Research Activities on Acid Mine Drainage Treatment in South Africa (1998–2025): Trends, Challenges, Bibliometric Analysis and Future Directions

Abstract

Acid mine drainage (AMD) remains a critical environmental challenge in South Africa due to its severe impact on water quality, ecosystems and public health. Numerous studies on AMD management, treatment and resource recovery have been conducted over the past 20 years. This study presents a comprehensive review of research activities on AMD in South Africa from 1998 to 2025, highlighting key trends, emerging challenges and future directions. The study reveals a significant focus on passive and active treatment methods, environmental remediation and the recovery of valuable resources, such as iron, rare earth elements (REEs) and gypsum. A bibliometric analysis was conducted to identify the most influential studies and thematic research areas over the years. Bibliometric tools (Biblioshiny and VOSviewer) were used to analyse the data that was extracted from the PubMed database. The findings indicate that research production has increased significantly over time, with substantial contributions from top academics and institutions. Advanced treatment technologies, the use of artificial intelligence and circular economy strategies for resource recovery are among the new research prospects identified in this study. Despite substantial progress, persistent challenges, such as scalability, economic viability and policy implementation, remain. Furthermore, few technologies have moved beyond pilot-scale implementation, underscoring the need for greater investment in field-scale research and technology transfer. This study recommends stronger industry–academic collaboration, the development of standardised treatment protocols and enhanced government policy support to facilitate sustainable AMD management. The study emphasises the necessity of data-driven approaches, sustainable technology and interdisciplinary cooperation to address AMD’s socioeconomic and environmental effects in the ensuing decades.

1. Introduction

Acid mine drainage (AMD) is a persistent environmental issue in South Africa, primarily resulting from historical and ongoing mining activities [1,2]. Water supplies become severely contaminated when sulphide minerals in mine waste oxidise, producing extremely acidic water that is teeming with heavy metals and sulphates [3]. Aquatic ecosystems, agricultural output and human health are all severely threatened by this occurrence, especially in mining-intensive areas such as the Witwatersrand Basin, Mpumalanga and Limpopo [4,5]. AMD has exacerbated ongoing disputes over the distribution of water resources and increased treatment expenses, contributing to water scarcity over time [6,7]. To address AMD and lessen its long-term effects, appropriate legislative frameworks must be combined with scientific and technical initiatives [4,8].

A substantial amount of study has been devoted to the development of treatment technologies, sustainable management strategies and regulatory measures due to the magnitude and complexity of AMD contamination [9,10,11]. To neutralise acidity and recover important metals from AMD, research has investigated chemical, biological and membrane-based treatment approaches [12,13]. To support sustainable mining and post-mining rehabilitation, policy-driven research has also sought to inform industry practices and governmental laws [14,15]. Despite a considerable amount of study in this area, the high cost of treatment methods, the lack of widespread large-scale application and the requirement for interdisciplinary cooperation are some of the issues that persist [3,16,17]. Examining the development of AMD research in South Africa can reveal important insights into current gaps and potential future directions [5].

Researchers, legislators and business stakeholders in South Africa have worked hard to comprehend and control AMD [18,19]. From early characterisations and treatment trials, research activities have progressed to more integrated strategies, including resource recovery, sustainable mine closure and the concepts of the circular economy [20]. The scientific knowledge base has been expanded through government-led programmes, such as those run by the Department of Water and Sanitation (DWS), the Water Research Commission (WRC) and partnerships with organisations, including the Council for Scientific and Industrial Research (CSIR), Mintek and universities [21]. However, AMD is still poorly handled in many areas despite advancements in technology and heightened awareness, highlighting the need for creative and scalable solutions [22].

A bibliometric analysis provides a methodical way to assess major contributors, research trends and the influence of scientific publications on AMD [23]. To identify influential publications, emerging themes and potential areas for further research, bibliometric studies analyse research output over time, citation patterns and collaboration networks [24]. Such an analysis can also reveal the extent of international cooperation and the contributions of South African scholars to AMD research worldwide. This approach is crucial for understanding the evolution of knowledge in AMD management, treatment and policy formation, which will ultimately inform future research objectives and environmental remediation innovation [25].

The purpose of this research is to provide a critical review and bibliometric analysis of AMD-related research activity in South Africa between 1998 and 2025. It examines the development of scientific research in this area, identifies significant patterns and gaps and highlights the most notable research, organisations and publishing houses. In addition to the analysis, key technological, environmental and policy issues are discussed, and tactical suggestions for future lines of inquiry are proffered. Previous studies have primarily focused on specific treatment methods without contextualising them within regional research progress, policy alignment or innovation trends. This review fills that gap by providing a comprehensive analysis of peer-reviewed literature, identifying underexplored technologies and mapping research collaboration networks. By integrating bibliometric analysis with a technical review, a unique and valuable perspective is offered to guide future research, policy development and the practical implementation in AMD remediation.

Moreover, this study aims to educate policymakers, researchers and industry stakeholders on the advancements in AMD treatment and management by providing a comprehensive summary of AMD research conducted in South Africa. The results will support the development of creative and economical AMD remediation options, promote interdisciplinary cooperation and assist in identifying priority areas for future research. The study aims to bridge the knowledge gap between research and application by promoting sustainable methods for mitigating the adverse effects of AMD on South Africa’s ecosystems and water resources.

2. Acid Mine Drainage’s Impact on Water Quality and Ecosystems

In 1980, the discharge of AMD into public streams was permitted by the Department of Water Affairs’ pollution control division, provided the water was neutralised [4,25]. By 2000, however, stricter regulations required that mine water be desalinated before being released into public waterways [26]. A significant milestone was achieved in 2005 with the completion of the eMalahleni wastewater treatment facility, the world’s first plant to supply purified mine water to a municipality [4,26]. This facility utilised the High Recovery Precipitating Reverse Osmosis (HiPRO^®) Process, designed and constructed by Keyplan, to effectively treat AMD. In 2018, the concept of zero-waste was adopted to further safeguard groundwater from contamination, particularly from leachate leaking through damaged plastic liners in evaporation ponds and pyrite-rich solid waste dumps [27,28]. The cost of disposing of solid and liquid hazardous waste ranges between R1,000 and R2,500 per tonne, with an additional R1,000 per tonne for transportation [29]. Furthermore, environmental legislation enforced since August 2019 prohibits hazardous landfill sites from accepting waste with a moisture content above 40% (Government Notice 636, Regulation 5, 2013). To meet these requirements, comprehensive mine water treatment approaches now include both neutralisation and desalination. A key advantage of implementing a zero-waste strategy is the elimination of mixed sludge disposal, such as gypsum and metal hydroxides, by instead recovering valuable, saleable products from the treatment process [30].

In South Africa, AMD continues to pose a significant risk to ecological integrity and water quality [20]. Degradation of aquatic habitats, biodiversity loss and contamination of essential freshwater supplies have all been caused by the release of highly acidic water loaded with dissolved metals, such as iron, aluminium and manganese, into rivers and groundwater systems [31,32]. High metal concentrations, low pH and sulphate loading have been reported in numerous studies conducted between 1998 and 2025 in impacted water bodies, particularly in mining-intensive areas such as Gauteng and Mpumalanga [33,34]. The deteriorating ecological health of the Olifants and Vaal River catchments due to AMD inputs has been the subject of extensive research [35,36,37].

AMD affects human livelihoods and terrestrial habitats in addition to aquatic ecosystems, particularly in communities that depend on these water supplies for household, drinking and agricultural purposes [38]. Metal toxicity and acidification damage the biological processes of aquatic creatures, interfere with nutrient cycles and make ecosystems less resilient to external stresses [39]. Long-term data indicate that ongoing AMD pollution continues to stress South Africa’s water infrastructure and ecological balance, despite the implementation of rehabilitation initiatives and monitoring systems [20]. This emphasises the necessity of sustainable treatment methods and integrated watershed management that are guided by current research and technological developments.

A study by Atangana [37] analysed surface water quality using an index-based water quality model in Vaalwaterspruit, Mpumalanga, South Africa. Temporal fluctuation for four years (January 2018–December 2021), sixteen physicochemical parameters, anions and trace metals were measured monthly to evaluate the surface water quality of Vaalwaterspruit. According to water quality indexes (WQIs) of the United States (US) and the Canadian Council of Ministers of the Environment (CCME), the stream is classified as bad (203–3072) for the US-WQI, good and fair (76.34–82.35) for the CCME WQI and fair status (0.61–0.87) for the comprehensive pollution index (CPI). The water quality, which deteriorated more in the downstream region over the four-year study period due to nutrients (including fluoride), trace metals and particulate matter, was best classified using CPI. To evaluate the data and pinpoint potential sources, multivariate techniques were used to analyse the relationship between the water quality data. Five eigenvalues were found to be more significant than one, accounting for 74% of the total variance. Apart from mining operations, the findings aligned with those of the principal component analysis (PCA), indicating a common source and revealing that sewage discharges and industrial effluents negatively affect the quality of surface water [37].

Sakala et al. [38] conducted a study on Geographic Information System (GIS)-based modelling of groundwater vulnerability, analysing the South African Witbank, Ermelo and Highveld coalfields. They found that to prevent additional effects on groundwater resources, it is necessary to identify groundwater zones that are susceptible to contamination from mine water. Some of these places have already been mined out, while others have not yet seen the commencement of mining. The findings of their analysis reveal a strong relationship between model values and pH, as well as groundwater sulphate concentrations. This demonstrates that the suggested strategy can indeed be applied in place of more conventional techniques for determining groundwater vulnerability [38].

Magagula et al. [39] conducted a study using various water quality indices to evaluate the effects of coal mining on water resources in Mpumalanga Province, South Africa. The evaluation was conducted over five years from 2017 to 2021 and examined specific chemical parameters throughout four distinct seasons, utilising integrated water quality indices. The CPI and the CCME WQI were used to assess the water quality conditions of surface water and groundwater. Some surface water monitoring locations have water quality ranging from low to fair, according to the combined indices. Additionally, groundwater monitoring stations were ranked from bad to fair. According to the CPI, specific locations have extremely low summertime water quality, above anticipated thresholds for the years 2017–2021. The PCA also revealed elevated chemical parameters and high levels of contamination at both surface water and groundwater sites. The outcomes were contrasted with the various water quality standards. To mitigate the impact on water resources, water management techniques must be effectively applied within a comprehensive monitoring programme [39].

