The importance of total inorganic carbon (TIC) chemistry in passive mine water treatment is well understood and is supported by numerous publications. However, there is less published information on how TIC effects active treatment both negatively and positively. For example, on raising the pH to promote iron oxidation with air, dissolved carbon dioxide is converted into bicarbonate/carbonate increasing alkali demand and sludge generation -effectively fighting against the process. Conversely with net alkaline waters the TIC can be beneficially used to maintain the pH reduce sludge generation provided an alternative oxidant (such as hydrogen peroxide) is used for iron precipitation.

The paper uses the following 4 mine water projects to demonstrates how by carrying out a series of carbon speciation analyses the plant performance can be improved:

Mine A – Highly acidic metalliferous mine water - Analysis revealed the water contained around 37 mg/L TIC present in the form dissolved CO_2. Modelling revealed that as a result around 50% of the added lime was consumed in converting this to bicarbonate and carbonate. In comparison the back calculated figure using actual plant data showed 47% wastage. Further calculations revealed that by air stripping the carbon dioxide, this could be reduced to less than 25%

Mine B – Circum neutral pH metalliferous mine water. Understanding the carbon chemistry for this application was crucial in reducing the lime demand and sludge generation rate 82.8% and 62.4% respectively.

Mine C – Circum neutral pH coal mine water with degassing followed by HDS iron precipitation. Understanding carbonate chemistry allowed a 50% reduction in lime by simply degassing prior to iron precipitation. Modelling demonstrated that by switching to peroxide iron oxidation, the water could be treated without the need for carbon dioxide stripping and lime addition- substantially reducing the operating cost

Mine D - circum neutral pH coal mine water with iron and manganese consents. By means of simply modelling the carbon chemistry it was possible to optimise the process at design stage, with the predicted savings subsequently confirmed by on-site pilot testing.

In conclusion, by carrying out carbon species analysis it is possible to not only understand the negative effect this can have on alkali demand and sludge generation but also develop an understanding of how these can be reduced these by working with thetic chemistry rather than fighting against it.

The Sustainable Active Treatment (SAT) system being developed by Cambrian Environmental Technologies (CET) presents a meaningful advancement in the treatment of circum-neutral mine water (CNMW). The process offers a rapid, sustainable, and cost-effective approach that not only improves water quality, but also provides the potential for the recovery of valuable metals.

The initial step of the SAT system is to degas dissolved CO₂ from CNMW, in order to elevate the pH to allow removal of Zn and other metals present via precipitation. The most recent advancements to the CET SAT system involved constructing a low-cost device to pre-strip carbon dioxide (CO₂) from compressed air before utilising the CO₂-depleted air to raise CNMW pH to 8.3.

Ocean acidification (pH reduction) is a well-known consequence of elevated gaseous CO₂ concentrations at the surface of water. The opposite phenomenon, removal of dissolved CO₂ from water, therefore, increases the pH. It was proposed that a steeper partial pressure differential be introduced to remove the dissolved CO₂ from CNMW. Initially this theory was tested using nitrogen gas rather than air as the gas supply in the SAT system high-shear degassing step to decarbonise the CNMW from Nant-y-Mwyn Mine, Mid-Wales.

The removal of CO₂ by chemical scrubber is a well-known practice to remove excess CO₂ in enclosed breathing spaces such as spacecraft, submarines or scuba-divers’ rebreathing apparatus. A ‘chemical scrubber’ using crushed waste concrete was constructed in the laboratory. Air was passed through the column to strip the CO₂ from compressed air by reaction with the residual lime in the concrete. Results demonstrated that degassing with this CO₂-depleted air more effectively increased the pH in CNMW from circum-neutral to a pH above 8 than using compressed air, leading to the precipitation of metal compounds. The use of waste concrete as the CO₂ scrubbing media was successful, further enhancing the system's sustainability.

This work highlights the potential of waste concrete as a sustainable reagent for gas scrubbing for high-shear degassing applications and the SAT system as a practical solution for mitigating environmental impacts of abandoned mine sites.

The mining activities in the Le Cetine di Cotorniano Sb-mine (Southern Tuscany, Italy) ceased in 1948 and the site was abandoned, but there are still residential buildings near the mine. The surface and drainage waters, that drained the mine, are convoyed into a reactive permeable barrier (PRB) with the aims to reduce the concentration of Sb.

