Sustainable Treatment of Acidic and Alkaline Leachates from Mining and Industrial Activities: Current Practice and Future Perspectives

*Thomas F. O'Dwyer, Bashir Ghanim, Ronan Courtney, Ashlene Hudson, J. Tony Pembroke and John G. Murnane*

### **Abstract**

Water resources are under continued pressure from anthropogenic sources, including acidic waste from abandoned mine sites and alkaline waste from a variety of industrial activities. Large quantities of mine and industrial wastes are typically stored in tailings facilities which can generate significant quantities of leachates due to weathering. If released untreated to the aquatic environment these have the potential to contaminate surface and ground waters. In addition, generation of leachates from abandoned or closed sites presents a major long-term environmental challenge where the generation of leachates is expected to continue for decades if not centuries post closure. An overview of leachate production and associated treatment technologies are described, with an emphasis on passive and potentially sustainable technologies. Measures to prevent the formation of acidic leachates and the potential for resource recovery from acidic and alkaline wastes and leachates are also discussed. Finally, technologies that require further development for long term and sustainable treatment are highlighted.

**Keywords:** mine and industrial wastes, acid mine drainage, alkaline leachates, passive treatment, resource recovery, sustainability, circular economy

### **1. Introduction**

The mining industry generates in excess of 6 billion tonnes of waste annually [1] with significant growth expected in the future. In the EU for example, annual mine waste comprised 26.3% (615 Mt) of the total waste generated in 2018 [2]. Mine waste is generally categorized as i) non-mineralized overburden (typically 2–20 cm diameter), which is removed to access valuable mine ores and stored in spoil heaps, and ii) tailings and process wastewater, arising from the extraction and processing of ores. Tailings from metal mining are enriched with heavy metals (metals with a density > 6 g cm−3),

whose extraction is no longer economically viable and are typically deposited indefinitely in storage lagoons, often referred to as tailings storage facilities. These storage facilities generate vast quantities of metal rich leachates, which if released to the aquatic environment, can result in elevated bioavailable metal concentrations and sediment loading leading to the stress and death of aquatic organisms and human health.

Leachates are often classified as acidic, alkaline or neutral depending on the geochemistry of the mine tailings and the processing steps utilized in the mining process. Acidic leachates and acid mine drainage (AMD) have a low pH (typically pH < 6) and are generated when sulfidic ores, most commonly pyrite ores (FeS2, often referred to as 'fool's gold'), which are normally stable in anaerobic underground conditions are exposed to oxygenated environments during mining operations causing the sulfides to oxidize. This process results in acidic conditions (sulfuric acid generated) with associated elevated levels of sulfate, heavy metals and metalloids (semi metals having metallic and non-metallic properties), which if released to the environment result in significant and long-term pollution. The most abundant and common metal in AMD is Fe(II) which reacts with dissolved oxygen to produce iron oxide precipitates. Alkaline leachates on the other hand have a high pH (typically pH > 10) and are generated at disposal sites of industrial by-products such as steel slag, coal ash, municipal waste incinerator ash and bauxite residue from the alumina processing industry. The high alkalinity of these leachates is typically generated from reagents used in industrial processes, such as sodium hydroxide (NaOH) and lime (CaO), and are often enriched with trace metals such as chromium (Cr), vanadium (V), molybdenum (Mo) and gallium (Ga). Neutral leachates (typically pH 6–10) are normally generated from mine wastes low in sulfides or when the oxidation of sulfides is weak or when waste is neutralized by carbonate content in the material [3]. Although neutral, these leachates can contain potentially toxic elements such as water-soluble forms of nickel (Ni), zinc (Zn), cobalt (Co), arsenic (As) and antimony (Sb).

An estimated 3.5 billion tonnes of bauxite residue, a byproduct of alumina refining and more commonly known as red mud, are deposited globally and this amount is increasing at rates of between 120 and 150 Mt. per annum. While it is difficult to accurately predict alkaline leachate quantities generated from these deposits, a global estimate is in the region of 150 million m3 per annum [4]. These leachates can contain elevated concentrations of metals such as aluminum (Al), potassium (K), sodium (Na), V, Mo, Ga and Ni, which are potentially toxic if released untreated to the aquatic environment. In addition to this, an estimated 30–40 Mt. of incinerated bottom ash and 2–6 Mt. of fly ash are generated annually from incineration of municipal solid waste (MSW). Most of these residues are generated in the EU (33%), China (29%), Japan (20%) and the USA (16%). Despite the varied, significant and potentially valuable metal content of these ashes, most are deposited to landfill with associated generation of metal rich alkaline leachate production [4].