The socioeconomic impacts of AMD in South Africa are deeply rooted in the legacy of extensive mining activities, particularly in the Witwatersrand and Mpumalanga regions [40,41]. Communities that rely on surface and groundwater resources for irrigation, drinking water and livestock production are directly affected by AMD contamination. The government and impacted communities are heavily burdened financially by the expense of obtaining alternative water sources or purifying tainted water [42]. Furthermore, AMD-induced biodiversity loss and soil degradation reduce agricultural production and restrict land-use alternatives, exacerbating poverty and food insecurity in vulnerable areas. Public health expenses and social vulnerability are also increased by the health concerns associated with prolonged exposure to heavy metals and acidic water [43,44]. With frameworks such as the National Water Act 36 of 1998 and the establishment of the Inter-Ministerial Committee on AMD, South Africa has made significant strides in recognising and managing AMD from a policy and governance standpoint [44].

3. Bibliometric Analysis

The bibliometric analysis provides a comprehensive summary of published research on AMD in South Africa from 1998 to 2025, utilising software tools to identify trends, research hotspots and pinpoint knowledge gaps systematically. Key topics covered in this analysis include limestone neutralisation, passive versus active treatment techniques, strategies for managing low-polluted water in small streams and acid leachate treatment, especially by pre-treating with coal dump leachate. It also includes membrane-based processes, biological sulphate removal (BSR), process modelling, magnetite recovery and beneficiation, lime treatment and plant operation. This thorough research mapping facilitates a better comprehension of South Africa’s challenges and advancements in tackling AMD [23].

3.1. Data Collection and Sources

A bibliometric analysis of research activities on AMD in South Africa from 1998 to 2025 was conducted using VOSviewer (version 1.6.20), RStudio (version 4.4.2) and reputable academic databases, such as PubMed. This platform was chosen because it indexes many technical papers, conference proceedings and peer-reviewed journals. The use of the PubMed database broadened the search to include grey literature, such as institutional reports. A methodical approach was employed, combining keywords and Boolean operators to ensure thorough coverage of relevant literature. Acid mine drainage, South Africa, AMD treatment and mine water pollution were the main search strings. These phrases were combined in various ways to encompass studies from multiple fields, including policy research, hydrology, geochemistry and environmental science. To identify new patterns, unmet research needs and potential avenues for AMD investigations in South Africa, the study utilised data spanning two and a half decades, from 1998 to 2025. Figure 1 illustrates the methodology employed in this study’s bibliometric analysis.

3.2. Trends in Acid Mine Drainage Research in South Africa (1998–2025)

3.2.1. Publication Trends over Time

The analysis of publication trends on AMD research in South Africa from 1998 to 2025 reveals a significant increase in scholarly output, particularly after 2010, driven by rising environmental concerns and policy interventions (Figure 2a,b). The past decade has witnessed the publication of a significant number of papers, indicating a heightened interest in sustainable AMD management among policymakers and researchers. This increasing tendency demonstrates the growing attention being paid by academia and industry to heavy metal removal, AMD treatment methods and water quality restoration.

Characterisation of AMD and its effects on the environment were the main emphasis of early research; however, studies conducted after 2015 indicate a move towards resource recovery, new treatment technologies and sustainable management techniques [2]. The increase in publications between 2018 and 2025 is indicative of growing industry and scholarly interest in mitigating the consequences of AMD, which is consistent with the implementation of stronger water quality standards and global sustainability goals. This increased trajectory is evidence that AMD is becoming more recognised as a significant water management issue in South Africa. Figure 2a,b shows the annual scientific distribution of AMD treatment in South Africa.

3.2.2. Leading Authors and Collaborative Networks

The bibliometric analysis reveals that leading authors in AMD research in South Africa (Figure 3) are primarily affiliated with major universities and research institutions specialising in environmental science, hydrology and mining engineering. Strong connections between South African researchers and their foreign counterparts, especially those from Europe, North America and Australia, are demonstrated via collaborative networks, which promote knowledge sharing and technological development. Funding and applied research initiatives are also greatly aided by government organisations and industry participants, including mining companies, the WRC, the National Research Foundation (NRF), the CSIR, the DWS, Mintek and others. Over the past 20 years, these partnerships have significantly advanced resource recovery, policy creation and AMD treatment options.

3.2.3. Thematic Analysis of Research Topics

Keyword analysis revealed dominant themes in the field, including water pollutants, mining, chemical analysis, adsorption, South Africa and hydrogen-ion concentration. According to a thematic analysis of research on AMD in South Africa from 1998 to 2025, key research themes have evolved from basic studies on AMD formation and environmental impact (1998–2010) to a greater emphasis on treatment technologies (2010–2015), including passive and active remediation methods. Recent studies (2020–2025) have focused more on policy and regulatory settings, evaluating the socioeconomic effects of AMD and the efficacy of government initiatives. This change marks a shift away from solely technical fixes towards a more comprehensive strategy that considers sustainability, viability and policy implications. The word cloud in Figure 4 represents the keywords related to AMD remediation in South Africa. The network visualisation in Figure 5 indicates that each node in the network denotes distinct keywords, while the lines represent the channels of association between keywords.

3.2.4. Journals Publishing Acid Mine Drainage Research

High-impact environmental and water science journals publish AMD research conducted in South Africa, guaranteeing widespread distribution and scholarly influence. Significant research on AMD treatment and remediation techniques has been published in prestigious journals, including Water Science and Technology, Environmental Management, Environmental Science and Health and The Science of the Total Environment (Figure 6). These periodicals support South Africa’s contributions to international AMD research by offering a forum for studies on novel treatment strategies, policy implications and case studies from mining locations throughout the country. Studies from South Africa are regularly published in these publications, demonstrating their scientific value and relevance to global water management issues.

3.2.5. Limitations of the Study

There are certain limitations to this study, especially about possible biases in the exclusion criteria and database coverage, which could compromise the thoroughness of the bibliometric analysis [45]. Relevant articles in regional or non-indexed journals, conference proceedings and industry reports may be underrepresented due to the dependence on specific databases, such as Web of Science and PubMed. Furthermore, database indexing practices and language limitations may cause essential research contributions to be overlooked. Trend analysis and collaborative mapping may be impacted by data gaps resulting from inconsistent author affiliations, differences in keyword usage and restricted access to articles. To ensure a more comprehensive understanding of AMD research in South Africa, future studies must consider a broader range of sources and approaches.

4. Treatment Technologies

Passive and active treatment methods are two fundamental approaches explored in South Africa’s research on AMD [46]. To treat AMD over time, passive treatment refers to low-maintenance, energy-efficient techniques that rely on natural processes, such as microbial activity, wetlands or limestone drains. In contrast, active treatment, which is typically used in constructed plants, utilises energy or chemicals, such as lime or sodium hydroxide, continuously to neutralise acidity and remove pollutants rapidly [47]. Studies conducted over the last few decades have examined the long-term viability, affordability and efficacy of both systems in various mining-impacted sites, underscoring the growing importance of hybrid solutions tailored to site-specific circumstances [48,49]. The combination of passive and active treatment technologies has garnered interest in South Africa as a comprehensive strategy to address the complex and site-specific characteristics of AMD [50,51,52]. Reverse osmosis (RO), oxidation and lime dosage are examples of active treatment techniques that provide quick and regulated neutralisation and heavy metal removal [3]. However, they frequently demand intense maintenance and have high operating costs [53].

The goal of recent research (after 2010) has been to combine these systems to minimise their drawbacks and maximise their virtues. An integrated approach provides a more sustainable solution, particularly for legacy AMD sites, where long-term water quality control is essential [54]. Active treatment, for instance, can be utilised to manage high pollutant loads during peak discharge events. At the same time, passive systems can be used to maintain compliance and polish effluents during periods of low flow [11]. Case studies from South Africa have demonstrated the effectiveness of integrating these techniques, particularly when supported by real-time monitoring tools and hydrological modelling [55,56]. The design of hybrid systems, the performance of passive units and the possibility of employing recovered elements from treatment sludge are all anticipated to be improved by future research, making integration a key component of AMD remediation techniques [16].

A study by Masindi et al. [57] assessed the sustainability of South Africa’s AMD treatment system. The life cycle assessment (LCA) methodology was used to analyse the environmental sustainability of AMD treatment at the semi-industrial scale. AMD from a South African coal mine was successfully treated at a semi-industrial scale using an integrated method that combines magnesite, lime, soda ash and CO₂ bubbling treatment. The system’s feasibility was demonstrated by a life cycle cost analysis (LCCA). Furthermore, this system’s adaptability allows it to be utilised in remote areas, in stand-alone mode or to treat AMD on an industrial scale, significantly enhancing community resilience at both the local and national levels. Regarding environmental sustainability, AMD has an environmental impact of 2.96 Pt/m³ or 29.6 kg CO₂eq per treated m³. Liquid CO₂ consumption and South Africa’s energy mix, which relies heavily on fossil fuels, were the primary environmental concerns. Using solar energy and gaseous CO₂ reduces the overall environmental footprint by 45% and 36%, respectively. Lastly, mineral recovery, or AMD sludge valorisation, can cut the overall environmental impact by as much as 12% [57].

A study by Zvimba et al. [58] investigated experimental and modelling studies on the passive neutralisation of AMD utilising basic oxygen furnace slag as the neutralisation material. After most of the characteristics were eliminated from the influent within the first ten days, the quality gradually improved. Under mildly acidic influent water conditions, the rapid release of alkalinity from slag minerals was responsible for the rapid removal kinetics of acidity. With overall percentage reductions of 88–100%, it was also found that the generation of alkalinity primarily governed the removal of metallic parameters through hydroxide production in the passive treatment system, which neutralised acidity. The legitimacy of the SO₄²⁻ removal kinetics experimental data was confirmed by modelling the removal kinetics for SO₄²⁻ using two different methods, which produced rate constant values of 1.56 and 1.53 L/(day mol), respectively. As part of addressing this issue in South Africa, the study’s conclusions offer valuable insights into the potential applications of slags and their limitations, particularly in the context of mine closure [58]. A summary table of lab-scale and pilot-plant studies on the treatment of AMD in South Africa, including key details such as location, method used, recovery or outcome and references, is given in

Table 1. Summary of selected lab-scale and pilot-plant studies on the treatment of AMD in South Africa.

