Modifications to improve the application of method D3987-85 in the extraction of EPT from the abandoned mine tailings El Fraile, Guerrero, México

Abstract

The method D3987-85 is one of the leaching procedures used worldwide to assess the hazardousness of mine tailings. In Mexico, this method has been made official in the Norma Oficial Mexicana NOM-141-SEMARNAT-2003. During its implementation, some inconsistencies have been reported, which could have significant impacts on human health and the environment. In the present study, we carried out modifications to the stirring times and mixing ratios stated in method D3987-85. In addition, efficient sampling strategies were designed to represent more appropriately the hazardousness posed by old mining waste deposited in the tailing ponds, at El Fraile, in Taxco, Guerrero, Mexico. The characterization of these tailings shows total concentrations (in mg/kg) of potential toxic elements (EPT) in the following ranges: As (1,719.0–3,412.1), Ag (27.8–55.7), Ba (152.1–527.3), Cd (15.6–49.7), Cu (89.7–204.5), Mn (243.6–2,195.5), Pb (3,084.9–4,930.1), Zn (449.0–5,558.7); and it also shows the occurrence of the following mineral phases, in decreasing order of abundance: quartz, gypsum, potassic jarosite, phyllosilicates, hematite, potassic feldspars (sanidine). Furthermore, three samples of acid effluents (pH: 2.0–2.6) were collected in the tailings, which reported concentrations (mg/L) of SO₄^2- (3,509.3–12,249.9), Fe (459.3–1,505.0) and potentially toxic elements (As: 0.7–1.3, Cd: 2.8–12.3, Cu: 6.1–23.7, Mn: 76.4–316, Zn: 334.5–1,262.5 ). The chemical composition of the acid effluents was compared to that of the synthetic leachates (SJA) prepared in the laboratory using the method D3987-85. According to the NOM-141-SEMARNAT-2003, the acid effluents are considered hazardous when the concentration of dissolved Cd exceed the maximum permissible limit (>1.0 mg/L). However, the synthetic leachates prepared according to the method D3987-85 (18 ± 0.25h and solid-extractant ratio of 1:20) are not considered hazardous because the concentrations of dissolved Cd ranged from 0.2–0.4 mg/L. Nevertheless, when changes were made to the method D3987-85, by increasing the stirring times (24 ± 0.25 h, 48 ± 0.25 h, 72± 0.25 h, 192 ± 0.25 h) and decreasing the mixing ratios (1:10, 1:3), the concentrations of dissolved Cd increased and ranged from 0.3–2.4 mg/L. Therefore, it was proved that the synthetic leachates obtained by the modified method better represent the geochemical composition of the acid effluents collected in the field, thus contributing to a more rigorous and accurate hazardousness assessment of the tailings.