Introduction
Acid mine drainage (AMD) is mining by-product generated by active and abandoned mines formed following the oxidation of pyrite (FeS2) and other sulphide materials in the presence of air, water, and oxidizing bacteria resulting in the production of acidic and metals-rich wastewater. Acid mine drainage contains elevated concentrations of aluminium (Al), iron (Fe) and manganese (Mn), moderate or low concentrations of zinc (Zn), copper (Cu), nickel (Ni) and arsenic (As), radioactive elements such as uranium (U) and very high concentrations of sulphate ions (SO42−) [1, 2]. In addition, the chemical composition of AMD is influenced by the geohydrological factors and the types of mining ores [3]. This unpleasant wastewater rich in various chemical species is very toxic and harmful to environment and human health if discharged untreated. Acid mine water lowers the pH of receiving water course, leading to an increase in most chemical species concentration and the toxicity of the entire aquatic ecosystem thereby affecting the suitability of water for myriads of defined uses [4, 5]. In addition to negative effects on the aquatic ecosystem, AMD also causes soil pollution and erosion thus render it unsuitable for agricultural use [6, 7]. The harmful and toxic effects of AMD are not only limited to its eco-toxicological effects but can be extended to human health due to the presence of radioactive substances, metalloids and toxic metal in AMD polluted areas, thus, increasing the risks of carcinogenic, mutagenic and teratogenic effects on humans and animals [7]. Overall, AMD is an unpleasant problem common to countries with advanced mining industries such as United States of America (USA), Canada, Chile, Australia, Brazil, South Africa, and China, amongst others [8, 9]. For instance, various studies have reported that to date, mega litres of AMD are still being generated annually by active and abandoned mines located in the Witwatersrand basin in the Republic of South Africa [10, 11]. This huge volume of AMD is an illustration of the nature of the severity of problems faced by South Africa and other countries with well-developed mining industries but seems to be less documented compared to global warming since the United Nations (UN) considers AMD as the world’s second-largest environmental problem after global warming.
As such, the prevention of AMD formation or the effective treatment of already generated AMD becomes fundamental to limit its environmental effects and safeguard human health. Various prevention technologies, including: mining tailings characterization [12], alkaline amendment [13], water flow diversion [14], oxygen barrier [15], bactericide [16], reduction of the volume of AMD generated [17], reclamation of contaminated land [18], co-disposal and blending [19], desulfurization [20] and surface passivation [21] have been designed and applied to suppress AMD formation. On the other hand, the already generated AMD is being treated using active methods or chemical treatment [22, 23], passive approaches or biotic systems [24–27], hybrid technology [28, 29], integrated systems [30, 31], emerging passive technology such as phytoremediation [25, 32], membrane technology and nanoremediation [33]. Amongst the aforementioned technologies, nanoremediation is very advantageous since it can reduce the overall cost and time of treating large volumes of AMD [34, 35]. In addition, nanoremediation significantly contributes to environmental sustainability since nanomaterials are used to remediate polluted soil, water, and air with reduced risk to human health [35–37]. For instance, Zinatloo-Ajabshir et al. [38] prepared a nanomaterial, a novel rod-like or [Cu(phen)2(OAc)].PF complex and successfully applied it for the photodegradation of methylene bleue, Eriochrome Black T, erythrosine and malachite green. Although the [Cu(phen)2(OAc)], was used for the removal of methylene blue, Eriochrome Black T, erythrosine and malachite green from aqueous solution, it can also be applied for the attenuation of inorganic contaminants in AMD. Moreover, besides its importance in environmental remediation, nanotechnology can be applied to improve human life by the diagnosis and treatment of neurodegenerative diseases [39], energy storage [40, 41], in the food industry to enhance food security by using nanosensors to detect pollutants in food [42, 43].
Although these technologies (prevention techniques) allow limiting the formation of AMD or significantly reducing pollutants in AMD (treatment technologies), they are deemed not suitable for lasting solutions since most prevention technologies have a very limited lifespan, are not always cost-effective, and sometimes lead to secondary environmental pollution, while the existing treatment technologies have variable efficacies in inorganic contaminants attenuation and also generate sludge and/or brines with secondary pollution if not properly managed [44,45]. Over and above that, the prevention and treatment technologies suffer from subpar performances, failures, and sustainability while managing AMD. It is therefore important to assess and evaluate the strengths, weaknesses, and economic and environmental sustainability of the existing AMD prevention and treatment technologies. As far as the authors of this manuscript are aware, the pros, cons, economic and environmental sustainability of AMD prevention and treatment technologies have never been evaluated. Despite the possibility of recovery valuable minerals and clean water from AMD using hybrid and integrated technologies, their sustainability remains a matter of great concern and has not been fully explored. Herein, the state-of-the-art in AMD prevention and treatment technologies is critically reviewed and discussed with a focus on their economic and environmental sustainability using life cycle assessment methodology (LCAM) thereby addressing the gaps of existing literature and contributing to the advancement of knowledge in AMD prevention and treatment technologies. This paper contains 11 sections, of which section 1 introduces the reader to the topic while section 2 discusses the formation process of AMD, section 3 highlights the environmental, human health, and socioeconomic effects of AMD, section 4 summarises AMD prevention techniques, section 5 describes different AMD treatment technologies, and section 6 highlights the importance of LCAM in AMD management and the LCA approach used to obtain different results. Section 7 presents and discusses different LCIA results obtained using LCAM methodology. Section 8 presents and discusses pros and cons of different treatment technologies. Section 9 highlights the beneficiation and valorisation of AMD while section 10 presents and discusses different factors hindering or obstructing the treatment of AMD, section 11 concludes and provides recommendations for future research avenues.
The Formation Process of Acid Mine Drainage
Acid mine drainage is acidic and inorganic contaminants-rich wastewater, which flows from active and abandoned mines. Acid mine drainage is the product of oxidation reactions of sulphide-bearing materials (e.g FeS2, CuS, ZnS and PbS) with atmospheric oxygen and water in the absence of neutralising materials as illustrated in Fig. 1.
Sulphide bearing materials are commonly found in coal ore [46], copper [6], gold [47] and other mineral deposits. Using pyrite (FeS2) as an example, the formation process is initiated by FeS2, which reacts with oxygen and water, leading to the formation of hydrogen ion (H+), ferrous ions (Fe2+), sulphate ions (SO42−) and soluble cations as illustrated in (Eq. (1)) [18, 19].
The FeS2 oxidation process occurs when enough oxygen is dissolved in the water and stimulates the reaction of soluble ferrous ions with oxygen to form ferric ions (Fe3+) as illustrated in [Eq. (2)].
During the formation process of AMD, FeS2 which is at the forefront, can get in contact with Fe3+ ions and the association leads to the dissolution of FeS2, formation of Fe2+ and SO42− thus leading to more acidity as illustrated in [Eq. (3)].
For the time being, if the pH of AMD is < 4 (pH < 4), Fe3+ are precipitated to form hydrated iron hydroxide (Fe(OH)3) as illustrated in [Eq. (4)] [48].
The above-equations clearly demonstrate the complexity of the AMD formation process and it follows that the process is completed through a series of oxidation reactions that release H+, SO42−, Fe3+ and Fe2+ thereby contributing to lowering the pH of the final aqueous solution to less than or equal to 4 (pH ≤ 4). These reactions occur simultaneously and are mediated by microorganisms and the geomorphological composition of the mining site, which all together influence the physicochemical composition of AMD [50, 51].
Various factors influence the formation process of AMD, and they include water, oxygen, microbiological activities, the mineralogy of the mine site, as well as the environmental conditions of the ore deposit. Minerals and rocks are suitable habitats for microorganisms, and their presence affects the dissolution, solubility, and speciation of the host rock thereby influencing the formation of AMD [52]. For instance, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans colonize sulphide-bearing minerals, mining tailing, and abandoned mines [53], thus, they stimulate or mediate the oxidation reaction of FeS2, which leads to the production of SO42− and H+ as illustrated in [Eq. (5)].
Overall, bacteria play a huge role in the AMD formation process. They stimulate the oxidation of Fe2+ to Fe3+. For instance, A. ferrooxidans, Leptospirillum spp, Acidimicrobiaceae spp, Metallibacterium and Ferroplasma, participate in the reduction of Fe3+ to Fe2+ while Acidiphilium, Acidisphaera, Acidimicrobiaceae spp, Metallibacterium, reduce sulpahte to sulfur (Desulfosporosinus) while sulfur oxidizing bacteria (A. ferrooxidans and A. thiooxidans) contribute to the oxidation of sulfur to sulphate.
Once generated, AMD water modify the physicochemical characteristics of receiving environment and those modifications constitute a starting point of myriads of severe environmental impacts associated with mine water pollution as shown in Fig. 2.
Environmental and Socioeconomic Effects of Acid Mine Drainage
Once generated, mine water has significant negative effects on different environmental compartments with ramifications to public health, economic, and social levels [55, 56]. Due to its physicochemical quality, mine water is toxic and once discharged untreated, it is responsible for devastating consequences on the receiving environment. Mine water causes soil corrosion, and as a direct consequence, the affected soil becomes unfit for a multitude of different uses [57, 58]. In addition, mine water-polluted soil can be transported by torrent to the nearest aquatic environment, leaving it devoid of most living creatures since it lowers the pH leading to the increase of metals and SO42− concentration to the level toxic to most aquatic life forms [4, 59]. The lowering of receiving aquatic environment pH, causes the migration of non-tolerant species, and only species capable of adapt themselves to new environmental conditions survive [60]. This situation creates an unbalance in the aquatic environment, and as a consequence, the symbiosis of the whole ecosystem is disrupted and can lead to the death of some species [61]. Several ecotoxicological and bioassay studies using lethal concentration (e.g, LC10, LC20, LC50) and lethal dosage (e.g LD10, LD20 and LD50) have revealed that mine water and specifically AMD is very toxic to all living forms, and this was confirmed through the exposure of various species, including plants and animals to raw AMD [62, 63]. The toxicological effects of mine water on living species can disrupt the metabolic functions of plants and animals [64], cause skin irritation, kidney damage, and neurological diseases in humans and increase the risk of skin cancer [7, 61, 65]. Metalloids, anions, and radioactive substances in mine drainage can find themselves in the food chain through the bioaccumulation in living organisms, leading to severe health effects such as skin redness, hair loss and acute radiation syndrome, amongst others, in humans and animals [66].
