Acid Mine Drainage Treatment and Control: Remediation Methodologies, Mineral Beneficiation and Water Reclamation Strategies

Abstract

Although mining plays a vital role in the economic development of many countries, devastating environmental repercussions are associated with it. The extraction of mineral resources inevitably results in the generation of acid mine drainage (AMD), which entails intricate oxidation interactions that occur under ambient conditions in abandoned and active mines. The arbitrary release of AMD can lead to a series of long-term environmental problems, degradation of aquatic habitats and health complications. Over the years, extensive progress has been made in the prevention and treatment of AMD, with some processes even progressing as far as the commercialisation level. This chapter therefore discusses the process of AMD formation, preventative and control measures and AMD treatment options applicable to both operating and developed mines, as well as to researchers interested in environmental remediation and rehabilitation. Advances in mineral beneficiation and water reclamation strategies employed in the AMD treatment processes are highlighted to shed light on strides being made towards promoting a circular economy in mining industries. The featured work therefore demonstrates the global progress towards environmental protection and water resource management. The challenges and loopholes associated with the current AMD treatment methods are deliberated and possible future prospects in the field are proposed.

1. Introduction

Although mining plays an important role in the global socio-economic development by generating wealth, providing employment opportunities and engendering foreign exchange, it often happens at the detriment of the environment. The mining industry is one of the greatest contributors to water pollution and environmental deterioration through the generation of acid mine drainage (AMD) [1]. Characterised by extremely acidic pH and high concentrations of dissolved metalloids and metals, AMD can severely contaminate surface and groundwater, as well as soils, posing a serious threat to ecological systems and human health [2]. This is because upon formation, it finds its way to the surrounding environment and water bodies through flooding of mines, runoff from open surface mining activities as well as seepage from mine residue, deposits and tailings.

Mining and mineral processing activities can generate large volumes of waste, including mill tailings, waste rock and mineral refinery waste. The exposure and oxidation of pyrites (iron disulfide) and other sulfide minerals (arsenopyrite, chalcopyrite, galena, pyrrhotite, and sphalerite) during mining and in mining waste results in the release of acidic water containing high concentrations of dissolved metals [3]. Since pyrite is the most abundant and widespread sulfide mineral, it is extensively considered as the predominant cause of AMD generation [4, 5].

The series of chemical reactions involved in the formation of AMD are shown in Figure 1. Briefly, when exposed to water and air, pyrite is oxidised in a reaction catalysed by microorganisms (mainly bacteria), releasing H⁺, SO₄²⁻, and Fe²⁺ ions. The Fe²⁺ ions are further oxidised to Fe³⁺ in the presence of water and air. At acidic pH, Fe³⁺ ions are hydrolysed and precipitated to form Fe(OH)₃, while part of the Fe³⁺ ions may continue to oxidise pyrite to sulfates and acid, forming AMD [6, 7]. The primary factors that determine the rate of acid generation therefore include; pH, oxygen concentration in the water phase; bacterial efficiency, degree of water saturation, surface area of exposed metal sulfide and the activation energy required to trigger acid generation amongst others [8].

Figure 1.

Chemical reactions related to AMD formation. Reprinted with permission [6].

The changes in the physicochemical properties, composition and pH of mine-impacted water deem it unsuitable for domestic, agricultural, and industrial uses [9]. Due to its distinctive bright colours, AMD alters the environmental aesthetic, causes habitat alteration and disruption of the nutrient cycle [10]. Moreover, due to its acidity, AMD induces corroding effects on infrastructure such as bridges and sculptures and contributes to the deterioration of roads around the impacted areas [11]. Additional impacts include the loss of biodiversity and decline of aquatic ecosystems [12]. Lastly, the oxidation of Fe²⁺ significantly reduces the amount of oxygen available in the water, which affects aquatic organisms [13]. The general ecological destruction caused by AMD generation is shown in Figure 2.

Figure 2.

Destruction of the ecosystem by mine wastes. Reprinted with permission [14].