As well as posing a serious threat to the environment and to human health, billions of euros worth of valuable metals contained in industrial and domestic wastes are disposed of in hazardous waste disposal sites [4]. However, metal recovery from these wastes and associated leachates is for the most part technically difficult, uneconomical and unsustainable, primarily because the metals tend to be present in low concentrations and in complex matrices. Nevertheless, resource recovery must remain a priority, particularly in an age where technological advancement is a key driver for global sustainability.

*Sustainable Treatment of Acidic and Alkaline Leachates from Mining and Industrial… DOI: http://dx.doi.org/10.5772/intechopen.103889*

The following sections will examine current treatment options with a focus on passive treatment of acid mine leachates and alkaline industrial leachates. An overview of recent attempts at resource recovery from these leachates will also be discussed before considering future requirements for treatment of acidic and alkaline leachates.

## **2. Treatment of acid mine leachates**

Source control techniques to prevent the formation of AMD is an ideal scenario for the mining industry, which would significantly reduce the environmental burden from mining operations. Source control operates by limiting the exposure of sulfidic waste to air, water or oxidizing bacteria (e.g. sulfide reducing or iron oxidizing bacteria) thereby preventing or reducing its acidification. The most common source control treatments include underwater storage of mine tailings or dry covering with non-reactive materials (oxygen barrier), co-disposal with acid consuming or alkaline producing materials, microencapsulation and passivation. Microencapsulation involves forming an iron hydroxide coating on the surface of the pyrite to inhibit pyrite oxidation and reduce the formation of AMD while addition of a passivation agent facilitates a series of reactions to form a dense inert layer on the surface of the metal sulfide materials which reduces contact with oxygen, water, microorganisms and metal sulfide materials and therefore reduces AMD formation. However, such methods do not always successfully prevent the formation of AMD and are difficult to implement in practice. While source control approaches are a focus for future research [5], a more realistic and common approach is to treat the generated AMD and leachates. Generation of AMD and leachate from tailings storage facilities at both active and historic mine sites is predicted to continue over a multi decadal time span and will therefore need corresponding long-term treatment. However, long-term treatment presents a difficulty, particularly where active treatment processes **Table 1** [6] require indefinite operational and maintenance inputs, which incur large costs, including long-term energy usage and treatment of metal rich sludges [7].

Given the costly, unsustainable and largely unknown operational timescale for active treatment processes for acid mine leachates, there has been a focus in the past 20+ years on passive treatment technologies, which tend to have lower capital


### **Table 1.**

*Summary descriptions of some active treatment processes for removal of heavy metals from acid mine leachates.*

construction costs and generally rely on gravity rather than pumped flow. They also require much lower operational and supervision inputs, although some level of maintenance will be required to ensure effective removal efficiency. Passive treatment technologies also do not require continuous chemical inputs and are therefore more sustainable than active treatment processes; however their ability to effectively treat mine waste streams in the long-term is largely unknown [7]. The key characteristics of passive treatment systems are their ability to produce alkalinity and to efficiently remove metals from the leachates. Some of the more promising passive treatment technologies are assessed below.

### **2.1 Neutralization**

Given that pH is an important influence on trace metal solubility, passive treatment by neutralization is sometimes used to remove metals from acid mine leachates. One such method is the installation of oxic or anoxic limestone drains where acidic mine leachate is directed through the bedding material and neutralized to a pH ≈ 6 by dissolution of the limestone. The alkalinity production and neutralization rates are important criteria when selecting the limestone, as a high carbonate content induces quicker neutralization rates. An operational drawback with limestone drains however is that long-term metal hydroxide precipitation tends to clog the limestone and reduce their flow capacity. This can be overcome somewhat by use of anoxic drains, which inhibit the formation of these precipitates; however, accumulation of other particulate material within the drains also contributes to reduced permeability of the drains over time [8]. In addition, removal of some metals, such as Zn and manganese (Mn), require a pH < 6, which is lower than the pH that can be naturally provided by passive limestone drains. In such cases, alternative or additional treatments such as Dispersed Alkaline Substrate (DAS) systems may be used. These may include application of fine-grained alkaline reagents to provide high neutralizing capacity, such as magnesium oxide (MgO) powder or limestone sand mixed with high porosity inert materials to ensure continuous flow through the medium [9, 10].

