal plant concentrations falling, in some cases, within the range considered as phytotoxic (>400 mg/kg). Nonetheless no visual signs of Mn phytotoxicity were observed. For animal diets, PHE concentrations in plant shoots were below the maximum allowable for pre-ruminants and ruminants, thus not preventing the use of plant shoots as feed.

Although additional assays should be done in order to improve the pasture yield, this study showed that is possible a secure environmental rehabilitation of rich-sulfide tailings through this designed Technosol and the economic valorization of these recovered areas for pastures of domestic animals.

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.