The discharge of acidic, metal and sulfate-rich water from tailings may continue for hundreds of years after mine closure if not managed properly [15]. Hence, the development of prevention and control methodologies as well as treatment technologies is vital, not only over the operational lifetime of a mine, but also during decommissioning or remediation of abandoned mine sites [16]. This will reduce the health and environmental impacts associated with AMD exposure and enhance environmental sustainability.

This chapter is thus intended to report on the current preventative, control and treatment methods applicable to operating and developed mines, as well as to researchers interested in environmental remediation and rehabilitation. The advantages and drawbacks associated with the methodologies are discussed to identify loopholes and inspire further innovative research in AMD prevention and remediation. With the aim of promoting a circular economy in mining industries, advances in mineral beneficiation and water reclamation are also deliberated. The featured work therefore highlights the global progress towards environmental protection and water resource management for enhanced ecological sustainability.

2. Prevention and control methodologies

In order to prevent the environmental damages, proper control and treatment methods need to be implemented to halt the formation of AMD. This is because, once AMD has formed and developed in mine sites, controlling its migration to the environment and surrounding water streams is rather difficult and requires costly infrastructure. For instance, Cacciuttolo et al. reported that Chile and Peru commonly use polymeric geomembranes, cut-off trenches, plastic concrete slurry walls, and grout curtain systems to control seepage at tailings storage facilities to control AMD migration [17]. Various methods have therefore been proposed to prevent the initial formation of AMD thereby retarding its environmental and health effects, mainly based on the management of rock waste and other mine tailings.

Since air, water and bacteria are key ingredients required for AMD formation, methods for controlling the formation of AMD usually function by limiting the exposure of mine areas and tailings to these resources [18]. The most commonly used method of control is minimising the penetration of air and water into the tailings waste by using dry covers; which are inorganic mineral materials that reduce the oxidation rate of sulfide minerals [19]. Dry covers are generally composed of materials with different granulation characteristics including compacted clay, portland cement and fly ash amongst others [20]. However, extensive analysis needs to be conducted during the selection of dry covers because alkaline covers like fly-ash can cause metal mobilisation from the waste, resulting in weathering of the tailing waste [21]. Alternatively, wet covers (water) have been proposed and used as an oxygen diffusion barrier. This is because, under normal conditions, the solubility of oxygen in water is low, which limits the rate of sulfide mineral oxidation when mine waste is covered with it [20].

The use of bactericides has also been reported to inhibit acid generation during hard rock and coal mining operations, resulting in an improvement in the quality of the reclaimed water [22]. Bactericides are however only effective on fresh tailings and are short-lived, and therefore do not serve as a permanent solution to AMD control [5]. Although they also have a short lifespan, organic materials such as wood waste have also been applied to cover mine tailings and the surface of mine waste, thereby preventing oxygen entry [23].

Lastly, the diversion of surface water flowing towards the mine tailings, prevention of groundwater infiltration into the tailings site and controlled placement of acid-generating waste can reduce the formation of AMD [8]. Even with such prevention measures in place, there is still a lot of mine- impacted water being generated globally due to the difficulty of maintaining some of these technologies overtime, thus research interest in the treatment of AMD has soared.

3. Treatment methods and technologies

Ideally, AMD treatment methods and technologies should have a high sulfate and metal removal efficiency, high water quality generation, low operational and labour costs, easy process design and control, minimal waste generation and low maintenance. As discussed next, several techniques have been employed for the removal of some chemical species from AMD. A description of the operation processes is provided, along with the primary advantages and disadvantages associated with the processes.

3.1 Adsorption

Adsorption is a surface phenomenon wherein atoms, ions, or molecules from gases, liquids, or dissolved solids adhere to the active sites of an adsorbent, forming a film of the adsorbate on its surface [24]. The adsorption technique offers several advantages, including affordability, high regeneration capacity, a wide pH range, the use of various raw materials, and a high metal binding capacity [25]. Although adsorption has been widely explored for treating AMD, there are significant drawbacks to consider; high selectivity and affinity of adsorption can hinder its effectiveness in decontaminating multi-charged wastewater like AMD. Rapid saturation leads to poor performance in highly concentrated solutions, further limiting its efficiency for AMD treatment. While the process has shown effectiveness in less concentrated solutions, it is primarily used as a polishing process. Moreover, the regenerates from the adsorption process are often highly mineralised and heterogeneous, making it challenging to obtain pure and high-quality minerals. Proper handling and disposal of regenerates also add to the overall cost [26]. Commonly used adsorbents for removing anionic and cationic pollutants through adsorption include activated carbon [27], zeolites [28], clay minerals [29], and ion exchange resins [30].