Sampling Location	Type of Method Used	Type of AMD	Target Elements/Treatment Goal	Recovery Efficiency/Outcome	Scale	Year	Reference
Randfontein mining areas, Gauteng and eMalahleni, Mpumalanga	Ion exchange using cationic resins (CHT4083, CHP4502 and CHP00712)	Coal and gold mine AMD	REEs	Up to 98% REE sorption (CHP4502 and CHP00712).	Lab-scale	2024	[59]
Krugersdorp, Gauteng Province, South Africa	Neutralisation with magnesite and precipitation	Gold mine AMD	Heavy metals, sulphates	Magnesite treatment for AMD raised pH and significantly decreased metal and sulphate concentrations.	Lab-scale	2015	[46]
Western Basin of Witwatersrand	Sequencing batch reactor (SBR) system for neutralisation with limestone	Mixed gold mine AMD	Heavy metals, iron (II)	In an SBR system, precipitated calcium carbonate was used to completely remove iron (II) in 90 min.	Lab-scale	2013	[31]
eMalahleni, Mpumalanga, Mintek	RO, ettringite precipitation, barium carbonate addition and BSR.	Coal AMD	Sulphates	This study compares the operating costs of four sulphate removal methods, RO, ettringite precipitation, barium carbonate addition and BSR, highlighting that their cost-effectiveness depends on specific site conditions, reagent availability and discharge regulations.	Pilot-scale	2021	[60]
Gauteng, South Africa	ABC desalination process	Coal AMD	Sulphates	Sulphate was reduced from 2250 to 200 mg/L in 90 min by employing a 1:1 molar ratio of Ba2+ to SO42−.	Pilot-scale	2015	[61]
Gauteng, South Africa	Neutralisation with MgO	Coal AMD	Sulphates, heavy metals, EC	The study demonstrated effective pH-dependent recovery of metals and removal of inorganic pollutants from AMD, achieving up to 100% removal of Cu and Ni, significant reductions in Fe, Al, Mn, Zn and SO42−, along with 82% and 85% decreases in EC and total dissolved solids, respectively.	Lab-scale	2024	[62]
Witwatersrand Basin, South Africa	Ettringite precipitation	Mixed gold mine AMD	Sulphates, heavy metals	SO42− reduced to <250 mg/L, metals were removed and pH increased.	Lab-scale	2008	[63]
Mintek, Ranburg	Mintek’s integrated cloSURETM technology	Coal AMD	Sulphates	At the laboratory-scale, the procedure produced 196 g/m3/d sulphate reduction rates with 87% sulphate removal and up to 98% sulphide removal.	Lab-scale	2021	[64]
Mpumalanga Province, South Africa	Treatment of AMD with struvite synthesis supernatant	Coal AMD	Sulphates, heavy metals	The study concluded that at a 1:9 AMD to struvite supernatant ratio, removal efficiencies ranked highest for sulphate (100%), followed by Mg, Fe, Pb, Ni, Cu, As, Al and Zn (>90%), with Ca, Mn and Cr showing slightly lower efficiencies (88–85.7%).	Lab-scale	2023	[65]
Johannesburg, South Africa	Biological sulphate reduction (BSR)	Coal AMD	Sulphate	Up to 80% sulphate removal.	Lab-scale	2020	[66]
Mpumalanga, South Africa	Treatment of AMD using fly ash (FA)	Coal AMD	Sulphate	This study demonstrated that FA can passively treat AMD by gradually producing alkalinity over an extended period.	Lab-scale	2008	[51]
CSIR, Gauteng	Neutralisation with lime and limestone	Coal AMD	Acidity, Fe, Al, sulphate	Acidity, Fe, Al and sulphate were all removed using the combined iron oxidation and limestone neutralisation procedure.	Lab-scale	1998	[48]
Gauteng, South Africa	CSIR ABC desalination process.	Gold mine AMD	Sulphates, heavy metals	AMD was neutralised by using the CSIR ABC (Alkali-Barium-Calcium) desalination process, which also reduced TDS from 2600 to 360 mg/L.	Pilot-scale	2010	[67]
CSIR, Gauteng	Limestone, lime and CO2 treatment	Coal AMD	Sulphate	AMD was neutralised effectively with limestone instead of lime. Moreover, sulphate was removed to 1900 mg/L (as SO4).	Lab- and pilot-scale	2003	[68]

.

5. Limestone and Lime Neutralisation

In South Africa, limestone neutralisation has been used extensively as an economical and ecologically friendly way to treat AMD [19,68]. The CSIR and mining companies’ pilot and full-scale treatment systems in the Witwatersrand Basin are a noteworthy example [67]. These systems utilise dolomitic and limestone materials to precipitate dissolved metals and effectively raise pH levels. For example, the Grootvlei Mine Project effectively demonstrated that aeration and the use of crushed limestone can lower acidity and remove iron and other metals at comparatively low operating costs [68]. The integration of locally obtained limestone into passive treatment systems with low-maintenance requirements was also demonstrated by experiments conducted in the Mpumalanga coalfields, providing a viable option for both current and abandoned mine sites [26]. These initiatives illustrate the possibility of neutralising limestone in AMD treatment; however, long-term sustainability still necessitates optimising issues such as carbonate scaling and sludge disposal [4,69].

Mohajane et al. [70] conducted a study using oxygen and limestone to treat AMD. The purpose of their research was to investigate the circumstances in which Fe²⁺ is extracted using limestone and concurrently oxidised with oxygen to Fe³⁺ in a pressure-filled polyvinyl chloride conduit. The oxygen pipe neutralisation (OPeN) process pilot-plant stops gypsum scaling by using rubber balls. The amount of recycled sludge, oxygen pressure, mixing rate, limestone dose and Fe²⁺ concentration all had an impact on the rate of Fe²⁺ oxidation, according to batch studies conducted in a pipe reactor. Ongoing studies in an OPeN process pilot-plant have demonstrated a 98% reduction in total acidity from synthetic gold mine water and a 100% elimination from synthetic coal mine water. Fe²⁺ was removed from gold mine water and synthetic coal water as Fe(OH)₃ sludge at oxygen pressures of 200 and 100 kPa, respectively, and at a value of approximately pH 7 [70].

Maree et al. [48] investigated acidic wastewater neutralisation by limestone, which includes the removal of metals and certain sulphates. In their study, a new method for neutralising acid water from coal mining and processing was described. Limestone is used to neutralise the leachate from a waste coal dump to remove sulphate, iron and aluminium. The results showed that the sulphate concentration decreased from 15,000 to 2600 mg/L (as SO₄) and the amounts of Fe(II), hydroxide, oxygen and suspended solids (SS) all affect the iron oxidation rate equation. The resulting sludge has a high solids content of 55% (m/v). This is higher than the usual 20% attained by the lime-based high-density sludge (HDS) technique [48].

A study by Strobos et al. [71] assessed a cost-effective dosing and makeup system for limestone. Lime’s drawbacks include the expense and maintenance of the slaking apparatus, as well as the associated risks of handling lime. In the Mpumalanga province, the price of CaCO₃ powder (a by-product) is between 40% and 50% of the cost of lime. A full-scale facility has been built, and a novel method for handling and dosing limestone has been designed. The limestone is held on an inclined slab as part of the dosing system and then rinsed into a makeup tank. Pilot-scale research identified the following ideal conditions: the slurry in the tank should have a working consistency of approximately 14%, the ratio of recycled to makeup water must be at least 4:1, and a nozzle may be made from an open-ended pipe. The full-scale design includes a makeup tank with a volume of 10 m³ and can accommodate 120 tonnes of limestone (enough for approximately 14 days’ worth of inventory) on a slab with a 5% fall. A level sensor was installed on the full-scale plant to measure the amount of slurry water in the tank. Consequently, rather than being a fixed point on the load cell, the density might be employed as a controllable variable [71].

Maree et al. [18] investigated a combined limestone neutralisation and Fe²⁺ oxidation of AMD. Powdered limestone was added to a bioreactor that treated simulated AMD to produce volumetric iron (II) oxidation rates more than 100 g/(l.d). Furthermore, partial sulphate removal and neutralisation were achieved. In the pH range of 5 to 6, the rate is highly dependent on the surface area exposed to the liquid reactor surface area (RSA), as well as the concentrations of OH⁻, oxygen, CaCO₃, suspended particles and iron (II), and less on specific surface area (SSA) and pressure. RSA and the quantities of OH⁻, oxygen and iron (II) determine the rate of chemical oxidation (pH greater than 6) [18].

Lime treatment has been one of the most widely applied methods for AMD neutralisation in South Africa [68]. In this active treatment method, lime (Ca(OH)₂ or CaO) is added to acidic water to elevate its pH, promoting the precipitation of dissolved metals and lowering sulphate concentrations [71]. The main areas of research currently being pursued are sludge handling, lime dosage optimisation, long-term efficiency assessment and lime treatment integration with other technologies, such as biological processes or magnetite recovery. Although lime-based and combination treatment methods are effective, concerns about sustainability, sludge formation and high operating costs have prompted more innovation in these approaches [19].

Akinwekomi et al. [47] conducted a study on combining Mg(OH)₂ or Mg(HCO₃)₂ with Ca(OH)₂ to treat AMD. This study utilised simulated AMD to investigate the impact of Mg(OH)₂ and Mg(HCO₃)₂ mole ratio dosages on Fe²⁺ levels, as well as factors influencing the rate of Fe²⁺ oxidation. Additionally, the effects of treating AMD with Mg(OH)₂/Mg(HCO₃)₂ initially, followed by Ca(OH)₂, were assessed. Aeration combined with stirring was shown to be the most effective way to completely remove Fe²⁺ and other metals in the form of metal hydroxide precipitate in less than 30 min, according to their results, which reveal a mole ratio of Mg(OH)₂/Mg(HCO₃)₂ to Fe²⁺ 2:1. Thus, after AMD treatment, gypsum and metal hydroxide were extracted independently. The significance of the treatment sequence and the potential for using the optimised approach at the plant scale were generally supported by the analysis results [47].

6. Recovery of Valuable Resources

One of the emerging focus areas in South Africa’s AMD research is the recovery of valuable resources from contaminated waters [5]. In addition to being harmful to the environment, AMD may be a source of essential minerals such as manganese, aluminium, iron (as haematite or magnetite) and REEs [72,73,74]. Innovative approaches to extracting these elements using techniques such as selective precipitation, solvent extraction, ion exchange and membrane-based separations have been investigated in several studies conducted between 2005 and 2025 [75,76,77,78]. The paradigm shift from waste management to resource recovery is facilitated by these techniques, which offer two key advantages: the rehabilitation of contaminated waters and the economic recovery of marketable resources [78,79]. The significance of process optimisation in improving selectivity, reducing chemical usage and ensuring environmental compliance has also been highlighted by the research [79]. Alkali-generating agents, such as Na₂CO₃, NaOH, CaCO₃, Mg(OH)₂ or industrial by-products, have demonstrated potential in neutralising AMD and recovering high-purity iron oxides appropriate for water treatment applications [79].

