The ferruginous district of Beni Bou Ifrour-Ouixane is located in northern Morocco. This mining area includes three abandoned iron mines: Ouixane, Axara, and Setolazar, where the tailings and rock waste have been oxidizing for over 32 years. The intensive mining of ore deposits has generated deep acidic pit lakes.

Geochemical and mineralogical techniques were used to characterize the tailings (Setolazar) and sediments from the open-pit lakes (Axara and Ouixane). Surface water samples were collected from the Axara open-pit lake to examine the hydrochemistry and heavy metal pollution characteristics. PHREEQC software was used to simulate the forms of heavy metals in the water and to better understand the interactions between the sediment and water in the open-pit lake.

The mineralogy of the tailings is dominated by jarosite (14–88%), gypsum (2–80%), and quartz (10–13%). Low quantities of hematite, magnetite, and tetanomagnetite were also identified. Lake floor sediments were composed mainly of gypsum (53-74%), quartz (10-32%), and langbeinite (15%). Wixane lake sediment contains morenosite (32%), bianchite (26%), and dolomite (9%), suggesting the presence of nickel and Zn-sulfates. Copiapite and jarosite are indicators of extreme acidic conditions. Ouiksane pit lake sediment contains more aluminosilicate minerals related to the local lithology, particularly the microdiorite. Fe and S constitute the main elements of the chemical composition of the studied samples. Mn is the predominant metal for mine tailings with values ranging from 198.49 to 638.65 µg/g, followed by As (606.93 to 16.12 µg/g); Cu (576.24 to 40.70 µg/g); Co (411.22 to 31.54 µg/g); Ni (144.70 to 16.94 µg/g); Zn (120.31 to 30.92 µg/g); and Pb (70.83 to 6.05 µg/g). The highest Mn, Zn, and Pb contents were recorded in lake sediments. Waters from Axara pit lake are characterized by acidic pH (3.12) with high conductivity (29.16 mS/cm), and very high sulfate contents (31506.2264 mg/l). High Fe (30.33 mg/l) and moderate Zn content (3.09 mg/l), Cu (1.15 mg/l), and Ni (1.59 mg/l) were recorded in Axara, indicating high contamination potential. This may be related to sulfate dissolution. PHREEQC modeling of sampled waters shows that Fe, Zn, and Cu exist in solution on FeSO4, FeCl, ZnSO4, and CuSO4 species, respectively. The dissolution kinetics of sulfate minerals control the distribution of the potentially toxic elements in water. The acquired data could be of great interest in managing potentially toxic element contamination and taking necessary remediation measures in this area.

The treatment of acid mine drainage (AMD) is an urgent issue due to mainly the economic burden of the treatment. The treatment of AMD using limestone emits CO₂ gas, which will become a major issue soon for carbon neutrality. Also, for carbon neutrality, expectations for enhanced rock weathering (ERW) have increased as negative emission technologies (NETs). The ERW has been applied in agricultural and marine sites which are moderately acidic to alkaline environments. In contrast, it is well known that the dissolution of most silicate minerals is enhanced by an order of magnitude in strongly acidic environments such as AMD. In this context, as a case study, geochemical reactive transport modeling of ERW with basalt at the AMD site in Japan was conducted to calculate the annual mass of weatherable minerals in the basalt and subsequent CDR. After that, a life cycle assessment (LCA) and techno-economic assessment (TEA) of ERW in the AMD were also conducted. This is the first study to evaluate both the effectiveness of ERW in AMD as one of NETs and its co-benefit in providing passive treatment for AMD with LCA/TEA.

Our research introduces an innovative approach by combining a one-dimensional reactive transport model (1D-RTM) of crushed basalt dissolution in AMD with LCA/TEA assessment to evaluate the effectiveness of basalt ERW in AMD. Additionally, arsenic adsorption by schwertmannite, a sulfuric iron oxyhydroxide known to precipitate under acidic conditions was predicted using a surface complex model (SCM) in 1D-RTM to assess the potential for passive treatment.

The results demonstrate that AMD substantialy accelerates weathering of basalt, leading to substantial net CO₂ removal within a year and ERW on AMD can be economical. Furthermore, the schwertmannite precipitation offers an efficient arsenic removal as an adsorbent, while the providing proton during schwertmannite precipitation could maintain the strongly acidic conditions for ERW. These two results indicate that passive treatment of AMD could be technically integrated with CDR by ERW in AMD.

