Environmental remediation of legacy mining sites in Portugal has been done since 2001 by EDM - Empresa de Desenvolvimento Mineiro, a state-owned company, including radioactive and sulfide polymetallic mines. One of the main focuses of the remediation design projects is the control and treatment of mine water using combined active and passive treatment systems. This paper presents the mine water treatment processes and monitoring that are implemented at Urgeiriça uranium legacy site. The treatment system is divided into two lines, active treatment and passive treatment. The active treatment includes pH neutralization with calcium hydroxide and addition of barium chloride followed by and sedimentation for solid-liquid separation. The secondary passive treatment system, includes several steps such as aeration, sedimentation, filtration in adsorbent media and phytoremediation in aerobic wetlands. Both systems are monitored before, in the intermediate steps and at the end of the treatment, and the water quality control programme includes in situ parameters (pH, electrical conductivity, temperature, redox potential, total dissolved solids and flow rate) and laboratory analysis of chemical and radiological parameters as total uranium, U238, U234, Ra-226, sulphates, chlorides, manganese, calcium and sodium. These elements were identified in previous studies as the best indicators of hydrogeochemical contamination related to the legacy uranium mining sites in Centro Region, Portugal. This paper will present an overview on the evolution of the mine water quality and treated volume since 2001, including a critical assessment on the effects of the implementation of the remediation of two tailings dams, waste rock piles, uranium mill and other affected areas in the Urgeiriça legacy mining site and the resulting water quality improvement, specific removal efficiency rates and compliance with regulatory limits for water discharge. Also, it will present the quantities of used chemical reagents and operation and maintenance costs of mine water treatment and how these processes can be further optimized.

In a small-scale column leaching study, treating pyritic waste rocks with silica (SiO2) proved suitable for preventing Acid Mine Drainage (AMD) formation by chemically passivating pyrite surfaces and maintaining the leachate pH at a circumneutral level. Aligned with the circular economy initiative and to ensure economic feasibility, treating pyritic waste rock to prevent AMD in the future should utilize readily available silica-bearing industrial residues as a potential source of dissolved Si.

However, preventing AMD by silica treatment on pyritic waste rock using industrial waste streams as a source for dissolved Si has yet to garner attention in previous research. Therefore, in this study, we will test several waste stream samples from extractive industries at various pH conditions to find the most optimum conditions to enhance their dissolution to leach out the silica and alkalinity. A batch leaching test will be conducted on several materials, i.e., waste rock containing a substantial amount of reactive silicate minerals (clays and micas), silicate-bearing desulfurized tailing, and siliceous slag. The materials will be tested at various experimental conditions, i.e., different pH levels and redox conditions, and as a mixture. Slag will be added to waste rock in a small proportion to achieve an alkaline pH. The leaching will also be conducted in an acidic pH environment to allow an acid-consuming reaction of the silicate phases, releasing silica and alkalinity.

Batch leaching using milli-Q water at circumneutral pH resulted in a low release of Si from these materials (i.e., 9 mg/L Si from the tailing, 9.6 mg/L Si from air-granulated slag, and 2.5 mg/L Si from water-granulated slag at a liquid-to-solid ratio 2:1) and an overall very low leachability of Si, despite the large surface area of the tested materials. Due to the unsatisfactory yield, we will test the leaching at various pH ranges to maximize the release of alkalinity and silica, promoting the silica precipitation on pyrite to control AMD.

This study contributes to solving environmental issues in mine waste management and AMD prevention using a novel method, i.e., chemical passivation. We will present more detailed results of our experiments, including batch leaching test results (i.e., physical parameters and chemistry of the leachates), solid phase analyses using Scanning Electron Microscopy (SEM), and implications will be discussed.

Carbonate aggregates such as limestone are commonly used in passive mine water treatment systems to neutralize acidity and generate alkalinity to remove metals. The service life of the aggregate is typically limited by the accumulation of metals solids which decreases both permeability and reactivity of the aggregate. These decreases normally occur well before the neutralizing capacity of the aggregate is depleted. Carbonate aggregate treatment effectiveness can be restored at lower cost than replacement through mechanical cleaning of the aggregate to remove the metals solids.