3.2 Chemical precipitation

Precipitation techniques are widely employed in the treatment of industrial wastewater, effluent water, and water affected by mining due to its simplicity [31]. The process entails the addition of suitable precipitating agents to the contaminated water, such as iron salts, limestone, lime, alum, or polymers. Chemical coagulants are used as a preliminary step to destabilise fine suspended particles and colloids, which then agglomerate into larger flocs. These precipitating agents react with heavy metal ions, converting them into insoluble precipitates, such as metal carbonates, metal hydroxides, and metal phosphates that can be easily separated from the water [32]. There are two main types of precipitation mechanisms used namely neutralisation precipitation and oxidative precipitation. Neutralisation precipitation involves the addition of alkaline materials to raise the pH of the water, making it more basic. On the other hand, oxidative precipitation involves the addition of oxidising agents into the contaminated water to convert the metal ions into insoluble forms [33]. The oxidising agents such as hydrogen peroxide (H₂O₂) facilitate the conversion of metal ions from their soluble reduced state to a less soluble oxidised state [34].

Both neutralisation and oxidative precipitation mechanisms can be effective in removing various heavy metals, such as lead, copper, zinc, and others, from industrial wastewater before its discharge into the environment. The choice of the specific precipitation mechanism and the type of alkaline material or oxidising agent used depends on the nature of the contaminants and the water chemistry involved in each particular case [35]. Following this step, the water is subjected to either sedimentation or filtration processes to separate the formed precipitates from the contaminated water. In sedimentation, the formed precipitates settle down under the influence of gravity, and the clear water can be decanted from the top [24, 36]. In filtration, the water is passed through a filter medium to separate the solid precipitates from the liquid phase [37]. In some cases, a secondary treatment step may be required to further purify the water. Once the water is sufficiently purified, it can be safely discharged into the environment if it meets regulatory standards. Alternatively, in water-scarce regions or in industries where water conservation is essential, the purified water can be recycled and reused for various purposes, reducing the overall water demand. This method offers several advantages for removing various metal ions from AMD. It proves to be effective in recovering valuable metals in the form of precipitates and can be optimised according to the specific pollutants present. However, there are certain drawbacks associated with this method. For instance, it leads to the generation of additional waste products. Moreover, pH adjustment may be required to achieve optimal precipitation, demanding careful monitoring. Additionally, the effectiveness of the method can be influenced by competing ions present in the water.

3.3 Filtration

Filtration is a widely used separation process that employs physical barriers to remove solid particles and contaminants from various fluids, including AMD [3, 26]. Filtration can use various types of media, depending on the specific application and the characteristics of the contaminants present in the AMD [26]. Commonly used filtration media include sand, activated carbon, diatomaceous earth, cellulose, anthracite coal, and various types of membranes such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes [38, 39]. Filtration media with smaller pores can remove finer particles, while larger particles may be effectively removed by media with larger pores. Filtration processes can be categorised into two main types: depth filtration and surface filtration. Depth filtration involves the penetration of contaminants into the filter media, whereas surface filtration retains particles on the top layer of the media. Depth filtration generally offers a higher contaminant-holding capacity compared to surface filtration [40]. Depth filtration has been a critical component of water purification technology, and also in bioprocessing [40].

Prior to filtration, some AMD may require pre-treatment to optimise the filtration process. Pre-treatment steps can include pH adjustment, coagulation, flocculation, and sedimentation to facilitate the removal of specific contaminants or to prevent clogging of the filtration media [23, 41, 42]. In some filtration systems, especially those using granular media like sand or activated carbon, periodic backwashing is necessary to remove accumulated solids and prevent clogging. Backwashing involves reversing the flow of fluid through the filter to dislodge and remove trapped contaminants [43, 44]. Filtration plays a crucial role in AMD treatment and is often combined with other processes like chemical precipitation, biological treatment, and ion exchange to achieve comprehensive and effective remediation. This method offers several advantages, such as efficiently removing particulate matter and solids, which leads to improved water clarity and reduced suspended pollutants. Moreover, it offers a relatively simple implementation and maintenance process. However, it does have some drawbacks, for instance, it may not be as effective in removing dissolved pollutants. Additionally, regular maintenance and replacement of the filter media are necessary. High flow rates could also lead to reduced filtration efficiency.