Other alkaline waste byproducts, such as fly ash (FA) and bottom ash (BA), flue gas desulphurization material, recycled concrete aggregate (RCA) and alkaline industrial byproducts have also been investigated as potential alternatives to traditional treatment materials. A leach test study to evaluate and compare the efficacy of RCA and FA in remediating AMD found that while RCA's were effective in neutralizing AMD and reduced concentrations of iron (Fe), Cr, copper (Cu) and Zn, FA's actually increased Fe, Cu and Mn concentrations. In addition, RCA with a higher calcium oxide content and finer grained particles had greater efficacy in increasing the pH and reducing concentrations of magnesium (Mg), Mn and Zn in the AMD [11]. In a separate laboratory based filtration study to investigate the effectiveness of alkaline industrial byproducts, namely drinking water treatment residuals (WTR's), to neutralize and remove metals from AMD, the authors reported irreversible removal of more than 99% of Fe, Al, Zn, lead (Pb), As, Mn and 44% of sulfate (SO4 2−) [12].

Natural neutralization of AMD has also been reported in two abandoned alum shale pit lakes, which originally contained acidic waters (pH < 4) with elevated levels of Na, K, Mg, calcium (Ca), Al, Mn, Fe, and sulfate. Inflow of leachates from an adjacent alkaline waste deposit gradually increased the pH from <4 to 8, which resulted in decreased concentrations of Fe, Al, Co, Ni and Zn in the lakes. However, accumulation of metal laden sediments in the lakes pose a long-term threat in the

*Sustainable Treatment of Acidic and Alkaline Leachates from Mining and Industrial… DOI: http://dx.doi.org/10.5772/intechopen.103889*

event that the lakes become re-acidified over time, leading to desorption/dissolution of trace elements. Thus, the long-term effects of changing lake chemistry in natural neutralization processes may alter the distribution and concentrations of trace elements with time in the lake outflows and this needs to be considered in the context of a sustainable solution [13].

### **2.2 Adsorption/biosorption**

Although considered by many as an active treatment process, adsorption, and in particular biosorption, is considered to be an efficient treatment method which uses abundantly available waste organic material and biomass to adsorb toxic contaminants, such as heavy metals, from wastewaters. Depending on the biomass used, biosorption of heavy metals occurs via electrostatic interactions or hydrogen bonding due to the formation of carboxyl groups on the adsorbents for binding cationic metals and amine groups for binding of either cationic or anionic metals [14]. Biosorption also offers the possibility of adsorbent regeneration and metal recovery while producing a minimal amount of chemical sludge. While many biosorption studies have examined the uptake of single metals with a variety of results (**Table 2**), adsorption of multiple metals is more difficult where competing ions reduce the capacity of the adsorbent to remove target metals [23]. For example, in a batch study to investigate the effectiveness of waste digested activated sludge (WDAS) as a biosorbent to remove and recover metals from AMD, the authors reported high (>70%) removal of V and Cu, and slightly lower (40–70%) removal of uranium (U), thorium (Th) and Cr; however the removal


### **Table 2.**

*Metal uptakes from acid mine drainage by a variety of natural biosorbents.*

rates were dependent on WDAS concentrations. The authors also noted that that there was no removal of Mn, Ni, Zn and yttrium (Y) at any WDAS concentration [24].

While many laboratory scale biosorption studies have been carried out for the removal of heavy metals from AMD, the development of full-scale biosorption treatment systems is at an early stage. Like many filtration systems, operational issues such as clogging of the adsorbent pore spaces and the need to recycle spent adsorbents are difficult issues to overcome and ultimately lead to increased maintenance. However, the possibility of resource recovery from such systems may eventually be a consideration for their full-scale development.