The practical implications are twofold: the potential for large-scale CO₂ sequestration and the simultaneous passive treatment of AMD. This dual benefit could offer substantial economic incentives, particularly for AMD sites dealing with arsenic, iron, and sulfuric acid contamination. Future research should apply to active treatment to compare the active treatment using limestone currently carried out at AMD treatment plants with the proposed active treatment that combines ERW.

Releasing toxic metals from mining waste heaps presents environmental challenges, particularly in regions with a long history of ores processing. The mining industry produces vast amounts of waste, often stored in open conditions and exposed to changing weather. For this reason, post-mining areas risk becoming sites of enhanced microbial activity, especially at acidic pH, where weathering and bioleaching occur. Microorganisms, when they adhere to solid surfaces, contribute to acid mine drainage (AMD) and acid rock drainage (ARD) by facilitating the release of toxic elements into surrounding ecosystems. Studying how bacteria interact with minerals within porous media is crucial to understanding and mitigating these processes.

There is still little information on the mechanism of metal transformations due to the acidophiles' activities in mixed wastes. Surfactants and biosurfactants can accelerate the leaching rate by changing solid-liquid interfacial properties such as surface charge and wettability. In bioleaching, where interactions occur at the interfaces between microbes, solids, solution, and other components, modification of the mineral surface, already applied in ore beneficiation, can improve the adhesion of bacterial cells, enhancing the leaching process. Depending on the surface-active agent, it can also inhibit AMD formation by increasing cell mobility in the heap.

Acidophilic microorganisms play a major role in the extraction of metals from mineral deposits, and one belonging to the genus Acidithiobacillus was dominant in the post-mining heaps studied. Bioleaching experiments were conducted to evaluate the potential for toxic element release and the possibility of using post-mining waste as a secondary resource. It has been shown that up to 1000 mg/L of arsenic can be leached depending on process conditions. Adhesion, movement and retention of microorganisms in the mineral bed can affect the release of metals and metalloids. Therefore, bacterial mobility was studied in untreated waste bed and the ones with adsorbed surfactants. Tests were performed at constant temperature. Experiments were carried out in a column filled with solid and closed circuits.

This insight into microbial-mineral interactions highlights the importance of controlling bacterial activity in mining waste management. An improved understanding of these processes could lead to more effective strategies for minimising environmental contamination from mining heaps and optimising metal recovery through biotechnological applications.

Coal holds an important position within South Africa's commodities due to its current role as the primary energy source. Nevertheless, the beneficiation and processing of South African coals result in substantial wastes in the form of discards and ultrafine coal slurries. Previous research has shown the potential of coal ultrafines to serve as an energy resource if suitably beneficiated, given their comparable quality to run-of-mine coal. The issue of long-term environmental impact from these ultrafine coals also comes to the forefront, particularly concerning the substantial sulfidic content in coal ultrafines, which can contribute to the generation of acid rock drainage (ARD). In addressing these complexities, froth flotation is seen as a promising technique for the beneficiation of ultrafine coal waste. However, this technique is not without its challenges. Coal ultrafines often encompass a significant proportion of clay minerals, with kaolinite being a prominent phyllosilicate clay mineral. The prevalence of kaolinite as a gangue mineral in South African coal ultrafines adds an intricate layer to the beneficiation process, as effectively depressing kaolinite during flotation can prove intricate. Efficient depression of kaolinite is thus necessary to separate it from ultrafine coal particles during flotation. Furthermore, considerations of pulp chemistries, such as process water quality in ultrafine coal flotation would be of paramount importance, especially given that there is water scarcity in regions within which coal mining occurs in South Africa. Because of this, the implementation of closed water circuits in flotation and effective dewatering of tailings for water recovery and dry stacking of tailings have gained much prominence owing to the need to minimise water consumption and enhance waste management practice in line with SDG 6, 9, 12, 13, 14 and 15. Therefore, it stands to reason that effective depression of kaolinite during ultrafine coal flotation is crucial in achieving high-quality coal products with reduced impurities with an understanding of the influence of different flotation chemistries. This research considers kaolinite depression and dewatering characteristics in inorganic electrolyte concentrated process water in view of process water recirculation in coal flotation and the need for the dewatering of tailings. Laboratory scale tests such as zeta potential measurements and depressant adsorption studies were considered. Understanding the behaviour of kaolinite in these contexts is vital for sustainable coal processing and environmental stewardship.