3.4 Bioremediation

Bioremediation effectively treats AMD by utilising living organisms to mitigate environmental damage [45, 46]. As shown in Figure 3, AMD bio-remediation techniques can be classified as active and passive treatment methods [47].

Figure 3.

Classification of AMD bio-remediation techniques: Active and passive treatment methods. Reprinted with permission [47].

Passive treatment systems use natural processes and materials to treat AMD wastewater without the need for active intervention such as mechanical or energy-intensive systems [48]. These systems typically involve constructed wetlands or ponds that promote vegetation growth and water interaction with various materials to remove contaminants. The commonly used passive treatment techniques for AMD wastewater are aerobic and anaerobic wetlands [10]. The passive treatment wetlands are designed to mimic natural wetland ecosystems and utilise plants, microbes, and soil to treat wastewater.

Unlike passive treatment, active treatment systems employ engineered processes to treat AMD. Active biological treatment of AMD involves the use of microorganisms to actively treat and remediate the acidic and metal-contaminated water. This approach utilises various biological processes to neutralise acidity, remove metals, and restore water quality [47]. Generally, implementation of the active treatment for treating AMD has enhanced contaminant removal and is a more reliable and efficient treatment method than the passive [49]. On the downside, active treatment methods can be expensive to implement and maintain, they often generate a significant amount of sludge or solid waste as a by-product, which can be challenging to dispose of [26].

The efficiency of bioremediation techniques varies depending on site-specific conditions and the characteristics of the AMD [50]. A comprehensive understanding of the site and thorough monitoring are crucial for successful implementation. Additionally, bioremediation is often used in combination with other treatment methods, such as chemical precipitation or physical filtration, to achieve optimal results in treating AMD.

3.4.1 Biosoprtion

Biosorption is a bioremediation process that involves the use of living or non-living biomass to remove pollutants from contaminated water. In the case of AMD, biosorption can be employed as a method to remove toxic metals from the acidic water [50]. This process relies heavily on the ability of certain materials, such as algae, bacteria, fungi, or even agricultural waste products, to bind and accumulate metals onto their surfaces [51, 52]. These biomass materials contain functional groups like carboxyl, amino, and hydroxyl groups, which can form complex bonds with metal ions [50, 51, 52]. The metals are thus removed from the solution and immobilised onto the biomass, effectively reducing their concentration in the water. As such, different types of biomass can be used, depending on the target metals and their concentrations in the AMD [51, 52]. Table 1 outlines some commonly used biomass materials.

Biomass Materials	Specifications	Ref.
Algae (e.g., Spirogyra, Chlorella)	Contains functional groups (e.g., carboxyl, amino) that can bind metal ions. Sensitive to environmental conditions (pH, temperature).	[53, 54]
Fungi (e.g., Aspergillus, Penicillium)	High surface area biomass with numerous binding sites. Resistant to harsh environmental conditions.	[55, 56]
Agricultural Waste Products (e.g., rice husks, coconut shells)	Cost-effective and readily available. Can be modified to enhance adsorption capacity. Often used as low-cost adsorbents for various pollutants.	[57]
Activated Carbon	High surface area with numerous micropores. Excellent adsorption capacity for a wide range of contaminants.	[58, 59, 60]
Clay Minerals (e.g., Bentonite)	High surface area and swelling capacity. Suitable for metal and dye removal.	[61, 62, 63]
Natural and Modified Chitosan	Contains amino groups for metal binding. Used for metal and dye removal.	[64, 65, 66]
Sawdust	Effective for dye and organic pollutant removal. Requires modification for enhanced adsorption.	[67]
Sugarcane Bagasse	Contains cellulose and hemicellulose for adsorption. Suitable for metal and dye removal.	[68]
Peat Moss	High organic matter content. Contains humic substances for metal binding. Used for metal and dye removal.	[69, 70]
Bark	Natural adsorbent with porous structure and good for organic pollutant removal. Requires pre-treatment	[69, 71, 72]
Magnetite	Magnetic properties for easy separation. Used for heavy metal removal.	[73, 74]
Hydrogels	High water retention capacity. Suitable for dye and metal removal. Can be tailored for specific applications.	[75, 76, 77]
Polymeric Materials	Versatile adsorbents with tunable properties. Effective for a wide range of contaminants.	[78, 79]
Carbon Nanotubes	High aspect ratio with nanoscale dimensions. Excellent adsorption capacity for organic pollutants.	[80, 81, 82, 83]
Soybeans Residues	Rich in proteins and amino acids. Requires modification for optimal adsorption.	[84, 85]