Notwithstanding these developments, problems with scaling up recovery systems, managing fluctuating AMD compositions and ensuring cost-effectiveness persist. However, the recovery of valuable minerals from AMD aligns with zero-waste and circular economy concepts, making it a crucial area for future research and industrial applications in South Africa [80,81]. Chemical desalination methods, such as CSIR alumina, barium and calcium (ABC), Tshwane University of Technology (TUT) magnesium barium oxide (MBO) and Mintek ettringite processes, can economically produce drinking water from AMD without creating a sludge disposal issue because the generated sludge is recovered to yield saleable by-products and feed chemicals [67,82,83].

Table 2. Recovery of valuable resources from AMD in South Africa (1998–2025).

Resource Recovered	Recovery Method	Application/Use	Notable Studies/Sites	Remarks	Reference
Gypsum (CaSO4·2H2O)	Lime/alkali neutralisation	Cement, soil amendment	Lab-scale studies at ROC water	Widely recovered during pH adjustment	[22,81]
Iron Oxides/Hydroxides	Precipitation during neutralisation	Pigments, water treatment media	Lab-scale using Sludge from AMD	Potential for commercial valorisation	[1,32,83]
Aluminium Hydroxide	Precipitation at low pH	Coagulant in water treatment	Lab-scale studies	Less explored commercially	[32]
REEs	Adsorption and selective extraction	Electronics, renewable energy tech	Lab-scale research in coal mine drainage	Low concentrations but high value	[59]
Sulphate	Crystallisation (e.g., ettringite)	Sulphuric acid, fertiliser production	Mintek, eMalahleni, Mpumalanga	Often treated as waste, potential for reuse	[82,84]
Calcium Carbonate	Precipitation with CO2 injection	Construction materials	R&D phase in Limpopo/Gauteng sites	Can aid in carbon mineralisation	[84]
Magnesium	Lime-soda ash softening	Magnesium-based fertilisers	Pilot-scale research at ROC Water Project	Economic recovery needs optimisation	[79]

shows a summary of selected studies on the extraction of critical and valuable materials from AMD, highlighting recovered products, methods used and potential applications.

A study by Masindi [85] combined magnesite, lime, soda ash, CO₂ and RO treatment methods to recover potable water and valuable products from AMD. The study investigated the potential for recovering drinking water and useful minerals from AMD. After 60 min of equilibration, AMD was neutralised, and metals were recovered. A simulated RO system was used to purify the water further, ensuring that it met SANS 241 drinking water quality criteria. The restored drinking water had a pH of about 6.5. The RO system’s metal removal effectiveness was almost 100% [85].

A study by Akinwekomi et al. [9] used sodium carbonate to synthesise magnetite from iron-rich AMD. Magnetite nanoparticle synthesis was assessed at different temperature gradients. Selective precipitation was the primary mechanism controlling the recovery of the metals. This was accomplished by adjusting the pH of the reaction mixture and the aeration rate. According to experimental data, the ideal parameters for recovering magnetite nanoparticles from AMD were a 2:1 mol ratio of Fe(II)/Fe(III), pH > 10 and temperatures between 25 and 100 °C. Al-removed magnetite had a purity of 24 (weight percentage), while magnetite synthesised without Al-removal had a purity of 28 (weight percentage). This study effectively demonstrated that iron-rich mine drainage may be used to synthesise magnetite nanoparticles [9].

Letjiane et al. [79] carried out a study on the pre-treatment of coal waste leachate with MgO to recover pigment. The goal of this study was to determine a method for extracting potable water and marketable goods, such as pigments, from iron-rich AMD. Iron was extracted using MgO as Fe(OH)₃, which was then dried at 80 °C to produce goethite (yellow) and 700 °C to produce haematite (red). The residual metals (Mn²⁺ and Ca²⁺) were removed as carbonates using Na₂CO₃. RO was used to desalinate the pre-treated water, yielding a brine with a total dissolved solids (TDS) amount of 85 069 mg/L and a permeate with a TDS of 105 mg/L. By using freeze crystallisation, the brine was further concentrated to a TDS of 336 820 mg/L. This is sufficiently concentrated to extract MgSO₄·7H₂O [79].

Akinwekomi et al. [1] conducted a study on a practical approach for the synthesis of goethite, haematite, magnetite and gypsum that is focused on the idea of a circular economy for the beneficiation of AMD. This study investigated novel methods for AMD beneficiation to produce valuable minerals with various industrial applications. To achieve this, goethite, haematite, magnetite and gypsum (product minerals) were synthesised using genuine AMD. This method is a zero liquid discharge (ZLD) process because drinking water was also recovered throughout the treatment phase. High-grade gypsum was created by adding lime to treated water. In addition, drinking water was recovered using RO in accordance with South African drinking water standards (SANS 241). If the operational costs of the treatment were partially offset by the sales proceeds of the product minerals, the process could become self-sustaining. Additionally, this will promote the idea of waste beneficiation and the circular economy, reducing the adverse effects of AMD on other environmental domains [1].

Mulopo [86] conducted a study on the recovery of elemental sulphur directly from drainage streams of gold acid mines. This study evaluates how CaSO₄/Mg(OH)₂ and BaSO₄/CaCO₃ produced during the water stage of the ABC desalination process are converted to sulphur. Africa is a significant importer of enormous quantities of sulphur, which are often expensive due to shipping costs, even if sulphur is still a reasonably priced commodity. According to thermal reduction research, a muffle furnace operating at around 1100 °C can convert BaSO₄/CaCO₃ and CaSO₄/Mg(OH)₂ sludges to BaS/CaS with duff carbon, yielding a BaS/CaS yield of 70–76%. After slurring the BaS/CaS in water, CO₂ was utilised to strip the sulphide, resulting in the formation of H₂S gas and a precipitate of BaCO₃/CaCO₃. Ferric sulphate and the H₂S produced interacted to produce elemental sulphur. Sulphur from the sulphate-rich wastes was recovered with a purity of 95.2% to 99.1% [86].

Ntumba et al. [59] conducted a study on the valorisation of AMD by employing cationic resins to extract REEs efficiently. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to analyse the levels of REEs in AMD samples from coal and gold mines. The sorption of these elements by several cationic resins (CHT4083, CHP4502 and CHP00712) was then assessed. By employing various concentrations of sulphuric acid (H₂SO₄) solutions to desorb the REEs from the resins and conducting batch experiments, the ideal conditions for the sorption of the REEs by these resins were identified. Compared to AMD from the gold mines (ΣREE 4.9705 mg/L), which had a pH of 3.21, coal mine drainage (CMD), which had a low pH of 2.37, contained more REEs (ΣREE 226.3732 mg/L). With CHP4502 and CHP00712 resins, a 98% REE sorption efficiency was achieved. Additional optimisation of CHP00712 revealed that a resin volume of 250 mL and a contact period of ten minutes were required to remove REEs from 500 mL of AMD effectively. Accordingly, the resin’s sorption capacities for the chosen REEs were 3.88 mg/g, 0.88 mg/g, 1.37 mg/g, 3.18 mg/g, 0.67 mg/g, 0.01 mg/g and 0.27 mg/g for Pr, Gd, Nd, Ce, Sm, Eu and Y. The REEs were desorbed by eluting the resin with a 0.5 N sulphuric acid solution [59].

Mulopo et al. [87] conducted a study on AMD using a pilot-scale bubbling column reactor to regenerate barium carbonate from barium sulphide. The degree of sulphate removal from AMD utilising the recovered BaCO₃ was examined. Additionally, the impact of essential process variables on carbonation, including the concentration of BaS slurry and the CO₂ flow rate, was also considered. In a bubbling column reactor, it was shown that the carbonation reaction rate for BaCO₃ regeneration rose significantly as the CO₂ flow rate increased. In contrast, the carbonation rate was not significantly impacted by the BaS slurry concentration, which was found to be between 5% and 10%. The BaCO₃ morphology was similarly affected by the CO₂ flow rate. When compared to commercial BaCO₃, the BaCO₃ recovered from the pilot-scale bubbling column reactor showed an efficient capacity to remove sulphates during AMD treatment [87].

Mulopo et al. [84] investigated calcium carbonate recovery from steelmaking slag and application as a pre-treatment for AMD. The technological viability of recovering calcium carbonate and using it to pre-treat AMD from coal mines was evaluated using batch reactors. The effects of essential process variables were considered, including pH, CO₂ flow rate and the acid-to-calcium molar ratio (HCl). An increase in the amount of hydrochloric acid was shown to increase the amount of calcium extracted from steelmaking slag dramatically. The carbonation reaction rate was also positively impacted by the CO₂ flow rate; however, for values below 2 L/min, the shape of the formed calcium carbonate remained unaffected. When compared to commercial laboratory-grade CaCO₃, the CaCO₃ recovered from the bench-scale batch reactor showed an excellent neutralisation ability during AMD pre-treatment [84].

Mogashane et al. [81] investigated iron-rich AMD for the recovery of drinking water and the production of nanoscale Fe₂O₃ pigment. Their study evaluated the reverse osmosis cooling (ROC) method for producing clean water and nanosized Fe₂O₃ pigment from leachate generated at a coal waste landfill. Following the recovery of nanosized Fe(OH)₃, the pH was increased to 4.5 to recover Al(OH)₃, then to 8.5 to remove the remaining metals. RO was then used for desalination. More water and Na₂SO₄ (s and aq) were recovered by treating the brine from the RO stage using freeze crystallisation. The pigment resulting from thermally treating Fe(OH)₃ had a particle size of less than 100 nm and could be utilised as a nanomaterial [81].

7. Modelling of Water Networks

The importance of water network modelling in creating efficient remediation plans has been shown by several successful AMD treatment projects in South Africa [2,34]. To optimise the transportation and treatment of contaminated water across interconnected mining basins, hydraulic modelling was utilised in the Witwatersrand Basin AMD Management Project [19].

Early warning systems and pumping schedule optimisation were implemented in the Eastern Basin using predictive modelling techniques to prevent uncontrolled decanting [5]. These case studies demonstrate how real-time monitoring, combined with water network models, can significantly enhance the effectiveness and affordability of AMD treatment systems [88,89]. They also demonstrate the importance of modelling for long-term strategic planning, particularly in areas with complex hydrogeological conditions, as well as aiding operational decision-making [44].