The issue of mine water on environment is very significant, specifically on freshwater resources which are an essential natural resources for the existence of life on planet earth since they are used for both domestic and economic activities. As a result of its environmental effects, mine water is drawing environmental activists and media attention worldwide [28, 67]. However, they seem to limit their focus on environmental effects and specifically effects on water resource likely because these effects are perceptible and the impacts on human health seem to be neglected probably because they are less visible but have long-term effects [67]. The effects of mine water are not only limited to the environment and human health, but extend to the economic level. Regions polluted by mine water face multiple economic challenges including the decline in fish of economic value, decline of touristic activities due to soil corrosion, natural habitat loss and pressure on endangered species [56, 68]. The treatment and management of mine water is not cost-effective and in cases where mining companies go defunct, it becomes the responsibility of the government to sustain the management and treatment cost, and this will likely have a negative effect on the GDP of a given country. Such a situation in the long-term may cause the recession of the economy, leading to a higher unemployment rate. Overall, AMD is hazardous to all environmental compartments with effects on physical, chemical, biological, ecological, and socio-economic aspects as shown in Fig. 3.
To prevent environmental, socioeconomic and human health effects resulting from mine water, it is advisable to prevent the formation of such mining by-products or to effectively treat the already generated AMD.
Acid Mine Drainage Prevention Techniques
Acid mine drainage prevention techniques refer to a set of methods applied to suppress AMD formation. In this regard, several technologies have been developed to prevent AMD generation from mine tailings, active and disused mines. Their mode of operation focuses on preventing the contact of tailings materials and mining areas with oxygen and/or water [46]. Prevention of AMD formation is an indirect approach to limit its effects on both human health and the environment since prevention inhibits the formation at source, and it has been done using various techniques as described below.
4.1. Mining Tailings Characterisation
The characterisation and quantification of waste materials is the initial step in AMD prevention, and it allows to have an insight about the quantity of sulphide materials present in mine tailings, while the geochemical characterisation of alkaline material identifies the threshold of alkaline material in respect to metals mobilization. Understanding the acid generation potential of mine tailings dictates, the design and waste storage methods to be applied. However, the material characterisation should precede the starting up of the mine, thus limiting its applicability since mine tailings or waste materials left over after the separation of valuable fractions from the uneconomic fraction of ore will mix with other pollutants present in the mining site [12].
4.2. Alkaline Amendment
Alkaline amendment technology is widely applied to prevent AMD formation. This technology consists of the addition of alkaline materials to different mine tailings that initiate AMD formation [41]. Alkaline materials added neutralise the acidic leachates and decrease dissolved metal concentrations. Commonly used alkaline materials include: brucite, hydrated lime, limestone, dolomite, soda ash [69], fly ash [70], magnesite [47] and eggshell [14] amongst others. Alkaline amendment is a very simple technique in its application, and it has proved to be efficient for a limited duration since its efficiency depends on the ability of alkaline material to continuously neutralise acidic mine tailings. As such, this technique has proved to be inefficient for a long-term solution of AMD prevention due to the limited potential of alkaline material. However, the findings of several studies have revealed that static test for acid generation potential (AGP) and acid neutralisation potential (ANP) can be applied to overcome the neutralising potential of alkaline materials [8,71].
4.3. Water Flow Diversion
Water flow diversion (WFD) method consists of interception and diversion of the flow direction of water away from the mine tailings in order to prevent the contact of water with sulphide materials. To achieve this, transverse materials are built using a combination of highly compacted clay and salvage to divert the water flow [72]. Low-permeability synthetic membrane liner or geomembranes such as polyvinyl chloride (PVC), and high density polyethylene (HDPE) are also widely used for water diversion [15]. This technique allows to separate water from materials containing heavy metals, thereby controlling erosion and sedimentation. However, it is not always cost-effective, and the cost varies according to the volume and flow rate of the water to be diverted. Furthermore, water diversion means creating a new waterway and this renders the water flow diversion more complex since it may lead to a new ecological and human health problem to be solved, thus increasing the cost and limits its application.
4.4. Oxygen Barrier
Oxygen barrier is one of the most used techniques to prevent AMD formation. It suppresses oxygen (O2) diffusion into sulphide-bearing materials by building a sort of wall made of non-reactive materials including fine mine residue and natural till [73], low sulphide tailings and natural soil [74], organic materials [75] to prevent the contact of O2 and mine tailings thereby preventing AMD formation. In addition, to prevent O2 diffusion, this technique also ensures water runoff upstream of mine waste since both O2 and H2O are primordial for the sulphide-bearing materials oxidation and AMD formation. Win et al. [76] assessed the efficacy of potential acidity producing (PAP) materials and non-acid producing (NAP) materials (fly ash) as oxygen barrier to prevent AMD formation in comparative study. The results revealed that PAP column led to the production of acidic leachate (pH < 3) while NAP column produced neutral mine drainage (pH 8.5–10.9). The efficiency of NAP to limit AMD formation was attributed to the decrease of O2 diffusion to mine waste. Applying the same technology but with different materials Neculita et al.[77] found that in the column with low sulphide tailings, an acidic leachate (pH ≤ 4) with elevated concentration of metals was produced while the column with capillary barrier effect (CBE) material produced mine drainage with neutral pH (8.5) and very low concentration of chemical species.
Other dry covers made of bottom ash, red mud, paper mill waste, pulp/paper residue and organic waste have also been reported to limit AMD formation [78]. For instance, the study by Demers et al. [74] revealed that the sludge materials recovered after treatment of AMD by neutralisation technique were used to make dry cover for prevention of AMD formation. Organic carbon-rich material has also been proved to be efficient material to make dry cover [16]. In fact, during the decomposition of carbon rich material, a huge quantity of O2 is consumed, as shown in the [Eq. (6)].
The consumption of huge amounts of O2 leads to less dissolved oxygen available to oxidize sulphide-bearing material, thus, limiting the AMD formation.
4.5. Bactericide
Bacteria play a vital role in AMD formation since they accelerate the oxidation of pyrite material. As such, the elimination of bacteria in sulphide-bearing materials will slow down the chemical reaction leading to AMD formation [79]. Taking into consideration this concept, various bactericides, including organic acids, food preservatives, and cleaning detergents amongst others, have been used to suppress the growth of Acidophilus bacteria, thus, limiting the formation of AMD [80]. According to Yang et al. [81], acidophilus bacteria thrive under acidic environments and mostly at pH < 3, however, they require neutral intracellular pH to survive. In order to achieve that goal, they use the cytoplasmic membrane to limit proton entry into the cell. The application of bactericide, such as anionic surfactant, opens a route to proton to enter freely into the bacteria cell membrane and disrupt the enzymatic functions [82,83]. The study by Singh and Bhatnagar [84] revealed that sodium lauryl sulphide (SLS) used as a bactericide to prevent AMD formation was able to inhibit FeS2 oxidation by Thiobacillus ferroxidans achieving more than 70% inhibition efficiency compared to mining waste without the application of SLS where this later stimulates the oxidation of Fe2+ to Fe3+ in a very short period. Despite being very promising in limiting AMD formation, bactericides cannot be considered as a sustainable solution to control AMD formation since they are soluble in water and easily being flushed out of the mine waste and as a consequences need to be replaced constantly. Moreover, the bactericides flushed out may end up in the open environment, thus leading to soil degradation, damage in gills, and alterations in mucous cells in fish [17,85].
4.6. Reclamation of Contaminated Land
This method is mostly applied to inhibit or stop the continuous formation of AMD in contaminated land. It consists of the use of uncontaminated topsoil to cover the surface of land polluted by AMD followed by the plantation of metals-accumulating plant species such as Vetiveria zizanioides [86]. This method allows modification of the slope of land surface, preserves the soil, and limit the infiltration of surface water, thereby preventing the contact of water and oxygen with contaminated material. This technique improves soil fertility and increases the productivity rate of the land. In addition, it contributes to maintaining good human health conditions in AMD polluted regions [19, 87]. However, land reclamation presents some drawbacks, including flooding, soil liquefaction, and change of the quality of the surrounding environment [88]. In addition, land reclamation is not always cost-effective and reclaimed soil is weak and may take many years to be stable and, as a consequence, not suitable for tall buildings and other heavy civil engineering activities.
4.7. Co-disposal and Blending
The co-disposal of mine tailings or mining waste with alkaline producing material or acid consuming material has appeared to be amongst the most promising technologies to control the AMD formation. Alkaline-producing materials including lime, limestone, fly ash, and mesa lime amongst others are co-disposed with mine tailings in order to consume acid and prevent AMD formation. The acid-consuming material prevent AMD formation by consuming H+ generated during the oxidation of sulphide-bearing material. The consumption of H+ has dual benefits since it leads to the precipitation of metals and metalloids and the formation of neutral mine drainage [82,89]. Using CaCO3 as an example, the process of acid-consuming material is illustrated in [Eq. (7)].
