Legacy mining wastes have polluted and continue to pollute the environment on decadal to millennial timescales [1
]. For example, in the UK, one of the most significant metal pollution contributors to fresh waters is legacy non-ferrous metalliferous mines [4
]. Several thousand mines are known to be discharging environmentally deleterious quantities of metal/metalloid pollutants into surrounding watercourses [4
]. It has been estimated that nine percent of rivers in England and Wales, and two percent in Scotland are at risk of failing to meet their European Water Framework Directive targets of chemical and ecological quality because of legacy mines [4
]. These rivers carry some of the largest quantities of contaminant metals, such as As, Cu, Cd, Pb, and Zn, into the seas surrounding the UK each year. Similar situations exist in most other locations with a metal mining legacy worldwide. Some examples include the USA and Canada, which have approximately 35,000 and 10,000 legacy metal mine sites, respectively, Japan with approximately 5500 legacy metal mines, and Sweden with around 1000 legacy metal mines [6
As well as causing environmental degradation, mine sites can also represent opportunities for resource recovery [7
]. Resources can be present in many forms, including as metals that can be leached and recovered; decontaminated residue that can be reused (for example as aggregate); or as a landscape resource. This latter resource value is recognised by planning designations based on its cultural or ecological value and the increase in heritage tourism [8
]. These unique geological, ecological and cultural designations would act as significant constraints to mine waste remediation and site reclamation if the existence of these features was to be adversely affected by such activities [5
]. The metal resources in these historical deposits are often not insubstantial but are generally not sufficient to present (planning issues aside) a justification for intervention for resource recovery alone [8
]. However, if designed correctly, remediation of such sites could be implemented where economically valuable metals are also recovered and used to offset the costs of such remediation activity.
Leaching-based technology can be applied to achieve the removal of metals and metalloids (hereafter, “metals”) for the purpose of remediation and/or recovery of resource(s), either through ex situ in-vessel soil washing or via in situ percolation leaching, with the latter technology being preferable to reduce capital and operational costs (e.g., of material handling). Percolation leaching could be carried out on the waste in situ (dump leaching), or the waste could be excavated, agglomerated, and placed on an engineered liner for leaching (i.e., heap leaching).
However, if the lixiviant used has the potential to cause environmental damage through leakage or run-off from residues, then heap leaching and soil washing might have limited practicability due to actual or perceived risk. Towards the goal of leaching with more environmentally acceptable (“greener” see for example (e.g., [9
]) lixiviants, this study focused on the efficacy of organic acids in remediation/value recovery (e.g., reference [12
]) because of the advantage that the conjungate base is biodegradable which should limit the environmental impact of accidental spill/leakage (e.g., references [13
]) and might be directly useful in capture/recovery systems (e.g., to alleviate metal toxicity and act as an electron donor in sulphate-reducing bioreactors (e.g., references [16
]) or microbial electrochemical systems.
There are several reasons why a biodegradable lixiviant is preferable for in-vessel soil washing, in situ and/or in heap leach scenarios. For soil washing, whilst the main process is contained within tanks, leaks/spills can occur, but more perhaps more importantly, when waste is replaced, there is the risk of environmental degradation from residual acidic leachate run-off. With in situ leaching of legacy mine wastes, it is highly unlikely that there is an engineered liner under the waste. Thus, for in situ dump leaching of waste piles, either there has to be a high-level of confidence that the underlying rock is impermeable, or a liner needs to be retrospectively installed. This is possible through the injection of cement-based grouts and chemical grouts or jet grouting. However, given that the use of such a technique for creating an impermeable barrier should ideally be limited to homogenous soils [18
] and that even well engineered liner systems can leak, the use of biodegradable lixiviant would add confidence that should leakage occur, biodegradation will retard the transport of the escaping plume. This is particularly the case in areas of non-carbonate rocks where the effect of acid escape is far more serious because of the lack of acid-neutralisation. Furthermore, even if the mine waste is placed on to a highly engineered liner, leakage can still occur through pinholes and shrinkage cracks in geomembrane and clay liners, respectively. The strong mineral acids hydrochloric (HCl) and sulfuric acid (H2
) are conventional hydrometallurgical reagents, and the latter, in particular, is extensively used as a lixiviant in heap leaching of Cu and U ores. Environmental concerns have arisen from the failure to contain process solutions within the heap leach circuit, which is compounded by large surface areas, the use of open drainage trenches (rather than enclosed in pipework) [19
]. Its strong acidity and environmental persistence also dictates that even after leach pad decommissioning, the leachate from the spent ore (ripios) has to be carefully managed for many years until residual acidity has been fully flushed by meteroic water.
A further disadvantage of using sulfuric acid as a lixiviant is the subsequent formation of sulfate precipiates (e.g., jarosite and gypsum) that consume sulfuric acid whilst simultaneously resulting in unwanted permeability loss.