Table 1.

Frequently employed bio-based materials.

The selected biomass is usually dried, ground, and sometimes chemically treated to enhance its metal-binding capacity. After the biosorption process, the metal-loaded biomass can be separated from the water through physical means, such as filtration or centrifugation [58]. The regenerated biomass may undergo a regeneration process, such as acid or alkali treatment, to restore its metal-binding capacity for future use. As such, biosorption is an attractive option for AMD treatment due to its cost-effectiveness and potential use of low-cost biomass materials. However, the efficiency of biosorption can be influenced by various factors, including pH, metal concentration, contact time, and biomass characteristics [52, 58, 86, 87]. Therefore, it is essential to optimise these parameters and conduct rigorous monitoring to ensure efficiency of the biosorption process in treating AMD.

3.4.2 Sulfate reducing bacteria

Sulfate-reducing bacteria (SRB) are a group of microorganisms that play a crucial role in the treatment of AMD. These bacteria can reduce sulfate ions (SO₄²⁻) present in AMD to sulfide ions (S²⁻), which can precipitate and immobilise heavy metals. The activity of SRB helps neutralise acidity and mitigate the environmental impact of AMD [14]. Moreover, SRB provide an effective and sustainable approach for AMD bioremediation [14]. As shown in Eq. (1), the typical SRB bioremediation process involved in the treatment of AMD involves sulfate reduction, which uses sulfate as an electron acceptor for its metabolism, converting sulfate to sulfide [88]. This metabolic activity occurs in an anoxic (oxygen-depleted) environment, typically found in the deeper layers of AMD-affected sites [14, 89].

$$S{O}_4^{2-}+2\left(C{H}_2\text{O}\right)={\text{H}}_2\text{S}+2HC{O}_3{}^{2-}$$E1

The H₂S can be oxidised to sulfate or sulphur (S^o) or can react with iron sulfide minerals in the sediment as shown in Eq. (2). The insoluble metal sulfides precipitate, thus making it easier to remove them from the wastewater.

$$F{e}_2{\text{O}}_3+4{\text{S}}_2\left(\text{mostly from}{\text{H}}_2\text{S}\right)+6{\text{H}}^{+}=2Fe{S}_2+3{\text{H}}_2\text{O}+2\text{e}$$E2

The treated water undergoes settling and filtering to separate solid precipitates from the liquid phase. Monitoring and discharge ensure compliance with regulatory standards and may involve additional treatment steps, such as pH adjustment or metal removal.

The SRB can form biofilms on solid surfaces, such as rocks or sediments, in AMD affected areas [90, 91]. These biofilms provide a favourable microenvironment for the growth and activity of SRB, allowing them to establish and sustain their sulfate-reducing capabilities [91]. As such, the efficiency of SRB in AMD treatment depends on various factors, including the availability of organic matter, nutrients, and suitable environmental conditions. Optimising these factors can enhance the activity of SRB and improve their potential for treating AMD [89, 91, 92].