Huisamen and Wolkersdorfer [90] conducted a study on modelling for the hydrogeochemical changes in mine water in an abandoned opencast coal mine. Together with calibration against current data, their approach proposed the geochemical modelling of transient pollutant release from opencast coal mines that have been rehabilitated. A directive received by a decommissioned opencast coal mine in Mpumalanga, South Africa, served as an example of the necessity for such an approach. Groundwater monitoring data, geochemical investigations, numerical flow modelling and geochemical modelling are used to model the hydrogeochemical evolution of mine water over time. Additionally, to better understand the site geochemistry, leaching experiments, acid-base accounting and mineralogical investigations were conducted. Using laboratory data calibrated against leaching test data, a geochemical model was developed to provide a statistically representative mineral assemblage. This collection was fed into a computational flow and transport model under field circumstances. This method may yield a more accurate estimate of how long the initial flush will take to complete, converting abandoned collieries into extensive groundwater reservoirs [90].

Maree et al. [69] conducted a study using process modelling to optimise wastewater treatment at a coal mine. An interactive steady-state model was used to simulate and audit a coal mine’s water network. According to simulations of the interactions, using calcium carbonate powder instead of lime to neutralise acid water can save 56% on reagent costs. In primary neutralisation and coal processing facilities, gypsum crystallisation lowers sulphate concentrations by 30% and 60%, respectively. The capital cost of a neutralisation/gypsum crystallisation facility is R3.0 million when treating coal waste leachate separately and the less polluted streams, whereas it is R10.3 million when treating both. In coal processing, sulphate removal from the input water can efficiently manage the over-saturation index (OSI) value. A flow rate of 222 m³/h must be used to reduce sulphate to 350 mg/L, thereby achieving an OSI value below 1. Coal that has been pre-washed will save operating and capital expenses [69].

More and Wolkersdorfer [89] conducted a study utilising analytics to forecast nonlinear systems using machine learning algorithms. The method presented in their research can help treatment facilities operate more efficiently and achieve their mine water management objectives. On an Anaconda 4.11.0 platform, the Python 3.7.1 programming language was used to compare neural networks and regression tree techniques. This was performed after the data had been subjected to statistical approaches for exploratory data analysis and robust data pre-processing [89]. Predicting mining-impacted water (MIW) parameters using this top-performing model was the primary goal. This strategy will enable the operators of treatment plants to understand the chemistry of MIW better, ultimately allowing them to plan the most effective chemicals and techniques for treating and managing polluted MIW. Using historical data (2016–2021) from Shaft 9 as a case study, the algorithms were trained and tested on water from the Westrand mine pool near Randfontein, South Africa. These algorithms included random forest regression trees, gradient boosting, artificial neural networks (ANNs) and deep neural networks (DNNs). The best-performing approach used multivariate long short-term memory (LSTM) to produce new data. A variety of data pre-processing techniques, such as anomaly identification and data interpolation, were investigated. These procedures, including machine learning, were used to emphasise data analytics, which is the most crucial component of finishing a project. Before attempting to create forecasting models, it is essential to do a thorough statistical analysis of the data [89].

More and Wolkersdorfer [91] conducted a study on eMetsi and machine learning graphical user interface (ML-GUI) as intelligent tools for mine water management. eMetsi, which translates to ‘electronic water’ in Setswana, is a framework for mine water sampling that combines mobile and web applications with near-field communication (NFC) technology. It includes the use of NFC microchips for bottle sampling, mobile applications for on-site data collection during sampling and online applications for data storage and display. eMetsi helps users meet sampling and sample analysis goals by facilitating quick data exchange between samplers, lab personnel and clients. One tool for creating predictive models is ML-GUI, a user interface powered by artificial intelligence (AI). Operators of mine water treatment plants can create ML models using ML-GUI even if they have no programming skills. These models can then be used for forecasting analysis to determine the chemicals and techniques that will be needed to manage MIW [91].

8. Biological Sulphate Removal

Several successful case studies on BSR have emerged in South Africa’s AMD treatment landscape between 1998 and 2025 [82,92,93,94]. One notable example is the pilot and full-scale operations at the eMalahleni Water Reclamation Plant, where biological processes were integrated with chemical treatments to remove sulphates and recover clean water for municipal use [95]. Research projects by institutions such as CSIR and Mintek have demonstrated that BSR technologies, utilising sulphate-reducing bacteria (SRB) and organic substrates, can effectively reduce sulphate concentrations while generating valuable by-products such as elemental sulphur [60,82,96]. Although there are still issues with maintaining ideal microbial activity and substrate supply, these studies demonstrate the benefits of BSR, including reduced operating costs and sustainable treatment under mild circumstances [96]. BSR’s potential as a crucial tactic in upcoming AMD management frameworks is highlighted by its effective implementation in various South African scenarios [66,86].

Mukwevho et al. [66] conducted a study on the assessment of BSR using response surface methodology in relation to pH, temperature and hydraulic retention time (HRT). Their work examined the impact of temperature, pH and HRT on BSR. Downflow-mode packed bed reactors were used for the experiments. The data were statistically analysed using response surface methods, which enable the development of statistical models that provide a comprehensive understanding of the individual impacts and interactions among the independent variables. The data fit the quadratic models well, as indicated by the analysis of variance results, which were also supported by a non-significant lack of fit. A considerable interaction was observed between temperature and HRT. Both effects were significant (p < 0.0001); however, the pH effect was insignificant (p > 0.05) [66].

A study by Du Preez and Marumo [82] assessed the viability of expanding a semi-passive BSR method to treat water impacted by high-sulphate mines. They used Mintek’s cloSURE^® technology to treat AMD. The first step of the process utilises the BSR method to remove metals, raise the pH and remove sulphate. The second step involved the biological oxidation of sulphide for sulphur recovery. Pilot-scale testing utilised 5 m³ reactors, while laboratory-scale testing was conducted in 1 m columns. Hay, cow manure and woodchips made up the organic material mix that was placed in both systems. To keep the sulphate-reducing reactors operating, a source of liquid organic carbon was added to a mine water sample with a low pH and a sulphate concentration of 3900 mg/L. To determine how scale-up affected performance, these data were compared. According to the data, sulphate was reduced by over 90% in both test work phases. According to the South African Water Quality Guidelines for Irrigation, the pH of the treated water was higher than 7.1, and the metal levels were within the acceptable limits for water suitable for reuse in irrigation [82].

Greben and Maree [97] investigated the impact on biological sulphate and sulphide removal rates of reactor type and residence duration. The study’s objectives were to determine the optimal reactor system for BSR, investigate whether a lower HRT could lead to a higher sulphate reduction rate and assess whether the sulphide oxidation rate would increase when reactor systems were tested with and without a clarifier. According to the study’s findings, the packed bed reactor’s sulphate reduction rate was 4.9 g SO₄/(o.d.) at a feed rate of 98 o/d, providing synthetic feed that contained ethanol as a source of carbon and energy. The maximum sulphate reduction rate achieved when feeding the same feedstock at 100 o/d was 4.8 g SO₄/(o.d) in a reactor design that was entirely mixed and 3.3 g SO₄/(o.d) in a reactor configuration that had fluidised beds at a feed rate of 90 o/d. A low ratio of sulphide created to sulphate eliminated was the outcome of reducing sulphate to sulphur via the formation of sulphide. The reactor systems with a clarifier installed had the best sulphide removal because air diffused into the reactor system from the top of the clarifier, oxidising more than one-third of the produced sulphide to elemental sulphur [97]. Greben and Maree [98] conducted a study using a laboratory-scale bioreactor to remove sulphate, metals and acidity from a nickel and copper mine effluent. According to the study’s findings, applying laboratory-scale BSR technology to nickel/copper mine wastewater steadily reduced sulphate concentrations from an average of 2000 to 450 mg/L and increased pH from 5.8 to 6.5. The HRT fluctuated between 24 and 12 h throughout this time [98].

9. Chemical Treatment

South Africa has been at the forefront of developing and testing a range of technologies for the treatment of AMD, both at laboratory and pilot scales, in response to its legacy of extensive mining activities [99,100]. One of the key technologies is the SAVMIN method, developed by Mintek, which utilises gypsum crystallisation, lime and barium carbonate to remove metals and sulphates from AMD [101]. Chemical softening and RO are combined in Mintek’s slurry precipitation and recycling RO (SPARRO^®) technology, which emphasises high water recovery and brine minimisation [44]. The ettringite technique, which has been demonstrated to be successful in AMD settings in the real world, utilises the creation of ettringite, a calcium aluminium sulphate mineral, to extract heavy metals and sulphates from mine water [82]. The CSIR ABC process focuses on the selective removal of contaminants through staged precipitation and is particularly suited for high-sulphate waters [67]. The ability of the MBO process to remove metals and sulphates while producing recoverable by-products has been investigated at TUT [30]. Passive and semi-passive methods for environmental stabilisation and long-term mine water treatment are also covered under Mintek’s Mine Closure Research Programme [64].

When combined, these technologies underscore South Africa’s commitment to reducing AMD through innovative, regionally tailored treatment approaches that prioritise resource recovery, water reuse and environmental sustainability [20]. Van Rooyen et al. [65] investigated technologies for removing sulphates from mine-impacted water. Operating expenses, based on utility and reagent consumption, were primarily compared. For a given combination of location and AMD composition, it was demonstrated that each process is subject to a distinct set of limitations that may favour one over the other. A major expense factor for any of the procedures under study would be the cost and availability of reagents. Additionally, the sort of effluents that could be released determines the total cost for each process [60].

10. Membrane Processes

The necessity for high-quality, sustainable water recovery solutions in areas affected by AMD is reflected in the increased interest in membrane technologies [18]. Research on membrane-based technologies, including forward osmosis (FO), RO and nanofiltration (NF), especially in relation to the selective removal of heavy metals and dissolved salts from AMD, has escalated [102,103,104]. Despite the significant purification efficiencies offered by these systems, problems with membrane fouling, high energy requirements and concentrate disposal still exist. To reduce fouling, recent research conducted in South Africa has examined pre-treatment techniques, enhanced membrane durability and combining membrane systems with pre-neutralisation procedures [30,105]. The development of inexpensive, AMD-resistant membrane materials remains a promising area for further study [106]. To increase longevity and performance, studies have also examined the integration of membrane systems with pre-treatment techniques, such as lime or biological processes.