Strong organic acids, such as some sulfonic acids, are a class of strong acids with readily (bio)degradable conjugate bases that have been demonstrated as generally less environmentally persistent than mineral acids, such as sulfuric acid. Methanesulfonic acid, CH3
H (pKa = −1.9), has been demonstrated as being highly efficient for the dissolution of a number of different heavy metals via the formation of soluble methanesulfonate complexes [20
]. Furthermore, the properties of methanesulfonic acid, such as its high conductivity, stability against volatilisation and hydrolysis, and low corrosiveness [20
] are advantageous for its widespread use in hydrometallurgy and chemical engineering. Another advantageous property of methanesulfonic acid is the stability of reduced metal ions in methanesulfonic acid solutions, which is best known in the Sn2+
system. Furthermore, methanesulfonic acid has a comparable conductivity to hydrochloric and sulfuric acid (299.6, 346.1 and 444.9 S cm2
, respectively) allowing for efficient recovery of metals from solution using electrowinning. Despite these attributes, the application of methanesulfonic acid to metal recovery from tailings has not been widely explored, aside from leaching of rare earth elements from bauxite residue [21
As a weak acid, citric acid (C6
) partially dissociates in water to form hydrogen ions and its conjugate base, citrate, which is readily biodegradable in aerobic and anaerobic environments [12
] and is commonly used as a raw material in the manufacturing/food and beverage industry and thus, is readily available as a bulk commodity and may be public acceptable due to common use in food products. The role of citric acid in metal mobilisation from tailings has been investigated more widely than methanesulfonic acid, for example, by Burckhard et al. [22
], and the efficacy of citric acid in remedial application has been previously investigated in relation to the leaching of metal-contaminated soils, e.g., in references [12
], and for hydrometallurgical application in Ni-bearing lateritic/saprolitic ores [24
Here, we present a preliminary study that assesses the comparative efficacy of citric, hydrochloric, methanesulfonic and sulfuric acids for the recovery of economically valuable and/or contaminant metals from mine tailings waste taken from a legacy Cu/As mine in the southwest England. The work was established in order to demonstrate the feasibility of using such lixiviants for the recovery of toxic and/or economically valuable metals from metalliferous mine waste.
The results demonstrate that CH3SO3H has relatively similar ability to remove Cu and As from mine waste as H2SO4 and HCl. As such, it is suggested that CH3SO3H is a suitable alternative for soil washing and heap leaching. C6H8O7 is a relatively weak organic acid (pKa = 2.79) and complexing agent, and has been demonstrated as secondary to the other acids but is still effective for the dissolution of Cu. However, it was demonstrated as being able to exhibit relatively high removal of As from mine tailings. This result is of particular significance because weak organic acids, such as C6H8O7, are an attractive alternative to stronger acids due to their lower acidity and subsequent lower environmental impact.
In soil washing and dump/heap leaching applications, the recovery of economically valuable and/or contaminant metals and metalloids is the primary objective. Such interventions must be conducted, however, whilst also ensuring that the treated residue is chemically stable (e.g., a low tendency to release aqueous contaminants). Certain mineral acids, such as HCl and H2SO4, which have strong acidity and persistence in the environment are therefore not ideal lixiviants for such applications. In contrast, the use of organic acids, such as CH3SO3H and C6H8O7, which exhibit lower acidity, but also lower environmental persistence, could provide an alternative which would enable targeted metal recovery with a lower detrimental impact on the environment, particularly for non-carbonate lithologies, such as in this study, which are very susceptible to acidification. An assessment of the feasibility of any proposed percolation or in-vessel leaching process based on the lixiviants studied herein would have to be made in light of (i) the ability of the lixiviant to achieve the site-specific remedial target concentrations in the residue, and (ii) a site-specific environmental risk assessment.
This study has demonstrated that H2SO4, HCl, CH3SO3H, and C6H8O7 solutions can be applied for the recovery of both Cu and As from mine tailings taken from the Devon Great Consols mine in southwest Devon, England. The leaching potential of each acid was investigated under changing operational factors—acid concentration, leaching time, mixing intensity, and solid–liquid ratio—in order to determine the optimum conditions for metal recovery. The following can be concluded:
Cu and As dissolution rates were determined to typically increase with an increase in the acid concentration, mixing speed, and liquid to solid ratio.
HCl, H2SO4 and CH3SO3H generally exhibited relatively similar leaching abilities for As despite their different pKa values, with removal percentages after 48 h of 58%, 56%, and 55% recorded for 1 M H2SO4, HCl and CH3SO3H respectively, compared to 44% exhibited by C6H8O7.
H2SO4 was generally shown to be the most effective acid type for Cu removal with 38% removal for 1 M solutions after 48 h, compared to 32%, 29%, and 22% recorded for HCl, CH3SO3H, and C6H8O7 respectively.
Overall the optimum leaching conditions was found to be 1 M acid concentration, 200 RPM mixing speed and a mixing time of 24 h, with only minor improvements in leaching efficacy recorded for concentrations greater than 1 M or time periods greater than 24 h.
The results therefore suggest that processes, such as in-vessel soil washing or percolation leaching, could be relatively low-cost, and in the case of percolation leaching, non-invasive metal recovery techniques that enable simultaneous contaminant and/or economically valuable metal recovery from mine tailing waste. In particular, the use of organic acids, such as CH3SO3H and C6H8O7, could provide similar As and Cu removal efficacies to H2SO4 and HCl but with a potentially lower environmental impact, especially in dump and heap leaching applications for environmental remediation and particularly, in cases where host rocks have low neutralising potential, which are more susceptible to acidification by non-degrading mineral acids. This work also provides a foundation which substantiates further research into the relative environmental performances and other co-benefits of the application of organic acids in metal mine waste remediation and value recovery, particularly for methanesulfonic acid, which, to the authors’ knowledge, has not previously been applied to metal recovery from mine tailings.