3.5 High density sludge (HDS) process

The high-density sludge (HDS) process has been implemented for decades as the standard in the mining industry for treating AMD, providing low metal levels in the final effluent, and reducing the generated waste sludge compared to other lime-based processes [93]. The HDS process entails addition of a neutralising reagent in the AMD to precipitate and remove metal ions from the wastewater, followed by the addition of a flocculant to aggregate fine particles, forming larger flocs. The mixture settles in a settling tank, forming a sludge layer that is concentrated with heavy metal contaminants. The treated sludge is then separated, and the clarified water can be discharged or further treated [94]. This essentially distinguishes the HDS process from other lime treatment processes i.e. the method of precipitating and separating the solids and the sludge that is formed differs. The metals can be recovered or beneficiated from the dewatered sludge. However, the current most common technique to dispose of the sludge is through landfilling, stabilisation, or recycling.

The advantage of the HDS process is that it is flexible and scalable, allowing for the treatment of various AMD sources with varying metal concentrations. The process can also be integrated into existing mine infrastructure, minimising the need for additional treatment facilities. Moreover, the process is widely accepted by regulatory agencies and has long-term stability, allowing controlled dewatering and disposal of the sludge [95].

The disadvantage of the HDS process is that it is expensive due to specialised equipment and chemicals, as well as the need for chemicals to precipitate and remove metal contaminants. It is also complex and requires skilled personnel and continuous monitoring for efficient operation [96]. Disposal of sludge, which contains concentrated heavy metals, can also be challenging and costly.

3.6 Hybrid and integrated treatment technology

Hybrid treatment technology involves combining two or more treatment methods to enhance the overall effectiveness of AMD remediation. The main objective is to take advantage of the strengths of different treatment processes to address multiple pollutants and achieve more efficient results [26]. Some common hybrid treatment approaches for AMD include a combination of chemical precipitation and filtration, biological and physical-chemical treatment, and constructed wetlands with secondary treatment [97]. Biological processes, such as bioremediation, can be combined with physical-chemical treatments to further remove contaminants from the water [98]. Constructed wetlands can be combined with secondary treatment methods like aeration or chemical addition to improve their ability to remove pollutants from AMD [49].

Integrated treatment technologies allow for a comprehensive approach to AMD treatment, targeting a wide range of contaminants [99]. One common integrated system is the multi-stage treatment system; consisting of multiple treatment stages, each targeting specific pollutants.

An example of patent technology is the Anoxic Limestone Drain/Anaerobic Bioreactor (ALD-AB) as discussed in the draft prepared by Tetra Tech Inc. [100]. The other method is membrane filtration with chemical treatment, where membrane filtration is used to remove fine particulates and residual pollutants, while chemical treatment is applied to adjust pH levels and remove dissolved contaminants. The electrochemical methods can be integrated with chemical precipitation to treat highly contaminated AMD, as they can help remove both dissolved and particulate pollutants effectively [101]. The WRC report number 940/1/04 discussed the electrochemical water treatment system called (ECODOSE) [102]. Other integrated include neutralisation combined with chemical desalination techniques i.e. the CSIR Alkaline-Barium-Calcium (ABC) and Magnesium-Barium-Hydroxide (MBO) processes. See Table 2 for the summary of some common patented hybrid or integrated technologies in South Africa.

Technology, Combined treatment techniques	Common name
High Recovery Precipitation, Reverse Osmosis	Nafasi Water’s HiPRO™
Magnesium Hydroxide and Barium Hydroxide	TUT MBO
Alkali Barium Calcium Process	CSIR-ABC
Metal Precipitation, Gypsum de-saturation, Ettringite precipitation, carbonation & recycle AL(OH)2	Mintek’s SAVMIN
Magnesite, Softeners, Reverse Osmosis and the Eutectic Freez	CSIR MASROE

Table 2.

Commercialised hybrid and integrated wastewater treatment technologies for AMD in South Africa.

4. Material and mineral beneficiation strategies

The recovery and reuse of valuable resources from AMD can offset the cost of AMD remediation. Mineral beneficiation strategies therefore aim to reduce the environmental impact of AMD by treating the harmful contaminants while simultaneously recovering valuable minerals that can be reused or commercialised.