Mogashane et al. [3] investigated technologies that can be applied to brine and wastewater treatment to recover drinkable water and marketable goods. They examined the technologies, ranging from neutralisation technologies used to treat acidic mine water to new and emerging technologies currently being marketed to recover valuable products and potable water from a range of wastewater types, employed to treat various wastewater streams. Some of the newest technologies include FO, freeze crystallisation and RO processes, which treat AMD to recover pigments and produce clean water [3].

A study by Mogashane et al. [80] assessed FO in the treatment of AMD. Their study investigated the potential of FO to treat sodium sulphate-rich brine using either NaCl or (NH₄)₂SO₄ as draw solutions. Utilising an Aquaporin Inside^® HFFO14 membrane module in a counter cross-flow setup, the system demonstrated that water successfully permeated the membrane if a sufficient osmotic pressure gradient existed. With continuous circulation of the draw solution and recycling of the feed, the feed volume was reduced by 73%; near steady-state conditions were achieved after 80% water recovery. The results indicate FO is a viable pre-concentration step for Na₂SO₄-rich AMD, supporting further water recovery processes [80].

A study by Ramothole et al. [107] assessed the performance of freeze crystallisation in extracting water and sodium sulphate from AMD. The objective of their study was to recover Na₂SO₄·10H₂O and drinkable water. The brine was cooled to freeze using a chiller. At a concentration of less than 45 g/L (as Na₂SO₄), the only substance generated was ice. Further ice removal caused Na₂SO₄·10H₂O to begin crystallising. The freezing point dropped from −2 °C at the beginning of ice formation to −4 °C when Na₂SO₄·10H₂O began to crystallise. TDS measurements showed an increase in ice purity from 2000 mg/L to 3000 mg/L. Energy consumption was 10 kWh/m³ during cooling to the freezing point and 100 kWh/m³ when ice and Na₂SO₄·10H₂O formed. These findings were observed as water recovery increased from 0 to 80% [107].

11. Brine Treatment

Brine treatment through freeze crystallisation has emerged as a promising and energy-efficient solution for managing hypersaline waste streams generated during AMD treatment in South Africa [108,109,110]. By utilising the idea that pure water freezes before salts, this method makes it possible to separate ice, or clean water, from concentrated brine under carefully monitored circumstances [107]. As it uses less energy and allows for the recovery of high-purity salts, such as sodium sulphate decahydrate (Na₂SO₄·10H₂O), freeze crystallisation has several advantages over traditional thermal evaporation or membrane-based techniques in the context of AMD [109,110].

To obtain almost minimal liquid discharge, recent studies and pilot-scale experiments have demonstrated that freeze crystallisation can be successfully combined with pre-treatment procedures such as RO and metal hydroxide removal [107,110]. Furthermore, the recovered salts can be utilised as raw materials in various sectors, making this strategy more economically viable [111]. As the procedure reduces the production of hazardous waste and the need for landfill disposal, it also supports South Africa’s zero-waste objectives [111,112].

Scalability, infrastructure costs, the effectiveness of ice-brine separation and operational difficulties in varying climates remain issues [112]. However, because of mounting regulatory pressure and growing interest in sustainable water management, freeze crystallisation is becoming more popular as a crucial method for treating brine in areas affected by AMD, helping to preserve the environment and recover resources [111]. The ROC process was developed by ROC Water Technologies to treat mine water and brines [27]. The ROC process is highly successful in lowering sulphate concentrations and recovering economically viable by-products, converting waste into opportunities for economic growth, as demonstrated by pilot operations in Mpumalanga [81]. In addition to mitigating their negative environmental impacts, these projects also demonstrate how to offset costs, which supports the sustainability of AMD management techniques and aligns with the principles of the circular economy [21].

The ROC process was developed to address key limitations of conventional mine water treatment methods, namely membrane scaling due to gypsum formation, challenges with brine and solid waste disposal and high operational costs. This integrated approach involves a pre-treatment step using Na₂CO₃ to remove metal hydroxides, followed by RO for desalination and brine concentration [29]. The resulting brine is then subjected to freeze crystallisation to recover Na₂SO₄·10H₂O. In a final thermal treatment stage, the recovered Na₂SO₄·10H₂O is reacted with coal at 1000 °C to regenerate Na₂CO₃ and recover elemental sulphur, effectively closing the loop and enhancing the sustainability and cost-efficiency of the process [78]. The process configuration of the ROC process, which allowed for several enhancements, is depicted in Figure 7. The enhancements may increase the recovery of valuable goods from mine water and lower treatment costs.

12. Selected Case Studies of Successful Acid Mine Drainage Treatment Projects

When science, policy and engineering intersect, several successful case studies in South Africa demonstrate the potential for effective AMD treatment [43,58,113,114]. A notable example is the Western Basin AMD Treatment Plant, located near Krugersdorp, which aims to reduce the amount of untreated AMD entering the Wonderfonteinspruit catchment area [115]. Over 30 million litres of AMD are treated daily at the Trans-Caledon Tunnel Authority (TCTA) plant, which uses HDS technology to neutralise acidity and precipitate metals [31,34]. In a similar vein, Springs’ Eastern Basin Treatment Plant has played a crucial role in mitigating the increasing AMD levels endangering the Blesbokspruit Ramsar wetland and its neighbouring villages [34].

A study by Lourenco and Curtis [34] assessed the impact of an AMD chemical treatment plant using HDS on the water quality of the Blesbokspruit Wetland in South Africa. One of the biggest HDS treatment facilities in the world, the Eastern Basin Chemical AMD Treatment Facility began operations in August 2016. The factory is located upstream of the Blesbokspruit Wetland, a former Ramsar Wetland of International Importance that is currently included on the Montreux Record, and close to the now-defunct Grootvlei Mine in Springs, South Africa. The surface water quality in the wetland region has been significantly impacted since the facility began operating. The unit was built to lessen the expected AMD water decanted into the Blesbokspruit Wetland from the defunct Grootvlei Mine [34]. Five historical Rand Water monitoring sites along the Blesbokspruit were examined using quarterly water quality data from 2013 to 2019 [34].

The downstream conductivity, chloride, magnesium, sodium and sulphate levels have all been adversely affected by the current HDS treatment procedure. The levels of these parameters have considerably increased since the treatment plant’s commissioning. The Blesbokspruit Forum’s framework, which is less strict than the national guidelines for aquatic ecosystems, defines conductivity and sulphate as ‘unacceptable’ in terms of management. This could influence the downstream salinisation of the Vaal Barrage. The findings underscore the need for further investigation into potential secondary treatment and desalination processes, as well as the prospective ecological and downstream water supply implications of rising salinity in the region [34].

Masindi et al. [49] investigated recent developments in the treatment of AMD using struvite made from municipal wastewater. In their work, struvite was formed because of the use of activated magnesia to remove phosphate and ammonia from municipal wastewater. The effectiveness of struvite in neutralising AMD and attenuating inorganic pollutants was ≥30% for SO₄²⁻ and ≥98.99% for metals. Other metal traces, including those of Zn, Cu, Ni, Pb and Cr, were much reduced. According to pH Redox Equilibria (PHREEQC), minerals such as metal hydroxides, oxy-(hydro)-phosphate, oxy-(hydro)-sulphates and other complexes would be eliminated. The viability of recovering ammonia and phosphate as struvite, which can be used to treat AMD, was validated by this investigation [49].

A study by Muliwa et al. [116] evaluated the effectiveness of eggshell waste material in treating AMD from coal dump leachate. They determined that eggshell powder (ES) was an inexpensive and biocompatible waste material for pre-treating AMD leachate from coal dumps. Energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM) confirmed the elimination of impurities. The treatment efficiency was improved by ES’s increased mass and decreased particle size. Mn and sulphates were a problem as they were only partially eliminated, even though total removal of Fe and Al was accomplished. The pH and purity of the mine water were also brought down to acceptable levels, and ES showed a better acid-neutralising action than limestone. While the removal of Mn followed an exothermic process, that of Fe and Al did not. Compared to the sludge from limestone powder, the kinetics were faster, and the ES powder sludge settled more rapidly. A cost analysis revealed that using ES material instead of limestone can lead to significant cost reductions. The results suggest that ES waste may be a cost-effective and practical material for pre-treating AMD effluent [116].

Masindi [117] conducted a study using cryptocrystalline magnesite tailings, a new technique for neutralising acidity and reducing harmful chemical species from AMD. The goal of their research was to turn two waste products into resources through beneficiation. AMD was exposed to magnesite tailings at a dosage of 1 g and an ideal equilibration duration of 30 min. When AMD encountered magnesite tailings, the pH level rose to greater than 10, and the inorganic pollutants decreased drastically to less than 1%, except for sulphate, which had an efficiency of more than 80%. One of the main processes identified as a significant rate-controlling step was pore diffusion. Geochemical modelling based on PHREEQC indicated that the following elements would be eliminated: basaluminite, boehmite, jurbanite and Al(OH)₃. Manganite (MnOOH) and rhodochrosite (MnCO₃) were the precipitated forms of Mn. Ca was extracted as dolomite and gypsum, and Mg as dolomite and brucite [117].

Kefeni et al. [118] investigated a two-step process for removing metals and sulphate from AMD using ferrite sludge and barium sulphate formation. Both simulated and actual AMD were subjected to the optimised approach. The effects of adding ferrite or ferrite sludge seeds to actual AMD samples and simultaneously removing metals and sulphate on the magnetic moment of ferrite sludge were examined. Without calcium hydroxide or barium ions, gypsum did not precipitate when AMD was neutralised with sodium hydroxide; thus, to extract the metals, a two-stage procedure was used. The first step involved using sodium hydroxide to generate ferrite sludge; the second step involved treating the filtrate with barium hydroxide or chloride to remove the sulphate as a barium sulphate precipitate. Real AMD was separated into 93%, 12% and 28% Fe, Mn and Co, respectively, with 99.6%, 57.5% and 47.5% Fe, Mn and Co in the pH range of 7–8.5, in the presence and absence of ferrite seeds, with a 20 min reaction time [118].

Mogashane et al. [21] conducted a study using calcium carbonate with a gypsum scale inhibitor for the recovery of ferric hydroxide from iron-rich AMD. This work focused on employing CaCO₃ to selectively neutralise and recover Fe(OH)₃ at pH 3.5, thereby enhancing the ROC process. Ferric hydroxide was isolated from gypsum and other metals in mine water using a particular inhibitor. The study found that gypsum crystallisation is influenced by factors such as supersaturation, seed concentration and temperature, and that effective inhibition depends on the presence of Fe(OH)₃ and CaCO₃, as well as the timing and dosage of the inhibitors. Fe(OH)₃ can be thermally converted into nanosized pigments, such as goethite and hematite. Using calcium-based alkalis significantly reduced TDS and treatment costs (ZAR29.43/m³) compared to sodium carbonate (ZAR48.46/m³), highlighting a more cost-effective approach [21].