During the co-disposal of mine tailings with acid-consuming materials, the oxidation of sulphide-bearing material is controlled by the precipitation of Fe3+ and other metals, which increase the pH to neutral (pH 6.5–8.5), thus disrupting the activity of bacteria, which usually promote the (Fes2) oxidation. In addition, the co-disposal or blending also promotes the formation of thin layer on the surface of sulphide-bearing materials. The thin layer prevents the contact of reactive surface with O2 and water, thereby, limiting the formation of AMD [90]. Despite its efficiency in AMD prevention, co-disposal technology requires the accurate stoichiometric balance between acid producing materials and acid-consuming materials. If not, the co-disposal will lead to the formation of AMD if there is an excess of acid-forming materials or in contrary strongly alkaline leachate will be formed in the presence of more acid-consuming materials [91]. The blending process of mine tailings with acid consuming materials is schematically illustrated in Fig. 4.
4.8. Desulfurization
Desulfurization is a preventive technique to manage sulfide wastes/tailings. This process separates sulphide minerals from tailings/ wastes into a low-volume stream, thereby leaving non-acid-generating waste with low sulfur content. The main advantage of desulfurization is its operation at ambient temperature and pressure with high selectivity, low gas emission and less generation of side products [92]. However, desulfurization leads to the severe corrosion of the equipment, thus reducing the lifespan of the whole system. In addition, the flue gas needs to be reheated after washing, thus incurring additional energy inputs, which results in high operational cost [92].
4.9. Surface Passivation
The minerals production site last for years leading to the continuous release of mine waste/tailings, which can remain AMD generation sources for decades or even centuries, thus, limiting the efficiency of water cover technique. To remediate the limitation of water cover, surface passivation emerged as an alternative. Surface passivation consists of using a passivate, which reacts with sulphide-bearing materials, resulting in the formation of hydrophobic film on the surface of sulphide-bearing materials, thus preventing their contact with oxygen [93]. According to the nature of passivation agents, there are inorganic passivation and organic passivation.
4.9.1. Inorganic passivation
It consists of the use of inorganic materials including carbonate, phosphate, and silicate amongst others. The inorganic passivates react with sulphide-bearing materials to form a hydrophobic film. For instance, the reaction of carbonate (CaCO3) with suphide materials leads to the formation of hydroxyl iron oxide (FeHO2), which coats the surface of sulphide-bearing materials. In addition to the formation of FeHO2 thin layer on the surface of sulphide-bearing materials, a significant quantity of arsenic can be absorbed if siderite is added to the solution at acidic pH (pH < 4) [91]. Despite the ability of carbonate passivates to limit AMD formation via FeHO2 thin layer, it is very slow and unstable. However, the study by Ji et al. [21] revealed that the use of phosphate to coat the surface of sulphide-bearing materials inhibits the oxidation of FeS2, thus, preventing AMD formation.
4.9.2. Organic passivation
Beside inorganic materials used to prevent AMD formation by passivation methods, organic materials have also been used as passivation materials for the coating of sulphide-bearing materials. Organic materials that prevent AMD formation include natural organic materials including humic acid (HA), oxalic acid (OA) and lignin [94,95] and silane materials such as tetramethoxysilane, methyltrimethoxysilane, tetraethoxysilane, g-amino-propyltrimethoxysilane and n-propyltrimethoxysilane [96–99]. Amongst them, HA is the most used due to its human health safety and eco-friendlier status since it is non-toxic [94]. The utilisation of HA as passivation materials leads to the generation of new compounds such as iron hydroxide [Fe(OH)2] and FeHO2 with high affinity and hydrophobicity during the oxidation process, thus inhibiting the AMD formation. The shortcomings associated with the use of natural organic materials are their decomposition and destruction by bacteria [100].
Despite the fact that AMD prevention techniques are very promising, they are not a lasting solution since they are only effective for a limited duration. Furthermore, they are not cost-effective and should be monitored regularly, preferably by skilled and experienced personnel, due to their fragility and complexity. This complexity in operation, together with their limited lifespan and regular, monitoring program hinders their application and sustainability. Considering the large numbers of active and disused mines globally, for example ≈ 6000 in South Africa, ≈ 50 000 in Australia, ≈ 500 000 in USA [54,58] and the huge volume of AMD generated throughout the year, e.g 400 000 ML produced only in the Witwatersrand basin in South Africa [10,11]. The successful prevention of AMD formation has multiple advantages compared to the treatment; however, their limited lifespan and secondary environmental effects are the major drawbacks hampering their application for a lasting solution. The limitations and inconveniences associated with AMD prevention and abatement technologies are a real sign that vigorous AMD treatment technologies are required to tackle the issues of AMD and its associated environmental and public health effects.
Acid Mine Drainage Treatment Technologies
Following the difficulties and challenges to prevent AMD formation, it is evident that active and abandoned mines will always be the sources of AMD. In order to protect public health and reduce its eco-toxicological effects, various treatment technologies have been developed and applied for AMD treatment and they include: active treatment or chemical treatment, passive treatment or self-renewable system, hybrid treatment or combination method, integrated or multistage treatment and emerging treatment technologies
5.1. Passive Treatment of Acid Mine Drainage
Passive treatment technologies are designed and implemented as an alternative solution to active treatment. They are self-renewable systems, do not require continuous monitoring as their active counterparts and they use spontaneous biochemical processes and microorganisms activities to remove pollutants in a controlled environment [8]. Passive treatment methods include constructed wetland systems (CWs), limestone leach beds (LSB), permeable reactive barriers (PRB), anoxic limestone drains (ALD) and sulphate-reducing bioreactors (SRB) amongst others.
5.1.1. Constructed wetlands
Constructed wetlands (CWs) are bioengineering system, which combine natural geochemical and biochemical processes and bacterial activities to remove pollutants from wastewater. Constructed wetlands are generally divided into aerobic wetlands and anaerobic wetlands. Aerobic wetlands are shallow ponds that collect wastewater and provide sufficient retention time for pollutants to settle down through the sedimentation process, allowing metals oxidation and hydrolysis [101, 102]. Contrary to aerobic wetlands, anaerobic wetlands are relatively deeper ponds with organic matter, which usually covers a layer of limestone gravel where wastewater percolates through the organic matter to the limestone bed and flows horizontally under high anoxic conditions [103]. Constructed wetlands have been used to treat municipal wastewater [104,105], domestic wastewater [106], agricultural wastewater [83] and AMD [24–26]. In anaerobic wetland treating AMD, the anoxic condition (without dissolved oxygen) resulting from high biochemical oxygen demand (BOD) promotes the development of SRB, which reduces sulphate to sulphide, thereby, producing alkalinity leading to metals precipitation. Furthermore, the alkalinity is produced at two levels: (i) the anaerobic chemical reaction occurring under anoxic conditions consumes H+ and reduces the acidity of the AMD; (ii) the decomposition of organic matter by microorganisms present in wetland substrate consumes dissolved oxygen (DO) and leads to the production of hydrogen sulphide (H2S) and pH increase [73]. In CWs technology, substrate, macrophytes, and external factors all contribute to wastewater quality improvement. Substrate serves as a medium for plant growth, provides energy sources for all biochemical reactions occurring in the wetland systems and ensures the sedimentation of heavy metals, hence facilitating their accumulation by macrophytes [24–26]. Plants need metals at acceptable concentrations for their myriads of metabolic functions. As such, plants accumulate heavy metals from the sediment using their roots, advocate the settling down of total suspended solid (TSS) and, lay out the finest conditions for the development of microorganisms [108]. Contrary to substrates and plants, which are directly involved in wastewater improvement in CWs systems, external factors, including evaporation, adsorption, biological assimilation, chemical transformation, and volatilisation also contribute indirectly to the improvement of wastewater in CWs systems [109].
Despite appearing as sustainable technology in AMD water treatment and management, CWs are unable to reduce pollutants to acceptable standards as set by regulatory bodies when operating in a stand-alone system with removal efficiency (RE) generally less than 50% in alkaline conditions [110]. The selection of plant species is another drawback associated with CWs since the selection of plant species to be used as wetland macrophytes should be done taking into account the ability of plant species to tolerate harsh, toxic and acidic wastewater such as AMD. Moreover, the high concentration of inorganic contaminants in AMD water can become toxic to plant and reduce or annihilate the ability of plants to accumulate non-toxic pollutants [111]. However, one of the main advantages of CWs is their self-renewable ability since they use natural energy to operate. They require less maintenance and do not need the service of skilled personnel to monitor their functioning and, as such, can be installed in remote areas [26].
5.1.2. Sulphate reducing bacteria
Sulphate reducing bacteria are mainly organic mixture rich in bacteria including Desulfovibrio, Desulfomicrobium, Desulfobacter and Desulfotomaculum, which reduce SO42− to H2S in the presence of suitable carbon source such as glucose and sucrose amongst others [112]. This passive method reduces SO42− ions concentration leading to the lowering of metal concentration and an increase in alkalinity. However, SRB is mostly applicable to abandoned mines generating AMD and its efficiency depends on the initial concentration of SO42−. The study by Bai et al. [113] revealed that the use of SRB for the treatment of real AMD was able to remove 61% of SO42− and significant reduction of metals ions (Fe2+, Cu2+ and Mn2+). Sulphate reducing bacteria method is mostly applied on a small scale for AMD treatment. However, some few applications of SRB for AMD treatment at the industrial level are presented below.
