Effects of organic/inorganic amendments on trace elements dispersion by leachates from sulfide-containing tailings of the São Domingos mine, Portugal. Time evaluation

•Sulfide mine wastes generate leachates with low pH and large elemental concentrations.•Leachate trace element dispersion is minimised by the addition of soil amendments.•The rate of amendment addition does not affect leachate characteristics.•Copiapite-group mineral efflorescent salts reflect the very low pH of the wastes.•Alunite-jarosite and apatite-group minerals control leachate elemental composition.

Greenhouse pot experiments were conducted over 13 months to evaluate the effect of two amendment mixture doses (30 and 75 Mg/ha) on the geochemical dispersion of trace elements by leaching hazardous mine wastes from the São Domingos mine. Mineralogical evolution of these materials was also evaluated. Amendment mixtures containing solid wastes from agriculture (plant remains + strawberry substrate and rockwool used for the strawberry crop) and from distillation of Ceratonia siliqua L. and Arbutus unedo L. fruits were used to improve the chemical characteristics of leachates from sulfide mine wastes. Sulfide mine wastes had acidic characteristics, as well as high electrical conductivity and total element concentrations (g/kg; Al: 54.8-61.2; Fe: 104.0-110.0; Pb: 9.0-13.8; S: 63.6-68.0; As: ≈ 1). These features contributed to the large capacity for leaching of hazardous elements during, at least, the first four months of incubation. In the seventh month of incubation, there was a significant decrease in the leachate concentration of the majority of hazardous elements. The addition of amendments minimised trace element dispersion in leachate percolation during the first seven months (25 to 99% reduction compared to control, depending on the element and sampling period). However, the leachate characteristics were not influenced by amendment doses and no significant differences were observed in leachate composition (control and amended treatments) after 13 months. Amendment application led to differences between the solid phases of the efflorescent salts formed on the surface of the control and the amended treatments. The efflorescent salts contained very soluble aluminium sulfates, together with alunite-jarosite-group solid phases in amended samples, and copiapite-group solid phases in control. In the core materials (5 to 10 cm in depth), the mineralogy was similar in both control and amended samples. The presence of various stable solid phases from alunite-jarosite-group, such as jarosite and beudantite (mainly in deeper materials), can explain the low concentrations of trace elements in the leachates after thirteen months.