Beauclair and Ambushe [13] conducted a study on combining biosorption with selective precipitation methods to treat AMD as a hybrid and incremental strategy for controlling environmental pollution and increasing AMD value. Their study investigated the use of MgO for selective precipitation and banana peels (BPs) for biosorption to treat and valorise AMD. The two separate steps of the treatment chain were selectively precipitating chemical species using MgO (Step 1) and polishing the pre-treated AMD with BPs (Step 2). MgO and 2.0 L of AMD from a coal mine were utilised in Step 1 for chemical species recovery and selective precipitation. With Fe(III) precipitating at pH ≤ 4, Al at pH ≥ 4–5, Fe(II), Mn and Zn at pH ≥ 8 and Ca and SO₄²⁻ precipitating across the pH range, the results showed that chemical species of interest were precipitated and recovered at various pH gradients. In Step 2, BPs were used to polish the AMD water that had already been treated. The pH increased from 1.7 to 10 overall, and the following chemical species were significantly removed: Al, Cu and Zn (100% each), ≥Fe and Mn (99.99% each), ≥Ni (99.93%) and ≥SO₄²⁻ (90%). While the biosorption process served as a polishing stage by eliminating any remaining contaminants, the chemical treatment step only partially eliminated the pollutants [13].

Azizi et al. [62] conducted an experimental investigation into the treatment of AMD and the recovery of metals through selective precipitation. The use of MgO in selective precipitation for the treatment of AMD and chemical species recovery was examined. For the experiment, 1500 mL of AMD from a coal mine was used. An overhead stirrer was used to gradually raise the pH of the AMD water, and a sequential precipitation approach was employed to recover chemical species at various pH gradients. The findings showed that inorganic pollutants were significantly removed in the following order: Cu (100%) = Ni (100%) ≥ Fe (99.9%) ≥ Al (99%) ≥ Mn (98.6%) ≥ Zn (97.2%) ≥ SO₄²⁻ (76%). Additionally, there were 82% and 85% decreases in EC and TDS, respectively. The fate of chemical species of pure MgO and sludge recovered at various pH gradients was supported and summarised by characterisation studies. The product water, which had a pH of 9.5, complied with the effluent discharge standards set by regulatory agencies [62].

Nguegang et al. [119] conducted a study using a vertically flowing wetland to treat AMD. Soil was used as the substrate, and Vetiveria zizanioides as a decontaminating medium to enrich the wetland. For 30 days, water percolated through the substrate; every five days, throughput samples were taken and described. The results showed a net decrease in metals and sulphate, as well as a tolerance index of 1.03 for Vetiveria zizanioides. This accumulation of chemical species on the soil through adsorption, precipitation and phyto-retention may be the reason why the used substrate further facilitated the removal of chemical species. The translocation assessment revealed that, except for manganese, which was transmitted in the shoot (67%), the distribution of chemical species was predominantly found in the roots [119].

The eMalahleni Water Reclamation Plant, a public–private partnership between Anglo American, BHP Billiton and the local municipality, is another noteworthy project [26,120]. This facility turns a hazardous waste stream into a lucrative water resource by treating AMD using RO and providing potable-quality water to locals and industrial users [4]. These initiatives demonstrate the viability of technologically sophisticated, community-focused AMD solutions and emphasise the significance of consistent funding, regulatory backing and cross-sector cooperation [71]. The effectiveness of AMD treatment has been further enhanced by advancements in FO technology, geochemical modelling and remote sensing [80,121].

The importance of South African research in developing internationally applicable solutions for sustainable mining water management is underscored by these case studies, which also emphasise the need for ongoing investments in cutting-edge treatment technologies and environmental preservation measures [16]. The knowledge gained serves as a guide for achieving success in other areas of the nation that have been impacted. The selected case studies of effective AMD treatment initiatives in South Africa are summarised in

Table 3. Summary of selected case studies demonstrating successful AMD treatment projects in South Africa.

Location	Main Finding	Technology/Method Used	Reference
Witwatersrand Basin	Effective neutralisation of AMD and recovery of metal salts	SBR system, lime and limestone neutralisation	[31,41]
eMalahleni (Mpumalanga)	AMD was treated and repurposed into potable water for municipal use	RO membranes and HiPRO® process	[4,35,60]
Krugersdorp (Gauteng)	Passive treatment is effective in removing Fe and Mn and improving pH	Vertically flowing wetlands	[119]
Sibanye Gold mine in Krugersdorp, Gauteng	Combined AMD treatment and resource recovery (gypsum, Fe oxide, Mg(OH)2)	MgO precipitation	[62]
Mpumalanga Coalfields	Use of waste products (ash and slag) to neutralise AMD	Passive treatment using industrial by-products	[51,122]
Eastern Basin chemical AMD treatment plant, Grootvlei Mine, Springs, South Africa	Daily treatment of AMD and discharge of compliant water	HDS process	[34]

.

13. General Discussion

Enhancing sustainability, cost-effectiveness and resource recovery have been the main goals of technological advancements in South Africa’s AMD treatment over the last 20 years [5,20]. These include bioremediation methods, especially SRB and artificial wetland systems, which have been extensively studied [12]. These biological systems provide low-energy, eco-friendly treatment options by promoting metal precipitation and sulphate reduction in anaerobic environments [123]. Research from South Africa has greatly advanced the knowledge of microbial consortia that can survive in environments with high pH and metal concentrations [74]. Using organic waste substrates and designed bioreactors to stabilise AMD discharges has shown encouraging results in pilot experiments conducted in the provinces of Mpumalanga and Gauteng [20].

While numerous studies have explored various treatment methods for AMD in South Africa, a critical examination reveals inconsistencies and gaps that warrant further investigation [16,28]. In contrast to promises of consistent performance, some studies reveal poor long-term effectiveness due to seasonal variations and metal deposition, even though passive treatment techniques, such as constructed wetlands, are commonly recommended for their sustainability [48]. Moreover, although several laboratory-scale studies demonstrate the efficiency of low-cost materials, such as coal fly ash (FA) and industrial by-products, few have progressed to pilot or full-scale applications, indicating a gap in technology transfer and implementation [62]. Differences in how AMD is described and performance criteria stipulated among studies hinder direct comparison and the development of standardised treatment protocols. These inconsistencies underscore the need for long-term monitoring data and integrated, site-specific evaluations to confirm treatment effectiveness in real-world settings. To advance workable and scalable solutions to AMD issues in South Africa, these gaps must be addressed [74].

Active treatment systems, such as lime neutralisation, HDS and chemical precipitation, offer high removal efficiencies and rapid treatment but involve high capital and operational costs, frequent maintenance and significant sludge generation [18,26,31]. Active systems provide more consistent and rapid treatment, achieving over 95% removal of Fe, Al and Mn under controlled conditions; however, this comes at the cost of high energy input, reagent consumption and complex sludge management [31,49]. Passive treatment systems, including constructed wetlands and permeable reactive barriers, are more cost-effective in the long term due to their low energy and maintenance requirements; however, they require larger land areas and exhibit variable performance depending on site conditions [56,64]. Emerging low-cost materials, such as coal FA and industrial by-products, show promising removal efficiency and environmental co-benefits through waste valorisation. However, scale-up and long-term stability remain challenges [51,68].

In response to the evolving complexities of AMD in South Africa, hybrid treatment technologies, which combine passive, active and resource recovery systems, have gained attention for their ability to address multiple contaminants simultaneously while optimising cost and environmental impact [11,30]. These integrated methods, including integrating membrane systems with constructed wetlands or BSR with chemical precipitation, offer improved treatment effectiveness and site-specific flexibility [30,56]. Compared to purely passive systems, which are cost-effective but often slower and less controllable, or active systems, which are highly efficient but energy- and chemical-intensive, hybrid technologies strike a balance by leveraging the strengths of each method [57]. According to comparative research, hybrids frequently provide better sludge manageability, an increased possibility for the recovery of REEs and a higher removal efficiency for metals and sulphates [72,73]. In situations where customised, sustainable and multipurpose remediation solutions are needed, hybrid systems usually perform better than single-mode treatments when evaluated using performance measures, including contamination removal, scalability, operational cost and environmental impact [5,16].

Hybrid systems offer a balance between cost and effectiveness but require integrated management and skilled operation. Hybrid systems that integrate passive and active components have demonstrated improved resilience and cost-effectiveness, achieving sulphate reductions of up to 90% and metal removals exceeding 99% in pilot studies [13,47,62]. Overall, regulatory support and lifespan assessments are essential for optimising the environmental and economic benefits of passive and hybrid techniques, even though they have significant promise for sustainable deployment in South Africa [65].

Table 4. Comparative assessment of passive and active AMD treatment technologies in South Africa.

Technology Type	Method/Process	Performance and Efficiency	Key Examples in SA	Advantages	Limitations	Reference
Passive	Constructed wetlands	Up to 71.25% Fe, Al, Ni, SO42−, Zn, Mn and Cu removal; pH increases to 7	Pilot projects in Gauteng	Low-cost, minimal maintenance, ecological co-benefits	Limited by flow rate, clogging, variable performance in dry/wet seasons	[88,119]
	Permeable reactive barriers (PRBs)	Up to 90% removal of Fe, Al and SO42− over several metres	Research-scale projects in Mpumalanga	Effective in subsurface AMD control, long operational lifespan	High initial installation cost, performance declines over time	[124]
	Anoxic limestone drains (ALDs)	pH increases from 3 to 6; Fe removal (70%)	Pilot studies near coal mines in the Highveld region	Simple design, passive alkalinity addition	Prone to armouring, limited for highly acidic AMD	[2]
Active	Lime neutralisation	>95% removal of Fe, Al, Mn; pH raised to 7–9	eMalahleni Water Reclamation Plant	Fast, reliable and effective across AMD types	High chemical and operational costs, sludge generation	[26,35]
	HDS process	>95% metal removal; reduces sludge volume by 50–70%	Pilot plant in CSIR, Gauteng	Produces cleaner effluent, compact sludge	Requires continuous chemical addition and skilled operation	[48,90]
	Chemical precipitation with soda ash/lime	Efficient for neutralisation and selective precipitation	Used in combination with biological steps in pilot projects	Adaptable, targets specific metals	High reagent costs, challenges in multi-contaminant AMD	[67,81]
Hybrid	BSR + lime/soda pre-treatment	>90% SO42− reduction, >99% metal removal; treated water meets irrigation standards	Mintek’s cloSURE® system, pilot-tested at high-sulphate coal mines	Combines the benefits of both passive and active systems	Needs a carbon source for BSR, scale-up challenges	[60,64]
	Constructed wetland + membrane filtration	>95% metal removal, 85% sulphate removal, high water quality	Laboratory-scale hybrid experiments in Gauteng	Improved water quality, dual treatment function	Membrane fouling, cost of membranes, and limited field application	[13]

presents a critical assessment of the effectiveness, performance information and limitations of the passive and active AMD treatment modalities used in South Africa.