5.1.2.1. CloSURE and BioSURE
CloSURE is an SRB AMD treatment developed by Mintek, South Africa. It is an integrated SRB system consisting of biological sulphate reduction (stage 1) and an oxidation step for H2S removal and biosulfur. In this integrated SRB system, H2S formed following the reduction of SO42− interact with free metals to form metal sulphides, thus facilitating their removal from the AMD while H2S is biologically removed from the system [114]. Like other passive AMD treatment methods, CloSURE is suitable for the treatment of AMD generated by abandoned mines since it uses anaerobic microorganisms from the natural environment. The main advantage of CloSURE is its efficacy to remove up to 85% of SO42− and significant attenuation of metals (Al, Fe, Ni and Zn) [115]. CloSURE is cost-effective and environmentally friendly technology since it requires low capital and operational costs, and generates less sludge, thus reducing secondary environmental footprints. However, the presence of H2S may lead to the corrosion of the equipment and associated human health effects. In fact, corrosion is a source of various health effects including skin irritation, respiratory problem amongst others [116–118]. In addition to CloSURE, BioSURE was also developed in South Africa by Rhodes University (RU) to biologically remove SO42− from AMD. The principle of AMD treatment using BioSURE is the mixture of SO42− rich AMD with sewage sludge, which acts as a carbon source and generator of microorganisms that reduce SO42− to H2S allowing the attenuation of close to 90% of SO42− while treating more than 10 ML (> 10 ML) of AMD water per day.
5.1.3. Integrated managed passive method
The integrated managed passive method (IMPM) is an integrated system combining four different stages for AMD treatment, of which stage 1 consists of layers made of organic materials providing microorganisms and carbon sources, which conditions the received AMD by removing dissolved oxygen (DO) to establish the desired redox condition while SO42− is reduced to H2S and attenuation of significant concentration of metals. The leachate produced in stage 1 flows into the H2S oxidizing bioreactor, where H2S is removed from the system (stage 2). The third step (stage 3) is a secondary SO42 reducing reactor where residual sulphate is continuously reduced to H2S using a unique carbon source, leading to the production of leachate with SO42− and metals concentration conforming to required standards for environmental discharge [119] while the leachate highly rich in H2S passes into the secondary H2S oxidising bioreactor where the residual H2S is oxidised to sulphur (S) and removed from the system [119, 120]. The IMPM is very promising in AMD treatment since it allows treating around 1 ML of AMD per day and achieving between 87 and 97% in pollutants attenuation. It is a self-renewable system using renewable energy from microbial activities and photosynthesis, amongst others [119, 120]. In addition, the IMPM is eco-friendlier AMD treatment technology with less sludge release thereby making it easy to manage environmental risks and impacts associated with it. However, the system is very fragile since the failure of one stage leads to the failure of the whole system. Furthermore, there is a high risk of cross-contamination.
5.1.4. Permeable reactive barrier
Permeable reactive barrier (PRB) is a layer made of alkaline materials placed perpendicularly to the flow direction of the water. The PRB process consists of filtering the water using a barrier where pollutants are retained while the liquid passes through, as shown in Fig. 5 [27]. Different materials, including fly ash, steel slag, and clay, amongst others, are commonly used as PRB materials [121]. The PRB method is relatively new, and from literature, it has proved to remove close to 85% of pollutants in AMD [27, 122]. The efficiency of PRB depends on many factors, including the nature of the materials used to make a barrier, the retention time and the flow rate [27]. As such, the PRB method is more suitable to treat AMD from abandoned mines since the flow rate and retention time, can be adjusted to meet the designer requirements [123]. In addition, materials used are easily saturated and need to be replaced frequently. The management of used materials is another shortcoming associated with this method since they are rich in toxic inorganic contaminants despite their relatively low capital and operation cost [71]. Furthermore, clogging, complexity, and armouring also hamper their application at an industrial level.
5.1.5. Other passive acid mine drainage treatment technologies
Other passive systems including slag leach beds (SLB), anoxic limestone drains (ALD), limestone diversion well (LDW), pyrolusite limestone beds (PLB), reducing alkalinity-producing systems (RAPS) have also been applied for AMD treatment. They mostly use limestone to neutralise AMD and precipitate metals. In ALD methods, layers of coarse clustered limestone are placed under a drain, which serves as a watercourse where AMD drains pass through and treated in an anaerobic medium [8]. However, the efficiency of this passive treatment is conditioned by the absence of O2 and Al in the channel since their presence may lead to the formation of Fe and Al hydroxides, which clog the system, leading to its failure. The treatment of AMD using passive methods is very promising; however, their full potential should be investigated further since they present some limitations as illustrated in Table 1.
5.2. Hybrid Treatment
A hybrid technology by definition is a combination of two or more different methods to form a mega system in order to exploit the advantages of each stand-alone system and achieve better efficiency [67]. In water engineering, hybrid technology is an association of two or more distinct and different approaches operating simultaneously for water treatment. In water and wastewater treatment, a hybrid approach is more efficient than stand-alone system due to the combination of advantages of each system [24]. In the quest to find lasting solutions for AMD treatment and management, active and passive systems have been associated or combined to treat AMD. It consists of neutralisation using alkaline chemicals (active) and polishing of product water using passive systems [67] or active and SRB [134]. This hybrid system has proven to be very promising in AMD management; however, the main drawbacks are the generation of polluted toxic sludge in neutralisation stage and the large land area required to implement it. Furthermore, for the hybrid system to be efficient, it should be constantly monitored, and routine maintenance should be done regularly if necessary. The flow rate of AMD must be significantly reduced to meet the hydrology ability of the passive system. Although it has proven to be promising, a lot of research should be conducted to investigate its full potential and upscaling it from pilot to industrial application [135, 136]. A schematic illustration of an AMD hybrid treatment system is presented in Fig. 6.
5.3. Integrated Technology or Multistage Treatment
Integrated technology in AMD water treatments is a sequential or fractional removal of pollutants at different stage of the treatment process. It integrates a series of active treatments or a combination of active and passive methods. In the case of a series of active treatments, metals are selectively precipitated with the increase of pH in a stepwise modular allowing the recovery of valuable minerals at different pH levels and reclamation of other natural resources, such as drinking water standards [48], or the merely neutralization of the AMD and precipitation of metals using alkaline material. Masindi [137] evaluated the integration of cryptocrystalline magnesite (MgCO3) and barium chloride (BaCl2) for AMD treatment. Cryptocrystalline magnesite was used to neutralise AMD and precipitate metals; the product water was then polished by BaCl2 to remove residual SO42−. In this specific case, 99% of metals and 99% of SO42− were removed. The integration of two or more active methods mostly involves the use of different chemical at different pH [138]. Active and passive approaches have also been integrated in synergistic mode where contaminants are removed gradually. For instance, Champagne et al. [139] assessed the integration of neutralization (active) and a series of passive system (bio filtration using bacteria to break down pollutants, SRB to reduce SO42− to H2S and anoxic limestone drains (ALD) to polish the water. For this integrated system, a specific function was assigned to each stage of the system and the removal efficiency (RE) was 99.9%, for each of the following metals: Al, Cu, Zn and 99.7%, 98.6%, 98.2% and 66.5% for Fe, Cu, Mn and Cd, respectively. Despite the high RE of the integrated system in pollutants attenuation, their application at large scale is complex due to high risks of system failure. The integration of two passive methods have also been used for the treatment of AMD. For instance, Zipper et al. [121] integrated SRB and an aerobic wetland for the treatment of AMD. In SRB cell, the acidity of the AMD was lowering following by the reduction of SO42− to H2S and the removal of Al, Cu, Zn and selenium (Se) and partial removal of Mn. The aerobic wetland was applied to polish the water by complete removal of Mn and Fe. This integrated system allowed to significantly improve the quality of AMD; however, it requires a large land area (≈ 5 hectares) which is sometimes very challenging and specifically in regions or countries with small land area. In addition, the loading rate of metals in the SRB step should be very low and should not exceed 0.3 moles of metals loading per day and per cubic meter in organic media [124]. A typical illustration of an integrated or multistage system treatment is presented in Fig. 7. Following the drawbacks associated with the aforementioned treatment methods, more researches is now directed to emerging treatment technologies.
5.4. Emerging Acid Mine Drainage Treatment Technologies
5.4.1. Emerging passive treatment technology: phytoremediation
Phytoremediation is the process of using selected plant species to extract and remove pollutants from contaminated soil or polluted water [140]. Both aquatic and terrestrial plants are used in the phytoremediation process, which involves various mechanisms: (i) phyto-extraction or phyto-accumulation which is the use of plants to remove pollutants from soil, sediments or water into harvestable biomass; (ii) phyto-stabilisation which is the process of using selected plants to reduce the mobility of pollutants in soil or water, and; (iii) rhizo-filtration or phyto-filtration, which involves absorption or precipitation of pollutants onto plants roots; (iv) phyto-stimulation which is the breaking down of pollutants by proteins and enzymes produced by plant species; (v) phyto-degradation or phyto transformation which is the transformation of organic pollutants from soil or water into less hazardous form; (vi) myco-remediation is a type of phytoremediation in which fungi and other lower plants such as algae are used to clean up a polluted site or water [141]. A phytoremediation is summarised as shown in Fig. S1 (see supplementary materials).
Phytoremediation has been widely applied to treat polluted soil and water. Various studies have revealed that plant species such as Vetiveria zizanoides [26, 32] Calamagrostis ligulata [142], Juncusim bricatus [142], Eichhornia crassipes [143], Pistia stratiotes [144], Typha latifolia [145], Phragmites australis [146] tolerated acidic mine water, however with a limited duration. Putri and Moersidik [147] investigated the potential of Typha sp and some algae (Pediastrum eudorina, Volvox melosira and Scenedesmus) growing in a soil polluted by AMD. The findings revealed that the above-mentioned plant species were accumulating heavy metals and stored them mostly in their roots system [148]. This demonstrates the auto-renewable abilities of this passive remediation system since the remediation process occurred naturally. Similarly, Bobadilla et al. [142] assessed the potential of Calamagrostis ligulata and Juncusim imbricatus growing in a natural aerobic wetland polluted by AMD. The findings showed that Calamagrostis ligulata and Juncus imbricatus were naturally treating the AMD-polluted wetland using the phytoremediation process. In phytoremediation technique, the bioremediation process is the main mechanism of pollutants removal and it operates through different mechanisms: (i) phytoremediation which uses plant species to remove pollutants and clean up contaminated soil, air and water [149]. Pollutants are absorbed and bio accumulated by plant species (phyto-extraction) and release them at very low concentration into the atmosphere (phytovolatisation); (ii) bio stimulation which modifies the environment and stimulate existing bacteria capable to bioremediate the contaminated water [149]; (iii) bio augmentation which is the introduction of bacterial culture to improve the degradation of the pollutants [150]; (iv) mycoremediation is the use of fungi-based remediation methods to decontaminate polluted sites [142].