The reclamation works has commenced last year. A geochemical characterization was conducted in 2011, while a geochemical survey has been ongoing since June 2024. In this work, full-scale and a laboratory-scale PRB models, the latter built by a 3D printer with inert material to the Sb-rich waters, are working in parallel. The PRB design comprises four cascade boxes, each one divided into three further boxes. Each box is filled with gravels and alternating layers of calcite and biochar. In addition, different packaging and materials are tested to improve the PRB efficiency up to 90%. Currently, laboratory experiments are going on to test whether the PRB is able to remove other chalcophile elements such as As, Tl, Hg and Se.

The 2024 geochemical characterization of the surface water near the ore bodies had showed Sb contents up to 700 µg/L, while 20 mg/L of dissolved Sb were measured in the water of a piezometer upstream of the tailing pile. Downstream of the mine, the Sb concentration is much lower, since the water of the Rosia River had concentrations of <10 µg/L. The laboratory-scale PRB models had showed that the filling material was found to reduce up to 75% of the initial Sb content.

If this technique succeeds in reducing the other chalcophilic elements, in addition to Sb, this technique could be exported to other sites in southern Tuscany where remediation has recently started or is about to start. This could minimize both the risk of exposure to people living nearby and the environmental impact to the aquatic ecosystem.

Iron ore has been periodically mined in Stripa, Sweden, since the Middle Ages. Mining operations ceased in 1977, and the 490-meter-deep mine was later repurposed by the Swedish Nuclear Fuel and Waste Management Company (SKB) as a research facility until 1991 Afterward, the mine was flooded, and the above-ground facilities were repurposed for industrial use. In 2006, the remaining structures were listed as buildings with historical value.

The area surrounding the mine is relatively densely populated by Swedish standards, and many private wells have been drilled in the vicinity. The possibility of using water from the old mine shaft prompted a scientific investigation summarized here. Water samples were collected from streams near the site, from the mine shaft overflow, and from within the mine shaft itself (down to a depth of 200 meters). All samples underwent general geochemical analysis, including measurements of pH, dissolved oxygen, metals, and anions. Given the site's history, a wealth of historical data is also available from previous reports and research articles.

From the literature, three types of groundwater have been identified in the area:

1. Deep anoxic water, with a Na-Ca-Cl composition, high salinity, and a pH above 9.
2. Intermediate anoxic water, characterized by Ca-HCO3, low salinity, and a pH of 7-8.
3. Shallow oxic water, with traces of surface water, low salinity, and a pH of 5-7.

The deeper water (below 330 meters) is 8,000 to 30,000 years old, while the shallow water is more recent and heavily influenced by infiltrating surface water.

During the most recent sampling campaign, several notable changes were observed compared to historical data.. Notably, uranium concentrations in the mine shaft have increased to nearly 1 mg/L at a depth of 200 meters, compared to previous averages of 10 µg/L. Dissolved oxygen measurements indicated relatively high oxygen saturation for such deep water, ranging from 37% at 15 meters to 23% at 200 meters depth, which suggest water mixing and oxidation of uranium(IV) minerals.

These findings suggest that the water quality in wells surrounding the site may be unsuitable for consumption, which requires further verification through analysis. The outflow of uranium from the mine is estimated at 10-100 kg annually, is likely driven by increased dissolved oxygen and poses a regional environmental concern.

The current study focused on identifying valuable elements that can be recovered from Lancaster Dam and gold mine dumps in Krugersdorp, a city in Gauteng Province, South Africa. The mine dumps considered are from closed and abandoned mines and have been existing for many decades. The area is also known for high volumes of acid mine drainage. The concentrations of valuable and base elements were measured. Rare earth elements (REEs), Ni, Co, Ag, Li, Cu and Cr among others were detected and found to be in concentrations that can be recovered to help meet their demand. Salvaging REEs and other valuable elements from mine waste can help improve the economies of many countries since, the elements are in demand due to improving technologies and growing population. Other elements including those that are toxic at very low concentrations were also detected. Thus, suitable techniques for the recovery of valuable elements in the study area, are those that have high selectivity and efficiency.