Margarita E. Gutiérrez-Ruiz

Taxco, Mexico, has centuries of polymetallic Ag–Zn–Pb mining. This review synthesized published information on potentially toxic element (PTE) geochemistry, environmental distribution, biological exposure, and ecological and human health risk along the Cacalotenango River. The maximum concentrations in La Concha tailings (mg/kg) were 11,800 As, 780 Cd, 43,700 Pb, and 37,994 Zn, whereas those in El Fraile were 5725, 384, 22,900, and 39,145, respectively. La Concha was neutral to alkaline and carbonate-rich, while El Fraile was more acidic and contained more secondary sulfates and Fe oxyhydroxides. Acidic conditions at El Fraile favored PTE mobility, although secondary minerals and cemented horizons provided attenuation. As and Pb were mainly associated with less labile fractions, whereas Zn and Cd were more labile. Soils and sediments near the impoundments showed the greatest contamination. Blood-Pb exceeded 5 µg/dL in 94% of children and 87% of women living relatively far from the impoundments, suggesting additional Pb sources. However, multiple sources, confounding stressors, particulate contamination, and reliance on total rather than bioaccessible concentrations limited attribution of biological effects. An integrated assessment combining source tracing, PTE speciation and bioaccessibility, biomonitoring, and longitudinal health studies is required to establish source–exposure–effect relationships and provide a scientific basis for a regional control program to protect human health.

This study focused on treating acid mine drainage (AMD) from a Zn-sulfide mine with a composition that includes Al = 445 mg/L, Fe = 263 mg/L, Mn = 364 mg/L, Cd = 2.8 mg/L, and Zn = 4,830 mg/L. After treatment with regional alkaline minerals, the pH increased from 3.0 to 6.3 and metal concentrations decreased mainly by precipitation, falling below Mexico’s permissible limits (MPL) for river discharges, except for Cd²⁺ and Zn²⁺, which exceeded their MPLs with concentrations of 0.4 and 1,110 mg/L, respectively. We tested the use of waste materials from a Mn mine containing Mn carbonates (R3) and Mn oxides (R6) for removal of these contaminants. Several Mn oxides have been reported as promising adsorbents and indeed the R6 waste was more efficient than the R3. At a ratio of 0.75 g/L of R6, with 30 h of contact or a ratio of 0.2 with 60 h of contact, Cd²⁺ and Zn²⁺ concentrations were decreased to below their MPLs. The evaluation of removal mechanisms was hindered by the complexity of the waste’s composition. However, contact with the AMD increased the zeta potential from negative to positive values, indicating a cation sorption process. Mn wastes can be used without prior alkaline mineral treatment, but their technical and economic viability is less. The results suggest that this process is suitable for treating AMD at abandoned mines sites. Additionally, the Mn wastes can potentially be sold as a sorbent material for other processes, offering a recycling option.

The study addresses Acid Mine Drainage (AMD) in Central Mexico from iron-sulfur ore mine which poses environmental risks. The initial passive treatment using alkaline minerals improved pH and reduced metal concentrations but exceeded permissible limits (MLP) for Zn2 + and Cd2+, set by Mexican regulations (Zn = 10 and Cd = 0.2 mg/L). This study aimed to improve the original process by utilizing waste materials from a manganese mine (Mn-wastes). These materials contain carbonates (R3) and oxides and clays (R6) with a high sorption capacity to reduce Zn²⁺ and Cd²⁺ concentrations below the permissible level (MPL) or to directly treat the AMD without using other alkaline materials. The original pH of the AMD was 2.99, with the following contents: Al (445 mg/L), Fe (263 mg/L), Mn (364 mg/L), Cd (2.82 mg/L), and Zn (4,830 mg/L). After the pre-treatment at pH 6.31, concentrations in mg/L were found to be below the limits of detection (LODs) for Al and Fe, while Mn = 279, Cd = 0.39, and Zn = 1,110 mg/L. The results indicated that using R3 and R6, with or without the alkaline pre-treatment, allows compliance with the MPL, but the technical-economic viability is greater with the pre-treatment. Waste R6 is more efficient than R3. The addition of R6 (Mn-waste) to pre-treated AMD in a ratio of 0.75 (R6/AMD) with 30 hours of contact or with a ratio of 0.2 and 60 hours, resulted in Cd²⁺ and Zn²⁺ soluble concentrations decreasing below their MPL. The Zeta Potential of both wastes after contact with AMD increased to positive values, indicating that sorption occurred during the removal of metal(oids).