5.4.2. Membrane technology
Recent findings have demonstrated that membrane technology can be a good alternative to conventional technologies currently applied for AMD treatment. Membrane technology widely uses different types of membranes, according to their pore sizes: Reverse osmosis, nanofiltration and ultrafiltration, which uses membranes with pore sizes between ≈ 0.1–1 nm, ≈ 1 – 10 nm and ≈ 10 – 100 nm, respectively.
5.4.2.1. Nanofiltration and ultrafiltration
Nanofiltration use different types of membranes namely NF90, NF99, NF270 and NF2540 to remove pollutants (metals) from AMD. Nanofiltration membranes are efficient to separate monovalent ions such K+, and Na+ from multivalent ions such as Al3+ [151], thus, making it a suitable membrane for a selective removal of pollutants from AMD. In addition, nanofiltration is cost-effective, requires low energy input and can be adapted to the chemical and physical instability of AMD.
Mintek, South Africa designed and applied a semi-permeable membrane (NICmembrane) made of bulk operationalized polyethersulfone (PES) powder for the treatment of AMD and obtained a product water that was used in mining processing plant. In addition to solutes, particulate matter and microorganisms were also significantly removed by the NICmembrane, which proved to consume less energy, affordable, and low fouling, thus, making it appropriate for the treatment of other types of wastewater such as municipal wastewater [152, 153]. The study by Fonseka et al. [154] in Pennsylvania, USA revealed that nanofiltration with NF90 membrane applied to treat AMD allowed to remove 95% of solute under the following conditions: 10 bar pressure and a flow rate of 3.5 gallons per minute. Moreover, Al-Zoubi et al. [155] applied a nanofiltration membrane (DK-4040F) fabricated from polyamide composite membrane for the polishing of pre-treated AMD from coal mine. The results revealed a significant removal of metals with 90% RE and 48% reduction in electrical conductivity (EC). Like nanofiltration membrane, ultrafiltration membranes are also cost effective, simple to operate, requires less energy input and eco-friendly [156]. Ultrafiltration is piloted by pressure difference and simultaneously purification, condensation and fractionation of macromolecules [157]. Ultrafiltration membranes are efficient to remove total dissolved solid (TDS), colloids and microorganisms with production of leachate of high quality [158]. However, they are also subject to fouling, thus, limiting their application at large scale or industrial level.
5.4.2.2. Reverse osmosis
In addition to nanofiltration and ultrafiltration membranes, reverse osmosis (RO) is another membrane technology driven by osmotic pressure, which push a solution to pass through a membrane where inorganics contaminant or solutes are retained [159]. There are some very patented RO technologies which have been applied to treat AMD.
Forward osmosis-reverse osmosis osmotic dilution process is a two membranes system consisting of seawater reverse osmosis (SWRO) and forward osmosis-reverse osmosis (FO-RO) designed and built by Gyu et al. [157], in Samcheok, Korea for the treatment of AMD from coal mine. The findings showed significant removal of pollutants from AMD. In addition, the combination of two different types of membranes allowed to reduce the RO membrane fouling due to the presence of FO as well as reduction of energy consumption to less than 15% compared to conventional wastewater treatment using desalination technique. Despite the production of relative good quality of leachate and the consumption of less energy, there are some shortcomings associated with this technology, which include the release of considerable volume of brine, thus, incurring with associated environmental footprints and additional cost.
In order to solve the problem of elevated concentration of calcium in product water treated using CaCO3, a technology named slurry precipitation and recycle reverse osmosis (SPARRO) was designed and applied for AMD treatment by Gjg Juby et al. [158] with the ultimate goal to get rid of drawbacks such as high energy input, sludge release, and high calcium SO42− recirculation rate associated with conventional treatment methods [160, 161]. Although calcium concentration was significantly reduced in leachate, high functioning cost and the huge quantity of chemical used will incur additional cost. Furthermore, the use of chemical will lead to significant environmental footprints due to the release of sludge. However, the application of RO for AMD treatment is hampered by some issues such as fouling, production of large volume of water with salt (brine), and high energy input amongst others [162].
Despite its advantages in AMD treatment, there are various drawbacks, which include membrane fouling, the efficiency and durability of nanofiltration depending on the thermal, chemical and mechanical properties of the membranes [163].
5.4.3. Nano-remediation of acid mine drainage
Nano-remediation is an emerging technology, which uses nanoparticles (with a diameter of less than 100 nm) to treat contaminated water and polluted soil [164]. Nano-remediation is very advantageous since besides organic and inorganics contaminant, it is also effective to remove biological toxins and pathogens microorganisms such as Vibrio cholerae (Cholerae) and Salmonella typhimurium (Typhoid fever). From literature, very few studies have been carried out to investigate the application of nano-remediation technology for the treatment of AMD. Ji et al. [165] assessed the use of polyetheleneimine-diatomaceous earth nanoparticles (PEI-DE-NPs) to remove Cu from freshwater polluted by AMD and it followed that (PEI-DE-NPs) was very effective in removing Cu from AMD polluted freshwater. The result further revealed that other metals concentration were found to be very low after the contact of PEI-DE-NPs with fresh water [165]. Furthermore, Nkosinathi et al. [166] used an eco-friendly approach to synthesize FeCu bimetallic NPs by polysaccharide bioflocculant (FeCuBNPs) and evaluated its application on coal mine water remediation. The authors showed that FeCuBNPs flocculated at low dosage (0.2 mg/mg/L), with high flocculation (99%) achieved at pH 7 and lower flocculation (95%) achieved at acidic pH (3) and alkaline pH (11). By definition, flocculation is water treatment technology applied to gather solids particles into large cluster and remove them from water [167, 168]. From the findings of the above-summarised studies, nanoparticles (NPs) can be applied to treat AMD if more researches are conducted to investigate their full potentiality. However, the application of NPs for AMD treatment is limited since NPs can easily spread and disperse in the nature, thus, increasing the risk of bio-accumulation in the living organisms to toxic level.
Overall, prevention and treatment technologies are associated with huge drawbacks which hinder their employability for a lasting solution thereby calling for the assessment of their sustainability using appropriate tools such as LCAM.
Importance of Life Cycle Assessment Methodology (LCAM)
While prevention and remediation techniques allow to limit the formation of AMD or to treat and manage the already generated AMD, various factors including outrageous cost and environmental impacts limit their application at large scale thereby rendering them less sustainable. As such, there is a dire need to evaluate their economic and environmental sustainability using LCAM. In environmental engineering and precisely in AMD management, LCAM is deeply informative approach used to identify environmental impact and cost of each prevention and treatment technology through their entire lifespan. LCAM revealed a trade-off to be used by scientists, policymakers and business promoters to set sustainable approach with the ultimate goal to reduce cost and environmental impacts while improving productivity [169]. LCAM considers various factors including: i) inputs (resources and energy consumed at every stage of the prevention or treatment plant during their entire lifespan); ii) outputs (contaminants and by-products released or emitted at each stage of the prevention or treatment plant during their entire lifespan); iii) environmental and human health effects of inputs and outputs. The challenge of AMD is its significant environmental and human health effects thus requiring well-thought out solutions. LCAM appears as the valuable and promising tools to assess the sustainability of each technology employed for effective management of AMD.
6.1. Methodology
6.1.1. Aim, scope and systems boundaries
The overall aim of this study is to assess the economic and environmental sustainability of different AMD prevention and treatment technologies using LCAM. The selected functional unit was the volume of AMD treated or in case of prevention technique the quantity of materials used or the area covered to avoid contact of tailing material with oxygen while the system boundaries differed from one technology to another. However, in most cases, the system boundaries were the inputs and output flows of materials, energy resources as well as the cost required for the construction and operational phase of each prevention or treatment technique. In the construction phase, raw materials acquisition, manufacturing and construction were considered while in the operational phase, the addition and replacement of materials used to treat or prevent AMD formation depending of the case and the handling of sludge if necessary. The LCAM was assessed by acceding data using OpenLCA modelling environment which is an open software source software developed by GreenData for LCA and sustainability assessment (https://www.openlca.org). In order to obtain the environmental and economic information and datasets, the Ecoinvent (version 3.5) (https://simapro.com/databases/ecoinvent) life cycle inventory (LCI) database was used since it is the largest, most reliable and globally accepted for LCA studies and assessment based on ISO 14040 and 14044.
6.1.2. Life cycle assessment
The possible environmental impacts were assessed using a ReCiPe method [170] at midpoint level Hierarchist (H) and end point level in order to have an insight about the different 17 midpoint indicators (environmental effects) and 3 midpoint indicators (human, ecosystem and resource availability) of each prevention and treatment technology [171]. The ReCiPe 2008 was used because it is the most suitable for this study due to the set of impacts categories and their associated characterization factors. Background LCAM uncertainty at ± 10% was applied in this study to consider the variabilities of data gathered from Ecoinvent 3.5 database since in LCAM, data sources are evaluated taking into consideration various factors including, data collection, modelling, choices, future change, spatial and temporal variability, variability between source and objects.
