In the Iberian Pyritic Belt (IPB), Hg-sulfides, often found in association with massive metal-sulfides, are responsible for generating Hg-rich fluxes of AMD that represents a potential focus related to water quality degradation. This study aims to monitor the spatial and temporal behavior of Hg in the watercourses drained by a huge pile of mining wastes at Caveira Mine (SW Portugal), and to test the most effective technology in retaining Hg.

Water from the watercourses were collected in two consecutive years (2022-2023). Total-Mercury was determined in samples using a mercury analyzer (NIC MA-3000). The sample with highest levels, was selected for laboratory-scale tests to evaluate the effectiveness of some nature-based materials (powders of carbonate rocks, terra rossa, iron oxides, clays, cellulose residues, acacia biochar, activated carbon from coconut shell), to be applied in the upstream sectors of the watercourses, to restore the quality of the hydrographic network.

The Hg levels in the water of consecutive years followed an identical pattern, with major values in areas near the tailings (2022: 16-18µgg^-1, 2023: 26-28µgg^-1), above the reference value, 0.3µgg^-1 (EU Regulation, 2009). The retention capacity of materials in relation to Hg, was determined by analyzing the ionic exchange capacity, kinetic tests (Hg-solution at pH5.5) and simulation of “ponds” containing different proportions of materials vs. mining water. While cellulose, limestone and marble powders were most effective in raising the water pH, increasing from pH1.56 to 6.6 after 7 days of contact, activated carbon showed the greatest retention capacity, retaining 99.99% of Hg after 15 minutes of contact with the synthetic Hg-solution, followed by bentonitic clay and cellulose. Analysis of Hg in the water from the “ponds” test, where mining water was used in its natural state, showed a greater efficiency in reducing the original levels (461µgg¹) for limestone, marble, activated carbon and cellulose (2µgg¹-3µgg¹). The slight difference observed is due to: (1) some materials increased the water pH when in contact, while others did not (e.g. activated carbon) and (2) chemical competition at the adsorption sites of materials between Hg and other elements also present in excess (As-Fe-Cu-Zn-Pb).

The remediation technologies should be applied in areas where the AMD flows, before entering the streams, following two stages (1) pond with a cellulose bottom, limestone or marble powders to increase pH, (2) new pond or reactive barrier with activated carbon to retain the Hg that has not precipitated with Fe-oxides in stage 1.

Acid mine drainage (AMD) derived from the oxidation of sulfide has become a major environmental issue facing the global mining industry. AMD can be highly acidic, rich in sulfates and metals, and poses long-term, large-scale pollution, with its treatment being complex, costly, and challenging. Although various remediation technologies have been proposed, high operational costs associated with large-scale applications warrant further development of low-cost, sustainable AMD treatment technologies.

This study proposes a novel method for preparing multifunctional remediation materials using natural attapulgite and industrial by-product (alkaline residues). These materials were applied to improve the filler in constructed wetlands (CWs), substantially enhancing the performance and stability of the wetland system in treating AMD. Additionally, through in-situ mineralization, this study not only effectively reduced pollution levels but also explored the resource utilization of AMD, contributing positively to the development of sustainable remediation concepts.

Compared to traditional CWs, those filled with the attapulgite-alkaline residue composites demonstrated a 30% improvement in sulfate removal and a 10-70% increase in the removal of metal ions such as iron, manganese, copper, zinc, cadmium, and lead. These metal ions were primarily retained in stable forms within the wetland filler, such as carbonate-, Fe/Mn (oxide) hydroxides-, and sulfides-bound forms. Moreover, the composites mitigated the adverse effects of AMD on wetland plants and microbial communities. Notably, the abundance of sulfate-reducing bacteria (such as Thermodesulfovibrionia and Desulfobacca) were increased, promoting the formation of metal sulfides on the surface of the composites. This allowed the saturated composite to regenerate and continue capturing metal ions. The synergy between adsorption by the composites and microbial sulfate reduction enabled the CW system to maintain long-term, efficient, and stable operation. While AMD is a major pollution source, it is also a potential resource for metals and sulfates. This study utilized sulfates and metal ions from field AMD to generate layered double hydroxides (LDHs) in-situ, effectively promoting the direct fixation of arsenic and antimony in AMD. Additionally, the study revealed the formation mechanism of arsenic- and antimony-loaded LDHs and their competitive effects on LDH surfaces.

Overall, the research demonstrates that integrating low-cost, sustainable materials with advanced treatment strategies can offer a viable pathway for the green and sustainable management of AMD. This approach could have broad applications in the mining industry, especially in regions facing severe environmental impacts from AMD, and could also be extended to other industrial wastewater treatment scenarios.

The significant costs involved in Acid Mine Drainage (AMD) remediation have led to a search for low-cost liming alternatives. The cost-effectiveness of concrete, which has enabled it to be the most dominant construction material, can be tapped into to provide an economic passive treatment for AMD. Laboratory Investigations on the use of pervious concrete (PERVC) have shown that PERVC is an effective treatment method for AMD. However, no pilot studies have been conducted on PERVC.

The aim of the study was to design and evaluate the application of a PERVC permeable barrier system for the remediation of AMD at an abandoned coal mine site. Furthermore, the study aimed to show that PERVC can effectively treat polluted mine water to meet the national limits applicable to wastewater discharge into a water resource. The PERVC pilot plant comprised a gravel pre-treatment zone (PTZ) overlying a PERVC reactive barrier zone. A 2000L precast tank was used as a distribution tank for the AMD. From the gravel PTZ, the AMD seeped below onto a PERVC bed for treatment. Thereafter, the treated AMD went into a collection chamber. The concentrations of Na, Mg, K, Ca, Cr, Mn, Fe, Co, Cd, Pb, Ni, Cu, Zn, and Al were determined using the Avio 200 Inductively Coupled Plasma Optical Emission Spectrometer. The concentration of SO₄ was determined using the Aquakem 250 Discrete Analyser. The temperature (T), pH, electrical conductivity (EC), and total dissolved solids (TDS), were recorded using a Hanna Combo pH/Conductivity/TDS Tester. Pervious concrete samples were analysed using the Pan Analytical X-X’pert PRO X-Ray diffractometer (XRD) and the ZEISS EVO Scanning Electron Microscope to identify mineral phases formed before and after exposure to the AMD.

The results showed that following treatment with PERVC, the pH increased from 2.6 to 12. Al, Fe, Zn, Ni, Co, Cu, and Mn were effectively removed from the mine water with efficiency levels of 98% to 100% within 24h of the experiment. Throughout the two years of monitoring, all pollution indicators showed significantly lower values than the South African limits for pollutant discharge into a water resource. Precipitation of heavy metals with an increase in pH, along with their possible adsorption onto calcium silicate hydrate, are the primary mechanisms for heavy metal removal by PERVC.

The successful demonstration of the PERVC treatment system offers a low-cost technology for polluted mine waters that can be marketed to the mining industry for practical implementation.