Conference Agenda

Effluents, surface water, and groundwater were analyzed, and a detailed mineralogical investigation was conducted aiming the hydrogeochemical characterization of acidic effluents and waters from areas surrounding a uranium waste rock pile. Additionally, gases circulating in the surface layer of the pile, such as O₂, CO₂, H₂, and Ar, were characterized to understand the influence of oxidation and mineral dissolution processes.

The conceptual model, through spectral analysis of hydrochemical samples, revealed that manganese (Mn), uranium (U), and sulfate begin to be released into the effluents after three months of exposure. These compounds, under supersaturation conditions, precipitate in the porous medium and accumulate during the dry season. With increased rainfall, these elements are mobilized and discharged. The pH of the waters remains between 3 and 4 throughout the hydrological year, exhibiting little variation until the onset of the next rainy season, indicating the prolonged effect of acidification processes.

The results showed that the primary contribution of effluents from the pile occurs through groundwater recharge rather than surface infiltration. Water percolating through the tailings mass carries dissolved compounds along the profile, releasing ions such as sulfate, iron, and aluminum. The western portion of the pile is particularly critical, exhibiting reduced pH and high concentrations of dissolved solids, iron, and sulfate, characteristics typical of acid mine drainage (AMD). The ions present in this region are found to be in a state of supersaturation or near saturation limits. In contrast, the waters circulating in the eastern portion exhibit characteristics of natural waters, with less influence from sulfide minerals and saturation in iron oxides. The difference in oxidation patterns between the two areas was corroborated by measurements of partial pressures of O₂, CO₂, and H₂, as well as the isotopic signature of argon, confirming distinct geochemical processes.

To evaluate the dynamics of these processes, a numerical model was developed and calibrated using the PHREEQC software. The model allowed for the simulation of different hydrogeochemical scenarios, validating the observed precipitation and mobilization patterns. A sensitivity analysis of the parameters was conducted to identify the most influential variables in the oxidation and transport processes. This research provides a detailed understanding of the hydrogeochemical processes in uranium tailings piles and highlights the importance of groundwater recharge in the generation and mobilization of effluents, contributing to more effective environmental management in the area.

This work aims to minimize Acid Mine Drainage influence on two mining areas located in the Iberian Pyritic Belt, a Volcanogenic Massive Sulfide province of high historical importance regarding the mining exploration activities. Both the Caveira mine (Grândola, Portugal) and the Trimpancho mining complex (Spain), located on opposite sides of the Portuguese South Zone, ceased operations in the 1960’s. Since then, the region has been under the influence of acid drainage from waste piles, left in aerial exposure with poor coverage, without supervision, which releases potential toxic elements (PTE) to waterways adjacent to the mining areas, subsequently leading to the dissemination of these PTE to more distant lands, causing scattered contamination.

In a first step, laboratory-scale tests have been conducted to evaluate the best conditions of environmental remediation of both mining areas, by applying mining water samples, in contact with different alternative materials such as paper, marble and limestone sludges, iron oxides, clays and industrial activated carbon. In this first step, it was verified that paper sludge had a great potential on pH increment, since a composed mining water with pH of 1.64 increased to 6.23 in 11 days of contact, in a proportion of 1g of material for 50mL of mining water.

Analysis of 18 water samples from Caveira and other 18 from Trimpancho, collected in February 2023 showed extremely high values of As, Fe, Cu, Pb, and Zn with peaks of 17.66 mg/L, 2831.7 mg/L, 33.1 mg/L, 2.3 mg/L and 146.0 mg/L, respectively, in Caveira’s mining waters, and <0.1 mg/L, 2958.0 mg/L, 206.6 mg/L, 0.2 mg/L and 2402.1 mg/L, in Trimpancho’s mining waters. The highest values are associated to pH of ≈ 1,22 (Caveira) and ≈ 1.11 (Trimpancho). In addition, we tested the removal of PTEs through column tests, with different combinations of geomaterials and wastes, simulating reactive barriers, to verify which set is most effective for containing and remediating these acidic drainage waters rich in toxic metals.

These laboratory-scale tests showed that the three sludges and the industrial activated carbon, are the most suitable materials for PTEs retention, such as As, Fe, Cu, Pb, and Zn. Also, due to the diversity of PTEs, there is geochemical competition between the alternative materials versus chemical elements, so a multilayered reactive barrier shows to be the most suitable way for future applications in a large-scale treatment out in the field, while attempting water remediation.

The oxidation of pyrite-bearing waste rock is a critical environmental concern, as it leads to the generation of acid mine drainage (AMD), which releases acidic and metal-rich waters into the surrounding environment. AMD poses a significant threat to water resources, making it essential to reduce the oxidation of sulfides, such as pyrite (FeS₂), to mitigate its environmental impacts. Stabilizing waste rock using Ordinary Portland Cement (OPC) is a promising approach, as it serves as a physical barrier against oxygen and moisture, thereby limiting pyrite oxidation and, consequently, reducing the risk of AMD generation. However, the high carbon emissions associated with cement production highlight its environmental drawbacks, making OPC less ideal for this stabilization technique. Thus, there is a need to identify and develop effective alternatives to OPC.

In this study, we explore the potential of slag-blended cement as an alternative to Ordinary Portland Cement (OPC) for stabilizing pyrite-bearing waste rock. Pyrite-bearing waste rock was fully encapsulated with OPC and slag-blended cement and then cured for 28 days under controlled conditions. The specimens were subsequently placed in a climate-controlled chamber for 64 days allowing interactions between the cement and waste rock.

Throughout a 64-day leaching experiment, OPC-treated waste rocks produced leachates with slightly higher pH levels and generally higher major ion release. This is likely due to the faster reactivity of OPC and the early-stage dissolution of soluble cement hydrate phases. In contrast, slag mix-treated waste rocks exhibited slower ion leaching, which may be attributed to the slower reactivity of slag, highlighting the need for a longer curing period. Some dissolved trace element concentrations (Cu, Mn, Ni, Pb) are slightly higher in the leachate from untreated waste rock signifying the early-stage performance of both cementitious materials in immobilizing metal(loids).

Further research involving extended leaching durations and microstructural analysis is required to evaluate the long-term effectiveness of slag-mix cement, particularly under conditions where waste rock oxidation occurs. This research seeks to provide a comprehensive understanding of the geochemical interactions within the cemented matrix, contributing to the identification of the most effective and sustainable alternative material for stabilizing sulfidic waste rock.