## **2.3 Constructed wetlands**

Constructed wetlands (CW's) are a passive wastewater treatment technology that combine biogeochemical and physical interactions between the wetland's soil matrix, vegetation and microbial communities. Constructed wetlands may be categorized in terms of their hydrology (whether surface or subsurface flow), their flow path (whether horizontal or vertical) and the type of macrophytic growth (whether free floating, submerged or emergent plant growth) [25]. When treating AMD, some or all of these components can be adjusted to suit the local and environmental conditions making CW's very flexible and efficient treatment systems. The key components and operating parameters for effective and efficient operation of a CW include: number of cells within the CW, substrate type and composition, plant type and planting density, hydraulic flow paths, hydraulic loading rate and hydraulic retention time. Wastewater pH is a key treatment indicator for AMD as it affects metal removal efficiencies. In a bench scale study to evaluate the performance of a CW using a mixed substrate of 75% soil, 20% powdered goat manure and 5% wood shavings, the pH of the AMD increased from 2.93 to 7.22 within 24 hours with corresponding enhanced removal rates for Fe (95%), Cu (90%), Zn (77%), Pb (89%), Co (70%), Ni (47%) and Mn (56%). In addition the sulfate content of the AMD decreased by an average 25% with an increase in alkalinity from 0 to 204 mg CaCO3 L−1 [26]. The authors attributed the sulfate reduction to the addition of biodegradable organic substrate to the soil (in the form of goat manure and wood shavings) which provided a carbon source for the anaerobic microbes to generate alkalinity, leading to sulfate reduction and associated metal removal.

Leachate metal removal by CW's include physical, chemical and biological processes which are both complex and interactive. The metal removing mechanisms include sedimentation, sorption, precipitation, cation exchange, photodegradation, phytoaccumulation, biodegradation, microbial activity and plant uptake [27]. During CW treatment of acidic and alkaline leachates, many metals are precipitated from solution, because of a change towards circumneutral pH (**Figure 1**). Once this happens, they settle through the liquid and into the substrate /sediment of the CW, provided flow conditions are sufficiently acquiescent. For example, Fe, Al and Mn can form hydroxides through hydrolysis and/or oxidation, which deposit in the substrate. The rate of change in pH varies as the effluent moves through the CW and this determines how quickly precipitation will occur and also the locations where most sedimentation occurs. If pH changes quickly it can be expected that metal accumulation in the sediment will occur at the inlet end of the CW and conversely if pH changes are slow then metal accumulation will be more dispersed. The retention time of CW's are therefore important design considerations. While settled *Sustainable Treatment of Acidic and Alkaline Leachates from Mining and Industrial… DOI: http://dx.doi.org/10.5772/intechopen.103889*

**Figure 1.** *Metal solubility versus solution pH for a selection of synthetic amphoteric metal solutions [unpublished data].*

metals will ideally remain in the sediment indefinitely, there is a risk that long term changes in the pH may result in resolubilisation and emission of high metals concentrations in the treated effluent. Such a risk is related to the composition of the substrate/sediment and the amount of organic matter it contains [26]. As well as precipitation and sedimentation, adsorption of the metals by the soil is probably one of the more significant metal removal processes in CW treatment. Metal adsorption occurs either by reversible cation exchange or by irreversible chemisorption. Adsorption to humic or clay colloids is more permanent than adsorption to soil organic matter which ultimately decomposes and releases the adsorbed metals back into solution. The role of biological processes for metal removal in CW's is an important one, and these are normally centered around the wetland plants. As well as providing direct uptake of metals from the wastewater, wetland plants generate organic particulate matter that contributes to sedimentation processes and to symbiotic bacterial processes. Plant species, particularly emergent macrophytes such as *Phragmites* or *Typha* species, also influence the rhizosphere. Root exudates and oxygen gradients within the sediment/substrate can facilitate diverse microbial communities that can influence the oxidation state in the sediment and partial pressures of CO2 or O2 in solution, influencing metal removal [28]. The rate of metal uptake by plants varies significantly, depending on plant type (emergent, surface floating, or submerged), species, density, and growth rates with maximum uptake observed in the roots [29].

The long-term performance of CW's to treat AMD is variable with effectiveness determined by variables such as metal types and concentrations in the influent and the quality and quantity of the wastewater. Typically, removal of the contaminants Fe, Al and Zn is highly effective in both the short (1–3 years) and longer term (c.10 years). However, other metals, such as Mn, are more problematic and lower removal rates have been reported [7].