Results and discussion
7.1. Economic and Environmental Sustainability of AMD Prevention and Treatment Technologies
Sustainability of AMD prevention and treatment techniques refer to their abilities to continuously prevent or treat AMD over time when taking into consideration cost and possible secondary environmental and human health effects.
7.1.1. Economic and environmental sustainability of AMD prevention techniques
Sustainability of AMD prevention techniques refer to the cost-effectiveness and the ability of those techniques to continuously prevent AMD formation or generation over time without any risk of causing environmental and human health damage. In accordance with, Broadhurst et al. [20] investigated the efficacy of desulfurization flotation approach in removing sulfide minerals from mine tailing with successful results. However, due to the complexity in operation, high capital and operational cost, and the limited lifespan of the system, it could not be considered as lasting solution to control AMD formation [20]. Sarkkinen et al. [172] investigated the economic and environmental sustainability of surface cover for the suppression of AMD formation using LCAM. Despite the promising results, the sustainability of this technique was hindered by the short lifespan, secondary environmental pollution, and the relatively high capital and operational cost [172]. Furthermore, Madzivire et al. [169] used the LCA methodology to assess the environmental sustainability of AMD prevention using the water diversion technique. The findings revealed that water diversion techniques has huge impacts on resource depletion, human health effects and ecosystem degradation since the transportation of construction materials leads to noise pollution, fossil fuels burning and the release of greenhouse gases, water pollution and its multiple direct and indirect effects on aquatic ecosystem [169]. Moreover, Ricardo Domingos et al. [173] used LCAM to investigate the economic and environmental sustainability of silica passivation suppressing AMD formation. The LCAM results identified less impact on soil quality, however, climate change due to silica transportation, processing in addition of energy used were the most noticeable environmental impacts. To mitigate environmental effects and reduce the risks to acceptable level, the authors proposed as alternative the sourcing of local silica or the use of recycled silica and the utilization of renewable energy source to render passivation more sustainable approach for AMD prevention.
One of the methods used to prevent AMD formation is the reclamation of land contaminated by mining waste. This technique consists of using uncontaminated topsoil to cover site containing mining waste in order to prevent the contact of mining waste with oxygen and limit the infiltration of water following by plantation of vegetation. This technique inhibits bacteria proliferation and slow AMD formation. The revegetation of the site allows plants to develop extensive roots, which stabilize soil quality by reversing the degradation process [174]. However, soil liquefaction, compaction, bulk density, change of the physicochemical quality of the surrounding areas leads to nutrients deficiency and flooding during rain fall are some of the limiting factors of land reclamation technique [88]. In addition, reclaimed land cannot be used for heavy civil engineering work [175, 176], while the selectivity in plant species to be used further limit the sustainability of this AMD prevention technique [177]. To remediate this situation, mining waste can be removed and relocated to a new site and treated, but it also constringing since it is difficult to find a new site where the treatment can be done without leading to new environmental problems. Overall, the efficiency of AMD prevention techniques is very limited due to their short lifespan, their high capital and operational cost, their complexity in operation, and above all, secondary environmental and human health effects associated with their operation [177].
7.2. Economic and Environmental Sustainability of AMD Treatment Technologies
7.2.1. Economic and environmental sustainability of passive treatment technologies
Passive treatment technologies are being considered as a viable alternative option to active treatment, however, their economic and environmental sustainability are not fully investigated. In agreement with that, Martinez et al. [178] assessed the LCA of a passive AMD treatment plant (alkaline substrate technology) to determine the environment effects throughout the life cycle of the treatment plant. It follows that the construction phase was responsible to more environmental effects, which reduced over time. The environmental effects were closely associated with the type of materials used and their transportation to the treatment plant. Nevertheless, the replacement of such materials significantly reduced the environmental effects of the treatment plant, thus rendering it more eco-friendlier [178]. Wang et al. [179] used LCAM to evaluate the cost and ecological effects of passive biological plant treating AMD. The results revealed less energy consumption by the biotic system and low capital and operational costs. In addition, the biological treatment plant was more environmentally friendly leading to fewer human health effects. However, the construction phase of the passive treatment plant resulted in more energy consumption (99%). The authors further highlighted that the use of less polluted and renewable materials could improve the sustainability of the biotic system. The findings from these studies denoted that the biotic system is very promising in AMD remediation if more research is conducted to investigate their full potential. Furthermore, passive treatments are self-renewable systems with low or zero energy inputs and uses non-hazardous materials for the construction of the treatment plant and are more environmentally friendly as compared to active treatment methods and they can be fully integrated with neighboring ecosystems [58]. Passive treatment technologies appear to be economically and environmentally sustainable for AMD treatment and management, yet they cannot be considered as viable alternative to conventional method (active or chemical treatment) due to their inefficacy to treat AMD to the required standard.
7.2.2. Economic and environmental sustainability of hybrid and integrated technologies
The drawbacks associated with passive treatment methods inspired the researchers to combine both active and passive in a hybrid approach or integrated approach with the aim to reduce their environmental footprints and capital investment while increasing their efficiency. Hengen et al. [180] assessed the sustainability of a hybrid approach [active (lime slaking) and passive (gravity-fed bioreactor)] treating AMD. Findings revealed that the passive treatment required no operational energy for the 17 years lifespan while the active treatment required 960 000 KWh of energy per year for the same duration. To construct each treatment plant, 58000 m3 and 2560 m3 of earth excavation were required for the active and passive treatment plant respectively. Construction materials were transported during the construction phase for both plants. However, for the active treatment, a total of 150 million kg of hydrated lime was transported 97 km away from the treatment plant during the operational phase. The transportation process led to air quality degradation via the emission of fossil-derived fuels, which can contribute to global warming. The transportation process also contributes to noise pollution, water pollution, leading to multiple direct and indirect effects on the environment and public health [180]. Masindi et al. [181] examined the economic and environmental sustainability of a multistage system treating AMD at semi-industrial scale. The integrated or multistage system treated AMD in a stepwise fashion in order to simultaneously remove inorganic contaminants and recover natural valuable resources. The findings showed promising results in terms of valuable resource recovery, however at the expense of high energy inputs, high capital and operational cost, secondary pollution due to the release of salty water (brine). Moreover, Miranda et al. [182] investigated the economic and environmental impacts of using industrial by-products to neutralise coal mine water and recover rare earth elements (REEs). It follows that industrial by-products had higher neutralising capacity and were efficient for the recovery of REEs. However, the process was responsible for various environmental effects from the construction to operational phase with most ecological effects occurring during the construction phase due to the transportation of materials to the site, earth excavation, deforestation, and oil spill. On the other hand, during the operational phase, environmental impacts were significantly reduced and the estimated effects on human health was reduced by applying the ReCiPe method for life cycle impact assessment (LCIA) to reduce ecological effects and yield a human health end points indicator of −2.0 × 10−3 disability-adjusted life years on ecosystems impacts of −5 × 10−6 species/year and a resource impact of 55 USD. The integration of basic oxygen furnace (BOF) slag, lime, soda ash and reverse osmosis (RO) for the treatment and valorisation of AMD was evaluated by Masindi et al. [30]. Their results revealed that integrating BOF, lime, soda ash and RO allowed to yield expected results. However, the sludge generated after the neutralisation process compromised the environmental sustainability of this process since it must be properly disposed of, thereby incurring additional cost and available spaces. Salted brine released can find its way into the watercourse leading to ecosystem damage such as anoxia, turbidity amongst others [183].
7.2.3. Economic and environmental sustainability of emerging technologies: Phytoremediation, membrane technologies and nanoremediation
Emerging technologies are appearing as viable options for AMD management and treatment since they are promising technologies that use plants (phytoremediation), nanomaterials (nanormediation), or membranes (membrane technology) to clean up contaminated sites or to treat wastewater. They are cost-effective; however, their environmental sustainability is still a matter of concern within the scientific community. Bearing that in mind, Espada et al. [184] examined the economic and environmental sustainability of phytoremediation technology decontaminating AMD-polluted soil in an old Spanish mine using LCAM. Two different types of plants species [Brassica juncea (brown mustard) and Medicago sativa (alfalfa)] were investigated with very promising results, achieving 30–100% of metals removal while presenting good environmental performance in addition to affordable cost since it used renewable energy and does not require routine monitoring. However, the main drawbacks associated with this technology were the slowness of the process (it requires a prolonged period to be effective) and the handling of contaminated plant species [184].
Bordbar et al. [185] assessed the environmental footprint of five treatment plants [multi-stage flash (MSF), hybrid reverse osmosis-MSF, hybrid nanofiltration (NF)-MSF, RO, and hybrid NFRO using LCAM. The parameters of concern were impacts on climate change, ozone depletion, fossil depletion, human health, and marine eutrophication. The results revealed that the hybrid NF-RO was the eco-friendliest with fewer environmental impacts. However, membrane fouling was the main drawbacks which hindering their sustainability.
Advantages and Disadvantages of Different Acid Mine Drainage Treatment Technologies
8.1. Advantages and Disadvantages of Passive Treatment Technologies
Passive treatment systems are very promising in treating AMD, however, there are some advantages and drawbacks that limit their application at an industrial scale. Passive systems are self-renewable approaches that, do not require routine maintenance by highly skilled personnel nor chemical inputs, more economical and environmentally sustainable and are therefore deemed more suitable for remote areas and abandoned mines [186]. Despite being very promising for AMD treatment, passive systems are relatively new and well-founded expertise is relatively scanty. In addition, they are inefficient to treat highly acidic AMD added to the low flow rate required for their operations, thus leading to long-term reliability with a high possibility of system failure. Their efficiency is low (less than 50%) and therefore mostly employed as a polishing stage in the treatment chain [25, 51]. Their efficacy is influenced by many factors, including: (i) weather (e.g winter season negatively affect the growth of plants in the case of CWs thereby reducing the removal efficiency of the treatment plant [116]; (ii) blockage of pipes draining water constraint their operations [26, 32]; (iii) they are highly dependent on the geographical and topography of the area [174]; (iv) their application in the circular economy concept is very challenging due to the impossibility to selectively precipitate metal; and (v) sludge released by passive systems is rich in minerals, and such sludge is difficult to valorise or treat [48]. Nyquist and Greger [187] assessed the feasibility of treating AMD using small-scale CWs erected near the Kristineberg mine in Northern Sweden. The treatment process lasted for one year from January 2006 to January 2007, water samples were collected every month, and the results revealed that CWs was inadequate to treat very harsh and acidic AMD generated by the Kristineberg mine. In addition, the study conducted by Noller et al. [188] showed that CWs planted with Typha latifolia partially reduce metal concentration in AMD generated directly by Northern territory mines in Australia, thus confirming that CWs are not suitable to treat very acidic mine to required standard.