By implementing methodologies such as pH adjustments, mineral precipitation into metal hydroxides, carbonates, or sulfides can occur. This requires the use of alkaline substances which neutralise acidity by reacting with sulphuric acid, increasing the pH level [103]. The precipitated minerals or metals can then be separated from the water through sedimentation or filtration processes. Selective precipitation is another commonly used method for retrieval of valuable minerals, especially for targeted removal of specific contaminants based on their solubility characteristics [104, 105]. These mineral beneficiation processes are often used consecutively. For instance, Akinwekomi et al. recovered Fe(III) and Fe(II) for use in the synthesis of goethite, haematite, and magnetite, using sequential precipitation in batch reactors and subsequently added lime to the treated water to generate high-grade gypsum [106].

The waste generated during the treatment of AMD can also be reused to value added materials. Amanda and Moersidik generated a sludge from a precipitate which was produced from AMD active treatment ponds. They proposed that the sludge could be utilised as an adsorbent material for removing pollutants in wastewater [107]. In addition, Liu et al. effectively synthesised M-type hexaferrite from AMD sludge which could be used for absorption purposes [108].

Furthermore, in addition to the minerals, value added chemicals i.e. sulphuric acid can also be recovered form AMD as proven by Martí-Calatayud et al., who reported on the recovery of sulphuric acid from AMD by means of a 3-compartment electrodialysis [109].

5. Water reclamation

The efficiency of water treatment methods and technologies is determined by their ability to generate water with a quality level that is acceptable for reuse. Water reclamation and reuse is of great importance in the mining industry as it reduces the utilisation of the limited clean water, it also provides water that can be reused in other industries [110]. For instance, Ricci et al. implemented and reported on sequential stages encompassing microfiltration, nanofiltration and reverse osmosis to recover sulphuric acid, separate noble metals, and produce high quality reusable water from gold mining effluents [111]. Peterson et al. has also reported on the initiative driven by Battelle to optimise its HydroFlex™ water purification technology to treat AMD for the purpose of supplying non-potable water for use in hydraulic fracturing activities [112]. The reuse of water within industrial processes is therefore fast becoming a norm as laws and regulations concerning environmental protection and water security are becoming increasingly stringent.

Water reclaimed from AMD treatment can however also be used for agricultural purposes (irrigation), or even polished to generate potable water [113].

5.1 Water reclamation for irrigation

Unfortunately, due to water scarcity, many agricultural areas make use of AMD contaminated streams, with elevated trace metal concentrations, for irrigation. For instance, Garrido et al. investigated the effects of irrigation using AMD contaminated water on soil and crops (potatoes). They found that the irrigation water contaminated the soil and potatoes with high concentrations of various ecotoxic metals. The levels of Cd, Pb, and Zn in the generated potatoes exceeded commercially sold vegetable guidelines, possibly presenting a potential health risk to subsistence farmers and other consumers [114]. Many researchers have therefore resorted to treating the AMD before embarking on such endeavours.

For instance, Martins et al. worked on generating water for irrigation from AMD by comparing the efficiencies of two bioremediation systems based on sulfate-reducing bacteria. System I consisted of a packed bed reactor containing calcite tailing followed by an anaerobic packed bed bioreactor. System II consisted of a settler, fed with AMD and treated water recycle, and a sulphidogenic anaerobic packed bed bioreactor, fed with clarified settler effluent. They reported that, in addition to not needing a neutralising agent, the system II had very high sulfate reduction (>99.0%) capacity, reaching levels much lower than legislated limits for irrigation water [115].

Additionally, Kalombe et al. constructed a pilot plant for the treatment of AMD using coal fly ash. The concentration of major contaminants (sulfate, Al, Fe, Ca, Mg), and minor contaminants in the treated AMD was significantly lowered (between one and four orders of magnitude) compared to the raw AMD with at least 66.6% (728.56 kg) of treated water being recovered. Their treated water met the target water quality range (TWQR) limit for agricultural irrigation in South Africa [116].

Moreover, Han et al. recently reported on the use of fertiliser driven forward osmosis (FDFO) to extract fresh water from AMD for irrigation using novel thin film nanofibrous composite (TFNC) membranes with selective layers. They were able to generate water that promoted plant growth without posing any potential health threat [117].