Carbonate aggregate cleaning methods can be divided into three general categories: mixing, pushing, and screening. The mixing and pushing methods utilize equipment to agitate the aggregate within the treatment cell to dislodge solids. The screening methods used specialized equipment to separate metals solids from cleaned aggregate outside the treatment cell.

To determine which carbonate aggregate cleaning method is most effective, five cleaning methods were evaluated by cleaning 16 oxic limestone beds in Pennsylvania, USA. All methods involve mechanically handling the aggregate to dislodge solids which are then carried away by flowing water to a detention pond. Short tern cleaning effectiveness was quantified by determine pre and post cleaning treatment effectiveness, alkalinity generation, porosity, and retention time. The composition of solids dislodged by the cleaning was also analyzed and the cleaning throughput per hour was estimated.

The evaluation found that all methods can effectively remove solids from the surfaces and pore spaces of the aggregate. The primary difference between the effectiveness of the methods was how much of the dislodged solids were removed from the treatment cell. The incomplete removal of solids has important implications for long-term operation as they will eventually require removal and disposal. While cost varied widely based on site characteristics, equipment availability, and local labor rates, in all cases the cost of cleaning aggregate was less than half of the cost of new aggregate.

Improvement of aggregate cleaning methods will lower operational costs for passive treatment systems and allow better prediction of long-term costs. Cleaning and reuse of existing aggregate also has lower environmental impact than removal and disposal of fouled aggregate followed by production, transport, and placement of new aggregate.

This work provided a promising methodology for removing iron ions and recovering copper ions from copper-containing AMD by incorporating the copper ions sulfide precipitation, iron ions biomineralization, and lime neutralization. The experimental results indicated that nearly all copper ions were removed in the sulfide precipitation process and 82.2% of iron ions were removed after biomineralization treatment. Additionally, the consumption of lime slurry was reduced compared to the conventional direct neutralization method. By integrating various techniques, it is possible to improve the removal efficiency of iron ions, reduce the consumption of lime, and recover the copper and sulfur from the AMD.

The contamination of water sources with potentially hazardous elements (PHEs), particularly in areas with active mining activities, represents a considerable risk to the environment and public health. The release of PHEs, including arsenic (As), lead (Pb), and cadmium (Cd), from both active and abandoned mines into surrounding water bodies has resulted in increased pollution levels and posed serious threats to ecosystems and human health. This study explores the potential of waste-derived carbohydrates as a viable and efficient material for passive plate heat exchanger removal from mining-impacted waters, with a preliminary case study involving arsenic.

Hydrochar, a carbonaceous material produced through the hydrothermal carbonisation of biomass, has demonstrated considerable promise due to the functionality-rich surface it exhibits, which facilitates the adsorption of various contaminants. This study examines the potential of hydrochar derived from a diverse range of biodegradable feedstocks, including unused woods, green waste, the organic fraction of municipal solid waste, grape bagasse, and other analogous materials, as a sustainable and effective material for the passive removal of PHEs from mining-influenced waters. The study places particular emphasis on a preliminary case study involving arsenic.

This study undertook a meticulous examination of the ability of hydrochars produced at a pilot scale to capture arsenic in its two prevalent forms, As(III) and As(V). This provided a comprehensive understanding of the material's adsorption capabilities. The adsorption process was subjected to detailed analysis in order to determine the efficiency and capacity of hydrochar in removing arsenic from contaminated water sources.

This preliminary assessment of arsenic behaviour during the adsorption process provides a robust foundation for further research into the effectiveness of hydrochar in conjunction with other PHEs. This places waste-derived hydrochar at the forefront of a versatile, cost-effective, and sustainable solution for both active remediation efforts and long-term passive systems, such as constructed wetlands or permeable reactive barriers, which are essential for mitigating the environmental impact of mining activities. The findings of this study indicate that hydrochar could play a pivotal role in developing innovative and sustainable strategies for water purification and environmental protection.