8.2. Advantages and Disadvantages of Hybrid and Integrated Systems
The hybrid and integrated systems are very promising in AMD treatment due to the high RE achieved since they can effectively treat AMD to the desired drinking water standard. Both systems are versatile and provide high RE in contaminants removal. In addition, integrated systems are more suitable to introduce reduce, reuse, recycle, and recover (4R) in AMD treatment, thus changing the paradigm surrounding AMD from significant environmental pollutants to beneficial mining effluent [31]. Acid mine drainage is recalcitrant wastewater containing elevated concentrations of various toxic chemical species. As such, hybrid and integrated systems are more indicated to treat mine wastewater to the required standard that can be reused for other purposes such as irrigation [28, 189]. However, the cost of the system has to be taken into consideration in terms of capital, operational, and maintenance. Moreover, the cost varies according to the sites, the concentration of chemical species in AMD and the nature of the combination. For instance, a hybrid system combining active and membrane technology may be very challenging due to high cost and the fouling of the membrane, added to high energy inputs in the active stage of the treatment chain. The integrated systems require frequent usage of different chemicals with different toxicity levels, therefore, employing integrated systems for AMD treatment will lead to the release of a mixture of toxic chemical in the water courses with possible ecotoxicological and human health effects unless the sludge is recycled. For instance, a semi-industrial AMD treatment plant designed and developed by Masindi et al. [31] is currently in operation at the premises of Council for Scientific and Industrial Research (CSIR). The treatment chain is made of four discrete steps: (1) neutralisation with magnesite to attenuate metal and partially remove SO42−; (2) limestone is added to remove residual SO42− and reduce the water hardness; (3) soda ash is added to remove residual calcium; (4) carbon dioxide (CO2) is added or dissolved into the water to adjust the pH (pH 8) and recover limestone. This semi-industrial system treats 3500 litres of AMD daily, producing high-quality of product water which is used for irrigation, however, at the expense of high energy inputs, production of high toxic sludge which can be valorised, but more research is needed for its feasibility.
In addition, the efficiency of hybrid and integrated treatment systems is influenced by weather conditions. For instance, in a hybrid system combining abiotic (active) and biotic (passive), temperature influences the efficiency of the biotic stage since the warm temperature is more suitable for the growth and reproduction of bacteria [190, 191]. Unlike warm temperatures, higher and colder temperatures may lead to more energy requirements to control the temperature of the treatment system thus affecting the cost and RE. In addition, colder temperature may affect the equipment by icing the pipes and the surface water directly exposed to the atmosphere, decrease anaerobic bacteria activity resulting to reducing efficiency [192]. Moreover, during raining seasons, the runoff may lead to additional influx of water into AMD treatment system and this can negatively influence the RE thus affecting the attenuation of inorganic contaminants.
8.3. Advantages and Disadvantages of Phytoremediation
The phytoremediation process removes a huge variety of chemical species without specificity and can be employed at large scale without the excavation of large contaminated soil as revealed by existing literature. For instance, the study by Nguyen et al. [193] showed that the aquatic plant: Eleocharis acicularis was able to simultaneously take up multiple metals in AMD-polluted soil in an abandoned mining site in Hokkaido, Japan after an experimental period of 11 months. Major metal (Al, Fe, Cr, Cu, Mn, Ni, Pb and Zn) commonly found in AMD and Silica (Si) were significantly accumulated by E. acicularis thereby demonstrating the potential of phytoremediation technology in mine water management. Phytoremediation is more economical and environmentally sustainable since it is cost-effective due to the use of renewable sources of energy, does not require routine maintenance by highly skilled personnel, is simple to operate, and does not release secondary pollutants. Acid mine water is highly toxic, and the phytoremediation process appears as a viable and eco-friendlier approach for a long-term solution. However, the phytoremediation technology is highly influenced by the ability of plant species to bioaccumulate various pollutants and the specificity of plants in pollutant removal. It is highly dependent on plant species and becomes ineffective when plants are disturbed by pets or diseases. Due to the harsh quality of AMD, elevated concentrations of inorganic contaminants can be toxic to plants thereby affecting the efficiency. Moreover, it is highly influenced by weather conditions, very slow process due to the slow growth of plants and low biomass, thus increasing the high risk of failure along the way [194]. Furthermore, plants used for phytoremediation are rich in inorganic contaminants and therefore must be properly disposed of to avoid the shifting of the problem [24, 26]. The aforementioned studies clearly demonstrated that AMD can be treated using phytoremediation technique; however, more research should be conducted to investigate its full potential. Operating in a stand-alone system, phytoremediation technology cannot be seen as a lasting solution for AMD management and should therefore be associated with or combined with other AMD treatment technologies.
8.4. Advantages and Disadvantages of Membrane Technology
Membrane technology is very sensitive provide accurate results in water and wastewater treatment since the full volume of liquid to be treated passes through the membrane filtration where all inorganic pollutants are trapped and removed [195, 196]. In addition, MT is highly flexible and can be used to remove large and small particles, does not require the use of chemicals and can remove close to 100% of microorganisms from aqueous samples and produce high-quality of product water [195]. However, several cons are associated with the application of MT for AMD treatment and they include: (i) fouling which is the blockage of membrane by colloidal solids and other substances; (ii) Membrane cost is another drawback since membranes used are not always cost-effective and some such as membrane bioreactors (MBRs) can be even more expensive and sometime challenging to find in the market; (iii) MT requires high energy inputs than chemical treatment (active method) thus limiting its application at industrial level; (iv) membrane cleaning requires the use of chemical to maintain its permeability, however, the used chemical may end up polluting the environment with possible eco-toxicological and human health effects; (v) MT is discontinuous technique since membrane must be replaced over time leading to the temporary cessation of the treatment process; (vi) and the production of brine (water rich in salt) or aqueous retentate (fluid rich in various components) that must be properly disposed of thus incurring additional cost, available space and some time the service of waste management expert.
8.5. Advantages and Disadvantages of Nanoremediation
Nano-remediation uses nanoparticles (NPs) including metals and carbon-based NPs which provide high surface area. Nanoremediation technique removes a wide variety of pollutants, including microorganisms [164]. In addition, nanoremediation is compatible with other treatment techniques and the cost is relatively low. Metals-based NPs have a high specific area thus allowing them to adhere, bind, and remove considerable concentrations of pollutants. Despite being very promising, the application of nanormediation can lead to negative effects on the environment and human health. Nanoparticles used for AMD treatment can easily spread and disperse in nature and therefore may contaminate different environmental compartments where they can damage plants by reducing the photosynthesis process, inducing cell death. Moreover, NPs can disrupt and even kill microorganisms, including those vital in industrial processes and wastewater treatment. The use of NPs for AMD treatment is very risky for human health and specifically the workers since exposure to NPs can cause skin irritation and respiratory damage [165]. Overall, the use of NPs for AMD and wastewater treatment may lead to problem shifting. Emerging technologies (MT and nanoremediation) are very promising for wastewater treatment; however, their utilisation for AMD treatment is very limited in literature. Specifically, nanoremediation mostly uses zero valent iron (nZVI) as a reductant since it is capable of sequestering a variety of contaminants found in AMD water. However, a high concentration of nZVI may form a cluster, thus losing its ability as NPs and becomes toxic to various environmental compartments. Due to their small size and their high persistence in the environment, NPs can easily spread and disperse in nature, thus increasing the risk of bioaccumulation in living organisms to toxic levels [197]. The nZVI which is the main reductant used in nanoremediation is a source of severe environmental effects as revealed by various studies [197, 198] For instance, Gomez-Sagatia et al. [198] studied the impacts of nZVI-NPs on bacteria communities and found out that the cytoxicity of nZVI-NPs was dose and species-dependent and affected by environmental conditions. The researchers added that oxidation of high concentrations of nZVI stimules the growth of reactive oxygen species (ROS) in animals and this may cause oxidative stress, damaging the cell membrane and finally lead to fatality. In plants, high concentration of nZVI affect plant metabolism by reducing the transfer of nutrients from roots to shoots, thus delaying the growth and ultimately the death of the plant [199–201]. In humans, exposure to NPs causes genotoxicity, inflammation, oxdative stress, lipid peroxidation and pulmonary disease [200, 201], thus rendering nanoremediation non-sustainable.
Overall, passive, combined and emerging technologies are becoming widely popular as alternative solutions to conventional AMD treatment technology (active or chemical treatment); however, their cost and environmental footprint are still matters of great concern, as reported in Table S1 (supplementary materials) where the cost and environmental attributes of selected passive, combined, and emerging technologies are meticulously compared.