5.2 Water reclamation for drinking water

The generation of potable water from AMD often requires complex treatment systems which involve filtration. For instance, Masindi generated drinking water from AMD by using magnesite to recover all the metals and increase the pH ≥ 9, lime to remove ≥90 of residual sulfate and soda ash to remove the residual calcium to form limestone. He further bubbled the treated water with CO₂ to remove residual sulfates and polished the treated water using a simulated Reverse Osmosis (RO) system to produce water that meets the drinking water quality as stipulated by the South African National Standard (SANS) 241 standards. The pH of the recovered drinking water was ≈ 6.5 and the metals removal efficiency of the RO system was ≈ 100% [118].

Additionally, Shingwenyana et al. reported on a developed and patented process by the Council for Scientific and Industrial Research (CSIR) for the treatment of AMD using an integration of a number of technologies. Their technology can reclaim drinking water while generating a number of valuable minerals such as goethite, haematite, magnetite, gypsum and limestone [119]. Lastly, Hutton et al. reported on the recovery of potable water from AMD using a Keyplan HiPRO process from the eMalahleni Water Reclamation Plant that was commissioned 2007. Although membrane health and performance are paramount in keeping this process going, they had targeted on generating 25ML/day of drinking water [120].

6. Challenges and future prospects

The closure and abandonment of mines is not usually caused by total depletion of resources, hence the continued illegal mining activities. Mine closure is typically associated with falling financial returns based on metal values, or social, political, and environmental restrictions [121]. The lack of stringent regulations for controlling abandoned mines however results in heightened environmental distress through many factors including AMD formation. Policy regulation, mine closure planning, rehabilitation and sealing of open shafts and sustainable practices are therefore essential for minimising AMD generation and preserving global water resources and ecosystems for future generations. Moreover, strengthening and enforcing environmental regulations related to mining and AMD discharge is essential to reduce pollution from operational mines and to hold responsible parties accountable.

The complexity of AMD makes its complete treatment using a single technique nearly impossible, therefore integrated treatment approaches are often implemented to generate reusable water. Essentially, addressing AMD therefore requires a multifaceted approach that accounts for the challenges associated with each technique while leveraging ongoing research and technological advancements to create more effective and sustainable treatment methods. Advancements in materials science, reactor design, and process optimisation however offer promising future prospects for more effective and sustainable AMD treatment methods. Moreover, collaborations between government entities, mining companies, environmental organisations, research associations and local communities are essential for the implementation of preventative and treatment solutions of AMD.

Ideally, mine management should consider sustainability, in which ecological restoration is an important part. Recently, Qi et al. suggested combining vegetation restoration with the covering of mine tailings as a synergistic technology for both pollution control and ecological restoration [122]. Also, with advances in AMD predictive tests and methodologies and integration with laboratory tools to measure mineralogical, geochemical, textural and geometallurgical properties, effective waste prevention and management during operation can be achieved, ultimately leading to less costly mine closure outcomes.

Although some AMD treatment processes give rise to new waste streams (e.g. brines and gypsum), finding useful applications and enhancing commercial value for products of AMD remediation is therefore an area worth investing in. For instance, Hedin et al. reported on the use of iron oxide sludge recovered from a drainage channel at an abandoned coal mine in the production of pigments, a venture which proved to be commercially lucrative [123]. Some researchers have also considered AMD as a potential low-cost raw material for the production of iron oxide particles (goethite, ferrihydrite, and magnetite), for potential application in industrial wastewater treatment [124]. Lastly, the high cost implications, energy requirements and low yield in the quantity and quality of purified water hamper the AMD treatment processes. It would thus be beneficial to invest in developing innovative technologies that will cater to these shortfalls.

7. Conclusions

This chapter has demonstrated the available technologies for the prevention, control and treatment of AMD and the urgency of furthering research and innovation in order to develop new technologies. With limitations such as high costs, low efficiency and selectivity as well as high by product (waste) generation in the currently employed methods of treatment, investments into innovative treatment processes is required. Stringent policy development and implementation is also required to regulate and limit the AMD released into the environment as well as promote a circular techno-economic strategy in the mining industry.

Acknowledgments

The authors wish to acknowledge Mintek for supporting this work as well as the Department of Science and Innovation (DSI), which supported this work through the DSI/Mintek, Nanotechnology Innovation Centre (NIC).

Conflict of interest

The authors declare no conflict of interest.