Furthermore, a key component of AMD treatment is still chemical neutralisation using alkaline agents, such as lime and magnesium oxide, and industrial by-products, such as FA and slag [125,126]. To lower operating costs and sludge generation, innovations have been made in the areas of automation, reaction kinetics and reagent dose optimisation [127]. Alternative neutralising agents derived from nearby industrial waste streams have been investigated in South African research, supporting the objectives of the circular economy [128,129]. The nation’s flexible and evolving approach to AMD management is reflected in these technological advancements, which, taken together, are anticipated to inform future initiatives, including hybrid systems, process automation and integration with resource recovery plans [19]. In the context of AMD research in South Africa,

Table 5. Technological advancements in AMD treatment in South Africa between 1998 and 2025.

Technology	Description	Key Applications	Advantages	Challenges	Notable Studies/Institutions	Reference
Bioremediation	Use of microorganisms and SRB to precipitate metals and neutralise AMD	Passive treatment in wetlands, bioreactors	Low-cost, environmentally friendly	Sensitive to pH, temperature and flow variations	WRC-funded projects, Mintek	[60,82,129,130]
Membrane Filtration	Techniques such as RO, NF and FO to separate contaminants	Treatment of AMD to produce potable or industrial reuse water	High removal efficiency, potential for resource recovery	Membrane fouling, high capital and operational costs	Mintek, ROC water	[3,64]
Neutralisation	Chemical dosing with lime, limestone or industrial by-products (e.g., FA, CaCO3)	Immediate pH correction and metal precipitation	Proven and effective, rapid response	Generates large sludge volumes; operational cost	Gold Fields, Anglo American, University of Pretoria	[51,131,132,133]

outlines technological advancements pertaining to bioremediation, membranes and neutralisation (1998–2025).

14. Feasibility

The feasibility of treating AMD in South Africa is influenced by a combination of technological, economic, environmental and regulatory factors [134,135]. South Africa has made significant strides in developing innovative treatment technologies, including active chemical processes, such as lime neutralisation, and advanced methods, such as SBR, RO and ZLD systems. The country has also pioneered projects such as the eMalahleni Water Reclamation Plant, as well as the implementation of sequencing batch reactors and the ROC process, demonstrating the potential for mine water to be transformed into a valuable resource.

However, high operational costs, the need for continuous energy and chemical inputs and the management of by-products, such as gypsum and metal sludge, pose ongoing challenges. Regulatory frameworks, particularly since the year 2000, have become more stringent, requiring desalination before discharge. Recent zero-waste legislation has increased pressure to recover usable products rather than dispose of waste [136]. Despite financial and logistical hurdles, the economic viability of mine water treatment improves with the recovery of marketable by-products, such as pigments, calcium carbonate and aluminium hydroxide, as well as the reuse of treated water in municipal, industrial and agricultural sectors [136,137,138].

The feasibility of scaling up these technologies relies on sustained investment, public–private partnerships, supportive policy and continuous research and innovation to improve process efficiency, reduce costs and ensure environmental sustainability. The study by Mogashane et al. [29] assessed the economic feasibility of the ROC process for treating iron-rich AMD through an integrated system involving neutralisation with CaCO₃ and Ca(OH)₂, RO and freeze crystallisation. The process effectively removed Fe³⁺, Al³⁺ and Mn²⁺, yielding valuable by-products such as marketable pigments from Fe(OH)₃ and sodium sulphate salts. A spreadsheet model based on a 1 m³/h demonstration plant estimated treatment costs at ZAR42.39/m³, with pigment recovery alone valued at ZAR96.78/m³. Compared to traditional methods that incur high brine and sludge disposal costs (ZAR432.99/m³), the ROC process offers a near-zero-waste alternative, with the potential to offset significant operational expenses through product recovery, making it a financially and environmentally sustainable solution [29].

15. Research Challenges and Future Studies

Despite significant advancements in AMD research in South Africa, several technical and scientific challenges persist. High operating costs, membrane fouling in filtration systems, sludge management and inefficiencies in metal recovery are some of the limitations of AMD treatment technology [5,54]. As many traditional techniques, including RO and lime neutralisation, produce secondary waste, more economical and environmentally friendly options are necessary [73]. Furthermore, the development of circular economy approaches, which could effectively extract and reuse essential metals such as iron, manganese and REEs, is hindered by gaps in resource recovery and sustainability research [83,137]. To guarantee that treatment technologies are both economically and environmentally feasible, lab-scale breakthroughs must also be better scalable to full-scale industrial applications [22].

Scaling up AMD treatment technologies in South Africa faces several challenges that extend beyond technical feasibility [29,47]. Due to high operating costs, insufficient infrastructure and restricted access to long-term funding, many innovative approaches are still only available at the laboratory- or pilot-scale [2]. Clogging, seasonal variations and metal accumulation can hinder the maintenance of treatment systems, especially passive ones such as wetlands, which lower long-term efficiency. Policy adoption is further constrained by fragmented regulatory frameworks and a lack of cohesive guidelines to support innovative or community-based solutions [33,94]. Furthermore, limited stakeholder engagement and community involvement often result in poor project acceptance and sustainability. Strengthening multi-sectoral collaboration, improving policy support mechanisms and incorporating community input are essential for successful large-scale implementation of AMD treatment strategies across the country [131].

Although electrochemical processes, membrane filtration techniques excluding RO and adsorption-based methods are widely explored in global AMD research, their application within the South African context has been relatively limited [12,52]. This is mainly because electrochemical and membrane-based systems can be challenging to employ in distant and resource-constrained mining regions due to their high operating costs, energy requirements and technical complexity [62]. Furthermore, although adsorption-based techniques, such as those utilising activated carbon or biosorbents, have shown promise in lab-scale research, they are still underutilised in South Africa at larger scales, frequently due to issues with sorbent regeneration, selectivity and cost-effectiveness [56]. As these systems are more practical for long-term, large-scale and cost-sensitive implementations, most local research has concentrated on passive, active and hybrid systems [51,67].

Novel technologies, including nanotechnology, bioremediation and hybrid treatment systems, are becoming increasingly important in the treatment of AMD and resource recovery [74,139]. By employing nanoparticles with high adsorption capacity for metal recovery, nanotechnology enhances the effectiveness of AMD treatment and offers potential alternatives [62]. Bioremediation methods that utilise microalgae and SRB have garnered interest due to their ability to neutralise acidity while promoting metal precipitation [97,140]. By maximising resource recovery and contaminant removal, hybrid treatment systems—encompassing chemical, biological and membrane-based processes—offer a sustainable solution [60]. Future studies should explore scalable and affordable methods to enhance the long-term viability of these technologies in South Africa’s AMD-affected areas [140,141].

Future studies should also focus on the use of big data and AI in AMD monitoring. Machine learning techniques could enhance treatment effectiveness, pollution dispersion and prediction modelling for AMD development [89]. AI-driven systems can analyse large datasets from real-time monitoring devices, remote sensing and lab research to enhance early warning systems and streamline treatment procedures. AI integration with automated monitoring networks may enable proactive AMD control, lower expenses and improve remediation results [91]. The development of reliable, AI-powered decision-making tools tailored to the mining and environmental contexts of South Africa should be the primary goal of this field’s research [89].

Despite significant scientific progress, regulatory obstacles and a lack of cooperation between researchers and industry partners continue to impede practical uses [142]. Future studies ought to investigate legislative frameworks that support industry-driven research projects and provide incentives for AMD remediation efforts [100,143]. Technology transfer, knowledge sharing and the commercialisation of AMD treatment advances can be facilitated through collaborative platforms that bring together mining firms, government agencies and academic institutions [144].

For AMD management, a shift towards sustainability and the concept of the circular economy presents a revolutionary opportunity [74,145]. To convert waste into useful resources, research should focus on zero-waste approaches that prioritise recovering essential metals, such as iron, copper and REEs, from AMD [73,146]. Long-term environmental restoration can also be facilitated by combining AMD therapy with environmentally friendly water reuse and land rehabilitation [134,147,148,149]. To ensure that AMD treatment aligns with South Africa’s sustainable development goals, future initiatives should prioritise life cycle assessments, techno-economic feasibility studies and the development of market-driven recovery procedures [150,151,152].

16. Conclusions and Recommendations

Research on AMD in South Africa has shown a notable shift in emphasis between 1998 and 2025, changing from traditional containment and neutralisation techniques to integrated and sustainable solutions. According to bibliometric analysis, the number of multidisciplinary research studies that incorporate resource recovery, materials science and environmental engineering is steadily increasing. In line with the ideas of the circular economy, recent developments include the successful recovery of REEs, precious pigments, such as haematite, and potable drinking water from AMD. Zero-waste strategies, where the commercial value of recovered items may offset or even surpass treatment costs, are increasingly being prioritised by emerging technologies. Although South African AMD research has influenced international debates, research effect assessments indicate that further integration of science, business and policy is still required to promote the widespread adoption of sustainable solutions. Despite these promising developments, challenges remain in scaling up pilot studies, reducing energy consumption and ensuring regulatory support.

The review also highlights that, while South Africa leads in AMD research on the African continent, more structured collaboration between academia, government and industry is urgently needed. The variability of AMD composition across mining sites complicates the development of universally applicable treatment solutions. Policy gaps, particularly in areas such as waste reuse and resource recovery, also pose significant barriers. Future studies should focus on cutting-edge technologies, such as AI-powered monitoring systems, nanomaterials and resource recovery strategies based on the circular economy. Guided by the findings of this study, future AMD research initiatives that encourage innovation will ensure that South Africa remains at the forefront of sustainable mine water management. Simultaneously, mining companies must be encouraged to adopt environmentally responsible practices and engage in partnerships that support the upscaling of innovative solutions.