Acid Mine Drainage Beneficiation and Valorisation
Despite being globally seen as hazardous pollutant of great significance to the environment and human health, AMD contains numerous metals, which can be precipitated and synthesised to recover valuable minerals. In addition to valuable minerals, a clean water can also be reclaimed indicating that AMD can be valorised, thus, changing the paradigm from hazardous mining by-products to valuable resources [48, 203]. The valorisation of AMD can allow reducing the environmental and human health effect, and reduce the treatment cost since minerals recovered can be sold to upset the treatment cost. In addition, the valorization of AMD can open routes for the implementation of 4R technology in the mining industry. In this regard, hybrid and integrated technologies appear to be the more promising technologies in AMD valorisation and beneficiation as reported by various studies [48, 138, 203, 204].
9.1. Reclamation of Freshwater
One way to reclaim water from AMD is by removing and recovering dissolved solids. This technique allows to reclaim a clean water, which can be used for various purposes, including industrial reuse and irrigation. The reclamation of water resources from AMD is done via different techniques. For instance, the study by hentati et al. [205] revealed that AMD treated by neutralisation technique with limestone was used for irrigation during the winter season in South Africa. Aguiar et al. [195] applied nanofiltration technology for AMD treatment and obtained a product water suitable to be reused in the mineral processing industry. However, the main challenge in reclaiming water from AMD is the difficulty to handle dissolved solids since they refer to toxic salts, metals, and ions [206].
A combination of neutralisation, filtration, and disinfection can be applied to recover possible drinking water from AMD with filtration being the polishing stage to remove dissolved solids while disinfection is applied to kill microorganisms. However, this integrated technology presents many drawbacks, such as the release of highly polluted toxic sludge in neutralisation step, challenges with membrane scaling and fouling in the filtration step [207, 208], and disinfection leads to the formation of by-products and its inefficacy to eliminate all microorganisms.
9.2. Recovery of Valuable Minerals
Acid mine drainage contains high concentrations of major metals (Al, Fe and Mn) and very elevated concentrations of SO42− suggesting the possibility of recovering valuable minerals from it. In accord with that, various techniques including electrodialysis, diffusion dialysis, distillation, acid retardation, freezing crystallisation, solvent extraction, and membranes technology, have been applied to recover sulphuric acid (H2SO4) from AMD [209–213]. Ricci et al. [214] successfully used microfiltration and nanofiltration membranes to recover H2SO4 and valuable metals from AMD. The recovery of other minerals (metals) has been widely explored using different techniques. For instance, selective precipitation has been widely applied to recover metals and gypsum [31, 215, 216], and REEs [189, 217], while bioelectrochemical technology has also been applied to recover metals from AMD [218, 219]. Besides metals, (Semi) metals such as goethite, hematite and magnetite have also been synthesized and recovered from AMD [220, 221]. These (semi) metals are very important in various industrial applications. For instance, hematite is widely used in pigments synthesis, and radiation shielding [221, 219]. Similarly, magnetite is used for ferro-fluid technology, storage of information, photodegradation for organic contaminants, biomedicine, and controlled drug delivery [219, 220], while Geothite is used in coating industries as well as adsorbents for pollutant removal in water and wastewater [221]. Despite being very promising in AMD valorisation, the recovery of valuable minerals from AMD presents various shortcomings. For instance, the cross-contamination is the main challenge in mineral recovery from AMD since it affects the purity of mineral thereby requiring the need to design and develop highly specific techniques, which require high capital investment.
Integrated treatment systems are the most suitable to be applied for the valorisation and beneficiation of AMD thus opening the route to implement 4R technology and ZLD in the mining industry. However, there are various barriers hampering their implementations; (i) legislative barriers: in most countries with intensive mining industries, there are various stringent overlapping environmental laws regulating mining operations. For instance, in South Africa, environmental regulations for mining include: Air Quality Act, National Water Act, Mine Health and Safety Act, Atmospheric Pollution Prevention Act, Nuclear Energy Act, Mineral and Petroleum Resources Development Act. In order to implement 4R technology and ZLD in the mining industry, a permit must be obtained from all sources of which pollutants will be discharged; (ii) insufficient knowledge of cleaner production. Many mines lack knowledge about 4R and ZLD and other cleaner’s technologies available on the market. In addition, the non-dissemination of information about 4R and ZLD by the policy marker leads to the stagnation of environmental policy; (iii) economic barriers: many mines lack financial assistance to invest in cleaner production and implement 4R and ZLD in their operation. As such, they are pleased with their conventional environmental compliance and find it difficult to shift to cleaner production. Furthermore, their environmental footprints, in fact, various chemical are used at every step when implementing 4R and ZLD, and this ultimately lead to the release of high polluted toxic sludge with eco-toxicological and human health effects thereby leading to problem shifting. The system is fragile since the damage or mal functioning of one stage leads to the failure of the whole system.
Factors Hindering or Obstructing AMD Treatment Technologies
Acid mine drainage has been treated using various technologies and researches are still ongoing to develop a sustainable and long-term technology to effectively manage and valorize AMD [203, 216, 221]. However, several factors exist that hinder or obstruct their sustainability and their application at an industrial scale. Currently, the limiting factors vary according to the type of technology being applied for AMD treatment and pertain to the quality of product water, the cost, and secondary environmental effects. Overall, the main challenges for AMD treatment greatly depend on the technology and can be summarised as follows:
The inability to treat AMD to required standards, the high cost associated with the treatment process, high energy input, secondary environmental problems including the release of highly polluted toxic sludge, environmental and human health effects associated with the transportation of construction materials for the treatment plant.
For passive treatment, they are ineffective to treat field AMD water and therefore more suitable used as a polishing step in the treatment process. In addition, to be efficient, the flow rate should be very low, and it becomes difficult or even impossible to use passive technology to treat AMD generated by active mines, thus, making it more suitable to treat AMD generated by disused or abandoned mines. Other passive methods, such as CWs require large land areas, thus hindering their applicability at an industrial scale and specifically in countries or region with limited land areas.
Hybrid technologies, in other hand combine active and passive methods for AMD treatment, however, the combined cost and energy input, the associated human health and environmental effects linked to the active step, the large land areas required, and high risks of failure of the passive step are the drawbacks associated with hybrid technologies, thus rendering their application at an industrial scale very complex.
For integrated processes, the high cost, complexity of the overall treatment process, high energy input, secondary environmental effects and associated human health impacts, the low quality of product water are the main effects hindering their application at industrial scale.
Conclusion and Recommendations
11.1. Conclusions
Mining activities are very significant to the environment and mostly via the generation of AMD and its ecological effects. In addition to its environmental effects, AMD is also responsible for severe human health effects and socioeconomic problems. Prevention of AMD is very difficult and complex while most AMD treatment technologies developed so far are inefficient to effectively address the problem caused by AMD due to limited efficacy, complexity in operation, high cost, and secondary environmental pollution. Considering the challenges in their application, existing AMD prevention and treatment technologies are perceived as non-sustainable, thus making it difficult to achieve the United Nations sustainable development goals (SDGs) while managing mine wastewater. Whereas existing literature largely focused on the limitations, pros and cons of AMD treatment technologies: active, passive, emerging, and combined approaches (hybrid and integrated system). However, there are limited literature on the advantages, disadvantages, and deficiencies of AMD prevention techniques. Moreover, in environmental engineering, the designing and implementation of each new technology must take into account the sustainability through the systematic analysis of all environmental impacts and costs over its entire lifespan. This paper reviewed the economic and ecological sustainability of selected AMD technologies (passive, combined. and emerging) using LCAM to quantify the environmental impacts and costs of the aforementioned treatment technologies. Passive technology appeared to be more environmentally friendly and cost-effective; however, fail to treat AMD to the required standard. Unlike passive systems, combined technologies (hybrid and integrated) are very efficient in contaminants removal providing high quality of product water but are not cost-effective and less eco-friendly. Phytoremediation is economically and environmentally sustainable but more efficient to treat AMD-polluted soil. Membrane technology is very fragile, not cost-effective, and leads to secondary pollution such as salty water, while there are potential human health risks and unintended consequences associated with the use of nanomaterials for AMD treatment.
11.2. Recommendations
The existing literature on the economic and environmental sustainability of AMD treatment technologies is limited and thus suggests that further research should be orientated in the direction to include LCA studies while designing a treatment technology approach. This will allow improving the environmental sustainability of mining activities. Different treatment technologies have been applied for AMD management and treatment worldwide, but they are mostly active. Passive treatments are self-renewable systems, more environmentally friendly but fail to treat AMD to required standards as set by regulatory bodies. To improve pollutant removal while reducing the environmental impact of mining activities to an acceptable level, there is a need to combine active and passive technologies through a hybrid system, which seems to be the viable option for managing AMD and its environmental effects. Hybrid technology allowed exploiting the benefit of each type of treatment process with very promising results. However, the application of a hybrid approach at an industrial scale is complex especially in areas with limited land spaces, but the size of wetland can be reduced according to the available land area, thus calling for the insertion of LCA methodology to support this approach in managing the environmental and human health impacts of mining activities. Integrated systems are more beneficial in terms of natural resource recovery but less eco-friendly. Like other treatment technologies, more research should be conducted, and LCAM should be incorporated to determine precisely to which extent those technologies are efficient to effectively manage and valorise AMD. To date, more research conducted focused on the raise of pH and reduction of chemical species to acceptable levels prior to environmental discharge. Research focused on effective treatment and valorization of AMD are very limited in literature. Acid mine drainage prevention and treatment technologies are not always sustainable and one way to ensure their economics and environmental sustainability is to orient future research on the following avenues: breaking the gap between pilot research findings and industrial application by introducing the LCAM to improve economic and environmental sustainability.
Designing an AMD treatment approach with the main objective to inverse the paradigm surrounding mine water so that AMD can be viewed as valuable wastewater rather than significant environmental and human health pollutant.
Carrying out detailed research to design and implement 4R technology in the mining industry to minimise and reduce AMD generation, effectively treat and reuse the product water for other purposes to ensure ZLD in the mining industry.
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